Freestanding Carbon Nanofibers Derived from Biopolymer (Kraft Lignin) as Ultra-Microporous Electrodes for Supercapacitors

Abstract

Lignin-derived carbon nanofibers (LCNFs) formed via the solution blowing of a biopolymer are developed here as a promising replacement for polyacrylonitrile (PAN)-derived carbon nanofibers (PCNFs) formed via electrospinning for such applications as supercapacitor (SC) electrodes. Accordingly, it is demonstrated here that a biopolymer (kraft lignin, which is, essentially, a waste material) can substitute a petroleum-derived polymer (PAN). Moreover, this can be achieved using a much faster and safer fiber-forming method. The present work employs the solution blowing of lignin-derived nonwovens and their carbonization to form electrode materials. These materials are characterized and explored as the electrodes in supercapacitor prototypes. Given the porosity importance of carbon fibers in SC applications, N2 gas adsorption tests were performed for characterization. LCNFs revealed the specific surface area (SSA) and capacitance values as high as 1726 m2/g and 11.95 F/g, which are about one-half of those for PCNFs, 3624 m2/g and 25.5 F/g, respectively. The capacitance values of LCNFs are comparable with those reported in the literature, but the SSA observed here is much higher. Moreover, no further post-carbonization activation steps were performed here in comparison with those materials reported in the literature. It was also found here that fiber pre-oxidation in air prior to carbonization and the addition of zinc chloride affect the SSA and capacitance values of both LCNFs and PCNFs. The electrochemical tests of the SCs prototypes were used to evaluate their capacitance at different charging rates, voltage windows, and the number of cycles. The capacitance of PCNFs decreased by about 47% during fast charging, while the capacitance of LCNFs improved during fast charging, bringing them to the level of only 21% below that of PCNFs. These changes were correlated with the packing density of the electrodes. It should be emphasized that LCNFs revealed a much higher mass yield, which was 4–5 times higher than that of PCNFs. LCNFs also possess a higher packing density, a lower price, and cause a significantly lower environmental impact than PCNFs. The best cell supercapacitor delivered a maximum specific energy of 1.77 Wh/kg and a maximum specific power of 156 kW/kg, surpassing conventional electrochemical supercapacitors. Remarkably, it retained 95.2% of its initial capacitance after 10,000 GCD cycles at a current density of 0.25 A/g, indicating robust stability. Accordingly, kraft lignin, a bio-waste material, holds great promise as a raw material for supercapacitor electrodes.