Effect and Prevention of Trace Ag⁺ Contamination from Ag/AgCl Reference Electrodes on CO₂ Reduction Product Distributions at Polycrystalline Copper Electrodes

The electrochemical reduction of CO2 at Cu electrodes is a promising way of converting an industrial waste product, CO2, into value-added products. However, trace metal contamination has a substantial impact on product distribution—both on the measured distribution and the variance in measured distributions from experiment to experiment. In this study, we quantify the effect of Ag+ contamination, a pervasive trace metal contaminant from commonly used Ag/AgCl/KCl(sat.) single-junction reference electrodes, on product distributions for the CO2 reduction reaction at Cu electrodes. We show that trace amounts of leaked Ag+ from the reference electrode deposit on Cu electrodes during the CO2 reduction reaction, resulting in increased production of CO compared to other products. The Cu electrode is extremely sensitive to Ag+ contamination, and only 10 ppb of Ag+ ions (equivalent to 92.7 nM) is enough to measurably change the CO2 reduction product distribution. We propose that using a double-junction reference electrode is a simple way to mitigate this contamination by effectively decreasing the rate of Ag+ leakage.