Defect-induced triple synergistic modulation in copper for superior electrochemical ammonia production across broad nitrate concentrations

Nitrate contamination is pervasive in industrial wastewater and groundwater due to fertilizer overuse and industrial emissions. Excess nitrate disrupts the nitrogen balance and poses health risks via conversion to nitrites. Electrochemical nitrate reduction offers a mild, environmentally friendly pathway to detoxify nitrate; producing ammonia adds value because NH3 is a key chemical and energy carrier. However, achieving simultaneously high Faradaic efficiency (FE) and high production rates across a broad span of nitrate concentrations (1–100 mM typical in waste streams) remains challenging. At low nitrate concentrations, vigorous hydrogen evolution reaction (HER) lowers FE and yield; at typical to higher concentrations, insufficient supply of active hydrogen (protons/adsorbed H) can limit industrial-level currents. The research question is how to engineer a catalyst that can (i) suppress HER at low nitrate levels, (ii) enhance proton supply for hydrogenation at higher nitrate levels, and (iii) strongly adsorb and activate nitrate across concentrations to deliver high FE and current density. The authors propose a defect-rich copper nanowire array electrode (V-Cu NAE) formed by in-situ electrochemical reduction of Cu3N nanowires. They hypothesize that defect sites on Cu provide a triple synergistic modulation—enhanced nitrate adsorption, promoted water dissociation (for active H supply), and suppressed HER—enabling superior ammonia production over a wide nitrate concentration range and in real wastewater.

Recent progress has focused on catalysts achieving high FE for nitrate-to-ammonia, often at higher nitrate concentrations, with limited performance at low concentrations where HER dominates. For low-concentration nitrate (e.g., 1–5 mM), Kang et al. reported NiFe LDH/Cu foam delivering 96.8% FE at 65 mA cm⁻² for 5 mM nitrate, but performance dropped at 1–2 mM (44–61% FE at 10–20 mA cm⁻²). He et al. synthesized a CuCoSP catalyst that improved FE across 1–100 mM but with modest current density (8–265 mA cm⁻²), potentially limiting practical application. Other studies highlight that strong HER at low nitrate concentrations restricts FE and yield, while at typical concentrations the lack of active H supply hinders reaching industrial currents. Prior work has also begun to explore real-wastewater scenarios, but reports remain rare and challenging. Collectively, literature points to the need for catalysts that simultaneously manage nitrate adsorption/activation, water dissociation for proton supply, and HER suppression across broad concentration regimes.

Catalyst synthesis and transformation:

• Cu(OH)2 nanowire arrays (NWs) were grown on pretreated Cu foam by immersing in ammonium persulfate/NaOH solution for 30 min, followed by washing and drying.
• Cu3N NWs were obtained by nitridation of Cu(OH)2 NWs at 300 °C for 2 h in NH3.
• Defect-free Cu NWs were prepared by annealing Cu(OH)2 NWs at 300 °C for 2 h in H2/Ar (5:95).
• Defect-rich Cu nanowire array electrode (V-Cu NAE) was formed via in-situ electrochemical reduction of Cu3N NWs in K2SO4 using cyclic voltammetry from -0.4 to 1.8 V at 200 mV s⁻¹ until stabilization, removing nitrogen and generating Cu vacancies.

Characterization:

• Structural/morphological: XRD, FESEM, HRTEM, aberration-corrected HAADF-STEM for vacancy visualization and intensity profiling; EDS mapping; XPS; synchrotron soft XANES (N K-edge, Cu L-edge); in-situ Raman during electroreduction of Cu3N to V-Cu.

Electrochemical evaluation (H-cell):

• Three-electrode H-type cell (separated by membrane); working electrode area 1 cm² (loading 3 mg cm⁻²), SCE reference, Pt counter; 0.5 M K2SO4 electrolyte (35 mL per compartment), pH ~6.92.
• Nitrate (KNO3) concentrations tested: 1, 2, 5, 10, 20, 30, 40, 50, 100, 500, 1000 mM as nitrate-N equivalent. LSV (0.2 to -0.6 V vs RHE), then 1 h potentiostatic electrolysis at set potentials with 300 rpm stirring. Cell resistance measured by PEIS.
• Quantification: UV-Vis methods for NO3-N (220/275 nm), NO2-N (Griess at 540 nm), NH3-N (Nessler at 420 nm); isotopic labeling with 15NO3⁻ and 1H NMR cross-validation using external standards; Cdl-based ECSA estimation.

Operando spectroscopy:

• Operando synchrotron radiation FTIR (BL01B beamline) in reflection mode with ZnSe window; background at OCP; spectra 4000–1250 cm⁻¹ collected under potentials from about -0.1 to -0.5 V vs RHE; analysis of NH3, NOx intermediates.

Theory and simulation:

• DFT (VASP, PAW, GGA-PBE, D3 dispersion), 400 eV cutoff; implicit solvation (VASPsol) and explicit water molecules on surfaces; constant potential double-referencing method; CHE framework for potential dependence; derivations for nitrate/nitrite adsorption free energies using thermodynamic cycles with CRC data; activation barriers via AIMD Slow-growth method; potential-dependent barriers (Butler-Volmer formalism with alpha=0.5); microkinetic simulations (MKMXXC) at 300 K to compute surface coverages and product rates.
• Models: Cu(111) and defective V-Cu(111) (surface Cu vacancy) with four explicit H2O; energies referenced at U ≈ -0.289 V vs RHE for comparisons.

Two-electrode flow electrolyzer and product extraction:

• Paired cathodic nitrate reduction (V-Cu NAE) and anodic glycerol oxidation (Ni foam anode, 0.1 M glycerol) to reduce cell voltage and add value via formate production.
• Real industrial wastewater (NO3-N ≈ 527 ppm) used as catholyte; performance assessed via LSV and long-term electrolysis; 1 L treated with 1 cm² cathode, 1.4 V cell voltage.
• Ammonia recovery via air stripping at 70 °C with gas flow; capture using HCl to form NH4Cl(s) or dissolution to make NH3·H2O(aq); product verification by XRD and 1H NMR; evaluation of conversion efficiencies across steps.

• Defect-rich V-Cu NAE delivers high performance for nitrate reduction to ammonia across a broad concentration range (1–100 mM): current density 50–1100 mA cm⁻² with Faradaic efficiency (FE) > 90%.
• In 0.5 M K2SO4 with 200 ppm nitrate, V-Cu NAE outperforms Cu foam, Cu NWs, and Cu3N NWs. At -0.3 V vs RHE: FE 95.1±2.3%, NH3 selectivity 97.2±1.8%, nitrate conversion 95.9±0.8% (1 h), NH3 production rate 7853±242 μg h⁻¹ cm⁻²; stable over 8 repeated 1 h cycles without decay.
• Simulated wastewater removal at -0.3 V: 200 ppm NO3⁻-N reduced to 2.1 ppm and NO2⁻-N to 0.52 ppm in 90 min, exceeding WHO standards; FE > 90%, NO3⁻ conversion ~99%, NH3 selectivity 99.7%.
• Broad-concentration practicality: at 10–50 mM NO3⁻-N (textile/industrial wastewater range) and -0.3 V, current density 200–800 mA cm⁻² with FE > 90%; at ultralow concentration (-0.2 V), high FE and conversion maintained; at high concentrations (0.1–1 M, -0.4 V), current density 1000–1300 mA cm⁻², FE > 95%, and 99.8% NO3⁻ conversion to drinking-water levels.
• ECSA of V-Cu NAE is ~5× greater than defect-free Cu NWs, indicating more active sites.
• Operando SR-FTIR detects increasing NH3 signals (∼3185 cm⁻¹) with more negative potentials, and intermediates assigned to NO2 and NH4⁺ (1582 and 1475 cm⁻¹), consistent with enhanced reaction rates.
• DFT reveals triple synergistic modulation at V-Cu(111): • Enhanced nitrate/nitrite adsorption: adsorption free energies (at U ≈ -0.289 V) of -1.62 eV (NO3⁻) and -1.34 eV (NO2⁻) vs Cu(111) at -0.33 eV and -0.71 eV. • Promoted water dissociation: lower water splitting activation energy 0.38 eV (V-Cu) vs 0.72 eV (Cu), enabling greater supply of active hydrogen for hydrogenation steps. • Suppressed HER: higher HER barrier 0.84 eV (V-Cu) vs 0.64 eV (Cu), improving FE by limiting competing hydrogen evolution.
• Optimal NO3RR pathway on both surfaces: NO3(l) → *NO3 → NO2 → *NO → *NHO → *NH2O → *NH2OH → *NH2 → *NH3 → NH3(g). Rate-determining step is protonation of *NH2OH with lower barrier on V-Cu (0.93 eV) than Cu (1.07 eV).
• Microkinetic simulations show faster *NO3 deoxygenation and faster protonation of oxygen species on V-Cu, leading to surface coverage dominated by *NO2 at steady-state and higher ammonia selectivity.
• Paired two-electrode system (V-Cu NAE cathode, Ni foam anode with 0.1 M glycerol): achieves 550 mA cm⁻² at about 1.34–1.4 V; treating 1 L real industrial wastewater (527 ppm NO3⁻-N) at 1.4 V yields 99.9% NO3⁻ conversion, 99.9% NH3 selectivity, FE > 80%, reducing NO3⁻-N to 0.52 ppm and NO2⁻-N to 0.38 ppm within 30 h. Stable operation demonstrated over 100 h (4×30 h cycles).
• Anodic glycerol oxidation produced formate with 81.3% FE, adding value and reducing cell voltage (~260 mV reduction to reach 500 mA cm⁻²).
• Ammonia recovery via air stripping and capture converts >80% of NH3 into NH4Cl(s) or NH3·H2O(aq); XRD and 1H NMR confirm high purity.

The study directly addresses the central challenge of achieving high FE and high current densities for nitrate-to-ammonia conversion across widely varying nitrate concentrations. By creating Cu vacancy defects via in-situ electroreduction of Cu3N, the catalyst attains a triple synergistic effect: stronger adsorption of nitrate/nitrite ensures efficient reactant capture even at low concentrations; facilitated water dissociation supplies active hydrogen to drive hydrogenation steps at higher rates; and a raised HER barrier suppresses the competing hydrogen evolution, especially critical at low nitrate levels. Operando SR-FTIR corroborates active intermediate formation and increased NH3 production with potential, while DFT and microkinetic analyses elucidate the favorable energetics and kinetics (lower barriers for water splitting and key protonation steps, faster deoxygenation, and surface coverage dynamics) on V-Cu relative to Cu. This mechanistic understanding explains the observed broad-range performance: FE > 90% with high current densities from 1 mM to 1 M nitrate sources. The successful integration into a two-electrode flow electrolyzer paired with glycerol oxidation demonstrates practical viability, lowering cell voltage, enabling treatment of real industrial wastewater to below drinking-water nitrate/nitrite limits, and co-producing valuable formate. Efficient ammonia stripping and chemical capture further highlight the end-to-end applicability.

A defect-rich copper nanowire array electrode (V-Cu NAE) produced by in-situ electroreduction of Cu3N delivers superior electrochemical nitrate reduction to ammonia across an exceptionally broad concentration range (1–100 mM, extended up to 1 M), achieving current densities up to ~1.1 A cm⁻² (and 1.0–1.3 A cm⁻² at high concentrations) with FE exceeding 90%. Operando SR-FTIR and DFT reveal a triple synergistic modulation—enhanced nitrate adsorption, promoted water dissociation, and suppressed HER—underpinning the performance. In a paired two-electrode flow system with glycerol oxidation, the catalyst achieves industrially relevant current densities at low cell voltages, converts real wastewater to below WHO limits with 99.9% nitrate conversion and 99.9% NH3 selectivity, and enables recovery of high-purity NH4Cl and NH3·H2O. Future work could explore scaling electrode areas and flow configurations, further durability optimization to mitigate nanowire detachment/aggregation, pairing with other value-added anodic reactions, and tolerance to complex wastewater matrices (e.g., competing anions/cations and organics).

• While the V-Cu NAE maintains structure as metallic Cu after long-term tests, SEM shows nanowire detachment, aggregation, and fractures after cycling, which may contribute to performance variations.
• In the paired two-electrode device, ammonia FE is over 80% (lower than >90% in the H-cell), indicating room to optimize system-level efficiency under practical flow conditions.
• Demonstrations used a 1 cm² electrode for 1–5 L scale tests; larger-scale, long-duration trials and robustness against complex wastewater compositions were not reported.
• Microkinetic simulations indicate nuanced trade-offs in intermediate coverages and rates; full alignment with all experimental rates across potentials and concentrations may require further model refinement.