STRUCTURE-ACTIVITY CORRELATIONS AND INTERFACIAL CHEMISTRY OF COBALT OXIDES AND OXYHYDROXIDES FOR OXYGEN EVOLUTION

Abstract

Cobalt-based oxide catalysts can improve the kinetics of oxygen evolution reaction (OER) and reduce the its overpotentials. Optimization and discovery of active, affordable and stable catalysts must follow rational design principles based on a solid understanding of the reaction mechanism and its correlations with catalyst properties. An investigation of H2O surface chemistry on Co3O4(111) and CoO(111) illustrated dissociative H2O adsorption on both surfaces. Additionally, a distribution in the desorption temperatures of H2O from these surfaces suggested different OH binding energies at various surface Co sites. Furthermore, the Co3O4(111) surface was demonstrated to be more active compared to the (100) surface of Co3O4, based on a comparison of catalyst activities of nanocrystals exposing these well-defined crystal orientations. Evaluation of catalyst stability revealed the (100) surface of Co3O4 nanocubes are more stable compared to the (111) surface of nanooctahedra, as a result of the lower surface energy of (100).

Transformation of cobalt oxides to CoOOH has been proposed, which provided excellent motivations to study their OER activities. Ni modification of CoOOH nanowires can significantly increase catalyst activity however, Mn modification did not promote faster kinetics. Impedance analysis revealed that Ni incorporation can reduce charge transfer resistances and improve the catalysts ability to stabilize surface intermediates, whereas Mn incorporation impedes intermediate stabilization. Using ambient pressure photoemission spectroscopy, we have directly observed greater stability of surface hydroxyl groups as a result of Ni incorporation. Additionally, extensive surface hydroxylation observed at mild conditions (27 °C, 1 torr H2O) indicated a low barrier of phase transformation to Co(OH)2. This prompted in situ investigation of catalyst structural transformation during OER using operando Raman spectroscopy. For the first time, the active structure of NiCoOxHy catalysts is identified as NiOOH-h-CoO2, which can be formed by an irreversible transformation of spinel Co3O4 to a-CoO, followed by a reversible conversion of the latter to NiOOH-h-CoO2. Incorporation of Ni into the active structure is necessary to improve the catalyst activity of NiCoOxHy, whereas Ni ions coordinated in a spinel structure is catalytically dormant.