GB2030349A - Process and Accumulator, for Storing and Releasing Electrical Energy - Google Patents

Abstract

A redox accumulator 10 has an anode compartment 16 and a cathode compartment 17 separated by a semi- permeable, ion-exchange membrane 11 with a solution permeable anode 12 comprising a thin, porous layer of electrically conductive and corrosion resistant powder bonded to one side of the membrane and a solution permeable cathode 13 comprising a thin porous layer of electrically conductive and corrosion resistant powder bonded to the other side of the membrane. A solution of an oxidisable compound is fed into compartment 16, and a solution of a reducible compound into compartment 17; a p.d. impressed between anode 12 and cathode 13 initiates the redox reaction. The oxidised anolyte and reduced catholyte solutions are stored, respectively, in 19 and 111. Electrical energy is released from the cell by circulating the oxidized anolyte through the anolyte chamber 16 and the reduced catholyte through the cathode chamber 17. Catholyte solutions may contain Cr<6+>/Cr<3+>, and anolyte solutions Cr<3+>/Cr<2+>. <IMAGE>

Description

SPECIFICATION Process and Accumulator for Storing and Releasing Electrical Energy It is well-known that an electrical storage accumulator can be made utilizing reduction and oxidation electrochemical reactions carried out substantially in the liquid phase with soluble salts of metals having different oxidation states. For instance, a chromium redox accumulator works according to the following electrochemical reactions: charge step: 6e 4Cr2(SO4)3+7H2O#6 CrSO4+ H2Cr207+ 6H2S04 discharge step: -6e 6CrSQ+H2Cr2O7+6H2SO4#4Cr2(SQ)3+7H2O with an open circuit cell voltage of about 1.74 V.An iron-titanium redox accumulator works according to the following electrochemical reactions: charge step: le FeCI2+TiCI3 ,Fe'Cl3+Ticl2 discharge step: --le FeCl3+Ticl2 yFeCI2+TiCI3 with an open circuit cell voltage of about 1.14 V.

Redox accumulators have a basic advantage in that the reactions occur entirely in solution with no chemical compounds depositing on or dissolving from the electrodes during both the charging and discharging stages. Moreover, no gases are released at the electrodes. Therefore, those problems commonly encountered in other types of electrical storage accumulators such as metallic deposit morphology on electrodes, gas evolution at the electrodes with associated bubble effect, gas withdrawal and storage are avoided.

Unfortunately, redox accumulators as now conceived are rather bulky and are therefore unsuitable as accumulators with high specific power. It is largely the presence of a separator or membrane defining two cell compartments and the necessity, for mass transfer considerations, of a rapid electrolyte flow across the electrodes to avoid parasitic electrodic reactions, such as hydrogen evolution at the anode or chlorine and oxygen evolution at the cathode, that calls for such electrodes and electrodic gap configurations that make the cells bulky. Moreover, the ohmic drops in the electrolyte are quite relevant and contribute considerably to lowering the efficiency of the accumulators.

In known redox accumulators, the elementary cell fundamentally consists of two compartments separated by a micro-porous, inert separator or preferably by an ion-exchange membrane. Generally, the electrodes in the compartments are placed a certain distance from the separator or membrane surface so that the electrolytes can be circulated directly in the spaces between the electrode surfaces and the membrane surfaces. Often porous electrodes are used and the electrolyte is caused to flow through the electrode. Therefore, the inter-electrodic gap is necessarily significant with a consequent high ohmic drop in the electrolyte.

It is an object of one aspect of the present invention to provide an improved redox process for storing and releasing electrical energy in a storage cell.

It is an object of another aspect of this invention to provide an improved redox accumulator with high power density and low ohmic losses.

Thus, according to one aspect of the present invention, we provide a process of storing and releasing energy in an electrochemical storage cell having an anode compartment and a cathode compartment separated by a semi-permeable, ion-exchange membrane with a solution permeable anode comprising a thin, porous layer of electrically conductive and corrosion resistant powder bonded to the anode side of the membrane and a solution permeable cathode comprising a thin, porous layer of electrically conductive and corrosion resistant powder bonded to the cathode side of the membrane, the process comprising storing energy in the cell by feeding into the anode compartment an anolyte solution of an oxidizable compound capable of remaining substantially soluble in the anolyte solvent and capable of being reduced when in its oxidized form, impressing an electric potential between the anode and cathode to oxidize the anolyte solution, withdrawing the oxidized anolyte solution from the anolyte compartment and storing said oxidized anolyte solution, simultaneously feeding a catholyte solution of a reducible compound capable of remaining substantially soluble in the catholyte solvent and of being oxidized when in its reduced form into the catholyte compartment, withdrawing reduced catholyte solution therefrom and storing said reduced catholyte solution and, thereafter releasing energy from the cell by circulating said oxidized anolyte solution through the anolyte chamber and said reduced catholyte solution through the cathode chamber while the resulting electric potential across the anode and cathode drives an electric current through an external load.

According to another aspect of the invention, a redox accumulator comprises a housing containing at least one cell comprising a cathode compartment and an anode compartment separated by an ion-exchange membrane, a porous permeable layer of electro-conductive, corrosion resistant material on each side of the membrane to form anode and cathode surfaces, means for evenly distributing a direct current over the anode and cathode surfaces, means for circulating an anolyte solution through the anode chamber and means for circulating a catholyte solution through the cathode chamber, said anolyte and catholyte solutions containing metal ions having different oxidation states.

The means for impressing a direct electric current on the electrodes are preferably grids contacting the electrodic surfaces at a plurality of evenly distributed points over the entire surface of each electrode and the grids are preferably electrically connected to back plates of the electrodic compartments.

The electrolyte is caused to flow continuously in contact with the surface of the porous and permeable electrode on the surface of the cell separator, that is the semi-permeable membrane.

Supply of the oxidisable and reducible compound, which preferably contains multi-valent metal ions to the sites of the electrode reaction is easily allowed by the porosity of the extremely thin layer of particulate material constituting the electrode and the electrode reaction occurs substantially at the interface between the particles and the ion exchange resin constituting the membrane onto which they are bonded. Therefore, the ohmic drop in the electrolyte is substantially eliminated.

Preferably, both the anode and the cathode are similarly constituted by a thin layer of the electrically conductive, powdered material bonded onto the opposite sides of the ion exchange resin membrane, although only one of the electrodes may be so constructed while the co-operating electrode may be a foraminous conventional self-supporting metal or graphite electrode, spaced from or pressed against, the other side of the membrane.

In a preferred embodiment of the accumulator in accordance with the invention, a number of elementary cells are arranged in series and are separated by bipolar partitions consisting of an electroconductive wall provided on both sides with projections suitable to contact, at a multiplicity of points, the anode fixed on the membrane surface of one of the elementary cells and the cathode fixed on the membrane surface of one of the elementary cells and the cathode fixed on the surface of the adjoining cell.

The anolyte and catholyte solutions may be contained in two respective tanks and can be circulated through the anodic and cathodic compartments by two circulation pumps. When this is done, the electrolyte solutions will flow along the surfaces of the porous and permeable electrodes fixed on the membrane surfaces so that the oxidation and reduction electrodic reactions occur substantially as the interface of the electrodic particles and the ion-exchange membrane itself. In this way, the ohmic drops both in the anolyte and the catholyte solution are substantially eliminated.

Moreover, the high specific surface of the electrodes which are constituted by the thin layers of powdered material bonded to the membrane with respect to the apparent surface effectively helps to reduce polarization and permits the use of high current densities.

The preferred redox couples which can be used in the accumulator of the invention are chromium-chromium, titanium-iron, vanadium-vanadium and vanadium-iron.

Examples of processes and of redox accumulators in accordance with the invention will now be described with reference to the accompanying drawings in which:~ Figure 1 is a diagrammatic outline of one example of a redox accumulator in accordance with the invention; Figure 2 is a partial cross-section to a larger scale of a basic cell of the accumulator of Figure 1 illustrating the reactions occurring; Figure 3a is an exploded cross-section of another practical example of an accumulator in accordance with the invention; and, Figure 3b is a cross-section of the accumulator of Figure 3a when assembled.

Referring to Fig. 1, the complete system of the redox accumulator includes at least one elementary cell generically labeled as 10 comprised of an ion exchange resin membrane 1 1 on the surface of which are incorporated the electrodes made of electroconductive and chemical corrosion resistant powdered materials. Examples of suitable materials are the platinum group metals, their alloys or intermetallic compounds, their conductive oxides, platinum black, palladium black, oxides of tin, lead, antimony, bismuth, cobalt and manganese, conductive oxides such as spinel-type oxides, perovskites and delafossites, graphite and acetylene black.

The anode 12 is bonded on one surface of the membrane and the cathode 13 is bonded on the opposite surface of the membrane. Metallic grid-shaped current collectors 14 and 1 5 are electrically connected to the walls 16 and 17, which act as negative and positive end plates, respectively, and the current collectors press against the electrodes. The two halves of the cell 1 6 and 1 7 are electrically insulated from each other by the insulating gasket 18. When the system is in operation, the anolyte solution contained in the tank 19 is continuously circulated through the anode compartment by recycle pump 1 10 and the catholyte solution contained in tank 1 1 1 is continuously circulated through the cathode compartment by recycle pump 1 12.

The cell electrodes are chemically inert, but act respectively as acceptor and donor of electrons to produce a potential dependent upon the concentration of ions in the solution which they are in contact with. For instance, to discharge a previously charged chromium accumulator, the hexavalent chromium containing catholyte solution contained in tank 111 is circulated into the cathode compartment and the bivalent chromium containing anode solution contained in tank 19 is circulated into the anode compartment. A reduction of hexavalent chromium to trivalent chromium occurs at the cathode while SO4-ions are released and migrate through the anionic membrane to reach the anode where bivalent chromium is oxidized to trivalent chromium.Therefore, the concentrations of the anolyte and the catholyte solutions will finally equalize themselves and the cell potential will become zero.

To charge the accumulator, the anode and the cathode are connected respectively to the negative and ptsitive pole of a direct current electrical source and the reactions are reversed, thus restoring a high concentration of hexavalent chromium with respect to trivalent chromium in the catholyte solution and conversely a high concentration of bivalent chromium with respect to trivalent chromium in the anolyte solution.

Fig. 2 schematically illustrates a cross section of the accumulator elementary cell consisting of the ion-exchange resin membrane 21 to which the anode 22 and the cathode 23 are bonded. The two metal screen current collectors contact the electrodes at a multiplicity of points, evenly spaced over the entire surface of the said electrodes. The figure is intended to particularly help understand the invention and the operation of the accumulator of the invention. A chromium storage is illustrated and the indicated reactions concern the discharge of the accumulator. The membrane 21 is of the anionic type and is resistant to sulfuric acid solutions and is preferably made of fluorinated resin containing quaternary ammonium groups.

Electrodes 22 and 23 preferably consist of a powdered mixture of graphite and platinum black bonded onto the membrane surface to form a porous layer easily permeable to the electrolyte. The current collectors 24 and 25 are screens, preferably made of a valve metal such as niobium, tantalum, titanium, zirconium and hafnium and most preferably galvanically coated with platinum or platinumiridium alloy. The current conductors 24 and 25 are pressed against electrodes 22 and 23 and are connected to the electrical load of the battery.

The hexavalent chromium containing catholyte solution is circulated into the cathode compartment in such a way that the solution will flow along the porous cathode surface and the current collector 25 is actually foraminous enough not to obstruct the flow of the solution along the cathode 23 surface. This favors a good mass transfer to and from the electrode surface and prevents depletion of the reacting ionic species at the electrode. The bivalent chromium containing anolyte solution is likewise circulated into the anode compartment to contact the porous anode 22 surface.

Thus, the two electrodes assume their respective potentials which are dependent upon the ratio of the concentrations of hexavalent and bivalent chromium ions in the respective solutions.

During discharge of the accumulator thus formed, electric current flows through the electrical load connected to the cell terminals, and the following reactions occur: at the cathode (positive pole): Cr20,23+ 1 4H@+6e9 > 2 Cr30++7H20 ionic transfer through the membrane (from the cathode to the anode side): 12 SQ2##12 > 12 SO423 at the anode (negative pole): 6 cr29 )6 Cr30++6e@ Overall reaction: H2Cr207+6CrSO4+6H2SQ-*4Cr2(SO4)3+ 7 H20 Conversely, during the recharge of the accumulator, the following reaction take place: at the cathode: 6Cr++++ > 6Cr++ ionic transfer through the membrane (from the anodic side to the cathodic side) 12 S04-- at the anode: 2Cr++++7 H20 Cr2O7#+14 H$+6e Overall reaction: 6F 7 H2O+4Cr2(S04)3#6CrS04+ H2Cr207+ 6H2SO4 The anodic and cathodic reactions practically occur at the interface between the particles constituting the electrodes and the ion exchange resin membrane, that is the catalytic sites in the electrodes are in direct contact with the ion exchanging radicals of the resin membrane.Thus, ionic conduction in the solution is substantially eliminated and ionic conduction takes place only through the thickness of the membrane and therefore the ohmic drop across the cell is effectively reduced. Meanwhile, the porosity and the thinness of the electrodes facilitate electrolyte renewal over the entire electrodic surface, thereby avoiding polarization effects and the multiplicity of electrical contacts with the current collectors affords low ohmic drop in the electronic conductive structures of the cell, that is in the porous particulated electrodes bonded to the membrane.

Figs. 3a and 3b illustrate a practical embodiment of the accumulator of the invention. Fig.

3b is a plan view of the accumulator described in Fig. 3a with the same parts of the accumulator being labeled with the same numbers in both figures.

The accumulator consists of an anode end plate 31, a cathode end plate 32, a bipolar partition 33 and two membranes 34a and 34b on whose surfaces the electrodes have been bonded. The accumulator is thus composed of two elementary cells connected in series, and it is intended that a nearly unlimited number of similarly connected elementary cells can be included between the two end plates by inserting a certain number of bipolar partitions and the electrodes bearing membranes between said plates. Preferably, also the anode 31 and cathode 32 end plates are made of the same material as the bipolar partition 33. The anode end plate 31 is provided on its inner surface with a central grooved area whose grooves 35 are hydraulically connected to anolyte inlet 36 and outlet 37.

The cathode end plate 32 is likewise provided on its inner surface with a central grooved area 38 whose grooves 39 are hydraulically connected to the catholyte inlet 311 and outlet 312 by connecting grooves 310. Both end plates are provided with means 313 for the electrical connection of the battery.

The bipolar partition 33 is provided on both sides with a central grooved area whose grooves 314 on the cathode side are connected with two holes 315 and 316 coaxial with inlet 311 and outlet 312 of the cathode end plate respectively, while grooves 317 on the anode side of the bipolar partition are in connection with two holes 318 (shown in Fig. 3a only) and 319, coaxial with anolyte inlet 36 and outlet 37 of the anode end plate, respectively.

The bipolar partition and the two end plates preferably consists of a conductive aggregate made of powdered graphite and a chemically inert resin. The membrane 34a of the first elementary cell consists of an ion exchange resin sheet on whose central area, corresponding to the grooved areas of the anode end plate and the cathode side of the bipolar separator 33, the electrodes have been incorporated with anode 320 on the side facing the anode end plate and the cathode 321 a on the side facing the cathode side of the bipolar separator 33, both electrodes being made of an electroconductive and corrosion resistant powdered material.Membrane 34b of the second elementary cell similarly consists of an ion exchange resin sheet on whose central area corresponding to the grooved areas of the anode side of the bipolar partition 33 and of the cathode end plate 32, anode 320b and the cathode 331 b have been incorporated, respectively, Suitable materials for the anodes 320a and 320b and the cathodes 321 a and 321 b are powders of graphite, noble metals such as platinum, ruthenium, rhodium, palladium, iridium, osmium, their alloys and intermetallic compounds, oxides of the same metals and oxides of tin, lead, antimony, bismuth, cobalt and manganese, conductive oxides such as spinel-type oxides, perovskites, delafossites, palladium black, platinum black and acetylene black.Both membranes 34a and 34b are respectively provided with four holes (6a), 37a, 311 a, 31 2a and (6b) 37b, 311 b, 31 2b, coaxial with the holes 36,37,311,312 of the anode 31 and cathode 32 end plates, respectively. The holes (6a) and (6b), not shown in the figures, are coaxial with the holes 36 of the anode end plate 31 and the holes 31 8 of the bipolar separator.

The anode end plate 31 and the cathode end plate 32, membranes 34a and 34b and bipolar partition 33 are provided with a series of holes 322 for assembly tie rods 323. The said tie rods 323 may be made of insulating material such as teflon, nylon, etc. or metal and in the latter case, an insulating plastic coating and washers must be provided to keep the tie rods, the end plates and the bipolar separator electrically insulated to prevent short circuits inside the cell. The tie rods are tied by nuts 324 and washers 325 and when the cell is assembled as shown in Fig. 3b, the peripheral areas of the end paltes 31 and 32 and the bipolar separator 33 are hydraulically sealed against the peripheral areas of membranes 34a and 34b.

All the surfaces of the bipolar separator and the two end plates, with the exception of the surfaces contacting the electrodes, are preferably insulated by a thick layer of resin not charged with electroconductive powdered material. This prevents electrodic reactions from occurring at the surfaces contacting the electrodic solution or the ion exchange resin for instance in correspondence of the seal surface owing to ions migrating throughout the membrane resin.

The projections of the central grooved area of the anode end plate 31 electrically contact anode 320a incorporated in the membrane 34a and the projections of the central grooved area of the bipolar partition 33 on the cathode side electrically contact cathode 321 a bonded on membrane 34a while the anode side projections of separator 33 contact the anode 320b and those of cathode end plate 32 contact cathode 321 b. A series of elementary bipolar cells is thus formed and their number can be augmented at will by inserting more bipolar elements in said series.

When the accumulator is working, the anolyte solution is pumped into the cell through anode end plate inlet 36 and the electrolyte distributes into the first anode compartment defined by the grooves in the central zone of anode end plate 31, then, through the passage defined by hole 36a in membrane 24a and hole 318 in bipolar partition 33, both holes being coaxial with inlet 36, the electrolyte flows into the second anode compartment defined by the grooves of the central area of the anode side of bipolar separator 33. The electrolyte leaves the cell through outlet 37 which, through hole 37a in membrane 34a and hole 31 9 in bipolar separator 33, is in communication with the second anode compartment. The catholyte solution is likewise circulated through the cathode compartments through inlet 311 and outlet 312 of the cathode end plate 32.

Various procedures may be followed to form and incorporate the electrode onto the membrane surface. According to one known method, the powder of electroconductive material is mixed with powdered polytetrafluoroethylene whose content in the mixture may be from 15 to 60% by weight, preferably between 15 and 20% by weight. A powder used satisfactorily is produced by DuPont under the commercial name of Teflon-T-30 but other resins unaffected by the electrolytes used in the storage battery may be used instead of polytetrafluoroethylene (PTFE). The mixture of powders is placed in a mold and heated to a sintered film which is then transferred to the membrane surface and hot-pressed to the same.

Another method is described in U.S. patent application Serial No. 878,906 filed Feb. 1 7, 1978 wherein the electrodes are formed by application to the membrane surface of a liquid suspension of electroconductive powder in an ion-exchange resin solution, said resin being compatible with the resin constituting the membrane. The suspension is then dried and heated to deposit and bond the powder containing resin on the' membrane surface. Preferably, the binder is an ion-exchange resin similar to the one forming the membrane.

The electroconductive powders used have an average particle diameter between 0.5 and 30 jum and preferably between 10 and 20,us. The electroconductive material loading is between 0.5 and 10 mg/cm2 and preferably between 1 and 5 mg/cm2 of electrode surface. The thickness of the electrodes fixed on the membrane surface is less than 0.1 mm and preferably between 0.010 mm and 0.06 mm.

The thinness of the electrodes together with their porosity is of importance as it has been in fact found that by reducing the electrodes thickness, side reactions are effectually hindered at the electrodes. This can be understood on the basis that the thinner and more permeable the electrodes are, the easier the transfer of the ions supporting the main electrodic reaction is made from the electrolyte bulk to the catalytic sites in the contact areas between the electrode particles and the ionexchange resin constituting the membrane. In this way, the diffusion of the ions supporting the main electrode reaction through the so-called electrode double layer is aided and in practice, this leads to a lower polarization fo the electrodes, and therefore to an improved voltage/current curve during the discharge of the accumulator and also to greater energy efficiency.The lower electrical plane conductivity of the electrodes made extremely thin for the aforesaid reasons is effectively compensated for by increasing the density of the contact points with the current collectors which can be achieved by the use, for instance, of very fine mesh grids. Coating of the current conductors contact areas with platinum further reduces ohmic drops.

The membrane may be either anionic or cationic, depending on the nature of the Redox couple in the solutions and consequently on the kind of ions which must pass through the membrane. In fact, the ionic conduction through the membrane may be due to the transfer of anions such as 5042 or CI or cations, typically H+.

Particularly well-suited cationic membranes consist of a fluorine-carbon polymer molded into thin sheets and containing acid radicals such as sulfonic acid or carboxylic acid groups. In the membranes containing sulfonic or carboxylic acid groups, the ion-exchange groups are the acid hydrated SO-3H.

H20 radicals inserted in the polymeric structures by sulfonation. The ion-exchange groups are not mobile within the membrane as they are chemically bonded to the polymeric chain so that their concentration is constant.

A specific class of such cationic membranes particularly resistant to acids and oxidizing agents is made by DuPont under the commercial name of Nafion and these membranes consist of hydrated copolymers of polytetrafluoroethylene and vinyl ether sulfonated polyfluoride containing sulfonic groups. Other suitable cationic membranes may be made of a styrene-divinylbenzene sulfonated copolymer, preferably supported by an inert and porous substrate such as asbestos paper or made of a tetrafluoroethylene/acrylic acid sulfonated copolymer.

Particularly suitable anionic membranes are made of a thin film of fluoro-carbon polymer or styrene-divinylbenzene copolymer containing basic radicals such as, for example, quaternary ammonium groups or pyridine or substituted The membrane must allow the migration of a particular type of ion (e.g. Cl- or 5042 or H+) while it must effectively prevent or minimize the passage and the mixture of other ionic species through the membrane. The thickness of the membrane is generally on the order of several tenths of millimeters. In the case of cross-linked thermosetting resins such as styrene-divinylbenzene copolymers copolymerized on an inert support such as asbestos paper, the membrane thickness may be one or more millimeters.

The charging or discharging process respectively of the accumulator connected to an external electrical load or to a direct current source can be respectively carried out by introducing the catholyte solution into the cathode compartment and the anolyte solution into the anode compartment and circulating said solutions through their respective tank and cell compartment. The flow rate is preferably between 100 and 500 cm3 per minute per 1,000 cm2 of electrode surface for 0.08 A/cm2 as the electric current drawn by the battery load.

In the following example there is described several preferred embodiments to illustrate the invention. However, it should be understood that the invention is not intended to be limited to the specific embodiments.

Example Test accumulators according to Figs. 3a and 3b were fabricated and installed as indicated in Fig.

1. The electrodes consisted of a powdered mixture of graphite and platinum black in a 9:1 ratio by weight bonded onto the surface of an IONAC 3475 M type anionic membrane. The electrodes had a thickness of about 0.05 mm which corresponded to a powder charge of between 2 and 3 mg/cm2. The powders had an average particle diameter of 20,u.

The accumulators used the CrVl/Crill Redox couple. The two tanks were loaded with a solution of 1 M chromium sulfate [ Cr2(S04)3 ] in 10% sulfuric acid. The accumulators was charged by connecting the cathode and the anode respectively to the positive and negative poles of a direct current source and circulating the two solutions in their respective cell compartments with a flow rate of 1000 cm3 per minute per 100 cm2 of electrode surface. The current density during the charging step was 0.1 A/cm2 and the charge was prolonged to correspond to about 40 Whr per kilogram of chromium in the system.

At this point, a resistance load was connected to the accumulators terminals and the accumulator was discharged at a 0.1 A/cm2 current density. The solution flow rate through the accumulator was kept at 1000 cm3/min. per 100 cm2 of electrode surface. The accumulator showed an open circuit cell voltage of 1.7 V while the closed circuit voltage was 1.5 V.

The power density was 5.8 k Wh/m2 and the energy efficiency through the full cycle was about 90%.

Claims (19)

Claims

1. A process of storing and releasing energy in an electrochemical storage cell having an anode compartment and a cathode compartment separated by a semi-permeable, ion-exchange membrane with a solution permeable anode comprising a thin, porous layer of electrically conductive and corrosion resistant powder bonded to the anode side of themembrane and a solution permeable cathode comprising a thin, porous layer of electrically conductive and corrosion resistant powder bonded to the cathode side of the membrane, the process comprising storing energy in the cell by feeding into the anode compartment an anolyte solution of an oxidizable compound capable of remaining substantially soluble in the anolyte solvent and capable of being reduced when in its oxidized from, impressing an electric potential between the anode and cathode to oxidize the anolyte solution, withdrawing the oxidized anolyte solution from the anolyte compartment and storing said oxidized anolyte solution, simultaneously feeding a catholyte solution of a reducible compound capable of remaining substantially soluble in the catholyte solvent and of being oxidized when in its reduced from into the catholyte compartment, withdrawing reduced catholyte solution therefrom and storing said reduced catholyte solution and, thereafter releasing energy from the cell by circulating said oxidized anolyte solution through the anolyte chamber and said reduced catholyte ssolution through the cathode chamber while the resulting electric potential across the anode and cathode drives an electric current through an external load.

2. A process according to Claim 1, wherein electric current is fed to and withdrawn from each of the electrodes at a plurality of points evenly spaced over the surface of the electrode.

3. A process according to Claim 1 or Claim 2, wherein during the storage of energy, the oxidizable anolyte solution circulated is a solution of a soluble salt of a metal having more than one valence state, the salt of said metal being in a lower valence state, and the catholyte solution is a solution of a soluble salt of a metal having more than one valence state, the salt of said metal being in a higher valence state.

4. A process according to Claim 3, wherein the or each metal is chromium, titanium, iron, or vanadium.

5. A process according to any one of the preceding Claims, wherein the electrically conductive and corrosion resistant powder is of at least one of the following materials: platinum, palladium, iridium, ruthenium, rhodium, osmium, alloys and intermetallic compounds of these metals, their oxides, platinum black, palladium black, graphite, acetylene black, oxides of tin, lead, antimony, bismuth, cobalt and manganese, spinel-type electro-conductive oxides, perovskites and delafossites.

6. A process according to any one of the preceding Claims, wherein the electrodes have a thickness of from 0.010 to 0.06 mm and the powder has an average particle size of from 0.5 to 30 ym.

7. A process of electrolytically oxidizing lower valence metal ions to higher valence metal ions in an anolyte solution, wherein the anolyte solution containing lower valence metal ions is caused to flow over an anode surface comprising thin porous anolyte-permeable layer of finely powdered electroconductive and corrosion resistant material bonded to the surface of a semi-permeable membrane separating an anolyte compartment from a catholyte compartment, while an electric potential is applied across said anode surface and a co-operating cathode sufficient to oxidize said metal ions to higher valence metal ions, and oxidized anolyte solution is recovered from said anolyte compartment.

8. A process according to Claim 7, wherein the finely powdered, electro-conductive and corrosion resistant material is of at least one of the following materials:- platinum, palladium, iridium, ruthenium, rhodium, osmium, alloys and intermetallic compounds of these metals, their oxides, platinum black, palladium black, graphite, acetylene black, oxides of tin, lead, antimony, bismuth, cobalt and manganese, spinel-type electro-conductive oxides, perovskites and delafossites.

9. A process according to Claim 7 or Claim 8, wherein the thin, porous, electrolyte-permeable layer bonded to the membrane surface has a thickness of from 0.010 to 0.06 mm and the powder has an average particle size of from 0.5 to 30 #m.

10. A process of electrolytically reducing higher valence metal ions to lower valence metal ions in a catholyte solution, wherein the catholyte solution containing higher valence metal ions is caused to flow over a cathode surface comprising a thin porous catholyte-permeable layer of finely powdered electro-conductive and corrosion resistant material bonded to the surface of a semi-permeable membrane separating a catholyte compartment from an anolyte compartment while an electric potential is applied across said cathode surface and a co-operating anode sufficient to reduce said metal ions to the lower valence metal ions, and reduced catholyte solution is recovered from said catholyte compartment.

11. A process according to Claim 10, wherein the electro-conductive and corrosion resistant finely powdered material is of at least one of the following materials:- platinum, palladium, iridium, ruthenium, rhodium, osmium, alloys and intermetallic compounds of these metals, their oxides, platinum black, palladium black, graphite, acetylene black, oxides of tin, lead antimony, bismuth, cobalt and manganese, spinel-type electro-conductive oxides, perovskites and delafossites.

12. A process according to Claim 10 or Claim 11, wherein the thin, porous, electrolyte permeable layer bonded to the membrane surface has a thickness of from 0.010 to 0.06 mm and the powder has an average particle size of from 0.5 to 30 ym.

13. A redox type accumulator comprising a housing containing at least one cell comprising a cathode compartment and an anode compartment separated by an ion-exchange membrane, a porous permeable layer of electro-conductive, corrosion resistant material on each side of the membrane to form anode and cathode surfaces, means for evenly distributing a direct current over the anode and cathode surfaces, means for circulating an anolyte solution through the anode chamber and means for circulating a catholyte solution through the cathode chamber, said anolyte and catholyte solutions containing metal ions having different oxidation states.

14. An accumulator according to Claim 13, wherein the electro-conductive and corrosion resistant material forming the electrodes is of at least one of the following materials:- platinum, palladium, iridium, ruthenium, rhodium, osmium, alloys and intermetallic compounds of these metals, their oxides, platinum black, palladium black, graphite, acetylene black, oxides of tin, lead antimony, bismuth, cobalt and manganese, spinel-type electro-conductive oxides, perovskites and delafossites.

15. An accumulator according to Claim 13 or Claim 14, wherein the electro-conductive powdered material is bonded on to each surface of the membrane with a resin binder chemically resistant to the cathodic and anodic solutions.

16. An accumulator according to any one of Claims 13 to 15, wherein each porous and permeable layer of electro-conductive material has a thickness from 0.010 to 0.05 mm and the powder has an average particle size of from 0.6 to 30 Mm.

17. An accumulator according to any one of Claims 13 to 16, wherein the metal ions of different oxidation states are Cr2+, Cr3+ and Cr6+.

18. A process according to Claim 1, substantially as described herein with reference to Figures 1 and 2 or Figures 3a and 3b of the accompanying drawings.

19. An accumulator according to Claim 13, substantially as described herein with reference to Figures 1 and 2 or Figures 3a and 3b of the accompanying drawings.