Highly Economical and Efficient Polyethylene Separator for Vanadium Redox Flow Battery

The low-cost and negligible protonic resistance of the polyethylene (PE) separator make it a potential alternative to the high-cost Nafion membranes. However, severe vanadium ion crossover through the micron size pores of the PE separator needs to be addressed. In the present study, the porous network of the PE separator is modified by applying the thermal treatment technique by exposing it to a high temperature within the range of 130–250 °C. Separator characterizations, such as the attenuated total reflectance–Fourier transform infrared spectroscopy (ATR–FTIR), thermogravimetric analysis (TGA), tensile strength, Brunauer–Emmett–Teller (BET), ionic resistance, and vanadium ion permeability, are discussed in detail. BET results show that the surface area and pore volume of the PE separator decrease with the increase in the temperature. The vanadium ion permeability of the PE pristine separator (217 × 10–7 cm2 min–1) reduced significantly to 9 × 10–7 cm2 min–1 for the PE-170 separator owing to the decrease in the pore size. Consequently, the self-discharge time of the thermally treated PE separators increased with an increase in the temperature. The PE separator heated at 170 °C exhibited a maximum self-discharge time of 94 h compared to the commercial Nafion 117 membrane (70 h) and PE pristine separator (24 h). As a result, the columbic efficiency (CE) of the separator reached up to 99% at 80 mA cm–2 owing to the decrease in the vanadium ion crossover. The vanadium redox flow battery (VRFB) cell equipped with the PE-140 separator demonstrated optimum results in terms of better capacity retention, CE (99%), and energy efficiency (EE, 70%). Further, the separator performance evaluated at a three-cell VRFB stack with an effective area increased to 228 cm2. Further, the feasibility of the treated separator is justified by a simple techno-economic analysis at the kilowatt (kW) level VRFB stack. Overall, the results exhibit that the low-cost thermally modified PE separator is suitable for the VRFB system.