Highly Efficient Hydrogen Evolution from Edge-Oriented WS2(1âx

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Highly efficient hydrogen evolution from edge-oriented WS2(1x)Se2x particles on three-dimensional porous NiSe2 foam Haiqing Zhou, Fang Yu, Jingying Sun, Hangtian Zhu, Ishwar Kumar Mishra, Shuo Chen, and Zhifeng Ren Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b03467 • Publication Date (Web): 02 Nov 2016 Downloaded from http://pubs.acs.org on November 3, 2016

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Highly efficient hydrogen evolution from edge-oriented WS2(1-x)Se2x particles on three-dimensional porous NiSe2 foam

Haiqing Zhou,†,‡ Fang Yu,†,‡ Jingying Sun,‡ Hangtian Zhu,‡ Ishwar Kumar Mishra,‡ Shuo Chen‡,* and Zhifeng Ren‡,*

†

‡

These authors contributed equally to this work.

Department of Physics and TcSUH, University of Houston, Houston, TX 77204, USA

*Address correspondence to [email protected] and [email protected].

ABSTRACT The large consumption of natural fossil fuels and accompanying environmental problems are driving the exploration of cost-effective and robust catalysts for hydrogen evolution reaction (HER) in water splitting. Tungsten dichalcogenides (WS2, WSe2, etc.) are promising candidates for such purpose, but their HER performances are inherently limited by the sparse catalytic edge sites and poor electrical conductivity. Here we demonstrate a highly active and stable HER catalyst by integrating ternary tungsten sulfoselenide WS2(1-x)Se2x particles with a 3D porous metallic NiSe2 foam, in which good electrical conductivity, good contact, high surface area, and high-density active edge sites are simultaneously obtained, thus contributing to good catalytic performance: large cathode current density (- 10 mA/cm2 at - 88 mV), low Tafel slope (46.7 mV/dec), large exchange current density (214.7 µA/cm2), and good stability, which is better than most reports on WS2 and NiSe2 catalysts. This work

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paves an interesting route for boosting HER efficiency of transition metal dichalcogenide catalysts.

KEYWORDS: Edge-oriented, hydrogen evolution reaction, porous electrocatalyst, tungsten disulfide, three-dimensional, water splitting


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Hydrogen (H2) is a promising, clean, and renewable energy carrier to relieve our reliance on natural fossil cells, and can reduce the growing global greenhouse effects.1 Hydrogen evolution reaction from water splitting is a straightforward and effective route to generate H2 from abundant water on earth,2,3 but the efficiency is too much dependent on the use of catalysts. Thus, the first and most important step is to develop efficient and durable catalysts from earth-abundant elements instead of the rare and precious Pt-group metals,4 so tremendous efforts are focused on finding alternative competitive catalysts with earth abundance and low cost. With the development of the synthesis of two-dimensional single crystals,5 layered transition-metal dichalcogenides (LTMDs such as MoS2, WS2, WSe2, etc) have attracted much attention as the promising catalysts for hydrogen evolution.6,7 However, the progress on the catalytic performance of tungsten dichalcogenides (WS2, WSe2, etc) is relatively slow.8 Only until recently, some breakthroughs have been made on their catalytic properties by engineering the nanostructures like nanosheets,9,10 nanotubes,11 or nanoribbons.12,13 Even so, the catalytic performance is still limited by the low-density active edge sites or poor electrical conductivity due to the semiconducting nature. In this sense, many other promising approaches14 have been used to improve the intrinsic activity by growing metallic 1T phase compounds,15,16 introducing substrate interactions to lower the hydrogen binding energy,17,18 tuning the electronic properties by Li+ ion intercalation,13,16,19 or heteroatom doping to activate S-edges.20 Up to now, the catalytic performance is still not satisfied, not to mention the complicated material synthesis. To the best of our knowledge, one of the most efficient strategies is to increase the number of active sites arising from the exposed edges21,22 and improve the electrical conductivity or the contact between the catalyst and the electrode,19,23 since in most cases the catalysts are semiconductors. Carbon-based materials (carbon cloth, carbon nanotube, and graphene) are promising supports for these catalysts due


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to the high surface area and improved electrical contact between the catalysts and electrode, but the performance is unsatisfactory.11,13,23-25 Especially, since the catalytic active sites of LTMDs are located at the edges rather than the catalytically inert basal planes, it is highly desirable to grow the tungsten LTMDs catalysts with a large number of exposed edges, which is easy to be realized by growing the catalysts on a rough surface.26,27 Moreover, three-dimensional (3D) porous architectures are attractive due to the high surface area for catalyst loading and fast reaction kinetics for hydrogen evolution.27-30 In view of these points, it is interesting and meaningful to introduce another 3D cost-effective and HER-active scaffold with porous and rough surface to support tungsten dichalcogenides, so that a large number of active edge sites, good electrical contact, and synergistic effects are simultaneously integrated in the same device, further boosting the catalytic properties of the resulting hybrid catalyst. In this work, we report a new efficient and durable hybrid catalyst composed of tungsten sulfoselenide WS2(1-x)Se2x particles supported by 3D porous NiSe2 foam from commercial Ni foam via thermal selenization.18,31 These particles disperse uniformly on NiSe2 surface with a large number of exposed edge sites. This WS2(1-x)Se2x/NiSe2 hybrid foam can be directly used as a 3D self-standing hydrogen-evolving electrode, which is demonstrated to exhibit good HER performance featured by relatively a large current density (- 10 mA/cm2 at only - 88 mV), low Tafel slope (46.7 mV/dec), large exchange current density (214.7 µA/cm2) and good electrochemical stability. The catalytic properties are superior to many MoS2, WS2, CoSe2, and NiSe2-based catalysts, which can be attributed to the synergistic effects of good conductivity and high surface area of porous NiSe2 foam, and a large number of active edge sites from ternary WS2(1-x)Se2x particles. Our strategy begins with the growth of porous NiSe2 foam from commercially available Ni foam by direct selenization (Figure 1a).18,31 For original Ni foam, it is composed of lots of Ni grains with the


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size in micrometers (Figure S1a). After thermal conversion of Ni foam into metallic NiSe2 foam, lots of additional porous structures with rough surface are generated (Figure S1b), and most of metallic Ni is converted to pyrite NiSe2 as confirmed by powder X-ray diffraction pattern (Figure S2), meaning that only a small amount of metallic Ni contributes to the total conductivity of porous NiSe2 samples. Then the as-grown NiSe2 samples were modified with (NH4)2WS4 precursor, followed by a second selenization at 500 oC in a tube furnace. According to the SEM images (Figure 1b,c), the WS2(1-x)Se2x particles are uniformly dispersed on a porous NiSe2 foam, which plays a significant role in the catalytic performance because of the increased active sites.18,32 Especially, with the assistance of high-resolution transmission electron microscopy (HRTEM), we can find that the layers of WS2(1-x)Se2x particles are mostly exposed on the surface of NiSe2 foam (Figure 1d and e), which may be attributed to the rough and curved surface of NiSe2 foam that is preferred for layer orientation of layered WS2(1-x)Se2x particles.27 These exposed layers of WS2(1-x)Se2x particles then provide a large number of active edge sites for the HER.33 Considering the metallic feature34 and porous structures of NiSe2 foam, and each layer of WS2(1-x)Se2x particles is probably in direct contact with the NiSe2 foam, the electrical contact between the WS2(1-x)Se2x catalyst and the electrode is greatly improved, which ensures quick electron transfer from the electrode to the WS2(1-x)Se2x particles. What's more, lots of porous structures provided by NiSe2 foam should quicken the proton transfer from the electrolyte to the catalyst surface because of high surface area. Thus, it is expected that our hybrid catalysts should simultaneously possess good electrical contact, high-density active edge sites, and 3D porous structures with high surface area, all of which contribute greatly to the electrocatalytic hydrogen evolution.


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Figure 1.The schematic diagram (a) and detailed morphologies (b-e) of edge-oriented WS2(1-x)Se2x particles supported on 3D porous NiSe2 foam. Low (b) and high (c)-magnification SEM morphologies of WS2(1-x)Se2x particles grown at 500 oC on 3D porous NiSe2 foam. (d,e) Typical HRTEM images showing a large number of exposed edge sites in WS2(1-x)Se2x particles grown on 3D porous NiSe2 foam.

X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were utilized to further characterize the chemical composition of the as-prepared catalysts. According to the XPS spectra collected on WS2(1-x)Se2x/NiSe2 hybrid material, all the Ni, W, S, and Se elements can be detected (Figure 2a-c, Figure S3), but it is difficult to distinguish whether the Se signal originates from the ACS Paragon Plus Environment

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WS2(1-x)Se2x particles or NiSe2 foam. To clarify it, we performed similar selenization at 500 oC by growing the tungsten compound on a Si substrate. In this case, we can clearly detect the presence of W, S, and Se elements in the relevant XPS data (Figure 2a-c). Alternatively, it is possible to detect the WS2(1-x)Se2x particles on a porous NiSe2 foam by Raman spectroscopy because of different vibration modes between NiSe2 and WS2(1-x)Se2x. As shown in Figure 2d, for pure NiSe2 foam, there are four vibration peaks ascribed to the Tg (153.6 cm-1), Eg (172.2 cm-1), Ag (217.7 cm-1), and Tg (243.7 cm-1) modes of NiSe2,31,35 while for pure WS2, two prominent Raman peaks are detected at 357.5 and 421.0 cm-1, which can be attributed to the E12g and A1g modes,11 respectively. Compared to pure WS2/NiSe2 foam, there is another broad peak appearing at around 257 cm-1 for WS2(1-x)Se2x,11 which can be clearly found on the Raman spectra of WS2(1-x)Se2x/NiSe2, and is associated with the corresponding WSe2-like E12g/2LA features. These observations in Raman and XPS data are consistent with previously reported results on WS2(1-x)Se2x system,11,13 confirming the formation of ternary WS2(1-x)Se2x particles on porous NiSe2 foam. In particular, based on Raman spectra, the factor x showing the atomic ratio between S and Se is around 0.3, which is further demonstrated by the X-ray energy dispersive spectroscopy measurement (Figure S4).


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Figure 2. Chemical composition analysis of the hybrid catalyst by XPS and Raman spectroscopy. (a) W 4f, (b) S 2p, and (c) Se 3d XPS spectra of the WS2(1-x)Se2x-based materials. (d) Raman spectra of the WS2 or WS2(1-x)Se2x particles on different substrates.

To investigate the electrocatalytic hydrogen evolution of the hybrid catalysts, we performed detailed electrochemical measurements in a three-electrode configuration in a N2-saturated 0.5 M H2SO4 electrolyte (Supporting Information Part 1).31 The loading of WS2 or WS2(1-x)Se2x particles on porous NiSe2 foam is around 5.4 mg/cm2. All the potentials reported here are referenced to the reversible hydrogen electrode (RHE). Figure 3a shows the polarization curves recorded on the as-prepared hybrid catalysts. For comparison, we have also included the curves collected on binary WS2/NiSe2, pure NiSe2 foam, and a Pt wire. Interestingly, the WS2(1-x)Se2x/NiSe2 hybrid catalyst can provide a geometric ACS Paragon Plus Environment

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current density of - 10 mA/cm2 at only - 88 mV, which is much lower than - 108 mV for WS2/NiSe2 and - 154 mV for pure NiSe2 foam. This performance outperforms many reported catalysts in the literatures including LTMDs MoS2/carbon nanotube forest (- 110 mV),32 WS2 nanosheets (- 142 mV),16 and WS2(1-x)Se2x nanoribbons

(-

173

mV),13 first-row

transition

metal

dichalcogenides

CoSe2

nanoparticles/carbon paper (- 139 mV),36 CoS2xSe2(1-x) nanowires/carbon fiber (104 mV)37 and NiSe2 (117 mV),38 and is comparable to some metal phosphide-based catalysts such as porous Mo-W-P nanosheet/carbon cloth (100 mV),39 Se-doped NiP2/carbon fiber (84 mV),40 and so on (Table S1), suggesting good catalytic performance of ternary WS2(1-x)Se2x particles/NiSe2 foam hybrids reported here for the HER. In addition, the Tafel slope of the WS2(1-x)Se2x/NiSe2 hybrid is only 46.7 mV/dec (Figure 3b), which is smaller than that of WS2 on NiSe2 foam (~ 54.7 mV/dec) and pure NiSe2 foam (46.8 mV/dec). This value is also much lower than many cheap HER catalysts reported previously in the same electrolyte (Table S1). What's more, the exchange current density (j0) is calculated to be around 214.7 µA/cm2, larger than most of the values reported on the well-known MoS2, WS2, and CoSe2 catalysts (Table S1). This may be due to the increased active edge sites from WS2(1-x)Se2x particles grown on porous NiSe2 foam. The as-prepared hybrid catalysts are also electrochemically stable in 0.5 M H2SO4. For example, after 1000 cycles, the polarization curve is almost the same as that of the initial one, indicating no observable degradation after long-term cycling tests (Figure 3c). In particular, the practical operation of the catalyst was examined in electrolysis at a fixed potential over a long period (Figure 3d). At a given overpotential of - 145 mV, there is no obvious decrease in the current density at ~ - 120 mA/cm2 for electrolysis over 8 h for the hybrid WS2(1-x)Se2x/NiSe2 catalyst, indicating its potential usage in water splitting for a long time.


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Figure 3. Electrochemical performance of as-prepared hybrid electrocatalysts in comparison with a Pt wire and pure NiSe2 support. (a) The polarization curves recorded on different catalysts: pure NiSe2 foam, WS2 on porous NiSe2 foam, WS2(1-x)Se2x on porous NiSe2 foam and a Pt wire. (b) The corresponding Tafel plots extracted from the curves shown in (a). (c) Polarization curves of WS2(1-x)Se2x/NiSe2 catalyst initially and after 1000 CV scans. (d) Time dependence of current density of the hybrid catalyst under a static overpotential of - 145 mV.

To evaluate the differences in the electrochemically effective surface areas of the catalysts studied here, the electrochemical double-layer capacitances (Cdl) were measured via a simple cyclic voltammetry (CV) method as displayed in Figure 4a, b, and Figure S5.31 By drawing the current difference between anodic and cathodic current densities (∆j = janodic - jcathodic) against each scan rate at a given potential of 0.15 V, linear fitting can be conducted, and then the Cdl is derived from the linearly ACS Paragon Plus Environment

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fitted curves, which is half the value of the linear slopes. As shown in Figure 4c, the Cdl values are extracted to be 256.9 mF/cm2 for the WS2(1-x)Se2x/NiSe2 hybrid catalyst, which is larger than 180.9 mF/cm2 of pure WS2/NiSe2 foam and nearly 28.5 times of 9.0 mF/cm2 of pure NiSe2 foam. Suppose that there is a linear relationship between the electrochemical surface area and the capacitance Cdl, we can derive the relative electrochemically active surface area (Table S2), which can be further used to normalize the exchange current density j0,normalized.29,38,40 As summarized in Table S2, the normalized exchange current density of WS2(1-x)Se2x/NiSe2 (8.54 µA/cm2) is larger than that of WS2/NiSe2 (6.46 µA/cm2), suggesting improved intrinsic catalytic activity by Se doping.40,41 Meanwhile, electrochemical impedance spectroscopy (EIS) was applied to study the electrode kinetics of the catalysts. Nyquist plots (Figure 4d) reveal a decrease of charge-transfer resistance (Rct) for the ternary WS2(1-x)Se2x/NiSe2 hybrid (0.8 - 1.3Ω) in contrast to the binary WS2/NiSe2 hybrid (3.2 Ω), and pure NiSe2foam (19.6 Ω). Furthermore, all the catalysts exhibit very small series resistances (~ 1 Ω), meaning that effective electrical integration is ensured by metallic NiSe2 foam. Based on the improved catalytic performance and EIS spectra, it is proposed that the substitution of S by Se atoms could affect the electrical conductivity of the ternary phase WS2(1-x)Se2x and thereby the hydrogen adsorption free energy.40,41 The enhanced electrical conductivity improves the electron transfer between WS2(1-x)Se2x catalyst and NiSe2 support. Consequently, Cdl measurements and EIS results confirm that WS2(1-x)Se2x/NiSe2 hybrid catalyst exhibit more facile electrode kinetics toward hydrogen evolution, which may be attributed to the good conductivity and porous structures of NiSe2 foam, and the active edge sites from WS2(1-x)Se2x particles.


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Figure 4. Double-layer capacitance (Cdl) measurements and Nyquist plots by EIS. (a) Electrochemical cyclic voltammogram of WS2/NiSe2 hybrid catalyst at different scan rates from 2 mV/s to 20 mV/s with an interval point of 2 mV/s. (b) Electrochemical cyclic voltammogram of WS2(1-x)Se2x/NiSe2 hybrid catalysts with the scan rates ranging from 2 mV/s to 18 mV/s with an interval point of 2 mV/s. (c) Linear fitting of the capacitive currents of the catalysts vs. the scan rates. (d) Nyquist plots showing the facile electrode kinetics of the hybrid catalysts WS2/NiSe2 and WS2(1-x)Se2x/NiSe2.

In conclusion, we introduced a simple and efficient strategy to greatly boost the catalytic HER activity of transition-metal dichalcogenides by making 3D porous architectures of layered WS2(1-x)Se2x particles on metallic NiSe2 foam. Good conductivity and porous structures of NiSe2 foam, and a large number of active edge sites from WS2(1-x)Se2x particles were achieved, which makes the hybrid catalyst


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highly active and efficient for HER, and stable in acid over a long period. This opens up a different avenue for the fabrication of robust and stable electrocatalysts for large-scale water splitting.

■ASSOCIATED CONTENT Supporting Information Available: The experimental details and characterization data. This material is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION Corresponding Authors *E-mail:[email protected] *E-mail: [email protected]. Author Contributions H.Z. and F. Y. contributed equally to this work. Notes The authors declare no competing financial interest.

■ACKNOWLEDGMENTS This work was supported by US Defense Threat Reduction Agency (DTRA) under grant FA 7000-13-1-0001 and also by the US Department of Energy under Contract No. DE-SC0010831. S. C. also appreciates the support from TcSUH as the TcSUH Robert A. Welch Professorships on High Temperature Superconducting (HTSg) and Chemical Materials (E-0001).


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Nano Letters

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