Cu UPD as a Diagnostic Technique for the Determination of Electrochemical Surface Area of Unsupported Pt, Pt/C, Pt/WOx, and PtRu Alloys

The study of Pt based electrocatalysts for use in fuel cells has lead to a wide expanse of materials: Pt-Mt (Pt-transition metal alloys), Pt/C (Pt supported carbons), Pt/CNT (Pt supported carbon nanotubes), Pt/MOx (Pt supported metal oxides), Pt-Mn (Pt-noble metal alloys), Pt core-shell materials and countless others. In order to characterize the electrochemically available surface area (ECA) for these materials research groups and industrial labs have primarily leaned on two methods: hydrogen underpotential deposition (HUPD) and CO stripping. The former can be used for any Pt, Pt-alloy, or Pt supported material where the alloying element or catalyst support does not convolute, or contribute to the HUPD region of Pt (e.g. most transitional metal Pt alloys and Pt supported carbons). The more time consuming CO stripping method is used in cases where the alloying metal or catalyst support obscures the otherwise obvious HUPD region of Pt to such a degree that the electrocatalyst double layer and hydrogen adsorption regions are no longer distinguishable. Consequently, ECA for PtRu alloys is most often determined via CO stripping. For the same reasons given above for CO stripping, Cu underpotential deposition (Cu UPD) and subsequent stripping has become a vogue technique to determine the overall electrocatalyst surface area (ECA) and surface ratios of Pt and Ru in PtRu alloys under fuel-cell-like conditions. We will show that the convolution of the "Cu-Pt stripping" peaks and the "Cu-Ru stripping" peak results in a misleading assessment of the Pt-Ru surface ratios and consequently the technique should only be used to determine the overall ECA of electrocatalysts. Additionally, we will demonstrate the usefulness of the Cu UPD technique in determining the ECA of Pt in Pt/WOx systems, where hydrogen intercalation into the tungsten oxide support prevents the use of HUPD.