Comparing the electrocatalytic reduction of CO2 to CO on gold cathodes in batch and continuous flow electrochemical cells

The electrocatalytic reduction of CO2 on gold cathodes was found to differ significantly between a standard batch electrochemical cell and a flow cell incorporating a porous gold cathode. While the well-known influence of KHCO3 concentration on product selectivity was observed in the batch cell, the selectivity of the CO2 reduction reaction was shown to be independent of KHCO3 concentration in the flow cell. The Faradaic efficiency for CO production was found to be 80-90% regardless of the KHCO3 concentration whereas in the batch cell it decreased from 75% to 35% as the KHCO3 concentration is increased from 0.05 to 0.5 mol L-1. The current density was found to be independent of the KHCO3 concentration and similar in both cells (-4 to -10 mA cm(-2) at -1.3 V vs Ag vertical bar AgCl). As the KHCO3 concentration effect is normally attributed to changes in the local pH at the cathode-electrolyte interface brought about by the buffering action of the electrolyte, the results found in this work suggest that pH buffering can be suppressed or manipulated in some cell/electrode configurations. In the flow cell used in this work, it is suggested that poor transport of the KHCO3 through the porous cathode support to the active surface results in higher local pH than that found at the surface of a cathode immersed in a traditional batch cell.