Solid State Flexible Asymmetric Supercapacitor Based on Carbon

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Solid State Flexible Asymmetric Supercapacitor Based on Carbon Fiber Supported Hierarchical Co(OH)xCO3 and Ni(OH)2 Debasis Ghosh, Manas Mandal, and Chapal Kumar Das Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b00649 • Publication Date (Web): 23 Jun 2015 Downloaded from http://pubs.acs.org on July 1, 2015

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Solid State Flexible Asymmetric Supercapacitor Based on Carbon Fiber Supported Hierarchical Co(OH)xCO3 and Ni(OH)2 Debasis Ghosh, Manas Mandal, Chapal Kumar Das* Materials Science Centre, Indian Institute of Technology Kharagpur, India Corresponding author email: [email protected]

Abstract: Conducting flexible carbon fiber (CF) cloth was used as a substrate for the hydrothermal growth of nickel hydroxide (Ni(OH)2) and cobalt hydroxy carbonate [Co(OH)xCO3] with unique hierarchical flowery architecture and then was used as flexible supercapacitor electrode. In a three electrode configuration in 6 M KOH aqueous electrolyte, the CF-Ni(OH)2 and CF-Co(OH)xCO3 electrode showed the maximum specific capacitance of 789 F/g and 550 F/g, respectively at 2A/g current accompanied by outstanding cycle stability by retaining 99.9% and 99.5% specific capacitance over 1500 consecutive charge discharge cycles at 5 A/g. However, the low cell voltage (0.4 V) restricted the respective specific energy to 4.38 Wh/kg and 3.05 Wh/kg at a specific power of 100 W/kg. To overcome the issue, two solid state

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flexible asymmetric supercapacitors were fabricated using the CF-Ni(OH)2 and CFCo(OH)xCO3 as the anode and sonochemically deposited CNT over carbon fiber as the cathode material in PVA-KOH gel electrolyte. The as fabricated flexible supercapacitors CF-Ni(OH)2//CF-CNT and CF-Co(OH)xCO3//CF-CNT were able to deliver high specific energy of 41.1 Wh/kg and 33.48 Wh/kg, respectively at high specific power of 1.4 kW/kg accompanied by excellent cycle stability (retaining 98% and 97.6% specific capacitance, respectively over 3000 charge discharge cycle at 5 A/g). Keywords: carbon fiber; specific energy; specific power; cycle life 1. Introduction Since, the early twenty first century the enormous demand of energy with developing civilization and the depleting fossil fuels has triggered ample researches in the field of alternative energy storage and conversion. Although the conventional batteries and capacitors are serving as the leading energy storage systems, the respective limitation of low power density and low energy density has restricted their applications in many fields. Supercapacitor can be considered as an alternative energy storage system, which can form the bridge between the two, with its unique properties of high energy density and high power density and can be used in such cases, where the demand of both high current at short time and low current for long time is required. The supercapacitor electrode material can store charge in two ways; It can store charge via the electron transfer faradaic reaction or by physical separation of charge at the electrode/electrolyte

interface

and

the

obtained

capacitance

2 ACS Paragon Plus Environment

is

known

as

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pseudocapacitance and electrical double layer capacitance respectively.1 Metal oxide/hydroxide, such as Co(OH)2, Ni(OH)2, Co3O4, NiO, MnO2, CeO2, etc. are well known for their redox activity and the electron transfer faradaic reaction that results in effective pseudocapacitance.2-7 High surface area graphene, CNT, activated carbon are the effective source of double layer capacitance. Amongst the pseudocapacitive electrode materials, Co(OH)2 and Ni(OH)2 have attracted severe attention due to their various morphologies, easy processibility, environmentally benign nature, high theoretical specific capacitance and over all low cost. Although, the theoretical specific capacitance of Co(OH)2 (3560 F/g) and Ni(OH)2 (2358 F/g) is very high,8-9 there is often a large difference between the theoretical and experimental specific capacitance due to the very low conductivity of these metal hydroxides, and only a very thin layer of electrode material over current collector may give the optimum result. Hence, processing of these metal oxides/hydroxides is always difficult when fabricating a devise, where large scale of material is required. However, if these materials grow on any conducting substrate, then the substrate containing the material can be used itself as current collector. Any conducting metal substrate can be useful in this case. But the heavy weight of metal substrate as well as stiffness causes a problem, when there is a large demand of flexible device. For the fabrication of supercapacitor device, it is essential that the device should be of light weight and flexible in nature in order to fit in any position. Nowadays, amongst many other conducting substrates flexible carbon fiber has attracted much attention due to their unique properties of high conductivity, high knittability, etc.10 Apart from that, due to


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the microsize it can effectively place the maximum amount of active material per unit substrate area. In our present work we have synthesized self-supported Ni(OH)2 and Co(OH)xCO3 grown on conducting carbon fiber (CF) in a large scale by a simple and cost effective hydrothermal method and applied it as potential electrode material for supercapacitor application. Both the materials shows high specific capacitance but they suffer from low cell voltage. In order to obtain a supercapacitor offering high energy density at a high power delivery rate, electrode assembly offering high cell voltage and high specific capacitance is needed. In order to fulfill this condition two solid state asymmetric hybrid type supercapacitor (SASs) have been fabricated using the CFNi(OH)2 and CF-Co(OH)xCO3 as the positive electrode and sonochemically deposited CNT over carbon fiber as the negative electrode. Although the recent investigated asymmetric supercapacitors mostly prefer activated carbon as an active material, it has certain disadvantage compared to that of other carbon based materials such as CNT, graphene, etc. In case of CNT, the mesopores are interconnected allowing continuous charge distribution and almost all the surface area is utilized. However, in case of activated carbon only the one third of the surface area is available for the ionic double layer formation. Besides, CNT with aspect ratio as high as 1000, tends to entangle with each other and form a durable and porous nanotube skeleton enabling easy electrolyte ion access. Moreover, due to the high diffusion behavior, low stacking tendency and overall lower cost, we have chosen it as an active material for negative electrode. However, this does not establish its superiority over other carbon material like graphene, rather this is a case study of flexible supercapacitor using CF-CNT as


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negative electrode.11 Here the pseudocapacitive electrode owing to their higher capacitance offers high energy density while the non-faradaic EDLC electrode provides high power density. The synthesis, characterization, electrode preparation and electrochemical properties have been portrayed elaborately. The hybrid supercapacitors exhibit high energy density at a high power density accompanied by excellent cycle life. 2. Experimental Section 2.1. Preparation of materials A piece of commercial carbon fiber cloth (CF) of dimension (2 cm × 6 cm), pretreated with 0.1M H2SO4, was sequentially washed with acetone, ethanol and distilled water. 20 ml 0.1 M Ni(NO3)2 solution was poured into a 50 ml capacity stainless steel autoclave and the pre-weighted cleaned carbon cloth was immersed into this solution around the autoclave wall for 20 minute. Then 20 ml 0.5 M urea solution was mixed with the Ni(NO3)2 solution in the autoclave slowly with gently stirring the autoclave and it was maintained at 120°C for 6h in a furnace. One side (1cm) of the CF cloth was wrapped by Teflon tape and kept outside the solution mixture. Then the autoclave was allowed to cool to room temperature. The CF was taken out carefully and a stream of distilled water was flown on the CF holding it vertically in order to remove any impurity and excess precipitation and was dried under vacuum. This process was repeated with 20 ml 0.1 M Co(NO3)2 instead of 20 ml 0.1 M Ni(NO3)2. The as prepared materials were labeled as CF-Ni(OH)2 and CF-Co(OH)xCO3, respectively.


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CNT was deposited over carbon fiber cloth by a dip coating method in CNT ink in ethanol solution containing 1% nafion, and the amount of CNT was controlled by manipulating the dipping period. The CNT ink was prepared by dispersing CNT by ultrasonication in 50 ml ethanol containing 1% nafion. The CNT deposited CF was hanged under air for 30 minute and then dried at 60°C. The as prepared material is labeled as CF-CNT. 3. Electrochemical set up The electrochemical characterizations of the CF-Ni(OH)2, CF-Co(OH)xCO3 and CF-CNT were carried out in a three electrode cell in 6M KOH electrolyte using the CF-Ni(OH)2 and CF-Co(OH)xCO3 as working electrode and pt electrode (1×1 cm2) as counter electrode. All the experiments were referred to the saturated calomel electrode. 3.1. Fabrication of asymmetric electrode Two solid state asymmetric flexible supercapacitors were fabricated using the CFNi(OH)2 and

CF-Co(OH)xCO3

as

positive

electrode

active

material,

and

sonochemically deposited CNT on carbon fiber as negative electrode active material and PVA-KOH gel as semisolid electrolyte. The PVA-KOH gel electrolyte was prepared using 3 g PVA and 3.36 g KOH. At first the PVA was dispersed into 30 mL deionized water by continuous stirring and heated to 85 °C. After some time when a clear viscous solution was achieved the solution was cooled down and the concentrated KOH (3.36 g) solution was added to the viscous solution slowly under constant stirring until a clear solution was obtained. Then the as prepared CF-Ni(OH)2, CF-Co(OH)2 and CF-CNT were immerged to the PVA-KOH gel and hanged for some


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time. Then the gel electrolyte covered CF-Ni(OH)2 and CF-Co(OH)xCO3 were individually pasted with the gel electrolyte coated CF-CNT and allowed to dry under mild heat. When the gel electrolyte solidifies it itself acts as a separator between the two electrodes. Silver paste was used for current connection between the carbon cloth and instrument. The as fabricated asymmetric supercapacitors were labeled as CFNi(OH)2//CF-CNT and CF-Co(OH)xCO3//CF-CNT, respectively. 4. Results and Discussion 4.1. X-ray diffraction analysis The as prepared materials were scratched from the carbon fiber and the XRD pattern of the scratched materials is shown in Figure 1. The XRD pattern in Figure 1a indicates the formation of Ni(OH)2 by urea hydrolysis in presence of Ni(NO3)2. The XRD pattern exhibits a sharp peak centered on 2θ = 12.3°, which can be indexed as the (003) plane. Three medium frequency peaks centered at 2θ = 24.7°, 33.4° and 59.5° indicates the (006), (101) and (110) plane, respectively. A broad hump can be observed within the range of 2θ = 34.8°- 38.8°, which are the indicative of the (101), (012) and (015) planes.12 All these peaks indicate the α phase of the as prepared Ni(OH)2 grown on CF. In contrary to the Ni(OH)2, the same experimental procedure with Co(NO3)2, instead of Ni(NO3)2 leads to the formation of the cobalt hydroxy carbonate with orthorhombic structure, which is in good agreement with a reported compound Co(OH)1(CO3)0.5,0.11 H2O (JCPDS card, no. 48-0083).13-15


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Figure 1. XRD pattern of as prepared Ni(OH)2 and Co(OH)xCO3.

4.2. Morphological analysis The morphological analysis of the as grown Ni(OH)2 and Co(OH)xCO3 on CF were carried out in terms of FESEM and TEM analysis. The FESEM micrographs of the CF-Ni(OH)2 are shown in Figure 2 at various magnifications. An aesthetically beautiful spherical flowery morphology with an average diameter of 2-5 µm appeared for the Ni(OH)2 grown on CF. Higher magnification images of the Ni(OH)2 flowers reveal that each flower comprised of several tiny nanopetals, which coils together to form flower like morphology. The nanopetals of the microflower look like nanowire. Close inspection indicates that these flowers are not separated from each other rather they are connected together via the petals. Thus they form an interconnected network. The growth of the network can be explained by the time dependent morphology analysis of the CF-Ni(OH)2 formed under hydrothermal condition. The FESEM


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images are shown in Figure S1. Nucleation center is created on CF for the growth of Ni(OH)2 nanopetals. A heterogeneous nucleation on the primary developed nenopetals results in further growth of the nanopetals in length. Then these nanopetals tied together to form porous flower. From a thermodynamics viewpoint, the surface energy of separable nanopetal is very high and to minimize the overall surface energy, the nanopetals are spontaneously self-assembled to form 3D hierarchical flowerlike Ni(OH)2.16

Figure 2. FESEM image of (a) CF-Ni(OH)2; FESEM image of the as grown Ni(OH)2 at various magnifications (b) and (c); (d) magnified view of the nanopetals forming interconnected network amongst the microflowers. The FESEM images of Co(OH)xCO3 (Figure 3) reveals its flower like architecture consisting of nanostructured acicular petals, which grows from the CF to the edge of


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the flower. The soaking of the CF in the Co(NO3)2 solution results in the generation of nucleation site on the CF, which then creates heterogeneous nucleation sites for the growth of the acicular petals and from the CF nucleation point a branch of acicular petal spreads out forming flowery morphology. The flowers with acicular petals grow quite uniformly all along the CF with hardly any bare surface. The FESEM images of CF-CNT (Figure 3e and 3f) clearly demonstrate the uniform distribution of the CNT over carbon fiber and there were hardly agglomeration. The average diameter of the used CNT was of ~15-25 nm.

Figure 3. FESEM image of CF-Co(OH)xCO3 at various magnifications (a, b and c); magnified view of the acicular nanopetals of the Co(OH)xCO3 flower (d); FESEM image of CF-CNT at various magnifications (e,f).


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The TEM micrographs in Figure 4 also supports the FESEM analysis of the microflower morphology of Ni(OH)2 consisting nanowire Ni(OH)2 as the building block element. The average diameter of the nanowire is ~5-10 nm. The TEM image of Ni(OH)2 in Figure 4a clearly indicates its porous architecture and several nanowires are engaged in forming an interconnecting network, as shown in Figure 4b. Even a sonication for 10 minute for TEM sample preparation did not result in tearing of the interconnecting nanowires as well as deformation of the microflowers indicating high flexibility as well as high mechanical strength of the nanowires forming the microflowers.

Figure 4. TEM image of (a) Ni(OH)2 microflower containing coiled nanopetals and (b) nanopetals interconnecting the microflower; (c) TEM image of acicular Co(OH)xCO3 nanopetals and (d) magnified view of the edge of one acicular nanopetal


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The TEM image of Co(OH)xCO3

in Figure 4c-d also supports the acicular petal

morphology of a flower where from the nucleation center a branch of acicular petals spread out to the edge. The petals are of ~ 5-8 µm in length and 50-80 nm in diameter and ends with a sharp edge. The Schematic representation of the formation of CF-Ni(OH)2 and CF-Co(OH)xCO3 by hydrothermal procedure is shown in Figure 5.

Figure 5. Schematic diagram of the formation of CF-Ni(OH)2 and CF-Co(OH)xCO3

4.3. Electrochemical characterizations Both the CF-Ni(OH)2, and CF-Co(OH)xCO3 electrodes were first electrochemically characterized in a three electrode cell in terms of cyclic voltammetry (CV) and galvanostatic charge discharge (GCD) test. The CV plots of the CF-Ni(OH)2, CFCo(OH)xCO3 are shown in Figure 6a and 6b, respectively, at various scan rates of 2, 5, 10 and 20 mV/s within the potential range of 0 V– (+)0.4V. A pair of redox peaks can be observed in both the CV plots indicating typical pseudocapacitor behavior of the 12 ACS Paragon Plus Environment

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electrode materials. With increasing scan rates the peak current also increases with the negative shifting of the cathodic peaks and positive shifting of the anodic peaks. The increasing peak current with scan rate is significant for the pseudocapacitive electode material and the peak shifting can be attributed to the influence of internal resistance. However, the deviation from rectangular shape in the CV plots for both electrodes signifies the deviation from ideal behavior of the electrode materials.

Figure 6. CV plot of (a) CF-Ni(OH)2 and (b) CF-Co(OH)xCO3 at various scan rates; GCD plots of (c) CF-Ni(OH)2, (d) CF-Co(OH)xCO3 at different currents.


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The redox reaction associated with CF-Ni(OH)2 electrode in KOH can be expressed as:15,17-19 Ni(OH)2+ OH- ↔ NiOOH + H2O + e-

(1)

In case of the CF-Co(OH)xCO3, when used as positive electrode, after the initial cycling an irreversible reaction occurs to lose the carbonate ions and to form cobalt hydroxide. Then a reversible transformation occurs between Co2+↔Co3+ and the redox reaction can be expressed as follow:20-21 Co(OH)2 + OH-= CoOOH + H2O + e-

(2)

CoOOH +OH-= CoO2 + H2O + e-

(3)

The formation of cobalt hydroxide is also confirmed by the XRD analysis of the CF-Co(OH)xCO3 after a few initial CV cycles (supporting information Figure S4e). The maximum specific capacitances obtained from the CV experiment were 1012 and 605 F/g, respectively for the CF-Ni(OH)2 and CF-Co(OH)xCO3 at 2 mV/s scan rate. The commercial capacitors are mainly characterized by their charging and discharging capacity. In order to find the suitability of our as prepared electrodes for commercial purpose the GCD test of both the CF-Ni(OH)2, CF-Co(OH)xCO3 was carried out at various current of 2, 3 and 5 A/g and the plots are shown in Figure 6c and 6d, respectively. For both the electrodes there is an initial potential drop in the discharge plot and with the increasing current density the potential drop increases following the Ohm’s law. The maximum specific capacitance of 789 F/g and 550 F/g was calculated for the CFNi(OH)2 and CF-Co(OH)xCO3, respectively at 2 A/g current, which were equivalent to the respective areal capacitance of 1.136 F/cm2 and


0.665 F/cm2. The specific

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capacitance decreased with increasing current from 2 A/g to 5 A/g. Still high specific capacitance of 562 F/g (0.831 F/cm2) and 412 F/g (0.499 F/cm2) at 5 A/g current demonstrate their good rate capability. The higher surface area and porosity also play a crucial role behind the higher specific capacitance of CF-Ni(OH)2 over CFCo(OH)xCO3 (Figure S2). It is generally accepted that a real world symmetric capacitor consisting two identical electrode generally gives one fourth of specific capacitance obtained from a single electrode in a three-electrode configuration.22 The maximum specific energy obtained from the CF-Ni(OH)2 and CF-Co(OH)xCO3 was 4.38 Wh/kg and 3.05 Wh/kg at a specific power of 100 W/kg. Although the specific capacitance was high, the low cell voltage restricts the specific energy and specific power. EDLC active materials, such as CNT, graphene, activated carbon, etc. are well known for their high rate of charge discharge, which increase the power density.23-25 In order to combine the high specific capacitance of pseodocapacitor with the high charge discharge rate of EDLC active materials, two SASs were fabricated using the CF-Ni(OH)2 and CF-Co(OH)xCO3 as the positive electrode and sonochemically deposited CNT onto carbon fiber as negative electrode. Figure 7a and 7b represents the CV plots at different scan rates and the GCD plots at different currents of the CFCNT electrode in a three electrode configuration within the potential range of (-) 1V to 0V in 6M KOH electrolyte. The CV plots resembles to that of rectangular shape with a little asymmetric behavior. The linear charge discharge plot of the CF-CNT also suggests its excellent reversibility. The maximum specific capacitance exhibited by the CF-CNT was 142 F/g at 2 A/g current. The comparative CV plots of the CF-CNT, CFNi(OH)2 and CF-Co(OH)xCO3 at 20 mV/s scan rate are shown in Figure 7c. The


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electrochemical performance in an asymmetric electrode configuration depends on the proper charge balance between the cathode and anode following the relationship q+ = q-, where q is the charge stored by each electrode, and can be calculated as a product of the specific capacitance of the individual electrode, operating potential of charge discharge and electrode mass. Hence the proper mass ratio of the anodic (m+) and cathodic electrode material (m-) for the best electrochemical utilizations should be m+/m- =(C-*V-)/(C+*V+) Based on the above equation the preferred mass ratio of the CF-Ni(OH)2 to CF-CNT should be 0.449 and CF-Co(OH)xCO3 to CF-CNT should be 0.645 in asymmetric electrode configuration. This is very close to the used electrode mass ratio of 0.46 and 0.67. The area of the each working electrode was 10 cm2. For the CF-Ni(OH)2 and CF-Co(OH)xCO3 tested in three electrode testing method the mass loading of the active material was 1390 µg/cm2, 1210 µg/cm2, respectively. For the CF-Ni(OH)2//CFCNT and CF-Co(OH)xCO3//CF-CNT asymmetric flexible supercapacitor the loading of active material in positive electrode was 1390 µg/cm2 and 1220 µg/cm2, whereas the load of active material in negative electrode was 3020 µg/cm2 and 1800 µg/cm2, respectively. The CV plots of the two SASs, CF-Ni(OH)2//CF-CNT and CFCo(OH)xCO3//CF-CNT are shown in Figure 7d and 7e, respectively, at different scan rates of 2, 5, 10, and 20 mV/s. It is noteworthy to mention that both the SASs were able to sweep within a large potential range of 0 V-1.4 V. With increasing scan rate the current response increases without any major deformation of the CV plot indicating excellent reversibility of both the SASs, where the CF-CNT plays the major role behind the high reversibility.


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Figure 7. (a) CV plots of CF-CNT at various scan rate, (b) GCD plot of CF-CNT at various currents; (c) comparative CV plots of CF-Ni(OH)2, CF-Co(OH)xCO3 and CFCNT at 20 mV/s scan rate; CV plots of (d) CF-Ni(OH)2//CF-CNT and (e) CFCo(OH)xCO3//CF-CNT at various scan rates of 2, 5, 10, and 20 mV/s, (f) variation of specific capacitance as a function of scan rate.


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Unlike, the CV plots of CF-Ni(OH)2 and CF-Co(OH)xCO3 the CV plots of both the SASs are rather flat indicating the redox phenomenon occurring at a pseudoconstant rate over the entire volumetric cycle. The maximum specific capacitance of 171 and 131 F/g was achieved at 2 mv/s scan rate, respectively for the CF-Ni(OH)2//CF-CNT and CF-Co(OH)xCO3//CF-CNT. The variation of specific capacitance with scan rates of

all

CF-Ni(OH)2,

CF-Co(OH)xCO3,

CF-Ni(OH)2//CF-CNT

and

CF-

Co(OH)xCO3//CF-CNT are shown in Figure 7f. The GCD plots of the CF-Ni(OH)2//CF-CNT and CF-Co(OH)xCO3//CF-CNT electrodes at various currents of 2, 3, and 5 A/g is shown in Figure 8a and 8b, respectively. Unlike the GCD plot of CF-Ni(OH)2 and the CF-Co(OH)xCO3, the GCD plots of the two SASs exhibited almost linear discharge plot with a little potential drop during the initial discharging, indicating the low equivalent series resistance of the SASs. With the increasing current the potential drop showed a slight increase for all the electrodes. The symmetrical and linear GCD plots indicate excellent reversibility of both the CF-Ni(OH)2//CF-CNT and CF-Co(OH)xCO3//CF-CNT SASs. The maximum specific capacitance obtained from the CF-Ni(OH)2//CF-CNT and CFCo(OH)xCO3//CF-CNT was 151 F/g and 123 F/g, respectively at 2 A/g current, which was equivalent to the areal capacitance of 0.332 F/cm2 and 0.185 F/cm2, respectively. An increase in current density from 2-5 A/g resulted in a gradual decrease of specific capacitance; however, high specific capacitance retention of 86 F/g (0.189 F/cm2) and 64 F/g (0.097 F/cm2) at high current of 5 A/g signifies the good rate capability of both the SASs.

The decreasing specific capacitance with increasing current is a

consequence of the fact that high current decreases the electrolyte ion diffusion and


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migration within the electrode material and as a result only a part of the active material remains active. The excellent reversibility was also supported by the linear charge discharge plot even at high current of 5 A/g within a large operating potential of 1.4 V.

Figure 8. GCD plots of (a) CF-Ni(OH)2//CF-CNT and (b) CF-Co(OH)xCO3//CF-CNT at various currents; (c) Variation of specific capacitance with mass normalized current (black plot) and current density (blue plot); (d) variation of specific capacitance retention as a function of cycle life at 5 A/g current.

The

variation

of

areal

capacitance

of

CF-Ni(OH)2,

CF-Co(OH)xCO3,

CF-

Ni(OH)2//CF-CNT and CF-Co(OH)xCO3//CF-CNT at different mass normalized 19 ACS Paragon Plus Environment

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current (black plot) and current density (blue plot) are shown in Figure 8c. The maximum specific energy of 41.1 Wh/kg and 33.45 Wh/kg was obtained from the CFNi(OH)2//CF-CNT and CF-Co(OH)xCO3//CF-CNT, respectively at a comparatively higher specific power of 1.4 kW/kg. Generally, increasing energy density results in a rapid loss of power density. However, at high specific power of 3.5 kW/kg both the SASs was still able to deliver high specific energy of 23.41 Wh/kg and 17.42 Wh/kg, respectively. The specific energy of both the SASs were higher than many other Ni and cobalt oxide or hydroxides based asymmetric supercapacitors (calculated energy and power per unit mass of active material), such as, Graphene–nickel cobaltite nanocomposites//AC (19.5 Wh/kg),26 Nickel cobalt layered double hydroxides (LDHs) on conducting Zn2SnO4 (ZTO)// active carbon (23.7 Wh/kg at 284.2 W/kg),27 Ni–Co binary hydroxides//chemically-reduced graphene (26.3 Wh/kg at 320 W/kg),28 NiCo2O4//AC (17.72 Wh/kg at 2542 W/kg),29 CNT/Ni(OH)2//AC (25.8 Wh/kg at 2800 W/kg),30 α-Ni(OH)2//AC (26.9 Wh/kg at 1100 W/kg),31 Ni(OH)2//AC (35.7 Wh/kg at 490 W/kg),32 Co(OH)2/USY//AC (30.625 Wh/kg at 520 W/kg),33 NiO//Carbon (15-20 Wh/kg),34 Ni(OH)2/UGF//a-MEGO (13.4 Wh/kg at 6500 W/kg).35 In order to analyze the cycle life of the as prepared CF-Ni(OH)2, CF-Co(OH)xCO3, the GCD test was continued upto 1500 consecutive GCD cycles at 5 A/g. Almost no loss of specific capacitance was observed for the CF-Ni(OH)2, whereas a meager amount of specific capacitance loss of 0.5% was observed for the CF-Co(OH)xCO3. To understand the devise application of the two SASs, the GCD plots were continued upto 3000 cycles at 5A/g current and very high specific capacitance retention of about 98.0% and 97.6% was achieved, respectively for CF-Ni(OH)2//CF-CNT and CF-Co(OH)xCO3//CF-CNT.


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The cycle life performances of both the SASs were also investigated at high current by continuing the GCD test upto 10000 consecutive cycles at 12 A/g and respective specific capacitance retention of 93.1% and 91.7% was achieved, which is high enough to promote them as supercapacitor with very high cyclability (see supporting information Figure. S3 e). Interestingly all the electrodes exhibited an initial increase in specific capacitance up to some cycles and then showed a slow fall. The initial increase in specific capacitance is due to the slow diffusion of the electrolyte ions within the electrode material. The very high specific capacitance retention of the CFNi(OH)2 electrode is associated with its unique flexible microflowery morphology and the flexible interconnected nanowire network, which releases the strain during the consecutive charging and discharging. The flexible CF also plays a key role in the very high specific capacitance retention. Apart from its flexibility and high conductivity, it is very useful as current collector or as a substrate for the growth of metal hydroxides due to its micro diameter, which offers maximum surface area to grow electrode material. The comparative low cycle life of the CF-Co(OH)xCO3 can be attributed to the carbonate ion loss from the Co(OH)xCO3 and some deterioration of the acicular nanopetals.15 The SASs with CF-CNT as cathode material is also responsible in retaining very high specific capacitance. The CNTs being flexible in nature release the strain involved during the consecutive charging and discharging without any major structural change thereby ensure the high cycle stability of the SASs. In order to understand the electrochemical behavior of the as fabricated SASs under flexible condition the CV test of both the CF-Ni(OH)2//CF-CNT and CF-Co(OH)xCO3//CFCNT was carried out at 30 mV/s scan rate under extreme bending condition where the


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supercapacitor was shaped in the form of an arc with the distance between the two end of 2.5 cm (Figure 9a).

Figure 9. Schematic diagram of the SAS under normal condition and bending condition; (b) devise performance of the CF-Ni(OH)2//CF-CNT which was able to lighten up a low power commercial LED lamp; Comparative CV plots of CFNi(OH)2//CF-CNT (c) and CF-Co(OH)xCO3//CF-CNT (d) under normal and bending condition at 30 mV/s scan rate; (e) Nyquist plot of CF-Ni(OH)2, CF-Co(OH)xCO3, CFNi(OH)2//CF-CNT and CF-Co(OH)xCO3//CF-CNT with the fitted equivalent electrical circuit shown inset.


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The comparative CV plots at 30 mV/s under normal condition and under bending condition of both the CF-Ni(OH)2//CF-CNT and CF-Co(OH)xCO3//CF-CNT are shown in Figure 9c, and 9d, respectively. Under bending condition the performance of both the SASs were comparable to that of their normal condition, with the maximum specific capacitance of 46 and 32.9 F/g at 30 mV/s scan rate. This confirms the effective use of both the SASs as flexible supercapacitor. The devise performance was also tested for the CF-Ni(OH)2//CF-CNT and it was able to lighten up a low power commercial LED lamp, the digital image is shown in Figure 9b. In order to investigate the various resistive parameters involved with the electrochemical performance of the as prepared CF-Ni(OH)2, CF-Co(OH)xCO3, CFNi(OH)2//CF-CNT, CF-Co(OH)xCO3//CF-CNT, the EIS analysis was carried out within the frequency range of 100 kHz and 1 Hz at an open circuit voltage of 5 mV. The EIS has been represented in terms of Nyquist plot after fitting with the equivalent electrical circuit, shown in Figure 9e. All the plots show similar nature of initial semicircle of small radius in the high frequency region followed by a straight line making an angle of around 45° with the real impedance axis in the low-frequency region. The post semicircle straight line indicates the diffusion of electrolyte ions at the electrode interface, and is called the Warburg line. In the equivalent electrical circuit Rs+Q/(Rct+W) the term RS, Rct, Q, and W represents the bulk solution resistance or ESR, double layer capacitance, charge transfer resistance, constant phase element and Warburg element. The Rct stands for charge transfer resistance due to Faradaic redox processes involving the exchange of OH− ions. The solution resistance CF-Ni(OH)2, CF-Co(OH)xCO3, CF-Ni(OH)2 //CF-CNT and CF-Co(OH)xCO3 //CF-


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CNT was 1.12, 1.35, 2.1 and 2.34 ohm, respectively, whereas, their respective charge transfer resistance

was 0.6, 1.9, 5.4 and 6.3. It can be seen that the semisolid

electrolyte showed higher resistance than the liquid one, probably due to the restricted movement of the electrolyte ions. 5. Conclusions Flexible carbon fiber cloth supported hierarchical Ni(OH)2 and Co(OH)xCO3 porous flowery architecture were prepared via facile hydrothermal approach. When used as electrode material for supercapacitor application, both the electrodes showed high specific capacitance accompanied by exceptional cycle life. Two solid state asymmetric flexible supercapacitors were fabricated using the carbon fiber supported Ni(OH)2 and the Co(OH)xCO3 as positive electrode and sonochemically deposited CNT on carbon fiber cloth as negative electrode. The as fabricated CF-Ni(OH)2//CFCNT and CF-Co(OH)xCO3//CF-CNT SASs were competent to deliver high specific energy of 38.65 Wh/kg and 33.48 Wh/kg, respectively at 1.4 kW/kg specific power accompanied by long term cycle stability. The low fabrication cost of the as fabricated SASs accompanied by their superior electrochemical performances over many other similar asymmetric supercapacitors endorses their suitability in lightweight flexible devise application. Acknowledgements The authors are thankful to UGC INDIA for financial support and IIT Kharagpur for instrumental help.


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Notes †Electronic Supplementary Information (ESI) available: [Materials and instruments, BET analysis, equation used, FESEM image of CF-Ni(OH)2, formation mechanism of Ni(OH)2 and Co(OH)xCO3 via urea hydrolysis, Post GCD cycle FESEM images of CFNi(OH)2 and CF-Co(OH)xCO3, Raman spectra, electrochemical performance of CF-CNT flexible supercapacitor, cycle life performance of SASs at high current] References (1) Conway, B. E. Electrochemical supercapacitors. Dordrecht/New York: Kluwer Academic Publishers/Plenum Press; 1999. (2) Mondal, C.; Ganguly, M.; Manna, P. K.; Yusuf, S. M.; Pal, T. Fabrication of Porous β-Co(OH)2 Architecture at Room Temperature: A High Performance Supercapacitor. Langmuir 2013, 29, 9179-9187. (3) Ghosh, D.; Giri, S.; Mandal, A.; Das, C. K. H+, Fe3+ Codoped Polyaniline/MWCNTs Nanocomposite: Superior Electrode Material for Supercapacitor Application. Appl. Surf. Sci. 2013, 276, 120-128. (4) Padmanathan, N. ; Selladurai, S.; Razeeb, Kafil M. Ultra-fast rate capability of a symmetric supercapacitor with a hierarchical Co3O4nanowire/nanoflower hybrid structure in non-aqueous electrolyte. RSC Adv., 2015, 5, 12700-12709 (5) Jiang, Y.; Chen, D.; Song, J.; Jiao, Z.; Ma, Q.; Zhang, H.; Cheng, L.; Zhao, B.; Chu, Y. A Facile Hydrothermal Synthesis of Graphene Porous NiO Nanocomposite and Its Application in Electrochemical Capacitors. Electrochim. Acta 2013, 91, 173-178.


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