Toward Beyond-Volcano-Top Performance in Oxygen Reduction Reaction on Pt from First-Principles and Kinetic Calculations

Abstract

While first-principles investigations are suitable for exploring oxygen reduction reaction (ORR), they are not straightforward in estimating the final performance of a fuel cell. We are interested in how the important properties of ORR can be extracted and bridged to fuel cell performance. We have calculated the redox potentials and activation energies of the elementary electrochemical reactions of ORR, and evaluated the cyclic and linear sweep volatmograms from rate and diffusion equations. Our results demonstrate how ORR properties affect fuel cell performance and indicate a future direction for fuel cell investigations. The adsorption energy of the intermediates must not only be tuned (a point already made by many researchers from what is referred to as the volcano plot), but the activation energy of the elementary reactions must also be decreased to obtain much higher fuel cell performance beyond the volcano top performance. We took first-principles molecular dynamics (FPMD) to simulate two elementary steps of the electrochemical oxygen reduction reaction on Pt(111); O(ad) + H+ + e−--> OH(ad) and OH(ad) + H+ + e−--> H2O In order to control the electrode potential, i.e. Fermi energy, we used constant-µ scheme [1] of the effective screening medium (ESM) method [2]. The redox potentials of these reactions were determined by changing the OH(ad) and O(ad) coverages. The activation energies of the reactions were determined using the blue-moon ensemble (bond constrained) method [3]. Using the parameters obtained, cyclic and linear sweep voltammograms were obtained.  We found that it is necessary not only to tune the adsorption energy of the intermediates, O(ad) and OH(ad), which are the origin of the so-called volcano plot, but also to reduce the activation energy of the second elementary step.  FPMD calculations were performed with “STATE” code [4], which uses density functional theory for electronic structure calculation with plane wave basis, ultrasoft pseudo potentials, and the GGA-PBE correlation functional. A unit cell of 8.38Å×9.67Å×26.04Å in size was established under the periodic boundary conditions, consisting of a slab of three layers of the Pt(111) plane, each containing 12 Pt atoms and about 32 water molecules on it. The details of the calculations are given elsewhere [5]. Computations were performed using the supercomputers at HPCC in Hokkaido University, IMR in Tohoku University, and ITC in the University of Tokyo. This work was partly supported by the NEDO (New Energy and Industrial Technology Development Organization) projects. [1] N. Bonnet, T. Morishita, O. Sugino, and M. Otani, Phys. Rev. Lett. 2012, 109, 266101. [2] M. Otani and O. Sugino, Phys. Rev. B 2006, 73, 115407. [3] M. Sprik and G. Ciccotti, J. Chem. Phys. 1998109, 7737. [4] Y. Morikawa, K. Iwata, and K. Terakura, Appl. Surf. Sci. 2000. 169-170, 11. [5] O. Sugino, I. Hamada, M. Otani, Y. Morikawa, T. Ikeshoji, and Y. Okamoto, Surf. Sci. 2007, 601, 5237; M. Otani, I. Hamada, O. Sugino, Y. Morikawa, Y. Okamoto, and T. Ikeshoji, J. Phys. Soc. Jpn. 2008 77, 024802; T. Ikeshoji, M. Otani, I. Hamada, O. Sugino, Y. Morikawa, Y. Okamoto, Y. Qian, and I. Yagi, AIP Adv. 2012, 2, 032182.