Self‐supported thin‐film electrode consisting of transition metal borides for highly efficient hydrogen evolution

Abstract

Transition metal borides (TMBs) are a new class of promising electrocatalysts for hydrogen generation by water splitting. However, the synthesis of robust all‐in‐one electrodes is challenging for practical applications. Herein, a facile solid‐state boronization strategy is reported to synthesize a series of self‐supported TMBs thin films (TMB‐TFs) with large area and high catalytic activity. Among them, MoB thin film (MoB‐TF) exhibits the highest activity toward electrocatalytic hydrogen evolution reaction (HER), displaying a low overpotential (η₁₀ = 191 and 219 mV at 10 mA cm⁻²) and a small Tafel slope (60.25 and 61.91 mV dec⁻¹) in 0.5 M H₂SO₄ and 1.0 M KOH, respectively. Moreover, it outperforms the commercial Pt/C at the high current density region, demonstrating potential applications in industrially electrochemical water splitting. Theoretical study reveals that both surfaces terminated by TM and B atoms can serve as the active sites and the H* binding strength of TMBs is correlated with the p band center of B atoms. This work provides a new pathway for the potential application of TMBs in large‐scale hydrogen production.

INTRODUCTION

Efficient electrocatalysis for the conversion of sustainable power into chemical energy has attracted much attention owing to its promising applications in high-purity hydrogen generation by water splitting.^1–4 However, electrochemical hydrogen evolution reaction (HER) is restricted by sluggish kinetics and non-spontaneous thermodynamics. Thus, the key challenge is to develop highly active and stable electrocatalysts to improve the HER efficiency.^5–7 Commercially, platinum (Pt)-based materials are well known as the state-of-the-art electrocatalysts for HER.⁸ However, natural scarcity and high cost severely limit their large-scale practical applications.^9–11 Therefore, it is very attractive to develop low-cost, highly active, and stable HER catalysts made from earth-abundant elements.^12,13

Up to now, transition metal (e.g., Mo, W, Fe, Co, and Ni) oxides, sulfides, borides, carbides, phosphides, and other compounds have been extensively investigated as promising non-noble-metal-based electrocatalysts.^14–18 Among them, transition metal borides (TMBs) have attracted much attention based on the following reasons: (1) the unique crystal structure, especially the strong ionic bonds of metal atoms which makes them featured as high electric conductivity^19–21; (2) the covalent bonds between boron atoms contribute to good chemical stability and high mechanical strength^22–24; (3) the suitable d-band center can be electronically modified by the coupling of d-sp orbitals, thus providing an optimized surface adsorption and catalytic ability.^25–28 Considering the above-mentioned advantages, TMBs have been demonstrated to be good candidates for HER. For instance, the Schottky g-C₃N₄/α-MoB catalyst shows a superior HER activity with a low Tafel slope of 46 mV dec⁻¹, which is much better than that of pristine MoB.²⁹ Zou et al.³⁰ and Park et al.³¹ recently reported that binary α-MoB₂ and ternary α-Cr_0.4Mo_0.6B₂ exhibited excellent HER activity at the high current density. Additionally, the superior HER performances of vanadium borides³² and tungsten borides³³ has been reported too.

From the viewpoint of practical application, it is highly desirable to directly grow electrocatalysts on the current collector to form a robust self-supported monolithic cathode. Such an electrode is beneficial for decreasing the interface resistance and facilitating the transport of charged species between the electrode and electrolyte interface.^34–36 However, the synthesis of self-supported monolithic TMBs-based electrodes is still a critical challenge owing to the high formation temperature. Herein, we report a generalized method to successfully synthesize self-supported monolithic TMB thin films (TMB-TFs, TM = Mo, W, V, Nb, and Ta) with large area on the commercially available TM foils. Among them, MoB-TF exhibits the highest intrinsic HER activity and outperforms the commercial Pt/C catalysts at the high current density region. Meanwhile, it keeps good long-term stability at the high current density of 600 mA cm⁻² for 16 h in an acidic solution. Therefore, the present work provides an effective synthetic route to design self-supported TMB-TFs with high HER performances for practical water electrolysis.

EXPERIMENTAL SECTION

Sample preparation

Transition metal foils (e.g., Mo, W, V, Ta, and Nb with purity~99.99%, thickness~0.2 mm, and diameter~12 mm) were used as metal source, and the sodium borohydride (NaBH₄) was used as boron source to synthesize the TMBs thin films. The metal foil was buried into the NaBH₄ powder (weight of 1.6 g) and pressed into a tablet under the pressure of 10 MPa. Then, it was vertically placed in a quartz tube and heated to the target temperature of 950–1150°C for 90 min keeping a high vacuum condition by mechanical pump. Finally, the furnace was naturally cooled to room temperature and the single-phase TMB-TFs were obtained without any purification. MoB-TFs with different thickness were also synthesized by the same synthesis method, and the thickness was controlled by adjusting the weight of NaBH₄ at 1.2, 0.8, 0.4, and 0.2 g, in which the corresponding thickness was 40, 28, 8.5, 4 and 2.5 μm, respectively.

Characterizations

The phase composition, surface morphology, cross section, and valence states of synthesized TMB-TFs were characterized by X-ray Diffraction (BRUKER D8 ADVANCE), field emission scanning electron microscopy (FESEM, HITACHI SU-8010), and X-ray photoelectron spectra (XPS) with Al X-rays excitation source. The microstructure of MoB and WB powders scratched down from thin film and the corresponding elemental mapping images from high-angle annular dard field-scanning transmission electron microsocpy (HAADF-STEM) was observed by transmission electron microscopy (TEM) (JEM-2100F).

Electrochemical measurements

All the electrochemical measurements were performed at an electrochemical workstation (CHI 760E, Chenhua) using a three-electrode framework. TMB-TFs with an area of 1.168 cm² or 0.584 cm² were used directly as the working electrode, the graphite rod was used as the counter electrode, and the Ag/AgCl and Hg/HgO electrodes were used as reference electrodes for acidic and alkaline media, respectively. Platinum on graphitized carbon (20 wt.% Pt/C) and Nafion® perfluorinated resin solution were purchased from Sigma-Aldrich. All potentials were corrected with IR to a reversible hydrogen electrode (RHE), and the linear sweep voltammetry (LSV) polarization curves were measured at a scan rate of 2 mV s⁻¹. Tafel slopes were calculated from the corresponding linear polarization curves, and the electrochemically active surface area (ECSA) was estimated from cyclic voltammetry (CV) curves measured at scan rates of 20−200 mV s⁻¹ and in the range of −0.167 to 0.039 V versus RHE. Electrochemical impedance spectroscopy (EIS) measurements were carried out from 100 kHz to 0.1 Hz.

Density functional theory (DFT) calculations

All DFT calculations were conducted based on the Vienna ab initio simulation package (VASP).^37,38 Electronic exchange-correlation interactions were modeled using the Perdew–Burke–Ernzerhof (PBE) within the generalized gradient approximation functional (GGA).³⁹ The kinetic energy cutoff for the plane-wave expansion was set to 500 eV. All configurations were relaxed until the atomic forces were smaller than 0.02 eV Å⁻¹, and the total energy fluctuation was converged to 10⁻⁴ eV. The Brillouin zones were sampled by Monkhorst-Pack 9 × 9 × 9, 9 × 9 × 2, and 5 × 5 × 1 k-point grid for all geometric optimizations of pure metal, VB₂/NbB₂/TaB₂, and WB/MoB. The DFT-D3 method was adopted for a better description of the van der Waals interactions between reactants and substrates.^40,41 The slab models were constructed with boron-termination along the (111) for Pt, WB/MoB, and VB₂/NbB₂/TaB₂, and metal-termination along the (011) for WB/MoB, and VB₂/NbB₂/TaB₂. The thickness of the vacuum layer was set to 15 Å to avoid interactions between mirror images. All the upper half of the atom layers were fully relaxed while the remaining were kept frozen during the slab calculations.

The adsorption energy of atomic hydrogen (ΔE_H*) on different sites is defined as 1$$\Delta {E}_{{\rm{H}}* }=E(\text{support}+{\rm{H}})-E(\text{support})-\frac{1}{2}E({{\rm{H}}}_{2}),$$where E(support + H), E(support), and E(H₂) represent the energies of the adsorption system, the individual support, and the H₂ molecular, respectively.

The adsorption free energy of atomic hydrogen (ΔG_H*) is defined as 2$$\Delta {G}_{{\rm{H}}* }=\Delta E+\Delta ZPE-T\Delta S,$$where ΔE and ΔZPE represent the change of the total energy and zero-point energy; the T and ΔS are the temperature and entropy change, respectively.

The p-band center (ε_p) can be obtained by the following equation: 3$$\varepsilon {p}_{z}=\frac{{\int }_{-\infty }^{0}ED(E)dE}{{\int }_{-\infty }^{0}D(E)dE}$$where D(E) is the density of states (DOS) of B p band at a given energy E.^42,43

RESULTS AND DISCUSSION

TMB-TFs including MoB, WB, VB₂, NbB₂, and TaB₂ were prepared by the solid-state reaction between NaBH₄ and corresponding TM foils (e.g., Mo, W, V, Nb, and Ta). The typical synthesis process is illustrated in Figure 1A. A compacted mixture of TM foil and NaBH₄ was vertically placed in a vacuumed quartz tube and heated at temperatures ranging from 900°C to 1150°C (details in Section 2). After the reaction, the shiny surface of the TM foil became black, indicating that the TM foil has reacted with NaBH₄ to form TMB layer on the surface of the TM foil (right side of Figure 1A). The representative XRD patterns of reaction products (Figure 1B) confirm that the boride layers grown on Mo and W foils are single phase with the tetragonal structure (space group of I41/amd) of MoB (PDF#06-0636) and WB (PDF#06-0635), respectively.³⁰ Differently, the boride layers grown on V, Nb, and Ta foils are the hexagonal structure (space group of P6/mmm) of VB₂ (PDF#38-1463), NbB₂ (PDF#08-0120), and TaB₂ (PDF#38-1462), respectively (Figure S1). Figures S2 and S3 display the XRD patterns of MoB-TF and WB-TF prepared at different reaction temperatures, revealing that all the synthesized TFs have a single phase of MoB and WB, respectively.

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XPS was used to analyze the chemical state and surface composition of the representative MoB, WB, VB₂, and NbB₂ TFs. As shown in Figure 1C, the profiles of Mo 3d can be deconvoluted into two peaks assigned to Mo-B species at 227.96 and 231.07 eV (Mo⁰ from MoB), and two peaks attributed to Mo-O species at 232.35 and 235.42 eV (Mo⁶⁺ from MoO₃),⁴⁴ respectively. As depicted in Figure 1D, the W 4f peaks at 32.2 and 34.35 eV are assigned to W-B species (W⁰ from WB),⁴⁵ and the peaks at 36.18 and 38.34 eV correspond to W-O bonds (W⁶⁺ from WO₃).⁴⁶ The XPS spectrum of V 2p (Figure 1E) reveals that the peaks at 512.8 and 520.36 eV are attributed to V-B species (V⁰ from VB₂),⁴⁷ while the peaks at 203.98 and 207.48 eV in the XPS spectrum of Nb 3d (Figure 1F) are similarly attributed to Nb-B species (Nb⁰ from NbB₂).⁴⁸ Figures S4A–D shows the corresponding XPS spectra of B 1s for MoB, WB, VB₂, and NbB₂ TFs, displaying typical deconvoluted peaks assigned to B-TM species and B-O species.^45–48 As for the presence of B and TM oxidation states, it should result from the exposure to air and the surface oxidation of TMB-TFs.^49,50 In addition, it can be found from the XPS spectra of Ta 4 f (Figure S5) that the peaks at 23.18 and 25.08 eV belong to Ta-B bonds (Ta⁰ from TaB₂),⁵⁰ and the peak at 26.48 eV belongs to Ta-O bonds (Ta⁵⁺ from Ta₂O₅).⁵¹ The above XPS analysis further confirms the successful synthesis of TMB-TFs.

Figure 2A displays the FESEM image of cross section of MoB-TF, which indicates that the MoB layer with a thickness of 40 μm has been vertically grown on the Mo substrate. The uniform distribution of Mo and B elements over the cross section (Figure 2B) confirms again the formation of single-phase MoB. In addition, the cross-section observation for other TMB-TFs, such as WB, VB₂, NbB₂, and TaB₂ was also performed by FESEM. The results shown in Figures S6–S9 reveal that these as-synthesized TMB-TFs have a thickness on the micron scale. FESEM image of surface morphology and corresponding elemental mappings (Figures S10–S14) suggest that TMB-TFs are composed of micron-sized grains, and the corresponding TM and boron elements are uniformly dispersed on the surface. As shown in Figure 2C and 2E the microstructures of representative MoB and WB powders scratched from thin film surface are further analyzed via TEM. The selected area electron diffraction (SAED) pattern in the inset of Figure 2C confirms the (008), (217), and (204) crystal planes in MoB, which is consistent with the standard PDF card (No. 06-0636). Moreover, the high-resolution TEM (HRTEM) image of MoB in Figure 2D shows the (008) crystal plane with lattice fringe of d = 0.21 nm, demonstrating the crystalline nature of MoB. The elemental mapping (Figure 2G) displays the uniform distribution of Mo and B elements across the sample, revealing the formation of a single phase. Similarly, the typical crystal planes of (004), (215), and (112) of WB are found from the SAED pattern (inset of Figure 2E), and the clear lattice fringes with d = 0.42 nm are indexed to (224) crystal plane (Figure 2F). Correspondingly, the W and B elements are also homogeneously distributed in the WB sample (Figure 2H). All the above TEM characterizations further demonstrate the successful synthesis of MoB-TF and WB-TF. On one hand, this integration of TMB-TFs with TM foils can improve the electron transporting property at the interface of electrode and electrolyte owing to their excellent electronic conductivity;^35,36 and on the other hand, the formed TM-B bonds imply a strong hybridization of the B-sp orbitals with TM-d orbitals, which may modify the d-band center of TM and optimize the free energy of H adsorption.⁵²

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The HER electrocatalytic activity of MoB, WB, VB₂, NbB₂, and TaB₂ TFs was evaluated using a three-electrode system at a scan rate of 2 mV s⁻¹ with IR-drop compensation. First of all, the activity of the as-prepared TMB-TFs at different reaction temperatures was investigated, as shown in Figures S15–S17. It is found that MoB-TF prepared at 1050°C has the best HER activity with the smallest onset overpotential in acidic solutions. Meanwhile, a comparative study of the HER activity between commercial Pt/C and TMB-TFs was also performed under the same condition. Figure 3A depicts LSV curves of MoB, WB, and VB₂ TFs in 0.5 M H₂SO₄ aqueous solution (pH = 0.5, Figure S18), followed by Pt/C, Mo foil, and V foil as references. The polarization curves display that, as expected, the Pt/C has the highest catalytic activity with a low overpotential of 33 mV at 10 mA cm⁻², which is consistent with other reported values (Table S1). MoB-TF exhibits a notably improved HER activity with a low overpotential of 222 mV at the current density of 10 mA cm⁻², compared to WB-TF (249 mV), VB₂-TF (322 mV), Mo foil (366 mV), W foil (366 mV), and V foil (557 mV). The overpotential of MoB-TF is also lower than those of previously reported MoB bulk and MoS₂-based thin film catalysts in the literature, including MoB bulk (300 mV at 3.5 mA cm⁻²),⁵³ α-MoB₂ (226 mV at 10 mA cm⁻²),⁵⁴ MoO₃-MoS₂ nanowires grown on FTO (270 mV at 10 mA cm⁻²),⁵⁵ mesoporous MoS₂ film (230 mV at 10 mA cm⁻²),⁵⁶ and so on (Table S2). Figure S17 shows the LSV curves of NbB₂-TF, TaB₂-TF, Nb foil, and Ta foil, indicating that their overpotentials are 382, 505, 621, and 628 mV at 10 mA cm⁻², respectively.

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To further clarify the enhanced HER activity of MoB-TF, in the acidic electrolyte, the Tafel slopes of all samples are calculated by using the data of potential (η) and corresponding current density (j) (Figure 3A) to fit the Tafel equation.⁵⁷ The commercial Pt/C shows a very low Tafel slope of 28.7 mV dec⁻¹ (Figure 3B) in an acidic medium, suggesting that the Tafel reaction is the rate-determining step (Tafel slope of ~30 mV dec⁻¹).⁵⁸ By comparison, it is observed that MoB-TF also shows a smaller Tafel slope of 66.41 mV dec⁻¹, significantly lower than the results obtained for WB-TF (~73.41 mV dec⁻¹), VB₂-TF (~100.85 mV dec⁻¹), Mo foil (~112.23 mV dec⁻¹), and V foil (229.6 mV dec⁻¹), suggesting the faster reaction kinetics of MoB-TF. The ECSA of these thin films was also compared by testing the electrochemical double-layer capacitance (C_dl) at the solid/liquid interface, which is calculated using the typical CV method.⁵⁹ In detail, the CV curves were measured at various scanning rates from 20 to 200 mV s⁻¹ with an interval of 20 mV s⁻¹ in a potential region from −0.167 to 0.039 V versus RHE in 0.5 M H₂SO₄ (Figure S19). By fitting the capacitive current density versus the scan rate, the C_dl values of MoB-TF, WB-TF, and VB₂-TF were calculated to be 2.09, 3.88, and 0.415 mF cm⁻² (Figure 3C). Among them, MoB-TF shows a moderate C_dl value, indicating that the highly catalytic HER activity of MoB-TF could originate from the intrinsic electronic structure. The EIS values⁶⁰ of MoB, WB, and VB₂ TFs were measured in 0.5 M H₂SO₄ solution at the overpotential of 222, 249, and 322 mV, respectively, in the frequency range from 0.1 to 100,000 Hz. The Nyquist plots of MoB, WB, and VB₂ TFs are shown in Figure 3D. By fitting the experimental fitting data, it is found that MoB-TF has the lowest charge transfer resistance (Rct) value of 4.93 Ω, much lower than that of WB-TF (~5.76 Ω) and VB₂-TF (~10.84 Ω). This means a faster charge transfer process of MoB-TF on the electrode surface.

Catalytic stability is also an important factor to evaluate the catalyst in practical HER application. As depicted in Figure 3E, MoB-TF keeps a stable current density of 10 mA cm⁻² for 10 h, and no obvious changes in morphology and crystal phase are observed (Figures S20 and S21). And after the chronoamperometry (i-t) measurement over 10 h, there is no significant change of overpotential at the current density of 50 mA cm⁻² (Figure 3F), confirming excellent electrochemical stability in acidic electrolytes. As for the outstanding electrocatalytic stability of MoB-TF, it may be interpreted by the following reasons: (1) both MoB-TF and Mo foils with high hardness and good electron conduction can provide high mechanical strength and low electron transferring resistance; (2) MoB-TF as a corrosion resistance layer grown on the Mo foil efficiently provides a stable structure for the long-term HER operation.⁶¹

To further study the dependence of HER activity on TF thickness, MoB-TFs with different thicknesses have been fabricated by adjusting the amount of boron penetrant (NaBH₄), in which the synthetic details are described in the experimental section of supporting information. As shown in the FESEM image of Figure S22, the thickness of MoB-TF is controlled to be 40, 28, 8.5, 4, and 2.5 μm, respectively. The corresponding XRD patterns (Figure S23) reveal that these synthesized TFs have a single phase of MoB. As depicted in LSV polarization curves (Figure 4A), the overpotential of MoB-TF gradually reduces from 222 to 191 mV, as the thickness is decreased from 40 to 4 μm. However, when the thickness is further decreased to 2.5 μm, the overpotential increases contrarily, suggesting that the optimal thickness of MoB-TF is 4 μm. Figure 4B gives the comparison result of LSV curves for MoB-TF with the thickness of 4 μm and commercial Pt/C. MoB exhibits much higher electrocatalytic activity toward HER than Pt/C at the high current density above ~490 mA cm⁻². Moreover, it has the smallest Tafel slope of 60.25 mV dec⁻¹ among all the MoB-TFs (Figure 4C). As shown in Figures S24 and S25, MoB-TF with a thickness of 4 μm also has the highest C_dl values of 2.02 mF cm⁻², indicating a higher active surface area as well as more exposed active sites. The Nyquist plots depicted in Figure 4D reveal that MoB-TF with a thickness of 4 μm possesses the lowest R_ct value of 3.73 Ω. Figure 4E shows the multistep chronoamperometric curve for MoB-TF with a thickness of 4 μm measured at the current density changing from 100 to 400 mA cm⁻² with an interval of 100 mA cm⁻². It indicates that the current density keeps a stable value at each testing overpotential region. Meanwhile, an H-type water-splitting device using MoB-TF as cathode and NiFe-layered double hydroxide (NiFe-LDH) as anode shows relatively good durability for 40 h, remaining a stable high current density at 600 mA cm⁻² for 40 h (Figure 4F). This indicates that MoB-TF has great potential application in industrial usage, which requires long-term operation at the large current density.^30,62,63 These results fully demonstrate that decreasing the thickness of MoB-TFs has significantly improved the HER activity, especially at the high current density.

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The HER activity of MoB-TF with different thicknesses was further evaluated in an alkaline medium (pH = 13.82, Figure S26). As depicted in Figure 5A, MoB-TF with a thickness of 4 μm has the lowest overpotential of 219 mV at 10 mA cm⁻². Interestingly, MoB-TF with a thickness of 4 μm also shows comparable activity to Pt/C at the high current density in an alkaline medium, especially when current density is higher than 545 mA cm⁻² (Figure 5B). Moreover, the Tafel slope of MoB-TF with a thickness of 4 μm is 61.91 mV dec⁻¹, much smaller than that of other MoB-TFs (Figure 5C). Figure 5D shows the calculated C_dl values for MoB-TFs with different thicknesses from the CV curves measured in an alkaline medium (Figure S27). The result shows that MoB-TF with a thickness of 4 μm has a higher active surface area as well as more exposed electrochemically active sites than other MoB-TFs. Correspondingly, MoB-TF with a thickness of 4 μm also has the lowest R_ct value ~9.75 Ω in 1 M KOH (Figure 5E). At the high current density of 100 mA cm⁻², MoB-TF with a thickness of 4 μm also keeps good stability for 12 h in alkaline conditions (Figure 5F).

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To further elucidate the catalytic mechanism of TMB-TFs for HER, DFT calculations were conducted to unveil the hydrogen-adsorption free energy (ΔG_H*) for all the TMB-TFs. The Gibbs free energy of H* adsorption for an ideal catalyst is closer to zero.^64–66 Too strong or too weak binding of H* to the catalyst surface is not conducive to the formation of H₂.⁶⁷ First, the possible adsorption sites for the H atom on the TM-terminated (001) surfaces of WB/MoB/VB₂/NbB₂/TaB₂ were explored (Figure S28). As shown in Figure 6C, it is evident that the magnitude of ΔG_H* on the MoB surface is 0.35 eV, close to that of Pt (0.11 eV), followed by WB (0.41 eV), VB₂ (0.54 eV), TaB₂ (0.75 eV), and NbB₂ (0.85 eV). This result is consistent with the observed HER activity in the aforementioned experiments. Among them, MoB with the smallest absolute ΔG_H* value exhibits the best HER electrocatalytic activity. To investigate how the B atom improves the HER catalytic activity of TMBs, a series of (111) surfaces of TMBs including MoB, WB, VB₂, NbB₂, and TaB₂ were constructed (Figure 6A), and the exposed boron atoms on the (111) surface of TMBs were considered as adsorption sites. Figure 6D shows ΔG_H* values for the B active sites of various TMBs. The B sites of VB₂, NbB₂, and TaB₂ have a strong H* adsorption strength with free energy ΔG_H* = −1.06 ~ −1.40 eV, while the B sites at MoB and WB exhibit adequate H* adsorption strength, ΔG_H* = −0.25 and −0.35 eV for MoB and WB, respectively. The ΔG_H* (−0.11 eV) for the benchmark Pt(111) surface catalyst was also given for comparison. Intuitively, the order of H* adsorption strength follows Pt(111) < MoB < WB < NbB₂ < VB₂ < TaB₂, matching well with the HER activity observed in the above-mentioned experiment results. Moreover, the charge density difference of H adsorption on B sites of MoB(111) further demonstrates the electron transfer from B atoms to H* species, as displayed in Figure 6B. For both the TM-terminated surface and B-terminated surface, MoB shows the smallest absolute ΔG_H* value, thus exhibiting the best HER electrocatalytic activity. To further characterize the catalytic activity of the TMBs, taking MoB and WB as examples, the kinetic process of water dissociation was examined. The adsorption energy of H₂O on MoB(111) and WB(111) is −0.82 and −0.89 eV, respectively, indicating that H₂O adsorption is thermodynamically favorable on these surfaces. The corresponding energy barrier of water dissociation is 2.20 and 2.11 eV (Figure S29), respectively, much higher than the value of Pt (0.8 eV),⁶⁸ agreeing well with the experimental results.

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To gain deeper insights into the B atom in enhancing HER activity of TMB-TFs, we conducted a comparative study on the projected density of states (PDOS) and p-band centers of the surface B atoms in (111) surfaces for TMBs. According to the extended Hückel theory,⁶⁹ deeper valence states will lead to stronger H* binding strength. The binding strength of various TMBs is correlated with the p band center of B atoms in (111) surfaces of TMBs, as demonstrated in Figure 6E. Overall, TMBs with a deeper p band center relative to the Fermi level provide stronger H* binding strength. Figure 6F shows a clear linear relationship between ε_p and ΔG_H* for TMBs. MoB with the shallow p band center (−4.16 eV) shows closer ΔG_H* to 0 eV than WB (−4.53 eV), VB₂ (−4.69 eV), NbB₂ (−4.71 eV), and TaB₂ (−4.96 eV), thus showing the best HER performance among all TMB-TFs. This is consistent with the p band theory for other non-metal active site catalysts, such as N-doped graphene supported by MXene monolayers,⁴² TM oxides and carbides substrates covered by N-doped graphitic sheets,⁴³ 2D MBene with B active surface,⁵² and TM filled boron nitride nanotubes.⁷⁰ We also considered the d band center of metal atoms in (111) surfaces of TMBs, as demonstrated in Figure S30. It is in contrast to the d band theory for transition metals,⁷¹ but in the same trend of the valence band energy level or p band center of B atoms in TMBs. Therefore, the HER behavior could be attributed to the deep p band center of TMBs, which allows the bonding and antibonding states of TMBs with the H* adsorbate to be fully occupied.

CONCLUSIONS

In summary, we reported a universal boronization strategy to synthesize the TMB-TFs grown on low-cost and commercially available TM foils. Among them, MoB-TF with a thickness of 4 μm exhibits the highest intrinsic HER activity in both acidic and alkaline media. Especially, it has superior electrocatalytic HER activity to the commercial Pt/C at the high current density. DFT calculations reveal that the HER activity of TMB-TFs is rooted in the p band center of B atoms on the surface of TMBs. Therefore, this work not only provides a facile and effective route for synthesizing self-supported TMB-TFs but also presents a high-performance HER catalyst for large-scale hydrogen production.

ACKNOWLEDGMENTS

This work is financially supported by the National Natural Science Foundation of China (No. 52172058), the Outstanding Youth Fund of Natural Science Foundation of Inner Mongolia Autonomous Region (Grant No.2023JQ15), the Fundamental Research Funds for the Inner Mongolia Normal University (Grant No. 2022JBBJ010 and 2022JBTD008), the Major Project Cultivation Fund for the Inner Mongolia Normal University (2020ZD01), the Funds for Reform and Development of Local Universities Supported by The Central Government (Cultivation of First-Class Disciplines in Physics), the Postdoctoral Fellowship Program of CPSF under Grant (No. GZB20240101), and the China Postdoctoral Science Foundation under Grant (No. 2024M750304).

CONFLICT OF INTEREST STATEMENT

The authors declare that there are no conflicts of interests.

References

Dresselhaus, MS, Thomas IL. Alternative energy technologies. Nature. 2001;414(6861):332‐337.

Crabtree GW, Dresselhaus MS, Buchanan MV. The hydrogen economy. Phys Today. 2004;57(12):39‐44.

Kuang M, Zheng G. Nanostructured bifunctional redox electrocatalysts. Small. 2016;12(41):5656‐5675.

Liu XL, Jiang YC, Huang JT, et al. Bifunctional PdPt bimetallenes for formate oxidation‐boosted water electrolysis. Carbon Energy. 2023;5(12): [eLocator: e367].

Görlin M, Chernev P, Ferreira de Araújo J, et al. Oxygen evolution reaction dynamics, faradaic charge efficiency, and the active metal redox states of Ni‐Fe oxide water splitting electrocatalysts. J Am Chem Soc. 2016;138(17):5603‐5614.

Wang Y, Kong B, Zhao D, Wang H, Selomulya C. Strategies for developing transition metal phosphides as heterogeneous electrocatalysts for water splitting. Nano Today. 2017;15:26‐55.

Shi Y, Zhang B. Recent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reaction. Chem Soc Rev. 2016;45(6):1529‐1541.

Xue Q, Wang Z, Ding Y, Li F, Chen Y. Chemical functionalized noble metal nanocrystals for electrocatalysis. Chin J Catal. 2023;45:6‐16.

Lv H, Li D, Strmcnik D, Paulikas AP, Markovic NM, Stamenkovic VR. Recent advances in the design of tailored nanomaterials for efficient oxygen reduction reaction. Nano Energy. 2016;29:149‐165.

Morales‐Guio CG, Stern LA, Hu X. Nanostructured hydrotreating catalysts for electrochemical hydrogen evolution. Chem Soc Rev. 2014;43(18):6555‐6569.

Faber MS, Jin S. Earth‐abundant inorganic electrocatalysts and their nanostructures for energy conversion applications. Energy Environ Sci. 2014;7(11):3519‐3542.

Jamesh MI. Recent progress on earth abundant hydrogen evolution reaction and oxygen evolution reaction bifunctional electrocatalyst for overall water splitting in alkaline media. J Power Sources. 2016;333:213‐236.

Ge ZX, Ding Y, Wang TJ, et al. Interfacial engineering of holey platinum nanotubes for formic acid electrooxidation boosted water splitting. J Energy Chem. 2023;77:209‐216.

Gong T, Liu Y, Cui K, et al. Binary molten salt in situ synthesis of sandwich‐structure hybrids of hollow β‐Mo₂C nanotubes and N‐doped carbon nanosheets for hydrogen evolution reaction. Carbon Energy. 2023;5(12): [eLocator: e349].

Sun Y, Zhang T, Li C, Xu K, Li Y. Compositional engineering of sulfides, phosphides, carbides, nitrides, oxides, and hydroxides for water splitting. J Mater Chem A. 2020;8(27):13415‐13436.

Swain G, Sultana S, Parida K. A review on vertical and lateral heterostructures of semiconducting 2D‐MoS₂ with other 2D materials: a feasible perspective for energy conversion. Nanoscale. 2021;13(22):9908‐9944.

Cao Y. Roadmap and direction toward high‐performance MoS₂ hydrogen evolution catalysts. ACS Nano. 2021;15(7):11014‐11039.

Yu J, Dai Y, Zhang Z, et al. Tailoring structural properties of carbon via implanting optimal Co nanoparticles in N‐rich carbon cages toward high‐efficiency oxygen electrocatalysis for rechargeable Zn‐air batteries. Carbon Energy. 2022;4(4):576‐585.

Tang H, Gao X, Zhang J, et al. Boron‐Rich molybdenum boride with unusual short‐range vacancy ordering, anisotropic hardness, and superconductivity. Chem Mater. 2020;32(1):459‐467.

Mohammadi R, Turner CL, Xie M, et al. Enhancing the hardness of superhard transition‐metal borides: molybdenum‐doped tungsten tetraboride. Chem Mater. 2016;28(2):632‐637.

Shao Y, Shao M, Kawazoe Y, Shi X, Pan H. Exploring new two‐dimensional monolayers: pentagonal transition metal borides/carbides (penta‐TMB/Cs). J Mater Chem A. 2018;6(22):10226‐10232.

Mori T. Thermoelectric and magnetic properties of rare earth borides: boron cluster and layered compounds. J Solid State Chem. 2019;275:70‐82.

Bao W, Liu D, Duan Y. A first‐principles prediction of anisotropic elasticity and thermal properties of potential superhard WB₃. Ceram Int. 2018;44(12):14053‐14062.

Pan Y, Lin Y. Influence of B concentration on the structural stability and mechanical properties of Nb‐B compounds. J Phys Chem C. 2015;119(40):23175‐23183.

Liu X, Jiao Y, Zheng Y, Qiao SZ. Isolated boron sites for electroreduction of dinitrogen to ammonia. ACS Catal. 2020;10(3):1847‐1854.

Guo F, Wu Y, Chen H, et al. High‐performance oxygen evolution electrocatalysis by boronized metal sheets with self‐functionalized surfaces. Energy Environ Sci. 2019;12(2):684‐692.

Chen D, Liu T, Wang P, et al. Ionothermal route to phase‐pure RuB₂ catalysts for efficient oxygen evolution and water splitting in acidic media. ACS Energy Lett. 2020;5(9):2909‐2915.

Li J, Chen H, Liu Y, Gao R, Zou X. In situ structural evolution of a nickel boride catalyst: synergistic geometric and electronic optimization for the oxygen evolution reaction. J Mater Chem A. 2019;7(10):5288‐5294.

Zhuang Z, Li Y, Li Z, et al. MoB/g‐C₃N₄ interface materials as a schottky catalyst to boost hydrogen evolution. Angew Chem Int Ed. 2018;57(2):496‐500.

Chen, Y, Yu G, Chen W, et al. Highly active, nonprecious electrocatalyst comprising borophene subunits for the hydrogen evolution reaction. J Am Chem Soc. 2017;139(36):12370‐12373.

Park H, Lee E, Lei M, Joo H, Coh S, Fokwa BPT. Canonic‐like HER activity of Cr_1‐xMo_xB₂ solid solution: overpowering Pt/C at high current density. Adv Mater. 2020;32(28): [eLocator: 2000855].

Jothi PR, Zhang Y, Yubuta K, Culver DB, Conley M, Fokwa BPT. Abundant vanadium diboride with graphene‐like boron layers for hydrogen evolution. ACS Appl Energy Mater. 2019;2(1):176‐181.

Zhang Z, Cui ZH, Jimenez‐Izal E, Sautet P, Alexandrova AN. Hydrogen evolution on restructured B‐rich WB: metastable surface states and isolated active sites. ACS Catal. 2020;10(23):13867‐13877.

Hua, W, Sun HH, Xu F, Wang JG. A review and perspective on molybdenum‐based electrocatalysts for hydrogen evolution reaction. Rare Met. 2020;39(4):335‐351.

Yin, J, Li Y, Lv F, et al. Oxygen vacancies dominated NiS₂/CoS₂ interface porous nanowires for portable Zn‐air batteries driven water splitting devices. Adv Mater. 2017;29(47): [eLocator: 1704681].

Chen GF, Ma TY, Liu ZQ, et al. Efficient and stable bifunctional electrocatalysts Ni/Ni_xM_y (M = P, S) for overall water splitting. Adv Funct Mater. 2016;26(19):3314‐3323.

Kresse G, Hafner J. Ab initio molecular dynamics for liquid metals. Phys Rev B. 1993;47(1):558‐561.

Kresse G, Furthmüller J. Efficient iterative schemes for ab initio total‐energy calculations using a plane‐wave basis set. Phys Rev B. 1996;54(16):11169‐11186.

Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phys Rev Lett. 1996;77(18):3865‐3868.

Grimme S. Semiempirical GGA‐type density functional constructed with a long‐range dispersion correction. J Comput Chem. 2006;27(15):1787‐1799.

Grimme S, Antony J, Ehrlich S, Krieg H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT‐D) for the 94 elements H‐Pu. J Chem Phys. 2010;132(15): [eLocator: 154104].

Zhou S, Yang X, Pei W, Liu N, Zhao J. Heterostructures of MXenes and N‐doped graphene as highly active bifunctional electrocatalysts. Nanoscale. 2018;10(23):10876‐10883.

Pei W, Zhou S, Bai Y, Zhao J. N‐doped graphitic carbon materials hybridized with transition metals (compounds) for hydrogen evolution reaction: understanding the synergistic effect from atomistic level. Carbon. 2018;133:260‐266.

Lan L, Fan X, Zhao C, et al. Two‐dimensional MBenes with ordered metal vacancies for surface‐enhanced Raman scattering. Nanoscale. 2023;15(6):2779‐2787.

Li Q, Wang L, Ai X, et al. Multiple crystal phases of intermetallic tungsten borides and phase‐dependent electrocatalytic property for hydrogen evolution. Chem Commun. 2020;56(90):13983‐13986.

Martin‐Litas I, Vinatier P, Levasseur A, Dupin JC, Gonbeau D, Weill F. Characterisation of r.f. sputtered tungsten disulfide and oxysulfide thin films. Thin Solid Films. 2002;416(1‐2):1‐9.

Sadeghi E, Peighambardoust NS, Aydemir U. Tailoring the morphology of cost‐effective vanadium diboride through cobalt substitution for highly efficient alkaline water oxidation. Inorg Chem. 2021;60(24):19457‐19466.

Escamilla R, Huerta L. X‐ray photoelectron spectroscopy studies of non‐stoichiometric superconducting NbB_2+x. Supercond Sci Technol. 2006;19(6):623‐628.

Liu X, Zhu T, Gong Y. Efficient removal of Azo‐Dyes in aqueous solution by CeB₆ nanocrystals. ACS Appl Nano Mater. 2019;2(9):5704‐5712.

Wu Z, Long Y, Lin HT, Zhang F. Effect of tantalum on phase transition and thermal stability of metastable tungsten tetra‐boride. Ceram Int. 2020;46(11):17217‐17223.

Moo JGS, Awaludin Z, Okajima T, Ohsaka T. An XPS depth‐profile study on electrochemically deposited TaO_x. J Solid State Electrochem. 2013;17(12):3115‐3123.

Yang X, Shang C, Zhou S, Zhao J. MBenes: emerging 2D materials as efficient electrocatalysts for the nitrogen reduction reaction. Nanoscale Horiz. 2020;5(7):1106‐1115.

Park H, Encinas A, Scheifers JP, Zhang Y, Fokwa BPT. Boron‐dependency of molybdenum boride electrocatalysts for the hydrogen evolution reaction. Angew Chem Int Ed. 2017;56(20):5575‐5578.

Park H, Zhang Y, Lee E, Shankhari P, Fokwa BPT. High‐current‐density HER electrocatalysts: graphene‐like boron layer and tungsten as key ingredients in metal diborides. ChemSusChem. 2019;12(16):3726‐3731.

Chen Z, Cummins D, Reinecke BN, Clark E, Sunkara MK, Jaramillo TF. Core‐shell MoO₃‐MoS₂ nanowires for hydrogen evolution: a functional design for electrocatalytic materials. Nano Lett. 2011;11(10):4168‐4175.

Kibsgaard J, Chen Z, Reinecke BN, Jaramillo TF. Engineering the surface structure of MoS₂ to preferentially expose active edge sites for electrocatalysis. Nat Mater. 2012;11(11):963‐969.

Li H, Hu M, Cao B, et al. Multi‐elemental electronic coupling for enhanced hydrogen generation. Small. 2021;17(11): [eLocator: 2006617].

Luo Z, Miao R, Huan TD, et al. Mesoporous MoO_3‐x material as an efficient electrocatalyst for hydrogen evolution reactions. Adv Energy Mater. 2016;6(16): [eLocator: 1600528].

Jothi PR, Zhang Y, Scheifers JP, Park H, Fokwa BPT. Molybdenum diboride nanoparticles as a highly efficient electrocatalyst for the hydrogen evolution reaction. Sustain Energy Fuels. 2017;1(9):1928‐1934.

Liu, X, Gong Y. α‐MoB₂ nanosheets for hydrogen evolution in alkaline and acidic media. ACS Appl Nano Mater. 2022;5(8):10183‐10191.

Zhang J, Deng C, Song J, Deng C, Liu M, Zhou K. MoB‐CoCr as alternatives to WC‐12Co for stainless steel protective coating and its corrosion behavior in molten zinc. Surf Coat Technol. 2013;235:811‐818.

Tang R, Yang Y, Zhou Y, Yu XY. Rational design of heterostructured Ru cluster‐based catalyst for pH universal hydrogen evolution reaction and high‐performance Zn‐H₂O battery. Adv Funct Mater. 2023;34(5): [eLocator: 2301925].

Wang X, Yao H, Zhang C, et al. Double‐tuned RuCo dual metal single atoms and nanoalloy with synchronously expedited Volmer/Tafel kinetics for effective and ultrastable ampere‐level current density hydrogen production. Adv Funct Mater. 2023;33(40): [eLocator: 2301804].

Hammer B, Nørskov JK. Theoretical surface science and catalysis‐calculations and concepts. Adv Catal. 2000;45:71‐129.

Hinnemann B, Moses PG, Bonde J, et al. Biomimetic hydrogen evolution: MoS₂ nanoparticles as catalyst for hydrogen evolution. J Am Chem Soc. 2005;127(15):5308‐5309.

You B, Tang MT, Tsai C, Abild‐Pedersen F, Zheng X, Li H. Enhancing electrocatalytic water splitting by strain engineering. Adv Mater. 2019;31(17): [eLocator: 1807001].

Nørskov, JK, Bligaard T, Logadottir A, et al. Trends in the exchange current for hydrogen evolution. J Electrochem Soc. 2005;152(3):J23‐J26.

Skúlason E, Karlberg GS, Rossmeisl J, et al. Density functional theory calculations for the hydrogen evolution reaction in an electrochemical double layer on the Pt(111) electrode. Phys Chem Chem Phys. 2007;9(25):3241‐3250.

Hoffmann R. An extended hückel theory. I. hydrocarbons. J Chem Phys. 1963;39(6):1397‐1412.

Zhou S, Yang X, Xu X, Dou SX, Du Y, Zhao J. Boron nitride nanotubes for ammonia synthesis: activation by filling transition metals. J Am Chem Soc. 2020;142(1):308‐317.

Nørskov JK, Abild‐Pedersen F, Studt F, Bligaard T. Density functional theory in surface chemistry and catalysis. Proc Natl Acad Sci USA. 2011;108(3):937‐943.