An aqueous polysulfide redox flow battery with a semi-fluorinated cation exchange membrane

Abstract

The vast availability and environmental benignity of polysulfide–ferricyanide redox flow batteries (PSFRFBs) have attracted much deserving attention. However, the commercial scalability of polysulfide-based batteries is hindered by the expensive commercial ion exchange membrane and also the sluggish kinetics of polysulfide. Herein, we report an economically viable, thermally annealed PVDF-co-HFP-based cation exchange membrane (T-CEM). Thermal densification of the membrane mitigated the cross-contamination of polysulfide and ferro/ferri species across the membrane, whereas controlled sulfonation allowed smooth conduction of charge carriers. The diffusion coefficient values were 4.57 × 10⁻¹¹ and 3.05 × 10⁻¹² dm² s⁻¹ for polysulfide and ferricyanide, respectively, better than those of commercial separators. The polarization curve experiment depicted a power density of 220 mW cm⁻² at 400 mA cm⁻² current density. The flow battery exhibited capacity retention of 88% with average capacity decay of 0.12% per cycle, 99.4% coulombic efficiency and 63.0% energy efficiency over 250 uninterrupted charge/discharge cycles at 40 mA cm⁻² current density, and the long durability characteristic revealed high efficacy and the best usability in PSFRFBs. Furthermore, the facile densification strategy demonstrated in this work can be employed to fabricate better ion exchange membranes for energy device applications and separation/purification.