Highly selective composite membranes using ladder-like structured polysilsesquioxane for a non-aqueous redox flow battery

Nanoporous composite membranes coated on the microporous support with a ladder-like ionic polymer were prepared and the structural selectivity effect of the membrane on a non-aqueous vanadium redox flow battery (RFB) was investigated. Crosslinkable ladder-like ionic polysilsesquioxane (L-PSQ) was synthesized and a Celgard 2400 membrane was used for the support. An L-PSQ/Celgard composite membrane was prepared by the thermal crosslinking reaction of crosslinkable L-PSQ after the dip-coating process. The permeability of ionic L-PSQ/Celgard composite membranes decreased by two orders of magnitude upon thin coating while maintaining its high ion conductivity of the porous support, which provides higher charge/discharge capacity and cell efficiencies of RFB at all current densities compared to those of the RFB with the porous Celgard 2400 membrane. Due to the reduction of the crossover, the coulombic and energy efficiencies of non-aqueous vanadium RFB increase and reached 62.7% and 43.7%, respectively, for the RFB with the membrane containing 5.4 wt% of L-PSQ. These values are higher than those obtained for the Celgard membrane and the commercial Neosepta AHA membrane. This result implies that structure control of the membrane is a practical solution for the adaptation of the non-aqueous RFB for the future energy storage systems. •The structural permselective effects of the membranes used in non-aqueous redox flow batteries (RFBs) was studied.•Nanoporous composite membranes were prepared with crosslinkable ladder-like ionic polysilsesquioxane.•A selectivity parameter was defined for evaluating the membranes in RFB.•A non-aqueous RFB with a nanoporous composite membrane shows higher efficiency than that with a commercial membrane.•Controlled pores in the ladder-like ionic polymer layer on a porous membrane enable successful RFB application.