The Effect of Oxygen Vacancies on Surface and Bulk Charge Storage in La0.5Sr0.5FeO3−δ

Abstract

Oxygen vacancies are widely used to tune the electrochemical performance of transition metal oxides, yet their isolated contribution to charge storage remains difficult to quantify because vacancy concentration and morphology typically change simultaneously. Here, we investigate the effect of oxygen deficiency on charge storage in La0.5Sr0.5FeO3−δ (δ = 0.02–0.38) while keeping crystal structure, morphology, and specific surface area essentially unchanged. Hollow spherical particles were synthesized by ultrasonic spray pyrolysis and reduced under controlled conditions. Electrochemical measurements in 3 M KOH reveal a pronounced maximum in specific capacity (0.2 Ah/g at 1 A/g), faradaic current, and heterogeneous electron transfer rate at δ ≈ 0.05; higher vacancy concentrations degrade performance. Step potential electrochemical spectroscopy and electrochemical impedance spectroscopy were used to separate surface and bulk charge storage contributions. X-ray photoelectron spectroscopy and electron paramagnetic resonance show that increasing δ sequentially reduces Fe4+ to Fe3+ and Fe2+, collapsing ferromagnetic double-exchange clusters and electronic percolation. Complementing these experimental findings, density functional theory (DFT) calculations on La0.5Sr0.5FeO3-δ reveal a progressive suppression of the Fe 3d and O 2p partial densities of states at the Fermi level with increasing oxygen vacancy concentration, leading to a metal-to-insulator transition at δ ≈ 0.25, consistent with the observed degradation of electronic conductivity at high defect levels. The optimal electrochemical response thus arises from a compromise between vacancy-enhanced electrolyte ions insertion and preserved electronic conductivity. These findings provide a rational basis for defect engineering in perovskite electrodes.