Potentiostated, three-electrode, solid polymer electrolyte (SPE) gas sensor having highly invariant background current characteristics with temperature during zero-air operation

#The instant invention relates to an electrochemical gas or vapor sensor and, more particularly, to an electrochemical sensor of the SPE type which is very stable with temperature at zero-air operation.

#In a contemporaneously filed application, Ser. No. 773,136, (52-EE-0-248) in the name of A. B. LaConti, et al. entitled

#Self-Humidifying, Potentiostated, Three-Electrode, Solid Polymer Electrolyte (SPE) Gas Sensor

#The novel features which are believed to be characteristic of this invention are set forth in the appended claims. The invention itself, however, both as to organization and mode of operation, together with further objectives and advantages thereof, may be best understood by reference to the following description taken in connection with the accompanying drawings in which:

#FIG. 1 is an exploded view of a gas sensor cell according to the instant invention;

#FIG. 2 is a partially broken away view of the solid polymer electrolyte membrane of FIG. 1;

#Operation of the thermally stable, electrically biased, potentiostated, three electrode, electrochemical gas sensor is based on the oxidation or reduction of the constituent to be detected at the catalytic sensing electrode. The sensing electrode is maintained at a potential to produce rapid oxidation or reduction of the gaseous constituent to be detected as, for example, in the case of carbon monoxide oxidation or carbon monoxide to carbon dioxide. The sensing electrode is also biased to maintain the potential at or above the rest potential of an electrode for oxygen or air so that oxidation or a reduction of air has minimal effect on the output of the cell. This may be achieved by utilizing the reference electrode at the rest potential for oxygen and maintaining the sensing electrode at a higher potential than the reference electrode by a fixed amount. The sensing electrode potential must, however, be maintained below that potential at one at which water is dissociated to produce hydrogen and oxygen. For example, in the case of carbon monoxide, the electrode potential for the CO.sub.2 /CO redox couple is -0.12 volts with reference to platinum/hydrogen reference electrode. By maintaining the potential of the sensing electrode in the range of 1.0 to 1.3 volts, there is rapid and almost immediate oxidation of the carbon monoxide reaching the sensing electrode in accordance with the following reactions at the sensing and counter electrodes. ##EQU1##

#It may be seen from the above reactions that as carbon monoxide is oxidized to carbon dioxide, electrons are released which flow in the external circuit and hydrogen ions are transported through the electrolyte (along with some water molecules) to the counter electrode and are reduced there to form molecular hydrogen or water. The current flowing in the external circuit as a result of this rapid oxidation of carbon monoxide is thus directly proportional to the carbon monoxide concentration.

#In the case of CO sensing and with the particular catalyst used here, and presently to be described, by limiting the sensing electrode to potential to 1.3 volts, the potential at the sensing electrode is not sufficiently anodic to oxidize water and introduce errors due to current produced by the oxidation of the O.sub.2,H.sup.+ /H.sub.2 O couple. That is, the theoretical oxygen/water redox couple is 1.23 volts. However, due to the high overvoltage at the electrode the oxidation of water takes place at some voltage greater than 1.23 volts. For the platinum-5% iridium catalyst used in the instant sensor, there is minimal oxidation of water at 1.3 volts, thereby ensuring that current flow in the sensing cell is due exclusively to the oxidation of the incoming gaseous constituent, such as carbon monoxide. It should also be appreciated that the precise voltage potential at the sensing electrode, reference electrodes, and the range of voltages will differ for differing gaseous constituents to be detected, depending on what the redox for differing gaseous constituents to be detected, depending on what the redox couple for the particular gaseous constituent is to oxidize and reduce that couple, and the range of overvoltages for the particular catalytic electrode.