Proton water/aqueous networks

The effective conductivity of the membrane depends on its random heterogeneous morphology—namely, the size distribution and connectivity of fhe proton-bearing aqueous pafhways. On fhe basis of the cluster network model, a random network model of microporous PEMs was developed in Eikerling ef al. If included effecfs of varying connectivity of the pore network and of swelling of pores upon water uptake. The model was applied to exploring the dependence of membrane conductivity on water content and... 

The difficulties in implementing such a coherent approach stem from the fact that one has to deal with complex interactions in a heterogeneous many particle system and that proton transfer in water is a genuinely complicated many-particle phenomenon itself. In the beginning of everything lies the fundamental phenomenon of PT in aqueous networks. 

This section provides a systematic account of proton transport mechanisms in water-based PEMs, presenting studies of proton transport phenomena in systems of increasing complexity. The section on proton transport in water will explore the impact of molecular structure and dynamics of aqueous networks on the basic mechanism of proton transport. The section on proton transport at highly acid-functionalized interfaces elucidates the role of chemical structure, packing density, and fluctuational degrees of freedom of hydrated anionic surface groups on concerted mechanisms and dynamics of protons. The section on proton transport in random networks of water-filled nanopores focuses on the impact of pore geometry, the distinct roles of surface and bulk water, as well as percolation effects. 

Structure and water sorption characteristics of fuel cell media determine their transport properties. The dynamic properties of water determine microscopic transport mechanisms and diffusion rates of protons in PEM and CLs. Protons must be transported at sufficiently high rates, away from or toward the active Pt catalyst in anode and cathode catalyst layers, respectively. Effective rates of proton transport in nanoporous PEM and CLs result from a convolution of microscopic transport rates of protons with random network properties of aqueous pathways. Accounting for the geometry of these materials, namely, their external surface area and thickness, gives their resistances. 

In the hydrated ionomer membrane, liquid-like water acts as the pore former, pore filler, and proton shuffle. The wafer disfribufion and the random network morphology of aqueous pafhways determine proton conduction at... 

Structural diffusion is favored by conditions that enhance the stiffness of the hydrogen-bonded network between water molecules low temperatures and low acid concentration. The decrease in water content leads to an effective increase in the concentration of acid protons, which in turn suppresses the contribution of structural diffusion, as found in aqueous acidic solutions. This agrees with the finding of an enhanced contribution of vehicular transport in PEMs at low hydration. Such an observation is also supported by recent studies of molecular mechanisms of proton transport in PEMs at minimal hydration. ... 

Both the experimental and theoretical studies indicate that the interactions between acetic acid and water molecules are more competitive in dilute aqueous solution. However to our knowledge, the specific interactions between acetic acid and water molecules are still not well understood, especially in such as the nature of hydrogen bonding, the bonds networking, the rule in architecture of larger hydration compounds, deprotonation of acetic acid in solution, stability of the hydrated proton, the local structure of its aqueous solution, and so on. In the present work, we have performed ab initio calculations on multi-hydrates (rich water hydration compounds) of acetic acid, and ab initio Car-Parrinello molecular dynamics (CPMD) [20] simulations on acetic acid monomer and water system (at dilute aqueous solution condition) to find something helpful for interpreting the nature of acetic acid aqueous solution. 

Beyond these very particular symmetrical cases, we may wonder whether such transfers of protons are of practical importance in chemistry, biology or physics. The object of this section is to show that the answer to such a question is, yes they are important, and we often have to deal with them, without however always recognizing them, hi order to support such an assertion, we shall examine some well-known mechanisms in which they are central and which clearly show that they may even be fundamental and it will also clearly appear that we still have a lot to learn about them. In reality, they are not found in such simple systems we have described in the preceding section. They scarcely occur in single H-bonds and most of the time they occur in systems that display a well-developed H-bond network , such as liquid water or aqueous media, which are more complex systems with an appreciable number of closely lying and interacting H-bonds. 

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