Electrochemistry on the computer: Understanding how to tailor the metal overlayers for the oxygen reduction reaction

Abstract

2008 Published by Elsevier B.V. In recent years computational methods based on density functional theory (DFT) have been extremely successful in describing chemical processes taking place at metal surfaces. It has become possible to account quantitatively for the dynamics of gas–surface interactions, and the kinetics of surface catalyzed reactions have been described in some detail [1–4]. A similar treatment of chemical transformations at the surface of electrodes has been hampered by the complexity of the electrified solid–liquid interface. A complete theoretical description would have to include the solid surface, the liquid, the solvated ions and the effect of changes in the chemical potential of the electrons in the solid. Several very interesting approaches have been proposed recently [5–12] but they are still quite approximate and in addition they are often also quite cumbersome. In an exciting paper in this issue Nilekar andMavrikakis take a much simpler approach to understand the electrochemical oxygen reduction reaction (ORR) [13]. The ORR is the cathode reaction in e.g. PEM fuel cells, and since most of the energy loss is associated with this reaction even over the best catalyst, Pt, there is an enormous impetus to find better (and cheaper) electrode catalysts [14]. The ORR involves the sequential addition of protons and electrons to adsorbed O2. Rather than calculating the free energy of the protons in solution and the electrons (H + e (U)) at a potential, U, Nilekar and Mavrikakis exploit the fact that if U is measured relative to the so-called Standard Hydrogen electrode, the free energy of (H + e (U)) is the same as the free energy of 1/2 H2 at standard conditions corrected for the effect of the difference in potential, –eU, relative to the potential U = 0 where the reaction (H + e )M 1/2 H2 is at equilibrium [15]. In this way they can get all reaction energies as a function of potential for the elementary electrochemical reactions from standard adsorption energy calculations of the intermediates, O, O2, OH, OOH, HOOH, and H2O. While solvation of H in the liquid is included implicitly, solvation effects at the surface are not. They can be included, but generally amount to a few tenths of an eV for the H-containing species and considerably less for adsorbed O [16]. Nilekar and Mavrikakis provide an overview of the reaction energetics for several possible reaction paths. Even more importantly, they use the calculations to investigate the effect of having an overlayer of one metal on top of another. Recent experiments have shown that Pt deposited on a number of metals (Au, Pd, Rh, Ir, and Ru) [17] has large changes in ORR activity compared to pure Pt. The calculations explain this in a detailed way. In fact, the calculations allow a systematic approach to understand trends in ORR activity with potential. They also identify an important descriptor of the electrocatalytic activity, the oxygen adsorption energy [15]. Fig. 1 shows that when Elsevier B.V. 45 4593 2399. ).