A Three Electrode Mixed Potential Sensor for Gas Detection and Discrimination

Abstract

Mixed potential solid state electrochemical sensors potentially serve as devices for detection of pollutants from industrial and automotive exhaust and can screen dangerous explosive chemicals in security sensitive areas. In these sensors the difference in catalytic activity between two electrode materials produces a voltage from the difference in the mixed potential that is established at steady state. By integrating multiple electrodes on a single substrate, each pair tuned to be sensitive to different gases, a device can be produced which is selectively sensitive to a variety of gases. The three-electrode sensors were fabricated by ESL ElectroScience with a high temperature co-firing technique on an insulator-YSZ-insulator substrate and Pt, La0.8Sr0.2CrO3 (LSCO), and Au0.5Pd0.5 electrodes were applied by screen printing. A porous YSZ layer over the electrodes serves as the electrolyte, and a length of Pt is printed on the back to serve as an integrated heater. The heating is applied by a control board which imposes a constant resistance, maintaining the temperature during the testing at various flow rates. The sensors were tested at 450-500oC in a 10% O2 atmosphere to different test gases which include NOx, CO, C3H8, (Figure 1) and mixtures of these gases. The Au/Pd+Pt pair is found to be most sensitive to NOx and CO, the LSCO+Pt pair is most sensitive to NOx and C3H8, and the Au/Pd+Pt pair is sensitive only to CO. Against 50-200 ppm backgrounds of NOx and C3H8, the introduction of 100 ppm CO to the gas stream yields a 30 mV response on the Au/Pd+Pt and Au/Pd+LSCO pairs, but produces less than 10 mV shift on the LSCO+Pt pair. Once sets of data consisting of voltage responses to blends of gases were collected, we successfully trained artificial neural networks to associate voltage responses with both binary gas mixture ratios and absolute concentrations of gases, as well as identify which mixtures of gases were present. Figure 1. Response of the three-electrode sensor at 450oC to different gases, where a preferential selectivity towards CO is seen in Au/Pd+Pt or Au/Pd+LSCO pairs and a selectivity towards C3H8 is seen in LSCO+Pt. Figure 1