Metal-Organic Frameworks as Porous Templates for Enhanced Cobalt Oxide Electrocatalyst Performance

Water oxidation to dioxygen is an essential energy capture process in biological systems mediated by the oxygen-evolving complex. It continues to attract interest as a half-reaction in electrochemical water-splitting to hydrogen and dioxygen in abiotic systems. There is a corresponding interest in improving the activity of nonprecious metal electrocatalysts for this reaction. One of the most well-known water oxidation catalysts is amorphous cobalt oxide. This catalyst can operate in water at neutral pH with low overpotentials and is known to continually reform under catalytic conditions, a type of self healing. The active sites for the electrocatalytic reaction are thought to be adjacent octahedral cobalt sites in the bulk material, meaning that only the surface of the material is active. Thus, catalytic activity could be improved by increasing the surface area to volume ratio of the active material. To this effect, codeposited microporous materials such as metal organic frameworks (MOFs) can be used to control the morphology of cobalt oxide thin films. This study employs a highly stable MOF, UiO-66, to act as a templating agent for an electrodeposited cobalt oxide film. By depositing this porous crystalline MOF on the electrode surface prior to the electrochemical growth of the catalyst, we show the catalytic current density of the cobalt oxide material can be increased relative to the unmodified material. Through cyclic voltammetry (CV), controlled potential electrolysis (CPE), and electron microscopy studies, we show that the increase in current density is likely due to an increase in the effective surface area of the catalyst resulting from catalyst deposition around the MOF particles.