POTENTIOMETRY OF THE INTERFACE BETWEEN A SOLID SODIUM-CONDUCTING ELECTROLYTE AND SMCO1-XMXO3 (M = TI, MN) IN THE PRESENCE OF OXYGEN

The potentiometry method is employed to study the behavior of the elec trochemical cell SmCo1-xMxO3\Na+-SE/Na0.65CoO2 in mixtures of molecula r oxygen and argon. The cell contains a sodium-conducting solid electr olyte (Na+-SE) and an oxygen electrode based on a binary Sm-Co oxide d oped by oxides of Ti and Mn. The beta-phase of a Na-Co bronze, Na0.65C oO2, is selected as the reference electrode. With compounds prone to h ydration, such as beta-Al2O3, beta ''-Al2O3, and Na5GdSi4O12, used as Na+-SE, the solid-phase system is reversible towards oxygen at tempera tures exceeding 150 degrees C. The oxygen reduction is shown to procee d at these temperatures as a single-electron process, probably, with t he formation of the superperoxide ion O-2(-). The reversibility by oxy gen can be ensured even at room temperature, provided finely divided p latinum (similar to 5 wt %) is deposited on the oxide electrode surfac e. In the low-temperature region, the oxygen reduction mechanism is de termined by the dopant M in the oxygen electrode composition. With M = Ti, the process involves two electrons and leads to the O-2(2-) ion f ormation, whereas with M = Mn, the process is a four-electron one and yields the OH- ion. The concentration dependence of the cell's emf is described by the Nernst equation up to 80 vol % of oxygen.