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Technical Article


CONDUCTIVE CARBON MICROFIBERS DERIVED FROM WET- SPUN LIGNIN/NANOCELLULOSE HYDROGELS

We introduce an eco-friendly process to dramatically simplify carbon microfiber fabrication from biobased materials. The microfibers are first produced by wet-spinning in aqueous calcium chloride solution, which provides rapid coagulation of the hydrogel precursors comprising wood-derived lignin and TEMPO-oxidized cellulose nanofibrils (TOCNF). The thermomechanical performance of the obtained lignin/TOCNF filaments are investigated as a function of cellulose nanofibril orientation (WAXS), morphology (SEM) and density. Following direct carbonization of the filaments at 900 °C, carbon microfibers (CMF) are obtained with remarkably high yield, up to 41%, at lignin loadings of 70 wt% in the precursor microfibers (compared to 23% yield for those produced in the absence of lignin). Without any thermal stabilization or graphitization steps the morphology, strength and flexibility of the CMFs are retained to a large degree compared to those of the respective precursors. The electrical conductivity of the CMFs reaches values as high as 103 S cm-1, making them suitable for microelectrodes, fiber-shaped supercapacitors and wearable electronics. Overall, the cellulose nanofibrils act as structural elements for fast, inexpensive and environmentally sound wet-spinning while the high carbon yield and density of lignin endows the system with electrical conductivity.

 

 

INTRODUCTION

The demand of carbon fibers (CFs) is increasing annually at a 10% rate and is expected to reach 89,000 tons by 2020.1 This demand has been met by the supply of petroleum-based precursors, such as polyacrylonitrile (PAN) and mesoporous pitch, which make about 90% of the market.2 Therefore, renewable materials that can be converted into CFs are critically urgent in the near future, especially if they can be processed by using simple routes. Cellulose and lignin can be considered as promising alternative precursors to PAN and mesoporous pitch because they are abundant, relatively inexpensive and renewable.

In this work, we propose for the first-time wet spinning of lignin and cellulose from aqueous suspensions to produce composite filaments and then, upon carbonization, the respective carbon microfibers (CMF), with no need for melting nor dissolution.

 

 

RESULTS AND CONCLUSIONS

Carbon microfibers (CMF) were obtained by wet spinning of bicomponent precursors containing lignin and cellulose nanofibrils, followed by one-step carbonization at 900 ℃ (Figure 1). Filaments were produced from the respective hydrogel dopes with lignin loadings of up to 70 wt% and with no need for strong solvent dissolution, volatile antisolvents nor high temperature melting. Lignin significantly influenced the morphology and properties of the bicomponent microfibers, including fibril orientation, mechanical strength and thermal stability. It was possible to carbonize the microfibers without stabilization treatment, and the obtained CMF retained circular cross sections with no signs of inter-fiber fusion. Highly dense CMF were obtained at high mass yield (41%) and displayed a significant electrical conductivity (103 S cm-1). Overall, widely available lignin can be processed by wet spinning followed by a one-step carbonization for a simple, scalable and energy-efficient alternative to conductive carbon microfibers. They are promising for fiber-shaped capacitors, microelectrodes and for composites and wearable electronics.

 

 

ACKNOWLEDGMENTS

Funding support from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (ERC Advanced Grant agreement No 788489, “BioElCell”) is gratefully acknowledged. Business of Finland (former Tekes) is also acknowledged through project Design Driven Value Chains in the World of Cellulose II. 

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