Modification of NASICON Solid Electrolyte for NO x Measurements

The construction and sensing performance of solid-state potentiometric NOx sensors using a natrium superionic conductor ~NASICON! as solid electrolyte are reported. Dense ceramic pellets with a pure NASICON crystal phase were produced by a solid-state reaction method using a modified procedure. The NASICON surface on the Pt sensing electrode side was modified with a NaNO3 layer as an auxiliary phase using a melting-quenching method. By the same method, the other surface of the NASICON was modified with an AgCl layer and was connected with an Ag wire as a reference electrode. An air/Pt quasi-reference electrode was fabricated by exposing the Pt black attached NASICON surface to the ambient. Both sensor devices using air/Pt and Ag/AgCl reference electrodes were investigated for NOx sensing. The two sensor designs exhibited Nernstian behavior, fast response, stability, and reproducibility. The Ag/AgCl reference electrode provides an all-solid-state design for the sensor device without the need for a reference gas with fixed composition or a hermetic seal separating the reference electrode from the sample gas. A NO x sensing mechanism is discussed. © 2004 The Electrochemical Society. @DOI: 10.1149/1.1647577# All rights reserved. Nitrogen oxides ~primarily NO and NO2 , abbr. NOx) exhausted from high-temperature combustion processes are major air pollutants. Rugged and reliable gas sensors capable of detecting NOx in harsh environments are at present in high demand for pollution monitoring and control. Ceramic electrochemical gas sensors are promising for this purpose due to their good sensitivity and selectivity, fast response, relatively simple structure, and the potential to operate in harsh environments. Both potentiometric and amperometric yttria-stabilized zirconia ~YSZ! solid electrolyte oxygen sensors with reliable sensing performance have been used widely for air/fuel ratio control in internal combustion engines to optimize engine performance and reduce polluting emissions. 1 This success has encouraged research into solid ionic conductor ~SIC! based gas sensors for the detection of many other gases at elevated temperatures. Two distinct approaches for potentiometric gas sensors using SICs have emerged over the last 10 years: mixed-potential gas sensors 2-10 and surface-modified solid electrolyte gas sensors. 11-18 The two types of sensors can be differentiated because the latter one exhibits a thermodynamically controlled equilibrium sensor response while the former exhibits a catalytically or kinetically controlled steady-state sensor response. Both sensor designs are SIC-based potentiometric devices and can be compared to the yttria-stabilized zirconia ~YSZ! oxygen sensor structure in which either the sensing electrode or the surface of the SIC is chemically modified. The mixed-potential gas sensor is based on the difference in catalytic activities for the oxidation of the targeted gas under the same conditions between sensing and reference electrodes that are made from different materials. The reference electrode that exhibits a low catalytic activity toward the target gas provides a near constant electrode potential under various concentrations of analyte. These sensors may be regarded as metal-oxide-modified solid electrolyte sensors because mostly transition metal oxides have been investigated as sensing electrode materials. There is a wide selection of metal oxide or mixed metal oxide materials as the electrode materials and their catalytic oxidation properties to the target analyte is of prime importance. The catalytic properties of the metal oxide electrode play an important role in not only sensitivity but also selectivity to the target gas. The sensitivity, defined as the slope of the electromotive force ~emf! vs. log @conc.# plot, is commonly unpredictable, sub-Nernstian, and