Insight into the Effect of the Continuous Testing and Aging on the so2 Sensing Characteristics of a YSZ (Yttria-stabilized Zirconia)-Based Sensor Utilizing ZnGa2O4 and Pt Electrodes

Abstract

IntroductionSO2 concentration detection in harsh environment is still a challenge because the present technology is based on large-scale instrument, which is expensive and can’t monitor in real time. Recently, gas sensor with low cost and the ability of in-situ measurement has been used to SO2 detection. Due to high sensing performance, reliable stability and acceptable humidity resistance, the mixed potential sensor based on YSZ was expected to achieve the real-time detection in harsh environment [1]. Method Fabrication and measurement of the gas sensor The sensor was fabricated utilizing 8mol% Y2O3-doped YSZ plate (2 mm×2 mm square, 0.3 mm thick, provided by Anpeisheng Corp. China). A point-shaped and a narrow stripe-shaped Pt electrode (PE) were formed on two ends of the YSZ plate using a commercial Pt paste (Sino-platinum Metals Co. Ltd.), and sintered at 950°C. The paste which was mixed by a minimum quantity of deionized water and the sensing materials ZnGa2O4. Next, the resultant paste was applied on the point-shade Pt to form stripe-shaped ZnGa2O4 electrode (ZE). Afterward, the device was annealed at 800°C to make a good contact between the sensing electrode and electrolyte. A Pt heater and a linear DC Power Supply were used to provide required heat to regulate the operating temperature of the sensor. Sulfuration process In order to further investigate the sensing mechanism of the sensor, we added a sulfuration experiment. The sulfuation process is as following: The sensor was put into a 1L chamber with a mixture of air and 100 ppm SO2 for 24 hrs, the temperature of the sensor maintain at 650°C during this process. When time is up, put the sensor into a 1L chamber with fresh air and begin to measure the sensing performance of the sensor after sulfuration. Results and Conclusions YSZ-based mixed potential sensor with ZnGa2O4 and Pt electrodes (ZE and PE) was developed for the SO2 detection. The effects of long-term aging and continuous test on various properties of the electrode materials had been investigated and characterized. Some interesting phenomenon were found that when the sensor exposed to the relative high concentration SO2 with air, the response direction was opposite to that exposing to low concentration SO2 as shown in Fig. 1. Moreover, with the last of the testing time, the responses of the sensor to all concentration SO2 were opposite to that in the original state. Hence, giving a reasonable explanation is one of the most important purposes in this paper. We have proved that the PtS was produced during the testing and aging process. According to the Y. shimizu’s work [2], most of metal mono-sulfides performed a high electrochemical catalytic activity to SO2. Hence, we speculated that the mono-sulfides PtS also has a high electrochemical catalytic activity to SO2. In this case, when measuring lower concentration SO2, the diffusion rate is a slow value. Due to the Pt’s high catalytic activity to SO2, SO2 react with Pt electrode and changed to PtS, which adhere to the Pt electrode surface. The process was clearly exhibited in the bottom half of Fig. 1. This process sharply decreased the SO2 concentration reaching TPB of PE, and then caused the ΔVPE approaching 0. On the other hand, ZnGa2O4 electrode had a response to SO2 and according to the mixed potential mechanism, the ΔVZE should be a negative value. However when measuring higher SO2 concentration, Pt has changed to PtS, which decreased the contact area, and then decreased the catalytic activity to SO2. Because of the much higher electrochemical catalytic activity of PtS at PE to SO2 than that of ZnGa2O4, the response at PE was much higher than that at ZE. Hence, the reversed response characteristics occurred as shown in the second response and recovery transient of Fig. 1. Furthermore, with continuous testing and aging, more and more sulfur element was deposited to the Pt electrode and the critical concentration deceased, finally, all of the ΔV were changed to the positive values as shown in the third of Fig. 1. Moreover, the sensor after sulfuration process (the detail of the sulfuration process was given in the last part) also performed similar sensing properties to the sensor with continuous testing and aging process, which indicated that the produced PtS should be the reason for that the sensor performed reversed sensing performances. In addition, the sensor after sulfuration can detect 0.05-500 ppm SO2 with the sensitivity being 5 mV/decade to 0.05-1 ppm and 41 mV/decade to 1-500 ppm. The sensor also had a reliable stability during the continuous measurement.