Patent Application
20080044716
The following discussion of the embodiments of the invention directed to a bipolar plate for a fuel cell having a thick hydrophilic layer and a thin adhesion layer deposited on the bipolar plate is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses.
A cathode side flow field plate or bipolar plate 28 is provided on the cathode side 12 and an anode side flow field plate or bipolar plate 30 is provided on the anode side 14. The bipolar plates 28 and 30 are provided between the fuel cells in the fuel cell stack. A hydrogen reactant gas flow from flow channels 32 in the bipolar plate 30 reacts with the catalyst layer 26 to dissociate the hydrogen ions and the electrons. Airflow from flow channels 34 in the bipolar plate 28 reacts with the catalyst layer 22. The hydrogen ions are able to propagate through the membrane 16 where they carry the ionic current through the membrane 16. The end product is water, which does not have any negative impact on the environment.
In this non-limiting embodiment, the bipolar plate 28 includes two stamped metal sheets 36 and 38 that are welded together. The sheet 36 defines the flow channels 34 and the sheet 38 defines flow channels 40 for the anode side of an adjacent fuel cell to the fuel cell 10. Cooling fluid flow channels 42 are provided between the sheets 36 and 38, as shown. Likewise, the bipolar plate 30 includes a sheet 44 defining the flow channels 32, and a sheet 46 defining flow channels 48 for the cathode side of an adjacent fuel cell. Cooling fluid flow channels 50 are provided between the sheets 44 and 46, as shown. The bipolar plates 28 and 30 can be made of any suitable conductive material that can be stamped, such as stainless steel, titanium, aluminum, etc.
The bipolar plate 28 includes a layer 52 and the bipolar plate 30 includes a layer 54 that makes the plates conductive, corrosion resistant, hydrophilic and/or stable in a fuel cell environment. According to one embodiment of the present invention, the layers 52 and 54 are a film of a hydrophilic material that has been deposited on the bipolar plates by a Sol-gel process. Particularly, the layers 52 and 54 are deposited on the bipolar plates as a colloidal suspension of hydrophilic particles in a suitable solvent, such as ethanol. One non-limiting example is silicon dioxide nano-particles suspended in ethanol that is a commercially available product referred to as Nano-X. In an alternate embodiment, the colloidal suspension can include a conductive material, such as gold particles, that makes the layers 52 and 54 both hydrophilic and electrically conductive for the fuel cell environment. The colloidal suspension is deposited on the bipolar plates by a suitable low cost process, such as dipping the bipolar plate in the solution or spraying the solution on the bipolar plate. The bipolar plate is then allowed to dry or be cured so that the solvent evaporates to form a hydrophilic film on the bipolar plates.
As discussed above, the layers 52 and 54 can be a film of silicon dioxide (SiO2) nano-particles. However, other metal oxides can be used for the hydrophilic layers including, but not limited to, titanium dioxide (TiO2), hafnium dioxide (HfO2), zirconium dioxide (ZrO2), aluminum oxide (Al2O3), tin oxide (SnO2), tantalum pent-oxide (Ta2O5), niobium pent-oxide (Nb2O5), molybdenum dioxide (MoO2), iridium dioxide (IrO2), ruthenium dioxide (RuO2) and mixtures thereof.
The metal oxides can be doped to make them electrically conductive. Suitable dopants can be selected from materials that can create suitable point defects, such as N, C, Li, Ba, Pb, Mo, Ag, Au, Ru, Re, Nd, Y, Mn, V, Cr, Sb, Ni, W, Zr, Hf, etc. and mixtures thereof. In one particular embodiment, the doped metal oxide is niobium (Nb) and tantalum (Ta) doped titanium oxide (TiO2) and fluorine (F) doped tin oxide (SnO2). The amount of dopant in the coatings can be in the range of 0-10% of the composition of the coatings.
According to one embodiment of the present invention, the hydrophilic material is removed from the lands 56 and 58 between the flow channels 32 and 34, respectively, by any suitable process, such as sanding, so that the metal part of the bipolar plate is in electrical contact with the diffusion media layers so that electricity is effectively conducted through the fuel cell. Alternately, a masking process can be used to block the lands 56 and 58 when the layers are deposited on the bipolar plates.
As discussed above, the deposition of a thick hydrophilic dispersion on a metal substrate by Sol-gel processes typically has a poor adhesion of the hydrophilic film to the substrate. According to the invention, a thin inter-layer/adhesion promoter layer is first deposited on the bipolar plates 28 and 30 before the layers 52 and 54 to increase the adhesion of the layers 52 and 54 to the bipolar plate.
In one embodiment, the thin layer 62 is the same material as the hydrophilic material in the thick layer 62, such as silicon dioxide. When the thick layer 60 is deposited on the thin layer 62 by the Sol-gel process discussed above to provide the film, the thick layer 60 has good adhesion to the thin layer 62 through covalent bonding. For example, if the hydrophilic material in both the thick layer 60 and the thin layer 62 is silicon dioxide, then the material forms Si—O—Si covalent bonds 66. Particularly, the outside surface of the layers 60 and 62 are exposed to air and produce SiOH. When the outside surfaces of the layers 60 and 62 come in contact, the SiOH bond together to form Si—O—Si.
The thin layer 62 can be any suitable material for the purposes described herein. For example, if the thin layer were titanium dioxide, then the bonds between the thick layer 60 and the thin layer 62 would be Si—O—Ti. Other materials, such as ruthenium oxide can also be used.
According to another embodiment of the invention, the thin layer can be an organic material deposited on the plate substrate by a suitable process. Non-limiting examples of suitable organic materials include amines, sulphites, sulphates, thiols and carboxylates.
According to another embodiment of the present invention, the thin layer 62 is formed by modifying the surface of the stainless steel substrate. For example, a suitable metal, such as titanium (Ti), Zirconium (Zr), tantalum (Ta), halfnium (Hf), chromium (Cr), tungsten (W), iridium (Ir), ruthenium (Ru) and mixtures thereof, are deposited on the substrate. These layers are then oxidized to form oxides on the substrate that provide a suitable bond to the thick layer 60.
The layers 52 and 54 are deposited on the bipolar plates 28 and 30, respectively, to a thickness, such as in the range of 100 nm-1000 nm, so that the hydrofluoric acid etch that occurs during operation of the fuel cell will not completely etch away the layers 52 and 54 over the desired lifetime of the fuel cell. Particularly, hydrofluoric acid is generated as a result of degradation of the perfluorosulfonic ionomer in the membrane during operation of the fuel cell. The hydrofluoric acid has a corrosive effect on the bipolar plates that make them electro-chemically unstable.
The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.
