Symposium ES12—Redox-Active Oxides for Creating Renewable and Sustainable Energy Carriers

Currently over 95% of hydrogen is made from fossil fuels through natural gas reforming or coal gasification. A major part of reducing carbon emissions must therefore include sources of renewable hydrogen. In addition, capture of existing carbon dioxide sources and subsequent conversion to higher value chemicals also requires a hydrogen source. Using hydrogen from traditional sources for this purpose does not make environmental sense, since the net effect is just shifting where the carbon dioxide is emitted geographically. Large scale, cost competitive sources of renewable hydrogen are critical in moving to more sustainable processes worldwide.

Low temperature electrolysis has decreased significantly in cost while increasing in scale over the last several years, and there continue to be large opportunities for additional cost reduction and performance improvements. With the advent of low cost renewable electricity, particularly in times of excess capacity when storage is most needed, electrolysis is reaching the production and cost scale to provide a viable solution. However, as research activities have increased, differences in testing conditions, reporting comparisons, cell designs, and other factors convolute data interpretation and can lead to misleading conclusions. In addition, insufficiently controlled procedures can mask effects. This talk will describe some of the complex interactions that need to be considered, and current efforts in developing community-wide tests ranging from ex-situ material screening protocols to realistic device testing. This work is part of an ongoing program across all of the major advanced water splitting technologies, the subject of this symposium and the parallel high temperature water splitting symposium.

Cost competitive hydrogen generation at scale using sunlight and water can play a significant role in decarbonization of our society. In recent years, photoelectrochemical (PEC) water splitting has made significant progress in materials discovery, mechanistic understanding, as well as in achieving un-precedent solar to hydrogen conversion efficiencies in the laboratory scale. To further advance this technology and reach the bench scale (0.1 kg/day) and sub-scale (2 kg/day) operational platforms, development of standard protocols and best practices for benchmarking materials and devices as a community is critical. Standardized methodologies and best practices also provide significant benefit in accelerated RD&D of advanced materials and cross-cutting efforts.

In this presentation, I will talk about recent community-based efforts in defining targets, best practices and gaps in PEC hydrogen production in HydroGEN Energy Material Network (EMN) consortium. Specific challenges related to PEC will be discussed including scale up challenges, integration challenges at the component level, lack of performance data and test protocols for photoelectrodes under real-world operating conditions and lack of fundamental understanding of photoelectrode corrosion mechanisms and accelerated test protocols. In addition, I will also present a briefly summary and status reports on lab EMN nodes/capabilities for PEC water-splitting. Key takeaways from a recent HydroGEN Benchmarking&Protocols Workshop and DOE PEC Working Group Meeting will also be discussed.

Hydrogen production via water electrolysis is considered to be an efficient way to smoothen the fluctuating power output of renewable energy sources and avoid oversupplies. This presentation will provide the status of the standards and protocols development for high temperature electrolysis (HTE) technology within the US DOE HydroGEN Energy Materials Network Consortium. The current material standards, advanced characterization techniques, best practices and testing protocols from screening of fundamental material properties to device level benchmarking will be discussed to address key challenges and leverage technical advancements across the different water splitting technologies.

This presentation will provide the status of standards and protocol development within the US DOE HydroGEN Energy Materials Network (EMN) Consortium for advanced water splitting using concentrated solar thermochemical redox cycles, generally referred to as STCH, solar thermochemical hydrogen,.

Cost competitive water splitting to produce hydrogen at scale using concentrated sunlight can contribute significantly to a net CO₂ neutral society. In recent years, STCH has made significant progress. Working as a community will further advance STCH technology with a goal of reaching demonstration scales of multiple kilowatts with promising redox active materials. The development of standard protocols, definitions, metrics, best practices, and roadmaps for benchmarking materials and reactor designs is critical to accelerate progress and lower barriers for new researchers.

In this presentation, we will discuss recent HydroGEN EMN community-based efforts in defining metrics, best practices, and gaps in STCH. In addition, we will present a brief summary of EMN nodes/capabilities as well as key conclusions from a recent questionnaire and from a recent HydroGEN Benchmarking & Protocols Workshop specifically for STCH.

Solar thermal water splitting (STWS) is a promising renewable approach to converting water into hydrogen using concentrated sunlight. The central process in STWS is the high temperature reduction and oxidation of a metal oxide where H₂ is produced by reaction of H₂O with the reduced material. For this process to be viable, the thermodynamics and kinetics of reduction and oxidation of the redox active material must produce sufficient H₂ at practical rates and at temperatures at which the active material is stable. In this talk we will present our results using ab initio and machine learned models to discover candidate perovskite STWS materials. This includes predicting the stability of these materials at the operating temperature, whether these materials form as perovskites and in what specific phase, the energy of oxygen vacancies and other defects that mediate the redox process, and finally the viability of these metal oxides as STWS redox materials. The Gibbs energy, G, determines the equilibrium conditions of chemical reactions and materials stability. Unfortunately, G has been tabulated for only a small fraction of known inorganic compounds, thus impeding an analysis of whether materials are stable and synthesizable at operating conditions. We used a statistical learning approach to identify a simple and accurate descriptor to predict G for stoichiometric inorganic compounds with ~50 meV/atom resolution, and minimal computational cost, for temperatures between 300 K and 1800 K. We then applied this descriptor to screen various compositions for their stability at STWS temperatures. Candidate materials that form as perovskites were then analyzed using a machine learned one-dimensional tolerance factor, τ, that we developed that correctly predicts 92% of compounds as perovskite or nonperovskite to predict which of the predicted stable compounds form perovskites. Finally, we applied DFT on the remaining viable candidates to predict the thermodynamics of STWS reactions. This requires predicting the correct perovskite phase and the correct spin and charge state of the defect mediating the oxidation and reduction processes at the operating conditions.

The ability to assess and compare the performance of solar thermochemical water splitting materials individually and against other materials and even to other advanced water splitting pathways is increasingly a critical need in the field of sustainable hydrogen production. Technical assessment and comparisons include evaluating mass loss of samples during thermal gravimetric analysis (TGA) in controlled oxidizing and reducing environments; the results from TGA measurements have evolved into a widely reported metric. However, the lack of universal reference materials and calibration protocols has made comparisons between materials developed at different institutions unmanageable. We are developing standard samples and testing protocols for the solar thermochemical water splitting community as part of HydroGEN, also applicable to carbon dioxide splitting, and to solar thermochemical energy storage. We report here on the synthesis of standard materials for high temperature (>1500°C) and intermediate temperature (>1200°C) applications, the development of standard testing protocols for TGA, and benchmarking of behaviors in high temperature XRD. We will cover plans for both standard material curation and availability.

There are well-established concepts for describing and understanding the gas phase dependence of defect concentrations in oxides (Brouwer diagrams, etc.). Accordingly, detailed knowledge on bulk defect concentrations has become available for many redox-active oxides. Much less clear is the effect of the gas phase on the rate of reactions taking place on oxide surfaces, for example the oxygen exchange reaction, which is required for establishing the bulk defect concentrations. Also the partial pressure (and overpotential) dependence of oxidation and reduction reactions in solid oxide fuel or electrolysis cells is often not truly understood. The empirical dependences of the corresponding reaction rates on experimental parameters may be monitored by tools such as impedance spectroscopy, current-voltage measurements, tracer diffusion or time dependent weight or color changes. However, mechanistic interpretation of the data is impeded by the fact that the gas phase not only affects adsorbates on oxides but also ionic and electronic defects in the oxides and thus several reactants.
In the first part of the talk this lack of mechanistic understanding of oxygen surface exchange reactions is addressed. It is discussed whether partial pressure dependences of reaction rates indicate the relevant species in the rate limiting step (atomic or molecular), and what can be concluded from an exponential or non-exponential shape of a current-voltage curve in an electrochemical cells with redox-active electrodes. Moreover, it is shown why surface exchange coefficients (k-factors) from impedance measurements (k^q) and from tracer diffusion experiments (k^*) usually differ. Further factors affecting the oxygen surface exchange reaction and its degradation are exemplified by measurements with appropriate tools such as in-situ impedance PLD or microelectrodes. The usefulness of measuring the chemical capacitance for analyzing the defect chemistry of electrode materials is shown.
Effects resulting from the exposure of an oxide to light are even less understood than the surface exchange kinetics of oxides. This is addressed in the second part of the talk. It is demonstrated why such an exposure to light with energy beyond the band gap can change the defect concentration in an oxide and examples are shown for SrTiO₃ upon UV exposure. Illumination by light is thus a forth option for changing the stoichiometry of an oxide, in addition to the well-known approaches by partial pressure variation, applying of a voltage in electrochemical cells, and enhancing the temperature in thermochemical cells.

Ceria is used widely as an oxygen exchange material due to its phase stability and facile oxygen release and uptake. The performance of ceria in particular applications depends on the oxygen vacancy formation energy. Ceria is often doped with Zr to lower the oxygen vacancy formation energy, but with the exception of Hf, no other single dopant has been shown to significantly alter the reduction energy, though some dopant introduce intrinsic vacancies. This is because tetravalent dopants are required to prevent intrinsic vacancy formation. Here we present the use of paired charge compensating dopants to “engineer” tetravalent dopants from non-tetravalent cations. Specifically, by co-doping 3+/5+ cations into ceria, the 3+ dopant behaves like a tetravalent dopant, thereby expanding the oxygen vacancy formation energies possible. We show that this novel ceria doping strategy using both computational (density functional theory calculations) and experimental results within the context of solar thermochemical water splitting.

Perovskites have shown great promise as efficient redox materials for solar-to-fuel production due to higher oxygen exchange capacity, which in turn results in higher fuels yields compared to state-of-the-art ceria. Additionally, perovskites can be easily modified through extrinsic doping leading to new compositions with altered kinetics, redox capacity and reduction temperatures, which positively impacts the overall syngas production.¹ Alternatively, reduction temperatures can be lowered down by using methane as reductive gas, which has the additional benefit of leading to syngas production during the reduction half-cycle by the so-called methane partial oxidation², in which oxygen lattice of the perovskite reacts with methane producing H₂ and CO in a 2:1 ratio, ideal for further syngas processing into liquid fuels. In the second step, the reduced perovskite can be reoxidized by CO₂, H₂O or a mixture thereof, resulting in the production of fuel for both steps, which increases the overall solar-to-fuel process efficiency. This technology also known as solar chemical looping reforming, requires materials with high oxygen capacity and fast redox kinetics. Traditionally, cermets based on ceria or perovskites composed by a metallic catalyst impregnated onto the oxide surface have been explored for improving reaction rates, gas conversion and syngas selectivity². However, high temperatures can lead to metal catalyst agglomeration and coarsening, decreasing its chemical activity and cermet stability. Motivated by this fact, we have explored processing of perovskite-cermet structures through the exsolution method as an alternative to impregnation, which was successfully demonstrated to control the catalysis e.g. for (La,Sr)(Ti,Ni)O₃ solid oxide fuel cell anodes.^3,4 Through this work we explore suited perovskite compositions towards faster reduction rates and improved syngas selectivity for thermochemical redox cycles, and also compare to traditionally infiltration processed materials of same chemistry. The high surface affinity between the metallic nanoparticles and the perovskite backbone, in which they are anchored, offers relevant advantages in terms of microstructural stability for high temperature thermochemical fuel production.
In the first part of the talk we will cover the materials design guidelines that result in exsolved-perovskite structures with increased particle coverage and size distribution, exploring the chemical nature of the nanoparticles and interfaces of the cermets. Secondly, their performance for syngas production tested under chemical looping reforming cycles will be evaluated and benchmarked with state-of-the-art materials, revealing the improved syngas production performance of exsolution-promoted perovskites and the high stability of such materials under prolonged cycling.⁵ These results show great promise for exsolution cermet processing route in improving fuel production kinetics, syngas yields and materials stability, opening new avenues for materials design and process optimization.
References:
1 M. Kubicek, A. H. Bork and J. L. M. Rupp, J. Mater. Chem. A, 2017, 5, 11983–12000.
2 P. T. Krenzke, J. R. Fosheim and J. H. Davidson, Sol. Energy, 2017, 156, 48–72.
3 J. T. S. Irvine, D. Neagu, M. C. Verbraeken, C. Chatzichristodoulou, C. Graves and M. B. Mogensen, Nat. Energy, 2016, 1, 15014.
4 T. Zhu, H. E. Troiani, L. V. Mogni, M. Han and S. A. Barnett, Joule, 2018, 2, 478–496.
5 A.J. Carrillo, K. Kim, Z. Hood, A.H. Bork, J.L.M. Rupp, in review, 2018

Metal oxides capable of supporting excess charge-carriers play an important role in current and future advancement in energy conversion and storage technologies. Colloidal semiconductor nanocrystals offer advantages of inexpensive, scalable solution-processing and size-tunable quantum properties, but structure-property elucidation can be hindered by inherent polydispersity in such materials. Polyoxometalate clusters are molecular quantum-dot analogues that can be easily reduced to contain high electron densities. Assembly of these clusters with transition-metal linkers into doped, electronically coupled arrays thus offers the opportunity to realize well-defined solid-state metal oxide materials via inexpensive, solution-phase methods. Post-synthetic modulation of electron density and the ability to tune material properties via transition metal will be discussed.

A broad range of highly active doped ternary oxides, including perovskites, are desirable materials in electrochemical energy conversion, catalysis and information processing applications. At elevated temperatures related to synthesis or operation, however, the structure and chemistry of their surfaces can deviate from the bulk. This can give rise to large variations in the kinetics of reactions taking place at their surfaces, including oxygen reduction, oxygen evolution, and splitting of H₂O and CO₂. In particular, aliovalent dopants introduced for improving the electronic and ionic conductivity enrich and phase separate at the surface perovskite oxides. This gives rise to detrimental effects on surface reaction kinetics in energy conversion devices such as fuel cells, electrolyzers and thermochemical H₂O and CO₂ splitting. This talk will have three parts. First, the mechanisms behind such near-surface chemical evolution will be discussed. Second, the dependence of surface chemistry on environmental conditions, including temperature, gas composition, electrochemical potential and crystal orientation will be described. Third, modifications of the surface chemistry that improve electrochemical stability and actvity, designed based on the governing mechanisms, will be presented. Guidelines for enabling high performance perovskite oxides in energy conversion technologies will be presented.

The transition towards a carbon neutral economy requires the development of new technologies for the production of alternative fuels. Thermochemical redox reactions are one established low carbon method for producing fuels to be used in existing portable and stationary applications, such as fuel cells. Current efforts have sought new redox-active metal oxides that enhance the fuel production volumes and lower the high operating temperatures set by the state-of-the-art ceria-based materials (>1200 °C).¹ Perovskites have gained the attention of the research community due to their ability to assimilate large changes in oxygen non-stoichiometry and the large number of possible compositions, leaving the field relatively unexplored. This work presents research conducted on a new family of perovskite materials for thermochemical water splitting at more industrially useful temperatures.

B-site doped strontium cobalt oxides are a promising perovskite family previously studied for IT-SOFC cathodes.^2-4 Thermogravimetric results demonstrate topotactic reversible redox cycling in inert and oxidising environments at intermediate temperatures (480-900 °C) significantly lower than those of ceria. The large observed changes in oxygen non-stoichiometry demonstrates the potential for significant fuel production volumes, although preliminary water splitting results show only trace hydrogen production due to strontium segregation forming a surface blocking layer. Future analysis by low energy ion scattering will aim to study the effect of surface modification with less-reducible B-site cations.⁵ This work intends to stabilise the perovskite surface structure and achieve improved cyclability and production volumes at intermediate temperatures.

1. C. Muhich et al., WIREs Energy Environ 2016, 5, 261–287
2. A. Aguadero et al., Chem. Mater., 2010, 22, 789-798
3. A. Aguadero et al., Chem. Mater., 2012, 24, 2655-2663
4. V. Cascos et al., Int. J. Hy. En., 2014, 39, 14349-14354
5. N. Tsvetkov et al., Nat. Mater., 2016, 15, 1010-1017

The greatest challenge facing Solid oxide cells (SOC), in both fuel and electrolysis cell modes (i.e SOFCs and SOECs) is to deliver high, long-lasting electrocatalytic activity while ensuring cost and time-efficient electrode manufacture. Ultimately, this can best be achieved by growing appropriate nanoarchitectures under operationally relevant conditions, rather than through intricate ex situ procedures.

In our approach, metal particles are grown directly from the oxide support though in situ redox exsolution. We demonstrated that by understanding and manipulating the surface chemistry of an oxide support with adequately designed bulk (non)stoichiometry, one can control the size, distribution and surface coverage of produced particles. We also revealed that the emergent particles are generally epitaxially socketed in the parent perovskite which appears to be the underlying origin of their remarkable stability, including unique resistance of Ni particles to agglomeration and to hydrocarbon coking, whilst retaining catalytic activity
We also present the growth of a finely dispersed array of anchored metal nanoparticles via electrochemical poling on an oxide electrode, yielding a sevenfold increase in fuel cell maximum power density. Both the nanostructures and corresponding electrochemical activity show no degradation over 150 hours of testing. These results not only prove that in operando redox treatments can yield emergent nanomaterials, which in turn deliver exceptional performance, but also provide proof of concept that electrolysis and fuel cells can be unified in a single, high performance, versatile and easily manufacturable device. This opens exciting new possibilities for simple, quasi-instantaneous production of highly active nanostructures for reinvigorating Solid oxide cells during operation.

The ultimate practicality of solar thermal chemical hydrogen production (STCH) depends on the reversibility and lifetime of the reacting materials. This necessitates an understanding of the nature of the actual defects vs process parameters. In these materials, the effective defect phase diagram encompasses a wide range of behaviors, including oxygen vacancy formation, cation site interchange, polymorphic phase transitions, and phase separation. Here we map the phase behavior of YMnO₃ using high temperature in-situ XRD under a range of oxidizing and reducing environments. We will report on the observation of phase separation and change in manganese oxidation state of YMnO₃ into MnO and Y₂O₃ at high temperatures and reducing conditions. The reversibility of this transition is evaluated during subsequent oxidizing high temperature anneals. This in-situ defect mapping is key for understanding the dynamic behavior of potential STCH materials in real thermodynamic cycling environment to establish reversibility/lifetime for STCH materials and is key early data for emerging materials.

Perovskite oxides are of great scientific interest due to their compositional versatility and the resulting breadth of physical properties they exhibit. Perovskite oxides are widely studied for numerous applications including solid oxide fuel cells, colossal magnetoresistors, multiferroics, chemical looping, and solar thermochemical hydrogen production. Currently, there are several hundred confirmed, experimentally synthesized inorganic perovskites. However, recent density functional theory (DFT) and machine learning (ML) screening efforts suggest that there are yet hundreds more stable perovskite or perovskite-related oxides remaining to be discovered and synthesized. Furthermore, because the perovskite lattice can generally accommodate significant concentrations of various A and B-site dopants, the range of possible multi-doped, perovskite solid-solution compositions (e.g. ABO₃ → A_xA_1-x’B_yB_1-y’O_3-d and so on) is virtually limitless. With such vast degrees of compositional freedom, how can we guide the design of new perovskite compositions to target specific properties or applications of interest?

In this presentation, we will discuss two application areas (solar thermochemical water splitting and intermediate temperature fuel cells) where we are combining high-throughput computation and high-throughput experiment to accelerate the search for new redox-active perovskite materials and to improve our scientific understanding of the composition-property-performance relationships that can guide future perovskite materials optimization.

For solar thermochemical hydrogen production (STCH), we have implemented a high-throughput density-functional theory (DFT) based screening strategy to identify existing and new redox-active oxides (mostly perovskite and perovskite-related) that may be promising high-temperature water-splitters based on pass/fail tests of several thermodynamic criteria. In parallel, we use high-throughput experimental screening methods to rapidly validate promising compositional spaces. Together, these techniques have enabled us to discover several promising new STCH-active oxides, including a new, previously unknown layered perovskite with remarkable water-splitting activity.

For fuel cell applications, we have recently developed a remarkable new redox-active oxide electrode based on a multi-doped perovskite, BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3-δ (BCFZY4411), that catalyzes both the oxygen evolution and oxygen reduction reactions in ceramic fuel cells and electrolyzers. While this oxide exhibits excellent properties, it represents only one point in the quaternary Co-Fe-Zr-Y space that describes the possible B-site compositions of the BCFZY system. We have begun to explore this space via high-throughput experimentation using combinatorial pulsed laser deposition coupled to high-throughput characterization tools, to allow spatially-resolved composition and property measurements of these films under various conditions. This system produces large datasets (we have already characterized more than 1,000 distinct compositions within the BCFZY system) that are particularly well-suited to an informatics approach. We have partnered with Citrine Informatics, an industry leader in materials informatics, to extract insight and construct meaningful models from these data based on a set of chemical “features” or descriptors. Initial results show that the models accurately fit and predict trends in the data, suggesting that the chemical features provide an appropriate basis set to describe these materials’ behavior. We are now using these models to pinpoint the optimal composition within the BCFZY system and understand property-feature relationships.

Computational driven design of materials has provided guidelines for designing functional oxide materials with desired properties. However, the current computational approaches are not able to predict reaction pathways passing through intermediate or metastable phases, which have the potential to exhibit desired properties. Consequently, repeated iteration is usually required to find the reaction conditions needed for realizing targeted materials, thus inhibiting the materials discovery process. To address this challenge, we combine theory and experiments to establish a framework that guides the synthesis of intermediate/metastable phases, as well as validate this framework using hydrothermal synthesis of manganese oxide [1,2], which is highly active in electrochemical reactions.

This framework has the ability to predict the multistage crystallization along reaction pathways, and the influence of particle size and solution composition on polymorph stability. In the experimental demonstration, we monitored the evolution of manganese oxide clusters and polymorphs occurring during the synthesis by using a combination of in-situ X-ray absorption spectroscopy and X-ray wide-angle scattering. Synthesis pathways with varying conditions were studied, including potassium ion concentrations and pH values.

The results demonstrated that our computed size-dependent phase diagrams capture the metastable/intermediate polymorphs to appear, the order of their appearance, and their relative lifetimes [3]. We also show that the intermediate phases can be isolated from reaction pathways by quenching the reaction, stabilizing the phases at room temperature. Overall, this combination of computational and experimental methodology offers a more rational and systematic paradigm for targeted synthesis of metal oxides. We envision that this “synthesis by design” approach is promising for general binary and ternary oxides systems, and will contribute to the discovery of redox-active oxides.

[1] W. Sun, S.T. Dacek, S.P. Ong, G. Hautier, A. Jain, W.D. Richards, A.C. Gamst, K.A. Persson, G. Ceder, Sci. Adv. 2 (2016) e1600225–e1600225.
[2] D.A. Kitchaev, S.T. Dacek, W. Sun, G. Ceder, J. Am. Chem. Soc. 139 (2017) 2672–2681.
[3] B.R. Chen, W. Sun, D.A. Kitchaev, J.S. Mangum, V. Thampy, L.M. Garten, D.S. Ginley, B.P. Gorman, K.H. Stone, G. Ceder, M.F. Toney, L.T. Schelhas, Nat. Commun. 9 (2018) 2553.

A large solid-state entropy of reduction increases the thermodynamic efficiency of metal oxides for two-step thermochemical water splitting (TWS). We present a high-throughput density functional theory (HT-DFT) study of more than 11,000 oxide compounds to screen for thermodynamically favorable two-step TWS materials, and identify many materials as potential new candidates for TWS applications. In addition to the configurational entropy associated with oxygen off-stoichiometry, we also examine a different source of entropy, the on-site electronic configurational entropy arising from excitations of electrons distributed over a large number of multiplet states (as in f-electron cations). We find that this electronic entropy is sizable in all lanthanides, and reaches a maximum value of ~4.7 kB per oxygen vacancy for Ce⁺⁴ / Ce⁺³ reduction. The unique and large positive electronic entropy in ceria contributes to its excellent water-splitting properties.

The ideal material for solar thermochemical hydrogen (STCH) generation, which has yet to be discovered, must satisfy stringent conditions for the free energy of reduction, including, in particular, a sufficiently large positive contribution from the solid-state entropy. By inverting the commonly used relationship between defect formation energy and defect concentration, it is shown here that charged defect formation causes a large electronic entropy contribution manifesting itself as the temperature dependence of the Fermi level. This result is a general feature of charged defect formation and motivates new materials design principles for solar thermochemical hydrogen production [1].

Since the total entropy (electronic, configurational, vibrational) equals the temperature dependence of the O chemical potential, the electronic contribution can help substantially to narrow the temperature window between reduction and oxidation, and to increase the H₂ yield (H₂/steam ratio) in the oxidation step. The charged defect thermodynamics description is applied to CeO2 as a model case, using first principles calculations for the defect formation energy and the electronic structure.

[1] S. Lany, J. Chem. Phys. 148, 071101 (2018)

The ability to dissociate H₂O and CO₂ to produce H₂ and CO, respectively, is vitally important in energy science because H₂ and CO are among the primary feedstocks to synthesize chemicals and fuels. This would offer the promising prospects of turning CO₂ from a liability to an asset and help decarbonize our global energy system. To achieve this, one would require processes at the Gigatonne scale, i.e. at the same order of magnitude as CO₂ emissions. It is worth noting that today’s chemical infrastructure at the Gigatonne scale relies almost exclusively on thermochemical transformations. Hence, this dominance warrants research in identifying materials and creating processes for thermochemical dissociation of H₂O and CO₂, both thermodynamically uphill reactions, that are infrastructure compatible. Motivated by this purpose and building upon past research on two-step thermochemical processes, we have been exploring a relatively new class of materials - polycation oxides - whose thermodynamics of phase transitions can be uniquely tuned and leveraged for redox activity. Based on this, we have demonstrated both H₂O and CO₂ dissociation at thermodynamic capacities often 5-10 times higher than state-of-the-art materials such as doped ceria, spinel ferrites and perovskites. Some of our discoveries defy conventional wisdom regarding concentrations of redox-active cations, but can be explained via calculations of phase diagrams. Our current work is focused on reducing the temperatures of redox reactions to make them infrastructure compatible, while understanding and tuning the kinetics to increase reactions rates, both of which critical to eventually make them cost competitive with traditional approaches. This talk will provide an overview of our research in this area.

Solar Thermochemical Hydrogen production (STCH) technologies employing two-step metal oxide water-splitting cycles are emerging as a viable approach to renewable and sustainable solar fuels. However, materials innovations that overcome thermodynamic constraints native to the current class of solar-thermal water splitting oxides are required to increase solar utilization and process efficiency. Increasingly, the search for new STCH cycle materials has turned towards perovskite-structured oxides. Perovskites have many desirable traits, including high structural tolerance for non-stoichiometry, tunable point-defect thermodynamics, good chemical stability and a long history of application in related fields that require oxygen exchange functionality (such as solid oxide fuel cells, chemical looping, and electrochemical water splitting). While research into perovskite-based STCH materials is still in its infancy, several compounds including Sr_xLa_1-xMn_yAl_1-yO_3-δ (SLMA) and La_0.6Ca_0.4Mn_1-yAl_yO_3-δ (LCMA) have shown promise.
However, not only is the compositional space of these quaternary cation perovskites vast (the number of elemental variations that will form solid-solutions of the type A_xA′_1-xB_yB′_1-yO_3-δ is thought to be in the tens of thousands), but the same cation combinations can form different structures. Such is the case for the recently reported Ce_xSr_2-xMnO₄ (CSM) layered perovskite and the related simple perovskite Sr_1-xCe_xMnO₃ (SCM). The former is a Ruddlesden-Popper phase (K₂NiF₄-type) where the simple perovskite layers are separated by simple oxide rock-salt SrO layers.
In this presentation, we will discuss the results of a thermodynamic study of these two structural families. Preliminary extent-of-reduction measurements hint at different reduction behavior between the two structures, even when accounting for cation and oxygen ratio differences. Specifically, the SCM family is much easier to reduce than the RP-phase CSM. Thermogravametric techniques are employed to capture the reduction thermodynamics, including defect-reaction specific enthalpy and entropy, Δh°_i and Δs°_i, as well as the δ-dependent partial molar enthalpy and entropy of oxygen, Δh°_O and Δs°_O. Additional DFT modeling is used to make oxygen-site specific predictions of the oxygen vacancy formation energy to explore the role of the rock-salt layers in CSM reduction.

Separation of O₂ from air to produce a high-purity stream of N₂ is considered via the aluminum-doped strontium ferrite perovskite (SrAl_yFe_1-yO₃) reduction/oxidation reactions in a two-step solar cycle. The cycle encompasses 1) the solar-driven thermal reduction of SrAl_yFe_1-yO₃; followed by 2) the re-oxidation with O₂ from air to produce high-purity N₂.
SrAl_yFe_1-yO₃ samples were synthesized and doped with controlled levels of aluminum on the B-site via the sol-gel method. Thermogravimetry was used to determine the non-stoichiometry at chemical equilibrium for temperatures between 673 and 1373, gas flows between 1 and 90% O₂–Ar, and aluminum dopant levels of between 0.0 and 0.2. A thermodynamic model based on the compound energy formalism was applied to thermogravimetric measurements to predict equilibrium non-stoichiometry as a function of temperature, O₂ partial pressure, and non-labile aluminum concentration. The predictions were used to calculate partial reaction enthalpies and standard entropies as functions of nonstoichiometry and aluminum concentration. Aluminum doping consistently increased in the enthalpy of reduction and, therefore, the oxygen affinity of the perovskite for all values of non-stoichiometry.
The thermodynamic characterization was used to inform a thermodynamic cycle analysis of an ideal air separation cycle. The analysis showed that, for a solar reactor operating at 1073 K, 99% purities were possible for oxide to air molar ratios of up to 0.8 to 1 in the air separator. The cycle efficiency showed a strongly-inverse relationship with N₂ purity. However, the addition of a recuperation stage significantly mitigated this decline. These findings will be used inform the study of more complex, concurrently or sequentially reducing B-site cations and dopants, as well as the tuning of compositions with other dopants and cycle operating conditions in future air separation cycles.

Recent developments in solar-thermal power generation aim as well to achieve higher temperatures to increase the efficiencies of the power cycles as to store the solar energy to enable baseload power generation from a transient energy source.
Thermochemical redox processes are an option to store large amounts of solar energy in a compact storage system. The enthalpy effects of these reversible chemical reactions can be exploited. Oxides of multivalent metals in particular, capable of being reduced and oxidized under air atmosphere with significant heat effects are perfect candidates for air-operated Concentrated Solar Power plants since in this case air can be used as both the heat transfer fluid and the reactant (O₂) and therefore can come to direct contact with the storage material (oxide).
Porous ceramic structures like honeycombs and foams are favorable for heat exchange applications, the idea of employing such structures either coated with or entirely made of a redox material like manganese oxide and cobalt oxide, as a hybrid sensible-thermochemical solar energy storage system in air-operated Concentrated Solar Power plants has been set forth and tested up to a prototype scale on DLR’s solar tower in Juelich, Germany. By cascading different redox materials heat can be stored even more efficiently than by single material systems,

HydroGEN (https://www.h2awsm.org/) EMN is a six-lab consortium, led by the National Renewable Energy Laboratory (NREL). It currently comprises six core national laboratories: NREL, Sandia National Laboratory (SNL), Lawrence Berkeley National Laboratory (LBNL), Idaho National Laboratory (INL), Lawrence Livermore National Laboratory (LLNL), and Savannah River National Laboratory (SRNL). HydroGEN aims to accelerate the discovery and development advanced water splitting materials (AWSM) for sustainable, large-scale hydrogen production, in order to more effectively enable the widespread commercialization of hydrogen and fuel cell technologies. With the rollouts of fuel cell electric vehicles (FCEVs) by major automotive manufacturers underway, enabling AWS technologies for the widespread production of affordable, sustainable hydrogen becomes increasingly important.

The HydroGEN Consortium offers an extensive collection of materials research capabilities for addressing RD&D challenges in efficiency, durability and cost. Leveraging the HydroGEN Consortium’s staff of leading technical experts and broad collection of resource capabilities is expected to advance the maturity and technology readiness levels in all the advanced water splitting technologies, including advanced electrolysis (low and high temperature), photoelectrochecmical (PEC) and solar thermochemical (STCH) routes. Currently, there are 20 HydroGEN seedling projects, and one project focused on benchmarking advanced water splitting technologies. These 21 new projects utilized over 40 unique capabilities across the six HydroGEN core labs, from computational modeling to material synthesis to advanced characterization. HydroGEN is indeed a national innovation ecosystem that comprises 11 national labs, 7 companies, and 30 universities. This presentation will provide an overview of the HydroGEN EMN consortium and highlight some advanced water electrolysis and PEC projects as well as a few unique HydroGEN capability nodes that the community can leverage.

Converting solar energy efficiently into hydrogen is a key element to develop a sustainable and affordable hydrogen economy. The presentation will give an insight in how concentrated solar radiation can be coupled into hydrogen production processes. It will discuss the benefits and challenges of using the sunlight directly instead of converting it into other energy vectors.
The main focus will be on technologies with the perspective of large scale production at very high temperatures. Therefore solar tower systems for such production processes will be presented. Also the different components like concentrator, receiver, and reactor of the solar production plants will be described, possible locations will be discussed, and synergies with other R&D efforts on using high temperature heat will be shown. Hybrid solutions e.g. from the sulfur industry will demonstrate how concentrated solar radiation can contribute even today to actual industrial business models.
As many of the addressed processes have to be operated continuously high temperature heat storage will also be introduced. Especially thermochemical heat storage has the potential for being the ideal technology for heat provision in high temperature production processes.
The European Union is supporting the development in its research framework programs since the 1980s. However until 2002 only a few projects were carried out. Since then a continuous improvement took place which was accelerated by the implementation of the European Fuel Cell and Hydrogen Joint Undertaking in 2008. This private public partnership lead by industry is in charge to bring technologies faster into the market than research programs lead by the European Commission would. The presentation will give an overview of the actions carried out so far and the perspective of the European efforts.

Widespread adoption of inexpensive renewable-electricity sources brings with it many exciting challenges and opportunities, the most significant being an overabundance of low-cost electricity from renewables as well as cheap photovoltaics. Thus, there is an opportunity to use virtually free electrons and photovoltaics to produce fuels and chemicals using electrochemical and photoelectrochemical devices, respectively with water as a reactant. Electrochemical hydrogen production using water electrolysis is a commercial technology, barriers remain in terms of cost and efficiency improvements for its widespread adoption; while alternative water-splitting technologies that have the long-term potential to produce inexpensive hydrogen include photoelectrochemical (PEC) approaches, are still in their infancy. In either case materials development, interface development and cell design are critical in advancing the efficiency and cost of water splitting devices. Benchmarking protocols help to identify promising materials, facilitate translation of materials from one technology readiness level to another, and can provide clear technology crosscutting opportunities. This talk will cover benchmarking activities performed within the HydroGEN Energy Materials Network on: low temperature electrolysis (LTE) electrocatalysts, development of benchmarking cells for photoelectrochemical (PEC) water splitting devices, and crosscutting opportunities between LTE and PEC with respect to the hydrogen evolution and oxygen evolution catalysts activity and durability.

Solar-driven thermochemical production of chemical fuels using redox active oxides has emerged as an attractive means for storing solar energy for use on demand. In this process a reactive oxide is cyclically exposed at high temperatures to an inert gas, which induces partial reduction of the oxide, and to an oxidizing gas of either H₂O or CO₂, which reduces the oxide, releasing H₂ or CO. The capacity for fuel production is dictated by the thermodynamic properties of the oxide, specifically the enthalpy and entropy of reduction. Here we report the thermodynamic properties of a range of previously unexplored ABO₃-type compounds and identify those with promising characteristics for efficient thermochemical fuel production.

High temperature electrolysis of water provides an economic and efficient pathway for large-scale hydrogen production that not only utilizes steam from nuclear energy but also allows using concentrated solar energy to accelerate renewable and sustainable energy utilization. In this technical contribution, a newly developed proton conducting solid oxide electrolysis cell (SOEC) offering the opportunity of producing dry, high purity and pressured hydrogen will be presented. Experimental results pertaining to the electrochemical performance of electrochemical cells containing developed electrolyte and electrode formulations will be presented and discussed. Bulk, interface and surface morphological and chemical stability, studied using scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) and focused ion beam-transmission electron microscopy (FIB-TEM) techniques, for the electrodes and electrolyte will be presented. Approaches for the minimization of cell performance degradation associated with electrode poisoning will be discussed.

Solid oxide electrolysis cells (SOECs) can efficiently convert electrical energy (e.g. surplus wind power) to energy stored in fuels such as hydrogen or other synthetic fuels. Hydrogen can be produced via steam electrolysis and synthesis gas via co-electrolysis of steam and carbon dioxide followed by catalytic reaction to produce to synthetic fuels.
Firstly, this presentation will provide an overview of the development and improvement of SOECs over the last 15 years at Risø Nat. Lab./DTU Energy. The SOEC improvements have involved a change from composite LSM/YSZ (lanthanum-strontium-manganite/yttria-stabilized-zirconia) electrodes to MIEC based oxygen electrodes leading to an increased oxygen electrode performance, analysis of and elimination of impurities in the feed streams, optimization of processing routes and of the Ni/YSZ based fuel electrode structure. These improvements have led to a decrease in long-term degradation rate from ~40 %/kh to ~0.4 %/kh for steam electrolysis at -1 A/cm², while the initial area specific resistance has been decreased from 0.44 Ωcm² to 0.15 Ωcm² at -0.5 A/cm² and 750 °C. Advanced and complementary characterization techniques have been developed during these 15 years of SOEC improvements and these will be included in the talk.
Secondly, a couple of critical degradation issues for state-of-the-art SOECs will be exemplified and discussed. This includes: 1) impurity sensitivity, where even impurities in the ppb-range can play a major role in detrimental degradation of the SOECs; 2) studies of carbon deposition and its relation to the specific operation conditions and the resulting overpotential of the SOEC; 3) segregation of phases and structural changes e.g. morphology changes and coarsening of the electro-catalytic and electron conducting nickel in the fuel electrode; and 4) observed Ni migration in the active fuel electrode leading to significant reduction in the percolating triple-phase-boundary length and changed properties of the remaining percolating structures.
Thirdly, and last, this talk will touch upon four selected case studies illustrating on-going SOEC research. This will include: 1) New results on improved mechanical reliability of SOEC via mechanically stronger stabilized zirconia. This is obtained through modifications (decreased yttria content or Ce-Y co-doping) of the traditional 3YSZ giving rise to a significant increased fracture strength and toughness alongside improved resistance against both high and low temperature aging. 2) Interesting results on the kinetics of LSF (lanthanum-strontium-ferrite based) electrodes will be discussed. Here tremendous performance increase has been obtained through thermal cycling – and thereby surface modifications - of model LSF electrodes. Furthermore addition of iron(III) and Zr(IV) ions to the LSF electrode surfaces have shown very promising initial performance at temperatures as low as 400 °C. 3) Recently completed thorough microstructural analysis of the Ni/YSZ electrode of a pristine and a 1-year tested SOEC stack has provided new insight into the nature of microstructural changes including Ni coarsening and mobility. In this context, phase field modelling has been used for simulating the microstructure evolution of three-dimensional complex Ni/YSZ electrodes and additional studies now reveals how critical parameters and driving forces such as contact angles and fuel electrode overpotential (pO₂ gradients) influences e.g. Ni morphology changes and Ni migration. 4) As the degradation of the Ni/YSZ based fuel electrode is the dominating degradation issue for S-o-A SOEC on-going research activities to overcome these degradation phenomena will also be discussed. This include results from long-term testing of SOEC with pre-reduced and infiltrated Ni/YSZ electrodes as well as more explorative work on Ni-free fuel electrodes and experiments with doping of the NiO during manufacturing the fuel electrode.

Renewable electric energy has become a significant source of the nation’s electricity. However, wind and solar power technologies are intermittent and fluctuating, and have to be balanced for electric grid stability purposes. Over-generation can lead to grid congestion or “curtailment”. Relieving the grid of excess renewable electricity can be accomplished by powering a water electrolyzer to produce hydrogen. The hydrogen can be temporarily stored and then used in filling stations, industrial applications, or converted back to electricity when shortages occur. It can also be injected into the natural gas grid either directly or after methanation as “renewable fuel”. There are a number of technologies that can be used to electrolytically split water to form hydrogen. Solid oxide cells (SOC) offer the highest overall efficiency as the high operating temperature significantly reduces the amount of electrical energy required to produce the hydrogen.

The biggest challenge for SOCs is the rapid material degradation and the limited long term stability due to their high operating temperatures. This paper will discuss the degradation processes in SOC electrodes.

Multiple button cells were tested using customary SOC materials: yttria-stabilized zirconia (YSZ) electrolyte, Ni/YSZ negative electrode, and (La,Sr)MnO₃ (LSM)-YSZ or (La,Sr)(Co,Fe))₃ (LSCF) positive electrodes. Long-term electrochemical performance was recorded using a multichannel potentiostat over a range of operating conditions. For each cell, losses associated with ohmic and electrodic processes were separated using electrochemical impedance spectrometry. Open-circuit voltage was periodically recorded to verify the quality of seals in a high steam environment as well as the consistency in steam production. Following termination of electrochemical tests, LSM/YSZ surfaces and cross-sections of individual cells were analyzed using scanning electron microscopy and energy dispersive spectroscopy (SEM/EDS).

Cells with LSM/YSZ anodes operated at higher overpotentials for similar H₂ production and degraded more rapidly when compared to cells with the LSCF electrodes. The SEM/EDS analysis revealed microstructural changes near the electrochemically active interface such as LSM/YSZ electrode delamination and embrittlement of the YSZ electrolyte. While this was consistent with the previous findings reported by other groups, our results indicate that cell failure can occur at low current densities (low oxygen flux through the electrolyte). One of the possible degdation mechanisms is the perovskite lattice expansion that leads to delamination at the oxygen evolution electrode.
SOC with LSCF positive electrodes operated at 0.9 A/cm² near or below thermoneutral voltage for over 1000 hours. Some performance degradation was observed during the initial 200 hours and was explained by an increase in both ohmic and polarization resistances. Subsequently, a rather stable performance (degradation rate < 0.4%/1000 hours) was observed with only a minimal increase in ohmic resistance and no changes in the electrode polarization resistances. The improved electrode stability was attributed to the presence of the ceria-based barrier layer between YSZ and LSCF that suppresses the formation of the SrZrO₃ phase during fabrication and is known to contribute to the anode delamination in SOC.

Metal oxide redox cycles offer a pathway for converting heat into chemical energy via the thermal reduction of a metal oxide. The reduced metal oxide can then be used in a number of applications by re-oxidizing it in a second step, utilizing the stored chemical energy and returning the oxide to its original state. One such application is thermochemical fuel production, where the reduced oxide is reacted with steam and/or carbon dioxide to produce hydrogen and/or carbon monoxide which form syngas, a valuable energy carrier. This process requires high temperature (> 1400 °C) and is most often coupled with high flux concentrated solar power systems, for converting solar energy to chemical fuels.
To understand the potential of such a fuel production process, it is important to start by determining the overall physical constraints of such cycles. In this presentation an overview of the limiting thermodynamic constraints of two-step thermochemical redox cycles will be presented, with a focus on water and carbon dioxide splitting cycles for fuel production. The current understanding of thermochemical redox materials, and the implications this has on the redox thermodynamics are also discussed. Finally, the trend towards ceria as the state of the art material for such processes is discussed with consideration for both thermodynamics and practical constraints.
The general thermodynamic principles formulated are then utilized to develop a streamlined approach of screening materials for thermochemical redox applications. The screening process is broken into two parts; (a) determine the redox thermodynamics of a large set of materials and add them to a database, (b) plug each materials thermodynamic properties into a simplified process model, to determine reaction conversion extents and a figure of merit such as efficiency. The performance parameters can then be used to rank the materials for a given application.
According to this methodology, a large family of perovskite solid solutions with the general composition A_xB_1-xM_yN_1-yO_3-δ with A, B = Ca , Sr and M, N = Ti, Mn, Fe, Co, were investigated experimentally. The redox enthalpies and entropies of these perovskites can be tuned by adjusting their composition, making them ideal for such a broad screening processes. As well as an experimental screening, an even broader range of perovskites were investigated using theoretical data gathered via density functional theory (DFT) calculations. In a joint effort between the German Aerospace Center and the Lawrence Berkeley National Laboratory, the data was used to create a search engine for redox materials based upon the infrastructure of Materials Project. A thermochemical fuel production process model is also included, which can be used to rank the materials for a given set of operation parameters.

Thermochemical H₂O and CO₂ splitting is a promising approach to convert and store solar energy. Further processing of the produced H₂ and CO via Fischer-Tropsch-Synthesis facilitates a path to renewable carbon based fuels. For efficient cycling of the active redox material (e.g. CeO₂) and high solar-to-fuel efficiencies low oxygen partial pressures during reduction are a necessity. We report an approach to overcome the present energy penalties of high vacuum pumping or intensive inert-gas sweeping, using perovskite based oxides as a thermochemical oxygen pump. Our results show, that Ca-Mn-based perovskites are able to effectively increase the reduction extent of the state-of-the-art splitting material CeO₂ through absorption of the oxygen released by the CeO₂ during reduction. That offers a way to increase the efficiency of the solar-to-fuel process, while minimizing energy penalties since this application can be implemented down-stream of the main reactor in a concentrated solar power (CSP) plant. To forecast and contribute to an actual implementation of the application into a CSP plant, our work includes a study of the cyclic behavior of structured specimens offering insight to strengths, challenges and prospects concerning structural integrity of manufactured parts. This incorporates investigation on the thermal expansion, considering structural changes and expansion due to oxygen vacancy formation, and thermal stress resistance.

Transition-metal oxides that can tolerate a large number of oxygen vacancies, specifically systems containing the redox-active Ce (CeO₂), Mn (LaMnO₃), and Fe (Fe₃O₄) atoms, are crucial ingredients for generating renewable fuels via two-step, oxide-based solar thermochemical (STC) reactors. However, any predictive modeling, such as using density functional theory (DFT) calculations, needs to accurately describe the energetics of redox reactions (and, subsequently, defect formation) involving transition metals, if new candidate materials are to be found. Recently, the strongly constrained and appropriately normed (SCAN) exchange-correlation (XC) functional was developed, which impressively satisfies all the 17 known constraints for the behavior of any XC functional. However, the ability of SCAN to describe redox energies involving transition metal atoms thus far has remained unexplored. Hence, in the first part of the talk, we will explore the need for a Hubbard U correction, along with SCAN, to account for some of the discrepancies in the thermodynamic and electronic properties of transition-metal and rare-earth oxides.

Importantly, theoretical models that can accurately predict the extent of off-stoichiometry, i.e., the extent of oxygen deficiency within the oxide, will be crucial in identifying promising candidates for STC applications. Thus, the second part of the talk will focus on using the sub-lattice formalism in combination with SCAN+U calculations to accurately predict off-stoichiometries in CeO₂ and Zr-doped CeO₂. Note that the sub-lattice formalism is a common tool used to thermodynamically assess experimental data and build temperature-composition phase diagrams in a variety of binary, ternary, quaternary, etc. systems. Although the sub-lattice model has been primarily used to “fit” to existing experimental data, the framework is general and can be used alongside DFT-based data. Hence, we build a sub-lattice model entirely based on SCAN+U calculations and observe fair agreement between experimental and predicted oxygen off-stoichiometries in CeO₂ and Zr-doped CeO₂. Finally, we will touch upon some of the future challenges in generalizing the use of sub-lattice models across chemical systems.

Solar Thermochemical Hydrogen Production (STCH) is an emerging technology that uses concentrated
solar heat to split water and produce hydrogen. STCH leverages the ability of some transition metal
oxides to reversibly change their oxygen stoichiometry with changes in temperature and oxygen partial
pressure. The potential impact of this technique is significant because it enables the use of solar radiation
as a carbon-free energy source to produce industrial scale quantities of hydrogen, which is widely used in
steel production, ammonia synthesis for fertilizers, and as an alternative renewable fuel.

The high reduction temperatures necessary for the current state-of-the-art STCH material, CeO 2 (ceria),
results in solar utilization and process efficiencies that are too low for practical application, leading to a
broad effort to replace it. However, the thermodynamic constraints of lowering the reduction temperature
while retaining a defect formation energy large enough to facilitate water splitting is challenging. The
tunable point-defect thermodynamics, good chemical stability, and structural tolerance for non-
stoichiometry inherent in transition-metal perovskite oxides makes them attractive candidates. The recent
development of Sr x La 1-x Mn y Al 1-y O 3-δ (SLMA) and BaCe 0.25 Mn 0.75 O 3 (BCM) as active STCH oxides
demonstrate the potential of the perovskite materials space for this application.

Following the success of BCM, a Sr-containing variant has been synthesized, Sr 1-x Ce x MnO 3 (SCM).
Unlike BCM, the SCM family contains Ce on the A-site and it is not a line compound, instead
accommodating a wide compositional range of approximately 0-50% Ce substitution. This allows for a
level of defect tunability that is not possible in BCM.

For this work, we will discuss the results of a thermodynamic study of this compositional family and the
effects of Ce content on thermal reduction. Thermogravametric techniques are employed to capture the
reduction thermodynamics, including defect-reaction specific enthalpy and entropy, Δh° i and Δs° i , as well
as the δ-dependent partial molar enthalpy and entropy of oxygen, Δh° O and Δs° O . Extent of reduction and
water-splitting hydrogen production results will also be shared.

In this paper, we present our efforts to scale up proton-conducting ceramic fuel cells (PCFC) into small stacks. Encouraging stack performance is observed under hydrogen and methane fuels at intermediate temperatures. The membrane-electrode assembly features a BaCe_0.2Zr_0.6Y_0.2O_3-δ (BCZY26) electrolyte on a Ni-BCZY26 anode support. The cathode material is BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3−δ (BCFZY), with mixed ionic-electronic conduction, enabling high performance at 550 °C. Cell active area is ~ 5 cm². The cells are packaged within ThyssenKrupp’s Crofer 22 APU ferritic-steel interconnects. A three-cell stack operating at 550 °C under internally reformed methane-steam fuel achieved 0.25 A / cm² at 0.75 V.

Stack development brings concerns of materials compatibility, mating novel proton-conducting ceramics with high-chrome ferritic steels and glass-ceramic sealants. While lower operating temperature should reduce degradation rates, it also brings questions on scale composition, materials interactions, and other sources of performance degradation. In an effort to gain insight on root causes behind degradation in our protonic-ceramic stacks, we distribute numerous voltage taps throughout the electrical interfaces across the unit-cell stack. We trace stack degradation to an increase in electrical resistance across the cathode interconnect. Following testing, this interconnect displays a scale with high iron content, rather than the desired (Mn,Cr)₃O₄ scale. We present early results on the effectiveness of interconnect pre-oxidation in reducing degradation. We also explore stack performance when using Sandvik’s SANERGY 441 interconnect, with and without protective Ce-Co coatings.

In this paper, we present our efforts to identify, quantify and mitigate sources of degradation in proton-conducting ceramic electrochemical devices. Proton-conducting ceramic membranes present an exciting new class of electrochemical materials that are now being harnessed to address societal challenges in electricity generation, energy storage, and fuels synthesis. Many materials-compatibility questions arise as we move protonic ceramics from lab-scale, “button cell” architecture to commercial kW-scale assemblies. For the first time, we are packaging protonic ceramics within multi-cell stacks, mating novel perovskites with high-chrome ferritic-steels and advanced glass materials. The compatibility and stability of these materials within tightly packaged assemblies over thousands of hours of operation is unclear. Additionally, the unique operating conditions found with protonic ceramics differ substantially from those for which the packaging materials were initially designed. Both fuel cells and electrolyzers based on protonic-ceramics feature high water-vapor content at the air / steam electrode. Operating temperatures may be 200 °C lower than that of the oxygen-ion-conducting equivalent. While this lower-temperature operation should be beneficial for long-term operation, many questions remain.

In this talk, we present our approaches to capturing degradation effects at the protonic-ceramic air / steam electrode. We have developed precise experimental tools and fabrication protocols to accurately quantify the small performance changes that may be manifested over thousands of hours operation. We utilize symmetric cells for these experiments, where a BaCe_0.7Zr_0.1Y_0.1Yb_0.1O_{3- δ} (BCZYYb) electrolyte is sandwiched between BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3−δ (BCFZY) air-steam electrodes, and exposed to steam-air mixtures at ~ 550 °C. In addition to identifying stable operating conditions, we are exploring the compatibility of commercial ferritic steels, such as ThyssenKrupp’s Croferr 22 H and Sandvik’s SANERGY 441 materials, with these novel protonic-ceramic materials. Experiments are executed under continuous potentiostatic electrochemical impedance spectroscopy (EIS) through a Gamry Reference 600 instrument. While degradation is found to be high at 50% steam in air, far-lower degradation is observed at 10% steam. Preliminary results reveal that packaging of symmetric cells within uncoated, untreated ferritic steels can lead to substantial performance degradation. We will also present on the effectiveness of degradation-mitigation strategies to enable long device lifetime.

Ammonia is an important industrial precursor [1] and a promising energy carrier due to its low liquefaction pressure. In this paper, we report high electrochemical ammonia-synthesis rates using proton-conducting ceramics with nitrogen and steam feedstocks. Ammonia synthesis is enabled through use of a novel barium-calcium-aluminum oxide support with a low-loading ruthenium catalyst (Ru-B2CA) [2]. This catalyst and support have been patented by Starfire Energy, Denver, Colorado, USA, for both ammonia synthesis and ammonia cracking. The mating of protonic-ceramic electrochemical cells with advanced Ru-B2CA catalysts enables efficient, cost-effective storage of intermittent renewable electricity in the form of a commodity chemical. The cell-catalyst system is also reversible, and demonstrates high power density under ammonia fuel.

The protonic-ceramic electrochemical cells feature a BaCe_0.7Zr_0.1Y_0.1Yb_0.1O_3-δ (BCZYYb) electrolyte on a Ni-BCZYYb fuel-electrode support, and BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3−δ (BCFZY) air-steam electrode. Through this solid-state electrochemical-synthesis process, we achieved an ammonia production rate of 1.0 x 10^-8 mol.s^-1.cm^-2 at ambient pressure from steam (carried by humidified air) and nitrogen feed streams. This rate is among the highest electrochemical ammonia-production rates ever reported, regardless of electrolyte type [3-6]. Other reported rates reach 1.1 x 10^-8 mol.s^-1.cm^-2 using Nafion electrolyte and SFCN as ammonia formation catalyst [3], and 1.0 x 10^-8 mol.s^-1.cm^-2 using molten NaOH-KOH as electrolyte and nano-Fe₂O₃ as catalyst [4]. An ammonia-production rate of 8.2 x 10^-9 mol.s^-1.cm^-2 has been reported using GDC electrolyte and Ag-Pd catalyst [5]. The effects of cell architecture, operating temperature and pressure on ammonia-synthesis rate will be discussed in this talk.
<!--![endif]---->
<!--[endif]---->When operated in fuel-cell mode, the same Ru-B2CA catalyst serves to crack the ammonia feed upstream of the protonic-ceramic fuel cell. We achieved a peak power density of 620 mW.cm^-2 at 600 °C using direct ammonia fuel with air oxidizer. Button cells and unit-cell stacks were tested under direct ammonia fuel for up to about 500 h. Integration of the Ru-B2CA NH₃-cracking catalyst upstream of the protonic-ceramic fuel cell was found to dramatically decrease degradation rates under ammonia fuel.

[1] I.A. Amar, et al., Journal of Solid State Electrochemistry (2011) 15 (9), 1845.
[2] Starfire Energy, Barium Calcium Aluminum Oxide Catalyst Support for NH₃ Synthesis, Pending patent PCT / US18 / 32759.
[3] G. Xu, et al., Science in China, Series B: Chemistry (2009) 52, 1171.
[4] S. Licht, et al., Science (2014) 345 (6197), 637.
[5] R.-Q. Liu, et al., Solid State Ionics (2006) 177 (1), 73.
[6] C.-Y. Yoo, et al., ACS Sustainable Chemistry & Engineering (2017) 5 (9), 7972.

Solar-thermochemical (STC) carbon dioxide and water splitting is a pathway for the direct thermal production of solar fuels—an alternative to direct thermolysis. Currently, the most widely researched implementation is the two–step cycle. Reducing two–step cycles to practice is challenging, and there is a substantial ongoing effort to improve reactor designs, and to synthesize redox active materials that exceed the STC potential of ceria, the current state–of–the–art redox active material. We have analyzed the underlying general thermodynamic boundaries that lead to materials performance tradeoffs between heat of reduction, productivity, and yield. These boundaries apply to all nonstoichiometric oxides without phase changes during redox cycles, or where the phase changes are energetically negligible. We quantify the tradeoffs between the desirable decrease of the heat of reduction and penalties such as decreasing yields and increasing cycling temperature range, and show that operating parameters pose firm limitations on cycle performance and on the desirable materials properties space.

Solid oxide electrochemical cells have the potential to play a major role in our renewable energy future. They efficiently store electrical energy as synthetic chemical fuels such as H2, CO and hydrocarbons by electrolysis of H2O and/or CO2. They can also operate as fuel cells to produce electricity again from those fuels. By reversible operation between electrolysis and fuel-cell modes, they can function as rechargeable flow batteries with extremely low cost storage media.

A major obstacle to commercialization is insufficient device lifetime due to loss of electrode performance. Nearly all cells employ nickel-based fuel-electrodes, which experience gradual deactivation - mainly due to the mobility of nickel - and can even be completely destroyed by oxidation or carbon deposition. Alternative electrodes made of mixed ionic-electronic conductor (MIEC) oxides are known to avoid these issues, but at the expense of typically much lower electrochemical activity.

We have been developing nanostructured ceria-based fuel-electrodes that overcome these issues and outperform conventional nickel-based electrodes. In the H2/H2O/CO/CO2 fuel environment, acceptor-doped ceria is a MIEC with a high concentration of oxygen vacancies, which provide a high density of surface reaction sites. This presentation will focus on how these electrodes achieve:
- exceptional activity: electrode polarization resistance <0.1 ohm cm^2 at 650°C
- exceptional stability: inhibiting carbon deposition even during CO2 electrolysis in thermodynamic carbon deposition conditions, as well as the ability to re-activate electrode performance by periodically shifting to oxidizing conditions and back again

Mixed ionic-electronic conducting (MIEC) oxide ceramics are grabbing attention towards a variety of applications such as oxygen sensors, catalyst bed for combustion of fossil fuels, cathodes for solid oxide fuel cells, oxygen separation membranes, oxygen storage devices etc. These MIECs can transport oxide ions through their lattice when an oxygen partial pressure gradient is applied across the material¹. They also have reversible oxygen storage capabilities at elevated temperatures. Transition metal Perovskite oxides (general formula ABO₃) and related structures are notable candidates in this regard. They have a relatively open structure with variable oxide ion vacancies that can be tuned by synthesis methods and chemistry. Cobalt-based perovskites have been studied extensively as oxygen separation membranes due to the coexistence of cations with different oxidation states. SrCoO₃ (Perovskite, Pm-3m) is one of such material which reversibly transforms to SrCoO_2.5 (Brownmillerite, Ima2) phase at 350°C with the evolution of oxygen². Herein, we report the utilization of BM SrCoO_2.5 for oxygen enrichment.
Brownmillerite (BM) SrCoO_2.5 phase has been stabilized to date only by liquid nitrogen quenching³. Herein, a cost-effective method of quenching with the use of an Al foil pad is adapted for the synthesis of BM SrCoO_2.5. A simple home-built volumetric setup has been fabricated for studying the Oxygen storage property of this material⁴. The sample was pre-treated with a higher partial pressure of oxygen at 673 K to form the oxygen-rich perovskite phase and this phase was heated at lower pressure to study the desorption characteristics. The pressure change observed when sample releases oxygen is used to find the oxygen storage capacity. Desorption characteristics of the sample treated at varying oxygen partial pressure have been studied. The results indicate 15.28 cm³ g^-1 of oxygen can be reversibly stored in the sample at STP. The effect of chemical modification on oxygen storage property was analyzed by isovalent substitution (Ca and Ba in Sr site). It was found that the substitution by Ca or Ba deteriorates the oxygen storage capacity of BM SrCoO_2.5.
References
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One of the key challenges identified in solar–thermochemical (STC) fuels production, is the need for low oxygen pressures in the thermal reduction step. These pressures must be reached by energetically efficient and economically affordable methods–currently unavailable by either vacuum pumping or inert gas sweeping. In an effort to address this unresolved high–impact challenge in STC fuels, we have analyzed a promising new thermally–driven approach, featuring high theoretical efficiencies, comparatively low operating temperatures, and exceedingly simple and robust devices–including implementations without moving parts. Our analysis shows that, even under conservative practical assumptions, our thermally–driven pumping approach has the potential to substantially exceed state–of–the–art pumping efficiencies, especially in the low–pressure regime, in inherently simple and potentially low cost devices.