Abstract: Harmful substances in automobile exhaust mainly include CO, HC, NOx, SOx, and some particulate matter, posing a serious threat to the atmospheric environment upon which humans depend for survival. Using oxygen sensors to regulate the air-fuel ratio of automobile engines and control the combustion process can achieve the dual goals of reducing pollution and saving energy. Currently, there are three main types of sensors suitable for automobile air-fuel ratio control: oxide semiconductor type (TiO2 sensor), concentration cell type (ZrO2 oxygen sensor), and limiting current type. This paper introduces the principles and structures of these three types of automotive oxygen sensors, focusing on a novel limiting current type oxygen sensor—the dense diffusion barrier layer limiting current type oxygen sensor—and briefly analyzes its development trend. Keywords : Oxygen sensor; Oxide semiconductor type; Oxygen concentration cell type; Limiting current type I. Introduction With the increasing demand for automobiles, they have gradually become necessities of life. However, the resulting pollution and energy shortages are becoming increasingly serious. The main source of harmful emissions from automobiles is engine exhaust. The harmful substances in automobile exhaust include CO, HC, NOx, SOx, and particulate matter (lead compounds, soot, oil mist, etc.). The emission of these pollutants threatens the environment upon which humanity depends for survival. Therefore, various measures must be taken to reduce the content of toxic substances in automobile exhaust, while simultaneously maximizing the completeness of the combustion process to achieve energy conservation and reduce environmental pollution. This goal is achieved through oxygen sensors. By adjusting the air-fuel ratio (A/F) of the automobile engine using an oxygen sensor, the combustion process in the engine can be controlled, solving exhaust purification problems and improving fuel combustion efficiency, thus saving energy. II. The combustion process cannot function without oxygen. For automobile engines, the completeness of fuel combustion depends on the A/F ratio. The oxygen sensor used to control the A/F ratio of an automobile engine is installed in the exhaust pipe. It detects the oxygen content in the exhaust gas. Based on the correlation between oxygen content and A/F, the measured oxygen content determines the A/F value. Therefore, the signal obtained from the oxygen sensor can be fed back to the control system to fine-tune the fuel injection quantity, keeping the A/F ratio at its optimal level, which greatly reduces emissions and saves energy. Currently, the air-fuel ratio controlled by automotive oxygen sensors is mainly concentrated at the stoichiometric air-fuel ratio and in the lean combustion zone. The output voltage of the stoichiometric air-fuel ratio sensor changes drastically near the stoichiometric air-fuel ratio due to changes in the oxygen partial pressure within the device. This characteristic makes it very suitable for use in three-way catalytic converter systems for stoichiometric air-fuel ratio control. This system was first used in Volvo and subsequently adopted by Japanese and American companies to reduce harmful gas emissions from vehicle exhaust, and is currently a major measure for controlling vehicle emissions to meet standards. However, the sensitivity of this sensor will decrease significantly for combustion conditions deviating from the stoichiometric air-fuel ratio. For lean combustion systems, the lean air-fuel ratio sensor is required to detect the oxygen concentration in vehicle exhaust within a wide air-fuel ratio range. This system was first used by Toyota in 1984. Of the three types of oxygen sensors, only the limiting current type oxygen sensor can be used in lean combustion systems. 1. Classification of automotive oxygen sensors According to different working principles, automotive oxygen sensors can be divided into three categories: oxide semiconductor type (TiO2 oxygen sensor), oxygen concentration cell type (traditional ZrO2 voltage type oxygen sensor), and limiting current type oxygen sensor (electrochemical pump oxygen type). (1) Oxide semiconductor type oxygen sensor Oxide semiconductor type oxygen sensor is based on the oxidation or reduction reaction of oxide semiconductors (TiO2, Nb2O5 and CeO2) according to the partial pressure of the surrounding atmosphere, which causes the resistance of the material to change. The representative metal oxides are TiO2 and Nb2O5. At room temperature, oxide semiconductors have high resistance. Once oxygen is insufficient, defects will appear in its lattice, thus reducing the resistance. Oxide semiconductor type oxygen sensor is made by utilizing the characteristic that the resistance value of oxide semiconductor material changes with the oxygen content in the exhaust gas. TiO2 series oxygen sensors are the most studied and mature among various metal oxide materials and have been put into practical use. TiO2 is a structurally stable and lead-resistant sensitive material. It does not exhibit oxygen-sensing properties at room temperature, only showing significant oxygen-sensing characteristics at high temperatures, and its temperature coefficient is relatively large, requiring temperature compensation. Nb2O5 sintered bodies produced using conventional ceramic processes do not have ideal response characteristics. Fabricating them into thin-film elements with a large specific surface area can improve surface activity and response performance. TiO2-Nb2O5 composite oxides outperform single TiO2 and Nb2O5 in terms of air-fuel ratio and temperature characteristics, representing a direction for the development of oxide semiconductor oxygen sensors. Thin-film oxide semiconductor sensors can improve sensor performance and reduce costs, and research in this area is very active. These sensors are fabricated using thin-film and micromechanical processes. Their advantages include fast response speed and normal operation at low temperatures. Thick-film oxygen sensors are also a very active field, offering the advantage of rapid response. Generally, these sensors use TiO2 as the material. In summary, thin-film and thick-film sensors are cheaper and offer superior performance (output linearity, operating temperature, A/F range, etc.) than other types. However, the lifespan of thin-film sensors is still an unsolved problem. Oxide semiconductor oxygen sensors are characterized by simple structure, light weight, low cost, fast response speed and strong resistance to lead pollution. However, the resistance of this type of oxygen sensor changes drastically near the theoretical air-fuel ratio, and the output voltage also changes drastically, which limits its application in the entire lean combustion zone. Moreover, its lifespan and sensitivity are not as good as those of zirconia sensors, and the input and output signal processing equipment is relatively expensive, so its application is not as widespread as that of zirconia oxygen sensors. (2) Oxygen concentration cell type oxygen sensor Among various types of oxygen sensors, the ZrO2 concentration cell type oxygen sensor is the earliest practical oxygen sensor, which has been in use for more than 20 years and is basically mature. Compared with the TiO2 oxygen sensor, which is also used in practice, the biggest advantage of the ZrO2 oxygen sensor is its high sensitivity and reliability. Current research mainly focuses on improving its performance, such as miniaturization and low temperature performance. In addition to ZrO2 being used as an electrolyte, LaCaO3 also has high oxygen ion conductivity after being doped with Sr and Mg. The voltage-type oxygen sensor prepared with it also has good performance below 600 K [6]. The following mainly introduces the ZrO2 concentration cell type oxygen sensor. Working principle of ZrO2 concentration cell type oxygen sensor: One side of the ZrO2 solid electrolyte material is exposed to the automobile exhaust, and the exhaust oxygen partial pressure is Po2; the other end is exposed to the reference atmosphere, and its oxygen partial pressure is fixed at Pref. In this way, there will be a potential difference in oxygen concentration or pressure on both sides. Oxygen will be conducted from the high concentration side to the low concentration side in the form of oxygen ions through the ZrO2 solid electrolyte with a large number of oxygen vacancies, thereby forming oxygen ion conductivity. Thus, an oxygen concentration difference potential E is generated on the electrodes on both sides of the solid electrolyte, forming a concentration cell structure [7]. Since it is used in automobiles, the environmental conditions are harsh and the lifespan requirement is long. In order to prevent impurities in the exhaust gas from corroding the platinum film, a layer of porous ceramic is covered on the platinum film of the ZrO2 sensing element as a coating. The oxygen sensor's inner side is open to the atmosphere, while its outer side is in direct contact with the exhaust gas. The exhaust gas temperature varies between 300 and 950°C. To ensure the sensor operates at a stable temperature, a heater must be inserted inside the U-tube. When a rich mixture burns, the oxygen in the exhaust is extremely scarce, resulting in a large difference between Po2 and Pref, which generates a large electromotive force. When a lean mixture burns, because there is more oxygen, Po2 and Pref are very close, resulting in a small oxygen concentration difference and almost no voltage. Therefore, near the stoichiometric air-fuel ratio, the ratio of oxygen concentrations on both sides of the electrolyte in a ZrO2 concentration cell type oxygen sensor changes drastically, causing a sharp change in the output voltage. ZrO2 concentration cell type oxygen sensors offer advantages such as high accuracy, fast response, wide applicability, and long lifespan when used near the stoichiometric air-fuel ratio. However, because its signal is logarithmically related to the partial pressure of oxygen, similar to oxide semiconductor type oxygen sensors, the signal change is very small throughout the lean combustion region, making it less sensitive. (3) Limiting Current Type Oxygen Sensor Both oxygen concentration cell type oxygen sensors and oxide semiconductor type oxygen sensors can only detect the theoretical air-fuel ratio. They have slow response and low sensitivity in the lean combustion zone, especially when A/F > 20. However, in order to reduce pollution and save energy, it is required that the sensor can continuously detect the air-fuel ratio in the lean combustion zone. Thus, the limiting current type oxygen sensor was developed. Working principle of limiting current type oxygen sensor: When a voltage is applied to the solid electrolyte ZrO2, O2 will gain electrons on the inner electrode (cathode) to form O2-. O2- is discharged on the outer electrode (anode) through the transfer effect of ZrO2, and O2- becomes O2 again. In this way, oxygen is pumped from the cathode to the anode through the solid electrolyte. This cell is usually called a pumped oxygen cell, the applied voltage is called the pump voltage, and the generated current is called the pump current. During the oxygen pumping process, the increase in pump current caused by the increase in external pump voltage will gradually decrease, and finally the pump current will remain unchanged or change very little within a certain voltage range [8]. The current reaches saturation, and this current is called the limiting current [9]. In order to obtain a limiting current that is related to the oxygen concentration in the ambient atmosphere and is relatively stable, a porous diffusion barrier layer is generally added to the cathode surface of the zirconia oxygen sensor to restrict the transmission of oxygen to the cathode. The diffusion of oxygen through the barrier layer will become the control link of the pumping current. When the voltage increases beyond a certain value, the current will no longer increase and will reach the limit. The magnitude of the limiting current is not related to the voltage that continues to increase, but depends on the diffusion rate of oxygen into the chamber and is proportional to the oxygen partial pressure in the measured environment. Limiting current type oxygen sensors all have similar characteristic curves, that is, the output current of the sensor is linearly related to the external oxygen partial pressure, so it can continuously detect the air-fuel ratio in the lean combustion zone over a wide range. In recent years, research on limiting current type oxygen sensors has been very active. Limiting current type oxygen sensors suitable for a wide range of air-fuel ratios have become an important development direction for automotive exhaust gas sensors, and are also an indispensable component for energy saving and automotive bench testing. Currently, two types of physical diffusion barrier limiting current oxygen sensors are used in the automotive industry: the orifice diffusion limiting current oxygen sensor and the porous diffusion layer limiting current oxygen sensor. ① Orifice Diffusion Limiting Current Oxygen Sensor The orifice diffusion limiting current oxygen sensor is the earliest type of current-measuring oxygen sensor. This type of oxygen sensor has two structures: a single-cell structure and a dual-cell structure. Only a brief introduction to the structure (see Figure 1) and principle of the single-cell orifice diffusion limiting current oxygen sensor will be given. Its gas diffusion control shroud is made of alumina and other ceramic materials, with a small hole at the center of the top. External oxygen diffuses into the enclosed space through the small hole. Due to the small diameter of the hole, the diffusion rate is limited. Therefore, when the applied voltage increases to a certain value, the current reaches its maximum (limiting current value). At this point, the magnitude of the current changes only with the change in the external oxygen concentration, and its theoretical equation is: Where: F—Faraday constant; S—cross-sectional area of ​​the diffusion orifice; D—diffusion coefficient of oxygen in the mixed gas; R—gas constant; T—absolute temperature (K); L—length of the diffusion orifice; Po2—oxygen partial pressure in the mixed gas. The orifice-limiting current type oxygen sensor utilizes a small orifice to control gas diffusion to achieve the oxygen concentration difference between the inside and outside of a closed chamber, without using any reference gas. However, prolonged use can easily cause changes in the orifice size or even blockage, thus affecting its oxygen sensitivity and operational stability. Furthermore, the manufacturing process is complex and the price is high. ② Porous diffusion layer-limiting current type oxygen sensor The orifice-limiting current type oxygen sensor mentioned above can be understood as having only one small orifice. However, since the solid electrolyte and electrodes of the porous diffusion layer-limiting current type oxygen sensor are porous, we can consider the entire sensor to be composed of an "infinitely many" small orifice structure (see Figure 2). The structural feature of this type of sensor is the addition of a porous coating on the electrodes to hinder the diffusion of O2-. This allows a reference oxygen partial pressure to be formed on one side of the electrode, eliminating the need for any reference gas. While porous diffusion layer limiting current type oxygen sensors are easy to fabricate, their porosity is difficult to control. Due to contamination from dust and other particles, the porosity changes over prolonged use, affecting the sensor's response performance and lifespan. All of the aforementioned oxygen sensors include a heater to maintain a constant operating temperature. However, adding a heater makes the sensor structure bulky and complex. Therefore, a dual-cell structure oxygen sensor has been designed, connecting two sheet-like porous diffusion layer limiting current type oxygen sensors together to achieve the goal of making the sensor output signal unaffected by temperature changes. Recently, research on thin-film and thick-film sensors has been very active in electrochemical pump-type oxygen sensors. Examples include thin-film oxygen sensors deposited on sapphire substrates and oxygen sensors with dual-cell structures fabricated using lamination and screen printing techniques. Takahashi et al. fabricated a limiting current type sensor, which assembles a thin-film solid electrolyte battery on a porous alumina substrate, thus expanding the measurement range to some extent. In addition, there are other forms of oxygen sensors, such as the limiting oxygen sensor developed by Ishibash, where the anode and cathode are located on the same side of the solid electrolyte surface; and the limiting oxygen sensor developed by Suguki, which integrates the stoichiometric air-fuel ratio and the lean air-fuel ratio. 2. Dense Diffusion Barrier Layer Limiting Current Type Oxygen Sensors Due to the aforementioned drawbacks of small-pore diffusion limiting current type oxygen sensors and porous diffusion layer limiting current type oxygen sensors, their practical applications are somewhat limited. To overcome the shortcomings of these two limiting current type oxygen sensors, extend their lifespan, and optimize their testing performance, research has begun abroad on limiting current type oxygen sensors using solid-state electron-ion hybrid conductors as dense diffusion barrier layers. Solid electron-ion hybrid conductor materials have certain oxygen permeability properties. Using them as chemical diffusion barrier layers can solve the problems of porous physical diffusion layers [17]. Its oxygen ion transport is accomplished through lattice defects (such as oxygen vacancies). There will be no pore blockage during use, so the performance is stable and the operation is reliable. Because it has a high electronic conductivity, the potential gradient is very small. Therefore, the migration of oxygen ions is not caused by the potential gradient, but by the chemical potential difference between the two sides of the hybrid conductor. The diffusion rate of oxygen in the solid hybrid conductor is much slower than in the gas phase. This will slow down the oxygen transport rate to the sensor cathode and limit the oxygen diffusion flow. Therefore, the hybrid conductor can act as a diffusion barrier layer, making the electrical output performance of the sensor linear. The limiting current type oxygen sensor with a dense diffusion barrier layer is composed of a solid electrolyte such as ZrO2 (Y2O3) with oxygen ion conduction and a layer of electron/oxygen ion hybrid conductor. Two electrode leads connected to the power supply are led out from the composite interface and the outer surface of the solid electrolyte, respectively (see Figure 3). When an external voltage is applied to the sensor, the negative terminal connects to the hybrid conductor layer. Oxygen molecules are converted into adsorbed oxygen under the catalytic action of the hybrid conductor material and gain electrons at the negative electrode (i.e., the cathode of the sensor) to generate oxygen ions. Due to the difference in oxygen concentration between the two surfaces of the hybrid conductor, the oxygen concentration at the interface between the hybrid conductor and the ZrO2 electrolyte is lower. Therefore, driven by the oxidation potential gradient, oxygen ions diffuse from the negative terminal to the ZrO2/hybrid conductor interface, and then diffuse to the positive electrode (i.e., the anode of the sensor) through the oxygen vacancy defects of the ZrO2 solid electrolyte. The oxygen ions are then discharged and converted back into oxygen molecules. In this way, oxygen is pumped from the cathode to the anode of the battery through the ZrO2 solid electrolyte. When the amount of oxygen diffused into the sensor through the hybrid conductor layer is equal to the amount of oxygen pumped away by the external current, diffusion reaches a steady state, and the current reaches its saturation value, i.e., a limiting current is formed. Obviously, the magnitude of the limiting current is directly related to the oxygen concentration in the environment. This is the oxygen measurement principle of the dense diffusion barrier layer limiting current type oxygen sensor. In 1998, Fernando et al. used ZrO2 stabilized by 8% Y2O3 as the solid electrolyte and La0.84Sr0.16MnO3 as the diffusion barrier layer for a limiting current oxygen sensor. They successfully fabricated a dense diffusion barrier layer (LSM) oxygen sensor by using magnetron sputtering and screen printing techniques to prepare a dense diffusion barrier layer on the surface of a YSZ substrate. However, the LSM dense diffusion barrier layer produced by magnetron sputtering was relatively thin, affecting the sensor's testing performance in high oxygen concentration atmospheres. Screen printing can avoid this problem. LSM powder and a certain amount of organic solvent are mixed evenly to form a slurry. An alumina ceramic sheet is used as the substrate, and a YSZ sheet is placed on it. A film is then printed onto the YSZ sheet using a screen printing machine and dried. Then, platinum paste is evenly coated onto one side of the thick film and the other side of the YSZ sheet, platinum wire is attached, and the mixture is sintered at a certain temperature to obtain the sensor sample. Sensors fabricated using this method can have their oxygen measurement range broadened and performance improved by varying the thickness of the LSM film. A cubic fluorite-structured chemical diffusion barrier containing TbO1.75 (Tb4O7) has also been developed: a dense Tb-YSZ film made from a 16 mm thick compound Y0.2Tb0.24Zr0.56O2-y. This compound, with appropriate Tb and Y doping, exhibits good electronic conductivity and oxygen ion mobility. Oxygen sensors using this diffusion barrier layer show a linear response within an oxygen concentration range of 0.2%–4.5%. Li Fushen et al. successfully prepared a limiting current type oxygen sensor using 8% Y2O3-stabilized ZrO2 as the solid electrolyte and a La0.8Sr0.2CoO3 mixed conductor material as the diffusion barrier layer through separate sintering, platinum paste bonding, and peripheral sealing. Platinum paste bonding involves bonding pre-fired mixed conductors and solid electrolyte sheets together with platinum paste, followed by sintering of the platinum paste to enhance the mechanical properties of the bond. This avoids co-firing of the two materials, preventing cracks caused by mismatched sintering shrinkage rates, and also prevents reactions between them. Peripheral sealing involves sprinkling a sealing material (glass powder) around the periphery of the composite sheet, followed by sintering to completely seal the gap at the junction of the two sheets. Sealing the contact gap between the two sheets prevents oxygen leakage, which could affect experimental results. Finally, platinum paste is evenly coated on both sides of the LSCo/YSZ composite sheet, and platinum wire is attached to obtain the block sensor sample (Figure 4). This sensor can provide a good limiting current plateau across the entire air-fuel ratio range. Xia Hui et al. also successfully fabricated a dense diffusion barrier layer limiting current type oxygen sensor using a platinum paste bonding method, with ZrO2 stabilized by 8% Y2O3 as the solid electrolyte and La0.8Sr0.2MnO3 as the diffusion barrier layer. This sensor avoids the problem of mismatched shrinkage rates between YSZ and LSM materials during sintering and provides better oxygen sensitivity. Yang Mei et al.'s oxygen sensor prepared using La0.8Sr0.2FeO3 showed good oxygen sensitivity in the full oxygen concentration range of 0-21%, with a good linear relationship between the limiting current and oxygen concentration, and a correlation coefficient greater than 0.997. In a lean-burning mixed atmosphere, the limiting current value corresponds one-to-one with the air-fuel ratio A/F (14.5-25), which can detect the air-fuel ratio in the lean-burning zone. The dense diffusion barrier layer limiting current oxygen sensor overcomes the shortcomings of the pinhole and porous layer limiting current oxygen sensors, exhibiting better oxygen sensitivity and broadening the sensor's oxygen measurement range. It boasts advantages such as simple structure, fast response, reliable operation, and low cost, making it a promising limiting current oxygen sensor. In summary, compared to the other two types of sensors, the limiting current oxygen sensor has the following advantages: firstly, it can continuously monitor the air-fuel ratio in the lean combustion zone, saving energy; secondly, it has high sensitivity and good control accuracy; thirdly, it has low temperature dependence; fourthly, it does not require a reference electrode; fifthly, it is simple to fabricate and easy to miniaturize; and sixthly, it has a fast response speed. Therefore, the limiting current oxygen sensor has become the main development trend of lean combustion zone automotive exhaust gas sensors. III. Research Directions for Automotive Oxygen Sensors Automotive oxygen sensors adjust the air-fuel ratio by detecting the oxygen content in engine exhaust gas, making the actual air-fuel ratio closer to the theoretical air-fuel ratio, thereby achieving complete combustion, the lowest exhaust pollutant content, and optimal engine power performance and economy. Globally, the development of automotive oxygen sensors is particularly rapid, with nearly 100 million units used in automobiles annually. Currently, its research progress mainly focuses on the following directions: 1. Research on low-temperature oxygen sensors. Since existing oxygen sensors must operate at relatively high temperatures to function properly, this causes many inconveniences in manufacturing and use. Therefore, research on low-temperature oxygen sensors is very active, leading to the introduction of many new structures and materials, such as CeO2, CoO, SrTiO3, and LaCaO3, which are increasingly widely used. 2. Expanding the air-fuel ratio control measurement range. Achieving wide-range air-fuel ratio measurement and control has been a hot research direction in recent years. This allows the oxygen sensor to continuously meter and control the entire state from the rich region to the ideal air-fuel ratio and then to the lean combustion region, achieving feedback control. 3. Thin-film and miniaturized sensors. Miniaturized oxygen sensors fabricated using thin-film and micromechanical processes have excellent performance, low cost, and are easy to integrate, achieve all-solid-state processing, and become multifunctional. 4. Research and improvement of protective layer materials. The protective layer and electrodes of sensors are often clogged by dust, oil, silicon, and other components, significantly affecting sensor performance. Therefore, improving the protective layer material, refining the manufacturing process, and enhancing the sensor's resistance to degradation are important research directions. In conclusion, with the rapid development of my country's automotive industry, the advancement of automotive technology, and the improvement of sensor manufacturing processes, automotive oxygen sensors will continue to improve and develop, with a very broad development prospect.