This article via: Structure and dynamics
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In the third report, Devanathan, Venkatnathan, and Dupuis computed the dynamical properties of water molecules and hydronium ions in Nafion and related them to the structural changes reported previously. They confirmed other researchers’ finding that the behavior of water molecules within nanoscale pores and channels of PEMs, especially at low hydration levels, is remarkably different from that of molecules in bulk water.
At low hydration, fewer than 20% of the water molecules are free (bulklike). With increasing hydration, the diffusion coefficients of hydronium ions and water molecules increase, and the mean residence time of water molecules around sulfonate groups decreases. These results provide a molecular-level explanation for the proton and water dynamics observed in neutron scattering experiments.
Because the structure and dynamics of the membrane under different levels of hydration cannot be directly observed in experiments, there is no universally accepted model of the structure of Nafion. This research makes a significant step toward that goal and toward the development of the next generation of PEMs.
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Characteristics of the ideal PEM include high proton conductivity at low hydration levels; thermal, mechanical, and chemical stability; durability under prolonged operation; and low cost. None of the existing membranes meet all these requirements, and developing new membranes requires a molecular-level understanding of membrane chemistry and nanostructure. Molecular dynamics simulations like these, together with experiments, are laying the foundation for future breakthroughs in fuel cells.
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A fundamental understanding of the relationship between membrane nanostructure and the dynamics of water molecules is needed for the development of efficient, reliable, and cost-effective membranes to advance PEM fuel cell technology. The structure and dynamics of the polymer membranes under different levels of hydration cannot be directly observed in experiments, but they can be modeled in molecular dynamics simulations, as shown in a series of three papers published in the Journal of Physical Chemistry B by Ram Devanathan, Arun Venkatnathan, and Michel Dupuis of Pacific Northwest National Laboratory (PNNL).
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