Rational Design of Bi Nanoparticles for Efficient Electrochemical CO₂ Reduction: The Elucidation of Size and Surface Condition Effects

Abstract

We report an efficient electrochemical conversion of CO 2 to CO on surface-activated bismuth nanoparticles (NPs) in acetonitrile (MeCN) under ambient conditions, with the assistance of 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim][OTf]). Through the comparison between electrodeposited Bi films (Bi-ED) and different types of Bi NPs, we, for the first time, demonstrate the effects of catalyst’s size and surface condition on organic phase electrochemical CO 2 reduction. Our study reveals that the surface inhibiting layer (hydrophobic surfactants and Bi 3+ species) formed during the synthesis and purification process hinders the CO 2 reduction, leading to a 20% drop in Faradaic efficiency for CO evolution (FE CO ). Bi particle size showed a significant effect on FE CO when the surface of Bi was air-oxidized, but this effect of size on FE CO became negligible on surface-activated Bi NPs. After the surface activation (hydrazine treatment) that effectively removed the native inhibiting layer, activated 36-nm Bi NPs exhibited an almost-quantitative conversion of CO 2 to CO (96.1% FE CO ), and a mass activity for CO evolution (MA CO ) of 15.6 mA mg –1, which is three-fold higher than the conventional Bi-ED, at −2.0 V (vs Ag/AgCl). This work elucidates the importance of the surface activation for an efficient electrochemical CO 2 conversion on metal NPs and paves the way for understanding the CO 2 electrochemical reduction mechanism in nonaqueous media.