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Issue 44, 2020
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Hardwood versus softwood Kraft lignin – precursor-product relationships in the manufacture of porous carbon nanofibers for supercapacitors

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Abstract

The process of stabilization is essential in the production of carbon fibers from lignins. During stabilization, the initially thermoplastic lignin polymer is converted to a thermoset polymer allowing for high-temperature treatment without a change in shape. In this work, hardwood (HKL) and softwood (SKL) Kraft lignins were stabilized in air at temperatures between 190 and 340 °C before carbonization at 800 °C in a nitrogen atmosphere. Due to the differences in side-chain linkages, functional groups and molar mass, the lignins exhibit different structural changes upon stabilization and hence develop different porosities upon carbonization. Both lignins undergo major crosslinking reactions in the side chains at low temperatures and degradation reactions at high temperatures during stabilization. Crosslinking gives rise to narrow pore size distributions with mainly (sub-) nanometer pores, whereas degradation reactions lead to a more open pore structure with additional mesoporosity (>2 nm). When both types of reactions take place simultaneously, highly accessible (sub-) nanoporosity can be effectively created, which boosts the performance of supercapacitors operating in 6 M KOH(aq). This effect terminates when the crosslinking reactions cease and mainly degradation reactions take place, which occurs in HKL at 340 °C. SKL shows both a lower degree of crosslinking and degradation and hence develops less specific surface area. The optimum performance in an aqueous alkaline supercapacitor is achieved with HKL stabilized at 310 °C. It shows a specific gravimetric capacitance of 164 F g−1 at 0.1 A g−1 and 119 F g−1 at 250 A g−1 with a capacitance retention of more than 90% after 10 000 cycles.

Graphical abstract: Hardwood versus softwood Kraft lignin – precursor-product relationships in the manufacture of porous carbon nanofibers for supercapacitors

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Supplementary files

Article information


Submitted
16 Sep 2020
Accepted
21 Oct 2020
First published
22 Oct 2020

This article is Open Access

J. Mater. Chem. A, 2020,8, 23543-23554
Article type
Paper

Hardwood versus softwood Kraft lignin – precursor-product relationships in the manufacture of porous carbon nanofibers for supercapacitors

P. Schlee, O. Hosseinaei, C. A. O' Keefe, M. J. Mostazo-López, D. Cazorla-Amorós, S. Herou, P. Tomani, C. P. Grey and M. Titirici, J. Mater. Chem. A, 2020, 8, 23543
DOI: 10.1039/D0TA09093J

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