Preparation And Performance Of NiO Electrode For Zirconia-Based NO_x Sensor

NO_x(the mixture gas of NO2and NO) is the chief air pollutant which leads tophotochemical smog and acid rain. Most of NO_xcomes from automobile exhaust. With theincreasing demand for environmental protection and improving fuel efficiency, variouscountries and regions are re-enacted the vehicle exhaust emission regulations, and theregulation for NO_xemissions has become stricter. Therefore, great efforts have been takento develop NO_xsensor. Compared with other type of electrochemical sensors, thezirconia-based NO_xsensors seem to have advantages due to good mechanical, thermal andchemical stability as well as the opportunity to work at high temperatures in a harshenvironment. So it has become a research hotspot.Using NiO, Ni and YSZ powders as raw materials, NO_xsensor NiO electrode slurrieswere fabricated. The effect of electrode paste formulations on the slurry rheology, and theeffect of process routes and the ratios of materials on the cofiring matching ability ofelectrode and electrolyte were investigated. Sensing element was prepared using NiO(YSZ)as sensing electrode, Pt as reference electrode and YSZ as solid electrolyte. Thesensitivities under different temperatures of the elements were carried out, and theelectrochemical impedance spectroscopy was used to analyze the sensing mechanism.Special attentions were paid on the effect of the electrode paste formulation onslurries rheological properties. Results showed that the pseudoplastie behavior wasappeared when the solid content was70wt.%. With the increase of the YSZ additionamount, the viscosities of the slurries increased. And they decreased with the temperatureincreased. And the thixotropy of Ni slurries was poorer than that of NiO slurries.The effects of process routes and the ratios of raw materials on the cofiring matchingability were analyzed. The results revealed that the NiO electrodes made by NiO pasteusing ordinary sintering at1400℃were compact and contained a lot of cracks. Bycontrast, the NiO electrodes prepared by Ni paste adopting reaction sintering were porous and no cracks were observed. The addition of YSZ had a strong particle refining effect onNiO grains, and enhanced interface adhesion between NiO electrode and YSZ electrolyteand increased the amount and length of three phase boundary.The sensitivities of NiO(YSZ)/YSZ/Pt sensing elements were discussed. The resultsillustrated that the EMF response of electrode prepared by NiO slurry was small since theamounts of TPB were limited, while the response signal of electrode prepared by Ni slurrywas larger because more TPBs were generated by reaction sintering. The largest signalwas achieved by adding10vol.%YSZ into NiO slurry.The electrochemical impedance spectroscopies under different atmospheres ofsensing elements were studied. Equivalent circuits were obtained by Zview software. Theresults showed that the equivalent circuits of NiO(YSZ) elctrode systems wasRs((WQ)Rp), and the equivalent circuits of Ni(YSZ) elctrode systems was Rs(QRp). Rprepresented polarization resistance. The sensitivities using Rp characterizing wereconsistent with the sensitivity tests.