The finite element model was constructed in COMSOL 6.1, based on mathematical descriptions of thermodynamics, enzyme kinetics and mass transport for H2ase, FDH and CA in IO-TiO2 electrodes. A three-domain model accounting for bulk solution, electrode diffusion layer and IO-TiO2 was used to predict trends in the current densities, local pH, and local CO2 concentrations of the systems (Detailed information in Supplementary Notes Sections 2-4).