Electrochemical gas sensor

Details Electrochemical gas sensor Electrochemical gas sensor

2001-11-15

2004-06-08

Noguerola, Alex (Department: 1753)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S001001

Reexamination Certificate

active

06746587

ABSTRACT:

FIELD OF THE INVENTION
The present invention concerns an electrochemical gas sensor for sensing a target gas, the sensor comprising a working electrode and a counter electrode, and particularly relates to a sensor in which a reagent gas needs to be supplied to or removed from the counter electrode. Applications of the sensor include the detection of toxic or other gases in an atmosphere, particularly the detection of carbon monoxide in air, where it is important that oxygen is supplied to the counter electrode.
BACKGROUND OF THE INVENTION
In recent years the need for detection and measurement of toxic, noxious and irritating gases in the environment has increased as public awareness of the potential dangers and corporate awareness of potential accidents and death by gas poisoning has increased.
Numerous sensing systems have been developed to measure such gaseous components. Although direct chemical methods exist, other analytical techniques such as gas chromatography, infrared absorption and molecular fluorescence have all been used to quantify toxic or noxious gas concentrations in the atmosphere.
However, chemical methods of gas detection continue to dominate. These chemical methods depend on measuring a change in a property of a sensing material. Conductivity, dielectric constant, mass change or semiconductor behaviour have all been used to monitor toxic gas concentration, either qualitatively or quantitatively. Colourimetric detection tubes (see K Grosskop “Angew.Chemie”, vol 63, pages 306-308, 1962) have been used for several decades. However, the above techniques suffer from either a lack of discrimination of the gas species or from excessive power demands in the case of solid state semiconductor devices which must operate at typically 500° C.
The most popular gas detection method is wet electrochemistry using an electrochemical gas sensor in the form of an electrochemical cell comprising a working electrode and a counter electrode. A target gas or vapour reacts electrochemically with the working electrode of an electrochemical gas sensor. The result of the reaction at this working electrode can be determined by measuring either a voltage change (potentiometry) or a current generated by the reaction (amperometry).
In electrochemical gas sensors, the target gas or vapour reacts at the working electrode of the electrochemical cell and the current generated by this electrochemical reaction is balanced by a reaction at the counter electrode, with an intervening body of electrolyte between the electrodes providing electrolytic continuity. The electrochemical cell must be part of a complete electrical circuit, so that the current generated at the working electrode must be conducted away from the cell to a circuit to measure this current. In addition, the electrochemical cell will usually include a third electrode (constituting a reference electrode) which sets a characteristic potential of the electrochemical cell, to assist in regulating the electrochemical reaction at the working electrode. In amperometry an external potentiostat circuit measures the reference electrode potential relative to the working electrode and supplies adequate current into the counter electrode, forcing a reaction at the counter electrode to balance the working electrode reaction. The potentiostat circuit not only provides the current required for the counter electrode but also ensures that the working electrode is operating at the same potential as the reference electrode, or if a bias voltage is inserted, then at a controlled bias potential relative to the reference electrode.
Like the working electrode, the counter and reference electrodes also require conductors either to allow electrical monitoring of the reference electrode potential or to complete the electric circuit to the counter electrode. As noted above, at the counter electrode a reaction occurs that is opposite to the reaction at the working electrode, i.e. if the working electrode is oxidising a gas then the counter electrode will be reducing another species. This reaction at the counter electrode will generate a by-product which may be a gas, a solution species or precipitate an ion. The creation or consumption of chemical components at both the working and counter electrodes requires that these required or resultant components are transported either to or away from the electrochemically surface active.
One major concern with electrochemical amperometric gas sensors is ensuring that the electrolyte maintains continuous contact with the electrodes and between the electrodes. This has been done by changing the state of the electrolyte; alternative forms include electrolyte gels and solid state ion conductive membranes. However, the most popular method for transporting electrolyte within electrochemical cells, including fuel cells, batteries and gas sensors, is the use of a wicking or wetting material which immobilises the electrolyte on to the surface of the material. The most popular geometry for wicking or wetting material is fibres, with glass fibre and asbestos traditionally serving these roles. Examples of this technology can be found as early as Billiter who used an asbestos matrix for ionic control in chlorine generating plants in the 1920s. More recently Binder in U.S. Pat. No. 4,036,724 has taught the use of asbestos fibres or aluminum oxide powder to immobilise electrolytes such as perchloric, sulphuric or phosphoric acid. Shaw in U.S. Pat. No. 3,755,125 extended this knowledge to include the use of planar separator discs made from glass fibre with a wick extension that dipped into a liquid electrolyte reservoir. This allowed continuous electrolyte access by the use of separator discs between the electrodes. Separator discs touched at the periphery of their surfaces to allow vertical wicking and four wick extensions into the reservoir allowed excess material to replenish evaporated electrolyte or excess electrolyte in high humidity applications to be stored in the reservoir. Chan et al. (GB 2,094,005) tried to simplify the wicking method, since Shaw used four planar separators, with each displaced 90° relative to the next. Chan et al. employed a hole in the counter electrode with a single annular wick: a simpler design with the added advantage that the wick connecting the reservoir to the working electrode was continuous and did not require pressure between the planar separators to ensure good contact for vertical capillary transport. However, both of these solutions have an excessive number of hydrophilic components to allow adequate electrolyte transport and the single wick in the Chan design is difficult to manufacture and wasteful of electrode material.
Other problems considered in the design of electrochemical gas cells relate not only to the reaction at the working electrode but also to the opposite reaction at the counter electrode and the stabilising reaction at the reference electrode.
The reaction at the reference electrode must be stable with time but does not require generation of a significant current. Therefore the reference electrode can be small and the reaction rates can be slow, so long as the potential resulting from the various reactions is constant. Therefore the reference electrode does not require a large flux of chemicals to create an electrochemically stable environment. The counter electrode, however, must produce a current equal and opposite in sign to the current created by the working electrode. The use of a potentiostatic circuit helps since a potentiostatic circuit is designed specifically to maintain the working electrode at a fixed potential and allow the counter electrode to vary its potential until it finds an electrochemically active species to reduce or oxidise.
When oxidation occurs at the working electrode, the most common reaction at the counter electrode is oxygen reduction but an alternative reaction is hydrogen gas generation. These two reactions are shown below.
O
2
+ 4H
+
+ 4e
2H
2
O
E
0
= 1.227 V
2H
+
+ 2e
−

H
2
E
0
= 0.0 V
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