The Potential of Zero Total Charge Predicts Cation Effects for the Oxygen Reduction Reaction

Cation effects are frequently observed for electrochemical reactions that take place at strongly reducing potentials. But a clear understanding of when cation effects will be observed for chemistries like the oxygen reduction reaction (ORR), which occurs at more mildly reducing potentials, has not been developed. Here, based on the results of experimental and computational studies, we propose that the potential of zero total charge (PZTC) predicts whether ORR rates will be influenced by alkali metal cation size. For metals whose PZTC is positive of the ORR potential window (Pt, Ir, Ru, and Au), the surface is negatively charged during catalysis, allowing cations to accumulate in the double layer and influence the stability of reaction intermediates. For these metals, ORR rates increase with cation size (Li⁺ < Na⁺ < K⁺ < Cs⁺). We argue that interfacial cations decrease *OH poisoning over strongly binding catalysts whose rates are limited by product desorption and decrease the apparent activation barrier for O₂ adsorption over weakly binding catalysts. Conversely, for metals whose PZTC is negative of the ORR potential window (Ag and Pd), the surface is positively charged; therefore, cations are electrostatically repelled from the surface under reaction conditions. Their corresponding ORR rates are insensitive to the electrolyte composition.