An electrospinning carbon nanofiber composite electrode with gradient porous structure for deep eutectic solvent electrolyte-based iron‑vanadium redox flow battery

This work designs and prepares a composite electrode that is made of triple-layer micropore/mesopore electrospinning carbon nanofiber (ECNF) felts with gradient pore distributions and graphite felt (GF) as backing layer. The micropore ECNF felt can provide sufficient reactive surface area as well as reduce the charge transfer resistance near the membrane side. Meanwhile, the other two mesopore ECNF felts prepared by two nozzle vertical same-side blending spinning method are sandwiched between GF backing layer and micropore ECNF felt, thus making a trade-off between the reducing charge transfer resistance and the decreasing electrolyte flow resistance. Experimental results indicate that the deep eutectic solvent electrolyte-based vanadium‑iron redox flow battery (RFB) assembled with this gradient porous composite electrodes has the energy efficiency of 82.02 % at 5 mA·cm−2, which is 42.57 % higher than that of conventional RFB with mono-layer GF electrode. Even at 10 mA·cm−2, its energy efficiency can still be above 70 %, which is 70.62 % higher than that of a conventional RFB. In addition, the peak power density is also 77.44 % higher than that of the GF electrode. It has been demonstrated that the performance of non-aqueous RFB can be greatly improved by using carbon nanofiber composite electrodes with gradient pore structure.