Synthesis of superconducting cobalt oxyhydrates using a novel method: Electrolyzed and oxidized water

Abstract

By deintercalation of Na+ followed by inserting bilayers of water molecules into the host lattice, the layered cobalt oxide of γ-Na0.7CoO2 undergoes a topotactic transformation to a layered cobalt oxyhydrate of Na0.35(H2O)1.3CoO2−δ with the c-axis expanded from c≈10.9Å to c≈19.6Å. In this paper, we demonstrate that the superconducting phase of c≈19.6Å can be directly obtained by simply immersing γ-Na0.7CoO2 powders in electrolyzed/oxidized (EO) water, which is readily available from a commercial electrolyzed water generator. We found that high oxidation–reduction potential of EO water drives the oxidation of the cobalt ions accompanying by the formation of the superconductive c≈19.6Å phase. Our results demonstrate how EO water can be used to oxidize the cobalt ions and hence form superconducting cobalt oxyhydrates in a clean and simple way and may provide an economic and environment-friendly route to oxidize the transition metal of complex metal oxides.