Flexible Asymmetrical Solid-State Supercapacitors Based on


Dec 22, 2015 - In this study, a flexible asymmetrical all-solid-state supercapacitor with high electrochemical performance was fabricated with Ni/MnO2...
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Flexible Asymmetrical Solid-State Supercapacitors Based on Laboratory Filter Paper Leicong Zhang,†,‡,# Pengli Zhu,*,†,⊥,# Fengrui Zhou,† Wenjin Zeng,§ Haibo Su,† Gang Li,† Jihua Gao,‡ Rong Sun,*,† and Ching-ping Wong*,∥,⊥ †

Shenzhen Institutes of Advanced Technology, Chinese Academy of Science, Shenzhen 518055, China Shenzhen Key Laboratory of Special Functional Materials, College of Materials, Shenzhen University, Shenzhen 518060, China § School of Materials Science and Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210003, China ∥ School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ⊥ Department of Electronics Engineering, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, China ‡

S Supporting Information *

ABSTRACT: In this study, a flexible asymmetrical all-solid-state supercapacitor with high electrochemical performance was fabricated with Ni/MnO2filter paper (FP) as the positive electrode and Ni/active carbon (AC)filter paper as negative electrode, separated with poly(vinyl alcohol) (PVA)−Na2SO4 electrolyte. A simple procedure, such as electroless plating, was introduced to prepare the Ni/MnO2−FP electrode on the conventional laboratory FP, combined with the subsequent step of electrodeposition. Electrochemical results show that the as-prepared electrodes display outstanding areal specific capacitance (1900 mF/cm2 at 5 mV/s) and excellent cycling performance (85.1% retention after 1000 cycles at 20 mA/cm2). Such a flexible supercapacitor assembled asymmetrically in the solid state exhibits a large volume energy density (0.78 mWh/cm3) and superior flexibility under different bending conditions. It has been demonstrated that the supercapacitors could be used as a power source to drive a 3 V light-emitting diode indicator. This study may provide an available method for designing and fabricating flexible supercapacitors with high performance in the application of wearable and portable electronics based on easily available materials. KEYWORDS: filter paper, electroless plating, MnO2, polymer electrolyte, flexible supercapacitor he novel concepts of flexible displays, artificial skins, and intelligent glasses were proposed some years ago, and a few of these have been successfully realized; the market of personal electronic products is sharply developed.1−4 The quick development of flexible electronic technology has drawn wide attention and activated the growing demand for energy-storage devices to be ultrathin, flexible, miniaturized, and high-efficiency.5,6 Compared with conventional energy conversion and storage devices, a flexible power source needs to not only ensure good stability, for example, constant power supplies, high security and reliability, long cycle life, and other basic requirements, but also possess good mechanical properties, such as rigorous mechanical flexibility, stretchability, etc.7−10 Therefore, good electrochemical performance and deformable mechanical properties are the key requirements for flexible energy storage devices.11 Among them, flexible supercapacitors, also known as electrochemical capacitors or ultracapacitors, have attracted more and more attention and research interest due to their series of advantages, such as fast

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© 2015 American Chemical Society

charge−discharge rate, high power density, large capacity, low cost, and much longer cycling life.12,13 Moreover, flexible supercapacitors are flexible, safe, reliable, lightweight, widely available, and have less environmental impact.14,15 In the past few years, many efforts have been devoted to searching for the efficient and facile methods to prepare thin, flexible, and lightweight solid-state supercapacitors, which are regarded as one of the candidates for power sources for portable electronic devices.16 At present, besides the application of polymeric gels as electrolytes, the methods to successfully prepare flexible supercapacitors mainly focus on the following aspects: (1) Directly apply the 1D or 2D active supercapacitor materials to prepare the self-supporting flexible electrode films via simple filtering17,18 or other dipping methods19,20 or grow electrochemical active materials directly on the porous metal Received: October 22, 2015 Accepted: December 16, 2015 Published: December 22, 2015 1273

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Figure 1. Schematic illustration for the fabrication of Ni/MnO2−FP electrode.

Herein, flexible Ni/MnO2−FP electrodes were fabricated, combined with the electroless plating, and followed an electrochemical deposition process. For the common laboratory filter paper, their surfaces are extremely rough and porous, and abundant hydroxyl groups exist along the cellulose chains; nickel ions could be easily absorbed, diffused, and reduced to metal nickel. So the first electroless plating strategy could help the formation of highly conductive nickel layers on the surface of wooden fibers of the filter papers with good adhesion. Therefore, porous nickel and MnO2 layers can be easy electrodeposited on it in the next steps. The method for fabricating Ni/MnO2−FP electrodes is convenient and simple, and the obtained electrodes exhibit excellent areal specific capacitance and outstanding cycling performance even at a high current density. In addition, using the Ni/MnO2−FP as a positive electrode, Ni/AC−FP as a negative electrode, and poly(vinyl alcohol) (PVA)−Na2SO4 gel as an electrolyte, a solid-state flexible asymmetrical supercapacitor could be easy assembled. This supercapacitor exhibits large volume energy density, extremely high operating voltage, and superior flexibility under different bending conditions. The present studies represent significant progress in the development of low-cost and environmentally friendly flexible energy storage devices.

substrates, such as nickel foams21 or stainless steel wires.22 (2) Coat active materials on the flexible substrates to obtain composite electrodes. Such flexible substrates are usually common in daily life, nonconductive, or have excellent stress−strain mechanical properties.23 For example, Yu et al. coated hybrid graphene/MnO2 nanostructures on textiles by a facile dip-electrochemical deposition method,24 and Si et al. developed the interdigital micro-supercapacitor equipped with hybrid MnOx/Au multilayers, which was directly deposited on polyethylene terephthalate (PET) film through electron beam evaporation;25 Xu et al. transferred and buckled the laminated ultrathin graphene film prepared by chemical vapor deposition (CVD) on polydimethylsiloxane substrates through a prestraining-then-buckling strategy;26 Qiu et al. reported the preparation of vertically aligned carbon nanotubes directly grown on carbon nanofiber cloth by combining electrospinning with pyrolysis technologies.27 The quantitative filter papers (FPs), one of the common lab consumables, which are made from wooden fibers and have well-developed porosity, are widely available, lightweight, biodegradable, flexible, and low-cost.20,28 However, similar to other cellulose papers or printing papers, the poor electrical conductivity and low electrochemical activity of the FP itself are primarily bottlenecks for their direct used in electronic applications.28 So in order to overcome the above limitations in the filter papers, several ingenious strategies have been developed and successfully prepared for functional hybrid paper electrodes. For example, Weng et al. fabricated a graphene−cellulose paper membrane by vacuum filtration of a graphene nanosheet suspension through a piece of filter paper;29 Yuan et al. developed highly conductive polypyrrolecoated paper as a flexible electrode by a simple “soak and polymerization” method,30 and Feng et al. prepared hybrid paper-based electrodes through a simple method involving pencil-drawing and electrodeposition strategies.31 Overall, in the above works, the active electrochemical materials or electrical conductivity materials are simply coated on the surface of these paper substrates. In order to make full use of the inner mesoporous and microfibril structures of the FP, new hybrid electrode structures are needed to be developed.

RESULTS AND DISCUSSION The design and manufacturing process of a flexible Ni/MnO2− FP electrode is illustrated in Figure 1, which includes the following three steps: (i) The conductive Ni(I)−FP was prepared through the traditional electroless nickel plating method; after this process, a homogeneous Ni layer was first plated on the fiber surface of the filter paper, which could extremely increase the electrical conductivity of bare filter paper and form the initial conductive networks. The specific mechanism of preparation process of Ni(I)−FP is described as follows: the cellulose fibers of bare filter paper contain abundant hydroxyl functional groups on their surfaces. Thus, Sn2+ ions could be easily absorbed on the surface of cellulose fibers through the sensitization process. Then, small Pd 1274

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Figure 2. SEM images of (a) bare FP, (b) Ni(I)−FP, and (c) porous Ni(II)−FP; insets are the corresponding SEM image at larger magnification.

Figure 3. SEM images of (a) Ni/MnO2(4)−FP, (b) Ni/MnO2(8)−FP, (c) Ni/MnO2(10)−FP, (d) Ni/MnO2(12)−FP, and (e) Ni/MnO2(15)− FP electrodes, and their corresponding magnified SEM images of Ni/MnO2−FP are provided in the insets.

for the electroless plating, and electrodeposition of the nickel layer and MnO2 electrochemical active materials successfully obtained the complex Ni/MnO2−FP electrode with a larger area, which provides an available method for designing and fabricating the flexible electrodes. Further, scanning electron microscopy (SEM) measurement was employed to identify the morphology of bare FP, Ni(I)− FP and Ni(II)−FP. As revealed in Figure 2a, for the bare filter paper, the multiple individual cellulose fibers are tightly stacked and intertwined with each other and form the basic framework of the paper. Pores, voids, and microfibrils between cellulose fibers are visible. Also, the rough surface structure of each cellulose fiber and the distinct cross-linked network structure formed between microfibrils and macrofibrils can be seen from the magnified SEM image provided in the inset of Figure 2a. Compared with bare FP with a visibly rough surface, after the electroless nickel plating process, the cellulose fibers were intimately coated by a Ni layer, which shows a slightly smooth surface similar to that for a single fiber (Figure 2b). In terms of the whole Ni(I)−FP, it clearly indicates that the Ni layers uniformly and continuously formed and grew along the cellulose fiber; therefore, the diameter of fiber becomes slightly thicker and fiber embosses more obviously than in bare FP

particles, which could act as the catalytic active sites, might be generated through the following activation treatment via the reaction between Sn2+ and Pd2+. When the treated filter paper is immersed into the aqueous solution containing Ni2+ ions and the subsequently added reducing agent, a very small amount of Ni2+ ions are reduced and attached on the surface of previously formed Pd particles. Also, the initial metal Ni particles perform as seed crystals to accelerate the development of continuous and uniform Ni layers. The formation of Ni is an automatically continuous process due to the self-catalytic behavior of electroless plating, so the following Ni reduction process could last until Ni2+ ions or reducing agent are exhausted in the plating solution. (ii) In order to further improve the electrical conductivity of Ni(I)−FP, the second Ni layer was coated through an electrodeposition technique. The good electrical conductivity of the Ni layer is very favorable to improve the electrochemical performance of electrode. (iii) A thin MnO2 layer used as an electrochemical active material was subsequently coated on the surface of Ni(II)−FP by an anodic electrodeposition method to fabricate a flexible Ni/MnO2−FP electrode (details in the Materials and Methods section). Using this method, the common laboratory filter paper with a diameter as larger as 8 cm could be used as a flexible substrate 1275

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ACS Nano (inset in Figure 2b). After the electroless plating process, the electrical conductivity (κ) of the Ni(I)−FP is 1.2 × 103 S/m, extremely increased by the existence of metal Ni and the change of the filter paper from insulative to conductive, which is good for the electrodeposition method. To further increase the electrical conductivity of the Ni(I)−FP, the Ni was deposited again. As shown in Figure 2c, porous lunar craterlike Ni layers were formed due to the abundant bubbles that were generated as templates during the reduction reaction. Compared with Figure 2b, the electrodeposition of metal nickel particles filled all the pores and voids among the cellulose fiber and completely covered the Ni(I)−FP. The thickness of the Ni layer in Ni(II)−FP is estimated to be about 23 μm (Figure S1, Supporting Information). The thick and continuous Ni layers further improve the electrical conductivity of the FP to 1.5 × 105 S/m. The magnified SEM images of Ni(II)−FP show that the electrodeposited Ni layer is composed of a large numbers of accumulated Ni particles and also many crack spaces between the Ni particles exist. The porous structures together with the rough surface of the electrical conductive electrodeposited Ni(II) layer would facilitate the mass loading of electrochemical material and enhance the electrochemical reactions and provide short diffusion paths for ions.31 Based on the Ni(II)−FP with good electrical conductivity, a thin MnO2 layer used as an electrochemical active material was subsequently electrodeposited on the surface via the anodic electrodeposition method. The morphology of the Ni/MnO2− FP electrodes prepared by different deposition time (4, 8, 10, 12, and 15 min) is illustrated in Figure 3. From Figure 3a−c, it can be seen that, with the increase of deposition time, the gap between nickel spheres is gradually decreased and MnO2 grows constantly and uniformly on the surface of the nickel layer; therefore, the mass loading of MnO2 is gradually increased. Besides, when increasing the deposition time to as long as 12 min, the lunar crater-like morphology disappears and crack space becomes apparently enlarged (Figure 3d). However, with prolonged electrodeposition time, more crack spaces appear and they gradually merged into larger spaces. Meanwhile, due to the extensive deposition of MnO2, the MnO2 layer together with the inner porous Ni layer might be peeled off from the paper substrates and broke the electrode structures. Magnified SEM images of the Ni/MnO2−FP inset in Figure 3 indicate that all the electrodeposited MnO2 possesses the nanosheet structure with negligible differences. The SEM analysis revealed that the optimum electrodeposition time for the MnO2 is 10 min, which could produce a compact and uniform MnO2 layer. In order to identify the composition of the sample, the electrode of Ni/MnO2(10)-FP was tested by X-ray diffraction (XRD), as shown in Figure 4. The XRD pattern clearly shows that the MnO2 phase is really formed (marked in red) after the anode deposition. Nevertheless, the MnO 2 peaks are characterized by broad peaks located at 36.6 and 65.7° due to the poorly crystallized and sharp intensity of Ni peaks. Furthermore, three obvious diffraction peaks at 44.5, 51.8, and 76.4° are attributed to the (111), (200), and (222) planes, respectively, which is in good agreement with the face-centered cubic Ni (JCPDS no. 04-0850). X-ray photoelectron spectroscopy (XPS) measurement was performed to study the composition and oxidation state of the as-prepared MnO2 layer, and the results are shown in Figure 5, which presents the full spectrum scan and multiplet splitting peaks for the Mn 2p, Mn 3s, and O 1s regions. The survey spectra presented in Figure 5a for the Ni/MnO2(10)−FP

Figure 4. XRD pattern of the Ni/MnO2(10)−FP electrode.

electrode show that the peaks located at the binding energy of 642.1, 531.1, and 83.4 eV correspond to Mn 2p, O 1s, and Mn 3s signals, respectively, which can be attributed to manganese dioxide.32 Additionally, the C 1s peak with a binding energy at 285.2 eV possesses the characteristics of carbon bonded to oxygen in cellulose macromolecules,33 associated with the presence of filter paper substrates. Also, the Ni 2p peak with a binding energy of 855.8 eV presents an extremely weak signal, which is in agreement with the fact that the Ni layer is completely coated by the MnO2 nanosheet layer. Figure 5b shows a high-resolution image of the Mn 2p spectrum; two peaks centered at 642.3 and 654.1 eV can be well ascribed to Mn 2p3/2 and Mn 2p1/2, from which the binding energy separation between the two peaks is calculated to be 11.8 eV, exactly consistent with the previously reported data for MnO2.32,34 For Mn 3s, Toupin et al. reported that the separation of peak energies (ΔE) of the Mn 3s component is linearly related to the mean oxidation state of Mn in MnOx,35 and the ΔE values of about 4.7 and 5.4 eV are in agreement with Mn4+ and Mn3+ oxides, respectively.32,35,36 Hence, according to these studies and the ΔE value of 5.16 eV calculated from Figure 5c, the average valence of Mn in the Ni/ MnO2(10)−FP electrode is determined to be 3.34. Moreover, the average manganese oxidation state can also be estimated from the O 1s core level spectrum (Figure 5d), with the intensities of the Mn−O−Mn and Mn−OH components: XMn =

IV ∗(SMn−O−Mn − SMn−OH) + III∗SMn−OH SMn−O−Mn

(1)

where S stands for the signal of the different components of the O 1s spectrum.32 The deconvoluted result from Figure 5d shows that the signals of Mn−O−Mn and Mn−OH components are 45.65 and 23.98 area %; as a result of the analysis, the valence of Mn is 3.47, which is almost consistent with that estimated from the Mn 3s analysis. The electrochemical performance of the Ni/MnO2−FP electrodes was studied by a three-electrode configuration using 1 M Na2SO4 as the aqueous electrolyte. Compared with Ni(II)−FP, the Ni/MnO2−FP electrode shows much larger current density and integral area, as shown in Figure 6a, indicating that MnO2 is the major contributor for the electrochemical capacitance. Figure S2 shows CV curves (at scan rates of 5−100 mV/s) of the Ni/MnO2−FP electrode prepared at different electrodeposition time. It can be seen that all of the composite paper electrodes show typical squareshaped CV curves, which indicate good supercapacitor 1276

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Figure 5. (a) XPS full spectrum of MnO2 deposited on the Ni layer, and the deconvoluted Mn 2p (b), Mn 3s (c), and O 1s (d) core level XPS spectra of MnO2.

behavior. The areal specific capacitances of Ni/MnO2−FP electrodes calculated from CV curves (Figure S2a−e) are collected and compared in Figure S2f,g (Supporting Information). When the MnO2 electrodeposition time is 4 min, the areal specific capacitance of the Ni/MnO2(4)−FP electrode is calculated to be 1160 mF/cm2 at a scan rate of 5 mV/s. With increasing electrodeposition time, the capacitance of the electrode increases greatly and obtains the maximum value of 1900 mF/cm2 at 10 min. However, with prolonged reaction time, the capacitance decreases. The morphology variation in Figure 3 might give the reason for the increase and decrease of the areal specific capacitance of the electrochemical properties and prove again that the optimum electrodeposition time for the MnO2 layer is 10 min. Figure 6b presents the CV curves of the Ni/MnO2(10)−FP electrode prepared with a deposition time of 10 min with scan rates from 5 to 100 mV/s; all of the CV curves exhibit a nearly symmetrical rectangular shape even at a high scan rate of 100 mV/s and reveal remarkable rate capability and good reversibility of the paper electrodes, which benefits from the fast ion diffusion rate and electron transfer rate. The calculated areal specific capacitances of the Ni/MnO2(10)−FP versus scan rate of 5, 10, 20, 50, and 100 mV/s are plotted in Figure 6c. The areal specific capacitance is 1900 mF/cm2 at 5 mV/s and 506 mF/cm2 at 100 mV/s, resulting in a capacitance retention ratio of 26%. Galvanostatic charge−discharge (GCD) curves of the Ni/ MnO2−FP electrode prepared at a deposition time of 10 min at different current densities are shown in Figure 6d, and they all are symmetrical, demonstrating that the electrode has outstanding reversible redox reaction performance and fast charge−discharge rate. Even with a discharge at a current density of 25 mA/cm2, the discharge time is still as long as 49 s.

The areal specific capacitance that depended on different current densities of the Ni/MnO2(10)−FP electrode calculated from Figure 6d is plotted in Figure 6e, suggesting a high areal specific capacitance of 1676 mF/cm2 when the current density is 5 mA/cm2 and a good rate capability. The capacitance retention is as high as 85.1% (Figure 6f) after 1000 cycles, which is measured in aqueous solution. Nyquist plots of the electrochemical impedance spectroscopy (EIS) for Ni/MnO2− FP in the frequency range from 10 mHz to 100 kHz is shown in Figure S2h; a semicircle at the high-frequency region is followed by an approximate straight line at the low-frequency region, indicating the good electric conductivity and fast ion diffusion rate of the electrode. The previous report proves that the intercept at the real part of the high frequency for the semicircle represents the equivalent series resistance (Rs), combined with the sum of the ionic resistance of the electrolyte, the intrinsic resistance of the electrode, and the contact resistance at the electrolyte/electrode interface.37,38 The EIS data show that Rs for the Ni/MnO2−FP electrode can be evaluated to be about 4.6 Ω, which is low enough to ensure the fast transfer of electrons. The straight line at the lowfrequency range represents the fast ion diffusion behavior in the electrode. In a word, the excellent performance of the as-prepared Ni/ MnO2−FP electrode may be primarily attributed to the following reasons: (1) the porous and hygroscopic filter paper allows Ni2+ ions to quickly and steadily diffuse into the paper fibers and be reduced to metal Ni, which enhances the electric conductivity of the substrates and boosts the electron transfer rate and the electrochemical performance of the electrode; (2) the rough and porous surface of the Ni layer increases the adhesion between Ni and MnO2 and could 1277

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Figure 6. (a) CV curves of Ni/MnO2(10)−FP electrode and porous Ni(II)−FP at a scan rate of 100 mV/s. (b) CV curves of Ni/MnO2(10)− FP electrode at different scan rates of 5−100 mV/s. (c) Areal specific capacitance of Ni/MnO2(10)−FP electrode calculated from CV curves as a function of scan rate. (d) GCD curves of Ni/MnO2(10)−FP electrode at different current densities of 5−25 mA/cm2. (e) Areal specific capacitance of Ni/MnO2(10)−FP electrode calculated from GCD curves as a function of current density. (f) Capacitance retention of Ni/ MnO2(10)−FP electrode as a function of 1000 cycles.

supercapacitor devices, PVA and Na2SO4 were chosen to prepare a gel-like electrolyte (details in the Materials and Methods section). In Figure 7a, the Ni/MnO2−FP and Ni/ AC−FP (the electrochemical performance of the as-prepared Ni/AC−FP electrode is shown in Figure S3, and more detailed discussion is given in the Supporting Information) are individually developed as positive and negative electrodes to assemble a sandwich-like flexible asymmetrical all-solid-state supercapacitor (FAAS) device. The electrical double-layer capacitor material (AC) and pseudocapacitor material (MnO2) are assembled to a hybrid supercapacitor, and ion adsorption (a purely electrostatic process) and a Faradaic process separately occur on the negative and positive electrodes

shorten the length of the electron transmission path in the electrode; (3) the lunar crater-like morphology and a large number of pores on the electrode increase the contact area between electrode and electrolyte, which results in an enhanced efficiency of electrolyte diffusion. Overall, the good porosity feature of the Ni/MnO2−FP electrode and hygroscopic property of the FP fibers can provide available diffusion channels for electrolyte solution, reduce the diffusion path, and result in the excellent electrochemical performances of this flexible paper supercapacitor. The above electrochemical results indicate that as-prepared Ni/MnO2−FP electrodes exhibit excellent electrochemical performance. In order to fabricate the all-solid-state flexible 1278

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Figure 7. (a) Schematic illustration for the Ni/MnO2−FP//Ni/AC−FP FAAS device. (b) Ni/MnO2−FP//Ni/AC−FP FAAS device in a bending state. (c) LED indicator (3 V) lighted by two Ni/MnO2−FP//Ni/AC−FP FAASs in series. (d) CV curves of Ni/MnO2−FP//Ni/AC− FP FAAS in different potential windows at a scan rate of 100 mV/s. (e) CV curves of Ni/MnO2−FP//Ni/AC−FP FAAS at different scan rates. (f) Discharged curves of Ni/MnO2−FP//Ni/AC−FP FAAS at different current densities. (g) Relation curve of energy density and power density of Ni/MnO2−FP//Ni/AC−FP FAAS. (h) CV curves of Ni/MnO2−FP//Ni/AC−FP FAAS at different bending angles at 100 mV/s.

the FAAS calculated by active area and volume are 0.7 F/cm2 and 2.0 F/cm 3 at 5 mV/s (Figure S4b, Supporting Information), respectively. Figure 7f shows the discharged curves of the FAAS device at various current densities; the long discharge time reveals the good capacity behavior, and the volume-specific capacitance is calculated to be 1.4 F/cm3 at a current density of 2.5 mA/cm3. In addition, we demonstrated that a LED indicator (3 V) could be powered by two FAAS in series, as shown in Figure 7c. Figure 7g shows the relation of volume energy and power density of the as-prepared FAAS calculated by eqs 4 and 5; it demonstrates that the maximum energy density can reach as high as 0.78 mWh/cm3 at a power density of 2.5 mW/cm3, and this value is considerably higher than most of the reported flexible supercapacitors (more details in Table S1, Supporting Information). More importantly, in order to further demonstrate flexibility of the FAAS device, the CV test was conducted at a different bending state, with a radius of curvature of 0.75 cm (Figure S5, Supporting Information), as shown in Figure 7h. There is no obvious change under various bending situations; even the bending angle is 180°, and a flexible test of the as-assembled FAAS devices is shown in Movie S1. The two FAAS devices were used to light a red 3 V LED indicator under a bending state, as shown in Figures S6 and S7 (Supporting Information), and

when electrical energy is stored. Both optimal power and energy densities are realized. The electrochemical active area of a fabricated FAAS is 0.6 cm2, and the volume of the whole device is about 0.21 cm3, which consists of the packaged PET films, electrodes, separate membrane, and gel electrolyte. The practical as-prepared Ni/MnO2−FP//Ni/AC−FP FAAS device is shown in Figure 7b and can be freely bent. Figure 7d shows the CV curves of FAAS device in different potential windows, measured at a scan rate of 100 mV/s with a two-electrode system; they all exhibit roughly rectangular shape, and the enclosed area of the CV curve becomes larger, and there are no obvious changes in shape with an increased potential window from 1.0 to 2.5 V; the largest potential window can achieve 2.5 V, which confirms the fact that the gel electrolyte can enlarge the operating voltage. The calculated specific capacitance value (at a scan rate of 100 mV/s) of the FAAS device increases with the increase of potential window from 1.0 to 2.5 V (Figure S4a, Supporting Information). Obviously, the nearly linear relationship between the capacitance value and potential window can be observed. Thus, the larger potential window is employed, and more electrical energy is stored. The detailed CV curves of the FAAS device all exhibit an approximate rectangular shape at different scan rates, as shown in Figure 7e, indicating good capability and fast ion diffusion rate. The specific capacitance of 1279

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piece of the as-prepared Ni(I)−FP and Pt plate were employed as the cathodic electrode and anodic electrode, respectively. The electrodeposition solution was composed of 0.15 M NiSO4·6H2O and 0.12 M NH4Cl, and the deposition process was conducted at a DC voltage of 2.0 V for 8 min. The as-prepared sample was rinsed with deionized water several times and dried at 60 °C for 2 h. The porous Ni filter paper after electrochemical deposition was named Ni(II)−FP. Fabrication of the Ni/MnO2−FP Electrode. The Ni/MnO2−FP electrode was fabricated by an anodic electrodeposition method. The deposition process was operated on an electrochemical station (Zennium, Zahner, Germany) by a three-electrode configuration in a 0.5 M Mn(CH3COO)2·4H2O and 0.5 M Na2SO4 aqueous electrodeposition solution with a 3.0 V DC voltage at different deposition time. The porous Ni(II)−FP (0.2 × 2.0 cm2), Pt plate, and saturated calomel electrode (SCE) were used as working, counter, and reference electrodes, respectively, and the length of working electrode inserted in the electrolyte was 1.5 cm. According to the different deposition times from 4 to 15 min, the as-prepared composite electrodes were named Ni/MnO2(4)−FP, Ni/MnO2(8)−FP, Ni/MnO2(10)−FP, Ni/ MnO2(12)−FP, and Ni/MnO2(15)−FP, respectively. Fabrication of the Ni/AC−FP Electrode. The Ni/AC−FP electrode was prepared by adding 90 wt % of active carbon and 10 wt % of PVDF as binder to N-methylpyrrolidinone to form a homogeneous slurry mixture. Then the slurry was coated on the asprepared Ni(II)−FP and dried in air. Assembly of the Flexible Asymmetrical All-Solid-State Supercapacitors. The solid gel electrolyte was fabricated by adding 3 g of PVA into 30 mL of deionzied water, and the whole mixture solution was heated at 85 °C by vigorous stirring until the solution became clear. One piece of Ni/MnO2−FP electrode, Ni/AC−FP electrode, and a FP as separator were immersed into the PVA gel for 3 h, and they were assembled into a flexible all-solid-state device and dried at room temperature for 2 h. Then the device was immersed into 1 M Na2SO4 aqueous solution for 12 h. After having excess water evaporated and being packaged by PET film, the flexible asymmetrical all-solid-state supercapacitor was finished. Materials and Electrochemical Characterization. The morphologies of the as-prepared samples were characterized by fieldemission scanning electron microscopy (FEI Nova Nano SEM 450). The phase analysis and structure were investigated by X-ray diffraction measurements (Rigaku D/Max 2500). The surface chemical element composition and chemical states of the elements in the samples were identified by X-ray photoelectron spectroscope (MICROLAB 350). The electrochemical properties and capacitance measurements of the Ni/MnO2−FP electrodes were characterized with CV, GCD, and EIS using an electrochemical station (Zennium, Zahner, Germany) with a three-electrode configuration in a Na2SO4 aqueous electrolyte. Ni/ MnO2−FP, platinum plate, and SCE were used as working, counter, and reference electrodes, respectively. The EIS was conducted at varying frequency from 100 kHz to 10 mHz with an amplitude of 5 mV at an open-circuit voltage. The CV and GCD were tested between 0 and 0.8 V, and the areal specific capacitance was calculated from CV curves and discharge curves using the following equations:

there was no obvious variation in brightness during the repeated bending process, which reveals that the remarkable flexibility and confirms a good combination of electrodes, polymer electrolyte, and PET films can protect the ion transfer and adsorption process from interference that comes from a bending test. Additionally, the stability of the FAAS device has been further proven by mechanical bending cycles, which was quantitatively studied by tracing capacitance of the FAAS device in the bending process. The FAAS device showed 96.3% retention after 500 bending cycles according to the initial capacitance, indicating good cycling stability and flexibility (Figure S8, Supporting Information).

CONCLUSIONS In summary, flexible, porous, and thin-film Ni/MnO2−FP electrodes were successfully prepared using the common laboratory quantitative FP as a substrate by the method of electroless plating combined with the following electrodeposition. The flexible thin-film electrode exhibits an excellent areal specific capacitance of 1900 mF/cm2 at a scan rate of 5 mV/s and maintains its capacity up to 85.1% over 1000 cycles at a current density of 20 mA/cm2. The assembled asymmetric solid-state flexible Ni/MnO2−FP//Ni/AC−FP supercapacitor device could be operated in a much broader voltage window of 2.5 V and reach high energy density of 0.78 mWh/cm3 at a power density of 2.5 mW/cm3. Owing to these outstanding performances, our strategy of fabricating the Ni/MnO2−FP electrode is a promising candidate for assembling ultrathin and flexible supercapacitors for wearable and portable electronic applications. MATERIALS AND METHODS Materials. Tin chloride (SnCl2) and palladium dichloride (PdCl2) were purchased from Aladdin Industrial Co., Ltd. and used as sensitizing agent and activating agent, respectively. Nickel(II) sulfate hexahydrate (NiSO4·6H2O), citric acid monohydrate (C6H8O7·H2O), sodium hypophosphite monohydrate (NaH2PO2·H2O), ammonia chloride (NH4 Cl), manganese(II) acetate tetrahydrate [Mn(CH3COO)2·4H2O], sodium sulfate (Na2SO4), and poly(vinyl alcohol) (1750 ± 50) were obtained from Sinopharm Chemical Reagent Co., Ltd. All other chemicals were of analytical grade and used without further purification. Ammonia−water (NH3·H2O) and hydrochloric acid (HCl) were used as received without further purification. Synthesis of the Electroless Ni Plating Filter Paper (Ni(I)− FP). A common electroless plating method was applied on the in situ formation of the original nickel layer on the wood fiber surface of the FP to improve its electrical conductivity. The typical process was carried out as follows: First, one piece of FP was immersed into a sensitizing solution (0.05 M SnCl2, 0.12 M HCl) at 30 °C for 10 min, then washed by deionized water three times. Second, the abovementioned paper was immersed into an activation solution (100 μg/ mL PdCl2, 0.03 M HCl) at 30 °C for 15 min and rinsed well with deionized water three times. Finally, the paper was immersed into the electroless plating solution A (0.05 M NiSO4·6H2O, 0.10 M C6H8O7· H2O) and then heated to 70 °C, and ammonia−water was added to solution A at the same time. The solution was kept at a temperature of 70 °C and a pH of 10.0. The electroless plating solution B (0.10 M NaH2PO2·H2O) was slowly added to the electroless solution A and kept for 30 min. After the electroless plating process, the sample was thoroughly rinsed by deionized water and alcohol (95%) three times and dried in air for further use. The electroless nickel plating filter paper was named Ni(I)−FP. Preparation of Porous Electrochemical Deposition Ni Filter Paper (Ni(II)−FP). To further improve the electrical conductivity of Ni(I)−FP, the electrochemical deposition method was employed. One

C=

C=

∫ i(V )dV 2s ·ν·ΔV

(2)

I ·Δt s ·ΔU

(3)

where ∫ i(V)dV is the integral areas of the CV curves, v is the scan rate, s is the geometry area of the working electrode, and ΔV is the operating potential window in the CV curves; I is the discharge current, ΔU is the potential window in the discharge process, and Δt is the discharge time. The energy and power density (E and P) were calculated by the following equations: E= 1280

1 2 CV 2

(4) DOI: 10.1021/acsnano.5b06648 ACS Nano 2016, 10, 1273−1282

Article

ACS Nano P=

E Δt

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(5)

where C is the volume specific capacitance, V is the potential window in the discharge process, and Δt is the discharge time.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b06648. Additional experimental details, figures, and table (PDF) Movie S1 (AVI)

AUTHOR INFORMATION Corresponding Authors

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

L.C.Z. and P.L.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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