The materials set for oxygen reduction reaction (ORR) electrocatalysts is limited in proton exchange membrane fuel cells (PEMFCs) due to the acidic, oxidizing conditions  existing at the cathode, which leads to corrosion of almost all but noble metals.  Practical ORR electrocatalysts comprise carbon-supported nanoparticles of Pt and Pt-alloys, for which the ORR activity has largely been correlated to the energy of the Pt d-band center, an 'internal' electronic property determined by the nanoparticle size and morphology.  We have taken a different approach to increasing the ORR activity of Pt and even Au nanoparticles; namely by hosting them on oxide and phosphate supports, and thus taking advantage of  'external' metal-support interactions (MSIs).  Determining the mechanism for the observed increases in ORR activity has been quite difficult because of the spectroscopic opacity associated with resolving O2 reduction intermediates on nanoparticles in the presence of an aqueous electrolyte.  In this talk, we will describe how we have successfully implemented in situ X-ray absorption spectroscopy (XAS) to understand the ORR mechanism on carbon-supported Au on tin oxide (Au-SnOx/C) and Pt on tantalum oxy phosphate (Pt-[TaOPO4/C]) in combination with critical examination of cyclic voltammetry (CV) and ORR measurements by rotating disk electrode (RDE) voltammetry.  The key to successful use of in situ XAS data is reduction with the delta mu (?�) method, in which the difference, ?�, between the XAS cross-section for an active catalyst containing intermediate ORR adsorbates, �(A/M), and the background, �(M), is extracted over a range of electrode potentials.  The resulting ?� signatures are identified by comparison to theoretical ?� calculated using simple molecular models of A/M. We conclude that Au is activated for the ORR once O2 adsorbs on the nearby tin oxide, and Pt is activated by enhanced proton conduction on the surface of tantalum oxyphosphate.

Unraveling metal support interactions in oxygen reduction electrocatalysts through electrochemistry and in situ X-ray absorption spectroscopy

247th National Meeting (2014)

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