WO2002015317A1

WO2002015317A1 - 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: WO2002015317A1
Application number: PCT/IT2000/000342
Authority: WO
Inventors: Barry Michael Broman
Original assignee: Squirrel Holdings Ltd
Current assignee: Squirrel Holdings Ltd
Priority date: 2000-08-16
Filing date: 2000-08-16
Publication date: 2002-02-21
Anticipated expiration: 2003-02-16
Also published as: CA2420014A1; AU2000267249A1; IL154098A0; TR200300178T2; KR20030034146A; NZ523752A; TW531934B; BR0017317A; MXPA03001330A; CN1502141A; JP2004519814A; EP1310008A1

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

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 REDOXBATTERY

BACKGROUND OF THE INVENTION

This invention relates in general to renewable electrochemical energy storage in redox flow battery systems and more in particular to so called all- vanadium redox secondary batteries.

The vanadium redox flow battery also referred to as the all-vanadium redox cell or simply the vanadium redox cell or battery, employs V(II)/N(III) and N(IN)/N(N) as the two redox couples, in the negative (sometime referred to as the anolyte) and positive (sometime referred to as the catholyte) half-cell electrolyte solutions, 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% vanadium ions with an oxidation state of +4.

The electrolyte is generally divided into two equal parts that are respectively placed in the positive and the 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 null.

When a current is forced through the battery by an external source of sufficiently high output voltage, the N⁺⁴ (50%) in the negative electrolyte will reduce to N⁺³ and at the same time the N⁺³ (50%) in the positive electrolyte will oxidize to N⁺⁴.

At a certain time the negative electrolyte, continuously circulated through the respective electrode compartments of the battery by a negative electrolyte circulation pump will contain only N ^"3 and the positive electrolyte circulated through the respective electrode compartments of the battery by a positive electrolyte circulation pump will contain only N^1"4.

In this condition the battery is said to have a null State Of Charge (SOC) and the open circuit voltage of the battery will be approximately 1.1 Nolt.

By continuing to force a "charging" current through the battery, at the negative electrode the N⁺³ will be reduced to N⁺² and at the positive electrode the N⁺⁴ will oxidize to N⁺⁵. When this transformation is completed (at the end of a charging process) the battery will have an open circuit voltage of about 1.58 N and the battery is said to have a SOC equal to 100%.

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

The storage capacity of an all 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 proportional to the volume of the two electrolytes.

Evidently there is a need to produce acid solutions of vanadium suitable as electrolyte for first filling the two circuits of a redox battery system and/or for expanding the storage capacity of an existing battery installation, using commercially available vanadium pentoxide (or ammonium vanadate) as starting (feed) material.

The process of preparation of a vanadium electrolyte is therefore a process that consists of dissolving N₂O₅ in sulfuric acid (or other acid) and reducing it to the required mixture of N⁺³ (approx. 50%) and V^1"4 (approx. 50%).

Finely divided (powdery) solid vanadium pentoxide is only slightly soluble in water or in an acid such as for example sulfuric acid and a simple process of preparing the electrolyte by dissolving N O₅ in acid is not possible.

In order to dissolve N₂O₅ it is necessary to first reduce it to a lower (more readily soluble) oxidation state.

Various methods have been proposed for the dissolution and reduction of the N⁺⁵, mainly by using reducing compounds, or complicated electrolytic and chemical processing methods.

EP-A-0 566 019 discloses a method for producing a vanadium electrolytic solution by chemical reduction of vanadium pentoxide or ammonium vanadate in concentrated sulfuric acid, followed by a heat treatment of the precipitate.

WO 95/12219 and WO 96/35239 disclose an electrochemical-chemical process of preparing a vanadium electrolytic solution from solid vanadium pentoxide and a method of stabilizing it. Dissolution of vanadium pentoxide is performed on a special louvered cathode of an ion exchange membrane cell by letting a vanadium pentoxide slurry run down in contact with the louvered cathode.

The method and techniques so far developed for preparing a suitable vanadium acid electrolyte are rather complex and costly. On the other hand, for the overall economics of an all vanadium flow redox battery system the availability of a vanadium electrolyte solution at a relatively low costs is an important factor of cost-benefit evaluation of a vanadium redox battery compared to other energy storage systems.

Fundamental to meet these requisites is to use the relatively cheap solid vanadium pentoxide as feed material.

OBJECT AND SUMMARY OF THE INVENTION

An outstandingly simple and inexpensive method of readily dissolving and reducing vanadium pentoxide in an acid electrolyte has now been found.

The invention is particularly useful for preparing a vanadium electrolyte from vanadium pentoxide (or ammonium vanadate) feed and is implemented by the use of extremely simple and low costs electrolytic cells while reducing to a minimum ancillary treatments of the solution.

Nevertheless the method of the invention remains quite efficient even from the point of view of energy consumption.

The method of this invention is intrinsically a continuous method whereby to a certain volume of circulating vanadium electrolyte solution are continuously fed solid vanadium pentoxide (N₂O₅) in a finely divided or powdery form, acid and water to maintain a certain molarity of the solution, while continuously bleeding off an equivalent volume of electrolyte solution, containing N⁺³ and V^1"4 in substantially similar or other desired concentrations.

The bled stream of electrolyte solution represents the yield of the process.

Basically the method of the invention consists in

passing the electrolyte solution in contact with the cathodes of a plurality of electrolytic cells hydraulically in cascade to progressively reduce part or all the N⁺⁴ content of the solution entering the first cell to N⁺³ and eventually in a minor amount even to N⁺² in the electrolyte solution at the outlet of the electrolyte solution from the last electrolytic cell of the plurality of cells in cascade;

reacting the so reduced vanadium content of the electrolyte solution at the exit of the last of the electrolytic cells with a stoichiometric quantity of vanadium pentoxide (N₂O₅) in a dissolution vessel provided with stirring means, obtaining an electrolyte solution containing a correspondent amount of dissolved vanadium that may almost completely be in a N⁺⁴ state;

adding acid, sulfuric acid or any other equivalent acid and water to the vanadium electrolyte solution ( e.g. close to V⁺⁴ ) to maintain a certain molarity thereof;

recycling the electrolyte solution through the cascade of electrolytic cells while bleeding a stream of electrolyte solution containing N^"1"3 and N⁺⁴ , preferably in substantially similar concentrations, at the exit 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 disposition such to establish on the anode surface a current density from 5 to 20 times greater than the current density on the cathode surface and oxygen is evolved at the anode surface.

In practice the cathode may be a carbon felt or an activated carbon felt or similar material providing a relatively large surface area and may have a tubular or even a channel-shaped form, while the anode may be in the form of a thin rod disposed along geometrical axis of the tubular or channel-shaped cathode.

The comparatively large specific active area of the cathode compared to the specific active area of the anode and their projected area ratio are such to determine a current density on the active anode surface from 5 to 20 times larger than the current density on the geometrically projected cathode surface.

By operating with a cathodic current density in the order of about one to few hundreds A/m² of projected area and by dimensioning the diameter of the anodic rod "concentrically" disposed in respect to a tubular or to an equivalently, at least partly enveloping, cathode, an anodic current density in excess of 1000A/m² or even much higher can be established.

Under these conditions of markedly disproportionate current densities and of relatively high anodic current density, while the current forced on the electrolyte cells is adjusted such to ensure that the cathodic reduction of N^1"4 to V⁺³ remains almost completely the sole cathodic half-cell reaction (that is limiting the maximum current density such to prevent parasitic reactions such as hydrogen evolution), the anodic half-cell reaction becomes primarily supported by oxygen evolution (water electrolysis) reaction. In fact, the thermodynamically privileged anodic half-cell reaction of oxidation of N^1"3 to N⁺⁴ is practically and effectively impeded by a grossly insufficient rate of migration and eventually of diffusion of N⁺³ ions from the bulk of the electrolyte filling the gap between the anode and the cathode surfaces toward the anode surface of the cell.

A further important impediment to a migration and/or diffusion of vanadium ions toward the anode surface is represented by the presence of oxygen gas bubbles that are vigorously evolved over the anode surface at such relatively high current densities.

The current forced in electrical series through the plurality of electrolytic cells, hydraulically in cascade, may be adjusted in function of the flow rate of the electrolyte through the cascade of cells, in order to produce a practically complete reduction of all the N⁺⁴ to N^1"3 in the electrolyte leaving the last cell of the cascade.

Of course, this remains an ideal condition, indeed a minimum (residual) amount of N^1"4 may be present in case of a defect of current been forced through the cells or, viceversa, in case of an excess of current, an incipient reduction of N⁺³ to N⁺² may occur so that in the electrolyte exiting the last cell a minor amount of N⁺² may be present together with N⁺³.

The anode has an electrocatalytic surface of low oxygen overpotential to promote oxygen evolution and above all is resistant to the acid electrolyte under conditions of anodic polarization and of 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 nonpassivating active coating of an oxygen discharge electrocatalyst.

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

In a dissolution vessel provided with ordinary mechanical stirring, the electrolyte solution exiting the last cell is contacted with a stoichiometric amount (referred to the amount of N⁺³(N⁺²) contained in the reduced electrolyte solution) of solid vanadium pentoxide, in a finely divided (powdery) form, prepared by milling and/or sieving solid vanadium pentoxide such to introduce particles with a maximum size of not more than lOOμm.

The decanted or filtered solution is recovered in a reservoir and any undissolved vanadium pentoxide particle may be recycled back into the dissolution vessel.

The so enriched solution contains vanadium substantially in a V^1"4 state although a relatively very small amount of dissolved vanadium may be present as N⁺⁵.

Acid that most commonly and preferably is sulfuric acid, and water are added to the vanadium enriched and filtered electrolyte solution to maintain a certain molarity and the electrolyte solution. Of course, the higher molar content of vanadium the higher will be the ratio of power/total volume of electrolyte, however problems with the stability of the solution under critical temperature conditions may be encountered at relatively high molar concentrations. Most preferably, in case of a sulfuric acid solution, the molar content of vanadium may range between 2 to 5 molar.

The solution is pumped back to the inlet of the first cell of the cascade of cells to undergo electrochemical reduction of the N⁺⁴ (and of any residue of N⁺⁵ ) to N⁺³ and eventually to N^1"2.

The yield of the electrolyte production plant is a solution containing approximately the same amount of N ^"3 and N ^"4 that can be bled off the main stream of recirculating solution at the exit of one of the cells of the cascade of cells.

The disproportionally large current density at the anode surface causing a massive oxygen evolution and a correspondingly minor oxidation of V^"3 to N⁺⁴, is a condition that is surprisingly sufficient to maintain the overall efficiency of the process at more than acceptable levels, also considering the relatively minor weight that the cost of electrical energy has on the overall economics of any process of preparation of a vanadium acid electrolyte.

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

The screen or microporous separator produces an effective a "confinement" of the oxygen bubbles rising by buoyancy in the electrolyte as they continuously grow on and detach from the anode surface, thus minimizing convective motions in the bulk of the electrolyte contained in the space between the screen and the cathode and further reducing the ability of reduced vanadium ions (N*³) to migrate and eventually reach the anode.

A most effective microporous separator may be a glass frit tube closed at its bottom end and enveloping the rod anode (in this case entering the cell from the top), whereby the evolved oxygen bubbles once they surface from the electrolyte may readily exhaust out of the cell through a vent. Alternatively, a suitable microporous separator may be a felt of polypropylene fibers of about 1 mm thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 depicts a plant for preparing a vanadium electrolyte solution from a solid N₂O₅ 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 action of an alternative embodiment of the vanadium reduction cell.

Figure 4 is a basic scheme of an all vanadium flow redox battery system including a vanadium reduction cell of the invention in the circuit of the positive electrolyte for rebalancing functions.

DESCRIPTION OF PREFERRED EMBODIMENTS 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 electrolytic vanadium reduction cells Cl, C2, C3, ..., C6, hydraulically connected in cascade and powered in electrical series by an appropriate DC supply Rl.

^' The solution leaving in the last cell C6 of the cascade collects in a dissolution vessel Tl, provided with stirring means SI.

Vanadium pentoxide (V₂O₅) is introduced in the dissolution tank Tl in an appropriate amount by way of the example of a conventional feed hopper and a motor driven controlled feeding mechanism.

The vanadium enriched solution, eventually containing residual solid particles of undissolved vanadium pentoxide flows out of the dissolution vessel Tl through a level discharge port and is decanted in the settler vessel T2.

A pump P2 eventually recycles back to the dissolution vessel Tl the separated residual solid particles of vanadium pentoxide that eventually collects at the bottom of the settler vessel T2.

The vanadium enriched and filtered solution is eventually collected in the reservoir T3.

The vanadium content of the enriched solution that collects in the reservoir T3 will substantially contain vanadium in a N^1"4 state. The content correspond to the sum of the amounts of N⁺³ and eventually V⁺² present in the electrochemically reduced solution flown out of last reduction cell (C6) of the cascade and of the equivalent reduced amount of dissolved and reduced N^1"5. Indeed, a residual unreacted amount of ⁵ may also be present together with V⁴⁴ in the so enriched solution collecting in T3.

The solution is continuously circulated by the pump PI through the cascade of the vanadium reduction electrolytic cells after having added acid, typically H₂SO₄, and water, H₂O, in relative amounts appropriate to maintain a desired molarity of vanadium electrolyte solution.

Therefore the vanadium electrolyte solution entering the first reduction cell Cl will substantially contain N^1"4 and possibly a residual amount of N^1"5.

At the negative electrodes (cathodes) of the reduction cells Cl, C2, C3, ..., C6, the main reaction is:

V⁺⁴ + e^~ = N⁺³ (or more exactly NO⁺² + e^~ + 2 H⁺ === N*³ + H₂O)

another reaction, if vanadium with an oxidation state of +5 is present, is

V⁺⁵ + _e- === N⁺⁴ (or more exactly NO₂ ⁺ + e^~ + 2 H⁺ === NO⁺² + H₂O).

No other reaction occurs at the negative electrode. Hydrogen evolution (thermodinamically favoured half-cell reaction) does not occur because the carbon felt electrode has a relatively high hydrogen overvoltage and the effective current density on the cathode surface is maintained at a sufficiently low value.

At the positive electrode, the main reaction should theoretically be the oxidation of any vanadium ion present, with a lower oxidation- state (+4, +3 and +2) to pentavalent vanadium (V^1"5) (the thermodynamically favored half-cell reaction).

Indeed, the vanadium ions close to the anode surface will be oxidized immediately to V^1"5 and so will be any low-oxidation-state vanadium ion that will eventually migrate and diffuse to the anode. However, as the vanadium ions in the proximity of the positive electrode are transformed to V^1"5 (consumed), the anodic half-cell reaction will begin to be supported more and more by the only other possible half- cell reaction, that is the discharge and consequent evolution of oxygen gas according to the reaction:

H₂O = O₂ + 2 H⁺ + 2 e^"

In the asymmetric cell of the invention, the oxidation of vanadium is not practically excluded as in prior art systems employing an ion exchange membrane cell and separate circuits of the vanadium containing catholyte and of a supporting acid anolyte. In practice, any vanadium ions that is able to reach the surface of the anode of the cell will be readily oxidized up to N⁺⁵.

However, the peculiar disproportion that is created in electrodic current densities makes the anode work at relatively very high current densities that are orders of magnitude higher than the migration and diffusion processes of vanadium ions in the electrolyte solution toward the anode surface may support. As a consequence, a massive oxygen evolution is promoted on the anode surface and the presence of a vigorous evolution of oxygen gas bubbles creates a "mechanical" barrier to the migration of N⁺³ ions toward the anode.

This intervening impediment to the diffusion of cathodically reduced vanadium ions to the anode may be greatly enhanced by using a screen or a pervious (microporous) diaphragm for confining the oxygen bubbles population near the anode and thus preventing induction of strong convective motions in the bulk of the electrolyte contained in the space between the gas confinement screen and the cathode surface, rich of reduced vanadium ions.

The use of a low oxygen overvoltage anode simply promotes oxygen evolution.

The overall faradic efficiency markedly increases when using a relatively tight microporous separator in place of a more permeable screen or diaphragm, however the cell voltage also increases. Therefore, a best compromise may 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 over 40% may be easily ensured with a cathodic/anodic current density ratio of about 5, and by increasing this current density ratio up to 20 the efficiency may reach 80% and even a higher level. A marked enhancement of these figures may be obtained by using a gas confinement screen and even more by using a relatively tight microporous separator.

In the dissolution vessel Tl the N contained in the electrolitically reduced vanadium electrolyte solution is reacted with the solid vanadium pentoxide N₂O₅ (or ammonium vanadate) to dissolve and reduce it to V⁴⁴ according to the reactions:

(N⁴⁵ + N⁴³ == 2 V⁴⁴) or more precisely

N₂O₅ + 2 N ³ + 2H⁴ — 4NO⁺² + H₂O (l)

and (if N⁴² is also present)

(2 N⁴⁵ + N^1"2 = 3 N⁴⁴) or more precisely

The cross action of an asymmetric cell according to this invention, used in the vanadium electrolyte preparation plant of the invention, is shown in Fig. 2.

The laboratory test cell depicted in Fig. 2 is composed of a cylindrical tubular body 1, typically of a metal chemically resistant to the electrolyte of a nonconductive, acid resistant plastic, such as PNC, closed at the bottom by a plug 2, and having an inlet port 3 in the lower portion of the tubular body 1 and an upper overflow port 4.

A cylindrical cathode that may consist of a carbon felt 5 with thickness of several millimeters may be disposed on and suitably anchored to the inner cylindrical surface of the tube 1. The felt cathode may be provided with an appropriate terminal 6 for electrical connection of the cell in the DC powering circuit. In the laboratory test cell shown in Fig 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") coated with a mixed oxide of indium and tantalum and has a length immersed in the electrolyte of approximately 250 mm.

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

In the laboratory test cell so defined, the projected area of the carbon felt cathode is approximately 353 cm , while the titanium rod anode surface is of about 47 cm .

With an electric current forced through the cells of 7A, the current density on the titanium anode surface is of approximately 0.1485 A/cm² = 1500 A/m² while the current density on the projected area of the carbon felt is of 0.022A/cm² = 220A/m². However, by virtue of the open and readily permeated morphology of the cathode, in the form of a felt of carbon fibers, the real or effective cathodic current density on the carbon may be estimated to be from two to ten times smaller than the current density calculated on the geometrically projected cylindrical area of the carbon felt cathode.

In Figure 3 is depicted the cross action of vanadium reduction cell according to an alternative embodiment.

The only difference is represented by the presence of a fluid pervious screen or diaphragm or microporous separator 8, interposed between the cylindrical cathode surface and the coaxially disposed rod anode, defining a cylindrical space around the rod anode 1, in which to maintain substantially confined the buoyant oxygen bubbles growing on and eventually detaching from the anodic surface into the surrounding electrolyte.

The screen-diaphragm 8 substantially prevents the induction of strong convective motions in the body of electrolyte closer to the cathodic surface at which the desired reduction of N⁴⁴ to N⁺³ and eventually to N⁺² occurs.

A plastic tube with small densely and uniformly distributed holes may be a satisfactory gas bubbles confinement screen, however the oxygen gas bubbles confinement screen 8 may alternatively be a fine mesh of a resistant material such as for example a mesh of titanium wire or of a woven fabric of plastic fiber. More preferably the gas confinement screen 8 may be a porous or microporous tube, for example of a glass frit, or of a resistant metal particles such as sintered titanium.

EXAMPLE

K V% liter glass beaker with an internal diameter of 8 cm was used to prove the validity of the technique of the invention.

A carbon felt with a thickness of about 6 mm ( ") was placed around the internal wall of the beaker and electrically connected to the negative pole of a DC power supply.

A IrO_x- ZrOy mixed oxide coated titanium rod with an outer diameter of about 6 mm (V") was positioned vertically along the geometrical axis of the beaker and electrically connected to the positive pole of the DC supply.

The ratio between projected cathode area and anode area was about 10,7.

A polypropylene felt of about 1 mm thickness was formed in the shape of a round tube, closed at the bottom, of about 12 mm inner diameter and placed in the beaker, concentrically around the coated titanium rod anode.

The beaker was filled with 473 ml of a solution of 5 molar sulfuric acid and 90.9 g (0.5 Moles) of vanadium pentoxide powder. The total volume of the mixture was 0.5 1.

Theoretically, 26.8 Ah are required to reduce 1 mole of vanadium from the state of oxidation +5 to the state of oxidation +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 a DC current of 8 A was forced to flow trough the cell. The positive electrode (anode) current density was approximately 5 '013 A/m2 and the negative electrode (cathodic) current density on projected area of the carbon felt was approximately 468 A/m2.

The cell voltage remained practically constant at about 3.8-4.0 Volt.

The suspension was gently stirred with the magnetic stirrer and after passing the current for 5.26 hours the yellow powder appeared to be completely dissolved.

The blue solution so obtained was analyzed and found to contain 2 moles of vanadium (2 Molar solution) and the oxidation state of vanadium was +3.55

The Faraday (current) efficiency of the process was estimated to be 92.28 %.

The test was repeated at a reduced current of 5 A and the time required was of 9,87 hours. The Faraday (current) efficiency had decreased to about 78.74%, but so had the cell voltage to about 2.8 N.

By substituting the felt with a thin woven polypropylene cloth, the current efficiency decreases to about 47%, and without any peraieable confinement element to about 20-25 %.

Even in these not optimized laboratory setup test conditions (in a glass beaker with stirring, power consumption in the order of 0.2 to 0.5 kWh per liter of yielded vanadium electrolyte, represents a rather low cost figure in the overall economy of producing a vanadium electrolyte.

The ability of the asymmetric vanadium electrolyte reduction cell of the invention to efficiently and inexpensively modify the state of oxidation of the dissolved vanadium content of an acid electrolyte solution makes the relatively simple and low cost, substantially undivided, asymmetric cell of the invention ideally suited for rebalancing the state of charge of the positive and negative vanadium electrolytes of an operating battery without having to perfomi costly and time consuming processing in an off-service condition of the redox battery plant, every time the battery reaches a no longer tolerable unbalance.

In order to appreciate more the nature of the problems that are likely to arise in operating a vanadium battery energy storage system, a brief recollection of the main mechanisms that lead to a progressively marked unbalance may be useful.

In theory, assuming that the only process occurring during charging and discharging of a vanadium redox battery is the electrochemical oxidation and reduction of vanadium and that no other side reactions are taking place, the process of charging and discharging a vanadium battery is a symmetric process.

During charging, the electric current flowing through the battery will oxidizes the N⁴⁴ to N⁺⁵ in the positive electrolyte compartments and, at the same time and at the same rate, will reduce the N⁺³ to N⁴² in the negative electrolyte compartments. The opposite oxidation and reduction reactions occur in the positive and negative electrolyte compartments during discharge.

Unfortunately, in practice the situation is different.

The electrochemical oxidation and reduction of vanadium is not the only process taking place. The following side reactions are likely to occur under critical conditions of operation:

1) electrochemical evolution of hydrogen gas at the negative electrode

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

3) chemical oxidation of N⁺² to N⁴³

4) chemical reduction of N⁴⁵ to N⁺⁴

(*) If the positive electrode is made of Carbon the evolution of oxygen is partially or totally replaced by the evolution of carbon dioxide

Reactions 1) and 2) become the only ones once the 100% state of charge is reached. In practice, after all the N⁴⁴, present in the electrolyte of the positive compartment, is oxidized to N⁴⁵, the only reaction on the positive electrode that may support the current, is the evolution of oxygen (or carbon dioxide). Similarly, when all the N^1"3, present in the electrolyte of the positive compartment, is reduced to V ², the only reaction on the negative electrode that may support the current, is the evolution of hydrogen. These reactions will begin to occur during the charging of the battery though in a relatively small amount when the state of charge becomes higher than 90%.

The voltage at which vanadium is oxidized or reduced increases proportionally with the ratio between the species produced and the species consumed (Νernst equation), therefore, at a high state of charge, the cell voltage rises to the voltage of evolution of hydrogen and oxygen (water electrolysis) of approximately 1.5 Nolts. The reactions 1) and 2) will also occur though in a relatively small amount, during discharging of the battery if the discharging occurs at an excessively high rate (current).

As the current density approaches the limiting current, evolution of hydrogen and oxygen will begin to occur as a side (parasite) electrode reaction.

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

Reaction 3), the oxidation of N⁺² to N⁺³ is a most recurcent side reaction during the operation of a vanadium battery. N⁴² is readily oxidized to N⁺³ in presence of air. Therefore, unless atmospheric air is strictly prevented to come in contact with the negative electrolyte (by nitrogen gas blanketing or by covering the surface of the electrolyte with wax, etc.), such a side reaction will readily take place. Because of the above side reactions, after many cycles of operation of the battery, symmetry may begin to be substantially lost.

Another reason for the electrolytes to become unbalanced, is because the membranes used are not perfect separators. Anionic membranes are inevitably penneated also by a small fraction of positive ions (H⁴ and N⁴").

Cationic membranes are generally preferred as cell separators of the battery because of their higher mechanical and chemical resistance when compared with anionic membranes.

Indeed, cationic membranes are mainly permeable to hydrogen ions (the diffusion rate of H⁴ is much higher than that of vanadium ions).

During the charging of the battery hydrogen ions, generated in the positive compartment according to the reaction:

NO²⁴ + H20 ===== VO₂ ⁺ + 2H⁴ + e^~

readily migrate to the negative compartment through the membrane together with a smaller fraction of lesser mobile vanadium ions.

Migration of vanadium ions will oxidize a corresponding amount of reduced vanadium ions present in the negative compartment (N⁴³ and N⁴²), but the process is not completely reversible because vanadium ions of different state of oxidation coordinate themselves differently with the solvent molecules (water, sulfuric acid) and have different mobility in the cation exchange resin of the membrane. Indeed, during the subsequent discharging phase, the number of vanadium ions that cross the membrane in the opposite direction will not be exactly the same number that had migrated during the charging phase.

A progressive unbalance between the electrolytes cause numerous problems among which:

1) the capacity of the battery (in terms of kWh liter of electrolyte) proportionally decreases;

2) during charging one of the two electrolyte may become completely charged while the other remains partly uncharged.

In practice, especially for small batteries where elimination of air from the negative compartment is often imperfect, the vanadium ions in the positive compartment may be completely oxidized to N⁴⁵ while in the negative compartment remains a substantial amount of N⁴³. This situation is very critical because, if the state of oxidation is not carefully controlled in the distinct electrolytes, but merely by measuring the open circuit voltage, charging will be continued to the point of reaching a complete oxidation of the V⁴⁴ to N⁴⁵. In this condition, massive evolution of oxygen on 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 (negative and positive) are mixed, the oxidation state is measured and, if found to be different from +3.5, is chemically adjusted to +3.5.

In practice when stopping the battery and mixing together the electrolytes, a vanadium oxidation state higher than +3.5 is always found (mainly because of the influence of the preponderant effect of side reaction 3)).

The electrolyte is readjusted to a vanadium oxidation state of +3.5 by adding a reducing agent (oxalic acid, sulfite, etc.).

Thereafter, a substantial amount of energy must be spent to bring back the system to a zero state of charge (N⁴³ in the negative electrolyte and N⁴⁴ in the positive electrolyte).

This amount of energy that is periodically spent, represents a net loss of the energy storage process. This not negligible loss may be greatly reduced according to an aspect of the present invention, by installing a relatively small vanadium reduction asymmetric cell of the present invention in the negative or more preferably in the positive electrolyte circuit as schematically depicted in Fig. 4.

As shown, the positive electrolyte may be circulated wholly or in part (in the latter case by using for example an adjustable three-way valve or by any other means) through a relatively small asymmetric vanadium reduction cell Red.

The cell Red can be operated according to needs, either continuously or discontinuously in order to keep a symmetric vanadium oxidation state configuration.

Because of the possibility offered by the presence of such an auxiliary reduction cell Red, the need of mixing together the two electrolytes, adjust the oxidation state to about +3.5 and subject the battery to a precharge in order to recover a zero state of charge can be eliminated or made only exceptionally necessary.

Claims

C L A I M S

1. A method of producing an acid vanadium electrolyte solution containing N⁴³ and N⁴⁴ in a desired concentration ratio from solid vanadium pentoxide fed into the electrolyte solution, consisting in 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 a N⁴ state of oxidation or lower; 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 N⁴⁴ state of oxidation; adding acid and water to maintain certain molarity of the solution; continuously recycling the electrolyte solution through the cascade of electrolytic cells while bleeding a stream of yielded electrolyte solution containing N⁴³ and N⁴⁴ in the desired concentrations at the exit of one of the cells of said cascade; each cell 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 grater than the current density on the projected cathode surface and evolve oxygen at the anode.

2. The method of claim 1, wherein said electrolyte solution is a sulfuric acid solution and the molar content of vanadium is comprised between 1 and 5.

3. The method of claim 1, wherein the current density on the projected cathode surface is comprised between 100 and 300 A/m² and the current density on the anode surface is comprised between 1000 and 8000 A/m².

4. The method of claim 1 , wherein said vanadium oxide is in powder form and has a particle size not greater than 100 μm.

5. The method of claim 1, wherein after reacting the reduced vanadium electrolyte solution with said stoichiometric quantity of vanadium pentoxide, the solution is separated from any residual undissolved particle of vanadium pentoxide.

6. A plant for preparing vanadium electrolyte solution containing N⁴³ and N⁴⁴ in a desired concentration ratio from a solid vanadium pentoxide feed, consisting of a plurality of vanadium reduction electrolytic cells hydraulically connected in cascade and electrically powered in series from a regulated DC source; a dissolution tank collecting the reduced electrolyte solution exiting from the last cell of said cascade of cells, having mechanical stirring means and a feed mechanism of a controlled amount of vanadium pentoxide in powder forni; means for separating the vanadium enriched solution outflowing from said dissolution vessel from residual solid particles of vanadium pentoxide; means for adding to the enriched vanadium solution sulfuric acid and water to maintain certain molarity of the solution; pump means for recycling the electrolyte solution through the cascade of vanadium reduction electrolytic cells; tap means for bleeding a stream of yielded electrolyte solution containing N⁴ and N⁴⁴ in the desired concentration ratio at the exit of one of the cells of said cascade of cells; each cell 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 grater than the current density on the projected cathode surface and evolve oxygen at the anode.

7. An electrolytic cell for reducing N⁴ and/or N⁴⁵ ions in an acid vanadium electrolyte aqueous solution to N⁴³ and/or N⁴² 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 grater than the current density on the projected cathode surface and evolve oxygen at the anode.

8. The electrolytic cell of claim 7, composed of a cylindrical tubular body of a nonconductive acid resistant material having an inlet port 3 and an overflow port; a carbon felt cathode disposed on the inner cylindrical surface of the tubular body, provided with a terminal for electrical connection of the cell; a valve metal rod anode coated with a nonpassivating electrocatalytic coating disposed along the axis of the cylindrical carbon felt cathode.

9. The electrolytic cell of claim 7, characterized by the fact that it includes electrolyte penneable means of confinement of buoyant oxygen bubbles rising in the electrolyte around or near the anode.

10. The electrolytic cell of claim 9, wherein said permeable means of confinement belong to the group composed mesh screens, woven cloths, felts, porous and microporous glass frits and sintered bodies, all of a material chemically resistant to the electrolyte solution.

11. The electrolytic cell of claim 7, wherein said anode is a valve metal coated with a mixed oxide of iridium and tantalum or zirconium.

12. A method of rebalancing the state of relative oxidation state of the two separate vanadium electrolytes circulating in an all vanadium flow redox battery system without placing it off-service, consisting in circulating part of one of the two distinct electrolytes in a vanadium reducing electrolytic cell, external to the battery, made according to any of the claims from 7 to 11 and forcing a current through the electrolytic cell for the time needed to reestablish a balance of the state of oxidation of the vanadium content in the two electrolytes of the flow redox battery system.

13. The method of claim 12, wherein said electrolyte is the positive electrolyte circulating in the positive electrode compartments of the battery.