INVESTIGATIONS OF ELECTRODES FOR OXYGEN SENSORS BASED ON LANTHANUM TRIFLUORIDE AS SOLID ELECTROLYTE

Electrodes of low-temperature oxygen sensors based on LaF3 as solid electrolyte are studied in relation to contact properties and electrode reaction mechanisms. The experimental techniques used are electrochemical impedance spectroscopy, measurements of electromotive forces (e.m.f.) and XPS and UPS photoelectron spectroscopies. Criteria for good reference electrodes are derived from results of impedance measurements on solid and liquid reference contacts. Solid reference electrodes with Ag/AgF show low ohmic resistance, but are unstable at elevated temperatures, whereas Sn/SnF2 reference electrodes show good thermal stability, but very high impedance. From XPS, UPS, work function and impedance measurements we derive a model of the thermodynamics and the microscopic reaction mechanisms of gaseous oxygen and water molecules at porous Pt electrodes on LaF3. Partly reduced oxygen is formed at the LaF3 surface by cathode reduction or treatment with oxygen and water at elevated temperatures and is stabilized by coadsorbed water as hydrogen peroxide. © 1990.