Membrane and interface modifications for improving the performance of Zinc/Bromine redox flow batteries

Abstract

Non-flammable and eco-friendly aqueous batteries, especially zinc bromine redox flow battery (ZBB) is one of the most promising candidates for gird-scale ESS due to its low cost, high power and energy density. However, dendritic zinc growth and severe bromine crossover practically limits the cycling stability of ZBB. Therefore, we tried to engineer the characteristics of membrane and interfaces in the battery to improve the ZBB performances. Firstly, we checked the applicability of Nafion materials as a separator in ZBBs. Scaling the water cluster size of Nafion membranes by a pre-hydration treatment can achieve not only a high ionic conductivity but also a bi-ionic transport, with significantly reduced bromine crossover. Compared with a conventional porous membrane, the properly treated Nafion membrane has higher energy efficiency indicating that the dense structured ion exchange membrane can be used for ZBBs by scaling the water cluster size. Secondly, with rational design of Nafion solution for Nafion impregnation into a porous PP separator, we fabricated a void-free 16 μm-thick Nafion-filled porous composite membrane by minimizing the use of Nafion. It showed comparable bromine crossover to conventional separator in spite of smaller membrane thickness, and exhibited higher energy efficiency. It demonstrates that ion exchange membrane can outperform the conventional porous membrane by reducing the membrane thickness with inexpensive porous substrate. Last chapter shows that the introduction of a non-conductive, highly porous, and zincophilic glass fiber (GF) layer on top of a carbon felt electrode notably suppresses dendritic Zn growth. The polar functional groups of the GF facilitate fast Zn ion transport and enable homogeneous ion distribution in the GF layer. A modified GF layer decorated with negatively charged polymer achieves superiorly uniform Zn deposition and noticeable cycling stability in ZBBs operation at 200 mA cm-2 and 50 mAh cm-2, supporting the validity of this approach.