Tungsten Carbide Hollow Microspheres with Robust and Stable Electrocatalytic Activity toward Hydrogen Evolution Reaction.

Here, we report a stable tungsten carbide hollow microsphere (W2C-HS) electrocatalyst with robust electrocatalytic activity toward hydrogen evolution reaction fabricated from carburization of tungsten oxides at 700 °C with CH4/H2 flow, which demands overpotentials of 153 and 264 mV to deliver 10 and 100 mA cm-2 ascribing to the hollow structures beneficial for interfacial charge transfer as well as releasing of hydrogen molecular. Meanwhile, the W2C-HS electrocatalyst exhibits undetectable degradation after 20 000 potential cycles indicative of extraordinary durability; in contrast, overpotential@100 mA cm-2 is dramatically increased from 128 to 251 mV after only 2000 potential cycles for benchmark platinum electrocatalyst.

Introduction

Exploration of an efficient hydrogen evolution reaction (HER) electrocatalyst has been tremendously carried out because of the ever-increasing environmental pollution as well as shortage in traditional fossil fuels.[1−3] It has been proved that platinum exhibits the most efficient HER electrocatalytic activity, which is applicable as the cathodic electrocatalyst in the current water-splitting technology. For large-scale industrial commercialization of water-splitting, development in an efficient and cost-effective HER electrocatalyst as a substitution of Pt electrocatalyst is of paramount desirability.[4−7] Intensive attention has been paid to transitional-metal sulfides,[1,8,9] carbides,[10−12] nitrides,[13−15] phosphides,[16−21] oxides,[22−25] and selenides[26,27] because the strong interaction between the transition metal and H* could be loosened after the incorporation of the above-mentioned elements, resulting in a neither too strong nor too weak interaction between the electrocatalyst with H* triggering efficient hydrogen evolution.[28]

Transition-metal carbides (TMCs), especially molybdenum carbide (MoC) and tungsten carbide (WC), have been investigated as HER electrocatalysts because a new d band of molybdenum or tungsten on the surface appears after combination with carbon, which was similar to the Pt d band suggesting that the exterior TMCs possessed analogous electronic structures compared to Pt and a comparable electrocatalytic activity to Pt was predicted.[29] Robust HER activity of TMCs was variously achieved via morphology design to create more active sites,[30−32] heteroatom doping to tailor the electronic structure of Mo[33−35] or W[36,37] resulting in a suitable interaction between H* and active sites, and hybridization with carbon materials to improve its electronic conductivity.[38−40] However, these TMCs only possessed low overpotentials for delivering a catalytic current density of 10 mA cm–2, which is far from the commercialization of water-splitting requiring massive H2 production at a high current density above 100 mA cm–2.[41] It is generally accepted that electrocatalysts with porous structures facilitated the diffusion of active species as well releasing of generated products;[42] thus, a porous electrocatalyst is favorable for boosting HER performance at high current density because the substantially produced H2 bubbles could be efficiently removed, resulting in less coverage of active sites and degradation in electronic conductivity.

Motivated by the above-mentioned consideration, here, we synthesized novel tungsten carbide hollow microspheres (W2C-HS) as HER electrocatalysts by carburization of tungsten oxide fabricated via the solvothermal method reported by Sun et al.[43] The W2C-HS electrocatalyst could efficiently produce H2 at high current density owing to its hollow structures as schematically shown in Figure a. To the best of our knowledge, this is the first report on using tungsten carbide with hollow microsphere structures as an HER electrocatalyst.

Figure 1

(a) Schematic illustration of synthetic routine of the W2C-HS electrocatalyst. (b) XRD patterns of WO3, WO2, W, W2C, and WC electrocatalysts. (c) N2 adsorption/desorption isothermal curves of WO3, W2C, and WC electrocatalysts.

Results and Discussion

The as-synthesized WO3 precursor was fabricated via a solvothermal process of sodium tungstate dihydrate (Na2WO4) and oxalic acid in water/isopropanol reported by Sun et al.[43] Subsequently, WO3 was carburized at various temperatures. In order to identify the structures of the electrocatalysts prepared at different temperatures, X-ray diffraction (XRD) test was carried out and the related XRD patterns are shown in Figure b, in which the XRD pattern of the WO3 precursor was in good agreement with the standard PDF card of WO3 (JCPDS: 32-1395) because the peaks appeared at 23° and 24° attributed to the (020) and (200) planes of WO3. Diffraction peaks at 26° and 37° were the (011) and (200) planes of WO2 (JCPDS: 32-1393), indicating that WO3 was transferred to WO2 after heating to 600 °C under a CH4/H2 atmosphere well agreed with the previous report.[44] When the temperature was increased to 650 °C, WO2 was further reduced to metallic W proved by the observed diffraction peaks at 40°, 58°, and 73° originating from the (110), (200), and (211) facets of metallic W (JCPDS: 04-0806). The carburization started at 700 °C and W2C was formed as characteristic diffraction peaks at 39°, 34°, and 38° were identified to (102), (002), and (200) planes of W2C according to the standard PDF card (JCPDS: 20-1315). If the temperature was increased to 800 °C, more carbon atoms entered into the lattices of W to form WC evidenced by the XRD peaks at 31°, 35°, and 48° stemming from the (001), (100), and (101) facets of WC (JCPDS: 65-8828).[45] Thus, it was concluded that WO3 was consequently transferred to WO2, W, W2C, and WC when the temperature increased from 600 to 800 °C under a CH4/H2 atmosphere.[46] As shown in Figure c, the specific surface areas of WO3, W2C, and WC estimated from the N2 adsorption/desorption isothermal curves were 13.0, 14.8, and 10.5 m2 g–1, respectively, which was indicative of stable structure during the carburization process. Continuously, the morphologies of the prepared electrocatalysts were measured by scanning electron microscopy (SEM) as shown in Figure a,b, suggesting that WO3 and W2C possessed perfect hollow spherical structures with a diameter of 5 μm. Additionally, WO2, W, and WC exhibited similar structures shown in Figure S1. The well-retained structure depicted that the carburization process negligibly affected the electrocatalyst structure. (The samples were defined as M-HS, where HS represents hollow microspheres.) Besides, from the SEM and transmission electron microscopy (TEM) images shown in Figure b,c, the W2C-HS electrocatalyst obtained a hollow structure because of the clearly observed pores in the SEM images and distinctly different brightness in the TEM image. Moreover, the high-resolution TEM (HR-TEM) image depicted that a lattice space of 0.23 nm was observed because of the main (102) plane of W2C-HS in good accordance with the XRD pattern. In order to understand the chemical environment and bonding configuration of the electrocatalyst, an X-ray photoelectron spectroscopy (XPS) test was conducted as shown in Figure S2. The two peaks observed at 37.7 and 35.6 eV were originated from W 4f5/2 and W 4f7/2 for WO3-HS, manifesting the presence of the W(VI) species (Figure d), whereas the two more peaks appeared at 33.9 and 31.8 eV were assigned to the W–C bond from W2C-HS (Figure e), similar to previous reports, and peaks at high binding energies of 37.7 and 35.5 eV were due to the unavoidable exterior oxidation of W2C exposed to air.[47] Also, W 4f peaks centered at 37.8 and 35.5 eV, indicating the successful formation of WC-HS as shown in Figure f. Additionally, the C 1s peak of W2C-HS was observed and peaks at 283.2, 284.5, and 286.5 eV were assigned to the C–W, C=C, and C–O bonds in electrocatalysts (Figure S2), which was also an indication of successful formation of W2C-HS.

Figure 2

SEM images of WO3-HS (a) and W2C-HS (b) electrocatalysts. (c) TEM image of the W2C-HS electrocatalyst and HR-TEM image as an inset. Deconvoluted W 4f spectra of WO3-HS (d), W2C-HS (e), and WC-HS (f) electrocatalysts.

Electrocatalytic activity toward HER was evaluated in N2 deoxygenated 0.5 M H2SO4 electrolyte with a catalyst loading of 0.285 mg cm–2. As shown in Figure a, benchmark Pt/C with low overpotentials of 31 and 128 mV to achieve catalytic current densities of 10 and 100 mA cm–2. Not surprisingly, WO3-HS, WO2-HS, and W-HS exhibited low electrocatalytic activity toward HER with overpotentials 558, 261, and 297 mV to deliver a cathodic current density of 10 mA cm–2. W2C-HS required 153 and 264 mV to attain 10 and 100 mA cm–2, respectively. W2C-HS exhibited a better HER performance compared to WC-HS as hydrogen ions strongly adsorbed on the WC-HS surface owing to a lower ΔGH (Gibbs free energy of hydrogen adsorption) of −0.56 eV compared to the W2C-HS (−0.31 eV) reported by Li et al.[48] A highly active HER electrocatalyst should have ΔGH close to 0 eV, representing a neither too strong nor too weak interaction between hydrogen ions and active sites. Besides, W2C-HS outperformed most of the WC-based electrocatalysts, for instance, WC/CNT,[49] W4MoC nanowire,[36] W2C@WC,[50] and WC.[51] In order to confirm the high HER activity of the W2C-HS electrocatalyst, the HER activity was conducted in H2-saturated electrolyte shown in Figure S3, in which a comparable linear sweep voltammetry (LSV) curve was obtained for the W2C-HS electrocatalyst. To quantitatively analyze the active sites of the obtained electrocatalysts, the electrochemical surface areas (ECSAs) calculated from the double-layer capacitances (Figure S4) were 21.1, 37.5, 43.9, 88.3, and 12.5 mF cm–2 for WO3-HS, WO2-HS, W-HS, W2C-HS, and WC-HS (Figure b), respectively, in good agreement with the order of HER performance. It should be noted that the double-layer capacitance was estimated from the simulated slope of Δcurrent density@0.14V versus scan rate shown in Figure b. The Tafel slope was extracted from LSV curves to study the HER kinetics, and commercial Pt/C with a Tafel slope of 29.5 mV dec–1 is shown in Figure c similar to previous report.[52] W2C-HS exhibited a much lower Tafel slope of 67.8 mV dec–1 compared to WO3-HS (107.4 mV dec–1), WO2-HS (98.8 mV dec–1), W-HS (80.7 mV dec–1), and WC-HS (75.4 mV dec–1) electrocatalysts, suggesting that W2C-HS followed the Volmer–Heyrovsky HER mechanism, in which the desorption of a hydrogen molecule is the rate-determining step (H3O+ + *Hads + e– → H2 + H2O + *, * represents the active site).[53] Additionally, the interfacial charge-transfer resistance (Rct) also significantly affected the HER activity, and electrochemical impedance spectroscopies (EIS) of all electrocatalysts were shown in Figure d, in which W2C-HS exhibited the lower Rct (60 Ω) compared to WC-HS (600 Ω), which could be due to the more metallic W in the electrocatalyst. It should be noted that all the EIS tests were performed at overpotential@10 mA cm–2 to study the electron transfer during the HER test. The high ECSA and low interfacial charge-transfer resistance co-contributed to the robust HER electrocatalytic activity of the W2C-HS electrocatalyst. It should be noted that the intrinsic electrocatalytic activity and the number of active sites play the significant role of HER performance. Because of the lowest ECSA of WO3-HS, WO3-HS showed the worst HER performance and the ECSA order (WO3-HS < WO2-HS < W-HS < WC-HS < W2C-HS) well coincided with the HER performance. As well known, the massively generated hydrogen bubbles at high overpotential cover the active sites resulting in deterioration in HER electrocatalytic activity. In order to emphasize the advantage of the W2C-HS electrocatalyst, HER performances of commercial Pt/C and W2C-HS electrocatalysts were measured using the rotating disk electrode. As well known, mass-transfer resistance could affect the HER performance at high current density because of the limited interfaces between electrocatalyst and electrolyte as well as coverage of active sites byproducts. As shown in Figure e,f, the HER performance of commercial Pt/C became better, especially for overpotential@100 mA cm–2 decreased from 128 to 105 mV with the rotation speed increased to 900 rpm because the strongly adhered H2 bubbles were removed releasing more active sites similar to previous reports;[54,55] in contrast, W2C-HS revealed a stable HER activity with almost negligible decrement (2 mV) for overpotential@100 mA cm–2 because of the hollow structure of W2C-HS beneficial for the release of hydrogen molecular resulting in no deterioration in ECSA and interfacial charge resistance. Thus, the hollow structure was beneficial for HER electrocatalytic activity at high current density. Durability is another important issue for HER electrocatalyst. As shown in Figure a, overpotentials@10 and 100 mA cm–2 were sharply increased from 31 and 128 to 46 and 251 mV for commercial Pt/C because of the aggregation/coalescence of Pt nanoparticles revealed by the decreased ECSAs shown in Figure S5 lost by ∼10%, although the W2C-HS electrocatalyst showed almost on degradation in the HER electrocatalytic activity (Figure b) because of the comparable ECSA (85.1 mF cm–2, Figure c) as well as a similar interfacial charge transfer resistance before and after the durability test (Figure S6). Meanwhile, W2C-HS could maintain a stable catalytic current density of 10 mA cm–2 for 10 h with an applied voltage of −153 V versus RHE as shown in Figure d. In addition, the overpotential@100 mA cm–2 (264 mV) for W2C-HS was comparable to that of commercial Pt/C after durability test (251 mV), indicating that the W2C electrocatalyst performed a similar HER activity to commercial Pt/C. W2C-HS retained a similar structure after durability test and the EDS mappings revealed its high durability (Figure e–g). The TEM image shown in Figure S7 depicted that the structure was well maintained after durability test confirmed by the lattice spacing of 0.23 nm assigned to the dominant (102) facet of W2C. Meanwhile, peaks at 33.9 and 31.8 eV (Figure S8) were attributed to the W–C bond from W2C-HS coincided with TEM test. Continuously, the HER performance of W2C-HS was tested in alkaline and neutral media shown in Figure S9, in which the overpotentials for delivering 10 mA cm–2 were 240 and 735 mV versus RHE in 1 M KOH and 1 M phosphate-buffered saline (PBS) electrolyte, respectively. The lower HER activities tested in 1 M KOH and 1 M PBS compared to the HER performance in acidic medium was due to the lower ECSA calculated from double-layer capacitance (11.5 and 5.6 mF cm–2 in 1 M KOH and 1 M PBS). Thus, W2C-HS performed high HER electrocatalytic activity and ultrahigh durability in acidic medium, indicating that W2C-HS could be a potential substitution of Pt/C for large-scale industrial water-splitting.

Figure 3

LSV curves (a), double-layer capacitances (b), Tafel slopes (c), and EIS (d) of WO3-HS, WO2-HS, W-HS, W2C-HS, and WC-HS electrocatalysts. HER performances of Pt/C (e) and W2C-HS (f) electrocatalysts with different rotation speeds.

Figure 4

HER performances of W2C-HS (a) and commercial Pt/C (b) electrocatalysts before and after the durability test. (c) Double-layer capacitance of W2C-HS before and after the durability test. (d) Chronoamperometric test of the W2C-HS electrocatalyst at −153 mV vs RHE. SEM image (e) and related EDS mappings (f,g) of W2C-HS electrocatalysts after the durability test.

Conclusions

In summary, we have synthesized novel W2C hollow microspheres (W2C-HS) as HER electrocatalysts. W2C-HS required 153 and 264 mV to deliver cathodic current densities of 10 and 100 mA cm–2. Moreover, W2C-HS showed an ultrahigh durability with almost no degradation in the HER activity after 20 000 potential cycles and performed a similar overpotential@100 mA cm–2 to commercial Pt/C after 2000 potential cycles. The high HER electrocatalytic activity of W2C-HS attributed to the hollow structures favorable for releasing of H2. The further improvement in the HER performance of the W2C-HS electrocatalyst is undergoing in our laboratory by optimization of size and shell thickness of the W2C-HS electrocatalyst.

Experimental Section

Synthesis of WO3

Sodium tungstate dihydrate (1.5 g, Sinopharm, ≥99.5%) and 1.0 g of oxalic acid (Sinopharm, ≥99.8%) were dissolved in 30 mL of water with 40 mL of isopropanol (Sinopharm, ≥99.7%), and a moderate amount of 2 M HCl (Sinopharm, ≥36.0–38.0%) was added. Then, the solution was transferred to a 100 mL Teflon-lined stainless steel autoclave and heated up to 80 °C for 24 h. After cooling naturally, the as-prepared samples were calcined at 300 °C in a tube furnace with an Ar atmosphere.

Synthesis of WC-HS

The WO3 was heated to a specified temperature (700 and 800 °C) for 2 h with CH4 (20 sccm) and H2 (80 sccm) in a tube furnace. The final black product was filtered, washed, and dried for 5 h.

Electrochemical Measurements

The HER performance was measured in 0.5 M H2SO4 electrolyte based on a three-electrode cell, in which a glass carbon electrode, a carbon rod, and a saturated calomel electrode were used as working, counter, and reference electrodes, respectively. The catalyst loading was 0.285 mg cm–2 and LSV curve was measured with a scan rate of 5 mV s–1 and CV was carried out with 50 mV s–1 ranging from −0.3 to 0.2 V versus RHE. EIS was measured from 100 kHz to 0.01 Hz by VMP3 (Bio-Logic Science Instruments).