Constructing 1D Boron Chains in the Structure of Transition Metal Monoborides for Hydrogen Evolution Reactions

: The forms of boron atoms are many and varied in the structure of transition metal borides (TMBs). The form of boron atoms determines the structure, morphology, and properties of borides. Herein, transition metal monoborides (CrB and WB) with different arrangement of one-dimensional (1D) boron chains were synthesized under high pressures and high temperatures. The 1D boron chains between the interlayers of CrB are parallel to one another, while the 1D boron chains between the interlayers of WB are perpendicular to one another. The morphologies of CrB and WB also show large differences due to the difference in 1D boron chain arrangement. As electrocatalysts for hydrogen evolution reactions (HERs), CrB and WB show good catalysis activity and durability. WB has the smallest overpotential (210 mV) and Tafel slope (90.09 mV dec − 1 ), which is mainly attributed to the intercrossing boron chains improving the electrical properties of WB, as well as the 5d electrons of W being more chemically active. The TOF value of WB is 1.35 s − 1 , proving that WB has a higher intrinsic catalytic activity during HERs. This work provides a data reference for the development of high-efﬁciency electrocatalysts. of


Introduction
Hydrogen, as a non-carbon energy carrier with high energy density and zero-pollution emissions, has attracted widespread attention in the past decade [1][2][3]. Although hydrogen is the most abundant element in the universe, it is mainly stored as a chemical compound in the most profuse resource on Earth, i.e., water. To this end, hydrogen from water splitting is the key element to develop hydrogen energy in the future. Water electrolysis could meet all the requirements of clean, renewable energy sources, while the electricity could be driven by solar, wind, or other renewable energy sources [4,5]. In order to improve the conversion efficiency of water electrolysis, an appropriate electrocatalyst is necessary. As we know, noble metals (e.g., Pt, Pd, etc.) have the best hydrogen evolution reaction (HER) performance in acidic solutions, but their high cost and rarity are not suitable for large-scale application [6]. Therefore, it is urgent to develop low-cost, high-efficiency, and stable electrocatalysts to replace noble metals. Over the past decade, more and more catalysts have been developed, including carbides, sulfides, nitrides, phosphides, borides, etc. [7][8][9]. 2

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Among them, transition metal borides (TMBs) have attracted extensive attention in the field of electrocatalytic water splitting, because of their excellent electrical properties and abundant structural forms-especially for HER. TMBs such as MoB, CoB, NiB, FeB, etc., have been reported [10][11][12][13][14][15][16][17][18][19][20][21][22][23], showing excellent catalytic performances in the watersplitting field. While some of these catalysts cover the intrinsic mechanism of TMBs for HERs, they have no specific structures other than amorphous. Although several TMBs with certain structures also indicate superior HER performance, others do not. One of the reasons for this is that, in addition to the transition metal, the form of boron atoms in the compound also has a certain influence on the catalytic performance. The most typical catalyst influenced by the form of boron atoms is molybdenum diboride (MoB 2 ), which has two phase structures, only differing in the arrangement of boron atoms. In the metastable phase (AlB 2 -type), the present form of boron atoms is a two-dimensional (2D) graphenelike borophene structure, while in the stable phase, boron atoms also exist in the form of a puckered boron layer as well as the 2D graphene-like borophene structure. Experimental and theoretical results confirm that the 2D graphene-like borophene structure is the main factor that improves the catalytic performance during the HER process [24]. However, the intrinsic influence of other forms of boron atoms is rarely reported, e.g., one-dimensional (1D) boron chains or 3D boron skeletons. Uncovering the inner mechanism of TMBs for HERs, based on the form of boron, is significant. In the boride system, 3D boron skeletons are always difficult to fabricate, because of boron's lack of electrons. Nevertheless, boron atoms can also exist in the form of onedimensional (1D) boron chains in the structure of transition metal monoborides. In our previous report, we found that MoB has two phase structures, with the difference being in the arrangement of 1D boron chains. Therefore, the synthesis of metastable-phase MoB with parallel boron chains requires a very high reaction temperature, which presents difficulties in the experimental operation [25]. However, by changing transition metal elements, compounds with two kinds of boron chains can be obtained under relatively mild conditions, and the influence of 1D boron chains on electrocatalytic activity can be explored.
Therefore, in this work, we synthesized pure phases of CrB and WB under high pressure and high temperature (HPHT). The structures and morphologies of CrB and WB were confirmed. In addition, the electrocatalytic activity of transition metal monoborides for HERs was conducted in acidic solution, and the differences in the HER performance of CrB and WB were analyzed. The influence of 1D boron chains and the effect of different metal atoms for HERs were studied. The above results prove that the transition metal borides are potential catalysts to replace noble metals, and provide data to support the development of hydrogen energy in the future.

Results and Discussion
Transition metal monoborides were synthesized by HPHT as described in our previous report (Scheme 1; for more details, see the Section 3). The samples obtained resembled tablets in appearance. The crystal structures of transition metal monoborides were obtained via powder X-ray diffraction (XRD). As can be seen from Figure 1a, CrB and WB were successfully synthesized; the crystal structures of CrB and WB belong to orthorhombic (Cmcm) and tetragonal (I4 1 /amd) systems, respectively, which is consistent with previous reports [26][27][28][29][30]. As described in these papers, the structural formulae of CrB and WB are shown in Figure 1b,c, respectively. CrB and WB have similar crystal structures, composed of a 3D metal frame and 1D boron chains. However, the interlayer 1D boron chains are arranged in parallel for CrB, while they are perpendicular to one another in WB.  The surface morphology of the samples was investigated via field-emission scanning electron microscopy (FESEM). The surface of CrB is composed of grains of micron size, and it exhibits a step-growth mechanism that shows two-dimensional steps, as shown by the blue dotted line in Figure 2a. When zooming in on the surface of CrB, a dense surface is observed, as with all previous samples prepared under HPHT ( Figure  2b). Meanwhile, WB has a relatively flat surface with pores of different sizes ( Figure 2e). As shown in Figure 2f, the internal morphology of WB was observed through the pores, and it was found that WB is composed of nanoparticles with grain sizes of ~200 nm. The existence of pores may be due to the fact that WB has stronger mechanical properties, preventing the elimination of air to the surface of the sample under HPHT. The difference in grain sizes between CrB and WB lies in the different arrangement of the 1D boron chains. According to the PBC theory, there is a strong covalent bond along the direction of the 1D boron chain, so there is a faster growth rate in this direction [31][32][33]. In CrB, the 1D boron chains between the interlayers are arranged in parallel to one another, so they grow in step mode, and eventually produce micrometer-size morphology. However, the 1D boron chains between the interlayers of WB are perpendicular to one another; thus, in the process of grain growth, the 1D boron chains between the interlayers inhibit one another's growth rates, limiting the growth of grains and forming nano-scale grains. High-resolution transmission electron microscopy (HRTEM) was used to observe finer structural information. Both CrB and WB show good crystallinity in Figure 2c,g, respectively. The crystal surface information was characterized by inverse fast Fourier transformation (IFFT) and line-scanning intensity profile. The line-scanning intensity profile obtained from the area is highlighted by the green frame and purple frame in the inset of Figure 2d,h, respectively. The spacing of 0.279 nm corresponds to the (110) crystal plane in CrB, and the spacing of 0.307 nm matches the (101) crystal plane in WB. All of the results confirm that CrB and WB were successfully synthesized under HPHT.  The surface morphology of the samples was investigated via field-emission scanning electron microscopy (FESEM). The surface of CrB is composed of grains of micron size, and it exhibits a step-growth mechanism that shows two-dimensional steps, as shown by the blue dotted line in Figure 2a. When zooming in on the surface of CrB, a dense surface is observed, as with all previous samples prepared under HPHT ( Figure  2b). Meanwhile, WB has a relatively flat surface with pores of different sizes ( Figure 2e). As shown in Figure 2f, the internal morphology of WB was observed through the pores, and it was found that WB is composed of nanoparticles with grain sizes of ~200 nm. The existence of pores may be due to the fact that WB has stronger mechanical properties, preventing the elimination of air to the surface of the sample under HPHT. The difference in grain sizes between CrB and WB lies in the different arrangement of the 1D boron chains. According to the PBC theory, there is a strong covalent bond along the direction of the 1D boron chain, so there is a faster growth rate in this direction [31][32][33]. In CrB, the 1D boron chains between the interlayers are arranged in parallel to one another, so they grow in step mode, and eventually produce micrometer-size morphology. However, the 1D boron chains between the interlayers of WB are perpendicular to one another; thus, in the process of grain growth, the 1D boron chains between the interlayers inhibit one another's growth rates, limiting the growth of grains and forming nano-scale grains. High-resolution transmission electron microscopy (HRTEM) was used to observe finer structural information. Both CrB and WB show good crystallinity in Figure 2c,g, respectively. The crystal surface information was characterized by inverse fast Fourier transformation (IFFT) and line-scanning intensity profile. The line-scanning intensity profile obtained from the area is highlighted by the green frame and purple frame in the inset of Figure 2d,h, respectively. The spacing of 0.279 nm corresponds to the (110) crystal plane in CrB, and the spacing of 0.307 nm matches the (101) crystal plane in WB. All of the results confirm that CrB and WB were successfully synthesized under HPHT. The surface morphology of the samples was investigated via field-emission scanning electron microscopy (FESEM). The surface of CrB is composed of grains of micron size, and it exhibits a step-growth mechanism that shows two-dimensional steps, as shown by the blue dotted line in Figure 2a. When zooming in on the surface of CrB, a dense surface is observed, as with all previous samples prepared under HPHT (Figure 2b). Meanwhile, WB has a relatively flat surface with pores of different sizes ( Figure 2e). As shown in Figure 2f, the internal morphology of WB was observed through the pores, and it was found that WB is composed of nanoparticles with grain sizes of~200 nm. The existence of pores may be due to the fact that WB has stronger mechanical properties, preventing the elimination of air to the surface of the sample under HPHT. The difference in grain sizes between CrB and WB lies in the different arrangement of the 1D boron chains. According to the PBC theory, there is a strong covalent bond along the direction of the 1D boron chain, so there is a faster growth rate in this direction [31][32][33]. In CrB, the 1D boron chains between the interlayers are arranged in parallel to one another, so they grow in step mode, and eventually produce micrometer-size morphology. However, the 1D boron chains between the interlayers of WB are perpendicular to one another; thus, in the process of grain growth, the 1D boron chains between the interlayers inhibit one another's growth rates, limiting the growth of grains and forming nano-scale grains. High-resolution transmission electron microscopy (HRTEM) was used to observe finer structural information. Both CrB and WB show good crystallinity in Figure 2c,g, respectively. The crystal surface information was characterized by inverse fast Fourier transformation (IFFT) and line-scanning intensity profile. The line-scanning intensity profile obtained from the area is highlighted by the green frame and purple frame in the inset of Figure 2d,h, respectively. The spacing of 0.279 nm corresponds to the (110) crystal plane in CrB, and the spacing of 0.307 nm matches the (101) crystal plane in WB. All of the results confirm that CrB and WB were successfully synthesized under HPHT. The valence and bonding modes of CrB and WB were characterized via X-ray photoelectron spectroscopy (XPS). Figure 3a,d show the XPS spectra of CrB and WB. The associated peaks of elements appeared in both CrB and WB. In addition, O peaks were observed, which may be from the result of surface oxidation [34,35]. The determination of metal-B bonding refers to the previous literature, where it was reported that the binding energy position of metal-B bonding is slightly higher than that of pure metal [36]. Hence, the bonding of Cr-B located at 574.44 and 583.69 eV corresponds to 2p3/2 and 2p1/2, respectively ( Figure 3b)-higher than that of pure metal Cr (574.2 eV) [37]. The other pairs of peaks at 576.9 and 586.47 eV belong to Cr-O bonding [38], whereas the B shows two peaks of B-Cr and B-O at 187.63 and 192 eV, respectively ( Figure 3c). As shown in Figure  3e, a pair of binding energies slightly higher than the peaks of pure metal W are located at 31.18 and 33.32 eV, which are attributed to to 4f7/2 and 4f5/2 of W-B bonding, respectively [39]. The pair of peaks at the higher binding energy belong to W-O bonding [40]. Similar B peaks also appeared in WB. The peak at a higher binding energy position was comparable to the B peak of CrB, indicating that the 1D boron chain with perpendicular arrangement needs less charge transfer to exist stably, compared with the 1D boron chain with parallel arrangement. The valence and bonding modes of CrB and WB were characterized via X-ray photoelectron spectroscopy (XPS). Figure 3a,d show the XPS spectra of CrB and WB. The associated peaks of elements appeared in both CrB and WB. In addition, O peaks were observed, which may be from the result of surface oxidation [34,35]. The determination of metal-B bonding refers to the previous literature, where it was reported that the binding energy position of metal-B bonding is slightly higher than that of pure metal [36]. Hence, the bonding of Cr-B located at 574.44 and 583.69 eV corresponds to 2p 3/2 and 2p 1/2 , respectively (Figure 3b)-higher than that of pure metal Cr (574.2 eV) [37]. The other pairs of peaks at 576.9 and 586.47 eV belong to Cr-O bonding [38], whereas the B shows two peaks of B-Cr and B-O at 187.63 and 192 eV, respectively ( Figure 3c). As shown in Figure 3e, a pair of binding energies slightly higher than the peaks of pure metal W are located at 31.18 and 33.32 eV, which are attributed to to 4f 7/2 and 4f 5/2 of W-B bonding, respectively [39]. The pair of peaks at the higher binding energy belong to W-O bonding [40]. Similar B peaks also appeared in WB. The peak at a higher binding energy position was comparable to the B peak of CrB, indicating that the 1D boron chain with perpendicular arrangement needs less charge transfer to exist stably, compared with the 1D boron chain with parallel arrangement.
The electrochemical HER performance of CrB and WB at 0.5 M H 2 SO 4 (pH 0) was determined by employing a standard three-electrode system on a CH Instruments 760E electrochemical analyzer. For comparison, B powder, Cr sheets, and W wire were also studied under the same conditions. The polarization curves of HER for different catalysts are shown in Figure 4a. The HER activity of B powder was negligible. The overpotential of CrB and WB at a current density of 10 mA cm −2 (η 10 ) was 347 and 210 mV, respectively, while those of metal Cr and W were 582 and 526 mV, respectively. The performance of WB was consistent with Zou's report [41]. The Tafel slopes were calculated from the polarization curve (Figure 4b), showing that the Tafel slopes of CrB and WB were 110.59 and 90.06 mV dec −1 , respectively-smaller than those of B powder (191.05 mV dec −1 ), Cr sheets (150.37 mV dec −1 ), and W wire (139.93 mV dec −1 ). The results suggest that the HER kinetics of metal Cr and W are accelerated after boronization. Meanwhile, the exchange current density (j 0 ) of CrB and WB was calculated according to the linear region of the extrapolated Tafel slope to characterize the intrinsic HER activity. The j 0 of CrB and WB was 0.017 and 0.776 mA cm −2 , respectively. Both Tafel slopes and j 0 indicated that WB has more favorable kinetics than CrB for HERs. Electrochemical impedance spectroscopy (EIS) also revealed that the electron transfer resistance (R ct ) of WB is much lower than that of CrB (Figure 4c), further demonstrating that WB has much faster charge transfer during the HER process, and accelerates the hydrogen evolution reaction. Figure 4d is obtained using η 10 , Tafel slope and R ct as functional coordinates, indicating that WB is the optimal electrocatalyst with various indices. energy position of metal-B bonding is slightly higher than that of pure metal [36]. Hence, the bonding of Cr-B located at 574.44 and 583.69 eV corresponds to 2p3/2 and 2p1/2, respectively (Figure 3b)-higher than that of pure metal Cr (574.2 eV) [37]. The other pairs of peaks at 576.9 and 586.47 eV belong to Cr-O bonding [38], whereas the B shows two peaks of B-Cr and B-O at 187.63 and 192 eV, respectively ( Figure 3c). As shown in Figure  3e, a pair of binding energies slightly higher than the peaks of pure metal W are located at 31.18 and 33.32 eV, which are attributed to to 4f7/2 and 4f5/2 of W-B bonding, respectively [39]. The pair of peaks at the higher binding energy belong to W-O bonding [40]. Similar B peaks also appeared in WB. The peak at a higher binding energy position was comparable to the B peak of CrB, indicating that the 1D boron chain with perpendicular arrangement needs less charge transfer to exist stably, compared with the 1D boron chain with parallel arrangement. The electrochemical surface area (ECSA) was used to assess the number of HER active sites on CrB and WB [42,43]. The double-layer capacitance (C dl ) of the catalyst is proportional to the value of ECSA [44,45]. Thus, the C dl values of CrB and WB are shown in Figure 4e. The C dl value of WB is 8.36 mF cm −2 , which is higher than that of CrB (1.72 mF cm −2 ). The larger ECSA is attributed to the pores on the WB surface (Figure 2e,f), which are conducive to the immersion of electrolytes, enabling the exposure of more active sites to participate in the HER. In order to investigate the intrinsic HER activity of each active site, we further calculated the H 2 turnover frequency (TOF) of CrB and WB (Figure 4f). When the overpotential is −0.3 V vs. RHE, the TOF value of WB is 1.35 s −1 , while that of CrB is only 0.17 s −1 . The results show that the intrinsic activity of WB is much better than that of CrB, which may be due to the fact that the chemical activity of 5d electron in W is higher than that of 3d electron in Cr, and it can undergo a rapid adsorption/desorption reaction in the hydrogen evolution process. Furthermore, the amount of hydrogen generated was tested with different times for CrB and WB (Figure 4g). The results indicate that WB has a higher hydrogen generation rate than CrB. For electrocatalysts, another important evaluation index is long-term stability. At a constant current density of 10 mA cm −2 , the overpotential of CrB and WB were maintained for approximately 12 h without significant activity loss (Figure 4h). To sum up, the transition metal monoborides synthesized under HPHT show good HER activity under acidic conditions, while WB shows excellent catalytic activity.

Materials Synthesis
Transition metal monoborides were synthesized under HPHT. For CrB, known amounts of metal chromium powder (99.9%) and amorphous boron powder (99.9%) were mixed at a molar ratio of 1:1.05, and the powders were mixed in an agate mortar for more than 3 h to ensure a uniform mixture. The mixed powder was cold-pressed into a tablet with a diameter of 4 mm and a height of 3 mm. The tablet was then placed in a cubic anvil HPHT apparatus (6 × 14,400 KN) at 5 GPa and 1600 °C for a holding time of 15 min. The details of the HPHT apparatus and assembly are provided in our previous report [46]. For the synthesis of WB, the operation was similar to that of CrB, except that the chromium powder was replaced with metal tungsten powder, the molar ratio of the starting material was 1:1, and the reaction temperature was 1400 °C.

Characterization
The phased structure of the transition metal monoborides was determined via powder X-ray diffraction (XRD) using Cu Kα (λ = 1.5418) radiation in a Rigaku SmartLab

Materials Synthesis
Transition metal monoborides were synthesized under HPHT. For CrB, known amounts of metal chromium powder (99.9%) and amorphous boron powder (99.9%) were mixed at a molar ratio of 1:1.05, and the powders were mixed in an agate mortar for more than 3 h to ensure a uniform mixture. The mixed powder was cold-pressed into a tablet with a diameter of 4 mm and a height of 3 mm. The tablet was then placed in a cubic anvil HPHT apparatus (6 × 14,400 KN) at 5 GPa and 1600 • C for a holding time of 15 min. The details of the HPHT apparatus and assembly are provided in our previous report [46]. For the synthesis of WB, the operation was similar to that of CrB, except that the chromium powder was replaced with metal tungsten powder, the molar ratio of the starting material was 1:1, and the reaction temperature was 1400 • C.

Characterization
The phased structure of the transition metal monoborides was determined via powder X-ray diffraction (XRD) using Cu Kα (λ = 1.5418) radiation in a Rigaku SmartLab SE instrument. The surface composition and chemical states of the synthesized samples were analyzed via X-ray photoelectron spectroscopy (XPS) on an ESCA LAB 250 X-ray photoelectron spectrometer with Al Kα radiation (hυ = 1486.6 eV) as the X-ray source. Field-emission scanning electron microscopy (FESEM, FEI Magellan 400 operating at 18 kV) and high-resolution transmission electron microscopy (HRTEM, JEM-2200FS operating at 200 kV) were used to observe the surface morphology and micromorphology of the synthesized samples.

Preparation of Working Electrode
The preparation of the working electrode was similar to that in our previous report [14]. The sample tablet was anchored on the L-shaped copper electrode with conductive silver slurry. After the slurry was dried, the exposed silver slurry and copper electrode were wrapped and sealed with modified acrylate adhesive. The test area of the working electrode was determined using a Leica M125 C instrument.

Electrochemical Measurements
All electrochemical measurements were performed using a CH Instrument (Model 760E) with a standard three-electrode system under 0.5 M H 2 SO 4 electrolytes (pH 0). A high-density graphite rod and saturated calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively. During the experimental testing, the electrolyte needed to be saturated with N 2 . The potential was calibrated against the reversible hydrogen electrode (RHE) in this work. During the linear scan voltammogram (LSV) measurements, the scan rate was set to be 2 mV s −1 , and the experimental data were iR corrected. The Tafel slope and exchange current density were obtained by extrapolating the polarization curve according to the following formula: η = a + blogj [47][48][49]. Electrochemical impedance spectroscopy (EIS) was carried out at −0.3 V vs. RHE with 5 mV amplitude, and at a frequency range from 10 6 to 1 Hz. The electrochemical surface area was investigated via cyclic voltammograms (CV) at various scan rates (2,4,8,12,16, and 20 mV s −1 ). The value of double-layer capacitance (C dl ) was obtained by calculating the slope between the ∆j = j anodic − j cathodic and scan rates. The turnover frequency (TOF) was calculated as described in previous reports [50]. Since the catalytic active site is uncertain, according to previous reports, we conservatively considered the top site of the metal to be the catalytic active site [41].

Conclusions
In conclusion, we can report that CrB and WB were successfully synthesized under HPHT. The only structural difference between CrB and WB is the arrangement of the 1D boron chains. In CrB, the boron chains between interlayers are parallel to one another, while in WB, the boron chains are perpendicular to one another. It is because of the different arrangement of the 1D boron chains that the micromorphology is characterized by micron size and nanometer size in CrB and WB, respectively. The transition metal monoborides exhibit excellent catalytic activity and durability for HERs under acidic conditions. Of the two, WB has a smaller overpotential of 210 mV, a smaller Tafel slope (90.09 mV dec −1 ), a smaller R ct (326.5 Ω), a larger ECSA (8.36 mF cm −2 ), and better durability. TOF results also confirmed that the HER activity of the active site in WB is higher than that of CrB. The best catalytic activity is generally shown by nanomaterials rather than bulk materials at present. Combined with the results of this work, transition metal monoboride catalysts have tremendous potential for improvement. Transition metal borides will become a promising kind of electrocatalyst candidate for the development and utilization of hydrogen energy in the future.