Universal Activity Volcano for 2e- and 4e- Reduction of Oxygen on Metal Surfaces

Oxygen reduction reaction (ORR) has been a subject of major interest in electrochemistry owing to its technological and scientific importance. The fundamental understanding of this reaction has been largely based on the surface science approach to electrocatalysis.[1] The surface science approach has allowed the bridging of theory and experiment by enabling direct theoretical investigations of the reaction mechanism of oxygen reduction reaction on model systems. In this talk, we will address the issue of 2e − versus 4e − reduction of oxygen on metal surfaces. Using a detailed thermodynamic analysis based on density functional theory calculations, we show that to a first approximation an activity descriptor, ΔG OH* , the free energy of adsorbed OH*, can be used to describe trends for the 2e− and 4e− reduction of oxygen. We demonstrate the existence of a universal activity volcano shown in the Figure and find quite a remarkable agreement between the predictions of the model and experimental results spanning nearly 30 years.[2-4] References [1] Fuel Cell Catalysis: A Surface Science Approach, Marc Koper and Andrzej Wieckowski, (2009) [2] V. Viswanathan, H. A. Hansen, J. Rossmeisl, and J. K. Nørskov, ACS Cat., (2012) 2, 1654-1660. [3] V. Viswanathan, H. A. Hansen, J. Rossmeisl, and J. K. Nørskov, J. Phys. Chem. Lett., (2012) 3, 2948-2951. [4] H. A. Hansen, V. Viswanathan and J. K. Nørskov, (submitted). Fig 1: Activity volcanoes for the 2e − and 4e − reduction of oxygen are shown in red and blue respectively. The source of the experimental data for the 2e − and 4e − reduction of oxygen are listed in ref. [3]. The experimental value of the limiting potential, U L , is determined by the half-wave potentials for the 4e − reduction and by the onset of ring current for the 2e − reduction. The activity of (111) and (100) surfaces has been plotted in circles and squares, respectively.