YSZ-Based Mixed Potential Type NH3 Sensor Attached with Ag Doped FeVO4 Sensing Electrode

Abstract

Introduction In an automobile selective catalytic reduction (SCR) system, the monitoring of NH3 concentration is of vital importance. NH3 detection in such harsh environment of high temperature and humidity is an almost impossible task for most testing methods. Nevertheless, YSZ-based mixed potential type gas sensor is an incomparable choice. Moreover, the development and utilization of sensing electrode materials is an effective method to develop high performance YSZ-based mixed potential type ammonia gas sensor [1]. FeVO4, a n-type semiconductor with direct band gap, is considered as a promising sensing electrode material due to its special three-dimensional layered structure [2]. Ag was reported to have excellent catalytic activity. Through Ag collaborating with the sensing electrode, the response of the NH3 sensor could be enhanced accompanied by the improved sensitivity and detection limit [3]. In this study, we prepared FeVO4 through the sol-gel method. Then, Ag doped FeVO4 in different molar ratio (1.5, 2.5, 3.5 and 4.5 mol.%) was synthesized through the sodium borohydride reduce method and used as sensing electrode material of the YSZ-based mixed potential type NH3 gas sensor. The effects of different molar ratios Ag doped on the NH3 sensing performance were studied. The sensor attached with 3.5 mol.% Ag doped FeVO4 displays high response to target NH3 gas, short response and recovery time, satisfying humidity resistance and good repeatability, indicating its potential for in-situ ammonia monitoring for automotive applications [4]. Method Fabrication and measurement of the gas sensor The sensor was fabricated on a YSZ plate (8 mol. % Y2O3 doped, 2 mm × 2mmsquare, 0.2 mm thickness), provided by Anpeisheng Corp. China. A point-shaped electrode and a narrow stripe-shaped electrode were formed on the two sides using a commercial Pt paste (Sino-platinum Metals Co. Ltd.), and sintered at 1000°C. The sensing material was mixed with a minimum quantity of deionized water and the resultant paste was applied on the point-shaped Pt to form stripe-shaped SE and then sintered at 800 °C for 2 h. The testing method of the sensor was as showed in our previous work [5]. Results and Conclusions A series of sensing tests showed the sensor attached with 3.5 mol.% Ag doped FeVO4-SE had the highest response to NH3, as Fig.1a shows. The reason that the sensor attached with 3.5 mol.% Ag doped FeVO4-SE have the highest sensing performance is due to best electrochemical catalytic activity to NH3 of the sensing electrode materials. The best electrochemical catalytic activity of the material will be proved by the polarization curves and complex impedance test in the future. The NH3 sensing performance of the sensor attached with 3.5 mol.% Ag doped FeVO4 was studied at different operating temperatures and the optimal operating temperature was determined at 550 °C, as Fig.1b shows. The sensor could even detect as low as 200 ppb NH3 with the response of -3.2 mV, as shown in Fig.1 c. Also, the sensor had a good reproducibility as shown in Fig.1 d. Moreover, the sensor displayed good selectivity to NH3 when compared to other interfering gases such as NO2, SO2, C2H2 and CO, as shown in Fig.1 e. For accurate and rapid detection of NH3, the response and recovery time is a vital indicator. In the present research, the typical 90% response time to 50 ppm NH3 of the sensor was less than 3 s as shown in Fig.1 f. In the actual working condition, the exhaust gas is always of high humidity. Therefore, the environmental humidity’s influence on the NH3 sensor was studied and the result in F ig.1g and h showed that the sensor had an acceptable humidity stability. Based on the above results, the sensor we fabricated is prospective in automobile selective catalytic reduction (SCR) exhaust gas treatment system.