Phase transformation of the brownmillerite SrCoO2.5 thin film through alkaline water electrolysis

A phase transformation from insulating brownmillerite SrCoO2.5 to conducting perovskite SrCoO3 through electrochemical oxidation has been demonstrated for thin films of SrCoOx on a SrTiO3 (001) substrate. The cobalt-oxide film strongly favors the brownmillerite phase of SrCoO2.5 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 SrCoO2.5 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 SrCoO2.5. 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.