Oxygen and carbon monoxide role on the electrical response of a non-Nernstian potentiometric gas sensor; proposition of a model
Introduction
Previous works have established that a potentiometric gas sensor may be developed using a solid electrolyte associated with two different electrodes located in the same gas mixture [1], [2], [3], [4]. Experimental results obtained with pellets of sintered β-alumina powder associated with two metallic electrodes (one in platinum, the other in gold) have revealed a large and complex influence of temperature and oxygen pressure on the output voltage at low oxygen pressure [5]. The aim of this previous work was to model the oxygen role in the output voltage delivered by the device at different temperatures. The model was based first on the competitive adsorption of two charged oxygen species on the device; one of the oxygen species should be characterised by an endothermic formation process. Then, such a charge species combined to a capacitive effect occurring at the metal–solid electrolyte interface were regarded as responsible for initiating the electrostatic potential developed on each electrode.
This mechanism gives rise to a mathematical solution driving an accurate simulation of the results obtained versus the oxygen pressure and the temperature. However, several aspects such as the existence of endothermal oxygen species, the charge and the exact location of the adsorbed species remain an open issue. Recent works were focused on the study of the oxygen role in the output voltage delivered by the device at different temperatures. A calorimetric study [6] has evidenced the formation of an endothermal oxygen species adsorbed on the β-alumina surface. Surface potential measurements [7] revealed the existence of at least two different oxygen species on the device: the first one, located on the solid electrolyte surface, is neutral whereas a charged one seems to be produced at the solid electrolyte/metallic electrode interface. On the other hand, the capacitive model was validated by studies about the influence of the nature and the size of the metallic electrodes on the sensor’s response versus oxygen pressure and temperature [8].
More recently, our device has been tested and evaluated in the presence of oxidising and reducing gases such as carbon monoxide and nitrogen dioxide. The sensor performs sufficiently well to be used in automotive applications. In the framework of European contracts [9], [10] including car manufacturers as partners, we have developed gas sensor prototypes produced by thick film technology [11]. These prototypes were first tested under laboratory conditions and we mainly directed our investigations to the role of oxygen and carbon monoxide on the device response.
The first part of the present paper describes the main particularities of the model said to account for the electrical responses of the device to oxygen pressure and temperature. This model is based on the previous one [5] but, in light of the results of the recent works, it describes more accurately the electrochemical mechanisms yielding the electrical response. Simulations carried out were fitted on the responses of the sensors made through thick film technology, verifying the validity of the model on such a device.
The second part of the paper concerns the study of the sensor’s response to the action of carbon monoxide. A kinetic model based on the consumption of the weakly chemisorbed oxygen species by carbon monoxide is proposed to account for the complexity of the sensor’s response. Simulations will then be compared to the experimental results observed under carbon monoxide as a function of both temperature and oxygen pressures.
Section snippets
Experimental device
The sensor prototypes described in this paper result from the collaboration between the different Econox I project’s partners [9], [10]. The sensing device is composed of a solid electrolyte (Na-β-alumina) associated with two metal electrodes, one in platinum the other in gold (Fig. 1).
For industrial applications (automotive exhaust systems), it was decided to produce sensors through thick film technology [11], [12]. A particular ink developed by the Department of Materials Science and Chemical
Experimental set-up
A tubular furnace was used for heating the sensor instead of the sensor’s built-in heating element in order to obtain the best experimental conditions possible as regards temperature monitoring and control, service temperature homogeneity on the test device and near environment.
Two types of experiment were conducted:
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First, measurements made with oxygen only were used to assess the thermodynamic constants particular to the new device. Experiments were conducted under equilibrium conditions and
Results of oxygen experiments
Since the device used for the experiments was made using a new manufacturing process, the electrical responses of the screen-printed sensors were compared to the sintered devices ones. The potential difference ΔV read between the platinum electrode and the gold electrode on the screen printed sensors as a function of oxygen pressure at various temperatures on the one hand (Fig. 3a), and as a function of temperature under various oxygen pressures on the other hand (Fig. 3b) are shown. Generally
Results of carbon mono oxide experiments
Fig. 5 shows the influence of the various parameters which play a role in the system reveals its complexity. The influence is evidenced through the results—oxygen pressure (Fig. 5a) and temperature (Fig. 5b) versus carbon monoxide concentration. In order to focus on the CO effects, the sensor’s response R to CO action is shown on the next graph (Fig. 6). R is defined as the deviation between the recorded value and the value obtained for a null CO partial pressure.As for
Modelisation of the process
The modelling of sensor’s response in the presence of CO with variable oxygen content is paramount for the automobile application considered. Then, a kinetic model compatible with the previous thermodynamic model had to be developed. The starting hypothesis was the following: when carbon monoxide is present in the gas mixture, oxidation to carbon dioxide will occur preferably with the weakly-bounded oxygen species which is considered as being responsible for initiating the electrostatic
Simulations
A computerised simulation method able to validate the model proposed has therefore been chosen. Here, four parameters (K40, ΔH4)Pt and (K40, ΔH4)Au had to be adjusted in expression (2′).
Obviously, the whole experimental results obtained versus gases partial pressure and temperature have to be simulated by expression (2′). As previously reported, these results are very complex and the main difficulty is relative to the simulation of the response’s inversion observed versus temperature and oxygen
Conclusion
Performances of a gas sensor made of a solid electrolyte fitted with two metallic electrodes, namely gold and platinum entirely produced through thick film technology were studied in this work. Electrical responses of these sensors were compared to those obtained on the previous devices made from sintered β-alumina pellets for both oxygen pressure and temperature variations. Very close results were observed, allowing us to validate on such a new type of sensors a model previously proposed to
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