Synthesis and Properties of Sulfonated Ethylene-Conjugated Alkene Copolymers for High-Performance Proton Exchange Membranes

The synthesis and properties of a family of high-performance polyethylene-proton exchange membranes (PE-PEMs) are discussed. The combination of copolymerization of ethylene with conjugated alkenes (including isoprene, butadiene, and myrcene) by scandium catalysts and further transformation of the carbon-carbon double bonds of the resulting copolymers into sulfonic acid groups and cross-linked structures efficiently, and in an environmentally friendly manner, affords PE-PEMs with well-controlled molecular structures. A systematic structure-property study of PE-PEMs indicates that (i) the block distribution of sulfonic acid groups in long polyethylene blocks, (ii) the incorporation of sulfonic acid groups by carbon-carbon double bonds of 3,4-polyisoprene, and (iii) the introduction of an alkyl chain thiol cross-linked structure are beneficial to the formation of a developed phase-separation structure, which is advantageous in improving the dimension and oxidation stabilities, conductivity, proton/methanol selectivity, and mechanical properties of PE-PEMs. The most desirable PE-PEMs with 30 mol % block distributed sulfonic acid groups and a 6 mol % cross-linked structure affords a good combination of desirable properties, including conductivity (188 mS cm(-1)), swelling ratio (23%), fuel barrier properties (proton/methanol selectivity: 0.71 x 10(5) S s cm(-3); hydrogen penetration: 11.1 Barrer), mechanical properties (tensile strength of 45 MPa, elongation of 273%), and oxidation stability (maintain over 73.8% conductivity with 24 h). A single cell with desirable PE-PEM shows open-circuit voltage and peak power density of 0.98 V and 424 mW cm(-2), respectively.