A Significant Increase in the Capacity of a Ferrite Perovskite Material in a Neutral Electrolyte Upon Electrochemical Protonation
Abstract
Herein, we report on the electrochemical reduction and structural rearrangement of La₀.₅Sr₀.₅FeO₃ (LSF) under electrochemical treatment in a neutral phosphate buffered electrolyte. The body of experimental observations with a variety of techniques (cyclic voltammetry, XRD, NMR, TGA/MS-spectroscopy and EDX, XPS) provide convincing evidence in favor of bulk reductive protonation of LSF. The extent of proton insertion is governed by the concentration of the phosphate buffer in the electrolyte. At low buffer concentrations, protonation is confined to the surface. At higher concentrations, we observe a structural rearrangement of LSF with the formation of an amorphous-crystalline structure. The main contribution to the Faradaic process is associated with protonation-induced oxidation-reduction reactions on iron ions. It is suggested that the partial amorphization, facilitated by the interaction of the protonated oxide with the phosphate groups, significantly enhances charge storage. The electrochemically-treated LSF displays an increase in capacitance, reaching 0.092 Ah/g at 0.6 A/g for a cathode with a surface area of 17.3 m²/g. Reproducible cyclic voltammetry and resilience upon heating in air to 500 °C demonstrate the remarkable stability of the amorphous-crystalline composite. Our work demonstrates electrochemical protonation in neutral buffered electrolytes as a powerful tool for engineering highly capacitive oxides.