Synthesis, Characterization and Stability of Anion Exchange Membranes (AEMs) with Perfluorinated Backbone

Bosnjakovic, Admira, Marek Danilczuk, and Shulamith Schlick

Despite the remarkable success of proton exchange membrane fuel cells (PEMFCs) as an alternative power source, their commercialization has been limited due to required use of costly noble metal (Pt) catalysts known for their resistance to low pH.1 Recently alkaline fuel cells (AFCs) with solid polymer anion exchange membranes (AEMs) as the electrolyte have received renewed attention as a low-cost power source.2 AEMs consist of hydrocarbon or perfluorinated backbones that are covalently attached to cations, typically quaternary ammonium ions. Due to its stability properties, the perfluorinated membrane known as Nafion, in sulfonyl fluoride (SO2F) form (Nafion precursor), has been explored as an anion exchange membranes for fuel cell application.2

 

We present the synthesis of AEMs based on 3M fluorinated membranes in the precursor form.3 The precursor was obtained in powder form and sheets of thickness in the range of 75-100 microns and diameter ~70 mm were prepared using hot-pressing at ~230°C for 1 min. The 3M-based AEMs were synthesized by covalent attachment of trimethyl ammonium  (TMA), 1,4-dimethylpiperazinium (DMP), and quaternized 1,4-diazabicyclo[2.2.2]octane (DABCO) cations to the sulfonyl fluoride (SO2F) group of the 3M precursor membrane. The characterization and chemical stability of 3M-based AEMs were determined by Fourier Transform Infrared (FTIR) spectroscopy. The FTIR spectra of all quaternized membranes showed the characteristic bands at 3078-3050, 2808-2797 and 1488-1466 cm-1 assigned to CH2 and CH3 vibrations of amine groups.4,5

 

The chemical stability of AEMs was determined by their exposure to (1) KOH at concentrations of 0.1, 0.5, and 1M, and (2) hydroxyl radicals obtained by UV irradiation of 3% H2O2. The FTIR spectra of all AEMs exposed to 0.1 M KOH showed the gradual disappearance of the amine groups with the time, indicating that the AEMs degrade in KOH solution. It is interesting that after exposure of all AEMs to KOH a sharp peak around 3670 cm-1 was seen, which is characteristic for OH- ions.6 We propose that the OH- ions are trapped inside the membrane, most probably in water clusters.

 

The FTIR spectra of AEMs with TMA cation showed that the amine bands remained intact after exposure to hydroxyl radical, while the FTIR spectra of the other two AEMs with DMP and DABCO cations showed lower intensity of the amine bands. 

 

Acknowledgements

This research was supported by Polymers Program of the U.S. National Science Foundation.

We are grateful to Steven Hamrock of 3M for the supply of 3M precursor.

 

References

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