Solar Thermochemical Hydrogen Production Research Articles

High‐Entropy Perovskite Oxides for Thermochemical Solar Fuel Production

The increasing global demand for energy, coupled with the need to mitigate climate change, has spurred significant interest in renewable energy sources. Among these, solar energy holds particular promise due to its abundance and potential to be converted into clean fuels through thermochemical cycles. High‐entropy perovskite oxides (HEPOs) have emerged as promising materials for solar thermochemical hydrogen (STCH) production, offering advantages over traditional materials like ceria due to their enhanced thermal stability, flexibility in composition, and lower operating temperatures. Herein, the advantages of HEPOs, including their stability under extreme thermal conditions which is critical for repeated redox cycling in H2 production, are highlighted. The inherent configurational entropy allows for a broader range of element incorporation, leading to improved tunability of physical properties. However, challenges remain, particularly in terms of cost and scalability. To address this, strategies such as the use of more abundant elements and optimized synthesis are discussed. Additionally, the future potential of HEPOs, including their integration into advanced solar reactors, is explored, and how computational methods can be employed to predict new high‐entropy compositions with improved performance is examined. The development of HEPOs for STCH offers a promising pathway toward sustainable hydrogen production, addressing both environmental and economic challenges.

Perovskite Oxide Materials for Solar Thermochemical Hydrogen Production from Water Splitting through Chemical Looping.

Solar-driven thermochemical hydrogen (STCH) production represents a sustainable approach for converting solar energy into hydrogen (H2) as a clean fuel. This technology serves as a crucial feedstock for synthetic fuel production, aligning with the principles of sustainable energy. The efficiency of the conversion process relies on the meticulous tuning of the properties of active materials, mostly commonly perovskite and fluorite oxides. This Review conducts a comprehensive review encompassing experimental, computational, and thermodynamic and kinetic property studies, primarily assessing the utilization of perovskite oxides in two-step thermochemical reactions and identifying essential attributes for future research endeavors. Furthermore, this Review delves into the application of machine learning (ML) and density functional theory (DFT) for predicting and classifying the thermochemical properties of perovskite materials. Through the integration of experimental investigations, computational modeling, and ML methodologies, this Review aspires to expedite the screening and optimization of perovskite oxides, thus enhancing the efficiency of STCH processes. The overarching objective is to propel the advancement and practical integration of STCH systems, contributing significantly to the realization of a sustainable and carbon-neutral energy landscape.

Manganese-based A-site high-entropy perovskite oxide for solar thermochemical hydrogen production

The A-site high-entropy perovskite oxide (La1/6Pr1/6Nd1/6Gd1/6Sr1/6Ba1/6)MnO3 with enhanced hydrogen production, phase stability, and surface oxygen exchange kinetics, offering the potential for tailoring properties in the STCH application.

(Invited) Water Splitting Materials for Solar Thermochemical Hydrogen Production

Solar thermochemical hydrogen (STCH) production uses concentrated sunlight to produce hydrogen using reduction/oxidation of metal oxides. Typically, the metal oxide is heated to high temperature (>1400 oC) causing it to release oxygen, then it is cooled to a lower temperature (<1000 oC) in steam whereby it re-oxidizes, stripping oxygen from water molecules and producing hydrogen. Due to improved stability, non-stoichiometric oxides that do not change phase during the STCH process are typically used, even though the reversible oxygen content is less than phase changing materials. HydroGEN (h2awsm.org) is an Energy Materials Network supported by the U.S. Department of Energy’s Hydrogen and Fuel Cell Technologies Office, and is focused on the development of new water splitting materials for STCH by fostering collaboration between U.S. national laboratories and external entities such as academia and industry. In this presentation, recent progress in developing new water splitting materials led by Sandia National Laboratories (SNL) will be presented. STCH performance of select materials predicted from a defect Graph Neural Network trained on crystal structures derived from density functional theory, subsequently synthesized and characterized using thermogravimetric analysis and flow reactor measurements, will be presented. Additionally, there will be discussion of benchmarking metrics and characterization of exemplar STCH materials. Lastly, trends in water splitting materials and durability considerations and challenges for commercialization will be discussed.SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525

Assessment of Bulk Oxygen Capacity and Transient Redox Behavior of Foamed Lanthanum Strontium Manganese Perovskites

Abstract Synthesis of solar thermochemical hydrogen (STCH) production redox materials with engineered structures, for example, replica foams, can enable efficient heat and mass transport and are critical for scaled-up systems. Prior work has motivated the use of lanthanum strontium manganese (LSM)-type perovskites as foamed STCH materials, but the effect of their morphology on bulk and kinetic behavior has not been reported. In this work, replica and direct foamed samples of La0.65Sr0.35MnO3-δ (LSM35) were fabricated and compared to synthesized powders and dense monoliths, and similarly synthesized CeO2-δ (ceria) foams, regarding their specific reaction rates and bulk oxygen capacity/H2 yields. Changes in oxygen capacity (Δδ) and reaction rates were measured between 1200 °C and 1400 °C by using fixed ratios of steam and hydrogen during both reduction and oxidation steps, allowing for analysis under practical high conversion conditions. Results suggest bulk behavior and reaction rates of the foamed LSM materials are comparable to their powder analogues. Differences in reaction rates were observed only when replica foamed samples were subjected to rapid laser heating (emulating conditions expected in solar furnaces), which is expected but has not been demonstrated at such a small scale. Foamed samples were further subjected to 50 redox cycles at 1400 °C to evaluate their stability. Results show no statistically significant decrease in hydrogen production for any of the foamed samples, but the direct foamed samples became brittle with time. Together, these results demonstrate the viability of replica foamed LSM perovskites for integration in scaled-up STCH systems.

(Invited) The U.S. Department of Energy’s HydroGEN Consortium for Advanced Water Splitting Materials

The goals of the Hydrogen and Fuel Cell Technologies Office (HFTO) support those of the Biden Administration to build a clean and equitable energy economy and address the climate crisis through development and deployment of hydrogen technologies to the benefit of all Americans. HFTO is dedicated to meet the goals of DOE’s Hydrogen Shot,[1] which targets affordable clean hydrogen production at $1/kg within the decade, and the H2@Scale Initiative,[2] which aims to advance affordable hydrogen to enable decarbonization and revenue opportunities across multiple sectors.HFTO’s HydroGEN consortium (https://www.h2awsm.org/) for advanced water splitting materials, part of the DOE’s Energy Materials Network (EMN), includes a focus on multiple clean hydrogen production pathways, including several relevant to solar hydrogen production. Since the consortium’s establishment in 2016, HydroGEN researchers have made significant progress in the areas of low and high temperature electrolysis, as well as photoelectrochemical (PEC) and solar thermochemical (STCH) hydrogen production pathways. The HydroGEN consortium facilitates collaborations between national laboratories, academia, and industry bringing together world-class technical expertise with state-of-the-art facilities made available to project partners. HydroGEN offers an extensive collection of materials research capabilities for addressing R&D challenges. Project participants can choose from over 40 unique capability nodes. Each capability comprises—a combination of a tool, technique, and expertise—that is unique to the national laboratory system.HydroGEN has supported over 30 competitively selected R&D projects with industry and academic partners by providing streamlined access to national lab resources and expertise. The consortium is also credited with contributing over 100 publications, fostering cross-cutting activities to advance material performance and durability through community engagement and protocol development, and initiating critical laboratory-led efforts to address specific R&D gaps in the AWS portfolio. Despite the progress, there is still significant ground to be covered in the water splitting materials R&D space to enable low-cost and large-scale hydrogen production, especially in the direct solar hydrogen production pathway of artificial photosynthesis.Current consortium efforts include photoelectrochemical (PEC) hydrogen production among other advanced water splitting pathways. The consortium seeks to discover and design new low-cost materials and increase the efficiency and durability of current systems for carbon-free hydrogen production. The consortium is also engaged in connecting the research community by developing a data platform for secure data transfers and public data sharing. Additionally, HydroGEN supports benchmarking performance and protocols of electrolyzer systems to ensure apples-to-apples comparisons across academia and industry. A new cohort of solar fuels focused projects were recently selected from the Funding Opportunity Announcement (FOA) Number: DE-FOA-0002792 with additional funding in this area expected in the future.[1] Hydrogen Shot | Department of Energy, https://www.energy.gov/fuelcells/hydroge-shot[2] H2@Scale | Department of Energy, https://www.energy.gov/eere/fuelcells/h2scale Figure 1

Defect graph neural networks for materials discovery in high-temperature clean-energy applications.

We present a graph neural network approach that fully automates the prediction of defect formation enthalpies for any crystallographic site from the ideal crystal structure, without the need to create defected atomic structure models as input. Here we used density functional theory reference data for vacancy defects in oxides, to train a defect graph neural network (dGNN) model that replaces the density functional theory supercell relaxations otherwise required for each symmetrically unique crystal site. Interfaced with thermodynamic calculations of reduction entropies and associated free energies, the dGNN model is applied to the screening of oxides in the Materials Project database, connecting the zero-kelvin defect enthalpies to high-temperature process conditions relevant for solar thermochemical hydrogen production and other energy applications. The dGNN approach is applicable to arbitrary structures with an accuracy limited principally by the amount and diversity of the training data, and it is generalizable to other defect types and advanced graph convolution architectures. It will help to tackle future materials discovery problems in clean energy and beyond.

Accuracy of DFT computed oxygen-vacancy formation energies and high-throughput search of solar thermochemical water-splitting compounds

The computation of the oxygen vacancy formation energy using density functional theory is a critical factor in applications like solar thermochemical hydrogen production. However, when we calculate it using structures that are dynamically unstable, results in artificially reduced values and lack of convergence with cell size. By comparing the calculated values with experimental data, the authors can clearly see the importance of using dynamically stable structures for accurate calculations. This comparison also validates the reliability of density functional theory calculations. Furthermore, using a high-throughput approach, the authors perform such calculations to identify new candidates for solar thermochemical hydrogen production among ABO${}_{3}$ perovskite materials, demonstrating the striong influence of B-site cations on the oxygen vacancy formation energy.

Neural network and experimental thermodynamics study of YCrO3-δ for efficient solar thermochemical hydrogen production

ABO3-type perovskites have been demonstrated as promising redox materials for solar thermochemical H2 production. In this work, a new energy material screening method based on the neural network system is designed as a feasible way to search for promising H2 production materials. The predicted oxygen vacancy formation energy of the selected YCrO3-δ is 4.199 eV, hinting excellent H2 production potential. Thermogravimetric analysis shows that the doping of Zr into YCrO3-δ improves oxygen formation capacity, leading to the maximum δ of 0.106. The molar enthalpy and entropy of YCr0.75Zr0.25O3-δ have the positive relationship with δ, and the maximum values of which are 273.7 kJ mol−1 and 164.9 J mol−1 K−1 respectively. Based on the equilibrium thermodynamic principle, the peak H2 yield is predicted to be 444.6 μmol g−1. Considering material kinetic limitation, gas-solid heat recovery and parameter sensitivity, the maximum H2 production efficiency of YCr0.75Zr0.25O3-δ is 17.3%. The combination of neural network and material thermodynamics provides a new pathway to design promising H2 production materials, and the screened YCrO3-δ presents excellent solar thermochemical H2 production capacity.

Formation of Ba3Nb0.75Mn2.25O9-6H during thermo-chemical reduction of Ba4NbMn3O12-12R.

The resurgence of inter-est in hydrogen-related technologies has stimulated new studies aimed at advancing lesser-developed water-splitting processes, such as solar thermochemical hydrogen production (STCH). Progress in STCH has been largely hindered by a lack of new materials able to efficiently split water at a rate comparable to ceria under identical experimental conditions. BaCe0.25Mn0.75O3 (BCM) recently demonstrated enhanced hydrogen production over ceria and has the potential to further our understanding of two-step thermochemical cycles. A significant feature of the 12R hexa-gonal perovskite structure of BCM is the tendency to, in part, form a 6H polytype at high temperatures and reducing environments (i.e., during the first step of the thermochemical cycle), which may serve to mitigate degradation of the complex oxide. An analogous compound, namely BaNb0.25Mn0.75O3 (BNM) with a 12R structure was synthesized and displays nearly complete conversion to the 6H structure under identical reaction conditions as BCM. The structure of the BNM-6H polytype was determined from Rietveld refinement of synchrotron powder X-ray diffraction data and is presented within the context of the previously established BCM-6H structure.

Accurate prediction of oxygen vacancy concentration with disordered A-site cations in high-entropy perovskite oxides

Entropic stabilized ABO3 perovskite oxides promise many applications, including the two-step solar thermochemical hydrogen (STCH) production. Using binary and quaternary A-site mixed {A}FeO3 as a model system, we reveal that as more cation types, especially above four, are mixed on the A-site, the cell lattice becomes more cubic-like but the local Fe–O octahedrons are more distorted. By comparing four different Density Functional Theory-informed statistical models with experiments, we show that the oxygen vacancy formation energies ({E}_{V}^{f}) distribution and the vacancy interactions must be considered to predict the oxygen non-stoichiometry (δ) accurately. For STCH applications, the {E}_{V}^{f} distribution, including both the average and the spread, can be optimized jointly to improve Δδ (difference of δ between the two-step conditions) in some hydrogen production levels. This model can be used to predict the range of water splitting that can be thermodynamically improved by mixing cations in {A}FeO3 perovskites.

Design of gas-liquid two-phase separation device with application in solar hydrogen production system

Currently, solar thermochemical energy conversion technology has attracted more and more attention because of the increasing global need for clean fuel and chemical energy demand. The thermochemical hydrogen production technology uses solar radiation with concentrated high-density as a high-temperature heat source to produce hydrogen by decomposing water through two-step cycle redox reactions. It is an environment-friendly technology based on artificial photosynthesis utilizing water vapor as the reaction gas. However, limited by the separation efficiency, current gas-liquid separation devices cannot output stable high-purity water vapor and sophisticate the experimental processes. In this research, intensive numerical investigations were carried out on the widely-used corrugated plate steam separators. The structural parameters are designed by the multi-objective optimization method combining the artificial neural network and the genetic algorithm. Nine optimized structures are designed by adding hydrophobic hooks, streamlined, and reducing the plate distance optimization methods. The results give engineering application conditions of different optimized separators. The streamlined single-hook corrugated plate separator with rounded corners exhibited the best separation efficiency under the heat state condition, which is 97.27%. It can also completely separate droplets over 28.5 μm. The applicability of the cyclone steam separators indicated that it is more suitable for pretreatment. The benchmark experimental system was built for the measurement of the separation efficiency resulting in only a 0.73% error. The designed steam separator was successfully applied to the solar thermochemical hydrogen production experiment, which ensures its practicability and pertinence in the process of solar fuel synthesis.

Synchrotron-based techniques for characterizing STCH water-splitting materials

Understanding the role of oxygen vacancy–induced atomic and electronic structural changes to complex metal oxides during water-splitting processes is paramount to advancing the field of solar thermochemical hydrogen production (STCH). The formulation and confirmation of a mechanism for these types of chemical reactions necessitate a multifaceted experimental approach, featuring advanced structural characterization methods. Synchrotron X-ray techniques are essential to the rapidly advancing field of STCH in part due to properties such as high brilliance, high coherence, and variable energy that provide sensitivity, resolution, and rapid data acquisition times required for the characterization of complex metal oxides during water-splitting cycles. X-ray diffraction (XRD) is commonly used for determining the structures and phase purity of new materials synthesized by solid-state techniques and monitoring the structural integrity of oxides during water-splitting processes (e.g., oxygen vacancy–induced lattice expansion). X-ray absorption spectroscopy (XAS) is an element-specific technique and is sensitive to local atomic and electronic changes encountered around metal coordination centers during redox. While in operando measurements are desirable, the experimental conditions required for such measurements (high temperatures, controlled oxygen partial pressures, and H2O) practically necessitate in situ measurements that do not meet all operating conditions or ex situ measurements. Here, we highlight the application of synchrotron X-ray scattering and spectroscopic techniques using both in situ and ex situ measurements, emphasizing the advantages and limitations of each method as they relate to water-splitting processes. The best practices are discussed for preparing quenched states of reduction and performing synchrotron measurements, which focus on XRD and XAS at soft (e.g., oxygen K-edge, transition metal L-edges, and lanthanide M-edges) and hard (e.g., transition metal K-edges and lanthanide L-edges) X-ray energies. The X-ray absorption spectra of these complex oxides are a convolution of multiple contributions with accurate interpretation being contingent on computational methods. The state-of-the-art methods are discussed that enable peak positions and intensities to be related to material electronic and structural properties. Through careful experimental design, these studies can elucidate complex structure–property relationships as they pertain to nonstoichiometric water splitting. A survey of modern approaches for the evaluation of water-splitting materials at synchrotron sources under various experimental conditions is provided, and available software for data analysis is discussed.

Synthesis and structure of high-purity BaCe0.25Mn0.75O3: an improved material for thermochemical water splitting

Solar thermochemical hydrogen production (STCH) via redox-active metal oxides is an approach for direct solar-driven hydrogen generation typically using a high-temperature redox cycle involving refractory oxides and steam. Typical cycles involve high-temperature reduction of oxides to form oxygen vacancies, followed by lower temperature reaction between oxygen vacancies and steam where the oxide is re-oxidized and the steam is reduced to hydrogen. Only a few materials have demonstrated reversible cycling under the typically harsh STCH conditions (e.g. 1500°C reduction, 900°C re-oxidation) and critical questions remain on the true reversibility of non-stoichiometric multi-cation oxide systems, significantly hampered by the lack of single-phase samples for these material systems. To date, most STCH processes have relied on CeO2 as a benchmark active material, but more recently, the 12R phase of BaCe0.25Mn0.75O3 (BCM) has demonstrated greater hydrogen-generation potential at lower peak temperatures. However, previous reports of 12R-BCM have included large fractions, &gt; 10 wt%, of secondary phases, which complicate analysis of the stability and performance. A comprehensive understanding of the redox mechanism and reversibility of the process in BCM can only be achieved with nearly single-phase samples which, to date, have been difficult to produce. Here two approaches to BCM synthesis are reported: solid state and sol–gel-based routes. It is demonstrated that both routes can be tuned to produce the 12R structure with &gt; 97 wt% yield when annealed ≥1450°C. Herein synchrotron-based diffraction measurements of rhombohedral 12R-BCM enabled characterization of the anisotropy between thermal expansion along the c-axis and within the ab plane. The impact of high-temperature redox cycling on the stability and phase fraction of the 12R-BCM polytype was also investigated. These results offer two viable routes for synthesis of high-purity 12R-BCM critically needed for evaluating the efficacy of BCM as a STCH material and validate its ability to split water at lower temperatures over extended numbers of redox cycles.

Probing Electronic and Structural Transformations during Thermal Reduction of the Promising Water Splitting Perovskite BaCe0.25Mn0.75O3

In this report, we investigate the thermal reduction of the octahedral perovskite BaCe0.25Mn0.75O3 (BCM) using in situ electron energy loss spectroscopy (EELS) in an aberration-corrected transmission electron microscope (TEM). The 12R-polytype of BCM is known to demonstrate high solar thermochemical hydrogen production capacity. In situ EELS measurements show that Mn is the active redox cation in BCM, undergoing thermal reduction from Mn4+ to Mn3+ during heating to 700 °C inside the TEM under a high vacuum. The progressive reduction of Mn4+ during oxygen vacancy (Ov) formation was monitored as a function of temperature. Additionally, atomic-resolution scanning transmission electron microscopy identified two different types of twin boundaries present in the oxidized and reduced form of 12R-BCM, respectively. These two types of twin boundaries were shown, via computational modeling, to modulate the site-specific Ov formation energies in 12R-BCM. It is concluded that these types of atomic defects provide sites more energetically favorable for Ov formation during thermal reduction.

Modulating the O-binding Energies of Mn-based Perovskites to Maximize Solar Thermochemical Hydrogen Production

Modulating the O-binding Energies of Mn-based Perovskites to Maximize Solar Thermochemical Hydrogen Production

Reticulated porous lanthanum strontium manganite structures for solar thermochemical hydrogen production

Porous ceramic foams are utilized in various fields and applications due to their advantageous properties such as high porosity, high specific surface area, and controlled permeability. Non-stoichiometric oxide foams are of particular interest for the solar thermochemical production of hydrogen in directly irradiated solar receivers due to their enhanced heat transfer capabilities and structural integrity. We report the fabrication of lanthanum strontium manganite replica foams and their solar thermochemical water splitting performance. Highly porous (>80%) foams were produced with controlled structure and tested in a high temperature water splitting furnace to demonstrate stability and overall performance. The temperature profile of the burnout and sintering of the foam structure were altered based on simultaneous differential scanning calorimetry and thermal gravimetry analysis. Two-step sintering was used both to retain micron-sized pores (D50 = 0.4 μm) on the foam's struts and to limit grain growth to a grain size of ∼1 μm. The structural characteristics were confirmed by porosimetry and scanning electron microscopy. The foams were repeatedly cycled, producing approximately 200 μmol/g of H2 per cycle. Water splitting performance was tested for 50 cycles and approximately 35 h at 1400 °C. No statistical difference in the effective hydrogen production per cycle was observed, which is promising for long-term redox cycle stability.

Predicting the formation of fractionally doped perovskite oxides by a function-confined machine learning method

Fractionally doped perovskites oxides (FDPOs) have demonstrated ubiquitous applications such as energy conversion, storage and harvesting, catalysis, sensor, superconductor, ferroelectric, piezoelectric, magnetic, and luminescence. Hence, an accurate, cost-effective, and easy-to-use methodology to discover new compositions is much needed. Here, we developed a function-confined machine learning methodology to discover new FDPOs with high prediction accuracy from limited experimental data. By focusing on a specific application, namely solar thermochemical hydrogen production, we collected 632 training data and defined 21 desirable features. Our gradient boosting classifier model achieved a high prediction accuracy of 95.4% and a high F1 score of 0.921. Furthermore, when verified on additional 36 experimental data from existing literature, the model showed a prediction accuracy of 94.4%. With the help of this machine learning approach, we identified and synthesized 11 new FDPO compositions, 7 of which are relevant for solar thermochemical hydrogen production. We believe this confined machine learning methodology can be used to discover, from limited data, FDPOs with other specific application purposes.

Formation of 6H-Ba3Ce0.75Mn2.25O9 during Thermochemical Reduction of 12R-Ba4CeMn3O12: Identification of a Polytype in the Ba(Ce,Mn)O3 Family.

The resurgence of interest in a hydrogen economy and the development of hydrogen-related technologies has initiated numerous research and development efforts aimed at making the generation, storage, and transportation of hydrogen more efficient and affordable. Solar thermochemical hydrogen production (STCH) is a process that potentially exhibits numerous benefits such as high reaction efficiencies, tunable thermodynamics, and continued performance over extended cycling. Although CeO2 has been the de facto standard STCH material for many years, more recently 12R-Ba4CeMn3O12 (BCM) has demonstrated enhanced hydrogen production at intermediate H2/H2O conditions compared to CeO2, making it a contender for large-scale hydrogen production. However, the thermo-reduction stability of 12R-BCM dictates the oxygen partial pressure (pO2) and temperature conditions optimal for cycling. In this study, we identify the formation of a 6H-BCM polytype at high temperature and reducing conditions, experimentally and computationally, as a mechanism and pathway for 12R-BCM decomposition. 12R-BCM was synthesized with high purity and then controllably reduced using thermogravimetric analysis (TGA). Synchrotron X-ray diffraction (XRD) data is used to identify the formation of a 6H-Ba3Ce0.75Mn2.25O9 (6H-BCM) polytype that is formed at 1350 °C under strongly reducing pO2. Density functional theory (DFT) total energy and defect calculations show a window of thermodynamic stability for the 6H-polytype consistent with the XRD results. These data provide the first evidence of the 6H-BCM polytype and could provide a mechanistic explanation for the superior water-splitting behaviors of 12R-BCM.

A Thermogravimetric Temperature-Programmed Thermal Redox Protocol for Rapid Screening of Metal Oxides for Solar Thermochemical Hydrogen Production

As combinatorial and computational methods accelerate the identification of potentially suitable thermochemically-active oxides for use in solar thermochemical hydrogen production (STCH), the onus shifts to quickly evaluating predicted performance. Traditionally, this has required an experimental setup capable of directly carrying out a two-stage thermochemical water-splitting process. But this can be a difficult endeavor, as most off-the-shelf equipment cannot adequately deal simultaneously with the high temperatures, varying oxygen partial pressures, and high H2O partial pressures required; achieving sufficient temporal sensitivity to accurately quantify the kinetics is also a major challenge. However, as proposed here, a less complicated experiment can be used as a first screening for thermochemical water splitting potential. Temperature-Programmed Thermal Redox (TPTR) using thermogravimetry evaluates the thermal reduction behavior of materials. This technique does not require water splitting or CO2-splitting analogs but can nonetheless predict water-splitting performance. Three figures of merit are obtained from the TPTR experiment: reduction onset temperature, extent of reduction, and extent of recovery upon reoxidation. These metrics can collectively be used to determine if a material is capable of thermochemical water-splitting, and, to good approximation, predict whether the thermodynamics are favorable for use under more challenging high-conversion conditions. This paper discusses the pros and cons of using TPTR and proposes a protocol for use within the STCH community.