CA1307819C - Composite electrode for use in electrochemical cells - Google Patents
Composite electrode for use in electrochemical cellsInfo
- Publication number
- CA1307819C CA1307819C CA000580269A CA580269A CA1307819C CA 1307819 C CA1307819 C CA 1307819C CA 000580269 A CA000580269 A CA 000580269A CA 580269 A CA580269 A CA 580269A CA 1307819 C CA1307819 C CA 1307819C
- Authority
- CA
- Canada
- Prior art keywords
- electrode
- composite electrode
- porous composite
- ion
- zone
- Prior art date
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- Expired - Lifetime
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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
- H01M4/90—Selection of catalytic material
- H01M4/9075—Catalytic material supported on carriers, e.g. powder carriers
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
- C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
-
- 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
-
- 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
- H01M4/90—Selection of catalytic material
- H01M4/9091—Unsupported catalytic particles; loose particulate catalytic materials, e.g. in fluidised state
-
- 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
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M8/00—Fuel cells; Manufacture thereof
- H01M8/10—Fuel cells with solid electrolytes
- H01M8/1004—Fuel cells with solid electrolytes characterised by membrane-electrode assemblies [MEA]
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M8/00—Fuel cells; Manufacture thereof
- H01M8/10—Fuel cells with solid electrolytes
- H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
- H01M8/1018—Polymeric electrolyte materials
- H01M8/102—Polymeric electrolyte materials characterised by the chemical structure of the main chain of the ion-conducting polymer
- H01M8/1023—Polymeric electrolyte materials characterised by the chemical structure of the main chain of the ion-conducting polymer having only carbon, e.g. polyarylenes, polystyrenes or polybutadiene-styrenes
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M8/00—Fuel cells; Manufacture thereof
- H01M8/10—Fuel cells with solid electrolytes
- H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
- H01M8/1018—Polymeric electrolyte materials
- H01M8/1039—Polymeric electrolyte materials halogenated, e.g. sulfonated polyvinylidene fluorides
-
- 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)
- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Manufacturing & Machinery (AREA)
- Sustainable Development (AREA)
- Sustainable Energy (AREA)
- Life Sciences & Earth Sciences (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Inert Electrodes (AREA)
Abstract
COMPOSITE ELECTRODE FOR USE IN ELECTROCHEMICAL CELLS
ABSTRACT OF THE INVENTION
A porous composite electrode for use in electrochemical cells. The electrode has a first face and a second face defining a relatively thin section therebetween. The electrode is comprised of an ion conducting material, an electron conducting material, and an electrocatalyst. The volume concentration of the ion conducting material is greatest at the first face and is decreased across the section. while the volume concentration of the electron conducting material is greatest at the second face and decreases across the section of the electrode. Substantially all of the electrocatalyst is positioned within the electrode section in a relatively narrow zone where the rate of electron transport of the electrode is approximately equal to the rate of ion transport of the electrode.
ABSTRACT OF THE INVENTION
A porous composite electrode for use in electrochemical cells. The electrode has a first face and a second face defining a relatively thin section therebetween. The electrode is comprised of an ion conducting material, an electron conducting material, and an electrocatalyst. The volume concentration of the ion conducting material is greatest at the first face and is decreased across the section. while the volume concentration of the electron conducting material is greatest at the second face and decreases across the section of the electrode. Substantially all of the electrocatalyst is positioned within the electrode section in a relatively narrow zone where the rate of electron transport of the electrode is approximately equal to the rate of ion transport of the electrode.
Description
~3~ 3 COMPOSITE ELECTRODE FOR USE IN ELECTROCHEMICAL CELLS
BAC~GROUND OF THE INVE~TI OM
The prefient in~ention relates to a ~ompo~ite electrode for use in electro~hemical cell~, and more particularly.
to a composite electrode for u~e in ~uel cell~, ~ueh a6 05 hydrogen/oxygen fuel cell~.
Certai~ fuel cell~ conYert chemical energy into electrical energy by reacting diffeeent ga~es a~
electro~atalytic surface~ on anode and cathod~ electrod~
which are ~o~itionea on oppo~ite side6 of an ion exc~ange membrane. Genetally, ~he ga~ introduced to the anode i~
categorized afi a ~uel while t~e ga6 i~trodu~ed to ~he cathode i~ an oxidant. Utilizing hydrogen/oxyge~ ~olid electrolyte fuel cell6 as illustrative, hydrogen i8 introduced ~ia a ga~eous 6tream to the anode ~ide ~ an io~ e~chang~ membrane and i~ electrochemically oxidized in he presence of a ~uitable cataly~t. 6uch as platinum or a : Qlaeinum alloy, in a~cordance with the following general reaction:
20~ H2 = 2~ ~ 2 ele~tron~ (l) : ~ ~
~:
:
~xyoen ~ lntroducod to ~he cDthode ~lde o~ the lon-con~uctlng ~e~bcane ID ~ ~eCODd ~D8eOU~ ~tCea~ an~ i-~lect~oche~lc~lly reduced ~n ~ccor~nce v1t~ the follo~ing general reactlon:
05 2 ~ ~H ~ ~ Qlectron~ ~ 2H2O (2) Conventlonally, f~el cell~ are de~igaed by po~l~lonlng an ion exchange ~embr~ne withiD ~ react~n~ cha~be~ to define ~ anode co~part~ent o~ one side of the ~on exchange ~e~brane and a cathode co~part~ent on the other side of ~he ~on e~change ~e~brane. Electrodes are positioned in each co~pa~t~ent a~d t~e cha~ber ~6 equ~pped vith ~uitable ~anifold~ for the introduction of qa~eou6 ~tcea~ into ~he anode and catbode co~part~ents. T~e reactant cha~ber also ha6 outlet~ for product ~ater and any unreacted gase~. The ~OD exchange oe~brane is a per~6elecelye ~on trao~porting ~e~brane~ i.e., selecti~ely tran6pocts onlr anio~6 or ca~ion6 depending upon ~he charge of group~ bouna ~ithi~ the polymeric ~atrix of t~e ion exchanqe ~e~bcane. In a hydrogen/orygen fuel cell, proton6 liberated during electrochemical o~idation of ~ydrogen are 6electively tran~ported ~rough the ion exchange ~embcanè-. ~ra-ditionally, the electrode~ u6ed in ~uel cell6 bave been coated w~tb or ~re compri6ed of an electroca~caly~t, 6uch a~ plat~num or a platinum containing 25 co~pound, to increace the raee o~ the electrochemical oxidation and reduction reaction6 occurring in the anode and cathode co~partment6, ce6pectivelY- The elect~on6 qenerated in the ~node sompart~ent are collected by a current collector and are ~ran6ported through an e~te~nal 30 circuit which ~ontain~ a load to t~e cathode coDIpart~ent.
3 i~ 78~9 Curr~nt ~ollecto~ can be con~ruct~d of Dny ~ultable ~lectrlcally conductlvQ ~at~rial, ~uch ~ any ~t~blo ~etal or 8 carbon blac~. Often, the ~aterlal o~ uhlch the re~ct~nt ch~ber is constructed functions a~ tbe current 05 collector. A6 conventlonally con~tructed, fuel ~ell6 can operate at approxi~ately 60~ efficiency, l.e., convert app~oxi~ately 60~ of the ~va~lable che~cal ~nerqy ~n t~e reacting fuel to electricity while the re~aini~g 40t i8 converted to ther~al energy.
Puel cell operation involve~ three ~eparate ~ode~ of tran~port. Pir~t, the introduction o~ reactant gase6 into the fual cell, for e~cample, a hydeogen/o~tyge~ ~uel cell.
from a supply source and ~ove~ent of these ga6eL ~hrough the uel cell durin~ operation involves molecular tran6port. Al60, no~e~ent of water ~or~ed as a result of the electcoche~ioal ~eduction of oxygen i~ accordance vith reaction (2) ~ithin the hydrogen/oxygen fuel cell involves ~oleculat tran~port. Secondly, transport OCCUlB as protons, i.e.. ~ydrogen ion~, produced as a result of reaction (1). ~Ye fco~ th~ anoda co~p~re~ent electrode, through the ion e~change ~e~brane, and into the cath~de compart~ent electrode of the fuel cell vhere electcoche~ical reduction occur6. La~tly, electron6 genera~ed in accordance ~ith t~e electroche~ical oxidation reaction (1) are t~a~6posted ~ia a curren~ collector through an external conduc~ive path having an electrical load to tbe cathode co~partment to 6erve as a reactant for tbe electrochemical reduction of o~ygen. Puel cell performaDce is reduced by any internal i~pedance to any one of the three ~ode~ of tran~port vhich occur during opera~ion thereof.
~,~
~'"`' `~
In ~n 4f0rt to r~duce the lnt~rnal re~l~tance to transport, ~articularly ~roton ~nd ol~ctron tran~port, and tbe~eby lnc~ease fusl cell pelfor~nce, fuel cells de~l~n6 have evolved to a ~zero-qap- configuratlon ~herein the 05 reaction cha~bec, electrode~ ~nd ion exchange ~e~brane are po~tioned ln a contiguous rela~ion6hip. To achieve a ~zero-gap~ configuratlon and the a~tendant reductlon ln internal re~i0tance to tran~port, conventional electrode ~tructure~, such ~8 a ~ire ~e~h Bcleen coated ~ith an electrocataly~t, have been e~bedded on each 0urface of the ion exchange ~embrane. Additionally, compo~ite electrode~
of ion exc~ange polyoers, ~etal~ or carbon co~pound~, and electrocatalytic co~pound6 have been ueilized in an effort to con~truct a~ electrode wherein an electron conductor, a proton conductor, and an electrocataly6t are incorporated into a three-pha$e lnterface to ~i~iaize internal re6i6tance to tran~port.
Solid ~on e~change polymer~ haYing fixed anionic sites in the fora of ~orbed anions or chemioally bonded anion6 ha~e been u~ed to for~ thc per~s~lactive ion exch~nge j ~e~brane used in fuel cell6. Recently, polymer6 po~se~6iny 60rbed or grafted ion6 of stro~g proton acids, for example, 6ulfonic or p~osp~onic acid6, ha~e been employed a~ ion exchange ~e~brane6 in ~uel cell~ due to the relatively rapid protoD tranfif er thereof.
Nafion0, a perfluorosulfonic acid ~embrane, ha6 gained increa6ing popularity as an ion excbange ~embrane for fuel cells. Typically, the ion exchange ~e~brane i~
con~truc~ed of a relatively thin, e.g., 0.002-0.01~ in.
30 thick, 6heet or fil~ of an active ion e~change polymer.
Several separate sheet6 or il~s of acti~e i~n excbange polymer~, each of ~hich posse6fies different conducting , ~ .
. . .
5 ~ 3 ~nd~or ~ettin~ ~op~rt1e~, c~n be ther~ally la~inated to foca a unltary loa exc~ange ~e~brano for uce ln fu~l cc118.
~ lth the advent of the unitary, rel~tively thln lon exc~ange ~e~brane~ ~nd electlode ~ase~blies, uel cellc 05 ~ave been arra~ged in a stacked configur~tion. AB ~uch, the plate or housing ~eparating ~he anode ~nd catbode co~part~ent~ of t~o ad~acent ~uel cell~ ~lectrically connect~ the fuel cells in Berle~ by conducting electrons qenerated in t~e anode of one fuel cell dirQctly to the cathode of an adlacent fuel cell. This common plate i~
refecred to a~ a ~bipolac pl~eea and additionally ~erve~
a6 a po~itive barrier to prevent ~ixing of snode and cat~ode gas flov~ bet~een adjacent cells. Thus, the relatively low volta~e ~enerated by a ~ingle fuel cell, 0.5-O.g V, can be added in ~eries ~o obtain u~eful voltaqes of, for e~a~ple, 120 V.
As previou~ly ~entioned, co~po~ite ~lectrode6 coapri6ing a ho~ogeneou~ Lixture of an ion eschange : poly~er, an electrical conductor. and an electrocataly~
~ have been 6ugge~ted foc u~e ln fuel cell~. The ion exchange poly~er, for example/ Nafion0, serve6 the function of a composite binder and an eiectrolyte for conducting cations. An Qlectfical co~duc~or, ~uch a6 cacbon powder, function6 not only ~o conduct electron6 but al60 a6 a catalyst support. The6e co~po~ites are formed with a filler ~aterial which i6 ~ub6equently re~oved by application of specific aqueou6 601ution6 or heat to proYide a porou~ electrode ~atrix which allow ga6eou6 reactant6 to flow therethrough. Accordingly, an electrode ~atri~ ~tructure i6 provided vhich for~6 ~ conductive skeleton for tran~port o~ bot~ electron~ and proton~
relatively unifoc~ throughout the entire electrode.
;~, ~ 'A~
,.~ .
~7~
Ho~eve~, loa~ing cat~ly~t throughout the ~ropo~ed co~po~lt~ ct~ode ~lgniflc~ntly lnc~ease~ t~e a~ount of catalyst ln the electrode thereby pro~otlng fuel c~ll c4taly8t lnefflciency ~ince only a portlon of the loaded 05 catalyst 18 utillzed 1~ the electroche~ic~l re~ct~ons, Accordin~ly, it i~ ~ object of the pleBent invention to construct a co~po~it~ ~lectrode for uae n electrochemical cells ~n vhich sub~tanti~lly ~11 of tbe elec~ro~ataly~t u~ed in fabricating the eleceroche~ical cell is positioned only alon~ ~ zone wit~in tbe electrode where the tran~port eate6 of electrons and protons are approxi~ately equal.
Another object of the pre~en~ inventio~ i~ to provide a co~p~6ite electrode for use in electroche~ical cell~
whicb i6 constructed to ~ave increa~ing electronic conduct~ity fro~ the catalyst loadi~g ~one to a current collectoc on one face of the elec~roae, and to have increa~i~g protonic co~ductivi~y from the zone of cataly~t loading to the face of the electroae ~hich engages the ion 20 excha~ge ~embrane of the el~ctrochemical cel1.
It is ~ further objec~ of the pre6en~ invention to provide a compo~ite electrode for use in electrochesical cell~ wherein the a~ount of electrocataly~t nece~sarY to ac~ieve the ~ighe6t electroche~ical cell perfor~ance, a~
~ea~ured by the value of watts peL ~g of electrocatalyst.
i6 ~ini~ized.
It is ~till a further object of ~he pre6ent invention to provide a composite electrode for u6e in electrochemical cell6 vherein ~he electrocataly~t is ~ore efficien~ly utilized, a~ ~ea6ured by the voltage of a fiingle electrochemical ~ell.
~3q:!~7~
~HMARY OF 1~ 1NV~N~
To achl~v~ the foregoing and ot~r ob)~ct~, and 1 ~ccordance ~lt~ t~e ~ur~oxe~ o~ tn~ ~r~ent i~Yentlon~ a8 e~bodied a~d broadly desc~lbed herein, one 05 characterlz~tion of the present ~n~ention ~ay co~pr~e a porou~ ~o~po~te electro~e for use ln electroche~ical cells. The electrode ha~ a fir~t face and ~ ~econd f~ce defi~i~g A relatively t~n sect~on t~erebetween. The co~pos~te electrode ha6 ~ean6 ~or co~ducting ion6, ~eans for conducting electrons, and a~ electrocatalyst. The ~olume concentration of the ion conducti~g ~eans i~
greatest at the fir~t face a~d i~ decreased across ~he ~ection, while ~he Yolu~e concentration of the electron conducting oean~ i~ greate6t at ~he second face and decrea~es acros~ the 6ection of the elec~rode.
Sub~tantially all of the electrocatalyst ifi po~itioned vithin the electrode section in a relati~ely narrov zone ~here the rate of electron transport of tbe electrode i8 ~pproxi~ately equal to the rate of ion transport of the 20 electrode.
In a~other characterization of the pre6ent inYention, a porou6 compo6ite electrode for use in electroche~ical cell6 i~ pro~ided which i~ co~prised of three zone6. A
fir~t zone conduct~ both electron6 and ions liberated by ~25 electrochemical reaction6 occurring vithin the : electrochemical cell. The ~ate at ~hi~h the fir6t zone t~ansport6 electrons is greater than ~he rate at which the : firs~ zone tran6port6 ions. A ~econd zone conducts both electron~ and ion~ and ha6 an ion tran6port rate greater than t~e ele~tron tran6po~t rate thereof. ~ third zone i6 contiguou6 with both the ir6t and ~econd zone6 and ha6 approxirately equal ~on and electron transport rate~. An '~
' ~ .
electroc~ly~t for lnceQa~lng t~ rate of t~e ~l~ctro~e~lcal reactlon~ i8 ~l~o~t ~nalrely po~ltloned witbln the t~rd 20ne.
In yet anot~er c~araceer~zatlon to the present 05 lnvention, an electrocbe~ical cell coaprlses a current collector ~eans for conducting electron~ ~enerated by el~ctroc~e~ical reactions of fuel vit~ln the electroc~e~ical cell, an ion conductor ~eans for selec~ively conduct~ng preselected ion~ liberaeed by the electroc~e~ical reaction~, and a co~po~ite electrode po6~tloned be~ween the collector ~ean6 and t~e ion conducto~ ~eans. The co~po~ite elec~rode has ion and electron transport eates which Yary acro~ tbe ~ection thereof and has an electcocataly~t whi~h i~ po~itioned ~long ~ zone vithin the section of the electrode Yherein the ion and electro~ tran6port rates are substantially equal.
B~IEF_DESCRIPTION OF THE D~A~INGS
The acco~pdnyi~g drawings, which are incorporated in and ~or~ a p~rt ~f th~ sp~ tlon, ~11UBtrate the e~bodiaent~ of the pre6ent inventioD and, together vith the de~cription, serve to explain t~e principles of the inventio~ he drawing~:
~lGURE 1 is a partially cu~avay cros~-sectional ~5 pictorial ~iev depicting the volu~e fraction of the co~ponents o a co~po6ite electrode of the pre6ent invention a6 asse~b1ed in an electroche~ical cell;
~IGURE 2 i6 a cro~6-~ectional pictorial ~ie~ of a preferred co~po~ite elec~rode of t~e pre~ent invention;
PIGURE 3 i~ a partially cutaway cross-sectional pictorial vie~ of a compoBite anode of the present invention as assemb1ed ~itb an solid ion exchange ~e2brane o a fuel cell: and , j.~"
.~ ;;, , 9 ~ 3~7~3 ~ IGUR~ ~ ~8 a partl~lly CUt~Wdy cro~ ectlonal plctorlal ~ of ~ co~po~ite ~thode of t~e present lnvention as as~e~bled vit~ the co~poaite anode and a ~olid lon axchange ~e~brane of ~ fuel cell ~s lllu~Crated 05 in Pigure 3.
pETAILED DESCRIPTION OF THE PREFERQeD EMBODIMENTS
~ eferriag ~o~ ~o ~igure 1, the co~poslte electrode of the pr~s~nt ~nventlon i8 illustrated generally a6 10 ~nd i~ in~erposed betveen ~ 801 id ion ~xchange ~e~br~ne lQ ~nd a current ~ollector l2, a6 i~ ~ell ~nown ln the srt.
Current collector l~ ~ay be a portion of a reactor ve~sel of an el2ctrochemical cell, a bipolar plate of ~ ~erie~ of ~tacked, individual fuel cell~, or another suitable electrical ~onductor. Current collector l2 can be con6tructed of any guita~le electron conducting ~aterial.
~uch as ~ickel, ~o~ other ~etal~. grap~ite (a conducti~q for~ of carbon), or a qraphite pla6tic co~po~ite vhich posse~se~ sufficient electron conduction for u~e ~ith the electrode of t~e present invention. as hereinafter de6cribed. Curr~n~ collector 1~ ~6 pre~er~bly con6tructed of a corrocion re~istant ~etal. such as ~ic~el, or a graphite pla~tic compDsite ~hich include~ a binder. for example, a eo~po~i~e of 80 wt ~ car~on black, ~uch a6 Vulcan XC-720 ~anufactured by Cabot Corporation. and 20 wt ~ polyvinylidene fluoride, such a~ ~ynar~
~anufactured by Pennvalt Corporatio~.
~ e~brane 18 can be fabricated f~om any ~uitable ion exchange poly~er ~icb ~unctions a~ a selective ion tran~porting ~aterial, a6 i~ ~ell SnovD in the electrochemical act. Me~brane l~ can be con6tructed of Na~ion ~17~, a 0.007 in. thick polyperfluoro~ulfonic acid membrane ~anufactured ~y Du Pont Corporation.
, .
10 ~3~ 3 Poly~erfluorocarbo~yl~c ~cld ~oly~er~ ~re al~o cui~ble lon axchange poly~ers for u8e ~ ~e~br~ 8. ~olld co~pos~te ~l~ctrode 1~ o the pre~ent in~ent~G~ ~r relatlvely thln ~beet, fll~, or vafer, ~.g., O.O10 in., 05 defining a gener~lly leetiline~r conigura~ion. ~lectrode 13 is co~pr~ed of ~n lon exc~ange poly~er 17 ~ich ba~ a che~ical ~o~po~ition and transpor~ propertle6 ~l~ilar to the ion excbange poly~er u~ed to con6truc~ the lo~
exohange me~brane 18. a cuerent conductiny ~dteelal 15.
~uch a6 tbe ~aterial utilized to coA6truct cur~nt collector 12, and an electrocataly6t 16. Electrocataly~t 16 function~ in part to acceler~te the electro~ transport ~eces~ary in the operatio~ of the co~posi~e electrode of the pre6ent i~veution. Suitable electrocatalyst can be ~elected fro~ the noble ~etal group, i~ particular.
platinum, rhodiu~. palladium or alloy~ thereof. or fro~
~etalorganic co~pound6, ~uch ~5 iron or cobale porphyrin co~pounds or iron or cobalt phthalocyanine co~pound~, that have been ther~ally treated. in a conven~ional aanner.
The elect~oc~t~lr6t ~n ba amploye~ either a6 lelatiVely s~all, aetallic particle6, i.e., ~etal black6, or a~ c~all particle6 supported on conducting substra~e~. Preferably, pla~inum part~les having a cro~ ectional di~6ion of ; 100-200 ~ 10-~ c~ depo&ited on larger carboD particle~
are utillzed a6 the electrocataly~t in the compo~ite :~ electrode of the pre6ent ~nvention.
Polypecfluoro~ulfonic acid~, typified by the Nafion~ cla~ of polymers ~anufactured by DU Pont, are prepared by the copolymerization of tetrafluoroethylene and a vinyl et~er ~oDomer that ter~inate~ wi~ a 6ulfuryl fluoride group. i.e., -S02P. The re6ulting p~lymer product can be ~eat proces~ed in~o 6heet~ or other de6ired .
:
1 1 ~3~ L9 ~hape~ by convent~onal ~etbod~. The proc~ d poly~rs are then ~ydeoly~od ln ~ concentr~t~d aqu~ou~ b~se ~olutio~ ~t 0l~Y~ted t~peratuc~ .g., ~ 30 ~t ~ XOH
~olutlon ~t 9OC. Durlng hydroly~is, the nonionic 05 ~ulfuryl group 1~ ~heolcally convert2a ~o ~ ~ulfonic acid an~on, -SO3 . ~fter hydroly~ he ~oly~er~ contain bound ~n~on~c groups t~at ~t~ract aY~llable ~ation~ ~o for~ an lo~ exc~an~e ~etwor~. Ther~al ~ropertles of the hydrolyzed poly~er are slt~c~d 3uch that t~e ~yd~olyzed poly~er cannot be heat ~loce6~ed. At el~vated fuel cell operating te~peratures, i.e., greater ~an lO~DC, dehydration of the ~e~brane, electrode intecface occurs which can be detrioental to lo~ Sran~port. Accordingly, at ~uch elevated te~perature~. other ~aterials ~hich exhibit relatively high ~onic conduction ~t high te~peratures, $uch as phosphori~ acid oe ceetain ~et~l o~ides, for exa~ple, ~rid~u~ oxide or tung~ten oxide can be selected as an io~ e~change ~aterial i~ lieu of the ion e~change poly~er l7 for utilizatlon In the composite electroae of ~e pre6ent inven~ion.
The present invention i~ based upon the di~covery ~hat by c~ea~i~g a relati~ely ~arrow zone Ol plane ~ithin a compo~ite fuel cell electrode wherei~ the rate of electron tran~poet i6 approxi~ately equal to the rate of proton tcan6port and by loading sub6tantially ~ll of the electrocataly~t utilized in the co~po6ite electrode only within t~i6 zone~ electrocataly6t use ~nd fuel cell perfor~ance, a~ ~ea6ured by the number of ~atta generated per ~g of ~lectrocataly6t o~ by vol~aqe efficiency, are ~ignificaDtly increa~ed. Po6itioning t~e electrocataly t along a zone of aub~tantially equal protonlc and electronic t~a~port also p~ovides for ~axi~u~ utilization 12 ~3~'71~
ot ehe ~vail~ble olectrocataly~t ~ due to t~e re~ultlng decrea~ ld~nce ti~e of ~ r~actlng gas ~olecule on Dny guch site.
The effectlv~ rate ~t which ~aterlal ~5 conduct~
05 ~l~ctric~l cutrent. i.e.. ~lectron~, ~nd the ~ate ~t vhlc~
~on exchange poly~er 17 conduct~ proeon~ .. hydrogen ~on flux, ace directly proport~onal to the volu~
concentratio~ of each of these ~ate~ia~ w~thi~ electrode 13. The electron flux, i.e.~ the current lfflol~s-c~2~, increases to~ard curren~ collector 12, while the protoD
flux, i.e., ~e ~onic flo~ (~ol~s-c~Z), ~ncreaseR to~ard oe~brane 18 si~ce each separat~ electrocatalytic site can serve as th~ locu6 of ~lectron tran~fer. and each ion ~U6t ~ove toward the ~e~br~ne. ACCDrding1Y, ehe volu~e 15 co~centration of current conductinsl loaterial 15 and poly~er 17 i~ qraded through the cro6s section of electrode 13 to corre~pond ts ~he varying proto~ and electron flus. Current conducti~g ~aterial 15 has tbe greate~t volu~e concentration ~thin electrode 13 along the interface of the ~lectrodc with the curren~ collector 12. ~he concentration of the current conductinq ~aterial 15 decrea6e~ acro66 the cross 6ection of electrode 13.
The Yolume concentration of ~he ion exchange poly~er 17 ~it~in elect~ode 13 is greatest a~ the interface of electrode 13 with De~brane 18 and i6 dec~ea~ed acros6 the section of elec~rod~ 13. Electroca~alyst 16 is concentrated at that plane or zone alo~g the ~ection of electrode 13 vherein the tran~port rate of eleGtron6 ~as deter~ined by the electron conductivity and tlle Yolu:lle 30 ~raction of naterial 15) and the tran~;poct rate of pro~ons ~a~ dete~mined Iby the p~oton conducti~ y and the volu~e feaction o~ ~aterial 17) are sub6tantially equal.
:
."~, .
13 ~3~
Solld co~po~lt~ ctrode 13 of th~ pre~ent lnventlon provides v~rled tran~ort rate~ for bot~ el~ctron~ and protons (in a hydro~en~oxygen fuel cell) acros~ ~he ~ection th~reo. The electro~ conductivity inc~ea~es S toward the electrode. cucrent collector interface, whLle the proton conductivltY increases toward the electrode, lon exc~ange membrane interface. Th~ electrocataly~t 1~
concentrated intermed1ate ln the elactrode sectioD ln that relatively narrow plane oc zone 16 where the transpor~
cate of electrons. e%pre~sed on ~ volume basi~, 1n appro~i~ately equal to the t~anspo~t rate o proton~, expressed on a volume basls.
Electron and proton transfer ~ate~ ace generally expre~sed u~in~ tha value o conductivity for a glven ~aterlal, l racip~ocally, a~ the value of it~
re~istivi~y. The basic unit of ~esistiviey i~ oh~c~ per c~2. aesistancs of a ~i~en ~aeerlal to either electron o~ p~oton t~ansport i~ ~easured by deter~ining the volta~e drop acro~s that ~aterial while a ~cnown f lux occur~
through that ~aterial and ~caling that resi~tance to the Yolume fraction of that ~ater~al within a partlcular composition. The resl~tance of a glven materlal to el~hec ~on~ or electrona can be defi~ed a~ ~caled to the cocrespondin~ volume fraction by the express~on ~et forth below. Thu~, the electronlc re~lstance, ~ in any volume element, i, i~ related to the bul~ resls~ance, ab, by ~he expre~sion:
;~e(1) ~ Rb/k~ (3) :30 whera f is the volume fraction, and k i~ an empiclcal ~3~
coeficlent t~at ~ccounts ~or ~onlin~ar ~f~ct~ ln YolU~e ~i~lng. Accordi~ly, ~qual ~loctron ~d proton conductivlty can be ~xpr~s~ed a~:
~ 2) (a~) ~4) wherein ~ he t~ckne~ of ebe path of ele~tron conducta~ce ro~ the zone of ~gual electron and peoton conduct~vlty to the current collector interface, ~2 i~
th~ ~ickne~s of t~e p~th o~ proton conductivley ~ro~ the zone of ~qual ~lectron and proton conductlv~ty to the ~e~brane interface, ae i8 tbe effective tran~fer resi~tance of electeon6 throu~h the electro~ tran~fer path, and ~p 18 the effecti~e tca~sfe~ re~ ance ~f proton6 through the proton tran&fer path. T~UB it can be ~ppreciat~d that ~ d ~2 ~u~t be ad3usted to account for difference6 in ~ ~nd Rp. A8 the rates of electron transport are typically faster t~an the rate of pcoton tEansport ~ X2 au8t be designed to be les~ than ~1~ Accordingly, the ~onc of relativ~lr ~qual pro~on and electron tean6port rateg, i.e., the zone of high electroca~aly~t loading, is located closer to ~he ~e~braae interface than the curren~ collec~or ~nterface of the co~po6ite electrode 13. The e~act po6itioning of this zo~e in a~ opti~ized co~pofiite electrode is a function of the Be and ~p value6, a6 deter~ined by the e~act mateeial~ used to fabrica~e elect~ode 13.
The co~po6ite electrode o~ the present inventi~n ~u6t be fabricated 6uc~ that reacting ~oleculeG of fuel ga6e~
introduced into the fuel cell have acce~sibility to electrocatalyt~c ~ites ~roug~out the ~lectrode.
Efficient ~as tran~port rate~ ar~ directly dependent upon `
~3~'7~
~d~gu~t~ ~oro~ity throughout the ~l~ctrode. ~ccocdlngly, the co~po~lt~ ~loctrode of the pr~sent ln~ontion i~
prov~ded v~tb ~ poro#ity ~ufflclent to peralt tbe uninh~blted flow of reactant g~ae~ and vat~r ~n ~he for~
05 of ~tea~. ~he bulk poro~lty of electrode 13, deflned ~8 t~e volu~e of gas ln el~ctrode 13 dl~ided by the total volume of electrode 13, i~ ~bout 0.6 to abou~ 0.7. Thi~
bulk porosity provide~ for a relatlvely unifor~
distribu~ion of ga~eous reactant~ througbou~ the ~tructure of composite electrode 13. ~he void DeCesBary to create such poro0ity can be ~ab~icated by u~e of porou~ ~heet~ or geld~ of ~aterial~, a6 herei~after described. or by u~e of filler ~aterialB which are dis~olved or ~her~ally re~oved after fabrlcation of electrode 13.
._ The porosity of el¢ctrode 13 can vary acros~ t~e ~ection thereof and, ~8 ~llustrated in Pigure 1. should lncrease at the el~ctrode, current collector ~nterace to per~it qaseou6 reactants acces~ to the zone Yhere the ele~troc~taly6t iB concentrated. The electrocatalyRt i6 di~per6e~ through~ut thi~ 20nQ to for~ ~hinO hig~-6urface area layer6 on the electro~ ~onducti~g solid6 ~ithin this zone. By incr~asi~g ~he poro6ity i~ this zone where cataly~t loadi~g ls concentrated. qa~ flow (~olecular transport) i8 ~axi~ized in that area of electrode 13 wherein reacti~ity i~ ~axi~ized. Plow~ of ga~eou~
reactant~ are i~roduced to electrode 13 fro~ an external source by utili~ing flov ~anifold~ and other 6tructural confiqurations which are well known in the art.
In one embodi~ent, the compo6ite electrode 13 of the 30 pre6ent invention i6 forloed of three separate layer6 or zone6 a6 illu~tra~ed ger,erally in Figure 2 a6 22, 24, and 26. Each layer or 20lle comprises a aiYture of carbon ~3~
black, ~latinu3 or ot~er ~ultable ~lectlocatalyst dl~per~ed and ~uppo~t~d on carbon blac~, ~olytetrafluoroethylene ~ a binder, ~nd ~ ~u~table ionic conductin~ ~at~rial, ~uch ~B polyperfluoro~ulfoDic ~cid.
~ Su~table carbon blac~ for u~e in ~anufacturlng the composite ~lectrode of the pre~ent invention po~esae~ a relatlvely high surf~ce area, a hlgh electr~cal conductivity, a~d a low che~ical reactiviey. Vulcan XC-72 ~anufactured by Cabot Corporation ~8 a pre~erred ~arbon blac~. It i~ preferred to utilize a relatively ~igh surface area platinum (e.g., 20 ~2~g~) in the for~ of a carbo~ blac~ vl~b approxi~ately 15 vt platiou~ loaded on tbe surace thereof.
Polytetr~fluoroethylene ifi co~mercially available a~ a fine su~pe~s~on, i.e., ~ fi~e powder di~persed ~n a ~olvent, ~uch as water. Polyperfluoro~ulfo~ic ac~d or ~ polyperfluorocae~oxyl~c acid poly~ers are avail~ble ~8 : ~u~pe~ions in ~olYents, $uch as ~i~ture6 of vater and ~: ~ethanol or a~ unbydrolyzed for~6. Tbe latter for~
per~it~ the u~e of ~u~h p~ly~er~ ~s a ther~al pla~tic binder. The co~po~itional and dimen6io~al para~eter6 for each layer or zone are set forth below in ~able 1.
~, ~,.~ .. . .
~ ~o'~o ~C C) ~ ~ C-~
~ ~o~
~D ~ ~ ~,,~
,~ ,~
D~ O -I O ~ ~ ~
~ V c ~ r~ O . O
~ ~ 1 ~ ~
o o o o ~ ~ ~o ~r ~ C u~ ~
~ o o ~ ~ t:
: ~J ~
a I
~C
V K K K
~ .
o ~ e .
e N ~ .0 ~; ' - : ~:
:: :
`
' Thu~, tne th1ckn~s of the co~poslt~ ol~ctro~
1.5 ~ 10-3c~. ~ac~ ~one 1~ ~repared ~y co~b~ning the con~tituent~ di~persed ~n a ~ultable dl~er~ant. 8UC~ ~
~exane or other lov boilin~ ~ol~t llquld~. in accor~ance S wieh the for~ul~tion set fort~ ~bove, and sprayln~ a layer of the resultin~ d~per~ion on ei~her current collec~ol 12 or ~embrane 1~. Accurate layer ~hickne~ ~ay be rea~ily achieved by one skilled in ~he ~pray co~ing ar~ util~zing ~onventional apparatu6. ~t lea~t one zone iR applied to each of the ~ex~ra~e 18 and collector 12, and thereafter, the ~embrane 18. collec~or 1~, snd layers 22, 24, and 26 are thecmally bonded to each ot~er to for~ the co~posite electrode of the pre6ent invention.
Referring now to Figure 3, a ~tructure of a coEp~site anode electrode or u~e in a fuel cell, ~uch as a hydrogen/oxyge~ uel cell, i~ illu~rated. The co~po~ite anode i6 illu6trated in Figure 3 generally as 30 and comprise6 layer¢ 32~ 3~, 36, and 3~. Layer 32 i~ an 0.05 in. thic~ carbon grid con~tructed o graphitized carbon ~n~ po~6afi6inq relatiYely high phy~ic~l strength, e.~., cru6h ~trength grea~er than 300 psi, ~nd electrical ee~istivity, e.g., 0.01 o~m-~. Lay~r 32 i8 con6tructed vitb pore openinq6 (~e6h) of approxi~ately 50-100 ~ 10 ~ c~ to permit efficient ~i~ration of ~- 25 reac~ant ga~e~ unifor~ly through the layer. The :electrical conductivity of layer 32 per~its rapid ~ran~port of electron6 to the current colleceor 12 ~ith low re~i6eance 1066. Layer 32 i6 treated with ~ydrophobic ~aterial6, ~u~h a~ elemental 1uorine. to decrease the veteing tendencie~ th~reof.
Layer 34 is a porous carbon ~hee~ having a thic~ne~$
of about 1-2 ~ cm. Layer 34 may be fabricated by ~oe pre66ing a ~i3ture of a co~ductive carbon black and an .
.,., ~,.,~ . . ..
unhydroly2ed lon ~xch~nge ~olp~er, ~uch a~ unhydrolyzed Na10n polymer ln po~der for~. Tbe conc2ntratio~3 of carbon black and unbydtolyzed loa 2xchange ~oly~er are ~el~cted ~o ~hat layer 3~ po~se~e~ aa ~lectron tr3n~port 05 rate ~hich ~s gr~ter than, e.g., approxl~a~ely t~ice, its proton tran~port rate. Lay2r 3~ ubsequ~ntly heated to ~elt the ion exchange poly~er which functlon6 a6 a b~nder therein.
Layer 36 i~ compri6ed of the sa~e ~ateri~ls as layer 34 but the concentration~ of carbon black and the unhyd~olyzed ion e~cchange poly~ee ~re ~l~ered ~o that the electron tran~poct rate of layer 36 iB approxi~ately egual to the proton tran~port raSe thereof. Follo~i~g hot pre66i~g t~e ~ix~ure a~ de~cribed with respect ~o layer 34, layer 36 i~ sprayed with a solutioa containiQg platinum, rutheniu~, or a ~i~ture thereof. ~or sxa~ple, lay~r 36 ls ~prayed ~ith aD alco~ol ~olution of pla~inu~
and rutbenium chloride~ at a~bient te~pecature and pLe~6ure. Layer 36 is ~prayed vith an amnunt of solution nece~6ary to obtain a cataly~t loading of l ~g~c~ or le68. Sub~equently, the cataly~t ~6 c~emically reduced to con~ere the catalytic ~e~al~ ~o the~r ele~e~tal for~.
Such chemical reduction ~ay ~equire the U6e ~ hydrazine or another ~oderate reducing agent which can ~e applied as a liquid.
Layer 3~ i8 al60 con6tructed of the 6a~e ~aterial6 as layer 39 except that the concen~ration6 of carbon black and unhydrolyzed ion exchanqe polymer are varied 60 that the protoD tran6port rate of layer 38 i6 greater than, e.g., approxi~ately t~ice, the electron transport rate of lay~r 3~. Layer 38 is prepared by ho~ pre~ing a thin (lO ~ lO 4 cm) ~ix~ure of carbon black and a~
unhydeolyzed ion excbange poly~er.
, , ~ .`i 2~ ~3C~7~9 Solid co~poclte anode 30 13 then as~e~bl~d by etac~l~g l~yer6 1~ ag~ln~t a ~heet o~ ~afer of unhydrolyzod ion ~xchange po~y~er ~ ~a8 lllu~trated), ~uch ~ unhydroly~ed N~flon poly~er, ~avlng a thickne~s of between about S 0.076 c~ - about 0.013 c~, and hot pre~slDg the stacked layers to bond the resultAnt co~po~ite anode 30 together and to the thicker sheet or wafer of unhydrolyzed Nafion0 polyme~. The resultant l~ ate ~ is~r~ed ln an aqueous bage, e.g., 6 ~olar ~od~u~ hydroxide, ~t approxioately ~0-100C to hydrolyze ~he io~ exchange polymer, thereby formi~g an active ion e~chan~e ~aterial.
This treatoent i~ co~tlnued u~tll ~ub6tantially all of the sulfuryl fluoride qeoup6 are hydrolyzed to the ~ulfonic acid anion. The resulting la~inate i6 ~ubsequently ~a~hed in deionized ~ate~ and submerged ~ a dilute electrocatalyst-containi~g solution. ~.g., an ~queou6 ~olution of a plati~u~ ~alt. The la~iaate i~ treated ~ith a chemical reduclng agent to reduce the platinu~ both in the ion exchanqe ~e~brane 18 ~nd the electrode 30. It i~
i~portant to note th3t th~ ~Mount of ~lectrocataly~t loaded throug~ou~ the lami~ate i~ this ~tep ~ay be le6~
: ~han 5 wt ~ of the amount of ~lectroca~alyst loaded onto intermediate layer 36 of co~pofiite anode 30.
Referri~q now to ~igure ~, a ~olid co~posite cathode of the pre~en~ invent~on i8 illu~trated generally a~ 40 and i8 co~pri6ed generally of layer6 ~2, qg, and ~8.
Layer 42 is a co~posite of a unhydrolyzed ion exchange polymer, 6uch as unhydrolyzed Nafion0, and a precatalyzed graphite powder. The graphite powder. a conductiYe carbon, ~s fir8t pretreated by thermal treat~ent i~ a controlled environmen~ u6ing well-~no~n carbon technology to control the o~idative ~ability thereof. Subsequently, the carbon powder is treated with plat~nu~ ~alt ~olution ~nd che~1cally reduced to yl~ld a thln co~tlng o~ ~latlnu~ on the ~urface of the c~rbon.
~elatlv~ly ~aall concentrat~ons of ~l~tinu~ ~re re~uired ln thl~ layer, i.e., ~ubstDne1ally le~ ~han 05 0.1 ~g/a6se~bled c~2 of electrode area. Thi~ cataly2ed carbon 1~ ~lxed with unhydrolyzed lon exchan~e polyDer and then hot pres6ed into a thi~ sheet or fil~. The conceneration6 of graphlte powder and unhydrolyzed ion e~change poly~er are ~elec~ed to prov~de l~yer ~2 vlth A
proton transport ~ate ~hic~ iB greater than, e.g., ~pproximately t~ice, lts electron transport rate.
Layer ~4 i~ a co~posite blend that i~ pr~pared by veavi~g a cloth of strand~ of the unhydrolyzed ion e~change poly~er which are ~pun-cast i~to fiber~ and of beat treated carbon fiber~. ~ela~ively thin f iber~ of each ~a~erial are e~ployed ~o that the resul~ant fabric hafi a relati~rely large nulaber of ~l~ter~ectioD~ between the t~o ~ypes of fib~r~. The concentratlons of carbon fiber6 and ion exchange polymec fibers are selected to provide layec 44 with ~ proton tran~port r~te ~ppro~ioately equal to it6 electron ~ran6port rate. An organic ~acrocycle CO~pOund contai~i~g iron or cobalt i6 utilized to catalyze thi6 fabric. A 6uitable elec~rocataly6t, a6 pre~iously described, ~6 depo6ited by either ~olution deposition or carbon pretreat~ent. Utilizing 601ution depo~ition, a compound, such a~ a cobalt ~etramethoxyt~traphenylpo~phyrin, i~ di6601ved i~ a solvent, ~uch a6 tetrahydrofuran. The fabric i~ i~mer6ed ~into thi6 solu~ion and the porp~yrin adsorb~ onto the ;30 carbon surface6. Thi6 immer~ion ca~ be repeated. The fabric i~ sub~equently dried a~ approxi~ately 100C to re~ove any exce~ organi~ ~olvent, and thereafter heat treated to bind the porphyrin to the fabric. A rbort . ~, , ~ 7~3~L9 p~riod o~ lnt~n~ hq~tln~ (aso-sso~c) ~ro~ ~n ~n~car~d ~oucc~ uch ~IIB an lnten~ lR l~s~r, w~ ct~lvQly h~at th~ carbo~ black ~urace~ conw~ctlng ~or~tlyrl~ to lt~
c~t~lytlc for~ ,-nd blndlng ~t to ~acbon. Utlllzlng c-rbon 05 ~eetre~ts~nt, the carbon f ib~r~ ~ro f ~r~t coatod ~ltb the porp~yrl~ befot~ vea-.rln~. For ~xa~ple~ th~ carbon flbers ~r~ r~ed in ~ ~ol~ltlon of cobalt te~ca~e~bor5~tetragheclylpol~hyrin dl~ol~r~d in ~etr~hyd~ofuran ~nd ~r~ pyrolyzed (B50-950) to ~onver~ tbe porphyrln to 1 t8 ca~alytlc 20cn and bind lt to carbo~. The re~ulting f iber 1~ WoYe~ s~lth the unbydsolyzed ion ~xchange polyloer to for~ the fabric uttlized hB ~ayer ~.
Layec .Q co~prl~ed of a l~yer of bighly ~vnduct~ve gcaph~tl2e~ ~lotb co~taining ~ r~latively s~all a~ount of UnhYdrO1Y2Qd 10Q s~change pol~erc whlch i~ treat~a vlth a los~ ~latlnul~ loadlng ~n a E~anner ~lallar to ~he tre~t-en~
of layeL 42. The ~oncentratlol~s of ~Iraphit~
earboD) and ~on exchange polyner ~re ~elacted to proYlde l~yer ~ rith an el~ctron eransport rate great~r than, ;i e. g ., appro~ aately t~ice, it8 proton tra~port c~te .
Thi6 layer aay 8160 be tre~l:ed b~lt~ hyd~ophobiC ~aterials to control ~ettlng ln a ~an~eE de6cribea here~nhbove with re~pect ~o layelc 32 of solld co~apo~t~ anode 30. Layer~
~2, ~4, and 4B are ~tacked alld ~eated under pre66ure fS0-60 p6i) UDt~l the unhydroly2ed io~ excllan~e polymer ~elt6 to bond ths layer~ together. T~e ~e~ultant la~inate oo~posite 1~ ~ydrolyzed ~n ~e ~anner descrlbed berei~
with re~pect to a~ode 30 until sub6tantially all of the ~ulfuryl g~oupa ara ~ydroly2ed to ~ulfonic ac~d anlon SlCoup6 .
As 111~6tr~tea ln Flgure~ 3 and 4, the preferr~d ~olid co~poslte ~nod~ 30 and ~olld ~at~ode ~10 of t~e ~re~ent ~..,..~.. ~.....
2~ ~L3~7~
in~ent~on can be asse~bl~d tog~ther by flr~t h~t1ng a~
alcollol go1utioD t~f a l~ydrolyz~d ILon ~xchansQ pols~er~
~ucA ~8 Na~ion , whl~ for~ed by heatlng that ~aterlal ~n an ~lcohol fiolutlon and ~pray coating the 05 heated solutlon onto the lon exchange ~e~brane integrally for~ed wlth the co~po~ite anode structure 30 ~nd the expo~ed face ~2 of the co~po~i~e cat~ode 40. The two ~tructur~s are subseQuently ~eated and bonded together by ~dhering t~e exposed face of layer 42 to the expo~ed face of t~e io~ exchange ~embrane 18.
Througbout the description, the co~posite electrode of the present inv~ntioQ ~as been ~haracter~zed ~s suitable for a~e~bly in a fuel cell, such a6 a ~ydrogen/o~ygen fuel cell. A~ will be evident ~o ~hose ~illed in the art, ~e co~posite electrode of the pr~6ent i~vention can be utilized in electroche~ical device6 vhich produce electri~al poYer or che~ical compound~. For e~a~ple, the present invention i~ applicable to ~y6t~6 vberein vater in liguid or vapor state i8 electrolyzed to generate hydrogen and oxygen. to the 6ynt~esig of chlorine ~hich i~
driven by electrical energy, or to the 6ynthesis of other ~aterial6 of commercial intere~t, ~uch as the production of organic acid6 Pro~ alkane~. In general. the pre~ent invention caa be utilized to fabricate electrode~ u6eful for the electrochemical generation of electrical powe~
fro~ the ~on~umption of r~acting ga6e~ or liquid6 or the electroche~ioal generation of chemical co~pounds from the con6umption of electrical po~er.
The fore~oing de~cription of the preferred embodiment6 of tbe in~ention have ~een presented for purpo~e~ of illu~tration and de6cription. It i~ not intended to be exhau6tive or to li~it the invention to t~e precise forn disclo6ed, and obviously ~any ~odifica~ion~ and variations :~, -24 ~l3~)71~
~re pos~ible ln li~ht of tbe above t~aching. The e~bodi~ent~ w~c~ cho~en and desceibed ln order to be~t explaln the princlple~ of the inventlon ~nd lt~ practlcal appllc~tlon to thereby anable others ~kllled ln tbe art to 05 best utlllze the lnvention ln varlouB e~bodi~ents and vith variou~ ~odiflcatlons a~ are ~uited to the ~articular use contempla~ed. It 1~ intended that t~e ~cope of the invention be defined by t~e clai~6 appended hereto.
,~
,, ., ~ .
BAC~GROUND OF THE INVE~TI OM
The prefient in~ention relates to a ~ompo~ite electrode for use in electro~hemical cell~, and more particularly.
to a composite electrode for u~e in ~uel cell~, ~ueh a6 05 hydrogen/oxygen fuel cell~.
Certai~ fuel cell~ conYert chemical energy into electrical energy by reacting diffeeent ga~es a~
electro~atalytic surface~ on anode and cathod~ electrod~
which are ~o~itionea on oppo~ite side6 of an ion exc~ange membrane. Genetally, ~he ga~ introduced to the anode i~
categorized afi a ~uel while t~e ga6 i~trodu~ed to ~he cathode i~ an oxidant. Utilizing hydrogen/oxyge~ ~olid electrolyte fuel cell6 as illustrative, hydrogen i8 introduced ~ia a ga~eous 6tream to the anode ~ide ~ an io~ e~chang~ membrane and i~ electrochemically oxidized in he presence of a ~uitable cataly~t. 6uch as platinum or a : Qlaeinum alloy, in a~cordance with the following general reaction:
20~ H2 = 2~ ~ 2 ele~tron~ (l) : ~ ~
~:
:
~xyoen ~ lntroducod to ~he cDthode ~lde o~ the lon-con~uctlng ~e~bcane ID ~ ~eCODd ~D8eOU~ ~tCea~ an~ i-~lect~oche~lc~lly reduced ~n ~ccor~nce v1t~ the follo~ing general reactlon:
05 2 ~ ~H ~ ~ Qlectron~ ~ 2H2O (2) Conventlonally, f~el cell~ are de~igaed by po~l~lonlng an ion exchange ~embr~ne withiD ~ react~n~ cha~be~ to define ~ anode co~part~ent o~ one side of the ~on exchange ~e~brane and a cathode co~part~ent on the other side of ~he ~on e~change ~e~brane. Electrodes are positioned in each co~pa~t~ent a~d t~e cha~ber ~6 equ~pped vith ~uitable ~anifold~ for the introduction of qa~eou6 ~tcea~ into ~he anode and catbode co~part~ents. T~e reactant cha~ber also ha6 outlet~ for product ~ater and any unreacted gase~. The ~OD exchange oe~brane is a per~6elecelye ~on trao~porting ~e~brane~ i.e., selecti~ely tran6pocts onlr anio~6 or ca~ion6 depending upon ~he charge of group~ bouna ~ithi~ the polymeric ~atrix of t~e ion exchanqe ~e~bcane. In a hydrogen/orygen fuel cell, proton6 liberated during electrochemical o~idation of ~ydrogen are 6electively tran~ported ~rough the ion exchange ~embcanè-. ~ra-ditionally, the electrode~ u6ed in ~uel cell6 bave been coated w~tb or ~re compri6ed of an electroca~caly~t, 6uch a~ plat~num or a platinum containing 25 co~pound, to increace the raee o~ the electrochemical oxidation and reduction reaction6 occurring in the anode and cathode co~partment6, ce6pectivelY- The elect~on6 qenerated in the ~node sompart~ent are collected by a current collector and are ~ran6ported through an e~te~nal 30 circuit which ~ontain~ a load to t~e cathode coDIpart~ent.
3 i~ 78~9 Curr~nt ~ollecto~ can be con~ruct~d of Dny ~ultable ~lectrlcally conductlvQ ~at~rial, ~uch ~ any ~t~blo ~etal or 8 carbon blac~. Often, the ~aterlal o~ uhlch the re~ct~nt ch~ber is constructed functions a~ tbe current 05 collector. A6 conventlonally con~tructed, fuel ~ell6 can operate at approxi~ately 60~ efficiency, l.e., convert app~oxi~ately 60~ of the ~va~lable che~cal ~nerqy ~n t~e reacting fuel to electricity while the re~aini~g 40t i8 converted to ther~al energy.
Puel cell operation involve~ three ~eparate ~ode~ of tran~port. Pir~t, the introduction o~ reactant gase6 into the fual cell, for e~cample, a hydeogen/o~tyge~ ~uel cell.
from a supply source and ~ove~ent of these ga6eL ~hrough the uel cell durin~ operation involves molecular tran6port. Al60, no~e~ent of water ~or~ed as a result of the electcoche~ioal ~eduction of oxygen i~ accordance vith reaction (2) ~ithin the hydrogen/oxygen fuel cell involves ~oleculat tran~port. Secondly, transport OCCUlB as protons, i.e.. ~ydrogen ion~, produced as a result of reaction (1). ~Ye fco~ th~ anoda co~p~re~ent electrode, through the ion e~change ~e~brane, and into the cath~de compart~ent electrode of the fuel cell vhere electcoche~ical reduction occur6. La~tly, electron6 genera~ed in accordance ~ith t~e electroche~ical oxidation reaction (1) are t~a~6posted ~ia a curren~ collector through an external conduc~ive path having an electrical load to tbe cathode co~partment to 6erve as a reactant for tbe electrochemical reduction of o~ygen. Puel cell performaDce is reduced by any internal i~pedance to any one of the three ~ode~ of tran~port vhich occur during opera~ion thereof.
~,~
~'"`' `~
In ~n 4f0rt to r~duce the lnt~rnal re~l~tance to transport, ~articularly ~roton ~nd ol~ctron tran~port, and tbe~eby lnc~ease fusl cell pelfor~nce, fuel cells de~l~n6 have evolved to a ~zero-qap- configuratlon ~herein the 05 reaction cha~bec, electrode~ ~nd ion exchange ~e~brane are po~tioned ln a contiguous rela~ion6hip. To achieve a ~zero-gap~ configuratlon and the a~tendant reductlon ln internal re~i0tance to tran~port, conventional electrode ~tructure~, such ~8 a ~ire ~e~h Bcleen coated ~ith an electrocataly~t, have been e~bedded on each 0urface of the ion exchange ~embrane. Additionally, compo~ite electrode~
of ion exc~ange polyoers, ~etal~ or carbon co~pound~, and electrocatalytic co~pound6 have been ueilized in an effort to con~truct a~ electrode wherein an electron conductor, a proton conductor, and an electrocataly6t are incorporated into a three-pha$e lnterface to ~i~iaize internal re6i6tance to tran~port.
Solid ~on e~change polymer~ haYing fixed anionic sites in the fora of ~orbed anions or chemioally bonded anion6 ha~e been u~ed to for~ thc per~s~lactive ion exch~nge j ~e~brane used in fuel cell6. Recently, polymer6 po~se~6iny 60rbed or grafted ion6 of stro~g proton acids, for example, 6ulfonic or p~osp~onic acid6, ha~e been employed a~ ion exchange ~e~brane6 in ~uel cell~ due to the relatively rapid protoD tranfif er thereof.
Nafion0, a perfluorosulfonic acid ~embrane, ha6 gained increa6ing popularity as an ion excbange ~embrane for fuel cells. Typically, the ion exchange ~e~brane i~
con~truc~ed of a relatively thin, e.g., 0.002-0.01~ in.
30 thick, 6heet or fil~ of an active ion e~change polymer.
Several separate sheet6 or il~s of acti~e i~n excbange polymer~, each of ~hich posse6fies different conducting , ~ .
. . .
5 ~ 3 ~nd~or ~ettin~ ~op~rt1e~, c~n be ther~ally la~inated to foca a unltary loa exc~ange ~e~brano for uce ln fu~l cc118.
~ lth the advent of the unitary, rel~tively thln lon exc~ange ~e~brane~ ~nd electlode ~ase~blies, uel cellc 05 ~ave been arra~ged in a stacked configur~tion. AB ~uch, the plate or housing ~eparating ~he anode ~nd catbode co~part~ent~ of t~o ad~acent ~uel cell~ ~lectrically connect~ the fuel cells in Berle~ by conducting electrons qenerated in t~e anode of one fuel cell dirQctly to the cathode of an adlacent fuel cell. This common plate i~
refecred to a~ a ~bipolac pl~eea and additionally ~erve~
a6 a po~itive barrier to prevent ~ixing of snode and cat~ode gas flov~ bet~een adjacent cells. Thus, the relatively low volta~e ~enerated by a ~ingle fuel cell, 0.5-O.g V, can be added in ~eries ~o obtain u~eful voltaqes of, for e~a~ple, 120 V.
As previou~ly ~entioned, co~po~ite ~lectrode6 coapri6ing a ho~ogeneou~ Lixture of an ion eschange : poly~er, an electrical conductor. and an electrocataly~
~ have been 6ugge~ted foc u~e ln fuel cell~. The ion exchange poly~er, for example/ Nafion0, serve6 the function of a composite binder and an eiectrolyte for conducting cations. An Qlectfical co~duc~or, ~uch a6 cacbon powder, function6 not only ~o conduct electron6 but al60 a6 a catalyst support. The6e co~po~ites are formed with a filler ~aterial which i6 ~ub6equently re~oved by application of specific aqueou6 601ution6 or heat to proYide a porou~ electrode ~atrix which allow ga6eou6 reactant6 to flow therethrough. Accordingly, an electrode ~atri~ ~tructure i6 provided vhich for~6 ~ conductive skeleton for tran~port o~ bot~ electron~ and proton~
relatively unifoc~ throughout the entire electrode.
;~, ~ 'A~
,.~ .
~7~
Ho~eve~, loa~ing cat~ly~t throughout the ~ropo~ed co~po~lt~ ct~ode ~lgniflc~ntly lnc~ease~ t~e a~ount of catalyst ln the electrode thereby pro~otlng fuel c~ll c4taly8t lnefflciency ~ince only a portlon of the loaded 05 catalyst 18 utillzed 1~ the electroche~ic~l re~ct~ons, Accordin~ly, it i~ ~ object of the pleBent invention to construct a co~po~it~ ~lectrode for uae n electrochemical cells ~n vhich sub~tanti~lly ~11 of tbe elec~ro~ataly~t u~ed in fabricating the eleceroche~ical cell is positioned only alon~ ~ zone wit~in tbe electrode where the tran~port eate6 of electrons and protons are approxi~ately equal.
Another object of the pre~en~ inventio~ i~ to provide a co~p~6ite electrode for use in electroche~ical cell~
whicb i6 constructed to ~ave increa~ing electronic conduct~ity fro~ the catalyst loadi~g ~one to a current collectoc on one face of the elec~roae, and to have increa~i~g protonic co~ductivi~y from the zone of cataly~t loading to the face of the electroae ~hich engages the ion 20 excha~ge ~embrane of the el~ctrochemical cel1.
It is ~ further objec~ of the pre6en~ invention to provide a compo~ite electrode for use in electrochesical cell~ wherein the a~ount of electrocataly~t nece~sarY to ac~ieve the ~ighe6t electroche~ical cell perfor~ance, a~
~ea~ured by the value of watts peL ~g of electrocatalyst.
i6 ~ini~ized.
It is ~till a further object of ~he pre6ent invention to provide a composite electrode for u6e in electrochemical cell6 vherein ~he electrocataly~t is ~ore efficien~ly utilized, a~ ~ea6ured by the voltage of a fiingle electrochemical ~ell.
~3q:!~7~
~HMARY OF 1~ 1NV~N~
To achl~v~ the foregoing and ot~r ob)~ct~, and 1 ~ccordance ~lt~ t~e ~ur~oxe~ o~ tn~ ~r~ent i~Yentlon~ a8 e~bodied a~d broadly desc~lbed herein, one 05 characterlz~tion of the present ~n~ention ~ay co~pr~e a porou~ ~o~po~te electro~e for use ln electroche~ical cells. The electrode ha~ a fir~t face and ~ ~econd f~ce defi~i~g A relatively t~n sect~on t~erebetween. The co~pos~te electrode ha6 ~ean6 ~or co~ducting ion6, ~eans for conducting electrons, and a~ electrocatalyst. The ~olume concentration of the ion conducti~g ~eans i~
greatest at the fir~t face a~d i~ decreased across ~he ~ection, while ~he Yolu~e concentration of the electron conducting oean~ i~ greate6t at ~he second face and decrea~es acros~ the 6ection of the elec~rode.
Sub~tantially all of the electrocatalyst ifi po~itioned vithin the electrode section in a relati~ely narrov zone ~here the rate of electron transport of tbe electrode i8 ~pproxi~ately equal to the rate of ion transport of the 20 electrode.
In a~other characterization of the pre6ent inYention, a porou6 compo6ite electrode for use in electroche~ical cell6 i~ pro~ided which i~ co~prised of three zone6. A
fir~t zone conduct~ both electron6 and ions liberated by ~25 electrochemical reaction6 occurring vithin the : electrochemical cell. The ~ate at ~hi~h the fir6t zone t~ansport6 electrons is greater than ~he rate at which the : firs~ zone tran6port6 ions. A ~econd zone conducts both electron~ and ion~ and ha6 an ion tran6port rate greater than t~e ele~tron tran6po~t rate thereof. ~ third zone i6 contiguou6 with both the ir6t and ~econd zone6 and ha6 approxirately equal ~on and electron transport rate~. An '~
' ~ .
electroc~ly~t for lnceQa~lng t~ rate of t~e ~l~ctro~e~lcal reactlon~ i8 ~l~o~t ~nalrely po~ltloned witbln the t~rd 20ne.
In yet anot~er c~araceer~zatlon to the present 05 lnvention, an electrocbe~ical cell coaprlses a current collector ~eans for conducting electron~ ~enerated by el~ctroc~e~ical reactions of fuel vit~ln the electroc~e~ical cell, an ion conductor ~eans for selec~ively conduct~ng preselected ion~ liberaeed by the electroc~e~ical reaction~, and a co~po~ite electrode po6~tloned be~ween the collector ~ean6 and t~e ion conducto~ ~eans. The co~po~ite elec~rode has ion and electron transport eates which Yary acro~ tbe ~ection thereof and has an electcocataly~t whi~h i~ po~itioned ~long ~ zone vithin the section of the electrode Yherein the ion and electro~ tran6port rates are substantially equal.
B~IEF_DESCRIPTION OF THE D~A~INGS
The acco~pdnyi~g drawings, which are incorporated in and ~or~ a p~rt ~f th~ sp~ tlon, ~11UBtrate the e~bodiaent~ of the pre6ent inventioD and, together vith the de~cription, serve to explain t~e principles of the inventio~ he drawing~:
~lGURE 1 is a partially cu~avay cros~-sectional ~5 pictorial ~iev depicting the volu~e fraction of the co~ponents o a co~po6ite electrode of the pre6ent invention a6 asse~b1ed in an electroche~ical cell;
~IGURE 2 i6 a cro~6-~ectional pictorial ~ie~ of a preferred co~po~ite elec~rode of t~e pre~ent invention;
PIGURE 3 i~ a partially cutaway cross-sectional pictorial vie~ of a compoBite anode of the present invention as assemb1ed ~itb an solid ion exchange ~e2brane o a fuel cell: and , j.~"
.~ ;;, , 9 ~ 3~7~3 ~ IGUR~ ~ ~8 a partl~lly CUt~Wdy cro~ ectlonal plctorlal ~ of ~ co~po~ite ~thode of t~e present lnvention as as~e~bled vit~ the co~poaite anode and a ~olid lon axchange ~e~brane of ~ fuel cell ~s lllu~Crated 05 in Pigure 3.
pETAILED DESCRIPTION OF THE PREFERQeD EMBODIMENTS
~ eferriag ~o~ ~o ~igure 1, the co~poslte electrode of the pr~s~nt ~nventlon i8 illustrated generally a6 10 ~nd i~ in~erposed betveen ~ 801 id ion ~xchange ~e~br~ne lQ ~nd a current ~ollector l2, a6 i~ ~ell ~nown ln the srt.
Current collector l~ ~ay be a portion of a reactor ve~sel of an el2ctrochemical cell, a bipolar plate of ~ ~erie~ of ~tacked, individual fuel cell~, or another suitable electrical ~onductor. Current collector l2 can be con6tructed of any guita~le electron conducting ~aterial.
~uch as ~ickel, ~o~ other ~etal~. grap~ite (a conducti~q for~ of carbon), or a qraphite pla6tic co~po~ite vhich posse~se~ sufficient electron conduction for u~e ~ith the electrode of t~e present invention. as hereinafter de6cribed. Curr~n~ collector 1~ ~6 pre~er~bly con6tructed of a corrocion re~istant ~etal. such as ~ic~el, or a graphite pla~tic compDsite ~hich include~ a binder. for example, a eo~po~i~e of 80 wt ~ car~on black, ~uch a6 Vulcan XC-720 ~anufactured by Cabot Corporation. and 20 wt ~ polyvinylidene fluoride, such a~ ~ynar~
~anufactured by Pennvalt Corporatio~.
~ e~brane 18 can be fabricated f~om any ~uitable ion exchange poly~er ~icb ~unctions a~ a selective ion tran~porting ~aterial, a6 i~ ~ell SnovD in the electrochemical act. Me~brane l~ can be con6tructed of Na~ion ~17~, a 0.007 in. thick polyperfluoro~ulfonic acid membrane ~anufactured ~y Du Pont Corporation.
, .
10 ~3~ 3 Poly~erfluorocarbo~yl~c ~cld ~oly~er~ ~re al~o cui~ble lon axchange poly~ers for u8e ~ ~e~br~ 8. ~olld co~pos~te ~l~ctrode 1~ o the pre~ent in~ent~G~ ~r relatlvely thln ~beet, fll~, or vafer, ~.g., O.O10 in., 05 defining a gener~lly leetiline~r conigura~ion. ~lectrode 13 is co~pr~ed of ~n lon exc~ange poly~er 17 ~ich ba~ a che~ical ~o~po~ition and transpor~ propertle6 ~l~ilar to the ion excbange poly~er u~ed to con6truc~ the lo~
exohange me~brane 18. a cuerent conductiny ~dteelal 15.
~uch a6 tbe ~aterial utilized to coA6truct cur~nt collector 12, and an electrocataly6t 16. Electrocataly~t 16 function~ in part to acceler~te the electro~ transport ~eces~ary in the operatio~ of the co~posi~e electrode of the pre6ent i~veution. Suitable electrocatalyst can be ~elected fro~ the noble ~etal group, i~ particular.
platinum, rhodiu~. palladium or alloy~ thereof. or fro~
~etalorganic co~pound6, ~uch ~5 iron or cobale porphyrin co~pounds or iron or cobalt phthalocyanine co~pound~, that have been ther~ally treated. in a conven~ional aanner.
The elect~oc~t~lr6t ~n ba amploye~ either a6 lelatiVely s~all, aetallic particle6, i.e., ~etal black6, or a~ c~all particle6 supported on conducting substra~e~. Preferably, pla~inum part~les having a cro~ ectional di~6ion of ; 100-200 ~ 10-~ c~ depo&ited on larger carboD particle~
are utillzed a6 the electrocataly~t in the compo~ite :~ electrode of the pre6ent ~nvention.
Polypecfluoro~ulfonic acid~, typified by the Nafion~ cla~ of polymers ~anufactured by DU Pont, are prepared by the copolymerization of tetrafluoroethylene and a vinyl et~er ~oDomer that ter~inate~ wi~ a 6ulfuryl fluoride group. i.e., -S02P. The re6ulting p~lymer product can be ~eat proces~ed in~o 6heet~ or other de6ired .
:
1 1 ~3~ L9 ~hape~ by convent~onal ~etbod~. The proc~ d poly~rs are then ~ydeoly~od ln ~ concentr~t~d aqu~ou~ b~se ~olutio~ ~t 0l~Y~ted t~peratuc~ .g., ~ 30 ~t ~ XOH
~olutlon ~t 9OC. Durlng hydroly~is, the nonionic 05 ~ulfuryl group 1~ ~heolcally convert2a ~o ~ ~ulfonic acid an~on, -SO3 . ~fter hydroly~ he ~oly~er~ contain bound ~n~on~c groups t~at ~t~ract aY~llable ~ation~ ~o for~ an lo~ exc~an~e ~etwor~. Ther~al ~ropertles of the hydrolyzed poly~er are slt~c~d 3uch that t~e ~yd~olyzed poly~er cannot be heat ~loce6~ed. At el~vated fuel cell operating te~peratures, i.e., greater ~an lO~DC, dehydration of the ~e~brane, electrode intecface occurs which can be detrioental to lo~ Sran~port. Accordingly, at ~uch elevated te~perature~. other ~aterials ~hich exhibit relatively high ~onic conduction ~t high te~peratures, $uch as phosphori~ acid oe ceetain ~et~l o~ides, for exa~ple, ~rid~u~ oxide or tung~ten oxide can be selected as an io~ e~change ~aterial i~ lieu of the ion e~change poly~er l7 for utilizatlon In the composite electroae of ~e pre6ent inven~ion.
The present invention i~ based upon the di~covery ~hat by c~ea~i~g a relati~ely ~arrow zone Ol plane ~ithin a compo~ite fuel cell electrode wherei~ the rate of electron tran~poet i6 approxi~ately equal to the rate of proton tcan6port and by loading sub6tantially ~ll of the electrocataly~t utilized in the co~po6ite electrode only within t~i6 zone~ electrocataly6t use ~nd fuel cell perfor~ance, a~ ~ea6ured by the number of ~atta generated per ~g of ~lectrocataly6t o~ by vol~aqe efficiency, are ~ignificaDtly increa~ed. Po6itioning t~e electrocataly t along a zone of aub~tantially equal protonlc and electronic t~a~port also p~ovides for ~axi~u~ utilization 12 ~3~'71~
ot ehe ~vail~ble olectrocataly~t ~ due to t~e re~ultlng decrea~ ld~nce ti~e of ~ r~actlng gas ~olecule on Dny guch site.
The effectlv~ rate ~t which ~aterlal ~5 conduct~
05 ~l~ctric~l cutrent. i.e.. ~lectron~, ~nd the ~ate ~t vhlc~
~on exchange poly~er 17 conduct~ proeon~ .. hydrogen ~on flux, ace directly proport~onal to the volu~
concentratio~ of each of these ~ate~ia~ w~thi~ electrode 13. The electron flux, i.e.~ the current lfflol~s-c~2~, increases to~ard curren~ collector 12, while the protoD
flux, i.e., ~e ~onic flo~ (~ol~s-c~Z), ~ncreaseR to~ard oe~brane 18 si~ce each separat~ electrocatalytic site can serve as th~ locu6 of ~lectron tran~fer. and each ion ~U6t ~ove toward the ~e~br~ne. ACCDrding1Y, ehe volu~e 15 co~centration of current conductinsl loaterial 15 and poly~er 17 i~ qraded through the cro6s section of electrode 13 to corre~pond ts ~he varying proto~ and electron flus. Current conducti~g ~aterial 15 has tbe greate~t volu~e concentration ~thin electrode 13 along the interface of the ~lectrodc with the curren~ collector 12. ~he concentration of the current conductinq ~aterial 15 decrea6e~ acro66 the cross 6ection of electrode 13.
The Yolume concentration of ~he ion exchange poly~er 17 ~it~in elect~ode 13 is greatest a~ the interface of electrode 13 with De~brane 18 and i6 dec~ea~ed acros6 the section of elec~rod~ 13. Electroca~alyst 16 is concentrated at that plane or zone alo~g the ~ection of electrode 13 vherein the tran~port rate of eleGtron6 ~as deter~ined by the electron conductivity and tlle Yolu:lle 30 ~raction of naterial 15) and the tran~;poct rate of pro~ons ~a~ dete~mined Iby the p~oton conducti~ y and the volu~e feaction o~ ~aterial 17) are sub6tantially equal.
:
."~, .
13 ~3~
Solld co~po~lt~ ctrode 13 of th~ pre~ent lnventlon provides v~rled tran~ort rate~ for bot~ el~ctron~ and protons (in a hydro~en~oxygen fuel cell) acros~ ~he ~ection th~reo. The electro~ conductivity inc~ea~es S toward the electrode. cucrent collector interface, whLle the proton conductivltY increases toward the electrode, lon exc~ange membrane interface. Th~ electrocataly~t 1~
concentrated intermed1ate ln the elactrode sectioD ln that relatively narrow plane oc zone 16 where the transpor~
cate of electrons. e%pre~sed on ~ volume basi~, 1n appro~i~ately equal to the t~anspo~t rate o proton~, expressed on a volume basls.
Electron and proton transfer ~ate~ ace generally expre~sed u~in~ tha value o conductivity for a glven ~aterlal, l racip~ocally, a~ the value of it~
re~istivi~y. The basic unit of ~esistiviey i~ oh~c~ per c~2. aesistancs of a ~i~en ~aeerlal to either electron o~ p~oton t~ansport i~ ~easured by deter~ining the volta~e drop acro~s that ~aterial while a ~cnown f lux occur~
through that ~aterial and ~caling that resi~tance to the Yolume fraction of that ~ater~al within a partlcular composition. The resl~tance of a glven materlal to el~hec ~on~ or electrona can be defi~ed a~ ~caled to the cocrespondin~ volume fraction by the express~on ~et forth below. Thu~, the electronlc re~lstance, ~ in any volume element, i, i~ related to the bul~ resls~ance, ab, by ~he expre~sion:
;~e(1) ~ Rb/k~ (3) :30 whera f is the volume fraction, and k i~ an empiclcal ~3~
coeficlent t~at ~ccounts ~or ~onlin~ar ~f~ct~ ln YolU~e ~i~lng. Accordi~ly, ~qual ~loctron ~d proton conductivlty can be ~xpr~s~ed a~:
~ 2) (a~) ~4) wherein ~ he t~ckne~ of ebe path of ele~tron conducta~ce ro~ the zone of ~gual electron and peoton conduct~vlty to the current collector interface, ~2 i~
th~ ~ickne~s of t~e p~th o~ proton conductivley ~ro~ the zone of ~qual ~lectron and proton conductlv~ty to the ~e~brane interface, ae i8 tbe effective tran~fer resi~tance of electeon6 throu~h the electro~ tran~fer path, and ~p 18 the effecti~e tca~sfe~ re~ ance ~f proton6 through the proton tran&fer path. T~UB it can be ~ppreciat~d that ~ d ~2 ~u~t be ad3usted to account for difference6 in ~ ~nd Rp. A8 the rates of electron transport are typically faster t~an the rate of pcoton tEansport ~ X2 au8t be designed to be les~ than ~1~ Accordingly, the ~onc of relativ~lr ~qual pro~on and electron tean6port rateg, i.e., the zone of high electroca~aly~t loading, is located closer to ~he ~e~braae interface than the curren~ collec~or ~nterface of the co~po6ite electrode 13. The e~act po6itioning of this zo~e in a~ opti~ized co~pofiite electrode is a function of the Be and ~p value6, a6 deter~ined by the e~act mateeial~ used to fabrica~e elect~ode 13.
The co~po6ite electrode o~ the present inventi~n ~u6t be fabricated 6uc~ that reacting ~oleculeG of fuel ga6e~
introduced into the fuel cell have acce~sibility to electrocatalyt~c ~ites ~roug~out the ~lectrode.
Efficient ~as tran~port rate~ ar~ directly dependent upon `
~3~'7~
~d~gu~t~ ~oro~ity throughout the ~l~ctrode. ~ccocdlngly, the co~po~lt~ ~loctrode of the pr~sent ln~ontion i~
prov~ded v~tb ~ poro#ity ~ufflclent to peralt tbe uninh~blted flow of reactant g~ae~ and vat~r ~n ~he for~
05 of ~tea~. ~he bulk poro~lty of electrode 13, deflned ~8 t~e volu~e of gas ln el~ctrode 13 dl~ided by the total volume of electrode 13, i~ ~bout 0.6 to abou~ 0.7. Thi~
bulk porosity provide~ for a relatlvely unifor~
distribu~ion of ga~eous reactant~ througbou~ the ~tructure of composite electrode 13. ~he void DeCesBary to create such poro0ity can be ~ab~icated by u~e of porou~ ~heet~ or geld~ of ~aterial~, a6 herei~after described. or by u~e of filler ~aterialB which are dis~olved or ~her~ally re~oved after fabrlcation of electrode 13.
._ The porosity of el¢ctrode 13 can vary acros~ t~e ~ection thereof and, ~8 ~llustrated in Pigure 1. should lncrease at the el~ctrode, current collector ~nterace to per~it qaseou6 reactants acces~ to the zone Yhere the ele~troc~taly6t iB concentrated. The electrocatalyRt i6 di~per6e~ through~ut thi~ 20nQ to for~ ~hinO hig~-6urface area layer6 on the electro~ ~onducti~g solid6 ~ithin this zone. By incr~asi~g ~he poro6ity i~ this zone where cataly~t loadi~g ls concentrated. qa~ flow (~olecular transport) i8 ~axi~ized in that area of electrode 13 wherein reacti~ity i~ ~axi~ized. Plow~ of ga~eou~
reactant~ are i~roduced to electrode 13 fro~ an external source by utili~ing flov ~anifold~ and other 6tructural confiqurations which are well known in the art.
In one embodi~ent, the compo6ite electrode 13 of the 30 pre6ent invention i6 forloed of three separate layer6 or zone6 a6 illu~tra~ed ger,erally in Figure 2 a6 22, 24, and 26. Each layer or 20lle comprises a aiYture of carbon ~3~
black, ~latinu3 or ot~er ~ultable ~lectlocatalyst dl~per~ed and ~uppo~t~d on carbon blac~, ~olytetrafluoroethylene ~ a binder, ~nd ~ ~u~table ionic conductin~ ~at~rial, ~uch ~B polyperfluoro~ulfoDic ~cid.
~ Su~table carbon blac~ for u~e in ~anufacturlng the composite ~lectrode of the pre~ent invention po~esae~ a relatlvely high surf~ce area, a hlgh electr~cal conductivity, a~d a low che~ical reactiviey. Vulcan XC-72 ~anufactured by Cabot Corporation ~8 a pre~erred ~arbon blac~. It i~ preferred to utilize a relatively ~igh surface area platinum (e.g., 20 ~2~g~) in the for~ of a carbo~ blac~ vl~b approxi~ately 15 vt platiou~ loaded on tbe surace thereof.
Polytetr~fluoroethylene ifi co~mercially available a~ a fine su~pe~s~on, i.e., ~ fi~e powder di~persed ~n a ~olvent, ~uch as water. Polyperfluoro~ulfo~ic ac~d or ~ polyperfluorocae~oxyl~c acid poly~ers are avail~ble ~8 : ~u~pe~ions in ~olYents, $uch as ~i~ture6 of vater and ~: ~ethanol or a~ unbydrolyzed for~6. Tbe latter for~
per~it~ the u~e of ~u~h p~ly~er~ ~s a ther~al pla~tic binder. The co~po~itional and dimen6io~al para~eter6 for each layer or zone are set forth below in ~able 1.
~, ~,.~ .. . .
~ ~o'~o ~C C) ~ ~ C-~
~ ~o~
~D ~ ~ ~,,~
,~ ,~
D~ O -I O ~ ~ ~
~ V c ~ r~ O . O
~ ~ 1 ~ ~
o o o o ~ ~ ~o ~r ~ C u~ ~
~ o o ~ ~ t:
: ~J ~
a I
~C
V K K K
~ .
o ~ e .
e N ~ .0 ~; ' - : ~:
:: :
`
' Thu~, tne th1ckn~s of the co~poslt~ ol~ctro~
1.5 ~ 10-3c~. ~ac~ ~one 1~ ~repared ~y co~b~ning the con~tituent~ di~persed ~n a ~ultable dl~er~ant. 8UC~ ~
~exane or other lov boilin~ ~ol~t llquld~. in accor~ance S wieh the for~ul~tion set fort~ ~bove, and sprayln~ a layer of the resultin~ d~per~ion on ei~her current collec~ol 12 or ~embrane 1~. Accurate layer ~hickne~ ~ay be rea~ily achieved by one skilled in ~he ~pray co~ing ar~ util~zing ~onventional apparatu6. ~t lea~t one zone iR applied to each of the ~ex~ra~e 18 and collector 12, and thereafter, the ~embrane 18. collec~or 1~, snd layers 22, 24, and 26 are thecmally bonded to each ot~er to for~ the co~posite electrode of the pre6ent invention.
Referring now to Figure 3, a ~tructure of a coEp~site anode electrode or u~e in a fuel cell, ~uch as a hydrogen/oxyge~ uel cell, i~ illu~rated. The co~po~ite anode i6 illu6trated in Figure 3 generally as 30 and comprise6 layer¢ 32~ 3~, 36, and 3~. Layer 32 i~ an 0.05 in. thic~ carbon grid con~tructed o graphitized carbon ~n~ po~6afi6inq relatiYely high phy~ic~l strength, e.~., cru6h ~trength grea~er than 300 psi, ~nd electrical ee~istivity, e.g., 0.01 o~m-~. Lay~r 32 i8 con6tructed vitb pore openinq6 (~e6h) of approxi~ately 50-100 ~ 10 ~ c~ to permit efficient ~i~ration of ~- 25 reac~ant ga~e~ unifor~ly through the layer. The :electrical conductivity of layer 32 per~its rapid ~ran~port of electron6 to the current colleceor 12 ~ith low re~i6eance 1066. Layer 32 i6 treated with ~ydrophobic ~aterial6, ~u~h a~ elemental 1uorine. to decrease the veteing tendencie~ th~reof.
Layer 34 is a porous carbon ~hee~ having a thic~ne~$
of about 1-2 ~ cm. Layer 34 may be fabricated by ~oe pre66ing a ~i3ture of a co~ductive carbon black and an .
.,., ~,.,~ . . ..
unhydroly2ed lon ~xch~nge ~olp~er, ~uch a~ unhydrolyzed Na10n polymer ln po~der for~. Tbe conc2ntratio~3 of carbon black and unbydtolyzed loa 2xchange ~oly~er are ~el~cted ~o ~hat layer 3~ po~se~e~ aa ~lectron tr3n~port 05 rate ~hich ~s gr~ter than, e.g., approxl~a~ely t~ice, its proton tran~port rate. Lay2r 3~ ubsequ~ntly heated to ~elt the ion exchange poly~er which functlon6 a6 a b~nder therein.
Layer 36 i~ compri6ed of the sa~e ~ateri~ls as layer 34 but the concentration~ of carbon black and the unhyd~olyzed ion e~cchange poly~ee ~re ~l~ered ~o that the electron tran~poct rate of layer 36 iB approxi~ately egual to the proton tran~port raSe thereof. Follo~i~g hot pre66i~g t~e ~ix~ure a~ de~cribed with respect ~o layer 34, layer 36 i~ sprayed with a solutioa containiQg platinum, rutheniu~, or a ~i~ture thereof. ~or sxa~ple, lay~r 36 ls ~prayed ~ith aD alco~ol ~olution of pla~inu~
and rutbenium chloride~ at a~bient te~pecature and pLe~6ure. Layer 36 is ~prayed vith an amnunt of solution nece~6ary to obtain a cataly~t loading of l ~g~c~ or le68. Sub~equently, the cataly~t ~6 c~emically reduced to con~ere the catalytic ~e~al~ ~o the~r ele~e~tal for~.
Such chemical reduction ~ay ~equire the U6e ~ hydrazine or another ~oderate reducing agent which can ~e applied as a liquid.
Layer 3~ i8 al60 con6tructed of the 6a~e ~aterial6 as layer 39 except that the concen~ration6 of carbon black and unhydrolyzed ion exchanqe polymer are varied 60 that the protoD tran6port rate of layer 38 i6 greater than, e.g., approxi~ately t~ice, the electron transport rate of lay~r 3~. Layer 38 is prepared by ho~ pre~ing a thin (lO ~ lO 4 cm) ~ix~ure of carbon black and a~
unhydeolyzed ion excbange poly~er.
, , ~ .`i 2~ ~3C~7~9 Solid co~poclte anode 30 13 then as~e~bl~d by etac~l~g l~yer6 1~ ag~ln~t a ~heet o~ ~afer of unhydrolyzod ion ~xchange po~y~er ~ ~a8 lllu~trated), ~uch ~ unhydroly~ed N~flon poly~er, ~avlng a thickne~s of between about S 0.076 c~ - about 0.013 c~, and hot pre~slDg the stacked layers to bond the resultAnt co~po~ite anode 30 together and to the thicker sheet or wafer of unhydrolyzed Nafion0 polyme~. The resultant l~ ate ~ is~r~ed ln an aqueous bage, e.g., 6 ~olar ~od~u~ hydroxide, ~t approxioately ~0-100C to hydrolyze ~he io~ exchange polymer, thereby formi~g an active ion e~chan~e ~aterial.
This treatoent i~ co~tlnued u~tll ~ub6tantially all of the sulfuryl fluoride qeoup6 are hydrolyzed to the ~ulfonic acid anion. The resulting la~inate i6 ~ubsequently ~a~hed in deionized ~ate~ and submerged ~ a dilute electrocatalyst-containi~g solution. ~.g., an ~queou6 ~olution of a plati~u~ ~alt. The la~iaate i~ treated ~ith a chemical reduclng agent to reduce the platinu~ both in the ion exchanqe ~e~brane 18 ~nd the electrode 30. It i~
i~portant to note th3t th~ ~Mount of ~lectrocataly~t loaded throug~ou~ the lami~ate i~ this ~tep ~ay be le6~
: ~han 5 wt ~ of the amount of ~lectroca~alyst loaded onto intermediate layer 36 of co~pofiite anode 30.
Referri~q now to ~igure ~, a ~olid co~posite cathode of the pre~en~ invent~on i8 illu~trated generally a~ 40 and i8 co~pri6ed generally of layer6 ~2, qg, and ~8.
Layer 42 is a co~posite of a unhydrolyzed ion exchange polymer, 6uch as unhydrolyzed Nafion0, and a precatalyzed graphite powder. The graphite powder. a conductiYe carbon, ~s fir8t pretreated by thermal treat~ent i~ a controlled environmen~ u6ing well-~no~n carbon technology to control the o~idative ~ability thereof. Subsequently, the carbon powder is treated with plat~nu~ ~alt ~olution ~nd che~1cally reduced to yl~ld a thln co~tlng o~ ~latlnu~ on the ~urface of the c~rbon.
~elatlv~ly ~aall concentrat~ons of ~l~tinu~ ~re re~uired ln thl~ layer, i.e., ~ubstDne1ally le~ ~han 05 0.1 ~g/a6se~bled c~2 of electrode area. Thi~ cataly2ed carbon 1~ ~lxed with unhydrolyzed lon exchan~e polyDer and then hot pres6ed into a thi~ sheet or fil~. The conceneration6 of graphlte powder and unhydrolyzed ion e~change poly~er are ~elec~ed to prov~de l~yer ~2 vlth A
proton transport ~ate ~hic~ iB greater than, e.g., ~pproximately t~ice, lts electron transport rate.
Layer ~4 i~ a co~posite blend that i~ pr~pared by veavi~g a cloth of strand~ of the unhydrolyzed ion e~change poly~er which are ~pun-cast i~to fiber~ and of beat treated carbon fiber~. ~ela~ively thin f iber~ of each ~a~erial are e~ployed ~o that the resul~ant fabric hafi a relati~rely large nulaber of ~l~ter~ectioD~ between the t~o ~ypes of fib~r~. The concentratlons of carbon fiber6 and ion exchange polymec fibers are selected to provide layec 44 with ~ proton tran~port r~te ~ppro~ioately equal to it6 electron ~ran6port rate. An organic ~acrocycle CO~pOund contai~i~g iron or cobalt i6 utilized to catalyze thi6 fabric. A 6uitable elec~rocataly6t, a6 pre~iously described, ~6 depo6ited by either ~olution deposition or carbon pretreat~ent. Utilizing 601ution depo~ition, a compound, such a~ a cobalt ~etramethoxyt~traphenylpo~phyrin, i~ di6601ved i~ a solvent, ~uch a6 tetrahydrofuran. The fabric i~ i~mer6ed ~into thi6 solu~ion and the porp~yrin adsorb~ onto the ;30 carbon surface6. Thi6 immer~ion ca~ be repeated. The fabric i~ sub~equently dried a~ approxi~ately 100C to re~ove any exce~ organi~ ~olvent, and thereafter heat treated to bind the porphyrin to the fabric. A rbort . ~, , ~ 7~3~L9 p~riod o~ lnt~n~ hq~tln~ (aso-sso~c) ~ro~ ~n ~n~car~d ~oucc~ uch ~IIB an lnten~ lR l~s~r, w~ ct~lvQly h~at th~ carbo~ black ~urace~ conw~ctlng ~or~tlyrl~ to lt~
c~t~lytlc for~ ,-nd blndlng ~t to ~acbon. Utlllzlng c-rbon 05 ~eetre~ts~nt, the carbon f ib~r~ ~ro f ~r~t coatod ~ltb the porp~yrl~ befot~ vea-.rln~. For ~xa~ple~ th~ carbon flbers ~r~ r~ed in ~ ~ol~ltlon of cobalt te~ca~e~bor5~tetragheclylpol~hyrin dl~ol~r~d in ~etr~hyd~ofuran ~nd ~r~ pyrolyzed (B50-950) to ~onver~ tbe porphyrln to 1 t8 ca~alytlc 20cn and bind lt to carbo~. The re~ulting f iber 1~ WoYe~ s~lth the unbydsolyzed ion ~xchange polyloer to for~ the fabric uttlized hB ~ayer ~.
Layec .Q co~prl~ed of a l~yer of bighly ~vnduct~ve gcaph~tl2e~ ~lotb co~taining ~ r~latively s~all a~ount of UnhYdrO1Y2Qd 10Q s~change pol~erc whlch i~ treat~a vlth a los~ ~latlnul~ loadlng ~n a E~anner ~lallar to ~he tre~t-en~
of layeL 42. The ~oncentratlol~s of ~Iraphit~
earboD) and ~on exchange polyner ~re ~elacted to proYlde l~yer ~ rith an el~ctron eransport rate great~r than, ;i e. g ., appro~ aately t~ice, it8 proton tra~port c~te .
Thi6 layer aay 8160 be tre~l:ed b~lt~ hyd~ophobiC ~aterials to control ~ettlng ln a ~an~eE de6cribea here~nhbove with re~pect ~o layelc 32 of solld co~apo~t~ anode 30. Layer~
~2, ~4, and 4B are ~tacked alld ~eated under pre66ure fS0-60 p6i) UDt~l the unhydroly2ed io~ excllan~e polymer ~elt6 to bond ths layer~ together. T~e ~e~ultant la~inate oo~posite 1~ ~ydrolyzed ~n ~e ~anner descrlbed berei~
with re~pect to a~ode 30 until sub6tantially all of the ~ulfuryl g~oupa ara ~ydroly2ed to ~ulfonic ac~d anlon SlCoup6 .
As 111~6tr~tea ln Flgure~ 3 and 4, the preferr~d ~olid co~poslte ~nod~ 30 and ~olld ~at~ode ~10 of t~e ~re~ent ~..,..~.. ~.....
2~ ~L3~7~
in~ent~on can be asse~bl~d tog~ther by flr~t h~t1ng a~
alcollol go1utioD t~f a l~ydrolyz~d ILon ~xchansQ pols~er~
~ucA ~8 Na~ion , whl~ for~ed by heatlng that ~aterlal ~n an ~lcohol fiolutlon and ~pray coating the 05 heated solutlon onto the lon exchange ~e~brane integrally for~ed wlth the co~po~ite anode structure 30 ~nd the expo~ed face ~2 of the co~po~i~e cat~ode 40. The two ~tructur~s are subseQuently ~eated and bonded together by ~dhering t~e exposed face of layer 42 to the expo~ed face of t~e io~ exchange ~embrane 18.
Througbout the description, the co~posite electrode of the present inv~ntioQ ~as been ~haracter~zed ~s suitable for a~e~bly in a fuel cell, such a6 a ~ydrogen/o~ygen fuel cell. A~ will be evident ~o ~hose ~illed in the art, ~e co~posite electrode of the pr~6ent i~vention can be utilized in electroche~ical device6 vhich produce electri~al poYer or che~ical compound~. For e~a~ple, the present invention i~ applicable to ~y6t~6 vberein vater in liguid or vapor state i8 electrolyzed to generate hydrogen and oxygen. to the 6ynt~esig of chlorine ~hich i~
driven by electrical energy, or to the 6ynthesis of other ~aterial6 of commercial intere~t, ~uch as the production of organic acid6 Pro~ alkane~. In general. the pre~ent invention caa be utilized to fabricate electrode~ u6eful for the electrochemical generation of electrical powe~
fro~ the ~on~umption of r~acting ga6e~ or liquid6 or the electroche~ioal generation of chemical co~pounds from the con6umption of electrical po~er.
The fore~oing de~cription of the preferred embodiment6 of tbe in~ention have ~een presented for purpo~e~ of illu~tration and de6cription. It i~ not intended to be exhau6tive or to li~it the invention to t~e precise forn disclo6ed, and obviously ~any ~odifica~ion~ and variations :~, -24 ~l3~)71~
~re pos~ible ln li~ht of tbe above t~aching. The e~bodi~ent~ w~c~ cho~en and desceibed ln order to be~t explaln the princlple~ of the inventlon ~nd lt~ practlcal appllc~tlon to thereby anable others ~kllled ln tbe art to 05 best utlllze the lnvention ln varlouB e~bodi~ents and vith variou~ ~odiflcatlons a~ are ~uited to the ~articular use contempla~ed. It 1~ intended that t~e ~cope of the invention be defined by t~e clai~6 appended hereto.
,~
,, ., ~ .
Claims (43)
1. A porous composite electrode for use between an electron conductor and an ion exchange membrane of an electrochemical cell, said electrode having a first face and a second face defining a relatively thin section therebetween, said composite electrode comprising:
an ion conducting material which selectively conducts certain ions therethrough, said electrode having a volume concentration of said ion conducting material which is greatest at said second face and which is decreased across the section Or said electrode thereby defining a transport rate of ions which correspondingly varies across the section of said electrode;
an electron conducting material having a volume concentration within said electrode which is greatest at said first face and which is decreased across the section of said electrode thereby defining a transport rate of electrons which correspondingly varies across the section of said electrode; and an electrocatalyst positioned along a zone within the section of said electrode wherein said transport rate of electrons and said transport rate of ions are substantially equal.
an ion conducting material which selectively conducts certain ions therethrough, said electrode having a volume concentration of said ion conducting material which is greatest at said second face and which is decreased across the section Or said electrode thereby defining a transport rate of ions which correspondingly varies across the section of said electrode;
an electron conducting material having a volume concentration within said electrode which is greatest at said first face and which is decreased across the section of said electrode thereby defining a transport rate of electrons which correspondingly varies across the section of said electrode; and an electrocatalyst positioned along a zone within the section of said electrode wherein said transport rate of electrons and said transport rate of ions are substantially equal.
2. The porous composite electrode of Claim 1 wherein said zone of substantially equal electron and ion transport rates is located at a distance closer to said second face than said first face.
3. The porous composite electrode of Claim 1 wherein said electrode has a porosity which varies across the section of said electrode and which is the greatest at said first face.
4. The porous composite electrode of Claim 1 wherein said electrode has a bulk porosity of from about 0.6 to about 0.7.
5. The porous composite electrode of Claim 1 wherein said electron conducting material is a stable metal, a graphite or a graphite plastic composite.
6. The porous composite electrode of Claim 1 wherein said ions are protons and said ion conducting material is and ion exchange polymer selected from the group consisting of polyperfluorosulfonic acid polymers and polyperfluorocarboxylic acid polymers.
7. The porous composite electrode of Claim 1 wherein said electrocatalyst is a noble metal or a metalorganic compound.
8. The porous composite electrode of Claim 7 wherein said electrocatalyst is a noble metal selected from the group consisting of platinum, rhodium, palladium, and alloys thereof.
9. The porous composite electrode of Claim 7 wherein said electrocatalyst is a metalorganic compound selected from the group consisting of cobalt porphyrin compounds, iron porphyrin compounds, cobalt phthalocyanine compounds, iron phthalocyanine compounds, or mixtures thereof.
10. An electrochemical cell comprising:
current collector means for conducting electrons generated by electrochemical reactions of fuel within said electrochemical cell:
ion conductor means for selectively conducting preselected ions liberated by said electrical reactions;
and a composite electrode positioned between said collector means and said ion conductor means, said composite electrode having varying ion and electron transport rates therethrough and having an electrocatalyst positioned within a zone in the section of said electrode wherein said rates are substantially equal.
current collector means for conducting electrons generated by electrochemical reactions of fuel within said electrochemical cell:
ion conductor means for selectively conducting preselected ions liberated by said electrical reactions;
and a composite electrode positioned between said collector means and said ion conductor means, said composite electrode having varying ion and electron transport rates therethrough and having an electrocatalyst positioned within a zone in the section of said electrode wherein said rates are substantially equal.
11. The electrochemical cell of Claim 10 wherein said composite electrode has first face and a second face defining a relatively thin section therebetween and is positioned between said current collector means and said ion conductor means such that said first face is contiguous with said current collector means thereby defining a first interface and said second face is contiguous with said ion conductor means thereby defining a second interface.
12. The electrochemical cell of Claim 11 wherein said composite electrode comprise:
an ion conducting material which selectively conducts said certain ions therethrough, said ion conducting material having a volume concentration which is greatest at said second interface and which is decreased across the section of said electrode thereby defining a rate of ion transport which correspondingly varies across the section of said electrode; and an electron conducting material having a volume concentration which is greatest at said first interface and which is decreased across the section of said electrode thereby defining a rate of electron transport which correspondingly varies across the section of said electrode.
an ion conducting material which selectively conducts said certain ions therethrough, said ion conducting material having a volume concentration which is greatest at said second interface and which is decreased across the section of said electrode thereby defining a rate of ion transport which correspondingly varies across the section of said electrode; and an electron conducting material having a volume concentration which is greatest at said first interface and which is decreased across the section of said electrode thereby defining a rate of electron transport which correspondingly varies across the section of said electrode.
13. The electrochemical cell of Claim 12 wherein said zone of substantially equal electron transport and ion transport is located at a distance closer to said second interface than said first interface.
14. The electrochemical cell of Claim 12 wherein said electrode has a porosity which varies across the section of said electrode and which is the greatest at said first face.
15. The electrochemical cell of Claim 12 wherein said electrode has a porosity of from about 0.6 to about 0.7.
16. The electrochemical cell of Claim 12 wherein said electron conducting material is a stable metal, a graphite, or a graphite plastic composite.
17. The electrochemical cell of Claim 12 wherein said ions are protons and said ion conducting material is an ion exchange polymer selected from the group consisting of polyperfluorosulfonic acid polymers and polyperfluorocarboxylic acid polymers.
18. The electrochemical cell of Claim 12 wherein said electrocatalyst is a noble metal or a metalorganic compound.
19. The electrochemical cell of Claim 18 wherein said electrocatalyst is a noble metal selected from the group consisting of platinum, rhodium, palladium, and alloys thereof.
20. The electrochemical cell of Claim 18 wherein said electrocatalyst is a metalorganic compound selected from the group consisting of cobalt porphyrin compounds, iron porphyrin compounds. cobalt phthalocyanine compounds, iron phthalocyanine compounds. or mixtures thereof.
21. A porous composite electrode for use in an electrochemical cell having a first face and a second face defining a section therebetween, said composite electrode comprising:
ion conductor means defining an ion transport rate through said electrode. said electrode having a volume concentration of said ion conductor means which is the greatest at said first face and which is decreased across said section;
electron conductor means defining an electron transport rate through said electrode. said electrode having a volume concentration of said electron conductor means which is the greatest at said second face and which decreased across said section: and an electrocatalyst positiond within said section in a relatively narrow zone where the rate of electron transport is approximately equal to the rate of ion transport.
ion conductor means defining an ion transport rate through said electrode. said electrode having a volume concentration of said ion conductor means which is the greatest at said first face and which is decreased across said section;
electron conductor means defining an electron transport rate through said electrode. said electrode having a volume concentration of said electron conductor means which is the greatest at said second face and which decreased across said section: and an electrocatalyst positiond within said section in a relatively narrow zone where the rate of electron transport is approximately equal to the rate of ion transport.
22. The porous composite electrode of Claim 21 wherein said zone of substantially equal electron and ion transport rates is located at a distance closer to said first face than said second face.
23. The porous composite electrode of Claim 21 wherein said electrode has a porosity which varies across the section of said electrode and which is the greatest at said second face.
24. The porous composite electrode of Claim 21 wherein said electrode has a porosity of from about 0.6 to about 0.7.
25. The porous composite electrode of Claim 21 wherein said conductor means is constructed of an electron conducting material selected from the group consisting of a stable metal, a graphite, and a graphite plastic composite.
26. The porous composite electrode of Claim 21 wherein said ions are protons and said ion conductor means is constructed of an ion exchange polymer selected from the group consisting of polyperfluorosulfonic acid polymers and polyperfluorocarboxylic acid polymers.
27. The porous composite electrode of Claim 21 wherein said electrocatalyst is a noble metal or a metalorganic compound.
28. The porous composite electrode of Claim 27 wherein said electrocatalyst is a noble metal selected from the group consisting of platinum, rhodium, palladium, and alloys thereof.
29. The porous composite electrode of Claim 27 wherein said electrocatalyst is a metalorganic compound selected from the group consisting of cobalt porphyrin compounds, iron porphyrin compounds, cobalt phthalocyanine compounds, iron phthalocyanine compounds, or mixtures thereof.
30. a porous composite electrode for use in an electrochemical cell comprising:
a first zone for conducting both electrons and ions liberated by electrochemical reactions in said electrochemical cell, the rate at which said first zone transports said electrons being greater than the rate at which said first zone transports ions:
a first zone for conducting both electrons and ions liberated by electrochemical reactions in said electrochemical cell, the rate at which said first zone transports said electrons being greater than the rate at which said first zone transports ions:
31 a second zone for conducting both electrons and ions liberated by said electrochemical reactions, the rate at which said second zone transports said ions being greater than the rate at which said second zone transports electrons;
a third zone contiguous with said first and said second zones and having an ion transport rate and an electron transport rate which are approximately equal; and an electrocatalyst for increasing the rate of said electrochemical reactions, substantially all of said electrocatalyst being positioned within said third zone.
31. The porous composite electrode of Claim 30 wherein at least 95 wt % of said electrocatalyst is positioned within said third zone.
a third zone contiguous with said first and said second zones and having an ion transport rate and an electron transport rate which are approximately equal; and an electrocatalyst for increasing the rate of said electrochemical reactions, substantially all of said electrocatalyst being positioned within said third zone.
31. The porous composite electrode of Claim 30 wherein at least 95 wt % of said electrocatalyst is positioned within said third zone.
32. The porous composite electrode of Claim 30 wherein each of said first, said second, and said third zones comprises a mixture of carbon black and an ion exchange polymer.
33. The porous composite electrode of Claim 32 wherein said ion exchange is a polyperfluorosulfonic acid polymer.
34. The porous composite electrode of Claim 33 wherein each of said first, said second, and said third zones further comprises a binder.
35. The porous composite electrode of Claim 34 wherein said binder is polytetrafluoroethylene.
36. The porous composite electrode of Claim 32 wherein said electrocatalyst is a metalorganic compound selected from the group consisting of cobalt porphyrin compounds, iron porphyrin compounds, cobalt phthalocyanine compounds, iron phthalocyanine compounds, or mixtures thereof, said electrocatalyst being supported on the surface of said carbon black.
37. The porous composite electrode of Claim 30 further comprising:
a fourth zone for conducting electrons and being contiguous with said first zone.
a fourth zone for conducting electrons and being contiguous with said first zone.
38. The porous composite electrode of Claim 37 wherein said fourth zone is a relatively thick graphite grid having a mesh of approximately 50-100 X 10-4 cm.
39. The porous composite electrode of Claim 30 wherein said third zone comprises a mixture of a precatalyzed graphite and an ion exchange polymer.
40. The porous composite electrode of Claim 30 wherein said second zone comprises strands of an ion exchange polymer and fibers of heat treated carbon woven together.
41. The porous composite electrode of Claim 30 wherein said first zone comprises strands of an ion exchange polymer and fibers of graphite woven together.
42. The porous composite electrode of Claim 30 wherein the rate at which said first zone transports electrons is approximately twice the rate at which said first zone transports ions.
43. The porous composite electrode of Claim 30 wherein the rate at which said second zone transports ions is approximately twice the rate at which said second zone transports electrons.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US07/109,133 US4804592A (en) | 1987-10-16 | 1987-10-16 | Composite electrode for use in electrochemical cells |
| US109,133 | 1993-08-19 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1307819C true CA1307819C (en) | 1992-09-22 |
Family
ID=22325969
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000580269A Expired - Lifetime CA1307819C (en) | 1987-10-16 | 1988-10-14 | Composite electrode for use in electrochemical cells |
Country Status (4)
| Country | Link |
|---|---|
| US (1) | US4804592A (en) |
| JP (1) | JPH01143151A (en) |
| CA (1) | CA1307819C (en) |
| DE (1) | DE3835336A1 (en) |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3423247A (en) * | 1963-06-07 | 1969-01-21 | Union Carbide Corp | Porous conductive electrode having at least two zones |
| US4602426A (en) * | 1985-06-28 | 1986-07-29 | Union Carbide Corporation | Method of producing a gas diffusion electrode |
-
1987
- 1987-10-16 US US07/109,133 patent/US4804592A/en not_active Expired - Fee Related
-
1988
- 1988-10-14 CA CA000580269A patent/CA1307819C/en not_active Expired - Lifetime
- 1988-10-17 DE DE3835336A patent/DE3835336A1/en not_active Withdrawn
- 1988-10-17 JP JP63261282A patent/JPH01143151A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| JPH01143151A (en) | 1989-06-05 |
| DE3835336A1 (en) | 1989-04-27 |
| US4804592A (en) | 1989-02-14 |
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