Multi-fractal Nanoporous Carbon Sphere-Decorated Graphite Felt Electrodes for Vanadium Redox Flow Batteries

We report a novel electrode design based on sustainable fructose-derived porous carbon spheres (F-PCS) uniformly deposited on graphite felt (GF) through a simple hydrothermal method, enabling an enhanced performance in vanadium redox flow batteries (VRFBs). The F-PCS architecture simultaneously provides a high surface area, electrical conductivity, and abundant electroactive sites. Comprehensive structural and electrochemical characterizations demonstrate improved redox reaction kinetics, lower charge-transfer resistance, and enhanced capacity retention. Importantly, quantitative image analysis reveals that the electrode surface exhibits a multifractal morphology and a complex, scale-invariant structure characterized by a broad spectrum of local singularities. This multifractality underpins the hierarchical pore distribution and edge-rich domains observed in the SEM and HR-TEM images, providing interconnected ion-transport pathways and localized electric field enhancement. The synergistic effects of nanoscale graphitization, surface doping, and multiscale architecture result in a superior electrochemical performance. These findings highlight the critical role of multifractal geometry in governing charge storage and transport, offering a design principle for next-generation redox-active materials. Also, this study on porous carbon spheres toward all VRFBs will route a way to developing a sustainable, metal-free, cost-effective, and ecofriendly lead for large-scale commercialization.