Working principle and application characteristics of electrochemical gas sensor

Earlier electrochemical sensors date back to the 1950s, when they were used for oxygen monitoring. In the mid-1980s, small electrochemical sensors began to be used to detect a variety of different toxic gases in the PEL range, and showed good sensitivity and selectivity. At present, various electrochemical sensors are widely used in static and mobile applications for personal safety.

First, the working principle

An electrochemical sensor works by reacting with the gas under test and producing an electrical signal proportional to the concentration of the gas. A typical electrochemical sensor consists of a sensing electrode (or working electrode) and a reverse electrode, separated by a thin electrolytic layer. The gas first reacts with the sensor through tiny capillary holes, then a hydrophobic barrier, and finally reaches the electrode surface. This method allows the appropriate amount of gas to react with the sensor electrode to form a sufficient electrical signal while preventing electrolyte leakage from the sensor.

The gas diffused through the barrier reacts with the sensing electrode, which may adopt an oxidation or reduction mechanism. These reactions are catalyzed by electrode materials designed for the gas under test. A current proportional to the measured gas concentration flows between the positive and negative electrodes through a resistor connected between the electrodes. The gas concentration can be determined by measuring the current. Because of the current generated in this process, electrochemical sensors are often referred to as current gas sensors or micro-fuel cells.

In practice, due to the continuous electrochemical reaction on the electrode surface, the sensor electrode potential cannot be kept constant, which will lead to the degradation of sensor performance after a long period of time. Reference electrode is introduced to improve the sensor performance. The reference electrode is installed in the electrolyte, adjacent to the sensing electrode. A fixed stable constant potential is applied to the sensing electrode. The reference electrode can maintain this fixed voltage value at the sensing electrode. No current flows between the reference electrodes. The gas molecules react with the sensing electrode, and the reverse electrode is measured at the same time. The measurement results are usually directly related to the gas concentration. The voltage value applied to the sensing electrode enables the sensor to target the gas.

Two, the electrochemical sensor contains the following main components:

A. Breathable membranes (also known as hydrophobic membranes) : Breathable membranes are used to cover sensing (catalytic) electrodes and in some cases to control the molecular weight of gases reaching the electrode surface. Such barriers are usually made of Teflon films with low porosity. Such sensors are called coating sensors. Alternatively, it can be covered with a high-porosity Teflon film, while the capillary controls the molecular weight of the gas reaching the electrode surface. This type of sensor is called capillary sensor. In addition to providing mechanical protection for the sensor, the film also filters out unwanted particles. In order to transfer the correct molecular weight of the gas, it is necessary to select the correct pore size of the film and the capillary. The aperture size should allow sufficient gas molecules to reach the sensing electrode. The aperture size should also prevent liquid electrolyte leakage or rapid dryness.

B. Electrode: The choice of electrode material is very important. The electrode material shall be a catalytic material capable of performing semi-electrolytic reactions over long periods of time. Typically, the electrodes are made of precious metals, such as platinum or gold, which react effectively with gas molecules when catalyzed. Depending on the design of the sensor, the three electrodes can be made of different materials to complete the electrolytic reaction.

C. Electrolyte: The electrolyte must be sufficient to facilitate electrolytic reactions and effectively transfer ionic charges to the electrode. It must also form a stable reference potential with the reference electrode and be compatible with the materials used in the sensor. If the electrolyte evaporates too quickly, the sensor signal will weaken.

D. Filter: Sometimes a wash filter is installed in front of the sensor to remove unwanted gas. The selection of filters is limited, each filter has a different degree of efficiency. Most commonly used filter material is activated carbon, activated carbon can filter out most chemicals, but not carbon monoxide. By selecting the right filter material, the electrochemical sensor can be more selective to its target gas.

Electrochemical sensors can be manufactured in a variety of ways, ultimately depending on the gas to be detected and the manufacturer, however, the main characteristics of the sensors are very similar in nature. Some common characteristics of electrochemical sensors are described as follows:

1. On a three-electrode sensor, a jumper wire is usually used to connect the working electrode to the reference electrode. If it is removed during storage, it takes a long time for the sensor to remain stable and ready for use. Some sensors require bias between the electrodes, and in this case, the sensors are shipped with nine-volt battery-powered electronics. It takes 30 minutes to 24 hours for the sensors to stabilize and up to three weeks for them to remain stable.

2. Most toxic gas sensors require a small amount of oxygen to function properly. The sensor has an air hole on the back for this purpose. It is recommended to perform a recheck with the manufacturer in applications where non-oxygen background gas is used.

3. The electrolyte of the cell in the sensor is a aqueous solvent, which is isolated by a hydrophobic barrier, which prevents the leakage of the aqueous solvent. However, like other gas molecules, water vapor can cross the hydrophobic barrier. Under high humidity conditions, prolonged exposure may cause excess water to build up and lead to leakage. Under low humidity conditions, the sensor may dry out. Sensors designed to monitor high concentrations of gases have a low porosity barrier to limit the molecular weight of the gas passing through, so it is not affected by humidity. Like sensors used to monitor low concentrations of gases, this sensor has a high porosity barrier and allows the free flow of gas molecules.

Three, pressure and temperature

The electrochemical sensor is very little affected by the pressure change. However, because the pressure difference within the sensor can damage the sensor, the same pressure must be maintained throughout the sensor. Electrochemical sensors are also very sensitive to temperature, so internal temperature compensation is usually taken. But it is better to keep the standard temperature as much as possible. In general, the sensor reading is higher when the temperature is above 25°C; Below 25°C, readings are low. The temperature impact is typically 0.5% to 1.0% per degree Celsius, depending on the manufacturer and sensor type.

Iv. Selectivity

Electrochemical sensors are usually highly selective to their target gases. The degree of selectivity depends on the sensor type, the target gas, and the concentration of the gas the sensor is to detect. The best electrochemical sensor is the one that detects oxygen, which has good selectivity, reliability and long life expectancy. Other electrochemical sensors are susceptible to interference from other gases. Interference data are calculated using relatively low gas concentrations. In practical applications, the concentration of interference can be high, resulting in incorrect readings or false alarms.

Life. Expectancy

The life expectancy of an electrochemical sensor depends on several factors, including the gas to be detected and the environmental conditions in which the sensor is used. Generally, the prescribed life expectancy is one to three years. In practice, life expectancy depends mainly on the total amount of gas exposed during sensor use and other environmental conditions such as temperature, pressure and humidity.

Vi. Summary

Electrochemical sensors have very low requirements for working power supply. In fact, of all the sensor types available for gas monitoring, they have the lowest power consumption. Therefore, this sensor is widely used in mobile instruments containing multiple sensors. They are the most used sensors in finite space applications. The life expectancy of the sensor is predicted by its manufacturer based on conditions they consider normal. However, the expected life of a sensor depends heavily on environmental pollution, temperature and the humidity to which it is exposed.