CA2099886C

CA2099886C - Method and apparatus for removing water from electrochemical fuel cells

Info

Publication number: CA2099886C
Application number: CA002099886A
Authority: CA
Inventors: Henry H. Voss; David Pentreath Wilkinson; David S. Watkins
Original assignee: Ballard Power Systems Inc
Current assignee: Ballard Power Systems Inc
Priority date: 1991-01-15
Filing date: 1992-01-15
Publication date: 1998-04-14
Anticipated expiration: 2012-01-15
Also published as: DE69219758T2; DE69219758D1; EP0567499B1; AU1164292A; JPH06504403A; JP2703824B2; DE69219758T3; CA2099886A1; US5441819A; WO1992013365A1; US5260143A; AU660446B2; EP0567499B2; EP0567499A1

Abstract

A method and apparatus is provided for removing water accumulated at the cathode (22) of an electrochemical fuel cell incorporating a solid polymer ion exchange membrane (12). Liquid water accumulated at the cathode can be removed by maintaining a partial pressure of water vapor in the hydrogen-containing gas supply below the saturation pressure of water vapor therein such that water accumulated at the cathode is drawn by a concentration gradient toward the anode (20) across the membrane and is absorbed as water vapor into the hydrogen-containing gas supply between the inlet and the outlet. In one embodiment, the partial pressure of water vapor in the hydrogen-containing gas supply is maintained below the saturation pressure of water vapor therein by imparting a pressure drop between the inlet and the outlet sufficient to draw water accumulated at the cathode toward the anode. In another embodiment, the partial pressure of water vapor at the inlet of the hydrogen-containing gas supply is maintained at less than the saturation pressure of water vapor therein. Liquid water accumulated at the cathode can also be removed by maintaining a partial pressure of water vapor in the oxygen-containing gas supply below the saturation pressure of watervapor therein such that water accumulated at the cathode is drawn by a concentration gradient and is absorbed as water vapor into the oxygen-containing gas supply between the inlet and the outlet. Liquid water accumulated at the cathode can also be removed by absorbing water vapor into both the hydrogen-containing gas supply and the oxygen-containing gas supply.

Description

W O ~2~1336~ 2 0 S ~ PCT/CA92/00017 METHOD AND APPARATUS FOR REMOVING W~TER
~ROM ELE~TROC~EMICAL FUEL CEL~S

~IELD OP T~E INV~NTION

The present invention relates to electrochemical fuel cPlls. More particularly, the presen~ invention relates to a method and apparatus for removing water accu~ulated at the cathode o~ electrochemical fuel cell~ employing solid polymer ion exchange membranes.

R~c~r~ouND OF T~E L~V~h~ ION

Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. In electrochemical fuel cell~
employing hydrogen a the fuel and oxygen-containing ~as as the oxidant, the reaction product is water. Such fuel cells generally contain a membrane electrode a~ e~bly (~MEA") con~isting of a solid polymer electrolyte or ion ~Ychange membrane dl~posed between two electrodes formed of porous, electrically conductive sheet material. The electrodes are typically for~ed of carbon fiber paper. The MEA contains a layer of cataly~t at each membrane/electrode interface to induce the desired electroche~ical reaction. The ~E~ is in turn dispo~ed bet~en two plate~ in which at le~st one flo~ passage is engraved or milled. ~hese fluid flow field plates are typically formed of graphite. The fluid flow field Fa~sage direct the fuel and oxidant to the re~pective electrode~, namely, the anode on the - SU~ JTE SHEET

W O 92/1336~ PCT/CA92/0~017

2~9~

fuel side and cathode on the oxidant side. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes.
At the anode, the fuel permeates the elec~rode and reac~s at the catalyst layer to form cations, which migrate through the membrane to the cathode. At the cathode, the oxygen-containing gas supply reacts at the catalyst layer to form anions. The anions formed at the cathode react with the cations to form a reac~ion product. ~n electrochemical fuel cells employing hydrogen as the fuel and oxygen-containing air (or pure oxygen) as the oxidant, a ca~alyzed reac~ion at the anode produoe~ hydrogen ca~ion~ from the fuel supply. The ion exchange ~embrane facilitates the migration of hydrogen ions from the anode to the cathode. In addition to conducting hydrogen cations, the membrane isolates the hydrogen fuel stream from the o~idant stream comprising oxygen-containing air. At the cathode, oxygen reacts at the cataly~t layer to ~orm anions. The anions formed at the cathode reaot with the hydrogen ions that have crossed the membrane to form liquid water as the reaction product.
Perfluorosulfonic ion exchange membranes, such as those sold by DuPont under its Nafion trade designa~ion, must be hydrated or saturated with water molecule3 for ion transport to oocur. It iq generally accepted that such perfluoroqulfonic me~branes transport cations using a "water pumpin~" phen~ ?non.
Water ~ ,ing involves the traAsport of cations in conjunc~ion with water ~olecule~, resulting in a net flow of water from the anode ~ide of the me~brane to the cathode side. Thus, membranes e~hibit~ng the water pu~ping Fh~r -ron can dry out on the anode ~ide if water tran ported along with hydrogen ion~ (protons) is not repleni3hed. In addition, fuel cells e~ploying such membranes require water to be removed from the cathode (oxidant) SU~ JTE: SHE:ET

W O 92/1336~ PCT/CA92/00~17 ~9~3~i3 side, both as a result of the water transported across the membrane from the water pumping phenomenon and product ~ater formed at the cathode from the reaction of hydrogen ions with oxygen.
The accumulation of water at the cathode is problematic for several reasons. ~irst, ~he presence of liquid water in the vicinity of the catalyst layer reduces the accessibility of the catalyst to the reactants, resulting in a reduction in the power of the fuel cell. This phenomenon is ~ometimes referred to as "flooding" of the catalyst site. Secondly, the accumulation of liquid water at the ~athode interferes with the permeation o~
reactan~s through the cathode to the catalyst, again resulting in a loss of power to the fuel cell. Thirdly, the accumulation of liquid water at the cathode can impart physical changes to the adjacent membrane, causing localized gwelling and e~pansion of the ~embrane.
Conventional water removal techniques generally involve conducting water accumulated at the cathode away from the cathode catalyst layer and toward the oxidant ~trea~ exiting the cathode flow field plate. One conventional water re~oval technique is wicking, or directing the ac. lated water away from the cathode using capillarie~ incorporated in th~ cathode. Another related water removal technique employs creens or m~shes within the cathode to conduct water away from the ca~alyst layer. Still another conventional water removal technique ls to incorporate hydrophobic substances, such as polytetrafluoroethylene (trade na~e ~eflon~, ints the cathode sheet ~a~erial to urgc accumulated water away from the cathode. The conventional water removal method~ can be disadvantageous because ~1) conventional methods involve limi~ed access to the catalyst site ~ince accumulated water is re~oved in liquid form, and ~2) the additional pre ence of re~oved water vapor in the oxidant gas ~tream decreases the - - 513~STITUTF SHEElr W O 92~l336~ PCT/CA92/Q0017 ~.'3'.~9~

~ole fraction of o~ygen in the stream.
It has been ~ound that a new type Oe experimental perfluorosulfonic ion exchange ~embranes, sold by Dow under the trade designation XU5 13204.10, does not appear to significantly S exhibit the water pumping phenomenon in connection with the transport of hydrogen ions across the membrane. Thus, the transport of water molecules across these Dow experimental membranes does not appear to be necessary for the transport of hydrogen ionq as in the Nafion-~ype membranes. This absence of water pumping in the Dow experimental membranes avoids the accumulation of transported water at the cathode, and, more importantly, permits ~he trancport of product ~ater across the membrane, in a direction counter to the ~low o~ hydrogen ions across the membrane, for removal on the anode side of the ~embrane electrode assembly. Water removal on the anode side can also be prac~iced with Nafion-type me~branes. ~owever, the degree of water pumping of such Nafion-type me~branes must be con~idered in determining the net flux of water across the membrane.
Thus, removing water at the Inode side of the fuel cell, a~ opposed to ~he cathode side, relieve flooding of the catalyst ~ite since transported water does not accumulate in addition to product water at the cathode. ~oreover, removing water at the anode side of the fuel cell permit~ oxygen to flow ~1nimreded to the cathode catalyqt layer.

O~JECTS 0~ T~E LNV~;N1ION

An objec~ of the invention is to provide a method and apparatus for removing acctlm~lated water from electrochemical fuel cells to overco~e the deficiencies of conventio~al water removal ~ethods and apparatuse~.

- 51J~ JTE SHF.ET

WO g2/13365 PCT/CA92/~On17 2 ~

Another object of the invention is to provide a method and apparatus for removing accu~ulated water from ~lectrochemical ~uel cells to prevent the ~looding of the cathode catalyst laye~
from water transported across the membrane in conjunction with the transport o~ hydrogen ions in addition to product ~ater formed at the cathode.
Ye~ another object of the in~ention is to provide a method and apparatus ~or removing water accumulated at the cathode catalyst layer by drawing it as vapor into the reactan~
gas stream on the anode ~ide of the fuel cell.
A ~urther object of the invention i3 to provide a method and apparatus for removing water accumulated at the cathode cataly~t layer by drawing it a~ vapor into the reactant gas stream on the cathode side of the fuel cell.
lS A still further object of the in~ention i9 to provide a method and apparatu~ for removing water accu~ulated at the cathode catalyst layer by drawing it a~ vapor into the reactant gas strea~s on both the anode ~ide and the cathode side of the fuel cell.

S~MMARY OF T~E l~vhh,ION

These and other objects are achieved by a method and apparatus for removing water ac. lated at the cathode of an electrochemical fuel cell. The fuel cell comprise~ an anode ha~ing a catalyst as~ociated therewith ~os producing hydrogen ions fro~ a hydrogen con~aining gas ~upply. The hydrogen-containi~g gas 3u~ply has an inle~ and an ou~let, and a fluid flow pa~a~e connecting the inlet and the outlet. The passage ~ nicates with the anode along it~ extent. The fuel cell fsrther compriseY a cathode having a catalyst a~30ciated therewith for produciny anions from an oxygen-containing gas - SU~ TE: 5HEE:T

WO 9~13365 P~T/CA92~Q017 supply. The anions react with the cations to form li~uld water at the cathode. A solid polymer ion exchange membrane is disposed between the anode and the cathode. The membrane facilitates the migra~ion of hydroqen ions from the anode to ehe cathode and isolates the hydrogen-containing gas supply from the oxygen-containing gas supply. The membrane is permeable to water. The fuel cell further compri~es an electrical path ~or conductin~ the electrons ~ormed at the anode to the cathode.
The method of removing water at anode side of the fuel cell compri~e~ removing liquid water accumulated at the cathode by maintaining a partial pre~sure of water vapor in the hydrogen-containing gas supply below the saturation pressure of water vapor therein such that water ac~ ated at the cathode is drawn by a concentration gradient toward the anode across the membrane and is absorbed as water v~por into the hydrogen-containing ga~ supply.
In one ~ nt of the method, the partial pres~ure of water vapor in the hydrogen-containing ga3 supply is maintained belo~ the ~aturation pres ure o~ water vapor therein by imparting a pre~sure drop between the inlet and the outlet ~uffici~nt to draw water ac_ lnted at the cathode toward the anode. Such a pres~ure drop can be imparted by (a) a shaped orifice at the inlet, ~b) extending the leng~h of the fluid flow p~s3age, (c) vaxying the cros~-sectional area of the fluid flow 2S pa sage, (d) increasing the friction factor of at least a po~tion of the interior 3urface o~ the fluid flow paa~age, and (e) maint~ining the ~low rate of the hydrogen-containing gas supply in the pa~sage ~ubstantially hig~er than the rate at which the hydrogen-containing gas supply is converted to cation3 at ~he anode.
In another ~ t of the method, the hydrogen-cont~ining ga~ ~upply i~ introduced into the passage at the inlet SU~a 11 a ~DTE S~E:ET

WO g2/13365 P~/CA92/0~017 2 ~ g .S~ $~'3 ~

having a partial pressure Oe water vapor less than the saturation pressure o~ water vapor therein at the operaeing temperature of the fuel cell. In this controlled humidification embodiment, the hydrogen-containing qas supply either ~a) is saturated with water S vapor prior to the inlet at a temperature less than the operating temperature of the fuel cell, or (b) comprises a fi~st portion saturated with water vapor prior to the inlet at substantially the operating temperature of the fuel cell and a second substantially unhumidified portion. In fuel cells where the hydrogen-containlng gas ~upply iq recirculated between the outlet and the inlet, water vapor can be re~oved from the hydrogen-containing ga~ supply prior ~o the inlet by reducing the temperature of the hydrogen-containing gas supply, condensing a portion of the water vapor in the recirculated hydrogen-containing gas ~upply, and removing the condensate from there~aining gaseous constituents. In such recirculated system~, water vapor can also be removed from the hydrogen-containing gas ~upply prior to the inlet by pa~ing the hydrogen-containing gas ~upply over a de~iccant. Water ~apor can al o be removed from the recirculated hydrogen-containing ~as supply prior to the inlet by p2s~ing the hydrogen-containing gas supply over one side of a water permeable membrane having a relatively drler gas ~upply on the opposite ide '~hereof.
In fuel cells wherein the ~e~brane requires the presence of water between the major ~urface~ thereof to facilitate the migration of the hydrogen ions, the water removal method fu~ther co~pri3es maintaining su Eicient water between the major surface3 o~ the me~brane to hydrate the membrane.
In addition to the remoYal of ac~, 1ated water at the anode ~ide of the fuel cell, liquid water ~c_ ~ated at the cathode can also be simultaneou~ly absorbed into the oxygen-containing gas supply by maintaining a partial pre~sure of water - - 5118~TITlJTE SHEE:'r 2 ~

vapor in the oxygen-containing gas supply below the saturation pressure of water vapar therein such that water accumulated at the cathode is drawn by a concentration gradient from the cathode and is absorbed as water vapor into the oxygen-coneaining gas supply.
A ntethod of removing water ac~, lated at the cathode into the oxygen-containing gas supply is also provided. ~he method comprises rentoving liquid water accumulated at the cathode by mainta ning a partial pressure o~ water vapor in the oxygen-containing gast supply below the saturation pres3ure of watervapor therein ~uch that water accu~tulated at the cathode is drawn by a concentration gradient from the cathode and is ab~orbed as water vapor into the o~ygen-cont~ining gas supply between the inlet and the outlet.
lS One embodiment of the cathode ~ide water removal method comprise~ i~tparting a preLtsure drop between the inlet and the outlet of the oxygen-containing ga~ supply ~ufficient to draw water ac~umulated at the cathode into the oxygen-containing gas supply, using ~tub~tantially the same ~tear~t~ e~ployed to i~tpart a pressure drop in the hydrogtn containiny 9~ supply.
A~tother entbodiment cf the cathode side water re~oval ~t~thod co~pri3es maintaining the partial pressure of water vapor at the inlet of th2 oxygen-containing ga~ ~upply at le~s than the saturatio~ pre~ure of water Yapor therein. In fuel cells wherein the oxygen-containing gas cupply is reciroulated, the me~ns for re~toving water vapor from the oYygen-containing gas ~upply prior to the inlet are the saMe a tho~e for removing water vapor fro~t th~ hydrogen-containi ng ga3 ~upply.
A ~oneralized ~tethod of removing reaction product from an electrochemical fuel cell i~t al~o provided. The fu~l cell compri~e3 an anode having a catalyst ~sociated therewith for prodttcin~ reactive cation3 from a first ga~eous reactant supply.

SU~ JTE SHEET

W O g2/1336~ P~T/CA~2~G~017 2~9(~3 ~

The first reactant qupply has an inlet and an outlet, and a Eluid elow passage connecting the inlet and the outlet. The fuel cell also comprises a cathode having a catalyst associated therewith ~or producing anions from a ~econd reactant supply, The anions S react with the cations to ~orm a liquid reaction product at the cathode. An electrolyte is disposed between the anode and the cathode. The electroly~e facilitate~ the ~igration Oe cations from the anode to the cathode and i~olate~ the first reactant supply from ~he second reactant supply. The electrolyte is permeable to the reaction product~ An electrical path conducts the electrons formed at the anode to the cathode.
~ he qeneralized method of removing reaction product from an electrochemical fuel cell method comprises removing the liquid reaction product a~ ated at the cathode by main~aining a vapor pres~ure of reaction product in the first reactant supply below the qaturation vapor pre~ure of reaction product therein such that liquid reaction product ~e~ lated at the cathode is drawn toward the anode through the electrolyte and i~ absorbed as vapor into the fir3t r~actant ~upply. In one e~bodiment of the generalized method~ the partial pre~qure of reaction product in the first reactant ~upply i5 maintained below the saturation pressure of reaction product therein by i~parting a pres3ure drop between the inle~ and the outlet ~ufficient to draw reaction product a _ 1ated at the cathode toward the anode. In another e '~ rt of the generalized method, the partial pres~ure of reaction product at the inlet of the fir~t reactant supply is ~aintained at leqs than ~he sa uration pres~ure of reaction product therein. In addition to removing reaction product on the ansde ~ide of the euel cell, liquid reaction product ac~ 1ated at the cathode can al~o be ~i~ultaneou~ly re~oved at the cathode side by maintaining a partial pressure of reaction produet in the second reactant supply below the saturation pre3sure of reaction - - SU~S'I I a LITE 5HE:ET

WO 92/13365 PC~/CA92/00017 product therein such that reaction product accumulated at the cathode i9 drawn by a concentration gradiene from the cathode and i9 ab~orbed as vapor into the second reactant supply.
A generalized method o~ removing liquid reaction produce solely erom the cathode side o~ the fuel cell is also pro~ided. The method comprises maintaining a partial pressure of reaction product in the second reactant supply below the saturation pressure of reaction product therein such that r action product accumulated at the cathode is drawn from the cathode and iq absorbed as vapor into ~he seoond reactant supply.

B~IEF DESCRIPTION OF T~E D~AWIN~S

FI5. 1 is a schematic diagram of an electrochemical fuel cell illu~trating in cros~-~ection a typical membrane electrode ag9A bly;
~IG. 2 i~ a plot of the ~aturation pre~sure of water vapor as a function of temperature;
FIG. 3 is a plot of the pressure drop a3 a function of hydrogen flow ra~e for (1~ a fuel cell e~ploying a standard anode flow field and (2) a fuel cell e~ploying an anode flow field having half the ~tandard groove depth.
FIG. 4 is a plot of the pres~ure drop a~ a function of flow rate for (1) a ~uel c~ll e~ploying a single pa~sage anode flow field configuration and (2) a fuel cell employing a ~wo passage anode flow field configuration;
FIG. 5 is a plot of thc pressure drop a~ a ~unction of oxidant flow rate for (1) a fuel cell having a si~gle paqsage cathode flo~ field configuration and (2) a fuel cell having a ten passage ca~hode flow field configuration;
FIG. 6 i~ a plo~ o~ ~tabilized cell voltage as a function of hydrogen flow rate in a fuel cell employing the Dow - SU3~ JTE SHEE~

2 ~ '3 ~

ex~eri~en~al ~e~brar.e (trade desiqnation :~US 1320~.10) and the standard anode flow field configuration o~ FIG. 3 operated at a constant 2~0 a~ps (1.0~6 a~peres per square centineter) using humidifiad air and hydrogen reac~ant streams;
.-IG. 7 is a plot of stabilized cell voltage as a runc~ion or hydrogen Clow rate ~or a ~uel cell e~ploying the 3cw exper ~en al ~embrane ar.d the anode flow field configurations of -IG. 3 s;~o-~ing the ef ect of decreasinq the flow field groove de?th in lcwering the peak stabilized cell voltage;
.-IG. 8 is a plot of st~bilized cell voltage as a f-~nc ion of hydrogen flow rate for a fuel cell e~ployed in the e.Yperi~ent of FIG. 6 operated at a constant 250 amps (1.076 ampe_es per square centi~eter) using dry unhumidified ai_ and hu~id-f-ed hydrogen raac~~nt s reams;
1~ -_G. 9 is a plot of cell voltage as a funct-on of hyd-_gen flow -ate for a fuel cell e~ploying a Nafion 117 ~e~brahe ar.d the standa;d anode flow field configuration of rIG.

3 ope_ated at a constant 250 amps (o~a6l amperes per square centi.et-r) using hum-di ied ai~ and hydrogen reactant streams;
.IG. 10 is a standard polarization plot of cell voltase as 2 func_ion of cur_ent for a ~embrane electrode asse3bly ncor?oraring the Dow e~peri~ental ~e brane (trade designation XUS 13204.10);
FIG. 11 is a polarization plot of voltage as a function of current for a fuel cell e~ploying the Dow experimental ~e~brane (t~ade desiqnation XUS 13204.10) superi~posing the standard polarization plot of FIG. 10 with the plot of peak s.abilized cell voltages obtained at each current in the excess hydrogen flow rate experi~ents such as that illustrated in FIG.
6;
FIG. 12 is a polarization plot of cell Yoltage as a function of current density in a fuel cell e~ploying the Dow . ,~

SU~STJTUTE SHEET

W O 92/1336~ PCT/CA92/00017 2~933 ~

experimental membrane (trade designation XUS 13204.10) and the standard anode flow field configuration of FIG. 3, operated at a constant 250 amps (1000 a~peres per square foot) using hu~idified air and hydrogen reactant stream~;
FIG. 7 is a plot of stabili~ed cell voltaqe as a function o~ hydrogen flow ra~e for a fuel cell employing the Dow experimental me~brane and the anode Elow field con~igurations of FIG. 3, showing the effect of decreasing the flow field groove depth in lowering the peak tabilized cell voltage;
~IG. ~ is a plot of stabilized c811 voltage as a function of hydrogen flow rate for a fu~l cell employed in the experiment of PIG. 6 operated at a conYtant 250 a~ps (1000 amperec per square foot) using dry, unhumidified air and humidified hydrogen reactant ~trea~s;
FIG. 9 i3 a plot of cell voltage as a function of hydrogen flow rate for a fuel c011 employing a Nafion 117 me~brane and the standard anode flow field con~iguration of FIG.
3, operated at a con tant 250 a~ps ~800 a~peres per square foot) using humidified air and hydrogen reactant ~treams;
FIG. 10 is a standard polarization plot of cell voltage as a function of current for a ~embrane electrode a~embly incorpora~ing the Dow e~peri~ental ~e~brane ttrade designation XUS 13204.10);
~IG. 11 iq a polarization plot of voltage a~ a function oE current for a fuel cell employing the Dow experimen~al membrane (trade designation X~S 13204.10) superimposing the ~tandard polarization plot of FIG. 10 with ths plot of psak stabilized cell voltage~ obtained at each current in the excess hydrogen flow rate e~periment3 ~uch a~ that illustrated in FIG.
6;
FIG. 12 i~ a polarization plot oE cell valtage as a ~unction o~ current den ity in a fuel cell e~ploying the Dow CAN~FI I Fn 5U~ JTE SHIEEl~

W O 92/l33fi~ PCT/CA92/00017 2~9~$

experimental membrane and different anode and cathade flow ~ield configurations, qhowing the effect of improved water removal on the cathode side, a~ well as water removal on both the anode and cathode sides of the fuel cell;
FIG. 13 is a polari~ation plot of cell voltage as a function of current density in a fuel cell employing the Nafion 112 me~brane, ~uperimposing the standard polarization plot with the plot of peak stabilized cell voltages obtained at each current in the excesR hydrogen flow rate experiments;
FIG. 14 is a top view of an anode flow field plate with a sin~le passage flow field configuration;
~$G. 15 ia a schematic diagram of an apparatus for removing water from an electrochemical fuel cell in which the partial pres_ure of wa~er vapor at the inlet of the fuel and/or oYidant gas streams iq ~aintained at less than the satu~ation pressure of water va~or in the streams through mixing of dry and saturated ga~ treams;
~ IG. 16 is a schematic diagram of an apparatus for removing water from an electrochemical fuel cell in which the partial pr~ur2 of water vapqr at the inlet of the fuel and/or o~idant ga~ ~tream~ is maintained at less than the saturation pre3sure oÇ water vapor in the stream~ through te~perature control of the ga strea~ hu~idifiers.

DETAI~E~ DESCRIPTION O~ T~E DRAWINGS

Turning fir~t to PIG. 1, a schematic diagra~ of an electrochemical fuel cell 10 illustrate3 in cross-section a typical me~bran2 electrode asse~bly 11. Membsane electrode a4~ y 11 comprises an electrolyte in the form of a solid - polymsr ion ~yrh~n~e membrane 12 dispo~ed between a pair of porous electrically conductive sheets 18. In addition to SIJ~ iJ rE SHE :ET

-2~9~3~

?erfor inq an ion exc~.~sqe f~nc' ion, ~e~r~n,e 12 isolates the hyd-oqen-con~alni.sg gas supply rrom t'.e o~fc,en-containing gas supoly .
Us-.~ranes t".at have been found sul~able eor 5 e!ectroche~ic~l ~uel cell applications ars ?erfluorosulfonic ion sxchanc,e ,.e~-~nes suc~ zs thos2 sold b,v ~uPont under the t~~c,e designation ~a~ion and zn sx~eri::lental me~hr~ne sold by Dow unde-t'ne .rade desiqnation :C~iS 13204.10. U.e~rLbr~ne 12 ~ay have a _Aic.~ness o-- about 0 . 03 centi~eters or less rec~use it has been 10 ~ound t'na. ~hinner mer~orar.es slgnif icantly i-?rove fuel cell e f 'ic i ency.
8~.eats 13 are prefe-ably for~ed Or carbon fiber pa?er.
Sh.ests 13 :lav also be for:~ed or other sui~able elect-ically ~_n~uc=ive s;.eet ~ate-i~1, such as c~~-on c!oth, gr~phite c13~h, 15 ca-~on ~oa:~, znd other ?orous carbon-bas2:L ~atarizls. .~ sui_a'o}-:~a.-rizl -c- car_on 'i~er pzper shee~s 13 is sold by Toray undsr the t~ade ~esi~nation "TG~". The prefer~sd c~-~on fiber ?aoer snee~s 13 have a thickn2ss of approxi~ately 0 . 27 mm and are d2si~nated "~P-90". ûther carbcn fiber papers could also be 20 used, such as PC206 f_o~ Stackpole cor?oration or XGF-2C0 by :~ureha. T'~.e ?referred thickness of .he ca_bon fiber paper is in .Ae ange of 0.1 ~ to 0 . ~5 ~m and ?ref2rably about 0 . 3 ~. The carbon fiber paper has a bu13~ density in th2 range of about 0.25 grams per cubic centi:: eter to abcut 0 . 50 g-a~s per c~bic 2 5 centi;~leter .
~ he carbon f iber paper sheets 18 are usually ~ pregnated .~ith a hydropnobic poly~er, such as TE~LON brand polytet-~f!uoroethylene, to render the car on fiber paDer sAeets 1 3 hydr-phobic as well as to i~part additional mechanical 30 str-ngth to sheets 18 so that shee,ts 13 can properly suppor-c ~er~brane 12 . The polytetraf luoroethylene is applied to sheets 18 as a slurry in water, and typically includes a dispersing agent.
. ";

SU:~STlTUT~ S~ f 2~3a33~

.~ resin dis?ersins asen; sald by DuPont under the ~rade ~esignation ~ '0~ 303 ~~~ has been ~'sund to ~e suita~le Cor a??lyins t.ie ?ol~f.a~-afluoroethylene ?olymer.
-ach car_on .'l_er ,oaper sr.se; 13 is providsd ~ h a c-a;ins or !~er OrL' cac~lftic mater~a' for operacion as an anode 20 ar.d a ca;hcde 22, -es?ecti~tely. ~he prefer-ed cataly.ic ~atsrial is ?i3~inu~ in ~inley co~oinu:ed for~, sometimes ~sferred to as p!atinu~ ~lack.
.~ pair o' elec_rically conductive ~low field plates 2 ~0 a~s ?rov ad on ths side of each sheet of car~on fi'er ?aFer 13 aci..g away ~-o ~em~r3ne 12. rlow -'ield plates 24 ars p.q-erably ~or ed of g-a?hice. ~low field ?lates 24 are each -rovided rr~~h at lsast one sroove or _hannel 25 for di_sctins the ~el and o~i~2nt gases to the anode a-.~ cathode respec~i;e'y.
:, Ch2nnels 2; also serte as passageways --or the removal of accl3ulatad .;a;er ~rom cathode 22 and anode 20. .1_~ ~is'd _la~as 2; u~~her serve as ;he conr.ec~ions .o an ex~ernal elec~~ical ci-_uit 2~3 throuqh which ~:e electrons for~ed a; the anode ~l,ow, as indic~ted ~y the arro~s in .-IG. 1.
In o?e-ation, the hydrose.n-cont~ini.ig gas supply (sesi~nate~ uel" in FIG. i) per~eaees c~rbon fiker ~aper shee~
:3 and reac~s at t;Le catalys~ layer of anode 20 to for~ cations (hydroqen ions). The hydrosen ians ~igrate across membrane 12 to cathode 22. .~t cathode 22, the cations react with the oxygen-containing gas supply (designated "oxidant" in FIG. 1) at the catalyst layer to for~ liquid water. ~he hydrogen ions ~hich c-oss the ~embrane to the cathode undergo an elect~ochemical rezction wi;h o~sen at the cathode ca;alyst l~yer to for~ liquid ~ater as ~he reaction product.
'~'hile the fuel cell 10 illust-ated in FIG. 1 contains cnly one me~brane electrode asse~Dly 11, it will be appreciated .hat fuel cell 10 can comprise a plurality of membrane electrode SU~;TlTlJTE S'd~r r 2 ~ 3 ~

A resin dis?ersi.~g agent so!d by DuPont under the ~rade desi~nat on T-.~ 303 ~~ has been f~und to be suitable 'or applying ehe ?olyt~t-afl-lor3ethylene ?oly~er.
-~ch car_an f _er oape_ sr.se~ 13 is provided ~i~h a coati.g or l~e; o c~tal~' c ~atQrial for operation as an anode 20 ar.d a cathace 22 .es?ecti-/e!y. The ?refer_ed catalytic -atarial is ?laein~ in finley co~minuted for~ somet-mes -efe-red to as platinu~ black.
.~ pair o. electrically conductive flow field plates 2 ~o a-e ?rov ~ed on the siJe Ot- e~ch sheet of car on fiber pa~e 13 aci.-g a-~ay f-3~ ~em~rane 12. rlow field plates 24 are ?referablv for~ed of g-a?hite. .-low field plates 24 are each ?rovided ~' h at least one groove or channel 25 for direct ng the el and o~ dant gases to the anode and cathode respectiie'y.
:~ Chan..els 2~ also ser-~e as ?assagewavs -or the re~oval of ~ccl~ulat-d ~ater ~_om cathode 22 and anode 20. -lo~ field p'a.es 2~ ~~r~her serve as the conr.ec_~ons to an external elec~-~cal ci;cuit 23 throu~h ~hlc~ e elec~rons Lor~sd at the anode flow as indic~ted ~y the arro~s in FIG. 1.
In o?e-ation the hydrogen-containing gas supply (designate~ el" in FIG. 1) per-~ea~es car~on fiber paoe~ sheet '3 ar.d reacts at the catalyst layer of anode 20 to for~ cations (hyd-ogen ions). The hydrogen ions ~igrat2 across membrane 12 to cathode 22. At cathode 22 the cations react ~ith the oxygen-containing gas supply (designated ~'oxidant" in ~I5. 1) at the catalyst layer to for~ liquid water. ?he hydrogen ions which c-oss the membrane to the c~thode undergo an elect;ochemical reac~ion wi.h oxygen at the cathode ca.alyst layer to for~ liquid ~ater as the reaction product.
While the fuel cell 10 illust-ated in FIG. 1 contains only one membrane electrode asse~Dly 11 it ~ill be appreciated ehat fuel cell 10 can comprise a plurality of me~rane electrode . ;-c~S~lTUT~: 5~

W O 92/133C~ PCTtCA92/00017 assemblies ll connected in series with suitable separator plates between adjacent membrane electrode a~emblies ll. Such a series o~ assemblies ll is sometimes referred to as a "fuel cell stack".
In fuel cells of the type illustrated in FIG. l, water S accumulates at the cathode as a result of the ~ormation of product water from the reaction of hydrogen ions and oxygen at the cathode. In addition, if the membrane employed in the fuel cell exhibits ~he ~ater pumping phenomenon in the transport of hydrogen ions across the membrane from the anode to the cathode, such transported watsr will accumulate at the cathode along with product water. Such accumulated water must be removed, preferably with the reactant gas streams eYiting the fuel cell, in order to avoid flooding of the catalyst sites and the resulting degradation of ~uel cell per~ormance.
The ability of the reactant gas streams to absorb and carry water vapor is directly related to the te~perature and pres~ure of the gas streamR. Under thermodynamic principles, the ratio of the partial pre~ures of water vapor and reactant gas i9 equal to the ratio of the molar rate~ of flow of water vapor and reactant gas. The molar rate of reaceant gas flow is, in turn, directly related to the operating ~toichiometry of and current generated by the fuel cell.
The saturation pre~sure of water vapor in a reactant gas st~eam i~ very ~trongly dependent upon the te~perature of the gas strea~. FIG. 2 ~hows the ~atura~ion pressure of water vapor a3 a function of te~perature.
During conventional fuel cell operation, a portion of the reactant gas is consu~ed by the electrochemical reaction. If the temperature and pre~sure of the reactant gas ~treamq re~ains constant, and the reactant gas ~trea~ enters the fuel cell fully - saturated with water vapor, then the ccr- ,~ion of a portion of the reactant ga~ would result in the conden~ation of a portion of 'I ~ d ~" '. ~

5~ ulTE 5HEE:T

W ~ 92/1~365 PCT/CA9~/~0017 20~3~

th~ ~ater vapor within the reactant gas strea~. Thus, ~he water ab~orption abllity o~ a reactant gag stream decreases as the gas is consllmed.
If, however, the reactant gas undergoes a pre~sure drop as it passes between the inlet and the outlet of the ~low field, then the water absorption abLlity o~ the ga~ stream will increace. In other words, the water absorption ability of a gas stream increase~ a~ the pressure of the gas stream drGps. As an example, assume the ~ollowing:
mvap = molar flow raee of water vapor in reactant gas ~tr~am;
as = ~olar ~low rate of reactant gas in reactant gas stream;
PVap = water vapor pre~3ure in reactant ga~ stream;
Pga~ = reactant ga~ pressure in reactan~ gas stream;
P~otal = total pre~3ure in reactant gas stream = p ~ P
vap ga~
A~ discu~sed above, under ~hermodynamic principle mvap Pvap Pvap Pvap O = or mvap = mga~ mgas mga Pgas Pgas Ptotal Pvap This value for mvap repre~ent~ the r-Yi amount of water that can be carried in a reactant ga stream at a given te~perature and pre ~ure.
2~ I~ the temperature of the reactan~ ga~ ~tre~ remains a constant 70 degrees C., ~IG. 2 ~hows that the sa~uration pres~ure of water v~por in the 3tream ~ill be 4.5 psia. Thus, where the inlet pre qure of the reactant ~a~ strea~ is 50 pRig (65 psia), CAN~

- - 5UBS7'1T~JTE SHFE: T

2~9~"~

~he ~ater vapor ~irhin the reactant gas stream. Thus, the ~atar a~sorp~ on abi!ity o~ a reactanc gas s~-ea~ decreases as the gas is consumed.
rf~ however, the reactant gas undergoes a pressure drop S as it passes bet~een ~he inLet and the outlet of the flow field, then the ~2~er a~sorption abilit~ or t~e gas stream will increas2. ~n other wor~s, ;he water absorption ability of a gas stream increases as the pressurs o~ ths gas stream drops. .~s an e~a~pla, assume the ~ollowing:
~,~ = molar flow ~ate o~ water vapor in reactant gas stream;
~zu = molar flow rate of -eac-ant gas in reactant gzs stream;
?,~ = water vapor pressu~e in reac ~nt gas stream;
1~ ?z~ = ~eac~ant qas pressure in -eac-ant gas s~~eam;
?~ = total pressure in reactant g2s stream p~,p + P~

.~s discussed above, under the~odynamic principles, ~v P~p Pv~ PV~p = or ~,~ = mzu ~ *
P~ ?~u ?~ P~

This value for m,~ re~resents the maximum amount of water that can ~e carried in a reactant gas stream at a ~iven temperature and pressure.
If .he temperature of the reactant gas stream remains a constant 70 degrees C., FIG. 2 shows that the saturation pressure of water vapor in the stream will ~e 309,989 dynes/c~'. Thus, where the inlet pressure of the reactant gas stream is 4,477,516 5~l~5TlTl3~ S~ T

2099~8~3 dynes/c:l-, .he ~olar =!ow -a:a of water ~apor at t~e inlet ls calculated fro~ the above e~aeions as follows:
~,~ (at inlst) = ~u (at Lnlet) ~ t3o9~9a9/(4~-~77~5l7 -30~,9~9)) = ~u (a~ inlet) * o 074 ~he-e 'he -eac~ant qas st-eam undergoes a 2,066,~92 dynes/cm-~ressure d-~p (to 2,~11,02~ d-~nes/cm-) bet-~een t.ie inLet and .he outlet, then the ~olar flow ate of ~ater vapor at the ouelet can be simila-ly calc~latsd f-o~ _he above equations as follows:
o ~,~ (at ou_let) = ~u (a. outlet) * (309,9-39/(2,411,01~ -309~9a9)) = ~u (at outlet) * 0.1~8 _f the molar flow -ate of reactant gas is sufficiently iiqh tha~ ~he !ate of reac~ant gas consu~ption is negligibl2, ~:.en ~ ~il' ~emain constant be~een the inlet and the outlet, and t.ie above c~lculatlons ssow t:~at tie ~olar ~low rate of ~asar vapor in ~he qas st~eam w l! doubl2 as the qas stream underqoes a 2,060,~2 dvnes/c~- pressure drop oet~een the inlet and the outlet. ~hus, the capacity of the aas s.rea~ .o absorb wate~
~o vzpor siqnificantly inc-easas as the s~ream undergoes a pressure drop bet-~een tha inlet and the outlet.
~he qeneral for~.ula for pressure drop in a pipe containing a flowing fluid, sometimes rererred to as Darcy's equation, ls as follows:
P ~ f * l * v' P = , where 0.0034 ~ d * 2g p = fluid density i~ grams per cubic centimeter, f = friction factor, l = length of pipe in cantimeters, ; ,~

5~i351-l~UTE S~EET

2ns(~ $

-la -~ 3 velocit~ of ~low in centi~eters per second, d -- internal diaoeter of pi?e in centi~eters, and g = accsleration of gr~vity = 975 centime~ers per s~cond per second.

.~ccordinq to Darc~'s for~ula, t~e pressure drop bet~een the in!ec and the ou.let of a gas strea~ in a fuel cell fluid flow '--1d increases ~i~h increasing fluid density, friction factor, flow passaqe lenqth and fluid velocit~. Conversely, the press~re d-op ketween the inlet and the outlet of a gas s~_oam~o dec-eases ~ith lncreasing flow passage (groove) diameter.
.-IG. 3 is a plot of the pressure drop as a function of hydrogen rlow rate for (1) a f~el cell employing a standard anode flow field and (2) a Luel c911 e~ploying an anode flow field havir.g h~lC the standard groove de?th. -IG. 3 shows that the press'.~ OD ~etween the inlet and the outlet or the anode f low field sig"i~icantly increases for a given hydroqen flow rate, consi,.~n~ ~ith Darcy's for ula, when the groove de?th ls decreas2d ~o ~0~ of the original ("standard") groave dapth.
rIG. 4 is a plot of the pressure drop as a function of flow ra~- for (1) a fuel cell employing a single passage anode flow field configuration and (2) a fuel cell employing a two passage anode flow field configuration. In both configura~ions, the c-oss-;ectional area of the individual flow field passages are the sa~e. In a one passage flaw field configuration, the 2~ length of the passage is approxi~ately twice the length of a two passase configuration. FIG. 4 shows that the pressure drop ~et~een the inlet and the outlet of the anode flow field significantly increases fcr a given hydrogen flow rate, consis~ent with Dar~y's for~ula, when the length of the flow passage is increased by reducing the nu~ber of flow passages fro~
two to one and also reducing the total cross-sectional area of ~.,;

S~ TI~T~ 5~

20~98~

the passages.
cIG. 5 is a plo~ of ~he pressu~e drop as a eunction of o~idant ~low rate eor (') a fuel cell having a single pass ca.hods flcw eield con~iguration and ~2) a fuel cell having a ten 5 ?ass cathcde flow .iald configuration. ~s with the one passage and two passase anode conrisur~t~on or FIG. 4, the one passage cathode flow field con~i~uration is approxinateiy ~en t-mes the len~h of the ten passaqe cathode ~low rield conriquration. FIG.
5 shows that the ?ressure drop between the inlet and the outlet of the cathcde 'low field d~a~tically increases for a given oxidant flow rate, consistent with Darcy's fortula, when t~e length or th.e flow oassage is increased, and the total cross-sectior.al a_ea a. tne flow passages decreased, ~y reducing the number of .low ?assaqes from ten to one.
I_ has been de~tonstrate~ that bv passing hydrocen .uel sas t.irough the fuel c-ll at high rlow rates, therebv -esult n~
in a pres,u~3 drop ac-oss the flow f~eld on the anode (ruel) side of the memDrane elect-ode assemgly and an increase in the ~ater absor?tion ability of .he fuel gas s~ream, the perfor~ance of the .uel cell is siqnificantly enhanced. The enhancement is believed due to the drawing of water accumulated at the cathode across t~e membrane by a concsnt-ation gradient and its subsequent absorption as water vap~r into the hydrogen-containing gas supply betwe_n the inlet and the outlet.
2~ ~IG. 6 is a plot of stabilized c811 voltaqe as a function of hydrogen flow rate in a fuel cell employing the Dow experimental me~brane (trade designation XU5 13204.10) and the standard anode flow field configuration of ~IG. 3, operated at a cons~ant 250 a~ps (1.076 amperes per square centimeter) using humidified air and hydrogen reactant strea~s. In the experiment, the following constant operating conditions were maintained: cell temDerature = 70 degrees C., ~,444,320/3,4~4,320 dynes/c~- air/H.
'; '' SU~ 3~U~E 5tl~~~

2~9g~ ~c~

(i~let pressures), air se~ic;~io~et-y = 2.0, ,u" stoichiomeery variablQ, air and u, humld-fied at cell te~peraturs, Oow exoeri~en~al ~e~brane ~US 13204.10. .As shown in FIG. 6, the s.aDi1ized cell volt~ae inc-eases slightly with inc~s~sing hydrogen flow f-o~ an initial value o~ approxi~ately 0.~6 volts, ar.d e~hibi;s a pea.~ staDilized cell voltaqe o~ 0.52 volts at about 37 to 38 lieers ?e; minute of hydrogen. Cell resistance ~egins ~o nc~ease sharply bevond the 'low rate ~or peak voltaae, probably ~ue to t~.e drying out of the membrane as a result o~ the reooval fYo~ the ~.embrane of ~ore water than is produced at t~e c~thode. Thus, F-~G. 6 establishes that employing a hiah flow rate of hyd-oaen sufficient to impart a pressure dr~p to draw ?roduct rat~r ac-oss the ~embrane fro~ the cathode to the anode -~ill ee~.ance 'uel cell perfor~ance by producing a qreater cell 1~ voltage for a gi-en curren~ (250 amps/1.076 amps per sauare cen~i~eter in cIG. 6).
~ -_G. 7 is a plot or s;abili~ed cell voltage as a func~ion of hydrogen flow rate for a fuel cell enploying the Dow e~?erimen.al ~e~brane and the anode flow field confi~urations o.
FTG. 3. FIG. 7 shows that decreasing the flow field groove depth by ~0% signiflcantly lowers the hydrogen flow rate required to achieve peak stabilized call voltage. Thus, FIG. 7 establishes that by reducing the depth of the flow passage, and thereby increasing the prsssure drop between the inlet and the outlet of the hydrogen gas stream, the capacity of the stream to absorb water accumulated at the cathode catalyst layer i5 Lncreased, resulting in a peak stabilized cell voltage at a lower hydrogen flow rate.
~IG. 8 is a plot of stabilized cell voltage as a function of hydrogen flo~ rate for a fuel cell employed in the experiment of FIG. 6 operated at a constant 25~ amps tl.076 amoeres per square centimeter) using dry, unhu~idified air and ':-slJ ~S

2 ~

~h~ miii~iad hyd~oqen reac~nt s~-eams. It was anticipated tAat the use Oe d-y, unhumidlfisd air would have the effec~ o~
educins the ~otal amount of water accumulated at the cachode c~calyst layer because o~ tAe absence ce condensate wate_ Chat ac_umulates at the c~thode when humidified air is e~ployed. .~s snc~-n in .-IG. 3, ;;~e U52 of dry, unhumidified air has t~e effec~
O'- ~2d~c~nq _ne ;~. flcw .~te r~sui~ed for peak s.aDilized cell voltase, ?robably Decause there is less accumula~ed water to remove eron the cathode in the absence of condensate wa~er. The 13 peak stabil1zed cell vol~aqe in .-IG. 3 occurred at appraxi~a~elv 3' liters ?er minute of hydrogen, as oppased to a??roxi~ately 3~-i3 litars ?er ~inute when humidified air was employ~d (see FIG.
6). -t is helieved that the Deak stabili~ed voltaqe did not dec--ase fu-_:.er becausa condensate water ~epresent3 only a smal 1~ ?e~can~qe of -he tot~l amount of water acc-~ulated at the -athcde.
~he 'o1'owinq table s;~ows the hydroqen flow _at-s -sc;ui_ed -~r ?eak s~abilized volta~es at varyinq clrrents, wi~h bc~h ai~ and ~. qases humidified, and wAere the same oper~tin~
2~ cor.ditions as t~e e~?eriment of FIG. 6 were e~ployed:

SU~5~TlJ~: 5~EE~:J

2 ~

.~3L~ l _ d ogen Flow ?~e~ui ed .or Deak Stabilize~ ~oltaqe .it Varying Currents ~2 ~low For ~eak 2eak Cur-ent S~abilized Voltage Voltages (2m2s) (~ i~ers oe ~inu~e~ (volts) 12~ no peak no peak 20a 31 - 3' 0.602 250 36 - 3O 0.602 o 30a 36 - 33 0.~63 35~ 38 - 39 0.~92 '00 ~2 - 43 0.~30 i50 ~6 - ~0 0.360 .~IG. 9 shows the results of an ~. e~cess experiment at 1~ 200 a-ps (0.361 amos per square centimeter) for a fuel cell ooerated at the same conditions as in the experiaent of FIG. 6, wi~h the e~ception that a Nafion 117 me brane was e~ployed in ?l ce of the Dow experimental 2e brane XUS 13204.10. ~s sho~-n in rIG. 9, the s.aDilized cell voltage increased with increasing hydrogen flow, in a manner similar to the Dow experimental membrane. The hYdrogen flow rate for peak voltage occurred at about lS lite-s per minute for the Nafion 117 me brane as compared to about 31-34 liters per minute for the Dow membrane at 200 amps. In addition, the shape of the resistance plot for the 2~ Nafion 117 membrane is similar to the resistance plot at higher currents for the Dow membrane. These differencas can be attributed to the chemical and structural differences bet~aen the Dow and Nafion membranes, particularly the water pumping phenomenon in the Nafion membrane which results in the acc~mulation of transported water at the cathode in addition to ~ . .~

S~ iT~T13T~ S~lC ET

2 ~

or~duct ~a~er ~nd conde~sa~e. water. Tbe mechanism o~ hydrogen ~on t~ansport is believed not to siqnificantly Lnvolve the concurren~ t~ans?or~ of water ~olecules ~roo the anode to the cathode. rhus, .~I5. 9 establishes that the removal o~
acc~ulated water ac-oss the Oe~Dr~ne ~nd into the hydroqen-containing gas suDoly enhances the pe_formance of fuel cells e~ployinq Nafion membranes.
Turning now to FIG. 10, a standard polariza~ion plot for a ~e~brane electrode assembly incorporating the ~ow e.xperi~ental ~e~r~ne (t~ade designation XUS 13204.10) is shown.
The fuel cell was 03erated at a te~perature of 70 degrees C., 3,4~,320/3,~44,320 dynes/cm-air/H. and stoichiometries of 2.0/1.15 air/H,. as shown in FIG. 10, fuel CPll voltage dec~eases 25 cur-ent increases, and drops off dramatically above '~ 250 a~ps. Conversely, fuel cell ~ssistance inc-eases as cu--ent increases, and inc~eases dr~atic~lly above 250 amps. The ~egrzdation in Luel call perfor~ance above 250 a~ps is believe~
due .o flooding of the ca~hode ca~alyst sites as a result of the zc_u~ulation of e~cessive amounts of water from the ir.creased rate of reaction bet-~een hydrogen cations and anions at the c~thode catalyst layer.
FIG. 11 shows a su~mary polarization plot of the peak stabilized cell ~roltages obtained in the hydrogen flow rate experiments using the Dow experimental ~e~rane XUS 13204.10, as su~marized above in Table 1, alon~ with the co~parative standard polarization plot sho~n in FIG. 10. As shown in FIG. 11, a significant increase in cell voltage for a given current is achieved in experiments where high hydrogen flow rates were employed. As indicated previously, use o~ high hydrogen flow rates creates a pressure drop between the inlet and the outlet of the anode flow field such that water accunu1ated at the cathode catalyst layer is drawn across the me2brane, absorbed as water .. ,~ .

SU~51 ITU~ SHF~:T

?.~98~,~

2~

vaDor in~a ~he h~dro~en gas st-ea~, a~d romoved at the anode s e of he fuel cell.
r TG . 12 is a polarlzatlon plot of cell voltaqe as a func~ion of curr2nt donsity ~n a fuel cell enploying the Dow e~peri~ental oe.~brane and differen~ anode and cathode flow field configur tions. T~.e --el cell ~as ooe~ated at a temperature o~
30 d3greos C., Z,066,~92/2,0~6,~2 dynes/cm- air/H. and stoichiorletries of 2.0/1.15 air/:~,. In the standard polarization olot sho-~n at the lowe- portion of rIG. 12, the cathode fluid '0 -low field contained ten passages for the oxidant and the anode fluid flo~ field contained two passages for the hydroqen (fuel~, resulting in a relatively low pressure droD in both the hydrogen anc oxidant gas strea~s. ~n the ~iddle plot of FIG. 12, the peak stabilized cell voltaqes are plot ed aqainst current densisv for tho ~el cell in ~~hich the cathode flow field contained one passage for the oxidant ~nd the anode fluid flow field contair.ed swo passages for the hydrogen (fuel). .~s shown in FIG. 12, a r8~"C ~ion in the number of oxidan~ flow passages, resulted in an inc-ease in cell voltaqe for a ~i;en current density. This inc-sas2 in power is believed due to the absor?tion o~ water vapor into the oxidant qas stream, ir~ibiting the accumulation of water at the ca;hode cat~lyst layer. In the upoer polarization plot of FIG. 12, the peak stabilized cell voltaseâ are plctted against c-lrrent density for the fuel cell in which the cathode 2~ flow field contained one passage for the oxidant and the anode fluid flow field contained one passage for the hydrogen (fuel).
In t.~is confiquration, a reduction in the nu~ber of anode flow oassages frsm t-~o to one, resulted in a further increase in cell voltage for a given current density. This additional increase in power i5 believed due to the concurrent absorption of water vapor into both the oxidant and hydroqen gas streams. FIG. 12 thus demonstrates t~e effect of imoroved water removal on the cathode S~S~ IT~: S~ ET

2 ~

-~5-side, as well as bo~h the ca~hode and anode sides Oe the fuel call.
FIG. 13 is a polarizaeion plot of cell voltage as a ~unction o~ current density in a ~uel cell employing the Nafion 112 membrane. The fuel cell was operated at a temoerature o~ 70 degrees C., 3,~,320/3,~.,320 dynes/cm- air/'~, and seoichio~ee-iss of 2.0/1.1~ air/~.. The standard polarization plot is shown at the lower por~ion of FIG. 13. The peak s.abilized cell voltage obtained at each currsnt density in lo excess hy~rogen flow rate experiments ~s shown in the upper plo~
of F;G. 13. FIG. 13 de~onstrates that increased power is obtained in fuel cells employing Nafion type ~embranes by imposinq a pressure drop betweon the inlet and the outlet of the hydrogen gas stream such that water is absorbed and re~oved from 1~ ~he anode side of the fuel cell.
--G. 1~ is a top view of the anode ~low field plate 90 with a sinqle passaqe 9s e~ployed in the excess hydrogen f'ow r~te experi~ents discussed above.
FIG. 15 is a schematic diagra~ of an apparatus ~or removing water from an electrochemical fuel cell. In the aoparatus of rIG. 1~, fuel gas supply 110 is fed to fuel flow .,eter 112. A portion of fuel gas st-eam 11~ is directed in dry (unhumidified) form .o flow mixing valve 120. The other portion of stream 114 is directed to fuel humidifier 116, the te~perature of which is controlled by ~emperature control 118. The humidified fuel stream exiting humidifier 116 is mixed with the dry fuel stream 114 at flow mixing valve 120. Fuel stream 122 exits f1OW mixing valve 120 and is fed to the fuel cell hydrogen gas supply inlet ~24 with a partial pressure of water vapor less than the saturation pressure o~ water vapor therein such that water accumulated at the cathode is drawn by a conc~ntration gradient toward the anode across membrane 130 and is absorbed as I ., .

SlJ~STlT~lTE SH~:ET

~l~9~5 wace~ ~aoor in~a the hydrosen qas supoly be~w2en inlet 124 and ou~le~ 126 ~-uel ~low requlator 128 regulates the ~low of .2ual ~hro~gh fuel cs11 100 rn c~e apparatls Oe FIG lS oxidant gas supply lt0 s .ed to oxidant flow ~ecer 1~2 ~ portion of oxidant gas s.-eam 1~ is d rected in dry (unhumidified) for~ to flow mixlr~ valve 1~0 The other portio~ of stream 1~4 is direc~sd ~o oxidant humidifior 146 the temperature of which is cont.olled by te~perature c~ntrol 1~8 The humidified oxidant s~ream exiting hu3idifisr 1 6 is mixed with the dry oxidant s.ream 14. at flow m xinq valve 150 Oxidant st~eam 152 exits flow mixinq valve 1~0 and is fed to the oxidant cell hydrogen gas supply inle~ 1~4 wit~ a partial pressure of water vapor less than the satu-~t on prsssure of water vapor therein such that water acolmulat-d at the cathode is dra~-n by a concent-ation gradieno and is absorbed as -~ater vapor into the oxidant gas suoply between inl-t 1~4 and outlet 1~6 Oxidant low regulato~ 153 egulates the flow or oxi~ant througA fuel cell L00 In the appara~us of FIG 1~ flow mixing valves 120 and 1~0 control the mlxing or the dry and humidlfied gas s.reams and .herefore the water vapor absor?~ion capaci~y of the gas s;reams based upon ehe operating conditions and current of ruel cell 100 ~ I5 15 is a schematic diagram of another apparatus for removing water fro~ an electrochemical fuel cell In the appaxatus of FIG 16 fuel gas supply 210 is fed to fuel flow meter 212 Fuel gas stream 214 is directed to fuel humidifier 216 the temperatur2 o~ which is controlled by te~perature controller 218 The saturated ~uel strea~ exiting humidifier 216 is fed to the fuel cell hydrogen gas supply inlet 224 with a partial pressure of waler vapor less than the saturation pressure of water vapor therein at the fuel cell operating te~perature such that water accumulated at the cathode is drawn by a concentration gradient toward the anode across membrane 230 and is absorbed as water vapor in~o the hydrogen gas supply between SU~S~iTUT~ Sl~ T --2 ~

!-!et 22-~ and outlst 226 cuel ~lo-~ re~ulator 22a requlates the ~!ow ce flel throu~h fuel cell 200. In the~aDpara~us of FIG. 16, o~idant qas supDly 2~0 is fed to oxidant flow ~eter 242. Oxidant qas s~rean 1~ is di-ectzd to oxidant humidifier 2~6, the te~pera~u~- Oe ~hich is con~-olled by te~perature controller 2 ~a .
~he hu~idified oxidant stre~m exiting humidifier 246 is fed to the oxidan~ cell hydroqen qas supply inlet 254 with a partial ~ressu-e of water vapor less than the saturation pressure of ~ater vapor therein at the fuel cell operating te~perature such .hat wate_ accu~ulated at the cathode is drawn by a concentra~ion gradient and is absorbed as wat2r vapor into the oxidant gas supply between inlet 254 and outlet 256. Oxidant flow re~ulator 2~3 regulates the flow of oxidant through ~uel cell 200. In the appa-~tus of FIG. 15, te.~perature con~rollers 218 and 24,3 1~ re~ulate the te.~per~eure of fuel and oxidant hu~idifiers 216 and 2,6, resDec~ivelv, the~eby cont-olling the partial pressure of watsr vapor at the inlet of the fuel and oxidant streams, based uDon the oDerating conditions and current of fuel cell 200.
rn either the case of re~oving water at the anode side of the fuel cell or removing water at the cathode side, fuel cell - pe-for~ance is enhar.ced in that higher voltages are achieved at particular current densities than in previous desiqns. The inventors believe that the perfor~ance enhance~ent is due, at least in part, to the ability of the oxidant to ~ore readily contact the catalyst. The presence of accuculated, unremoved water in prior art designs oay have rendered the catalyst inaccessible to a portion of the oxidant. The ~o~e efficient re~oval of water using the present method and apparatus increases the availability of the catalyst to the reactants at the cathode, and thereby increases the voltages achievable at given fuel cell currents. Such an increase in voltage at given currents increases the net power available from the fuel cell. Moreover, the extension of the polarization plot increases the stability of SLJ,~ST~TUT~: S~
-2 ~

the fuel ce!l in operat!ng ranges that were-previo~lsly sensitive to wa~er ~looding. Small changes in current, tempera~ures and membralle elect-ode assem~ly s~ructure no longer result in dramatic changes in voltaqe in the sensi~ive re~ion of the S polarization plot around 200 to 250 amps, namely, the ~nee of the ?olarization plot.
In addition to voltage enhancement, the present method and apparatus provide a diagnostic tool for evaluatins proposed modifica~ions to membrane elec;rode assemblies and associated flow field structures. In this ragard, the hydrogen excess experiments per~it the quantitative deter~ina~tion of effect of a modification on water removal and thus fuel cell per~or~ance.
.-or example, modifications that facili_ate the removal of water ~-om the ca~hcde should effect a lowering of ;he hydrogen flow lS -ate or anode flow field pressure drop at which peak staoilized voltage is achieved.

S~STIT~31 E Sr~ET

Claims

What is claimed is:

1. A method of removing water accumulated at the cathode of an electrochemical fuel cell, said fuel cell comprising an anode having a catalyst associated therewith for producing cations from a hydrogen-containing gas supply, said hydrogen-containing gas supply having an inlet, an outlet and a fluid flow passage connecting said inlet and said outlet, said passage communicating with said anode along its extent; a cathode having a catalyst associated therewith for producing anions from an oxygen-containing gas supply, said anions reacting with said cations to form liquid water at said cathode; a solid polymer ion exchange membrane disposed between said anode and said cathode, said membrane facilitating the migration of cations from said anode to said cathode and isolating said hydrogen-containing gas supply from said oxygen-containing gas supply, said membrane permeable to water; and an electrical path for conducting the electrons formed at said anode to said cathode; said method comprising:
removing liquid water accumulated at said cathode by maintaining a partial pressure of water vapor in said hydrogen-containing gas supply below the saturation pressure of water vapor therein such that water accumulated at said cathode is drawn by a concentration gradient toward said anode across said membrane and is absorbed as water vapor into said hydrogen-containing gas supply, wherein the partial pressure of water vapor in said hydrogen-containing gas supply is maintained below the saturation pressure of water vapor therein by imparting a pressure drop between said inlet and said outlet.

2. The method of claim 1 wherein said pressure drop is imparted by at least one shaped orifice at said inlet.

3. The method of claim 1 wherein said pressure drop is imparted by extending the length of said fluid flow passage.

4. The method of claim 1 wherein said pressure drop is imparted by varying the cross-sectional area of said fluid flow passage.

5. The method of claim 1 wherein said pressure drop is imparted by increasing the friction factor of at least a portion of the interior surface of said fluid flow passage.

6. The method of claim 1 wherein said pressure drop is imparted by maintaining the flow rate of said hydrogen-containing gas supply in said passage substantially higher than the rate at which said hydrogen-containing gas supply is converted to cations at said anode.

7. The method of claim 1 wherein said membrane further requires the presence of water between the major surfaces thereof to facilitate the migration of said cations and said method further comprises maintaining sufficient water between the major surfaces of said membrane to hydrate said membrane.

8. The method of claim 1 wherein said oxygen-containing gas supply has an inlet, an outlet and a fluid flow passage connecting said inlet and said outlet, said passage communicating with said cathode along its extent, and said method further comprises removing liquid water accumulated at said cathode by maintaining a partial pressure of water vapor in said oxygen-containing gas supply below the saturation pressure of water vapor therein such that water accumulated at said cathode is drawn by a concentration gradient from said cathode and is absorbed as water vapor into said oxygen-containing gas supply.

9. A method of removing water accumulated at the cathode of an electrochemical fuel cell, said fuel cell comprising an anode having a catalyst associated therewith for producing cations from a hydrogen-containing gas supply;
a cathode having a catalyst associated therewith for promoting an electrochemical reaction between said hydrogen ions and an oxygen-containing gas supply to form liquid water at said cathode, said oxygen-containing gas supply having an inlet, an outlet and a fluid flow passage connecting said inlet and said outlet, said passage communicating with said cathode along its extent; a solid polymer ion exchange membrane disposed between said anode and said cathode, said membrane facilitating the migration of hydrogen ions from said anode to said cathode and isolating said hydrogen-containing gas supply from said oxygen-containing gas supply; and an electrical path for conducting the electrons formed at said anode to said cathode; said method comprising:
removing liquid water accumulated at said cathode by maintaining a partial pressure of water vapor in said oxygen-containing gas supply below the saturation pressure of water vapor therein such that water accumulated at said cathode is drawn by a concentration gradient from said cathode and is absorbed as water vapor into said oxygen-containing gas supply, wherein the partial pressure of water vapor in said oxygen-containing gas supply is maintained below the saturation pressure of water vapor therein by imparting a pressure drop between said inlet and said outlet.

10. The method of claim 9 wherein said pressure drop is imparted by at least one shaped orifice at said inlet.

11. The method of claim 9 wherein said pressure drop is imparted by extending the length of said fluid flow passage.

12. The method of claim 9 wherein said pressure drop is imparted by varying the cross-sectional area of said fluid flow passage.

13. The method of claim 9 wherein said pressure drop is imparted by increasing the friction factor of at least a portion of the interior surface of said fluid flow passage.

14. The method of claim 9 wherein said pressure drop is imparted by maintaining the flow rate of said oxygen-containing gas supply substantially higher than the rate at which said oxygen-containing gas supply is converted to anions at said cathode.

15. The method of claim 9 wherein said membrane further requires the presence of water between the major surfaces thereof to facilitate the migration of said cations and said method further comprises maintaining sufficient water between the major surfaces of said membrane to hydrate said membrane.