The Potential\nof Zero Total Charge Predicts Cation\nEffects for the Oxygen Reduction Reaction

Cation effects are frequently observed for electrochemical\nreactions\nthat take place at strongly reducing potentials. But a clear understanding\nof when cation effects will be observed for chemistries like the oxygen\nreduction reaction (ORR), which occurs at more mildly reducing potentials,\nhas not been developed. Here, based on the results of experimental\nand computational studies, we propose that the potential of zero total\ncharge (PZTC) predicts whether ORR rates will be influenced by alkali\nmetal cation size. For metals whose PZTC is positive of the ORR potential\nwindow (Pt, Ir, Ru, and Au), the surface is negatively charged during\ncatalysis, allowing cations to accumulate in the double layer and\ninfluence the stability of reaction intermediates. For these metals,\nORR rates increase with cation size (Li<sup>+</sup> < Na<sup>+</sup> < K<sup>+</sup> < Cs<sup>+</sup>). We argue that interfacial\ncations decrease *OH poisoning over strongly binding catalysts whose\nrates are limited by product desorption and decrease the apparent\nactivation barrier for O<sub>2</sub> adsorption over weakly binding\ncatalysts. Conversely, for metals whose PZTC is negative of the ORR\npotential window (Ag and Pd), the surface is positively charged; therefore,\ncations are electrostatically repelled from the surface under reaction\nconditions. Their corresponding ORR rates are insensitive to the electrolyte\ncomposition.