Perovskite
oxides are promising electrocatalysts due to their rich
composition, facile synthesis, and favorable stability under oxidizing
conditions. Despite extensive research on doping strategies, the impact
of cation nonstoichiometry on electrocatalytic performance is less
understood. Here, we reveal that A-site cation nonstoichiometry significantly
influences the phase evolution of Bax(Co,
Fe, Zr, Y)O3−δ, transitioning from a single
cubic perovskite (x = 1) to a nanocomposite comprising
a major cubic perovskite phase and a minor hexagonal swedenborgite
phase (0.80 ≤ x ≤ 0.95). The nanocomposite
with a nominal chemical composition of Ba0.80Co0.7Fe0.1Zr0.1Y0.1O3−δ showed markedly enhanced electrocatalytic performance for the oxygen
evolution reaction (OER) in alkaline solutions due to the synergistic
effect of the two strongly interacting phases, promoting a lattice-oxygen-participating
OER pathway. Further optimizing cation nonstoichiometry allowed the
design of nanocomposites with controlled phase concentrations. The
optimal candidate, with an increased content of the swedenborgite
phase, demonstrated further boosted OER performance.