PEDOT Nanocomposites on Contra Wires

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Assembly of NiO/Ni(OH)2/PEDOT Nanocomposites on Contra Wires for Fiber-Shaped Flexible Asymmetric Supercapacitors Huiling Yang, Henghui Xu, Ming Li, Lei Zhang, Yunhui Huang, and Xianluo Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09526 • Publication Date (Web): 28 Dec 2015 Downloaded from http://pubs.acs.org on December 29, 2015

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ACS Applied Materials & Interfaces

Assembly of NiO/Ni(OH)2/PEDOT Nanocomposites on Contra Wires for Fiber-Shaped Flexible Asymmetric Supercapacitors

Huiling Yang,a Henghui Xu,a Ming Li,a Lei Zhang,b Yunhui Huang,a and Xianluo Hua,*

a

State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials

Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China b

Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia

* To whom correspondence should be addressed. E-mail: [email protected].

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ACS Paragon Plus Environment


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ABSTRACT: Fiber-shaped solid-state supercapacitors have aroused much interest in the fields of portable devices because of their attractive features such as high flexibility and safety, tiny volume, and high power density. In this work, NiO/Ni(OH)2 nanoflowers encapsulated in three-dimensional interconnected poly(3,4-ethylenedioxythiophene) (PEDOT) have been fabricated on contra wires through a mild electrochemical route. The as-formed hybrid electrode made of NiO/Ni(OH)2/PEDOT delivered a high specific capacitance of 404.1 mF cm–2 (or 80.8 F cm–3) at a current density of 4 mA cm–2, and a long cycle life with 82.2% capacitance retention after 1000 cycles. Furthermore, a fiber-shaped flexible all-solid-state asymmetric supercapacitor (ASC) based on the resulting hybrid electrode was assembled. The energy density of 0.011 mW h cm–2 at a power density of 0.33 mW cm– 2

was achieved under an operating voltage window of 1.45 V. This work provides an effective strategy

to fabricate high-performance electrodes for fiber-shaped flexible asymmetric supercapacitors through a facile and low-cost route.

KEYWORDS: Nanocomposites; Supercapacitor; Flexible; Electrochemical performance; PEDOT

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Introduction Flexible and wearable electronic devices have received great commercial and research interests for next generation portable electronics.1–2 Fiber-shaped solid-state supercapacitors offer great promise in portable applications because of their attractive properties including high flexibility and safety, tiny volume and high power density.3–5 However, these supercapacitors still suffer from relatively low capacitance and high production cost, which cannot meet the ever-increasing demand in flexible devices. Therefore, it is highly desirable to fabricate fiber-shaped supercapacitors with high performances. Transition-metal oxides/hydroxides based on Faradaic reactions have been widely explored for supercapacitors, enabling higher capacity and energy in comparison to carbon-based materials.6–8 To date, pseudoactive materials with high theoretical capacitance, such as RuO2,9 NiO,10 Co3O4,11 Ni(OH)212–13 and MnO2,14–16 have been investigated. Among them, nickel oxides or hydroxides, have attracted widespread attention for supercapacitor applications because of their intriguing properties including high theoretical specific capacitance (2584 F g−1 for NiO and 2081 F g−1 for Ni(OH)2 with 0.5 V), low cost, and environmental friendliness. However, the intrinsically poor electrical conductivity and poor stability limit their actual electrochemical performances.17 In recent years, much effort has been made to overcome these issues. One effective method is to rationally design unique nanostructures. For instance, highly porous structures in the form of nanotubes or nanorod arrays,18–20 nanoflowers21–22 or hollow spheres are fabricated.23 The porous nanoarchitectures featuring high active area, fast transfer paths for ions and electrons, and good strain accommodation, exhibit superior charge-storage capabilities.18 In addition, composites containing highly conductive materials such as nickel,24 graphene,25–26 carbon nanotubes27–28 and some conducting polymers29–30 are synthesized for supercapacitor electrodes. Tang et al. fabricated nano-architectured electrodes made of Ni(OH)2/carbon nanotubes (CNTs) for supercapacitors with a high specific capacitance of 3300 F g−1 and a high areal capacitance of 16 F cm−2.28 More recently, 3-dimenstional (3D) NiO flowers have been encapsulated within polypyrrole as high-capacity supercapacitor electrodes with a specific capacitance of 595 F g–1 at 1 A g–1.31 3



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Here we report the fabrication of PNC/PEDOT nanoflowers on contra wires (CWs). Contra is a kind of a Cu-Ni alloy with the nickel content from 1% to 44%. Since the lattice constants of Cu and Ni are nearly the same, a solid solution of Cu-Ni with a whole compositional range can be formed. The flower-like porous NiO/Ni(OH)2 composite (PNC) possesses a 3D structure and high surface area, prompting the ion migration of the electrolyte. Also, PEDOT is a conducting polymer with ultrahigh stability. It was covered to the surface of PNC as a protective thin layer to enhance the conductivity and structural stability of the composite.32–33 As a consequence, the resulting composite electrode of PNC/PEDOT exhibits a high specific capacitance of 404.1 mF cm–2 (or 80.8 F cm–3) at a current density of 4 mA cm–2. Furthermore, we have fabricated a fiber-shaped flexible all-solid-state asymmetric supercapacitor (ASC) by using a polymer gel as the electrolyte, the as-abtained PNC/PEDOT electrode as the positive electrode, and the ordered mesoporous carbon (CMK-3) fiber as the negative electrode, respectively. The fiber-shaped ASC exhibits an output voltage as high as 1.5 V. A high specific capacitance of 31.6 mF cm–2 (or 3.16 F cm–3) and a high specific energy density of 0.011 mW h cm–2 (or 1.1 mW cm–3) are achieved.

Experimental Section Materials synthesis The contra wires (CuNi44) with a diameter of 0.2 mm were purchased from Jiangyin Chengxin Alloy Material Co. Ltd. The nanoporous nickel (npNi) wires were prepared in a three-electrode cell containing a mixed solution of 1 M NiSO4, 0.01 M CuSO4, and 0.5 M H3BO3 (pH = 4) by an electrodeposition and dealloying method. The contra wire, platinum plate, and Ag/AgCl electrode were used as the working, counter, and reference electrodes, respectively. First, a layer of CuNix film was electrodeposited on the contra wire at a constant potential of –0.85 V for 8 min. After etching copper from CuNix at 0.5 V for 10 min, nanoporous nickel was generated. Porous PNC flower-like nanostructures were obtained by anodizing the npNi wires (powered by a SS1796C DC stable power supply) in a mixed solution of 0.5 wt% KOH, 5 wt% H2O and 95 wt% glycerol at 20 V for 5 min, with 4


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a platinum plate as the counter electrode. Then, the as-prepared PNC flowers were encapsulated by PEDOT via chemical polymerization in an acetonitrile solution containing 0.005 M FeCl3 at room temperature. The CMK-3 slurry was prepared by adding the mixed powder (85% CMK-3 and 15% acetylene black) into the 5% nafion solution.32 Then, the CMK-3 slurry was then painted on the as-prepared npNi wire to form a cable-like electrode of npNi@CMK-3.

Fabrication of fiber-shaped flexible all-solid-state ASCs For the electrolyte, two aqueous stock solutions of polyvinyl alcohol (PVA, Mw = 89,000–98,000, 2g dissolved in 20 mL of deionized water at 90 ºC) and KOH (2 g dissolved in 5 mL of deionized water) were first prepared. Then, they were mixed and vigorously stirred at 90 ºC until the mixture turned into clear. After immersion of the PNC/PEDOT and npNi@CMK-3 electrodes in the above PVA/KOH mixture for 5 min, they were taken out and were continuously kept at ambient conditions for several hours, and the excess water could be removed. Finally, an all-solid-state ASC was successfully obtained by pressing the electrolyte-pregnant electrodes under a slight pressure.

Materials characterization A scanning electron microscope (SEM, FEI, Sirion 200) coupled with a X-ray energy-dispersive (EDX) spectrometer (Oxford Instruments), and a transmission electron microscope (TEM, JEOL 2100F) were used to characterize the shape and microstructure of the samples. X-ray diffraction (XRD) analyses were performed using a X'Pert PRO diffractometer (PANalytical B.V., Holland) with Cu Kα1 irradiation (λ = 1.5406 Å). X-ray photoelectron spectra (XPS, Thermo VG Scientific, VG MultiLab 2000) were measured to characterize the composition and the chemical valence state of the products. The products were characterized by Fourier-translation infrared spectroscopy (FTIR, VERTEX 70).

Electrochemical measurements A three-electrode system was utilized to study the electrochemical performances of individual electrodes, whereby the counter electrode of Pt and the reference electrode of Ag/AgCl in an electrolyte of KOH (6 M) were used. Cyclic voltammetry (CV) and galvanostatic charge–discharge 5



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profiles, and electrochemical impedance spectra (EIS) were obtained on an electrochemical work station (CHI 660D). The EIS were measured at a frequency range between 0.01 Hz and 100 kHz with a 5-mV potential window.

Results and Discussion Typically, three main steps are involved for fabricating the hybrid electrode of PNC/PEDOT. First, a layer of nanoporous nickel (npNi) was grown on the contra wire through electrodeposition and dealloying (Step 1),35 followed by anodizing the as-prepared npNi to the porous PNC nanocomposite (Step 2). In order to enhance the conductivity, PEDOT was then chemically polymerized and encapsulated on the surface of the porous PNC nanocomposite (Step 3). Meanwhile, the CMK-3 slurry was painted onto the as-prepared npNi wire and used as the negative electrode (Step 4). Figure 1 shows the schematic illustration for the fabrication process of the ASC device. Figure 2a shows the SEM images for the npNi wire (0.2 mm in diameter) that was obtained through the electrodeposition and dealloying processes. Clearly, the surface of the contra wire became rough and porous. The SEM images (Figure 2a and 2b) at a higher magnification show that a large amount of flower-like porous arrays exist on the surface of the wire, and the thickness of the array layer is about 1µm (inset of Figure 2b). Interestingly, each array comprise several nanotubes with diameters of about 60~100 nm. Figure 2d illustrates the SEM image of the porous PNC composite that was evolved from the as-formed npNi wire by anodizing. It is observed that the morphology of the npNi can be maintained. The high-resolution TEM (HRTEM) image in Figure 2e shows that the inside of npNi film is not completely oxidized. The lattice fringes can be clearly seen, and the distance of 2.1 Å corresponds to the crystal plane (111) for Ni.36 The PNC electrode possesses a flower-like array nanoarchitecture. It is expected that the improved surface area of the electrode would enhance the overall interface between the active material and the electrolyte, and prompt the ion transfer,37 especially at current densities. Figure 2f shows the SEM image of the PEDOT-modified PNC nanocomposite. After PEDOT coating, no significant structure change could be detected in the composite. This suggests that the mechanical strength of the PNC arrays is strong. The PEDOT layer possesses a 3D network structure with a 6


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thickness of a dozen nanometers (Figure 2g). The 3D framework in the electrode is beneficial to the electronic transport. Meanwhile, the mechanical stress resulted from the volume change during continuous charge/discharge cycling could be effectively relieved, benefiting from the 3D framework as a buffer layer. The selected-area electron diffraction (SAED) pattern of the PNC/PEDOT hybrid is shown in the inset of Figure 2g. Obviously, three sets of diffraction rings from the inner to the outer can be indexed to the lattice planes (111), (200) for Ni, and (220) for NiO, respectively.

Figure 1. Schematic illustration for the fabrication process of the ASC device.

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Figure 2. (a–c) SEM images of the obtained npNi wire, indicating that a 3D porous Ni layer is formed on the contra wire by electrodeposition and dealloying. (d) SEM image of the PNC prepared by anodizing. (e) HRTEM image of the PNC (inset: enlarged lattice fringe of Ni). (f) SEM image of the PNC/PEDOT composite. (g) HRTEM image of the PNC/PEDOT composite (inset: SAED pattern).

Figure 3a shows the energy dispersive X-ray (EDX) spectrum for the wire electrode of the contra-supported PNC/PEDOT composite. There exist the elements of Ni, O, S, Cu, and C in the product. The elemental mappings of Ni, O and S (Figure 3b) clearly display that the PNC is evenly distributed on the surface of the contra wire and the PEDOT layer is coated uniformly on the surface of the PNC arrays. The FTIR spectrum for the PNC/PEDOT composite indicates that two peaks at 972 cm–1 and 1388 cm–1 are assigned to the C–S and C–O–C bonds of PEDOT, respectively (Figure S1). The XRD patterns of the products show that the PNC/PEDOT composite is amorphous in nature 8


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(Figure S2). Furthermore, XPS is conducted to determine the surface states and valence states of the products. The survey XPS spectra for the PNC and PNC/PEDOT composites are shown in Figure S3. The existence of S (S 2p) confirms that PEDOT was successfully coated on the surface of the PNC arrays. Three peaks for Ni 2p3/2 at 850–867 eV and three peaks for Ni 2p1/2 at 867–885 eV appear in the high-resolution XPS spectrum of Ni 2p (Figure 3c). The strong peak at 855.7 eV for Ni 2p3/2 and 873.5 eV for Ni 2p1/2 are assigned to Ni2+, while the weak satellite peaks at 852.5 and 869.8 eV are attributed to the Ni-Ni bond, respectively. Also, the peaks at 861.5 and 880.1 eV are satellite peaks attributed to the Ni 2p3/2 and Ni 2p1/2 spin orbit levels of NiO or Ni(OH)2. The high-resolution XPS spectrum of O 1s (Figure 3d) suggests four kinds of oxygen contributions. The signal at 529.5 eV is originated from for the Ni–O bonds. The other three peaks at 531.1, 531.8 and 532.8 eV may be related to the Ni-OH bonds, defects and surface oxygen-based species.24

Figure 3. (a) EDX spectrum and (b) corresponding elemental mapping from a single contra-supported PNC/PEDOT wire (the signal of Fe originates from the impurity in the contra wire. The Pt peak is

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from the Pt coating by sputtering to minimize charging effects under SEM imaging conditions). High-resolution XPS spectra for (c) Ni 2p and (d) O 1s of the PNC/PEDOT composite.

The electrochemical properties of the electrodes were explored using a three-electrode system. Figure 4a displays the cyclic voltammetry (CV) curves of the CW/PEDOT, CW/PNC, and CW/PNC/PEDOT electrodes. As expected, the hybrid CW/PNC/PEDOT electrode exhibits a larger current density than that of the CW/PNC electrode, suggesting the enhanced electrochemical capacitance through PEDOT coating. In comparison, the capacitance contribution from the bare PEDOT is negligible. The CV curves at different scan rates (Figure 4b) were obtained to analyze the capacitance behavior of the CW/PNC/PEDOT electrode. All the CV curves exhibit a pair of well-defined redox peaks (between 0.15 V and 0.35 V), corresponding to the reversible conversion between Ni2+ and Ni3+. The related electrochemical reactions can be summarized in the following equations:20 NiO + OH– ↔ NiOOH + e– Ni(OH)2 + OH– ↔ NiOOH + H2O + e–

(1) (2)

Figure 4. (a) CV curves of different electrodes collected at 100 mV s–1 in 6 M KOH. (b) CV curves for the CW/PNC/PEDOT electrode at different scan rates (10, 20, 50, 100 and 200 mV s–1). (c) 10


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Charge-discharge curves of the CW/PNC/PEDOT electrode at various current densities (ranging from 2 to 16 mA cm–2). (d) Dependence of the areal and volumetric capacitances on the charge/discharge current density for the CW/PNC/PEDOT electrode. (e) Cycling performance of the CW/PNC and the CW/PNC/PEDOT electrodes. (f) Nyquist plots of the pristine CW/PNC, and CW/PNC/PEDOT electrodes. The corresponding equivalent circuit is showing in the inset.

To further investigate the capacitance performance of the resulting CW/PNC/PEDOT electrode, galvanostatic charge-discharge (GCD) tests were carried out at different current densities in the voltage range of 0 ~ 0.4 V. As shown in Figure 4c, the discharge plateaus are located in the range of 0.15–0.20 V, which is identical to the CV discharge peaks. When the current density was increased from 4 to 32 mA cm–2, the areal capacitance is decreased from 404.1 to 310.5 mF cm–2, indicating a good rate capacitance (Figure 4d). The unique flower-like porous array structure may contribute to the high rate capability and the large specific capacitance, which endows good mechanical property and electrical conductivity, and offers rich redox reactions. Furthermore, we studied the cycling performance of the CW/PNC and CW/PNC/PEDOT electrodes at 4 mA cm–2 (Figure 4e). It is

obvious that the capacitance performance and cyclability were both enhanced after the modification of PEDOT. Over 1000 discharge/charge cycles, the specific capacitance increased in initial 50 cycles. The increased inceptive capacitance is probably due to the gradual oxidation of the residual Ni in the surface layer of the porous arrays.20 Figure 4f shows the EIS data for the composite electrodes. A vertical curve in the low-frequency region and an arc in the high-frequency region were observed from the plot. The semicircle is usually assigned to a charge-transfer process, and the straight slope arises from a diffusion-limited process.33 From the Nyquist plots, the charge-transfer resistance of the CW/PNC/PEDOT electrode is smaller than that of the CW/PNC electrode. The equivalent series resistance (ESR) for the PEDOT-modified PNC electrode is about 1.5 Ω, which is lower that of the uncoated PNC electrode (2 Ω). This clearly reveals that the conductivity of the PNC composite can be improved by coating with a PEDOT layer.

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Figure 5. (a) CV curves of the CW/PNC/PEDOT and CW/CMK-3 electrodes at a scan rate of 100 mV s–1. (b) CV curves of the assembled solid-state asymmetric supercapacitor device with a cell voltage of 1.45 V at different scan rates. (c) Galvanostatic charge/discharge curves of the asymmetric supercapacitor device collected at different current densities. (d) Areal and specific capacitances calculated from the charge/discharge curves as a function of current density. (e) Cycling performance of the asymmetric supercapacitor device. (f) Ragone plots of the device compared to other reported fiber supercapacitors.

The CW/PNC/PEDOT electrode was further evaluated for practical applications. Typically, an ASC device made of the CW/PNC/PEDOT wire as the positive electrode was fabricated. The CW/CMK-3 fiber and the PVA-KOH gel were used as the negative electrode and the solid electrolyte, respectively. The morphology and electrochemical characterization of the CW/CMK-3 fiber electrode are presented in Figure S4. Two different electrodes with a matching capacitance were chosen by the △U value, as shown in the CV curves of Figure 5a.38 The CV curves of the as-fabricated solid-state ASC at different voltage windows (Figure S5) depicts that the electrochemical potential windows up to 1.5 V is stable. Figure 5b shows the typical CV curves at different scan rates of 10–200 mV s–1 during the potential window ranging from 0.0 to 1.45 V. A 12


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combination of both pseudocapacitance and electrical double layer (EDL) capacitance characteristics could be clearly observed at different scan rates, which is consistent with the charge-discharge investigation. Even at a high scan rate up to 200 mV s–1, the shape of the CV profiles would not be evidently distorted, suggesting that the ASC device has the high rate capability. Figure 5c displays the galvanostatic charge/discharge curves at different current densities of the as-assembled ASC device. A large areal specific capacitance of 31.6 mF cm–2 (3.16 F cm–3 for the specific volumetric capacitance) can be achieved at a current density of 0.4 mA cm–2. Even at a higher current density of 8 mA cm–2, the specific capacitance of 15.9 mF cm–2 (or 1.59 F cm–3) could be maintained (Figure 5d). The quick ion transfer of the device can be further confirmed by EIS (Figure S6). The low ESR value of about 2Ω and a vertical curve suggest a high power density and energy density, which is also displayed in Figure 5f. Figure 5e shows the cyclability at a current density of 3.2 mA cm–2 for 1400 cycles. The as-fabricated ASC only has a slight fluctuation in the whole process, demonstrating the outstanding stability of the device. The digital photos (inset of Figure 5e) show the as-prepared CW/PNC/PEDOT and CW/CMK-3 electrodes, as well as the ASC device under the bending state, indicating the good flexibility. Small changes occur when the device was bended at different angles (Figure S7). The ASC device displays a high energy density of 0.011 mW h cm–2 at a power density of 0.33 m W cm–2. Even at a high power density of 7.8 mW cm–2, the energy density of the ASC device could be kept at 6.1 µW h cm–2, which is better than most of those previously reported fiber-shaped supercapacitors at the same power level (Figure 5f).39–43 The enhanced performances may be attributed to the attractive nanoarchitecture of the as-fabricated hybrid electrode and the synergistic effects of porous NiO/Ni(OH)2 and the modification layer of PEDOT. On one hand, the metallic contra wire may serve as a highly conductive support. On the other hand, the flower-like porous NiO/Ni(OH)2 composite possesses a 3D structure and high surface area, which is advantageous for the ion transport of the electrolyte. Importantly, the modification layer of PEDOT with an interconnected 3D framework can construct the electrical conductive pathways. Meanwhile, the protective PEDOT servers as the buffer layer to relieve the mechanical stress arising from the volume change during continuous charge/discharge cycling. Benefitting from the synergistic effects 13



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of the porous NiO/Ni(OH)2 and PEDOT, therefore, the outstanding electrochemical performances of the ASC device is achieved.

Conclusions In summary, the porous PNC nanoflowers with surface modification of PEDOT on contra wires have been successfully fabricated through a mild electrochemical route. The resulting CW/PNC/PEDOT electrode exhibits a high specific capacitance of 404.1 mF cm–2 (or 80.8 F cm–3) at a current density of 4 mA cm–2, and high stability of 82.2% capacitance retention after 1000 cycles. A fiber-shaped ASC device with the CW/PNC/PEDOT fiber as the positive electrode and the CW/CMK-3 fiber as the negative electrode can work steadily at a voltage of 1.45 V, and an energy density of 0.011 mW h cm–2 at a power density of 0.33 mW cm–2 can be achieved. This work provides a facile strategy to fabricate nanostructured active materials on micrometer-sized metal wire as high-performance electrodes for flexible all-solid-state supercapacitors. It can be extended to fabricate other nanostructured metal oxides/hydroxides-based hybrid electrodes for energy-storage devices.

Acknowledgements This work was supported by National High-tech R&D Program of China (863 Program, No. 2015AA034601), National Natural Science Foundation of China (No. 51522205, 51472098 and 21271078), and Program for New Century Excellent Talents in University (No. NECT-12-0223).

ASSOCIATED CONTENT Supporting Information Available: FTIR spectra, XRD patterns, XPS spectra, SEM image, and electrochemical performances. This material is available free of charge via the Internet at hppt://pubs.acs.org.

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PEDOT Nanocomposites on Contra Wires

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