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solution at room temperature. After several cycles of potential sweeps, nanoscale thick conformal MoSx layer was formed at the surface of RGO fibers, where the amount of deposition was precisely controllable by cycle number. Synergetic effect from the large surface area of electrically conductive RGO fibers and high electrochemical activity of amorphous MoSx renders the MoSx/RGO hybrid fibers to attain highly efficient catalytic per-formance for HER with a good stability.

Schematic illustration of MoSx/RGO fiber fabrication pro-cess is described in Scheme 1. First, RGO fiber was obtained by wet spinning from 2 wt% GOLC aqueous dispersion (Figure S1, Supporting Information) into 5 wt% of CaCl2 in ethanol/water (1:3, v/v) coagulation bath and subsequent hydriodic acid (HI) treatment. During the wet-spinning process, rotating driven drawing was performed to increases the alignment of GO. End of tubing was placed 2.3 cm away from the coagulation bath center, and the bath was rotated with a speed of 17 rpm to achieve 1.3 draw ratio. We observed the morphology of gel-state GO fiber with a polarized optical microscope (Figure 1a). Between two cross-polarizers, get-state fiber with a typical diameter of ≈350 μm showed vivid brownish color originated from the high alignment of graphene layers along the fiber axis. After drying, we could obtain GOLC fibers, composed of tightly stacked and aligned GO flakes. Its microstructure was confirmed by cross-sectional scanning electron microscope (SEM) observation (Figure 1b). It showed plenty of dentate bends derived from the packing of GOLC structure in the form of disclinations. After chemical reduction with HI, significant morphology change was observed along with volume shrinkage (Figure 1c,d). Removal of water and oxygen functional groups between GO layers are responsible for the volume shrinkage. While other chemical/thermal reduction methods usually result in volume expansion and disruption of compact layered struc-ture due to the release of gas, HI reduction shrinks the volume of bulk RGO due to its unique reduction mechanism based on the halogenation substitution reaction and water generation.[12] Noteworthy that initial large diameter of gel-state GO fiber is crucial for the large area, highly rough surface geometry for-mation of the final RGO fiber. Previously, graphene fiber spin-ning works have mostly employed fine size nozzle less than 250 μm diameter for the uniform and smooth fiber surface formation.[6,9] In this work, we adopted 500 μm large diameter nozzle to induce significant volume shrinkage and subsequent wrinkling at fiber skin. Consequently, 1D fiber geometry with high surface roughness enhances the total surface area. Con-currently, high degree of graphene alignment toward fiber axis and their dense packing yield a good electrical conductivity of 231 S cm−1.

K. E. Lee, Dr. S. P. Sasikala, H. J. Lee, G. Y. Lee, S. H. Koo, T. Yun, H. J. Jung, I. H. Kim, Prof. S. O. KimNational Creative Research Initiative (CRI) Center for Multi-Dimensional Directed Nanoscale AssemblyDepartment of Material Science and EngineeringKAISTDaejeon 34141, Republic of KoreaE-mail: sangouk.kim@kaist.ac.kr

DOI: 10.1002/ppsc.201600375

Amorphous Molybdenum Sulfide Deposited Graphene Liquid Crystalline Fiber for Hydrogen Evolution Reaction Catalysis

Kyung Eun Lee, Suchithra Padmajan Sasikala, Ho Jin Lee, Gil Yong Lee, Sung Hwan Koo, Taeyeong Yun, Hong Ju Jung, InHo Kim, and Sang Ouk Kim*

Graphene oxide liquid crystal (GOLC) is a promising building block for versatile orientation controlled graphene-based mate-rial.[1] Our research group first discovered GOLC behavior that sufficiently purified GO dispersions in aqueous media demonstrate stable colloidal LC phase arising from the dis-cotic shape anisotropy of GO flakes with high aspect ratio and electrostatic repulsion among them.[2] Taking advantage of mesophase ordering, GOLC has been widely exploited in various applications, such as electronics,[3] energy conversion/storage,[4] and environmental treatment.[5] Recently, graphene-based carbon fibers spun from GOLC phase are getting a great deal of research attention as an emerging candidate to comple-ment the current expensive, energy-consuming carbon fiber spinning process.[6] By means of facile direct wet spinning from aqueous LC dispersion, GO flakes gain unidirectional alignment enforced by the strong shear force generated within spinneret.[7] Subsequent coagulation and chemical/thermal reduction may produce reduced graphene oxide (RGO) fibers with remarkable thermal conductivity,[8] electrical conductivity, and tensile strength[9] arising from the highly ordered graphite stacking layers uniaxial oriented along fiber axis.

In this study, we utilize RGO fiber as effective catalytic sup-port for hydrogen evolution reaction (HER). To date, electro-catalyst based on graphene-based fiber has not been reported yet. GOLC fibers were spun from aqueous dispersion and chemically reduced for the successful fabrication of 1D carbon electrodes with a high surface roughness. We chose amor-phous molybdenum sulfide (MoSx) as efficient HER catalyst by coupling with the RGO fiber. It is well known that many unsaturated defect sites of polymeric amorphous MoSx offer abundant catalytic active sites particularly for HER.[10] None-theless, the intrinsic low electrical conductivity of amorphous MoSx is a major drawback for the catalytic applications. In this regard, judicious material design is required.[11] We per-formed electrodeposition of amorphous MoSx at the surface of electrically conductive RGO fiber to prepare catalytic hybrid fiber. Electroconductive RGO fiber allowed straightforward deposition of MoSx by simple potential sweep in the precursor

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For a straightfoward hybridization of MoSx with RGO fiber, a simple electrodeposition method was selected. RGO fiber was immersed in 2 × 10−3 m (NH4)2MoS4 precursor solution and cyclic voltammetry was performed for several cycles at room temperature. As shown in Figure 2a, current responses during the deposition of MoSx shows distinct one oxidation peak and one reduction peak around −0.5 V and −0.8 V versus SCE, respectively. Those peaks are responsible for oxidative conver-sion of MoS4

2− to MoS3 and reductive conversion to MoS2.[13] Also, repetitive deposition of MoSx following CV cycle was con-firmed by increasing integrated current density value. There-fore, loading amount of MoSx catalyst can easily be controllable by varying the number of deposition cycles. In this study, we designated MoSx/RGO fiber according to respective deposition cycle as MoSx-30, MoSx-50, MoSx-70, MoSx-100, MoSx-130, and

MoSx-150/RGO fiber. As a reference, Indium tin oxide (ITO) substrate was used as a flat catalytic support.

Morphology of MoSx/RGO fibers was observed through SEM (Figure 2b; Figure S2, Supporting Information). For a direct observation of MoSx layer, a MoSx/RGO fiber was fractured in liquid nitrogen and the mechanically delaminated part was focused. Figure 2b shows MoSx-50/RGO fiber surface composed of MoSx layer, which is intimately deposited at wrinkled RGO fiber surface. When deposition was performed only 30 cycles, RGO fiber was covered with nanometer-thick amorphous mate-rial regardless of the concave or convex part of uneven fiber surface (Figure S2a, Supporting Information). As the deposi-tion number increases, MoSx layer was clearly observed in a high-magnification image and its thickness reached to 100 nm in MoSx-100/RGO fiber (Figure S2b, Supporting Information).

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Scheme 1. Schematic illustration of fabrication process of amorphous MoSx/RGO fiber through wet-spinning and electrodeposition method.

Figure 1. a) Polarized microscope image of wet-spun GO hydrogel fiber. b) SEM observation of cross-sectional image after drying. c) Cross-section of RGO fiber and d) its high-magnification image. Scale bar: a–c) 500 μm, d) 2 μm.

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Energy-dispersive X-ray spectroscopy (EDS) is a useful tool for elemental composition analysis. We performed EDS on MoSx/RGO fiber to clarify the chemical composition and coverage of MoSx layer. In the surface SEM image of MoSx-100/RGO fiber, it was hard to distinguish RGO fiber and top MoSx layer because of resolution limit and low contrast between two mate-rials (Figure S3a, Supporting Information). Only several delam-inated part provided visible existence of MoSx top layer. When EDS mapping was performed, molybdenum and sulfur was clearly detected in the entire RGO fiber surface (Figure S3b, Supporting Information). From these result, we confirmed that controlled thickness MoSx can be intimately deposited on the rough RGO fiber surface, which is highly desirable for effective charge transfer to less electroconductive MoSx.

Further, the chemical composition of MoSx layer was analyzed with X-ray photoelectron spectroscopy (Figure 2c,d; Figure S4, Supporting Information). Mo 3d scan showed major Mo4+ peaks

at 229.3 and 232.5 eV, which may be attributed to spin-coupled doublet of Mo 3d5/2 and Mo 3d3/2, respectively. Also, defective Mo(V) O bond peaks were observed at 230.4 and 233.9 eV. Deconvolution of S 2p scan gave S2

− 2p doublet at 162.1 and 163.5 eV and also S2

2− at 162.8 and 164.5 eV, representing the amorphous structure of electrochemically deposited MoSx layer. From the intensity of each peak, atomic ratio between Mo and S was calculated to be 2.3. Figure 2e shows Raman spectroscopy results of RGO fiber and MoSx/RGO fiber. Both RGO fiber and MoSx/RGO fiber showed typical graphitic Raman peaks such as D peak at 1351 cm−1, G peaks at 1581 cm−1, and broad 2D peak ≈2700–3000 cm−1 range. In the case of MoSx/RGO fiber, none of in-plane mode and out-of-plane mode of crystalline MoS2 (at 383 and 408 cm−1) was observed, which also supports the amor-phous structure of MoSx

[14] (Figure 2f).Figure S5 (Supporting Information) shows the CV

curve of each MoSx/RGO fiber with 100 mV s−1 scan rate

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Figure 2. a) Cyclic voltammetry (CV) of RGO fibers during the deposition of MoSx films for 70 cycles. b) SEM image of MoSx film deposited on RGO fiber. c) Mo 3d region and d) S 2p region. e) Raman spectroscopy of RGO fiber (black) and MoSx/RGO fiber (red). f) Raman spectroscopy result of MoSx/RGO fiber in the region where crystalline MoS2 in-plane (E2g) mode and out-of-plane (A1g) peaks rise.

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in −0.1 versus +0.1 versus SCE potential range. Square shape of the curves proved non-Faradic capacitive current density response in this potential range. The areal integration of curve was calcu-lated the specific capacitance (C) value. Table 1 summarizes all the obtained values. After 30 deposition cycles, 5 mF cm−2 was obtained, which is three times higher than the C value of MoSx on flat ITO (1.6 mF cm−2) prepared in the same deposition condi-tion. This result exhibits that RGO fiber can support the amount of catalyst significantly higher than usual flat electrodes. Further-more, the value gradually increased along with cycle number and finally reached up to 23.2 mF cm−2 after 150 cycles.

Electrocatalytic HER activity of each MoSx/RGO fibers was systematically investigated in 0.5 m H2SO4 solution with three electrode system (Figure 3). Figure 3a shows iR-corrected linear sweep voltammetry (LSV) results for the MoSx-based catalysts with different catalytic support and deposition number. Catalytic activity of MoSx was significantly increased when it is deposited on RGO fiber surface. When MoSx was deposited on flat ITO

electrode (ITO-30, black dot), it exhibited 266 mV overpoten-tial to achieve 10 mA cm−2 current density. By contrast, over-potential value for the same condition significantly decreased down to 229 mV when MoSx was deposited on RGO fiber (MoSx-30/RGO fiber, black line), and this catalyst fiber showed higher current density in the entire negative potential range. Such a high C value of MoSx/RGO fiber compared to MoSx/ITO directly influenced their respective catalytic activity. We systemati-cally investigated the effect of deposition number on the catalytic activity of MoSx/RGO fiber. It showed that when the deposition number increased, onset potential gradually decreased. How-ever, too much high deposition can inhibit catalytic activity by low electrical conductivity of MoSx and diffusion limit of elec-trolyte into porous MoSx layer. In this regard, the overpotential to reach 10 mA cm−2 may be higher for MoSx-150/RGO fiber (210 mV) compared to MoSx-130/RGO fiber (204 mV) (Table 1).

In Figure 3b, the linear portion of Tafel plot (η = b log j + a, where η is the overpotential, b is the Tafel slope, and j is the current density) was fitted to investigate the HER mechanism of our catalyst. A reference sample, MoSx-30/ITO showed the smallest Tafel slope (39.3 mV per decade), comparable to the previous literature values of ≈40 mV per decade.[10b,c] Theoretical study suggests that HER mechanism can be understood from the Tafel slope as Volmer–Heyrovsky and free energy barrier for the first step is compensated.[15] In the case of MoSx/RGO fiber, the Tafel slope slightly increased from 46.6 to 55.1 mV per decade along with deposition number. Figure 3c shows the Nyquist plot of each MoSx/RGO fiber at 160 mV overpotential. Two distinct semicircles were observed in the high and medium frequency range, respectively. Semicircle at high frequency

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Table 1. Electrochemical properties of MoSx/ITO and MoSx/RGO fibers.

Support ITO RGO fiber

Deposition [times] 30 30 50 100 130 150

C [mF cm−1] 1.6 5 10.8 17.3 21.8 23.2

ηa) at 10 mA cm−1 [mV] 266 229 216 214 204 210

Tafel slope [mV per decade] 39.3 46.4 45.7 49.3 54.0 55.1

Rctb) [Ohm] 5461 772.6 457.0 304.4 188.3 229.5

a)Overpotential; b)Charge-transfer resistance.

Figure 3. a) Linear sweep voltammetry (LSV) and b) the Tafel plot of MoSx/RGO fiber with various deposition cycle numbers in 0.5 m H2SO4. Scan rate is 5 mV s−1. c) Nyquist plot of MoSx/RGO fiber at η = 160 mV in 0.5 m H2SO4. d) Stability test result of 130 cycle deposited MoSx/ITO (dotted line) and RGO fiber (solid line). Black lines show polarization curves after activation and red lines indicate polarization curve after CV 50 times scanning.

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range was independent of overpotential, implying non-Faradic origins such as contact resistance between MoSx/RGO or capacitance/resistance of RGO fiber (Figure S6, Supporting Information).[11a,16] The semicircle at medium range, which is dependent on overpotential, is chosen to fit using the equiva-lent circuit (Figure S6, Supporting Information, inset) and thereby the charge-transfer resistance (Rct) value was calcu-lated. In consistent with LSV result, Rct dramatically reduced in the high deposition number catalysts, but increased in MoSx-150/RGO fiber. Significantly, clear 45° line was observed at high frequency range in the high catalyst loading samples due to the thick porous diffusive layers of MoSx.

For the evaluation of catalytic stability, we compared iR-corrected LSV results after 10 and 50 times of CV scan. In Figure 3d, MoSx-130/ITO showed a significant degradation of catalytic activity after 50 times of CV scan. Overpotential to reach 10 mA cm−2 current density increased from 230 (black dot) to 396 mV (red dot). By contrast, MoSx/RGO fiber showed a much higher stability compared to MoSx on ITO. After 50 cycles, overpotential for 10 mA cm−2 slightly increased from 204 (black line) to 212 mV (red line). Increased stability of MoSx deposited on RGO fiber was also observed in chronoampero-metric responses (Figure S7, Supporting Information). When constant potential 0.2 V versus RHE was applied for 15 000 s, current density of MoSx-130/ITO significantly dropped to 7% while MoSx-130/RGO fiber maintained its HER activity about 75%. This confirms that not only the advantage of RGO fiber as catalytic support is large loading amount, but also, it gives a higher electrochemical stability of MoSx. Such a positive effect on catalytic stability could be originated from the tight inter-facial adhesion between MoSx and RGO surface given by the surface functional groups at RGO, which makes it possible to endure the intense generation of hydrogen bubbles.

In conclusion, we have demonstrated novel hybrid fiber-type HER catalyst composed of nanoscale thick MoSx layers and RGO LC fibers. Straightforward hybridization by wet fiber spinning and subsequent electrodeposition at room temperature realizes uniform conformal deposition of MoSx layers on rough RGO fiber surface. Geometrically favorable RGO fiber synergistically couples with the hundreds of nanometers thick catalytic MoSx layer, yielding excellent HER catalytic activity and stability. Our facile fabrication of multifunctional graphene-based fiber can be readily utilized for various combinations of functional materials and greatly enlarge the potential applications of graphene LC fibers for electrocatalysis and energy storage/conversion.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsThis research was supported by Nano-Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (2016M3A7B4905613) and the National Creative Research Initiative

(CRI) Center for Multi-Dimensional Directed Nanoscale Assembly (2015R1A3A2033061).

Received: November 23, 2016Revised: December 23, 2016

Published online:

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