Bi-layer graphite felt as the positive electrode for zinc-bromine flow batteries: Achieving efficient redox reaction and stable mass transport

Zinc-bromine flow battery (ZBFB) is one of the most promising energy storage technologies due to their high energy density and low cost. However, their efficiency and lifespan are limited by ultra-low activity and stability of carbon-based electrode toward Br2/Br− redox reactions. Herein, chitosan-derived bi-layer graphite felt (CS-GF) with stable physical structure and abundant active sites is proposed and developed as cathodes for ZBFBs. In this uniquely developed electrode, the catalyst layer possesses oxygen and nitrogen co-doped surface enables high activity for redox reaction, whilst the wrapping structure enhances bonding force between catalyst layer and supporting layer to keep the stability against electrolyte flow. Moreover, the closely-bonding and partially-cracked catalyst layer ensures low electrical resistance between two layers and further emerges more inside active sites for bromine ions reactions. Consequently, the bi-layer electrode is demonstrated high electrochemical activity towards bromine ion redox reactions by cyclic voltammetry and electrochemical impedance spectra test. In battery charge-discharge measurements, a ZBFB employing CS-GF electrode yields a high current density of 120 mA cm−2, with an ultra-high coulombic efficiency of 99 % and improved energy efficiency of 69 %. Furthermore, the proposed bi-layer electrode enables stable battery performance, which cycle for more than 100 cycles with minor 0.057 % energy efficiency decay per cycle. This work provides a promising strategy to develop high-activity and high-stability electrode that possesses great application potentials in ZBFBs and other energy storage systems.