Highly Efficient Hydrogen Evolution from Seawater by

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A Highly Efficient Hydrogen Evolution from Seawater by Biofunctionalized Exfoliated MoS2 Quantum Dots Aerogel Electrocatalyst That Is Superior to Pt I-Wen Peter Chen, Chien-Hsuan Hsiao, Jheng-Yi Huang, Yu-Hong Peng, and Chia-Yu Chang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02582 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 23, 2019

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ACS Applied Materials & Interfaces

A Highly Efficient Hydrogen Evolution from Seawater by Biofunctionalized Exfoliated MoS2 Quantum Dots Aerogel Electrocatalyst That Is Superior to Pt I-Wen Peter Chen,* Chien-Hsuan Hsiao, Jheng-Yi Huang, Yu-Hong Peng, and Chia-Yu Chang Department of Applied Science, National Taitung University, 369, Sec. 2, University Rd., Taitung City (95092), Taiwan. E-mail: [email protected] KEYWORDS (Word Style “BG_Keywords”). Hydrogen evolution reaction, molybdenum disulfide, quantum dots, liquid phase exfoliation, chlorophyll, aerogel

ABSTRACT: As a source of clean and sustainable energy, reliable hydrogen production requires highly efficient and stable electrocatalysts. In recent years, molybdenum disulfide (MoS2) has been demonstrated as a promising electrocatalyst for hydrogen evolution reactions (HER). Here we demonstrate that a three-dimensional (3D) MoS2 quantum dots (MoS2QD) aerogel is an efficient cathode electrocatalyst that can be used to enhanced the HER in acid, neutral, and alkaline (e.g. real seawater) environments. In studying the effects of the exfoliated MoS2 dimension for the HER, we found that the biofunctionalized exfoliated MoS2QD shows much higher cathodic density, a more lower energy input, and a lower Tafel slope for the HER than the larger size of the chlorophyll-assisted exfoliated MoS2, highlighting the importance of the size of the MoS2 aerogel support for accelerating the HER performance. Moreover, the electrocatalytic activity of MoS2QDaerogel is superior to that of Pt in neutral conditions. In real seawater, MoS2QD-aerogel sample exhibits stable HER performance after consecutive scanning for 150 cycles, while the HER activity of the Pt dramatically decreases after 50 cycles. These results showed for the first time how the 3D MoS2 configuration in MoS2 aerogel can be used to effectively produce hydrogen for clean energy applications.

Introduction As the world uses several hundred million barrels of crude oil every day, and the global climate system changes around the world, research has been committed to search for alternative energy sources.1 Renewable energies are possible routes because we believe that this may be a suitable way out of environmental crisis and energy source crisis.2 Solar energy, wind energy, geothermal energy, and biomass have all declared that they are the alternative energy sources of the future. Hydrogen, which is the friendliest fuel for the ecosystem and has the highest energy density per mass of any fuel, attracts more attention owing to its negligible greenhouse gases emissions. The hydrogen evolution reaction (HER), where two hydrogen ions are reduced to molecular hydrogen, is an important process for electrocatalytic water splitting.3 Pt is still considered to be one of the most efficient electrocatalysts for HER owing to its superior catalytic performance and stability. However, due to its scarcity, Pt is expensive ($824 per ounce) and this unaffordable price severely hinders in the widespread application and commercialization. Recently, much effort has paid more attention on developing alternatives catalysts, e.g., catalysts based on non-precious metals and their compounds, for efficient

generation of hydrogen.4-10 earth-abundant low-priced electrocatalytic activity is efficiency electrocatalysts production of hydrogen.

Therefore, the exploration of materials that have excellent urgent for developing highneeded in the large-scale

Two-dimensional (2D) materials, such as transitionmetal dichalcogenides (TMDs), have drawn significant attention in various areas, such as energy generation and storage.11-12 Molybdenum disulfide (MoS2) thin sheets have gained enormous attention as a non-precious HER electrocatalyst. Previous studies show that as MoS2 thin sheets possess more edges of sulfur (S), they exhibit superior electrocatalytic activity (Tafel slopes of 55~60 mV/dec), which favors the HER process by reducing overpotential and enhancing current densities.13 However, the basal planes of the MoS2 are electrocatalytically inert.1314 Therefore, the MoS thin sheets with reduced size and 2 few layers could have better electrocatalytic performance due to the existence of abundant exposed sulfur edge sites.15-19 In addition, another critical obstacle for the practical application of MoS2 thin sheets in the field of HER is their poor electrical conductivity; however, this poor electrical conductivity can be improved in terms electrical conductivity via the addition of high electrical conductivity

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materials20 and thus the injection of the electrons from the working electrode and transportation to the MoS2 thin sheet active sites can be more efficient. Therefore, it is of importance to make the synthesis the exfoliated monolayered or few-layered MoS2 thin sheets scalable to achieve a large amount of the exposed sulfur sites around the MoS2 surface, and each exfoliated MoS2 thin sheet should avoid sheet restacking when performing the HER. Various synthesis routes for the preparation of MoS2 thin sheets have been developed. Owing to the high manufacturing costs and limited scalability, methods such as mechanical exfoliation limit the use of MoS2 materials for academic research and specific applications.21 The method of chemical vapor deposition may be an alternative approach to synthesis MoS2 thin sheets. However, this technique is not only hard to precisely control the stoichiometry ratio of the precursors and the effect of temperature gradient of the tube furnace, this technique is also expensive and requires low packing density of the asobtained MoS2.22-23 According to the aforementioned techniques, one method is widely used for the production of MoS2 thin sheets: exfoliant-assisted liquid-phase exfoliation. 22, 24-25 In exfoliant-assisted liquid-phase exfoliation, pristine MoS2 is first dispersed in a solvent containing exfoliant to reduce the strength between the MoS2 layers.24-27 Subsequently, an external force such as sonication or shearing is applied to proceed the exfoliation of bulk MoS2 into exfoliated MoS2 thin sheets. Then the exfoliant-assisted liquid-phase exfoliation greatly enhances the concentration of the exfoliated MoS2 thin sheets and which is beneficial for practical applications.28 Currently, most of the reported HER results used drop casting to deposit MoS2 thin sheets to fully cover the working electrode. MoS2 thin sheets are a poor electrical conductive material, therefore the electrons are difficult to transfer during electrochemical measurement. As a consequence, only the working electrode/MoS2 interfacial layers possess the most effective electrocatalytic performance.15 However, the electrocatalytic performance of the MoS2 thin sheets, which is far from the working electrode, was less catalytically active because of large resistant and less exposed sulfur active sites. Therefore, controlling the electrode structure to preferentially expose as much of the active sites while electrons are efficiently transported to the electrocatalytic surface is still an urgent and unaccomplished goal.29-30 Herein, we demonstrate an environmentally sustainable method to develop a three dimensional (3D) MoS2 aerogel via a facile one-pot hydrothermal process with an ultralow overpotential of 53 mV (at the current density of 7 mA/g), a Tafel slope of 41 mV/dec, and over 100 hours of electrochemical stability in 0.5 M H2SO4. Moreover, the hydrothermal 3D MoS2 aerogel is superior to the commercial Pt electrocatalyst under neutral and alkaline environments at a high overpotential. In real seawater, the hydrothermal 3D MoS2 aerogel exhibits a more stable HER performance after scanning for 150 cycles, while the HER activity of the commercial Pt electrocatalyst dramatically decreases after scanning 50 cycles. These results represent a significant breakthrough

in HER performance for a non-precious electrocatalyst system. Experimental Chemicals. Molybdenum (IV) sulfide (MoS2; 99% metals basis; ~325 mesh powder) and American bacteriological agar was purchased from Alfa Aesar and Conda-Pronadisa (Spain). Platinum (Pt; 99.9 %) was supplied by Leesan Precious Metal Co.. Acetone and sulfuric acid (H2SO4, 99.5%, HPLC grade) were purchased from ECHO chemical Co., Ltd. All solvents were used without further purification. Extraction of Chlorophyll. 20 g of sapium sebiferum leaves were placed in a mortar. Then, 100 mL of liquid nitrogen was poured into the mortar and the pestle was used to grind leaves for 10 min. The leaves extracts were rinsed with acetone and transferred to a serum glass bottle to exactly 500 ml volume. The extracts were filtered via a polyvinylidene fluoride membrane (0.22 μm pore size; Millipore) to remove impurities. Then, the filtered solutions were centrifuged (Model: Z326; Hermle) at 2,000 rpm for 30 min to collect the supernatant of the chlorophyll extracts solution. The results of the chlorophyll extracts were identical to our previous reported. These values were also consistent with the reported results.31-33 Preparation of Chlorophyll-assisted exfoliated MoS2 thin sheets suspension. 5 g of the MoS2 powder, 50 ml of the chlorophyll extracts solution, and 950 mL of acetone were placed in a 1000 mL beaker and sonicated with the ultrasonicator (Model: Q700; Qsonica) with a high gain horn in pulse mode for 5 h. The temperature of the sonicated solution was controlled at 10 °C. The concentration of the exfoliated MoS2 thin sheets suspension was ~0.5 mg/ml. The exfoliated MoS2 thin sheets were characterized by transmission electron microscopy (TEM), Raman spectroscopy, and x-ray photoelectron spectroscopy. MoS2 aerogel preparation. 1 g of agar was dissolved in 100 ml of deionized water to obtain the concentration of a 10 mg/ml agar solution. The electrical conductivity of the agar was in the range of 0.05 to 0.1 S/m which is several orders higher than the MoS2 thin sheets.34-35 Afterwards, 50 ml of the exfoliated MoS2 suspension was mixed with 100 ml of agar to form gel-like suspension. Then, the beaker was transferred to a -20 oC refrigerator to freeze the mixture. The frozen solution was then placed in a 10-3 Torr vacuum chamber (freeze-dried) to produce a MoS2 aerogel. The sample was name MoS2-aerogel-50. Hydrothermal synthesis of MoS2 quantum dots (MoS2 QD) aerogel. The exfoliated MoS2 thin sheets suspension was placed in a Teflon-lined stainless steel autoclave reactor at 120 °C for 8 h. The statistical distribution of the diameter of the exfoliated MoS2 thin sheets and the hydrothermal MoS2 thin sheets was characterized using a dynamic light scattering (DLS) instrument, which is indicated to be probable hydrodynamic size of an equivalent sphere calculated by the rotating of the MoS2. 1 g of agar was dissolved in 100 ml of deionized water to


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ACS Applied Materials & Interfaces obtain the concentration of a 10 mg/ml agar solution. Afterwards, 100 ml of the hydrothermal MoS2 suspension was mixed with 100 ml of agar. Then, the mixer (L5M-A; Silverson) head was then immersed into the mixture and the shear mixing speed gradually increased to the speed of 5000-7000 rpm. The mixer was held at this speed for 1 h at room temperature. After mixing, the mixture was transferred to a Teflon-lined stainless steel autoclave reactor and then keep at a controlled temperature in an oven at 120 °C for 3 h. After standing for overnight at room temperature, the beaker was then cooled to -20 oC in a refrigerator to freeze the solution. Then, the frozen solution was placed in a 10-3 Torr vacuum chamber (freezedried) to produce an aerogel. The sample was named MoS2QD-aerogel-100. Electrochemical Characterization. An electrochemical workstation (CHI7927) was used for electrochemical measurements in 0.5 M H2SO4 aqueous solution. A Pt and a KCl-saturated Ag/AgCl electrode were used as the counter and reference electrode, respectively. The MoS2 aerogels were cut into small pieces and used as the working electrodes. The current densities were determined in terms of the cut mass of the MoS2 aerogel electrodes. For example, systematic studies were also performed by loading the catalysts, such as MoS2-aerogel-50, MoS2aerogel-100, MoS2-aerogel-150, and pure agar aerogel. The curves of the linear scanning voltammetry (LSV) were measured by sweeping the potential from 0 to −0.6 V (vs. RHE) at a scan rate of 5 mV/s. The stability tests were measured in 0.5 M H2SO4 by potential scanning between 0 and -0.13 V (vs. RHE) at a sweep rate of 100 mV/s for 1000 cycles. Chronoamperometric measurements of the MoS2 aerogels were used to monitor current-time responses for 100 h. The calibration of the Ag/AgCl electrode were measured in a three-electrode system with two bulk Pt as the working and counter electrodes, respectively, and the Ag/AgCl electrode as the reference electrode. Cyclic voltammograms (CVs) were run at the scan rate of 100 mV/s, and the potentials at which the current passed zero point was used as the thermodynamic potential for the starting point of the hydrogen electrode reactions. In 0.5 M H2SO4, E(RHE) = E(Ag/AgCl) + 0.224 V (Figure S1). Impedance measurements were carried out at frequencies ranging from 1 Hz to 100 kHz with an amplitude of 5 mV at the overpotential of each MoS2 aerogel. Characterization. The exfoliated MoS2 thin sheets were examined by transmission electron microscope (TEM; JEOL JEM-2100) and a high-resolution transmission electron microscope (HRTEM; Hitachi H-7100) by drying a droplet of the exfoliated MoS2 solutions on Cu-grids with Formvar carbon film. The morphologies of the aerogel sample were characterized by a field emission scanning electron microscope (FESEM; JEOL JSM-7600F) with an energy dispersive spectroscopy (EDS). A tapping mode atomic force microscope (TMAFM; Innova/Bruker, Santa Barbara, CA) was utilized to scan the morphology of the exfoliated MoS2 thin sheets. Raman spectra were collected using a 532 nm laser source with an iHR550 spectrometer (Horiba Jobin Yvon). DLS were used with a Malvern

Zetasizer Nano ZS90 with an avalanche detector, using a 633 nm laser. Results and discussion

Figure 1. Characterization of chlorophyll-assisted exfoliated MoS2 thin sheets. a) Pure chlorophyll extracts solution with the concentration of 28 μg/ml. b) The chlorophyll-assisted exfoliated MoS2 suspension. c) The as-prepared MoS2 suspension after standing for 30 days. d) and e) TEM images. f) HRTEM images of the edge of the exfoliated MoS2 thin sheets. g) Fast Fourier transform (FFT) image of the exfoliated MoS2 thin sheets. h) Raman spectrum. i) XPS spectrum showing the peak regions of Mo 3d core level for the exfoliated MoS2 thin sheets.

The exfoliated MoS2 thin sheets were synthesized via the exfoliant of the chlorophyll extracts in acetone.36 Figure 1b shows the bulk MoS2 powder was exfoliated by chlorophyll extracts after sonicating 5 h. No significant precipitation was observed in the Figure 1c after standing for one month in the exfoliated MoS2 suspension. To characterize the morphologies and sizes of the exfoliated MoS2 thin sheets, the surfaces of the exfoliated MoS2 thin sheets were examined by TEM and HRTEM. Figures 1d and 1e show that the exfoliated MoS2 thin sheets can be achieved at the micrometer scale in lateral size, which is about two orders of magnitude higher than lithium intercalated methods.3738 Figures 1f and 1g show the high resolution image of the edge of the exfoliated MoS2 monolayer and the Fourier transform (FFT) pattern, respectively. Figure S2 shows the morphology of the exfoliated MoS2 thin sheets obtained by TMAFM from chlorophyll-assisted exfoliated MoS2 suspensions. The height profile showed that the exfoliated MoS2 sheet has an apparent thickness of ~ 1.1 nm, identical to similar results from monolayer MoS2 thin sheet by other groups.38 Figure 1h shows typical Raman shift of 385 and 405 cm-1 for E12g and A1g, respectively, which indicates the exfoliated MoS2 thin sheets remain as a thermodynamic stable semiconducting type.36 The shoulder peak appearing at 300 cm-1 in the Raman shift indicates the chlorophyll extracts signal (Figure S3). X-ray photoelectron



spectroscopy (XPS) characterization was used to detect the oxidation states of the Mo in the exfoliated MoS2 thin sheets (Figure 1i). Figure 1i shows the XPS binding energy of the Mo 3d spectrum of the exfoliated MoS2 film. The Mo 3d5/2 and Mo 3d3/2 show peaks centered at 229.6 and 232.7 eV, corresponding to the Mo4+ components of the exfoliated MoS2. Mo6+ signal at 235.5 eV is also measured in the exfoliated MoS2 samples, which indicates the MoO3 existence in the exfoliated MoS2 surface. The oxide form is thought to be due to the Mo atoms which can be easily oxidized in air. When MoO3 dissolves in solutions, the oxidation form of Mo would not influence the HER activity of the MoS2 sample.39 Figure S4 shows that the binding energies of 162.3 and 163.5 eV serve as S 2p3/2 and S 2p1/2, respectively. These results match well with the reported values of the MoS2 crystal.40-42 Subsequently, the exfoliated MoS2 thin sheets were mixed with agar solution, and then the mixture was treated non-hydrothermally and hydrothermally to prepare MoS2-aerogel-x and MoS2QDaerogel-x (where x represents the utilization amount of the exfoliated MoS2 suspension), respectively. In this way, the exfoliated MoS2 thin sheets were encapsulated and covered by agar to have a uniform distribution of the MoS2 framework.

hydrogen electrode (RHE) at the current density of 10 mA/g. The resulting Tafel slope of MoS2-aerogel-100 is 45 mV decade-1, indicating the Volmer-Heyrovsky mechanism as the path way to generate hydrogen. According to the related literature, HER can be enhanced by exposing more S atoms at the edges of the MoS2 surface.43 A hydrothermal method was used to reduce the size of the chlorophyll assisted exfoliated MoS2 thin sheets. Under the condition of high temperature and pressure, the chlorophyll assisted exfoliated MoS2 thin sheets were cut into small sizes (Figure S5). The statistical distribution of the diameter of the exfoliated MoS2 thin sheets and the hydrothermal MoS2 thin sheets was characterized to be ~167 and ~29 nm, respectively. Figure 3c shows the overpotential (at the current density of 7 mA/g) and the Tafel slope of the MoS2QD-aerogel-100 are 53 mV and 41 mV decade-1, respectively, which is the best result for pure semiconducting type MoS2 electrocatalysts up until the present.13, 29, 44-45 During the applied potential in the HER region, hydrogen gases were dramatically generated on the surface of MoS2QD-aerogel-100 (Movie S1).

Figure 2a shows a digital image of the light-weighted MoS2 aerogel sample. A typical FESEM images of the MoS2aerogel-50 shows that interconnected 3D porous network with various uniform macropores of several micrometers in diameter was clearly observed (Figure 2b-c). The porous networks would benefit mass transfer and reduce transportation of the electrolyte at the cathode. The EDS mapping analysis of the MoS2-aerogel-50 hybrid sample shows the homogeneous distribution of elemental C (Figure 2d), Mo (Figure 2e) and S (Figure 2f), further confirming that agar was evenly anchored on the surface of MoS2 thin sheets.

Figure 3. Hydrogen evolution performance. a) LSV curves of MoS2-aerogel-50, MoS2-aerogel-100, MoS2-aerogel-150, and pure agar, b) Corresponding Tafel plots. c) LSV curves of MoS2-aerogel-100 and MoS2QD-aerogel-100, and d) corresponding Tafel plots.

Figure 2. a) Photograph of the MoS2-aerogel-50. b)-c) FESEM images of the MoS2-aerogel-50. The elemental mapping images of d) C, e) Mo and f) S.

The electrocatalytic activity of MoS2-aerogel-100 was first investigated in 0.5 M H2SO4 solution using a threeelectrode system. For comparison, the HER performance of MoS2-aerogel-50, MoS2-aerogel-100, MoS2-aerogel-150, and pure agar were also measured under the same conditions. As shown in Figure 3a and 3b, MoS2-aerogel-100 shows a very small overpotential of 63 mV versus the reversible

To unveil the role of the hydrothermal treatment, electrochemical impedance spectroscopy (EIS) measurements were executed under the same HER operational conditions (Figure 4). In high-frequency region, the MoS2QD-aerogel-100 shows a minimum semicircle, representing it has a charge-transfer resistance (RCT) of 14 Ω, while the RCT value for MoS2-aerogel-100 (27 Ω) is almost two times higher. The low RCT value of MoS2QD-aerogel-100 indicates fast charge-transfer rates during HER and, consequently, effective kinetics at the interface of the MoS2QD-aerogel-100 aerogel and electrolyte. In addition, Figure S6 shows that the MoS2 aerogel can sustain large-strain deformations under compression and recover to nearly its original volume without damaging the structure, indicating that the


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ACS Applied Materials & Interfaces sample was strong and flexible. The nitrogen adsorptiondesorption isotherm of the MoS2QD-aerogel-100 hybrid sample indicates that it contains a typical microporous and mesoporous configuration and that the Brunauer-EmmettTeller (BET) average pore size is 5.3 nm, which is sufficient for electrolyte diffusion and enhancing the contact between the electrolyte and the catalyst. Operational stability is another vital criterion to consider when evaluating the long-term performance of the electrocatalyst.

Figure 4. Nyquist plots of a) MoS2-aerogel-100 and b) MoS2QD-aerogel-100 recorded in 0.5 M H2SO4 with a bias of 223 mV versus the RHE and a frequency range of 1 Hz to 100000 Hz.

Figure 5a shows that the LSV curve measured after 10000 cycles was shifted only 3 mV less than the initial curve at the current density of 150 mA/g, affirming the highly stability of the MoS2QD-aerogel-100 aerogel. We measured the stability of the MoS2QDaerogel-100 aerogel as the HER electrocatalyst electrode by recording its chronopotentiometric response for 100 h. High durability manifested with the current density and showed no significant change after operation for 100 h at 10 mA g-1, as shown in Figure 5b. The aforementioned properties of high activity, favorable kinetics, and long-term stability serve as solid evidence that MoS2QD-aerogel-100 aerogel is a promising water-splitting electrocatalyst material.

Figure 5. Durability of the MoS2QD-aerogel-100. a) LSV curves after 1 and 10000 cycles under the same HER operation condition. b) Stability tests at an applied potential of -60 mV in 0.5 M H2SO4 solution.

To minimize the detrimental environmental impacts, the HER performance of the aerogel in neutral electrolyte condition was carried out. Figure 6a shows that the MoS2QD-aerogel-100 aerogel possess a much lower HER potential than the Pt electrode. The MoS2QD-aerogel-100 exhibits long-term stability regarding hydrogen generation in KCl electrolyte (Figure S7). Importantly, the Tafel slope of 166 mV decade-1 for MoS2QD-aerogel-100 aerogel is smaller than that of the Pt (223 mV decade-1 as determined in the overpotential range of 0.36 and 0.54 V vs RHE; Figure 6b).46-47 It has been reported that if the dissociation step of water is the rate-determining step, the theoretical calculation Tafel slope is around 118 mV decade-1.47-48 The Tafel slope of MoS2QD-aerogel-100 is only a step off to this theoretical prediction, indicating the role of the water dissociation step for HER in MoS2-based aerogel system. The part beyond the Tafel slope of the 118 mV decade-1 is probably due to the low mass transport process in a neutral electrolyte. However, as a single salt neutral solution does not exist in a real world environment, the HER performance of the prepared aerogel was tested in a real world sample (such as seawater) and demonstrates its electrocatalyst behavior. However, based on the ultimate goal of seawater electrolysis, HER is still an ongoing challenge. A core issue is that most of the electrocatalysts tend to be deactivated and/or corroded in the seawater, usually showing inferior HER performance and instability. Therefore, seawater, the most abundant electrolyte in the world, is used to demonstrate the aerogel HER performance to further expand practical application. In this case, we investigated the HER performance of the MoS2QD-aerogel-100 electrocatalyst in seawater. The real seawater was obtained from the Pacific Ocean and the pH was 8.8. In real seawater, the 10th LSV curve of the MoS2QD-aerogel-100 and Pt is shown in Figure 6c, while the activities of Pt gradually decline decrease after scanning for 150 cycles (Figures 6c ~ 6g). After scanning 150 cycles, Figure 6g depicts the applied potential of the MoS2QD-aerogel-100 as 0.15 V more positive than that of Pt at the current density of 80 mA/g. Figure 6h shows that the MoS2QD-aerogel-100 almost remains at an identical potential level at the current density of 80 mA/g via various LSV cycling, while commercial Pt electrocatalyst decrease significantly. It has been demonstrated that the electrocatalytic reactivity of MoS2QD-aerogel-100 for HER in a real seawater environment is superior to the state-ofthe art commercial available precious metal “Pt.” According to the above electrocatalytic examination and Tafel slope analysis, we envision that the porous configuration may assist ions adsorption and/or the subsequent water dissociation onto the surface of the MoS2 thin sheets.



Figure 6. a) LSV curves and b) Tafel slope of the MoS2QD-aerogel-100 aerogel and Pt in neutral electrolyte. c) - g) LSV curves of the MoS2QD-aerogel-100 aerogel and Pt in real seawater. h) Potential of the MoS2QD-aerogel-100 aerogel and Pt in real sea water at different cycles.

Conclusions In summary, we have demonstrated that MoS2QDaerogel is an easy-to-prepare, highly efficient, costless, long-term stable, and seawater feasible HER electrocatalyst. The 3D MoS2QD-aerogel-100 exhibits the HER overpotential of 53 mV (at the current density of 7 mA/g) and a Tafel slope of 41 mV decade-1 in acidic electrolyte, which is the best catalytic activity for MoS2 thin sheets to date. Moreover, MoS2QD-aerogel exhibits excellent HER performance which is superior to the commercial Pt electrocatalyst under neutral electrolyte and real seawater conditions at high overpotential. This work presents a new idea to fabricate a 3D porous configuration that provides many conductive pathways to enhance electrons and ions transportation during the electrochemical HER process. It can also be expected that the MoS2QD-aerogel might provide an up-and-coming pathway to incorporate into the industry of chlorine-alkali with hydrogen production.

ASSOCIATED CONTENT This material is available free of charge via the Internet at http://pubs.acs.org. Experimental characterizations (PDF).

AUTHOR INFORMATION Corresponding Author * [email protected]

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT This study is supported by MOST, Taiwan (MOST 107-2628M-143-001-MY2; 107-2813-C-143-028-M). The authors gratefully thanks to Ms. S.-J. Ji, C.-Y. Chien and P. Y. Huang of Precious Instrument Center for the assistance in SEM, TEM, and HRTEM experiments.

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