Interfacial Structure and Proton Conductivity of Nafion at the Pt-Deposited Surface.

Abstract

Understanding the Nafion-Pt interface structure is important because fuel cell reactions occur at the three-phase boundary. Infrared (IR) p-polarized multiple-angle incidence resolution spectrometry (p-MAIRS) technique was used to investigate the in-plane (IP) and out-of-plane (OP) spectra in the identical substrate. Our previous study revealed that the proton conductivity of the Nafion thin films decreased at the MgO and SiO2 surfaces. We proposed that the origin for the lower proton conductivity can be derived from the highly oriented structure at the interface. However, the interface structure of the Nafion-Pt interface remains unclear. In this study, Nafion thin films were prepared by spin-coating on a Pt-deposited MgO substrates. The IP spectrum exhibited a well-known spectrum, but the OP spectrum was quite differed considerably from the IP spectrum. Furthermore, thickness dependence of the degree of orientation for this OP band was observed at the Nafion-Pt interface. This OP band can be assigned as the vibration mode of the mixture of the CF2 and sulfonic acid groups. At the low-RH region, proton conductivity of the Nafion thin film on the Pt-deposited surface was 1 order of magnitude higher than that on the SiO2 surface. Furthermore, the activation energy was 0.4-0.5 eV, which is lower than that of the SiO2 surface. These results, which suggest that the Pt surface influenced the proton transport property of Nafion thin film, can contribute to understand the relationship between the proton transport property and thin film structure on the Pt-deposited surface at the three-phase boundary for fuel cells.