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

We report an efficient electrochemical conversion of CO2 to CO on surface-activated bismuth nano particles (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 CO2 reduction. Our study reveals that the surface inhibiting layer (hydrophobic surfactants and Bi3+ species) formed during the synthesis and purification process hinders the CO2 reduction, leading to a 20% drop in Faradaic efficiency for CO evolution (FECO). Bi particle size showed a significant effect on FECO when the surface of Bi was air-oxidized, but this effect of size on FECO 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 CO2 to CO (96.1% FECO), 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 CO2 conversion on metal NPs and paves the way for understanding the CO2 electrochemical reduction mechanism in nonaqueous media.