Hydrogen peroxide generation and hydrogen oxidation reactions of vacuum-prepared Ru/Ir(111) bimetallic surfaces

From the viewpoint of the application of Ir-Ru alloys for the anode of proton exchange membrane fuel cells (PEMFCs), hydrogen peroxide (H₂O₂) generation and the hydrogen oxidation reaction (HOR) properties of well-defined Ir-Ru bimetallic surfaces (Ru/Ir(111)) have been investigated using scanning electrochemical microscopy (SECM). Using thermal inter-diffusion of vacuum-deposited Ru and substrate Ir atoms, the topmost surface atomic ratios of Ru/Ir(111) were controlled via changing the substrate temperature (x) during the deposition of 1 monolayer (ML)-thick Ru. Low-energy ion scattering spectroscopy (LE-ISS) estimated the Ru/Ir ratio to be 1 : 1 (x = 673 K), 1 : 2 (x = 773 K), and 1 : 4 (x = 873 K). The H₂O₂ generation property of Ru/Ir(111) was similar to that of clean Ir(111) and under the detection limit in the potential region of 0.06-0.3 V, while clean Ru(0001) generated H₂O₂ in this potential region. The results suggest that the Ir sites contribute to the reduction of H₂O₂ intermediates generated at neighboring Ru sites. In contrast, the HOR activity of Ru/Ir(111) correlated with the probabilities of ensembles, such as Ir₂ dimers and Ir₃ trimers: the ensemble probabilities were calculated under the assumption of random solute Ir and Ru atoms at the topmost surfaces. Such a close correlation suggests that the Ir ensemble sites strongly contribute to the HOR. In conclusion, the Ir sites play a key role in the suppression of H₂O₂ generation and high HOR activity, which is essential for next-generation PEMFC anode catalysts.