Textile-based RGO-muffled cobalt (II, III) oxide hybrid nano-architectures for flexible energy storage device

Applied Surface Science 485 (2019) 238–246

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Textile-based RGO-muffled cobalt (II, III) oxide hybrid nano-architectures for flexible energy storage device

T

Promita Howlia,1, Karamjyoti Panigrahib, Anuradha Mitraa, Nirmalya Sankar Dasb,2, ⁎ Kalyan Kumar Chattopadhyaya,b, a b

Department of Physics, Jadavpur University, Kolkata 700032, India School of Materials Science and Nanotechnology, Jadavpur University, Kolkata 700032, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Supercapacitor Flexible carbon fabric Cobalt (II, III) oxide Solid state device Specific capacitance Energy density

Recent century is enduring a passionate development of hardback electronic devices that promised to be more reliable power sources with higher energy density and long-term stability. To push up the energy density limit of solid-state symmetric supercapacitor (SSC) devices here we report RGO-muffled cobalt (II, III) oxide nanowires hybrid nanostructures as a potential aspirant in the field of flexible and portable electronics. The eco-friendly chemical route was followed to synthesize RGO enfolded cobalt (II, III) oxide nanowires on flexible carbon fabric substrate (CONW-RGO), which disclosed prevailing electrochemical performances rather than cobalt (II, III) oxide nanowires on flexible carbon fabric (CONW). The specific capacitance of CONW-RGO electrode we achieved 1110 F/g at current density 1 A/g, measured in 3-electrode configuration. Also solid state symmetric device performances were investigated which provide extensive potential window of 0 to 1.5 V. As-fabricated device dispensed higher energy and power density 34.78 Wh/kg (0.214 mWh/cm3) and 3.6 kW/kg (23 mW/cm3) respectively and exhibited outstanding cyclic performance by retaining 86.4% capacitance after 10,000 cycles. This is the very first time report of using CONW-RGO electrode as binder-free flexible SSC device with moderately widen potential window.

1. Introduction The ever-intensifying greenhouse gas emissions and the future expeditious impoverishment of fossil-fuel reserves have made an urgent necessity to evolve environmental friendly renewable energy technologies [1,2]. In parallel, to consummate the power and energy demand of recent on-going transportable, low cost and high durability electronic gadgets, intensive efforts should give for the manufacture of flexible, lightweight and eco-friendly energy storage devices [3]. The supercapacitor is one of the most requiring electrochemical energy storage devices due to its long life stability, reversibility and high power density, so it has been used as an energy storage system in consumer electronics and bake up power sources [4–8]. It seems to be a connective bridge in between high specific energy density batteries and high specific power density conventional capacitors [9,10]. However to encounter ever-swelling energy demand supercapacitors need to upgrade their energy density [11–13]. On this aspect, tremendous efforts have been dedicated for the synthesis of tailored nanostructured

materials of large surface area [14–16]. With recent inventions, numerous inorganic metal oxides/hydroxides have already made a strong platform in the field of electrochemistry. Among them, less-expensive, naturally abundant, low toxic transition metal oxides such as RuO2, MnO2, CoOx, NiOx, Fe2O3, V2O5 etc. and also some ternary/quaternary metal oxides/sulphides like NiCo2O4, Cu2NiSnS4 are acknowledged as promising electrode materials for supercapacitor [17–28]. Due to multiple redox states they belong in the group of pseudocapacitor. However, the poor electrical conductivity profoundly effects on their electrochemical performances. In order to overwhelm this hurdle incorporation of high surface area, high conductive carbon materials with above-mentioned metal oxides are proposed as a practicable solution [29,30]. Predominantly, it is well-approved that hierarchical nanostructures directly grown on conductive, flexible substrates are the potential candidates in the field of flexible electronics. Ductile carbon textile having excellent mechanical strength can resist several bending and twisting and also eminent as because of their good electrical conductivity [31]. So many competitive requirements can be fulfilled such

⁎

Corresponding author at: Department of Physics, Jadavpur University, Kolkata 700032, India. E-mail address: [email protected] (K.K. Chattopadhyay). 1 Present address: Department of Physics, Prabhu Jagatbandhu College, Jhorhat, Andul, Howrah 711302, India. 2 Present address: Department of Basic Science and Humanities, Techno India Batanagar, Kolkata 700141, India. https://doi.org/10.1016/j.apsusc.2019.04.216 Received 12 November 2018; Received in revised form 16 April 2019; Accepted 23 April 2019 Available online 24 April 2019 0169-4332/ © 2019 Published by Elsevier B.V.

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fabric was activated before deposition. Henceforth, a 100 mL homogeneous precursor solution was prepared with the ingredients 0.05 M Co(NO3)2.6H2O, 0.1 M NH4F and 0.25 M urea and the achieved pink solution was transferred into 100 mL stainless steel autoclave with inserting the activated CF into the solution, as here CF was used for growth platform of nanostructures. Henceforth, the autoclave was moved to a hydrothermal chamber for 6 h at 120 °C temperature. After the reaction, CF was rinsed several times by using distilled water and ethanol respectively. Thereafter, CF was inserted into an air furnace at 400 °C for 3 h maintaining the heating rate at 5o C/min. Homogeneous black coloured Co3O4 was formed over the CF. Subsequently, the incorporation of RGO with the nanostructure was done by ex-situ method. Synthesis of graphene oxide (GO) was given in detail as above mentioned paper [46]. Above all, 100 mL homogeneous and uniform GO solution was made by using 5 mg GO powder into 100 mL distilled water and sonicated for well dispersion. Then the asprepared CF with Co3O4 nanowire was dipped into the above solution. Eventually, 5 μL hydrazine hydrate and 35 μL NH3.H2O were added to the solution for reduction of the graphene oxide. The reducing agents were added dropwise and very slowly with vigorous stirring and keeping the solution temperature at 90 °C until the solution transformed into black coloured. Finally, the obtained sample was washed by DI water and then dried in an air oven at 80 °C for 12 h.

as ion diffusivity, ion accessibility in electrode-electrolyte interface by using carbon fabric as an ideal scaffold for the growth of nanostructures. Thus for lightweight, bendable and portable power devices, carbon fabric is used as a current collector. In this prospect, considerable attempts have been paid by many researchers to fabricate nanostructured electrode materials on carbon fabric substrate [32]. Co3O4 is one of uttermost pseudocapacitor material. There are several reports on regard to tune the morphological structure of cobalt oxide, as because the surface area is a key point for enhancing the capacitive behaviour [33]. Limited voltage window and poor rate capability may hinder the performance of Co3O4 and have bounded it to some extent in the empirical field as a solo performer. Over a few past decades, various carbonaceous materials like graphene oxide or reduced graphene oxide, carbon nanotube, aerogel, activated carbon etc. are used as electrode materials [34–37]. Although they suffer for low specific capacitance value which suppresses the energy density of the electrode, but their excellent electrical conductivity enhances their cyclic performance which in a commercial point of view is a promising characteristic for energy devices. Among those conductive additives, graphene/reduced graphene oxide secures considerable recognition because of its low resistive 2D nanosheet structure with oxygen functional group [38]. So more recently, hybrid nanostructures which are made of inorganic metal oxides composite with graphene/reduced graphene oxide have drawn biggest attention by reason of their superior electrochemical implementations that repress the disabilities like poor electrical conductivity and inferior cyclic stability of metal oxide electrodes. In order to achieve higher energy density by enlarging the voltage window, several groups are endeavouring to make supercapacitor devices in asymmetric configuration (ASSC) [39–42]. As well as so many reports of energy storage materials on basis of symmetric configuration are also remarkably proclaimed recently [43–45]. Here we report, synthesis of mesoporous architecture of Co3O4 composite with reduced graphene oxide over activated carbon textile (CONW-RGO) and its excellent performance as an electrochemical cathode material. All electrochemical measurements were done by using aqueous 3 M KOH electrolyte in three electrode system. In result from galvanostatic charge-discharge curve CONW-RGO procured high specific capacitance value 1110 F/g, measured at current density 1 A/g. The electrode exhibited excellent rate capability and retained 94.2% of its initial capacitance after 2000 cyclic performances. Further, to implement this nanostructured material in practical field, the solid state symmetric supercapacitor device was fabricated by using two CONWRGO electrodes both as cathode and anode. From experimental outcomes, the as-prepared device can deliver sufficiently high energy and power densities 34.78 Wh/kg or 214 mWh/cm3 and 3.6 kW/kg or 23 W/cm3 respectively.

2.2. Structural characterizations X-ray diffraction (XRD) pattern of as-prepared nanostructure sample was recorded by using Cu Kα radiation (λ = 1.5406 Å) (XRD, D8 Advanced, Bruker). Raman spectra were measured by using a confocal Raman spectrometer (alpha 300, Witec, Germany) with the excitation of a 532 nm laser source (WITECH). The morphological study was done by field emission scanning electron microscope (FESEM, S-4800, Hitachi) and further, the lattice structure was inspected by high-resolution transmission electron microscope (HRTEM, JEOL, JEM 2100). 2.3. Electrochemical tests All electrochemical characterizations (cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD)) were measured in three electrode cell of Gamry Interface 1000 (Potentiostat/Galvanostat/ZRA). Here CONW-RGO, platinum wire and Ag/AgCl electrode were used as working, counter and reference electrode respectively. In working electrode, deposited active area of the carbon fabric was 1 cm2 and the remaining bare part was out of electrolyte for electrical connection. The potential window was kept at −0.2 to 0.7 V for typical CV curves and various scan rates were applied in the range of 5 mV/s to 100 mV/s. Next, GCD was conducted at current densities 1 A/g to 10 A/g. All measurements were performed in 3 M KOH electrolyte. Eventually, to verify the practical validation of the as-prepared sample, we fabricated solid state device in symmetric configuration i.e., CONW-RGO both as cathode and anode with gel electrolyte (PVA/KOH) and Whatman filter paper as a separator in between two electrodes. Firstly, the gel electrolyte was prepared using weight ratio PVA: KOH = 2:1, dissolved in 30 mL DI water. The mixture was stirred vigorously with increasing temperature and was constrained to a fixed temperature of 90 °C until a clear and homogeneous solution was obtained. After cooling the solution to room temperature (30 °C), two pieces of CONW-RGO and the filter paper were soaked into it and left for drying. Finally, they were assembled and tested in the same electrochemical workstation (Gamry Interface 1000, Potentiostat/ Galvanostat/ZRA).

2. Experimental section 2.1. Methods 2.1.1. Materials Cobalt nitride, ammonium fluoride, urea, hydrazine hydrate, ammonia solution, potassium permanganate and conc. Sulphuric acid were of analytical grade, were bought from Sigma-Aldrich and used as received without further purification. Graphite flake (SP-1 graphite, ~150 μm size) was purchased from Bay Carbon Corporation. Throughout the experiments, de-ionized (DI) water was used in the laboratory. 2.1.2. Synthesis of RGO-cobalt (II, III) oxide hybrid nanostructures The synthesis process was already mentioned in our previous published work [46]. In short, at first, Co3O4 was grown on carbon fabric (CF) substrate and then after RGO was incorporated with this. Here the commercially available CF was used in the experiment and cleaned it by proper procedure before use, then by using KMnO4 solution carbon

3. Result and discussions As mentioned earlier, a simple, eco-friendly wet chemical route was used to synthesize this nanostructure. Carbon textile used as a current 239

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Fig. 1. (a) FESEM image of cobalt (II, III) oxide nanowire on carbon fabric (CONW). (b) RGO wrapped over porous cobalt (II, III) oxide nanowire (CONW-RGO) (c) HRTEM image of CONW, highly distributed pores are observed. (d) CONW after incorporation of RGO sheet. (e) Small nanoparticle of cobalt oxide circumambient by thin RGO sheet. Red arrows indicate the RGO sheet. (f) RGO sheet with lattice of cobalt oxide, measured 0.283 nm representing the (220) plane.

Morphology evolution of the prepared sample was done by FESEM, is displayed in Fig. 1. Fig. 1a reveals a dense entanglement of nanowires over the cylindrical surface of a single carbon fibre and also the protocol of scalable synthesis is confirmed from this image. The nanowires of length approximately 3–3.5 μm are highly porous with sharp edges and tip diameter is nearly 10–12 nm. Fig. 1b shows the magnified image of RGO incorporation over nanowires. Thin layers of RGO muffle over the vertexes of nanowires and make a keen protrusions which are clearly observed from the image. Again HRTEM was carried out for further detailed study regarding this microstructures. The low magnification TEM image of highly porous CONW is displayed in Fig. 1c, which affirms the mesoporous configuration of nanostructures and also the pores are symmetrically well-organised all over the area. The plausible growth mechanism of such structure was discussed elaborately in our previous report [46]. This porosity arises due to the thermal decomposition of CO2 and H2O at the time of annealing, which plays an imperative role in electrochemical performance. Surface area

collector has lots of advantages like excellent electrical conductivity with good mechanical strength. But the hydrophobicity of CF may be an obstacle for uniform and homogeneous growth, which can be eliminated by KMnO4 activation-conciliated seed layer deposition. Wellknown physically deposited seed layer techniques always have a tendency of elevated deposition at the front face of curvy fibres, which may responsible for non-homogeneous growth. Although the growth of nanostructures over any substrate dynamically depends upon several parameters like reagent, reaction atmosphere, but well-aligned growth is directed by controlled seeding mechanism [47]. Also, the homogeneous growth of electro-active material is a crucial point for good electrochemical performances. Further RGO was introduced by ex-situ method, so the RGO nanosheets gently muffled over the well-aligned mesoporous nanostructures. As-composed CONW-RGO electrode showed eminent structural integrity after suffering by several bending and twisting, above all which is the first standpoint for the flexible electronics (Fig. S1 in supporting information). 240

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magnification TEM images (Fig. 1e & f) also clearly reveal that RGO sheet interlinked in between the nanoparticles and made a conducting connection. The thin layer of RGO covering up the nanoparticles side by side is specified by red arrows in those pictures. The assessed interplanar spacing between two lattice fringes found in Fig. 1f is 0.28 nm, which represents the lattice plane (220) of spinel Co3O4. Phase identification of the as-composed sample was verified by Xray powder diffraction pattern as shown in Fig. 2a. The intense diffraction peaks in the pattern indicate the higher degree crystallinity of as-produced nanostructures. Strong diffraction peaks at 2θ values of 18.9, 31.3, 36.9, 38.5, 44.9, 55.6, 59.5, and 65.3° are attributed to the lattice planes of (111), (220), (311), (222), (400), (422), (511) and (440) respectively which are well-matched with JCPDS card no: 421467. A broad peak/hump at 2θ = 26o and a small peak approximately at 43o (red ‘*’ in the XRD curve) indicate the presence of amorphous carbon. The (002) plane for CF and that of graphene results X-ray diffraction peak in the same 2θ value and hence we can't get any clear notification of RGO wrapping over the nanostructure by from XRD data. For further justification the RAMAN spectra analysis of three different samples were performed and the combined results are displayed in Fig. 2b. The first spectrum for only bare carbon fabric shows the vibration mode of D-band at 1354 cm−1 and G band at 1600 cm−1. Also another peak at 2676 cm−1 is due to the 2nd order of D-band (2D). Now after chemical synthesis, Co3O4 nanowire was grown over the fabric that shows only peaks arising for the presence of Co3O4. D, G & 2D vibrational bands disappeared from the spectrum, giving a clear indication of uniform and homogeneous growth of nanostructures over the entire surface of the fabric. Then again the RAMAN spectrum was recorded after incorporation of RGO sheets, where the D, G & 2D vibrational bands are reappeared. The higher value of the ratio of the intensity of D to G-band (in our verification we get ID/IG ~ 1.2) implies more disorder generated and the abundance of sp2 bonding become lesser [48]. Also, the vibrational peaks due to Co3O4 are present but those are less intense with respect to the peaks of CONW. The vibrational peaks (marked in the dotted box) in Fig. 2b are for the Co3O4, which have been discussed in detail in Fig. S2 of the supporting information. Finally, from RAMAN analysis we can draw a strong conclusion about the existence of RGO sheet. 3.1. Electrochemical studies Fig. 2. (a) X-ray diffraction pattern CONW-RGO with different lattice plane. Red ‘*’ represents for RGO. (b) RAMAN spectra of three different sample: bare carbon fabric, CONW, CONW-RGO.

Electrochemical analysis in three electrode system was done in aqueous 3 M KOH electrolyte. To explore the pseudo-capacitive behaviour of CONW-RGO, CV survey was verified. Voltammograms were taken at different scan rates 5, 10, 20, 50, 100 mV/s in voltage window −0.2 V to 0.7 V (Fig. 3a). Two prominent redox peaks are observed in both oxidation and reduction parts of the curve indicating the change of states of Co2+/Co3+ and Co3+/Co4+, following the Eqs. (1a) & (1b) in basic electrolyte solution are given below:

and as well as the path of ion movement enhance due to this porosity and effectively modulate the capacitance of the nanostructures. Now Fig. 1d shows the TEM image of CONW-RGO, regular and equable coverage of RGO sheet over the whole nanowire is confirmed. High

Fig. 3. Three electrode measurements: (a) CV curves of CONW-RGO at scan rates 5–100 mV/s. (b) GCD curves of CONW-RGO at various current densities 1–10 A/g. (c) Change of specific capacitances with current densities. In inset, 2000 cyclic performance of CONW-RGO, exhibits 94.2% retention. 241

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Co3 O4 + OH− + H2 O = 3CoOOH + e−

CoOOH +

OH−

= CoO2 + H2 O +

e−

(1a)

calculate gravimetric, surface and volumetric capacitances of the asfabricated SSC device are given below:

(1b)

With increasing scan rates the shift of redox peaks at the rightward and leftward depending upon oxidation and reduction respectively are observed, which may be for irreversible electrode reaction and polarization effect occurred at high scan rates [49]. But the presence of prominent redox peaks as same as at higher scan rate stipulates the fact that the electrode material is low resistive, so feasible ion movement can be possible. To investigate the charging-discharging property of CONW-RGO electrode, we charged it up to 0.45 V and then discharged to −0.2 V. Corresponding GCD curves at various current densities 1, 2, 4, 6, 8, 10 A/g are depicted at Fig. 3b. Specific capacitance value was calculated from the GCD curve by using the following Eq. (2):

Cs =

I × Δt m × ΔV

CG =

I × Δt m × ΔV

(3)

CA =

I × Δt A × ΔV

(4)

CV =

I × Δt ϑ × ΔV

(5)

where, m is effective mass of electroactive material of SSC electrode, A is effective area of the SSC electrode, ϑ is volume of as-fabricated device and I, Δt and ΔV are corresponding discharge current, discharge time and voltage window of discharging. Maximum gravimetric (CG), surface (CA) and volumetric (CV) capacitances we obtained are 111.35 F/g, 33.4 mF/cm2 and 685 mF/cm3 respectively. Relevant plot of gravimetric and volumetric capacitances with respect to current densities is illustrated in Fig. 4c. Gund et al. proclaimed flexible supercapacitors based on MnO2 and Fe2O3 both as symmetric (0–1 V) and asymmetric (0–2 V) devices, got maximum specific capacitances 91 F/g at 0.61 A/g and 75 F/g at 1.28 A/g respectively [54]. Zhang et al. reported MnOOH/CC as symmetric supercapacitor (0–1.7 V) possesses capacitance 81.1 F/g at 1 A/g [55]. Also Patil et al. fabricated symmetric device based on βNiS (0–1.2 V) encounter specific capacitance less than 50 F/g (3 mA) [56] and MnO2/MnO2 symmetric (0–1.6 V) device was invented by Chodankar et al., got capacitance 110 F/g (5 mV/s) [57]. On basis of above symmetric device survey, CONW-RGO/SSC can manifest substantially superior specific capacitance. However in this work we have claimed CONW-RGO/SSC as flexible electrode, so to substantiate its appositeness in flexible electronics we examined the same CV and GCD tests at bending condition. Fig. 4e is shown the digital image of the device at bending posture, of which the corresponding CVs and GCDs compare with normal posture are plotted in Fig. 4d & f. No significant changes were identified in obtained results between normal and bending posture, verifying its behaviour as a flexible electrode. To accomplish the cyclic stability of the CONW-RGO/SSC device, 10,000 charge-discharge cycles were conducted at current density 10 mA/cm2, the as-composed device showed noticeable retention of 86.4% after cycling (Fig. 5a), which confirms long life stability of our manufactured device and also far predominating rather some recent reports such NiCoO2@rGO/NF-6 h//PrGO/NF (71% after 2000 cycles) [58], rCoNi2S4//AC (80% after 10,000 cycles) [59], M-Co3O4//AC ASC (86.4% after 8000 cycles) [60], (Ni, Co)0.85Se//porous graphene film ASC (85% after 10,000 cycles) [61], Ni3S2/CoNi2S4/NF//AC/NF (92.8% after 6000 cycles) [62] and Cu3Mo2O9 NCAs//AC (93.7% after 2000 cycles) [40]. More importantly, electrochemical impedance spectra (EIS) of CONW-RGO/SSC device before and after 10,000 cycling are demonstrated as Nyquist plots in Fig. 5b & c. From EIS study, we can postulate the feasibility of ionic movement in our device. Impedance measurements were carried out in frequency range of 0.01 Hz to 1 MHz. At the low frequency region, the axis of imaginary part of impedance is indicating nearly ideal capacitive nature of the device [63] and at high frequency portion, the intercept of the x-axis represent series resistance (ESR), arises mainly due to the ingrained resistance of electroactive material and additionally due to contact resistance in between electrode-electrolyte interface. We got the ESR value 1.68 Ω for CONWRGO/SSC device, which increased at 2.8 Ω after cyclic performance of 10,000 cycles. This accounts low resistive nature of the device that in turn lower the power consumption. Also there should appear a semicircle in the high frequency region, which is presumed to be promoted by charge transfer resistance (Rct) emerges for the enhanced electron transfer due to fast diffusion [51,64]. In the case of CONW-RGO/SSC device semi-circle was not completely appeared in high frequency region, suggesting excellent ionic conductivity and diffusivity in between electrode and electrolyte. Lower interfacial resistance and intensifying conductivity of the device make it appreciative claimant for electronic

(2)

where I is discharge current, Δt is relative discharge time, m is active mass of the electrode (weight of electroactive material that was deposited on the CF) and ΔV is the potential window between that range we checked the charge-discharge properties. The values of specific capacitance we evaluated are, 1110, 1107, 1076, 1061, 1046, 1038 F/g at current densities 1, 2, 4, 6, 8, 10 A/g. The respective curves of variation of capacitance with current density are displayed in Fig. 3c. Mostly it has been observed to decrease capacitance value with increase of current density, because of rapid movement of ions and lesser interaction with electroactive material. This issue is also escalate if the material itself is resistive by nature. Here it is perceived that CONW-RGO electrode can preserve its specific capacitance value 93.5% of initial value with 10 times enhancement of current density. In comparison with CONW, we got highest specific capacitance value 764 F/g at current density 1 A/g and the capacitance retention was 70.6% at 10 A/g in our previous published work [50]. 2000 random charging-discharging cycles were afforded to explore its cyclic performance and CONW-RGO cathode exhibits excellent retention of 94.2% after cycling (% retention vs. cycle number plot is displayed in inset of Fig. 3c). Comparison of CV and GCD performances in between CONW-RGO and CONW are shown in Fig. S3 (supporting information), where the voltammetric curve for both samples at scan rate 50 mV/s and the charge-discharge data at current density 3 A/g are unveiled. The obtained results were significantly enhanced after RGO consolidate into the nanostructures. 3.2. Solid state device performance A demo test was also taken by fabricating symmetric solid-state supercapacitor device based on two CONW-RGO electrodes (CONWRGO/SSC) with PVA/KOH gel electrolyte and filter paper as a separator, which confirmed its practicability in the commercial market. The cyclic voltammograms of CONW-RGO/SSC device at scan rate 50 mV/s at different potential window 0.0–0.9 V, 0.0–1.1 V, 0.0–1.3 V, 0.0–1.5 V are shown in Fig. 4a. Alike of CV curves measured in the 3electrode aqueous system, these curves are not showing any redox peaks, which manifest the electric double layer capacitive (EDLC) nature. The pseudocapacitive behaviour is inconsequential in case of SSC device performance, as the two measuring configurations are totally different [51]. In aqueous 3-electrode system potential window is limited as because of water dissociation voltage (1.23 V), although recently few reports have been published where they modulated the overpotential value by using suitable cathode and anode materials to increase the dissociation voltage of the water and they got the practically suitable voltage window in aqueous supercapacitor device [52,53]. But a huge drawback of supercapacitor device in aqueous medium is due to a cumbersome packaging to use such device in daily life. Now, GCD performances of CONW-RGO/SSC device at current densities 0.3, 0.6, 0.9, 1.5 mA/cm2 at potential window 0 to 1.5 V were recorded and are shown in Fig. 4b. The equations that were used to 242

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Fig. 4. Solid state device performance of CONW-RGO/SSC: (a) CV cycles at scan rate 50 mV/s in different potential windows. (b) GCD at different current densities 0.3, 0.6, 0.9, 1.5 mA/cm2. (c) Calculated gravimetric and volumetric specific capacitances with different current densities. (d) & (f) CV and GCD were taken at scan rate 50 mV/s and current density 2 mA/cm2 respectively, comparing the normal and bending condition of the device. (e) Digital photograph of SSC device at bending condition. Photograph courtesy Promita Howli.

4.6 mW/cm3. Also the maximum power density the device can supplied is 3.6 kW/kg or 1.2 mW/cm2 or 23 mW/cm3 at energy density 24.68 Wh/kg or 0.007 mWh/cm2 or 0.16 mWh/cm3. This values are surprisingly better and comparable with so many recent published reports. Comparative results are displayed in Table 1. Comparative Ragone plot showing the energy density vs. power density of a conventional battery, capacitor and asymmetric/symmetric supercapacitor device with the value obtained from our device is displayed in Fig. 6. Thus, fabricated CONW-RGO/SSC device is an excellent aspirant to compete with recent asymmetric devices and it has sufficient energy density comparable with commercial battery [75]. A schematic pictorial depiction of ion movement in the electrode is shown in Fig. 7. Also here we try to interpret these superior electrochemical performances of CONW-RGO by following some particulars: (1) Structural dependency: Cobalt (II, III) oxide is one of remarked electroactive material, by modulating the morphology of Co3O4, capacitive behaviour can be tuned. Vertically aligned, well-oriented growth of porous nanostructures was possible by adapting the controlled

devices. A demonstration of LED lighting is also shown at inset picture of Fig. 5a. Moreover, the intercontinental market for supercapacitor is forecasted to grow at an increasing rate that stimulate the research and development regarding supercapacitor device for further advancement of their performances including energy and power density most predominantly. Despite of high power density the unappreciable energy density has held them back, which should be modified [13]. Whatever, mostly of the literature concerned about gravimetric parameters (capacitance, energy and power density) of the device but in a practical point of view for commercialization surface and volumetric parameters are need to be scrutinized. Habitually, very high gravimetric specific capacitance can be achieved due to ultralow mass loading of electroactive material which may devastate the surface/volumetric parameters. Keeping that in mind, we have displayed here all parameters with respect to mass, area and volume. Corresponding equations for calculation are supplied in supporting information. Our SSC device delivers much higher energy density 34.78 Wh/kg or 0.0104 mWh/cm2 or 0.214 mWh/cm3 at power density 750 W/kg or 0.225 mW/cm2 or

Fig. 5. Solid state device performance of CONW-RGO/SSC: (a) Cyclic stability up to 10,000 cycles. In inset, digital image of demonstration of lighting the LEDs. (b) & (c) Electrochemical impedance study before and after cycling. 243

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Table 1 Comparative study on basis of energy density and power density of previous published work. Materials

Electrolyte used

AuNP/Nano-Co3O4//AC Co-M@SrGO//AC Co3O4/NHCS//AC CPY//C-G/AFC MLG-PANI BFO-RGO NCGs-800//NCGs-800 ZnCo2O4//AC CoFe2O4/rGO (Fe,Cr)2O3//MnO2 (Fe,Cr)2O3//MnO2 β-NiS(FSS-SCS) MnO2/MnO2 CoNW/CF CONW-RGO/SSC

2 M KOH 1 M KOH 2 M KOH PVA-KOH Gel PVA-H2SO4 Gel 3 M KOH + 0.1 M K4[Fe(CN)6] 1 M Na2SO4 3 M KOH 1 M KOH 1 M KOH 1 M Na2SO4 PVA-LiClO4 Gel PVP-LiClO4 Gel PVA-KOH Gel PVA-KOH Gel

Configuration

Voltage window (V)

Maximum energy density E (Wh/kg)

Maximum power density P (kW/kg)

Asymmetric Asymmetric Asymmetric Asymmetric Symmetric Symmetric Symmetric Asymmetric Asymmetric Asymmetric Asymmetric Symmetric Symmetric Symmetric Symmetric

0–1.5 0–1.5 0–1.5 0–1.4 0–0.8 0–1.0 0–1.8 0–1.6 0–1.5 0–1.6 0–1.6 0–1.2 0–1.6 0–1.0 0–1.5

25 23.3 34.5 18.3 17.0 18.62 12.7 27.78 12.14 12.36 20.89 7.97 23.0 6.7 34.78

11.25 36.6 3.8 8.4 3.61 1.9 15.126 0.92 – 1.7 2.17 0.666 7.69 5.0 3.6

Ref

[65] [66] [67] [68] [69] [70] [71] [72] [73] [74] [74] [56] [57] [50] This work

synthesis procedure. Generally, the mesoporous structures are most desirable nanoarchitecture in the application of pseudocapacitor material. Diffusivity of ions in between the electroactive materials of the electrode, is one of an essential point to keep in mind. Optimized porosity will minimize the ion diffusion and facilitate the uninterrupted ion movement at the interface of the electrode. (2) Growth substrate: Typically, for powder electroactive material we have to choose another external substrate like Ni foil, stainless steel foil, Cu foil etc. as current collector and also nonconducting binder is used to coat the material on the substrate. This is a big issue for increasing the contact resistance as well as the material resistance that may effect on electroactive performances. In our experiment, we have directly grown the nanostructure on the current collector (carbon fabric). Adhesion of nanostructure with the fibre is very strong, so that the bending and twisting do not affect the integrity of the structure. Excellent electrical conductivity, flexibility and robust nature of CF make it a preferred candidate for flexible electronics. (3) Effect of RGO: Thin RGO nanosheet enfolded over the apexes of the nanowire, makes a conjugative connection in between the nanowires and also assists as physical support of metal oxides. High specific surface area of RGO sheets maximizes the carbon-metal oxide interfacial area, which is one of the prime parameters that decides the charge transfer ability of the composite electrode. Also the thickness of the nanosheets should be thin as comparable with depth of ion insertion to fully utilize the electroactive material for pseudo-capacitive performances, as in that case redox reaction occurs on the outer surface area of metal oxide and this type of attenuated structure of nanosheet was confirmed from our FESEM/HRTEM results. The performance of the supercapacitor deliberately depends on the electrical conductivity of the electrode. Here the RGO sheets tune the conductivity of the hybrid structure which reduces the interfacial resistance. Currently, the symmetric supercapacitor device has a crucial drawback of low energy density due to the workability of the material in limited voltage window which we have been revamped by producing RGO integrated hybrid nanostructures.

Fig. 6. Ragone plot: Comparative study of energy density and power density of CONW-RGO/SSC with electrochemical capacitor, batteries, asymmetric supercapacitor and symmetric supercapacitor.

4. Conclusions A novel hydrothermal based scalable synthesis strategy was approached to synthesize RGO wrapped cobalt (II, III) oxide nanoporous wires over carbon textile. Here CONW-RGO was used as cathode material in 3-electrode electrochemical performance. As-prepared cathode showed significantly enhanced specific capacitance (1110 F/g at current density 1 A/g) in comparison with CONW and after 10-times enhancement of current density, the value becomes 0.935 times of initial capacitance (1038 F/g). Such magnificent outcomes are due to the incorporation of RGO, as it enhances the overall surface area of the electrode which provides an electrically conducting large ionic

Fig. 7. Schematic representation of ionic movement in the electrode CONWRGO. 244

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movement path. Also, the feasibility of the as-prepared nanostructure in the practical application was confirmed by fabricating binder-free solidstate symmetric supercapacitor device which provided maximum gravimetric (CG), surface (CA) and volumetric (CV) capacitances of 111.35 F/g, 33.4 mF/cm2 and 685 mF/cm3. Highest energy density and power density that the device can deliver are 34.78 Wh/kg or 0.0104 mWh/cm2 or 0.214 mWh/cm3 and 3.6 kW/kg or 1.2 mW/cm2 or 23 mW/cm3 respectively. Hence, high-performance CONW-RGO electrode can open up new possibilities in the field of flexible energy devices.

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Acknowledgements [25]

The author (PH) would like to thank the University Grants Commission (UGC), the Government of India, for awarding her a research fellowship during the execution of the research work. One of the authors (KP) would like to thank Council of Scientific & Industrial Research (CSIR) for awarding him research fellowship during the tenure of this work. We also thankful to the UGC also for the ‘University with Potential for Excellence (UPE-II)’ scheme for providing instrument facility.

[26]

[27]

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Appendix A. Supplementary data

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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.04.216.

[30]

References

[31]

[1] B.C. Steele, A. Heinzel, Materials for fuel-cell technologies, in: V. Dusatre (Ed.), Materials for Sustainable Energy: A Collection of Peer-reviewed Research and Review Articles From Nature Publishing Group, World Scientific, 2011, pp. 224–231. [2] M. Dresselhaus, I. Thomas, Alternative energy technologies, Nature 414 (2001) 332. [3] L. Mai, X. Tian, X. Xu, L. Chang, L. Xu, Nanowire electrodes for electrochemical energy storage devices, Chem. Rev. 114 (2014) 11828–11862. [4] G. Nagaraju, S. Chandra Sekhar, L. Krishna Bharat, J.S. Yu, Wearable fabrics with self-branched bimetallic layered double hydroxide coaxial nanostructures for hybrid supercapacitors, ACS Nano 11 (2017) 10860–10874. [5] X. Xiao, T. Li, P. Yang, Y. Gao, H. Jin, W. Ni, W. Zhan, X. Zhang, Y. Cao, J. Zhong, Fiber-based all-solid-state flexible supercapacitors for self-powered systems, ACS Nano 6 (2012) 9200–9206. [6] J. Tao, N. Liu, W. Ma, L. Ding, L. Li, J. Su, Y. Gao, Solid-state high performance flexible supercapacitors based on polypyrrole-MnO 2-carbon fiber hybrid structure, Sci. Rep. 3 (2013) 2286. [7] X. Lu, M. Yu, G. Wang, Y. Tong, Y. Li, Flexible solid-state supercapacitors: design, fabrication and applications, Energy Environ. Sci. 7 (2014) 2160–2181. [8] L. Wang, Q. Wu, Z. Zhang, Y. Zhang, J. Pan, Y. Li, Y. Zhao, L. Zhang, X. Cheng, H. Peng, Elastic and wearable ring-type supercapacitors, J. Mater. Chem. A 4 (2016) 3217–3222. [9] M. Winter, R.J. Brodd, What Are Batteries, Fuel Cells, and Supercapacitors, ACS Publications, 2004. [10] P. Simon, Y. Gogotsi, B. Dunn, Where do batteries end and supercapacitors begin? Science 343 (2014) 1210–1211. [11] X. Xia, Y. Zhang, D. Chao, C. Guan, Y. Zhang, L. Li, X. Ge, I.M. Bacho, J. Tu, H.J. Fan, Solution synthesis of metal oxides for electrochemical energy storage applications, Nanoscale 6 (2014) 5008–5048. [12] D. Chen, Q. Wang, R. Wang, G. Shen, Ternary oxide nanostructured materials for supercapacitors: a review, J. Mater. Chem. A 3 (2015) 10158–10173. [13] M. Zhi, C. Xiang, J. Li, M. Li, N. Wu, Nanostructured carbon–metal oxide composite electrodes for supercapacitors: a review, Nanoscale 5 (2013) 72–88. [14] S. Giri, D. Ghosh, C.K. Das, Growth of vertically aligned tunable polyaniline on graphene/ZrO2 nanocomposites for supercapacitor energy-storage application, Adv. Funct. Mater. 24 (2014) 1312–1324. [15] Z. Yu, L. Tetard, L. Zhai, J. Thomas, Supercapacitor electrode materials: nanostructures from 0 to 3 dimensions, Energy Environ. Sci. 8 (2015) 702–730. [16] Y. Shao, M.F. El-Kady, L.J. Wang, Q. Zhang, Y. Li, H. Wang, M.F. Mousavi, R.B. Kaner, Graphene-based materials for flexible supercapacitors, Chem. Soc. Rev. 44 (2015) 3639–3665. [17] P. Chen, H. Chen, J. Qiu, C. Zhou, Inkjet printing of single-walled carbon nanotube/ RuO2 nanowire supercapacitors on cloth fabrics and flexible substrates, Nano Res. 3 (2010) 594–603. [18] Q. Lv, S. Wang, H. Sun, J. Luo, J. Xiao, J. Xiao, F. Xiao, S. Wang, Solid-state thinfilm supercapacitors with ultrafast charge/discharge based on N-doped-carbontubes/Au-nanoparticles-doped-MnO2 nanocomposites, Nano Lett. 16 (2015) 40–47. [19] Y. Gao, Y.S. Zhou, M. Qian, H.M. Li, J. Redepenning, L.S. Fan, X.N. He, W. Xiong, X. Huang, M. Majhouri-Samani, High-performance flexible solid-state

[32]

[33]

[34]

[35]

[36] [37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

245

supercapacitors based on MnO2-decorated nanocarbon electrodes, RSC Adv. 3 (2013) 20613–20618. F. Chen, X. Liu, Z. Zhang, N. Zhang, A. Pan, S. Liang, R. Ma, Controllable fabrication of urchin-like Co3O4 hollow spheres for high-performance supercapacitors and lithium-ion batteries, Dalton Trans. 45 (2016) 15155–15161. M. Pal, R. Rakshit, A.K. Singh, K. Mandal, Ultra high supercapacitance of ultra small Co3O4 nanocubes, Energy 103 (2016) 481–486. X. Ren, C. Guo, L. Xu, T. Li, L. Hou, Y. Wei, Facile synthesis of hierarchical mesoporous honeycomb-like NiO for aqueous asymmetric supercapacitors, ACS Appl. Mater. & Interfaces 7 (2015) 19930–19940. Y. Qian, R. Liu, Q. Wang, J. Xu, D. Chen, G. Shen, Efficient synthesis of hierarchical NiO nanosheets for high-performance flexible all-solid-state supercapacitors, J. Mater. Chem. A 2 (2014) 10917–10922. H. Jiang, H. Ma, Y. Jin, L. Wang, F. Gao, Q. Lu, Hybrid α-Fe2O3@ Ni(OH)2 nanosheet composite for high-rate-performance supercapacitor electrode, Sci. Rep. 6 (2016) 31751. B. Saravanakumar, K.K. Purushothaman, G. Muralidharan, Interconnected V2O5 nanoporous network for high-performance supercapacitors, ACS Appl. Mater. & Interfaces 4 (2012) 4484–4490. X. Zhou, Q. Chen, A. Wang, J. Xu, S. Wu, J. Shen, Bamboo-like composites of V2O5/ polyindole and activated carbon cloth as electrodes for all-solid-state flexible asymmetric supercapacitors, ACS Appl. Mater. & Interfaces 8 (2016) 3776–3783. N. Jabeen, Q. Xia, M. Yang, H. Xia, Unique core–shell nanorod arrays with polyaniline deposited into mesoporous NiCo2O4 support for high-performance supercapacitor electrodes, ACS Appl. Mater. & Interfaces 8 (2016) 6093–6100. S. Sarkar, P. Howli, U.K. Ghorai, B. Das, M. Samanta, N.S. Das, K.K. Chattopadhyay, Flower-like Cu2NiSnS4 microspheres for application as electrodes of asymmetric supercapacitors endowed with high energy density, CrystEngComm 20 (2018) 1443–1454. R.R. Salunkhe, H. Ahn, J.H. Kim, Y. Yamauchi, Rational design of coaxial structured carbon nanotube–manganese oxide (CNT–MnO2) for energy storage application, Nanotechnology 26 (2015) 204004. C.-C. Tu, L.-Y. Lin, B.-C. Xiao, Y.-S. Chen, Highly efficient supercapacitor electrode with two-dimensional tungsten disulfide and reduced graphene oxide hybrid nanosheets, J. Power Sources 320 (2016) 78–85. S. Das, S. Saha, D. Sen, U.K. Ghorai, D. Banerjee, K.K. Chattopadhyay, Highly oriented cupric oxide nanoknife arrays on flexible carbon fabric as high performing cold cathode emitter, J. Mater. Chem. C 2 (2014) 1321–1330. P. Yang, X. Xiao, Y. Li, Y. Ding, P. Qiang, X. Tan, W. Mai, Z. Lin, W. Wu, T. Li, Hydrogenated ZnO core–shell nanocables for flexible supercapacitors and selfpowered systems, ACS Nano 7 (2013) 2617–2626. K.K. Lee, W.S. Chin, C.H. Sow, Cobalt-based compounds and composites as electrode materials for high-performance electrochemical capacitors, J. Mater. Chem. A 2 (2014) 17212–17248. J. Li, X. Wang, Q. Huang, S. Gamboa, P. Sebastian, Studies on preparation and performances of carbon aerogel electrodes for the application of supercapacitor, J. Power Sources 158 (2006) 784–788. C. Lai, Y. Sun, X. Zhang, H. Yang, B. Lin, High-performance double ion-buffering reservoirs of asymmetric supercapacitors based on flower-like Co3O4-G > N-PEGm microspheres and 3D rGO-CNT > N-PEGm aerogels, Nanoscale 10 (2018) 17293–17303. D. Kim, G. Shin, Y.J. Kang, W. Kim, J.S. Ha, Fabrication of a stretchable solid-state micro-supercapacitor array, ACS Nano 7 (2013) 7975–7982. B. Li, X. Cao, H.G. Ong, J.W. Cheah, X. Zhou, Z. Yin, H. Li, J. Wang, F. Boey, W. Huang, All-carbon electronic devices fabricated by directly grown single-walled carbon nanotubes on reduced graphene oxide electrodes, Adv. Mater. 22 (2010) 3058–3061. C. Ogata, R. Kurogi, K. Hatakeyama, T. Taniguchi, M. Koinuma, Y. Matsumoto, Allgraphene oxide device with tunable supercapacitor and battery behaviour by the working voltage, Chem. Comm. 52 (2016) 3919–3922. T. Gao, Z. Zhou, J. Yu, D. Cao, G. Wang, B. Ding, Y. Li, All-in-one compact architecture toward wearable all-solid-state, high-volumetric-energy-density supercapacitors, ACS Appl. Mater. & Interfaces 10 (2018) 23834–23841. S.M. Cha, S. Chandra Sekhar, R. Bhimanaboina, J.S. Yu, Achieving a high areal capacity with a binder-free copper molybdate nanocone array-based positive electrode for hybrid supercapacitors, Inorg. Chem. 57 (2018) 8440–8450. J. Zhang, F. Liu, J. Cheng, X. Zhang, Binary nickel–cobalt oxides electrode materials for high-performance supercapacitors: influence of its composition and porous nature, ACS Appl. Mater. & Interfaces 7 (2015) 17630–17640. X.-F. Lu, X.-Y. Chen, W. Zhou, Y.-X. Tong, G.-R. Li, α-Fe2O3@PANI core–shell nanowire arrays as negative electrodes for asymmetric supercapacitors, ACS Appl. Mater. & Interfaces 7 (2015) 14843–14850. F. Zhang, T. Zhang, X. Yang, L. Zhang, K. Leng, Y. Huang, Y. Chen, A high-performance supercapacitor-battery hybrid energy storage device based on grapheneenhanced electrode materials with ultrahigh energy density, Energy Environ. Sci. 6 (2013) 1623–1632. Q. Wang, X. Wang, B. Liu, G. Yu, X. Hou, D. Chen, G. Shen, NiCo2O4 nanowire arrays supported on Ni foam for high-performance flexible all-solid-state supercapacitors, J. Mater. Chem. A 1 (2013) 2468–2473. D. Guo, Y. Luo, X. Yu, Q. Li, T. Wang, High performance NiMoO4 nanowires supported on carbon cloth as advanced electrodes for symmetric supercapacitors, Nano Energy 8 (2014) 174–182. P. Howli, S. Das, S. Saha, B. Das, P. Hazra, D. Sen, K.K. Chattopadhyay, RGO enveloped vertically aligned Co3O4 nanowires on carbon fabric: a highly efficient prototype for flexible field emitter arrays, RSC Adv. 6 (2016) 91860–91869. K. Panigrahi, S. Das, S. Saha, B. Das, D. Sen, P. Howli, K.K. Chattopadhyay,

Applied Surface Science 485 (2019) 238–246

P. Howli, et al.

[48] [49]

[50]

[51]

[52]

[53]

[54]

[55]

[56]

[57]

[58]

[59]

[60]

[61]

Chemically activated growth of CuO nanostructures for flexible cold cathode emission, CrystEngComm 18 (2016) 3389–3398. S. Eigler, C. Dotzer, A. Hirsch, Visualization of defect densities in reduced graphene oxide, Carbon 50 (2012) 3666–3673. S. Sarkar, P. Howli, B. Das, N.S. Das, M. Samanta, G.C. Das, K.K. Chattopadhyay, Novel quaternary chalcogenide/reduced graphene oxide-based asymmetric supercapacitor with high energy density, ACS Appl. Mater. & Interfaces 9 (2017) 22652–22664. P. Howli, S. Das, S. Sarkar, M. Samanta, K. Panigrahi, N.S. Das, K.K. Chattopadhyay, Co3O4 nanowires on flexible carbon fabric as a binder-free electrode for all solidstate symmetric supercapacitor, ACS Omega 2 (2017) 4216–4226. G. Wang, H. Wang, X. Lu, Y. Ling, M. Yu, T. Zhai, Y. Tong, Y. Li, Solid-state supercapacitor based on activated carbon cloths exhibits excellent rate capability, Adv. Mater. 26 (2014) 2676–2682. R. Sahoo, D.T. Pham, T.H. Lee, T.H.T. Luu, J. Seok, Y.H. Lee, Redox-driven route for widening voltage window in asymmetric supercapacitor, ACS Nano 12 (2018) 8494–8505. W.S.V. Lee, T. Xiong, G.C. Loh, T.L. Tan, J. Xue, Optimizing electrolyte physiochemical properties toward 2.8 V aqueous supercapacitor, ACS Applied Energy Materials 1 (2018) 3070–3076. G.S. Gund, D.P. Dubal, N.R. Chodankar, J.Y. Cho, P. Gomez-Romero, C. Park, C.D. Lokhande, Low-cost flexible supercapacitors with high-energy density based on nanostructured MnO2 and Fe2O3 thin films directly fabricated onto stainless steel, Sci. Rep. 5 (2015) 12454. Y. Zhang, Z. Hu, Y. An, B. Guo, N. An, Y. Liang, H. Wu, High-performance symmetric supercapacitor based on manganese oxyhydroxide nanosheets on carbon cloth as binder-free electrodes, J. Power Sources 311 (2016) 121–129. A. Patil, A. Lokhande, N. Chodankar, V. Kumbhar, C. Lokhande, Engineered morphologies of β-NiS thin films via anionic exchange process and their supercapacitive performance, Mater. Des. 97 (2016) 407–416. N.R. Chodankar, D.P. Dubal, G.S. Gund, C.D. Lokhande, A symmetric MnO2/MnO2 flexible solid state supercapacitor operating at 1.6 V with aqueous gel electrolyte, J. Energy Chem 25 (2016) 463–471. H. Zhang, X. Deng, H. Huang, G. Li, X. Liang, W. Zhou, J. Guo, W. Wei, S. Tang, Hetero-structure arrays of NiCoO2 nanoflakes@ nanowires on 3D graphene/nickel foam for high-performance supercapacitors, Electrochim. Acta 289 (2018) 193–203. Z. Li, D. Zhao, C. Xu, J. Ning, Y. Zhong, Z. Zhang, Y. Wang, Y. Hu, Reduced CoNi2S4 nanosheets with enhanced conductivity for high-performance supercapacitors, Electrochim. Acta 278 (2018) 33–41. G. Wei, Z. Zhou, X. Zhao, W. Zhang, C. An, Ultrathin metal–organic framework nanosheet-derived ultrathin Co3O4 nanomeshes with robust oxygen-evolving performance and asymmetric supercapacitors, ACS Appl. Mater. & Interfaces 10 (2018) 23721–23730. C. Xia, Q. Jiang, C. Zhao, P.M. Beaujuge, H.N. Alshareef, Asymmetric

[62]

[63]

[64]

[65]

[66]

[67]

[68]

[69]

[70]

[71]

[72]

[73]

[74]

[75]

246

supercapacitors with metal-like ternary selenides and porous graphene electrodes, Nano Energy 24 (2016) 78–86. W. He, C. Wang, H. Li, X. Deng, X. Xu, T. Zhai, Ultrathin and porous Ni3S2/CoNi2S4 3d-network structure for superhigh energy density asymmetric supercapacitors, Adv. Energy Mater. 7 (2017) 1700983. Y. Liu, J. Zhou, J. Tang, W. Tang, Three-dimensional, chemically bonded polypyrrole/bacterial cellulose/graphene composites for high-performance supercapacitors, Chem. Mater. 27 (2015) 7034–7041. M. Wang, L.D. Duong, N.T. Mai, S. Kim, Y. Kim, H. Seo, Y.C. Kim, W. Jang, Y. Lee, J. Suhr, All-solid-state reduced graphene oxide supercapacitor with large volumetric capacitance and ultralong stability prepared by electrophoretic deposition method, ACS Appl. Mater. & Interfaces 7 (2015) 1348–1354. Y. Tan, Y. Liu, L. Kong, L. Kang, F. Ran, Supercapacitor electrode of nano-Co3O4 decorated with gold nanoparticles via in-situ reduction method, J. Power Sources 363 (2017) 1–8. M. Qorbani, T.-c. Chou, Y.-H. Lee, S. Samireddi, N. Naseri, A. Ganguly, A. Esfandiar, C.-H. Wang, L.-C. Chen, K.-H. Chen, Multi-porous Co3O4 nanoflakes@sponge-like few-layer partially reduced graphene oxide hybrids: towards highly stable asymmetric supercapacitors, J. Mater. Chem. A 5 (2017) 12569–12577. T. Liu, L. Zhang, W. You, J. Yu, Core–shell nitrogen-doped carbon hollow spheres/ Co3O4 nanosheets as advanced electrode for high-performance supercapacitor, Small 14 (2018) 1702407. H. Fan, R. Niu, J. Duan, W. Liu, W. Shen, Fe3O4@ carbon nanosheets for all-solidstate supercapacitor electrodes, ACS Appl. Mater. Interfaces 8 (2016) 19475–19483. G. de Souza Augusto, J. Scarmínio, P.R.C. Silva, A. de Siervo, C.S. Rout, F. Rouxinol, R.V. Gelamo, Flexible metal-free supercapacitors based on multilayer graphene electrodes, Electrochim. Acta 285 (2018) 241–253. D. Moitra, C. Anand, B.K. Ghosh, M. Chandel, N.N. Ghosh, One-dimensional BiFeO3 nanowire-reduced graphene oxide nanocomposite as excellent supercapacitor electrode material, ACS Appl. Energy Mater. 1 (2018) 464–474. L. Wang, C. Wang, H. Wang, X. Jiao, Y. Ouyang, X. Xia, W. Lei, Q. Hao, ZIF-8 nanocrystals derived N-doped carbon decorated graphene sheets for symmetric supercapacitors, Electrochim. Acta 289 (2018) 494–502. Y. Shang, T. Xie, C. Ma, L. Su, Y. Gai, J. Liu, L. Gong, Synthesis of hollow ZnCo2O4 microspheres with enhanced electrochemical performance for asymmetric supercapacitor, Electrochim. Acta 286 (2018) 103–113. K.V. Sankar, R.K. Selvan, D. Meyrick, Electrochemical performances of CoFe2O4 nanoparticles and a rGO based asymmetric supercapacitor, RSC Adv. 5 (2015) 99959–99967. P. Deshmukh, Y. Sohn, W.G. Shin, Electrochemical performance of facile developed aqueous asymmetric (Fe, Cr)2O3//MnO2 supercapacitor, Electrochim. Acta 285 (2018) 381–392. N. Choudhary, C. Li, J. Moore, N. Nagaiah, L. Zhai, Y. Jung, J. Thomas, Asymmetric supercapacitor electrodes and devices, Adv. Mater. 29 (2017) 1605336.