Modeling Study of pH Distribution and Non-Equilibrium State of Water in Hydrogen Evolution Reaction

pH distribution and non-equilibrium state of water in hydrogen evolution reaction are studied using continuum models. In the first model, we analyze the pH distribution in a rotating ring disk electrode system, where the hydrogen evolution occurs on the disk electrode. The model predicts a pH distribution comparable to the experimental data and the nonequilibrium state of water (cH*cOH>1.0 x 10(-14)) in a small portion of the diffusion layer (ca. 5 mu m in thickness) adjacent to the bulk electrolyte under forced convection. The second model explores the pH distribution on an electrode with nanovoids in hydrogen evolution reaction in an acidic media. The value of cH*cOH shifts significantly when close to the electrode surface, e.g., <= 2 mu m, indicating pH is not viable to assess its impact on an electrochemical reaction involving hydroxide ions. Modeling results also prove that, for an electrode with nanovoids, the concentration gradient of hydroxide between the plain field and the bottom of the nanovoid is minimal. Therefore it should not be the root cause for the differential kinetics of metal electrodeposition inside/outside the nanovoids. (C) The Author(s) 2019. Published by ECS.