CA1303123C - Electrode substrate for fuel cell and process for producing the same - Google Patents
Electrode substrate for fuel cell and process for producing the sameInfo
- Publication number
- CA1303123C CA1303123C CA000563352A CA563352A CA1303123C CA 1303123 C CA1303123 C CA 1303123C CA 000563352 A CA000563352 A CA 000563352A CA 563352 A CA563352 A CA 563352A CA 1303123 C CA1303123 C CA 1303123C
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- resin
- electrode
- carbon fiber
- substrate
- carbon fibers
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
- H01M4/96—Carbon-based electrodes
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/30—Hydrogen technology
- Y02E60/50—Fuel cells
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- Chemical & Material Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Electrochemistry (AREA)
- General Chemical & Material Sciences (AREA)
- Inert Electrodes (AREA)
- Fuel Cell (AREA)
- Ceramic Products (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A porous electrode substrate for fuel cell and a process for producing the same, which contains 35 to 60 % carbonized resin by weight, and has pores with a mean pore size of 20 to 60 µm and a porosity of 60 to 80 %, and a compression ratio of 20 % or less. chopped carbon fibers with fiber lengths of 3 to 20 mm and fiber diameters of 4 to 9 µm are dispersed in random directions within substantially 2-dimensional plane, and bound with an organic binder to make an carbon fiber mat. The carbon fiber mat is impregnated with a resin to make prepreg and hot pressing the prepreg under a temperature of 120 to 200°C and a pressure of 2 to 10 kg/cm2. Then, the press molded product is carbonized by heat treatment under a temperature of 1300 to 3000°C in an inert atmosphere or in vacuum to obtain an electrode substrate excellent in various characteristics such as gas permeability, flexural strength, compression strength, etc. Further, for improvement of electroconductivity, it is preferable to use a mixture of a curable resin and a non-curable resin as the resin impregnated into the carbon fiber mat.
A porous electrode substrate for fuel cell and a process for producing the same, which contains 35 to 60 % carbonized resin by weight, and has pores with a mean pore size of 20 to 60 µm and a porosity of 60 to 80 %, and a compression ratio of 20 % or less. chopped carbon fibers with fiber lengths of 3 to 20 mm and fiber diameters of 4 to 9 µm are dispersed in random directions within substantially 2-dimensional plane, and bound with an organic binder to make an carbon fiber mat. The carbon fiber mat is impregnated with a resin to make prepreg and hot pressing the prepreg under a temperature of 120 to 200°C and a pressure of 2 to 10 kg/cm2. Then, the press molded product is carbonized by heat treatment under a temperature of 1300 to 3000°C in an inert atmosphere or in vacuum to obtain an electrode substrate excellent in various characteristics such as gas permeability, flexural strength, compression strength, etc. Further, for improvement of electroconductivity, it is preferable to use a mixture of a curable resin and a non-curable resin as the resin impregnated into the carbon fiber mat.
Description
13(J3123 TITLR OF THR INVKNTION
Electrode Sub~trate for Fuel Cell and Process for Producing the Same BACKGROUND OF T~l~ INV~NTION
Thi~ invention relates to an electrode substrate suitable for fuel cell, particularly phosphoric acid fuel cell and a process for producing the same.
Phosphoric acid fuel cell i8 constituted of a matrix layer impregnated with a phosphoric acid solution which is an electrolyte, and positive and negative electrodes sandwiching the matrix layer from both surfaces. Electrodes are constituted by forming catalyst layers on the surfaces of electrode substrates which are contacted with the mntrix layer.
The electrode substrates to be used for such phosphoric acid fuel cell are demanded to have variou~
electrical, chemical and mechanical characteristics, including high electroconductivity as a matter of courqe, chemical stability at high temperatures, excellent ga~ permeability, and also hi~h flexural strength, etc.
More specifically, the electrode ~ubstrate for phosphoric acid fuel cell, since it is required to haYe the function to feed a fuel gas (hydrogen gaY, etc.) or oxidative gas (air or oxygen gas) to the catalyst layer adjacent thereto, is first required to have basically high gas permeability. Gas permeability of the electrode substrate is given by porous property of the electrode sub~trate, namely by the electrode substrate having interconnected pores, and gas permeability performance i9 determined by the pore size and poro~ity, etc. If interconnected pores having large l3a~
pore sizes are formed in the electrode substrate for improvement of ga~ permeabilitv of the electrode substrate, the phosphoric acid ~olution will be readily scattered therethrough, whereby excessive constlmption of the solution occurs. On the contrary, if interconnected pores having ~mall pore si~es are formed, the phosphoric acid solution penetrate to the electrode substrate from matrix layer through the capillary phenomenon, whereby its gas permeability may be lowered or the cell performance may be lowered due to shortage of the electrolyte in the matri~ layer.
Thus, setting of the pore si7e and porosity within optimum value ranges is one of important matters which influences electrode performances.
Also, a phosphoric acid fuel cell, in order to obtain a power enough to be provided for commercial use, is stacked up with a large number of cell units shaped in flat plate, and therefore the electrode substrate must have high mechan;cal strength, particularly flexural strength. High mechanical strength is also necessary for performing easily workings of wetproof treatment of the electrode substrate or coating of the catalyst, assembling working of cell units, etc. This demand for mechanical strength becomes greater as the sub~trate area of the fuel cell becomes larger. Further, for improvement of electroconductivity between the contacted surfaces of the respective cell units, the stacked cell units are clamped with a given compressive force in the stacking direction. Accordingly, the electrode substrate of each unit mu~t have uniform characteri~tics relative to compression. If the amounts to be compressed when a constant compreYsive force is permitted to act differ 13~1J~lZ3 depending on the electrode ~ubstrate~ of the respective unit~, the characteri~tic~ of ga~ permeability of the electrode sub~trate~ of the re~pective cell unit~ will be al~o different, whereby variance occurs in cell characteristic~ between the units, and the height~ of the re~pective unit~ become irregular.
For the electrode aub~trate of a pho~phoric acid fuel cell comprising a large number of cell unit~
~tacked up, the ~urface roughne~ i 9 al~o an important problem. More ~pecifically, the electrode of a pho~phoric acid fuel cell comprise~ a catalyst layer con~i~ted of p]atinum or platinum alloys ~upported on a carrier coated on the electrode ~ub~trate ~urface, and the coating thickne~ of the cataly~t is generally 50 to 500 ~m. Therefore, when the ~urface roughne~ of the electrode ~ubstrate i~ great, planar variance of the coated amount of the cataly~t i~ not negligible and power generation irregularity occur~ to cau~e lowerinR
in power Reneration efficiency. Al~o, for performing practical power generation, a large number of cell unit~ may be ~tacked up electrically in ~erie~. In thi~ ca~e, ~ince an electroconductive gas separator i~
~andwiched between the adjacent unit cel]~, one ~urface of the electrode ~ubstrate i~ as~embled 80 a~ to be contacted with the gas ~eparator. If the ~urface of the electrode ~ub~trate i~ rough, the contact between the ~ubstrate and the ga~ ~eparator become~
in4ufficient, whereby electrical re~i~tance at the contacted ~urface is increased to lower power generation efficiency.
A~ the ~ub~trate con~tituting the electrode of fuel cell as de~cribed above, there have been known in the art tho~e compri~ing ~hort carbon fiber~ di~per~ed in random directions within substantially 2-dimensional plane bonded to each other with carbonized or graphitized re~in by means of paper makin~, etc., as disclosed in the literatures such as Japanese Patent Publications Nos. 63-18603 and 53-43920, Japanese Laid-Open Patent Publications Nos. 57-129814, 57-166354 and 60-44963 and others.
All of these substrate~ of the prior art described in theRe literatureR had one advantage and one shortcoming in gas permeability, mechanical stren~th, compres~ive characteristic, etc. ~uch that some Rre good in ga~ permeability but involve a problem in mechanical strength, or on the contrary others are high in mechanical strength but inferior in gas permeability, etc. Thu~, no electrode sub~trate has been obtained, which can sufficiently satisfy the requirements in all of these characteristics.
On the other hand, Japane~e laid-Open Pntent Publication No. 58-68881 disc]oses a method for obtaining an electrode substrate all at once according to moldi~g fabrication of a mi~ture of chopped carbon fiber~ and a carbonizable re~in without recourse to paper making. Since this method relies on molding fabrication, it i~ neceYR~ry to u~e chopped carbon fibers with extreme]y short fiber length of, for example, 1 mm or le~9 ~ and therefore mechanical strength i~ remarkably inferior as compared with the ~ub~trate prepared by the paper making method, whereby there is the problem that handlin~ of the electrode becomes difficult in workings of wetproof treatment of the electrode sub~trate, coating of the cataly~t, assembling of cell units, etc. as described above.
13V31~;~
OllJ~3CT~3 AND ~UMMARY OF TI~R I NV~NTION
Primary object of the pre~ent invention i~ to provide a ~ubstrate and a proce~s for producing the ~ame, which is ~uitable for con~tituting the e]ectrode of a fuel cell, particularly the electrode of a phosphoric acid fuel cell, having excellent chemical ~tability at hi~h temperature~ and electroconductivity, as well a~ var;ou~ excellent characteri~tics ~uch as ga~ permeability, flexural ~trength, compres~ive characteristic, etc., and yet being well balanced in these various characteri~tic~.
Another object of the present invention is to provide an electrode substrate for fuel cell and a proces~ for producing the ~ame, which has higher electroconductivity and yet excellent mechanical strength.
Still another object of the present invention is to provide an electrode sub~trate with ~mooth ~urfaces and a process for producing the ~ame, which i~ improved in power generation efiiciency by precluding power generation irregularity and electrical contact inferiority caused by the ~urface roughnes~ of electrode substrate.
In accordance with the present invention, there is provided a porous electrode ~ubstrate for fuel cell comprising ~hort carbon fibers disper~ed in random directions within sub~tantially 2-dimen~ional plane and carbonized resin for mutually bonding the carbon fiber~. The individual fiber of the carbon fibers ha~
a diameter of from 4 to 9 ~m and a length of from 3 to 20 mm. The content of the carbonized re~in i~ in the range of from 3h to 60 X by weight, and the sub~trate of the present invention haY pore~ with a mean pore ~3~
si~e of from 20 to 6Q ~m and a porosity of from 60 to ~0%, and a compression ratio not more than 20%.
Preferably, a surface of the substrate of the present in~ention has a surface roughness of not more than 100 ~m, defined as the ma~imum undulation of the rolling circle tracing the shape of the substrate surface.
The electrode substrate of the present invention is produced according to the following process. Chopped carbon fibers each of which has a length of from 3 to 20 mm and a diameter of from 4 to 9 ym are mixed by stirring together with a dispersion medium containing an organic binder, and then a paper-like sheet is made from the mixture to obtain a carbon fiber mat.
Alternatively, chopped carbon fibers are mixed by stirring together with a dispersion medium containing no organic binder, after that a paper-like sheet is made from the mixture, and then the paper-like sheet obtained is treated with an organic binder to obtain a carbon fiber mat. The carbon fiber mat is impregnated with a resin to obtain a prepreg, i.e., a mat impregnated with a resin. After the impregnation, the prepreg is hot pressed under a temperature of form 120 to 200C and a pressure of 2 to 10 kg/cm2 to obtain a molded substrate. Then, the molded substrate is heat treated to carbonize the resin under a temperature of from 1300 to 3000C in an inert atmosphere or vacuum atmosphere to obtain an electrode substrate excellent in gas permeability, flexural strength, compression strength, etc.
13(~3~Z3 - 6a - 72465-5 Preferably, the carbon fiber ma~ is dried by heating, and the amount of the organic binder attached to the ~arbon fiber mat after completion of the drying step is in the range of from 5 to 30~ by weight based on the total weiqht of the carbon fiber mat.
Also, in the hot pre~in~ ~tep, if desired, a plural number of sheets of the prepre~ are laminated and subjected to hot pressing in the hot pre4si.n~ step.
The re~in impregnated into the carbon fiber mat may contain a resin which i~ curable in the Aub~equent hot pressing ~tep and a resin which i~ not curable in the subsequent hot pressing step, and the mixin~ ration may be preferably 10 to 500 part~ by weight of the non-curable resin based on 100 parts by weight of the curable resin. The curable re~in is at least one selected from the group consi~ting of resol type phenolic resins, polyphenyl quinoxalines and thermosetting polyimide~, and the non-curable resin is at least one selected from the group consi~ting of novolak type phenolic resins, pitch, furan resins, epoxy resin~ and polyphenylene oxides.
The above and other objects, features and advantages of the present invention will become more apparent from the ensuinR detailed description taken in conjunction with the accompanyin~ drawing~.
BRIFF DFSCRIPTION OF T~F DRA~INGS
Fig. ~ i~ an exploded perspective view showing the cell ~tructure of a phosphoric acid fuel cell, particularly a flat plate electrode fuel cell for which the electrode substrate of the present invention is applied;
Fi~. 2 is an exploded perspective view showing the ~ell structure of a phosphoric acid fuel cell, particul.arly a ribbed electrode fuel cell for which the electrode sub~trate of the present invention is applied;
Fig. 3 is a graph showing the relationships of the ~3'~31Z3 diameter _ of the individual chopped carbon fibers used in the electrode substrate with the mean pore size Pd and pressure loss Pl when a gas is flowed throu~h the thickness direction of the sub~trate, in the case of us;ng only a resol type pheno~ic resin as the resin impregnated to the carbon fiber mat;
Fig. 4 is a graph showing the relationships of the fiber length 1, of the chopped carbon fiber~ used in the electrode substrate with the mean pore size Pd and the flexural strength Bs of the substrate, in the case of using only a resol type phenolic resin as the resin ;mpregnated to the carbon fiber mat;
Fig. 5 is a graph showin~ the relationships of the carbonized resin content C in the electrode substrate with the resistivity Rt in the thiclcness direction and the pressure lo~s Pl in the thickne~s direction, in the case of using only a re~ol type phenolic resin as the resin impregnated to the carbon fiber mat;
Fig. 6 i~ a graph showing the relationships of the pressure applied P during hot pressing of the mat prepreg with the mean pore size Pd of the substrate and the compre~sion ratio Cr when a load of 5 kgfJcm2 is applied on the substrate, in the case of u~ing only a resol type phenolic resin as the resin impregnated to the carbon fiber mat;
Fig. 7 is a graph showing the relationships of the porosity Pr of the substrate with the pressure loss Pl in the thickness direction and the resistivity Rt in the thickness direction, in the case of using only a resol type pheno~ic resin a~ the resin impregnated to the carbon fiber mat;
Fig. 8 is a grap} showing the surface roughness of the electrode substrate of the present ;nvention; and ~3~3~23 Fi~. 9 i~ a graph showing the ~urface roughne~ of the electrode substrate of the compara-tive example.
DFTAILRD DUS~PIPTION
The pho~phoric acid fuel cell for which the electrode sub~trnte of the pre~ent invention i8 preferably applied may be one having a cell ~tructure a~ ~hown in Fi~. 1 and Fig. 2. Fig. 1 ~how~ one in which an electrode ~ub~trate ~haped in flat plate i~
used, and Fig. 2 one in which a ribbed electrode ~ub~trate i8 used.
In Fig. 1, on one surface of a matrix layer 10 containing pho~phoric acid electrolyte, an oxidative gas tair or oxygen ga~) electrode, namely an air electrode 11 is ~uperpo~ed, while on the other ~urface a fuel ga~ (hydrogen ga~, etc.) electrode, namely a fuel electrode 12 i~ ~uperpo~ed. The electrode~ 11 and 12 are each constituted by the electrode 3ubstrate 11a, 12a, and cataly~t layer 11b, 12b of platinum or platinum alloys supported on a carrier which i~ to be contacted with the matrix layer 10. Pho~phoric acid fuel cell i~ constituted by ~tacking up a lar~e number of cell un:it~, each includ;ng matrix 1ayer 10, air electrode 11 and fuel electrode 12, with gas ~eparator~
13, 14 interpo~ed between the adjacent cell units. In the flat plate electrode type ~hown in Fig. 1, the oxidative gas and the fuel ga~ are re~pectively introduced into the groove~ 13a (13b), 14a (14b~
between the ribs formed on the gas ~eparators 13, 14.
Al~o in the ribbed electrode cell structure shown in Fig. 2, ~imilarly as the flat plate electrode of Fig. 1, air electrode 11' and fuel electrode 12' are superposed on both surface~ of the matrix layer 1~' so 13V3~L,.,~
1 o as to sandwich ;t therebetween. The electrodes 11' and 12' are each constituted by the electrode sub~trate 11'a, 12'a, and catalyst layer 11'b, 12'b which is to be contacted with the matrix layer 10'. The ribbed electrode cell i~ constituted by stacking up a large number of units, each including matrix layer 10', air electrode 11' and fuel electrode 12' through gas separator~ 13', 14' shaped in flat plate, which are interposed between the ad~jacent cell units. In the case of the ribbed electrode cell, the oxidative gas and the fuel ga~ are respectively introduced into the grooves 11'c, 12'c between the ribs formed on the electrode substrates 11'a and 12'a.
In the following, the electrode sub~trate of the present invention i~ de~cribed along with its preferable production process.
I~ the present invention, the electrode sub~trate is constituted of a porou~ material, comprising short carbon fibers dispersed in random direotions within Yubstantially 2-dimensional plane bonded to each other with carboni~ed resin.
The ~hort carbon fiber may compri.~e any of polyacrylonitrile cnrbon fiber, pitch carbon fiber, rayon carbon fiber, etc. However, polyacrylonitrile carbon fiber with relatively higher mechanical stren~th i8 preferred. The Yhort carbon fibers are generally obtained by cutting and splitting continuous fiber bundles. Fiber bundle~ are generally attached with a ~ynthetic sheafing agent such as epo~y resin, etc., and ~uch sheafing agent should be preferably removed for improvement of dispersibility in the paper making step as described below. When use of a ~heafing a~ent is indispensable, it is preferable to use a water-soluble 13V31~3 ~heafing agent ~uch as polyvinyl alcohol, polyethylene glycol, starch, etc.
The individual f;ber of the chopped carbon fiber~
u~ed ha~ a diameters of from 4 to 9 ~m. Prefernble the diameter is in the range of from 4 to 8 ~m. Here, the diameter, when the individual fiber ha~ an ellipsoidal ~ectional shape, is represented by the ~imple avera~e of its longer diameter and ~horter diameter.
The diameter of the individual chopped carbon fibers affect~ me~n pore size, ~a~ permeability, etc.
of the sub~trate. In other word~, a~ ~hown in Fig. 3, the mesn pore ~ize Pd ( ~m) of the ~ubstrate will be monotonously increa~ed a~ the diameter d t ~m) of the individual chopped carbon fiber~ i8 greater. On the other hand, the pre~sure 108~ Pl (mmH20/mm) when a gas i3 flowed through the thicknes~ direction of the substrate will become acceleratedly ~maller as the fiber diameter d is greater. Small pre~ure 1088 means good ga~ permeability. Good 8as permeability i~
preferable for electrode substrate for pho~phoric acid fuel cell, but if the mean pore size becomes too large, phosphoric acid solution a~ an electrolyte tend~ to be scattered as described above to be excessively consumed. On the other hand, if the fiber diameter becomes smaller, the avera~e pore size Pd becomes smaller in proportion thereto. Wherea~, if the avera8e pore size become~ ~maller, the pre~sure lo~s will become greater a~ a matter of cour~e. In addition, phosphoric acid solution i~ penetrated a~ de~cribed above to lower further gas permeability of the electrode substrate, or the electrolyte in the matrix layer tends to become in shortage, thereby lowering cell performance. However, the~e inconvenience~ can be i3()31Z;:~
made negligible if the mean ~ore size is controlled within the range of from 20 to fiO ~m. The diameter for obtaining the menn pore size of from 20 to fiO ~m i~ in the ran~e of from 4 to 9 ~m from Fig. 3. ~lowever, mean pore size and ~as permeability are dependent not only on the diameter of chopped carbon fibers, but on the various conditions as described below. ~lere, mean pore size i8 defined by the vallle of the maximum volume of the pore di~tribution measured by the known mercury pressure penetration method. On the other hand, gas permeability is evaluated by the pres~ure 1099 at gas flowing. More ~pecifically, a sample of about 47 mm in diameter is cut out from the substrate provided for the test with a thickne~s of t (mm), and the sample is ~et on a sample holder. Next, the pressure difference ~P
(mm~l20) between the opposite sides of the ~ample is measured while nitrogen gas i.q cau~ed to flow through the ~ample at a flow rate of t4 cm/sec. And, the pressure 108~ (mml~20/mm) during gas flowing is calculated from the following formula. The gas flowing throu~h area is 11.94 cm2.
Pressure 10~9 during ga~ flowing = ~P/t Fiber length of chopped carbon fibers affects mechanical strength such as flexural strength, mean pore size, distribution of pores, etc. of the electrode substrate. In other words, as shown in Fig. 4 for the case when the carbonized resin content in the substrate is 45 % by weight, flexural strength Bs (kgf/cm2) and mean pore size Pd ~ ~m) will be increased as the fiber length L (mm) of chopped carbon fibers becomes longer.
However, if the fiber length is too long, it is difficult to disperse the fibers uniformly9 and therefore flexural strength will not be substantial 1~Y
13~
increased. If dispersion of the fibers is non-uniform, ~izes of pore~ will be irregular, and/or distribution of the pores will be non-uniform. On the other hand, if the fiber length L i~ shorter, not only ~he flexural ~trength B~ is greatly lowered, but al~o the fibers are too densely packed, failing to maintain the mean pore ~ize of from 20 to 60 ~m and the porosity of from 60 to 80 %. In the present invention, by use of chopped carbon fibers with fiber lengths of from 3 to 20 mm, ~uch inconveniences are avoided. Preferable fiber length i~ in the range of from 5 to 15 mm.
Mea~urement of porosity is conducted a~ described below. That is, previou~ly, the re~pective weights of the gla~s bottle for measurement and the sample are weighed, and the measurement resu1ts are re~pectively made Wb and W9. Further, ethanol is filled in the glass bottle and weighed, and the weight We of the ethanol filled i8 calculated. Next, the ~ample is placed in the glasq bottle, the internal pressure of the gla~s bottle ig decreased to a vacuum of 1 mm~lg or lower, and then ethanol i~ filled in the bottle, followed by weighing. The result of weighing at this time iQ made Wbem, and the value obtained by detractin~
Wb from thi~ value Wbem is made Wm (= Wbem - Wb). When the bulk density of the sample is made pm, and the density of ethanol pe, the porosity Pr (~) can be given by the following formula:
Pr = (l - (We-Wm+Ws) x pm / (Ws x pe)) x 100 However, the bulk density pm (g/cm3) is calculated according to the following formula:
pm = Ws x l0/(t x A) Here, t i~ the thickness of the sample (mm), and A i~
the area of the qample (cm3).
~3~31Z3 Now, the chopped carbon fibers as described above are made into carbon fiber mat by means of continuously or batch wise.
This carbon fiber mat is practiced by mixing and stirring chopped carbon fibers in a dispersion medium containing a binder for paper making, and the mixture is filtered onto a fabric or a wire net, etc. By means of paper making, the chopped carbon fibers are dispersed in random directions within substantially 2-dimensional plane, and bound to each other with the binder to be formed into a mat or a sheet having self-form retentivity. Here, as the organic binder for paper making, polyvinyl alcohol, hydroxyethyl cellu-lose, polyethylene oxide, polyacrylamide, polyester, etc. can be used, and the binder is diluted with a dispersion medium such as water or methanol, etc. The amount of the binder diluted may be in the range of from about 1 to about 30 % by weight. The chopped carbon fibers and the dispersion medium may be mixed at a ratio which may also depend on the kind of the dispersion medium, etc., but preferably at a ratio of from about 0.01 to about 0.1 % by weight of the chopped fibers.
After paper making, the dispersion medium is removed by drying with heatiny. The amount of the remaining binder with respect to the total weight of the carbon fiber mat dried may, after drying, be in the range of from 5 to 30 ~ by weight, prefer-ably 5 to 20 % by weight.
The carbon fiber mat can be also obtained by the method other than that described above. For example, the above-mentioned chopped carbon fibers are dispersed in water as the dispersion medium to 0.01 to 0.1 % by weight thereof, if desired, surfactants and an agent of ~3~ 3 increa~ing a visco~ity of the di~persant such a~ ~odium acrylate, sodium glycolate, etc. are added to improve a disper~ibility, and paper making is carried out in the ~ame manner as described above. Then, the above binder diluted with the ~olvent is attached to the re~ultant paper material by impregnation or ~praying, thereby obtaining the carbon fiber mat.
Next, the carbon fiber mat which the chopped carbon fiber are bound to each other with a binder, make into a prepreg by mean~ of impregnatin~ a re~in carbonizable by heat treatment. The re~in may be, for example, a resol type phenolic resin. A~ the Yolvent for such resin, methanol and water can be uqed. This ~olvent i8 removed by drying with heating after impregnation.
Further, to produce an electrode ~ub~trate further improved in electroconductivity by lowering resistivity and yet also excellent in mechanical strength, it i8 preferable to use, as a resin to be impregnated, Q
mixture of a ~elf-curable re~in and non-~elf-curable re~in mixed at an appropriate ratio. The ~elf-curable resin i8 a re~in which can be cured through conden~ation and/or addition reaction only by heating, while the non-~elf-curable re~in iY a reYin which can be lowered in vi~c08ity but not cured only by heating becau~e it contain~ no curing agent.
A~ the self-curable re~in, which is not particularly limited, re~ol type phenolic re~in~, thermo~ettin~ type polyimide~, polyphenyl quinoxalines, etc. may be preferably used. Aq the non-self-curable re~in, resins which will not be cured unle~Y a curing agent and heating are u~ed, and the re~in~ which are not particularly limited, may include preferably ~3~3~
novolak type phenolic resins, pitch, furan resins, epoxy resins, polyphenylene oxides, etc.
When a non-self-curable re~in is impre8nated together with a self-curable resin into the carbon fiber mat, electroconductivity of the ~ubYtrate can be remarkably improved. The rea~on for remarkable improvement of electroconductivity may be that the non-self-curable resin i~ not cured even by heatin~ but has fluidity with lowered visco~ity as described above, and therefore even if voids are formed around the chopped carbon fibers by scattering of the binder which has bound the chopped carbon fiber~ to each other in the heat treatment ~tep a~ described later, the non-self-curable resin enters into the voids, thus embedding the voids.
It is preferable that the non-self-curable resin should be mixed at a proportion of 10 to 500 parts by weight based on 100 parts by weight of the ~elf-curable resin. At a level less than lO parts by wei~ht, electroconductivity may not be improved sufficiently.
On the other hand, if such a large amount as exceeding 500 part~ by weight is used, the mixed resin cannot become ~ufficiently stiff even in the subsequent hot pressing ~tep or hear treatment step, whereby the substrate become~ to have tackiness and adhered with other members, etc. to become difficult in handling.
More preferably, the non-self-curable re~in should be mixed at a proportion of 50 to 300 part~ by weight based on 100 parts by weight of the ~elf-curable resin.
Impregnation of the resin into the carbon fiber mat may be practiced by dippiny the carbon fiber mat into a resin solution dissolved in a solvent such as water, methanol, tetrahydrofuran, dioxane, 13~31;~
N-methylpyrrolidone, etc., or ~praying the above resin solution onto the carbon fiber mat. The resin concentration of the solution used is preferably controlled to about 5 to 50 % by weight, in view of easines4 of impregnation. Then~ the solvent is removed by heating.
The amount of the resin impregnated into the carbon fiber mat may be preferably about 80 to 600 parts by weight baYed on 100 parts by weight of the chopped carbon fibers in the carbon fiber mat. More preferable impregnation amount is in the range of from 100 to 350 parts by weight based on 100 parts by weiYht of the ohopped carbon fibers.
The preferable range of the impregnated amount (attached amount) of the resin of the carbon fiber mat as described above is finally determined from the preferable ran~e of the carbonized resin amount relative to the total amount of chopped carbon fibers.
To de~cribe this more specifically, the carbonized resiD content in the electrode s~lbstrate affects electroconductivity, pre~sure lo~s, etc. of the substrate. More specifically, in the case of using only a resol type phenolic resin~ as shown in Fig. 5, with the increase of the carbonized resin content C
while the resistivity Rt ( Q cm~ in the thickness direction is acceleratedly lowered, the pressure loss Pl (mmH2O~mm) during gas flowing throu~h the thickness direction will be accelerat&dly increased. Although lowering in resistivity iY preferable, increase of pressure loss should be avoided because it will lower gas permeability as described above. Also, as the carbonized resin content becomes lowered, bindability between the chopped carbon fibers will be lowered, and ~3~31~
al~o mechanical strength of the e]ectrode ~ub~trate will be lowered. In the pre~ent invention, ~uch inconvenience~ a~ described above are avoided by controlling the carbonized resin amount to 35 to 60 X
by weight ba~ed on the total a~ount of the chopped carbon fibers and carbonized re~in. Thu~, the amount of the re~in attached on the carbon fiber mat is required to be controlled such that the carbonized resin content in the electrode ~ub~trate may become 35 to 60 X by wei~ht. The re~istivity Rt ( Q cm) in the thickness direction i8 calculated according to the following formula from the voltage drop E (V) when the substrate with the thicknes~ _ (mm) is sandwiched between mercury electrode~ with a certain area S (cm2) and a certain current I (A) is passed between the electrodeY:
Rt = ~E x S) x 10 / (I x t) Also, when electricity generated i~ varied within the electrode plane, there is a fear that exces~ively great current may be passed through a part of the electrode to expedite con~umption of the electrode.
For solving this problem, the re~i~tivity in the planar direction of the electrode should be preferably 20 m Q cm or less, more preferably l5 m Q cm or less.
The resistivity in the planar direction is measured a~ described below. That is, a sample of 25 x 50 mm i~ cut out from the ~ubstrate provided for the test, two measurement electrodes, each having a length of 25 mm and made of a copper plate plated with gold, are placed on the upper ~urface of the sample in parallel to the shorter side of the sample with an interval of 25 mm between the electrodes, a current of 1 A is made flow between the measurement electrode~
13~31~;~
I g with a load of 5 kgf/cm2 npplied on each of the electrodes, and the voltage drop F (mV) between the electrodes is mea~ured. From the voltage drop measured, the resistivity Ra (m Q cm) i9 calculated from the following formula:
Ra = (R/I) x t x 10~1 Here, t is the thickness of the sample ~mm), I is current value pas~ed between the two measurement electrodes (A), namely 1 A, and the measurement plane is 25 mm x 25 mm. The resistance in the plane direction depends on the length of chopped carbon fibers, bindability mutually between the chopped carbon fibers, etc.
The mat prepreg after resin impregnation is molded by hot pressing under a pre~ure of 2 to 10 kgf/cm2 at 120 to 200 C for about 5 to 60 minutes to cure the resin. At this stage, if desired, a plural number of mat prepregs are laminated on one another ~o a~ to obtain the necessary thickness. When a mixture of a ~elf-curab]e and non-self-curable resins i8 used to obtain the mat prepreg, the self-curable resin of the mixture will be cured in the hot pressing step. The applied pressure during hot pressing affect~
compre~sion ratio, mean pore size, etc. of the electrode substrate. Fig. 6 ~hows the relationship of the pressure P ~kgftcm2~ applied at hot pressing, with mean pore size Pd ( ~m) and the compre~sion ratio Cr (X~ measured under a condition of 5 kgf/cm2 being applied to the substrate. As shown in Fig. 6, it was discovered that the compression ratio Cr becomes greater as the applied pressure P is higher. On the other hand, the mean pore size Pd becomes abruptly smaller as the applied pressure P becomes greater 13~31~
within the ranRe when the applied pre~sure i~
relatively lower, but wi]l not be changed 80 much thereafter. And, from this Fig. 6, it can be understood that an electrode ~ub~trate having a mean pore si~e of 20 to 60 ~m and a compre~3ion ratio of 20 % or le~ can be obtained when the applied force i8 2 to 10 kgf/cm2. In Fig. 6, the pre~sure applied to the ~ubstrate, i.e., 5 k~f/cm2, i~ determined in view of the force applied when electrode ~ubstrates are practically ~uperpo~ed and u3ed by clampin~ the3e.
Compres~ion ratio Cr i~ determined a~ follows. The electrode ~ub~trHte i 8 cut to 3 cm ~quare, and 20 ~heets thereof are superpo~ed on one another to be placed on a metal plate, The thickne~ses t1, t2, t3, and t5 of the sheet3 are mea~ured under loading pressures of 1, 2, 3, and 5 kgf~cm2, re~pectively, and the initial thickness to under no load~ i~ obtained by ~traight line approximation from the thickne~se~ t1, t2, and t3 measured, Cr is calculated according to the following formula:
Cr = [~to ~ t5)/to] x 100 The molded ~ubstrate after hot pre3sing is then heat treated in an inert atmo~phere ~uch a~ nitrogen or argon or in vacuum atmo3phere to 1300 to 3000C. By thi~ heat treatment, the re~in previou~ly impregnated is carbonized to become carbon or graphite. At thi~
~tage, the binder u~ed during paper makin~ will be pyrolyzed and di3~ipated. In the ca~e of uYing a mixture of self-curable and non-~elf-curable re~ins, for example, a mixture of a re~ol type phenolic re~in and a novolak type phenolic re~in, the novolak type phenolic resin probably enter~ into the void~ formed by dissipation before it is carboni~ed. Thu~, the ~3V31~3 electrode sub~trate is obtained.
Fig. 7 shows, in the case of using only a re~ol type phenol;c resin as the resin impregnated to the carbon fiber mat, the relation~hips of the porosity Pr ~X) with the pre~sure 108~ Pl (mm~12O/mm) in the thickness direction of the substrate and the resi~tivity Rt ( Q cm) in the thickne~s direction. A~
i~ apparent from thi~ Fig. 7, the pressure 1098 Pl will become acceleratedly smaller as the porosity Pr becomes larger to improve ~a~ permeability, while on the other hand, the resi~tivity Rt becomes acceleratedly greater to lower greatly the electroconductivity. On the other hand, if the porosity Pr exceeds 80 %, bindability mutually between the chopped carbon fibers will be lowered, whereby not only electroconductivity but also mechanical strength will be lowered. To be used for the electrode substrate, the poro~ity is required to be 60 to 80 %.
It i~ important to avoid power generation irregularity caused by planar variance of the amount of the cataly~t coated on the electrode substrate surface, and avoid an increa~e in electrical resistance due to contact inferiority between the electroconductive ~as separator intervening between the ad~jacent cell units and the electrode ~ubstrate when a large number of cell units are ~tacked up electrically in ~erie~. To thi~
end, the surface roughnes~ of the electrode ~ubstrate should preferably be 100 ~m or less at the maximum undulation of the rolling circle.
The surface roughnes~ of the substrate is related to diameter of individual chopped carbon fibers, length of individual chopped carbon fiber~, dispersibility of chopped carbon fibers, etc. If the fiber diameter is 13(?31Z3 large, the ~urface roughness become~ larger due to the planar variance of the amount attached during paper making of chopped carbon fibers. If the fiber length i8 too long, di~per~ibility during paper making also becomes poor to make the surface rough. When short, the above problem can be ~olved but the ~trength of the substrate is weakened. Short carbon fiber~ are generally obtained by cutting and splitting continuous fiber bundle~, and the ~heafing a~ent attached on the fiber bundles will generally worsen disperYibility in the paper making step, to con~equently roughen the sub~trate ~urface. Preferably, no sheafing agent should be used, or it is preferable to u~e water or water-401uble sheafing agent. Also for realizing the above surface roughnes~ of 100 ~m or le~s, it is preferable that the chopped carbon fibers should have a diameter of from 4 to 9 ~m and a length of from 3 to 20 mm in the individual fibers.
Mea~urement of the maximum undulation ~WEM) of the rolling circle on the substrate surface can be practiced according to the following method. A~ the measuring instrument, for example, a univer~al surface shape measuring instrument MODEL SR-3C produced by Kabushiki Kaisha Kosaka kenkyu~ho may be uYed. The contact needle which forms the detecting portion of this measuring instrument has a contact needle tip end radius of 800 ~m and a contact apex angle of 60 D ~ with the contact needle measuring load during mea4urement being 0.4 gf or les~.
By Eixing the sub~trate to be measured on a checker board with ~mooth surface and moving the above contact needle about 7 cm along the substrate surface, the shape of the surface can be measured. The 13(~3123 difference between the maximum value and the minimum value of the wave form at the interval of 5 cm at the central portion of the movement length of the contact needle iY defined as the maximum undulation of the rolling circle.
The e]ectroconductive sub~trate thu~ prepared can be worked. For example, by formin~ grooves in parallel to each other on one ~urface of the Rubstrate, a ribbed electrode ~ubstrate for pho~phoric acid fuel cell can be provided.
~XAMPL,~S
Example 1 A~ ~hown in Table 1, variou~ electrode substrates were prepared by varying the diameter of the individual chopped carbon fibers, fiber length of the individual chopped carbon fiber~, amount of binder attached during paper making of carbon fiber mats, amount of resin attached, applied pre~ure during hot pressing, and heat treatment temperature. More specifically, for the ~ub~trates of the present invention No~ 8 and comparative ~ub~trate~ No~. C1 - C6, polyacrylonitrile carbon fiber "TORAYCA" T300 (mean diameter of single filament: 7 ~m, number of filaments: 6000) manufactured by Toray Industries, Inc. was u~ed, for the ~ubstrate No. 9 of the present invention a polyacrylonitrile carbon fiber "TORAYCA" T800 (mean diameter of ~in~le filament: 5 ~m, number of filaments: 6000) manufact~red by Toray Industries, Inc., for the comparative substrate No. C7 a carbon fiber "Thornel" P25-W (mean diameter of ~ingle filament: 11 ~m, number of filaments: 4000) manufactured by Union Carbide Corp., U.S.A., respectively, and the polyacrylonitrile carbon fibers were cut to the re~pective fiber lengths shown Jrc~ Je -mc~k ~L3f(~3123 is Table 1, followed by sufficient fiber splitting.
These chopped fibers were stirred in water, subjected to paper makin~ on a wire net, and after dryin~, impregnated with an aqueou~ polyv;nyl nlcohol solution, followed by dryin~ with heating to obtain an carbon fiber mat. The amounts of the polyvinyl alcohol attached were as shown in Table 1.
Next, each carbon fiber mat was impregnated with a 10 % by wei~ht of methanolic solution of a re~ol type phenolic resin (PR-g183B, manufactured by Sumitomo Durez K.K., repre~ented by R in the table) to have the phenolic resin in part~ by weight shown in Table 1 based on 100 parts by weight of the carbon fiber mat, and after drying at 90C, subjected to hot pressing under the pressure and the temperature ~hown in Table 1 for 15 minutes to cure the phenolic resin.
~ Subsequent]y, the above molded substrate with the cured phenolic resin was carbonized by heat treatment at the temperature shown in Table 1 in ar~on atmoæphere to obtain an electrode substrate having the thickness, the bulk density, the mean pore size, the porosity, the compression ratio, the pressure loss, the planar resi~tivity, the re~istivity in the thickness direction, the roughne~s, the flexural strength, and the heat resistance shown in Table 2.
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'C L~
Electrode Sub~trate for Fuel Cell and Process for Producing the Same BACKGROUND OF T~l~ INV~NTION
Thi~ invention relates to an electrode substrate suitable for fuel cell, particularly phosphoric acid fuel cell and a process for producing the same.
Phosphoric acid fuel cell i8 constituted of a matrix layer impregnated with a phosphoric acid solution which is an electrolyte, and positive and negative electrodes sandwiching the matrix layer from both surfaces. Electrodes are constituted by forming catalyst layers on the surfaces of electrode substrates which are contacted with the mntrix layer.
The electrode substrates to be used for such phosphoric acid fuel cell are demanded to have variou~
electrical, chemical and mechanical characteristics, including high electroconductivity as a matter of courqe, chemical stability at high temperatures, excellent ga~ permeability, and also hi~h flexural strength, etc.
More specifically, the electrode ~ubstrate for phosphoric acid fuel cell, since it is required to haYe the function to feed a fuel gas (hydrogen gaY, etc.) or oxidative gas (air or oxygen gas) to the catalyst layer adjacent thereto, is first required to have basically high gas permeability. Gas permeability of the electrode substrate is given by porous property of the electrode sub~trate, namely by the electrode substrate having interconnected pores, and gas permeability performance i9 determined by the pore size and poro~ity, etc. If interconnected pores having large l3a~
pore sizes are formed in the electrode substrate for improvement of ga~ permeabilitv of the electrode substrate, the phosphoric acid ~olution will be readily scattered therethrough, whereby excessive constlmption of the solution occurs. On the contrary, if interconnected pores having ~mall pore si~es are formed, the phosphoric acid solution penetrate to the electrode substrate from matrix layer through the capillary phenomenon, whereby its gas permeability may be lowered or the cell performance may be lowered due to shortage of the electrolyte in the matri~ layer.
Thus, setting of the pore si7e and porosity within optimum value ranges is one of important matters which influences electrode performances.
Also, a phosphoric acid fuel cell, in order to obtain a power enough to be provided for commercial use, is stacked up with a large number of cell units shaped in flat plate, and therefore the electrode substrate must have high mechan;cal strength, particularly flexural strength. High mechanical strength is also necessary for performing easily workings of wetproof treatment of the electrode substrate or coating of the catalyst, assembling working of cell units, etc. This demand for mechanical strength becomes greater as the sub~trate area of the fuel cell becomes larger. Further, for improvement of electroconductivity between the contacted surfaces of the respective cell units, the stacked cell units are clamped with a given compressive force in the stacking direction. Accordingly, the electrode substrate of each unit mu~t have uniform characteri~tics relative to compression. If the amounts to be compressed when a constant compreYsive force is permitted to act differ 13~1J~lZ3 depending on the electrode ~ubstrate~ of the respective unit~, the characteri~tic~ of ga~ permeability of the electrode sub~trate~ of the re~pective cell unit~ will be al~o different, whereby variance occurs in cell characteristic~ between the units, and the height~ of the re~pective unit~ become irregular.
For the electrode aub~trate of a pho~phoric acid fuel cell comprising a large number of cell unit~
~tacked up, the ~urface roughne~ i 9 al~o an important problem. More ~pecifically, the electrode of a pho~phoric acid fuel cell comprise~ a catalyst layer con~i~ted of p]atinum or platinum alloys ~upported on a carrier coated on the electrode ~ub~trate ~urface, and the coating thickne~ of the cataly~t is generally 50 to 500 ~m. Therefore, when the ~urface roughne~ of the electrode ~ubstrate i~ great, planar variance of the coated amount of the cataly~t i~ not negligible and power generation irregularity occur~ to cau~e lowerinR
in power Reneration efficiency. Al~o, for performing practical power generation, a large number of cell unit~ may be ~tacked up electrically in ~erie~. In thi~ ca~e, ~ince an electroconductive gas separator i~
~andwiched between the adjacent unit cel]~, one ~urface of the electrode ~ubstrate i~ as~embled 80 a~ to be contacted with the gas ~eparator. If the ~urface of the electrode ~ub~trate i~ rough, the contact between the ~ubstrate and the ga~ ~eparator become~
in4ufficient, whereby electrical re~i~tance at the contacted ~urface is increased to lower power generation efficiency.
A~ the ~ub~trate con~tituting the electrode of fuel cell as de~cribed above, there have been known in the art tho~e compri~ing ~hort carbon fiber~ di~per~ed in random directions within substantially 2-dimensional plane bonded to each other with carbonized or graphitized re~in by means of paper makin~, etc., as disclosed in the literatures such as Japanese Patent Publications Nos. 63-18603 and 53-43920, Japanese Laid-Open Patent Publications Nos. 57-129814, 57-166354 and 60-44963 and others.
All of these substrate~ of the prior art described in theRe literatureR had one advantage and one shortcoming in gas permeability, mechanical stren~th, compres~ive characteristic, etc. ~uch that some Rre good in ga~ permeability but involve a problem in mechanical strength, or on the contrary others are high in mechanical strength but inferior in gas permeability, etc. Thu~, no electrode sub~trate has been obtained, which can sufficiently satisfy the requirements in all of these characteristics.
On the other hand, Japane~e laid-Open Pntent Publication No. 58-68881 disc]oses a method for obtaining an electrode substrate all at once according to moldi~g fabrication of a mi~ture of chopped carbon fiber~ and a carbonizable re~in without recourse to paper making. Since this method relies on molding fabrication, it i~ neceYR~ry to u~e chopped carbon fibers with extreme]y short fiber length of, for example, 1 mm or le~9 ~ and therefore mechanical strength i~ remarkably inferior as compared with the ~ub~trate prepared by the paper making method, whereby there is the problem that handlin~ of the electrode becomes difficult in workings of wetproof treatment of the electrode sub~trate, coating of the cataly~t, assembling of cell units, etc. as described above.
13V31~;~
OllJ~3CT~3 AND ~UMMARY OF TI~R I NV~NTION
Primary object of the pre~ent invention i~ to provide a ~ubstrate and a proce~s for producing the ~ame, which is ~uitable for con~tituting the e]ectrode of a fuel cell, particularly the electrode of a phosphoric acid fuel cell, having excellent chemical ~tability at hi~h temperature~ and electroconductivity, as well a~ var;ou~ excellent characteri~tics ~uch as ga~ permeability, flexural ~trength, compres~ive characteristic, etc., and yet being well balanced in these various characteri~tic~.
Another object of the present invention is to provide an electrode substrate for fuel cell and a proces~ for producing the ~ame, which has higher electroconductivity and yet excellent mechanical strength.
Still another object of the present invention is to provide an electrode sub~trate with ~mooth ~urfaces and a process for producing the ~ame, which i~ improved in power generation efiiciency by precluding power generation irregularity and electrical contact inferiority caused by the ~urface roughnes~ of electrode substrate.
In accordance with the present invention, there is provided a porous electrode ~ubstrate for fuel cell comprising ~hort carbon fibers disper~ed in random directions within sub~tantially 2-dimen~ional plane and carbonized resin for mutually bonding the carbon fiber~. The individual fiber of the carbon fibers ha~
a diameter of from 4 to 9 ~m and a length of from 3 to 20 mm. The content of the carbonized re~in i~ in the range of from 3h to 60 X by weight, and the sub~trate of the present invention haY pore~ with a mean pore ~3~
si~e of from 20 to 6Q ~m and a porosity of from 60 to ~0%, and a compression ratio not more than 20%.
Preferably, a surface of the substrate of the present in~ention has a surface roughness of not more than 100 ~m, defined as the ma~imum undulation of the rolling circle tracing the shape of the substrate surface.
The electrode substrate of the present invention is produced according to the following process. Chopped carbon fibers each of which has a length of from 3 to 20 mm and a diameter of from 4 to 9 ym are mixed by stirring together with a dispersion medium containing an organic binder, and then a paper-like sheet is made from the mixture to obtain a carbon fiber mat.
Alternatively, chopped carbon fibers are mixed by stirring together with a dispersion medium containing no organic binder, after that a paper-like sheet is made from the mixture, and then the paper-like sheet obtained is treated with an organic binder to obtain a carbon fiber mat. The carbon fiber mat is impregnated with a resin to obtain a prepreg, i.e., a mat impregnated with a resin. After the impregnation, the prepreg is hot pressed under a temperature of form 120 to 200C and a pressure of 2 to 10 kg/cm2 to obtain a molded substrate. Then, the molded substrate is heat treated to carbonize the resin under a temperature of from 1300 to 3000C in an inert atmosphere or vacuum atmosphere to obtain an electrode substrate excellent in gas permeability, flexural strength, compression strength, etc.
13(~3~Z3 - 6a - 72465-5 Preferably, the carbon fiber ma~ is dried by heating, and the amount of the organic binder attached to the ~arbon fiber mat after completion of the drying step is in the range of from 5 to 30~ by weight based on the total weiqht of the carbon fiber mat.
Also, in the hot pre~in~ ~tep, if desired, a plural number of sheets of the prepre~ are laminated and subjected to hot pressing in the hot pre4si.n~ step.
The re~in impregnated into the carbon fiber mat may contain a resin which i~ curable in the Aub~equent hot pressing ~tep and a resin which i~ not curable in the subsequent hot pressing step, and the mixin~ ration may be preferably 10 to 500 part~ by weight of the non-curable resin based on 100 parts by weight of the curable resin. The curable re~in is at least one selected from the group consi~ting of resol type phenolic resins, polyphenyl quinoxalines and thermosetting polyimide~, and the non-curable resin is at least one selected from the group consi~ting of novolak type phenolic resins, pitch, furan resins, epoxy resin~ and polyphenylene oxides.
The above and other objects, features and advantages of the present invention will become more apparent from the ensuinR detailed description taken in conjunction with the accompanyin~ drawing~.
BRIFF DFSCRIPTION OF T~F DRA~INGS
Fig. ~ i~ an exploded perspective view showing the cell ~tructure of a phosphoric acid fuel cell, particularly a flat plate electrode fuel cell for which the electrode substrate of the present invention is applied;
Fi~. 2 is an exploded perspective view showing the ~ell structure of a phosphoric acid fuel cell, particul.arly a ribbed electrode fuel cell for which the electrode sub~trate of the present invention is applied;
Fig. 3 is a graph showing the relationships of the ~3'~31Z3 diameter _ of the individual chopped carbon fibers used in the electrode substrate with the mean pore size Pd and pressure loss Pl when a gas is flowed throu~h the thickness direction of the sub~trate, in the case of us;ng only a resol type pheno~ic resin as the resin impregnated to the carbon fiber mat;
Fig. 4 is a graph showing the relationships of the fiber length 1, of the chopped carbon fiber~ used in the electrode substrate with the mean pore size Pd and the flexural strength Bs of the substrate, in the case of using only a resol type phenolic resin as the resin ;mpregnated to the carbon fiber mat;
Fig. 5 is a graph showin~ the relationships of the carbonized resin content C in the electrode substrate with the resistivity Rt in the thiclcness direction and the pressure lo~s Pl in the thickne~s direction, in the case of using only a re~ol type phenolic resin as the resin impregnated to the carbon fiber mat;
Fig. 6 i~ a graph showing the relationships of the pressure applied P during hot pressing of the mat prepreg with the mean pore size Pd of the substrate and the compre~sion ratio Cr when a load of 5 kgfJcm2 is applied on the substrate, in the case of u~ing only a resol type phenolic resin as the resin impregnated to the carbon fiber mat;
Fig. 7 is a graph showing the relationships of the porosity Pr of the substrate with the pressure loss Pl in the thickness direction and the resistivity Rt in the thickness direction, in the case of using only a resol type pheno~ic resin a~ the resin impregnated to the carbon fiber mat;
Fig. 8 is a grap} showing the surface roughness of the electrode substrate of the present ;nvention; and ~3~3~23 Fi~. 9 i~ a graph showing the ~urface roughne~ of the electrode substrate of the compara-tive example.
DFTAILRD DUS~PIPTION
The pho~phoric acid fuel cell for which the electrode sub~trnte of the pre~ent invention i8 preferably applied may be one having a cell ~tructure a~ ~hown in Fi~. 1 and Fig. 2. Fig. 1 ~how~ one in which an electrode ~ub~trate ~haped in flat plate i~
used, and Fig. 2 one in which a ribbed electrode ~ub~trate i8 used.
In Fig. 1, on one surface of a matrix layer 10 containing pho~phoric acid electrolyte, an oxidative gas tair or oxygen ga~) electrode, namely an air electrode 11 is ~uperpo~ed, while on the other ~urface a fuel ga~ (hydrogen ga~, etc.) electrode, namely a fuel electrode 12 i~ ~uperpo~ed. The electrode~ 11 and 12 are each constituted by the electrode 3ubstrate 11a, 12a, and cataly~t layer 11b, 12b of platinum or platinum alloys supported on a carrier which i~ to be contacted with the matrix layer 10. Pho~phoric acid fuel cell i~ constituted by ~tacking up a lar~e number of cell un:it~, each includ;ng matrix 1ayer 10, air electrode 11 and fuel electrode 12, with gas ~eparator~
13, 14 interpo~ed between the adjacent cell units. In the flat plate electrode type ~hown in Fig. 1, the oxidative gas and the fuel ga~ are re~pectively introduced into the groove~ 13a (13b), 14a (14b~
between the ribs formed on the gas ~eparators 13, 14.
Al~o in the ribbed electrode cell structure shown in Fig. 2, ~imilarly as the flat plate electrode of Fig. 1, air electrode 11' and fuel electrode 12' are superposed on both surface~ of the matrix layer 1~' so 13V3~L,.,~
1 o as to sandwich ;t therebetween. The electrodes 11' and 12' are each constituted by the electrode sub~trate 11'a, 12'a, and catalyst layer 11'b, 12'b which is to be contacted with the matrix layer 10'. The ribbed electrode cell i~ constituted by stacking up a large number of units, each including matrix layer 10', air electrode 11' and fuel electrode 12' through gas separator~ 13', 14' shaped in flat plate, which are interposed between the ad~jacent cell units. In the case of the ribbed electrode cell, the oxidative gas and the fuel ga~ are respectively introduced into the grooves 11'c, 12'c between the ribs formed on the electrode substrates 11'a and 12'a.
In the following, the electrode sub~trate of the present invention i~ de~cribed along with its preferable production process.
I~ the present invention, the electrode sub~trate is constituted of a porou~ material, comprising short carbon fibers dispersed in random direotions within Yubstantially 2-dimensional plane bonded to each other with carboni~ed resin.
The ~hort carbon fiber may compri.~e any of polyacrylonitrile cnrbon fiber, pitch carbon fiber, rayon carbon fiber, etc. However, polyacrylonitrile carbon fiber with relatively higher mechanical stren~th i8 preferred. The Yhort carbon fibers are generally obtained by cutting and splitting continuous fiber bundles. Fiber bundle~ are generally attached with a ~ynthetic sheafing agent such as epo~y resin, etc., and ~uch sheafing agent should be preferably removed for improvement of dispersibility in the paper making step as described below. When use of a ~heafing a~ent is indispensable, it is preferable to use a water-soluble 13V31~3 ~heafing agent ~uch as polyvinyl alcohol, polyethylene glycol, starch, etc.
The individual f;ber of the chopped carbon fiber~
u~ed ha~ a diameters of from 4 to 9 ~m. Prefernble the diameter is in the range of from 4 to 8 ~m. Here, the diameter, when the individual fiber ha~ an ellipsoidal ~ectional shape, is represented by the ~imple avera~e of its longer diameter and ~horter diameter.
The diameter of the individual chopped carbon fibers affect~ me~n pore size, ~a~ permeability, etc.
of the sub~trate. In other word~, a~ ~hown in Fig. 3, the mesn pore ~ize Pd ( ~m) of the ~ubstrate will be monotonously increa~ed a~ the diameter d t ~m) of the individual chopped carbon fiber~ i8 greater. On the other hand, the pre~sure 108~ Pl (mmH20/mm) when a gas i3 flowed through the thicknes~ direction of the substrate will become acceleratedly ~maller as the fiber diameter d is greater. Small pre~ure 1088 means good ga~ permeability. Good 8as permeability i~
preferable for electrode substrate for pho~phoric acid fuel cell, but if the mean pore size becomes too large, phosphoric acid solution a~ an electrolyte tend~ to be scattered as described above to be excessively consumed. On the other hand, if the fiber diameter becomes smaller, the avera~e pore size Pd becomes smaller in proportion thereto. Wherea~, if the avera8e pore size become~ ~maller, the pre~sure lo~s will become greater a~ a matter of cour~e. In addition, phosphoric acid solution i~ penetrated a~ de~cribed above to lower further gas permeability of the electrode substrate, or the electrolyte in the matrix layer tends to become in shortage, thereby lowering cell performance. However, the~e inconvenience~ can be i3()31Z;:~
made negligible if the mean ~ore size is controlled within the range of from 20 to fiO ~m. The diameter for obtaining the menn pore size of from 20 to fiO ~m i~ in the ran~e of from 4 to 9 ~m from Fig. 3. ~lowever, mean pore size and ~as permeability are dependent not only on the diameter of chopped carbon fibers, but on the various conditions as described below. ~lere, mean pore size i8 defined by the vallle of the maximum volume of the pore di~tribution measured by the known mercury pressure penetration method. On the other hand, gas permeability is evaluated by the pres~ure 1099 at gas flowing. More ~pecifically, a sample of about 47 mm in diameter is cut out from the substrate provided for the test with a thickne~s of t (mm), and the sample is ~et on a sample holder. Next, the pressure difference ~P
(mm~l20) between the opposite sides of the ~ample is measured while nitrogen gas i.q cau~ed to flow through the ~ample at a flow rate of t4 cm/sec. And, the pressure 108~ (mml~20/mm) during gas flowing is calculated from the following formula. The gas flowing throu~h area is 11.94 cm2.
Pressure 10~9 during ga~ flowing = ~P/t Fiber length of chopped carbon fibers affects mechanical strength such as flexural strength, mean pore size, distribution of pores, etc. of the electrode substrate. In other words, as shown in Fig. 4 for the case when the carbonized resin content in the substrate is 45 % by weight, flexural strength Bs (kgf/cm2) and mean pore size Pd ~ ~m) will be increased as the fiber length L (mm) of chopped carbon fibers becomes longer.
However, if the fiber length is too long, it is difficult to disperse the fibers uniformly9 and therefore flexural strength will not be substantial 1~Y
13~
increased. If dispersion of the fibers is non-uniform, ~izes of pore~ will be irregular, and/or distribution of the pores will be non-uniform. On the other hand, if the fiber length L i~ shorter, not only ~he flexural ~trength B~ is greatly lowered, but al~o the fibers are too densely packed, failing to maintain the mean pore ~ize of from 20 to 60 ~m and the porosity of from 60 to 80 %. In the present invention, by use of chopped carbon fibers with fiber lengths of from 3 to 20 mm, ~uch inconveniences are avoided. Preferable fiber length i~ in the range of from 5 to 15 mm.
Mea~urement of porosity is conducted a~ described below. That is, previou~ly, the re~pective weights of the gla~s bottle for measurement and the sample are weighed, and the measurement resu1ts are re~pectively made Wb and W9. Further, ethanol is filled in the glass bottle and weighed, and the weight We of the ethanol filled i8 calculated. Next, the ~ample is placed in the glasq bottle, the internal pressure of the gla~s bottle ig decreased to a vacuum of 1 mm~lg or lower, and then ethanol i~ filled in the bottle, followed by weighing. The result of weighing at this time iQ made Wbem, and the value obtained by detractin~
Wb from thi~ value Wbem is made Wm (= Wbem - Wb). When the bulk density of the sample is made pm, and the density of ethanol pe, the porosity Pr (~) can be given by the following formula:
Pr = (l - (We-Wm+Ws) x pm / (Ws x pe)) x 100 However, the bulk density pm (g/cm3) is calculated according to the following formula:
pm = Ws x l0/(t x A) Here, t i~ the thickness of the sample (mm), and A i~
the area of the qample (cm3).
~3~31Z3 Now, the chopped carbon fibers as described above are made into carbon fiber mat by means of continuously or batch wise.
This carbon fiber mat is practiced by mixing and stirring chopped carbon fibers in a dispersion medium containing a binder for paper making, and the mixture is filtered onto a fabric or a wire net, etc. By means of paper making, the chopped carbon fibers are dispersed in random directions within substantially 2-dimensional plane, and bound to each other with the binder to be formed into a mat or a sheet having self-form retentivity. Here, as the organic binder for paper making, polyvinyl alcohol, hydroxyethyl cellu-lose, polyethylene oxide, polyacrylamide, polyester, etc. can be used, and the binder is diluted with a dispersion medium such as water or methanol, etc. The amount of the binder diluted may be in the range of from about 1 to about 30 % by weight. The chopped carbon fibers and the dispersion medium may be mixed at a ratio which may also depend on the kind of the dispersion medium, etc., but preferably at a ratio of from about 0.01 to about 0.1 % by weight of the chopped fibers.
After paper making, the dispersion medium is removed by drying with heatiny. The amount of the remaining binder with respect to the total weight of the carbon fiber mat dried may, after drying, be in the range of from 5 to 30 ~ by weight, prefer-ably 5 to 20 % by weight.
The carbon fiber mat can be also obtained by the method other than that described above. For example, the above-mentioned chopped carbon fibers are dispersed in water as the dispersion medium to 0.01 to 0.1 % by weight thereof, if desired, surfactants and an agent of ~3~ 3 increa~ing a visco~ity of the di~persant such a~ ~odium acrylate, sodium glycolate, etc. are added to improve a disper~ibility, and paper making is carried out in the ~ame manner as described above. Then, the above binder diluted with the ~olvent is attached to the re~ultant paper material by impregnation or ~praying, thereby obtaining the carbon fiber mat.
Next, the carbon fiber mat which the chopped carbon fiber are bound to each other with a binder, make into a prepreg by mean~ of impregnatin~ a re~in carbonizable by heat treatment. The re~in may be, for example, a resol type phenolic resin. A~ the Yolvent for such resin, methanol and water can be uqed. This ~olvent i8 removed by drying with heating after impregnation.
Further, to produce an electrode ~ub~trate further improved in electroconductivity by lowering resistivity and yet also excellent in mechanical strength, it i8 preferable to use, as a resin to be impregnated, Q
mixture of a ~elf-curable re~in and non-~elf-curable re~in mixed at an appropriate ratio. The ~elf-curable resin i8 a re~in which can be cured through conden~ation and/or addition reaction only by heating, while the non-~elf-curable re~in iY a reYin which can be lowered in vi~c08ity but not cured only by heating becau~e it contain~ no curing agent.
A~ the self-curable re~in, which is not particularly limited, re~ol type phenolic re~in~, thermo~ettin~ type polyimide~, polyphenyl quinoxalines, etc. may be preferably used. Aq the non-self-curable re~in, resins which will not be cured unle~Y a curing agent and heating are u~ed, and the re~in~ which are not particularly limited, may include preferably ~3~3~
novolak type phenolic resins, pitch, furan resins, epoxy resins, polyphenylene oxides, etc.
When a non-self-curable re~in is impre8nated together with a self-curable resin into the carbon fiber mat, electroconductivity of the ~ubYtrate can be remarkably improved. The rea~on for remarkable improvement of electroconductivity may be that the non-self-curable resin i~ not cured even by heatin~ but has fluidity with lowered visco~ity as described above, and therefore even if voids are formed around the chopped carbon fibers by scattering of the binder which has bound the chopped carbon fiber~ to each other in the heat treatment ~tep a~ described later, the non-self-curable resin enters into the voids, thus embedding the voids.
It is preferable that the non-self-curable resin should be mixed at a proportion of 10 to 500 parts by weight based on 100 parts by weight of the ~elf-curable resin. At a level less than lO parts by wei~ht, electroconductivity may not be improved sufficiently.
On the other hand, if such a large amount as exceeding 500 part~ by weight is used, the mixed resin cannot become ~ufficiently stiff even in the subsequent hot pressing ~tep or hear treatment step, whereby the substrate become~ to have tackiness and adhered with other members, etc. to become difficult in handling.
More preferably, the non-self-curable re~in should be mixed at a proportion of 50 to 300 part~ by weight based on 100 parts by weight of the ~elf-curable resin.
Impregnation of the resin into the carbon fiber mat may be practiced by dippiny the carbon fiber mat into a resin solution dissolved in a solvent such as water, methanol, tetrahydrofuran, dioxane, 13~31;~
N-methylpyrrolidone, etc., or ~praying the above resin solution onto the carbon fiber mat. The resin concentration of the solution used is preferably controlled to about 5 to 50 % by weight, in view of easines4 of impregnation. Then~ the solvent is removed by heating.
The amount of the resin impregnated into the carbon fiber mat may be preferably about 80 to 600 parts by weight baYed on 100 parts by weight of the chopped carbon fibers in the carbon fiber mat. More preferable impregnation amount is in the range of from 100 to 350 parts by weight based on 100 parts by weiYht of the ohopped carbon fibers.
The preferable range of the impregnated amount (attached amount) of the resin of the carbon fiber mat as described above is finally determined from the preferable ran~e of the carbonized resin amount relative to the total amount of chopped carbon fibers.
To de~cribe this more specifically, the carbonized resiD content in the electrode s~lbstrate affects electroconductivity, pre~sure lo~s, etc. of the substrate. More specifically, in the case of using only a resol type phenolic resin~ as shown in Fig. 5, with the increase of the carbonized resin content C
while the resistivity Rt ( Q cm~ in the thickness direction is acceleratedly lowered, the pressure loss Pl (mmH2O~mm) during gas flowing throu~h the thickness direction will be accelerat&dly increased. Although lowering in resistivity iY preferable, increase of pressure loss should be avoided because it will lower gas permeability as described above. Also, as the carbonized resin content becomes lowered, bindability between the chopped carbon fibers will be lowered, and ~3~31~
al~o mechanical strength of the e]ectrode ~ub~trate will be lowered. In the pre~ent invention, ~uch inconvenience~ a~ described above are avoided by controlling the carbonized resin amount to 35 to 60 X
by weight ba~ed on the total a~ount of the chopped carbon fibers and carbonized re~in. Thu~, the amount of the re~in attached on the carbon fiber mat is required to be controlled such that the carbonized resin content in the electrode ~ub~trate may become 35 to 60 X by wei~ht. The re~istivity Rt ( Q cm) in the thickness direction i8 calculated according to the following formula from the voltage drop E (V) when the substrate with the thicknes~ _ (mm) is sandwiched between mercury electrode~ with a certain area S (cm2) and a certain current I (A) is passed between the electrodeY:
Rt = ~E x S) x 10 / (I x t) Also, when electricity generated i~ varied within the electrode plane, there is a fear that exces~ively great current may be passed through a part of the electrode to expedite con~umption of the electrode.
For solving this problem, the re~i~tivity in the planar direction of the electrode should be preferably 20 m Q cm or less, more preferably l5 m Q cm or less.
The resistivity in the planar direction is measured a~ described below. That is, a sample of 25 x 50 mm i~ cut out from the ~ubstrate provided for the test, two measurement electrodes, each having a length of 25 mm and made of a copper plate plated with gold, are placed on the upper ~urface of the sample in parallel to the shorter side of the sample with an interval of 25 mm between the electrodes, a current of 1 A is made flow between the measurement electrode~
13~31~;~
I g with a load of 5 kgf/cm2 npplied on each of the electrodes, and the voltage drop F (mV) between the electrodes is mea~ured. From the voltage drop measured, the resistivity Ra (m Q cm) i9 calculated from the following formula:
Ra = (R/I) x t x 10~1 Here, t is the thickness of the sample ~mm), I is current value pas~ed between the two measurement electrodes (A), namely 1 A, and the measurement plane is 25 mm x 25 mm. The resistance in the plane direction depends on the length of chopped carbon fibers, bindability mutually between the chopped carbon fibers, etc.
The mat prepreg after resin impregnation is molded by hot pressing under a pre~ure of 2 to 10 kgf/cm2 at 120 to 200 C for about 5 to 60 minutes to cure the resin. At this stage, if desired, a plural number of mat prepregs are laminated on one another ~o a~ to obtain the necessary thickness. When a mixture of a ~elf-curab]e and non-self-curable resins i8 used to obtain the mat prepreg, the self-curable resin of the mixture will be cured in the hot pressing step. The applied pressure during hot pressing affect~
compre~sion ratio, mean pore size, etc. of the electrode substrate. Fig. 6 ~hows the relationship of the pressure P ~kgftcm2~ applied at hot pressing, with mean pore size Pd ( ~m) and the compre~sion ratio Cr (X~ measured under a condition of 5 kgf/cm2 being applied to the substrate. As shown in Fig. 6, it was discovered that the compression ratio Cr becomes greater as the applied pressure P is higher. On the other hand, the mean pore size Pd becomes abruptly smaller as the applied pressure P becomes greater 13~31~
within the ranRe when the applied pre~sure i~
relatively lower, but wi]l not be changed 80 much thereafter. And, from this Fig. 6, it can be understood that an electrode ~ub~trate having a mean pore si~e of 20 to 60 ~m and a compre~3ion ratio of 20 % or le~ can be obtained when the applied force i8 2 to 10 kgf/cm2. In Fig. 6, the pre~sure applied to the ~ubstrate, i.e., 5 k~f/cm2, i~ determined in view of the force applied when electrode ~ubstrates are practically ~uperpo~ed and u3ed by clampin~ the3e.
Compres~ion ratio Cr i~ determined a~ follows. The electrode ~ub~trHte i 8 cut to 3 cm ~quare, and 20 ~heets thereof are superpo~ed on one another to be placed on a metal plate, The thickne~ses t1, t2, t3, and t5 of the sheet3 are mea~ured under loading pressures of 1, 2, 3, and 5 kgf~cm2, re~pectively, and the initial thickness to under no load~ i~ obtained by ~traight line approximation from the thickne~se~ t1, t2, and t3 measured, Cr is calculated according to the following formula:
Cr = [~to ~ t5)/to] x 100 The molded ~ubstrate after hot pre3sing is then heat treated in an inert atmo~phere ~uch a~ nitrogen or argon or in vacuum atmo3phere to 1300 to 3000C. By thi~ heat treatment, the re~in previou~ly impregnated is carbonized to become carbon or graphite. At thi~
~tage, the binder u~ed during paper makin~ will be pyrolyzed and di3~ipated. In the ca~e of uYing a mixture of self-curable and non-~elf-curable re~ins, for example, a mixture of a re~ol type phenolic re~in and a novolak type phenolic re~in, the novolak type phenolic resin probably enter~ into the void~ formed by dissipation before it is carboni~ed. Thu~, the ~3V31~3 electrode sub~trate is obtained.
Fig. 7 shows, in the case of using only a re~ol type phenol;c resin as the resin impregnated to the carbon fiber mat, the relation~hips of the porosity Pr ~X) with the pre~sure 108~ Pl (mm~12O/mm) in the thickness direction of the substrate and the resi~tivity Rt ( Q cm) in the thickne~s direction. A~
i~ apparent from thi~ Fig. 7, the pressure 1098 Pl will become acceleratedly smaller as the porosity Pr becomes larger to improve ~a~ permeability, while on the other hand, the resi~tivity Rt becomes acceleratedly greater to lower greatly the electroconductivity. On the other hand, if the porosity Pr exceeds 80 %, bindability mutually between the chopped carbon fibers will be lowered, whereby not only electroconductivity but also mechanical strength will be lowered. To be used for the electrode substrate, the poro~ity is required to be 60 to 80 %.
It i~ important to avoid power generation irregularity caused by planar variance of the amount of the cataly~t coated on the electrode substrate surface, and avoid an increa~e in electrical resistance due to contact inferiority between the electroconductive ~as separator intervening between the ad~jacent cell units and the electrode ~ubstrate when a large number of cell units are ~tacked up electrically in ~erie~. To thi~
end, the surface roughnes~ of the electrode ~ubstrate should preferably be 100 ~m or less at the maximum undulation of the rolling circle.
The surface roughnes~ of the substrate is related to diameter of individual chopped carbon fibers, length of individual chopped carbon fiber~, dispersibility of chopped carbon fibers, etc. If the fiber diameter is 13(?31Z3 large, the ~urface roughness become~ larger due to the planar variance of the amount attached during paper making of chopped carbon fibers. If the fiber length i8 too long, di~per~ibility during paper making also becomes poor to make the surface rough. When short, the above problem can be ~olved but the ~trength of the substrate is weakened. Short carbon fiber~ are generally obtained by cutting and splitting continuous fiber bundle~, and the ~heafing a~ent attached on the fiber bundles will generally worsen disperYibility in the paper making step, to con~equently roughen the sub~trate ~urface. Preferably, no sheafing agent should be used, or it is preferable to u~e water or water-401uble sheafing agent. Also for realizing the above surface roughnes~ of 100 ~m or le~s, it is preferable that the chopped carbon fibers should have a diameter of from 4 to 9 ~m and a length of from 3 to 20 mm in the individual fibers.
Mea~urement of the maximum undulation ~WEM) of the rolling circle on the substrate surface can be practiced according to the following method. A~ the measuring instrument, for example, a univer~al surface shape measuring instrument MODEL SR-3C produced by Kabushiki Kaisha Kosaka kenkyu~ho may be uYed. The contact needle which forms the detecting portion of this measuring instrument has a contact needle tip end radius of 800 ~m and a contact apex angle of 60 D ~ with the contact needle measuring load during mea4urement being 0.4 gf or les~.
By Eixing the sub~trate to be measured on a checker board with ~mooth surface and moving the above contact needle about 7 cm along the substrate surface, the shape of the surface can be measured. The 13(~3123 difference between the maximum value and the minimum value of the wave form at the interval of 5 cm at the central portion of the movement length of the contact needle iY defined as the maximum undulation of the rolling circle.
The e]ectroconductive sub~trate thu~ prepared can be worked. For example, by formin~ grooves in parallel to each other on one ~urface of the Rubstrate, a ribbed electrode ~ubstrate for pho~phoric acid fuel cell can be provided.
~XAMPL,~S
Example 1 A~ ~hown in Table 1, variou~ electrode substrates were prepared by varying the diameter of the individual chopped carbon fibers, fiber length of the individual chopped carbon fiber~, amount of binder attached during paper making of carbon fiber mats, amount of resin attached, applied pre~ure during hot pressing, and heat treatment temperature. More specifically, for the ~ub~trates of the present invention No~ 8 and comparative ~ub~trate~ No~. C1 - C6, polyacrylonitrile carbon fiber "TORAYCA" T300 (mean diameter of single filament: 7 ~m, number of filaments: 6000) manufactured by Toray Industries, Inc. was u~ed, for the ~ubstrate No. 9 of the present invention a polyacrylonitrile carbon fiber "TORAYCA" T800 (mean diameter of ~in~le filament: 5 ~m, number of filaments: 6000) manufact~red by Toray Industries, Inc., for the comparative substrate No. C7 a carbon fiber "Thornel" P25-W (mean diameter of ~ingle filament: 11 ~m, number of filaments: 4000) manufactured by Union Carbide Corp., U.S.A., respectively, and the polyacrylonitrile carbon fibers were cut to the re~pective fiber lengths shown Jrc~ Je -mc~k ~L3f(~3123 is Table 1, followed by sufficient fiber splitting.
These chopped fibers were stirred in water, subjected to paper makin~ on a wire net, and after dryin~, impregnated with an aqueou~ polyv;nyl nlcohol solution, followed by dryin~ with heating to obtain an carbon fiber mat. The amounts of the polyvinyl alcohol attached were as shown in Table 1.
Next, each carbon fiber mat was impregnated with a 10 % by wei~ht of methanolic solution of a re~ol type phenolic resin (PR-g183B, manufactured by Sumitomo Durez K.K., repre~ented by R in the table) to have the phenolic resin in part~ by weight shown in Table 1 based on 100 parts by weight of the carbon fiber mat, and after drying at 90C, subjected to hot pressing under the pressure and the temperature ~hown in Table 1 for 15 minutes to cure the phenolic resin.
~ Subsequent]y, the above molded substrate with the cured phenolic resin was carbonized by heat treatment at the temperature shown in Table 1 in ar~on atmoæphere to obtain an electrode substrate having the thickness, the bulk density, the mean pore size, the porosity, the compression ratio, the pressure loss, the planar resi~tivity, the re~istivity in the thickness direction, the roughne~s, the flexural strength, and the heat resistance shown in Table 2.
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~5i ,~o ~3~el~sqns ,~o ~e~sqns 13V31;~3 The substrates Nos. 1 and 2 of the present invention differ in thickne~s of the substrate, the substrates NQS. 1, 3, 4 of the present invention and comparative ~ubstrates No~. C1, C2 in the carbonized re~in content in the substrate prepared, the substrates Nos. 1, 5, ~ of the pre~ent invention and comparative substrates No~. C3, C4 in applied pressure during hot pressing, the sub~trates Nos. 1 and 7 of the present invention in heat treatment temperature, the substrates Nos. 1 - 9 of the present invention and comparative substrates Nos. C5 - C7 in lenth and/or diameter of individual chopped carbon fibers from each other, respectively.
For the substrate No. 1 of the present invention and comparative substrate No. C7 shown in Table 1, the substrate surface roughness was measured by use of the universal surface shape measuring instrument MODEL SE-3C produced by Kabushiki Kaisha Xosaka Kenkyusho as mentioned above. The substrate surface shape of each sample was subjected to measurement of the respective two points at an~ de~ired direction of the substrate surface ~0 ) and at the direction perpendicular thereto, from which the averaged maximum undulation of the rolling circle was determined. Fig. 8 and Fig. 9 show the substrate ~urface ~hapes of the ~ubstrate No.
1 of the present invention and comparative substrate No. C7, respectively. The avera~ed maximum undulation WEM of the rolling circle of the substrate No. 1 of the present invention was 65 ~m, when the rolling circle radius was 800 ~m. On the other hand, the averaged maximum undulation WEM of the rolling circle of the comparative substrate No. C7 was 125 ~m.
Heat resistance of the substrate provided for the test wa4 evaluated by the weight reduction of the sample determined a~ described below. More specifically, by use of a super-heating weight reduction te3ting device (Thermal Analyzer BT-30 produced by Shimazu Corp.), the sample was maintained in an air atmosphere of 390~ for 12 hours and the weight reduction of the sample wa~ measured. The initial weight of the mea~ured sample wa~ set at around 10 mg.
Flexural ~trength was measured according to the three point flexural test by use of "Autograph" IS2000 produoed by Shimazu Corp. under the condition~ shown in the following table.
~ample Span Indenter Fulcrum Loading Speed Thickness Diameter Diameter of Loaa mm mm mm mmimin 0.2 mm or less 50 3 3 0.2 mm or more 7 0.75 0.75 5 .
Here, span is the di~tance between fulcrums, indenter diameter i~ the radius of curvature of the indenter for applying the load on the sample at the contacted portion with the sample, the fulcrum diameter i8 the radius of ourvature of the fulcrum for supporting the sample at the contacted portion with the sample, and the loading ~peed of load iY the falling speed of the crosshead supporting the indenter, and the load corresponding to the falling amount i~ applied through the indenter on the sample. Flexural stren~th was determined from the load on breakin~ of the sample.
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m ~ ~0 ~ sqns 13t~3~
Example 2 As shown in Table 3, various electrode substrate~
were prepared by varying the resin impregnated into the carbon fiber mats formed by means of paper makin~ and the heat treatment temperature. More specifically, for all of these substrate~, the polyacrylonitrile carbon fiber "TORAYCA" T300 manl~factured by Toray Industries, Inc. as described above was u~ed, and ~ufficiently splitted after cutting into len~th of 12 mm. The splitted chopped fiber~ were dispersed to 0.04 X by weight thereof in water, subjected to paper makin~ on a wire net, further the resultant paper wa~ dipped in an aqueous 10 X by weight of polyvinyl alcohol solution, pulled up and dried to obtain a carbon fiber mat with attachment of about 30 X by weight of polyvinyl alcohol as a binder.
Next, various mixed re~ins at mixin~ ratios shown in Table 3 were impregnated into the carbon fiber mats, to have about 155 parts by wei~ht of mixed resins attached per 100 parts by weight of -the chopped carbon fibers in all of the carbon fiber mats. For the ~ub~trates Nos. 11 - 19 of the present invention, mixed resins of a resol type phenolic resin (PR-9183B
manufactured by Sumitomo Durez K.K. as mentioned above) and a novolak type phenolic resin (KP-759S manufactured by Arakawa Kagaku Kogyo K.K. represented by N in the table), and for the substrates No. 20 and No. 21 of the present inventiont mixed resins of a resol type phenolic resin (PR-9183B manufactured by Sumitomo Durez K.K. a~ mentioned above~ and pitch were employed, respectively. For the substrates Nos. 22 and 23 of the pre~ent invention, a resol type phenolic re~in (KP-743K
manufactured by Arakawa Kagaku Kogyo K.K. represented 13~3~
by A in the table) was sinRly employed. The carbon fiber mat was dipped in tO % by wei~ht of methanolic solution of these reqins to be impregnated with the resin(s). However, for the substrates Nos. ZO and 21 of the present invention, 15 % by weight of tetrahydrofuran solution of the above mixed resins was impregnated. Next, after the impre~nation, those were dried at 90~C for 3 minutes to obtain a prepreg, and then two sheets of the prepreg were superposed on one another and sub~jected to hot pressing under a temperature of 170C by application of a pressure of 5 kgf/cm2 for 15 minutes to cure the resol type phenolic resin to obtain a molded substrate.
Next, the above molded sub~trate was carbonized by heat treatment at the temperature shown in Table 3 in argon atmosphere for 30 minutes, and obtain the electrode substrates having the thickne~s, the bulk den~ity, the mean pore size, the porosity, the compre~sion ratio, the pressure loss, the re~istivity in the planar direction, the resistivity in the thickness direction, the roughnesY, the flexural strength, and the heat resistance shown in Table 4.
All of the subYtrates No~ 23 of the present invention ~hown in Table 3 and Table 4 satisfy the ranges of carbonized resin content, pore size, porosit~
and compression ratio as defined by the pre~ent invention. Further, electroconductivity of the electrode sub~trate of the present ;nvention could be improved by increa~ing the mixing ratio of the non-~elf-curable resin in the re~in solution impregnated into the carbon fiber mat.
3~ o o o o o o o o o o o o o o o o _ _ _ _ c~ c~ ~ ~ a~ ~r _ ~ ~ ~ ~ ~ ~ ~
o ~ o o o o o o o o o o o o o o ~-~ - - - - -~ ~ _ ~ æ _ ~ ~ ~ _ æ _ __ ~ ~ ~ ~
_ __ c~ _ c~ c~ ~r ~ ~o ~ oo a~ E: ~ ~ S ~ ~,o, ~
i~l ~ Z 0 2 O :~ ~ O O ~ O :~ ~ 0 2 m a~ uol~U~J~uI uoslledulo~
~5i ,~o ~3~el~sqns ,~o ~e~sqns 13V31;~3 The substrates Nos. 1 and 2 of the present invention differ in thickne~s of the substrate, the substrates NQS. 1, 3, 4 of the present invention and comparative ~ubstrates No~. C1, C2 in the carbonized re~in content in the substrate prepared, the substrates Nos. 1, 5, ~ of the pre~ent invention and comparative substrates No~. C3, C4 in applied pressure during hot pressing, the sub~trates Nos. 1 and 7 of the present invention in heat treatment temperature, the substrates Nos. 1 - 9 of the present invention and comparative substrates Nos. C5 - C7 in lenth and/or diameter of individual chopped carbon fibers from each other, respectively.
For the substrate No. 1 of the present invention and comparative substrate No. C7 shown in Table 1, the substrate surface roughness was measured by use of the universal surface shape measuring instrument MODEL SE-3C produced by Kabushiki Kaisha Xosaka Kenkyusho as mentioned above. The substrate surface shape of each sample was subjected to measurement of the respective two points at an~ de~ired direction of the substrate surface ~0 ) and at the direction perpendicular thereto, from which the averaged maximum undulation of the rolling circle was determined. Fig. 8 and Fig. 9 show the substrate ~urface ~hapes of the ~ubstrate No.
1 of the present invention and comparative substrate No. C7, respectively. The avera~ed maximum undulation WEM of the rolling circle of the substrate No. 1 of the present invention was 65 ~m, when the rolling circle radius was 800 ~m. On the other hand, the averaged maximum undulation WEM of the rolling circle of the comparative substrate No. C7 was 125 ~m.
Heat resistance of the substrate provided for the test wa4 evaluated by the weight reduction of the sample determined a~ described below. More specifically, by use of a super-heating weight reduction te3ting device (Thermal Analyzer BT-30 produced by Shimazu Corp.), the sample was maintained in an air atmosphere of 390~ for 12 hours and the weight reduction of the sample wa~ measured. The initial weight of the mea~ured sample wa~ set at around 10 mg.
Flexural ~trength was measured according to the three point flexural test by use of "Autograph" IS2000 produoed by Shimazu Corp. under the condition~ shown in the following table.
~ample Span Indenter Fulcrum Loading Speed Thickness Diameter Diameter of Loaa mm mm mm mmimin 0.2 mm or less 50 3 3 0.2 mm or more 7 0.75 0.75 5 .
Here, span is the di~tance between fulcrums, indenter diameter i~ the radius of curvature of the indenter for applying the load on the sample at the contacted portion with the sample, the fulcrum diameter i8 the radius of ourvature of the fulcrum for supporting the sample at the contacted portion with the sample, and the loading ~peed of load iY the falling speed of the crosshead supporting the indenter, and the load corresponding to the falling amount i~ applied through the indenter on the sample. Flexural stren~th was determined from the load on breakin~ of the sample.
~ 7~ccoJe~ k 13(~
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~ C~ C`l ~ ._ _ ~ ~ _ ~ e~' O ~ ~ ~ C`J ~ ~ C~ C" ~ ~ er ._ ll ll ll ll ll ll ll ll ll ll ll X Y Y C~ C2!: ~ ~ X X ~: a:
~ :2: 2 2 :7: 2: Z :Z: ;2: Z Z ~
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~ ~ _ C~ O O O O O O O O O O O O
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m ~ uol ~uaAUI
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r- ~ C~ ~ ~ 88 ~ ~a ~3 ~ ~ ''~; ~ ~ -c~ _ ~3 _ ~o~ _ _ _ . _ ~ ~ ~ C~ g~ g~
_ _ _ __ ~ ..~.~ oc~ ~ a~ oo ~ r- ~ ~ ~ O ~ ~ r-.~ ~ _ _æ
~ ~ ~ U~ ~ ~Y ~ ~ ~ ~ ~3 2~; ~ ~i ~3 ~
~ o o o o o o o o o o o o o ~ _ _ _ _ o a~ r- r- ~ 90 ~o ~ Oo r- _ o oo r-_ .~ c~ ~r ~r =e _ _ _ o ~ ~ ~ ~ ~ ~g _,~ o o o o o o o o o o o o o ~ ~ _ c~ ~3 _ ~ a~ ~ ~ ~3 c~ ~3 _ ~ o o o o o o o o o o o o æ ~ ~
_ _ __ _ _ c~ c~ ~r In ~D t- CO ~ ~ ~
~r o 2 2: ~ 2: z ~ - ;z a ~ z ~z 1~1 ~ UF ~,u~AUI
m ~ ~0 ~ sqns 13t~3~
Example 2 As shown in Table 3, various electrode substrate~
were prepared by varying the resin impregnated into the carbon fiber mats formed by means of paper makin~ and the heat treatment temperature. More specifically, for all of these substrate~, the polyacrylonitrile carbon fiber "TORAYCA" T300 manl~factured by Toray Industries, Inc. as described above was u~ed, and ~ufficiently splitted after cutting into len~th of 12 mm. The splitted chopped fiber~ were dispersed to 0.04 X by weight thereof in water, subjected to paper makin~ on a wire net, further the resultant paper wa~ dipped in an aqueous 10 X by weight of polyvinyl alcohol solution, pulled up and dried to obtain a carbon fiber mat with attachment of about 30 X by weight of polyvinyl alcohol as a binder.
Next, various mixed re~ins at mixin~ ratios shown in Table 3 were impregnated into the carbon fiber mats, to have about 155 parts by wei~ht of mixed resins attached per 100 parts by weight of -the chopped carbon fibers in all of the carbon fiber mats. For the ~ub~trates Nos. 11 - 19 of the present invention, mixed resins of a resol type phenolic resin (PR-9183B
manufactured by Sumitomo Durez K.K. as mentioned above) and a novolak type phenolic resin (KP-759S manufactured by Arakawa Kagaku Kogyo K.K. represented by N in the table), and for the substrates No. 20 and No. 21 of the present inventiont mixed resins of a resol type phenolic resin (PR-9183B manufactured by Sumitomo Durez K.K. a~ mentioned above~ and pitch were employed, respectively. For the substrates Nos. 22 and 23 of the pre~ent invention, a resol type phenolic re~in (KP-743K
manufactured by Arakawa Kagaku Kogyo K.K. represented 13~3~
by A in the table) was sinRly employed. The carbon fiber mat was dipped in tO % by wei~ht of methanolic solution of these reqins to be impregnated with the resin(s). However, for the substrates Nos. ZO and 21 of the present invention, 15 % by weight of tetrahydrofuran solution of the above mixed resins was impregnated. Next, after the impre~nation, those were dried at 90~C for 3 minutes to obtain a prepreg, and then two sheets of the prepreg were superposed on one another and sub~jected to hot pressing under a temperature of 170C by application of a pressure of 5 kgf/cm2 for 15 minutes to cure the resol type phenolic resin to obtain a molded substrate.
Next, the above molded sub~trate was carbonized by heat treatment at the temperature shown in Table 3 in argon atmosphere for 30 minutes, and obtain the electrode substrates having the thickne~s, the bulk den~ity, the mean pore size, the porosity, the compre~sion ratio, the pressure loss, the re~istivity in the planar direction, the resistivity in the thickness direction, the roughnesY, the flexural strength, and the heat resistance shown in Table 4.
All of the subYtrates No~ 23 of the present invention ~hown in Table 3 and Table 4 satisfy the ranges of carbonized resin content, pore size, porosit~
and compression ratio as defined by the pre~ent invention. Further, electroconductivity of the electrode sub~trate of the present ;nvention could be improved by increa~ing the mixing ratio of the non-~elf-curable resin in the re~in solution impregnated into the carbon fiber mat.
Claims (18)
1. A porous electrode substrate for fuel cell comprising short carbon fibers dispersed in random directions within substantially 2-dimensional plane and carbonized resin for mutually bonding said carbon fibers, the individual fiber of said carbon fibers having a diameter of from 4 to 9 µm and a length of from 3 to 20 mm, the content of said carbonized resin being in the range of from 36 to 60 % by weight, said substrate having pores with n mean pore size of from 20 to 60 µm and a porosity of from 60 to 80 %, and a compression ratio not more than 20 %.
2. The electrode substrate according to Claim 1, wherein an individual fiber of said carbon fibers has a diameter of from 4 to 8 µm.
3. The electrode substrate according to Claim 1, wherein an individual fiber of said carbon fibers has a length of from 5 to 15 mm.
4. The electrode substrate according to Claim 1, wherein a surface of said substrate has a surface roughness of not more than 100 µm, defined as the maximum undulation of the rolling circle tracing the shape of said substrate surface.
5. The electrode substrate according to Claim 1, wherein a resistivity in the planar direction of said substrate is not more than 20 m .OMEGA. ?cm.
6. A process for producing a porous electrode substrate for fuel cell containing in the range of from 35 to 60 X carbonized resin by weight, and having pores with a mean pore size of from 20 to 60 µm and a porosity of from 60 to 80 %, and a compression ratio of not more than 20 %, comprising the steps of:
dispersing chopped carbon fibers each of which has a length of from 3 to 20 mm and a diameter of from 4 to 9 µm, in random directions within substantially 2-dimensional plane;
binding said carbon fibers with an organic binder to obtain an carbon fiber mat;
impregnating said carbon fiber mat with at least one resin to obtain a prepreg;
hot pressing said prepreg under a temperature of from 120 to 200°C and a pressure of 2 to 10 kg/cm2 to obtain a molded substrate; and heat treating said molded substrate to carbonize said resin under a temperature of from 1300 to 3000°C
in an inert atmosphere or vacuum atmosphere.
dispersing chopped carbon fibers each of which has a length of from 3 to 20 mm and a diameter of from 4 to 9 µm, in random directions within substantially 2-dimensional plane;
binding said carbon fibers with an organic binder to obtain an carbon fiber mat;
impregnating said carbon fiber mat with at least one resin to obtain a prepreg;
hot pressing said prepreg under a temperature of from 120 to 200°C and a pressure of 2 to 10 kg/cm2 to obtain a molded substrate; and heat treating said molded substrate to carbonize said resin under a temperature of from 1300 to 3000°C
in an inert atmosphere or vacuum atmosphere.
7. The process according to Claim 6, wherein said dispersing and binding steps comprise mixing said chopped carbon fibers by stirring together with a dispersion medium containing an organic binder, and then making paper from the mixture to obtain said carbon fiber mat.
8. The process according to Claim 6, wherein said dispersing and binding steps comprise mixing said chopped carbon fibers by stirring together with a dispersant containing no organic binder, after that making paper from the mixture, and then treating the paper obtained with an organic binder to obtain said carbon fiber mat.
9. The process according to Claim 6, wherein a step of drying is provided between said binding and impregnating steps, said drying step comprises heating said carbon fiber mat, and the amount of said organic binder attached to said carbon fiber mat after completion of said drying step is in the range of from 5 to 30 % by weight based on the total weight of said carbon fiber mat.
10. The process according to Claim 6, wherein said resin is at least one selected from the group consisting of phenolic resins, epoxy resins, furan resins and pitch.
11. The process according to Claim 6, wherein a plural number of sheets of said prepreg are laminated and subjected to hot pressing in said hot pressing step.
12. The process according to Claim 6, wherein said resin contains a resin which is curable in said subsequent hot pressing step and a resin which is not curable in said subsequent hot pressing step.
13. The process according to Claim 12, wherein said resin contains 10 to 500 parts by weight of said non-curable resin based on 100 parts by weight of said curable resin.
14. The process according to Claim 13, wherein said resin contains 50 to 300 parts by weight of said non-curable resin based on 100 parts by weight of said curable resin.
15. The process according to Claim 12, wherein said curable resin is at least one selected from the group consisting of resol type phenolic resins, polyphenyl quinoxalines and thermosetting polyimides, and said non-curable resin is at least one selected from the group consisting of novolak type phenolic resins, pitch, furan resins, epoxy resins and polyphenylene oxides.
16. An electrode adapted for a phosphoric acid fuel cell, which comprises, an electrode substrate of a plate shape, and a catalyst layer formed on a surface of the electrode substrate, wherein the electrode substrate is as defined in any one of claims 1 to 5.
17. The electrode according to claim 16, wherein the catalyst layer comprises platinum or platinum alloy and has a thickness of 50 to 500 µm.
18. A phosphoric acid fuel cell, comprising a matrix layer impregnated with a phosphoric acid solution as an electrolyte, and positive and negative electrodes each of a plate shape sandwiching the matrix layer, wherein the electrodes comprises electrode substrates and catalyst layers formed on surfaces of the substrates that are contacted with the matrix layer, and the substrates are as defined in any one of claims 1 to 5.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP62-88129 | 1987-04-10 | ||
| JP62088129A JPS63254669A (en) | 1987-04-10 | 1987-04-10 | Electrode substrate for fuel cell |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1303123C true CA1303123C (en) | 1992-06-09 |
Family
ID=13934311
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000563352A Expired - Fee Related CA1303123C (en) | 1987-04-10 | 1988-04-06 | Electrode substrate for fuel cell and process for producing the same |
Country Status (5)
| Country | Link |
|---|---|
| US (1) | US4851304A (en) |
| EP (1) | EP0286945B1 (en) |
| JP (1) | JPS63254669A (en) |
| CA (1) | CA1303123C (en) |
| DE (1) | DE3881941T2 (en) |
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| US7297445B2 (en) | 2000-01-27 | 2007-11-20 | Mitsubishi Rayon Co., Ltd. | Porous carbon electrode substrate and its production method and carbon fiber paper |
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| JP3166133B2 (en) * | 1992-05-19 | 2001-05-14 | ソニー株式会社 | Optical shaping method and apparatus |
| JP2933443B2 (en) * | 1992-06-10 | 1999-08-16 | 日本電気アイシーマイコンシステム株式会社 | Positive and negative waveform separation circuit |
| JPH0644963A (en) * | 1992-07-24 | 1994-02-18 | Japan Storage Battery Co Ltd | Paste filling equipment |
-
1987
- 1987-04-10 JP JP62088129A patent/JPS63254669A/en active Granted
-
1988
- 1988-04-05 DE DE88105396T patent/DE3881941T2/en not_active Expired - Lifetime
- 1988-04-05 EP EP88105396A patent/EP0286945B1/en not_active Expired - Lifetime
- 1988-04-06 CA CA000563352A patent/CA1303123C/en not_active Expired - Fee Related
- 1988-04-08 US US07/179,286 patent/US4851304A/en not_active Expired - Lifetime
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US7297445B2 (en) | 2000-01-27 | 2007-11-20 | Mitsubishi Rayon Co., Ltd. | Porous carbon electrode substrate and its production method and carbon fiber paper |
Also Published As
| Publication number | Publication date |
|---|---|
| EP0286945A2 (en) | 1988-10-19 |
| EP0286945B1 (en) | 1993-06-23 |
| DE3881941T2 (en) | 1994-02-10 |
| JPS63254669A (en) | 1988-10-21 |
| DE3881941D1 (en) | 1993-07-29 |
| EP0286945A3 (en) | 1990-07-18 |
| JPH0544779B2 (en) | 1993-07-07 |
| US4851304A (en) | 1989-07-25 |
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