Carbon layer-confined sphere/fiber hierarchical electrodes for efficient and durable vanadium flow batteries

It is essential to boost the catalytic activity and cycling stability of the electrode for the ultra-long-term operation of vanadium flow battery (VFB). Herein, we report a novel hierarchical electrode by coating carbon spheres on the graphite felt (CS/GF) employing silicon dioxide nanospheres as templates and phenol formaldehyde resin as carbon sources. CS/GF possess larger surface areas and more defects than the thermally activated graphite felt, leading to better wettability and more active sites. As a result, the electrochemical activity and kinetic reversibility of the CS/GF electrode toward the VO 2 + /VO 2+ and V 3+ /V 2+ redox couples are promoted. Meanwhile, the novel carbon layer-confined structure ensures the superior durability of the CS/GF electrode during the ultra-long-term cycling test. The VFB assembled with CS/GF electrode exhibits outstanding rate performance at current densities of 50–300 mA cm −2 as well as excellent stability over 5000 charge-discharge cycles at a high current density of 150 mA cm −2 . The facile method in this work provides a new idea for developing efficient and durable VFB electrodes.