Citation

Zhang Zunjie, Liu Mengran, Ge Bingcheng, Yang Tianfang, Wang Shuaitong, Liu Yang, Gao Shuyan. In-situ reconstructed Cu/NiO nanosheets synergistically boosting nitrate electroreduction to ammonia[J]. Chin. Chem. Lett., 2025, 36(8): 110657  

Zunjie Zhang^a, Mengran Liu^b, Bingcheng Ge^b, Tianfang Yang^a, Shuaitong Wang^a, Yang Liu^b, Shuyan Gao^b,*     

Ammonia (NH₃) plays a crucial role as a vital chemical feedstock in contemporary industrial production, finding extensive applications across diverse sectors including agriculture for fertilizers, food industry, and pharmaceuticals [1-3]. The industrial-scale production of NH₃, primarily carried out through the Haber-Bosch process, not only demands significant amounts of energy but also leads to substantial environmental pollution [4, 5]. Consequently, it has become inadequate in meeting the requirements of sustainable development. The electrochemical synthesis of NH₃ from small nitrogenous molecules under mild conditions has garnered significant attention [6-8]. In contrast to the N₂ reduction reaction, which requires overcoming high dissociation energy (941 kJ/mol) and low solubility, nitrate (NO₃⁻) possesses lower dissociation energy, higher solubility, and faster reduction kinetics [9, 10]. Moreover, the accumulation of NO₃⁻ in surface and groundwater can not only contaminate water resources but also pose a potential threat to human health [11, 12]. Therefore, it is more industrial practical, and environmentally friendly to use the electrochemical nitrate reduction reaction (NO₃RR) process to convert NO₃⁻ into NH₃ with economic value [13].

The NO₃RR process involves a complex pathway consisting of nine proton-coupled eight-electron transfers [14, 15]. Moreover, a significant overpotential is necessary to drive the reaction, leading to the competition with hydrogen evolution reaction (HER) and subsequently reducing the Faradaic efficiency (FE) of NO₃RR [16, 17]. Therefore, HER-inert catalysts such as Cu- [18, 19], Co- [20, 21], Ti- [22, 23], and carbon-based materials, as well as single-atom catalysts [24, 25], have garnered significant attention for the NO₃RR in recent years. Among these candidates, Cu-based catalysts stand out due to their remarkable activity and selectivity towards NH₃ production, coupled with their cost-effectiveness [26]. Unfortunately, Cu exhibits limited adsorption capacity for hydrogen and is unable to supply a sufficient amount of protons for effective hydrogenation during NO₃RR [27-31]. Therefore, some precious metals with excellent hydrogen adsorption–desorption capacity (such as Ru [32-34], Rh [30, 35], Au [36], Ir [37], and Pd [38-40]) are employed in conjunction with copper to ensure an ample supply of protons for the reaction. Nevertheless, the exorbitant cost constrains the utilization of noble metals. Density functional theory (DFT) calculations have shown that NiO nanoclusters can greatly accelerate volmer-step kinetics and balance the adsorption/desorption of hydrogen intermediates [41, 42]. Therefore, the appropriate distribution of NiO at Cu sites is speculated to compensate for the limited hydrogenation ability of bare Cu and enhance the performance of NO₃RR.

Inspired by the above, we synthesized a range of CuO/NiO_x-CP nanocomposite catalysts with low NiO content that grow in-situ on carbon paper to catalyze NO₃RR in 0.5 mol/L Na₂SO₄ + 0.1 mol/L KNO₃. Due to the significant adsorption of copper to nitrate and the remarkable H₂O dissociation ability of nickel oxide to generate hydrogen free radicals (^•H), the optimized sample of CuO/NiO_2.3%-CP (2.3% represents the molar ratio of Ni/Cu) exhibited superior performance with a FE of 97.9% and an NH₃ yield rate (YR) of 391.5 µmol cm⁻² h⁻¹ at −0.2 V vs. RHE. Furthermore, the dynamic evolution of the exact active site during the reaction was investigated through ex-situ X-ray diffraction (XRD) and in-situ Raman spectroscopy, revealing the structural transformation from CuO to Cu on CuO/NiO_2.3%-CP, which confirmed that Cu served as the active species involved in the NO₃RR. The electron paramagnetic resonance (EPR) verified that the presence of NiO facilitates the generation of ^•H, which can be promptly consumed by the reaction intermediate. Theoretical calculations revealed that NiO species in CuO/NiO_x-CP can boost the dissociation of H₂O to release more protons, hence lowering the reaction energy barrier of ^*NO hydrogenation on Cu surface and benefiting the formation of NH₃.

The synthesis procedure of CuO/NiO_x-CP is illustrated in Fig. 1a. Firstly, the precursor is prepared by hydrothermal method, followed by calcination in air to obtain the final sample. The morphology of CuO/NiO_2.3%-CP was characterized by scanning electron microscopy (SEM), which displays a sphere-like structure of the stacked nanosheets (Figs. 1b–d). SEM images of the other samples are shown in Fig. S1 (Supporting information). Transmission electron microscopy (TEM) was adopted for further analyzing the morphology and composition of CuO-CP and CuO/NiO_2.3%-CP. The results reveal that CuO-CP depicts a sheet-like morphology with a distinct lattice fringe of 0.253 nm, which is indexed to the (002) plane of CuO (Fig. S2 in Supporting information). TEM and high-resolution TEM (HRTEM) results of CuO/NiO_2.3%-CP also demonstrate a sheet-like image with a lattice spacing of 0.253 and 0.242 nm, which can be ascribed to the (002) and (111) crystal plane of CuO and NiO, respectively (Figs. 1e–g). In addition, the energy-dispersive X-ray spectroscopy (EDS) elemental mapping images (Fig. 1h) reveal a homogeneous distribution of Cu, Ni, and O elements within the CuO/NiO_2.3%-CP, providing evidence for the uniform combination of NiO with CuO nanosheets.

Figure 1

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Fig. 1. (a) Schematic illustration of the preparation of CuO/NiOx-CP. (b–d) SEM images of CuO/NiO2.3%-CP. (e-g) TEM and HRTEM images of CuO/NiO2.3%-CP. (h) EDS elemental mapping images of CuO/NiO2.3%-CP.

Fig. 2a shows the XRD pattern of the as-prepared samples. The diffraction peaks of CuO-CP show a series of characteristic peaks of CuO (JCPDS No. 70-6829). CuO/NiO_0.7%-CP and CuO/NiO_2.3%-CP exhibit the same peak as CuO-CP, which is attributed to the low Ni content [43, 44]. With the increases of Ni content, new diffraction peaks located at 37.2°, 43.3°, and 62.9° appear in CuO/NiO_6.7%-CP and CuO/NiO_10.6%-CP, corresponding to the (111), (200), and (220) plane of NiO (JCPDS No. 71-1179). Raman spectroscopy of CuO-CP shows three characteristic peaks of CuO located at 290, 334, and 625 cm⁻¹ (Fig. 2b) [45, 46]. With the addition of Ni, two new characteristic peaks of NiO centered at 547 and 723 cm⁻¹ appear (Fig. 2b) [47]. The X-ray photoelectron spectroscopy (XPS) full spectra of the catalysts (Fig. S3 in Supporting information) reveal the presence of Cu, Ni, O and background C. Fig. 2c displays the high-resolution Cu 2p XPS spectra, which present two peaks at binding energy of 933.2 and 953.0 eV and the corresponding strong satellite peaks that reveal the formation of Cu²⁺ [48, 49]. Additionally, in high-resolution Ni 2p XPS spectra (Fig. 2d), the peaks of about 855.3 and 873 eV could be assigned to Ni²⁺ [50, 51]. The characteristic peaks at 529.6 and 531.4 eV in the O 1s spectra can be attributed to lattice O (O_lattice) and O atoms in the vicinity of vacancies (O_defect), which confirm that the copper/nickel species exist as oxide (Fig. 2e).

Figure 2

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Fig. 2. (a) XRD patterns. (b) Raman spectra. High-resolution XPS spectra of (c) Cu 2p, (d) Ni 2p, (e) O 1s. (f) XRD pattern of CuO/NiO2.3%-CP after electrochemical reduction of different duration at −0.2 V. (g) Electrochemical in-situ Raman spectra of CuO/NiO2.3%-CP. (h) Cu 2p XPS spectra of CuO/NiO2.3%-CP. (i) HRTEM image of CuO/NiO2.3%-CP after electrochemical reduction.

Since metal oxides could be reduced during the electrocatalytic reaction, XRD, in-situ Raman, XPS and TEM were performed to investigate the reduction products. XRD results (Fig. 2f) show that the initial diffraction peak of CuO gradually disappears, while the diffraction peak of Cu gradually increases with the extension of reaction time. Fig. 2g shows the in situ Raman test results at different potential ranges from open circuit potential (OCP) to −0.35 V. At −0.15 V, Raman peaks of Cu₂O at 148 cm⁻¹ appear [52], and then disappear as the potential continued to become negative. Raman peaks of the initial CuO gradually disappear with decreasing potential. The remaining peaks centered at 547 and 723 cm⁻¹ at all applied potentials are assigned to NiO, while two peaks at 977 and 1048 cm⁻¹ correspond to SO₄²⁻ and NO₃⁻ in the electrolyte [53, 54], respectively. After electrochemical reduction, XPS characteristic peaks of Cu⁰/Cu⁺ exist without the presence of Cu²⁺ (Fig. 2h). Furthermore, Auger electron spectroscopy (AES) was used to confirm the valence state of Cu. As shown in Fig. S4 (Supporting information), the characteristic Auger peaks of Cu²⁺ (917.5 eV) were exclusively observed on the initial CuO/NiO_2.3%-CP surface. Following electrochemical reduction, only Cu⁰ (918.6 eV) was detected on CuO/NiO_2.3%-CP surface [55]. The lattice spacings of 0.209 nm in the HRTEM image (Fig. 2i) can be indexed to Cu (111) [56, 57]. The aforementioned results indicate the in-situ electrochemical reduction of CuO/NiO_x to form Cu/NiO_x, which served as active species.

The electrochemical NO₃RR activities of the as-prepared catalysts were evaluated in a typical H-type electrolytic cell in a neutral electrolyte (0.5 mol/L Na₂SO₄ + 0.1 mol/L KNO₃). The LSV curves in the electrolyte containing KNO₃ solution (Fig. 3a) demonstrate that CuO/NiO_2.3%-CP possesses a higher current density than that of other samples, indicating its superior activity in NO₃RR process. Moreover, CuO/NiO_10.6%-CP catalyst demonstrates the highest current density for HER activity without the presence of KNO₃ solution in the electrolyte, as depicted in Fig. 3b. By comparing the LSV result in different electrolytes (with/without KNO₃), CuO/NiO_2.3%-CP exhibits significantly enhanced catalytic activity for NO₃RR compared to HER (Fig. S5 in Supporting information). These findings indicate that the combination of NiO with CuO can significantly boost the NO₃RR activity of CuO.

Figure 3

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Fig. 3. (a) LSV curves in the electrolyte of 0.5 mol/L Na2SO4 + 0.1 mol/L KNO3. (b) LSV curves of all samples in 0.5 mol/L Na2SO4. (c) Tafel slopes derived from LSV analysis. (d) EIS plots. (e) FE and YR of NH3 at various potentials. (f) YR of NH3 and NO2− after reaction of CuO/NiO2.3%-CP in electrolyte with/without the presence of KNO3 at −0.2 V vs. RHE. (g) The 14NH3 and 15NH3 detections by 1H NMR spectra. (h) Cycling stability of CuO/NiO2.3%-CP at −0.2 V vs. RHE.

To gain more insight into the impact of nickel oxide on electrocatalytic activity, Tafel slope and electrochemical impedance spectroscopy (EIS) were explored. As shown in Fig. 3c, the Tafel slope of CuO/NiO_2.3%-CP (151.8 mV/dec) is significantly lower than that of CuO-CP (184.7 mV/dec), CuO/NiO_0.7%-CP (249.7 mV/dec), CuO/NiO_6.7%-CP (265.4 mV/dec) and CuO/NiO_10.6%-CP (291.5 mV/dec), demonstrating that CuO/NiO_2.3%-CP exhibits a more efficient kinetic process for NO₃RR [58]. According to the results of EIS in Fig. 3d, a simulation model (inserted in Fig. 3d) including solution resistance (R_s), charge transfer resistance (R_ct), and a constant phase element (CPE) was proposed. Fig. S6 (Supporting information) displays the fitting results of R_s and R_ct, with nearly identical R_s values (~2.7 Ω) across all samples. In addition, CuO/NiO_2.3%-CP presents the lowest R_ct (4.7 Ω) compared to that of the other samples, suggesting that the introduction of NiO enhanced charge transfer and promoted electrocatalytic kinetics in NO₃RR.

Then, we investigated the selectivity of CuO/NiO_2.3%-CP for the electrocatalytic NO₃RR at different potentials. The concentration of NH₃ in the electrolyte after electrocatalytic reaction was determined by the indophenol blue colorimetry method. The FE and YR of NH₃ under various potentials were determined by chronoamperometry and ultraviolet-visible (UV−vis) spectrophotometer absorbance tests (Fig. S7 in Supporting information). As shown in Fig. 3e, the observed FE of NH₃ production for CuO/NiO_2.3%-CP gradually increases and then declines as the applied voltage gets increasingly negative. The maximum FE of NH₃ on CuO/NiO_2.3%-CP is 97.9% at the potential of −0.2 V, at which the YR of NH₃ reaches 391.5 µmol h⁻¹ cm⁻². To assess the intrinsic activity of various samples, we determined the electrochemically active surface area (ECSA) through double-layer capacitance (C_dl) measurements and subsequently normalized the NH₃ partial current density to ECSA values. As shown in Fig. S8 (Supporting information), the partial current density of NH₃ for CuO/NiO_2.3%-CP is higher than that of the other samples, demonstrating good intrinsic activity of nitrate reduction to ammonia. The origin of the product was verified through a set of control tests performed with CuO/NiO_2.3%-CP in 0.5 mol/L Na₂SO₄ solution, as shown in Fig. 3f and Fig. S9 (Supporting information). In the absence of NO₃⁻, NH₃ and NO₂⁻ are almost undetectable after 30 min of electrolysis at −0.2 V, negating the possibility that NH₃ and NO₂⁻ come from testing system contamination, which verifies that both NH₃ and NO₂⁻ detected in the electrolyte should come from the conversion of NO₃⁻. Additionally, the isotope (¹⁴N/¹⁵N) labeling experiments were performed for further confirming the source of NH₃. As shown in Fig. 3g, in the ¹⁵NO₃⁻ electrolyte, all the NH₃ generated from electroreduction is ¹⁵NH₄⁺ without any ¹⁴NH₄⁺, confirming that the detected NH₃ is definitely derived from NO₃RR rather than environmental contamination. Moreover, the YR of NH₃ calculated by UV–vis and NMR methods demonstrates a significant level of resemblance (Fig. S10 in Supporting information). Next, the cycling stability of CuO/NiO_2.3%-CP for NO₃RR was investigated by cyclic tests. As shown in Fig. 3h, the FE and YR of NH₃ remain stable during 56 cyclic tests (147 h). Additionally, the current density exhibited no evident decline throughout the cycling tests, indicating good stability in continuous cycle experiments (Fig. S11 in Supporting information). The structural characterizations of CuO/NiO_2.3%-CP after the cyclic stability test are depicted in Fig. S12 (Supporting information). The XRD pattern of CuO/NiO_2.3%-CP exhibits three characteristic peaks of copper at 43.3°, 50.4°, and 74.1°, which correspond to those observed in the initial sample; SEM and TEM results demonstrate negligible changes in both morphology and crystalline phase; EDS elemental mapping images confirm the continued even distribution of Cu and Ni elements on the surface of CuO/NiO_2.3%-CP after long-term stability testing. These findings collectively indicate that CuO/NiO_2.3%-CP possesses good durability and stability for NO₃RR towards NH₃ production. The electrocatalytic activity of CuO/NiO_2.3%-CP for NO₃RR is then compared to that of other Cu-based and Ni-based catalysts previously reported (Table S1 in Supporting information), and it can be seen that CuO/NiO_2.3%-CP demonstrated a favorable ammonia yield and Faradaic efficiency.

Electrochemical experiments demonstrate that the synergistic action of Cu and NiO achieves a more effective electrocatalytic conversion of nitrate to ammonia than Cu. This can be attributed to the introduction of NiO, which promotes the dissociation of H₂O to produce H, thereby hastening the hydrogenation process of NO₃RR. We performed EPR measurements using DMPO as ^•H trapping reagent to monitor the formation of ^•H when performing HER/NO₃RR on the surface of CuO-CP and CuO/NiO_2.3%-CP. Upon electrolysis without NO₃⁻, CuO/NiO_2.3%-CP shows stronger DMPO-H signals than that of CuO-CP, suggesting the greater capability of CuO/NiO_2.3%-CP to drive H₂O dissociation to produce ^•H (Fig. 4a). After adding NO₃⁻, the DMPO-H signals of CuO-CP and CuO/NiO_2.3%-CP could hardly be detected (Fig. 4b). These experiment results indicate that NiO in CuO/NiO_2.3%-CP can enhance the generation of ^•H, and the generated ^•H can be rapidly consumed to promote the NO₃RR hydrogenation process.

Figure 4

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Fig. 4. (a) DMPO-H test without NO3−. (b) DMPO-H test with NO3−. (c) Gibbs free energy profile of the NO3RR. (d) The reaction Gibbs free energy changes (ΔG). (e) The kinetic barriers of H2O-dissociation process.

DFT calculations were employed to investigate the reaction Gibbs free energy of NO₃RR on Cu/NiO_x samples, considering NiO and Cu as the actual active species. The reaction pathways from NO₃⁻ to NH₃ were considered as follows: NO₃⁻ → ^*NO₃ → ^*NO₂ → ^*NO → ^*NOH → ^*NH₂OH → ^*NH₂ → ^*NH₃ → NH₃ [30]. The optimized models of the reaction intermediates adsorbed on Cu/NiO_x surface were depicted in Table S2 (Supporting information), showcasing the most favorable arrangements for adsorption. The step of ^*NO to ^*NOH exhibits the highest reaction Gibbs free energy changes (0.42 eV) during NO₃RR for Cu, showing that the hydrogenation reaction of ^*NO is the potential-determination step (PDS) on the Cu surface (Fig. 4c and Fig. S13 in Supporting information). The ΔG values of ^*NO to ^*NOH for Cu/NiO_0.7%, Cu/NiO_2.3%, Cu/NiO_6.2%, and Cu/NiO_10.6% are 0.34, 0.16, 0.27 and −0.36 eV, respectively. The formation of ^*NO₃ on Cu, Cu/NiO_0.7%, Cu/NiO_2.3%, Cu/NiO_6.2%, and Cu/NiO_10.6% surface requires overcoming a reaction energy barrier of 0.26, 0.36, 0.33, 0.58, and 0.55 eV, respectively (Fig. 4d). Clearly, the introduction of NiO into Cu effectively reduces the energy barrier of ^*NO to ^*NOH in NO₃RR process, which facilitates the formation of ^*NOH by hydrogenating ^*NO. However, with increasing NiO content, the formation of ^*NO₃ becomes difficult. According to the ΔG values of ^*NO to ^*NOH and NO₃ to ^*NO₃, Cu/NiO_2.3% demonstrates the lowest reaction energy barrier of 0.33 eV, which is consistent with the results of electrochemical tests. To further elucidate the underlying factors contributing to the enhanced hydrogenation process, we conducted an investigation into the dissociation process of H₂O on NiO and Cu. The computational results indicate that NiO exhibits a superior ability in promoting H₂O dissociation compared to Cu (Fig. 4e) [59], thereby suggesting its potential for supplying a greater number of protons for ^*NO hydrogenation.

The aforementioned DFT calculations indicate that NiO can supply an ample number of protons for hydrogenation. By combining Cu with a suitable quantity of NiO, it is possible to realize a balance between proper ^*NO₃ adsorption and an adequate proton supplement for hydrogenation. Consequently, the synergistic effect of Cu and NiO will result in a heightened selectivity of the NO₃RR for NH₃ production [26, 60].

In summary, CuO/NiO_x-CP nanocomposite catalysts have been designed and prepared to enhance the adsorption and hydrogenation performance of ^*NO_x intermediates, thereby significantly improving the NO₃RR for NH₃. Cu/NiO_x-CP was in-situ produced via dynamic reconstruction of CuO/NiO_x-CP during electrochemical NO₃RR process. Due to the synergistic effect of Cu and NiO, Cu/NiO_2.3%-CP exhibited the optimal NO₃RR performance (391.5 µmol h⁻¹ cm⁻² at −0.2 V with FE of up to 97.9%). EPR spectra tests indicate that more hydrogen radicals are produced after the introduction of NiO. Theoretical calculations demonstrate that NiO species in Cu/NiO_x-CP can facilitate H₂O dissociation to supply more protons to reduce the reaction energy barrier for ^*NO hydrogenation on the Cu surface, thereby benefiting the production of NH₃. The present study offers a straightforward and viable approach to address the issue of limited selectivity in the nitrate reduction of commonly employed Cu-based materials.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Zunjie Zhang: Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation. Mengran Liu: Software, Formal analysis, Data curation. Bingcheng Ge: Writing – review & editing, Supervision. Tianfang Yang: Formal analysis. Shuaitong Wang: Formal analysis, Data curation. Yang Liu: Writing – review & editing, Formal analysis. Shuyan Gao: Writing – review & editing, Writing – original draft, Supervision, Funding acquisition.

This work was supported by the National Natural Science Foundation of China (No. U22A20253). The DFT calculations were performed on Tianhe-2 at the Shanxi Supercomputing Centre in China.

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110657.