  Solar thermochemical hydrogen generation (STCH) is a promising approach for
eco-friendly H2 production, but conventional STCH redox compounds often suffer
from thermodynamic and kinetic limitations with limited tunability. Expanding
from the nascent high-entropy ceramics field, this study explores a new class
of compositionally complex perovskite oxides
(La0.8Sr0.2)(Mn(1-x)/3Fe(1-x)/3CoxAl(1-x)/3)O3 for STCH. In situ X-ray
diffraction demonstrates the phase stability during redox cycling and in situ
X-ray photoelectron spectroscopy shows preferential redox of Co. The extent of
reduction increases, but the intrinsic kinetics decreases, with increased Co
content. Consequently, (La0.8Sr0.2)(Mn0.2Fe0.2Co0.4Al0.2)O3-{\delta} achieves
an optimal balance between the thermodynamics and kinetics properties. The
combination of a moderate enthalpy of reduction, high entropy of reduction, and
preferable surface oxygen exchange kinetics enables a maximum H2 yield of 395
+- 11 {\mu}mol g-1 in a short 1-hour redox duration. Entropy stabilization
expectedly contributes to the structure stability during redox without phase
transformation, which enables an exceptional STCH stability for >50 cycles
under harsh interrupted conditions. The underlying redox mechanism is further
elucidated by the density functional theory based parallel Monte Carlo
computation, which represents a new computation paradigm first established
here. This study suggests a new class of non-equimolar compositionally complex
ceramics for STCH and chemical looping.
