Oxygen Reduction at Model Pt Single Crystal Surfaces

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Oxygen reduction reaction ORR is the main cathodic problem in electrochemistry and has a deep influence in the development of technologically relevant applications such as fuel cells. It is a complex reaction involving a nonpolar molecule with double bond breaking, 4 electron transfer and 4 O-H bond formation in different scenarios. In the reaction mechanism a possible important role can be played by intermediate peroxo species, after the exchange of only 2 electrons. To enhance reactivity, the understanding of the reaction mechanism at the Pt surfaces is necessary. Models can be used to simplify the complex reaction and from the experimental point of view such models are related to the use of single crystal surfaces with well defined orientations. For Pt(111) electrodes, the activation region is quite narrow despite the high overpotential and the process readily becomes transport controlled. Moreover, in the activation region the platinum electrode is covered by oxygenated species and it is well known the influence that these species can play in the electrode surface order, the surface stability being dependent a structure sensitive process. Possible influence of the surface oxygen-containing species, particularly as intermediates in the ORR is not clearly understood and only has been addressed at Pt(111), which likely is the most stable surface upon oxidation . On the other hand the ORR is structure sensitive, the Pt(111) being the most active in alkaline media [1] in contrast to that observed in acidic solution in which Pt(110) and Pt(hkl) surfaces with small two-dimensional domains are able to reduce oxygen at lower potentials. It is obvious that surface effects play an important role in the activation region as deduced from the marked anion effect that should be less important in alkaline solutions due to the shift in the surface charge on the metal. Another aspect that should be considered is the reactivity of hydrogen peroxide at these platinum surfaces, because its possible role as reaction intermediate [2]. In this respect if the ORR necessarily took place through the peroxo intermediate, a possible strategy would be based in the inhibition of peroxide oxidation while keeping constant ORR. Acknowledgements Financial support from MICINN (Spain) through project CTQ2010-16271 and Generalitat Valenciana (PROMETEO/2009/045, FEDER) are greatly acknowledged.