ORR on the Different Surfaces with Different Adsorbents: First Principles Molecular Dynamics Simulations

Oxygen Reduction Reaction (ORR) is the hard task in the fuel cell operations and has been studied extensively not only from practical point of view but also from fundamental one. Although various calculations by both MO and DFT calculations were reported, many of them are under the static conditions (i.e. at 0 K) or they are on simple surfaces. We are currently working for it with more realistic models at a certain temperature under the electric bias. First Principles Molecular Dynamics (FPMD) combined with Effective Screening Medium (ESM) [1,2] to apply the potential on the electrode in the similar manner as the real double layer region. We first used it for the simple electrode reaction of the first step of the hydrogen evolution reaction (Volmer reaction) on Pt(111) [3] and then we are extending it to ORR [4,5,6] on various surfaces with various adsorbents.

Electronic structure calculations in the FPMD are based on DFT with ultrasoft-pseudopotentials and PBE functional for the exchange correlation. Program code "STATE" is used. MD is performed at constant temperature 353 K by a velocity scaling thermostat. Electrode potential is controlled by the charges added by the ESM method shown in the figure.

The last step of ORR (=the first step of oxygen evolution) is

H₃O⁺ + OH(ad) + e^– --> 2 H₂O

The reversibility of this reaction was confirmed on Pt(111) surface with surface charge density σ = 7 µC/cm²(corresponding to ca. 0.6–0.7 V vs. SHE) [4,5] with a pre-adsorbed OH. With higher or lower σ, reaction was remained in only one side. If we introduce a defect (a pit) on the surface, the reaction did not take place [6].

In order to see an effect of the different surface, we simulated a full ORR from O₂adsorption to H2O formation on (111), (110) and (100) surfaces. The main reaction scheme observed in the several time simulations for each surface is (dissociation scheme):

O₂ --> 2O(ad)

O(ad) + H₃O⁺ + e^– --> OH(ad) + H₂O

H₃O⁺ + OH(ad) + e^– --> 2 H₂O

In some cases, following H2O2 formation takes place as an intermediate species (association scheme).

2O(ad) + e^– + H₃O⁺ --> H₂O₂

We are also testing another kind of coadsorption species as a reaction promoter.

In summary, we could confirm that water liquid layer (not only single or bilayer) on the Pt surface plays an important role to proceed to a complete ORR.

[1] M. Otani and O. Sugino, Phys. Rev. B 73, 115407 (2006). [2] O. Sugino, I. Hamada, M. Otani, Y. Morikawa, T. Ikeshoji Y. Okamoto, Surf. Sci., 601 (2007) 5237. [3] M. Otani, I. Hamada, O. Sugino, Y. Morikawa, Y. Okamoto, T. Ikeshoji, J. Phys. Soc. Jpn.. 77 (2008) 024802, Phys. Chem. Chem. Phys., 10 (2008) 3609. [4] T. Ikeshoji, M. Otani, I. Hamada and Y. Okamoto, Phys. Chem. Chem. Phys., 13 (2011) 20223. [5] T. Ikeshoji, M. Otani, I. Hamada, O. Sugino, Y. Morikawa, Y. Okamoto, Y. Qian, and I. Yagi, AIP Advances, 2, 032182 (2012). [6] Y. Qian, I. Hamada, M. Otani, and T. Ikeshoji, Catal. Today, 202, 163-167 (2013).