EXPERIMENTAL AND THEORETICAL-STUDY OF THE RESPONSE OF ZRO2 OXYGEN SENSORS TO SIMPLE ONE-REDUCING-GAS MIXTURES

The emf of commercial automotive zirconia oxygen sensors exposed to O- 2,/CO/CO2/N-2 or O-2/H-2/H2O/N-2 gas mixtures was measured as a functi on of R' = (2P(O2)/P-CO) or (2P(O2)/P-H2) at several temperatures. A s tep-like transition from high to low emf occurs at the switch-point R' (sO), that is almost independent of temperature. The high emf (at say R' = 0.5) first increases and then decreases with temperature for O-2/ CO/CO2/N-2 gas mixture, whereas it only decreases for O-2/H-2/H2O/N-2. These results may be analyzed using the steady state model of gas sen sors [A.D. Brailsford, M. Yussouff, E.M. Logothetis, M. Shane, Steady- state model of a zirconia oxygen sensor in a simple gas mixture, Senso rs and Actuators, B 24/25 (1995) 362-365]. Two lumped parameters, R'(s ) and xi, are needed in the model to compute the emf. R'(s) represents the theoretical switch-point and xi determines the theoretical high e mf. To obtain good fit with experiment, we choose R'(s) close to the o bserved R'(sO) and then vary xi. The quantity xi depends upon the fund amental processes taking place on the sensor electrode; the temperatur e dependence of its fitted values (and hence the high emf) may be unde rstood in terms of rate constants for adsorption, desorption and react ion. (C) 1997 Elsevier Science S.A.