ABSTRACT
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.
