In this work, we have studied the impact of metallic cationic contaminants (Ce, Co and Ni) in the catalyst layer ionomer on the oxygen reduction reaction (ORR) of a Pt microelectrode in various conditions. A well-established microelectrode mini-cell set-up was used for the electrochemical measurements of the ORR activity at the Pt-ionomer interface with and without the presence of cationic contaminants in concentrations observed during fuel cell operation. Nafion was doped with cation contaminant and subsequently deposited on Pt micro-electrodes. X-ray fluorescence (XRF) was used to verify that the contaminant concentrations in the thin films was equivalent to experimentally determined concentrations in the CL ionomer .
Results obtained using electrochemical and other characterization techniques will be presented to further understand the role of the contaminants in the performance of Pt electrodes for the ORR. These techniques include laser profilometry to measure the thin film thickness, conductivity cells and impedance measurements to understand the impact of contaminants on the ionomer resistance. Results indicate that in presence of Ni, we observe a decrease in ionomer conductivity (Nafion® 211), from 102 mS cm-1 in proton form to 35 mS cm-1 with 5.2 mgNi cm-3 at 80°C, 100% RH. This performance correlates with a decrease in ORR performance, most notably lower limiting currents 1.1x10-1 mA cm2 in proton form vs. 6.3x10-2 mA cm-2 with 7 mgNi cm-3and lower ORR on-set potentials at 25°C, 100% RH.
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