Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Uniting two separate solid materials
Reexamination Certificate
1999-11-08
2001-08-21
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Uniting two separate solid materials
C427S115000, C429S047000, C029S623500
Reexamination Certificate
active
06277261
ABSTRACT:
BACKGROUND OF THE INVENTION
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 patent publication “K. Kordesch, Günther Sinadar: “FUEL CELLS AND THEIR APPLICATIONS”, VCH Weinheim, 1996. In this publication 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 application with the official serial number 196 38 928.3-45. The manufacture of gas diffusion electrodes by way of a spray process is disclosed in the printed publication EP 0 687 024 A1.
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 (a product of E.I. Dupont De Nemours) 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 (a product of E.I. Dupont De Nemours) 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 (a product of E.I. Dupont De Nemours) membrane).
The printed publication U.S. Pat. No. 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 U.S. Pat. No. 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.
It is the object of the present invention to provide a cost effective manufacturing method for an electrode-electrolyte unit.
SUMMARY OF THE INVENTION
In a method for the manufacture of an electrode-electrolyte unit with a catalytically active layer a metal salt solution is placed layer-like between an electrolyte layer and an electrolyte and the metal in the metal salt solution is precipitated from the metal salt in situ between the two layers.
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 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 H
2
PtCl
6
or Pt(NH
3
)
4
Cl
2
. Such a salt is then mixed with a solvent.
As solvents, for example, acids such as HCl, H
2
SO
4
, HClO
4
are suitable.
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 cm
2
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 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 consequently minimized.
The catalytically active metal is deposited directly at the three-phase zone. The catalyst material is therefore applied to the electrochemically active area related to the predetermined 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 consisting 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 embodiment 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 considered.
With respect to the known state of the art, this embodiment 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 (a product of E.I. Dupont De Nemours) is used, preferably dissolved NAFION (a product of E.I. Dupont De Nemours) 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 electrochemically active catalyst layer on a suitable carrier, which catalyst layer is suitable as a gas diffusion electrode for electrochemical applications such as in fue
Divisek Jiri
Oetjen Hans-Friedrich
Schmidt Volkmar M.
Bach Klaus J.
Forschungszentrum Julich GmbH
Gorgos Kathryn
Nicolas Wesley A.
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