Electrochemical CO2 reduction enables carbon-neutral fuels and chemicals to be produced directly from renewable electricity. Monometallic Cu is the only electrocatalyst capable of reducing CO2 into hydrocarbons and alcohols. Unfortunately, Cu exhibits insufficient activity and selectivity for industrial implementation. Furthermore, CO2 reduction is incredibly susceptible to concentration polarization due to the sluggish rate of diffusion through the hydrodynamic boundary layer. However, these effects have only been systematically studied for electrocatalysts that produce CO, such as Ag and Au. In the present study, we utilize an electrolyte recirculation reactor to systematically investigate the impact of mass transfer on the activity and selectivity of CO2 reduction over Cu for the first time. This reactor enables the mass transfer resistance to be directly controlled by varying the electrolyte flow rate. The resulting mass transfer coefficient is quantified by measuring the diffusion limited rate of ferricyanide reduction. We find that reducing the mass transfer resistance initially improves the generation rate of hydrocarbons, such as ethylene. However, reducing the mass transfer resistance further suppresses the rate of formation of these products, despite an overall increase in the rate of CO2 consumption. In order to understand the origin of this phenomena, we investigated the impact of mass transfer on the activity and selectivity of CO reduction in alkaline media. Under these conditions, the rate of hydrocarbon formation only improves as the mass transfer resistance is reduced, in stark contrast to what was observed during CO2 reduction. We utilize 1D continuum modeling to understand the origin of this discrepancy.
