CA1143432A

CA1143432A - Redox process and accumulator

Info

Publication number: CA1143432A
Application number: CA000331386A
Authority: CA
Inventors: Alberto Pellegri; Placido M. Spaziante
Original assignee: Oronzio de Nora Impianti Elettrochimici SpA
Current assignee: De Nora SpA
Priority date: 1978-07-10
Filing date: 1979-07-09
Publication date: 1983-03-22
Also published as: DE2927868A1; JPS5524389A; FR2431196A1; FR2431196B1; IT1212303B; GB2030349A; GB2030349B; IT7825511A0

Abstract

IMPROVED REDOX PROCESS AND ACCUMULATOR
ABSTRACT OF THE DISCLOSURE

It is described a method of storing and releasing energy in an electrochemical cell having an anode compartment and a cathode compartment separated by a semi-permeable, ion-exchange membrane with a solution permeable anode comprised of a thin, porous strata of electrically conductive and corrosion resistant powder bonded to the anode side of the membrane and a solution permeable cathode comprised of a thin, porous strata of electrically conductive and corrosion resistant powder bonded to the cathode side of the membrane comprising 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 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 therefrom and storing said reduced catholyte solution and thereafter releasing energy from the said cell by circulating said oxidized anolyte through the anolyte chamber and said reduced catholyte through the cathode chamber while the resulting electric potential across the anode and cathode drives an electric current through an external load; a novel accumulator and novel methods of electrolytically oxidizing and reducing multivalent metal ions in an electrolyte solution.

Description

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I STATE OF THE ART
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¦ It i5 well-known that an accurnulator can be made utilizing reduction and oxidation electrochemical reaction carried out s~bstantially in liquid phase with soluble salts of metals having different oxidation states. Eor instance, a chromiurn redox accumulator works according to tne following electrochemical o l reaction:
charge step 4Cr2(S04)3 + 7HzO-- ~6 CrS04+~2Crz07 + 6Hz504 I discharge step: 6CrS04 -~ H2Cr207 * 6 HzSo4~4Cr2(s04)3 ~ 7 H20 ;~~ with an open circuit c~eIl voltage of about 1~ 74 V. An il;on-titanium redox accumulator works according to the following electrochetIlical reactions:
cnarge step O FeC12 ~ TiC13 _ -~ FeC13 + TiC12 discharge step FeC13 + TiC12~FeC12 + TiC13 with an open circuit cell voltage of about 1. 14 V.
Redox accumulators have a basic advantage in tnat the reaction occurs entirely in solution with no chemical compc~unds depositing on or dissolving from the electrodes during botn the ¦ chargir~ and discharging stages. Moreover, no gases are released ¦ at the electrodes. Therefore, those problems commonly encountered in othex types of accumulators such as metallic deposit morpnology ~5 ¦ on electrodes, gas evolution at the electrodes with associated bubble effect, gas withdrawl and storage ~re avoided.
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¦ Unfortunately, the redox accwnulators as now con-celved are rather bulky and unsuitable for making accumula-¦ tors with high specific power. Furtherr(lore, the presence ¦ o~ a separator or membrane derining the two cell compartnlent I and the necessity, ~or mass transrer considerations~ of a ¦ rapid electrolyte rlow across the electrodes to avoid para-j sitic electrodic reactions such as hydrogen evolution at the ¦ anode or chlorine and oxygen evolution at the cathode requlr~
¦ such electrodes and electrodic gap conrigurations that the l cells are quite bulky. Moreover, ~he ohmic drops in the electrolyte are quite relevant and contrlbute considerably to lowering the erficiency Or the accumulator.
¦ In known accumulators, the elernentary cell ~unda-¦ mentally consists of two compartments separated by a micro-Iporous~ inert separator or preferably by an ion-exchange ¦membrane. Generally, the electrodes in the compartments are ¦placed a certain distance from the separator or mernbrane Isurface so that the electrolytes can be circulated directly ¦in the spaces between the electrode surface and the membrane ¦sur~ace. Often porous electrodes are used and the electro-¦lyte is flowed through the electrode. ~herefore, t}le inter-¦electrodic gap is necessarily significant with a consequent high ohmic drop in the electrolyte.

OBJECTS OF THE INVENTION

! It is an ob~ect o~ the invention to provide a _ _ _ _ _ _ ¦n~w an6 im ~ ved r-do- ccumulal G wlth hlgh p~wer density . ' , , ,. ~ . , '.
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and low ohmic losses. It i9 another object of the invention to provide an improved redox accumulator wherein the electrodes are in contact with, preferably bonded to, the membrane. It is an additional object of the invention to provide a novel, improved method of storing and/or releasing energy in an electrochemical cell.
Further objects and advantages of the lnvention will become obvious from the following detailed description.

SUM~RY OF THE INVENTION
_ The present invention provides a method of storing and releasing energy, which comprises storing energy by feeding an anolyte solution of an oxidable compound capable of remaining substantially soluble in the anolyte solvent and of being reduced when in its oxidized forrn, into the anode compartment of an electrochemical 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 strata of electrically conductive and corrosion resistant powder bonded to the cathode side of said membrane, impressing an electric potential between anode and cathode to oxidize the anolyte solution, withdrawing the oxidized anolyte solution from the anolyte compartment and storing said oxidized anolyte solution, feeding a catholyte of a reducible compound capable of remaining substantially soluble in the catholyte solvent and o being oxidized when in its reduced form, into the catholyte compartment, withdrawing reduced catholyte there-from and storing said reduced catholyte solution and thereafter releasing said energy by circulating said oxidized anolyte : ~ .
dm~ 4 -, through the anolyte chamber and said reduced catholyte through the cathode chamber, while the resulting electric potential across the anode and cathode drives an electric current through an external load.
The present invention also provides for an improvement in the method of electrolytically oxidizing lower valence metal ions to higher valence metal ions in an anolyte solution comprising flowing the anolyte solution containing lower valence metal ions over an anode surface comprised of thin porous anolyte-permeable layer of finely powdered electroconductive and corrosion resistant material bonded to the surface of a semipermeable membrane separating the anolyte compartment from the catholyte compartment, while applying an electric potential across said anode surface and the cooperating cathode sufficient to oxidize said metal ions to higher valent metal ions and recovering oxidi2ed anolyte solution from said anolyte compar~ment.
The present invention also provides for an improvement in the method of electrolytically reducing higher valence metal ions to lower valence metal ions in a catholyte solution compri.sing flowing the catholyte solution containing higher valence metal ions over a cathode surface comprised of a thin porous catholyte-permeable layer of finely powdered electro-conductive and corrosion resistant material bonded to the surface o a semi-permeable membrane separating the catholyte compartment from the anolyte compartment while applying an electric potential across said cathode surface and the cooperating anode sufficient to reduce said metal ions to the lower valence metal ions, and recovering reduced catholyte dm~ ~ 5 ~
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The electrodes are in direct contact with the surfac~s of the ion exchange membrane and are made of a thin, porous permeable layer of a finely powdered, electroconductive, corrosion resistant material secured to the membrane surface.
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 the electrodes and the grids are preferably electrically connected to the back plates of the electrodic compartments.
The electrolyte is continuously flowed in contact with the surface of the porous and permeable electrode embedded on the surface of the cell separator, that is the semi-permeable membrane.
Supply of the multivalent 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 substantiaIly at the interface between the particles and the ion exchang~
resin constituting the membrane onto which they are embedded.
Therefore, the ohmic drop in the electroyte is substantially eliminated.
Preferably, both the anode and the cathode are similarly constituted by a thin layer of the electrically conductive, powdered material bonded ont~ the opposite sides of the ion exchange resin membrane, althouyh only one of the electrodes may be so constructed while the ~ operating electrode may be a foraminous conventional self-supportin~ metal or graphite electrode, spaced or pressed against the other side of the membrane.

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.` ~ 32 ~ , . ' ' . .' ¦. The redox type accumulator of the inventiorl :I.s . comprised of a housing containing at least one cell com- ' prised of a cathode compartment and an anode compar~Ment . separated by an ion-exchange membrane, porous permeable .
layers of electroconducti.ve, corrosion reslst;ant mal;erial .
on each slde of the membrane to form anode and cathode surfaces, means for evenly distributing a direct current on the anode and cathode surf'aces, means for circulating anolyt solutions through the anode chamber and means for circulatin catholyte solution through the cathode chamber~ said anolyte and catholyte containing metal ions having different oxida-tion states.
. I In a pre~'erred embodlment Or the accumulator of . u ¦ the invention, a number of elementary cells are arranged in .
¦ series and are separated by b:lpolar partitions consisting Or ¦ an electroconductive wall provided on both sides with pro-¦ jections suitable to contact, on a multipilicity of points, ¦ the anode fixed on the membrane surface of one of the elemen . ¦tary cells and the cathode fixed on the membrane surface of-I one of the elementary cells and the cathode fixed on the . ¦surface of the adjoi.n~ng cell. ' . ¦ TAe anolyte and catholyte solutions may be con-' ¦tained in t~ro respective tanks and can be circulated through ¦the anodic and cathodlc compartments by two circulation pump Therefore, the electrolyte solutions will flow along the sur faces of the porous and permeable electrodes fixed on the . ,, , .
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membrane surfaces so that the oxidation and reduction elect~
( rodic reactions occur substantially at the interface of the electrodic particles and the ion-exchange membrane itself.
¦ In this way, the ohmic drops both in t~le anolyte and the ¦ catholyte solution are substantially eliminated. Moreover, ¦the high specific sur~ace of the electrodes which are con~
I stituted by the thln layer of powdered rnaterial bonded to ¦ the membrane with respect to the apparent surface effectivel ¦ helps to reduce polarization and permits the use of high I current densities.
¦ Whereas the novel reatures which are peculiar to the invention are set ~orth in the appended claims, the ¦ in~ention, both in its essence and its practical embodiment, ~ ¦is herelnafter desc~ibed in detail and its understanding can ( 15 ¦be made easier by reference to the attached figures.
¦ The preferred redox couples which can be used in ¦the storage battery of the invention are chromium-chromium~
¦titanium-iron, vanadium-vanadium and vanadium-iron.
Fig. 1 is a ~chematic outline of a complete arrangement of a redox accumulator of the invention.
Fig. 2 is an enlarged partial cross-section of a ¦basic cell of the accumulator of Fig. 1 illustrating the ¦reactions occurring.
¦ Fig. 3a is an exploded cross-section of a typical embodiment of the accumulator Or the invention, and Fig. 3b is a cross-section of the assemb~.ec~ accumu-~ lator of Fig. 3a.

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¦ ~c~erring to I~`lg. 1~ the complete system Or the ¦ redox accumulator inc]udes at least one elementary cell ( ¦ generically labeled as 10 compr-lsed Or an ion exchange resln ¦ membrane 11 on the sur~ace of which are incorporated the ¦ electrodes made of electroconductive and chemical corrosion . ¦ resistant powdered materials. Examples of suitable materia-L
¦ are the platinum group metals, their alloys or intermetallic ¦ compounds, their conductive oxldes, platinum black, palladiu ¦ black, oxides o~ tin, lead, antimony, bismuth~ co~alt and ¦ manganese, conduct~ve oxides such as splnel-type oxides, ¦ perovskites and delarossites, grap~ite and acetylene black.
¦ The anode 12 is bonded on one sur~ace of the membrane and the cathode 13 is bonded on the opposite sur~ac lor the membrane. Metallic grid-shaped current collectors .~ ~ and 15 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 valves of the cell 16 and 17 are electrically insulated l from each other by the insulating gasket 18. When the systen ¦ is in operation, the anolyte soIution contained in the tank ¦ 19 is continuously circulated through the anode compart~.-nt ¦ by recycle pump 110 and the catholyte solution contained in ¦ tank 111 is continuously circulated through -~he cathocie com-~partment by recycle pump 112.
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~ 3~La2 , ., . The cell ele'ctrodes are chemically inert, but act . respectlvely as acceptor and donor of` electrons to procluce ( a potential dependent upon the concentration of lons in the solution w~lich they are in contact ~Jith. ~or instance, to disc~arge a previously charged chromium accumulator, the hexavalent chroMium contalning catholyte solution contained in tank 111 is circulated into the cathode compartment and the blvalent chromium containing anode solution contained in tank 19 is circulated into the anode compartment. A reduc- .
tion of hexavalent chromium to trivalent chromlum occurs at .
the cathode l~hile S~4-- ions are released and migrate throug~ .
. the anlonlc membrane to reach the anode where blvalent chromium is oxldlzed to trlvalent chromium. Therefore~ the ~- concentrations of t`he anolyte and the catholyte sol.utions .~1.5 will finally equalize themselves and the cell pQtential will .
become zero. . .
: . To charge the'accumulator,.the anode and the cathode are:connected respectively to the negative and p'osl~
tive pole of a direct current electrical source and the . -reactions are reversed, thus restoring a high concentratlon' ~:
of hexavalent chromlum with respect to trivalent chromium in the catholyte solution and converserly a high concentra-tion of bivalent chromium with respect to trivalent chromium in the anolyte solution. ' ~,. .. .

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¦ Fig. 2 schematically illustrates a cross section o~ the accumulator elemertary cell consisting of the ion-exchange resin membrane 21 to ~hich the anode 22 and the cathode 23 are bonded. The two metal screen current collec~
tors contact the electrodes at a multlplicity of points5 evenly spaced over the entire surrace of the said electrodes.
The f:Lgure is intended to particularly help understand the invention and the operation of the accumu:Lator o~ the inven-tion. ~ chromium storage is illustrated and the indlcaked reactions concern the discharge of the accumulator. The membrane 21 is of the anionic type and is reslstant to sul-furic acid solutions and is prererably made Or rluorinated resin containing quaternary amrnonium groups.
Electrodes 22 and 23 pre~erably con~ist of a ; ;C15 powdered mixture Or graphite and plat:lnum b:Lack bonded onto the membrane surface to form a porous layer easily permeable to the electrode. The current collectors 24 and 25 are screens, pre~erably made o~ a valve metal such as niobium, tantalum, titanium, zirconium and hafnium and most pre~erabl~
galvanically coated with pla-tinum or platinum-iridium alloy.
The current conductors 24 and 25 are pressed against elect-rodes 22 and 23 and are connected to the electrical load o~
the battery.
The hexavalent chromium containing catholyte solution ls circulated ~nto the cathode compartment ln such a ~ay that the solution will flow along the porous cathode ( ¦ surrace and the curr t colleotor 25 is actunlly roraminous ¦

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enough not to obs~ruct the flow Or the solution along the cathode 23 surface. This favors a good mass transfcr to and (~ rrom the electrode surface and prevents depletlon c~ the reacting lonic species at the electrode. The bivalent chro~
mium containing anolyte solution is likewi.se circu].ated into .
the anode compartment to contact the porous anode 22 sur~ace ¦ Thus, the two electrodes assume their respective potentlals which are dependent upon the ratio of the concentratiorls of : hexavalent and bivalent chromium ions in the respective solutions. :
During discharge of the accumulator thus f'ormed~
electric current flows through the electrical load connected to the cell terminals~ and the following reactions occur~
at the cathode (positive pole):
Cr2072 ~ -~ 14H ~ I- 6e ~ ~ 2 Cr3 ~ ~ 7H20 .-ionic transfer through the membrane (~rom the cathode to the anode side):
12 S0112 ~ ~--~ 12 S04 2 a-t the anode tnegative pole):
6 Cr2 ~ 6 Cr3 ~ + 6e ~ :
Overall reaction:
. H2cr2o7+6cr~sol~ ~6~l2S4 3 1 2~ 4 3 2 Con~ersely~ during the recharge of the accumulator . ¦ the following reactlon take place:

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at the cathode: 6Cr~ 6e~ _ ~6Cr~l ionlc transf~er through the membrane (from the . .' ( c~nodic side to the cathodlc side~
12SO~ .' at the.anode: 2Cr -~ 7 H20 - -~ Cr207~~ +
14 H~ + 6e-Overall reaction:
7 H20 * IICr2(SOI~)3 7 6CrS0~ 2 2 7 , , , 1 6H2Sol~

The anodic and eathodie reactions practically OCC-I
. at the interface between the parti.cles eonstituting the electrodes and the ion exchange resin membrane, that is the ~ ;i- catalytic sltes in ~Fne electrodes are in direct contact with ;~ (. the ion exchanging radicals of the resin membrane. Thus, : 15 ionic conduction in the solution is substantially eli~inated . 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 electl~olyte renewal : 20 over the entire electrodic surface, thereby avoiding polariz . ation effects and the multiplicity of electrical contacts . ~ . with the current collectors affords low ohrnic' drop in the . electronic conductive structures of the cell, that is in the porous particulated electrodes bonded to the membrane. .
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F~gs. ~a and 3b illustrate a practical embod:Lment Or the accumulator of the invention. Fig. 3b i.s a plan vlew of the acc~lmulator descrlbed ln Fig. 3a wlth the same parts Or the accumulator being labeled with the same numbers in .
both figures. .
The accumulator consi.sts o~ an anode end plate 31 a cathode end plate 32~ a bipolar partition 33 and tWo mem-branes 31la and 34b on whose surfaces the electrodes have been bonded. The accumulator is -thus composed of two elemen-tary cells connected in series, and it is intended that anearly unlimited number o}' similarly connected elementary cells can be included bet~een the two end plates by insert-lng a certain number of bipolar partitions and the elect--rodes b~aring memb~nes between said plates. Preferably, also the anode 31 and cathode 32 end plates are made of the same material as the bipolar parti~ion 33. m e 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 ~nner surface with a central grooved area 38whose grooveC
39 are hydraulically connected to the catholyte inlet 311 and outlet 312 by connecting grooves 310. Both end plates are provided with means 313 f'or the electrical connection Or~the battery. The bipolar partition 33 i5 provided on both sides with a central grooved area whose grooves 314 on the ~a-hode :lde Qre nected With tw~ hole~ 317 and 3l6 ~' . ~ . .
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3~32 coaxial with inlet 311 and outlet 312 of' ~he cathode end plate respectiYely, ~hile grooves 31'1 on the anode side of ( ¦the blpolar parti.tion are in connection ~ith two holes 318 ¦(shown in Fig. 3a only) and 319, coaxial with anolyte inlet 36 and outle~ 37 of' the anode end plate, respectively.
¦ ~he bipolar partitlon and the two end plates pre-~erably consist o~ a conductive aggregate made of' powdered ¦graphite and a chemically inert resin. The membrane 34a o~
the ~lrst elementary cell consists Or an ion exchange resin ¦sheet on whose central area, corresponding to the grooved l jareas of the anode end plate and the cathode side Or the - I bipolar separator 33, the electrodes have been incorpor-ated ~ith anode 320 on the side racing the anode end plate and ¦the cathode 321a on the side facing the cathode side of the ~,; I .~ , f 15 l bipolar separator 33, both electrodes being made-of an elec-¦troconductlve and corrosion resistant powdered material.
Membrane 34b of the second elementary cell similarly consists ¦of an ion exchange resin sheet on ~Ihose central area corr~es-I ponding to the grooved areas of the anode side of the bipolar l partition 33 and of the cathode end plate 32, anode 320b and the cathode 331b have been incorporated, respectively.
¦ Suitable materlals ~or the anodes 320a and 320b . land the cathodes 321a and 321b 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 oxldes o~ tin, lead, antlmony, bismuth, cobalt and manganese, conductive oxides such as ~. . . .
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spinel-type oxides, perovskites, dela~oBsites~ palladium black, platinum black and acetylene black. Both membranes 34a and 3llb are respectlvely provided with four holes (6a), 37a, 311a, 312a and (6b) 37b, 311b, 312b, coaxial vith the .
holeB 36, 37, 311, 312 of thc anode 31 and cQthode 32 end plates, respectively. The holes (6a~ and (6b), not shown in the ~lgures, are coaxial ~Jith the holes 36 of the a.node end plate 31 and the holes 318 Or the bipolar separator. .
The anode end plate 31 and the cathode end plate .

32, membranes 34a ancl 34b and bipolar partition 33 are provided t~ith a series Or 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 ln the .
latter case, an in~.~lating plastic coàting 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. ~he 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 plates 31 and 32 ...

and the bipolar separator 33 are hydraulically sealed against the peripheral areas of membranes 34a and 34b.
A11 the surfaces of the bipolar separator and the two end plates~ with the exception Or the surfaces contact-ing the electrodes, are preferably insulated by a thiclc laye of.resln not char~ed ~lith electroconductive po~dered material Thls prevents electrodic reactions from occurring at the sur faces contactin~ the electrodic solution or the lon exchange ..

resin for instance in correBpondence Or the seal surface ..... _- I . .
I o~llng to ions ~Sratin~ tl~ ~out tlhe n:srbrane l~sln.
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~ ~¦ ' The proJections of the central grooved area of the ;' anode end plate 31 e].ectrically contact anode 320a ( incorporated in the membrane 3lla and the projections ~1' t,he central grooved area of the bipolar partition 33 on the cathode side electrically contact cathode 321a bonded on membrane 31la while the anode side projections of separator 33 contact the anode 320b and those of cathode end plate 32 contact cathode 321b. ~ series of elementary bipolar cells is thus formed and their number can be augmented at ~rill by inserting more bipolar elements in said series.
~hen the accumulator is working, the anolyte solu-tion is pumped into the cell through anode end plate inlet 36 and the electrolyte distributes into the first anode com~
partment derined by the grooves in the central æone of anode ( end plate 31, then, through the passage defined by hole 36a in membrane 2lla and hole 318 in bipolar partition 33, both holes being coaxial ~ith inlet 36, the electrolyte flows int the second anode compartment defined by the grooves of the central area of the anode side Or bipolar separator 33. The.
electrolyte leaves thecell through outlet 37 which, through hole 37a in membrane 34a and hole 319 in bipolar separator 33, is in communication wlth the second anode compartment.
~he catholyte solution is likewise circulated through the cathode compartments through inlet 311 and outlet 312 o~ , z5 j the oathode end pl~te . I

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r'"~''' ,' , , '''' ",,. ~, - . , : . ' ' Various procedllres may be followed to form and incorporate the electrode onto the membrane surface. According to one known method, the powder of electroconduc-tive material ls 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 sa'tisfactorily is produced by DuPont under the commercial name of Teflon -T-30 but other resins unaffe'cted by the electrolytes used in the storage battery may be used instead of polytetrarluoroethylene (PTFE). The mixture of powders is placed in a mold and lleated to a sintered film which is then transferred to the membrane surface and hot-pressed to the same.
In another method the electrodes are formed hy application to the membrane surface of a liquid suspension of electroconductive powder iTI an ion-exchange resin solution, sai'd resin being compatible with the resin constituting the membrane. The suspension is then dried and heated to deposit and bond the powder con-taining 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 ~m and preferably between 10 and 20 ~m. The electroconductive material loading is between 0.5 and 10 mg/cm~ and preferably between 1 and dm~
' Il . , , , ' 5 rng/em2 of electrode surface. 'rhe thlckness of the eleet-¦ rodes fixed on the membrane surf'ace is less t;han 0.1 r~n and ( preferably bet~een 0.010 mm and 0. o6 mm.
The thinness of the electrodes ~ogether with their .- ¦ porosity is of importance as lt has been in fact f'ound that I by reducing the electrodes thlckness, side reactions are effectua],ly hindered at the electrodes. Ttlis can be under- .
¦ stood on the basls that the thinner and more permeable the ¦ electrodes are, the easier the transfer.of the.ions support-¦ ing the main electrodie reaction is made from the electro-¦ lyte bulk to the ca'talytie sites in the eontact areas betwee ¦ the elee~rode partie].es and the ion-exehange resin eonstiut- . ' ¦ ing the membrane. In this way, the diffusion of the ions supporting the ma.;.n eleetrode reaction through the so-cal'led 15 electrode double la~er is aided and in practice~ this leads (, ¦ to a lower polarization of the eleetrodes, and ~herefore to ¦ an improved voltage/eurrent eurve during the diseharge of ¦ the accu~ulator and also to greater energy efficieney. The lower electrical plane eonduetivity of the eleetrodes made ¦ extremely thin for the aforesaid reasons i5 effeetively eom-p.,ensated:..for'by inereasing the density of the contact points with the current eollectors whieh can be aehieved by the use, for instanee, of very fine mesh grids. Coating of the eurrent conduetors eo~taet areas with platinu~ further reduees ohmie drops.
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¦ ~he membrane may be elther anionic or cationic~
depending on the nature Or t~le ~edox couple in the solutions ( and consequently on the klnd Or ions which ~ust pass through ¦ the membrane. In fact, the ionic conduction through the 5 ¦ membrane may be due to the transfer of anions such as S04 ¦ or Cl or cations~typically H+.
¦ Particularly well-suited cationic membran~ consist ¦ of a M uorine-carbon polymer molded 1nto thin sheets and ¦ containing acid radicals such as sulfonlc acid or carboxylic acid groups. In the membranes containing sul~onic or carboxylic acid groups, the ion-exchange groups are the acid hydrated S03H . H20 radicals inserted in the polymeric struc-¦ tures by sulronation. ~he ion-exchange groups are not mobile ~- within the membrane as they are chemically bondecl to the polymeric chain so that their concentration is constant.
A speci~ic class of such cationic membranes particularly resistant to acids and oxidizing agents is made ¦ by DuPont under the commercial name oY Na~ion*and these membranes consist o~ hydrated copolymers of polytetrafluoro-20 ¦ ethylene and vinylether sulfonated poly~luoride containing ¦ sulfonic groups. Other suitable cationic membranes may be ¦ made of a styrene-divinylbenzene sulfonated copolymer, pre-¦ ~erably supported by an inert and porous substrate such as ¦ asbe~tos paper or made o~ a tetrafluoroethylene~acrylic acid 25 I sul~onated copolymer.
Parti~ularly suitable anionic membranes are made ! of a thin flim o~ fluoro-carbon polymer or styrene-divlnyl-( ¦ benzene copolymer containing basic radicals such as, ~or examplej quaternary ammonium groups or pyridine or substituted ¦ pyridine groups. Such membranes are produced by lonics. .
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, The,membrane must allow the rnigratlon of a particular ! type of ion (e.g. Cl- or S011 or ~1 ) while it must c~fec-¦ tively prcvent or minirnize the passage ~lnd the mixture Or ¦ other ionlc specles through the membrane. The thickness of I the membrane is generally on the order of several tenths of ¦ millimeters. In the case of cross-lin~ed thermosetting resins such asstyrene-divinylbenzene copolymers copolyrnerized on an inert support such as asbestos paper, the membrane I thickness may be one or more rnillimeters.
I The charging or discharging process respectively Or the accumulator connected to an external electrica:L load I or to a direct current source can be respectively carried ¦ out by lntroducing the catholyte solution lnto the cathode j compartrnent and the anolyte so]ution into the anode'cornpart-~5 ¦ ment and clrculating said solutions through thelr respective tank and cell compartment. The rlow rate is preferably between 100 and 5000 cm per minute per:l,000 cm2 of electrod surface for 0. o8 A~cm2 as the electrlc current drawn by the battery load.
In the following example there is described several prererred embodiments to illustrate the invention.
However, lt should be understood that the lnvention is not intended to be limlted to the specl~lc embodlments.

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ll EXAMPI,E

( I Test accumulators accordin~ to Flgs. 3a and 3b wer~
fabricated and installed as indicated in Fig. l. The elect-rodes consisted Or a powdered mixture Or graphite and platinum black in a 9:l ratio by weight bonded onto the sur-¦ ~ace Or an IONAC 3475 M type anionic membrane. The elect-j rodes had a thickness of about 0.05 mm which corresponded to a powder charge Or between 2 and 3 mg/cm2. The powders had I an average particle diameter of 20 ,u.

I The accumulators used the Cr I /Cr~II Redox couple The two tanks were loaded with a solution o~ l M chromium sulfa~e rCr (SO4) ] in l0~ sul~uric aGid. The accumulators was charged by connecting the cathode and the anode respec-¦ tively to the positl~e and negative poles o~ a direct curren C 15 l source and circulating the two solutions in their respective ¦ cell compartments ~ith a flow rate of lQ00 cm3 per minute l per l00 cm of electrode surface. The current density duri ¦ the charging step was 0.l A/cm2 and the charge was prolongedto correspond to about 40 Whr per kilogram Or chromium in 20 the system.
At this point, a resistance load was connected to the àccumulntors terminals and the accumulator was discharged at a 0.l A/cm current density. The solution flow rate l through the accumulator was kept at l000 cm3~min. per l00 ¦ cm2 of electrode surface. The accumulator showed an open circuit cell voltage of l.7 V while the closed circuit voltage was l.5 V.

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The power densit~y was 5.8 Ic Wh~m2 and the energy erficiency ~, ( through the ~ull cycle was.about 90%.
:,~ . Variou~. modiflcations Or the accumula~or of the :~
invention may:be made without departi.ng rrom the spirit or:
scope thereor and it is to be understood that the lnvention .
.. is to be limited only as de~ined in the appended:claims.
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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AM EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of storing and releasing energy in an electrochemical cell having an anode compartment and a cathode compartment separated by a semi-permeable ion-exchange membrane, an anode in said anode compartment and a cathode in said cathode compartment, which comprises storing energy 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 and 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, while impressing an electric potential between the anode and cathode to oxidize the anolyte solution and to reduce the catholyte solution, withdrawing the oxidized anolyte solution from the anode compartment and storing said oxidized anolyte solution, withdrawing the reduced catholyte solution from the cathode compartment and storing said reduced catholyte solution, thereafter releasing energy in said cell by circulating said oxidized anolyte solution through the anode compartment and said reduced catholyte solution through the cathode compart-ment while the resulting electric potential across the anode and the cathode drives an electric current through an external load, the method characterized by:
a) bringing the electrolyte solutions into contact with porous, electrolyte permeable particulate anode and and cathode electrodes bonded to and embedded in the membrane on the side facing the respective anode and cathode compartments, whereby the catalytic sites in the electrodes are in contact with the ion exchanging sites of the membrane so that electrodic reactions take place directly at the membrane-electrode interfaces;
b) supplying to and collecting current from the electrodes through current collectors in physical contact with the electrodes at a plurality of points evenly spaced over the surface of said electrodes.

2. The method of claim 1, wherein the porous, electrolyte permeable, particulate electrodes bonded to and embedded in the membrane have a thickness between 0.010 and 0.06 mm.

3. The method of claim 1 wherein electric current is fed to and withdrawn from the electrodes at a plurality of points evenly spaced over the surface of said electrode.

4. The method of claim 3 wherein during the storing energy cycle, 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.

5. The method of claim 4 wherein the metals are selected from the group consisting of chromium, titanium, iron and vanadium.

6. The method of claim 1 wherein the electrically conductive and corrosion resistant powder is a powder of at least one of the materials selected from the group consisting of platinum, palladium, iridium, ruthenium, rhodium osium, their alloys and intermetallic compounds, their oxides, platinum black, palladium black, graphite, acetylene black, oxides of tin, lead, antimony, bismuth, cobalt and manganese, spinel-type electroconductive oxides, perovskites and delafossites.

7. In a method of electrolytically oxidizing lower valence metal ions to higher valence metal ions in an anolyte solution, the improvement comprising flowing the anolyte solution containing lower valence metal ions over an anode surface comprised of thin porous anolyte-permeable layer of finely powdered electroconductive and corrosion resistant material bonded to the surface of a semipermeable membrane separating the anolyte compartment from the catholyte compart-ment, while applying an electric potential across said anode surface and the cooperating cathode sufficient to oxidize said metal ions to higher valent metal ions and recovering oxidized anolyte solution from said anolyte compartment.

8. The method of claim 7 wherein the finely powdered, electroconductive and corrosion resistant material is selected from the group consisting of platinum, palladium, iridium, ruthenium, rhodium, osmium, their alloys and intermetallic compounds, their oxides, platinum black, palladium black, graphite, acetylene black, oxides of tin, lead, antimony, bismuth, cobalt and manganese, spinel-type electroconductive oxides, perovskites and delafossites.

9. The method of claim 7, wherein the thin, porous, electrolyte-permeable layers bonded to the membrane surface have a thickness between 0.010 and 0.06 mm and the powder has an average particle diameter between 0.5 and 30 µm.

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

11. An accumulator of claim 10 wherein the electro-conductive and corrosion resistant material forming the electrodes is selected from the group consisting of platinum, palladium, iridium, ruthenium, rhodium, osmium, their alloys and intermetallic compounds, their oxides, platinum black, palladium black, graphite, acetylene black, oxides of tin, lead antimony, bismuth, cobalt and manganese, spinel-type electroconductive oxides, perovskites and delafossites.

12. An accumulator of claim 10 wherein the electro-conductive powdered material is bonded onto the surface of the membrane with a resin binder chemically resistant to the cathodic and anodic solutions.

13. An accumulator of claim 12 wherein the porous and permeable layer of electroconductive material bonded onto the membrane surface has a thickness between 0.010 and 0.06 mm and the powder has an average particle diameter between 0.5 and 30 µm.

14. An accumulator of claim 10 wherein metal ions of different oxidation states are Cr2+, Cr3+ and Cr6+.