Unifying kinetic and thermodynamic analysis of 2 e- and 4 e - reduction of oxygen on metal surfaces

Abstract

Oxygen electrochemistry has played a key role in developing the foundational understanding of surface electrocatalysis. In this work, we develop a detailed microkinetic model for the oxygen reduction reaction based on density functional theory calculations and molucular dynamics simulations. The developed microkinetic model has a reaction order of 1 in O2 and a 59 mV/dec Tafel slope in the potential range where the Pt(111) surface is dominated by OH adsorption. Using calculated scaling and BrØnsted-Evans- Polanyi relations, we extend this analysis to calculate the activity on materials with a varying OH binding energy. We demonstrate the existence of a kinetic activity volcano which is in close agreement with the thermodynamic activity volcano derived earlier. Both of these analyses identify an activity optimum around 0.1 eV weaker binding of OH relative to Pt(111). The predictions of the kinetic activity volcano are in close agreement with several RDE experiments on metals and Pt alloys. On the basis of rate control analysis and Sabatier analysis, we demonstrate the close connection between the kinetic and thermodynamic formulations for oxygen reduction. We further examine trends in 2e- reduction of O2 to H2O2 and show a similar correspondence between the thermodynamic and kinetic activity volcano with the activity optimum around 0.3 eV weaker OH binding than Pt(111). On the basis of rate control analysis, we show that many elementary steps play a key role in determining the selectivity. This emphasizes the importance of detailed kinetic analysis for addressing trends in selectivity. © 2014 American Chemical Society.