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 (λw,thre) for the onset of proton conductivity, above which long-range water transport may occur. Below λw,thre, slow and fast water motions are characterized by two independent confined-sphere diffusion models (CSDM), while above λw,thre, the slow and fast water motions follow jump diffusion and CSDM, respectively. We further propose the ratio of local to macroscopic water diffusivities (Rt) as a measure relevant to the local-to-bulk structural tortuosity, which is as high as ∼137 below λw,thre, revealing the prohibition of long-range water transport, in contrast to the notably low Rt values above λw,thre that are observed in both graft-type PEMs and Nafion, suggesting similar structural tortuosity for water transport.