Autocatalytic mechanism of H2O2 reduction on Ag electrodes in acidic electrolyte: experiments and simulations

It is shown that the cathodic reduction of hydrogen peroxide (H2O2) on silv er electrodes in acidic electrolyte can proceed by two parallel mechanisms: first, by the 'normal' mechanism that has been discussed in the literature , second, by a novel mechanism proceeding at a significantly more positive potential. It is proposed that the second mechanism involves the activating adsorbate (OH)(ad), that forms in the course of the H2O2 reduction reactio n as an unstable intermediate. The coverage of the electrode with (OH)(ad) increases with the rate of H2O2 reduction, i.e., the process is autocatalyt ic. At more negative potentials the coverage decreases as the rate of adsor bate reduction/desorption rises. This leads to a potential region of negati ve differential charge-transfer resistance and thus to complex dynamic phen omena, in particular to electrochemical oscillations. Model calculations ba sed on these considerations yield the potential dependent OH-adsorption, th e N-shaped current/voltage curves and current oscillations that agree well with the experimental findings. (C) 1999 Elsevier Science Ltd. All rights r eserved.