We present the results of the atomistic molecular dynamics modeling of different protonation states of Nafion at varying hydration levels. Previous experiments have shown that the degree of deprotonation (DDP) of the sulfonic acid groups in a Nafion membrane varies significantly upon hydration. Our goal is to provide insights into the effects of variable protonation states and water content on the internal structure and vehicular transport inside the Nafion membrane. The Nafion side chain lengths showed a weak increasing trend with increasing DDP at all hydration levels, exposing more of the sulfonic acid groups to the hydrophilic/water phase. The water-phase characteristic size/diameter decreased with increasing DDP, but, interestingly, the average number of water molecules per cluster increased. The probability of water–hydronium hydrogen bond formation decreased with increasing DDP, despite an increase in the total number of such hydrogen bonds. The water diffusion was largely unaffected by the state of deprotonation. In contrast to that, the hydronium ion diffusion slowed down with increasing DDP in the overall membrane. The hydronium ion residence times around the sulfonic acid group increased with increasing DDP. Our simulations show a strong connection between the morphology of the water domains and protonation states of Nafion. Such a connection can also be expected in polyelectrolyte membranes similar to Nafion.

Nafion nanocomposites for energy-related applications are being used extensively because of the attractive properties such as enhanced water retention, low unwanted crossover of electrolytes, and high proton conductivity. We present the results of the molecular dynamics modeling of Nafion films confined between two walls (substrates) of different polymer–wall interaction strengths and of different separation distances to model Nafion nanocomposites. Our goal is to provide insights into the effects of varying hydrophilicity and volume fraction of fillers/nanoparticles on the internal structure and water transport inside the Nafion membrane. The sulfur–sulfur radial distribution function first peak distance and the sulfur–oxygen (water) coordination number in the first hydration shell were negligibly affected by the wall (substrate) hydrophilicity or the film thickness. The Nafion side chains were found to bend toward the substrates with high hydrophilicity which is in qualitative agreement with existing experiments. The amount of bending was observed to reduce with increasing film thickness. However, the side-chain length did not show any noticeable variation with wall (substrate) hydrophilicity or film thickness. The water clusters became smaller and more isolated clusters emerged for highly hydrophilic substrates. In addition, the water cluster sizes showed a decreasing trend with decreasing film thickness in the case of hydrophilic substrates, which has also been observed in experiments of supported Nafion films. The in-plane water diffusion was enhanced considerably for hydrophilic substrates, and this mechanism has also been proposed previously in experiments. The in-plane water diffusion was also found to be a strong function of the substrate selectivity toward the hydrophilic phase. Our simulations can help provide more insights to experimentalists for choosing or modifying nanoparticles for Nafion nanocomposites.

The annealing of polymer electrolyte membranes is known to affect the membrane structure and proton conductivity. The observed changes depend drastically upon the annealing temperatures and cooling rates. In this study, we have performed a fully atomistic classical molecular dynamics simulation of hydrated Nafion at various hydration levels and annealing rates. The simulations show the compression of hydrophobic Nafion domains by larger water clusters, with a strong antiplasticization effect upon hydration, demonstrated by increasing the glass-transition temperature. The close-range proximity of sulfonate−sulfonate groups of Nafion pendant side chains remains unchanged with the simulated cooling rates. The water clusters in hydrated Nafion become more disconnected and larger in size with slower cooling rates/increased annealing time. This results in the decrease of water and hydronium diffusivity and the corresponding conductivity, thereby explaining qualitatively the experimental observations.

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In this work, we elucidated the nanostructure and dynamics of Nafion-doped graphene-oxide (GO) systems from molecular dynamics simulations at varying hydration levels and temperature. It was found that the presence of GO resulted in the formation of Nafion layers along a direction normal to the GO surface. Chain conformations in the Nafion layers close to the GO interface were characterized by a backbone preferably oriented parallel to the GO plane, whereas the size of the formed nanochannels was found to be commensurate to the average dimensions of the Nafion side chains. The mechanism of water cluster growth was found to change drastically upon introduction of Nafion chains, although addition of GO in the membranes did not impart further measurable changes at the examined temperatures. Hydronium ions were found to adsorb partly onto the GO surface, whereas the pertinent adsorption/desorption rate increased significantly with hydration. Translational dynamics of water molecules was much slower close to the GO surface compared to that at distances far from GO. In the temperature range examined, the dynamics of the effectively confined water molecules was found to follow an Arrhenius-like dependence. Water retention at the Nafion/GO interface appears only at high hydration levels of Nafion.