Tannic Acid-Assisted Surface Encasing of Bismuth Nanoparticle on Carbon Felt for High-Performance Vanadium Redox Flow Batteries

Achieving superior catalytic ability and robust mechanochemical stability in metal-catalyzed nanoparticles deposited on electrode surfaces is essential for advancing the commercial viability of high-performance flow battery technologies. Herein, we propose a type of carbon felt electrode surface encased with bismuth nanoparticles (Bi NPs) via a tannic acid-assisted robust immobilization, which can catalyze the V²⁺/V³⁺ redox reaction with high efficiency and outstanding durability. The hydroxyl groups in tannic acid serve as polydentate ligands, coordinating with bismuth ions to form an exceptionally stable protective framework around the Bi NPs generated during carbothermic reactions. These enchased Bi NPs are well-preserved under electrolyte flow, surpassing that of Bi NPs electrodes fabricated via single carbothermic reduction or electrodeposition methods. The results demonstrate that the VRFBs equipped with the present electrode achieve energy efficiencies of 77.68% and 72.11% at current densities of 300 mA cm⁻² and 400 mA cm⁻², which are remarkably increased by 4.90% and 8.53% than those of batteries integrated with pristine carbon felt. Furthermore, at 400 mA cm⁻², the discharge capacity increases by as much as 136%. Moreover, the novel structured catalytic electrodes enable the VRFBs to sustain high performance even after 2000 cycles at 300 mA cm⁻², exhibiting an energy efficiency retention rate of 99.97% and a discharge capacity decay rate of 0.014% per cycle.