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Title: New oxides for oxygen evolution catalysis from hydrothermal synthesis
Author: Burnett, David L.
ISNI:       0000 0004 6350 9736
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2016
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The hydrothermal synthesis of complex oxides of ruthenium and iridium with potential application as oxygen evolution reaction catalysts in polymer electrolyte membrane fuel cells is reported. The materials M0.15Ru0.85O2, where M = Zn, Mg, Ni, Co or Cu, have been synthesised from peroxide reagents and potassium perruthenate. Structural refinement against powder neutron diffraction data shows these materials adopt the rutile structure with space group P42/mnm with the metals substituting ruthenium and no evidence of oxide vacancies. X-ray absorption near-edge structure spectra recorded at the Ru K-edge show that to compensate for the inclusion of these metals, the ruthenium is oxidised above +4. New oxides based on (Ca0.59Na0.27)2Ir2O6·0.66H2O were produced with substitutions made on both the A and B-site. All materials were shown to adopt the pyrochlore structure with space group Fd m. The level of B-site substitution was found to be dependent on the substituent element (Sb, Ru, Rh, Mn or Zr), with maximum substitution levels ranging from 30-100 %. The zirconium substituted material, (Ca0.58Na0.32Zr0.12)2(Ir0.56Zr0.44)2O6·0.97H2O, shows significant deviation from the average structure at the local scale. The synthesis of the pure iridium material was further investigated and it was found both the A-site composition and particle size could be controlled. Treatment in concentrated H2SO4 at elevated temperature yielded materials with vacant A-sites. The hydrothermal synthesis of a number of other mixed metal oxides is reported. These include the perovskites Na(Ta1-xMx)O3, Na(Nb1-xIrx)O3and SrRuO3, where M = Ir or Ru, and x < 0.15, the hexagonal perovskite 4H-BaRuO3, Sr2.85Ir3O11 a material with a KSbO3-type structure and a barium iridate with an unknown structure. In electrochemical tests, performed in membrane electrode assemblies, all materials outperform the benchmark materials, iridium tantalum oxide and ruthenium iridium oxide under acidic conditions. The substituted rutile materials are highly active, but not as durable or selective towards the oxygen evolution reaction as the iridate materials. In situ studies of catalyst layers using X-ray absorption fine structure spectroscopy at the Ir LIII, Ru K, Rh K and Sb K-edges show that both iridium and ruthenium participate in redox chemistry at oxygen evolution conditions, however antimony and rhodium are redox inactive.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry