The Mechanism of Electrochemical Oxygen Reduction: A Combined DFT and in-Situ ATR-IR Study on Model Semiconductor Surfaces Ge(100) and ZnO

The electrochemical oxygen reduction reaction (ORR) is a fundamental reaction with relevance in many fields including fuel cells and corrosion.  The reaction mechanism is controversially debated and strongly dependent on the electrocatalytic surface, electrolyte, pH and applied potential.  Many studies have focused on high-performance noble metal catalysts, however, semiconducting oxides often form at the metal/electrolyte interface.  Therefore we have studied the ORR mechanism at model semiconductor electrodes, using a combination of in-situ attenuated total reflection infrared (ATR-IR) spectroscopy and DFT modeling.  We have identified several reaction intermediates on the n-type Ge(100) surface [1,2] and on zinc oxide by their characteristic vibrational modes. 
The reaction mechanism in terms of elemental steps of electron transfer and proton transfer and hence including charged intermediates is studied with DFT. The model is based on small clusters representing the surface interacting with the solvated ORR intermediates.  Solvent effects were treated by a combination of explicit water molecules and a self-consistent reaction field approach.  The influence of pH and electrode potential on the energetic course of the ORR reaction is analysed.  We identify the catalytically active surface site and propose a detailed catalytic cycle in terms of electron transfer, proton transfer and bond breaking/forming as elemental steps.  The calculated IR-frequencies agree well with our experimental results. 
[1]    S. Nayak, P.U. Biedermann, M. Stratmann, A. Erbe, Pys. Chem. Chem. Phys. 15, 5771 (2013)
[2]    S. Nayak, P.U. Biedermann, M. Stratmann, A. Erbe, Electrochimica Acta 106, 472 (2013)