Selenourea-assisted synthesis of selenium-modified iridium catalysts: evaluation of their activity toward reduction of oxygen

Highlights: • Enhancement of catalytic activity of Ir with low Se coverages toward O₂ reduction. • Formation of oxo-groups on iridium is suppressed upon exposure to selenourea. • Slight increase of the H₂O₂ formation after deposition of selenium on iridium. • Higher tolerance of Se-modified Ir to presence of methanol and ethanol. • In presence of formic acid, decreased selectivity of Ir after modification with Se. - Abstract: Carbon-supported selenium-modified iridium nanoparticles have been synthesized using IrCl₃ and selenourea serving as a precursor of selenium and nitrogen atoms. Here nanostructured iridium is chosen as model base metal for fundamental catalytic considerations because it exhibits interfacial properties resembling both platinum and ruthenium. The systems' electrocatalytic properties have been studied in sulfuric acid electrolyte toward reduction of oxygen and formation of hydrogen peroxide intermediate in comparison to bare iridium and platinum catalysts. To get insight into the reaction dynamics and mechanisms, such electrochemical diagnostic techniques as cyclic voltammetry and rotating ring-disk electrode voltammetry have been considered. To mimic operation of catalysts in real fuel cells, additional experiments utilizing gas diffusion electrode have also been performed. Materials are subjected to surface analytical, structural and microscopic characterization using X-ray photoelectron (XPS), fluorescence (EDX), and diffraction (XRD) methods as well as transmission and scanning electron microscopies. At low (optimum) coverages of selenium on surfaces of iridium nanoparticles, the reduction of oxygen tends to proceed at more positive potentials in comparison to bare iridium under analogous conditions. Apparently, strong affinity of bare iridium to form oxo groups on its surface (known as inhibiting oxygen reduction) is largely suppressed in presence of selenium or nitrogen atoms. But the optimum selenium-modified iridium system produces somewhat higher amounts of the hydrogen peroxide intermediate presumably due to partial physical blocking of iridium (metal and metal oxo) sites (that would otherwise be active toward the reduction of the H₂O₂ intermediate). High tolerance (during reduction of oxygen) of the optimum Ir-based catalyst (functionalized using selenourea) to the parasitic (e.g. in polymer membrane fuel cells) simultaneous oxidation of organic fuels (e.g. methanol or ethanol) should be mentioned as well.