Controlling the selectivity of CO2 electroreduction on copper: The effect of the electrolyte concentration and the importance of the local pH

• The buffer capacity of the electrolyte determines the activity and selectivity of Cu.
• Our observations are attributed to a different local pH in different electrolytes.
• H2 and CH4 formation rate is improved when the local pH close to neutral.
• Ethylene's formation rate shows Tafel relation independently of local pH.

In the present study we demonstrate that the activity and selectivity of copper during the CO2 electrochemical reduction can be tuned by changing the concentration of the bicarbonate electrolyte. Comparing the absolute formation rate and Faradaic selectivity of H2, CH4, CO, and C2H4 as a function of the applied electrode potential, we show that variations in the bulk buffer capacities of the electrolyte have substantial impact on absolute product formation rates and relative faradic selectivity. We find that high concentrations of bicarbonate improve the overall Faradaic CO2 electroreduction activity, largely due to higher absolute formation rates of H2 and CH4. In lower-concentrated bicarbonate electrolytes with their lower overall activity, the selectivity toward ethylene was drastically enhanced. Following earlier theoretical work, we hypothesize the pH near the copper electrode interface to largely account for the observed effects: diluted KHCO3 solutions allow for more alkaline local pH values during CO2 electroreduction. Our study highlights the controlling role of the interfacial pH on the product distribution during CO2 reduction over a wide electrode potential range.

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