Enhanced sensing response of solid-electrolyte gas sensors to toluene: Role of composite Au/metal oxide sensing electrode

In this study, YSZ-based potentiometric gas sensors of the type Au*/YSZ/Pt, where Au* represents either dense (d-Au), porous Au (p-Au) or porous Au/4MOx (4 wt% CeO2, Gd2O3, La2O3, Pr6O11, Sm2O3, Pr0.1Ce0.9O2 or Ce0.8Zr0.2O2) composite sensor electrodes (SEs), were fabricated, and their toluene-sensing properties examined over the temperature range of 400-500 degrees C. The roles of microstructure of the SEs, and improvements in sensing properties due to MO additions were examined with respect to their impact on catalytically enhanced toluene oxidation as well as electrochemical reactions at the SE/electrolyte interface. Au(4 MO)/Pt sensors showed larger response to toluene than d-Au/Pt, with the Au(4CeO(2))/Pt sensor showing the largest response among all of the Au(4 MO)/Pt sensors. While the response of Au(4CeO(2))/Pt was smaller than that of p-Au/Pt, its recovery time was much shorter. These observations were shown to result from Au(4CeO(2)) exhibiting the largest catalytic activity of toluene oxidation among all of the oxides tested, leading to enhanced toluene oxidization during its diffusion through the SE, as well as the removal of residual toluene at TPBs, during the recovery process. This resulted in decreased response, but at the same time, faster recovery. The results demonstrated that both high electrochemical activity of toluene oxidation and low electrochemical activity of oxygen reduction are indispensable for achieving large sensor response. The Au(4CeO2)/Pt sensor also showed a nearly linear response vs logarithm of toluene concentration (0.5-50 ppm), with strong response down even to 0.5 ppm toluene (ca. +55 mV) in dry air. (C) 2017 Elsevier B.V. All rights reserved.