Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
1998-12-18
2001-09-18
Brouillette, Gabrielle (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000
Reexamination Certificate
active
06291091
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an improved method for manufacturing a continuous multi-layer laminated electrolyte and electrode assembly (“laminated assembly”) for an electrochemical fuel cell.
BACKGROUND OF THE INVENTION
Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (“MEA”) in which an electrolyte in the form of an ion-exchange membrane is disposed between two electrode layers. The electrode layers are made from porous, electrically conductive sheet material, such as carbon fiber paper or carbon cloth. In a typical MEA, the electrode layers provide structural support to the membrane, which is typically thin and flexible.
The MEA contains an electrocatalyst, typically comprising finely comminuted platinum particles disposed in a layer at each membrane/electrode layer interface, to induce the desired electrochemical reaction. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes through an external load.
During operation of the fuel cell, at the anode, the fuel permeates the porous electrode layer and reacts at the electrocatalyst layer to form protons and electrons. The protons migrate through the ion-exchange membrane to the cathode. At the cathode, the oxygen-containing gas supply permeates the porous electrode material and reacts at the cathode electrocatalyst layer with the protons to form water as a reaction product.
In conventional fuel cells, the MEA is disposed between two electrically conductive plates, each of which typically has at least one flow passage formed therein. The flow passages direct the fuel and oxidant to the respective electrodes, namely, the anode on the fuel side and the cathode on the oxidant side. In a single cell arrangement, fluid flow field plates are provided on each of the anode and cathode sides. The fluid flow field plates act as current collectors, provide fuel and oxidant to the respective anode and cathode catalytic surfaces, and provide channels for the removal of exhaust fluid streams.
One known method for fabricating an MEA for use in an electrochemical fuel cell, is to use a heat-press to join together the MEA components. A disadvantage of this method is that heat-pressing the entire assembly, which comprises the porous electrode layers, catalyst material and solid polymer electrolyte, subjects each of these components to undesirable mechanical and thermal stresses. Such mechanical and thermal stresses can diminish the performance and lifetime of an MEA in an operating fuel cell. Another disadvantage relates to the suitability of this known method for mass production. The heat-pressing procedure is typically a discontinuous or “batch” process. While the press is being heated, a layered structure comprising electrodes, catalyst layers, and a solid ion-exchange membrane, is typically inserted in a press, pressed therein, and subsequently removed from the press. This conventional batch procedure involves inefficient, costly and time-consuming process control.
German Patent DE 195 09 748 C2, discloses a generic process for producing a composite laminate comprising an electrode material, a catalyst material and a solid electrolyte material. The component materials are arranged on an electrostatically charged surface, and an external heater heats the solid electrolyte material until the upper side of the electrolyte material becomes soft. While the upper side of the electrolyte material is still soft, it is applied under pressure to the catalyst material for bonding the catalyst material to the polymer electrolyte. After the bond has set, the composite laminate is removed from the surface. A problem with this procedure is that the dimensions of the electrostatically charged surface limits the size of the MEA produced. A continuous sheet can not be manufactured according to this method.
Accordingly, there is a need for a continuous process for manufacturing a continuous laminated electrolyte-electrode assembly. A continuous process is desirable to improve efficiency by increasing productivity and the speed of the manufacturing process, thereby reducing production costs.
SUMMARY OF THE INVENTION
An objective of the present invention is to provide an improved continuous manufacturing method for producing a continuous multi-layer laminated assembly for use in electrochemical energy converters, particularly fuel cells, but also electrolyzers, and storage media (for example double layer capacitors). The laminated assembly includes first and second pre-formed electrode layers, an electrolyte layer interposed between the first and second electrode layers, a first catalyst layer interposed between the first electrode layer and a first major surface of the electrolyte layer, and a second catalyst interposed between the second electrode layer and a second major surface of the electrolyte layer. The electrolyte layer comprises an ionic electrolyte or an electrolyte precursor, which may be converted into an ionic form suitable as an electrolyte. The method involves forming at least one of the catalyst and electrolyte layers in situ and using the in situ formed layer as a laminating medium to bond the in situ formed layer with the two immediately adjacent pre-formed layers of the laminated assembly, thereby bonding two pre-formed layers of the laminated assembly together. The in situ formed layer preferably acts as the laminating medium while it is still soft or in a formable state when the layers of the laminated assembly are pressed together.
In one embodiment of the method, the first and second catalyst layers, and the electrolyte layer are all formed in situ by a tri-layer extrusion process. The opposing major surfaces of the tri-layer extrusion act as the laminating media for bonding the tri-layer extrusion to the immediately adjacent pre-formed electrode layers. The tri-layer extrusion may be produced by a multiple channel, single slit extrusion die. Suitable catalyst mixtures and electrolyte materials (or electrolyte precursor materials) may be continuously fed into separate intake chambers of an extrusion assembly. These materials are preferably in the form of granules or a paste. Each of the intake chambers is fluidly connected with one of the channels in the extrusion die.
The material selected for the electrolyte layer is suitable as an ion-exchange layer in an electrochemical cell, or is a precursor electrolyte material that is transformable into a ion-exchange layer. For example, a precursor material could be converted from a non-ionic to an ionic form after being extruded, using known methods such as, for example, hydrolysis.
The manufactured electrolyte layer preferably has the following properties:
substantially impermeable to reactant fluids;
ion conductor;
electrical insulator; and
substantially inert in an electrochemical fuel cell environment.
In the laminated assembly, the electrolyte layer may assume a solid form or remain in the form of a gel or a paste. The electrolyte material is preferably polymeric, but it could also be non-polymeric so long as it has the above-identified properties. Some examples of some preferred electrolyte materials are perfluorosulfonic acid based materials such as Nafion™, polyetheretherketoneketone (poly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene), commonly referred to as PEEKK) based ion conducting polymers, sulfonated polytrifluorostyrene based ion conducting polymers, such as BAM™ polymer.
The catalyst mixture may comprise catalyst particles and a binder material. The binder material is preferably compatible for bonding with the electrolyte layer and may be the same as a material used in the electrolyte layer. The catalyst mixture may also include additives that enhance the desired properties of the catalyst layer, such as fluid transport properties, permeability, ionic conductivity, and electrical conductivity. Using the described method it is possible to use different catalyst mi
Hahn Alfred
Hedrich Peter
Preischl Christine
Ballard Power Systems Inc.
Brouillette Gabrielle
McAndrews Held & Malloy Ltd.
Wills M.
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