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Inhomogeneous transport in model hydrated polymer electrolyte supported ultra-thin films


In collaboration with colleagues of the Liten and the National Research Council of Canada, we have given a significant contribution to the understanding of Nafion supported ultra-thin films by computer simulations, and they found evidences of strongly heterogeneous behavior in different regions of the film.


Published on 18 September 2013

Performances of the membrane-electrode assembly in polymer electrolyte fuel cells strongly depend on the features of the catalyst layer. This is a complex interface region which forms during the fabrication process, when the ionomer mixes with carbon supported platinum nanoparticles, self-organizing into heterogeneous ultra-thin films. These are expected to depend on substrate properties such as chemical composition, geometry and, ultimately, wetting behavior. The features of the films have impact on charge distributions, proton conductivity, diffusion of reactants and products, degradation kinetics of platinum and carbon support, modifying electro-catalytic properties.

Inac scientists, in collaboration with colleagues of the Liten and the National Research Council of Canada, have given a significant contribution to the understanding of Nafion supported ultra-thin films by computer simulations. The approach used is typical of statistical mechanics of complex matter, with quite limited emphasis on chemical details but enhanced attention at grasping main universal physical features. The core idea is to consider a mean-field-like interaction ionomer/substrate, precisely controlling the hydrophilic character of the latter by means of a tunable control parameter. In doing so, the researchers have been able to investigate in a unique framework a variety of environments peculiar of the catalyst layer, ranging from hydrophobic to hydrophilic (representative of carbon and platinum, respectively). They have explored self-assembly and thin-film formation of a detailed model for the Nafion ionomer and focused on transport properties of water molecules, finding evidences of strongly heterogeneous behavior in different regions of the film.

This work will have impact on the interpretation of available experimental results and provide useful information to be taken into account for the optimization of actual fuel cell catalyst layers.

Snapshot of hydrated Nafion ultra-thin film in contact with an hydrophobic support (at the bottom, not shown), produced by Molecular Dynamics simulation. One observes the formation of water pools (blue), separated from the polymer matrix (gray) by the sulfonic groups interface (green).

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