Compositionally Complex Perovskite Oxides for Solar Thermochemical Water Splitting

Abstract

Solar thermochemical hydrogen (STCH) generation is a promising approach for eco-friendly H<sub>2</sub> production, but conventional STCH redox compounds cannot easily achieve desirable thermodynamic and kinetic properties and phase stability simultaneously due to a rather limited compositional space. Expanding from the nascent high-entropy ceramics field, this study explores a new class of compositionally complex perovskite oxides (La<sub>0.8</sub>Sr<sub>0.2</sub>)(Mn<sub>(1–x)/3</sub>Fe<sub>(1–x)/3</sub>Co<sub>x</sub>Al<sub>(1–x)/3</sub>)O<sub>3</sub> with new non-equimolar designs for STCH. In situ X-ray photoelectron spectroscopy shows preferential redox of Co. The extent of reduction increases, but the intrinsic kinetics decreases, with increasing Co content. Consequently, (La<sub>0.8</sub>Sr<sub>0.2</sub>)(Mn<sub>0.2</sub>Fe<sub>0.2</sub>Co<sub>0.4</sub>Al<sub>0.2</sub>)O<sub>3</sub> achieves an optimal thermodynamic and kinetic balance. The combination of a moderate enthalpy of reduction, a high entropy of reduction, and preferable surface oxygen exchange kinetics enables a maximum H<sub>2</sub> production of 89.97 mmol mol<sub>oxide</sub><sup>–1</sup> in a short 1 h redox duration. Entropy stabilization may contribute to the phase stability during redox cycling without phase transformation, which enables STCH production for >50 cycles under harsh interrupted conditions. The underlying redox mechanism is further elucidated by a density functional theory-based parallel Monte Carlo computation approach. This study suggests a new class of non-equimolar compositionally complex ceramics for STCH and thermochemical looping.