Mesoporous carbon soft-templated from lignin nanofiber networks

ab Flexible electrodes with supercapacitance were developed from highly mesoporous carbon ﬁ bers synthesized from lignin. Polyvinyl alcohol (PVA) facilitated the electrospinning of aqueous solutions of lignin and was used as a sacri ﬁ cial polymer. Most importantly, PVA produced phase-separated domains for extreme surface area (>2000 m 2 g (cid:1) 1 ) and mesoporous volume (0.7 cm 3 g (cid:1) 1 ). An optimized sequential thermal treatment that initially included stabilization at 250 (cid:3) C, allowed the formation of ﬂ exible, freestanding carbon networks upon PVA evolution to the gas phase and carbonization of the as-spun lignin-based ﬁ bers. Their main morphological and chemical characteristics were assessed by ﬁ eld emission scanning microscopy, transmission electron tomography reconstructions and Raman spectroscopy. The carbon ﬁ ber networks were used directly as electrodes with electrochemical double layer capacitance as determined by cyclic voltammetry and galvanostatic charge/discharge methods. Excellent electrochemical performance was demonstrated from the measured high rate capability and long-term cycling stability. The determined speci ﬁ c capacitance ( (cid:4) 205 F g (cid:1) 1 in 0.5 M Na 2 SO 4 electrolyte) is one of the highest recorded for electrodes obtained from biopolymer precursors. Moreover, the electrical conductivity of the carbon ﬁ ber network (386 S m (cid:1) 1 ) was signi ﬁ cantly higher, by two-orders of magnitude, than that obtained from the precursor (non-ﬁ brous, powder) sample (2.47 S m (cid:1) 1 ). The remarkable performance of the synthesized electrodes is ascribed to the robust network morphology and mesoporosity obtained by soft-templating from the phase-separated sacri ﬁ cial polymer. This is a demonstration of lignin valorization for novel application


Introduction
Electric double layer capacitors (EDLC), the most common and current devices for energy storage, are based on the electrical adsorption of ions at the electrode/electrolyte interface.EDLCs have a number of desirable characteristics, such as high power and energy density as well as rapid charge/discharge capability.Supercapacitors have the potential to compete with or replace batteries for energy conversion and storage, this is most relevant to transportation, including hybrid electric vehicles and metro-train transportation. 1,2Carbon materials, including activated carbon, [3][4][5][6][7] and those derived from aerogels, 8 nanotubes, 9,10 templated porous systems, [11][12][13] and nanonobers, 14 have been considered for related purposes.This is because of advantages that include low cost, easy processing, non-toxicity, high specic surface area, good electric conductivity, high chemical stability, and a wide range of operational temperatures. 15owever, there are still technical challenges that need to be overcome for developing high electrical capacitance.
Carbons for double-layer type supercapacitors should ideally have three main properties, namely, (1) high specic surface areas, of the order of 1000 m 2 g À1 , (2) good intra-and interparticle conductivity in porous matrices, and (3) good electrolyte accessibility to the intrapores. 16Several methods have been developed to obtain large specic surface areas as well as suitable porosity in carbon materials, including activation in alkaline or acidic media, [17][18][19] steam 20,21 or carbon dioxide treatments, 22 hard-templating via lling of inorganic particles within an organic precursor, organic-organic assembly via sotemplating, 23,24 among others.Particularly, so-templating methods are suitable for developing mesoporosity in carbon materials towards conductive electrodes.This is mainly because of the small amounts of impurities le behind aer carbonization.
Prior studies have reported successfully on the fabrication of ordered mesoporous carbons by using so-templating synthesis with thermosetting network polymers.This has included phenolic-type resins as a carbon source and block copolymers; resorcinol-formaldehyde and diblock copolymers (PS-b-P4VP); 25 resorcinol-formaldehyde and Pluronic surfactants 26,27 and PANb-PMMA. 28In these studies, the pore size developed from the precursor structure was well controlled, at the molecular scale.
However, some drawbacks exist in these systems, such as the complex procedure used for polymer synthesis, timeconsuming protocols and relatively high cost.Furthermore, additional limitations include the choice of both carbon and porogens precursor materials, to avoid the collapse of the porous structure during high-temperature treatment.In fact, only few materials meet associated requirements. 23In this context, lignin has attracted attention as a precursor of carbon materials owing to its high carbon content (more than 60% in phenyl propane groups), as well as its thermal stability, and favorable stiffness. 29,30Thus, we propose lignin as a widely available (exceeding 300 billion tons 31 as a byproduct of the pulp and paper industries and bioreneries), 32 renewable and inexpensive aromatic polymer for the generation of carbon electrodes and as an attractive option to replace synthetic thermosetting polymers as source of mesoporous carbon.Sotemplating, on the other hand, can be achieved by addition of a sacricial polymer, polyvinyl alcohol (PVA).
The few, recent studies related to supercapacitors by using activated carbon from lignin are listed in Table 1.A capacitance as high as 268 F g À1 (ref.33) was recorded in the presence of a conductive additive in the working electrodes.
In this study we developed a highly mesoporous nano-and micro-carbon ber network as conductive electrode for supercapacitance by using lignin as a carbon precursor.The distinct advantage of the proposed approach is the synthesis of a mesoporous carbon network that can be utilized directly as an electrode with no need of additives and even in the absence of binders.
Recently, we demonstrated that the sub-micron bers can be synthesized easily by electrospinning of aqueous solution of lignin.We also concluded that the physical properties (ber diameter, morphology, etc.) can be tuned by the addition of polyvinyl alcohol (PVA) as a minor component. 37,38Moreover, our previous studies indicated microphase separation between lignin and PVA, which spontaneously segregated during the electrospinning process. 38For the system containing 75 wt% lignin (PVA, 25 wt%), discontinuous lignin/PVA phases formed within a lignin-rich, external phase.Based on these results, we hypothesize that highly mesoporous carbon ber networks are feasible upon carbonization and by templating the two-phase morphology, from microphase separation of lignin and PVA.Here, the main lignin domain would act as carbon scaffold, while the mesopore domain would result from the PVA phases acting as a so template.Fig. 1 illustrates the proposed component distribution in lignin/PVA electrospun bers.
Free-standing brous carbon mats can be useful as performance conductive electrode in supercapacitors.A continuous brous morphology would also enable mechanical robustness, exibility and stability.Furthermore, exible electrodes, which may operate awlessly even upon twisting, are desirable in the design of light and exible devices, which are demanded in applications such as exible circuits, 39 displays, 40 solar cells, 41,42 and pressure sensors. 43,44][47] Such process balances electrostatic repulsive forces in the polymer solution and an externally-applied electric led.Additionally, it can be scaled up to ensure low production costs.Here, we propose that high surface area electrospun mats obtained from lignin/PVA solutions, which are exible and  mechanically robust, are suitable, directly aer carbonization, as freestanding electrodes.Therefore, we produced electrospun mats from lignin/PVA (75 : 25) as carbon precursor to fabricate highly mesoporous carbon networks using so-templating.The morphology and porous structure of the resultant carbon network were investigated.In addition, electrochemical measurements for supercapacitance was performed to afford new storage devices from such mesoporous system.Overall, this contribution attempts to demonstrate the use of a renewable biopolymer to synthesize brous networks for free-standing and exible carbon mats that can be used as supercapacitor electrodes.

Results and discussion
Aqueous solutions of lignin/PVA, at given concentrations, were found to be suitable in the production of defect-free electrospun bers. 37In this study, a lignin/PVA mass ratio of 3 : 1 (w/w) and 17.4% (lignin + PVA) polymer concentration was chosen as precursor system for electrospinning.Mat 1, Mat 2, and Mat 3 are used herein to indicate the carbon ber networks obtained by the respective carbonization protocol, as indicated in Table 2.The as-spun brous mats were subjected to carbonization at 900 C for 2 h under nitrogen ow followed by different combinations of isothermal stabilization and temperature gradient.When the as-prepared electrospun mat was subjected to isothermal treatment at 250 C, which is above T g of lignin, 48 the yield of the nal carbon product was improved, up to 13%; in addition, the ber morphology remained unchanged upon carbonization at 900 C for 2 h (Mat 3).Without the stabilization step, the yield proved to be too low (Mat 0, $0% yield).The condensation reactions between phenolic units, which introduced the main lignin domain endowed ber thermal stability. 49,50Without the stabilization step, the effect of decomposition, chain scission of lignin as well as PVA resulted in network disintegration (for a $0% yield upon carbonization at 900 C).Aer the isothermal treatment, an equal amount of potassium hydroxide was used as a chemical activator to increase the specic surface area. 17,51The chemical activation was achieved during carbonization (up to 900 C) under nitrogen ow.The heating rate used in the carbonization inuenced the yield of the nal product.A high heating rate, e.g., 20 C min À1 , was not suitable to produce a carbonaceous material (Mat 4, ca.$0% yield).However, a moderate heating rate (4-10 C min) afforded yields between 10 and 13% (Mat 1, 2, and 3); it was also effective in retaining the nanostructure as well as brous network morphology.Aer many lignin carbonization tests, it was determined that freestanding brous carbon mats with preserved network structure can be achieved by thermal treatment (250 C) for stabilization followed by heating to 900 C (using a temperature gradient of 4 or 10 C min À1 ).The three conditions for thermal treatment applied to the as-spun ber mat afforded the highest efficiency in terms of yield and processing time.Also, compared to other conditions, they produced lms with better exibility and mechanical robustness.
The morphology of the as-spun bers, before and aer carbonization, is shown in Fig. 2. Fibers with a radius of 148 AE 8 nm were observed as a randomly oriented network.Fig. 2c and  d show the morphology for Mat 3 aer carbonization followed by isothermal treatment and chemical activation.Some bers adopted a higher curvature and fused into the network structure possibly due to the melting of PVA distributed in the bers during the isothermal treatment (250 C).The diameter of the bers remained unchanged aer carbonization, as concluded from repeated experiments under the same conditions.A micrograph obtained with high magnication revealed numerous small pores on the surface of the bers, which could be accessible to the electrolyte.The carbon mats could be bent easily (Fig. 2e) and recovered the original at shape aer the stress was release, indicating that the mats were mechanically strong and the bers formed an entangled and interconnected network.As a reference, lignin powder was subjected to carbonization by using the same conditions as the ones used for Mat 3 (herein indicated as "Powder" sample).A lignin/PVA solid lm prepared by solvent casting was also prepared for comparison ("Solid lm").Note that the lignin/PVA solid lm is expected to be closer to thermodynamic equilibrium because of a "slow" consolidation upon solvent evaporation. 38In this slow process, the interfacial tension between lignin and PVA drives phase segregation to yield PVA domain sizes as large as 500 nm. 38The effect of the characteristic time of treatment on the porous structure of the derived carbon material (from electrospun ber mats or solid lms) is discussed later.Both the powder sample (powder) and the solid lm (Solid lm) presented a higher yield (26% and 45%, respectively) compared to the brous mats (Table 2).
Nitrogen adsorption measurements (BET) were carried out in order to examine the specic surface area, pore volume, and pore size distribution of the carbon brous mats, powder and solid lms (Table 3).The NLDFT model was used to analyze the BET isotherm. 52The pore size distributions for the powder and the mat samples are shown in Fig. 3.For the brous mats, highly specic surface areas were recorded (1689, 1387, and 2005 m 2 g À1 for Mat 1, 2 and 3, respectively).Also, a high pore volume was determined (0.60, 0.71, and 0.70 cm 3 g À1 ) with a dominant mesoporosity (58, 77, and 66%, respectively).The isotherms for the mat samples and the solid lm presented a typical type-IV isotherm with a hysteresis loop in the P/P 0 ¼ 0.45-0.6relative pressure region, which is attributed to mesoporous structure (Fig. S1a †).The isotherm displayed a rapid adsorption rise in the low-pressure region, indicating the presence of micropores.In contrast, the powder sample exhibited a BET specic surface area of 1236 m 2 g À1 with total pore volume of 0.25 cm 3 g À1 along with a half of mesoporous volume (51% mesopore).Besides, the isotherm of the powder sample showed type-I isotherm.These results suggest that the powder sample possessed a highly microspore structure (Fig. -S1b †).The brous mats and the powder sample exhibited two prominent peaks in the pore size distribution, in the micropore (<2 nm) and mesopore (2-50 nm) regions.An interesting observation is that compared to the powder sample, the brous carbon mats presented a broader distribution in the mesopore region, centered around 2.5 nm (Mat 1 and Mat 3) or 2.9 nm (Mat 2).The BET surface area and pore size distribution conrmed the presence of highly mesoporous structures with high specic surface area.The pore size distribution can be explained in terms of microphase separation of the precursor electrospun bers.In the 75 : 25 lignin/PVA system, the twophase morphology arises from discontinuous PVA domains separated from the lignin, main phase. 38As the electrospun bers were subjected to carbonization, the lignin scaffold was carbonized while the PVA phase evolved into gas, leaving behind a porous network.Further insights into the effect of the PVA domain size on the mesopore distribution can be gained from the solid lms prepared by the evaporation-casting   method.In this process, the characteristic time for densication is much longer than in electrospinning, which leads to conditions that are closer to equilibrium.In turn, this induces large phase-separated domains, with sizes of the order of 500 nm.BET surface and pore volume determinations for the solid lms revealed signicantly reduced values compared to the mat samples.Moreover, the isotherm exhibited type-IV prole characteristic of mesoporous structures (Fig. S1c †).
The results suggest a lower pore volume for the lms compared to the electrospun bers, owing to the larger PVA domains in the former structure.The PVA domains were effective in introducing the mesoporous structure in the solid lm, depending on the domain size.The mesoporous structure of the carbon brous mat is evident in TEM micrographs and transmission electron tomography reconstructions (Fig. 4), which indicate the presence of numerous randomly distributed mesopores (the lighter areas in Fig. 4b) with diameter of $5 nm.They can be ascribed to the PVA domains that so-templated as pores that, in turn, would allow electrolyte access from the outer surfaces (Fig. 4c).The results of SEM and TEM analyses are consistent with the results of nitrogen adsorption.It was later noticed that the capacitance strongly depended on the specic surface area as well as the pore size distribution of the electrode materials.Since higher accessibility of the electrolyte can contribute on higher charge accumulation at the electrodeelectrolyte interface. 53aman spectroscopy was applied to estimate the degree of graphitization of the carbonized samples (carbon brous mat as well as powder sample) (see Fig. S2 †).Two Raman bands in the region between 1100 and 1800 cm À1 were distinctively observed for both the mat and powder samples.A strong band at 1590 cm À1 , the G-band, and another band at 1350 cm À1 , the D-band, were apparent.A similar peak position of the D and G peaks for the different samples was noted.The presence of graphitic bonding due to the periodic sp 2 valance in crystalline carbonaceous material would strengthen the intensity of the G-band, while the D-band would reect the presence of disorder and defective carbonaceous constituents.Therefore, the relative D-to-G band intensity ratio, I D /I G , can be related to the perfection of the graphitic layered structure, reecting the graphitization degree of the material. 54The I D /I G ratios were similar for the samples tested, around 1.25-1.26,which indicates that the carbonization condition and the morphology of the samples (brous mat or powder) did not inuence the graphitization.
A high electrical conductivity is of great signicance in the development of electrodes for supercapacitors.The electrospun carbon mats derived from the lignin/PVA bi-component bers were not completely graphitized; however, the electrical conductivity of the mat samples (386 S m À1 ) was signicantly larger, by two orders of magnitude, than the one measured for the powder sample (2.47 S m À1 ).This can be explained by the morphology of the ber network that provides a continuous pathway for effective electron transport and for a reduced electrochemical resistance in the electrode.In contrast, the powder samples contain micron/nano-sized carbon grains that may not have enough entanglement to yield a proper electron conductivity.

Electrochemical measurements
The supercapacitance of the carbon brous mats and powder samples was studied by a three-electrode cell conguration.Cyclic voltammograms (CV) were obtained at cell potential window from À0.2 to 0.8 V (vs.Ag/AgCl) at scan rates (n) from 5 to 100 mV s À1 in 0.5 M Na 2 SO 4 electrolyte.The as-prepared carbon mats were used directly as the working electrode, while in the case of the power sample a modied glassy carbon electrode was produced with a drop of carbon ink.proles exhibited a nearly symmetrical rectangular shape at low scan rates.This corresponds to an ideal reversible capacitive behavior with good performance characteristics, fast ion diffusion and electron transfer rates for EDLC.Nevertheless, a distortion of the CV curves was observed at high scan rates, likely due to limitations in electrolyte transport and diffusion.Although this phenomenon can be seen quite oen in carbonbased electrode materials, further studies are needed to overcome this effect.It is noticeable that the current density for Mat 3 was higher than that of the powder sample.
The capacitance values at a scan rate of 5 mV s À1 was calculated; under this condition a prominent rectangular shape in the CV plots, with maximum plateau atness for both mat and powder samples, was observed.As expected, Mat 3 produced a higher specic capacitance, 205 F g À1 , than that of the powder, 93 F g À1 .The other samples, Mat 1 and Mat 2, exhibited capacitance values of 204 F g À1 and 155 F g À1 , respectively.The remarkably high supercapacitance of the mat samples can be attributed to the high surface area and welldeveloped mesoporous structure, with open pores accessible to ion/electrolyte transport.Thus, the freestanding carbon network structure in the brous mats provides effectively more active sites due to high specic surface area and mesoporosity, as well as highly electrical conductivity.The featured high capacitance and the exibility of the free-standing brous carbon mats offer promising advantages for use in electrochemical supercapacitor devices.At the same time, they involve facile and fast synthesis.As comparison, we further studied the Solid lm that was prepared by solvent casting and noted a quite low capacitance (6.4 F g À1 ).Fig. 5c and d show typical charge-discharge proles of Mat 3 and powder samples in 0.5 M Na 2 SO 4 solution at current densities from 0.3 A g À1 to 2 A g À1 .The charge-discharge curves were similar in shape between 0 and 0.6 V, indicating that the double layer capacitance can form reversibility in a wide range of current densities, for both mat and powder samples.Mat 3 electrode, with a large BET specic surface area, exhibited a longer discharging interval, which suggests a higher electrical capacity.The results were in accordance with the CV proles.However, voltage drops at the turning point of charge-discharge can be seen, especially in the measurement with higher current density, i.e., 2 A g À1 , for both mat and powder samples.This indicates energy losses due to the internal resistance by diffusion-limited mobility of the electrolyte ions in the electrode pores.
The cycling stability of the Mat 3 and powder samples was also examined by CV measurements over 1500 cycles, at a scan rate of 10 mV s À1 , and the supercapacitance retention was monitored as a function of cycling number (Fig. 6).Mat 3 kept $83% of the initial supercapacitance aer 1500 cycles, indicating good electrochemical stability as a supercapacitor electrode material.The brous carbon network in mat sample can work as a support matrix, which provide structural robustness and stability.For the powder sample, a steep decline in supercapacitance was observed, which may be due to the disintegration of active material and mechanical detachment from the glassy carbon electrode.
Lignin/PVA blend electrospun bers Lignin/PVA solutions were prepared according to the method described in our previous report. 37Briey, aqueous PVA solutions (15 wt% concentration) were prepared and lignin was added to obtain solutions with lignin/PVA dry mass ratio of 75 : 25.Distilled water was added to the lignin/PVA solution to obtain a viscosity suitable for electrospinning.The solution was kept under vigorous mechanical agitation at 60 C for 1 h, followed by cooling to room temperature under stirring for 4 h.Solutions stored for less than one-week were used to produce the electrospun bers.For electrospinning, the lignin/PVA solution was loaded into a 10 ml plastic, disposable syringe with a 22-gauge needle.The needle was connected to the positive terminal of a voltage generator designed to produce a voltage up to 30 kV DC (Spellman SL30, USA).A thin aluminum foil covering a 15 cm diameter aluminum plate was used as a collector.The plate was connected to the negative electrode of the power supply (ground) and set at a working distance of 20 cm.An operating voltage of 17 kV was used.A constant 8 or 10 ml min À1 ow rate was maintained during electrospinning using a computer-controlled syringe pump.Electrospinning was performed at room temperature and at 35-45% relative humidity.The collected electrospun mats were kept in a desiccator containing anhydrous silica gel.

Solid lm preparation
Solid lms were prepared by solution casting from lignin/PVA with a 75 : 25 dry mass ratio, same as used for the electrospinning.The aqueous solution was poured onto a clean Teon plate and dried overnight at room temperature in a dust-free atmosphere.The resulting lms were ca.40 mm in thickness as detected by optical microscopy.

Carbonization
The carbonization procedure was optimized aer a series of experiments.Firstly, stabilization of as-spun ber was performed by heat treatment (heating rate of 4 C min À1 ) at 250 C for 1 or 2 h in a nitrogen atmosphere followed by a second heating treatment at 600 C for 1 h.The stabilized ber mats were impregnated with KOH in aqueous solution (ber : KOH mass ratio of 1 : 1).The impregnated sample was dried in an oven at 60 C for 2 h and then carbonized in a metal chamber inside a muffle furnace.Typically, the sample was heated to 900 C for 2 h (heating rate of 4 C min À1 ) under nitrogen gas ow (400 ml min À1 ).Aer the carbonization, the samples were down to room temperature under nitrogen gas ow and washed with distilled water repeatedly until neutral pH followed by 1 M HCl rinsing and nal washing with distilled water.As a last step, the samples were washed with ethanol and dried in an oven.Lignin powder was used as a control sample along with the respective solid lms, both of which were subjected to the same procedure as indicated for the brous mats.These samples are referred to as "Powder" and "Solid lm".

Material characterization
The morphology of the electrospun nanobers and powder samples were examined using a eld emission scanning electron microscope (FE-SEM) (Zeiss SigmaVP, Germany) operating at 1.6 kV and a working distance 4 mm.A small piece of the nanober mat was xed on a carbon tape and then sputtered with Pt. transmission electron microscopy (TEM) and electron tomography (ET) was carried out using JEOL JEM 3200FSC eld emission microscopy operated at 300 kV in bright eld mode with an omega-type zero-loss energy lter.For TEM measurements, the sample was dispersed in ethanol by sonication, and evaporated on a lacey carbon grid.ET tilt series acquirement was carried out with the SerialEM-soware package by tilting the sample between AE70 with 2 increment steps.Image alignment was done with IMOD. 55Maximum entropy reconstruction scheme was carried out with MEM program 56 on Linux cluster with regularization parameter value d ¼ 5 Â 10 À2 .The images were binned twice to reduce noise and computation time.The specic surface area of the samples was analyzed by the Brunauer, Emmett, and Teller (BET) method while the micropore volume was estimated using the non-local density functional theory (NLDFT).Raman spectroscopy was used to detect the graphitic carbon structure and the Raman spectra of the carbonized samples were obtained by exposure to laser light (633 nm wavelength).The DC electrical conductivity of the carbonized samples (both carbon ber mat and powder) was measured using a Jandel four-point probe system.The electrical conductivity s was determined using the equation s ¼ L/AR S where R s is the sheet resistance, A is the cross-sectional area, and L is the distance between the electrodes (6 Â 10 À3 m in our Jandel instrument).

Electrochemical activity
The carbonized electrospun mats were used directly as a freestanding working electrode.For the powder sample, 40 mg ml À1 catalyst ink was prepared by dispersing the powder sample in ethanol containing ion exchange ionomer, FAA3, (Fuma-Tech) and deposited on a glassy carbon electrode followed by oven drying at 60 C. The working electrodes were assembled into a three-electrode cell conguration with Ag/AgCl reference electrode and platinum plate or rod counter electrodes.Electrochemical evaluations were performed using an Autolab PGSTAT12 potentiostat (EcoChemie, The Netherlands) controlled by the GPE soware.Cyclic voltammetry (CV) measurements were carried out at a potential window from À0.2 V to 0.8 V (0.5 M Na 2 SO 4 electrolyte).Galvanostatic charge/ discharge tests were also performed using various current densities (between 0.3 to 2 A g À1 ).The specic capacitance C m was calculated from the discharging curves according to the following equation: , where I is the constant discharge current and t is the discharge time, m is the mass of active materials (g), and DV is the potential window of discharge.The cycling stability of the brous carbon mat and powder samples was carried out for over 1500 cycles.

Conclusions
Freestanding mats comprising carbon nanobers were prepared successfully by using an abundant natural polymer, lignin, via electrospinning followed by carbonization at 900 C. According to the morphology and pore analyses, the freestanding carbon mat exhibited a network structure consisting of carbon micro/nano bers with high surface area, up to 2005 m 2 g À1 , as well as large pore volume, 0.7 cm 3 g À1 with a dominant 70% mesopore content.The lignin-based carbon brous network afforded an excellent specic double layer capacitance, 205 F g À1 , and maintained their capacity up to 83% over 1500 cycles.This outstanding electrochemical performance can be explained by the broad pore distribution in the mesopore range of the carbon bers, which was induced by the combination of microphase separation between lignin and polyvinyl alcohol.A schematic representation of the microphase separation of the electrospun lignin/PVA bers and the mesoporous structure in the carbon ber is proposed (Fig. 1).This type of so polymer templating allows the formation of mesoporous carbon nano-bers via electrospinning combined with chemical activation.Overall, we demonstrate that lignin can be easily converted into a high-end material for efficient energy storage technologies.

Fig. 2
Fig. 2 SEM images of electrospun lignin-based fiber mats before (a and b) and after carbonization at 900 C, for 1 h at a rate of 10 C min (c and d).Included also is a photograph of a flexible carbon mat (3 Â 3 cm 2 ) after carbonization (e).

Fig. 5
Fig. 4 TEM micrograph of carbon fibrous (a); cross section slice from 3D (ET) reconstruction (b); isosurface model from segmented 3D reconstruction of Mat 3 (c).Scale bar is 100 nm in (a) and 40 nm in (b and c).Black dots are fiducial gold markers used for image alignment purposes in the ET reconstruction.

Table 1
Specific capacitance of materials obtained from mixtures with lignin carbon a a PVDF ¼ polyvinylidene uoride; PTFE ¼ polytetrauoroethylene; NEt 4 BF 4 ¼ tetramethyl ammonium tetrauoroborate; CB ¼ carbon black."Carbon" in the "Electrode" column refers to the material obtained from lignin.

Table 2
Carbonization conditions, yield, and the resultant morphology for lignin-based electrospun mat, lignin powder, and lignin-based solid film

Table 3
Pore volume and specific surface area (SSA) of the carbon materialsSampleV total , cm 3 g À1 V micro , cm 3 g À1 V meso , cm 3 g À1 Mesopore ratio, % BET SSA, m 2 g À1