Sensitive element of electrochemical sensor for carbon monoxide in gas mixtures

IPC classes for russian patent Sensitive element of electrochemical sensor for carbon monoxide in gas mixtures (RU 2326375):

G01N27/406 -

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FIELD: measuring devices.

SUBSTANCE: sensitive element of the electrochemical sensor for carbon monoxide in gas mixtures is made from a solid oxide electrolyte with polarised and non-polarised electrodes on its surface. The electrolyte is in the form of plates. The comparison non-polarised electrode is put on the surface of the electrolyte in form of a paste of silver oxide, while the operating polarised electrode is put on the surface of the electrolyte in form of a fine-dispersed platinum powder or palladium with a dope of up to 10% of the electrolyte powder. Both electrodes are baked, and the operating electrode is activated.

EFFECT: lowering of operating temperature of the sensor down to possibility of measuring non-equilibrium values of carbon monoxide in the presence of free oxygen, exclusion of division of the gas space of the electrochemical cell, which greatly simplifies the structure of the measuring cell and improves its operational characteristics.

1 ex, 1 tbl, 1 dwg

The invention relates to analytical chemistry and instrumentation and can be used both in laboratory practice and in various industries, in particular in thermal power plants, where severe problems of ecology, energy saving and fuel economy, in other cases, requiring optimization of combustion processes with minimal emissions of carbon monoxide.

It is known device [A.S. No. 646243 the USSR, G01N 27/46, "high Temperature electrochemical cell, Mvhline, Blazin, Bvecome, Asclepion], where as a sensitive element, an electrochemical sensor is used tablet of porous refractory material, impregnated with a solid electrolyte based on zirconium dioxide, with primechanie from opposite sides of the electrodes. Tablet with electrodes tightly inserted in eloundou tube, so the cell is obtained from the separated gas space one electrode (reference) air, another worker is in the atmosphere of the test gas. Knowing the temperature and EMF of a cell on the corresponding Nernst equation defines the partial concentrations of CO, CO₂or other oxygen-containing gas.

Significant disadvantages of this device are the circumstances that this device is very complicated to manufacture, operates at high is erature up to 1000° C and above, this requires total separation of the gas spaces. Measurement of carbon monoxide is only possible in the absence of free oxygen in the sample gas. In General, the working conditions of the sensor must be capable of measuring thermodynamic EMF of the cell.

Common symptoms are known and the inventive device is that both using a solid oxide electrolyte based on zirconium dioxide coated with electrodes on its surface.

The closest analogue (prototype) to the claimed invention's technical solution is the sensitive element of the solid electrolyte cell containing a solid electrolyte with the measuring and reference electrodes [A.S. No. 1427280 the USSR, G01N 27/46 "method for determining the oxygen-containing components in gas media", Mvhline]. The cell consists of the solid electrolyte tubes coated inside two electrodes - nepostizhimy measuring and polarizable measurement, are analyzed in the atmosphere. The third reference electrode is located outside of the tube and surrounded by air. Running the cell at a temperature C. By measuring the electromotive force between the measuring electrodes and the reference nepalaikomas electrode on the basis of the Nernst equation calculate the quantity of the analyzed component.

The disadvantages of this the disorder are significant complexity, the need for additional measurements, titration process and the associated additional devices, you also need a clear separation of the gas spaces. The carbon monoxide in the sample gas can be determined only when the absence of free oxygen. Does the sensing element at high temperatures, where the observed thermodynamics.

Common features of the prototype and the claimed device are the presence of the solid oxide electrolyte is coated on the surface of the electrodes and the measurement of analog signals between the polarizable and nepalaikomas electrodes.

An object of the invention is to reduce the temperature of the sensor to the measurement of non-equilibrium values of carbon monoxide in the presence of free oxygen, the exception to the separation of gas space of the electrochemical cell, which dramatically simplifies the design of the measuring cell and improves its operational characteristics.

The problem is solved due to the fact that as the sensor uses the record of stable solid solution of zirconium dioxide, while the reference electrode is made by coating on the surface of the electrolyte paste of silver oxide, the working electrode is made by coating on the surface of electroly is as fine powder of platinum or palladium with the addition of up to 10% of the powder electrolyte, both electrode pripuceny, and the working electrode is activated. As records of the electrolyte used sufficiently thin samples is of the order of 0.2 mm, an area of approximately 0.2-0.3 cm².

For reference electrode used silver paste, made from fine silver oxide and pounded in ethanol with the addition of polyvinylbutyral. The paste was applied on the surface of the plate and was burnt at 800°C for 30 minutes, and the silver oxide is decomposed (˜300° (C) and pure fine powder of silver was prepenalty to the electrolyte. This electrode was indifferent to the existence of the analyzed gas and was used as depolarizing reference electrode.

The working electrode was prepared so that its interphase was adsorbiroval mainly carbon monoxide, with its concentration corresponded to the contents in the analyzed gas. The potential of the working electrode with the growth of the carbon monoxide is shifted relative to the reference in the negative direction. For this purpose, we used the fine powders Pd or Pt with the addition of up to 10% of the electrolyte powder, which was ground in an alcohol solution polyvinylbutyral, was applied to the appropriate side of the tablet electrolyte and hot at 1100°within an hour. The working electrode was subjected to activation and, to increase the number of active sites capable of adsorbing molecules of carbon monoxide.

Comparative analysis of the present invention with the prototype allows to conclude that the claimed technical solution is different from the known fact that the electrolyte used is not the test tube from the electrolyte, and a thin plate covering 0.2-0.3 mm²that for the manufacture of electrodes used the paste of silver oxide and paste from fine powder of platinum or palladium powder electrolyte, which was burnt to the ground sides of the tablet oxide electrolyte. Additionally, the working electrode is appropriately activated.

The result is a sensitive sensors operating in the temperature range from 300 to 550°and concentrations - from 10 to 10⁵ppm.

The invention can be illustrated by an example.

Example. The electrolyte used solid solution of ZrO₂+ 10% Y₂About₃. The record of this electrolyte thickness ˜0.2 mm polished diamond circle, washed with distilled water and alcohol, and was annealed at 700°C for 30 minutes.

For reference electrode used silver paste, made from fine silver oxide by rubbing it with ethyl alcohol with the addition of polyvinylbutyral. PA is the applied on the polished surface of the electrolyte, was dried and burnt at 800°C for 30 minutes. This electrode was indifferent to the existence of the analyzed gas.

For preparation of working (indicator) electrode used a paste made of fine powders Pd or Pt with the addition of up to 10% of the electrolyte powder, which was ground in an alcohol solution polyvinylbutyral. The paste was applied on the corresponding ground side of the tablet electrolyte was dried and burnt at 1100°C for one hour. Primechanie electrodes activated.

The sensing element was placed in a glass tube, the electrodes drew silver electrodes, the ends of which are brought out and connected to the voltmeter. Through a glass tube with a constant controlled speed blew four-component gas mixture consisting of nitrogen, oxygen, carbon dioxide and carbon monoxide with a given concentration. The concentration of the latter was replaced by nitrogen or carbon dioxide, because their concentration is practically no influence on the electrode potentials of the cell.

The drawing shows the dependence of the deflection potentials (E) of palladium electrodes on the concentration of carbon monoxide in the gas phase cells (P_CO) at a temperature of C. Curves 1, 2 belong to different cells and shown for each p the pit and reverse changes in the content. These curves show a good reproducibility of the results. Shows similar results and platinum working electrodes.

Because electrochemical sensors operate at low temperatures, when the Nernst equation is not applicable, the necessary calibration of the sensing element. The table below shows an example of calibration of the platinum sensor using a calibration gas mixtures. Calibration was as follows. Analyzed the gas of specific composition was passed through the cell with Pt sensor, while the measured stationary potentials (E) of the working electrode, which is shown in the table, relatively indifferent silver electrode. In the coordinates of the E - ln P_COthe obtained dependence is close to linear. This allows us to mathematically describe the results that plays a significant role in the development of the gas analyzer for carbon monoxide on the basis of this sensor and its calibration.

At all temperatures and concentrations of carbon monoxide performance of the electrochemical element is 4-8 seconds, the sensitivity of better than 10 ppm at the beginning of the range of concentrations FROM stationary signal is stable over time. The sensors can operate in the following atmospheres: not containing oxygen, nitrogen, inert gases; containing bound oxygen - CO₂N₂2and containing free oxygen concentrations from 0 to 21% (air). Temperature range sensor 300-550°With the range of measured concentrations from 0.001 to 10%vol.

Electrochemical sensors miniature, have absolutely no fear of thermal shock, have a long and continuous mode of operation, storing them is not limited by time.

Thus, the above data confirm that the combination of stated characteristics of the sensing element, an electrochemical sensor WITH gas mixtures can solve the problem.

Calibration gas	Pt sensor
P_CO, ppm	E, mV
50	25,7
100	to 49.3
200	72,9
500	104,0
1000	uniforms, 127.6

The sensor element of an electrochemical sensor carbon monoxide in gas mixtures made of solid oxide electrolyte coated on the surface of the polarizable and nepalaikomas electrodes, characterized in that the electrolyte is made in the form of a plate, with nepostizhimy the reference electrode deposited on the surface of the electrolyte in paste form of silver oxide, and polarisable the working electrode deposited on the surface of the electrolyte in the form of a fine powder of platinum or palladium with the addition of up to 10% of the powder electrolyte, both electrode pripuceny, and the working electrode is activated.