Investigations of gaseous electrochemical reactions on zirconia electrolytes using amperometric sensors.

Sensors incorporating zirconia electrolytes have been widely used for monitoring oxygen concentration and the air-to-fuel ratio of combustion systems. The aim of this work was to investigate the extension of this technology to other gases and to gas mixtures.
Initial work was done on single zirconia. discs with porous metal electrodes on each face. Platinum, silver and gold electrodes were tested in controlled
Atmospheres at temperatures between 300*C and 850*C. It was shown, as expected, that in 02/N2 mixtures electrode activity/conductance decreased as oxygen concentration and temperature were reduced. In CO/CO2/N, 2 atmospheres reproducibility was poor. Interestingly, an increase in electrode conductance of 1-2 orders of magnitude was observed as the temperature was reduced through around 700*C which does not appear to have previously been reported. Is is thought to be
related to the corrresponding change in the thermodynamic stability of CO at this temperature, ie 2CO ; ==t C1 + C02.
Zirconia pump-gauges with small enclosed volume (< Imm3) and laser drilled diffusion holes were constructed with the novel addition of reference electrodes. These were operated with current-interruption circuitry and rapid subsequent data aquisition. This enabled, for the first time, the overvoltage due to charge transfer on the internal pumping electrode to be isolated: interesting behaviour was revealed. The overvoltage increased as oxygen-containing gases were, reduced and then. in the case of NO, decreased as the reduction reached completion. The gauge EMF remained remarkably stable while NO was being reduced. Reduction of S02 was shown to generate electronic conductivity in the yttria-stabilised zirconia.
A potentially important result was obtained when operating a sensor in CO/CO2/02/N2 mixtures where there was sufficient 02 present to oxidise all the CO present to C02. With the application of zero pumping current the gauge EMF showed a substantial, non-zero, value. It was suspected that a mixed potential developed on the external electrode while the diffusion hole prevented its formation
on the internal electrode.
Based upon'the results obtained, two sensor designs are proposed. The first is a lean-bum combustion sensor eliminating the possibility of an ambiguous response in rich- and lean-bum conditions. By reducing the activity of the external pumping electrode while maintaining a highly active internal pumping electrode the response in rich-bum conditions would be insignificant while the response in leanbum conditions would be maintained.
The second design concerns multi-gas sensing; it is based upon previous designs but the need for minimising electrochemical leakage was pinpointed by this work. Because overvoltages on the internal electrodes were shown to reach high values electrode activitity should be high and a gauge cell should be incorporated to provide information about the oxygen partial pressure within the sensor. If operated by sweeping the voltage applied it was shown that because of the slow electrode
response the sweep rate would need to be less than 0.2mV/s. Consequently, it is proposed that an array of sensors operating at a range of fixed pump voltages would greatly reduce the response time.
A computer-controlled apparatus was built with which to perform the experimental work. The apparatus controlled the furnace temperature, gas mixing apparatus and a potentiolstat/gaivanostat. The latter was purpose built so that all current and voltage ranges could be selected by the computer and current interruption performed automatically. Using software written in Turbo Pascal the apparatus was capable of performing a wide range of tests, unattended over periods of months.

A thesis submitted to Middlesex University in partial fityllment of the requiremeilts for the degree of Doctor of Philosophy.
November 1993