Mechanistic Studies of the Oxygen Evolution Reaction by a Cobalt-Phosphate Catalyst at Neutral pH

Department of Chemistry, 6-335, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307, United States.
Journal of the American Chemical Society (Impact Factor: 11.44). 10/2010; 132(46):16501-9. DOI: 10.1021/ja106102b
Source: PubMed

ABSTRACT The mechanism of the oxygen evolution reaction (OER) by catalysts prepared by electrodepositions from Co(2+) solutions in phosphate electrolytes (Co-Pi) was studied at neutral pH by electrokinetic and (18)O isotope experiments. Low-potential electrodepositions enabled the controlled preparation of ultrathin Co-Pi catalyst films (<100 nm) that could be studied kinetically in the absence of mass transport and charge transport limitations to the OER. The Co-Pi catalysts exhibit a Tafel slope approximately equal to 2.3 × RT/F for the production of oxygen from water in neutral solutions. The electrochemical rate law exhibits an inverse first order dependence on proton activity and a zeroth order dependence on phosphate for [Pi] ≥ 0.03 M. In the absence of phosphate buffer, the Tafel slope is increased ∼3-fold and the overall activity is greatly diminished. Together, these electrokinetic studies suggest a mechanism involving a rapid, one electron, one proton equilibrium between Co(III)-OH and Co(IV)-O in which a phosphate species is the proton acceptor, followed by a chemical turnover-limiting process involving oxygen-oxygen bond coupling.

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