CO2, the substrate for FDH, forms the CO2/HCO 3 & (pK1= 6.34) and HCO 3 & /CO 3 2& (pK2 = 10.32) buffer equilibria when dissolved in H2O (Supporting Information, Equilibria). However, on the relatively short timescales within the diffusion layer at the electrode, the slow kinetics of CO2 hydration result in minimal interconversion of CO2 and HCO 3 & in response to the shifting equilibria. Therefore the CO2 concentration is almost unchanged by differing pH within the diffusion layer. As such the buffering effect of CO2/HCO 3 & , and similarly the formation of aqueous CO2 from HCO 3 & , is limited in its mitigation of the local concentration gradients. In heterogeneous and heterogenised molecular catalyst systems, the poor buffering of the CO2/HCO 3 & equilibrium benefits the system by enhancing the local pH change, which slows down the competing HER, improving selectivity for the CO2RR. However, in enzymatic CO2RR, the HER is negligible due to high selectivity and low overpotential requirements preventing the HER from occurring directly at the metal oxide electrode. Therefore, increasing the buffer capacity would be beneficial, and so an additional, faster-buffering species can be introduced. Three electrolyte solutions were therefore chosen, consisting firstly of a 'bicarbonate only' electrolyte (BC) and then two alternative electrolytes which include an additional 100 mM buffer as follows: BC-MES and BC-MOPS. A pH of 6.54 was again selected as at this pH the buffer capacity of electrolyte BC-MES and BC-MOPS is equal (Figure ), and also the CO2 equilibria species concentrations do not change between all the electrolytes (Figure ).