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

Abstract

The standard Gibbs energies of formation of IrO₂, CaIrO₃, Ca₂IrO₄ and Ca₄IrO₆ 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 pressure of 0.1 MPa as the reference electrode. For the design of appropriate working electrodes, phase relations in the ternary system Ca-Ir-O were investigated at 1300 K. The only stable oxide detected along the binary Ir-O was IrO₂. Three ternary oxides. CaIrO₃, Ca₂IrO₄, and Ca₄IrO₆, compositions of which fall on the join CaO-IrO₂, were found to be stable. Each of the oxides coexisted with pure metal Ir. Therefore, five working electrodes were prepared, consisting of mixtures of Ir + IrO₂, Ir + IrO₂ + CaIrO₃, Ir + CaIrO₃ + Ca₂IrO₄, Ir + Ca₂IrO₄ + Ca₄IrO₆, and Ir + Ca₄IrO₆ + CaO. These mixtures unambiguously define unique oxygen chemical potentials under isothermal and isobaric conditions. A novel apparatus, incorporating a buffer electrode between 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 accurate data. The standard Gibbs energies of formation of the compounds, obtained from the emf of the cells, can be represented by the equations IrO₂Δ_fG°/J mol^-1 = -239, 230 + 172.19T (±240) CaIrO₃Δ_f(ox)G°/J mol^-1 = -29, 460 + 1.39T (±380) Ca₂IrO₄Δ_f(ox)G°/J mol^-1 = -35, 145 + 0.81T (±410) Ca₄IrO₆Δ_f(ox)G°/J mol^-1 = -37, 400 + 2.01T (±630) where Δ_f(ox)G° represents the standard Gibbs energy of formation of the ternary compound from its component binary oxides CaO and IrO₂. Based on the thermodynamic information, chemical potential diagrams for the system Ca-Ir-O were developed.