Electrocatalysis of the Oxygen Reduction Reaction at Platinum

An overarching requirement for Pt-based catalysts is that they must strike the optimal balance between performance and durability. Deconvoluting the mechanism of 

The ORR is notoriously known as a complex reaction. An oxygen molecule needs to be associated with four electrons and four protons  

In the process, electrons lose potential energy by occupying energy levels in the newly formed water molecules. The total Gibbs energy change for all four electrons is -4.92 eV, which corresponds to 1.23 eV per electron. 

The ORR proceeds at fte catalyst surface, where electrons are readily available, at a concentration that is determined by the electronic density of states of the metal. Protons are supplied from the electrolyte, with a concentration determined by the composition of the electrolyte and by the distribution of the electrolyte potential. The ORR involves, at least, three surface-adsorbed intermediate species. They are, surface oxide, Oad, hydroxide, OHad, and superoxide, OOHad- Surface processes that transform these species into one another and ultimately into water involve kinetic barriers that determine the net rate of the overall reaction. In the electrode potential range of interest, from 0.6-1.0 V versus SHE at the cathode, the formation of surface oxides as intermediates of the ORR interferes with the formation of surface oxides from the splitting of water. 

Thanks to a plethora of efforts in theory and experiment, the ORR has lost some of its enigmatic appearance. Especially, electronic structure calculations at the level of DFT have brought about tremendous advances in understanding of surface electrochemical processes at metallic catalysts. In this section, the following questions will be explored Does a cohesive picture of the ORR pathway and mechanisms exist Is the significance of electronic structure effects, formation of surface-adsorbed reaction intermediates, and kinetic limitations for the overall process understood  

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Specific processes involved in the ORR will be dealt with in the section Electrocatalysis of the Oxygen Reduction Reaction at Platinum in Chapter 3. Here, the focus is on basic phenomenological concepts that are required to incorporate electro-catalytic reactions in device-level modeling of PEFCs. For a more detailed treatment of fundamental concepts in electrocatalysis, see the recent edition of the textbook by Schmickler and Santos (2010), as well as the classical textbook by Bard and Faulkner (2000). 

Pathways, mechanisms, and corresponding kinetic parameters of the ORR have been discussed in the section Electrocatalysis of the Oxygen Reduction Reaction at Platinum. In a highly simplified picture, derived originally on the basis of a series of experimental studies by Damjanovic and coworkers (Damjanovic, 1992 Gatrell and MacDougall, 2003 Sepa et al., 1981,1987), it was proposed that the rate-determining reaction step is the initial adsorption. 

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