NOx Sensor Development

Abstract

Objectives• Develop an inexpensive, rapid-response, high-sensitivity and selective electrochemical sensor for oxides of nitrogen (NO x ) for compression-ignition, direct-injection (CIDI) exhaust gas monitoring• Explore and characterize novel, effective sensing methodologies based on impedance measurements and designs and manufacturing methods that could be compatible with mass fabrication• Collaborate with industry in order to (ultimately) transfer the technology to a supplier for commercialization Approach• Use an ionic (O 2− ) conducting ceramic as a solid electrolyte and metal or metal-oxide electrodes• Correlate NO x concentration with changes in impedance by measuring the cell response to an ac signal• Evaluate sensing mechanisms and aging effects on long-term performance using electrochemical techniques• Collaborate with Ford Research Center to optimize sensor performance and perform dynamometer testing Accomplishments• Modified sensor designs to successfully improve mechanical robustness for withstanding engine vibrations and prevent failure during engine/vehicle dynamometer testing• Developed a preliminary strategy to mitigate major noise factors and improve accuracy by quantifying effects of temperature, water, and oxygen to generate a numerical algorithm• Publications/Presentations: Future Directions• Continue developing more advanced prototypes suitable for cost-effective, mass manufacturing• Continue evaluating performance of prototypes, including long-term stability and cross-sensitivity, in laboratory, dynamometer, and on-vehicle tests• Continue efforts to initiate the technology transfer process to a commercial entity Because the need for a NO x sensor is recent and the performance requirements are extremely challenging, most are still in the development phase. [4][5][6] Currently, there is only one type of NO x sensor that is sold commercially, and it seems unlikely to meet more stringent future emission requirements.Automotive exhaust sensor development has focused on solid-state electrochemical technology, which has proven to be robust for in-situ operation in harsh, high-temperature environments (e.g., the oxygen stoichiometric sensor). Solid-state sensors typically rely on yttria-stabilized zirconia (YSZ) as the oxygen-ion conducting electrolyte and then target different types of metal or metal-oxide electrodes to optimize the response. [2][3][4][5][6] Electrochemical sensors can be operated in different modes, including amperometric (a current is measured) and potentiometric (a voltage is measured), both of which employ direct current (dc) measurements. Amperometric operation is costly due to the electronics necessary to measure the small sensor signal (nanoampere current at ppm NO x levels), and cannot be easily improved to meet the future technical performance requirements. Potentiometric operation has not demonstrated enough promise in meeting long-term stability requirements, where the voltage signal drift is thought to be due to aging effects associated with electrically driven changes...