Therefore, the current density increased to -8.0 mA cm -2 , producing 83 μmol cm -2 of H2 at a FE of 92% with a H2ase-based turnover number (TON) for H2 of 3.2×10 5 and turnover frequency (TOF) of 9 s -1 . The TON and TOF was calculated from the product quantification and the enzyme loading, and since the number of active enzymes in direct electron transfer with the surface is unknown, the TON and TOF of these systems therefore represent the lower limits of the actual values. Due to the significantly higher current densities obtained for this system, even greater bubble effects are expected, which can be observed in the decrease in current density over the one hour-long experiment. The stability of CO2 reduction by the mesoITO|FDH52 electrode, with the same loading (52 pmol), was demonstrated over 10 hours producing 22 μmol cm -2 h -1 of formate with a FE of 103% (Figure ). As with H2ase, the increased mass transport of the macroporous IO structure can be observed by the lower current density of IO-TiO2|FDH52 at -0.6 V vs. SHE. On increasing the local proton consumption by again doubling the enzyme loading (104 pmol -IO-TiO2|FDH104) and applying a more reductive potential (-0.8 V vs. SHE), the local pH increases, raising the current density to -3.0 mA cm -2 . However, the local pH then increases beyond the optimum due to the significantly higher current densities, therefore the bulk pH must again be lowered to 4.59 to further increase the enzyme activity. As the increase in local pH is slower in the less-confined IO structure, the local pH change can be observed in the chronoamperometry as an initial increase in current density, peaking at 1.2 h. The final electrolyte solution demonstrates current densities of -3.6 mA cm -2 , producing 0.55 ± 0.043 mmol cm -2 of formate (55 ± 4.3 μmol cm -2 h -1 ) at a FE of (96 ± 8.7)% with a FDH-based TON for formate of 1.0×10 and TOF of 28 s -1 . Thus demonstrating the significant 18-fold improvement that can be achieved compared to previously reported systems (IO-TiO2|FDH38, 38 pmol, 0.2 mA cm -2 ) by simple adjustment of the experimental parameters.