CO₂ 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 CO₂ 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 CO₂ 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 (KHCO₃). Bis(trifluoromethylsulfonyl)imide [NTF₂]^- or dicyanamide [DCA]^- as anion were chosen due to their significantly different hydrophobicity and CO₂ 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 N₂-saturated electrolytes with DCA anions, a current density ~44 mA/cm² (regardless of the cation type) was observed at -1.12 V vs. RHE. By saturating the DCA electrolytes with CO₂, 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, NTF₂-based salts showed a lower HER activity compared to DCA-salts. Among different cations with NTF₂ anion, Na[NTF₂] showed a higher HER compared to [EMIM][NTF₂] and [BMIM][NTF₂]. According to the cyclic voltammetry in N₂-saturated electrolytes with NTF₂ anions, NaNTF₂ showed a current density of ~14 mA/cm² at -1.12 V vs. By saturating the NTF₂ electrolytes with CO₂, the total current density increased, in opposite to DCA-salts. This can show that NTF₂ salts are able to enhance CO2ER. This can be due to their high hydrophobicity and CO₂ absorption capacity. The best performance was observed for [BMIM][NTF₂]. [BMIM][NTF₂] showed a high faradaic efficiency (38.7%) for formate at -0.92 V vs. RHE. Electrochemical impedance spectroscopy (EIS) showed that the charge transfer resistance is more impacted by the anion nature. DCA salts showed a lower charge transfer resistance compared to NTF₂ salts probably due to the enhanced HER which is kinetically faster than CO2ER. Moreover, an inductive loop was observed in EIS for both [EMIM][NTF₂] and [BMIM][NTF₂] additives (not for Na[NTF2]) which can be indicative of the interaction of CO₂ with imidazolium cations at the interface which can facilitate formate formation.