JP2004519814A - Preparation of Vanadium Electrolyte Using Asymmetric Vanadium Reduction Cell and Use of Asymmetric Vanadium Reduction Cell to Rebalance the State of Charge in Electrolyte of Running Vanadium Redox Battery - Google Patents

Abstract

An acidic vanadium electrolytic solution containing V ⁺³ and V ⁺⁴ at a desired concentration ratio from solid vanadium pentoxide supplied to the electrolytic solution is used to connect at least a portion of the vanadium dissolved in the acidic electrolytic solution to a plurality of vertically connected electrolytic solutions. By circulating between the electrolysis cells, the electrochemical reduction to at least partly the state of V ⁺³ is carried out, and the vanadium content thus reduced at the electrolyte outlet from the last cell of said electrolysis cell is theoretically reduced. Reaction with a vanadium pentoxide in a value amount to obtain an electrolytic solution containing vanadium in a substantially V ⁺⁴ state, adding an acid and water to maintain the solution at a constant molarity, and connecting the electrolytic solution in a vertically connected electrolytic cell. one to continuously recycled, preferable electrolytic solution stream produced containing V ⁺³ and V ⁺⁴ in concentration, thereby flowing out from one of said longitudinal connecting cell More it is generated. Each cell has a cathode and an anode, each having a surface morphology and geometric and mutual interaction such that the current density at the anode surface is 5 to 20 times the current density at the protruding cathode surface and oxygen is generated at the anode. Extremely asymmetric, such as having an associated arrangement. An asymmetric cell of this type can be used in one of the operating battery's yin and catholyte circuits to rebalance the respective oxidation state of these vanadium contents.

Description

[0001]
Background of the Invention
The present invention relates generally to renewable electrochemical energy storage devices in redox flow battery devices, and more particularly to so-called all-vanadium redox secondary batteries.
[0002]
All vanadium redox cells, or simply vanadium redox cells or batteries, called Vanadium Redox Flow Batteries, are referred to as V (II) / V in negative (sometimes called anolyte) and positive (sometimes called catholyte) half-cell electrolytes, respectively. (III) and V (IV) / V (V) are used as two redox couples.
A typical electrolyte used in a vanadium battery is a mixture of 50% +3 oxidation state vanadium ions and 50% +4 oxidation state vanadium ions.
[0003]
Usually the electrolyte is divided into two identical parts, which are respectively provided in the positive and negative electrolyte chambers of the battery, and more particularly in the respective flow circuits. In this starting state, the battery has a virtually zero open circuit voltage.
When a current is forcibly applied to the battery by an external power supply having a sufficiently high output voltage, V in the negative electrolyte is reduced.⁺⁴(50%) is V⁺³At the same time as V in the positive electrolyte⁺³(50%) is V⁺⁴Is oxidized.
After a certain time, the negative electrolyte continuously circulated in the respective electrolyte chambers of the battery by the negative electrolyte circulation pump is V⁺³The positive electrolyte circulated in each electrolyte chamber of the battery by the positive electrolyte circulation pump is V⁺Only have.
[0004]
In this state, the battery is said to be in a zero charge state (SOC), and the open circuit voltage of the battery is approximately 1.1 volts.
By continuing to pass the "charge" flow through the battery, V⁺³Is V⁺²And at the positive electrode V⁺⁴Is V⁺⁵Is oxidized. When this conversion is complete (at the end of the charging process), it can be said that the battery has an open circuit voltage of 1.58 volts and the battery has an SOC equal to 100%.
Vanadium can be purchased as vanadium pentoxide (or ammonium vanadate). In any case, it is commercially available, usually in the +5 oxidation state.
[0005]
The storage capacity of all vanadium redox battery devices is determined by the amount of vanadium dissolved in the acidic electrolyte. For a given molarity of the electrolyte, the storage capacity is directly proportional to the volume of the two electrolytes.
Obviously, commercially available vanadium pentoxide (or ammonium vanadate) was started to initially fill the two circuits of the redox battery device and / or to increase the storage capacity of existing battery equipment ( It is necessary to prepare an acidic solution of vanadium suitable as an electrolyte solution to be used as a feed) agent.
Therefore, the step of preparing the vanadium electrolyte is to add sulfuric acid (or other acid) to V₂O₅Is dissolved and reduced to obtain V⁺³(About 50%) and V⁺⁴(Approximately 50%) of the required mixture.
[0006]
Finely divided (pulverized) solid vanadium pentoxide is only slightly soluble in water or acids such as sulfuric acid,₂O₅Cannot be taken by simply dissolving the compound in an acid to prepare an electrolytic solution.
V₂O₅To dissolve it, one must first reduce it to a lower (more soluble) oxidation state.
V⁺⁵Various methods have been proposed for dissolving and reducing the compound, mainly using a reducing compound or using complicated electrolytic and chemical processing methods.
[0007]
EP-A-0566019 discloses a method for producing a vanadium electrolytic 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-chemical method for preparing a vanadium electrolytic solution from solid vanadium pentoxide and a method for stabilizing it. Vanadium pentoxide is dissolved on a special vane-shaped cathode consisting of cells of an ion-exchange membrane, and is carried out by flowing a slurry of vanadium pentoxide to contact the vane-shaped cathode.
[0008]
The methods and techniques developed to date for developing suitable acidic vanadium electrolytes have been rather complex and expensive. On the other hand, the availability of vanadium electrolytes at relatively low prices is an important consideration in the cost-benefit assessment of vanadium redox batteries compared to other energy storage devices in the overall economics of all vanadium flow redox battery devices. Element.
The basis for meeting these requirements is the use of relatively inexpensive solid vanadium pentoxide as a feed.
[0009]
Object and Summary of the Invention A remarkably simple and inexpensive method for easily dissolving and reducing vanadium pentoxide in acidic electrolytes has now been discovered.
The present invention is particularly useful for preparing a vanadium electrolyte from a vanadium pentoxide (or ammonium vanadate) feedstock, using a very simple and low cost electrolysis cell while reducing the solution to minimal processing. Implemented by
Nevertheless, the method of the invention remains quite effective from an energy consumption point of view.
[0010]
The process of the present invention is essentially a continuous process, in which a fixed amount of circulating vanadium electrolyte is continuously cut into finely divided or powdered solid vanadium pentoxide (V₂O₅) And supply of acid and water to maintain the solution at a constant molarity, while maintaining approximately the same or other desired concentration of V⁺³And V⁺⁴The same amount of electrolytic solution containing is continuously discharged.
The effluent electrolyte stream indicates the product of the process.
[0011]
Basically, the method according to the invention comprises:
The electrolytic solution is brought into hydraulic contact with the cathodes of a plurality of vertically connected electrolytic cells, and the V of the solution introduced into the first cell is increased.⁺⁴Part or all of the contents are sequentially V⁺³Finally, a small amount of V in the electrolytic solution⁺²Until passing through the electrolyte solution outlet from the last electrolysis cell of the plurality of vertically connected cells,
The vanadium content of the electrolytic solution at the outlet of the final electrolytic cell thus reduced is converted to a theoretical amount of vanadium pentoxide (V) in a dissolving tank having stirring means.₂O₅), And the corresponding amount of V⁺⁴Obtain an electrolytic solution containing dissolved vanadium that will be in a state,
(Eg V⁺⁴Adding acid, sulfuric acid or other equivalent acid and water to the vanadium electrolytic solution (close to) to maintain the solution at a constant molarity,
While the electrolytic solution is recirculated between the vertically connected electrolytic cells, V⁺³And V⁺⁴Flowing out of the outlet of one of the vertically connected cells.
[0012]
The essential aspects of an electrolytic cell are that the cathode and anode each have a surface morphology and geometric and mutual geometry such that the current density at the anode surface is 5 to 20 times the current density at the protruding cathode surface and oxygen is generated at the anode. Is to have an arrangement related to
In practice, the cathode can be made of carbon felt or activated carbon felt or similar material providing a relatively large surface area, and may even be tube-shaped or groove-shaped, while the anode is the geometric axis of the tube-shaped or groove-shaped cathode May be in the form of a thin bar arranged along.
[0013]
The specific activity area of the cathode, which is relatively large compared to the specific activity area of the anode, and the ratio of these projecting areas is such that the current density at the active anode surface is 5 to 20 times the current density at the geometrically projecting cathode surface. Is set to
The current density of the projecting area of the cathode is from about 1 to several hundred A / m²Operating at about 1000 A / m by setting the diameter of the anode placed "coaxial" with the cathode, at least partially encased, at least partially in the form of a tube or equivalent.²Anode current densities in excess of or above are achieved.
[0014]
Under these conditions of extremely unbalanced current density and relatively high anode current density, the current through the electrolytic cell is V⁺⁴To V⁺³The anode half cell is tuned to ensure that the cathodic reduction to is almost completely the only cathode half cell reaction (ie, limiting the maximum current density to prevent parasitic reactions such as hydrogen evolution). The reaction is primarily assisted by an oxygen evolution (water electrolysis) reaction.
In fact, the thermodynamically favorable V⁺³To V⁺⁴The anodic half-cell reaction, oxidation to, is a process in which a large amount of electrolyte fills the gap between the anode surface and the cathode surface from V to the anode surface of the cell.⁺³The very poor rate at which ions migrate and eventually diffuse is substantially and effectively impeded.
[0015]
An even more important obstacle to the migration and / or diffusion of vanadium ions towards the anode surface is manifested in the presence of oxygen gas bubbles that are actively generated over the anode surface at such relatively high current densities.
All V of the electrolyte discharged from the last cell of the vertically connected cells⁺⁴To V⁺³For almost complete reduction, the current forced hydraulically in series between a plurality of vertically connected electrolysis cells may be adjusted as a function of the flow rate of the electrolyte flowing through the vertically connected cells. Good.
Of course, this is only an ideal condition, and in fact, if there is a defect such as current flowing between cells at the time of overcurrent or vice versa, the minimum (residual) amount of V⁺⁴May exist and V⁺³To V⁺²Initial reduction to the electrolyte and a small amount of V⁺²Is V⁺³May be present with
[0016]
To promote oxygen evolution, the anode has a low oxygen overvoltage electrocatalyst surface, and is particularly resistant to acidic electrolytes with anodic polarization and oxygen elimination.
For example, the anode may be a rod made of a valve metal such as titanium or tantalum or an alloy thereof having resistance to damage to the anode, and having a non-passive active coating made of an electrocatalyst that emits oxygen. Good.
The coating may be a mixed oxide or a mixture of at least a noble metal oxide such as iridium, rhodium and ruthenium and at least a valve metal oxide such as titanium, tantalum and zircon. The active coating may alternatively be platinum, iridium or rhodium, or a dispersion of these metals in a conductive oxide matrix.
[0017]
In a dissolution tank having an ordinary mechanical stirrer, the amount of the electrolytic solution discharged from the final cell is the theoretical value (the amount of V contained in the reduced electrolytic solution).⁺³(V⁺²) Is ground and / or sieved to react with solid (powder) solid vanadium pentoxide having a maximum dimension of less than 100 μm.
The supernatant or filtered solution may be collected in a reservoir and any undissolved vanadium pentoxide may be recycled to the dissolution vessel.
The vanadium-concentrated solution has a relatively small amount of dissolved vanadium.⁺⁵May exist as⁺⁴Contains vanadium in a state.
The acid, most commonly and preferably sulfuric acid, is added to the vanadium-concentrated filtered electrolyte solution to maintain a constant concentration of the electrolyte solution. Of course, the higher the molar amount of vanadium, the higher the power to total volume ratio of the electrolyte, but problems with the stability of the solution under critical temperature conditions can occur at relatively high molar concentrations. Most preferably, in the case of a sulfuric acid solution, the molar amount of vanadium may be between 2 and 5.
[0018]
The solution is returned by the pump to the inlet of the first cell of the vertically connected cell,⁺⁴To (and any V⁺⁵Residue) to V⁺³And eventually V⁺²Electrochemically reduced to
The products of the electrolytic solution production apparatus have approximately the same amount of V⁺³And V⁺⁴And a solution that can flow out of the mainstream of the recirculated solution from the outlet of one of the vertically connected cells.
Massive oxygen evolution and corresponding V⁺⁴To V⁺³The disproportionately high current density at the anode surface, which causes a small amount of reduction to, furthermore takes into account the relatively light burden of electrical energy on the overall economics of the preparation process of any acidic vanadium electrolyte. It is surprisingly sufficient to maintain the overall efficiency of the process above an acceptable level.
[0019]
As another example, the efficiency can be further increased by including a screen or microporous separator between the rod anode and the surrounding cylindrical cathode.
The screen or microporous separator continuously grows on the anode surface and becomes an effective "containment" for oxygen bubbles that rise away from the electrolyte by buoyancy and are contained in the space between the screen and the cathode. Convection motion in a large amount of electrolyte can be suppressed to a minimum, and further reduced vanadium ions (V⁺³) Can be reduced and gradually reach the anode.
The most effective microporous separator can be a glass frit tube closed at the bottom end and enclosing a rod-shaped anode (in this case inserted into the cell from the top), and the oxygen bubbles generated once the air bubbles rise above the electrolyte. , Can be immediately discharged from the cell through the vent. Instead of a suitable microporous separator, a felt of polypropylene fibers of about 1 mm thickness may be used.
[0020]
Description of the preferred embodiment according to the invention
Referring to the functional schematic diagram of FIG. 1, a vanadium electrolyte preparation device according to the present invention is a plurality of electrolyte solutions that are hydraulically connected in a vertical connection and are electrically powered in series from a suitable DC power source R1. It consists of vanadium reduction cells C1, C2, C3,... C6.
The solution discharged from the last vertically connected cell C6 is collected in a dissolving tank T1 having stirring means S1.
[0021]
Vanadium pentoxide (V₂O₅) Is introduced into the dissolution vessel T1 by a suitable quantity, for example a conventional feed hopper and a feed machine controlled by a motor drive.
The vanadium-enriched solution gradually contains residual solid particles of undissolved vanadium pentoxide, flows out of the dissolution tank T1 through the liquid level outlet, and the supernatant is removed in the settling tank T2.
Pump P2 recycles the remaining solid particles of vanadium pentoxide, which are gradually collected at the bottom of settling tank T2, to dissolution tank T1.
The vanadium-enriched, filtered solution is eventually collected in reservoir T3.
[0022]
The vanadium content of the vanadium-enriched solution collected in reservoir T3 is substantially V⁺⁴Contains vanadium in a state. The contents are the V present in the electrochemically reduced solution flowing out of the cascade final reduction cell (C6).⁺³And the V that will eventually exist⁺²And the same amount of dissolved and reduced V⁺⁵And the sum of In fact, V⁺⁵Of the vanadium-enriched solution thus collected in T3⁺⁴It may be present with.
This solution contains a relative amount of acid, typically H 2, suitable to maintain the desired molarity of the vanadium electrolyte.₂SO₄And water H₂After adding O, it is continuously circulated through the vertically connected vanadium reduction electrolytic cell by the pump P1.
Therefore, the vanadium electrolytic solution entering the first reduction cell C1 is approximately V⁺⁴, And V⁺⁵May be present in a residual amount, but contains.
[0023]
At the negative electrodes (cathodes) of the reduction cells C1, C2, C3,.
V⁺⁴+ E⁻= V⁺³  (More precisely, VO⁺²+ E⁻+ 2H⁺= V⁺³+ H₂O) and when vanadium in the +5 oxidation state is present, the other reaction is:
V⁺⁵+ E⁻= V⁺⁴  (More precisely, VO⁺+ E⁻+ 2H⁺= VO⁺²+ H₂O).
[0024]
No other reaction takes place at the negative electrode. Since the carbon felt electrode has a relatively high hydrogen overvoltage and the effective current density on the cathode surface is suppressed to a sufficiently low value, hydrogen generation (a thermodynamically advantageous half-cell reaction) does not occur.
At the positive electrode, theoretically, the main reaction is from the low oxidation state (+4, +3, and +2) of any vanadium ions present to the pentavalent vanadium (V⁺⁵) (A thermodynamically favorable half-cell reaction).
In fact, vanadium ions near the anode surface immediately⁺⁵The vanadium ions in the low oxidation state, which move and diffuse toward the anode, are also oxidized. However, the vanadium ions present near the positive electrode are V⁺⁵And the anode half-cell reaction is initiated with the help of the only other possible half-cell reaction, namely the evacuation and consequent generation of oxygen gas by the next reaction.
H₂O = O₂+ 2H⁺+ 2e⁻
[0025]
In the asymmetric cell of the present invention, oxidation of vanadium is substantially eliminated as in the conventional apparatus using an ion exchange membrane cell and a separate circuit of a vanadium-containing catholyte and an auxiliary acidic anolyte. is not. Vanadium ions that can reach the anode of the cell in practice are immediately V⁺⁵Is oxidized to
[0026]
However, due to the unique imbalance in electrode current density, the anode operates at relatively high current densities, which are higher than can be supported by the process in which vanadium ions in the electrolyte move and diffuse toward the anode surface. Range. As a result, generation of a large amount of oxygen is promoted on the anode surface, and active generation of oxygen gas bubbles is caused by V⁺³It provides a "mechanical" barrier for ions to move towards the anode.
Barriers that impede the diffusion of cathode-reduced vanadium ions to the anode use screens or permeable (microporous) membranes, which confine the oxygen bubble population near the anode and thus between the gas-filled screen and the cathode surface. This can be greatly enhanced by preventing strong convective motion from being generated in most of the reduced vanadium ion-enriched electrolyte present in the space.
Using a low oxygen overvoltage anode only promotes oxygen evolution.
[0027]
The use of a separator with relatively tight pores instead of a more permeable screen or membrane significantly increases the overall Faraday effect, but also increases the cell voltage. Therefore, the best compromise is sought considering that energy consumption is the product of current and voltage.
When the cathode-to-anode current density ratio is about 5, a Faraday effect of 40% or more can be easily secured, and if this current density ratio is increased to 20, the effect reaches a level of 80% or more. Was. The use of gas-filled screens and separators with relatively tight micropores can further increase these figures significantly.
[0028]
In the dissolving tank T1, V contained in the electrolytic reduced vanadium electrolytic solution⁺³Is solid vanadium pentoxide V₂O₅(Or ammonium vanadate), which is dissolved according to the following reaction,⁺⁴To be reduced to
(V⁺⁵+ V⁺³= 2V⁺⁴) Or more specifically
V₂O₅+ 2V⁺³+ 2H⁺= 4VO⁺²+ H₂O (1)
And (V⁺²If present),
(2V⁺⁵+ V⁺²= 3V⁺⁴) Or more specifically
V₂O₅+ V⁺²+ 4H⁺= 3VO⁺²+ 2H₂O
[0029]
A cross-sectional view of the asymmetric cell according to the invention used in the vanadium electrolyte preparation of the invention is shown in FIG.
The experimental test cell illustrated in FIG. 2 comprises a tubular tubular body 1 which is typically made of a metal which has chemical resistance to the electrolyte or a non-conductive and acid-resistant plastic such as PVC. The bottom is closed by a plug 2 and has an inlet 3 and an upper overflow 4 in the lower part of the tubular body 1.
A cylindrical cathode, which may consist of a few millimeters thick carbon felt 5, may be provided on the inner cylindrical surface of the tube and suitably fastened. The felt cathode may be provided with a suitable terminal 6 to electrically connect the cell to a DC power supply circuit.
[0030]
In the experimental test cell shown in FIG. 2, the inner cylindrical surface area of the cathode has a diameter of about 50 mm and a height of approximately 250 mm for contact with the electrolyte.
Anode 7 is a titanium rod 6.3 mm (1/4 inch) in diameter, coated with a mixed oxide of iridium and tantalum, and immersed in an electrolyte approximately 250 mm long.
The anode 7 of the coated titanium rod is arranged along the axial direction of the cathode of the tubular carbon felt.
The test cell for experiments defined in this way has a projecting area of the carbon felt cathode of about 353 cm.²While the surface of the titanium rod anode is about 47 cm²It is.
[0031]
For a 7 A current forced through the cell, the current density at the titanium anode surface is approximately 0.1485 A / cm²= 1500A / m²On the other hand, the current density of the projected region of the carbon felt is 0.022 A / cm²= 220A / m²It is. However, thanks to the form in which the cathode takes the form of carbon fiber felt, which is open and easily penetrated, the actual or effective cathode current density of carbon is reduced by the geometrically protruding tube of the carbon felt cathode. It can be estimated that it is 2 to 10 times smaller than the current density calculated from the shape area.
[0032]
FIG. 3 shows a sectional view of a vanadium reduction cell according to another embodiment.
The only difference is represented by the presence of a fluid permeable screen or membrane, or a separator 8 having micropores, which is provided between the cylindrical cathode surface and the coaxially arranged rod-shaped anode, And a buoyant oxygen bubble generated on the anode surface and gradually moving away from the surface toward the surrounding electrolyte is maintained substantially confined in the cylindrical space.
Screen-membrane 8 is V⁺⁴To V⁺³To V⁺²It is possible to substantially prevent the electrolytic liquid near the cathode surface from undergoing strong convection movement where desired reduction to the cathode occurs.
[0033]
A plastic tube with small, dense and evenly distributed holes may be sufficient as a gas bubble-filled screen, but instead of an oxygen bubble-filled screen 8 woven, for example, with reticulated titanium wire or plastic fibers A fine net made of a resistance material such as cloth may be used. More preferably, the gas-filled screen 8 may be a porous or microporous tube made of, for example, glass frit or resistive metal particles such as sintered titanium.
[0034]
An example
To prove the effectiveness of the technique of the present invention, a 1/2 liter glass beaker with an inner diameter of 8 cm was used.
Approximately 6 mm (1/4 inch) thick carbon felt was placed along the inner wall of the beaker and electrically connected to the cathode of a DC power supply.
IrO_x-ZrO_yAn approximately 6 mm (1/4 inch) outside diameter titanium rod coated with oxide loaded with was placed vertically along the geometric axis of the beaker and electrically connected to the anode of a DC power supply.
The ratio of the projected cathode area to the anode area was about 10,7.
[0035]
1 mm thick polypropylene was formed into a circular tube having an inner diameter of about 12 mm closed at the bottom and placed coaxially around the coated titanium rod anode in a beaker.
The beaker was filled with 473 ml of a solution of 5 mol of 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 Ah is required to reduce 1 mole of vanadium from the +5 state to the +4 state.
[0036]
The mixture was stirred with a magnetic stirrer and the yellow powder of vanadium pentoxide remained almost undissolved for several days.
The DC power was turned on and its output voltage was adjusted to force a DC current of 8 A into the cell. The current density of the anode (anode) is about 5,013 A / m²The current density of the cathode (cathode) in the projected area of the carbon felt is approximately 468 A / m.²Met.
The cell voltage was substantially constant between 3.8 and 4.0 volts.
The suspension was gently agitated with a magnetic stirrer and subjected to a current for 5.26 hours and the yellow powder appeared to be completely dissolved.
[0037]
Analysis of the blue solution thus obtained contained 2 moles of vanadium (2 mole solution), indicating that the oxidation state of vanadium was +3.55.
The Faraday (current) efficiency of this step was estimated to be 92.28%.
When the test was performed again at a lower current of 5 A, the required time was 9.87 hours. Faraday (current) efficiency was reduced to about 78.74%, but the voltage of the battery was also reduced to 2.8 volts.
Using a thin polypropylene woven fabric instead of felt reduced the current efficiency to about 47% and to about 20-25% without any permeable encapsulation element.
[0038]
Even under these non-optimal laboratory conditions (called stirring in a glass beaker), power consumption in the range of 0.2 to 0.5 kWh per liter of vanadium electrolyte produced was noted. , A relatively low cost figure for the overall economics of producing vanadium electrolytes.
The ability of the asymmetric vanadium electrolyte reduction cell of the present invention to efficiently and inexpensively alter the oxidation state of vanadium dissolved in acidic electrolytes means that the redox battery device must be recharged whenever the battery is no longer in an unacceptable imbalance. Rebalancing the state of charge of the positive and negative vanadium electrolytes in a running battery without performing expensive and time consuming steps of shutting down the operation is a relatively simple, low cost, nearly An undivided, asymmetric cell of the invention makes it ideally suited.
[0039]
To better understand the nature of the challenges involved in operating a vanadium battery energy storage device, it may be helpful to briefly recall the main mechanisms leading to an increasingly prominent imbalance.
In theory, the only steps during charging and discharging of a vanadium redox battery are the electrochemical oxidation and reduction of vanadium, and assuming that no other side reactions occur, the charging and discharging steps of a vanadium battery are symmetric. Process.
[0040]
During charging, the current flowing through the battery is V in the positive electrolyte chamber.⁺⁴To V⁺⁵And at the same time, at the same rate, V⁺³To V⁺²To be reduced to During discharge, opposite oxidation and reduction reactions take place in the positive and negative electrolyte chambers.
Unfortunately, the situation is actually different.
The steps being performed are not limited to the electrochemical oxidation and reduction of vanadium. The following side reactions tend to occur at critical conditions of operation:
1) Electrochemical generation of hydrogen gas at the cathode
2) Electrochemical generation of oxygen gas at the anode (*)
3) V⁺²To V⁺³Chemical oxidation to
4) V⁺⁴To V⁺⁵Chemical reduction to
(*) If the anode is made of carbon, the generation of oxygen is partially or completely replaced by the generation of carbon dioxide.
[0041]
Reactions 1) and 2) are the only reactions once the state of charge is achieved at 100%. In practice, all V present in the electrolyte in the positive electrolyte chamber⁺⁴Is V⁺⁵Once oxidized, the only reaction at the anode that will assist the current is the evolution of oxygen (or carbon dioxide). Similarly, all V present in the electrolyte in the negative electrolyte chamber⁺³Is V⁺²Once reduced, the only reaction at the cathode that would assist the current is the evolution of hydrogen. These reactions begin to occur during the charging of the battery, but become relatively small when the state of charge exceeds 90%.
Since the voltage at which vanadium is oxidized or reduced increases in proportion to the ratio between the species produced and the species consumed (Nernst equation), the cell voltage can be approximately 1.5 volts at high charge. The voltage rises to the voltage at which hydrogen and oxygen are generated (water electrolysis). Reactions 1) and 2) also occur, although in relatively small amounts, when discharge occurs at a very fast rate (current).
As the current density approaches the limiting current, hydrogen and oxygen begin to occur as secondary (parasitic) electrode reactions.
[0042]
The limiting current is a current when the rate of oxidation or reduction of vanadium on the electrode surface coincides with the rate at which vanadium ions diffuse from most of the electrolyte to the electrode surface through the layer depleted.
Reaction 3), ie V⁺²To V⁺³Oxidation to is the most frequently repeated side reaction during the operation of a vanadium battery. V⁺²Easily V in the presence of air⁺³Oxidized to
.
Therefore, such side reactions occur immediately unless the atmosphere is strictly prevented from coming into contact with the negative electrolyte (by covering with nitrogen gas or covering the surface of the electrolyte with wax or the like).
Due to the side reactions mentioned above, the symmetry will start to be considerably lost after many cycles of battery operation.
[0043]
Another reason for electrolyte imbalance is that the membrane used is not a perfect separator. Anionic membranes also necessarily have a small amount of cation (H⁺And V^{+ N}) Is also transmitted.
Cationic membranes are generally preferred as battery cell separators because of their higher mechanical and chemical resistance compared to anionic membranes.
In fact, cationic membranes are mainly composed of hydrogen ions (H⁺Is much higher than the diffusivity of vanadium ions).
While charging the battery,
VO²⁺+ H₂O = VO₂ ⁺+2⁺+ E⁻
The hydrogen ions generated in the positive electrolyte chamber according to the above reaction easily move through the membrane to the negative electrolyte chamber together with a small amount of vanadium ions having a lower mobility.
[0044]
Due to the movement of vanadium ions, reduced vanadium ions (V⁺³And V⁺²) Is oxidized, but this step is not completely reversible. Because vanadium ions in different oxidation states coordinate themselves differently with solvent molecules (water, sulfuric acid) and have different mobilities in the cation exchange resin of the membrane. In fact, in the subsequent discharge phase, the number of vanadium crossing the membrane in the opposite direction is not exactly the same as the number moved to the charge phase.
The gradual imbalance of the electrolyte causes a number of problems, including:
1) The capacity of the battery (converted to kWH / liter of electrolyte) decreases proportionately.
2) During charging, one of the two electrolytes is fully charged while the other is only partially discharged.
[0045]
Indeed, the vanadium ions in the positive electrolyte chamber can cause V.sub.2, especially in small batteries where the air removal from the negative electrolyte chamber is often incomplete.⁺⁵While oxidizing completely to significant amounts of V⁺³May remain in the negative electrolyte chamber. This situation is very critical. If the oxidation state is not carefully controlled in a separate electrolyte and only the open circuit voltage is measured, V⁺⁴To V⁺⁵This is because charging continues to the point where complete oxidation to is reached. In this situation, the mass production of oxygen at the carbon electrode will oxidize and damage the electrode.
The usual approach is to mix the two (negative and positive) electrolytes after a certain number of charge and discharge cycles, measure the oxidation state and, if different from +3.5, change the chemical state to +3.5. It is adjusted to.
[0046]
In practice, in shutting down the battery and mixing the electrolyte, the vanadium oxidation state is always +3.5 or higher (mainly due to the dominant effect of side reaction 3).
The electrolyte is readjusted by adding a reducing agent (oxalic acid, sulfite, etc.) to the +3.5 oxidation state.
This is followed by a considerable amount of energy expending the device in a zero charge state (V⁺³V in the positive electrolyte⁺⁴).
This amount of energy expended regularly indicates the net loss of the energy storage process.
[0047]
In accordance with aspects of the present invention, this notable loss is greatly enhanced by installing the relatively small vanadium-reducing asymmetric cell of the present invention in the yin and more preferably in the positive electrolysis circuit shown schematically in FIG. Can be reduced.
As shown, circulating the positive electrolyte entirely or partially (in the latter case, for example, by using an adjustable three-way valve or other means) through a relatively small asymmetric vanadium reduction cell Red. Can be.
In order to maintain the symmetric vanadium oxidation state arrangement, the cell Red may be operated continuously or discontinuously as required.
Thanks to the possibilities opened up by the presence of such an auxiliary reduction cell Red, the two electrolytes are mixed, the oxidation state is adjusted to about +3.5 and the battery is precharged to restore the zero charge state The need to state can be eliminated or only required in exceptional cases.
[Brief description of the drawings]
FIG.
FIG. 1 shows a solid V according to the invention.₂O₅1 shows an apparatus for adjusting a vanadium electrolyte from a supply material.
FIG. 2
FIG. 2 is a sectional view of a vanadium reduction cell according to the present invention.
FIG. 3
FIG. 3 is a sectional view of another embodiment of the vanadium reduction cell.
FIG. 4
FIG. 4 is a basic schematic diagram showing a vanadium flow redox battery device in an electrolyte circuit for a rebalancing function including a vanadium reduction cell of the present invention.

Claims (13)

A method for producing an acidic vanadium electrolytic solution containing a desired concentration ratio of V ⁺³ and V ⁺⁴ from solid vanadium pentoxide supplied to the electrolytic solution,
Electrochemically reducing at least a portion of the vanadium dissolved in the acidic electrolytic solution to an oxidation state of at least V ⁺³ or less by circulating the electrolytic solution between a plurality of vertically connected electrolytic cells;
Reacting the vanadium content thus reduced at the electrolyte outlet from the last cell of the electrolysis cell with a theoretical amount of vanadium pentoxide to obtain an electrolyte solution containing vanadium in a substantially V ⁺⁴ oxidation state When,
Adding acid and water to maintain the solution at a constant molarity;
Continuously recirculating the electrolyte solution between the cascaded electrolysis cells while allowing a generated electrolyte solution stream containing V ⁺³ and V ⁺⁴ at a desired concentration to flow out of one of said cascade cells When,
Each cell has a cathode and an anode, each having a surface morphology and geometric and mutual interaction such that the current density at the anode surface is 5 to 20 times the current density at the protruding cathode surface and oxygen is generated at the anode. A method comprising having an associated arrangement. The method according to claim 1, wherein the electrolytic solution is a sulfuric acid solution, and the molar amount of vanadium is 1 to 5. The current density of the protruded cathode surface is 100 to 300A / ^{m 2,} The method of claim 1 current density is 1000 to 8000 A / ^{m 2} at the anode surface. The method of claim 1, wherein the vanadium oxide is in powder form and has a particle size of less than 100 μm. The method of claim 1 wherein after reacting the reduced vanadium electrolytic solution with a stoichiometric amount of vanadium pentoxide, the solution is separated from undissolved residual vanadium pentoxide particles. Consisting of a plurality of vanadium reduction cells hydraulically connected in tandem and powered in series by a controlled DC power supply, containing V ⁺³ and V ⁺⁴ at a desired concentration ratio from a solid vanadium pentoxide feed. In a device for adjusting a vanadium electrolytic solution,
A dissolution tank having mechanical stirring means and a supply device for supplying a controlled amount of vanadium pentoxide in powder form, and for collecting an electrolytic solution discharged from a final cell of the vertical-linkage electrolytic cell;
Means for separating the residual solid particles of vanadium pentoxide and the vanadium-enriched solution overflowing from the dissolution tank,
Means for adding acid and water to the vanadium-enriched solution to maintain the solution at a constant molarity;
Pump means for recirculating the electrolyte solution between the vertically connected vanadium reduced electrolyte cells,
Tap means for allowing a generated electrolyte stream containing V ⁺³ and V ⁺⁴ at a desired concentration ratio to flow out of an outlet of one of said cascading cells;
Each cell has a cathode and an anode, each having a surface morphology and geometric and mutual interaction such that the current density at the anode surface is 5 to 20 times the current density at the protruding cathode surface and oxygen is generated at the anode. A device comprising a related arrangement. V ⁺⁴ and / or V ⁺⁵ ions are reduced to V ⁺³ and / or V ⁺² in an acidic aqueous vanadium electrolytic solution, and have a cathode and an anode. The current density on the anode surface becomes 5 to 20 times the current density on the protruding cathode surface. Electrolytic cells each having a surface morphology and a geometric and interrelated arrangement to generate oxygen at the anode. A tubular tube-shaped main body made of a non-conductive and acid-resistant material and having an inflow port 3 and an overflow port;
A carbon felt cathode provided on the inner cylindrical surface of the tube-shaped body and having terminals for electrical connection of the cells,
8. The electrolytic cell according to claim 7, comprising an anode of a valve metal rod covered with a non-passivated electrocatalytic coating and disposed along the axis of the tubular carbon felt cathode. The electrolytic cell according to claim 7, further comprising an electrolytic permeation means for containing buoyant oxygen bubbles generated in the electrolyte around or around the anode. 10. The method according to claim 9, wherein said containment and permeation means belongs to a group consisting of a reticulated screen, a woven fabric, a felt, a porous and microporous glass frit and a sintered body, all of which are made of a substance having chemical resistance to an electrolytic solution. The electrolytic cell as described. The electrolytic cell of claim 7, wherein the anode is a valve metal coated with a mixed oxide of iridium and tantalum or zirconium. The relative oxidation state of two separately circulating vanadium electrolytes in a vanadium flow redox battery can be achieved by charging a portion of one of the two separate electrolytes outside the battery without deactivating the battery. Circulating the vanadium-reduced electrolysis cell produced by any of 7 to 11 and allowing the cell to re-establish the balance of the oxidation state of the vanadium content in the two electrolytes of the flow redox battery device; A method of rebalancing consisting of forcing current through. 13. The method of claim 12, wherein the electrolyte is a positive electrolyte circulating in an anolyte compartment of a battery.

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