Electronic Structure Modulated by B-Doped Cu Promotes Electrocatalytic Nitrate Reduction for Ammonia Production

Abstract

Electrocatalytic nitrate reduction for ammonia (eNIRR) is an ammonia production process that simultaneously removes nitrate contaminants from water. However, the lack of activity of cathode catalysts used as eNIRR catalysts is the main limiting factor for its development. Motivated by this fact, born-doped copper (BDCu) was obtained by using ZnO, which was easily removed at high temperature, as a dispersant, combined with weakly reducing boron clusters (closo-[B₁₂H₁₂]^2–) as a reducing agent and B source during high-temperature pyrolysis. Impressively, BDCu demonstrated a Faradaic efficiency of 96.58% and a yield rate of 25741.51 μg h^–1 mg_cat^–1 toward ammonia production at −1.8 V (vs saturated calomel electrode). The ammonia yield rate of BDCu was twice as high as in the case of undoped B. Evolutionary behavior of NO₃^– to NH₃ conversion detected by in situ Fourier-transform infrared (in situ FT-IR) and electrochemical in situ mass spectrometry (in situ DEMS). Experimental and density functional theory (DFT) calculations explained that the activation of water was enhanced by B-doped Cu, and the adsorption of proton *H was weakened, which made it easy for *H to migrate away from the catalyst to NO₃^– as a proton required for NO₃^– reduction. In addition, the electron-deficient of B provides conditions for electron transfer between B and Cu. The electron transfer from Cu to B in BDCu led to a decrease in the center of the d-band of Cu, which modulated the electronic properties of Cu and altered the behavior of the NO₃^– to NH₃ transition on the Cu surface. Compared with Cu undoped B as well as unreduced CuO, BDCu lowered the energy barrier of the rate-determining step (*NO → *N), allowing for a smoother conversion of NO₃^– to NH₃. This study provides a strategy to change the electronic structure of transition metals by B-modification and thus improve the performance of ammonia synthesis.