CO/CO2 ELECTROCHEMISTRY ON ZIRCONIA ELECTROLYTE WITH PLATINUM-ELECTRODES IN RELATION TO AMPEROMETRIC OXYGEN SENSORS

An amperometric zirconia oxygen sensor operated in the flue of a gas-burning combustion system showed a current linearly related to oxygen partial pressure under excess-air conditions (diffusion-controlled) and a current of the same sign but sensitive to sensor operating temperature (kinetically-controlled) for fuel-rich operation. Hence the sensor did not unambiguously distinguish the two sides of stoichiometry. Electrochemical investigations were carried out in CO/CO2/N2 and CO2/O2/N2 mixtures to help resolve this problem. Electrode conductance values in CO/CO2/N2 Mixtures were sometimes higher when approached from the anodic compared with the cathodic direction suggesting electrode blocking by a reduced and non-reversibly adsorbed species (CO or C). An abrupt decrease of more than an order of magnitude in conductance was observed as the temperature was raised through a critical temperature (near 700-degrees-C). This was thought to be associated with the change in stability of CO at that temperature via the disproportionation reaction (2CO reversible CO2 + C). Results indicated that the problem of distinguishing the two sides of stoichiometry could be ameliorated by constructing an amperometric sensor having a high ratio of cathode to anode 3-phase boundary lengths and operating at relatively low temperatures.