Manipulation of highly durable sulfonated poly(ether ether ketone) thin-film composite membranes by polyamide interfacial layer thickness for vanadium flow battery

This study reports on the preparation of sulfonated poly (ether ether ketone) (SPEEK) thin-film composite membranes (SPEEK/PA), with SPEEK acting as the substrate and piperazine and trimesoyl chloride acting as the reactive monomers for the polyamide (PA) thin layer, via interfacial polymerization. Controlled PA layer thickness of 119, 223, 283 and 396 nm were developed for the SPEEK/PA composite membranes, which showed excellent vanadium ion resistance and thickness dependence. The formed interfacial acid-base interaction between the SPEEK and PA layers enables stable proton transport, thus ensuring excellent conductivity. Compared to Nafion 212 and SPEEK, SPEEK/PA-283 exhibits much higher proton conductivity (29.7 mS cm −1 ), ion selectivity (35.2 × 10 3 S min cm −3 ), and much lower vanadium ion permeability (8.44 × 10 −7 cm 2 min −1 ). Furthermore, SPEEK/PA-283 displays remarkable single-cell performance, including higher coulombic efficiency and energy efficiency, with maximum values of 96.5%–98.4% and 82.7%–71.1%, respectively, at current density of 100–200 mA cm −2 . Manipulating the interfacial PA layer on the SPEEK surface also provides satisfactory cyclic stability, and the superior self-discharge time of 122.3 h demonstrates that the anchored PA layer guarantees the structural stability and durability of the SPEEK matrix. This provides an easy way to modulate the conductivity and permeability of the SPEEK composite membrane, and opens up the possibility of exploring highly structurally stabilized proton exchange membranes using the interfacial polymerization technique.