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Refined First-Principles Computational Scheme for the Electrode Dynamics

M. Otani, N. Bonnet, and O. Sugino

The problem of the bias-potential induced charge-transfer dynamics and associated structural change at the solid-liquid interface has attracted renewed attention. This problem, known as the electrode problem, has been recognized since the invention of the fuel-cell in 19th century and is focused today as a key issue in the development of clean energy-conversion devices. Atomic scale details, however, have remained only imperfectly elucidated. The authors worked together until several years ago as a member of this institute and developed a method to apply the bias-potential to the interface. The method, which is called the Effective Screening Medium (ESM) [1], was implemented to a first-principles molecular dynamics (FPMD) simulation code and was used to successfully detail the atomistic processes on the platinum electrode, where the hydrogen-evolution is initiated by an exchange of the electron and the hydrated proton (or the hydronium ion: H3O+) [2]. Because of the generation change of the supercomputer, the computational speed has been increased by two orders of magnitude since that time, and it has now become possible to dramatically enhance the accuracy of the simulation. In this context, the authors developed schemes to sophisticate the ESM simulation.

Fig. 1. Schematic configurations of (a) original and (b) smooth ESM. By introducing the improved algorithm, we can remove undesirable vacuum region (hatched region in (a)).

There are two major improvements. One is related to the ESM modeling of bulk water. In the original ESM, the water molecules beyond those explicitly treated were modeled by the dielectric continuum, but a thin vacuum region (hatched area in fig. 1(a)) had to be inserted in between because of an algorithmic reason. The existence of vacuum region degraded the simulation hindering the interpretation of the calculated results. The vacuum region is found dispensable when the border is smoothed by introducing an improved algorithm. The refined ESM, called smooth ESM, allows us to construct more realistic modeling of the electrode/electrolyte interface (see fig1.(b)). It is coded and tested to show that the description of the border greatly improved.

The other improvement is on the FPMD simulation. It is desirable to conduct the simulation keeping the bias-potential constant as does a real electrode, but our former simulation was done under the constant charge condition. To switch the scheme from the constant charge to the constant bias-potential, we introduce a double thermostat to keep both bias-potential and temperature constant. When implemented in our code to test an interface, the scheme is found to work indeed without making the simulation unstable.

Our joint group is selected as a priority subject in the “K-computer” project. With the refined simulation scheme, we will soon start using the extraordinary computational facility to attack the electrode problem.


References
  • [1] M. Otani and O. Sugino, Phys. Rev. B 73, 115407 (2006).
  • [2] M. Otani, I. Hamada, O. Sugino, Y. Morikawa, T. Ikeshoji, and Y. Okamoto, J. Phys. Soc. Jpn. 77, 024802 (2008).
Authors
  • M. Otania, N. Bonnet, and O. Sugino
  • aNational Institute of Advanced Industrial Science and Technology