Importance of defect site concentration on carbon electrodes over heteroatom functionalities for vanadium redox kinetics in redox flow batteries

Heteroatom doping is a well-established approach to improving the electrocatalytic performance of carbon electrodes in vanadium redox flow batteries (VRFBs). This study investigates the primary kinetic factors driving the enhanced vanadium redox kinetics on heteroatom-doped carbon electrodes. Graphene samples doped with heteroatoms, including p-type (boron) and n-type (nitrogen, oxygen, and phosphorous) species, were prepared by a sequential impregnation and thermal treatment process, with variations in the dopant type and structural configuration. Comprehensive analyses of structural and electrocatalytic properties revealed a strong correlation between the intrinsic vanadium redox kinetics and the concentration of surface defects on carbon electrodes. In contrast, the impact of heteroatom functionalities, including their surface concentration and structural configuration, was found to be minimal. These finding underscore the pivotal role of the defect sites over heteroatom functionalities in enhancing the vanadium redox kinetics on carbon electrodes. Additionally, the study demonstrated that depositing graphene with abundant surface defects onto conventional carbon paper electrodes led to marked improvements in the energy storage capacity and charge-discharge efficiencies of VRFBs. This highlights the potential of defect-engineered carbon materials for advancing VRFB performance.