In Situ Lignin Sulfonation for Highly Conductive Wood/Polypyrrole Porous Composites

To address the rising need of sustainable solutions in electronic devices, the development of electronically conductive composites based on lightweight but mechanically strong wood structures is highly desirable. Here, a facile approach for the fabrication of highly conductive wood/polypyrrole composites through top‐down modification of native lignin followed by polymerization of pyrrole in wood cell wall. By sodium sulfite treatment under neutral condition, sulfonated wood veneers with increased porosity but well‐preserved cell wall structure containing native lignin and lignosulfonates are obtained. The wood structure has a content of sulfonic groups up to 343 µmol g−1 owing to in situ sulfonated lignin which facilitates subsequent oxidative polymerization of pyrrole, achieving a weight gain of polypyrrole as high as 35 wt%. The lignosulfonates in the wood structure act as dopant and stabilizer for the synthesized polypyrrole. The composite reaches a high conductivity of 186 S m−1 and a specific pseudocapacitance of 1.71 F cm−2 at the current density of 8.0 mA cm−2. These results indicate that tailoring the wood/polymer interface in the cell wall and activating the redox activity of native lignin by sulfonation are important strategies for the fabrication of porous and lightweight wood/conductive polymer composites with potential for sustainable energy applications.


Introduction
To develop sustainable solutions in the energy sector, high energy and power densities are a necessity, however, with the spread application of batteries and supercapacitors, environmentally friendly alternatives that are also abundant and low cost can be a needed resort. [1]In this context, the design and fabrication of polymer composites from renewable resources, such as wood and wood components, and intrinsically conducting polymers, are of great interest.Among the conductive polymers, polypyrrole (PPy) There have also been attempts to convert wood to a conductive substrate, utilizing its micro/nanopores for the transport of electrons and ions in applications for electrodes, sensors, lighting devices, and photovoltaics. [4]These attempts have involved either the impregnation of wood with conductive materials, including polymers, metal oxides, and metal-organic frameworks, or the carbonization of wood. [13]Through such top-down approaches, pyrrole has also been polymerized directly inside the lumen of native wood structures to fabricate wood-based electrodes for supercapacitors. [14]To further activate the wood cell wall for monomer, prepolymer, or polymer infiltration, lignin is often removed while preserving the hierarchically ordered structure of natural wood. [15]Subsequently, functionalizations inside the wood cell wall have been carried out and led to advanced materials with improved mechanical, [16] optical, [17] thermal, [18] photothermal, [19] and fluidic [20] properties.In this fashion, conductive delignified wood/PPy composite materials for electromagnetic interference shielding [21] and solar steam generation devices [22] have been developed.However, to further obtain higher conductivity and optimized capacitive performance, tailoring microstructures of wood/PPy composites while preserving the wood hierarchical structure and anisotropy remains a major challenge.Particularly, in situ sulfonation of lignin in wood structure has not been exploited in the preparation of wood/PPy composites.
Previously, we developed a facile strategy to perform lignin sulfonation and preserve the obtained lignosulfonates inside the wood structure followed by infiltration of PEDOT:poly(styrene sulfonate) (PSS) polymer. [23]Sulfonation of lignin with sodium sulfite, previously used in acidic conditions to remove lignin from softwood in sulfite pulping process, [24] was carried out under neutral conditions to generate highly charged sulfonated wood veneers (WVs) with high porosity.A homogeneous and uniform coating of inner surfaces (lumen) of the cell wall with the PEDOT:PSS polymer was obtained, but the porous cell wall was not infiltrated with the polymer due to its large particle size (≥30 nm).Herein, we further scaled up the reaction vessel to accommodate larger WV samples of (50 mm × 40 mm, Longitudinal × Tangential) for sulfonation and optimized the reaction condition to preserve high content of lignin and lignosulfonates in the wood cell wall.The degree of sulfonation (i.e., content of sulfonic acid) and specific surface area of the sulfonated WVs were controlled by varying the reaction time of the sodium sulfite treatment step, providing accessibility of the cell wall for the infiltration and oxidative polymerization of pyrrole (Scheme 1).Subsequently, pyrrole was polymerized in the sulfonated WVs with iron(III) chloride (FeCl 3 ) and 0.04 m hydrochloric acid (HCl) in water.In the obtained sulfonated WV/PPy composites, we exploited the wood anisotropic structure for electrolyte transport and the important charge storage capacity of quinone moieties in lignin.The composites were characterized and showed robust mechanical properties as well as high conductivity and capacitive performances.

Structure of Sulfonated WVs
Sulfonation of lignin is an established method in sulfite pulping process, [25] which has not been previously investigated from the wood modification perspective.In this work, sodium sulfite sulfonation at pH 7 using subcritical water was applied to pine WV and the effect of treatment time was studied to optimize the content of lignosulfonates and cell wall porosity to allow subsequent infiltration and polymerization of pyrrole.The degree of sulfonation was determined by conductometric titration, quantifying the amount of acid groups.According to SCAN-CM 65:02, uronic acid side groups in xylan were assigned to "weak acid," while the sulfonic acids groups arising after sulfonation were assigned to "strong acids." As shown in Figure 1a, the content of weak acid groups (carboxylic acid) remained in the range between 100 and 150 µmol g −1 during the sulfonation treatment.The content of strong acid groups was increased significantly during the first 3 h, from 100 µmol g −1 at 30 min to 280 µmol g −1 at 3 h, and then more slowly increased to 350 µmol g −1 at 14 h.FTIR spectra of the wood veneer samples were also recorded and presented in Figure 1b, confirming successful sulfonation.After 1 h treatment, the SO stretching of the sulfonic group was detected at 1205 cm −1 and the intensity of this peak increased with increasing reaction time, indicating higher degree of sulfonation.14c,26] The composition of cellulose, hemicelluloses (glucomannan and xylan), and lignin of the sulfonated WVs were determined from sugar analysis and gravimetric Klason lignin content, and the results are summarized in Table 1.The sulfonated WVs Scheme 1. Schematic illustration for the preparation of conductive pine wood veneer (WV).
were coded as SWV-x, where x denotes the treatment time.The lignin was gradually removed from the WVs, from the native lignin content of 28.9 wt%, down to 20.6 wt% after 5 h of treatment and 15.1 wt% after 14 h.The gradual removal of lignin is ascribed to the progressive debranching of lignin moieties as more hydrophilic and soluble lignosulfonates are formed.Compared to our previous study, [23] lignin was better preserved when using a bigger vessel with the same reaction time.Cellulose and xylans remained mostly preserved while the content of glucomannan decreased slightly with longer sulfonation reaction times, which was confirmed by FTIR analysis.Therefore, most of the weight loss in the treated samples, as measured from oven-dried samples before and after treatment (Table 1), are derived from the partial removal of lignin while the holocellulosic content remained unaltered, securing mechanical integrity of the sample.
It is also important to understand how micro-and nanostructures of WVs are influenced by the sulfonation treatment using field emission scanning electron microscopy (FESEM).The morphological analysis (Figure 2) revealed that longer reaction times influenced the wood microstructure mostly in the middle lamella and cell wall corner regions (lignin-rich), similar to previous studies. [24]The erosion started in the earlywood region from 3 h when the lignin content started to decrease, and proceeded to the latewood region with thicker cell walls after 5 h.The partial erosion of the lignin-rich areas contributed to a 20-fold increase in the specific surface area (SSA) and a decisive increase in porosity (Figure S1, Supporting Information).Native WV has an SSA value of 0.84 m 2 g −1 which increased to 17 m 2 g −1 for the sulfonated WV after 14 h sulfonation reaction (Table 1).These values were measured by using critical point dried (CPD) samples and calculated from Brunauer-Emmett-Teller (BET) isotherms recorded by N 2 physisorption at −196 °C.A partial softening of the WV samples became more pronounced with longer treatment times, making the WVs more flexible and bendable, as shown in Figure S2 (Supporting Information).The integrity of the WVs was not compromised.At the same time, the higher porosity made the sulfonated WVs an ideal substrate for polymerization, allowing better accessibility of the pyrrole monomer inside the wood cell wall structure.

Structure of Sulfonated WV/PPy Composites
A straight forward water-based polymerization of pyrrole was initiated directly inside sulfonated WV samples using FeCl 3 as an oxidant.The polymerization was carried out at low temperature in order to obtain longer PPy polymer chains with better electrochemical performance. [9,27]The WV samples were impregnated with the filtrate of a reactive PPy polymerization solution containing both pyrrole and oxidant and kept for 3 h.This "prepolymerization" method was adopted from that reported Table 1.Effect of sulfonation reaction time on the composition of cellulose, hemicellulose (glucomannan and xylan), and lignin, as well as weight loss and BET specific surface areas for the WV samples.
Native WV 41.8 ± 0.5 16.0 ± 0.4 5.7 ± 0. by Huang et al., [8] aiming to obtain a controlled homogeneous coating of the cellulose microfibrils with PPy inside the cell wall in the wood structure.The WV samples that were sulfonated for 3 and 14 h were used to prepare wood/PPy composites with native and delignified WVs as the control.After polymerization, the weight increases were recorded by measuring in triplicates using oven-dried samples (Table S1, Supporting Information).
The SWV-14 h sample with higher porosity and higher surface area resulted in a weight percent gain of PPy polymer of 35 wt%, five times as that for native WV (7 wt%) and almost twice as that for the SWV-3 h (19 wt%).This value is even higher than that for low density wood, such as delignified balsa (24 wt%) reported in recent work by Gan et al. [21] Indeed, the delignified WV (DWV) with an SSA value of 90.9 m 2 g −1 had a weight percent gain of only 20 wt%.Thus, we can speculate that the higher compatibility between PPy and sulfonated WV was not only a mere capillarity effect driven by higher porosity as in the delignified WV, but also had a decisive contribution from the sulfonation chemistry.This favorable interaction was visible for sulfonated WV samples after a few minutes from the initiation of the polymerization, as the sulfonated WV sample quickly turned black.This is a sign of successful impregnation of the monomer and subsequent faster polymerization as compared to native and delignified WV samples (Figure S3, Supporting Information).The presence of PPy polymer inside the WV samples could be inferred by the naked eye, as the sample turned black.This was further confirmed by FTIR analysis of the composites as compared to the neat PPy polymer (Figure 3).The spectra of SWV/PPy composites presented the typical peaks of PPy polymer.The bands at 1525 and 1430 cm −1 can be assigned to the fundamental stretching vibrations in the pyrrole ring.14a,22b,28] This indicates wellcoated microfibrils in the cell wall as the characteristic peaks of the wood components are not visible.
To further investigate the distribution of the PPy polymer inside the cell wall, energy-dispersive X-ray (EDX) maps were collected, and the images are presented in Nitrogen is an element present in pyrrole but not in native WV.It is obvious that nitrogen was distributed throughout the cell wall and middle lamella of the SWV-3 h/PPy and SWV-14 h/PPy composite samples.Meanwhile, the presence of Cl in the same regions of N demonstrated the successful doping with FeCl 3 during the polymerization step.N content was higher in the middle lamella region (where more lignin is present) and in the SWV-14 h/PPy sample.EDX compositional analysis (Table S2, Supporting Information) showed 0, 4.1, and 7.3 at% of nitrogen in the WV/PPy, SWV-3 h/PPy, and SWV-14 h/PPy composite samples, respectively.These values were averaged on the whole imaged section.The point values for cell wall and middle lamella regions, referring to the same images (and marked with a star on the pictures) were also collected (Table S2, Supporting Information) for all samples.The content of N in the cell wall corners was higher than that in the cell wall for the SWV/PPy samples.For the WV/PPy sample, the amount was 0 at% for both regions.
From FESEM images of the transverse and longitudinal cross sections of the composite samples (Figure 5), the morphology and distribution of PPy polymer deposited in the wood cell wall were characterized.PPy was deposited mostly in the lignin-rich regions such as middle lamella and cell wall corners, and such distribution was particularly distinct for the SWV-14 h/PPy sample (Figure 5c).The regions previously eroded by the sulfonation step were filled by PPy, strengthening the cell wall structure while the lumen channels remained open for possible electrolyte transport when used for electrodes in energy storage applications.The effect of the integration of PPy polymer on mechanical properties was visible in a macroscopic stiffening of the WV samples that, nonetheless remained flexible in wet conditions (Figure S2, Supporting Information).The FESEM images of the longitudinal sections revealed that cellulose microfibrils in the cell wall lumen surface were covered with the PPy polymer.In the WV/PPy composite sample (Figure 5d), the deposited PPy appeared as nanoparticles of smooth surface and heterogeneous size (50-200 nm in diameter).In the SWV-3 h/PPy sample (Figure 5e), the PPy polymer appeared as rather homogeneous nanoparticles of ≈20 nm in diameter and aggregated into larger nanoparticles of similar size as in the WV/PPy sample.With the highest degree of sulfonation in SWV-14 h/PPy composite (Figure 5f), the dispersion of PPy polymer was the most homogeneous with no visible formation of aggregates, owing to the stabilizing effect of the sulfonic groups introduced by sulfonation treatment of WVs.Such homogeneous distribution and smaller nanoparticles are also speculated to have a positive impact on the overall conductivity  of the composite, allowing better charge transport between PPy nanoparticles. [9]o better understand the mechanism behind the effect of sulfonation of wood veneer on PPy polymer distribution and morphology, it is important to understand the interactions at the molecular scale.In the wood structure, the superior absorption of FeCl 3 in delignified wood was attributed to the hydroxyl groups of cellulose and hemicellulose that are able to coordinate the Fe 3+ ions. [21]It was also argued that PPy polymer could directly link to the hydroxyl groups on the surfaces of cellulose fibers. [29]These mechanisms are valid for our sulfonated wood veneers.Furthermore, the presence of the stronger sulfonic acid groups provided additional electrostatic interaction with the oxidized pyrrole monomer as well as with the PPy polymer chain.Such beneficial contribution has already been exploited with other conductive polymers such as PEDOT:poly(styrene sulfonate) (PSS) using lignosulfonates. [12,23]Hence, both morphological and chemical structure analysis revealed that pyrrole was able to be homogeneously polymerized inside the sulfonated wood cell wall with high porosity and well preserved lignosulfonates.

Mechanical Properties
The mechanical properties of the dry native WV, SWV-3 h, and SWV-3 h/PPy samples were evaluated with three-point bending  2. The sulfonated WV (SWV-3 h) sample showed a more ductile behavior and a lower flexural strength (81 MPa) compared to native WV (115 MPa).This is ascribable to the partial removal of lignin in the middle lamella after sulfonation (Figure 2) which is speculated to have caused a loss of cohesion between fibers.However, an improvement in strength and a slightly more brittle behavior was shown after the integration of polypyrrole (94 MPa) and can be attributed to the beneficial polymerization of pyrrole in the eroded regions (Figure 5) which partially restored the fiber-fiber cohesion, bringing the strength back up to a value comparable to the native WV.In all fabrication steps, the mechanical properties remained in a satisfactory range as the native hierarchical structure and anisotropic alignment of wood fibers were preserved.

Electrical and Electrochemical Properties
10b] Macromolecular anions such as alkylsulfonates or alkylbenzenesulfonates have been shown to contribute to improved electrochemical performance mainly for two reasons: 1) by a reduction in mobility of the doping anions within polypyrrole structure, preventing degradation of the electron conduction and 2) by stabilizing the polymer dispersion in water.As the second contribution was clearly visible in the morphological studies of the sulfonated WV/PPy composites, the effect of degree of sulfonation in sulfonated WV on electrical and electrochemical performance of the composites was studied.The WV/PPy, SWV-3 h/ PPy, and SWV-14/PPy samples showed similar thicknesses of 1.03 ± 0.02, 1.07 ± 0.03, and 1.04 ± 0.02 mm, respectively.The SWV-3 h/PPy and SWV-14 h/PPy samples showed an electrical conductivity of 172.0 and 186.0 S m −1 , respectively (Table 3).These values were not only more than 15 times higher than that for the WV/PPy sample (11.0 S/m) but also more than four times higher than the previously reported value for PPy-coated delignified wood (39 S m −1 ). [21]Such significant improvement in conductivity was attributed to the better infiltration and homogeneous distribution of PPy polymer inside the wood cell wall with well-preserved lignosulfonates bearing the sulfonic groups.The mechanism of action is related to the electrostatic interactions between the negative-charged sulfonate groups in lignosulfonate and the positive-charged PPy + moieties in the polymer chain of PPy.During the polymerization of PPy, the sulfonate groups behave as a counterion, which ultimately interacts and additionally dopes the monomer. [11]s a result, the areas of CV curves for SWV-3 h/PPy and SWV-14 h/PPy samples were evidently larger than that for WV/PPy (Figure 7a), indicating higher charge storage capacity as electrodes in energy storage devices.Based on the discharge times (Figure 7b), the specific capacitances of WV/PPy, SWV-3 h/PPy, and SWV-14 h/PPy were determined to be 0.19, 1.31, and 1.71 F cm −2 , respectively.The corresponding equivalent series resistances (ESR) of the electrodes were 30.3, 19.7,  and 18.5 Ω, respectively.5b,7,14] To lower the ESR resistances, the voltage drop can be limited by reducing the potential range.Surprisingly, when compared to the CV curve of a delignified pine WV/PPy composite (DWV/PPy in Figure 7a), the WV/ PPy composite showed even a higher charge storage capacity.Lignin was completely removed in the delignified wood by using 1 wt% of sodium chlorite in sodium acetate buffer solution at pH 4.6 and 80 °C. [30]The DWV/PPy composite was prepared with the same condition as the SWV/PPy sample (Figure S3, Supporting Information).These results show that the higher specific capacitance of the SWV/PPy composite electrodes did indeed benefit from the lignosulfonates which are in close vicinity with PPy in the wood cell wall.The access of PPy to lignosulfonate facilitates redox activities of lignosulfonate and thus utilizes its pseudo-capacitance for the charge storage in combination with PPy, as widely reported in the literature. [11,12,31]  The SWV-3 h/PPy sample was selected for further study of the electrochemical properties as it had comparable electrical conductivity and specific capacitance to the SWV-14 h/PPy sample but with 40% lower amount of PPy in the composite and less energy consumption for production.The CV curves of the SWV-3 h/PPy sample are presented in Figure 7c,d.At lower scan rates (Figure 7c), the curves had a pair of redox reaction peaks, which disappeared in curves at higher scan rates (Figure 7d).14a,32] The galvanostatic charge-discharge curves of the SWV-3 h/PPy electrode exhibited characteristics corresponding to its CV curves as shown in Figure 7e.At a lower current density, the curve had a longer discharge time, leading to a higher specific capacitance.Accordingly, SWV-3 h/PPy has a specific capacitance of 1.31 F cm −2 (203 F g −1 ) at a current density of 8.0 mA cm −2 (1.2A g −1 ).The obtained capacitance is comparable to other reported PPy-or PANI-modified wood supercapacitor electrodes (213 F g −1 at 0.21 mA cm −12 , [14a] 360 F g −1 at 0.2 A g −1 , [14c] 216 F g −1 at 0.05 A g −1 , [33] 384.69 F g −1 at 0.3 A g −1 [34] ), which were obtained at even lower current densities.The CV cyclic stability of SWV-3 h/PPy electrode was 47% after 300 cycles, which is low but reasonable for an electrode fully made of pure redox-active polymers (Figure S4, Supporting Information). [28,35]Although the focus of this work was not to prepare high-performance supercapacitor electrodes, [36] the poor cycling stability should be improved in the future attempts for optimized capacitive performance.Common strategies to improve the electrode stability are hybrids with active and electrochemically stable materials, such as carbon derivatives, or copolymerization with other conductive polymers which can avoid the depletion and aggregation of electrode material during the charge-discharge process. [14,32,37]

Conclusion
In summary, we have successfully fabricated highly conductive wood/PPy composites of unique composition and micro/ nanostructures.The route was via a facile sulfonation approach to obtain a pine wood veneer substrate with well-preserved lignin and lignosulfonates.This is followed by oxidative polymerization of pyrrole that was confirmed to be located inside the sulfonated wood cell wall.Sulfonated wood veneer with high content of sulfonic groups (343 µmol g −1 ) and porous cell walls with a specific surface area of 17 m 2 g −1 was prepared by optimizing the reaction time.Pyrrole was polymerized inside the porous wood cell wall which was facilitated by the presence of well-preserved lignosulfonates, achieving a polymer weight gain up to 35 wt%, five times as that for the reference composite from native wood substrate (7 wt%).The synthesized PPy polymer nanoparticles were rather homogeneous and their deposition in the cell wall was very uniform due to the high content of sulfonic acid groups.Furthermore, the complete infiltration of PPy polymer inside the cell wall made most of the important capacitive contribution of lignosulfonates during electrochemical processes.The preserved wood structure with low tortuosity channels provided extra pathways for ion conductivity, leading to self-standing composites with high conductivity and good capacitive behavior, along with tunable porosity and good mechanical properties.These results showed the advantage of in situ lignin modification approach for wood structure as a multifunctional platform for controlled and improved conductive polymer integration toward sustainable and low-cost alternatives for energy applications.
Sulfonation of WVs: Pine (Pinus sylvestris) veneers (50 × 40 × 0.75 mm 3 , Longitudinal × Tangential × Radial) were wetted under vacuum prior to modification.The wet veneers were sealed in an acid digestion vessel of 45 mL capacity, filled with an aqueous solution of 0.7 m Na 2 SO 3 , which was adjusted to pH 7 with the addition of H 2 SO 4 at room temperature.The vessel was heated in an oven for 1 h at 80 °C to allow the impregnation.The temperature was then increased to 165 °C and kept for different reaction times: 30 min, 1 h, 3 h, 5 h, and 14 h.To stop the reaction, the vessel was quenched in an ice water bath, and the samples were washed with acetone and deionized water (three times each) by vacuum filtration.The samples were stored in water until further use.
Oxidative Polymerization of Pyrrole in Sulfonated WVs: Equimolar solutions of pyrrole and FeCl 3 in deionized water (0.5 m) were mixed with the addition of 37% HCl (160 µL for 50 mL of total reaction solution).The mixture was stirred in an ice water bath and a black PPy precipitate started to form after a few minutes.The reaction solution was filtered to remove the coarsely polymerized PPy particles after 30 min and a clear filtrate was used to infiltrate the sulfonated WV samples.The veneer samples and reaction solution were kept in an ice water bath for 3 h under gentle stirring and subsequently washed several times with deionized water.
Structural Characterizations: The microstructure of the samples was analyzed by FESEM (Hitachi S-4800, Japan) at an acceleration voltage of 1 kV and a working distance of 8 mm.The samples were sputtered with a platinum/palladium conductive layer using a sputter coater (Cressington 208HR, UK).EDX was performed at an acceleration voltage of 6 kV and a working distance of 15 mm with an Oxford Instruments, X-MAX N 80, UK.Clean sections of the sample's surface were obtained prior to analysis with a sliding microtome (SM 2010R, Leica, Germany).The determination of total acidic group content was performed in duplicates by conductometric titration according to SCAN-CM 65:02 using a Metrohm 856 Conductimeter.Before the analysis, samples were disintegrated using a Wiley mill to obtain a fine powder.Specific surface area and pore size distribution were evaluated by nitrogen physisorption and calculated according to BET and Barrett-Joyner-Halenda methods, respectively, using a Micromeritics 3Flex.Prior to the measurement, the samples were degassed at 110 °C for 24 h.Lignin content (Klason lignin) was determined according to TAPPI method (TAPPI Test Methods T222 om-02) by acid hydrolysis with 72% sulfuric acid.Quantification of the neutral sugars was performed on a Dionex ICS-3000 ion-exchange chromatograph (Thermo Fisher Scientific Inc., USA) after hydrolysis.The samples were analyzed in duplicates and anhydrous factors were used for the monosaccharides (0.88 for xylose and arabinose, 0.90 for glucose, mannose, and galactose).The weight percentage of cellulose and hemicellulose were calculated using Meier's correlations. [38]The FTIR spectra were recorded on a Spectrum 100 FT-IR spectrometer (PerkinElmer Inc.) equipped with a Golden Gate diamond ATR (Gaseby Specac Ltd., UK).The spectra were recorded at room temperature in the range between 4000 and 600 cm −1 .The three-point bending tests were performed using an Instron 5944 (USA) equipped with a 500 N load cell.The tests were carried out with a span of 2.0 mm and a crosshead speed of 0.1 mm s −1 .A preload of 0.5 N and 10 mm min −1 extension rate was applied.The samples were cut into strips of 30 × 5 mm 2 (Longitudinal × Tangential) and preconditioned for 48 h in a room at a controlled temperature of 22 °C and 50% relative humidity.
Conductivity Measurement: The SWV/PPy sample was glued on top of four chromium/gold electrodes by carbon paste and measured using a four-probe technique.The Keithley 2400 sourcemeter was used to supply the current to the two outer electrodes, and the voltage between the two inner electrodes was recorded using a four-wire sensing mode.Based on the recorded resistance, the electrical conductivity (σ) and sheet resistance (R s ) of SWV/PPy sample were calculated using the equations below where R is the obtained resistance, L is the distance between the two inner electrodes, A is the cross-sectional area of the specimen (CWVs), and t is the thickness of the sample.
Electrochemical Measurement: The self-standing SWV/PPy sample was connected to carbon fibers using carbon paste before wrapping a part of the carbon fiber with paraffin wax and Kapton tape, respectively.The electrochemical measurement was performed in a three-electrode configuration system using NaCl 1 m as electrolyte with a potentiostat/ galvanostat (by Biologic, SP-200) coupled to a computer.The areal/ mass-specific capacitances and ESR of a single pseudocapacitor electrode were calculated from its GCD profiles by the following equations [7,12,14] where I is the discharge current, ∆t is the discharge time, S is the effective area of the electrode, ∆V is the difference between operation potential, the voltage drop (V d ), and m is the mass of the electrode.

Figure 1 .
Figure 1.a) Content of acidic groups as measured by conductometric titration and b) FTIR spectra of the sulfonated pine WV as compared to the native WV.

Figure 4 .
N and Cl cross-sectional maps are shown along with C for comparison.

Figure 4 .
Figure 4. FESEM images of cell wall corners in the composites of a) WV/PPy, e) SWV-3 h/PPy, and i) SWV-14 h/PPy with corresponding EDX maps for b,f,j) carbon, c,g,k) nitrogen, and d,h,l) chlorine.

Table 3 .
Conductivity and sheet resistance of WV/PPy and sulfonated WV/PPy composite samples.