Comprehensive Understanding of Water Transport in Graft-Type Polymer Electrolyte Membranes at Low Hydration Levels through Local Dynamics and Structural Tortuosity

The conduction reduction of proton exchange membranes (PEMs) under low-humidity conditions is one of the most severe problems, leading to significance performance loss in conventional fuel cells. Recently, we identified that radiation-grafted PEMs possess excellent conductivity under high-temperature and low-humidity conditions, compared with the benchmark material Nafion. In this study, we aim to gain a deep understanding of water transport in graft-type PEMs, which is the dominant factor for conductivity, by taking into account two distinctive parameters: water dynamics and ion-channel structural tortuosity. Especially, water dynamics are distinctly detected using quasielastic neutron scattering (QENS), together with the ion-channel structure using small-angle neutron scattering, owing to the newly prepared deuterated polystyrene sulfonic acid grafting onto a fully fluorinated base polymer, which minimizes the incoherent scattering from polymers. The QENS results show three types of water molecules within the ion-conducting channels in PEMs: immobile water tightly bound to ionic groups and two diffusive water molecules, slow and fast, near and relatively far from ionic groups, respectively. There exists a threshold hydration number (λ w,thre ) for the onset of proton conductivity, above which long-range water transport may occur. Below λ w,thre, slow and fast water motions are characterized by two independent confined-sphere diffusion models (CSDM), while above λ w,thre, the slow and fast water motions follow jump diffusion and CSDM, respectively. We further propose the ratio of local to macroscopic water diffusivities ( R t ) as a measure relevant to the local-to-bulk structural tortuosity, which is as high as ∼137 below λ w,thre, revealing the prohibition of long-range water transport, in contrast to the notably low R t values above λ w,thre that are observed in both graft-type PEMs and Nafion, suggesting similar structural tortuosity for water transport.