Sustainable lignin nanofiber electrodes for vanadium redox flow batteries

¹ Department of Chemical Engineering, Faculty of Engineering and the Built Environment, Cape Peninsula University of Technology, Symphony Way, Bellville, Cape Town, 7535, South Africa.
² Department of Civil Engineering, Faculty of Engineering and the Built Environment, Cape Peninsula University of Technology, Symphony Way, Bellville, Cape Town, 7535, South Africa.
³ South African Institute of Advanced Materials Chemistry, Faculty of Natural Science, University of the Western Cape, Robert Sobukwe, Bellville, Cape Town, 7535, South Africa.
⁴ Department of Chemical Engineering Technology, Faculty of Engineering and the Built Environment, University of Johannesburg, P.O Box 17011, Doornfontein, Johannesburg 2088, South Africa.

^* Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Abstract

The development of sustainable, high-performance electrode materials is a challenge in advancing vanadium redox flow battery (VRFB) technology for large-scale electrochemical energy storage. This study investigates the fabrication of lignin-based carbon nanofibers (LCNFs) via electrospinning as a renewable and cost-effective alternative to conventional PAN-based carbon electrodes. Alkali lignin blended with polyvinylpyrrolidone PVP in a 1:1 mass ratio was dissolved in N, N-dimethylformamide (DMF) at total polymer concentrations of 15, 17.5, 20, and 21.70 wt%, and electrospun under systematically varied process conditions using a central composite design (CCD) framework. Scanning electrode microscopy (SEM) characterisation revealed a clear concentration-dependent evolution in fiber morphology: beaded, discontinuous fibers at 15 wt% transitioned to well-developed, bead-free, interconnected nanofiber networks at 20 wt% and above, with mean fiber diameters ranging from 163 nm to 476 nm. Thermal stabilisation in air at 250°C and subsequent carbonisation under nitrogen at 600°C preserved the fibrous structure while imparting electrical conductivity to the mats. Electrochemical characterisation by cyclic voltammetry in a 0.05 M vanadium electrolyte showed a quasi-rectangular profile indicative of a mixed capacitive-faradaic charge storage mechanism. The LCNF electrode exhibited stable current response scaling with scan rate from 10 to 500 mV/s, with current density increasing from ±0.5 mA/g to ±6 mA/g, confirming good rate capability and reversible charge storage behaviour.