Carbon-encapsulated tungsten oxide nanowires as a stable and high-rate anode material for flexible asymmetric supercapacitors

Abstract

Herein, we report an effective carbon encapsulation strategy to boost the capacitive performance of WO_3−x nanowires as a high-energy and stable anode material for flexible asymmetric supercapacitors (ASCs). The carbon-encapsulated WO_3−x nanowires exhibited a remarkable areal capacitance of 786.8 mF cm⁻² and excellent durability. Furthermore, an ASC device with high energy and power density was also fabricated from the as-prepared carbon encapsulated WO_3−x nanowire anode.

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To meet the demands and needs of specific environments, the developing trend in electronics is moving towards tiny, lightweight and portable concepts.^1–3 In particular, flexible electronics are essential for the advancement in personalized electronics due to their interesting application potentials in highly integrated on-body equipment.^4–6 As indispensable parts to power personalized electronics in wearable systems, recently, there has been a rapid increase in the research of flexible energy devices because they are lightweight and can be easily woven into textiles or other structures.^1,7–10 Amongst all the energy storage devices, flexible asymmetric supercapacitors (ASCs) have drawn considerable attention for their impressive features of having high energy and power densities, fast charge/discharge capability as well as excellent cycling stability.^11–17 However, due to low capacitance of the developed negative electrodes, the energy density of the most current flexible ASCs is yet insufficient.^2,18,19 To increase the energy-density limit of ASCs, a state-of-the art anode material with large capacitance and long cycling lifetime is highly desired.

As a typical metal oxide with multiple oxidation states, tungsten oxide (WO_3−x) has attracted considerable interest as an electrode material for SCs due to its significant advantages of having high theoretical capacitance and good conductivity, as well as being low-cost and environmentally friendly.^20–26 For example, W₁₈O₄₉ nanowires with an areal capacitance of 20 mF cm⁻² were obtained via a facile hydrothermal method.²⁵ Park et al. reported a 3D urchin-like W₁₈O₄₉ nanostructured electrode via a simple solvothermal approach and delivered a high specific capacitance of ∼235 F g⁻¹ at 20 A g⁻¹.⁶ Mai and coworkers also developed a nanoflower-like WO₃ electrode with a good electrochemical performance through a facile and effective electrodeposition method, which could achieve an areal capacitance of 458 mF cm⁻² at a current density of 14 mA cm⁻².²⁷ However, the capacitive performances of these WO_3−x electrodes are yet unsatisfactory, especially their cycling stabilities. It is still a great challenge to fabricate high-performance stable WO_3−x electrodes.

Herein, we present a smart and effective approach to markedly enhance the electrochemical performance and durability of WO_3−x nanowires by encapsulating them in an amorphous carbon shell. Carbon-encapsulated WO_3−x (denoted as C@WO_3−x) nanowires were obtained by a facile hydrothermal growth and thermal treatment. Their unique structure not only allows for fast charge transport and ion diffusion, but also readily prevents structural pulverization, thus resulting in an excellent electrochemical behavior and cycling performance. The C@WO_3−x nanowires exhibited a remarkable areal capacitance of 786.8 mF cm⁻² at a high current density of 20 mA cm⁻². More importantly, when compared to the cycling stability of pristine WO₃ nanowires, the cycling stability of the C@WO_3−x nanowires was substantially improved. The former showed a capacitance retention of 7.4% after 10 000 cycles, whereas the latter showed 87.7%. Additionally, when using the C@WO_3−x nanowires as anode, a flexible high-performance ASC device with a remarkable energy density of 1.9 mW h cm⁻³ and a remarkable power density of 4.1 W cm⁻³ was demonstrated.

The schematic of carbon-encapsulated WO_3−x nanowires preparation is illustrated in Fig. 1a, and involves two main steps. First, free-standing WO₃ nanowires were directly grown on a conductive carbon cloth substrate via a simple hydrothermal process. As shown in Fig. 1b, numerous smooth WO₃ nanowires with a diameter of 50–80 nm and a length of up to several micrometers were uniformly coated onto each carbon fiber. The X-ray diffraction (XRD) patterns of the as-prepared WO₃ nanowires, as shown in Fig. 1d (lower), suggest that they possess a hexagonal structure (JCPDS = #33-1387) with good crystallinity. Second, these WO₃ nanowires were coated with a carbon shell via a hydrothermal method, and then further annealed at 800 °C in a N₂ atmosphere for 1 h to obtain the C@WO_3−x nanowires. During the calcination process, the hydrothermally derived carbon was pyrolyzed into an amorphous carbon shell of the nanowires, and the WO₃ was reduced to WO_3−x, which is confirmed by the XRD pattern of the annealed sample, as shown in Fig. 1d (upper). Obviously, all the diffraction peaks are well indexed to monoclinic W₁₈O₄₉ (JCPDF #05-0392), indicating that the WO₃ nanowires have been successfully converted into W₁₈O₄₉ nanowires. Scanning electron microscopy (SEM) images, as shown in Fig. 1c, demonstrate that the nanowire-like morphology and smooth surface of these WO₃ nanowires are preserved after encapsulating them in a carbon shell.

Fig. 1 (a) Schematic of the preparation process of the C@WO_3−x nanowires. SEM images of the as-prepared (b) WO₃ and (c) C@WO_3−x nanowires. (d) XRD patterns of the WO₃ and C@WO_3−x nanowires.

Transmission electron microscopy (TEM) was carried out to study the detailed structure of the as-prepared samples. The representative TEM image of the WO₃ nanowire is depicted in Fig. 2a, which shows that the smooth WO₃ nanowire has a diameter of about 50 nm. Moreover, the bright and ordered diffraction spots in the selected-area electron diffraction (SAED) pattern confirm the single-crystalline nature of the WO₃ nanowires. Fig. 1b is a high-resolution TEM (HRTEM) image of the WO₃ nanowire. Clear lattice fringes with d-spacings of 0.36 and 0.39 nm are observed, which agree best with the (110) and (001) planes of hexagonal WO₃ (JCPDS = #33-1387). Fig. 1c displays a typical TEM image for a C@WO_3−x nanowire. It can be seen that a carbon shell was uniformly coated over the whole nanowire. The SAED pattern inserted in Fig. 2c confirms that the WO_3−x nanowire is also highly crystalline. Fig. 2d displays the HRTEM image of a C@WO_3−x nanowire. An amorphous carbon layer of ca. 6.7 nm in thickness is uniformly developed on the surface of WO_3−x nanowires. The lattice fringes with d-spacings of 0.33 nm and 0.38 nm are well indexed to WO_3−x after calcination. Energy dispersive X-ray spectroscopy (EDS) mapping was also conducted, and is shown in Fig. 2e–g. Obviously, W and C are equally distributed on the nanowire, and the distribution of C is mainly located at the outer part of the nanowire, which again demonstrates that the C@WO_3−x nanowire has a core–shell structure.

Fig. 2 (a) TEM image and (b) HRTEM image of the WO₃ nanowire. (c) TEM image, (d) HRTEM image, (e) HAADF-STEM image, (f) W elemental mapping and (g) C elemental mapping of the C@WO_3−x nanowire.

X-ray photoelectron spectroscopy (XPS) analysis was performed to study the composition and surface state of the C@WO_3−x nanowires. Fig. 3a compares the XPS survey spectra of the WO₃ and C@WO_3−x samples. In comparison to the pristine WO₃ sample, the C single of the C@WO_3−x sample is substantially enhanced whereas the W and O singles are weaker, confirming that the carbon shell is successfully deposited onto the WO_3−x nanowires. Fig. 3b presents the high-resolution W 4f XPS spectra of the WO₃ and C@WO_3−x samples. For WO₃ nanowires, two well-defined peaks located at about 35.8 and 37.9 eV, corresponding to W 4f_7/2 and W 4f_5/2, respectively, are observed. These values are consistent with those of the W⁶⁺ state in oxides,^25,28 and agree well with the XRD results. After coating with the carbon shell, two different peaks at the low binding energies of 34.1 eV (W 4f_7/2) and 36.1 eV (W 4f_5/2) are detected, which is associated with the lower oxidation states of W.^28–30 Moreover, as shown in Fig. 3c, the substantially decreased intensity of the O1s peak for the C@WO_3−x nanowires again reveals that the carbon shell was uniformly coated onto the WO_3−x nanowires.

Fig. 3 (a) XPS survey spectra, (b) W 4f core level XPS spectra and (c) O1s core level XPS spectra of the WO₃ and C@WO_3−x nanowires.

To investigate the capacitive performance of our samples, we first conducted the electrochemical measurements in a three-electrode cell with a Pt electrode as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode in 5 M LiCl aqueous electrolyte. Cyclic voltammograms (CV) of the WO₃ and C@WO_3−x nanowire electrodes at a scan rate of 100 mV s⁻¹ were obtained, as shown in Fig. 4a. Remarkably, the C@WO_3−x electrode exhibited a substantially larger current density and a more rectangular shape than the pristine WO₃ electrode, demonstrating the dramatically improved capacitive performance of the C@WO_3−x nanowires. Fig. S1† shows the CV curves for the C@WO_3−x electrode obtained at different scan rates. The unchanged shapes of these CV curves, even at 100 mV s⁻¹, indicates the good capacitive reversibility and rate capability of the C@WO_3−x electrode. Fig. 4b displays the galvanostatic charge and discharge (GCD) curves of the WO₃ and C@WO_3−x electrodes, obtained at a current density of 12 mA cm⁻². The longer discharge time of the C@WO_3−x electrode compared to that of the WO₃ electrode again confirmed its superior capacitive performance. Furthermore, the IR drop for the C@WO_3−x electrode was only 0.06 V, which is much smaller than that of the WO₃ electrode (0.16 V), shows the improved conductivity of the C@WO_3−x electrode. In Fig. 4c the areal capacitances of the WO₃ and C@WO_3−x electrodes are plotted against different discharge current densities. The areal capacitances of the C@WO_3−x electrode are considerably larger than those of the WO₃ electrode. For example, the areal capacitance of the C@WO_3−x electrode at a current density of 20 mA cm⁻² was 786.8 mF cm⁻², whereas that for the WO₃ electrode was only 336.2 mF cm⁻². The presented values of areal capacitance for the C@WO_3−x electrode are substantially higher than the values for recently obtained tungsten-based electrodes^{23–25,27,31,32} such as WO_3−x/MoO_3−x core shell nanowires (∼203 mF cm⁻² at 20 mA cm⁻²),³³ porous WON nanowires (∼134 mF cm⁻² at 20 mA cm⁻²)³⁴ and other carbon-coated electrodes.^33,34 More importantly, the C@WO_3−x electrode also exhibited an outstanding rate capability. When the current density increases from 2 to 20 mA cm⁻², the C@WO_3−x electrode was able to retain more than 66.8% of its initial capacitance, which is significantly higher to that of the WO₃ electrode (50.1%) and other recent electrodes. Considering that the thick carbon layer can restrain the ions in the electrolyte to penetrate through it and react with the active materials,^35,36 we also studied the influence of the shell thickness on the capacitance of the C@WO_3−x electrode. By prolonging the hydrothermal time to 9 h, we were able to obtain a type of C@WO_3−x nanowires with a carbon shell thickness of about 14.5 nm (Fig. S2a–2c). We found that the capacitance of these C@WO_3−x nanowires obviously decreased when the thickness of carbon shell increased to 14.5 nm (Fig. S2d†). Thus, a carbon shell with the appropriate thickness is important for improving its capacitive performance.

Fig. 4 (a) CV curves at 100 mV s⁻¹ and (b) GCD curves at 12 mA cm⁻² obtained for the WO₃ and C@WO_3−x nanowires. (c) Areal capacitances of the WO₃ and C@WO_3−x nanowires calculated based on GCD curves as a function of current density. (d) Nyquist plots of the WO₃ and C@WO_3−x nanowires.

To better understand the effect of phase transformation and carbon coating on the electrochemical properties of the C@WO_3−x electrode, electrochemical impedance spectroscopy (EIS) analyses were performed. Fig. 4d compares the Nyquist plots of WO₃ and C@WO_3−x electrodes. Both electrodes exhibited a semicircle in the high frequency region and a straight line in the low-frequency region, which correspond to the electron-transfer-limited processes and diffusion-limited electron transfer processes, respectively. Compared with the WO₃ electrode, the C@WO_3−x electrode exhibited a much smaller charge transfer resistance (the diameter of the semicircle in the impedance spectrum), which indicates the enhanced conductivity of the C@WO_3−x electrode. Moreover, the larger slope on the Nyquist plot of C@WO_3−x electrode, when compared to that of the WO₃ electrode, also reveals its superior ion transport.

The long-term stability of the WO₃ and C@WO_3−x nanowire electrodes were evaluated at 100 mV s⁻¹ for 10 000 cycles. As shown in Fig. 5a, the WO₃ electrode suffered from fast and drastic capacitance decay, which agrees well with the electrochemical behavior of the previous WO₃ electrodes. After 10 000 cycles, only 7.4% of the initial capacitance remained. This markedly decreased capacitance is primarily due to the structural breakdown of the WO₃ nanowires during the charge/discharge cycling process, which is further confirmed by the SEM image (Fig. 5b). The WO₃ nanowires basically disappeared and only some nanoparticles were anchored onto the carbon fibers after 10 000 cycles. In contrast, the C@WO_3−x electrode delivered a prominent long-term cycling durability, with more than 87.7% capacitance retention after 10 000 cycles. This value is also comparable to the values of other WO_3−x-based electrodes.^6,27,31 SEM images in Fig. 5 and TEM images in Fig. S3† show that the nanowire-like morphology of the C@WO_3−x electrode is mostly retained after long-term cycling, clearly demonstrating that this thin carbon layer can significantly protect the nanowires from the structural pulverization. Such remarkably enhanced capacitive performance and durability of the C@WO_3−x nanowires in a neutral electrolyte is attributed to the synergistic effect of the thin amorphous carbon shell and the fact that WO_3−x is more conductive than WO₃.

Fig. 5 (a) Cycling performance of the WO₃ and C@WO_3−x nanowires at 100 mV s⁻¹ as a function of cycle number. SEM images of the (b) WO₃ and (c) C@WO_3−x nanowires after 10 000 cycles.

To test the potential application of the C@WO_3−x nanowires as a negative electrode in a ASC device, a simple ASC device was designed by using TiN/MnO₂ nanorods as the positive electrode, the as-prepared C@WO_3−x nanowires as the negative electrode and PVA/LiCl gel as the electrolyte (denoted as TiN/MnO₂//C@WO_3−x). TiN/MnO₂ core–shell nanorods were prepared by electrodepositing amorphous MnO₂ onto the surface of TiN nanowires (ESI†). Representative SEM and TEM images of the TiN/MnO₂ nanorods are shown in Fig. S4,† which confirms the core–shell structure of the TiN/MnO₂ nanorods. Moreover, an XPS analysis was also performed to confirm that these nanorods are made of TiN and MnO₂ (Fig. S4d†). Before the fabrication of the ASC device, the charge between the C@WO_3−x electrode and the TiN/MnO₂ electrode had to be adjusted. From Fig. 6a, the C@WO_3−x electrode area to TiN/MnO₂ electrode area ratio was determined to be 1 : 1. Since C@WO_3−x and TiN/MnO₂ possess stable voltage windows from 0 to 0.8 V and from 0 to −1 V, respectively, it was expected that the operating cell voltage could be extended to 1.8 V if assembled into an ASCs. Fig. 6b shows the GCD curves for our optimized TiN/MnO₂//C@WO_3−x ASC, obtained at 8 mA cm⁻² with different voltage windows. Indeed, this device was not stable when the voltage exceeded 1.8 V (Fig. S5†), and its maximum stable electrochemical voltage was 1.8 V. In addition, as the voltage increased from 1.0 V to 1.8 V, the volumetric capacitance considerably increased from 2.1 F cm⁻³ to 3.47 F cm⁻³ (Fig. 6c), whereas the energy density was remarkably enhanced from 0.28 mW h cm⁻³ to 1.56 mW h cm⁻³ (both based on the volume of the entire device).

Fig. 6 (a) CV curves of C@WO_3−x and TiN/MnO₂ electrodes collected at 100 mV s⁻¹. (b) GCD curves collected at 8 mA cm⁻² with various potential windows, and (c) volumetric capacitance and energy density collected at 8 mA cm⁻² as a function of potential window of the TiN/MnO₂//C@WO_3−x-ASC device. (d) CV curves at different scan rates, (e) GCD at different current densities and (f) volumetric capacitance and capacitance retention of the TiN/MnO₂//C@WO_3−x-ASC device.

CV and GCD curves with a range of potential windows between 0 and 1.8 V at various scan rates and current densities were obtained to thoroughly assess the electrochemical performance of the TiN/MnO₂//C@WO_3−x ASC device. All CV curves, shown in Fig. 6d, presented rectangle-like shapes, revealing their perfect capacitive performance and excellent charge/discharge capability. Furthermore, as displayed in Fig. 6e, the GCD curves collected at various current densities showed symmetrical triangular shapes, again confirming the outstanding electrochemical properties of the as-fabricated ASC device. The volumetric capacitances calculated based on the discharge current densities and rate performances of the TiN/MnO₂//C@WO_3−x ASC device are shown in Fig. 6d. Significantly, our ASC device reached a remarkable volumetric capacitance of 4.28 F cm⁻³ at 2 mA cm⁻², which is substantially higher than the volumetric capacitance achieved by most of the recently reported ASCs at the same current density^{25,27,34,37,38} such as TiO₂@C@PPy//WO₃ (1.47 F cm⁻³),²⁷ MnO₂//Ti–Fe₂O₃ PEDOT-ASCs (∼2.0 F cm⁻³),³⁷ RGO@MnO₂//VOS-ASCs (1.62 F cm⁻³),²⁵ MnO₂//WON-ASCs (2.2 F cm⁻³ at 2 mA cm⁻²),³⁴etc. Additionally, a volumetric capacitance of more than 3.04 F cm⁻³, which is about 71% of its initial capacitance, was still retained when the current density increased to 20 mA cm⁻², demonstrating the superior rate capability of this ASC.

Ragone plots of the TiN/MnO₂//C@WO_3−x-ASC device along with those of some latest ASC devices are shown in Fig. 7a, to comprehensively compare the energies and power densities. Note that our TiN/MnO₂//C@WO_3−x-ASC device was able to deliver a maximum volumetric energy density of up to 1.9 mW h cm⁻³ at a power density of 3.33 W cm⁻³ (with a current density of 2 mA cm⁻²). This value is comparable or higher than the values obtained for recently reported ASC devices^{25,33,34,37,39–45} such as ZnO@MnO₂//RGO-ASCs (0.24 mW h cm⁻³),³⁹ MnO_2−x//RGO-ASCs (0.25 mW h cm⁻³),⁴¹ MnO₂//N–Fe₂O₃-ASCs (0.41 mW h cm⁻³),⁴² TiO₂@C@PPy//WO₃-ASCs (0.53 mW h cm⁻³),²⁷ MnO₂@Ti–Fe₂O₃@PEDOT-ASCs (0.89 mW h cm⁻³),³⁷ VO_x//VN-ASCs (0.61 mW h cm⁻³),⁴³ Fe₃O₄@MnO₂//Fe₃O₄@Fe₂O₃-ASC (0.83 mW h cm⁻³),⁴⁴ RGO@MnO₂//VOS-ASCs (0.87 mW h cm⁻³),²⁵ TiN@MnO₂//electrochemically-activated carbon cloth (EACC)-ASCs (1.5 mW h cm⁻³),⁴⁵ MnO₂/WNO-ASCs (1.06 mW h cm⁻³)³⁴ and PANI//WO_3−x/MoO_3−x (1.9 mW h cm⁻³).³³ In addition, our fabricated ASC device achieved a maximum volumetric power density of 4.1 W cm⁻³ at a high energy density of 1.24 mW h cm⁻³, which is considerably higher than that of other ASCs.^{25,33,34,37,39–45} The long term cycling performance test for the TiN/MnO₂//C@WO_3−x-ASC device was conducted at 100 mV s⁻¹ for 10 000 cycles. As shown in Fig. S5,† the ASCs exhibited a remarkable stability with a 89.4% retention of the initial capacitance. Finally, to illustrate the advantages of our ASC device as a flexible energy-storage device, CV curves of the TiN/MnO₂//C@WO_3−x-ASC device under different bent conditions were obtained, and are shown in Fig. 7b. There is no obvious variation among these CV curves; thus, the as prepared ASC holds great promise in flexible electronics.

Fig. 7 (a) Ragone plots of the TiN/MnO₂//C@WO_3−x-ASC. The values reported for other ASC devices are added for comparison.^{25,28,33,34,37,39,41–45} (b) CV curves of the TiN/MnO₂//C@WO_3−x-ASC device under different bent conditions at 100 mV s⁻¹.

Conclusion

In summary, we have demonstrated that the capacitive performance and cycling stability of the WO_3−xnanowires could be significantly increased by carbon encapsulation and their great potential as a high-performance anode for flexible ASCs. The areal capacitance of the C@WO_3−xnanowires was achieved (786.8 mF cm⁻²) at a high current density of 20 mA cm⁻², which is much higher than that of the pristine WO₃ nanowires. Furthermore, the C@WO_3−xnanowires exhibited an extraordinary high rate capability and long-term durability, with more than 87.7% retention after 10 000 cycles. In addition, a flexible ASC device with an excellent electrochemical performance was successfully fabricated, based on the as-obtained C@WO_3−xnanowires (as the anode) and a TiN/MnO₂ cathode. The TiN/MnO₂//C@WO_3−x-ASC device was able to deliver a remarkable volumetric energy density of 1.9 mW h cm⁻³ and a prominent charge/discharge capability. Such findings offer new opportunities to design other high-performance and stable electrode materials for energy storage applications.

Acknowledgements

We thank the Natural Science Foundation of China (Grant No. 21576230) for the financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ta08274b

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