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Article

One-Pot Synthesis of W2C/WS2 Hybrid Nanostructures for Improved Hydrogen Evolution Reactions and Supercapacitors

1
Hybrid Materials Center (HMC), Sejong University, Seoul 05006, Korea
2
Department of Nano and Advanced Materials Engineering, Sejong University, Seoul 05006, Korea
3
Division of Electronics and Electrical Engineering, Dongguk University-Seoul, Seoul 04620, Korea
4
Department of Physics, Sejong University, Seoul 05006, Korea
5
Center of Research Excellence in Nanotechnology (CENT), King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabia
*
Author to whom correspondence should be addressed.
Nanomaterials 2020, 10(8), 1597; https://doi.org/10.3390/nano10081597
Received: 11 June 2020 / Revised: 12 August 2020 / Accepted: 12 August 2020 / Published: 14 August 2020

Abstract

:
Tungsten sulfide (WS2) and tungsten carbide (W2C) are materialized as the auspicious candidates for various electrochemical applications, owing to their plentiful active edge sites and better conductivity. In this work, the integration of W2C and WS2 was performed by using a simple chemical reaction to form W2C/WS2 hybrid as a proficient electrode for hydrogen evolution and supercapacitors. For the first time, a W2C/WS2 hybrid was engaged as a supercapacitor electrode and explored an incredible specific capacitance of ~1018 F g−1 at 1 A g−1 with the outstanding robustness. Furthermore, the constructed symmetric supercapacitor using W2C/WS2 possessed an energy density of 45.5 Wh kg−1 at 0.5 kW kg−1 power density. For hydrogen evolution, the W2C/WS2 hybrid produced the low overpotentials of 133 and 105 mV at 10 mA cm−2 with the small Tafel slopes of 70 and 84 mV dec−1 in acidic and alkaline media, respectively, proving their outstanding interfaced electrocatalytic characteristics. The engineered W2C/WS2-based electrode offered the high-performance for electrochemical energy applications.

Graphical Abstract

1. Introduction

To overcome the ever-increasing energy necessities, researchers have devoted considerable attention to designing and developing new and eco-friendly materials for electrochemical energy production and storage uses [1,2]. Among the various electrochemical storage devices, supercapacitors (SCs) are highly favored, owing to their quick charge–discharge ability, great power density, robust cycling constancy, and simple configuration [3]. SCs are mainly divided into pseudocapacitive and electrical double-layer capacitive (EDLC) behaviors. In the EDLC, energy is filled by the accretion of ions at the junction of electrode/electrolyte. In pseudocapacitors, storage operation is ensued by rapid revocable Faradaic operation between the electro-active species of electrolyte and electrode [3,4,5]. The commonly used electrode materials for EDLCs have some limitations because of low conductivity or low specific capacitance. The improved capacitance and energy density are to be attained from pseudocapacitive property, owing to the rapid Faradic reaction. On the other hand, one of the water electrolysis processes of hydrogen evolution reaction (HER) is a prominently accomplished route for green energy. The efficient HER electrocatalysts are improve the rate of electrolysis and produce low overpotential, to reach a specific current density [6,7]. Precious platinum (Pt) is a highly proficient HER electrocatalyst, but costliness and low stability obstruct its commercial use [8,9]. Due to the economic issue, researchers are keen on developing inexpensive and earth-abundant electrode materials [3,10,11].
Transition metal dichalcogenides (TMD) materials, especially metal sulfides (MS2, M = W, Mo, Co, etc.) are received greater consideration for energy storage and HER devices because of its many beneficial properties such as covalently bonded S−M−S with feeble van der Waals relations between the each layer, efficient mass transport, high specific area, robust edges, and chemical stability [1,12,13]. Moreover, the layered 2D TMD materials own excessive potential as the SC electrode materials, due to their excellent electronic structure, rapid ion intercalation, and preferred pseudocapacitive behavior. However, the low intrinsic conductivity and inactive basal planes are greatly limited to their widespread use [14,15]. In order to circumvent the electrical conductance limitations and to enhance the structural stability and efficiency, TMDs can be hybridized with other high-conducting materials. Carbonaceous materials (graphene or CNT) and transition metal carbides (TMCs) are suitable partner materials for hybridization. TMCs, such as Mo2C and W2C, which are newer than graphene and CNT, could be useful electrodes for supercapacitors and HER [16,17,18]; however, their performance was not widely appealing. The previous reported hybrid materials are MoSe2/Mo2C [9], WS2/reduced graphene oxide hybrids [14,19], MoS2/Ti3C2Tx hybrid [20], MoS2/WS2/graphene heterostructures [21], MoS2/Mo2C hybrid nanosheets [22], WS(1-x)Sex decorated 3D graphene [12], graphene supported lamellar 1T′-MoTe2 [23], and MoS2/reclaimed carbon fiber [24]. Recently, the W2C/WS2 has been used as a potential candidate to perceive the enriched catalytic properties for high hydrogen evolution characteristics [25,26]. Furthermore, Wang et al. [27] fabricated Wx[email protected]2 heterostructure via carbonizing WS2 nanotubes, which produced the overpotential of 146 mV at 10 mA cm−2 and Tafel slope of 61 mV dec−1. Chen et al. [28] also claimed the Tafel slope of 59 mV dec−1 with overpotential of 75 mV at 10 mA cm−2 in alkaline medium for a novel eutectoid-structured WC/W2C heterostructure. The detailed literatures are suggested to combine the W2C and WS2 and thereby to progress the electronic and electrochemical properties of resulted hybrid material.
Hence, in this work, W2C was chosen as a partner material for the hybridization with WS2, to form W2C/WS2 hybrid. This study focused on a simple one-pot strategy to synthesize W2C/WS2 as efficient and durable electrodes for electrochemical HER and SCs. So far, few reports are available for W2C/WS2 electrocatalysts for HER [26,27,29]. W2C/WS2 hybrid, for the first time, is employed as an SCs electrode in this work for improved storage behavior. W2C/WS2 possessed an excellent specific capacitance of ~1018 F g−1 at 1 A g−1 and high cycling stability with 94% retention. Furthermore, symmetric W2C/WS2 supercapacitor owned the high energy density of 45.5 Wh kg−1 at a low power density of 0.5 kW kg−1. As HER electrocatalyst, low overpotential of 133 and 105 mV was exhibited in acidic and alkaline media, respectively. Synergetic chemical coupling effects between the conducting W2C and semiconducting WS2 are believed to contribute significantly improving the electrochemical properties.

2. Materials and Methods

2.1. Synthesis of WS2 and W2C Nanostructures

For WS2 synthesis, 0.5 g tungsten chloride (WO3) was dispersed in the 2:1 volume ratio of ethanol and DI water mixture, and then the solution was stirred for 30 min, using a magnetic stirrer. Then, 1 g of thiourea was dissolved in aqueous solution, blended, and placed on a hot plate, at 90 °C, with vigorous stirring for 2 h, and then endorsed, to realize the room temperature. Consequently, the dark solution was segregated out by centrifuge, and sediment was cleansed with deionized (DI) water and ethanol and parched at 100 °C, in oven, overnight. Lastly, the synthesized powder was sulfurized, using CVD tubular furnace using argon (Ar) carrier gas at 600 °C for 120 min.
The reported simple chemical reduction route was employed to produce the W2C nanoparticles [18]. Briefly, 1 g of commercial W2C powder was dispersed in the ethanol solution (50 mL), with stirring for 3 h, at room temperature. Subsequently, 25 mL of ammonia liquid was poured in the bath mixture and kept to vigorous stirring at 85 °C for 5 h. Then, resulted sediment, after being cleansed using DI water by centrifuge, was kept in an oven, overnight, at 60 °C. The product powder was kept under the gas mixture of H2 (80 standard cubic centimeters per minute, sccm), CH4 50 (sccm), and Ar environment in the tubular furnace at 850 °C for 3 h annealing process. Finally, the W2C powder was collected after the tube attained room temperature.

2.2. Synthesis of W2C/WS2 Hybrids

About 1 g of commercial W2C powder (Sigma Aldrich, Seoul, Korea; CAS number: 12070-12-1) was dispersed in the ethanol solution (50 mL), with stirring, at room temperature, for 3 h. Then 0.25 g WO3 (Sigma Aldrich, Seoul, Korea; CAS number: 1314-35-8) was dispersed in ethanol and DI water mixture solution. Then 0.5 g of thiourea (Sigma Aldrich, Seoul, Korea; CAS number: 62-56-6) dissolved aqueous solution was blended with W2C solution and stirred. After that, 2 mL of hydrazine solution and 30 mL of liquid ammonia were mixed with the one-pot solution and stirred for 5 h at 85 °C. The deposit was parted, cleansed with DI water, and dehydrated in a hot oven, overnight, at 60 °C. The powder was post-annealed in a furnace, at 850 °C, for 3 h, under the CH4 (50 sccm), H2 (80 sccm), and Ar atmosphere. Figure 1 shows the illustration for the synthesis of a hierarchically W2C/WS2 hybrid nanostructure.

2.3. HER Performance

For the working electrode preparation, polyvinylidene fluoride (PVDF), active material (W2C, WS2, and W2C/WS2), and carbon black at 10:80:10 mass ratio were mixed, and N-methyl-2-pyrrolidone (NMP) was added drop-wise. The paste was layered on Ni foam (NF) and overnight dehydrated at 100 °C. For the reference electrode, Ag/AgCl and Hg/HgO were used in acidic and alkaline media performance, respectively, with a graphite counter electrode. We recorded iR corrected linear sweep voltammetry (LSV) by using an electrochemical system (model: 660D; company: CH Instruments, Inc., Austin, TX, USA) in 1 M KOH and 0.5 M H2SO4 media, with a scan speed of 10 mV s−1. Electrochemical impedance spectroscopy (EIS) results were noted at the frequencies of 0.01 Hz–100 kHz in acid and alkaline media. The HER potential values were converted for reversible hydrogen electrode (RHE) by the following formula: E (vs. RHE) = E (vs. Ag/AgCl) + E0 (Ag/AgCl) + 0.0592 × pH for acidic medium and E (vs. RHE) = E (vs. Hg/HgO) + E0 (Hg/HgO) + 0.0592 × pH for alkaline medium.

2.4. Supercapacitor Performance

A biologic science instrument (SP150, Seyssinet-Pariset, France) was used to analyze the supercapacitor properties. The fabrication process for working electrode was same as for HER. The electrochemical performance was employed in 2 M KOH aqueous for three-electrode (denoted as half-cell) and two-electrode (denoted as symmetric) measurements. The active materials (W2C, WS2, and W2C/WS2) loaded NFs were employed as the working electrodes, along with Ag/AgCl as a reference electrode and a Pt wire as a counter electrode for three electrode measurements. For symmetric, a cut of Whatman filter paper was socked for 2 h in 2 M KOH and then dried, to remove the excess water. As-prepared filter paper was positioned between the couple of similar W2C/WS2 electrodes and pressed to make a sandwich structure. Cyclic Voltammetry (CV) and galvanostatic charge–discharge (GCD) scans were noted from −0.8 to + 0.2 V (vs. Ag/AgCl), at different sweep rates and current densities. The capacitance (C, F g−1), energy density (E, Wh kg−1), and power density (P, Wkg−1) were derived by using the Equations (1)–(3), respectively [3,30].
C = ( I × Δ t ) ( m × Δ V )
E = ( C × Δ V 2 ) ( 2 × 3.6 )
P = ( E × 3600 ) / Δ t
where m is the mass, I is the current, Δt is the discharging time, and ΔV is the potential. EIS studies were accomplished at 10 mV AC amplitude, in an open circuit, in the frequency region of 0.01 Hz to 200 KHz.

2.5. Characterization Details

Field-emission scanning electron microscopy (FESEM) (HITACHI S-4700, Tokyo, Japan) was used to explore the morphological studies. The atomic structures were analyzed by a JEOL-2010F transmission electron microscopy (TEM) with an operation voltage of 200 keV. The Raman spectroscopy (Renishaw inVia RE04, Gloucestershire, UK) measurements were performed in ambient conditions, using the 512 nm Ar laser source with a laser spot size of 1 µm and a scan speed of 30 s. The structural properties were characterized by Rigaku X-ray diffractometer (XRD) (Tokyo, Japan) with Cu-Kα radiation (0.154 nm), at 40 kV and 40 mA, in the scanning range of 10–80° (2θ). For chemical composition and binding energy, the X-ray photoelectron spectroscopy (XPS) measurements were carried out by using an Ulvac PHI X-tool spectrometer (Kanagawa, Japan) with Al Kα X-ray radiation (1486.6 eV). A Brunauer–Emmet–Teller (BET) study to calibrate the surface area of nanostructures was performed, using a N2 adsorption/desorption medium at 77 K (Micromeritics, Norcross, GA, USA). Pore size distribution was measured with the Barrett–Joyner–Halenda (BJH) analysis.

3. Results and Discussion

3.1. Materials Characteristics

The Raman spectroscopy were performed to measure the crystalline quality and phonon vibration mode properties of W2C, WS2, and W2C/WS2. Figure 2a displays Raman profiles for W2C, WS2, and W2C/WS2. For W2C, Raman spectrum reveals the solid peaks at 693 and 808 cm−1, which relates to the W–C mode of vibration [31,32]. The sp2-hybridized graphitic G and defective carbon D related bands observed at 1582 and 1353 cm−1, respectively [32]. For WS2, the characteristic bands exhibit at 354 and 420 cm−1, which relates to the E2g and A1g mode of vibration, respectively. The additional peaks in the lower frequency regions enabled at 137, 188, and 258 cm−1, referring to J1, J2, and Ag mode, support the formation of 1T′ phase WS2 [33,34]. Interestingly, the synchronized W2C and WS2 characteristic peaks appear for W2C/WS2 hybrid. The relatively higher intensity for A1g mode than the E2g mode credits to the edge exposed TMD structure, which facilitates the high electrocatalytic activity [35,36].
The materials structure was validated further by XRD. In Figure 2b, W2C spectrum shows polycrystalline structure. The (020), (002), (220), (041), (123), (004), (142), and (322) lattice directions observe at 31.5°, 35.1°, 48.8°, 64.1°, 65.8°, 73.2°, 75.6°, and 77.1°, respectively (JCPDS: 89-2371). The diffraction signals at 2θ values of 14.2°, 33.2°, 43.0°, 49.6°, 60.4°, 66.7°, and 67.6° correspond to the (002), (101), (103), (105), (112), (114), and (200) lattices of hexagonal WS2, respectively (JCPDS: 87-2417). Similar to Raman observation, XRD pattern of W2C/WS2 hybrid also produced the cumulative XRD peak positions from W2C and WS2 phases, which are indexed with black and red color, respectively. From the structural outcomes, the blended nature of W2C and WS2 phase is evidently proved in hybrid with preferential orientation of (220) and (002) lattice planes of W2C, which might be originated by the dominating behavior of W2C and their rich presence. Further, the full width at half maximum (FWHM) values were derived from the XRD peaks. The crystallite size was estimated by using FWHM by Scherrer relation [37,38]. The derived FWHM and crystallite values are provided in the Supplementary Materials Tables S1–S3 for W2C, WS2, and W2C/WS2, respectively. The estimated mean size (τ) are at 22.9, 18.7, and 18.1 nm for W2C, WS2, and W2C/WS2, respectively. The reduced-nanosize crystallites for hybrid can be originated by interconnection mechanism between W2C and WS2 and their modified crystallographic structure. The d-spacing values were estimated by the Bragg’s law (2d sinθ = nλ), and their values are well correlated with the standard results [25]. The extracted values are provided in the Supplementary Materials Tables S1–S3 for W2C, WS2, and W2C/WS2, respectively. The observed d-spacing values are considerably strained for hybrid compared with pristine, which might be due to the interfacial bonding nature.
The BET surface area was examined to assess the area and pore size distributions by using an N2 adsorption/desorption medium at 77 K. Figure 2c shows the N2 isotherms. W2C/WS2 possessed a maximum surface area of 6.368 m2g−1, equated with WS2 (3.405 m2g−1) and W2C (1.75 m2g−1). The measured total pore volume was 0.009, 0.016, and 0.020 cm3g−1 for W2C, WS2, and W2C/WS2, respectively (Supplementary Materials Figure S1). The high porosity of W2C/WS2, compared with pure, is believed to give the significant contribution of enhancing the electrocatalytic activity by promoting the electrolyte diffusion into the electrode.
XPS was employed to describe the composition and valence states of constructed material. The survey profiles are given in the Supplementary Materials Figure S2, to prove the coexistence of all the elements. The high-resolution XPS profiles for W 4f, C 1s, and S 2p states are provided in Figure 2d–f. From a W 4f XPS profile of W2C (Figure 2d), the deconvoluted peaks reveal the W2+ (31.4 and 33.1 eV), W4+ (31.8 and 35.69 eV), and W6+ (37.0 and 38.3 eV) doublets. For WS2, the W4f core level peaks are located at 31.7 and 33.9, due to W4f7/2 and W4f5/2, respectively. W 4f region of W2C/WS2 reveals the peaks at 31.7 eV (W4f7/2) and 33.8 eV (W4f5/2) with the W6+ couplets (37.6 and 36.2 eV) [39,40]. Figure 2e shows the C 1s profile and explores the graphitic sp2 carbon peak at 284.7 eV and W-C peak at 283.0 eV for W2C [6]. For the W2C/WS2 hybrid, C 1s spectrum produced the C-C1 (284.4 eV), C-C2 (285.2 eV), W-C (282.9 eV), C-O (286.3 eV), and C=O (288.9 eV) peaks [29,41,42]. Figure 2f deconvolution peaks reveal the S 2p couplets of S2p1/2 and S2p3/2 at 163.1 and 161.8 eV for WS2, whereas, they are at 163.6 and 162.3 eV for the W2C/WS2 hybrid, respectively [43,44]. The atomic percentage of W2C/WS2 hybrid is determined to be 32.13%, 22.20%, and 45.67% for W, C, and S atoms, respectively, which is well correlated with EDX results (discussed later). The observed elemental confirmation proved the formation of W2C/WS2 hybrid.
Surface characteristics were further elaborated by FESEM and TEM examinations. Figure 3 shows the FESEM micrographs of W2C, WS2, and W2C/WS2. FESEM micrographs clearly picture the formation of different sizes of nanograins by chemical reduction process in the W2C nanoparticles (Figure 3a). Nano-spherical-shaped agglomerated grains exhibit for WS2 (Figure 3b). The sizes of the grains are considerably varied in the nanoscales, due to the bulk agglomeration process during the annealing. In the hybrid, the spherically shaped W2C particles seem to cover the WS2 particles (Figure 3c) due to the interconnected mechanism. Reduced sizes of the grains appear with cauliflower like agglomerated grain bunches and well-interconnected domain structure for W2C/WS2 hybrid. To prove the hybrid formation, EDX spectrum for W2C/WS2 clarifies the elemental composition, as shown in Figure 3d. Furthermore, the mapping images are provided to confirm the equal distribution of all the elements in the hybrid (Figure 3e–h).
TEM measurements were carried out for W2C/WS2 hybrid (Figure 4). The different magnification TEM images are provided in Figure 4a–c. Vertically aligned nano-stirpes-like structures are broadly exhibited for the hybrid. The interconnection between the layered fringes and finger-printed structures is clearly visualized. Due to the polycrystalline lattices for W2C/WS2 hybrid, the different widths of the lattice fringes are obviously demonstrated in the TEM images (Figure 4b,c). A higher magnification TEM image (Figure 4d) explores the layer structure with the cross-section of different lattice fringes in the W2C/WS2 hybrid (inset—fast Fourier transform (FFT), left panel). The phase profile spectrum, extracted by point mask mode and inverse FFT (iFFT, right panel) pattern of inset Figure 4d, shows 6.2 nm spacing which related to (002) WS2 lattice orientation (Figure 4e). The fingerprint structured grains interface with layered WS2 (Figure 4f–g) [44]. The phase profile spectrum, extracted from the iFFT pattern of inset Figure 4g, elevates 2.9 nm spacing, which is related to (020) W2C lattice orientation (Figure 4h).

3.2. Hydrogen Evolution Studies

Active-materials-coated NFs were engaged as working electrodes to appraise the HER activities in 0.5 M H2SO4 and 1 M KOH electrolytes, at room temperature. Figure 5a explores iR-recompensed LSV polarization profiles in 0.5 M H2SO4, using 10 mV s−1 sweep speed. The W2C and WS2 produce 171 and 242 mV to attain 10 mA cm−2, respectively. In contrast, the W2C/WS2 hybrid produces an overpotential of 133 mV at 10 mA cm−2 (51 mV @ 10 mA cm−2 for Pt/C). The exhibited low overpotential credits to interfacial active edges sharing and rapid electron conductivity in the W2C/WS2 which proves the importance of hybrid formation. In the 1 M KOH media (Figure 5b), the W2C/WS2 electrode also produces highly dynamic HER behavior with a small overpotential of 105 mV at 10 mA cm−2 than WS2 (189 mV) and W2C (123 mV). The HER performance of W2C/WS2 is superior to most of the hybrid-based electrodes (Figure 5c) [45,46,47,48]. Li et al. [29] have prepared the nanocomposite of N, S-decorated porous carbon matrix encapsulated WS2/W2C (WS2/W2[email protected]), which delivered the small overpotential of 126 and 205 mV in 0.5 M H2SO4 and 1.0 M KOH, respectively. In addition, Nguyen et al. [26] have reported the 170 mV of onset potential with 55.4 mV dec−1 of Tafel slope in 0.5 M H2SO4 for W2[email protected]2 nanoflowers synthesized by hydrothermal method.
Tafel slope is a factor to indicate the inherent electrocatalytic activity of the electrode. The Tafel slope values of Pt/C, bare NF, W2C, WS2, and W2C/WS2 electrocatalysts, are 36, 168, 86, 138, and 70 mV dec−1, respectively, in H2SO4 medium (Figure 6a). The outputs prove the outstanding electrocatalytic activity of the W2C/WS2 hybrid. Exchange current densities (j0) assessed by extrapolation Tafel lines to X-axis and their values observe at ~1.03, 1.02, 0.55, and 0.19 mA cm−2 for Pt/C, W2C/WS2, WS2, and W2C, respectively. W2C, WS2, and W2C/WS2 produce the Tafel slopes of 141, 127, and 84 mV dec−1, respectively, in KOH medium (Figure 6b). The small Tafel slope of W2C/WS2 also supports high HER behavior of hybrid electrode in the KOH electrolyte. The extrapolated j0 is 0.93, 0.72, and 0.38 mA cm−2 for W2C, WS2, and W2C/WS2, respectively, in KOH electrolyte. The exhibited Tafel slope range suggests that HER involves a Volmer–Heyrovsky mechanism for W2C/WS2 hybrid, with electrochemical desorption as the rate-regulatory direction [44,49,50,51]. Outstanding HER activity in the W2C/WS2 hybrid could be explained with electrode kinetics by accumulated electrocatalytic edge facets and a high ratio of charge transfer. The observed HER parameters are provided in Supplementary Materials Table S4 for all the measured electrodes. The j0 and Tafel values are superior to most reported hybrid electrocatalysts (Figure 6c,d and Supplementary Materials Table S5) [46,48,52]. The low overpotential, large j0 value, and small Tafel slope in the W2C/WS2 hybrid also confirm the importance of hybrid formation for efficient HER electrocatalytic activity in KOH and H2SO4 medium.
CV profiles were acquired in the non-Faradaic region, to estimate the double-layer capacitance (Cdl) (Supplementary Materials Figure S3a–c). The Cdl by linear fitting (Supplementary Materials Figure S3c) are 3.81 mF cm−2 (in H2SO4) and 3.33 mF cm−2 (in KOH) for the W2C/WS2 hybrid. Electrochemical surface areas are 108 and 83 cm2 in H2SO4 and KOH, respectively. A stability and durability evaluation of W2C/WS2 electrode was carried out by using chronoamperometric response at a persistent 133 and 105 mV overpotential in H2SO4 and KOH (Figure 6e,f), respectively. No significant decline is observed over 20 h in H2SO4 the medium, whereas slight deterioration is exhibited for the KOH medium. Note the excellent robustness of W2C/WS2 electrode in the H2SO4, rather than KOH, medium for HER. LSV curves at initial and after 20 h of continuous HER operation are shown in the inset of Figure 6e,f, respectively.
To probe insights for electrocatalytic activity of materials, EIS was performed in the H2SO4 and KOH (Supplementary Materials Figure S4). The observed EIS plot revealed the low charge-transfer resistance (Rct) and swift electron transfer via the electrolyte–electrode interface for W2C/WS2. The charge-transfer resistances, Rct, of W2C/WS2 (~1.8–2.2 Ω) in the H2SO4 and KOH media are lower than those of W2C and WS2. Moreover, the small series of resistances (~1.5–2.5 Ω) of all the electrodes suggests that the active materials are well integrated with the porous NF. The high electrocatalytic properties and robust solidity of the W2C/WS2 hybrid support it as a potential material to substitute Pt in HER application.

3.3. Supercapacitor Performances

Electrochemical storage properties were elucidated by CV and GCD tests in 2 M KOH electrolyte, using three electrodes, as explained in the experimental part of the manuscript. The CVs were recorded with the potential interval of −0.8 to 0.2 V vs. Ag/AgCl at 10 mV s−1 sweep speed for the W2C, WS2, and W2C/WS2 electrodes (Figure 7a). All the electrodes produce the identical CV loops with Faradaic-adsorptions-blended EDLC operations [53]. The W2C/WS2 hybrid electrode shows a wider electrochemical area than the W2C and WS2. Moreover, the W2C/WS2 hybrid shows a couple of redox peaks (−0.61 and −0.25 V), indicating the reversible reaction from W4+ to W6+, corresponding to the proton’s absorption/desorption into the WS2 interlayers [54]. Figure 7b shows CVs of various scan rates (10–50 mV s−1), and their shapes are maintained, indicating good electrochemical capacitive characteristics and high-rate performance. The results for WS2 and W2C at various scan speeds are shown in Supplementary Figure S5a,b, respectively. Successive 100 CV cycles were executed in the potential region of −0.8 to 0.2 V, to assess the stability for the W2C/WS2 electrode, and its output is presented Figure 7c. Due to the EDLC-combined Faradaic storage mechanism, it possesses good stability with minimal degradation over the repeated cycles [55].
The electrochemical storage performance was further tested by GCD curves, as shown in Figure 8a–d. The slightly distorted triangle like the GCD curve is exhibited for the W2C/WS2 and WS2 electrodes, due to the redox reaction. For the W2C electrode, a square-structured GCD curve is exhibited, with a voltage drop, due to the easy oxidation characteristic. The W2C/WS2 electrode exposes an outstanding specific capacitance (estimated by using the Equation (1)) of 1018 F g−1, as compare to the WS2 (~158 F g−1) and W2C (~133 F g−1), at the current density of 1 A g−1. GCD analysis displays similar curves at the different current densities. Observe the specific capacitances of 133, 90, 66, and 50 F g−1 for W2C (Figure 8b) and 158, 120, 87, and 80 F g−1 for WS2 (Figure 8c) at 1, 2, 3, and 5 A g−1 current density, respectively. For W2C/WS2, a high specific capacitance of 1018, 866, 816, and 660 F g−1 exhibits at the 1, 2, 3, and 5 A g−1 current density, respectively (Figure 8d). The specific capacitance changes with current density, as shown in Supplementary Materials Figure S6. The significant enhancement of capacitances for W2C/WS2 electrode credits to the mutual interactions between W2C and WS2, large surface area, rich active edge facets of WS2, and high electronic conductivity of W2C to enable the rapid transference of electrons during a charge–discharge process. Supplementary Materials Table S6 provides the extended comparison of W2C/WS2 SCs performance with the previously reported hybrid electrodes.
Cyclic stability is an essential property for the supercapacitor electrodes. In the case of the W2C/WS2 electrode, 94% of primary capacitance was perceived after 5000 cycles (Figure 8e), suggesting long-term stability. EIS measurements were performed to prove the charge-transfer characteristics (Figure 8f). Fitted curve from the Nyquist profile is inserted in Figure 8f, where Cdl is the double-layer capacitance, Rct is the charge-transfer resistance, Rs is the series resistance, Wo the Warburg impedance at open circuit voltage, and Rc and Cc are the capacitive resistance and capacitive capacitance, respectively. The output curve indicates that the charge-transfer resistance significantly reduces for W2C/WS2 hybrid electrode, as compared to their pristine. A lower Rct (~1.0 Ω) was obtained by the hybrid electrode, as compared to the WS2 (~13.9 Ω) and W2C (10.1 Ω), respectively.
To assess the practical application, the symmetric supercapacitor (SSC) was assembled with two identical W2C/WS2 hybrid electrodes. The electrochemical CV measurement of SSC was measured by using the similar potential range of half-cell measurements. Figure 9a shows the CV curves for W2C/WS2 symmetric cell device. The modified rectangular shape of CV curve for the W2C/WS2 hybrid SSC reveals slightly shouldered redox reaction, confirming the key contribution of EDLC characteristics. The constructed SSC device gives an excellent current response and larger integral area, compared to previous TMDs-based materials [56,57]. The different scan rates, using performed CV curves, prove the high rate of capability of the prepared SSC devices. Figure 9b shows the different current densities, using prepared SSC GCD curves, indicating the excellent electrochemical rate capability. The charge/discharge time considerably decreases for the SSC device due to its direct intercalation and extraction of ions through the solid electrolyte, compared with its half-cell outcomes. The capacitance value of symmetric device was valued from the GCD curve, using Equation (1) [58]. The W2C/WS2 hybrid delivers the higher symmetric capacitance of 328, 306, 255, and 220 F g−1 at 1, 2, 3, and 5 A g−1 of current densities, respectively, as presented in Figure 9c. Interestingly, our symmetric device results show the enhancing capacitance, compared to other symmetric capacitor results [56,59,60]. The specific energy and specific power values are significant for the practical uses, which were weighed by relations 2 and 3, respectively. The symmetric device carries the energy densities of 45.5, 42.5, 35.4, and 30.5 Wh kg−1 at 0.5, 1.0, 1.5, and 2.0 kW kg−1 power density, respectively, as given in the Figure 9d.
The exhibited energy density of symmetric W2C/WS2 capacitor is superior to the recently reported symmetric devices using TMDs and TMCs electrodes, MoS2 sheets (18.43 Wh kg−1) [60], Ti3C2Tx/MWCNT (3 Wh kg−1) [61], s-MoS2/CNS (7.4 Wh kg−1) [62], 3D-graphene/MoS2 (24.59 Wh kg−1) [63], MoS2/RCF (22.5 Wh kg−1) [24], MoS2/RGO/MoS2@Mo (6.22 Wh kg−1) [56], and MoS2 sponge (6.15 Wh kg−1) [59]. Overall, our findings demonstrate that the inclusion of carbide-based material with TMDs deliberately improves the conductance of the hybrid material, assists swift conveyance of electrons/ion, and improves the stability of electrode material.

4. Conclusions

We have successfully engineered the W2C/WS2 hybrid electrode by a simple cost-effective one-pot chemical reaction. Highly conductive W2C-supported WS2 hybrids were designed to promote high electrocatalytic activity for HER and SCs by accumulating the number of active edges and facilitating the swift electron transport. In the case of HER, the interfaced W2C/WS2 hybrid produced the small overpotentials of 133 and 105 mV, to achieve the 10 mA cm−2 current density with the Tafel slope of 70 and 84 mV dec−1 in H2SO4 and KOH media, respectively, which proved the outstanding electrocatalytic HER characteristics. Half-cell measurements unveiled the remarkable specific capacitance of ~1018 F g−1 at 1 A g−1 with the rate competency nature and robust responses for W2C/WS2 hybrid electrode. W2C/WS2-based symmetric supercapacitor exposed the specific energy of 45.5 Wh kg−1 at 0.5 kW kg−1 specific power with a capacitance of 328 F g−1 at 1 A g−1 current density. The suggested low-cost methodology of one-pot reaction is highly feasible to fabricate the efficacious nanostructured hybrids and has larger-scale production capability because of its controlled synthesis process. Hence, the developed hybrid material and methodology have a broad scope for the future electrochemical applications.

Supplementary Materials

The following are available online at https://www.mdpi.com/2079-4991/10/8/1597/s1. Figure S1: Pore diameter versus pore volume variations for the nanostructures. Figure S2: Survey XPS spectrum of (a) W2C, (b) WS2, and (c) W2C/WS2. Figure S3: CV spectra in the non-Faradaic region with different scan rates for W2C/WS2 hybrid HER electrodes in (a) 0.5 M H2SO4 and (b) 1 M KOH electrolyte and (c) their current differences. Figure S4: EIS spectra of Pt, W2C, WS2, and W2C/WS2 hybrids electrodes in (a) 0.5 M H2SO4 and (b) 1M KOH electrolyte. Figure S5: CV curves for the (a) WS2 and (c) W2C electrodes at various scan rates, using half-cell (10–50 mVs−1). Figure S6: The specific capacitance variations at different current densities for W2C, WS2, and W2C/WS2 electrodes by half-cell measurements. Table S1: Microstructural parameters of W2C. Table S2: Microstructural parameters of WS2. Table S3: Microstructural parameters of W2C/WS2. Table S4: Comparison of electrochemical parameters for different electrocatalysts. Table S5: HER catalytic performances TMDs and TMCs-based electrocatalysts. Table S6: Performances of TMDs- and TMCs-based electrodes for supercapacitors.

Author Contributions

Conceptualization, S.H. and J.J.; methodology, S.H.; formal analysis, I.R. and A.F.; software, M.A.; validation and investigation, D.V. and S.H.; resources, Y.-S.S., H-S.K., and S.-H.C.; data curation, M.A., I.R., and A.F.; writing—original draft preparation, S.H.; writing—review and editing, D.V., H.-S.K., and J.J.; supervision and funding acquisition, J.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Nano Material Technology Development Program and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, and the Science and ICT (2017R1C1B5076952, 2016M3A7B4909942, and 2020R1A6A1A03043435). This research was also sponsored by the Korea Institute of Energy Technology Evaluation and Planning and the Ministry of Trade, Industry and Energy of the Republic of Korea (20172010106080).

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. Graphic representation for the synthesis of W2C/WS2 hybrid with its derived structure.
Figure 1. Graphic representation for the synthesis of W2C/WS2 hybrid with its derived structure.
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Figure 2. Structural studies of W2C, WS2, and W2C/WS2. (a) Raman profiles, (b) X-ray diffraction patterns, (c) N2 sorption isotherms, (d–f) XPS curves, (d) W 4f, (e) C 1s, and (f) S 2p regions.
Figure 2. Structural studies of W2C, WS2, and W2C/WS2. (a) Raman profiles, (b) X-ray diffraction patterns, (c) N2 sorption isotherms, (d–f) XPS curves, (d) W 4f, (e) C 1s, and (f) S 2p regions.
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Figure 3. FESEM images of (a) W2C, (b) WS2, and (c) W2C/WS2 hybrid. (d) EDX spectrum of W2C/WS2 hybrid; (e) mapping image of W2C/WS2 hybrid and its elements, (f) W, (g) C, and (h) S.
Figure 3. FESEM images of (a) W2C, (b) WS2, and (c) W2C/WS2 hybrid. (d) EDX spectrum of W2C/WS2 hybrid; (e) mapping image of W2C/WS2 hybrid and its elements, (f) W, (g) C, and (h) S.
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Figure 4. HRTEM images for W2C/WS2 hybrid. (a) Low- and (b,c) high-resolution TEM images. (d) Layered WS2 structure with the inset of FFT and iFFT patterns. (e) Phase profile spectrum for (002) lattice orientation of WS2 with 6.2 nm spacing in the W2C/WS2 hybrid. (f,g) High-resolution TEM images for W2C related portion in the hybrid with inset of FFT and iFFT patterns. (h) Phase profile spectrum for (020) lattice orientation of W2C with 2.9 nm spacing in the W2C/WS2 hybrid.
Figure 4. HRTEM images for W2C/WS2 hybrid. (a) Low- and (b,c) high-resolution TEM images. (d) Layered WS2 structure with the inset of FFT and iFFT patterns. (e) Phase profile spectrum for (002) lattice orientation of WS2 with 6.2 nm spacing in the W2C/WS2 hybrid. (f,g) High-resolution TEM images for W2C related portion in the hybrid with inset of FFT and iFFT patterns. (h) Phase profile spectrum for (020) lattice orientation of W2C with 2.9 nm spacing in the W2C/WS2 hybrid.
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Figure 5. (a,b) Hydrogen evolution polarization profiles for Pt/C, bare NF, W2C, WS2, and W2C/WS2 at 10 mV s−1 sweep speed in (a) 0.5 M H2SO4 and (b) 1 M KOH media. (c) Overpotential comparison of different electrocatalysts.
Figure 5. (a,b) Hydrogen evolution polarization profiles for Pt/C, bare NF, W2C, WS2, and W2C/WS2 at 10 mV s−1 sweep speed in (a) 0.5 M H2SO4 and (b) 1 M KOH media. (c) Overpotential comparison of different electrocatalysts.
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Figure 6. (a,b) Tafel plots for Pt/C, bare NF, W2C, WS2, and W2C/WS2 hybrid at 10 mV s−1 sweep speed in (a) 0.5 M H2SO4 and (b) 1 M KOH media; comparison of (c) Tafel slope and (d) exchange current density with different electrocatalysts; chronoamperometric profile of W2C/WS2 hybrid for 20 h continuous hydrogen evolution reaction (HER) operation in (e) 0.5 M H2SO4 and (f) 1 M KOH electrolyte (inset: LSV curves before and after 20 h operation).
Figure 6. (a,b) Tafel plots for Pt/C, bare NF, W2C, WS2, and W2C/WS2 hybrid at 10 mV s−1 sweep speed in (a) 0.5 M H2SO4 and (b) 1 M KOH media; comparison of (c) Tafel slope and (d) exchange current density with different electrocatalysts; chronoamperometric profile of W2C/WS2 hybrid for 20 h continuous hydrogen evolution reaction (HER) operation in (e) 0.5 M H2SO4 and (f) 1 M KOH electrolyte (inset: LSV curves before and after 20 h operation).
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Figure 7. Supercapacitor Cyclic Voltammetry (CV) for three electrode measurement: (a) CV curves for W2C, WS2, and W2C/WS2 electrodes; (b) different scan rate CV curves for the W2C/WS2 hybrid; (c) multiple cycle CV curves for the W2C/WS2 hybrid.
Figure 7. Supercapacitor Cyclic Voltammetry (CV) for three electrode measurement: (a) CV curves for W2C, WS2, and W2C/WS2 electrodes; (b) different scan rate CV curves for the W2C/WS2 hybrid; (c) multiple cycle CV curves for the W2C/WS2 hybrid.
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Figure 8. (a–d) Galvanostatic charge–discharge (GCD) profiles for three electrode measurement; (a) GCDs at 1 Ag−1 for W2C, WS2, and W2C/WS2; GCDs at different current densities for (b) W2C, (c) WS2, and (d) W2C/WS2; (e) stability performance of W2C/WS2 hybrid; (f) EIS curves for W2C, WS2, and W2C/WS2 (inset-fitted circuit).
Figure 8. (a–d) Galvanostatic charge–discharge (GCD) profiles for three electrode measurement; (a) GCDs at 1 Ag−1 for W2C, WS2, and W2C/WS2; GCDs at different current densities for (b) W2C, (c) WS2, and (d) W2C/WS2; (e) stability performance of W2C/WS2 hybrid; (f) EIS curves for W2C, WS2, and W2C/WS2 (inset-fitted circuit).
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Figure 9. Symmetric supercapacitor performance of W2C/WS2. (a) Different scan rate CV curves for symmetric W2C/WS2 hybrid; (b) GCDs for symmetric W2C/WS2 at different current densities; (c) specific capacitance at various scan rates for W2C/WS2 by symmetric measurements; (d) Ragone plots of W2C/WS2 for symmetric device.
Figure 9. Symmetric supercapacitor performance of W2C/WS2. (a) Different scan rate CV curves for symmetric W2C/WS2 hybrid; (b) GCDs for symmetric W2C/WS2 at different current densities; (c) specific capacitance at various scan rates for W2C/WS2 by symmetric measurements; (d) Ragone plots of W2C/WS2 for symmetric device.
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MDPI and ACS Style

Hussain, S.; Rabani, I.; Vikraman, D.; Feroze, A.; Ali, M.; Seo, Y.-S.; Kim, H.-S.; Chun, S.-H.; Jung, J. One-Pot Synthesis of W2C/WS2 Hybrid Nanostructures for Improved Hydrogen Evolution Reactions and Supercapacitors. Nanomaterials 2020, 10, 1597. https://doi.org/10.3390/nano10081597

AMA Style

Hussain S, Rabani I, Vikraman D, Feroze A, Ali M, Seo Y-S, Kim H-S, Chun S-H, Jung J. One-Pot Synthesis of W2C/WS2 Hybrid Nanostructures for Improved Hydrogen Evolution Reactions and Supercapacitors. Nanomaterials. 2020; 10(8):1597. https://doi.org/10.3390/nano10081597

Chicago/Turabian Style

Hussain, Sajjad, Iqra Rabani, Dhanasekaran Vikraman, Asad Feroze, Muhammad Ali, Young-Soo Seo, Hyun-Seok Kim, Seung-Hyun Chun, and Jongwan Jung. 2020. "One-Pot Synthesis of W2C/WS2 Hybrid Nanostructures for Improved Hydrogen Evolution Reactions and Supercapacitors" Nanomaterials 10, no. 8: 1597. https://doi.org/10.3390/nano10081597

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