ABSTRACT
Electrochemical gas sensors based on ceramic materials with high ionic conductivity have been technologically improved over the last twenty years and have been successfully used in many practical applications [1, 2]. A detailed mathematical model of the transport of both physical and electrochemical processes on the surface and within the sensing electrode (SE) as well as on the triple-phase boundary (TPB) YSZ-SE-gas can be a powerful tool for further development of these sensors. The activity in the field of computer-aided optimum design in engineering of the electrochemical gas sensors has also been increasing steadily over the last decade. A vast range of models exists today, varying in complexity and in the number of assumptions employed. There is no doubt that the considerable progress in computational modeling of the YSZ-based gas sensors over the last two decades has led to an improved understanding of the relevant physical, electrical, and chemical phenomena. However, the emphasis in a majority of the models has been either on the transport processes or on the electrochemical processes. Sometimes, the lack of complete understanding of the complexity of various reactions occurring on and within the SE and on the TPB has been substituted by the oversimplified sensing mechanism. A clear example of this approach is the zirconia-based mixed-potentialtype gas sensors. These sensors with the oxide SE and Pt reference electrode (RE) have been developed for nitrogen oxide (NO
), carbon monoxide (CO), and hydrocarbon (C
H
) detection at high temperatures of 500-900°C [3-28]. The oversimplified mixed-potential sensing mechanism has often been used in publications in order to explain the sensor behavior [11, 14, 18]. The domination of the mixedpotential theory has led to a situation where the potentially interesting results or phenomena obtained during experiments have not been reported simply because they could not be explained by the widely accepted mixed-potential theory. Furthermore, some of the results, which have recently been published, completely contradict the mixed-potential theory [29], showing that the sensor’s output to both hydrocarbons CH
and C
H
changes polarity from negative to positive for the NiO-SEs sintered at high temperatures of 1300°C and 1400°C. Fortunately,
another gas-sensing mechanism, “differential electrode equilibria” [27, 28], has also been proposed to explain the NO
sensitivity that is caused not only by the electrochemical reactions, but also by the different electrocatalytic activity and/or sorption-desorption behavior of two electrodes. Although the proposed mechanism has enhanced our knowledge in relation to the NO
sensing, it does not explain the complexity of the electrochemical and physic-chemical processes on and within the SE. Both molecular and dissociative adsorption of gaseous NO
, O
, and H
O are observed on many oxide and transitional metal SEs. As a consequence of dissociative adsorption, a variety of surface species such as (NO)
, N
O, and N and O adatoms have been found on the surfaces under different reaction conditions [30]. Apart from the competition between molecular and dissociative adsorption, the situation becomes even more complex when the surface topology changes during adsorption at high temperatures in excess of O
. Consequently, the most important developments in the improvement of the mathematical models of the zirconia-based gas sensors must be based on understanding the mechanism of detailed electrochemical reactions and by accounting for the complex heat-masstransfer processes occurring at the microscale level.
