Designing La-Based Non-Equimolar High-Entropy Metal Oxides (HEMOs) for Superior Oxygen Evolution Reaction

La 0.7 Sr 0.3 CoO 3 (denoted as P1) and dual-substituted non-equimolar high-entropy perovskites, LaSrCo 1− x − y Ni x Fe y O 3, were synthesized by the solution combustion method and evaluated as OER electrocatalysts. XRD confirms the formation of a crystalline perovskite phase for all samples, while FE-SEM images reveal a porous network of interconnected nanoparticles that can enhance surface exposure and catalytic activity. All as-synthesized compounds were evaluated electrochemically in alkaline medium, among which LaSrCo 0.5 Ni 0.12 Fe 0.38 O 3 (denoted as P7) exhibited the highest current density, lowest onset potential, and good stability. The P7 catalyst exhibits a 3.6-fold higher ECSA-normalized current density ( j ECSA ) than the parent P1, demonstrating the synergistic effect of Ni and Fe dual substitution. HR-TEM analysis of the P7 sample reveals homogeneous elemental distribution and slight lattice distortion relative to P1, confirming the successful incorporation of Ni–Fe into the perovskite framework while preserving its structural integrity. XPS analysis, before and after OER, revealed surface oxidation of Co, Ni, and Fe species and the formation of oxyhydroxide (OOH) groups, suggesting the generation of high-valent active sites. Gas chromatography measurements confirm that the P7 catalyst exhibits highest O 2 Faradaic efficiency among all samples. MEA testing in a zero-gap electrolyzer demonstrates efficient hydrogen production with a faradaic efficiency of ∼59% and energy efficiencies of ∼44% (HHV) and ∼37% (LHV), highlighting effective electrochemical energy conversion. These findings demonstrate the potential of Fe and Ni dual-substituted LaSrCoO 3 perovskites as efficient and stable OER catalysts in alkaline media.