KR20030034146A - Vanadium electrolyte preparation using asymmetric vanadium reduction cells and use of an asymmetric vanadium reduction cell for rebalancing the state of charge of the electrolytes of an operating vanadium redox battery - Google Patents

Abstract

An acid vanadium electrolyte solution containing V ⁺³ and V ⁺⁴ at a desired concentration from solid vanadium pentoxide supplied to the electrolyte solution,

Circulating the electrolyte solution through a plurality of series electrolysis cells to electrochemically reduce the vanadium dissolved in the acidic electrolyte solution to an oxidation state of V ⁺³ or below,

Reacting the vanadium content thus reduced in the electrolyte solution from the final cell of the electrolysis cell with any stoichiometric vanadium pentoxide to obtain an electrolyte solution containing vanadium with an oxidation state of V ⁺⁴ ,

Adding acid and water to maintain the molarity of the solution,

Continuously recycling the electrolyte solution through the in-line electrolysis cell while squeezing out a stream of the resulting electrolyte solution containing the desired concentrations of V ⁺³ and V ⁺⁴ exiting one of the series cells,

Are manufactured. Each cell is then very asymmetrical, with cathodes and anodes with their respective surface structures, shapes, and mutual arrangements, which build up at the anode surface a current density of 5-20 times greater than the current density of the projected cathode surface. Release oxygen. Asymmetric cells of this kind can be used in either the positive or negative electrolyte circuit of a battery in operation to rebalance the oxidation state of the vanadium content, respectively.

Description

VANADIUM ELECTROLYTE PREPARATION USING ASYMMETRIC VANADIUM REDUCTION CELLS AND USE OF AN ASYMMETRIC VANADIUM REDUCTION CELL FOR REBALANCING THE STATE OF vanadium electrolyte OF THE ELECTROLYTES OF AN OPERATING VANADIUM REDOX BATTERY}

Vanadium redox batteries are referred to as all-vanadium redox cells or simply vanadium redox cells or batteries, which are V (II) / V (III) and V in the negative half-cell electrolyte solution and the positive half-cell electrolyte solution, respectively. Use (IV) / V (V) as two redox couples.

A typical electrolyte used in vanadium batteries consists of a mixture of 50% vanadium ions with an oxidation state of +3 and 50% of vanadium ions with an oxidation state of +4.

The electrolyte is divided into two equal parts, each placed in the positive and negative compartments of the battery, and more precisely, in two equal parts, each placed in a relative flow circuit. In this starting environment, the battery has an open circuit voltage that is substantially zero.

When current flows through the battery by an external source of a sufficiently high output voltage, V ⁺⁴ (50%) of the negative electrolyte will be reduced to V ⁺³ , while V ⁺³ (50%) of the positive electrolyte Will be oxidized to V ⁺⁴ .

A negative electrolyte that continually circulates through each electrode compartment of the battery by a negative electrolyte circulation pump will have only V ³⁺ at any moment and continually circulates through each electrode compartment of the battery by a positive electrolyte circulation pump. The positive electrolyte will only have V ⁴⁺ at any moment.

In this situation the battery is said to have a Stated Of Charge (SOC) of zero, and the open circuit voltage of the battery will be approximately 1.1 volts.

By continuing to send a "charge" current to the battery, V ⁺³ at the negative electrode will be reduced to V ⁺² , and V ⁺⁴ at the positive electrode will be oxidized to V ⁺⁵ . When this conversion is complete, the battery will have an open circuit voltage of 1.58V and the battery is said to have 100% SOC.

Vanadium is commercially available as vanadium pentoxide (or ammonium vanadate). In either case, they are generally traded in an oxidation state of +5.

The storage capacity of all vanadium redox battery plants is determined by the amount of vanadium dissolved in the acidic electrolyte. For an electrolyte solution of a given molarity, the storage capacity is directly proportional to the two electrolyte volumes.

Using commercial vanadium pentoxide (or ammonium vanadate) as a starting material, an appropriate vanadium acidic solution is used as an electrolyte to expand the storage capacity of existing battery equipment or to first fill the two circuits of a redox battery system. I need to make it.

Thus, the process of preparing the vanadium electrolyte is a process of dissolving V ₂ O ₅ in sulfuric acid (or other acid) and reducing it to a mixture of V ⁺³ (about 50%) and V ⁺⁴ (about 50%).

Finely divided (powdered) vanadium pentoxide is dissolved in very small amounts in acids or water, such as sulfuric acid, and a simple process for preparing an electrolyte by dissolving V ₂ O ₅ in an acid solution is not possible.

In order to dissolve V ₂ O ₅ , it is first necessary to reduce V ₂ O ₅ to a lower (more easily soluble) oxidation state.

Several methods have been proposed for the dissolution and reduction of V ⁺⁵ , mainly by using reducing compounds, or by using complex electrolytes or chemical treatment methods.

EP 0 566 019 discloses a process for producing a vanadium electrolyte solution by chemically reducing vanadium pentoxide or ammonium vanadate in concentrated sulfuric acid, and then heat treating the precipitate.

WO 95/12219 and WO 96/35239 disclose an electrochemical process for preparing a vanadium electrolyte solution from solid vanadium pentoxide and a method of stabilizing the electrolyte solution. The vanadium pentoxide dissolution is performed by bringing the vanadium pentoxide solution into contact with the cathodes of the small pores in the cathode with the special small pores of the ion exchange membrane cell.

The methods and techniques developed so far to produce suitable vanadium acid electrolytes are complex and expensive. On the other hand, in terms of the overall economics of all vanadium flow redox battery systems, the cost advantage of vanadium redox batteries over other energy storage systems becomes an important factor because the vanadium electrolyte solution is available at a relatively low cost.

The basic requirement for meeting these requirements is to use a relatively inexpensive solid vanadium pentoxide as feed material.

The present invention relates to a rechargeable electrochemical energy storage device in a redox battery system. In particular it relates to a so-called all-vanadium redox secondary battery.

1 is a diagram of a plant for preparing vanadium electrolyte solution from a solid V ₂ O ₅ feed material according to the present invention.

2 is a cross-sectional view of a vanadium reduction cell of the invention.

3 is a cross-sectional view of an alternative embodiment of a vanadium reduction cell.

4 is a basic diagram of all vanadium redox battery systems including the vanadium reduction cell of the invention in a circuit of positive electrolyte for functional balance control.

A very simple and inexpensive method of dissolving and reducing vanadium pentaoxide directly in an acidic electrolyte is now presented.

The present invention is particularly useful for preparing vanadium electrolytes from vanadium pentoxide (or ammonium vanadate) feed materials, and is implemented by using an electrolysis cell at a very simple and low cost with minimal reduction in incidental processing of this solution.

Nevertheless, the method of the present invention remains very efficient in terms of energy consumption.

The method of the present invention continuously supplies solid vanadium pentoxide (V ₂ O ₅ ), acid, and water in the form of a finely divided powder to maintain the molarity of the solution until a volume of circulating vanadium electrolyte solution is obtained. . Then squeeze the electrolyte solution containing V ⁺³ and V ⁺⁴ at similar or different desired concentrations continuously in equal volumes.

The stream squeezed out of the electrolyte solution determines the yield of this process.

Basically, the method of the invention

-At the outlet of the electrolyte solution from the final electrolysis cell of the multiple in-line cells, reduce all or part of the V ⁺⁴ content of the solution entering the first cell in the electrolyte solution to V ⁺³ and ultimately produce a small amount up to V ⁺² Contact the cathodes of the plurality of series electrolysis cells with the electrolyte solution,

React the vanadium content of the electrolyte solution so reduced at the outlet of the final electrolysis cell with a stoichiometry of vanadium pentoxide (V ₂ O ₅ ) in the dissolution vessel provided with a means to stir, and to be almost completely at V ⁺⁴ . Obtaining an electrolyte solution containing a corresponding amount of dissolved vanadium,

Add an acid, such as sulfuric acid or other suitable acid, to the vanadium electrolyte solution (close to V ^{+ 4} ) to maintain some molarity,

Regenerating the electrolyte solution through an in-line electrolysis cell while squeezing a stream of electrolyte solution containing similar concentrations of V ⁺³ and V ⁺⁴ (preferred) at the outlets of the multiple cells connected in series,

It consists of the above steps.

A key element of the electrolysis cell is that the cathode and the anode have a surface structure, form, and mutual arrangement, such as building a current density on the anode surface that is 5 to 20 times greater than the current density on the cathode surface. Oxygen is released.

In practice, the cathode may be a carbon felt or an activated carbon felt that provides a relatively large surface area, and may have a tubular or grooved shape. At the same time the anode may be in the form of an elongated rod disposed along the geometric axis of the tubular / grooved cathode.

The relatively large specific active area of the cathode relative to the specific active area of the anode and its projected area ratio is equivalent to determining the current density on the active anode surface that is 5-20 times greater than the current density on the geometrically projected cathode surface.

By operating as a cathode current density of from 1 to several hundred A / m ² level of the projected area, and a tube form or a jam of the anode rod diameter is arranged in a concentric circle form on the cathode of the groove forms, 1000A / m anode current density of greater than ² Can be obtained.

Under these conditions, with very unbalanced current densities and relatively high anode current densities, the current forced in the electrolysis cell is reduced by the cathode reduction from V ⁺⁴ to V ⁺³ (which limits the maximum current density to prevent hydrogen-like vortex reactions). The anode half-cell reaction is mainly supported by the oxygen release (water electrolysis) reaction, while being adjusted to ensure that the independent cathode half-cell reaction is nearly complete.

Indeed, the thermodynamically preferred anode half-cell reaction of oxidation from V ⁺³ to V ⁺⁴ results in a large rate of migration and diffusion of V ⁺³ ions from the mass of electrolyte filling the gap between the anode and the cathode towards the cell's anode surface. It is practically hindered by lack.

Another important obstacle to the movement and diffusion of vanadium ions toward the anode surface is manifested by the presence of oxygen gas bubbles that are vigorously released to the anode surface at this relatively high current density.

The current flowing through multiple electrolysis cells connected in series is regulated as a function of the flow rate of the electrolyte through the series cells to substantially completely reduce all V ⁺⁴ to V ⁺³ in the electrolyte leaving the final cell of the series cells. Can be.

Of course, this remains an ideal condition, and if there is a lack of current in the cell, a minimum residual amount of V ⁺⁴ may remain, and vice versa, if the current is excessive, a small amount of V ⁺² in the electrolyte leaving the final cell will result in V ^+. An initial reduction from V ⁺³ to V ⁺² can occur so that it can be present with ³ .

The anode has a low oxygen electrocatalyst surface with high oxygen release potential and, above all, does not react to acidic electrolytes under conditions of anode polarization and oxygen release.

For example, the anode may be a rod of valve metal that is resistant to anode attack, such as titanium, tantalum, or alloys thereof with a non-passivating active coating of an oxygen releasing electrocatalyst.

The coating may be a mixture of oxides of one or more inert metals such as iridium, rhodium, and ruthenium and oxides of one or more valve metals such as titanium, tantalum, and zirconium, or may be mixed oxides. The active coating may consist of an inert metal coating such as platinum, iridium, or rhodium or a coating of the same metal interspersed with a conductive oxide matrix.

In a dissolution vessel provided with normal agitation, the electrolyte solution exiting the final cell is a fine form of solid vanadium pentoxide chemistry prepared by grinding or filtering solid vanadium pentoxide to insert particles up to 100 microns in size. Contact a quantity (called a quantity of V ⁺³ (V ⁺² )).

The transferred or filtered solution is recovered in a reservoir and any undissolved vanadium pentoxide particles can be regenerated in the dissolution vessel.

The solution thus obtained mostly contains vanadium of V ⁺⁴ , and relatively small amounts of dissolved vanadium may be present as V ⁺⁵ .

The most frequently used and preferred acid is sulfuric acid, and water is added to the vanadium-filtered electrolyte solution to maintain the molarity and electrolyte solution. Of course, the higher the vanadium molar concentration, the higher the power / total volume ratio of the electrolyte, but the problem of stability of the solution under critical temperature conditions may appear at relatively high molar concentrations. For sulfuric acid solutions the vanadium molar content ranges between 2 and 5 moles (preferred).

The solution to ⁺⁴ V (and the residual amount of the ⁺⁵ V) to V ^+3, and eventually is pumped back to the inlet of the first cell in the series cell for electrochemical reduction to V ^+2.

The product of the electrolyte production plant is a solution containing the same amount of V ⁺³ and V ⁺⁴ , which can be squeezed into the main stream of recycle solution at the outlet of one of the cells of the series cell.

Disproportionately large current densities at the anode surface causing a large amount of oxygen generation and thus a small amount of oxidation from V ⁺³ to V ⁺⁴ are sufficient conditions to maintain the total efficiency of the process above acceptable levels, electrical energy It also makes us think about the relatively small weights that cost has to the total economic cost of the manufacturing process of the vanadium acid electrolyte.

In an alternative embodiment, efficiency may be increased by including a microporous separator or screen between the rod anode and the surrounding cylindrical cathode.

Screens or porous separators continue to appear at the anode surface and effectively "limit" the oxygen bubbles rising by the buoyancy of the electrolyte as they fall off the anode surface. Thus, the convection of the electrolyte portion in the screen-cathode space is minimized, reducing the likelihood that the reduced vanadium ions (V ⁺³ ) migrate and eventually reach the anode.

The most effective microporous separator may be a glass tube plugged underneath and covering the rod cathode (in this case entering the cell from the top). Oxygen bubbles generated in the electrolyte can be immediately discharged out of the cell through the exhaust. Alternatively, a suitable microporous separator may be a felt of polypropylene fiber about 1 mm thick.

In Figure 1, the vanadium electrolyte production facility according to the present invention comprises a plurality of electrolyte vanadium reduction cells C1, C2, C3, ..., C6 connected in series, which are electrically powered in series by a suitable direct current supply R1. Supplied.

The solution leaving the final cell C6 in the serial cell is collected in a dissolution vessel T1 with agitation means S1.

Vanadium pentaoxide (V ₂ O ₅ ) enters the dissolution tank T1 in an appropriate amount using, for example, a conventional feed hopper and a motorized drive mechanism.

Solutions containing vanadium, especially those containing residual solid particles of undissolved vanadium pentoxide, exit from the dissolution vessel T1 through the discharge port and enter the settler vessel T2.

The pump P2 recycles the residual solid particles of vanadium pentoxide that collect at the bottom of the settler vessel T2 back to the dissolution vessel T1.

The filtered solution containing vanadium eventually collects in Rezyba T3.

The vanadium content of the solution collected in Reservoir T3 will contain vanadium in the V ⁺⁴ state. This content is equal to the amount of V ⁺³ present in the electrochemically reduced solution flowing out of the final reduction cell (C6) and finally V ⁺² and the amount of dissolved and reduced V ⁺⁵ which is equivalently reduced. Corresponds to the sum. In addition, an unreacted residual amount of V ⁺⁵ may be present with V ⁺⁴ in the solution that collects in T3.

This solution was continuously circulated by pump P1 through a vanadium reduction series electrolysis cell after addition of the added acid, typically H ₂ SO ₄ and water H ₂ 0, in a relative amount suitable to maintain the desired molar concentration of the vanadium electrolyte solution. do.

The vanadium electrolyte solution flowing into the first reduction cell C1 will thus contain V ⁺⁴ and possibly residual amount of V ⁺⁵ .

In the negative electrodes (cathodes) of the reducing cells C1, C2, C3, ..., C6, the key reaction is as follows.

V ⁺⁴ + e- === V ⁺³ (more precisely VO ⁺² + e- + 2H ⁺ === V ⁺³ + H ₂ O)

In the presence of vanadium with an oxidation state of +5, another reaction is as follows.

V ⁺⁵ + e- === V ⁺⁴ (more precisely VO ₂ ⁺ + e- + 2H ⁺ === VO ⁺² + H ₂ O)

No other reaction takes place at the cathode. Hydrogen evolution (thermodynamically preferred half-cell reaction) does not occur. This is because 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 value.

At the anode, the main reaction should be the oxidation of the vanadium ions present (+4, +3, +2 vanadium to +5 vanadium) (thermodynamically preferred half-cell reaction).

In addition, vanadium ions close to the anode surface will be immediately oxidized to V ⁺⁵ , so that vanadium ions in the low oxidation state will migrate and diffuse to the anode. However, as vanadium ions close to the anode are converted to V ⁺⁵ , the anode half-cell reaction begins to be increasingly supported by the remaining possible half-cell reactions. This is the release of oxygen gas according to the reaction below.

H ₂ O === O ₂ + 2H ⁺ + 2e-

In the asymmetric cell of the invention, vanadium oxidation is not practically excluded from existing systems using ion exchange membrane cells and separate circuits of vanadium containing negative electrolytes and auxiliary acidic positive electrolytes. In fact, any vanadium ions that can reach the cell anode surface will be immediately oxidized to V ⁺⁵ .

However, certain imbalances produced at electrode current densities allow the anode to operate at a much higher level of current density than can be supported by the vanadium ion transport and diffusion process of the electrolyte solution towards the anode. As a result, the release of large amounts of oxygen at the anode surface is enhanced, and the vigorous release of oxygen gas bubbles creates a "physical" barrier to the migration of V ⁺³ ions towards the anode.

Obstacles to diffusing cathode reduced vanadium ions to the anode can be greatly improved by using a screen or microporous diaphragm to limit the oxygen bubble distribution near the anode, thus reducing cathode surface and gas enriched with reduced vanadium ions Induction of strong convection in the electrolyte embedded in the space between the screens can be prevented.

Oxygen release is enhanced by using a low oxygen overvoltage anode.

The use of very dense microporous separators instead of more permeable screens or diaphragms greatly increases the overall induction efficiency, but also increases the cell voltage. Thus, considering the energy consumption is proportional to the product of current and voltage, an optimal compromise can be found.

Induction efficiencies above 40% can easily be assured with a cathode / anode current density ratio of about 5, and the efficiency may reach 80% or more by increasing this current density ratio to 20. Notable improvements of these features can be obtained by using a gas confinement screen and by using a relatively dense microporous separator.

In the dissolution vessel T1, V ⁺³ contained in the reduced vanadium electrolyte solution reacts with the solid vanadium pentoxide V ₂ O ₅ to dissolve vanadium and reduce it to V ⁺⁴ according to the reaction below.

V ⁺⁵ + V ⁺³ === 2V ⁺⁴

V ₂ O ₅ + 2V ⁺³ + 2H ⁺ === 4VO ⁺² + H ₂ O (1)

And (if V ⁺² is also present)

2V ⁺⁵ + V ⁺² === 3V ⁺⁴

V ₂ O ₅ + V ⁺² + 4H ⁺ === 3VO ⁺² + 2H ₂ O (1)

A cross section of an asymmetric cell according to the invention for use in the inventive vanadium electrolyte production facility is shown in FIG. 2.

The experimental test cell shown in FIG. 2 is a cylindrical conduit (1) in which a metal that is chemically resistant to electrolyte of a non-conductive acid-resistant plastic such as PVC clogged down by a plug, and a suction port (3) in the lower part of the conduit (1) And an upper overflow port (4).

Cylindrical cathodes, which may consist of carbon felt 5 with a thickness of several millimeters, may be deposited or appropriately hung on the inner cylindrical surface of the tube 1. The felt cathode may be provided with suitable terminals 6 for electrically connecting the cells to the direct current supply circuit.

In the experimental test cell shown in Fig. 2, the inner cylindrical surface area of the cathode is 50 mm in diameter and 250 mm in height in contact with the electrolyte solution.

The anode 7 is a titanium rod 6.3 mm (1/4 inch) in diameter coated with a mixed oxide of iridium and tantalum and is about 250 mm long immersed in an electrolyte.

A coated titanium rod anode 7 is disposed along the axis of the cylindrical carbon felt cathode.

In this defined test cell, the projected area of the carbon felt cathode is approximately 353 cm ² and the titanium rod anode surface area is approximately 47 cm ² .

With a current forced through a cell of 7 A, the current density on the titanium anode surface is approximately 0.1485 A / cm ² and the current density on the projected area of the carbon felt is 0.022 A / cm ² = 220 A / m ² . However, due to the easy structure of the permeation of the cathode in the form of carbon fibers, the effective cathode current density in carbon is estimated to be 2 to 10 times smaller than the current density calculated from the geometrically projected cylindrical area of the carbon felt cathode.

3 is a cross section of a vanadium reduction cell according to an alternative embodiment.

The difference is that there is a fluid permeable screen or diaphragm or microporous separator 8 sandwiched between the cylindrical cathode surface and the rod anode co-located. They form a cylindrical space around the rod anode 7 and serve to define oxygen bubbles that grow on the anode surface and eventually fall off from the anode surface to the surrounding electrolyte.

The screen-diaphragm 8 prevents the induction of strong convection in the electrolyte close to the cathode surface where a process of reducing V ⁺⁴ to V ⁺³ and eventually V ⁺² occurs.

While plastic tubes with dense and uniform holes may be satisfactory gas bubble confinement screens, oxygen gas bubble confinement screens 8 may be fine meshes of resistive materials such as fabrics of plastic fibers or meshes of titanium wires. The preferred gas confined screen 8 may be a resistant metal particle such as sintered titanium or a (micro) porous tube such as glass.

Yes

A half-liter glass beaker with an inner diameter of 8 cm is used to demonstrate that the technology of the invention is valid.

A 6 mm thick carbon felt is placed around the inner wall of the beaker and electrically connected to the cathode of the DC power supply.

Titanium rods coated with a 6 mm outer diameter IrO _x -ZrO _y mixed oxide are vertically located along the geometric axis of the beaker and are electrically connected to the anode of the DC power supply.

The projected casso and anode area ratio is about 10.7.

Approximately 1 mm thick polypropylene felt is formed in the form of a round tube with a membrane underneath, its inner diameter is 12 mm, placed in a beaker and arranged concentrically around the coated titanium rod anode.

The beaker is filled with 473 ml of 5 molar sulfuric acid and 90.9 g of vanadium pentoxide powder (0.5 mole). The total volume of the mixture is 0.5 liters.

In theory, 26.8 Ah is required to reduce 1 mole of vanadium from an oxidation state of +5 to an oxidation state of +4.

The mix is stirred with an electromagnetic whip and the yellow powder of vanadium pentoxide remains insoluble for several days.

By supplying DC power to regulate the output voltage, 8A DC current flows into the cell. The anode (anode) current density is approximately 5013 A / m ² , and the negative electrode (cathode) current density in the carbon felt projection area is approximately 468 A / m ² .

The cell voltage is kept constant at 3.8-4.0 volts.

The solution is gently stirred with a magnetic stirrer and the yellow powder is completely dissolved only after passing a current for 5.26 hours.

The resulting blue solution appears to contain 2 moles of vanadium (2 molar concentration solution), and the oxidation state of vanadium is +3.55.

Induction current efficiency of this process was estimated to be 92.28%.

The experiment is repeated again with the current reduced to 5 A and the required time is 9.87 hours. Induction current efficiency is reduced to 78.94%, but the cell voltage has 2.8 volts.

By converting the felt into a thin polypropylene fabric, the current efficiency is reduced to about 47% and to 20-25% without any permeability confinement elements.

Even under these unoptimized experimental conditions (while stirring glass beakers), the power consumption of 0.2-0.5 kWh per liter in the resulting vanadium electrolyte shows a very low cost effect in terms of the overall cost of producing vanadium electrolyte.

The ability of the inventive asymmetric vanadium electrolyte reduction cell to efficiently and inexpensively modify the oxidation state of the vanadium content dissolved in an acidic electrolyte solution is a relatively simple and inexpensive asymmetric cell that operates a positive vanadium electrolyte and a negative vanadium electrolyte. It is suitable for rebalancing the charging state of the battery and does not require costly and time consuming processing under off-service conditions of the redox battery installation until the battery reaches an unbalance that can no longer be tolerated.

In order to better understand the nature of the problems that are likely to occur when operating a vanadium battery energy storage system, a simple recollection of the main mechanism leading to particularly notable imbalances can be useful.

Theoretically, the charging and discharging process of the vanadium battery is a symmetrical process, assuming that the only process that occurs during the charging and discharging of the vanadium redox battery is the electrochemical oxidation and reduction of vanadium and no other incidental action occurs.

During charging, the current flowing through the battery will oxidize V ⁺⁴ to V ⁺⁵ in the positive electrolyte compartment and at the same time reduce V ⁺³ to V ⁺² in the negative electrolyte compartment. During discharge, the opposite oxidation and reduction reactions occur in the positive and negative electrolyte compartments.

Unfortunately, the situation is different.

Electrochemical oxidation and reduction of vanadium is not the only developmental process. The following additional reactions are likely to occur.

1) Electrochemical Release of Hydrogen Gas from the Cathode

2) Electrochemical Release of Oxygen at the Anode (*)

3) chemical oxidation of V ⁺² to V ⁺³

4) chemical reduction of V ⁺⁵ to V ⁺⁴

(*) When the anode is made of carbon, oxygen emissions are replaced by carbon dioxide emissions.

Reactions 1) and 2) are reactions that occur only when the 100% state of charge is reached. In fact, when all V ⁺⁴ present in the positive electrolyte compartment is oxidized to V ⁺⁵ , the only reaction at the anode that can support the current is oxygen release (or carbon dioxide release). Likewise, when all V ⁺³ present in the positive electrolyte compartment is reduced to V ⁺² , the only reaction at the cathode that can support the current is hydrogen evolution. These reactions begin to occur during charging of the battery in relatively small amounts when the state of charge exceeds 90%.

The voltage at which vanadium is oxidized or reduced increases in proportion to the ratio between the species produced and the species consumed. Thus, at high charge, the cell voltage rises to a voltage of about 1.5 volts of hydrogen and oxygen (water electrolysis) release. Reactions 1) and 2) will occur in relatively small amounts during the discharge of the battery if the discharge occurs at a very high rate.

When the current density approaches the limit current, hydrogen and oxygen release will begin to occur in the secondary (vortex) electrode reaction.

The limit current is the electrical current when the rate of vanadium oxidation or reduction at the electrode surface is equal to the rate at which vanadium ions diffuse through the depleted layer from the electrolyte to the electrode surface.

The reaction of oxidizing V ⁺² to V ⁺³ (reaction 3) is the most frequent side effect of vanadium battery operation. In air atmosphere V ⁺² is easily oxidized to V ⁺³ . Thus, this side effect can easily occur if the atmosphere is not strictly limited in contact with the negative electrolyte.

Because of this side effect, after several battery operating cycles, symmetry begins to lose.

Another reason for the electrolyte imbalance is that the membrane used is not a complete blower. Anionic membranes are inevitably permeated by small amounts of cations (H ⁺ and V ^{+ n} ).

Cationic membranes are preferred as cell separators in batteries because of their high physical / chemical resistance compared to anionic membranes.

In addition, cationic membranes can easily permeate hydrogen ions (the diffusion rate of H ⁺ is greater than that of vanadium ions).

Hydrogen ions generated in the positive compartment according to the invention during battery charging

VO ²⁺ + H ₂ O === VO ²⁺ + 2H ⁺ + e-

A small amount of foot is willing to move through the membrane to the negative compartment with slow vanadium ions.

The movement of vanadium ions will oxidize the corresponding amounts (V ⁺³ , V ⁺² ) of the reduced vanadium ions present in the negative compartment, but vanadium ions of different oxidation states are separated from the solvent molecules (water, sulfuric acid) This process is not completely reversible because it is arranged differently and has different mobility in the cation exchange resin of the membrane. In addition, during the subsequent discharge process, the number of vanadium ions crossing the membrane in the opposite direction will not be exactly the same as the number moved during the charging step.

The gradual imbalance between electrolytes produces a number of problems.

1) The battery capacity (kWh / liter of electrolyte) decreases proportionally accordingly.

2) During charging, one of the two electrolytes may be fully charged while the other is not partially charged.

Indeed, for small batteries where air removal in the negative compartment is often incomplete, the vanadium ions in the positive compartment can be fully oxidized to V ⁺⁵ when maintaining a positive V ⁺³ in the negative compartment. . This situation is very important. Because if the oxidation state is not carefully controlled in a separate electrolyte, charging will continue until the complete oxidation is reached from V ⁺⁴ to V ⁺⁵ by simply measuring the open circuit voltage. In this condition, the large release of oxygen at the carbon electrode will oxidize and destroy the electrode.

According to a common approach, after some charge and discharge cycle, the two electrolytes (negative and positive) are mixed, the oxidation state is measured and chemically adjusted to +3.5 if determined to be different from +3.5.

In fact, when the battery is stopped and the electrolyte mixed, a vanadium oxidation state higher than +3.5 is always found.

The electrolyte is readjusted to a vanadium oxidation state of +3.5 by the addition of a reducing aid (oxalic acid, sulfurous acid, etc.).

Subsequently, a significant amount of energy must be consumed to return the system to a zero state of charge (V ⁺³ for the negative electrolyte and V ^{+4 for the} positive electrolyte).

The amount of energy consumed periodically represents the net loss of the energy storage process.

By installing the relatively small vanadium reduced asymmetric cell of the present invention in a negative or positive electrolyte circuit as shown in FIG. 4, the non-negligible losses can be greatly reduced according to one aspect of the present invention.

As shown, the positive electrolyte may be purified completely or partially through the relatively small asymmetric vanadium reduction cell Red.

Cell Red may operate as needed continuously or discontinuously to maintain a symmetric vanadium oxide state structure.

Because of the potential provided by the presence of this auxiliary reducing cell Red, the need to mix the two electrolytes together adjusts the oxidation state to +3.5 and requires the battery to be in a precharge state to recover the zero state of charge. The necessity may be eliminated or only needed exceptionally.

Claims (13)

A method for producing an acidic vanadium electrolyte solution containing V ⁺³ and V ⁺⁴ at a desired concentration from solid vanadium pentoxide supplied to an electrolyte solution, Circulating the electrolyte solution through a plurality of series electrolysis cells to electrochemically reduce the vanadium dissolved in the acidic electrolyte solution to an oxidation state of V ⁺³ or below, Reacting the vanadium content thus reduced in the electrolyte solution from the final cell of the electrolysis cell with any stoichiometric vanadium pentoxide to obtain an electrolyte solution containing vanadium with an oxidation state of V ⁺⁴ , Adding acid and water to maintain the molarity of the solution, Continuously recycling the electrolyte solution through the series electrolysis cell while squeezing a stream of the produced electrolyte solution containing the desired concentrations of V ⁺³ and V ⁺⁴ exiting one of the series cells, The above steps include each cell having a cathode and an anode having respective surface structures, shapes, and mutual arrangements, thereby constructing at the anode surface a current density of 5-20 times greater than the current density of the projected cathode surface. A method for producing an acidic vanadium electrolyte solution containing V ⁺³ and V ⁺⁴ at a desired concentration from solid vanadium pentoxide supplied to an electrolyte solution, characterized by releasing oxygen at the anode. The method according to claim 1, wherein the electrolyte solution is a sulfuric acid solution, and the molar content of vanadium is between 1 and 5. The method of claim 1 wherein the current density on the projected cathode surface is 100-300 A / m ² and the current density of the anode surface is 1000-8000 A / m ² . The method of claim 1, wherein the vanadium oxide is in powder form and has a particle size of 100 microns or less. The method of claim 1, wherein after reacting the reduced vanadium electrolyte solution with the stoichiometric vanadium pentoxide, the solution is separated from the remaining undissolved vanadium pentoxide particles. A vanadium electrolyte solution containing V ⁺³ and V ⁺⁴ at a desired concentration ratio is obtained from a solid vanadium pentoxide feed material consisting of a plurality of vanadium reduced electrolysis cells connected in series in a fluid series and powered in series from a DC power supply. As a facility for manufacturing, this facility, A dissolution tank for collecting the reduced electrolyte solution from the final cell of the series cell, the dissolution tank having a feeding mechanism for supplying a controlled amount of vanadium pentoxide in powder form and a mechanical stirring means, Means for separating the vanadium-rich solution flowing out of the dissolution vessel from residual solid particles of vanadium pentoxide, Means for adding sulfuric acid and water to the vanadium-rich solution to maintain the molarity of the solution, Pump means for recycling the electrolyte solution through a series vanadium reduction electrolysis cell, Tap means for squeezing a stream of the resulting electrolyte solution containing V ⁺³ and V ⁺⁴ at a desired concentration, exiting one of the series cells Wherein each cell has a cathode and an anode having respective surface structures, shapes, and mutual arrangements, so that at the anode surface and build a current density of 5-20 times greater than the current density of the projected cathode surface To release oxygen at V ^{+ at a} desired concentration ratio from a solid vanadium pentoxide feed material consisting of a plurality of vanadium reduced electrolysis cells connected in series in a fluid series and powered in series from a direct current power supply. A facility for producing a vanadium electrolyte solution containing ³ and V ⁺⁴ . An electrolysis cell for reducing V ⁺⁴ and V ⁺⁵ to V ⁺³ and V ⁺² in an aqueous solution of acidic vanadium electrolyte, which contains cathodes and anodes with respective surface structures, shapes, and mutual arrangements. Having a current density of 5-20 times greater than the current density of the projected cathode surface to build up at the anode surface and release oxygen at the anode. An electrolysis cell according to claim 7, consisting of a cylindrical conduit of non-conductive acid resistant material having a suction port (3) and an operation port, A carbon felt cathode disposed on a cylindrical inner surface of the conduit, having a terminal for electrical connection of the cell, and A valve metal rod anode coated with a non-passivating electrocatalyst coating disposed along the axis of the cylindrical carbon felt cathode Electrolysis cell comprising a. 8. The electrolysis cell of claim 7, comprising electrolyte permeation limiting means for spatially confining oxygen bubbles emerging from or near the anode. 10. The method of claim 9 wherein the permeable confinement cell belongs to the group consisting of mesh screens, fabrics, felts, porous and microporous glass, and sintered fuselage, all of which are chemically resistant to the electrolyte solution. Electrolysis cell. 8. The electrolysis cell of claim 7, wherein said anode is a valve metal coated with a mixed oxide of iridium and tantalum or zirconium. A method of rebalancing the relative oxidation states of two separate vanadium electrolytes circulating in all vanadium redox battery systems. Circulating one part of two distinct electrolytes in a vanadium reduction electrolysis cell, external to the battery, made according to any of claims 7 to 11, and oxidation of vanadium in the two electrolytes of the redox battery system The balance of the relative oxidation states of two separate vanadium electrolytes circulating in all vanadium redox battery systems, characterized by the above process, which flows the current flowing through the electrolyte cell for the time required to rebuild the state balance. How to readjust. 13. The method of claim 12 wherein the electrolyte is an amount of electrolyte circulating in the positive compartment of the battery.