This work investigates the mechanical and water transport properties of Nafion, a fully fluorinated ion conducting polymer used as a membrane material in proton exchange membrane fuel cells (PEMFCs). Both of these properties are extremely important to the short and long term operation of fuel cells.
Nafion is a viscoelastic material, responding to stress in a time-dependant manner. The result is that Nafion flows under stress and responds dynamically to changes in hydration and stress. Stresses applied to the membrane of a PEM fuel cell, both from clamping as well as strain from changing levels of hydration, cause Nafion to flow. This results in thinning in spots and sometimes leads to the development of pinholes or contact problems between membrane and electrode. Temperature and water content strongly affect Nafion’s viscoelastic response, of direct importance for operating PEM fuel cells.
The viscoelastic response of Nafion was measured over a range of temperature and hydration using viscoelastic creep. A specially designed creep apparatus with environmental controls was used. It was found that the effects of temperature and hydration on Nafion’s viscoelastic response are very complicated. Around room temperature, water acts to plasticize Nafion; elastic modulus and resistance to creep decrease with increasing hydration. As temperature increases, water has the opposite effect on mechanical response; hydration acts to stabilize the material. Mechanical property values are reported over a range of temperature and hydration germane to the operation of PEMFCs. Additionally, the data is used to infer molecular level interactions and the effects of temperature and hydration on microstructure.
Hydration of Nafion and other PEMFC materials is required for the high proton conductivity needed for fuel cell operation. Uptake of water by Nafion results in volumetric swelling. Water transport through Nafion was directly measured by permeation. Both liquid and vapor phase transport was measured as a function of temperature for several membrane thicknesses. Diffusion and interfacial mass transport were considered. Diffusion was found to increase with membrane hydration. Permeation was much greater from liquid than vapor. Interfacial mass transport at the membrane/vapor interface dominated liquid permeation at all temperatures and for all membrane thicknesses. Diffusion dominated for vapor permeation and was found to increase with temperature.
|School Location:||United States -- New Jersey|
|Source:||DAI-B 68/12, Dissertation Abstracts International|
|Subjects:||Chemical engineering, Mechanical engineering, Materials science|
|Keywords:||Creep instrument, Hydration, Hydration and temperature effects, Mechanical properties, Nafion, PEM fuel cells, Viscoelastic response, Water transport|
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