RU2471169C1 - Method to register variation of surface tension of solid electrodes that contact with high-temperature electrolytes - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: to record variation of surface tension of solid electrodes that contact with high-temperature electrolytes, an electrochemical cell is used from an investigated solid electrode, by means of a sound guide connected with a piezoelement, and also an electrode of comparison, a jacket with a thermocouple, a fire-resistant cooled test tube. The assembled cell is connected to the system of registration of surface tension variation in solid electrodes. At the same time an investigated solid electrode connected with a piezoelement, as well as a comparison electrode, a jacket with a thermocouple and a container of electrolyte are assembled into a separate module, which is placed into a fire-resistant test tube.

EFFECT: simplified assembly of an electrochemical cell, higher reliability of its structure, reduced material intensity and cheaper trial, possibility to use a test tube and a module many times, reduced sources of contamination for an investigated system.

7 cl, 7 dwg

Description

The invention relates to electrochemistry and can be used to study the interfacial boundaries between conductive solid electrodes in contact with molten, mainly high-temperature electrolytes, such as salts, hydroxides, slags.

There is a method of measuring variable surface tension (estans) of solid electrodes in contact with molten high-temperature electrolytes (Pastukhov Yu.G. Electrochemistry, 1994, Volume 30, No. 8, p. 997-1007) [1]. The method is carried out by means of an electrochemical cell, which is placed in a sealed chamber of a vacuum post and connected to a variable surface tension registration system. The meniscus is formed by vertical movement of the crucible - a container with molten electrolyte inside the chamber. The known method is characterized by high complexity, cost and cumbersome through the use of a vacuum post, the need to alter the sealed chamber of the vacuum post for mounting a high-temperature furnace and water cooling, the difficulty of isolating the entire installation from mechanical vibrations of the building.

A known method of measuring the estance of solid electrodes interacting with a high-temperature melt (Stepanov VP Physical chemistry of the surface of solid electrodes in salt melts. Yekaterinburg: Ural Branch of the Russian Academy of Sciences, 2005, p.196) [2]. The known method includes the assembly of an electrochemical cell and its connection to the registration system of the estans of solid electrodes. To do this, the studied solid electrode is placed in the cell, which is mechanically connected via a sound guide to a piezoelectric element located separately from the cell. In addition, a reference electrode, a thermocouple case, a fused silica tube 25-30 cm long, in which the melt is placed, are placed in the cell. The introduction of the melt into the test tube requires a reserve of free volume in the cell, since the solid electrolyte at room temperature has to be redistributed in the volume of the test tube, taking into account the need to place other cell elements. This complicates its assembly and significantly increases the electrolyte consumption, which is essential for expensive electrolytes, such as, for example, high-purity salts. Expensive fused silica tubes require replacement after 3-5 melts, since they lose transparency, become porous and can be a source of uncontrolled impurities. The fact that the piezoelectric element in the cell used is located separately from the cell on a special design makes it difficult to vertically move the electrode to form a hanging meniscus. The polarization voltage of the electrode under study, supplied through a separate conductor located in the hot zone, reduces the reliability of the structure, since thin metal wires in the environment of salt vapors at high temperature are prone to embrittlement and are often destroyed by mechanical stress. The cell neck is cooled by air flow, because the formation of more efficient water cooling of a quartz tube is associated with great technical difficulties. Practice has shown that air cooling is not efficient enough at temperatures above 800 ° C. In this case, the closing plug made of vacuum rubber acts as a source of additional pollution of the studied system.

Thus, the installation and use of the cell in the known method of measurement is associated with significant labor costs, and the percentage of failed experiments is unreasonably high. Due to the increased material consumption of the experiment, difficulties in assembling and using the cell, the known method for detecting the surface tension of a solid electrode has not been widely used.

The claimed method of detecting the surface tension of solid electrodes, comprising assembling an electrochemical cell from a solid electrode under study, by means of a sound guide connected to a piezoelectric element, a reference electrode, a thermocouple cover, a heat-resistant cooled test tube and connecting the assembled cell to a system for recording changes in the surface tension of solid electrodes. The method is characterized in that the test solid electrode connected to the piezoelectric element, the reference electrode, the thermocouple case, and the electrolyte container are assembled in a separate module, which is placed in a heat-resistant cooled test tube.

The assembly of the elements of the electrochemical cell - the solid electrode under study, the electrolyte container, the reference electrode, the thermocouple cover, connected to the piezoelectric element, in a separate module, which is placed in a heat-resistant cooled tube, reduces the used electrolyte volume, since only a small container capacity is required to be filled. In addition, excluded contact of the electrolyte with the walls of the tube, which is not always chemically compatible with the test substances and can be a source of contamination of the system. In addition, the modular principle of cell assembly simplifies its final stage, since in this case the orientation and fixing of the cell elements, loading of the electrolyte takes place outside the tube, and the operation of redistributing the electrolyte around the cell elements requiring visual inspection is excluded.

In the particular case of the execution of the method, a piezoelectric element connected to the solid electrode under investigation by means of a sound guide is placed in a separate metal case on the test tube cover on the movable part of a cylindrical glass section. As a result, the studied L-shaped electrode has a free vertical stroke, sufficient to bring it to the surface of the melt and the formation of the meniscus. In addition, they use a sound guide made of a ceramic tube, inside of which a current-conducting wire is passed and having a cut in the upper part, through which a current-conducting wire is brought out through a pressure lead in the piezoelectric element body. In this case, the current supply to the test electrode is located inside the sound guide, which allows you to freely move the electrode vertically inside the cell without the risk of disturbing electrical contact and possible distortion of the electrode due to the elasticity of the outer conductor. Thus, the need for a separate current supply in the cell structure disappears.

The electrical connection of the electrolyte container with the output of the potentiostat allows it to be used as an auxiliary electrode for polarization of the studied conductor by direct and alternating current, thereby simplifying the design of the cell. The use of heat-resistant steel tubes is advantageous for several reasons: a longer service life compared to quartz and a lower price; less gas permeability compared to quartz; the possibility of organizing water cooling of the cell neck; shielding and leveling the temperature field inside the cell due to the electrical and thermal conductivity of the metal walls. In addition, the tube is sealed with a viscous fast-hardening silicone sealant of sufficient viscosity to hold it at the place of sealing without draining down.

A new technical result achieved by the claimed method consists in simplifying the assembly procedure of the electrochemical cell before the experiment and during repair, increasing the reliability of the cell design, reducing the material consumption and making the experiment cheaper, the possibility of reusing both the test tube and the module placed inside it, and reducing the sources of contamination of the test system.

The invention is illustrated by drawings, in which Fig. 1 shows a general view of an electrochemical cell, Fig. 2 shows a device for vertical supply of an electrode with a piezoelectric element, Fig. 3 shows a diagram of the connection of devices in an installation, and Fig. 4 shows the dependences of the module, estance phase and current polarization of the gold electrode in a potassium chloride melt, in Fig. 5 - dependences of the module, estance phase and polarization current of a silver electrode in a potassium chloride melt, Fig. 6 - dependences of the module, estance phase and polarization current of a copper electrode in a eutectic ces chloride melt Ia and sodium, in Fig.7 - the dependence of the modulus, the estance phase and the polarization current of the glassy carbon electrode in the molten potassium bromide.

The cell for measuring the surface tension of solid electrodes contains a test tube 1 made of heat-resistant steel, for example, grade Х18Н9Т, equipped with a water cooling jacket 2, fitting 3. The diameter of the pipes and supply pipes to the cooling jacket, the water circulation device must allow the cooling liquid to circulate in the laminar flow, so that avoid the occurrence of noise affecting the registration of estance. Two cuts 4 of sufficient size were made in the neck of the tube for insertion between the cap and neck of the tip of a bench screwdriver. The test tube is closed with a metal cap 5 with heat shields 6, fixed with pins 7 by means of dividing sleeves 8. Under the nuts of the pins, plates 9 with holes for hanging a container with an electrolyte 10 made of a standard platinum crucible equipped with platinum suspensions 11 with a diameter of 1 mm welded to the edge crucible. One of the suspensions is welded to the current lead 12, which is connected to the output of the auxiliary electrode of the potentiostat. For electrical separation of the circuits of the potentiostat and the grounded mass of the tube are corundum insulating plates 13, tube 14, crucible 15.

A cover with a thermocouple 16, a reference electrode 17, a sound guide 18, on which the test electrode 19 is mounted, is passed through the openings in the lid and screens. The length of the sound guide is selected based on the cell size and usually it is 25-30 cm. The vertical electrode is supplied using a cylindrical section with motionless 20 and mobile 21 parts. On the movable part of the thin section, a housing 22 is installed containing a piezoelectric element and electrical leads (not indicated). The clamp 23 limits the vertical displacement of the body of the piezoelectric element, and the rack 24 prevents its rotation. A tube 25 for supplying gas is passed through a cover with shields.

The device body of the piezoelectric element (figure 2) contains a metal housing 26 with a cover 27, which are additionally screens from spurious interference. The inner part of the glass thin section 21 is glued into the housing 26, the outer part 20 is fixed in the cap of the test tube 5. Vacuum grease is applied to the thin section. Piezoelectric element 28 is a factory-made sound-emitting element of the type ZP-1. It is an elastic plate with a diameter of 30 mm with a glued thin piezoceramic disc with plates. The amplitude of a signal recorded from such a piezoelectric element under resonance conditions reaches an order of millivolts. The piezoelectric plate is fixed in the housing using a spring clip 29 on the studs 30. One ceramic lining is grounded to the housing 26, from the second lining the signal is passed through the conductor through the hole 31 in the lid 27, and a selective coaxial cable 32 is fed to the preamplifier of a selective nanovoltmeter. The hole is filled with epoxy resin along with the end of the cable to achieve tightness. The braid of the cable is connected by soldering to the cover 27. On the lower side in the center of the piezoelectric element plate, a brass screw clamp 33 is attached to the sound guide 18. The sound guide 18 is made of a ceramic material, for example corundum, stable in the form of measurements, in the form of a tube or a two-channel straw 3 mm in diameter . The upper part of the sound guide 18, which enters the clamp, has a flange, in the plane of which the clamp screw 34 abuts. Inside the sound guide 18, a platinum wire 35 is passed through which the constant and alternating components of the polarization voltage of the test electrode 19 are supplied. The wire 35 has electrical contact with the test electrode 19 in the lower part of the sound guide 18, its other end is passed through a cut 36 in the upper part of the sound guide 18, and then through the hole 37, filled with epoxy resin, is connected to the potentiostat. The studied electrode 19 is mounted on the sound guide 18 with a clamp 38 from 5-10 turns of wire, preferably platinum.

The cell assembly is as follows. A module is assembled, consisting of a cover with screens, an electrolyte container, a case with a thermocouple, a reference electrode, a test electrode mounted on a sound guide with a piezoelectric element. In the case of studies of systems with alkali metal halides, it is convenient to use a lead reference electrode. For reproducibility of the data, it is advisable to load a new portion of the electrolyte into the housing of the reference electrode before each experiment. In a container of electrolyte located in a previously prepared module, crushed salt is loaded, after which the module is placed in a test tube with an insulating crucible. All joints and gaps in the structure are filled with quick-hardening silicone sealant of sufficient viscosity to hold it at the place of sealing without swelling down. The lower part of the tube is placed in a resistance furnace and connected to the gas system through a tube 25 and a fitting 3 (Fig. 1). Before the experiment, the tube is heated and vacuum to a residual pressure of 1-10 Pa to remove moisture and air. The duration of this procedure depends on the moisture content of the electrolyte. The experiment is conducted in the purge mode with purified inert gas at a speed of 3-5 liters per hour.

The cell oven should be installed on a solid base, isolated from external vibrations. To do this, as practice has shown, it is advisable to use layers of paper and rubber with a thickness of 10-20 cm on which a stove with a stove is installed as damping material. In another embodiment, use a plate weighing 50-100 kg on rubber supports. Combinations of the base device are possible depending on the intensity of external vibrations.

To obtain diagrams of the estans of solid electrodes in contact with high-temperature melts, the cell assembled as described above is connected to a system for recording changes in the surface tension of solid electrodes, a diagram of which is shown in FIG. 3. According to this scheme, an alternating signal with an amplitude of 10-30 mV, depending on the quality factor of the electrode - sound guide - piezoelectric element from the generator is mixed with a constant voltage in the potentiostat and fed to the electrode under study. The potentiostat can work in potential stabilization mode, which will correspond to the measurement of the φ-estance, or in the current stabilization mode, then the q-estance will be measured. The potential sweep should be carried out in a cyclic mode, in this case it is easy to check the stability of the state of the system by the coincidence of the curves in successive cycles. To form a cyclic sweep, a reference signal is applied to the potentiostat in the form of a linearly varying voltage from the minimum value of the desired polarization potential to the maximum. The rate of change varies from millivolts per second to hundreds, depending on need. The alternating signal from the piezoelectric element is amplified by a broadband preamplifier and output to an oscilloscope to search for the maximum amplitude at the resonant frequency, then it is amplified at the resonance frequency by a selective nanovoltmeter. The amplitude of the rectified signal is a modulus of estance. The phase meter records the phase difference between the selectively amplified signal and the original voltage of the generator. Simultaneously record in time the polarization potential, polarization current, estance module, estance phase using a multi-channel analog-to-digital converter connected to a computer.

To disassemble the cell, at the end of the experiment, the tips of the screwdrivers are inserted into the cuts 4 and rotated around its axis, detaching the lid from the tube, after which the assembly is removed, holding it by the lid with screens and the melt is poured out of the container 10. It is undesirable to solidify the melt in the container due to its deformation as a result of electrolyte shrinkage during solidification.

Processing the experimental results consists in interpreting the diagrams of the dependences of the current on the polarization voltage, the modulus, and the estance phase on the polarization voltage.

In the following figures, the numbers denote: 1 - polarization current density, 2 - modulus of the surface tension derivative with respect to the potential (| ∂γ / ∂φ |) - estans modulus, 3 - estans phase in degrees. Figure 4 shows a diagram of the estance of a gold electrode in a potassium chloride melt. The estance module has 3 minima, accompanied by a phase change by an amount close to 180 °, this indicates a 3-fold change in the sign of the charge on the electrode surface as a result of specific anion adsorption. Figure 5 shows a diagram of the estance of a silver electrode in a cesium chloride melt, showing an overcharge of the silver surface at anodic polarization, similar to a gold electrode. 6 is a diagram of the estance of a copper electrode in a eutectic melt of cesium and sodium chlorides, showing the absence of a specific interaction of copper with halide melts, accompanied by a change in the sign of the surface charge, since the estance phase does not change in the anode potential region. 7 is a diagram of the estans of a glassy carbon electrode in contact with a molten potassium bromide, showing the absence of surface overcharging during anodic polarization.

The claimed method allows to reduce the material consumption of the experiment, to simplify the assembly and use of the cell, and can be used, as can be seen from the example, to obtain estance diagrams of metal, carbon electrodes in contact with high-temperature melts of electrolytes.

Claims (7)

1. A method for detecting changes in the surface tension of solid electrodes in contact with high-temperature electrolytes, including the use of an electrochemical cell from the studied solid electrode, by means of a sound guide connected to a piezoelectric element, a reference electrode, a thermocouple cover, a heat-resistant cooled test tube, and connecting the assembled cell to the surface change change registration system tension of solid electrodes, characterized in that the studied solid electrode connected to the piezoelectric element , a reference electrode, a thermocouple case, and also an electrolyte container are collected in a separate module, which is placed in a heat-resistant test tube. 2. The method according to claim 1, characterized in that the piezoelectric element connected to the studied solid electrode by means of a sound guide is placed in a separate metal housing on the test tube lid on the movable part of a cylindrical glass section. 3. The method according to claim 1, characterized in that they use a sound guide made of a ceramic tube, inside of which a current-carrying wire is passed. 4. The method according to claim 1, characterized in that they use a sound guide having a cut in the upper part, through which a lead wire is led out through a pressure lead in the piezoelectric element housing. 5. The method according to claim 1, characterized in that the electrolyte container is electrically connected to the output of the potentiostat. 6. The method according to claim 1, characterized in that the module is placed in a test tube of heat-resistant steel with water cooling of the neck. 7. The method according to claim 1, characterized in that the test tube is sealed with a viscous fast-hardening silicone sealant.

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