Membranes in non-aqueous redox flow battery: A review

Redox flow battery (RFB) is promising in grid-scale energy storage, and potentially applicable for facilitating the harvest of the intermittent renewable power sources, like wind and solar, and stabilizing the power grid. Early RFBs are based on aqueous electrolytes and the all-vanadium as well as Zn/Br systems have been demonstrated in close commercial scale. Non-aqueous RFBs (NARFBs) are the second-generation flow batteries based on organic solvent which have potentially much wider electrochemical window, and thus possible much higher energy density, and temperature window than those of the aqueous systems. As a crucial component of NARFBs, the membrane serves to prevent the crossover of the positive and negative active species whilst facilitating the transfer of the supporting electrolyte ions. However, the membranes utilized in the state-of-the-art publications still need great improvements in performance. In this article, the fundamentals, classifications, and performances of the membranes in NARFB are introduced. The recent progresses and challenges on the innovation of NARFB membranes are summarized. A perspective on the near future developments of NARFB membranes are presented. The composite membranes are likely the promising direction to forward the development of the NARFB technologies.