Sulfonated multiblock copolynaphthalimides for polymer electrolyte fuel cell application

Sulfonated multiblock copolynaphthalimides (multiblock co-SPIs) were prepared by two-pot polymerization method from 1,4,5,8- naphthalenetetracarboxylic dianhydride, sulfonated diamine of 4,4′-bis(4-aminophenoxy)-3,3′-bis(4-sulfophenyl)biphenyl (BAPSPB) and nonsulfonated diamine of 4,4′-diaminophenyl hexafluoropropane. The multiblock co-SPI (BA1) with hydrophilic/hydrophobic block length of 20/10 and ion exchange capacity (IEC) of 1.67 meq g^-1 exhibited larger water uptake, larger in-plane and through-plane proton conductivity ( ^σ∥ and ^σ, respectively) than the random co-SPI with the similar IEC. The multiblock co-SPI (BA2) with the longer block length of 20/20 exhibited the large ^σ∥ and ^σ comparable to those of BA1, in spite of the smaller IEC of 1.35 meq g^-1. Both the multiblock and random co-SPIs showed the moderate anisotropic proton conductivity (σ/σ_//≒ 0.70) as well as anisotropic membrane swelling with about three times larger through-plane swelling than in-plane swelling. The TEM observation revealed that BA2 had an isotropic and inhomogeneous morphology with indistinct microphase-separated structure, whereas the random co-SPIs had a homogeneous morphology. The behavior of BAPSPB-based multiblock co-SPI membranes were quite different from that of the multiblock co-SPIs based on 2,2′-bis(4- sulfophenoxy)benzidine, which was due to the presence of two flexible ether bonds in BAPSPB moiety of the main chain. Even under the low humidification of 27/27% RH at 90 °C and 0.2 MPa, BA2 exhibited the fairly high PEFC performance; namely, cell voltage of 0.67 V at load current density of 0.5 A cm^-2 and maximum output of 0.51 W cm^-2, which were much larger than those of BA1 and the random co-SPI (RA1) with IEC of 1.84 meq g ^-1, and have the high potential as PEM for PEFC applications.