In our daily life, we utilize many electrochemical reactions that occur on the electrode, for example, the electrolysis of water, the fuel-cell reactions to convert chemical energy to electric energy, and the plating and etching to modify metal surfaces in nano-meter scale. Research of the electrochemistry may be traced back to 230 years ago when Luigi Galvani discovered the bioelectricity, but the atomic scale detail has not been well understood compared with other chemical reactions. The electrochemical reaction is so complicatedly affected by the bias voltage and the solvation effect of water, hampering comprehensive understanding for centuries.
This is even the case for the hydrogen and oxygen evolution reaction that pupils learn at elementary school. How do the electrons change their accommodation from the electrode to the ion, and vice versa? How do neighboring water molecules and the electrolyte ions play a role? This topic is a problem of physics as well as chemistry, and is important both scientifically and technologically. Recently an educational-industrial complex team, consisting of the University of Tokyo, Osaka University, NEC Corporation, and the National Institute (AIST), performed a large-scale simulation utilizing a newly developed simulation tool and explained the atomic scale detail on the electrochemical reaction on the platinum cathode. The research was published on February 2008 issue of the Journal of the Physical Society of Japan (JPSJ).
2. Hydrogen Evolution Reaction and Simulation
The cathodic reaction is considered to proceed in double steps. First, the positively charged hydrated proton (H3O+) is discharged on the cathode and is dissociated into a neutral hydrogen adsorbate and a water; this reaction is called Volmer step. Second, the hydrogen molecule is produced when the hydrogen adsorbates meet after surface diffusion (the Tafel step) or when the hydrated proton is discharged at the hydrogen adsorbate (the Heyrovsky step).
To simulate the Volmer step, a model interface was prepared using 36 Pt atoms, 32 water molecules, and an additional hydrogen atom, as shown in Fig. 1. At 80 degree Celsius and under a negative bias voltage, the simulation was performed. At each time step of the simulation, the electronic states were calculated using the density functional theory to obtain the interatomic force, which was used to move the atoms. This step was repeated for thousands of times to follow the time-evolution of the reaction. That is, the first-principles molecular dynamics simulation was used to trace the reaction.
Figure 1: Computational cell used for the simulation. The red and while spheres represent the oxygen and hydrogen atoms, respectively, and the gold sphere corresponds to the platinum. The green line represents the hydrogen bond.
The simulation utilizes a newly developed algorithm called the Effective Screening Medium (ESM) to apply the bias voltage between the electrode and the solution. When biased, excess charge is accumulated at the interface causing divergent the total energy. ESM introduces a conductor offshore to keep charge neutrality in the whole system, thus enabling the simulation of the charged interface.
The positively charged hydrated proton（H3O+） is attracted by the negatively biased Pt surface and is discharged afterwards. The Volmer step was successfully simulated as shown in Fig. 2. The simulation also shows how the electronic structure changes with time, from which we can learn how the electrons transfer from the electrode to the ions in the course of this electrochemical reaction.
Figure2: The adsorption process of the hydrated proton. The figures correspond to the snapshots, 5.27, 5.29, and 5.32 ps after the simulation. The hydrogen atom, shown with the arrow, of the hydrated proton (H3O+), shown with yellow, is adrobed on the Pt (111). The dynamics can be seen in the movies:
(1) Initiall, the hydrated proton (H3O+) approaches the electrode and the water molecules are arranged to screen the electric field.
(2) The electron-transfer reaction, together with the reorganization of the water molecules that occur afterwards. Diffusion of the adsorbed hydrogen can also been seen.
(3) Time evolution of the charge population of each atom associated with the electron-transfer reaction.
4. Future Prospects
The present simulation has demonstrated the most fundamental and simplest electrochemical reaction, or the Volmer step of the hydrogen evolution reaction (HER). The same method can be applied to simulate the whole process for HER, with which to show why the platinum electrode is so effective towards HER and how the platinum may be substituted by other materials. Continuous efforts would finally lead to a comprehensive understanding of the electrochemical reaction process. The theoretical knowledge would be particulatly important in the present day in that the electrochemical process may be used for the future energy supply system.