Iridium-platinum alloy nanoparticles: Composition-dependent electrocatalytic activity for formic acid oxidation

Ir(x)Pt(100) (x) alloy nanoparticles with varied compositions (x = 100, 75, 67, 50, 34, and 0) were synthesized by a thermolytic process at varied ratios of the IrCl(3) and PtCl(2) precursors. High-resolution transmission electron microscopic (HRTEM) measurements showed that the nanoparticles all exhibited well-defined crystalline structures with the average core diameters around 2 nm; and the elemental compositions were determined by X-ray photoelectron spectroscopic (XPS) measurements. The electrocatalytic activities of the Ir(x)Pt(100-x) nanoparticles toward formic acid oxidation were then examined by electrochemical measurements, including cyclic voltammetry (CV), chronoamperometry, and electrochemical impedance spectroscopy (EIS). In the CV studies, it was found that both the current density of formic acid oxidation and the tolerance to CO poisoning were strongly dependent on the composition of the iridium-platinum alloy nanoparticles, with the best performance found with the Ir(50)Pt(50) nanoparticles. Due to heavy CO poisoning, Pt nanoparticles exhibited the lowest catalytic performance among the series of nanoparticles (excluding Ir nanoparticles). The Ir(50)Pt(50) nanoparticles also showed the maximum current density and stability in chronoamperometric measurements. Consistent results were obtained in electrochemical impedance spectroscopic studies of the electron transfer kinetics involved. Notably, of all the nanoparticle electrocatalysts, an inductive component, i.e. negative impedance, was observed in the potential windows where CO was removed by electro-oxidation; and the charge transfer resistance was found to be the lowest with the Ir(50)Pt(50) nanoparticles. Based on the current density and peak potential of formic acid oxidation, the ratio of the anodic peak current density to the cathodic peak current density measured in CV studies, the stability manifested in chronoamperometric measurements and the tolerance to CO poisoning displayed in EIS measurements, the electrocatalytic performance of the IrPt alloy nanoparticles was found to decrease in the order of Ir(50)Pt(50) > Ir(67)Pt(33) > Ir(75)Pt(25) > Ir(34)Pt(66) > Pt. That is, among the series of IrPt alloy nanoparticles, the sample with the Ir/Pt atomic ratio of 1 : 1 showed the highest electrocatalytic activity towards formic acid oxidation, which might be ascribed to the bifunctional reaction mechanism of bimetallic alloy electrocatalysts.