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Article

Growing Nanocrystalline Ru on Amorphous/Crystalline Heterostructure for Efficient and Durable Hydrogen Evolution Reaction

by
Quanbin Huang
,
Xu Zhang
,
Li Tong
,
Yipu Liu
* and
Shiwei Lin
*
Key Laboratory of Pico Electron Microscopy of Hainan Province, School of Materials Science and Engineering, Hainan University, Haikou 570228, China
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(5), 434; https://doi.org/10.3390/catal15050434
Submission received: 31 March 2025 / Revised: 24 April 2025 / Accepted: 27 April 2025 / Published: 29 April 2025
(This article belongs to the Section Photocatalysis)
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Abstract

:
The design of efficient hydrogen evolution reaction (HER) catalysts to minimize reaction overpotentials plays a pivotal role in advancing water electrolysis and clean energy solutions. Ru-based catalysts, regarded as potential replacements for Pt-based catalysts, face stability challenges during catalytic process. The precise regulation of metal–support interactions effectively prevents Ru nanoparticle degradation while optimizing interfacial electronic properties, enabling the simultaneous enhancement of catalytic activity and stability. Herein, we design an amorphous/crystalline support and employ in situ replacement to develop a Ru-NiPx-Ni structure. The crystalline Ni phase with ordered atomic arrangement ensures efficient charge transport, while the amorphous phase with unsaturated dangling bonds provides abundant anchoring sites for Ru nanoclusters. This synergistic structure significantly enhances HER performance, which attains overpotentials of 19 mV at 10 mA cm−2 and 70 mV at 100 mA cm−2 in 1 m KOH, with sustained operation exceeding 55 h at 100 mA cm−2. Electrochemical impedance spectroscopy analysis confirms that the Ru-NiPx-Ni structure not only has a high density of active centers for HER, but also reduces the charge transfer resistance at the electrode–electrolyte interface, which effectively enhances HER kinetics. This study presents new directions for designing high-efficiency HER catalysts.

1. Introduction

Green hydrogen production plays a crucial role in realizing carbon neutrality, supplying a clean energy source essential for the global shift towards industrial transformation and sustainable development [1,2]. Employing renewable energy-driven electricity for water electrolysis to produce hydrogen is a key method for achieving green hydrogen [3,4,5]. However, water electrolysis for hydrogen production faces high overpotentials from both the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER), requiring efficient electrocatalysts to reduce these overpotentials and minimize energy consumption [6,7,8]. At present, Pt/C and IrO2 are the benchmark catalysts for HER and OER, respectively, owing to their outstanding catalytic efficiency [9,10,11]. Nevertheless, their scarcity and high cost constrain their large-scale industrial applications [12,13,14,15]. Therefore, the design of cost-effective catalysts is essential to accelerate the development of water electrolysis.
Ru-based catalysts are considered viable replacements for Pt-based catalysts due to their comparable hydrogen adsorption energy, lower water dissociation barriers, and cost advantages [16,17,18,19]. For example, Yin et al. synthesize Ru-cluster-loaded NiFe-LDH (Ruc/NiFe-LDH) via a one-step solvothermal method [20]. The Ru-O-Ni/Fe bridge bonds formed between Ru and the LDH support effectively anchor Ru clusters, modulating the local coordination environment and optimizing the electronic structure. This synergy achieves a balanced adsorption/desorption energy for reaction intermediates, enabling Ruc/NiFe-LDH to exhibit outstanding HER activity with an overpotential of only 26 mV at 10 mA cm−2. In addition, Ya et al. introduce oxygen vacancies into NiFe-LDH by utilizing H2O2 etching and subsequently anchor Ru clusters onto its surface via an impregnation technique, yielding Ru/d-NiFe-LDH [21]. The introduction of oxygen vacancies improves charge transfer at the Ru-O-Ni interface and induces metastable Ru2+δ sites, destabilizing the half-filled 4d5 orbital configuration. This electronic modulation optimizes intermediate adsorption/desorption, thereby boosting HER activity. Ru/d-NiFe-LDH achieves 10 mA cm−2 at a low overpotential of 18 mV in 1 m KOH. However, Ru-based catalysts in the HER process undergo Ostwald ripening, aggregation, dissolution, and detachment, which limit their long-term stability [22,23]. Metal–support interaction (MSI) is recognized as a crucial strategy for stabilizing active metals, reducing the content of noble metal, and maintaining catalytic activity, thereby enabling cost-effective water electrolysis [24,25]. Thus, precise modulation of the MSI between Ru and the support is essential for suppressing Ru nanoparticle aggregation and dissolution, thereby enhancing the catalytic stability. Additionally, the design of Ru-based catalysts has increasingly shifted toward integrating sustainability with performance optimization [26,27]. Environmentally friendly methods, particularly electrodeposition and impregnation at ambient temperature, provide low-energy scalable routes that fulfill the demands of green chemistry and industrial feasibility [28,29,30].
Herein, we develop a heterojunction support consisting of amorphous NiPx and crystalline Ni nanoparticles (NiPx-Ni). The introduction of Ru is governed by a surface Ni replacement reaction, ensuring its stable anchoring to NiPx-Ni and substantially reinforcing Ru stability (denoted as Ru-NiPx-Ni). X-ray photoelectron spectroscopy (XPS) results indicate that Ru reconstructs the electronic structure of the NiPx region, while the Tafel slope suggests that Ru facilitates the Volmer step during HER. Electrochemical impedance spectroscopy (EIS) analysis further confirms that Ru-NiPx-Ni exhibits a high coverage of hydrogen intermediates (H*) at corresponding surface structure and a quite low charge transfer resistance at the electrode–electrolyte interface. The Ru-NiPx-Ni exhibits outstanding HER activity in 1 m KOH, requiring overpotentials of 19 mV at 10 mA cm−2 and 70 mV at 100 mA cm−2, respectively. Furthermore, the catalyst operates steadily at 100 mA cm−2 for at least 55 h, highlighting its outstanding long-term stability.

2. Results and Discussion

Electrodeposition provides an ideal pathway for designing and synthesizing amorphous/crystalline support structures due to its unique kinetic regulation and non-equilibrium deposition characteristics [31]. Based on its features, we synthesize a short-range ordered amorphous NiPx matrix enriched with metallic Ni clusters (denoted as NiPx-Ni), through the co-reduction of Ni2+/H2PO2− pairs under negative potential conditions. Subsequently, the electrode potential difference between Ru3+ and Ni is strategically employed to drive the transformation from Ni0 to Ru0, achieving the synthesis of Ru-NiPx-Ni heterostructures via an in situ replacement approach (the detailed process is provided in Section 3). To further optimize the synthesis parameters, we determine the optimal deposition conditions for NiPx-Ni by comparing the HER performance (Figure S1). Unless otherwise specified, NiPx-Ni refers to the material synthesized under these optimized conditions.
The basic structural characteristics of the Ru-NiPx-Ni heterostructure are first investigated. For precise structural identification, the NiPx-Ni structure is also examined as a control counterpart. According to the X-ray diffraction (XRD) results (Figure 1a), NiPx-Ni exhibits characteristic peaks at 44°, 52°, and 76°, corresponding to the (111), (200), and (220) planes of Ni (PDF#04-0850) [32]. The XRD pattern of Ru-NiPx-Ni exhibits no characteristic diffraction peaks of Ni, while the broad peak appearing within 40–55° may result from short-range structural order and nanocrystalline contributions [33]. Additionally, the diffraction peaks located at ~26° in both NiPx-Ni and Ru-NiPx-Ni are assigned to the carbon cloth substrate [34]. Regarding the microscopic morphology of the Ru-NiPx-Ni structure, the optical microscopy image (Figure 1b inset) reveals that the Ru-NiPx-Ni fully retains the three-dimensional networked fiber structure of the carbon cloth (CC). This architecture could facilitate rapid electrolyte transport and efficient hydrogen release, further ensuring uniform loading of Ru-NiPx-Ni nanoparticles (Figure 1b). As shown in the corresponding scanning electron microscope (SEM) image (Figure 1c), Ru-NiPx-Ni exhibits a nanoparticle-assembled morphology, with particle sizes ranging from 30 to 60 nm (Figure 1c inset). Compared with the dense nanostructure of NiPx-Ni (Figure S2), this morphology significantly increases the specific surface area of Ru-NiPx-Ni. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDS) (Figure 1d) further confirm that Ru-NiPx-Ni is loaded onto CC in the form of nanoparticles, with Ru, Ni, P, and O elements uniformly distributed at an approximate atomic ratio of 1:2:0.5:0.3.
Transmission electron microscopy (TEM) imaging and selected area electron diffraction (SAED) pattern analysis further support the amorphous/nanocrystalline nature of Ru-NiPx-Ni. As revealed by the TEM image in Figure 2a, irregularly shaped crystallized nanoparticles with a broad size distribution are uniformly dispersed within the amorphous matrix. The crystalline regions in Ru-NiPx-Ni are identified as Ni and Ru through lattice spacing analysis, with the electron diffraction image (Figure 2b) conclusively confirming this assignment. Additionally, the crystalline regions are interconnected by amorphous structures. As shown in Figure 2c, the characteristic halo diffraction pattern lacks observable crystalline spots, demonstrating the amorphous characteristics of A region. The size distribution analysis reveals that Ni crystallites predominantly fall within 3~8 nm, whereas Ru crystallites exhibit a narrower range of 2~4 nm (Figure 2d,e).
The surface valence states of NiPx-Ni and Ru-NiPx-Ni are analyzed based on X-ray photoelectron spectroscopy (XPS) (Figure S3). The incorporation of Ru significantly modifies the chemical environment of NiPx-Ni. As shown in the P 2p spectrum of NiPx-Ni (Figure S3a), the peak at 129.4 eV corresponds to P-Ni bonds, while the peak at 133.2 eV is associated with residual H2PO2− from electrodeposition [35,36]. TEM results (Figure S4) indicate that the phosphide in NiPx-Ni presents in an amorphous form. Interestingly, upon Ru incorporation, Ru-NiPx-Ni exhibits only P-O bonds attributed to PO43−, with no detectable P-Ni bonds [37]. These findings suggest that Ru incorporation breaks a portion of the P-Ni bonds, resulting in P primarily existing as P-O bonds. The Ni 2p spectrum of NiPx-Ni (Figure S3b) further reveals a peak at 852.5 eV, corresponding to Ni0 species, and a peak at 856.1 eV, attributed to Ni2+ species, confirming the coexistence of metallic and oxidized Ni states in NiPx-Ni [38,39,40]. The XPS etching analysis of the Ni 2p spectrum (Figure 2f) indicates that Ni0 is absent on the Ru-NiPx-Ni surface but reappears after etching. This finding suggests that Ru incorporation predominantly occurs via a surface Ni replacement reaction, which acts as the key driving force for stabilizing Ru on NiPx-Ni. The O 1s spectrum (Figure S5a) exhibits three distinct peaks at 530.24, 531.85, and 533.38 eV, corresponding to surface oxygen, oxygen vacancies, and M–O, respectively [36,41,42]. As shown in Figure S5b, the proportion of oxygen species exhibits slight changes after etching. Notably, observable shifts in binding energies are observed for both Ru and P after etching, specifically manifesting as a downward shift in Ru binding energy and an upward shift in P binding energy (Figure 2g,h), suggesting tri-phase electron transfer dynamics in Ru-NiPx-Ni. The XPS and TEM results demonstrate that surface Ni undergoes a replacement reaction with Ru3+, facilitating the preferential exposure of the more catalytically active Ru on the surface of Ru-NiPx-Ni. Additionally, Ni, in the bulk phase, contributes to the catalytic system by providing an excellent charge transfer pathway, thereby boosting the overall catalytic performance.
The HER activity of Ru-NiPx-Ni is evaluated using a conventional three-electrode setup in 1.0 m KOH. A highly crystalline nickel phosphide (denoted as c-Ni-P) is also selected as a reference material, with Ru loading achieved via the impregnation method, which is denoted as c-Ru-Ni-P [43] (details in Section 3). The XRD results (Figure S6) indicate that c-Ni-P mainly exhibits diffraction peaks associated with Ni2P and Ni5P4. The addition of Ru does not affect the crystal structure of c-Ni-P, ensuring that c-Ru-Ni-P preserves its original structural features. The LSV curves of Ru-NiPx-Ni, c-Ru-Ni-P, NiPx-Ni, c-Ni-P, Pt/C, and pristine carbon cloth are compared (Figure 3a). Ru-NiPx-Ni demonstrates the best catalytic performance in 1.0 m KOH, achieving a current density of 10 mA cm−2 and 100 mA cm−2 at an overpotential of only 19 and 70 mV, respectively, which surpasses that of c-Ru-Ni-P (η100 = 114 mV), Pt/C (η100 = 88 mV), NiPx-Ni (η100 = 264 mV), c-Ni-P (η100 = 277 mV), and pristine carbon cloth (η100 = 506 mV).
The reaction kinetics of the HER process are analyzed using Tafel plots fitted according to the (η = a + b log|I|, with I representing the current density) [44] (Figure 3b). The Tafel slope of Ru-NiPx-Ni is 41 mV dec−1, which is close to that of Pt/C (45 mV dec−1), indicating that its HER process follows the Volmer–Heyrovsky mechanism [45]. The incorporation of Ru significantly lowers the Tafel slopes of NiPx-Ni and c-Ni-P from 110 mV dec−1 and 107 mV dec−1 to 41 mV dec−1 and 60 mV dec−1, respectively, implying that Ru sites play a crucial role in promoting the sluggish Volmer step kinetics. These results further reinforce the importance of Ru in enhancing HER catalytic efficiency. The double-layer capacitance (Cdl) serves as a key parameter for evaluating HER catalytic activity. The Cdl value of Ru-NiPx-Ni is 65.35 mF cm−2, significantly higher than that of c-Ru-Ni-P (36.16 mF cm−2), NiPx-Ni (21.62 mF cm−2), and c-Ni-P (12.76 mF cm−2) (Figure 3c). This result indicates that Ru-NiPx-Ni possesses a higher electrochemically active surface area (ECSA). Furthermore, the intrinsic activity of Ru-NiPx-Ni is demonstrated to be twice that of c-Ru-Ni-P and 20 times higher than both NiPx-Ni and c-Ni-P (Figure 3d). This significant difference further confirms the exceptional intrinsic catalytic activity of Ru-NiPx-Ni for HER. Furthermore, a comparison with previously reported Ru-based catalysts (Figure 3e) demonstrates the superior HER activity of Ru-NiPx-Ni compared with other counterparts.
Stability is an important criterion for assessing electrocatalyst performance [46]. To evaluate the stability of Ru-NiPx-Ni, a 55-h constant current test was performed at a current density of 100 mA cm−2 (Figure 3f). The results demonstrate that Ru-NiPx-Ni maintains its catalytic activity without noticeable decay, with no potential shift in the LSV curve observed after stability testing, suggesting its outstanding durability in the alkaline electrolyte (Figure 3g). The catalytic activity of c-Ru-Ni-P deteriorates within 5 h, with the overpotential at a current density of 100 mA cm−2 increasing from 281 mV to 442 mV. This result further confirms the critical role of the amorphous structure in maintaining the catalytic stability of Ru-based structures. The morphological analysis of the Ru-NiPx-Ni sample after the stability test (Figure 3h) reveals that its microstructure maintains nanoparticle morphology. Additionally, the XRD and XPS results of Ru-NiPx-Ni after the stability test are carefully evaluated. The XRD patterns (Figure S7a) show negligible changes before and after catalysis, indicating that the crystalline phases in Ru-NiPx-Ni remain stable. XPS (Figure S7b–d) analysis reveals minimal changes in the Ru 3p and Ni 2p spectra, whereas the P signal disappears after HER, suggesting that the NiPx phase might transform into NiOx during HER. Given the unchanged catalytic performance of Ru-NiPx-Ni, it is further speculated that the exact composition of the amorphous matrix may not be critical, as the resulting NiOx phase still functions to anchor Ru clusters, thereby preserving the catalytic stability. The above findings indicate that Ru-NiPx-Ni offers both high catalytic activity and remarkable long-term stability, positioning it as a highly promising HER electrocatalyst.
To further elucidate the origin of enhanced HER performance in Ru-NiPx-Ni and the synergistic effects between Ru and the amorphous/crystalline support, electrochemical impedance spectroscopy (EIS) analysis is conducted. The EIS impedance spectra of Ru-NiPx-Ni and comparative materials are provided in Figure 4a, indicating that the charge transfer ability of Ru-NiPx-Ni is markedly superior to that of c-Ru-Ni-P and NiPx-Ni. The Nyquist and Bode plots for Ru-NiPx-Ni, c-Ru-Ni-P, NiPx-Ni, and Pt/C are presented in Figure S8. An equivalent circuit model based on the Volmer–Heyrovsky mechanism derived from Tafel analysis is constructed to simulate the EIS data [47,48,49,50] (Figure 4b), with the fitting results presented in Tables S2–S4 [25]. More specifically, the equivalent circuit model is composed of three elements: (i) R1 represents the uncompensated resistance of the electrolyte; (ii) R2 and Q2 correspond to the resistance and active surface area during the charge transfer process for HER, representing the Heyrovsky step; (iii) R3 and C3 primarily model the accumulation process of the reaction intermediates (H* and OH*) at the corresponding surface structure, corresponding to the Volmer step [45,51]. Ru-NiPx-Ni exhibits lower R2 and R3, while Q2 and C3 increase significantly compared with NiPx-Ni, indicating that the incorporation of Ru markedly enhances HER catalytic kinetics (Figure 4c,d and Figure S9). Ru-NiPx-Ni shows a larger Q2 and a smaller R2 than c-Ru-Ni-P within the potential range of 24 mV to −96 mV vs. RHE, suggesting that the amorphous/crystalline structure increases the active site density and decreases the charge transfer resistance at the electrode–electrolyte interface. At the same time, Ru-NiPx-Ni demonstrates increased C3 and reduced R3, implying a higher H* coverage, which significantly enhances the HER process [52]. The above results further confirm that the amorphous/crystalline framework enhances the MSI of Ru, kinetically accelerating the HER process and outperforming the single-phase crystalline structures.
To elucidate the charge transfer process of Ru-NiPx-Ni for HER, the Bode plot is analyzed to track the corresponding dynamic evolution [53]. The Bode plot of Ru-NiPx-Ni (Figure S8) exhibits a smaller phase angle than that of c-Ru-Ni-P at the same applied potential, indicating that more electrons participate in HER catalysis rather than exhibiting capacitive behavior [54]. Moreover, the electron transfer ability from the substrate to the electrode–electrolyte interface can be determined using the Bode plot. The electron migration from the CC substrate to surface active sites is reflected in the high-frequency region. Comparisons of Bode plots at −6 mV and 9 mV vs. RHE reveal no significant differences in internal electron transfer capabilities among Ru-NiPx-Ni, c-Ru-Ni-P, and NiPx-Ni (Figure 4e) [45]. In contrast, the Pt/C electrode, due to its spray-coating fabrication process, shows a slightly lower internal electron transfer ability than these as-mentioned self-supported electrodes (Figure S10).
To explore the influence of P on the system, a-Ni, a-RuNi, and Ru@CC were also prepared on the CC substrate, with the specific synthesis methods outlined in Section 3. As shown in the polarization curves (Figure 4f), a-RuNi and Ru@CC achieve 100 mA cm−2 at 100 and 130 mV, respectively, while a-Ni requires an overpotential of 307 mV to achieve a current density of 100 mA cm−2, exhibiting the lowest HER performance. The HER performance of Ru-NiPx-Ni is superior to both a-RuNi and Ru@CC, demonstrating the critical role of NiPx in enhancing catalytic performance. Moreover, the reduced performance of Ru@CC in comparison with a-RuNi highlights the essential role of the Ni-based support in tuning the electronic environment of Ru. Remarkably, the performance enhancement of Ru-NiPx-Ni upon Ru incorporation is far more pronounced than all counterparts. The outstanding performance of Ru-NiPx-Ni arises from two primary aspects. On the one hand, the NiPx-Ni substrate actively regulates the electronic configuration of Ru crystallites, leading to improved intrinsic catalytic performance [55,56]. On the other hand, the amorphous NiPx phase with high defect density facilitates the anchoring of Ru nanoclusters, and the crystalline Ni component establishes a conductive pathway that significantly enhances charge transportation [57]. These synergistic effects collectively reinforce the HER activity of Ru-NiPx-Ni, leading to superior catalytic performance.

3. Materials and Methods

3.1. Chemicals and Reagents

The carbon cloth (CC) is purchased from Suzhou Zhengtai Rongxin Materials Co., Ltd. (Suzhou, China). Ruthenium (III) chloride (RuCl3) is obtained from Hefei Momentum Conservation Green Energy Co., Ltd. (Hefei, China). Sulfuric acid (H2SO4) is supplied by Xilong Scientific (Shantou, China). Nickel (II) nitrate hexahydrate [Ni(NO3)2·6H2O], Nafion 117 solution (5%), sodium hypophosphite monohydrate (NaH2PO2·H2O), and nickel (II) chloride hexahydrate (NiCl2·6H2O) are obtained from Aladdin (Shanghai, China). Ammonium fluoride (NH4F), potassium hydroxide (KOH), and urea are provided by Macklin (Shanghai, China). Deionized water with a resistivity greater than 18 mΩ cm is obtained using a Heal Force water purification system (Heal Force Bio-meditech Holdings Limited, Shanghai, China).

3.2. Surface Hydrophilization of CC

The carbon cloth is pretreated by immersing it in a three-electrode system with a 0.5 m H2SO4 solution, followed by 25 cycles of cyclic voltammetry (CV) scanning within a voltage range of 1.5~2.0 V (vs. Ag/AgCl) at a scan rate of 0.01 V s−1.

3.3. Preparation of NiPx-Ni and Ru-NiPx-Ni

The self-supported NiPx-Ni electrocatalyst is prepared via electrodeposition in a three-electrode system. A carbon plate serves as the anode, a pretreated CC (1 cm × 3 cm) as the cathode, and an Ag/AgCl electrode as the reference. The deposition occurs at room temperature in a 0.2 m NaH2PO2 and 0.2 m NiCl2 electrolyte under −1.5 V (vs. Ag/AgCl) for 400 s, forming NiPx-Ni. The electrodeposited NiPx-Ni is immersed in a 3 mg mL−1 RuCl3 solution for 12 h to obtain the Ru-NiPx-Ni electrocatalyst.

3.4. Preparation of C-Ni-P and C-Ru-Ni-P

c-Ru-Ni-P is synthesized via a hydrothermal method followed by a phosphorization process. In a typical procedure, 1.16 g of Ni(NO3)2·6H2O, 1.2 g of urea, and 0.45 g of NH4F are dissolved in 50 mL of water under continuous stirring until fully homogeneous, followed by transfer into a reaction autoclave. The Ni(OH)2 precursor is obtained through a hydrothermal reaction at 120 °C for 12 h. Subsequently, phosphorization is carried out in an Ar atmosphere using NaH2PO2·H2O. During this process, 400 mg of NaH2PO2·H2O and Ni(OH)2 are positioned at different crucibles within a tube furnace, with thermal treatment at 350 °C for 2 h, yielding c-Ni-P. c-Ni-P is immersed in a 3 mg mL−1 RuCl3 solution for 12 h to obtain the c-Ru-Ni-P electrocatalyst.

3.5. Preparation of Pt/C, A-RuNi, and Ru@CC

The Pt/C electrode is prepared by dispersing 10 mg of Pt/C in a mixture of 0.5 mL of water and isopropanol, along with 50 μL of Nafion solution, followed by ultrasonication for 2 h. The resulting suspension is drop cast onto the pretreated carbon cloth, achieving a loading of 2 mg cm−1. The synthesis of a-RuNi follows the same procedure as Ru-NiPx-Ni, except that NaH2PO2 is excluded from the electrodeposition solution. Ru@CC is obtained by directly immersing the pretreated carbon cloth in a Ru solution for 12 h.

3.6. Characterization

The phase structure of synthesized samples is characterized by X-ray diffraction (XRD, Smart Lab, Rigaku, Japan) using Cu Kα radiation at 5° min−1 from 10° to 80°. The optical microscope images are captured using an optical microscope (XSP-01/02, Yimei Scientific and Educational Instruments Co., Ltd., Shenzhen, China). Field emission scanning electron microscopy (FESEM, ZEISS Sigma 360, Carl Zeiss AG, Oberkochen, Germany) is employed to analyze the surface topography and particle distribution at a nanoscale resolution. Transmission electron microscopy (TEM, FEI Tecnai G2 F30, FEI Company, Hillsboro, OR, USA) is utilized to investigate the internal microstructure, crystallinity, and phase distribution, equipped with energy-dispersive X-ray spectroscopy (EDS) for elemental mapping. The chemical states and elemental composition of the samples are analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific Nexsa, Hillsboro, OR, USA). Depth profiling is performed by Ar ion sputtering with an etching depth of 10 nm.

3.7. Electrochemical Measurements

Electrochemical measurements are performed using a standard three-electrode system on a German Zahner electrochemical workstation. The as-prepared materials (1 cm × 1 cm) are used as the working electrode, while a carbon rod and an Hg/HgO electrode serve as the counter and reference electrodes, respectively. All recorded potentials versus Hg/HgO are converted to the reversible hydrogen electrode (RHE) scale using the Nernst equation (ERHE = EHg/HgO + 0.098 + 0.059 × pH). Polarization curves are recorded via linear sweep voltammetry (LSV) at a scan rate of 2 mV s−1, with 95% iR compensation applied unless stated otherwise. The electrochemical double-layer capacitance (Cdl) is determined from cyclic voltammetry (CV) measurements conducted at scan rates ranging from 20 to 100 mV s−1. The electrochemical surface area (ECSA) is then estimated using the equation ECSA = Cdl/Cs × ASA, where Cs is 40 μF cm−2 and ASA is the actual electrode area. Electrochemical impedance spectroscopy (EIS) measurements are conducted over a frequency range of 0.1 Hz to 100 kHz with a 10 mV AC amplitude and a DC bias potential range from −96 mV to 10 mV vs. RHE to characterize the electrode–electrolyte interface properties.

4. Conclusions

In this study, a NiPx-Ni support with amorphous/nanocrystalline heterojunction features is designed and prepared via electrodeposition, followed by the synthesis of the Ru-NiPx-Ni structure through an in situ replacement strategy. The stability comparison between Ru-NiPx-Ni and c-Ru-Ni-P confirms that the amorphous/crystalline NiPx-Ni heterostructure stabilizes Ru nanoclusters through the high defect density of the amorphous phase. Electrochemical impedance spectroscopy demonstrates that the MSI between Ru and NiPx-Ni reduces interfacial charge transfer resistance, enhances the electrochemical active surface area, and increases the coverage of hydrogen adsorption intermediates on the Ru-NiPx-Ni surface during the catalytic reaction. The NiPx-Ni amorphous/nanocrystalline heterojunction support cooperatively improves both the HER catalytic activity and the stability of Ru nanoclusters. Benefiting from these characteristics, Ru-NiPx-Ni shows overpotentials of only 19 mV and 70 mV at current densities of 10 and 100 mA cm−2, respectively, and operates stably for more than 55 h at 100 mA cm−2. This work not only investigates an efficient Ru-based catalyst for alkaline HER but also introduces a new strategy for regulating the crystal/amorphous surface composition of catalysts, providing new directions for energy related studies.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15050434/s1, Figure S1: Optimization of NiPx-Ni electrodeposition parameters: (a) NiCl2 concentration, (b) NaH2PO2 concentration, (c) deposition voltage, (d) deposition time; Figure S2: SEM image of NiPx-Ni; Figure S3: (a) P 2p and (b) Ni 2p spectra of NiPx-Ni and Ru-NiPx-Ni. (c) Ru 3p spectra of Ru-NiPx-Ni; Figure S4: TEM image of NiPx-Ni; Figure S5: (a) O 1s XPS spectra and (b) corresponding surface oxygen species distribution in Ru–NiPx–Ni before and after etching; Figure S6: XRD patterns of c-Ni-P and c-Ru-Ni-P; Figure S7: (a) XRD patterns of Ru-NiPx-Ni before and after 55 h stability test. (b) Ni 2p, (c) Ru 3p, and (d) P 2p spectra of Ru-NiPx-Ni after 55 h HER stability test (100 mA cm−2) in 1 m KOH solution; Figure S8: Nyquist plots and corresponding Bode plots for (a,b) NiPx-Ni, (c,d) Pt/C, (e,f) c-Ru-Ni-P and (g,h) RuNiPx-Ni measured at applied potentials ranging from 0.024 to -0.096 V (V vs. RHE); Figure S9: The simulated C3 values for NiPx-Ni, c-Ru-Ni-P and Ru-NiPx-Ni; Figure S10. Bode Plots of Pt/C and Ru-NiPx-Ni at 0.009 V and -0.006 V (V vs. RHE); Table S1: Summary of HER performance of reported Ru-based electrocatalysts; Table S2: Fitted parameters of the EIS data of NiPx-Ni catalyst for HER; Table S3: Fitted parameters of the EIS data of c-Ru-Ni-P catalyst for HER; Table S4. Fitted parameters of the EIS data of Ru-NiPx-Ni catalyst for HER. Refs. [58,59,60,61] are cited in the Supplementary Materials.

Author Contributions

Methodology, Q.H. and X.Z.; Validation, X.Z. and L.T.; Resources, S.L.; Data curation, Q.H.; Writing—original draft, Q.H. and Y.L.; Writing—review & editing, L.T., Y.L. and S.L.; Supervision, X.Z. and Y.L.; Project administration, Y.L. and S.L.; Funding acquisition, Y.L. and S.L. All authors have read and agreed to the published version of the manuscript.

Funding

The authors acknowledge financial support from the Natural Science Foundation of China (Grant No. 22369003), Hainan Provincial Natural Science Foundation of China (Grant No. 223QN185), Scientific Research Starting Foundation of Hainan University (Grant No. KYQD(ZR)-22022), and the Hainan Provincial Innovative Research Project of Postgraduates (No. Qhys2023-173, Qhyb2023-48).

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) XRD patterns of Ru-NiPx-Ni and NiPx-Ni. (b) Low-magnification SEM image of Ru-NiPx-Ni. Inset: optical microscopy image of Ru-NiPx-Ni. (c) High-magnification SEM image of Ru-NiPx-Ni. Inset: particle size distribution of Ru-NiPx-Ni. (d) HAADF-STEM and corresponding EDS mapping images of Ru-NiPx-Ni.
Figure 1. (a) XRD patterns of Ru-NiPx-Ni and NiPx-Ni. (b) Low-magnification SEM image of Ru-NiPx-Ni. Inset: optical microscopy image of Ru-NiPx-Ni. (c) High-magnification SEM image of Ru-NiPx-Ni. Inset: particle size distribution of Ru-NiPx-Ni. (d) HAADF-STEM and corresponding EDS mapping images of Ru-NiPx-Ni.
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Figure 2. (a) HRTEM image of Ru-NiPx-Ni (Region A shows an amorphous region, with white lines representing Ru particles and green lines representing Ni particles). The SAED pattern of (b) Ru-NiPx-Ni and (c) region A. The particle size distribution of (d) Ru nanocrystalline and (e) Ni nanocrystalline. (f) Ni 2p, (g) Ru 3p, and (h) P 2p XPS spectra of Ru-NiPx-Ni and Ru-NiPx-Ni before and after etching.
Figure 2. (a) HRTEM image of Ru-NiPx-Ni (Region A shows an amorphous region, with white lines representing Ru particles and green lines representing Ni particles). The SAED pattern of (b) Ru-NiPx-Ni and (c) region A. The particle size distribution of (d) Ru nanocrystalline and (e) Ni nanocrystalline. (f) Ni 2p, (g) Ru 3p, and (h) P 2p XPS spectra of Ru-NiPx-Ni and Ru-NiPx-Ni before and after etching.
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Figure 3. (a) Polarization curves with 95% iR correction, and (b) corresponding Tafel plots. (c) The double-layer capacitance, and (d) comparison of ECSA-normalized specific activities. (e) Performance comparison of Ru-NiPx-Ni with reported catalysts at 100 mA cm−2 current density in 1 m KOH. (f) Chronopotentiometric curve of Ru-NiPx-Ni and c-Ru-Ni-P. (g) Polarization curves of Ru-NiPx-Ni before and after stability testing with 95% iR correction. (h) SEM image of Ru-Nix-P with after stability testing.
Figure 3. (a) Polarization curves with 95% iR correction, and (b) corresponding Tafel plots. (c) The double-layer capacitance, and (d) comparison of ECSA-normalized specific activities. (e) Performance comparison of Ru-NiPx-Ni with reported catalysts at 100 mA cm−2 current density in 1 m KOH. (f) Chronopotentiometric curve of Ru-NiPx-Ni and c-Ru-Ni-P. (g) Polarization curves of Ru-NiPx-Ni before and after stability testing with 95% iR correction. (h) SEM image of Ru-Nix-P with after stability testing.
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Figure 4. (a) Nyquist plots and (b) equivalent circuit model of c-Ni-P, NiPx-Ni, c-Ru-Ni-P, and Ru-NiPx-Ni. (c) Simulated resistance values for catalysts and (d) the simulated Q2 values. (e) Corresponding Bode plots in different potentials. (f) Polarization curves of a-Ni, a-RuNi, and Ru@CC with 95% iR correction.
Figure 4. (a) Nyquist plots and (b) equivalent circuit model of c-Ni-P, NiPx-Ni, c-Ru-Ni-P, and Ru-NiPx-Ni. (c) Simulated resistance values for catalysts and (d) the simulated Q2 values. (e) Corresponding Bode plots in different potentials. (f) Polarization curves of a-Ni, a-RuNi, and Ru@CC with 95% iR correction.
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Huang, Q.; Zhang, X.; Tong, L.; Liu, Y.; Lin, S. Growing Nanocrystalline Ru on Amorphous/Crystalline Heterostructure for Efficient and Durable Hydrogen Evolution Reaction. Catalysts 2025, 15, 434. https://doi.org/10.3390/catal15050434

AMA Style

Huang Q, Zhang X, Tong L, Liu Y, Lin S. Growing Nanocrystalline Ru on Amorphous/Crystalline Heterostructure for Efficient and Durable Hydrogen Evolution Reaction. Catalysts. 2025; 15(5):434. https://doi.org/10.3390/catal15050434

Chicago/Turabian Style

Huang, Quanbin, Xu Zhang, Li Tong, Yipu Liu, and Shiwei Lin. 2025. "Growing Nanocrystalline Ru on Amorphous/Crystalline Heterostructure for Efficient and Durable Hydrogen Evolution Reaction" Catalysts 15, no. 5: 434. https://doi.org/10.3390/catal15050434

APA Style

Huang, Q., Zhang, X., Tong, L., Liu, Y., & Lin, S. (2025). Growing Nanocrystalline Ru on Amorphous/Crystalline Heterostructure for Efficient and Durable Hydrogen Evolution Reaction. Catalysts, 15(5), 434. https://doi.org/10.3390/catal15050434

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