Objectif
After having worked on understanding the operation, we attempted to apply the existing models in the literature with a view to finding a relationship between the electrical current and voltage parameters and the physico-chemical parameters of the electrochemical cells.
To do this, we studied in the main two measuring methods used in laboratories, namely: the application of a current pulse and the monitoring of the potential, and the application of a voltage pulse, or an AC voltage, and the monitoring of the current. We then tried to apply them to the existing electrochemical cells.
The main questions we asked ourselves were these:
- Can ageing of the cells by dehydration be measured?
- Is discrimination of the gases possible?
To create a current pulse, we transmit gas of constant and known content at a moment in time to = 0, and we observe the modification of the voltage once the current has stabilised. This method enabled use to study the ageing of the cells by dehydration, and to see whether the type of gas has any influence on the voltage response.
It seems that fairly simple processing of the voltage of the auxiliary electrode could make it possible to identify the gas present. The ageing, on the other hand, is more delicate to follow; ageing is noted over the first few tens of hours, but nothing is noticed, with due allowance for the accuracies, in the long term. It would be necessary to carry out an actual on-site study, with data recording, in order to be able to draw a conclusion.
The tests in which a voltage pulse is applied have not enabled worthwhile results to be obtained because of the much too high double-layer capacitance. We know, however, that under equilibrium conditions this method makes it possible to improve the selectivity; we have demonstrated this on the CO/H2 four-electrode cell.
The tests in which an AC voltage is applied also did not provide any worthwhile practical results. They confirm that the model of the electrochemical cell is very complex, and simply does not enable resistive and capacitive elements to be used.
In conclusion, we think that this study has been very useful in demonstrating that the use of electrochemical cells could be more reliable if the signal of the auxiliary electrode were tracked. To interpret this tracking, a long learning process is needed. For cost reasons, the most sensible solution therefore would be to record the data of the cells at actual sites; to do this, modification of the appliances used is essential. The more judicious solution therefore would be to collaborate with an industrial manufacturer and a user.
As for the techniques of modifying the voltage at the reference electrode, it is not conceivable that these techniques can be applied on site without a more advanced laboratory study since the electronic assemblies used so far do not make it possible eliminate the double layer capacitance loading time.
Electrochemical detectors are being increasingly used in mines for the detection of the contents of toxic gases, particularly carbon monoxide, or for monitoring the oxygen deficiency.
The appliances designed hitherto do not have all the necessary qualities. For example, they are neither sufficiently reliable nor sufficiently accurate for the monitoring of slow temperature rises in coal mass by measurement of CO, or for inerting monitoring by measurement of O2... They suffer from a lack of selectivity which makes the measurements much less reliable than those made by analysers. However, the cost of analysers and their delicate maintenance place them at a big disadvantage; users are turning more and more to electrochemical detectors.
There is therefore one major problem: how to increase the reliability of these appliances?
Several improvements are desirable:
- obtaining a facility for self-diagnosis of ageing.
- obtaining at least partial selectivity,
- obtaining a short response time.
There were three main lines of research:
- physical improvement of the electrochemical cells, vis-a-vis the problems encountered during their use,
- the use of signal processing techniques, including matrix or neuronal techniques, linked to a sensor or a set of sensors,
- the use of more elaborate measurement techniques, by modification of laboratory electrochemical techniques.
The present study has been centred on the third line of approach: adaptability of laboratory electrochemical techniques to gas detectors.
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60550 Verneuil-en-Halatte
France
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