Quantifying effects of surface morphology and functional groups of carbon fibers on mass transfer coefficient in vanadium redox flow batteries

Optimization of porous electrodes has emerged as a fascinating alternative to improve the power density of redox flow batteries. While numerous studies have demonstrated the significant reduction in overpotentials due to electrode modifications, there has yet to be research that elucidates the underlying mechanism. The developed fitting model in the present study enables efficient mass transfer coefficient characterization for redox flow battery systems with sluggish reactants. Based on the newly proposed fitting model and experimental data, the present study explores the mechanism of how changes in electrode morphology and functional groups affect the mass transfer coefficient. It is found that micro-scale pores on the fiber surface, when fibers were thermally treated at 300 C-degrees, successfully enhanced mass transfer of the reactants in the electrode likely owing to the shortened diffusion distance, while nano-scale pores, when fibers thermally treated at 400 C-degrees, showed minor effects on mass transfer enhancement. Besides, the increment of the oxygen containing functional groups also enhanced the mass transfer rate in the diffusion layer likely attributable to the improved electrode hydrophilicity. Last, the power-law correlations for Sherwood number and Reynolds number for different electrode samples were established, enabling frontend screening in future's electrode development campaigns.