Insight on Pure vs Air Exposed Hydroxide Ion Conductivity in an Anion Exchange Membrane for Fuel Cell Applications

Understanding the role of water in anion exchange membrane (AEM) used in fuel cells is essential for commercial application.¹ Many literatures published have emphasized the important of water to optimize membrane properties such as ionic conductivity, ion transport, and mechanical and chemical stability.^1-3 Recently, the idea of using thin layers of catalyst and solubilized ionomer is very common and demands the greater understanding of hydration behavior of AEMs.⁴ Several techniques have been used to understand the diffusion of water in membranes. Elabd et al⁵ have summarized the diffusion coefficient values from different methods and mentioned the in-consistency among different methods. A time resolved Fourier transform infrared- attenuated total reflectance (FTIR-ATR) spectroscopy is a less common method to measure the water diffusion through AEMs although this method has been recently used in other materials.^{6, 7}

 FTIR-ATR spectra were collected using Nexus 4700 (Thermo-fisher) with an ATR accessory (Specac, Inc). A liquid nitrogen cooled mercury cadmium telluride (MCT) detector was used to collect a spectrum with 128 scans every 25 seconds with a resolution of 4 cm^-1. A custom built design was used to control RH of the membrane sample during experiments. In order to verify the experimental setup, the diffusion coefficient of water through Nafion 117® membrane was observed at the same order of magnitude (1.63±0.27 x 10^-7 cm²/s vs 3.71x 10^-7 cm²/s observed by Elabd et al⁷) when the relative humidity of the sample was changed from 0 to 100 %RH. Water diffusion coefficient was then measured for AEMs as a function of temperature and RH. The Fickian water diffusion coefficient for AEMs was observed to be lower than proton exchange membranes by one order of magnitude. Figure 1 shows the comparison of water diffusion kinetics between one of the anion exchange membranes and Nafion 117® at 30 ^oC.

In this work,  water behavior of different AEMs as a function of temperature and RH will be presented, the energy barrier for the water diffusion through these membrane will be discussed and the science behind a slow water diffusion through AEMs will be explored.

Figure 1. Time-resolved normalized absorbance for O-H stretching vibration collected using FTIR-ATR at 30^oC when RH was increased from 0 to 100%.

Acknowledgement

The authors would like to thank the US Army Research Office for the funding under the MURI #W911NF-10-1-0520and the purchase of FTIR microscope from ARO DURIP #W911NFNF-0462

References

 

1. Merle, G.; Wessling, M.; Nijmeijer, K., Anion exchange membranes for alkaline fuel cells: A review. Journal of Membrane Science 2011, 377, 1-35.
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