Note: Descriptions are shown in the official language in which they were submitted.
. - CA 02238945 1998-OS-28
- .
GAS DIFFUSION ELECTRODE FOR
POLYMER ELECTROLYTE MEMBRANE FUEL CELLS
The invention relates to polymer electrolyte membrane
fuel cells, in particular gas diffusion electrodes for
fuel cells or electrolysis cells, a method of making a
gas diffusion electrode for fuel cells or electrolysis
cells, a method of coating gas diffusion electrodes with
a catalytically active layer, and a method of making a
membrane and electrode unit.
In polymer electrolyte membrane fuel cells, a gas dif-
fusion mat is used as electrode between polymer electro-
lyte membrane and currant collectors, such as e.g. bi-
polar plates. This mat has the function of dissipating
the current produced in the membrane, and it has to
allow diffusion of the reaction gasses to the catalytic
layer. Moreover, the gas diffusion electrode has to be
hydrophobic at least in the layer facing the membrane,
in order to prevent that water formed in the reaction
process floods the pores of the gas diffusion electrode.
For many applications, for example aerospace, it is im-
portant furthermore that the materials employed for
building the cellstacks are of light weight and consume
little space. An as inexpensive as possible fabrication
of the materials is always of interest.
So far, mats of graphitized fabric are used for such gas
diffusion electrodes, which are available from a density
of 116 g/mz. The gas diffusion mats of graphitized
CA 02238945 2005-04-12
CA 02238945 1998-05-28
' ~ '
-2-
fabric often do not permit sufficiently good diffusion
of oxygen, in particular Oz from the air under low
pressure, and moreover they are relatively heavy. Fabri-
cation thereof r~.ecessitates high temperatures, resulting
in a correspondingly high consumption of energy and high
prices.
It is the object of the invention to make available a
gas diffusion electrode .which is inexpensive to manu-
IO facture and of light weight and which permits good dif-
fusion of oxygen, in particular from the air under a
slight pressure above atmospheric, and which furthermore
displays the required high electrical conductivity and
is hydrophobic.
IS
A further object of the.invention is to make available a
polymer electrolyte membrane fuel cell comprising such a
gas diffusion electrode_
20 An additional object of the invention consists in in-
dicating a method of making such a gas diffusion elec-
trode. .
Another obj ect of the a.nvention is to make available a
25 method of coating a gas diffusion electrode with a cata
Iytically active layer.
A still further object of the invention consists in in-
dicating a method of making a membrane electrode unit.
35
CA 02238945 2005-04-12
- CA 02238945 1998-05-28
-3-
Figure 1 shows a polymer electrolyte membrane fuel cell.
The gas diffusion electrodes according to the invention
are suitable. ~or fuel cells, in particular polymer elec-
trolyte membrane fuel cells, and polymer electrolyte
IO rnernbrane electrolysis cells. In polymer electrolyte fuel
cells, the gas diffusion electrodes according to the
invention can be utilized both as anode anal as cathode,
whereas in electrolysis cells they can be employed only
on the hydrogen side, as oxidation would take place on
the oxygen side. The gas diffusion electrodes according
to the invention can be used in especially advantageous
manner in polymer electrolyte membrane fuel cells using
liydrogeri -as -fuel gas arid- air -as 'oXZdan t ' arid - opc~z-atei3. at
low pressure of less than 0.5 bar, preferably less than
0.1 bar. Particularly preferred are operating pressure
differences in the order of magnitude of about L0 mbar.
As starting material for the gas diffusion electrodes
according to the invention, a very lightweight carbon
fiber .nonwoven fabric, preferably of carbonized fibers
is used. Particularly suitable are carbonized carbon
fiber nonwoven fabrics having mass-area ratios of up to
60 g/m2, typically 30 g/m2. Carbonized carbon fibers can
be produced with much lower expenses than graphitized
fibers, since their manufacture requires considerably
lower temperatures_
For fabrication of a gas diffusion electrode according
to the invention, a suspension a.s prepared first from
soot and at least one liquid, e.g. a suspension of
Vulcan. XC '72 and water. For reducing the surface
- CA 02238945 1998-OS-28
-4-
tension, additives such. as e.g. isopropanol may be
added. Such additives improve the making of the sus-
pension since they effect better wettability of the soot
and thus better miscibility of soot and suspension
liquid. This liquid is mixed intensively with a suspen-
sion of PTFE in at least one liquid, preferably water.
PTFE and soot are employed preferably in a mass ratio of
1:10 to 1:1_ Typical is 25 to 40 0 of PFTE related to
the weighed-in amount of soot. The carbon fiber nonwoven -
IO fabric is impregnated with this mixture, or this mixture
is evenly applied to the carbon fiber nonwoven fabric,
respectively, so that the carbon fiber nonwoven fabric
is impregnated in substantially homogeneous manner_
Thereafter, the carbon fiber nonwoven fabric is dried,
with the temperatures required for drying being depen-
dent upon the type of the liquids used. As a rule,
drying at higher temperatures than room temperature is
of advantage, e.g. about 110 °C or above in. case of
mainly aqueous suspensions. Impregnating and drying of
the carbon fiber nonwoven fabric can be repeated once or
several times. The thus impregnated carbon fiber non-
woven fabric is finally sintered at a temperature of at
least 200 °C. Preferably, sintering takes place for half
an hour at temperatures of about 300 °C to 400 °C.
The thus obtained electrode of carbon fiber nonwoven
fabric is particularly homogeneous, porous and light,
but nevertheless is mechanically very stable. It permits
better diffusion of oxygen than the graphitized fabrics
used so far and due to its lower weight as compared to
graphitized fabrics contributes in reducing the overall
weight of fuel cells. An important factor furthermore
consists in the savings in the manufacturing process of
the gas diffusion electrodes according to the invention
as compared to graphitized fabrics: the manufacture of
the carbon fiber nonwoven fabrics requires lower tem-
CA 02238945 2005-04-12
CA 02238945 1998-05-28
_$_
peratures than in case of graphitized fabrics, which.
results in savings of energy and costs. Due to their
share of soot and polytetrafluoroethylene, the gas dif-
fusion electrodes according to the invention have the
required high conductivity for electrical current, and
they are hydrophobic.
The gas diffusion electrode fabricated as described
hereinbefore, can now be installed in a polymer electro- ..._
lyre membrane fuel cell. Due to the fact that the elec-
trode does not contain a catalytically active layer, a
membrane coated with a catalyst has to be used. As an
alternative, it is however also possible to coat the gas
diffusion electrode according to the invention with a
catalyst. The catalytic layer has to be gas permeable
and have electrical conductivity as well as H~'-ion con-
ductivity and, of course, has to catalyze the desired
reaction. These properties are obtained with a very thin
layer containing a mixture of ion conductive material,
e.g. nafion'~polymer and noble metal catalyst. The pre-
ferred noble metal catalyst used is platinum on carbon
carrier. A very favorable platinum load is about 0.2
mg/cmz of the gas diffusion electrode. The mass ratio of
platinum on the carbon carrier to nafion typically is in
the range from 2:I. to 4:1. The carbon carrier is elec-
trically conductive and porous, so that sufficient con-
ductivity and gas permeability of the catalytic layer is
ensured. The polymer at the same time serves as a binder
for the layer. The low layer thickness in the order of
magnitude of about 20 ~,m ensures short transport. paths
for electrons, H'~-ions and gas.
According to the invention, a gas diffusion electrode is
coated with a eatalytically active layer as follows:
noble metal catalyst on carbon carrier, e.g. 20 % Pt,
80 % C, is mixed intensively with ion conducting polymer
- CA 02238945 1998-OS-28
-6-
in solution or suspension. As ion conducting polymer,
e.g. nafion dissolved in alcohols and water may be used.
The suspension possibly may be diluted with a suitable
liquid, e.g. water. The suspension of catalyst and poly-
mer i.s applied onto a surface of the gas diffusion elec-
trode, and the layer applied is then dried. In most
cases it is advantageous before application of the sus-
pension to evaporate part of the alcohols, possibly at a
slightly increased temperature. Evaporation of part of
the alcohols serves to increase the surface tension of
the suspension, for in case of a too low surface tension
there is the risk, that the impregnated carbon fiber
nonwoven fabric will soak in the suspension. However,
the aim consists in obtaining a thin catalyst layer on
the surface of the impregnated carbon fiber nonwoven
f abric .
The catalyticall:y active-layer-can be -a~piied e:-g: icy
spraying, screen printing or by application with a
brush. Particularly good adhesion of the catalytically
active layer is obtained when the application and drying
steps are repeated once or several times. The formation
of cracks in the layer can also be reliably avoided in
this manner. The catalytically active layer need not
necessarily be homogeneous through its entire thickness,
rather it is in most cases more favorable when there is
a concentration gradient with respect to electrically
and ion conducting material perpendicularly to the
layer. When the layer is applied in several steps, it is
easily possible by selection of the suitable 'concen-
trations of the respective suspension of carbon and po-
lymer to obtain layers which are rich in carbon on the
carbon fiber nonwoven fabric, but rich in polymer on the
side facing the membrane later on. Such a distribution
of electron conducting carbon and ion conducting polymer
is of advantage in so far as it is adapted to the dif-
CA 02238945 1998-OS-28
ferent concentration of electrons and ions in the cata-
lytically active layer. For example, when looking at the
anode, the fuel gas passing from the carbon fiber non-
woven fabric into the catalytically active layer is
ionized in increasing manner on its' path- through the
layer towards the polymer electrolyte membrane, so that
the concentration of ions and thus the need for ion con-
ducting material in portions of the catalytically active
layer near the membrane is higher than in the portions
adjacent the carbon fiber nonwoven fabric. The concen-
tration of electrons and thus the need for electron con-
ducting carbon, however, is lower in the portions near
the membrane, since it is not the total quantity of the
released electrons that passes these portions, but only
the electrons released during the ionization of the
neutral remaining gas that is still left in the respec-
tive portion. Analogously therewith, the oxidation gas
is increasingly ionized in the catalytically active
layer on its way through the layer by absorption of
electrons, so that here too, the ion concentration is
higher in portions near the membrane and the electron
concentration is lower than in portions remote from the
membrane.
The method can be utilized with any non-catalyzed gas
diffusion electrode.
The gas diffusion electrode can be reinforced by a con-
ductive grid. Particularly suitable for the grid is a
nickel square mesh fabric having a mesh aperture of 0.4
to 0.8 mm and a wire gauge of 0.12 to 0.28 mm. Nickel is
a favorable material in so far as it is chemically inert
for the conditions in the fuel cell and has a con-
siderably lower transition resistance to impregnated
carbon fiber nonwoven fabric than e.g. stainless steel.
Upon assembly of the fuel cell, the grid is installed on
CA 02238945 1998-OS-28
_g_
the side of the gas diffusion electrode facing away from
the membrane. The function of the grid consists in en-
suring very good current dissipation from the gas dif-
fusion electrode and in urging the electrode uniformly
against the membrane at the same time.
If necessary, it is also possible to combine several
carbon fiber nonwoven fabrics after impregnation and
sintering so as to form a gas diffusion electrode. The
use of several impregnated carbon fiber nonwoven fabrics
on top of each other reduces the risk that the grid
and/or parts of the current collectors, e.g. of the bi-
polar plates, push through up to the membrane and damage
the same. Typically, two to three impregnated carbon
fiber nonwoven fabrics are combined with each. other. The
use of more than four carbon fiber nonwoven fabrics on
top of each other may result in a no longer suff~.cient
gas diffusion, which makes itself felt in the U-I-cha-
racteristic. For obtaining good adhesion of the impreg-
nated carbon fiber nonwoven fabrics to each other, the
desired number of impregnated and sintered carbon fiber
nonwoven fabrics can be subjected to pressing, with
pressures of up to 500 bar and temperatures of up to
400 °C being applied preferably. Typical conditions are
a pressure of about 200 bar and a temperature of about
1a0 °C. Coating of a surface of such a gas diffusion
electrode with a catalyst is carried out best after
pressing.
The gas diffusion electrode according to the invention
can be combined with a polymer electrolyte membrane so
as to form a membrane and electrode unit. Depending on
whether or not the gas diffusion electrode carries a
catalytically active layer, either a membrane without or
with a catalytically active layer has to be used. For
fabrication of a membrane and electrode unit, a gas dif-
CA 02238945 1998-OS-28
_g_
fusion electrode which may be composed of one or several
impregnated carbon fiber nonwoven fabrics, is disposed
on one side ofa polymer electrolyte membrane present in
its H+-form and is then pressed on at pressures of up to
500 bar and temperatures of up to 250 °C. Typical con-
ditions are a pressure of about 200 bar and a tempera-
ture of about 125 °C. When the gas diffusion electrode
contains the catalytically active layer, it must be
pressed onto the membrane such that the catalytically
active layer is in contact with the membrane. This can
be performed for both sides of the membrane, so that
both the anode and the cathode can be fabricated in this
manner_ By such pressing-on, electrical contact between
the catalyst layer on the membrane and the carbon fiber
nonwoven fabric or between the catalyst layer on the
carbon fiber nonwoven fabric and the membrane, respec-
tively, is improved considerably as compared to loose
clamping together thereof. Prior to installation of the
membrane and electrode unit in a polymer electrolyte
membrane fuel cell, the gas diffusion electrodes on the
side facing away from the membrane can be reinforced by
the addition of a grid.
A particularly preferred embodiment of a fuelcell with
a gas diffusion electrode according to the invention is
shown in Fig. 1. Anode 1 and cathode 1' are constituted
by impregnated carbon fiber nonwoven fabrics 3 and 3'_
Anode 1 and cathode- 1', on their sides facing the poly-
mer electrolyte membrane 5, each carry a catalyst layer
4 and 4', respectively. Anode 1 and cathode 1' together
with polymer electrolyte membrane 5 constitute the mem-
brane and electrode unit 6 and 6', respectively. Anode 1
and cathode 1', on their sides facing away from the mem-
brane, are reinforced by conductive grids 2 and 2', re-
spectively. The bipolar plates 7 and 7' confine the cell
on the anode and cathode sides, respectively.
CA 02238945 2005-04-12
CA U2238945 1998-05-28
- 10-
An example of the fabrication of a gas diffusion elec-
trode according to the invention:
45 g soot (Vulcan~~XC 72) is suspended in 450 ml water
and 495 ml isopropanol. This suspension is mixed in-
tensively with 32.27 g of a PTFE suspension (60 % Hosta-
flon fibers in aqueous suspension). The resulting mix-
ture is .evenly brushed onto a carbonized carbon fiber
nonwoven fabric (30 mg/mz), and the nonwoven fabric then
is dried at a temperature of about 70 °C_ Brushing on
and drying is repeated twice. After the last drying
step, the impregnated carbon fiber nonwoven fabric is
sintered for about 30 minutes at 400 °C. One thus ob-
tams a carbon fiber nonwoven fabri-c that is uniformly
impregnated with Vulcan~XC 72 ar~d Hostaflon.
An example for coating a gas diffusion electrode with a
catalytically active layer:
0.6 g of noble metal catalyst on carbon carrier (20
PT, 80 %C) are intensively mixed with 4.0 g of a 5-per-
cent nafion solution (nafion dissolved in low aliphatic
alcohols and water) and 10.0 g water. Thereafter, 2 g of
the alcohols contained therein are evaporated at 50 °C
so as to increase the surface tension of the suspension.
The suspension now is sprayed onto an impregnated car-
bon fiber nonwoven fabric and thereafter dried at 80 °C.
The spraying and drying steps are repeated twice. The
result hereof is a gas diffusion electrode coated with a
catalyst.
The thus fabricated gas diffusion electrode permits a
better diffusion of oxygen than graphitized fabric, dis-
plays high electrical conductivity due to its soot con-
tent and is hydrophobic due to its PTFE content. In
j
CA 02238945 1998-OS-28
- 11 -
addition thereto, it can be fabricated in less expensive
manner, is very homogeneous and has a lower mass-area
ratio than the graphitized fabrics with soot known so
far.
