Effect of ionomer dispersions on the performance of catalyst layers in proton exchange membrane fuel cells

Abstract

In this study, various ionomer dispersions having different equivalent weights, length of side chains, and dispersing solvents have been extensively analyzed to investigate the effect of the microstructure of catalyst layers on the performance of membrane-electrode assembly in proton exchange membrane fuel cells. Five different commercially available perfluorinated sulfonic acid ionomer dispersions have been used to prepare laboratory-made propylene glycol-based ionomer dispersions. The thickness of self-assembled ultra-thin films on silicon wafer for all types of ionomer dispersions has been measured and correlated mainly with the type of dispersing solvents (i.e., water, 2-propanol, and propylene glycol), the length of ionomer side chain, and the wettability. As a result, the short side chain and the propylene glycol-based ionomer dispersions form thicker films due to the smaller average size of their ionomer aggregates. The thick-ionomer wrapped catalyst layers especially prepared by the propylene glycol-based ionomer dispersions result in no influence on activation loss but approximately 42% less Ohmic loss and 43% higher limiting current density in proton exchange membrane fuel cells. In addition, it is confirmed that the slightly higher performance is exhibited as the thickness of ultra-thin ionomer film increases due to the effect of the length of side chains. The catalyst layers with the ionomer film over 100 nm show the higher performance than those below 100 nm. The MEA using Nafion D2020 having hydrophobic surface shows the lower increasing ratio (2.26%) in HFR than the one using Aquivion D98-25BS having hydrophilic surface due to the difference of the water-retaining ability.