Sustainable conversion of alkaline nitrate to ammonia at activities greater than 2 A cm-2

Abstract

Nitrate (NO₃^‒) pollution poses significant threats to water quality and global nitrogen cycles. Alkaline electrocatalytic NO₃^‒ reduction reaction (NO₃RR) emerges as an attractive route for enabling NO₃^‒ removal and sustainable ammonia (NH₃) synthesis. However, it suffers from insufficient proton (H⁺) supply in high pH conditions, restricting NO₃^‒-to-NH₃ activity. Herein, we propose a halogen-mediated H⁺ feeding strategy to enhance the alkaline NO₃RR performance. Our platform achieves near-100% NH₃ Faradaic efficiency (pH = 14) with a current density of 2 A cm^-2 and enables an over 99% NO₃^--to-NH₃ conversion efficiency. We also convert NO₃^‒ to high-purity NH₄Cl with near-unity efficiency, suggesting a practical approach to valorizing pollutants into valuable ammonia products. Theoretical simulations and in situ experiments reveal that Cl-coordination endows a shifted d-band center of Pd atoms to construct local H⁺-abundant environments, through arousing dangling O-H water dissociation and fast *H desorption, for *NO intermediate hydrogenation and finally effective NO₃^‒-to-NH₃ conversion.

Fig. 1. Cl mediated H⁺ feeding.

Schematic diagram of Cl mediated H⁺ feeding to boost *NO intermediate hydrogenation and finally achieve efficient NO₃^‒-to-NH₃ conversion in alkaline NO₃RR over Pd-Cl/Cu₂O.

Fig. 2. Theoretical prediction.

a Gibbs free energy change of H⁺ supply over catalysts. b The charge density difference between *H intermediate and catalysts. The isosurface level was 0.005 e·bohr^-3. The yellow and cyan colors represented positive and negative charge regions, respectively. 0.13 e was the electrons transferred from Pd-Cl/Cu₂O to *H, and 0.09 e was the electrons transferred from Pd/Cu₂O to *H. c The local density of states of Pd 3d orbital for the Pd/Cu₂O and Pd-Cl/Cu₂O catalysts. d Gibbs free energy diagram of various intermediates generated during NO₃RR over Pd-Cl/Cu₂O at the potential of −0.6 V vs. RHE for pH = 14.

Fig. 3. Synthesis and structural characterizations of Pd-Cl/Cu₂O.

SEM (a), EDX mapping (b), and AC-HAADF-STEM images (c) of Pd-Cl/Cu₂O. Pd K-edge XANES spectra (d), and FT k²-weighted EXAFS spectra (e) of Pd-Cl/Cu₂O and reference samples. f The fitting EXAFS spectra of Pd-Cl/Cu₂O. Inset: fitting model.

Fig. 4. NO₃^‒ intermediates hydrogenation.

a In situ Raman spectra of Pd-Cl/Cu₂O. b Schematic diagram of the local H⁺-abundant environment construction over Pd-Cl/Cu₂O. In situ Raman spectra of Pd-Cl/Cu₂O (c), corresponding peak area (d) and Raman shift (e) of various interfacial H₂O structures. In situ ATR-IR spectra of Pd-Cl/Cu₂O (f) and Pd/Cu₂O (g) catalysts. Si-O signal was derived from the reduction of surface SiO₂ on the Si semi-cylindrical prism substrate under the applied potentials.

Fig. 5. Alkaline NO₃RR performance.

a NH₃ yield rate and NH₃ FE of catalysts in a 1 M KOH with 56 mM NO₃^– electrolyte (pH = 14) for 1 h electrolysis. Catalyst mass loading: 3 mg cm⁻². Resistance of catalyst: 0.156 Ω cm⁻². Resistance of electrolyte: 1.45 Ω. b Kinetic isotopic effect (KIE) diagram for the ratio of NH₃ yield rate in H₂O to D₂O solvent in a 1 M KOH with 56 mM NO₃^– electrolyte at −0.4 V vs. RHE. c NO₃^‒ removal of catalysts measured in a 1 M KOH with 56 mM NO₃^– electrolyte (equals 790.3 μg mL⁻¹ NO₃^––N) at −0.4 V vs. RHE. After 1 h electrolysis, only 7.1 μg mL⁻¹ of NO₃^––N and 0.85 μg mL⁻¹ of NO₂^––N remained, both below the WHO regulations for drinking water (NO₃^––N < 11.3 μg mL⁻¹ and NO₂^––N < 0.91 μg mL⁻¹). d NH₃ partial current densities of Pd-Cl/Cu₂O in a 1 M KOH electrolyte with 1000 mM NO₃^– under the potential range from −0.2 to −0.6 V vs. RHE. e NH₃ yield rate of Pd-Cl/Cu₂O in a 1 M KOH electrolyte with different NO₃^– concentrations for 1 h of electrolysis. f NO₃RR performance comparison of reported electrocatalysts. g Schematic of the ammonia product synthesis process from 1000 mM NO₃^– electrolyte to NH₄Cl for 5 h electrolysis at −0.6 V vs. RHE. h The conversion efficiency of different steps for the ammonia product synthesis process. Numbers on the x-axis indicated the corresponding conversion steps in panel g. Error bars indicate the relative standard deviations of the mean (n = 3). See “Methods” for experimental details.