Influence of the Nature of Ionic Additives in Aqueous Electrolyte on CO2 Electroreduction over Cu Catalysts
Authors: Sharifi Golru Samaneh , Biddinger Elizabeth J. 
Published: July 2022

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.