Elucidation of interface reactions between water and electrode metal by large-scale simulation

Hidetoshi Kizaki

Graduate School of Engineering,　Osaka University

First-principles simulations based on quantum　mechanics, which make progress　dramatically, have played a huge role in　figuring out various phenomena in the field　of materials science. Recently, first-principles　simulations for mechanisms of electric　chemical reactions have also made progress　dramatically. Since we could not observe　elementary steps of reactions directly in　traditional experiments of electric chemical　reactions, so far reactions mechanisms have　been discussed by considering reaction　kinetics between reactants and products.　However, the dramatic progresses of　first-principles simulations enable us to　investigate elementary steps of reactions　under atomic-scale criteria, traces of those　on electrode interface and its dependence　on electrode potentials and electrode　metals.
For example, it is important to study the　interface reactions between water and the　electrode metal in the field of fuel cell.　Therefore reaction simulations that take　an influence of complicated water structures　and electrode interfaces into　account have been performed. As a result,　the adsorption mechanism of H atom on a　Pt surface in transferring electronic　charge from hydronium ion, H₃O+, to　Pt-electrode, which is Volmer mechanism　as a first step of hydrogen generation　reaction, has been reproduced. This　enables us to obtain a significant key of　solving the hydrogen generation reaction　mechanism by investigating an interaction　between H₃O+ and a metal surface　and investigating H-adsorption dependence　on behavior of water and electric　potential.
However, due to the limitation of computational　resources, the dependence on　super cell size of periodic boundary conditions　remains in the traditional simulations　of electrode interface. Moreover, in　order to take statistically adequate sampling　of infinite geometry of metastable　water and investigate the reaction path　of H-adsorption dependence on the electric　potential and the electrode metal, a　large amount of calculation is required. To　solve the problem, large-scale fast parallel　computations are indispensable. By using　the K computer and more realistic　models, we aim to solve the whole electrode　reactions of fuel cell.