Note: Descriptions are shown in the official language in which they were submitted.
W093/04222 2~3~577 PCT/US91/05420
XIG~ ~TILIZATION SUPPORTED C~TA~YTIC METAL-CONTAINING
GA~-DIFFUSION E~ECTRODE, PROCESS FOR MARING IT,
AND CEL~S UTI~IZING IT
Technical Field
This i~vention relates to methods for making gas-diffusion
electrodes with low or modest loadings of expensive catalytic
metals especially suited for use in solid polymer electrolyte
(s.p.e.) electrochemical cells, e.g. solid polymer electrolyte
fuel cells. An aspect of this invention relates to the gas-
;0 diffusion electrodes (GDE~s) themselves and the electrochemicalcells into which they are incorporated. Still another aspect of
this invention relates to cells (e.g. fuel cells) in which the
solid polymer electrolyte comprises a fluorinated cation-exchange
polymer obtained from an unsaturated fluorinated monomer. Still
another aspect of this invention reiates to a GDE structure
comprising partlcles or fibers or sintered particles or fibers
of an elect~ically conduc~ing or semiconducting material such as
carbon, an electrically-conducting oxide of titanium, or the
like, and a combination of s.p.e. and supported catalytic metal
20 deposited on ~he electrically conducting or semiconducting
material.
Description of the Prior Art
It has long been known in the art of electrochemistry that
the electrolyte o~ an electrochemical cell does not have to be
an aqueous liquid such as sulfuric acid, phosphoric acid, a
solution of an alk~li metal hydroxide, a water solution of a
salt, or the like. Indeed, it is not necessary that the
electrolyte be a liguid o~ any sort, aqueous or non-aqueous.
Solid ion exchange polymers can provide the necessary ionic
30 pathways between cathodes and anodes. Accordingly, many years
ago it was discovered that the ion exchange polymers which serve
as membranes dividing electrolytes into catholyte and anolyte can
... . . .
themselves serve as the cell electrolyte. For an example of the
type of poly~er which can function as a solid polymer
. , .. , . ~ , . , .. ., , .. , , , , ... . . ...... .. ~ .
electrolyte, see U.S. Patent 3,282,875 ~Connolly et al), issued
November 1, 1966. For an example of the use of such ion exchange
polymers as mem~ranes to separate ~he anode and cathode
.. . :
2 0 ~ 55 7 7 Pcr/usgl/0s420_
compartments of an electrolysis cell, see U.S. Patent 4,s44,4s8
~Grot et al), issued October l, 1985. An electrochemical fuel
ce l utilizing a layer of fluorinated acid-containing polymer
_onded to the catalytic surface of an electrode is disclosed in
U.S. Patent 4,610,938 (Appleby), issued September 9, 1986. An
s.p.e./electrode assembly formed by applying to the electrode
structure a fluorinated ion exchange resin is disclosed in
British published application 2,101,160 (Diamond Shamrock Corp.),
published January 12, 1983.
The use of solid polymer electrolytes (s.p.e.~s) in fuel
cells and other electrochemical cells has important implications.
For example, many of the difficulties and hazards posed by liquid
electrolytes are eliminated. Moreover, fuel cells with solid
polymer electrolytes tend to have good ~cold start" properties
and can be operated at very modest temperatures. This low-
temperature feature provides a sharp contrast to the molten
carbonate class Or fuel cells, which are only operative at high
temperatures. Even the technologically mature and reliable
"phosacid" fuel cell (which utilizes an aqueous phosphoric acid
electrolyte) tends to operate more effectively at ~emperatures
above 50 or even 80' C.
Despite the potential advantages of solid polymer
electrolyte fuel cells, their development has been impeded by the
need for ~igh concentrations of expensive catalytic metals (e.g.
precious metals such as gold and silver or noble metals of ~roup
VIII of the Periodic System of the Elements) on the
electrocatalytic surfaces of the fuel cell electrodes. In the
case of state-of-the-art phosacid cells,~ so-called "supported"
catalytic metals can be used on the catalytic surfaces of the
electrodes thereby decreasing the requirements for these
expensive metals to-2.0 mg/cm2 or less, typically ~l mg/cm2.
Supported noble and precious metal catalysts hav~ been proposed
- for- use in the electrodes of solid poly~er electrolyte fuel
cells, but attempts to ùse supported catalytic metals have not
- b~en notably succêssful. As a result, typical state-of the-art
solid polymer elèctrolyte fuel cells utilize unsupported noble
~etals such as "platinum black~. The size of the "platinum black"
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2 0 85 ~ 7 7 PCT/US91/0~2~
particles is relatively large, e.g. 10 nanometers and larger--
at least two or three times (more typically 6 to 10 times) the
size of platinum particles used in ~supported~' platinum
electrocatalysts For particles the size of, say, 10 nanometers
(100 Angstroms)l the specific surface area is small, e.g. 25-30
m2/g. Consequently, the fraction of surface atoms is less than
10% and hence the maximum catalyst utilization is only 10%.
As a result, a state-of-the-art s.p.e. fuel cell electrode
typically contains about 4 mg Pt/cm2, and the catalyst
utilization is low.
A possible reason for the difficulties of using supported
platinum electrocatalysts in solid polymer electrolyte fuel cells
is demonstrated in a study carried out by McBreen, "Voltametric
Studies of Electrodes in Contact with Ionomeric Membranes", J.
Electrochem. Soc. 132:112 ~1985). According to McBreen~s simple
yet elegant study, platinum supported on carbon would not be in
ionic contact with the s.p.e.. As a result, this supported
platinum would not be electrochemically actiYe for oxygen
reduction or hydrogen oxidation.
Subsequent to HcBreen's study, solùtions were proposed for
the problems c~nnected w~th obtaining an electrochemically active
supported platinum suitable for use in a solid polymer
electrolyte cell. It has been suggested, for example, that the
solid polymer electrolyte could be applied as a liquid
composition, due to the solubility of many solid polymer
electrolyte in polar organic liquid solvents, alone or in
combination with water. See, for example, U.S. Patent 4,433,082
(Grot), issued February 21, 1984. It has been further suggested
that the solid polym-r electrolyte solution could be painted or
sprayed onto or otherwise applied to the catalytic face of a ga~-
-diffusion electrode containing supported platinum or some other
supported no~le ~etal.- See U.S. Patent 4,876,11S (Raistnich),
-issued Octob~r 24, 1989 and the abstract entitled "Modified Gas
Diffusion-Electro~es for Proton Exchange Fuel Cells" published
in the Extended Abstracts of the ElectrochemiCal Society of
Boston, Massachusetts (1986). Raistrick has proposed painting
or spraying a solution o~ solubilized NAFIO~ (trade~ark for ion-
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W093/~222 ~ 55 7 7 PCT/US91/0~20
exchange polymer) into a carbon-supported platinum GDE prior to
presslng the electrode against the s.p.e.. The ultimate result
of this approach is a hydrogen/oxygen fuel cell with a
performance (for NAFIO ~ impregnated electrodes containing 0.S mg
Pt/cm2 supported on high surface area carbon) comparable to
state-of-the-art solid polymer electrolyte fuel cell electrodes
containing 4 mg of low surface area unsupported platinum per
square centimeter of the electrode. It may be noteworthy that
in th~ actual practice of the Raistrick approach, a layer of
o platinum is optionally sputtered onto the NAFION-impregnated
electrode prior to its operation in a solid polymer electrolyte
fuel cell. One can only speculate regarding the importance of the
sputtered layer of platinum as compared to the carbon-supported
platinum of the Raistrick electrode, but it may be reasonable to
conclude that in this optional embodiment the carbon-supported
platinum plays a les~er role than the sputtered layer of
platinum.
Sin~e it is extremely difficult to quantify or control the
extent of NAFIO~ impregnation using the painting technique, and
20 since platinum sputtering may also present scale-up problems,
manufacturability, ~urther improvements in solid polymer
electrolyte fuel cells are still being sought.
Th~ ~st o~ applying catalytic metal only to specific
portions of support material has undergone some development
outside the field of s.p.e. fuel cells, e.g. in the case of
phosphoric aci~ fuel c~lls, as described in U.S. Patent 3,979,227
(Katz et al), issued September 7, 19?6.
. . . ~
It has now been discovered that a fully catalyzed, highly
30 efficient ga~-diffusion electrode (GDE) suitable for use in a
solid polymer electrolyte cell can be made from a GDE which is
initially free of noble and precious metals and wherein, after
catslyzation (or further catalyzation) with noble or precious
metal the loading o~ this expensive catalyticjmetal is either
very low (e.g. less than 1 m~/cm2) or is somewhat higher but also
capable of unusually high performance. Prior to treatment
according to the process of this invention, a conventional GDE
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2 ~
~093/04222 PCT~US91/05420
structure is provided, minus the usually employed expensive
catalytic metal (i.e. minus a platinum group metal or the like). ~ -
These conventional structures have an essentially gas-permeable
hydrophobic layer and a catalytic layer or catalyzable layer
comprising a support material made up of fibers or particles or
sintered fibers or particles of an ele~trically conductive
material such as carbon, a non-noble and non-precious particulate
metal (such as nickel), a metalloid, an electrically conducting
or semiconducting ceramic or transition metal oxide, or the like.
It is this catalytic or catalyzable layer to which the relatively
rare or expensive catalytic metal particles and solid polymer
electrolyte are to be applied. For convenience of description,
the GDE structure which is free or essentially free of noble or
precious metals and their compounds (e.g. free of metals and
compounds of Groups VIII, second and third triads, of the ~,
Periodic Table or gold or silver) is hereafter referred to as
"the uncatalyzed GDE", even though this GDE may have some
catalytic acti~ity by virtue of its ~catalyzable layer"
comprising high sur~ace area carbon and/or a non-noble transition
metal and/or transition metal compound (e.g. Ni, a sub-oxide of
titanium, etc.). tE~en~uncatalyzed carbon particles can e~hibit
electrocatalytic activity, depsite a significant lack o~ H-H
bond-stretching capability~) In other words, the ~uncatalyzed
GDE" is a ;ow perfor~ance ~c40 mA/cm2 at lO0 mV) GDE, while the
"catalyzed GDE" $s a high performance GDE, capable of producing
current densitie~ in excess of 50 mA/c~2 at 400 to 600 mV. The
method of this invention comprises:
a. impregnating into the catalytic layer of the uncatalyzed
GDE a solution comprising an ion-exchange polymer (the solid
polymer ~lectrolyte), until this solution has watted the
catalytic layer of the uncatalyzed GDE structure and has
penetrated part/whole way into the cross-sect~on of this
structure (prior to this impregnation step, th~ uncatalyzed GDE
tructure can be totally ~ree of catalytic metal on the surface
of the support material and can be totally free of ion-exchange
polymer),
b. inserting the thus-treated GDE, along with a
., ., , . ~ . ~ : -
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.
W093~04~2 PCT/US91/0~20
~ 7 6 "~~
counterelectrode, in a plating bath containing io~s, at leas~
some of these ions being an oxidized form of the catalytic metal
to be applied, e.g. cations of a metal or metals of Group VIII
or I-~ of the Periodic Table or, less preferably, anions
containing one of these metals in some other oxidized form,
c. applying a pulsed direct c~rrent to t~e thus-treated
GDE, so that well dispersed catalytic metal particles not larger
than about 10 nanometers tl A), e.g. <5 nanometers (~soA) in
average particle size deposit on the support material of the
o catalytic face of the treated GDE, but essentially only on sites
where support material is in contact with ion-exchange polymer
which was deposited there during the impregnating ~tep (step a).
The current applied to the thus-treated GDE is preferably neither
a continuous, constant direct current tD.C.), which would result
in plating or agglomeration effects nor a pulsed-D.C. of short-
duration pulses. This applied current is perhaps best described
as an "interrupted" direct current characterized by on-times of
long duration (>1 second, e.g. 0.1 to 2 minutes), orj if desired,
a single on-time of up to 2 minutes. If low levels of noble or
precious metal deposition (e.g. 0.05 mg/cm2) are desired, the
singl~ on-time ~single interruption) approach can b~ used so long
as the current is interrupted before plating or agglomeration
e~ects ca~ occur.
Because many types of ion-exchange polymer useful as solid
polymer electrolytes will dissolve in polar solvents, the
solution used during the impregnating step can co~prise 0,1 to
50 part~ by weight (preferably 1-10 parts per weight) of solid
polymer electrolyte di~solved in a relatively volatile polar
organic liquid solvsnt such as a low molecular weight
monofunctional or difunctional alcohol, ether, or the like, alone
or in admixture with other polar solvents such as water. It is
preferred that all or most traces of the solvent be removed after
the complëtion o~ the impregnating step so that the solid polymer
electrolyte will be present in the GDE structure as solid,
esse~tially dry material. Because the preferred solid polymer
ele~trolytes used in this invention have relatively low
solubility in water at ordinary cell-operation te~peratures, the
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~093/~222 2 0 ~ 7 PCT/US91/0~20
t~us-deposited solid polymer electrolyte is substantially
immobilized and will not migrate during the pulsed direct current
deposition step; moreover, t~e interface between electrocatalyst, -~
gaseous reactant and solid polymer electrolyte will remain
substantially in the same area during cell operation.
During step c (the interrupted-current, electro-deposition
step), the peak current density is moderately hi~h ~preferably ~ ;
above 5 mA/cm2), but the on-time of the pulse, though relatively
lengthy, should be less than 5 minutes, more preferably less than
_3 2 minutes.
A high-performance GDE made by the process of this invention
can be used as the cathode and/or the anode of a solid polymer
electrolyte cell, e.g. a solid polymer electrolyte fuel cell, or
a fuel cell in which the solid polymer electrolyte coating or
coatings on the electrode or electrodes are in contact with a
liquid electrolyte.
D~TAILED D~8CRIPTION OF T~ INvENTIoN
Although ~his invention is not bound by any theory, the
principles o~ operation of a gas-di~fusion electrode (GDE) are
considered to be fairly well understood and generally relevant
to the principles of this invention. A key aspoct o~ any GDE is
its ability to provide a complicat~d interface between a gaseous
reactant, a heterogeneous electrocatalyst ~such as a particulate
~oble metal), and an electrolyte. Two different kinds of
electrical pathways ~ust ~e provided to this three-way interface,
an ionic pathway (through the electrolyto) and an electronic
pathway (typically through an electrically conductive support
material, in the ca~e of a support~d electrocatalyst). Any
electrocatalyst particlo which is not at a three-way interface
or which is-not provided with the requisite eloctrical pathways
is ~ssentially usel4s~ and not utilized in the dosired fuel cell
reaction. If-the electrocatalyst comprises a r~re or expensive
--metal suc~ as a nobl~ ~etal of Group VIII (e.g. platinum) or gold
or silver, the pre~ence of a su~stantial number of useless
electroc~talyst particles in the GDE structure has serious
i~plications, including under-utilization of one of the most
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W093/W222 2 0 8~ 7 7 PCT/USgl/0~20~
~ 8
expensive components of the electrode structure and perhaps less
than poor optimization or poor overall cell performance.
In the present invention, it is preferred that no Group VIII
or Group I-B metal be present in the GDE structure until the
support material has been thoroughly wetted with a solution
containing the solid polymer electrolyte. The solvent used to
prepare this solution preferably has a vapor pressure at room
temperature comparable to or greater than that of water (e.g. >
10 mm of mercury at room temperature, more typically 10-500 mm
iO of mercury at 23 C.) and will evaporate from the treated
(impregnated) GDE at room temperature or with a very modest
application of heat. The thus-dried, previously impregnated GDE
may not have perfect contact between solid polymer electrolyte
deposits and support material, but there will be at least some
direct physical contact between deposited soli~ polymer
electrolyte and fi~ers or particles or sintered fibers or
particles of support material. Accordingly, during the
interrupted-current electrodeposition of catalytic metal, the
metal catalyst particles (since they must be obtained from ions
such as metal c~tions) will not form on the surface of the
support material unless that suppor~ material is in contact with
an ion~c pathway provided by the solid deposits of solid polymer
electroly~. Moreover, no electrodeposition of the rare or
expenslve catalyt~c metal will occur anywhere along an ionic
pathway unless th~t pathway leads to or comes in contact with
electrically conducting or semiconducting support material.
Because the elQctrodeposition step is carried out with a
interr,upt~d current governed by a wave form and/or on-time
favoring ~all p~rticle formation, e~sentially all of ~he
electrodeposited particles of rare or expensive catalytic metal
will be very small, averaging less t~an lOO A in size, more
typically-less than 50~A. :Accordingly, essentially all of the
electrodeposited c~taly~ic metal,will have extremely high surface
area and will bç provided with both an electronic pathway and an
ionic pathway. All that will be needed to complete the
utilization of the deposited particle will be the presence of the
gaseous reactant. Virtually no noble or preciou5 metal particles
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~093/W222 9 ~ ~ 8~5 7 ~ PCT/~S91/0~20
isolated or insulated from electrically conductive support
material will be present, similarly, virtually no such particles
isolated from solid polymer electrolyte will be present Those
electrc ~posited catalytic metal particles which are present will
generally be utilized when the cell is in operation Loadings
of noble metal (or gold or silver) as low as 0 01 mg per
geometric square centimeter of electrode surface will provide a
useful level of electrocatalytic activity, and electrodes
containing 0 05 m~/cm2 electrocatalytic metal can provide a
performance comparable to advanced to state-of-the-art electrodes
containing about 10 times as much as the same electrocatalytic
metal Indeed, it is a reasonable rule of thumb to consider
electrodes of this invention to be generally about an order of
magnitude better than advanced state-of-the-art in their
u~ilization of rare or expensive catalytic metals such as
platinum, palladium, ruthenium, gold, silver and the like
Electrodes of this invention are up to about 80 times better in
this regard as compared to aYailable solid polymer electrolyte
fuel cell GD~'s having 4 mg Pt/cm2
It is important to the concept of this invention that the
electrodeposition o~ rare or expensive catalytic metal particles
takes place on the support material after the support material
is in place on a fully fabricated (but uncatalyzed or low-
performance) gas-~iffusion electrode and after that GDE has been
impregnated with the ion-exchange polymer (pre~erably a solid
polymer electrolyt-) Accordingly, an understanding of
conventional GDE d-sign provides important background information
which und-rlies th- principles of this invention
T~ VNTR~AT~D GA~-D~FV8TON ~CTaOD~
~he amount of scientific and patent literature which
describes th~ typical gas-diffusion electrode (GDE) i~ vast By
way of a brief summary, most of the~é known GDE structures
.. . ..
contain a hydrophobic polymer obtained by fluorinating a
hydrocarbon poly~er or by polymerizing an unsaturated, partially
or fully fluorina~ed-~~~ monomer (tetrafluoroethylene,
hexafluoropropene, trifluorochloroethylene, less preferably
vinylidene fluoridq, etc ) and a paxticulate and/or ~ibrous,
~ -- , ;, . . ~- , . .
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W093/04222 2 0 ~ 7 lo PCT/US91/0~20~_
relatively inexpensive electrically conductive material which may
or may not be pressed and/or sintered. The preferred electrically
conductive material is usually carbon, which may if desired be
present in two forms: as a fibrous sheet backing material for
structural integrity and as a very finely divided mass of
particles which provides the support for a highly active
electrocatalytic metal. Since finely divided carbon is itself
an electrocatalyst in some circumstances, e.g. oxygen reduction
in alkaline electrolyte, useful low-performance gas-permeable
10 electrodes have been prepared which contain essentially only
carbon and the hydrophobic polymer and no electrocatalytic metal
whatsoever. For high performance fuel cells, however, it is
virtual}y essential that the carbon/hydrophobic polymer GDE be
further catalyzed with a metal of Group VIII or Group I-3 of the
Periodic ~able, particularly a metal of the second and third
triads of Group VIII.
In the typical conventional GDE structure, the concentration
of hydrophobic polymer in the structure increases to a very high
level at one Sac~ and drops off to a relatively low level at the
opposite face. The face provided with the higher level of
hydrophobic polyner is permeable to gases, but the hydrophobic
polymer protects against flooding of the GDE by a liquid
electrolyte. Accordingly, the face with the high concentration
of hydrophobic polymer is the gas-permeable (hydrophobic) face
which has direc~ access to the flow of gaseous reactant. This
hydrophobic face is sometimes referred to as the "gas" side of
the electrode. On the opposite face, where the amount~of
hydrophob~c polymer is relatively low, the amount of finely
divided support material i5 very high. This opposite face can be
referred to as the ~catalytic~ side or face. A greatly magnified
cross-section of this typical GDE structure would reveal a
fibrous mat protected with hydrophobic polymer on the "gas" side
and a mass of tiny support particles on the "catalytic" side.
- It is known to treat the support material with Group VIII
metal at any one of various stages of fabrication of the GDE. For
example, particles of support material can be suspended in a
reaction ~edium oontaining a Group VIII me~al in some dissolved
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~093/~222 2j0~5~ PCT/US9l/0~20
11 `
form (e.g. anions of a salt of chloroplatinic acid), and the
resulting treated s~port particles can be subjected tO an
electrochemical or chemical reduction step, so that tiny
particles of Group VIII metal form on the surface of the support
particles. The GDE should be substantially fully fabricated
(except for the treatment with electrocatalytic metal) prior to
use in this invention.
A variety of fibrous carbon materials (carbon cloth, carbon
paper, etc.) are commercially available for use as the backing
3 material on the hydrophobic side of the GDE. Several types of
very high surface area carbon particles, both graphitized and
non-graphitized are available for use as the support material.
The surface area of these available support materials can range
all the way from as low as So m2/g to more than 1000 mZ/g (e.g.
up to 2000 m2/g). A more typical range of surface area is 200-
1200 m2/g. When the carbon support material is non-graphitized
it may be more subject to corrosion or attack when the fuel cell
is in use. On the other hand, non-graphitized forms of carbon
are more wettable and can be easier to work' with. The
graphitizsd forms of carbon tend to be relatively resistant to
attack in the pr-sence of acidic and even basic electrolytes.
The person skilled in this art is not limited to the use of
car~on, either as a backing material or as a support material.
Other corrosion-resistant, electrically conductive or
semiconducting materials are known and are available or can be
made available in high surface area forms and/or as sheet
material~, ~.g. as sheets of sintered particles. Typically,
th~se corrosion-r-sistant,' porous sheet-like and/or particulate
material~ are inorganic and may even be metals or metalloids.
3C For example, r ney nickel has been used as a support material for
noble metals such as plAtinum. So-called conductive ceramics have
also b~en used in electrode structurès.~ Not 'all of these
materials ~re c~r~mics in the strict sense of the ter~, but some
~ have natural ~ineral analogs such 'as pero~skite. ' Another
interësting c;lass of ~aterials includes the Magneli Phan sub-
oxides of tita~iu~, Tixo2Xl, where x is >2: the suboxides in which
x~4 and S app~ar'~ost conductive. Many o~ these ~aterials are
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W093/04~2 2 0 ~ ~ 5 7 7 12 PCT/US91/0~20!
transition metal oxid~s and some are highly corrosion resistant
as well as being electrically conductive. Typically, these
~ompounds contain chemically combined oxygen and one or more
transition metals of Groups IVB, VB, VIB, or VIII of the Periodic
Table. These materials, in particulate form, can be sintered or
fused to form rigid porous particles which may themselves have
some electrocatalytic activity and may serve as the substrate for
an electrocatalytic metal. Since it is desirable for the sheet-
like backing layer of the GDE to have a hydropho~ic face, it ~s
i~ preferable to use carbon paper or carbon cloth as the sheet-like
element of the GDE, and if conductive ceramics are to be used,
they would more typically be in the form of finely divided
particles having a surface area of at least 10 to 50 m2/g. These
fine particles can be mixed with particles of hydrophobic
polymer, if desired.
It is generally preferred in this invention that the support
material have low resistivity at ordinary temperatures of cell
operation (e.g. from 0 to lOO-C). Metals and metalloids
typically have resistivities less than 1 x 10'4 ohm-cm at
ordinary temperatures (e.g. 10-30- C) and are considered to be
highly conductive. Solid graphite has one or two orders of
magnitude greater resistivity as compared to metals and
metalloids but is nevertheless an excellent conductor, as are
most other- economically attractive forms of carbon such as
activated carbon and carbon fibers. The semiconductors have
greater resistivitie~ as compared to most forms of carbon and are
typically at loast one, in some cases two orders oS magnitude
higher in electrical resistivity. For purposes of this invention,
non-conductor~ can be considered to have a resistivity greater
than 10 ohm-cs at ordinary temperatures. True insulators have
resistivlties in the hundreds or thousands of ohm-cm.
Electrically conductive or semiconducting support materials
having a resistivlty greater than 0.1 ohm-cm are not preferred.
As will be apparent from the foregoing discussion, the
preferred hydrophobic polymers used in the GDE structure are
fluorin~ted ethylene polymer (FEP), polytetrafluoroethylene
(PTFE), polytrifluorochlorethylene, and the like, FEP or PTFE and
~. , . .. : - . :,, - . .
W093/04222 2 0 8~5 ~ 7 7 PCT/~S91/0~20
13
its copolymers being particularly preferred. The essentially
fully fabricated but untreated GDE used as a starting material
in this invention can be treated on one side with hydrophobic
polymer and pressed and/or subjected to sintering temperatures
prior to use in the impregnation step described subsequently.
~XE PROCE88
The process of this invention converts a GDE which is
essentially fully fabricated but can be totaIly free of noble or
precious metals into a high-utilization noble and/or precious
i0 metal-containing GDE suitable for use in an electrochemical cell
provided with an ion-exchange electrolyte, such as a fluorinated
solid polymer electrolyte. An extremely important step in the
process involves impregnation of the low-performance GDE, i.e the
GDE which is essentially free of noble or precious metal. (As
noted above, it is convenient to refer to the starting material
of the process as an ~'uncatalyzed~ GDE, even though an
"uncatalyzed" support material such as high surface area carbon
may have some catalytic activity of its own.) The low
per~ormance GDE, as noted above, has a hydrophobic layer or "gas
side" and a catalyzable layer or side. ~he "gas side" of the GDE
should permit relatively free influx of a reactant gas. The
catalytic layer of the GDE is porous so as to facilitate the
formation o~ complicated interfaces between reactant gas,
electralyte, and el~ctrocatalyst. Accordingly, the impregnation
step of thi~ invention must be carried out with great care so
that the ion-exchange material used as the electrolyte penetrates
i~to the catalyzable layer reasonably well but does not
significantly change the gaq-permea~ility properties of the gas
side o~ the electrode. Accordingly, the impregnation proceeds
from th~ outermost ~urfaces of the catalytic face inward, partway
into the cross-section of the untreated GDE, but generally not
so far as the backi-ng sheet which provides the ~as-permeable
regions for the reactant gas to enter from the gas side. Fully
trea~ed elcctrode~ prepar~d according to this invention have been
- - examined microacopically and it has been found that little or no
solid polymer electrolyte penetration of the bac~ing sheet or
b~cking layer occur~. `
- : -
-
- : ',, , ', `
~ . .:, : , .. .
W O 93/04222 2 ~ 8 ~ 5 7 7 14 P~r/VS91/05420~_
The preferred technique for impregnating selectively, so
that the hydrophobic face is not substantially affected, involves
bringing the catalyzable face of the GDE into face-to-face
contact with a solution containing dissolved ion-exchange
polymer, e.g. the upper surface of the solution or a pre-
impregnated sponge, pad, felt, etc., containing this solution.
The simplest way to carry out this technique is to float the GDE,
catalyzable face down, on the surface of an s.p.e. solution. At
room temperature (15-25~C) or moderately elevated temperatures
below lOO C, impr~gnation proc~eds fairly rapidly from the
catalyzable face inward, but only part way through the cross-
section of the GDE. When the catalyzable face comprises carbon
and fluorinated hydrophobic polymer, after only a few seconds of
impregnation the loading of s.p.e. on the catalytic face of the
GDE is more than 0.5 mg/cm2; after 30 seconds this loading has
increased to 1 to 2 mg/cm2. The loading continues to increase
fox a period of hours (e.g. up to 24 hours), but loadings of
s.p.e. ot 0.5 to 2.0 mg/cm2 (e.g. 1.5 mg/cm2) are adequate, such
loading~ being readily achieved in less than 12 hours.
The concentration of s.p.e. in the solution can range from !~
0~1 to 50 parts by waight per lO0 parts by weight of solution,
preferably 1-10 w~ight-%, based on the weight of solution. This
preferrod~range provides ease of impregnation and rapid drying
of the i~pregnated electrode at ambient temperatures after
impregnation.
The dissolved solid polymer electrolyte is absorbed into the
mass of finely divided support material on the surface of the
; catalytic face through capillary action. Thus, for ease of
impregnating it ~ prefered that carbon support materials have
some sur~ce functionality, (e.g. oxygen-containing groups such
as OH, -C00~, ~tc.), which is already the case for many
commercially-ava~lable high surface area carbons. Graphitized
c~rbon ~aterials can, if necessary, be provided with surface
- functionality through treatmen~ wi~h oxidizing agents or, less
preferably, by el~ctrochemical treatment.
The s~l-ction of solvenk for the solid polymer electrolyte
or ion-exchange polymer can also play a role in promoting ease
- , I , -
.:
.
,. ; . .. . .
~. -
, . . . ~ , .
2 ~ 8 ~ ~ 7 7
`V093/04222 `` `` PCT/US9l/0~20
of impregnation. 3ecause ionic or ionizable functional groups
(e.g. -S03H, -S03Na, -503NHL, etc.) are present in t~ese
polymers, the polymers themselves have some degree of
compatibility with polar organic solvents and with water. As
pointed out by W. G. Grot in U.S. Patent 4,433,082, preferred
liquid organic polar solvents include lower aliphatic (C~-C~,
especially (Cl-C4) alcohols, particularly primary and secondary
alcohols, lower alkoxy-substituted lower alcohols, lower
aliphatic glycols and glycol ethers and diglycol ethers, other
lower aliphatic ethers (such as diethyl ether), cyclic ethers
such as dioxane, and lower aliphatic nitriles such as
acetonitrile. Typical cOD ercially available perfluorinated ion
exchange polymers having sulfo groups and an equivalent weight
in the range of 1025 to 1500 can be dissolved in a mixture
comprising 20-90 wt.-% water and 10 to 80% by weight of one or
a combination of the above-described polar organic solvents.
Formation of the solution from the s.p.e. and the solvent mixture
takes place at about 180 to 300'C.
The thus-fl~at~d GDE is care~ully removed from the surface
20 of the s.p.e. solution so as not to wet the "gas ~ide" (the
fluid-permeable face) of the ~lectrode with solution. As
indicated earlier, impregnation of s.p.e. solution into the gas
side coul~ block gas pores and potentially hinder the gas
transport properties Or tho electrode.
Excess solution is allow~d to drain off the catalytic face,
and the t~lus-treated GDE is preferably cured in air or in an
inert gas at room tQmperature (15-25-C) or elevated temperatures
for 0.5 to 24 hours, e.g. 8 to 16 hours. It is generally most
convenient to imprognate in a single step and air-cure or air-dry
30 in a single s~ep, rather than intersperse a plurality of
incr~ental impregnations and incremental dryings, but
increm~ntal impregnation and drying are also oper~tiYe.
The preferred ~ion-exchange ~poly~ers .have more than 3
repeating units and are solid at room temperature. ~ese polymers
.,
can be e~ther homopoly~ers or copolymers made from two, three,
or ~ven four d~ff~rent monomers. At least one of the monomers
preferably is partially or fully fluorinated, conta~ns At least
, .
. . - - :, . .
WO93/~222 ~ f ~ PCT/US91/0~20J_ ;
16
one u~saturated site, and is provided with a pendent group
terminated with an ionic or ionizable radical. The pendent group
can be a side chain terminated with an acidic, basic, or salt-
like radical, and the side chain can be in the nature of a
halohydrocarbon or a halohydrocarbon ether.`
Thus, a particularly preferred monomer is either
Z-CF(Rf)CF(Rf)O[CF(R,)CF20]n.CRf=CFRf (I)
or .`
Z - ~ CF ( Rf ) ] ~CRf =CFRf ( I I )
where Z is the ionic or ionizable group, preferably an
acidic radical, ,,
the R~ radicals, which are the same or different, are F,
perfluoroalkyl, or Cl, preferably F or lower (C~-C6)
perfluoroalkyl, i
n is a number ranging from 0 ~o 10, and
m is a number ranging from 1 to 10.
Particularly when monomer (II) is selected, by itself or in
combination with another monomer, care should be taken to insure
that the resulting s.p.e. can be dissolved in a mixture of lower
aliphatlc alcohols and water at temperatures below 300'C.
Co-monomer~ useful with monomers (I) and (II) include
CFtRt)~CFR~ ~III)
wherfL ~t is as definQd previously And is pre~erably R or
CF3.
Again, cAre mu~t be taken so that the number o~ repeating
units derlved from monomer (III) does not totally impair ease of
colubility in polar solv-nts.~~~
SinCQ it i5 particularly-preferred that Z have cation-
exchanse propertie~, Z is typically an acidic radical such as
-SO3~. Other ac~ic radicals are disclosed in the s.p.e. art,
including -COOH, -PO3Hz, -P(R,)O~, -B(R~)OH, and -B(ORt)OH, where
R~ i5 as definéd previously.- -
- A p rticularly -preferr~d-commercially ~available s.p.e.
contains-~-fiuorinated rep~ating units; and pendent groups ~?.
ter~inated with -S03H radicals and is sold under the trademark
NA~IO~D by E. I. du~on~ de Nemours and Company o~ Wilmington,
Delaware, U.S.A.;
.,
~093/~ ~2 2 0 8 55 ~ ~ PCT/US91~0~20
~FIO ~ (trademark) is soluble in mixtures of aliphatic alcohols
and water and is available in solution form.
The curing or drying of the impregnated electrode fixes the
s.p.e~ in place as a kind of membrane on the surface of the
càtalytic face which is in highly intimate contact with the
support material and even includes snake-like or tree limb-like
branching incursions into the cross-section of the electrode.
The thus-deposited s.p.e. will not be attacked or re-dissolved
by the a~ueous electrolyte at ordinary or moderately elevated
lO temperatures (e.g. 20-lOO C); accordingly, the intimate contact
with the support material is not adversely affected when the
treated GDE is immersed in a plating bath.
In the electrodeposition step, the GDE (treated with s.p.e.
as described above) is assembled in a parallel flow
electroplating flow cell. The electrolyte or plating bath
typically contains dissolved noble metal or precious metal in a
concentration of about 1-50, more typically l to 20 g/l. When the
s.p.e. impregnated into the catalytic face of the GDE is proton-
exchan~ing (as is the preferred NAFIO ~ polymer), it is preferred
20 that the noble metal or precious metal-containing ions in the
plating bath be cations, e.g. Pt~, Au~, Au~, Ag~, Pd~, Ir~,
Rh~, Ru~, Os~, etc., rather than anions such as AUCl~ or
PtCl~'. If the catalytic metal to be applied to the
"uncatalyzed" GDE is a non-noble metal, Ni~ Cu~ Cu-, or Hg~ can
be employed, or, less preferably, ions o~ manganese, tungsten,
molybdenum, tantalum, or one of the rare earths. The most
preferr~d cation is Pt~ Alloying of metals can be achieved in
situ by using a plating bath containing a mixture of cations.
It is preferred that the plating bath be circulated though
30 the electroplating flow cell at a rate which will minimize mass
transport limitations. The electrodeposition of noble or
precious metal (or noble and/or precious metal alloys) is
prefera~ly carried out at room~ temperature -(15-25-C) or
- moder~tely elevated ~emperatures well ~elow the-boiling polnt of
water.
A pulse current rectifier provides the desired interruptions
in current flow or imposes a wave form with the desired peak
~ i , - , . .. .
wo g3/~4222 2 0 ~ ~ ~ 7 7 PCr/US91/05420~
18
current density, on-times, off-times, and pulse periods (pulse
period = reciprocal of frequency = on-time I off-time).
Interrupted direct current offers important advantages,
particularly in the control of nucleation and growth of noble
metal or precious metal particles, in improved adherence to the
support (carbon) of the electrodeposited metal particles which
are formed, in the hi~h surface area resulting from the uniformly
small size of these catalytic metal particles, and in lower
internal stress. Recently it has also been discovered that
C interrupted D.C. can initiate secondary nucleation, produce very
fine crystallites, and form more porous deposits. Although this
invention is not bound by any theory, these effects can probably
be attributed to the fact that the interrupted or waveform-shaped
current modifies the mass transport processes, enhances the rate
of nucl~ation, and improves the kinetics of the reaction.
High instantaneous current densities (above about 5 mA/cm2,
though less than 1000 mA/cm2, preferably 10 to 100 mA/cm2)
generally a~sist in controlling nucleation and preventing
agglomeration or smooth plating effects. A low duty cycle (on-
20 time di~ided by "pul~e" or on-time period) and a "pulse" period
shorter than 5 minute~ is preSerred. It has been found that the
presence o ion-exchange polymer on the catalytic face also
inhi~its ~gglomeration of metal particles and/or metal particle
growth. The nucleation dQnsity of the electrodeposited metal is
directly proportional to the applied current density, and the
instantaneous current density can be 2 or 3 ord~rs of magnitude
greater than the DC limiting current density. The length of the
on-time i~ beli-v-d to control the growth of deposited metal
nuclei and to h-lp keep the-electrodeposit-d metal particles
dispersed throughout the support material, hence on-times shouid
be short enough to~keep the size of the metal particles below
100A (preferably ~s0A) and to a~oid formation of agglomerated
metal particle? which may lose discr2teness, leading to an
undesired ~gglomeration or film-forming -effect. The metal-
containing ions must traverse the solid polymer electrolyte to
be depositQd on nuclei for the forma~ion of electrodeposited
metal particles. On-ti~es of 0.005 to 5 minutes, more typically
,. " . . . . . . .... ., . ., - .... . ..... .
-, . . . . .. .......... ...... ..
: -. . , , ~ .
' ~'''-' , '''' . '.' ' ,''; '' ", ~ ` ' '' " '' ' ' '` '` . ,
~o g3/04~2 19 ` : 2 ~ ~5 ~ ~ 7CT/US9ltO5420
0.~1 to 2 minutes can be used, about 0.1 to o.5 minute being
especially preferred. Because the solid polymer electrolyte
present in the treated GDE tends to inhibit agglomeration,
plating or film-forming effects, on-times of up to 2 minutes can
be used without obser~ing metallic particle growth to sizes
significantly larger than 50A (5 nm). On-times of less than 0.1
minute may possibly produce particles smaller than 15A (1.5 nm) -
-smaller than necessary for use in this invention. The optimum
size of the deposited catalytic metal particles is generally in0 the range of about 20 to about 40~ (2 t~ 4 nm).
~EC~ROC~EMICA~ C~
The fully treated and fully catalyzed GDE~s of this
invention can be used as cathodes or anodes in electrochemical
cells which utilize reactant fluids fed to the fluid-permeable
face o~ the cathode or anode and have one or more polymeric ion-
exchange electrolyte media disposed between the cathode and anode
to provide the icnic pathway or pathways between cathode and
anode. ~ypically, the membrane-like layer of so;lid polymer
electrolyte formed on the catalytic face of the anode and cathode
may be su~icient by itself to serve as the electrolyte of the
cell, but, if des~red, the electrode may be bondèd onto either
side of a solid polymer electrolyte membrane. Alternatively, a
liquid el~trolyte can be interposed between the solid polymer
electrolytc-coated lectrodes in which case ions (preferably
protons) formed at catalytic sites on the catalytic layer of the
anode are tran~ported through the solid-polymer-electrolyte
impregnation/coating on that layer of the anode to the liquid
electrolyte, and ions (preferably protons) from the liquid
electrolyte are transported through the solid-polymer-electrolyte
impregnation/coating on the catalytic side of the cathode to
catalytic si~s on t~e cathode, where water or the like can be
formed fro~ oxygen or the like. ~referred liquid electrolytes
are aquëous ~olutions of acids, especially inorganic acids such
as phosphoric acid, sulfuric acid, hydrochloric acid, perchloric
acid, etc. ~n this solid polymer~agueous acid~solid polymer
electrolyte configuration, the anions of the aqueous acid do not
co~e into contact with any catalytic sites; th~ only electrolyte
W093~ 2 0 8 ~ ~ 7 7 PCT/US91/0~2~-
in contact with the noble metal or precious metal anode
electrocatalyst is immobilized, fully solidified ion-exchange
polymer. If a liquid electrolyte is used, it is particularly
important that the cathode be s.p.e.-treatPd, even if the anode
is not:.
Techniques for attaching leads or external circuitry to
electrodes of this invention are conventional and known to anyone
skilled in the art. The external circuit from cathode to anode
can include a load (such as an electric motor or electric light),
G a galvanometric measuring device, switching means, etc.
Anodes made according to this invention can provide a
surface for electrocatalytic oxidation of a variety of reactant
fluids, including hydrogen, organic compounds (e.g. alcohols,
alkenes, etc.), ammonia, hydrazine, volatile or volatilizable
hydrides (borane, etc.), oxides of low or intermediate oxidation
state (e.g~ SO2), etc. It is preferred that these reactants be
fed to the fluid-permeable face of the anode in gaseous form
(alone or in admixture with a carrier gas), but techniques for
feeding liquids or ~queous solutions to the "gas" side of a GDE
are known. When the essential purpose of the cell is to produce
electric power, the preferred reactant gases are "fuels" such as
hydrogen, lower alcohols (use~ directly or reformed),
hydrocarbQns (especially when reformed), ammonia or hydrazine,
volatile hydride~, and the like. If the purpose o~ the cell is
to proYide electrochemical synthesis (i.e. limited oxidation),
the preferred rQactant gases are organic compounds, SO2 ~which is
convertible to S0~ or H2SO,), and the like. ,~
Cathodes made according to this invention cAn provide a
surface for electrocatalytic reduction of fluids such as air,
oxygen, halogens, peroxides such as H202, nitric oxide, and
organic compounds. Again,-it is preferred that these fluids be
in a gaseou~ state. If tbe cell is a fuel cell, the preferred
- reactant gas ~oxidant) is air or oxygen.- Electrochemical
synthesis can also take place at the cathode for example, nitric
oxide can ~e reduced to nitrous oxide, hydroxylamine, or ammonia,
aldehydes or nitriles can be reductively dimerized, etc.
' '. '` ' ': " , `' :. ' ,~ ~ .
5 7 7
~Vos~04222 PCT/US9l/0~420
21
Electric power can be produced in the synthesis mode as well
as t~e f~el cell mode, and electric power can also be produced
if the cathode is a GDE of this invention but the anode is an
oxidizable metal such as lead, iron, zinc, etc. operating
temp~ratures of electrochemical cells of this invention can range
from below roo~ temperature (and even below O C, due to the
excellent cold-start capability of s.p.e. fuel cells) to well
above lOO-C, e.g. about lSO-C, which is still well below the
decomposition temperature of most s.p.e.'s.
For fuel cell use, the loading of noble metal on cathodes
and anodes made according to this invention can be far below the
4 mg/cm2 typical of many state-of-the-art H2/O2 s.p.e. fuel cells.
Loadings <2 mg/cm2, more preferably <1 mg/cm2 are sufficient. In
fact, an s.p.e. fuel cell of this invention with a loading of
only O.05 mg/cm2 of pl~_inum exhibited the same performance as a
conventional H2/O2 fuel cell with a 0.5 mg Pt/cm2 loading, i.e.
an order of magnitude improvement over conventional cells and
almost two orders of magnitude of improvement compared to most
s.p.e. fuel cells. Thus, with s.p.e. fuel cells of this
invention, one has the choice o~ using a lesser noble metal
loading to get the same per~ormance as a conventional cell or
obtaining higher performance at a given loadins.
This improvement in the utilization of noble metal in the
GDE catal~st has ~ajor implications ~or govsrnment and industry,
particularly in view of the self-heating feature of s.p.e. fuel
cells which enhanc2s their cold-start and cold weather
performance. Commercial and governmental applications of s.p.e.
fuel cell tochnology include auxiliary or primary power for ships
or submQrsibles, offshore platforms, remote navigation systems,
regenerativo energyreceptors for space, highway bus applications
and other electric vehicles (including automobiles).
Th~ principle and practice of this i m ention ic illustrated
in the following non-li~iting Example.
... ., . - - :
, ,
.. . ..
,, , ,- -
'' . ~` ''' '
' '.`
.. . ' . : .
'
W093/04222 2 ~ 8 ~ ~ 7 7 PCT/US91/05420~
22
EXAMPLE
~ATIO~ Im~eanation of Hiah Surface Area Carbon
Gas Diffusion Electrode tGDE)
A plain carbon, planar gas diffusion electrode was prepared
using high surface area KETJENBLACX EC300J carbon (Azko-Chemie
America) which was sieved through -170 mesh screen. The carbon
was dispersed in an acidified aqueous solution. 30 wt% TEFLO~
binder was added using a dilute suspension of TFE-30 (DuPont) and
the suspension was mixed with stirring and sonification. The
resulting slurry was filtered and transferred onto a hydrophobic
polymer-treated porous carbon paper substrate (Stac~pole PC206)
to form a uniform layer on the carbon paper. The hydrophobic
polymer used to treat the porous carbon paper substrate was
fluorinated ethylene-propylene (FEP). The electrode was then
cold pressed, -hot pressed and sintered at 300 C. The resulting
electrode had a gas-permeable face characterized by a high
concentration o~ FEP and an opposite (catalyzable) face having
a high concentration of KETJENBLACK.
The plain carbon GDE electrode sample was floated, carbon
face down, on the surface of a solution o~ solubilized NAFIO~D in
a shallow beaker. The solution was made up o~ 5 w/o NAFI0~D llO0
EW dissolved in a mixture of lower aliphatic alcohols and water
(Solution Technologie6, Inc.). The NAFI0 ~ was absorbed into the
. .
carbon layer of the catalytic face through capillary action. The
solution impregnation period was 30 seconds.
The electrode was then carefully removed from the NAFI0
solution so as to not wet the gas-permeable side of the electrode
with solution. Excess solution was allowed to drain o~f of the
electrode surfacQ and the eiectrode was subsequently air cured
at- room temperature ~or 12 hr. The NAFIO~D loading in the
electrode was determined by difference (mass of electrode after
impregnation - mas~ of electrode before impregn~tion).
The ~mpr~gnation perlod of jo seconds providQd a lO0 cm2-
size NAFIO ~ i~pregnated gas diffusion elsctrode with a MAFI0
lo~ding of 1.5 mg/cm2. The N~FIO~impregnatæd elsctrode was
ex~mined using scanning electron microscopy (SEM) in order to
gain ~nsight as to the extent of impregnation in the electrode
.,; ~-
, . . .~
., , , .. .. ., , .. , . , . .. ....... ....... ~.. ..
,. , ... - ~ - - -; ~ ; . ' .
208~77
--/og3/~222 II PCT/US91/0~20
23
structure. Micrographs confirmed the presence of NAFIo~
throughout high surface area carbon layer on the carbon paper
substrate. (The NAFIO~treated, exposed surface of this layer is
hereafter referred to as the ~catalytic face".) Regions of
NAFION~ incorporated into the high surface area carbon layer
structure were apparent at all magnifications. However,
examination of the carbon fiber paper backing o~ the electrode
showed no evidence of NAFIO ~ present.
The effect of solution impregnation time on NAFIO ~ loading
was also investigated. A longer solution impregnation period (12
hours) was also used and the resulting NAFIO~ loading was
determined. The longer impregnation period nearly doubled the
NAFIO~ loading in the electrode --from 1.5 mg/cm2 to 2.5 mg/cm2.
The catalyzation step was done using the lower NAFIO~¢~
impregnated electrode.
Catalvzation of NAFIO ~ Coated GDE Usina Interruted D.C.
The NAFIO ~ impregnated plain carbon, planar GDE treated for
30 seconds (1.5 mg NAFIO ~ per cm2) was assembled in a parallel
plate flow cell with platinum screen as the counter-electrode.
The electrolyte was an acid platinum plating bath solution
(Technic ~nc.) with a metal concentration o~ lO g/l. A
peristaltic pump was used to circulate the electrolyte through
the cell to minimize mass transport limitations. The catalyzation
was carried out at room temperature. The electrode size was SO
c~2. A puls~ current rectifier (Rraft Dynatronix) was used to
polarize the electr~de galvanostatically to produce long Hpulses"
(on-time~), i.e. interrupted D.C. of desired pea~ current
density, on-time and duty cycle. The pea~ Hpulse" current density
rang~d from lO to SO mA/cm2; and the on-time of the pulse varied
from 0.02 to 2.0 min. By this procedure, the catalyzable
(uncatalyzed carbon) face of the GDE was catalyzed (or further
catalyzed, if the carbon is considered catalytic) with metallic
platinu~.
The pl~tinum catalyzation procedure involvsd: (l)
installation of the elec~rodes and se~up of the parallel plate
flow cell; (2) starting the pump and circulating the electrolyte
through the cell ensuring to remove any trapped gas bubbles
;, ~ . ' :, -
':
`
W093/04222 2 0 8 ~ ~ 7 7 PCT/US91/0542~
24
inside the cell; (3) the setup of controls at the pulse current
rectifier to generate the desired PC current waveform; (4)
polarization of the electrode for the desired length of time; (5~
removal of the electrode fro~ the cell; (6) washing and drying
of the electrode The Pt particle size and the nature of
electrodeposit obtained was determined by carefully scraping the
electrocatalyst surface and transfering the powder ~o the TEM
support grid for analysis.
The loading of Pt catalyst was determined gravimetrically
1o by placing a known area of an electrode sample in a tared quartz
crucible and transferring it into the heated zone of a tube
furnace. The sample was oxidized in air at 950-C for 2 hr. The
weight of the ~t residue (i.e., Pt) was determined by difference
using an ultra-microbalance and the loadin~ of the platinum was
determined from the weight and area measurements. An electrode
sample of a known platinum content was first used to check the
val~dity of the test procedure.
Results of transmission electron micrographs (SEM's) are
summarized below.
.
~.. ..
.. . -- - ~ . .
: , : ,. -
- ~o 93/~2n 2 ~ 7 7 PCT/US91/0~20
Peak current
On-Time min Densitv (mA/cm2) Resul~
-- All carbon and NAFIO~,
no Pt in samples
. .
conventional fuel 10 wt -% Pt, 0 5 mg
cell electrode, Pt/cm2, average Pt
colloidal deposition particle size about 20
of Pt on high surface (2 nm)
area carbon
0 01 10 Pt particle size = 5 to
10 ~ (0.5 - 1 nm) ' `
0 5 10 Dispersed Pt, average Pt
particle size is 20-30A
(2-3 nm)
2 0 10 Average Pt particle size
still about 20 - 30
(2-3 nm)
_~ '.
Polarlzation ~-asur~ments were dono in a half cell in
sulfuric acid-~ ~edia to determine the activity of our
-electrocatalyst for th~ tuel cell reactions of interest, namely,
~oxygen reduction and hydrogen oxidation
The hal~ cell testing procedure utilized a sweeping method
for evaluating ~l-ctrodes The current-was swept at 1 0 mA/s
-- betw~en-pre~t li~it~ whiIe the potential was monitored The
el~ctrod~ t~ting--was- done in 2 5H sulfuric acid and the
- temper~tures rang~d ~ro~ 2~ to 70 C ~A PAR (Princ-tsn Applied
Research) ~odel 363 po~entiostat was used in the galvanostatic
~ode for the te~t Readings of the current, electrode potential
and iR potential were recorded every 5 seconds during the first
'.
.
W093/~222 2 0 8 ~ 7 ~ PCT/US91/0~2 ~
26
30 seconds of the scan, then every 10 seconds thereafter. T~e
iR-corrected electrode potential was plotted versus current
density on the computer's screen in real time and the
polarization data stored for later use.
Polarization measurements were done with oxygen, air and
hydrogen. Plots of iR-corrected potential versus current density
and mass activity were generated from the polarization data and
the catalyst loading data for the different gases to examine the
electrode performance and the electrochemical activity of the
electrocataly5t.
Gravimetric analysis showed that the conventional 10 wt.-~
Pt-on-C electrode loaded with 5 mg/cm2 of Pt+C (a theoretical
loading of 0.5 mg Pt/cm2) had an actual loading of 0.53 mg
Pt/cm2. The NAFIO ~ treated electrodes of this invention treated
further in the Pt-catalyzation with pulsed-D.C. having a peak
current density of 10 ~A/cm2 and a pulse period of 2 minutes,
assuming an __lQ% a current efficiency, should have had a
theoretical Pt loading of approximately 0.1 mg/cm2, Gravimetric
analysis showed an actual loading o~ 0.05 mg Pt/cm2.
Comparison plots of iR-corrected oxygen reduction
performance on oxygen and air for the above-described GDE of this
invention ~0.05 mg Pt/cm2) compared to a standard electrode
prepar-~ with a NAF~O ~ impregnatod 10 percent platinum on carbon
electrocataly~t (~0.5 mg Pt/cm2) lndicated no difference in
oxygen reduction performance for oxygen between the standard
electrode ~nd the electrode of this invention even though the
electrode oS th~s invention contains one tenth the platinum as
the standard l~ctrode. T~e perrormance o~ both el~ctrodes is
nearly eguiY~l~nt on air ~ well, with the electrode of this
invention only slightly worse than t~e standard electrode at
higher current densitios. - -
An logous half cell polarization eXperi~entc were conducted
to measur~ the hydrogen oxidation activity of the electrode of
this in~ention in co~parison to the standard NAFIO ~ i~pregnated
10 p~rcent pl~tinum-on-carbon electrocatalyst. A co~parison plot
of iR-c4rr¢ct~d hydrogen oxidation performance versus current
density show~ si~ilar perfor~ance behavior for both the electrode
., .. ... . ~ ~ . . . ,,, ... .. . . . . ...... ~
-. - - -, - . -- . s,
,: ... . , .. : - ~.: ,- ..
, . . :
,- . ,. , :
~093/~222 2 0 8 ~ 5 ~ 7 PCT/USgl/0~20
27
of this invention and the standard electrode although the
standard electrode contains ten times more platinum. The above-
described results are attributed to a much higher utilization of
the platinum due to the more selective catalyzation which more
effectively locates the platinum catalyst particles at the three-
way interface of the GDE's ~f this invention. The implication of
this demonstrated increase in the utilization of the platinum
catalyst lies in the eventual production of less expensive s.p.e.
fuel cells through either the decrease of th~ catalyst loading
or the increase in cell performance with electrodes of this
invention. In practicing this invention and obtaining this high
utilization of electrocatalytic metal, results are reliably
reproducible, hence good quality control is obtainable.
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