MXPA03001330A

MXPA03001330A - 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.

Info

Publication number: MXPA03001330A
Application number: MXPA03001330A
Authority: MX
Inventors: Barry Michael Broman
Original assignee: Squirrel Holdings Ltd
Priority date: 2000-08-16
Filing date: 2000-08-16
Publication date: 2004-12-13
Also published as: EP1310008A1; TR200300178T2; JP2004519814A; AU2000267249A1; NZ523752A; WO2002015317A1; IL154098A0; TW531934B; CN1502141A; CA2420014A1; KR20030034146A; BR0017317A

Abstract

An acid vanadium electrolyte solution containing V+3 and V+4 in a desired concentration ratio from solid vanadium pentoxide fed into the electrolyte solution, is produced by electrochemically reducing at least partly of the dissolved vanadium in the acid electrolyte solution by circulating the electrolyte solution through a plurality of electrolytic cells in cascade to at least partly to a V+3 state; reacting the so reduced vanadium content in electrolyte solution outlet from the last of said electrolytic cells with a stoichiometric quantity of vanadium pentoxide obtaining an electrolyte solution containing vanadium substantially in a V+4; adding acid and water to maintain a certain molarity of the solution; and continuously recycling the electrolyte solution through the cascade of electrolytic cells while bleeding a stream of yielded electrolyte solution containing V+3 and V+4 in the desired concentrations at the exit of one of the cells of said cascade. Each cell is highly asymmetric, having a cathode and an anode with respective surface morphologies, geometry and mutual disposition such to establish on the anode surface a current density from 5 to 20 times greater than the current density on the projected cathode surface and evolve oxygen at the anode. An asymmetric cell of this type may be used in the circuit of one of the positive and negative electrolytes of a working battery for rebalancing the respective states of oxidation of their vanadium content.

Description

PREPARATION OF VANADIUM ELECTROLYTES USING ASYMMETRIC VANADIUM REDUCTION CELLS AND USE OF AN ASYMMETRIC VANADIUM REDUCTION CELL TO REPLACE THE STATE OF CHARGE OF THE ELECTROLYTES OF A REDUCTION OPERATING BATTERY - VANADIUM OXIDATION BACKGROUND OF THE INVENTION This invention relates generally to storage of renewable electrochemical energy in reduction / oxidation (redox) flow battery systems and more particularly to so-called secondary vanadium redox secondary batteries. The vanadium redox flow battery also referred to as the vanadium completely redox cell or simply the vanadium redox cell or battery, employs V (II) / V (III) and V (IV) / V (V) as the two redox couplets, in the electrolytic solutions of negative cell median (sometimes referred to as the anolyte) and positive (sometimes referred to as the catholyte), respectively. The typical electrolyte used in a vanadium battery 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 generally divided into two equal parts that are placed respectively in the positive and negative compartments of the battery or more precisely in the relative flow circuits. In this starting condition, the battery has an open circuit voltage that is practically zero. When a current is forced through the battery by an external force of sufficiently high output voltage, the V + 4 (50%) in the negative electrolyte will be reduced to V + 3 and at the same time the V + 3 (50%) ) in the positive electrolyte will be oxidized to V + 4. At some point the negative electrolyte, made continuously circulating through the respective compartments of the battery electrode by a negative electrolyte circulation pump will contain only V + 3 and the positive electrolyte circulated through the respective electrode compartments of the battery by a positive electrolyte circulation pump will contain only V "4. In this condition, the battery is said to have a zero charge state (SOC) and the open circuit voltage of the battery will be approximately 1.1 volts. By continuing to force a "charge" current through the battery, at the negative electrode V + 3 will be reduced to V + 2 at the positive electrode, V + 4 will be oxidized at V + 5. When this transformation is completed ( at the end of a charging process) the battery will have an open circuit voltage of approximately 1.58 V and the battery is said to have a SOC equal to 100% Vanadium is commercially available In any case, vanadium pentoxide (or also as ammonium vanadate) is normally marketed with an oxidation state of +5. The storage capacity of a completely vanadium redox battery plant is given by the amount of vanadium dissolved in the acid electrolyte. For a given molarity of the electrolyte solutions, the storage capacity is directly to provide the volume of the two electrolytes. Obviously, there is a need to produce acid vanadium solutions suitable as an electrolyte to first fill the two circuits of a redox battery system and / or to expand the storage capacity of an existing battery installation, using commercially available vanadium pentoxide ( or ammonium vanadate) as the starting material (food). Therefore, the process of preparing a vanadium electrolyte is a process consisting of dissolving V2Os in sulfuric acid (or other acid) and reducing it to the required mixture of V + 3 (approximately 50%) and V + 1 (approximately 50%). The finely divided solid vanadium pentoxide (powder) is only slightly soluble in water or in an acid such as for example sulfuric acid and a simple process for preparing the electrolyte by dissolving V205 in acid is not possible. In order to dissolve V205, it is necessary to reduce it first to a lower oxidation state (more easily soluble). Several methods have been proposed for the dissolution and reduction of V + 5, mainly when using reducing compounds, or complicated chemical and electrolytic processing methods. EP-A-0 566 019 describes a method for producing an electrolytic solution of vanadium by chemical reduction of vanadium pentoxide or ammonium vanadate in concentrated sulfuric acid, followed by thermal treatment of the precipitate. WO 95/12219 and WO 96/35239 describe an electrochemical-chemical process for preparing an electrolyte solution of vanadium from solid vanadium pentoxide and a method for stabilizing it. The vanadium pentoxide solution is carried out in a special cathode in splints of an ion exchange membrane cell by placing a slurry of vanadium pentoxide placed in contact with the cathode in splints. The method and techniques developed in this way to prepare a suitable vanadium acid electrolyte are rather complex and expensive. On the other hand, for reasons of economy as a whole of a vanadium flow redox battery system completely, the availability of a vanadium electrolyte solution at a relatively low cost is an important factor in the evaluation of the cost / benefits of a vanadium redox battery compared to other energy storage systems. The key to meeting these requirements is to use relatively inexpensive solid vanadium pentoxide as a feedstock.

OBJECT AND Brief Description of the Invention A remarkably simple and inexpensive method for dissolving and easily reducing vanadium pentoxide in an acid electrolyte has now been found. The invention is particularly useful for preparing a vanadium electrolyte from vanadium pentoxide feed (or ammonium vanadate) and is implemented by the use of extremely simple electrolyte cells and owing cost while minimizing auxiliary treatments. of the solution. However, the method of the invention remains completely efficient even from the point of view of energy consumption.

The method of this invention is intrinsically a continuous method whereby a certain volume of the vanadium electrolyte circulation solutions is continuously fed with solid vanadium pentoxide (V205) in a finely divided form or in powder, acid and water to maintain a certain embodiment of the solution, while continuously purging an equivalent volume of the electrolyte solution, containing V + 3 and V + 4 at substantially similar concentrations or other desired concentrations. The purge current of the electrolyte solution represents the process performance. Basically, the method of the invention consists in: passing the electrolyte solution in contact with the cathodes and a plurality of electrolytic cells, hydraulically in cascades to progressively reduce part or all of the V + 4 content of the solution entering the first cell at V + 3 and possibly in an amount less than one V + 2 in the electrolyte solution at the outlet of the electrolyte solution of the last electrolytic cell of the plurality of cells in cascade; reacting the vanadium content reduced in this way from the electrolyte solution to the outlet of the last of the electrolytic cells by a stoichiometric amount of vanadium pentoxide (V205) in a dissolution vessel provided with a stirring medium, obtaining an electrolyte solution containing a corresponding amount of dissolved vanadium which may be almost completely in a V + 4 state; adding acid, sulfuric acid or any other equivalent acid and water to the vanadium electrolyte solution (for example, close to V + 4) to maintain a certain modality thereof; recycling the electrolyte solution through the cascade of electrolytic cells while purging a stream of electrolyte solution containing V + 3 and V + 4, preferably at substantially similar concentrations, at the outlet of a cell of the plurality of cells in cascade. An essential aspect of the electrolytic cells is that their cathode and anode have respective surface morphologies, geometry and mutual arrangement such that a current density of 5 to 20 times greater than the current density at the cathode surface is established on the anode surface. and oxygen is emitted at the anode surface. In practice, the cathode can be a carbon felt or an activated carbon felt or similar material that provides a relatively large surface area and can be tubular or even channel-shaped, while the anode can be in the shape of a thin rod placed along the cathode axis of tubular or channel shape. The comparatively large specific active area of the cathode in comparison to the specific active area of the anode and its projected area ratio are such as to determine a current density at the anode active surface of 5 to 20 times greater than the current density in the anode. geometrically projected cathode surface. By operating with a cathodic current density in the order of approximately a few hundred A / m 2 of the projected area and by sizing the diameter of the anodic rod placed "centrically" with respect to a tubular cathode or equivalent, at least partially enveloping, anodic current density in excess of 1000A / m2 or even greater can be established. Under these conditions of markedly disproportionate current densities and relatively high anodic current density, while the forced current in the electrolyte cells is adjusted to ensure the cathode reduction of V + 4 to V + 3, the reaction of half cathodic cell, alone (ie, which limits the maximum current density such as to prevent parasitic reactions such as hydrogen emission), the half-cell anodic reaction becomes supported mainly by the oxygen emission reaction (water electrolysis) ). In fact, the half-cell, anodic, thermodynamically privileged reaction of oxidation from V + 3 to V + 4 is practically and effectively prevented by a totally insufficient migration speed and eventually the diffusion of V + 3 ions from the electrolyte volume which fills the gap between the anode and cathode surfaces towards the anode surface of the cell. A major impediment, in addition to the migration and / or diffusion of the vanadium ions to the anode surface, is represented by the presence of oxygen gas bubbles that emanate vigorously on the anode surface at relatively high current densities. The forced current in electrical series through the plurality of electrolytic cells, hydraulically cascaded, can be adjusted depending on the flow rate of the electrolyte through the cascade of cells, in order to produce a practically complete reduction of all the Vt4 to V + 3 in the electrolyte left by the last cell of the cascade. Of course, this retains an ideal condition, in fact a minimum (residual) amount of V + 4 can be present in the case of a defect that the current has been forced through the cells or, vice versa, in the case of an excess of current, an incipient reduction of V + 3 to V + 2 may occur so that in the electrolyte leaving the last cell a smaller amount of V + 2 may be present together with V + 3. The anode has an electrocatalytic surface of low on oxygen potential to promote the emission of oxygen and above all it is resistant to the acid electrolyte under conditions of anodic polarization and oxygen discharge. For example, the anode may be a rod of a valve metal resistant to anodic attack such as titanium, tantalum or alloys thereof provided with a non-passive active coating of an oxygen discharge electrocatalyst. The coating may be a mixed oxide or a mixture of oxides of at least one noble metal such as iridium, rhodium and ruthenium and at least one valve metal such as titanium, tantalum and zirconium. The active coating may alternatively consist of a coating of noble metal such as platinum, iridium or rhodium or the same metals dispersed in a conductive oxide matrix. In a dissolution vessel provided with ordinary magnetic stirring, the electrolyte solution that exists in a last cell is contacted with a stoichiometric amount (referred to as the amount of V + (V2) contained in the reduced electrolyte solution) of the pentoxide of solid vanadium, in a finely divided form (powder) prepared by grinding and / or sieving the solid vanadium pentoxide such as to introduce particles with a maximum size of not more than 100 m. The decanted or filtered solution is recovered in a container and any undissolved vanadium pentoxide particle can be recycled back to the dissolution vessel. The solution enriched in this manner contains substantially in a V + 4 state although relatively small amounts of dissolved vanadium such as V + 5 may be present. The acid that is more common and preferred than sulfuric acid and water is added to the filtered electrolyte solution enriched with vanadium to maintain a certain molarity of the electrolyte solution. Of course, the greater the vanadium molar content, the higher the total power / volume ratio of electrolyte, however, problems can be encountered with the stability of the solution under critical temperature conditions at relatively high molar concentrations. More preferably, in the case of a sulfuric acid solution, the molar content of vanadium can vary from 2 to 5 mol. The solution is pumped back to the entrance of the first cell of the cascade of cells to undergo electrochemical reduction of V + 4 (and of any residue of V + 5) to V + 3 and eventually V + 2. The performance of the electrolyte production plant is a solution that contains approximately the same amount of V + 3 and V + 4 that can be drained from the main stream of the recirculation solution at the outlet of one of the cells in the cascade of cells. The disproportionately large current density at the anode surface that causes a massive emission of oxygen and a corresponding lower oxidation from V + 3 to V + 4, is a condition that is surprisingly sufficient to maintain the overall efficiency of the process at levels more than acceptable, also considering the relatively lower weight that the cost of electric power has in the total economy of the preparation process of a vanadium acid electrolyte. Even efficiency can be increased by including, as an alternative embodiment, a sieve or even a microporous separator between the rod anode and the surrounding cylindrical cathode. The microporous sieve or separator produces an effective "confinement" of the oxygen bubbles that arise by the flotation in the electrolyte as they continuously grow and separate from the surface of the anode, thus minimizing the connective movements in the volume of the contained electrolyte in the space between the sieve and the cathode and further reducing the capacity of the reduced vanadium ions (V + 3) to migrate and eventually reach the anode. A more effective microporous separator can be a glass frit tube closed at its bottom end and that surround the rod anode (in this case introducing the cell from the top) so that the bubbles emanate oxygen once in the The electrolyte surface can easily be discharged from the cell through a vent. Alternatively, a suitable microporous separator may be a felt of approximately 1 mm thick polypropylene fibers.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 depicts a plant for preparing a vanadium electrolyte solution of a V205 solid feed, according to the present invention. Figure 2 is a cross section of a vanadium reduction cell of the invention. Figure 3 is a cross section of an alternative embodiment of the vanadium reduction cell.

Figure 4 is a basic scheme of a fully vanadium flow reduction-oxidation battery system that includes a vanadium reduction cell of 1 invention in the positive electrolyte circuit to rebalance the functions.

Description of Preferred Modalities of the Invention With reference to the functional scheme of Figure 1, a vanadium electrolyte preparation plant according to the present invention is composed of a plurality of vanadium reducing electrolytic cells Cl, C2, C3, ..., C6, hydraulically connected in cascade and operated in electrical series by an appropriate Rl supply of CD. The solution leaving the last cell C6 of the cascade is collected in a dissolution vessel TI, provided with an agitating medium SI. Vanadium pentoxide (V205) is introduced into the solution tank TI in an appropriate amount by means of the example of a conventional feed hopper and a motor driven, controlled feeding mechanism. The solution enriched with vanadium, which eventually contains residual solid particles of undissolved vanadium pentoxide, flows out of the solution tank IT through a level discharge orifice and decant into the settling vessel T2. A pump P2 eventually recycles back to the dissolution tank TI, the solid, residual particles separated from the vanadium pentoxide that are eventually collected from the bottom of the settlement container T2. The solution filtered and enriched with vanadium is eventually collected in tank T3. The vanadium content of the enriched solution that is collected in tank T3 will substantially contain vanadium in a V + 4 state. The content corresponds to the sum of the amounts of V + 3 and eventually V + 2 present in the electrochemically reduced solution flowing out of the last cell (C6) of reduction of the cascade and of the reduced amount, equivalent to V + s dissolved and reduced. In fact, an unreacted residual amount of V + 5 together with V + 4 may also be present in the solution enriched in this way which is collected in T3. The solution is circulated continuously by the pump Pi through the cascade of the electrolytic vanadium reduction cells after acid has been added, typically H2SO4, and water, H20 in appropriate relative amounts to maintain a desired molarity of vanadium electrolyte solution. Therefore, the vanadium electrolyte solution entering the first reduction cell Cl will contain substantially V + 4 and possibly a residual amount of V + 5. In the negative electrodes (cathodes) of the cells Cl, C2, C3, ..., C6, of reduction, of the main reaction is: V + 4 + e "=== V + 3 (or more exactly VO + 2 + e '+ 2H "=== V + 3 + H20), another reaction, if vanadium is present with an oxidation state of +5, is V + 5 + e- === V + i (or more exactly V02 ++ e "+ 2H + === V + 2 + H20) No other reaction occurs at the negative electrode No hydrogen emission (thermodynamically favored half cell reaction) occurs because the felt electrode Carbon has a relatively high hydrogen overvoltage and the effective current density of the cathode surface is kept at a sufficiently low value.At the positive electrode, the main reaction must aeriodically oxidize any vanadium ion present, with a from the lowest oxidation (+4, +3 and +2) to pentavalent vanadium (V + 5) (the thermodynamically favored half-cell reaction). vanadium near the surface of the anode will be oxidized immediately to V + 5 and thus any vanadium ion of low oxidation state will migrate and eventually diffuse to the anode.

However, as the vanadium ions in the vicinity of the positive electrode are transformed to V + 5 (consumed), the anodic half-cell reaction will begin to be supported more and more by just another possible half-cell reaction, which is the discharge and consequent emission of oxygen gas according to the reaction: H20 === 0 + 2 H + + 2e "In the asymmetric cell of the invention, vanadium oxidation is practically not excluded as in prior art systems they employ an ion exchange membrane cell and separate circuits of the catholyte containing vanadium and an anolyte of support acid In practice, any vanadium ion that is capable of reaching the anode surface of the cell will readily oxidize to V +5. However, the peculiar disproportion that is created in the electrode current densities causes the anode to work at relatively high current densities that are orders of magnitude higher that what the processes of migration and diffusion of the vanadium ions can support in the electrolyte solution to the surface of the anode. As a consequence, a massive emission of oxygen is promoted at the anode surface and the presence of a vigorous emission of oxygen gas bubbles creates a "mechanical" barrier to the migration of V + 3 ions to the anode. This impediment of intervention to the diffusion of cathodically reduced vanadium ions to the anode can be greatly improved by using a sieve or a permeable (microporous) diaphragm to confine the bubble population of oxygen near the anode and thereby prevent induction of strong connective movements in the volume of the electrolyte contained in the space between the gas confinement screen and the cathode surface, rich in reduced vanadium ions. The use of a low-oxygen oxygen overvoltage anode simply promotes the emission of oxygen. The total faradic efficiency increases remarkably when a relatively tight microporous separator is used instead of a more permeable screen or diaphragm, however the cell voltage is also increased. Therefore, a better arrangement can be sought, considering that the energy consumption is proportional to the product of current and voltage. It has been found that a faradic efficiency of more than 40% can easily be ensured with a cathodic / anodic current density ratio of approximately 5 and by increasing this current density ratio up to 20, efficiency can reach 80% and even a higher level. A marked improvement of these figures can be obtained by using a gas confinement screen and even more by using a relatively tight microporous separator. In the dissolution vessel TI, the V + 3 contained in the electrolytically reduced vanadium electrolyte solution is reacted with the solid V205 vanadium pentoxide (or ammonium vanadate) to dissolve it and reduce it to V + 4 according to the reactions : (V + 5 + V + 3 == 2V + 4), or more precisely V205 + 2V + 3 + 2H + ==== 4VO + 2 + H20 (1) and (if V + 2 is also present) (2V +5 + V + 2 == 3V ÷ 4), or more precisely V205 + V + 2 + 4H + ==== 3VO + 2 + 2 H20 The cross action of an asymmetric cell according to this invention, used in the plant of the vanadium electrolyte preparation of the invention is shown in Figure 2. The laboratory test cell shown in Figure 2 is composed of a tubular, cylindrical body 1, typically made of a chemically resistant electrolyte metal of a plastic. acid resistant, non-conductive, such as PVC, encased in the bottom by a plug 2, and having an inlet hole 3 in the lower portion of the tubular body 1 and an upper 4 hole of overflow.

A cylindrical cathode which may consist and a carbon felt 5 with a thickness of several millimeters may be appropriately positioned and fixed to the inner cylindrical surface of the tube 1. The felt cathode may be provided with an appropriate terminal 6 for the connection Electrical cell in the DC power distribution circuit. In the laboratory test cell shown in Figure 2, the inner cylindrical surface area of the cathode has a diameter of approximately 50 mm and a height in contact with the electrolyte solution of approximately 250 mm. The anode 7 is a titanium rod with a diameter of 6.3 mm (1/4 inch) coated with a mixed oxide of iridium and tantalum and has a submerged length in the electrolyte of approximately 250 mm. The anode 7 of the titanium-coated rod is positioned along the cathode axis of the cylindrical carbon felt. In the laboratory test cell defined in this way, the protected area of the carbon felt cathode is approximately 353 seconds2, while the surface of the titanium rod anode is approximately 47 cm2. With a forced electric current through the 7A cells, the current density at the titanium anode surface is about 0.1485 A / cm2 = 1500 A / cm2 while the current density of the carbon felt protected area is 0.022 A / cm2 = 220 A / m2. However, by virtue of the open and easily permeable morphology of the cathode, in the form of a felt of carbon fibers, the actual or effective cathodic current density in the carbon can be estimated to be two to ten times smaller than the current density calculated in the geometrically protected cylindrical area of the carbon felt cathode. Figure 3 shows the cross-action of the vanadium reduction cell according to an alternative modality. The only difference is represented by the presence of a fluid permeable screen or diaphragm, or microporous separator 8, interposed between the surface of the cylindrical cathode and the coaxially placed rod anode, which defines a cylindrical space around the rod anode 7, which are substantially confined the floating oxygen bubbles that arise and eventually separate from the anodic surface in the surrounding electrolyte. The sieve-diaphragm 8 subsequently prevents the induction of strong convective movements in the body of the electrolyte closest to the cathodic surface in which the desired reduction of V + 4 to V + 3 and eventually to V + 2 occurs. plastic with small holes distributed in a dense and uniform manner can be a satisfactory sieve for confining the gas bubbles, however, the sieve 8 for confining the oxygen gas bubbles can alternatively be a fine maya of a resistant material such as, for example, a titanium wire maya or a woven plastic fiber cloth. Preferably, the gas confinement screen 8 can be a porous or microporous tube, for example, of a glass frit, or of resistant metal particles such as sintered titanium.

Example A ½ liter glass beaker with an internal diameter of 8 cm was used to test the validity of the technique of the invention. A carbon felt with a thickness of approximately 6 mm (¼ inch) was placed around the inner wall of the beaker and electrically connected to the negative pole of a CD energy minister. A titanium rod coated with IrOx-ZrCL mixed oxide with an outer diameter of about 6 mm (¾ inch) was placed vertically along the geometric axis of the beaker and electrically connected to the positive pole of the DC supply. The ratio between the protected area of the cathode and the anode area was approximately 10.7. A polypropylene felt of approximately 1 mm, closed at the bottom, of an internal diameter of approximately 12 mm was formed and placed in the bead of concentrics, concentrically around the anode of coated titanium rod. The beaker was filled with 473 ml of a solution of 5 molar sulfuric acid and 90.9 g (0.5 mole) of vanadium pentoxide powder. The total volume of the mixture was 0.5 1. Theoretically, 26.8 Ah is required to reduce 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 powder of vanadium pentoxide remained substantially undissolved for a few days. By turning on the DC power supply and adjusting its output voltage, an 8 A DC current was forced to flow 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 (cathodic) electrode in the protected area of the carbon felt was approximately 468 A / m 2. The cell voltage remained practically constant at approximately 3.8-4.0 volts. The suspension was stirred gently with the magnetic stirrer and after passing the stream for 5.26 hours, the yellow powder appeared to be completely dissolved. The blue solution obtained in this way was analyzed and found to contain 2 moles of vanadium (2 molar solution) and the oxidation state of the vanadium was +3.55. The Faraday (current) efficiency of the process 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 efficiency Faraday (current) has increased approximately 78.74%, but has cell voltage to approximately 2.8 V. When replacing the felt with a thin woven polypropylene fabric, the current efficiency decreased to approximately 47% and without any permeable confinement element at approximately 20-25%.

Even in these test conditions of laboratory equipment, not optimized (in a glass beaker with agitation, energy consumption in the order of 0.2 to 0.5 kWh per liter of electrolyte produced from vanadium, represents a cost figure rather than low in the total economy of the production of a vanadium electrolyte The capacity of the vanadium electrolyte reduction asymmetric cell of the invention to efficiently and inexpensively modify the oxidation state of the dissolved vanadium content of an electrolyte solution The acid makes the asymmetric, relatively simple and inexpensive, substantially undivided cell of the invention ideally suitable for rebalancing the charge state of the positive and negative vanadium electrolytes of an operation battery without having to perform an expensive and time-consuming processing in an out-of-service condition of the reduction-oxidation battery plant n, whenever the battery reaches a tolerable imbalance no longer. In order to further appreciate the nature of the problems that are likely to arise in the operation of a vanadium battery energy storage system, a brief collection of the main mechanisms leading to a progressively marked imbalance may be useful.

In theory, assuming that the only process that occurs during the loading and unloading of a vanadium reduction-oxidation battery is the oxidation and electrochemical reduction of vanadium and that other side reactions are not taking place, the process of loading and unloading A vanadium battery is a symmetric process. During charging, the electric current flowing through the battery oxidizes the V + 4 to V + s in the positive electrolyte compartments and at the same time and at the same speed, will reduce the V + 3 to V + 2 in the compartments of the negative electrolyte. The opposite oxidation and reduction reactions occur in the positive or negative electrolyte compartments during discharge. Unfortunately, in practice the situation is different. The oxidation and electrochemical reduction of vanadium is not the only process that takes place. Probably the following side reactions will occur under critical operating conditions: 1) electrochemical emission of hydrogen gas at the negative electrode; 2) electrochemical emission of oxygen at the positive electrode (*) 3) chemical oxidation from V + 2 to V + 3 4) chemical reduction from V + 5 to V + 4 (*) If the positive electrode is made of carbon, the Oxygen emission is partially or totally replaced by the emission of carbon dioxide. Reactions 1) and 2) become the only ones once the state of charge of 100% is reached. In practice, after everything in V + i, present in the electrolyte of the positive compartment, oxidizes V + s, the only reaction in the positive electrode that can withstand the current, is the emission of oxygen (or carbon dioxide) ). Similarly, when all the V + 3, present in the electrolyte of the positive compartment, is reduced to V + 2, the only reaction in the negative electrode that supports the current is the emission of hydrogen. These reactions will begin to occur during the charging of the battery through a relatively small amount when the charge state becomes greater than 90%. The voltage at which vanadium is oxidized or reduced increases proportionally between the species produced and the species consumed (Nernst equation), therefore, at a high state of charge, the cell voltage increases to the hydrogen and oxygen emission voltage (water electrolysis) of approximately 1.5 volts. Reactions 1) and 2) will also occur, albeit in a relatively small amount, during discharge of the battery if the discharge occurs at an excessively high (current) rate. As the current density reaches the limit current, the emission of hydrogen to oxygen begins to occur as a lateral (parasitic) reaction of the electrode. The limiting current is the electric current at which the rate of oxidation or reduction of vanadium at the surface of the electrode is equal to the rate at which the vanadium ions are diffused from the volume of the electrolyte to the surface of the electrode, through of the exhausted layer. Reaction 3), the oxidation of V + 2 to V + 3 is a more recurrent side reaction during the operation of a vanadium battery. V + 2 is easily oxidized to V + 3 in the presence of air. Therefore, unless the atmospheric air is prevented from coming into contact with the negative electrolyte (by blanket of nitrogen gas or by covering the surface of the electrolyte with wax, etc.), this side reaction will readily occur. Due to the above lateral reactions, after many cycles of battery operation, the symmetry can begin to be lost substantially. Another reason for the electrolytes to become unbalanced is because the membranes used are not perfect separators. Inevitably, the anionic membranes are also permeated by a small fraction of positive ions (H + and V + n). Cationic membranes are generally preferred as cell cell separators due to their greater chemical and mechanical resistance compared to anionic membranes. Actually, the cationic membranes are mainly permeable to hydrogen ions (the diffusion rate of H + is greater than that of the vanadium ions). During the charging of the battery hydrogen ions, generated in the positive compartment according to the reaction: V02 + H20 ==== V (V + 2H + + e "will easily migrate the negative behavior through the membrane together with a fraction smaller vanadium ions less mobile.The migration of vanadium ions will oxidize a corresponding amount of reduced vanadium ions in the negative compartment (V + 2 and V + 2), but the process is not completely reversible because the ions vanadium from different oxidation states will coordinate themselves differently with the solvent molecules (water, sulfuric acid) and have different mobility in the cation exchange resin of the membrane.In reality, during the subsequent discharge phase, the number of vanadium ions that cross the membrane in the opposite direction will not be exactly the same number that has migrated during the loading phase. The electrolyte causes numerous problems among which: 1) the capacity of the battery (in terms of kWh / liter of electrolyte) decreases proportionally; 2) during the charging of one of the two, the electrolyte can be fully charged while the other remains partially discharged. In practice, especially for small batteries where the removal of air from the negative compartment is often imperfect, the vanadium ions in the positive compartment can be completely oxidized to V + 5 while the negative compartment remains a substantial amount of V + 3 . This situation is very critical because, if the oxidation state is not carefully controlled in the different electrolytes, if not only by measuring the open circuit voltage, the load will be continued to the point of reaching a complete oxidation of V + 4 to V + 5 In this situation, the massive emission of oxygen at the carbon electrode will oxidize and destroy the electrode. According to a common approach, after a certain number of charging and discharging cycles, the two electrolytes are mixed (negative and positive), the oxidation state is measured and, if found to be different from +3.5, it is chemically adjusted to +3.5. In practice when the battery is stopped and the electrolytes are mixed together, a vanadium oxidation state greater than +3.5 is always found (mainly due to the influence of the preponderant factor of the lateral reaction 3). The electrolyte is readjusted to a vanadium oxidation state of +3.5 by adding a reducing agent (oxalic acid, sulfite, etc.). Therefore, a substantial amount of energy can be spent to bring the system back to zero charge (V + 3 in the negative electrolyte V + 4 in the positive electrolyte). This amount of energy that is spent periodically represents a net loss of the energy storage process. This non-essential loss can be greatly reduced according to one aspect of the present invention, by installing a relatively small vanadium reduction asymmetric cell, of the present invention in the negative electrolyte circuit more preferably of the positive as shown schematically in Figure 4. As shown, the positive electrolyte can be circulated completely or in part (in the latter case by using for example an adjustable tri-directional valve or by any other means) through a cell Network of Vanadium reduction, asymmetric, relatively small. The network cell can be operated according to the needs, either continuously or discontinuously in order to maintain a symmetric configuration of the vanadium oxidation state. Due to the possibility offered by the presence of this cell of auxiliary reduction, it is possible to eliminate or make only exceptionally necessary the need to mix together the two electrolytes, to adjust the oxidation state to approximately +3.5 and subject the battery to a preload in order to recover a zero state of charge.

Claims

CLAIMS 1. A method for producing a vanadium electrolyte solution containing V + 3 and V + 4 in a desired concentration ratio of solid vanadium pentoxide fed into the electrolyte solution, which consists of: electrochemically reducing at least partially the vanadium dissolved in the acid electrolyte solution by circulating the electrolyte solution through a plurality of cascaded electrolytic cells to at least one oxidation state V + 3 or less; reacting the vanadium content reduced in this manner in the electrolyte solution transferred from the last of the electrolytic cells with a stoichiometric amount of vanadium pentoxide to obtain an electrolyte solution containing vanadium substantially in an oxidation state of V + 4; add acid and water to maintain a certain molarity of the solution; continuously recycle the electrolyte solution through the cascade of electrolytic cells while purging a stream of the produced electrolyte solution containing V + 3 and V + 4 at the desired concentrations at the exit of one of the cells of the electrolyte. the waterfall; each cell having a cathode and an anode with respective morphologies of the surface, geometry and mutual arrangement such as to establish on the surface the anode a current density of 5 to 20 times greater than the current density on the protected surface of the cathode and emit oxygen at the anode. The method according to claim 1, wherein the electrolyte solution is a sulfuric acid solution and the molar vanadium content is between 1 and 5. The method according to claim 1, wherein the current density in the protected surface of the cathode is comprised between 100 and 300 A / m2 and the current density of the anode surface is between 1000 and 8000 A / m2. 4. The method according to claim 1, wherein the vanadium oxide is in powder form and has a particle size no greater than 100 μm. The method according to claim 1, wherein after reacting the reduced vanadium electrolyte solution with a stoichiometric amount of vanadium pentoxide, the solution is separated from any residual or dissolved vanadium pentoxide particle. 6. A plant for preparing vanadium electrolyte solution containing V + 3 and V + 4 at a desired concentration ratio of a solid feed of vanadium pentoxide, which consists of a plurality of vanadium reducing electrolytic cells hydraulically connected cascaded and electrically driven in series from a regulated source of CD; a dissolution tank that collects the reduced solution of the electrolyte leaving the last cell of the cascade of cells, which has a mechanical means of agitation and a mechanism for feeding a controlled amount of vanadium pentoxide in powder form; means for separating the enriched vanadium solution leaving the dissolving vessel of residual solid particles of vanadium pentoxide; a means for adding to the solution enriched vanadium, sulfuric acid and water to maintain a certain molarity of the solution; a pump means for recirculating the electrolyte solution through the cascade of electrolytic vanadium reduction cells; a shunt means for purging a stream of the produced electrolyte solution containing V + 3 and V + 4 to the desired concentration ratio at the output of one of the cells of the cascade of cells; each cell having a cathode and an anode with respective surface morphologies, respective geometries and mutual arrangement such as to establish on the surface of the anode a current density of 5 to 20 times greater than the current density on the protected surface of the cathode and emanate oxygen at the anode. 7. An electrolytic cell to reduce V + 4 and / or V + 5 ions in an aqueous solution of vanadium electrolyte, acid to V + 3 and / or V + 2 having a cathode and an anode with respective surface morphologies, respective geometry and mutual arrangement such as to establish on the surface of the anode a current density of 5 to 20 times greater than the current density on the protected surface of the cathode and emanate oxygen at the anode. The electrolytic cell according to claim 7, comprised of a cylindrical tubular body of an acid-resistant, non-conductive material having an inlet orifice 3 and an outlet orifice; a carbon felt cathode placed on the inner cylindrical surface of the. tubular body, provided with a terminal for the electrical connection of the cell; an anode of the valve metal rod coated with a non-passive electrocatalytic coating positioned along the axis of the cylindrical carbon felt cathode. The electrolytic cell according to claim 7, characterized in that it includes a permeable electrolyte means for confining the floating oxygen bubbles that arise in the electrolyte around or near the anode. 10. The electrolytic cell according to claim 9, wherein the permeable confinement means corresponds to the group composed of mesh screens, woven fabrics, felts, porous and microporous glass frits and sintered bodies, all of a chemically resistant material. electrolyte solution. 11. The electrolytic cell according to claim 7, wherein the anode is a valve metal coated with a mixed oxide of iridium and tantalum or zirconium. 12. A method for rebalancing the relative oxidation state of the two separate vanadium electrolytes circulating in a vanadium flux reduction-oxidation battery system without putting it out of service, which consists of circulating part of one of the two different electrolytes in an electrolytic vanadium reduction cell, the battery external, made according to any of claims 7 to 11 and forcing a current through the electrolytic cell for the time necessary to re-establish a balance of the oxidation state of the vanadium content in the two electrolytes of the reduction-oxidation flow battery system. The method according to claim 12, wherein the electrolyte is the positive electrolyte that circulates in the positive electrode compartments of the battery. SUMMARY OF THE INVENTION An acid solution of vanadium electrolyte containing V + J and v + 4 in a desired concentration ratio of solid vanadium pentoxide fed into the electrolyte solution, is produced by at least partially reducing the vanadium dissolved in electrochemically. the acid solution of electrolyte by circulating the electrolyte solution through a plurality of electrolytic cells in cascade to at least partially a state of V + 3; reacting the content of vanadium reduced in this way in the electrolyte solution transferred from the last of the electrolytic cells with a stoichiometric amount of vanadium pentoxide, obtaining an electrolyte solution containing vanadium substantially in a V + 4; adding acid and water to maintain a certain morality of the solution; and continuously recycling the electrolyte solution through the cascade of electrolytic cells while purging a stream of the produced electrolyte solution containing V + 3 and V + 4 at the desired concentrations at the exit of one of the cells of the electrolyte. the waterfall. Each Celtic is highly asymmetric having a cathode and an anode with respective surface morphologies, geometry and mutual arrangement such as to establish on the anode surface a current density of 5 to 20 times greater than the current density at the projected cathode surface and allow oxygen at the anode. An asymmetric cell of this type can be used in the circuit of one of the positive and negative electrodes of a working battery to pre-equilibrate the respective oxidation states of its vanadium content.