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Computational Electrochemistry

(Cramer, Dupuis, Gao, Garrett, Schenter, Truhlar, and Valiev)

Many chemical reactions include steps where a single electron is transferred, actually or formally, from one reacting partner to another, resulting in oxidation or reduction. Such reactions are important in environmental chemistry, where many strategies for cleaning up contaminated soils and other media involve oxidations or reductions of contaminants. Another example is creation of electron/hole pairs and subsequent selective transport of protons and ions through membranes, which is a key part of photo-catalytic processes for hydrogen production in fuel cells. (Fuel cells are the final and key step in a hydrogen power economy, in that the chemical energy of the fuel, such as hydrogen or methanol, is transformed into electrical energy.)

Oxidation-reduction reactions typically effect a significant change in the charge distribution of a reacting system, and hence they are very sensitive to condensed-phase surroundings. Attempts to model the energetics of such reactions thus face a number of challenges such as those that arise in modeling of open-shell species. Second, there is the need to accurately model the very strong interactions that a surrounding solvent will have with reactant and product ions. Third, to model the kinetics of electron transfer, one must adequately account for different timescales over which various solute and solvent relaxations to equilibrium take place during and following the generally non-equilibrium electron-transfer event. Finally, theoretical descriptions of charge transport processes after the electron-transfer event (e.g., to model fuel cell operation) are difficult because of the diversity of transport mechanisms and their dependence on local structure in complex systems.

A comprehensive description of an electrochemical process may be modeled by sets of coupled kinetic equations describing the rates of all involved reactions. We will develop tools that expand greatly our ability to generate the relevant parameters that enter such kinetic models. The currently available databases of kinetic parameters will be supplemented with very high level electronic structure calculations, and we will construct and validate highly cost‑effective computational protocols for predicting accurate kinetic parameters. We will consider the effects of aqueous solvation using an economical continuum solvent model, and we will further calibrate the kinetic parameters against reliable experimental data.

A particular problem arises by the proton transfer in fuel cells, which requires accurately captures of the quantum mechanical characteristics of light-atom motion in simulations of large-scale systems. Classical mechanics as used in molecular dynamics simulations does not describe quantum mechanical effects, such as zero-point energy constraints and tunneling, which are often very significant in these processes. However, fully quantum mechanical treatment of large condensed-phase systems is beyond current computational capabilities. We will develop computational tools that treat the quantum dynamics of light-atom motion reliably within large-scale simulations of molecular materials, in a way that can be folded into multi-scale modeling of kinetics. These advances will permit development of new materials with desired properties, including control of transport processes.

Updated: Feb. 18, 2008