Bio-inspired multiscale-pore-network structured carbon felt with enhanced mass transfer and activity for vanadium redox flow batteries

Simultaneously achieving fast mass transfer and high electrochemical activity has been a central issue of the porous electrode design of vanadium redox flow batteries (VRFBs). In this work, we present a bio-inspired multiscale-pore-network structured carbon felt electrode via chemistry of metal extraction for VRFBs. For the uniquely developed electrode, the micron-scale pores formed by the intersected carbon fibers are favorable for electrolyte permeation while the vanadium ion diffusion and redox reactions are promoted by the submicron-scale and nanoscale pores on the carbon fiber surfaces. Besides, oxygen and nitrogen functional groups are also introduced onto electrode surfaces to synergistically improve the catalytic activity and wettability. The superb structural and surface characteristics of the present multiscale electrode result in an encouraging VRFB performance with an energy efficiency of 81.9% at a current density of as high as 320 mA cm(-2), which is 15.2% higher than that of pristine (single-scale) carbon felt electrodes. Furthermore, the battery with the multiscale electrodes can stably operate for 500 cycles without an obvious efficiency decay, verifying the excellent stability of such a structure. In principle, the multiscale electrode design can also be extended to other types of flow-cell architectured electrochemical systems that involve multiscale processes.