RU2251763C2 - Preparation of vanadium electrolyte with aid of asymmetric vanadium-reducing electrolyzer and use of asymmetric vanadium-reducing electrolyzer for reducing electrolyte charge state balance in operating reduction-oxidation vanadium battery - Google Patents

Abstract

FIELD: renewable electrochemical devices for energy storage in reduction-oxidation batteries.

SUBSTANCE: novelty is that acid vanadium electrolyte liquor that has in its composition V⁺³ and V⁺⁴ in desired concentration ratio introduced in electrolyte solution is produced from solid vanadium pentoxide by electrochemical method while at least partially reducing dissolved vanadium in acid electrolyte liquor; for the purpose electrolyte liquor is circulated through plurality of cascaded electrolyzers at least partially to V⁺³ degree; in this way reduced vanadium incorporating electrolyte liquor leaving the last of mentioned electrolyzers enters in reaction with stoichiometric amount of vanadium pentoxide to produce electrolyte liquor incorporating in effect vanadium in the V⁺³ form; acid and water are introduced to ensure definite molarity of liquor and the latter is continuously circulated through cascaded electrolyzers; stream of electrolyte liquor produced in the process that incorporates V⁺³ and V⁺⁴ in desired concentrations is discharged at outlet of one of electrolyzers of mentioned cascade. Each electrolyzer is distinguished by high degree of asymmetry and has cathode and anode of relevant surface morphology, geometry, and relative arrangement for setting current density on anode surface exceeding by 5 to 20 times that on projected cathode surface, oxygen being emitted from anode surface. Asymmetric electrolyzer of this type can be used in one of electrolyte circuits, positive or negative, of operating battery (cell) for reducing balance of respective oxidation degrees of their vanadium content.

EFFECT: facilitated procedure and reduced coast of vanadium electrolyte preparation.

10 cl, 4 dwg, 1 ex

Description

State of the art

The invention generally relates to a renewable electrochemical device for storing energy in systems of a redox flow battery (accumulator) and, in particular, to so-called fully vanadium redox secondary batteries (accumulators).

A vanadium reduction and oxidation flow battery, also called a fully vanadium reduction and oxidation cell or simply a vanadium reduction and oxidation cell or battery, uses V (II) / V (III) and V (IV) / V (V) as two reduction -oxidative pairs in negative (sometimes called anolyte) and positive (sometimes called catholyte) semi-electrolyte solutions, respectively.

The usual electrolyte used in a vanadium battery consists of 50% vanadium ions with an oxidation state of +3 and 50% vanadium ions with an oxidation state of +4.

The electrolyte is usually divided into two equal parts, which are respectively placed in the positive and negative chambers of the battery, and more precisely - in the corresponding circulation circuits. In this initial state, the battery has an open circuit voltage of almost zero.

When the current is passed through the battery by an external source with a sufficiently high output voltage, V ⁺⁴ (50%) in the negative electrolyte will be restored to V ^+3, and at the same time, V ⁺³ (50%) in the positive electrolyte will oxidize to V ⁺⁴ .

At a certain time, the negative electrolyte constantly circulating through the corresponding electrode chambers of the battery due to the circulation pump of the negative electrolyte will contain only V ⁺³ , and the positive electrolyte circulating through the corresponding electrode chambers of the battery using the circulating pump of the positive electrolyte will contain only V ⁺⁴ .

In this state, the battery is considered to have a zero charge state (C3), and the open circuit voltage for such a battery will be approximately 1.1 V.

With continued supply of the “charging” current through the battery on the negative electrode, V ^{+3 will be} restored to V ⁺² , and on the positive electrode, V ⁺⁴ will oxidize to V ⁺⁵ . Upon completion of this conversion (at the end of the recharging process), the battery will have an open circuit voltage of about 1.58 V, and the battery will be considered to have an SOC of 100%.

Vanadium is commercially available as vanadium pentoxide (or also ammonium vanadate). In any case, it is usually realized in an oxidation state of +5.

The charging capacity of the installation of a fully vanadium redox battery is determined by the amount of vanadium dissolved in the acidic electrolyte. For a given molarity of electrolyte solutions, the charging capacity is directly proportional to the volume of two electrolytes.

There is an obvious need to obtain acidic vanadium solutions suitable as an electrolyte for the first charging of two circuits of a reduction-oxidation battery system and / or to increase the charging capacity of an active battery using commercially available vanadium pentoxide (or ammonium vanadate) as a starting material (raw material).

The method of preparing a vanadium electrolyte is therefore a method according to which V ₂ O ₅ is dissolved in sulfuric acid (or in another acid) and reduced to the desired mixture of V ⁺³ (about 50%) and V ⁺⁴ (about 50%).

Finely ground (powder) solid vanadium pentoxide is only slightly soluble in water or in an acid such as, for example, sulfuric acid; therefore, a simple method for preparing an electrolyte by dissolving V ₂ O ₅ in acid is not possible.

To dissolve V ₂ O _{5 it is} first necessary to restore it to a lower (easier soluble) oxidation state.

Various methods have been proposed for dissolving and reducing V ⁺⁵ , mainly through the use of reducing compounds, or sophisticated electrolytic or chemical processing methods have been proposed.

Document EP-A-0566019 discloses a method for producing a vanadium electrolyte solution by chemical reduction of vanadium pentoxide or ammonium vanadate in concentrated sulfuric acid, after which the precipitate is heat treated.

Documents WO 95/12219 and WO 96/35239 disclose an electrochemical-chemical method for preparing a vanadium electrolyte solution from solid vanadium pentoxide, as well as a method for stabilizing it. Vanadium pentoxide is dissolved at the special louvre cathode of the ion-exchange membrane electrolyzer while passing through a suspension of vanadium pentoxide during contact with the louvre cathode.

The methods and methods developed to date for the preparation of the corresponding vanadium acidic electrolyte are quite complex and expensive. On the other hand, for the overall cost-effectiveness of a system of a fully vanadium in-line reducing and oxidizing battery, the presence of a relatively low cost vanadium electrolyte solution is an important factor in evaluating the profitability of a vanadium reducing and oxidizing battery compared to other energy storage systems.

To fulfill these requirements, it is first necessary to use a relatively inexpensive solid vanadium pentoxide as a raw material.

The purpose and essence of the invention

A very simple and inexpensive method was found for easily dissolving and reducing vanadium pentoxide in an acidic electrolyte.

This invention is particularly suitable for the preparation of a vanadium electrolyte from a raw material in the form of vanadium pentoxide (or ammonium vanadate), and it is carried out by using very simple and inexpensive electrolyzers while minimizing the auxiliary processing of the solution.

However, the method according to this invention is still quite efficient in terms of energy consumption.

The method according to this invention is essentially continuous, and according to it, solid vanadium pentoxide (V ₂ O ₅ ) is continuously added in a finely divided or powdered form, acid and water into a certain volume of the circulating vanadium electrolyte solution to maintain a certain molarity of the solution; in this case, an equivalent volume of an electrolyte solution containing V ⁺³ and V ⁺⁴ in substantially the same or other desired concentration is continuously withdrawn.

The discharge flow of the electrolyte solution is an indicator of the performance of the method.

Basically, the method of the present invention is that:

- an electrolyte solution is passed in contact with the cathodes of a plurality of electrolysers in the hydraulic cascade in order to sequentially restore part or all of V ⁺⁴ contained in the solution entering the first electrolyzer to V ⁺³ , and sometimes in small quantities even to V ^{+ 2} , into the electrolyte solution at the outlet of the electrolyte solution from the last cell of the plurality of cells in the cascade;

- reacting thus restored vanadium contained in the electrolyte solution at the outlet of the last cell with a stoichiometric amount of vanadium pentoxide (V ₂ O ₅ ) in a dissolution vessel equipped with a stirring means, thereby obtaining an electrolyte solution containing an appropriate amount of dissolved vanadium, which can be almost completely in a state of V ⁺⁴ ;

- introduce acid - sulfuric or other equivalent acid and water into a vanadium electrolyte solution (eg, close to V ⁺⁴ ) to maintain its specific molarity;

- carry out the recirculation of the electrolyte solution through the cascade of electrolysis cells, while diverting the flow of the electrolyte solution containing V ⁺³ and V ⁺⁴ , preferably in similar concentrations, at the outlet of one cell from the plurality of cells in the cascade.

An essential feature of electrolyzers is that their cathode and anode have appropriate surface morphologies, geometry, and relative positions to establish a current density on the anode surface that is 5 to 20 times higher than the current density on the cathode surface; and oxygen is released on the anode surface.

In practice, the cathode may be carbon felt or activated carbon felt, or a similar material providing a relatively large surface area, and may have a tubular or even grooved shape, and the anode may be a thin rod mounted along the geometric axis of the tubular or grooved cathode.

The relatively large specific active area of the cathode compared to the specific active area of the anode, and their ratio of the projected area are such that they determine the current density on the active anode surface, 5 to 20 times higher than the current density on the geometrically projected cathode surface.

When working with a cathode current density of the order of one several hundred A / m ^{2 of the} projected area and by selecting the diameter of the anode rod “concentrically” located relative to the tubular or equivalent at least partially covering the cathode, it is possible to create an anode current density in excess of 1000 A / m ² or much higher.

Under these conditions, significantly disproportionate values of current densities and a relatively high density of the anode current, when the current supplied to the electrolytic cells are regulated to ensure that the cathodic reduction of V ⁺⁴ to V ⁺³ remains almost completely the only cathodic semi-element reaction (i.e., limiting the maximum current density, in order to eliminate such adverse reactions as hydrogen evolution), the anode half-element reaction begins to be provided primarily by the oxygen evolution reaction (electrolysis water).

In fact, the thermodynamically privileged anode half-element reaction of the oxidation of V ⁺³ to V ^{+4 is} practically and effectively restrained by the essentially insufficient rate of migration and, ultimately, diffusion of V ⁺³ ions from the bulk of the electrolyte, filling the gap between the anode and cathode spaces, to the anode surface of the cell.

Another significant obstacle to the migration and / or diffusion of vanadium ions to the anode surface is the presence of gaseous oxygen bubbles, which are intensively released on the anode surface at such relatively high current densities.

The current passed sequentially through the set of electrolysers in the hydraulic cascade can be adjusted depending on the flow rate of the electrolyte through the cascade of electrolysis cells to obtain almost complete restoration of all V ⁺⁴ to V ⁺³ in the electrolyte leaving the last electrolyzer in the cascade.

Of course, this is an ideal condition, since a minimum (residual) amount of V ⁺⁴ may be present in case of a lack of current flowing through the electrolysis cells, or, conversely, in the case of excess current, an embryonic restoration of V ⁺³ to V ⁺² can occur, as a result which, in the electrolyte leaving the last cell, a small amount of V ⁺² may be present together with V ⁺³ .

The anode has an electrocatalytic surface with low oxygen overvoltage to facilitate oxygen evolution, and, most importantly, it is resistant to acidic electrolyte under conditions of anodic polarization and oxygen evolution.

For example, the anode may be a rod of valve metal resistant to anodic destruction, for example, titanium, tantalum or their alloys, with a non-passivating active coating of the oxygen evolution electrocatalyst.

The coating may be a mixed oxide or a mixture of oxides consisting of at least a noble metal such as iridium, rhodium or ruthenium, and at least a valve metal such as titanium, tantalum and zirconium. The active coating may also consist of a coating with a noble metal such as platinum, iridium or rhodium, or the same metals dispersed in an electrically conductive oxide matrix.

In a dissolution vessel equipped with a conventional mechanical stirrer, the electrolyte solution exiting the last electrolyzer is contacted with a stoichiometric amount (indicated as the amount of V ⁺³ (V ⁺² ) contained in the reduced electrolyte solution) of solid vanadium pentoxide in finely divided (powder) form prepared by grinding and / or sieving solid vanadium pentoxide so as to introduce particles with a maximum size of not more than 100 μm.

The drained (decanted) or filtered solution is removed into the tank, and undissolved particles of vanadium pentoxide can be sent back to the dissolution tank.

The solution enriched in this way contains vanadium in a substantially V ^{+4 state} , although a relatively very small amount of dissolved vanadium may be present as V ⁺⁵ .

An acid, which is most often and preferably sulfuric acid, and water are introduced into the vanadium-rich and filtered electrolyte solution to maintain a certain molarity of the electrolyte solution. Of course, the higher the molar content of vanadium, the higher the ratio of energy to the total volume of the electrolyte, but problems of solution stability at critical temperatures can also occur at relatively high molar concentrations. Most preferably, in the case of a sulfuric acid solution, the molar content of vanadium is in the range of 1 to 5, and even more preferably 2 to 5 moles (moles / liter).

The solution is pumped back into the inlet of the first electrolyzer of the cascade of electrolyzers in order to carry out the electrochemical reduction of V ⁺⁴ (or some residual V ⁺⁵ ) to V ⁺³ and sometimes to V ⁺² .

The product of such an installation for the production of electrolyte is a solution containing almost the same amount of V ⁺³ and V ⁺⁴ , which is discharged from the main stream of the recycle solution at the outlet of one of the electrolyzers of the cascade of electrolyzers.

A disproportionately high current density on the anode surface, causing significant oxygen evolution and, accordingly, insignificant oxidation of V ⁺³ to V ⁺⁴ , is a condition that is very effective for ensuring the overall efficiency of the method at more than acceptable levels, while taking into account a relatively small fraction, which the cost of electricity takes in the general economic indicators of any method of preparing a vanadium acidic electrolyte.

Efficiency can even be improved, as in an alternative embodiment, by using a shielding mesh or even a microporous separator between the rod anode and the surrounding cylindrical cathode.

A screening mesh or microporous separator provides effective “retention” of oxygen bubbles rising due to buoyancy in the electrolyte as they constantly grow and come off the anode surface, thereby minimizing convective movements in the main volume of electrolyte contained in the space between the screening grid and cathode, and also lowering the ability of the reduced ions (V ⁺³ ) vanadium to migrate and, ultimately, to reach the anode.

The most effective microporous separator can be a glass frit tube, closed at its lower end and covering the rod anode (in this case, entering the cell from above), as a result of which oxygen bubbles, separated from the surface of the electrolyte, can escape from the cell through the outlet. Or, a suitable microporous separator can be felt made of polypropylene fibers with a thickness of about 1 mm.

Brief Description of the Drawings

Figure 1 illustrates the installation for the preparation of a vanadium electrolyte solution from a solid source of V ₂ O ₅ according to this invention.

Figure 2 - cross section of the electrolytic cell for the restoration of vanadium according to this invention.

Figure 3 is a cross section of an alternative embodiment of an electrolytic cell for vanadium reduction.

Figure 4 is a schematic diagram of a system of a fully vanadium in-line reducing and oxidizing battery containing an electrolytic cell for reducing vanadium according to this invention in a positive electrolyte circuit for rebalancing.

Description of preferred embodiments of the invention

Turning to the functional diagram in FIG. 1, the apparatus for preparing a vanadium electrolyte according to this invention consists of a plurality of vanadium recovery electrolyzers C1, C2, C3, ... C6 hydraulically connected to the cascade and fed in series with the corresponding DC source R1.

The solution, leaving the last electrolyzer C6 of the cascade, is collected in a dissolution vessel T1 having a stirring means S1.

Vanadium pentoxide (V ₂ O ₅ ) is introduced into the dissolution container T1 in an appropriate amount using, for example, a conventional feed hopper and an engine driven by a controlled feed mechanism.

A vanadium-enriched solution containing residual solid particles of undissolved vanadium pentoxide flows out of the dissolution vessel T1 through an outlet and is discharged into a settling tank T2.

Pump P2 feeds separated residual solids of vanadium pentoxide, which is then collected at the bottom of sedimentation tank T2, back into the dissolution tank T1.

Enriched with vanadium and filtered solution is ultimately collected in a T3 tank.

Vanadium contained in the enriched solution, which is collected in a T3 tank, will be vanadium in the V ^{+4 state} . Its content corresponds to the sum of the amounts of V ⁺³ , and sometimes V ⁺² , present in the electrochemically reduced solution resulting from the last recovery cell (C6) of the cascade, and is equivalent to the reduced amount of dissolved and reduced V ⁺⁵ . A residual unreacted amount of V ⁺⁵ may also be present together with V ⁺⁴ in the thus enriched solution collected in T3.

The solution is constantly circulated by pump P1 through a cascade of electrolysis cells for vanadium recovery after the introduction of an acid, usually H ₂ SO ₄ , and water in relative amounts corresponding to the desired molarity of the vanadium electrolyte solution.

Therefore, the vanadium electrolyte solution entering the first reduction electrolyzer C1 will essentially contain V ⁺⁴ and possibly a residual amount of V ⁺⁵ .

At the negative electrodes (cathodes) of the electrolysers C1, C2, C3, ..., C6, the following main reaction occurs:

V ⁺⁴ + e ^- == V ⁺³ , (or rather VO ⁺² + e ^- + 2H ⁺ == V ⁺³ + H ₂ O)

and one more reaction if vanadium is present in the oxidation state +5:

V ⁺⁵ + e ^- == V ⁺⁴ (or rather VO $\genfrac{}{}{0ex}{}{\text{+}}{\text{2}}$ + e ^- + 2H ⁺ == VO ⁺² + H ₂ O).

At the negative electrode, no other reactions occur. Hydrogen evolution (a thermodynamically favorable semi-element reaction) does not occur, since the carbon felt electrode has a relatively high hydrogen overvoltage, and the effective current density on the cathode surface is kept at a sufficiently low level.

At the positive electrode, the main reaction theoretically should be the oxidation of any vanadium ion present with a reduced oxidation state (+4, +3 and +2) to pentavalent vanadium (V ⁺⁵ ) (thermodynamically favorable semi-element reaction).

Of course, vanadium ions located closer to the anode surface will oxidize directly to V ⁺⁵ , as well as any vanadium ion in a low oxidation state that will migrate to the anode and diffuse to the anode. But as vanadium ions near the positive electrode turn into V ⁺⁵ (are absorbed), the anode half-element reaction will be increasingly supported by the only other possible half-element reaction, i.e. the formation and subsequent evolution of gaseous oxygen in accordance with the following reaction:

Н ₂ О == О ₂ + 2Н ⁺ + 2е ^-

In the asymmetric electrolyzer according to this invention, the oxidation of vanadium is practically impossible, as in systems of the prior art using an ion-exchange membrane electrolyzer and individual circuits containing vanadium-containing catalite and an auxiliary acid anolyte. In practice, any vanadium ion capable of reaching the surface of the anode of the electrolyzer will be easily oxidized to V ⁺⁵ .

But the special imbalance that is created in the electrode current densities makes the anode work at relatively very high current densities, which are orders of magnitude higher than those that can be provided by the migration and diffusion of vanadium ions in the electrolyte solution k and on the surface of the anode. This contributes to a significant release of oxygen on the anode surface, and the presence of intense release of gaseous oxygen bubbles creates a “mechanical” barrier for the migration of V ⁺³ ions to the anode.

This obstacle for the diffusion of cathode-reduced vanadium ions into the anode can be significantly enhanced by using a screening mesh or a permeable (microporous) diaphragm to hold the oxygen bubbles formed near the anode and, thus, to exclude the induction of strong convective motions in the main electrolyte volume, located in the space between the gas-retaining screening grid and the cathode surface, and enriched with reduced vanadium ions.

The use of an anode with a low oxygen overvoltage simply increases oxygen evolution.

The total Faraday current efficiency is significantly increased when using a relatively dense microporous separator instead of a more permeable shielding grid or diaphragm, but the cell voltage also increases. Therefore, it is possible to find the most optimal compromise, taking into account the fact that energy consumption is proportional to the product of current and voltage.

It was found that the Faraday current efficiency above 40% can be easily achieved with a ratio of the cathode / anode current density of about 5; and with the increase of this ratio over 20, the efficiency can reach 80% and higher. A significant increase in these indicators can be achieved through the use of a gas-separating screening grid and, to an even greater extent, using a relatively dense microporous separator.

In dissolution vessel T1, V ⁺³ contained in an electrolyte solution with electrolytically reduced vanadium reacts with solid vanadium pentoxide V ₂ O ₅ (or with ammonium vanadate) to dissolve it and recover to V ^{+ 4} according to the following reactions:

(V ⁺⁵ + V ⁺³ == 2V ⁺⁴ ) or more precisely:

and (if V ^{+2 is} also present)

(2V ⁺⁵ + V ⁺² === 3V ⁺⁴ ), or more precisely:

V ₂ O ₅ + V ⁺² + 4H ⁺ == 3VO ⁺² + 2H ₂ O

The cross-section of an asymmetric electrolyzer according to this invention used in a plant for preparing a vanadium electrolyte according to this invention is shown in FIG. 2.

The laboratory test cell depicted in FIG. 2 consists of a cylindrical tubular body 1, typically made of a metal chemically resistant to electrolyte, of a non-conductive acid-resistant plastic, such as PVC, closed at the lower end with a plug 2 and having an inlet 3 in the lower parts of the tubular body 1 and the upper overflow hole 4.

A cylindrical cathode, which may consist of carbon felt 5 with a thickness of several millimeters, can be mounted on the inner cylindrical surface of the tube 1 and attached to it accordingly. The carbon felt cathode may have a corresponding terminal 6 for electrically connecting the electrolyzer to the DC power circuit.

In the laboratory test cell depicted in FIG. 2, the surface of the inner cylindrical surface of the cathode has a diameter of about 50 mm and a height in contact with the electrolyte solution of about 250 mm.

Anode 7 is a titanium rod with a diameter of 6.3 mm, coated with a mixed oxide of iridium and tantalum, and its immersion length in the electrolyte is approximately 250 mm.

Anode 7, which is a coated titanium rod, is mounted along the axis of a cylindrical carbon felt cathode.

In the indicated laboratory test cell, the projected area of the carbon felt cathode is approximately 353 cm ² and the surface of the anode, which is a titanium rod, is about 47 cm ² .

When applying an electric current of 7A through the electrolysers, the current density on the surface of the titanium anode is approximately 0.1485 A / cm ² = 1500 A / m ² ; the current density on the projected area of the carbon felt is 0.022 A / cm ² = 220 A / cm ² . But due to the open and easily permeable morphology of the cathode in the form of carbon fiber felt, the actual or effective current density of the cathode on carbon can be estimated to be two to ten times lower than the current density calculated on the geometrically projected cylindrical area of the cathode, which is carbon felt.

FIG. 3 is a sectional view of a vanadium reduction electrolyzer according to an alternative embodiment of the present invention.

The only difference is the presence of a fluid permeable shielding grid or diaphragm, or microporous separator 8, mounted between the cylindrical surface of the cathode and the coaxially spaced rod anode, defining a cylindrical space around the rod anode 7, in which pop-up oxygen bubbles generated on the anode surface are essentially held and separated from it into the surrounding electrolyte.

The diaphragm screen 8 essentially eliminates the induction of strong convective motions in the main volume of the electrolyte near the cathode surface, on which the necessary restoration of V ⁺⁴ to V ⁺³ and then to V ⁺² takes place.

A plastic tube with small densely and uniformly distributed openings can serve as a satisfactory shielding mesh for holding gas bubbles, but the shielding mesh 8 for holding gaseous oxygen bubbles can be either a fine mesh mesh made of a resistant material such as, for example, a mesh made of titanium wire or woven plastic fiber material. More preferably, the gas retention screen 8 may be a porous or microporous tube, for example of fritted glass, or of particles of a resistant metal such as agglomerated titanium.

EXAMPLE

A glass beaker with a capacity of 1/2 liter with an inner diameter of 8 mm was used to verify the effectiveness of the methodology according to this invention.

Carbon felt about 6 mm thick was placed around the inner wall of the beaker and was electrically connected to the negative pole of the DC power source.

A titanium rod coated with a mixed oxide IrO _x -ZrO _y with an outer diameter of about 6 mm was mounted vertically along the geometrical axis of the cup and was electrically connected to the positive pole of the DC power source.

The ratio of the projected cathode area to the anode area was about 10.7.

A polypropylene sheet with a thickness of about 1 mm was made in the form of a round tube, closed from the bottom, with an inner diameter of about 12 mm and placed in a beaker concentrically around the coated anode in the form of a titanium rod.

The glass was filled with 473 ml of a solution of 5 molar sulfuric acid and 90.9 g (0.5 mol) of vanadium pentoxide powder. The total volume of the mixture was 0.5 L.

Theoretically, 26.8 A / h are required to recover 1 mole of vanadium from the oxidation state +5 to the oxidation state +4.

The mixture was stirred with an electromagnetic stirrer, and the yellow vanadium pentoxide powder remained essentially insoluble for several days.

After applying DC power and adjusting its output voltage, a direct current of 8 A was passed through the cell. The current density of the positive electrode (anode) was approximately 5.013 A / m ² , and the current density of the negative electrode (cathode) on the projected area of the carbon felt was approximately 468 A / m ² .

The cell voltage remained almost constant in the value of about 3.8-4.0 V.

The suspension was stirred vigorously with a magnetic stirrer, and after passing the current for 5.26 hours, the yellow powder seemed to completely dissolve.

The blue solution thus obtained was analyzed and found to contain 2 moles of vanadium (2 molar solution), and the oxidation state of vanadium was +3.55.

The Faraday current efficiency of the method was estimated to be 92.28%.

The test was repeated at a reduced current of 5 A, and the required time was 9.87 hours. The Faraday current efficiency decreased to approximately 78.74%, and also the voltage of the cell to approximately 2.8 V.

After replacing the felt with a thin woven polypropylene material, the Faraday current efficiency decreases to about 47%, and without permeable retention, to about 20-25%.

Even under these unoptimized laboratory test conditions (in a glass beaker with stirring), energy consumption of the order of 0.2–0.5 kWh / l of the obtained vanadium electrolyte gives a rather low value in terms of general economic indicators for the production of vanadium electrolyte.

The ability of an asymmetric electrolysis cell to recover a vanadium electrolyte in accordance with this invention to effectively and inexpensively modify the oxidation state of a dissolved vanadium content of an acid electrolyte solution makes the relatively simple and inexpensive, substantially undivided, asymmetric electrolyzer of the present invention ideally suited for rebalancing (restoring equilibrium) a positive and negative charge electrolytes of vanadium in an active battery (battery nore) without the need for costly and lengthy processing, in a state out of service, of a redox battery installation whenever the battery reaches a more unacceptable imbalance.

To explain the nature of the difficulties that may arise during the operation of a vanadium battery in an energy storage system, a brief description of the main mechanisms that give rise to an increasingly noticeable imbalance is provided.

If it is theoretically assumed that the only process occurring during the recharging and discharging of a vanadium reduction-oxidation battery is the electrochemical oxidation and reduction of vanadium, and that any other adverse reactions do not occur, then the process of recharging and discharging a vanadium battery is a symmetrical process.

During recharging, the electric current passing through the battery will oxidize V ⁺⁴ to V ⁺⁵ in the positive electrolyte chambers and at the same time will restore V ⁺³ to V ⁺² in the negative electrolyte chambers. The opposite oxidation and reduction reactions occur in the positive and negative chambers of the electrolyte during discharge.

In practical terms, the situation is different.

Electrochemical oxidation and reduction of vanadium is not the only process that occurs. It is likely that under critical conditions the following adverse reactions will occur:

1) electrochemical evolution of hydrogen gas at the negative electrode

2) electrochemical oxygen evolution at the positive electrode (*)

3) chemical oxidation of V ⁺² to V ⁺³

4) chemical reduction of V ⁺⁵ to V ⁺⁴

(*) if the positive electrode is made of carbon, then the evolution of oxygen is partially or completely replaced by the evolution of carbon dioxide.

Reactions 1) and 2) become the only reactions upon reaching a 100% state of charge. In practice, after oxidation of the entire positive chamber V ⁺⁴ present in the electrolyte to V ^{+5, the} only reaction on the positive electrode that can provide current is the release of oxygen (or carbon dioxide). Similarly, after the restoration of the entire positive chamber V ⁺³ present in the electrolyte to V ^{+2, the} only reaction on the negative electrode that can provide current is the evolution of hydrogen. These reactions will begin during battery recharging, albeit relatively in a small volume, when the state of charge exceeds 90%.

The voltage at which vanadium is oxidized or reduced proportionally increases with the ratio between the obtained object and the consumable object (Nernst equation), and therefore, when the charge is high, the voltage of the electrolyzer increases to a voltage of hydrogen and oxygen evolution (electrolysis of water) of approximately 1.5 V . Reactions 1) and 2) will also occur in a relatively small volume during battery discharge, if the discharge is too fast (current).

When the current density reaches the limiting current, then the evolution of hydrogen and oxygen begins as a side (unnecessary) reaction.

The limiting current is an electric current at which the rate of vanadium oxidation or reduction on the electrode surface is equal to the speed at which vanadium ions diffuse from the bulk of the electrolyte into the electrode surface through the depletion layer.

Reaction 3), the oxidation of V ⁺² to V ⁺³ , is the most repeated adverse reaction during the operation of the vanadium battery. V ^{+2 is} easily oxidized to V ⁺³ in the presence of air. Therefore, if atmospheric air in contact with a negative electrolyte is not strictly excluded (due to the fact that the surface of the electrolyte will be covered with a layer of nitrogen gas or coated with wax, for example), then this side reaction will easily occur.

Due to the side reactions mentioned above, after many cycles of battery operation, it is possible that the symmetry will begin to essentially disappear.

Another reason why electrolytes become unbalanced: the membranes used are not perfect separators. A small fraction of positive ions (H ⁺ and V ^{+ n} ) inevitably penetrate through anionic membranes.

Typically, cationic membranes are preferred as battery separators because of their higher mechanical and chemical resistance compared to anionic membranes.

Indeed, cationic membranes are mainly permeable to hydrogen ions (the diffusion rate of H ^{+ is} much higher than that of vanadium ions).

During battery recharging, hydrogen ions formed in the positive chamber by the reaction:

VO ²⁺ + H ₂ O === VO $\genfrac{}{}{0ex}{}{\text{+}}{\text{2}}$ + 2H ⁺ + e ^- ,

easily migrate into the negative chamber through the membrane along with a smaller fraction of less mobile vanadium ions.

Migrating vanadium ions will oxidize the corresponding amount of reduced vanadium ions present in the negative chamber (V ⁺³ and V ⁺³ ), but the process is not completely reversible, since vanadium ions of a different oxidation state coordinate differently with solvent molecules (water, sulfuric acid ), and have a different mobility in the cation exchange resin of the membrane. Of course, during the subsequent discharge phase, the number of vanadium ions passing through the membrane in the opposite direction will not be exactly the number that was carried away during the charging phase.

The growing imbalance between electrolytes causes numerous problems, including:

1) the battery capacity (in units of kWh / l of electrolyte) is proportionally reduced;

2) during recharging, one of the two electrolytes can be fully recharged, while the other remains partially uncharged.

In practice, especially in the case of small batteries, in which the removal of air from the negative chamber is often incomplete, the vanadium ions in the positive chamber can completely oxidize to V ⁺⁵ , and a significant volume of V ⁺³ will remain in the negative chamber. This situation is very critical since if the degree of oxidation is not carefully regulated in different electrolytes, but only to measure the open circuit voltage, then recharging will continue until the complete oxidation of V ⁺⁴ to V ^{+5 is} reached. Under these conditions, significant oxygen evolution at the carbon electrode will oxidize and destroy the electrode.

In accordance with the generally accepted methodology, after a certain number of recharging and discharging cycles, two electrolytes (negative and positive) are mixed, the oxidation state is measured, and if it is not +3.5, then it is chemically controlled to +3.5.

In practice, when the battery is stopped and the electrolytes are mixed, the oxidation state of vanadium always exceeds +3.5 (mainly due to the influence of the prevailing side effect of side reaction 3)).

The electrolyte is regulated to a vanadium oxidation state of +3.5 by introducing a reducing substance (oxalic acid, sulfite, etc.).

Then a significant amount of energy should be spent on bringing the system back to the zero state of charge (V ⁺³ in the negative electrolyte, and V ⁺⁴ in the positive electrolyte).

This periodically spent amount of energy represents the net loss of the energy storage process.

This, which is not negligible, the energy loss can be significantly reduced in accordance with the feature of the present invention, i.e. by installing a relatively small asymmetric electrolysis cell for vanadium reduction in accordance with this invention in the negative, or more preferably, positive electrolyte circuit according to FIG. 4.

According to the indicated image, the circulation of the positive electrolyte can be carried out in whole or in part (in the latter case, using, for example, an adjustable three-way valve or other means) through a relatively small asymmetric electrolysis cell for vanadium reduction - (Vosst).

The electrolyzer (Vosst) can act in accordance with the need either continuously or periodically in order to maintain a symmetric oxidation state of vanadium.

Due to the possibility provided by the presence of this auxiliary recovery electrolyzer (Vosst), it is possible to eliminate the need for mixing two electrolytes, regulating the oxidation state to approximately +3.5 and pre-charging the battery to restore the zero state of charge, or to perform them in cases of exceptional need.

Claims (13)

1. A method of obtaining an acidic vanadium electrolyte solution containing V ⁺³ and V ⁺⁴ in the desired concentration ratio from solid vanadium pentoxide supplied to the electrolyte solution, which consists in the electrochemical reduction of at least partially dissolved vanadium in acid an electrolyte solution by circulating an electrolyte solution through a plurality of electrolyzers in a cascade at least to an oxidation state of V ⁺³ or to a lower oxidation state; reacting thus restored vanadium contained in the electrolyte solution leaving the last of these electrolysis cells with a stoichiometric amount of vanadium pentoxide, thereby obtaining an electrolyte solution containing vanadium, essentially, in the oxidation state V ⁺⁴ ; acid and water are introduced to maintain a certain molarity of the solution; carry out continuous recycling of the electrolyte solution through the cascade of electrolyzers, while diverting the flow of the resulting electrolyte solution containing V ⁺³ and V ⁺⁴ in the desired concentrations at the outlet of one of the electrolyzers of the specified cascade; moreover, each cell has a cathode and anode with corresponding surface morphologies, geometry and relative position to establish a current density on the anode surface that is 5 to 20 times higher than the current density on the projected cathode surface, and oxygen evolution on the anode. 2. The method according to claim 1, characterized in that said electrolyte solution is a sulfate solution, and the molar content of vanadium is from 1 to 5. 3. The method according to claim 1, characterized in that the current density on the projected cathode surface is from 100 to 300 A / m ² and the current density on the anode surface is from 1000 to 8000 A / m ² . 4. The method according to claim 1, characterized in that the said vanadium oxide is in the form of a powder and has a particle size of not more than 100 microns. 5. The method according to claim 1, characterized in that after the reaction of the reduced vanadium electrolyte solution with the indicated stoichiometric amount of vanadium pentoxide, the solution is separated from any residual undissolved particles of vanadium pentoxide. 6. Installation for the preparation of a vanadium electrolyte solution containing V ⁺³ and V ⁺⁴ in the desired concentration ratio, from solid raw materials in the form of vanadium pentoxide, consisting of many electrolytic cells for vanadium recovery, hydraulically connected to a cascade and electrically powered in series from an adjustable constant source current; a dissolution tank collecting the reconstituted electrolyte solution emerging from the last electrolyzer of said cascade of electrolysis cells and having mechanical mixing means and a feeding mechanism for an adjustable amount of vanadium pentoxide in powder form; means for separating a vanadium-enriched solution exiting said dissolution vessel from residual solid particles of vanadium pentoxide; means for introducing sulfuric acid and water into the vanadium-enriched solution to maintain a certain molarity of the solution; pumping means for recycling the electrolyte solution through a cascade of electrolyzers to restore vanadium; outlet means for diverting the flow of the resulting electrolyte solution containing V ⁺³ and V ⁺⁴ in the desired concentration ratio, at the outlet of one of the electrolyzers of the indicated cascade of electrolyzers; moreover, each cell has a cathode and anode with corresponding surface morphologies, geometry and relative position to establish a current density on the anode surface that is 5 to 20 times higher than the current density on the projected cathode surface, and oxygen evolution on the anode. 7. An electrolyzer for the reduction of V ⁺⁴ and / or V ⁺⁵ ions in an acidic vanadium electrolyte aqueous solution to V ⁺³ and / or V ⁺² , having a cathode and anode with appropriate surface morphologies, geometry, and relative position to be installed on the anode surface a current density of 5 to 20 times the current density on the projected cathode surface, and oxygen evolution on the anode. 8. The electrolyzer according to claim 7, consisting of a cylindrical tubular body of a non-conductive acid-resistant material having an inlet 3 and an overflow hole; a cathode of carbon felt, located on the inner cylindrical surface of the tubular body, equipped with a terminal for electrical connection of the cell; a rod anode of valve metal coated with a non-passivating electrocatalytic coating and installed along the axis of the cylindrical cathode of carbon felt. 9. The electrolyzer according to claim 7, characterized in that it contains permeable to the electrolyte means for holding floating oxygen bubbles rising in the electrolyte around or near the anode. 10. The electrolyzer according to claim 9, characterized in that the said permeable retention means is selected from the group consisting of mesh screens, woven materials, felts, porous and microporous glass frits and agglomerated bodies, which are all made of a material having chemical resistance to solution electrolyte. 11. The electrolyzer according to claim 7, characterized in that said anode is made of valve metal coated with a mixed oxide of iridium and tantalum or zirconium. 12. A method of restoring the balance of the state of the relative oxidation state of two separate vanadium electrolytes circulating in the system of a fully vanadium in-line reduction and oxidation battery without decommissioning it, according to which part of one of two different electrolytes is circulated to the vanadium recovery electrolyzer, which is external to in relation to the battery, made in accordance with any one of paragraphs.7-11, and supply current through the electrolyzer for the time necessary to restore Ia balance degree of oxidation of the vanadium contained in the two electrolytes system redox-flow battery. 13. The method according to p. 12, characterized in that the electrolyte is a positive electrolyte circulating in the chambers of the positive electrode of the battery.

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