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12 Mar 2010

This article via: Hydration and temperature

In this paper the researchers set out to create simulations that examined the impact of hydration and temperature on the positively charged hydrated protons and water molecules. Understanding these dynamics could lead to polymer membranes that are better engineered for transporting protons while controlling electrode flooding by the water molecules. One of the goals is to develop PEM membranes that need little water.

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Using classical molecular dynamics simulations, the research team investigated the impact of four levels of hydration and two different temperatures. The scientists calculated structural properties such as radial distribution functions, coordination numbers, and dynamical properties such as diffusion coefficients of hydronium ions (H3O+) and water molecules.

The results of their calculations showed that protons and water molecules are bound to sulfonate groups in the membrane at low hydration levels. As the hydration level increases, the water molecules become free and form a network along which protons can hop (Figure 2). This leads to a dramatic increase in proton conductivity. Temperature was found to have a significant effect on the absolute value of the diffusion coefficients for both water and hydronium ions. These findings have helped in interpreting experimental results that indicate a major structural change taking place in the membrane with increasing hydration.

Dynamics of Water and Hydronium in Nafion Nanopores Examined by

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In the second paper, the authors used all-atom molecular dynamics simulations to systematically examine eight different levels of membrane hydration to closely mirror two experimental studies. They also simulated bulk water to develop a novel criterion to identify free water in Nafion. This enabled them to quantify the fraction of free, weakly bound, and bound water molecules in the membrane as a function of hydration.

The researchers found that at low hydration levels, strong binding of hydronium ions to sulfonate groups prevents transport of protons. Multiple sulfonate groups surrounding the hydronium ions in bridging configurations hinder the hydration of the hydronium and the structural diffusion of protons (Figure 3). As the hydration level increases, the water molecules mediate the interaction between hydronium ions and sulfonate groups, moving them farther apart. These results provide atomic-level insights into structural changes observed in Nafion by infrared spectroscopy.

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