Influence of the Nature of Ionic Additives in Aqueous Electrolyte on CO 2 Electroreduction over Cu Catalysts

CO 2 electroreduction (CO2ER) has attracted considerable attention due to its promising results in producing valuable chemicals and fuels and mitigating global warming. Among different electrolytes, water-based solutions are the most common electrolytes for CO2ER due to their low cost, abundance, and eco-friendliness. However, the product selectivity and activity in aqueous electrolytes are poor due to their low CO 2 solubility and the presence of the competing hydrogen evolution reaction (HER). Using ionic additives such as inorganic salts and ionic liquids can enhance CO2ER by controlling the water and CO 2 concentration at the interface. Ionic additives can also impact the intermediate stability on the surface. In this study, the effect of additive anion and cation on CO2ER has been studied. A series of salts (10 mM) with different anions and cations were added to 0.1 M potassium bicarbonate (KHCO 3 ). Bis(trifluoromethylsulfonyl)imide [NTF 2 ] - or dicyanamide [DCA] - as anion were chosen due to their significantly different hydrophobicity and CO 2 absorption capacity. Sodium (Na + ), potassium (K + ), 1-ethyl-3-methylimidazolium [EMIM] + , and 1-butyl-3-methylimidazolium [BMIM] + were used as cation. Results showed that the effect of anion is more significant on CO2ER compared to cations. Adding DCA-based salts, regardless of the cation type significantly enhanced HER and suppressed CO2ER. According to the cyclic voltammetry in N 2 -saturated electrolytes with DCA anions, a current density ~44 mA/cm 2 (regardless of the cation type) was observed at -1.12 V vs. RHE. By saturating the DCA electrolytes with CO 2 , the total current density decreased. Regarding the product selectivity, [DCA]-based salts also had a high faradaic efficiency (FE) for hydrogen and a very low FE for hydrocarbons even at high overpotentials. This can be justified by high hydrophilicity and strong adsorption of DCA-salts on the surface. The strong adsorption of DCA-salts was also confirmed by X-ray photoelectron spectroscopy (XPS) and In-situ electrochemical quartz crystal microbalance (EQCM). Strongly adsorbed DCA ions on the surface can promote hydrogen evolution reaction, destabilize the intermediates and suppress CO2ER. On the other hand, NTF 2 -based salts showed a lower HER activity compared to DCA-salts. Among different cations with NTF 2 anion, Na[NTF 2 ] showed a higher HER compared to [EMIM][NTF 2 ] and [BMIM][NTF 2 ]. According to the cyclic voltammetry in N 2 -satura