Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode

TLDR

The authors developed a method based on electronic structure calculations to identify the origin of the overpotential in the oxygen reduction reaction. Using density functional theory, they mapped the free‑energy landscape of O₂ reduction on Pt(111) and examined alternative proton/electron transfer pathways, finding that a peroxide mechanism dominates on the most noble metals. They found that adsorbed oxygen and hydroxyl are highly stable near equilibrium, that the calculated proton/electron transfer rates match observed kinetics, that adsorption‑energy trends explain rates across many metals, that the peroxide pathway dominates on noble metals, and that the model offers routes to improve cathode performance.

Abstract

We present a method for calculating the stability of reaction intermediates of electrochemical processes on the basis of electronic structure calculations. We used that method in combination with detailed density functional calculations to develop a detailed description of the free-energy landscape of the electrochemical oxygen reduction reaction over Pt(111) as a function of applied bias. This allowed us to identify the origin of the overpotential found for this reaction. Adsorbed oxygen and hydroxyl are found to be very stable intermediates at potentials close to equilibrium, and the calculated rate constant for the activated proton/electron transfer to adsorbed oxygen or hydroxyl can account quantitatively for the observed kinetics. On the basis of a database of calculated oxygen and hydroxyl adsorption energies, the trends in the oxygen reduction rate for a large number of different transition and noble metals can be accounted for. Alternative reaction mechanisms involving proton/electron transfer to adsorbed molecular oxygen were also considered, and this peroxide mechanism was found to dominate for the most noble metals. The model suggests ways to improve the electrocatalytic properties of fuel-cell cathodes.