Solid-state cells with buffer electrodes for the measurement of thermodynamic properties of IrO2, CaIrO3, Ca2IrO4, and Ca4IrO6

The standard Gibbs energies of formation of IrO2, CaIrO3, Ca2IrO4 and Ca4Ir O6 have been measured in the temperature range 940-1350 K using solid-state cells with (Y2O3) ZrO2 as the electrolyte and pure oxygen gas at a pressur e of 0.1 MPa as the reference electrode. For the design of appropriate work ing electrodes, phase relations in the ternary system Ca-Ir-O were investig ated at 1300 K. The only stable oxide detected along the binary Ir-O was Ir O2. Three ternary oxides, CaIrO3, Ca2Ir4, and Ca4IrO6, compositions of whic h fall on the join CaO-IrO2, were found to be stable. Each of the oxides co existed with pure metal Ir. Therefore, five working electrodes were prepare d, consisting of mixtures of Ir + IrO2, Ir + IrO2 + CaIrO3, Ir + CaIrO3 + C a(2)IrO4, Ir + Ca2IrO4 + Ca4IrO6, and Ir + Ca4IrO6 + CaO. These mixtures un ambiguously define unique oxygen chemical potentials under isothermal and i sobaric conditions. A novel apparatus, incorporating a buffer electrode bet ween reference and working electrodes to absorb the electrochemical flux of oxygen through the solid electrolyte, was used for measurement. The buffer electrode prevented polarization of the measuring electrode and ensured ac curate data. The standard Gibbs energies of formation of the compounds, obt ained from the emf of the cells, can be represented by the equations IrO2 Delta(f)G degrees/J mol(-1) = -239, 230 + 172.19 T (+/- 240) CaIrO3 Delta(f(ox))G degrees/J mol(-1) = -29, 460 + 1.13 T (+/- 380) Ca2IrO4 Delta(f(ox))G degrees/J mol(-1) = -35, 145 + 0.81 T (+/- 410) Ca4IrO6 Delta(f(ox))G degrees/J mol(-1) = -37, 400 - 2.01 T (+/- 630) where Delta(f(ox))G degrees represents the standard Gibbs energy of formati on of the ternary compound from its component binary oxides CaO and IrO2. B ased on the thermodynamic information, chemical potential diagrams for the system Ca-Ir-O were developed. (C) 1999 The Electrochemical Society. S0013- 3651(98)07-104-3. All rights reserved.