Cotton fabrics coated with lignosulfonate-doped polypyrrole for flexible supercapacitor electrodes

Abstract

Polypyrrole/lignosulfonate (PPy/LGS) coated cotton fabrics have been prepared via in situ oxidation polymerization of pyrrole in the presence of lignosulfonate as both template and dopant. The mass loading on the fabric samples decreases dramatically with the increased LGS content. The electrical conductivity of the coated fabrics achieved 3.03 S cm⁻¹ under the optimized conditions. The electrochemical properties of the coated fabrics were investigated using cyclic voltammetry and galvanostatic charge–discharge measurements. The specific capacitance of the coated fabrics can be as high as 304 F g⁻¹ at a current density of 0.1 A g⁻¹ in aqueous electrolyte. These novel fabrics are desirable for applications in wearable supercapacitors.

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1. Introcduction

The high proliferation of portable electronic devices in various fields, such as flexible electronics and wearable/stretchable electronics, has promoted the increasing demand for cheap, flexible, and lightweight energy storage devices (such as batteries and supercapacitors).^1–7 As a result of these demands, great efforts have been dedicated to develop new flexible electrode materials as alternatives to the materials used in contemporary batteries and supercapacitors.^4,5

Cotton fabric is a flexible and porous material made by weaving natural cotton fibers, which have a hierarchical structure with complicated surface morphology, functional groups such as hydroxyl groups, and high porosity.⁸ For example, each cotton fiber is comprised of multiple individual fibrils, which are in turn composed of multiple microfibrils bundled together. The microfibrils are made of poly-D-glucose chains, generally arranged in crystalline or partially crystalline domains. This structure allows the fibers to absorb large amounts of polar chemicals, causing the fibers to swell when placed in solutions. Cotton fabric can be used in flexible electrodes to increase the surface area of the active material and reduce the weight of the substrate.⁹ Recently, carbon materials such as carbon nanotubes (CNTs) and graphene have been integrated into fabrics to prepare flexible and lightweight electrodes for batteries and supercapacitors.^8,10–13 Pasta et al. fabricated a flexible and lightweight fabric supercapacitor electrode by impregnating single-walled CNTs into cotton fabric. The resulting electrode achieved a high specific capacitance of ∼70–80 F g⁻¹ at 0.1 A g⁻¹.¹² Liu et al. reported a flexible and easily processed electrode via a simple “brush-coating and drying” process using cotton fabric as the platform and a stable graphene oxide (GO) suspension as the ink, which exhibited a high specific capacitance of 81.7 F g⁻¹ in aqueous electrolyte.¹³ Despite their attractive electrochemical performance, the relatively high price of CNTs and elaborate preparation procedures of graphene would more or less limit their further application in large-scale fabrication.

Conducting polymers (CPs) are particularly interesting materials for flexible electrodes due to their relatively high theoretical capacities, inherent fast redox switching, good conductivity, mechanical flexibility, and low weight.^14–16 A wide variety of CPs, such as polypyrrole (PPy), polyaniline (PAn), polythiophene (PT), polyethylenedioxythiophene (PEDOT) and its derivatives, have been investigated as electrode materials in supercapacitors and batteries.^14–16 CPs can store energy by accumulating/releasing counter charges under the electric field. These materials show higher conductivity with doping, thus it might largely simplify the structure and reduce the cost of power sources if these materials were directly coated onto flexible substrates and served as both electrodes and current collectors.¹⁷ However, one main problem for CPs-based devices is their low capacitance due to the low surface area of the polymer layers and consequently the mass transport limitations. Hence, depositing nanoscale CPs in the form of a thin film on large surface area substrates has been widely used to render CPs with the preferred capacitance.^18–22 Mihranyan et al. developed a porous electrode material based on a chemically deposited thin (∼50 nm) layer of PPy coated on a large surface area substrate of dispersed nanofibrous cellulose from the Cladophora algae, which could be used in paper-based supercapacitors with charge capacities of 33 mA h g⁻¹.² Yue et al. demonstrated the performance of PPy-coated nylon lycra fabric electrode in supercapacitor application and studied the electrochemical performance while stretching.¹⁸ Recently, our group prepared a textile-based electrode material by chemically depositing PPy nanoparticles on cotton fibers, with mixed anionic and cationic surfactants as soft template.²¹

A variety of anionic dopants have been used to afford CPs with high conductivity, including Cl⁻, ClO₄⁻, BF₄⁻, p-toluenesulfonate, dodecylbenzenesulfonate, and polystyrene sulfonate.^22,23 Natural polymeric acids, the derivatives of the natural polymers such as lignin²⁴ and cellulose,²⁵ have attracted more and more attention as the dopants and stabilizers of CPs due to their biocompatibility and biodegradation. Lignosulfonate (LGS) (Fig. 1), a cheap byproduct produced by sulfonation or sulfomethylation of the lignin (the second most abundant naturally occurring biopolymer) from pulp processing industries, has become increasingly valuable for its versatility in many fields.²⁴ Until now, there have been several reports on the preparation of PPy with LGS as both polymeric template and dopant.^24,26–28 Only very few works reported just few electrochemical properties of PPy doped with LGS in an open cell configuration with aqueous electrolyte for battery/supercapacitor application.²⁹

Fig. 1 Typical monomer unit of LGS.

In this work, the fabrication and electrochemical characterization of cotton fabrics coated with LGS-doped PPy were reported. The possibility of directly using the coated fabrics as supercapacitor electrode materials (without the need for any metal substrate support) were examined, with a view towards further research on totally flexible supercapacitors.

2. Experimental

2.1 Preparation of PPy-coated cotton fabrics

The plain cotton fabrics were washed with aqueous solution of scouring agent and Na₂CO₃ for 1 h to remove grease and other impurities attached on them, followed by rinsing with deionized water till neural pH and drying in air. Deposition of PPy on the fabrics was carried out by in situ chemical oxidation polymerization of pyrrole (Py) in the presence of fabrics, following the method described in the previous report.²¹ The fabric samples (∼0.42 g) were immersed in 100 ml of aqueous solution containing Py and sodium lignosulfonate (Table 1), and stirred at 5 °C for 1 h. Then, 100 ml of 0.5 M FeCl₃ aqueous solution was added dropwise to initiate polymerization. The reaction was performed at 5 °C for 2 h. The resulting fabrics were washed with water and dried at room temperature. In this way, a series of PPy-coated fabrics were obtained, which were labeled as S0, S1, S2, S3, S4 and S5 (Table 1), respectively.

Table 1 Mass loading and electrical conductivity of the coated fabrics obtained with different LGS concentration

No.	Pyrrole (ml)	LGS (g)	Mass loading (wt%)	Conductivity (S cm^−1)
S-0	6.9	0	51.9	1.15
S-1	6.9	0.1	26.6	3.03
S-2	6.9	0.5	7.0	1.05
S-3	6.9	1.0	2.0	0.59
S-4	6.9	1.5	0.7	0.05
S-5	6.9	2.0	0.2	0.05

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2.2 Characterization of PPy-coated cotton fabrics

Mass loading was determined by weighing the coated fabric, and then subtracting the mass of substrate. Surface conductivity was measured by a four-point probe resistivity system (RTS-9, 4Probes Tech. Co., China) with copper electrodes and a 5 N pressure at 20 °C and 65%. Fourier transform infrared (FTIR) spectra were recorded using a TGA-FTIR spectrometer (Tensor 27, Bruker, Germany) in the range from 4000–600 cm⁻¹ with 8 scans and a resolution of 2 cm⁻¹. Morphologies of the samples were examined by scanning electron microscopy (SEM, JSM-6510LV, JEOL, Japan) at various magnifications. Double-sided conductive carbon tape was used to attach the samples to the microscope stage. The samples were sputtered with gold to get good electrical contact and avoid charging. The composition of the samples was determined by Energy Dispersion Microanalysis (EDS), using an X-Max 50 detector from Oxford Instruments.

2.3 Electrochemical properties of PPy-coated cotton fabrics

Electrochemical measurements were carried out at room temperature using Autolab PGSTAT302N (Metrohm AG, Switzerland). Cyclic voltammograms were recorded from −0.3 to +0.5 V at various rates. Electrochemical impedance spectra (EIS) were measured in the frequency range from 0.01 Hz to 100 kHz, with AC amplitude of 10 mV. For the test in a three-electrode configuration, the fabric samples with a 0.5 cm² of area were used as the working electrode; a platinum foil and a KCl saturated calomel electrode (SCE, CH Instrument, China) were used as reference and counter electrodes, respectively.

Galvanostatic charge–discharge cycling measurements were carried out using a multichannel battery-testing device (LAND CT2001A, Wuhan Landian Electronic Co., China) in a two-electrode setup. The electrodes were charged and discharged automatically according to the pre-set cut-off potentials at various current densities. The mass of the active material was determined by weight measurements before and after PPy coating.

All the measurements were performed in 2.0 M aqueous solution of NaCl.

3. Results and discussion

The mass loading and electrical conductivity of the fabrics as a function of LGS contents are summarized in Table 1. The mass loading decreased exponentially with the increased LGS content, where only a mass loading of 0.2 wt% was obtained for the LGS content of 2.0 g. It was well-known that the large amount of hydroxyl groups of cellulose could interact with the imine groups of pyrrole to form hydrogen bonds, which ensure the compact distribution of pyrrole on the surface of cellulose fibers. However, LGS would interact with pyrrole through the formation of hydrogen bonds between the pyrrole imine groups and the carbonyls or hydroxyl groups present in the LGS, as suggested by Yang and Liu,²⁴ which might depress the adsorption of PPy on the cotton fibers. Thus, the mass loading decreased with an increase of the LGS feed ratio. On the other hand, the conductivity of the fabrics increased from 1.15 to 3.03 S cm⁻¹ with the introduction of LGS, because LGS could play the role of dopant as well as template. However, the conductivity of the fabrics decreased with the increased exorbitant amounts of LGS, mainly due to the dramatically decreased PPy adsorbed on the fabrics and the long chain characteristics of LGS possibly preventing a number of sulfate anions in LGS from doping the PPy moiety.²⁴

The SEM morphologies of the coated fabrics prepared with different LGS contents are shown in Fig. 2. The surface of the uncoated fabric was very clean and smooth (as shown in the inset of Fig. 2a). In the case of the PPy-coated fabric (S-0), the PPy coatings were quite loose and aggregated into lumps with diameters ranging from about 300 nm to 1 μm on the fiber and fabric surfaces (Fig. 2a and b). For the coated fabric (S-1) prepared with 0.1 g LGS, it was clearly found that the PPy loading on the fabric was decreased (Fig. 2c). Meanwhile, the PPy coatings became looser than those for S-0, and were well formed as a three-dimensional network of small conducting granular particles (Fig. 2d) due to the template effect of LGS. The porous structure of the S-1 fabric would maximize the electroactive surface area and facilitate ion transport, possibly in favor of the electrochemical performance of PPy. However, the PPy coatings which were composed of smaller particles became relatively compact when the LGS content increased to 0.5 g (Fig. 2f). For the fabrics prepared with even higher LGS content, the smooth surface similar to that of the uncoated fabric was observed (Fig. 2g and h) due to the very low PPy loading on the fabrics.

Fig. 2 SEM images of the coated fabrics. (a) and (b): S-0; (c) and (d): S-1; (e) and (f): S-2; (g): S-3; and (h): S-4. The inset in (a) is of the uncoated fabric.

FTIR spectroscopy in combination with attenuated total reflectance (ATR) has become an extremely powerful tool for surface characterization of materials. Fig. 3 shows the FTIR spectra of the PPy-coated fabric (S-0), PPy/LGS-coated fabric (S-1), pure LGS, and cotton fabric. The spectrum related to the uncoated cotton fabric showed strong absorption bands at 1055 and 1029 cm⁻¹, as a result of the overlapping bands attributed to functional groups of cellulose, namely the C–C, C–O and C–O–C stretching vibrations. The spectrum for the PPy-coated fabric (S-0) was characterized by the typical features of PPy.^30,31 In the range of 4000–1800 cm⁻¹ there was the characteristic tail of the electronic absorption³² associated with the electrical conductivity of PPy³⁰ (only the beginning of this band is shown in Fig. 3). The broad bands at 1526 cm⁻¹ and 1454 cm⁻¹ were assigned to the C–C and C–N stretching vibrations in pyrrole ring, respectively. The band at 1302 cm⁻¹ was attributed to the C–H and C–N in-plane deformation modes. The peak at 1161 cm⁻¹ was due to the breathing vibration of the pyrrole ring. The weak peak at 1092 cm⁻¹ was assigned to the in-plane deformation vibration of NH⁺ groups which are formed in the PPy chains by protonation.³⁰ The band of N–H in-plane deformation vibration was situated at 1026 cm⁻¹. Finally, the peak at 789 cm⁻¹ due to the C–H out-of-plane ring deformation was observed. The broad intense bands from 1200 to 900 cm⁻¹ attributed to cellulose appeared attenuated in the spectrum of the PPy-coated fabric, revealing that the fibers are covered by the coating. For pure LGS,³³ the strong peak at 1589 cm⁻¹ originated from the C=C aromatic skeletal vibration. The bands at 1414 cm⁻¹, 1198 cm⁻¹ and 1123 cm⁻¹ were assigned to the C–O stretching vibration. The S=O stretching at 1045 cm⁻¹ also appeared. In the case of PPy/LGS-coated fabric (S-1), the changes in the typical features of PPy represented the doping state of PPy by LGS.

Fig. 3 FTIR spectra of the PPy-coated fabric (S-0), PPy/LGS-coated fabric (S-1), and LGS.

The microstructural composition of the PPy/LGS-coated fabric (S-1) was analyzed by the EDS spectrum as shown in Fig. 4. The S element was found in the aggregation region of granular particles (Fig. 4a), further confirming the LGS species in the coating. However, there was no S element on the fiber surface (Fig. 4b), indicating that LGS was mainly interacted with PPy through the formation of hydrogen bonds, and the PPy/LGS composite aggregated on the fiber.

Fig. 4 EDS study of the PPy/LGS-coated fabric (S-1).

EIS measurements were carried out in order to further compare the charge transfer performance of the fabric electrodes prepared with different LGS concentration. The Nyquist plots recorded for the fabric electrodes at an equilibrium open circuit potential are shown in Fig. 5. In general, the diameter of the compressed semicircle at the high to medium frequency region is related to the charge transfer resistance (R_ct) in the electrochemical system. The smaller the diameter of the semicircle, the smaller the charge transfer resistance and the higher the conductivity.²² From comparing the diameters of the semicircles, the charge transfer resistance of the S-1 fabric electrode was significantly lower than those of the other fabrics, which were ranked in the order of S-1 < S-0 < S-2 < S-3. This result was consistent with the conductivity values for these coated fabrics as shown in Table 1, further confirming the influence of LGS on the conductivity of PPy-coated fabrics.

Fig. 5 Nyquist plots of the fabric electrodes in 2.0 M NaCl. The area of each electrode is 0.5 cm².

To investigate the electrochemical behavior of the coated fabric, cyclic voltammetry (CV) curves were recorded at different scan rates for the S-1 fabric electrodes. The S-1 fabric showed stable electrochemical performance at the potential window ranging from −0.3 to 0.5 V vs. SCE in 2.0 M NaCl aqueous solution (Fig. 6). The CV curves exhibited a distorted rectangular shape at low scan rate, which indicated good capacitance behavior and equivalent series resistance (ESR). However, the rectangular CV shape became deformed significantly when the scan rate reached 1.0 mV s⁻¹. This might be explained by entering into/ejecting and diffusion of counterions being too slow compared to the transfer of electrons in the PPy matrix at high scan rates.³⁴ Rectangularly shaped voltammograms have been obtained for chemically synthesized PPy-coated nylon lycra fabric,¹⁸ which is in agreement with the expected behavior for an ideal double-layer capacitor.³⁵ However, it could be inferred from the CV curves that the charging and discharging of the present fabric involve both surface confined electrochemical reactions and double layer charging.²

Fig. 6 Cyclic voltammetry curves for the S-1 fabric electrodes in 2.0 M NaCl at different scan rates.

The galvanostatic charge–discharge curves of the S-1 fabric electrodes at different current densities in the potential range from 0 to 0.8 V are shown in Fig. 7. All curves exhibited almost the same triangle shape, indicating the reversible and sustainable behavior of the ideal capacitor in the broad current range. The deviations from linearity were attributed to the pseudocapacitive nature of PPy.³⁶ The specific capacitance (C_m) was calculated from the discharge curve using the following equation

where I is the charge–discharge current (A), t is the time of discharge (s), ΔV is the voltage difference between the upper and lower potential limits (V), IR_drop is the voltage drop at the beginning of the discharge (V), and m is the total mass of the active PPy and LGS electrode material (g). The specific capacitance obtained for the S-1 fabric electrodes at a current density of 0.1 A g⁻¹ was 304 F g⁻¹, with an energy density of 20.6 W h kg⁻¹. It was noted that the specific capacitance was decreased with the increase of the current density. When a current density of 0.2 A g⁻¹ was applied on the fabric electrodes, the corresponding specific capacitance was changed to 287 F g⁻¹, with the capacitive retention of 94.3%. Even the current density was increased to 0.8 A g⁻¹, there was 77.6% of specific capacitance maintained (236 F g⁻¹). The result indicated the fabric electrodes have good rate capability. The specific capacitance values obtained for the S-1 fabric electrodes are much higher than the values reported for CNTs and graphene coated fabrics,^12,13 and even larger or comparable with the values reported for PPy coated fabrics.^9,18

Fig. 7 Charge–discharge curves of the S-1 fabric electrodes at various current densities.

The normalized specific capacitance (discharge) at 0.2 A g⁻¹ current density for the S-1 fabric electrodes versus the cycle number are plotted in Fig. 8. After 300 cycles, about 24% loss was observed. The cycling stability of the present S-1 fabric electrodes were better than those reported for PPy coated fabrics.¹⁸ It has been reported that the reduction of the stability of the PPy materials was due to the volume expansion of the materials and the structure relaxation of the polymer matrix chain during the long-time charge–discharge runs in aqueous solution.³⁷ These observations revealed that the addition of the LGS could improve the stability of the electrodes, and LGS played an important role in lower the volume expansion and the structure relaxation of the polymer matrix chain. Further analyses are under progress to improve the cycling stability of these PPy/LGS-coated fabric electrodes.

Fig. 8 Cycling stability of the S-1 fabric electrodes at a current density of 0.2 A g⁻¹.

4. Conclusions

In summary, we have successfully prepared the PPy-coated cotton fabrics with LGS as both polymeric template and dopant. The mass loading on the fabric samples decreases dramatically with the increased LGS content. The electrical conductivity of the coated fabrics achieves 3.03 S cm⁻¹ under the optimized condition. The coated fabrics can be used in supercapacitors as both electrode and current collector, which has high specific capacitance (304 F g⁻¹ at 0.1 A g⁻¹) and good cycling performance in 2.0 M NaCl electrolyte. These fabric electrodes are promising materials for fabricating flexible and wearable supercapacitors since all the chemicals used are compatible with the human body.

Acknowledgements

This work was supported by the Natural Science Foundation of China (no. 51003082 and no. 61108033), the Natural Science Foundation of Hubei Province (no. 2012FFA098 and no. 2013CFB064), the Young People Project of Wuhan Science and Technology Bureau (no. 201271031381), the Science and Technology Research Project of Education Department of Hubei Province (Q20132601), and Doctoral Research Fund of Hubei University of Arts and Science (2013B008).

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