Unlocking high-current-density nitrate reduction and formaldehyde oxidation synergy for scalable ammonia production and fixation

Abstract

Nitrate electroreduction to ammonia holds great promise in sustainable green ammonia synthesis, yet faces the dearth of competent electrocatalysts adapting varying nitrate concentrations, and the inadequate ammonia fixation. Herein, we present a high-performance Ag single-atom-decorated Cu2O nanowires catalyst (Ag1@Cu2O) that exhibits concentration-universal high-rate nitrate reduction, achieving >90% to near-unity ammonia Faradaic efficiency (FE) across nitrate concentrations from 0.01 to 0.5 M. Notably, at 0.5 M nitrate concentration, it attains a two-ampere-level current density (2.3 A cm−2) at ‒1 V vs. RHE, resulting in a leading ammonia yield rate of 184.4 mgNH3 h−1 cm−2. In-situ studies combined with theoretic calculations elucidate an Ag-Cu inter-site synergistic catalytic mechanism, in which single-atom Ag serves as an accelerator for active hydrogen generation and stabilization on Cu sites to boost the hydrogenation kinetics of N-containing intermediates, thus smoothing the energy barriers for ammonia production via the favorable *NHO pathway. Additionally, Ag1@Cu2O demonstrates near-unity formate FE for formaldehyde oxidation, reaching a 300 mA cm−2 current density at merely 0.31 V vs. RHE. Motivated by this exceptional bifunctionality, we demonstrate an innovative tandem electrochemical-chemical strategy for upgrading ammonia into high-value ammonium formate by coupling electrolysis of nitrate reduction and formaldehyde oxidation, followed by straightforward chemical combination and isolation. Practice in a membrane electrode assembly (MEA) electrolysis at 1.6 V for 100 h successfully outputs 10.7 g of high-purity ammonium formate. Furthermore, the commonality of this strategy is validated by application to various nitrate/aldehyde pairs. This work blazes a new trail for scalable, cost- and energy-efficient green ammonia production and fixation from nitrate reduction.