(402b) Radiation Induced Grafted Fep-g-Polystyrene Membranes: Chemical Degradation and Quantum Chemical Simulation

Authors: 
Li, X., Tsinghua University
Xie, X., Tsinghua University
Zhao, Y., Tsinghua University
Li, W., Tsinghua University
Wang, S., Tsinghua University
Ramani, V., Washington University in St. Louis

Polymer
electrolyte membrane fuel cells (PEMFCs) have received significant attention in
recent years due to high efficiency and low emissions. The proton exchange
membrane (PEM) is one of the key components that of significant importance to
the fuel cell performance, which is electron insulating while proton conductive
and should prevent the crossover of fuels and oxidant, as well as stable in the
fuel cell operation environment[1].
Radiation induced graft (RIG) is a versatile method to modify pre-existing
polymers to introduce a variety of desired functionalities, and has been
studied to prepare PEM[2].

In
this work, PEM was prepared by RIG polymerization of styrene onto FEP base
polymer followed by sulfonation using chlorosulfonic acid. The graft
polymerization was performed by immersing the FEP films into a series of
styrene solutions and submit to the 60Co radiation source. The sulfonated
FEP-g-polystyrene membranes exhibited much higher proton conductivity than
Nafion 211 as shown in Fig.1(a). And the membrane prepared in 40%v/v solution
showed better fuel cell performance than Nafion 211 under the same test
conditions in Fig.1(b).

Fig.1 (a) Proton conductivity of the
sulfonated FEP-g-polystyrene grafting in solutions of different concentration
in comparing with Nafion 211. The corresponding degrees of graft were listed
alongside. (b) Polarization curve of the sulfonated FEP-g-polystyrene in
comparison with Nafion 211, catalyst loading is 0.6mgPt/cm2
and H2/air was used in the fuel cell test.

Also elucidation of the mechanism of
membrane degradation is an active area of research. Current papers have
proposed a degradation mechanism that involves attack of oxygen radicals on
susceptible groups in the polymer backbone [3].In
this work, Fenton¡¯s test was conducted using 30% H2O2 and
10ppm Fe2+. And the membranes were also tested in fuel
cell operation conditions. The resulting solution of Fenton test and water
collected at the anode and cathode were analyzed for fluoride ion content using
an ion chromatography system. XPS, FTIR and SEM were also used to characterize
the degraded membrane.

Previous work has suggested that hydroxyl free radicals can attack
polymer end groups having H-containing terminal bonds (such as -CF2COOH)
that are formed during membrane processing[4].
Here we would use quantum chemical simulations to elucidate degradation
mechanism of the sulfonated FEP-g-polystyrene membrane suffered to the ¡¤OH and ¡¤OOH radical attack by searching the
transition state of different reaction pathways.

Keywords: radiation induced graft,
FEP-g-polystyrene, chemical degradation, quantum chemical calculation

Acknowledgements: This work was
supported by the Intergovernmental International Scientific and Technological
Innovation Cooperation Key Projects (2016YFE0102700) and National Natural
Science Foundation of China (51573083).




References:

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PFSA Ionomers used in PEM Fuel Cells[J]. Fuel Cells, 2005, 5(2):302-308.

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