Apparatus for reduction of shunt current in bipolar electrochemical cell assemblies

Shunt currents which flow between the electrodes of adjacent cells through the moving conductive fluid and the fluid pool in the manifold of a bipolar cell assembly are minimized by introducing the conductive fluid at the top of an elongated outlet manifold. This results in cascaded flow which interrupts the current path. Shunt currents between the fluid manifold walls of the conductive bipolar elements in the series connected electrochemical cell assemblies are minimized by insulating the manifold walls with insulating, elastomeric sealing grommets. This prevents current flow between the manifold walls through the electrically conductive fluid in the manifold and provides an edgeseal between bipolar plates.

The instant invention relates to a process and apparatus for 
electrochemical cell assemblies and more particularly, for reducing shunt 
current in series connected bipolar assemblies. 
While the instant invention will be described specifically in connection 
with a chlorine electrolyzer cell assembly, the invention is by no means 
limited thereto and may be utilized with any electrochemical system which 
utilizes a conductive fluid. For example, it is applicable to fuel cell 
batteries utilizing a plurality of conductive bipolar elements between 
fuel cells which utilize conductive halogen/hydrogen feed stocks. 
Construction of an electrolyzer as a cell stack operating in an electrical 
series arrangement and comprising a plurality of bipolar elements 
separated by ion transporting membranes having electrodes bonded to 
opposite surfaces thereof offers a number of advantages in terms of 
efficient space and material utilization and permits fluid manifolding to 
be an integral part of the bipolar plates. 
If the fluid (such as an aqueous solution of HCl or brine, for example), is 
itself a good electrical conductor it is possible for a fraction of the 
electrical current applied to the stack to follow a path through the fluid 
in the manifold rather than through the electrolytic cells. These currents 
are usually called "shunt currents" and are parasitic since they are not 
used in the cell reactions and obviously cause the electrolyzer assembly 
to be less efficient. 
When the individual bipolar cell elements are fabricated from an 
electrically conductive material such as graphite, the interior wall of 
each manifold is exposed to the conducting fluid. In a series connected 
bipolar assembly, voltage differences exist between individual cells and 
these differences may be 2 or 3 volts per cell. Although the periphery of 
the major faces of the conductive bipolar elements are separated by the 
thickness of an insulating film or a gasket, the conductive interior walls 
of the manifold are exposed to the fluid and large shunt currents can flow 
between the interior manifold walls of adjacent cell through the 
conductive fluid in the common manifold. 
Another source of a parasitic shunt current that exists in a bipolar series 
connected cell assembly is current flow between the conductive electrodes 
bonded to the membranes of adjacent cells through the moving fluid streams 
in contact with the electrodes which pass into a manifold and into the 
pool of conductive fluid in the common manifold. Such currents perform no 
useful function in the electrochemical cell and are therefore a parasitic 
current which reduces the efficiency of the electrolyzer or other 
electrochemical assembly. 
Applicant has discovered a method and means for minimizing both sources of 
the parasitic shunt currents. First, by insulating the manifold walls of 
the individual bipolar elements by means of an elastomeric, insulating and 
sealing grommet to form an insulated pipe down the length of the manifold, 
and secondly by bringing the conductive liquid out of each of the cells 
into the top of an elongated manifold to provide cascaded, gravitational 
flow which interrupts the flow path so that this shunt current is 
minimized. It is therefore a primary objective of this invention to 
provide a method and apparatus for minimizing shunt current flow in series 
connected, bipolar electrochemical cell assemblies. 
Another objective of the invention is to provide a method and apparatus for 
minimizing shunt current flow in a bipolar cell stack which utilizes 
common manifolding for conductive fluids. 
Still another objective of the invention is to minimize shunt current flow 
in a filter press, bipolar cell assembly by insulating the fluid manifold 
walls from the fluid. 
Yet another objective of the invention is to minimize shunt current flow in 
a bipolar series connected cell assembly by providing interrupted flow 
paths for a conductive fluid into the fluid outlet manifold. 
Other objectives and advantages of the instant invention will be become 
apparent as the description thereof proceeds. 
The various advantages and objectives of the invention are achieved in a 
series connected multicell assembly utilizing conductive graphite bipolar 
plates which incorporate outlet manifolds for the conductive fluids 
directly in the bipolar plate. The manifolds are elongated and lined with 
an insulating elastomeric grommet to eliminate conduction of current flow 
between the conductive flow in the manifold. In addition, shunt currents 
between adjacent cell electrodes through the flowing conductive fluids are 
minimized by introducing the conductive fluid (anolyte or catholyte) at 
the bottom of the bipolar plates so that they flow upwards through the 
cell. The excess fluid is introduced into the top of the outlet manifold 
so that it cascades into a pool of the conductive liquid at the bottom of 
the manifold. The cascade flow interrupts the flow path of the conductive 
fluid sufficiently raising the resistance of the path sufficiently to 
minimize shunt current flow between the conductive electrodes of various 
cells.

FIG. 1 illustrates a multicell bipolar HCl electrolyzer assembly which 
consists of conductive anode and cathode endplates 10 and 11 clamped 
together by suitable bolts or tie rods 12. Endplates 10 and 11 are 
respectively connected to the positive and negative terminals of a power 
source. Positioned between the endplates are a plurality of bipolar 
elements 13 separated by ion transporting membranes 14; to be described in 
greater detail in connection with FIG. 2. Catalytic anode and cathode 
electrodes are bonded to opposite sides of membranes 14. Conductive 
projections on opposite sides of the bipolar elements contact the 
electrodes bonded to the major surfaces of an adjacent pair of membranes. 
The anolyte feed stock is brought into the electrolyzer through an inlet 
conduit 15 and excess feed stock is removed through an outlet conduit 16. 
Outlet conduit pairs 17 and 18 communicate respectively with the inlet and 
outlet manifolds of the bipolar elements to remove the electrolysis 
products from the anode and cathode chambers of each cell as well as 
depleted anolyte and catholyte. The outlet conduit pairs respectively 
remove the gaseous electrolysis products and the fluid. 
FIG. 2 shows a partially exploded view of a four cell bipolar electrolyzer 
19 in which shunt currents are minimized. Electrolyzer 19 consists of a 
plurality of conductive bipolar elements 20, and a plurality of cation 
transporting membranes 21 positioned between anode and cathode endplates 
10 and 11. An anode electrode 22 is bonded to the central portion of 
membranes 21 and corresponding cathode electrodes, not shown, are bonded 
to the other side of each membrane. The conductive, bipolar current 
collecting and fluid distributing elements 20 contact the anode electrode 
22 of one membrane and the cathode electrode, not shown, of the adjacent 
membrane to form a plurality of series connected cells in which the 
electrochemical reactions, (electrolysis, fuel cell power conversion, 
etc.) take place. 
Each conductive, bipolar, element 20 include a central chamber 23 
containing a plurality of parallel, conductive electrode contacting 
projections 24. The parallel projections also define fluid conducting 
channels 25 through which the conductive fluids, as well as the 
electrolysis products, in the case of an electrolyzer, are transported. 
The conductive anolyte, such as an aqueous solution of HCl is introduced 
into the central chamber through inlet manifolds 26 which are lined by 
insulating liners 27. Insulating liners 27 and inlet manifolds 26 include 
a plurality of passages 28 which communicate between the inlet manifolds 
and central chamber 23. The anolyte and evolved chlorine pass through 
fluid distribution channels 25 to anode collecting channel 30 at the top 
of the central chamber. Channel 30 communicates through opening 31 to a 
anolyte outlet manifold 32. A similar outlet manifold chamber 33 is 
provided on the other side of the bipolar collector which communicates 
with the fluid distribution channels on the other side of the bipolar 
element, not shown. The current conducting projections on the other side 
of the bipolar elements are oriented at right angles on the anode 
contacting side, and are similar to those which may be seen in cathode 
endplate 11 which shows a plurality of horizontal fluid channels 34. 
Manifolds 32 and 33 of the various bipolar collector elements are, in 
accordance with one aspect of this invention, insulated by means of the 
elastomeric, insulating grommets 35 to minimize shunt current flow between 
the conductive manifold walls of adjacent bipolar units. As pointed out 
previously, in a filter press assembly the electrodes (anode and cathode) 
of the series connected bipolar cells, are at different potentials, with 
the electrodes of the cells closest to the anode endplate being at a 
higher potential than those of cells closer to the cathode endplate. As a 
result, current can flow between the conductive manifold walls of adjacent 
cells through the conductive fluid at the bottom of the outlet manifolds. 
The insulating grommets line the manifold walls and interpose a 
non-conductive barrier between the fluid and the conductive walls. 
A thin, preferably 5 mil or less insulating film 36 is attached to one face 
of each bipolar plate to prevent a short circuit between the conductive 
plates. The film is preferably a fluorocarbon polymer such as polytetra 
fluoroethylene of the type sold by DuPont under the trade designation 
TEFLON or polyvinilidene fluoride sold under the trade designation KYNAR. 
Insulating film 36 is fastened to the face of the bipolar element by a 
suitable adhesive. One form of such suitable adhesive is a polyvinilidene 
adhesive sold under the trade designation TEMPER-TAPE by the Howard Rubber 
Company of Bridgeport, CT. 
The elastomeric manifold sealing grommets 35 are preferably fabricated of 
any suitable insulating material which is capable of withstanding the 
environment in a particular system. Thus in an HCl chlorine electrolyzer 
the grommet may be fabricated of a fluorocarbon material or any other 
material which is resistant to HCl and to the evolved chlorine. One 
example of such a material is an elastomeric fluorocarbon such as 
polyhexafluoroprophlene rubber which is sold by the DuPont Company under 
its trade name VITON. For a chlorine and HCl resistant formulation, VITON 
having a Parker Compound No. V 834-70 is preferred. 
Grommet 35 consists of a body 38 which lines the interior walls of the 
manifolds and flanges 39 and 40 which form an edge sealing arrangement 
when the bipolar cells are assembled. Sealing flange 40 includes a sealing 
bead or lip, not shown, on its underside which fits into sealing groove 42 
in the bipolar element. As will be described in greater detail later, the 
sealing bead fits into the groove and is retained in the groove by flange 
39 of the adjacent grommet. The two flanges seal against each other to 
prevent conductive fluid and gasous electrolysis products from escaping 
between the bipolar plates. 
As may be seen most clearly in FIG. 4, when assembled the walls of 
manifolds 32 are lined by grommet body 38. The sealing flange 40 of the of 
the grommet includes a sealing bead or lip 43 which fits into groove 42 on 
the side of the bipolar element which contacts the cathode electrode. 
Flange 39 of the grommet lining the manifold of the adjacent cell is 
seated in notch 50 in the bipolar plate and bears against flange 40 and 
compresses that flange and sealing bead 43 to form an edge seal between 
adjacent bipolar plates thereby preventing gas and fluid leakage. Also 
positioned on the side of the bipolar plates contacting the cathode of 
electrodes of each of the cells are o-ring seals 44 which are seated in 
o-ring grooves in the bipolar element. The combination of insulating film 
37, o-ring seals 44 and insulating flanges 35 and 40 insure that there is 
no direct contact between the faces of the bipolar elements. The grommets 
when assembled thus form an insulating pipe down the manifold thereby 
eliminating shunt currents between the conductive manifold walls of 
adjacent bipolar elements. 
FIG. 3 illustrates the manner in which shunt currents which may flow 
between the conductive electrodes of adjacent cells through the flowing 
fluid and the conductive fluid pool at the bottom outlet manifold are 
minimized. This aspect of the invention will be described in connection 
with the anolyte outlet manifold of an electrolyzer. It will however be 
clear to the man skilled in the art, that it applies with equal force to 
the catholyte outlet manifold of any electrochemical cell assembly 
utilizing conductive fluids and conductive bipolar elements. 
To this end, the depleted conductive anolyte fluid 45 and chlorine from the 
anode chamber of each cell passes into collection channel 30 and through 
passages 46 in the bipolar plates and openings 47 in grommet 35 to the top 
of the anolyte outlet manifold. Fluid stream 45 thus cascades from the top 
of the manifold into the fluid pool 48 at the bottom of the manifold. By 
forcing the conductive fluid to fall from the top of the manifold 
vertically into the pool, the conductive current path of the fluid is at 
least interrupted thereby increasing the resistance of the path of the 
fluid sufficiently to minimize shunt current flow from the anode of one 
cell through fluid stream 45 and pool 48 to the fluid stream 45 of an 
adjacent cell. 
FIG. 5 illustrates the variable depth collection channel 49 on the cathode 
side of each bipolar collector. Channel 49 communicates with the fluid 
distributing channels 34 on the cathode side of the bipolar collector and 
communicates through the opening 31 shown in FIG. 1, with the cathode side 
outlet manifold 33. The depth of the channel increases towards the 
manifold so that the volume of the collection channel increases to 
accomodate all the fluid flowing toward the exit manifold. 
The conductive bipolar current collector, fluid distributing elements 20 
are, in the case of a HCl electrolysis system, preferably a bonded 
aggregate of graphite and fluorocarbon polymeric particles. The 
fluorocarbon particles may be of any sort although polyvinilidene fluoride 
polymers such as those sold by the Pennwalt Corporation under the 
tradename KYNAR are preferred. In the instance of the chlorine 
electrolyzer utilizing aqueous solution of hydrochloric acid as the 
anolyte a conductive molded graphite plate has been described as the 
preferred embodiment. However, the invention is by no means limited 
thereto and is equally applicable to any conductive bipolar element. 
The ion transporting membranes 21 to which electrodes are physically bonded 
are preferably perfluorosulfonic acid cation transporting membranes of the 
type sold by the DuPont Company under its trade designation Nafion. These 
membranes allow transport of hydrogen cations, in the case of a HCl 
system, from the anode to the anode chamber where they are discharged at 
the cathode electrode to form hydrogen while chlorine is generated in the 
anode chamber. The electrodes 22 which are bonded to the major surfaces of 
the membranes are in the case of the anode electrode preferably a bonded 
mixture of the oxides of a platinum group metal such as platinum, iridium, 
ruthenium, etc. with a fluorocarbon particles such as 
polytetrafluoroethylene as sold under the trade name TEFLON. The 
electrodes are gas and liquid permeable, electroconductive, and 
catalytically active to evolve chlorine from the anolyte. The precise 
manner in which these electrodes are fabricated, their preferred 
constituents, and the manner in which the membrane and bonded electrode 
system are fabricated are explained in detail in U.S. Pat. No. 4,224,121 
issued Sept. 23, 1980 and assigned to the assignee of the present 
invention. The above identified patent is hereby specifically incorporated 
by reference for a complete and detailed showing of the details of the 
membrane, the electrode, the manner of fabricating the electrode and the 
manner of applying the same of the membrane. 
While any number of manifold geometries will serve to remove fluid, a 
geometry which has a form factor such that the vertical axis is 
substantially greater than the horizontal axis, and introducing the 
conductive fluid at the top of the manifold is the preferred approach. By 
making the vertical height of the grommet and the manifold greater than 
its width, and by removing fluid from the bottom of the manifold at a rate 
sufficient to insure that the manifold is partially empty of fluid, 
cascade flow of the fluid into the manifold fluid pool occurs thus 
interrupting the fluid path, increasing its resistance thereby minimizing 
the shunt current. This aspect of the invention, the reduction of the 
shunt current by use of an internal cascade to cause fluid discontinuity 
and increase path length is applicable to any electrolyzer or 
electrochemical cell system which employs a conductive fluid, independent 
of insulating or conductive nature of the manifold walls or cell plates. 
Grommet shapes and manifold shapes approaching squares or circles can also 
achieve the effect depending on the size and diameter of the device but 
such manifolds must be very large to permit a sufficient cascade height. 
This would not be an efficient use of space and material and for this 
reason the above described elongated shapes are the preferred embodiment. 
Since the excess fluids from the cell are introduced at the top of the 
outlet manifold to allow cascaded flow, the fluid must pass upwardly 
through the cell. This requires that the fluids be pressurized. 
Introducing the fluids at 15-15 psig is more than adequate with the 
precise pressure range depending on the height of the cell. 
It will be apparent from the foregoing that a very effective arrangement 
has been provided for eliminating shunt current in series connected 
electrochemical cells which include a plurality of conductive bipolar 
current conducting elements positioned between ion transporting membranes. 
While the instant invention has been shown in connection with the preferred 
embodiment, the invention is by no means limited thereto, other 
modifications of the instrumentality employed and the steps of the process 
may be made and fall within the scope of the invention. It is contemplated 
by the appended claims to cover any such modifications that fall within 
the true scope and spirit of this invention.