A kinetic descriptor for the electrolyte effect on the oxygen reduction kinetics on Pt(111)

Proton-exchange membrane fuel cells demand efficient electrode-electrolyte interfaces to catalyse the oxygen reduction reaction (ORR), the kinetics of which depends on the energetics of surface adsorption and on electrolyte environment. Here we show an unanticipated effect of non-specifically adsorbed anions on the ORR kinetics on a Pt(111) electrode; these trends do not follow the usual ORR descriptor, that is *OH binding energy. We propose a voltammetry-accessible descriptor, namely reversibility of the *O <-> *OH transition. This descriptor tracks the dependence of ORR rates on electrolyte, including the concentration/identity of anions in acidic media, cations in alkaline media and the effect of ionomers. We propose a model that relates the ORR rate on Pt(111) to the rate of the *O to *OH transition, in addition to the thermodynamic *OH binding energy descriptor. Our model also rationalizes different trends for the ORR rate on stepped Pt surfaces in acidic versus alkaline media.