Behavior of reversible intercalation of oxygen anions into porous SrFeO_3-delta as a pseudocapacitor electrode material

As promising electrode materials for pseudocapacitors, perovskite-typed oxides have been widely investigated because of their excellent electrochemical stability, high energy storage capacity, and fast charge/discharge rate. Herein, we investigated the intercalation/deintercalation behaviors of oxygen anions in porous polyhedral SrFeO_3-δ particles that were prepared by a hard-templating method. The hydrothermally synthesized Sr₃Fe₂(OH)₁₂ transformed into a solid dispersion in which SrCO₃ is embedded in the perovskite matrix after thermal treatment in air. By removing carbonate by acid leaching, porous polyhedral particles were obtained. The porous perovskite (δ = 0.04) has a specific surface area of 35.8 m2/g and the average particle size is around 2.6 μm. The electrochemical energy storage was evaluated in a three-electrode system. In 1 M aqueous KOH solution, the fresh sample exhibits a specific capacitance of 438 F/g at a scan rate of 2 mV/s. After subtracting the contribution of the electric double-layer contribution, the specific capacity is 581C/g that approximately corresponds to a change of one in the average valence state of bulk iron (Fe4+↔Fe3+). The intercalation/deintercalation processes are governed by the diffusion of oxygen ions (O2−) in solids, which most likely evolve from/into free hydroxide ions (OH−) via _⁎OH−↔_⁎OOH−↔_⁎O−↔O_int2− at the electrode-electrolyte interface. In the cycling stability test, the specific capacitance first decreased and then quickly stabilized, which was also observed for a non-porous SrFeO_3-δ in 1 M aqueous KOH solution and the porous one in 1 M K₂CO₃ aqueous solution. The porous one in 1 mol/L K₂CO₃ aqueous solution, likely arising from a gradual increase in the average oxidation state of iron at the end of deintercalation during initial charge/discharge cycles.