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Article

Correlating Boron Existence Morphologies with Electrocatalytic HER Activity in Ni-B Compounds Synthesized via High Pressure and High Temperature

by
Xinrong Guo
1,
Rui Bao
1,
Jiawen Lv
1,
Li Bai
1,
Guiqian Sun
2,
Huilian Liu
1,
Pinwen Zhu
2,
Yanli Chen
1,*,
Maobin Wei
1,* and
Qiang Tao
2,*
1
Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Changchun 130103, China
2
Synergetic Extreme Condition High-Pressure Science Center, State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China
*
Authors to whom correspondence should be addressed.
Catalysts 2026, 16(1), 65; https://doi.org/10.3390/catal16010065
Submission received: 10 December 2025 / Revised: 28 December 2025 / Accepted: 4 January 2026 / Published: 6 January 2026
(This article belongs to the Special Issue 15th Anniversary of Catalysts: Feature Papers in Electrocatalysis)
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Abstract

Nickel boride compounds have attracted considerable attention in the field of electrocatalysis due to their unique electronic structures and excellent chemical stability. However, the difficulty in obtaining single-phase products under traditional experimental conditions hinders the analysis of their intrinsic catalytic performance. Herein, we report the successful synthesis of three single-phase nickel boride compounds (Ni2B, Ni4B3, and NiB) via a high pressure and high temperature (HPHT) method. The configurations of B in their respective structures are distinct. Their electrocatalytic hydrogen evolution reaction (HER) performance was systematically evaluated. The results demonstrate that NiB exhibits the lowest overpotentials of 182 mV (in acidic electrolyte) and 234 mV (in alkaline electrolyte) at a current density of 10 mA cm−2, accompanied by the smallest Tafel slope, the lowest electron transfer resistance (Rct), and the largest double-layer capacitance (Cdl). This superior HER activity is primarily attributed to the presence of strong B-B covalent bonds in NiB, which weaken the Ni-B interaction and reduce the orbital hybridization between Ni 3d and B 2p orbitals. Consequently, the hydrogen adsorption intermediate (H*) achieves the optimal adsorption strength on the NiB surface. This work provides a novel insight for the design of high-performance transition metal boride electrocatalysts.

Graphical Abstract

1. Introduction

Hydrogen is a high-energy-density clean energy carrier that can effectively reduce global economic development’s reliance on fossil fuels and lower carbon emissions [1,2]. Electrocatalytic water splitting is the process of decomposing water using electrical energy to produce hydrogen and oxygen (hydrogen evolution reaction (HER) at the cathode, oxygen evolution reaction (OER) at the anode), which is a pivotal process for large-scale production of green hydrogen [3,4,5,6]. However, to break the H-O bonds in water molecules requires a very high energy barrier. This necessitates the use of appropriate catalysts to reduce the energy barrier for H-O bond cleavage [7,8]. The state-of-the-art platinum-based catalysts can effectively reduce the energy barrier of HER, while their high cost and limited reserves severely hinder their large-scale commercial application [9,10,11,12]. Therefore, developing earth-abundant, low-cost, and high-performance non-noble metal catalysts has become a research frontier in electrocatalysis.
Among the various catalysts, transition metal borides (TMBs) have emerged as promising alternatives to Pt-based catalysts owing to their unique electronic structure, high electrical conductivity, and robust chemical stability [13,14,15,16,17]. Meanwhile, nickel borides (Ni-B compounds) have attracted particular attention due to the earth abundance of Ni and B and the synergistic interaction between Ni (active sites) and B (electron modulator) [18]. The d-orbital electrons of Ni can form appropriate interactions with reaction intermediates during the catalytic process, which is beneficial to the progression of the reaction [19,20,21]. Additionally, the existing form of B in the structure plays a pivotal role in regulating catalytic performance. In our previous reports on the Mo-B system, with an increase in the B atomic stoichiometric ratio, B in the structure gradually transforms from a zero-dimensional (0D) form to two-dimensional (2D) and quasi-three-dimensional (quasi-3D) configurations. Experimental results demonstrated that the catalytic performance of the system is progressively enhanced with increasing B content, and attains an optimum when B exists in a 2D boronene-like structure [22]. These findings illustrate that the existing form of B in such metal–boron systems directly modulates the charge transfer between B and the metal component. Notably, the interaction between metal sites (serving as catalytic active centers) and reaction intermediates exerts a decisive effect on catalytic performance [23]. Therefore, investigating the influence of B’s existing forms in the Ni-B system on its catalytic performance constitutes a key aspect for the development and design of high-performance Ni-B catalysts.
To date, numerous Ni-B-based catalysts have been reported for water splitting. Surface-oxidized nickel boron compounds were prepared by a simple chemical reduction method. The optimized Ni−B−O@Ni3B catalyst achieves a current density of 10 mA cm−2 at an overpotential of 64 mV for the HER, and 264 mV for the OER in alkaline electrolyte [24]. Hao et al. employed a mild electroless plating method to prepare amorphous NiB on the nickel foam (NF). This electrocatalyst exhibited excellent hydrogen evolution reaction (HER) performance in an alkaline electrolyte [25]. Nickel boride (Ni2B) film was uniformly deposited on activated NF via an electroless plating technique. The as-prepared Ni2B/NF catalyst is an amorphous product, after annealing at 300 °C, diffraction peaks corresponding to Ni2B and other impurities were observed. For the OER in 1 M KOH, this catalyst exhibits an overpotential of 274 mV to reach a current density of 10 mA cm−2 [26]. To date, most reported Ni-B-based electrocatalysts are amorphous or composites, primarily due to the difficulty in synthesizing single-phase Ni-B compounds with well-defined stoichiometries. This not only obscures the catalytic mechanism but also leaves the exact role of B in the boride structure unclear. To obtain clear answers to these questions, it is necessary to obtain single-phase Ni-B compounds.
However, the synthesis of single-phase Ni-B compounds remains challenging via conventional methods, limiting mechanistic understanding. Notably, high pressure and high temperature (HPHT)—an effective strategy for TMB synthesis, which is a powerful technique in materials science—offers a unique solution to the above challenges [27,28,29]. Under high pressure, the distance between atoms can be shortened, the electron cloud overlap can be increased, the interaction between atoms can be enhanced, and the reaction barrier can be reduced [30]. Additionally, it can also promote the rearrangement of atoms to obtain crystal structures that are difficult to obtain under normal pressure [31]. Compared with conventional methods, HPHT synthesis enables precise control over phase composition by tuning pressure and temperature, which is ideal for preparing a series of Ni-B compounds with well-defined B contents.
In this work, we successfully synthesized three single-phase Ni-B compounds, Ni2B (I4/m cm), Ni4B3 (C2/c), and NiB (Cmcm), with specific stoichiometric ratios via HPHT, which addresses the long-standing challenge of narrow synthesis scope for single-phase Ni-B materials. The existence forms of B atoms differ in the three structures. By studying the HER performance of the three single-phase Ni-B compounds, the decisive role of the existence form of B atoms in the HER performance is revealed. Among them, NiB possesses the highest boron content, exhibits the best catalytic performance in both acidic and alkaline electrolytes, and can achieve a current density of 10 mA cm−2 with only 182 and 234 mV, respectively. The research results indicate that the interactions between boron atoms in the structure can regulate the degree of orbital hybridization between Ni and B, thereby facilitating the progress of the HER. This study not only provides a new strategy for the phase-controllable synthesis of Ni-B compounds but also offers fundamental insights into the rational design of high-performance TMB-based HER catalysts.

2. Results and Discussion

2.1. Structures and Morphologies

The crystal structure of the single-phase Ni-B compound prepared via HPHT treatment was characterized by XRD. Figure 1a presents the XRD pattern of Ni2B. The results demonstrate that all diffraction peaks are consistent with the PDF card No. 82-1697, and no impurity peaks are detected. This indicates that single-phase Ni2B with a tetragonal crystal system can be successfully synthesized by reacting precursors at a Ni:B molar ratio of 2:1.2 under the conditions of 5 GPa and 1800 °C for 20 min. The inset of Figure 1a presents the crystal structure of tetragonal Ni2B. As observed from its crystal structure, Ni2B comprises metallic and ionic bonds. Owing to the large spacing between B atoms and the absence of covalent bonds, B atoms are present as isolated entities within the Ni metallic framework. The corresponding existence state of B is illustrated in Figure 1d. Figure 1b presents the XRD pattern of Ni4B3. All the diffraction peaks matched well with those in PDF card No. 73-1794, indicating that the as-prepared Ni4B3 belongs to the monoclinic crystal system. The corresponding crystal structure and existence state of B atoms are presented in the inset and Figure 1e. As observed from the crystal structure, as the B atom content increases, a small number of covalent bonds are also present in addition to metallic and ionic bonds. According to previous work, these covalent bonds adopt a chair-like configuration with symmetric bond parameters, featuring two distinct bond lengths (1.92 Å and 1.85 Å) and a bond angle of 113.95° at the bend [32]. Figure 1c presents the XRD pattern of NiB. The results demonstrate that single-phase NiB has been successfully synthesized. All diffraction peaks match well with those in PDF card No. 74-1207, and no impurity peaks are detected. The crystal structure of the as-prepared NiB belongs to the orthorhombic crystal system, with the corresponding crystal structure shown in the inset (Figure 1c). As observed from the inset, B atoms in the structure exist as one-dimensional (1D) zigzag-type boron chains, with these chains arranged in parallel. High-magnification observation of the 1D boron chain reveals a B-B bond length of 1.73 Å (Figure 1f), which is shorter than the covalent bond length in Ni4B3, and a bond angle of 118.21°. These results indicate that B-B covalent bonding is more pronounced in NiB, which weakens the Ni-B interfacial interaction. Figure 1g–i presents the scanning electron microscopy (SEM) images of the three single-phase Ni-B compounds. As observed from the images, the morphology of all three samples consists of closely connected micron-sized grains. This is primarily attributed to the fact that HPHT-synthesized samples undergo enhanced grain growth and densification driven by the combined effects of pressure and temperature. Planar scanning elemental mapping of the three single-phase Ni-B compounds were characterized, as presented in Figure 1g1–i2. The results demonstrate that all three samples comprise both Ni and B elements, with both elements being uniformly distributed in the plane.
The micromorphology and local crystal structure of the three single-phase Ni-B compounds (Ni2B, Ni4B3, NiB) were further characterized via transmission electron microscopy (TEM) and high-resolution TEM (HRTEM), with results presented in Figure 2. Figure 2a displays the TEM image of Ni2B, revealing a compact block-like micromorphology composed of closely packed grains—consistent with the SEM observation of micron-sized grain aggregation (Figure 1g). The HRTEM image of the blue selected area in Figure 2a (Figure 2d) shows well-ordered lattice fringes, with a measured interplanar spacing of 0.248 nm. This value corresponds precisely to the (200) crystal facet of tetragonal Ni2B, which is consistent with the XRD result (Figure 1a), further confirming the successful synthesis of Ni2B. Given that both Ni4B3 and NiB are synthesized via HPHT conditions (same as Ni2B), their grains also exhibit micrometer-scale dimensions. This observation is verified by the TEM images of Ni4B3 and NiB (Figure 2b,c). The HRTEM images of Ni4B3 and NiB were acquired from magnified local regions of Figure 2b and Figure 2c, respectively. As observed from these HRTEM images, the lattice fringes are arranged regularly, indicating excellent crystallinity of the samples. The measured interplanar spacings are 0.289 and 0.368 nm, corresponding to the (112) crystal plane of Ni4B3 and the (020) crystal plane of NiB, respectively. These results are consistent with the aforementioned XRD data (Figure 1b,c), reaffirming that HPHT is an effective approach for synthesizing the single-phase Ni-B compounds.
The surface electronic states and electron transfer of the Ni-B compounds are characterized by X-ray photoelectron spectroscopy (XPS). To eliminate the influence of the surface oxidation state on the binding energy position of the samples, all the sample surfaces are subjected to 180 s of argon ion bombardment before the XPS test, in order to remove the surface oxide layer. Figure 3a depicts the high-resolution Ni 2p XPS spectrum of Ni2B. The peaks at binding energies of 853.1 and 870.3 eV correspond to the Ni 2p3/2 and Ni 2p1/2 orbitals of Ni2B, respectively [33,34]. The other two peaks located at 856.9 and 874.3 eV are assigned to shake-up satellite peaks [35,36]. For the B 1s XPS spectrum of Ni2B (Figure 3d), a main peak is observed at 188.2 eV, which corresponds to B-Ni chemical bonds [37,38]. Figure 3b,c present the high-resolution Ni 2p XPS spectra of Ni4B3 and NiB, respectively. For Ni4B3, the peaks at 853.4 and 870.6 eV are attributed to the Ni 2p3/2 and Ni 2p1/2 orbitals, while the Ni 2p3/2 and Ni 2p1/2 peaks of NiB are located at 853.2 and 870.4 eV. Corresponding to their B 1s XPS spectra (Figure 3e,f), both Ni4B3 and NiB exhibit a main peak at 188.4 eV, which is assigned to B-Ni chemical bonds in their respective structures. From the aforementioned results, the Ni 2p3/2 peaks of all Ni-B compounds are positively shifted relative to that of metallic Ni (852.6 eV) [39], while the B 1s peaks are also higher than those of elemental B (186.4 eV) [40]. Collectively, these binding energy shifts, combined with the higher electronegativity of B (2.04) compared to Ni (1.91), indicate that B is more prone to accepting electrons. Thus, in Ni-B compounds, Ni acts as an electron donor and B as an electron acceptor.
Furthermore, the crystal structure analysis (Figure 1d–f) reveals distinct bonding configurations among the three compounds: In Ni2B, only metallic bonds and ionic bonds are present, with no covalent bonds detected. In contrast, Ni4B3 and NiB exhibit a coexistence of metallic bonds, ionic bonds, and covalent bonds—with NiB featuring the strongest B-B covalent interactions. Correlating with the XPS results, in Ni2B, all electrons donated by Ni act exclusively on B atoms to form Ni-B ionic/metallic interactions. For Ni4B3 and NiB, however, the electrons donated by Ni not only participate in Ni-B bonding but also contribute to the formation of B-B covalent interactions. Notably, NiB exhibits the strongest B-B interactions, meaning a larger fraction of the electrons provided by Ni are allocated to B-B bond formation. Consequently, the Ni-B interaction is weakened in NiB compared to Ni2B and Ni4B3. Such electronic state modulation and bond interaction tuning exert a pivotal effect on the catalytic hydrogen evolution performance of Ni-B compounds.

2.2. HER Performance

The electrocatalytic HER performance of the Ni-B compounds was evaluated using a three-electrode system, the schematic diagram is presented in Figure 4a. The linear sweep voltammetry (LSV) curves for the HER of the Ni-B compounds are shown in Figure 4b. As depicted in this figure, NiB exhibits the optimal HER activity in the acidic electrolyte (0.5 M H2SO4). At a current density of 10 mA cm−2, NiB requires an overpotential of only 182 mV, which is significantly lower than those of Ni2B (357 mV) and Ni4B3 (330 mV). In the alkaline electrolyte (1 M KOH) (Figure 4c), NiB remains the most efficient HER electrocatalyst, requiring an overpotential of 234 mV to achieve a current density of 10 mA cm−2, which is markedly lower than those of Ni2B (360 mV) and Ni4B3 (285 mV). Similarly, the overpotentials of the three catalysts at a higher current density of 100 mA cm−2 were compared (Figure 4d and Table 1), with the results demonstrating that NiB still exhibits the smallest overpotential: 334 mV in the acidic electrolyte and 396 mV in the alkaline electrolyte, respectively. Collectively, these results confirm that NiB delivers the superior HER catalytic activity in both acidic and alkaline media.
To further elucidate the origin of NiB’s superior electrocatalytic performance, the LSV curves of the three catalysts in the acidic electrolyte were analyzed by fitting to obtain tafel slopes, with results presented in Figure 4e. The tafel slopes of Ni2B, Ni4B3, and NiB are 130.5, 120.6, and 93.2 mV dec−1, respectively. A smaller tafel slope indicates more favorable electrocatalytic kinetics, as it reflects the requirement of a lower overpotential to achieve the same current density. Thus, NiB exhibits the fastest HER kinetics among the three catalysts. Additionally, electrochemical impedance spectroscopy (EIS) measurements were performed to evaluate the electron transfer behavior of the catalysts, and the Nyquist plots are displayed in Figure 4f. Ni2B shows the largest semicircular arc, corresponding to the highest electron transfer resistance (Rct) of 21.99 Ω during the catalytic process. In contrast, NiB exhibits the smallest semicircular arc, indicating the fastest electron transfer rate with an Rct of 18.71 Ω. The Rct of Ni4B3 (19.12 Ω) falls between those of NiB and Ni2B, consistent with its intermediate catalytic activity. The electrochemical surface area (ECSA) of the catalysts was estimated from the double-layer capacitance (Cdl), as shown in Figure 4g. NiB possesses the largest Cdl value of 0.59 mF cm−2, followed by Ni4B3 (0.32 mF cm−2) and Ni2B (0.18 mF cm−2). A larger Cdl implies a higher ECSA, which enables the exposure of more catalytic active sites to participate in the HER, thereby enhancing catalytic performance. A comprehensive comparison of the key catalytic performance metrics (overpotential, tafel slope, Rct, and Cdl) for Ni2B, Ni4B3, and NiB is summarized in the radar chart (Figure 4h and Table 1), clearly demonstrating that NiB is the most efficient HER catalyst. Stability is another critical criterion for evaluating catalyst performance, as it directly affects the catalyst’s service life and reaction durability. Chronoamperometric stability tests were performed on NiB in the acidic and alkaline electrolytes at a fixed overpotential of 182 and 234 mV, respectively (Figure 4i). Following 15 h of continuous operation, the results demonstrate that NiB exhibits superior stability in the alkaline electrolyte compared to the acidic electrolyte. Meanwhile, a comparison of its performance with that of various other transition metal borides is summarized in Table 2, which shows that NiB exhibits excellent performance.
Based on comprehensive structural, microstructural, and surface electronic state characterizations of the Ni-B compounds, the correlation between their structure and electrocatalytic HER performance can be summarized as follows: With the increase in B content (from Ni2B to NiB), the Ni-B interaction is gradually weakened, which in turn reduces the hybridization degree between Ni’s 3d orbitals and B’s 2p orbitals. Consequently, the d-band center of Ni is upshifted (most significantly in NiB), which enhances the orbital overlap between Ni and the hydrogen adsorption intermediate (H*). This endows H* with an optimal adsorption energy on the NiB surface, thereby synergistically promoting the overall HER catalytic kinetics.

3. Experimental Section

3.1. Chemicals and Material

Nickel powder (Ni, 99.8 wt%) and amorphous boron powder (B, 98.0 wt%) were purchased from Thermo Fisher Scientific Inc. (Shanghai, China). Potassium hydroxide (KOH, ≥85.0 wt%) and sulfuric acid (H2SO4, 98.0 wt%) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemical reagents used were of analytical grade and utilized without further purification.

3.2. Synthesized Single-Phase Ni-B Compounds

Single-phase Ni-B compounds were synthesized using the HPHT method. The synthesis procedures for Ni2B, Ni4B3, and NiB were consistent with our previous report [32]. The stoichiometric ratios of the starting materials Ni and B were 2:1.2, 4:3.6, and 1:1.2, respectively. Excessive B was added due to the occurrence of a eutectic reaction during the sample reaction process. The growth process of Ni-B compounds is melting Ni which diffuses to cover B grains, then to form Ni-B compounds under HPHT. That is, after the metal Ni melts, it encapsulates a portion of B. Therefore, the peritectic reaction occurring in local regions will hinder the chemical reaction between Ni and B [32]. Therefore, an excess amount of B was required to compensate for the unreacted B during the eutectic reaction [32]. The Ni powder and B powder were mixed in agate mortar by a manual grinder for more than 1.5 h, and then cold-pressed into cylindrical pellets with a diameter of 4 mm and a thickness of 2.5 mm. The pressed pellets were subsequently placed into a HPHT apparatus. In a Chinese-made cubic press (six-anvil type, 6 × 14,400 KN), the single-phase samples were synthesized under a constant pressure of 5 GPa with tailored temperature–time parameters: Ni2B and NiB were reacted at 1800 °C for 20 min, while Ni4B3 required a reaction temperature of 2200 °C for 10 min.
The anvils are made of tungsten carbide, with a square front end (23.5 mm × 23.5 mm). High temperature is provided by a graphite heater. The insulation outside the graphite heater is an MgO tube. A W5% Re-W26% Re thermocouple was used to calibrate the temperature, and the relationship between temperature and power was established prior to the experiments. The experimental temperature was then determined based on the power of the apparatus. The pressure was also calibrated before the experiments via the abrupt changes in electrical resistivity caused by the phase transitions of Bi (2.55 GPa, 2.69 GPa) and Ba (5.5 GPa). The quasi-hydrostatic pressure was generated by a cubic pyrophyllite pressure-transmitting medium (32.5 mm × 32.5 mm × 32.5 mm).

3.3. Materials Characterization

The crystal structure and purity of the as-synthesized single-phase Ni-B compounds (Ni2B, Ni4B3, and NiB) were characterized by X-ray diffraction (XRD, Rigaku D/Max-2500, Tokyo, Japan) with Cu Kα radiation (λ = 1.5406 Å) (Rigaku, Tokyo, Japan). The surface morphology and elemental distribution of the samples were observed via scanning electron microscopy (SEM, FEI Magellan 400 L) equipped with an energy-dispersive X-ray spectroscopy (EDS) mapping system. SEM images were acquired at an accelerating voltage of 18 kV to avoid sample damage, while EDS elemental mapping (Ni and B) was conducted at 18 kV to verify the uniform distribution of constituent elements and rule out elemental segregation. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) measurements were carried out on a JEM-2200FS TEM system (JEOL, Tokyo, Japan) operating at 200 KV, which can investigate the microstructural details and crystal lattice characteristics. X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250, Waltham, MA, USA) with Al Kα radiation (hν = 1486.6 eV) was employed to analyze the chemical states and valence configurations of Ni and B in the samples. All binding energies were calibrated using the C 1s peak at 284.8 eV as the reference. High-resolution XPS spectra of Ni 2p and B 1s were deconvoluted using Gaussian–Lorentzian fitting to identify the characteristic peaks corresponding to different chemical environments, providing insights into the electron transfer between Ni and B.

3.4. Electrochemical Measurements

Electrochemical measurements were conducted on a CHI 760E electrochemical workstation (Chenhua Instrument Co., Ltd., Shanghai, China) using a standard three-electrode configuration in 1 M KOH or 0.5 M H2SO4 electrolyte. The prepared samples were pasted on L-shaped electrodes and the HER performance was evaluated under a fixed exposed surface area. The working electrode (WE) was composed of single-phase Ni-B compounds, a saturated calomel electrode (SCE) and Hg/HgO electrode (Hg/HgO|1 M KOH, SCE|0.5 M H2SO4) served as the reference electrode (RE), and a graphite rod was used as the counter electrode (CE). All potentials reported herein were calibrated to the reversible hydrogen electrode (RHE).
The fabrication process of the working electrode was consistent with our previous reports [22,23]. Briefly, the cross-section of single-phase Ni-B compounds pellets was used as the active surface for the HER. Each sample was fixed onto an L-shaped copper current collector using conductive silver paste, and the non-active areas were sealed with a modified acrylate adhesive to avoid electrolyte leakage. The effective exposed surface area of each sample was measured using a Leica M125 C stereomicroscope (Leica, Wetzlar, Germany) [45].
Linear sweep voltammetry (LSV) measurements were performed at a scan rate of 5 mV s−1 to evaluate the HER catalytic activity. The catalytic performance of different samples was primarily compared based on the overpotential (η), defined as the difference between the actual HER onset potential and the thermodynamic equilibrium potential of the HER. Cyclic voltammetry (CV) measurements were carried out in the non-Faradaic potential region (1.02–1.12 V vs. RHE) to estimate the electrochemical double-layer capacitance (Cdl), which is proportional to the effective electrochemical surface area (ECSA) of the catalysts. The ECSA of the material can be calculated by dividing Cdl by the specific capacitance (Cs) of the material (ECSA = Cdl/Cs). A Cs value of 0.035 mF/cm2 was used herein. A series of CV curves were recorded at various scan rates (20, 40, 60, 80, 100, 150, and 200 mV s−1), and each scan cycle was repeated 40 times to ensure measurement consistency. Electrochemical impedance spectroscopy (EIS) tests were performed at a constant potential of −0.25 V vs. RHE to investigate the charge transfer kinetics at the electrode–electrolyte interface. During EIS measurements, a sinusoidal voltage with an amplitude of 5 mV was applied, and the frequency was scanned from 105 Hz to 1 Hz.

4. Conclusions

In summary, three single-phase Ni-B compounds (Ni2B, Ni4B3, and NiB) were successfully synthesized via HPHT method. With increasing B content (from Ni2B to NiB), the existence form of B gradually evolves from isolated atoms (in Ni2B) to chair-like B-B covalent configurations (in Ni4B3) and finally to zigzag one-dimensional (1D) boron chains (in NiB). These distinct B existence forms directly modulate the surface electronic states of the Ni-B compounds by regulating Ni-B interaction strength and orbital hybridization. Electrochemical tests demonstrated that NiB exhibits the most superior electrocatalytic HER performance, with the lowest overpotentials (182 mV in 0.5 M H2SO4 and 234 mV in 1 M KOH at 10 mA cm−2), the smallest Tafel slope (93.2 mV dec−1), the smallest Rct, and the largest Cdl. This exceptional performance is primarily attributed to the strong B-B covalent interactions within the zigzag 1D boron chains in NiB, which weaken the Ni-B interaction and reduce the hybridization degree between Ni 3d and B 2p orbitals. Consequently, the d-band center of Ni is optimized to an ideal position, endowing NiB with the most suitable H* adsorption energy, and ultimately enhancing the overall catalytic kinetics. This study provides a novel strategy for designing high-performance transition metal boride HER catalysts by regulating the existence form of non-metallic elements (e.g., B) to tailor electronic structures and catalytic active sites.

Author Contributions

Conceptualization: X.G. and Y.C.; methodology, P.Z. and X.G.; validation, M.W.; investigation, X.G.; data curation, X.G., R.B., J.L., L.B. and G.S.; visualization: R.B., J.L., L.B. and H.L.; formal analysis: P.Z.; writing—original draft, X.G. and Y.C.; writing—review and editing: Y.C. and Q.T.; funding acquisition: Y.C., H.L., P.Z., M.W. and Q.T. All authors have read and agreed to the published version of the manuscript.

Funding

The authors acknowledge funding support from the Science and Technology Development Project of Jilin Province (YDZJ202201ZYTS308) and the Open Research Fund of Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education (Jilin Normal University, 202405).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors acknowledge the “Solid Environment High Pressure and High Temperature (SEHPHT) Station” for all high-pressure experiments.

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 16 00065 g001
Figure 1. (a) The XRD pattern of Ni2B, (b) the XRD pattern of Ni4B3, (c) the XRD pattern of NiB, (d) the existence state of B atoms in Ni2B, (e) the existence state of B atoms in Ni4B3, (f) the existence state of B atoms in NiB, (g) the SEM image of Ni2B (where (g1) and (g2) denote the elemental mapping images obtained from planar scanning of the area in (g)), (h) the SEM image of Ni4B3 (where (h1) and (h2) denote the elemental mapping images obtained from planar scanning of the area in (h)), and (i) the SEM image of NiB (where (i1) and (i2) denote the elemental mapping images obtained from planar scanning of the area in (i)).
Figure 1. (a) The XRD pattern of Ni2B, (b) the XRD pattern of Ni4B3, (c) the XRD pattern of NiB, (d) the existence state of B atoms in Ni2B, (e) the existence state of B atoms in Ni4B3, (f) the existence state of B atoms in NiB, (g) the SEM image of Ni2B (where (g1) and (g2) denote the elemental mapping images obtained from planar scanning of the area in (g)), (h) the SEM image of Ni4B3 (where (h1) and (h2) denote the elemental mapping images obtained from planar scanning of the area in (h)), and (i) the SEM image of NiB (where (i1) and (i2) denote the elemental mapping images obtained from planar scanning of the area in (i)).
Catalysts 16 00065 g001
Catalysts 16 00065 g002
Figure 2. (a) The TEM image of Ni2B, (b) the TEM image of Ni4B3, (c) the TEM image of NiB, (d) the HRTEM image of the blue selected area in (a,e) the HRTEM image of the orange selected area in (b,f) the HRTEM image of the green selected area in (c).
Figure 2. (a) The TEM image of Ni2B, (b) the TEM image of Ni4B3, (c) the TEM image of NiB, (d) the HRTEM image of the blue selected area in (a,e) the HRTEM image of the orange selected area in (b,f) the HRTEM image of the green selected area in (c).
Catalysts 16 00065 g002
Catalysts 16 00065 g003
Figure 3. (a) The high-resolution Ni 2p XPS spectrum of Ni2B, (b) the high-resolution Ni 2p XPS spectrum of Ni4B3, (c) the high-resolution Ni 2p XPS spectrum of NiB, (d) the high-resolution B 1s XPS spectrum of Ni2B, (e) the high-resolution B 1s XPS spectrum of Ni4B3, and (f) the high-resolution B 1s XPS spectrum of NiB.
Figure 3. (a) The high-resolution Ni 2p XPS spectrum of Ni2B, (b) the high-resolution Ni 2p XPS spectrum of Ni4B3, (c) the high-resolution Ni 2p XPS spectrum of NiB, (d) the high-resolution B 1s XPS spectrum of Ni2B, (e) the high-resolution B 1s XPS spectrum of Ni4B3, and (f) the high-resolution B 1s XPS spectrum of NiB.
Catalysts 16 00065 g003
Catalysts 16 00065 g004
Figure 4. (a) The schematic diagram of the three-electrode system, (b) the LSV curves for HER of Ni2B, Ni4B3, and NiB in 0.5 M H2SO4, (c) the LSV curves for HER of Ni2B, Ni4B3, and NiB in 1 M KOH, (d) the comparison of overpotentials at 10 and 100 mA cm−2, (e) the tafel slopes, (f) the Nyquist plots, (g) the double-layer capacitances, (h) the radar chart comparing the comprehensive HER catalytic performance of Ni2B, Ni4B3, and NiB, and (i) the time-dependent potential curves at 10 mA cm−2.
Figure 4. (a) The schematic diagram of the three-electrode system, (b) the LSV curves for HER of Ni2B, Ni4B3, and NiB in 0.5 M H2SO4, (c) the LSV curves for HER of Ni2B, Ni4B3, and NiB in 1 M KOH, (d) the comparison of overpotentials at 10 and 100 mA cm−2, (e) the tafel slopes, (f) the Nyquist plots, (g) the double-layer capacitances, (h) the radar chart comparing the comprehensive HER catalytic performance of Ni2B, Ni4B3, and NiB, and (i) the time-dependent potential curves at 10 mA cm−2.
Catalysts 16 00065 g004
Table 1. HER performance of Ni2B, Ni4B3, and NiB.
Table 1. HER performance of Ni2B, Ni4B3, and NiB.
SamplesOverpotentials
at 10 mA cm−2 (mV)
Acidic/Alkaline		Tafel Slope
(mV dec−1)		Double-Layer Capacitance
(Cdl) (mF cm−2)		Electron Transfer Resistance
(Rct) (Ω)
NiB182/23493.20.5918.71
Ni2B357/360130.50.1821.99
Ni4B3330/285120.60.3219.12
Table 2. The overpotentials (mV) of transition metal light element compounds (TMLEs) at corresponding current density, J, of electrocatalysts for HER in 0.5 M H2SO4 and 1 M KOH.
Table 2. The overpotentials (mV) of transition metal light element compounds (TMLEs) at corresponding current density, J, of electrocatalysts for HER in 0.5 M H2SO4 and 1 M KOH.
CatalystElectrolyteCurrent Density, J (mA cm−2)Overpotential at Corresponding J (mV)Ref.
NiB0.5 M H2SO410182This work
Ni2B0.5 M H2SO410357This work
Ni4B30.5 M H2SO410330This work
Ni3B-9000.5 M H2SO410175[41]
Ni2B0.5 M H2SO410252[41]
Ni3B-9500.5 M H2SO410287[41]
NiB0.5 M H2SO450290[13]
Ni3B-10000.5 M H2SO410467[41]
NiB1 M KOH10234This work
Ni2B1 M KOH10360This work
Ni4B31 M KOH10285This work
Ni2B 1 M KOH10262[42]
NiB1 M KOH10307[43]
Ni2B 1 M KOH10707[44]
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Guo, X.; Bao, R.; Lv, J.; Bai, L.; Sun, G.; Liu, H.; Zhu, P.; Chen, Y.; Wei, M.; Tao, Q. Correlating Boron Existence Morphologies with Electrocatalytic HER Activity in Ni-B Compounds Synthesized via High Pressure and High Temperature. Catalysts 2026, 16, 65. https://doi.org/10.3390/catal16010065

AMA Style

Guo X, Bao R, Lv J, Bai L, Sun G, Liu H, Zhu P, Chen Y, Wei M, Tao Q. Correlating Boron Existence Morphologies with Electrocatalytic HER Activity in Ni-B Compounds Synthesized via High Pressure and High Temperature. Catalysts. 2026; 16(1):65. https://doi.org/10.3390/catal16010065

Chicago/Turabian Style

Guo, Xinrong, Rui Bao, Jiawen Lv, Li Bai, Guiqian Sun, Huilian Liu, Pinwen Zhu, Yanli Chen, Maobin Wei, and Qiang Tao. 2026. "Correlating Boron Existence Morphologies with Electrocatalytic HER Activity in Ni-B Compounds Synthesized via High Pressure and High Temperature" Catalysts 16, no. 1: 65. https://doi.org/10.3390/catal16010065

APA Style

Guo, X., Bao, R., Lv, J., Bai, L., Sun, G., Liu, H., Zhu, P., Chen, Y., Wei, M., & Tao, Q. (2026). Correlating Boron Existence Morphologies with Electrocatalytic HER Activity in Ni-B Compounds Synthesized via High Pressure and High Temperature. Catalysts, 16(1), 65. https://doi.org/10.3390/catal16010065

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