Symmetrical potentiometric gas sensors in which a potential difference is generated between identical metal electrodes by a single reaction but involving different activities of the same chemical species at each of the electrodes are well known. Cells of this sort with stabilized zirconia electrolytes are widely used to monitor the oxygen partial pressure in vehicle exhaust systems. When the oxygen partial pressures over the two electrodes of such a cell are equal then there is no potential difference between them. However, at temperatures lower than is normal for zirconia oxygen cells the potentials of electrodes of two different materials on a zirconia electrolyte in a common atmosphere are, in general, not equal. The potential difference arising in such a system is extremely sensitive to the presence of reducing gases (carbon monoxide, hydrogen, methane etc.) in a backwood atm~ph~e of air. Such a system can be used to monitor the presence of reducing gases in air. The measured signal arises principally from a difference in mixed potential between the electrodes but additional contributions to the change in potential resulting from a change in atmospheric ~ompo~tion could arise from surface states and from ~~ovolta~es.
The nse of a solid electrolyte cell in which one electrode was held in a reference atmosphere inside a tube with one closed end has enabled contributions to the gas response of the external electrode to be studied in isolation. In particular the apparatus allows us to show that the con~butions from the two electrodes are additive in the potential response of a single gas space device. The atmosphere-dependent modulation of potential in such devices is robust and reproducible, though temperature dependent. Since the mixed potential depends on the electrocatalytic activity of the electrode material there are some prospects for selective response between more and less reactive gases. Thus the variation of response with temperature for methane is quite different from those for carbon monoxide and hydrogen.