Sulfur Mediated Interfacial Proton-Directed Transfer Boosts Electrocatalytic Nitric Oxide Reduction to Ammonia over Dual-Site Catalysts

Abstract

Electrocatalytic nitric oxide reduction reaction (NORR) for ammonia (NH₃) synthesis represents a sustainable strategy that simultaneously realizes the nitrogen cycle and resource integration. The key issue hindering the NORR efficiency is accelerating proton (*H) transfer to facilitate NO hydrogenation while inhibiting the hydrogen evolution reaction (HER). Herein, we demonstrate an interface-engineered sulfur-mediated Cu@Co electrocatalyst (S-Cu@Co/C) that boosts NORR performance through dual modulation of electronic structure and proton transfer on active sites. A comprehensive program of experimental and theoretical calculations was employed to discover that sulfur incorporation induces electron redistribution in the Cu-Co interface, creating electron-rich sulfur and electron-deficient metals. This electronic configuration synergistically enhances NO adsorption on Cu sites and promotes water dissociation on Co sites. More critically, sulfur could direct the rapid transfer of *H from Co to Cu sites, thereby accelerating the NO hydrogenation and suppressing HER. Consequently, S-Cu@Co/C achieves an NH₃ yield rate of 655.3 µmol h^-1 cm^-2 in a flow cell and a Faradaic efficiency of 92.4% in an H-cell. Remarkably, the catalyst could maintain continuous electrolysis tests and steady NH₃ yield up to 100 h. This work provides innovative insights into the fabrication of efficient electrocatalysts via heteroatom-mediated interfacial engineering strategies.

Keywords: Ammonia synthesis; Dual‐site catalysts; Electrocatalysis; Nitric oxide reduction; Sulfur‐mediated.

Figure 1

Morphology and structure characterization. a) XRD patterns of the catalysts; b) Raman spectra of the catalysts. c,d) HRTEM image of S‐Cu@Co/C catalyst. The inset in (c) is the particle size distribution, and in (d) is the calculated lattice fringes distance. e) AC‐HAADF‐STEM image of S‐Cu@Co/C catalyst and the three‐dimensional atomic topology image of the marked region.

Figure 2

Characterizations of catalysts. a) Cu 2p and b) Co 2p XPS spectra of S‐Cu@Co/C and Cu@Co/C; c) The charge density difference analysis for S‐Cu@Co/C and Cu@Co/C (yellow and cyan colors represent electron accumulation and depletion regions, respectively); d) Cu K‐edge XANES spectra of S‐Cu@Co/C and Cu@Co/C. The inset is the enlarged pre‐edge region; e) Co K‐edge XANES spectra of S‐Cu@Co/C and Cu@Co/C. The inset is the enlarged pre‐edge region; f) The FT‐EXAFS spectra of Cu foil, Cu₂O, CuS, S‐Cu@Co/C and Cu@Co/C; g) The FT‐EXAFS spectra of Co foil, CoO, CoS₂, S‐Cu@Co/C and Cu@Co/C; (h‐i) WT contour plots of Cu and Co K‐edge EXAFS signals of S‐Cu@Co/C.

Figure 3

NORR performance. a) LSV curves of prepared catalysts under NO and Ar in H‐cell; b) NH₃ yield rate and FE over S‐Cu@Co/C at series potential; c) NH₃ yield rate and FE over prepared catalysts at −0.6 V vs. RHE in H‐cell; d) Comparison of the NORR performance of S‐Cu@Co/C with the reported catalysts; e) NORR performance of S‐Cu@Co/C under series conditions (OCP and CP represent open circuit potential and carbon paper); f) NMR spectra of isotope labeling experiments on S‐Cu@Co/C; g) LSV curves of S‐Cu@Co/C under NO and Ar in flow cell; h) NH₃ yield rate and FE over S‐Cu@Co/C in flow cell; i) Current density and NH₃ yield of S‐Cu@Co/C for stability test of NORR in flow cell at −0.5 V vs. RHE. All tests were conducted in PBS electrolytes. (Error bars mean the standard deviations of three independent experiments, the center value of the error bars means the average of three independent experiments).

Figure 4

NO adsorption and water dissociation investigations. a) NO‐TPD of S‐Cu@Co/C and Cu@Co/C; b) Calculated NO adsorption energy on Cu and Co sites in S‐Cu@Co/C and Cu@Co/C; c) Operando ATR‐IRAS spectra of NO adsorption on S‐Cu@Co/C without applying bias potential in PBS electrolyte; d) Gaussian‐fitted peaks of in situ Raman spectra of S‐Cu@Co/C; e) Population of interfacial water of S‐Cu@Co/C; f) Gaussian‐fitted peaks of in situ Raman spectra of Cu@Co/C; g) Calculated water dissociation energy on Cu and Co sites in S‐Cu@Co/C and Cu@Co/C; h) Quasi‐in situ electrochemical EPR signals for S‐Cu@Co/C in PBS electrolyte at −0.6 V vs. RHE; i) Comparison of ·H signal degradation rates on S‐Cu@Co/C and Cu@Co/C.

Figure 5

Reaction pathway and mechanism. a) The Gibbs free energy diagram of *H adsorption on S, Cu and Co sites in S‐Cu@Co/C; b) The calculated PDOS of S‐Cu@Co/C and Cu@Co/C; c) The COHP of *NO on S‐Cu@Co/C and Cu@Co/C; d) Operando ATR‐IRAS spectra of electrocatalytic NORR under − 0.6 V vs. RHE over S‐Cu@Co/C; e) In situ Raman spectra of electrocatalytic NORR under − 0.6 V vs. RHE over S‐Cu@Co/C; f) DEMS spectra of NORR over S‐Cu@Co/C; g) The Gibbs free energy diagram of the NORR over S‐Cu@Co/C and Cu@Co/C. The insets are the diagram of the adsorption of intermediates on S‐Cu@Co/C after structural optimization and the ΔG of the RDS; h) Schematic of NORR mechanism over S‐Cu@Co/C.