Tin dioxide gas sensors. Part 3.—Solid-state electrochemical investigations of reactions taking place at the oxide surface

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Abstract
The use of standard electrochemical measurements effected through solid oxide electrolytes has proved to be a useful method for observing the surface chemistry of semiconducting oxides in gaseous atmospheres. Open-circuit potential measurements across a stabilized zirconia ceramic with electrodes of tin dioxide and platinum, respectively, yield non-zero values over most of the temperature range between ambient and ca. 400 °C. Such measurements are made without the separation of a reference atmosphere and finite potential differences are attributable to non-equilibrium processes by which differences in surface oxygen potential may arise on the two electrode materials. Open-circuit potentials measured under these circumstances are extremely sensitive to the presence of reducing gases in an air ambient. Current–potential measurements with a relatively small contact area between tin dioxide and the electrolyte allow the derivation of exchance current densities at this interface, and these too are sensitively affected by the presence of reducing gases in air. Measurement of the exchange current density over a range of temperatures has enabled the activation energy for the surface oxygen reaction to be derived (1.4 eV) . This value is in tolerable agreement with the activation with the activation energy obtained from conductivity measurements (1.1 eV) as described in part 1 of this series.

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