Bismuth iridium oxide oxygen evolution catalyst from hydrothermal synthesis

Abstract

We present a detailed characterization of the pyrochlore Bi 2Ir 2O 7 prepared by a one-step hydrothermal synthesis route from aqueous sodium hydroxide solution of NaBiO 3·2H 2O and IrCl 3·5H 2O in the presence of Na 2O 2 at 240 °C. Using 5 M NaOH solution as the reaction medium, a fine powder of polycrystalline Bi 2Ir 2O 7 with an average crystal size of 10 nm and surface area of ∼46 m 2 g -1 is produced. Structure refinement against powder neutron diffraction reveals a stoichiometric pyrochlore with no evidence for significant oxide-ion defects. X-ray absorption near-edge structure (XANES) spectra recorded at both metal L III-edges show that, although Bi is present solely as Bi 3+, there is evidence for the oxidation of iridium slightly beyond +4. This would suggest some surface oxidation of iridium, which is also shown by X-ray photoelectron spectroscopy (XPS) measurements. Magnetization data, as a function of temperature, show that the system is paramagnetic down to a temperature of 2 K, while the electrical conductivity shows hydrothermal Bi 2Ir 2O 7 to be a metallic conductor. In electrochemical tests, performed on rotating disk electrodes fabricated from the powdered iridate and Nafion solution, the material shows oxygen evolution activity in acidic solution, comparable to the most active precious-metal oxide materials, with reproducibility over >1000 cycles, demonstrating the formation of robust electrodes. © 2012 American Chemical Society.