Origin of Superior Activity of Ru@Pt Core-Shell Nanoparticles towards Hydrogen Oxidation in Alkaline Media
Authors: Schwämmlein Jan Nicolas , El-Sayed Hany A , Stühmeier Björn Marcel , Wagenbauer Klaus F , Dietz Hendrik , Gasteiger Hubert A. 
Published: February 2020

Platinum shows an exceptionally high activity for the hydrogen oxidation reaction (HOR) in acidic environment,[1] enabling ultra-low Pt-loadings on the anode side of proton exchange membrane fuel cells (PEMFCs).[2] Unlike in acid, this reaction is about two orders of magnitude slower on platinum and other platinum metals in alkaline electrolyte,[3]; [4] which has recently been suggested to be due to entropic effects.[5] Amongst other issues, the low HOR activity of Pt-based electrocatalysts in alkaline environment hinders the development of anion exchange membrane fuel cells (AEMFCs).[4]

Bimetallic Pt-Ru catalysts were demonstrated to exhibit significantly higher HOR activities in acid[6]; [7] and base[8]–[10] compared to Pt catalysts. One hypothesis for the high HOR activity of Pt-Ru alloys in base is a bi-functional mechanism,[9]; [10] with hydrogen being adsorbed on Pt and hydroxide being supplied by more oxophilic Ru, whereby the presence of hydroxide in the vicinity of Pt-Hads would accelerate the rate of the hydrogen oxidation. The second hypothesis considers a modification of the electronic structure of platinum by ruthenium,[8] leading to a lower Pt-Hads binding energy and ultimately to a higher activity of platinum towards the oxidation of hydrogen. While the exposure of ruthenium on the surface of the catalyst is absolutely mandatory for a bi-functional mechanism, this is not the case for an activity enhancement due to a modification of the electronic structure of platinum. Based on this fundamental difference, we have attempted to identify the actual HOR mechanism on bimetallic Pt-Ru catalysts by evaluating Ru@Pt core-shell nanoparticles with various Pt-coverages and shell thicknesses. Pt-coverage and shell thickness were determined electrochemically via the Ru-oxide reduction peak and COads stripping voltammetry, a method established by El Sawy et al.[11] Considering a pure, bi-functional mechanism where hydroxide on Ru reacts with Hads on Pt, ruthenium is required to be on the surface of the catalytically active particle. Hence, we expect the highest activity at submonolayer Pt-coverage.

For the bifunctional mechanism, covering the whole Ru-surface with Pt would result in a performance equal or similar to that of pure Pt (s. red line in Fig. 1). In contrast to that, for a purely electronic effect, Ru surface atoms would not be required for effective HOR catalysis and one would expect the best performing Ru@Pt catalysts to have a fully Pt-covered Ru surface or even multilayers of Pt; the optimum Pt-shell thickness would mainly depend on the range of the electronic effect (s. black line in Fig. 1). Based on this concept, we will present data to disentangle the mechanism of the hydrogen oxidation on bimetallic Pt-Ru catalysts. The activity of the developed catalysts will be compared to Pt catalysts prepared by the same method with a similar particle size as that of the Ru@Pt core-shell nanoparticles.

Acknowledgment

The authors of this work thank the microanalytical laboratory at TUM for quick and reliable elemental analysis.

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Figure 1