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An on-board particulate matter (PM) sensor, consisting of a gas-permeable electrochemical cell with a porous yttria-stabilized zirconia solid oxide electrolyte, was developed to assist the on-board diagnostics (OBD) system of a vehicle. Exhaust is pumped from the anode side to the cathode side and PM deposited on the anode is instantly oxidized by the catalytic effects of the metal component of the electrode at temperatures higher than 350°C. The PM oxidation reaction occurs at the three-phase boundary between the anode, electrolyte and gas phase, and causes a slight change in the bulk average oxygen concentration, which produces electrochemical polarization by the difference in oxygen partial pressures between the anode and cathode. The developed PM sensor has a detection limit of 2 mg/m₃, at which level will enable PM detection in the OBD system according to the EURO VI regulation.

To comply with increasingly stringent emission regulations, there is a high demand for A/F sensors with shorter light-off time and higher accuracy. Improvement of sensor accuracy requires reduction in sensor output shift induced by unburned hydrogen in the exhaust gas. The sensor output shift can effectively be reduced by placing a catalyst at the gas inlet of the sensing element. However, this involves the challenging technical task of developing a catalyst material with high durability in the sensor operating environment. By using a theoretical analysis technique, we studied a new catalyst material with superior catalytic performance and high durability, and have successfully improved A/F sensor accuracy, as reported in the following paper.