Due to the change in local proton concentration as the catalysis occurs, the pH at the electrode instantly increases (Figure ). The peak activity of proton reduction by DvH [NiFeSe] H2ase in solution assays was shown to be at pH 6, with a reasonably sharp decrease in activity on deviation from the optimum (Figure ). Therefore as the local pH increases away from the optimum, a decrease in the rate of catalysis of the enzyme will occur. However, the change in local proton concentration is reduced when using an electrolyte solution with a high buffer capacity at that local pH (Figure ). Therefore, we observe a lower local pH change in the electrodes for the MOPS buffer (pKa = 7.2) compared with MES (pKa = 6.3) at a local pH of >7.5 (ΔpH = 1.24 and 1.76 for MOPS and MES, respectively). Consequently, the enzyme activity is higher with the MOPS electrolyte solution and therefore higher experimental current densities were observed than using MES solution (Figure ). This can additionally be observed in the decrease in current density between the first and second scan of the initial voltammetry scans (Figure ). The combination of experimentation and computation was therefore able to demonstrate that porous electrodes with confined enzymes exhibit a significant local pH change, thereby affecting the current and product output. Consequently, the bulk pH should not be the optimum pH for the enzyme, but instead should be aligned as to give a local pH at the optimum. It can also be concluded that the pKa of the electrolyte buffer should be matched to the local pH, not the bulk pH. Qualitatively, for DvH H2ase HER the optimum pH is 6 (Figure ), and therefore a lower bulk pH of about 4-5 would likely be appropriate with a buffer pKa of 6 (for example MES).