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.

The distribution and the chemical states of water in the electrolyte membrane of proton-exchange membrane fuel cells (PEMFCs) directly affect the performance and stability during cell operation. Coherent anti-Stokes Raman scattering (CARS) spectroscopy was used to investigate the transient behavior of water inside a Nafion membrane with a high time resolution of 0.5 s. After a current-density jump from 0.1 to 1.0 A cm −2 , the cell voltage rapidly dropped, then increased, and reached equilibrium in 7 s, whereas the ohmic resistance immediately dropped due to the increase in the water species that contributed significantly to the proton conductivity. The number of water molecules per sulfonic acid group, λ, at the membrane surface of the cathode side initially overshot and reached equilibrium after 7 s. This synchronous change of the cell voltage and λ is suggested to be caused by the change in oxygen partial pressure near the catalyst layer. This CARS technique is expected to be useful for analyzing many transient phenomena in materials science.