Solid state electrochemical characterization of thermodynamic properties of sodium-metal-oxygen systems (2005)

Sodium-metal-oxygen systems (Na-Me-O) being potential electrode materials for potentiometric solid state galvanic cells having very low sodium activities, are usually characterized electrochemically. In this technique the thermodynamic stability data might be erroneous due to the electronic transference through the solid electrolyte especially sodium ion conductors, if the cell is operated under extreme conditions and coexistence of phase mixture can not be checked in situ. In order to solve the problem, phase equilibria comprising compounds of the ternary Na-Me-O systems with Me = Mo, Ti, Nb have been selected. The characterization of the thermodynamic stability of these equilibria has been performed by potentiometric measurements on solid electrolyte galvanic cells of the following types: (I) Pt, O2, CO2 | Na2CO2 (Au) | Na-Me-O (Au) | YSZ | O2, (CO2) Pt (II) Pt, O2, CO2 | Na2CO2 (Au) | NBA (Au) | Na-Me-O (Au) | YSZ | O2, (CO2) Pt (III) Pt, O2,CO2 |Na2CO2(Au) |NBA (Au)|YSZ|NBA (Au)| Na-Me-O(Au)O2,(CO2)Pt The measuring principle of these cells is based on an oxygen concentration chain with yttria stabilized zirconia (YSZ) as oxygen ion conducting solid electrolyte. In cells of type (I) and (II), the oxygen potential, established at the interface YSZ/Na-Me-O and ending up in the cell voltage, provides an information about the activity of Na2O resulting from the Na-Me-O phase mixture while the sodium activity being kept constant. Different levels of the activity have been established by different gas compositions thereby varying the sodium activity over three orders of magnitude. In the cells of type (III) the measuring principle consists in making use of the thermodynamically fixed relationship between the oxygen and the sodium potential. Thus, the measurement of an oxygen potential difference comes down to a comparison of a known sodium chemical potential with the unknown sodium chemical potential of the Na-Me-O phase mixture to be characterized. For estimating the extent of the electronic transference in a usual sodium concentration chain, the following cell was studied: (IV) Pt, O2, CO2 | Na2CO2 (Au) | NBA | Na2MoO4/Na2Mo2O7 (Au) | O2, (CO2), Pt The compounds for the phase mixtures under investigation have been synthe-sized by solid state reaction. The identity of the compounds used, the phase distribution and particularities of the thermal behavior of some of the compounds have been characterized by X-ray diffraction, by chemical analysis, by optical and electron microscopy and by thermal analysis. The studies on the system Na2MoO4/Na2Mo2O7 have been concentrated on the ap-plication of all types of galvanic cells out of the spectrum considered in the present work. The consideration of the inherent particularities and of possible errors of each of these techniques reveals that the application of cell configuration (II) promises to deliver the most reliable information about the difference of the standard Gibbs energies of formation of the molyb-dates under study. The phase equilibrium Na2Ti3O7/Na2Ti6O13 has been studied almost as compre-hensive as the Na-Mo-O system. However, the results obtained from the different cell configurations are more contradictory. The thermodynamic characterization of the phase mixture Na2Ti6O13/TiO2 by means of measurements on cell type (I) and (II) is completely impossible, since in both cases an equilibrium is not established. From these findings the employment of the studied phase mixture as electrode for galvanic cells appears to be quite questionable. All phase mixtures of the type NaaNbbOc/NaxNbyOz investigated by potentiometric measurements on cell configuration (I) have exhibited the thermodynamic be-haviour of a univariant system with a distinct plateau in the Na2O activity. For NaNb3O8/Na2Nb8O21 this has been confirmed by additional measurements on a cell of type (II). Moreover, concerning Na3NbO4/NaNbO3 and NaNbO3/ Na2Nb4O11 data have also been determined by means of cell configuration (III) in order to check the reproducibility of the previous findings. Up to the present, no reliable data on the standard Gibbs energy of formation of any of the single Na-Nb-O phases have been known in the literature. Only an esti-mation exists concerning the thermodynamic stability of Na3NbO4 and NaNbO3. Using this data and based on the experimental data of the present work, values on the standard Gibbs energy of formation of the Na-Nb-O compounds under investigation have been determined for the first time.