English

• Ammonia (NH₃) plays a pivotal role in the development of human society and has a vital impact on the global economy [1,2]. Recently, NH₃ has also been recognized as a crucial energy storage medium [3]. Currently, NH₃ production depends on the Haber-Bosch (H-B) process under harsh conditions. This process not only consumes a significant amount of global energy but also contributes to over 2% of global CO₂ emissions [4]. Therefore, it is imperative to explore green methods for ammonia synthesis under milder conditions. One promising alternative is the NRR at ambient conditions using N₂ and H₂O, which could potentially replace the energy-intensive H-B process [5,6]. Presently, NRR catalysts are categorized into noble metal-based [7-9], non-noble metal-based [10,11] and carbon-based materials [12]. Despite these advancements, achieving commercial viability for NRR remains difficult due to challenges in N₂ adsorption and dissociation, along with the competing hydrogen evolution reaction (HER) [13,14].

  Ruthenium (Ru), as a second-generation NH₃ catalyst, has been widely considered as an alternative to iron (Fe) catalyst in the H-B process at lower temperatures [15]. Meanwhile, Ru exhibits favorable activity for NRR due to its position at the top of volcano distribution map [16]. However, according to scaling relations, catalysts that strongly adsorb N₂ often inhibit the hydrogenation of nitrogen-related intermediates [17,18]. Density functional theory (DFT) calculations highlight that Ru is limited by the reductive desorption of intermediates to form NH₃ [18]. Experimental investigations further indicate that the robust binding interaction between N-containing intermediate and the Ru surface leads to decreased NRR performance [19].

  Improving the NRR performance of Ru-based electrocatalysts requires to weaken the adsorption strength of intermediates on Ru surfaces. Constructing multi-site electrocatalysts through alloying presents a promising approach for enhancing NRR performance [20-22]. Notably, nickel (Ni) metal possesses a relatively strong hydrogen adsorption energy and weak interaction with N₂ [23]. For instance, Jiang et al. reported that Ni₃Mo exhibits separated active sites, enabling simultaneous activation of N₂ and hydrogenation of N-containing species, resulting in efficient NRR [24]. Similarly, Li et al. demonstrated that inorganic Au-Ni couples boost the FE of Au electrocatalysts to 67.8%, where Ni is regarded as an electron donor [25]. Additionally, Xu et al. developed the NiSb alloy as a promising electrocatalyst by optimizing the adsorption/desorption of intermediates [26].

  Inspired by these concepts, we fabricated a RuNi catalyst through a facile unipolar pulse electrodeposition method for electrocatalytic NRR under ambient conditions. Compared to unitary Ru and Ni, RuNi achieved an impressive NH₃ yield rate under neutral conditions. In situ surface enhanced spectroscopic techniques revealed that its superior NRR performance is traced back to the H* generated on the Ni sites of RuNi, while the activation/dissociation of N₂ occurs at the Ru site, thereby synergistically enhancing the catalytic performance of NRR. Meanwhile, theoretical investigations reveal that specific adsorption of N₂H* intermediates of RuNi is optimized in the presence of Ni. We designed a combined device for the neutral electrosynthesis of NH₃ from N₂.

  The RuNi alloy was electrodeposited on CP using a unipolar pulse electrodeposition approach [27]. To ensure the reduction of Ru³⁺ and Ni²⁺, a deposition potential of −1.4 V vs. Hg|Hg₂SO₄ was applied for 1 s [28]. An open circuit potential (OCP) was maintained for 1 s to allow the recovery of metal ion concentration. As a control, monometallic Ru and Ni were obtained using the same method but without the precursor of Ni²⁺ or Ru³⁺. XRD patterns of RuNi/CP, Ru/CP and Ni/CP are shown in Fig. 1a. The sharp and distinct diffraction peaks of Ni/CP were observed, identifying the presence of face-centered cubic (fcc) structured Ni [29]. However, no characteristic diffraction peaks corresponding to Ru were detected in Ru/CP, indicating a low content of Ru [27]. The addition of Ru into the Ni lattice caused the diffraction peaks of RuNi/CP to shift negatively compared to Ni/CP, confirming the formation of RuNi alloys [30,31]. Additionally, a diffraction peak was assigned to carbon at 2θ = 54.4°.

  Figure 1

  

  Figure 1.  (a) XRD pattern of RuNi/CP, Ru/CP, and Ni/CP. (b) HRTEM images and (c) HAADF-STEM image and elemental mapping images of RuNi. (d) XPS survey spectra of RuNi, (e) Ru 3p region of Ru and RuNi, (f) Ni 2p region of Ni and RuNi.

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  SEM images revealed agglomerate-like particles uniformly distributed on the CP (Fig. S1a in Supporting information). Further magnification showed that these particles were aggregates of smaller nanoparticles of approximately 100 nm in size (Fig. S1b in Supporting information). High-resolution TEM images (Fig. 1b) showed spacings of 0.223 nm, which is larger than that of the (111) crystal plane of Ni [32]. Elemental mapping analysis in Fig. 1c displayed the existence of Ru and Ni and homogeneous element distribution in the RuNi catalyst. According to HRTEM image of the pure Ru and Ni samples, the lattice fringes with 0.210 nm attributed to the (002) planes of Ru (Fig. S2b in Supporting information); for Ni samples with lattice fringes with a spacing of 0.201 nm corresponded to the (111) lattice planes Ni (Fig. S3b in Supporting information). Moreover, the synthesized materials exhibited a particle size distribution concentrated around 2–3 nm (Figs. S1d, S2c and S3c in Supporting information). The XPS full spectrum of RuNi was displayed in Fig. 1d where the peaks of Ni 2p, Ru 3p, and Ru 3d were distinctly detected. As show in Fig. 1e, the Ru 3p spectrum of RuNi exhibited peaks at 462.3 and 484.6 eV, corresponding to metallic Ru [33]. Compared with pure Ru, the binding energies of Ru 3p shifted 0.21 eV in a positive direction. Two peaks located at 855.7 and 852.5 eV were assigned to the 2p_3/2 peaks of Ni²⁺ and Ni⁰ [27]. Notably, the Ni 2p spectrum exhibited a negative shift compared to pure Ni (Fig. 1f). The positive shift of the Ru 3p spectrum and the opposite shift of the Ni 2p spectrum confirmed the existence of electron transfer between Ru and Ni [27]. X-ray absorption spectroscopy (XAS) was performed to reveal the local atomic environment of Ni and Ru in the dual-site structure. The Ni K-edge XANES spectra of RuNi determined that the valence state of Ni was close to 0 (Fig. S4a in Supporting information). The Ni K-edge EXAFS spectrum (Fig. S4b in Supporting information) showed that the peak at 2.3 Å could be originated from Ni-Ru interaction [30,34].

  The catalytic performance of the RuNi electrocatalyst was tested in an H-type cell at room temperature. To prevent contamination from environmental nitrogen sources such as NH₃ and NO_x, the nitrogen feed gas with 99.999% purity was pre-bubbled through 0.5 mol/L H₂SO₄ and 1.0 mol/L KOH (Fig. S5 in Supporting information). To assess the NRR activity of RuNi alloys, LSV were recorded in N₂-saturated versus Ar-saturated 0.1 mol/L Na₂SO₄ electrolyte. Notably, clear differences in current density were observed, indicating a possible NRR activity under a N₂ atmosphere (Fig. 2a).

  Figure 2

  

  Figure 2.  (a) LSV curves of RuNi at different atmosphere. (b) NH₃ yield rate and corresponding FE at each applied potential. (c) ¹H NMR spectra of the products after electrolysis at −0.2 V vs. RHE using different feed gases. (d) The cycling tests of electrochemical NRR by RuNi.

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  The possible products, NH₃ and hydrazine, were analyzed using spectrophotometric methods (Figs. S6 and S7 in Supporting information). Chronoamperometric tests were conducted under different potentials for 7200 s to explore NRR performance (Fig. S8a in Supporting information). The current density increased with more negative potentials. The UV–vis absorption spectra were obtained after the electrocatalytic reaction (Fig. S8b in Supporting information). The most prominent absorbance peak at 665 nm was detected at −0.2 V vs. RHE, indicating the maximum concentration of NH₃. The NH₃ yield rate and FE of RuNi at various potentials were calculated using a specific formula and the results are presented in Fig. 2b The RuNi catalyst exhibited a significant NH₃ yield rate of 5.07 µg h^-1 cm^-2 at −0.2 V vs. RHE. Additionally, it achieved a FE of 26.2% at −0.1 V vs. RHE. During potential decay, both FE and NH₃ yield rate decreased, suggesting competitive factors such as the HER [35]. This optimal NH₃ yield rate at a low potential was comparable to previously reported work (Table S1 in Supporting information). Moreover, no discernible hydrazine was detected, as shown by the same absorbance before and after electrolysis (Fig. S9 in Supporting information).

  Isotopic experiments and multistep comparative experiments were conducted to confirm the origin of NH₃ generated from NRR over the RuNi catalyst. In Fig. 2c, the ¹H NMR spectra of the electrolyte obtained after NRR experiment under a ¹⁴N₂ atmosphere exhibited triplet peaks, responding to ¹⁴NH₄⁺. While the NMR spectra showed doublet coupling peaks under a ¹⁵N₂ atmosphere, representing ¹⁵NH₄⁺ [36]. This provided a strong evidence that the ammonia was indeed produced via the electrochemical NRR process [37]. Moreover, UV–vis spectra of the electrolyte and corresponding NH₃ contents were tested under different conditions (Figs. S10 and S11 in Supporting information). Contrast experiments at OCP with N₂ supply and at −0.2 V vs. RHE in an argon-saturated electrolyte were also conducted. The UV–vis spectra of the electrolytes closely overlapped with the 0.1 mol/L Na₂SO₄ background, suggesting no NH₃ contamination. Furthermore, the absence of ammonia was observed in the electrolyte after electrolysis of a pristine CP electrode, indicating that the CP neither possessed NRR activity nor caused contamination. Additionally, the UV–vis spectra of ultrapure water, Na₂SO₄ electrolyte, and the electrolyte bubbled with purified N₂ were determined; no NH₃ contaminants were detected (Fig. S11). These results suggested that NH₃ was generated from N₂ sources via the NRR process. The electrocatalytic stability of RuNi was further examined at −0.2 V vs. RHE for six cycles (Fig. 2d). By adjusting the pulse duration, the atomic ratio of Ru/Ni first decreases and then increases. The optimal catalytic performance is achieved when the pulse duration is set to 4000 cycles (Fig. S12 in Supporting information). Similarly, the RuNi catalyst exhibits optimal catalytic performance when the deposition potential is −1.4 V vs. Hg|Hg₂SO₄ (Fig. S13 in Supporting information). The flat and repeatable i-t curves exhibited negligible degradation during the cycling process. The corresponding NH₃ yield rate and FE showed slight fluctuations, indicating that the RuNi maintained good stability (Fig. S14a in Supporting information). Long-term stability was tested at −0.2 V vs. RHE for 24 h, with the current density profile against time exhibiting negligible attenuation (Fig. S14b in Supporting information). The NRR activity and stability of RuNi catalyst was also carried out in acidic sodium sulfate electrolytes (pH 2.7, 0.1 mol/L Na₂SO₄) and 0.1 mol/L NaOH. As shown in Fig. S15 (Supporting information), the RuNi catalyst showed a remarkable performance and excellent stability under neutral conditions (0.1 mol/L Na₂SO₄).

  As illustrated in Fig. 3a, the current response of the RuNi electrode in N₂-saturated electrolyte was significantly higher compared to that of the unitary Ru and Ni electrodes under the same conditions, confirming its superior NRR performance. Ru (1.88 µg h^-1 cm^-2, 9.42%) and Ni (0.36 µg h^-1 cm^-2, 2.86%) exhibited significantly lower NRR activity. Compared with counterparts, the NH₃ yield rate of RuNi was the highest, nearly 2.7 and 14.1 times greater than those of pristine Ru and Ni, respectively (Fig. 3b). Utilizing the synergistic effect from Ru and Ni contributed to improvement of NRR performance. The reasons for improved NRR performance of RuNi were further analyzed through the conductivity and electrochemically active surface area (ECSA) of different samples. As illustrated in Fig. 3c, the RuNi catalyst possessed significantly lower charge transfer resistance (R_ct) than both Ru and Ni, as determined by electrochemical impedance spectroscopy (EIS) analysis. This lower R_ct could facilitate charge transfer and thus promote NRR kinetics [38]. The ECSA was assessed by the double-layer capacitance (C_dl) (Fig. S16 in Supporting information) [39]. As shown in Fig. 3d, RuNi (16.20 mF/cm²) exhibited a larger ECSA than Ru (5.01 mF/cm²) and Ni (0.17 mF/cm²), revealing the presence of more active sites for RuNi. The poor stability of RuNi, related to dissolution or corrosion, restrict its large-scale applications. To address this issue, introducing non-metal dopants such as C and N has been proposed as an effective strategy to improve the stability of the catalyst [40].

  Figure 3

  

  Figure 3.  (a) LSV curves of Ru, Ni, and RuNi in N₂-saturated 0.1 mol/L Na₂SO₄. (b) Comparison of NH₃ yield rate and corresponding FEs of different catalysts. (c) EIS (inset: the equivalent circuit) and (d) ECSA profiles of N₂ electrolysis on Ru, Ni, and RuNi.

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  To gain further insights, in situ surface-enhanced Raman spectroscopy was performed to monitor possible reaction intermediates in N₂ electrolysis conditions at different potentials. In Fig. 4a, with increasing potential from OCP to −0.3 V vs. RHE, two vibration bands at approximately 1170 and 1527 cm^-1 could be detected on RuNi, which attributable to the N—N vibration in -N₂H_y and -NH, respectively [37,41]. The intensity of two peaks increased with the rising potential, reaching a maximum at −0.2 V vs. RHE. The features disappeared at −0.3 V vs. RHE due to HER. Meanwhile, the N—N vibration and -NH were observed for Ru (Fig. S17a in Supporting information). These operando Raman results demonstrate that Ru serves as active centers for N₂ activation in the NRR [42]. However, no such phenomenon appeared for Ni at low potential (Fig. S17b in Supporting information), indicating negligible chemisorption of N species on Ni, which is less active for the NRR. Moreover, two peaks, attributed to hydrated nickel, were observed at 800 cm^-1 and 1864 cm^-1 on RuNi catalyst, with intensities increasing as the overpotential increased (Fig. 4a) [43]. Similar features were detected on the Ni catalyst as well, suggesting that H₂O decomposes into H, which are subsequently adsorbed on Ni sites (Fig. S17b). Further, in situ Raman spectra of RuNi catalyst were obtained in deuterium reagents (Na₂SO₄/D₂O). Ni-D stretching vibration peaks were detected between 1400 cm^-1 and 1500 cm^-1, accompanied by the emergence of N-D peaks (Fig. 4b) [44]. These results indicated that hydrogen radicals generated at Ni sites directly contribute to the NH₃ formation.

  Figure 4

  

  Figure 4.  In situ surface enhanced Raman spectra on RuNi under N₂ atmosphere: (a) Na₂SO₄/H₂O and (b) Na₂SO₄/D₂O.

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  To further verify the influence of Ni, EPR technology was tested to detect the existence of hydrogen radicals using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a radical-trapping reagent. The electrocatalysis was operated in 0.1 mol/L Na₂SO₄. To rule out possible interference, DMPO was tested before the electrochemical test and no EPR peak was detected (Fig. S18 in Supporting information). Nine EPR peaks were observed with an intensity ratio of 1:1:2:1:2:1:2:1:1 and hyperfine coupling constants calculated as A_H = 22.5 G and A_N = 16.7 G when electrocatalysis was run under an Ar atmosphere for Ni and RuNi [45]. These peaks were assigned to DMPO—H [44,46]. Ru exhibited a weaker DMPO—H signal than Ni under the Ar atmosphere (Fig. 5a). This observation demonstrated that massive *H generated by water splitting on Ni [47]. However, a new signal corresponding to *OH was presented for RuNi under a N₂ atmosphere, revealing that the H* produced by Ni was consumed in the NRR process (Fig. 5b) [48]. The adsorption energy is an important descriptor for evaluating the strength of the interaction between intermediates and electrocatalyst surfaces, which is used to assess trends in catalytic activity [49]. However, linear scaling relations inhibit advancements in catalytic performance [50]. According to the Sabatier principle, optimal performance is achieved with moderate adsorption energies of intermediates on the electrocatalysts [51]. Thus, the adsorption properties of the key intermediate N₂H* on various systems [including the Ni (111), RuNi and Ru (002) surfaces] were calculated to confirm an optimal electrocatalyst using DFT calculations [52]. We first investigate three possible adsorption sites for N₂H* on RuNi surface: the top, bridge, and hollow sites (Fig. S19 in Supporting information). The top site of RuNi exhibits a stronger adsorption capacity than the other sites, indicating that the favorable adsorption site for N₂H* is the top site. Additionally, the adsorption properties of N₂H* on the top sites of both pristine Ru and Ni surfaces were calculated (Fig. 5c). The results suggest that pure Ru has stronger N adsorption, whereas RuNi possesses weaker N adsorption strength and achieves destabilization of the reaction intermediates. Thus, RuNi circumvents the restriction of linear scaling relations and achieves good performance for NRR. In addition to the efficient nitrogen dissociation, an excellent catalyst for nitrogen fixation should exhibit facile desorption of NH₃ [53]. Therefore, the desorption of NH₃ for all the catalysts was calculated. Compared to mono-metallic Ru or Ni, the desorption energies of NH₃ of RuNi was only 0.56 eV (Fig. S20 in Supporting information), facilitating NH₃ release of the active site for new reactions.

  Figure 5

  

  Figure 5.  (a) EPR spectra of different samples under Ar. (b) EPR spectra of RuNi under different gases. (c) Calculated adsorption energies of *N₂H values of various systems, including the Ni, RuNi and Ru surfaces.

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  In summary, a dual-site catalyst, RuNi, was developed using a straightforward unipolar pulse electrodeposition method for the electrocatalytic synthesis of NH₃. Remarkably, RuNi demonstrated an exceptionally high NH₃ yield rate of 5.07 µg h^-1 cm^-2 at −0.2 V vs. RHE and a FE of 26.2% at −0.1 V vs. RHE, greatly surpassing performance of Ru in a neutral aqueous electrolyte. The superior performance of the dual-site system can be attributed to the synergistic effects of N₂ adsorption and activation on the Ru site, coupled with the formation of hydrogen radical on the Ni sites and decrease of the adsorption energy of *N₂H on RuNi. This work not only introduces a novel design strategy for alloy nanocatalysts but also provides valuable insights for the application of similar approaches to other small molecules electrocatalysis in renewable technology.

  Declaration of competing interest

  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.

  CRediT authorship contribution statement

  Tan Zhang: Writing – review & editing, Writing – original draft, Investigation, Formal analysis, Data curation, Conceptualization. Zhikai Che: Visualization, Validation, Investigation, Formal analysis. Yuru Song: Visualization, Investigation, Formal analysis, Data curation. Jinping Li: Writing – review & editing, Supervision, Project administration, Investigation, Funding acquisition. Yuhan Sun: Writing – review & editing, Supervision, Resources, Investigation. Guang Liu: Writing – review & editing, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (Nos. U22A20418, 22075196), the Research Project Supported by Shanxi Scholarship Council of China (No. 2022–050).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111295.

  
  
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  Figure 1  (a) XRD pattern of RuNi/CP, Ru/CP, and Ni/CP. (b) HRTEM images and (c) HAADF-STEM image and elemental mapping images of RuNi. (d) XPS survey spectra of RuNi, (e) Ru 3p region of Ru and RuNi, (f) Ni 2p region of Ni and RuNi.

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  Figure 2  (a) LSV curves of RuNi at different atmosphere. (b) NH₃ yield rate and corresponding FE at each applied potential. (c) ¹H NMR spectra of the products after electrolysis at −0.2 V vs. RHE using different feed gases. (d) The cycling tests of electrochemical NRR by RuNi.

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  Figure 3  (a) LSV curves of Ru, Ni, and RuNi in N₂-saturated 0.1 mol/L Na₂SO₄. (b) Comparison of NH₃ yield rate and corresponding FEs of different catalysts. (c) EIS (inset: the equivalent circuit) and (d) ECSA profiles of N₂ electrolysis on Ru, Ni, and RuNi.

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  Figure 4  In situ surface enhanced Raman spectra on RuNi under N₂ atmosphere: (a) Na₂SO₄/H₂O and (b) Na₂SO₄/D₂O.

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  Figure 5  (a) EPR spectra of different samples under Ar. (b) EPR spectra of RuNi under different gases. (c) Calculated adsorption energies of *N₂H values of various systems, including the Ni, RuNi and Ru surfaces.

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