Phase transformation of the brownmillerite SrCoO_2.5 thin film through alkaline water electrolysis

A phase transformation from insulating brownmillerite SrCoO_2.5 to conducting perovskite SrCoO₃ through electrochemical oxidation has been demonstrated for thin films of SrCoO_x on a SrTiO₃ (001) substrate. The cobalt-oxide film strongly favors the brownmillerite phase of SrCoO_2.5 instead of the perovskite phase of SrCoO₃ on a SrTiO₃ (001) substrate due to its low lattice mismatch. Therefore, the phase transformation has its own retention. The alkaline water electrolysis occurs between the copper cathode and the SrCoO_2.5 film anode. The H⁺ ions are attracted to the cathode and generate H₂ gas. The OH^- ions are attracted to the film's surface and generate a rich amount of oxygen to fill the oxygen vacancy channel of brownmillerite SrCoO_2.5. The phase transformation was verified from the change in the out-of-plane lattice constant and the change in the resistivity of the electrolyzed film.