Poly(arylene ether ketone) proton exchange membranes grafted with long aliphatic pendant sulfonated groups for vanadium redox flow batteries

Abstract

As an energy storage system (ESS), all-vanadium redox flow batteries (VRFBs) have drawn much attention for their long life time, flexible design, low maintenance cost, and high energy capacity. The proton exchange membrane, one of the core components of VRFBs, is prepared from poly(arylene ether ketone) (PAEK) with long pendant sulfoalkyl groups. The chemical structure of PAEK is identified by using ¹H-NMR and FT-IR. The essential properties as a membrane for VRFB application such as proton conductivity, ion exchange capacity, and vanadium ion permeability along with the thermal, mechanical and chemical stability are tested for all membranes synthesized, and are compared with those of a commercial Nafion 117® membrane. As the proton conductivity increases with the degree of sulfonation (DS), the sulfonated PAEK membrane with a DS of 40% (S-PAEK-40) shows even higher proton conductivity than the Nafion 117® membrane. All of the S-PAEK membranes are thermally stable up to 200 °C and no chemical degradation is observed in electrolyte solution in the present experiment. The long aliphatic side chains prohibit the approach (attack) of vanadium ions to the polymer backbone (phenyl rings) and thus it results in lower VO²⁺ permeability than Nafion 117. While the ion cluster dimensions of S-PAEK membranes are larger than that of the Nafion 117® membrane, S-PAEK membranes display higher coulombic efficiency than the Nafion 117® membrane. In addition, S-PAEK membranes show excellent stability during the cycling test with 100 cycles.