Symposium EC4 : Materials, Devices and Systems for Sustainable Conversion of Solar Energy to Fuels

Mesoscopic photovoltaics have emerged as credible contenders to conventional p-n junction photovoltaics. Separating light absorption from charge carrier transport, dye sensitized solar cells (DSCs) were the first to use three-dimensional nanocrystalline junctions for solar electricity production, reaching currently a power conversion efficiency (PCE) of over 14% in standard air mass 1.5 sunlight. Meanwhile, large-scale production and commercial sales have been launched on the multi-megawatt scale. Recently, the DSC has engendered the meteoric rise of perovskite solar cells (PSCs) which have presently attained a power conversion efficiency (PCE) over 22 %, certified exceeding the PCE of polycrystalline silicon solar cells. Methylammonium lead iodide (CH₃NH₃PbI3) and related pigments have emerged as powerful light harvesters. Carrier diffusion lengths in the 100 nm to micron range have been measured for solution-processed perovskites. These photovoltaics shows intense electroluminesence matching the external quantum efficiency of silicon solar cells. As a consequence, very high Voc values close to 1.25 V for a 1.55 eV band gap material have been obtained under standard reporting conditions. These high Voc values render perovskite-based mesoscopic photosystem very attractive for use in tandem cells and for generation of fuels from sunlight mimicking natural photosynthesis.

Photoelectrochemical (PEC) and photovoltaic (PV) two-absorber tandem configurations are a promising concept for solar-driven water splitting into hydrogen and oxygen, enabling the generation of sufficient driving force for standalone water splitting while efficiently utilizing the sunlight spectrum. It generally consists of three parts: photocathode generating hydrogen, counter electrode generating oxygen (or vice versa), and PV component providing a bias voltage. Most recent works on these components of PEC-PV configuration have only been carried out on the small scale below 1 cm². Furthermore, there are no reports on the performance and characterization of large scale PEC electrode, until now. Research on the large scale development of PEC electrodes is important toward eventual implementation of solar-driven water splitting in the future.
Therefore, in this study, we focus on the large scale cuprous oxide (Cu₂O) photocathode with an active area of 50 cm² for PEC-PV tandem demonstrator because it has attracted much attention as a promising photocathode material due to proper band position for hydrogen generation, abundance, non-toxicity and scalability. Although our planar Cu₂O photocathode with AZO/TiO₂ overlayers and RuO_x hydrogen evolution reaction catalyst shows photocurrent density reaching up to almost 5 mA cm^-2 at 0V vs RHE in pH 5 electrolyte in the small scale, the performance was notably decreased with poor fill factor when scaled up to 50 cm² due to increased charge resistance. To improve the performance on the large scale, we design several types of large scale Cu₂O photocathode with metal grids, as well as one with Cu₂O nanowire structure. In addition, we study the large scale counter electrode with oxygen evolution reaction catalysts, which is also important in the PEC-PV tandem demonstrator for solar driven water splitting.

Various tandem cell configurations have been reported for highly efficient and spontaneous hydrogen production from photoelectrochemical (PEC) solar water splitting. However, there is a contradiction between two main requirements of a front photoelectrode in a tandem cell configuration, namely, high transparency and high photocurrent density. Here we demonstrate a simple yet highly effective method to overcome this contradiction by incorporating a hybrid conductive distributed Bragg reflector on the back side of the transparent conducting substrate for the front PEC electrode, which functions as both an optical filter as well as a conductive counter-electrode of the rear dye-sensitized solar cell. The hybrid conductive distributed Bragg reflectors were designed to be transparent to the long-wavelength part of the incident solar spectrum (λ>500 nm) for the rear solar cell, while reflecting the short-wavelength photons (λ<500 nm) which can then be absorbed by the front PEC electrode for enhanced photocurrent generation. The tandem device with the hybrid conductive distributed Bragg reflector shows unassisted hydrogen evolution with a solar-to-hydrogen conversion efficiency of 7.1%, which is the best performance reported to date for a PEC/solar cell tandem device.

Direct photoelectrochemical (PEC) hydrogen production aims to provide a clean and cost-effective solar fuel. Solar-to-hydrogen (STH) conversion efficiency is central to evaluating and comparing research results, and it largely establishes the prospect for successfully introducing commercial solar water-splitting systems. Present measurement practices do not follow well-defined standards, and common methods potentially impact research results and their implications. We demonstrate underestimated influence factors and experimental strategies for improved accuracy[1].

Our focus is tandem devices that have the prospect for greater STH efficiency[2], but increased complexity that requires more careful consideration of characterization practices. We perform measurements on an advanced version of the classical GaInP/GaAs design[3] while considering (i) calibration and adjustment of the illumination light-source; (ii) confirmation of the consistency of results by incident photon-to-current efficiency (IPCE), and (iii) definition and confinement of the active area of the device.

We initially measured 21.8% STH efficiency using a tungsten white-light source, a calibrated GaInP photovoltaic reference cell, and epoxy-encased photocathodes. In contrast, integrating experimental IPCE over the AM 1.5G solar irradiance showed that less than 10% STH conversion appeared conceivable. We then performed a set of on-sun measurements that gave 16.1% STH, before eliminating indirect light coupled to the sample by using a collimating tube and 13.8% STH efficiency thereafter. However, the value still vastly exceeded the current density expected according to the quantum efficiency measured via IPCE. Finally, suspecting that the illuminated area is poorly defined by epoxy, we use a compression cell for an epoxy-free area definition, resulting in 9.3% STH efficiency – a number also compatible with our IPCE results.

We propose applying the following standards for future PEC performance reporting: (i) traceable disclosure of the illumination-source configuration (lamp, filters, optics, PEC configuration) and/or its measured spectral distribution; (ii) thorough device-area definition (including confinement of the illumination area and avoidance of indirect light paths); (iii) complementary IPCE confirmation of the solar-generation potential; and (iv) proper consideration of faradaic efficiency.

[1] H. Döscher, J. L. Young, J. F. Geisz, J. A. Turner, and T. G. Deutsch, “Solar to hydrogen efficiency: Shining light on phoelectrochemical device performance,” Energy Environ. Sci. 2015.
[2] H. Döscher, J. F. Geisz, T. G. Deutsch, and J. A. Turner, “Sunlight absorption in water – efficiency and design implications for photoelectrochemical devices,” Energy Environ. Sci. 2014.
[3] O. Khaselev and J. A. Turner, “A Monolithic Photovoltaic-Photoelectrochemical Device for Hydrogen Production via Water Splitting,” Science. 1998.

With the escalation of energy consumption, consequent carbon dioxide footprint and fossil fuel reserves depletion, the worldwide nations are increasingly aware of the necessity to take advantage of renewable energy sources. Also, in the last years, new energy policies are being developed and some were already enforced aiming at more energy-sustainable behavior. This means that over the next decade, the energy supply of buildings will be increasingly dominated by renewable sources perfectly integrated in buildings, aiming at energy generation where it is consumed.
Among the different renewable energy sources, solar energy can easily provide enough power for all of human needs if it can be efficiently harvested[1]. However, its intermittent nature and its increasing contribution to electricity generation are craving for efficient storage technologies. There are several battery technologies that can be used for electric power storage but redox flow batteries (RFBs) are a very at-tractive technology for their low cost, high charge-discharge energy efficiency, fast response time, modularity and flexibility[2].
In this work, it is proposed an innovative storage system, based on the combination of RFB with photoelectrochemical (PEC) cells – Solar-RFB. Two different systems were explored. In the first system, the possibility of storing solar energy in a simple way using a vanadium redox flow battery and a CdS photoanode is demonstrated; unbiased charged of a solar redox flow battery (RFB) (CdS(s)|V³⁺, VO²⁺||V³⁺, V²⁺|Carbon Felt(s), E⁰ = 0.6 V^NHE) was achieved[3]. Different strategies to improve the low CdS stability were explored and CdSe protective overlayer had a twofold increase with photocurrents up to 1.4 mA cm^–2. A tandem system battery using a dye-sensitized solar cell (DSC) was also studied to provide the necessary photovoltage to charge a standard all vanadium redox flow (DSC/CdS(s)|VO²⁺, VO₂⁺||V³⁺, V²⁺|Carbon Felt(s), E⁰ = 1.2 V_NHE); in this configuration, the observed photocurrent was 1 mA cm^–2.
For practical applications, non-acidic electrolytes combined with stable and efficient semiconductors are of critical importance. A new RFB was developed based on anthraquinone-2,7-disulphonate and ferrocyanide redox pairs[4]. This new battery displays high energy and coulombic efficiencies. It is shown the possibility of direct solar charging ferrocyanide/anthraquinone redox pairs in an alkaline RFB with a hematite photoanode, which is low cost and stable. Novel surface treatments of the hematite photoanodes showed improved performance. The use of organic and inorganic redox pairs in alkaline and acidic aqueous media offers a wide range of possibilities to form synergetic combinations with different semiconductors for direct solar charging RFBs.

References:
1. D. Abbott, Proc. IEEE. 98 (2010) 42–66.
2. A.Z. Weber et al., 41 (2011) 1137–1164.
3. J. Azevedo et al., Nano Energy. 22 (2016) 396–405.
4. K. Wedege et al., Angew. Chem. (2016) 1–9.

Solar to fuel conversion could provide an alternative to mankind’s currently unsustainable use of fossil fuels. Solar fuel generation by photoelectrochemical (PEC) methods is a potentially promising approach to address this fundamental challenge.
The key materials science challenges that need to be addressed to enable practical and scalable solar fuel devices will be outlined. The analysis will focus on a fundamental requirement of any sustainable solar conversion technology, which is to generate more energy over its useful lifetime than was required to manufacture and maintain it. In this analysis, both solar to fuel conversion and device lifetime are crucial. For example, in the case of PEC-driven solar water splitting, using very general assumptions regarding the primary energy cost of manufacturing, operational lifetimes on the order of years are required for a positive energy payback [1].
The reported laboratory solar to hydrogen (STH) conversion efficiencies of PEC water splitting devices range from <1% to over 20% [2,3]. However, there are very few reports of operational stability beyond a few days. While there has been little mechanistic work on the precise failure mechanisms, corrosion of the photoactive material, particularly for oxygen-evolving photoanodes, is frequently observed. The use of carrier-selective protection layers is one strategy to mitigate this effect [4,5], and the prospects of achieving long-term operation with this approach will be discussed.
There has been substantial recent progress in solar driven PEC CO₂ reduction, with some reports of energy conversion efficiencies exceeding 5% [6]. Similar to the case of PEC water splitting, an approach with long-term operational stability has not yet been demonstrated. The devices operate in near-neutral pH as opposed to the extreme acid or basic conditions typically used in PEC water splitting, which relaxes some materials constraints. However, new materials challenges arise. For example, the large overpotentials required on the cathode (or photocathode) require reductively stable materials. Also, many CO₂ reduction catalysts (e.g. Cu) deactivate on time scales much shorter than would be required for a positive energy return. Substantial improvements in operational lifetime are required for PEC-based CO₂ reduction to be sustainable.

This material is based upon work performed by the Joint Center for Artificial Photosynthesis, supported through the Office of Science of the U.S. Department of Energy. Support from the Singapore Berkeley Research Institute for Sustainable Energy (SinBeRISE) is also acknowledged.

(1) Sathre et al., Energy Environ. Sci. 2014, 7, 3264–3278; Energy Environ. Sci. 2016, 9, 803.
(2) Ager et al. Energy Environ. Sci. 2015, 8 (10), 2811–2824.
(3) Rongé et al. Chem. Soc. Rev. 2014, 43, 7963–7981.
(4) Chen et al. J. Am. Chem. Soc. 2015, 137, 9595.
(5) Hu et al. J. Phys. Chem. C 2015, 119, 24201.
(6) Schreier et al. Nat. Commun. 2015, 6, 7326.

Widespread deployment of solar fuel generators requires the development of efficient and scalable functional materials, especially for photoelectrocatalysis of the oxygen evolution reaction. Metal oxides comprise the most promising class of photoanode materials, but no known material meets the demanding photoelectrochemical requirements. Copper vanadates have recently been identified as a promising class of photoanode materials with several phases exhibiting an indirect band gap near 2 eV and stable photoelectrocatalysis of the oxygen evolution reaction in a pH 9.2 electrolyte. By employing combinatorial inkjet printing of metal precursors and applying both calcination and rapid thermal processing, we characterize the phase behaviour of the entire CuO-V2O5 composition space for different thermal treatments via automated analysis of approximately 100 000 Raman spectra acquired using a novel Raman imaging technique. These results enable the establishment of structure-property relationships for optical absorption and photoelectrochemical properties, revealing that highly active photoelectrocatalysts containing alpha-Cu2V2O7 or alpha-CuV2O6 can be prepared using scalable solution processing techniques. An additional discovery results from the formation of an off-stoichiometric beta-Cu2V2O7 material that exhibits high photoelectroactivity in the presence of a ferri/ferrocyanide redox couple with excellent stability in a pH 13 electrolyte, demonstrating that copper vanadates may be viable photoanodes in strong alkaline electrolytes.

We have shown that p-type CuRhO₂ (E_g = 1.9 eV) is an excellent photocathode for H₂ evolution.¹ With a band structure that straddles both water reduction and oxidation, CuRhO₂ enables water reduction at an underpotential of ~0.2 V and possesses a strong resistance to being reduced to metallic copper in air-saturated basic electrolyte. Because of its promising intrinsic properties, improving the photoactivity of CuRhO₂ by hole-doping is of significant interest. A series of doped CuRhO₂ samples were synthesized by reacting various ratios of CuO and Rh at 900 °C for 36 hours under flowing mixed gas atmosphere (O₂:Ar = 1:99) with various amounts of Mg(OH)₂ or RuO₂, after which time their identities were confirmed by powder X-ray diffraction (XRD). The dopant was probed using X-ray photoelectron spectroscopy (XPS). Hall effect mobilities and four probe resistivity measurements were used to determine carrier concentrations, which were shown to be correlated with photoresponses. Depending on the dopant identity, the photocatalytic activity of CuRhO₂ is found to either increase or decrease. This finding provides insight into the mechanism of water splitting on this and related delafossite materials.

1. Gu, J.; Yan, Y.; Krizan, J. W.; Gibson, Q. D.; Detweiler, Z. M.; Cava, R. J.; Bocarsly, A. B. J. Am. Chem. Soc. 2014, 136, 830.

Thin films of copper vanadate (CVO), a new candidate as an n-type semiconductor photoanode for driving the oxygen evolution reaction (OER), were prepared via reactive co-sputtering in an oxygen environment. A series of different CVO phases were synthesized by varying the Cu:V stoichiometric ratio during the sputtering process. Among these phases, Cu₃V₂O₈ (McBirneyite) and Cu₁₁V₆O₂₆ (Fingerite) possess promising characteristics for photoelectrochemical applications, including (photo)chemical stability, visible light absorption, and low dark current. Under PEC testing conditions, both Cu₃V₂O₈ and Cu₁₁V₆O₂₆ generated anodic photocurrent responses when illuminated with simulated solar radiation and remained stable for at least 20 h of continuous illuminated operation at 1.23 V vs. RHE in pH 9.2 buffer solution. However, obtained photocurrent densities were significantly lower than anticipated based on their bandgaps, even in the presence of sacrificial hole acceptor. In order to understand PEC performance limitations, measurements of basic optical and electrical properties, together with photocarrier dynamics, were performed. Variable angle spectroscopic ellipsometry was used to establish optical constants and determine that both phases possess indirect bandgaps, making them relatively weak absorbers across the visible spectral range. The relaxation dynamics of excited-state carriers in CVO films were studied via time-resolved spectroscopy. The transient absorption signals of free carriers were characterized by dominant sub-picosecond relaxation kinetics associated with rapid carrier trapping processes. Consistent with this finding, PEC response measurements as a function of film thickness revealed an upper bound for the minority carrier diffusion length of ~50 nm. As a result of mismatch between optical absorption depth and minority carrier diffusion length, low charge extraction efficiencies at the semiconductor/electrolyte interface are obtained. However, ongoing work is dedicated to determining the origin of rapid photocarrier trapping processes; elimination of this relaxation channel could greatly enhanced PEC performance characteristics. In addition, differences between the native catalytic activities of Cu₃V₂O₈ and Cu₁₁V₆O₂₆ were observed. Electrodeposition of cobalt borate (CoBi), a known OER catalyst, onto CVO photoelectrodes yields favorable shifts of photocurrent onset potentials and increases in photocurrent densities, suggesting that the semiconductor interface is defect tolerant.

The design of highly efficient, nonbiological energy conversion “machines” that generate fuels directly from sunlight, water, and carbon dioxide is both a formidable challenge and an opportunity that, if realized, could have a revolutionary impact on our energy system and efforts to address climate change. In the past five years, considerable progress has been made in scientific discovery of key materials and mechanisms needed to realize artificial solar fuels generators for hydrogen generation by water splitting, and advances in materials, modeling and design have yielded a conceptual paradigm for a solar fuels generator and working prototypes. While we still lack sufficient knowledge to design solar-fuel generation systems with the ultimate efficiency, scalability, and sustainability to be economically viable, considerable advances have been made, particularly for water-splitting solar fuels devices. Recent progress from JCAP, and practical limits to water splitting devices will be discussed. Understanding of CO₂ reduction catalysis is an outstanding scientific challenge for artificial photosynthesis structures for generation of fuels and chemicals by reduction of CO₂. I will discuss recent advances by JCAP in understanding approaches to achieving selectivity and activity in CO₂ reduction electrocatalysis and photocatalysis.

Artificial photosynthetic systems can store solar energy and chemically reduce CO₂. We developed a hybrid water splitting-biosynthetic system based on a biocompatible earth-abundant inorganic catalyst system to split water into H₂ and O₂ at low driving voltages. When grown in contact with these catalysts, Ralstonia eutropha consumed the produced H₂ to synthesize biomass and fuels or chemical products from low CO₂ concentration in the presence of O₂. This scalable system has a CO₂-reduction energy efficiency of ~50% when producing bacterial biomass and liquid fusel alcohols, scrubbing 180 grams of CO₂ per 1 kWh of electricity. Coupling this hybrid device to existing photovoltaic systems would yield a CO₂ reduction energy efficiency of ~10%, exceeding natural photosynthetic systems.

The majority of photoelectrochemical (PEC) water splitting cells cannot drive the overall water splitting reactions without the assistance of an external power source. In order to provide added power, the cells are usually connected to photovoltaic (PV) devices in a tandem arrangement or to external power sources. These two methods suffer from severe disadvantages. In the tandem arrangement, the PEC cell is connected in series to the PV cell and the overall current is typically limited by the saturation current of the PEC component. Thus, the operating point of the PV cell is often far from optimal and the overall system efficiency tends to be low. Alternatively, when PV cells are connected to the PEC cells through a DC-DC converter, the operating point of each sub-system can be chosen independently. However, this comes at the cost of increased losses and additional balance of system costs associated with the DC/DC conversion. To overcome the limitations imposed by these two traditional configurations, we propose a multi-terminal hybrid PV and PEC system (HPEV). As for the case of tandem arrangements, the PEC cell is optically connected in series with the PV cell. However, a second back contact is used to extract the PV cell surplus current and allow parallel production of both electrical power and chemical fuel. This multi-terminal approach allows each of the electrical and chemical components to operate at nearly independent electrical operating points, thereby increasing the overall solar energy conversion efficiency of the system. Furthermore, the amount of fuel or electricity produced can be chosen in real-time according to demand. Devices consisting of three-terminal silicon photovoltaic cells coupled to titanium dioxide water splitting layers are simulated and fabricated. The cells are shown to produce electricity with little reduction in the water splitting current, surpass the current mismatch limits, and increase the overall system efficiency.

The direct conversion of solar energy into fuels, H₂ in particular, is still a challenge. Recently, organic and hybrid organic-inorganic photoelectrochemical systems emerged as an alternative to the usual transition metal oxides or more costly III-V semiconductors. Here we present different suitable architectures for hybrid organic-inorganic photoelectrochemical devices for the conversion of solar energy to H₂. Starting from a prototypical P3HT:PCBM blend as photoactive element, we focused our attention on different interfacial layers and their influence on the photocathode performances. The photoelectrocatalityic activity and long-term stability of a simple, catalysed, bulk heterojunction is proven and the effect on hydrogen generation performances of properly engineered selective contacts is investigated. Introduction of an electron selective layer is found to increase the photocurrent response, while a hole blocking layer shifts the onset potential towards positive voltages allowing operation in a electrical region compatible with a tandem photoanode and/or a PV cell. The relevance of our findings can be summarized in few key points: (i) high performances with a maximum photocurrent of 8 mA/cm² at 0 V vs RHE and 50% IPCE; (ii) 100% faradaic efficiency along the whole electrode’s lifetime; (iii) excellent energetics with onset potential as high as +0.7 V vs RHE; (iv) promising operational activity of several tens of hours and (vi) by-design compatibility for implementation in a tandem architecture [Fumagalli et al. JMCA (2016)]. Moreover, we investigated the influence of photocathode nanostructuration, developing a 3D multi-layered system based on a nanostructured hole selective scaffold, in host/guest architecture. This allowed us to demonstrate efficient devices with an active layer thickness as low as 20 nm, thanks to the orthogonalization of light absorption and carrier collection. Finally, we demonstrate the realization of an efficient and cost-competitive “all-solution processed” photocathode with and without a non-precious catalyst. Such a system exhibits 4mA/cm² at 0 V vs RHE, an onset potential of 0.4 V at RHE and stability of over 1hr. Collectively, this set of features establish the hybrid architecture we developed well ahead of existing reports on organic photoelectrochemical systems and suggest the potential of the hybrid organic-inorganic photoelectrochemical (HOPEC) concept as real contender to the traditional inorganic counterpart. From the knowledge learnt, we anticipate that stable operation at photocurrents close to the maximum achievable with OPV, beyond 10 mA/cm² at positive bias, are achievable. This work opens up the way to the exploration of the rich library of organic semiconductors developed for OPVs in photoelectrochemistry and to the realization of a new generation of large area, solution processed tandem water splitting devices for renewable and low cost direct conversion of solar energy into hydrogen.

CO₂-derived fuels present an attractive way towards a sustainable energy system. Mimicking natural photosynthesis by synthesizing carbon-based energy carriers using power from the sun allows for closing the anthropogenic carbon cycle and therefore represents an attractive way to store solar energy, a challenge that has not yet found a satisfying solution.
The widespread use of solar fuels will require large surfaces of absorbers and catalysts, which should therefore be fabricated from abundant materials. In this context, we show the application of low-cost and scalable Cu₂O photocathodes in combination with molecular rhenium catalysts, both in solution¹ and covalently bound to a modified photoelectrode surface.² From both approaches, we observe substantial photocurrents and photovoltages, demonstrating protected Cu₂O photocathodes as viable candidates for solar-driven CO₂ reduction processes. Investigating the charge transfer behaviour on these systems allows us to observe unexpected effects, which provide insight into the mechanism of Re-based CO₂ reduction catalysts.
Moving from organic solvents into aqueous systems, we demonstrate the unassisted and sustainable splitting of CO₂ into CO and O₂ using CH₃NH₃PbI₃-based photovoltaics as light absorbers, reaching an efficiency of 6.5 %.³ Building up on this work, we show how ALD modification of CuO nanowires can lead to a bifunctional and low-cost catalyst both for CO evolution from CO₂ and for the oxygen evolution reaction. By ALD modification, the wide product distribution of Cu-based catalysts could be impressively narrowed to yield predominantly CO. Investigations into the microkinetics on these electrodes indicate that the observed change is due to the suppression of H₂ evolution, while the rate of CO production remained similar. Together with the use of a bipolar membrane, allowing for separating product gases while maintaining a sustained pH gradient, we used these electrodes to demonstrate long-term solar CO production at an efficiency of 14.2 %, driven by a single 3-junction GaInP₂/GaAs/Ge photovoltaic.⁴
Another way to modify the selectivity and activity of CO₂ reduction electrodes has been identified in the use of co-catalysts. By investigating structure-activity relationships between imidazolium co-catalysts and CO₂ reduction activity on silver cathodes, we discovered that the imidazolium C4 and C5 protons play an important role, extending the state-of-the-art understanding of the mechanism of this process.⁵ These results will also be presented.

(1) Schreier, M. et al. Energy Env. Sci 2015, 8 (3), 855.
(2) Schreier, M. et al. J. Am. Chem. Soc. 2016, 138 (6), 1938.
(3) Schreier, M. et al. Nat. Commun. 2015, 6, 7326.
(4) Schreier, M. et al. in preparation
(5) (Lau, G. P. S.; Schreier, M.)‡ et al. J. Am. Chem. Soc. 2016.
‡ equal contribution

Processes that convert CO₂ into fuels and chemicals using reducing equivalents from renewable energy are essential for building an environmentally benign energy economy. In order to be implemented on a significant scale, these methods must be competitive with fossil fuel–based syntheses. To this end, the key chemical challenge is to convert CO₂ into multi-carbon (C₂₊) products because these targets have higher value, greater energy density, and more applications than C₁ compounds. Our strategy is to leverage existing solar energy conversion processes and develop light-independent CO₂ conversions that make C–C bonds. This talk will highlight recent results in our development of heterogeneous CO₂ reduction catalysts. I will describe “defect-rich” nanoparticles for electrochemical CO₂ and CO reduction and carbonate-based catalysts for CO₂ hydrogenation. These catalytic materials provide complementary routes from CO₂ to high-value C₂₊ oxygenates. The fundamental materials chemistry will be discussed in the context of the challenges that must be addressed to develop scalable technologies.

The electrochemical conversion of CO₂ into carbon-based fuels is an attractive strategy for utilizing atmospheric CO₂. For achieving this goal, the essential step is to develop a cheap, stable, and efficient catalyst with high selectivity for a desired product. Over the past few decades, several catalyst materials with the capability of reducing CO₂ electrochemically in CO₂ saturated-aqueous solutions have been identified. It has been demonstrated that polycrystalline Au is capable of reducing CO₂ to CO with a high faradaic efficiency (FE) of ~ 87% at -0.74 versus the reversible hydrogen electrode (RHE). While Au is currently the most efficient electrocatalytic surface for CO₂ reduction to CO, the low abundance and high cost of Au may prevent its large-scale applications. To find a cost-effective and stable catalyst with high selectivity and efficiency remains a challenge for achieving practical utilization of CO₂ reduction to CO.
In this work, the selective electrocatalytic reduction of CO₂ to CO on oxide-derived Ag electrocatalysts is presented. By a simple synthesis technique, the overall high faradaic efficiency for CO production on the oxide-derived Ag was shifted by >400 mV towards a lower overpotential compared to that of untreated Ag. Notably, the Ag resulting from Ag oxide is capable of electrochemically reducing CO₂ to CO with approximately 80% catalytic selectivity at a moderate overpotential of 0.49 V vs. RHE, which is much higher than that (~4%) of untreated Ag at identical conditions. Electrokinetic studies show that the improved catalytic activity is ascribed to the enhanced stabilization of COOH* intermediate. Furthermore, a highly nanostructured Ag is likely able to create a high local pH near the catalyst surface, which may also facilitate the catalytic activity for the reduction of CO₂ with suppressed H₂ evolution.
In addition, the effect of Cu nanowire morphology on the selective electrocatalytic reduction of CO₂ is presented. Cu nanowire arrays were prepared through a two-step synthesis of Cu(OH)₂ and CuO nanowire arrays on Cu foil substrates and a subsequent electrochemical reduction of the CuO nanowire arrays to Cu nanowire arrays. By this simple synthesis method, Cu nanowire array electrodes with different length and density were able to be controllably synthesized. We show that the selectivity for hydrocarbons (ethylene, n-propanol, ethane and ethanol) on Cu nanowire array electrodes at a fixed potential can be tuned by systematically altering the Cu nanowire length and density. The nanowire morphology effect is linked to the increased local pH in the Cu nanowire arrays and a reaction scheme detailing the local pH induced formation of C₂-products is also presented via a preferred CO dimerization pathway.
Overall, we show two disntict systems that offer selective and efficient electrochemical reduction of CO2 to CO.

Copper (I) based catalysts, such as Cu₂X (X=S,Se), are considered to be very promising materials for their light harvesting properties and possible applications in photocatalytic CO₂ reduction [1]. A common synthesis route for Cu₂X via cation exchange from CdS nanocrystals requires Cu (I) precursors and restrictive conditions, such as organic solvents and neutral atmosphere.

We report a novel cation exchange method in which we harness the reducing potential of photoexcited electrons in the conduction band of CdX nanocrystals to fabricate Cu₂X nanostructures from CdX using Cu²⁺ precursors. In contrast to other cation exchange methods, this photoinduced process can proceed in an aqueous environment and under aerobic conditions, yet preserves the shape and the crystallinity of the original crystals and enables complete conversion of CdX nanocrystals [2].

We demonstrate that the as-prepared Cu₂S nanorods can be efficiently used for the reduction of CO₂ to carbon monoxide and methane. At the same time the, the competing water reduction, known to be very efficient for CdS [3], is suppressed.

We also show the applicability of the light-induced approach to preparation of other shapes of Cu₂X nanocrystals, e.g. quantum dots [4]. The process opens new pathways for the preparation of new efficient photocatalysts from readily available nanostructured templates.

References
[1] S.N. Habisreutinger, L. Schmidt-Mende, J.K. Stolarczyk, Angew. Chem. Int. Ed. 2013, 52, 7372-7408.
[2] A. Manzi, T. Simon, C. Sonnleitner, M. Döblinger, R. Wyrwich, O. Stern, J.K. Stolarczyk, J. Feldmann, J. Am. Chem. Soc. 2015, 137, 14007–14010.
[3] T. Simon, N. Bouchonville, M.J. Berr, A. Vaneski, A. Adrovic, D. Volbers, R.Wyrwich, M. Döblinger, A.S. Susha, A.L. Rogach, F. Jäckel, J.K. Stolarczyk, J. Feldmann, Nature Mater. 2014, 13, 1013-1018.
[4] A. Manzi, F. Ehrat, et al. in preparation

Electrochemical reduction of carbon dioxide (CO₂) is one of the possible candidates to combat with the exhaustion of fossil fuels by creating useful hydrocarbons from renewable energy sources and wasted CO₂. One of the recent research trends of CO₂ reduction has been nanoparticle or nanostructured catalyst, especially combined with carbon support. However, there is a problem regarding the product selectivity of such catalysts, which tend to favor hydrogen (H₂) production compared to that in planer form. Therefore, there need to reduce competing hydrogen production by developing a novel catalyst structure to deliver sufficient CO₂ to the catalyst. Also, the simplicity for the fabrication process is important in terms of practical application of the nanoparticle catalysts.
Here, we report on copper-nanoparticles embedded in carbon nanofiber (Cu/CNF) as a catalyst designed to ensure both the catalytic property of Cu and the mass-transport of CO₂ through the catalyst with simple fabrication procedure. Cu/CNF catalyst was fabricated with electrospinning method from the mixture of CuO nanoparticles (average diameter: 20 nm) and polyimide precursor, where the ratio of CuO nanoparticles in CNF was 1.5wt%. After the electrospinning process, the catalyst was calcinated in argon atmosphere at 800 °C. During the calcination, CuO was reduced to Cu and nanofiber was carbonized. 0.5 M potassium chloride (KCl) solution was used for the electrolyte of CO2 reduction.
First, we compared the result of Cu/CNF under constant potential at -1.7 V vs. Ag/AgCl with that in planer Cu electrode and CNF (without Cu nanoparticles) of the same geometric surface area. The operation currents of Cu/CNF and Cu plate were the same order, whereas that of CNF was one-magnitude lower. In addition, Cu/CNF catalysts exhibited similar product distribution as that in Cu plate. After an hour of experiment, Cu nanoparticles were still stabilized at the surface of CNF though slight aggregation was observed. This work will enlarge the possibility of Cu-based catalyst for the application to more practical CO₂ reduction system such as PEM-like and/or gas diffusion reactors. In the presentation, we will discuss the mechanism for CO₂ reduction associated with the unique characteristic of CNF for CO₂ adsorption.

Photoelectrochemical conversion of CO₂ using semiconductor photocatalysts is an efficient method for generating hydrocarbon fuels using renewable energy. This can be a promising alternative for fossil fuels while reducing carbon dioxide emissions, a major contributor to the greenhouse effect and global warming. Carbon dioxide’s reduction potential is only 20 mV positive of water reduction, hence carbon dioxide reduction competes with hydrogen generation. An ideal catalyst should then have a high hydrogen overpotential, allowing a higher selectivity for the reduction of carbon dioxide. Here, we report the use of titania nanotubes decorated with copper nanofibers for the selective photocatalysis of carbon dioxide to hydrocarbon fuels. Copper nanofibers were fabricated using the electrospinning technique. The fibers were collected over titania nanotubes and then annealed in oxygen at 500 °C for 4 hours. After annealing, the polymer decomposed leaving a porous-clustered copper nano-structure. The Cu nanofibers-decorated titania nanotubes were prepared via a facile method that is applicable for large-scale industrial applications. The photocatalytic performance of the material was measured using a three electrode cell, where carbon dioxide gas was bubbled for 30 min until saturation in 1 M KHCO₃ solution. Under illumination of the sample with a 500 W xenon light source; light intensity, 100 mW cm^-2, the reduction peak showed a positive shift. This confirms the photo-enhanced CO₂ reduction on Cu nanofibers-decorated titania nanotubes. The results of this study confirm that the fabrication of nanofibers as a photocatalyst via electrospinning is a promising approach to catalysis for solar driven applications.

Solar fuels have been considered as one of the most promising technological concepts due to their high potential use and environmental suitability. The development of devices able to photocatalytic split water into O₂ and H₂ is currently of primary interest. In such framework, in 2011, the EUropean FCH JU Initiative financed the ARTIPHYCTION project (contract nr. 303435) with the aim to build a water splitting prototype, able to maximise the fraction of the solar light spectrum that can be captured and converted into hydrogen (ca. 3 g/h H₂), with a targeted solar to hydrogen (STH) conversion efficiency of 5%, a prospected system durability of 10.000 h lifetime, and without using expensive noble metals or materials and via assembling techniques amenable for mass production.
The final Artiphyction prototype (area of 1.6 m²) was designed, optimized, manufactured and tested in the last six months of the project. Key features of the ArtipHyction photo-electro-chemical (PEC) device will be pointed out in this talk as listed in following:
- Efficiency target partially (50% - 66%) achieved: The best results with a CoPi-catalysed Mo-doped BiVO₄ photo-anode and a Co nanoparticles-based cathodic electro-catalyst in the Final Artiphyction prototype show a potential of 3% overall sunlight into hydrogen conversion efficiency. However, due to mass-transfer and kinetics limitations phenomena (bubbles formation and accumulation in the electrodes surface) the performance decrease up to about 2% conversion efficiency during long-term operation.
- Life testing fully achieved: The final prototype module constituted by 5 PEC-PV units has been tested for 1000h and a limited reduction of efficiency (less than 5%) has been observed when operating at a 2% STH efficiency. Indeed, a trade-off between STH efficiency and stable operation has been pointed out in order to maintain the stability with the prospective durability of 10,000 h.
- No use of noble metals achieved: Attention have been addressed mostly on the anode water-splitting side of the cell were noble metal catalysts are not present. The synthesis techniques (i.e. spin-coating, electrodeposition, spray coating) adopted for BiVO4 and Co-based anodic and cathodic catalysts are amenable for mass production.
- Modularity achieved to fit various production needs: Special attention was paid on manufacture a modular system. The final ARTIPHYCTION prototype is composed of 20 modules, each of them with 5 PEC-PV units that can be assembled to reach different total areas.
- 100 W (ca. 3 g/h H₂ equivalent) partially achieved (66%): The system was proved to be able to operate at 3% of STH efficiency, but a strategic decision was taken in favour of the prototype performance stability, limiting the STH efficiency to 2%. Therefore, the maximum overall H₂ production of the prototype is slightly higher than 1 g/h.
- The objective of scaling up the photo electro-chemical conversion to 1.6 m² irradiated surface was fully achieved.

The hydrogen molecule has many important uses in industry and will play a key role as a clean, sustainable energy vector and effective means of storing energy. On Earth, hydrogen is mainly found in water and organic compounds such as hydrocarbons and carbohydrates. Currently, about 96% of the global annual production of hydrogen is obtained from fossil fuels, primarily by high temperature steam methane reforming. In this process, hydrogen is liberated with formation of CO₂ emissions. In current industrial plants, about 10 kg of CO₂ is released per kg of hydrogen produced. Developing new H₂-production processes based on renewable feedstock and energies will be a key contribution to energy transition and abatement of CO₂.
With its Blue Hydrogen commitment, Air Liquide is moving towards a gradual decarbonization of its hydrogen production dedicated to energy applications. The splitting of water achieved with solar energy and semiconductor materials is a route for generation of renewable hydrogen with less carbon footprint. Many pathways at different technology readiness levels exist for the absorption of photons and for the electrocatalytic water splitting. The simplest sun-to-hydrogen pathway is the combination of photovoltaic cells with an electrolyzer interfaced with power conditioning device. In another approach based on photoelectrochemical water splitting, the photon absorption by semiconductor material creates a pair of electron-hole but instead of collecting electron and using it in an external load, electrons and holes are directly transferred to aqueous electrolyte to respectively reduce/oxidize water to hydrogen/oxygen. This solar-to-hydrogen pathway has received great attention but many issues and challenges remain to have it efficient and cost competitive.
The aim of this paper is to present the industrial view of the different approaches of photoelectrochemical water splitting with semiconductors for H₂ production, to discuss recent progress and global activity, and to provide a relative performance of the most important identified technologies. Recommendations in areas where research efforts should be continued will be proposed.

NiMo and NiFe catalysts have been explored for the hydrogen and oxygen evolution reaction. By combining the two catalysts, full water splitting was demonstrated with a potential as low as 1.55 V-1.60V at 10 mA/cm2. The NiMo and NiFe catalysts were combined with silicon technology into devices for solar hydrogen production. Both spatially separated PV-electrolysers and integrated monolithic systems were constructed. A reasonably cheap, stable, scalable, monolithic, earth abundant system with a solar-to-hydrogen (STH) efficiency above 10 % was demonstrated. With further engineering and optimisation, STH-efficiencies as high as 15 % are within reach. We further analyse how the costs compare between the different device architectures and show that even if these systems today not can compete with the hydrogen cost from steam reforming of natural gas, they are competitive compared to what previously have been seen in the solar hydrogen community.

In the efforts to strengthen Europe’s position in the field of solar fuels, the PECDEMO consortium aims at developing a hybrid photoelectrochemical-photovoltaic (PEC-PV) tandem device for solar water splitting. The project targets solar-to-hydrogen (STH) efficiency of 8-10%, large scale (≥ 50 cm²), and long-term stability (1000 hours). Despite the emergence of several devices with reported efficiency beyond 10%, they are limited to small-area electrodes (~1 cm²) and short-term stability. We focus on improving known high-performing metal oxide photoelectrodes (e.g., BiVO₄, Cu₂O, Fe₂O₃) deposited with scalable methods. These oxides are then combined with a solar cell using innovative photon management strategies and cell design to form a highly efficient PEC-PV tandem device.

In this talk, specifically, the progress in developing BiVO₄ thin film photoelectrodes will be outlined. By combining doping strategies, hydrogenation treatments, and photon management, we obtained an AM1.5 photocurrent of ~5 mA/cm² at 1.23 V vs RHE, which is one of the highest reported value for a non-nanostructured BiVO₄. The role of hydrogenation was also investigated by means of time-resolved microwave and DC conductivity, and contrasted to typical donor doping (e.g., with W or Mo). This high-performing thin film photoelectrode was coupled with a silicon solar cell in a simple stacked tandem configuration to form a hybrid PEC-PV device with an STH efficiency of 6.3% (based on the lower heating value of hydrogen). Finally, we fabricated a large area (~50 cm² active area) BiVO₄ photoelectrode via spray pyrolysis, and found that the photocurrent is limited by the ohmic losses in the conducting substrate. The implications of this finding and strategies to overcome this limitation will be discussed. Selected results from the PECDEMO consortium on Cu₂O and Fe₂O₃ photoelectrodes, as well as techno-economic analysis will also be presented.

Hydrogen production from photoelectrochemical (PEC) water splitting is a promising pathway for converting solar energy to chemical fuels. To achieve efficient and durable solar to hydrogen conversion, PEC photoelectrodes desirably need the following features: effective and broad band light absorption, rapid charge separation, and superior stability. N-type semiconductor oxides (e.g., TiO₂, Fe₂O₃, and BiVO₄) are popular candidates for constructing robust photoanodes due to their excellent chemical stability. The main drawbacks of these oxides are the limited light absorption and poor charge separation efficiency due to their large band gap and high trapping density. There are two predominant strategies to enhance the separation of electron-hole pairs in photoanodes: reducing the crystal size to the scale of the hole diffusion length; and increasing the carrier conductivity by morphology and crystallography control. Nevertheless, both strategies are restricted by the limit of synthesis procedures.
Permanent electrical polarization induced by ionic displacement (e.g., ferroelectric and piezoelectric potential) has shown great promises in engineering the interfacial band structure and manipulating the charge transfer property of heterostructures. PEC water splitting is an electrochemical system driven by similar energy discontinuity at the electrode/electrolyte interface. One can expect positive (maybe significant) performance gain when ferroelectric or piezoelectric polarization is appropriately introduced to such a system. Here, we report two PEC photoanodes with ferroelectric- and piezoelectric-enhanced charge separation efficiency on the basis of a TiO₂/barium titanate (BTO) core/shell nanowire (NW) array and a Ni(OH)₂ decorated ZnO photoanode systems, respectively. For TiO₂/BTO electrode, the 5 nm BTO-coated TiO₂ NWs achieved 67% photocurrent density enhancement compared to pristine TiO₂ NWs. By numerically calculating the potential distribution across the TiO₂/BTO/electrolyte heterojunctions and systematically investigating the light absorption, charge injection and separation properties of TiO₂ and TiO₂/BTO NWs, the PEC performance gain was proved to be a result of the increased charge separation efficiency induced by the ferroelectric polarization of the BTO shell. Similarly, for ZnO/Ni(OH)₂ system, appreciably improved photocurrent density of sulfite and hydroxyl oxidation reactions were obtained by physically deflecting the photoanode. Both theoretical and experimental results suggested that the performance enhancement was a result of the piezoelectric polarization-endowed enlargement of the built-in electric field at the ZnO/Ni(OH)₂ interface, which could drive additional amount of photoexcited charges from ZnO toward the interface for water oxidation. These studies evidence that the permanent electrical polarizations hold great promises in improving the performance of PEC systems in addition to chemistry and structure optimization.

Dr Savio Moniz,* Dr Junwang Tang

Solar Energy & Advanced Materials Group, Department of Chemical Engineering, University College London, UK


The most important factors dominating the efficiency of solar hydrogen synthesis over semiconductor photocatalysts include (1) light absorption, (2) charge separation and transport, and (3) surface chemical reactions (charge utilization).[1] Furthermore, no single photocatalyst is efficient for both water oxidation and reduction. This talk will cover 2 strategies to improve charge separation; firstly, a Z-scheme, principally for suspension-based systems and secondly, thin film heterojunction photoelectrodes for photoelectrochemical (PEC) water splitting.

The Z-scheme is comprised of 2 semiconductors sandwiched in between a redox shuttle that effectively transfers charge between the materials for either oxidation or reduction reactions. Using this approach, we recently demonstrated an organic-inorganic Z-scheme system comprised of a metal oxide coupled with structure controlled, robust g-C3N4, capable of stoichiometric H2 and O2 evolution under visible light.[2] This shows that g-C3N4 can be integrated into a nature-inspired water splitting system, analogous to PSII and PSI in natural photosynthesis. Recently, we have developed other photoactive polymers that could be used in such a system.[3]

The materials design strategy is critically important to the mechanism for charge transfer. In terms of developing a water splitting device, we have shown that a 1D junction cascade photoelectrode consisting of nanoparticulate BiVO4 on 1D ZnO nanowire arrays with a cobalt phosphate (Co-Pi) surface OEC exhibits a dramatic cathodic shift in onset potential, 12-fold increase in photocurrent and STH value of 1% for PEC water oxidation.[4] The reasons for the enhanced activity will be discussed in light of the limiting factors described earlier.

1. Moniz et al. Energy Environ. Sci., 2015, 8, 731-759.
2. Moniz et al., J. Am. Chem. Soc., 2014, 136, 12568.
3. Moniz et al., 2016, submitted
4. Moniz et al., Adv. Energy Mater., 2014, 4, 1301590.

Direct H₂ formation by photoelectrochemical H₂O splitting theoretically provides high light (photon) to fuel (H₂) efficiency. From a theoretical analysis of the boundary conditions of efficient H₂ production by photoelectrochemical cells a number of elementary processes as well as their coupling to each other must be optimized without severe losses in the number and the chemical potential of the originally generated electron-hole pairs. From a consideration of the given thermodynamic and kinetic energetic condition to split H₂O only tandem or multifunction structures with a sufficient splitting of the quasi-Fermi level as evidenced by the achievable photo voltage of the photovoltaic component will be able to produce H₂O in reasonable amounts. Thus the first duty is to optimize light converting devices structure in a similar way as it has to be done for electricity producing solar cells. This means that the transport lengths of charge carriers must exceed the thickness of the device, where the thickness of the absorber layer has to exceed the absorption length. Subsequently the separated charge carriers must be transferred to the reactants without loss in chemical potentials (photo voltage) and loss in photocurrent (recombination) across the solid electrolyte interface. For the involved multi electron transfer reactions usually solid state electro catalysts must be involved which stabilize reactive and energetically unfavourable intermediates. For minimizing loss mechanisms the energetic positions of the semiconductor band edges, of the electro catalyst’s Fermi level, and electrolyte density of states must arrange in a proper way under illumination. Thus the involved charge transfer steps must be coupled to the selective multi-electron transfer catalysts allowing electron transfer reactions with a nearly isoenergetic coupling of their electronic states which needs appropriate surface engineering strategies.
Because of these boundary conditions research and development of advanced thin film absorber materials for tandem and multijunction photovoltaics and efficient photoelectrochemical cells need in part the same research efforts and are limited by similar shortcomings. In our opinion with multi junction devices based on thin film absorber materials promising research and development routes to highly competitive PV and PEC cells are given which need, however, further systematic materials research efforts e. g. by combining theoretical design concepts and high throughput experimental validation.

The development of complex mesoscale (nm - µm) materials used for a wide range of solar fuel applications requires comparable evolution in the analytical instruments and techniques in order to understand the physical and chemical structure-property relationships underlying their performance. Conventional “hard” X-ray (i.e., > 10 keV) scattering has received considerable attention due to the fact that it is a high-resolution nondestructive structural probe that can interrogate a statistically significant 3-dimensional sample area. However, the physical nature of the scattering process at these energies limits its applicability to materials that possess significantly different electron densities. Unfortunately, the performance of many electrochemical materials often hinges on subtle heterogeneities, which do not possess sufficiently distinct electron densities to provide significant contrast. To help address this challenge, resonant soft X-ray scattering (RSoXS) uses tunable “soft” X-rays (i.e., 100 - 1500 eV) to dramatically enhance the scattering cross sections from heterogeneous materials when the X-ray photon energy is judiciously chosen to coincide with favored transitions near a material’s absorption edges. This combination of absorption spectroscopy and scattering enhances the scattering contrast due to specific chemistries over a large sample volume. In recent years, RSoXS performed near the carbon K-edge has proven to be a very useful tool for the soft matter researchers interested in defining chemical structure and molecular orientation of complex materials with nm-scale spatial sensitivity.¹ Still, the full potential of RSoXS to study these processes is far from being fully explored by inorganic material scientists and electrochemists.

In this presentation, we will show some of the first experimental results demonstrating how RSoXS can be a powerful tool for the solar fuels community due to its chemical sensitivity, large accessible size scale, and polarization control.Specifically, we will reveal its ability to simultaneously interrogate the bulk, surfaces, and buried interfaces of low-Z element materials (including transition metals), such as those used as nanostructured photoelectrodes, catalysts, and ion exchange membranes. In addition, we will present recent developments we have made, on both the instrumental and device level, to enable operando and in-situ RSoXS characterization of electrochemical materials under reaction conditions in liquid and gaseous environments. The practical challenges and limitations of conducting such experiments will also be discussed, with the goal of developing a novel time-resolved reciprocal space probe that should ultimately be applicable to a broad range of scientific problems facing the solar fuels community.

[1] C. Wang, D.H. Lee, A. Hexemer, M.I. Kim, W. Zhao, H. Hasegawa, H. Ade, T.P. Russell, Nano Lett, 11 (2011) 3906-3911.

In response to the rising energy demand and corresponding pollution issues, H₂ is one of the most promising renewable and sustainable energy candidates. Extracting H₂ from H₂O through solar water splitting enables recycle of H₂O, and allows to collect and store solar energy into the simplest chemical bond, as nature accomplishes through photosynthesis. Among existing strategies, photoelectrochemical (PEC) water splitting promises a direct route for solar-to-chemical energy conversion. Successful implementations of the reaction often require the integration of catalysts with light absorbers, especially on photoanodes where four electron water oxidation reaction occurs with sluggish kinetics. Nevertheless, it is challenging to uncover the main causes of the performance enhancement by surface catalysts owing to their cumulative effects and the interface complexities, making it difficult to further improve these systems for practical applications. Here, we systematically compare two water-oxidation catalysts (WOCs) on a hematite (α-Fe₂O₃) photoelectrode by the combination of intensity modulated photocurrent spectroscopy (IMPS), photoelectrochemical impedance spectroscopy (PEIS) and kinetic isotope effects (KIE) measurements, aiming to shed light on working mechanism of the photoelectrode/catalyst interface. It is already known that the energetics of photoelectrode surface or charge transfer kinetics (or both) could be altered by the application of catalysts. And, significant research efforts have been devoted into heterogeneous WOCs due to their abundancy. However, their molecular structures and working mechanisms are poorly understood. In terms of molecular WOCs, their functionalities are well studied but how they affect photoelectrodes are still unclear. In this work, we found that when hematite photoanode is anchored with a thin layer of heterogenized molecular Ir WOC (het-WOC), the improved PEC performance is attributed to higher charge transfer rate monitored by IMPS while surface recombination rate remains almost the same. On the contrary, when hematite is decorated with a denser layer of heterogeneous oxide WOC (IrO_x), the surface recombination rate is dramatically reduced. It is indicative that the het-WOC provides additional charge-transfer routes across the Fe₂O₃|H₂O interface, while IrO_x and analogous bulk metal-oxide catalysts replace the interface with a fundamentally different one. PEIS analysis of IrO_x and het-WOC decorated hematite exhibiting larger capacitance of WOCs and smaller charge transfer resistance, respectively, provide additional strong support. In the end, KIE measurements highlight rate-determining steps for water oxidation by the het-WOC and IrO_x are distinct.

Atomic layer deposited (ALD) TiO₂ protection layers may allow for the development of both highly efficient and stable photoanodes for solar fuel synthesis; however, the very different conductivities and photovoltages reported for TiO₂-protected silicon anodes prepared using similar ALD conditions indicate that mechanisms that set these key properties are, as yet, poorly understood. In this report, we study hydrogen-containing annealing treatments and find that post-catalyst-deposition anneals at intermediate temperatures reproducibly yield decreased oxide/silicon interface trap densities and high photovoltage. A previously-reported insulator thickness-dependent photovoltage loss in metal-insulator-semiconductor Schottky junction photoanodes is suppressed. This occurs simultaneously with TiO₂ crystallization and an increase in its dielectric constant. At small insulator thickness, a record for a Schottky junction photoanode of 623 mV photovoltage, is achieved, yielding a photocurrent turn-on at 0.92 V vs NHE or -0.303 V with respect to the thermodynamic potential for water oxidation.

Many high efficiency photoelectrode and photovoltaic materials are unstable in electrochemical environments, and thus require a protection layer to be deposited on their surface. Recently, the use of an amorphous titanium dioxide (TiO₂) interfacial layer has been shown to extensively extend the durability of the photocathode material under photocatalytic conditions. Limited attention however has been given to the understanding of the corrosion mechanism and its correlation with the charge transfer process occurring at the TiO₂/electrolyte interface. In our study, we found that the photocurrent of an amorphous silicon carbide (a-SiC) photocathode increases dramatically upon electrochemically reducing the TiO₂ front layer, which we attributed to the high surface activity of the Ti³⁺ on the uncatalyzed amorphous TiO₂. We investigated the effect of various electrochemical reduction treatment on the Ti³⁺ donor density, which is confirmed by x-ray photoelectron spectroscopy, and showed variation of photocurrent density.

Additionally, we examined the correlation of in situ- and post-thermal treatment of the Ti³⁺-doped amorphous TiO₂ with its electrical conductivity as well as the catalytic activity. Using electrochemical impedance spectroscopy, we estimated the bulk electronic properties and analyzed the charge transfer kinetics which are responsible for the significant gain of onset potential and photocurrent density. Finally, we showed the prolonged stability of the a-SiC protected by the reduced-TiO₂ and clarified its Faradaic efficiency. This high performance, very thin, earth-abundant and uncatalyzed photocathode is very promising for integration with smaller band gap solar absorbers to form a bias-free multi-junction photoelectrode for overall water splitting.

A short overview of the pros and cons of various tandem devices [1,2] will be given. The large band gap semiconductor needs to be in front, but apart from that we can chose to have either the anode in front or back using either acid or alkaline conditions. There seems to be a severe lack of good semiconductors that can provide an open circuit potential approaching E_G-0.5V. Even those that can, which are usually small band gap materials like Si, are very prone to corrosion the advantage of using buried junctions and using protection layers shall be discussed [3-5]. In particular we shall show how doped TiO₂ is a very generic protection layer for both the anode and the cathode [6]. Next we shall discuss the availability of various catalysts for being coupled to these protections layers and how their stability and amount needed may be evaluated [7, 8, 9]. Here we shal demonstrate how mass selected clusters can be used for illuminating the activity of the various materials such as RUO₂, Pt, and NiFe Oxides [8,9,10]. Examples of half-cell reaction using protection layers for both cathode and anode will be discussed though some of recent examples both under both alkaline and acidic conditions. Notably NiO_x promoted by iron is a material that is transparent, providing protection, and is a good catalyst for O₂ evolution [10,11]. Si is a very good low band gap semiconductor and the optimal thickness of this in a tandem device will be discussed [12]. Finally if time allows we shall also discuss the possibility of making high energy density fuels by hydrogenation of CO₂ instead of hydrogen evolution [13]. We shall here show how we can investigate the recent ethanol synthesis on oxygen derived Cu found by Kanan et al. and show how acetaldehyde seems to be an important intermediate [14].

References
[1] A. B. Laursen et al., Energy & Environ. Science 5 5577 (2012)
[2] B. Seger et al. Energy & Environ. Science 7 2397 (
[3] B. Seger, et al. Angew. Chem. Int. Ed., 51 9128 (2012)
[4] B. Seger, et al., JACS 135 1057 (2013)
[5] B. Seger, et al., J. Mater. Chem. A, 1 (47) 15089 (2013)
[6] B. Mai et al. J. Phys. Chem. C 119 15019 (2015)
[7] R. Frydendal, et al. Chem.Elec.Chem 1 2075 (2014)
[8] E. A. Paoli, et al. Chemical Science, 6 190 (2015)
[9] E. Kemppainen et al. Energy & Environmental Science, 8 2991 (2015)
[10] B. Seboek et al. In Preparation (2016)
[11] B. Mei, et al. J. Phys. Chem. Lett. 5 1948 (2014)
[12] B. Dowon et al. Energy & Environ. Sci. 8 650 (2015)
[13] A. Verdaguer-Casadevall et al. J. Am. Chem. Soc. 137 9808 (2015)
[14] E. Bertheussen et al. Angewandte ChemieInt. Ed. 55 1450 (2016)

Manufacturing of batteries, solar cells, sensors and fuel cells have greatly depended on core/shell nanorods arrays and their coated forms due to the defect-free interface concerns, fabrication of the conformal shell layers on nanorod arrays has been a great challenge. Monte Carlo (MC) simulations to estimate the most favorable deposition condition to produce a conformal shell coating. For this reason, MC methods became a favored simulation technique for physical vapor deposition. We studied different height effects of the arrays on the conformality of the coating. Our results indicate that conventional PVD techniques, which offer low cost and large scale thin film fabrication, can be utilized for highly conformal and uniform shell coating formation in device applications.

Hydrogen appears as a next-generation clean energy source to replace fossil fuels. One of the most promising ways to produce hydrogen is photoelectrochemical (PEC) water splitting. However, the existing photoelectrodes such as Si with noble metal catalysts still suffer from low efficiency and poor stability and the extremely high cost of the noble metal catalysts limits the wide use of water splitting photoelectrodes. Therefore, a novel approach is necessary to make a breakthrough for highly efficient PEC water splitting.
We have synthesized wafer-scale transferable molybdenum disulfide (MoS₂) thin-film catalysts by using the thermolysis method for the first time. We have demonstrated that wafer-scale, transferable, and transparent thin-film catalysts based on MoS₂, which consists of cheap and earth abundant elements, can provide the low overpotential of 1 mA/cm² at 0.17 V versus a reversible hydrogen electrode and the high photocurrent density of 24.6 mA/cm² at 0 V for a p-type Si photocathode. c-Domains with vertically stacked (100) planes in the transferable 2H-MoS₂ thin films, which are grown by a thermolysis method, act as active sites for the hydrogen evolution reaction, and photogenerated electrons are efficiently transported through the n-MoS₂/p-Si heterojunction.
Our results show that the MoS2 thin-film catalysts not only reduce the overpotential at electrolyte/solid interfaces but also stabilize the surface of solids for efficient water splitting using p-type semiconductor photocathodes including Si, InP, GaAs, and GaP. Our approach could be applied to the synthesis of various 2-dimensional transition metal dichalcogenides (TMDs) and the catalytic performance of the materials would be further enhanced using substitutional doping, defect engineering, and n-TMD/p-TMD heterojunctions.

Closing the photosynthetic cycle on the shortest possible length scale – the nanoscale – is an important design principle of photosynthesis for minimizing side reactions, efficiency degrading charge transport processes and resistance losses. For completing the catalytic cycle of CO₂ reduction by H₂O on the nanoscale in an artificial photosystem, the half reactions need to be separated by an ultrathin proton transmitting, gas impermeable membrane. We are developing Co₃O₄-silica core-shell nanotube arrays in which each nanotube operates as an independent photosynthetic unit by oxidizing water on the inside while reducing carbon dioxide on the outside. The spaces of O₂ evolution and reduced product are separated by the 2-3 nm thick dense phase silica shell. Charge transport across the silica layer is accomplished by embedded p-oligo(phenylenevinylene) molecular wires (OPPV, 3 aryl units).
To quantify electron transport through the embedded OPPV molecules, we conducted visible light sensitized photo-electrochemical measurements using cm² sized planar Co₃O₄-silica/wire films prepared by atomic layer deposition on Pt.^1,2 STEM-EDX, XPS and grazing angle ATR-FT-IR showed covalent attachment of the wires to a 5 nm uniform Co₃O₄ layer at a density of 1 wire per nm². Photoelectrochemical measurement of visible light sensitized charge (hole) injection in short circuit configuration using sensitizer molecules with different redox potentials revealed that the HOMO and LUMO energetics of the wire molecules controls electron flow and allowed us to quantify the charge flux through this new nanoscale membrane for artificial photosynthesis. To understand the detailed charge transfer pathway through the membrane, ultrafast optical spectroscopy was conducted using spherical Co₃O₄-silica/wire particles in aqueous solution. Femtosecond excitation of a sensitizer adsorbed on the silica membrane revealed hole injection within a ps, which was monitored by the rise of the wire radical cation absorption band at 550 nm. Photocatalytic activity results of the core-shell nanotube construct will be presented.

Edri, E.; Frei, H. J. Phys. Chem. C 2015, 119, 28326.
Kim, W.; McClure, B. A.; Edri, E.; Frei, H. Chem. Soc. Rev. 2016, 45, 3221.
Frei, H. Curr. Opin. Chem. Eng. 2016, 12, 91.

This work explores the effects of photonic structuring and localized surface plasmon resonance (LSPR) and on CO₂ reduction reactions (CO2RR). We design photonic crystals where the photonic bandgap is tuned by changing the periodicity of the inverse opal cavities. The photonic bandgap of an inverse opal structure can be tuned by the material and pore size/periodicity. These photonic properties control how the incoming light is reflected and refracted within the material. We investigate a variety of photonic crystal materials, such as metal oxides, by infilling the voids of an opal template of polystyrene (PS) spheres to obtain an inverse opal photonic crystal by subsequently removing the PS spheres.
Similarly, the LSPR can be tuned in metal or degeneratively doped semiconductor nanoparticles by tuning the size, shape, and material composition. This increases absorption near the resonance peak. Additionally, hot carriers can be generated as the LSPR decays, providing the potential to access kinetic pathways normally difficult for desired products in CO2RR chemistry. Since we can tune the photonic bandgap simply by changing the size of the PS spheres in the template, we can investigate how matching the photonic bandgap with the plasmonic resonance of a metal particle affects CO₂ reduction. We then study the effects of combining both of these physics phenomena on chemical transformation and selectivity in CO₂ reduction. From this basis, one could potentially tune CO2RR processes to a desired hydrocarbon fuel product.
We use a combination of SEM, TEM, and XRD to analyze the morphological structure and crystallinity of our materials as well as material composition via EDX. Surface analysis of our structures is done using XPS. We characterize the films’ optical properties, monitoring shifts in the photonic bandgap and LSPR, via UV-Vis-NIR spectroscopy. We monitor CO2 reduction products using in line GC for gas products and NMR for liquid products.

Electrochemical reduction of carbon dioxide (CO₂) is one of the key technologies which can create fuels like methane (CH₄) combined with renewable energy sources such as solar and wind energy. However, copper (Cu) is the only catalyst which produces CH₄ as a major product. To maximize the selectivity and production rate of CH₄ on Cu, it is quite important to obtain systematic insights for the effect of reaction parameters in CO₂ reduction, such as CO₂ pressure, temperature, stirring speed (or flow rate) and electrolyte, on the mass transport of CO₂ and resulting CH₄ production.
Based on such a concept, the authors constructed a system with combinatorial reactors which enables multiple and simultaneous measurements under various experimental conditions. Previously, we clarified the role of CO₂ pressure, stirring speed and temperature for the quantified CO₂ supply (J_lim) and CH₄ Faradaic efficiency with this system.¹ Here, the scope was extended to the effect of electrolyte, especially between potassium chloride (KCl) and potassium bicarbonate (KHCO₃), on the rate of CO₂ supply and CH₄ production from CO₂.
Experiments are performed under 9 conditions in total, selecting CO₂ pressure (1.3, 2, 3 atm) and the electrolyte (0.5 M KCl, 0.25 M KHCO₃, 0.5 M KHCO₃) under ambient temperature and no stirring condition. At each condition, current density dependences of Faradaic efficiencies of reaction products were obtained. In KHCO₃, the current density dependences were different from those in KCl, where the peak of CH₄ selectivity decreased but made long tail toward higher current density. However, the decreased peak of CH₄ selectivity turned to increase with the CO₂ pressurization in KHCO₃ solutions. Our analysis clarified that the J_lim increased in KHCO₃ compared to KCl, which means that more CO₂ participate in the reaction in KHCO₃ even under the same CO₂ pressure. The analysis also showed that the KHCO₃ concentration could be the important factor for controlling J_lim. In the presentation, we will discuss the possible mechanisms for the increase of CO₂ supply in KHCO₃ as well as the comparison of electrolyte better for CH₄ production from the practical viewpoint.

¹ H. Hashiba et al., ACS Comb. Sci., 2016, 18 (4), pp 203–208

Third-generation conjugated polymers are functionalized polymers for a specific purpose introducing a new property on top of the semiconducting and conducting properties. As an example here, for the catalytic function we prepared polythiophenes, bearing Rhenium carbonyl complexes and pyridinium pendant groups as active sites to drive the photoelectrochemical reduction of CO₂. Cyclic voltammetry and controlled potential electrolysis experiments were performed in CO₂-saturated acetonitrile and acetonitrile-water solutions under illumination as well as in dark. The formation of CO as main product was confirmed with IR spectroscopy and quantified with gas chromatography in the case of poly-[Re-(4-methyl-4'-(7-(thiophen-3-yl)heptyl)-2,2'-bipyridyl)tricarbonyl chloride] (P[3HRe(bpy)CO₃Cl-Th]) giving a max. Faradaic efficiency of 2.5% and a TON of 20. In the case of poly- [4-(7-(thiophen-3-yl)heptyl)pyridine] (P[3HPyr-Th]) which was supposed to catalyse CO₂ to MeOH reaction, no products were observed.This strongly indicates that the pyridinium ion should be mobile in order to act as an electron shuttle leading to 6e^- reduction. With this study, we introduce a method in which can be applied to other widely used molecular homogeneous catalysts in the field in order to pave the way to use them as heterogeneous ones.

Nowadays, efficient solar conversion of carbon dioxide (CO₂) to hydrocarbon fuels, such as carbon monoxide, formic acid, methanol, and methane, has been a holy grail with pressing environmental issues that are presumed to result from CO₂ emission. Relatively staggering progress in the current state-of-the-art conversion efficiency and selectivity is a ramification of chemical stability and low solubility of CO₂ as well as complicated multi electron-involving intermediate reactions. Recently, delafossite materials (CuMO₂, M=Fe, Cr, Al, etc) have been reported as a promising class of p-type semiconductors due to its narrow bandgap, favorable band edge position and good stability. It has been highlighted in previous studies that CuO/CuFeO₂ heterostructures formed by one-step air annealing process is highly stable and efficient for conversion of CO2 into formate even without an external bias. However, it is still ambiguous as to what causes the enhancement in activity and stability, since the heterostructures have not been comprehensively characterized in terms of its crystallinity and morphology. To elucidate the role of structures in the structure-property relationship in detail, we started with the synthesis and analysis of CuFeO₂ and CuO.
PEC experiments were performed in a three-electrode setup, with a Ag/AgCl/KCl(sat.) reference electrode (E=+0.210 V versus NHE) and a platinum counter electrode, using an aqueous potassium bicarbonate (0.1 M KHCO₃) electrolyte. A simulated solar light (AM 1.5G; 100 mW/cm²) was irradiated after purging with CO2 gas for 30 min. CuFeO₂ photocathode showed negligible photocurrent, 15 uA/cm² with severe spike evolution unlike the previous report. After addition of CuO, photocurrent reached to 74 uA/cm² with significantly reduced spike. Despite of this huge enhancement, the photo-activity was still too low compared to the reported one. Surprisingly, by post heat-treatment under air atmosphere, the photocurrent increased further up to 150 uA/cm². This was closely related to the surface reconstruction of CuFeO₂ into CuO and CuFe₂O₄, which can make the surface more active toward CO_2. In addition, it was also revealed by impedance spectroscopy that heterojunction between these materials facilitates charge separation. In conclusion, CuFeO_2, itself, is not a good choice for CO₂ reduction due to its inactive surface. However, surface modification by controlled post heat-treatment can make this shortcoming overcome. Even though the photocurrent density is still much lower than the state-of-the-art one, we clearly demonstrated that the importance of surface engineering of delafossite materials for CO₂ reduction. Based on these findings, we believe that these findings may pave a new way of how delafossite materials should be engineered to be an efficient CO₂ conversion photocathode.

Metal oxide semiconductors are under intense investigation for photoelectrochemical (PEC) water splitting due to their stability in aqueous electrolyte, which is critical for the advancement of PEC technology. Tungsten trioxide is one of the few stable oxide semiconductors with a band gap small enough to absorb visible light (E_g = 2.5 eV) and is therefore frequently investigated as a photoanode in PEC water splitting. However, like other smaller bandgap oxides, the position of the conduction band of WO₃ is below the thermodynamic potential for proton reduction of 0 V vs. RHE, while the valence band position provides much more driving force than is required for the oxidation of water (too much overpotential). The photovoltage generated by the oxide could be better utilized by shifting the band edge energies through the introduction of a dipole layer on the semiconductor-liquid interface, a strategy that has been applied successfully for TiO₂ and Si.
In this work, we show our effort to tune the band edges of a model oxide (WO₃) by modifying the semiconductor surface with organic dipoles. We used simple methods to adsorb a monolayer of substituted phenylphosphonic acids on electrodeposited WO₃. Impedance spectroscopy and Mott-Schottky analysis were used to measure the flat band potential. A series of substituted phenylphosphonic acids shifted the bands by up to +300 mV. These results suggest that the choice of a suitable organic dipole molecule could allow unassisted water splitting by WO₃, if these dipole molecules could be stabilized on the surface.

Picosecond time-resolved X-ray techniques, such as time-resolved X-ray diffraction, scattering, and spectroscopy, utilize the pulsed nature of synchrotron radiation from storage rings, and are becoming general and powerful tools to explore structural dynamics in various materials. This method enables to produce “atomic structural movies” at picosecond temporal resolution. It will be fascinating to apply such capability to capture ultrafast structural dynamics in advanced materials of strongly-correlated electron systems, photochemical catalytic reaction dynamics in liquid or on solid surface, light-induced response of photo-sensitive proteins, etc.
For example, ultrafast X-ray spectroscopy offers new opportunities for elucidating ultrafast structural dynamics both in solid and solution. X-ray absorption fine structure (XAFS) spectroscopy reveals details about the local geometric and electronic structure around selected atoms. Extended X-ray absorption fine structure (EXAFS) provides accurate bond distances, while X-ray absorption near-edge structure (XANES) delivers additional information about unoccupied orbitals and spin configurations.
Photon Factory Advanced Ring (PF-AR) at the High Energy Accelerator Research Organization (KEK), Tsukuba, Japan is a 6.5-GeV electron storage ring dedicated for single-bunch operation and is suitable for such time-resolved X-ray studies. An in-vacuum undulator beamline NW14A at the PF-AR was designed and constructed to conduct a wide variety of time-resolved X-ray measurements, such as time-resolved X-ray diffraction, scattering and spectroscopy. Successful examples of time-resolved XAFS studies applied to photocatalyst will be presented.

Solar-driven water-splitting (2H₂O → 2H₂ + O₂) has the potential to address the global energy demand. Most artificial solar water-splitting processes are mediated by semiconductors that absorb solar radiation and facilitate photo-generated charge carriers to their respective active sites to generate H₂ and O₂. However, no known semiconductor is capable of satisfying the stringent material requirements for cost-effective implementation of solar-driven water-splitting. The cocatalyst-loaded (Ga_1-xZn_x)(N_1-xO_x) (GZNO) solid solution absorbs visible-light, exhibits greater operational stability than its binary component parents, and displays the largest apparent quantum yield among visible-light-driven water-splitting photocatalysts.¹ Despite its proof-of-concept performance, the apparent quantum yield of GZNO is far from its theoretical limit. Here we show that photocatalytic inefficiencies may be attributed defects resulting from the loss of Zn during the nitridations process. Incorporation of a ZnO buffer layer during the nitridation process suppresses Zn volatilization to yield GZNO with similar Zn content as the parent material. In addition, we have synthesized Sn-doped GZNO microcrystals and investigated the role of Sn in defect passivation. Neutron and X-ray diffraction techniques were used to elucidate the average (via Rietveld analysis) and local structure (via pair distribution function analysis and reverse Monte Carlo modeling) of GZNO and Sn-doped GZNO microcrystals. Structural investigations were complemented with spectroscopic techniques and photochemical measurements, with the aim of developing structure-property relationships towards a better understanding of photocatalysts.

References
1. (a) Ohno, T.; Bai, L.; Hisatomi, T.; Maeda, K.; Domen, K., Photocatalytic Water Splitting Using Modified GaN:ZnO Solid Solution under Visible Light: Long-Time Operation and Regeneration of Activity. Journal of the American Chemical Society 2012, 134 (19), 8254-8259; (b) Fabian, D. M.; Hu, S.; Singh, N.; Houle, F. A.; Hisatomi, T.; Domen, K.; Osterloh, F. E.; Ardo, S., Particle suspension reactors and materials for solar-driven water splitting. Energy & Environmental Science 2015, 8 (10), 2825-2850.

As an intrinsic n-type semiconductor, Ta₃N₅ has the flat-band potential around -0.2 to 0 V vs. normal hydrogen electrode (NHE), meaning that over 1 V of photovoltage can be potentially harvested from this material for solar water splittin. Together with the theoretical photocurrent of 12.9 mA/cm², 15.9% solar to hydrogen efficiency is possible. Indeed, the benchmark photocurrent of 12.1 mA/cm² at 1.23 V vs reversible hydrogen electrode (RHE) has been reported recently. However, the turn-on potentials for water oxidation in almost all literature still remain positive than 0.6 V vs RHE even with intentional doping, and the long-term stability for water oxidation remains poor. In this presentation, we report a detailed study on how the surface oxidation of Ta₃N₅ and the intermediates produced during water oxidation process result in the Fermi level pinning, which greatly reduces the measurable photovoltages in Ta₃N₅. Based on the results of detailed X-ray photoelectron spectroscopy and photoelectrochemical measurement in aqueous/nonaqueous system, the origin of the low photovoltage for water oxidation is quantitatively identified as the formation of surface states on the Ta₃N₅/water interface. These states pin the surface Fermi level and lead to a photovoltage loss of 0.4 V. The instability of Ta₃N₅/water interface under the PEC water oxidation condition presents challenges for the selection of suitable protective material and oxygen evolution catalyst. This study provides important guidelines for future development of nitride semiconductors for solar energy harvesting/storage applications.

With limited access to fossil fuels, increasing world energy consumption will become a key challenge for future society.¹ One of the sustainable solutions to this problem is the utilization of solar energy. However solar energy is an intermittent source, so storage also becomes an issue.

A switchable energy storage technology known as MOST (molecular solar thermal)² can store and release solar energy on demand by using a well-designed photoswichable molecule. Exposing one such compound to sunlight generates a photoisomer with a potentially long storage lifetime. When energy is required, the photo isomer can be back converted to its initial state by use of a catalyst or heat. The stored solar energy is then released in the form of extra heat.

Our work mainly focuses on three parts: a). The development of new molecules whose absorption matches well to the solar spectrum, possess a long half-life and have low molecular weight. b). The development of new catalysts for heat release. c). Design and implementation of MOST systems in functional devices.³ Here we synthesized a new compound with a high energy density (396 kJ/kg), and combined it with a new improved device.⁴ This new material has been tested in real outdoor conditions and showed high potential for future use.



1. K. Moth-Poulsen, D. Coso, K. Borjesson, N. Vinokurov, S. K. Meier, A. Majumdar, K. P. C. Vollhardt and R. A. Segalman, Energy & Environmental Science, 2012, 5, 8534-8537.

2. Z.-i. Yoshida, Journal of Photochemistry, 1985, 29, 27-40.

3. K. Borjesson, D. Dzebo, B. Albinsson and K. Moth-Poulsen, Journal of Materials Chemistry A, 2013, 1, 8521-8524.

4. V. Gray, A. Lennartson, P. Ratanalert, K. Borjesson and K. Moth-Poulsen, Chem Commun (Camb), 2014, 50, 5330-5332.

Photocatalytic water-splitting is an ideal method for solar energy harvesting. Since Honda and Fujishima first reported the water-splitting using TiO₂ with Pt photoelectrodes, some photocatalysts that can split water under UV light have been discovered. However, development of visible-light sensitive photocatalysts is indispensable due to the effective utilization of incoming solar energy. On the other hand, semiconducting iron disilicide (β-FeSi₂) has been attracting a great deal of attention as a photo-detector and Si-based light emitter operating at wavelengths suitable for optical fiber communications (1.55 μm). This is because β-FeSi₂ has a band gap of approximately 0.80 eV and a very large optical absorption coefficient over 10⁵ cm⁻¹ at 1 eV. Moreover, this semiconducting material is composed of the elements which are naturally abundant and less toxic than the elements used in conventional compound semiconductors.

In this paper, we report on the novel fabrication method of β-FeSi₂/3C-SiC composite powder by using metal-organic chemical vapor deposition (MOCVD) method which is general in semiconductor process technology. Moreover, we repot on the hydrogen evolution over this composite powder under irradiation of not only UV but also visible light and near-infrared light from methyl-alcohol aqueous solution.

10-nm-thick gold (Au) was deposited on the surface of 3C-SiC powder, average diameter of 60nm, at room temperature by rf-sputtering. β-FeSi₂ was deposited on the Au-coated 3C-SiC powder by using MOCVD. Iron pentacarbonyl [Fe(CO)₅] and monosilane (SiH₄) were used as sources, and the deposition temperature and deposition rate were 923 K and 1.6 nm/min, respectively. XRD θ-2θ scan profile showed the formation of poly-crystalline β-FeSi₂ phase for the powder after MOCVD deposition of 1 hour. From the SEM observation of this composite powder, the inhomogeneous grains with sizes of the order 10 nanometers were confirmed to be formed on the 3C-SiC surface. The hydrogen gas was evolved by irradiation of UV light (250-430 nm) from methyl-alcohol aqueous solution. Moreover, similar hydrogen evolution was confirmed under visible light (420-650 nm) and near-infrared light (1050-1600 nm) light irradiation.

We demonstrate, for the first time, the synthesis of highly ordered niobium oxynitride microcones as an attractive class of materials for visible-light-driven water splitting. As revealed by the ultraviolet photoelectron spectroscopy (UPS), photoelectrochemical and transient photocurrent measurements, the microcones showed enhanced performance (~1000% compared to mesoporous niobium oxide) as photoanodes for water splitting with remarkable stability and visible light activity. Finally, this proposed platform of microcones array films holds promise for a variety of applications of the future design of optoelectronic devices.

Hematite has been widely investigated for application in photoelectrochemical cell (PEC) as photoanode due to its physical and chemical characteristics. This work describes a systematic investigation on hematite photoanode preparation via hydrothermal microwave-assisted process. The photoanodes were growth varying the synthesis time (from 1 to 5 hours) onto commercial glass substrate covered with fluorine doped-tin oxide conductive coating. To complete the electrode preparation, it was subjected to an additional thermal treatment at 750°C for 30 min in different atmosphere (air or N₂). Then, all photoanodes were evaluated using a combination of techniques to observe the impact of the synthesis time and atmosphere of thermal treatment on its performance. Top view scanning electron microscopy images exhibit photoanodes composed by nanorods homogeneously distributed onto glass substrate area with thickness varying from 80 to 400 nm. From X-ray diffraction patterns of pure hematite phase with nanorods showed a preferentially oriented toward to (110) diffraction plane. The highest current density response under sunlight illumination was obtained for hematite photoanode obtained at 4 hours of synthesis time and thermal treated in N₂ atmosphere. The impedance spectroscopy (EIS) was carried out in dark conditions and Nyquist plot showed that the hematite thermal treated with N₂ exhibit lowest resistance for charge transfer. Additionally, the EIS data were analyzed by Mott-Schottky plot and the donor density was estimated around 3.1 10¹⁹ cm^-3 and 1.3 10²⁰ cm^-3 for hematite photoanodes treated in air and N₂, respectively. In fact, Finally, the catalytic efficiency was investigated under sunlight irradiation in presence of electrolyte H₂O₂-modified. Hematite photoanodes with thermal treatment in presence of N₂ showed an integrated catalytic efficiency around 77%. This finding showed that the N₂ atmosphere could favor the chemical reaction at hematite surface enhancing it catalytic performance under sunlight irradiation.

Acknowledgements
We gratefully acknowledge financial support from the Brazilian agencies of FAPESP (Grants 2011/19924-2 and 2014/11736-0).

A photoelectrochemical cell (PEC) is an elegant and feasible way to convert solar energy into fuel.¹ In a PEC system, a semiconductor photocatalyst captures solar energy and subsequently splits water into hydrogen (H₂) and oxygen (O₂). The O₂ is generated in the photoanode (PA) and the H₂ in the cathode or photocathode. Hematite (α-Fe₂O₃), an n-type semiconductor with high photoelectrochemical stability, is considered the most promising material to be used as a PA. However, its use presents many challenges, but the main is electron/hole recombination that can occur in bulk, in solid-solid and solid-liquid (electrolyte) interfaces.^2,3 It is clear that the variety of recombination pathways is related to the thin film morphology of hematite PA, and the morphological complexity makes it harder to identify the dominant losses process and to assess how these processes impact the final PA performance. Therefore, we used the simplest hematite PA, i.e., a sintered polycrystalline hematite electrode and we will show how the addition of Sn⁴⁺ (as dopant) and the heat treatment atmosphere modifies the electrical properties of the sintered ceramic pellet as well as impacts the electrochemical and photoelectrochemical properties in this PA. The results obtained allowed us to conclude that the addition of Sn⁴⁺ decreases the grain boundary resistance of the hematite polycrystalline electrode. Heat treatment in a nitrogen (N₂) atmosphere also contributes to a decrease of the grain boundary resistance, supporting the evidence that the presence of oxygen is fundamental for the formation of a voltage barrier at the hematite grain boundary. The N₂ atmosphere affected both doped and undoped sintered electrodes. We also observed that the heat treatment atmosphere modifies the surface states of the solid-liquid interface, changing the charge-transfer resistance.

The consumption of the world’s fossil fuels and their hazardous pollution impact on the atmosphere, makes the photoelectrocatalytic reduction of carbon dioxide a promising option to recycle this major green house gas. In the reduction of aqueous carbon dioxide, water would be the source of both the electrons and hydrogen atoms. However, the protons in solution compete to form hydrogen. This process can be made more selective with the use of metal catalysts. We report on the novel use of bi- and tri-metallic catalysts deposited on titania nanotubes using ALD and their photoelectrocatalytic activity for energy efficient, highly selective, and less competitive reduction of aqueous CO₂. Titania nanotubes were fabricated by anodization then coated with Cu, Cu-Pt and Cu-Pt-Pd atomic layers using ALD. The objective of this setup is to combine the catalytic activity of metals with the stability of semiconducting photocathodes. The heterojunction electrodes were fully characterized by FESEM, TEM, XRD and XPS techniques. Under illumination with a 500 W xenon light source; light intensity, 100 mW cm^-2, the onset reduction voltage showed a positive shift for all the prepared samples. The results showed a synergistic effect between the Cu-Pt and Cu-Pt-Pd catalysts. Results clearly demonstrated the potential of a suitable technology for the reduction of CO₂.

Hydrogen as fuel is gaining more momentum with time due to the rapid progress in materials science and the urgent need for clean sources of energy. Here we report on the photocatalytic reforming of formic acid using pure and Ca doped Bi ferrite, BFO and BCFO, respectively. Nanostructures BFO and BCFO were synthesized using facile sol-gel method with calcined temperature of 550 °C, characterized using XRD, TEM, UV-vis, etc. Ca doping decreased particles size from around 70 nm down to 20 nm and increased surface area by 3 times. The calculated band gap of BFO was found to be 2.5 eV making it good visible light photocatalyst. Formic acid concentration was 10 mmol and the photocatalytic reaction was carried out in a double jacket quartz glass photoreactor connected to a quadrupole mass spectrometer (QMS) for gas analysis. Reforming of formic acid gives H2 and CO2. The use of a co-catalyst and/or doping is expected to enhance the H2 production.

In the field of renewable synthetic fuel production, photoelectrochemical (PEC) water splitting takes an increasingly important role in providing routes to hydrogen and other hydrocarbons. One of the main challenges around the oxygen evolution reaction (OER) arises from the incompatibility of most photoanode materials with the highly oxidizing conditions under operation in strongly basic or acidic aqueous media.
By protecting the photoanode material, this problem may be resolved. As Lewis [1],[2] has shown, TiO₂ can be used as such a material. It exhibits excellent stability in both acidic and basic aqueous solutions and shows hole conductivity when a metal catalyst such as Ir, Pt or Ni is deposited. Without the addition of these metal overlayers, the TiO₂ is blocking.
Our goal is to investigate if the “leakiness” of ALD-deposited TiO₂ is compatible with other materials, specifically polymeric hole-conductors and molecular catalysts. Having reproduced the hole-conductive character of ALD-TiO₂ with sputtered Pt, we report our results of the TiO2-molecule system, using ferrocene anchored via a phosphonate group as a model compound for a molecular water oxidation catalyst.

[1] Hu, S., Shaner, M., Beardslee, J., Lichterman, M., Brunschwig, B., Lewis, N., Science, 2014, 6187, 1005-1009.
[2] Lichterman, M., Carim, A., McDowell, M., Hu, S., Gray, H., Brunschwig, B., Lewis, N., Energy Environ. Sci., 2014, 7, 3334-3337.

Photoelectrochemical water splitting (PEC) is one of the alternative ways to produce energy using sun light. Thus, intensive work has been devoted to produce an efficient photoelectrode by revisiting the state-of-the-art materials or tailoring new ones. In this study, we report the synthesis, characterization and optical properties of transition metal-doped WO₃ (TM-W). In addition, we attempt to synthesis composites of WO₃/perovskite phase (W-P) and WO₃/carbon nanotubes (W-CNT) in order to establish a heterojunction that is expected to improve the photoconversion efficiency through enhancing the visible light absorption and minimizing charge recombination. Wet chemistry methods have been adopted to prepare highly surface area compositions. Alot of efforts have been taken during composites preparation to assure a good contact between the particles within the composite matrix and the synergistic interaction between the oxide phases and the CNT is explored. All the powders have been characterized by XRD, IR, TG and SEM. The prepared materials are tested under illumination and their optical properties have been thorouly studied. A correlation between the microstructure and the electrical and the optical behaviour is discussed.

Following the report of Fujishima and Honda in 1972,[1] photoelectrochemical (PEC) water splitting has been developed considerably as a promising method to convert solar energy into clean hydrogen fuel. To address the issue of stability in aqueous media, numerous efforts have focused on oxides, particularly n-type semiconductors which serve as photoanodes.[2] Fe₂TiO₅ is a robust and abundant oxide with a small band gap (~2.1 eV), capable of absorbing a significant part of the visible spectrum. Moreover, the conduction band is significantly more negative than the more widely studied hematite, leading to a much earlier onset of photocurrent.[3] Here we demonstrate the synthesis of stoichiometric Fe₂TiO₅ thin films via spin-coating on transparent conductive substrates and employ them as efficient photoanodes exhibiting good photocurrents and durability. Various strategies for enhancing PEC performance have been explored, including extrinsic doping, catalyst loading, nanostructuring and hetero-junction fabrication.


References
[1] Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature, 1972, 238, 37-38.

[2] Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev., 2010, 110, 6446– 6473.

[3] Liu, Q.; He, J.; Yao, T.; Sun, Z.; Cheng, W.; He, S.; Xie, Y.; Peng, Y.; Cheng, H.; Sun, Y.; Jiang, Y.; Hu, F.; Xie, Z.; Yan, W.; Pan, Z.; Wu, Z.; Wei, S. Aligned Fe₂TiO₅-Containing Nanotube Arrays with Low Onset Potential for Visible-Light Water Oxidation. Nat. Commun., 2014, 5: 5122.

A mild solution approach enabled the growth of 1-dimensional CdS nanostructures having high-density and uniform morphology on FTO substrates. It has been well known and proved that vertically aligned 1-D nanostructures can provide the enormous reaction sites and fast charge transport, resulting in the improved photocatalytic performance compared to thin films. Subsequently, the CdO shell was formed on nanorods successfully via the dip coating method. We controlled the thickness of CdO shell on CdS nanorods to obtain the optimum photoelectrochemical performance. The combination of CdS and CdO introduced the Z-scheme energy band structure that caused the increase of PEC performance due to the effective charge separation and higher photovoltage in comparison with bare CdS nanorod electrodes. When the shell thickness was ~3 nm, CdS/CdO nanorod electrodes showed the photocurrent density of ~4.3 mA/cm² at 0 V (vs. SCE). As the shell thickness increased, however, the photocurrent onset potential moved toward the cathodic direction and the photocurrent density became lower in the overall range of the voltage.
To understand the carrier transport behavior between CdS and CdO regarding the shell thickness, we estimated the depletion region width of CdO and the band diagrams with respect to the shell thickness. When the thickness is smaller than ~3 nm, only the space charge region exists in CdO shell, which results in the facile charge separation. When the shell thickness becomes larger than ~3 nm, however, there exists the bulk state which requires the external voltage for effective carrier transportation.
To conclude, the optimum thickness of CdO shell on CdS nanorod electrodes for the effective Z-scheme band structure was estimated to be ~3 nm. Due to the shell thickness smaller than the width of space charge region, the electrons and holes in CdO can be easily separated, which accelerates the photocatalytic reactions on each electrode surface.

Metal oxides have emerged as attractive candidates for photoelectrochemical water splitting, mainly due to their good stability in aqueous solutions, easy synthesis, and low cost. However, in the past, metal oxide photoanodes, such as BiVO₄ or Fe₂O₃, suffered from a low charge injection efficiency. To enhance the performance of these metal oxides for water splitting, their surfaces are usually functionalized with water oxidation catalysts (e.g. CoPi, RhO₂, FeOOH, NiFeO_x, MnO_x). However, the true nature of semiconductor/catalyst/electrolyte interactions and the improvement mechanisms are still unclear, and require further attention.
In this study, we used intensity modulated photocurrent spectroscopy (IMPS) to examine the surface charge carrier dynamics of spray-deposited bismuth vanadate (BiVO₄) photoanodes and to understand the role of a co-catalyst layer and its relation to the semiconductor’s limitation. A LED was used to illuminate the sample with a modulated intensity, and the real and imaginary parts of the opto-electrical impedance were recorded. To interpret the resulting spectra, we used a model developed by Peter et al. that allows one to distinguish the rate constants for surface recombination and charge transfer at the electrode surface. By comparing the bare and the co-catalyzed photoanode systems, we found that the photocurrent of BiVO₄ is mainly limited by surface recombination. The surface recombination was significantly reduced, when CoPi and Ni_xMn_yO_z were deposited on the BiVO₄ photoanodes, whereas the charge transfer did not increase. This result suggests that BiVO₄ is already a good oxidation catalyst by itself, originating from its high thermodynamic driving force. This observation shows that the role of those catalysts on our BiVO₄ is mainly that of a surface passivation layer. Based on this we predict that the modification of BiVO₄ surface with RuO_x, a well-known (non-passivating) oxygen evolution catalyst, does not improve the performance; our experiments show that this is indeed the case. These results allow us to develop a modified carrier dynamics model, which will be discussed here. We further compared BiVO₄ prepared by various deposition techniques—both chemical and physical routes—to demonstrate their influence on the nature of the surface of BiVO₄. The semiconductor/co-catalyst/electrolyte interface was also investigated by depositing CoPi onto those differently prepared BiVO₄ photoanodes. Finally, the diverse effects of porous (ion-permeable) and dense (non-ion-permeable) co-catalyst on the photoelectrochemical performance of such semiconductor/co-catalyst systems will be discussed.

There has been much interest in hot carrier science in the field of renewable energy generation and solar capture. These carriers are generated by illumination and are useful because of their high excess energy which can be extracted as electrical work or for direct fuel generation. However, there has been little work in measuring the energy profile of hot carriers generated in different materials and with different illumination. We propose a general device to retrieve such energy profiles with the use of a sharp electron energy high-pass filter. The device is a multilayer stack of TiO2-Al2O3-TiO2 structure which minimizes smearing effects and transmits a sharp energy cut-off of electrons by direct injection.

With approximately 89 000 TW of usable energy continuously striking the
earth surface, the sun is by far the principal clean energy source
able of overcoming the human energy needs.[1] Photovoltaic (PV) still
the best technology to convert solar into electrical energy; however,
it is strongly dependent on the intermittence of solar irradiance and
complementary technologies are needed to store solar energy; the
conversion into chemical energy in the form of a fuel is a promising
path to overcome this limitation.[2]
Photoelectrochemical (PEC) cell combines solar energy harvesting and
water electrolysis in a single device converting solar radiation and
water into hydrogen and oxygen.[3] Usually PEC devices have a
transparent front window, which is placed between the sunlight and the
photoelectrode, through which the sunlight reaches the surface of the
photoelectrode. The evolved gas bubbles should slip easily through
this front window. If the evolved gases get trapped, the amount of
light intensity reaching the photoelectrode is smaller, affecting the
system overall efficiency. Moreover, to maximize the sunlight
harvesting, PEC devices should operate tilted making the sunlight
obstruction by the evolved gas bubbles a pertinent issue.
A titanium dioxide coating was firstly used due to its photocatalytic
effect to prevent the accumulation of organic fouling that delay
bubbles rolling up, reducing the window’s transparency.[4]
Additionally, TiO2 presents a photo-induced superhydrophilicity state
when exposed to UV light radiation;[5] the latter phenomenon occurs
due to the formation of hydroxyl groups at TiO2 surface.[6] Glass
windows (soda lime glass 2 mm thick) were then coated with a thin film
of TiO2 deposited by spin coating and annealed at 465 oC for 45
minutes. The prepared thin films showed high transparency, crack-free
surface and good adhesion to the glass substrate. The superhydrophilic
behaviour was confirmed measuring the water contact angle (WCA); a WCA
of 0° was obtained after irradiating the sample surface for 30 min
with UV-light (365 nm, 2 Wm-2). For both samples, coated and uncoated,
the transmittance is ca. 90 % in the spectrum range between 350 nm and
800 nm. An experimental setup was assembled to measure the
transparency during the evolution of H2/O2 for coated and uncoated
samples. Higher photocurrent densities were recorded when TiO2 coated
glass windows were used: ca. 10 % higher than the uncoated sample for
hydrogen and oxygen evolutions. The developed TiO2 coating was
assessed for 10 h showing very good stability.
_1._Miller, E. L. (2010). _John Wiley & Sons_, ch. 1: 3-32.
_2._Enteria, N. and A. Akbarzadeh, _Sol Ener Sc and Eng Ap_. 2013: CRC
Press.
_3._Lopes, T., et al., _Sol Ener Mat and Sol Cel_, 128 (2014),
399-410.
_4._Zhang, L., et al., _Ener & Env Sc_, 5 (2012), 7491 - 7507.
_5._Simonsen, M.E., Z. Li, and E.G. Søgaard, _Ap Surf Sc_, 255
(2009), 8054 - 8062.
_6._Hashimoto, K., H. Irie, and A. Fujishima, _Jap Journ of Ap Phys_,
44 (2005) 8269 - 8285.

Hydrogen treatment (H-treatment) on photocatalysts (by simple post annealing under H₂ atmosphere) has received much attention, since Chen et al. reported the dramatically improved water splitting and dye degradation performance on hydrogenated black TiO₂ in 2011 (1). Recently, the Li group reported that this H-treatment can also be applied to improve the photoelectrochemical (PEC) water splitting performance of various metal oxides such as TiO₂, WO₃, Fe₂O₃, and BiVO₄ (2-3). Despite these reported improvements and proposed explanations, it is still not clear to what extent the lattice and electronic structure of the oxides are modified upon H-treatment to obtain such a performance enhancement.
To elucidate the effect of H-treatment in oxides, here we compared the photocurrent and photoconductivity of H-treated BiVO₄ and tungsten (W)-doped BiVO₄. Doping increases the charge carrier density and conductivity, but at the same time it may produce defect states that can act as recombination sites. As a result, one would expect the saturated photocurrent increase, but the onset potential to shift to the positive direction (i.e., decreased photovoltage) due to decreased carrier lifetime. This is exactly what we observed for W-doped BiVO₄, which is also consistent with our previous work (4). However, we observed the opposite for the H-treated BiVO₄; the onset potential shifted to the negative direction, and the saturated photocurrent increased. This suggests that while H-treatment may increase the charge carrier density (and conductivity), it also increases the carrier lifetime, which is atypical for doping. We further verified this by performing time-resolved microwave conductivity (TRMC) and direct conductivity measurement. Finally, we succeeded to detect hydrogen in the BiVO₄ lattice by using nuclear reaction analysis (NRA). The ability to do this will no doubt advance the understanding behind the true enhancement mechanism of H-treatment and provide additional pathways for highly efficient photoelectrodes.

References
(1) X. Chen, L. Liu, P. Y. Yu, S. S. Mao, Science 2011, 331,746-750.
(2) G. Wang, Y. Ling, Y. Li, Nanoscale 2012, 4, 6682-6691.
(3) G. Wang et al. J. Phys. Chem. C 2013, 117, 10957–10964.
(4) F.F. Abdi et al., J. Phys. Chem. Lett. 2013, 4, 2752–2757.

Photoelectrochemical (PEC) water splitting is one of the most promising ways to convert solar energy directly into fuel. PEC water splitting has three main components such as an anode, a cathode, and an electrolyte. In particular, it is achieved by light absorbing semiconductors driving the oxygen evolution and hydrogen evolution reactions. However, most solar water splitting devices have suffered from low solar-to-hydrogen efficiency in spite of the theoretical efficiency of as high as 30% for a single semiconductor light absorber. Thus the primary focus of this field is to engineer the materials for more efficient solar energy conversion to chemical fuel. Facile strategies to improve the efficiency in PEC devices include (1) increasing the photocurrent by engineering the semiconductor absorber layer to yield better light absorption and (2) reducing the rate of electron–hole recombination by surface state passivation. The former is to functionalize the semiconductors electrode with plasmonic metal nanoparticles by localized surface plasmon resonance (LSPR) that can significantly increase the light absorption. And the latter is to passivate a photoanode with 2-dimensional (2D) materials, which facilitate low surface recombination and low corrosion rates.
Here, we present the enhanced performance of PEC water splitting cell based on ZnO nanowires (NWs) photoanode with Ru nanoparticles (NPs) and 2D SnS₂ passivation layer. ZnO NWs with about ~25 nm in diameter and about ~1 μm in length were synthesized by a hydrothermal method. In order to uniformly decorate Ru NPs on the ZnO NWs, an atomic layer deposition (ALD) was used with bis(ethylcyclopentadienyl)ruthenium (Ru(EtCp)₂) as a Ru precursor and an ammonia plasma as a reactant at the deposition temperature of 400 °C. With the size control of Ru NPs, we achieved visible range LSPR effect because of its intrinsic free carrier concentration above 10²² cm^-3. Lastly, ZnO NWs with Ru NPs were conformally passivated by few-layers SnS₂ to improve the kinetics at catalytic layers. The more detailed results of solar water splitting using the ZnO NWs photoanode will be discussed in the Meeting. In addition, the ZnO NWs photoelectrode was created by the ALD method, which offers great compatibility and reproducibility with the fabrication of visible-light water splitting.

The geometrical parameters of metallic nano-structures determine their optical responses. It is appealing to think about a universal light device, in which the plasmonic modes at different frequencies are excited onto a large piece of nano-structure metallic network and harness the electromagnetic (EM) field. However, fabrication of such a metallic network is challenging because it demands fine structures at the nanoscale over a large-scale. Herein we report on a direct strategy to prepare pure, scalable nanoporous metallic networks by physical vapor deposition (PVD). Such nanoporous networks are made of interconnected pure metallic nano-size ligaments of about10-100 nm and multimodal connective nano-pores. Several metallic and metal-oxide networks have been fabricated among them Cu, Ag, Au, Al, Pt, Ti, Fe and TiO₂. The coinage metallic networks are colored and translucent. We characterize their opto-electronic responses, and demonstrate reduction process of molecules adsorbed onto the network due to generation of hot-electrons.
The nano-porous metallic films are panchromatic light absorbers and are capable of absorbing a large fraction of the solar spectrum by plasmonic excitation, which can be followed by generation of hot electrons and holes. The energetic carriers can then be injected to a semiconductor catalyst to carry out a useful chemical processes. Such a metallic network, because of its catalytic activity, can be used both as light absorber and catalyst and thus increase the efficiency of the process.

Direct conversion of solar power to chemical fuel using photoelectrochemical (PEC) water splitting cells provides a promising path for solar energy conversion and storage. Previous work in our group explored resonant light trapping in ultrathin film (~20-30 nm) hematite photoanodes on metallic (Ag) specular back-reflectors as a promising path to achieve high photocurrents.¹ Our next goal is to couple these photoanodes with photovoltaic (PV) cells in order to construct a stand-alone tandem system for solar water splitting.
This work explores an innovative tandem cell design that couples ultrathin film hematite photoanodes with Si PV cells using wavelength-selective dielectric mirrors instead of metallic specular reflectors. Toward this end we develop distributed Bragg reflectors (DBRs) that reflect short wavelength photons (400 < λ < 550 nm) back into the hematite photoanode while transmitting long wavelength photons (550 < λ < 1100 nm) to the PV cell. This design enables simple monolithic integration and the use of more compatible and cheaper materials than Ag. The DBR comprises a multilayer stack of SiO₂ and Nb₂O₅ with an ITO transparent electrode on top. The stack is designed, using optical modeling, to achieve the desired spectral response. Preliminary results of the very first specimens revealed that ~8 nm thick hematite photoanodes on DBR structures reached photocurrent enhancement of ~40% compared to similar films on transparent substrates. Theoretical predictions show that by optimizing the DBR and adding a V-shape cell configuration that retraps back-reflected light, enhancement of up to ~210% in photocurrent can be achieved (compared to a planar photoanode without DBR). Theoretical and empirical results will be presented at the meeting.
¹ Dotan et al., Nature Mater. 12 (2013) 158 – 164.

The design of efficient hydrogen-evolving photocathodes for dye-sensitized photoelectrochemical cells (DSPECs) requires the incorporation of molecular light absorbing chromophores that are capable of delivering reducing equivalents to molecular proton reduction catalysts at rates exceeding those of charge recombination events. Here, we report the functionalization and kinetic analysis of a nanostructured NiO electrode with a modified perylene-3,4-dicarboximide chromophore (PMI) that is stabilized against degradation by atomic layer deposition (ALD) of thick insulating Al₂O₃ layers. Following photoinduced charge injection into NiO, films with Al₂O₃ layers demonstrate longer charge separated lifetimes as characterized via femtosecond transient absorption spectroscopy and photoelectrochemical techniques. The photoelectrochemical behavior of the electrodes in the presence of Co(II) and Ni(II) molecular proton reduction catalysts is examined, revealing reduction of both catalysts. Under prolonged irradiation, evolved H₂ is directly observed by gas chromatography supporting the applicability of PMI embedded in Al₂O₃ as a photocathode architecture in DSPECs.

No single material has shown to be able to efficiently, stable, and scalable split water by photoelectrochemical approaches. Therefore, a decoupling of the required functions into different materials and components has been pursued resulting in extremely complicated multi-component system parts. Photoelectrodes (PEs) are typical examples being composed of multiple photoabsorbers, catalysts, conducting interconnects, and protecting layers. The arrangement of these components is usually not regularly layered but complex and stochastic, resulting from two main reasons: i) desired nano/micro structuring of materials in order to provide an approach to circumvent some of the critical limitations in bulk material properties, and ii) less controllable, cheap and fast processing routes such as dip or slurry coating of electrodes by photocatalytic particles. For the latter, the ability to vary the morphology and arrangement of the single particles allows for adapting the performance of the PEs. In addition, inter-particle necking procedure, surface passivation and co-catalyst deposition can further improve the performance. The impact of morphology, arrangement, and material combinations on multi-physical transport phenomena and, consequently, solar to hydrogen efficiency must be understood to provide design guidelines for high-performing particle-based PE.
In this talk, I will describe multi-scale, experimental-computational characterization and optimization of such morphologically complex multi-component photoelectrodes made of LaTiO₂N. Photocurrent measurements of LaTiO₂N were performed using front and back illumination with amorphous TiO₂ inter-particle necking, NiO_x/CoO_x/Co(OH)₂ catalysts and Ta₂O₅ passivation layer. These measurements were compared with a macroscopic 2D numerical model combining electromagnetic wave propagation, charge transport and conservation in the particles, semiconductor-electrolyte charge transfer and approximating the morphology by equivalent pillars. The morphological and transport characteristics relevant for the macroscopic model are determined based on particle-scale investigations. These investigations consist of obtaining the exact morphology of the PEs via experimental 3D imaging techniques (e.g. focused ion beam tomography) and the subsequent use of the digitalized structure in direct numerical simulations (Monte Carlo and finite volume methodologies) for the characterization of the morphology and transport.
I will show that this multi-scale, experimental-numerical approach can be used for a systematic understanding of the influence of the morphology and material combination (co-catalyst deposition, surface passivation, necking procedure, modification of bulk material properties, etc.) of the photoelectrodes on their performance, for identifying material’s related bottlenecks, and for optimizing the PEs

The key reaction step for solar water splitting on photoanodes is the reduction of water by photo-generated holes. In order for the oxygen evolution reaction (OER) to be efficient, bulk recombination of photo-generated holes must be minimized. Another detrimental process is the recombination of holes and electrons at the surface of the photoanode.

The charge transfer efficiency, η_ct, is the ratio between transferred holes across the photoanode/electrolyte interface and the total number of holes reaching the surface. It helps to differentiate between surface and bulk recombination effects and to identify the performance limiting factors for the efficient operation of photoanodes. Its value varies for material choices and operation conditions such as bias potential, light spectrum, light intensity and electrolyte composition.

In this contribution, the three main methods to determine the charge transfer efficiency of photoanodes are introduced and reviewed critically for the case of hematite photoanodes. Measurements with hole scavengers have been shown to eliminate surface recombination and yield the bulk-limited photocurrent [1]. However, they also change the electrochemical reaction which may modify the surface charge which may in turn modify the bulk-limited photocurrent due to changes in the space charge width and band bending. Therefore, these measurements may lead to inaccurate assessment of the charge transfer efficiency.

The spikes in chopped light measurements indicate the hole current and the difference to the steady-state value has been ascribed to surface recombination [2]. However, simulations show that flawless results require fast shutters and data acquisition. Another issue with chopped light measurements is that large amplitudes in light intensity can lead to changes in band bending and therefore the measurement is not performed in steady-state.

Intensity modulated photocurrent spectroscopy (IMPS) overcomes this problem with a small signal light intensity perturbation. This measurement technique is becoming increasingly more popular. However, the interpretation of the results is currently based on a simplified model that does not account for nonlinearities in the recombination current with respect to light intensity. Additionally, the LEDs mostly used for IMPS measurements have a different light spectrum and intensity as compared to sunlight. Our analysis will demonstrate how these factors affect the calculated charge transfer efficiency.

For a comprehensive assessment of the possibilities and limitations of all three techniques, we will compare results obtained for model hematite photoanodes. This contribution also discusses theoretical background and experimental setup. We will further provide best-practice guidelines for conducting measurements and analyzing the results.

[1] Dotan et al, Energy Environ. Sci., 2011, 4, 958.
[2] Le Formal et al, J. Phys. Chem. C 2012, 116, 26707.

The spatial collection efficiency of photovoltaic and photoelectrochemical devices has critical influence on their performance. Defined as the fraction of photogenerated charge carriers created at a specific point within the device that eventually contribute to the collected photocurrent, measurement of the spatial collection efficiency would help to extract the diffusion length of charge carriers and may shed new light on different charge transport mechanisms. We present a non-destructive empirical method to extract the spatial collection efficiency out of Incident Photons to Current Efficiency (IPCE) measurements. Equipped with knowledge on the optics of the system, which can be accurately measured and modeled, the spatial collection efficiency can be extracted from IPCE measurements with high spatial resolution and without a priori assumptions on the charge transport mechanism.
In this contribution, we demonstrate the method by extracting the spatial collection efficiency profiles of amorphous silicon solar (PV) cells and thin film hematite photoanodes for solar water splitting. The method is validated theoretically by modeling amorphous silicon solar cells where the spatial collection efficiency is calculated directly and extracted from simulated IPCE measurements. Next, we show how the spatial collection efficiency profiles of such cells can be extracted out of available experimental data. Last, the spatial collection efficiency profiles of thin film hematite photoanodes are extracted out of IPCE measurements at several bias potentials, giving new insights on the photoanodes performance. The relative simplicity of the method along with its applicability to probe in operando performance make this method an important tool for analysis and design of new photovoltaic and photoelectrochemical devices.

Impedance spectroscopy is a preferred method of characterization for photoelectrochemical operation of semiconductor photoelectrodes and catalytic layers that enhance the reaction rates. Monitoring the details of charge transfer is still challenging due to a combination of different effects such as recombination and surface trapping/detrapping of carriers that need to be understood for the optimization of the onset photovoltage of the photoanodic reaction and the maximum achievable photoanodic current in water splitting applications. In this contribution we outline the development of a full characterization of the impedance spectroscopy of surface processes, that combines the analysis of capacitances and resistances and the correspondent equivalent circuit models, to give insight into the dominant mechanisms of the operation of semiconductor electrodes in photoelectrochemcial cells.

The rising societal and environmental costs of fossil fuels have driven a resurgence of intense research interest into artificial photosynthesis. Recently, the role of surface states in artificial photosynthesis using a typical semiconductor-water junction has gained considerable research interest. Surface states can work as an effective recombination center for the photogenerated carriers and hinder the minority carrier transport to the liquid, or on the other hand, surface state mediated reaction can facilitate water oxidation at photoanode and reduction at photocathode. In this work, we present a semiclassical method to study the equilibration process of surface states present at semiconductor-liquid junction. It is shown that the commonly used concept of perfect equilibration of surface state alone with semiconductor might not necessarily true. Non-trivial electrostatics, for example, shifting of Mott- Schottky plateau might arise when deep-level surface states in a wide band gap semiconductor partially equilibrates with the liquid. We also make an attempt to tie up recently reported experimental results with the prediction from our model in order to explain the non-linearity appears in the Mott-Schottky plot. In general, the results of this work are intended to drive the development of comprehensive engineering tools for the optimization of photocatalytic performance in solar fuel generation.

Semiconductor water splitting photoelectrode is typically a Schottky junction device, with its energy conversion efficiency largely depending on the mean free path of the photogenerated minority carriers. To date, the trade-off between photon collection and minority carrier delivery remains a persistent obstacle for maximizing the performance of a water splitting photoelectrode. In this talk I will demonstrate that the PEC water splitting efficiency for an n-SrTiO₃ (n-STO) photoanode can be improved very significantly despite its weak indirect band gap optical absorption, by widening the depletion region that is responsible for minority carrier delivery. Featuring a graded doping profile with the bulk heavily doped but the surface lightly doped, the architecture pushes the depletion region and minority carrier mean free path beyond 500 nm, which helps the n-STO photoanodes to achieve high quantum efficiencies (>70%) for the weak indirect transition. This simultaneous optimization of the light absorption, minority carrier (hole) delivery, and majority carrier (electron) transport by means of a graded doping architecture may be useful for other indirect band gap photocatalysts that suffer from a similar problem of weak optical absorption.

With the development in the last few decades, photovoltaic (PV) solar cells convert solar energy to electricity with increasing efficiency and decreasing cost. However, the intermittent nature of sunlight necessitates the storage of the photo-generated electricity, therefore further large-scale deployment of solar energy also depends on scalable and inexpensive energy storage solutions. We will first discuss our recent studies on enhancing the efficiency of photoelectrochemical (PEC) solar energy conversion devices for water splitting to generate hydrogen as the clean high density energy storage carriers. Not only highly efficient electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) and high performance semiconductors need to be employed, but also their effective integration into photocathodes or photoanodes need to carefully studied. High quality catalyst-semiconductor interface has been achieved using various strategies and the optical transparency of the electrocatalysts was also found to be very important. Furthermore, we have developed a novel type of hybrid energy conversion and storage devices by integrating liquid junction regenerative solar cells with electrochemical redox flow batteries (RFBs) using the same redox couples as a potentially more practical near term alternative to PEC water splitting. By using redox couples that have facile kinetics in both liquid junction cells and RFBs batteries, not only can highly efficient solar conversion be achieved, but also it enables the efficient storage of the converted solar energy into chemical energy in charging of the flow batteries that can be readily discharged to provide electricity in a simple cyclable device. We demonstrate this novel concept by first building integrated functional solar cell/redox flow battery devices based on aqueous organic redox couples and mature PV semiconductors. We have demonstrated that such an integrated PEC-RFB device can be charged under solar illumination without external electric bias and deliver a high discharge capacity comparable with state-of-the-art RFBs over many cycles. We further evaluate and demonstrate their excellent energy conversion efficiency, energy storage density and efficiency, and cyclability.

The Dye Sensitized Photoelectrosynthesis Cell (DSPEC) integrates molecular light absorption and catalysis with high band gap oxide semiconductors for solar water splitting into H₂ and O₂ or reduction of CO₂ to carbón-based fuels. Significant advances have been made in this área based on designed chromophore-catalyst assemblies, core/shell oxide structures, and the stabilization of surface binding of phosphonate-derivatized assemblies by atomic layer deposition (ALD) of oxide overlayers.

Efficient water splitting using light as the only energy input is a high impact concept that requires stable semiconductor electrodes with favorable energetics for the water oxidation and proton reduction reactions. Strategies to tune electrode potentials using molecular dipoles adsorbed to the semiconductor surface have been pursued for decades but are often based on weak interactions that are unstable under highly alkaline or acidic conditions relevant to sustained photoelectrolysis. Here, we will discuss our recent work on the covalent attachment of fluorinated aromatic molecules to GaAs(100) surfaces and show that these strong dipoles can tune the photocurrent onset potentials of the photocathodes and reduce the energy required for water splitting. Results indicate that photocurrent onset potentials for proton reduction can be shifted nearly 100 mV and that the modified photocathodes exhibit extended operation in pH –0.5 electrolyte under 1 sun illumination. Related work on interfacial photochemistry of other III–V as well as Si electrode surfaces will be discussed.

During this talk, I will present our progress toward a 20% solar-to-hydrogen (STH) efficiency water splitting photocathode based on III-V semiconductors. We incorporated several key solid-state technological advances to achieve unprecedented efficiencies exceeding 16% STH. The first improvement was to increase the device current via a non-lattice-matched 1.2 eV InGaAs grown using the inverted metamorphic multijunction (IMM) technique developed by NREL. The second modification that led to device improvement was to add a thin n-GaInP2 layer to p-GaInP2 to generate a "buried junction", which increased the photocurrent onset or Voc of the device by several hundred mV and enabled 14% STH efficiency. Finally, we increased the top junction photon conversion efficiency by adding an AlInP "window layer", which is commonly used in solid-state PV devices to reduce surface recombination. Through the use of a collimating tube, we measured our devices outdoors under direct solar illumination and verified over 16% STH conversion efficiency. I will also briefly discuss common experimental pitfalls that can influence the accuracy of measured STH efficiencies of mulitjunction absorbers.

Despite excellent materials properties and record-high solar-to-hydrogen efficiency, existing challenges including high material cost and fast degradation of performance make it hard to justify the use of III-V compound semiconductors in photoelectrochemical solar fuel generation. Here we present an alternative approach that can circumvent many of these difficulties by exploiting a novel heterogeneously integrated electrode platform based on epitaxially grown III-V compound semiconductors in conjunction with printing based materials assemblies. A thin film stack of GaAs-based epitaxial materials with a buried solid-state junction is released from the growth wafer and printed onto a non-native transparent substrate to form integrated photocatalytic electrodes for solar-driven water splitting. Bifacial electrode configurations together with specialized epitaxial design effectively decouple materials interfaces for light absorption and electrocatalysis, and therefore allow independent control and optimization of light absorption, carrier transport, charge transfer, as well as materials stability, all of which synergistically contributed to the significant enhancement of the system efficiency and life time of GaAs-based photocathodes in solar water splitting. Unassisted water splitting at a high solar-to-hydrogen efficiency is enabled with a series-connected tandem configuration of integrated GaAs photocathodes and GaAs photoanodes.

This study demonstrates a novel method to prepare high aspect ratio p-type macroporous GaP photocathodes by anodic etching of an undoped, instrinsically n-type GaP(100) wafer followed by thermal drive-in doping with Zn from ZnO films prepared by atomic layer deposition (ALD). Under 100 mW cm^-2 white light illumination, the resulting Zn-doped macroporous p-type GaP consistently exhibit strong cathodic photocurrent when measured in aqueous electrolyte containing methyl viologen, with photocurrent density larger than its planar featureless counterpart prepared using the same method. Wavelength-dependent external quantum yield measurements of the Zn-doped macroporous GaP revealed enhanced collection efficiency at wavelengths longer than 460 nm, indicating the ALD doping step was effective in rendering the entire material p-type and promoting preferential migration of photogenerated minority carriers to the GaP/electrolyte interface. Collectively, this work presents a new, straightforward doping strategy with potentially high degree of controllability for high aspect ratio III-V materials. This study therefore demonstrates the power of ALD for doping for the assembly of photoelectrochemical systems – an important advancement in the development of high performance photocathodes for solar fuel generation.

Photoelectrochemical water splitting offers the possibility to convert solar energy directly into a chemical fuel and therefore is a promising candidate for a sustainable energy solution in the future. GaInP₂ with a direct bandgap of 1.8–1.9 eV show so far the highest reported solar-to-hydrogen conversion efficiencies. Nevertheless, the fast photocorrosion of III–V semiconductors in aqueous solutions presents a major obstacle for their use as efficient and stable photoelectrodes. So, it is important to study the interaction of these semiconductors with aqueous electrolyte solutions under photoelectrochemical conditions to gain insight into the reaction mechanisms at the semiconductor/electrolyte interfaces. To better understand the processes that occur during water splitting at the GaInP₂/electrolyte interface, we studied the interaction of GaInP₂(100) surfaces with different aqueous solutions. Using synchrotron-radiation photoemission spectroscopy we perform quasi-in-situ analysis of the reactions occurring at the p-GaInP₂(100)/aqueous solutions interfaces under (photo)(electro)chemical conditions.
On interaction with 1M aqueous HCl solution the amount of oxides left on the surface is reduced. On exposure under open circuit potential the etched p-GaInP₂(100) surface remains in equilibrium with the solution and its composition does not changes even after prolonged exposure to the solution. Once a cathodic bias less than –1.0 V vs. RHE is applied in the dark, the amount of oxides on the surface decreases. An increase of the cathodic bias to –1.8 V results in a further decrease in the oxides content and in a gain of the OH-related component of the O 1s photoemission. At the same time, traces of metallic gallium appear at the surface. The application of a cathodic bias of –0.7 V under visible light irradiation leads to an increase in the OH-related component of the O 1s spectrum, in metallic Ga accumulation and in a decrease in the oxides content at the surface. Further increase of the cathodic bias to –1.8 V results in a considerable increase in both the oxide and the OH-related components of the O 1s spectrum with metallic gallium accumulation. So, the electrochemical water splitting at the p-GaInP₂(100)/HCl_aq interface is always accompanied by metallic gallium accumulation. Water splitting efficiency increases with increased bias and applied illumination. However, under these conditions an oxidation of the semiconductor surface can take place.
Exposure of the oxide-free GaInP₂ surfaces to acidic (0.1M H₂SO₄) and alkaline (0.1M KOH) aqueous solutions results in the formation of surface phosphates and oxides/hydroxides. Thus, a clean GaInP₂ surface is not stable in aqueous solutions even for a short period. The self-passivating phosphate/hydroxide layer is formed at the GaInP₂/aqueous solution interface under open circuit potential, but under (photo)electrochemical conditions additional measures are necessary for surface stabilization.

High-efficiency direct solar-to-fuel energy conversion can be achieved using a photoelectrochemical (PEC) device consisting of an n-type photoanode in tandem with a p-type photocathode. However, the development of robust and inexpensive photoelectrodes are needed to make PEC devices economically viable. In this presentation our laboratory’s progress in the development new materials for economically-prepared, high performance photoelectrodes will be discussed along with the application toward overall PEC water splitting tandem cells for H₂ production. Specifically, this talk will focus on the application of ternary oxide CuFeO₂ and 2D transition metal dicalcogenides as solution-processed photoelectrodes.

With respect to CuFeO₂, in our recent work [1] we demonstrate state-of-the-art sacrificial p-type photocurrent with optimized nanostructuring. Recent results addressing interfacial recombination by the electrochemical characterization of the surface states will be presented along with approaches to overcome the limitations of this material.

In addition, two-dimensional (2-D) transition metal dicalcogenides (TMDs) generally have intriguing electronic properties making them promising candidates for high-efficiency solar energy conversion. However, it is notoriously difficult to fabricate thin films of 2-D TMDs over the large areas required to convert solar energy on a practical scale. We recently developed a simple method to fabricate high-quality thin films of 2-D layered TMDs at low cost and with good efficiency towards solar-to-fuel energy conversion [2]. The challenges with charge transport, separation and water redox catalysis in these systems will also be discussed with respect to the 2D flake size.


References
[1] Prevot, M. S.; Li, Y.; Guijarro, N.; Sivula, K. J. Mater. Chem. A 2016, 4, 3018-3026.
[2] Yu, X.; Prevot, M. S.; Guijarro, N.; Sivula, K., Nat. Commun. 2015, 6, 7596.

Photoelectrochemical water splitting is a promising route to convert solar energy into chemical energy for the quest of a clean energy conversion system. Currently the most efficient photocathodes are based on Si, GaInP₂, GaP and copper indium gallium sulphide/selenide (CIGS). Although these materials exhibit high photocurrents, they either constitute rare metals or require high cost processing techniques. However in order to rival the traditional photovoltaic plus electrolysis, there is a need to develop highly efficient photoelectrochemical cells using cheap and earth abundant materials.
In this work, copper sulphide (Cu_2-xS) photocathodes were prepared using a simple sulphurization of copper oxide (Cu₂O). X-ray diffraction (XRD) and scanning electron microscopy (SEM-EDX) indicate that the films were Cu_2-xS. These films were found to be highly degenerate from the Mott-Schottky analysis (on the order of 10²² cm^-3). Photoelectrochemical cells (PEC) were fabricated using these films by using CdS as a n-type electron extraction layer followed by a protective TiO₂ layer and a Pt catalyst. Photocurrents of ~ 1.8 mA/cm² were obtained from this PEC device at 0 V vs RHE in a pH 5 phosphate buffer of 0.5 M Na₂SO₄. Various strategies to improve the photocurrent and stability of these PECs will be discussed.

An attractive way of converting solar energy to storable hydrogen fuel is a photoelectrochemical (PEC) water splitting using light absorbing semiconductors. A careful selection of the semiconductor photoelectrode is necessary to ensure efficient and stable evolution of the hydrogen. Chalcogenide compound, Cu(In,Ga)Se₂ (CIGS) is a promising photocathode given its successful application as a light absorber in thin film solar cells. Here, we investigated PEC performance and stability of the CIGS photocathode with functional overlayers using multiple scanned linear sweep voltammetry (LSV). Various functional overlayers such as CdS, Zn_xSn_yO_z, TiO₂, and combinations of the aforementioned were prepared on to the CIGS photocathode to enhance the performance and the stability. Among these, the CIGS/CdS/TiO₂/Pt photocathode showed the best current-potential characteristics and half-cell solar-to-hydrogen efficiency (HC-STH) of 2.63 %. However, the repeated LSV resulted in the degradation of the PEC performance. Through careful chemical analysis we have found that the degradation was due to not only the photocorrosion, but also the redistribution of atomic elements constituting photocathode stacks. Details of analysis will be presented and discussed.

Cupric oxide (CuO) is a promising absorber material for the fabrication of scalable, low cost solar energy conversion devices, due to the high abundance and low toxicity of copper. It is a p-type semiconductor with a band gap of around 1.5 eV, absorbing a significant portion of the solar spectrum. One of the main challenges in using CuO as solar absorber in an aqueous system is its tendency towards photocorrosion, generating Cu₂O and metallic Cu. Although there have been several reports of CuO as a photocathode for hydrogen production, it is unclear how much of the observed current actually corresponds to H₂ evolution, as the inevitability of photocorrosion is usually not addressed.

We investigated the effect of the deposition of overlayers onto CuO thin films for the purpose of enhancing its photostability as well as performance for water splitting applications. CuO thin films were fabricated by electrodeposition of metallic copper onto gold-coated FTO substrates, followed by annealing in air at 600 ^oC. Photoelectrochemical measurement of the bare CuO film in 1M phosphate buffer (pH 7) under simulated AM 1.5 sunlight (100 mW cm^–2) showed a current density of ca. 1.5 mA cm^-2 (at 0.4 V_RHE), which photocorroded to Cu metal upon prolonged illumination. This photocorrosion could be suppressed by deposition of 50 nm-thick TiO₂, deposited by atomic layer deposition. In addition, we found that insertion of an n-type CdS layer, deposited by chemical bath deposition, between the CuO and TiO₂ layers was able to enhance significantly the photocurrent compared to without the CdS layer. Photocurrents exceeding 2 mA cm^-2 (at 0 V_RHE) and relatively positive onset potential (ca. 0.9 V_RHE) were observed using the photocathode stack FTO/Au/CuO/CdS/TiO₂/Pt. Structural, electrochemical, and photostability characterizations of the photocathode as well as results on various overlayers will be presented.

The computational design of solid catalysts has become a field of great interest during the last decades. With the recent increase in computational tool performance, new insights can be obtained by modelling the electronic structure of catalytic materials. Specifically, the electronic structure implications on catalysis for NiOOH, one of our best water oxidation catalysts will be discussed. Key issues include: band edge positions, band edge chemical character, metal-oxygen bond iconicity, and catalytic overpotential. Our results suggest that chemical activity of NiOOH can be controlled by composition design strategies.

References:
J. Zaffran and M. Caspary Toroker, “Metal-oxygen bond ionicity as an efficient descriptor for doped NiOOH photocatalytic activity”, ChemPhysChem 17, 1 (2016).
J. Zaffran and M. Caspary Toroker, “Designing efficient doped NiOOH catalysts for water splitting with first principles calculations”, ChemistrySelect 1(5), 911 (2016).
V. Fidelsky and M. Caspary Toroker, “Engineering band edge positions of nickel oxyhydroxide through facet selection”, J. Phys. Chem. C 120, 8104 (2016).
V. Butera and M. Caspary Toroker, “Electronic properties of pure and Fe-doped beta-Ni(OH)2: New insights using density functional theory with a cluster approach”, J. Phys. Chem. C, in press (2016).

Ni is an important element widely applied in structural alloys, solid oxide fuel cells, lithium-ion batteries, capacitors, and photonic/electrochemical catalysts, where the electrochemical stability of the involved Ni metals/compounds play a key role. Electrochemical stability can be well described by Pourbaix diagram, i.e., phase equilibria with respect to electrode potential and solution pH. Remarkably, a reliable Ni Pourbaix diagram is still lacking, although it has been simulated using multiple levels of materials theory for decades, owing in part to inaccurate experimental formation energies used as input for the calibration of prior simulations. In this work, the performances of various density functional theory (DFT) methods in obtaining formation energies and electrochemical stabilities of Ni compounds (oxides, hydroxides, and oxyhydroxides) are systematically benchmarked and explained. With this understanding, we construct a first-principles Ni Pourbaix diagram that agrees with many reported electrochemical phenomena. The stabilities of NiO and Ni(OH)2 in solutions at pH 2.9 through 14 were examined using electrochemical impedance spectroscopy (EIS) and surface-enhanced Raman spectroscopy (SERS). Thes modern experimental results further validate our new Ni Pourbaix diagrams. The accurate electrochemical stabilities of Ni metal and compounds calculated here can facilitate the understanding and design of Ni-based materials/devices in many fields.

This work was sponsored by the U.S. Office of Naval Research (ONR) under the MURI program “Understanding Atomic Scale Structure in Four Dimensions to Design and Control Corrosion Resistant Alloys” through grant no. N00014-14-1-0675.

Solar-based production of organic fuels (e.g. methanol) through the reduction of carbon dioxide (CO₂) had attracted great attention in the last decades. One of the challenges is to find Earth abundant materials that meet at the same time the properties of efficient sunlight absorption and suitable band edge potentials, facilitating photon-driven charge transfer reactions. In this work, we have employed quasi-particle theory and time-dependent density functional theory to achieve fundamental understanding of the electronic structure and optical properties of two relevant photocatalysts, namely C₃N₄ in the graphitic structures (g-h-triazine and g-h-heptazine) and Ta₂O₅. The aim is to assess the photocatalytic power of these compounds and establish structure-properties relationships. The calculations have been carried out within the framework of semilocal and van der Waals (vdW) exchange-correlation functionals for DFT. The quasi-particle energies were calculated by the non-self-consistent GW approximation. The calculated band gaps are 2.92 and 2.94 eV for g-h-triazine and g-h-heptazine, respectively, which are in good agreement with previous theoretical findings. Our results show that graphitic g-h-triazine displays the band edge potentials at suitable positions for direct CO₂ reduction to formic acid with a catalytic power of about 1 eV. We have also carried out a thorough investigation of surface states of Ta₂O₅ assessing the band alignments for different non-polar low-index facets. We have found that the conduction band lies between 2.68 and 3.2 eV from the vacuum level, depending on the orientation. These results provide the basis for further improvement of the materials properties for application as suitable photocatalyst.

Selecting suitable material(s) for water splitting is an intricate dilemma, as materials with high solar-to-hydrogen conversion are typically not stable in aqueous environment and/or are scarce, whereas stable and abundant materials often exhibit unacceptable performance for commercialization [1]. For example, Fe₂O₃ is an abundant n-type semiconductor that has excellent stability in neutral and alkaline electrolytes, but so far the reported solar-to-hydrogen conversion efficiency has not exceeded 3% [2]. A major factor hampering the performance Fe₂O₃, is the high charge recombination rate inside the semiconductor. Thus, there is a need to develop methods and designs to reduce charge recombination.

Recently we showed that by joining two different oxides we were able to control the charge recombination rate [3]. The control mechanism relies on the formation of dipole-like electric fields at the interface which, depending on the field direction, attract or repel minority carriers from the interface. Here we investigate the built-in electric field generated at the interface of Fe₂O₃/TiO₂ using first-principles methods. The results show how electronic band alignment and defects doping at the interface determine the direction and strength of the built-in field. Our understanding of the oxide nanostructures can be employed for designing and improving the performance of water-splitting photoelectrodes.


[1]: A. Fujishima, X. Zhang and D. A. Tryk, Surf. Sci. Rep., 2008, 6, 515-582.
[2]: D. K. Bora, A. Braun, E. C. Constable, Energy Environ. Sci., 2013, 6, 407-425.
[3]: B. Iandolo,B. Wickman, E. Svensson, D. Paulsson, and A. Hellman, Nano Lett., 2016, 16, 2381–2386.

Realizing large-scale hydrogen generation from photoelectrochemical (PEC) water splitting requires engineering devices that are both efficient and robust. This implies long-term stability of both the electronic and optical properties of all layers in the device to yield the desired charge transfer and catalytic activity. Dual-absorber hybrid photoelectodes that rely on p-type absorbers and n-type contact layers are among the most promising designs for maximizing the solar-to-hydrogen (STH) efficiency in PEC devices, but the influence of hydrogen incorporation on their long-term performance remains poorly understood. Here we use hybrid functional calculations to investigate the behavior of hydrogen-related point defects and their impact on the electronic structure for a large class of candidate chalcopyrite absorbers within the group I-III-VI₂ system (I=Ag,Cu ; III=B,In,Ga,Al ; VI=S,Se). We identify materials that may be more tolerant to hydrogen incorporation and discuss associated changes in the electronic structure that may influence overall device performance. Our results help identify possibly absorber candidates for realizing high-efficiency STH conversion in these types of devices.


This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and funded by the Department of Energy office of Energy Efficiency & Renewable Energy (EERE).

The fabrication of porous nanocomposites is key to the advancement of energy conversion and storage devices that interface with electrolytes. Bismuth vanadate, BiVO₄, is a promising oxide for solar water splitting where the controlled fabrication of BiVO₄ layers within porous, conducting scaffolds has remained challenging. Thus far all reports have suffered from either non-uniform depositions or used cathodic deposition that limits the use of hole-blocking layers at heterojunctions. Here ALD brings a significant advantage in that controlled films may be grown on arbitrary 3D porous scaffolds. The ALD of bismuth vanadates are reported here from commercial reagents. The resulting films have tunable stoichiometry and may be crystallized to form the photoactive scheelite structure of BiVO₄. A selective etch process was used with vanadium-rich depositions to enable phase-pure BiVO₄ after spinodal decomposition. BiVO₄ thin films were measured for photoelectrochemical performance under AM 1.5 illumination and demonstrated efficient charge separation with a hole-scavenging sulfite electrolyte. The capability to deposit conformal BiVO₄ will enable a new generation of nanocomposite architectures for solar water splitting.

Bismuth vanadate (BiVO₄) is one of the most efficient metal oxides for solar water splitting applications. It is an n-type photoanode material, with a bandgap energy of 2.4 eV, a theoretical efficiency of ~9% STH, and is made of cheap, earth abundant, non-toxic elements. However, it suffers from substantial recombination losses that limit its performance to well below its theoretical maximum. While deposition of a co-catalyst¹, doping², nanostructuring³ and passivation⁴ have been reported to successfully improve the overall PEC activity of BiVO₄, they add extra steps to the material processing and introduce significant complexity in optimizing a practical PEC water splitting device.

In our recent work⁵ we have reported for the first time on a photoelectrochemical procedure called photocharging (PC), that enables to successfully address limitations of BiVO₄ photoanodes. We have demonstrated that BiVO₄ photoanodes immersed in an aqueous electrolyte under open circuit conditions and exposed to simulated solar irradiation for prolonged time can greatly increase its solar water oxidation efficiency via a reduced onset potential and an increased maximum photocurrent density. According to our results, the presence of a liquid electrolyte is essential to the PC effect, and no PC can take place without it. These findings indicate that the PC effect should be studied in the context of the semiconductor-liquid junction (SLJ), the key interface in any PEC system

Herein we provide new insights on the possible mechanisms of photocharging in BiVO₄ photoanodes, and how it directly leads to improved PEC performance. We show that alkaline conditions favor the PC effect, specifically BiVO₄ photoanodes subjected to PC treatment in pH 10 achieve a record high photocurrent for undoped and uncatalyzed BiVO₄ of 4.3 mAcm^-2 @ 1.23 V_RHE, an outstandingly low onset potential of 0.25 V_RHE, and a very steep photocurrent onset. Alkaline conditions also facilitate excellent external and internal quantum efficiencies of 75 and 95 % respectively (average in the 440 nm > λ > 330 nm range). Moreover, impedance spectroscopy and in-situ X-ray absorption study suggests that electronic, structural and chemical properties of the bulk of these films remain fairly unchanged during the PC treatment, however, appreciable changes in the surface-related properties take place. Ultimately, our results indicate that the improved activity of PC-BiVO₄ is enhanced by surface reaction pathways of the SLJ, which is directly correlated to the electrochemical environment it is modified in.

1. D.K. Zhong, S. Choi, D.R. Gamelin, J. Am. Chem. Soc. 2011, 133, 18370
2. F. F. Abdi, N. Firet, R. van de Krol, ChemCatChem 2013, 5, 490
3. T. W. Kim and K. Choi, Science 2014, 343, 990
4. M. T. McDowell et al., J. Phys. Chem. C 2014, 118, 19618−19624
5. B. J. Trzesniewski and W. A. Smith, J. Mater. Chem. A 2016, 4, 2919-2926

Artificial photosynthesis relies on the availability of stable semiconductors that efficiently harness solar radiation and convert it into chemical energy for sustainable production of solar fuels. Because of the thermodynamic and kinetic challenges associated with oxygen production from water and the harsh conditions in which this reaction is performed, much attention has been devoted to the study of stable n-type metal oxide semiconductors. Among available candidates for photoanodes, bismuth vanadate (BiVO₄) is a promising and widely utilized metal oxide that absorbs in the visible range and possesses valence and conduction band positions that are well suited for water oxidation. In this work¹, we present a detailed study on the corrosion and photocorrosion of BiVO₄ through analysis of changes in morphology, bulk, and surface composition as a function of the photoelectrochemical conditions. We find that BiVO₄ degradation is accelerated, in decreasing order, by light, increasing pH, and applied bias. Electron microscopy and in-situ electrochemical atomic force microscopy indicate that degradation initiates at and propagates from surfaces and grain boundaries at solid/liquid interfaces. From a thermodynamic perspective, it is expected that BiVO₄ should self-passivate via formation of a chemically stable and less catalytically active bismuth oxide at its surface. In contrast, we observe the dissolution of both Bi and V ions into the electrolyte solution, and conclude that self-passivation does not prevent corrosion. By using computational methods, we surmise that the degradation of this material cannot be described by thermodynamic considerations alone and that kinetic limitations must be taken into account to understand this process. In addition, our findings suggest that the accumulation of photogenerated holes at the surface accelerate degradation. Our methodology can be applied for the investigation of stability of new functional materials under operating conditions, where kinetic factors should be considered in the search for stable and visible-light-absorbing novel semiconductors for solar-to-fuel conversion.

1 F. M. Toma et al, Nature Communications, 2016, in press

BiVO₄ has become the top-performing semiconductor among photoanodes for photoelectrochemical water oxidation, which is important for overall water-splitting and artificial photosynthesis. However, BiVO₄ photoanodes are still limited to a fraction of the theoretically-possible photocurrent at low applied voltages because of poor electron transport properties and a trade-off between light absorption and charge separation efficiencies. Here, we investigate photoanodes composed of thin layers of BiVO₄ coated onto Sb-doped SnO₂ (Sb:SnO₂) nanorod-arrays (Sb:SnO₂/BiVO₄ NRAs) and demonstrate a new record for the product of light absorption and charge separation efficiencies (η_abs × η_sep) of ~ 52% for at an applied voltage of 0.6 V versus the reversible hydrogen electrode, as determined by integration of the quantum efficiency over the standard AM 1.5G spectrum, and η_abs × η_sep of ~76.2% at 0.6 V_RHE for monochromatic light with wavelength of 450 nm. To the best of our knowledge, these are among the highest η_abs × η_sep efficiencies achieved to date at this voltage for any BiVO₄ photoanode. Moreover, although WO₃ has recently been extensively studied as a core material for core/shell BiVO₄ photoanodes, the Sb:SnO₂/BiVO₄ NRAs generate larger photocurrents, especially at low applied voltages. In addition, we present control experiments on planar Sb:SnO₂/BiVO₄ and WO₃/BiVO₄ heterojunctions, which indicate that Sb:SnO₂ is more favorable as a core material. These results indicate that integration of Sb:SnO₂ nanorod cores with other successful strategies such as doping and coating with oxygen evolution catalysts, can move the performance of BiVO₄ and related photocatalysts closer to their theoretical potential.

L. Zhou, C. Zhao, B. Giri, P. Allen, X. Xu, H. Joshi, Y. Fan, L. V. Titova, and P. M. Rao, “High Light Absorption and Charge Separation Efficiency at Low Applied Voltage from Sb-Doped SnO₂/BiVO₄ Core/Shell Nanorod-Array Photoanodes”, Nano Letters, 16 (6), 3463–3474, 2016.

Metal oxides are particularly attractive as photoelectrode materials for solar water splitting, mainly due to their general aqueous stability and low cost. However, there is a deficiency in their semiconducting properties due to poor carrier transport (small polarons) and high defect densities. This is especially true for complex metal oxides, where even small deviations from the ideal cation stoichiometry can generate high defect concentrations (e.g., 0.1% deviation can result in 10¹⁹ defects per cm³). In addition, the presence of oxygen vacancies and non-crystalline phases that are often formed when depositing complex oxides at moderate temperatures can cause further complications. One way to alleviate these limitations is to apply a high temperature treatment in an oxygen-rich environment [1,2]. However, systematic studies on the effect of such heat treatments on the defect properties and their influence on the photoelectrochemical (PEC) performance are scarce in the literature.

In this work, we investigate the defect behaviour of BiVO₄ powder and films at high temperature. BiVO₄ is currently the highest performing metal oxide photoanode [3,4], yet little is known on the effect of high temperature treatments. Upon heating to 700 °C in air, we observed crystal lattice restructuring. X-ray diffractogram still showed the monoclinic scheelite phase, but the crystal orientation changed from (1 2 1) to (0 4 0). Raman spectroscopy also revealed a shift of the symmetric V-O stretching mode peak at ~825 cm^-1. To investigate this further, we performed a mass spectrometry coupled thermogravimetric analysis and found that vanadium species (in the form of V, VO and VO₂) leave the lattice at this temperature. XPS analysis showed a consistent decrease of the V/Bi ratio after the high temperature treatment, indicating the presence of vanadium vacancies. We propose that these vacancies are responsible for the intraband absorption that is observed above 500 °C with in-situ UV-vis measurements. We found that the loss of vanadium can be suppressed by performing the annealing in a V-rich environment. This strategy offers a general approach to prevent changes in the cation stoichiometry during high temperature treatment of complex metal oxides. Finally, the implications of the high temperature treatment on the conductivity and PEC activity of BiVO₄, as well as the influence of annealing conditions, will be discussed.
[1] Thalluri et al., Ind. Eng. Chem. Res. 52 (2013) 17414
[2] Sivula et. al., J. Am. Chem. Soc. 132 (2010) 7436
[3] Pihosh et al., Sci. Rep. 5:11141 (2015)
[4] Abdi et al., Nat. Commun. 4:2195 (2013

Solar induced water splitting is a promising route to produce green and renewable hydrogen fuel. Photoelectrodes for solar water splitting must employ a semiconductor with unique combination of visible light absorption, stability in aqueous solutions, low cost and abundance. Iron oxide (a-Fe₂O₃, hematite) is one of few materials meeting these criteria, but its poor transport properties and fast recombination present challenges for efficient solar hydrogen production.
We developed an innovative photoelectrode design which overcomes these challenges using ultrathin (20-30 nm) quarter-wave films on specular back reflectors [1]. Our simple optical cavity effectively traps the light in otherwise nearly translucent ultrathin films, amplifying the intensity close to the surface wherein photo-generated charge carriers can reach the surface and split water before recombination takes place. This design enables us to explore optimization strategies from thin film photovoltaics for the development of efficient hematite photoelectrodes for water splitting.
In this talk I will present new advances in the development of high efficiency thin film hematite photoelectrodes, including the effect of different dopants and doping levels [2]; pn junctions [3]; heteroepitaxial thin films of high crystalline quality and specific orientations [4]; concentrated solar water splitting [5]; and integration with PV cells for tandem PEC-PV solar cells.

References:
[1] Hen Dotan, … Avner Rothschild, Resonant light trapping in ultrathin films for water splitting, Nature Materials 12, 158-164 (2013).
[2] Kirtiman Deo Malviya, Hen Dotan,… Avner Rothschild, Systematic comparison of differentto dopants in thin film hematite (a-Fe₂O₃) photoanodes for solar water splitting, Journal of Materials Chemistry A 4, 3091-3099 (2016).
[3] Asaf Kay, Hen Dotan, … Avner Rothschild, Heterogeneous doping to improve charge separation and collection in thin film hematite photoanodes for solar water splitting (in preparation).
[4] Daniel Grave, Hen Dotan, … Avner Rothschild, Heteroepitaxial hematite photoanodes as a model system for solar water splitting, Journal of Materials Chemistry A 4, 3052-3060 (2016).
[5] Gideon Segev, Hen Dotan, … Avner Rothschild, High solar flux concentration water splitting with hematite (a-Fe₂O₃) photoanodes, Advanced Energy Materials 6, 1500817 (2016).

Solar energy is the largest and most widespread source of renewable energy. The market for photovoltaics (PV) is growing around 40 % per year; however, a PV cell only works during daylight time making critical the development of efficient energy storage technologies ¹. The direct production of chemical fuels from sunlight is a promising route and, in particular, hydrogen generated via photoelectrochemical (PEC) water splitting represents a clean and energy-dense fuel.
A photoelectrochemical cell can be used for converting sunlight and water into hydrogen, but developing an energy efficient and stable photoelectrode has revealed to be a great challenge. Hematite is emerging as one of the most attractive materials, offering a favorable combination of low price, abundance, non-toxicity, good visible light absorption thanks to its bandgap of ca. 2.1 eV (maximum thermodynamic efficiency of 16.8 %) and excellent chemical stability ^2,3. However, the performance of hematite photoanodes is very sensitive to the deposition method, either in terms of efficiency and stability. In this work, bare hematite thin films were prepared by spray pyrolysis and systematically optimized following a design of experiments approach. The optimized hematite photoelectrode is ca. 19 nm thick and revealed to be stable over 1000 h of PEC operation under 1-sun of simulated sunlight – a record-breaking result with no evidences of hematite film degradation nor current density loss.
Despite these encouraging characteristics, the use of hematite in practical PEC devices is limited mainly by its photopotential and photocurrent. The present work improved the low photopotential combining an 800 °C annealing with the use of a RuO₂ co-catalyst. The optimized RuO₂-coated hematite photoelectrode enabled a turn-on potential of 0.52 V_RHE and a photopotential of 0.95 V. A photocurrent density of ca. 0.98 mA/cm² was obtained at 1.23 V_RHE, corresponding to around 50 % increase compared with bare hematite. RuO₂ proved to be a highly active co-catalyst for the water oxidation on hematite photoanodes, allowing fast charge transfer and reduced electron-hole recombination at its surface. The developed hematite photoelectrodes allows tandem integration with a DSC or PSC photovoltaic cell for unbiased water splitting.

(1) Krol, R. In Photoelectrochemical Hydrogen Production; van de Krol, R., Graetzel, M., Eds.; Springer US: 2012; Vol. 102, p 3.
(2) Sivula, K.; Le Formal, F.; Graetzel, M. ChemSusChem 2011, 4, 432.
(3) Warren, S. C.; Voïtchovsky, K.; Dotan, H.; Leroy, C. M.; Cornuz, M.; Stellacci, F.; Hébert, C.; Rothschild, A.; Graetzel, M. Nat Mater 2013, 12, 842.

Solar-powered photoelectrochemical (PEC) water splitting has been a promising candidate for producing hydrogen energy in a clean and renewable way. Photoelectrodes are key components in PEC cells for efficient and stable hydrogen generation because they play crucial roles in photon harvesting, charge separation and transport, as well as the catalytic reactions with water. A variety of nanostructured metal oxides for photoelectrodes have been intensively explored as they offer a lot of advantages including effective charge transport and large surface area. However, large band gap energies of typical metal oxides impose a limitation on light absorption in the visible wavelength region.
Fe₂O₃ (hematite) has been widely studied for efficient PEC water splitting because its band gap energy is small (2.0-2.2eV), which can absorb visible solar irradiation, as well as it is cheap and earth-abundant, and has an appropriate band edge position for water oxidation. However, it has critical limitations such as low carrier mobility and short diffusion length of minority carriers. Many efforts have been made to overcome the drawbacks through using a thin Fe₂O₃ layer, which causes another problem of poor light absorption.
In this study, three-dimensional SnO₂ nanohelixes (NHs) were fabricated as scaffolds for a thin Fe₂O₃ light-harvesting layer on fluorine-doped tin oxide glass substrate by using oblique angle deposition method. SnO₂ NHs based photoelectrode can simultaneously enhance light absorption and charge separation and transport for efficient PEC water splitting by trapping the incident light through strong light scattering effect as well as by providing pathways for charge separation and transport and large surface area. In addition, SnO₂ NHs are easy to be hybridized with Fe₂O₃ through only simple solution-based spin-coating method due to its high porosity and aspect ratio. Very thin Fe₂O₃ layer was coated along the surface of SnO₂ NHs forming a type-II band hetero-junction and these Fe₂O₃ coated SnO₂ NHs are significantly desirable for efficient PEC water splitting by simultaneously improving light absorption, charge separation and transport. Highly efficient PEC performance of our hybridized photoelectrode (including approximately 4.0mAcm^-2 photocurrent density at 1.23V_RHE which is a significantly excellent performance for Fe₂O₃ based photoelectode) will be investigated and the advantages of three-dimensional SnO₂ NHs as scaffolds for very thin Fe₂O₃ layer as well as its potential for practical application will be discussed in detail based on electrochemical analysis, FEM simulation, angular-dependent reflectance, PEC analysis at different intensity of incident light, etc.

Iron oxide (α-Fe₂O₃, hematite) is a promising photoanode candidate for solar water splitting, but fast recombination and slow transport of charge carriers are critical challenges that limit the photocurrent. In this talk, we present a systematic study investigating heterogeneous doping profiles in hematite thin film photoanodes on FTO-coated glass substrates with an SnO₂ underlayer. The films are deposited by pulsed laser deposition (PLD), a method known for excellent structural and thickness control. First, homogenous Ti-doped, undoped, and Zn-doped hematite films were identified with conclusive evidence to be n-type, weak n-type, and p-type semiconductors, respectively and are thus termed n, i (intrinsic), and p throughout this talk. We show that utilizing a combination of these layers in order to create proper doping profiles such as a p-i-n structure can significantly improve the photoanode performance. We observe enhancement in the plateau photocurrent by almost 20% and cathodic shift of the onset potential by 150 mV in p-n and p-i-n structures compared to their homogenously doped counterparts. These results are encouraging for the development of highly efficient α-Fe₂O₃ photoanodes for water splitting

Photo-electrochemical cells that are being used for solar powered hydrogen production attract major academic interest due to their challenging combination of tandem photovoltaic and catalytic semiconductor layers. As the semiconductor layer must be from industrial reasons cost effective, studies focus on hematite (F₂O₃) or simply rust electrodes. However, despite the vast amount of investigations the efficiency is not yet reached technological feasibility nor the mechanism of water splitting is understood. By modifying Salvador's mechanism [J. Chem. Phys. 89, 3863 (1985)] for Titanium electrodes and by experimentally controlling hydrogen peroxide (H₂O₂) in the electrolyte, we propose a distinct model for photo-anodic water splitting reactions. Specifically, we highlight two main features that agree well with empirical observations: (i) the gradual decrease in current density with the increase of voltage, and (ii) the abrupt effect of hydrogen peroxide addition (even at small concentrations) to the electrolyte. We believe that the proposed pathways provide a basic step toward improving the efficiency of the photo-anode.

Hematite (alpha-Fe₂O₃) is a promising photoanode material for harvesting solar energy by splitting water into hydrogen and oxygen. It has a favorable bandgap energy (2.1 eV), good catalytic activity for water oxidation, low cost, is chemically stable in alkaline solutions and environmentally benign. However, its water splitting efficiency is limited by the short life time of photogenerated charge carriers resulting in a short charge collection length of only 2 to 20 nm,^[1] much short than the extinction length of visible light in hematite (~1 µm). Our approach to overcome this problem uses ultrathin (~20-30 nm thick) hematite films on specular back reflector substrates that give rise to resonant light trapping in the hematite film.^[2] This approach gives rise to new challenges, creating the need for a new fabrication process and deposition conditions. The hematite is typically deposited at high temperatures (> 500 °C) in oxygen atmosphere, conditions that oxidize metallic back reflectors such as Al and tarnish noble ones such as Ag. This contribution reports on a novel fabrication method employing film transfer and flip-over process, thereby avoiding the degradation of the metallic back reflector. The presented method gives rise to new challenges, especially in the mechanical design of the stack, but it also opens up new opportunities such as the possibility to transfer the hematite photoanodes not only to rigid substrate but also to flexible ones. Our recent efforts and achievements in these directions including photoelectrochemical testing of different hematite dopants, the effect of microstructure and thin film thickness will be reported.

References
[1] J. H. Kennedy, K. W. Frese, J. Electrochem. Soc. 1978, 125, 709.
[2] H. Dotan, O. Kfir, E. Sharlin, O. Blank, M. Gross, I. Dumchin, G. Ankonina, A. Rothschild, Nat. Mater. 2013, 12, 158.

Acknowledgements: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 656132, the European Union's Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n. [617516] and European Commission’s Framework Project 7 cofunded by Europe’s Fuel Cell and Hydrogen Joint Undertaking (FCH JU) under PECDEMO Grant 621252.

Decades ago, K.L Hardee and A.J. Bard researched hematite (α-Fe₂O₃) as a potential material for photoelectrochemistry devices due to its ability to absorb in visible light as well as its chemical stability in an alkaline medium and the abundance of this element. Since then, much research has been devoted to improve the water splitting efficiency of hematite photoanodes. For the last four years, our research group has been working on the colloidal nanocrystal deposition (CND) process to prepare hematite and other semiconductor oxide photoanodes with excellent activity for water splitting. In this study, we are demonstrating a significant advance in the CND process to obtain hematite thin films with a high photoelectrochemical performance. Using this non-aqueous deposition route, we produced undoped and Sn-doped hematite photoanodes with excellent photocurrents of 1.4 mA.cm^-2 at 1.23 V_RHE and 2.7 mA.cm^-2 at 1.23 V_RHE, respectively, under a standard AM 1.5 G solar light simulator. The introduction of an external magnetic field during the CND process allowed to obtain thin film with controlled thickness in a single cycle of dip-coating and sintering. In addition, we will address also a critical analysis about the role of doping, film thickness, crystallographic orientation and morphology in the photoelectrochemical properties of hematite photoanode. We will summarize and describe what we have been learned about hematite photoanode based on the use of CND process.

In this work, effects of post treatments which consist of oxygen plasma treatment and a sequential annealing process on hematite (α-Fe₂O₃) photoanodes have been investigated. The hematite photoanodes have been fabricated by plasma enhanced chemical vapor deposition (PECVD) following by high temperature annealing (750 ^oC). The oxygen plasma treatment was conducted by means of PECVD which had been utilized for the formation of FeOx layer with varied process duration (10, 20 and 30 min). After that, samples were heat treated for short period of time. All the hematite samples post-treated exhibited enhanced photocurrent density values and the highest photocurrent density (1.31 mA/cm² at 1.23 V vs. RHE) was observed from the sample treated with 20 min of oxygen plasma and short annealing showing ca. 2-fold enhancement comparing to the pristine hematite photoanode. Evaluation of the prepared samples has been done by an X-ray photoelectron spectroscopy (XPS) and a scanning electron microscope (SEM) to investigate the effects of the post treatments on PEC performance. In addition, a transient absorption spectroscopy (TAS) measurement will be given to have an insight of charge carriers’ behavior in the post-treated samples.

The orientation dependence on the photoelectrochemical properties of Sn-doped hematite photoanodes was studied by means of heteroepitaxial film growth. Nb-doped SnO₂ (NTO) was first grown heteroepitaxially on single crystal sapphire substrates in three different orientations as a transparent conducting electrode. Hematite was then grown in the (001), (110), and (100) orientations on the NTO films. The structural, morphological, optical, and photoelectrochemical properties of the photoanodes were studied. The hematite films possessed high crystallinity and smooth surfaces and were found to be suitable model films for an investigation into the effect of anisotropy.

Hole scavenger measurements revealed that bulk photocurrent was not significantly affected by orientation. Cathodic shifts of up to 200 mV were observed in the water oxidation performance of the hematite photoanodes for (110) and (100) oriented films as compared to (001) oriented films. These results suggest that the varying of the orientation of thin film hematite photoelectrodes primarily affects charge transfer into the electrolyte due to the surface properties rather than affecting charge transport through the bulk. These results are further supported by Mott-Schottky analysis and Kelvin force probe microscopy measurements as well as an additional electrochemical characterization technique used to probe the effect of surface charging.

Sustainable development directives for the 21^st century recommend that all countries must have access to affordable and clean energy, with the urgent need for increasing the efficiency and sufficiency.¹ Solar energy is the most abundant renewable energy source and hydrogen produced from renewable sources is a sustainable, low pollutant and very convenient vector of energy.²
Photoelectrochemical (PEC) cells arranged in light series with a photovoltaic (PV) cell (tandem device/PEC-PV) has the ability to convert solar energy into storable hydrogen upon solar water splitting. Together with solar thermochemical water splitting, the PEC approach are considered the most efficient and low cost processes to convert sunlight into hydrogen.³
Following, worldwide effort is being undertaken to bring PEC-PV water splitting to a commercially stage. Besides the development of stable and cost-effective photoelectrodes displaying high current densities and photopotentials, the keystone on this endeavour must address the development of innovative PEC-PV cell designs that provide high solar-to-hydrogen conversion efficiency and low production costs. Optimised tandem cells should minimise the charge transport resistances and the overpotential losses at photo- and counter-electrodes, in a compact and low cost design. This work addresses these questions presenting different designs where embedded metal lines in the transparent electrode (ETCO) are used to minimise the TCO electrical resistance and where nanoparticulated counter-electrodes deposited directly in an anion exchange membrane, in a similar way as in PEMFC, are used to minimize the ionic transport resistance and the overpotential at the counter-electrode side. This assemble was named half-membrane electrode assembly (HMEA), because the membrane has just one face coated with an electrode. The photoelectrode width (directly related to the mean path length for the ionic species) and the area of the HMEA were optimized to delivery the least ionic transport resistance and counter-electrode overpotential, maximizing the hydrogen production.
The use of concentrated solar radiation, up to 30-sun, is also discussed as an effective way to minimize the PEC-PV solar hydrogen costs. Efficient heat dissipating strategies are presented, needed to avoid the overheating of the photoelectrode under concentrated sunlight; CFD results supporting the proposed designs will be presented.
Optimized PV current bias produces a surplus of electricity that should be used for the electrolyte pumping, hydrogen dehumidification and compression for storage and transport. A compact, cost-effective PEC-PV plant, using concentrated sunlight, is presented and discussed paving the way for the solar refinery concept in opposition to the unsustainable fossil fuel refinery.

References:
1. OWG, United Nations 2014
2. Jacobsson, T. J., et al. 2014 Energy Environ. Sci., 7, 2056-2070
3. J. A. Herron, et al. 2015 Energy Environ. Sci. 8, 126–157

In this presentation, we will discuss an oxygen permeable membrane reactor that utilizes solar energy to produce hydrogen from water thermolysis and partial oxidation of methane. This technology has been studied in lab scale [1, 2]. Yet thermodynamic studies on the reactor and system scale are needed to guide the design and integration of this system with hydrocarbon processing and solar energy utilization.

Mixed ionic-electronic conductive perovskites such as BaCo_xFe_yZr_1-x-yO_3-δ and La_0.9Ca_0.1FeO_3-δ are used at elevated temperatures. In this study, a monolith plug flow reactor is modelled, and we use one-step surface reactions and Nernst-Planck equation for charged species diffusion through the dense membrane. The surface kinetics and charge conductivities are derived from literature and previous studies.

Parameters such as channel height, width, length and gas flow rate are studied, and different membrane materials such as BaCo_xFe_yZr_1-x-yO_3-δ and La_0.9Ca_0.1FeO_3-δ are compared. Sensitivity analysis will be carried out to study the effects of membrane surface area, permeation and surface activities on the reactor performances. As the overall reaction is endothermic, concentrated solar power is used to supply high temperature heat source. Therefore, an integrated system with solar energy is analyzed and the overall efficiency of the reactor and receiver is discussed.

[1] Wu, X. Y., Chang, L., Uddi, M., Kirchen, P., and Ghoniem, A. F., 2015, "Toward enhanced hydrogen generation from water using oxygen permeating LCF membranes," PCCP, 17(15), pp. 10093-10107.
[2] Wu, X. Y., Uddi, M., and Ghoniem, A. F., 2016, "Enhancing co-production of H2 and syngas via water splitting and POM on surface-modified oxygen permeable membranes," invited to submit AIChE Journal, under review.

Solar water splitting is a promising approach to convert and store solar energy in the form of stable chemical bonds. Technoeconomic analyses¹ on photoelectrochemical device concepts indicate that particle-suspension reactor designs have the potential to produce hydrogen at a low levelized cost even when operated at less than 10 % solar-to-hydrogen efficiency. To this end, a vertically stacked particle-suspension reactor design² has been proposed to effect solar water splitting. Hydrogen and oxygen are produced in the top and bottom reaction compartments in the presence of a redox shuttle through a Z-scheme mechanism. Porous separators in between the reaction compartments facilitate ion transport while reducing product gas crossover. A robust, two-dimensional, transient numerical model has been developed to understand the interrelated charge transport processes in the semiconductor particles and mass transport in the electrolyte, both with and without forced convection to continuously circulate the redox shuttle. Moreover, transport processes at the semiconductor—electrocatalyst—solution interfaces are considered to provide insights on the effects of particle size, electrolyte redox potential, and surface reaction rate constants on the energy conversion efficiencies. Model results are used to guide device design and operation by identifying reaction compartment heights for hydrogen and oxygen evolution, concentrations of the semiconductor particles and the redox shuttle, and the convective flow rates required to optimize reactor performance. Results indicate sustainable solar-to-hydrogen conversion efficiencies of at least 1% for a chosen reactor design with purely diffusive transport and state-of-the-art photocatalytic materials.

References
1. Pinaud, B. A. et al. Technical and economic feasibility of centralized facilities for solar hydrogen production via photocatalysis and photoelectrochemistry. Energy Environ. Sci. 6, 1983 (2013).
2. Fabian, D. M. et al. Particle suspension reactors and materials for solar-driven water splitting. Energy Environ. Sci. 8, 2825–2850 (2015).

Sustainable development directives for the 21^st century recommend that all countries must have access to affordable, reliable and sustainable energy.[1] The solar PV technology is now a well-established way to convert solar into electric energy but the daily and seasonal variability of solar irradiance cannot assure a stable and constant power supply.[2] A practical way to convert sunlight into a storable energy form is using photoelectrochemical (PEC) cells that split water into hydrogen and oxygen by light-induced electrochemical processes.[3] Hydrogen produced from solar water splitting is considered one of the most promising routes for long-term renewable energy production.[4] The design of efficient PEC cells is of critical importance to turn this technology marketable, including the following challenges: (a) construction cost and robustness; (b) thermal management; (c) photoelectrodes and counter-electrodes configuration; (d) evolved gases separation and bubbles removal. Presently, most of PEC reactors were developed for testing new photoelectrodes at lab scale and they are unable to overcome the challenges that industrial production of hydrogen requires to become competitive.[5]
The present work presents a scaled-up PEC device based on an innovative PEC cell design disclosed by the authors that goes beyond the state of the art. This cell comprehends a front illuminated photoelectrode working simultaneously as window. The electrolyte container was designed to minimize the ohmic losses with the counter-electrodes placed side by side to the photoelectrode. The cell was engineered targeting significant weight reduction, cost-effectiveness, continuous electrolyte feeding and efficient separated gases collection. This configuration allows a tandem arrangement with PV cells placed in the back of the photoelectrochemical cell. Tests performed under concentrated solar irradiance (10-sun) demonstrated that the electrolyte flow rate combined with the external air temperature were enough to cool down the cell. Under 1000 mW.cm^-2 a maximum difference of only 3 °C in the electrolyte was recorded between the inlet and the outlet. These results are in accordance with CFD simulations performed for different working scenarios.

1. González-Eguino, M., Energy poverty: An overview. Renewable and Sustainable Energy Reviews, 2015. 47: p. 377-385.
2. Edwards, P.P., et al., Hydrogen and fuel cells: Towards a sustainable energy future. Energy Policy, 2008. 36(12): p. 4356-4362.
3. Dias, P., et al., Extremely stable bare hematite photoanode for solar water splitting. Nano Energy, 2016. 23: p. 70-79.
4. Sharma, S. and S.K. Ghoshal, Hydrogen the future transportation fuel: From production to applications. Renewable and Sustainable Energy Reviews, 2015. 43: p. 1151-1158.
5. Carver, C., et al., Modelling and development of photoelectrochemical reactor for H 2 production. International Journal of Hydrogen Energy, 2012. 37(3): p. 2911-2923.

Photoelectrochemical (PEC) water splitting is a promising path to solar hydrogen production. However, separation of the H₂ and O₂ gas products as well as hydrogen collection and transport in large solar fields becomes an overwhelming technical challenge. State of the art water splitting technologies make use of single cell units, separated into anode and cathode compartments by membranes. This implies that millions of PEC cell units in the solar field would have to be hermetically sealed and fitted with membranes, gas tubes and tube adaptors in order to separate and collect the hydrogen gas, resulting in a very complicated and expensive construction. The hydrogen would then have to be transported to the end user, either by pipelines or by high-pressure/liquid-H₂ vessels. These obstacles, in addition to efficiency and stability challenges, render PEC hydrogen production economically questionable.
In the present report, we aim to solve these problems by totally separating the H₂-generation electrochemical cell from the O₂-generation PEC solar cell. This is achieved by introducing an additional set of electrodes, called the auxiliary electrodes. These are Ni(OH)₂/NiOOH electrodes, commonly used in rechargeable alkaline batteries, which can be cycled many times with minimal energy loss. By placing a "charged" (NiOOH) auxiliary electrode in the oxygen cell, and electrically connecting it to a "discharged" (Ni(OH)₂) auxiliary electrode in the hydrogen cell, electrolysis can be performed in two separate cells. During electrolysis, one auxiliary electrode charges while the other discharges. Thereafter, the process can be repeated by cycling the auxiliary electrodes between the charged/discharged states. Using suitable photoanodes, the PEC cell can generate O₂, which can then be discharged to the atmosphere, alleviating the need for sealing and piping. Since the separate cells are connected to each other by metal wires only, the H₂ can be generated at any location, for example, directly at the end user place.

Photoelectrochemical (PEC) water splitting is a promising method to store solar energy in chemical bonds. Tantalum nitride (Ta₃N₅) has emerged as a promising candidate to drive the PEC water oxidation half reaction of overall water splitting. Prior studies of Ta₃N₅ begin with a similar synthetic route, consisting of an initial oxidation of Ta(0) metal to Ta(V), followed by ammonolysis at high temperatures ( > 850 °C). Despite the simplicity of this method, there are also several drawbacks. Importantly, the high temperature ammonolysis limits the utilization of transparent conductive oxide (TCO) substrates which are necessary for fundamental photoelectrochemical measurements. In addition, lack of viable TCOs thwart the implementation of Ta₃N₅ as the top electrode in a tandem PEC cell.
As a solution to the above problems, the synthesis and behavior of Ta₃N₅ films on TCO substrates will be presented. We have found Ta-doped TiO₂ (TTO) to be a sufficiently stable TCO material under the reducing atmospheres employed, and we will present the first example of its synthesis via atomic layer deposition (ALD). Thin films of Ta₃N₅ were subsequently deposited on TTO substrates either via ALD or a new solution based method. Both synthetic methods allow subsequent in-situ deposition of water oxidation co-catalysts on the surface. The photoelectrochemical properties of the resultant TTO/Ta₃N₅/catalyst films were investigated, and the results will be presented. The excellent material control derived from the methodologies developed allow for a detailed material structure–function relationship to be determined and a path to optimized performance of this material in a tandem device architecture elucidated.

Perovskite-type oxynitride materials are increasingly promising candidates as photoanodes for solar water splitting. Especially suitable in this regard are Nb-based perovskite oxynitrides, including SrNbO₂N, which has a narrow band gap (< 2 eV). There are reports in the literature on the photocatalytic performances of these electrodes prepared by particle-transfer or electrophoretic methods [1, 2]. However, there have been few experimental studies on the fundamental characteristics of Nb-based perovskite oxynitrides, such as their absorption coefficients, optical transitions and electrical properties. In this study, we carried out epitaxial growth of SrNbO₂N thin films and characterized their semiconducting properties.
We grew 100-nm-thick films on undoped and 0.5 wt % Nb-doped SrTiO₃ (001) substrates by RF reactive sputtering in an ambient mixture of Ar, O₂ and N₂. We used an Sr₂Nb₂O₇ targets. The crystalline quality of the SrNbO₂N film was then investigated by X-ray diffraction (XRD). The 2θ-ω scan profile shows only the 00h diffractions of SrNbO₂N. The full widths at half-maximum value of the rocking curve of the 002 diffraction and 103 diffraction were 0.15° and 0.49°, respectively. Hall-effect measurements reveal n-type conduction and a low carrier concentration of 6×10¹⁵ cm^–3. These results indicate that the SrNbO₂N (001) film had grown epitaxially on the SrTiO₃ (001) substrate and had good crystalline quality. We measured the absorption coefficient spectrum of the SrNbO₂N epitaxial thin film by spectroscopic ellipsometry. Plotting (αhν)^1/2 vs. hν gave a straight-line segment, indicating that SrNbO₂N film has an indirect transition characteristic. The indirect band gap was determined to be 1.8 eV. We carried out the electrochemical measurements using a three-electrode cell in 0.1M H₂SO₄ solution (pH 1). The resultant Mott-Schottky plot shows the flat-band potential of the SrNbO₂N (001)/Nb-doped SrTiO₃ (001) electrode to be 0.34 V vs. RHE. The onset potential of photocurrent was about 0.85 V vs. RHE, making it compatible with the flat-band potential.
In this study, we determined the indirect band gap to be 1.8 eV and the flat-band potential to be 0.34 V vs. RHE for SrNbO₂N (001) epitaxial film, showing that the valence band edge is more positive than the water oxidation potential and that the conduction band edge is very close to, or even more positive than, the water reduction potential. Thus, SrNbO₂N has suitable band gap and band-edge positions for use as a photoanode. Our results represent a first step toward a further understanding of the semiconducting properties of SrNbO₂N.

Acknowledgement
This work was supported by Advanced Research Program for Energy and Environmental Technologies of the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

References
[1] K. Maeda et al., J. Am. Chem. Soc., 2011, 133, 12334-12337.
[2] M. Kodera et al., J. Mater. Chem. A, 2016, 4, 7658-7664.

We measured near edge X-ray absorption fine structure spectroscopy (NEXAFS) and performed first principles calculations of mixed anion semiconductors as promising new photoanode materials. Mixed anion materials such as oxynitrides have the potential to decrease the optical band gap of transition metal oxides and simultaneously give nitrides chemical stability, thus addressing two shortcomings in the separate materials. Thin film composition spreads of Ta-La-ON were fabricated using reactive sputtering followed by rapid thermal annealing. Tuning of the band gap was observed from changes in color and a 2 eV red-shifted absorption spectrum of the known wide band gap material La₃TaO₇ after N doping. Results will be presented on the acid stability, the presence of a passivation layer, and first principles calculations of NEXAFS spectra with DFT giving information about the N chemical bonding state. We show that N doped La₃TaO₇ deserves further attention by the photoelectrochemistry community as a promising photoanode for the oxygen evolution reaction.

We present, herein, visible-light water splitting using earth-abundant zirconium-based nanostructured photoanodes. ZrO₂/ZrON core/shell arrays were fabricated via the atomic layer deposition (ALD) of various ZrN layers on anodically synthesized hexagonal ZrO₂ nanotubes. Compositional analysis of the composite photoanodes showed the transformation of the nitride layers to oxynitride phases, with the sample made of 95 ALD cycles having a structure nearest to stoichiometry. Optical analysis showed visible light absorption within the oxynitride layers with an estimated band gap of 2.6 eV, as compared to 3.8 eV for the bare oxide nanotubes. This decrease in band gap was attributed to the cathodic shift of the valence band maximum (VBM), as confirmed by X-ray photoelectron spectroscopy valence band and photoluminescence spectra. The core/shell photoanodes made of 10–95 cycles of ZrN showed photocurrent enhancements over the bare nanotubes, with samples having 95 ALD cycles exhibiting a photocurrent density of 1.2 mA/cm² at an applied potential of 1 V versus Ag/AgCl reference electrode under AM 1.5 illumination. Further increase in deposition cycles resulted in photocurrent deterioration, which was attributed to the increased surface states. The electrochemical impedance spectra (EIS) revealed electron lifetimes in the core/shell electrodes that are 2 orders of magnitude longer than those in the bare oxide nanotube samples. Finally, Mott–Schottky analysis confirmed the cathodic shift of the valence band maximum, as evidenced by a very small anodic shift in the conduction band minimum. The results attained in this study compose a step toward earth-abundant, visible-light absorbing photoanodes for solar water splitting.

Photocatalytic water splitting using solar energy has been studied extensively as a means of renewable hydrogen production on a large scale [1,2]. A solar-to-hydrogen energy conversion efficiency (STH) of 5% or higher is considered to be necessary to make this process economically feasible. The scalability of water splitting systems is also important when solar hydrogen is used globally [3]. Development of particulate photocatalysts that are active in the water splitting reaction without an external power supply is considered to have advantages in the fabrication cost and scalability.

Z-scheme water splitting that uses two different photocatalysts for hydrogen and oxygen evolution is suited to effective utilization of the solar energy, because narrow band gap photocatalysts that are active either in the hydrogen evolution reaction or the oxygen evolution reaction can be applied [1]. It is critical to establish efficient electron transfer between a hydrogen evolution photocatalyst (HEP) and an oxygen evolution photocatalyst (OEP).

The authors’ group recently developed photocatalyst sheets consisting of a hydrogen evolution photocatalyst (HEP) and an oxygen evolution photocatalyst (OEP) embedded into conductive materials for Z-scheme water splitting [4,5]. La- and Rh-codoped SrTiO₃ (SrTiO₃:La,Rh) and BiVO₄ or Mo-doped BiVO₄ (BiVO₄:Mo) were employed as HEP and OEP, respectively, and embedded into Au thin layer by a particle transfer method [6]. The SrTiO₃:La,Rh/Au/BiVO₄ photocatalyst sheet exhibited water splitting activity six times higher than the suspension of the SrTiO₃:La,Rh and BiVO₄ photocatalysts [4]. The SrTiO₃:La,Rh/Au/BiVO₄:Mo photocatalyst sheet achieved an apparent quantum efficiency of 33% at 419 nm and a STH of 1.1% through optimization of sheet preparation and water splitting reaction conditions [5]. The photocatalyst sheet exhibits high activity in pure water without any additives or solution mixing. On the photocatalyst sheet, HEP and OEP particles are physically embedded into conductive layer in the immediate vicinity. This configuration is effective in suppressing the generation of concentration overpotentials of H⁺/OH^- and IR drops between the hydrogen and oxygen evolution sites. Therefore, the photocatalyst sheet with high activity is scalable as is.

In this talk, the factors controlling the activity of the photocatalyst sheets will be discussed. Our recent effort in the development of photocatalyst sheets based on narrow band gap photocatalysts will also be presented.

[1] Hisatomi et al. Chem. Soc. Rev. 2014, 43, 7520.
[2] Hisatomi et al. Catal. Lett. 2015, 145, 95.
[3] Maeda et al. J. Phys. Chem. Lett. 2010, 1, 2655.
[4] Wang et al. J. Catal. 2015, 328, 308.
[5] Wang et al. Nat. Mater. 2016, 15, 611.
[6] Minegishi et al. Chem. Sci. 2013, 4, 1120.

Materials based on n-type metal oxides such α-Fe₂O₃ or WO₃ have gained relevance for application in hydrogen production by water splitting (photoelectrochemical cells) [1] due to their low cost, fairly easy preparation, synthesis and high stability in aqueous media. Hematite has been the of the most studied materials due to its narrow band gap (2.1-2.2 eV), collecting up to 40 % of the solar spectrum energy, and its high stability [2]. However, hematite has limitations due to poor electron mobility (0.01-0.1 cm²V^-1s^-1) that results into high electron-hole recombination rate, and short hole diffusion length (2-4 nm). Therefore, tungsten trioxide is becoming more attractive as it shows higher electron mobility (6 cm²V^-1s^-1) and hole diffusion length (150 nm), though possesses a slightly broader band gap (2.7-2.8 eV) than hematite. Besides, nanostructuring techniques have been proven useful for increasing the performance of the WO₃ photoresponse [3]. In this work, nanostructured WO₃ photo-electrodes for photoelectrochemical water splitting were developed. In particular, WO₃ nano-square plates multi-layers were successfully grown onto transparent TCO substrates by jet-spraying a WO₃ nanoparticle suspension. This simple and straightforward deposition method offers new opportunities to develop the scale-up of efficient photo-electrode materials for solar water splitting. Afterwards, the as-prepared WO₃ samples were annealed in a slow-step temperature thermal annealing in order to obtain crystalline WO₃ photo-electrodes. Scanning electron microscopy (SEM) was used to assess the WO₃ multilayer morphology; nanostructured features such as square shape and dimension of the nanoparticles were unveiled. The XRD structural analysis discloses that the WO₃ photo-electrodes grow in the monoclinic crystalline phase with a preferential direction (002). Photoelectrochemical characterization of the WO₃ photo-electrodes revealed a photocurrent density of 1.6 mA/cm² at 1.23 V/V_RHE (pH = 0) under AM 1.5G (100 mWcm^-2). It is believed that this improved photocurrent is mainly due to (i) the square-like plate’s morphology that leads to WO₃ films layers with intrinsic porosity (increasing the photo-active surface area) and (ii) the slow-step thermal annealing. As conclusion, an increase in photocurrent of more than 100 % compared to the conventional fast-step thermal annealing was achieved.
References:
[1]- M Gratzel, Photoelectrochemical cells, Nature, 414 (2001) 338–344.
[2]-P Dias, A Vilanova, T Lopes, L Andrade, A Mendes, Nano Energy, 2016. 23: p. 70-79.
[3]-T Zhu, M N Chong and E S Chan, ChemSusChem 2014, 7, 2974 – 2997

An electrochemical method is presented to study the nature of the defect states in sub-stoichiometric tungsten oxide nanoflake photoanodes used in water splitting. First, stoichiometric/sub-stoichiometric tungstate nanoflake arrays were deliberately developed via annealing under different atmospheres (air, O₂, and H₂) in different sequences. UV-Vis diffuse reflectance spectra and Tauc analysis indicated the presence of oxygen vacancies, which was also confirmed via XRD and Raman analysis, with samples annealed in air/O₂ sequence resulted in the most stoichiometric monoclinic structures. A Defect Sensitivity Factor is proposed to explain the nature of defects whether they are deep or shallow. Mott-Schottky analysis is used to confirm the expected defect donor densities, as well as to confirm the nature of the developed oxygen vacancy defect states. The tungstate photoanodes were tested in photoelectrochemical water splitting cells and their photoconversion efficiency is demonstrated and discussed in details.

The intermediates formed during the synthesis of iridium oxide colloids (IrO_x●nH₂O) by alkaline hydrolysis of Ir(III) or Ir(IV) salts were characterized using electrochemical and spectroscopic methods and modeled in TDDFT calculations. In air-saturated solutions the monomers exist in a mixture of Ir(III) and Ir(IV) oxidation states, where the most likely formulations at pH 13 are [Ir(OH)₅(H₂O)]^2- and [Ir(OH)₆]², respectively. The results from this study address a very important issue in IrO_x●nH₂O–based PECs: how does the chemistry of the catalyst and its interface with the semiconductor influence the photoresponse of the cell? The careful preparation, purification and characterization of both IrO_x●nH₂O catalysts and photoanodes in this work provide reasonable explanations to previously observed discrepancies in PECs that utilize IrO_x●nH₂O colloids as co-catalysts on the photoanode electrode. The monomeric anions strongly adsorb onto TiO₂, and they promote the adsorption of ligand-free IrO_x●nH₂O colloids onto mesoporous titania photoanodes. However, the reversible adsorption/desorption of electroactive monomers effectively short-circuits the photoanode (TiO₂ or W-BiWO₄) redox cycle and thus dramatically degrades the photoelectrochemical performance of the cell.¹

(1) Zhao, Y.*; Vargas-Barbosa, N. M.*; Strayer, M. E.; McCool, N. S.; Pandelia, M.-E.; Saunders, T. P.; Swierk, J. R.; Callejas, J. F.; Jensen, L.; Mallouk, T. E. Understanding the Effect of Monomeric Iridium(III/IV) Aquo Complexes on the Photoelectrochemistry of IrO_x●nH₂O-Catalyzed Water-Splitting Systems. J. Am. Chem. Soc. 2015, 137 (27), 8749–8757.