Effect of Cations and Anions in the Electrolyte on CO2 electroreduction over Cu Catalysts

Abstract

CO2 electroreduction has been reported as a promising method to reduce the CO2 level in the atmosphere as well as producing valuable products. Aqueous electrolytes have been reported as the most common solutions for CO2ER due to their low cost, abundance and eco-friendliness. However, CO2 electroreduction in aqueous electrolytes is not efficient enough due to the presence of the competing hydrogen evolution reaction (HER). Among techniques to improve the catalytic performance in CO2ER, electrolyte design has attracted considerable attention. Electrolyte additives such as salts or ionic liquids (ILs) can alter the surface CO2 concentration or the intermediates stability on the surface. In this study, the effect of cations and anions of additives on CO2electroreduction over copper in aqueous electrolytes has been investigated.Buffer electrolytes containing 10 mM ofsodium (Na+) or 1-butyl-3-methylimidazolium [BMIM]+ as cation and bis(trifluoromethylsulfonyl)imide [NTF2]- or dicyanamide [DCA]- as anion were added to 0.1 M KHCO3. Electrochemical impedance spectroscopy (EIS) showed that both cations and anions impact the solution resistance and charge transfer resistance. Comparing to [BMIM]+ containing electrolytes, Na-based salts showed a lower solution resistance due to the small size of Na+ions and their high conductivity. However, the charge transfer resistance is more impacted by the anion nature. DCA salts showed a lower charge transfer resistance compared to NTF2salts. The selectivity results showed that NTF2-based electrolytes showed a high faradaic efficiency (FE%) for formate probably due to high hydrophobicity and CO2 absorption capacity of the NTF2-salts. Maximum FE (38.7%) for formate was observed for [BMIM][NTF2]. In addition to the anion effect, this can be due to the interactions of [BMIM]+ with CO2 molecules and stabilizing the intermediates on the surface. In contrast, [DCA]-based salts showed a high FE for hydrogen and a very low FE for hydrocarbons even at high overpotentials. We attributed this observation to the strongly adsorption of [DCA]- on the surface. X-ray photoelectron spectroscopy (XPS) also confirmed the strong adsorption of the [DCA]- anions on the surface. Strongly adsorbed DCA ions on the surface can promote hydrogen evolution reaction, destabilize the intermediates and suppress CO2ER. In-situ electrochemical quartz crystal microbalance (EQCM) also showed a higher mass loss for DCA-salts probably due to the displacement of water molecules by ions. This study showed that additives can significantly alter the selectivity and activity in CO2ER in aqueous electrolytes.