Mean-field microkinetic modeling (MKM) was performed to estimate the current densities as a function of the potential and ion concentrations at OHP. We considered a differential reactor model and used the PyMKM solver including the effect of potential and pH for estimation of current densities. The Gibbs free reaction and activation energy equations under CHE approximation for H 3 O + and water as proton sources were derived from literature (Supporting Information Section 1.1.1) . Current densities from MKM were used to calculate mass flux boundary conditions at OHP for a transport model implemented in COMSOL Multiphysics 6.0. More details regarding the elementary steps and specific reactor equations can be found in the Supporting Information Section 1.1 .