Modifying Copper Local Environment with Electrolyte Additives to Alter CO2 Electroreduction vs Hydrogen Evolution

Abstract

CO2 electroreduction (CO2ER) by using renewable energy resources is a promising method to mitigate the CO2 level in the atmosphere as well as produce valuable chemicals. Local environment at the electrode–electrolyte interface plays a key role in CO2ER activity and selectivity along with its competing hydrogen evolution reaction (HER). In addition to the catalyst and reactor design, electrolyte also has a significant impact on the interface. Herein, electrolyte additives were used to modify the local environment around the Cu catalyst during CO2ER. For this purpose, 10 mM ionic additives with bis(trifluoromethylsulfonyl)imide ([NTF2]−) and dicyanamide ([DCA]−) as anions and 1-butyl-3-methylimidazolium ([BMIM]+), potassium (K+), or sodium (Na+) as cations have been added to an aqueous potassium bicarbonate solution (0.1 M KHCO3). COMSOL Multiphysics was also used to calculate the local pH and CO2 concentration at the electrode–electrolyte interface in different electrolytes. Results showed that the local environment modifications by the electrolyte additives altered the activity and selectivity of Cu in CO2ER. It was found that the CO2ER activity at −0.92 V was enhanced when using anions with high CO2 affinity and high hydrophobicity, such as [NTF2]−. Among [NTF2]−-based additives, [BMIM][NTF2] had a higher faradaic efficiency (FE) for formate (38.7%) compared to K[NTF2] (23.2%) and Na[NTF2] (18.5%) at −0.92 V likely due to the presence of imidazolium cations that can further stabilize the intermediates on the surface and enhance CO2ER. Electrolytes containing [DCA]−-based additives with high hydrophilicity and low CO2 affinity had a very high HER selectivity (>90% FEH2) and low CO2ER selectivity regardless of the cation nature. This observation is attributed to the presence of hydrophilic [BMIM][DCA] in the vicinity of the catalyst, which impacts the microenvironment around the catalyst. We observed that [DCA]− anions have a high affinity to adsorb on Cu catalysts as soon as the catalyst is submerged in the electrolyte. Although FTIR showed that [DCA]− anions desorb from the surface at negative potentials, it is likely that [DCA]− anions still remain in the proximity of the electrode, next to the adsorbed cations, impacting the transport of H2O and CO2, and altering the product selectivity. COMSOL calculations showed that the local pH is directly proportional to the H2 evolution activity. Also, hydrophilic salts such as those with the [DCA]− anion had a more alkaline local pH, which led to a lower CO2 concentration in the vicinity of the catalyst.