Evaluation of Nafion Film SPR Properties and its Humidity Dependence for Analysis of Transport Phenomena in PEFC Catalyst Layers

Nafion (ionomer) used in the catalyst layer of polymer electrolyte fuel cells (PEFCs) provides a transport path for protons, but also causes resistance in the transport of oxygen to catalysts such as platinum. Therefore, understanding its structure and transport properties is crucial for improving PEFC performance. In this study, we characterize the properties of Nafion thin films using surface plasmon resonance (SPR) imaging, which observes nanoscale structure. SPR refers to a phenomenon where collective oscillations of surface free electrons on a metal surface couple with an electromagnetic field and propagate along the surface. By illuminating a prism at a specific angle, SPR is excited, reducing reflected intensity. This enables observation of nanoscale structure near the gold surface. We formed nano-scale Nafion thin films on substrates, and developed an experimental system for SPR measurements of Nafion films with various thickness and controlled humidity. The SPR observation and the SPR curve calibration showed that the fabricated Nafion film is uniform and has a thickness of 9.2 nm. The SPR observation results under humidity-controlled condition showed that the change in thickness of the Nafion film due to humidity change can be captured. To investigate the effect of water content on the optical properties of Nafion, we prepared an evaluation system using bulk Nafion films, which are thick enough to minimize the influence of film thickness on the SPR response. This approach is expected to improve the accuracy of optical constant estimation and contribute to more precise analysis of Nafion film structure.