Gas sensing technology platforms have allowed for the improvement of crucial industrial processes and the decreased emissions of environmentally harmful gases that have led to deleterious outcomes. Industries where gas control has become critical are petrochemical, refinery, food and beverage, and power generation. These industries utilize gases such as ethylene, sulfur, carbon dioxide (CO 2 ), and oxygen (O 2 ) to produce gasoline, carbonated beverages, and electricity. [1] During various processes, harmful gases such as carbon monoxide (CO), CO 2 , sulfur dioxide (SO 2 ), and nitrogen dioxide (NO 2 ) and volatile organic compounds (VOCs) such as ethanol (C 2 H 5 OH), acetone (CH 3 COCH 3 ), toluene (C 7 H 8 ), and acetylene (C 2 H 2 ) can be emitted. [2][3][4][5][6] The emission of these environmentally toxic gases and VOCs from both industrial processes and automobiles has been linked to the nitrous oxides (NO x ), CO, and hydrocarbon emissions responsible for acid rain in the 1960s. Legislation was introduced to address and mitigate this acidic precipitation and the emission of environmentally toxic gasses by requiring the use of gas sensors and catalytic converters. [2] Gas sensing technology can be found in two leading commercial platforms: electronic (lambda sensors) and optical (nondispersive infrared (NDIR) sensors and optrodes). Lambda sensors are made with electrolytes or gas-sensitive films that exploit the ionic conductivity of electrolytes or the adsorption of gases and bonding of hydroxyl groups on metal oxide (MO) materials. State-of-the-art, automotive lambda sensors, also known as "automotive oxygen sensors," use a solid yttria-stabilized zirconia (YSZ) electrolyte in wideband sensor design geometries. The wideband sensor design was adapted from the early narrowband YSZ sensor design that utilized a simple Nernst cell. The wideband sensor design offers improved sensor linearity when compared to the narrowband designs across a wide range of air-to-fuel ratio values. Though the electrolytic lambda sensors' physical design has changed in previous years, the sensor has not seen an improvement in terms of electrolytic materials.Lambda sensors have also been based on gas-sensitive oxide films, instead of electrolytes, that employ the use of MO materials. MO materials include a wide range of oxidized transitional metals. These MOs undergo electronic changes when gases or VOCs are chemisorbed or bound to hydroxyl groups on the material's surface, thereby increasing or decreasing the MO's resistivity. The gas-material interaction only occurs at high temperatures, called the "activation temperatures," required by the MO.High activation temperatures are common in both electrolytic and MO gas sensors. Bringing the electrolytic and MO gas sensors to these high temperatures requires electronic heater elements. The reduction of the activation temperature of a lambda sensor can be achieved by using promoter materials and catalytic metals. Some material platforms, such as metal-organic frameworks (MOFs), are als...
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