for DvH H2ase H2 evolution on the mesoITO electrode. By iteration of the solution properties in the FEM, the system could be optimised to give the highest predicted current densities, taking into consideration the contrasting effects of MES concentration on enzyme activity (Supporting Information, Enzyme Current Density) and buffer capacity for an optimised local pH. The optimal solution determined by the FEM was a MES concentration of 240 mM and a bulk pH of 4.24, which predicted a resulting local pH of 5.94, closely matching the optimum (pH 6) for HER with DvH H2ase, and a simulated current density of 4.4 mA cm -2 .