Proton transport in water

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. 

The molecular mechanism of proton tranter in water was unraveled in the mid-1990s. The recent history is reviewed in Marx (2006). The invention of the Car-Parrinello technique of molecular dynamics (Car and Parrinello, 1985) opened the field of atomistic computer simulations for a myriad of applications in 

More specifically, the detailed spectroscopic analysis by Agmon found that the timescale of water molecule rotation, which takes 1-2 ps at room temperature, is similar to proton hopping times determined from the analysis of the resonance in NMR studies by Meiboom (1961). Using the hopping time of Tp = 1.5 ps and a hopping length of Ip = 2.5 A, an estimate of proton mobility in three-dimensional networks can be obtained from the Einstein relation, Dh+ = Ip/6rp = 7.0 10 cm s . This estimate is close to the experimental value of 9.3 10 cm s . From this closeness, Agmon concluded that proton mobility in water is an incoherently Markovian process. 

Combining spectroscopic insights and CPMD simulations by Tuckerman et al., the following molecular mechanism of proton transport was conceived, as depicted in 

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The effect of solvents on PT is complicated by the fact that usually the solvents which exhibit a strong activity are both proton donor and proton acceptor, like water, and thus can participate in the reactions. A model of proton transport in water which uses the EVB model has been recently published by Schmidt and Voth [28]. 

Schmitt, U.W. and Voth, G.A. (1998). Multistate empirical valence bond model for proton transport in water. J. Phys. Chem. B 102, 5547-5551... 

Schmitt, U. W., Voth, G. A., Multistate Empirical Valence Bond Model for Proton Transport in Water, f Phys. Chem. B 1998, 102, 5547-5551. 

Schmidt, R. Brickmann, J. (1998) Molecular Dynamics Simulation of the Proton Transport in Water, Ber. Busenges. Physical Chemistry 101, 1816-1827... 

Electro-osmotic drag phenomena are closely related to the distribution and mobility of protons in pores. The molecular contribution can be obtained by direct molecular dynamics simulations of protons and water in single ionomer pores, as reviewed in the sections Proton Transport in Water and Stimulating Proton Transport in a Pore. The hydrodynamic contribution to nd can be studied, at least qualitatively, using continuum dielectric approaches. The solution of the Poisson-Boltzmann equation... 

The main processes are electrochemical reactions at electrified metal-electrolyte interfaces reactant diffusion through porous networks proton transport in water and at aggregates of ionomer molecules electron transport in electronic support materials water transport by gasous diffusion, hydraulic permeation, and electro-osmotic drag in partially saturated porous media and vaporization/condensation of water at interfaces between liquid water and gas phase in pores. 

G. Karlstrom, Proton transport in water modeled by a quantum chemical dielectric cavity model, J. Phys. Chem., 92 (1988) 1318. 

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