WO2012042288A1

WO2012042288A1 - Frameless electrochemical cell stack having self centering rigid plastic bushings in aligned through holes of interconnects and membrane assemblies

Info

Publication number: WO2012042288A1
Application number: PCT/IB2010/002499
Authority: WO
Inventors: Krisada Kampanatsanyakorn; Suradit Holasut
Original assignee: Krisada Kampanatsanyakorn; Suradit Holasut
Priority date: 2010-10-01
Filing date: 2010-10-01
Publication date: 2012-04-05

Abstract

A frameless multi-cell electrochemical stack, filter-press sealed between two end headers, comprising an alternate succession of planar, electrically conductive inter-cell interconnects and membrane assemblies having aligned through holes along perimeter portions for composing inlet and outlet manifolds (4-3; 3-4,...) of parallel flow or ziz-zag flow paths of serial flow, respectively, for circulating distinct electrolyte solutions in respective flow compartments of all the cells, separated by the permionic membrane (M) of a membrane assembly sandwiched between two electrically conductive inter-cell interconnects (I), the perimeter portion of the membrane (M) being held between rear planar surfaces of two elastomeric gaskets (G1, G2) assembled back-to-back and similarly bas-relief patterned over their front surfaces for defining separate flow paths (6) of the electrolyte solutions in respective flow compartments coinciding with a central window of one and of the other of said two bas-relief patterned gaskets, has simple rigid plastic bushings (13, B) in the through holes of the planar electrically conductive inter-cell interconnects (I) and in the aligned through holes (4-3, 3-4), of the two back-to-back assembled, bas-relief patterned elastomeric gaskets of each membrane assembly for cooperatively providing a self-aligning function (13g+Cr1) and for implementing an O-ring type seal function (Or+Cr2) that enhances the hydraulic seals of the back-to-back assembled gaskets within the aligned through holes (4-3, 3-4,...) thereof. Optionally, counter-opposed comb like strip inserts (8s) of rigid plastic along inlet and outlet sides of the cell compartments improve the hydraulic seal on the permionic membrane sandwiched between the back-to-back assembled gaskets.

Description

"FRAMELESS ELECTROCHEMICAL CELL STACK HAVING SELF CENTERING RIGID PLASTIC BUSHINGS IN ALIGNED THROUGH HOLES OF INTERCONNECTS AND MEMBRANE ASSEMBLIES"

5

TECHNICAL FIELD

The present disclosure relates generally to redox flow battery systems employing multicell stack reactors.

10 BACKGROUND AND DISCUSSION OF PRIOR ART

The so-called redox flow battery systems or briefly redox batteries, store energy in acid electrolyte solutions, namely a positive and a negative solution, that are flown through respective electrode compartments of the cells of a multicell electrochemical reactor during charge and discharge phases. The unlimited

15 possibility of storing large volumes of positively and negatively charged electrolyte solutions containing ions of the so-called redox couple, make these systems exceptionally suitable for load-leveling (peak-shaving) in electric power generation and distribution industry, as storage battery in self standing wind farms or solar photovoltaic conversion plants as well as for powering vehicles. Redox

20 flow battery systems may employ either a multi-cell bipolar or monopolar stack.

In any case, when dealing with extremely large power ratings as would be needed in renewable source energy plants requiring large energy storage capacities, in peak-shaving applications and alike, the active cell area (width of the flow compartments hydraulically separated by a permionic membrane and of 25 the electrodes of opposite polarity contained therein) must be as large as possible.

In the prior patent applications No. PCT/IB2010/001651 (filed on June 29, 2010), No. PCT/IB2010/002004 (filed on August 13, 2010) and No. PCT/IB2010/002231 (filed on September 7, 2010), Redoxl l, 12 and 13 of the present applicants, a particularly effective multi-cell stack architecture is described that does not employ any plastic frame but only substantially planar electrical inter-cell interconnects (that may even work per se as either monopolar or bipolar electrodes) of substantially homogeneous electrical conductivity having a perimeter that exceed the outermost perimeter of any other stackable elements and whenever useful for the particular application may have a protruding "lug portion" that projects beyond the outer perimeter side of the other stacked elements, which, therefore, may have an externally contactable area sufficiently large for the power (current rating) of the electrical tap (that, in case of a bipolar cell stack, would be at an intermediate voltage relative to the voltage difference between the end terminals of the stack), to be electrically connected to an external circuit. Consequently, also the planar inter-cell interconnects have through holes that provide continuity of internally defined manifolds or ducting for a parallel or serial flow of the two electrolyte solutions.

In case of internal manifolds for parallel flow embodiments, the hole surface and planar surfaces of the conductive interconnect in the perimeter area of abutment in hydraulic sealing with a perimeter elastomer gasket are rendered electrically non conductive by a hole lining and surface coating of an insulating material. Electrical isolation of surfaces in contact with electrolyte solutions may be established by inserting a lining ring of a suitable plastic material, for example a lining ring of polyvinyl chloride (PVC) inside the through hole and thereafter coating the perimeter areas that will be exposed to contact the electrolyte solutions on opposite sides of the planar electrical interconnect, with an adherent film of a suitable plastic material glued or hot laminated thereon to bond onto the end surfaces of the lining ring and onto said perimeter areas.

Accordingly, a substantially frameless multi-cell bipolar or monopolar electrochemical reactor may be composed of stackable elements between two headers, hydraulically sealed by compressing (in a filter-press fashion) the stacked elements, including planar electrical conductive inter-cell interconnects alternated to permionic membrane assembly fixtures adapted to separate anodic and cathodic solution flow compartments defined between said permionic membrane separator and one and the other of the inter-cell interconnects and through which the two distinct electrolyte solutions are respectively flown, comprising two identical parallelepiped elastomer gaskets defining a central aperture and having at least two through holes along two opposite perimeter sides thereof, a fiat perimeter seal surface on a backside and on the opposite or front side thereof: a bas-relief patterned perimeter seal area contouring one every two through holes along said two opposite perimeter sides and two similar pluralities of patterned seal areas defining there between flow channels extending from a rim region of non- contoured through holes to the edge of the nearest juxtaposed side of said central aperture of the gasket.

The permionic membrane has its perimeter edge portions sealingly held between a substantially flat perimeter seal surfaces of the two identical gaskets, disposed back-to-back.

FIG.l of the above cited prior patent application PCT/IB2010/001651 is an exemplary embodiment of the basic frameless membrane assembly of the stack architectures described in the above cited prior _^patent applications of the same applicants, and is replicated as FIG. 1 of the present application for pointing out certain problems that could be encountered, under certain circumstances, in practicing the inventions of the three prior patent applications and for illustrating the manner in which these potential problems may be prevented, according to the new invention of the same applicant.

The bas-relief patterned seal areas over the front side of the two identical gaskets, press against the planar surfaces of respective electrically conductive elements in alignment with the through holes of the gaskets for hydraulically sealing the perimeter of the two flow compartments that are thus in communication with the through holes non-contoured by the perimeter seal area of the base-relief patterned front side of the respective gasket. This architecture lends itself to greatly simplify construction and to enhance compactness of multi-cell stacks. However, when assembling stacks of cells of relatively large active area, the hydraulic seal between the rear planar surfaces of the two back-to-back compressed elastomer gaskets, sandwiching there between the perimeter portion of the permionic membrane separator of the two flow compartments of the cell, may be unsatisfactory, because of the fact that in correspondence with these seal areas, the bas-relief flow path patterns present on the front side of the two gaskets may not be perfectly aligned to each other and cause localized deflections of the base sheet of elastomer that leads to leakages of the electrolyte solutions between the two back-to-back compressed gaskets. Even if of limited amount, intermixing decreases the overall electrochemical conversion efficiencies both during a charge phase and during a discharge phase of a redox flow battery storage system. Further increasing compression may not entirely resolve and sometimes even worsen the situation by causing encroachment of the base sheet of one gasket under raised portions thereof into the base sheet of the other gasket under troughs thereof. Another drawback of excessively compressing the gaskets of the membrane assemblies is that the bas-relief patterned features (ridges and troughs) of their front sides tend to shift sideways because of the elastic behavior of the elastomer, aggravating the problems. SUMMARY OF THE INVENTION

To the above discussed baffling problems of ensuring a reliable hydraulic seal that would prevent any intermixing of the two electrolyte solutions, after laborious tests and after abandoning several ineffective approaches, the applicants have found an outstandingly effective solution based on the introduction of simple rigid plastic inserts that may be manufactured by injection molding and easily introduced in the through holes of the planar electrically conductive inter-cell interconnects and in the aligned through holes of the two back-to-back assembled, bas-relief patterned elastomer gaskets of each membrane assembly, before alternately stacking the elements. The insert in the through holes of the planar electrically conductive inter- cell interconnects is an enlarged annular liner insert that prevents contact of the stream of electrolyte solution with the electrically conductive interconnect (as contemplated in the above-mentioned prior application No. PCT/IB2010/001651 filed on June 29, 2010). Such a purposely enlarged rigid plastic bushing that may have the same thickness of the planar inter-cell interconnect for retaining the planarity of the opposite surfaces of the conductive inter-cell interconnect, over the planar surface facing bas-relief split flow channels in communication with the flow hole as defined on the front side of the membrane assembly, has a "v" cross sectional circular groove, concentrically surrounding the central hole of the rigid plastic bushing.

With such a rigid plastic hole lining bushing, grooved on the side of the cell compartment in communication with the flow-through hole in the electrically conductive inter-cell interconnect, cooperates a rigid plastic bushing that is inserted through the aligned through holes of the two elastomeric gaskets bas- relief patterned on front side of the membrane assembly, for satisfying a self- centering function of the two rigid plastic inserts and for implementing an O-ring type seal function when the membrane assembly becomes compressed between two inter-cell interconnects that solves the problem deriving from an otherwise unconstrained elastic sideway expansion of the elastomer that could occur upon excessively increasing stack compression.

Such a cooperating rigid plastic bushing inserted through the aligned through holes of the two back-to-back assembled elastomeric gaskets of the membrane assembly has a terminal flange adapted to bear against a purposely recessed circular rim area of the elastomeric base sheet of the bas-relief profiled front side of the gasket, around a through hole in hydraulic communication with the flow compartment of the cell of the same gasket that perimetrally seals it, and a conical tubular part that extends as far as lining the aligned through holes of the same gasket and of the other gasket of the membrane assembly that, according to an important feature of its novel geometry, has on its bas-relief patterned front side a raised rim portion (substantially a molded O-ring feature) that remains confined between the inner wall surface of the central hole of the rigid plastic bushing of the inter-cell interconnect that bears on the second gasket and the outer surface of the conical tubular part of the plastic bushing inserted through the aligned holes of the two gaskets of the membrane assembly.

Over the surface of the flange of the plastic bushing inserted through the aligned holes of the two gaskets that faces toward the compressing inter-cell interconnect are present an inner circular crown of spaced protrusions having flat co-planar top surfaces and an outer circular crown of "v" tipped teeth.

The "v" tipped teeth of said outer circular crown fit inside the "v" cross sectional circular groove for ensuring alignment of the through holes of the electrically conductive inter-cell interconnects and of the membrane assemblies, and the flat co-planar top surfaces of the spaced protrusions of the inner circular crown press the raised annular rim portion on the front side of the gasket on which bears a planar inter-cell interconnect with said rigid plastic hole lining bushing, grooved on the opposite side, which is laterally confined between the inner wall surface of the central hole of the rigid plastic bushing of the inter-cell interconnect and the outer surface of the conical tubular part of the plastic bushing inserted through the aligned through holes of the two back-to-back assembled gaskets of the membrane assembly. Compression of said raised annular rim portion causes its expansion that establishes an O-ring type seal around the outer conical surface of the tubular part of the flanged rigid plastic bushing encroaching there through and on the wall of the central hole the bushing lining the through hole of the interconnect. The arrangement has proven itself to be outstandingly effective in ensuring a reliable hydraulic seal preventing seepage of electrolyte solutions between the compressed gaskets of the membrane assemblies which would cause a certain intermixing to occur and could eventually reach the edges of the permionic membrane sandwiched between the two gaskets around the central window closed by the permionic separator of the two flow compartments of the cell.

Of course, in view of the fact that the inner diameter of the flanged rigid plastic bushing with conical tubular extension defines the lumen of flow, the through holes that are formed in the perimeter portion of the conductive planar interconnects and of the elastomer gaskets will have a correspondingly enlarged diameter adapted to accommodate the respective tubular inserts of rigid plastic, according to this disclosure.

Preferably, the comb-like linear arrays of protrusions of the bas-relief patterned front sides of the two back-to-back assembled gaskets, at the outlet or at the inlet of distribution channels of the electrolyte solution into and out of the flow compartment of the cell, along the opposite side edges of the central window of the respective gaskets, may be substituted by strip inserts of rigid plastic. The planar base strip of rigid plastic of each comb-like strip, resting over the base sheet of elastomer of the bas-relief patterned front sides of the two back-to-back assembled gaskets, ensure compression of the opposite side edges of the central window of the respective gaskets between two substantially planar base strips of the comb-like plastic inserts. This further enhances reliability of the hydraulic seal of the perimeter of the permionic membrane separator sandwiched there between.

The invention is defined in the annexed claims, the recitation of which is to be intended constituting part of the specification and the peculiar features and advantages of the stack architecture of this disclosure will become even more clear in the ensuing description of exemplary embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 is an exemplary embodiment of a typical frameless membrane assembly of the stack architectures described in the above cited prior patent applications of the same applicants;

FIGURE 2 is a cross sectional view of a seven tier cell stack according to a preferred embodiment of the present invention that includes both essential and optional inserts, as described below.

FIGURE 3 is a schematic exploded partial detail view from the other side of the essential cooperating rigid plastic bushings that are inserted in the though holes of the electrically conductive inter-cell interconnects and of the back-to- back assembled elastomeric gaskets of the membrane assemblies, showing also optional comb-like strip inserts of rigid plastic on the front surface of the gaskets.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring to FIG. 1, the permionic membrane M, commonly a flexible film of an ion exchange polymer adapted to exchange anions, cations or both, depending on the destination of use of the electrochemical reactor, has its perimeter portion held between two identical parallelepiped gaskets Gl and G2 of a natural or synthetic elastomeric material adapted to withstand contact with the acid solutions being used, disposed back-to-back. The so composed membrane assembly is eventually compressed between two planar electrical inter-cell interconnects (not shown in FIG. 1) upon tightening the stacked elements together, according to any of the stack architectures described in the above cited prior applications of the same applicants.

The two identical gaskets Gl and G2 define a central aperture or window closed by the membrane M that provides for the required hydraulic separation between the flow compartments of the cell, on one side and on the opposite side of the permionic membrane M. Therefore, the active cell area practically corresponds to the area of the central aperture defined by the two gaskets Gl and G2.

In the exemplary embodiment shown in FIG. 1, the gaskets have four through holes, 1, 2, 3, 4, that, coherently to the fact that the two gaskets are identical but disposed back-to-back, are indicated with different corresponding numbers. The four holes, once the stack is completed and tightened, form, together with similarly aligned through holes in the inter-cell interconnects, inlet and outlet internal manifolds for circulating in parallel the two electrolyte solutions in the respective cell compartments of all the cells of a multicell stack.

As observable for the visible face of the gasket Gl, the "front side" (as opposed to the backside) of the gaskets has a bas-relief patterned perimeter seal area 5 that forms loops adapted to contour completely the through hole 2 and the diametrically opposite through hole 4. Along two opposite side of the central aperture or window there are two similarly bas-relief patterned areas of the elastomer gasket with defined thereon elongated split flow channels 6 that extend from a rim region 7 of non-contoured through holes 1 and 3 at the other diagonally opposite corners of the membrane. In a parallel flow embodiment of FIG. 1, the parallel split flow channels 6 are tortuously elongated at art in order to comprise at least a narrow elongated tract along which the electrolyte solution is forced to flow for reaching the "inlet zone" along one side edge of the central window of the gasket and entering the flow compartment of the cell, passing through the gaps of a comb-like linear array 8 of relatively short teeth of elastomer, and vice versa along a similar structured , "outlet zone" at the opposite side of the central window of the gasket.

All the seal areas at the top of the salient portions defined over the bas-relief patterned front side of the elastic gaskets have the same height, being destined to press against a substantially planar surface of a electrically conductive inter-cell interconnect. In any case, the patterned salient parts of elastomer over the front side of the gaskets besides establishing a hydraulic seal over the counter-opposed surface of the planar inter-cell interconnects, define electrolyte flow ducting channels and the central compartment void through which the electrolyte solution flows. Each membrane assembly is contoured by plastic spacers 9 having a thickness corresponding to a designed maximum compression of the elastomer gaskets between the inter-cell interconnects, adapted to reliably secure all hydraulic seals defined by the bas-relief patterned elastomer gaskets, form leak proof internal manifolds and split flow ducting 6, and at the same time avoid localized over compression of the elastomer gaskets and/or compressible mat or felt electrodes if present there between, making the inter-cell interconnects perfectly parallel to each other and equally spaced.

An exemplary embodiment of this invention is illustrated by way of a cross sectional view of a seven tier cell stack of FIG. 2 and of a tridimensional exploded fragment view of the stack of FIG. 3.

The sectional plane of FIG.2 intercepts the inlet (or outlet) manifolds that distribute the two distinct electrolyte solutions in parallel to the respective flow compartments of all the cells of the stack, by way of example the aligned through holes 4 and 3 of the typical frameless stack assembly architecture of FIG. 1, described in the cited prior patent applications of the same applicants, whose content is intended to be herein incorporated by express reference.

For a quicker reference and comparison of certain novel geometrical features according to the present invention, the component parts and geometrical features functionally equivalent to those of FIG. 1 , are identified with the same numerals and letters in the cross sectional view of FIG. 2 and in the fragment view of FIG. 3.

The solution that has been found by the applicants to the potential risk of undue leakages of the electrolyte solutions out of their defined flow paths through the stack assembly and consequent intermixing of minor amounts of the two distinct electrolyte solutions, and practical difficulties in ensuring a perfect alignment of the through holes in the inter-cell interconnects I and in the back-to- back assembled elastomer gaskets Gl and G2, that sealingly hold there between the perimeter portion of the permionic membrane cell separator M, consists in a cooperative action of each rigid plastic bushing 13, (contemplated also in the structures described in said prior patent applications of the same applicants), the outer diameter of which may be eventually enlarged, purposely provided with a "v" shaped circular groove 13g. on one side ((the downstream side relative to the direction of flow of the electrolyte solution in the aligned through holes), and of a rigid plastic bushing B that are inserted into the aligned through holes of the two back-to-back assembled gaskets Gl and G2, of each membrane assembly.

The cooperating rigid plastic bushing B has a terminal flange Bf bearing against a purposely depressed circular region dA of the elastomenc base sheet of the bas-relief profiled front side of the gasket Gl (for the aligned through holes 4) and G2 (for the aligned through holes 3) and a conical tubular part Be that extends as far as to line the aligned through holes of the gasket on which bears the flange Bf and of the other gasket of the membrane assembly that, according to an important feature of this novel geometry, has on its bas-relief patterned front side a raised rim portion Or (substantially a "molded O-ring feature") that is confined between the inner wall surface of the central hole of the rigid plastic bushing 13 of the inter-cell interconnect and the outer surface of the conical tubular part Be of the plastic bushing B.

Over the surface of the flange Bf are present an inner circular crown Cr2 of spaced protrusions having flat co-planar top surfaces and an outer circular crown Crl of'v" tipped teeth.

The "v" tipped teeth of said outer circular crown Crl fit inside the "v" cross sectional circular groove 13g for ensuring alignment of the through holes of the electrically conductive inter-cell interconnects and of the membrane assemblies, and the flat co-planar top surfaces of the spaced protrusions of the inner circular crown Cr2 press the raised annular rim portion Or on the front side of the gasket on which bears a planar inter-cell interconnect with said rigid plastic hole lining bushing 13, grooved on the opposite side, which is laterally confined between the inner wall surface of the central hole of the rigid plastic bushing of the inter-cell interconnect and the outer surface of the conical tubular part of the plastic bushing inserted through the aligned through holes of the two back-to-back assembled gaskets of the membrane assembly. Compression of the raised annular rim portion Or causes its expansion that establishes an O-ring type seal around the outer conical surface of the tubular part Be of the flanged rigid plastic bushing B encroaching there through and on the wall of the central hole the bushing 13 lining the through hole of the interconnect I.

The arrangement has proven itself to be outstandingly effective in ensuring a reliable hydraulic seal preventing seepage of electrolyte solutions between the compressed gaskets of the membrane assemblies which would cause a certain intermixing to occur and could eventually reach the edges of the permionic membrane sandwiched between the two gaskets around the central window closed by the permionic separator of the two flow compartments of the cell.

According to the preferred embodiment shown in FIG.2 and FIG. 3, the comb-like linear arrays 8 of protrusions of the bas-relief patterned front sides of the two back-to-back assembled gaskets Gl and G2, at the outlet or at the inlet of distribution channels 6 of the electrolyte solution, respectively into and out of the flow compartment of the cell, along opposite side edges of the central window of the respective gaskets, may optionally be substituted by strip inserts 8s of rigid plastic. As shown, the planar base strip of rigid plastic of each comb-like strip 8s , may be accommodated in a purposely recessed area Ar of the base sheet of elastomer of the bas-relief patterned front sides of the two back-to-back assembled gaskets Gl and G2. As may be observed in the cross sectional view of FIG 2, the opposite side edges of the central window of the two gaskets are thus compressed between two substantially rigid planar base strips of the respective comb-like plastic inserts 8s. This further enhances reliability of the hydraulic seal of the perimeter of the permionic membrane separator M that is sandwiched there between.

Claims

1. Frameless multi-cell electrochemical stack, filter-press sealed between two end headers, comprising an alternate succession of planar, electrically conductive inter-cell interconnects and membrane assemblies having aligned through holes along perimeter portions for composing inlet and outlet manifolds of parallel flow or ziz-zag flow paths of serial flow, respectively, for circulating distinct electrolyte solutions in respective flow compartments of all the cells, separated by the permionic membrane of a membrane assembly sandwiched between two electrically conductive inter-cell interconnects, the perimeter portion of the membrane being held between rear planar surfaces of two elastomeric gaskets assembled back-to-back and similarly bas-relief patterned over their front surfaces for defining separate flow paths of the electrolyte solutions in respective flow compartments coinciding with a central window of one and of the other of said two bas-relief patterned gaskets, further comprising

- rigid plastic bushings in all through holes of said electrically conductive inter-cell interconnects, over the planar surface facing bas-relief split flow channels in communication with the hole, the plastic bushing having a "v" cross sectional circular groove around a central flow through hole of the plastic bushing;

- rigid plastic bushings with an end flange and a conical tubular part adapted to be inserted into the aligned through holes of the back-to-back assembled elastomeric gaskets of each membrane assembly, over the surface of the flange facing bas-relief split flow channels in communication with the flow through hole being present an inner circular crown of spaced protrusions with flat co-planar top surfaces and an outer circular crown of spaced "v" tipped teeth;

the "v" tipped teeth of said outer circular crown fitting into said "v" cross sectional circular groove for selfaligning the through holes of the electrically conductive inter-cell interconnect and of the back-to-back assembled elastomeric gaskets of the membrane assembly and said flat co-planar top surfaces of the spaced protrusions of said inner circular crown compressing a raised lip portion of the bas-relief patterned elastomeric gasket lining said central flow through hole of the plastic bushing, acting as an O-ring like seal around said conical tubular part of said flanged rigid plastic bushing inserted there through.

2. The frameless multi-cell electrochemical stack of claim 1, further comprising rigid plastic comb-like strips having a flat base part resting on a planar recessed area of the bas-relief patterned front surfaces of both back-to-back assembled elastomer gaskets, along opposite sides of the central window thereof.

3. The frameless multi-cell electrochemical stack of claim 1, wherein said rigid plastic bushing are of polyvinylchloride.

4. The frameless multi-cell electrochemical stack of claim 1, wherein said elastomeric gaskets are of a natural or synthetic elastomeric material adapted to withstand contact with acid solutions.

5. The frameless multi-cell electrochemical stack of claim 1, wherein said planar electrically conductive aggregate of graphite and/or carbon particles and a resin binder.

6. The frameless multi-cell electrochemical stack of claim 1, wherein the stack is composed of monopolar cells, said electrically conductive planar interconnects alternately distributing and collecting electric current from active porous electrodes contained in the respective flow compartments of two adjacent cells of the stack and being individually connected alternately to a positive rail and a negative rail of an external DC bus.

7. The frameless multi-cell electrochemical stack of claim 1, wherein the stack is composed of bipolar cells, said electrically conductive planar interconnects distributing and collecting electric current from active porous electrodes contained in the respective flow compartments of two adjacent cells of the stack, the end interconnects of which are connected to an external circuit.

8. The frameless multi-cell electrochemical stack of claim 1, wherein the aligned through holes of said planar interconnects and of said membrane assemblies define inlet and outlet internal manifolds for the two electrolyte solutions circulated in parallel through the respective flow compartments of all the cells of the stack.

9. The frameless multi-cell electrochemical stack of claim 1 , wherein the aligned through holes of said planar interconnects and of said membrane assemblies define distinct internal zig-zag flow paths for the two electrolyte solutions circulated in series through the respective flow compartments of all the cells of the stack.