Improvement in Response/Recovery Characteristics of Mixed-Potential-Type Zirconia-Based CO Sensor Using ZnCr2O4 Added with Au Particles-Sensing Electrode

Abstract

In order to fulfill strict environmental regulations regarding air pollutions such as carbon monoxide (CO), nitrogen oxides (NOx), and sulfur dioxide (SO2), high performance gas sensors have been widely investigated for the last decade. Among air pollutions, CO is well known as colorless and toxic pollutant. Currently, the sensitive and selective detection of CO at high operating temperatures remains a challenging task. Recently, we have reported that the CO response of a mixed-potential-type yttria-stabilized zirconia (YSZ)-based sensor using a ZnCr2O4-sensing electrode (SE) was augmented by the dispersion of nano-sized Au (nano-Au) particles into the SE layer. However, the 90% response and recovery times against 800 ppm CO at 550°C for the sensor using ZnCr2O4 (+nano-Au)-SE were more than 40 and 160 s, respectively. From a practical point of view, it is very important to improve such poor response and recovery characteristics to CO and to achieve fast response and recovery behavior. In this study, therefore, to improve the response and recovery characteristics against CO for the sensor using ZnCr2O4 (+ nano-Au)-SE, Au particles with three different sizes (nano-Au, subμ-Au, and μ-Au) were separately added to ZnCr2O4-SE, and then the CO sensing performances of the resulting sensors were examined. The SE materials were prepared by mixing commercial ZnCr2O4 powder (Kojundo Chemical Lab. Co., Ltd., Japan) and one of the following Au particles (nano-Au: 1-4 nm; subμ-Au: 0.3-0.6 μm; μ-Au: 2-5 μm) in an agate mortar with ethanol, which was subsequently dried to evaporate the dispersant solution. The sensors were assembled in a tubular configuration using a commercial hemispherically-terminated YSZ tube (8 mol.% Y2O3-doped ZrO2, Nikkato Corp., Japan). A YSZ layer was firstly formed on the outer surface of the YSZ tube using a YSZ paste, which was made by mixing a commercial YSZ powder (Tosoh Corp., Japan) and α-terpineol, to improve the mechanical and electrochemical stability of the interface between the SE layer and the YSZ tube. After drying the YSZ layer, the SE material pastes, which were also made by mixing with α-terpineol, were applied on the YSZ layer to form SE. A commercial Pt paste was applied on the inner surface of the closed-end YSZ tube to form a Pt/air-reference electrode (RE). Finally, the painted and assembled YSZ tube was sintered at 1100°C for 2 h in air to fabricate the final sensing device. Gas sensing measurements were carried out in a conventional gas-flow apparatus equipped with a furnace operating at 550°C. The SE was exposed to the humidified base gas (synthetic air + 5 vol.% H2O) or sample gases with a flow rate of 100 cm3/min. The Pt/air-RE was always exposed to an atmospheric air. The difference in electromotive force (emf) between SE and RE was measured as the sensing signal by means of a digital electrometer. The effect of the addition of Au particles with different sizes into the ZnCr2O4−SE on the CO sensing characteristics of the mixed-potential-type YSZ-based sensors was investigated by measuring the emf response against the inner Pt/air−RE after exposure to 20 - 800 ppm CO. The response transients indicate that the sensitivity toward CO decreases with increasing Au particle size. Among the three SEs tested, the ZnCr2O4(+subμ-Au)-SE gave relatively high sensitivity and good response/recovery against CO. Although CO sensitivity of the ZnCr2O4(+subμ-Au)-SE became lower than that of the ZnCr2O4(+nano-Au)-SE, the 90% recovery time to 800 ppm CO was improved and it was approximately 80 s. Additionally, to examine the influence of Au additive on the sensitivity and response/recovery behavior to CO, the content of subμ-Au additive to ZnCr2O4 was varied from 1 to 10 wt.%. As a result, the CO sensitivity increased with increasing content of subμ-Au, as expected. Examinations of the detailed CO sensing characteristics for the present sensor are now in progress.