Electrochemical processes molecular dynamics

M. R. Philpott, J. N. Glosli. Molecular dynamics simulation of interfacial electrochemical processes electric double layer screening. In G. Jerkiewicz, M. P. Soriaga, K. Uosaki, A. Wieckowski, eds. Solid Liquid Electrochemical Interfaces, Vol. 656 of ACS Symposium Series. Washington ACS, 1997, Chap. 2, pp. 13-30. 

The several theoretical and/or simulation methods developed for modelling the solvation phenomena can be applied to the treatment of solvent effects on chemical reactivity. A variety of systems - ranging from small molecules to very large ones, such as biomolecules [236-238], biological membranes [239] and polymers [240] -and problems - mechanism of organic reactions [25, 79, 223, 241-247], chemical reactions in supercritical fluids [216, 248-250], ultrafast spectroscopy [251-255], electrochemical processes [256, 257], proton transfer [74, 75, 231], electron transfer [76, 77, 104, 258-261], charge transfer reactions and complexes [262-264], molecular and ionic spectra and excited states [24, 265-268], solvent-induced polarizability [221, 269], reaction dynamics [28, 78, 270-276], isomerization [110, 277-279], tautomeric equilibrium [280-282], conformational changes [283], dissociation reactions [199, 200, 227], stability [284] - have been treated by these techniques. Some of these... 

Although this chapter would not be complete without a discussion of the contribution of molecular dynamics computer simulation to the study of electrochemical processes at the liquid/liquid interface, this subject has been extensively reviewed recently, and so here we limit ourselves to a complete listing of the publications in this area. 

Following the early studies on the pure interface, chemical and electrochemical processes at the interface between two immiscible liquids have been studied using the molecular dynamics method. The most important processes for electrochemical research involve charge transfer reactions. Molecular dynamics computer simulations have been used to study the rate and the mechanism of ion transfer across the water/1,2-dichloroethane interface and of ion transfer across a simple model of a liquid/liquid interface, where a direct comparison of the rate with the prediction of simple diffusion models has been made. ° ° Charge transfer of several types has also been studied, including the calculations of free energy curves for electron transfer reactions at a model liquid/liquid... 

The use of molecular dynamics and Monte Carlo simulations to study electrochemical processes at the interface between two phases is only in its preliminary stages. The need to provide a molecular-level understanding of structure and dynamics at the interface to help in interpreting the new microscopic level of experimental data will increase. However, many important basic issues remain to be understood before these computational methods become routine research tools. 

As discussed in Section 13.2.4, when one of the two rings of a catenane carries two different recognition sites, the dynamic processes of one ring with respect to the other can be controlled. In particular, if redox units are incorporated into the catenane structure, there is the possibility of controlling these processes upon electrochemical stimulation. Catenanes that exhibit such a behavior can be seen as electrochemically driven molecular rotors. An example is offered by catenane 384+ (Fig. 13.33a), which incorporates macrocycle 2 and a tetracationic cyclophane comprising one bipyridi-nium and one trans-l,2-bis(4-pyridinium)ethylene unit.19,40... 

What is next Several examples were given of modem experimental electrochemical techniques used to characterize electrode-electrolyte interactions. However, we did not mention theoretical methods used for the same purpose. Computer simulations of the dynamic processes occurring in the double layer are found abundantly in the literature of electrochemistry. Examples of topics explored in this area are investigation of lateral adsorbate-adsorbate interactions by the formulation of lattice-gas models and their solution by analytical and numerical techniques (Monte Carlo simulations) [Fig. 6.107(a)] determination of potential-energy curves for metal-ion and lateral-lateral interaction by quantum-chemical studies [Fig. 6.107(b)] and calculation of the electrostatic field and potential drop across an electric double layer by molecular dynamic simulations [Fig. 6.107(c)]. 

Elucidation of the mechanism of an electrochemical process implies knowledge of the structure of the activated complex, and the way in which such a transition state is reached. Hence, one looks for the important reaction coordinates these are the coordinates that critically determine the free energy of the system (molecule + electrode). Progress in this difficult field of science requires molecular dynamic simulations [18, 19], experimental data, and common sense. Here, we briefly discuss the important reaction coordinates for typical ECIT and ECET processes. 

The results obtained clearly demonstrate that the Marcus model for ECL processes may be used for qualitative as well as for quantitative descriptions of this kind of electron transfer reactions. The more sophisticated approach, taking into account the vibronic excitation in the reaction products (important in the inverted Marcus region), solvent molecular dynamics (important in the case of large values of the electronic coupling elements), as well as the changes in the electron transfer distance, should be used. The results indicate that the Marcus theory may also be used for predicting the ECL efficiency, provided that some conditions are fulfilled. Especially, during the ECL process, only the annihilation of ions should occur, without any competitive reactions. The necessary rate constants can be evaluated from pertinent electrochemical and spectroscopic data. 

Mechanistic views and theoretical formalism of molecular STM processes are addressed in some detail in Chapter 8. Views of single molecule electron transport are rooted in theories of interfacial electrochemical electron transfer but offers new theoretical features and even phenomena. At the same time puzzles remain, resolution of which requires substantial computational efforts in the form of molecular dynamics and quantum chemical computations. Efforts along such lines are only just beginning. We provide here a few observations of immediate relevance to the data and images shown above. 

Philpott MR, GlosU JN (1997) Molecular dynamics simulation of interfadal electrochemical processes electric double layer screening. In Jerkiewicz G, Soriaga MP, Uosaki K, Wieckowski A (eds) Solid-liquid electrochemical interfaces. American Chemical Society, Washington... 

The same approach, using the simple solvation model, could be used to compare the deposition of one specific metal ion on various metal substrates. However, a more important problem is the realistic description of the whole process of metal deposition, including the desolvation of the metal ion as it reproaches the surface. In principle, the latter aspect can be treated by molecular dynamics, and first results have already been obtained a few years ago [81]. What is missing is the incorporation of such simulations into a fiumewoik that contains all of the electronic interactions. In this way, we should be able to understand what has been termed the enigma of metal deposition [78] Why is the deposition of certain metal ions so fast The deposition of silver, for example, is one of the fastest electrochemical reactions known, even though the ion looses about 6 eV of solvation energy during the process. So we close this chapter on an optimistic note we believe we now have the tools at hand to answer such fundamental questions. 

The last section was devoted to a range of real-world applications treated with ab initio molecular dynamics simulations. Results of gas to liquid phase transition simulations, structural and dynamical properties of liquids such as common solvents as well as the emerging neoteric media of ionic liquids were presented. After a short discussion of chemical reactions concerning homogeneous catalysis, we presented an overview of electrochemical reactions and related processes. 

Nanoscale modeling methods, such as molecular dynamics (MD) and DFT, aim to understand the kinetics of the electrochemical and chemical reactions on the electrode surfaces and at the interfaces between the electrodes and electrolyte, and the conduction processes in the electrolyte and the electrodes. 

Molecular Dynamics Simulation of Interfacial Electrochemical Processes Electric Double Layer Screening... 

The status of computer simulations of electric double layers is briefly summarized and a road map for solving the important problems in the atomic scale simulation of interfacial electrochemical processes is proposed. As examples efforts to simulate screening in electric double layers are described. Molecular dynamics simulations on systems about 4 nm thick, containing up to 1600 water molecules and NaQ at IM to 3M concentration, displayed the main features of double layers at charged metal surfaces including bulk electrolyte zone, diffuse ionic layer that screens the charge on the electrode and a layer of oriented water next to the surface. 

Xiao, Y., Yuan, J. Sundn, B. Process based large scale molecular dynamic simulation of a fuel cell catalyst layer. J. Electrochem. Soc. 159 (2012), B251-B258. 

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