Solid-state cells with buffer electrodes for accurate thermodynamic measurements: system NdIrO

Abstract

A new design for the solid-state cell incorporating a buffer electrode for high-temperature thermodynamic measurements is presented. The function of the buffer electrode, placed between the reference and working electrodes, is to absorb the electrochemical flux of the mobile species through the solid electrolyte caused by trace electronic conductivity. The buffer electrode prevents polarization of the measuring electrode and ensures accurate data. The application of this novel design and its advantages are demonstrated by measurement of the standard Gibbs energies of formation of Nd 6Ir 2O 13 (low-temperature form) and Nd 2Ir 2O 7 in the temperature range from 975 to 1450 K. Yttria-stabilized zirconia is used as the solid 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 NdIrO were investigated at 1350 K. The two ternary oxides, Nd 6Ir 2O 13 and Nd 2Ir 2O 7, compositions of which fall on the join Nd 2O 3IrO 2, were found to coexist with pure metal Ir. Therefore, two working electrodes were prepared consisting of mixtures of Ir+Nd 2O 3+Nd 6Ir 2O 13 and Ir+Nd 6Ir 2O 13+ Nd 2Ir 2O 7. These mixtures unambiguously define unique oxygen chemical potentials under isothermal and isobaric conditions. The standard Gibbs energies of formation (Δ G° f (ox)) of the compounds from their component binary oxides Nd 2O 3 and IrO 2, obtained from the emf of the cells, can be represented by the equations: Nd 6 Ir 2 O 13: ΔG° f (ox) / J mol −1 = −115 890+7.67 T/ K ( ±2640) Nd 2 Ir 2 O 7: ΔG° f (ox) / J mol −1 = −87 690+6.23 T/ K ( ±1460) Based on the thermodynamic information, chemical potential diagrams for the system NdIrO are developed.