ALL-VANADIUM REDOX BATTERY AND ADDITIVES TECHNICAL FIELD This invention relates to an all-vanadium redox battery having a catholyte and an anolyte, a positive carbon electrode and a negative carbon electrode and which includes stabilizing/kinetic enhancing ions in the catholyte and hydrogen inhibiting ions in the anolyte. The positive carbon electrode in this battery can be impregnated with Au, Mn, Pt, Ir, Ru, Os, Re, Rh, Sb, Te, Pb and/or Ag. The negative carbon electrode in this battery can be impregnated with Pb, Bi , Tl , Hg, Cd, In, Ag, Be, Ga, Sb, As, Zn, Ca and/or Mg. The invention also concerns an all-vanadium redox battery having a catholyte and an anolyte, a positive carbon electrode and a negative carbon electrode wherein the positive carbon electrode in this battery is impregnated with Au, Mn, Pt, Ir, Ru, Os, Re, Rh, Sb, Te, Pb and/or Ag and the negative carbon electrode is impregnated with Pb, Bi , Tl , Hg, Cd, In, Ag, Be, Ga, Sb, As, Zn, Ca and/or Mg. The invention further relates to an all-vanadium redox battery having a catholyte and an anolyte, a positive carbon electrode and a negative carbon electrode wherein the positive carbon electrode in this battery is impregnated with Au, Mn, Pt, Ir, Ru, Os, Re, Rh, Sb, Te, Pb and/or Ag and the anolyte contains hydrogen inhibiting ions. The invention additionally relates to an all-vanadium redox battery having a catholyte and an anolyte, a positive carbon electrode and a negative carbon electrode and which includes stabilizing/kinetic enhancing ions in the catholyte and wherein the negative carbon electrode is impregnated with Pb, Bi, Tl , Hg, Cd, In, Ag, Be, Ga, Sb, As, Zn, Ca and/or Mg.

BACKGROUND ART The cycle life of the all-vanadium redox flow battery is currently limited by the slow disintegration of he positive carbon or graphite electrode during charging.

The slow disintegration of carbon or graphite anodes has been a problem in many industrial electrolytic processes (e.g. chlor-alkali process) and has led to its replacement by dimensional ly stable anodes. This disintegration is mainly due to the slow side reaction:

C + 2H ₂ 0 = C0 ₂ + 4H ⁺ + 4e~ which occurs during oxygen evolution as well as the reaction between the carbon surface and evolved oxygen.

A graphite felt and graphite collector plate have been used as the positive electrode in the all-vanadium redox flow battery. As a result of the above reactions which occur towards the end of the charge cycle, however, the positive electrode slowly disintegrates, thus reducing the cycle life of the battery. This problem can be alleviated to some extent by avoiding overcharge and restricting the battery charging to up to about 90% state-of-charge. However, the latter precautions do not address the central cause of the degradation problem which is apparently associated with the oxygen transfer during the vanadium charging reactions described by:

V0 ²⁺ + 1/20 ₂ + e ^" = VO* V0 ²⁺ + H _£ 0 = VO* + 2H ⁺ + e ^"

During the oxidation of V (IV) to V (V), oxygen transfer occurs and it has been postulated that oxygen- bridging with the electrode substrate is involved. Thus at a carbon electrode it is further postulated that C-0 bonds would be broken at the surface when V (IV) is oxidized to V (V) and this in turn leads to disruption of the carbon surface structure and disintegration.

The inventor has found that another difficulty with an all-vanadium redox battery is that as a result of the above oxygen transfer the kinetics of the V (II))/V (III) at a positive carbon electrode in such a battery is less than optimum with the result that the Derfor ance of the battery is

below optimum.

In addition hydrogen evolution can occur at the negative electrode near the end of the charging cycle, resulting in a loss of coulombic efficiency.

OBJECTS OF INVENTION

An object of this invention is to provide an all-vanadium redox battery having a catholyte and an anolyte, a positive carbon electrode and a negative carbon electrode and which includes stabilizing/kinetic enhancing ions in the catholyte and hydrogen inhibiting ions in the anolyte. The positive carbon electrode in this battery can be impregnated with Au, Mn, Pt, Ir, Ru, Os, Re, Rh, Sb, Te, Pb and/or Ag. The negative carbon electrode in this battery can be impregnated with Pb, Bi , Tl , Hg, Cd, In, Ag, Be, Ga, Sb, As, Zn, Ca and/or Mg. Another object is to provide an all-vanadium redox battery having a catholyte and an anolyte, a positive carbon electrode and a negative carbon electrode wherein the positive carbon electrode in this battery is impregnated with Au, Mn, Pt, Ir, Ru, Os, Re, Rh, Sb, Te, Pb and/or Ag and the negative carbon electrode is impregnated with Pb, Bi , Tl , Hg, Cd, In, Ag, Be, Ga, Sb, As, Zn, Ca and/or Mg. A further object is to provide an all-vanadium redox battery having a catholyte and an anolyte, a positive carbon electrode and a negative carbon electrode wherein the positive carbon electrode in this battery is impregnated with Au, Mn, Pt, Ir, Ru, Os, Re, Rh, Sb, Te, Pb and/or Ag and the anolyte contains hydrogen inhibiting ions. Yet a further object is to provide an all-vanadium redox battery having a catholyte and an anolyte, a positive carbon electrode and a negative carbon electrode and which includes stabilizing/kinetic enhancing ions in the catholyte and wherein the negative carbon electrode is impregnated with Pb, Bi , Tl , Hg, Cd, In, Ag, Be, Ga, Sb, As, Zn, Ca and/or Mg.

DISCLOSURE OF INVENTION

The present invention has found that introduction of active sites on the carbon surface facilitates the oxygen transfer during the oxidation process, not only stabilizes the carbon surface, but also improves the kinetics of the V (IV)/V (V) reaction.

Experiments by the inventor have shown, however, that the kinetics of this reaction are much improved on a number of metallic electrodes namely, Au, Mn, Pt, Ir, Ru, Os, Re, Rh, Sb, Te, Pb and/or Ag although the fabrication of electrodes wholly from these metals would be costly in practice. The inventor has found, however, that the same effect can be achieved by adding traces ^" (ppm) of Au, Mn, Pt, Ir, Ru, Os, Re, Rh, Sb, Te, Pb and/or Ag salts to the positive electrolyte. During cycling their oxides are deposited at the surface of the carbon or graphite felt or plate, providing active sites for the V (IV)/V (V) reaction, thus eliminating any interaction with the carbon. Similar effects have been achieved using a positive carbon electrode impregnated with Au, Mn, Pt, Ir, Ru, Os, Re, Rh, Sb, Te, Pb and/or Ag.

The inventor has also found that hydrogen evolution during charging is inhibited at a negative carbon electrode in an all-vanadium redox battery by adding traces (ppm) of Pb, Bi , Tl , Hg, Cd, In, Ag, Be, Ga, Sb, As, Zn, Ca and/or Mg ion-s to the anolyte. Similar effects have also been achieved by using a negative carbon electrode impregnated with Pb, Bi , Tl , Hg, Cd, In, Ag, Be, Ga, Sb, As, Zn, Ca and/or Mg.

According to a first embodiment of this invention there is provided an all-vanadium redox battery having: a positive compartment containing a catholyte having tetravalent and/or pentavaleπt vanadium ions and stabilizing/kinetic enhancing ions selected from the group consisting of Au, Mn, Pt, Ir, Ru, Os, Re, Rh, Sb, Te, Pb and Ag ions in electrical contact with a positive carbon electrode

the stabilizing/kinetic enhancing ions being at a concentration sufficient to suppress etching of the positive carbon electrode during conversion of tetravalent vanadium ions to pentavalent vanadium ions at the positive carbon electrode, a negative compartment containing an anolyte having divalent, trivalent and/or tetravalent vanadium ions and hydrogen inhibiting ions selected from the group consisting of Pb, Bi , Tl , Hg, Cd, In, Ag, Be, Ga, Sb, As, Zn, Ca and Mg ions in electrical contact with a negative carbon electrode the hydrogen inhibiting ions being at a concentration sufficient to increase hydrogen overvoltage and suppress hydrogen coevolution at the negative carbon electrode during conversion of divalent ions to trivalent and/or tetravalent ions, 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.

The positive and negative carbon electrodes of the first embodiment can be carbon or 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; and cellulose. The positive electrode can also -be carbon or 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, Mn, Pt, Ir, Ru, Os, Re, Rh, Sb, Te, Pb and/or Ag. The negative electrode can also 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 and carbon impregnated polyvinylidenechloride, impregnated with and/or coated with Pb, Bi , Tl , Hg, Cd, In, Ag, Be, Ga, Sb, As, Zn, Ca and/or Mg. Other types of carbon electrodes can also be used.

According to a second embodiment of this invention there is provided an all-vanadium redox battery having a positive compartment containing a catholyte having tetravalent and/or pentavalent vanadium ions in electrical contact with a positive carbon electrode impregnated with and/or coated with Au, Mn, Pt, Ir, Ru, Os, Re, Rh, Sb, Te, Pb and/or Ag to suppress etching of and enhance kinetics at the positive carbon electrode during conversion of tetravalent vanadium ions to pentavalent vanadium ions at the - positive electrode, a negative compartment containing an anolyte having divalent, trivalent and/or tetravalent vanadium ions in electrical contact with a negative carbon electrode impregnated with and/or coated with Pb, Bi , Tl , Hg, Cd, In, Ag, Be, Ga, Sb, As, Zn, Ca and/or Mg to increase hydrogen overvoltage and suppress hydrogen coevolution at the negative carbon electrode during conversion of divalent vanadium ions to trivalent and/or tetravalent ions, 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.

According to a third embodiment of this invention there is provided an all-vanad um redox battery having a positive compartment containing a catholyte having tetravalent and/or pentavalent vanadium ions in electrical contact with a positive carbon electrode impregnated with and/or coated with Au, Mn, Pt, Ir, Ru, Os, Re, Rh, Sb, Te, Pb and/or Ag to suppress etching of and enhance kinetics at the positive carbon electrode during conversion of tetravalent vanadium ions to pentavalent vanadium ions at the positive electrode, a negative compartment containing an anolyte having divalent, trivalent and/or tetravalent vanadium ions and hydrogen inhibiting ions selected from Pb, Bi , Tl , Hg, Cd, In, Ag, Be, Ga, Sb, As.

Zn, Ca and/or Mg ions in electrical contact with a negative carbon electrode the hydrogen inhibiting ions being at a concentration sufficient to increase hydrogen overvoltage and suppress hydrogen coevolution at the negative carbon electrode during conversion of divalent vanadium ions to trivalent and/or tetravalent ions, 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.

According to a fourth embodiment of this invention there is provided an all-vanadium redox battery having a positive compartment containing a catholyte having tetravalent and/or pentavalent vanadium ions and stabilizing/kinetic enhancing ions selected from Au, Mn, Pt, Ir, Ru, Os, Re, Rh, Sb, Te, Pb and/or Ag ions in electrical contact with a positive carbon electrode the stabilizing/kinetic enhancing ions being at a concentration sufficient to suppress etching of and enhance kinetics at the positive carbon electrode during conversion of tetravalent vanadium ions to pentavalent vanadium ions at the positive electrode, a negative compartment containing an anolyte having divalent, trivalent and/or tetravalent vanadium ions in electrical contact with a negative carbon electrode impregnated with and/or coated with Pb, Bi , Tl , Hg, Cd, In, Ag, Be, Ga, Sb, As, Zn, Ca and/or Mg to increase hydrogen overvoltage^ and suppress hydrogen coevolution at the negative carbon electrode during conversion of divalent vanadium ions to trivalent and/or tetravalent ions, 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.

According to a fifth embodiment of this invention there is provided a process for recharging a discharged or partially discharged all-vanadium redox battery according to the first, second, third or fourth embodiments which process comprises providing electrical energy to the positive and

negative electrodes to derive divalent vanadium ions in the anoiyte and pentavalent vanadium ions in the catholyte.

According to a sixth embodiment of this invention there is provided a process for the production of electricity from a charged all-vanadium redox battery of the first, second, third or fourth embodiments which process comprises withdrawing electrical energy from the redox battery by loading an external circuit in electronic communication with the positive and negative electrode.

An all-vanadium redox battery system is also provided consisting of a combination of the all-vanadium redox battery of the first, second, third or fourth embodiments and an anolyte reservoir for storing anolyte coupled to the negative compartment by anolyte supply and return lines via a pump and a catholyte reservoir for storing catholyte coupled to the positive compartment by catholyte supply and return lines via a pump.

Another all-vanadium redox battery is provided which consists of a combination of the all-vanadium redox battery of the first, second, third or fourth embodiments and an anolyte charge reservoir having anolyte charge supply and return line or lines for charging further anolyte which is to be delivered to the negative compartment and a catholyte charge reservoir having catholyte charge supply and return line or lines for charging further catholyte which is to be delivered to the positive compartment an anolyte storage reservoir having anolyte storage supply and return line or lines for storing anolyte from the negative compartment and a catholyte storage reservoir having catholyte storage supply and return line or lines for storing catholyte from the positive compartment and pumping means associated with the anolyte storage line or lines and/or the anolyte charge line or lines and with the catholyte storage line or lines and/or the catholyte charge line or lines for pumping:

⁽ , ⁾ the catholyte through the catholyte storage line or lines, the

positive compartment and the catholyte charge line or lines; and

(ii) the anolyte solution through the anolyte solution storage line or lines, the negative compartment and the anolyte solution charge line or 1 ines.

The catholyte can also include Pb, Sb and/or Te ions to enhance reversibility at the positive carbon electrode.

The positive and negative carbon electrodes of the second, third and fourth embodiments 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; and cellulose. Other types of carbon electrodes can also be used.

The positive and negative carbon electrodes can be any shape desired. It is preferred that the positive and negative carbon electrodes are rectangular-plate shaped.

According to a seventh embodiment of this invention there is provided an all-vanadium redox battery having a positive compartment containing a catholyte having tetravalent and/or pentavalent vanadium ions in electrical contact with a positive electrode consistin-g of a di ensionally stabilized anode (DSA - Ti or Ti alloy core, coated at least partially with titanium dioxide which coating is coated in turn with a noble metal coating selected from the group consisting of Mn, Pt, Pd, Os, Rh, Ru, Ir and alloys thereof), a negative compartment containing an anolyte having divalent, trivalent and/or tetravalent vanadium ions and hydrogen inhibiting ions selected from Pb, Bi , Tl , Hg, Cd, In, Ag, Be, Ga, Sb, As, Zn, Ca and/or Mg ions in electrical contact with a negative carbon electrode the hydrogen inhibiting ions being at a concentration sufficient to increase hydrogen overvoltage ard suppress hydrogen coevolution at the negative carbon

electrode during conversion of divalent ions to trivalent and/or tetravalent ions, and an ionically conducting separator disposed between the positive and r-gative compartments and in contact with the catholyte and the anolyte to provide ionic communication therebetween.

According to an eighth embodiment of this invention there is provided an all-vanadium redox battery having a positive compartment containing a catholyte having tetravalent and/or pentavalent vanadium ions in electrical contact with a positive electrode consisting of a dimensionally stabilized anode (DSA - Ti or Ti alloy core, coated at least partially with titanium dioxide which coating is coated in turn with a noble metal coating selected from the group consisting of Mn, Pt, Pd, Os, Rh, Ru, Ir and alloys thereof), a negative compartment containing an anolyte having divalent, trivalent and/or tetravalent vanadium ions in electrical contact with a negative carbon electrode impregnated with and/or coated with Pb, Bi, Tl , Hg, Cd, In, Ag, Be, Ga, Sb, As, Zn, Ca and/or Mg to increase hydrogen overvoltage and suppress hydrogen coevolution at the negative carbon electrode during conversion of divalent vanadium ions to trivalent and/or tetravalent ions, 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.

The electrolyte is typically an aqueous solution which includes H ₂ S0 ₄ , trifluoro ethanesulphonic acid, Na ₂ S0 ₄ , ^--^ H ₃ P0 ₄ , Na ₃ P0 ₄ , K ₃ P0 ₄ , HNO3, KNO3, NaN0 ₃ , C--C _]4 arylsulphonic acid such as p-toluenesulphonic acid monohydrate, C,-C _β alkylsulphonic acid such as methylsulphonic acid and ethylsulphonic acid, acetic acid or mixtures thereof in a concentration of from 0.01M to 5.0M. It is especially preferred to use H«S0 ₄ in a concentration of from 0.25M to 3.5M, more preferably 0.5M to 3.25M.

The all-vanadium redox battery of the invention is typically a

battery 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 hydraulically-impermeable in contrast to a diaphragm (typically asbestos) which allows restricted electrolyte transfer between compartments. Thus the ionically conducting separator can be a microporous 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 Sele ion (Trade Mark) material as manufactured by Asahi Glass Company.

The all-vanadium redox battery of the invention include moncpolar and bipolar type cells. A bipolar cell typically includes a plurality of positive compartments 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 cell is typically of the flat plate- or filter press-type.

An all-vanadium redox battery of the invention can be operated over a broad temperature range, e.g. -5°C to 99°C but is preferably operated in the temperature range 15°C to 40°C.

The stabilizing/kinetic enhancing ions and the hydrogen inhibiting ions can be formed in the electrolyte by dissolving salts such as oxides, sulphates, phosphates, nitrates, halogenides or other salts or complexes which are soluble in the electrolyte. The concentration of the stabilizing/kinetic enhancing ions and the hydrogen inhibiting ions can be very low (in the order of ppm) where there is a high electrolyte volume to electrode area ratio (e.g. electrolyte volume: electrode area = 10:1). For high electrolyte volume to electrode area ratios the concentration of the

stabilizing/kinetic enhancing ions and the hydrogen inhibiting ions is preferably in the range 0.1 ppm - 100,000 ppm, more preferably 1 ppm - 10,000 ppm and even more preferably 1 ppm - 100 ppm. Salts or complexes containing the stabilizing/kinetic enhancing ions and the hydrogen inhibiting ions can be of low solubility in the electrolyte since low concentrations are effective in suppressing etching of the positive carbon electrode and enhancing the kinetics of the reduction of pentavalent vanadium ions to tetravalent vanadium ions.

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 all-vanadium redox battery system;

Fig. 2(a) shows three scans of a cyclic voltammogram of a glassy carbon electrode in 1M V0S0 ₄ in 2M H ₂ S0 ₄ (Standard Solution = S.S.) at a scan rate of 4V/min.;

Fig. 2(b) shows three scans of a cyclic voltammogram of a glassy carbon electrode in S.S. + 0.2ml 0.001M AuCI, in 2M H„S0, at a scan rate of 4V/min;

Fig. 3 shows three scans of a cyclic voltammogram of a glassy carbon electrode in S ^' .S. + 0.2ml 0.001M SbCl., in 2M H ₂ S0 ₄ at a scan rate of 4V/min:

Fig. 4 shows three scans of a cyclic voltammogram of a glassy carbon electrode in S.S. + 0.2ml 0.001M TeCl ₄ in 2M H ₂ S0 ₄ at a scan rate of 4V/min;

BEST MODE AND OTHER MODES OF PERFORMING INVENTION

Referring to Fig. 1 an all-vanadium redox battery system 10 includes an all-vanadium redox battery 11 which has a positive compartment 12 containing a catholyte 13 in electrical contact with positive carbon electrode 14. Catholyte 13 has stabilizing/kinetic enhancing ions in a

concentration of 0.1 ppm to 100,000 ppm to suppress etching of and enhance kinetics at positive carbon electrode 14 and tetravalent and/or pentavalent vanadium ions. The stabilizing/kinetic enhancing ions are selected from Au, Mn, Pt, Ir, Ru, Os, Re, Rh, Sb, Te, Pb and/or Ag ions. The vanadium and stabilizing/kinetic enhancing ions in catholyte 13 are prepared by dissolving an oxide, sulphate, phosphate, nitrate, halogenide or other salt or complex which is soluble in catholyte 13 which is 0.01M to 5.0M H _? S0 ₄ , trifluoromethanesulphonic acid, Na ₂ S0 ₄ , K ₂ S0 ₄ , H ₃ P0 ₄ , Na ₃ P0 ₄ , K ₃ P0 ₄ , H 0 ₃ , N0 ₃ , NaN0 ₃ or mixtures thereof. As far as the vanadium salt is concerned it is especially preferable to dissolve vanadyl sulphate in 0.5M to 3.5M H _? S0 _4<

Battery 11 has a negative compartment 15 which includes an anolyte 16 in electrical contact with a negative carbon electrode 17. Anolyte 16 contains hydrogen inhibiting ions selected from Pb, Bi , Tl , Hg, Cd, In, Ag, Be, Ga, Sb, As, Zn, Ca and/or Mg ions in a concentration of 0.1 ppm to 100,000 ppm and divalent, trivalent and/or tetravalent vanadium ions. The vanadium and hydrogen inhibiting ions in anolyte 16 are prepared by dissolving an oxide, sulphate, phosphate, nitrate, halogenide or other salt or complex which is soluble in anolyte 16 which is 0.01M to 5.0M H ₂ S0 ₄ , trifluoromethanesulphorvic acid, a ₂ S0 ₄ , K ₂ S0 ₄ , H ₃ P0 ₄ , Na ₃ P0 ₄ , K-P0 ₄ , HN0 ₃ , KN0 ₃ , aN0 ₃ or mixtures thereof.

Positive and negative carbon electrodes 14 and 17 are fabricated from 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; or cellulose.

Battery 11 includes ionically conducting separator 18 disposed between positive and negative compartments 12 and 15 and is in contact with

catholyte 13 and anolyte 16 to provide ionic communication therebetween. A catholyte reservoir 19 is coupled to positive compartment 12 via pump 20 and supply line 21 and return line 22. An anolyte reservoir 23 is coupled to negative compartment 15 via pump 24 and supply line 26 and return line 25.

Positive carbon electrode 14 and negative carbon electrode 17 are electrically coupled to load 27 via switch 28 and to power source 29 via switch 30.

In a typical application 0.5M to 2.5M vanadyl sulphate and Ippm AuCl ₂ are dissolved in aqueous 0.25M to 5.0M H,S0 ₄ which is added to catholyte reservoir 19 and positive compartment 12 and 0.5M to 2.5M vanadyl sulphate and Ippm PbS0 ₄ are dissolved in aqueous 0.25M to 5.0M H ₂ S0 ₄ which is added to the anolyte reservoir 23 and negative compartment 15. Catholyte T3 is then pumped through positive compartment 12 and catholyte reservoir 19 via catholyte supply and return lines 22 and 21 by pump 20 and simultaneously anolyte 16 is pumped through positive compartment 15 and anolyte reservoir 23 via anolyte supply and return lines 26 and 25 by pump 24. Battery 11 is charged by providing electrical energy from power source 29 to positive and negative carbon electrodes 14 and 17 by closing switch 30 and open-ing switch 28 to derive pentavalent vanadium ions in catholyte 13 and divalent vanadium ions in anolyte 16.

Electricity can be derived from battery 11 by opening switch 30, closing switch 28 and withdrawing electrical energy across load 27.

Battery 11 is recharged by opening switch 28, closing 30 and providing electrical energy to positive and negative electrodes 14 and 17 to derive pentavalent vanadium ions in catholyte 13 and divalent vanadium ions in anolyte 16.

The above steps are usually conducted in sealed positive and negative compartments 12 and 15 and/or under an inert atmosphere such as nitrogen.

argon, helium or neon or mixtures thereof, in positive and negative compartments 12 and 15.

Positive carbon electrode 14 in battery 11 can be impregnated with Au, Mn, Pt, Ir, Ru, Os, Re, Rh, Sb, Te, Pb and/or Ag. Negative carbon electrode 17 in battery 11 can be impregnated with Pb, Bi , Tl , Hg, Cd, In, Ag, Be, Ga, Sb, As, Zn, Ca and/or Mg.

Another preferred embodiment of the invention consists of an all-vanadium redox battery system 10 as depicted in Fig. 1 except catholyte 13 does not include stabilizing/kinetic enhancing ions, anolyte 16 does not contain hydrogen inhibiting ions, positive carbon electrode 14 is impregnated with and/or coated with Au, Mn, Pt, Ir, Ru, Os, Re, Rh, Sb, Te, Pb and/or Ag to suppress etching of and enhance kinetics at positive carbon electrode 14 whilst charging battery 11 for converting tetravalent vanadium ions to pentavalent vanadium ions in positive compartment 12 and negative carbon electrode 17 is impregnated with and/or coated with Pb, Bi , Tl , Hg, Cd, In, Ag, Be, Ga, Sb, As, Zn, Ca and/or Mg to increase hydrogen overvoltage and suppress hydrogen coevolution at negative carbon electrode 17 whilst discharging battery 11 and converting divalent vanadium ions to trivalent and/or tetravalent vanadium ions in negative compartment 15.

A further preferred embodiment is the same as system 10 as depicted in Fig. 1 except catholyte 13 does not include stabilizing/kinetic enhancing ions and positive carbon electrode 14 is impregnated with and/or coated with Au, Mn, Pt, Ir, Ru, Os, Re, Rh, Sb, Te, Pb and/or Ag to suppress etching of and enhance kinetics at positive carbon electrode 14 whilst charging battery 11 and converting tetravalent vanadium ions to pentavalent vanadium ions in positive compartment 12.

Another preferred embodiment is the same as system 10 as depicted in Fig. 1 except anolyte 16 does not contain hydrogen inhibiting ions and negative carbon electrode 17 is impregnated with and/or coated with Pb, Bi .

Tl, Hg, Cd, In, Ag, Be, Ga, Sb, As, Zn, Ca and/or Mg to increase hydrogen overvoltage and suppress hydrogen coevolution at negative carbon electrode

17 whilst discharging battery 11 and converting divalent vanadium ions to trivalent and/or tetravalent vanadium ions in negative compartment 15.

EXAMPLE 1

A polished glassy carbon electrode was immersed in a solution of 1M V0S0 ₄ in 2M H ₂ S0 ₄ and cycled between - 1.2 and + 1.4 volts for 24 hours. In Fig. 2(a) three scans of a cyclic voltammogram of a glassy carbon electrode in 1M V0S0 ₄ in 2M H,S0 ₄ (Standard Solution = S.S.) at a scan rate of 4V/mϊn immediately after the glassy carbon electrode had been placed in a S.S. On removal from the solution after 24 hours of cycling, the surface of the glassy carbon appeared visibly etched. The same experiment was carried out in the same solution, to which 10 M AuCU had been added. Fig. 2(b) shows three scans of a cyclic voltammogram of a glassy carbon electrode in S.S. + 0.2ml 0.001M AuCU in 2M H ₂ S0 ₄ at a scan rate of 4V/min. immediately after the glassy carbon electrode had been placed in the S.S. + 0.2ml 0.001M AuCU in 2M H ₂ S0 ₄ . On removal of the electrode after 24 hours cycling, no etching of the surface could be observed, showing that the presence of traces of Au ions do -stabilize the carbon surface. The kinetics of the V(IV)/V(V) couple.was also enhanced as shown in the cyclic voltammogram of Fig. 2(b) as compared with the cyclic voltammogram of Fig. 2(a).

In a practical battery electrolyte the additive concentration would need to be higher (e.g. up to 1.0%) so that sufficient protection would be provided to a high surface area carbon felt which is employed in the positive half-cell .

EXAMPLE 2

A polished glassy carbon electrode was immersed in a solution of 1M V0S0 ₄ in 2M H ₂ S0 ₄ and cycled between - 1.2 and + 1.4 volts for 24

hours. In Fig. 2(a) three scans of a cyclic voltammogram of a glassy carbon electrode in 1M V0S0 ₄ in 2M H ₂ S0 ₄ (Standard Solution = S.S.) at a scan rate of 4V/min immediately after the glassy carbon electrode had been placed in a S.S. On removal from the solution after 24 hours of cycling, the surface of the glassy carbon appeared visibly etched. The same experiment was carried out in the same solution, to which 10 M SbCU had been added. Fig. 3 shows three scans of a cyclic voltammogram of a glassy carbon electrode in S.S. + 0.2ml 0.001M SbCU in 2M H ₂ S0 ₄ at a scan rate of 4V/min. immediately after the glassy carbon electrode had been placed in the S.S. + 0.2ml 0.001M SbC in 2M H ₂ S0 ₄ . On removal of the electrode after 24 hours cycling, no etching of the surface could be observed, showing that the presence of traces of Sb ions do stabilize the carbon surface. The kinetics of the V(IV)/V(V) couple was also enhanced as shown in the cyclic voltammogram of Fig. 3 as compared with the cyclic voltammogram of Fig. 2(a).

EXAMPLE 3 A polished glassy carbon electrode was immersed in a solution of 1M V0S0 ₄ in 2M H ₂ S0 ₄ and cycled between - 1.2 and + 1.4 volts for 24 hours. In Fig. 2(a) three scans of a cyclic voltammogram of a glassy carbon electrode in 1M V0S0 ₄ in 2M H ₂ S0 ₄ (Standard Solution = S.S.) at a scan rate of 4V/min immediately after the glassy carbon electrode had been placed in a S.S. On removal from the solution after 24 hours of cycling, the surface of the glassy carbon appeared visibly etched. The same experiment was carried out in the same solution, to which 10 M TeCl ₄ had been added. Fig. 4 shows three scans of a cyclic voltammogram of a glassy carbon electrode in S.S. + 0.2ml 0.001M TeCl ₄ in 2M H ₂ S0 ₄ at a scan rate of 4V/min. immediately after the glassy carbon electrode had been placed in the S.S. + 0.2ml 0.001M TeCK in 2M H_S0 ₄ . On removal of the electrode after 24 hours cycling, no etching of the surface could be

observed, showing that the presence of traces of Te ions do stabilize the carbon surface. The kinetics of the V(IV)/V(V) couple was also enhanced as shown in the cyclic voltammogram of Fig. 4 as compared with the cyclic voltammogram of Fig. 2(a).

EXAMPLE 4 Cyclic voltammograms were obtained between -1.6 and +1.6 volts at a glassy carbon electrode in 1M V0S0 ₄ in 2M H ₂ S0 ₄ solution both with and without hydrogen suppressing additives. A current of 6.25 mA for H ₂ evolution occurs around 1.25 volts. Adding 10 ^" M SbC to the solution results in a shift in the hydrogen evolution reaction towards a more negative potential of -1.55 volts. Table I shows the suppressing effect of several other additives during the negative potential scan corresponding to the charging of the negative electrolyte in the all-vanadium redox battery.

TABLE I

S.S. + PdCl.

*: A very noise result was obtained due to the damage on the

hydrogen and oxygen evolution voltage would be a true value. **: At a hydrogen evolution current of 6.25 A.

Thus, a significant increase in H ₂ overvoltage was achieved with Pb, Sb, Te and borax.

EXAMPLE 5 Cyclic voltammograms were obtained at a glassy carbon electrode between -0.4 and +1.6 volts in 2M V0S0 ₄ in 2M H ₂ S0 ₄ both with and without kinetic enhancing additives. Table II shows the effect of each additive in reducing the peak separation, corresponding to an improvement in the kinetics of the V(IV)/V(V) redox couple.

TABLE II

Conditions forking electrode Glassy carbon electrode. Reference electrode Standard Calomel Electrode. Counter electrode Graphite Rod. Standard solution 1M V0S0 ₄ /2M H ₂ S0 ₄ . Volume of additive 2 ml was added to 20 ml of the Standard solution.

Scan rate 4 V/min.

*: The peak separation potential could not be presented because both anodic and cathodic current peaks were very low after 24 and 48 hours cycle. It was observed after 48 hours, the anodic peak was very low and no anodic peak was observed. Thus, reversibility enhanced significantly by Au and Sb, and to a lesser extent by Pb, Te and borax.

INDUSTRIAL APPLICABILITY

Adding traces (ppm) of Au, Mn, Pt, Ir, Ru, Os, Re, Rh, Sb, Te, Pb and/or Ag salts to the positive electrolyte all-vanadium redox battery has been found to have a stabilizing and/or kinetic enhancing effect at carbon and DSA electrodes. Similar effects have also been achieved by using a negative carbon electrode impregnated with Au, Mn, Pt, Ir, Ru, Os, Re, Rh, Sb, Te, Pb and/or Ag.

Hydrogen evolution during charging can be inhibited at a negative carbon electrode in an all-vanadium redox battery by adding traces (ppm) of Pb, Bi , Tl , Hg, Cd, In, Ag, Be, Ga, Sb, As, Zn, Ca and/or Mg ions to the anolyte. Similar effects have also been achieved by using a negative carbon electrode impregnated with Pb, Bi , Tl , Hg, Cd, In, Ag, Be, Ga, Sb, As, Zn, Ca and/or Mg.