Characterization of Low EW Ionomers in Fuel Cells

Perfluorosulfonic-acid (PFSA) ionomers play a key role in the polymer-electrolyte fuel-cell (PEFC), in which they are primarily used as the proton-exchange membrane (PEM) with their remarkable transport properties and chemical/mechanical stability, but they also function as the nanometer thick "thin" electrolyte film in the catalyst layers binding the catalytic sites and providing proton transport pathways. In the thin-film regime, an ionomer exhibits vast deviations in its properties which are controlled by its interfaces with the air and the catalytic particles, i.e., carbon and platinum. While bulk membrane is responsible mainly for the ohmic resistances, in the catalyst layers, transport resistance in the ionomer film resulting from its interfacial interactions has direct implication on the mass-transport limitations in the PEFC, especially at higher current densities. Thus, it is imperative to understand the origins of ionomer-related transport resistances in fuel-cell catalysts, and how they changes with ionomer's chemistry and equivalent weight (EW).

In this talk, we present our findings on the impact of EW and hygrothermal environmental stressors the structure and properties of ionomers across the lengthscales, i.e., from bulk membrane (PEM) to catalyst ionomer film in CLs. It will be shown through X-Ray scattering studies how hygrothermal ageing induces morphological changes in the catalyst ionomer films, and subsequently alters their swelling and sorption behavior as investigated by ellipsometry and quartz crystal microbalance (QCM). From the collected data, structure-swelling-uptake correlations are generated for PFSA ionomers of various EWs, both as bulk membrane and thin films. From the grazing-incidence X-ray scattering (GIXS), changes in the size and orientation of water domains in the ionomer film are determined in response to humidity, heat-treatments and hygrothermal ageing, which result in substantial changes in these morphological features. Nanostructrual anisotropy, phase-separation and sorption kinetics are all found to be related and affected by the substrates the ionomer film is interacting with. The investigation on the impact of equivalent weights (EWs) and PFSA side-chain chemistries on ionomer’s response demonstrated a key role EW plays in controlling the structure/transport relationship of the ionomer, especially in the catalyst layers. Impact of ageing and heat-treatments on ionomer’s structure and properties are similar for both bulk and thin films, but found to be more pronounced for the latter. Our results provide new insights into the environmental and operational stressors controlling ionomer's functionalities, especially as thin-films in fuel-cell catalysts.