Macromolecules
COMMUNICATION TO THE EDITOR
In summary, we have developed a versatile and facile approach
for the preparation of a family of new ionomers with rigid
aromatic backbones and pendant perfluoroalkyl sulfonic acid
side groups. These ionomers exhibited comparable or even
superior proton conductivity to Nafion over a wide humidity
range at elevated temperatures, while maintaining other out-
standing properties of aromatic polymers, e.g., high Tg, low gas
permeability, excellent thermal and chemical stability, and good
mechanical properties. The prepared membranes showed better
performance than Nafion in initial fuel cell tests at 120 °C
without any optimization, indicating the potential of the aromatic
ionomers to meet the demands of elevated temperature PEM
fuel cells.
’ ASSOCIATED CONTENT
Figure 3. H2/air fuel cell performance of PAE-BP and Nafion 115 at
120 °C with anode and cathode gas streams humidified to 80% RH.
S
Supporting Information. Experimental details pertaining
b
to the synthesis and characterization. This material is available
coefficient (>10ꢀ5 cm2/s, Table 1), which are on a par with that
of Nafion. The proton diffusion coefficient (D) was calculated by
the following equation, D = RTσ/F2C(Hþ), where R is the gas
constant, T is the absolute temperature, σ is proton conductivity,
F is the Faraday constant, and C(Hþ) is the concentration of the
proton ions.10
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: wang@matse.psu.edu.
The humidity dependence of proton conductivity was deter-
mined at 80 and 120 °C. As depicted in Figure 2, PAE-BP and
PAE-NA possessed conductivities comparable (<50% RH) or
superior (>50% RH) to Nafion 115. For instance, a proton
conductivity of 116 mS/cm was measured for PAE-BP at 80%
RH and 120 °C, while Nafion 115 showed a value of 95 mS/cm.
For comparison, we also prepared and measured the conductivity
of an aromatic ionomer with the same backbone as PAE-BP and
hydrocarbon alkyl sulfonate side chain (PAE-BPHC, inset of
Figure 2a). The proton conductivity of PAE-BPHC was almost 1
order of magnitude lower than the conductivity of the aromatic
ionomers with perfluoroalkyl sulfonic acid groups and Nafion.
Furthermore, the conductivity of PAE-BPHC declined precipi-
tously with decreasing RH, which is a common trend for
conventional aromatic ionomers.2b,5 This observation further
proves the benefit of perfluorosulfonic acid groups on the proton
conduction of the ionomer membranes.
’ ACKNOWLEDGMENT
This work was supported in part by the National Science
Foundation (CBET-0932740).
’ REFERENCES
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Membraneꢀelectrode assemblies (MEAs) of these new
aromatic ionomers are now under investigation to evaluate
their potential application in hydrogen/air fuel cell devices at
elevated temperatures. The initial fuel cell performance was
tested at 120 °C with reactant gases humidified at 113 °C. To
isolate only the effect of the membrane, MEAs of PAE-BP
employed the Pt/C catalyst layers without using Nafion
binders. Figure 3 presents the polarization and power density
curves for PAE-BP and Nafion 115 membranes with similar
thickness (∼125 μm). Encouragingly, it was found that PAE-
BP membrane outperformed Nafion under the test conditions.
The maximum power density and current density at 0.5 V of
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branes is their low gas permeability. The measured hydrogen
crossover current density of 0.7 mA/cm2 from PAE-BP was
half that observed for Nafion 115 (1.6 mA/cm2). Further work
is ongoing to optimize MEA fabrication and evaluate the long-
term performance of new ionomer membranes in elevated
temperature fuel cells.
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dx.doi.org/10.1021/ma201188e |Macromolecules 2011, 44, 4605–4609