Nitrogen-doped carbon nanofibers confined bismuth oxide nanocrystals boost high single-pass CO2-to-formate conversion in large area membrane electrode assembly electrolyzers

The Faradaic efficiency (FE) of CO2 electroreduction to formate has been close to 100% by developing efficient catalysts and optimizing the structure of electrolyzers, however, the single-pass conversion rate (SPCR) of CO2- to-formate is generally less than 10% (relative to the volume of CO2 feeding). Herein, the small-sized Bi2O3 nanocrystals (-20 nm) were grown uniformly on the surface of N-doped carbon nanofibers (N-CNFs) by virtue of the confinement of N-CNFs supports. Bi2O3/N-CNFs achieve a formate partial current density (jHCOO -) of-540 mA cm-2 with a formate FE of-93% in a flow cell and a long-term stability of 100 h with a current density of-210 mA cm-2 in a 3 x 3-cm2 MEA (membrane electrode assembly) cell. In addition, a high SPCR of CO2-to-formate of 33.4% with a reaction current of-12.7 A (-160 mA cm-2) is demonstrated in a 9 x 9-cm2 MEA cell, which promotes the formate yield rate to 0.18 M h-1. DFT calculations reveal that the electronic interaction between N-doped carbon layers and Bi2O3 endows the Bi sites with an electron-rich environment, which en-hances the adsorption capacity of CO2*, optimizes the binding strength of OCHO*, and accelerates the charge transfer kinetics.