Molecular Structure of Buried Perfluorosulfonated Ionomer/Pt Interface Probed by Vibrational Sum Frequency Generation Spectroscopy

Abstract

Perfluorosulfonated ionomer (PFSI), such as Nafion, in polymer electrolyte fuel cells (PEFCs) has been recognized as an important component to shuttle protons during the electrocatalytic reactions, especially the oxygen reduction reaction (ORR) at the cathode. However, a molecular structure of PFSIs inside catalyst layers in PEFCs has been unclear, since the polymers surrounding the gas-diffusion electrode with meso-to-macroporous structures have been considered to be much more complicated to resolve. Recent progress in the environmental electron microscopic technology clarified the real thickness of PFSI films to be only several nanometers, which can be analyzed by spectroscopic techniques and simulation modelings with molecular insights. Infrared and Raman spectroscopies were promptly applied to obtain the molecular arrangement of Nafion on the surface of Pt catalysts, but the thicknesses of the Nafion films seemed to be much thicker than the real thicknesses of PFSI in catalyst layers. In the present study, the preparation method of an ultrathin film of Nafion on Pt surface with <1 nm thickness was established and the vibrational sum frequency generation (VSFG) spectroscopy, which is an inherently surface-selective method, was applied to extract the interfacial molecular structure even at relatively thick Nafion films on Pt surfaces. By comparing thickness dependencies of IR and VSFG spectra, the molecular arrangement of Nafion at buried Nafion/Pt interface was confirmed to adsorb via sulfonate terminals of side-chains.