Pseudocapacitive Charge Storage in Perovskite Oxides as a Function of Particle Size: Case Study of SrFe0.5Co0.5O3–δ

Systematic trends in anion-based pseudocapacitive properties are demonstrated for a perovskite oxide SrFe0.5Co0.5O3–δ, prepared using four different synthesis methods that result in a gradual decrease of the particle size. The mechanism of the pseudocapacitive charge storage, involving the hydroxide adsorption and oxide anion intercalation, is confirmed using ex situ X-ray photoelectron spectroscopy (XPS) and X-ray diffraction. The latter shows a decrease in the unit cell volume of the material in the charged state compared to the zero-potential state, as confirmed by Rietveld refinements. The XPS studies at both the zero-potential and charged states demonstrate the oxidation of iron and cobalt upon charging, consistent with the oxide intercalation. The essential role of hydroxide adsorption in initiating the oxide intercalation process is confirmed by comparative experiments in KOH and KNO3. The four synthesis methods lead to products that show a systematic increase in pseudocapacitive properties as the particle size decreases progressively from the solid-state synthesis (SS) product to those of sol–gel (SG), autocombustion (AC), and polymeric-steric entrapment (PSE). The contributions from diffusive and capacitive processes to electrochemical properties are also evaluated for the four samples. We note that this study is not aimed at maximizing the reported capacity; instead, its objectives are the elucidation of fundamental mechanisms and the impact of synthetic control of the particle size on pseudocapacitive properties, providing foundational insight into this process.