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Flexible Micro-Supercapacitors using Silk and Cotton Substrates Chayanika Das, and Kothandam Krishnamoorthy ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10431 • Publication Date (Web): 07 Oct 2016 Downloaded from http://pubs.acs.org on October 8, 2016

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ACS Applied Materials & Interfaces

Flexible Micro-Supercapacitors using Silk and Cotton Substrates Chayanika Das and Kothandam Krishnamoorthy* Polymers and Advanced Materials Laboratory, CSIR-National Chemical Laboratory, CSIRNetwork of Institutes for Solar Energy, Dr Homi Bhabha Road, Pune 411008, Maharashtra, India.

Flexible micro-supercapacitors are needed to power ultra-small wearable electronic devices. Silk cocoons comprise microfibers of silk, which are attractive natural resource to fabricate micro-supercapacitors (MSCs). These fibers are insulators, hence they must be converted to conducting surfaces. Polyphenols from green tea has been used as protective layer that also acted as a reducing agent to silver ion. The reduction of silver ion resulted in the formation of silver nanoparticles that subsequently reduced gold ions to gold. The gold film imparts conductivity to the silk fiber without affecting the mechanical strength of the silk fiber. The mechanical strength of uncoated silk fiber and gold coated silk fiber were found to be 5.2 GPa and 5 GPa, respectively. A pseudoecapacitive polymer, poly(3,4-ethylenedioxythiophene), was used as active material to fabricate MSCs. The MSCs showed an impressive gravimetric capacitance of 500 F/g and areal capacitance of 62 mF/cm2. The power and energy densities were calculated to be 2458 W/kg and 44, Wh/kg respectively. The device was coiled on a cylinder and the performance of the device was found to be same as that of the uncoiled device.

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In order to demonstrate that the approach is not specific to silk, we also coated gold on cotton fibers using the protocol used to coat gold on silk. Coiled and uncoiled supercapacitors were fabricated using PEDOT coated cotton fibers. The gravimetric capacitance was found to be 250 F/g with energy and power densities of 5.5 Wh/kg and 1118 W/kg, respectively. We have also demonstrated that the devices can be connected in parallel and series to improve the miniaturized devices.

1. INTRODUCTION Micro-supercapacitors (MSCs) are required to power miniaturized electronic devices.1-7 Towards this objective, polymers and its composites were deposited on microelectrodes to fabricate MSCs.8-13 Flexible MSCs are desired for the fabrication of wearable electronics.14-19 Furthermore, a method devoid of micro patterning facilitates easier and cost effective fabrication of MSCs. In fact, MSCs with the shape of threads are desirable for easy integration with flexible clothes.20-28 Towards this objective, biscrolled yarns comprising carbon nanotubes and conjugated

polymers

were

prepared.29,30

Niobium

yarns

coated

with

poly(3,4-

ethylenedioxythiophene) showed a volumetric capacitance of 158 F/cm3.31 Polyaniline is a well known pseudocapacitive material, which upon coating on top of CNT yarn showed areal capacitance of 38 mF/cm2.32 Often, carbon fibers are used as current collector and base substrate to fabricate yarn supercapacitors.20-22,24,33-41 In these experiments, carbon nanotubes are converted to its yarns using variety of simple and involved approaches. Silk fibers (SFs) that form cocoons are few tens of microns in diameter and few centimeter in length.42 These fibers are mechanically very strong.42-44 The silk is naturally available in abundance. Thus, SFs could be an useful substrate for the fabrication of MSCs.


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However, the SFs are insulators. Deposition of metals to impart conductivity by conventional approaches affect the mechanical stability of SFs.45-47 We have used polyphenols from green tea as protective as well as reductive coating to prepare gold film on top of SFs.47 One of the very stable and commercially exploited conjugated polymers, poly(3,4-ethylenedioxythiophene) (PEDOT), was used as pseudocapacitive material.48-51

The MSCs showed highest specific

capacitance of 500 F/g, areal capacitance of 62 mF/cm2 and volumetric capacitance of 120 F/cm3. Coiled devices were also prepared to demonstrate the suitability of MSCs as flexible devices. The performance of the coiled devices was same as that of uncoiled devices. To demonstrate that the diameter is not restricted to microns and the substrate doesn't have to be single fiber like SFs, we used cotton threads as base to coat gold. Cotton threads comprise bundle of smaller threads and they are insulators.52 The polyphenol based gold coating was used to prepare gold coated cotton threads (GCTs). These threads were used for the deposition of PEDOT. While using GCTs as substrate and PEDOT as active material, a specific capacitance of 250 F/g was observed. The best energy and power densities were found to be 5.5 Wh/kg and 1118 W/kg. The performances of coiled devices are comparable to that of uncoiled devices, which indicates that the charge storage properties are unaffected upon bending the devices. Carbon substrates were prepared using cotton as source. On those substrates, inorganic charge storage materials such as Co-Al layered double hydroxides were deposited.53 The carbon prepared from cotton was also used to deposit CoO and NiO core shell nano structures.54 These devices exhibited an impressive capacitance of 150 F/g. Using the same substrate and hierarchical core-shell nanostructure of NiCo2O4, a supercapacitor with capacitance of 1929 F/g (based on the weight of NiCo2O4) was fabricated.55 Interestingly, the ternary oxide based


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Figure 1 a) CV of 1 mM ferrocenemethanol recorded using GSF as working electrode. The inset shows the linear plot of square root of scan rate vs peak current. b) Stress-strain curve for GSF and PEDOT@GSFs c) SEM image showing the morphology of PEDOT@GSF_15s. d) Enlarged SEM image showing the morphology of PEDOT in PEDOT@GSF_10s. supercapacitor was prepared on porous graphene paper, the capacitance was found to be 1768 F/g.56 Commercially available carbon microfibers have been used to deposit Zn2SnO4/MnO2 core-shell nanostructure to fabricate supercapacitors with capacitance of 621 F/g based on MnO2 weight.57 To avoid the high temperature carbonization process to convert cotton to conducting carbon substrate, the approach reported in this report is an alternate. Furthermore, the silk could also be used as a carbon source. Silk comprises fibroin protein, hence carbonization of silk will provide doped carbon.58 This is an added advantage.


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2. RESULTS AND DISCUSSION Silk cocoons were used as a source of SFs. The SFs of defined length was dipped in green tea extract to obtain a colourless coating of polyphenol. The polyphenol is capable of reducing silver ions to silver.59,60 Thus, silver nanoparticles could be easily synthesized by immersing the polyphenol coated SFs in ammoniacal silver nitrate solution. This process results in the formation of silver nanopartcile decorated SFs. Silver nanoparticles can reduce gold ions due to the commensurate electrochemical potential. Thus, further immersion of silver nanoparticle coated SFs in gold plating solution results in gold coated SFs (GSFs). The gold coating was confirmed by EDAX analysis as well as SEM imaging (Figure S1). The conductivity of the GSF was measured and it was found to be 2 x 103 S/cm. The GSFs were used as working electrode to record cyclicvoltammograms (CVs) and study the electroactivity of the surface. CVs were recorded in 1 mM ferrocenemethanol using 0.1 M LiClO4 as supporting electrolyte. The well defined redox peaks of ferrocenemethanol indicate that the GSFs are conducting. The CVs were recorded at various scan rates and the plot of square root of scan rate vs current was found Table 1. Length, areal, volumetric and gravimetric capacitances of uncoiled and coiled PEDOT@GSFs-10 devices. Current density

Uncoiled device

Coiled device

Cs

Cl

Cvol

Careal

Cs

Cl

Cvol

Careal

(F/g)

(mF/cm)

(F/cm3)

(mF/cm2)

(F/g)

(mF/cm)

(F/cm3)

(mF/cm2)

0.0012

500

0.6

120

62

500

0.6

120

62

0.002

333

0.3

80

41

333

0.3

80

41

0.004

214

0.3

51

26

230

0.3

55

28

0.005

182

0.2

43

22

200

0.25

48

24

0.006

161

0.2

38

20

166

0.2

40

21

(mA/cm)


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to be linear (Figure 1a). This indicates the redox species don't adhere on the surface of the GSFs. These experiments encourage us to use GSFs as substrate to fabricate MSCs. From our previous experiments, we are aware of the oxidation potential (1.3 V vs Ag/AgCl) of 3,4ethylenedioxythiophene.46,47 Therefore, PEDOT was coated on top of GSFs by applying a constant potential of 1.3 V vs Ag/AgCl in EDOT monomer solution. PEDOT coated GSFs were prepared at three polymerization times, (i) 5 s, (ii) 10 s and (iii) 15 s. This variation permits us to identify the optimum PEDOT thickness to achieve best MSCs performance. PEDOT coated GSFs prepared at various polymerization will be mentioned as PEDOT@GSFs-5 (5 s), PEDOT@GSFs-10 (10 s) and PEDOT@GSFs-15 (15 s). Together, they will be mentioned as PEDOT@GSFs. Before fabrication of MSCs, the mechanical stability of the SFs, GSFs and PEDOT@GSFs were studied using tensile measurements (Figure 1b). The Youngs modulus of SFs was found to be 5.2 GPa. This decreased marginally upon coating gold (5.0 GPa) and further decreased to 4.7 GPa for PEDOT@GSFs-5. The lowest Young's modulus of 3.5 GPa was measured for PEDOT@GSFs-15. Thus, the gold coating and thinner PEDOT coatings don't affect the mechanical strength of SFs. However, thicker PEDOT coating does affect the mechanical strength. Despite the decrease in mechanical strength, we are able to fabricate MSCs using PEDOT@GSFs-15. The SEM images of the GSFs and PEDOT@GSFs (Figure 1c and Figure S2) were recorded to study the morphology of surfaces. The PEDOT coating impart roughness to the surface of GSFs. PEDOT films usually have polydisperse spheres on the surface.61 Similar morphology is observed for the PEDOT@GSFs (Figure 1d).

The redox

characteristics of PEDOT@GSFs were studied using cyclicvoltammetry (CV). The potential was swept between 0 and 0.8 V vs Ag/AgCl, while using the PEDOT@GSFs as working electrode. The CVs didn't show Faradaic peaks (Fig S5 a-c). From the box type CVs, areal capacitance


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Figure 2 a) Overlay of charge-discharge profiles of PEDOT@GSFs in 1 M LiClO4 at 1 A/g. b) Nyquist plot of PEDOT@GSFs showing high frequency intercept. c) CCD of PEDOT@GSF_10s recorded in gel electrolyte, d) CV recorded in gel electrolyte using PEDOT@GSF_10s at 5 mV/s and e) Ragone plot for uncoiled and coiled devices based on PEDOT@GSFs-10. f) Plot showing the variation in specific capacitance as a function of length of the PEDOT@GSFs-10 device. could be calculated, which was found to be 37 mF/cm2 (PEDOT@GSFs-5), 53 mF/cm2 (PEDOT@GSFs-10) and 93 mF/cm2 (PEDOT@GSFs-15) at 5 mV/s scan rate. The capacitance was found to increase linearly as a function of polymerization time. With this information in hand, the PEDOT@GSFs were used as electrode in a MSCs. In the first set of experiments, PEDOT@GSFs were used as working electrode with GSFs as counter electrode. The electrolyte was 1 M LiClO4. The charge discharge experiments were carried out between 0 and 0.8 V at various current densities (Figure S5d-f). PEDOT@GSFs-5 showed symmetrical charge discharge profiles. On the other hand, PEDOT@GSFs-15 showed unsymmetrical charge discharge profile,


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which indicates increased IR drop. This is due to the poor conductivity of the thick PEDOT film. Furthermore, the 15 s polymerization results in the formation of uneven and discontinuous film (Figure 1c), which also contributes to the increased resistance. In Figure 2a, the charge discharge profiles of PEDOT@GSFs-5, PEDOT@GSFs-10 and PEDOT@GSFs-15 at 1 A/g is shown. The highest specific capacitance was measured for PEDOT@GSFs-10 (500 F/g) at a current density of 1 A/g (Table S2). The impedance spectra of PEDOT@GSFs were recorded using the experimental conditions that are used for capacitance measurements. A linear line in the nyquist plot indicates ion transport (Figure 2b). The resistance was calculated from high frequency intercept and it was found to be 0.2 Ω (PEDOT@GSFs-5), 0.3 Ω (PEDOT@GSFs-10) and 0.4 Ω (PEDOT@GSFs-15). Thus, the resistance related to ion transport is not significantly different among the PEDOT@GSFs. Among the three PEDOT@GSFs, PEDOT@GSFs-10 is the best performing electrode. Therefore, we carried out all the forthcoming experiments using this electrode. Furthermore, gel electrolyte comprising poly(vinyl alcohol) and 1M LiClO4 was used in all the forthcoming experiments. The MSCs were fabricated by immersing PEDOT@GSFs-10 and GSFs in gel electrolyte, which were then scrolled to form the device. Cyclicvoltammograms of the device were recorded at various scan rates (Fig S7a). The device metrics are provided in table S4. At a scan rate of 5 mV/s, the areal and volumetric capacitances are 51 mF/cm2 and 128 F/cm3, respectively. The volumetric capacitance is calculated by taking the whole electrode comprising gold coated silk fiber as well as PEDOT into account. These values decreased upon increase in scan rate. The charge discharge experiments were carried out using the MSCs between 0 and 0.8 V (Figure 2c). The charge discharge curves were symmetric triangles. The slope due to IR drop could be observed during the discharge cycle. The specific capacitance was calculated based on the length of the PEDOT@GSFs-10, which was found to be 0.6 mF/cm


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Figure 3 a) SEM image of GCTs. b) Charge discharge profiles of PEDOT@GCTs-10 recorded in gel electrolyte at 1 A/g. c) Stability study showing the variation if specific capacitance of PEDOT@GCTs-10 based coiled and uncoiled devices as a function of number of chare discharge cycles. d) Ragone plot showing the change in energy and power density of PEDOT@GCTs-10 for uncoiled and coiled devices. e) The CV of three PEDOT@GCTs-10 based devices connected in parallel. f) The charge discharge profile of three PEDOT@GCTs10 based devices connected in series. while discharging at 0.001 mA/cm (1 A/g). The capacitance decreased to 0.2 mF/cm while discharging at 0.006 mA/cm (5 A/g). Upon increase in discharge current densities, the specific capacitances have been found to decrease. We also calculated areal and volumetric capacitances which are tabulated in table 1. Often, flexible devices are fabricated using conjugated polymers. The device measurements have been carried out using uncoiled devices. In order to test the devices' flexibility, coiled MSCs were prepared. For this purpose, 10 cm MSCs were coiled around a plastic cylinder. Then, the performance of the device was measured. The


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cyclicvoltammograms (Figure S7b) were found to be devoid of any Faradaic peaks. From the box type CVs, the areal and volumetric capacitance was calculated. At a scan rate of 5 mV/s, areal and volumetric capacitance was found to be 51 mF/cm2 and 127 F/cm3, respectively. These values are same as that observed for uncoiled devices (Figure 2d). Indeed, the areal and volumetric capacitances are same for coiled and uncoiled devices irrespective of scan rate (Table S4). Thus, the coiling doesn't affect the performance of MSCs. With this information in hand, we proceeded to study the charge discharge profiles of the coiled MSCs. The charge discharge experiments were carried out between 0 and 0.8 V. The charge discharge curves were not perfectly symmetric with IR drop during the discharge cycle (Figure S7d). The volumetric and areal capacitance of the device was found to be 120 F/cm3 and 62 mF/cm2, respectively, while discharging at 0.001 mA/cm (1 A/g). This is exactly same as that of uncoiled device. The coiled device showed comparable device performance with that of uncoiled device as a function of discharge current density (Table 1). Furthermore, we studied the device metrics as a function of various bending angles. The devices were placed on a protector and the bending was carried out based on the angles marked in the protector (Figure S8). The device performance was studied while the GSFs based devices were bent at 30°, 60°, 90°, 120° and 150° (Figure S9). The lowest capacitance was observed for the device bent at 150° (495 F/g while discharging at 1A/g), which is 1% less than that of the unbent device (0°).In fact, the capacitance of most bent devices are equal to that of the unbent device (Table S5). We attribute this impressive performance to the conductivity imparted to the SFs by gold. Please note that the gold coating is possible on SFs using polyphenols extracted from green tea. The stability of the uncoiled and coiled devices were studied over 10000 cycles (Figure S7f). It is gratifying to note that the capacitance decrease was only 15% for both coiled and uncoiled devices. The device's capacitance was monitored while


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charge-discharge experiment was carried out using a device that was bent at 90° and 0° subsequently for 100 cycles. The charge-discharge curves at both these bending angles were overlapping irrespective of the cycle number indicating that the capacitance didn't vary significantly (Figure S10). In fact, at 90° bending, there was only 0.6 % variation in capacitance after 100 cycles. This is due to multiple factors including the stability and conductivity of the GSFs and PEDOT. The Ragone plot that compiles the energy and power density of a energy device is shown in Figure 2e. Highest energy density of 44 Wh/kg and power density of 2667 W/kg was calculated for the silk fiber based uncoiled devices. The energy and power density didn't vary significantly upon coiling (Figure 2e). The other factor that is likely to affect the performance of MSCs is length of the device. The performances of the devices were measured by varying the length of the device between 2 cm and 10 cm. The capacitance as a function of length was found to be 0.9 mF/cm for 2 cm device, which decreased to 0.6 mF/cm for 10 cm device, while discharging at 0.001 mA/cm (Figure 2f). A decrease of 34% in capacitance for a 80% increase in length is small. Thus, the device performs well despite increase in length. Another advantage of GSFs is their softness. Many materials used in charge storage applications undergo volume change.62 Gold being malleable and ductile and silk being soft, the substrate in this report can accommodate volume change of the active materials. Cotton threads were coated with gold by following the procedure used for SFs (SEM image shown in Figure 3a). The conductivity of the gold coated cotton (GCT) was found to be 1.6 x 103 S/cm. PEDOT was coated by applying a constant potential of 1.3 V vs Ag/AgCl for 10 s (SEM image shown in Figure S11b). This electrode will be mentioned as PEDOT@GCTs-10. The duration of the polymerization was chosen based on our previous experiments using GSFs as substrate. For device fabrication, the GCTs were used as counter electrode. The electrolyte


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comprises of poly(vinyl alcohol) and 1 M LiClO4. The cyclicvoltammograms of the device showed no Faradaic peaks in the potential window of 0 and 0.8 (Fig. S11d). From the box type CV, the capacitance as a function of length was calculated to be 0.82 mF/cm while sweeping the potential at 5 mV/s. This value decreased to 0.63 mF/cm upon increasing the scan rate to 100 mV/s. The charge discharge experiment was performed between 0 and 0.8 V at various current densities (Figure S11e). The charging curves were not linear but the discharge curves were linear with IR drop. The specific capacitance was found to be 250 F/g while discharging at 1 A/g (Figure 3b). This decreased to 166 F/g upon increase in discharge current density to 10 A/g. To study the device performance upon coiling, the devices were coiled around a plastic cylinder. The length of the device was 10 cm. The device showed a specific capacitance of 250 F/g while discharging at 1 A/g. This decreased to 153 F/g upon increase in discharge current density to 10 A/g. At higher current densities, the specific capacitance decreased by ~40%. The decrease is likely due to the bundle of threads, which has several twisting points. At these places, the ions transport is hindered at higher current densities. This issue is not present in SFs, hence the device performance doesn't vary as a function of coiling. We also studied the GCTs based device performance as a function of different bend angles. The device metrics were measured at 30°, 60°, 90°, 120° and 150° (Figure S12). The lowest capacitance was measured for a device bent at 150° (237 F/g while discharging at 1 A/g). This capacitance is 5% less that of the unbent device. As was found in case of GSFs based devices, the capacitance of many bent devices are same as that of the unbent devices (Table S7). The coiled and uncoiled device's stability was studied by monitoring the specific capacitance as a function of cycle number. The specific capacitance decreased by 20% after 10000 cycles (Figure 3c). The variation in energy and power density for coiled and uncoiled devices are shown in Ragone plot (Figure 3d). Highest energy and power


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densities of 5.5 Wh/kg and 1118 W/kg, respectively were observed for uncoiled PEDOT@GCTs-10. We also studied the device's stability while the device was bent at a angle of 90° (Figure S13). The capacitance didn't vary significantly (~3 %) as a function of bending angle and cycle number. The PEDOT@GCTs-10 based devices were connected in series and parallel. While the devices are connected in parallel, the current is expected to increase due to addition of current as a function of number of devices. Indeed, we do observe linear increase in current while connecting two and three devices. A single device showed a current of 0.7 mA in CV which increased to 2 mA on connecting three such devices in parallel. On the other hand, the potential will increase upon connecting the devices in series. This was demonstrated by recording charge discharge curves. The voltage was found to increase as function of number of devices. While a single device could be charged to 0.8 V, three devices when connected in series could be charged to 2.4 V. 3. CONCLUSIONS In summary, we have demonstrated the feasibility of coating gold on top of silk and cotton fibers. The coating process doesn't affect the mechanical stability of the naturally occurring fibers. The conducting fibers were used to electrochemically deposit PEDOT, which was used as active material to fabricate MSCs. The MSCs showed an impressive capacitance of 500 F/g and areal capacitance of 62 mF/cm2. It should be noted that the performance of the MSCs were not altered upon coiling of the devices. The cotton based supercapacitors showed a slightly decreased performance due to weaving of several fibers in the thread. The movement of ions as well as gold coating in the twisting points have contributed to the decrease in the performance of the devices. Despite these deficiencies, the device showed energy and power


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densities of 5.5 Wh/kg and 1118 W/kg, respectively. The devices were connected in parallel and series to improve the overall performance of the devices.

ASSOCIATED CONTENT Supporting Information. Experimental section, figures and tables are provided. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Kothandam Krishnamoorthy [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Council of Scientific and Industrial Research (TAPSUN -NWP 54) ACKNOWLEDGMENT KK acknowledges the funding from CSIR through TAPSUN (NWP 54) project. CD thanks UGC india for the fellowship. REFERENCES


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Table of Contents

Twisting G

GSF

PEDOT@GSFs-10

Device

Silk and cotton fibers are coated with gold by electroless gold coating procedure using polyphenols from green tea as a protective coating. PEDOT has been used as pseudocapacitive material to fabricate micro-supercapacitors with impressive efficiencies.


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Flexible Microsupercapacitors Using Silk and ... - ACS Publications

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