Hydrolysis of Electrolyte Cations Enhances the Electrochemical Reduction of CO₂ over Ag and Cu

Abstract

Electrolyte cation size is known to influence the electrochemical reduction of CO over metals; however, a satisfactory explanation for this phenomenon has not been developed. We report here that these effects can be attributed to a previously unrecognized consequence of cation hydrolysis occurring in the vicinity of the cathode. With increasing cation size, the pK for cation hydrolysis decreases and is sufficiently low for hydrated K, Rb, and Cs to serve as buffering agents. Buffering lowers the pH near the cathode, leading to an increase in the local concentration of dissolved CO. The consequences of these changes are an increase in cathode activity, a decrease in Faradaic efficiencies for H and CH, and an increase in Faradaic efficiencies for CO, CH, and CHOH, in full agreement with experimental observations for CO reduction over Ag and Cu.

CO2 Reduction: From the Electrochemical to Photochemical Approach

“…It was not until very recently that their roles in CO 2 RR received careful investigations. Based on their experimental observation, Bell and co‐workers proposed that the p K a for cation hydrolysis decreased from Li + to Cs + in their bicarbonate electrolyte, and larger cations such as K + , Rb + , and Cs + had p K a values sufficiently low that they could act as buffering agents and lower the local pH near the cathode, leading to increased ethylene selectivity and lowered Faradaic efficiency for H 2 on electropolished Cu foils 60. The CO 2 RR performance of Cu can also be tuned with the addition of halide anions to the electrolyte.…”

Promoter Effects of Alkali Metal Cations on the Electrochemical Reduction of Carbon Dioxide

“…A number of studies have shown that one way of influencing the product distribution is through changes in the electrolyte cation. [8][9][10][11][12][13] The role of cations is particularly interesting because a number of studies have shown that both the activity and selectivity of Ag, Hg, and Cu for the CO 2 reduction reaction (CO 2 RR) are influenced significantly by the size of the alkali metal cation in the electrolyte. [8][9][10][11][12][13] Similar effects have also been observed for the electrochemical reduction of oxygen, the oxidation of hydrogen, and the oxidation of low molecular weight alcohols.…”

“…[8][9][10][11][12][13] The role of cations is particularly interesting because a number of studies have shown that both the activity and selectivity of Ag, Hg, and Cu for the CO 2 reduction reaction (CO 2 RR) are influenced significantly by the size of the alkali metal cation in the electrolyte. [8][9][10][11][12][13] Similar effects have also been observed for the electrochemical reduction of oxygen, the oxidation of hydrogen, and the oxidation of low molecular weight alcohols. [14][15][16][17][18] For Ag and Hg, which are selective for CO 2 reduction to carbon monoxide (CO) and formate anions (HCOO -), respectively, increasing the alkali metal cation size increases the rates of formation of these products.…”

“…8,10,11 For Cu, increasing alkali metal size leads to higher selectivities to C 2 products, e.g., ethylene and ethanol. 9,12,13 Explanations for these phenomena have been attributed to differences in local pH, or differences in kinetic overpotentials due to the electrochemical potential in the outer Helmholtz plane being affected by cation size. 8,10,11,13 4…”

“…9,12,13 Explanations for these phenomena have been attributed to differences in local pH, or differences in kinetic overpotentials due to the electrochemical potential in the outer Helmholtz plane being affected by cation size. 8,10,11,13 4…”