The electrochemical reduction of carbon dioxide (CO₂) may provide a future sustainable and profitable way to utilize the most abundant greenhouse gas to produce high-value chemicals and fuels. Recently, the application of gas-diffusion electrodes (GDE) for CO₂ reduction reaction (CO₂RR) has rapidly increased and can boost the maximum reaction rate of CO₂RR by over ten-fold, surmounting the mass transport limitations and concentration polarizations, which was previously a result of low CO₂ availability at the surface of the catalyst in H-cells.

Understanding electrochemical CO₂ reduction and catalytic activity in gas-diffusion systems is complicated by the large number of interconnected factors including the catalytic material and composition, active surface area, mass transport of reagents and products and the choice of electrolyte. By using controlled copper nanostructures we show that despite differences in CO₂ reduction activity, selectivity and working electrode potential on copper catalysts at low current densities, experimental performance is nearly identical in 1 M KOH, KHCO₃ and KCl at current densities of 300 mA/cm². Experimentally, we demonstrate ethylene selectivities between 50-55% in all three electrolytes at local-pH-corrected working potentials of -0.9 to  1.0 V vs a reversible hydrogen electrode. By modeling both gas-phase and liquid phase transport of reagents and products in a gas-diffusion electrode, we find that the local reaction environment for all electrolytes converges, explaining the relative trends in performance at both low and high current densities. Further, using experimental selectivity and gas-channel concentrations as an input for the upgraded model, we determine that the local CO concentration at the catalyst’s surface approaches that of CO₂ due to accumulating concentration of CO in the gas-channel and transport limitations through the gas-diffusion layer. The insights gained from these experimental and modeling results are then used experimentally to show that manipulating local dissolved CO₂ and CO concentrations by varying CO₂ flow rates can almost linearly change the Faradaic efficiency of oxygenate production at 200 mA/cm². More importantly, these findings suggest that newly-considered operational performance metrics, such as CO₂ single-pass conversion efficiencies, will come to play an extremely important role in C₂/C₃ product selectivities on copper-based catalysts by substantially changing the ratio of CO₂/CO at a catalyst’s surface.