Fabrication and performance studies of a wire like

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Fabrication and performance studies of a cable-type flexible asymmetric supercapacitor† S. T. Senthilkumar and R. Kalai Selvan* In the present work a novel cable-type asymmetric supercapacitor was fabricated using plate-like b-Ni(OH)2 as the positive and activated carbon as the negative electrode, with polyvinyl alcohol–KOH (PVA–KOH) as the gel polymer electrolyte. The b-Ni(OH)2 plates were prepared by a reflux method and the activated carbon was derived from Tamarindus indica fruit shell by chemical activation. The working

Received 5th March 2014, Accepted 6th May 2014

voltage of the fabricated cable-type asymmetric supercapacitor (ASC) was 1.4 V, and it achieved

DOI: 10.1039/c4cp00955j

Besides, the fabricated ASC delivered a maximum per-unit-length (gravimetric) energy density of

per-unit-length and gravimetric capacitances of 40.7 mF cm1 and 37.5 F g1 respectively at 2 mA. 10.7 mW h cm1 (9.8 W h kg1) at a power density of 169 mW cm1 (154 W kg1). In addition, it exhibited

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a better capacitance retention, of 88% over 1000 cylces and 76% over 2000 cycles.

Introduction Electronic devices such as sensors, transistors, solar or photovoltaic cells and thermoelectric generators are very important devices in current society. Generally, conventional electronic components were very rigid, heavy and bulky, so it was difficult to accommodate them for modern flexible applications. Therefore in the last few decades, lightweight, flexible and portable devices have been developed. Very recently, flexible wire/cable type electronic gadgets were developed since these can be used in any shape, with more flexibility than conventional flexible electronic devices.1–4 However, to power a cable-type sensor and transistor as well as to store the produced energy from solar cells and thermoelectric generators, similar kinds of cable-type energy storage devices are needed.5,6a,b Hence, cable-type energy storage and conversion devices have attracted much more attention in recent times. Flexible supercapacitors (SCs) are essential energy storage devices, which can deliver a higher power density than batteries and a higher energy density than classical capacitors.7,8 Additionally, they have the benefits of a long cyclic life, i.e. more charge–discharge cycles, and a faster charge–discharge rate. They are also lightweight and safer than any other energy storage devices.8,9 Previously, flexible SCs were fabricated as planar structures using metal substrates like nickel foil, and foam and stainless steel mesh as flexible current collectors. However, these exhibited limited

Solid State Ionics and Energy Devices Laboratory, Department of Physics, Bharathiar University, Coimbatore, 641 046, India. E-mail: [email protected] † Electronic supplementary information (ESI) available: SEM images of the fiber electrode at normal and bent positions, videos etc. available. See DOI: 10.1039/ c4cp00955j

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flexibility and increased the weight of devices. In the recent past, plastic, paper, carbon fabric and cotton/textile-based substrates were used in SCs to make them more lightweight, with good flexibility.10a Currently, cable/fiber-like SCs are being fabricated to maximize flexibility.6,10b,11a Notably, a fiber supercapacitor has been fabricated using MnO2 coated ZnO nanowire, which achieved a specific capacitance of 2.4 mF cm2 (0.2 mF cm1).11b Peng et al. used two aligned MWCNT/MnO2 fibers as electrodes for a fiber supercapacitor which showed a specific capacitance of 3.58 mF cm2 (0.018 mF cm1).11c Following this, the same group has developed MWCNT/PANI11d and MWCNT/ordered mesoporous carbon (OMC)11e based fiber electrodes. Correspondingly, specific capacitances of 39.67 mF cm2 (1.9 mF cm1) for the MWCNT/PANI, and 263 mF cm1 for the MWCNT/OMC were reported. Moreover, a novel type of coaxial fiber supercapacitor was developed recently, in which the coaxial structure decreased the contact resistance between the two electrodes and delivered a better specific capacitance (59 F g1) than twisting two CNT fibers electrodes together (4.5 F g1).11f However, to the best of our knowledge, no reports are available on the fabrication of asymmetric flexible cabletype SCs. Asymmetric SCs are developed by grouping pseudo(or battery) and capacitor-type electrodes in the same cell, to increase the energy performance of the SCs. Generally, symmetric SCs possess a lower energy density than batteries. On the other hand, in asymmetric SCs the pseudo (or battery) electrodes provide a high energy density through a faradaic process and the capacitor electrodes retain their power density. The aim of the development of asymmetric SCs is to increase their energy density to a value closer to that of batteries, while also keeping a better power density than batteries.

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Moreover, the energy density is increased via the sum of the maximum voltage window of the negative and positive electrodes. Similarly, polymer gel electrolytes are another attractive part of flexible SCs, as liquid electrolytes have numerous drawbacks such as leakage, corrosion and packing problems. Additionally, they can avoid the use of a separator. Furthermore, a polymer gel electrolyte has the advantage of a better ionic conductivity (103 to 104 S cm1) than a solid electrolyte (107 to 108 S cm1), because there is a large amount of water in the polymer matrix that predominately enhances its ionic conductivity. Mostly, poly(vinyl alcohol) (PVA), poly(vinyl chloride), poly(ethylene oxide) (PEO), poly(vinylidene carbonate) and poly(vinylidene fluoride) are used as polymer gel electrolytes. Among these, PVA is a widely used polymer due to its low cost, good electrochemical stability, good mechanical properties and non-toxic nature.12 Keeping the above requirements in mind, here we present the very first attempt to fabricate a cable-type asymmetric supercapacitor (ASC) using bio-waste derived activated carbon as the negative electrode, plate-like b-Ni(OH)2 as the positive electrode and PVA–KOH as the polymer gel electrolyte. The electrodes are prepared by a simple brush coating method and the fabricated cable-type ASC is operated at a maximum potential of 1.4 V. This approach to the fabrication of cable-type ASCs is therefore very attractive for cable-type flexible electronic applications in the near future.

Results and discussion Electrode materials such as b-Ni(OH)2 were prepared by a facile reflux method, and activated carbon (AC) was derived from the bio-waste of tamarind shell by a chemical method. Fig. 1a shows the XRD pattern of the b-Ni(OH)2. The observed diffraction planes reveal the formation of single phase, hexagonal and highly crystalline structures of b-Ni(OH)2 (JCPDS#14-0117). Similarly, the observed broad peak (Fig. 1b) between B201 and B301 infers that AC has a disordered carbon structure. A small, broad peak is also observed at B441 that is ascribed to the existence of sp2 hybridized carbon, from the aromatic structures in the AC. Fig. 1b (inset) shows the Raman spectrum for the AC. Two prominent peaks are obtained at 1349 cm1 and 1562 cm1, corresponding to a D-band, which is due to the

Fig. 1

presence of a disordered carbon structure, and a G-band, which indicates the sp2 hybridized carbon of the aromatic structures or a graphitic nature.9,13 The morphological features of the b-Ni(OH)2 and AC were analyzed using scanning electron microscopy (SEM). Fig. 2a and b show the SEM images of the b-Ni(OH)2 and AC. It can be seen that b-Ni(OH)2 formed with a plate-like morphology, with a thickness of 30–60 nm. Interestingly, the prepared AC has a honeycomb-like porous morphology, with a pore size between 10–60 nm. Fig. 2c and d show the N2 adsorption–desorption isotherm and the pore size distribution of the AC. The obtained hysteresis loop below P/P0 o 0.5 and above P/P0 = 0.9 in the adsorption–desorption isotherm indicates the existence of mesopores (2–50 nm), micropores (o2 nm) and macropores (450 nm).14,15 Furthermore, the obtained pore size ranges of 1.6–3 nm and 3–80 nm in the graph of pore size distribution (Fig. 2d) also indicate the presence of micro-mesopores and meso-macropores in the AC. The calculated BET surface area of the AC is 657 m2 g1. To fabricate the flexible cable-type ASC, PVA–KOH gel electrolyte was prepared (see Experimental methods). The masses of the positive and negative electrodes were balanced based on the gravimetric capacitance calculated from the CV curves, to maintain a mass ratio between the positive and negative electrodes of B0.24 (see Experimental methods). The detailed fabrication procedure of the cable-type ASC is given in Scheme 1. Initially, the PVA–KOH gel electrolyte was coated on both the positive and negative fiber electrodes individually and dried at the desired temperature (Step 1 and 2). Subsequently, both the electrodes were inserted into a rubber tube with a simultaneous injection of PVA–KOH gel electrolyte for better contact (Step 3 and 4), followed by drying. Finally, the fabricated ASC is shown in Step 5. The corresponding SEM images of the prepared AC and Ni(OH)2 fiber electrodes are given in Fig. 3(a and b), and those of the PVA–KOH gel electrolyte-coated fiber electrodes are given in Fig. 3(c and d). These corroborated the uniform coating of polymer gel electrolyte on both the electrodes. Fig. 3(e–g) shows photographic images of a fabricated cable-type ASC and its flexibility behaviour. Fig. S1 (ESI†) shows SEM images of the PVA–KOH gel electrolyte-coated and uncoated AC and Ni(OH)2 electrodes in bending conditions. Some cracks can be observed on the AC electrode before the coating of the gel electrolyte,

XRD pattern of the (a) Ni(OH)2 and (b) AC (inset: Raman spectrum for the AC).


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Fig. 2

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SEM images of the (a) Ni(OH)2 and (b) AC, (c) N2 adsorption–desorption isotherm of the AC and (d) pore size distribution of the AC.

Scheme 1

Schematic representation of the fabrication of the cable-type ASC.

but this is rectified after the coating of the electrolyte. On the other hand, smooth surfaces are observed on both the uncoated and coated Ni(OH)2 electrode. Hence, it is strongly believed that coating of the PVA–KOH gel electrolyte maintains the electrode active material as more stable on the surface of the substrates. The thickness of the measured PVA–KOH polymer gel electrolyte coating is 180 mm and the distance between the two electrodes in the ASC is B520 mm (Fig. S2a, ESI†). The electrochemical properties of the individual electrodes and the fabricated ASC was studied using CV, galvanostatic charge–discharge (GCD), electrochemical impedance spectroscopy (EIS) and a cycle stability test.


Fig. 3 SEM images of the (a) AC and (b) Ni(OH)2 fiber electrodes. SEM images of the PVA–KOH gel electrolyte-coated (c) AC and (d) Ni(OH)2 electrodes. Photographic image of (e) the fabricated cable-type ASC and (f and g) its flexible states.


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Fig. 4 (a) CV curves for the Ni(OH)2 and AC fiber electrodes at 5 mV s1 and (b and c) CV and GCD curves for the cable-type ASC (Ni(OH)2/PVA/KOH/AC) with various potential ranges, at 5 mV s1 and 2 mA.

The electrochemical properties and working potential window of the prepared b-Ni(OH)2 and activated carbon fibre electrodes were electrochemically tested using cyclic voltammetry (CV) through a three-electrode system in 2 M KOH electrolyte at 5 mV s1. As shown in Fig. 4a, it was found that the b-Ni(OH)2 fibre electrode works at a positive potential, from 0 to 0.6 V (vs. Hg/HgO), and the redox peak indicates the occurrence of an electron charge transfer reaction between the hydroxyl ions (OH) and the b-Ni(OH)2, as given in eqn (1).16 In detail, the positive peak potential at 0.44 V reveals the oxidation process of Ni(II) to Ni(III) and the negative peak potential at 0.13 V represents the reduction process of Ni(III) to Ni(II). The activated carbon fibre electrode shows a rectangular CV curve within a potential range from 0 to 0.8 V (vs. Hg/HgO), inferring that it stores the charges by the simple electrosorption of electrolytic ions on the surface of the electrode, as given eqn (2).17a b-Ni(OH)2 + OH 2 NiOOH + H2O + e

(1)

AC + xH+ 2 ACJxH+

(2)

Hence, based on the observed potential window of the b-Ni(OH)2 and the activated carbon fibre electrodes, the ASC was fabricated with the cell voltage of 1.4 V. The specific capacitance of the fibre electrodes was calculated by integrating the area of the CV curve. Moreover, the calculated length capacitances of the activated carbon and b-Ni(OH)2 fibre electrodes are 0.1 F cm1 and 0.54 F cm1, respectively, at 5 mV s1. Additionally, the corresponding calculated gravimetric capacitances are 87 F g1 and 481 F g1 for the activated carbon and b-Ni(OH)2 fibre electrodes, respectively.


The recorded CV curve at 5 mV s1 is given in Fig. 4b with different cell voltages from 0.8 to 1.4 V. The current area of the CV curve increases with increasing voltage, indicating an increase in charge or energy storage. Also, no predominating oxygen evolution peak is observed, even at a high cell voltage of 1.4 V, signifying the better electrochemical properties of the cable-type ASC. As expected, the per-unit-length and gravimetric capacitances are increased from 14.3 to 47 mF cm1 and 13.3 to 43 F g1 at 5 mV s1, respectively, with an increase in cell voltage from 0.8 to 1.4 V. Similarly, GCD was also carried out at 2 mA with different cell voltages from 0.8 to 1.4 V (Fig. 4c), and no other overcharge curve was observed, which infers a better cell performance. The GCD result also shows an increase in charging and discharging time with an increasing cell voltage, thus indicating the increasing capacitance or energy performance of ASC. This result coincides with the CV result. Fig. 5a and b show the CV curves of the cable-type ASC at scan rates from 5 to 60 mV s1 and GCD curves at currents from 2 to 10 mV. No significant change is observed in the CV curves even at a high scan rate of 60 mV s1, which infers the better rate capability of the cable-type ASC. The obtained per-unitlength capacitance is 47 mF cm1 at a low scan rate of 5 mV s1 and 23.6 mF cm1 at a high scan rate of 60 mV s1, while the gravimetric capacitance is 43.5 F g1 at 5 mV s1 and 21.8 F g1 at 60 mV s1. To further substantiate the CV results, the perunit-length and gravimetric capacitances are calculated using GCD, i.e., 40.7 mF cm1 and 37.5 F g1 at 2 mA, respectively, and 25 mF cm1 and 23 F g1 at 10 mA. Fig. 5c and d show the GCD and CV curves of the cable-type ASC in normal and bending conditions, where it is noted that the observed GCD


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Fig. 5 (a) CV curves of the cable-type ASC at 5–60 mV s1, (b) GCD curves of the cable-type ASC at 2–10 mA, (c) GCD curve for normal and bending conditions at 2 mA, (d) CV curves for normal and bending conditions at 5 mV s1, (e) Ragone plot and (f) Nyquist impedance spectrum for the cable-type ASC.

and CV curves are almost overlapping without any significant distortion. It indicates their good electrochemical reliability even in flexible conditions. Besides, the fabricated cable-type ASC delivered a maximum length (gravimetric) energy density of 10.7 mW h cm1 (9.8 W h kg1) at a power density of 169 mW cm1 (154 W kg1) (Fig. 5e) and this value is higher than those of symmetric fiber/cable supercapacitors11e,17b and closer to planar supercapacitors.18–20 Moreover, the flexibility performance of the cable-type ASC was also tested in bending conditions. Fig. 5f displays the Nyquist impedance spectrum of the cable-type ASC. The obtained equivalent resistance (ESR) is B24 O, which mainly arises from contact resistance between the electrolyte and the electrode as well as the resistance of the current collector. In order to further validate the electrochemical performance, another cable-type ASC was fabricated with a separation between the electrodes of B1030 mm (Fig. S2b, ESI†), and the corresponding CV is given in Fig. S3 (ESI†). It can be seen that the measured current area is decreased with a increase in


distance between the electrodes. The calculated per-unitlength and gravimetric capacitances are also decreased from 47 to 40 mF cm1 and 43.5 to 36.2 F g1 at 5 mV s1, respectively, and this is may be due to the increasing ionic contact resistance of the electrolytes.11f Moreover, the cycling test of the supercapacitor is very important for practical applications. The cycling test was carried out at 5 mA (Fig. 6) and the fabricated cable-type ASC delivered 76% of the capacitance retention after 2000 cycles. Also, the function of the cable-type ASC was established for practical applications, using two serially connected cable-type ASCs. These cable-type ASCs successfully powered a red light emitting diode (LED), even in bending conditions (see Video, ESI†) as shown in Fig. 6b and c. This serial ASC could power the LED for more than one minute. In addition, Fig. 6d shows the brightness variation of the powered LED at different times. It is noted that the powered LED is initially very bright but this decreases slightly after 45 seconds. However, good brightness is observed even after a minute.


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Fig. 6 (a) Cycle life of the cable-type ACS at 5 mA, red LED powered by two charged serial cable-type ACSs in (b) normal and (c) bending conditions and (d) brightness variation of the LED with time.

Experimental Preparation and characterization of the plate-like b-Ni(OH)2 and the AC In a typical synthesis of b-Ni(OH)2, 3 g of Ni(NO3)26H2O was dissolved in 10 mL of distilled H2O and mixed with 20 mL of ethylene glycol under constant stirring. After 1 hour, ammonia solution was added drop-wise to increase the pH to 8 and the mixture was kept at 80 1C for a few hours under slow stirring. Then, the green precipitate was separated from the solution by centrifugation and washed with distilled H2O and ethanol. Finally, the washed product was dried at 70 1C. Similarly, the activated carbon (AC) was prepared from tamarind fruit shell using our previous method as reported.21 In short, the tamarind fruit shell was pulverized and preheated at 200 1C for 24 h. Then, the desired amount of preheated sample was activated in 50% KOH for 24 h. Following this, the activated sample was carbonized at a temperature of 700 1C for 4 h in an Ar atmosphere. The obtained sample was washed with distilled water and the desired amount of 1 M HCl, until the pH reached B7, and was finally dried at 100 1C overnight. The prepared samples of b-Ni(OH)2 and AC were subjected to various characterizations such as powder X-ray diffraction spectroscopy (XRD, X’Pert PRO X-ray DiffractometerPANalytical), Raman spectroscopy (Jobin-Yvon ISA T 64000), scanning electron microscopy (SEM) (Quanta 200 ESEM, FEI, USA) and BET surface area as well as pore size analysis. Gel electrolyte preparation The PVA–KOH polymer electrolyte was prepared as follows. 1 g of PVA was mixed with 20 mL of hot (70 1C) water with constant stirring, and was kept for 2 h to form a clear solution. Then, 10 mL of 2 M KOH was added and it was left to form a gel-like solution. Preparation of the fibre electrodes and fabrication of the cable-type asymmetric supercapacitor To prepare the b-Ni(OH)2 and AC fibre electrodes, Cu and stainless steel thread-like wires were used as current collectors with diameters of 0.62 mm and 0.50 mm, respectively. The slurry was prepared by mixing b-Ni(OH)2 or AC (24 mg), carbon black (3 mg)


and polyvinylidene fluoride (PVDF, 3 mg), then the slurry was coated on the Cu and stainless steel thread-like wires for 8 cm by individual brush coating and then drying at 80 1C for overnight. The mass of active material was 9 mg b-Ni(OH)2 on the Cu fibre and 7 mg AC on the stainless steel fibre. The cable-type ASC was fabricated using the following procedure. Firstly, the prepared fibre electrodes were coated with PVA–KOH gel electrolyte by a dip coating method and were dried at 70 1C for 2 hours. Subsequently, the electrolyte coated fibre electrodes were inserted into a rubber tube with an injection of PVA–KOH gel electrolyte and were subjected to drying at 70 1C for a few hours. Electrochemical studies The electrochemical properties of the b-Ni(OH)2 and AC fibre electrodes were studied by cyclic voltammetry using a threeelectrode system, with b-Ni(OH)2 or AC as the working electrode, a Hg/HgO electrode as the reference electrode and Pt wire as the counter electrode in 2 M KOH solution. In addition, the electrochemical properties of the cable-type ASC were studied via cyclic voltammetry at a potential of 1.4 V, at different scan rates from 5 to 60 mV s1; a galvanostatic charge–discharge test at a potential of 1.4 V, at different current densities from 2 to 10 mA; and electrochemical impedance spectroscopy (EIS) done at open circuit potential (OCV) by applying alternating current potential with a 10 mV amplitude, in a frequency range from 10 mHz to 1 MHz. Additionally, the cycle life of the cable-type ASC was tested at a current density of 5 mA for 1000 cycles. All of the above electrochemical studies were carried out at room temperature using a Bio-Logic SP150 electrochemical workstation. The fibre electrode capacitance was calculated from the CV curve using the following equation, C¼

Id SDV

(3)

where Id is the discharge current and DV is the range after ohmic potential drop. Next, balancing the mass of active material between the negative and positive electrodes was very important to equate the charges (q = q+; where, q and q+ are stored charges on the negative and positive electrodes, respectively)


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of the individual electrodes. In this regard, the mass ratio of positive to negative electrode was kept at 0.24, based on the theoretical mass ratio calculation given in eqn (4), mþ C þ DV ¼ m C DV

(4)

where m, m+, C, C+, DV and DV+ are the mass of active material, the gravimetric capacitance and the potential window for the positive and negative electrode respectively, obtained in the three-electrode system. Similarly, the capacitance of the cable-type ASC was calculated from the following equation, C¼

I ðDV=DtÞ

(5)

where I is the applied current and DV/Dt is the average slope value. Moreover, the per-unit-length or gravimetric specific capacitances of the electrodes and the cable-type ASC are obtained by dividing the capacitance C by the length (L) of the electrode, or by the total mass (M) of the active material; therefore CL = C/L or CM = C/M. The energy (E) and power (P) density were calculated using the following equations, Ð Ð I VðtÞdt I VðtÞdt and E ¼ (6) E¼ M L P¼

E Dt

(7)

Ð where I is the applied current density, V(t)dt is the integral area of discharge curve, Dt is the discharge time, L is the length of the electrode or device and M is the total mass of the active material.

Conclusions In summary, the very first attempt was made to fabricate a cable-type ASC for flexible electronic applications, using platelike b-Ni(OH)2 as the positive electrode and activated carbon as the negative electrode, together with PVA–KOH as the gel polymer electrolyte. Moreover, the fabricated cable-type ASC delivered per-unit-length (gravimetric) capacitances of 40.7 mF cm1 (37.5 F g1) at 2 mA and 25 mF cm1 (23 F g1) at 10 mA. Additionally, it exhibits a maximum per-unit-length (gravimetric) energy density of 10.7 mW h cm1 (9.8 W h kg1) at a power density of 169 mW cm1 (154 W kg1). Based on the present results, it is strongly believed that the cable-type ACS can be used for various flexible applications in the near future.

Acknowledgements The authors are grateful to the Department of Atomic EnergyBoard of Research in Nuclear Sciences (DAE–BRNS), Government of India (No. 2010/37P/46/BRNS/1443) for their financial support.

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