Solid-State Electrochemical Protonation of SrCoO_2.5 into H_xSrCoO_2.5 (x = 1, 1.5, and 2)

Among many transition-metal oxides (TMOs), strontium cobalt oxide (SrCoOx) is a promising active material for advanced memory devices due to the versatile valence state of cobalt ions. Several SrCoOx-based electrochemical devices have been proposed, but solid-state protonation from SrCoO2.5 to HxSrCoO2.5 (x = 1, 1.5, and 2) at room temperature has not been demonstrated thus far due to the absence of an appropriate solid electrolyte. Here, we demonstrate a solid-state electrochemical protonation of SrCoO2.5 using mesoporous amorphous 12CaO·7Al2O3 (CAN) film as the solid electrolyte. The crystalline phase discretely changed from SrCoO2.5 to HSrCoO2.5 (phase A), H1.5SrCoO2.5 (phase B), and H2SrCoO2.5 (phase C) through formation of an intermediate phase of H1.25SrCoO2.5. H1.5SrCoO2.5 (phase B) was colorless transparent and showed weak ferromagnetism. The present results indicate that the CAN film can be used as the solid electrolyte for the protonation treatment of TMOs.