Effect of Hygrothermal Ageing on PFSA Ionomers' Structure/Property Relationship

Perfluorinated sulfonic-acid (PFSA) ionomers are widely used in polymer-electrolyte fuel cells (PEFCs) as electrolyte membranes for both separating reactant gases and transporting protons from anode to cathode. Ionomer properties like conductivity are largely affected by hydration (water uptake) and temperature. Thus, it is desirable for the membrane to operate at higher humidity and temperature. However, during practical operation, where there is high temperature and humidity, these ionomers frequently undergo hygrothermal ageing, which may, for example, result in the formation of anhydrides through a condensation reaction of the sulfonic-acid groups[1]. As a result, the properties and morphology change accordingly, which can significantly impact PEFC operation. Although there are some reports on the effects of hygrothermal ageing [2-4], much remains to be explored in terms of possible changed of the membrane’s underlying structure/property relationship. In this talk, ionomers subjected to hygrothermal ageing will be analyzed in terms of their properties to understand the role of hygrothermal ageing in membrane degradation.

In this talk, the structure/property relationship of ionomers such as Nafion 212, 3M and Nafion XL will be examined both as received as well as after undergoing hygrothermal ageing at different conditions so as to correlate the separation-phased nanostructure with the membranes’ physical and transport properties. We will present the effect of pretreatment, temperature, humidity and purge flowrate on ageing and subsequent changes in mechanical, transport, uptake, and structural properties. It will be shown how the impact of ageing depends nonlinearly on humidity. We will also show how pretreatment like boiling and hot-pressing affect the impact of the ageing process. In addition, the possible effect of contamination will be discussed. Our findings provide new insight into how hygrothermal ageing affects the structure/property relationship of ionomers under different conditions.

References
[1] F.M. Collette, C. Lorentz, G. Gebel, F. Thominette, J. Membr. Sci., 330 (2009) 21-29.
[2] S. Naudy, F. Collette, F. Thominette, G. Gebel, E. Espuche, J. Membr. Sci., 451 (2014) 293-304.
[3] S. Shi, G. Chen, Z. Wang, X. Chen, J. Power Sources, 238 (2013) 318-323.
[4] F.M. Collette, F. Thominette, H. Mendil-Jakani, G. Gebel, J. Membr. Sci., 435 (2013) 242-252.

Acknowledgements

The authors would like to thank Kyle T. Clark for his help with using some of the diagnostics equipment. AK and AZW thank C.G. Gittleman of General Motors for insightful discussions. SAXS/WAXS experiments were performed in beamline 7.3.3 at the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory, which is a national user facility funded by the Department of Energy, Office of Basic Energy Sciences. We thank Chenhui Zhu and Dr. Eric Schiable for their assistance during facilitating the use of equipment at ALS. Shouwen Shi greatly thanks China Scholarship Council (CSC) for financial support during his visit to Lawrence Berkeley National Laboratory. This work was funded by the Assistant Secretary for Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U. S. Department of Energy under contract number DE-AC02-05CH11231 (LBNL) and program managers Donna Ho and Nancy Garland.