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 a dearth of competent electrocatalysts adapted to varying nitrate concentration, and inadequate ammonia fixation. Herein, we present a high-performance Ag single-atom-decorated Cu₂O nanowire 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 mg_NH3 h⁻¹ cm⁻². In situ studies combined with theoretical 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 a mere 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 coupled electrolysis of nitrate reduction and formaldehyde oxidation, followed by straightforward chemical combination and isolation. In practice, 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.