Chemical States of Water Molecules Distributed Inside a Proton Exchange Membrane of a Running Fuel Cell Studied by Operando Coherent Anti-Stokes Raman Scattering Spectroscopy
Hiromichi Nishiyama, Shogo Takamuku, Katsuhiko Oshikawa, Sebastian Lacher, Akihiro Iiyama, Junji Inukai
Abstract
On the performance and stability of proton exchange membrane fuel cells (PEMFCs), the water distribution inside the membrane has a direct influence. In this study, coherent anti-Stokes Raman scattering (CARS) spectroscopy was applied to investigate the different chemical states of water (protonated, hydrogen-bonded (H-bonded) and non-H-bonded water) inside the membrane with high spatial (10 μm φ (area) × 1 μm (depth)) and time (1.0 s) resolutions. The number of water molecules in different states per sulfonic acid group in a Nafion membrane was calculated using the intensity ratio of deconvoluted O–H and C–F stretching bands in CARS spectra as a function of current density and at different locations. The number of protonated water species was unchanged regardless of the relative humidity (RH) and current density, whereas H-bonded water molecules increased with RH and current density. This monitoring system is expected to be used for analyzing the transient states during the PEMFC operation.