From the FEM evaluation of the local electrode environment, it is apparent that the local pH is significantly reduced at the electrode with electrolyte BC-MES and BC-MOPS compared to BC (Figure ). The highest buffer capacity at the electrode (BC-MOPS) again causes the lowest local pH change (Figure ). A pH of 7.1 has been shown in solution assays to be the optimum pH of the FDH for CO2 reduction, and therefore increasing the pH beyond this optimum decreases enzyme activity (Figure ). Whilst the increased initial buffer capacity of BC-MES improves upon the BC system, the higher pKa of MOPS enables a high buffer The experimental data shows continued functionality of the FDH in electrolyte BC despite the local pH increase to 8.4 at which the solution assay suggests <20% activity. However the CO2 concentration would have changed dramatically above pH 7.5 in the assay as the addition of 50 mM of NaHCO3 was used to provide CO2, rather than saturated CO2. Therefore, the enzyme may in fact function better at a higher bulk pH than the assay suggests, as supported by the slightly lower predicted current densities from the FEM model in electrolyte BC. The influence of low CO2 concentration as opposed to the pH effect on the solution assay is further corroborated by the continued oxidation activity of the enzyme at high pH. These results further demonstrate the importance of considering the effect of pH on CO2 when designing experiments and suggest that solution assays may be more accurate with saturated CO2.