Engineering Two-Dimensional Mass-Transport Channels of MoS2

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Energy, Environmental, and Catalysis Applications

Engineering Two-Dimensional Mass-Transport Channels of MoS2 Nanocatalyst towards Improved Hydrogen Evolution Performance Ge Wang, Jingying Tao, Yijie Zhang, Shengping Wang, Xiaojun Yan, Congcong Liu, Fei Hu, Zhiying He, Zhijun Zuo, and Xiaowei Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07163 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 6, 2018

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ACS Applied Materials & Interfaces

Engineering

Two-dimensional

Mass-Transport

Channels of MoS2 Nanocatalyst towards Improved Hydrogen Evolution Performance Ge Wanga, Jingying Taoa, Yijie Zhanga, Shengping Wanga, Xiaojun Yana, Congcong Liua, Fei Hua, Zhiying Hea, Zhijun Zuob,*, Xiaowei Yang a,* a

Institute for Regenerative Medicine, Shanghai East Hospital, School of Materials Science and Engineering, Tongji University, Shanghai 200123, China.

b

Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province,

Taiyuan University of Technology, Taiyuan 030024, Shanxi, China.

ACS Paragon Plus Environment

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ABSTRACT

In addition to the intrinsic catalytic activity, the mass transport should be taken into adequate account in order to realize the superior performance of electrocatalysts. Here, we engineer the interstitial space between MoS2 nanosheets via the introduction of “spacer” to construct twodimensional (2D) channels for favorable mass transport. The nano-sized spacers effectively separate MoS2 nanosheets, generating open and connective channels to fulfill timely reactant supply and rapid gas release. Besides, the spacer served as physical support can prevent the collapse of 2D channels. Due to the engineering of nanostructured channels, the reduction in the overpotential by approximately 100 and 360 mV at -10 and -100 mA cm−2, a decrease in the Tafel slope from 66.7 to 39.4 mV dec-1 and a more stable operation can be achieved, respectively. After being integrated by carbon paper (CP), a further improved performance of 198 mV at -200 mA cm−2 and 36 mV dec-1 can be obtained. This work emphasizes the importance of mass-transport channels and paves a way to enhance the hydrogen evolution reaction (HER) performance.

KEYWORDS: mass transport; two-dimensional channel; spacer; molybdenum disulfide; hydrogen evolution revolution; electrochemical water splitting

INTRODUCTION Electrochemical hydrogen/oxygen evolution reactions (HER/OER), which hold keys to various energy conversion and storage technologies, have gathered great attention in recent years.1-3 Traditionally, critical components in these reactions are basic electro-catalysts, thus enormous efforts have been focused on the improvement of catalytic activity to achieve


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favorable reaction kinetics.4-6 However, with the reactions proceeding, interfacial reactants (H+, OH-, or H2O) are gradually depleted while substantial gases (H2 or O2) are simultaneously generated,7-9 which possibly suppress continuous catalytic reactions, leading to a compromised performance. Notably, this issue will be significantly pronounced for the highly active catalysts, since faster reaction kinetics mean rapid consumption of reactants and generation of gases.10-13 Therefore, maintaining timely reactant supply and rapid gas release for the catalyst is highly desirable. With a fundamental understanding of reaction process engineering, both the flow of produced gases from the inner of catalyst to bulk solutions and the penetration of reactants from the electrolyte into catalyst interfaces take place through the channels/pores.14 The channels in the catalyst play an important role to guarantee the efficiency of reactant supply and gas release. It is of great significance to construct mass-transport channels for superior catalytic properties. Two-dimensional (2D) layered nanocatalysts have attracted increasing interests due to their promising intrinsic property and abundant active sites, and notable successes have been achieved in these two aspects till now.15-17 For further enhanced catalytic performance, more attentions should be paid to design the transport channels within these nanosheets. In fact, the interstitial space between neighboring 2D sheets can be treated as natural lamellar channels for the mass transport.18, 19 It has been evidenced by the progresses in various energy storage technologies such as the supercapacitor and lithium ion battery, where the transport of ions and liquid electrolytes among 2D channels plays an important role in the electrochemical performance.20-27 It is presumable that by taking the advantage of such 2D channels, the rapid diffusion of water molecule and fast release of gaseous products could also come true. In regard to the use of these 2D channels for HER and OER, not only the liquid diffusion, but the gas transportation should


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also be taken into account. In addition, the drastic gas evolution would collapse the layered structure of catalyst, causing the damage to the 2D mass transport channels. To resolve these issues and make full use of the space between adjacent 2D nanosheets, here, we incorporate spacers into their layers to separate the sheets largely, leading to the generation of open and broad 2D channels with the transporting capability for both liquid and gas. Moreover, the spacer between 2D nanosheets can provide physical support to maintain the layered structure in the harsh reaction environment. We chose MoS2 nanosheet as the model, an attractive HER material with the 2D structure, to study the channel engineering and achieve enhanced mass transportation. Remarkably, the channel-engineered MoS2 catalyst exhibited a reduction in overpotential by approximately 100 and 360 mV at -10 and -100 mA cm−2, respectively, a decrease of Tafel slope from 66.7 to 39.4 mV dec-1, and a more stable working state than that without modified. After being integrated by carbon paper (CP), a further improved performance of 198 mV at -200 mA cm−2 and 36 mV dec-1 can be achieved. Our work focused on the engineering of 2D mass-transport channels will pave a way to superior HER performance. RESULTS AND DISCUSSION MoS2 nanosheets were prepared via the modified hydrothermal method described by Wang et al.28 In short, a mixture of sodium molybdate and thiourea in hydrochloric acid solution was heated in Teflon-lined stainless steel autoclave (see the Experimental Section for detailed preparation method). As shown in Figure S1, the synthesized MoS2 nanosheets can be characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The channel engineered MoS2 (MS/C) was prepared by dispersing carbon black particles into MoS2 dispersion uniformly and then drop-casting together


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onto the electrode. After drying, the 2D MoS2 nanosheets should be restacked and the dispersed carbon black particles will be trapped in the lamellar assembly layers as illustrated in Figure S2. TEM image (Figure 1a) clearly shows carbon black particles coupled with MoS2 sheets, also confirmed by the SEM image of cross-section of the electrode (shown in Figure S3) (shown in Figure S3). These particles intercalated between neighboring MoS2 layers may serve as pillars to maintain the structure of 2D channels. In addition, due to the tiny size of carbon black (size range 20-70 nm, Figure S4), the active sites of MoS2 nanosheets would be negligibly concealed. To further confirm the function of spacers, we measured the XRD of the catalyst electrodes of MS/C and pure MoS2 (MS), together with that of MoS2 powders. As shown in Figure 1b and S1b, no detectable peak is found at (002) in MS/C, furthermore, the electrode shows much weaker diffraction peaks than its corresponding powder as the result of intercalated Nafion. Since the (002) peak reflects the restacking of MoS2 nanosheets,

22, 29

its absence in MS/C may

suggest that the intercalation of spacers between sheets can prevent the restacking, which is beneficial to form wider lamellar channels for fast mass transportation (Figure 1c).


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Figure 1. (a) TEM image of MS/C. (b) XRD patterns of MS/C and MS. (c) Schematic illustration of engineered 2D channels in MoS2 nanosheets. To examine the characteristics of engineered channels, we studied the electrochemical properties of MS/C and MS in a standard three-electrode setup with the same MoS2 loading in 0.5 M H2SO4, which reveals that superior mass-transport channels can be achieved by the introduction of spacer. From the scan curve of CV, MS/C delivers a specific capacitance of 48.0 F g-1 (Figure 2a), higher than that of MS (14.5 F g-1), demonstrating that more appropriate channels are created by the spacers for the better access of electrolyte into the inner of catalyst layer. Capacitance retentions of these two electrodes were carried out to study the ion response at


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the different current densities (Figure 2b). Significantly, the specific capacitance of MS/C is less affected by the charge/discharge rates while that of MS drops substantially when the applied current increases. The outstanding high-rate capability suggests the formation of highly open and continuous larger 2D channels that prominently facilitate the ion and other mass transport.30, 31

Figure 2. Electrochemical characterizations of MS/C and MS. (a) CV curves obtained at 0.025 V s-1. (b) Capacitance retention ratio obtained at various charge/discharge currents. (c) Nyquist plots. (d) Bode plots of the frequency response of the capacitance The facilitated ion diffusion by channel engineering is further demonstrated by the Nyquist plots (Figure 2c). The length of the Warburg-type line (the slope of the 45° portion of the curve) reflects the ion diffusion process from outside electrolyte into the inner of MoS2 catalysts layer. The Warburg-type line of the MS/C is much shorter, suggesting faster ion diffusion than MS.


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Bode plots on the frequency response of capacitance (Figure 2d) also confirm the influence of the channel in ion transport rate. We approximated the operating frequency as the frequency where the capacitance is 50% of its maximum value.32 The operating frequency of MS/C and MS are 303.0 and 6.2 Hz, respectively, corresponding to the characteristic relaxation time constants τ0 = 3.3 and 161.3 ms, showing an improvement of ion diffusion by 50 times. The above experiments significantly point out the important role of spacer in the formation of superior 2D mass-transport channels with enlarged accessible surface area and facilitated ion diffusion. Although we have demonstrated that rational mass-transport channels can be constructed via introducing the spacer, whether these modified channels can facilitate the catalytic performance need to be further studied. Linear sweep voltammetry (LSV) plots of these two electrodes are shown in Figure 3a and their comparison with Pt/C is shown in Figure S5. In the initial region, these two electrodes exhibit different hydrogen evolution performances where the MS/C gives a lower overpotential (η=178 mV) at a current density of 10 mA cm-2, while that of MS is 269 mV. Considering the active component was taken from the same hydrothermal products with a same mass loading, the different HER activity may probably be caused by a different exposure of active sites, undoubtedly owing to the modified channels within MS/C. Furthermore, the differences of catalytic performance become more obvious at higher current densities. It is noticeable that the current density of MS/C increases really faster than that of MS: a current increasing as rapid as 2.5 mA cm−2 per mV was achieved in the linear region of high currents while only 0.31 mA cm−2 per mV for MS. As a result, a significant reduction in the overpotential by 363 mV at -100 mA cm−2 can be observed. Notably, both the conductivity and mass transport turn out to be the limiting factors of fast catalytic reactions at the high current densities.33 Since the system resistances are similar in these two electrodes (6.7 ± 0.3 Ω), the superior large-


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current-density catalytic performance may be mainly attributed to the enlarged mass-transport channels, which well meets the requirement of rapid electrolyte access and gas evolution for the high current density working condition. Moreover, note that the plot of MS turns out to fluctuate by bubble disturbing at the high current densities while that of MS/C still maintains smooth. A conclusion could be drawn that the channel engineered MoS2 might also be in favor of bubble detachment, confirming its advantage for the high-current water splitting. Corresponding Tafel plots of above MoS2 electrodes were transferred from LSV plots to study the kinetic process and have been shown in Figure 3b. The MS possess a Tafel slope of 66.7 mV dec-1, closely to several recent works on MoS2.34-38 By contrast, the reaction kinetics are substantially improved in MS/C, as indicated by the lower Tafel slopes of 39.4 mV dec-1. This low Tafel slope shows the Volmer-Heyrovsky mechanism responds the reaction, where the desorption of hydrogen is the rate-determining step.39 To our knowledge, the Tafel slope of MS/C is among the lowest values for MoS2-based electrocatalysts. It is believed that our channel engineered catalyst is beneficial for practical applications, especially for the high current applications such as water electrolyzers.35, 40


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Figure 3. Evaluation on the catalytic performance affected by 2D channel engineering. (a) LSV plots. (b) Tafel plots. (c) Plots showing the extraction of the Cdl. (d) Nyquist plots. To further investigate the possible origins of such unique performance, CV (Figure S6) and EIS are carried out to indicate the electrochemically active surface area (ECSA) and electrode kinetics, respectively. As shown is Figure 3c, the MS/C exhibits a Cdl of 0.51 mF, which is more than 4 times that of the MS (0.12 mF). Since the Cdl of carbon black is really small (0.01mF, Figure S7), the large increase of Cdl in MS/C can be attributed to the carbon black acting as spacers to separate MoS2 nanosheets largely, resulting in a better contact of MoS2 nanosheets with electrolyte. As a consequence, the large ECSA arising from channel engineering provides abundant active sites for catalytic reaction, which definitely exhibits intensified HER performance. By the way, note that the MoS2 catalysts on different electrodes possess the same mass loading, but the resultant effectively exposed active sites are quite different. It is really


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important to pay attention to the rational channel construction of nanocatalyst at electrode level toward the further utilization of nanocatalyst. Comparing to the improvement of ECSA, the contribution of channel engineering becomes more remarkable in electron transport and catalytic kinetics illustrated by Figure 3d. Nyquist plots are fitted with Randles equivalent circuit models (Figure S8, Table S1). The semicircle at lower frequencies represents the charge transfer process (Rct), while the semicircle at higher frequencies represents a time constant which is probably related to the electrical contact between electrode and catalyst as well as catalysts themselves (Ri).41 As shown in Figure 3d, the radius of semicircles for both Ri and Rct are sharply reduced when the carbon black is introduced. MS/C possesses a much smaller Rct of 4.3 Ω, in contrast to the MS (60.5 Ω). This small Rct affords faster HER kinetics, which is corresponded to its low Tafel slope. Additionally, the Ri of MoS2 catalyst also decrease from 20.0 Ω (MS) to 1.4 Ω (MS/C), suggesting the enhancement of electronic transport from current collector to catalyst surface. The decrease in Ri can be attributed to the inhibition of restacking of MoS2 sheets, as the restack will seriously hinder the vertical charge transport. The Cdl and EIS results confirm that our channel construction leads to a further exposure of MoS2 active sites and more facile electrode kinetics, achieving the favorable HER performance. Notably, compounding MoS2 with carbon materials will induce improved HER performances due to the enhanced the conductivity. To highlight the role of channel engineering in the improved HER performance, we also used the semiconducting Si particles to replace the conductive carbon black particles as spacers to fabricate the channel-engineered MoS2 electrode (MS/Si), achieving an improved HER performance compared to the pure MoS2 (MS) too (shown in Figure S9). It proves the irreplaceable effect of channel engineering on the HER performance.


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As mentioned above, we have revealed the impact of spacers on the formation of rational mass-transport channels and achieved promising MoS2 electrode with high activity. In addition to that, long-term electrochemical stability considered as another important criterion for catalytic electrode is also needed to be examined. We performed chronopotentiometry experiment (at 10, 100 mA cm-2) to evaluate the stability of our channel constructed electrodes. As shown in Figure 4a, when a constant low current density of -10 mA cm-2 is applied, the overpotentials of these two electrodes exhibit negligible degradation. However, when the applied current density rises to -100 mA cm-2, the MS possess a rather poor stability in contrast to MS/C. As the electrochemical active component of catalyst is same, the significant degradation of MS is probably caused by the physical structure deformation in its catalyst layer. Indeed, when applying in a large-currentdensity working condition, catalysts coated on the electrode will be easier to peel off due to the vigorous gas evolution.42 Besides, the drastic gas evolution would also collapse the layered structure of catalyst, causing a damage to 2D mass transport channels. The degradation occurred in MS may well verify that point. However, the stability of MS/C seems to be not affected by the high current at all. We attribute this outstanding stability performance to the broadened diffusion channels, which is beneficial for releasing trapped H2, decreasing the pressure within catalyst layer and smoothing the flow of gas at large-current-density working condition. In addition, the spacer between 2D nanosheets can provide physical support to maintain the layered structure in harsh reaction environment. We further prolong the test time of MS/C in the same current density of 100 mA cm-2 and also cycle it continuously for 1000 cycles. After a 12 h test, only a degradation of 9 mV of overpotential can be observed, meanwhile, the MS/C affords similar CV curves as before at the end of cycling (Figure S10). These results suggest the practical future of such channel engineering for a stable large-current-density electrocatalysis.


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Figure 4. (a) Chronopotentiometry curves of MS and MS/C at constant current densities of 10 (inset) and 100 mA cm-2. (b) LSV plots of channel-constructed MoS2 recorded on carbon paper. To be extended to practical application, where porous 3D conductive substrate is usually used as the catalyst support, the strategy of channel engineering is further evaluated by depositing the catalysts onto 3D carbon paper (CP). The result confirms that this strategy applied to 3D porous substrate could achieve a same tendency as the results tested on the GC electrode (Figure 4b). We compare the performance of our channel-engineered MoS2 with previous notable works of MoS2 catalyst in 2D and 3D substrate respectively (Figure S11, 12, Table S2, 3), finding that our samples upon channel engineering are even comparable to the previous works. Notably, the η = 198 mV versus RHE for j = −200 mA cm−2, among the best performances for MoS2-based catalyst to our knowledge, is achieved in our channel engineered MS/C-CP. Therefore, besides the optimization of nanocatalysts, constructing rational channels for mass transportation is also of great importance to enhance catalytic performance and should be paid more attention. CONCLUSION In summary, based on the facilitation of mass transport, using MoS2 nanosheets as a typical model, we engineered the 2D channels by easily incorporating carbon black as spacers and


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achieved enhanced catalytic performance. The channel engineered MoS2 nanocatalyst offers highly exposed active sites and accelerated mass transportation, leading to an improvement in both catalytic activity and stability. Studies on 3D catalyst support further demonstrate the applicability of this strategy. This work points out the requirements of channel engineering for faster mass transport and paves a new way to develop more efficient electrodes for the water splitting, especially for the large current density use. EXPERIMENTAL SECTION Materials. Sodium molybdate dihydrate (Na2MoO4·2H2O), sulfuric acid (H2SO4) and hydrochloric acid (HCl) were purchased from Sinopharm Chemical Reagent Co., Ltd. Thiourea (SC(NH2)2) and Nafion were purchased from Sigma-Aldrich. Carbon black was purchased from TIMCAL. Synthesis of catalyst. The molybdenum disulfide catalyst was prepared via a hydrothermal method. Typically, 2.5 mmol Na2MoO4·2H2O and 10 mmol SC(NH2)2 were dissolved in 50 mL deionized water. After being stirred to form a homogeneous mixture, the solution was added with 20 mL of 1 M HCl and transferred into a 100 ml Teflon-lined stainless steel autoclave. The autoclave was heated at 200 °C for 3 h and then cooled to room temperature naturally. Thereafter, the products were centrifuged several times with deionized water to remove redundant hydrochloric acid and unreacted reagents. The resulting products were collected and maintained in water. Characterization. Powder XRD patterns were recorded on an X-ray diffract meter (D2 PHASER, BRUKER). A JEM-2100 electron microscopy with an accelerating voltage of 200 kV


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was performed for TEM measurements. XPS measurement was carried out on a Thermo Scientific Escalab 250Xi X-ray photoelectron spectrometer using Al as the exciting source. Electrochemical Measurements. Electrochemical measurements were performed in a threeelectrode electrochemical cell with Bio-Logic potentiostat (VMP3). For the preparation of control sample (MS), 1 mL of water-isopropanol solution (volume ratio, 4:1) containing 3 mg MoS2 catalyst was mixed with 60 µL Nafion by sonicating for 30 min to form a homogeneous ink. Then 5 µL of the ink was dropped onto the glassy carbon electrode with 3 mm diameter (catalyst loading: 0.21 mg cm-2). The MS/C sample was prepared following the same electrode process in addition that the catalyst was a mixture of MoS2 and carbon black (MoS2: carbon black = 5:1), where the active component of MoS2 is still 3 mg. The MS/Si was prepared in a same method with MS/C. For measurements on carbon paper, the working electrode was prepared by loading 1 mg cm-2 of MoS2 on carbon paper from aforementioned two kinds of catalyst ink. All measurements were performed in N2-saturated 0.5 M H2SO4 aqueous solution using a graphite rod as the counter electrode, saturated calomel electrode as the reference electrode and the glassy carbon electrode dropped with different catalysts as the working electrode. For characterizing the properties of two-dimensional (2D) channels, the capacitance was estimated by CV with a scan rate of 0.025 V s-1 from 0.25 to 0.35 V vs SCE. The EIS measurements were carried out at 0 mV overpotential in the frequency range from 105 to 0.01 Hz. The galvanostatic charge/discharge tests were carried out between 0.25 and 0.35 V vs SCE at current densities between 1 and 15 A g -1.


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To evaluate the HER performance, the LSV with a scan rate of 5 mV s-1 was conducted for the characterization of HER activity and subsequent Tafel analysis. The double layer capacitance was estimated by CV with various scan rate (25, 50, 100, 200, 400 mV s-1) in the range of 0.250.35 V vs SCE. The EIS measurements were carried out at 250 mV overpotential in the frequency range from 105 to 0.01 Hz. To investigate the electrochemical stability, chronopotentiometry experiment (at 10, 100 mA cm-2) and CV (-0.3-0.1 V vs RHE at 50 mV s-1) were both conducted. All data in this work were corrected for iR. All potentials were calibrated to a reversible hydrogen electrode (RHE), E (RHE) = E (SCE) + 0.241 V.

ASSOCIATED CONTENT Supporting Information. The XRD, TEM, SEM, XPS, diagrammatic sketch and electrochemical data. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]; [email protected]. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by grants from the National Natural Science Foundation of China (21303251 and 21776197), Innovation Program of Shanghai Municipal Education Commission (16SG17).

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TOC

Two-dimensional (2D) channels in MoS2 nanocatalyst, achieved by easily introducing carbon black particles as spacers into neighboring 2D sheets, make a significant contribution with enhanced mass transportation to the improvement of HER performance.


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Engineering Two-Dimensional Mass-Transport Channels of MoS2

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