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Interface Designing over WS2/W2C for Enhanced Hydrogen Evolution Catalysis Yang Li, Xin Wu, Huabin Zhang, and Jian Zhang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00550 • Publication Date (Web): 28 Jun 2018 Downloaded from http://pubs.acs.org on June 29, 2018

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ACS Applied Energy Materials

Interface Designing over WS2/W2C for Enhanced Hydrogen Evolution Catalysis Yang Li,† Xin Wu,† Huabin Zhang,* ‡ Jian Zhang.* †

†

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of

Matter, Chinese Academy of Sciences, 350002 Fuzhou, P. R. China. ‡

School of Chemical and Biomedical Engineering, Nanyang Technological University, 62

Nanyang Drive, Singapore 637459, Singapore.

ACS Paragon Plus Environment

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ABSTRACT: Interface engineering is a promising strategy for boosting the catalytic performances via the optimized coordination. Herein, we developed a top-down strategy to in situ obtain the nanocomposite of N, S-decorated porous carbon matrix encapsulated WS2/W2C (WS2/W2C@NSPC). The as-synthesized hybrid is characterized by excellent interface coupling in atomic-level, good electrical conductivity, and high active surface area. Electrochemical measurements show that the optimized catalyst exhibits remarkable electrocatalytic activity for hydrogen evolution in both acidic and alkaline media. These results should be attributed to the abundant active sites existing in the different phase boundaries, resulting from a synergistic effect of the activated WS2/W2C heterostructure and the highly conductive carbon matrix. This strategy opens new avenues toward understanding the relationship between chemical structure and catalytic performance in molecular level, and thus providing a rational way to fabricate high efficient and durable electrocatalysts.

KEYWORDS: WS2/W2C heterostructure, MOF-derived electrocatalysts, in-situ integration, interface coupling, active sites, hydrogen evolution reaction.


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1. INTRODUCTION Electrochemical reduction of water to molecular hydrogen is a plausible alternative to satisfy the growing global energy demand and curb environmental pollution.1-4 Owing to a superb hydrogen evolution rate, Pt and Pt-alloys have been extensively applied as the electrocatalysts for hydrogen evolution reaction (HER).5-8 Nevertheless, its large-scale industrial applications are greatly hindered by the high cost and limited resources.9 Therefore, developing efficient and durable alternatives consisting of earth-abundant elements is attractive and serves as an ideal channel to replace noble metal-based catalysts.10-11 Tungsten disulfide (WS2), a typical two-dimensional material, is a promising nonprecious catalyst for electrocatalytic hydrogen evolution. The main challenge associated with WS2 is the relatively low hydrogen evolution activity caused by the low intrinsic electric conductivity and thermodynamic stable but relatively inactive basal plane.12-15 Numerous methods have been developed to either increase the number of active edge sites or activate the basal planes, but the catalytic performances are still far from satisfactory.16-18 Interface engineering has been proven as a promising strategy for improving the electrochemical performance of electrode materials and has received more and more attention recently.19-23 The phase interface present in an integrated composite often leads to lattice mismatches in the interface region, which could effectively modify the coordination environment of inserted S atom from the basal plane of WS2 and thus realize the activation of inserted S atom.24 Besides, interactions at the coupled interface are also beneficial for the electrons transformation, which could lead to accelerated reaction kinetics of the hybrid.21, 25-28 Hence, the construction of interfacial structures may represent a useful strategy to promote the performance of WS2. Other than tuning the interface between the active components, the catalytic activity can


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also be boosted through the manipulation of the architectures. Hybridizing heteroatom-doped carbon matrix with highly active HER catalysts could further improve the performance because facile electron conductivity and high HER activity are combined together.29-34 Herein, we first develop a bottom-up strategy to confine interface optimized WS2/W2C heterostructure into the heteroatom-doped porous carbon matrix (denoted as WS2/W2C@NSPC). By taking advantage of the periodic pores, regular architecture and modifiable active interface, W-based hybrid zeolitic imidazolate framework (HZIF-W) are selected as the sole-precursor for fabrication of porous functional hybrid, followed by confined carburization and sulfuration under Ar atmosphere. The HZIF-W precursors decompose at high temperature (900 oC) to form W2C and N-doped porous carbon species (W2C/NPC). With the introduction of a trace amount of S in this process, W2C edge-function site in the carbon frameworks can be effectively converted into WS2 active sites in the form of WS2/W2C heterogeneous structure decorated onto N, S co-doped carbon matrices. The as-synthesized WS2/W2C@NSPC nanocomposite presents a strong interplay between the generated interfaces. Such a coupled heterogeneous interface may activate the insert sites and facilitate the conductivity of electrons, producing synergistically enhanced electrochemical

performance.

Impressively,

the

obtained

WS2/W2C@NSMC

hybrid

demonstrates outstanding HER performance in both acid and alkaline media. 2. EXPERIMENTAL SECTION 2.1 Synthesis of HZIF-W The hybrid zeolite imidazolate framework-W (HZIF-W) was applied as the precursor for the synthesis of WS2/W2C@NSPC nanosheets. The synthesis of HZIF-W was based on a previous procedure with some modifications. Typically, Zn(CH3COO)2·2H2O (0.17 g, 0.001


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mmol), 2-methylimidazole (2-mim, 0.09 g, 0.001 mmol), tungstic acid (0.05 g, 0.0002 mmol), and N, N-dimethylformamide (4 mL) were mixed in a 25 mL Teflon-line airtight reactor and then heated at 160 oC for 24 h. After cooling to room temperature, the product was separated by centrifugation and washed thoroughly with water and ethanol, and finally dried overnight at 60 o

C.

2.2 Synthesis of WS2/W2C@NSPC and W2C/NPC nanosheets In a typical procedure, sulfur powder (5 mg) and dried HZIF-W were grinded to powder, respectively, and then placed in two porcelain boats. The porcelain boat with sulfur powder was put in the front end of the port of tubular furnace while the porcelain boat with HZIF-W was put in the middle. Then the boats were heated to 900 oC at a heating rate of 2 oC min-1 and kept at 900 oC for 5 h under flowing N2 atmosphere, finally cooled to room temperature naturally to obtain porous carbon nanomaterials. In this process, the zinc component was reduced by carbon to generate evaporative Zn at such a high temperature, whereas the W component was reacted with carbon and sulfur to produce nanosized tungsten carbide particles and layered WS2 nanosheets. Then the product was etched thoroughly with 0.5 M HCl aqueous solution at room temperature, which was followed by repeated filtrations and washing with lots of water to yield WS2/W2C@NSPC catalysts. The W2C/NPC catalyst was prepared with the same treatment except sulfur powder was not introduced during the annealing process. 2.3 Characterization of the samples Powder X-ray diffraction patterns were recorded on a MiniFlex 600 diffractometer with Cu Kα radiation (λ= 1.54056 Å). Raman scattering spectra were recorded on a laser Raman microscope system (LabRAM HR) with an excitation wavelength of 532 nm. The specific surface areas were determined by a surface area analyzer (ASAP 2010 analyzer) with the


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Brunauer-Emmett-Teller method. A surface chemical analysis was performed using X-ray photoelectron spectroscopy (XPS, ESCALAB 250xi). Transmission electron microscopy (TEM) and elemental mapping were performed using JEM-2010 instrument. The scanning electron microscopy images were recorded on a field-emission scanning electron microscope (SEM, JEOL-6700F). 2.4 Electrochemical measurements All the electrochemical experiments were conducted on the CHI 760E electrochemical work station with a standard three-electrode system in N2-saturated 0.5 M H2SO4 solution or 1 M KOH at room temperature. The catalyst-coated glassy carbon rotating disk electrode (diameter 3 mm) was used as the working electrode, and Ag/AgCl electrode in saturated KCl solution (4 M) and graphite rod were used as the reference and counter electrode, respectively. To prepare the working electrode, 5 mg of the catalyst was dispersed in 0.5 mL of ethanol/Nafion (9:1, v/v), and then sonicated for 6 h to form a well dispersed black ink. Then, 1.5 uL well-dispersed catalysts were covered on the glassy carbon electrode with drying naturally for the test (0.21 mg). Linear sweep voltammetry was conducted from 0.2 to -0.6 V with a scan rate of 5 mV s-1. Before testing, the catalyst was cycled numerous times by cyclic voltammogram until a stable cyclic voltammogram curve was obtained. All potentials were referenced to a reversible hydrogen electrode (RHE). Electrochemical impedance spectroscopy (EIS) measurements were carried out from 100 kHz to 0.1 Hz at -0.25 V on a rotation disk electrode under 1600 rpm. 3. RESULTS AND DISCUSSIONS


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Figure 1. (a) FESEM image of HZIF-W. Scale bar: 100 nm. (b) FESEM image of HZIF-Wderived hybrid WS2/W2C@NSPC. Scale bar: 500 nm. (c, d) TEM and HRTEM images of W2C/NPC. Scale bar: 20 nm (c), 2 nm (d). (e, f, g, h) TEM and HRTEM images of WS2/W2C@NSPC. (i) High-angle annular dark-field (HAADF)-STEM image of as-synthesized WS2/W2C@NSPC and energy dispersive spectrometer (EDS) elemental mapping of the selected area revealing the homogeneous distribution of: carbon (i1), nitrogen (i2), sulfur (i3) and tungsten (i4). Scale bar: 100 nm (e), 2 nm (f), 2 nm (g), 2 nm (h), 25 nm (i). The precursor HZIF-W has a similar structure with zeolitic imidazolate framework-8 (ZIF-8) except for the partial substitution of Zn(im)42- units by WO42- nodes.35 Field-emission scanning electron microscopy (FESEM) image shows that the obtained HZIF-W possesses the morphology of nanosheet. (Figure 1a and Figure S1 in the Supporting Information). To prepare the electrocatalyst, the precursor HZIF-W was calcinated in the presence of sulfur vapor at a high temperature under N2 atmosphere. In this process, layered WS2 nanosheets were simultaneously


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generated over the surface of the newly formed W2C nanoparticles, resulting in optimized interface WS2/W2C with close coupling and strong interaction. At the same time, the heteroatomdoped (N and S) carbon matrix with high conductivity and porosity was also constructed, hybridized with the WS2/W2C heterostructure. The zinc component in the nanocomposite was partly reduced by carbon to give evaporative Zn at such high temperature, while the unevaporated zinc component was etched by an aqueous solution of HCl. For reference, catalysts W2C/NPC organized by W2C nanoparticles and N-doped porous carbon matrix, N, S co-doped porous carbon (NSPC) and N-doped porous carbon (NPC) have also been constructed using similar strategy. As confirmed by the typical FESEM images, the original nanosheet morphology of parent HZIF-W is still maintained after heating and etching treatments, suggesting that the HZIF-W serves as both a precursor and self-sacrificing template for the formation of WS2/W2C@NSPC (Figures 1b and S2). Transmission electron microscope (TEM) image of WS2/W2C@NSPC nanocomposite reveals

that WS2 nanosheets are homogenously distributed without

agglomeration (Figure 1e). High-resolution TEM (HRTEM) image further demonstrates that the widely separated fringes with the closest spacing of 0.62 nm are consistent with the (002) planes of 2H-WS2 (Figures 1f and 1g).36 A closer examination in the margin of the nanosheets reveals the fringes with the lattice distance of 0.227 nm and 0.267 nm, corresponding to the plane of (101) for W2C and WS2, respectively (Figure 1g).24 Furthermore, the numerous small pits denoted by the white dash line illustrate the formation of defects on WS2/W2C@NSPC (Figures 1g and 1h). Concomitant with the internal defect structure, the subtle lattice distortions appear between the (101) plane of W2C and the neighboring (101) plane of WS2, which constitutes atomic-level nanointerface (Figure 1h). Both the nanointerfaces and inner defects can not only


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activate the basal planes of WS2 but also can expose more active edge sites of W2C.37-38 Besides, the successful integration of W2C/WS2 with high conductivity carbon matrix can effectively accelerate charge transfer, which would be beneficial for improving catalysis kinetics. Highangle annular dark-field (HAADF) image and STEM-EDX mappings (Figures 1i-1i4) further indicate that the C, N, S, and W are uniformly distributed in the nanocomposite. For reference, the TEM and HRTEM images of W2C/NPC reveal only numerous W2C nanoparticles with the lattice distance of 0.227 nm, corresponding to (101) plane, are embedded in the porous carbon matrix (Figures 1c and 1d). Powder X-ray diffraction (PXRD) measurements were performed to investigate the bulk structure of WS2/W2C@NSPC, which reveal the co-existence of WS2 and W2C phases in the nanocomposite (Figure S3). A prominent peak at 14.3o (d = 0.62 nm) corresponds to the (002) planes of 2H-WS2. For W2C/NPC, the main phase is indexed as W2C (Figure S4). Notably, for both WS2/W2C@NSPC and W2C/NPC, an obvious diffraction peak at about 26.2o is assigned to (002) plane of graphitic carbon, which may be attributed to the complete conversion of HZIF-W precursor to heteroatom-doped porous carbon matrix (Figures S3 and S4). The structure information of WS2/W2C@NSPC was further analyzed by using Raman spectrum. As shown in Figure 2a, two main Raman peaks located at 697 and 804 cm-1 are observed for both WS2/W2C@NSPC and W2C/NPC, corresponding to stretching mode of W-C.39 The E12g and A1g phonon modes of WS2 have also been detected in the WS2/W2C@NSPC with Raman peaks located at around 350.1 and 415.2 cm-1.40 Compared with the reported bulk WS2, a small blue shift of E12g and A1g phonon modes and a relatively higher peak intensity ratios (E12g/A1g) are observed for WS2/W2C@NSPC (1.02). This result clearly indicates that the strong coupling of WS2 and W2C phase can weaken the interlayer interaction of WS2 and activate the basal plane of


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WS2.7 The Raman spectroscopy also reveals the different graphitization degree of the carbon phase in all the samples.41 The inset of Figure 2a and Figure S5 display the ID/IG ratio of WS2/W2C@NSPC (0.80) and W2C/NPC (0.79), which is lower than those of NSPC (1.02) and NPC (0.99).39 Brunauer-Emmett-Teller (BET) surface area of the WS2/W2C@NSPC catalyst was measured to be 379.4 m2g-1 (Figures 2b, S6, S7, S8 and Table S1), providing sufficient active sites for the catalytic reaction. X-ray photoelectron spectroscopy (XPS) analyses and inductively coupled plasma atomic emission spectrometry (ICP-AEM) were also conducted to probe the elemental composition and chemical status of as-prepared catalysts (Table S2). In WS2/W2C@NSPC catalyst, the overview survey spectrum clearly shows the concomitance of C, N, S and W elements (Figures 2c and S9). The deconvolution of C1s XPS spectrum can be mainly ascribed to C-C/C=C (284.47 eV) and C-N/C-S (285.49 eV) (Figure S10). The existence of O-C=O at the high binding energy of 288.45 eV indicates the partial oxidation of surface carbon.42 The high-resolution N1s spectrum reveals the presence of three main types of nitrogen species, pyridinic N at 398.17 eV, pyrrolic N at 400.15 eV and graphitic N at 401.11 eV (Figures 2d and S11).43 Particularly, a new peak at lower binding energy of 196.6 eV in WS2/W2C@NSPC and W2C/NPC is attributed to the N-W bond, which may come from the interface interaction between W2C and the N-doped carbon.39 As shown in Figure 2e, W4+ 4f7/2 and W4+ 4f5/2 with the binding energy centered at 32.6 and 34.72 eV are observed in the XPS W4f core-level spectrum of WS2/[email protected] Compared with pure WS2 nanosheets, these two peaks shift to lower binding energy, further indicating the mutual interaction of WS2 and W2C would weaken the W-S bonding.44 Two other weak peaks centered at 31.8 and 33.97 eV should be attributed to the W-C bonding (Figure S12). The S 2p spectrum of WS2/W2C@NSPC reveals two major peaks centered at 162.24 and 163.42 eV, which can be assigned to S 2p3/2 and


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S 2p1/2, respectively. (Figures 2f and S13). Particularly, a new peak at lower binding energy (161.16 eV) can be ascribed to the electronic redistribution around the sulfur atom via charge transfer,24 which strongly suggests the establishment of the coupling interface between W2C nanoparticles and WS2 nanosheets.

Figure 2. (a) Raman spectra of W2C/NPC and WS2/W2C@NSPC. (b) N2 adsorption-desorption isotherms for HZIF-W and WS2/W2C@NSPC. (c) XPS survey spectra of WS2/W2C@NSPC and W2C/NPC. The high resolution of (d) N 1s, (e) W 4f and (f) S 2p of WS2/W2C@NSPC, respectively.


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The electrocatalytic activities of as-prepared catalysts toward HER have been evaluated in 0.5 M H2SO4 and 1.0 M KOH, respectively. A convenient three-electrode system was utilized with graphite rod as the counter electrode, catalyst powders loaded on glassy carbon as the working electrode and Ag/AgCl (saturated KCl-filled) as the reference electrode. As a reference, the HER performance of commercial Pt/C catalyst has also been investigated. During the test, the same loading amount of approximately 0.21 mg cm-2 was applied for WS2/W2C@NSPC, W2C/NPC, NSPC, NPC and commercial Pt/C to ensure reasonable comparison. To exclude the influence of electrode, blank glassy carbon electrode as the working electrode was also employed to investigate the HER activities.

Figure 3. (a) Polarization curves for HER of various electrodes in 0.5 M H2SO4. (b) The corresponding Tafel plots for various catalysts. (c) Comparison of the exchange current density and the overpotential at the current density of 10 mA cm-2 among different samples.


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Linear sweep voltammetry curve shows that the onset potential of WS2/W2C@NSPC (≈10 mV, vs. reversible hydrogen electrode (RHE), all the potentials are referenced to RHE in this work) is comparable to that of Pt/C ( ≈ 0 mV), and notably more positive than those of W2C/NPC (≈ －34 mV), NSPC (≈ －80 mV), NPC (≈ －141 mV) and blank glassy carbon electrode (≈ －420 mV) (Figures 3a and S14). To deliver a 10 mA cm-2 HER current density, WS2/W2C@NSPC requires a small overpotential of 126 mV, roughly 44, 334 and 504 mV lower than those of W2C/NPC (170 mV), NSPC (460 mV) and NPC (630 mV), respectively. These results indicate that chemical doping can effectively increase the catalytic active sites and the interface optimizing between WS2 nanosheet and W2C can induce a synergistically enhanced HER activity (Figures 3a and 3c). The HER kinetics can be revealed by the Tafel plots derived from linear sweep voltammetry curves (Figure 3b). The WS2/W2C@NSPC catalyst has the smaller Tafel slope of 68 mV dec-1 than other reference samples such as W2C/NPC (80 mV dec1

), NSPC (170 mV dec-1) and NPC (158 mV dec-1), indicating the more favorable kinetics in

WS2/W2C@NSPC. Mechanistically, three major reactions are typically involved for HER in acid solution, including a primary discharge step (Volmer reaction, with a Tafel slope of 120 mV dec1

), electrochemical desorption step (Heyrovsky reaction, with a Tafel slope of 40 mV dec-1) or a

recombination step of adsorbed hydrogen on catalysts (Tafel reaction, with a Tafel slope of 30 mV dec-1). In the present study, the HER mechanism of the WS2/W2C@NSPC catalyst follows the Volmer-Heyrovsky reaction where the electrochemical desorption is the rate-determining step.7, 45 Moreover, the exchange current density (j0), as one of the most important parameters to reflect the intrinsic catalytic activity of the electrode material, is calculated by extrapolation of the Tafel plots to J axis. As expected, WS2/W2C@NSPC exhibits a higher j0 of 0.501 mA cm-2 than those of W2C/NPC (0.31 mA cm-2), NSPC (0.063 mA cm-2), and NPC (0.022 mA cm-2)


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(Figure 3c, Table S4). The low Tafel slopes and high exchange current density of WS2/W2C@NSPC confirm the most favorable HER kinetics of WS2/W2C@NSPC, accounting for the best catalytic efficiency of WS2/W2C@NSPC.

Figure 4. (a) Polarization curves for HER of various electrodes in 1.0 M KOH. (b) The corresponding Tafel plots for various catalysts. (c) Cyclic voltammetry curves of WS2/W2C@NSPC nanosheet under different scan rates. These data were used to present the plots showing the extraction of the Cdl. (d) the capacitive current as a function of scan rate for WS2/W2C@NSPC and W2C/NPC. The electrocatalytic HER activities of the catalysts in N2-saturated basic medium (1 M KOH) were also investigated. As shown in the polarization curve (Figure 4a), WS2/W2C@NSPC shows a small overpotential of 205 mV at a current density of 10 mA cm-2, which is much lower


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than that of W2C/NPC (297 mV) catalyst and even comparable with those of other reported nonnoble metal HER electrocatalysts in basic solution (Table S5). The Tafel plots calculation shows WS2/W2C@NSPC catalyst holds the lowest Tafel slope of 72 mV dec-1 compared with those of W2C/NPC (87 mV dec-1), NSPC (150 mV dec-1) and NPC (260 mV dec-1) (Figure 4b). Electrochemical double-layer capacitance (Cdl), which is believed to be positively proportional to the electrochemically active surface area (ECSA), has also been investigated to further illustrate the superior HER performance of the as-prepared WS2/W2C@NSPC. (Figures 4c and S15).25 As observed, the Cdl of WS2/W2C@NSPC (0.1144 mF cm-2) is substantially higher than that of W2C/NPC (0.0689 mF cm-2), which implies the larger ECSA of WS2/W2C@NSPC. (Figure 4d).

Figure 5. (a) Cycling stability of WS2/W2C@NSPC before and after 2000 CV cycles in 0.5 M H2SO4 medium at the scan rate of 100 mV s-1. (b) Cycling stability of WS2/W2C@NSPC and W2C/NPC before and after 2000 CV cycles in 1.0 M KOH medium at the scan rate of 100 mV s-1.


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(c) Time dependence of cathodic current density over WS2/W2C@NSPC under a fixed overpotential of -250 mV in 1.0 KOH medium. (d) electrochemical impedance spectra at 0.25 V of different catalysts (inset: applied equivalent circuit model). Durability is an important parameter in the evaluation of electrocatalyst. To evaluate the stability of the WS2/W2C@NSPC electrocatalyst, cyclic voltammetry curves were recorded from -50 to -250 mV for 2000 cycles in both acidic and alkaline medium. At the end of cycling, the WS2/W2C@NSPC delivered similar polarization curve with the initial cycle, demonstrating the long-term durability of the material under the operating conditions (Figures 5a and 5b). Moreover, during long-term stability tests at a fixed potential, the current density of WS2/W2C@NSPC maintained very well and even increased slightly in acidic media, which may be attributed to the electrochemical activation, demonstrating the actual electrochemical performance of the WS2/W2C@NSPC nanosheet (Figures 5c and S16). These performances are superior to that of W2C/NPC, which exhibits an obvious sluggish electrochemical performance after 2000 CV cycles and gives only 70% current density compared with the initial values over 9 h continuous operations (Figures S17 and S18). Moreover, the electrocatalytic kinetics is also investigated by the electrochemical impedance spectra (EIS) measurements. (Figure 5d, S19, Table S3) The lower charge transfer resistance (Rct) obtained from the semicircle in the lowfrequency zone of Nyquist plot confirms a faster charge transfer rate. Obviously, the Rct value of WS2/W2C@NSPC (79.2 Ω) is much smaller than those of W2C/NPC (122.8 Ω), NSPC (148.5 Ω) and NPC (179.4 Ω), which can further account for the best excellent HER activity of WS2/W2C@NSPC. 4. CONCLUSIONS


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In summary, we have presented a simple approach for developing earth-abundant and highactivity HER catalysts through interface engineering. The optimized WS2/W2C heterostructure with abundant interfaces exhibits remarkable electrocatalytic activity and stability for HER in both acid and alkaline media, including low onset overpotential, a small Tafel slope, high exchange current density and good cycle stability. The basal planes of WS2 can be effectively activated by the coupled W2C component at the nanointerface. This method opens a new avenue to create maximally active sites at atomic and molecular level, and thus provide a way to fabricate high-performance electrocatalysts rationally. ASSOCIATED CONTENT Supporting Information. Experimental details of steady-state measurements. The Supporting Information is available free of charge on the ACS Publications website. AUTHOR INFORMATION Corresponding Author *Jian Zhang; E-mail: [email protected] *Huabin Zhang; E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by NSFC (21425102, 21473202, 21521061, and 21673238) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000).


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