Molecular dynamics simulations of proton transport

To date, our reactive molecular dynamics simulations of proton transport have been limited to bulk water. However, the extension of Ae RMD algorithm to proton transport in PFSA membranes is analogous to what has been done in bulk water and simi-... 

Smondyrev, A.M. and Voth, G.A. (2002). Molecular dynamics simulation of proton transport near the surface of a phospholipid membrane. Biophys. J. 82, 1460-1468... 

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... 

In addition to enhancing surface reactions, water can also facilitate surface transport processes. First-principles ab initio molecular dynamics simulations of the aqueous/ metal interface for Rh(l 11) [Vassilev et al., 2002] and PtRu(OOOl) alloy [Desai et al., 2003b] surfaces showed that the aqueous interface enhanced the apparent transport or diffusion of OH intermediates across the metal surface. Adsorbed OH and H2O molecules engage in fast proton transfer, such that OH appears to diffuse across the surface. The oxygen atoms, however, remained fixed at the same positions, and it is only the proton that transfers. Transport occurs via the symmetric reaction... 

S. Izvekov and G. A. Voth (2005) Ab initio molecular-dynamics simulation of aqueous proton solvation and transport revisited. J. Chem. Phys. 123, 044505... 

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

Cui and co-workers performed classical molecular-dynamics simulations of two dilferent perfluorosulfonic-acid (PFSA) membranes to investigate the hydrated morphology and the hydronium-ion dilfusion. They put special emphasis on the water content of the membrane (5% to 20%) and compared the properties for two dilferent lengths of the side chains carrying the sulfonic-acid groups. The short side chains lead to a more disperse distribution of water clusters inside the membrane. At low water content this results in a more connected water-channel network, which enhances the proton transport. 

Ab Initio Molecular Dynamics Simulation of the Structure and Proton Transport Dynamics of Methanol-Water Solutions. 

FIGURE 2.29 Ab initio molecular dynamics simulation of a triflic acid monohydrate crystal, (a) shows the structure of the native crystal, (b) shows the intermediate state with two delocalized protons is 0.3 eV higher in energy than the ordered conformation of the native crystal (a). (Reprinted from Chem. Phys. Lett, 368, Eikerling, M. et al. Defect structure for proton transport in a triflic acid monohydrate solid, 108-114, Figure 1,2,4, Copyright (2003) Elsevier. With permission.)... 

In this work, we have approaehed the understanding of proton transport with two tasks. In the first task, deseribed above, we have sought to identify the moleeular-level stmeture of PFSA membranes and their relevant interfaees as a funetion of water content and polymer architecture. In the second task, described in this Section, we explain our efforts to model and quantify proton transport in these membranes and interfaces and their dependence on water content and polymer architecture. As in the task I, the tool employed is molecular dynamics (MD) simulation. A non-reactive algorithm is sufficient to generate the morphology of the membrane and its interfaces. It is also capable of providing some information about transport in the system such as diffusivities of water and the vehicular component of the proton diffusivity. Moreover, analysis of the hydration of hydronium ion provides indirect information about the structural component of proton diffusion, but a direct measure of the total proton diffusivity is beyond the capabilities of a non-reactive MD simulation. Therefore, in the task II, we develop and implement a reactive molecular dynamics algorithm that will lead to direct measurement of the total proton diffusivity. As the work is an active field, we report the work to date. 

Ah initio molecular dynamics simulations allow to identify the limiting step of the proton transport, between the transfer and the reorganization. An understanding of the transport mechanisms helps design new materials, whose structure is based on well-known materials, but whose proton conductivity may be improved considerably. 

In general, molecular dynamics simulations, in the framework of the Born-Oppenheimer or Car-Parrinello approximation, are of great importance for the understanding of materials dedicated to proton transport. Especially for materials, where interactions are dominated by covalent or hydrogen bonds, ab initio molecular dynamics provide a proper description. The results obtained by such methods give details at the atomic level, which are not accessible by experimental investigations. Nevertheless, the choice of the model system has to be done in a very careful way in order to consider the manifold possibilities of structures and mechanisms. 

Parallel to the developments achieved in methodology and hardware, the conventional methods and some of the new approaches have been employed to study several types of photoinduced processes which are relevant mainly in biology and nanotechnology. In particular, important contributions have been made related to the topics of photodissociations, photostability, photodimerizations, photoisomerizations, proton/hydrogen transfer, photodecarboxylations, charge transport, bioexcimers, chemiluminescence and bioluminescence. In contrast to earlier studies in the field of computational photochemistry, recent works include in many cases analyses in solution or in the natural environment (protein or DNA) of the mechanisms found in the isolated chromophores. In addition, semi-classical non-adiabatic molecular dynamics simulations have been performed in some studies to obtain dynamical attributes of the photoreactions. These latter calculations are however still not able to provide quantitative accuracy, since either the level of theory is too low or too few trajectories are generated. Within this context, theory and hardware developments aimed to decrease the time for accurate calculations of the PESs will certainly guide future achievements in the field of photodynamics. 

In this chapter we will review the recent developments in simulating and modelling proton transport. We will put a special emphasis on studies employing classical and quantum molecular-dynamics simulations, but also include basic studies that have focussed on model systems using accurate quantum-chemical methods. Proton-transport and dilfusion phenomena in liquids - such as water, inorganic acids, or organic liquids - will be discussed as well as in biomolecules, solid-state materials, and at the solid-liquid interface. Many of these materials are used in proton-transporting fuel-cell membranes, so that membrane materials will be the focus of the last section. 

The system, for which proton-transfer reactions are investigated best, is very simple and complex at the same time liquid water. Numerous theoretical studies - mainly based on different types of molecular-dynamics simulations - have been published in the last decades that try to reveal the secrets behind the proton-transport properties of water. Generally, these studies make use of an excess proton which might be solvated in two different ways either as a so-called Eigen ion (or Eigen complex) H9O/ or as a so-called Zundel ion (Zundel complex) H502i In the first, the excess proton is complexed by... 

This section presents a review of atomistic simulations and of a recently introduced meso-scale computational method to evaluate key factors affecting the morphology of CLs. Most of the effort in molecular dynamics simulations for PEFCs has concentrated on dynamic motion of proton and water through the hydrated membrane [96-104], Little attempt has been made to employ MD techniques for elucidating the structure and transport of CLs, particularly in three-phase systems of carbon/Pt, ionomer, and gas phase. In the following subsections, we discuss various MD simulations to study the transport and dynamic behavior of CLs in terms of water and proton diffusivity, Pt-supported electrocatalyst, and microstructure formation. 

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