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Improving electrochemical active area of MoS2 via attached on 3D-ordered structures for hydrogen evolution reaction
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Improving electrochemical active area of MoS2 via attached on 3D-ordered structures for hydrogen evolution reaction Kisun Kim 1, Anand P. Tiwari 1, Gayea Hyun, Travis G. Novak, Seokwoo Jeon* Department of Materials Science and Engineering, Advanced Battery Centre, KAIST Institute for the Nanocentury, KAIST, Daejeon, 34141, Republic of Korea
highlights A
graphical abstract
well-ordered
3-dimensional
metal (3D-Ni) nanostructures is developed. 2D-MoS2
sheets
are
homoge-
neously grown on the 3D-Ni by solvothermal method. Homogeneously attached 2D-MoS2 on 3D-Ni abundant active sites for hydrogen evolution. Well-ordered 3D-Ni ensures stability and high electrochemical conductivity. The 2D/3D structure combo can be used
in
other
electrochemical
TMDC applications.
article info
abstract
Article history:
To date, researchers have revealed that the electrocatalytic activity of 2-dimensional (2D)
Received 8 May 2019
layered transition metal dichalcogenides (TMDCs) such as MoS2 can be improved by
Received in revised form
making free standing vertical structures to expose edge sites for efficient water splitting.
6 September 2019
However, poor electrical conductivity and structural instability restrict the practical
Accepted 9 September 2019
application of vertical structures for efficient electrocatalytic activities. Here, a homoge-
Available online 30 September 2019
neously attached MoS2 structure on well-ordered 3-dimensional nickel (3D-Ni) is reported for efficient hydrogen evolution reaction (HER). This homogeneously attached structure of
Keywords:
MoS2 leads to abundant active sites and well-ordered 3D-Ni structures, solving the con-
3D nanopatterning
ductivity issue of MoS2 and ensuring the structural stability during electrocatalytic pro-
Hydrogen evolution reaction
cesses. By controlling the amount of MoS2 on the 3D-Ni, it is found that the electrochemical active area (ECSA) is increased by 5 times (50 cm2 of active sites) compared to normal MoS2
* Corresponding author. E-mail address: [email protected] (S. Jeon). 1 Equally Contributed. https://doi.org/10.1016/j.ijhydene.2019.09.071 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 8 1 4 3 e2 8 1 5 0
MoS2
grown on 2D-Ni (9 cm2 of active sites). It is also found that the charge transfer resistance
Proximity field nanopatterning (PnP)
(Rct) of attached MoS2 structures on 3D-Ni (1 U) is 16 times lower than MoS2 grown on 2D-Ni (16 U). In addition, the proposed attached structure of MoS2 is stable in acidic electrolytes for continuous electrocatalytic activity and can be mass producible for practical applications. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In order for hydrogen (H2) to be a real alternative to fossil fuels, a renewable production method is needed. Electrochemical hydrogen evolution reaction (HER) is regarded as one of the most promising clean and renewable techniques for developing H2 as an energy source [1e4]. Noble metal electrocatalysts based on platinum (Pt) are known to catalyze HER very efficiently due to the exceptionally low overpotential, which is close to the thermodynamic potential for hydrogen generation [5,6]. However, the high cost and scarcity of noble metal electrocatalysts have greatly hindered their potential utilization in commercial applications. There has been much research attention dedicated to finding HER electrocatalysts that are both effective and based on earthabundant elements. In order to find efficient electrocatalysts for HER, the hydrogen adsorption free energy (DGH) is a good descriptor for the rate of HER, with an optimal binding DGH y 0 eV [7], which means an efficient electrocatalyst for HER will thus be one that binds hydrogen neither too strongly nor too weakly. Recently, remarkable advances have been made in the utilization of earth-abundant transition-metal-dichalcogenides (TMDCs), MoS2, MoSe2, MoTe2, WS2, etc., as noblemetal-free electrocatalysts for the HER [8e13]. Typical among them is MoS2, which has relatively low adsorption free energy for protons (DGH ¼ 0.08 eV) near its edge sites [14], making it comparable to Pt for HER. However, the HER activity of MoS2 is linearly proportional to the number of edge sites and electrical hopping between layers of MoS2 [15] In this regard, extensive efforts have been made to developing highly efficient MoS2 and other TMDC electrocatalysts by maximizing the active edge sites, including the use of nanoparticles [3], nanowires [16], thin films [17,18], nanosheets [19,20], and defect-rich nanosheets [21]. It has been also reported that vertically aligned MoS2 nanostructures with exposed, highly active edge sites exhibited higher electrocatalytic activities than bulk MoS2 [22,23]. However, the poor electrical conductivity of vertical MoS2 along the adjacent interlayers hinders electron transfer, which is essential for highly efficient HER. Motivated by this understanding, various works have sought to improve HER of MoS2 through growth on various conductive substrates, such as nanoporous carbon [24], carbon nanotube (CNT) [25], nanoporous metal [26] or graphene [27] to facilitate electron transfer at the interfaces. Even though significant improvements have been made from free standing as well as grown on different conductive substrate vertical nanostructures of
MoS2, poor stability as well as sluggish kinetics due to edge defects and difference between surface tension of the conductive substrate and MoS2 [28] hinders the practical applications. MoS2 sheets decorated on different 3dimensional (3D) conductive structures could solve the stability issues of MoS2 heterostructures with conductive materials. In this respect, MoS2 has been grown on 3D graphene [29], 3D nickel foams [30e33] and 3D silica [19] structures for efficient water splitting. However, inhomogeneity in these structures leads aggregation of MoS2 sheets, which results in less exposure of active edge sites and reduced activity for HER. Herein, we demonstrate a significant HER enhancement of MoS2 by making a homogeneously attached structure on well-ordered conductive 3D nickel (3D-Ni) nanostructures [34e40]. First, 3D-Ni nanostructures are prepared by a wellestablished templating method based on the combination of PnP and electrodeposition [36]. Attached MoS2 sheets on the 3D-Ni nanostructures are synthesized by solvent-assisted hydrothermal technique followed by subsequent thermal annealing. We observed that homogeneously attached MoS2 structures on the 3D-Ni showed significantly better HER performance (onset potential 237 mV vs. NHE at 10 mA/cm2) than the MoS2 on 2D-Ni (onset potential 283 mV vs. NHE at 10 mA/cm2). More importantly, we found that homogeneously attached MoS2 structures on 3D-Ni have an electrocatalytic active area (50 cm2 per active sites) that is 5 times higher than MoS2 on 2D-Ni (9 cm2 per active sites). The attached structure of MoS2 on 3D-Ni is stable at higher applied voltage (0.60 V and 0.70 V vs. Ag/AgCl) for 48 h of continuous electrocatalytic reaction and can be scalable for the practical applications. Therefore, we conclude that the attached MoS2 structures on well-ordered conductive 3D-Ni enhances the electrocatalytic activity of MoS2 and this homogeneous structure solves the stability issues of heterostructures in acidic electrolytes.
Materials and methods Preparation of the well-ordered 3D-epoxy template A layer of Au (50 nm) and Cr (5 nm) was deposited on a SiO2/Si substrate as a metal seed layer using electron-beam evaporator (SNTEK) (the Au layer is on the top of Cr layer). The Cr/ Au-deposited substrate was cleaned using an air plasma (CUTEMP, Femtoscience) for 2 min (50 sccm, 40 mTorr, 60 W. An epoxy-based photoresist film (SU-8, Microchem) with
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thickness of 10 mm was spin coated on the substrate as a template. After that the photoresist-coated substrate was soft-baked at 95 C for 10 min. Then, a polydimethylsiloxane (PDMS) (Sylgard 184, Dow Corning) phase mask that consisted of a square array of holes with a diameter of 600 nm, a depth of 420 nm, and a periodicity of 600 nm was placed on the photoresist-coated substrate. The collimated laser (355 nm) (Nd:YAG, Advanced optowave) was went through the phase mask with 15 mJ/cm2 of exposure dose. And the substrate was post-baked at 65 C for 7 min 30 s. Finally, the unexposed regions in photoresist were selectively removed by developing solution (SU-8 developer, Microchem) and rinsed with ethanol.
Preparation of the well-ordered 3D-Ni A 3D-Ni was electrodeposited on 3D nanostructured epoxy template by a conventional two-electrode system using VersaSTATE3 (principle-applied research). The electrodeposition system was consisted of a Ni electrodeposition solution (Hantech) as electrolyte, the seed layer under the 3D-epoxy template as the working electrode, and nickel plate as a counter electrode. Then, the Ni was alternatively deposited in the 3D-epoxy template for 3 h by 2 mA/cm2 (5 s) and 0 mA/cm2 (5 s). After the electrodeposition, the 3D-epoxy template was removed using an ashier (STP compat, Muegge).
Synthesis of MoS2 on 3D-Ni A mixture of 10 mg of (NH4)2MoS4 (Sigma aldrich) and 40 ml of DMF (Sigma aldrich) was sonicated for 10 min. Then, the 3D-Ni was soak in the solution and 50 ml of N2H4$H2O (Sigma Aldrich) was added into the solution. After sonicating for another 10 min, the mixed solution with 3D-Ni was transferred into an autoclave, which was sealed and heated in an oven at 200 C for 24 h. Then, the autoclave was cooled to room temperature naturally and MoS2 on 3D-Ni was rinsed with ethanol. The dried MoS2 on 3D-Ni was annealed in 1-inch tube furnace with Ar (100 sccm) and H2 (200 sccm) at 450 C for 1 h with rate of 3 C/min and Ar (100 sccm) without H2 at 450 C for 1 h. Then, the annealed MoS2 on 3D-Ni was cooled to room temperature naturally.
Structural and chemical composition characterization X-ray diffraction (XRD) of samples are performed with using Cu Ka radiation (Ultima IV, Rigaku). Chemical compositions of samples are confirmed by X-ray photoemission spectroscopy (Thermo VG Scientific, K-alpha). The Raman analysis of the samples has been done with laser 532 nm. The microstructure analysis is done by field emission scanning electron microscopy (SEM) (S-4800, Hitachi).
Electrochemical analysis Electrochemical analyses are done by using VersaSTAT3 (Principle-applied research) electrochemical workstation. The electrochemical performances are carried out via threeelectrode system. The working electrode is as synthesized samples on Au/Cr, the reference electrode is Ag/AgCl, 3M KCl
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and the counter electrode is Pt wire. 0.5M H2SO4 is used as electrolyte. Nitrogen bubbling is employed during measurements in the electrolyte solution to remove oxygen and kept continuously throughout measurements.
Results Fig. 1 shows the fabrication process of attached MoS2 structures on the well-ordered 3D-Ni nanostructure. At first, Au/Cr are deposited on Si/SiO2, afterwards the epoxy template is prepared by PnP, which produces periodic 3D nanostructures in a single exposure by Talbot interference from a conformal phase mask [41e44]. The well-ordered 3D-Ni is produced by electrodeposition of Ni onto the 3D-epoxy template by a conventional two-electrode method (for more details please see materials and methods section) [36]. The attached MoS2 structures are synthesized by solvent-assisted hydrothermal technique on well-ordered 3D-Ni followed by subsequent thermal annealing. For reference we have also synthesized MoS2 sheets on 2D-Ni. Moreover, by changing the amount of precursor to synthesize MoS2, we optimized the MoS2 loading level for the attached structures. Fig. 1b and c shows the scanning electron microscopy (SEM) images for top and crosssectional views of the attached MoS2 on the well-ordered 3DNi. It can be seen from cross sectional view of the SEM image that MoS2 sheets are homogeneously attached on the 3D-Ni. It is also revealed from top view of SEM image that MoS2 sheets are not aggregated on the surface of 3D-Ni. From the SEM images we have found that 3D-Ni structures did not collapse during synthesis of attached MoS2. By contrast, SEM images of MoS2 grown on 2D-Ni (shown in supporting information Fig. S1) shows that MoS2 sheets are aggregated on the surface of 2D-Ni. Transmission electron microscopy (TEM) images of as-synthesized structure reveals 2-dimensinoal sheets of MoS2. Moreover, fast Fourier transform (FFT) from TEM shows a hexagonal pattern, revealing the well crystalline structures A (shown in Fig. 1d). of MoS2 with lattice parameter of 2.7 The X-ray diffraction (XRD) patterns of the as-prepared attached MoS2 on the 3D-Ni along with MoS2 on the 2D-Ni are shown in Fig. 2a. The XRD patterns of MoS2 in Fig. 2a are indexed as hexagonal 2HeMoS2 (JCPDS 37-1492). The single (002) reflection at 2Ө ¼ 14.2 corresponds to a d-spacing of 0.615 nm, indicating a layered structure of MoS2 for both attached MoS2 on the 3D-Ni and MoS2 on the 2D-Ni. It is worth noting that the (100) and (110) reflections at 2Ө ¼ 32.2 and 2Ө ¼ 34.1 are present in the attached MoS2 on 3D-Ni, while absent in the MoS2 on the 2D-Ni, revealing that MoS2 sheets are aggregated on the 2D-Ni but well attached on the 3D-Ni. Furthermore, the peaks for Ni and gold (Au) for both 3D-Ni and 2D-Ni are observed. For further confirmation of quality of MoS2, the possible vibrational modes of the attached MoS2 on 3D-Ni and MoS2 on 2D-Ni have been confirmed by Raman spectroscopy, as shown in Fig. 2b. It can be seen that the Raman spectra of the attached MoS2 on 3D-Ni have E2g (at 377 cm1) and A1g (at 407 cm1) peaks of MoS2 in-plane and out-of-plane vibrations, confirming that the MoS2 sheets are well-attached on 3D-Ni. It is revealed that E2g and A1g peaks of MoS2 on 2D-Ni are slightly shifted towards a higher wavenumber, which would be possible due to aggregation of MoS2
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Fig. 1 e Study concept. (a) Schematic illustration of the MoS2 on well-ordered conductive 3D nickel (3D-Ni). (b, c) Top and cross-sectional SEM images of the optimized MoS2 on 3D-Ni. (d) TEM image of the MoS2 on 3D-Ni. sheets on 2D-Ni [45] and is in good agreement with the XRD and SEM results. To determine the chemical compositions of the MoS2 on 3D-Ni and MoS2 on 2D-Ni, X-ray photoelectron spectroscopy (XPS) measurement is performed (shown in Fig. 2c). It can be seen from elemental scan that the peaks at 162.14 eV and 163.32 eV correspond to the S 2p3/2 and S 2p1/2 states of S, respectively, confirming the formation of MoS2 on 3D-Ni and MoS2 on 2D-Ni. The peaks at 228.24 eV, 232.56 eV, and 235.73 eV indicate the Mo4þ 3d5/2, Mo4þ 3d3/2 and Mo6þ 3d3/ 2 states of Mo, confirming the bond between Mo and S. It is noted that there are no additional peaks of Mo or S sources present in the XPS spectrum, confirming that MoS2 on 3D-Ni as well as MoS2 on 2D-Ni do not have any impurities or additional phases. In order to evaluate the electrochemical performances of MoS2 on 3D-Ni and MoS2 on 2D-Ni as well as reference samples for HER activity, a conventional three-electrode cell is implemented. The electrochemical experiments for all
samples are carried out under the same cell configuration in 0.5 M H2SO4 electrolyte (see the materials and methods section for details). First, HER activities of attached MoS2 grown on 3D-Ni and MoS2 on 2D-Ni are investigated by polarization curves obtained with linear sweep voltammetry (LSV) and shown in Fig. 3a. LSV curves of as-synthesized samples are obtained at the scan rate of 10 mV/s in N2-saturated 0.5 M H2SO4 electrolyte. The MoS2 grown on different Ni structures can be applied directly into the electrolyte without an additional binder, producing solid contact with other electrodes. It can be seen in Fig. 3a that the attached MoS2 on 3D-Ni sample has the smallest onset potential of 237 mV at 10 mAcm2 current density. However, the onset potential of the MoS2 on 2D-Ni at 10 mAcm2 is 283 mV, which clearly shows that the growth of attached MoS2 on 3D-Ni has enhanced the electrocatalytic activity due to homogeneous and non-aggregated MoS2 sheets on the 3D-Ni. For control experiments we have varied the MoS2 amount on the 3D-Ni. It
Fig. 2 e Characterization of the MoS2 on 2D-Ni and 3D-Ni. (a) XRD spectra of the MoS2 on 2D-Ni and 3D-Ni. (b) Raman spectra of the MoS2 on 2D-Ni and 3D-Ni. (c) XPS of the MoS2 on 2D-Ni and 3D-Ni.
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Fig. 3 e Electrochemical performances. (a) Liner sweep voltammetry curves of the MoS2 on 2D-Ni and 3D-Ni with bare 2D-Ni and 3D-Ni. (b) Tafel plots of the MoS2 on 2D-Ni and 3D-Ni with bare 2D-Ni and 3D-Ni. (c) Plots for the determination of the double-layer capacitance (Cdl) for the MoS2 on 2D-Ni and 3D-Ni. (d) Nyquist plots for the MoS2 on 2D-Ni and 3D-Ni.
has been found that decreasing the amount of MoS2 (to half the value of the optimized sample) is not effective at enhancing the electrocatalytic activity of MoS2 (onset potential 288 mV at 10 mAcm2). Moreover, using a higher amount of MoS2 (double the optimized sample) on 3D-Ni has very low electrocatalytic activity (onset potential 313 mV at 10 mAcm2), even lower than aggregated MoS2 on 2D-Ni (shown in supporting information Fig. S5). The kinetics of the HER process were investigated by Tafel slope, in which a small value of the Tafel slope indicates more HER activity per incremental increase of potential. The Tafel plots of the as-synthesized MoS2 on 3D-Ni and MoS2 on 2D-Ni along with reference samples are obtained from LSV curves and shown in Fig. 3b. The Tafel slope of attached MoS2 on 3D-Ni sample has the lowest value (110 mV/dec) among the asprepared samples. However, the MoS2 on 2D-Ni sample has a Tafel slope of 118 mV/dec. It is known that the Volmer step, Heyrovsky step, and Tafel step have Tafel slopes of 118 mV/dec, 40 mV/dec and 30 mV/dec respectively [46]. In this work the attached MoS2 on 3D-Ni sample with Tafel slope (110 mV/dec) belongs to the Volmer mechanism, correlating well with enhancement of electrocatalytic behavior of MoS2 from the homogeneous and well-ordered attached structures of MoS2, which expose the maximum number of active sites. Furthermore, double-layer capacitances (Cdl) are calculated from cyclic voltammetry (CV) to evaluate the effective electrochemically active surface area (ECSA) of attached MoS2 on 3D-Ni as well as MoS2 on 2D-Ni. As shown in Fig. 3c, the calculated Cdl value of attached MoS2 on 3D-Ni (40 mF/cm2) is fivefold higher than MoS2 on 2D-Ni (7 mF/cm2) (CV of samples are shown in supporting information Fig. S4), which implies that attached MoS2 on 3D-Ni provides higher rate of active sites for efficient electrocatalytic activity than normal 2D MoS2 nanostructures
(summarized in Table 1). To compare the intrinsic activity difference between samples, we have normalized the current density of the as-prepared samples with electrochemical active surface area (ECSA). It is revealed from the normalized current density that MoS2 on the 3D-Ni possesses highest electrochemical activity among as-prepared samples, which is consistent with our report (shown in supporting information Fig. S2). We have also performed electrocatalytic activity of MoS2 on the 3D-Ni sample in the neutral media. It is revealed from the liner sweep voltammetry (LSV) curves that the over potential of the sample to gain 10 mA cm2 current density in neutral media is higher than in acidic media, which is well known (shown in supporting information Fig. S3). The electrochemical impedance spectroscopy (EIS) is conducted to confirm changes of the surface properties MoS2 grown on different 2D-Ni and 3D-Ni nanostructures. The Nyquist plots of the impedance spectrum of attached MoS2 on 2D-Ni and 3D-Ni at 0.60 V vs. Ag/AgCl are shown in Fig. 3d. Both plots have a semi-circular domain, where the diameter corresponds to electron transfer resistance (Rct). It is revealed from the plot that attached MoS2 on 3D-Ni has an extremely low Rct of 1U,
Table 1 e Electrochemical performances of the assynthesized samples along with reference samples. Samples
Pt_wire Bare 2D-Ni Bare 3D-Ni MoS2 on 2D-Ni MoS2 on 3D-Ni
Onset potential Tafel slope Double layer (at 10 mA/cm2 capacitance vs. NHE) (Cdl) 22 mV 380 mV 324 mV 283 mV 237 mV
33 mV/dec 161 mV/dec 158 mV/dec 118 mV/dec 110 mV/dec
e e e 7 mF/cm2 40 mF/cm2
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Fig. 4 e Durability and Scalability Test. (a) Chronoamperometry measurements (j-t) of the MoS2 on 3D-Ni with continually applied voltage for 48 h. (b) Linear sweep voltammetry curves of the MoS2 on 3D-Ni with the different size of sample area.
which is 16 times lower than that of MoS2 on 2D-Ni (16 U). This result indicates that charges can move faster in the attached MoS2 on 3D-Ni than in MoS2 on 2D-Ni, which enhances the catalytic activity of MoS2 for HER. The stability is a significant factor of electrocatalysts for efficient HER activity. The durability of attached MoS2 on 3DNi is investigated by chronoamperometry (jet) measurements with different applied voltages (shown in Fig. 4a). The chronoamperometry process is first performed at applied potential of 0.60 V vs. Ag/AgCl for 24 h, afterword we increased the applied potential to 0.70 V vs. Ag/AgCl for next 24 h of continuous operation. The current density of attached MoS2 on 3D-Ni sample increased gradually over 24 h of continuous operation, which might be caused by increased access of the electrolyte to the sample interfaces [47]. However, after initial increase in the current density, it is stable over the next 24 h at higher applied voltage (0.70 V vs. Ag/AgCl). An SEM image of sample after 48 h continuous electrolysis revealed no change or collapse in the structures, indicating significant stability and robustness of the attached structure of MoS2 (shown in supporting information Fig. S6). The phase stability is also confirmed by XRD analysis (shown in supporting information Fig. S6), where the major peaks corresponding to MoS2 and Ni are clearly still present after the electrochemical tests. The scalability of the attached MoS2 on 3D-Ni is obtained by LSV curves of the samples with the different area (shown in Fig. 4b). It can be seen from LSV curves that the value of current at same applied voltage is proportionally increased with the size of sample area, which is well agree with homogeneity of the sample, revealing that the attached MoS2 on 3D-Ni structures can be produced on a large scale for efficient electrocatalytic activity for practical applications. However, the larger area sample can be tested by bigger cell configuration.
Conclusions In summary, we propose a new method to improve electrocatalytic activity of MoS2 by synthesizing an attached MoS2 structure on 3D-Ni for HER. Our results demonstrated that the enhancement in the electrocatalytic activity of attached MoS2 on 3D-Ni is attributed to the homogeneous and well-ordered structure, which leads to abundant active sites for excellent HER performance. Furthermore, the attached MoS2 on 3D-Ni
shows a low onset potential of 237 mV at 10 mA/cm2 current density, which is lower than MoS2 grown on the 2D-Ni (283 mV at 10 mA/cm2). The higher electrochemical activity of attached MoS2 on 3D-Ni is attributed to the synergetic effects of well-attached, homogeneous and non-aggregated structures of MoS2, which yield a highly accessible electrochemical active area (5 times more than MoS2 on 2D-Ni) for efficient HER activity. In addition, our proposed attached structure of MoS2 is stable during continuous electrocatalytic activity and can be scalable for practical applications. Therefore, in general our strategy could be applicable for other layered materials for improving their electrocatalytic activity for efficient HER.
Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2017R1D1A1B03032791, No. 2016R1E1A1A01943131, No. 2017M3D1A1039558 and 2017M3A7B4049507).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.09.071.
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Improving electrochemical active area of MoS2 via attached on 3D-ordered structures for hydrogen evolution reaction Kisun Kim 1, Anand P. Tiwari 1, Gayea Hyun, Travis G. Novak, Seokwoo Jeon* Department of Materials Science and Engineering, Advanced Battery Centre, KAIST Institute for the Nanocentury, KAIST, Daejeon, 34141, Republic of Korea
highlights A
graphical abstract
well-ordered
3-dimensional
metal (3D-Ni) nanostructures is developed. 2D-MoS2
sheets
are
homoge-
neously grown on the 3D-Ni by solvothermal method. Homogeneously attached 2D-MoS2 on 3D-Ni abundant active sites for hydrogen evolution. Well-ordered 3D-Ni ensures stability and high electrochemical conductivity. The 2D/3D structure combo can be used
in
other
electrochemical
TMDC applications.
article info
abstract
Article history:
To date, researchers have revealed that the electrocatalytic activity of 2-dimensional (2D)
Received 8 May 2019
layered transition metal dichalcogenides (TMDCs) such as MoS2 can be improved by
Received in revised form
making free standing vertical structures to expose edge sites for efficient water splitting.
6 September 2019
However, poor electrical conductivity and structural instability restrict the practical
Accepted 9 September 2019
application of vertical structures for efficient electrocatalytic activities. Here, a homoge-
Available online 30 September 2019
neously attached MoS2 structure on well-ordered 3-dimensional nickel (3D-Ni) is reported for efficient hydrogen evolution reaction (HER). This homogeneously attached structure of
Keywords:
MoS2 leads to abundant active sites and well-ordered 3D-Ni structures, solving the con-
3D nanopatterning
ductivity issue of MoS2 and ensuring the structural stability during electrocatalytic pro-
Hydrogen evolution reaction
cesses. By controlling the amount of MoS2 on the 3D-Ni, it is found that the electrochemical active area (ECSA) is increased by 5 times (50 cm2 of active sites) compared to normal MoS2
* Corresponding author. E-mail address: [email protected] (S. Jeon). 1 Equally Contributed. https://doi.org/10.1016/j.ijhydene.2019.09.071 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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MoS2
grown on 2D-Ni (9 cm2 of active sites). It is also found that the charge transfer resistance
Proximity field nanopatterning (PnP)
(Rct) of attached MoS2 structures on 3D-Ni (1 U) is 16 times lower than MoS2 grown on 2D-Ni (16 U). In addition, the proposed attached structure of MoS2 is stable in acidic electrolytes for continuous electrocatalytic activity and can be mass producible for practical applications. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In order for hydrogen (H2) to be a real alternative to fossil fuels, a renewable production method is needed. Electrochemical hydrogen evolution reaction (HER) is regarded as one of the most promising clean and renewable techniques for developing H2 as an energy source [1e4]. Noble metal electrocatalysts based on platinum (Pt) are known to catalyze HER very efficiently due to the exceptionally low overpotential, which is close to the thermodynamic potential for hydrogen generation [5,6]. However, the high cost and scarcity of noble metal electrocatalysts have greatly hindered their potential utilization in commercial applications. There has been much research attention dedicated to finding HER electrocatalysts that are both effective and based on earthabundant elements. In order to find efficient electrocatalysts for HER, the hydrogen adsorption free energy (DGH) is a good descriptor for the rate of HER, with an optimal binding DGH y 0 eV [7], which means an efficient electrocatalyst for HER will thus be one that binds hydrogen neither too strongly nor too weakly. Recently, remarkable advances have been made in the utilization of earth-abundant transition-metal-dichalcogenides (TMDCs), MoS2, MoSe2, MoTe2, WS2, etc., as noblemetal-free electrocatalysts for the HER [8e13]. Typical among them is MoS2, which has relatively low adsorption free energy for protons (DGH ¼ 0.08 eV) near its edge sites [14], making it comparable to Pt for HER. However, the HER activity of MoS2 is linearly proportional to the number of edge sites and electrical hopping between layers of MoS2 [15] In this regard, extensive efforts have been made to developing highly efficient MoS2 and other TMDC electrocatalysts by maximizing the active edge sites, including the use of nanoparticles [3], nanowires [16], thin films [17,18], nanosheets [19,20], and defect-rich nanosheets [21]. It has been also reported that vertically aligned MoS2 nanostructures with exposed, highly active edge sites exhibited higher electrocatalytic activities than bulk MoS2 [22,23]. However, the poor electrical conductivity of vertical MoS2 along the adjacent interlayers hinders electron transfer, which is essential for highly efficient HER. Motivated by this understanding, various works have sought to improve HER of MoS2 through growth on various conductive substrates, such as nanoporous carbon [24], carbon nanotube (CNT) [25], nanoporous metal [26] or graphene [27] to facilitate electron transfer at the interfaces. Even though significant improvements have been made from free standing as well as grown on different conductive substrate vertical nanostructures of
MoS2, poor stability as well as sluggish kinetics due to edge defects and difference between surface tension of the conductive substrate and MoS2 [28] hinders the practical applications. MoS2 sheets decorated on different 3dimensional (3D) conductive structures could solve the stability issues of MoS2 heterostructures with conductive materials. In this respect, MoS2 has been grown on 3D graphene [29], 3D nickel foams [30e33] and 3D silica [19] structures for efficient water splitting. However, inhomogeneity in these structures leads aggregation of MoS2 sheets, which results in less exposure of active edge sites and reduced activity for HER. Herein, we demonstrate a significant HER enhancement of MoS2 by making a homogeneously attached structure on well-ordered conductive 3D nickel (3D-Ni) nanostructures [34e40]. First, 3D-Ni nanostructures are prepared by a wellestablished templating method based on the combination of PnP and electrodeposition [36]. Attached MoS2 sheets on the 3D-Ni nanostructures are synthesized by solvent-assisted hydrothermal technique followed by subsequent thermal annealing. We observed that homogeneously attached MoS2 structures on the 3D-Ni showed significantly better HER performance (onset potential 237 mV vs. NHE at 10 mA/cm2) than the MoS2 on 2D-Ni (onset potential 283 mV vs. NHE at 10 mA/cm2). More importantly, we found that homogeneously attached MoS2 structures on 3D-Ni have an electrocatalytic active area (50 cm2 per active sites) that is 5 times higher than MoS2 on 2D-Ni (9 cm2 per active sites). The attached structure of MoS2 on 3D-Ni is stable at higher applied voltage (0.60 V and 0.70 V vs. Ag/AgCl) for 48 h of continuous electrocatalytic reaction and can be scalable for the practical applications. Therefore, we conclude that the attached MoS2 structures on well-ordered conductive 3D-Ni enhances the electrocatalytic activity of MoS2 and this homogeneous structure solves the stability issues of heterostructures in acidic electrolytes.
Materials and methods Preparation of the well-ordered 3D-epoxy template A layer of Au (50 nm) and Cr (5 nm) was deposited on a SiO2/Si substrate as a metal seed layer using electron-beam evaporator (SNTEK) (the Au layer is on the top of Cr layer). The Cr/ Au-deposited substrate was cleaned using an air plasma (CUTEMP, Femtoscience) for 2 min (50 sccm, 40 mTorr, 60 W. An epoxy-based photoresist film (SU-8, Microchem) with
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thickness of 10 mm was spin coated on the substrate as a template. After that the photoresist-coated substrate was soft-baked at 95 C for 10 min. Then, a polydimethylsiloxane (PDMS) (Sylgard 184, Dow Corning) phase mask that consisted of a square array of holes with a diameter of 600 nm, a depth of 420 nm, and a periodicity of 600 nm was placed on the photoresist-coated substrate. The collimated laser (355 nm) (Nd:YAG, Advanced optowave) was went through the phase mask with 15 mJ/cm2 of exposure dose. And the substrate was post-baked at 65 C for 7 min 30 s. Finally, the unexposed regions in photoresist were selectively removed by developing solution (SU-8 developer, Microchem) and rinsed with ethanol.
Preparation of the well-ordered 3D-Ni A 3D-Ni was electrodeposited on 3D nanostructured epoxy template by a conventional two-electrode system using VersaSTATE3 (principle-applied research). The electrodeposition system was consisted of a Ni electrodeposition solution (Hantech) as electrolyte, the seed layer under the 3D-epoxy template as the working electrode, and nickel plate as a counter electrode. Then, the Ni was alternatively deposited in the 3D-epoxy template for 3 h by 2 mA/cm2 (5 s) and 0 mA/cm2 (5 s). After the electrodeposition, the 3D-epoxy template was removed using an ashier (STP compat, Muegge).
Synthesis of MoS2 on 3D-Ni A mixture of 10 mg of (NH4)2MoS4 (Sigma aldrich) and 40 ml of DMF (Sigma aldrich) was sonicated for 10 min. Then, the 3D-Ni was soak in the solution and 50 ml of N2H4$H2O (Sigma Aldrich) was added into the solution. After sonicating for another 10 min, the mixed solution with 3D-Ni was transferred into an autoclave, which was sealed and heated in an oven at 200 C for 24 h. Then, the autoclave was cooled to room temperature naturally and MoS2 on 3D-Ni was rinsed with ethanol. The dried MoS2 on 3D-Ni was annealed in 1-inch tube furnace with Ar (100 sccm) and H2 (200 sccm) at 450 C for 1 h with rate of 3 C/min and Ar (100 sccm) without H2 at 450 C for 1 h. Then, the annealed MoS2 on 3D-Ni was cooled to room temperature naturally.
Structural and chemical composition characterization X-ray diffraction (XRD) of samples are performed with using Cu Ka radiation (Ultima IV, Rigaku). Chemical compositions of samples are confirmed by X-ray photoemission spectroscopy (Thermo VG Scientific, K-alpha). The Raman analysis of the samples has been done with laser 532 nm. The microstructure analysis is done by field emission scanning electron microscopy (SEM) (S-4800, Hitachi).
Electrochemical analysis Electrochemical analyses are done by using VersaSTAT3 (Principle-applied research) electrochemical workstation. The electrochemical performances are carried out via threeelectrode system. The working electrode is as synthesized samples on Au/Cr, the reference electrode is Ag/AgCl, 3M KCl
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and the counter electrode is Pt wire. 0.5M H2SO4 is used as electrolyte. Nitrogen bubbling is employed during measurements in the electrolyte solution to remove oxygen and kept continuously throughout measurements.
Results Fig. 1 shows the fabrication process of attached MoS2 structures on the well-ordered 3D-Ni nanostructure. At first, Au/Cr are deposited on Si/SiO2, afterwards the epoxy template is prepared by PnP, which produces periodic 3D nanostructures in a single exposure by Talbot interference from a conformal phase mask [41e44]. The well-ordered 3D-Ni is produced by electrodeposition of Ni onto the 3D-epoxy template by a conventional two-electrode method (for more details please see materials and methods section) [36]. The attached MoS2 structures are synthesized by solvent-assisted hydrothermal technique on well-ordered 3D-Ni followed by subsequent thermal annealing. For reference we have also synthesized MoS2 sheets on 2D-Ni. Moreover, by changing the amount of precursor to synthesize MoS2, we optimized the MoS2 loading level for the attached structures. Fig. 1b and c shows the scanning electron microscopy (SEM) images for top and crosssectional views of the attached MoS2 on the well-ordered 3DNi. It can be seen from cross sectional view of the SEM image that MoS2 sheets are homogeneously attached on the 3D-Ni. It is also revealed from top view of SEM image that MoS2 sheets are not aggregated on the surface of 3D-Ni. From the SEM images we have found that 3D-Ni structures did not collapse during synthesis of attached MoS2. By contrast, SEM images of MoS2 grown on 2D-Ni (shown in supporting information Fig. S1) shows that MoS2 sheets are aggregated on the surface of 2D-Ni. Transmission electron microscopy (TEM) images of as-synthesized structure reveals 2-dimensinoal sheets of MoS2. Moreover, fast Fourier transform (FFT) from TEM shows a hexagonal pattern, revealing the well crystalline structures A (shown in Fig. 1d). of MoS2 with lattice parameter of 2.7 The X-ray diffraction (XRD) patterns of the as-prepared attached MoS2 on the 3D-Ni along with MoS2 on the 2D-Ni are shown in Fig. 2a. The XRD patterns of MoS2 in Fig. 2a are indexed as hexagonal 2HeMoS2 (JCPDS 37-1492). The single (002) reflection at 2Ө ¼ 14.2 corresponds to a d-spacing of 0.615 nm, indicating a layered structure of MoS2 for both attached MoS2 on the 3D-Ni and MoS2 on the 2D-Ni. It is worth noting that the (100) and (110) reflections at 2Ө ¼ 32.2 and 2Ө ¼ 34.1 are present in the attached MoS2 on 3D-Ni, while absent in the MoS2 on the 2D-Ni, revealing that MoS2 sheets are aggregated on the 2D-Ni but well attached on the 3D-Ni. Furthermore, the peaks for Ni and gold (Au) for both 3D-Ni and 2D-Ni are observed. For further confirmation of quality of MoS2, the possible vibrational modes of the attached MoS2 on 3D-Ni and MoS2 on 2D-Ni have been confirmed by Raman spectroscopy, as shown in Fig. 2b. It can be seen that the Raman spectra of the attached MoS2 on 3D-Ni have E2g (at 377 cm1) and A1g (at 407 cm1) peaks of MoS2 in-plane and out-of-plane vibrations, confirming that the MoS2 sheets are well-attached on 3D-Ni. It is revealed that E2g and A1g peaks of MoS2 on 2D-Ni are slightly shifted towards a higher wavenumber, which would be possible due to aggregation of MoS2
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Fig. 1 e Study concept. (a) Schematic illustration of the MoS2 on well-ordered conductive 3D nickel (3D-Ni). (b, c) Top and cross-sectional SEM images of the optimized MoS2 on 3D-Ni. (d) TEM image of the MoS2 on 3D-Ni. sheets on 2D-Ni [45] and is in good agreement with the XRD and SEM results. To determine the chemical compositions of the MoS2 on 3D-Ni and MoS2 on 2D-Ni, X-ray photoelectron spectroscopy (XPS) measurement is performed (shown in Fig. 2c). It can be seen from elemental scan that the peaks at 162.14 eV and 163.32 eV correspond to the S 2p3/2 and S 2p1/2 states of S, respectively, confirming the formation of MoS2 on 3D-Ni and MoS2 on 2D-Ni. The peaks at 228.24 eV, 232.56 eV, and 235.73 eV indicate the Mo4þ 3d5/2, Mo4þ 3d3/2 and Mo6þ 3d3/ 2 states of Mo, confirming the bond between Mo and S. It is noted that there are no additional peaks of Mo or S sources present in the XPS spectrum, confirming that MoS2 on 3D-Ni as well as MoS2 on 2D-Ni do not have any impurities or additional phases. In order to evaluate the electrochemical performances of MoS2 on 3D-Ni and MoS2 on 2D-Ni as well as reference samples for HER activity, a conventional three-electrode cell is implemented. The electrochemical experiments for all
samples are carried out under the same cell configuration in 0.5 M H2SO4 electrolyte (see the materials and methods section for details). First, HER activities of attached MoS2 grown on 3D-Ni and MoS2 on 2D-Ni are investigated by polarization curves obtained with linear sweep voltammetry (LSV) and shown in Fig. 3a. LSV curves of as-synthesized samples are obtained at the scan rate of 10 mV/s in N2-saturated 0.5 M H2SO4 electrolyte. The MoS2 grown on different Ni structures can be applied directly into the electrolyte without an additional binder, producing solid contact with other electrodes. It can be seen in Fig. 3a that the attached MoS2 on 3D-Ni sample has the smallest onset potential of 237 mV at 10 mAcm2 current density. However, the onset potential of the MoS2 on 2D-Ni at 10 mAcm2 is 283 mV, which clearly shows that the growth of attached MoS2 on 3D-Ni has enhanced the electrocatalytic activity due to homogeneous and non-aggregated MoS2 sheets on the 3D-Ni. For control experiments we have varied the MoS2 amount on the 3D-Ni. It
Fig. 2 e Characterization of the MoS2 on 2D-Ni and 3D-Ni. (a) XRD spectra of the MoS2 on 2D-Ni and 3D-Ni. (b) Raman spectra of the MoS2 on 2D-Ni and 3D-Ni. (c) XPS of the MoS2 on 2D-Ni and 3D-Ni.
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Fig. 3 e Electrochemical performances. (a) Liner sweep voltammetry curves of the MoS2 on 2D-Ni and 3D-Ni with bare 2D-Ni and 3D-Ni. (b) Tafel plots of the MoS2 on 2D-Ni and 3D-Ni with bare 2D-Ni and 3D-Ni. (c) Plots for the determination of the double-layer capacitance (Cdl) for the MoS2 on 2D-Ni and 3D-Ni. (d) Nyquist plots for the MoS2 on 2D-Ni and 3D-Ni.
has been found that decreasing the amount of MoS2 (to half the value of the optimized sample) is not effective at enhancing the electrocatalytic activity of MoS2 (onset potential 288 mV at 10 mAcm2). Moreover, using a higher amount of MoS2 (double the optimized sample) on 3D-Ni has very low electrocatalytic activity (onset potential 313 mV at 10 mAcm2), even lower than aggregated MoS2 on 2D-Ni (shown in supporting information Fig. S5). The kinetics of the HER process were investigated by Tafel slope, in which a small value of the Tafel slope indicates more HER activity per incremental increase of potential. The Tafel plots of the as-synthesized MoS2 on 3D-Ni and MoS2 on 2D-Ni along with reference samples are obtained from LSV curves and shown in Fig. 3b. The Tafel slope of attached MoS2 on 3D-Ni sample has the lowest value (110 mV/dec) among the asprepared samples. However, the MoS2 on 2D-Ni sample has a Tafel slope of 118 mV/dec. It is known that the Volmer step, Heyrovsky step, and Tafel step have Tafel slopes of 118 mV/dec, 40 mV/dec and 30 mV/dec respectively [46]. In this work the attached MoS2 on 3D-Ni sample with Tafel slope (110 mV/dec) belongs to the Volmer mechanism, correlating well with enhancement of electrocatalytic behavior of MoS2 from the homogeneous and well-ordered attached structures of MoS2, which expose the maximum number of active sites. Furthermore, double-layer capacitances (Cdl) are calculated from cyclic voltammetry (CV) to evaluate the effective electrochemically active surface area (ECSA) of attached MoS2 on 3D-Ni as well as MoS2 on 2D-Ni. As shown in Fig. 3c, the calculated Cdl value of attached MoS2 on 3D-Ni (40 mF/cm2) is fivefold higher than MoS2 on 2D-Ni (7 mF/cm2) (CV of samples are shown in supporting information Fig. S4), which implies that attached MoS2 on 3D-Ni provides higher rate of active sites for efficient electrocatalytic activity than normal 2D MoS2 nanostructures
(summarized in Table 1). To compare the intrinsic activity difference between samples, we have normalized the current density of the as-prepared samples with electrochemical active surface area (ECSA). It is revealed from the normalized current density that MoS2 on the 3D-Ni possesses highest electrochemical activity among as-prepared samples, which is consistent with our report (shown in supporting information Fig. S2). We have also performed electrocatalytic activity of MoS2 on the 3D-Ni sample in the neutral media. It is revealed from the liner sweep voltammetry (LSV) curves that the over potential of the sample to gain 10 mA cm2 current density in neutral media is higher than in acidic media, which is well known (shown in supporting information Fig. S3). The electrochemical impedance spectroscopy (EIS) is conducted to confirm changes of the surface properties MoS2 grown on different 2D-Ni and 3D-Ni nanostructures. The Nyquist plots of the impedance spectrum of attached MoS2 on 2D-Ni and 3D-Ni at 0.60 V vs. Ag/AgCl are shown in Fig. 3d. Both plots have a semi-circular domain, where the diameter corresponds to electron transfer resistance (Rct). It is revealed from the plot that attached MoS2 on 3D-Ni has an extremely low Rct of 1U,
Table 1 e Electrochemical performances of the assynthesized samples along with reference samples. Samples
Pt_wire Bare 2D-Ni Bare 3D-Ni MoS2 on 2D-Ni MoS2 on 3D-Ni
Onset potential Tafel slope Double layer (at 10 mA/cm2 capacitance vs. NHE) (Cdl) 22 mV 380 mV 324 mV 283 mV 237 mV
33 mV/dec 161 mV/dec 158 mV/dec 118 mV/dec 110 mV/dec
e e e 7 mF/cm2 40 mF/cm2
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Fig. 4 e Durability and Scalability Test. (a) Chronoamperometry measurements (j-t) of the MoS2 on 3D-Ni with continually applied voltage for 48 h. (b) Linear sweep voltammetry curves of the MoS2 on 3D-Ni with the different size of sample area.
which is 16 times lower than that of MoS2 on 2D-Ni (16 U). This result indicates that charges can move faster in the attached MoS2 on 3D-Ni than in MoS2 on 2D-Ni, which enhances the catalytic activity of MoS2 for HER. The stability is a significant factor of electrocatalysts for efficient HER activity. The durability of attached MoS2 on 3DNi is investigated by chronoamperometry (jet) measurements with different applied voltages (shown in Fig. 4a). The chronoamperometry process is first performed at applied potential of 0.60 V vs. Ag/AgCl for 24 h, afterword we increased the applied potential to 0.70 V vs. Ag/AgCl for next 24 h of continuous operation. The current density of attached MoS2 on 3D-Ni sample increased gradually over 24 h of continuous operation, which might be caused by increased access of the electrolyte to the sample interfaces [47]. However, after initial increase in the current density, it is stable over the next 24 h at higher applied voltage (0.70 V vs. Ag/AgCl). An SEM image of sample after 48 h continuous electrolysis revealed no change or collapse in the structures, indicating significant stability and robustness of the attached structure of MoS2 (shown in supporting information Fig. S6). The phase stability is also confirmed by XRD analysis (shown in supporting information Fig. S6), where the major peaks corresponding to MoS2 and Ni are clearly still present after the electrochemical tests. The scalability of the attached MoS2 on 3D-Ni is obtained by LSV curves of the samples with the different area (shown in Fig. 4b). It can be seen from LSV curves that the value of current at same applied voltage is proportionally increased with the size of sample area, which is well agree with homogeneity of the sample, revealing that the attached MoS2 on 3D-Ni structures can be produced on a large scale for efficient electrocatalytic activity for practical applications. However, the larger area sample can be tested by bigger cell configuration.
Conclusions In summary, we propose a new method to improve electrocatalytic activity of MoS2 by synthesizing an attached MoS2 structure on 3D-Ni for HER. Our results demonstrated that the enhancement in the electrocatalytic activity of attached MoS2 on 3D-Ni is attributed to the homogeneous and well-ordered structure, which leads to abundant active sites for excellent HER performance. Furthermore, the attached MoS2 on 3D-Ni
shows a low onset potential of 237 mV at 10 mA/cm2 current density, which is lower than MoS2 grown on the 2D-Ni (283 mV at 10 mA/cm2). The higher electrochemical activity of attached MoS2 on 3D-Ni is attributed to the synergetic effects of well-attached, homogeneous and non-aggregated structures of MoS2, which yield a highly accessible electrochemical active area (5 times more than MoS2 on 2D-Ni) for efficient HER activity. In addition, our proposed attached structure of MoS2 is stable during continuous electrocatalytic activity and can be scalable for practical applications. Therefore, in general our strategy could be applicable for other layered materials for improving their electrocatalytic activity for efficient HER.
Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2017R1D1A1B03032791, No. 2016R1E1A1A01943131, No. 2017M3D1A1039558 and 2017M3A7B4049507).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.09.071.
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