A gradient electrospinning electrode structure both in the in/through-plane directions for non-aqueous iron-vanadium redox flow battery.

• The work proposes an electrode with three-dimensional gradient pore structure. • The electrode has continuous gradient nanofiber size prepared by electrospun. • Energy efficiency of this electrode improves 74.2 % at 10 mA cm−2 compared to GF. To boost the performance of non-aqueous redox flow batteries (RFBs), it is important to synergistically improve the flow/mass transfer efficiencies and the uniformity of overpotential distribution into the porous electrode. In this work, electrostatic spinning technology is developed to propose a novel porous electrode with gradient pore distribution both in the in-plane and through-plane directions, and applied in deep eutectic solvent (DES) electrolyte-based iron-vanadium RFB. On the one hand, the new in-plane gradient design modifies the distribution of reactive species of electrode near the membrane side, resulting in the decreasing polarization loss. On the other hand, the increasing porosity of electrodes from the flow field side to the membrane side attains a trade-off between the charge transfer and the electrolyte flow resistances. According to the experimental results, compared to the graphite felt electrode, the energy efficiency of this RFB with three-dimensional gradient electrode improves by 74.2 % at a current density of 10 mA·cm−2. Moreover, the numerical simulation reveals the reactive transfer behaviors of three-dimensional gradient porous electrode. The results show that the proposed three-dimensional gradient design can enhance the uniformity of overpotential distribution and achieve the decrease of polarization resistance, thus improving the performance of DES electrolyte-based iron-vanadium RFB effectively. [ABSTRACT FROM AUTHOR]