CA1246140A

CA1246140A - Coke filled separator plate for electrochemical cells

Info

Publication number: CA1246140A
Application number: CA000484389A
Authority: CA
Inventors: Gregory J. Sandelli; William A. Taylor
Original assignee: United Technologies Corp
Current assignee: RTX Corp
Priority date: 1984-12-24
Filing date: 1985-06-18
Publication date: 1988-12-06
Also published as: EP0186611A3; EP0186611A2; BR8506263A; ZA859226B; JPS61161666A; US4643956A

Abstract

Abstract Coke Filled Separator Plate For Electrochemical Cells An electrochemical cell separator plate formed from a mixture of carbonizable resin and coke particles, wherein the plate is molded from the resin and coke particles, carbonized, and graphitized. The particle size distribution of the coke is selected so that expansion occurring during graphitization minimizes the rupture of carbonized bonds within the separator plate. The separator plates have improved properties over separator plates formed from graphite particles including increased density, decreased porosity and increased corrosion resistance. The coke particle plates are more energy efficient to manufacture than graphite particle plates.

Description

:~L2~6~0 Description Coke Filled Separator Plate For Electrochemical Cells Technical Field The field of art to which this invention pertains is molded articles produced from carbonizable material, particularly adapted for use in electroche~ical cells.

Background Art Graphitized electrochemical separator plates eor use in ~uel cells are well known in the ~uel cell industry, A typical fuel cell comprises a plurality of individual cells, each cell comprising an anode, a cathode and an electrolyte. The fuel cells are typically stacked together to produce a unit with the desired electrical operating characteristics. The cells are separated from each other by separator plates.
Typically, separator plates are thin, molded structures formed from a composite of graphite and phenol-formaldehyde resin. It is critical to select a resin which is carbonizable. After the plate has been molded, the plate is heat treated to carbonize the resin, and then further heat treated to graphitize the molded plate. These yraphitized separator plates are particularly useful in fuel cells wherein the electrolyte is phosphoric acid and corrosion resistant separator plates are required.

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The components of a phosphoric acid fuel cell, in which the reactants are hydrogen and oxygen, are subjected to severe operating conditions and require certain physical, chemical, and elect rical characteristics in order to perform adequately and provide the fuel cell with a reasonably long life expectancy. The plate should be thin and should have a low permeability, especially to hydrogen gas. The plate should have high thermal conductivity and low tru-plane and in-plane elect-rical resistance. The plate should have high corrosion resistance as indicated by threshold corrosion po-tential and resistance to oxidation.
The plate should have high stxuctural integrity, in par-ticular, flexural s-trength. Flexural strength is related -to the lon~evity of a separator plate in an operational uel cell.
Graphitized separator plates manufactured from carbonizable resin and graphite particles for use in electrochemical cells are known in the art, see U.S. Patent No. 4,301,222, and have been successfully used as fuel cell separator plates.
~ethods of manufacturing graphitized electrochemical cell separator pla-tes from carbonizable resin and graphite particles for use in fuel cells are similarly known in the art, see U.S. Paten-t No.
4,360~4~5. Although the electrochemical cell separator plates of the prior art produced from graphite performed well, -there is always a constant search for plates with improved properties.

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For example, it is advantageous to have separator plates with improved thermal conductivity to increase the rate at which heat is removed from a fuel cell.
It is also advantageous to have an electrochemical cell separator plate with increased retention of flexural strength as a result of improved corrosion resistance in order to prolong the life of the separator plates and fuel cell. In addition, separator plates having decreased electrical resistivity resulting from higher density would increase the efficiency of a fuel cell.
Electrochemical separator plates having increased corrosion resistance due to the use of materials o~
construction having lower levels Oe impurities would prolong the useful life of fuel cells.
Electrochemical separator plates having increased density and decreased porosity are desirable.
Electrochemical separator plates manufactured from low cost components and with a lower expenditure of energy during the overall manufacturing process are similarly advantageous.
Accordingly, what is needed in this art are improved separator plates having increased density, decreased porosity, increased thermal conductivity, decreased electrical resistivity, improved corrosion resistance, and which are less costly to manufacture.

Disclosure of Invention It has now been found that electrochemical cell separator plates comprising the molded, carbonized and graphitized product formed from a composition comprising about 30 wt.% to about 60 wt.% of coke ~2~6~

particles and about 30 wto% to about 60 wt.% of carbonizable resin have advantages over conventional plates manufactured from graphite particles. The molded plates are carbonized and then graphitized resulting in graphitized separator plates having increased density, decreased porosity, and increased corrosion resistance over conventional separator plates formed from graphite particles.
Another aspect of this invention is a fuel cell of the type comprising a plurality of cells, each cell comprising an anode, a cathode, an electrode and separator plates, wherein the fuel cell incorporates improved carbonized and ~raphitized separator plates formed from a composition comprising about 30 wt,~ to about 60 wt.% of coke particles, and about 30 wt.~ to about 60 wt.~ of carbonizable resin. The separator plates have increased density, decreased porosity, and increased corrosion resistance over separator plates formed from graphite particles.
Another aspect of this invention is a method of manufacturing electrochemical cell separator plates wherein the plate is molded at sufficient heat and pressure to densify the plate, the plate is then heated at a sufficient temperature to carbonize the resin in the molded plate, and then the plate is heated to graphitize the entire plate, wherein an improved plate is produced by using as the molding composition a mixture comprising about 30 wt.% to about 60 wt.% of coke particles and about 30 wt.~ to about 60 wt.~ of carbonizable resin. The separator plates produced by this method have increased density, decreased porosity, and increased corrosion resistance over separator plates formed from graphite particles.
The foregoing, and other features and advantages of the present invention will become more apparent from the following description.

Description of Preferred Embodiments The coke used in the practice of this invention will preferably be petroleum coke. It is particularly preferred to use high purity petroleum coke, Petroleum coke is typically made by heating petroleum pitch and then calcining the pitch to produce a thermal degradation or decomposition of the hydrocarbons in the pitch to produce what is known as coke. The coke is then granulated and is typically purchased as a powder. The coke useful in the practice of this invention will typically have a~
sufficient particle size distribution such that minimal fracture of carbonized resin bonds occurs for a given separator plate thickness. The particle size is directly related to the size of the component to be molded. T~e coke useful in the practice of this invention will typically have a particle size distribution wherein about 2% to about 10% oE the coke 25 particles are about 74 microns to about 149 microns, about 17% to about 35% of the coke particles are about 44 microns to about 74 microns, and at least about 55%
to about 80~ of the coke particles are less than about 44 microns, It is preferred that at least about 55%
of the coke particles are less than about 44 microns ~2~4~

and that 100~ of the coke particles are less than 150 microns. The coke particles will ha~e an aspect ratio sufficient to produce a graphitized separator plate having sufficient flexural strength and structural characteristics. Aspect ratio is defined as the ratio of the difference between the length and width of a particle divided by the length of that particle.
Typically the aspect ratio will be less than about 0.5 and preferably about 0.35.
It is optional to use prepuffed coke in the practice of this invention. Normally when the molded coke resin composite is graphitized, the coke particles tend to increase in volume i.e., tend to "puf". This can create problems such as stress cracks and the breaking of the coke resin bonds in the yraphitized plate. Prepuffing is defined as a process wherein the coke particles are exposed to sufficient heat to pre-expand the particles prior to molding.
Prepuffing of coke is an extra step in the manufacturing process requiring significant energy inputO It should be noted, however, that if the correct particle size is chosen, surprisingly and unexpectedly puffiny is not a problem and no significant fracture of carbonized resin bonds is observed to occur during graphitization. It is critical in the practice of this invention to ascertain the correct coke particle size for a given thickness of molded object in order to use coke which has not been prepuffed. The use of coke particles which have not been prepuffed yields separator plates having a higher density than graphite plates. The ~2~

coke particle size distribution is similarly deter-mined in accordance with the component to be molded.
The petroleum coke which can be used in the practice of this invention includes AircoTM Grade 90 petroleum coke manufactured by Airco Carbon Company, Saint Mary's, Pennsylvania, and Asbury CF70-W coke manufactured by Asbury Graphite Mills, Asbury, New Jersey. The coke is preferably of high purity, for example, a precursor of a high or intermediate purity graphite.
The resins which can be used in the practice in this invention include the thermosetting phenol-formaldehyde resins, both novolacs and resols. It is crtical that the resin selected be lS capable of virtually eomplete carbonization. While thermosetting phenolic resins are preferred, it is possible to use other resinous ma-terials such as coal tar or petroleum pitch resins, furfural resins, etc~ Phenol-formaldehyde resins are well known in the art and are typically manufactured by reacting phenol with aqueous formaldehyde in the presence of a basie eatalyst. When the proeess is varied and an acid catalyst is used, a novolac resin is produced.
A resin which may be used in the practice of this invention is Reichhold Chemical Company (Niagara Falls, New York) grade 24-655 phenolic resin or grade 2~-810 phenolic resin. Other resins that can be used to manufacture the separator plates of this invention include Plastics Engineering Company brand phenolic resin number 1339 and phenolic resin number 1442 manufactured by Plastics Engineering Company, Sheboygan, Wisconsin.

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Other additives conventionally used in the art for compression molding phenolic resins may be used to manufacture the electrochemical plates of this invention. For example, sufficient amounts of lubricants, mold release agents, etc. may be included in the molding compositions of this invention to improve the molding process parameters.
To manufacture the separator plates of this invention, initially the dry coke is mixed with the dry powdered phenolic resin for a sufficient period of time to achieve a uniform mix. Any dry mixing process can be utilized to accomplish the mixing of the components. Typlcally the components are mixed for about three to about five minutes, more typically about three minutes to abou-t Eour minutes, and preferably about three minutes to about three and one-half minutes to achieve a homogeneous mixture in a mixing means such as a Littleford blender, manufactured by Littleford Company, Covington, Kentucky or a Nauta blender manu-factured by J.H. Day Company in Ohio. It is desirable, althougn not necessary, to further com-pound the dry molding mixture to obtain sufficient homogeneity and sufficient consistency in the molded plate. This is typically done by hot milling and pelletizing using processes and equipment conven-tional and known in the art for producing phenolic molding compounds. Whether or not further compound-ing is required depends upon the particular characteristics of the resin selected.

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g Compounding and hot milling of phenolic molding compounds is disclosed in Polymer Chemistry. An Introduction, Seymour, R.V., and Carraher, Jr., C., P. 225, Marcel Dekker, Inc., New York, 1981.
The molding composition is then molded in conventional compression molding equipment with sufficient heat and pressure and for a sufficient time period to provide a molded plate with a density of about 1.58 grams/cc to about 1.62 grams/cc., more typically about 1.58 grams/cc to about 1.60 grams/cc, and preferably about 1.5g grams/cc to about 1.60 grams/cc. The plates are -typically molded at a temperature of about 250F to about 350F, more typically abou-t 275F to about 350F, and preferably about 3000F to about 3500F, at a pressure typically about 500 psig. to abou-t 1,500 psig., and preferably about 600 psig. to about 1,500 psig. for a time period of about 2 minutes to about 10 minutes, more typically about 2 minutes to about 6 minutes, and preferably about 2 minutes to about 5 minutes. Typical of the compresison molding presses used in the practice of this are is a Williams White M 1500 ton compresion molding press manu-facturecl by Williams White Company, located in Moline, Illinois 61265.
The molded plates are then surface ground, if necessary, to achieve a uniform thickness.
Surface grinding is done with grinding equipment conventional in the art. Coarse and fine (about 180 grit) grinding media should be used to minimize blinding of the grinding media. It is important that the pla~e be molded to the most uniform thickness economically feasible to minimize or eliminate surface grinding.
Once the plates have been ground to the size required, the next step in the process is the carbonization of the phenolic resin. That is, the phenolic resin must be converted to carbo~ by controlled decomposition of the carbon hydrogen bonds in the resin. The carbonization process is accomplished in a conventional gas fired convection oven with a retort and computerized controls. The plates are typically stacked in the oven in such a manner that the plate5 are restrained to maintain flatness, the retort is then purged with nitrogen or an inert gas, and the plates are exposed to a controlled heat-up cycle wherein the temperatures are typically about 1,200F to about 2,000F~ more typically about 1,200F to about 1,850F, and preferably about 1850F for a sufficient amount of time to achieve carbonization without adversely affecting the plate characteristics by permitting the gaseous by-products of the carbonization process to slowly be removed ~rom the plate interior. The pre~erred cycle time is about 100 hours to about 200 hours, Once the plates have been carbonized, the next step is the graphitization process. The plates are graphitized at temperatures of about 2,100C to about 3,000C, more typically about 2,200C to about 3,000C, and preferably about 2,650C to about

2,850C. The graphitization process is an electrical ~24~4~

heating process in which the plates are connected to a current source, and, sufficient current is passed through the resistive load to produce the graphitization temperatures. The process used is the Acheson process in which a standard graphitization cycle, known in the art, is used tc graphitize the plate. The finished plates have a nominal thickness of about .025 inch to about .036 inch, more typically about .028 inch to about .033 inch, and preferably about .030 inch. The molded plate, prior to carbonization and graphitization, is approximately 15%
oversized to compensate for shrinkage during the carbonization and graphitiza~ion processes. The electrochemical separator plates of this invention typically can have a nominal size Oe up to about 16 square feet, although, nominal sizes of about 5" X 5"
to about 24" X 27" are particularly useful.
The separator plates of this invention have higher densities than the graphite particle plates of the prior art and exhibit improved properties. Due to the significant quantities of heat generated during the operation of a typical fuel cell it is important that the cell have high thermal conductivity. Thermal conductivity is defined as the rate of heat conduction per unit area degree fahrenheit. The separator plates of the present invention have thermal conductivities of about 40 BTU/hr ft2F to about 120 BTU/hr ft2F, more typically about 40 BTU/hr ft2oF to about 70 BTU/hr ft F. Preferably, the thermal conductivity is at least about 40 BTU/hr ft2F.

The separator plates of the present invention will typically have an in-plane electrical resistivity less than about 1 X 10 2 ohm-cm, more typically less than about 0.5 X 10 ohm-cm, and preferably less than about 0.25 X 10 2 ohm-cm. The plates of the present invention will typically have a thru-plane resistivity o~ less than about 2 X 10 2 ohm-cm, more typically less than about 0.75 X 10 2 ohm-cm, and preferably less than about 0.4 X 10 2 ohm-cm.
The plates of the present invention will have sufficient structural integrity to withstand typical fuel cell operation of about 40,000 hours at a temperature o~ about 400F. The primary parameter which is a good indicator of the structural longevity of a separator plate is the retention of flexural strength as a result of good corrosion resistance.
Flexural strength is defined as the upper limit of plate bending ~ithout cracking~ The plates of the present invention have initial flexural strengths of typically about 4,000 psi to about 9,000 psi, and more typically about 5,000 psi to about 7,000 psi.
As previously mentioned, it is important to have separator plates with high initial flexural strength and good corrosion resistance thereby enabling the plate to retain its structural strength and improve longevity during operation. The separator plates of the present invention have increased resistance to corrosion and to oxidation. Corrosion resistance can be predicted by measuriny the threshold corrosion potential. The threshold corrosion potential is the ~46~

electrochemical potential at which there is a breakdown of the carbon in a plate, to form carbon monoxide and carbon dioxide indicated by a rapid increase in the magnitude of current. The magnitude of the threshold corrosion potential is related to the purity of the components used to manufacture the plate as well as the degree of graphitization. Typically, the threshold corrosion potential of the plates o~
this invention will be about 1,150 millivolts to about 10 1,210 millivolts, more typically about 1,165 millivolts to about 1,200 millivolts and generally about 1,190 millivolts to about 1,200 millivolts.
Another indication of corrosion resistance is the open porosity o~ the plate. The open porosity will typically be about 4~ to about 6~ Eor pores greater than 0.004 micron. Open porosity refers to surface pores in the plate. Open porosity is measured by mercury intrusion porosimetry using a conventional yorosimeter.
It is believed that the use of coke in separator plates rather than graphite particles results in a decrease in the residual impurities in the graphitized plate. It is thought that impurities are inherent in graphite powder because of the additional processing done by the manufacturer. The reduction or elimination of the impurities by using coke particles improves the corrosion resistance.
The graphitized electrochemical plates of this invention will typically have a density of about 1.88 g/cc to about l.9~ g/cc, more typically about 1.88 g/cc to about 1.92 g/cc, and typically greater than f~

about 1.88 g/cc. It should be noted that increased density results in a plate with decreased wetted areaO
Wetted area is related to corrosion threshold.
Increased density and resulting decreased wetted area result in lower measured corrosion current at the same current density. It should be noted that oxidation and corrosion rates are generally related to increased active or wetted area of the plate as well as impurities in the plate.
It is contemplated that the separator plates of this invention will have multiple uses in addition to use as separator plates in fuel cells. The plates can be used as battery separators, the plates can also be used as ionic membrane cell separators, or the plates can be used in any system or chemical process requiring a separating barrier constructed of an inert conductive material.
The following example is illustrative of the principles of practice of this invention although not ,20 limited thereto. Parts and percentages where used are parts and percentages by weight, EXAMPLE
An electrochemical plate was made by initially mixing 50 wt.% of coke particles with 50 wt,% of phenolic resin until a homogeneous mixture was produced. The coke was Asbury CF70W petroleum coke manufactured by Asbury Graphite Mills, Asbury, New Jersey. The coke particles had an average aspect ratio of less than 0.4; the particles had a particle size distribution such that 80% of the particles were 6~

less than 44 microns in size and 100% of the particles were less than 150 microns in size. The resin used was powdered Reichhold Grade 24-655 phenolic resin manufactured by Reichhold Chemical Company, Niagra Falls, New York. The coke and resin were mixed for about five minutes in a Littleford brand mixer.
The molding mixture was then molded into a separator plate having a length of about 6 inches, a width of about 6 inches and a thickness of about 0.05 inch. The plate was molded in a 50 ton laboratory compression molding press manufactured by ~aldwin-De~iance, Inc., Broomal, PA 16008 at a -temperature of about 300F, a pressure of about 5,000 psiy. ~or about 3 mlnu-tes.
The molded plate was then packed in a LindbergTM brand electric convection oven with a retort manufactured by 5O1a-Basic Industries, Chicago, IL. The plate was carbonized by purging the retort with nitrogen and slowly hea-ting to about 1850F in a conventional controlled heating carbon-ization cycle with a cycle time of about 160 hours.
The plate was next graphitized using a conventional Acheson Graphitization Process by plac-ing the plate in a conventlonal Acheson graphit-ization furnace and passing a suf~icient current through the plate until a temperature of ~,650C was reached. The plate was held at this temperature for at least one hour.
The graphitized plate had a length of 5 inches, a width of 5 inches and a thickness of 0.040 inch.

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A comparison of the plate characteristics of the coke particle plates with the graphite particle plates of V. S. Patent No. 4,301,222 is pres,anted in Table I.

TABLE I

Graphite Coke Particle Plate Particle Plate Density 1.88 g/cc 1.93 g/cc % Open Porosity 8.7% 5.6%
Flexural Strength 6,773 psi 8,780 psi Electrical Resi~ivity In-plane 1.7 X 10-3 ohm-cm 2.4 X 10-3 ohm-cm Thru-plane 8.76 X 10-3 ohm-cm 7.2 X 10-3 ohm-cm Thermal Conductivity In-plane 59 BTU/hr ftF 38 BTU/hr ftF

Corrosion Threshold 1,140 mv 1,165 mv A test of corrosion resistance was conducted by immersing the plate in 105% phosphoric acid maintained at a temperature of 400F for 5,000 hours and maintaining a 950 mv potential on the plate. The corrosion test results are present in Table II.

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TABLE II

Coke Particle Plate Corrosion Threshold Initial 1,165 mv Final 1,135 mv - Flexural Strength Initial 8,780 psi Final 8,71S p~i Weight Change ~ 0.85~

The coke particle separator plates of the present invention have surprising and unexpected improved properties over the graphite particle separator plates of the prior art The plates of the present invention have a higher density, and decreased open plate porosity. The plates have increased corrosion resistance as shown by improved corrosion threshold potential and increased resistance to oxidation. It is expected that, overall, the electrical resistivity and the thermal conductivity of coke particle plates will be improved over graphite particle plates due to the increased density of the coke particle plates.

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It should be noted that coke particle separator plates are more economical to manufacture since coke tyically has a co~t below that of graphite due to the high energy inpu~ required to manufacture graphite.
The process of the present invention is energy efficient since the coke is converted to graphite during the separator plate graphitization process.
The graphite particle plates of the prior art must still be graphitized resulting in a wasted expenditure of energy required to bring the graphite particles up to the graphitizing temperature. A person skilled in the art would realize that the overall manufacturing process is more energy efficient when coke particles are used in the separator plates versus graphite particles.
Although this invention has been shown and described with respect to detailed embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the spirit and scope of the claimed invention.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. An electrochemical cell separator plate comprising the molded, carbonized and graphitized product formed from a composition comprising:
about 30 weight % to about 60 weight % of coke particles;
said coke particles having a size less than 150 microns; and about 30 weight % to about 60 weight % of carbonizable resin;
wherein the separator plate has increased density, decreased open porosity and increased corrosion resistance over separator plate formed from graphite particles.

2. The separator plate of claim 1 wherein the coke has a particle size distribution such that about 55% to about 80% of the coke particles are less than about 44 microns in size.

3. The separator plate of claim 2 wherein the resin is a carbonizable phenolic resin selected from the group consisting of phenol-formaldehyde resols and phenol-formaldehyde novolacs.

4. A fuel cell comprising at least one anode, at least one cathode, electrolyte material and at least one separator plate, wherein the improvement comprises as the separator plate a carbonized and graphitized separator plate formed from a composition comprising:
about 30 weight % to about 60 weight %
coke particles;

said coke particles having a size less than 150 microns; and about 30 weight % to about 60 weight %
carbonizable resin;
the separator plate having increased density, decreased open porosity and increased corrosion resistance over separator plates formed from graphite particles.

5. The fuel cell of claim 4 wherein the coke has a particle size distribution such that about 55%
to about 80% of the coke particles are less than about 44 microns in size.

6. The fuel cell of claim 4 wherein the resin is a carbonizable phenolic resin selected from the group consisting of phenol-formaldehyde novolacs.

7. An electrochemical cell separator plate produced from a carbonizable and graphitizable mold-ing composition by molding at sufficient heat and pressure to densify the plate, and then heating at a sufficient temperature to carbonize and then graphitize the plate, wherein the improvement com-prises using as the molding composition a mixture comprising:
about 30 weight % to about 60 weight % of coke particles;
said coke particles having a size less than 150 microns; and about 30 weight % to about 60 weight % of carbonizable resin;

wherein the carbonized plate has increased density, decreased open porosity, and increased corrosion resistance over separator plates formed from graphite particles.

8. The electrochemical cell separator plate of claim 5 wherein the resin is a carbonizable phenolic resin selected from the group consisting of phenol-formaldehyde resols and phenol-formaldehyde novolacs.

9. An electrochemical cell separator plate of claim 5 wherein the coke has a particle size distri-bution such that about 55% to about 80% of the coke particles are less than about 44 microns in size.