Poly(phenylene) block copolymers bearing tri-sulfonated aromatic pendant groups for polymer electrolyte fuel cell applications

Novel poly(tri-sulfonated phenylene)-block-poly(arylene ether sulfone) copolymers (PTSP-b-PAESs) were synthesized by Ni(0)-catalyzed copolymerization of 2,5-dichloro-3′-sulfo-4′-((2,4-disulfo)phenoxy)-benzophenone and chlorobenzophenone-endcapped oligo(arylene ether sulfone). Their physical properties, morphology and polymer electrolyte fuel cell (PEFC) performance were investigated compared to those of poly(mono-sulfonated phenylene)-block- poly(arylene ether sulfone) and the corresponding random copolymers. They had a low ion exchange capacity (IEC) of 1.1-1.2 meq. g^-1 and showed very low water uptake and in-plane dimensional change in water. They exhibited a more well-defined microphase-separated structure composed of hydrophilic and hydrophobic domains, where the hydrophilic domains were well-connected to each other to form the channels for proton conduction, than the mono-sulfonated one. This led to the relatively high proton conductivity under low relative humidities. The corresponding random copolymers exhibited a homogeneous morphology and much lower proton conductivity in spite of a high IEC of 2.0-2.1 meq. g^-1. Even under the low humidification of 30% RH at 90°C and 0.2 MPa, they exhibited high PEFC performance and durability; for example, a cell voltage of 0.69 V at a load current density of 0.5 A cm^-2 and a maximum output of 0.73 W cm^-2, which was comparable to that of the mono-sulfonated one with a much higher IEC of 1.8 meq. g^-1 and much higher than those of the corresponding random copolymers. PTSP-b-PAESs have high potential as polymer electrolyte membranes for fuel cell applications.