Bi/ZIF-8 catalysts: the important role of ZIF-8 for enhanced electrochemical N2-to-NH3 conversion using a neutral electrolyte

Abstract

Low NH₃ yield and faradaic efficiency (FE) are the main technical bottlenecks of the electrocatalytic nitrogen reduction reaction (NRR) to synthesize ammonia. Herein, Bi/ZIF-8 series catalysts were successfully designed and synthesized through the polyol reduction method. The NH₃ yield and FE of Bi/ZIF-8 catalysts were found to be greatly improved. Among them, the 8%Bi/ZIF-8 catalyst obtained the highest NH₃ yield (34.53 μg h⁻¹ mg_cat.⁻¹) and FE (23.27%), which were better than those of most of the reported Bi-based NRR catalysts. Experiments and density functional theory (DFT) calculations revealed the following important roles of ZIF-8 on the enhanced NRR performance of Bi/ZIF-8 catalysts: (I) the high specific surface area of ZIF-8 promoted the high dispersion of Bi active sites, thereby exposing more Bi sites; (II) the abundant and ordered porous structure of ZIF-8 facilitated the mass transfer diffusion of N₂-/N-related intermediates; (III) ZIF-8 facilitated full contact of the highly dispersed Bi with highly exposed Zn, resulting in more charge transfer from Zn to Bi. This effective charge transfer not only improved the adsorption capacity of Bi/ZIF-8 catalysts for N₂ but also enhanced the p-electron feedback ability of Bi active sites and improved the Bi 6p-N₂ π* interactions, thereby promoting the transfer of more occupied Bi 6p orbital electrons to π* of N₂, which weakened the strength of NN bonds and reduced the rate-determining step (RDS) (*NN → *NN–H, 0.16 eV); and (IV) the introduction of ZIF-8 appropriately improved the charge transfer efficiency and increased the hydrogen adsorption free energy (ΔGH*: 0.35 eV), which effectively suppressed the hydrogen evolution reaction (HER).