Multiblock-copolymerisation-derived sulfonated-poly(p-phenylene)-based polymer electrolyte membranes with simultaneously enhanced proton conductivity and mechanical strength

Abstract

To achieve cooperative improvements in the proton conductivity and mechanical properties of sulfonated hydrocarbon-type polymer electrolyte membranes (PEMs) for fuel cell applications, a series of hydrophilic-hydrophobic multiblock copolymers with controlled unit ratio and ion-exchange capacity (IEC) was synthesised using a four-step technique: ([1] sulfonation of a monomer with a protecting group to prepare the hydrophilic segment, [2] synthesising dichloro-terminated oligomers with controlled chain length via the SNAr reaction to obtain the hydrophobic segment, [3] direct copolymerisation of the two products generated in the preceding steps by Ni(0)-catalysed coupling, and [4] cleavage of the protecting group of the copolymers). Six types of sulfonated poly(4-phenoxybenzoyl-1,4-phenylene)-b-poly(arylene ether ketone) multiblock copolymers, S-6X (n) x:y, were obtained by varying the sulfonated-monomer/hydrophobic-oligomer feed ratio. The IEC and weight-average molecular weight of the copolymers were 0.98-2.02 meq g−1 and 47 300-239 000 g mol−1, respectively. The proton conductivity of the S-6X specimens at 80 °C and 90% relative humidity (RH) was 2.2 × 10−2-1.3 × 10−1 S cm−1. The tensile strength and ultimate elongation of S-6X under similar conditions were 19.3-40.7 MPa and 21.6-210%, respectively. S-6X exhibited superior conductivity and mechanical properties with suppressed excess swelling compared to those of conventional hydrocarbon-based PEMs. Electrochemical atomic-force-microscopy images of S-6X revealed a phase-separated morphology with continuous hydrophilic domains. In fuel cell tests conducted at 80 °C and 100% RH, S-6X exhibited an optimal maximum-power-density of 611 mW cm−2 and a limiting current density of 1800 mA cm−2, which are comparable to those of perfluorosulfonic-acid-type PEMs.