Unraveling the factors affecting electrochemical performance of MoS2

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Energy, Environmental, and Catalysis Applications

Unraveling the factors affecting electrochemical performance of MoS-carbon composite catalysts for hydrogen evolution reaction: Surface defect and electrical resistance of carbon supports 2

Yuhwan Hyeon, Su-Ho Jung, Wonseok Jang, Mansu Kim, Byung-Sung Kim, Jaehyun Lee, Koteeswara Reddy Nandanapalli, Namgee Jung, and Dongmok Whang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19072 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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ACS Applied Materials & Interfaces

Unraveling the factors affecting electrochemical performance of MoS2-carbon composite catalysts for hydrogen evolution reaction: Surface defect and electrical resistance of carbon supports Yuhwan Hyeon,†, ‖ Su-Ho Jung,†, ‖ Wonseok Jang,‡ Mansu Kim,‡ Byung-Sung Kim,§ Jae-Hyun Lee,¶ Koteeswara Reddy Nandanapalli,‡ Namgee Jung,⁋, * and Dongmok Whang†, ‡, *

†SKKU

Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 16419, Republic of Korea

‡School

of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea

§Department

of Engineering Science, University of Oxford, Oxford OX1 3PJ, United Kingdom

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

¶Department

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of Energy Systems Research and Department of Materials Science and

Engineering, Ajou University, Suwon 16499, Republic of Korea

⁋Graduate

School of Energy Science and Technology (GEST), Chungnam National University, Daejeon 34148, Republic of Korea

Keyword: MoS2-carbon composite catalyst, carbon support, surface defect, electrical resistance, hydrogen evolution reaction

ABSTRACT

In MoS2-carbon composite catalysts for hydrogen evolution reaction (HER), the carbon materials generally act as supports to enhance the catalytic activity of MoS2 nanosheets. The carbon support provides a large surface area for increasing the MoS2 edge site density and its physical structure can affect the electron transport rate in the composite catalysts. However, despite the importance of the carbon materials, direct observation of the effects of the physical properties of the carbon supports on the HER activity of MoS2-


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carbon composite catalysts has been hardly reported. In this work, we conduct an experimental model study to find the fundamental and important understanding of the correlation between the structural characteristics of carbon supports and the HER performance of MoS2-carbon composite catalysts using surface-modified graphitic carbon shell (GCS)-encapsulated SiO2 nanowires (GCS@SiO2 NWs) as support materials for MoS2 nanosheets. The surface defect density and the electrical resistance of GCS@SiO2 NWs are systematically modulated by control of H2 gas flow rates during the carbon shell growth on the SiO2 NWs. From in-depth characterization of the model catalysts, it is confirmed that the intrinsic catalytic activity of MoS2-carbon composites for the HER is improved linearly with the conductance of the carbon supports regardless of the MoS2 edge site density. However, in the HER polarization curve, the apparent current density increases in proportion to the product of the number of MoS2 edge sites and the conductance of GCS@SiO2 NWs.


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1. INTRODUCTION

As global warming becomes more serious due to increased CO2 emissions by combustion of fossil fuels, concerns about energy resources are continuously growing.1,2 Proton exchange membrane fuel cells (PEMFCs) using H2 gas as a fuel are one of the alternative energy conversion devices which can meet the increased energy demands while preserving the environment because the fuel cells produce electricity without any pollutant.3,4 However, in PEMFCs operating at low temperature (60 ~ 80 ℃), highly pure H2 fuel should be supplied into the anode because Pt catalysts can be easily poisoned by a few CO residues contained in H2 gas reformed from hydrocarbons, resulting in a severe performance degradation.5,6 Therefore, water electrolysis, as a carbon-free H2 production method, has been intensively studied to directly produce pure H2 gas from water.7 However, since very expensive Pt is the most efficient catalyst to convert proton to H2 gas in the present water electrolysis system, non-noble catalysts such as MoS2, Ni2P, and Mo2C have been widely studied to replace the Pt catalyst.8-10 Among them, MoS2 has


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been considered as one of the best candidates because it has an interesting electronic structure showing a similar H binding energy as Pt.11,12

Technical strategies to design highly efficient MoS2-based catalysts focus on two key factors, i.e. (i) increasing the number of active edge sites; (ii) enhancing the electron transport property. First, to extend the active edge sites of MoS2, various edgeengineering strategies have been extensively proposed. For example, Kibsgaard et al. developed a highly ordered double-gyroid MoS2 bicontinuous architecture with preferentially exposed active edge sites that led to outstanding improvement of HER activity.13 In addition, Xie et al. reported an interesting synthetic strategy controlling the concentration of precursors to fabricate defect-rich MoS2 sheets with a number of edge sites.14 Second, to enhance the conductivity of MoS2 itself, one can modify the basic MoS2 structure from a non-conductive 2H-MoS2 to a conductive 1T-MoS2 phase.15,16 The electrical conductivity of MoS2 can be affected by the structural arrangement of Mo and S atoms, which results in the intrinsically higher HER performance of 1T-MoS2 (octahedral coordination) compared to 2H-MoS2 (trigonal prismatic coordination). However, the


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technology using 1T-MoS2 catalysts should be further developed since the phase transform of MoS2 still requires a complicated process and 1T-MoS2 has a metastable structure.17

Alternatively, the poor electrical conductivity of MoS2 can be improved through the combination of conductive carbon-based materials (e.g., graphene, carbon nanotubes (CNTs), carbon nanofibers (CNFs), and so on) with MoS2.8,18,19 This approach also prevents the restacking and aggregation of active 2D materials such as MoS2, which greatly enhances the catalytic activity of MoS2 nanosheets. In this strategy, it is very important simultaneously to extend the active surface area of MoS2 for HER on the carbon surface and to ensure the conductivity of the composite materials. However, most of researchers have focused only on the increase in the number of edge-exposed MoS2 nanosheets loaded on the carbon supports, regardless of the electrical property of the carbon materials, since it is well known that the HER activity correlates linearly with the number of active edge sites on the MoS2 catalyst.12


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As typical methods for surface functionalization of carbon supports, for example, acid treatment or plasma etching process have been generally adopted to produce a number of functional groups (or defects sites) as nucleation and anchoring sites for MoS2 on the carbon surface.20,21 However, in many cases, despite the surface structure (e.g. defect site) and physical property (e.g. thermal and electrical conductivities) of carbon supports might be significantly changed by the surface modification,22-25 only the HER curves of the MoS2-carbon composite catalysts have been simply compared to show their catalyst performance without considering the impact of the structural change of carbon materials. Therefore, to develop MoS2-carbon composite catalysts suitable for HER, it is necessary to investigate the effects of the physical property and structure of carbon supports on the MoS2 formation and the catalytic performance as a model study since the number of active sites and the electron transport rate can be simultaneously changed depending on the carbon structure.


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Scheme 1. Schematic illustration of experimental strategy to study the effects of carbon support structure on MoS2 catalyst properties in HER.

In this work, we conducted an in-depth study to find the correlation between the surface structure of carbon supports and the HER performance of MoS2-carbon composite catalysts using surface-modified graphitic carbon shell (GCS)-encapsulated SiO2 nanowires (NWs) (GCS@SiO2 NWs) as support materials for MoS2 nanosheets (MoS2/GCS@SiO2). As shown in Scheme 1, the surface defect density and electrical resistance of GCS@SiO2 NWs were systematically controlled by changing the partial


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pressure of hydrogen (H2) gas during the growth of carbon layers on SiO2 NWs. The surface defect density of GCS had a great effect on the formation of catalytic active sites of MoS2 for the HER, and the electrical resistance of GCS@SiO2 NWs were dramatically changed depending on the carbon shell structure. Based on these experimental results, we have found a fundamental and important understanding that the intrinsic catalytic activity of MoS2-carbon composite catalyst for HER is more influenced by the surface structure and resistance of carbon supports than the active site density of MoS2. On the other hand, the apparent HER activity of MoS2-carbon composite catalysts is closely related to both the number of MoS2 active sites and the electrical property of carbon supports. Therefore, the present study will provide important information for the development of novel carbon support materials for high-performance MoS2 hybrid catalysts.

2. EXPERIMENTAL

2.1 Materials


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Titanium foil (Ti, 99.7 %, 0.25 mm thickness) and ammonium tetrathiomolybdate, (NH4)2MoS4 (99.97 %), were purchased from Sigma-Aldrich. Dimethylformamide (DMF, 99.8 %) was obtained from Daejung Chemical & Metals Co. Ltd. All reagents were used as received, without further purification.

2.2 Preparation of GCS@SiO2 NWs

Silicon NWs (Si NWs) were grown on Ti foil (2 cm × 2 cm) using the vapor–liquid–solid (VLS) method, as described in our previous articles reported elsewhere.26-28 5 nm of gold (Au) was deposited on the Ti foil by thermal evaporation and loaded into low pressure chemical vapor deposition (LPCVD) system. Au-catalyzed Si NWs were grown by silane (SiH4, 10 % in helium) gas at 500 ℃ and 40 Torr for 20 min. The as-grown structures were loaded into a muffle furnace and annealed at 800 ℃ in air ambient for overnight (~12 h) (Figure S1 shows the TEM images of before and after oxidizing Si NW). Then, the structures were transferred into LPCVD chamber. Subsequently, the temperature was increased to 1050 ℃ with a rate of 20 ℃/min under the flow of Ar (99.999 %) gas. The graphitic carbon layers were deposited by injecting methane (CH4, 99.999 %) and


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hydrogen (H2, 99.999 %) gases for 40 min, after the Ar gas was turned-off. The amount of H2 gas is 0, 1, 2, and 5 sccm, which were denoted as G0, G1, G2, and G5, respectively.

2.3 Synthesis of MoS2 nanosheets on GCS@SiO2 NWs

MoS2 nanostructures were synthesized on GCS@SiO2 NWs by using the solvothermal method. Initially, 10 mg of (NH4)2MoS4 was added to 20 mL of DMF, and the solution was transferred into a 40 mL Teflon liner after vigorous stirring for 30 min. The as-grown GCS@SiO2 structures were gently immersed in the solution, and then the Teflon liner was placed in a stainless-steel autoclave. The temperature of the autoclave was raised to 200 ℃ by keeping it in a muffle furnace and maintained for 12 h. After cooling down to room temperature, the samples were rinsed with copious deionized water, ethanol, and dried in a vacuum oven at 60 ℃. The same procedure was followed for all other samples.

2.4 Physical characterization

The structural morphology of GCS deposited SiO2 NWs and MoS2 nanostructures grown GCS@SiO2 core-shell NWs were analyzed with transmission electron microscopy (TEM,


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JEOL JEM ARM 200F), high resolution TEM (HRTEM), and selected area electron diffraction (SAED) techniques. Characteristics of GCS deposited on SiO2 NWs were examined by Raman spectroscopy (WITec Alpha300 M) using the 532 nm wavelength of a laser and probe-station attached with semiconductor characterization system (Keithley 4200-SCS). Energy dispersive X-ray spectrometer (EDS, Oxford AZtec) attached with TEM and X-ray photoelectron spectroscopy (XPS, ESCA 2000 with an Al Kα source) were used to estimate the elemental composition and chemical states of MoS2 nanostructures grown on GCS@SiO2 core-shell NWs along with their binding energies. To evaluate the electrical properties of GCS@SiO2 NWs, single nanowire-based devices were fabricated, and their I-V curves were obtained at room temperature. The resistances of the GCS@SiO2 NWs as support materials were calculated from the I-V curves using Ohm’s law.

2.5 Electrochemical characterization

The electrochemical properties of the samples were evaluated in a three-electrode system connected to an electrochemical station (Autolab PGSTAT302N). A saturated


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calomel electrode (SCE) and graphite rod were used as reference and counter electrodes, respectively. The as-prepared samples were used as working electrodes. To conduct electrochemical analysis, copper tape was fastened on the prepared sample using silver paste. All surfaces except the 0.5 × 0.5 cm2 areas were insulated using twopart epoxy resin. The potentials were corrected with that of a reversible hydrogen electrode (RHE). To calculate the active sites of MoS2 catalysts, cyclic voltammograms (CVs) were measured in pH = 7 phosphate buffer solution. After phosphate buffer solution was deaerated by N2 gas purging for 30 min, the potential was swept with a scan rate of 50 mV/s between -0.2 and 0.6 V. The HER polarization curves were measured in the N2saturated 0.5 M H2SO4 with a scan rate of 5 mV/s. All the data were recorded without IR compensation. Electrochemical impedance spectroscopy (EIS) was measured in 0.5 M H2SO4 at -0.2 V (vs. RHE) in the frequency range between 105 and 10-2 Hz.

3. RESULTS AND DISCUSSION


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In order to realize 3D nanostructured electrodes, SiO2 NWs were grown on a conductive Ti substrate, and GCSs were then coated on the SiO2 NWs through CVD techniques. The formation mechanism of the GCSs can be explained by the non-catalytic thermal pyrolysis of carbon sources (e.g. CH4) and their self-nucleation on the SiO2 surface.28-31 As shown in Figure 1a, these carbon atoms randomly nucleate on the SiO2 NW surfaces and start to form graphene domains (step 1). As the domains rapidly grow further, graphene edges are generally produced on the surface due to the misalignments between adjacent graphene domains (step 2). As the carbon layers are gradually stacked through the continuous growth of graphene, the top-most carbon layer covers the inner layers including the exposed graphene edges (step 3). Finally, after further prolongation of growth time, thick graphitic carbon shells with large and smooth ripples are entirely coated on the SiO2 NWs (step 4).

Meanwhile, we could successfully modify the surface defect density and thickness of GCS formed on the SiO2 NWs through the controlling the growth rate of the carbon shell by introduction of H2 gas during the GCS formation. As shown in Figure 1b-e, with


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increasing the H2 gas flow rate from 0 to 5 sccm, the number of GCS layers gradually decreases while the surface roughness increases due to the high contents of graphene edges. It is well known that H2 gas etches activated carbon sites during the fabrication of graphitic carbon layer, so the introduction of H2 molecules in GCS formation process slows down the growth rate of carbon layers.32 In addition, the graphene edges have hydrogen-terminated structures when the partial pressure of H2 gas increases, and the grain boundaries of the graphene layers can be more easily overlapped.33 That is, the higher the H2 gas flow, the slower the growth rate of carbon layers and the higher the ratio of graphene edges (defects) on the surface. Therefore, the number of graphene edges on the GCS surface could be systematically modulated by the control of H2 gas flow rate during the GCS formation on SiO2 NWs.


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Figure 1. (a) Schematic representation of growth steps of carbon layers and formation of edge sites on the surface of the SiO2 NW. TEM images of carbon layers grown on SiO2 NW under the H2 flow rate of (b) 0, (c) 1, (d) 2, and (e) 5 sccm, respectively. (f) Raman spectra of GCS grown on SiO2 NWs at different flow rates of H2.


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The Raman spectra show the characteristic peaks of sp2-hybridized carbon: D peak (~1350 cm-1), G peak (~1580 cm-1), and 2D peak (~2700 cm-1) (see Figure 1f). The intensity ratio of D and G peaks, ID/IG, of the GCS@SiO2 NW (G0) fabricated without addition of H2 gas implies that the carbon structure possesses a certain number of defect sites.34 As mentioned above, it was also revealed that the defect density in the GCS@SiO2 NWs increases due to the surface etching effect and hydrogen termination of graphene with increasing the flow rate of H2 gas used in the GCS formation. Based on the stage theory of Ferrari et al., all of the GCSs belongs to “stage 2” which is the intermediate form between amorphous and nanocrystalline carbon (ID/IG ratio → ~1, I2D/IG → ~0).35 This means that all the GCSs have lots of defect sites (e.g. vacancy and graphene edge). However, since the unavoidable defect sites can serve as nucleation and anchoring sites of active catalysts without any additional process to modify the carbon surface (e.g. acid or plasma treatments),20,21 it was considered that the number of defect site of the GCS@SiO2 NW (G0) was enough to be used as a support for realization of a hybrid structure with MoS2 nanosheets. In addition, from the change in the ID/IG ratios, it


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was clearly demonstrated that the surface defect densities of the GCS@SiO2 NWs were properly controlled depending on the H2 gas flow rates.

To fabricate the 3D-nanostructured electrodes for HER, MoS2 nanosheets were synthesized on the GCS@SiO2 NWs with different surface structures through the solvothermal reaction at 200 ℃. As shown in Figure 2a-d, it was confirmed that the defect densities of the GCS@SiO2 NWs had significant effects on the growth and structure of MoS2 since the number of edge-exposed MoS2 seemed to be quite different in the TEM images. The number of S-Mo-S edge sites is an important factor for improving the HER activity of MoS2 since the edge site is one of the most active sites in MoS2 nanosheets for the HER.12 Although, in MoS2 with a low crystallinity, the S vacancy on the basal plane as well as the edge site can be active for the HER, the number of S-Mo-S edge sites mainly contributes to the total number of active sites.36 That is, the total number of active sites is almost proportional to the number of edge sites. As expected, the higher the carbon defect density on the GCS@SiO2 NWs, the larger the amount of the exposure of the S-Mo-S edges. Although many edge sites of MoS2 were exposed even on the


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MoS2/GCS@SiO2 NW (G0) due to the existence of carbon defect sites, MoS2 edge structures were observed much more in the MoS2/GCS@SiO2 NW (G5) than MoS2/GCS@SiO2 NW (G0). This shows that the defect sites of the GCS@SiO2 NWs act as effective nucleation site for MoS2 and influence the formation of active edge sites of MoS2.

Figure 2. TEM images of MoS2 on the (a) GCS@SiO2 (G0), (b) GCS@SiO2 (G1), (c) GCS@SiO2 (G2), and (d) GCS@SiO2 (G5), respectively. (e) Cyclic voltammograms of


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MoS2/GCS@SiO2 electrodes in phosphate buffer solution with scan rate of 50 mV/s. (f) Voltammetric charges of MoS2/GCS@SiO2 electrodes calculated from (e).

To more clearly prove the change in the density of the active sites on the GCS@SiO2 NWs with different carbon surfaces, the electrochemically active surface area of MoS2 catalysts were investigated by measuring the CVs of the samples, as shown in Figure 2e.36 From the CVs and voltammetric charges of MoS2/GCS@SiO2 electrodes (Figure 2f), it was confirmed that the surface area of active MoS2 gradually increases from MoS2/GCS@SiO2 NW (G0) to MoS2/GCS@SiO2 NW (G5) (see the supporting information how to calculate the active sites). Therefore, we could physically control the density of the active sites by using the GCS@SiO2 NWs with different surface structures in the synthesis of MoS2. However, it was required to identify the chemical structure of MoS2 formed on the different carbon supports since the chemical states of the synthesized MoS2 catalysts might be also changed depending on the carbon surface structures.


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Therefore, the chemical bonding structure of MoS2 catalysts grown on the GCS@SiO2 NWs were thoroughly studied by XPS measurement (Figure 3a and S2). First, the XPS survey scan of the samples shows the existence of C, Mo, and S elements along with O (Figure S2). As shown in Figure 3a, the Mo 3d core-level XPS spectra of the samples exhibit two characteristic peaks centered at 228.8 and 232 eV, which are corresponding to the Mo 3d5/2 and Mo 3d3/2 binding energies of Mo4+. Similarly, the peaks indicated at 161.9 and 163 eV in the S 2p core-level XPS spectra belong to S 2p3/2 and S 2p1/2 binding energies of S2-. Also, the atomic ratios of S to Mo in MoS2/GCS@SiO2 (G0, G1, G2, and G5) samples were 2.13, 2.13, 2.15, and 2.13, respectively. Accordingly, it was thought that all the prepared MoS2 catalysts had similar chemical bonding and elemental composition despite the existence of different surface structure of GCSs,37,38 and we expected that the catalytic activity of MoS2 itself would be almost same.


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Figure 3. (a) Comparison of XPS spectrum among MoS2/GCS@SiO2 electrodes. (b) STEM image and corresponding EDS elemental line scan profiles of MoS2/GCS@SiO2 (G0) electrode. (violet: Si, black: C, cyan: O, red: Mo, and yellow: S) (c) HRTEM image of MoS2/GCS@SiO2 (G0) showing the lattice fringes of MoS2.

As shown in Figure 3b, from the EDS line profile measured by the HAADF STEM, the core-shell structure of the GCS@SiO2 NW were clearly identified, and it was confirmed


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that MoS2 sheets were properly loaded on the carbon shells. Meanwhile, the appearance of a sharp peak at the center of the profile revealed the existence of non-oxidized Si in the middle of SiO2 NWs. In addition, the prepared MoS2 sheets had the lattice constant of 0.62 nm, which is consistent with that of well-known edge layers in MoS2 (Figure 3c). Therefore, it was concluded that the synthesis of MoS2 through the solvothermal reaction hardly affected the chemical structure of MoS2 while only the density and distribution of MoS2 edge sites on the GCS@SiO2 NWs were physically tuned depending on the carbon surface structures.


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Figure 4. (a) HER polarization curves of MoS2/GCS@SiO2 electrodes. (Inset: HER polarization curve of MoS2 nanoparticles and GCS@SiO2 (G0)) (b) Graphs of the current density at 250 mV and the number of active sites. (c) I-V curves of GCS@SiO2 NWs measured from single NW device. (d) The relationship between the turnover frequency (TOF) at 250 mV and resistance of GCS@SiO2 NW.

Finally, the HER polarization curves of the MoS2/GCS@SiO2 NWs were obtained in 0.5 M H2SO4 electrolyte solution to study the effects of the carbon surface structure on the catalytic performance of the composite catalysts (Figure 4a). As expected, the HER performances of all the MoS2/GCS@SiO2 NWs were significantly improved due to the increase in the number of active edge site and the enhanced electrical conductivity compared to MoS2 nanoparticles without carbon support (the preparation of MoS2 nanoparticles is provided in supporting information). On the other hand, the GCS@SiO2 NWs without MoS2 nanosheets didn’t show HER activity in the same potential range, which means that the GCS@SiO2 NWs were used only as support materials. However,


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the overpotentials at the current density of 10 mA/cm2 were 205, 196, 186, and 208 mV for MoS2/GCS@SiO2 (G0, G1, G2, and G5) samples, respectively. While the HER activities gradually increased due to the increase in the number of active site from MoS2/GCS@SiO2 (G0) to MoS2/GCS@SiO2 (G2), MoS2/GCS@SiO2 (G5) with extremely extended active edge sites showed the lowest performance. In the structural point of view, this is an interesting result because the HER activity of the MoS2-carbon composite catalyst with much more MoS2 edge sites was significantly reduced although that of MoS2 itself should increase linearly with the increase in the number of active edge site.12

To clearly compare the effect of the number of active sites on the HER performance, the relation between the number of active sites and the current densities at 250 mV of the samples were plotted in Figure 4b. The current densities at 250 mV of MoS2/GCS@SiO2 NWs indicated a volcano shape with increasing the H2 gas flow rate used in the GCS@SiO2 fabrication while the number of active sites per geometric area almost linearly increased from 5.8 × 10-8 to 20.0 × 10-8 mol/cm2. If we do not consider other factor which can additionally affect the HER performance, the activity enhancement of the samples


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from MoS2/GCS@SiO2 (G0) to MoS2/GCS@SiO2 (G2) can be explained simply by the increase in the number of active sites. However, it is difficult to understand the dramatically decreased performance of MoS2/GCS@SiO2 (G5) despite the greatest number of active sites.

Accordingly, to find more critical evidence, we needed to study the correlation between the TOFs of MoS2/GCS@SiO2 composite catalysts and the resistance of the GCS@SiO2 NWs. First, to understand the electrical properties of GCS@SiO2 NWs, single nanowirebased devices were fabricated (Figure S3), and their I-V curves were obtained at room temperature (Figure 4c). The resistances of the GCS@SiO2 NWs as support materials were calculated from the I-V curves using Ohm’s law. As a result, the electrical resistance increased from 24 to 94 kΩ along with increasing the H2 gas flow rate used in GCS@SiO2 fabrication (Figure S4, the physical meaning of high resistance values for GCS@SiO2 single nanowires is described in detail in supporting information). From the measurement, it was thought that the increase in the resistance of GCS@SiO2 NWs might be mainly due to the decrease in thickness as well as the increase in defect density of the GCSs.


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For instance, in case of MoS2/GCS@SiO2 (G5) with much large active MoS2 sites, the GCSs had a thinner layer thickness due to the etching effect of the carbon surface by H2 gas compared to those of other MoS2/GCS@SiO2 catalysts since it had been grown on the SiO2 NWs under a much higher H2 partial pressure (H2 gas flow rate = 5 sccm), as observed in TEM images of Figure 1.

Based on the change in the electrical resistance of the support materials, we made Figure 4d to show the correlation between the electrical resistance and the turnover frequencies (TOFs) calculated at 250 mV. TOF is usually used as an important indicator of the intrinsic activity of the catalyst since it represents the activity per active site (how to calculate the TOFs is provided in supporting information). From the correlation graph, we were able to identify a key factor that reduces the HER performance of MoS2/GCS@SiO2 (G5) with very large active MoS2 sites.

MoS2/GCS@SiO2 (G0) and MoS2/GCS@SiO2 (G1) had similar TOF values because their electrical resistances were almost same although they had different number of active sites. Actually, the number of active MoS2 site have caused the difference in the apparent


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current densities shown in Figure 4b. On the other hand, in MoS2/GCS@SiO2 (G2) and MoS2/GCS@SiO2 (G5), the TOF values significantly decreased along with the increased resistances of the support materials while the number of active sites was linearly increased.

Interestingly, as shown in Figure 5a, the ratio of TOF of each sample to that of MoS2/GCS@SiO2 (G0) changed linearly by the conductance (a reverse parameter of resistance) ratio of the GCS@SiO2 NWs. For example, MoS2/GCS@SiO2 (G2) indicated a ~40 % reduction in the TOF value of MoS2/GCS@SiO2 (G0) when the electrical conductance of GCS@SiO2 (G2) decreased to ~40 % of GCS@SiO2 (G0). Therefore, it can be concluded that the TOFs of MoS2-carbon composite catalysts is linearly reduced with decreasing the conductance of carbon supports, regardless of the amount of active MoS2 sites on the carbon. In addition, as shown in Figure 5b, the HER current of all the samples increased in proportion to the product of the number of active sites and the conductance of GCS@SiO2 NWs. From these results, the best performance of


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MoS2/GCS@SiO2 among the samples can be now explained by the compensation effect between the number of active sites and the electrical conductance of support materials.

Figure 5. (a) Linear correlation between the ratio of TOFs and the ratio of electrical conductance of support materials. (b) Linear correlation between the ratio of current


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density and the ratio of the product of “the number of active sites” and “the electrical conductance of support materials”. The TOFs and current densities were obtained from 0.2 V to -0.3 V.

Furthermore, the effect of the conductance (or resistance) of the support materials on the charge transfer resistance of the composite catalysts was confirmed by electrochemical impedance spectroscopy (EIS) measurements. As shown in Figure S5 and Table S2, MoS2/GCS@SiO2 composite catalysts indicated similar series resistance (Rs) values between 4 Ω and 5 Ω. Although Rs includes various resistances arising from wiring, Ti substrate, GCS@SiO2 nanowires, and solution, the solution resistance mainly contributes to the magnitude of Rs and causes IR loss.39 That is, while the effect of the use of GCS@SiO2 on IR loss is negligible during the HER, the charge transfer resistances (Rct) are significantly affected by the electrical property of GCS@SiO2 supports as well as the number of active sites. As a result, the charge transfer resistances of the samples showed exactly the opposite tendency to their HER activities. Especially, a noticeable


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increase in the charge transfer resistance of MoS2 nanoparticles without support materials implies that the catalytic activity depends on the electrical conductivity of materials.8,36 Accordingly, it was confirmed that the GCS@SiO2 nanowires had electrical conductivity suitable for use as a support material. Moreover, in this work, the resistance of the GCS@SiO2 nanowires were systematically changed within an acceptable range, and the effect of the electrical resistance of carbon support materials on the HER activity of MoS2-carbon composite catalysts was properly investigated. Consequently, it can be concluded that one should consider both the surface defect density and electrical property of carbon supports to enhance the HER performance in the development of MoS2-carbon composite catalysts.

4. CONCLUSION

The correlation between the surface structure of carbon supports and the HER activity in MoS2-carbon composite catalysts were thoroughly investigated using surface-tuned GCS@SiO2 NWs as support materials. The surface defect density and electrical


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resistance of GCS@SiO2 NWs were systematically modulated by control of the H2 gas flow rate during the GCS formation on SiO2 NWs. The higher the surface defect density of the GCSs, the larger the number of catalytic active sites of MoS2 while MoS2 nanosheets grown on the GCS@SiO2 NWs with different surface structures had all the same chemical structures. However, the electrical resistance of GCS@SiO2 NWs as well as the active site formation of MoS2 was dramatically changed depending on the carbon shell structure. From in-depth physical characterization and electrochemical analyses, it was confirmed that the intrinsic catalytic activity (TOFs) of MoS2-carbon composites for the HER increased linearly with the conductance of the carbon supports regardless of the MoS2 edge site density. Furthermore, the HER current densities measured in I-V curves increased in proportion to the product of the number of active MoS2 sites and the conductance of GCS@SiO2 NWs. Consequently, in MoS2-carbon composite catalysts for HER, the intrinsic catalytic activity is more influenced by physical properties of carbon support than the number of active sites, and the HER performance can be improved by simultaneously considering the number of active sites and the electrical resistance. These


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fundamental and important findings will provide new insights into the development of carbon supports suitable for MoS2-carbon composite catalysts for HER.

ASSOCIATED CONTENT

Supporting Information.

The following file is available free of charge. Calculation of active sites & turnover frequency (TOF); Preparation of MoS2 nanoparticles as working electrode; TEM images of Si and SiO2 NWs; XPS spectra of MoS2/GCS@SiO2 (G0); SEM image of single nanowire-based device. EIS Nyquist plots and Rs and Rct values for the samples.

AUTHOR INFORMATION

Corresponding Author

*E-mail addresses: [email protected] (N. Jung), [email protected] (D. Whang)

Author Contributions


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‖These

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authors contributed equally.

ACKNOWLEDGMENT

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2017R1A2B2010663, 2018M1A2A2061991).

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Table of Contents/Abstract Graphic


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Unraveling the factors affecting electrochemical performance of MoS2

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