Abstract
The influence of the ionomer type in DMFC anode catalyst layers (ACLs) on the properties of primary and secondary pores was investigated. Two hydrocarbon ionomers, Fumion®SO-340 and Fumion®S-360, were compared with Nafion®. Electrochemical properties of MEAs and ACLs were determined by galvanostatic U/i-characteristics, methanol stripping and impedance spectroscopy. Structural properties and chemical compositions of the ACLs were determined by SEM/EDS and automated standard porosimetry. Substitution of Nafion® by hydrocarbon ionomers causes a decrease of maximum power density and active surface area by up to 48% and 91%. Ionomer phases formed by Fumion® ionomers have a mean tortuosity twice as high as Nafion® ionomer phases. The relative percentage of primary pore volume decreases in the order: Nafion® > Fumion®SO-340 > Fumion®S-360, indicating an increased blocking of primary pores by Fumion® ionomer films. The proton conductivities of the primary pores are a factor of 2–5 higher than those of the secondary pores. The results are explained by an enhanced film-forming ability of Fumion® ionomers compared to that of Nafion®, leading to a higher tortuosity of the ionomer phase. By encapsulating primary pores, Fumion® ionomer films block active sites and reduce the active surface and the performance of ACLs and MEAs.
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