Multiphysics modelling of lignin-derived porous electrodes for enhanced mass transport in vanadium redox flow batteries

¹ Department of Chemical Engineering, Faculty of Engineering and the Built Environment, Cape Peninsula University of Technology, Bellville, 7535, South Africa
² Department of Chemical Engineering Technology, Faculty of Engineering and the Built Environment, University of Johannesburg, Doornfontein, South Africa

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

Abstract

This study examines the influence of microstructure, including porosity, permeability, and tortuosity of lignin-derived electrodes (LDEs) on mass transfer behaviour in vanadium redox flow batteries (VRFBs) using a three-dimensional modelling approach. A physics-based model for the LDE was developed in COMSOL Multiphysics that couples the Navier-Stokes, Brinkman, Nernst-Planck, and Butler-Volmer equations to resolve electrolyte hydrodynamics, species transport, and electrochemical kinetics across the electrode, membrane, and flow channel domains. Model validation against published polarisation curves demonstrates strong predictive capability, with a coefficient of determination, R² = 0.914, root mean square error, RMSE = 0.0398 V, and mean absolute percentage error, MAPE = 2%. In addition, hydrodynamic validation shows excellent agreement for the interdigitated flow field, R² = 0.999 and RMSE = 2.16 mbar, while the parallel configuration exhibits larger deviations, R² = 0.889 and RMSE = 31.78 mbar, highlighting limitations in capturing flow resistance. Results show that increasing porosity enhances permeability and reduces tortuosity (≈ 1.05 – 1.6), promoting uniform species distribution governed by coupled convective-diffusive-migrative transport, while improving electrochemical utilisation through more homogeneous Butler-Volmer kinetics. Furthermore, the Sherwood number increases with Reynolds number, indicating that enhanced convection associated with higher-porosity structures improves local interfacial mass transfer.