The effect of electrolyte composition on the electroreduction of CO2 to CO on Ag based gas diffusion electrodes

Abstract

The electroreduction of CO₂ to C₁–C₂ chemicals can be a potential strategy for utilizing CO₂ as a carbon feedstock. In this work, we investigate the effect of electrolytes on the electroreduction of CO₂ to CO on Ag based gas diffusion electrodes. Electrolyte concentration was found to play a major role in the process for the electrolytes (KOH, KCl, and KHCO₃) studied here. Several fold improvements in partial current densities of CO (j_CO) were observed on moving from 0.5 M to 3.0 M electrolyte solution independent of the nature of the anion. j_CO values as high as 440 mA cm⁻² with an energy efficiency (EE) of ≈ 42% and 230 mA cm⁻² with EE ≈ 54% were observed when using 3.0 M KOH. Electrochemical impedance spectroscopy showed that both the charge transfer resistance (R_ct) and the cell resistance (R_cell) decreased on moving from a 0.5 M to a 3.0 M KOH electrolyte. Anions were found to play an important role with respect to reducing the onset potential of CO in the order OH⁻ (−0.13 V vs. RHE) < HCO₃⁻ (−0.46 V vs. RHE) < Cl⁻ (−0.60 V vs. RHE). A decrease in R_ct upon increasing electrolyte concentration and the effect of anions on the cathode can be explained by an interplay of different interactions in the electrical double layer that can either stabilize or destabilize the rate limiting CO₂˙⁻ radical. EMIM based ionic liquids and 1 : 2 choline Cl urea based deep eutectic solvents (DESs) have been used for CO₂ capture but exhibit low conductivity. Here, we investigate if the addition of KCl to such solutions can improve conductivity and hence j_CO. Electrolytes containing KCl in combination with EMIM Cl, choline Cl, or DESs showed a two to three fold improvement in j_CO in comparison to those without KCl. Using such mixtures can be a strategy for integrating the process of CO₂ capture with CO₂ conversion.