Effect of Particle Size on Electrocatalytic Properties of Perovskite Oxide Sr2FeCoO6-δ for Alkaline Water-Splitting

Oxygen-deficient perovskites have emerged as a promising family of electrocatalytic materials, particularly for water-splitting. Investigation of the parameters affecting their electrocatalytic properties is an important endeavor. In this work, four different synthesis methods have been employed to synthesize oxygen-deficient perovskite Sr₂FeCoO_6-δ (δ = 0.5). The four synthesis methods lead to systematically varying particle sizes, providing an opportunity to investigate the particle size effect on electrocatalytic properties for both half-reactions of water-splitting, namely oxygen-evolution reaction (OER) and hydrogen evolution reaction (HER). The four materials were evaluated by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), iodometric titrations, transmission electron microscopy (TEM), impedance spectroscopy, and electrocatalytic studies. As evident from Rietveld refinements, all four synthesis methods led to the same cubic crystal structure. Iodometric titrations confirmed the concentration of oxygen-vacancies (δ = 0.5). XPS was used for determination of the transition metal oxidation states and ex-situ evaluation of the stability. Detailed investigations using a particle size analyzer, combined with TEM, showed the material synthesized by polymeric steric entrapment (PSE) had the smallest particle size, followed by the materials obtained from autocombustion (AC), sol–gel (SG), and solid state (SS) synthesis techniques. The investigation of electrocatalytic properties for OER and HER revealed systematic trends, where the electrocatalytic activity had an inverse relationship with particle size. This trend was evident from the overpotentials of OER and HER, Tafel kinetics, charge transfer resistances, and turnover frequencies. The most active material, PSE, showed a remarkable OER overpotential (293 mV at 10 mAcm^–2) that was even lower than those of noble metal catalysts, IrO₂ and RuO₂. Importantly, this catalyst was highly stable under the OER conditions, as revealed by chronopotentiometry experiments, as well as X-ray diffraction and XPS studies before and after the OER.