The Importance of Acid-Base Equilibria in Bicarbonate Electrolytes for CO2 Electrochemical Reduction and CO Reoxidation Studied on Au(hkl) Electrodes

Among heterogeneous electrocatalysts, gold comes closest to the ideal reversible electrocatalysis of CO2 electrochemical reduction (CO2RR) to CO and, vice versa, of CO electroxidation to CO2 (COOR). The nature of the electrolyte has proven to crucially affect the electrocatalytic behavior of gold. Herein, we expand the understanding of the effect of the widely employed bicarbonate electrolytes on CO2RR using gold monocrystalline electrodes, detecting the CO evolved during CO2RR by selective anodic oxidation. First, we show that CO2RR to CO is facet dependent and that Au(110) is the most active surface. Additionally, we detect by in situ FTIR measurements the presence of adsorbed COtop only on the Au(110) surface. Second, we highlight the importance of acid-base equilibria for both CO2RR and COOR by varying the electrolyte (partial pressure of CO2 and the concentration of the bicarbonate) and voltammetric parameters. In this way, we identify different regimes of surface pH and bicarbonate speciation, as a function of the current and electrolyte conditions. We reveal the importance of the acid-base bicarbonate/carbonate couple, not only as a buffering equilibrium but also as species involved in the electrochemical reactions under study.

Automated summary

Gold is the closest to the ideal reversible electrocatalyst for CO2 electrochemical reduction, and the nature of the electrolyte crucially affects its behavior. Research has shown that CO2RR to CO is facet-dependent, with Au(110) being the most active surface, and the acid-base equilibria play a significant role in both CO2RR and COOR. The bicarbonate/carbonate couple is not only a buffering equilibrium but also a species involved in the electrochemical reactions studied.