Identifying the active sites and intermediates on copper surfaces for electrochemical nitrate reduction to ammonia

Abstract

Copper (Cu) is a widely used catalyst for the nitrate reduction reaction (NO₃RR), but its susceptibility to surface oxidation and complex electrochemical conditions hinders the identification of active sites. Here, we employed electropolished metallic Cu with a predominant (100) surface and compared it to native oxide-covered Cu. The electropolished Cu surface rapidly oxidized after exposure to either air or electrolyte solutions. However, this oxide was reduced below 0.1 V vs. RHE, thus returning to the metallic Cu before NO₃RR. It was distinguished from the native oxide on Cu, which remained during NO₃RR. Fast NO₃⁻ and NO reduction on the metallic Cu delivered 91.5 ± 3.7% faradaic efficiency for NH₃ at −0.4 V vs. RHE. In contrast, the native oxide on Cu formed undesired products and low NH₃ yield. Operando shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) analysis revealed the adsorbed NO₃⁻, NO₂, and NO species on the electropolished Cu as the intermediates of NH₃. Low overpotential NO₃⁻ and NO adsorptions and favorable NO reduction are key to increased NH₃ productivity over Cu samples, which was consistent with the DFT calculation on Cu(100).

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1. Introduction

Ammonia (NH₃) is imperative for agricultural fertilizers and hydrogen-carrying fuel.^1–3 However, the excessive use of artificial fertilizer has disrupted the nitrogen cycle, causing nitrates (NO₃⁻) to drain into groundwater and rivers, and nitrous oxide (N₂O) emissions.^4–6 There is growing interest in using reactive nitrogen pollutants as an NH₃ source to address environmental concerns and explore clean energy alternatives. Electrochemical NO₃⁻ reduction reaction (NO₃RR) in water is the representative method, offering additional advantages over the Haber–Bosch process in terms of mild reaction conditions (room temperature, atmospheric pressure, and no need for natural gas feedstock),^7–12 and outperforming electrochemical N₂ reduction in kinetics.^13–15

NO₃RR involves eight electron-transfer processes with a thermodynamic reduction potential (E⁰) of +0.69 V vs. the reversible hydrogen electrode (RHE).^16–18

NO₃⁻ + 9H⁺ + 8e⁻ → NH₃ + 3H₂O E⁰ = 0.69 V vs. RHE

Computational simulations envisioned a sequential deoxygenation process from NO₃⁻ to nitric oxide (*NO, where the asterisk symbol indicates surface adsorption of the species) or nitride (*N), followed by hydrogenation to yield NH₃.^18–20 Two key steps determine activity and selectivity; (i) the initial NO₃⁻ reduction to nitrite (*NO₂) is the rate-determining step. The sluggish process caused significant onset overpotential.^21,22 (ii) The *NO binding strength on the catalyst significantly impacts selective NH₃ production. Weak NO adsorption leads to NO dissolution or NO–NO coupling, yielding NO, N₂O, NH₂OH, or N₂ byproducts.^{17,18,21–24}

Various catalysts have been investigated to optimize *NO₃⁻ and *NO adsorption. Copper (Cu) emerges as the most economical choice in addition to its ability to create a significant potential gap between NO₃⁻ reduction and hydrogen evolution reaction (HER). Both Cu(111)^25–28 and (100) facets^9,29 exhibited reasonable activity in overall processes, and their performance was further enhanced when forming alloys or bimetallic structures with Ru, Rh, Pd, or Ir.^11,30–34 However, the vulnerable nature of the Cu surface has posed challenges in identifying the actual active sites and their roles; the metallic Cu undergoes quick oxidation in the air or an electrolyte solution. Conversely, the Cu oxides are electrochemically reduced. However, degrees of oxidation and reduction vary depending on the conditions and have not been easily controlled. For instance, CuO nanostructures transformed into Cu/Cu₂O heterostructures through in situ reduction.¹² Although the heterostructure showed better NH₃ productivity, identifying the true active sites has proven difficult due to the presence of defect/strain structures with varying surface roughnesses^9,35–37 and the complexity of valence states at Cu/Cu₂O sites.³⁸

Here, we focused on the Cu surface states that are essential as the active sites to determine NO₃RR activity and selectivity. Three surface states of Cu foil were prepared, representing the native oxide-covered Cu, partially etched native oxide, and metallic Cu surface with a predominant (100) facet. It was found that the metallic Cu surface was oxidized by air or electrolyte solution. However, this oxide layer was rapidly eliminated below 0.1 V vs. RHE (before starting NO₃RR). The reverted metallic Cu surface facilitated NO₃⁻ and NO reductions and offered 91.5 ± 3.7% of NH₃ faradaic efficiency at −0.4 V vs. RHE. In contrast, native oxide on Cu was not fully removed under the same electrochemical condition, providing inferior NH₃ productivity. Operando Raman spectroscopy analysis revealed *NO₃⁻, *NO₂, and *NO species on metallic Cu as the intermediates of NH₃. Subsequent reductions of *NO₃⁻ and *NO are associated with NH₃ productivity, which is further supported by the DFT calculation. In contrast, the native oxide on Cu did not clearly show these intermediates and appeared *NO with higher overpotential.

2. Results and discussion

We prepared three types of Cu foils using different surface treatments (Scheme S1 and Experimental details in ESI†). First, as-received Cu foil was washed with acetone, isopropanol, and de-ionized (DI) water sequentially using bath sonication for 5 min each. The resulting Cu foil was named w-Cu. The second and third methods eliminated the native oxide layer of w-Cu.^39,40 The w-Cu was soaked in glacial acetic acid for 5 min,⁴¹ which was indicated as a-Cu. Separately, w-Cu was electrochemically polished by applying 2 V for 5 min in 85 wt% H₃PO₄ solution,^42,43 denoted as e-Cu. All these treatments were conducted just before surface characterizations or NO₃RR tests.

The as-received Cu surface state and morphology remained intact in w-Cu (Fig. 1a). Scanning electron microscope (SEM) and atomic force microscope (AFM) displayed stripe patterns on the Cu surface, attributed to root-mean-square (RMS) roughness at 14.4 nm in 100 μm² area. While a-Cu had a similar surface pattern, a surface RMS increased to 25.4 nm (Fig. 1b). It indicated non-uniform and incomplete surface etching. Electron backscatter diffraction (EBSD) analysis revealed a prevalent Cu(100) facet (red) over both w-Cu and a-Cu surfaces, while various polycrystals also appeared with small domains. Chemical analyses using Auger spectra (Fig. 1) and X-ray photoelectron spectroscopy (XPS, Fig. S1†) showed intense Cu⁺ and Cu²⁺ signals for both samples, i.e., the presence of Cu₂O and CuO.⁴⁴ Quantitatively, the surface of w-Cu was covered by 81% Cu⁺ and 21% Cu²⁺ (Fig. S2†). There were no metallic Cu⁰ present in the Cu Auger spectra. In comparison, a-Cu had a partially metallic Cu surface (37%) due to etching, although the major species was Cu⁺ (63%). Native Cu oxide on w-Cu was further analysed using cross-sectional transmission electron microscopy (TEM), exhibiting CuO(111) and (002) with 0.23 and 0.25 nm d-spacings, respectively (Fig. 2a). The thicknesses were varied from 2 to 40 nm in multiple areas (Fig. 2b).

Fig. 1 Surface characteristics of Cu electrodes with different surface treatments through (a) solvent washing (top panel, w-Cu), (b) acid-etching (middle panel, a-Cu), and (c) electropolishing (bottom panel, e-Cu). Analyses of each sample showed schematic illustration, SEM image, electron backscatter diffraction (EBSD) image with a color indicator (right bottom), AFM image, and Auger Cu LMM spectrum (from the left to the right side). All scale bars in SEM and EBSD indicate 5 μm and 50 μm, respectively. RMS stands for root-mean-square roughness with nanometer unit. Dashed lines in Auger spectra indicate Cu⁺ (916.8 eV), Cu²⁺ (917.7 eV), and Cu⁰ (918.6 eV).

Fig. 2 Characteristics of native oxide on Cu foils and after air/electrolyte solution oxidation. (a and b) Cross-sectional TEM images of w-Cu in multiple areas, prepared by focused ion beam (FIB) milling. (a) d-spacing of CuO from native oxide and Cu(200) and corresponding FFT image (inset) (b) varied native oxide thicknesses of w-Cu. (c) Auger Cu LMM spectra of Cu foils after exposure to air and Ar-saturated 1 M KOH solution. (d and e) Cathodic linear sweep voltammogram (LSV) profiles of Cu foils in Ar-saturated 1 M KOH solutions. Potentials were swept from open circuit potential (0.53 V) for (d) the initial and (e) the second scan at a rate of 5 mV s⁻¹. The violet regions indicate hydrogen evolution reaction (HER).

In contrast, e-Cu had a stripe-free and even surface with Cu(100) facet (RMS: 1.9 nm, Fig. 1c). Auger and XPS analyses identified the prevalent metallic Cu⁰ (71% coverage) on the surface, demonstrating the complete elimination of the oxide layer (Fig. S1 and S2†).

Three types of Cu foils were susceptible to oxidation when electrochemical cells were assembled in ambient air and stabilized in an electrolyte solution. Thus, the actual surface states before NO₃RR tests should be identified. We analysed Cu surfaces after air-tight H-cell installation (details in Fig. S3†) and argon (Ar) gas purge into 1 M KOH solution for 10 min. Auger spectra revealed a growing Cu⁺ signal on e-Cu and a-Cu compared to a negligible change on w-Cu (Fig. 2c), indicating immediate oxidation arising from the metallic Cu sites. We also note that residual organic contaminants appearing on w-Cu were removed by soaking in the KOH solution (Fig. S1 and S4†).

However, the instantly oxidized Cu differs from the native oxide; the former was rapidly removed before or during the initial NO₃RR tests. We tracked different reduction potentials of these oxide layers in 1 M KOH solution. Cathodic linear sweep voltammetry (LSV) in Fig. 2d shows three potential regions: (i) 0.4–0.5 V (vs. RHE unless otherwise stated), (ii) 0.1–0.4 V, and (iii) −0.25–−0.45 V. The (i) includes the CuO reduction to Cu₂O, and the (ii) and (ii) regions indicate Cu(OH)₂/Cu₂O reduction to Cu⁰.^45–47 The cathodic signal of CuO was developed in (i) for all samples. However, the Cu(OH)₂/Cu₂O reduction in the (ii) region was applied for e-Cu only. With higher overpotentials (close to HER), the Cu⁺ reduction appeared for a-Cu and w-Cu, but not e-Cu, as shown in (iii) region.^46,48–50 The appearance of the cathodic peaks in (iii) is, therefore, likely due to the thick native oxide (up to 40 nm).^{45,46,48,49,51} During the second LSV scan, all oxide/hydroxide signals vanished (Fig. 2e). It indicates that the metallic Cu surface was recovered on e-Cu below 0.1 V, whereas the native oxide was reduced to below −0.4 V.

Next, NO₃RRs were examined with 50 mM KNO₃. Cathodic LSVs showed two waves for e-Cu and three for a-Cu and w-Cu (Fig. 3a–c). The initial (c₁) and second waves (c₂) appeared for all Cu foils, while the third one (c₃) at higher overpotentials only emerged for a-Cu and w-Cu (Fig. 3a–c). To identify each signal, NO₂⁻ or NO, which are key intermediates in NO₃RR, was added to a 1 M KOH solution without NO₃⁻.²⁹ Adding 50 mM NaNO₂ vanished the c₁ wave, identifying c₁ to the NO₃⁻ reduction to NO₂/NO₂⁻. NO gas was generated by Cu powder and neat nitric acid (HNO₃) reaction and introduced to the electrochemical cell through the Ar stream (Scheme S2†).⁵² Emerging NO cathodic waves notably relied on the Cu surfaces; the metallic Cu on e-Cu promoted the NO reduction at c₂ (−0.02 V) with a significant current density (J), whereas the native Cu oxide of w-Cu reduced NO at c₃ (−0.31 V). The a-Cu allowed both c₂ (−0.10 V) and c₃ (−0.28 V) due to the coexistence of both metallic Cu and Cu oxide.²¹ Consequently, metallic Cu offered more active sites for prompt NO reduction and caused the absence of c₃.

Fig. 3 Electrochemical NO₃RR for Cu foils in Ar-saturated 1 M KOH solutions with 50 mM KNO₃. (a–c) Cathodic LSVs for (a) e-Cu, (b) a-Cu, and (c) w-Cu at a scan rate of 5 mV s⁻¹. The c₁–c₃ indicate cathodic waves, and all scale bar indicates 10 mA cm⁻². NO₂⁻ and NO curves (gray) were acquired from 50 mM NaNO₂ and NO gas, respectively, instead of NO₃⁻. (d–f) Potential-dependent faradaic efficiency (FE, bar graphs with the left y-axis) of NH₃ (sky blue), NO₂⁻ (pale yellow), and undetected products (gray) by colorimetric analyses for (d) e-Cu, (e) a-Cu, and (f) w-Cu. The partial current density of NH₃ (J_NH3) is indicated as dots with the right y-axis.

For the potential aspect, e-Cu offered more positive (i.e., lower overpotentials) for c₁ (>100 mV) and c₂ (>50 mV) compared to those for a-Cu and w-Cu (Table 1). The rate-determining NO₃⁻ reduction to NO₂⁻ was significantly catalysed by the metallic Cu and improved overall reaction kinetics. Additionally, the Tafel slope of e-Cu was smaller (71.4 mV dec⁻¹) than that of a-Cu and w-Cu (>100 mV dec⁻¹) (Fig. S5†) and even superior to the ones of previously reported Cu-based catalysts (Table S1†).^{21,38,53–57}

Table 1 Evaluations of NO₃RR with different Cu foils

	Surface states Auger spectra	Cathodic potentials in LSV (V vs. RHE) | cathodic reactant(s)						FENH3 (%)		Yield rate ^b (mass yield rate) ^c	EENH3 (%)
		c 1		c 2		c 3		0 V	−0.4 V	−0.4 V	−0.4 V
a c 1 potentials were indicated from the 2nd derivatives of LSV curves. b The unit is μmol cm^−2 h^−1. c The unit is mg cm^−2 h^−1.
e-Cu	Cu^0 (71%)	0.15	NO3^−	−0.02	NO2^−, NO	NA	—	41.6	91.5	143.1 (2.43)	30.8
a-Cu	Cu^+ (63%)	0.05		−0.10	NO2^−, NO	−0.28	NO	0.7	84.8	95.5 (1.62)	28.4
w-Cu	Cu^+ (81%)	0.03		−0.08	NO2^−	−0.31	NO	0.1	56.9	43.8 (0.75)	19.0

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Furthermore, the active sites on e-Cu were stable. Cyclic voltammograms (CVs) exhibited constant NO-reducing waves during three cycles, although sample surfaces were roughened during NO₃RR (Fig. S6 and S7†). To depict electrochemical active surface area (ECSA), we estimated the roughness factors from two methods: the RMS surface roughness from the AFM measurement and the electric double layer capacitance (EDLC) relative to the specific capacitance, as shown in Table S2 and S3.† Both methods consistently resulted in the lowest roughness factor and ECSA for the as-prepared e-Cu. During NO₃RR, the EDLC of e-Cu increased more than a-Cu and w-Cu (Fig. S8a†). However, the geometric-area-normalized current densities were constant during CV cycling (Fig. S6†). In addition, e-Cu still exhibited the highest ECSA-normalized current density in the fourth cycled CV (Fig. S8 and Table S3†). It corroborated the stable and preserved active sites on e-Cu regardless of the increased surface roughness.

The selectivity and yield of NH₃ production was assessed by chronoamperometry (CA) tests and colorimetric analyses of the electrolyte solution (NO₂⁻ and NH₃ detection, Fig. S9–S11†).^12,18,21 For e-Cu, faradaic efficiency of NH₃ (FE_NH3) was 41.6% at 0.0 V and maximized to 91.5% at −0.4 V during 1 h reaction (Fig. 3d). Isotope labeling tests demonstrated that NO₃⁻ was the exclusive N source to form NH₃ (Fig. S12†). Declining FE_NH3 below −0.4 V was attributed to HER included in the increased undetected species. By comparison, w-Cu and a-Cu generated <1% FE_NH3 at 0.0 V, where NO₂⁻ was the primary product (Fig. 3e and f). A very small current density hindered a precise quantification (>100% FE), and this result is explained by an insignificant current density at c₁ in the LSV (Fig. 2b and c). High charge-transfer resistances obtained at 0.0 V and −0.1 V also illustrated sluggish NO₃⁻ reduction on the native Cu oxide (Fig. S13†). At −0.4 V, although FE_NH3 was developed to 84.8% (a-Cu) and 56.9% (w-Cu), the efficiency was still inferior to e-Cu. We also found that the undetected species became the maximum at −0.3 V from w-Cu and decreased toward the negative potential, distinct from the e-Cu trend (Fig. 3f). Undesired products, such as N₂O, NH₂OH, and N₂, were produced from w-Cu (Fig. S14†) due to weak *NO adsorption on the native oxide.^58,59 A subsequent decline of undetected species indicated a slow HER from the oxide layer that was not wholly eliminated during NO₃RR. The a-Cu showed an in-between tendency as existing both Cu⁰ and native oxide (Fig. 3e). Still, the contribution of metallic Cu for FE_NH3 was notable at −0.2–−0.4 V. As a result, e-Cu showed an excellent NH₃ yield rate and half-cell energy efficiency (Table 1 and Fig. S15†), verifying the best NH₃ selectivity.

To further shed light on the NO₃RR process, e-Cu and w-Cu surfaces were observed by operando electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) using Au@SiO₂ nanoparticles and Nafion binder (Fig. 4a, S16 and S17, and details in ESI†). Spectra of as-prepared e-Cu and w-Cu surfaces showed peaks of Cu₂O (430 cm⁻¹), Cu(OH)_x (489 cm⁻¹), and CuO (630 cm⁻¹) (the left panel of Fig. 4b, c and Table S4†). However, the oxide/hydroxide peaks from e-Cu weakened at 0.0–−0.2 V, validating a recovery of the metallic Cu surface during NO₃RR (Fig. S18†). The trend is more evident when comparing peak-height intensity with potential change, as shown in Fig. S18.† During NO₃RR, e-Cu showed that the peaks of Cu(OH)_x and CuO were almost completely absent, while Cu₂O species exhibited a relatively small decrease. In sharp contrast, w-Cu preserved all oxide peaks in 0.0–−0.5 V. At the end of NO₃RR, the peaks of Cu oxides remained on w-Cu but not on e-Cu.

Fig. 4 Operando SHINERS analysis for NO₃⁻ reduction process on e-Cu and w-Cu electrodes. (a) Schematic illustration of an electrochemical cell. The right scheme illustrates the SERS hotspot defined by the SHIN (Au@SiO₂) particle placed on top of the Cu surface undergoing a chemical reaction. (b and c) SERS spectra for (b) e-Cu and (c) w-Cu, before the test, 0.0 V–−0.5 V vs. RHE, and after the test (from bottom to top). The potential was negatively scanned with −100 mV step. Vertical dashed lines indicate vibrational frequencies of CuO, Cu₂O, Cu(OH)_x, NO₃⁻ (black), NO₂⁻ (blue), NO (violet), HNO/NH/NH₂/NH₃ (red). For the details of peak assignment, see Tables S4 and S5.† The orange, violet, and gray box indicate *NH₂/*NH₃, *NO, and byproduct intermediates, respectively. (d–f) Reaction free energy diagrams of (d) *NO₂ → *N, (e) *NO₂ → H*NO, and (f) *NO₂ → *NOH on Cu(100), Cu₂O (100), and CuO(100) at pH = 14, calculated by DFT. The insets showed the optimized structures of three final intermediates adsorbed on Cu(100) (brown, blue, red, and white represent Cu, N, O, and H, respectively).

NO₃RR intermediates were observed in 900–1700 cm⁻¹ region (the right panel of Fig. 4b, c and Table S5†), where peaks of Nafion binder and residual citrate used for the synthesis of Au nanoparticles (Fig. S19†) also appeared, marked as ● and ○, respectively. Before applying a potential, a strong symmetric stretching peak of NO₃⁻ (ν_s(NO₃⁻), 1047 cm⁻¹) in the solution emerged. At 0.0 V, this intensity weakened on e-Cu, while asymmetric NO₂ of NO₃⁻ peak (ν_as(NO₂), 1354 cm⁻¹)^60,61 was intensified, possibly due to *NO₃⁻ adsorption (the right panel of Fig. 4b). Concurrently, peaks of *NO₂ intermediates were observed as the result of NO₃⁻ reduction,^9,35–37 assigned to ν_as(*ONO*) (1290 cm⁻¹) and ν_as(*ON = O) (1367 cm⁻¹). More importantly, the appearance of *NO (1608 cm⁻¹) at 0.0–−0.1 V signified the suitable binding strength with e-Cu and the occurrence of subsequent hydrogenation. It caused the appearance of peaks of *NH₂ (δ_s(H*NH), 1320 cm⁻¹) at 0.0–−0.3 V and growing *NH₃ associated δ_as(H*NH) (1578 cm⁻¹) at −0.3 V. Rapid NH₃ production at −0.4 V then attenuated all Raman peak intensities, and HER obscured signals at −0.5 V.

In comparison, w-Cu showed intense peaks of *NO₂ (1367 cm⁻¹ of *ON=O and 1450 cm⁻¹ of *ON*=O) at 0.0–−0.2 V (the right panel of Fig. 4c). Unlike the e-Cu case, new hydrogenated species emerged, such as δ(NH₂) (1160 cm⁻¹) possibly arising from NH₂OH,⁶²δ(*NH) (1520 cm⁻¹),^62,63 and δ(H*NO) (1534 cm⁻¹), with strong intensities. We presume that they are intermediates of byproducts (N₂O, NH₂OH, and N₂) rather than those of NH₃. Meanwhile, *NO had not yet emerged in this potential range, suggesting that the accessible NO is weakly absorbed on w-Cu or reacts undesirably with re-dissolved NO, which was supported by the increased undetected species in Fig. 3f. The apparent *NO peak was observed at −0.3 V, which aligned with the NO reduction potential (c₃) in the LSV (Fig. 3c) and the increased FE_NH3 below −0.3 V (Fig. 3f).

DFT calculations also supported the notable influence of the Cu valence state on *NO₂ and *NO reductions using three Cu surface models: Cu(100), Cu₂O(100), and CuO(100) (Fig. S20 and Computational details in ESI†). We first examined the formation of *NO from the reduction of *NO₂ and calculated Gibbs free energy changes (ΔG) on different Cu surfaces at pH = 14 (Tables S6–S8†). The ΔG value is positive on all Cu surfaces, implying a thermodynamically energy-consuming reduction process. However, each surface has only a small difference in ΔG, around 0.75 eV.

Subsequent reduction of *NO through the ΔG was calculated. Simulated intermediates were suggested as *N, H*NO, and *NOH,^11,18,64,65 which are different in deoxygenation, hydrogenation, and final product determination steps.^65–67Fig. 4d–f shows each intermediate's Gibbs free energy diagrams on Cu surfaces at pH = 14. The optimized structures of three intermediates adsorbed on each surface were presented in the inset and Fig. S21–S23.† The Cu(100) surface had the most favourable ΔG value of −0.027 eV for forming *N from *NO. In comparison, the other oxide surfaces had significantly large positive values (>1.8 eV) due to the thermodynamic barrier toward *N, which was suggested to be a highly NH₃-selective intermediate.⁶⁵ The ΔG for the formation of H*NO and *NOH are also calculated through *NO reduction. Cu₂O(100) and CuO(100) surfaces showed the lowest ΔG for H*NO compared to *N and *NOH, consistent with the appearance of H*NO species in the Raman spectrum at −0.1–−0.2 V (Fig. 4c). Nonetheless, the energy barrier for H*NO and *NOH formation with *NO reduction was lower at the Cu(100) surface than at Cu oxides. It suggests that all intermediates from the *NO reduction can be produced more easily on the metallic Cu surface, consistent with the available experimental results.

3. Conclusions

In summary, we studied Cu surface states for NO₃RR activity and selectivity. The native oxide layer on Cu involved slow NO₃⁻ and NO reduction and produced undesired products. In contrast, metallic Cu with an even surface produced NH₃ with better activity and selectivity. Although the metallic Cu was immediately oxidized in air or electrolyte solution, this oxide was rapidly reduced below 0.1 V. Therefore, the metallic Cu⁰ surface was recovered in contrast to the persistent native oxide. Electrochemical tests, operando Raman spectroscopy, and DFT calculations consistently demonstrated the metallic Cu as the active sites of NO₃RR, attributed to suitable *NO₃⁻ and *NO adsorptions and following their rapid reductions.

Data availability

All data associated with this publication are provided in the ESI.†

Author contributions

Y. K. and H. R. B. designed the experiments; Y. K. and M. S. conducted electrochemical experiments; J. K. contributed to DFT calculations on free energy change under the supervision of Y. J.; J. P., H. S, and Z. H. K. advised to setup and interpret SHINERS experiments.; H. S. simulated the Raman peak position of intermediates under the supervision of Z. H. K; Y. K., H. R. B., J. K., and Y. J. wrote the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. NRF-2019R1A2C2007551 and 2021R1A5A1030054).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3sc05793c

‡ Contributed equally to this work.

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