Electrospun lignin fibers and their application
Xiangyu You1), Junlei Duan1), Keiichi Koda2), Tatsuhiko Yamada3), and Yasumitsu Uraki2)
1) Graduate School of Agriculture, Hokkaido University, Sapporo 060-8589, Japan.
2) Research Faculty of Agriculture, Hokkaido University, Sapporo 060-8589, Japan.
3) Division of Biomass Chemistry, Forestry and Forest Products Research Institute, Tsukuba 305-8687, Japan.

Introduction
Lignin has attracted much attention as an alternative raw material to fossil resources. Recently, lignin-basedcarbon fibers are being focused on as one of value-added functional materials. So far they have been prepared by melt
spinning or dry spinning followed by thermostabilization and carbonization. In this study, we attempted
electrospinning and melt-electrospinning of different lignin preparations, such as acetic acid lignin (AL), soda lignin(SL) and poly(ethylene glycol)-lignin (PEGL) [1, 2], to prepare thin fibers as precursory fibers for carbon fibers(CFs) and activated carbon fibers (ACFs). Herein, we demonstrate preparation routes for CFs and ACFs, especiallyusing modified method of thermostabilization in order to shorten its treatment time. In addition, we would like toreport a new utilization of lignin-based CFs and ACFs as electrodes for electrical double layer capacitor (EDLC).

Experimental
Preparation of AL fibers by dry-electrospinning
Hardwood acetic acid lignin (HAL) was dissolved in acetic acid (AcOH) or AcOH/CCl4 (weight ratio, 8:2), andthen hexamethylenetetramine (hexamine) with different amounts was added to the solution. The mixture solution wassubjected to electrospinning of lignin fibers by using a homemade electrospinning machine, and it was carried out
under the following conditions: applied voltage, 38 kV; solution flow rate, 5 ml/min; tip-collector distance, 13 cm.
Preparation of SL fibers by dry-electrospinning
A mixture of softwood soda lignin (SSL) and poly(ethylene glycol) (PEG, Mw= 500 kDa) was dissolved in
N,N-dimethylformamide (DMF), DMF/acetone and DMF/ethanol, and followed by heating at 80°C for 2.5 h. The asprepared lignin solutions were then directly electrospun under these conditions: applied voltage, 25 kV; solution flow
rate, 0.5 ml/h; tip-collector distance, 13 cm.
Preparation of PEGL fibers by melt-electrospinning
PEGL alone derived from softwood or its mixture with different solvents, such as acetone, ethanol, and DMF, indifferent weight ratio, was melt-electrospun by a homemade spinning machine under the following conditions:Nozzle temperature, 145-150 oC; applied voltage, 20 kV; winding rate, 50 m/min.
EDLC assembly and characterization
An EDLC assembly process is shown in Fig. 1. As a first process, the electrospun lignin fibers as mentionedabove were converted to CFs and ACFs. The lignin fibers were thermostabilized under air atmosphere by heating up
to 250 oC at different heating rates. Then, the thermostabilized fibers were, in turn, carbonized under a nitrogenstream at 1000 °C to yield lignin based CFs. Finally, the resultant CFs were steam activated at 900 oC. Thus, lignin
based ACFs were obtained.
In the case of melt-electrospun
fibers, the lignin fibers were first
immersed in 6 M HCl aqueous solution
at 100 oC to give chemically stabilizedfibers.

Thesesubjectedfiberstowere

furtherthermostabilization,

carbonization and activation by the sameprocesses as other electrospun fibers.
The resultant CFs and ACFs weregrounded to powder, and suspended inaqueous carboxymethyl cellulose (CMC)
solution.

These

suspensions

wereultrasonicated to give homogeneousslurry. Then, 5% of conductive carbon
black(CB)carbonaceous

wasadded

tosample/CMC

thesuspen-sions, and the mixtures were spread onan Al foil to yield electrode. The
electrodes and a piece of paper sheet(Mitsubishi paper Mills Ltd. Tokyo,Japan) as a separator were immersed intriethylmethylammonium

tetrafluoro-borate (TEAMBF4)/propylene carbonate(PC) solution. An Al foil was placed at
the bottom of electrode flat cell, and thecellulosic separator was put on the foil.
Afterwards, the other Al foil was placedon the separator to give EDLC.

Fig. 1 A scheme for lignin-based ACF preparation andEDLC assembly

The performance of our EDLC was evaluated by cyclic voltammetry (CV), electrochemical impedance
spectroscopy (EIS), and galvanostatic charge/discharge (GCD) method using an electrochemical workstation(Autolab PGSTAT302N FRA32M, Metrohm Autolab B.V., Japan).

Results and discussion
Preparation of lignin based fibers
The structure of lignins varies depending on the plant source and the method of isolation. Furthermore, they also
affect lignin fusibility and solubility in different solvents. Herein, both dry-electrospinning and melt-electrospinningof different lignin preparations were attempted to obtain lignin fibers. Dry-electrospinning is a very simple method,which enables spinning by electrical force, and applied to most of polymer solutions to produce fine fibers, while
melt-electrospinning can be applied only to polymer melt. The latter spinning costs rather low because of no use ofsolvent. So the requirements of two methods are basically different. AL electrospun fibers (Fig. 2-A) could beobtained by dry-electrospinning from 35 wt% of AL/AcOH solution. An average fiber diameter was 1.30 μm. SL
electrospun fibers (Fig. 2-B) were also prepared by dry-electrospinning from 1 wt% of PEG. On the other hand,
PEGL fibers was able to be obtained by melt melt-electrospinning. However, the fiber diameter (18.0 μm) wasrelatively large. The diameter could be decreased by mixing PEGL with 30 wt% of DMF (Fig. 2-C). Therefore, we
provided relatively feasible routes for preparing lignin-based thin fibers.

Fig. 2 Images of A) AL electrospun fibers, B) SL electrospun fibers, C) PEGL melt-electrospun fibers, and D)
thermostabilized fibers, E) CFs and F) ACFs derived from AL electrospun fibers.
EDLC assembly and electrochemical characterization
The obtained fibers were converted into CFs and ACFs as a raw material for electrode of EDLC by stabilization,carbonization and activation (Fig. 1). In conventional thermostabilization process, it is carried out at 250 °C in air
atmosphere for sufficient time to render them infusible by introducing oxygen molecules into lignin molecules. In
general, a low heating rate is impractical because very long periods are required (i.e. for 2 days), while a high heating
rate causes violent exothermic reactions resulting in a melt. Therefore, optimum stabilization conditions (heating rate,heating time and heating temperature) should be established. Thus, thermostabilization is very critical in the ACFs
production process. To overcome this subject, we tried to add hexamine to the lignin solution, which was expected toact as a crosslinker to suppress thermal mobility of lignin. When hexamine was added to AL/AcOH solution, noelectrospun fiber was obtained because of rapid curing before spinning. Therefore, when the reagent was applied to amixture solvent of AcOH and CCl4, the dry-electrospinning of AL mixture was carried out. When the resultant fibers
with 10% hexamine (on solution) were subjected to thermostabilization under air atmosphere, the infusible fibers (Fig.
2-D) were obtained by 3 h treatment. In addition, the resultant CFs (Fig. 2-E) obtained a large BET specific surface
area of 1287 m2/g even without activation. The CFs were easily converted into ACFs with the surface area of 2185
m2/g (Fig. 2-F) by steam activation.
EDLC was assembled with the resultant CFs or ACFs as electrode materials and cellulosic paper as a separator,
which contained TEAMBF4/PC as an organic electrolyte, where commercial activated charcoal (AC) powder wasalso used as a reference.

As shown in Table 1, the specific electrostatic capacitance and charge transfer impedance (Rct) of ACF-based
EDLC were 25.1 F/g and 37.2 Ω, respectively. These electrochemical properties were remarkably superior to those ofEDLC assembled with AC powder. The excellent performance was attributed to large specific surface area of ACFs.
However, these values were quite low, compared to those of EDLC previously reported[3]. By the addition of CB,
EDLC performance was drastically improved; Rct of ACF-CB based EDLC was decreased to 12 Ω and thecapacitance was remarkably increased to 133.3 F/g, which was comparable among the reported work[4]. Therefore,
lignin-based ACFs can be a promising electrode material for preparing EDLC.

Acknowledgement
This work was supported by a grant from the Ministry of Agriculture, Forestry and Fisheries of Japan,"Development of Technologies for Biofuel Production Systems in Rural Areas (2012-2015)".

Table 1. The electrochemical properties of EDLC based on different electrode materials.

SampleSpecific
capacitance1)(F/g)

Specific capacitance2)(F/g)

Charge transfer resistance(Ω)

AC powder-0.1>4000

AC powder-CB

74.8

90.9

2.4

AL-ACF-2

27.9

25.1

37.2

AL-ACF-CB-2

142.0

133.3

12.0

PEGL-CF-CB

87.5

94.3

7.01)

Specific capacitance was obtained by cyclic voltammetry2)

Specific capacitance was obtained by galvanostatic charge and discharge method.

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