Snowflake-Like Dendritic CoNi Alloy-rGO Nanocomposite as a Cathode Electrode Material for an All-Solid-State Flexible Asymmetric High-Performance Supercapacitor Device.

Flexible all-solid-state supercapacitors having high mechanical stability and foldable features are crucial to meet the growing demands for a large number of portable electronic devices such as wearable electronics, displays, touch screens, detectors, etc. Here, we report the fabrication of such a flexible all-solid-state asymmetric supercapacitor device by using a nanocomposite composed of a snowflake-like dendritic CoNi alloy and reduced graphene oxide ((CoNiD)60-rGO40) as the positive electrode and pure rGO as the negative electrode for the first time. In this device, a polyvinyl alcohol (PVA) gel containing 3 M KOH and 0.1 M K4[Fe(CN)6] was used as the electrolyte cum separator. This supercapacitor device offers a high energy density value of 52.8 Wh kg-1 at a power density of 2000 W kg-1. The values of these two key performance parameters are superior to the many commercially available supercapacitors and reported values in the literature. In addition, this device also exhibits retention of ∼95% of its initial specific capacitance value after 4000 cycles at a current density of 2.5 A g-1, displaying its high cycling stability. This supercapacitor is so flexible that no mechanical deformation occurs even after bending at different angles and folding up to 180°, and its specific capacitance value practically remains unaffected when the device was twisted at different bending angles. This flexible all-solid-state asymmetric supercapacitor device can power a light-emitting diode (LED) and demonstrates its promise to meet the practical applications in energy storage technology.

Introduction

The escalating demand for fossil fuels with the growth of the global population and economy has led to the exhaustion of energy sources for future generations. Electrochemical energy storage (EES) technologies have emerged as the leading models worldwide in the energy sector. In this context, supercapacitors have gained considerable attention in recent years owing to their capability of high power density, long life cycle, short charge/discharge time, and better safety as compared to batteries.[1,2] Supercapacitors are widely used in hybrid platforms of trucks and buses, electric vehicles, heavy-duty vehicles, light rails, load-leveling technology, etc. According to the charge storage mechanisms, supercapacitors are generally categorized as (i) electrical double-layer capacitors (EDLCs) and (ii) pseudocapacitors.[3,4] Various carbon-based materials (e.g., porous carbon, graphene, reduced graphene oxide (rGO), etc.) are EDLC materials, and different metal oxide (e.g., RuO2, MnO2, CoO, Co3O4, AB2O4, transition metal oxides, etc.) and conducting polymers are pseudocapacitive materials.[5−7] Generally, the electrochemical performances of the electrode materials and devices are evaluated by constructing three- and two-electrode setups. A three-electrode setup provides important information about the active electrode material, such as its redox behavior, suitable operating potential window, specific capacitance, rate capability, etc. However, the two key performance parameters of a device, energy and power densities, can only be realized in a two-electrode assembly.[5] A two-electrode system consists of a closed spaced pair of electrodes, which are fabricated by using active electrode materials, and these electrodes are separated by a separator that is electrically insulating but capable of ion permeation. In a given electrolyte, the electrochemical performances of a two-electrode cell largely depend on both the electrodes. Depending upon the nature of the active electrode material, which is used to construct each of the electrodes, the two-electrode supercapacitor cells can be categorized as symmetric and asymmetric supercapacitors. In a symmetric device, both the positive and the negative electrodes are constructed with the same active material with the same mass loading. However, the symmetric supercapacitors generally suffer from a limited working window due to the thermodynamic breakdown of water at ∼1.2 V.[5] When aqueous electrolyte systems are used, symmetric cells operate in a potential window of ≤1.0 V. Therefore, symmetric cells with aqueous electrolyte exhibits low energy density, which limits its practical applications. Though nonaqueous electrolytes and ionic liquids operate in a wider stable window (operating window ∼4 V), they suffer from lower ionic conductivity, relatively high cost, poor safety, non-ecofriendliness, toxicity, etc.[8−10] Therefore, the use of an aqueous electrolyte is a preferred option. To overcome the limitation associated with the limited working potential window of a symmetric cell with aqueous electrolyte, the development of asymmetric supercapacitor (ASC) devices has become an attractive approach. In an asymmetric supercapacitor, two electrodes are made up of two different types of electrode active materials, i.e., one pseudocapacitive and another EDLC material. As an ASC device combines two different electrode materials, which operate at two different potential windows, it operates at an extended potential window and achieves a higher energy and power density than those of the symmetric cell.[11,12]

However, the conventional supercapacitors are associated with the limitation of uncompressible package materials. Moreover, the use of liquid electrolytes sometimes results in the leakage of electrolyte and dislocation of electrodes. To address these issues, recently, the development of all-solid-state asymmetric supercapacitors has gained immense attention. Currently, for the application of an all-solid-state supercapacitor in portable flexible wearable electronics, displays, touch screens, detectors, etc., an enormous amount of effort has been put to develop mechanically flexible all-solid-state supercapacitors.[13−17] These all-solid-state flexible supercapacitor devices consist of flexible electrodes, solid-state electrolyte, and a separator.[18]

Today, many cutting edge electrical/electronic devices demand supercapacitors having high power performances with high energy density. However, the commonly available commercial supercapacitors exhibit an energy density of 3–5 Wh kg–1 and a power density of 3000–10,000 W kg–1.[2,19−21] Therefore, there is an urgent need to develop a high-performance supercapacitor, preferably an all-solid-state device, which is capable of exhibiting high energy and power densities along with high rate capability. Several researchers have reported varieties of electrode materials for supercapacitor applications. Recently, Surendran et al. have reported the superior electrochemical properties of CoNi alloy to its monometallic counterpart.[22] Ni- and Co-based materials exhibit high specific capacitance and proved their potential as the high energy density cathode material with high power delivery. Wu et al. have reported that NiCo2O4-based materials exhibited an energy density of 33 Wh kg–1 and a power density of 41.25 kW kg–1.[23] Li et al. have fabricated a CoAl-LDH- and rGO-based flexible asymmetric supercapacitor, which is capable of delivering an energy density of 0.71 mWh cm–2 at a power density of 17.05 mWh cm–2.[11] Wang et al. have reported the energy density of 37.4 Wh kg–1 at a power density of 163 W kg–1for an asymmetric supercapacitor fabricated by using porous nickel cobalt oxide nanowires.[24] Wang et al. have fabricated a nickel-cobalt oxide-reduced graphite oxide composite material-based asymmetric supercapacitor, which exhibited an energy density of 23.32 Wh kg–1 at a power density of 324.9 W kg–1.[25] Tang et al. have reported that an asymmetric full-cell supercapacitor, which was constructed by using porous Ni-Co oxide, can deliver an energy density of 10.8 Wh kg–1and a power density of 474.4 W kg–1.[26]

Our recent investigations revealed that a nanocomposite composed of 60 wt % snowflake-like dendritic CoNi alloy and 40 wt % reduced graphene oxide ((CoNiD)60-rGO40) is a promising electrode active material for the fabrication of a high-performance supercapacitor.[27] The electrochemical performance measurements of this nanocomposite by using a three-electrode system showed that it possessed a high specific capacitance (Cs) of 501 F g–1 at a current density of 6 A g–1. It has also exhibited a significantly high (∼85%) retention of capacitance of up to 4000 cycles. However, when a two-electrode symmetric cell was constructed using (CoNiD)60-rGO40 as the electrode material, the device exhibited relatively low values of energy density (9–4 Wh kg–1) at the range of power density of 1200–8000 W kg–1. These results motivated us to develop an asymmetric supercapacitor (ASC) device using (CoNiD)60-rGO40 as one of the electrode materials.

In this paper, we report the fabrication of a mechanically flexible all-solid-state asymmetric supercapacitor (ASC) device where (CoNiD)60-rGO40 and pure rGO were used as electrode materials for the preparation of cathode and anode electrodes, respectively. In this all-solid-state device, a gel composed of a mixture of PVA, 3 M KOH, and 0.1 M K4[Fe(CN)6] was used, which acted as both the separator and electrolyte. This device exhibited a significantly high value of energy density of 52.8 Wh kg–1 at a power density of 2000 W kg–1, along with ∼95% retention of capacitance over 4000 cycles. Moreover, this flexible all-solid-state ASC device also showed almost the same electrochemical performances when subjected to different bending angles and demonstrated its mechanical flexibility. To the best of our knowledge, this is the first time that the development of a high-performance mechanically flexible all-solid-state ASC device, using (CoNiD)60-rGO40 as an active electrode material for cathode electrode, has been reported.

Results and Discussion

Formation of (CoNiD)60-rGO40 and Its Structural Characterizations

A “one-pot” synthesis route was employed to prepare the (CoNiD)60-rGO40 nanocomposite. Recently, we have published the synthesis of (CoNiD)60-rGO40 by this one-pot method elsewhere.[27]Scheme illustrates the preparation of the (CoNiD)60-rGO40 nanocomposite. In this route, the chloride salts of Co2+ and Ni2+ were used as the source of metals and CTAB was used as the microstructure directing agent. In the alkaline medium, N2H4 acted as a reducing agent and transformed M2+ to M0. Simultaneously, it reduced some of the surface-attached functional groups of GO and transformed graphene oxide (GO) to rGO. These reactions can be presented as followswhere M2+ = Co2+, Ni2+.

Scheme 1

Schematic Presentation of Preparation of the (CoNiD)60-rGO40 Nanocomposite

The synthesized (CoNiD)60-rGO40 nanocomposite and GO were characterized by using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy-dispersive spectra analysis (EDS), Fourier Transform infrared spectroscopy (FTIR), and Raman spectroscopy. XRD patterns of GO and (CoNiD)60-rGO40 are shown in Figure a. In the XRD pattern of pure GO, the diffraction peaks at 9.76° and 42.14° corresponding to the (001) and (101) plane, respectively, are present.[28] The XRD pattern of the (CoNiD)60-rGO40 nanocomposite exhibits the diffraction peaks at 2θ = 41.71°, 44.71°, 47.54°, and 76.19°, which can be attributed to the (100), (002), (101), and (110) planes of CoNi alloy having an hcp (hexagonal closed-packed) structure of Co, respectively.[29] In this one-pot synthesis route, CTAB plays an important role in the development of a snowflake-like dendritic microstructure of CoNi alloy (CoNiD) in the nanocomposite. CTAB influences the formation of the (100), (110), and (010) planes by controlling the growth of the dendritic structure via the adsorption–desorption process.[29] It can also be noted that, in the XRD pattern of (CoNiD)60-rGO40, the characteristic diffraction peaks of GO (i.e., 2θ = 9.76° and 42.14°) are absent.[28] This indicates that, in this one-pot method, GO is converted to rGO during the formation of the (CoNiD)60-rGO40 nanocomposite. The FESEM micrograph of this nanocomposite is presented as Figure b, which illustrates the presence of a snowflake-like dendritic-structured CoNiD nanoparticle on the surface of nanometer-thin rGO sheets. EDS (Figure S1) analysis reveals that this nanocomposite is composed of Co, Ni, and C. In the FTIR spectra of GO (Figure c), the appearance of bands at 3397, 1740, 1387, 1226, and 1061 cm–1 indicates the presence of −OH, carbonyl, carboxyl, and epoxy groups on the surface of GO. In the FTIR spectra of (CoNiD)60-rGO40 (Figure c), the disappearance/reduction of intensities of the peaks at 1740, 1387, 1226, and 1061 cm–1 indicates that, during the formation of (CoNiD)60-rGO40, GO is reduced to rGO.

Figure 1

(a) XRD pattern, (b) FESEM micrograph, (c) FTIR spectra, and (d) Raman spectra of the GO and (CoNiD)60-rGO40 nanocomposite.

The Raman spectra of GO and (CoNiD)60-rGO40 are shown in Figure d, which shows the appearance of D and G bands of GO at 1342 and 1582 cm–1, and these bands are shifted to 1336 and 1585 cm–1 for the (CoNiD)60-rGO40 nanocomposite. The ID and IG ratio for GO and nanocomposite are 0.99 and 0.83, respectively, which indicates the decrease in the average size of sp2 domains due to the reduction of GO during the (CoNiD)60-rGO40 formation.

Electrochemical Measurements

In our previous study, the evaluation of electrochemical performances of (CoNiD)60-rGO40 in the three-electrode setup (where (CoNiD)60-rGO40 was the working electrode and a Hg/HgO electrode and a Pt wire were used as the reference and counter electrode, respectively) revealed the pseudocapacitive nature of (CoNiD)60-rGO40 in aqueous 3 M KOH electrolyte.[27] In the CV curve of (CoNiD)60-rGO40, the presence of distinct anodic and cathodic peaks suggested its pseudocapacitive nature (Faradic behavior) and the identical nature of these peaks indicated the reversibility of the electrochemical process, which occurred on the surface of the electrode.[30,31] The electrochemical reactions that occurred on the surface of the (CoNiD)60-rGO40 electrode in the presence of aqueous KOH electrolyte solution can be presented as[30−33]

We have observed that the addition of 0.1 M K4[Fe(CN)6] solution to the 3 M KOH significantly enhanced the specific capacitance (Cs) of (CoNiD)60-rGO40 from 130 to 501 F g–1 at 6 A g–1.[27] The enhancement of Cs occurred due to the addition of K4[Fe(CN)6] because [Fe(CN)6]4–/[Fe(CN)6]3– offered an additional redox reaction and acted as an electron buffer source in the electrochemical process occurring at the electrode/electrolyte interface.[27,34−36] Therefore, in the present study, we have used an aqueous mixture of 3 M KOH and 0.1 M K4[Fe(CN)6] as the electrolyte system. We had also observed that the symmetric supercapacitor, fabricated by using the (CoNiD)60-rGO40 nanocomposite as active electrode materials for both the electrodes, exhibited a Cs of 405 F g–1 at a current density of 3 A g–1 but with an energy and power density of only 9 and 1200 W kg–1, respectively.[27]

Electrochemical Performances of the Asymmetric Supercapacitor (ASC) Device

In the present study, to achieve a superior energy and power density, we have fabricated an asymmetric supercapacitor (ASC) device using (CoNiD)60-rGO40 and pure rGO as electrodes (Figure a). Figure b shows the individual CV curves of these electrodes, which were obtained from the three-electrode measurement systems with the working potential windows of 0 to 0.6 and −1 to 0 V for (CoNiD)60-rGO40 and pure rGO electrodes, respectively, at a scan rate of 10 mV s–1. The shape of the CV profiles indicated the pseudocapacitance nature of (CoNiD)60-rGO40 and EDLC nature of pure rGO. The CV profiles of the ASC were obtained for the different working potential window at a scan rate of 10 mV s–1 and shown in Figure c. It was observed that the current response and occlusive area of CV curves increased when the operating window was extended up to +1.6 V. When the working window was operated beyond 1.6 V, in the CV curve, an undesired peak due to oxygen evolution was observed. In Figure d, the GCD curve at 1.7 V also showed, at the end of the charging curve, a tail (as marked by a red circle), indicating the side reaction. Therefore, we have used the potential window of 0 to +1.6 V for further electrochemical measurements. The CV measurements of this ASC device were performed at different scan rates; Figure e shows the representative CV curves obtained at sweep rates of 10, 50, and 100 mV s–1. The shape of the CV profile was found to be well retained even at a higher scan rate of 100 mV s–1, suggesting the good rate performance of the ASC device. The nearly symmetric triangular shape of the GCD curves (Figure f) indicated the well-balanced charge storage and superior electrochemical reversibility of this ASC device. The values of the specific capacitance (Cs) with changing current density were determined from GCD measurements, and the highest Cs value obtained was 151.25 F g–1 at a current density of 2 A g–1. Cs was decreased with increasing current density because of the incomplete redox reaction at the electrode surface/electrolyte interface due to the sluggish ion diffusion at the comparatively higher current densities. Approximately 67% retention of Cs at a current density of 5 A g–1 indicated its good rate capability. The values of Rct (charge transfer resistance) and RS (equivalent series resistance) of this all-solid-state ASC device were obtained from the Nyquist plot (Figure S2) and found to be 0.931 and 0.732 Ω, respectively. The fitting parameters are listed in Table S1. The two important parameters of the ASC device, energy and power density, were evaluated based on the GCD measurements; Figure S3 shows the Ragone plot of the (CoNiD)60-rGO40//rGO ASC device. A maximum energy density of ∼53.8 Wh kg–1 was obtained at a power density of 1600 W kg–1 with aqueous electrolyte (3 M KOH + 0.1 M K4[Fe(CN)6]. This energy density is significantly higher than that of symmetrical cell, i.e., ((CoNiD)60-rGO40//(CoNiD)60-rGO40)[27] and commercial supercapacitor (3–9 Wh kg–1 at 3000–10,000 W kg–1).[28]

Figure 2

(a) Schematic illustration of the asymmetric supercapacitor cell (ASC), (b) CV profiles of pure rGO, and (CoNiD)60-rGO40 measured in a three-electrode setup. (c) CV curves of ASC at 10 mV s–1 in a varying voltage window. (d) GCD profiles of ASC at different potential windows from 1.4 to 1.7 V at a current density of 4 A g–1. The inset shows the plot of specific capacitance vs applied voltage that increases to 104 F g–1 at an applied voltage window of 1.6 V. (e) CV curves at a different sweep rate and (f) GCD curves at different current densities.

Electrochemical Performances of the Mechanically Flexible All-Solid-State ASC Device

As the ASC device (CoNiD)60-rGO40//rGO demonstrated incredible charge storage characteristics, we expected that it could also exhibit excellent energy storage capability in the form of a flexible all-solid-state ASC device. The fabrication of this flexible all-solid-state ASC device has been described in the Experimental Section (subsection ), and a schematic illustration is presented in Figure a. To determine the Cs, the energy and power density of this all-solid-state ASC device, CV and GCD measurements, were performed. As discussed in the previous section, the potential window of 0–1.6 V is a suitable working window for the ASC, so in this study, we have also used this potential window. Figure b depicts the CV curves of this flexible ASC device at different scanning rates from 10 to 100 mV s–1 in the potential window of 0–1.6 V. The shape of the CV curve was almost remained the same even at a scan rate of up to 100 mV s–1. GCD curves obtained at different current densities are shown in Figure c; the Cs values were calculated from those GCD measurements. The maximum Cs value of 149 F g–1 was obtained at a current density of 2.5 A g–1, and Cs values were found to be decreased with increasing current densities. The values of Rct (charge transfer resistance) and RS (equivalent series resistance) of this all-solid-state ASC device were obtained from the Nyquist plot (Figure S4) and found to be 1.08 and 0.35 Ω, respectively. The shape and phase angle (Bode plot; Figure S5) indicated the capacitive behavior of this device. The fitting parameters are listed in Table S2.

Figure 3

(a) Schematic illustration of the fabricated all-solid-state flexible asymmetric supercapacitor cell. (b) CV profiles at a different sweep rate. (c) GCD curves at different current densities. (d) Cycling performance (inset, GCD curves of 10 cycles as representative after 2000 cycles). (e) XRD pattern and (f) SEM micrograph of (CoNiD)60-rGO40//rGO after 5000 charge–discharge cycles.

The cycling performance of this all-solid-state ASC device was evaluated at a current density of 2.5 A g–1; the results are shown in Figure d. This device displayed excellent cycling stability with ∼95% of the initial Cs even after 5000 cycles. To examine whether any changes in the phase and microstructure of (CoNiD)60-rGO40 occur or not after long electrochemical charge storage cycles, powder XRD and FESEM analysis were performed on the samples collected from the electrode after 5000 cycles. No additional diffraction peak was observed in the XRD pattern of the collected sample (Figure e), indicating the phase stability of the material. The FESEM micrograph (Figure f) also revealed no significant change in the microstructure. These results clearly indicate the stability of (CoNiD)60-rGO40 during the electrochemical process in the presence of an alkaline electrolyte.

The values of energy and power density were calculated from GCD measurements. Figure a displays the Ragone plot, which shows the relationship between the energy and power density of this flexible all-solid-state ASC device. It was observed that a significantly high energy density of 52.8 Wh kg–1 was obtained at a power density of 2000 W kg–1 with an aqueous electrolyte system. These results are comparable and in many cases superior to the reported values in the literature as well as to many commercial supercapacitors (Table ).

Figure 4

(a) Ragone plot of an all-solid-state flexible asymmetric ((CoNiD)60-rGO40//rGO) cell supercapacitor device compared with some of the reported results from the literature. (b) Digital image of red LED lighted by the flexible asymmetric ((CoNiD)60-rGO40//rGO) supercapacitor device. (c) CV scans of flexible asymmetric cell obtained at different bending conditions of 0°, 90°, 180°, and the twisted state.

Table 1

Comparison of Electrochemical Energy Storage Parameters of the All-Solid-State Flexible Asymmetric (CoNiD)60-rGO40//rGO Supercapacitor Device with Literature

S.no.	device	electrolyte	working potential(V)	power density(W kg–1)	energy density(Wh kg–1)	retention	reference
1	NiCo2O4octahedronnanoparticle//GPPCS	EMIMBF4/PVDF	0 to 3	2800	41		(1)
2	CoS//AC	3.5 M KOH	0 to 1.8	1800	5.3	92%(5000 cycles)	(37)
3	Ni/Co-MOF//AC	2 M KOH	0 to 1.6	800	20.9	85%(5000 cycles)	(38)
4	NiO//rGO	6 M KOH	0 to 1.5	1081.9	45.3	91.9(5000 cycles)	(39)
5	CuO//AC	3 M KOH	0 to 1.4	700	19.7	96% (3000 cycles)	(40)
6	NiO//carbon	6 M KOH	0 to 1.3		10	74% (1000 cycles)	(41)
7	Ni-Co oxide//AC	1 M KOH	–1 to 0	1902.9	7.4	85% (2000 cycles)	(26)
8	Co(OH)2 nanowires//AC	6 M KOH	0 to 1.6	153	13.6	86.3% (5000 cycles)	(42)
9	Ni/Co-MOF//AC	3 M KOH	0 to 1.7	1132.8	30.9	80.1 (3000cycles)	(43)
10	NiO//NiCo2O4	6 M KOH	0 to 2	1000	23.2	83.4%(5000 cycles)	(44)
11	α-MnO2/FSS//h-CuS/FSS	PVA-LiClO4	0 to 1.8	32,000	18.9	93.3%(5000 cycles)	(45)
12	Co3O4 NSs-rGO//AC	2 M KOH	0 to 1.45	2166	13.4	89%(1000 cycles)	(46)
13	NiCo2O4 NSs@HMRAs//AC	1 M KOH	0 to 1.5	7800	15.42	106%(2500 cycles)	(47)
14	Co3O4@MnO2//MEGO	1 M LiOH	0 to 1.6	158,000	17.7	95%(5000 cycles)	(48)
15	MnCo2CO4//AC	1 M LiOH	0 to 1.6	1551	19	98%(3000 cycles)	(49)
16	WO3-MnO2//WO3	CMC-Na2SO4 gel	0 to 1.4	915	24.13	95%(2500)	(8)
17	NiCo2S4 nanotube//AC	NKK, MPF20AC-100 + 2 M KOH	0 to 1.7	409	33.9	78%(3000 cycles)	(50)
18	NixCo1–x LDH–ZTO//activated carbon	2 M KOH	0 to 1.2	284.2	23.7	92.7%(5000 cycles	(25)
19)	NiCo3O4//AC	2 M KOH	0 to 1.6	163	37.4	82.8%(3000 cycles)	(24)
20)	(CoNiD)60-rGO40//rGO	6 M KOH + 0.1 M K4[Fe(CN6)]	0 to 1.6	3000	52.8	95%(5000 cycles)	this work

To demonstrate the mechanical stability of this all-solid-state ASC device, its CV profiles were measured at different positions of 0°, 90°, 180°, and twisted and shown in Figure b. The nearly overlapping of the CV curves, which were obtained at various bending positions indicated the negligible effect of bending angles on the capacitance of this device and thus exhibited its mechanical flexibility. The demonstration of the practical application of this fabricated all-solid-state flexible asymmetric device was carried out by lighting the blue LED (Figure c, which showed its potential as a flexible supercapacitor.

Conclusions

In summary, a bendable, foldable, and twistable all-solid-state asymmetric supercapacitor device was fabricated by using the (CoNiD)60-rGO40 nanocomposite and pure rGO as a positive and negative electrode, respectively, a PVA gel containing 3 M KOH, and 0.1 M K4[Fe(CN)6] aqueous mixture as electrolyte cum separator. (CoNiD)60-rGO40 was synthesized by employing a simple one-pot methodology. In the (CoNiD)60-rGO40 nanocomposite, the surface of nanometer-thin rGO sheets is decorated with snowflake-like dendritic CoNi alloy nanoparticles.

This flexible all-solid-state asymmetric device offers a significantly high energy density of 52.8 Wh kg–1 at a power density of 2000 W kg–1 with aqueous 3 M KOH + 0.1 M K4[Fe(CN)6] electrolyte. It also exhibits excellent cyclic stability. Investigations on mechanical flexibility reveal that this device delivers almost similar value of Cs at various bending angles (e.g., 0°, 90°, 180°, and twisted). The present flexible all-solid-state asymmetric device demonstrates its high mechanical stability with superior charge storage features in the presence of an aqueous electrolyte and exhibits its potential to be used in portable energy storage technology in the coming days.

Experimental Section

Chemicals

Ethanol, acetone, hydrazine hydrated (N2H4·H2O), polyvinylpyrrolidone (PVP), sodium hydroxide (NaOH), polyvinyl alcohol (PVA), sodium nitrate (NaNO3), sulfuric acid (H2SO4), hydrochloric acid (HCl), cobalt(II) chloride hexahydrate (CoCl2·6H2O), and nickel(II) chloride hexahydrate (NiCl2·6H2O) were purchased from Fisher Scientific. Potassium permanganate (KMnO4) and ethylene glycol were purchased from Merck, India, and graphite powder (mean particle size of <20 mm) were purchased from Sigma-Aldrich. All the chemicals were used without further purification. Distilled water was used throughout the experiment.

Synthesis of (CoNiD)60-rGO40

A one-pot coprecipitation reduction methodology was adopted to synthesize the (CoNiD)60-rGO40 nanocomposite with snowflake-like dendritic morphology.[26] The formation of dendritic-structured CoNi alloy particles (CoNiD) and transformation of GO to rGO occur simultaneously. In CoNiD-rGO nanocomposites, the dendritic CoNiD particles were immobilized on the surface of rGO. The formation mechanism for the synthesis of the CoNiD-rGO nanocomposite is presented as Scheme . The detailed protocol of synthesis of (CoNiD)60-rGO40 is provided in the Supporting Information.

Characterization

The synthesized materials were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), energy-dispersive spectra analysis (EDS), and Raman spectroscopy. The detailed information about the chemicals and the instruments used for this purpose are provided in the Supporting Information.

The electrochemical performances of the as-prepared (CoNiD)60-rGO40 nanocomposite were investigated by fabricating first an asymmetric supercapacitor (ASC). Finally, a mechanically flexible all-solid-state asymmetric supercapacitor device was also constructed. Cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) measurements, and electrochemical impedance spectroscopy (EIS) were conducted by using the workstation IVIUMSTAT (10 V/5A/8 MHz) to evaluate the electrochemical performance. For an asymmetric cell setup ((CoNiD)60-rGO40//rGO), the CV measurements were carried out in a potential window of 0–1.6 V. The GCD measurements were carried out at different current densities ranging from 1 to 8 A g–1. The equations used to determine specific capacitance (Cs), power density, and energy density are mentioned in the Supporting Information (eqs S1 to S3). Electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range of 0.01–10,000 Hz at open-circuit potential with an alternating current amplitude of 0.01 V.

Fabrication of Supercapacitor Devices

Construction of an Asymmetric Supercapacitor (ASC) Cell

To construct an ASC cell, positive and negative electrodes were fabricated by using (CoNiD)60-rGO40 and pure rGO as active electrode materials and the Ni form as an electron collector. Electrodes were prepared by first making a viscous paste of 10 wt % poly(vinylidene fluoride), 10 wt % acetylene black, and 80 wt % active material in N-methyl-2-pyrrolidinone. This paste was then cast on one side of the nickel foam (1.5 cm × 1.5 cm). After casting the paste, the residual solvent was removed by drying the electrodes at 60 °C for 24 h. The ASC was constructed by assembling the positive and negative electrodes using a Whatman-42 filter paper as a separator. This separator was soaked with the aqueous solution of the electrolyte (i.e., a mixture of 3 M KOH and 0.1 M K4[Fe(CN)6] solutions). During the preparation of electrodes, the charge balance theory (q+ = q– where q+ is the stored charge at the positive electrode and q– is the stored charge at the negative electrode) and was used to estimate the mass ratio of the negative electrode to the positive electrode. The voltammetric charges (Q) were calculated based on the following equationswhere m is the mass of the electrode (g), ΔV is the potential window (V), and Csingle is the specific capacitance (F g–1) of each electrode measured in a three-electrode setup (calculated from cyclic voltammograms at a scan rate of 10 mV s–1).

Considering the charge/mass ratio for both anode and cathode, the balancing of the charge was carried out by substituting above equation as

For constructing this asymmetric cell, the mass ratio of (CoNiD)60-rGO40:rGO used was 0.41.

Fabrication of an All-Solid-State Flexible ASC Device

To fabricate this device, the positive and negative electrodes were prepared by using (CoNiD)60-rGO40 and pure rGO as active electrode materials, respectively. The protocol used to construct these electrodes has been discussed in the above section. PVA gel containing an aqueous solution of 3 M KOH + 0.1 M K4[Fe(CN)6] was used as the electrolyte as well as a separator. The device was fabricated by pouring the PVA gel on the surface of the electrodes and placing the electrodes one over the other. Then, the PVA gel was allowed to solidify so as to form a thin layer, which was sandwiched between the two working electrodes.