Tuning mobility of intermediate and electron transfer to enhance electrochemical reduction of nitrate to ammonia on Cu2O/Cu interface

Abstract

• R-Cu 2 O/Cu/CF achieved high ammonia yield rate of 2.17 mg cm -2 h −1 with selectivity of 94.4%. • The faradaic efficiency of electrochemical reduction of nitrate to ammonia (ERNA) was 84.36%. • ERNA was dependent on surface mobility of adsorbed NO 2 - and electron transfer to NO 3 - . • Cu 2 O/Cu interface alleviated NO 2 - adsorption to improve surface diffusion of adsorbed NO 2 - . • Cu 2 O/Cu interface upshifted d band center of Cu to enhance electron transfer to NO 3 - . Compared to traditional haber–bosch process for synthetic ammonia, electrochemical reduction of nitrate to ammonia (ERNA) offers a promising and sustainable technology to generate ammonia at ambient temperature. However, the performance of ERNA is impeded by lacking of sound strategy for designing high-performance electrocatalyst. Here, a Cu based electrode with Cu 2 O/Cu interface was prepared by pulse electrodeposition and electroreduction, achieving the fast rate constant of 0.14 min −1 for reducing nitrate with a high ammonia yield rate of 2.17 mg cm -2 h −1 (faradaic efficiency: 84.36%) and ammonia selectivity of 94.4% at −0.25 V vs. RHE. Particularly, density functional theory (DFT) calculations for the adsorption energy indicated that Cu 2 O/Cu interface alleviated adsorption energy of NO 2 - from −2.02 eV (pure Cu) to −1.59 eV, improving surface diffusion of adsorbed NO 2 - . And DFT calculations for electronic structure implied that Cu 2 O/Cu interface upshifted the d band center of Cu (-2.25 eV vs. −2.48 eV of pure Cu), boosting electron transfer to NO 3 - . Additionally, in-situ infrared spectroscopic analysis confirmed vital intermediates of NO 2 - and NH 2 OH on Cu 2 O/Cu to identify reaction pathway and Gibbs free energy diagram clarified this interface decreased reaction barrier of crucial step (reducing *NO 2 to *NO) from 0.31 eV (pure Cu) to −0.77 eV for the enhancement of ERNA. Thereby, these findings demonstrated that Cu 2 O/Cu interface tunes mobility of NO 2 - and electron transfer to NO 3 - on Cu based electrocatalyst is an efficient method to promote performance of ERNA with high efficiency and rate.