Effect of Electrolyte Composition on CO2 electroreduction over Cu Electrodes
Authors: Sharifi Golru Samaneh , Biddinger Elizabeth J. 
Published: July 2021

CO2 electroreduction (CO2ER) is one of the most promising methods to convert waste CO2 to valuable chemicals such as fomate, CO, and hydrocarbons. Cu is reported as the only catalyst for producing energy-intensive products such as hydrocarbons due to its optimum binding energy towards the intermediates. However, the presence of the parasitic hydrogen evolution reaction (HER), the requirement for high overpotentials and poor product selectivity have made CO2ER on copper inefficient. Numerous strategies including altering catalyst morphology, electrolyte, and cell design, have been proposed to control the selectivity and enhance the catalytic activity. Electrolytes have been shown to play an important role in CO2ER. Electrolytes can influence the selectivity and activity by changing the CO2 solubility, CO2 concentration at the surface, pH, conductivity, and viscosity. Using additives in the electrolyte not only can alter the physical/chemical properties of the electrolyte, but it can also influence the reaction intermediate adsorption and stability behavior. The performance of the salt additives in CO2 electroreduction is highly dependent on the properties of cation-anion pairs such as CO2 absorption capacity, hydrophilicity, functional group, and size. In this study, the electrochemical reduction of CO2 and hydrogen evolution reaction on copper in a buffer solution containing salt additives has been investigated.
Aqueous electrolytes containing 0.1 M KHCO3 and 10 mM of different salts with sodium (Na+) or 1-butyl-3-methylimidazolium [BMIM]+ as cation and bis(trifluoromethylsulfonyl)imide [NTF2]- or dicyanamide [DCA]-as anion were used. Electrochemical impedance spectroscopy (EIS) showed that both cations and anions significantly impact the charge transfer resistance. [DCA]--based salts showed a smaller charge transfer resistance compared to [NTF2]--based salts. This observation can be due to the smaller size and stronger adsorption ability of the [DCA]-ions. The effect of cation on charge transfer resistance was different in [NTF2]- and [DCA]- salts. [BMIM][NTF2] had a higher charge transfer resistance compared to Na[NTF2]; however, [BMIM][DCA] and Na[DCA] showed a relatively similar charge transfer resistance. This observation can show that in [DCA]--based salts, the interface is mainly impacted by the anion rather than cation. Electrochemical quartz crystal microbalance (EQCM) in N2-saturated electrolytes also showed that a mass loss was observed in the interface by applying a negative potential (-0.92 V vs. RHE). This mass loss is likely due to the substitution of water molecules by ions on the copper surface. The maximum mass loss was observed for [BMIM][NTF2] likely due to its large cation and anion which can cause more water molecules to leave the surface. Regarding the product selectivity, the results showed that the presence of [BMIM]+ cations in the salt can enhance formate formation in CO2 electroreduction. In particular, [BMIM][NTF2] and [BMIM][DCA] had a 115% and 100% increase in faradaic efficiency (FE) for formate at -1.02 V vs RHE compared to Na+ salts. This can be attributed to the acidic hydrogen at C2position which can stabilize CO2
.- intermediates and promote CO2ER. However, in [NTF2]- -based salts, we observed that the FE% for C2products are higher in Na+ cations compared to [BMIM]+ cations. This shows that the presence of bulky cations decreases the amount of available active sites and also prevents the approach of CO2radical intermediates which is required to form C2products. Regarding the effect of anion, the presence of [DCA]- anions in the electrolyte significantly increased hydrogen formation and decreased CO2ER regardless of the type of cation. X-ray photoelectron spectroscopy (XPS) results and also discoloration of the electrodes after running the experiments in [DCA]--based salts showed the strong adsorption of the [DCA]- anions on the surface. It has been previously reported that the anions which are strongly adsorbed on the surface can promote hydrogen evolution reaction, destabilize the intermediates and suppress CO2ER. Moreover, [DCA]- is a highly hydrophilic anion. Therefore, they can bring more water molecules to the surface. This study showed that how ions in the electrolyte can determine the selectivity in CO2 electroreduction by manipulating the interfacial structure.