Protection for Double-cell Type Zirconia Oxygen Sensors from Deterioration by means of Auxiliary Pumping

Abstract

A double-cell type zirconia oxygen sensor which consists of two solid electrolyte cells, a pumping cell and a sensing cell, has a capability to measure an air-to-fuel ratio widely extended from rich to lean. When the sensors operated in air, solid electrolyte of some of the sensors deteriorated. The sensors which can not operate in air have few applications, because air is the only reference gas with constant oxygen concentration easily obtained everywhere.
To clarify the cause of the deterioration, polarographic characteristics of the oxygen sensors are studied carefully in air. The study shows the difference between deteriorated sensors and durable sensors. Electromotive force of sensing cells increase more slowly for deteriorated sensors when pumping current are increased gradually. The slow increase implies that oxygen partial pressure are extremely low locally at the cathode of the pumping cell.
To protect the sensors from the deterioration, methods to increase oxygen partial pressure at cathode are studied. Auxiliary pumping from the sensing electrode is found to be effective, and the optimum auxiliary pumping method is to leak pumping current from the pumping cell. Oxygen sensors with auxiliary pumping were not deteriorated in a continuous operating test in air.