Multi-layers coating for protecting metallic substrates

Details Multi-layers coating for protecting metallic substrates Multi-layers coating for protecting metallic substrates

2002-05-20

2004-12-07

LaVille, Michael (Department: 1775)

Stock material or miscellaneous articles

All metal or with adjacent metals

Composite; i.e., plural, adjacent, spatially distinct metal...

C428S681000, C428S684000, C428S685000, C428S334000, C428S220000

Reexamination Certificate

active

06828040

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to improvements in the field of fuel cells. More particularly, the invention relates to an improved composite used as a bipolar separator plate (BSP) and a process for preparing said composite.
BACKGROUND OF THE INVENTION
As environmental concerns rise amongst the population, new less polluting energy sources are developed. Proton exchange membrane fuel cells offer an easy way to produce electricity from hydrogen and oxygen with water and heat as by-products. So far, fuel cells have been proposed as an alternative to combustion motors in vehicles as well as for many other applications. A proton exchange membrane (PEM) fuel cell (see
FIG. 1
) comprises a thin polymer film as electrolyte replacing the liquid electrolyte found in alkaline fuel cells, a cathode on one face of the membrane electrolyte and an anode on the other face of the membrane-electrolyte. In order to increase the fuel cells voltage, these cells are assembled in series. In such a case, a new component, the bipolar separator plate (BSP), is therefore required to separate each cell (see FIG.
1
).
A BSP has two working faces, one confronting the anode of one cell and the other confronting the cathode on the next adjacent cell in the stack, and electrically conducts current between the adjacent cells. It is imperative that the BSP be as conductive as possible to minimise resistive losses throughout the stack (F. Barbir, J. Baun, J. Neutzler,
J. New Mat. Electrochem Systems
2, 1999, 197; R. L. Borup, N. E. Vanderborgh,
Mater. Res. Symp. Proc
. 1995, 393). Since the BSP also separates the anodic and cathodic compartments, the BSP material should not allow hydrogen or oxygen to permeate it. In a typical stack, the BSP also contains the flow channels for distributing gases on the entire surface of the cell. On top of those properties, BSP materials should be able to survive being assembled to form the fuel cell stack and transported on site. Once in its final form, the BSP should have some basic mechanical strength and be to some degree shock resistant. Furthermore, if the flow channel design is complicated, the material used for making BSP should be easy to machine or be simply processed in its final form, by compression moulding for example.
BSP materials must also be resistant and even practically inert to constant contact with highly acidic environment such as conditions found in PEM fuel cells. The acidity of the membrane (Nafion®) is roughly equivalent to a solution containing 0.1 M of H
+
(S. Gottesfeld, T. A. Zawodzinski,
Adv. Electrochem. Sci. and Eng
, 5, 1997, 195. ). The pH of water coming out of the anodic and cathodic compartments ranges between 3 and 5. In such acidic conditions, most metals will either form passivating non-conductive oxides or be dissolved like steel. Passivating oxides will decrease the electrical conductivity of the BSP to intolerable levels. On the other hand, ions leached during the dissolution of ferrous materials will contaminate Nafion® that ultimately leads to poor performance (A. S. Woodman, E. B. Anderson, M. C. Kimble, “Sensitivity of Nafion® to Metal Contaminants for Proton Conducting Membrane Fuel Cells”, The Electrochemistry Society Meeting Abstract, 99-2, 1999). Finally, the material used should also be a good thermal conductor to help redistribution of heat generated inside the stack.
Large scale commercialisation of fuel cells is possible if their production costs are lowered. One of the most expensive components in the proton exchange membrane fuel cell hardware is the BSP. Up to now, the material that has been widely used in making bipolar plates is graphite. Precision machining of these plates is expensive and to ensure that they are impermeable to gases and strong enough, the graphite bipolar plates are rather thick. To replace graphite, the new material must be low cost, easy to shape, light, compact and corrosion resistant. Furthermore, its electrical and thermal conductivity must be high. New processes as well as new materials must therefore be developed to fulfil all these requirements.
Recent Areas of Research on Low Cost BSP Materials and Production Processes
New metallic alloys can be developed to withstand the fuel cell conditions. Also new methods of producing graphite BSP such as injection moulding are being actively pursued. Composites made of metals and graphite are also studied. The latter category encompasses the use of metallic powder in graphite blends that are later processed in many different ways.
New Metallic Alloys
Since the only requirement that most metallic materials fail to meet is chemical stability in an acidic environment, the use of various metals alloys and metallic coatings for making BSP have been studied. There are generally two main approaches pursued to get around the chemical stability problem. First, a noble metallic coating can be applied on a less expensive substrate. The coatings presented by Woodman et al. in “Development of Corrosion-Resistant Coatings for Fuel Cell Bipolar Plates, Proc. AESF SUR/FIN Annu. Int. Tech. Conf. 1999”, 717-725, that are gold over aluminium and gold over nickel over copper over aluminium are a good example of this approach. L. Ma et al. in
J. New Mat. Electrochem. Systems
, 3, 2000, 221, have also studied other coating materials such as TiN. Secondly, existing corrosion resistant alloys have been tested in a fuel cell environment to assess their chemical stability and new metallic alloys have been developed. Austenitic stainless steels containing small amounts of copper like 904 L (N 08904) and N 08926 were investigated by D. P. Davies et al. (
J.Power Sources
, 86, 2000, 237
; J. Appl. Electrochem
, 30, 2000, 101) and R. C. Makkus et al. (
J.Power Sources
, 86, 2000, 274). Also, R. Homung and G. Kappelt (
J. Power Sources
, 72, 1998, 20) studied a novel iron and nickel-based alloys that appear to be promising.
Even if the use of new metallic alloys and metallic coated alloys appears interesting, there are nonetheless a few unanswered questions. Gold plating complicated patterns on BSP is expensive especially if the coating is similar to the best coating produced by Woodman et al. in “Development of Corrosion-Resistant Coatings for Fuel Cell Bipolar Plates, Proc. AESF SUR/FIN Annu. Int. Tech. Conf. 1999”, 717-725 which is gold over nickel over copper over aluminium. New, alloys offer a simple solution to the corrosion problem but they also comprise many major potential problems. Complicated alloys containing more than 50% non-ferrous additives are costly. Furthermore, all of these alloys would produce multivalent cations if dissolved in the fuel cell, causing contamination of the Nafion® membrane that will cause a decrease in cell performance. Since it is impossible to ensure that a single bipolar plate would not corrode in an entire stack, there will always be a risk when exposed metal is in contact with the electrode.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a composite useful in the fabrication of BSPs. A further object of the present invention is to provide a process for the preparation of the composite.
According to a first aspect of the invention, there is provided a composite comprising:
a steel substrate having a carbon coating thereon, the carbon coating comprising a carbon layer derived by pyrolysis of an acetylenic polymer having a content of carbon of at least 90 weight %, the carbon layer protecting the substrate against corrosion and improving long term stability of contact resistivity of the substrate, the polymer being soluble at a temperature below 110° C. in an organic solvent, and the carbon layer is contacting the steel substrate.
Chlorobenzene, chloroform, o-dichlorobenzene, N,N-dimethylacetamide, N,N-dimethylformamide, 1,1,2-trichloroethane, 1,1,1,2-tetrachloroethane 1,1,2,2-tetrachloroethane, tetrahydrofuran, xylene and mixtures thereof, are non-limitative examples of organic solvents that can be effective to dissolve the polymer.
According to a second aspect of th

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