A New Synthetic Approach to Cobalt Oxides: Designed Phase Transformation for Electrochemical Water Splitting

Abstract The structural phase transformations of non-native hexagonal CoO (h-CoO) into various other cobalt oxides, such as cubic rock-salt CoO, spinel Co3O4, trigonal Co(OH)2, and trigonal CoOOH, were examined by controlling the reaction conditions. The transformation pathway of h-CoO for each cobalt oxide was fully elucidated. The prepared cobalt oxides were studied as catalysts for oxygen evolution reaction (OER) to clarify the structure-activity relationship. Among the cobalt oxide materials, spinel Co3O4 showed the best catalytic activity. Moreover, spinel Co3O4 was deposited directly via the oxidative phase transformation of h-CoO particles on a conductive substrate without additional coating processes. The efficient chemical adhesion of spinel Co3O4 via phase transformation indicated fast oxidation of Co3+ to Co4+, which resulted in improved OER activity.