| The increasing atmospheric pollution caused by diesel exhaust, mainly NOx, has resulted in stricter emission regulations in the world, and the Selective Catalyst Reduction (SCR) system was widely used to reduce NOx emission. To maximize the NOx conversion and to avoid NH3 slips in the SCR system, a high-performance onboard NH3 sensor is an inevitable trend. Compared with various NH3 sensors, the YSZ-based potentiometric NH3 sensors based on the non-equilibrium electrochemical principle meet all the requirements: potential of economic cost, mass manufacturability, durability, and hostile high temperature exhaust environment, are considered as the perfect choice. The sensing material with high NH3 adsorptivity and catalytic activity is the key to obtain a high-performance NH3 sensor. As selective catalytic oxidation (SCO) catalysts of NH3, Mg2CuxFe03.5+x mixed metal oxides meet the basic requirement of NH3 sensors for sensing material, show a great research potential and application prospect. In this thesis, the Mg2CuxFe03.5+x mixed metal oxides were first used as sensing material to fabricate YSZ-based potentiometric NH3 sensors, and the NH3 sensitivity and selectivity against various interfering gases were evaluated, and the influence mechanism of the NH3 sensitive performance were systematically studied from several aspects such as:Cu-content, sintering temperature, electrode-thickness and the Pt/YSZ mixing by the polarization curve and electrochemical impedance spectroscopy methods.Firstly, Mg2CuxFe03,5+x (x=0,0.25,0.50,1) mixed metal oxides powders were synthesized by co-precipitation method. The phase characteristic of the Mg2CuxFe03.5+x mixed metal oxides powders were performed by X-ray diffraction (XRD). The Mg2CuxFe03.5+x mixed metal oxides powders consisted of poor-crystalline periclase (MgO) phase and magnesioferrite (MgFe2O4) phase mainly, and there was copper oxide (CuO) phase and a small amount of cuprous oxide (CU2O) phase in the Cu Containing powders, and the content of copper oxide (CuO) phase increases with the increase of Cu-content. The microstructures of the Mg2CuxFeO3.5+x mixed metal oxides powders were analyzed by an Environmental Scanning Electron Microscope (ESEM), these powders were porous spherical particles, larger particles were aggregated by many smaller particles, the size of the larger particles are 1-18 ?m, and the size of the smaller particles are 100-500 nm.Then, the YSZ-based potentiometric NH3 sensors based on Mg2CuxFeO3.5+x electrodes (x= 0,0.25,0.50,1) were designed to evaluate NH3 response performance. The effects of Cu-content on the phase composition, the microstructures, the NH3 sengsing performances and the electrochemically catalytic-activity of Mg2CuxFeO3 5+x electrodes were studied to gain more insight into mechanism that influences the NH3 sensing performances. In the 1150 ? sintered Mg2CuxFe03.5+x electrodes, the magnesium copper oxide (Mgo.78Cu0.22O) phase was generated when x=0.25,0.5, whereas the magnesium dicopper oxide (Cu2MgO3) was generated when x=1. As the Cu-content increased, the electrode grain size increased and the specific surface area (SBET) decreased, the electrochemical catalytic activity increased first and then decreased. The NH3 response of the Mg2CuxFe03.5+x electrodes decreased as the working temperature increased at the range of 350-500 ?, and increased first and then decreased as Cu-content increased. Adding a moderate amount of Cu can significantly improve the NH3 response of the Mg2FeO3.5 electrode, and reducing the sintering temperature can effectively improve the NH3 sensitivity of the Mg2CuFeO4.5 electrode. The Mg2Cuo.25FeO3.75 electrode shows the highest NH3 response (50 ppm NH3,124 mV), the highest NH3 sensitivity (220 mV/decade NH3), the shortest response time (17-30s) and recovery time (19-31 s) at 350 ?. The above results indicate that the Magnesium copper oxide (Mgo.78Cuo.22O) may be a functional phase that improved NH3 sensitivity of Mg2CuxFeO3.5+x electrodes, but the magnesium dicopper oxide (Cu2MgO3) played an opposite role as a suppressive phase.In order to investigate the practical applicability for NH3 sensor, the sensitivity and selectivity against various interfering gases (NH3, O2, NO, NO2, CO, CH4, and C3H8) of the NH3 sensor with the 1150 ? sintered Mg2Cuo.25FeO3.75 electrode were evaluated at 350 ?, and the selectivity mechanism were studied by the polarization curve in charpter 3 emphatically. It can be seen that the NH3 sensor exhibits larger response to 50 ppm NH3 (124 mV) than NO (-4 mV), NO2 (-24 mV), CO (16 mV), CH4 (2 mV) and C3H8 (7 mV). Furthermore, the gas concentration at the range of 50-400 ppm, the NH3 sensor shows the highest NH3 sensitivity (220 mV/decade NH3) than orther interfering gases, and the NH3 sensitivity are stable in different concentration of O2 atmosphere. Polarization curves show that the NH3 anodic polarization current is higher than other gases, and the change of the NH3 anodic polarization current under different concentration of O2 atmosphere is not obvious. Therefore, the NH3 sensor with the 1150 ? sintered Mg2Cu0.25FeO3.75 electrode has excellent NH3 selectivity and good resistance to O2 interference at 350 ?.And then, the Mg2Cu0.25FeO3.75 electrodes were sintered at various temperatures (1000 ?,1100 ?,1200 ?, and 1300 ?) to obtain different microstructures, and the different thickness Mg2Cu0.25FeO3.75 electrodes were prepared by the method of control the screen-printing times. The influences of the microstructures on the NH3 sensing properties and the electrochemically catalytic activity of the Mg2Cu0.25FeO3.75 electrodes were investigated to explain the mechanism that influences the NH3 sensing performances by the the polarization curve and the electrochemical impedance spectroscopy methods. As the sintering temperature increased, all of the electrodes have the same phase composition, the electrode grain size increased and the specific surface area (SBET) decreased. The electrodes sintered at 1200 ? and 1300 ? show sintered three-dimensional network electrode structure, but the electrodes sintered at 1000? and 1100 ? show the unsintered packing structure. The electrochemical catalytic activity of the Mg2Cu0.25FeO3.75 electrodes increased first from 1000 ? to 1200 ? and then decreased at 1300 ?. The NH3 sensing performances of the Mg2Cu0.25FeO3.75 electrodes decreased as the working temperature increased at the range of 400-550 ?, and increased first and then decreased as the sintering temperature increased. The enhanced NH3 sensing performances of the Mg2Cu0.25FeO3.75 electrodes may be attributed to electrodes' larger electrochemically catalytic activity and more quantity of gas-YSZ-Pt triple-phase boundary (TPB) of the sintered three-dimensional network electrode structure with unblocked diffusion channel for NH3. As the electrode thickness increased, the NH3 response and the NH3 sensitivity of the 1200 ? sintered Mg2Cu0.25FeO3.75 electrodes decreased at the range of 400-550 ?, and the response/ recovery time increased. From the electrochemical impedance spectroscopy results, the interface resistance of the gas-YSZ-Pt TPB decreased as the concentration of NH3 increased, and it increased as the electrode thickness increased. The adsorbed NH3 was consumed by the catalytic oxidation reaction during the diffusion process of the thick electrode, and the amount of NH3 reach the gas-YSZ-Pt TPB for the electrochemical reaction was reduced, which then lead to the NH3 sensitive degradation.At the last, this thesis tries to change the TPB amount by adding Pt/YSZ to Mg2Cuo.25FeO3.75 electrode. The influences of the mixing amount of Pt/YSZ on the phase composition, the microstructures, the NH3 sensing properties of the Mg2Cuo.2sFeO3.75 electrode were studied. The influence mechanism of the mixing amount of Pt/YSZ on the NH3 sensitive performances were investigated by the electrochemical impedance spectroscopy method. The mixing amount of Pt/YSZ at range of 0-20 wt.% does not affect the phase composition and the particle size of the Mg2Cu0.25FeO3.75 electrode. Adding Pt/YSZ can increase electrode roughness; but adding 10 wt.% of Pt or 20 wt.% of YSZ, the Pt or YSZ particles will cover the electrode particles. Adding 5 wt.% of Pt or 5,10 wt.% of YSZ can improve the NH3 sensitivity, but adding 10 wt.% of Pt or 20 wt.% of YSZ would reduce the NH3 sensitivity obviously. From the electrochemical impedance spectroscopy results, adding 5 wt.% of Pt can improve the electrochemically catalytic activity of the Mg2Cuo.25FeO3.75 electrodethe, and adding 5,10 wt.% of YSZ can increase the amount of gas-YSZ-Pt TPB, which then lead to the electrochemical reaction enhanced. But too much Pt or YSZ particles would impede function phase (Mg0.78Cuo.22O) particles of the electrode to adsorb NH3, then the amount of NH3 reach the gas-YSZ-Pt TPB for the electrochemical reaction was reduced, and caused the NH3 sensitive degradation... |
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