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 (similar to 20 mu m) offer pathways for electrolyte flow, the nanoscale pores (similar to 20 nm) render sufficient active sites for electrochemical reactions, while the mesoscale pores (similar to 0.5 mu m) 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 m(2) 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.