Investigations On Solid Electrolyte Type NO₂and H₂Sensors Using Nano-structured Sensing Materials

The solid electrolyte type gas sensors with simple structure, good selectivity and highsensitivity, especially excellent chemical and physical durability at elevated temperatures,have the widespread prospect of application. It is important to study the design andsynthesis of sensing materials which were the key factors for improving the performanceof the sensors. In the present work, the solid electrolyte type NO₂and H₂sensors werefabricated using nano-structured sensing materials which were prepared in the poroussolid electrolyte layers by impregnating method. The composition and microstructure ofthe sensors were characterized by XRD, SEM and EDX. The sensing performances andmechanisms were investigated by potentiodynamic method, potentiostatic method, ACimpedance spectroscopy and pulse potential method.Nano-structured perovskite-type oxide La_0.75Sr_0.25Cr_0.5Mn_0.5O_3-δ（LSCM）as sensingelectrode was prepared in the porous Ce_0.9Gd_0.1O_1.9（5CGO）layer by impregnating methodusing nitrate salts precursor solution. The LSCM particles in the range of50–150nm werehomogeneously dispersed in the porous CGO layer. With calcining temperature increasing,LSCM particles gradually grew up. An amperometric NO₂sensor using nano-structuredLSCM sensing electrode with CGO electrolyte was fabricated. The sensor showed goodsensitivity to NO₂at400⁶00℃. The response current was linearly related to theconcentration of NO₂. The sensor showed good response–recovery characteristics,reproducibility and stability. Negligible effects of O₂and CO₂on the sensor response wereobserved.The sensing performance and mechanism of the NO₂sensor using nano-structuredLSCM sensing electrode were investigated by AC impedance spectroscopy. The resultsshowed that the impedancemetric NO₂sensor was sensitive to NO₂.With the increase inthe NO₂concentration, the total impedance values （|Z|） linearly decreased. Anequivalent-circuit model analysis for the impedance spectra showed that sensor responseto NO₂was attributed to the processes occurring at the triple-phase boundary. Amperometric NO₂sensors using nano-structured composite sensing electrodes（LSCM/Ag and LSCM/CGO）with CGO electrolyte were fabricated. Compared withsingle LSCM sensing electrode, the sensors with composite sensing electrodes showedextraordinarily higher response currents, sensitivities and response rates. The responsecurrent was linearly related to the concentration of NO₂at500⁷00℃. The sensorsshowed good response–recovery characteristics, stability and anti-interference ability tothe coexistence gases.Novel NO₂sensors were fabricated using nano-structured spinel-type oxides MCr2O4（M=Cu, Ni, Zn） as sensing electrodes prepared in the porous yttria-stabilized zirconia（YSZ） layer by impregnating method. The sensing performances of the NO₂sensors wereinvestigated using different working modes, including amperometric, impedancemetricand pulse potential mode. The sensor showed good sensitivity and response–recoverycharacteristics to NO₂. The response current was linearly related to the concentration ofNO₂. Compared with the potentiostatic mode, the sensors using pulse potential modeshowed extraordinarily higher response current and sensitivity which could reach1×10-3Aand1.5μA ppm-1, respectively. The response time of the sensors was0.1s. A negligibleeffect on the sensor response was observed when O₂concentration varying from0to20vol.%.Amperometric hydrogen sensors using nano-structured sensing electrodes and aproton conductor CaZr_0.9In_0.1O_3-δas electrolyte were fabricated. A bilayer CaZr_0.9In_0.1O_3-δelectrolyte including both a dense layer and a porous layer was prepared by conventionalsolid state reaction method. During the preparation of electrolyte, sintering aid （ZnO） wasintroduced into CaZr_0.9In_0.1O_3-δfor promoting its sintering. CaZr_0.9In_0.1O_3-δwith highdensified structure, good conductivity and excellent chemical stability was prepared aftersintered at1350℃for5h. The nano-structured ZnO and SnO₂particles were in-situprepared in the porous CaZr_0.9In_0.1O_3-δbackbone by impregnating method. The sensorshowed good sensitivities, response–recovery characteristics, reproducibility and stability.