Solid state cells with buffer electrodes for measurement of chemical potentials and Gibbs energies of formation: system Ca–Rh–O

Abstract

The isothermal section of the phase diagram for the system Ca–Rh–O at 1273 K has been determined by equilibrating 12 compositions in the ternary and identifying phases present in quenched samples by optical microscopy, powder X-ray diffraction (XRD), and energy-dispersive analysis of X rays (EDX). One ternary compound, CaRh₂O₄, was found to be stable. It coexists with CaO and metallic Rh. All the alloy compositions are in equilibrium with CaO. The standard Gibbs energy of formation of CaRh₂O₄ and the oxygen chemical potential corresponding to the three-phase field Rh+CaO+CaRh₂O₄ were determined using novel designs of solid state cells incorporating yttria-stabilized zirconia as the electrolyte: Pt–Rh, CaO+CaRh₂O₄+Rh/(Y₂O₃)ZrO₂/Rh+Rh₂O₃, Pt–Rh,Pt–Rh, CaO+CaRh₂O₄+Rh/(Y₂O₃)ZrO₂/O₂(0.1 MPa),Pt–Rh. Buffer electrodes were introduced between reference and working electrodes to dissipate the electrochemical flux of oxygen through the solid electrolyte caused by the difference in chemical potential of oxygen at the electrodes. Polarization of the electrodes was prevented by the use of buffer electrodes. For the reaction 2Rh+Cao+3/2O₂→CaRh₂O₄, ΔG°=3/2 Δμ°O₂=-493.340+288.67T (285) J/mol. The Gibbs energy of formation of the interoxide compound from the component binary oxides is β-Rh₂O₃+CaO→CaRh₂O₄,ΔG°=−101,670+10.6T (±290) J/mol. Using the Neumann–Kopp rule to estimate the heat capacity of CaRh₂O₄, the standard enthalpy of formation and standard entropy of the compound at 298.15 K are evaluated as −1142.3(±2) kJ/mol and 103.2(±2) J/mol·K, respectively.