Invited: Tailoring Platinum Group Metals Towards Optimal Activity for Oxygen Electroreduction to H2o and H2O2: From Extended Surfaces to Nanoparticles
Authors: Stephens Ifan Erfyl Lester 
Published: February 2020

The slow kinetics of the 4-electron reduction of oxygen to H2O imposes a bottleneck against the widespread uptake of low temperature fuel cells in automotive vehicles. High loadings of platinum are required to drive the reaction; the limited supply of this precious metal limits the extent to which fuel cell technology could be scaled up.(1) The most widely used strategy towards decreasing the Pt loading is to alloy Pt with other late transition metals, in particular Ni or Co. (2-5) However, when tested in a fuel cell, these alloys are often susceptible towards degradation via dealloying.(6, 7)

At our laboratory, we have developed a different class of Pt alloy for oxygen electroreduction: alloys of Pt with rare earths, such as Y or Gd.(1, 8, 9) The strong interaction between Pt and the rare earth elements should make these compounds inherently less prone towards dealloying. We first demonstrated the very high activity of Pt3Y and Pt5Gd on extended polycrystalline surfaces. However, we have more recently shown that model, size-selected nanoparticles of PtxY exhibit up to 3 Ag-1at 0.9 V. These promising results provide a strong impetus towards the large scale synthesis of these catalysts, so that they can be implemented in fuel cells.

In most fuel cell applications, the production of H2O2 during oxygen reduction is an unwanted side reaction, to be avoided at all cost. However, H2O2 is a very useful chemical in its own right, whose annual global production exceeds 3 M tons. At present, H2O2 is produced via the anthraquinone process, a complex, batch process, conducted in large scale facilities. The electrochemical production of H2O2 would enable on-site small scale production of hydrogen peroxide, closer to the point of consumption. The viability of the process would require a catalyst that is active, stable and selective for H2O2 production. We recently discovered a set of electrocatalysts that showed an unprecedented combination of all three of these desired properties: alloys of Pt, Ag or Pd with Hg.(10, 11)

I will present data collected a wide range of different methods, including electrochemical measurements, ex-situ physicochemical characterisation techniques (X-ray photoelectron spectroscopy, transmission electron microscopy, X-ray diffraction and X-ray absorption spectroscopy) and density functional theory calculations.

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The figure shows transmission electron miscroscopy images of 9 nm diameter Pt
x
Y nanoparticles, based on high angle annular dark field –scanning transmission electron microscopy (left) and Y, Pt and combined Pt+Y X-ray energy dispersive X-ray spectroscopy elemental maps. (a) as-prepared catalyst and (b) after oxygen reduction reaction. The Pt and Y EDS intensity line profiles extracted from the spectrum image data cube , demarcated by the purple line.