Multiple crystal phases of intermetallic tungsten borides and phase-dependent electrocatalytic property for hydrogen evolution

Article abstract of DOI:10.1039/d0cc06072k

Four stoichiometric W-B intermetallic phases, including W2B, WB, WB2 and WB3, are synthesized, and their hydrogen-evolution electrocatalytic properties and electronic structures are investigated comparatively. The electrocatalytic activity for the hydroge

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ARTICLE TYPE
Multiple crystal phases of intermetallic tungsten borides and phase-
dependent electrocatalytic property for hydrogen evolution
Qiuju Li,‡ Lina Wang,‡ Xuan Ai, Hui Chen, Jiayun Zou, Guo-Dong Li* and Xiaoxin Zou*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
active, nonprecious metal material.16, 17 Our further comparative
50 investigation of multiple crystal phases of intermetallic ruthenium
borides revealed that the boat-like covalent boron sheets in RuB2
were the crucial structural subunits, and their electronic interaction
with Ru atoms generated highly efficient hydrogen-evolution
electrocatalytic active sites.19
Four stoichiometric W-B intermetallic phases, including W2B,
WB, WB2 and WB3, are synthesized, and their hydrogen-
evolution electrocatalytic properties and electronic structures
are investigated comparatively. The electrocatalytic activity
10 for the hydrogen evolution reaction is found to first increase
from W2B to WB2 and then decrease; and this activity trend
can be rationalized based on their different degree of
hybridization between d orbitals of W and sp orbitals of B.
55
W-B intermetallics contain multiple crystal phases with
different boron substructures. Considering the unique advantages
of nonprecious WB2,16, 17 a deeper study on the W-B intermetallics
as electrocatalysts for HER is important. However, the study is
challenging due to the difficulty in precise synthesis of different
Intermetallic borides are a class of important functional inorganic
15 solids that cover a wide range of constituent metals, compositions
and crystal structures.1-4 Some of them exhibit exceptional bulk
properties, including permanent magnetism, thermoelectricity,
superhardness and superconductivity.5-8 These outstanding bulk
properties of intermetallic borides are generally attributed to the
20 unique crystal structures in which ionic, covalent and metallic
bonding types coexist, as well as the tunable d-sp orbital
hybridization between metals and boron atoms. The multiple types
of bonding and the metal-boron interplay also endow intermetallic
borides with some exciting surface catalytic functions (e.g.,
25 electrocatalytic water splitting and N2 fixation as well as catalytic
hydrogenation),1, 9-13 but the relevant studies emerged relatively
recently. The main bottleneck restricting the development of
boride catalysts lies in the lack of enough knowledge about
synthetic chemistry, surface adsorption and electronic properties
30 as well as structure-bonding-function correlations of intermetallic
borides.
60 W-B intermetallics. In this work, we aim to selectively synthesize
multiple crystal phases of intermetallic tungsten borides, and to
comparatively investigate their crystal and electronic structures as
well as their phase-dependent hydrogen evolution electrocatalytic
performances. We also want to reveal the effect of hybridization
65 of W d orbitals and B sp orbitals on surface catalytic behaviors of
intermetallic tungsten borides.
We first compared the crystal structures of four intermetallic
tungsten borides including W2B, WB, WB2 and WB3. As the ratio
of tungsten to boron increases, the boron substructures in these
70 borides evolve from isolated atoms to 1D chains, and then to 2D
networks. W2B possesses the lowest boron content (Fig. 1a), and
the isolated boron atoms is located in the central void of a square
antiprism composed of eight tungsten atoms (Fig. S1 in
Supplementary Information (SI)). As for WB (Fig. 1b), the B-B
75 distance significantly decreases, and the boron atoms form zig-zag
chains which alternate orthogonally. In WB2 (Fig. 1c), the boron
atoms are covalently bonded to form two types of 2D networks,
graphene-like boron layers (top of Fig.1e) and puckered boron
layers (middle of Fig.1e), which are stacked alternately with
80 tungsten layers. WB3 (Fig.1d) also contains graphene-like boron
layers similar to WB2 (bottom of Fig.1e). However, the tungsten
layers in WB3 can be viewed as the result of removing one third of
tungsten atoms (marked by the blue dotted circles) from close-
packed tungsten layers.21
In this context, several synthetic methods without using high
pressures have been developed to access intermetallic borides that
14-17
are compatible with the needs of catalysis studies.10,
But
35 progress is not easy, because precision synthesis of pure and/or
metastable crystal phases of intermetallic borides is still
challenging. Success has been limited to the synthesis of either a
small group of intermetallic borides with a fixed stoichiometry
(e.g., TM:B = 1:2; TM = transition metal)16-18 or multiple crystal
40 phases of a few of transition metal-boron systems (e.g., Ru-B
system).19, 20 For instance, a family of transition metal diborides
comprising group IVB-VIII metals were synthesized by our group
based on borothermic reduction reaction or quasi solid-state
metathesis reaction, and their trend in electrocatalytic activity for
85
In order to better understand the bonding of these four
intermetallic tungsten borides, the average distance of adjacent W-
W and B-B atoms, as well as electron location function (ELF) were
calculated. As shown in Fig. 1f, the B-B distance decreases with
the increase of boron content, while the W-W distance increases
45 the hydrogen evolution reaction (HER) was investigated
17
experimentally and theoretically.16,
Among transition metal
90 from W2B to WB2. Due to the similar crystal structures, WB3
possesses a W-W distance close to WB2. ELF analysis (Fig. 1g and
Fig. S2 in SI) reveals that there are relatively large ELF values
diborides, RuB2 was found to be the most efficient catalyst with
Pt-like activity for HER, and WB2 was identified as the highest
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(0.8-0.9) between two adjacent B atoms in WB, WB2 and WB3,
metallic W.
indicating the existence of B-B covalent bond in them. The B-B
distance is too long to form B-B bonds in W2B. The ELF values
between W-W atoms are in the range of 0.4 to 0.5, reflecting the
formation of W-W metallic bond. Owing to the difference in ELF
around adjacent W and B atoms, the W-B bond is ionic. This result
is further supported by the Bader charge transfer from W atom to
its adjacent B atoms (Table S1 in SI).
5
Fig. 2 (a) COHP curves of WB2. (b) The integrated COHP (-ICOHP) of
W2B, WB, WB2 and WB3 for different interactions. (c) Calculated DOS
40 and pDOS of W, W2B, WB, WB2 and WB3.
After investigating the crystal structures and electronic
properties of these intermetallic tungsten borides, we considered
their thermodynamic stability via the formation energies (Table S3
in SI). Fig. S4a (SI) presents the calculated convex hull diagram of
45 the W-B system. The four intermetallic tungsten borides have
negative formation energies, indicating that these structures are
deemed thermodynamically stable and synthesizable in principle.
Among four intermetallic tungsten borides, the formation energy
of WB is the smallest, implying that WB is the most stable phase.
50 However, the formation energies of these intermetallic tungsten
borides are similar to each other, making their selective synthesis
difficult.
We employed WCl6 as tungsten source, MgB2 as boron source,
and Mg as additive, to selectively synthesize these four
55 intermetallic tungsten borides based on the following three
adjustments: (i) the ratio of tungsten source and boron source; (ii)
the amount of Mg additive; (iii) the reaction temperature. The
details are provided in Experimental Section and Table S4 in the
SI. As shown in the powder X-ray diffraction (XRD) patterns (Fig.
60 S4b in SI), the as-synthesized samples are phase-pure intermetallic
tungsten borides mentioned above. Transmission electron
microscopy (TEM) images show that these intermetallic tungsten
borides are composed of nanoparticles ranging in size from 15 to
150 nm (Fig. S4c-f in SI). Scanning electron microscopy-energy
65 dispersive X-ray spectrometry (SEM-EDS) mapping images (Fig.
S5 in SI) confirm the uniform distribution of W and B in tungsten
borides. Additionally, the Brunauer-Emmett-Teller (BET) surface
areas of these intermetallic tungsten borides are in the range of 5.5
to 18.6 m2 g−1 (Table S5 in SI). The X-ray photoelectron
70 spectroscopy (XPS) spectra show that the strength of W-B bonding
enhances with the increase of boron content (Fig. S6 in SI).22 These
results illustrate that we have successfully synthesized multiple
crystal phases of intermetallic tungsten borides.
10 Fig. 1 Crystal structures of (a) W2B, (b) WB, (c) WB2 and (d) WB3. (e) Top
view of graphene-like (top) and puckered (middle) boron layers of WB2,
and graphene-like boron layer of WB3 (bottom). The W atoms deficiency
in WB3 is marked with the blue dotted circle. (f) The average distance of
adjacent W-W and B-B atoms of four intermetallic tungsten borides. (g)
15 ELF images of different planes for WB2.
The interatomic interactions (W-B, W-W and B-B) can also be
supported by Crystal Orbital Hamilton Population (COHP) curves.
As shown in Fig. 2a and Fig. S3 in SI, the results confirm the
metallic W-W and ionic W-B bonding nature in all four
20 intermetallic tungsten borides, as well as the covalent B-B bonding
nature in WB, WB2 and WB3. Moreover, the integrated COHP (-
ICOHP), which is used as a measure of bonding strength, was also
analyzed (Fig. 2b and Table S2 in SI). Generally, the larger the -
ICOHP value, the stronger the bonding strength. As the boron
25 content increases, the -ICOHP value of W-W decreases, while that
of B-B increases. This is contrary to the trends in W-W and B-B
distances. The density of states (DOS) and projected density of
states (pDOS) of metallic W and intermetallic tungsten borides
were calculated and displayed in Fig. 2c. All four intermetallic
30 tungsten borides have metallic properties, with the DOS crossing
the Fermi level. In addition, the pDOS reveals that there is the DOS
overlap between the W d orbitals and the B sp orbitals in
intermetallic tungsten borides, also suggesting the orbital
hybridization between W and B atoms. As a result, intermetallic
35 tungsten borides generally possess broader d-bands compared with
Next, we studied the electrocatalytic activities of these
75 intermetallic tungsten borides toward HER in 0.5 M H2SO4
solution (See details in Experimental Section (SI)). For comparison,
we also evaluated the activities of metallic W nanoparticles
(Figures S7 in SI) and 20 wt% Pt/C under the same testing
condition. Fig. 3a shows the polarization curves of four
80 intermetallic tungsten borides, W particles and 20 wt% Pt/C. As
the benchmarking HER catalyst, Pt/C has excellent activity and
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-2
only needs a low overpotential of 17 mV to reach 10 mA cmgeo
.
long-term stability (Fig. 3e) shows that WB2 can stably
Meanwhile, intermetallic tungsten borides also show good 40 electrocatalyze HER in 0.5 M H2SO4 solution for at least 70 h,
catalytic activities as nonprecious metal materials. WB2, WB and
W2B exhibit better activities than metallic W, achieving 10 mA
cmgeo-2 current density at overpotentials of 198, 206 and 259 mV,
respectively. WB3 requires a larger overpotential (328 mV) than W
demonstrating its good catalytic stability. WB2 also exhibits a
nearly 100ꢀ% Faradaic efficiency (Fig. S11 in SI), indicating a
complete electricity-to-hydrogen conversion during HER catalysis.
Moreover, intermetallic tungsten borides display medium HER
5
(296 mV) at the current density of 10 mA cmgeo-2. To further 45 performance in 1 M KOH solution, and the activity trend is
compare the specific activities of four intermetallic tungsten
borides and W, we normalized the measured currents by their
10 electrochemical surface areas (ECSAs, estimated from the value of
electrochemical double-layer capacitance, Fig. S8 in SI). As shown
consistent with which in acidic media (Fig. S12 in SI).
The hydrogen adsorption free energy (ΔGH*) has been
determined as an effective descriptor for HER activity, with a
value closer to zero indicating an optimal activity.24 In order to
in Fig. 3b, the comparison result demonstrates that their specific 50 study the relationship between catalytic activity and crystal phase,
activities follow the same trend as the activities normalized by
geometric area. That is, WB2 > WB > W2B > W > WB3. The
15 activity trend can be further confirmed by electrochemical
impedance spectroscopy Nyquist plots (Fig. S9 in SI).
we calculated ΔGH* values for the most stable H adsorption sites
on W-terminated (001) surfaces of intermetallic tungsten borides
(Fig. 4a and Fig. S13, Table S6-S8 in SI). For WB2, there are two
types of W-terminated surfaces, denoted as WB2-I (Fig. 4b) and
55 WB2-II (Fig. 4c), whose W layers are bond to graphene-like boron
layers and puckered boron layers, respectively. The ΔGH* values
for Pt (111) and W (110) surfaces were also included for
comparison. The benchmark catalyst Pt holds a near-zero ΔGH*
value. Among the four intermetallic tungsten borides, WB2-I has
60 the smallest absolute ΔGH* values, followed by WB. W2B exhibits
a moderate ΔGH* value similar to W, while the ΔGH* value of WB3
is far negative than W. Such change trend of ΔGH* value is
consistent with that of HER activity observed in the
aforementioned experiment.
65
We noticed a significant difference in ΔGH* values of WB2-I (-
0.40 eV) and WB2-II (-0.73 eV) surfaces, suggesting that WB2-I is
the catalytic surface with higher HER activity in WB2. In order to
explore the difference in activity of the two catalytic surfaces, we
calculated the d-band centers of the surface W atoms in the slab
70 models for W, WB2-I and WB2-II. As shown in Fig. 4d and Fig.
S14 (SI), the d-band center of WB2-I downshifts from -1.02 eV to
-1.51 eV compared with that of W, while the d-band center of
WB2-II upshifts to -0.91eV. The more negative d-band center in
WB2-I means that the antibonding states reduce in energy and
75 become more occupied, resulting in a weaker H adsorption energy
and better ΔGH* value.25 These results, together with the
experimental result that WB2 has the best catalytic activity among
four intermetallic tungsten borides, suggest that WB2-I should be
the dominant catalytic surface in WB2.
Fig. 3 (a) Polarization curves with 85% iR-compensation (the currents were
normalized by geometric area of the electrode). (b) Comparison of ECSAs
20 and specific activities (the currents were normalized by ECSAs) at 0.25
VRHE. Error bars indicate standard deviation from five measurements. (c)
Tafel plots derived from (a). (d) The short-term stability. The x-axis and y-
axis are the overpotentials required to reach 10 mA cmgoe−2 current density
at times t = 0 h and t = 2 h. The dashed line represents the ideal stable
25 catalyst response. (e) Chronopotentiometric curves of WB2 at 10 mA cmgoe
2. All catalysts (including Pt/C, W, W2B, WB, WB2 and WB3) were tested
for HER in 0.5 M H2SO4 solution.
80
We next attributed the variations of d-band center to local
changes in the adsorption sites, including strain effect and ligand
effect. 18, 25 On the one hand, the incorporation of B atoms in W
framework causes lattice expansion, which can be demonstrated by
the W-W bonding in WB2 being longer than metallic W (2.76 Å).
-
85 This tensile strain will reduce the W d-orbital overlap and band
width, thereby upshifting the d-band center (called the strain
effect). On the other hand, the orbital hybridization between W and
B atoms always results in the broadening of d-band width and
downshift of d-band center (called the ligand effect). Generally,
90 the higher the coordination number, the more negative the d-band
center. For WB2-I and WB2-II surfaces, the B coordination number
of surface W atoms is 6 and 4 (insets in Fig.4b and 4c),
respectively. This implies that WB2-I surface has a stronger
interatomic d-sp orbital hybridization than WB2-II surface, which
95 can be further proved by the -ICOHP values per unit cell (Table S9
in SI). These results indicate that ligand effect has a dominant role
Additionally, the calculated Tafel plots of the four intermetallic
tungsten borides are closed to that of W nanoparticles (Fig. 3c),
30 ranging from 50 to 80 mV decade-1. The result indicates that the
HER for these intermetallic tungsten borides occurs via a Volmer-
Heyrovsky mechanism, with electrochemical desorption of
hydrogen as the rate-determining step.23 We evaluated the
electrochemical stabilities of intermetallic tungsten borides and
35 metallic W using the chronopotentiometric test. As exhibited in the
short-term stability diagram (Fig. 3d), after 2 h of constant
polarization, WB2 and W have no obvious activity loss, whereas
the activities of WB, W2B and WB3 decrease slightly. Further
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in the modulation of d-band structure for WB2-I, while for WB2-II
strain effect is the main aspect. Therefore, WB2-I and WB2-II show
different variations of d-band center, compared with W surfaces.
21621001) and the 111 Project (B17020) for additional financial
support. We thank the LvLiang Cloud Computing Center of China,
and the calculations were performed on TianHe-2.
Conflicts of interest
50 There are no conflicts to declare.
Notes and references
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry,
College of Chemistry, Jilin University, Changchun 130012, P. R. China;
E-mail: xxzou@jlu.edu.cn; lgd@jlu.edu.cn
55 † Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI:10.1039/b000000x/
‡ Q. Li and L. Wang contributed equally to this work.
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15 explained by the fact that the d-band center is a simple but not
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휀W = 휀 + 푊 /2
descriptor,
, the upper band-edge energy proposed
d
d
d
by Nørskov at al.25 As shown in Table S10 in SI, the upper band-
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I. Since the upper band-edge energy intrinsically controls the
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ΔGH*) becomes weaker correspondingly. There is a linear
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In summary, we have synthesized four crystal phases of
intermetallic tungsten borides, and investigated their crystal and
35 electronic structures as well as phase-dependent hydrogen
evolution activities. We have also demonstrated the influence of
interatomic d-sp orbital hybridization on catalytic activities in
intermetallic tungsten borides. Our results have provided scientific
basis for the precise synthesis of other intermetallic borides and the
40 study of boron-regulated catalytic processes for various chemical
reactions.
X. Z. thanks the financial supports from the National Natural
Science Foundation of China (NSFC) Grant No. 21771079 and
21922507 and the Fok Ying Tung Education Foundation, Grant
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