Platinum Metals Rev., 1977, 21, (2), 58
Exhaust Gas Sensors Aid Emission Control
Zirconia Devices Utilise Platinum Electrodes
The purification of automobile exhaust gases by platinum group catalysts can be most consistently achieved when the engine is operated with an exactly stoichiometric air/fuel ratio. One way of achieving this state is by monitoring the oxygen content of the exhaust gas and using the information to control the input of air to the engine. This requires an instrument which can reliably detect the variations in oxygen concentration.
Exhaust sensors consisting of a ceramic tube of stabilised zirconia closed at one end and having porous platinum electrodes on both the inner and the outer surfaces have been developed for this purpose. The sensor is inserted in the exhaust system so that the exhaust gases flow over the outer platinum anode while the platinum cathode on the inner surface is open to atmosphere. As the sensor is heated by the engine exhaust the ceramic becomes conducting to oxygen ions and, as the partial pressure of the oxygen on the two sides of the device is different, an electrical potential is generated between the two electrodes. A change in voltage occurs whenever the composition of the exhaust changes, for example from ‘rich’ to ‘lean’ or vice versa. This signal is a measure of the air/fuel ratio and can be used, via a closed loop circuit, to control any departure from stoichiometry.
While the sensor voltage can be calculated from the appropriate thermodynamic relationship, actual sensor behaviour can differ from this ideal, and such departures are detrimental to the performance of the systems they control. In a recent article W. J. Fleming of General Motors Corporation, Research Laboratories (J. Electrochem. Soc., 1977, 124, (1), 21–28) describes work to derive a physical model of a non-ideal sensor. The physical processes involved in the function of zirconia exhaust gas sensors are examined and a theory to account for the departure from ideal behaviour is presented.