Electrochemical ammonia synthesis by reduction of nitrate on Au doped Cu nanowires

Abstract

Electrochemical nitrate reduction reaction (NO₃⁻RR) to synthesize valuable ammonia (NH₃) is considered as a green and appealing alternative to enable an artificial nitrogen cycle. However, as there are other NO₃⁻RR pathways present, selectively guiding the reaction pathway towards NH₃ is currently challenged by the lack of efficient catalyst. Here, we demonstrate a novel electrocatalyst for NO₃⁻RR consisting of Au doped Cu nanowires on a copper foam (CF) electrode (Au–Cu NWs/CF), which delivers a remarkable NH₃ yield rate of 5336.0 ± 159.2 μg h⁻¹ cm⁻² and an exceptional faradaic efficiency (FE) of 84.1 ± 1.0% at −1.05 V (vs. RHE). The ¹⁵N isotopic labelling experiments confirm that the yielded NH₃ is indeed from the Au–Cu NWs/CF catalyzed NO₃⁻RR process. The XPS analysis and in situ infrared spectroscopy (IR) spectroscopy characterization results indicated that the electron transfer between the Cu and Au interface and oxygen vacancy synergistically decreased the reduction reaction barrier and inhibited the generation of hydrogen in the competitive reaction, resulting in a high conversion, selectivity and FE for NO₃⁻RR. This work not only develops a powerful strategy for the rational design of robust and efficient catalysts by defect engineering, but also provides new insights for selective nitrate electroreduction to NH₃.

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Ammonia (NH₃) is not only an essential chemical and the cornerstone of the large and ever-growing fertilizer industry, but also considered as an important energy storage medium and carbon-free energy carrier.^1–4 Currently, most of the ammonia synthesis in the world is implemented via the Haber–Bosch process, which consumes about 5.51 EJ of energy every year (∼38 GJ/t_NH3) and emits over 450 million metric tons of CO₂ (∼2.9 t_CO2/t_NH3), this is because the process requires substantial driving force and hydrogen gas (e.g., H₂), which is produced from natural gas or coal through steam reforming, accounting for about half of CO₂ emissions in the entire process.^5–8 Nitrogen gas (N₂) from air was identified as one major nitrogen source for this renewable route via electrochemical nitrogen reduction reaction (NRR), however, the faradaic efficiency (FE) is greatly hampered by the high dissociation energy of N≡N tripe bond (941 kJ mol⁻¹) and poor solubility of N₂ in electrolytes and the competitive reaction of H₂ evolution.^9–11 While exciting progresses in NRR catalyst development have been made, in many cases it is still challenging to firmly attribute the detected NH₃ to NRR process rather than contaminations due to the extremely low NH₃ production rate (mostly <200 μg h⁻¹ mg_cat.⁻¹).^12,13 Thus, developing a new route for ammonia synthesis under benign conditions is urgently desired.

It is common knowledge that, nitrate pollution in surface water and groundwater is widespread in the world.¹⁴ High concentrations of nitrate in aquatic ecosystems pose a serious threat to ecological balances and human health. To minimize such adverse effects, many approaches including biological denitrification,¹⁵ reverse osmosis,¹⁶ ion exchange,¹⁷ electrodialysis,¹⁸ membrane filtration,¹⁹ electrocatalytic denitrification^20–22 and so on have been adopted to dispose of nitrate contamination to produce clean water, among them, electrocatalytic denitrification driven by “green” electricity generated from renewable resources is the most likely practical alternative, which can overcome these limitations. Compared with the NRR, the nitrate reduction reaction (NO₃⁻RR) to NH₃ is not limited by the low solubility of N₂ in water environment and its thermodynamically more favourable because of lower dissociation energy of N=O bond (204 kJ mol⁻¹) than the N≡N tripe bond (941 kJ mol⁻¹).^23,24 Therefore, it is an frontier field that needs in-depth study.

Herein, we utilized a facile three-step method to fabricate the Au doped Cu nanowires on a copper foam (CF) (denoted as Au–Cu NWs/CF) electrode for the selective nitrate electroreduction to ammonia. The Au–Cu NWs/CF sample exhibited an exceptional performance with the NH₃ yield rate of 5336.0 ± 159.2 μg h⁻¹ cm⁻² and the FE of 84.1 ± 1.0% at −1.05 V (vs. RHE) for the electrocatalytic NO₃⁻RR under neutral conditions. ¹⁵N isotopic labelling experiments were performed to confirm the origin of ammonia, which was quantified by both ¹H nuclear magnetic resonance (NMR) spectra and colorimetric methods. The XPS analysis and in situ infrared spectroscopy (IR) spectroscopy characterization results indicated that the oxygen vacancies in Au–Cu NWs/CF can weaken the N–O bonding,²⁵ moreover, the electron transfer between Cu and Au interface could inhibit the competitive reaction of the hydrogen evolution reaction (HER),¹¹ resulting in high NH₃ yield rate and FE of NO₃⁻RR.

Fig. 1a shows the schematic illustration of the growth of the Au doped Cu nanowires on a copper foam electrode. As illustrated in Fig. 1a, Au–Cu NWs/CF can be prepared by a three-step method. In the first step, the Cu(OH)₂ NWs/CF was prepared via a facile wet-chemical oxidation method. Subsequently, NWs/CF was directly immersed into 10 mM HAuCl₄·3H₂O solution for 12 h, dried at 60 °C under vacuum for 4 h, the Au–Cu(OH)₂ NWs/CF was annealed under Ar atmosphere to obtain Au–CuO NWs/CF. Finally, the Au–Cu NWs/CF was obtained by in situ electrochemical reduction of the resultant Au–CuO NWs/CF. The scanning electron microscopy (SEM) images of CF (Fig. 1b) and Au–Cu NWs/CF (Fig. 1c) demonstrate that the nanowires have been successfully generated on CF. After cation exchange reaction with Au precursor and subsequent thermal treatment and electrochemical reduction, the morphology of nanowires was largely maintained on the Au–Cu NWs/CF with the diameters of ∼100 nm (Fig. 1c). Fig. 1d shows the X-ray diffraction (XRD) patterns of CF and Au–Cu NWs/CF samples. As shown, similar diffraction peaks at 2θ = 43.3°, 50.4° and 74.1° can be observed for these two samples, corresponding to (111), (200) and (220) plane of metallic Cu (JCPDS no. 04-0836), respectively.^26–28 While besides of typical diffraction peaks of metallic Cu, the Au–Cu NWs/CF sample also displays the weak characteristic peaks of Au nanoparticles at 2θ = 38.2°, 44.3°, 64.6° and 77.5°, suggesting the formation of fcc Au phase on Cu nanowires with low loading content.²⁹ The actual loading of Au was calculated to be 5.6 wt% by inductively couple plasma atomic emission spectroscopy (ICP-AES). High-resolution TEM (HR-TEM, Fig. 1e) images show the lattice fringes of 0.24 and 0.27 nm, corresponding to the (111) and (200) planes of Cu, respectively, in good accord with the XRD results.^27,28 In addition, the corresponding element mapping analysis of Au–Cu NWs/CF reveals that Au was homogeneously dispersed over the whole Cu foam (Fig. 1f).

Fig. 1 (a) Schematic illustration and corresponding structure of products. SEM images of (b) bare CF substrate and (c) Au–Cu NWs/CF. (d) XRD patterns for CF and Au–Cu NWs/CF. (e) HR-TEM image of Au–Cu NWs/CF. (f) EDS images of Au–Cu NWs/CF.

The X-ray photoelectron spectroscopy (XPS) measurement was performed to investigate the surface composition and valence state of Au–Cu NWs/CF. For comparison, we also performed the XPS characterization of CF sample. The XPS survey spectra and high-resolution XPS spectra of Au 4f verified the existence of doped Au in the Au–Cu NWs/CF (Fig. 2a and b). The high-resolution XPS spectra of Cu 2p in bare CF substrate is shown in Fig. 2c, where peaks of Cu 2p_3/2 and Cu 2p_1/2 appear at 932.5 and 952.3 eV.^26–28 The two characteristic peaks confirms the presence of Cu⁰/Cu¹⁺.^26–28 Note that after Au doping, the binding energy of Cu 2p_3/2 and Cu 2p_1/2 shifted by 0.5 eV and 0.4 eV towards the lower binding energy of 932.0 and 951.9 eV in Au–Cu NWs/CF (Fig. 2c), due to the transfer of electrons between Cu and Au via chemical binding, which led to an increase in charge density and is conducive to electrocatalysis.^27,28 Additionally, the new peak at binding energy of 934.2 eV was attributed to Cu²⁺ in Au–Cu NWs/CF. Based on previous reports,^27,28 we further used Auger Cu LMM spectra to confirm the coexistence of Cu⁰ and Cu¹⁺. It can be clearly observed in the Fig. S1† (ESI) that the Auger kinetic energy peak is wide and asymmetric in the range of 906 eV to 924 eV. The two asymmetric peaks with centers located at the position around 916.5 and 918.7 eV, 916.1 eV and 918.4 eV can be assigned to Cu¹⁺ and Cu⁰ in the CF and Au–Cu NWs/CF, respectively.^27,28 In the O 1s XPS spectra (Fig. 2d), 530.9 and 532.5 eV, 530.6 eV and 531.8 eV correspond to lattice oxygen and oxygen vacancy in the CF and Au–Cu NWs/CF, respectively.²⁶ The significantly increased oxygen vacancy after doping is favourable for weakening the N–O bond and inhibiting the formation of by-products in the electrocatalytic nitrate reduction reaction, thereby improving the selectivity of ammonia.²⁵

Fig. 2 (a) Surface survey XPS spectra of Au–Cu NWs/CF and CF samples. High-resolution XPS spectra of (b) Au 4f, (c) Cu 2p and (d) O 1s in Au–Cu NWs/CF and CF samples.

We evaluated the NO₃⁻RR performance of the as-fabricated Au–Cu NWs/CF catalysts in a 0.1 M Na₂SO₄ + 10.0 mM KNO₃ solution (pH = 5.6) using a three-electrode configured two-compartment cell. In all experiments, the Au–Cu NWs/CF catalyst was used as working electrode, Ag/AgCl (saturated KCl solution) and Pt mesh were used as the reference electrode and counter electrode, respectively. Colorimetric methods were adopted determine the concentration of NO₃⁻, NO₂⁻ and NH₄⁺ (Fig. S2, S3 and S4, ESI†). The liner sweep voltammetry (LSV) curves of Au–Cu NWs/CF electrocatalysts was conducted in 0.1 M Na₂SO₄ electrolytes with and without 10.0 mM KNO₃. As shown in Fig. 3a, the current density increased obviously with the present of KNO₃, suggesting that NO₃⁻ in solution participated in the reduction reactions. Note that the LSV of Au–Cu NWs/CF tested in the presence of NO₃⁻ exhibits a remarkable reduction peak at −0.6 V (vs. RHE), which may be due to the electrochemical reduction of NO₃⁻. Chronoamperometry (CA) measurements of Au–Cu NWs/CF were conducted at different potentials for 2 h with continuous argon gas (Ar) bubbling. Fig. S5a† (ESI) shows the chronoamperometry curves at each given potential for 2 h electrolysis from −0.7 V to −1.1 V (vs. RHE). The concentration of NH₃ product was measured using indophenol blue method (Fig. S5b, ESI†). The calculated NH₃ yield rates and FEs based on three repeated experiments are given in Fig. 3b. It is worth noting that the Au–Cu NWs/CF achieved the highest NH₃ yield rate (R_NH3) of 5336.0 ± 159.2 μg h⁻¹ cm⁻² and the FE of 84.1 ± 1.0% at −1.05 V (vs. RHE). The selectivity of NH₃ (S_NH3) and R_NH3 show the same trend with the increase of potential, and highest S_NH3 was 90.6 ± 3.2% (Fig. 3c). In addition, the conversion of nitrate increases slowly with the increase of potential, and 100% conversion can be achieved at −0.95 V (vs. RHE) (Fig. 3d). When the potential further increased to −1.1 V (vs. RHE), the R_NH3 and S_NH3 decreased due to the competitive hydrogen evolution reaction (HER).³⁰ Although the electrodynamic potential of NO₃⁻ to NO₂⁻ is higher than that of NO₃⁻ to NH₃, NO₂⁻ is easily detected an main by-product of NO₃⁻RR.³¹ As shown in Fig. S6† (ESI), few NO₂⁻ is detected after electrocataytic reduction at −1.05 V (vs. RHE), further demonstrating the high selective reduction of nitrate to NH₃. We also tested R_NH3 and FE of bare CF with 0.1 M Na₂SO₄ + 10.0 mM KNO₃ solution at −1.05 V (vs. RHE) to exclude the influence of substrate. As shown in Fig. S7† (ESI), the highest R_NH3 and FE for bare CF were 1777.7 μg h⁻¹ cm⁻² and the FE of 49.9%, much lower than Au–Cu NWs/CF. The corresponding equivalent circuit diagrams of bare CF and Au–Cu NWs/CF are shown in Fig. S8† (ESI). The much lower ohmic resistances (R_s) and charge-transfer resistance (R_ct) from Au–Cu NWs/CF confirms its high electrical conductivity, which could be an important attribute for the achieved high R_NH3 and FE.

Fig. 3 (a) LSV curves of Au–Cu NWs/CF catalyst in 0.1 M Na₂SO₄ and 0.1 M Na₂SO₄ + 10.0 mM KNO₃ solution. (b) NH₃ yield rate and faradaic efficiency of Au–Cu NWs/CF catalyst obtained at different potentials for 2 h NO₃⁻RR measurement. (c) NH₃ selectivity and (d) NO₃⁻ conversion of Au–Cu NWs/CF catalyst obtained at different potentials. (e) ¹H NMR spectra of Au–Cu NWs/CF catalyst using ¹⁴NO₃⁻/¹⁵NO₃⁻ as nitrogen source for NO₃⁻RR and standards (¹⁴NH₄)₂SO₄/(¹⁵NH₄)₂SO₄. (f) Recycling tests for Au–Cu NWs/CF catalyst during NO₃⁻RR at −1.05 V (vs. RHE).

¹⁵N isotope labeling with ¹H nuclear magnetic resonance (NMR) is usually required in NO₃⁻RR experiments to confirm that the detected NH₃ indeed originates from the electrochemical nitrate reduction to rule out contaminations. We carried out chronoamperometry measurement at −1.05 (V vs. RHE) for 2 h in the electrolyte with K¹⁵NO₃ and K¹⁴NO₃ as N source, respectively. As shown in Fig. 3e, when the electrolysis was carried out in solution with K¹⁴NO₃, the ¹H NMR spectra of the obtained products displayed typical peaks of ¹⁴NH₄⁺. In contrast, when the K¹⁵NO₃ was used as nitrogen source, the ¹H NMR spectra showed typical double peaks of ¹⁵NH₄⁺. Such results indicated that the produced NH₃ was entirely derived from the nitrate in the electrolyte, rather than from the contaminations. Meanwhile, the electrolyte without nitrate addition is also tested. The electrochemical measurement in blank 0.1 M Na₂SO₄ electrolyte produced ignorable NH₃ (Fig. S9, ESI†), further confirming that the produced NH₃ orginated from nitrate electroreduction. The durability of the Au–Cu NWs/CF electrocatalyst for NO₃⁻RR was subsequently assessed by consecutive recycling electrolysis at −1.05 V (vs. RHE), no noticeable decay in the cathodic current density and UV-vis absorptions (Fig. S10, ESI†). As shown in Fig. 3f, the R_NH3 and FE are stable after 6 consecutive recycling tests, indicating the good durability of Au–Cu NWs/CF.

After electrolysis, the high-resolution Au 4f and Cu LMM XPS spectra were carried out to analyze the electronic properties of Au–Cu NWs/CF before and after NO₃⁻RR measurement (Fig. S11, ESI†). Interestingly, after NO₃⁻RR, the Au 4f_7/2 shifted slightly to a lower binding energy by 0.2 eV after electrolysis (Fig. S11a, ESI†). Similarly, the Auger peak of Cu²⁺ shifted to a lower binding energy by 0.3 eV, while the Auger peaks of Cu⁰ and Cu¹⁺ shifted to the higher binding energy by 0.2 eV and 0.3 eV after electrolysis (Fig. S11c, ESI†), indicating the existence of charge transfer between Au, Cu²⁺ and Cu^0,+1 during NO₃⁻RR process. Inaddition, the oxygen defect increased significantly after electrolysis (Fig. S11d, ESI†). In a word, the high electronic density of Cu⁰ and oxygen vacancy decreased the reduction reaction barrier and inhibited the generation of hydrogen in the competitive reaction, resulting in a high conversion, selectivity and FE of Au–Cu NWs/CF for NO₃⁻RR.^31,32

To gain a deeper understanding of the NO₃⁻RR mechanism over Au–Cu NWs/CF catalysts, we utilized in situ infrared spectroscopy (IR) spectroscopy characterization to detect intermediates and monitor the reaction. Fig. 4a display the in situ IR spectra of Au–Cu NWs/CF under various potentials. As shown, without the applied potential, there is no any infrared peak in the in situ IR spectra. In the investigated potential range from −0.7 to −1.1 V (vs. RHE), the new infrared bands at ∼1541 cm⁻¹ was assigned to the −NO_x intermediates.^33,34 In addition, the bending mode of –NH₂ is also found at ∼1457 cm⁻¹.^34,35 Clearly, as the applied potential increased, the peak intensity of −NO_x intermediates and –NH₂ gradually increased (Fig. 4a). Fig. 4b shows the in situ IR measurements for the NO₃⁻RR at −1.05 V (vs. RHE). The IR intensity of the peaks at around 1457 cm⁻¹ and 1541 cm⁻¹, corresponding to –NH₂ and −NO_x intermediates is increased obviously from 4 to 36 min, implying that the NO₃⁻RR takes place gradually with reaction time under the given electrocatalytic conditions. Evidenced by the in situ IR results, the NH₃ synthesis by NO₃⁻RR is successfully achievable (Fig. S12, ESI†), supportable for the electrocatalytic experimental results aforementioned.

Fig. 4 (a) In situ IR spectroscopy measurements under various potentials for Au–Cu NWs/CF in 1.0 M Na₂SO₄ + 10.0 mM KNO₃ electrolyte. (b) In situ IR spectroscopy measurements of Au–Cu NWs/CF in 1.0 M Na₂SO₄ + 10.0 mM KNO₃ electrolyte at −1.05 V (vs. RHE) for NO₃⁻RR.

In conclusion, Au doped Cu nanowires on a copper foam electrode was synthesized via a facile three-step method, which further generated the oxygen vacancies in Au–Cu NWs/CF can weaken the N–O bonding, moreover, the electron transfer between Cu and Au interface could inhibit the competitive reaction, resulting in high conversion, selectivity and FE of Au–Cu NWs/CF for NO₃⁻RR. The Au–Cu NWs/CF exhibited significantly enhanced NO₃⁻RR activity with an NH₃ yield rate of 5336.0 ± 159.2 μg h⁻¹ cm⁻² and the FE of 84.1 ± 1.0% at −1.05 V (vs. RHE) in neutral electrolyte. The in situ IR spectroscopy measurements confirm the successful realization of NH₃ synthesis by NO₃⁻RR over Au–Cu NWs/CF. Our work would be helpful to design and develop high-efficiency NO₃⁻RR electrocatalysts for ambient electrosynthesis of ammonia.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by Anhui Provincial Natural Science Foundation (Grant No. 2108085QB60 and 2108085QB61), CASHIPS Director’s Fund (Grant No. YZJJ2021QN18 and YZJJ2021QN21), China Postdoctoral Science Foundation (Grant No. 2020M682057), Special Research Assistant Program, Chinese Academy of Sciences.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details of experimental process, calibration curves and electrochemical measurement results. See DOI: https://doi.org/10.1039/d3ra00679d

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