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The ultra-high proton/vanadium ion selective composite membranes for vanadium redox flow battery (VRB) consisting of ZrO2 nanotubes (ZrNT) with perfluorosulfonic acid (Nafion) and polyoxometalate coupled with a graphene oxide-sulfonated poly(arylene ether ketone) (SPAEK) were designed and fabricated. The main goal of introducing ZrNT and PW-mGO fillers in Nafion and SPAEK membrane are to provide high proton conductivity of the composite membrane as well as to partially block the porous structure of Nafion and SPAEK pristine membranes to reduce the vanadium ion permeability by tortuous pathway effect. The incorporation of zirconium oxide nanotubes in the Nafion matrix exhibited high proton conductivity (95.2 mS cm-1), low vanadiumion permeability (3.2 × 10-9 cm2 min-1) and superior ion selectivity (2.95 × 107 S min cm-3).
Similarly, the superior proton conductivity of SPAEK-PW mGO composite membrane exhibits 20-times lower vanadium ion crossover than a pristine Nafion membrane. The fabricated composite membranes, Nafion-ZrNT and SPAEK/PW-mGO were used as an electrolyte membrane for VRB showed low self-discharge rate (open circuit voltage was maintained above 1.3 V after a period of 330, 441 h, respectively) relative to a pristine Nafion membrane (29 h).
After 100th charge-discharge cycle, the capacity retention for Nafion-ZrNT and SPAEK/PW-mGO composite membrane exhibits 66 and 78%, respectively, which are much better compared with a Nafion membrane (38%). The high performance of composite membranes can be more obviously observed in the coulombic efficiency, voltage efficiency and energy efficiency of the VRB with Nafion-ZrNT and SPAEK/PW-mGO composite membranes compared with a Nafion membrane at a 40 mA cm-2 current density. The detailed overview are reported in results and discussion part.
The improved performance is attributed to the superior proton conductivity, low vanadium permeability and strong chemical stability of Nafion-ZrNT and SPAEK/PW-mGO composite membranes. ⓒ 2017 DGIST
