A New Synthetic Approach To Cobalt Oxides: Designed Phase Transformation For Electrochemical Water Splitting

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 structureactivity 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.