Asymmetric electrode design incorporating N-doped positive and Bi-modified negative electrodes for enhanced energy efficiency and durability in vanadium redox flow batteries

Minimizing polarization losses and enhancing stability through electrode engineering are essential for improving the efficiency of the vanadium redox flow battery (VRFB) systems. In this work, we present an asymmetric VRFB featuring a nitrogen-doped graphite felt (N-GF) with a carbon nanofiber network as the positive electrode, and a bismuth-decorated graphite felt (Bi-GF) as the negative electrode. The N-GF enhances the redox kinetics of the V 5+ /V 4+ pairs and provides structural durability, while the Bi-GF mitigates the hydrogen evolution reaction and facilitates the V 3+ /V 2+ reaction. As a result, the asymmetric cell achieves an energy efficiency of 90.72% at 100 mA cm −2 and maintains 80.52% at 250 mA cm −2 . After 200 cycles at 250 mA cm −2 , the energy efficiency exhibits a minimal average decay rate of only 0.017% per cycle, while the capacity retention remains at 73.47%. Long-term cycling analysis further reveals that N-GF preserves its nanofiber network and surface chemistry, whereas the pristine graphite felt suffers from serious carbon corrosion. This work demonstrates that rational asymmetric electrode design is an effective strategy to achieve both high energy efficiency and long-term durability in VRFBs.