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Tuned Polymer Electrolyte Membranes Based on Aromatic Polyethers for Fuel Cell Applications
journal contribution
posted on 2007-04-04, 00:00 authored by Kenji Miyatake, Yohei Chikashige, Eiji Higuchi, Masahiro WatanabePoly(arylene ether sulfone)-based ionomers containing sulfofluorenyl groups have been
synthesized for applications to polymer electrolyte membrane fuel cells (PEMFCs). In order to achieve
high proton conductivity and chemical, mechanical, and dimensional stability, the molecular structure of
the ionomers has been optimized. Tough, flexible, and transparent membranes were obtained from a series
of modified ionomers containing methyl groups with the ion-exchange capacity (IEC) ranging from 1.32 to
3.26 meq/g. Isopropylidene tetramethylbiphenylene moieties were more effective than the methyl-substituted
fluorenyl groups in giving a high-IEC ionomer membrane with substantial stability to hydrolysis and oxidation.
Dimensional stability was significantly improved for the methyl-substituted ionomer membranes compared
to that of the non-methylated ones. This new ionomer membrane showed comparable proton conductivity
to that of the perfluorinated ionomer membrane (Nafion 112) under a wide range of conditions (80−120 °C
and 20−93% relative humidity (RH)). The highest proton conductivity of 0.3 S/cm was obtained at 80 °C
and 93% RH. Although there is a decline of proton conductivity with time, after 10 000 h the proton
conductivities were still at acceptable levels for fuel cell operation. The membranes retained their strength,
flexibility, and high molecular weight after 10 000 h. Microscopic analyses revealed well-connected ionic
clusters for the high-IEC membrane. A fuel cell operated using the polyether ionomer membrane showed
better performance than that of Nafion at a low humidity of 20% RH and high temperature of 90 °C. Unlike
the other hydrocarbon ionomers, the present membrane showed a lower resistance than expected from its
conductivity, indicating superior water-holding capability at high temperature and low humidity.