Bifunctional framework activated carbon graphite felt composite electrodes for high-performance vanadium redox flow battery

The limited electrochemical and transport performance of the electrode in vanadium flow batteries (VRFBs) leads to low power density, and high stacking costs, and therefore hinder their commercialization. In the present work, we propose to use a bifunctional frame activated carbon (BFAC) graphite felt (GF) composite electrode to improve the power density of VRFBs. The proposed electrode is fabricated in two steps: (1) in-situ growth of multi-edge carbon framework carbon on GF surface is first achieved using sodium citrate as a precursor and (2) the skeleton carbon and exposed GF fibers are then activated by KOH. The multi-edge carbon framework carbon produced in step (i) increases the active sites for the vanadium ion reaction and enables the regulation of V²⁺/V³⁺ and VO²⁺/VO₂⁺ redox reaction, while the oxygen-containing functional groups and micropores induced in step (ii) not only further increases the number of active sites, but also enhances the mass transfer process. Electrochemical characterization and contact angle analysis jointly demonstrate that the BFAC GF electrode exhibits superior electrocatalytic activity and enhanced mass transfer capabilities relative to pristine GF. The underlying mechanism behind the electrochemical performance improvement is further elucidated via the first-principle calculations at the atomic scale. Benefiting from these improvements, VRFBs assembled with BFAC GF achieve a high energy efficiency (EE) of 78.6 % at 200 mA cm⁻², and can operate at a high current density of 300 mA cm⁻² while maintaining an EE of 69.7 %. Moreover, the VRFBs demonstrate a remarkable long-term cycle performance, retaining 91.7 % EE after 500 cycles at 200 mA cm⁻². This work paves the way to developing low-cost and high-performance electrodes for VRFBs.