Symposium EN02—Sustainable Routes to Fuels and Commodity Chemicals Production via Electrochemical Methods

Recent work has shown intriguing possibilities for non-equilibrium carrier generation in metal nanostructures at semiconductor interfaces to modify the rates and pathways for photochemical reactions at the nanoscale, suggesting that light can play a new role as an agent of photocatalytic selectivity – beyond its traditional role of charge carrier generation. Key open questions in this field include: i) how hot are the hot carriers? ii) how hot carriers are transported and whether they modify interfacial reactions? iii) ultimately, how efficiently can these processes generate chemical products from sunlight? We show that hot carrier photocatalytic reactions at the Au/p-GaN and Cu/NiO nanoparticle interfaces alter the Faradaic efficiency for carbon dioxide reduction relative to dark electrocatalysis. Notably, at both of these interfaces, charge carrier separation occurs via photoexcited hot hole injection into the p-type wide bandgap semiconductor, localizing electrons in the metal catalyst nanoparticles. An exciting new result is the observation of self-sustaining, unassisted photocatalytic gas phase carbon dioxide reduction in metal/p-GaN structures, where the CO₂ reduction reaction is balanced by water oxidation on gallium nitride. In this work, we have address carrier generation, transport and interface dynamics with first-principles theory, and experimental observation of carrier dynamics via hot-carrier photocurrent spectroscopy, ultrafast transient absorption spectroscopy, and photoelectrochemical measurements.
Another key future photocatalyst challenge for solar fuels from carbon dioxide reduction is design of earth-abundant tandem photocatalysts with both appropriate bandgaps and sufficiently high photovoltages to efficiently drive carbon dioxide reduction reactions at solar photocurrent densities, with validated durability and long lifetime. We survey options for photocatalysts comprised of metal oxide, metal nitride and tandem heterostructure components.

Results of theoretical calculations of electrochemical CO2 reduction to formate, hydrocarbons and alcohols will be presented. The mechanism for the formation of various products is established and the rate evaluated and compared with experimental measurements. The rate of the main side reaction, the hydrogen evolution reaction, is also estimated. The calculations are based on a detailed atomistic model of the electric double layer (metal slab and water layer) and density functional theory calculations to evaluate not only the free energy of intermediates as a function of applied voltage but also the activation energy for each elementary step, both Heyrovsky and Tafel reactions [1]. Comparison is also made with calculations using an implicit solvation model [2]. A range of close packed metal surfaces are compared, including Cu, Ag, Au, Ni, Fe, Rh, Ir and Pt. The resuls are in remarkably good agreement with the available measurements. A two parameter descriptor is established that can help identify improved catalysts for this important reaction. The elucidation of the reaction mechanism of CO2 electroreduction to hydrocarbons and alcohols as well as the competing hydrogen evolution reaction is an important step towards the design of a selective and energy efficient catalyst for small scale, decentralized fuel production using renewable energy sources and CO2 as reactant.

[1] J. Hussain, H. Jónsson and E. Skúlason, ACS Catalysis 8, 5240 (2018).

[2] M. Van den Bossche, E. Skúlason, C. Rose-Petruck and H. Jónsson, J. Phys. Chem. C 123, 4116 (2019).

Electrochemical Reduction of CO₂ (ER-CO₂) is a very attractive alternative to tackle Global Warming.[1] Cu-based materials have shown increased yields of hydrocarbon and oxygenate products, while its selectivity towards CO is low.[2] Inspired by the thermocatalytic process, a traditional co-precipitation method was employed to synthesize CuZnAl-based oxide catalysts with a mesoporous structure. This CuZnAl catalyst was tested for the first time for the ER-CO₂ under ambient conditions. The Physico-chemical properties of the catalysts were studied by several characterization techniques (e.g., XRD, XPS, BET, SEM, TEM) and electrochemical impedance spectroscopy at different applied-potentials to understand the role of the modification of the catalyst components during operation in the final selectivity and activity. Results revealed that adding amphoteric metal oxides like ZnO and Al₂O₃ to the CuO-based catalyst contributed to promote CO formation over H₂. XPS measurements on the fresh samples revealed that the ternary CuZnAl catalyst presented a lower percentage (5%) of Cu⁰ + Cu¹⁺ mixture on the surface than the other catalysts, being mainly constituted by Cu⁺². This material reached a Faradaic efficiency towards syngas of almost 95% at -0.89 V vs. RHE. Nevertheless, the highest production rate of syngas was obtained at the most negative applied potential (∼ 17 µmol h^-1 cm^-2 at -1.14 V vs. RHE).). A tunable H₂/CO ratio was achieved by applying different potentials, reaching lower values by increasing applied negative potential (CO current density increased). In fact, a syngas with a H₂/CO ratio of ∼2 was obtained at -0.89 V vs. RHE, which is a suitable raw material for further methanol synthesis. The enhanced performance for syngas production of the developed CuZnAl catalyst is demonstrated to be attributed to its surface properties (i.e., alkalinity and the oxidation state on the surface, its lowest diffusional mass transfer resistance, its highest total pore volume, and the lowest Cu crystals size among the prepared catalysts.

Acknowledgments
This work has received funding from the European Union’s Horizon 2020 Research and Innovation Action program under the Project SunCoChem (Grant Agreement No 862192).

Reference
[1] H. Guzmán, M.A. Farkhondehfal, K. Rodulfo Tolod, N. Russo, S. Hernández, Photo/electrocatalytic hydrogen exploitation for CO₂ reduction toward solar fuels production, in: Sol. Hydrog. Prod. Process. Syst. Technol., Elsevier Inc., 2019: p. 560. https://doi.org/10.1016/C2017-0-02289-9.
[2] J.J. Velasco-Vélez, T. Jones, D. Gao, E. Carbonio, R. Arrigo, C.J. Hsu, Y.C. Huang, C.L. Dong, J.M. Chen, J.F. Lee, P. Strasser, B. Roldan Cuenya, R. Schlögl, A. Knop-Gericke, C.H. Chuang, The Role of the Copper Oxidation State in the Electrocatalytic Reduction of CO₂ into Valuable Hydrocarbons, ACS Sustain. Chem. Eng. 7 (2019) 1485–1492. https://doi.org/10.1021/acssuschemeng.8b05106.

The development of molecular catalysts for CO₂ electroreduction within electrolyzers requests their immobilization on the electrodes. Several methods have been explored for the heterogenization of homogeneous complexes, we here report a novel approach using a hierarchical porous carbon material, derived from a Metal Organic Framework, as a support for the well-known molecular catalyst [Re(bpy)(CO)₃Cl] (bpy = 2,2’-bipyridine). This cathodic hybrid material, named Re@HPC, has been tested for CO₂ electroreduction using a mixture of an ionic liquid (1-Ethyl-3-methylimidazolium tetrafluoroborate, EMIM) and water as the electrolyte. The present study reveals that thanks also to the hierarchical porosity and structure of the porous carbon, the Re@HPC is a remarkable catalyst, enjoying excellent activity (high current densities and high turnover numbers) and good stability. Interestingly, it catalyzes the conversion of CO₂ into a mixture of carbon monoxide and formic acid, with a selectivity that depends on the applied potential. These results emphasize the advantages of integrating molecular catalysts onto such porous carbon materials for developing novel, stable and efficient, catalysts for CO₂ reduction.

Identifying the chemical state of the catalytic surface at the electrode-electrolyte interface is of significant interest, but the in situ characterization of the active surface during the CO₂ reduction reaction (CO₂RR) remains challenging due to the formation of bubbles from gaseous products and mass transport limitations. These hurdles limit state-of-art in-situ/operando techniques from characterizing catalyst surfaces under the reaction conditions. In particular, there is great interest in whether Cu oxide species exist at the surface or near-surface of Cu-based CO₂RR catalysts under these conditions, and if such species play a role in the catalytic pathway to multi-carbon products.
Here, we conducted in-situ characterization of the atomic and electronic structure of the near-surface of a Cu thin-film using synchrotron X-ray methods under the high current CO₂RR conditions relevant for device operation. The structural evolution of the near-surface region of polycrystalline Cu electrodes under in situ conditions was investigated through a combination of grazing incidence X-ray absorption spectroscopy (GIXAS) and X-ray diffraction (GIXRD). The in situ GIXAS shows that the surface oxide is fully reduced to metallic Cu before the onset potential for CO₂RR, and the catalyst surface retains the metallic state across the potentials relevant to the CO₂RR. In addition, in-situ GIXRD results showed that potential-dependent surface reconstruction from polycrystalline Cu to a Cu(100)-like surface was observed only in the presence of CO₂ reactants. Previously believed to be a soley potential-induced effect, our findings suggest that surface reconstruction is increased by the CO₂RR chemistry and that this reconstruction occurs across the macroscopic (mm-scale) surface. These findings show that the surface of Cu electrocatalysts is dynamic during the CO₂RR, and underscore the need for in-situ studies to accurately understand the catalyst surface state during catalysis.

The exponential increase in the concentration of greenhouse gasses in the atmosphere is considered one of the most important reason for climate change. Carbon Dioxide is the most significant anthropogenic gas that contributes in global warming.
CO2 capture and storage (CCS) has been proposed as one of the most important invention to mitigate CO2 emissions. Moreover, conversion of carbon dioxide into energy-rich chemicals is a viable approach to reducing the global carbon footprint. The most common techniques to remove CO2 from gas streams are the chemical and physical adsorption by liquid solvents. Traditionally, aqueous amine solutions have been used as chemical solvents because of their high selectivity, high reactivity and low price. Unfortunately, they present also many disadvantages associated with the high energy demand required for the solvent regeneration, corrosion issues and loss of solvent because of their high volatility. Hence, in the need to find more efficient solvents for CO2 capture and conversion, Ionic Liquids (ILs) have been highlighted as very good alternatives to common amine solution. (1)

Within this field lies this research, which in turn is part of a much broader European project called SunCoChem. For this project we are testing the stability and performance of various ionic liquids, provided by Iolitec Ionic Liquids technologies GMBH, and in particular their ability to capture and electrochemically convert a pure CO2 stream to CO with high efficiencies.

The ionic liquids were tested in a two-compartment H-type electrochemical cell. In the anodic chamber a nickel mesh electrode was immersed in a solution of potassium hydroxide and in the cathodic one a silver foil cathode was employed in a solution of CH3CN and ionic liquid. An organic solvent was used to favor the dissolution of the ionic liquid and the homogenization of the solution. The Ionic Liquids (ILs) tested so far have a cationic part based on imidazole, which is expected to stabilize and lower the activation energy for the reduction of CO2, namely [BMIM][BF4], [BMIM][CH3CO2], [BMIM][CF3CO2] and [BMIM][CF3SO3]. This trend was confirmed by a shift to more positive potentials of the onset (~ 0.5V) for the CO2 reduction reaction in the presence of these Ionic Liquid.
During the CP studies, some ILs evidenced a decrease in the applied potential indicating the increase of the electrolyte conductivity on difference to the behavior of the most commonly used [BMIM][BF4] IL in the same organic solvent.

Our results evidence relevant current density values, a good stability during chronopotentiometry (CP) tests and a high selectivity towards the target product: CO, which however change depending on the used IL.
AKCNOWLEDGMENT
This work has received funding from the European Union’s Horizon 2020 Research and Innovation Action programme under the Project SunCoChem (Grant Agreement No 862192).

References:
(1) Shokat Sarmad et al, Carbon Dioxide Capture with Ionic Liquids and Deep Eutectic Solvents: A New Generation of Sorbents, 2016, https://doi.org/10.1002/cssc.201600987

The need to reduce greenhouse gas emissions and increase our energy supply makes the electrochemical reduction of CO₂ (CO₂R) a very attractive alternative to produce non-fossil-based fuels or chemicals. Copper-based catalysts is one of the catalyst that most efficiently promote the formation of species with one or more carbon-carbon bonds from the electrochemical reduction of CO₂ [1]. Because the catalyst preparation method has an influence on the physicochemical properties and on the electrocatalytic performance[2], in this work, it was decided to evaluate the effect of the ultrasound application (US) on the shape and size of the particles obtained, its electrocatalytic activity and its selectivity to products of interest. For this purpose, sonication was carried out at different percentage amplitudes of ultrasonic power (23, 30 and 37%) during the aging time of the synthesis. Physical characterization was carried out by using different techniques including X-ray diffraction, BET and filed-emission scanning electron microscopy (FESEM). Electrochemical tests for CO₂ reduction were done under ambient conditions. Regarding the physical characteristics, we found that pore size distribution is narrower by increasing the US amplitude. On the other hand, there is no significant difference in morphology and dimension of particles. However, the surface area increased with the use of ultrasound, this is attributed to a better dispersion created by acoustic cavitation. Ultrasound has also an effect on Copper-based catalysts performance; in this case, the selectivity towards H₂ and C₁ products (CO and formate) was enhanced. In addition, an increase in productivity of CO₂R products was obtained with respect to the synthesized catalysts that were not assisted by ultrasound (> 3-fold). These results motivate us to further explore in what other ways acoustic cavitation phenomenon can influence the physical characteristics of the catalysts and, in turns, their performance for the electrochemical reduction of CO₂.
Acknowledgement
This work has been performed with the financial support of Eni SpA and the R&D Program Energy Transition (Cattura e Utilizzo CO₂).
References
[1] Hilmar Guzmán; M.Amin Farkhondehfal; Kristine-Rodulfo Tolod; Simelys Hernández; Nunzio Russo, Chapter 11: Photo/electrocatalytic hydrogen exploitation for CO₂ reduction toward solar fuels production. In Solar Hydrogen Production, Francesco Calise, M. D. D. A., Massimo Santarelli, Andrea Lanzini, Domenico Ferrero, Ed. Elsevier Inc.: 2019; pp 365-418.
[2] Dasireddy, V. D. & Likozar, B., 2019. The role of copper oxidation state in Cu/ZnO/Al₂O₃ catalysts in CO₂ hydrogenation and methanol productivity. Renewable Energy, Volumen 140, pp. 452-460.

To reverse the impact of increasing CO₂ emissions would need not only curbing the reliance on fossil fuels but also developing effective strategies to capture and utilize CO₂. There are also increasing attempts to consider CO₂ as a resource and a business opportunity rather than a waste with a cost of disposal. Among the several available strategies, the electrochemical reduction of CO₂ to produce value-added chemicals becomes a feasible mechanism of storing intermittent renewable energy that is entering the stage of gradual implementation [1]. When electrochemical reduction integrates a light source to provide energy for redox reactions, we have a photoelectrochemical device, which have attracted an intense attention in recent years [2]. In principle, such integration enables higher energy efficiency by reducing the requirements of external energy and by reducing losses in transporting electricity to the electrolysis cell. Ideally, these systems may be completely operated under the sun without the need of an external bias between cathode and anode compartments, which may additionally reduce system capital costs [3].
Since CO₂ is a thermodynamically stable molecule, its multistep reduction confronts many fundamental technical hurdles. The kinetics for CO₂ reduction are also, in general, more sluggish than the thermodynamically favorable two-electron H₂ evolution reaction, which competes with CO₂ reduction. The electro-and-photoelectro-chemical processes are thus highly dependent on the development of efficient materials capable of selectively reducing CO₂ at low overpotentials. Of the myriad cathode materials that have been used, copper and copper oxides are regarded unique with both intermediate hydrogen overpotentials and CO adsorption [4] allowing further reduction of CO₂ to fuels and commodity chemicals such as methanol [5] or ethylene [6] at moderate reaction rates.
With this communication, the author presents the research activities in the electro-and-photoelectro-chemical conversion of CO₂ to alcohols and hydrocarbons at copper-based systems in the DePRO research group of the University of Cantabria, to provide new insights and guidance to the field. A filter-press dual-chamber cell is used for the continuous conversion of CO₂ under different phases involved (i.e. gas or liquid-phase), electrode/cell designs (e.g. GDE, MEA or photoanode/photocathode) and key operating conditions. Many of these factors are often intertwined, which complicates advances in the field, but with continued research efforts these sustainable technologies for CO₂ conversion might become technically and economically feasible in the near future.

ACKNOWLEDGEMENT
The author gratefully acknowledge the financial support from the Spanish Ministry of Science and Innovation (MICINN) through the project PID2019-104050RA-I00 as well as Ramón y Cajal programme (RYC-2015-17080).

References
[1] Chauvy, R., Meunier, N., G. De Weireld, T. Selecting emerging CO₂ utilization products for short- to mid-term deployment. Appl. Energ. 236, 2019, 662–680.
[2] Vignesh, V., Bartlett, J., Pillai, S. C. Photoelectrochemical conversion of carbon dioxide (CO₂) into fuels and value-added products. ACS Energy Lett. 5, 2, 2020, 486–519
[3] Castro, S., Albo, J., Irabien, A. Photoelectrochemical reactors for CO₂ utilization. ACS Sustain. Chem. Eng. 6, 12, 2018, 15877-15894.
[4] Garza, A. J., Bell, A. T., Head-Gordon, M. Mechanism of CO₂ reduction at copper surfaces: Pathways to C2 products. ACS Catal. 8, 2, 2018, 1490–1499.
[5] Albo, J., Alvarez-Guerra, M., Castaño, P., Irabien, A. Towards the electrochemical conversion of carbon dioxide into methanol. Green Chem. 17, 2015, 2304-2324.
[6] Ma, W., Xie, S., Liu, T., Fan, Q., Ye, J., Sun, F., Jiang, Z., Zhang, Q., Cheng, J., Wang, Y. Electrocatalytic reduction of CO₂ to ethylene and ethanol through hydrogen-assisted C–C coupling over fluorine-modified copper. Nat. Catal. 3, 2020, 478–487.

The chemical industry propelled human progress by using energy from fossil fuels and, in the process, inadvertently contributed to anthropogenic climate change. This industry outputs >70,000 products (1.2 billion tons in total), impacts more than 25% of the US GDP, and is responsible for ~5% of the US primary energy consumption (4.5 Quads). Thermochemical processes in this industry account for >93% of this energy consumption (>87% in the form of fossil-fuel-derived heat). Decarbonization of this industry would represent a giant step towards mitigating global warming. This urgent transition must integrate renewable energy sources with chemical manufacturing, ultimately resulting in the electrification of this industry via large-scale implementation of electrochemical manufacturing. Currently, however, two major challenges prevent the deployment of electrosynthesis reactors at scale: their low selectivity and their low production rates. This presentation will discuss reaction engineering opportunities to enhance the performance of organic electrosynthesis reactors. Specifically, I will present our work on understanding and improving the electrohydrodimerization process for the production of adiponitrile (ADN), a precursor to Nylon 6,6. Although this model reaction is the largest and most successful organic electrosynthesis implemented in industry, it faces many challenges owing to its limited energy conversion and selectivity. Through a combination of experimental electroanalytical characterization and Bayesian machine learning, we elucidate guidelines for the optimal operation of ADN electrosynthetic reactors. Our results provide insights into mass transport limitations that affect the selectivity of organic electrosynthesis processes and on how to dynamically control electrode processes to mitigate them.

Electrocatalytic CO₂ reduction has the dual-promise of neutralizing carbon emissions in the near future, while providing a long-term pathway to create energy-dense chemicals and fuels from atmospheric and waste CO₂. The field has advanced immensely in recent years, taking significant strides towards commercial realization. Gas diffusion electrodes (GDEs), composed of solid electrocatalysts on porous supports positioned near the interface of a conducting electrolyte and CO₂ gas, have been able to demonstrate the substantial current densities needed for future commercialization. These higher reaction rates have often been ascribed to the presence of a three-phase interface, where solid, liquid, and gas provide electrons, water, and CO₂, respectively. Conversely, mechanistic work on electrochemical reactions implicate a fully two-phase reaction interface, where gas molecules reach the electrocatalyst’s surface by dissolution and diffusion through the electrolyte. Here, we first outline the macro, micro and atomistic phenomena occurring within a gas-diffusion electrode to provide a focused introduction to the architecture of the often-discussed three-phase region for CO₂ electrolysis.
In addition, we use a 2-D model to examine the concentration gradients in the gas and electrolyte flow channels across a GDE, which provides longitudinal information along the length of a flow cell. In doing so, we quantify the extent of concentration overpotentials and ohmic drops throughout the catalyst layer across a range of applied potentials and flow rates. Various process parameters were modified to explore the effects on the CO₂ mass transfer-limited current density, conversion, and outlet concentrations of CO₂, CO and H₂. Our model suggests that ohmic losses largely determine the current density distributions at low conversion rates where small gradients in concentration exist. However, as the concentration profiles become less uniform (e.g. high conversion rates), the non-electrochemical consumption of CO₂ starts to overtake electrochemical conversion, and leads to non-uniform performance across the electrode. In addition, while higher flow rates allow the ability to substantially increase current density, this comes at the expense of ohmic drops and losses in selectivity, which become more prominent at high conversion rates. The ability to understand the interplay between process conditions and local environment will be necessary to further develop the science and technology of electrochemical CO₂ reduction.

Nanoporous (NP) transition metals offer a high surface area and edge-rich catalytic environment capable of selective electrolysis of carbon dioxide (CO₂) when utilized as a catalyst in a gas diffusion electrode (GDE). While the copper and gold have been extensively explored as catalysts for the carbon dioxide reduction reaction (CO₂RR) in both aqueous and vapor fed electrolyzes, a fully nanostructured catalytic environment remains unexplored. Here, we report the development and electrochemical performance of NP gold and copper GDEs deposited on a carbon or polytetrafluoroethylene (PTFE) microporous layer (MPL). NP GDEs are synthesized by vapor deposition of an alloy onto the MPL followed by a chemical or electrochemical dealloying process that yields a continuous porous electrode with nanoscale dimensions forming a NP catalyst layer. The average ligament and pore sized can be tuned by varying the temperature, solvent and length of the dealloying process. We observe that carbon paper-based, NP GDEs are prone electrolyte flooding due to the hydrophilicity of the bare metal surface. The contact angle of the NP GDE can be tailored from 40^o to 150^o via a 2 step application of 1) Nafion ionomer and 2) Nafion:carbon black mixture applied to the NP GDE. This system is capable of electrochemical generation of carbon monoxide with a partial current density of 650 mA/cm² in a 250mM KHCO₃ electrolyte that can be maintained for over 48 hours of continuous electrolysis. We will also report results of experiments of NP Cu/Au on PTFE and the effects of ligament thickness, pore size, catalyst structure and hydrophobicity on CO₂RR. These studies demonstrate that nanoporous catalyst environments can play an important role in governing the product selectivity and achievable current density of CO₂RR in a GDE.

In order to achieve a sustainable society driven by the abundant solar energy, the intermittency of sunlight needs to be overcome. One attractive solution is to store solar energy in the form of chemical fuels, e.g., the generation of hydrogen and oxygen in solar-driven water splitting. To this end, tremendous efforts have been devoted to developing efficient semiconductors and electrocatalysts, which resulted in a significant improvement of the reported solar-to-fuel efficiency in the last couple of decades. As the next stage, designing device architectures that are scalable and efficient is paramount in order to fully translate the developed materials towards practical applications. Indeed, it has been shown that major performance losses can be obtained from ineffective design,^1,2 especially when scaling-up devices to larger area. A key component of the device operation is the fluid dynamics, as it plays an important role to determine the mass-transport phenomena in the device and the resultant energy efficiency. In this study, we highlight the buoyancy effects driven by (photo)electrochemical reactions and evaluate their influences on the ion distribution in the electrolyte solutions and product separation.

In the first part, pH changes during water splitting in a stagnant electrochemical cell with near-neutral electrolyte are monitored in-situ by a pH-sensitive fluorescence sensor foil placed between the anode and cathode. Electrolyte close to the anode and cathode gets more acidic and more alkaline, respectively, in unbuffered and buffered neutral-pH conditions. A closer look at the pH distribution within the cell further reveals that natural convection is generated by the electrochemical reactions under non-stirred conditions. Specifically, at low current densities (≤ 2 mA/cm²), the acidic and alkaline regions move to the top and bottom of the cell, respectively. Based on this observation, we develop a model that considers natural convection driven by buoyancy forces due to local changes in the density of the electrolyte, which reveals that buoyancy-driven convection significantly helps to stabilize local pH gradients. Our extended simulations further predict the great contribution of buoyancy-driven convection towards the production of value-added aqueous chemicals even at high current density range.

Buoyancy-driven fluid dynamics is also considered in terms of product separation in a solar-driven water splitting device. We consider a membrane-less flow cell as an attractive technique to separate gaseous products through the electrolyte flow without having any membrane-associated complexity and losses. We introduce Euler-Euler multiphase fluid dynamics simulations, which calculate the volume fraction of liquid and gas phases, to investigate the crossover in the presence of gas bubbles. We specifically consider realistic implementation of solar-driven water splitting device by taking into account the tilt orientation of the device. Our simulations reveal that gas bubbles, often ignored in previous studies,^2,3 contribute more to the crossover rather than dissolved gases. We also extensively evaluate various important parameters that affect the overall product crossover, e.g., the device tilt angle, bubble diameter, bubble formation efficiency, etc. For example, smaller gas bubbles are found to suppress the crossover due to the stronger momentum exchange between the liquid and gas phases. This suggests the potential benefit of using surfactants, which are known to decrease the bubble diameter, for efficient product separation in membrane-less devices. Overall, our study highlights the critical importance in understanding and controlling the bubble formation under operating conditions to design efficient membrane-less water splitting devices.

1. Ahmet, et al., Sustainable Energy Fuels, 2019, 3, 2366.
2. Abdi, et al., Sustainable Energy Fuels, 2020, 4, 2734.
3. Holmes-Gentle, et al., Sustainable Energy Fuels, 2017, 1, 1184.

In the context of the strategies needed to mitigate CO₂ emissions and combat climate change, the electrochemical CO₂ reduction represents a promising alternative. Among the different reactors, GDE-based ones are widely studied systems: here, the limitations shown by configurations with CO₂ dissolved in electrolyte solutions can be overcome by feeding CO₂ directly in gaseous form. In this work, the manufacturing process of the Cu-based gas diffusion electrode, namely the catalytic ink deposition on a porous carbon paper support, was carried out both by airbrushing (manual) and by spray-coating (automated) techniques. The characterization of the electrodes was performed by using X-Ray Diffraction, X-Ray Photoelectron Spectroscopy and Field Emission Scanning Electron Microscopy techniques. To assess electrodes behavior, cyclic and linear sweep voltammetry techniques were conducted. When comparing the achieved current densities at the highest applied potential, the electrode obtained with the spray coater displayed a better electrocatalytic activity (~10 mA/cm² higher at about -2.5 V vs Ag/AgCl), with respect to that fabricated with the airbrush. A thorough study of the GDEs performance was accomplished, testing the so obtained electrodes and thereby evaluating the effect of a variation of Nafion content in the productivity and selectivity results toward the desired products. A promising outcome was achieved when boron (B) has been incorporated in the Cu-catalyst material: indeed, in this case, the hand-made GDE revealed a FE of about 6% toward alcohols (mostly ethanol). The catalyst layer dispersion is a critical aspect of electrochemical CO₂ reduction and, confirming previous studies on the different deposition methods, a more uniform distribution of the catalyst particles enabled the spray coated GDEs to outperform the hand-made ones. Furthermore, the variation of Nafion content on the GDE structure had a relevant effect on the electrode performance, allowing to considerably reduce the side-production of hydrogen and increasing at the same time the CO generation. Finally, the role of local pH was taken into account: a preliminary model highlighted that a higher pH on the electrode surface may promote the hydration of CO₂ to carbonate and bicarbonate species, thus lowering CO₂ concentration at the triple-phase-boundary and hindering liquid products formation.
Acknowledgement
This work has received funding from the European Union’s Horizon 2020 Research and Innovation Action programme under the Project SunCoChem (Grant Agreement No 862192) and from the Piedmont Region under the Saturno project (https://saturnobioeconomia.it/).

Magnetron sputtering is a scalable and eco-friendly approach of developing high-performance non-noble electro(co)catalysts with strong adhesion for durability. Herein, a 3D triangular pyramid nickel nitride with tunable nitrogen vacancies was created through magnetron sputtering, which is different from conventional nitridation process with high temperature and hazardous resource. Continuous sputtering the Ni₃N/Ni catalysts with poor-crystalline and good wettability delivered 99 mV overpotential at 10 mA cm^-2, a Tafel slope of 28 mV dec^-1 and durability 75 hours, which is approximate to Pt sputtering on FTO and exceeds the other reported catalysts on the plannar substrates in alkaline media. Based on the theoretical calculations, it is further verified that the presence of nitrogen vacancies effectively enhances the adsorption of water molecules and ameliorates the adsorption-desorption behavior of intermediately adsorbed hydrogen, which leads to an advanced HER activity of Ni₃N/Ni. Then, the Ni₃N/Ni grown on c-Si photocathode showed the current density of 37.3 mA cm^-2 at 0 V RHE with 0.6 V onset potential, and applied bias photon-to-current efficiency (ABPE) 8.9% toward PEC-HER with stability in strong media 50 hours including the power on-off and continuous reaction. DFT calculations verified the surface structure with N-vacancies surface and high HER activity of the Ni₃N/Ni catalyst under working conditions. The demonstration has showed the potential for practical, large-scale manufacturing of the nitrides cocatalysts for composite photoelectrodes.

Electrochemical reduction of CO₂ offers an eco-friendly recycling strategy in producing economically valuable fuels and chemical feedstocks. This process is readily tuned by employing suitable electrocatalysts, electrolytes, and electrochemical cell configurations to achieve preferred products. The nature of the anions and cations used in the electrolyte, could either having neutral, promoting or detrimental effect on the CO₂ electroreduction; underlying the complexity of such electrocatalytic reaction. This talk will present two recent studies on how the electrolytes could significantly influence the CO₂ electroreduction performance. The dramatic influence of electrolyte alkalinity in widening the potential window for CO₂ electroreduction in a flow-cell system utilized SnS catalyst will be discussed. Alkaline electrolyte was found suppressing the H₂ evolution across all potentials particularly at the less negative potentials, as well as suppress CO evolution at more negative potentials. This in turn widens the potential window for formate conversion. Mechanistic aspects of the role of alkalinity in widening the electrochemical potential window will be elucidated. In another study, seawater was employed as an electrolyte for CO₂ electroreduction. Under the applied cathodic potential and in the presence of CO₂, calcium ions in the seawater resulting in detrimental effect of calcium carbonate deposition onto the catalyst surface. We proposed a strategy in preventing calcite deposition onto the catalyst, hence enabling seawater to be used as a viable electrolyte for CO₂ electroreduction.

The utilization of fossil fuels as the main energy source gives rise to serious environmental issues, including global warming caused by the continuously increasing level of atmospheric CO₂. The electrochemical conversion of CO₂ (CO₂RR) to chemicals and fuels driven by electricity derived from renewable energy has been recognized as a promising strategy towards sustainable energy.
In my talk I will provide examples of the dynamic transformations undergone by well-defined CO₂RR catalysts (e.g. Cu single crystals, Cu nanocubes) under reaction conditions and their influence on the catalyst selectivity. In particular, I will discuss how important morphological motives and chemical sites can be created and regenerated in potentiodynamic (pulsed) electrochemistry experiments. Additionally, the determining role of the electrolyte in the surface restructuring, reaction activity and selectivity will be illustrated.
In situ and operando characterization methods (e.g. EC-AFM, Liquid-TEM, LEEM/XPEEM, XAS, XPS, XRD) will be used to gain in depth understanding on the structure- and electrolyte-sensitivity of real CO₂RR catalysts under working conditions. Our results are expected to open up new routes for the reutilization of CO₂ through its direct selective conversion into higher value products such as ethylene and ethanol.

The Haber-Bosch process is widely used in the fertilizer industry for ammonia (NH₃) production, but it depends heavily on fossil fuels and involves very high temperatures and pressures. The electrochemical nitrogen reduction reaction (NRR) when combined with renewable sources of energy potentially offers a sustainable and fossil-fuels-free route for NH₃ production at moderate temperatures and pressures. However, for scalable NRR, a highly active, selective, and low-cost electrocatalyst is still needed, which motivates the search and rational design of NRR electrocatalysts. Recently, molybdenum disulfide (MoS₂), a layered transition metal dichalcogenide, has attracted significant interest as an NRR electrocatalyst, because of its highly catalytically active edges and defect sites on the basal plane. Yet, MoS₂ shows overall poor NRR activity and selectivity because of the high thermodynamic energy barriers, sluggish reaction kinetics, and relatively unfavorable thermodynamics than the competing hydrogen evolution reaction (HER). Here, we show that substitutional doping of MoS₂ monolayer with iron (Fe) dopants can improve significantly its NRR activity and selectivity. Using density functional theory (DFT) calculations, we study the influence of isolated Fe-dopant and clusters of Fe-dopants on the stability and local structure of MoS₂ basal plane and edges. Also, we study the thermodynamics of three competing reactions: NRR, HER, and electrochemical desulphurization reaction on the basal plane, and edges of undoped and Fe-doped MoS₂ monolayers. Key finding from our study is that Fe-doping promotes the formation of S-vacancy (V_S) sites on MoS₂ and clustering of Fe-dopants in the basal plane results in the spontaneous (thermodynamically favorable) formation of Fe-dopants and S-vacancy defect (Fe-V_S) complex. The Fe-V_S complex renders Fe and Mo atoms available for NRR reaction intermediates and substantially suppresses HER, increasing the NRR selectivity of MoS₂. Overall, our DFT calculations highlight the role of Fe-dopants in improving the NRR activity and selectivity of MoS₂ and rationalize the observations of recent experiments on Fe-doped MoS₂ nanoflowers that display significantly improved NRR faradaic efficiency and yield compared to undoped MoS₂ nanoflowers.

As an alternative to the cost-and energy-intensive Haber-Bosch process, the implementation of electrolytic ammonia synthesis from the dinitrogen molecule has been a long-sought goal. State-of-the-art electrocatalysts for nitrogen reduction reaction (NRR) face not only activity but also selectivity problems with the competitive hydrogen evolution reaction (HER). Recently, single-atom catalysts (SACs) have emerged as promising for various reactions as they combine the best of homogeneous and heterogeneous catalysts. The reason for their high activity compared to their bulk and nanoparticle counterparts are yet to be completely understood. Inspired by the structure of the nitrogenase FeMo cofactor, we studied 13 transition metals anchored on MoS₂ monolayer at Mo-top positions, as possible electrolytic NRR catalysts using first-principles methods. Employing the implicit solvation model, we calculated free energy barriers for proton abstraction by N₂ molecule in end-on configuration and adsorption free energy of hydrogen on all SACs. Based on these two parameters, Fe, Co, and Ru were found to be the most active and highly selective electrolytic NRR catalysts. Compared with other mechanisms, the limiting potentials (and hence activity) for enzymatic mechanisms were found to be higher on these three SACs, with Ru SAC having a very low overpotential of 0.38V versus Standard Hydrogen Electrode (SHE). Bader charge transferred from transition metal to N₂ molecule and group number of transition metals correlate strongly with the NRR activity and hence emerge as two key descriptors for catalytic activity. These intuitive principles for rational designing of promising alternatives to the currently used bulk Ru(0001) catalyst could accelerate the search for highly efficient and selective SACs for electrolytic NRR.

To advance the application of solar water splitting for renewable energy, tremendous works have been put on the development of state-of-the-art protective coatings for long-term stability and high efficiency, whereas more effort needs to be focused on the fundamental study of materials’ stability/degradation mechanisms. In this work, we report Si/GaN photocathodes for sustained hydrogen production via a self-improving mechanism. By employing state-of-the-art characterization techniques, we show the presence of an oxynitride-like layer, which behaves similarly to electrocatalyst for better mediating charge transfer. DFT simulation shows that replacing the nitrogen atoms by oxygen to form oxynitride can not only passivate the surface for further stability, but also provide more active sites for HER. This work also sets a prototype for fundamental understanding of stability mechanism of protective layers in PEC water splitting application, and the gained knowledge provides feedback for development of further improved design of durable and efficient photoelectrode.

Photoelectrochemical (PEC) water splitting is emerging as a technology that could potentially sustain a low-carbon hydrogen-based economy. Among the different photoactive materials suitable for operation in a PEC device, Cu(In,Ga)S₂ (CIGS) is drawing increasing attention as a photocathode owing to its outstanding optoelectronic properties and tunable band gap. However, serious bottlenecks for the implementation of this material arise from the costly fabrication of PV-grade thin-films using non-scalable vacuum techniques, and the need for overlayers/catalysts to extract charges and perform the hydrogen evolution reaction.

Recently, we reported an all-solution-processed CIGS photocathode that demonstrates a significant and stable photocurrent response towards water reduction, even in the absence of overlayers or co-catalysts. Although the photocurrent onset potential is substantially negative with respect to the flat-band potential, the bare CIGS photocathode offers an intriguing system for gaining fundamental insights. Motivated by its intrinsic catalytic properties and unsatisfactory photovoltage, we combined three complementary operando spectroelectrochemical techniques including photoelectrochemical impedance spectroscopy (PEIS), intensity-modulated photocurrent spectroscopy (IMPS) and operando Raman spectroscopy with density-functional theory (DFT) calculations to understand the fate of photogenerated charge carriers. Through PEIS, photoluminescence spectroscopy and DFT calculations, we resolved atom-specific surface trapping states that restrict the photovoltage of CIGS. In addition, we performed the first operando Raman study on CIGS photocathodes where we identified the catalytically active sites. Computational methods further confirm these sites requiring the least free energy to adsorb protons for hydrogen evolution reaction. Overall, these fundamental findings suggest the composition of an improved surface for defect passivation and active sites exposure, which paves the way for rational design of this material for PEC water reduction.

Hematite (α-Fe₂O₃) is a promising candidate for a photoanode material for photo-electrochemical hydrogen production, meeting several key criteria. A photoanode’s performance depends on optical absorption and electronic transport, which are manifestations of its electronic structure. In spite of hematite’s Earth abundance and deceptively simple chemical formula, the electronic and magnetic interactions arising from its Fe d⁵ orbitals and neighboring oxygens, and longer range interactions, is surprisingly intricate. Here we interrogate hematite’s electronic structure with Resonant Inelastic X-Ray Scattering (RIXS). After a brief introduction to the technique, we present our Fe L_III-edge RIXS spectra of heteroepitaxial hematite thin films, both undoped and variously doped, measured for several momentum transfers (q), at both low temperature (T=14K) and room temperature. Although there could be some weakly dispersive excitations present in the spectra, a possible sign of charge transfer excitations, we conclude that the bulk of the observed L_III-edge RIXS intensity in the visible range originates from ligand-field d-d excitations, which is corroborated with the help of spin-multiplet simulations. The temperature dependence, which showed some highly temperature-sensitive features, identically for undoped, 1% Sn-doped, and 1% Zn-doped samples, is discussed.

The cornerstone of the emerging hydrogen economy is hydrogen production by water electrolysis with concomitant oxygen generation. Incorporating a third element in metal phosphides can tune the crystalline and electronic structure, hence improving the electrocatalytic properties. In this work using first-principles theoretical calculations, Mn-substituted Ni2P has been explored as an excellent catalyst for water splitting. The catalytic activity of Ni2P improves drastically on Mn-substitution for hydrogen evolution reaction as we obtain a close to optimum ΔGH (-0.14 eV) and the work function of Ni1.5Mn0.5P which falls just below the hydrogen reduction potential. The oxygen evolution activity of Ni2P is also explored upon Mn-substitution. We find a 32% reduction in the overpotential (η) towards OER for Ni1.5Mn0.5P. The effect of surface oxygen (metal-oxides) species on the catalytic performance of Ni1.5Mn0.5P is further studied and the η further reduces thus improving the OER activity of the catalyst. We show that substitution of a second metal species in metal phosphides alters the electronic structure and induces synergistic interactions in the system leading to an enhanced catalytic activity towards water splitting.

Advances in first-principles calculations enable accurate predictions of charge carrier mobility and scattering mechanisms in semiconductors. However, many transition metal oxides of technological interest remain an open problem as they exhibit soft phonon modes due to structural phase transitions, strong electron-phonon (e-ph) interactions leading to polaron effects, and electron correlation due to open d shell, or combinations of these features.

This talk will present a range of new ab initio approaches for treating e-ph interactions and charge transport in oxides. We will first present methods to address soft phonon modes and large-polaron effects, and demonstrate their application to SrTiO3 perovskite as a paradigmatic case. A recent study of a wide range of oxides with room temperature mobilities ranging from low to high will then be presented, highlighting materials design rules for high mobility. The second part of the talk will focus on recent progress, including studies of e-ph interactions in the frameworks of DFT+U and dynamical mean field theory, and their application to understanding the transport properties of various transition metal oxides. If time permits, we will briefly discuss a new approach to predict the formation of self-localized (small) polarons in oxides with inexpensive e-ph calculations.

Photoelectrochemical (PEC) approaches for the processing of solar fuels and materials are interesting, provided they can be efficiently, stably, scalably, and sustainably implemented.
There is still significant process required on the development of earth abundant and efficient but also stable and robust materials. The rapid identification of novel and promising photoelectrode materials or components is needed in order to provide a faster turnover of potential novel candidate materials (and different synthesis processes). Identifying and quantifying the key parameters limiting the efficiency and the stability of the photoelectrodes is fundamental to providing material and mesostructural design guidelines. This quantification is experimentally not accessible given by the multi-physical nature of the processes taking place in the photoelectrodes. Computational modeling can provide the necessary insights but requires the detailed knowledge of material parameters that are often unknown for new photoelectrode materials and requires to account for the complex mesostructured of the photoelectrode.
First, I will discuss versatile and validated computational models that allow for the material characterization, material parameters optimization, and mesostructural optimization of photoelectrodes. I will distinguish between involved multi-dimensional and multi-scale models and rapid screening models based on analytical correlations. I will then show how models can be extended to incorporate degradation (specifically photocorrosion) processes in order to understand and increase stability. I will show how important the local reaction environment is for the stability of a component, and how heterogeneity in the operating variables (current density, species concentration, temperature, etc.) affect degradation. I will end with providing general guidelines on materials and component integration into efficient and stable photoelectrodes and full devices, while also discussing the robustness of material and device efficiency and stability when exposed to varying, non-design-point operating conditions (such as will be expected in on-sun operation).

Practical photoelectrochemical devices for solar water splitting should generate photovoltages in excess of 1.7 V to reach viable efficiencies. Currently, the only practical way to achieve this is to combine two or more absorbers in a tandem configuration. With silicon being a nearly ideal candidate as a bottom absorber, the main challenge is to find a suitable top absorber material that combines good chemical stability with a bandgap of around 1.9 eV. I will discuss recent progress on Cu₅V₂O₁₀, b-Cu₂V₂O₇, α-SnWO₄, and CuBi₂O₄, all of which have a bandgap that is close to the ideal value. We have synthesized all these materials with pulsed laser deposition in an attempt to obtain films of high electronic quality. Although all these materials show relatively modest photocurrents, some show surprisingly high photovoltages and reasonable stability under certain conditions. The opportunities for these materials and challenges that have to be addressed will be critically discussed. In the second part, I will show several examples of tandem water splitting devices. One is an all-oxide device based on p-CuBi₂O₄ and W:BiVO₄, the second is based on a top absorber of nanostructured WO₃/BiVO₄ with a flat silicon wafer as a bottom absorber. I will finish with some considerations on the scale-up of these devices and show some recent results on the importance of natural convection near electrode surfaces.

The development of efficient, stable photoanodes and photocathodes remains a primary materials challenge in the establishment of a scalable technology for artificial photosynthesis. The typical photoelectrode architecture consists of a semiconductor light absorber coated with a metal oxide that serves a combination of functions, including corrosion protection, electrocatalysis, light trapping, carrier transport, and elimination of deleterious surface recombination sites. We describe high throughput investigations of the variation in performance and photo-response of integrated photoanode and photocathode libraries. The photoanode assembly libraries consist of uniform BiVO₄ light absorbers coated with films varying in composition and loading and evaluated at pH 9 and 13. The photocathode assembly libraries, for example, consist of uniform CuBi₂O₄ light absorbers coated with film libraries varying in composition and loading. Photoelectrode assemblies based on promising coating compositions and loadings were characterized in detail.

Tandem devices are necessary architectures for unassisted photoelectrochemical water splitting because of their ability to maximize the power conversion efficiency and provide a high photo-voltage necessary to drive the reaction. The traditional tandem integration process involves a monolithic tandem stack where the wide bandgap top cell is grown onto the narrow bandgap bottom cell, which places limitations on material selection and fabrication processes due to restrictions such as deposition techniques and temperature. To overcome this challenge and provide flexibility in material selection and manufacturing processes, a semi-monolithic tandem device architecture is investigated.

The semi-monolithic tandem device architecture is a novel approach of tandem cell manufacturing aimed at providing cost-effective, scalable photoelectrochemical water splitting technology. This technique employs a lift off and transfer method to combines two independently processed cells into one cell architecture using a transparent conductive adhesive (TCA) to create a multi-junction photovoltaic stack. This allows combination of materials which a standard deposition technique would not allow due to bottom cell degradation during deposition.

In this communication, we report of our latest results to integrate wide bandgap chalcopyrite absorbers into semi-monolithic tandem devices for PEC water splitting. Being an essential steppingstone in the semi-monolithic tandem manufacturing process, characterization of the relative conversion efficiency of a single junction solar cell before and after the lift off process was investigated. The cell was exfoliated using a soda lime glass substrate and a TCA that exhibited ~90% transmittance from 300-2000 nm as well as an out-of-plane series resistance on the order of 10^-1 Ω-cm² (comparable to that of molybdenum in Cu(In,Ga)Se₂ solar cells). Using a 1.8 eV CuGa₂Se₅ cell integrated with CdS, ZnO and ITO, our results showed that approximately 100% of the baseline efficiency, short-circuit photocurrent density and open-circuit voltage was preserved after exfoliation. Then, the wide bandgap device was integrated with a 1.1eV narrow bandgap silicon solar cell using our lift-off/transfer technique to create a semi-monolithic tandem device. An open circuit voltage (Voc) of 1.2V was obtained, corresponding to over 83% of the sum of the sub-cells Voc. These promising results confirm the viability of the semi-monolithic approach to create efficient tandem devices for PEC water splitting.

The depletion of nonrenewable fossil fuels and its adverse impact on the environment has intensified the search for alternative, sustainable, and clean energy sources for the last several decades. Concurrently, the researchers have focused on designing and developing innovative materials to convert and efficiently store energy. Oxides and sulfides of transition metals have become the state-of-the-art material for energy conversion and storage, mainly attributed to their rich faradic reaction in electrode-electrolyte interface. These materials' potential applications have been extensively studied for electrode material in batteries, supercapacitors, electrocatalyst for water splitting, and fuel cell applications. In this project, metal molybdate CoMoO₄ was prepared via two different methods, binder-free/in situ and dip coating. The two different CoMoO₄ electrodes' specific capacitance was calculated at different current densities and scan rates. Sharp differences between the two electrodes were observed regarding how much specific capacitance they can deliver based on various current densities and scan rates. The in-situ electrode displayed a specific capacitance of 2853 F/g, whereas the dip-coated achieved 357 F/g, both recorded at 1 mV/s. When the OER performances of CoMoO₄ in situ and dip-coated electrodes were compared, in situ CoMoO₄ required a lower overpotential of 324 mV whereas, dip-coated electrodes exhibited 381 mV, both at 10 mA/cm². In situ CoMoO₄ showcased faster kinetics with a more downward Tafel slope of 70 mV/dec compared to dip-coated electrode with 90mV/dec. With the introduction of 0.5 M methanol in the 1M KOH electrolyte, an improved current density signal was observed in both the electrodes. A current density signal of 200 mA/cm² (@20 mV/s) and 25 mA/cm² (@20 mV/s) with and without 0.5 M methanol were observed in dip-coated CoMoO₄. With 0.5 M methanol in electrolyte, an improved current density signal of 135 mA/cm² was obtained in the binder-free electrode from an initial current density signal of 30 mA/cm² (@20 mV/s). Such findings indicate the facile construction of molybdate-based electrocatalysts for methanol oxidation.

The economies of the future and the sustainability of the planet’s ecosystems will rely on green energy technologies such as through the capture of sunlight to drive chemical transformations. In particular, light-harvesting photoelectrodes dictate the generation of photovoltage and photocurrent so understanding the underlying physics governing performance and stability limitations are critical to advance related technologies. Specifically, CuBi₂O₄ is a natively p-type semiconductor with 1.5 – 1.8 eV optical bandgap; as a photocathode it has been demonstrated to have a photocurrent onset potential greater than 1 V_RHE yet remains limited to about 1.5 mA cm^-2. In this investigation we characterize the electronic structure (valence and conduction band orbital composition) of the material using a suite of advances spectroscopic techniques in conjunction with DFT modeling. These results suggest the limited electron polaron diffusion length of 45 nm is related to charge localization at Cu 3d states in the conduction band. Photoelectrodes made of plasmonic nanostructures are interesting for energy conversion applications due their dual function acting both as catalytic centers and light absorbers thus localizing the light energy directly at the catalytic site. Hollow-gold nanospheres (HGNs), thin shells of gold which can be readily dispersed in water, have a plasmon resonance absorption which can be tuned from 520 nm to the near infrared based on the shell dimeter and thickness making them excellent structures to harvest a large portion of the terrestrial solar flux. After the plasmon is excited, the energy is rapidly converted to heat the particle. In this study, through a combination of transient absorption (TA) spectroscopy and full scale molecular dynamics (MD) on 30 nm particles surrounded by water, the spatiotemporal temperature and pressure profiles reveal that photoexcitation of the HGN creates a high temperature and high-pressure environment particularly suited for thermal catalysis. These studies taken together identify an unique opportunity for coupled reaction micro-environments to take advantage of photoelectrochemical and photothermal reaction mechanisms.

Artificial photosynthesis including water splitting and CO₂ fixation using photocatalysts is an attractive research field toward realizing sustainable society. The metal sulfides with narrow band gaps are a promising material group as a photocatalyst for hydrogen evolution under visible light irradiation. Although the metal sulfide is a favorable material for visible light absorption, the metals sulfide itself is oxidized by photogenerated holes (photocorrosion). We have successfully utilize metal sulfide photocatalysts in Z-schematic water splitting and CO₂ reduction upon using a reduced graphene oxide as a solid-state electron mediator.^1-3) In the present study, we will present the successfully developed and improved Z-scheme systems using the metal sulfides prepared by a flux method with various alkaline chloride salts.
A single phase of metal sulfides was successfully prepared by a solid-state reaction and a flux method with various flux alkaline chloride salts. The particle size and aggregation degree depended on the preparation temperature in the flux method. Highly crystalline large particles were obtained at high temperature. The particle size was smaller than that of a solid-state reaction. All samples showed photocatalytic activity for sacrificial H₂ evolution under visible light irradiation. The metal sulfides prepared by the flux method showed the higher activity for sacrificial H₂ evolution than that prepared by a conventional solid-state reaction. The metal sulfide prepared by the flux method also functioned as a H₂-evolving photocatalyst in the Z-scheme systems with a reduced graphene oxide as an electron mediator. The solar water splitting activity was higher than that using the metal sulfide prepared by a solid-state reaction.⁴⁾ The Z-scheme system was also active for CO₂ reduction under visible light irradiation.
Thus, we successfully developed and improved Z-scheme systems for water splitting and CO₂ reduction using photocorrosive metal sulfide photocatalysts under visible light irradiation.

1) K. Iwashina, A. Iwase, Y. H. Ng, R. Amal, A. Kudo, J. Am. Chem. Soc. 2015, 137, 604.
2) A. Iwase, S. Yoshino, T. Takayama, Y. H. Ng, R. Amal, A. Kudo, J. Am. Chem. Soc. 2016, 138, 10260.
3) T. Takayama, K. Sato, T. Fujimura, Y. Kojima, A. Iwase, A. Kudo, Faraday Discuss. 2017, 198, 397.
4) S. Yoshino, A. Iwase, Y. H. Ng, R. Amal, A. Kudo, ACS Appli. Energy Mater. 2020, 3, 5684.

Converting the water molecule that sits at the bottom of the free energy landscape into hydrogen can serve as a core-technology in the future sustainable society, if driven by renewable energy sources. Photocatalytic and photoelectrochemical water splitting falls in this category of technologies, which proceeds, upon the irradiation of light, via a complex photophysical and electrocatalytic process involving photon absorption, exciton separation, carrier diffusion, carrier transport, surface reaction, and mass-transport.^[1,2] Achieving solar hydrogen production at a high efficiency thus requires these steps to function efficiently and simultaneously despite of their differing time scales, however, the complex reaction network hinders its quantitative understanding and thus rational designing. Notably, by decoupling the steps, the photophysical and electrocatalytic events during water splitting can be independently addressed, which would shed light on the system and pave a way for its development. ^[1,2]
This contribution describes the electrocatalytic water splitting at near-neutral pH levels, whose milder condition than the extreme pH counterparts is compatible with photocatalytic and photoelectrochemical systems, with a particular focus placed on the electrolyte.^[3-7] Electrocatalytic water splitting at the near-neutral pH levels experiences a reactant-switching.^[3] The hydrogen evolution reaction (HER) proceeds as the reduction of proton at smaller reaction rates, which switches to that of the water molecule at higher reaction rates due to the diffusional constraints of the proton. Likewise, the oxygen evolution reaction (OER) proceeds as the oxidation of the hydroxide ion and the water molecule at lower and higher reaction rates, respectively. Adding the buffering species, e.g., phosphate, into the solution mitigates such local pH alteration by in-situ providing the proton and hydroxide ions, although beyond this simple buffering action, the added buffering species plays a variety of decisive roles.^[3-6] For instance, the molality of the buffering species needs to be sufficiently high to attain improved buffering capacity for maintaining the local pH levels, which however is counterbalanced by the enlarged viscosity at high molalities, resulting in a slowed mass-transport flux of the buffering species.^[3.4] Also, the identity of ions, either kosmotropic or chaotropic, has similarly large influences on the electrolysis. Particularly, at molalities higher than 0.1 mol kg⁻¹, the mean activity coefficient of the added ions is no longer unity, and the association of the ions need to be taken into account that is determined by the identity and combination of the ions.^[3,5] All in all, looking at the electrolysis from the viewpoint of electrolyte quantitatively elucidates the bottleneck of the water splitting at the near-neutral pH levels and provides the guideline to improve its performance, highlighting the significance of electrolyte engineering.^[3] The aspect of electrolyte engineering detailed herein can be extended to the regulation of the gas cross-over during water splitting and quantitative understanding of mass-transport during photocatalytic water splitting.
References:
[1] K. Takanabe, ACS Catal. 2017, 7, 8006-8022.
[2] T. Shinagawa, Z. Cao, L. Cavallo, K. Takanabe, J. Energy Chem. 2017, 26, 259-269.
[3] T. Shinagawa, K. Takanabe, ChemSusChem 2017, 10, 1318-1336.
[4] T. Shinagawa, K. Takanabe, J. Phys. Chem. C 2015, 119, 20453-20458.
[5] T. Shinagawa, K. Takanabe, J. Phys. Chem. C 2016, 120, 1785-1794.
[6] T. Shinagawa, K. Obata, K. Takanabe, ChemCatChem 2019, 11, 5961-5968.
[7] T. Naito, T. Shinagawa, T. Nishimoto, K. Takanabe ChemSusChem, accepted. DOI: 10.1002/cssc.202001886

Due to the rapid depletion of fossil fuels and related environmental issues, developing technologies for producing renewable, clean fuels for our future is of great importance. Artificial photosynthesis via CO₂ reduction offers an attractive and cost-effective route to achieve this goal. However, the uncontrollable chemical transformations of existing photoelectrocatalytic materials under operating conditions greatly limit their applications for artificial photosynthesis technologies. Understanding their chemical transformations is a prerequisite to achieve sustainable energy conversion and storage into chemical bonds. Herein, we unravel the transformation mechanisms of photoelectrodes by systematic chemical and photoelectrochemical analysis and consequently provide an efficient protection scheme for light driven CO₂ reduction, using cuprous oxide as a model system.

The depletion of fossil fuels and the increase in environmental pollution have quest the scientific society to develop renewable and environment-friendly resources that can fulfill the needs of energy demands. Amidst all the existing clean energy resources, hydrogen energy is the most economical and has already captured a prime place in recent technological developments. Having high energy density, hydrogen can stand as an alternative to traditional fossil fuels. Water splitting via the electrochemical process to generate hydrogen for a fuel cell is an economic and green approach to resolve the looming energy and environmental crisis. It involves two half-cell redox reactions i.e. cathodic hydrogen evolution reaction-HER (2H⁺ + 2e^- H₂) and anodic oxygen evolution reaction-OER (4OH^- O₂ + 2H₂O) but due to their sluggish kinetics, its desires to develop highly active electrocatalysts to accelerate the electron transfer process. So far, the precious noble metals like Pt (for HER) and RuO₂/IrO₂ (for OER) have been proven to be the best electroactive catalyst but their extortionate cost and dearth restrain the industrial-grade applications. Besides that, these noble metals (Pt, RuO₂, IrO₂) are incompetent to make their position in the same electrolyte for continuous overall water splitting. Developing cheap, stable, scalable, and efficient electrocatalyst is in need to explore the suitable cost-effective replacement of noble metal-based electrocatalyst for future scale-up industrial viability. Herein, we report a rationally design seamless super-hydrophilic homogeneous bimetallic phosphide of Ni₂P-CuP₂ on Ni-foam-Graphene-CNTs heterostructure using facile electrochemical metallization followed by phosphorization without the intervention of metal oxides or hydroxides. The bimetallic phosphide shows extraordinary water electrolysis activity in terms of ultra-low overpotential (HER-12 mV@10 mA/cm², 124 mV@100 mA/cm² in acid medium and OER-140 mV@20 mA/cm², 190 mV@50 mA/cm² in basic medium) and high turn-over frequency (1s^-1@100 mV for HER and 1s^-1@310 mV for OER). The excellent stability at least for 10 days at a high current density of 500 mA/cm² without much deviation, infers the practical utilization of the novel catalyst towards green fuel production. The low Tafel slope (41 mV/dec for both HER and OER), low charge transfer resistance (18 ohm-HER), large electrochemical active surface area (959 cm²-HER, 630 cm²-OER), and large active site density (4.26x10^-4 mol/cm²-HER, 2.12x10^-4 mol/cm²-OER) indicating super electrocatalytic properties. The versatility of the catalyst has been observed when it shows remarkable performance in overall water splitting with a very low cell voltage of 1.45 V @10 mA/cm² in 1M KOH solution and long-term stability up to 40 hours with minimum loss (3%) of current density. The theoretical density functional study reveals that H-binding energy distribution among P-sites of CuP₂ and Ni₂P and d-band sifting among metal atoms in the heterostructure (Ni₂P-CuP₂) favour the HER and OER activity, respectively. Besides, the catalyst demonstrates an alternate transformation of solar energy to green H₂ production using a standard silicon solar cell. This research unveils a new avenue to design and synthesize highly stable electrocatalyst against an attractive paradigm of commercial water electrolysis for renewable electrochemical energy conversion.

The oxidation of methane, driven by electrochemical potential, is desirable as a strategy to overcome the low selectivity in thermochemical catalytic reactions, but improving productivity remains a challenge. We demonstrate the electrochemical conversion of methane to methanol in a CuO/CeO₂ oxide catalyst. We confirm the highest methane oxidation activity at Cu:Ce=6:4. we achieve the highest production rate of 752.9 μmol/g_cat/hr (6 hr reaction) at ambient pressure; Among the oxygenates, a CH3OH selectivity of 79% is obtained. In a high pressure reaction at 10 bar, productivity of 1,832.2 μmol/g_cat/hr and 21,986.6 μmol/g_cat is achieved. We confirm the formation of methanol by oxidation of methane by active oxygen on the catalyst, and also present a side reaction pathway by further oxidation of methanol.

For the development of systems to convert CO₂ into useful energy-rich chemicals under sunlight or electrical energy, the use of metal-complex catalysts is one of attractive approaches because CO₂ selectivity and reaction rates can be controlled by ligand modification [1]. The combination with semiconductor photosensitizers is also one of promising approaches to promote the reaction under photoirradiation. To realize a CO₂ reduction system which utilizes water molecule as an electron donor in the CO₂ reduction reaction (CO₂RR), we previously proposed the a hybrid molecular-catalyst/semiconductor system for the visible-light driven CO₂RR, and demonstrated systems of particulate photocatalyst and photoelectrode utilizing Ru-bipyridine complex polymer catalysts linked with semiconductors of N-doped Ta₂O₅[2], GaP:Zn[3], InP:Zn[4], (CuGa)_0.8Zn_0.4S₂[5], (AgIn)_0.22Zn_1.56S₂[4], ZnS:Ni[4], etc. The photoelectrode system demonstrated a monolithic tablet-formed device, and it generated formate with a solar-to-chemical conversion efficiency (E_solar) of 4.6 % in an aqueous solution [4]. DFT calculation and related experiments clarified that the very low onset potential (-0.18 V vs RHE) for CO₂ reduction at the Ru-bipyridine complex is dependent on combination of carbon support and K⁺ in solution, which enabled the demonstration of the monolithic device [6].
One recent topic is a visible-light-driven Z-schematic CO₂ reduction using H₂O as an electron donor in aqueous particulate suspension which can be operated in a simple mixture of [Ru(4,4’-diphosphonate-2,2’-bipyridine)(CO)₂Cl₂] ([Ru(dpbpy)] modified (CuGa)_1-xZn_2xS₂ (CGZS) hybrid photocatalyst as a CO₂ reduction, BiVO₄ photocatalyst as a water oxidation and Co-complex as an electron mediator [7]. The CO₂ reduction activity was significantly dependent on the composition of CGZS, and utilization of [Ru(dpbpy)]/CGZS at x=0.7 (E_g = 2.36 eV) showed the highest Z-schematic CO₂ reduction activity for CO and HCOO^- production accompanying O₂ generation under visible-light irradiation. The very high CO₂ reduction selectivity exceeding 90 % under visible light irradiation as the aqueous particulate suspension suggests that particulate Z-schematic reaction is also applicable to the highly selective and efficient photocatalysis for solar fuel generation by CO₂ fixation.
As for the electrochemical system, our recent approach is the use of earth-abundant elements. As well as lowering system costs, reduction in CO₂ emissions from raw materials mining to final catalyst synthesis further confirms the feasibility of this earth-abundant systems. A silicon photovoltaic cell (Si-PV) was connected to a [Mn-bpy] cathode for CO₂RR [5] and an β-FeOOH anode combined with Ni hydroxide for OER [8, 9]. The system operated in a one-compartment reactor filled with pH 6.9 aqueous solution saturated with CO₂ [10], and exhibited E_solar of over 10 % in production of mixture of CO and H₂ as syngas.

- References
[1] K. Kamada, S. Saito et al., J. Am. Chem. Soc. 2020, 142, 10261−10266.
[2] S. Sato, T. Morikawa, et al, Angew. Chem. Int. Ed. 2010, 49, 5101-5105.
[3] S. Sato, T. Arai, T. Morikawa, et al., J. Am. Chem. Soc., 2011, 133, 15240-15243.
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Direct photoelectrochemical hydrogen production is a promising route towards a clean and sustainable energy supply. To obtain high solar-to-hydrogen (STH) efficiencies, a suitable semiconducting material with a band gap of 1.7 to 1.9 eV is needed as the top absorber in tandem solar water splitting devices [1]. An interesting candidate is α-SnWO₄, a ternary metal oxide with an ideal band gap of 1.9 eV and a flatband potential of ~0 V vs. RHE. Several deposition methods have been used for the deposition α-SnWO₄ thin films. The highest reported photocurrent so far of 0.75 mA/cm² has been achieved by pulsed laser deposited (PLD) α-SnWO₄ films covered with an NiO_x overlayer [2].
In this study, we provide an overview of our current understanding of α-SnWO₄ films prepared by pulsed laser deposition. First, although the deposition of NiO_x is crucial to obtain the above-mentioned record photocurrent, it limits the photovoltage as observed from the cyclic voltammetry and open circuit potential analysis (OCP). This suggests that the interface between α-SnWO₄ and NiO_x is not ideal, and understanding this interface is important to improve the performance further. We therefore present a thorough α-SnWO₄/NiO_x interface investigation by means of synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). These data are complemented with OCP analysis and Monte-Carlo-based photoemission spectra simulation. Our study reveals that the deposition of NiO_x with PLD introduces a strong upwards band bending (~400 meV) at the interface, which is favorable for charge separation. However, significant oxidation of Sn²⁺ to Sn⁴⁺ can be simultaneously observed at the interface with increasing NiO_x layer thickness. Our photoemission spectra simulation indicates that this can be attributed to the formation of SnO₂ at the interface. Control experiments show that such a layer can indeed be responsible for the reduction of photovoltage; its formation has to therefore be avoided to overcome the photovoltage limitation of α-SnWO₄. In the second part of this study, a systematic investigation of the photoelectrochemical stability of α-SnWO₄ is performed in electrolytes with a broad pH range (2 – 13) at various applied potentials. Inductively coupled plasma optical emission spectroscopy (ICP-OES) is combined with X-ray photoelectron spectroscopy, X-ray diffraction and electron microscopy to reveal the mechanisms behind the stability or degradation at respective pH and applied potential. Finally, we show the operation conditions in which a self-limiting passivation layer is formed on α-SnWO₄. While this layer blocks further charge transfer, such a self-limiting process relaxes the requirement for the protective overlayer, i.e., it allows for the presence of a limited number of pinholes. Overall, this overview suggests that despite the previously reported challenges, α-SnWO₄ is a promising material for photoelectrochemical water splitting. Further performance improvements can be achieved by optimizing the interfacial properties between α-SnWO₄ and the overlayer as well as taking advantage of the beneficial self-passivation mechanism.

[1] Seitz et al., ChemSusChem 7 (2014) 1372 – 1385
[2] Kölbach et al. Chem Mater. 30 (2018) 8322-8331

Metal halide perovskites have recently emerged as promising alternatives to commonly employed light absorbers for solar fuel synthesis.^[1,2] These semiconductors enabled photoelectrochemical (PEC) perovskite-BiVO₄ tandem devices which can perform unassisted water splitting,^[3,4,6] as well as the more challenging CO₂ reduction to syngas.^[5,7] While the bare perovskite light absorber is rapidly degraded by moisture, recent developments in the device structure have led to substantial advances in the device stability, from seconds to days.

In this contribution, we give an overview of the latest progress from the field of perovskite PEC devices, introducing design principles to improve their performance and reliability. For this purpose, we will discuss the role of charge selective layers in increasing the device photocurrent and photovoltage, by fine-tuning the band alignment and enabling efficient charge separation. A further beneficial effect of hydrophobicity is revealed by comparing devices with different hole transport layers (HTLs). A threefold increase in the lifetime of perovskite photocathodes is obtained by replacing a hydrophilic PEDOT:PSS HTL with an inorganic NiO_x HTL.^[3] A further leap in stability up to 96 h can be demonstrated by introducing a hydrophobic PTAA HTL, which acts as an additional barrier to lateral moisture infiltration while further increasing the onset potential for H₂ evolution to approximately 1.0 V vs. RHE.^[6]

On the manufacturing side, we will provide new insights into how appropriate encapsulation techniques can extend the device lifetime to a few days under operation in aqueous media.^[3,5] Many prototypes rely on low melting alloys as encapsulants, however the demand on rare elements can be detrimental for the overall cost and scalability of the tandems, whereas metals can suffer from chemical corrosion. To avoid these drawbacks, we introduce graphite epoxy paste as a conductive, hydrophobic encapsulant.^[6,8] This abundant, metal-free composite can reduce the device cost^[6] while enabling a more facile integration of perovskite devices with inorganic,^[6,7] molecular^[5] and bio-catalysts.^[4] The combined advantages of these approaches are demonstrated in a perovskite-BiVO₄ tandem configuration, leading to selective unassisted CO₂ reduction to syngas.^[7]


[1] Chen, J.; Dong, C.; Idriss, H.; Mohammed, O. F.; Bakr, O. M. Metal Halide Perovskites for Solar-to-Chemical Fuel Conversion. Adv. Energy Mater. 2019, 1902433.
[2] Samu, G. F.; Janáky, C. Photocorrosion at Irradiated Perovskite/Electrolyte Interfaces. J. Am. Chem. Soc. 2020, 142 (52), 21595.
[3] Andrei, V.; Hoye, Robert L. Z.; Crespo-Quesada, M.; Bajada, M.; Ahmad, S.; de Volder, M.; Friend, R.; Reisner, E. Scalable Triple Cation Mixed Halide Perovskite–BiVO₄ Tandems for Bias-Free Water Splitting. Adv. Energy Mater. 2018, 8, 1801403.
[4] Edwardes Moore, E.; Andrei, V.; Zacarias, S.; Pereira, I. A. C.; Reisner, E. Integration of a Hydrogenase in a Lead Halide Perovskite Photoelectrode for Tandem Solar Water Splitting. ACS Energy Lett. 2020, 5, 232.
[5] Andrei, V.; Reuillard, B.; Reisner, E. Bias-free solar syngas production by integrating a molecular cobalt catalyst with perovskite–BiVO₄ tandems. Nat. Mater. 2020, 19, 189.
[6] Pornrungroj, C.; Andrei, V.; Rahaman, M.; Uswachoke, C.; Joyce, H. J.; Wright, D. S.; Reisner, E. Bifunctional Perovskite-BiVO4 Tandem Devices for Uninterrupted Solar and Electrocatalytic Water Splitting Cycles. Adv. Funct. Mater., 2008182.
[7] Rahaman, M.; Andrei, V.; Pornrungroj, C.; Wright, D.; Baumberg, J. J.; Reisner, E. Selective CO production from aqueous CO2 using a Cu96In4 catalyst and its integration into a bias-free solar perovskite–BiVO4 tandem device. Energy Environ. Sci. 2020, 13 (10), 3536.
[8] Andrei, V.; Bethke, K.; Rademann, K. Adjusting the thermoelectric properties of copper(i) oxide-graphite-polymer pastes and the applications of such flexible composites. Phys. Chem. Chem. Phys. 2016, 18 (16), 10700.

Si-based photoelectrodes have attracted substantial interest due to their potential for cost-effective conversion of water into clean fuel using solar energy. However, Si-based photoanodes for the oxygen evolution reaction (OER) are still challenging due to the requirements of large overpotential and low stability in alkaline solutions. To achieve high efficiency and long-term stability, metal-insulator-semiconductor (MIS) structures have been widely explored. In Si-based MIS photoanodes, the insulator needs to prevent Si corrosion in alkaline solutions and also transport carriers efficiently. In this work, we demonstrate a series of highly manufacturable processes for fabricating Ni/SiO₂/Si MIS photoanodes with SiO₂ insulating layers up to 90nm thick and localized, non-lithographically fabricated Ni catalyst contacts. The resulting photoanode structures show high efficiency and long-term stability in alkaline solutions. Specifically, a Ni/SiO₂/n-Si photoanode yielded onset potential and photocurrent density of ~1.0 V versus RHE and ~25 mA/cm², respectively, while a Ni/SiO₂/p⁺n-Si photoanode yielded an onset potential of 0.7 V versus RHE and saturation current density of 32 mA/cm², all in 1 M KOH alkaline solutions. Moreover, in stability testing in 1 M KOH aqueous solution, a constant photocurrent density of ~15.5 mA/cm² was maintained at 1.3 V versus RHE for 48 hours. This approach yields an enhancement in OER performance of Si-based MIS photoanodes with highly manufacturable fabrication processes that are amenable to large-scale commercial fabrication.

Hydrogen is a versatile energy carrier. When produced by water splitting using renewable energy sources, it is a greenhouse gas-free alternative to fossil fuels. The industrialization process of this technology is currently dominated by electrolyzers powered by electricity from wind turbines or photovoltaic solar cells. This approach is, for now, the most advanced and feasible. However, there are indications that the long-term goal to produce Hydrogen on a TW-scale only by wind power could face physical limitations,^[1] not present for the solar route, which we will discuss.
Furthermore, it is debated in the community whether more integrated device designs for solar water splitting can optimize hydrogen production due to lower balance of system costs and a smarter thermal management.^[2,3,4] Such a more integrated device–with the photoabsorber immersed in the electrolyte or not–offers the opportunity to thermally couple the absorber and the electrolyte. In this way, heat losses in the photoabsorber can be turned into an efficiency boost for the device via simultaneously cooling the absorber, enhancing the catalytic performance of the water splitting reactions, and decreasing the ohmic losses.^[5]
However, the complex influence of different weather and climate conditions on the efficiency of such a thermally coupled device is poorly understood. Here, we introduce an open-source Python-based model that combines solar cell physics, electrochemistry, thermal fluxes, and climate data as part of the “YaSoFo” environment.^[6] This model allows us to find suitable device configurations and quantify the efficiency-benefits for multi-junction photoelectrochemical devices. In addition, we scrutinize our predictions under idealized laboratory conditions. Our modelling and experimental results give important insights into the chances and challenges for thermally coupled water splitting.

[1] A. Kleidon, L. Miller, F. Gans, Physical Limits of Solar Energy Conversion in the Earth System. In:
Tüysüz H., Chan C. (eds) Solar Energy for Fuels, Top. Curr. Chem. 2015, 371, Springer, Cham
[2] T. J. Jacobsson, Energy Environ. Sci. 2018, 11, 1977-1979
[3] R. van de Krol, B. Parkinson, MRS Energy Sustain. 2017, 4(e13), 1-11
[4] M. Reuß, J. Reul, T. Grube, M. Langemann, S. Calnan, M. Robinius, R. Schlatmann, U.
Rau, D. Stolten, Sustain. Energy Fuels 2019, 3, 801-813
[5] S. Tembhurne, F. Nandjou, S. Haussener, Nat. Energy 2019, 4, 399–407
[6] M. M. May, D. Lackner, J. Ohlmann, F. Dimroth, R. van de Krol, T. Hannappel, K.
Schwarzburg, Sustain. Energy Fuels 2017,1, 492-503

Since the industrial revolution, anthropogenic activities have impacted the planet’s carbon cycle by the emissions of large amounts of greenhouse gases (GHGs), shifting the equilibrium of human history. The electrocatalytic CO₂ reduction (EC CO₂R) is an interesting technology because, driven by renewable energy sources, can be used to store both renewable electricity and CO₂ in added-value products such as liquid fuels (ethanol and other high-octane alcohols (>C₂)).[1] To date, researchers have focused on observing the effects of surface modification (e.g., nano-structuring and surface tailoring) on catalyst selectivity and activity to produce C₂₊ products. Nonetheless, it still remains an ongoing challenge due to high C-C coupling barriers. Among these studies, incorporation of the light element boron (B) into the Cu catalyst has been reported to induce the formation and stabilization of Cu⁺¹/Cu interfaces and reduce the barrier of the *CO dimerization, promoting a high activity for EC CO₂R towards C₂ products.[2] We have developed a B-doped Cu oxide catalyst by the ultrasound-assisted co-precipitation method and B₂O₃ impregnation, which were tested for this technology. The catalyst materials have been fully characterized by different Physico-chemical methods like X-ray diffraction, BET, porosimetry, filed-emission scanning electron microscopy (FESEM), among others, before and after the catalytic tests. The results revealed that for producing C₂₊ alcohols, the catalyst must overcome the 1° RDS of *CO formation, and then this *CO can be converted by dimerization to superior products. Thus, Faradaic efficiency towards alcohols (> 95% selectivity to ethanol) increases, and B-doping reduces H₂ production up to an optimum B amount. Therefore, under the optimum synthesis condition, a partial current density of about 3 mA cm^-2 for alcohols was reached. The physical and chemical properties of the nanostructures synthesized materials define concerted efforts that can be manipulated to tune the performance of the electrochemical reaction. These interesting results could help find a suitable electrocatalyst to establish this technology at the industrial level.

Acknowledgments

This work has been performed with the financial support of Eni SpA and the R&D Program Energy Transition (Cattura e Utilizzo CO₂).

References
[1] H. Guzmán, M.A. Farkhondehfal, K. Rodulfo Tolod, N. Russo, S. Hernández, Photo/electrocatalytic hydrogen exploitation for CO₂ reduction toward solar fuels production, in: Sol. Hydrog. Prod. Process. Syst. Technol., Elsevier Inc., 2019: p. 560. https://doi.org/10.1016/C2017-0-02289-9.
[2] C. Chen, X. Sun, L. Lu, D. Yang, J. Ma, Q. Zhu, Q. Qian, B. Han, Efficient electroreduction of CO₂ to C₂ products over B-doped oxide-derived copper, Green Chem. 20 (2018) 4579–4583. https://doi.org/10.1039/c8gc02389a.

The conversion of the atmospheric CO₂ to value-added compounds is more and more attractive to the scientific community, since natural sink cannot keep up with the constant anthropogenic emission and amplification processes. Recently, CO₂ concentration in the atmosphere exceeds 410 ppm, and its growth has remained constant since the 50s^[1]. Renewable and green approaches to CO₂ recovery are aimed to minimize the worrying impact of its emission to the environment, and to drive the transition to a new circular economy approach in chemistry and energy production. Within the depicted scenario, electrochemical and photoelectrochemical CO₂ reduction are being widely investigated as promising methods to transform CO₂, under mild reaction conditions, into useful chemicals or fuels. For instance, alcohols, CO and HCOOH that can be exploited as renewable energy sources or as key intermediates for the chemical industry.
Among the non-precious metal oxides, Cu₂O is a cheap, abundant and intrinsically p-type semiconductor. Due to its narrow band gap (~ 2 eV) and the suitable positioning of conduction and valence bands, Cu₂O is an ideal photocatalyst for CO₂RR. Simultaneously, SnO₂ is an n-type direct band-gap semiconductor with noticeable electron mobility together with an intrinsic stability. It openly transpires the dual role of Cu₂O, as photoabsorber and forming a p-n junction with tin oxide.
In this work, the synthesis of photoactive copper-tin-oxide-based catalyst was optimized by a co-precipitation method^[2], employing Cu(NO₃)₂3H₂O and SnCl₄5H₂O into a stirred and heated reactor. A solution of Na₂CO₃ was added as a precipitant agent, while NaBH₄ as the reducing one^[3], in order to promote the Cu₂O formation. The work-up protocol, based on copious MilliQ water washings, was implemented and finally optimized. The characterization step included Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray Analysis (EDX) X-rays Diffraction Analysis (XRD), UV-Visible Spectroscopy analysis, among others, and allowed the morphological assessment, the porosity value estimation and the crystalline phase evaluation. With the latter, it has been found a correspondence to the cubic crystalline phase (cuprite) of Cu₂O and consequently confirmation that the reduction process has been successfully carried out.
The so obtained catalyst was then deposited it onto a GDL (Gas Diffusion Layer) and FTO-based substrates by spray coating of an ink containing: the catalysts, Vulcan carbon (to increase the electrode conductivity), Nafion as binder and Isopropanol as carrier. The photo-electrochemical activity for the CO₂ reduction reaction was tested in the dark and under sunlight simulated conditions by means of Linear Sweep Voltammetry (LSV) and Chrono-Potentiometry (CP) analyses. Relevant current density (j) values of up to 40 mA/cm² were observed, and from the products analysis during the CP a high Faradaic Efficiency to CO was obtained. The influence of the ink composition was accurately investigated in terms of interaction among all the components and with respect to the employed substrate, taking into account sun-light activity and stability of the prepared electrodes towards their future utilization in a device for the sun-driven CO₂ conversion to high-added value products.
AKCNOWLEDGMENT
This work has received funding from the European Union’s Horizon 2020 Research and Innovation Action programme under the Project SunCoChem (Grant Agreement No 862192).
^[1] X. Lan, B. D. Hall, G. Dutton, J. Mühle, and J. W. Elkins. (2020). Atmospheric composition [in State of the Climate in 2018, Chapter 2: Global Climate].
^[2] Schuth F., et al., Journal of Catalysis, (2008), 258
^[3] Angew. Chem. Int. Ed. 10.1002/anie.201808964

In conventional solar cell materials, above-bandgap photon absorption generates free electrons and holes, which travel freely through the material and contribute to the photocurrent. In metal oxides, however, light can excite electrons and holes into a variety of different electronic states, some of which are, or can eventually become mobile charge carriers, while others do not contribute to photocurrent. For example, the visible absorption spectrum of hematite (α-Fe₂O₃), one of the leading photoanode candidates for solar water splitting, is thought to be composed of both ligand-to-metal charge transfer (LMCT) bands which are responsible for the photocurrent and more localized d-d bands which do not generate free charge. As a result, much of the optical absorption is wasted. Since both the LMCT and d-d bands span the visible spectrum, the photogeneration yield, defined as the probability of an absorbed photon to generate mobile charge carriers, depends strongly on wavelength.
Here, we introduce a new method to empirically extract the mobile charge carrier photogeneration yield spectrum of semiconductor photoabsorbers using optical and external quantum efficiency measurements of ultrathin films. We apply this method to hematite photoanodes in photoelectrochemical cells under operando conditions and show agreement between the extracted photogeneration yield spectrum and the photoconductance spectrum measured by time-resolved microwave conductivity. We demonstrate that mobile charge carrier generation fundamentally limits the conversion efficiency of hematite photoanodes, and provides an upper limit to the achievable photocurrent that is lower than that predicted by its bandgap. We then extend the analysis more generally to other photoelectrode materials and demonstrate its value towards understanding optical absorption mechanisms and charge carrier generation in complex materials.

Conventional electrocatalysts used in fuel cells, electrolyzers, and photoelectrochemical systems are typically comprised of metallic nanoparticles that lower energy barriers for electrochemical reactions at the electrocatalyst/electrolyte interface. Typically, these nanoparticles are partially or fully exposed to a liquid phase that contains the reactants and products of interest. In contrast, this presentation describes an emerging class of (photo)electrocatalysts for which the active electrocatalyst sites are completely encapsulated by ultrathin layers of permeable oxides that can exhibit membrane-like properties. These so-called membrane coated electrocatalysts (MCECs)^[1] offer many potential advantages over conventional electrocatalysts, including the ability to simultaneously mitigate degradation,^[2] alter reaction selectivity,^[3] and improve reaction kinetics at the buried interface between the overlayer and active catalyst.^[4] This talk will focus on describing recent case studies of silicon oxide (SiO_x) and titanium oxide (TiO_x)-encapsulated Pt electrocatalysts that demonstrate the ability of nanoscopic overlayers to greatly modulate reaction selectivity, even in cases where the thermodynamics and/or kinetics of the undesired reaction are highly favored compared to the desired reaction. By systematically changing the properties of well-defined oxide overlayers, this work uncovers design principles that can be used to tune reaction selectivity for a wide range of electrochemical reactions of interest for the sustainable production and use of fuels and commodity chemicals.
References
[1] D.V. Esposito, “Membrane Coated Electrocatalysts—an Alternative Approach to Achieving Stable and Tunable Electrocatalysis“, ACS Catalysis, vol. 8, pp 457–465, 2018..
[2]. N. Y. Labrador, et al., “Enhanced Performance of Si MIS Photocathodes Containing Oxide-Coated Nanoparticle Electrocatalysts” , Nano Letters, vol. 16, 6452-6459, 2016.
[3] N. Y. Labrador, et al., “Hydrogen Evolution at the Buried Interface between Pt Thin Films and Silicon Oxide Nanomembranes”. ACS Catalysis, vol. 8, pp 1767–1778, 2018.
[4] J.E. Robinson, et al., “Silicon Oxide-Encapsulated Platinum Thin Films as Highly Active Electrocatalysts for Carbon Monoxide and Methanol Oxidation”. ACS Catalysis, vol. 8, pp 11423–11434, 2018.

Semiconductor/electrolyte interfaces attract intense interest to convert solar energy to chemical fuels. Although many analytical models describing the photocurrent-voltage response of these devices exist, they have difficulty to reproduce full numerical simulations under small anodic bias.
We recently derived an analytic model of a weakly absorbing n-type semiconductor/electrolyte interface with a slow rate of water oxidation reaction, fast recombination rate and under small voltage bias [1]. Excellent overlap of our model was demonstrated with full numerical simulations. Our model enabled us to simplify calculation of the impedance of the semiconductor/electrolyte interface derived in the work of Bertoluzzi et al[2]. The comparison of analytic and measured impedance allows to extract the reaction rate for redox reaction from the dark impedance and the direct bulk recombination constant of the semiconductor from the impedance under illumination.
This work provides easy-to-use recipe for the quantitative comparison of the recombination and reaction properties of the various semiconductor photoelectrodes by impedance spectroscopy.
References
[1] Cendula at el., 10.1021/acs.jpcc.9b07244
[2] Bertoluzzi et al., 10.1039/C5TA03210E
Acknowledgement
We acknowledge support of Operational Program Integrated Infrastructure 2014 - 2020 of the project: Innovative Solutions for Propulsion, Power and Safety Components of Transport Vehicles, code ITMS 313011V334, co-financed by the European Regional Development Fund.

Heterostructures of semiconductors can achieve photoinduced charge separation for a wide variety of applications in photocatalysis, including photoelectrochemical (PEC) water splitting. Heterostructures comprised of quantum dots (QDs) and MoS₂ nanoplatelets have demonstrated potential as architectures for PEC water splitting and hydrogen generation.¹ Linker-assisted assembly (LAA) is a promising synthetic method for these heterostructures. In LAA, colloidal QDs are pre-synthesized, affording substantial control over size and energetics, and then interfaced with the nanoplatelets. Moreover, charge-transfer properties of the heterostructures can be tuned with properties of the bridging ligand.
This presentation will focus on the effects of varying the length of molecular linkers between QDs and MoS₂ on the dynamics of charge transfer and the efficiency of redox photocatalysis. Mercaptoalkanoic acid ligands, ranging from 3 to 16 carbon atoms, as well as cysteine were used to attach the QDs to MoS₂. We hypothesized that tuning the alkyl chain length of the ligand would modulate the rate of electron transfer, due to the variable spatial separation between the components. LAA-derived QD/MoS₂ heterostructures were initially synthesized with cysteine as the linker and exhibited rapid electron transfer from the QDs to MoS₂ due to the type-II energetics. These heterostructures also produced oxidative photocurrent and demonstrated photoelectrochemical and photocatalytic hydrogen generation. The LAA-derived QD/MoS₂ heterostructures with mercaptoalkanoic acid linkers also exhibited dynamic quenching, relative to free QDs, in time-resolved emission experiments, which provided evidence for excited-state charge transfer.
Measured electron-transfer rate constants for the heterostructures were tunable over three orders of magnitude by varying the alkyl chain length. LAA-derived heterostructures oxidized lactic acid to pyruvic acid, and oxidative photocurrents varied by approximately 4-fold with the length of the bridging ligand. These results highlight the extent to which the properties of bridging ligands affect excited-state charge transfer and photocatalysis within QD-MoS₂ heterostructures, and, more generally, provide insight into the design of materials architectures for PEC water splitting.

1. Cho, J.; Suwandaratne, N. S.; Razek, S.; Choi, Y.-H.; Piper, L. F. J.; Watson, D. F.; Banerjee, S., Elucidating the Mechanistic Origins of Photocatalytic Hydrogen Evolution Mediated by MoS2/CdS Quantum-Dot Heterostructures. ACS Applied Materials & Interfaces 2020, 12 (39), 43728-43740.

Recently, Fe₂TiO₅ has emerged as a promising photoanode material for water photo-oxidation due to its suitable bandgap of 2.1 eV, high chemical stability, and scalability consisting of earth-abundant elements [1]. It has been reported that photoelectrochemical (PEC) performance is enhanced in heterostructure forms such as Fe₂TiO₅/α-Fe₂O₃ or Fe₂TiO₅/TiO₂ [2,3]. However, these investigations have been limited in polycrystalline nanostructures that hinders the elucidation of the intrinsic PEC properties of Fe₂TiO₅.
Building on our successful fabrication of single-phase Fe₂TiO₅ epitaxial thin films using pulsed laser deposition [4], here we report the comprehensive characterization of the intrinsic PEC properties of Fe₂TiO₅ using the most well-defined surface and bulk structures available. We demonstrate that Fe₂TiO₅ exhibits highly efficient charge transfer efficiency of 20% at the Fe₂TiO₅-electrolyte interface at 1.23 V vs. RHE, while it is only 1% for α-Fe₂O₃. This excellent surface property is further corroborated by forming heterostructures with α-Fe₂O₃ epitaxial thin films, in which the photocurrent enhances by an order of magnitude and the onset potential improves by ~300 mV compared to a pure α-Fe₂O₃ photoanode. Details of the epitaxial photoanode fabrication, photocurrent-voltage curves, transient photocurrent analysis, and impedance spectroscopy will be discussed in the presentation.

[1] D. S. Ginley, and M. A. Butler, J. Appl. Phys. 48, 2019 (1977).
[2] Q. Liu et al., Nat. Commun. 5, 1 (2014).
[3] P. S. Bassi et al., Nano Energy 22, 310 (2016).
[4] M. Osada et al., APL Mater. 6, 056101 (2018).
*Supported by DOE BES MSD (DE-AC02-76SF00515) and the LDRD program at SLAC National Accelerator Laboratory.

There has been a vast improvement in photovoltaic (PV) technology in the last couple of decades; however, solar PV technology has a lot of drawbacks, including that it is intermittent, broadly dispersed, and non-transportable. Hence, cost-effective solar energy storage is critical for widespread implementation of solar energy as the primary energy source. My project focuses on the development of tandem semiconductor microparticles which when placed in water under 1 Sun condition could spontaneously split water into hydrogen and oxygen with solar-to-hydrogen efficiency of >10%.
In the present work, we have acquired customized microwire array structures made on silicon (Si) wafer by deep reactive ion etching method and formed a diode on these particles via a doping process. We have also been able to produce a TiO2 tandem junction for additional photovoltage. Nickel, which serves multiple purposes in the design, has been deposited on Si via an electroless photo-deposition process. It acts as a hydrogen evolution catalyst while protecting the Si from oxidation. Secondly, owing to its ferromagnetic property, an electromagnet can orient these particles to achieve current-matching, in the series-connected Si and TiO2. This is hypothesized to achieve higher efficiency.
For proof-of-concept, the magnetic alignment analysis has been done on Ni coated Si microwire particles. Future work aims at replicating similar analyses for tandem microwire particles and testing its effect on the eventual solar-to-hydrogen efficiency.

Oxide-based photoelectrodes recently have been at the forefront of research for photoelectrochemical water splitting. While most oxide-based photoanodes suffer from severe electron-hole recombination, BiVO₄ photoanodes are known to achieve exceptionally high electron-hole separation efficiencies. However, an understanding of what features of BiVO₄lead to this desired property is currently lacking. In this study, we sought to elucidate these features by investigating PbCrO₄, which has electronic and structural similarities to BiVO₄. For this goal, we prepared PbCrO₄ as a high-quality photoanode and compared its photoelectrochemical properties and stability with those of BiVO₄. In this presentation, we will discuss the photoelectrochemical similarities and differences between PbCrO₄ and BiVO₄ in detail, which can provide useful guidelines to develop high-performance oxide-based photoanodes. Another topic to be discussed in this presentation is the effects of surface termination/composition of a ternary photoelectrode on the interfacial energetics. For ternary oxide photoelectrodes, there exist numerous ways to terminate the surface even for the same facet and their surface composition can be different from their bulk composition. However, the effects of surface termination/composition on a ternary oxide photoelectrode have not been systematically studied. In this presentation, we will compare BiVO₄ photoelectrodes with V-rich and Bi-rich surfaces and demonstrate that the surface composition has a considerable effect on the surface energetics and photocurrent generation of BiVO₄ even for the same facet. We will also discuss the atomic origin of the impact that the surface of photoelectrodes has on their surface energetics and photoelectrochemical properties.