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An Approach To Fabricate PDMS Encapsulated All-Solid-State Advanced Asymmetric Supercapacitor Device with Vertically Aligned Hierarchical Zn−Fe−Co Ternary Oxide Nanowire and Nitrogen Doped Graphene Nanosheet for High Power Device Applications Anirban Maitra, Amit Kumar Das, Ranadip Bera, Sumanta Kumar Karan, Sarbaranjan Paria, Suman Kumar Si, and Bhanu Bhusan Khatua* Materials Science Centre, Indian Institute of Technology Kharagpur, Kharagpur 721302, West Bengal, India S Supporting Information *

ABSTRACT: We highlight the design and fabrication of a polydimethylsiloxane (PDMS) encapsulated advanced allsolid-state asymmetric supercapacitor (ASC) device consisting of hierarchical mesoporous zinc−iron−cobalt ternary oxide (ZICO) nanowire coated nickel (Ni) foam (ZICO@Ni foam) as a promising positive electrode and nitrogen doped graphene coated Ni foam (N-G@Ni foam) as negative electrode in the presence of PVA−KOH gel electrolyte. Owing to outstanding electrochemical behavior and ultrahigh specific capacitance of ZICO (≈ 2587.4 F/g at 1 A/g) and N-G (550 F/g at 1 A/g) along with their mutual synergistic outputs, the assembled allsolid-state ASC device exhibits an outstanding energy density of ≈40.5 Wh/kg accompanied by a remarkable long-term cycle stability with ≈95% specific capacitance retention even after 5000 charge−discharge cycles. The exclusive hierarchical ZICO nanowires were synthesized by a facile two-step process comprising of a hydrothermal protocol followed by an annealing treatment on a quartz substrate. While Zn2+ gives the stability of the oxide system, Fe and Co ions provide better electronic conductivity and capacitive response under vigorous cyclic condition. The extraordinary performance of as-fabricated ASC device resembles its suitability for the construction of advanced energy storage devices in modern electronic industries. KEYWORDS: polydimethylsiloxane, asymmetric supercapacitor, ZICO, N-G, energy density

1. INTRODUCTION Over the past few decades, researchers have devoted themselves to develop a handy and safe energy storage device, a green energy harvesting device that can be considered as a potential member to substitute fossil fuel energy. In order to meet the requirement of energy consumption for modern generation, one should construct a device that can have a very high power density together with improved energy density and stability.1 Recently, supercapacitors or ultracapacitors have widely been studied for their tremendously high power and energy density along with improved cycle life.2−4 The increasing demand of electrochemical supercapacitors in portable electronic appliances and electronics industries explores the possibility of developing novel and efficient electrode materials with unique structure and morphologies.5 Conventionally, electrochemical capacitors can be categorized into two parts: electrochemical double layer capacitor (EDLC) that predominantly involves porous carbonaceous materials like activated carbon, graphene,6 carbon aerogel, etc.,7 and pseudocapacitors usually comprising of various transition metal oxides,8 sulfides,9,10 hydroxides,11 carbonates,12 etc. Faradaic or pseudocapacitor type electrode materials have most extensively been studied due to its improved © 2017 American Chemical Society

capacitance and exciting electrochemical behavior with respect to EDLCs.13 Profound interest has been dedicated to fabricate simple and binary metal oxide based positive or battery type electrodes, e.g., Co3O4,14 MnO2,15−17 NiCo2O4,18 ZnCo2O4,19,20 etc. In spite of having several advantages like easy processing, low cost of manufacturing, and high power density, these metal oxides also have some drawbacks like low electronic conductivities, sluggish ion diffusion rates, etc. The template assisted synthesis approach for hierarchical structures has found to play a crucial role in order to overcome these drawbacks. Threedimensional hierarchical structures provide better exposed surface area and enhanced permeability toward electrolyte ions.2 Adequate space in between the nanowires provides huge volume variation and thereby restricts the distortion and disintegration of the nanowires during repetitive electrochemical cycling. A hierarchical structure with improved electrical conductivity eventually reduces the equivalent series resistance (ESR) and enhances power densities.21 Furthermore, studies Received: October 18, 2016 Accepted: January 17, 2017 Published: January 17, 2017 5947

DOI: 10.1021/acsami.6b13259 ACS Appl. Mater. Interfaces 2017, 9, 5947−5958

Research Article

ACS Applied Materials & Interfaces

to accomplish compact packing with improved chemical, moisture, heat, impact, and scratch resistance, the ASC can be encapsulated by polydimethylsiloxane (PDMS) elastomer having an attractive aesthetic appeal. Our present study deals with the fabrication of PDMS encapsulated unique asymmetric supercapacitor (ASC) device with hierarchical ZICO nanowire coated nickel (Ni) foam (ZICO@Ni foam) as efficient positive electrode and nitrogen doped graphene (N-G) coated Ni foam (N-G@Ni foam) as negative electrode, operated in PVA−KOH gel electrolyte.35−37 The rationale behind choosing these electrode materials is to widen the working potential window of the ultimate device. The presence of Fe and Co ions improves the overall electrochemical performance and electronic conductivity while Zn ion provides structural stability during vigorous cycling conditions. Template assisted synthesis of hierarchical ZICO nanowire arrays has been executed in the presence of a quartz substrate by a facile hydrothermal protocol followed by an annealing treatment.2 Comprehensive morphological analysis of the constituent electrode materials reveals successful development of ZICO nanowire with an average diameter of 45−60 nm and wrinkled sheets of N-G with average thickness 20−30 nm. Electrochemical performance of the as-fabricated all-solid-state ASC device exhibits a maximum specific capacitance value of ≈129.74 F/g at 1 A/g current density with corresponding energy density and power density of 40.5 Wh/kg and 750 W/kg, respectively. The superb cycle stability with ≈95% retention of its original specific capacitance even after 5000 consecutive charge−discharge cycles and improved dimensional stability brands the fabricated ASC device as a promising candidate for advance energy storage applications. The device can instantly light up commercial light-emitting diodes (LEDs) and powers up several portable electronic appliances.

on improving the electrical conductivity introduce a new perception of direct coating (slurry coating) or growing appropriate metal oxides on a conductive current collector surface, i.e., Ni foam,3 Ni foil, carbon cloth,22 and carbon fiber23 surface that enhances overall electrochemical performance of the electrode materials.24 In 2014, Tu et al. reported a spinel manganese−nickel−cobalt ternary oxide nanowire that exhibits a specific capacitance value of 638 F/g at 1 A/g current density with outstanding cycling stability.4 Recently, Shen and Zhang et al. synthesized three-dimensional hierarchical zinc−nickel− cobalt based ternary oxide (ZNCO) nanowires directly on Ni-foam substrate having a very high specific capacitance of ≈2481.8 F/g at 1 A/g with excellent (91.9%) cycling stability.2 However, to date, reports on the studies of zinc−iron−cobalt ternary oxide (ZICO) and their electrochemical applications are relatively rare. Contrary to cobalt−iron−oxide based electrode materials, ZICO would significantly decrease the cost of manufacture and provide good safety performance. The complex chemical configuration and synergistic contributions of zinc, cobalt, and iron ions greatly enhance the electrochemical properties compared to particular metal constituent oxides. The electronic conductivity and capacitance behavior increase due to the presence of cobalt and iron ions while, eco-friendly zinc provides better electrical conductivity and structural stability.3,25 The higher exposed surface area together with enormous mesopores provides more channels for electrolyte transportation and inaugurate high amount of electrode−electrolyte interactions. In order to mitigate the requirements for device fabrication in portable electronics, the supercapacitor should have a wider working potential window along with higher energy and power density. The operating potential window of a cell can be expanded by fabricating asymmetric supercapacitors (ASC). Generally, the ASC device consists of a combination of typical EDLC type electrode material (delivers high power density) together with a pseudocapacitive or battery type electrode material (provides energy density) separated by an electrolyte. This type of device can provide the properties of both supercapacitors and batteries with an added advantage of wide working potential window.26−28 Nowadays, substantial research has also been dedicated to fabricate ASCs with improved electrochemical properties and environmental stability.29 In 2014, Li et al. have evaluated the electrochemical behavior of ZnCo2O4//activated carbon film based asymmetric supercapacitor in 3 M KOH electrolyte.30 In 2009, Gao et al. explained the capacitive performance of porous Ni−Zn−Co oxide/hydroxide//carbon electrodes in 6 M KOH, and they acquired a specific capacitance value of 136 F/g at 0.25 A/g operated at 1.5 V potential range.31 Recently, Alshareef et al. have reported long-term cycle stability with improved energy and power density for nickel−cobalt−sulfide//graphene film.10 Activated carbon32 and graphene27 have extensively been used as negative electrodes owing to their very high specific surface area, interconnected porous structure, and improved electrical conductivity. The interconnected porous structure provides channels for better transportation of the electrolyte ions. Recently, Kim et al. have reported dopant-specific unzipping of carbon nanotubes for intact crystalline graphene nanostructure and its ultrahigh-power supercapacitor performance.33 Effort has been put forward to fabricate an all-solid-state ASC with poly(vinyl alcohol)−potassium hydroxide (PVA−KOH) gel electrolyte membrane in order to achieve high output voltage and improved electrochemical activity. The gel electrolyte provides better durability and dimensional stability.34 Furthermore,

2. EXPERIMENTAL SECTION 2.1. Synthesis of Quartz Supported Hierarchical ZICO Nanowire Arrays. Quartz supported three-dimensional hierarchical mesoporous ZICO nanowires have been prepared by a simple and facile hydrothermal method followed by annealing treatment. A piece of quartz substrate (2 × 2 cm2) was first rubbed mechanically by using sand paper in order to make the entire surface uneven. After that, the dust-free quartz substrate was ultasonicated with acetone for 30 min and washed several times with deionized (DI) water. The washed quartz substrate with uneven surface was then dipped in a solution containing the precursors for 10 min. The precursor solution was prepared by mixing 0.5 M cobalt nitrate hexahydrate (Co(NO3)2·6H2O), 0.25 M zinc nitrate hexahydrate (Zn(NO3)2·6H2O), 0.25 M iron(III) nitrate nonahydrate [Fe(NO3)3·9H2O], 1 M urea (CO(NH2)2) and 0.3 M ammonium fluoride (NH4F) with 40 mL of DI water. The reaction mixture comprising the growth solution along with quartz substrate was shifted on a 50 mL Teflon sealed autoclave and heated in a muffle furnace at around 150 °C for 7 h. After cooling of the autoclave, the quartz supported sample was taken out, washed with DI water, and dried at 60 °C for ≈12 h. Finally, the obtained quartz supported reddish-brown powder was annealed in a furnace at 350 °C for 3 h in air to obtain hierarchical ZICO nanowires. Hereafter, ZICO powder was peeled off completely from the quartz substrate utilizing a powerful ultrasonic treatment.2 2.2. Synthesis of Quartz Supported Mesoporous Nitrogen Doped Graphene (N-G). Nitrogen doping of graphene has also been carried out by using a facile hydrothermal method. Initially, 0.15 g of graphene oxide (prepared by following modified Hummers’ method) was added in 30 mL of DI water and sonicated in ultrasound bath for 30 min to homogenize. After that, a piece of washed quartz substrate (2 × 2 cm2) with uneven surface was dipped in the suspension for 10 min. Finally, ∼8 mL of liquid ammonia was added dropwise into it, 5948


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Figure 1. Schematic illustration for the fabrication of ZICO@Ni foam electrode and the assembled PDMS encapsulated ZICO//N-G all-solid-state ASC device. and the entire solution was immediately transferred in a 50 mL Teflon sealed autoclave and heated in a muffle furnace maintained at 180 °C for 12 h. After cooling, the quartz supported sample was washed with DI water and dried at ∼80 °C for 10 h under vacuum. The obtained black powder was peeled off from quartz substrate by applying ultrasonic vibrations and labeled as N-G. 2.3. Fabrication of PDMS Encapsulated All-Solid-State ASC Device. A high performance all-solid-state ASC device has been fabricated by assembling hierarchical ZICO nanowires coated on Ni foam substrate (ZICO@Ni foam) as positive and N-G coated Ni foam (N-G@Ni foam) as negative electrode in the presence of PVA−KOH gel electrolyte membrane. (Electrodes fabrication and preparation of PVA−KOH gel electrolyte have been elaborated in the Supporting Information.) Then, the entire device was wrapped by a single layer of polypropylene (PP) tape. The tape protects the device during curing of PDMS and restricts the passage of liquid PDMS in to the interior of the device. Finally, the PP wrapped ASC device was dipped in a solution containing a mixture of PDMS and curing agent at a weight ratio of 10:1 (PDMS elastomer:curing agent). The entire system was kept in a vacuum oven at temperature of ∼80 °C for 45 min in order to obtain transparent and robust PDMS encapsulated all-solid-state ASC device with decent aesthetic appeal. The complete fabrication procedure of ASC device has been depicted schematically in Figure 1.

Figure 2. XRD patterns of (a) ZICO, (b) JCPDS card no. 006-4012, and (c) N-G.

crystal planes, respectively. The diffraction peak appearing at 2θ ≈ 38.3° signifies the minute existence of cubic Co3O4 (JCPDS card no. 078-1969).2 Moreover, a trace amount of Fe3O4 (JCPDS card no. 075-0449) as impurity can also be detected. The slight alteration of the acquired peak positions with respect to the standard one (Figure 2b) perhaps due to dissimilarities in ionic radius of Zn, Co, and Fe.38 Figure 2c illustrates the diffraction pattern of N-G, where two broad peaks at 2θ ≈ 25.7° (major) and 43.4° (weak) resemble graphene (002) and (100) planes, respectively. The average interlayer spacing between two successive (002) of N-G is found to be ∼3.46 Å. This calculated interlayer spacing is somewhat greater than commercial graphite powder (∼3.36 Å) which resembles doping with nitrogen and oxygen. However, the broadening of the obtained diffraction peaks suggests exfoliation and transformation of N-G. The intensity of the peak for (100)

3. RESULTS AND DISCUSSION X-ray diffraction (XRD) studies of ZICO and N-G (peeled off from quartz substrate) were carried out by mounting the electrode materials on a transparent glass fiber plate and inserting them in X-ray diffractometer instrument. Figure 2a−c represents the corresponding XRD patterns of the electrode materials. The obtained diffraction peaks of as-prepared ZICO (as represented in Figure 2a) ensures the development of cubic Zn−Fe−Co−oxide with Fd3m space group as it coincides well with the JCPDS card no. 006-4012. The intense peaks appearing at 2θ ≈ 30.9°, 35.5°, and 36.6° resemble (220), (311), and (222) crystal planes while other distinctive moderate-to-low intense peaks at 2θ ≈ 18.4°, 43.1°, 53.3°, 56.9°, 62.5°, and 66.0° correspond to (111), (400), (331), (511), (440), and (531) 5949


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temperature, the nanowire tends to coalesce with each other with a decrease in lateral spacing’s and porous architecture. An extended annealing treatment also leads to an agglomerated coarse surface with poor electrochemical activity.2 Additionally, altering the molar ratio of Zn, Fe, and Co will tend to provide different type microstructures and phases which will have diverse redox activity. Figure 3c,d depicts the FESEM micrographs of as-synthesized N-G electrode material grown on a quartz substrate at different magnifications. Typical crumpled-flake type morphology (as depicted in Figure 3d) provides higher exposed surface area and porous architectures for better electrode−electrolyte interactions. Transmission electron microscopic (TEM) studies also reveal analogous features as obtained from FESEM investigations. The bulk morphological features of the electrode materials obtained from TEM are represented in Figure 3e,f,h,i. The hierarchical nanowires of ZICO have been clearly visualized from Figure 3e,f. Random growth of nanowires with average diameter of ≈50−60 nm is evident (as shown in Figure 3f). Moreover, frequent mesopores with 2−4 nm pore diameters have been recognized. The occurrence of numerous mesopores provides better channel for faster electrolytic ion transfer and hence enhances the electrochemical properties.45 The selected area electron diffraction (SAED) pattern (Figure 3g) reflects the well-defined hexagonal rings of ZICO. The hexagonal spot diffraction pattern possibly indicates single crystalline nature of the tip regions (hexagonal structure) of the nanowires. The interplanar distance measured from SAED pattern for (111) plane is ∼0.52 nm, which is in good agreement with the XRD analysis. Bright signature of (222) crystal plane and less intense signatures of (440) and (531) planes can also be well recognized. Figure 3h,i depicts the TEM micrographs of porous crumpled flake-like N-G electrode material. The acquired SAED pattern (inset of Figure 3i and Figure S1d) also approves the presence of graphene (002) and (100) crystal planes. The corresponding elemental mappings of ZICO and N-G (as depicted in Figure S2) reveal a uniform distribution of the constituent elements. The existence of Zn, Fe, Co, and O in ZICO (Figure S2a) as well as N and C of N-G (Figure S2b) can be clearly visualized. The presence of the constituent elements can further be confirmed from energy dispersive X-ray line pattern obtained from FESEM studies (as depicted in the left panel of Figure S3). The corresponding weight and atomic percentage of the elements has also been tabulated in right panel of the same figure. Furthermore, the morphological features obtained after 5000 consecutive charge−discharge cycles have

plane is quite small, which indicates that all carbon atoms in six-membered honeycomb-like construction are not sp2 hybridized. This could be due to two possible reasons: some nitrogen may be linked with edge carbon atoms in the form of nitrogen-containing surface functional groups and probable formation of a closed ring structure during the hydrothermal reaction.39,40 The Scherrer equation (eq 1) was utilized to determine the average crystallite size of ZICO (Table S1, Supporting Information) which is further corroborated by transmission electron microscopic investigations. ZICO dhkl =

0.9λ 180° β cos θ π

(1)

where λ denotes the wavelength of Cu Kα, β = FWHM, and θ is the Bragg angle. The morphological features and microstructure of ZICO and N-G electrode materials were determined by field emission scanning electron microscopic (FESEM) and transmission electron microscopic (TEM) analysis. The detailed surface morphological features obtained from FESEM have been represented in Figure 3a−d. The FESEM images illustrated in Figure 3a,b exhibit nanowire arrays of ZICO with an average diameter of ≈45−60 nm. As evident from Figure 3b, growth of ZICO nanowires from individual nucleation centers on a quartz substrate with sharp ridge was taken place during the annealing process. The tips of the nanowires are hexagonal in shape as depicted in high resolution FESEM images (Figure S1, Supporting Information). The development of the nanowires emulates a core-spike kind of growth which is possibly induced by grain boundaries. The spikes grow longer and constructs extended network, during annealing treatment.41−43 This network-like architecture of vertically aligned ZICO with interconnected nanowires promotes transfer of electrons and/or electrolytic cations throughout the entire material, as reflected by its high specific capacitance value. The subsequent annealing treatment possibly associated with the release of H2O and CO2 gases which also stimulates the formation of 3D architecture. The presence of quartz substrate is assumed to be one of the driving factors for the development of hierarchical nanowires. The rough surface of mechanically scrubbed quartz substrate provides exposed area for nuclei formation and subsequent growth of the nanowires.44 It is noteworthy that an increase of the hydrothermal reaction time and annealing temperature eventually destructs the hierarchical structures. At higher

Figure 3. (a, b) FESEM images of hierarchical ZICO nanowire arrays and (c, d) wrinkled N-G at different magnifications. (e, f) TEM images of ZICO nanowires and (h, i) N-G. (g) and (i, inset) represent SAED patterns of ZICO and N-G, respectively. 5950


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appropriate wetting, activation of the electrodes, and superior electrode−electrolyte interaction. After achieving steady state, CV, GCD, and EIS measurements were performed. Cyclic voltammetry plots of ZICO are represented in Figure 4a with varying scan rates of 1, 2, 5, 10, 30, and 50 mV/s under a potential window of 0−0.5 V, accordingly. A pair of redox peaks has been identified in each of the CV curves of ZICO, signifying its pseudocapacitive response in aqueous 6 M KOH. It has been observed that the oxidation peak for ZICO appears at ∼0.32 V while the reduction peak appears at ∼0.11 V at 1 mV/s scan rate. The slight shifting of the redox peak positions with the increase of scan rate is possibly due to the presence of polarization.46 It is also noteworthy that overpotential plays a crucial role in the escalation of current response with the increase of scan rates.47 The appearance of the redox peaks is probably due to the following reactions happening between the electrode material and the electrolyte.

also been illustrated (Figure S4) which supports the stability of the constituent electrodes under vigorous cycling conditions. Raman spectroscopy (as explained in Figure S5 under S6 in SI), Fourier transformed infrared (FTIR) spectroscopy (Figure S6 under S7 in SI), X-ray photoelectron spectroscopy (XPS) (Figure S7 under S8 in SI) and Brunauer−Emmett− Teller (BET) analysis (as discussed in S9 and Figure S8) have been comprehensively elaborated. The electrochemical and capacitive performances of ZICO@ Ni foam and N-G@Ni foam have been investigated initially using a three-electrode system with 6 M aqueous KOH electrolyte. The influence of exposed specific surface area and morphology on the electrochemical performances of the as-synthesized electrodes has also been studied through cyclic voltammetry (CV), galvanostatic charge−discharge (GCD), and electrochemical impedance spectroscopy (EIS). The electrodes were charged and discharged for ∼100 repetitive cycles at a constant 5 A/g current density until a stable capacitance value was achieved, before performing the electrochemical experiments. Initially, the active electrode materials display some inconsistencies in their specific capacitance values due to inadequate wetting of the electrode surface by the electrolyte. After ∼100 cycles, the active materials exhibit steady capacitance values with slight or almost no variations. This is an indication of

Co2 + + 3OH− = CoOOH + H 2O + e−

(I)

CoOOH + OH− = CoO2 + H 2O + e−

(II)

Fe3 + + Co2 + = Fe 2 + + Co3 +

(III)

Figure 4. (a) Cyclic voltammetry plots of ZICO at different scan rate. Inset represents magnified view of the CV profile at 1 mV/s scan rate. (b) Variations of specific capacitance with scan rates. (c) GCD plots of ZICO at different current density. (d) Variations of specific capacitance with current densities. (e) Nyquist plot of ZICO. The equivalent circuit fitted to the corresponding Nyquist plot is demonstrated in the inset. (f) Cyclic stability profile of ZICO electrode obtained at 1 A/g. 5951


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ACS Applied Materials & Interfaces The Fe2+/Fe3+ redox peak possibly overlaps with the other high intense peak which appeared owing to its minute existence. Substantially higher current response of ZICO electrode material signifies improved ionic/electronic conductivity. A gradual escalation of current response with increasing scan rate reveals diffusional behavior of the electrolytic ions within the inner pores of electrode material.48 The measured area under the CV curve of ZICO is considerably wider, signifying its high specific capacitance value. Evidently, an immense current response with greater area under the CV profile implies superior capacitive performance of ZICO electrode and labels it as a prospective candidate for efficient electrode material. The specific capacitance value has been calculated from the CV plots using the equation12

the electrolytic ions rather weakens the overall electrochemical process and hence decreases the specific capacitance.53 The contribution of blank Ni foam (substrate material/current collector) toward the ultimate capacitive response is considered to be negligible in comparison with the as-prepared electroactive materials (as explored in Figure S9 under S10 in SI). In true sense, the capacitance contribution mainly originates from as-prepared electrode materials. Ni foam only acts as a good current collector and substrate. In order to investigate several resistance behavior offered by the ZICO electrode, electrochemical impedance spectroscopic (EIS) analysis was implemented within a frequency window of 1 MHz−100 mHz. The Nyquist plot for ZICO is illustrated in Figure 4e. The circuit diagram fitted with the acquired Nyquist plot is shown in the inset. The impedance plot exhibits a semicircle at high frequency region followed by a straight line at low frequency region. The obtained semicircle diameter represents the charge transfer resistance (Rct) while the existence of Warburg impedance (W) can be predicted by the steeper profile at low frequency regions. The frequency dependent ion diffusion process at the electrode−electrolyte interface can be ascribed by calculating the slope of the steeper profile.54,55 At high frequency region, the initial intersection point of the obtained semicircular curve with the real impedance axis defines solution resistance (Rs) characteristic which is a direct indication of several resistances associated with the electrode material and electrolyte such as the effective resistance generated between the current collector (Ni foam) and the active electrode material, resistance offered by the electrolytic ions, and inherent resistance of the active electrode material. It is noteworthy that the active electrode material does not exhibit an ideal capacitive nature. Hence, for better fitting, a constant phase element (CPE) has been instigated within the equivalent circuit diagram in place of pure capacitive element. It has been observed that ZICO reveals an Rs value of ∼0.24 Ω, with a corresponding Rct value 1.23 Ω. On the other hand, a moderately steeper profile at low frequency region demonstrates better surface accessibility for the electrolytic ions and low ion diffusion resistance. Hence, ZICO exhibits paramount capacitive performance. The cyclic stability measurement at a constant current density of 1 A/g reveals 95.5% specific capacitance retention after 3000 successive GCD cycles (as represented in Figure 4f). The mesoporous nature of the electroactive ZICO nanowires provides shorter ion and electron diffusion tracks and hence affords good rate performance. The electrochemical properties and capacitive behavior of N-G@Ni foam as negative electrode have also been explored (Figure S10). The electrochemical performance of the as-fabricated PDMS encapsulated ZICO//N-G all-solid-state asymmetric supercapacitor (ASC) device has been evaluated by introducing ZICO@Ni foam as positive electrode and N-G@Ni foam as negative electrode in the presence of PVA−KOH gel electrolyte. This gel electrolyte (membrane type) also acts as separator. Prior to constructing solid-state ASC device, these two electrodes were tested separately through CV (at 1 mV/s) and GCD (at 1 A/g) by using 6 M aqueous KOH electrolyte. Figures 5a and 5b represent the CV and GCD plots of the two electrodes, respectively, for proper elucidation of probable working potential window of the combined device. It can be inferred that the operating potential window of the device will be 1.5 V where ZICO reveals a higher cutoff potential (0.5 V) and N-G with a lower cutoff potential (−1 V).

V

specific capacitance (Cs) =

∫V 2 i(V ) dv 1

(V2 − V1)νm

(2)

Here, i represent the current in mA, V1 and V2 denote the lower and the upper potential limits in V, ν designates the scan rate in mV/s, and m is the effective mass (g) of the electrode material coated on Ni foam. The highest specific capacitance value obtained for ZICO electrode was 2117.1 F/g at 1 mV/s scan rate. The active mass of ZICO coated on Ni foam was 0.014 g. This elevated specific capacitance value of ZICO electrode is solely due to the existence of frequent redox-active sites. The variation of calculated specific capacitance with scan rate is demonstrated in Figure 4b. It is evident that the specific capacitance decreases moderately with the elevation of scan rates. The gradual decrease of specific capacitance is attributed to less accessibility of the redox active sites in the electroactive material by the electrolytic ions at high scan rates. Galvanostatic charge−discharge (GCD) analysis is the most reliable way to evaluate the specific capacitance of the electrode materials under constant current density. The specific capacitance of the as-prepared electrode materials has also been calculated from their respective GCD plots with the help of the equation14,49 specific capacitance (Cs) =

iΔt mΔv

(3)

where Cs is the calculated specific capacitances in F/g, (i/m) collectively denotes the current density in A/g, ΔV signifies the potential drop during the discharge process in volts, and Δt is the discharging time in seconds. The charge−discharge profile of ZICO electrode (Figure 4c) within a potential window of 0−0.5 V exhibits a nonlinear behavior with a tiny plateau region which is evident for the inherent pseudocapacitive nature under applied potential window. The calculated specific capacitance values for ZICO electrode are about 2587.4, 2401.6, 2096.0, 1888.0, 1759.2, and 1528.0 F/g at 1, 2, 5, 8, 10, and 15 A/g, respectively, based on the total mass of ZICO on Ni foam (0.014 g). The obtained specific capacitance values are comparatively higher than those reported in the literature, including brush-like cobalt oxide nanowires (1525 F/g at 1 A/g),50 ZnCo2O4@Ni foam (∼1400 F/g at 1 A/g),51 Mn−Ni−Co ternary oxide nanowire@Ni foam (638 F/g at 1 A/g),4 and ZnFe2O4@stainless steel (433 F/g at 1 A/g).52 The synergistic contribution of the constituent ions in as-prepared ZICO influences the ultimate capacitive response. The gentle decrease in specific capacitance values with the elevation of current density (as depicted in Figure 4d) is probably because of the increased voltage (I−R) drop. At higher current densities, the inaccessibility of redox active sites of the active electrode by 5952


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Figure 5. (a) CV profile of ZICO and N-G electrode at 1 mV/s scan rate. Inset diagrams represent the probable phenomenon occurring at each electrode. (b) GCD profile of ZICO and N-G electrode at 1 A/g current density.

performance even in moderately bent condition can be easily predicted as there is a little or almost no variation in the nature of the CV profiles. PDMS encapsulation offers marginal bendability due to its inherent flexibility with extreme impact and weather resistance.57 The variation of specific capacitance with applied scan rates (Figure 6c) reveals moderate decrease of specific capacitance with the elevation of scan rate. The GCD profiles of the ASC device at different current densities are depicted in Figure 6d. The shape of the GCD curves with a plateau region reveals excellent capacitive behavior of the assembled device. The specific capacitance values at various applied current densities were calculated by using eq 3, where m signifies the total mass (g) of the active electrode materials coated on Ni foam. An outstanding specific capacitance value of 129.74 F/g is achieved at 1 A/g current density which remarkably remains up to ≈60.6 F/g even at a high current density of 15 A/g. It is also noteworthy that the calculated specific capacitance decreases slowly with the elevation of current density (Figure 6e). This is an evidence of better electrochemical stability of the device. A simple schematic circuit diagram for the assembled ASC device connection is depicted in the inset. The Nyquist plot of the ASC device (as illustrated in Figure 6f) displays a small semicircular arc at high frequency followed by a steep line at low frequency region. The measured ESR value is ∼0.74 Ω. The equivalent circuit diagram fitted to the corresponding Nyquist plot is represented in the upper inset. The energy density (E) and power density (P) of the device have been calculated by utilizing the following equations:2,58

In agreement with the obtained outcomes, the Faradaic and EDLC response of the respective electrodes can cooperatively contribute to the overall electrochemical performance of the assembled all-solid-state ASC device. The fabricated ASC device (as depicted in Figure 1) has been subjected to repetitive charged and discharge at 5 A/g for ∼100 cycles until a stable capacitance value achieved, before performing the electrochemical tests. After achieving steady state, complete electrochemical measurements have been executed within a wide potential window of 0−1.5 V. The charge balance relation between positive and negative electrode can be represented as q+ = q−, where q+ and q− symbolize the charge of positive and negative electrodes, respectively. The charge (q) stored in the electrode and the optimized mass ratio among the two constituent electrodes can be calculated by the two equations56

q = C ΔVm

(4)

m+ C ΔV = − − m− C+ΔV+

(5)

where C, ΔV, and m represent the specific capacitance in (F/g), operating potential window (V), and mass of the respective electrodes (g). The specific capacitances acquired for ZICO and N-G have directly been introduced into eq 5 with their individual working potential window to compute the precise mass ratio. The optimized mass ratio between the positive and negative electrode (m+/m−) is ∼0.42 in the ASC device. The capacitive performances are the exclusive properties of the electroactive materials. Therefore, the mass ratio of the active electrode materials has only been considered in the present study. Figure 6a represents the CV plots of the as-fabricated ASC device with varying scan rates of 1, 2, 5, and 10 mV/s. The enlarged view of the CV profile (inset of Figure 6a) obtained at 1 mV/s reveals superior capacitive performance of the ASC device. The combined electrochemical contribution (EDLC and Faradaic) of two respective electrodes can also be perceived. The nature of the CV profile remains virtually similar at all scan rates. The specific capacitance at different scan rates is measured by employing eq 2, where m reflects the total mass of the two component electrode material in the device. The highest specific capacitance acquired from the CV curve of as-prepared ZICO//N-G all-solid-state ASC device is 106 F/g at 1 mV/s. Figure 6b represents the comparative CV plots of the encapsulated ASC device: formerly, after 5000 consecutive GCD cycles and under bending mode at 1 mV/s scan rate. A schematic overview of the slightly bent ASC device has been represented in the inset. Extreme stability of the device even after 5000 GCD cycles and superior electrochemical

EASC =

1 CASC(ΔV )2 2

PASC = EASC /T

(6) (7)

where EASC signifies the energy density of ZICO//N-G ASC device in Wh/kg, CASC is the total specific capacitance of the device in (F/g), ΔV represents the drop of potential during the discharge process in (V), PASC represents the power density of the ASC device in W/kg, and T signifies the discharge time. Figure 6g illustrates the Ragone plot of the fabricated ASC device acquired at a potential window of 0−1.5 V. The device provides an exclusive energy density of ≈40.5 Wh/kg at a power density of 750 W/kg (at 1 A/g), and the delivered energy density still remains at ≈26.7 Wh/kg even at a very high power density of 5997.6 W/kg (at 8 A/g). This exclusive features certifies its prospective in power device applications. A schematic diagram of PDMS encapsulated ZICO//N-G all-solid-state ASC device is represnted in the inset. In order to explore the cycling performance of the fabricated device, the 5953


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Figure 6. (a) Cyclic voltammetry plots of the solid-state ASC device at different scan rates (inset represents enlarged view of the CV profile at 1 mV/s). (b) Comparison among the CV curves of the ASC device: formerly, after 5000 GCD cycles and under bending mode at 1 mV/s (inset represents a schematic overview of the slightly bent ASC device). (c) Variation of specific capacitance with scan rates. (d) GCD plots of the ASC device at different current density. (e) Variations of specific capacitances with current densities (inset depicts a schematic circuit diagram for the ASC device connection). (f) Nyquist plot of the ASC device (inset represents the equivalent fitted circuit diagram). (g) Ragone plot of ASC device (inset depicts the fabricated all-solid-state ASC device). (h) Specific capacitance retention after 5000 consecutive charge−discharge cycles (inset depicts GCD profile from 1st to 10th cycle at 1 A/g).

Furthermore, in order to investigate the effect of pressure and temperature on the encapsulated ASC device, cyclic voltammetry has been accomplished by exposing the PDMS encapsulated ASC to different pressures originated by a human finger and palm. The device exhibits almost similar nature of CV profile under different pressure (Figure 7a). To explore the effect of temperature, the device was kept on a hot plate and subjected to different temperatures (room temperature to 80 °C). The nature of the CV curves at different temperature (Figure 7b) is almost comparable while the area under the CV

cycle stability experiment has been performed at 1 A/g constant current density within 0−1.5 V potential window for 5000 successive charge−discharge cycles. The GCD cycle stability profile (as depicted in Figure 6h) reveals significant retention (∼95%) in specific capacitance over 5000 successive charge−discharge cycles. The result reveals decent stability of ZICO//N-G ASC device under continuous GCD cycles. The uninterrupted GCD profile of the fabricated device up to ten sequential GCD cycles measured at 1 A/g is demonstrated in the inset. 5954


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Figure 7. (a) Cyclic voltammetry curves of the as-fabricated PDMS encapsulated all-solid-state ASC device under applied pressure at 30 mV/s scan rate (inset represents different types of finger and hand pressure applied on the device). (b) CV curves of the device at different temperatures.

Figure 8. (a) Schematic circuit diagram of the as-fabricated PDMS encapsulated ZICO//N-G all-solid-state ASC device. (b) Simple schematic illustration of the redox activity occurring on the two respective electrodes. (c−h) Intensity variation of a single glowing red LED at different time span. (i) Lightning of a blue and green LED with moderate intensity. (j) Simultaneous lightning of four red LED lights. (k) Illumination of mobile LED screen. (l) Power-up electronic wrist watch, (m) commercial digital calculator, and (n) portable speaker.

doping of graphene with nitrogen-like heteroatoms.59 Uniform slurry coating of hierarchical ZICO nanowire arrays and N-G over Ni foam substrate provides higher contact area between the electroactive material and current collector substrate for efficient electrolyte transportation. Ni foam itself acts as a good current collector and provides tracks for electrons transport. The contribution of blank Ni foam toward the overall specific capacitance is very negligible as compared to the synthesized electroactive materials. All these features cooperatively play a significant role to enhance the overall electrochemical properties of the PDMS encapsulated ZICO//N-G ASC device. Efforts have been put forward to utilize the power generated from the as-fabricated all-solid-state ASC device for practical application in commercial LED driving. A simple schematic circuit diagram of the assembled ASC device for practical application is proposed in Figure 8a. The ASC charges when switch NP and MO are connected while it discharges when switch RP and QO are connected and lighten four red round LED lights (as portrayed in the figure). The probable phenomenon occurring at each electrode and interface is schematically illustrated in Figure 8b. The intensity variation of a single commercial red LED lighting at different time span after 2 s charging are sequentially depicted in Figure 8c−h. Figure 8c depicts the initial condition when LED is not connected to the device. Figure 8d−h represents the consecutive

plots increases slightly with temperature. This may be due to facilitated ion transport at elevated temperature. Siloxane encapsulation thus provides greater mechanical integrity and restricts the device form evaporation of water from solid electrolyte. Additionally, we have investigated the electrochemical stability of the as-fabricated ASC device at different days to check the lifetime. The cyclic voltammetry plots of the device were evaluated after 2, 5, 8, 12, and 15 consecutive days at 1 mV/s scan rate. The nature of the CV profiles (Figure S11) was properly retained even after 15 days, which demonstrates a moderate decay in the specific capacitances and exceptional durability of the device. The incredible performance provided by the PDMS encapsulated ZICO//N-G all-solid-state ASC device is attributed to its superior constituent electrodes. The synergistic effect of Zn, Fe, and Co ions gives high capacitance and admirable electrochemical outcomes. Zn offers better structural stability during vigorous GCD cycles while Fe and Co provide enhanced electronic conductivity. Additionally, incorporation of Zn ions raises the number of active sites for electrochemical redox reaction. On the other hand, mesoporous nitrogen doped graphene also provides impressive electrochemical performance and contributes for high power density of the fabricated device. Surface reactivity and surface energy can be effectually increased by 5955


ACS Applied Materials & Interfaces variation/fluctuation of intensity of a glowing single red LED light connected with the single device initially and after 3, 10, 15, and 20 min, respectively, after the charging. It can also lighten a blue and green LED with adequate intensity (Figure 8i and inset) and simultaneously lighten four red LEDs (Figure 8j). The as-fabricated ASC device can able to power up mobile LED screen, electronic watch, commercial digital calculator, and portable speaker (Figure 8k−n). The overall feature revealed by our fabricated all-solid-state ASC device is demonstrated in a video clip in the Supporting Information.

ACKNOWLEDGMENTS

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b13259. Materials details and characterizations, preparation of PVA−KOH gel electrolyte, preparation of working electrodes, average crystallite sizes, FESEM images of ZICO, elemental mappings and EDS patterns, morphology after 5000 charge−discharge cycles, Raman, Fourier transformed infrared, and X-ray photoelectron spectroscopic analysis with EDAX, BET analysis, contribution of blank Ni foam, electrochemical performance of N-G as negative electrode in 6 M KOH and lifetime sustainability of the ASC device (PDF) Video S1 (AVI) Video S2 (AVI)

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We are very much thankful to the Indian Institute of Technology Kharagpur for financial support.

4. CONCLUSIONS This work demonstrates the fabrication of a unique and durable ASC device composed of PDMS encapsulated ZICO@Ni foam as positive electrode and N-G@Ni foam as negative electrode in PVA−KOH gel electrolyte. Hierarchical nanowires of ZICO have been synthesized by a facile hydrothermal protocol followed by subsequent annealing treatment on a quartz substrate. Crumpled N-G nanoflakes have also been prepared on quartz substrate by a facile hydrothermal method. Very high specific capacitance values of 2587.4 F/g (ZICO) and 550 F/g (N-G) were obtained at 1 A/g current density. Moreover, the fabricated ZICO//N-G ASC device exhibits a specific capacitance value of 129.74 F/g at 1 A/g current density with a high energy density (≈40.5 Wh/kg) and superior (≈95%) cycle stability over 5000 repeated charge−discharge cycles. Additionally, PDMS encapsulation provides resistance to leakage associated with the electrolytes and decent stability against impact and weather. The outstanding electrochemical performance revealed by the ZICO//N-G ASC device and its constituent electrodes are highly demandable and promising for next-generation supercapacitor and power device applications in various electronic industries.

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AUTHOR INFORMATION

Corresponding Author

*(B.B.Khatua) E-mail [email protected]; Tel +913222-283982. ORCID

Bhanu Bhusan Khatua: 0000-0002-1277-0091 Notes

The authors declare no competing financial interest. 5956


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