|Title of Invention||
A PROCESS FOR THE PRODUCTION OF A GAS DIFFUSION ELECTRODE
|Abstract||With a method for production of a gas diffussion electrode made of a silver catalysator no PTFE substratum reproducible results should be obtained under avoidance of the dis-advantages of the state of the art technic, where this is achieved thereby, that the poring system of the stiver catalysator is filled up with a moistening liquidity a dimension stable solid body with a grain size above that of the silver catalysator under the silver catalysator is mixed this accordingly compression stable substance is formed in a calender for a homogeneous catalystator band and in a second calender step an electrically conductive deflecting material is pressed-in in the catalysator band.|
|Full Text||The present invention pertains to a method for production of
porous gas diffusing electrodes of the class (sort) indicat-
ed in the patent claim no. 1. Such a gas diffusing electrode
can base on , for example , a catalytic-active silver or
silver alloys for the application in electro-chemical cells ,
especially of the chlorine (gas) - alkali - electrolysis or
on alkaline fuel cells.
In electrochemical cells the reduction of oxygen is imple-
mented at platinum, silver or also carbon. Platinum is appli-
cable both in acidic as well as in alkaline surrounding ,
whereas silver and carbon are corrosion resistant only in ..
alkalic electrolytes. However, in case of the silver catalyt-
ic agent also in alkaline media a rash deactivisation occurs,
which is explained by rearrangement of the oxldic surface of
the silver. ( Texas Instruments, U.S. 35 05 120 ). It has
been attempted on several occassions , to reduce the corro-
sive attack of the silver by suitable alloy partners. There
both alloys with precious materials like platinum, palladium gold and
mercury ( quick silver) ( DE 20 21 009 ) , but also with non
-precious materials (metals) , like nickel (DE 15 46 728), ,
copper and other metals are known. It has also been attempted
to achieve a stabilisation of the silver both by purification
as well by anodic corrossion -protection (local element),
In case of the corrosion, there builds at first a silver-
oxide-surface Since silver oxide is relatively good solu-
ble in alloys , with that a rearrangement of the silver cr-
ystals can take place. In the print 2 and 3 REM inclusion of
stiver electrodes before and after the operation are represen
-ted. It is possible to recognise very distinctly the shrink-
age of the inner porous structure. The catalytic activities
Besides the stabilisation the method for production of an
active silver catalytic agent (catalyser) must also ensure,
that the active surface of the silver is adequately large,
that means, the grain size of the silver is as far as possi-
ble small. Thus, for example, is known from the booklet (US
3 668 101), that the very active silver catalysers are
achieved in case of particle diameter of 5 to 10 µm size.
Further more methods are known, in case of which it is attem-
pted, to produce smallest particles of stable silver alloys.
Adequately small silver particles are generated during proced-
ures, of precipitation. Besides the control of p-H value, of
the temperature and of the supersaturation, there plays so
called crystallsing embryo an extraordinary role, in order
to produce smallest silver particles. Known is also a method
(EP 0 115 8^5)t in case of which a mixture from silver nltr -
ate and quick silver (mercury) nitrate is deposited on a
PTFE dispersion by addition of potassium hydroxide. Herewith
a silver amalgam is produced with smallest particle diameter .
In order to produce so called gas diffussion electrode from
this catalytic agents, as these are required in fuel cells
or in the chloride -alkali-electrolysis, the powder must be
processed for a homogeneous and laminar electrode. This elec-
trode must be electrically conductive and must render possi-
ble both the entrance of the electrolyte as well as also
gas. Even the spheres of the electrode must be in a position
to be moistened,whereas other spheres of moistening must be
protected. A solution of this problem was represented with
a bi-porous poring structure. The electrolyte can at first
penetrate without any trouble both in the small as well as
in the large pores. By en excess pressure of the gas or the
gravitational force the electrolyte is distanced again from the
larger pores.Such biporous structures operate only then satis-
factorily, when a difference of pressure between the gas cha-
amber and the electrolyte room can prevail. Whether such di-
fference of pressure can build automatically, when in the
electrolyte a membrane separates the anode and cathode from
one another, is questionable. Thereby, it is not possible that
these electrodes be related to the chlorlde-alkaline-electr-
lysis or in fuel cells with alkalic membrane.
Therefore it was attempted to construct on the material pro-
perties a bi-porous poring system. That means , one requires
hydrophile and hydrophob materials. Suitable hydrophobmate-
terials are some thermoplasts- asfor example , Polytetrafl-
uorethylene. The mentioned catalytic agents and also the
silver are always hydrophyl. If one mixes silver and PTFE
together and forms therefrom a plane electrode, then this
displays different spheres with hydrophil and hydrophob pro-
perties. Mostly an additional metalic conductor is integrat _
ed, in order to achieve better electrical properties.
Some methods are known, to produce mixtures from PTFE (Poly-
tetrafluorethylene) and catalytic agent a so called gas di-
ffussion electrode. Thus it is proposed, for example , in
the write-up (EP 0 115 845) to float (suspend) on the mate-
rials in the manner that one gets a paste, which in form
can be poured, passed and dried up. Disadvantageous in case
of such " pasted " electrodes is the homogeneous division
of the material - It can easily result in small holes ,
through which gas or electrolyte can penetrate. In order
to prevent this, the electrodes are produced at least 0.8 mm
thick. As a result, there prevails a very high quantity of
silver in the electrode (around 2 kg/m2), so that the ad-
vantage in respect of the silver is again gone lost. Since
these commercial oxygen electrodes with around 4-40 g/m2
Platinum/Carbon are known.
Two methods are known, in case of which out of such hydroph-
obes/ hydrophlle materials a thin , homogeneous gas dlffu-
ssion electrode is rolled. According to the method (EP-0 144
002, US 4 696 872) in one special mixer the catalytic parti-
cles and the PTFE are mixed with one another in such a manner,
that a finely mixing hydrophobe network system condenses on
the Catalysator. In a powder roller the loosely poured subs-
stance is rolled together to a folio of around 0.2mm. thick.
This method has proved Itself worthwhile for mixer of PTFE
and Carbon, or PTFE and Raney-Nickel. It is similarly possi-
ble, thereby as a result, to roll (rough) down a Ranny-Sil-
ver- Alloy with 80% Allumlnium for a porous foil. Such s
calender rolling mill is represented in the print no. 1.
However, it is not possible to process the ductile Silver.
In case of the necessary surface (contact)-pressure-around,
0.01 to 0.6 t/cm2 in such powder rollers PTFE and Silver are
pressed to a compact, gas and electrolyte impermeable foil.
The cirrent-voltage-characteristic line of such an electrode
has to be seen in Print no.5.
In order to be able to produce Silver electrodes, at first
a Silver Oxide/ PTFE mixture is processes in the pulverising
roller and subsequently reduced Electro-chemlcally/ (DE 37 10
168). The Silver oxide is stable enough, in order to resist
the surface pressure of the roller. Besides that, the volume
reduces itself during transition from Silver oxide to the Silver,
so that additional pores develop in thegas diffussion electrode.
By the parameter during reduction the grainsire of the particle
can very well be adjusted. Disadvantageous in case of this
method is that it is not yet known, how Siver Alloys with cata-
lytic properties can be reduced electrochemlcally. Thereby it
is also not possible to produce long time stable Silver elec-
trodes by the electrochemical reduction.
The task of the present invention exists therein , to make
readily available a method of production of a gas diffusion
electrode, with which not only the disadvantages of the state
of art technique is avoided, but also specially reproducible
results during methodical production adjusts itself.
With a method of the nature Indicated in the beglning this prob
lem is solved in accordance with the invention thereby , that
-the porous system of the Silver catalytic agent is
filled up with a moistening liquidity,
- a dimension stable rigid body is mixed with a grain
size above that of the Silver catalytic agent below
the silver catalytic agent
- this consequent compression-stable substance is formed
but in a calender homogeneous to a catalytic agent
- ina «econd calender step an electrically conductive
arresting material is pressed-in in the catalytic
The speciality of the method in accordance with the invention
lies therein, that inner porous system of the ductile mate-
rials has to be filled up with a liquidity. Since this liqui-
dity cannot be condensed and on the other hand is connected
through the capillar forces rigidly in the poring system, it
ls possible also in case of the present pressure of maximum
600 kg/ cm2 to distance the liquidity from the micro pores.
A further mixture of some Carbon powder or of the volatile
ammonium carbonate can furthermore receive (hold) the mecha-
nical pressure of the powder rolls. By means of these coarse
grainedjadditions of typical 10 - 100 µm grain diameter the
pouring system with larger pore diameter is protected from
the condensation. By means of a subsequent temperature pace
it is possible that both the liquidity as well as the solid
matter ammonium carbonate are operated from the electrode.
In this manner it is possible to receive large pores in the
gas diffusion electrode,which take care of the quick transport
of gas and smaller pores in a catalytic agent which permits
a homogeneous utlllisation of the catalysator ( catalytic agent).
A prefered implementation of this method is represented as
follows : at first Silver or a Silver alloy is produced by
a precipitation process. There it is advantageous,to imple-
ment the precipitation on a PTPE dispersion. The best expe-
riences are made with a mixture of 15% Teflon and 85% Silver.
With the help of addition of formaldehyde during precipitation
there converts itself in the alkalic surrounding the Silver
hydroxide immediately in a Silver crystal. The precipitation
substance is washed and dried up. A subsequent temperature
at 200 C improves the electrical contact between the Silver
particles and drives the residual liquidities out.
To this powder a volume of around 5% - 40 %t preferably
however 8 %% of a liquidity is given, which can penetrate
in the poring system of the PTPE and of the Silver. Because
of the hydrophobe character of the PTPE there come thereto
only Isopropanol Ethanol and Methanol in question. If the
powder is moistened and filled up with such solution agent,
then an exchange of liquidities can be connected at this .
If one brings, for example, a powder impregnated in Iso-
propanol in a water bath, or Glycerine, then there exchange
within hours the liquidities by diffussion. In this manner
there arrives a liquidity in the poring system of the PTFE,
which is usually knocked off (planned off) by the PTFE.
This thus moistened ( dampened) material behaves ltsdlf taa
in this manner outwards as a powder, since the liquidity
prevails only in the inner poring system.
Another class of the moistening medium are the so called
tenside. These penetrate both in the poring system, cover
also the surface of the catalytic agent and reduce thus its
surface roughness. This small surface roughness leads in
the rolling process thereto, that the Silver Catalytic
agent can deviate from the compression *one, whereas other
powder components, which have not been treated, remain in
the compression tone and thus then generate the electrode
connection level, in which the Silver Catalytic agent is
embedded (print 4). As such a powder there proffer (offer)
Ammonium carbonate or active coal, which is now mixed with
the Silver Catalytic agent in a crusher to a homogeneous
substance, as it is described in EP 0 144 002. Subsequently
the loose vibrated (shaken) substance is cogged (rolled out)
with the help of powder roll to approximately 0.2 mm thick
In a second pair of rolls a metallic supporting frame in
the form of a woven network or expanded metal can be rolled
in, and thereby the mechanical stability and the electri-
cal conductivity can improve. According to this procedure
the gas diffussion electrode is dried up. According to that
the electrode has a silver coating between 0.2 kg/m2 and 1.5
kg/m2. Conventionally a weight of around 0.5 kg/m2 is attem-
pted . Therewith upto 75% of the uptil now necessary silver
can be saved (spared). Despite the reduced silver weight
there follows with such electrodes current-voltage-charac-
teristic line as are to be seen from the print no.6.
This method may be obviously combined also with other methods.
Thus it is possible to ignore the environmentally harmful
formaldehyde during the precipitation and for this purpose
the reduction after the production of GDE can be completed
in electrochemical way. It may also similarly produce alloys
in which a co-precipitation of Silver and Quick silver (mer-
cury) , Titan, Nickel, Copper, Cobalt or Bismuth be imple-
Specially for the Chlorine alkali-electrolysis it is possi-
ble to under-talie changes at the finished gas diffussion
electrode, which render possible a better transport of the
developing caustic-sodat For this purpose, for example, the
induction of a coarse deflecting system would be advisable.
This is possible, if on the finished electrode a network
is pressed and subsequently is drawn out again. The negative
pressure of the network builds channels, in which the elec-
trolyte can flow off later parallel to the electrode surface.
Further features, details and advantages of the invention
result on the ground of the following prints. These show in
Pig. 1 j (Print 1) a functional diagram of a plant in
accordance with the Invention
Print 2 : a microscopic absorption of a silver electrode
before the application (use),
Print 3 * *n same kind of representation a silver electrode
after the application (use)
Print k t a PTPE-frame embedded in a Silver catalytic agent
Print 5 t a current/voltage diagram of a Chlorine alkali-
electrolysis as well as in
Print 6 $ the same curve according to the measure of the
1. A process for the production of a gas diffusion electrode from a silver catalyst
on PTFE substrate, wherein that.
- the poring system of the silver catalyst is filled with a wetting liquid,
- a dimensionally stable solid body with a grain size above that of the silver
catalyst is mixed among the silver catalyst,
- that mass which is thus stable in respect of compression is shaped in a
calendar to form a homogeneous catalyst strip, and
- in a second calendering step an electrically conductive discharge conductor
material is impressed into the catalyst strip.
2. A process as claimed in claim 1 wherein that 5% isopropanol is used as the
wetting liquid and 30% ammonium carbonate or ammonium hydrogen
carbonate is used as the solid and those two fillers are expelled after
electrode production by a heat treatment step at preferably 110°C.
3. A process as claimed in claim 1 wherein a tenside - preferably 5% Triton X
100 - is used as the wetting liquid, which both penetrates into the pore
system of the catalyst but also reduces surface friction so that the silver
catalyst can slide out of the compacting zone and the dimensionally stable
am-monium carbonate and the PTFE binder absorb the rolling pressure.
4. A process in particular as claimed in claim 1 wherein in the first calender step
a homogeneous catalyst strip of a thickness of 0.2 - 0.5 mm is produced.
5. A process as claimed in one of the preceding claims wherein the rolling gap is
set to 350 pm and the rolling advance is set to about 2 meters per minute.
6. A process as claimed in one of the preceding claims wherein a silver - plated
nickel wire mesh of a wire thickness of 0.15 mm and a mesh width of 0.45
mm with a silver covering of about 10 µm in thickness is used as the
electrical discharge conductor material.
Dated 13th DAY OF November 2003
With a method for production of a gas diffussion electrode made of a silver
catalysator no PTFE substratum reproducible results should be obtained under
avoidance of the dis-advantages of the state of the art technic, where this is
achieved thereby, that the poring system of the stiver catalysator is filled up with
a moistening liquidity a dimension stable solid body with a grain size above that
of the silver catalysator under the silver catalysator is mixed this accordingly
compression stable substance is formed in a calender for a homogeneous
catalystator band and in a second calender step an electrically conductive
deflecting material is pressed-in in the catalysator band.
|Indian Patent Application Number||1482/KOLNP/2003|
|PG Journal Number||16/2009|
|Date of Filing||13-Nov-2003|
|Name of Patentee||UHDE GMBH|
|Applicant Address||FRIEDRICH-UHDE-STRABE 15, 44141 DORTMUND|
|PCT International Classification Number||H01M 4/88|
|PCT International Application Number||PCT/EP2002/06706|
|PCT International Filing date||2002-06-18|