Proton conduction in PEMs

Proton Conduction in PEMs Complexity, Cooperativity and Connectivity... 

Although a considerable effort has been undertaken to understand proton conduction in PEMs, much remains to be understood in terms of how molecular chemistry and hydrated morphology dictate fuel cell performance. Molecular modeling of acidic functional groups, polymeric fragments, proton diffusion, and dielectric properties of the confined water in several different PEMs has suggested that the critical ingredients of proton conduction include complexity ... 

Proton conductivity in PEMs is usually a strong fimction of the degree of hydration. The maximum conductivity in Nafion ( 0.1 Scm ) is attained with water contents A, 15 (H2O SO3" ratio) imder typical operation conditions. Conductivity decreases monotonically towards lower water content. It usually exhibits a quasi-percolation transition for A, < 5 [21,22]. 

There are different ways to depict membrane operation based on proton transport in it. The oversimplified scenario is to consider the polymer as an inert porous container for the water domains, which form the active phase for proton transport. In this scenario, proton transport is primarily treated as a phenomenon in bulk water [1,8,90], perturbed to some degree by the presence of the charged pore walls, whose influence becomes increasingly important the narrower are the aqueous channels. At the moleciflar scale, transport of excess protons in liquid water is extensively studied. Expanding on this view of molecular mechanisms, straightforward geometric approaches, familiar from the theory of rigid porous media or composites [ 104,105], coifld be applied to relate the water distribution in membranes to its macroscopic transport properties. Relevant correlations between pore size distributions, pore space connectivity, pore space evolution upon water uptake and proton conductivities in PEMs were studied in [22,107]. Random network models and simpler models of the porous structure were employed. 

Liu Y, Ji C, Gu W, Baker DR, Jome J, Gasteiger HA (2010) Proton conduction in PEM fuel cell cathodes effects of electrode thickness and ionomer equivalent weight. J Electrochem Soc 157(8) B1154-B1162... 

Molecular modeling of PT at dense arrays of protogenic surface groups (SGs) demands ab initio quantum mechanical calculations. The starting point for the development of a viable model of surface proton conduction in PEM is the self-organized PEM morphology at the mesoscopic scale. Eigure 2.30a illustrates the random array of hydrated and ionized sidechains that are anchored to the surface of ionomer bundles. 

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