Porosimetry of MEAs made by "thin film decal" method and its effect on performance of PEFCs

Mercury porosimetry, scanning electron microscopy, and transmission electron microscopy have been used to characterize the pore size and distribution of carbon supports, with and without catalyst loading, and the catalyst layer of membrane electrolyte assemblies (MEAs) used in polymer electrolyte fuel cell (PEFC) applications. The majority of pores for carbon powders and catalyst loaded carbon powders concentrate in a range from 24 to 84 nm (in diam) with a small number of residual pores lying outside this range. The membrane electrode assembly catalyst layer prepared from recast Nation and carbon supported catalyst has a different pore structure, determined by the type of catalyst used, the preparation method, and the total Nation content. The catalyst layer has two distinctive pore categories with boundary of ∼ 17 nm in diam. The specific pore volume of both the primary (small) pores and the secondary (large) pores decreases with an increase of Nation content. Boiling the MEA tremendously increases the pore area. The Nation content in the catalyst layer affects the performance of MEAs over the entire current range. The optimum performance was obtained with a Nation content of 28 wt % for a Pt ₃Cr catalyst layer made by the thin film decal process.