Effect of Cerium, Cobalt and Nickel Contaminants on the Oxygen Reduction Reaction at Platinum Electrodes

Abstract

The performance and durability of proton exchange membrane fuel cells (PEMFCs) is of great interest for vehicular power applications. In traditional PEMFCs, Pt catalysts are used in both the anode and the cathode. However, efforts to improve PEMFCs often exposes the membrane electrode assembly of PEMFCs with other metallic cations. For example, reducing the cost of PEMFCs and in particular of Pt-based catalysts can be accomplished with Pt-Ni or Pt-Co alloying catalysts [1-3]. Additionally, Cerium (Ce) is incorporated in membranes in order to improve the chemical durability of the membrane [4]. While these materials effectively improve performance and durability, they have shown to leach out contaminants such as Ce3+, Co2+, and Ni2+ions into the ionomer [5-7]. Such leaching can reduce the efficacy of the additives, decrease ionomer conductivity and reduce cell performance. 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 [4]. 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.