Achieving gradient-pore-oriented graphite felt for vanadium redox flow batteries: Meeting improved electrochemical activity and enhanced mass transport from nano- to micro-scale

Developing high-performance electrodes that enable high redox activity and quick mass transport has been a central issue to enhance energy efficiency and current density in all-vanadium redox flow batteries (VRFBs). In this work, a gradient-pore-oriented graphite felt (gradient-pore GF) electrode that contains pores from nano- to micro-scale was proposed by a facile one-step etching method. In this uniquely developed electrode, the microscale pores (∼20 μm) offer pathways for electrolyte flow, the nanoscale pores (∼20 nm) render sufficient active sites for electrochemical reactions, while the mesoscale pores (∼0.5 μm) as a "bridge" between nano and micro scale pores both facilitate the formation of active sites and reduce the electrolyte diffusion resistance. Thanks to the multiscale-pore-architectured structure, the high specific surface area (21.16 m² g^-1), and the abundant oxygen functional groups (25.69%), gradient-pore GF has demonstrated high electrochemical activity towards vanadium ion redox reactions on both positive and negative sides. The battery assembled with the gradient-pore GF electrodes yields an energy efficiency as high as 79.74% at the current density of 200 mA cm^-2, 19.09% higher than that with pristine graphite felt electrodes. Additionally, its energy efficiency can reach 63.41% at high current densities up to 500 mA cm^-2. This work provides an effective way to develop a high-performance electrode that possesses great potential applications in VRFBs and other battery systems.