Improved Toluene Response of Mixed-Potential Type YSZ-Based Gas Sensors Using CeO2-Added Au Electrodes

Abstract

YSZ-based potentiometric sensors employing thin CeO2-added Au sensing electrodes (SEs) were fabricated by using a spin-coating method, and effects of the CeO2 addition on their toluene sensing properties were examined. The sensor response (ΔE) to 50 ppm toluene of the sensors using 8 wt% CeO2-added Au SEs tended to decrease with an increase in the number of spin-coating cycles (x: 5, 10, 15; i.e., the thickness of SEs). However, ΔE of the sensors using 24 wt% CeO2-added Au SEs showed an opposite behavior to what was observed for 8 wt% CeO2-added Au SEs. The sensor using the thickest SE (x: 15) showed the largest ΔE (e.g., ca. 212 mV for 50 ppm toluene) at 450 °C among all the sensors examined and responded even to 0.5 ppm toluene. ΔE exponentially increased with an increase in logarithmic toluene concentration in the range higher than 10 ppm. These obtained results indicate that electrochemical toluene oxidation proceeds at triple-phase boundaries in SE with gas (Au/CeO2/gas) in addition to conventional TPBs (SE/YSZ/gas) in addition to the conventional interface at SE/YSZ/gas, because CeO2 is a mixed ionic and electronic conductor. The electrochemical reactions at TPBs of the sensor were also discussed on the basis of the I-E characteristics and the electrochemical impedance properties.