Origin of Superior HOR/HER Activity of Bimetallic Pt-Ru Catalysts in Alkaline Media Identified via Ru@Pt Core-Shell Nanoparticles

Abstract

Compared to Pt/C, the HOR activity of Pt-Ru alloys in alkaline electrolyte is exceptionally high. Nevertheless, it remains unknown whether this enhancement is due to a bifunctional mechanism involving Pt and Ru as active sites or an electronic effect of Ru on Pt. In this study, we distinguish between those fundamental differences using Ru@Pt core-shell nanoparticles as a model system. Ru@Pt catalysts were prepared from submonolayer to multilayer Pt coverage. The exposure of Ru on the surface of the catalyst was analyzed by cyclic voltammetry, showing that Ru is solely exposed on the surface of Ru@Pt particles with low Pt-coverage. The thickness of the Pt-shell was characterized by CO stripping in H 2 SO 4 , allowing to distinguish between single and bilayered Ru@Pt catalysts. Determining the HOR/HER activity of these catalysts in 0.1 M NaOH revealed that fully Pt-covered Ru is more active than partially covered Ru@Pt nanoparticles. Hence, the participation of Ru as active site in a bifunctional mechanism is of minor importance with respect to the HOR/HER activity compared to its influence on the electronic structure of Pt. Similar to Pt-Ru alloys, the most active Ru@Pt core-shell nanoparticles show a 4 to 5-fold enhancement of the surface-normalized HOR/HER activity compared to Pt/C.

The role of adsorbed hydroxide in hydrogen evolution reaction kinetics on modified platinum

“…plot shown in Fig. 5 also helps to clarify the debate over the mechanism for the improved performance of Pt-Ru alloy materials [3][4][5] and Ru clusters and/or islands on Pt(111) (ref. 35 )-both weakened hydrogen binding and stronger hydroxide binding contribute.…”

“…This trend in activity, however, is also convoluted with the differing hydrogen binding strengths across these surfaces. Later work that examined the HER/HOR kinetics on Pt-Ru alloys 4 and Pt-Ru core shell nanoparticles 5 instead attributed the higher activity of these materials to a weakening, relative to that of pure platinum, of the hydrogen binding induced by ruthenium. This is expected to increase activity, as pure platinum sits on the too-strong binding side of the hydrogen volcano 1,2 .…”

“…There has since been substantial debate 12,13 on the role of the electric field 14 , PZC/PME 15 and near-surface water 16 , with studies that either support [17][18][19][20] or refute 5,21,22 a bifunctional mechanism that involves adsorbed hydroxide, in the HER/HOR. However, the majority of this work was performed using complex polycrystalline or metal (alloy) nanoparticles, which made it difficult to connect changes in activity to an atomic-scale picture of the reaction mechanism, as the surface structure and composition, especially in the electrochemical environment, are unknown.…”

Unifying the Alkaline Hydrogen Evolution/oxidation Reactions Kinetics by Identifying the Roles of Hydroxyl-Water-Cation Adducts

“…It is unclear why the HER/HOR rates of some transition metals including Pt, Ir, and Pd in base are two orders of magnitude slower than in acid, 3 and why they are sensitive to the surface structure. 4 While it is certain that mixing Pt with some metals such as Ru or Ni can improve the HER and/or HOR rates, [5][6][7][8][9][10][11] it is uncertain why. These puzzles also inhibit rational design of electrocatalysts for the alkaline HER/HOR.…”

Unifying the Alkaline Hydrogen Evolution/oxidation Reactions Kinetics by Identifying the Roles of Hydroxyl-Water-Cation Adducts

“…6,10 A key aspect of the HBE theory is that the second metal does not need to be on the surface to promote the HER/HOR. 6,10 However, unambiguous in situ evidence for Pt-bimetallic systems with clean Pt-surfaces free of the second metal during the HER/HOR is missing. 11 The HBE theory was also adopted to account for the pHdependent and structure-dependent HER/HOR rates.…”