Understanding Oxygen Reduction on Tantalum Oxyphosphate and Tantalum Oxide Supported Platinum by X-ray Absorption Spectroscopy
- George Washington University
- Naval Research Laboratory, Washington, D.C.
- ORNL
The Pt activity for the oxygen reduction reaction (ORR) is improved when Pt nanoparticles are supported on nanoscale layers of tantalum oxyphosphate (nominally, TaOPO{sub 4}) on Vulcan carbon (VC) and heated at high temperature (660 C) in reducing conditions. We attempt to explain the increased activity of the Pt-TaOPO{sub 4}/VC 'HT' by comparison to other less-active electrocatalysts comprising Pt on tantalum oxide (Pt-Ta{sub 2}O{sub 5}/VC) and Pt-TaOPO{sub 4}/VC heated to 200 C in air. Our toolbox for this analysis contains the rotating disk electrode methodology for characterization of the ORR and high-angle annular dark-field scanning transmission electron microscopy with high-resolution energy-dispersive X-ray spectroscopy (EDS) capabilities. The adsorption of molecular species on the Pt and Ta is determined from the {Delta}{mu} XANES (X-ray absorption near-edge structure) adsorbate isolation technique of X-ray absorption spectroscopy (XAS) data for electrocatalsyts in situ, whereby H and OH adsorption products from water activation can be used to infer how an electrocatalyst would react for oxygen reduction. The {Delta}{mu} XANES analysis at the Pt L{sub 2} edge suggests that interfacial hydrogen exists between the Pt and the support for Pt-Ta{sub 2}O{sub 5}/VC and Pt-TaOPO{sub 4}/VC 'HT' at potentials > 0.3 V vs RHE even after surface H is removed. More importantly, in the most active sample, Pt-TaOPO{sub 4}/VC 'HT', the onset potential for O(H) adsorption is highest in the {Delta}{mu} XANES. The XAS and microscopy/EDS results show that the Pt-TaOPO{sub 4}/VC 'HT' is Pt-Ta{sub 2}O{sub 5}/VC with polyphosphate groups directly associated with the Pt. From the body of results, we surmise that the nanoscale layer of polyphosphate on the Pt-TaOPO{sub 4}/VC 'HT' facilitates proton conduction to the Pt particles and hence moves the equilibrium for OH adsorption on the Pt to higher potentials.
- Research Organization:
- Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Shared Research Equipment Collaborative Research Center
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- DE-AC05-00OR22725
- OSTI ID:
- 1049860
- Journal Information:
- Journal of Physical Chemistry. C, Vol. 116, Issue 34; ISSN 1932-7447
- Country of Publication:
- United States
- Language:
- English
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