Mechanistic studies of the oxygen evolution reaction by a cobalt-phosphate catalyst at neutral pH.
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.
Surendranath
Yogesh
Y
Department of Chemistry, 6-335, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307, United States.
Kanan
Matthew W
MW
Nocera
Daniel G
DG
D002384:Catalysis; D003035:Cobalt / Q000737:chemistry*; D004563:Electrochemistry; D006863:Hydrogen-Ion Concentration; D015394:Molecular Structure; D010100:Oxygen / Q000737:chemistry*; D010710:Phosphates / Q000737:chemistry*
D010710:Phosphates; D003035:Cobalt; D010100:Oxygen