Tuning the ion-transport nanochannels of sulfonated poly (ether ether ketone) membranes for efficient aqueous organic redox flow battery

Sulfonated poly(ether ether ketone) (SPEEK) is a potential low-cost candidate to replace fluorine-based Nafion membranes in some redox flow batteries (RFBs). In this work, the ionic conductivity of SPEEK membranes is increased almost fourfold by conducting a well-controlled irreversible swelling of the membrane. Atomic Force Microscopy images and Small-Angle X-Ray Scattering curves effectively demonstrate that the size of the hydrophilic nanochannels in the swollen membranes can be precisely tuned by simply varying the solvent concentration. The modified SPEEK membrane (SPEEK45) is applied in an aqueous organic/organometallic RFB, using 1,1′-bis(3-sulfonatopropyl)-4,4′-bipyridinium (SPr2V) and ferrocyanide salts as anodic and cathodic materials, respectively. The boosted conductivity of the modified SPEEK membrane enables higher energy efficiencies than in the case of the Nafion membrane (N212) at various current densities. In addition, a 200-cycle charging-discharging test is carried out to evaluate the long-term stability of the modified SPEEK membrane. The results show that SPEEK45 exhibits higher energy efficiency than N212 (70.8 and 65.4% at 60 mA cm^-2, respectively), with comparable capacity retention. The well-controlled swelling method proves to be a promising and innovative membrane technology for promoting ion conductivities of membranes used for various RFBs.