Polymeric Active Materials for Redox Flow Battery Application

The redox flow battery (RFB) is one of the most promising systems for large scale electrochemical energy storage applications. The development of redox-active materials is an essential part of RFB research. Commercial RFBs utilize redox-active inorganic ions, which have several issues such as expensive and toxic active materials, crossover of redox species, and high cost of the ion exchange membrane. The incorporation of redox-active polymeric materials is an intriguing solution because low-cost polymeric redox-active materials and size-exclusion porous membranes formed by commodity polymers could be applied to replace expensive inorganic redox-active materials and ion exchange membranes, respectively. The development of polymer-based redox-active materials in RFB application (PRFB) has advanced rapidly in the past few years. In this review, the recent progress of PRFB is summarized. Three major categories based on materials are discussed: the aqueous redox-active polymers, the nonaqueous redox-active polymers and oligomers, and polymer suspension systems. This review not only provides comprehensive information on synthetic strategy of polymeric redox-active materials and their properties such as redox potential and solubility in electrolyte but also reports the performance of recent PRFB cells, including cycling stability, cell voltage, and implementation of size-exclusion porous membranes. Finally, a short future perspective of PRFB development is provided.