Competition between Initial CO2 Electroreduction and Hydrogen Evolution Reaction on Cu Catalysts in Acidic Media: Role of Specifically Adsorbed Halide Anions

The role of halide anions and competing mechanisms between initial CO 2 electroreduction pathways and hydrogen evolution reaction (HER) are systematically identified at halide anions modified Cu(111)/H 2 O interfaces based on density functional theory calculations in this paper. The present results show that halide anions modified Cu(111)/H 2 O interfaces can notably enhance electroreduction activity of CO 2 into CO. Simultaneously, it is concluded that the specifically adsorbed halide anions modified Cu electrodes can inhibit HER by studying competing HER mechanisms, and thus the enhanced CO 2 electroreduction activity can be ascribed to the suppressed HER. The origin of enhanced CO production activity and inhibited HER is further scrutinized. The present results show that the presence of halide anions can lead to stronger CO adsorption and the increased adsorption strength of CO can explain easier CO production based on the Sabatier principle. Interestingly, the calculated results show that the presence of halide anions does not exert an effect on H adsorption strength, which is regarded as a key descriptor of HER activity, implying that halide anions modified Cu electrodes may be not able to directly lead to the inhibited HER. However, the present results indicate that co-adsorbed CO can weaken adsorption strength between H and Cu electrodes and thus result in inhibited HER and decreased HER activity. The upshift of d-band centers of surface Cu atoms due to modification of halide anions may be a reason for stronger CO adsorption, whereas the downshift of the d-band center due to the presence of co-adsorbed CO can lead to a weakening effect on H adsorption strength. Our present insights into the role of halide anions can aid in designing an optimal electrolyte and developing electrocatalysts that are more selective toward CO 2 electroreduction than HER.