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Alkaline Stability of Benzyl Trimethyl Ammonium Functionalized Polyaromatics: A Computational and Experimental Study
journal contribution
posted on 2014-10-14, 00:00 authored by Yoong-Kee Choe, Cy Fujimoto, Kwan-Soo Lee, Luke T. Dalton, Kathy Ayers, Neil J. Henson, Yu Seung KimAlkaline stability of benzyl trimethylammonium
(BTMA)-functionalized
polyaromatic membranes was investigated by computational modeling
and experimental methods. The barrier height of hydroxide initiated
aryl-ether cleavage in the polymer backbone was computed to be 85.8
kJ/mol, a value lower than the nucleophilic substitution of the α-carbons
on the benzylic position of BTMA cationic functional group, computed
to be 90.8 kJ/mol. The barrier heights of aryl–aryl cleavage
(polymer backbone) are 223.8–246.0 kJ/mol. The computational
modeling study suggests that the facile aryl–ether cleavage
is not only due to the electron deficiency of the aryl group but also
due to the low bond dissociation energy arising from the ether substituent.
Ex situ degradation studies using Fourier transform infrared (FTIR)
and 1H nuclear magnetic resonance (NMR) spectroscopy indicated
that 61% of the aryl–ether groups degraded after 2 h of treatment
in 0.5 M NaOH at 80 °C. BTMA cationic groups degraded slowly
over 48 h under the same conditions. In situ degradation studies validate
the calculated results: anion exchange membrane fuel cells and water
electrolyzer using poly(arylene ether) membranes exhibit a catastrophic,
premature failure during lifetime tests, while no sudden performance
loss is observed with an ether-free poly(phenylene) membrane. Despite
the gradual performance loss due to the degradation of BTMA cation
functional group, the membrane electrode assembly using the poly(phenylene)
membrane exhibited a lifetime of >2000 h in the alkaline water
electrolyzer
mode at 50 °C.
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Keywords
performance lossExperimental StudyAlkaline stabilitydegradation studiesarylcleavagebond dissociation energyFTIR0.5 M NaOHwater electrolyzer modeNMRBTMA cationic groupsmembrane electrode assemblyBenzyl Trimethyl Ammonium Functionalized PolyaromaticskJpolymer backboneanion exchange membrane fuel cells
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