Integrated impedance based hydrocarbon gas sensors with Na‐zeolite/Cr₂O₃ thin film interfaces: Electrochemical mechanism of the impedance changes

Abstract

This study aims to clarify the mechanism of a hydrocarbon gas sensor effect, which involves impedance changes of a Na+ ion conducting Pt‐doped zeolite layer in contact with chromia (Cr2O3) films as electrodes. Based on our previous studies on sensing characteristics and model experiments with asymmetrical cells, a coherent picture of the sensing mechanism is derived: The impedance change is attributed to the process of Na+ ion insertion from the zeolite into the chromia film. The impedance of this interface process is a function of the electrical potential at the interface. Combined with the observation that gas sorption in the zeolite micropores causes changes of the electrochemical potential at the zeolite surface, these two effects explain the observed sensor effect. The sensor response as predicted from this theory agrees well with the experimental data of the original sensor. Implications for the development of integrated devices are discussed.

Exploitation of Unique Properties of Zeolites in the Development of Gas Sensors

“…The other group of molecules that has been extensively studied are hydrocarbons [ 58 – 60 ]. Impedance spectra of a device made by coating Cr 2 O 3 on top of an ion-conducting Pt-doped ZSM-5 exhibited several semicircles, with a hydrocarbon dependent semicircle in the medium frequency range [ 58 ] (though later studies from the same group showed that it is the low frequency part that exhibits the increase in the presence of the hydrocarbons) [ 59 , 61 ]. Measurements at a fixed frequency in the hydrocarbon sensitive region exhibited high sensitivity to different hydrocarbons and minimal interference to CO and H 2 , but with interference to NH 3 .…”

“…An alternative explanation has also been proposed to explain the impedance changes of the zeolite/chromia device in the presence of hydrocarbons [ 61 ]. Sorption of gas within the zeolite pores is proposed to alter the electrochemical potential at the zeolite surface.…”

“…However, if the formation of sodium chromate at the zeolite/chromia interface can be proven by careful electron microscopy studies, it would lend support to the second mechanism. Also, it is unclear to us how the second mechanism can explain the lack of interferences to other gases that can also adsorb within the zeolite, unless it is the reaction products of the hydrocarbon oxidation that are playing the key role, as has been alluded [ 61 ]. So experiments with water and maybe CO 2 may also be relevant in explaining the sensor mechanism.…”

Planar Zeolite Film-Based Potentiometric Gas Sensors Manufactured by a Combined Thick-Film and Electroplating Technique

“…In the case of the ammonia sensor, it is obvious that the bulk properties change with ammonia adsorption [ 19 ], whereas in some hydrocarbon sensors, electrode/bulk interactions seem to play a major role [ 20 ]. The detailed effect remains a subject of discussion [ 21 , 22 ].…”

“…The origin of the formation of the potential difference U between both electrodes is under study and will explicitly not be discussed here. The interested reader is referred to the literature [ 21 , 22 , 27 ].…”