Ultra-low vanadium ion permeable electrolyte membrane for vanadium redox flow battery by pore filling of PTFE substrate

The proton conductive membranes were prepared by impregnating sulfonated poly(arylene ether ketone) (SP) into the porous polytetrafluoroethylene (PTFE) substrates for vanadium redox flow battery application. To impregnate SP, the surface of porous PTFE was chemically hydrophilized with catechol and polyethyleneimine (PEI). The SP filled PTFE membranes (trPTFE/SP) showed significantly enhanced thermal, dimensional, and mechanical stability, compared with the pristine SP membranes. While the water uptake of trPTFE/SP membranes reduced by only 3–5%, the swelling ratio reduced to about a half and tensile strength increased by 5 times. Although this pore filling process slightly decreased the proton conductivity of the pure SP membranes by 10%, it hugely decreased the vanadium ion permeability about 5 times. Their low vanadium permeability, 1.37 ​× ​10⁻⁷ ​cm² ​min⁻¹–4.21 ​× ​10⁻⁷ ​cm² ​min⁻¹, was even 15 times at most lower than that of Nafion117, 20.28 ​× ​10⁻⁷ ​cm² ​min⁻¹. This low vanadium permeability led to high coulombic efficiency over 96% and energy efficiency around 84% during the 100 charge-discharge cycling tests, which are better than Nafion117 membrane.