Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PRODUCTION OF ELECTROLYTE UNITS BY ELECTROLYTIC DEPOSITION
OF A CATALYST
The invention relates to a method of producing an
electrode-electrolyte unit with a catalytically active
layer.
Electrochemically operating units consisting of
electrode-electrolyte-electrode are provided for example for
use in fuel cells, electrolysis cells, or cells for electro-
organic syntheses. The electrodes are preferably porous
throughout so that operating means such as air and hydrogen
can pass through the electrodes. In many cases, the
electrodes participating in the electrochemical reactions
must be activated by suitable catalysts.
For fuel cells whose operating temperatures are
0 -150 C, ion conductive solid electrolyte membranes are
used. The anodes for the hydrogen oxidation and the
cathodes for the oxygen reduction are coated mostly with
platinum, recently also with a platinum-ruthenium alloy.
The principle of such a membrane fuel cell is known from the
book by K. Kordesch, Gunter Simadar entitled "FUEL CELLS AND
THEIR APPLICATIONS", published by VCH Weinheim in 1996. In
this book furthermore various methods for producing
membrane-electrode units for fuel cells are described. For
example, the electrode can be activated by sputtering a thin
platinum layer onto the diffusion layer of the gas diffusion
electrode. Additional manufacturing methods are described
in the German patent document DE 196 38 928 Al published on
April 2, 1990. The manufacture of gas diffusion electrodes
by way of a spray process is disclosed in the printed
publication EP 0 687 024 Al.
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The main disadvantages of the known electrode-
electrolyte units with electrochemically active areas are
the high costs. The high price results essentially from
expensive membranes consisting for example of *Nafion and
from expensive catalysts consisting for example of platinum.
To avoid the high prices, it is being tried
therefore to deposit thin catalytically active layers in
electrochemically active areas. The electrochemical
processes in a fuel cell occur immediately at the contact
area between the gas diffusion electrode and the nafion
membrane. The catalyst is therefore preferably located at
these contact areas, in other words, at the three-phase zone
consisting of a gas distributor with electronic current
conductance, the place of the electrochemical reaction and
the electrolytes (in this case: nafion membrane).
The printed publication US 5 084 144 and the
printed publication, E.J. Taylor, E.B. Anderson, NR K.
Vilambi, Journal of the Electrochemical Society, Vol. 139
(1992) L 45-46" discloses a method for the manufacture of
gas diffusion electrodes with the object to achieve a high
platinum utilization for membrane fuel cells. In accordance
with that method, among others, a catalyst metal is
electrolytically deposited from a galvanic bath to form a
thin catalytically active layer.
The disadvantage of the method disclosed in
US 5 084 144, is that it requires expensive liquid galvanic
baths which must be reconditioned in a complicated and
expensive manner. Furthermore, the utilization of the
precious metal dissolved in the galvanic bath is very
limited so that the advantages obtained by the optimized
deposition are offset for example by rinsing procedures.
*Trade-mark
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It is the object of the present invention to
provide a cost effective manufacturing method for an
electrode-electrolyte unit.
According to the invention, there is provided a
method for the manufacture of an electrode-electrolyte unit
with a catalytically active layer, comprising the steps of
placing a solution including a dissolved metal salt as a
layer between an electrolyte layer and an electrode and
precipitating metal electrochemically, in situ, from the
metal salt on said electrode, whereby all of the metal salt
is used in the precipitation of the metal from the metal
salt for forming said catalytically active layer.
With the method according to the invention
dissolved metal salt is first sandwiched between an
electrolyte and an electrode. In this way, the dissolved
metal salt forms an intermediate layer in a multi-layer
(layer-) system. Subsequently, the metal is
electrochemically removed from the intermediate layer that
is from the dissolved metal salt.
Salts of a metal of the VIII group or of an I-B
metal of the periodic system may be provided as metal salts
from which catalytically active metal can be extracted.
If, for example, platinum is to be deposited as
the catalytically active metal, a suitable salt is for
example H2PtCl6 or Pt (NH3) 4C12. Such a salt is then mixed
with a solvent.
As solvents, for example, acids such as HCl, H2SO4r
HC104 are suitable.
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First, the metal salt solution may be applied as a
layer on the electrolyte layer of the electrode by spraying,
brush coating, screen printing, etc... Then the electrode, or
respectively, the electrolyte layer is disposed onto the
solution layer. In this way, a layer system is provided
which consists of an electrode, a metal salt solution and an
electrolyte.
The layer thickness that is the amount of metal
salt deposited between the electrolyte and the electrode is
for example so selected that up to 0.01-1 mg metal per cm2
can be deposited from the intermediate layer. In order to
generate the electric current required for the deposition,
for example, a second electrode which is also disposed
adjacent the electrolyte layer may be provided as an
additional current conductor. The electrolyte layer is then
disposed between two electrodes.
In the method according to the invention, no
liquid electrolyte is needed for the electrochemical
deposition. Consequently, expensive liquid galvanic baths
are eliminated. The
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complicated and expensive reconditioning and decontamination of
such galvanic baths is also eliminated. Only a thin layer of
the solution is applied. The consumption of expensive metals
such as platinum, ruthenium, rhodium or palladium is conse-
quently minimized.
The catalytically active metal is deposited directly at
the three-phase zone. The catalyst material is therefore ap-
plied to the electrochemically active area related to the pre-
determined utilization in a controlled manner.
As a result, the membrane with the catalyst deposited
thereon can be manufactured comparatively inexpensively.
If electrodes together with the intermediate layer con-
sisting of the metal salt solution are disposed at both sides
of the electrolyte layer, this electrode-electrolyte compound
structure can be used directly in a fuel cell.
For the manufacture of an alloy, in an advantageous em-
bodiment of the method, the solution includes several metal
salts, which are electrochemically deposited together. In this
way, an alloy of two or more metals or mixtures of metals and
metal oxides, that is, an alloy catalyst, is deposited. In
particular, ruthenium and platinum containing salts are consid-
ered.
With respect to the known state of the art, this embodi-
ment of the method according to the invention has the advantage
that alloy catalysts can be optimally deposited and produced at
the same time.
In another advantageous embodiment of the invention, the
solution contains an ion conductive polymer in a dissolved or
liquid state.
After completion of the process, an ion conductive polymer
in the solution should be firmly connected to the membrane
(electrolyte layer), that is it should be part of the membrane.
A polymer suitable to achieve this object is to be selected.
If for example, a solid electrolyte consisting of nafion is
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used, preferably dissolved nafion is used as ion conductive
polymer in the solution.
The ion-conductive polymer causes an increase of the
three-phase zone and, consequently improves the utilization of
the catalyst material.
With the above-mentioned embodiment of the invention,
catalytically active material is embedded in the solid material
electrolyte and is advantageously mechanically firmly connected
therewith.
The method facilitates the manufacture of an electrochemi-
cally active catalyst layer on a suitable carrier, which cata-
lyst layer is suitable as a gas diffusion electrode for elec-
trochemical applications such as in fuel cells, electrolysis
cells, or cells for electro-organic syntheses. With the
method electrodes with metal catalysts, alloys of metals or
mixtures of metal oxides and metals can be manufactured in a
simple manner. Only small amounts of the expensive catalyst
material are consumed with this method.
With each embodiment, the method according to the inven-
tion facilitates the use of the accurate amount of metal salts.
In this way, alloys or mixtures of metals and metal oxides of a
predetermined combination can be accurately manufactured. An
expert can to determine optimal mixture ratios by simple test
procedures.
With the electrochemical precipitation, the active layer
is formed on the diffusion layer in a controlled manner at the
three-phase zone between the gas space in the pores of the gas
diffusion electrode, the electro-active catalyst and the elec-
trolytes. As a result, the catalyst utilization in application
such as in fuel cells, electrolysis cells or cells for electro-
organic synthesis is optimized and the required total amount is
significantly reduced.
A fuel cells stack can be provided with pre-finished elec-
trodes at one side and electrodes prepared in accordance with
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the method of the invention mounted at the other side. The
electrolytic precipitation can be performed in the finished as-
sembled fuel cell unit.
For single uses, units consisting of an ion conductive
solid electrolyte, a manufactured gas diffusion electrode as
counter electrode and a prepared operating electrode may be
bolted together with a suitable seal or cemented together or
they may be encapsulated in a similar way. For the applica-
tion, the active electrode layer is formed by a short electro-
lyte precipitation. Possible contamination or residues of the
metal salt solution can subsequently be rinsed out.
Examples:
First example:
A diffusion layer for the technical gas diffusion elec-
trode consisting of a mixture of finely distributed carbon and
PTFE is manufactured. The diffusion layer contains no electro-
mechanically active material:
A solution of a preferably 5% solution of nafion in low-
molecular alcohols, preferably 1-propanol or 2 propanol and an
aqueous solution of hexachloroplatinum acid hydrate (H2PtCl6) is
prepared. The concentrations in the mixture of nafion solution
and platinum solutions can be so adjusted that the desired im-
pregnation with ion conductive nafion and the catalyst coating
for the technical gas-diffusion electrode are obtained (pref-
erably, 0.01 - 1 mg catalyst/c2 based on the geometric surface
of the electrode). The mixture is then applied to the elec-
trode by spraying brushing or screen-printing. As a counter
electrode, a suitable electrode is provided or a counter elec-
trode with an additional electrolyte layer is used. This stock
unit is clamped together in an arrangement as shown in the fig-
ure. By applying a current density in the particular applica-
tion of 0.1 - 10 mA/cmZ, for example 2 mA/cmz, and a voltage of
at least 1.23 V, for example, 2 V, the electrolysis is con-
ducted at room temperature or at a raised temperature (<100 C)
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until all the platinum is deposited on the porous electrically
conductive layer. By the addition of H20 as indicated in the
figure, it is insured that the polymer solid electrolyte does
not dry out and consequently is, or respectively remains, ioni-
cally conductive. Subsequently, the electro-chemically active
gas diffusion electrode so manufactured is treated for example
with hydrogen peroxide, water and sulfuric acid and is cleaned.
Membrane electrode units with the manufactured electro-
chemically active gas diffusion electrodes are used particu-
larly in PEM fuel cells, for example, with a platinum coating
of about 0.1 mg/cm2 for both the anode and the cathode. During
operation with pure hydrogen and oxygen, current densities of
more than 300 mA/cmz can be achieved at an operating tempera-
ture of 80 C and with a terminal voltage of 0.7 V.
Second example.
The operation corresponds to that of the first example.
Instead of a platinum salt solution however, a mixture of
platinum and ruthenium salt solutions (for example: H2PtC16 and
RuCl3in H2SO4) is used. In this way, platinum-ruthenium alloys
of a desired composition can be manufactured.
Third example.
The operation corresponds to that of the first example.
The nafion and metal salt containing solution is applied di-
rectly to the solid electrolyte membrane by spraying brushing
or screen-printing. Onto it a flexible graphite mesh or a
graphite paper with suitable electronic conductivity and suit-
able porosity for establishing electric contact is placed. The
subsequent steps are the same as in the first example.
Fig. 1 is a schematic cross-sectional view of an electro-
lyte layer 1 with a layer-like coating of a solution 2 disposed
thereon. Electrodes 3 and 4 are disposed at opposite sides of
the electrolyte layer 1. One of the electrodes 3 abuts the so-
lution coating 2 and the other electrode 4 abuts the opposite
side of the electrolyte layer 1. The electrode 4 includes a
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container-like recess 5. The container-like recess 5 is to be
filled with water. Passages 6 present in the electrode 4 ex-
tend from the container-like recess 5 to the membrane 1. The
membrane 1 is moistened by the water in the container-like re-
cess S. The moistened membrane remains electrically conduc-
tive. It is necessary that the membrane is electrically con-
ductive in order to achieve the electrochemical precipitation
of the metal from the solution. For the precipitation, a cur-
rent is applied in the manner as shown in Fig. 1.
The gases generated during the electrochemical precipita-
tion are discharged by way of the gas passages 7.
The container-like recess 5 may be provided with a closure
element, which is not shown. In that case, water vapors can be
generated in the container for keeping the membrane moist.
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