STEADY-STATE MODEL OF ELECTROCHEMICAL GAS SENSORS WITH MULTIPLE REACTIONS

We present an extension of the first-principles model for the steady s tate response of electrochemical (e.g. zirconia) gas sensors. In earli er work, it was assumed that only one electrochemical reaction couples the electrical and the chemical parts of the sensing system; i.e. in an O-2/CO/CO2 gas mixture, the emf is due to the reaction O-ads + V-0( 2+) (oxide)+ 2e(-) (electrode) <-> O-0 (oxide) that changes the concen tration of charged oxygen vacancies, V-0(2+) (oxide); while COads, ind irectly affects the emf by chemically reacting and changing the concen tration of O-ads, the adsorbed oxygen atoms on the metal electrode. Ho wever, CO may additionally react directly with oxygen in the metal oxi de: COads + O-0 (oxide) <-> CO2ads + V-0(2+) (oxide)+ 2e(-) (electrode ). In this paper, the model is extended to also include this reaction. For certain values of the parameters, it is shown that this additiona l reaction produces new features in the sensor emf versus gas-redox ra tio relationship in good agreement with the experimental results. This generalized model reveals a rich diversity of sensor behavior and pro vides insights that can be useful in developing improved gas sensors. With appropriate modifications, the formalism is readily adaptable to resistive type of gas sensors.