Solar-driven thermochemical water splitting using redox-active metal oxides has emerged as an intriguing approach for storing solar energy in chemical fuels. Specifically, the metal oxide is partially reduced after exposure to a certain high temperature (\~1400 °C) under oxygen-poor gas and is oxidized upon supplying H~2~O at a lower temperature (\~800 °C), releasing H~2~. The thermodynamic properties play a dominant role in determining the capacity of H~2~ generation, while reaction kinetics may also be significant in determining the actual thermochemical behavior only if it is a rate-limiting constraint. The state-of-the-art non-stoichiometric CeO~2-δ~, though is found to have favorable thermodynamics and fast reaction kinetics on H~2~ production, the high-temperature requirement to achieve a large reduction extent induces many challenges in solar reactors design. Perovskite-structured oxides have been proposed as alternatives to CeO~2-δ~, for the benefits of easy tuning of compositions, structures, and thermodynamics properties, lowering reduction temperature, and larger oxygen non-stoichiometry (δ), which enables more H~2~ production and fewer challenges in reactor design. Here, we explore the thermodynamic properties and thermochemical water splitting capability of a perovskite SrTi~0.5~Mn~0.5~O~3-δ~. Thermogravimetric analysis reveals favorable thermodynamics, specifically, much lower enthalpy and much higher entropy of reduction compared to known perovskite La~1-x~Sr~x~MnO~3-δ~. A sufficient high δ = 0.283 is expected under thermochemical cycling condition (1400 °C, 10 ppm O~2~),  consequently, a noteworthy H~2~ productivity of 12.5 mL (g of oxide)^-1^ is achieved in a thermochemical cycle of 1400 °C – 1000 °C, *p*H~2~O = 0.4 atm (*t*~Total~ = 540 min, long cycle), and 7.39 ± 0.07 mL (g of oxide)^-1^ with thermal reduction temperature of 1350 °C followed by oxidation at 1100 °C, *p*H~2~O = 0.4 atm (*t*~Total~ = 90 min, short cycle). In addition, we reveal the gas-phase rather than material-kinetic limitation on the H~2~ production rate and demonstrate the prediction of H~2~ production profiles using material thermodynamics implemented in a thermo-kinetic model.


High-Capacity Solar Thermochemical Splitting of Water From srti0.5mn0.5o3-\u03b4 Perovskite With Excellent Stability and Favorable Thermodynamics

Division / Sponsor:
Advanced Energy Systems Division (AESD) & Solar Energy Division (SED)

Paper Proceedings: https://asmedigitalcollection.asme.org/ES/ES2020/volume/83631

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