Low temperature response of the reaction-bonded zirconia oxygen sensor

Abstract

When operating the reaction-bonded zirconia sensor as an electrolytic cell, in the temperature range 250–500°C, a relationship is shown to exist between oxygen partial pressure at the external platinum electrode and the cell overpotential occurring as a result of an applied cathodic potential of between 0.1 and 4 V. To investigate the nature of this relationship, the variation in sensor resistance polarization Rp and cathodic current flow I with oxygen partial pressure is measured over the range of pO2 of 1 to 10−3 atm. The magnitude of current flowing through the sensor is related to a number of variables - I α pO2, T, (VB-η), 1/Rp, where pO2 is oxygen partial pressure at the cathode, T is temperature, VB is applied cathodic potential, and η is sensor overpotential. All response is recorded as a change from a “reference condition”, applying when anode and cathode are both at air atmosphere; this allows compensation to be made for polarization effects normally occurring when operating the sensor as an electrolytic cell. It is shown that there is a linear relationship between the cell overpotential η and oxygen partial pressure, and that the applied potential VB will determine both the magnitude of η and the electrode reaction rates of the sensor. In this mode of operation, the sensitivity of the electrode reaction to oxygen partial pressure is shown to be increased dramatically, such that usable response can be achieved well below the normal minimum operating temperature for zirconia sensors.