Effect of Electrode Composition and Microstructure on Impedancemetric Nitric Oxide Sensors Based on YSZ Electrolyte

Abstract

The role of metal (Au, Pt, and Ag) electrodes in YSZ electrolyte-based impedancemetric nitric oxide (NO) sensors is investigated using impedance spectroscopy and equivalent circuit analysis. The test cell consists of a rectangular block of porous YSZ with two metal wire loop electrodes, both exposed to the same atmosphere. Of the electrode materials, only Au was sensitive to changes in NO concentration. The impedance behavior of porous Au electrodes in a slightly different configuration was compared with dense Au electrodes and was also insensitive to NO. Ag showed no sensitivity to either O 2 or NO, and the measured impedances occurred at frequencies > 10 kHz, which are typically associated with ionic conduction in YSZ. Pt and porous Au showed sensitivity to O 2 , which was quantified using power-law exponents that suggest electrochemical rate-determining mechanisms occurring at the triple phase boundary. The behavior of the dense Au suggests different rate-determining processes (e.g., diffusion or adsorption) for the O 2 reaction. Although the exact mechanism is not determined, the composition and microstructure of the metal electrode seem to alter the rate-limiting step of the interfering O 2 reaction. Impedance behavior of the O 2 reaction that is limited by processes occurring away from the triple phase boundary may be crucial for impedancemetric NO sensing.

NOx Sensor Development

“…The approach is described in detail in previous reports and several publications (See Ref. [8][9][10][11]. Briefly, impedancemetric operation has shown the potential to overcome the drawbacks of other approaches, including higher sensitivity towards NO x , better longterm stability, potential for subtracting out background interferences, total NO x measurement, and lower cost materials and operation.…”

“…9-11 Impedancemetric sensing is possible with a variety of electrode materials, both metal and metal oxides, that meet general sensor criteria, which include a dense microstructure and appropriate composition to limit the catalytic activity towards oxygen. [10][11] Impedance is defined as the measured response to an alternating current and is a complex quantity with both magnitude and phase angle information. The phase angle has been found to provide a more stable response at higher operating frequencies and is chosen as the sensor signal.…”

“…The phase angle has been found to provide a more stable response at higher operating frequencies and is chosen as the sensor signal. [8][9][10][11] In previous work, impedancemetric sensing of either gold or strontium-doped lanthanum manganite (LSM), an electronically conducting metal oxide, electrodes was investigated in laboratory and engine testing. Preliminary results indicated that gold electrodes have good stability and the potential for low water cross-sensitivity, but also have a higher thermal expansion coefficient and lower melting temperature than the YSZ electrolyte, which limit processing flexibility.…”

“…Briefly, impedancemetric operation has shown the potential to overcome the drawbacks of other approaches, including higher sensitivity towards NO x , better longterm stability, potential for subtracting out background interferences, total NO x measurement, and lower cost materials and operation. [8][9][10][11] Past LLNL research and development efforts have focused on characterizing different sensor materials and understanding complex sensing mechanisms. [8][9][10][11] Continued effort has led to improved prototypes with better performance, including increased sensitivity (to less than 5 ppm) and longterm stability, with more appropriate designs for mass fabrication, including incorporation of an alumina substrate with an imbedded heater.…”

“…[8][9][10][11] Past LLNL research and development efforts have focused on characterizing different sensor materials and understanding complex sensing mechanisms. [8][9][10][11] Continued effort has led to improved prototypes with better performance, including increased sensitivity (to less than 5 ppm) and longterm stability, with more appropriate designs for mass fabrication, including incorporation of an alumina substrate with an imbedded heater.Efforts in the last year to further improve sensor robustness have led to successful engine dynamometer testing with prototypes mounted directly in the engine manifold. Previous attempts had required exhaust gases to be routed into a separate furnace for testing due to mechanical failure of the sensor from engine vibrations.…”