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Hierarchical carbon nanopapers coupled with ultrathin MoS2 nanosheets: Highly efficient large-area electrodes for hydrogen evolution
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Hierarchical carbon Nanopapers coupled with ultrathin MoS2 nanosheets: Highly efficient large-area Electrodes for Hydrogen evolution Tian-Nan Ye, Li-Bing Lv, Miao Xu, Bing Zhang, Kai-Xue Wang, Juan Su, Xin-Hao Li, Jie-Sheng Chen
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S2211-2855(15)00200-1 http://dx.doi.org/10.1016/j.nanoen.2015.04.033 NANOEN826
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Nano Energy
Received date: 2 February 2015 Revised date: 10 April 2015 Accepted date: 27 April 2015 Cite this article as: Tian-Nan Ye, Li-Bing Lv, Miao Xu, Bing Zhang, Kai-Xue Wang, Juan Su, Xin-Hao Li, Jie-Sheng Chen, Hierarchical carbon Nanopapers coupled with ultrathin MoS2 nanosheets: Highly efficient large-area Electrodes for Hydrogen evolution, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2015.04.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hierarchical Carbon Nanopapers Coupled with Ultrathin MoS2 Nanosheets: Highly Efficient Large-area Electrodes for Hydrogen Evolution Tian-Nan Ye, Li-Bing Lv, Miao Xu, Bing Zhang, Kai-Xue Wang, Juan Su, Xin-Hao Li* and Jie-Sheng Chen* School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China. Tel:+86 34201273. E-mail address: [email protected]; [email protected]. Abstract Design of large-area hybrid paper for directly using as electrodes in hydrogen evolution reaction (HER) can provide an efficient approach for the extension of electrochemical hydrogen industry. Here we construct MoS2 decorated hybrid carbon papers (MoS2-CPs) that consist of tiny MoS2 nanosheets coupled with 3D graphene-carbon nanofiber papers. MoS2CPs can function as large-area working electrodes for HER with a overpotential (at 10 mA/cm2) of 80 mV in acid media and 186 mV in basic media, surpassing the Mo-based catalysts ever reported thus far in acid and basic solution respectively. It is the highly coupled interface of carbon frameworks and MoS2 components that resulted in the formation of patched and few-layer MoS2 nanosheets with rather small size and thus ensured the abundance of exposed active edge sites. Stability tests through long-term potential cycles and extended electrolysis confirm the outstanding durability of MoS2-CPs in both acid and basic electrolytes. Keywords Molybdenum disulfide, Carbon paper, Large-area electrode, Hydrogen evolution, Graphenes.
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Introduction With the energy crisis becoming more and more serious, hydrogen is considered as one of the most promising candidates substituting the fossil energy [1]. The search for high stability and low-cost sustainable electrodes for the splitting of water into hydrogen gas is one of the noble missions of hydrogen energy [2,3]. Pt based catalyst, the best hydrogen evolution reaction (HER) catalyst, is often deposited on electrodes to promote electrochemical hydrogen production [4]. As the most electroactive catalyst, Pt catalyst just requires negligible overpotential, even at high reaction rates [5]. However, the high price and scarcity have critically impeded the extensive applications of these electrochemical techniques for practical preparation of hydrogen fuels. Recently, transition metal phosphides and chalcogenides as the non-precious metal catalysts have shown a striking electrocatalytic performance for HER [6-21]. Among these transition-metal based catalysts, layered molybdenum disulfide (MoS2) has been widely used in both industry and fundamental researches as a cheap alternative of platium with noblemetal-like catalytic activities [22-29]. There is a consensus that the catalytic sites of layered MoS2 mainly arises from the active edges of two-dimensional layers and the (0001) basal planes of MoS2 are catalytically inert. The bulk phase is relatively inert because the (0001) basal planes are the major on the surface [17]. Consequently, nanostructured MoS2 catalysts have been well engineered to maximize the amount of exposed edges of the MoS2 layers and thus boost their catalytic activity, including HER activity [30-32]. With respect to the practical use of those MoS2 nanocatalysts, large-area electrodes with mesoscale to microscale pores are essential for releasing hydrogen bubbles via HER. Considering the intrinsic low electrical conductivity of MoS2 with a band gap of 1.69 eV, the resistance of MoS2 nanomaterials could be further elevated by a factor of several orders of magnification due to the increased number of grain boundaries in nanoscale [33]. Current efforts are thus mainly focused on painting the ink of nanocatalysts with binder to a 2
conductive substrate (e.g. commercial carbon papers, Ni foams or even graphene-coated Ni foams) with microscale subunits to fabricate large-area MoS2 thick films as practical electrodes [34-36]. Such a process obviously lead to a decrease in the HER activity as compared with that over glass carbon electrode (GCE), presumably due to the poor distribution of the MoS2 nanocatalysts over the support [34]. Direct deposition of defect-rich and ultra-fine MoS2 nanocatalysts on large-area conductive paper composed of nanoscale subunits promises great room for significantly elevating the HER activity of noble-metal-free electrocatalysts for practical applications [35,37-39]. Herein, we highlighted the fabrication of ultra-thin MoS2 nanosheets decorated carbon papers (MoS2-CPs) as large-area electrodes for HER. The defect-rich MoS2 nanosheets with a size smaller than 15 nm and a mean layer number around 2-3 layers can function as electrocatalytic active sites here. The highly coupled MoS2-CPs hybrid papers show superb activity and stability in both acid and basic media when directly used as electrode for HER.
Experimental Section CPs aerogels synthesis The bacterial cellulose (BC) pellicles were kindly provided by Ms CY Zhong (Hainan Yeguo Foods Co., Ltd., Hainan, China). In a typical synthesis, the BC pellicles were first washed with deionized water and then dipped in a urea aqueous solution for 24 h. After frozen by liquid nitrogen and dried in a bulk dryer, white BC aerogels were generated. The resulting BC aerogels were transferred into a crucible with flowing-nitrogen and heated at 600 °C at a rate of 1 °C min–1, kept this temperature for 1 h. This was followed by further heating at a rate of 1 °C min–1 to reach a temperature of 1000 °C and maintaining at that temperature for 1 h. The sample was then cooled naturally to room temperature within nitrogen gas, resulting in black and ultralight carbon papers (CPs).
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Preparation of MoS2-CPs Briefly, 0.1 mL of N2H4·H2O and 55 mg of (NH4)2MoS4 were dispersed into 10 mL of DMF solution containing under sonication. The as-obtained CPs (weight: 11 mg) were dipped into the mixed solution. All these precursors were then transferred into a 100 mL Teflon-lined autoclave and heated at 200 °C for 12 h. The as-formed MoS2-CPs were washed with distilled water and ethanol repeatedly for 6 times to remove DMF and free-standing nanoparticles. Electrochemical measurements Firstly, the MoS2-CPs were used as direct electrodes for HER. A standard caliper was employed to define the 1 cm2 electrode area by cutting scissors. The loading of MoS2 species in MoS2-CPs was estimated to be 0.875 mg cm-2 via inductively coupled plasma (ICP) analysis. And the polytetrafluoroethene wapped platinum wire clip was used to connect the MoS2-CPs electrode with an external circuit. The electrolyte was 0.5 mol L-1 H2SO4 and 1.0 mol L-1 KOH solution, the counter and the reference electrodes were a platinum net and a saturated calomel electrode, respectively. Linear scanning voltammetry (LSV) measurements were performed in an N2-saturated electrolyte at a sweep rate of 0.5 mV s-1. Cyclic voltammetry (CV) was conducted from -0.2 to 0.2 V vs RHE at a scan rate of 10 mV s-1 in N2-saturated 0.5 mol L-1 H2SO4 solution to investigate the cycling stability. The electrochemical impedance spectroscopy (EIS) measurements for the MoS2-CPs and CPs 1000, 900, 800 were performed in N2-saturated 0.5 mol L-1 H2SO4 solution with the frequencies range from 10 KHz to 0.1 Hz with an AC voltage of 5 mV. For measurements on glassy carbon electrode, the MoS2-CPs were ground into fine powder. Then, 4 mg of the ground powder catalyst and 80 µL of 5 wt % Nafion solution were mixed in 1 mL of 4:1 v/v water/ethanol by sonication for 30 mins to form a homogeneous ink. Then, 7 µL of the slurry was loaded onto the surface of a glassy carbon (GC, 3 mm in diameter) electrode. And electrochemical tests were also performed in a three-electrode system like with MoS2-CPs
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measurement. All data of our bench-marked samples presented in this work were corrected for iR. All measurements were conducted at room temperature. Characterization The SEM measurements were performed on a FEI Nova NanoSEM 2300. The TEM and HRTEM measurements were taken with a JEM-2100F microscope operated at an acceleration voltage of 200 kV. The powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/Max 2550 X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). The XPS measurements were conducted on a Kratos Axis Ultra DLD spectrometer using a monochromated Al Kα radiation. The ICP measurements were conducted on a Perkin-Elmer Optima 3300DV inductively coupled plasma (ICP) spectrometer for the elemental analysis. Raman spectra were acquired using a Thermo Fisher DXR Raman microscope with a 532 nm wavelength incident laser.
Results and discussion MoS2-CPs were synthesized by direct transformation of biomass BC films (Figure 1a) into nitrogen-doped 3D graphene based carbon papers/films and facile solution-phase deposition of MoS2 nanosheets (Figure S1). The as-obtained composites are free standing films (Figure 1b), which can be further cut into wanted shapes and sizes (Figure S1) for specific tasks. The thickness of the paper was reduced from 2.5 mm to 1.5 mm after such a two-step process (Figure 1b), whilst the size of the as-formed film retained only 50% of its original size (BC pellicles). Scanning electron microscopy (SEM) image of the cross section of the as-obtained MoS2-CPs film (Figure 1c) exhibits a three-dimensional hierarchical structure composed of holey layers. From the face view of the film (Figure 1d), both nanofibers and wrinkled graphene nanosheets can be seen. The transition electron microscopy (TEM) image of the CPs sample indicated highly integrated structure of the nanofiber and graphene nanosheet (Figure S2). The graphene nanosheets grew along both sides of the carbon nanofibers to form a 5
complex structure in nanoscale. The highly integrated structure principally reduced the interface resistance of the nanofiber and graphene nanosheet in CPs for electron transfer in potential electrochemical reactions.
Figure 1 Characterization of MoS2-CPs catalysts. Photos of the pristine BC pellicles (a) and the as-obtained MoS2-CPs films (b). SEM images of the cross section (c) and surface (d-e) of MoS2-CPs film with a 3D interconnect hierarchical structure. Compositional line profile (f) across a MoS2/carbon nanofiber@graphene complex, the subunit of MoS2-CPs. Detailed observation via SEM reveals the formation of tiny nanosheets, presumably MoS2 nanosheets, with a size smaller than 10 nm (Figure 1e and Figure S3) on the surface of the carbon frameworks. The X-ray diffraction (XRD) pattern (Figure S4) of the sample unambiguously reveals the presence of MoS2 phases in MoS2-CPs, corresponding to MoS2 (JCPDS 77-1716). The broad diffraction peaks of MoS2 also reflect the tiny size of these MoS2 nanosheets. Energy-dispersive X-ray spectroscopy (EDX) analysis confirms the 6
homogeneous distribution of Mo and S atoms across over the surface of few-layer graphenes in MoS2-CPs (Figure 1f), which was further confirmed by the Raman 3D mapping images of 2D band of graphene and A1g band (the out-of-plane vibration band) of MoS2 layers (Figure S5). The weight percentage of MoS2 species in MoS2-CPs was around 21.88% according to the ICP analysis. The highly integrated carbon frameworks and the homogeneous distribution of tiny MoS2 nanosheets may benefit their potential applications as catalysts or electrocatalysts. Inspired by the unique complex nanostructure, we initially assessed this catalyst towards the HER on GCE using a typical three-electrode setup in 0.5 mol L-1 H2SO4 electrolyte. The linear sweep voltammetry (LSV) was performed to estimate the HER activity over MoS2-CPs and a series of control samples including bare carbon papers (CPs), MoS2 nanoparticles, a mechanical mixture of MoS2 and bare CPs (MoS2+CPs) and the commercial 20 wt% Pt/C catalyst (Pt/C). As shown in Figure S6, the overpotential (at 10 mA cm-2) of MoS2-CPs is more positive than the overpotentials in the following order: MoS2-CPs << MoS2+CPs << MoS2 << CPs, rather speaking for a synergetic effect of MoS2 nanosheets and CPs on the HER activity. As seen in the Tafel plot tested on GCE (Figure S7), MoS2-CPs yielded much higher activity with a Tafel slope of 41 mV per decade than that of MoS2+CPs. In brief, the complex structure of MoS2-CPs is crucial for their small overpotential at 10 mA cm-2 and low Tafel slope in acidic media for HER. The weight ratio of (NH4)2MoS4 precursor and CPs was further optimized to be 5:1 to achieve the best HER activity here (Figure S8). The carbon papers obtained at 1000 °C (CPs 1000) with less pyridine nitrogen heteroatoms (Figure S9) and thus the highest conductivity (Figure S10) were used as the best supports in this work for HER (Figure S11) [40]. All these results suggested an obvious synergistic effect of CPs and MoS2 components on significantly promoting the HER activity here.
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Figure 2 HER activity of MoS2-CPs catalysts. The photograph (a) taken during the electrocatalytic hydrogen reduction of the MoS2-CPs. Polarization curves (b) of MoS2-CPs and bare CPs in N2-saturated 0.5 mol L-1 H2SO4 electrolyte with a sweep rate of 0.5 mV s-1. Inset: Polarization curves of MoS2-CPs on GCE and directly use as work electrode. Polarization curves (c) of MoS2-CPs and bare CPs electrodes in N2-saturated 1.0 mol L-1 KOH electrolyte with a sweep rate of 0.5 mV s-1. EIS spectra (d) of MoS2-CPs and bare CPs at low frequency with -0.1 V vs RHE in N2-saturated 0.5 mol L-1 H2SO4 electrolyte. Inset: corresponding EIS spectra at high frequency. The loading of MoS2 species in MoS2-CPs: 0.875 mg cm-2. With respect to the practical application of nanocatalyts in HER, large-area films or papers are required to be directly used as electrodes. In our case, the as-obtained MoS2-CPs can function as large-area working electrodes for HER. MoS2-CPs obtained on CPs-1000 with a weight ratio of 5:1 ((NH4)2MoS4 precursor vs CPs) was used as the best electrocatalyst for all following tests. The size of MoS2-CPs was adaptable from several centimeters to tens of centimeters, only limited by the size of nitrogen oven used for synthesis. A large amount of H2 bubbles evolved on the surface of MoS2-CPs (Figure 2a and Movie S1), suggesting its excellent HER activity. The fact that bare CPs was inactive at low potential as shown in Figure 2b illustrated the proton reduction kinetics was mainly originated from the supported 8
MoS2 nanosheets. The amount of hydrogen gas evolved in the electrolysis cell was also measured with the Faraday efficiencies higher than 96% in both acid and base electrolytes (Figure S12, S13). Apart from acid and basic electrolyte, MoS2-CPs hybrid paper also offered good HER performance even in neutral solution (Figure S14). After excluding the obvious effect of sweep rate on HER activity (Figure S15, S16), we estimated the accurate overpotentials from polarization curve obtained at a slow sweep rate (0.5 mV s-1) with negligible background current for further comparison with bench-marked paper electrodes. Note that polarization curves tested under fast sweep rate showed a larger background current, but nearly the same current output at more negative potentials. Obviously, the background current was attributed to the accumulation of electrostatic charge at the interface as confirmed by further cyclic voltammogram results (Figure S15b, S16b) [35, 36]. Until now, most nanocatalysts for HER were mainly dispersed into liquid ink with the presence of binder and coated on commercial carbon paper composed of micrometer fibers. As a result, more negative overpotentials of these nanocatalysts were usually obtained on carbon paper as compared with the overpotentials tested on GCE. The increased boundary resistance of the thick layer of nanocatalysts in large-area paper electrode depressed their HER activity. In contrary, our MoS2-CPs exhibited a more positive overpotential, when directly used as working electrode, than that on GCE (Figure 2b inset). MoS2-CPs catalyst also afforded the most positive overpotential (-80 mV vs RHE) under 10 mA cm-2 (Figure S17a) and the highest mass activity (54 mA mg-1) at -150 mV vs RHE (Figure S17b) among all benchmarked chalcogenide based HER catalysts in the literature [34,37,38,41,42]. It should be noted that nanocatalysts available in the literature usually gave relatively lower HER activity in basic media because of the limited amount of hydrogen ions for proton reduction reaction in basic solution. Only several examples of nanocatalysts had been tested for HER in basic media. Our MoS2-CPs hybrid paper showed markedly higher HER activity (Figure 2c) as compared with those reported nanocatalysts (Figure S18) [43-45]. To be 9
precise, our hybrid catalysts only required ∼186 mV to achieve 10 mA cm-2, performing far better than other HER catalysts in basic solution (Figure S18). Thus, this work represented a new break for advanced MoS2 electrocatalysts highly performed in a basic media for HER.
Figure 3 HER stability of MoS2-CPs catalysts. Cycling stability (a) of MoS2-CPs at -0.2 V vs RHE in 0.5 mol L-1 H2SO4 electrolyte before and after 1,000 cycles. Cycling stability (b) of MoS2-CPs at -0.4 V vs RHE in 1.0 mol L-1 KOH electrolyte before and after 1,000 cycles. The MoS2 catalyst loading in MoS2-CPs: 0.875 mg cm-2. Durability of catalysts should be one of the most important aspects for their real applications. To investigate the electrochemical stability of MoS2-CPs catalysts, we carried out a long-term test in both acid and basic electrolytes. Note that the release of hydrogen gas bubbles from the surface of the MoS2-CPs electrode as shown in Figure 2a and Movie S1 slightly changed the total area of the electrode and thus slightly disturbed the current intenstiy. However, the stability and thus activity of MoS2-CPs electrode was not disturbed too much. As shown in Figure S19a, the polarization curves of MoS2-CPs exhibited negligible degradation after 1000 cycles, suggesting the superior stability of MoS2-CPs. Accordingly, the current density of MoS2-CPs decreased slightly to 97% of the initial value after 1000 cycles (Figure 3a). For use in a basic environment, MoS2-CPs catalysts could also retain the polarization curves nearly identical to those before potential cycling (Figure S19b). The cathode current density merely changed by 4.0 mA cm-2 after 1000 cycles for MoS2-CPs, indicating that degradation hardly occurred in basic electrolyte solution (Figure 3b). The XPS spectra (Figure S20) and XRD pattern (Figure S21) of used MoS2-CPs further confirmed their 10
electrochemical stability with the structure preverved well after HER test. All these results revealed the excellent durability of MoS2-CPs for HER in the both acid and basic solution.
Figure 4 Real structure of the MoS2-CPs catalysts. HRTEM image (a) of MoS2-CPs. HRTEM images (b) of MoS2-CPs with different layer numbers (defects were marked with yellow arrows; scale bar: 5 nm). The layer number distribution (c) of MoS2 nanosheets in MoS2-CPs. Schematic models (d) of MoS2-CPs. Raman spectra (e) and high resolution XPS Mo 3d spectra (f) of MoS2 and MoS2-CPs. To achieve a better understanding on the superior performance of MoS2-CPs catalysts in HER, we further analyzed the detailed structural features to elucidate the effect of the defect rich ultrathin MoS2 nanosheets and the highly coupled interaction with CPs for HER. High resolution TEM (HRTEM) images of MoS2-CPs revealed the formation of patched MoS2 nanosheets with sizes less than 20 nm (Figure 4a) on the surface of carbon nanofibers and graphene nanosheets (Figure S22). According to the detailed HRTEM observation (Figure 4b), the layer numbers of MoS2 were estimated to be 1 to 7 layers with 2-3 layered MoS2 nanosheets as the major (Figure 4c). This is to say that the mean thickness of deposited MoS2 11
nanosheets is 1-2 nm, which is much thinner than the thickness of MoS2 nanosheets deposited on reduced graphite oxides [46]. Moreover, the red shift of in-plane E12g vibration and the blue shift of out-of-plane A1g vibration in the Raman spectra of MoS2-CPs (Figure 4e) as compared with bare MoS2 further confirmed the formation of few-layered MoS2 sheets [47]. The orientation of MoS2 nanodomains are disordered and irregular (Figure 4a and Figure S22) with a great amount defects inside or on the surface of these MoS2 nanosheets (bottom image of Figure 4b). It is obvious that the defect-rich and tiny MoS2 nanosheets with ultra-thin structure (only 2-3 layers) as depicted in Figure 4d could ensure the abundance of exposed edges and defects of the MoS2 layers for HER. Most importantly, the strong coupled interface between MoS2 nanosheets and graphene layers may also contribute to the high HER activity of the MoS2-CPs. The strong interaction between MoS2 nanosheets and graphene layers was obviously reflected by the homogeneous distribution of the ultra-thin MoS2 nanosheets (thickness: 1-2 nm), whilst aggregates of large and thick MoS2 layers (thickness: 7-12 nm, size: 70-300 nm) formed without the presence of CPs support (Figure S23). The nitrogen heteroatoms (6 at.%, Figure S9) in the carbon frameworks of CPs could also act as electron-rich binding sites for constructing structurally tightly bonded interface of MoS2 nanosheets and CPs [48]. A rectifying contact between the semiconductor (MoS2 nanosheets) and semimetal (graphene) was possible with the electrons flowing at the interface to equilibrate the Fermi levels [49,50]. This can be confirmed by the XPS spectra of MoS2-CPs with Mo 3d (Figure 4f) and S 2p (Figure S24) peaks shifted toward lower energy as compared with those of bare MoS2. Such a highly coupled MoS2-CPs interface could not only lead to the formation of ultra-thin MoS2 nanosheets, but also reduce possible bounadry resistance and facilitate the electron transfer from the electrode via carbon CPs to MoS2 active cites [51-54]. More importantly, the hierachical structure of CPs could further promote mass transfer and hydrogen gas release. Indeed, Nyquist plots (Figure 2d) of the MoS2-CPs and CPs with small semi-circles (Figure 2d inset) at high frequency region 12
indicated that the introduction of MoS2 nanosheets didn’t disturb the conductivity of CPs obviously. Meanwhile, the electron transfer resistance decreased obviously with increasing overpotentials (Figure S25), rather speaking for a fast electron transfer process in MoS2-CPs [55]. At low frequency region, a vertical straight line of MoS2-CPs suggested the fast ion diffusion behavior of MoS2-CPs comparable to bare CPs. All these results directed the key importance of support effect to the formation of ultrathin MoS2 nanosheetsand thus the superior HER activity.
Conclusion In conclusion, we rationally constructed 3D MoS2-CPs with ultra-fine MoS2 nanosheets deposited on a highly integrated graphene-carbon nanofiber papers for highly efficient HER. The strong interaction between MoS2 species and graphene with tunable dopant concentration (exemplified with nitrogen heteroatom here) ensure the formation of patched few-layer MoS2 nanosheets and thus the abundance of exposed edges and defects as electrocatalytic active cites for HER. An obvious synergetic effect of active MoS2 nanosheets and highly conductive hierarchical carbon framework afforded the as-obtained MoS2-CPs superior HER activity for direct use as working electrode or on GCE, surpassing benchmarked Mo-based catalysts available for HER. Moreover, such a 3D hybrid paper catalyst also offered an excellent durability during long-term cycling both in acid and basic medium. The findings above open up new prospects for designing an ideal electrocatalytic device by strongly coupling highly active noble-metal like disulfide nanomaterials with highly conductive monoliths of graphenes and other carbon nanoallotropes into working electrodes, replacing commercial carbon paper supported Pt catalyst for HER. We also believe that a family of highly coupled metal disulfide-CPs nano-dyades can find wide applications in different fields of catalysis.
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Acknowledgements We gratefully acknowledge the financial support from the National Basic Research Program of China (2013CB934102, 2011CB808703), the National Natural Science Foundation of China (21331004, 21301116), SJTU-UM joint grant and Eastern Scholar Program.
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Highlights 3D hybrid papers of tiny MoS2 nanosheets coupled with graphene-carbon nanofiber paper (MoS2-CPs) can function as large-area cathodes for hydrogen evolution reaction (HER). The defect-rich MoS2 nanosheets and 3D-porous conductive frameworks make the MoS2CPs exhibit superior electrocatalytic performance and long-term operation stability in both acid and basic medium for HER. The MoS2-CPs electrode overcomes the drawbacks of presently GCE for HER and opens up a wide horizon of noble-metal-free HER catalysts for diverse practical usages.
Tian-Nan Ye received his B.S. degree in School of Material Science and Engineering from Tianjin Polytechnic University. And now he is a Ph.D. candidate in the School of Chemistry and Chemical Engineering at Shanghai Jiao Tong University in Prof. JieSheng Chen’s research group. His research interest is primarily focused on the inorganic nanomaterials synthesis specially carbon and graphene based functional nanomaterials and seeks to exploit their novel properties for energy and environmental science.
Li-Bing Lv completed his B.S. degree at the School of Chemistry and Chemical Engineering in Shanghai Jiao Tong University from 2009 to 2013. Then he joined the Prof. Jie-Sheng Chen’s group continuing his PhD from 2013 to now. His current scientific interest is mainly focus on the synthesis of graphene and its catalytic properties. 18
Miao Xu is a Ph.D. candidate in the School of Chemistry and Chemical Engineering at Shanghai Jiao Tong University. Her research interests include the development of novel photocatalysts for water splitting, electrocatalysis.
Bing Zhang received his B.S. degree in polymeric materials and engineering from Ocean University of China during 2006-2010. Then he received M.E. degree from Zhejiang University in 2013 and his research project was biomimetic polymer materials for hair follicle regeneration. Now he is a Ph.D. candidate majoring in chemistry in Shanghai Jiao Tong University and his research interests are catalysts for photochemical hydrogen evolution and biomass-based carbon catalysts for electrochemical application.
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Prof. Kai-Xue Wang obtained his PhD degree in inorganic chemistry from Jilin University in 2002 and first worked as a postdoctoral researcher at University College Cork, Ireland and then JSPS research fellow at National Institute of Advanced Industrial Science and Technology, Japan, during 2003 – 2009. Since 2009, he has been a professor in the School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University. His research focuses on the design and preparation of functional materials for energy storage and conversion.
Dr. Juan Su completed each of her B.S. degree at the Department of Chemistry of Jilin University from 2004 to 2013, receiving her PhD in 2013 with Professor JieSheng Chen on porous functional semiconductor materials. Since 2013, she joined Prof. Kai-Xue Wang group as a postdoctor at the school of chemistry and chemical engineering of Shanghai Jiao Tong University in Shanghai. His current scientific interest is mainly focused on the controllable preparation of porous semiconductor materials and the microstructure-regulating of their functions for energy and environmental science.
Prof. Xin-Hao Li completed each of his academic degrees at the Department of Chemistry of the Jilin University from 1999 to 2009, receiving his PhD in 2009 with Professor Jie-Sheng Chen. He then joined in Prof. Markus Antonietti’s group as an Alexander von Humboldt Research Fellowship for postdoctoral researchers at the Max-Planck Institute of Colloids and Interfaces from 2009 to 2012. Since 2013, he has been a professor in the School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University. His current scientific interest is mainly focused on the synthesis of carbon nitride and graphene based functional materials for energy and environmental science. 20
Prof. Jie-Sheng Chen received his PhD degree from Jilin University in 1989 and worked as a postdoctoral fellow in the Royal Institution of Great Britain, the United Kingdom, from 1990 to 1994, and as a professor in the Department of Chemistry, Jilin University from 1994 to 2008. Since 2008, he has been a professor in the School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University. His research interest is the synthesis of solid compounds and composite materials with new structures and functions.
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Hierarchical carbon Nanopapers coupled with ultrathin MoS2 nanosheets: Highly efficient large-area Electrodes for Hydrogen evolution Tian-Nan Ye, Li-Bing Lv, Miao Xu, Bing Zhang, Kai-Xue Wang, Juan Su, Xin-Hao Li, Jie-Sheng Chen
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S2211-2855(15)00200-1 http://dx.doi.org/10.1016/j.nanoen.2015.04.033 NANOEN826
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Nano Energy
Received date: 2 February 2015 Revised date: 10 April 2015 Accepted date: 27 April 2015 Cite this article as: Tian-Nan Ye, Li-Bing Lv, Miao Xu, Bing Zhang, Kai-Xue Wang, Juan Su, Xin-Hao Li, Jie-Sheng Chen, Hierarchical carbon Nanopapers coupled with ultrathin MoS2 nanosheets: Highly efficient large-area Electrodes for Hydrogen evolution, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2015.04.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hierarchical Carbon Nanopapers Coupled with Ultrathin MoS2 Nanosheets: Highly Efficient Large-area Electrodes for Hydrogen Evolution Tian-Nan Ye, Li-Bing Lv, Miao Xu, Bing Zhang, Kai-Xue Wang, Juan Su, Xin-Hao Li* and Jie-Sheng Chen* School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China. Tel:+86 34201273. E-mail address: [email protected]; [email protected]. Abstract Design of large-area hybrid paper for directly using as electrodes in hydrogen evolution reaction (HER) can provide an efficient approach for the extension of electrochemical hydrogen industry. Here we construct MoS2 decorated hybrid carbon papers (MoS2-CPs) that consist of tiny MoS2 nanosheets coupled with 3D graphene-carbon nanofiber papers. MoS2CPs can function as large-area working electrodes for HER with a overpotential (at 10 mA/cm2) of 80 mV in acid media and 186 mV in basic media, surpassing the Mo-based catalysts ever reported thus far in acid and basic solution respectively. It is the highly coupled interface of carbon frameworks and MoS2 components that resulted in the formation of patched and few-layer MoS2 nanosheets with rather small size and thus ensured the abundance of exposed active edge sites. Stability tests through long-term potential cycles and extended electrolysis confirm the outstanding durability of MoS2-CPs in both acid and basic electrolytes. Keywords Molybdenum disulfide, Carbon paper, Large-area electrode, Hydrogen evolution, Graphenes.
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Introduction With the energy crisis becoming more and more serious, hydrogen is considered as one of the most promising candidates substituting the fossil energy [1]. The search for high stability and low-cost sustainable electrodes for the splitting of water into hydrogen gas is one of the noble missions of hydrogen energy [2,3]. Pt based catalyst, the best hydrogen evolution reaction (HER) catalyst, is often deposited on electrodes to promote electrochemical hydrogen production [4]. As the most electroactive catalyst, Pt catalyst just requires negligible overpotential, even at high reaction rates [5]. However, the high price and scarcity have critically impeded the extensive applications of these electrochemical techniques for practical preparation of hydrogen fuels. Recently, transition metal phosphides and chalcogenides as the non-precious metal catalysts have shown a striking electrocatalytic performance for HER [6-21]. Among these transition-metal based catalysts, layered molybdenum disulfide (MoS2) has been widely used in both industry and fundamental researches as a cheap alternative of platium with noblemetal-like catalytic activities [22-29]. There is a consensus that the catalytic sites of layered MoS2 mainly arises from the active edges of two-dimensional layers and the (0001) basal planes of MoS2 are catalytically inert. The bulk phase is relatively inert because the (0001) basal planes are the major on the surface [17]. Consequently, nanostructured MoS2 catalysts have been well engineered to maximize the amount of exposed edges of the MoS2 layers and thus boost their catalytic activity, including HER activity [30-32]. With respect to the practical use of those MoS2 nanocatalysts, large-area electrodes with mesoscale to microscale pores are essential for releasing hydrogen bubbles via HER. Considering the intrinsic low electrical conductivity of MoS2 with a band gap of 1.69 eV, the resistance of MoS2 nanomaterials could be further elevated by a factor of several orders of magnification due to the increased number of grain boundaries in nanoscale [33]. Current efforts are thus mainly focused on painting the ink of nanocatalysts with binder to a 2
conductive substrate (e.g. commercial carbon papers, Ni foams or even graphene-coated Ni foams) with microscale subunits to fabricate large-area MoS2 thick films as practical electrodes [34-36]. Such a process obviously lead to a decrease in the HER activity as compared with that over glass carbon electrode (GCE), presumably due to the poor distribution of the MoS2 nanocatalysts over the support [34]. Direct deposition of defect-rich and ultra-fine MoS2 nanocatalysts on large-area conductive paper composed of nanoscale subunits promises great room for significantly elevating the HER activity of noble-metal-free electrocatalysts for practical applications [35,37-39]. Herein, we highlighted the fabrication of ultra-thin MoS2 nanosheets decorated carbon papers (MoS2-CPs) as large-area electrodes for HER. The defect-rich MoS2 nanosheets with a size smaller than 15 nm and a mean layer number around 2-3 layers can function as electrocatalytic active sites here. The highly coupled MoS2-CPs hybrid papers show superb activity and stability in both acid and basic media when directly used as electrode for HER.
Experimental Section CPs aerogels synthesis The bacterial cellulose (BC) pellicles were kindly provided by Ms CY Zhong (Hainan Yeguo Foods Co., Ltd., Hainan, China). In a typical synthesis, the BC pellicles were first washed with deionized water and then dipped in a urea aqueous solution for 24 h. After frozen by liquid nitrogen and dried in a bulk dryer, white BC aerogels were generated. The resulting BC aerogels were transferred into a crucible with flowing-nitrogen and heated at 600 °C at a rate of 1 °C min–1, kept this temperature for 1 h. This was followed by further heating at a rate of 1 °C min–1 to reach a temperature of 1000 °C and maintaining at that temperature for 1 h. The sample was then cooled naturally to room temperature within nitrogen gas, resulting in black and ultralight carbon papers (CPs).
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Preparation of MoS2-CPs Briefly, 0.1 mL of N2H4·H2O and 55 mg of (NH4)2MoS4 were dispersed into 10 mL of DMF solution containing under sonication. The as-obtained CPs (weight: 11 mg) were dipped into the mixed solution. All these precursors were then transferred into a 100 mL Teflon-lined autoclave and heated at 200 °C for 12 h. The as-formed MoS2-CPs were washed with distilled water and ethanol repeatedly for 6 times to remove DMF and free-standing nanoparticles. Electrochemical measurements Firstly, the MoS2-CPs were used as direct electrodes for HER. A standard caliper was employed to define the 1 cm2 electrode area by cutting scissors. The loading of MoS2 species in MoS2-CPs was estimated to be 0.875 mg cm-2 via inductively coupled plasma (ICP) analysis. And the polytetrafluoroethene wapped platinum wire clip was used to connect the MoS2-CPs electrode with an external circuit. The electrolyte was 0.5 mol L-1 H2SO4 and 1.0 mol L-1 KOH solution, the counter and the reference electrodes were a platinum net and a saturated calomel electrode, respectively. Linear scanning voltammetry (LSV) measurements were performed in an N2-saturated electrolyte at a sweep rate of 0.5 mV s-1. Cyclic voltammetry (CV) was conducted from -0.2 to 0.2 V vs RHE at a scan rate of 10 mV s-1 in N2-saturated 0.5 mol L-1 H2SO4 solution to investigate the cycling stability. The electrochemical impedance spectroscopy (EIS) measurements for the MoS2-CPs and CPs 1000, 900, 800 were performed in N2-saturated 0.5 mol L-1 H2SO4 solution with the frequencies range from 10 KHz to 0.1 Hz with an AC voltage of 5 mV. For measurements on glassy carbon electrode, the MoS2-CPs were ground into fine powder. Then, 4 mg of the ground powder catalyst and 80 µL of 5 wt % Nafion solution were mixed in 1 mL of 4:1 v/v water/ethanol by sonication for 30 mins to form a homogeneous ink. Then, 7 µL of the slurry was loaded onto the surface of a glassy carbon (GC, 3 mm in diameter) electrode. And electrochemical tests were also performed in a three-electrode system like with MoS2-CPs
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measurement. All data of our bench-marked samples presented in this work were corrected for iR. All measurements were conducted at room temperature. Characterization The SEM measurements were performed on a FEI Nova NanoSEM 2300. The TEM and HRTEM measurements were taken with a JEM-2100F microscope operated at an acceleration voltage of 200 kV. The powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/Max 2550 X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). The XPS measurements were conducted on a Kratos Axis Ultra DLD spectrometer using a monochromated Al Kα radiation. The ICP measurements were conducted on a Perkin-Elmer Optima 3300DV inductively coupled plasma (ICP) spectrometer for the elemental analysis. Raman spectra were acquired using a Thermo Fisher DXR Raman microscope with a 532 nm wavelength incident laser.
Results and discussion MoS2-CPs were synthesized by direct transformation of biomass BC films (Figure 1a) into nitrogen-doped 3D graphene based carbon papers/films and facile solution-phase deposition of MoS2 nanosheets (Figure S1). The as-obtained composites are free standing films (Figure 1b), which can be further cut into wanted shapes and sizes (Figure S1) for specific tasks. The thickness of the paper was reduced from 2.5 mm to 1.5 mm after such a two-step process (Figure 1b), whilst the size of the as-formed film retained only 50% of its original size (BC pellicles). Scanning electron microscopy (SEM) image of the cross section of the as-obtained MoS2-CPs film (Figure 1c) exhibits a three-dimensional hierarchical structure composed of holey layers. From the face view of the film (Figure 1d), both nanofibers and wrinkled graphene nanosheets can be seen. The transition electron microscopy (TEM) image of the CPs sample indicated highly integrated structure of the nanofiber and graphene nanosheet (Figure S2). The graphene nanosheets grew along both sides of the carbon nanofibers to form a 5
complex structure in nanoscale. The highly integrated structure principally reduced the interface resistance of the nanofiber and graphene nanosheet in CPs for electron transfer in potential electrochemical reactions.
Figure 1 Characterization of MoS2-CPs catalysts. Photos of the pristine BC pellicles (a) and the as-obtained MoS2-CPs films (b). SEM images of the cross section (c) and surface (d-e) of MoS2-CPs film with a 3D interconnect hierarchical structure. Compositional line profile (f) across a MoS2/carbon nanofiber@graphene complex, the subunit of MoS2-CPs. Detailed observation via SEM reveals the formation of tiny nanosheets, presumably MoS2 nanosheets, with a size smaller than 10 nm (Figure 1e and Figure S3) on the surface of the carbon frameworks. The X-ray diffraction (XRD) pattern (Figure S4) of the sample unambiguously reveals the presence of MoS2 phases in MoS2-CPs, corresponding to MoS2 (JCPDS 77-1716). The broad diffraction peaks of MoS2 also reflect the tiny size of these MoS2 nanosheets. Energy-dispersive X-ray spectroscopy (EDX) analysis confirms the 6
homogeneous distribution of Mo and S atoms across over the surface of few-layer graphenes in MoS2-CPs (Figure 1f), which was further confirmed by the Raman 3D mapping images of 2D band of graphene and A1g band (the out-of-plane vibration band) of MoS2 layers (Figure S5). The weight percentage of MoS2 species in MoS2-CPs was around 21.88% according to the ICP analysis. The highly integrated carbon frameworks and the homogeneous distribution of tiny MoS2 nanosheets may benefit their potential applications as catalysts or electrocatalysts. Inspired by the unique complex nanostructure, we initially assessed this catalyst towards the HER on GCE using a typical three-electrode setup in 0.5 mol L-1 H2SO4 electrolyte. The linear sweep voltammetry (LSV) was performed to estimate the HER activity over MoS2-CPs and a series of control samples including bare carbon papers (CPs), MoS2 nanoparticles, a mechanical mixture of MoS2 and bare CPs (MoS2+CPs) and the commercial 20 wt% Pt/C catalyst (Pt/C). As shown in Figure S6, the overpotential (at 10 mA cm-2) of MoS2-CPs is more positive than the overpotentials in the following order: MoS2-CPs << MoS2+CPs << MoS2 << CPs, rather speaking for a synergetic effect of MoS2 nanosheets and CPs on the HER activity. As seen in the Tafel plot tested on GCE (Figure S7), MoS2-CPs yielded much higher activity with a Tafel slope of 41 mV per decade than that of MoS2+CPs. In brief, the complex structure of MoS2-CPs is crucial for their small overpotential at 10 mA cm-2 and low Tafel slope in acidic media for HER. The weight ratio of (NH4)2MoS4 precursor and CPs was further optimized to be 5:1 to achieve the best HER activity here (Figure S8). The carbon papers obtained at 1000 °C (CPs 1000) with less pyridine nitrogen heteroatoms (Figure S9) and thus the highest conductivity (Figure S10) were used as the best supports in this work for HER (Figure S11) [40]. All these results suggested an obvious synergistic effect of CPs and MoS2 components on significantly promoting the HER activity here.
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Figure 2 HER activity of MoS2-CPs catalysts. The photograph (a) taken during the electrocatalytic hydrogen reduction of the MoS2-CPs. Polarization curves (b) of MoS2-CPs and bare CPs in N2-saturated 0.5 mol L-1 H2SO4 electrolyte with a sweep rate of 0.5 mV s-1. Inset: Polarization curves of MoS2-CPs on GCE and directly use as work electrode. Polarization curves (c) of MoS2-CPs and bare CPs electrodes in N2-saturated 1.0 mol L-1 KOH electrolyte with a sweep rate of 0.5 mV s-1. EIS spectra (d) of MoS2-CPs and bare CPs at low frequency with -0.1 V vs RHE in N2-saturated 0.5 mol L-1 H2SO4 electrolyte. Inset: corresponding EIS spectra at high frequency. The loading of MoS2 species in MoS2-CPs: 0.875 mg cm-2. With respect to the practical application of nanocatalyts in HER, large-area films or papers are required to be directly used as electrodes. In our case, the as-obtained MoS2-CPs can function as large-area working electrodes for HER. MoS2-CPs obtained on CPs-1000 with a weight ratio of 5:1 ((NH4)2MoS4 precursor vs CPs) was used as the best electrocatalyst for all following tests. The size of MoS2-CPs was adaptable from several centimeters to tens of centimeters, only limited by the size of nitrogen oven used for synthesis. A large amount of H2 bubbles evolved on the surface of MoS2-CPs (Figure 2a and Movie S1), suggesting its excellent HER activity. The fact that bare CPs was inactive at low potential as shown in Figure 2b illustrated the proton reduction kinetics was mainly originated from the supported 8
MoS2 nanosheets. The amount of hydrogen gas evolved in the electrolysis cell was also measured with the Faraday efficiencies higher than 96% in both acid and base electrolytes (Figure S12, S13). Apart from acid and basic electrolyte, MoS2-CPs hybrid paper also offered good HER performance even in neutral solution (Figure S14). After excluding the obvious effect of sweep rate on HER activity (Figure S15, S16), we estimated the accurate overpotentials from polarization curve obtained at a slow sweep rate (0.5 mV s-1) with negligible background current for further comparison with bench-marked paper electrodes. Note that polarization curves tested under fast sweep rate showed a larger background current, but nearly the same current output at more negative potentials. Obviously, the background current was attributed to the accumulation of electrostatic charge at the interface as confirmed by further cyclic voltammogram results (Figure S15b, S16b) [35, 36]. Until now, most nanocatalysts for HER were mainly dispersed into liquid ink with the presence of binder and coated on commercial carbon paper composed of micrometer fibers. As a result, more negative overpotentials of these nanocatalysts were usually obtained on carbon paper as compared with the overpotentials tested on GCE. The increased boundary resistance of the thick layer of nanocatalysts in large-area paper electrode depressed their HER activity. In contrary, our MoS2-CPs exhibited a more positive overpotential, when directly used as working electrode, than that on GCE (Figure 2b inset). MoS2-CPs catalyst also afforded the most positive overpotential (-80 mV vs RHE) under 10 mA cm-2 (Figure S17a) and the highest mass activity (54 mA mg-1) at -150 mV vs RHE (Figure S17b) among all benchmarked chalcogenide based HER catalysts in the literature [34,37,38,41,42]. It should be noted that nanocatalysts available in the literature usually gave relatively lower HER activity in basic media because of the limited amount of hydrogen ions for proton reduction reaction in basic solution. Only several examples of nanocatalysts had been tested for HER in basic media. Our MoS2-CPs hybrid paper showed markedly higher HER activity (Figure 2c) as compared with those reported nanocatalysts (Figure S18) [43-45]. To be 9
precise, our hybrid catalysts only required ∼186 mV to achieve 10 mA cm-2, performing far better than other HER catalysts in basic solution (Figure S18). Thus, this work represented a new break for advanced MoS2 electrocatalysts highly performed in a basic media for HER.
Figure 3 HER stability of MoS2-CPs catalysts. Cycling stability (a) of MoS2-CPs at -0.2 V vs RHE in 0.5 mol L-1 H2SO4 electrolyte before and after 1,000 cycles. Cycling stability (b) of MoS2-CPs at -0.4 V vs RHE in 1.0 mol L-1 KOH electrolyte before and after 1,000 cycles. The MoS2 catalyst loading in MoS2-CPs: 0.875 mg cm-2. Durability of catalysts should be one of the most important aspects for their real applications. To investigate the electrochemical stability of MoS2-CPs catalysts, we carried out a long-term test in both acid and basic electrolytes. Note that the release of hydrogen gas bubbles from the surface of the MoS2-CPs electrode as shown in Figure 2a and Movie S1 slightly changed the total area of the electrode and thus slightly disturbed the current intenstiy. However, the stability and thus activity of MoS2-CPs electrode was not disturbed too much. As shown in Figure S19a, the polarization curves of MoS2-CPs exhibited negligible degradation after 1000 cycles, suggesting the superior stability of MoS2-CPs. Accordingly, the current density of MoS2-CPs decreased slightly to 97% of the initial value after 1000 cycles (Figure 3a). For use in a basic environment, MoS2-CPs catalysts could also retain the polarization curves nearly identical to those before potential cycling (Figure S19b). The cathode current density merely changed by 4.0 mA cm-2 after 1000 cycles for MoS2-CPs, indicating that degradation hardly occurred in basic electrolyte solution (Figure 3b). The XPS spectra (Figure S20) and XRD pattern (Figure S21) of used MoS2-CPs further confirmed their 10
electrochemical stability with the structure preverved well after HER test. All these results revealed the excellent durability of MoS2-CPs for HER in the both acid and basic solution.
Figure 4 Real structure of the MoS2-CPs catalysts. HRTEM image (a) of MoS2-CPs. HRTEM images (b) of MoS2-CPs with different layer numbers (defects were marked with yellow arrows; scale bar: 5 nm). The layer number distribution (c) of MoS2 nanosheets in MoS2-CPs. Schematic models (d) of MoS2-CPs. Raman spectra (e) and high resolution XPS Mo 3d spectra (f) of MoS2 and MoS2-CPs. To achieve a better understanding on the superior performance of MoS2-CPs catalysts in HER, we further analyzed the detailed structural features to elucidate the effect of the defect rich ultrathin MoS2 nanosheets and the highly coupled interaction with CPs for HER. High resolution TEM (HRTEM) images of MoS2-CPs revealed the formation of patched MoS2 nanosheets with sizes less than 20 nm (Figure 4a) on the surface of carbon nanofibers and graphene nanosheets (Figure S22). According to the detailed HRTEM observation (Figure 4b), the layer numbers of MoS2 were estimated to be 1 to 7 layers with 2-3 layered MoS2 nanosheets as the major (Figure 4c). This is to say that the mean thickness of deposited MoS2 11
nanosheets is 1-2 nm, which is much thinner than the thickness of MoS2 nanosheets deposited on reduced graphite oxides [46]. Moreover, the red shift of in-plane E12g vibration and the blue shift of out-of-plane A1g vibration in the Raman spectra of MoS2-CPs (Figure 4e) as compared with bare MoS2 further confirmed the formation of few-layered MoS2 sheets [47]. The orientation of MoS2 nanodomains are disordered and irregular (Figure 4a and Figure S22) with a great amount defects inside or on the surface of these MoS2 nanosheets (bottom image of Figure 4b). It is obvious that the defect-rich and tiny MoS2 nanosheets with ultra-thin structure (only 2-3 layers) as depicted in Figure 4d could ensure the abundance of exposed edges and defects of the MoS2 layers for HER. Most importantly, the strong coupled interface between MoS2 nanosheets and graphene layers may also contribute to the high HER activity of the MoS2-CPs. The strong interaction between MoS2 nanosheets and graphene layers was obviously reflected by the homogeneous distribution of the ultra-thin MoS2 nanosheets (thickness: 1-2 nm), whilst aggregates of large and thick MoS2 layers (thickness: 7-12 nm, size: 70-300 nm) formed without the presence of CPs support (Figure S23). The nitrogen heteroatoms (6 at.%, Figure S9) in the carbon frameworks of CPs could also act as electron-rich binding sites for constructing structurally tightly bonded interface of MoS2 nanosheets and CPs [48]. A rectifying contact between the semiconductor (MoS2 nanosheets) and semimetal (graphene) was possible with the electrons flowing at the interface to equilibrate the Fermi levels [49,50]. This can be confirmed by the XPS spectra of MoS2-CPs with Mo 3d (Figure 4f) and S 2p (Figure S24) peaks shifted toward lower energy as compared with those of bare MoS2. Such a highly coupled MoS2-CPs interface could not only lead to the formation of ultra-thin MoS2 nanosheets, but also reduce possible bounadry resistance and facilitate the electron transfer from the electrode via carbon CPs to MoS2 active cites [51-54]. More importantly, the hierachical structure of CPs could further promote mass transfer and hydrogen gas release. Indeed, Nyquist plots (Figure 2d) of the MoS2-CPs and CPs with small semi-circles (Figure 2d inset) at high frequency region 12
indicated that the introduction of MoS2 nanosheets didn’t disturb the conductivity of CPs obviously. Meanwhile, the electron transfer resistance decreased obviously with increasing overpotentials (Figure S25), rather speaking for a fast electron transfer process in MoS2-CPs [55]. At low frequency region, a vertical straight line of MoS2-CPs suggested the fast ion diffusion behavior of MoS2-CPs comparable to bare CPs. All these results directed the key importance of support effect to the formation of ultrathin MoS2 nanosheetsand thus the superior HER activity.
Conclusion In conclusion, we rationally constructed 3D MoS2-CPs with ultra-fine MoS2 nanosheets deposited on a highly integrated graphene-carbon nanofiber papers for highly efficient HER. The strong interaction between MoS2 species and graphene with tunable dopant concentration (exemplified with nitrogen heteroatom here) ensure the formation of patched few-layer MoS2 nanosheets and thus the abundance of exposed edges and defects as electrocatalytic active cites for HER. An obvious synergetic effect of active MoS2 nanosheets and highly conductive hierarchical carbon framework afforded the as-obtained MoS2-CPs superior HER activity for direct use as working electrode or on GCE, surpassing benchmarked Mo-based catalysts available for HER. Moreover, such a 3D hybrid paper catalyst also offered an excellent durability during long-term cycling both in acid and basic medium. The findings above open up new prospects for designing an ideal electrocatalytic device by strongly coupling highly active noble-metal like disulfide nanomaterials with highly conductive monoliths of graphenes and other carbon nanoallotropes into working electrodes, replacing commercial carbon paper supported Pt catalyst for HER. We also believe that a family of highly coupled metal disulfide-CPs nano-dyades can find wide applications in different fields of catalysis.
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Acknowledgements We gratefully acknowledge the financial support from the National Basic Research Program of China (2013CB934102, 2011CB808703), the National Natural Science Foundation of China (21331004, 21301116), SJTU-UM joint grant and Eastern Scholar Program.
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Highlights 3D hybrid papers of tiny MoS2 nanosheets coupled with graphene-carbon nanofiber paper (MoS2-CPs) can function as large-area cathodes for hydrogen evolution reaction (HER). The defect-rich MoS2 nanosheets and 3D-porous conductive frameworks make the MoS2CPs exhibit superior electrocatalytic performance and long-term operation stability in both acid and basic medium for HER. The MoS2-CPs electrode overcomes the drawbacks of presently GCE for HER and opens up a wide horizon of noble-metal-free HER catalysts for diverse practical usages.
Tian-Nan Ye received his B.S. degree in School of Material Science and Engineering from Tianjin Polytechnic University. And now he is a Ph.D. candidate in the School of Chemistry and Chemical Engineering at Shanghai Jiao Tong University in Prof. JieSheng Chen’s research group. His research interest is primarily focused on the inorganic nanomaterials synthesis specially carbon and graphene based functional nanomaterials and seeks to exploit their novel properties for energy and environmental science.
Li-Bing Lv completed his B.S. degree at the School of Chemistry and Chemical Engineering in Shanghai Jiao Tong University from 2009 to 2013. Then he joined the Prof. Jie-Sheng Chen’s group continuing his PhD from 2013 to now. His current scientific interest is mainly focus on the synthesis of graphene and its catalytic properties. 18
Miao Xu is a Ph.D. candidate in the School of Chemistry and Chemical Engineering at Shanghai Jiao Tong University. Her research interests include the development of novel photocatalysts for water splitting, electrocatalysis.
Bing Zhang received his B.S. degree in polymeric materials and engineering from Ocean University of China during 2006-2010. Then he received M.E. degree from Zhejiang University in 2013 and his research project was biomimetic polymer materials for hair follicle regeneration. Now he is a Ph.D. candidate majoring in chemistry in Shanghai Jiao Tong University and his research interests are catalysts for photochemical hydrogen evolution and biomass-based carbon catalysts for electrochemical application.
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Prof. Kai-Xue Wang obtained his PhD degree in inorganic chemistry from Jilin University in 2002 and first worked as a postdoctoral researcher at University College Cork, Ireland and then JSPS research fellow at National Institute of Advanced Industrial Science and Technology, Japan, during 2003 – 2009. Since 2009, he has been a professor in the School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University. His research focuses on the design and preparation of functional materials for energy storage and conversion.
Dr. Juan Su completed each of her B.S. degree at the Department of Chemistry of Jilin University from 2004 to 2013, receiving her PhD in 2013 with Professor JieSheng Chen on porous functional semiconductor materials. Since 2013, she joined Prof. Kai-Xue Wang group as a postdoctor at the school of chemistry and chemical engineering of Shanghai Jiao Tong University in Shanghai. His current scientific interest is mainly focused on the controllable preparation of porous semiconductor materials and the microstructure-regulating of their functions for energy and environmental science.
Prof. Xin-Hao Li completed each of his academic degrees at the Department of Chemistry of the Jilin University from 1999 to 2009, receiving his PhD in 2009 with Professor Jie-Sheng Chen. He then joined in Prof. Markus Antonietti’s group as an Alexander von Humboldt Research Fellowship for postdoctoral researchers at the Max-Planck Institute of Colloids and Interfaces from 2009 to 2012. Since 2013, he has been a professor in the School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University. His current scientific interest is mainly focused on the synthesis of carbon nitride and graphene based functional materials for energy and environmental science. 20
Prof. Jie-Sheng Chen received his PhD degree from Jilin University in 1989 and worked as a postdoctoral fellow in the Royal Institution of Great Britain, the United Kingdom, from 1990 to 1994, and as a professor in the Department of Chemistry, Jilin University from 1994 to 2008. Since 2008, he has been a professor in the School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University. His research interest is the synthesis of solid compounds and composite materials with new structures and functions.
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