# Water-Based Production of Carbon Fibers from Lignin and Alginate

Olympe Longeras, Sébastien Gounel, Wilfrid Neri, Emmanuel Texier, Laurence Gbaguidi, Ahmed Bentaleb, Frédéric Peruch, Stéphane Grelier, Lou Delugeau, Nicolas Mano, et al.

► To cite this version:

Olympe Longeras, Sébastien Gounel, Wilfrid Neri, Emmanuel Texier, Laurence Gbaguidi, et al. Water-Based Production of Carbon Fibers from Lignin and Alginate. ACS Sustainable Resource Management, 2025, 2 (7), pp.1212-1219. 10.1021/acssusresmgt.5c00053. hal-05190911

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# Water-Based Production of Carbon Fibers
## from Lignin and Alginate

Olympe Longeras,<fnref n="1" /> Sébastien Gounel,<fnref n="1" /> Wilfrid Neri,<fnref n="1" /> Emmanuel Texier,<fnref n="1" /> Laurence Gbaguidi<fnref n="1" />, Ahmed Bentaleb<fnref n="1" />, Frédéric Peruch,<fnref n="2" /> Stéphane Grelier,<fnref n="2" /> Lou Delugeau,<fnref n="2" /> Nicolas Mano<fnref n="1" />,*. Philippe Poulin<fnref n="1" />,*

## Abstract

Lignin, a promising precursor of low cost carbon fibers, faces a significant challenge for environmentally friendly processing: Kraft lignin's insolubility in water. To address this issue, the enzyme Bilirubin oxidase from *Bacillus pumilus* has been employed to modify lignin's solubility. This treatment results in aqueous solubilization and condensation, evidenced by a nearly 5-fold increase in molecular weight. Crucially, the carbon content after pyrolysis remains intact, which is essential for carbon fiber production. Consequently, this modified lignin is well-suited for carbon fiber manufacturing. In this study, a solution of the treated lignin and alginate, acting as plasticizer, was wet-spun into calcium chloride and ammonium persulfate baths, producing fibers composed entirely of biopolymers. Following carbonization, without the need for a stabilization step, the resulting fibers exhibited mechanical properties that compare with properties of fibers produced in organic solvents or with oil-based plasticizers. The optimal mechanical properties were achieved with 67% lignin content, yielding a Young's modulus of 39 ± 6 GPa and a tensile strength of 457 ± 54 MPa. These findings demonstrate the successful development of 100% bio-sourced fibers under environmentally friendly conditions.

Keywords: Lignin, Enzyme, Wet Spinning, Carbon Fiber.

<footnote n="1">¹ Centre de Recherche Paul Pascal, CNRS - Université de Bordeaux, UMR 5031, 33600 Pessac, France</footnote>

<footnote n="2">² Univ. Bordeaux, CNRS, Bordeaux INP, LCPO, UMR 5629, F-33600, Pessac, France</footnote>

<footnote>philippe.poulin@crpp.cnrs.fr</footnote>

<footnote>nicolas.mano@crpp.cnrs.fr</footnote>
Introduction

Carbon fibers are essential materials for aerospace, automobile or energy domains.[1] Most carbon fibers
are today produced from polyacrylonitrile (PAN) precursors.[2, 3] PAN based fibers display high mechanical
performances. But PAN is an oil-based and costly polymer.[4, 5] Moreover, its fiber processing involves the
use of organic solvents, and results in toxic exhausts during carbonisation.[5] Therefore, significant research
efforts are underway to explore alternative methods for carbon fiber production, with cost reduction and
lowered environmental impact as primary objectives. Bio-sourced precursors [6] such as lignin are
particularly promising for this purpose. Lignin is the second most important bio-polymer found in vegetal
biomass after cellulose.[7] This polymer, primarily extracted from plant biomass,[9, 10] is currently a waste
of the paper industry.[9,10] Lignin has a high carbon atom content, making it favorable for carbon fiber
production.[6]. Several fiber spinning processes have been developed for lignin-based carbon fiber
production,[11] with lignin generally blended with a plasticizing agent to facilitate manipulation before
carbonization.[4,12] Melt spinning, which consists in extruding molten lignin that solidifies upon cooling, is
one of the most common process. This process is fast and does not involve the use of solvent.[12,13]
However, fiber melt spinning is energy intensive, and necessitates the use of fusible lignin. Precursor fibers
made of fusible lignin have to be thermally stabilized before carbonization. This stabilization generally
consists in slowly heating the fibers and maintaining them at an intermediate temperature so that they can
latter sustain the high temperature carbonization without melting.[14,15] Fiber wet spinning is another
scalable and traditional method used to produce lignin-based fibers. In this process, lignin is dissolved in a
solvent, and the resulting solution is injected into a coagulating medium to form fibers that solidify upon
drying.[16,17] One key advantage of wet spinning is the ability to use non-fusible lignin. Fibers made from
non-fusible lignin can be directly carbonized without the need for thermal stabilization. However, fiber wet
spinning also comes with limitations. Raw lignin is not water soluble and fiber wet spinning often involves
the use of organic solvents, for lignin dissolution or coagulation, and oil based plasticizing agents.[18–23]
Modifying lignin to render it water-soluble presents a promising starting point. However, the
functionalization of lignin often requires the use of organic chemicals.[4] Lignosulfonate derivatives exhibit
high water solubility, but their high sulphur content reduces their attractiveness for carbon fiber production.
It is therefore crucial to transition towards cleaner solvents and co-products to enhance the environmental
friendliness of lignin-based carbon fibers.

In this work, we report a water-based method for wet spinning lignin precursor fibers, which are
subsequently carbonized without thermal stabilization. We utilize Kraft lignin that has been enzymatically
treated with bilirubin oxidase (BOD). This enzymatic treatment makes the lignin soluble in water at neutral
pH. The treated lignin is then mixed in water with sodium alginate, a low-cost, biosourced polymer that serves
as a plasticizing agent. The resulting dope is coagulated in an aqueous solution of calcium chloride to form
lignin:alginate precursor fibers. An additional bath of an ammonium persulfate at an empirically optimized
concentration is used to limit the fusion of the filaments. Although currently demonstrated only at the laboratory scale, the present fiber production process closely resembles conventional wet-spinning techniques already employed in industry. The gel fibers are then washed, dried, and carbonized at high temperature in an inert atmosphere, without the need for thermal stabilization. While the mechanical properties of the resulting carbon fibers do not yet match those of PAN-based carbon fibers, they are comparable to other biosourced carbon fibers. Furthermore, this process exclusively employs green materials, avoiding the use of toxic solvents and oil-based plasticizing agents at all stages.

# Experimental Section

## Materials

Softwood lignin Kraft Indulin AT was provided by the CANOE platform. Alginic acid sodium salt from brown algae, medium viscosity, ammonium persulfate ($NH_4)_2SO_8$, calcium chloride ($CaCl_2$), hydrochloric acid (HCl) and ethylenediaminetetraacetic acid (EDTA) were all purchased from Sigma Aldrich.

## Enzymatic treatment and preparation of water soluble lignin

In a typical treatment experiment, three grams of lignin Kraft Indulin AT and 100 mL of 50 mM borate buffer (pH 10) are introduced into a flask. The mixture is stirred, and 10 mg of Bilirubin oxidase from *Bacillus Pumilus* (BOD enzyme) are added. BOD with an activity of 800 U/mg was produced and purified as already reported [24]. The system is then heated to 60 °C and stirred overnight under 50 cm³/min $O_2$ bubbling. The pH of the solution is neutralized by dialysis in distilled water. The lignin solution is first placed in a 10 kDa cellulose dialysis bag for 8 hours in water followed by a second dialysis in water overnight to achieve a neutral pH. After this treatment, most of the lignin is solubilized in water, but an insoluble fraction remains. This insoluble fraction is separated from the soluble fraction by centrifugation at 4700 rpm for 15 minutes. The soluble fraction is then freeze-dried for 4-5 days to yield the final material in a powder form. The solubilization yield is around 65-70% [25]. The treated and water-soluble lignin is referred to as "lignin<sub>sol</sub>" in the following text.

## Lignin characterisation

### Elemental analysis

C, N, H, O and S atom contents were characterized using a Thermo Fisher Flashsmart system. Two samples of lignin both before and after treatment were analysed.

### Thermogravimetric analysis

A TGA 5500 from Hewlett-Packard was employed to characterize lignin. All the measurements were performed under inert atmosphere with nitrogen gas flushed at a rate of 25 mL/min. Lignin samples were weighted in aluminium crucibles. After one hour of equilibration at 30°C, the samples were heated from 30 to 1000 °C at a heating rate of 5 °C/min.
## Size-exclusion chromatography

Lignin molar masses were determined by Size Exclusion Chromatography (SEC) using water at a pH of 12 (by NaOH addition) as the eluent. Measurements were performed with an Ultimate 3000 system from Thermoscientific equipped with a diode array detector. The system also includes a multi-angles light scattering detector and a differential refractive index detector from Wyatt technology. Polymers were separated on Tosoh PW (7.5\*300) columns (exclusion limits from 100 Da to 300 000 Da) at a flowrate of 1 mL/min. Columns temperature was held at 25 °C. Polystyrene sulfonates were used as standards. The flow marker was ethylene glycol.

## Preparation of precursor fibers

The dope solutions are aqueous mixtures of lignin and alginate, with their viscosity primarily controlled by the amount of alginate, which acts as a viscosifying agent. It was determined that an alginate concentration of 1 wt% in Milli-Q water is optimal for producing dopes suitable for fiber wet spinning. Below this concentration, precursor fibers become too thin and brittle, while concentrations above 1 wt% make the dope too viscous for easy injection into the coagulating medium. The addition of 2 wt% water-soluble lignin maintains the spinnability of the dope, allowing a sufficient lignin-to-alginate ratio to ensure carbon fiber formation during pyrolysis. The solutions are stirred overnight, followed by a 20-minutes centrifugation at 4700 rpm to remove air bubbles.

Wet spinning was performed using lab-scale equipment, as schematized in Figure 1. The dope was injected at a flow rate of 25 mL/h using a syringe pump. The spinning head has seven circular holes, each 90 µm in diameter, resulting in an average linear velocity of 9.3 m/min at the spinneret exit. Further coagulation and fiber drawing along the line were optimized by adjusting several parameters, including the chemical composition of the baths and the rotation speed of the rolls.

The dope was first injected into a coagulating bath measuring 97 cm in length and containing an aqueous $CaCl_2$ solution at a 0.5 wt% concentration. The polymer material begins to solidify through polymer gelation induced by calcium ions. Nevertheless, the amount of $CaCl_2$ was chosen minimal to allow for efficient fiber gelation without downgrading mechanical properties (SI 1). It was observed that the fibers could not be directly drawn in a single bath with a roll at a significantly higher linear velocity than that of the injected dope. This is likely due to fiber linear contraction during gelation. Therefore, the forming fiber was drawn with a first roll operating at a linear velocity of only 12.0 m/min. Drawing at higher velocities in the first bath led to frequent fiber ruptures. Gelation was further enhanced in a second bath, 67.5 cm in length, with the same composition as the first. The fiber was drawn through this bath at a linear velocity of about 22.0 m/min, which was maintained constant by the subsequent rolls until the final wounding bobbin.

In the third bath, the coagulated fiber was washed; however, calcium ions tended to cause the filaments to adhere to one another. To mitigate this phenomenon, ammonium persulfate ($NH_4_2SO_2O_8$), was added to the wash, helping to keep the fibers separated. As detailed further, the concentration of ammonium persulfate
was kept as low as possible to preserve optimal properties of the fibers. After washing, the fibers were dehydrated in an ethanol bath and then dried in an infrared oven at 90 °C. Finally, the dried fibers were wound onto a cylindrical bobbin at a linear velocity of 22.0 m/min.

<figure><img src="image_2.png" /><figcaption>**Figure 1: Fiber Wet Spinning Experiment Overview:** The polymer dope is extruded into a first coagulation bath, where the fibers begin to solidify. From there, the fibers are transported to a second coagulation bath for further solidification. After coagulation, the fibers are washed and then dried.</figcaption></figure>

## Fiber Carbonization

40 mm long lignin fiber samples were fixed with carbon paint on graphite frames. The fibers were carbonized in a tube furnace (TF1-1200) with a regulator CC-T1 from Carbolite Gero. Lignin<sub>sol</sub> is infusible and does not need oxidative stabilisation for carbonization. The latter is performed in argon 6.0 (150 cm³/min) under a reduced pressure of 20 mbar. The temperature is increased from 25 °C to 1000 °C at a heating rate of 5°C/min. This heating is performed without any oxidative stabilization treatment. The fibers are maintained during one hour at 1000°C before being cooled down to room temperature.

## Carbon Fiber Washing, and Ash Content

As produced carbon fibers still contain calcium based compounds and potentially traces also of sodium based compounds. Carbon fiber washing in EDTA solutions (1mM) or HCl (pH 2) during 10 min were performed to remove these compounds.

The ash content was determined by heating the fibers at 1000°C under O₂ atmosphere.

## Fiber Characterisation

### Mechanical properties

Tensile tests of the fibers were carried out by using a tensile machine, TN1S from Zwick Roell, at room temperature following the ISO5079 norm. Four fibers were analysed per sample, each fiber being composed of seven separated filaments. The samples were fixed on a 20 mm long paper frame.

### Scanning electron microscopy

The cross-section surface of the fibers were determined by scanning electron microscopy using a Hitachi TM-1000 microscope. The fibers were metallized during 45 seconds before the analysis.
# Results and discussion

## Lignin characterisation

Enzymatic treatment by BOD makes lignin water soluble at neutral pH, revealing thereby chemical modifications of the lignin. The mechanism by which BOD catalyzes lignin solubilization is not yet fully understood. In a recent study [25], we demonstrated the critical role of borate in this process and showed that enzymatic treatment induces oxidative coupling reactions. This was evidenced by a decrease in hydroxyl group content and the formation of 4-O-5' and 5-5' linkages, as confirmed by confirmed by ³¹P and ¹³C NMR spectroscopy. Although BOD may function similarly to laccases, this hypothesis remains to be confirmed. Ongoing investigations using small model compounds aim to further elucidate the underlying mechanism. Results of elemental analysis for five elements to compare raw and enzymatically treated lignin are given in Supplementary Information (SI 2). The fraction of soluble lignin contains a reduced content of sulphur compared to raw Kraft lignin. The amount of sulphur is reduced by an order of magnitude, from more than 1% to less than 0.1%. The enzymatic solubilisation contributes therefore to the purification of lignin,[11] which can be a significant advantage in applications where the presence of sulphur has to be limited. Additionally, the fraction of sulphur in treated lignin, which is below 0.1%, appears to be much lower than the fraction of sulphur in lignosulfonate which is in the range of 3.5-8%.[26]

Results of thermal gravimetric analyses in nitrogen atmosphere are shown in Figure 2.a and 2.b for both raw lignin and lignin<sub>sol</sub>. Weight losses of lignin<sub>sol</sub> below 100°C are greater than that of raw lignin. This behavior reflects a greater hydration of lignin<sub>sol</sub>. However, both lignins display substantial degradation above 200°C, with a similar inflexion point in the TGA curves at 340°C. Carbon residues at 1000°C are also comparable. They are in both cases of 41%, meaning that the enzymatic treatment does not impede the excellent intrinsic potential of lignin for making carbon materials.

<figure><img src="image_3.png" /><figcaption>Figure 2: TGA of raw lignin and lignin<sub>sol</sub> under nitrogen. Weight loss vs temperature (a), and derivative of weight loss vs temperature (b).</figcaption></figure>
Enzymatic treatments are known to potentially alter the molecular weight of lignin.[27,28] Such a potential alteration was checked by SEC analyses of raw lignin and lignin<sub>sol</sub>. The results of SEC measurements are shown in Figure 3. Before treatment, raw lignin has a molecular weight (MW) of 3500 g/mol. After enzymatic treatment, the MW is multiplied by a 4.8 factor to reach a value of 17000 g/mol.

<figure><img src="image_4.png" /><figcaption>Figure 3: SEC chromatograms of raw lignin and lignin<sub>sol</sub>.</figcaption></figure>

Overall, the above characterizations demonstrate several positive effects of the enzymatic treatment for the processing of fibers via a water-based process. First, the BOD treatment makes raw lignin water-soluble at neutral pH. It also induces condensation of lignin. Last, the treatment leads to a reduction of the sulphur content.
(Alginate:lignin<sub>sol</sub>) precursors fibers

Pure lignin cannot be easily spun into fibers, by melt or wet spinning processes, because pure lignin fibers
are too brittle for further uses or transformations. This is why lignin is generally blended with other species
for fiber spinning. Alginate, a biosourced polymer extracted from algae, is chosen in the present work.
Alginate is water soluble, non-toxic and biocompatible.[29] This biopolymer is employed for its gelation
properties in presence of divalent ions.[21, 30–32] Calcium ions (Ca<sup>2+</sup>) in particular are known to replace
sodium ions present in raw alginate.[21] This ion exchange induces gelation of the polymer. This gelation
mechanism is employed here to solidify alginate:lignin<sub>sol</sub> mixtures in the spinning process. Moreover, alginate
plays the role of plasticizer and makes the alginate:lignin<sub>sol</sub> fibers sufficiently flexible for easy manipulation.
Lignin<sub>sol</sub> and alginate were mixed in water, and fibers were spun using the line schematized in Figure 1. As
indicated in the experimental section, the dope is injected through a spinneret composed of seven circular
holes at a flow rate of 25 mL/h in a first coagulating bath. This bath is an aqueous CaCl<sub>2</sub> solution at a
concentration of 0.5 wt%. The fibers are further solidified in a second bath of CaCl<sub>2</sub>. It is observed that the
filaments tend to fuse as they circulate in the coagulating baths. To mitigate this phenomenon, an acidic bath
of (NH<sub>4</sub>)<sub>2</sub>SO<sub>8</sub>, instead of pure water, is used as third bath in the process shown in
Figure 1, motorized rolls R<sub>i</sub> guide and circulate the fibers along the spinning line. The fibers undergo a total
draw ratio of 2.4 from the extrusion to the final collection on a cylindrical winder. The linear velocity of the
first roll R<sub>1</sub> is 12m/min. The velocity of the other rolls, from R<sub>2</sub> to R<sub>5</sub>, is 22 m/min. The objective of the third
bath is to degrade the gel formed by crosslinks between filaments. Ammonium persulfate achieves this
objective through two effects. The persulfate ions have an oxidizing role that can degrade polymer chains.
Additionally, the monovalent ammonium ions can displace calcium ions and lead to a rupture in the gel at
the interface between two filaments.[33,34] These effects must be carefully controlled and limited to avoid
damaging the filaments themselves.

Several tests have been performed in order to empirically find the optimal composition of the washing bath.
SEM pictures of carbonized fibers obtained with (NH₄)₂SO₈ concentrations varied from 0.05wt% to 1wt% are
shown in Figure 4.
<figure><img src="image_5.png" /><figcaption>Figure 4: Scanning electron micrographs of the section of carbonized fibers obtained after washing the precursor fibers in bath with distinct amounts of (NH₄)₂SO₈: a) 0.05wt%, b) 0.1wt%, c) 0.25wt%, d) 0.5wt%, e) 1wt%. Scale bars a, b, c, e 20 µm, d 30 µm.</figcaption></figure>

It is observed that fibers washed with less than 0.25wt% (NH₄)₂SO₈ display fused filaments. In contrast, filaments are well-separated when washed in a bath containing 0.5 wt% or 1 wt% (NH₄)₂SO₈. The pH of these solutions was around 2, but the separation of the filaments is not due to the pH change. Tests using a washing bath with HCl at a similar pH resulted in completely fused filaments. However, disruption of the gelled polymer network between filaments results in a rough fiber surface and reduced mechanical properties. Therefore, a 0.5 wt% concentration was selected as a compromise, providing separated filaments without significantly compromising the fibers' structure and properties. Consequently, further characterization and discussion in this study focus on both precursor and carbonized fibers washed with an intermediate concentration of 0.5 wt% (NH₄)₂SO₈.

As shown in Figure 5, the seven produced filaments of precursor fibers are well separated at the end of the fiber spinning process. One fiber exhibits seven filaments, each with a diameter of 11.5 ± 1.2 µm.
<figure><img src="image_6.png" /><figcaption>Figure 5: SEM picture of (alginate:lignin<sub>sol</sub>) filaments containing 67 wt% of lignin<sub>sol</sub> and 33% of alginate.</figcaption></figure>

The mechanical properties of these precursor fibers are presented in Table 1. The (alginate:lignin<sub>sol</sub>) fibers have a stress at break of 167 ± 29 MPa and a Young's modulus of 9 ± 1 GPa. We can note that these values exceed that of other lignin-based fibers made by wet spinning. For example, Yang *et al.* produced fibers by wet spinning using a dope composed of polyvinyl alcohol (PVA) and lignin dissolved in a mixture of water and ionic liquid. [35] The mechanical properties of these fibers were 25-50 MPa for the stress at break and 3-5 GPa for the Young's modulus. The (alginate:lignin<sub>sol</sub>) fibers produced in this work exhibit a stress at break 3.3 to 6.7 times higher, and similar Young's modulus values, making their manipulation easier for further transformation. Other fiber composed of lignocellulose nanofibrils and alginate, at the ratio (97:3), were reported.[21] The fibers with higher lignin content (around 9% lignin and 91% of cellulose/hemicellulose) displayed a tensile strength close to 20 MPa and a Young's modulus of 0.6 GPa.[21] The mechanical properties reached by the precursors fibers produced in this work suggest that they are promising candidates for carbon fiber production.

<table><caption>Table 1: Mechanical properties of (alginate:lignin<sub>sol</sub>) fibers containing 67 wt% of lignin<sub>sol</sub>.</caption><thead><tr><th>Ratio<br>(alginate:lignin<sub>sol</sub>)</th><th>Lignin<sub>sol</sub><br>concentration</th><th>Fibers surface<br>wt%</th><th>Stress at break<br>μm<sup>2</sup></th><th>Young<br>modulus<br>MPa</th><th>Elongation<br>%</th></tr></thead><tbody><tr><td>(1:2)</td><td>67</td><td>732 ± 153</td><td>167 ± 29</td><td>9 ± 1</td><td>2.2 ± 0.3</td></tr></tbody></table>

## Carbon fibers

Carbonization procedure allows the increase of carbon content in a fiber by removing oxygen and hydrogen elements.[36] Lignin based fibers were directly carbonized at 1000 °C during 1 hour in a furnace tube with an heating ramp at 5 °C/min and under argon atmosphere. Carbonization was not formally optimized in this study; instead, it was carried out using the maximum heating rate of our furnace to minimize time and energy consumption. However, we acknowledge that optimization of this process is important and should be addressed in future work. We also stress that no oxidative stabilization stage was needed to carbonize the
fibers because enzymatically treated lignin was found to be infusible. Infusibility is deduced from the
resistance of the fibers to directly sustain heating treatment without breaking or deforming. The absence of
stabilization step is a significant advantage in terms of time and energy consumption.

SEM pictures of lignin<sub>sol</sub>-based carbon fibers are shown in Figure 4d, and in Figure 6 with a higher magnification. After carbonization, the fibers remained unfused. As for precursor (alginate:lignin<sub>sol</sub>) fibers, each filament are well defined and separated. One fiber exhibits seven filaments, each with a diameter of 5.9 ± 0.6 µm.

<figure><img src="image_7.png" /><figcaption>Figure 6: SEM pictures of carbon fiber obtained after carbonization of (alginate:lignin<sub>sol</sub>) fibers containing 67 wt% of lignin<sub>sol</sub>.</figcaption></figure>

Upon high-temperature treatment, the decomposition of alginate complexed with Ca²⁺ ions can yield calcium-based alkaline compounds such as Ca(OH)₂, CaCO₃, and CaO. The presence of remaining sodium ions can also result in the formation of NaOH, Na₂CO₃ or Na₂O. We have verified the presence of calcium and sodium compounds in unwashed fibers using X-ray analyses. The corresponding data are given in Supporting Information (SI 3). Several diffraction peaks reflecting the presence of calcium and sodium based compounds can be seen in unwashed fibers. We believe that most of these compounds are located on the fiber surface, as they can be effectively removed through washing with hydrochloric acid or EDTA solutions. Calcium based compounds are likely responsible for most of the ash content which is about 4±1% for fibers washed with EDTA solutions.

The mechanical properties of the obtained carbon fibers are given in Table 2. Stress at break and Young's modulus are of 457 ± 54 MPa and of 39 ± 6 GPa, respectively. These properties were found to be unaffected by the washing process, which is expected, as washing primarily removes calcium and sodium-based compounds located on the fiber surface. However, metals and other inorganic species embedded within the fiber core remain unaffected by this treatment.

Lignin is commonly combined with other polymers for carbon fiber production via wet spinning, with the best mechanical properties typically achieved when polyacrylonitrile (PAN) is used. Q. Li et al. investigated fibers composed of PAN and lignin derived from various biomass feedstocks.[18] The polymers were dissolved in N,N-dimethylformamide for wet-spinning and coagulated in a methanol bath. The authors observed a decrease in mechanical properties and spinnability as the lignin content increased. The best results were
obtained with a 1:1 weight ratio of lignin to PAN using sugar maple lignin. The resulting carbon fibers exhibited a Young's modulus of 40.4 GPa and a tensile strength of 475 MPa.[18] In another study, PAN was blended with wheat straw lignin in dimethyl sulfoxide, with lignin content up to 40 wt% relative to PAN.[37] The tensile strength of the resulting carbon fibers ranged between 300 and 500 MPa. Jin and Ogale also studied wet-spun PAN-lignin-based carbon fibers with lignin content up to 50 wt% relative to PAN.[38] The polymers were dissolved in dimethyl sulfoxide, and after carbonization at 1200 °C, the fibers demonstrated excellent properties, with a stress at break of 1200 ± 100 MPa and a Young's modulus of 130 ± 3 GPa. These properties were obtained in a process that involves mechanical tension during stabilization and carbonization. However, the large amounts of PAN used in these earlier studies limit the economic and environmental interest of the produced carbon fibers. To address these limitations, less expensive polymers than PAN have been explored in conjunction with lignin for carbon fiber production via wet-spinning. For instance, fibers composed of Kraft lignin and polyvinyl alcohol (PVA) at a 70:30 weight ratio have been produced by wet-spinning in dimethyl sulfoxide.[22] After carbonization at 1000 °C, the fibers exhibited a stress at break of 351 ± 108 MPa and a Young's modulus of 44.5 ± 9.6 GPa.[22] In another study, a solution composed of lignin, PVA and graphene oxide was used at a weight ratio of 66-29-5 for carbon fiber production.[23] Following carbonization at 1000 °C, the carbon fibers achieved a stress at break of 763 MPa and a Young's modulus of 52 GPa.[23] Additionally, bio-based fiber precursors have been investigated, such as cellulose-lignin fibers produced by wet-spinning.[6] The tensile strength of these fibers ranged from 480 to 1170 MPa, with a Young's modulus between 33 and 77 GPa. In a broader context [39], lignin-based carbon fibers can be produced by methods beyond wet-spinning. Melt-spinning, for instance, is a widely used technique, though it requires a time- and energy-intensive stabilization step. In this process, lignin is often blended with polymers such as PEO or PET. The resulting fibers can achieve a modulus exceeding 60 GPa and a tensile strength of up to 700 MPa.[40, 41] These properties can be further enhanced by performing stabilization and carbonization under mechanical tension.[42] Dry-spinning represents another promising alternative, although it often relies on volatile organic solvents.[43, 44] When combined with tension-assisted stabilization and carbonization, dry-spinning can produce fibers with moduli greater than 50 GPa and tensile strengths surpassing 900 MPa [39].

While the wet spun carbon fibers reported in this work do not yet reach the high performance levels of the best PAN-lignin-based fibers, they are comparable to several carbon fibers made by wet-spinning lignin with other oil-based polymers. It is also important to note that the carbonization temperature was limited to 1000 °C, which is likely insufficient to promote high graphitization, as indicated by the X-ray characterization results (Figure S2). The key distinctive feature of the present fibers lies in fact in their environmentally friendly processing. To our knowledge, this is the first process that utilizes only water as a solvent and coagulant, along with bio-sourced polymers as plasticizers. The resulting fiber precursors are 100% bio-based and renewable. Furthermore, the elimination of a stabilization step prior to carbonization reduces energy
consumption and may further improve the overall sustainability of the process. Nonetheless, despite following a green methodology, a comprehensive life cycle analysis is essential to rigorously validate the process's low environmental impact.

<table><caption>Table 2: Mechanical properties of carbon fibers obtained after carbonization of (alginate:lignin<sub>sol</sub>) fibers containing 67 wt% of lignin<sub>sol</sub>.</caption><thead><tr><th>Ratio<br>(alginate:lignin<sub>sol</sub>)</th><th>Lignin<sub>sol</sub><br>concentration</th><th>Fibers surface</th><th>Stress at break</th><th>Young<br>modulus</th><th>Elongation</th></tr><tr><th></th><th>wt%</th><th>µm<sup>2</sup></th><th>MPa</th><th>GPa</th><th>%</th></tr></thead><tbody><tr><td>(1:2)</td><td>67</td><td>193 ± 41</td><td>457 ± 54</td><td>39 ± 6</td><td>1.2 ± 0.3</td></tr></tbody></table>

## Conclusion

In this study, we demonstrated the successful development of lignin-based carbon fibers using a water-based process, highlighting a promising step forward in sustainable carbon fiber production. The enzymatic treatment of Kraft lignin with Bilirubin oxidase (BOD) resulted in a water-soluble lignin derivative, which exhibited a nearly 5-fold increase in molecular weight and a substantial reduction in sulfur content. This modified lignin was wet-spun with alginate, a bio-sourced plasticizer, into precursor fibers that were subsequently carbonized without the need for oxidative stabilization, owing to the infusibility of the treated lignin.

The resulting carbon fibers displayed promising mechanical properties, with a Young's modulus of 39 ± 6 GPa and a tensile strength of 457 ± 54 MPa. These properties are competitive with other bio-sourced carbon fibers and highlight the potential of this environmentally friendly process. While the mechanical properties do not yet match those of carbon fibers derived from PAN or other conventional methods, the use of 100% bio-sourced materials and the elimination of toxic solvents represent an advancement toward more sustainable carbon fiber production. Future work should focus on enhancing the mechanical properties, as demonstrated in other processes involving carbonization under mechanical tension. Additionally, scaling up enzyme production is a critical step toward the full technical validation of the process. Finally, while the methodology is designed to be green, its overall environmental impact must be confirmed through a comprehensive life cycle analysis.

## Acknowledgements

This study was supported by the French government in the framework of the University of Bordeaux's France 2030 program/GPR PPM, and by the ANR LIGNIN project 20-CE43-0006. The authors thank Sandrine Adach from SynBioN for elemental analysis, and the CANOE platform for providing lignin materials. We thank Emmanuelle Hamon for the TGA measurements.
Conflicts of interest

There are no conflicts to declare.
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