Quasi Elastic Neutron Scattering Study on Water Dynamics in Nafion Membrane

Abstract

Elucidation of the proton conductive mechanism is required for further improvement and/or development of a proton conductive membrane. Quasi elastic neutron scattering (QENS) is suited to reveal the water dynamics in the proton conductive membrane because the hydrogen has a huge incoherent neutron scattering cross section. In this paper, results of in–situ QENS measurements of a Nafion membrane under controlled water vapor pressure was introduced. The adsorbed water was categorized into two types; (1) strongly bound water, where a strong interaction occurred with the sulfonic group and the dynamics was frozen within the energy resolution and (2) weakly bound water which weakly interacted with the sulfonic group and showed one order of magnitude smaller diffusion coefficient. The relationship between the proton conductivity and the dynamics of weakly bound water was also discussed. The observed diffusion process of the weakly bound water could contribute to the proton conduction as a vehicle mechanism.