Probing the Real-World Properties of Thin Ionomer Films in Proton-Exchange Membrane Fuel Cell Catalyst Layers

PFSA ionomer thin films were prepared from realistic ionomer solution concentrations at variable thicknesses on SiO2, Pt, and C. The thin films, prepared from two different ionomers, were then characterised using Neutron Reflectometry and Dynamic Vapour Sorption.  All ionomer films exhibited the expected increase in thickness due to hydration on exposure to a high relative humidity. Use of a custom-built environmental control chamber enabled measurement of the fast hydration equilibration times of the films, unlike previous studies that used salt solutions for humidity control. The data obtained enabled the probing of the effect of ionomer film thicknesses, substrate, and ionomer type. The measurements show that the different ionomers, despite taking up similar amounts of water per sulfonic acid group, showed significant differences in film swelling in response to that water uptake. The effect of ionomer type is shown to be greater than differences due to substrate. This study therefore provides mechanistic insight into the differences performance of the catalyst layers containing these ionomers at different ambient humidities beyond traditional characterisation techniques.