Certain aspects of the interpretation of rotating ring-disk electrode, RRDE, data for the oxygen reduction reaction, ORR, in aqueous electrolytes have been examined. In particular, the classical mechanism proposed by Damjanovic et al. [ J. Chem. Phys. 45, 4057 (1966)] predicts that at a fixed potential, a plot of the ratio of the disk to the ring currents for the ORR vs -Ω-1/2 , where is Ω the rotation rate of the disk, should be linear. According to this analysis, the slope of this line is proportional to k 3 / X H2 O2 , where k3 is the rate constant for the reduction of solution phase hydrogen peroxide, H2 O2 (aq), and X H2 O2 is the fraction of the disk current that generates H2 O2 (aq), the magnitude of which can be determined from the line intercept. Illustrations of this theoretical framework are provided for ORR data reported in the literature for experiments involving Pt-based electrodes and collected in our laboratories for Pt in aqueous acidic solutions. Critically emphasized is the need to perform ring-disk measurements at multiple rotations rates in order to determine reliable values of X H2 O2, which can be used as figures of merit for ranking the efficacy of ORR electrocatalysts.
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