“…Proton-exchange membranes (PEMs), serving as proton-conductive media in PEMFCs, still need to be improved not only toward higher proton conductivity but also toward higher thermal and chemical durability. , Many physical/chemical phenomena are involved in the operation of PEMFCs, and the distribution and transport behavior of water species inside the PEMs are known to exert a large influence on these phenomena . Water transport in PEMFCs, such as the back-diffusion and electro-osmotic drag, as well as the water transport at the membrane/catalyst layer (CL) interfaces, is found to have a significant impact on both the performance and stability. , Nafion, a perfluorinated sulfonic acid polymer, is the most commonly used PEM for PEMFCs, and its water distribution has been extensively studied using various techniques, including vibrational spectroscopies, − nuclear magnetic resonance, neutron scattering/imaging, and X-ray scattering . Raman spectroscopy is a powerful technique for quantifying the distribution of water without interfering with the cell reaction. − Not only the distribution of the number of water molecules but also their chemical states inside the PEM are highly important.…”