Operating Principles of Electrochemical Gas Detectors
A large proportion of flammable, toxic and hazardous gases possess electrochemical activity and can be electrochemically oxidized or reduced. These reactions enable gas component identification and concentration measurement. Electrochemical gas sensors are divided into several subcategories:
① Galvanic Cell Gas Sensors (also known as voltaic cell sensors, fuel cell sensors or spontaneous cell sensors) Their working principle is similar to that of ordinary dry batteries, except that the carbon-manganese electrodes of dry cells are replaced with gas-sensing electrodes. Taking the oxygen sensor as an example, oxygen is reduced at the cathode, and electrons flow through an ammeter to the anode where lead metal undergoes oxidation. The magnitude of the generated current is directly proportional to the oxygen concentration. This type of sensor can effectively detect oxygen, sulfur dioxide, chlorine and other gases.
② Potentiostatic Electrolytic Cell Gas Sensors This sensor performs excellently in detecting reducing gases. Different from galvanic cell sensors, its electrochemical reaction is driven by an applied external current, making it a genuine coulometric analysis sensor. It has been widely and successfully applied for the detection of carbon monoxide, hydrogen sulfide, hydrogen, ammonia, hydrazine and other gases, and serves as the mainstream sensor for toxic and hazardous gas monitoring at present.
③ Concentration Cell Gas Sensors Electrochemically active gases will spontaneously generate concentration difference electromotive force on two sides of the electrochemical cell, and the electromotive force value correlates with gas concentration. Typical mature applications include automotive lambda oxygen sensors and solid electrolyte carbon dioxide sensors.
④ Limiting Current Gas Sensors A type of oxygen concentration sensor is developed based on the principle that the limiting current in the electrochemical cell is related to carrier concentration. It is applied for oxygen measurement in automobile exhaust and oxygen content detection in molten steel.
