Characterization of Catalyst Layer Ionomer Degradation in PEM Fuel Cells

The catalyst layer ionomer unavoidably gets involved in other components degradation processes since it is subjected to exposure to different operating effects, including the presence of the catalyst, catalyst support, and the porous nature of the electrode layer which includes 2-phase flow. PEMFC durability issues cannot be fully resolved without understanding the contribution of ionomer degradation in electrode to the performance decay in life time. However, catalyst layer ionomer is essentially chemically identical to the membrane ionomeric material, and is composed of low atomic number elements, making characterization difficult. In the present work, MEAs with different Nafion ionomer types: stabilized (S) and non-stablized (NS) ionomer in the electrode layer (Type I) and mixed membrane / ionomer MEAs (Type II) were designed to separate ionomer degradation from membrane degradation respectively. Stabilized and non stabilized ionomers were 5% Nafion ®ҏ solutions. The non-stabilized version is the typical Nafion chemical structure with carboxylic acid (-COOH) end groups; these end groups are thought to be a susceptible point of degradative peroxide attack Type I MEA fabrication follows the same procedure described as Ref Commercial Nafion ® 212 membrane was applied in Type I MEAs initially. The MEAs failed fast (< 200h) during the AST mainly due to membrane degradation. The short life time of the AST test provided limited time resolved ECSA loss data, with limited impedance characterization due to membrane thinning, which makes it difficult to unambiguously conclude much about the different ionomers' effect on the degradation process. Similar catalyst layer degradation analysis by XPS as Zhang et al The ratio of CF x /QRQ-fluorinated C decreased from 0.59 to 0.52 at the surface of NS electrodes, while that of S electrodes decreased from 0.53 to 0.49. The ratio of CF x to total carbon decreased from 0.373 to 0.345 and 0.346 to 0.331 for NS and S electrodes respectively. The higher decrease in the NS electrodes probably was due to the longer life time (~240h) compared to that of S electrodes (~192 h). The biggest change was observed for the ratio of IOXRULQDWHG &JUDSKLF LQ 16 HOHFWURGHV IURP WR 3.25, ~ 42% decrease, while the drop of S electrodes was from 3.88 to 3.63, only ~7%. This difference is not consistent with the testing time and longer durability testing may be required for better understanding of this difference. For a longer membrane life time during AST testing to obtain better time resolved analysis, MEAs were made using stabilized Nafion ® XL 100 membrane supplied by Ion Power. The MEA durability during the OCV AST test was extended over 400hr