Understanding Oxygen Reduction on Tantalum Oxyphosphate and Tantalum Oxide Supported Platinum by X-ray Absorption Spectroscopy

Abstract

The Pt activity for the oxygen reduction reaction (ORR) is improved when Pt nanoparticles are supported on nanoscale layers of tantalum oxyphosphate (nominally, TaOPO4) on Vulcan carbon (VC) and heated at high temperature (660 °C) in reducing conditions. We attempt to explain the increased activity of the Pt-TaOPO4/VC “HT” by comparison to other less-active electrocatalysts comprising Pt on tantalum oxide (Pt-Ta2O5/VC) and Pt-TaOPO4/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 Δμ 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 Δμ XANES analysis at the Pt L2 edge suggests that interfacial hydrogen exists between the Pt and the support for Pt-Ta2O5/VC and Pt-TaOPO4/VC “HT” at potentials > 0.3 V vs RHE even after surface H is removed. More importantly, in the most active sample, Pt-TaOPO4/VC “HT”, the onset potential for O(H) adsorption is highest in the Δμ XANES. The XAS and microscopy/EDS results show that the Pt-TaOPO4/VC “HT” is Pt-Ta2O5/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-TaOPO4/VC “HT” facilitates proton conduction to the Pt particles and hence moves the equilibrium for OH adsorption on the Pt to higher potentials.