The Research On Novel Sensing Electrodes For YSZ-based Potentiometric NO Sensors

NO and NO₂?referred as NO_x?are the main pollution gases in the air pollutants which mostly come from the vehicle exhaust emissions and other combustion processes,and cause acid rain,photochemical smog and many health problems.In these combustion processes,NO is the major component of NO_x at high temperatures,the percent of which is usually about 90%-95%.Developing high-performance sensing devices is in urgent demand to monitor NO_x gases in the emission process,and has drawn much attention from scientific workers whole the world.The amperometric NO_x sensor is the only commercial sensor,but some problems remains to be solved,such as complex structure,selectivity against O₂ and short service life,etc.Therefore,much attention has been widely paid to solid-state potentiometric sensors based on YSZ because of their high stability,cross-selectivity,simplified structure and low costs in recent years.At present,the study of such gas sensors is mainly focused on finding suitable sensing electrode materials and modifying them to improve their sensitivity.In this work,La₂CuO₄ was selected to be sensing electrode material due to its excellent sensing performance to NO.Ni was added into La₂CuO₄ via ball-milling method,and the properties of the electrode were enhanced significantly.At meanwhile,the insensitivity to the O₂ of the electrode is unchanged.In the course of the study,it was found that the new phase La₈Cu_5.6Ni_2.4O₂₀ produced by the introduction of Ni was the main reason for the improvement of the sensing performance.Consequently,La₈Cu_5.6Ni_2.4O₂₀ was synthesized specially by sol-gel method for further research.The main conclusions of this study are as follows.The pure La₂CuO₄ powder was prepared by citric acid-gel combustion method,and different Ni content were introduced into La₂CuO₄ by ball milling method.The effects of Ni addition on the microstructure was studied.The new phase La₈Cu_5.6Ni_2.4O₂₀ appeared in the electrode due to the Ni addition.When the Ni content reached 10wt%,the NiO phase began to appear.And with the increase of Ni content,the NiO content increased,while the La₈Cu_5.6Ni_2.4O₂₀ content decreased gradually.Besides,when the sintering temperature was elevated from 1000? to 1100?,the content of La₂CuO₄ decreased in the electrode with10 wt.%Ni,while the La₈Cu_5.6Ni_2.4O₂₀ content increased and became the main phase of the electrode.On the other hand,the introduction of YSZ inevitably leads to the formation of La₂Zr₂O₇ which is an insulation pyrochlorite phase and harmful for NO response.With the change of Ni addition,the relative content of the phase changed accordingly.The sensing properties of electrodes with different Ni addition were studied.With the increase of Ni content,the response of the electrode to NO showed a tendency to increase first and then decrease.With the increase of Ni content,the responses of the electrodes to NO increased first and then decreased.Particularly,the electrode with 10 wt.%Ni added showed the largest response to NO.The response value of the sensor to 200 ppm NO increased from 32.2 mV to 42.3 mV compared with the sensor without Ni addition,and the sensitivity raised from 19.7 mV/decade to 33.7 mV/decade.In addition,the positive function of added Ni is more dominant when the sintering temperature was elevated to1100? from 1000?.The response to NO of the electrode with 10 wt.%Ni addition was almost more than doubled.Furthermore,the response transients were almost reproducible in the successive runs.And the addition of Ni did not alter the insensitivity to O₂ of the La₂CuO₄ electrode.The La₈Cu_5.6Ni_2.4O₂₀ powder was synthesized by sol-gel method,and the microstructures of the electrodes were studied by changing sintering temperature.The change of the sintering temperature had little effects on the phase of the electrode.However,with the increase of the sintering temperature,the particle size of the electrode increased and also the porosity improved.The sensing performances of La₈Cu_5.6Ni_2.4O₂₀ sensing electrodes were studied,including the sensitivity,selectivity,repeatability and studies of polarization curve.In the sintering temperature range of 1000?-1300?,the electrode sintered at 1200? exhibited the best sensitivity to NO.The response value to 200 ppm NO is more than 60mV at 450? with the response time of about 20s and high repeatability.In addition,the selectivity of the electrode for NO was also greatly improved.The results of the polarization curves showed that the catalytic activity of the electrode increased first and then decreased with the increase of the sintering temperature,which is corresponding to the consequence of response.