Covalently Modified Electrode with Bismuth Nanoparticles Encapsulated in Ultrathin Porous Organic Polymer Linked by Amine Bonding for Efficient CO₂ Electroreduction

Bismuth-based materials in electrocatalytic CO₂ reduction (CO₂RR) usually face the problem of high overpotential. We first show a covalently modified electrode with Bi nanoparticles encapsulated in ultrathin porous organic polymer nanosheets (POPs) with amine linkages to effectively reduce the overpotential for the CO₂-to-formate conversion, which exhibits a high formate Faradaic efficiency (FE_HCOO-) of 98.5% and a partial current density up to 148.7 mA cm^-2 at -0.85 V in comparison with that of a bare bismuth electrode with a FE_HCOO- of 85% at -1.15 V (versus a reversible hydrogen electrode). Different from the reaction mechanism with *CO₂^•- radicals as the intermediate over bare Bi sites, in situ spectroscopic studies and density functional theory calculations reveal that the abundant amine linkages in the POPs backbone provide chemisorption sites to interact with enriched CO₂ molecules to form carbamates (*[-NCOO-]) intermediates with a low reaction barrier of 0.064 eV, which significantly reduces the free energy for the conversion process of CO₂ to formate. Moreover, the modified amine linkages promote water dissociation and the subsequent protonation reaction on the Bi surface with a reduced dissociation energy of -0.31 eV than that on the bare Bi surface of 0.11 eV. This work not only delivers a new mechanism for the CO₂-to-formate conversion but also offers a clean platform to investigate the influence of covalently modification.