Abstract: Electrochemical CO₂ conversion is one of the most promising technologies to achieve carbon neutrality. However, it still suffers from some nonnegligible challenges on low production rate and unsatisfied current densities for potential large-scale applications. Herein, we prepare ultrasmall Bi nanoparticles uniformly encapsulated in the carbon nanofibers through electrospinning techniques, which is denoted as Bi/CNFs-900. Gratifyingly, this Bi/CNFs-900 catalyst demonstrates excellent performance and stability on CO₂ electro-reduction in a broad potential window. Specifically, it can produce formate with a Faradaic efficiency over 90% and a high partial current density of -235.3 mA cm^-2 at -1.23 V vs. RHE in a flow-cell. Furthermore, the confinement effect of carbon nanofibers largely restricts the severe aggregation of bismuth nanoparticles during synthesis as well as electrolysis procedure, which greatly increases the accessible active sites and decreases the actual mass fraction of bismuth composition. Consequently, Bi/CNFs-900 not only achieves ultrahigh mass activity of -1.6 A mg_Bi^-1, but also possesses an unprecedented formate production rate of 4403.3 μmol h^-1 cm^-2. DFT calculations and in situ Raman spectroscopy further uncover the possible reaction mechanism for CO₂ reduction toward formate. These results could provide an economical and industrial-viable strategy for the preparation of electrocatalysts in CO₂ reduction.

Key words: CO₂ electro-reduction, Bismuth nanoparticles, Carbon nanofibers, Formate, Mass activity