Altering the selectivity of galvanostatic CO2 reduction on Cu cathodes by periodic cyclic voltammetry and potentiostatic steps

•Frequent stripping CV reduced CO2 reduction activity, contrary to previous literature.•Suppression of CO2 reduction is due to partial oxidation of adsorbed intermediates.•Short steps at mild cathodic potentials switched the selectivity from CH4 to CO.•The surface coverage of CO increased at the expense of H2 during the short steps.•Multiple surface coverages of adsorbed H and CO is proposed.

Analysis of the product selectivity and potential of Cu cathodes during galvanostatic CO2 reduction is reported. Initially, it is found that clean Cu cathodes are more selective for hydrogen evolution, but as the Cu is slowly poisoned by CO2 reduction products (most likely carbon) the cathode potential becomes more negative, which in turn drives the formation of CH4, C2H4 and CO. As the accumulation of surface poisons continues, the selectivity towards CH4 and C2H4 begins to decrease due to the loss in neighbouring reaction sites that support the hydrogenation of COads by Hads. In an attempt to avoid these changes in product selectivity, periodic cyclic voltammetry and potentiostatic steps were used throughout extended periods of galvanostatic CO2 reduction. Contrary to previous literature, it is demonstrated that temporarily interrupting galvanostatic CO2 reduction with short periods at potentials between −0.5 and −0.1 V vs Ag|AgCl suppresses the formation of CH4, CO and C2H4. It is proposed that this is due to the partial removal or oxidation of adsorbed CO2 reduction intermediates and that this “clean” cathode surface is more active for the hydrogen evolution reaction. However, when brief potentiostatic steps (84 and 200 s) were conducted at more negative potentials (−1.2 V vs Ag|AgCl), the CO2 reduction selectivity could be switched from CH4 to CO, and maintained for at least 2 hours. This change in selectivity is proposed to be caused by an increase in the surface coverage of COads (at the expense of Hads) during the brief −1.2 V steps, which then enables the Cu cathode to switch between multiple steady-state surface coverages when the cathodic current is re-applied.

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