Using Vapor-Grown RuxPty and RuxPdy Nanoparticles to Investigate the Hydrogen Oxidation Reaction Mechanisms in Alkaline Electrolyte

We have developed a method for the synthesis of metallic and alloy nanoparticles dispersed on carbon supports that is based on a chemical vapor deposition process. This technique is applied here to investigate the mechanism of the hydrogen oxidation reaction (HOR) in alkaline media for which competing theoretical frameworks have been proposed in the literature. Such frameworks either hypothesize that molecular hydrogen dissociative adsorption (Tafel step) is rate limiting or that electron transfer (Volmer/Heyrovsky step) is rate limiting.

Our objective was to elucidate the HOR reaction mechanism using Tafel slope data collected in alkaline media while using small changes in alloy composition to look for trends in the changes of M-H binding energy as a descriptor for activity changes. Such data will be used to inform future in situ investigations of adsorbate coverage and binding energy using x-ray spectroscopy.

Pt, Pd, Ru, as well as alloys of Ru_xPt_y and Ru_xPd_y nanotubes were synthesized from metal-organic precursors onto Vulcan carbon support. Standard electrochemistry was conducted in conventional three-electrode cell hardware with a working electrode deposited onto glassy carbon with a Pt wire counter electrode and a double-junction Ag/AgCl reference electrode. Electrochemical data were collected at 2500 rpm under 1 atm H₂ in 0.1M KOH at 1 mV/s to minimize the contribution from capacitive currents. HOR activity was measured using staircase voltammetry, averaging the sampled current over the last 20% of the dwell time. Data were fit according to the standard Tafel analysis approach to determine Tafel slope along with fits by specific electrochemical models particular to a Tafel or Volmer rate-determining step.

HAADF-STEM images of the nanoparticles and XRD data suggest alloyed nanoparticles with some intraparticle atomic clustering. Electrochemical charging currents were consistent with similar surface and bulk compositions. The activity results revealed that the rate-determining step depends upon the choice of alloy materials and that both mechanisms present themselves accordingly. Additionally, the Pt-rich alloys with Ru exhibited HOR activity several times higher than pure Pt and comparable to the mass-transport limited current. These results suggest not only that a low-cost, alkaline anode catalyst is possible, but also that it is tantalizingly close to reality.