On the role of electrolyte flow in Cu-catalyzed CO2 and CO electroreduction

The selectivity of CO2 and CO electroreduction reaction (CO2RR and CORR) is highly sensitive to changes in the local microenvironment at the electrode-electrolyte interface. This local microenvironment can be modulated by the continuous flow of the electrolyte, which renews the diffusion layers. Here, we explore the effect of electrolyte flow on Cu-catalyzed CO2RR and CORR in H-cells, flow cells, and membrane electrode assemblies (MEAs). We find that flowing electrolytes can halve the Faradaic efficiency (FE) toward multi-carbon (C2+) products in CO2RR compared to static electrolysis in H-cells. This phenomenon also exists in CORR using near-neutral electrolytes. Further kinetic analysis, operando Raman studies, and finite-element modeling (FEM) unravel that the FE reduction caused by the electrolyte flow in H-cells arises from the decrease in local pH due to the renewal of the diffusion layer, rather than the variation in reactant availability and rate-determining steps (RDS). Lowering the local pH favors the formation of H-2, CO, and formate, reducing the probability of carbon-carbon coupling. The issue can be mitigated by high current densities and catholyte-free structures that increase the OH- concentration at the cathode. Therefore, little to no FE differences caused by increasing the electrolyte flow rate are seen in flow cells and MEAs.