ATMOSPHERE-DEPENDENT POTENTIALS AT OXIDE INTERFACES

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 eac h of the electrodes are well known. Cells of this sort with stabilized zirconia electrolytes are widely used to monitor the oxygen partial p ressure in vehicle exhaust systems. When the oxygen partial pressures over the two electrodes of such a cell are equal then there is no pote ntial difference between them. However, at temperatures lower than is normal for zirconia oxygen cells the potentials of electrodes of two d ifferent materials on a zirconia electrolyte in a common atmosphere ar e, in general, not equal. The potential difference arising in such a s ystem is extremely sensitive to the presence of reducing gases (carbon monoxide, hydrogen, methane etc.) in a background atmosphere 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 mixe d potential between the electrodes but additional contributions to the change in potential resulting from a change in atmospheric compositio n could arise from surface states and from thermovoltages. The use of a solid electrolyte cell in which one electrode was held in a referenc e atmosphere inside a tube with one closed end has enabled contributio ns to the gas response of the external electrode to be studied in isol ation. In particular the apparatus allows us to show that the contribu tions from the two electrodes are additive in the potential response o f a single gas space device. The atmosphere-dependent modulation of po tential 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 select ive 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.