Without CA, the interconversion of CO2 and HCO3 -is slow, and the depleted CO2 and H + are predominantly replenished by diffusion from the bulk solution, as opposed to the equilibration of CO2 with HCO3 -. When CA is present, the rate of CO2 and HCO3 -interconversion is increased, allowing the CO2 and HCO3 -concentrations to follow the chemical equilibria in solution. The more basic pH formed within the electrode favors the conversion of CO2 to H + and HCO3 -, in competition with the depletion of CO2 favoring the reverse reaction. Although the decreased local pH marginally increases FDh activity (Figure ), the additional consumption of CO2 required to mitigate the local pH change decreases enzymatic CO2R (Figure ). These effects on enzyme activity counteract one another and are responsible for the lack of improved rates of fuel production with increased CO2 hydration kinetics. FEM explains why CA does not improve formate production in these solutions as CO2 acts as both the substrate and primary buffer, highlighting the trade-off in maintaining an optimal local pH and high substrate levels under reduced CO2 concentrations.