Fuel cells based on polymer electrolyte membranes (PEM) show promise as a means of energy conversion for a wide range of applications both in the transportation sector and for stationary power production due to their high charge density and low operating temperatures. While the structure and transport of bulk PEMs for fuel cell applications have been studied extensively, there has been little effort focused on these materials at interfaces and under confinement as they exist within the membrane electrode assembly (MEA) of a working PEM fuel cell. Using a combination of x-ray reflectivity, grazing-incidence small-angle x-ray scattering (GISAXS), quartz crystal microbalance (QCM), as well as polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) we have studied the structure, swelling, water solubility and water transport behaviour as a function of relativity humidity for the polyelectrolyte confined to thin films (< 220 nm). We observe that while the relative humidity-dependent equilibrium swelling ratio, volumetric water fraction, and effective diffusivity is relatively constant for films above ca. 60 nm, below this thickness there is a measurable suppression in the aforementioned materials properties. It is speculated that these confinement effects have an impact on the mass transport at interfaces and within the catalyst layer of the MEA and, therefore, the overall fuel cell performance. These studies clearly show that the behaviour of PEM materials under confinement can deviate significantly from the bulk. Current fuel cell modelling efforts rely on bulk property values, when considering the catalyst layers and interfaces, to predict structure, mass transport and fuel cell performance. With this new information, researchers will be able to more accurately model the performance of the MEA within a working fuel cell which could lead to improvements in MEA design and more efficient operating conditions.
Pub Type: Journals
polyelectrolytes, fuel cells, Nafion