Cation-controlled diffusion of chloride ions during electrochemical chlorine evolution in acidic media

Impurity ions pose a major challenge towards diversifying water usage for electrolysis. In particular, millimolar-level chloride impurities remaining in reverse osmosis filtrates significantly diminish the selectivity and longevity of water electrolyzers. Here, we show that alkali metal cations can modulate the diffusion coefficient of chloride ions, enabling suppression of chlorine evolution during water electrolysis at diffusion-limiting conditions. Evidence for the cation-dependent diffusion coefficient is provided by non-zero intercepts in both Levich and modified Koutecký−Levich plots using a rotating ring disk electrode, indicating the presence of an additional, cation-dependent diffusion layer that suppresses chloride diffusion. Numerical simulations based on the double diffusion model quantify this effect, resulting in a linear correlation between the cation-dependent diffusion barrier and the structural entropy of cation hydration. These findings suggest that the cation-dependent structuring of water significantly influences mass transport, which is particularly important at practical current densities where impurity ions are diffusion-limited.