1 Introduction
Carbon monoxide (CO) is colorless, tasteless gas, which has high toxicity to animals and humans. When CO reaches 400 × 10-6 volume fractions, there is a headache after 1 ~ 2 h; when the 1600 × 10-6 volume fraction, it will feel headache, dizziness, and after 1 h will lead to death.
Many CO gas detection system, use of potential, current, conductivity, refractive index of light, the light absorption wavelength of the physical properties of a physical sensor for detecting the like; the chemical reaction, electrochemical reaction, chemical adsorption, chemiluminescence detection chemical sensors. Electrochemical gas sensor having a detecting multiple gas species, a wide concentration range, small size, low price, high accuracy, can be used for field detection, etc., have been widely used in environmental monitoring and production safety. At present, Japan's Riken keiki company, New Cosmos Electvic, Germany Drager, the United States Gastech, British City CO sensors and other production companies in the international market. China started relatively late in terms of gas sensors, although some work has been done, but compared with international counterparts still a big gap.
Because conventional electrochemical gas sensor using a liquid electrolyte, and the pollution caused by evaporation of the electrolyte sensor signal attenuation or drop, short life, in general, only about one year life. To avoid this problem caused due aqueous electrolyte, it is natural to turn their attention to the solid electrolyte. Introducing the solid electrolyte, the traditional electrochemical gas sensor has entered a new era. Currently, the use of electrochemical sensors have been developed into an international hot spot study of electrochemical gas sensor body with polymer electrolyte (SPE). In this paper, the Nafion membrane developed solid CO gas sensor (hereinafter referred to as sensors), the catalyst and membrane treatment processes and issues to explore a series of improvements.
2, the working principle
Air flow through the diffusion into the cell containing CO sensor, an oxidation reaction occurs on the working electrode constant potential, while the electrode on the occurrence of the reduction reaction, a redox reaction to produce the corresponding limiting diffusion current, its size and the volume fraction into CO proportional, that is,
Where: i is the steady-state current diffusion; Z is the number of electrons transferred; F. Is the Faraday constant; reaction area S; D is the diffusion constant; [delta] is the thickness of diffusion layer; C measured volume fraction of CO.
Under operating conditions, Z, F, 5, D, δ are constants, and therefore, the measured diffusion current i, the volume fraction of CO can be obtained.
3, activation technique Electrodes
Sensitivity and response time of the sensor depends on the working electrode and the counter electrode, the working electrode and the counter electrode performance is determined by the activity of the noble metal catalyst to improve the activity of the noble metal catalyst is a key technology, and therefore a series of problems for the activation process Improve.
Currently, the most common is the phase of the direct reduction of chloroauric acid in water to give gold nanoparticles. In this paper, this method is improved, as the thiol compound selected gold nanoparticles stabilized ligand, while adding citrate and an amphiphilic surfactant, the particle size obtained by varying the controllable stabilizer gold nanoparticles and gold compounds. Using this catalyst with high activity, preparation of an electrode having high catalytic activity, which is shown in Figure 1 of AFM.
4, a liquid electrolyte Solidification techniques
Determined by the lifetime of the sensor electrolyte, liquid electrolyte dry faster, easy to leak, the seal is difficult, therefore, to extend the life of the sensor, the key technology is solidified electrolyte. Currently, the solid electrolyte is classified into inorganic solid electrolytes and polymer electrolytes. Conductivity film partially present in the polymer electrolyte (e.g., Nafion membrane wettability) is comparatively high, but it also adds additional conditions of use: the good must be controlled humidity and temperature of the external environment. Prepared by impregnation Nafion electrolyte membrane, formed of different moisture contents Nafion membrane electrolyte by controlling the reaction time and temperature. Hydrogen bonding (FIG. 2) because water is formed with the sulfonate Nafion membrane of branches, tightly locked in the internal structure of the Nafion membrane, Nafion membrane so that the electrolyte is solid. At the same time, a series of exploration for the Nation membrane treatment process moisture problem, solved the technical key solidified electrolyte.
5, anti-jamming, anti-poor research environment sensor technology
The lifetime of the sensor affected by the use environment. In order to cross the gas anti-interference, high resistance, temperature, humidity, shock, vibration or the like, has been designed in terms of structure and circuit. Structure of the sensor increases a grant electrodes (FIG. 3), in the atmosphere of O2 and Nafion membrane electrolyte H + synthetic H2O, to grant electrodes interfering substances H2 atmosphere is decomposed into H +, forming Nafion membrane electrolyte from pay, then , has been plagued by problems Nafion membrane electrolyte - because of long working hours and lead to water shortages, it will be solved.
Since the electrodes of the sensor using a noble metal material, catalyst poisoning occurs when the presence of high concentrations of gas, in order to avoid poisoning of the reference electrode, one end of the sensor using a cylindrical sealed structure, so that neither a reference electrode in contact with the gas, but also constant holding potential.
The operating principle of the sensor requires, to a working electrode at a potential kept constant at values (relative to the reference electrode potential) before normal operation. How to keep constant the potential of the working electrode is critical to the design, which may be achieved by an operational amplifier. Adjust the potentiometer to select a given potential. An operational amplifier in a differential mode of operation, while at the same potential when the phase-inverting input terminal and an inverting input terminal and an output terminal until a zero potential, so as to ensure a constant electrode potential of about 300 mV.
6. Conclusion
This sensor is in the 0 ~ 500 × 10-6 volume fraction, the sensitivity of 0.1μA / 10-6 volume fraction, signal loss of 1% a year. Nafion membrane was developed using the solid-state controlled potential electrolysis type gas sensor CO, the CO gas sensor is compared with the performance of the whole liquid has improved greatly, and its life has been greatly improved signal stability, electrochemical sensor miniaturization, long the direction of the development of a useful life of exploration.
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