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Molecular modeling in PEMs

The important impact of these experimental insights for molecular modeling is that the development of structure versus property relations of PEMs does not require multiscale approaches going all the way to the macroscopic scale. Rather, the main job is done if one arrives at the scale of several 10s of nanometers. Notably, operation at low hydration emphasizes even more the importance of (sub)nanoscale phenomena controlled by explicit interactions in the polymer-water-proton system. [Pg.359]

Molecular modeling of PT at dense interfacial arrays of protogenic surface groups in PEMs needs ab initio quantum mechanical calculations. In spite of fhe dramafic increase in computational capabilihes, it is still "but a dream" to perform full ab initio calculations of proton and water transport within realistic pores or even porous networks of PEMs. This venture faces two major obstacles structural complexity and the rarity of proton transfer events. The former defines a need for simplified model systems. The latter enforces the use of advanced compufahonal techniques that permit an efficient sampling of rare evenfs. ... [Pg.385]

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 ... [Pg.410]

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. [Pg.30]

In general, results of fully atomistic or mixed representations have confirmed the formation of a microphase-separated morphology in PEMs, albeit results on sizes, shapes, and distributions of phase domains have remained inconclusive. In comparison to experiment, molecular models were found to underestimate the sizes of ionic... [Pg.86]

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. [Pg.133]

Because of the complexity of hydrated PEMs, a full atomistic modeling of proton transport is impractical. The generic problem is a disparity of time and space scales. While elementary molecular dynamics events occur on a femtosecond time scale, the time interval between consecutive transfer events is usually 3 orders of magnitude greater. The smallest pore may be a few tenth of nanometer while the largest may be a few tens of nanometers. The molecular dynamics events that protons transfer between the water filled pores may have a timescale of 100-1000 ns. This combination of time and spatial scales are far out of the domain for AIMD but in the domain of MD and KMC as shown in Fig. 2. Because of this difficulty, in the models the complexity of the systems is restricted. In fact in many models the dynamics of excess protons in liquid water is considered as an approximation for proton conduction in a hydrated Nation membrane. The conformations and energetics of proton dissociation in acid/water clusters were also evaluated as approximations for those in a Nation membrane.16,19 20 22 24 25... [Pg.364]


See other pages where Molecular modeling in PEMs is mentioned: [Pg.351]    [Pg.368]    [Pg.385]    [Pg.93]    [Pg.272]    [Pg.123]    [Pg.388]    [Pg.33]    [Pg.34]    [Pg.1]    [Pg.382]    [Pg.1109]    [Pg.416]    [Pg.76]    [Pg.83]    [Pg.87]    [Pg.97]    [Pg.97]    [Pg.129]    [Pg.235]    [Pg.347]    [Pg.401]    [Pg.408]    [Pg.175]    [Pg.534]    [Pg.37]    [Pg.283]    [Pg.3006]    [Pg.19]    [Pg.729]   
See also in sourсe #XX -- [ Pg.83 ]




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