Impedancemetric technique for NOx sensing using a YSZ-based electrochemical cell

An impedancemetric technique for NOx sensing using a yttria-stabilized zirconia (YSZ) electrochemical cell is reported. The cell consists of a dense YSZ substrate disk with two YSZ/metal-oxide electrodes deposited on the same side. The cell is completely exposed to the test gas (no air reference). The NO, and 02 response of the cell were evaluated during constant-frequency operation at frequencies in the range from 1 to 1000 Hz. At 10 Hz, the NOx response (as measured by phase angle shift) is shown to be linear with concentration over the range from 8-50 ppm, with comparable response to both NO and NO2. A method of operation is described which enables compensation for the O-2 response at oxygen concentrations greater than approximately 4%. This mode of operation allows the sensor to provide sub-10 ppm detection of NOx irrespective of the O-2 concentration. The sensor exhibits good stability during continuous operation for more than 150 hr. It was observed that the O-2 response of the cell may be too slow to be of practical use, taking several minutes to equilibrate after changing the concentration by a few percent. However, data will be presented which demonstrate that this response is related to the metal oxide used for the electrode, and that more rapid response times can be achieved by modification of the electrode material.