Fast rechargeable vanadium redox flow batteries: The effect of Mo co-doping on F-doped SnO₂ nanoparticles deposited on graphite felt

Vanadium redox flow batteries (VRFBs) are promising rechargeable energy storage devices due to their use of nonflammable aqueous electrolytes. Accordingly, electrode surface modification strategies have been investigated to enhance ion and electron transfer kinetics during fast-rated vanadium-based redox reactions. Herein, Mo and F co-doped tin oxide nanoparticles were deposited on graphite felt (Mo-FTO/GF) using ultrasonic spray pyrolysis deposition. To demonstrate the effect of Mo co-doping on FTO/GF, bare GF and FTO/GFs were fabricated and compared. Notably, the Mo-FTO/GF exhibited tack-shaped pointed Mo-FTO nanoparticles (NPs) on the GF surface, increasing the effective surface area between the electrode and electrolyte. This nanostructure enabled a large number of hydrophilic functional groups to effectively interact with the electrolyte and enhanced fast ionic transfer kinetics. Additionally, Mo replaced Sn sites in the FTO NPs, generating extra free electrons due to its valence difference. This modification improved the electronic structure of the Mo-FTO NPs, thereby enhancing electrical conductivity and fast electron transfer kinetics. Therefore, the simultaneous enhancement in ion and electron transfer kinetics, attributed to the Mo co-doping effect on FTO/GF, resulted in fast rechargeability of VRFBs, with high energy efficiency (82.08 %) and longevity (retention rate of 100.00 %) after 300 cycles at 160 mA/cm².