CO Sensing Performance of Porous Thick Film Nasicon-Based Solid Electrolyte Gas Sensors

Abstract

We have reported the CO sensing properties of sintered disk-type NASICON(Na3Zr2Si2PO12)-based solid electrolyte gas sensors equipped with a metal oxide (MO)-added Pt sensing electrode (SE, Pt(nMO) (n: MO additive amount in wt%)) and a pristine Pt or another metal oxide (M’O)-added Pt counter electrode (CE, Pt or Pt(nM’O)) on the same side of the disk [1, 2]. The outstanding CO sensing characteristics of these NASICON-based sensors are 1) no reference atmosphere necessary (both SE and CE can be exposed to the same atmosphere, leading to a simple sensor structure), 2) capable of operation at room temperature and also at temperatures below the freezing point, 3) improvement of CO selectivity against H2 in humid environment at room temperature operation, whereas CO response decreased slightly, 4) CO response based on the mixed potential theory, etc. To realize a further simple manufacturing process of sensor elements and also excellent CO sensing properties, the present study was directed to establishing the optimum fabrication conditions of porous thick film NASICON-based solid electrolyte CO gas sensors. Pt paste (Tanaka Corp., TR-7907) mixed with 15 wt% of a metal oxide (Pt(15MO)) and pristine Pt paste were applied in a rectangular shape (1 × 4 mm) with a distance of 4 mm on the same surface of a quadrate alumina substrate (10 × 10 mm), then the same past was used to attach a Pt lead wire to each electrode, followed by drying at 100°C for 10 min in air. Thereafter, NASICON paste was applied on the alumina substrate equipped with a pair of the electrodes and dried at 100°C for 1 h, followed by calcination in the temperature range of 700 to 900°C for 0.5 h in air. The thickness of porous NASICON thick films fabricated was ca. 1 mm. The sensor thus fabricated is denoted as Pt(15MO)/Pt-T, where T represents the calcination temperature. Response properties to 300 ppm CO balanced with dry air of as-fabricated sensors and those subjected to the following aging treatment, heating at 400°C for 1 h in dry air and then exposure to 3,000 ppm CO balanced with air at 400°C for 0.5 h, were measured at 30°C and −10°C. The sensor subjected to the aging treatment is referred to as Pt(15MO)/Pt-Ta. The metal oxides tested were Bi2O3, Cr2O3 and CeO2. The electromotive force (E, mV) of the sensors was measured with a digital electrometer as a sensing signal. The magnitude of CO gas response was defined as a difference in E measured between in 300 ppm CO balanced with air and in base air. Figures 1 and 2 show CO response transients of Pt(15Bi2O3)/Pt-T and Pt(15Bi2O3)/Pt-Ta at 30°C. Even for porous thick film sensors, we could obtain CO response abilities as those observed for sintered disk-type sensors [1, 2]. When Bi2O3 was used as an additive to SE, the aging treatment improved sensing stability, enhanced CO response and shortened the response and recovery time, especially for the sensor calcined at 700°C. CO response properties were also much improved by the aging treatment in the case of the Cr2O3 addition, but were deteriorated in the case of the CeO2 addition. Similar aging effects were also observed at the operation of −10°C, whereas the excellent CO response properties were observed at different calcination temperature values. Another notable feature is the shift in E to a positive direction upon exposure to CO for both the Bi2O3 and Cr2O3 additions, irrespective of the calcination and operating temperatures, but to a negative direction for the Pt(15CeO3)/Pt-T sensors. The detailed CO sensing performance of these sensors will be delivered in my presentation.Reference[1] H. Takeda, T. Ueda, K. Kamada, K. Matsuo, T. Hyodo, Y. Shimizu, CO-sensing properties of a NASICON-based gas sensor attached with Pt mixed with Bi2O3 as a sensing electrode, Electrochimica Acta, 155, 8−15 (2015).[2] T. Ueda, H. Takeda, K. Kamada, T. Hyodo, Y. Shimizu, Enhanced CO response of NASICON-based gas sensors using oxide-added Pt sensing electrode at low temperature operation, Electrochemistry, 85(4), 174−178 (2017). Figure 1