Shell Nanowire Supercapacitor

High-Performance One-Body Core/Shell Nanowire Supercapacitor Enabled by Conformal Growth of Capacitive 2D WS2 Layers Nitin Choudhary,† Chao Li,† Hee-Suk Chung,§ Julian Moore,† Jayan Thomas,*,†,‡,∥ and Yeonwoong Jung*,†,‡ †

NanoScience Technology Center, ‡Department of Materials Science and Engineering, and ∥College of Optics and Photonics, University of Central Florida, Orlando, Florida 32816, United States § Analytical Research Division, Korea Basic Science Institute, Jeonju, Jeollabuk-do 54907, South Korea S Supporting Information *

ABSTRACT: Two-dimensional (2D) transition-metal dichalcogenides (TMDs) have emerged as promising capacitive materials for supercapacitor devices owing to their intrinsically layered structure and large surface areas. Hierarchically integrating 2D TMDs with other functional nanomaterials has recently been pursued to improve electrochemical performances; however, it often suffers from limited cyclic stabilities and capacitance losses due to the poor structural integrity at the interfaces of randomly assembled materials. Here, we report high-performance core/shell nanowire supercapacitors based on an array of one-dimensional (1D) nanowires seamlessly integrated with conformal 2D TMD layers. The 1D and 2D supercapacitor components possess “one-body” geometry with atomically sharp and structurally robust core/shell interfaces, as they were spontaneously converted from identical metal current collectors via sequential oxidation/sulfurization. These hybrid supercapacitors outperform previously developed any stand-alone 2D TMD-based supercapacitors; particularly, exhibiting an exceptional charge−discharge retention over 30,000 cycles owing to their structural robustness, suggesting great potential for unconventional energy storage technologies. KEYWORDS: 2D TMD, tungsten disulfide, WS2 core/shell nanowire, supercapacitor

T

electronic devices. As the advances of mobile/portable devices are progressing to incorporate additional functionalities, as in flexible/wearable energy technologies, it is compelling to develop supercapacitors which deliver high power under mechanical deformations for long time cycles. Performance of supercapacitors can be improved by optimizing the intrinsic properties of electrode materials as well as rationally engineering their electrode designs.7,8 From the material properties’ perspective, high electronic/ionic conductivities are essential for reducing capacitance losses, particularly at high scan rates/current densities. Accordingly, materials that are electrically conductive (for electron trans-

he ever-increasing consumption of nonrenewable energy sources and global concerns toward environment protection has compelled scientific communities to search for sustainable energy storage technologies. Supercapacitors are one of the most promising energy storage devices owing to their intrinsic performance advantages.1,2 They store energy in terms of charges by either ion adsorption (electrochemical double layer capacitors (EDLCs)) or surface faradic reactions (pseudocapacitors) on electrodes.3−7 High-performance supercapacitors should satisfy a set of required properties such as high specific capacitance, long cyclic stability, and large rate capability.7 The high power density of supercapacitors over other energy storage systems such as lithium ion batteries makes them suitable for a variety of applications where high bursts of power are instantly required, as in electric vehicles.5,6 Moreover, their fast charging−discharging and long-term cyclic stabilities make them attractive for power supplies in portable © 2016 American Chemical Society

Received: September 9, 2016 Accepted: October 12, 2016 Published: October 12, 2016 10726

DOI: 10.1021/acsnano.6b06111 ACS Nano 2016, 10, 10726−10735

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alone 2D TMD supercapacitors, particularly, impressively long cycle stabilities.

port), yet structurally nanoporous (for ion transport), are highly desired.9,10 From the electrode designs’ perspective, electrodes should be integrated on metal current collectors with mechanically/chemically stable interfaces (ideally, binder-free), which will ensure long cycle life as well as high electronic/ionic conductivities.11,12 Moreover, enlarging the surface area of electrodes will lead to improved specific capacitance owing to the increased amount of charges stored on their surfaces. Recently, substantive efforts have been focused to develop nanostructured electrode materials in order to satisfy the aforementioned requirements by bridging their superior material properties to more efficient electrode designs.13,14 For example, an array of electrochemically active 1D nanowires directly integrated on current collectors can offer multiple advantages.15−17 Their large surface areas with small diameters provide facile access and short diffusion paths for electrolyte ions, which can improve capacitances. Moreover, the direct integration of nanowires (active electrode materials) on metallic substrates (current collectors) without using binding materials can enhance mechanical robustness while reducing capacitive loss at their interfaces. In addition to the 1D nanostructures, 2D TMDs such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2) are recently being considered as promising capacitive materials due to their structural advantages.18−21 For example, the intrinsically layered structure of 2D TMDs enables the facile incorporation of ions in between 2D layers separated by subnanometer physical gaps, which favors a fast ionic adsorption/transport through them, which are free to expand.20 Their large surface area is another contributing factor for enhancing capacitance via the EDLC mechanism.21 Despite the projected advantages, most 2D TMDs do not present sufficiently high electrical conductivities unlike zero-bandgap graphene, which hampers the direct applications of stand-alone 2D TMDs for supercapacitors. Unlike the single materials-based approaches, incorporating multiple 1D and 2D materials with well-defined dimensions and distinct functionalities are anticipated to offer synergic advantages.22−24 These efforts include the combination of 2D TMDs with highly conductive carbonaceous 1D or 2D materials as well as conductive polymers, constructing hybrid composites for electrodes.25,26 Some demonstrations include 2D WS2 incorporated with 1D carbon nanotubes (CNTs)27 and reduced graphene oxide (rGO),28 as well as 2D MoS2 composites with carbons and conductive polymers.29 Despite enhanced specific capacitance in some cases, these 2D TMDsbased composite materials still suffer from limited performances. For example, significant capacitance decays are observed after a few hundred-to-thousand charge−discharge cycles,26,30 which are attributed to the poor structural integrity at the interfaces of randomly integrated dissimilar nanomaterials. Here, we develop high-performance flexible supercapacitors based on 1D/2D core/shell nanowires directly assembled on metal foils and explore their electrochemical performances. These hybrid nanomaterials satisfy all the aforementioned advantages inherent to 1D and 2D materials from materials’ and electrode designs’ perspectives. All nanowire supercapacitor components, i.e., highly single-crystalline tungsten trioxide (WO3) core and 2D WS2 shell, were fabricated by the spontaneous oxidation/sulfurization of a W foil, which leads to the “one-body” array of supercapacitors with seamless interfaces. These supercapacitors exhibit excellent electrochemical performances, surpassing previously reported stand-

RESULTS AND DISCUSSION Figure 1 shows an illustration to visualize the concept of the one-body array of 1D/2D core/shell nanowires directly

Figure 1. Schematic illustration for one-body array of core/shell nanowire supercapacitor. Each constituting component offers distinct functionalities as depicted in the cartoon.

integrated on metal current collectors. The presented design offers several distinct advantages desired for high-performance supercapacitors: (1) highly dense, vertically aligned nanowires provide enhanced surface areas for improved adsorption/ intercalation of electrolyte ions; (2) the nanowire core is composed of electrically conductive, highly single-crystalline WO3 for efficient carrier transports (detailed structures to be explained below); (3) the nanowire shell is composed of 2D WS2 with subnanometer physical gaps (van der Waals gaps) in between individual 2D layers for facile ion absorption from electrolytes; (4) the interfaces of the core/shell and the nanowire/current collector are chemically self-assembled without any binders or extra materials depositions, ensuring good structural integrity; and (5) all the constituting components, i.e., core, shell, and current collectors, are converted from one identical material, enabling one-body structures. Figure 2a is a schematic illustration for the fabrication process of the core/shell nanowires. The fabrication starts with the preparation of a metal current collector tungsten (W) foil, which is to be sequentially converted to W-based core/shell nanowires. The W foil is drop-casted with potassium hydroxide (KOH) and is subsequently oxidized at 650 °C, which leads to the formation of vertically aligned, single-crystalline WO3 nanowires in hexagonal (h-) structures.31 The nanowires are then sulfurized in a chemical vapor deposition (CVD) furnace under sulfur (S) environment, which converts the outer surface of the h-WO3 nanowires to 2D WS2. Accordingly, entire supercapacitor components are seamlessly connected since they are chemically converted from the same material without a physical addition of extra materials. The morphology of the hWO3/WS2 core/shell nanowires is characterized with scanning electron microscopy (SEM), X-ray diffractometer (XRD), and Raman spectroscopy. Figure 2b shows an image of as-prepared core/shell nanowires on a W foil under bending (left) and its corresponding SEM image (right). The SEM image reveals 10727


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Figure 2. Fabrication process and macroscopic structural characterizations of core/shell nanowires on W foils. (a) Schematic for the fabrication process of h-WO3/WS2 core/shell nanowires on a W foil. (b) Optical image of as-prepared core/shell nanowires on a W foil under mechanical bending (left). Corresponding SEM image (right) shows high-density, well-aligned nanowires along with their faceted surface (inset). The scale bar in the inset is 500 nm. (c, d) XRD pattern and Raman spectra from as-prepared h-WO3/WS2 core/shell nanowires on a W foil, respectively.

Figure 3. TEM characterizations of the crystalline structures of h-WO3/WS2 core/shell nanowires. (a−c) Plane-view TEM characterizations: (a) Low-magnification BF-TEM image of a h-WO3/WS2 core/shell nanowire. (b) ADF-STEM image of the nanowire corresponding to the red boxed region in (a). The inset shows the corresponding fast Fourier transform (FFT) where WS2 (002) and h-WO3 (002) crystalline planes are indexed. (c) ADF-STEM image to show a stack of 2D WS2 shell on the h-WO3 core and an atomically sharp h-WO3/WS2 interface. (d−f) Cross-sectional TEM characterizations: (d) Low-magnification ADF-TEM image of a cross-sectioned h-WO3/WS2 core/shell nanowire. (e,f) ADF-STEM images of the cross-sectioned nanowire corresponding to the green (e) and blue (f) boxed regions, respectively, revealing the conformal growth of 2D WS2 on the h-WO3. The inset of (e) shows an indexed FFT obtained from the core h-WO3 region in [001] zone axis. (g,h) Schematic illustrations for the crystalline structure of a h-WO3 nanowire along their corresponding ADF-STEM images. (g) Crosssectional view of the nanowire revealing the crystalline structure on its (001) basal plane. (h) Projected-view of the nanowire along its length, corresponding to the c-axis of h-WO3.

highly dense, well-aligned nanowires with diameters of ∼150− 200 nm and lengths of ∼8−10 μm, leading to an aspect ratio as

high as ∼50. Figure 2c shows the XRD patterns of the asprepared core/shell nanowires on the W foil. Most of the 10728


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Figure 4. Electrochemical characterizations of h-WO3/WS2 core/shell nanowires on a W foil in 0.1 M Na2SO4 electrolyte. (a) CV curves at various scan rates. (b) Trend of change in areal capacitance value as a function of scan rates. (c) GCD curves at various current densities. (d) Nyquist plot and its corresponding zoomed-in image near at −Z″ = 0 Ω. (e) Cyclic performacne measured at 100 mV/s.

transmission electron microscopy (TEM) characterizations were performed. Figure 3a shows the low-magnification bright-field (BF) TEM image of an isolated nanowire, revealing its uniform diameter (∼200 nm) along its entire length (>6 μm). Figure 3b,c contains plane-view dark-field (DF) TEM images which zoom in on the red boxed region in Figure 3a, depicting the detailed crystallinity of the nanowire in a side view. The crystalline structure of the nanowire is investigated in details by annual dark-field (ADF) high-resolution TEM (HRTEM) in a scanning TEM (STEM) mode. The ADFSTEM image in Figure 3b indicates that the nanowire is composed of two components: (1) highly single-crystalline hWO3 core, which maintains its crystallinity throughout the entire nanowire length along the [001] axis of h-WO3 and (2) 2D WS2 shell with a uniform thickness of 7−8 nm outside of the h-WO3 core. The zoomed-in ADF-STEM image in Figure 3c further shows that the shell is composed of 2D WS2 layers with uniformly spaced 2D layers as well as the atomically sharp core/shell interface of h-WO3/WS2. The morphology of the hWO3/WS2 core/shell nanowire is further investigated by crosssectional TEM characterizations. Figure 3d is the lowmagnification ADF-STEM image of a cross-sectioned hWO3/WS2 nanowire, and Figure 3e,f contains the highresolution ADF-STEM images corresponding to the green and blue boxed regions in Figure 3d, respectively. The images reveal that 2D WS2 is uniformly grown in stacks of ∼8−9 layers throughout the entire periphery of the h-WO3 core in a conformal manner, consistent with the plane-view TEM analysis. Figure 3g,h visualizes the crystalline structure of the h-WO3 core nanowire in schematic views and corresponding

distinguishable XRD peaks can be indexed to the standard diffraction patterns of single-crystalline h-WO3 (JCPDS no. 852459) without any impurities, consistent with previous reports.32,33 A small peak observed at 2θ = 57.8° can be attributed to the (200) lattice planes of pure W,34 which must be originated from the W foil. The absence of XRD peaks corresponding to 2D WS2 layers is obvious because of the small thickness of 2D WS2, (thus, difficult to be detected by XRD), and details are discussed and confirmed in proceeding sections. The XRD pattern of h-WO3 nanowires prior to sulfurization is shown in Supporting Information, Figure S1a. The structural quality of h-WO3/WS2 core/shell nanowires was further characterized by the Raman spectroscopy under 532 nm laser attained at room temperature. Figure 2d shows Raman spectra collected from h-WO3/WS2 nanowires on a W foil, revealing the presence of WS2 which was not detected by XRD. The Raman bands centered at 420.1 and 351 cm−1 confirm the W− S stretching from the WS2 corresponding to A1g and E′2g vibration modes, respectively.35 Apparently, Raman spectra reveals some additional peaks at 323.7 and 581.8 cm−1, which are indicative of the first-order and second order Raman modes of 2D WS2.36 A small peak at ∼700 cm−1 originates from the stretching modes of O−W−O bonds,37 indicating the presence of h-WO3 along with WS2. Indeed, the Raman characterization of as-prepared h-WO3 nanowires before sulfurization more clearly reveals the stretching modes of O−W−O bonds (Supporting Information, Figure S1b), which is also consistent with previous studies.38 To elucidate the crystalline structure and the interface morphology of the h-WO3/WS2 core/shell nanowires, various 10729


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onto the electrode surfaces.50 For comparison, pristine h-WO3 nanowires prior to sulfurization were also tested under CV measurements, as shown in Supporting Information, Figure S3b. The plots show 2 orders of magnitude lower capacitance for the pristine state in comparison to the core/shell structure, indicating that the 2D WS2 shell is critically responsible for enhancing the capacitive performance. Galvanostatic charge− discharge (GCD) curves of the core/shell nanowires were recorded at various current densities in the voltage range of −0.3 and 0.5 V, as shown in Figure 4c. Consistent with the CV curves, the GCD curves also show nearly symmetrical voltage curves, indicating highly reversible and fast responses. The electrochemical impedance spectroscopy (EIS) measurements were carried out to further assess the carrier transport efficiency of the nanowire supercapacitors, as shown in Figure 4d. The Nyquist plot in Figure 4d shows nearly linear and vertical characteristics, an indication of superior supercapacitor performance. A low equivalent series resistance (ESR = 5.1 Ω), as calculated from the GCD curves, confirms the low internal resistance and high-rate capacitive behavior of the core/shell nanowire supercapacitor. Figure 4e shows the cyclic stability of the core/shell nanowire supercapacitor recorded up to 30,000 charge−discharge cycles at a scan rate of 100 mV/s. It is interesting to note that the capacitance gradually increases up to ∼2500 cycles and remains nearly constant throughout the entire measurement. The initial increase of the capacitance is attributed to that the 2D WS2 shell becomes more electrochemically active with increasing cycles, as the number of 2D layers that come into contact with the electrolyte increases. A high areal capacitance of 74.25 mF/cm2 was observed (scan rate: 5 mV/s) after 2500 cycles (Supporting Information, Figure S3c), which surpasses the performance of all the previously reported 2D TMDs-based supercapacitors and is comparable to the state-of-the-art carbon-based supercapacitors.51−53 Although a slight decay of capacitance is observed after ∼25,000 cycles, the overall cyclic stability is well retained throughout further extended cycles, resulting in zero loss of initial capacitance even at 30,000 cycles. This high-capacitance retention is superior to most of the previously developed 2D TMDs-based supercapacitors (Table 1), and we believe that this is due to the excellent mechanical stability of the nanowire supercapacitors benefiting from their one-body geometry and seamless heterointerfaces. Indeed, TEM characterizations of core/shell nanowires which underwent >30,000 cycles reveal that the nanowires still maintain atomically sharp core/shell interfaces and highly single-crystalline h-WO3, while 2D WS2 shells turned partially amorphized (Supporting Information, Figure S4a). Raman characterizations of identical core/shell nanowires before and after 30,000 charge−discharge cycles do not reveal any noticeable changes in peak positions and relative intensities, further confirming the structural and chemical robustness of the nanowires (Supporting Information, Figure S4b). A comparison for the cyclic stabilities of 2D TMDs-based supercapacitors is summarized in Table 1.26,29,48,49,54−62 We also tested the performance of solid-state supercapacitors assembled with two identical core/shell nanowires on W foils and Na2SO4 electrolyte gels in two-electrode geometry. Figure 5a shows CV curves collected at various scan rates ranging from 5 to 100 mV/s in a voltage window of 0−0.8 V, exhibiting nearly symmetric shapes. The areal capacitance as a function of scan rate is shown in Supporting Information, Figure S5a. The solid-state device achieved a maximum areal capacitance of 18.3 mF/cm2 at a scan rate of 5 mV/s, which is much higher than

ADF-STEM images. Figure 3g left shows a schematic view of hWO3 constructed by the arrangement of the six tungsten oxide (W-O6) octahedral by sharing their corners along the [001] zone axis. The image indicates the presence of open hexagonal areas (∼5.36A° in diagonal) on its basal plane along c-axis,39 which is confirmed by the corresponding ADF-STEM image in Figure 3g, right. The stacking of such crystalline units along the c-axis suggests that core h-WO3 nanowires possess a long 1D hexagonal open channel along their lengths, as shown by the projected schematic image in Figure 3h, left. The ADF-STEM image in Figure 3h, right, corresponding to the schematic, shows a lattice spacing of ∼3.8A° which corresponds to (002) planes in h-WO3, consistent with previous studies.40 These open 1D channels are the intrinsic features of h-WO3 compared to other phases such as monoclinic (m-) WO3, and their advantages for pseudocapacitive reactions via enhanced ion/ carrier transports have previously been suggested elsewhere.40−42 The morphology of the h-WO3/WS2 interface was also investigated by BF-HRTEM and energy dispersive Xray spectroscopy (EDS)-STEM characterizations (Supporting Information, Figure S2). The BF-HRTEM images (Supporting Information, Figure S2a) also confirm the atomically sharp hWO3/WS2 interface, indicating the conformal growth of 2D WS2 layers on top of the outer surface of the h-WO3 core. The EDS elemental mapping images (Supporting Information, Figure S2b) reveal the localized distribution of W, O, and S throughout the nanowire, further supporting the core/shell structure. The EDS spectra (Supporting Information, Figure S2c) collected from each core and shell indicate the stoichiometric distribution of the elements in the corresponding regions. The electrochemical performance of the h-WO3/WS2 core/ shell nanowire on a W foil (effective area: 1 cm2) was evaluated in a three-electrode geometry using 0.1 M sodium sulfate (Na2SO4) solution as an electrolyte and the backside of the W foil as a working electrode. Figure 4a shows the typical cyclic voltammetry (CV) curves of the nanowire supercapacitor recorded at various scan rates, ranging from 5 to 100 mV/s in a voltage window of −0.3 V to 0.5 V. The CV curves exhibit near symmetrical loops for all the scan rates, indicative of dominant EDLC reactions over faradic adsorptions.43 However, the deviation from the ideal rectangular CV profile suggests some extent of redox reactions of alkali metal ions (Na+) intercalated into the van der Waals gaps of 2D WS2 at the various valence states of W, following WS2 + Na+ + e− = WS-Na+ as previously suggested44−46 (details are to be discussed below). Based on the CV curves, the specific areal capacitance of the nanowire supercapacitor was calculated as a function of scan rates, as shown in Figure 4b. Higher capacitance values were obtained with lowering scan rates in the tested range of 5 mV/s to 1000 mV/s, for example, 47.5 mF/cm2 at 5 mV/s and 33 mF/cm2 at 10 mV/s. These values are much higher than those from previously developed stand-alone 2D TMD-based supercapacitors and their composites obtained at comparable scan rates; such as, liquid-exfoliated MoS2 sheets (2.4 mF/cm2 at 10 mV/s),47 edge oriented-MoS2 (12.5 mF/cm2 at 50 mV/s),48 3D porous MoS2 (∼33.0 mF/cm2 at 5 mV/s),11 metallic (1T)WS2 (2813 μF/cm2) and semiconducting (2H)-WS2 (223.3 μF/cm2at 10 mV/s),49 and MoS2-graphene composite (11 mF/ cm2 at 5 mV/s).25 As the scan rate increases beyond 200 mV/s, the CV curves start to display skewed rectangular loops deviating from the symmetric shapes (Supporting Information, Figure S3a), attributed to the more active accumulation of ions 10730


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Wh/cm3, which outperforms almost all of the state-of-the-art technologies such as lithium thin-film batteries, porous graphene, electrolytic capacitors, and recently developed MXenes and 1T MoS2.51,59,64,66−70 The excellent electrochemical properties of the core/shell nanowires are mainly attributed to EDLC mechanism, but the possibility of pseudocapacitive reactions cannot be completely ruled out. In order to test this hypothesis and better clarify the governing mechanism, we examine the CV behaviors of the core/shell nanowires in various electrolytes, such as sulfuric acid (H2SO4), lithium chloride (LiCl), and potassium chloride (KCl), as shown in Supporting Information, Figure S6. It is clear that the CV curves more significantly deviate from EDLCtype symmetric shapes, as the size of the cations in the electrolytes decreases (K+ > Li+ > H+). This observation indicates enhanced pseudocapacitive reactions with smaller cations, as they are easier to get intercalated into the interlayers of 2D WS2 shells. This result, together with the TEM evidence of deformed 2D WS2 shells after extended cycles (Supporting Information, Figure S4a), suggests that the capacitive performances of our core/shell nanowires are likely the combined results of EDLC and pseudocapacitive reactions, although quantitatively decoupling one from the other is technically challenging. Moreover, the outstanding cyclic stability of our core/shell nanowires is attributed to the structural advantage of WS2 shells whose interlayer spacing is significantly larger than the diameters of electrolyte ions (∼3 times larger than that of Na+ used in this study). The interlayer interaction between WS2 layers is relatively weaker than those of other 2D TMDs with similar structures,71 which further enables the facile and reversible intercalation of ions into WS2 shells without a significant distortion of 2D layers for a long period of cycles.

Table 1. Comparison of Cyclic Performances of 2D TMDBased Supercapacitors material WS2/rGO 1T-2H-hybrid MoS2 WS2 nanoparticle/ amorphous CNT edge-oriented MoS2 MoS2-graphene WS2/SWCNT hybrid 1T WS2 nanoribbons mesoporous MoS2 1T MoS2 MoS2/Ppy MoS2/MWNT 3D MoS2/PANI MoS2/carbon h-WO3/WS2 nanowire

electrolyte

retention (%)

no. of cycles

scan rate/ current density

ref

KOH (0.5 M) KOH (6 M) KOH (3 M)

98.6

5000

2 A/g

54

92.2

1000

0.5 A/g

55

60

500

10 A/g

27

H2SO4 (0.5 M) Na2SO4 (1 M) LiPF6 (1 M) H2SO4 (1 M) Na2SO4 (1 M) Na2SO4 (0.5 M) KCl (1 M) Na2SO4 (1 M) H2SO4 (1 M) Na2SO4 (1 M) Na2SO4 (0.1 M)

83

20,000

50 mV/s

48

1 A/g

57

92.3

1000

47

500

10 mV/s

26

33

2000

10 mV/s

49

80

2000

150 mV/s

58

≥93

5000

2 A/g

59

85

4000

1 A/g

60

95.8

1000

1 A/g

61

79

6000

1 A/g

29

94.1

1000

1.6 A/g

62

100 mV/s

this work

≥100

30,000

other 2D material-based devices fabricated by various methods, such as spray painted MoS2 nanosheets (8 mF/cm2),46 onion like carbon- and graphene-based microsupercapacitors (0.5−3 mF/cm2),63,64 and activated carbon capacitors (13.5 μF/ cm2).65 The CV curves of the nanowire supercapacitor were also collected with different voltage windows at 100 mV/s (Supporting Information, Figure S5b), demonstrating their stable performance even at a higher voltage. The GCD curves obtained at various current densities in Figure 5b show nearly triangular shapes, suggesting a fast charge−discharge process. The Nyquist plots (Supporting Information, Figure S5c) exhibit nearly a vertical line in the low-frequency region with a low ESR (∼6.9 Ω: inset), indicative of excellent capacitive behavior and good conductivity. The electrochemical performance of the solid-state nanowire supercapacitor under mechanical deformation was tested by folding it at various angles ranging from 15° to 90°. Figure 5c shows that the supercapacitor remained over 98% of its initial capacitance even after folding at 90° for 100 times, revealing its excellent bendability. Figure 5d confirms that CV characteristics are well maintained even under the repeated mechanical bending of the nanowire supercapacitor. A demonstration of lighting a red light-emitting diode (LED) using nanowire supercapacitors is shown in Figure 5e. Three solid-state nanowire supercapacitors (1 × 3 mm) connected in series are used, charging up the LED at 1.6 V. Lastly, Figure 5f presents a Ragone plot to compare the energy vs power densities of our core/shell nanowires with other stand-alone 2D materials, including 2D transition-metal carbides/nitrides/carbonitrides (MXenes) and porous graphene as well as various energy storage devices. The maximum energy density of the core/shell structure in this work is 0.06

CONCLUSION In summary, we report the fabrication of h-WO3/WS2 core/ shell nanowires and their electrochemical performance for flexible supercapacitor applications. These hybrid materials present multifold advantages desired for high-performance supercapacitors from both material properties’ and electrode designs’ perspectives, i.e., large surface area, distinct functionalities from 1D and 2D components, and mechanical robustness. Synergistic effects of these attributes are confirmed by excellent capacitive performances, particularly, extremely long cycle stability during the course of galvanostatic charge− discharge. The material design principle suggested in this work can be extended to other material systems, implying its great potential for a variety of energy storage devices compatible with emerging flexible and wearable technologies. METHODS Core/Shell Nanowires Preparation. The preparation of the nanowire was carried out in two steps. In the first step, an array of vertically aligned h-WO3 nanowires was prepared on top of a thin (100 μm) W foil by its oxidation. First, the W foil was sequentially cleaned in an ultrasonic bath with acetone, hydrochloric acid, ethanol, and deionized water. A 10 wt % KOH solution was drop-casted on the top surface of the foil followed by spin coating at 2000 rpm. Then, the foil was kept in a thermal furnace which was subsequently heat up from room temperature to 650 °C at a ramp rate of 12 °C/min. The furnace was held for 2 h (dwell time) at atmospheric pressure, followed by a natural cool-down to room temperature. The sample was taken out of the furnace followed by rinsing in DI water and drying at room temperature. The sample (h-WO3 nanowires on W foil) was subsequently placed in the heating zone of a home-built CVD system 10731


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Figure 5. Electrochemical performances of solid-state core/shell nanowire supercapacitors. (a) CV curves at various scan rates. (b) GCD curves at various current densities. (c) Capacitance retention under repeated mechanical deformations at various bending angles. (d) CV curves obtained at different bending angles (0°−90°). (e) Demonstration of powering a LED. (f) Ragone plot to compare the performances of various technologies with the core/shell nanowires supercapacitor in this study. equipped with an 1 in. diameter quartz tube furnace. A ceramic boat containing pure sulfur (99.99% purity, Sigma-Aldrich) was placed inside the quartz tube located at the upstream of the CVD. The tube was then pumped down to a pressure of 10−3 mTorr and flushed with argon (Ar) gas repeatedly to guarantee a favorable growth atmosphere. With a flow of Ar (200 standard cubic centimeters per minute (sccm)), the furnace was heated up to 850 °C at a rate of 20 °C/min and was held for 40 min, followed by natural cooling. TEM Sample Preparation and Characterizations. For the plane-view TEM sample preparation, individual nanowires on W foils are directly transferred to lacey-carbon-coated Cu grids (Ted Pella) by gently rubbing them against the grids. For the cross-sectional TEM sample preparation, as-prepared nanowires were first coated with an amorphous carbon film of 100 nm thickness to protect their surfaces. Then, cross-sectioning of the sample was performed by a focused ion beam (FIB: Quanta 2D FEG, FEI) using a Ga ion milling (30 keV)

and a platinum (Pt) deposition until the target area becomes electron transparent (∼80 nm thick). The prepared sample was transferred to a Cu TEM grid by a micromanipulator (Omniprobe) inside the FIB. ADF-STEM characterizations were performed with JEOL ARM200F TEM/STEM with a spherical aberration corrector (CEOS GmbH) and an electron probe (size of ∼1 Å) at 200 kV. STEM-EDS characterizations were carried out with EDAX detector (SDD type 80T), and chemical analysis was performed with a analysis software (AZtecTEM, Oxford). Electrochemical Tests. The electrochemical measurements were conducted in 0.1 M Na2SO4, H2SO4, KCl, and LiCl electrolytes. For the three electrode tests, an array of core/shell nanowires on a W foil (area: 1 cm2) was used as the working electrode, silver/silver chloride (Ag/AgCl) or saturated calomel as the reference electrode, and a Pt foil as the counter electrode. Two electrode tests were performed by assembling two batches of the nanowires on W foils as positive and 10732


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negative electrodes. CV, GCD, and EIS tests were performed on a SP150 electrochemical workstation (Bio-Logic, USA).

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b06111. Additional electrochemical data, TEM data, XRD data, Raman data, and details for the calculation of areal capacitance, energy and power densities (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

J.T. and Y.J. conceived the idea, directed the experiments, and organized the manuscript. N.C. synthesized the nanowires and conducted SEM, Raman, and XRD characterizations. C.L performed the electrochemical measurements and fabricated the supercapacitor devices with the help of J.M. H.-S.C. performed TEM characterizations. N.C. and C.L. analyzed and interpreted the data under the guidance of J.T. and Y.J. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS Y.J. acknowledges the financial support from a start-up fund from the University of Central Florida and the use of Materials Characterization Facility, AMPAC, at the University of Central Florida. J.T acknowledges National Science Foundation (CAREER: ECCS-1351757) for the financial support. H.-S.C. was supported by Korea Basic Science Institute (grant no. C36958). REFERENCES (1) Wang, G.; Zhang, L.; Zhang, J. A Review of Electrode Materials for Electrochemical Supercapacitors. Chem. Soc. Rev. 2012, 41, 797− 828. (2) Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7, 845−854. (3) Qian, W.; Sun, F.; Xu, Y.; Qiu, L.; Liu, C.; Wang, S.; Yan, F. Human Hair-Derived Carbon Flakes for Electrochemical Supercapacitors. Energy Environ. Sci. 2014, 7, 379−386. (4) Wang, X.; Zhang, Y.; Zhi, C.; Wang, X.; Tang, D.; Xu, Y.; Weng, Q.; Jiang, X.; Mitome, M.; Golberg, D.; Bando, Y. Three-Dimensional Strutted Graphene Grown by Substrate-Free Sugar Blowing for HighPower-Density Supercapacitors. Nat. Commun. 2013, 4, 2905. (5) Simon, P.; Gogotsi, Y.; Dunn, B. Where Do Batteries End and Supercapacitors Begin? Science 2014, 343, 1210−1211. (6) Kim, H.; Park, K. − Y.; Hong, J.; Kang, K. All-Graphene-Battery: Bridging the Gap Between Supercapacitors and Lithium Ion Batteries. Sci. Rep. 2014, 4, 5278. (7) Yu, Z.; Tetard, L.; Zhai, L.; Thomas, J. Supercapacitor Electrode Materials: Nanostructures from 0 to 3 Dimensions. Energy Environ. Sci. 2015, 8, 702−730. (8) Yan, J.; Wang, Q.; Wei, T.; Fan, Z. Supercapacitors: Recent Advances in Design and Fabrication of Electrochemical Supercapacitors with High Energy Densities. Adv. Energy Mater. 2014, 4, 1300816. (9) Chen, T.; Dai, L. Carbon Nanomaterials for High-Performance Supercapacitors. Mater. Today 2013, 16, 272−280. (10) Hu, B.; Qin, X.; Asiri, A. M.; Alamry, K. A.; Al-Youbi, A. O.; Sun, X. Synthesis of Porous Tubular C/MoS2 Nanocomposites and 10733


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