Effect of Sulfonated Poly(ether ether ketone) Membranes with Different Sulfonation Degrees on the Performance of Vanadium Redox Flow Battery

Sulfonated poly(ether ether ketone) (SPEEK) materials were prepared by sulfonation of poly(ether ether ketone) (PEEK) in the concentrated H2SO4 (ca. 98%) under different time, and the SPEEK membranes were cast by a casting method and characterized by many methods. These key parameters were firstly obtained and reported, such as the ion exchange capacity (IEC), water uptake, mechanical property, proton conductivity of the membranes and vanadium(IV) ion permeability in SPEEK membranes with different degrees of sulfonation (DS). The following results were observed: (1) the membrane prepared by sulfonation for 12 h with DS of 46% had water uptake of 28%, and the vanadium(IV) ion permeability with the lowest proton conductivity of 0.02 S.cm(-1) in SPEEK had 1.2 x 10(-7) cm(2).min(-1), which was 1/24 of that in Nafion 117; (2) the membrane prepared by sulfonation for 96 h with DS of 79% had the proton conductivity of 0.16 S.cm(-1) which was 1.6 times higher than it in Nafion 1] 7, but had the worst mechanical property; (3) the membranes with sulfonation for 36 similar to 48 h, compared with Nafion 117, not only had good integration properties such as good property of mechanics, high IEC, proton conductivity, water uptake and better selectivity to V(IV) ion, but also had very low price, which was 1/13 of that of the Nafion 117. Based upon above properties, the SPEEK membranes prepared by appropriate time are more likely to be one of the substitutes of Nafion 117 for vanadium redox flow battery (VRF) system. The effects of the degree of sulfonation of PEEK polymer and the amount of H2SO4 were also discussed.