Unraveling the Surface Chemistry and Structure in Highly Active Sputtered Pt₃Y Catalyst Films for the Oxygen Reduction Reaction

Abstract

Platinum is the most widely used and best performing sole element for catalyzing the oxygen reduction reaction (ORR) in lowtemperature fuel cells. Although recyclable, there is a need to reduce the amount used in current fuel cells for their extensive uptake in society. Alloying platinum with rare-earth elements such as yttrium can provide an increase in activity of more than seven times, reducing the amount of platinum and the total amount of catalyst material required for the ORR. As yttrium is easily oxidized, exposure of the Pt−Y catalyst layer to air causes the formation of an oxide layer that can be removed during acid treatment, leaving behind a highly active pure platinum overlayer. This paper presents an investigation of the overlayer composition and quality of Pt 3 Y films sputtered from an alloy target. The Pt 3 Y catalyst surface is investigated using synchrotron radiation X-ray photoelectron spectroscopy before and after acid treatment. A new substoichiometric oxide component is identified. The oxide layer extends into the alloy surface, and although it is not completely removed with acid treatment, the catalyst still achieves the expected high ORR activity. Other surface-sensitive techniques show that the sputtered films are smooth and bulk X-ray diffraction reveals many defects and high microstrain. Nevertheless, sputtered Pt 3 Y exhibits a very high activity regardless of the film's oxide content and imperfections, highlighting Pt 3 Y as a promising catalyst. The obtained results will help to support its integration into fuel cell systems.

Surface Composition of a Highly Active Pt3Y Alloy Catalyst for Application in Low Temperature Fuel Cells

“…For both samples (as‐deposited and after acid treatment) there are three contributions to the overall signal: bulk metallic yttrium (Y 0 ) around 155.9 eV (3d 5/2 ), yttrium oxide (Y 3+ ) around 158 eV , . There is a third peak that we assign to substochiometric Pt 3 Y oxide (Y–O) around 157 eV; where the material composition and oxidation is different to Y 2 O 3 , . There are notable trends with photon energy for all core levels.…”

“…There is no carbon present during the film deposition and as yttrium is so oxyphilic, the presence of a very small amount of oxygen in the chamber (during deposition, or from sputter cleaning) could cause substochiometric oxide to form. Previously, we have performed calculations based on this which have shown that subsurface oxygen (which we call substochiometric oxide) will have a shift towards slightly higher binding energies, the sputter cleaning presented in this paper provides further evidence of the existence of subsurface and substochiometric oxide . The percent content of Y 0 to Y–O, showing the distribution at different photon energies, can be found in the supplementary information.…”

“…There are also complexities with this technique, however the gain in surface sensitivity is beneficial for examining the platinum overlayer. In this work, SRPES was used to investigate three important components of the catalytically activity of Pt 3 Y: (i) impurities that could affect the performance, (ii) the composition of the platinum overlayer, and (iii) subsurface substochiometric oxide [14]. To observe these three components, SRPES spectra were taken before and after acid treatment.…”

Pore-Scale Transport and Two-Phase Fluid Structures in Fibrous Porous Layers: Application to Fuel Cells and Beyond

“…The core of a PEMFC consists of the polymer electrolyte membrane sandwiched between two thin catalyst layers and two porous electrodes, which are called gas diffusion layers (GDLs). The catalyst layer has a great impact on the cost of fuel cells, and research efforts have been devoted to its improvement (Brown et al 2019). On the cathode catalyst layer surfaces, protons and oxygen participate in an exothermic chemical reaction the products of which are water and heat.…”

Simple chemical synthesis of intermetallic Pt2Y bulk nanopowder

“…3,4 However, it is a significantly challenging task to prepare alloy nanostructures for actual applications, mainly because yttrium has a very strong oxygen affinity to form Y 2 O 3 readily and the oxide is hardly reducible. 5 Therefore, most successful reports of Pt-Y nanoparticles have been made via a physical top-down approach, such as advanced sputtering techniques, 6,7 in which the nanoparticles were commonly prepared from the pure metals under a highly clean environment of oxygen-/moisture-tight conditions. 8 On the other hand, a more challenging chemical approach has recently been attempted ardently to obtain Pt-Y nanostructures because the approach has a more scalable potential.…”