An advanced design of the solid-state cell for accurate thermodynamic measurements on ternary oxides: system Sm–Pd–O

Abstract

An advanced design of the solid-state cell incorporating a buffer electrode has been developed for high temperature thermodynamic measurements. The function of the buffer electrode, placed between reference and working electrodes, was to absorb the electrochemical flux of the mobile species through the solid electrolyte caused by trace electronic conductivity. The buffer electrode prevented polarization of the measuring electrode and ensured accurate data. The application of the novel design and its advantages have been demonstrated by measuring the standard Gibbs energies of formation of ternary oxides of the system Sm–Pd–O. Yttria-stabilized zirconia was 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 Sm–Pd–O were investigated at 1273 K. The two ternary oxides, Sm₄PdO₇ and Sm₂Pd₂O₅, compositions of which fall on the Sm₂O₃–PdO join, were found to coexist with pure metal Pd. The thermodynamic properties of the ternary oxides were measured using three-phase electrodes in the temperature range 950–1425 K. During electrochemical measurements a third ternary oxide, Sm₂PdO₄, was found to be stable at low temperature. The standard Gibbs energies of formation (Δ_f(ox)G^o) of the compounds from their component binary oxides Sm₂O₃ and PdO, can be represented by the equations: Sm₄PdO₇: Δ_f(ox)G^o (J mol⁻¹)=−34,220+0.84T(K) (±280); Sm₂PdO₄: Δ_f(ox)G^o (J mol⁻¹)=−33,350+2.49T(K) (±230); Sm₂Pd₂O₅: Δ_f(ox)G^o (J mol⁻¹)=−59,955+1.80T(K) (±320). Based on the thermodynamic information, three-dimensional P–T–C and chemical potential diagrams for the system Sm–Pd–O were developed.