CA1282920C

CA1282920C - Stable carbon-plastic electrodes and method of preparation thereof

Info

Publication number: CA1282920C
Application number: CA000550214A
Authority: CA
Inventors: Calman Herscovici; Allen Charkey; Anthony Leo
Original assignee: Electric Power Research Institute Inc
Current assignee: Electric Power Research Institute Inc
Priority date: 1986-11-20
Filing date: 1987-10-26
Publication date: 1991-04-16
Anticipated expiration: 2008-04-16
Also published as: US4758473A; JPS63138659A; ATE77003T1; DE3779570T2; EP0268397B1; DE3779570D1; EP0268397A1

Abstract

ABSTRACT OF THE DISCLOSURE

Novel bipolar electrodes for electrochemical cells and methods for making same are provided. The bipolar electrodes comprise a pressure-molded composite of heat-treated graphite particles having a particle size distribution of 0 to 45µm and thermoplastic resin particles. The graphite and resin in the composite are in a weight ratio of from 1:5 to 1:1 and are formed by pressure-molding at 400 kg/cm2 or higher at 190°C.

Description

9~) STABLE CARBON-PLASTIC ELECTRODES
AND MET~IOD OF PREPARATION THEREOF

The present in~ention is directed to pressure molded bipolar electrodes for electrochemical cells, and more particularly ~or zinc bromide electrochemical cells.

BACKGROUND_OF THE INVENTION

There has been great interest in the development of a zinc bromide battery as an energy storage device because o~ the potential for its simplicity of design, high theoretical voltage, and low cost of reactants.
In such a battery the energy is stored by electrolyzing an a~ueous zinc-bromide cell on charge , to form zinc metal and bromine liquid. During charge bromine is evolved at the cathode and dissolved in the 1~ electrolyte while zinc metal is deposited on the anode. On discharge the two reactants are consumed to form zinc bromide.

An exemplary zinc bromide battery consists of a stack of flow frame assemblies wherein a carbon bipolar 20 electrode i5 bonded into each ~rame. The flow channels in the frames direct electrolyte past the anode and cathode side of each electrode. One side of each electrode i5 usually a flat surface on which zinc is deposited and consumed while the other side of the electrode may comprise a carbon felt to ~upport the ~, . .

., , bromine evolution and consumption reactions. A porous separator is maintained between the positive and negative sides of the adjacent electrodes to prevent bromine from diffusing from the positive electrolyte to the negative electrolyte, each of which is maintained in a eeparate flow system.

During charge of the battery a method of storage is required to remove generated bromine from the catholyte, to avoid increase in bromine concentration to levels of sel~discharge and corrosion of cell components. Thus, bromine is stored as a complex with a quaternary ammonium bromide salt 50 that up to four bromine molecules can reversibly complex with the ; salt. The unbrominated quaternary salt is soluble in aqueous electrolyte while the polybromide complex is insoluble and separates out in a heavier oil-like phase. The organic complexing agent flows in a separate flow loop and is not pumped through the cell stac~. contact between the catholyte and complexing agent is accomplished by dispersing the complexing agent into droplets in a mixer external to the cell ; stack, thus increasing the area for bromine transfer between the two phases. An exemplary zinc bromide battery stack utiliæing this system is disclosed in U.S. Patent No. 4,162,351.

It is, however, desirable to scale up current zinc bromide technology to meet costs and performance requirements on a large scale, as for example for a power utility load leveling mission. To meet these requirement6 it is important to improve not only the e~ficiency of the design o~ the battery but also to improve the system lifetime, and in particular the lifetime of components which are subjected to particularly stressful conditions. one of these ~X~29~

components is the bipolar electrode. ~he commonly used component for this purpose is a vitreous carbon electrode which, particularly when scaled to the sizes required to meet industrial uses, is brittle, difficult to bond into ~low ~rames and overall is one of the most expensive components in the cell stack.
In addition, chemical stability is an issue which must be addressed in order to improve the lifetime of this component.

Bipolar current collector-separators for electrochemical cells containing graphit~ d~f thermoplastic fluoropolvmers are disclosed ln~ Patent Nos. 4,214,969 and 4,339,332. A bipolar plate substrate for electrochemical cell containing glassy carbon and a plastic such ~asfp~o~v~nylidene fluoride homopolymer is disclosed in~atent No. 4,098,967. The above patents, however, are not directed to bipolar electrodes which meet the requirements of low cost, durability, and good electxical performance in a zinc bromide battery for large industrial application.

It is therefore an object of the present invention to provide novel bipolar electrodes which have improved stability to a stringent electrochemical cell environment, particularly to a zinc-bromide cell environment.

It is a further object of the present invention to provide novel bipolar electrodes which may be scaled up to large industrial applications while maintaining or improving physical strength, chemical stability to slectrochemical cell environment, electrical performance, and component longevity.
i It is another objec~ of the present invention to provide a method ~or manufacturing improved bipolar electrodes for electrochemical cells.

It i~ yet another object of the present invention to provide a novel combination of a bipolar electrode and carbon felt for use in electrochemical cells.

These and other objects will become apparent from the following description and appended claims.

SUMMARY OF THE INVENTION
;

Novel bipolar electrode elements for electro~hemical cells are provided as well as mPthods for producing same. The bipolar elements comprise a pressure-molded composite of heat-treated electrically conductive graphite particles having a particle size distribution (before molding) of o to 45~m the graphite being heat-treated at a temperature of at least 800C for 2 hours prior to ~orming the composite; and thermoplastic resin particles, tha graphite and resin in the composite being in a weight ratio of fromll:5 to l:l.
The composite i6 characterized by less than ~.0~
weight loss and less than 0.1% dimensional increase in length upon immersion at 60C into polybromide oil for 1600 hours. The composite is formed by pressure-molding a mixture comprising the graphite and resin at 400 kg/cm at 190 C.
A novel assembly is also provided comprising the above dQscribed bipolar current collecting element and a composite carbon ~elt held in interfacial contact with each other with an adhesive.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying figures:

! Fig. 1 is a battery design shown schematically ! incorporating the bipolar electrodes according to the 1 5 present invention.

Fig. 2 i8 a graph comparing the chemical stability of bipolar electrode matexial~ according to the present invention to a conventional rigid graphite plate and a non-graphitic carbon/plastic ~ormulation, with respect to change in weight.

Fig. 3 is a graph comparing the chemical stability of a bipolar electrode according to the present invention to a rigid plate and a non-graphitic carbon/plastic ~ormulation, with respect to change in length.

Fig. 4 is a flowchart illustrating the manufacturing process for bipolar electrodes according to the present invention.

DESCRIPTION OF THE INVENTION

In accordance with the invention, a pressure-molded bipolar electrode element is provided which is a composite o~ heat treated electrically conductive graphite particles and thermoplastic resin particles, in a weight ratio of from 1:5 to 1:1. ~he bipolar element is characterized by improved chemical stabillty to conditions within an electrochemical cell, particularly within zinc bromide electrochemical cell. This has improved stability as manifested by chemical ~tability in polybromide oil utilized in a zinc bromide cell, as for example, described in U.S.

J
: -6-Patent No. 4,162,351. At a temperature of 60C upon immersion into this oil for 1600 hours, the electrodes according to the present invention are characterized by less than 4.0% weight loss and less than 0.1%
dimensional increase in length.

The bipolar electrodes according to the present invention also are characterized by excellent physical strength and chemical characteristics Por an electrochemical cell. In particular the bipolar electrodes are characterized, after immersion in polybromide oil at 60C for 2300 hours, resistivity of less than 0.8 ohm-cm, and a ~lexural strength of at least 2400 psi.

One component of the bipolar electrodes according to the present invention is graphite having a particle size distribution o~ 0 to 45~m. The graphite is also heat treated at a temperature of at least 800C for 2 hours prior to being processed into a composite. It is critical that the graphite have the above characteristics and be heat treated in order to obtain the proper chemical, electrical and physical characteristics in accordance with the invention.
Graphite meeting the above characteristics of particle size distribution is available commercially from Asbury Graphite Mills, Inc. (Asbury, N.J.).

The thermoplastic resin particles may be any thermoplastic fluoropolymer, and in particular a polyvinylidene di~luorlde. Fluoropolymer resins such as tetra~luoropolyethylene, and the like, are commercially available and may be utilized in place of polyvinylidene difluoride although the polyvinylidene difluoride is pre~erable. A suitable polyvinylidene difluoride is available under the trade name Kynar, ~rom PennWalt Corporation. The particle size distribution of the thermoplastic resin particles is also critical, and have a particle size distribution in the range of about 0 to 45~m.
Typically the heat treated graphite particles will be dry mixed with the thermoplastic resin particles and compression molded into a composite under 400 kg/cm2 or higher pressure at 190C with gradual cooling.
Other alternate and preferred embodiments for preparing the electrodes will be described hereinbelow in connection with Figure 4.

The composite will contain graphite particles and res'n particles in the weight ratio of 1:5 to 1:1.
The preferred weight ratio is 1:1.

In one embodiment of the present invention, the bipolar electrode may be formed into an assembly with a composite carbon felt. The carbon felt will be held ; in interfacial contact with the bipolar electrode by an adhesive material which is stable to the conditions within the electrochemical cell. Preferably this adhesive will comprise carbon (carbon black or graphite), polyvinylidene difluoride resin and dimethylformamide. Carbon black is the preferred carbon in the adhesive. The composite carbon felt will be attached to the positive face of the bipolar electrode with the electrically conductive adhesive material. The attachment of this felt improves the performance over a loose assembly of the felt and the bipolar electrode.

While not lntending to be limited by any particular theory, it is believed that the bipolar electrodes made in accordance with the present invention with the 29~

above described graphite, exhibit a lower surface area than, for instance, an electrode made with carbon black and therefore exhibits less of a tendency for bromine absorption. Furthermore, it is believed that the thermoplastic resin shields the graphite particles from the effect of bromine and is heat resistant.

Further detail of preferred embodiments of the invention will be described in connection with the attached fiyures.

Referring to Figure 1 there is shown a schematic exploded view diagram of a cell component of an electrochemical battery for use of zinc bromide technology. Frame 10 is a typical flow frame showing the positive side of the frame which is utilized to hold cell components. Flow frame 10 is usually made from heat and chemical resistant injection molded material such as polypropylene. Bipolar electrodes 12A and 12B are plates made in accordance with the present invention. Bonded to plate 12A is a carbon felt cathode ~ubstrate 14. The assembly of the ; bipolar electrode 12A and carbon felt cathode substrate 14 comprises a preferred assembly according to the present invention. Adjacent to the carbon felt 14 is the catholyte flow. Then partially shown is a fllled gasket 16 area of the separator followed by the separator membrane 18. Bonded to separator membrane 18 is the anode grid spacer 20, which is adjacent to a space for the ~low of anolyte. Finally, there is the bipolar electrode 12B for the next adjacent cell, with the cell ~tack continuing, dependlng upon the number of cells desired in the battery stack.

The carbon elt cathode ~ubstrate 14 is an open structure which allows flowthrough of the catholyte.

" 11 ~8Z'9~2~
_ g .
Pref~rably the felt will be about 97% porous. It has be~n ~ound than an effective mean pore diameter in the ~elt of about 86 microns i6 preferable with a preferred range of porosity being 88% pores distributed betwe~n 10 and 70 microns.
. . .
Referring to Figure 2 there is shown a graph of the results of an accelerated chemical stability test of three composite materials (molded 15 inch by 15 inch pieces): a rigid carbon/carbonized resin plate ~represented by square data points) consisting of a , sheet o~ phenolic resin heat treated to ~orm a J, vitreous carbon: a plate formed by compression molding -~ a mixture of 60~ by weight Xynar and 40% by weight carbon black (data points represented by circles), and a plate according to the present invention formed from compression molding a mixture of 50% by weight Xynar and 50% by weight graphite having the characteristics as described herein (datapoints represented by triangles).

The three plates were immersed in 60~C polybromide oil (a quaternary ammonium bromide salt used to complex bromide in zinc bromide cells) at 60~ for 1600 hours.
The increase in weight of each of the plates in terms of weight percentage is shown versus exposure kime.
The plate made in accordance with the present invention exhibited a less than 4% increase in weight indicating an improved weight stability.

Referring to Figure 3 there is shown a graph of accelerated chemical stability of the same three plates as described in connection with Figure 2 under the same conditions except that changes in the length of the samples are compared. The electrode made in accordance with the present invention (data points ~233~32~

J
represented by tri~ngles) exhibited less than a 0.1 dimensional ~ncrease in length, indicating improved dimensional stability.

Referring to Figure 4 there is illustrated a ~lowchart of the preferred manufacturing processes preparing bipolar electrodes according to the present invention.
In one embodiment the graphite is first subjected to the step of heat treatment 30 at a temperature of at least 800C for a period of at least 2 hours. Then the heat treated graphite is subjected to the step 32 of screening to ensure the particle size distribution of 0 to 45 ~m. The graphite is then dry mixed in step 34 wi~h the appropria~e amount of the the~moplastic resin particles. The mixture is then screened in step 36 to ensure that the resin particles in the mixture are within the size distribution range of 0 to 45j~m.
The dry blend of graphite and resin is then compression molded in step 38 under pressure of at least 400 ~b/cm2 at l9o~C with cooling in a conventional compression mold apparatus.

In a second embodiment, referring to Figure 4, in the dry mixing ~tep 34 a small amount of a binder material is added to the dry mix. Preferably this will be a perfluorinated polymer material, preferably Teflon .
This will be added in an amount of about 2 weight percent of the total mixture. After screening ~n step 36 the dry blend is then wet mixed with a hydrocarbon carrier, such as Solvent 340 (Shell) in step 40 then calendered into sheets in step 42. The sheets are then dried in step 44 and submitted to the compression molding ~tep 38 ~s described above. In this embodiment, by loading the mold with 8 preform calendered sheet, the material uniformity over the ~8~92~
t ~

area of the electrode is improved and the inclusion of airborne particulates in the composite is reduced.

The following example is provided by way of illustration and is not intended to limit the invention in any way.
I

EXAMPLE

A mixture of 250 gms of p,olyvinylidene difluoride, sold under the trade ~a~Q of Xynar by the PennWalt Corporation and 250 gms of a graphite, (artificial), sold under the name A-99, by Asbury Graphite Mills, Inc., having been pretreated at 800C in air for 2 hours, were dry mixed. The graphite was previously screened through a 325 mesh screen to ensure the particle size distribution of 0 to 45~m. To the dry mixture of graphite and polyvinylidene difluoride resin was added ~ gms of particulate Teflon~. The dry mixture containing these three components was screened through a 20 mesh screen then mixed with 250 ml of Shell Solvent 340 as a hydrocarbon carrier. The wet mixture was calendered between rollers to form sheets.
The sheets were dried at 20C for 168 hours then placed into a compression mold and molded into 15X15 inch plates under 400 kg/cm2 pressure at 190C.
The mold was gradually cooled during compression. The formed plates had a compressed thickness of 40 mils.

Claims

1. A bipolar electrode element for electrochemical cells comprising:
a pressure-molded composite of heat-treated electrically conductive graphite particles having a particle size distribution before molding of 0 to 45µm, said graphite being heat-treated at a temperature of at least 800°C in air for 2 hours prior to forming said composite; and thermoplastic resin particles, said graphite and said resin in said composite being in a weight ratio of from 1:5 to 1:1;
said composite characterized by less than 4.0% weight loss and less than 0.1% dimensional increase in length upon immersion at 60°C into polybromide oil for 1600 hours; and said composite being formed by pressure-molding a mixture comprising said graphite and said resin at 400 kg/cm2 or above at 190° C.

2. A bipolar element according to Claim 1.
characterized by a resistivity of less than 0.8 ohm-cm and a flexural strength of at least 2400 psi upon immersion at 60°C into polybromide oil for at least 2300 hours.

3. A bipolar element according to Claim 1 further comprising up to 2% by weight of a perfluorohydrocarbon wherein said graphite, said resin and said perfluorohydrocarbon are calendered with a hydrocarbon carrier into preformed shapes prior to being subjected to said pressure molding.

4. A bipolar element according to Claim 3 wherein said perfluorohydrocarbon comprises polytetrafluoroethylene.

5. A bipolar element according to Claim 1 wherein said thermoplastic resin comprises polyvinylidene difluoride.

6. A bipolar element according to Claim 5 wherein said graphite and said resin in said composite are in a weight ratio of 1:1.

7. A bipolar assembly comprising a bipolar element according to Claim 1 and a composite carbon felt, said felt and said element being held in interfacial contact with an adhesive.

8. A bipolar assembly according to Claim 7 wherein said adhesive is formed from carbon, polyvinylidene difluoride resin and dimethylformimide.

9. A method of making a bipolar element for electrochemical cells comprising the steps of:
a) dry mixing electrically conductive graphite particles with thermoplastic resin particles, said graphite particles having a particle size distribution of 0 to 45µm, said graphite further being heat-treated at a temperature of at least 800° in air for 2 hours;
said graphite and said resin being in a weight ratio of from 1:5 to 1:1, with up to 2% by weight of a perfluorohydrocarbon binder;
b) screening the mixture from step a) to form a particulate mixture of average particle size of 0 to 45µm;
c) forming a wet mixture of the particulate mixture from step b) with a liquid hydrocarbon carrier and calendering said wet mixture into sheets;
d) drying said sheets from step c);

e) compression molding said sheets from step d) under a pressure of at least 400 kg/cm2 at 190°C to form said bipolar elements.

10. A method according to Claim 9 wherein said perfluorohydrocarbon in step a) comprises polytetrafluoroethylene.

11. A method according to Claim 9 wherein said thermoplastic resin comprises polyvinylidene difluoride.

12. A method of making a bipolar element for electrochemical cells comprising the steps of:
a) dry mixing electrically conductive graphite particles and thermoplastic resin particles, said graphite having a particle size distribution of 0 to 45µm, said graphite further being heat-treated at a temperature of at least 800°C for 2 hours: said graphite and said resin being in a weight ratio of from 1:5 to 1:1:
b) screening the mixture from step a) to form a particle mixture of average particle size of o to 45µm;
c) compression molding said mixture from step b) under a pressure of at least 400 kg/cm2 at 190 C to form said bipolar element.

13. A method according to Claim 12 wherein said thermoplastic resin particles comprise polyvinylidene difluoride.