Though density functional theory (DFT) studies of electrocatalytic systems are wide spread, the calculation of activation barriers for elementary steps involving electron and ion transfer remains challenging.  A simple and transferable DFT approach will be presented to estimate these barriers for inner sphere electrochemical reactions.  Our approach follows from the assumption that an electrocatalytic elementary step is inherently an inner-sphere reaction, for which attainment of the transition state requires nuclei rearrangement local to an adsorbedspecies, and electron transfer is rapid once the nuclei attain the transition state.  The Arrhenius concept (transition state theory) of the reaction rate then holds, and the Born-Oppenheimer approximation allows for evaluation of reaction rates without explicit consideration of electron conduction.  For electrocatalyst development, it is exactly these types of reactions that concern us.   We replace the need to locate a transition state for an electrochemical reaction (reduction of an adsorbed species by a proton/electron pair, for example) with the location of a transition state for an analogous non-electrochemical reaction (hydrogenation).  It is assumed that the transition states for these two reactions are identical at one specific electrode potential.  Once the transition state is located, the activation free energy, Gact0, may be assigned to the potential, U0, at which the chemical potential of the H* species is equivalent to the true H+ + e- reactant. The activation barrier can be extrapolated as a function of electrode potential using Butler-Volmer theory, with the symmetry factor estimated by examination of the transition state and reaction coordinate.The talk will further detail this approach and demonstrate that the method produces activation barriers consistent with experimental electrokinetics.  Example uses of this method will include elementary steps within the oxygen reduction, carbon dioxide reduction, and borohydride oxidation reactions.

Density functional theory approach to examining elementary kinetics of electrocatalytic reactions

246th National Meeting (2013)

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