Structural Requirements in Lithium Cobalt Oxides for the Catalytic Oxidation of Water

Abstract

The development of water oxidation catalysts (WOCs) to replace costly noble metals in commercial electrolyzers and solar fuel cells is an unmet need that is preventing the global development of hydrogen fuel technologies. Two of the main challenges in realizing catalytic water splitting are lowering the substantial overpotential that is required to achieve practical operating current densities in the O2-evolving halfreaction at the anode, and the use of earth-abundant elements for the fabrication of inexpensive electrodes that are free from noble metals. To meet these challenges, molecular catalysts that are based upon the cubic CaMn4Ox core within photosystem II in photosynthetic organisms, which is the gold standard of catalytic efficiency, have begun to appear. Among solid-state materials, several noble-metal oxides, which include IrO2 and RuO2, are already in use in industrial electrolyzers, but are not globally scalable. Aqueous solutions of cobalt phosphate form water-oxidation catalysts under electrolysis and photolysis that are suitable for the fabrication of noncrystalline electrode materials. Nanocrystalline spinel-phase metal oxides (AM2O4, M= transition metals) that are comprised of M4O4 cubical subunits and are active water oxidation catalysts have been developed. The catalytic activity of the spinel Co3O4 has been reported for Co3O4 nanorods that are incorporated into SBA-15 silica, as well as Co3O4 nanoparticles that are adsorbed onto Ni electrodes. NiCo2O4 spinel also oxidizes water when the nanoparticles are electrophoretically deposited onto a Ni electrode. Reports that examined the effect of lithium doping on the surface of Co3O4 electrodes in solutions of KOH attributed the higher evolution rate of O2 to better electrical conductivity. However, the oxidation of water by Co3O4 was strongly dependent on crystallite size and surface area and frequently necessitates high overpotentials and alkaline conditions to accelerate the rate of reaction. In contrast, we recently reported that the catalytically inert spinel LiMn2O4 gives spinel l-MnO2, which is an active water oxidation catalyst, upon topotactic delithiation. Thus, the importance of removing the A-site lithium for catalysis by the cubic Mn4O4 core of spinels was revealed. [11]