Investigation of Electrospun Lignin Fiber Separators for Supercapacitor Applications
Authors: R.K. Azega , Haque Mazharul , Enoksson Peter , Lundgren Per 
Published: December 2024

This research work investigates the potential of fossil-free electrospun lignin fibers (ELFs) as separators in supercapacitors, contrasting with commonly used commercial separators. We focus on ELFs derived from both hardwood and softwood wood derivatives, along with electrospun ELF with reduced graphene oxide (rGO), comparing their performance to eco-friendly cellulose separators.

While polyolefin-based separators are prevalent in the industry, they often suffer from high production costs and limited performance due to sluggish ionic transfer kinetics and poor thermal stability 1. Separators play a critical role in battery performance, providing a microchannel for ion migration and affecting mechanical properties and safety. Lignin, with its high biocompatibility and stable aromatic rings, presents an attractive alternative material for separators2. Previous studies have demonstrated the potential of lignin-based separators, exhibiting high-rate capability and capacity retention compared to commercial separators. Lignin-polymer composite fibers, especially those with polyacrylonitrile (PAN) as the host polymer, have shown improved thermal stability, electrolyte affinity, and electrochemical performance. In our work, we aim to explore further the suitability of lignin-derived separators, particularly fossil-free ELFs and lignin nonwovens, for supercapacitor applications. By synthesizing nanofibers via electrospinning, we create separators with desirable pore properties and mechanical strength3. Our initial electrochemical investigations indicate promising results for ELF derived from both softwood and hardwood lignin, exhibiting specific capacitances of 174 F/g and 170 F/g, respectively. The nonwovens on the other hand demonstrate notably high specific capacitances, with 187 F/g surpassing conventional cellulose nonwovens at 173 F/g. These findings suggest that ELFs perform comparably to cellulose nonwovens, while lignin-based nonwovens exhibit superior performance. In large-scale applications, it's crucial to reduce the overall weight of individual supercapacitor devices. This helps improve their efficiency and ultimately makes them more affordable. One way to do this is by reducing the weight of separators inside the devices. Notably, the reduced mass of ELFs, nearly five times less than nonwoven counterparts, presents an avenue for improving overall component mass. Furthermore, ELFs demonstrate satisfactory rate capabilities comparable to nonwoven materials, further highlighting their potential for various applications. Moreover, lignin's substantially lower raw material costs compared to cellulose, even at its highest quality priced at 100-300 USD/milli ton, position it as an economically feasible alternative for separator material exploration 4. Employing lignin in nonwoven separators, particularly when utilizing inexpensive variants, offers a cost-effective solution for large-scale production of simple nonwoven fabrics. However, electrospinning, despite its advantages such as high surface area and precise fiber morphology control, typically entails higher expenses. Nevertheless, with the declining cost of lignin raw materials, electrospinning may become a more viable option. However, a thorough cost analysis, encompassing all relevant factors, remains essential for evaluating specific cases comprehensively. Furthermore, we intend to discuss more about the electrolyte absorption, mechanical properties, and thermal stability of these lignin-based separators compared to the commercial counterparts with continued investigation of this study. Therefore, through detailed characterization and electrochemical testing, we seek to elucidate the performance of the different separators in supercapacitors. Our findings aim to contribute to the development of sustainable and high-performance energy storage devices, leveraging lignin-derived materials as eco-friendly alternatives to traditional separators.

References:

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Y. Yi et al., Energies, 15, 9450 (2022).

H. Y. Jung et al., Polymers, 14, 673 (2022).

R. Nadányi et al., Energies, 15, 6520 (2022).

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