BACKGROUND ART At present, electrolyte for the positive and negative half-cells of an all-vanadium battery is produced by dissolving VOSCL in 2 - 3M H ₉ S0«. To form the electrolytes, one of two methods is used:

(i) Twice the volume of solution is used in the positive half-cell.

On charging: at the +ve electrode: V (IV) →- V (v) + e at the -ve electrode: V (IV) + 2e ^■ * V (II)

Half the volume of the positive electrolyte is then removed from the battery tanks and the excess V (V) must be reprocessed.

(ii) The same volumes are used in both +ve and -ve half-cell containers, but sufficient charge is applied to convert V (IV) to V

(II) in the negative. This requires overcharging the positive side and generating ( during half the charge-wastes electricity but also degrades positive electrode.

OBJECT OF INVENTION An object of this invention is to provide a process for dissolving and reducing a reducible vanadium compound disposed in, but not wholly dissolved in, an aqueous electrolyte by utilizing an electrochemical cell and/or a chemical reductant.

DISCLOSURE OF INVENTION According to a first embodiment of this invention there is provided a process for dissolving and reducing a reducible vanadium compound disposed 'n, but not wholly disso ^' ved in. an.aαueous electrolyte bv utilizinq an

electrochemical cell which aqueous electrolyte is in electrical contact with a positive electrode and a negative electrode which process comprises providing electrical energy from an external circuit to the positive and negative electrodes to dissolve and reduce at least a part of the compound in the electrolyte.

According to a second embodiment of this invention there is provided a process for dissolving and reducing a reducible vanadium compound disposed in, but not wholly dissolved in, an aqueous electrolyte by utilizing an electrochemical cell having a positive compartment containing a catholyte in electrical contact with a positive electrode, a negative compartment containing an anolyte comprising an aqueous electrolyte in electrical contact with a negative electrode, and an ionically conducting separator disposed between the positive and negative compartments and in contact with the catholyte and the anolyte to provide ionic communication therebetween which process comprises adding the vanadium compound to the aqueous electrolyte or wherein the vanadium compound is predisposed in the aqueous electrolyte, and providing electrical energy from an external circuit to the Dositive and negative electrodes to dissolve and reduce at least a part of the compound in the aqueous electrolyte.

The aqueous electrolyte can include vanadium (II) and/or vanadium (III) ions predisposed therein. The vanadium compound is thus reduced and dissolved by the V(II)/V(III) ions on addition to the aqueous solution and resultant V(IV) ions can be reduced at the negative electrode to V(II)/V(III) ions. Generally from 0.05 to 2.5M typically 0.25M to 1.0M V(II)/V(III) ions are included in the aqueous electrolyte. By VCIDWIII) is meant V(II) ions alone or V(III) alone or a mixture of V(II) and V(III) ions.

According to a t i d embodiment of this invention there is provided a

disposed in, but not wholly dissolved in, an aqueous electrolyte which process comprises adding a chemical reductant to the electrolyte to dissolve and reduce the compound in the electrolyte.

According to a fourth embodiment of this invention there is provided a process for dissolving and reducing a reducible vanadium compound disposed in, but not wholly dissolved in, an aqueous electrolyte by utilizing a chemical reductant and an electrochemical cell having the aqueous electrolyte in electrical contact with a positive electrode and a negative electrode which process comprises:

(a) adding a chemical reductant to the electrolyte to assist in dissolution and reduction of the compound in the electrolyte; and

(b) providing electrical energy from an external circuit to the positive and negative electrodes to dissolve and reduce at least a part of the compound in the electrolyte.

According to a fifth embodiment of this invention there is provided a process for dissolving and reducing a reducible vanadium compound disposed in, but not wholly dissolved in, an aqueous electrolyte by utilizing a chemical reductant and an electrochemical cell having a positive compartment containing a catholyte in electrical contact with a positive electroαe, a negative compartment containing an anolyte comprising the aαueous electrolyte in electrical contact with a negative electrode, and an ionically conducting seDarator disposed between the positive and negative compartments and in contact with the catholyte and the anolyte to provide ionic communication therebetween which process comprises:

(a)adding a chemical reductant to the anolyte to assist in dissolution and reduction of the compound in the anolyte; and

(b)providing electrical energy from an external circuit to the Dositive and negative electrodes to dissolve and '-educe at least a Dart of the "ompound in the e'ectOiyte

In this specification and claims when reference is made to the electrolytes of the first, second, fourth and fifth embodiments the positive and negative electrolytes are referred to as the catholyte and anolyte respectively. This is opposite to normal convention used in electrolytic processes but for convenience and consistency with nomenclature relating to other all-vanadium redox battery patent applications by the present applicant the former arbitrary convention has been adopted.

The chemical reductant can be a V (II), V (III) or V (IV) compound, which is soluble in the electrolyte or an aqueous solution containing V (II), V (III) and/or V (IV) ions, particularly an aqueous solution of V0S0 ₄ -dihydrate, hydrated (V ₂ (S0 ₄ )-) and/or VS0 ₄ .7H-0, in an amount sufficient to dissolve and reduce the vanadium compound. It is particularly preferred that a V (II) or V (III) compound, or the the aqueous solution contains V (II) and/or V (III) ions.

The chemical reductant can also be KHC^.F O, K„C ₂ 0 ₄ , Na ₂ C ₂ 0 ₄ , (NH _{4 z} C ₂ 0 ₄ NH ₄ HC ₂ 0 ₄ .H ₂ 0, LiHC ₂ 0 ₄ .H ₂ 0, EDIA, NaHC ₂ 0 ₄ .H ₂ 0, Li,C ₂ 0 ₄ , S0 ₂ , H ₂ C ₂ 0 ₄₎ H ₂ S0 ₃ , NaHSO- Na ₂ S0-, Na-S-0-, a ₇ S ₂ 0 ₄ , N ₂ S ₂ 0 ₅ , Na^Og, Li ₂ S0 ₃ , i ₂ S0 ₆ , KHS0 ₃ , K ₂ S0 ₃ , K ₂ S ₂ 0 ₃ , K ₂ S ₂ 0 ₄ , K ₂ S ₂ 0 ₅ , K ₂ S ₂ 0 ₆ , NH ₄ HS0 ₃ , (NH^SO^ (NH^SO^ (NH ₄ )-S0 ₅ , ₂ H ₄ , H ₂ N ₂ H ₂ .H ₂ 0, H^H^I^SO^ (NH ₄ ) ₂ S0 ₆ , NaBH ₄ , LiBH ₄ , BH ₄ , Be(BH _{4 2} , D ₂ , T _£ , CaH ₂ , MgH ₂ , H ₂ or calcium and magnesium salts of sulphurous acid, alkal -hydrogen-phosphites (Li, K, Na) , alkali hypophosphites (Li, K, Na) , hydroxyl amines, pyrosulphurous acid and dithioneous acid. Other chemical reductants can be used. For example, in principle it should possible to use a reducing organic water-soluble compound such as a reducing organic water-soluble mercaoto group-contain ⁱ ng rnmDOund including SH-containing - ^~ - ^{~ ~}

secondary and tertiary alkyl alcohols), SH-containing C,-C,- primary, secondary and tertiary alkyl carboxylic acids, SH-containing C,-C, ₂ primary, secondary and tertiary alkyl amines and salts thereof, SH-containing C,-C,« primary, secondary and tertiary alkyl amine acids and di- or tripeptides such as 2-mercaptoethylamine hydrochloride, 2-mercaptoethanol , 2-mercaptopropionylglycine, 2-mercaptopropionic acid, cystenylglycine, cysteine, carbamoyl cysteine, homocysteine, glutathione, cysteine hydrochloride ethyl ester and acetylcysteine. In principle it should also be possible to employ photocatalytic reduction and photoreduction at a semiconductor photocathode.

Reductants such as (NH ₄ )-C ₂ 0 ₄ NH ₄ HC ₂ O ₄ .H ₂ 0, S0 ₂ , H ₂ C ₂ 0 ₄ , NH ₄ HS0 ₃ , (NH ₄ ) ₂ S0 ₃ , ( H^SO^ NH ₄ ) ₂ S0 ₅ , N _£ H ₄ , H ₂ N ₂ H ₂ .H ₂ 0, H _{£ 2} H ₂ .H ₂ S0 ₄ ,

(NH ₄ ) ₂ S0g and H ₂ are particularly advantageous as reductants since at least some of the reaction product is gaseous permitting higher concentrations of vanadium ions to be prepared and reducing further treatment of electrolyte to remove unwanted products.

The vanadium compound can be ammonium metavanadate (NH ₄ V0 ₃ ); ammonium vanadium sulphate (NH ₄ V(S0 ₂ ) ; barium pyrovanadate

(Ba ₂ V ₂ 0 ₇ ); bismuth vanadate ⁽ Bi~0 ₃ V ₂ V ^; cesium vanadium sulphate VCs S0 ₄ ) ₂ 12H ₂ 0); iron metavanadate (FeCVO,)-); lead metavanadate (Pb(VOr)-) ; potassium metavanadate (KV0-); potassium vanadium sulphate (KVS0 ₄ ); rubidium vanadium sulphate <RbV(S0 ₄ ) _? ) ; sodium eta vanadate (NaV0 ₃ ); meta vanadic acid (HV0-); sodium ort ovanadate (Na _^ VO ; sodium pyrovanadate (Na ₄ V ₂ 0 ₇ ); sodium hexavanadate (Na ₄ Vg0, ₇ ); thallium pyrovanadate (Tl ₄ V ₂ 0-_); thallium metavanadate (T1V0 ₃ ); thallium pyrovanadate (ThV-0-

6H20); vanadium pentoxide (V2-0o-); vanadium sulphate ⁽ V(S04.) ); and calcium and magnesium vanadates including calcium metavanadate and magnesium metavanadate Other vanadium salts and comolexe. can also be.

dissolved and reduced in an electrolyte by the processes of the invention. For example, in principle it should also be possible to dissolve and reduce vanadium salts occurring in vanadium-bearing minerals such as patronite, bravoite, sulvanite, davidite, roscoelite, carnotite, vanadinite, descloizite, cuprodescloizite, vanadiferous phosphate rock and titaniferous magnetite using the processes of the invention as well as for recovering vanadium from spent catalysts.

Vanadium salts or complexes such as ammonium metavanadate NH ₄ V0 ₃ ) and ammonium vanadium sulphate NH ₄ V S0 ₄ ) ₂ ) are particularly advantageous since at least some of the reaction product is gaseous permitting higher concentrations of vanadium ions to be prepared and reducing further treatment of electrolyte to remove unwanted products. Other vanadium salts to which the processes of the invention are particularly applicable are vanadium pentoxide (V„0r) and sodium meta vanadate (NaVO,).

The electrolyte is typically an aqueous solution which includes H _? S0 ₄ , trifluoromethanesulDhonic acid, Na ₇ S0 ₄ , K _? S0 ₄ , H ₃ P0 ₄ , a ₃ P0 ₄ , K ₃ P0 ₄ , HN0 ₃ , KN0-, N0 ₃ , C ₈ -C arylsulphonic acid such as p-toluenesulphonic acid monohydrate, sulphamic acid, C _η -Cg alkylsulphonic acid such as methylsulphonic acid and ethylsulphonic acid or acetic acid or mixtures thereof in a concentration of from 0.01M to 6.0M. It is especially preferred to use H ₉ S0 ₄ in a concentration of from 0.25M to 6.0M, more preferably 0.5M to 6.0M.

The processes of the invention are typically performed in the temperature range 5-60°C more typically 15-40°C.

During the processes of the invention the electrolyte is typically stirred or agitated preferably with a mechanical stirrer or by fluidization of the solid reactar-ts using electrolyte flow.

conducted under an inert atmosphere such as nitrogen, argon, helium or neon or mixtures thereof.

The positive and negative electrodes can be any shape desired. It is preferred that the positive and negative electrodes are rectangular-plate shaped although the positive electrode can be an expanded metal sheet to allow for zero gap from the membrane while facilitating escape of 0 ₂ gas.

The positive and negative electrodes can be carbon and graphite felt, mat, plate, rod, knit, fibre, and cloth; carbon impregnated teflon; carbon impregnated polyethylene; carbon impregnated polypropylene; carbon impregnated polystyrene; carbon impregnated polyvinylchloride; carbon impregnated polyvinylidenechloride; glassy carbon; non-woven carbon fibre material; cellulose; carbon and graphite felt, mat, plate, rod, knit, fibre, and cloth, carbon impregnated teflon, carbon impregnated polyethylene, carbon impregnated polypropylene, carbon impregnated polystyrene, carbon impregnated polyvinylchloride and carbon impregnated polyvinylidenechloride, impregnated with and/or coated with Au, Pt, Ir, Ru, 0s, Re, Rh and/or Ag; platinised Ti ; platinised Ru; platinised Ir; platinised Pd; Pt; Pt black; dimensionally stabilized anode (DSA - Ti or Ti alloy core, coated at least partially with titanium dioxide which coating is coated or doped in turn with a noble metal coating selected from the group consisting of Pt, Pd, 0s, Rh, Ru, Ir and alloys thereof); Au; Pd; Ir; Ru; 0s; Re; Rh; Hg; Ag; Tl ; Bi ; Pb; In; Cd; Ga; Sb; Zn; Pb/Hg; Pb/Bi ; Hg/In; Hg/Cd; or Hg/Ga or other suitable electrodes.

In particular the positive electrode can be selected from the group consisting of CIA; Pb; Pb allc ⁽ Ξg Pb-Bi alloy); platinised Ti ; platinised Ru; platinised Ir; and D _j - coated on Pb, Ti , Zr, Hf, Ta, W or Nb which are also suitable materials for use as positive charge electrodes in an all-vanadium redox . arge cell as has been disclosed in the applicant's cooending applioaro-; entitled 'VANADIUM ^" HARI.I G ?ELL AND

VANADIUM DUAL BATTERY SYSTEM" which has the same priority date as the present application. V ₂ 0 ₅ coated electrodes would be unsuitable negative electrodes as they would dissolve. A DSA electrode performs well as a positive or negative electrode.

Preferably a DSA, Pb, V O _ς on Pb or graphite anode is used. It is preferred that a Pb or graphite cathode is used.

The electrochemical cell is typically a cell of the "membrane-type", that is it employs a membrane rather than a diaphragm to separate a positive compartment from a negative compartment. The membrane employed is typically sheet-like and can transport electrolyte ions whilst at the same time being hydraul ical ly-impermeable in contrast to a diaphragm (typically asbestos) which allows restricted electrolyte transfer between compartments. Thus tne ionically conducting separator can be a icroporous separator or a membrane fabricated from a polymer based on perfluorocarboxylic acids or a proton exchange polymer such as sulphonated polystyrene, sulphonated polyethylene or a substantially fluorinated sulphonic acid polymer such as Nafion (Trade Mark) or membranes of Flemion (Trade Mark) or Selemion (Trade Mark) material as manufactured by Asahi Glass Company.

- The electrochemical cell includes monopolar and bipolar type cells. A bipolar cell typically includes a Dlurality of positive comoartments each having a positive electrode therein and a plurality of negative compartments each having a negative electrode therein and wherein each of the compartments are separated by a membrane. A bipolar ceT is typically of the flat plate- or filter press-type.

By the processes of the invention an electrolyte having vanadium ions in sufficient concentration in an aqueous electrolyte 0.25M to 3M, preferably 1M to 3M, and more preferably 1.5M to 2.5M which are suitable for use in a practical all-vanadium battery can be DreDared in a single ^~

electrolyte consisting of 50%M V(III) to 507.M V(IV) can be obtained. Equal volumes of this solution can then be used for each half-cell so that no overcharge of the positive side is required for the initial charging process.

The processes of the invention also allow a much less costly electrolyte to be produced for an all-vanadium battery. At present, the V _? 0r powder produced at the mineral processing plant must be converted to flake to facilitate handling. The flake is then used to produce V0S0 ₄ solution which is crystallized as V0S0 ₄ and H ₇ 0. The crystals are then dissolved in H _? S0 ₄ to form the vanadium battery electrolyte etc. The processes eliminate several steps including flake production and thus allows an all-vanadium battery electrolyte to be produced much more economically: e.g. ₂ 0 costs a $15/kg

V0S0 ₄ and H ₂ 0 crystals cost a $300/kg. BRIEF DESCRIPTION OF DRAWINGS Preferred embodiments of the invention are described below with reference to the following drawings in which:

Fig. 1 depicts schematically an electrochemical cell which does not have an ionically conducting separator for dissolving and reducing a reducible vanadium compound, the electrolyte produced comDrising typically a mixture of V(IV) and V(V) ions in the electrolyte;

Fig. 2 depicts schematically an electrochemical cell having an ionically conducting separator for dissolving and reducing a reducible vanadium compound;

Fig. 3 depicts schematically an apparatus for dissolving and reducing a reducible vanadium compound;

Fig. 4 depicts schematically an electrochemical cell which does not have an ionically conducting separator fo ^r dissolving and reducing a reducible vanadium compound, h ^p ie/ ι ol vte coduced o pri sing typical ¹ , v.

a mixture of V(IV) and V(V) ions in the electrolyte;

Fig. 5 depicts schematically an electrochemical cell having an ionically conducting separator for dissolving and reducing a reducible vanadium compound;

Fig. 6(a) depicts schematically an alternative electrochemical cell for dissolving and reducing a reducible vanadium compound;

Fig. 6(b) is a bottom cross-sectional view along the line I-I in Fig. 6(a);

Fig. 7(a) depicts schematically an all-vanadium redox battery system and flow loops for the ail-vanadium redox battery during normal operation;

Fig. 7(b) depicts schematically an all-vanadium redox battery system and flow loops for the all-vanadium redox battery during anolyte and catholyte mixing operation; and

Fig. 8 depicts schematically an alternative electrochemical cell having an ionically conducting separator for dissolving and reducing a reducible vanadium compound.

DESCRIPTION OF BEST MODE AND OTHER MODES OF PERFORMING INVENTION

Referring to Fig. 1 an electrochemical cell 10 for dissolving and reducing a reducible vanadium compound has a negative electrode 11 and a positive electrode 12 which are electrically coupled via electrical Dower source 13 and switch 14 which are connected in series. Power source 13 can be a dc battery capable of delivering at least about 0.5 to about 2.0 volts over and above resistance losses of cell 10. Cell 10 contains an aqueous electrolyte 15 which is in electrical contact with positive and negative electrodes 12 and 11. Electrolyte 15 is preferably 0.5M to 6M H„S0 _Λ and contains a reducible vanadium compound such as V„0,- in powder form.

To dissolve and reduce the Dowdered V ₂ 0 ₅ electrolyte 15 is stirred by teflon coated stirrer bar 16 which is driven by magnetic

electrolyte 15 via line 18 which delivers nitrogen from nitrogen gas cylinder 19 to which it is coupled. Nitrogen is vented from cell 10 via vent 101. Switch 14 is closed so as to deliver 0.5 to 2.0 volts between negative and positive electrodes 11 and 12. The following reactions take place at negative electrode 11 or as a consequence of reactions of ions formed by reactions which take place at negative electrode 11:

(a) V ⁵⁺ + 3e ^{~ •} * V ²⁺

(b) V ⁵⁺ + 2e~ →- V ³⁺

(c) V ⁵⁺ + e~ →- V ^→

(d) (i) V ²⁺ + V ⁴⁺ -> 2V ³⁺

(ii) V ²⁺ + ¹ / ₂ V ₂ 0 ₅ -> V ³⁺ /V ⁺ (iii) V ³⁺ + ¹ ₂ V ₂ 0 ₅ →- 2V ⁴⁺ (iv) V ⁴⁺ + ^] _{2 2} 0 ₅ →- V ⁵⁺ (in solution) + V ⁴⁺ From the above reactions the V (II), V (III) and V (IV) ions dissolved in electrolyte 15 reduce the V^O _ς powder and form dissolved and reduced V (II), V (III), V (IV) and V (V) ions. Powdered V,0 ₅ which comes into contact with negative electrode 11 is also reduced. At positive electrode 12 the following reactions take place:

(a) V ²⁺ →- V ³⁺ , V ⁴⁺ or V ⁵⁺ (in solution) + 1 , 2 or 3e ^~

(b) V ^J+ -> V ⁴⁺ or V ⁵⁺ (in solution) + 1 or 2e ^~

(c) V ⁴⁺ -> V ⁵⁺ (in solution) + le ^"

(d) H _-0 -> 0_,

From the immediately preceding reactions whilst V (II), V (III) and V (IV) ions are oxidised to higher oxidation states at positive electrode 12 the higher oxidation state vanadium ions thus formed remain in solution.

Referring to Fig. 2 an electrochemical cell 20 for dissolving and reducing a reducible vanadium compound has a positive compartment 24 containing a catholyte 25 in electrical contact with positive electrode 26. Cell 20 s a negat ⁱ ve "omp rt-Tien 21 containing an anolvte 22 in

electrical contact with positive electrode 23. Positive electrode 26 is electrically coupled to a negative electrode 23 via electrical power source 204 and switch 205 which are connected in series. Power source 204 can be a dc battery capable of delivering at least about 2.3 volts over and above resistance losses of cell 20. Ionically conducting separator 27 is disposed between positive and negative compartments 24 and 21 and is in contact with catholyte 25 and anolyte 22 to provide ionic communication therebetween. A preferred separator 27 is a Nafion membrane or Selmion CMV membrane. Anolyte 22 is preferably 0.5M to 6.0M H.,S0 _Λ and contains a reducible vanadium compound such as V _? 0 ₅ in powder form. ^τ o dissolve and reduce the powdered V ₂ 0 _ς , anolyte 22 is stirred by teflon coated magnetic stirrer bar 28 which is driven by magnetic stirrer 29 disposed below negative compartment 21. Nitrogen is bubbled through anolyte 22 via line 201 which delivers nitrogen from nitrogen gas cylinder 202 to which it is coupled. Nitrogen is vented from negative compartment 21 via vent 203. Switch 205 is closed so as to deliver 2.5 volts between negative and positive electrodes 23 and 26. The following reactions take place at negative electrode 23 or in positive compartment 21 as a consequence of reaction of ions formed by reactions which take place at negative electrode 23:

(a) V ⁵⁺ + 3e ^~ -> V ²⁺

(b) V ⁵⁺ + 2e ^~ →- V ^{3+ '} _■ - 4+

(c) V ⁺ + e -> y ^{÷ '} (d) (i) V ²⁺ + V ⁴⁺ →- 2V ³⁺

(ii) V ²⁺ + ¹ ₂ V ₂ 0 ₅ →- V ³⁺ /V ⁴⁺ (iii) V ³⁺ + ^] ,V ₂ 0 ₅ →- 2V ⁺ (iv ⁾ V ^{4+ ■} _- ^λ l-Vfls ^■ * ^■ V ^{5+ (} in solution) + V ⁴⁺ From the above reactions the V (II), V (III) and V (IV) ions dissolved in anolyte 22 reduce the V. . powder and form dissolved and

direct contact with negative electrode 23 is directly reduced and at positive electrode 26, water is decomposed producing 0 _? .

Referring to Fig. 3 a container 30 for dissolving and reducing ammonium metavanadate or vanadium pentoxide or other reducible vanadium salts or complexes is disposed on top of magnetic stirrer 31. For exemplification a process of dissolving and reducing ammonium vanadate is described. Container 30 contains an aqueous electrolyte 32 comprising 0.5M to 6.0M H ₂ S0 ₄ and ammonium vanadate in powder form.

To dissolve the powdered ammonium vanadate electrolyte 32 is stirred by teflon coated stirrer bar 33 which is driven by magnetic stirrer 31. Nitrogen is bubbled through electrolyte 32 via line 34 which delivers nitrogen from nitrogen gas cylinder 35 to which it is coupled. Nitrogen is vented from container 30 via vent 36. A chemical reductant preferably oxalic acid (0.05M to 5M) is added to electrolyte 32 from reservoir 37 by opening tap 39. Sufficient reductant is added to dissolve and reduce the ammonium vanadate powder in electrolyte 32.

Referring to Fig. 4 an electrochemical cell 40 for dissolving and reducing a reducible vanadium compound is the same as electrochemical cell 10 shown in Fig. 1 except it includes a reservoir 41 which contains a chemical reductant preferably oxalic acid (0.05M to 5M) . Electrochemical cell 40 can be used to dissolve and reduce a reducible vanadium compound such as V ₇ 0 _ς in the same way as electrochemical cell 10 in Fig. 1 is used except the chemical reductant is added to electrolyte 42 from reservoir 41 by opening tap 43. Sufficient reductant is added to assist in dissolving and reducing the V ₇ 0 powder which is in electrolyte 42 prior to further reduction.

Referring to Fig. 5 an electrochemical cell 50 for dissolving and reducing a reducible vanadium compound is the same as electrolyte chemical cell 20 in Fig. 2 except it has a reservoi ^" 51 which contains a chemical 'eductant preferably oxa^c .; f0.«.-5M o 5M ⁾ Electrochemical eel ¹ .50

can be used to dissolve and reduce a reducible vanadium compound such as ₉ 0 _c in the same way as electrochemical cell 20 in Fig. 2 is used except the chemical reductant is added to electrolyte 52 from reservoir 51 by opening tap 53. Sufficient reductant is added to assist in dissolving and reducing the ₂ 0ς powder which is in electrolyte 52.

Fig. 6(a) depicts an alternative electrochemical cell 60 for dissolving and reducing a reducible vanadium compound. Cell 60 has negative electrodes 61 and positive electrodes 62 which are electrically coupled via electrical power source 63 and switch 64 which are connected in series. Power source 63 can be a dc battery capable of delivering of at least 2.5 volts over and above resistance losses of cell 60. An aqueous electrolyte 65 is recirculated through cell 60 via recirculation line 66 which includes pump 67. Electrolyte 65 is preferably 0.5M to 6M H ₂ S0 ₄ and contains a reducible vanadium compound such as ₂ 0 ₅ or ammonium metavanadate in powder form in negative compartments 68 which contain the powdered compound by filter elements 69 located at either end of each negative compartment. Elements 69 can be glass frits or synthetic filter cloth. The compound containing electrolyte 64 can be added to negative compartments 68 from reservoir 601 by opening tap 602. Positive compartments 603 are separated from negative compartments 68 by ionically conducting separators or icroDorous separators 604 which are disposed in cell 60 at the bottom of positive compartment 603.

In use electrolyte 65 is recirculated through cell 60 and about 2.3 volts (excluding resistance losses) is applied between negative electrodes 61 and positive electrodes 62 respectively. Typically, the current

_ ₂ density at negative the cathode is from about 5 to about 50mA.cm , while at the anode, the current density is from about 10 to about

_ ₂ 300mA.cm . Flow of electrolyte 65 through negative compartments 68 agitates the compound therein and maintains the powder in a fluidized

reduced into electrolyte 65. The dissolution and reduction of the compound in electrolyte 65 can be assisted by adding a chemical reductant such as oxalic acid or bubbling a gaseous chemical reductant such as S0 ₉ through electrolyte 65.

Referring to Fig. 7(a) an all-vanadium redox battery system 70 includes an all-vanadium redox battery 71 having positive compartment 72 and negative compartment 73 separated by ionically conducting separator 74. During normal operation electrolyte from negative compartment 73 is recirculated via tank 75 and electrolyte from positive compartment 72 s recirculated via tank 76. The flow loops are schematically depicted in Fig. 7(a) by lines with arrows. In Fig. 7(b ⁾ an all-vanadium redox battery system 700 includes an all-vanadium redox battery 701 having positive compartment 702 and negative compartment 703 separated by ionically conducting separator 704. The anolyte and cathoylte are periodically mixed by recirculating electrolyte through positive compartment 702, negative compartment 703 storage tanks 705 and 706 and valves 800. The flow loops during anolyte and catholyte mixing operation are schematically depicted by broken lines with arrows. Mixing of the catholyte and anolyte can be beneficial where precipitation of V (V) has occurred since such mixing will cause dissolution of the precipitated V (V) species by reaction with V(II ⁾ or V(III). System 70 can simulate system 700 in Fig 7(b) by opening and closing appropriate valves 80 in system 70.

Precipitate redissolution and electrolyte regeneration can also be carried out by simply reversing the polarity of the battery stack, that is positive and negative terminals interchanged and battery cnarged. At the new negative electrode V(V) and precipitate is reduced to V(III) and then to V(II) thereby dissolving the preciptate in the electrolyte. At the new positive V(II) and V(III) are oxidi ^r d to V ⁽ V ⁾ in solution. deferring to Fig. 8 an e1ec i- --le ical cell 80 for dissolving an .

reducing a reducible vanadium compound has a negative compartment 83 containing an anolyte 84 in electrical contact with negative electrode 81 and a positive compartment 85 containing a catholyte 86 in electrical contact with positive electrode 82. Negative electrode 81 is electrically coupled to positive electrode 82 via electrical power source 87 and switch 88 which are connected in series. Power source 87 can be a dc battery, potentiostat or galvanostat capable of delivering at least about 2.3 volts over and above the resistance losses of cell 80. Ionically conducting separator 89 is disposed between positive and negative compartments 85 and 33 and is in contact with anolyte 84 and catholyte 86 to provide ionic communication therebetween. A typical separator is a Nation membrane or Selemion CMV membrane.

The use of V ₇ 0 ₅ slurry or suspension, as the reducible vanadium salt, for example, to produce vanadium ions dissolved in an aqueous electrolyte could cause difficulties, particularly where a continuous flow -process is employed. Difficulties in maintaining the V_,0r powder in a fluidized state, as well as problems due to blockage of the membrane 89 and any porous frits disposed at anolyte inlets and outlets by fine V„0 ₅ powder can readily occur in practice. The inventors have found much to their surprise that the dissolution rate of a reducible vanadium powder such as V.,0,- is enhanced when V(II) and/or V(III) ions are predisposed in anolyte 84.

To dissolve and reduce powdered ₂ 0 _ς , for example, anolyte 84 is typically stirred by teflon coated magnetic stirrer bar 800 which is driven by magnetic stirrer 801 disposed below negative compartment 83. Nitrogen is bubbled through anolyte 84 which has V(III) ions or a mixture of V(III) and V(IV) ions predisposed therein, via line 802 which delivers nitrogen from nitrogen gas cylinder 803 to which it is coupled. Nitrogen is vented from negative compartment 83 via vent 804. Switch 88 is closed

positive electrodes 81 and 82. The cathodic current density can be in the

_ range of from about 5 to about 50 A.cm while the anodic current

_2 density can vary from about 10 to about 300mA.cm . V _? 0,- powder is slowly added to anolyte 84. As the V _? Q- powder comes into contact with V(III) ions disposed in anolyte 84 it is converted to V(IV) ions which is in turn reduced back to V(III) at negative electrode 81.

Simultaneously H ₂ S0 ₄ is slowly pumped through negative compartment 83 from reservoir 804 via line 805, pump 806 and valve 807 into the bottom of negative compartment 83 and the dissolved V(III) containing anolyte 84 is pumped into reservoir 808 via line 809 and pump 810. The flow rate of

H ₂ S0 ₄ through negative compartment 83 and the rate of V ₉ 0 ₅ addition are adjusted together with the current through negative and positive electrodes 81 and 82 so as to obtain a desired concentration of vanadium (eg V(III) and V(IV)) dissolved in H ₂ S0 ₄ .

2 For instance, assume a lm negative electrode 81 area and an

_ ₂ operating current of 20mA.cm which gives a total current of 200A.

No. of electrons/vanadium ion to produce 50% V(III) + 50% V(IV) = 1.5.

Thus no. of moles V per second = I/nF = 200/(1.5x96500)

- _ -1

= 1.38x10 moles. sec

Since 1 mole V ₂ 0 ₅ produces 2 moles vanadium ions:

Rate of V _? 0 ₅ addition = 0.5 x 1.38 x 10 ^~3

= 0.69 x 10 ^~3 moles.sec ^-1 .

To produce a 2M solution of vanadium ions in 2M H ₂ S0 ₄ , for example:

H-S0 ₄ flow rate required = 1.38 x 10~ moles.sec ' / 2 moles v.r ¹

= 0.69 x 10 ^"3 l.sec ^"1 .

2_ Since 2 moles of vanadium ions require 2 moles S0 ₄ ions in excess of the 2M H,S0 ₄ supporting electrolyte so as to form vanadium sulphate in

H _n S0 ₄ then 4M H„S0 ₄ is required for let ir, negative compartment

33 from reservoir 804 via 'ine 805 D>ιmp 80S and «. lve 807.

In positive compartment 85 oxygen is evolved at positive electrode 82 as a result of H 0 oxidation and is vented therefrom via vent 811. As the process continues, however, vanadium ions will diffuse through membrane 89 into positive compartment 85 whereby the vanadium ion concentrations will eventually be the sa. e in anolyte 84 and catholyte 86. The breakdown of water by oxidation to produce water will also cause gradual concentration of H ₂ S0 ₄ in positive compartment 85 and a further imbalance in cell 80. For example, electrolysis of 1 mole V(V) to 50%M V(III) + 50%M V(IV) consumes about 0.75 moles or 13.5gms H ₂ 0. This difficulty can be overcome by feeding slightly diluted 4M H-S0 ₄ into positive compartment 85 from reservoir 812 via line 813 and pump 814 and pumping catholyte 86 after concentration has taken place to about 4M H ₂ S0 ₄ to negative compartment 83 via line 815, pump 816 and valve 807.

EXAMPLE 1 SO, was bubbled through a solution of 0.1M V,,0 _ς in 2M sulphuric acid produced V (IV), but due to the low solubility of V _? 0- , only very dilute solutions can be prepared in this manner. Such dilute solutions are not suitable for use in a practical all-vanadium battery.

Surprisingly and unpredictably it has been found that concentrated solutions of V (IV), V (III) and/or V (II) ions can be prepared as follows. T o moles of V-Oj- powder were suspended in 1 litre of 2M H ₂ S0 ₄ and SO- gas was bubbled through suspension, Substantially all the the V _? 0 ₅ powder dissolved and was reduced by the S0 ₂ to form V (IV) ions in solution initially. By continuing the S0 bubbling the V (IV) ions were further reduced. SO,, bubbling was continued until the potential of the solution reached that of a 50:50 mixture of V (III) and V ⁽ IV) (approx. 0.3 V vs S.C.E. for 2 M vanadium in 2 M H-S0 ₄ ) , which is a suitable electrolyte for use in a c-ractical all-vanadium battery.

EXAMPLE 2 An electrolyte consisting of 50:50 ratio of V (IV) and V (V) in 2M H ₂ S0 ₄ was also prepared by electrolyzing a slurry of 1M of V ₂ 0r powder in 1 litre 4M H _? S0 ₄ in a cell as depicted in Fig. 1.

The process was carried out without a separator between the anode and cathode. Since no separator was used, the 0 ₂ bubbles generated at the anode assist in fluidizing the powder, however some of the V(IV) produced at the cathode was reoxidised to V(V ⁾ at the anode. Cell Voltage ~ 2.5 Volts. Substantially all the V 0 ₅ powder dissolved and was reduced at the cathode to form V (IV) ions with some V(V) ions, in solution.

EXAMPLE 3 An electrolyte consisting of 50:50 ratio of V (III) and V (IV) in 2M H ₂ S0 ₄ was also prepared by electrolyzing a slurry of 2M of V,,0 ₅ powder in 1 litre 4M H ₂ S0 ₄ in a cell as depicted in Fig. 2.

The process was carried out with a separator between the anode and cathode. Since a separator was used, the electrolyte in contact with the cathode was continuously stirred. Cell Voltage ~ 2.5 Volts. Substantially all the the V-Oj- powder dissolved and was reduced at the cathode to form a mixture of V(III) and V (IV ⁾ ions in solution.

EXAMPLE 4 As for Example 2, except that S0 was bubbled through the electrolyte to assist in reduction of V„0 ₅ as well as to keep powder in suspension.

EXAMPLE 5 As for Example 3, except that SO,, was bubbled through the electrolyte to assist in reduction of V _o 0r as well as to keep powder in suspension.

EXAMPLE 6

An electrolyte consisting of 2M V(IV) in 2M H _? S0 ₄ was prepared by adding 1M oxalic acid to a suspension of 1M V D,- in 4M H ₉ S0 ₄ with continuous stirring. During reduction and dissolution of V _? 0 ₅ to VCIV) ions the oxalic acid is oxidized to C0 ₂ which substantially leaves the aqueous solution so that no products of the oxidation reaction are produced which must subsequently be removed.

EXAMPLE 7

As for Example 6 except 1M oxalic acid was added to a suspension of 2M ammonium metavanadate in 4M H„s.S04 _Λ .

Solutions prepared as in Examples 3 and 6 have been utilized as electrolytes in all-vanadium redox batteries and the charge/discharge characteristics of the batteries were the same as all-vanadium redox batteries prepared by dissolving 2M V0S0 ₄ in 2M H ₂ S0 ₄ -

EXAMPLE 8

Recent experiments conducted on the stability of V (V) solution have shown that concentrated solutions (greater than 1.8 M Vanadium) when subjected to temperatures greater than 40°C, slowly precipitate. This problem in use can be avoided by reducing the vanadium ion concentration to less than 1.8 M for applications where the temperature is likely to exceed 40°C and where the systems will be maintained in fully charged state for long periods. However in many applications it is not desirable to reduce the vanadium ion concentration below 2.5M since such a reduction effectively reduces the energy density of the battery.

If precipitation of the V (V) does occur however, it can easily be redissolved and reduced by combining the V (II) / V (III) catholyte with the anolyte containing suspension. This will result in a solution which is mixture of V (III) and V (IV) as in an uncnarged battery which can readily be recharged and return battery to its original state.

Occasional mixinα of the catholvte and anolvte is beneficial as it

assists in rebalancing the cell. Unequal rates of diffusion of the different ions of vanadium across a membrane gradually leads to a greater concentration of vanadium ions on one side, but by periodically mixing the catholyte and anolyte and equally dividing the volumes into the +ve and -ve tanks, a rebalanced cell can be readily achieved. Alternatively, reversal of polarity of terminals and charging caused redi ssolution of V(V) precipitate by reducing it to V(II) and/or V(II)/V(III) mixture.

TECHNICAL APPLICABILITY The processes of the invention permit the preparation of an aqueous electrolyte having vanadium ions in sufficient concentration that is, 0.25M to 3M which is suitable for use in a practical all-vanadium battery Further the vanadium ion containing electrolyte can be prepared in a single step process. In addition, the processes of the invention allow a much less costly electrolyte to be produced for an all-vanadium battery since they eliminate flake production and thus allow an all-vanadium battery electrolyte to be produced much more economically.