What is the structure and working principle of electrochemical gas sensor? What are the measurements?

　　To carry out a specific measurement work, we must first consider the principle of the sensor, which needs to be analyzed after many factors to determine. Because, even if it is to measure the same physical quantity, there are a variety of principles of the sensor to choose from, which principle of the sensor is more suitable, domestic or imported, whether the price can afford, or their own development. After considering the above issues, we can determine which type of sensor to choose, and then consider the specific performance indicators of the sensor.

　　1. Structure of electrochemical gas sensor

　　Electrochemical gas sensor, mainly using the chemical potential difference between two electrodes, one is to measure the concentration of gas in the gas, the other is a fixed reference electrode. The electrochemical sensor works by potentiostatic electrolysis and galvani cell. There are liquid electrolytes and solid electrolytes, and the liquid electrolytes are divided into potential and current types. Potentiometric type is measured by the relationship between electrode potential and gas concentration. The current model adopts the principle of limiting current, and uses gas diffusion through thin breathable film or capillary pores as the current limiting measure to obtain stable mass transfer conditions and produce limiting diffusion current proportional to gas concentration or partial pressure.

　　The electrochemical sensor has two - electrode and three - electrode structure, the main difference is there is no reference electrode. Two-electrode CO sensor without reference electrode, simple structure, easy to design and manufacture, low cost suitable for low concentration CO detection and alarm; The introduction of the reference electrode into the three-electrode CO sensor makes the sensor have a large range and good accuracy, but the introduction of the reference electrode increases the manufacturing process and material cost, so the price of the three-electrode CO sensor is higher than that of the two-electrode CO sensor, mainly used in the industrial field. The two-electrode electrochemical CO sensor is mainly composed of electrode, electrolyte, electrolyte holding material, filter material of outgoing interference gas, pin and other parts.

　　2. Working principle of the electrical sensor

　　Electrochemical gas sensor is a kind of chemical sensor, according to the working principle is generally divided into: a. When the interface of electrode and electrolyte solution is kept as a constant potential, the gas is oxidized or reduced directly, and the current flowing through the external circuit is taken as the output of the sensor. b. Ionic interaction of gaseous substances dissolved in electrolyte solution and ionized with ionic electrodes, and the resulting electromotive force is output as a sensor; c. The electrolytic current generated by the reaction of gas and electrolyte solution is output as the sensor; d. Sensors are made of organic electrolytes, organic gel electrolytes, solid electrolytes, solid polymer electrolytes and other materials instead of electrolyte solution.

　　Table 1 Comparison of various electrochemical gas sensors

　　Table 1 summarizes the types, detection principles, materials and characteristics of various electrochemical gas sensors.

　　2.1 Potentiostatic electrolytic gas sensor

　　The principle of the potentiostatic electrolytic gas sensor is that the interface between the electrode and the electrolyte solution is kept at a certain potential for electrolysis. By changing its set potential, the gas can be selectively oxidized or reduced, so as to detect various gases quantitatively. For a particular gas, the set potential is determined by its inherent REDOX potential, but also varies with the material of the electrode and the type of electrolyte. The relationship between electrolytic current and gas concentration is expressed as follows:

　　I=（nfADC）/σ

　　Where: I- electrolytic current；n-1mol The number of electrons produced by the gas；f- Faraday constant；A- gas diffusion area；D- diffusion coefficient；C- Concentration of electrolytic gas in electrolyte solution；σ-The thickness of the diffusion layer。

　　In A unified sensor, n, f, A, D and σ are fixed, and the electrolytic current is proportional to the gas concentration.

　　Since the emergence of CIDK electrode in the 1950s, controlled potential electrochemical gas sensors have developed greatly in terms of structure, performance and application. There have been 31 detectors on the market since the early 1970s. There have been CO, NxOY (nitrogen oxide), H2S testing equipment and other products. The sensitivity of these gas sensors is different, generally H2S "NO" NOb "Sq" CO, response time is generally a few seconds to tens of seconds, most less than 1min; Their life is very different, short only half a year, some CO monitor actual life has been nearly 10 years. The main factors affecting the life of this kind of sensor are: electrode flooding, electrolyte drying, electrode catalyst crystal growth, catalyst poisoning and sensor use method.

　　Taking CO gas monitoring as an example, the structure and working principle of the diaphragm working electrode are illustrated. An acting electrode h 'and a contrast electrode are arranged on the opposite walls of the container, which are filled with electrolyte solution to form a sealing structure. Watts in the chemical field by pole 3g hedge by pole AnljI constant potential difference to form a constant voltage circuit. At this time, the current between the acting electrode and the contrast electrode is I, and the basic structure of the potentiostatic electrolytic gas sensor can know the concentration of CO gas according to this current value. This type of sensor can be used to detect a variety of flammable gases and toxic gases, such as H2S, NO, NOb, Sq, HCl, Cl2, PH3, and can also detect oxygen concentration in the blood.

　　2.2 Ion electrode type gas sensor

　　The working principle of the ionic electrode type gas sensor is that the gaseous substance is dissolved in the electrolyte solution and dissociated, and the ions generated by dissociation act on the ionic electrode to produce an electromotive force, which is taken out to represent the gas concentration. This type of sensor is composed of an acting electrode, a contrast electrode, an internal solution and a diaphragm.

　　The detection of NH3 sensor is taken as an example to illustrate the working principle of this gas sensor. The acting electrode is a glass electrode that can measure pH, the reference electrode is A8 secondary electrode, and the internal solution is NIkCE solution. NEACt dissociates to produce ammonium ion NH4+, while water also weakly dissociates to produce hydrogen ion H+, and NH4+ is in equilibrium with H+. If the sensor is penetrated into NH3, NH3 will penetrate internally through the diaphragm, NH3 will increase, while H+ will decrease, that is, pH will increase. The NH3 concentration can be determined by measuring this change in PH with a glass electrode. In addition to NH3, the sensor can detect HCN (hydrogen cyanide), H2S, Sq, C02 and other gases.

　　Ionic electrode type gas sensor appeared earlier, through the detection of ion polarization current to detect the volume fraction of the gas, the main point of the electrochemical type gas sensor is the detection of gas sensitivity, good selectivity.

　　2.3 Coulometric gas sensor

　　The principle of the coulometric gas sensor is: the measured gas reacts with the electrolyte solution to generate electrolytic current, and the current is output as the sensor to detect the gas concentration. The acting electrode and the contrast electrode are Pt electrodes.

　　The detection of C12 is taken as an example to illustrate the working principle of this sensor. The bromine MBr (M is a monvalent metal) aqueous solution between two platinum electrodes is dissociated into a ratio, and water is dissociated into H+, and the appropriate voltage is added between the two platinum electrodes, and the current starts to flow, and H2 is generated due to the H+ reaction, and the polarization between the electrodes occurs. As a result, H2 in the electrode part is polarized and the current is generated. This current is proportional to the H2 concentration, so detecting this current will detect the Cl2 concentration. In addition to Cl2, this type of sensor can also detect NH2, H2S and other gases.

　　3. Sensor detection

　　Electrochemical gas sensors can be divided into galvanic cell type, controlled potential electrolysis type, electric type and ion electrode type four types. Galvanic gas sensors detect the volume fraction of gas by detecting the current. Almost all the commercially available instruments for detecting hypoxia are equipped with such sensors. Controllable electrolytic sensor is through the detection of electrolytic current flow to detect the volume fraction of gas, and galvanic cell type is different, need to be applied by the outside specific voltage, in addition to the detection of CO, NO, NO2, O2, SO2 and other gases, but also can detect the oxygen volume fraction in the blood. The coulometric gas sensor detects the volume fraction of the gas by the current generated by the reaction of the gas and electrolyte. Ion-electrode gas sensors appear earlier and measure the volume fraction of gas by detecting the ion polarization current. The main advantages of electrochemical gas sensor are high sensitivity and selectivity in gas detection.

　　To sum up, different kinds of gas sensors are suitable for different gas detection and control needs. With the development of modern industry, especially the continuous strengthening of the concept of green environmental protection, the development and application of gas sensor technology will have a very broad prospect for development. The two-electrode electrochemical CO sensor is the hot spot of research in recent years, which belongs to the advanced sensor technology in the world. Through the experimental research, in the electrode, filter layer, electrolyte and other materials selection and structure design, many technical problems affecting the life of the sensor have been overcome, and a new type of CO sensor with practical significance has been successfully developed. It will certainly play an active role in the field of CO gas detection.