Structural Effects of Proton Conducting Ionomers on Water Uptake

The proton exchange membrane fuel cell (PEMFC) is a type of fuel cell that is widely used in transport, stationary, and portable applications. It consists of an anode, a cathode, and an electrolyte composed of an ionomer. Ionomers are used to carry charges for protons, separate reactant gases, and prevent electrons from passing through. Additionally, they also serve as the electrolyte film in catalyst layers that bind to the catalyst sites, where electrochemical activity takes place. However, the ionomer is confined to a nanoscale size in the catalyst layer, where particles are not well understood. The mobility of water and polymer chains are constrained due to severe confinement and deviates from the bulk-like behavior seen in polyelectrolyte membranes. Using sodium lignin sulfonate, perfluoroimide aicd (PFIA), and 825 EW ionomer, we determined how the chemical structure, film thickness, and humidity affects their water uptake. We used the quartz crystal microbalance (QCM) to measure the frequency of the crystal containing the polymer film at various relative humidity. This was used to calculate the mass change from the Sauerbrey equation, which represents the amount of water absorbed by the film at set conditions. The percent water uptake from the results of these polymers show that they are associated with a series of factors such as acidity, water-ionomer interactions, and swelling. These factors are all affiliated with proton mobility and impact the proton conductivity of the cell.