<p>Impurity ions pose a major challenge towards diversifying water sources for electrolysis. In particular, chloride impurities in low-grade water diminish the selectivity and longevity of water electrolysers. Here we demonstrate that alkali cations can regulate chloride diffusion, allowing a marked improvement in the reaction selectivity of water oxidation. Rotating ring-disk electrode measurements exhibit anomalous positive intercepts in the Levich plot, indicating a diffusional barrier that is cation dependent yet independent of rotational speed. To rationalize this barrier, we propose a simple modification to the Levich model, in which the cation-dependent diffusion coefficient is at least two orders of magnitude lower than that of the bulk solution. The potential of maximum entropy and the structural entropy of hydration both indicate that the diffusion barrier increases when the first hydration shell is structurally rigid　(Li⁺ &gt; Na⁺ &gt; H⁺ &gt; K⁺ &gt; Cs⁺). Our findings offer a strategy to suppress impurity-driven side reactions at the high current densities relevant to water electrolysis.</p><p></p>