Solar thermochemical hydrogen (STCH) generation is a promising approach for eco-friendly H 2 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 0.8 Sr 0.2 )(Mn (1−x)/3 Fe (1−x)/3 Co x Al (1−x)/3 )O 3 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 0.8 Sr 0.2 )(Mn 0.2 Fe 0.2 Co 0.4 Al 0.2 )O 3 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 2 production of 89.97 mmol mol oxide −1 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.

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 ($${E}_{V}^{f}$$ E V f ) distribution and the vacancy interactions must be considered to predict the oxygen non-stoichiometry (δ) accurately. For STCH applications, the $${E}_{V}^{f}$$ 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.

Aliovalent doping on perovskite oxides can tune the oxygen vacancy formation energy. This work discovers normal vs. abnormal aliovalent doping effects on redox behaviors in medium-entropy compositionally complex perovskite oxides...