In contrast, the performance of IO-TiO2|FDH (100 pmol) electrodes depends on the CO2 hydration equilibrium through both the local H + (or pH) and CO2 concentration, as CO2 (not HCO3 -) is also consumed during catalysis (Fig. ) . Co-immobilisation of CA increased current densities from of 95% and TOFFDH of 6.2×10 4 h -1 (Fig. ). When FDH reduces CO2 to formate, it is thermodynamically favourable for CA to buffer the local pH increase by converting CO2 and H2O into HCO3 -and H + . Despite a corresponding reduction in FDH activity from a lower local CO2 concentration (from 24.5 mM to 2.9 mM, Fig. ), it is still above its Michaelis-Menten constant (KM = 0.42 mM) , and as such this factor is outweighed by an improved local pH (from 8.74 to 7.76, Fig. and Fig. ) closer to the enzyme's optimum (pH 7.1) .