Abstract: This article briefly describes the basic principles, characteristics, structure, applications in various fields, and recent development and future research directions of several types of electrochemical gas sensors. Keywords: electrochemistry; gas sensor; progress I. Introduction The environmental damage caused by the high development of human civilization is a serious and acute problem facing the 21st century. In order to survive and develop, strict control of pollutant emissions in the atmospheric environment has become the common call of people all over the world. Therefore, the development of effective gas detection equipment has become an urgent task. At present, the main methods for gas detection are as follows[1]: thermal conductivity analysis (commonly used in gas chromatography analysis), magnetic oxygen analysis, electron capture analysis, ultraviolet absorption analysis, fiber optic sensors, semiconductor gas sensors, chemiluminescence gas sensors, chemical analysis, and electrochemical sensors. Among numerous analytical devices, some, such as chemiluminescence gas analyzers, while possessing advantages like high detection sensitivity and accuracy, are limited in application due to their large size, inability to be used for real-time on-site monitoring, and high cost, exceeding the affordability of general users. Other analytical devices, such as semiconductor gas sensors (e.g., SnO2, ZnO, etc.) [2-4], while having relatively high sensitivity, suffer from poor stability, with operating temperatures mostly above 300℃ requiring heating devices, and are generally only used as alarms. In contrast, electrochemical sensors not only meet the sensitivity and accuracy requirements of general detection but also offer advantages such as small size, simple operation, portability, on-site monitoring, and low cost. Therefore, electrochemical sensors occupy a very important position among the various gas detection devices currently available. II. Classification of Electrochemical Gas Sensors Electrochemical gas sensors are a type of chemical sensor. According to their working principle, they are generally classified into the following types: (1) When the interface between the electrode and the electrolyte solution is kept at a constant potential, the gas is directly oxidized or reduced, and the current flowing through the external circuit is used as the sensor output; (2) The ions of gaseous substances dissolved in the electrolyte solution and ionized are applied to the ion electrode, and the electromotive force generated therefrom is used as the sensor output; (3) The electrolytic current generated by the reaction of the gas with the electrolyte solution is used as the sensor output; (4) The sensor is made without an electrolyte solution, but with organic electrolytes, organic gel electrolytes, solid electrolytes, solid polymer electrolytes, etc. Table 1 summarizes the types, detection principles and performance of electrochemical gas sensors that are currently in practical use. Table 1 Comparison of various electrochemical gas sensors III. Working principles and research progress of various sensors (1) Constant potential electrolytic gas sensor The principle of constant potential electrolytic gas sensor is: to keep the interface between the electrode and the electrolyte solution at a certain potential for electrolysis, and to selectively oxidize or reduce the gas by changing its set potential, thereby enabling quantitative detection of various gases. For a specific gas, the set potential is determined by its inherent redox potential, but it varies with the material of the electrode and the type of electrolyte during electrolysis. The relationship between the electrolysis current and the gas concentration is expressed by the following formula: Where, I—electrolysis current; n—number of electrons produced per mol of gas; F—Faraday constant; A—gas diffusion area; D—diffusion coefficient; C—concentration of electrolyzed gas in electrolyte solution; δ—thickness of diffusion layer. In the same sensor, n, F, A, D and δ are constant, so the electrolysis current is proportional to the gas concentration. Since the appearance of Clark electrode in the 1950s, the control potential electrochemical gas sensor has been greatly developed in terms of structure, performance and application[5]. Foreign reports on this topic appeared extensively in the 1970s. In the early 1970s, SO2 detection instruments were available on the market. Later, CO, CH3COOH, NXOY (nitrogen oxides), H2S detection instruments and other products appeared one after another[6-10]. These gas sensors have different sensitivities, generally H2S>NO>NO2>SO2>CO, and their response time is generally a few seconds to tens of seconds, most of which is less than 1 minute [9, 12]. Their lifespans vary greatly, with the shortest being only half a year, while the CO monitor produced by General Electric in the United States has an actual lifespan of nearly 10 years. The main factors affecting the lifespan of this type of sensor are: electrode flooding, electrolyte drying, growth of electrode catalyst crystals, catalyst poisoning, and sensor usage [13]. Taking CO gas detection as an example, we can explain the structure and working principle of this sensor. Its basic structure is shown in Figure 1 [14]. The active electrode and the control electrode are placed on opposite walls inside the container, which are filled with electrolyte solution to form a sealed structure. A constant potential difference is then applied between the active electrode and the control electrode to form a constant voltage circuit. CO gas passing through the diaphragm (porous polytetrafluoroethylene membrane) is oxidized on the active electrode, while O2 is reduced on the control electrode, so CO is oxidized to form CO2. At this time, the current between the active electrode and the control electrode is I according to equation (1), and the concentration of CO gas can be known from this current value. This type of sensor can be used to detect various combustible and toxic gases, such as H2S, NO, NO2, SO2, HCl, Cl2, PH3, etc. (2) Galvanic cell gas sensor The galvanic cell gas sensor is the same as the constant potential electrolytic type mentioned above, and detects the gas concentration by measuring the electrolytic current. However, since the sensor itself is a battery, it does not need to be supplied with voltage from the outside. This type of sensor is mainly used for the detection of O2, and almost all instruments for detecting oxygen deficiency use this type of sensor. Equation (1) which is applicable to the relationship between the electrolytic current and the gas concentration for constant potential electrolytic gas sensors is also applicable to this type of sensor. The structure and principle of this sensor will be explained by taking O2 detection as an example. Its basic structure is shown in Figure 2 [15]. A PTFE (polytetrafluoroethylene) membrane with good oxygen permeability with a thickness of 10μm to 30μm is placed on one side of the plastic container. A cathode (Pt, Au, Ag, etc.) is placed near the inner surface of the membrane. An anode (base metals with high ionization tendency such as Pb and Cd) is placed on other inner walls or in the space inside the container. KOH and KHCO3 are used as electrolyte solutions. When detecting high concentrations (1 to 100%) of O2, a PTFE membrane can be used; while for detecting low concentrations (several ppm to hundreds of ppm) of gas, porous polytetrafluoroethylene is used. O2 passing through the membrane dissolves in the thin layer of electrolyte solution between the membrane and the cathode. When the output terminal of this sensor is connected to a load circuit with a certain resistance, the oxygen reduction reaction occurs on the cathode and the oxidation reaction occurs on the anode. The lead on the anode is oxidized to lead hydroxide (a portion of which is further oxidized to lead oxide) and consumed. Therefore, there is a current flowing in the load circuit. This current generates a voltage change across the load circuit. Amplifying this voltage change can represent the concentration. The main factors affecting the lifespan of this type of sensor are passivation of the Pb negative electrode and evaporation of the electrolyte. Fujita Yuko and Ding Tou Hisashi in Japan have done a lot of work on how to improve the lifespan of galvanic cell oxygen sensors [16-17]. Seki Sadao and Kobayashi Nagao have also conducted detailed research on the performance of the sensor [18-19]. Galvanic cell gas sensors for detecting other gases are also being put into practical use. (3) Ion electrode gas sensor The working principle of the ion electrode gas sensor is: gaseous substances dissolve in electrolyte solution and dissociate. The ions generated by dissociation act on the ion electrode to generate an electromotive force. This electromotive force is taken out to represent the gas concentration. This type of sensor is composed of an active electrode, a reference electrode, an internal solution and a diaphragm. The working principle of this gas sensor is explained by taking the NH3 sensor as an example. Its basic structure is shown in Figure 3. The active electrode is a glass electrode that can measure pH value, the reference electrode is an Ag/AgCl electrode, and the internal solution is an NH4Cl solution. NH4Cl dissociates to produce ammonium ions NH4+, while water also slightly dissociates to produce hydrogen ions H+, and NH4+ and H+ remain in equilibrium. According to the Nernst equation, the electromotive force E generated by the H+ concentration can be expressed by the following formula: Where, E0—the standard electromotive force of the battery; R—thermodynamic parameter; T—absolute temperature; [H+]—hydrogen ion concentration. When the sensor is placed in NH3, NH3 will permeate through the membrane into the interior, [NH3] increases, while [H+] decreases, that is, the pH value increases. By detecting this change in pH value through the glass electrode, the concentration of NH3 can be known. In addition to NH3, this sensor can also detect gases such as HCN (hydrogen cyanide), H2S, SO2, and CO2 [20-21]. (4) Electrostatic gas sensor The principle of the electrostatic gas sensor is: the gas to be measured reacts with the electrolyte solution to generate an electrolytic current, and this current is used as the sensor output to detect the gas concentration. Its active electrode and control electrode are both Pt electrodes. The working principle of this sensor is explained by taking the detection of Cl2 as an example. An aqueous solution of bromide MBr (M is a monovalent metal) is placed between two platinum electrodes. It dissociates into Br-, and water also weakly dissociates into H+. When an appropriate voltage is applied between the two platinum electrodes, current begins to flow. Then, due to the reaction of H+, H2 is produced, polarization occurs between the electrodes, and the current stops flowing. If the sensor is then brought into contact with Cl2, Br- is oxidized into Br2, and Br2 reacts with the H2 produced by polarization. As a result, the H2 in the electrode part is depolarized, thereby generating current. This current is proportional to the concentration of Cl2, so measuring this current can detect the concentration of Cl2. In addition to Cl2, this type of sensor can also detect gases such as NH3 and H2S [21-22]. Japan has recently developed a coulometric Cl2 sensor. Through experiments, it has been proven that the product has a measurement range of 0 mg/m3 to 30 mg/m3 and has the advantages of fast response speed, high stability and good reproducibility [23]. (5) Concentration cell gas sensor The concentration cell gas sensor is based on the concentration potential generated by the solid electrolyte for measurement. Its basic structure is shown in Figure 4 [15]. The magnitude of its concentration potential can be obtained by using the Nernst formula: Where, E—sensor concentration potential; Po2(I)—gas reference oxygen partial pressure; Po2(II)—gas measured oxygen partial pressure. The concentration ZrO2 oxygen sensor is a relatively mature product and has been widely used in many fields, especially in the air-fuel ratio control of automobile engines [24]. IV. Development Direction Most of the sensors mentioned above use aqueous solutions as electrolyte solutions, which have the following problems: (1) Evaporation or contamination of the electrolyte often leads to signal attenuation and short service life (generally, the lifespan of electrochemical sensors is only about one year, and at most two years); (2) Long-term direct contact between the catalyst and the electrolyte can cause the effective reaction area, i.e., the gas-liquid-solid three-phase interface, to easily move, which will reduce the catalytic activity; (3) In a dry atmosphere, especially under ventilation conditions, the electrolyte in the sensor is prone to dehydration and drying out, causing the sensor to fail; (4) There are problems such as leakage and corrosion of electronic circuits; (5) In order to ensure a certain service life of the sensor, the amount of electrolyte used cannot be too small, thus limiting the miniaturization of this type of sensor. In order to avoid the above problems caused by aqueous electrolyte solutions, people have turned their attention to solid electrolytes. Currently, products such as organic gel electrolyte gas sensors and solid polymer electrolyte gas sensors have been launched. Organic gels are a type of electrolyte in which inorganic salts are doped into organic electrolytes to improve conductivity. The sensor made from it can work at room temperature, but the adsorption and desorption rate of gas on the electrode is slow. The response time to reach 90% response takes several minutes. It also has the disadvantage that the output takes a long time to return to zero after a single contact with a high concentration of gas. In addition to detecting hydrogen sulfide, this sensor can also detect gases such as HCN, NO2, and COCl2 (phosgene). Solid electrolyte is solid polymer electrolyte (SPE) [1]. In the past 20 years, this type of electrolyte has been extensively studied internationally and has been widely used in various fields such as chemical electrolysis, chemical power source, chemically modified electrode, and electrochemical sensor. Among them, the Nafion membrane (perfluorosulfonic acid ion exchange membrane) produced by DuPont is generally considered to be the best H+ ion conductive membrane. However, initially, the performance of this type of sensor made from Nafion membrane was very unstable. Furthermore, the Nafion membrane would expand or contract under conditions such as current flow and temperature change, and would gradually be damaged. The catalyst on the membrane would fall off. However, the addition of a thin layer of solid electrolyte has fundamentally changed the structure and performance of this type of sensor. Now, it has received much attention from researchers, and the research on the next generation of solid electrolyte gas sensors has become a hot topic in electrochemical sensor research. Wuhan University has developed a fully solid-state controlled potential electrolytic oxygen sensor using Nafion membrane. This sensor uses Nafion membrane instead of electrolyte as the supporting electrolyte, but because the conductivity of Nafion membrane is greatly affected by moisture, the sensor is constrained by the ambient humidity in actual operation and can only work in the range of 32% to 96% humidity. It has strict environmental requirements and cannot achieve the purpose of application. In order to solve the problem of the dependence of Nafion membrane conductivity on moisture, some people have added a water tank to the sensor design to adjust the humidity conditions required for the operation of the sensor. This is obviously not conducive to the miniaturization of the sensor. In recent years, the Changchun Institute of Applied Chemistry of the Chinese Academy of Sciences has carried out research on semi-solid and fully solid-state controlled potential electrolytic CO[25] or O2[26] gas sensors using Nafion membrane, polybenzimidazole (PBI) membrane doped with strong acid, and polystyrene anion exchange membrane as electrolytes, and has achieved good results. With further research and development of electrochemical sensors, the research on electrochemical gas sensors will move towards the following directions: high sensitivity, high stability, long lifespan, portability, miniaturization, and intelligence. It can be asserted that the future of electrochemical sensors is incredibly bright. References: [1] Yang Hui, Lu Wenqing, Applied Electrochemistry [M], Science Press, March 2001, 1st edition, 226-227. [2] Eddy DS, Semiconductor SnO2 gas sensors [J], IEEE Trans., VT-23, 1974, 23: 125-128. [3] Watson J., Tanner D., Semiconductor ZnO sensors [J], Radio Electronic Eng., 1974, 44: 336-339. [4] Azad A., Mhaisalkar S, Birkefeld L, Akbar S. et al, Behavior of a new ZrO2-MoO3 sensor for carbon monoxide detection [J], Electrochem.Soc., 1992, 139, 2913-2920. [5] Clark CL, Internal organs. [J], Trans Am Soc. ,1956,2:41. [6] Bay HW, Blurton Kf, Lieb HC,et al., Electrochemical means of blood alcohol levels[J], Nature, 1972,240:52～55. [7]Oswin HG, Blurton HF, [P],USP 3824167,1974. [8]Hollaurell CD, A new electrochemical analysis for nitric oxide [J], Anal. Chem., 1973, 45 (1): 63～72. [9] Seldlak JM, Blurton KFA, A new electrochemical analysis for nitric oxide and nitrogen dioxide [J], Talanta, 1976, 23 (11): 811～814. [10] Blurton KFA, Electrochemical determination of hydrogen sulphide in air [J],Talanta,1976,23(6): 455～459. [12] Todahi k. , Miyo S. ，[P], USP 4478704, 1984. [13] Gao Xiaoxia, ed., Introduction to Electroanalytical Chemistry [M], Beijing: Science Press, 1986: 423. [14] Ayumu Y., Takeo S., Electrochemical carbon monoxide sensor with a Nafion film [J]. Reactive & Functional Polymers, 1999, 41: 235～243. [15] Yang Bangchao, Jian Jiawen, Duan Jianhua et al., Principles and progress of oxygen sensors [J]. Sensor World, 2002, 1: 1～8. [16] Ding Teng Hisashi, Fujita Yuko. Development of long-life battery-type oxygen [J]. Electrochemistry and its industrial physical chemistry, 1989, 57 (6): 606～611. [17] Fujita Y., Ku D., Hisa S., et al., Galvanic Cell type oxygen sensor[P], USP 4495051, 1984. [20] Mitsuo Isobe, Development of ion electrode gas sensor[J], Measurement Technology, 1972, 4(7): 43-46. [21] Mitsuo Isobe, Development of electrostatic chlorine sensor[J], Chemical Equipment, 1979, 21(1): 35-39. [22] Jin Zhunzang, Wang Mingshi (eds.), Modern Sensor Technology[M], Beijing: Electronic Industry Press, March 1995, 1st edition. [23] Zhang Jiyu, Development of electrostatic Cl2 sensor[J], Environmental Monitoring Management and Technology, 2001, 13(3): 39-45. [24] Kenji Nishio (ed.), Sensors for Automobile Engine Control [M], Japan: Shin Nippon Printing Co., Ltd., 1st edition, 1991. [25] Lu Xiujuan, Wang Yujiang, Yang Hui et al., Research on all-solid-state CO sensor [J], Applied Chemistry, 1998, 15(4): 68-72. [26] Dong Feng, Wang Yujiang, Lü Xiangyu et al., Research on PBI semi-solid and all-solid-state oxygen sensor [J], Journal of Yunnan University (Natural Science Edition), 1997, 19(1): 104-107. About the authors: Chen Changlun, Master student of School of Chemical Engineering, Hefei University of Technology. Research direction: electrochemical sensor. He Jianbo, Professor of School of Chemical Engineering, Hefei University of Technology, Master supervisor, has long been engaged in electrochemical research. Liu Wei, Master student of Hefei Institute of Intelligent Machines, Chinese Academy of Sciences. Liu Jinhuai, Researcher of Hefei Institute of Intelligent Machines, Chinese Academy of Sciences, Doctoral supervisor, has long been engaged in gas-sensitive material research.