Metal-free Fabrication of Nitrogen-doped Vertical Graphene on Graphite Felt Electrodes with Enhanced Reaction Kinetics and Mass Transport for High-performance Redox Flow Batteries

["Graphite felt is commonly used in redox flow batteries, but the low specific surface area and poor catalytic activity cause unsatisfactory mass transfer and reaction kinetics. Here, nitrogen-doped vertical graphene is in-situ grown on graphite felt via a metal-free chemical vapor deposition method, which exhibits a high specific surface area and remarkable catalytic activity due to abundant exposed high-density sharp graphene edges and nitrogen doping. Multiphysical simulations reveal that the vertical-standing nanostructure promotes the accessibility of vanadium ions to electrode/electrolyte interfaces, effectively decreasing the mass transport resistance of active species. Density functional theory calculation evidence shows that nitrogen doping aids in the improvement of catalytic activity via boosting vanadium ions' adsorption and redox. Consequently, the nitrogen-doped vertical graphene/graphite felt electrode shows an energy efficiency of 87.1% at 200 mA cm-2, significantly higher than that of pristine (65.9%) and air-oxidize (73.1%) electrodes, an energy efficiency over 80.2% at 300 mA cm-2 during 1500 cycles, and a high-peak power density of 1308.56 mW cm-2, which are superior to previously reported carbon nanomaterial decorated electrodes for flow batteries. Significantly, the synthesis process only involves gas-phase reactions without metal catalysts to avoid hydrogen evolution reactions. This work provides an exciting pathway for developing high-performance electrodes for flow batteries.","In this work, nitrogen-doped vertical graphene is in situ grown on graphite felt by metal-free catalyst chemical vapor deposition for the first time, enabling the electrode to have fast reaction kinetics and efficient mass transfer. The present battery possesses the highest power density and most extended cycle life among the electrodes decorate by carbon nanomaterials for redox flow batteries.image"]