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We measured the self-diffusion coefficients of water in a Nafion membrane and two sulfonated polyethersulfone (SPES) membranes with varying ion-exchange capacities (IEC) in terms of relative humidity using the pulse field gradient NMR (PFG-NMR) technique. The self-diffusion coefficients were plotted against the number of water molecules per sulfonic acid group, λ, and compare these values with the results of molecular dynamics (MD) simulations. Classical MD simulations for all membranes were carried out using a consistent force field at λ = 3, 6, 9, 12, and 15. The dynamic properties of water (HO) and hydronium (HO) on a molecular level were estimated as self-diffusion coefficients and residence times around a sulfonate group ( $$ {\text{SO}}_3^{-} $$). The diffusion coefficients of HO and HO followed the order, Nafion > SPES with IEC = 1.4 > SPES with IEC = 1.0 > SPES with IEC = 0.75, which agreed with the experimental data. The residence time distribution of HO around $$ {\text{SO}}_3^{-} $$ in Nafion was in the range of 1-6 ps, whereas HO in the SPES exhibited a residence time of greater than 20 ps. [ABSTRACT FROM AUTHOR]

Abstract: Two types of polymer electrolyte membranes (PEMs), Nafion (NR-212) and sulfonated poly(4-phenoxybenzoyl-1,4-phenylene) (sPPBP), were conditioned under air flow in independently controlled temperature and relative humidity conditions. The effects of temperature and relative humidity on the diffusion behavior of water in the PEMs were investigated by pulsed field gradient (PFG) NMR. The two PEMs show different tendencies, especially with respect to their temperature effects. In a comparison of diffusion data with proton conductivity, mechanisms for proton conduction in the PEMs were considered. It was suggested that proton conduction by proton hopping occurs to a larger extent in NR-212 than in a hydrocarbon-based membrane. Temperature-affected microstructural changes below the glass transition temperature are also discussed. [Copyright &y& Elsevier]