Nano-cladding of natural microcrystalline cellulose with conducting polymer: preparation, characterization, and application in energy storage

Abstract

This study demonstrates the possibility to coat individual fibers of natural microcrystalline cellulose with polypyrrole using in situ chemical polymerization to obtain an electrically conducting continuous high charge storage capability composite with a nano-cladding structure. By manipulating the ordered nano-cladding nanostructure, conducting nanocomposites were achieved and outstanding electrical conductivity, as high as 48.7 S cm⁻¹, was obtained with the feeding mass ratio of sisal microcrystalline cellulose SMC : Py = 3 : 7. X-ray photoelectron spectroscopy (XPS) results showed that with the increase in SMC content, doping level of PPy in the composite increased. It was found that the PPy nanoparticles deposited on the surface of the SMC and connected to form a continuous cladding. The as-prepared SMC/PPy nanocomposites demonstrated a mass-specific capacitance (C_s) of 367 F g⁻¹ at a 0.2 A g⁻¹ current density in the supercapacitor application. Moreover, SMC/PPy electrode retained about 87.5% of the initial C_s after 1000 cycles, and about 57.8% of C_s when the current density increased five times. This study provides a straightforward method to utilize the renewable resource sisal microcrystalline cellulose to obtain a conducting composite, which could be applied in sensors, flexible electrodes, and flexible displays. It also opens a new field of potential applications of micrometre-scale natural microcrystalline cellulose.

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1. Introduction

Nowadays, there is a simultaneous and growing interest in developing bio-based products and innovative process technologies that are based on sustainable materials, which can reduce the dependence on fossil fuels.^1–3 Natural fibrous substances have held a key role in chemical and textile industries worldwide.^4,5 The unique morphologies of natural substances afforded by the self-assembly of the polymer chains provided them superior properties. Coating of an individual cellulose fiber with layers, such as metal oxides, polymers, and inorganics, became a method to control overall properties of composite materials.^6–9 Cellulose supported conducting polymers have received growing interest in recent years due to their better performance and new properties with increasing potential for the development of applications such as batteries, sensors, and electrical devices.

Polypyrrole (PPy) is one of the most widely investigated conducting polymers because of its good thermal and environmental stability and good electrical conductivity.^10,11 Unfortunately, the poor processability and inadequate mechanical properties of PPy limits its commercial application.^12,13 In order to overcome these problems, the deposition of PPy on the fiber surface of fabrics and yarn, such as cotton,¹⁴ silk,¹⁵ and cellulose derivatives,^16,17 has been widely investigated in the last few years. Recently, some research groups have explored the preparation of bacterial cellulose/conducting polymer composites.^18–20 However, the price of bacterial cellulose (BC) is high and the yield is low, which limits the commercial applications of BC.²¹ In the search for cheaper and more effective type of fibers, a special kind of cellulose, sisal microcrystalline cellulose, produced by Agave sisalana, has drawn particular attention. The sisal fiber is one of the most common and abundant natural fibers, and it is also the most widely used.^22,23 Sisal is a renewable resource par excellence and can form part of the overall solution to climate change. Measured over its life cycle, sisal absorbs more carbon dioxide than it produces, and during processing, it is 100 percent biodegradable. Among polymer fibers, sisal microcrystalline cellulose (SMC) is an attractive material to be applied as insulation matrix in conducting polymer composites. Note that SMC exhibits an ultrafine fibrous network, a highly crystalline structure, purity, low density and remarkable mechanical strength; furthermore, the raw material is easily obtainable. Thus, electrically conducting materials composed of SMC coated with PPy emerge as promising polymer composites that present a successful combination of the inherent properties of each single component e.g. high tensile strength, toughness, biocompatibility, high surface areas for SMC and cytocompatibility, electronic and chemical properties of PPy.

In this paper, a highly conductive SMC/PPy nanocomposite, with conductivity reaching 4870 S m⁻¹, was developed with SMC as the scaffold. The as-prepared nanocomposites were characterized by FTIR, XRD, TGA, SEM, XPS and electrochemistry techniques. The preparation conditions were optimized to obtain the highest electrical conductivity with well-controlled composite morphologies. The SMC/PPy nanocomposites were further evaluated for their applicability in electrochemical energy storage, and a high special capacitance of 367 F g⁻¹ was obtained at a 0.2 A g⁻¹ current density. In addition, the growth mechanism of PPy on the SMC scaffold was proposed.

2. Experimental section

2.1 Materials

Pyrrole, ammonium persulfate (APS), sodium p-toluene sulfonate (STS), sodium tetraborate, acetic acid, and nitric acid were purchased from Nacalai Tesque, Inc., Japan. Pyrrole monomer dehydrated with calcium hydride for 24 h was distilled under reduced pressure before use. Sisal fibres were provided by Guangxi Sisal Group Co., Ltd., China. All solutions were prepared in deionized water, and all other chemical reagents were of analytical grade.

2.2 Preparation of natural microcrystalline cellulose/polypyrrole nanocomposites

The sisal microcrystalline cellulose (SMC) was produced from sisal fibres as previously reported.²⁴ The treatment process was basically performed as follows: (I) sisal fibers were air-dried, milled using a knife mill and successively extracted with acetone in a Soxhlet apparatus for 8 h, followed by a hot water extraction (3 h at 100 °C), and then, the short sisal fibers were preprocessed with 0.1 M NaOH in 50% volume of ethanol at 45 °C for 3 h under continuous agitation; (II) treatment with 1.0% H₂O₂ at pH = 11.5 (buffer solution) and 45 °C for 3 h under continuous agitation; (III) treatment with 10% w/v NaOH-1% w/v Na₂B₄O₇·10H₂O at 28 °C for 15 h under continuous agitation; (IV) treatment with HNO₃, 70% + HAc, 80% (1/10 v/v) at 120 °C for 15 min; (V) washing with 95% ethanol; washing with water and another wash with 95% ethanol; (VI) drying at 60 °C in an oven until constant weight.

The SMC/PPy nanocomposite was prepared via in situ oxidative polymerization-induced adsorption onto SMC micro-rods. A typical synthesis was as follows: STS (4.16 g) was dissolved in 100 ml of deionized water, and then a certain amount of SMC was added (Table 1). Then, the mixture was ultrasonically dispersed, and pyrrole (1 ml) was added into the mixture with vigorous stirring. Afterwards, the mixture was mechanically stirred for 30 min at 0 °C, and then an aqueous solution (20 ml) of APS (0.90 g) was added drop by drop to the above mixture to instantly start the oxidative polymerization. The reaction was performed under mechanical stirring for 10 h, and the resulting precipitates were washed with deionized water and ethanol several times. Finally, the product was dried under vacuum at 60 °C for 24 h to obtain the desired SMC/PPy nanocomposite, as a dark powder.

Table 1 The conditions of the polymerization reactions investigated in this work

Sample	Py (ml)	SMC (mg)	Theoretical PPy content (%)	Yield (%)
S-1	1	50.9	95	66.7%
S-2	1	107.4	90	63.1%
S-3	1	241.8	80	69.9%
S-4	1	414.4	70	65.2%
S-5	1	967.0	50	68.6%

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2.3 Characterization and electrochemical measurements

The morphology and microstructure of the samples were investigated with a S-4800 field emission scanning electron microscope (SEM; HITACHI, Japan) at an accelerating voltage of 5.0 kV. The Fourier transform infrared (FTIR) spectroscopy measurements (Impact 400, Nicolet, Waltham, MA) were carried out with the KBr pellet method. Thermogravimetric results were obtained with a TA Instrument Q500 thermogravimetric analyzer at a heating rate of 10 °C min⁻¹ from 25 to 800 °C under nitrogen atmosphere. X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI 5000 Series (Shimadzu, Japan) equipped with a monochromatic Mg Kα X-ray source, and a resistive anode detector. The size of the X-ray spot was 1 mm × 1 mm, which correlated to an X-ray power of 400 W. The electrical conductivities of samples were measured using SDY-4 Four-Point Probe Meter (Guangzhou Semiconductor Material Academe) at ambient temperature. The pellets were obtained by subjecting the powder sample to a pressure of 30 MPa. The reproducibility of the results was checked by measuring the electrical conductivity three times for each pellet. Electrochemical experiments were performed on a CHI660D electrochemical workstation (Shanghai, China) with a conventional three-electrode system. The working electrode was prepared by mixing the active material with 15 wt% acetylene black and 5 wt% polyvinylidene fluoride (based on the total electrode mass) to form a slurry. Then, the slurry was cast on Ni foam. A platinum foil and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. The strong electrolyte, 0.5 M Na₂SO₄ solution, was used to ensure high ionic strength. Cyclic voltammetry studies were performed in the voltage range from −0.6 V to 0.4 V at scan rates of 1, 5 or 10 mV s⁻¹. Galvanostatic charge/discharge experiments were carried out in the potential range from −0.6 V to 0.4 V with an applied current density of 0.2, 0.4, 0.6 or 0.8 A g⁻¹. To eliminate dissolved oxygen, all suspensions and reagent solutions were degassed with pure nitrogen for 15 min before experiments, and all reactions were then carried out under nitrogen atmosphere.

3. Results and discussion

3.1 SEM observations

The morphology of SMC and PPy/SMC composites was observed by scanning electron microscopy. Images showed that the SMC surface (Fig. 1 a) was smooth. This observation clearly indicated that the pretreatment could remove the hemicellulose and lignin coverings from the SMC surfaces. In SMC/PPy composites, the presence of PPy particles coated on SMC can be observed (Fig. 1(c–f)). The thick layer of polypyrrole deposited on SMC increased and the PPy particles tend to agglomerate with increasing pyrrole concentration. Apparently, a uniform coating layer was achieved for SMC/PPy composites prepared with 30 wt% and 50 wt% SMC. Lower SMC contents promoted the agglomeration of PPy on the SMC fibers, where large PPy particles agglomerated and therefore SMC was embedded in PPy aggregates. The PPy layer thickness increased for SMC/PPy with high content of SMC, which resulted in the formation of a SMC/PPy composite with a nano-cladding structure, in which the SMC formed the inside and the polypyrrole formed the external layer. As the SMC content increased, a more homogeneous coating was formed. From SEM observations, it is possible to state that the initial content of SMC determines the thickness of the PPy layer, defining the final microstructure and coating quality.

Fig. 1 SEM images of SMC and SMC/PPy nanocomposites (a: SMC; b: S-2; c and d: S-4; e and f: S-5). Insets show low magnification SEM images for a, b, c, and e.

In previous reports, the coating of nanoparticles with PPy was performed by template polymerization using surfactant bilayers that were adsorbed on the nanoparticle surface. Pyrrole monomers were firstly condensed onto the bilayers, followed by their oxidative polymerization to form PPy shells. Unfortunately, the abovementioned approach is not applicable to cellulose fibers, because pyrrole monomers cannot be adsorbed or immobilized onto a cellulose surface. The formation process of PPy nanocoating is due to the advantage of the polymerization-induced adsorption, which is based on the adsorption of growing polymer chains from solution, and their subsequent immobilization to give thin films. By using a dilute polymerization solution (C_py = 1.2 × 10⁻⁴ mol ml⁻¹ in this work), a thin PPy layer can be deposited on the substrate. This would fit with observations that the PPy coatings on the SMC surface presented an obvious granular structure, and the coating was not very smooth. The main processes can be clearly seen in Fig. 2.

Fig. 2 Illustration of the SMC/PPy composite fabrication process via polymerization-induced adsorption.

3.2 FTIR spectroscopy

The FTIR spectra of SMC and SMC/PPy composites are shown in Fig. 3. The characteristic IR peaks of SMC are in good agreement with those from literature,²⁵ which includes a broad band at 3403 cm⁻¹, attributed to the O–H stretching vibration, and a band at 2897 cm⁻¹, assigned to the aliphatic C–H stretching vibration. The small peak at 1647 cm⁻¹ is attributed to carbonyl functional groups because of the natural ageing of cellulose. The SMC/PPy composites exhibited a band at 3429 cm⁻¹ for the N–H stretching vibration instead of the band at 3403 cm⁻¹ for the O–H stretching, which resulted from the complete wrapping of PPy around SMC fibers. A similar quenching of the aliphatic C–H stretching at 2897 cm⁻¹ was also observed. Moreover, the characteristic peaks for PPy were identical to those for SMC/PPy composites, which included a band at 1542 cm⁻¹ assigned to the C=C stretching vibration in the Py ring, and a band at 1459 cm⁻¹ corresponding to the C=N stretching vibration in the Py ring.²⁶ It is important to identify how PPy was incorporated into the SMC. We observed that the intensity of the 3400 cm⁻¹ absorption band, which was attributed to the –OH, was reduced in the SMC/PPy composite. This additional evidence to support this process is based on the adsorption of growing polymer chains from the solution. Therefore, more hydroxyl groups become accessible, which helps to form the uniform PPy particles coatings on the surface of SMC, and the result is confirmed in the SEM section. Moreover, the absorbance of the sulfonate S=O stretching at 1180 cm⁻¹ indicated that PPy had been doped with sodium p-toluene sulfonate. All composites have similar absorption bands, indicating that the doping degree of PPy was not affected by the presence of SMC molecules.

Fig. 3 FT-IR spectra of SMC, PPy and SMC/PPy composites.

3.3 X-ray diffraction analysis

Fig. 4 shows the XRD patterns of pristine SMC, PPy and SMC/PPy nanocomposite samples. As shown, the three main peaks of pristine SMC, located at 14.8°, 16.7°, and 22.5°, corresponding to the (101), (101) and (002) diffraction planes of cellulose I, disappeared in SMC/PPy nanocomposites.²⁷ However, the characteristic broad diffraction peak at 24.3° for amorphous PPy was clearly observed in the as prepared SMC/PPy nanocomposites.²⁸ It is reasonable to conclude that the crystalline behavior of the SMC fiber was hindered due to the complete coating with the amorphous PPy cladding.

Fig. 4 X-ray diffraction patterns of SMC, PPy and PPy coated SMC. (Inset shows the resolving of overlapping peak.)

3.4 Thermal analysis

Fig. 5 shows the TGA results of the composites compared to that of the SMC used as a reference. It could be seen that water was continuously evaporating up to a temperature of about 100 °C and that this resulted in a weight loss of 5 wt% for the SMC. The SMC underwent rapid decomposition in the interval between 250 and 380 °C in a process typical for cellulose pyrolysis, involving the formation of various anhydro-monosaccharides (including levoglucosenone, levoglucosan, and 1,6-anhydro-β-D-glucopyranose), carbon oxides, and char.^29,30 For the composites, the onset decomposition temperatures are lower than that of SMC. In the region between 200 °C and 360 °C, the composite lost weight, mainly as a result of the degradation process of the cellulose part in the composite material, but also partly caused by the thermal degradation of the polymer backbone in polypyrrole. Moreover, the process during which the counterion is expelled occurs before the polymer backbone degradation in PPy, which is probably responsible for shifting the main composite degradation step to have its maximum degradation temperature at a lower temperature (230 °C) than the maximum degradation temperature of the SMC degradation process (280 °C). The initial mass loss at a lower temperature is mainly due to the release of water from the SMC. Nevertheless, the composites did not exhibit this evapotranspiration effect. This also further supports that this process is based on the adsorption of growing polymer chains from solution.

Fig. 5 TGA curves of SMC and SMC/PPy composites.

3.5 X-ray photoelectron spectroscopy

The full spectrum for a virgin substrate shows that the surface is mainly constituted of carbon (signal at 284 eV) and oxygen atoms (signal at 532 eV) as shown in Fig. 6(a). No other surface elements, except for oxygen and carbon, were observed in SMC. As expected, after oxidative polymerization of pyrrole, two new peaks appear around 399 eV and 168 eV, corresponding to nitrogen and sulfur atoms, respectively.³¹ Fig. 6(b) shows high resolution N_1s and S_2p spectra for SMC/PPy composites, and we utilized XPS to determine the content of the dopant as well as the doping level of the resulting PPy/SMC composites. The S/N ratio of S-3 is 0.24, whereas the value of S-5 increases to 0.31. It should be noted that all composites have the same feeding ratio of pyrrole and STS. The increased S/N ratio indicates that sulfonate can be effectively inserted into the PPy composite in the presence of SMC during polymerization. With the increase in SMC content, the doping level of PPy in the composite increases, and this has also been verified by electrical conductivity measurements.

Fig. 6 XPS scan spectra for (a) SMC and SMC/PPy composites (b) N_1s and S_2p XPS spectra of SMC/PPy composites.

3.6 Electrical conductivity of SMC/PPy composites

Fig. 7 shows the weight increase of SMC as a function of electrical conductivity. The maximum electrical conductivity obtained was 48.7 S cm⁻¹, and this is a relatively high electrical conductivity for the prepared conductive composite and is comparable with values obtained for similar nanocoatings on insulants with PPy reported in the literature. As expected, with the increase in the SMC content, the conductivity first increases and then decreases. The doping level of PPy in the composite is higher than that in the neat PPy, which results in a high conductivity.³² In addition, the increase in the SMC content enhanced the compactness of the samples and decreased the conducting PPy content, which resulted in two contrary effects on the conductivity of the SMC/PPy composite. The composites prepared by aqueous solution polymerization show a typical percolation phenomenon in terms of electrical conductivity as a function of the PPy content.^33,34

Fig. 7 Conductivities of the SMC/PPy samples at room temperature.

3.7 Supercapacitor performance of SMC/PPy composites

The potential applications of the as-prepared SMC/PPy nanocomposites were explored by fabricating the samples into supercapacitor electrodes and characterizing with cyclic voltammograms (CVs), galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy (EIS). CVs response of SMC/PPy carried out at varied scan rates between −0.6 and +0.4 V using a 0.5 M Na₂SO₄ solution is shown in Fig. 8. The composite showed a high degree of electroactivity, with rectangular CV traces showing the transitions from reduced to oxidized forms, which demonstrated the retention of the important redox feature of conducting polymers in the as-synthesized SMC/PPy nanocomposites.

Fig. 8 (a) CV curves of the SMC/PPy composites electrodes at 5 mV s⁻¹ in a 0.5 M Na₂SO₄ solution; (b) CV curves of the SMC/PPy composite (S-3) electrode in a 0.5 M Na₂SO₄ solution at different scan rates.

The electrochemical redox reaction at the interface between the electrode and electrolyte can be expressed as follows:³⁵

The characteristic redox behavior of PPy is clearly seen with an oxidation peak at +0.2 V and reduction peak at −0.48 V vs. SCE (saturated calomel electrode), respectively. From Fig. 8(b) one can find that the curves have some deviations with the increment of potential scan rates from 5 to 50 mV s⁻¹. The deviation from rectangularity of the CV becomes obvious, which is mainly due to the resistance of the electrode. Moreover, this indicates that the adsorption of electrolyte cations such as Na⁺ on the SMC/PPy nanocomposites surface contributes to the capacitance of the supercapacitor.

Galvanostatic charge/discharge experiments for SMC/PPy nanocomposites were performed at current densities of 0.2, 0.4, 0.6 and 0.8 A g⁻¹ in a 0.5 M Na₂SO₄ solution. As illustrated in Fig. 9, during the charge and discharge process, the charge curve of SMC/PPy is almost symmetric to its corresponding discharge counterpart with a slight curvature, indicating the pseudo capacitance behavior. The discharge capacitance (C_s) may be calculated from the equation:

where I is the charge/discharge current, Δt is the discharge time, ΔV is the potential drop during discharge (1 V), and m is the mass of active material. The C_s values of SMC/PPy composites were calculated to be 195, 244, 367, 187, and 114 F g⁻¹, respectively, at the current density of 0.2 A g⁻¹. According to the literature, cellulose exhibited only negligible electroactivity. The improved capacitance of the nanocomposites might be mainly ascribed to the higher conductivity, and smaller particle sizes of the composite, which can shorten the ion diffusion length and cause higher materials utilization.³⁶ Galvanostatic charge/discharge C_s results for the S-3 sample were 367, 281, 227, and 212 F g⁻¹ performed at current densities of 0.2, 0.4, 0.8 and 1.0 A g⁻¹, respectively. About 57.8% of C_s was retained when the current density increased five times, which was attributed to the discrepant insertion–disinsertion behavior of the alkali ion from the electrode to PPy.

Fig. 9 (a) Galvanostatic charge/discharge curves of SMC/PPy composites at current densities of 0.2 A g⁻¹; (b) Galvanostatic charge/discharge curves of S-3 at current densities of 0.2, 0.4, 0.8, 1.0 A g⁻¹; (c) cycle life of S-3 at 0.2 A g⁻¹.

The stability and reversibility are also important for electrochemical supercapacitor applications. Furthermore, the electrochemical stability of SMC/PPy composites was also investigated (Fig. 9(c)). It was found that the SMC/PPy electrode retained about 87.5% (321 F g⁻¹) of the initial capacitance after 1000 cycles. This good stability may be ascribed to the well-ordered SMC/PPy composites and to the strong interaction between the SMC and PPy.

4. Conclusion

This work may offer a new convenient approach for making non-toxic, cheap, readily available and environmentally friendly electrode materials from natural cellulose. It has been shown that it was possible to manufacture an electrical conducting composite material composed of natural microcrystalline cellulose and polypyrrole by direct chemical polymerization of pyrrole on the sisal microcrystalline cellulose without the need for sophisticated techniques. It was found that the PPy particles uniformly deposited on the surface of SMC, connected to form a continuous nano-cladding by adopting the SMC template. The electrical conductivity can reach 48.7 S cm⁻¹ by controlling the feeding mass ratio of SMC/pyrrole. The as-prepared SMC/PPy nanocomposites had a specific capacitance reaching 367 F g⁻¹ at a 0.2 A g⁻¹ current density in the supercapacitor. The electric conductivity, thermal stability, and well-controlled microstructure of SMC/PPy composites pave the way towards promising applications for various electronic devices.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51303035), the Foundation of Guangxi Science and Technology Department (2013GXNSFBA019041), the Guangxi Funds for a Specially-Appointed Expert, and the Guangxi Small Highland Innovation Team of Talents in Colleges and Universities.

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