Ultramicroelectrode study of oxygen reduction at the platinum/ionomer interface at low relative humidity

FUEL 223

Jerzy Chlistunoff, jerzy@lanl.gov1, Francisco Uribe1, and Bryan S. Pivovar2. (1) MST-11, Los Alamos National Laboratory, P.O.Box 1663, MS D429, Los Alamos, NM 87545, (2) MST-11, Los Alamos National Lab, Los Alamos, NM 87544
The effects of temperature (20-80°C), relative humidity (~10-100%) and ionomer on the oxygen reduction reaction (ORR) at the platinum/ionomer interface were studied using an ionomer-covered platinum ultramicroelectrode. The cyclic voltammograms of oxygen exhibited a significant hysteresis between the forward and reverse scans, which increased with either humidity or temperature increases. For NafionŽ family of ionomers, the effects were found to increase with the decrease in the ionomer equivalent weight. Under certain conditions, no distortions were observed for the film made of 6F-40, a fluorinated ionomer with the more rigid structure than NafionŽ. The effects are attributed to surface restructuring of the ionomer, where sulfonic groups at the interface between the ionomer and platinum are replaced by the hydrophobic component of the ionomer. The restructuring is thermally activated and is also facilitated by the decrease of the electrode potential and by water absorbed by the ionomer.