English

• 0.   Introduction

  Hydrogen with high energy density and zero carbon emissions upon utilization shows great potential in alleviating the energy crisis and environmental pollution^[1-4]. Among the various energy conversion technologies, fuel cells are considered one of the most efficient ways to convert chemical energy from hydrogen into electricity directly. Particularly, hydrogen fuel cells have attracted significant attention for applications ranging from portable electronics to transportation and stationary power generation^[5-6]. Proton exchange membrane fuel cells (PEMFCs) dominate current commercial applications, while their reliance on expensive platinum-based catalysts and acidic operating conditions poses challenges in terms of cost and long-term durability^[7]. In contrast, anion-exchange membrane fuel cells (AEMFCs) operate under alkaline conditions, which allow the use of non-platinum group metal-based catalysts^[8-10]. Nevertheless, the kinetics of the hydrogen oxidation reaction (HOR) in alkaline media are significantly slower than in acidic environments, thereby seriously limiting the overall performance of AEMFCs^[11-12]. Therefore, there is an urgent need to develop efficient and cost-effective HOR catalysts for alkaline conditions.

  Among the various catalysts for alkaline HOR, ruthenium (Ru)-based materials with favorable hydrogen binding energy (HBE) and relatively low cost compared to platinum have garnered increasing attention^[13-14]. Ru exhibits limited HOR activity in acidic environments; its performance significantly improves under alkaline conditions, making it a promising candidate for alkaline HOR^[15-16]. Recent studies have shown that the catalytic performance of Ru-based materials is significantly influenced by its micro-stucture^[17-21]. Amorphous metals possess a high density of unsaturated coordination sites and disordered atomic arrangements^[22-23]. In contrast, crystalline metals typically offer superior electrical conductivity and structural stability^[24-25]. Meanwhile, the oxidation state plays a critical role in modulating its interaction with hydrogen and OH intermediates, influencing the catalytic activity. Thus, the fine modulation of the crystallinity degree and the oxidation state will provide room for the enhancement of the catalytic activity. To date, few studies have investigated how the crystallinity and oxidation state of Ru affect alkaline HOR activity.

  Crystalline phase engineering has emerged as an effective strategy to optimize the activity and stability of electrocatalysts by regulating their atomic arrangements, electronic structures, and surface coordination environments^[26-29]. He et al. have demonstrated that phase engineering of Ru can significantly enhance the alkaline HOR activity, achieving over 20-fold higher performance than Pt/C^[30]. In the context of the alkaline HOR, modulating the crystallinity of catalysts can influence key factors, such as hydrogen adsorption/ desorption kinetics, electronic conductivity, and the density of active sites^[31]. Amorphous materials often exhibit enhanced catalytic activity due to their disordered atomic structures and abundant low-coordination surface atoms, which can reduce energy barriers for intermediate adsorption and improve reaction kinetics^[32-33]. For instance, Wang et al. reported that local oxidation-induced amorphization of ultrathin Pt-Ru nanowires significantly enhanced alkaline HOR activity and CO tolerance, which was attributed to the formation of amorphous domains and abundant interfacial sites that optimized hydrogen and hydroxyl adsorption^[34]. Lee et al. systematically investigated the effect of particle size and crystallinity of Ru nanoparticles on alkaline HOR activity, and demonstrated that Ru catalysts with reduced crystallinity exhibited enhanced hydrogen oxidation performance due to optimized hydrogen adsorption behavior^[35]. However, amorphous materials may suffer from poor stability and lower conductivity. Conversely, highly crystalline materials usually offer better electrical transport and structural integrity, but may have fewer active sites or overly strong hydrogen binding, leading to sub-optimal HOR performance^[33]. As such, a trade-off exists between amorphous and crystalline phase materials, while identifying the optimal phase state for maximum HOR activity remains a central challenge.

  Herein, a series of Ru nanosheets with tunable crystallinity and oxidation degree were prepared. Electrochemical measurements showed that Ru with moderate crystallinity exhibited higher activity compared to amorphous and pristine crystalline materials. The mass activity of the optimized catalyst was 2.85, 5.7, and 2.27 times that of amorphous, crystalline counterparts, and commercial Pt/C, respectively. It was also found that partial oxidation is an effective strategy for enhancing the catalytic activity.

  1.   Experimental

  1.1   Chemicals and materials

  Ruthenium acetylacetonate (Ru(acac)₃) was purchased from Energy Chemical Co., Ltd. Potassium bromide (KBr), potassium hydroxide (KOH), and sulfuric acid (H₂SO₄, 98%) were obtained from Macklin Biochemical Co., Ltd. Cupric sulfate (CuSO₄) and ethanol (CH₃CH₂OH) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The Pt/C catalyst (20% Pt) was purchased from Yilongsheng Energy Co., Ltd. Nafion solution (5%) was obtained from the Shanghai Branch of DuPont China Holding Co., Ltd. Deionized water (18.2 MΩ·cm) was used throughout all experiments. All chemicals were used as received without further purification.

  1.2   Synthesis of the catalyst

  Briefly, 30 mg of Ru(acac)₃ and 60 mg of KBr were dissolved in a mixed solvent of 3 mL deionized water and 12 mL ethanol under ultrasonication. The resulting solution was dried in air. The obtained powder was ground and subsequently annealed in a tubular furnace at 280, 360, and 400 ℃ for 90 min under an Ar atmosphere. The product was repeatedly washed with a mixture of deionized water and ethanol, yielding Ru nanosheets, which were denoted as Ru-280, Ru-360, and c-Ru-400 according to the annealing temperature.

  The Ru-RuO₂-280 sample was synthesized using a similar procedure to that of Ru-280, except that the sample was annealed in air at 280 ℃ for 90 min and subsequently cooled under Ar protection. Pure RuO₂ was obtained by annealing the precursor entirely in air.

  1.3   Materials characterization

  Transmission electron microscopy (TEM) images were acquired using a JEOL JEM-2100F microscope (Japan) operated in imaging mode at an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) images were obtained with a Zeiss Merlin Compact (Germany). Powder X-ray diffraction (PXRD) patterns were collected on a Rigaku SmartLab diffractometer (Japan) using Cu Kα radiation (λ=0.154 06 nm) at 40 kV and 150 mA to analyze the crystal structure and phase composition, with data acquired in continuous scanning mode over a 2θ range of 5°-85°, a step size of 0.020°, and a scan rate of 10 (°)·min^-1. Electron paramagnetic resonance (EPR) spectra were recorded on a Bruker A300 spectrometer (Germany). X-ray photoelectron spectroscopy (XPS) analysis was performed using a Thermo Scientific Nexsa spectrometer (United States) equipped with a monochromatic Al Kα X-ray source (hν=1 486.6 eV). The spectrometer was operated at a voltage of 12 kV and a filament current of 6 mA. High-resolution spectra were recorded with a pass energy of 50 eV and a step size of 0.1 eV.

  1.4   Electrochemical measurements

  Electrochemical tests were conducted with a conventional three-electrode system using an Autolab PGSTAT204 workstation. A Hg/HgO electrode (1.0 mol·L^-1 KOH) served as the reference electrode, a glassy carbon rotating disk electrode (GC-RDE, with disk diameter of 5.0 mm and disk area of 0.196 cm²) as the working electrode, and a carbon rod as the counter electrode. The catalyst slurry was fabricated by dispersing 1 mg of the catalyst and 4 mg of carbon black into a mixed solvent of 145 μL ethanol, 350 μL water, and 5 μL of 5% Nafion, which was sonicated for at least 3 h to form a suspension and was drop-cast onto the GC-RDE to get the working electrode.

  Electrochemical hydrogen oxidation reaction (HOR) tests were conducted in 0.1 mol·L^-1 KOH electrolyte saturated with H₂ (bubbled for 30 min before measurements). Cyclic voltammetry (CV) was first performed for 20 cycles at a scan rate of 50 mV·s^-1 to obtain stable voltammogram curves. HOR polarization curves were then recorded in H₂-saturated 0.1 mol·L^-1 KOH at a sweep rate of 10 mV·s^-1 under different electrode rotation rates (400, 900, 1 600, and 2 500 r·min^-1). All tests were carried out at room temperature.

  1.5   Kinetic analysis

  The kinetic current density (j_k) was calculated from the Koutecky-Levich (K-L) equation:

  $ \frac{1}{j}=\frac{1}{{j}_{k}}+\frac{1}{{j}_{d}} $

  where j is the measured current density, j_d is the diffusion-limited current density obtained from the Levich equation:

  $ {j}_{d}=0.62nF{D}^{\frac{3}{2}}{v}^{-\frac{1}{6}}{C}_{0}{\omega }^{\frac{1}{2}}=B{C}_{0}{\omega }^{\frac{1}{2}} $

  Here, n is the number of electrons involved in HOR, F is the Faraday constant, D is the diffusion coefficient of H₂, v is the kinematic viscosity of the electrolyte, C₀ is the solubility of H₂ in the electrolyte, ω is the rotation rate, and B is the Levich constant.

  The exchange current density (j₀) was obtained by using the Butler-Volmer (B-V) equation:

  $ {j}_{k}={j}_{0}\left({e}^{\frac{\alpha F\eta }{RT}}-{e}^{-\frac{\alpha F\eta }{RT}}\right) $

  where α is the charge transfer coefficient, R is the universal gas constant, T is the temperature, and η is the overpotential.

  1.6   Estimation of electrochemically active surface area

  The electrochemically active surface area (ECSA) was estimated by Cu underpotential deposition (Cu-UPD) stripping. Firstly, CV curves were recorded at 50 mV·s^-1 in Ar-purged 0.5 mol·L^-1 H₂SO₄ to obtain a background curve. Cu-UPD stripping was then performed in 0.5 mol·L^-1 H₂SO₄ containing 4 mmol·L^-1 of CuSO₄ for 100 s under a potential of about 0.25 V (vs RHE). The ECSA was calculated using the equation:

  $ {\rm{ECSA}}=\frac{{Q}_{Cu}}{{Q}_{s}} $

  where Q_Cu is the measured charge, and Q_s is the surface charge density of 420 μC·cm^-2, corresponding to a monolayer of copper.

  2.   Results and discussion

  Various Ru catalysts were obtained by mixing Ru(acac)₃ with KBr followed by thermal treatment^[36-37]. The annealing procedure at different temperatures and atmospheres enabled effective regulation of the crystallinity and oxidation degree of Ru (Fig. 1). In this synthesis procedure, the temperature is a critical parameter for the formation of amorphous metal, which must be situated between the melting point of metal acetylacetonate and that of KBr. Higher annealing temperature results in the formation of crystalline Ru materials. The involved KBr is also essential for controlling the formation of amorphous material. Molten KBr suppresses crystallization kinetics and stabilizes the amorphous phase by confining Ru species during thermal decomposition. In addition, molten KBr acts as a template and spatial separator, facilitating the formation of ultrathin Ru nanosheets. Annealing the mixture in air will induce the formation of RuO₂.

  Figure 1

  

  Figure 1.  Schematic illustration for the synthesis of Ru catalysts

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  SEM images reveal that the synthesized Ru-280, Ru-360, and Ru-RuO₂-280 samples exhibited nanosheet-like structures with lateral dimensions of hundreds of nanometers to the micrometer scale (Fig. 2a-2c). TEM images further confirm the 2D sheet-like microstructure (Fig. 2d-2f). High-resolution TEM (HR-TEM) observation was then employed to investigate the microstructures (Fig. 3). The HRTEM image showed that the Ru-360 nanosheets are composed of crystalline and amorphous domains interconnected with obvious interfaces (Fig. 3a). The amorphous and crystalline domains can be clearly identified in different regions of the nanosheets. In the crystalline domain, the measured lattice spacing of 0.205 nm corresponds to the (101) plane of hexagonal Ru. The existence of diffraction rings and bright spots in the selected-area electron diffraction (SAED) pattern further confirms the crystalline phase in Ru-360 (Fig. 3d). In contrast, no discernible lattice fringes were observed in the HRTEM image of Ru-RuO₂-280 (Fig. 3b). The SAED pattern displayed very weak diffraction rings (Fig. 3e), verifying the typical amorphous nature of Ru-RuO₂-280. The contrast samples of Ru-280 and c-Ru-400 were also investigated for comparison. Both samples exhibited nanosheet-like morphologies; however, distinct structural features were observed. The HRTEM image for Ru-280 does not show lattice fringes (Fig. 3c). The weak diffraction rings in the SAED pattern confirm its amorphous nature (Fig. 3f). In contrast, the c-Ru-400 product displayed well-defined and uniform lattice fringes with an interplanar spacing of 0.205 nm across the entire nanosheet (Fig.S1a, Supporting information). Its SAED pattern exhibited sharp diffraction rings with bright spots, indicating high crystallinity (Fig.S1b).

  Figure 2

  

  Figure 2.  SEM images of (a) Ru-360, (b) Ru-RuO₂-280, and (c) Ru-280; TEM images of (d) Ru-360, (e) Ru-RuO₂-280, and (f) Ru-280

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  Figure 3

  

  Figure 3.  HRTEM images of (a) Ru-360, (b) Ru-RuO₂-280, and (c) Ru-280; SAED patterns of (d) Ru-360, (e) Ru-RuO₂-280, and (f) Ru-280

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  PXRD patterns showed that the Ru-280, Ru-360, and Ru-RuO₂-280 lacked distinct diffraction peaks, while c-Ru-400 exhibited sharp diffraction peaks that match well with the standard PDF card of metallic Ru (PDF No.70-0274) (Fig. 4a)^[38]. There is a discrepancy that no PXRD diffraction peaks were shown for Ru-360, while localized crystalline domains were observed in the HRTEM image. This is reasonable. The PXRD pattern reflects long-range structural ordering, whereas the TEM observation can probe short-range atomic arrangements. The low volume fraction of crystalline domains embedded in the amorphous matrix is insufficient to generate obvious PXRD signals, but can be clearly observed by local high-resolution imaging. The above results collectively demonstrate that the crystallinity can be effectively regulated by thermal treatment parameters. Electron paramagnetic resonance (EPR) spectroscopy analysis was carried out at room temperature for the samples (Fig. 4b). A strong EPR signal was observed at g=2.003, indicating the presence of abundant unpaired electrons on the Ru catalyst. With increased crystallinity, the EPR signals gradually weakened, suggesting that a large number of unpaired electrons are neutralized during the crystallization process^[39-40].

  Figure 4

  

  Figure 4.  (a) PXRD patterns of Ru-280, Ru-360, Ru-RuO₂-280, and c-Ru-400; (b) EPR spectra of Ru-280, Ru-360, and c-Ru-400; (c) XPS survey spectra of Ru-280, Ru-360, and Ru-RuO₂-280

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  XPS was employed to probe the oxidation states of elements in the catalysts. Because the Ru3d and C1s signals significantly overlap, the Ru3p orbital was analyzed instead of Ru3d (Fig. 4c). Two characteristic peaks were observed at 462.4 and 484.8 eV, corresponding to the Ru3p_1/2 and Ru3p_3/2 orbitals, respectively^[41-42]. High-resolution XPS spectra further revealed that all samples contained both Ru⁰ and Ru⁴⁺ species. The presence of Ru⁴⁺ is due to the surface oxidation of Ru upon exposure to air. The atomic ratio of Ru⁰ to Ru⁴⁺ in Ru-280 and Ru-360 was determined to be 2.06∶1 and 2.02∶1 (Fig. 5a and 5b). Considering the shallow probing depth of XPS (ca. 5 nm), this suggests that the majority of Ru exists in the metallic state. In contrast, the Ru-RuO₂-280 sample exhibited a significantly higher proportion of Ru⁴⁺ with a Ru⁰/Ru⁴⁺ atomic ratio of 1.65:1, confirming its greater degree of oxidation compared with Ru-280 (Fig. 5c).

  Figure 5

  

  Figure 5.  High-resolution XPS spectra: Ru3p spectra of (a) Ru-280, (b) Ru-360, and (c) Ru-RuO₂-280; O1s spectra of (d) Ru-280, (e) Ru-360, and (f) Ru-RuO₂-280

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  High-resolution XPS O1s spectra of the samples were then analyzed. All catalysts exhibited three typical components in the O1s region, which can be assigned to lattice oxygen (O^2-, 529.7-530.2 eV), oxygen species associated with surface hydroxyl groups or defective oxygen sites (531.1-531.7 eV), and adsorbed water or chemisorbed oxygen (532.5-533.0 eV). For Ru-280, the O1s spectrum was dominated by the defective/hydroxyl oxygen component, suggesting the presence of abundant surface disorders and oxygen-containing functional groups, which originate from the low crystallinity of the nanosheets (Fig. 5d)^[43]. Upon annealing to form partially crystalline Ru-360, the relative intensity of lattice oxygen slightly increased, while the proportion of defective oxygen evidently decreased, indicating that moderate crystallization partially reconstructs the local Ru-O coordination environment and suppresses surface disorder (Fig. 5e). In contrast, the Ru-RuO₂-280 showed a markedly enhanced lattice-oxygen signal together with a higher fraction of metal-oxygen bonds, consistent with the formation of RuO₂ domains and the higher oxidation state observed in the Ru3p spectra (Fig. 5f). Meanwhile, the increased amount of surface hydroxyl species suggests the coexistence of RuO₂, which may contribute to optimized hydrogen and hydroxide adsorption during HOR. These trends collectively demonstrate that the thermal- and atmosphere-modulated synthesis successfully tunes both the oxygen coordination and the oxidation degree of Ru, which play essential roles in regulating the electronic structure and interfacial adsorption behavior pertinent for HOR catalysis.

  The catalytic performance of the catalysts for HOR was then evaluated by using a rotating disk electrode (RDE) in a typical three-electrode system with H₂-saturated alkaline electrolyte (0.1 mol·L^-1 KOH). After activation within the potential range of -0.2 to 0.4 V (vs RHE), the electrocatalytic properties of the catalysts were measured. As shown in Fig. 6a, polarization curves recorded at 1 600 r·min^-1 in H₂-saturated KOH revealed that the Ru-360 product with moderate crystallinity and the Ru-RuO₂-280 product exhibited superior HOR activity compared to commercial Pt/C and other contrast samples. The Ru-360 product displayed a current density of 2.47 mA·cm^-2 at 0.3 V (vs RHE). HOR polarization curves were then collected at rotation rates ranging from 400 to 2 500 r·min^-1. As the rotation rate increased, the limiting current density exhibited a systematic rise, which can be attributed to enhanced H₂ mass transport (Fig.S2 and S3). In contrast, polarization curves obtained in N₂-saturated 0.1 mol·L^-1 KOH under identical conditions showed current density close to zero, confirming that the observed current densities in H₂-saturated electrolyte originate from hydrogen oxidation (Fig.S4).

  Figure 6

  

  Figure 6.  Catalytic performance of the Ru-280, Ru-360, Ru-RuO₂-280, c-Ru-400, RuO₂-280, and commercial Pt/C catalysts: (a) HOR polarization curves, (b) Tafel slopes; (c) Micro-polarization region fitting; HOR polarization curves of (d) Ru-360 and (e) Ru-RuO₂-280 at different rotating rates; (f) Kinetic current density and exchange current density comparison for various catalysts

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  Fig. 6b further illustrates the kinetic current density (j_k) calculated using the K-L equation. Among the three samples of Ru-280, Ru-360, and c-Ru-400, the Ru-360 product exhibited higher kinetic currents compared with the other samples. This indicates that a moderate crystallinity of Ru is favourable for the alkaline HOR. Between the two samples of Ru-280 and Ru-RuO₂-280, the Ru-RuO₂-280 product showed obviously higher catalytic current density, indicating that the presence of RuO₂ is favourable for the catalytic process. The Ru content of the samples was determined by the ICP-OES method. The mass-specific kinetic current density (j_{k, m}) of Ru-360 reached 61.78 mA·mg_Ru^-1, which was 2.17, 4.35, and 1.74 times higher than those of Ru-280, c-Ru-400, and Pt/C, respectively (Table 1). Remarkably, the Ru-RuO₂-280 product achieves a higher j_{k, m} value of 81.66 mA·mg_Ru^-1, corresponding to 2.85, 5.70, and 2.27 times relative to the Ru-280, c-Ru-400, and Pt/C products, respectively. Fig. 6d and 6e present the polarization curves collected on products of Ru-360 and Ru-RuO₂-280 at different rotation rates. The increase in rotation rate enhances the mass transport, thereby leading to higher current densities.

  Table 1

  

  Table 1.  Electrochemical performance of the catalysts

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  Sample	Metal loading / (g·cm-2)	ECSA / (cm2·mg-1)	jk, 50 mV / (mA·cm-2)	jk, m / (mA·mg-1)	j0 / (mA·cm-2)	j0, s / (mA·cm-2)
  Ru-280	4.97×10-5	51.08	1.42	28.52	0.79	0.31
  Ru-360	4.90×10-5	65.52	3.03	61.78	1.51	0.47
  Ru-RuO2-280	4.49×10-5	73.86	3.67	81.66	1.70	0.51
  c-Ru-400	4.95×10-5	77.01	0.71	14.34	0.50	0.13
  RuO2-280	4.16×10-5	91.66	1.99	47.74	1.07	0.28
  Pt/C	5.10×10-5	56.50	1.84	35.99	0.75	0.26

  The intrinsic catalytic activity of the catalysts is typically described by the exchange current density (j₀), which was estimated from the linear fitting of the micropolarization region. The j₀ values of Ru-360 and Ru-RuO₂-280 reached 1.51 and 1.70 mA·cm^-2, respectively, which were significantly higher than those of other reference samples and the Pt/C catalyst (j₀=0.75 mA·cm^-2). In addition, the ECSA of all samples was estimated using the Cu-UPD stripping voltammetry method (Table 1 and Fig.S5). The j₀ values were then normalized by ECSA, providing a more accurate and direct evaluation of the intrinsic activity of the catalysts. The ECSA-normalized j₀ value (j_{0, s}) of Ru-360 was 0.47 mA·cm^-2, which was 1.52, 3.62, and 1.81 times those of Ru-280, c-Ru-400, and Pt/C, respectively. The j_{0, s} value of Ru-RuO₂-280 reached 0.51 mA·cm^-2 (Fig. 6f). Table 2 shows the catalytic activity comparison for the prepared catalysts with the reported catalysts, further indicating the excellent catalytic performance. These results indicate that the modulation of crystallinity and oxidation degree is important for the alkaline HOR.

  Table 2

  

  Table 2.  Comparison of HOR activities with the reported catalysts

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  Catalyst	j0, s / (mA·cm-2)	Ref.
  Ru-RuO2-280	0.51	This work
  Ru-360	0.47	This work
  fcc Ru/C	0.85	[30]
  Sn-Ru/C	0.43	[44]
  RuCr/C	0.399	[45]
  Ru-RuO2	0.46	[46]
  fcc-RuW	0.67	[47]
  Ru/VOC	0.12	[48]
  fcc Ru/C	0.193	[49]
  Ru/Cu-Cu2O@C	0.17	[50]
  Ru3Sn7/C	0.263	[51]

  Durability is a critical parameter for a HOR catalyst. To evaluate the catalytic stability of the Ru-360 and Ru-RuO₂-280 products, accelerated durability tests were conducted using CV tests (Fig.S6a and S6c). The catalysts were subjected to 2 000 CV cycles between -0.1 and 0.4 V (vs RHE). The CV and linear sweep voltammetry (LSV) profiles were then recorded before and after the CV cycling for comparison. The Ru-360 and Ru-RuO₂-280 products exhibited a decrease in current density of 17% and 18%, respectively, after the long-time CV cycles. Meanwhile, the exchange current density decreased by about 9% and 10% (Fig.S6b and S6d). This finding demonstrates that the Ru-360 and Ru-RuO₂-280 products possess relatively higher durability.

  It is generally accepted that the HOR processes involve three elementary steps:

  Tafel step:

  $ \mathrm{H}_2+2 \mathrm{~h} \rightarrow 2 \mathrm{H}_{\mathrm{ad}} $

  (1)

  Heyrovsky step:

  $\mathrm{H}_2+\mathrm{OH}^{-}+\mathrm{h} \rightarrow \mathrm{H}_{\mathrm{ad}}+\mathrm{H}_2 \mathrm{O}+\mathrm{e}^{-} $

  (2)

  Volmer step:

  $ \mathrm{H}_{\mathrm{ad}}+\mathrm{OH}^{-} \rightarrow \mathrm{H}_2 \mathrm{O}+\mathrm{e}^{-}+\mathrm{h} $

  (3)

  According to the hydrogen binding energy (HBE) theory, the adsorbed H_ad species on the catalyst surface reacts with OH^- from the electrolyte to produce water^{[43, 52]}. An optimal HBE ensures a balance between adsorption and desorption of H_ad intermediates, thus promoting efficient catalysis in alkaline media. The HBE of the samples was then investigated using the H_UPD stripping method^[53]. The results show that the Ru-360 and Ru-RuO₂-280 products exhibited obvious negatively shifted hydrogen adsorption peaks. This indicates that the decreased HBE on the surface Ru sites would induce the enhancement of the catalytic activity (Fig.S7).

  The characteristics of electron transfer can be explored through Bode plots^[54-55]. EIS measurements were then conducted in H₂-saturated 0.1 mol·L^-1 KOH within the potential range of -0.06 to 0.08 V (vs RHE) (Fig. 7). Typically, the Volmer step manifests in the low-frequency region, whereas the Heyrovsky step appears in the mid-frequency region^[30].When the applied potential shifted to HOR potential (E > 0 V), for the Ru-360 and Ru-RuO₂-280 products, the majority of phase angle signals were distributed in the low-frequency region, with a much lower proportion in the mid-frequency region. This implies that fewer signals of the Heyrovsky step can be probed by HOR Bode plots. These results suggest the possibility of a Tafel-Volmer pathway on the two products, which is different from that on Ru-280. Notably, the Ru-280 product exhibited particularly intense signals in the mid-frequency range, suggesting the involvement of a Heyrovsky-Volmer pathway. This result indicates accelerated Volmer steps on the optimized Ru-360 and Ru-RuO₂-280 products, leading to moderate binding of hydrogen and OH and inducing the enhancement of the catalytic activity.

  Figure 7

  

  Figure 7.  Bode plots of (a) Ru-280, (b) Ru-360, (c) Ru-RuO₂-280 at different applied potentials

  θ represents the phase angle.

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  In combination, the enhanced HOR activity of Ru-360 and Ru-RuO₂-280 can be attributed to the synergistic effects of moderate crystallinity and partial oxidation. Moderate crystallinity balances the electronic conductivity and the structural stability, while maintaining abundant defect sites for hydrogen adsorption. Partial oxidation introduces Ru-O interfaces that optimize OH^- adsorption and lower the energy barrier for the Volmer step. These factors jointly promote a favorable Tafel-Volmer pathway, leading to accelerated reaction kinetics.

  3.   Conclusions

  This work demonstrates that controlling the crystallinity and partial oxidation of Ru is a powerful strategy to enhance their hydrogen oxidation reaction performance in alkaline media. Ru-based catalysts with moderate crystallinity (Ru-360) and partial oxidation (Ru-RuO₂-280) exhibit superior activity compared to the amorphous or highly crystalline Ru and commercial Pt-based references. The improved performance originates from optimized hydrogen binding energy and accelerated reaction pathways enabled by crystallinity and surface oxidation modulation. These findings highlight the importance of crystallinity tuning and surface oxidation in the rational design of efficient, durable, and cost-effective Ru-based catalysts for alkaline fuel cell applications.

  
  Acknowledgements: The authors are grateful to the key project of the Joint Fund from the Heilongjiang Natural Science Foundation (Grant No.ZL2025E008) and the project from the Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Grant No.2024-KF-24, Wuhan University of Technology). Author contributions: Liu Yuanjun: Conceptualization, Validation, Writing-original draft. Jiang JiaYang: Conceptualization, Data curation. Wang Rui: Data collection. Hu Jiangnan: Data collection. Yuan Chaoyi: Data curation, Resources. Guo Xingmei: Conceptualization, Resources. Zhang Junhao: Conceptualization. Zhu Guoxing: Supervision, Writing-review & editing, Project administration.
  Conflicts of interest: The authors declare that they have no conflict of interest.
  Supporting information is available at http://www.wjhxxb.cn
  
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  Figure 1  Schematic illustration for the synthesis of Ru catalysts

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  Figure 2  SEM images of (a) Ru-360, (b) Ru-RuO₂-280, and (c) Ru-280; TEM images of (d) Ru-360, (e) Ru-RuO₂-280, and (f) Ru-280

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  Figure 3  HRTEM images of (a) Ru-360, (b) Ru-RuO₂-280, and (c) Ru-280; SAED patterns of (d) Ru-360, (e) Ru-RuO₂-280, and (f) Ru-280

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  Figure 4  (a) PXRD patterns of Ru-280, Ru-360, Ru-RuO₂-280, and c-Ru-400; (b) EPR spectra of Ru-280, Ru-360, and c-Ru-400; (c) XPS survey spectra of Ru-280, Ru-360, and Ru-RuO₂-280

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  Figure 5  High-resolution XPS spectra: Ru3p spectra of (a) Ru-280, (b) Ru-360, and (c) Ru-RuO₂-280; O1s spectra of (d) Ru-280, (e) Ru-360, and (f) Ru-RuO₂-280

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  Figure 6  Catalytic performance of the Ru-280, Ru-360, Ru-RuO₂-280, c-Ru-400, RuO₂-280, and commercial Pt/C catalysts: (a) HOR polarization curves, (b) Tafel slopes; (c) Micro-polarization region fitting; HOR polarization curves of (d) Ru-360 and (e) Ru-RuO₂-280 at different rotating rates; (f) Kinetic current density and exchange current density comparison for various catalysts

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  Figure 7  Bode plots of (a) Ru-280, (b) Ru-360, (c) Ru-RuO₂-280 at different applied potentials

  θ represents the phase angle.

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  Table 1.  Electrochemical performance of the catalysts

  Sample	Metal loading / (g·cm-2)	ECSA / (cm2·mg-1)	jk, 50 mV / (mA·cm-2)	jk, m / (mA·mg-1)	j0 / (mA·cm-2)	j0, s / (mA·cm-2)
  Ru-280	4.97×10-5	51.08	1.42	28.52	0.79	0.31
  Ru-360	4.90×10-5	65.52	3.03	61.78	1.51	0.47
  Ru-RuO2-280	4.49×10-5	73.86	3.67	81.66	1.70	0.51
  c-Ru-400	4.95×10-5	77.01	0.71	14.34	0.50	0.13
  RuO2-280	4.16×10-5	91.66	1.99	47.74	1.07	0.28
  Pt/C	5.10×10-5	56.50	1.84	35.99	0.75	0.26

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  Table 2.  Comparison of HOR activities with the reported catalysts

  Catalyst	j0, s / (mA·cm-2)	Ref.
  Ru-RuO2-280	0.51	This work
  Ru-360	0.47	This work
  fcc Ru/C	0.85	[30]
  Sn-Ru/C	0.43	[44]
  RuCr/C	0.399	[45]
  Ru-RuO2	0.46	[46]
  fcc-RuW	0.67	[47]
  Ru/VOC	0.12	[48]
  fcc Ru/C	0.193	[49]
  Ru/Cu-Cu2O@C	0.17	[50]
  Ru3Sn7/C	0.263	[51]

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