Electrode Reaction and Response Characteristics of Zirconia Oxygen Gas Sensors

Abstract

In order to find out a guiding principle for the improvement of the response of zirconia oxygen gas sensors below 500°C, measurements were made on the steady-state (Figs.3 and 7)and transient-state EMF (Fig.4 and Table 1), electrode interface conductance, σ_Ee (Figs.1and 8), and electrode capacitance, C_E (Figs.6 and 9), of oxygen concentration cells using HF treated and non-treated stabilized zirconia with Pt electrodes. With non-treated zirconia, the EMF of the cell abnormally deviated from the Nernst relation when temperature was abruptly changed (Fig.4). The response was improved remarkably by the use of HF treated zirconia. The EMF of this cell obeyed the Nernst equation as low as 240°C. The results were discussed based on the theory proposed before, and the followings were concluded. ( 1 ) Improvement of the response and prevention of abnormal EMF can be mad e by increasing a, and reducing CE (Eqs.3 and 4). ( 2 ) The GE is determined by the length of zirconia/Pt/O₂(g) triple phase boundary. The alternation of neither the chemical composition of zirconia surface nor the thickness of the Pt paste electrode affects the value of σ_E. ( 3 ) The C_E is determined by the amount of adsorbed oxygen on the zirconia surface in contact w ith the closed pores left at the zirconia/Pt interface (Eq.5. ). It can be reduced either by making the interface closely contacted or by changing the chemical composition of zirconia surface with, for example, HF treatment. ( 4 ) The improvement by HF treatment is due to the decrease in C_E. ( 5 ) It is suggested that the chemical modification of zirconia surface with a material which is less adsorptive for oxygen would result in the improvement of the response.