Role of the field fluctuations in electrochemical reactions

Rate constants of electrochemical reactions, accompanying by electron transfer via the electrolyte–metal interface, exponentially depend on the electrode potential. We present Monte Carlo simulations illustrating the effect of fluctuations of the potential near the reactants on the rate of such reactions. The two models employed are based on the phenomenological Frumkin theory and aimed, respectively, at conventional electrolytes and polymer electrolytes widely used in fuel cells. In both cases, the role of spatio-temporal fluctuations is found to be minor if the dielectric constant in the double layer is large, ≃80 (as in the bulk water). If the double-layer dielectric constant is smaller, e.g. ≃30 as it often takes place in conventional and polymer electrolytes, the fluctuations result in appreciable increase of the apparent reaction rate constant. This effect is especially important (of a factor of 15) for polymer electrolytes. In the latter case, the reaction rate is actually controlled by the fluctuations. Implications of these findings for basic and applied electrochemistry are briefly discussed.