The tungsten formate dehydrogenase (FDh) from Desulfovibrio vulgaris Hildenborough was chosen as the CO2 reductase for its quantitative selectivity, electrochemical reversibility, nonlinear substrate dependence, and high affinity for CO2 (K M : 0.42 mM), which makes this FDh ideally suited for the direct utilization of low concentration CO2 streams (SI Figure ). FDh can also be co-immobilized with or without CA on mesoporous ITO electrodes (40 nm particle size, 8.5 µm thickness, and 0.19 cm 2 geometric surface area) at lower fuel forming enzyme concentrations than in the natural carboxysome (Table , 9.52 mM vs 0.63 mM, 40 pmol total concentration immobilized in the porous electrode) due to FDh's higher activity, with the nonredox active enzyme CA (40 pmol) at comparable concentrations to the carboxysome, catalyzing CO2 hydration kinetics within the porous electrode. The CO2 purged aqueous solutions used in this study have a total dissolved carbon concentration that is greater than that of CO2 alone due to its equilibration to form HCO3 -and CO3 2-(eq 1), species that cannot directly be utilized as substrates for FDh. Given its commercial availability and fast kinetics, CA from bovine erythrocytes was chosen to accelerate HCO3 -dehydration, allowing for the utilization of the total inorganic carbon pool to offset CO2 depletion within the porous electrode. Enzyme electrochemistry experiments informed the development of finite element models (FEM), which demonstrate how experimentally inaccessible components of the local electrochemical environment (compartmentalized CO2, HCO3 -, and H + concentrations) change with applied potential and electrolyte composition to improve formate production under reduced CO2 conditions.