Oxygen evolution mediated by co-based thin film electrocatalysts

Abstract

The electrocatalytic conversion of water to O₂ is the key efficiency-determining reaction for the storage of electrical energy in the form of liquid fuels. In this thesis, the simple preparation of a cobalt-based catalyst for the oxygen evolution reaction (OER) is described and details of its structure, valency, mechanism of action, and mechanism of formation at intermediate pH are elaborated. The catalyst is obtained as an electronically conductive, porous thin film by electrolysis of Co2 in aqueous phosphate, methylphosphonate, or borate electrolyte. Catalyst films prepared from phosphate are comprised of Co oxo/hydroxo clusters of molecular dimensions, as determined by X-ray absorption spectroscopy. The clusters consist of edge-sharing CoO6 octahedra arranged in a sheet-like pattern. The average cluster nuclearity increases as the film thickness increases. X-ray absorption near edge structure (XANES) spectra, EPR spectra, and electrochemical data support a catalyst film consisting predominately of Co(III) in the absence of an applied bias with minor populations of Co(II) and Co(IV) centers. As the film is polarized in the region of water oxidation, the population of Co(IV) centers increases systematically. The mechanism of the OER mediated by the catalyst was studied at neutral pH by electrokinetic and 180 isotope experiments. The catalyst exhibits an OER Tafel slope approximately equal to 2.3 x RT/F, an inverse first order dependence on proton activity, and a zeroth order dependence on phosphate for buffer strengths > 0.03 M. In the absence of phosphate, the Tafel slope increases ~3 fold and the overall activity is greatly diminished. These data point to an OER mechanism involving a rapid, one electron, one proton, equilibrium between Co"'-OH and CoWv-O in which a phosphate species is the proton acceptor, followed by a chemical turnover-limiting process involving oxygen-oxygen bond coupling. The mechanisms of nucleation, steady-state growth, and repair of the catalyst were studied at intermediate pH by electrokinetic, AFM and NMR methods. Catalyst nucleation is progressive with a non-zero-order nucleation rate law. Steady-state growth exhibits a Tafel slope approximately equal to 2.3 x RT/F, an inverse third order dependence on proton activity, and an inverse first order dependence on buffer strength. Together, the electrokinetic studies point to a mechanism involving a rapid one-electron, three-proton equilibrium oxidation of Co2+ coupled to phosphate dissociation from the catalyst surface, which is followed by a chemical rate-limiting process involving Co binding to the growing clusters. Consistent with the disparate pH profiles for the OER and catalyst formation, functional stability and repair are operative at pH > 6 whereas catalyst corrosion prevails at lower pH.