Towards uniform distributions of reactants via the aligned electrode design for vanadium redox flow batteries

Enhancing the hydraulic permeability of electrodes along both the through-plane and in-plane directions is essential in flow-field structured vanadium redox flow batteries, as it can promote uniform distributions of reactants, lower the concentration overpotential, and therefore improve battery performances. In this work, uniaxially-aligned carbon fiber electrodes with the fiber diameter ranging from 7 to 12 µm (average ~10 µm) are fabricated by electrospinning method. Attributed to the enhanced permeability of the aligned structure, the battery assembled with the prepared electrodes exhibits an energy efficiency of 84.4% at a current density of 100 mA cm⁻², which is 13.2% higher than that with conventional electrospun fiber electrodes. The permeability in the in-plane direction is further tailored by adjusting the orientation of aligned fibers against the flow channels. Results show that when the orientation of aligned fibers is perpendicular to the direction of flow channels, the battery delivers the largest discharge capacity and the highest limiting current density (~900 mA cm⁻²). Such an enhancement in the battery performance can be ascribed to the more uniform in-plane distribution of reactants and current by maximizing the permeability along the direction vertical to the flow channels, as evidenced by a three-dimensional model.