Sulfonated poly(aryl ether nitrile) proton exchange membranes containing acid–base pairs for achieving low methanol permeability and enhanced dimensional stability

Abstract

Sulfonated poly(aryl ether nitrile) (SPEN) as proton exchange membrane (PEM) typically possesses excellent properties, but its performance is overly dependent on the degree of sulfonation. Balancing the degree of sulfonation with the conductivity, mechanical properties, methanol permeability, and dimensional stability of membranes is crucial. Herein, aminodiol monomer (4-AmPHQ), a bisphenol monomer, is synthesized and further reacts with 4,4′-dihydroxybiphenyl (BP), 2,5-dihydroxybenzenesulfonate (SHQ), 2,6-difluorobenzonitrile (DFBN), K₂CO₃, N-methylpyrrolidone (NMP), and toluene to prepare SPEN with varying amounts of amino groups (AmSPEN-Y). The influences of the amino groups on the structure and properties of SPEN are investigated. The results show that the proton conductivity of AmSPEN-Y membranes at 80 °C ranges from 0.137 to 0.174 S·cm⁻¹, which is much higher than 0.08 S·cm⁻¹ of Nafion 117 and meets the requirements of direct methanol fuel cells. The selectivity of AmSPEN-10 reaches 3.93 × 10⁵ S·cm⁻³·s, which is 8.7 times higher than the selectivity of Nafion 117. At 80 °C, the swelling rate of AmSPEN-10 is only 14.22%, demonstrating that this strategy simultaneously achieves the enhancement of SPEN’s proton conductivity, methanol selectivity, and dimensional stability. Our investigation provides a new strategy for preparing high-performance copolymer membranes, which have potential applications in direct methanol fuel cells (DMFCs) and other fields.