Highly Active Mo₂C@WS₂ Hybrid Electrode for Enhanced Hydrogen Evolution Reaction

Abstract

Transition metal dichalcogenides (TMDs) are the auspicious inexpensive electrocatalysts for the hydrogen evolution reaction (HER) which has been broadly studied owing to their remarkable enactment, however the drought of factors understanding were highly influenced to hinder their electrocatalytic behavior. Recently, transition metal carbide (TMC) has also emerged as an attractive electrode material due to their excellent ionic and electronic transport behavior. In this work, Mo₂C@WS₂ hybrids have been fabricated through a simple chemical reaction method. Constructed heterostructure electrocatalyts presented the small Tafel slope of 59 and 95 mV per decade and low overpotential of 93 mV and 98 @10 mA·cm⁻² for HER in acidic and alkaline solution, respectively. In addition, 24-h robust stability with the improved interfacial interaction demonstrated the suitability of hybrid electrocatalyst for HER than their pure form of Mo₂C and WS₂ structures. The derived outcomes describe the generated abundant active sites and conductivity enhancement in TMC/TMD heterostructure along with the weaken ion/electron diffusion resistance for efficient energy generation applications.

1. Introduction

The swift development of global energy requirements and connected curb climate pollution, as well as environmental challenges has generated an imperative mandate to replace exhaustible fossil fuels for renewable energy sources. Hydrogen (H₂) is considered as a promising existent energy candidate for sustainable/environmental energy resources owing to their zero carbon emission and high energy density [1,2]. Currently, to obtain a superb H₂ evolution rate, the precious metals of Pt and, Pd based groups are considered as benchmark electrocatalysts for the hydrogen evolution reaction (HER) [1,2]. Due to the scarcity and high-cost, extensive efforts have been made to construct low-cost electrocatalysts, including transition metal carbide (TMC), transition metal chalcogenides (TMD), transition metal nitrides, and transition metal phosphides which have been performed well in acid or alkaline media [3,4,5,6]. The earth-abundant TMC have attracted tremendously by the research community due to low cost, easy availability and active and stable HER characteristics in both acidic and base medium [7]. Amongst, Mo₂C has considered as promising a substitutes for expensive Pt-based catalysts resembling that of Pt-like electronic structure, high corrosion resistance and superior electrical conductivity (~1.0 × 10² S·cm⁻¹) [8]. Due to the aggregation/entanglement and coalescence of nanoparticles and low surface areas, bare Mo_xC nanoparticles toward HER are considered as a premature electrocatalysts. Various nanocarbons (carbon nanotubes, graphene, or graphene oxide) are connected/blended with Mo_xC to prevent aggregation and propagation of active nanoparticles. TMDs have the chemical composition of MX₂ (M-Metal and X- chalcogen elements) with lamellar sandwich structure. TMDs layered bulk structure own the covalent bonds between M atoms and X atoms, whereas the layers of TMDs are bound to each other by weak van der Waals forces. Among the various TMD candidates, WS₂ is a highly favorable member for HER applications. Theoretical and experimental assumptions also confirm that the active electrocatalytic sites of WS₂ are primarily situated on metallic edges, whereas the basal surfaces are catalytically inert [9]. Due to large amount of edge sites, easy ion transport channel and large specific surface area of WS₂ have been broadly opened their use as HER catalysts. By taking advantage of numerous pores, edges shape architecture, and modifiable active interface of TMD or TMC, fabrication of TMD/TMC hybridize material is still appealing by research community. However, the study of TMD/TMC based hybrid for HER application is still infancy and there are only few articles deal with this topic. Quite recently, TMD/TMC has proven to be a powerful method to enhance the electrocatalytic activity, such as MoS₂/Mo₂C hybrid nanosheets [10], W_xC@WS₂ nanostructure [11], WS₂/W₂C hybrid [12,13], Ni-induced nitrogen-doped carbon@Mo₂C@MoS₂ [14], MoSe₂-Mo₂C hybrid [15], 3D Mo₂C@MoS₂ heterojunction [16], MoS₂@X₂C (X = Mo or W) hybrids [17].

The combination of both TMD/TMC structure present several advantages: (1) Mo₂C act as substrate and blended WS₂ nanomaterial consciously avoided the aggregation/restacking of Mo₂C; (2) defect-rich TMD nanosheets or introduction of sulfur (S) vacancy defects activates the WS₂ basal plane and creates the plentiful active sites [16]; (3) TMC improves surface area, conductivity and durability of the electrocatalysts, and also expands the interlayer spacing between TMD and TMC sheets, enables the transmission of mass/charge and reduces the diffusion resistance of ions in the electrode; (4) porous active structures weaken the diffusion resistance, reduce the interfacial contact resistance and promote the interface charge transfer; (5) hybrid improves the activity and reduces the Gibbs hydrogen adsorption free energy (ΔG_H) due to strong chemical binding and electrical coupling between 2D conductive networks [17]. Thus, the intrinsic catalytic property of WS₂ material has vital role. However, controlled growth of TMD/TMC hybrid via rationally designed architecture for an effective HER performance is still a challenge. Previously, our group have prepared MoS₂/Mo₂C and WS₂/W₂C based hybrids for various electrochemical applications [13,17]. However, this work is focused to form the dissimilar structures interacted WS₂@Mo₂C hybrid for the enhanced HER characteristics. In this report, we combined a chemical reaction and carburization method which was adopted to synthesize vertical Mo₂C@WS₂ nanosheets for HER application. Mo₂C@WS₂ hybrid electrode offers a low overpotential of 59 and 95 mV in acidic and alkaline medium, respectively, lower than the respective pure WS₂ and Mo₂C electrodes.

2. Experimental Section

Synthesis of Mo₂C@WS₂ Hybrid

Mo₂C (Commercial −325 mesh, 99.5%; CAS Number: 12069-89-5, Sigma-Aldrich, Gangnam-Gu, Seoul, Korea) powder was used to synthesize the reduced Mo₂C nanoparticles, as reported earlier [18,19]. The commercial Mo₂C powder was liquefied in ethanol blended ammonia, and then boiled under vigorous agitation using hot plate at 85 °C for 5 h. After cleansing with deionized (DI) water and ethanol via centrifugation, the ensuing blend was dried in an oven at 80 °C overnight. To finish, the ensuing dark powder was collected and then post-annealed at 850 °C under a gaseous mixture of Ar (30 sccm), H₂ (30 sccm) and CH₄ (30 sccm) for 3 h. A simple hydrothermal method was adopted to synthesis of WS₂ nanoparticle. All the chemicals were purchased from Merck Korea, Gangnam-Gu, Seoul, Korea. Typically, 0.003 mol of sodium tungstate (Na₂WO₄·2H₂O), 0.01 mol of hydroxylamine hydrochloride (NH₂OH·HCl) and 0.008 mol of thiourea (CH₄N₂S)) were dissolved in 50 mL deionized water under vigorous magnetic stirring. Hydrochloric acid (HCl) and ammonia water (NH₃·H₂O) were added stepwise to control the PH value to 6. The final solution was transferred into a 100 mL Teflon steel chamber, sealed into an autoclave and incubated tightly, heated at 180 °C for 12 h and then naturally cooled down to room temperature. The black precipitates were recovered from the solution by centrifugation (5000 rpm, 10 min) and cleansed with ethanol and DI water several times, lastly dehydrated in vacuum for 12 h at 80 °C.

For Mo₂C@WS₂ nanosheets synthesis, the commercial Mo₂C powder was firstly dispersed into ethanol (100 mL). Next, a mixture of Na₂WO₄·2H₂O (0.01 mol), NH₂OH·HCl (0.003 mol) and CH₄N₂S (0.008 mol) were added in 100 mL DI water separately, then calibrated to PH value 6 by dissolving the HCl and NH₃·H₂O. After the dispersion, Mo₂C were poured into WS₂ mixture at room temperature with continuous magnetic agitation. The intermediate blended solution was transferred to an autoclave. Furthermore, it was also sealed and heated at 200 °C for 12 h, then allowed cooling to room temperature. The dark solution was then cleaned with DI water and ethanol using a centrifuge, then dried overnight at 100 °C. Finally, the prepared black powders were annealed for 1 h at 850 °C in tubular furnace under a blended gas of H₂/Ar/CH₄. The detailed characterization information is provided in the supporting information.

3. Results and Discussion

The process for preparation of Mo₂C@WS₂ hybrid via one-step chemical treatment is illustrated in Figure 1. Raman spectroscopy was used to understand the structural properties of Mo₂C, WS₂ and Mo₂C@WS₂ hybrid. Figure 2a shows the Raman profiles of prepared nanostructures. The pure Mo₂C yields three well-documented Raman shifts at 662 cm⁻¹, 817 cm⁻¹, and 990 cm⁻¹ consents to the Mo₂C formation. For WS₂, two pronounced peaks centered at 350 and 420 cm⁻¹, can be assigned to E_2g and A_1g modes, respectively. From Mo₂C@WS₂ hybrid, both Mo₂C and WS₂ phase peaks are presented which suggesting the formation of Mo₂C@WS₂ heterogeneous structure. In addition, Mo₂C@WS₂ heterostructure produces the low intensity peaks at 132 cm⁻¹ and 259 cm⁻¹ region corresponds to J₁, and A_g mode 1T′ phase WS₂, respectively [20,21]. The slight peak shift might be a substantial amount of diffusion of C and S elements on W/Mo sides. Figure 2b shows the X-ray diffraction (XRD) profiles of Mo₂C, WS₂ and Mo₂C@WS₂ hybrid. The prepared Mo₂C reveals (201), (112), (103), (110), (102), (002) and (100) lattice planes along with (101) preferential orientation, which are indexed with a standard result (JCPDS–893014). The diffraction pattern from WS₂ nanosheets contains peak from (002), (004), (006), and (008) lattices, correlated with a standard result (JCPDS–080237). For Mo₂C@WS₂ hybrid, the amalgamated peaks from Mo₂C and WS₂ are observed which indicated the highly effective preparation for hybrid structure.

The Brunauer–Emmett–Teller (BET) surface area analysis and its pore size distribution of Mo₂C, WS₂ and Mo₂C@WS₂ hybrid were ascertained by the N₂ adsorption−desorption measurements. Figure 2c represents the BET sorption profiles for Mo₂C, WS₂ and Mo₂C@WS₂ hybrid. The improved BET area of 7.49 m²·g⁻¹ was retrieved for the Mo₂C@WS₂ hybrid, which is larger than that of WS₂ (4.24 m²·g⁻¹) and Mo₂C (0.83 m²·g⁻¹). Furthermore, the pore diameter in terms of pore volume profile (Figure 2d) depicts the mesoporous behavior of Mo₂C, WS₂, and Mo₂C@WS₂ with the pore diameter of 14.3, 8.01, and 19.1 nm, and the pore volume of 0.00297, 0.00741, and 0.0357 cm³·g⁻¹, respectively.

Furthermore, field emission scanning electron microscopy (FESEM) was used to analyze the surface modification of prepared nanostructures. Figure 3a–c displays the Mo₂C nanostructures FESEM images. The observation demonstrates the accumulated bulk structure grains which lump of spherical shaped nanoparticles. Figure 3d–f represents the FESEM images with different magnifications for WS₂ nanostructures. WS₂ also exposes the grain bunches through agglomerated nanoparticles. The prepared Mo₂C@WS₂ hybrids FESEM images are clearly pictured the formation bulk grains by the agglomerated nanoparticles as shown in the Figure 3g–i. The elemental mapping was performed for Mo₂C@WS₂ hybrids (Figure S1) to confirm the spatial distribution of Mo, W, C and S elements. The observed results are confirmed the uniform distribution of all the elements in the Mo₂C@WS₂ heterostructure.

Furthermore, X-ray photoelectron spectral (XPS) analysis was done to evaluate the chemical composition and electronic structures of Mo₂C@WS₂ hybrid electrocatalyst. Figure 4a displays the survey profile of Mo₂C@CNT hybrid. Figure 4b presents the W 4f binding energy spectrum for hybrid. The deconvoluted spectrum clearly exposes the W 4f_7/2 and W 4f_5/2 characteristic peaks along with 5p_3/2 peak [22]. Figure 4c displays the S 2p region which establishes the 2p_3/2 and 2p_1/2 doublets [22]. The C 1s profile (Figure 4d) shows a strong peak due to C–C relation in Mo₂C at 284.5 eV binding energy with the shoulder peaks at 283.5 and 286.4 eV due to Mo–C and C–O, respectively [7,23,24]. The high-resolution Mo 3d region (Figure 4e) exposes the peak at 228.3 eV and 231.6 eV ascribes to the Mo²⁺ 3d_5/2 and Mo²⁺ 3d_3/2, respectively for Mo₂C@WS₂ hybrid [15,25]. In addition, the strong bands related to Mo⁴⁺ 3d_5/2 and Mo⁴⁺ 3d_3/2 along with Mo⁶⁺ are observed for hybrid structure [26,27,28,29,30]. The presence of Mo⁴⁺ states indicates interaction of sulfur [17]. Hence, the observed results are clearly proved the hybrid formation.

Figure 5 represents the different magnifications of transmission electron microscopy (TEM) images for Mo₂C@WS₂ hybrid. A low magnification TEM image obviously reveals the decorated sheet like vertically aligned intersected layers (Figure 5a). Figure 5b,c reveal the exhibit of nano-fringes interaction with the vertically aligned layers. The high magnification images also clearly represent the well interaction between the Mo₂C and WS₂, which are outlined, in the hybrid structure. Figure 5d shows the FFT profile which proves crystalline direction of Mo₂C and WS₂ in the hybrid. Figure 5e shows the phase spectrum which reveals the 0.237 nm lattice spacing due to the (002) plane of Mo₂C with their inserted inverse FFT profile. Figure 5f explores phase profile of WS₂ (002) lattice direction with a spacing of 0.615 nm. The perceived highly interfaced fingerprint structures with the vertically aligned nano-stripes would be greatly supported to achieve the improved electrochemical characteristics [17,31].

The electrocatalytic performance of all catalysts (Pt/C, Mo₂C, WS₂ and Mo₂C@WS₂ hybrid) for HER were assessed using a standard three-electrode configuration using N₂-saturated 0.5 M H₂SO₄ and 1.0 M KOH electrolyte at room temperature. The active loading mass of materials was 2 mg·cm⁻². The polarization curves were collected from the iR-compensated linear sweep voltammetry (LSV) measurements at 10 mV·s⁻¹. Figure 6a,b show the polarization profiles for Pt/C, Mo₂C, WS₂ and Mo₂C@WS₂ hybrid in acidic and alkaline media, respectively. As expected, the commercial Pt/C has outstanding HER performance with an overpotential of 48 and 45 mV vs RHE, respectively to reach 10 mA·cm⁻² in acidic and alkaline medium. The hybrid Mo₂C@WS₂ catalyst produces a superior electrocatalytic performance with a overpotential of 93 mV compared to the Mo₂C (134 mV) and WS₂ (126 mV) mono-component in acidic medium. Likewise, the overpotentials of bare NF, Mo₂C, WS₂, and Mo₂C@WS₂ hybrid were 430, 116, 108, and 98 mV vs RHE achieving a current density of 10 mA·cm⁻² in alkaline solution, respectively (Figure 6b). The observed result is better than most of TMD and TMC based HER catalysts in acidic and alkaline medium (Table S1), such as W₂C/WS₂ hybrid ~133 and 105 mV@10 mA·cm⁻² [13], MoC-Mo₂C hetero nanowires ~126 & 120@ 10 mA·cm⁻² [32], MoC_x nano-octahedrons ~142 & 151@ 10 mA·cm⁻² [33], MoS₂/Mo₂C-NCNTs~145@ 10 mA·cm⁻² and MoS₂/WSe₂ heterostructures ~116@ 10 mA·cm⁻² in acidic medium [34]. The excellent performance of HER could be attributed by the highly active edge site of WS₂ and superior conductivity of Mo₂C or partial substitution of C/S atom owing to heterostructure formed by the mutual coordination between WS₂ and Mo₂C.

To illustrate the internal HER kinetic reaction mechanism, the Tafel plots were derived from its respective LSV profiles. The linear portion of Tafel plots are mounted to extract the Tafel slopes as shown in the Figure 6c,d. The observed slope values are at 36, 83, 138, 59 and 163 mV per decade for Pt/C, Mo₂C, WS₂, Mo₂C@WS₂ hybrid and bare NF, respectively in acid medium. In an alkaline medium, Tafel slopes of 45, 96, 127, 95, and 168 mV·dec⁻¹ were found Pt/C, Mo₂C, WS₂, Mo₂C@WS₂ hybrid and bare NF, respectively. Extensively described HER processes are implicated in the alkaline and acidic electrolyte, which follows by the leading release of H₃O⁺ and H_ads creation [35,36,37]. The following three phases of HER process is defined in acid media.

$$\mathrm{Herrovsky}: H_{ads}+H_3{O}^{+}+e^{-}\to H_{2\uparrow }+H_{2}O$$

(1)

$$\mathrm{Volmer}: H_3{O}^{+}+e^{-}\to H_{ads}+H_{2}O$$

(2)

$$\mathrm{Tafel}: H_{ads}+H_{ads}\to H_{2\uparrow }$$

(3)

HER kinetics is defined by subsequent phases in an alkaline solution:

$$\mathrm{Herrovsky}: H_{2}O+e^{-}+H_{ads}\to H_{2\uparrow }+O{H}^{-}$$

(4)

$$\mathrm{Volmer}: H_{2}O+e^{-}\to H_{ads}+O{H}^{-}$$

(5)

$$\mathrm{Tafel}: 2H_{ads}\to H_{2\uparrow }$$

(6)

Furthermore, the 59 and 95 mV·dec⁻¹ Tafel slope for Mo₂C@WS₂ hybrid suggests that the rate-determining step may be designated as an electrochemical desorption mechanism (Volmer−Heyrovsky kinetics reaction). To evaluate the inherent catalytic yield of Mo₂C, WS₂, and the Mo₂C@WS₂ hybrid, the exchange current density (j₀) was also studied by extrapolating from the Tafel plots. The normalized j₀ value of Mo₂C@WS₂ (1.54 and 1.19 mA·cm⁻²) is superior than Mo₂C (0.13 and 0.93 mA·cm⁻²) and WS₂ (0.83 and 0.91 mA·cm⁻²) for acidic and alkaline media, which signifying an enhanced rates of intrinsic electron transference among the catalyst and the electrolyte. The electrochemical impedance spectroscopy (EIS) study was implemented to understand the boundary reactions and electrode charge transport kinetics for HER. Figure 7a,b show Nyquist graphs of Mo₂C, WS₂, and the Mo₂C@WS₂ hybrid electrocatalysts in acid and base solution, respectively (inset equivalent circuit). The smaller R_ct value of the Mo₂C@WS₂ hybrid (~2.98 Ω & 2.08 Ω) electrocatalysts substantially boost the interfacial electron/ion diffusion compared to Mo₂C (~3.4 Ω & 4.1 Ω) and WS₂ (~3.3 Ω & 3.9 Ω) in acid and base media. The low R_ct of Mo₂C@WS₂ hybrid may be related to effect of intimate contact effect between the exposed edges sites (WS₂) and the reaction of the metallic superconductor Mo₂C sublayers that can reduce the resistance to electron transportation at the porous catalyst/electrolyte boundary and then produce the rapid response for HER. The observed electrochemical HER parameters are provided in the Table S1, supporting information.

Long-term robustness is another critical parameter of electrocatalysts for HER application. The long-term durability of Mo₂C@WS₂ and the ability to constantly catalyze H₂ production was tested in 24-h continuous chronoamperometric process in acid and base medium using constant potential of 93 mV and 98 mV vs RHE, respectively. Figure 7c displays the chronoamperometric curves of the Mo₂C@WS₂ electrocatalyst in the acidic medium. No significant loss of cathodic current density occurred over 24-h of electrolysis in an acidic medium, which implies stable performance with high charge transportation capability between Mo₂C@WS₂ hybrid. Figure 7d depicted the chronoamperometric curves (i-t) in the base electrolyte for the Mo₂C@WS₂ electrocatalyst. Slight degradation cathodic current density was observed due to the formation of H₂ bubble over 24-h of electrolysis in the base medium. For the HER reaction of Mo₂C@WS₂ hybrid, TMD WS₂ is a dominant supplier of active sites. During the HER reaction mechanism, the catalytic active sites of WS₂ are supposed to be moved to the terraces from the chalcogens under alkaline conditions, whereas the dominant active facets are shifted to edge sites in acid media [38]. These sensitive edge sites are more active than the oblivious terrace sites, thereby acidic medium offers the improved HER behavior rather than alkaline media [38,39]. The hybrid HER results are compared with the Mo₂C@WS₂ outcomes in Table S2. Furthermore, FESEM studies were performed to explore the stability of prepared Mo₂C@WS₂ hybrid after 24-h continuous operation which clearly proves their robust nature without any structural alteration/collapse as represented in Figure S2.

4. Conclusions

In this report, vertical Mo₂C@WS₂ nanosheets were synthesized by chemical reaction and carburization method. The constructed Mo₂C@WS₂ heterostructure was evidently confirmed by the Raman, XRD, XPS, FESEM and TEM studies. The prepared hybrid electrocatalyst revealed the low overpotential of 93 and 98 mV at 10 mA·cm⁻² and Tafel slope of 59 and 95 mV per decade in acidic and alkaline medium, respectively, outperforming the performance of pristine WS₂ and Mo₂C under the same condition. The robust stability was observed over 24-h HER operation under alkaline and acidic medium by Mo₂C@WS₂ hybrid electrocatalyst. This work highlights a new path to regulate the WS₂ based hybrid material through facile and economical approach for the enhanced HER activity.