Patent Application
20070287057
The following discussion of the embodiments of the invention directed to a bipolar plate for a fuel cell that includes a titanium oxide layer and a ruthenium oxide layer that makes the bipolar plate hydrophilic and electrically conductive in a fuel cell environment 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 18 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 18 and 30 are provided between the fuel cells in the fuel cell stack. A hydrogen reactant gas flow from flow channels 28 in the bipolar plate 30 reacts with the catalyst layer 26 to dissociate the hydrogen ions and the electrons. Airflow from flow channels 32 in the bipolar plate 18 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. The by-product of this electro-chemical reaction is water.
In this non-limiting embodiment, the bipolar plate 18 includes two sheets 34 and 36 that are formed separately and then joined together. The sheet 36 defines the flow channels 32 and the sheet 34 defines flow channels 38 for the anode side of an adjacent fuel cell to the fuel cell 10. Cooling fluid flow channels 40 are provided between the sheets 34 and 36, as shown. Likewise, the bipolar plate 30 includes a sheet 42 defining the flow channels 28, a sheet 44 defining flow channels 46 for the cathode side of an adjacent fuel cell, and cooling fluid flow channels 48. In the embodiments discussed herein, the sheets 34, 36, 42 and 44 are made of an electrically conductive material, such as stainless steel, titanium, aluminum, polymeric carbon composites, etc.
According to one embodiment of the present invention, the bipolar plate 18 includes a titanium or titanium oxide layer 50 and a titanium oxide/ruthenium oxide layer 52, and the bipolar plate 30 includes a titanium or titanium oxide layer 54 and a titanium oxide/ruthenium oxide layer 56 that makes the plates 18 and 30 conductive, corrosion resistant, hydrophilic and stable in the fuel cell environment. The titanium or titanium oxide layers 50 and 54 protect the plate substrate from aggressive ruthenium chloride ions and the titanium oxide/ruthenium oxide layers 52 and 56 make the plates 18 and 30 hydrophilic and electrically conductive. The hydrophilicity of the layers 52 and 56 causes the water within the flow channels 28 and 32 to form a film instead of water droplets so that the water does not significantly block the flow channels. Particularly, the hydrophilicity of the layers 52 and 56 decreases the contact angle of water accumulating the flow channels 32, 38, 28 and 46 so that the reactant gases deliver the flow through the channels at low loads. In one embodiment, the contact angle of water is below 30° and preferably about 10°.
By making the bipolar plates 18 and 30 more conductive, the electrical contact resistance between the fuel cells and the losses in the fuel cell are reduced, thus increasing cell efficiency. Also, an increase in the conductivity provided by the layers 52 and 56 provides a reduction in compression force in the stack, addressing certain durability issues within the stack.
Also, the layers 50, 52, 54 and 56 are stable, i.e., corrosion resistant. The hydrofluoric acid generated as a result of degradation of the perfluorosulfonic ionomer in the membrane 16 during operation of the fuel cell 10 does not corrode the layers 50, 52, 54 and 56.
When the layers 50-56 are deposited on the bipolar plate 30, they can be deposited on the sides of the sheets 42 and 44 where the cooling fluid flow channels 48 are provided so that the sheets 42 and 44 do not need to be welded together. This is because the ruthenium oxide provides a good ohmic contact between the sheets for the conduction of electricity. Therefore, instead of the laser welding that would bond the plates and provide the electrical contact between the sheets in the prior art, the sheets need only be sealed around the edges to seal the bipolar plates.
In an alternate embodiment, the layers 50 and 54 can be tantalum oxide and the layers 52 and 56 can be tantalum oxide/iridium oxide that also makes the plates 18 and 30 electrically conductive, corrosion resistant, hydrophilic and stable in the fuel cell environment.
The layers 50, 52, 54 and 56 can be deposited on the bipolar plates 18 and 30 by any suitable process.
Next, a ruthenium chloride solution is applied to the titanium coating. In one non-limiting embodiment, the ruthenium chloride solution is ruthenium chloride dissolved in an ethanol. The ruthenium chloride solution can be applied to the bipolar plates 18 and 30 by any suitable process, such as by brushing.
The titanium coating and the ruthenium chloride solution are then calcinated at a predetermined temperature for a predetermined period of time to develop a dimensionally stable ruthenium oxide/titanium oxide coating on the bipolar plates 18 and 30 that is hydrophilic and electrically conductive in the fuel cell environment. In one embodiment, the titanium and the ruthenium chloride solution are calcinated at about 450° C. for about 10 minutes. Titanium and ruthenium have almost identical crystalline structures. When the ruthenium chloride is deposited on the titanium or titanium oxide layer and calcinated, a bottom layer of titanium or titanium oxide and a top layer of titanium oxide/ruthenium oxide is formed. Because of the properties of the titanium oxide and the ruthenium oxide as discussed herein, the stainless steel used to make the bipolar plates 18 and 30 can be low grade. The thickness of the titanium or titanium oxide layer can be in the range of 10-500 nm and the thickness of the titanium oxide/ruthenium oxide layer can be in the range of 1 nm-50 nm.
After the calcination process, the different phases of the titanium oxide layer and the ruthenium oxide layer are combined as one phase, i.e., they form an extensive solid solution. The titanium oxide provides the hydrophilicity and the ruthenium oxide provides the conductivity. Further, because ruthenium oxide does not adhere well to stainless steel, the titanium or titanium oxide allows the ruthenium oxide to be deposited on stainless steel bipolar plates. Also, during the calcination process, the ruthenium chloride is converted to ruthenium oxide. The titanium layer is applied to the stainless steel prior to applying the ruthenium chloride to the stainless steel to protect the stainless steel against the aggressive nature of the chloride ions at the calcination temperature. Without the ruthenium chloride applied to the titanium layer, the process would also oxidize the titanium and convert it to titanium oxide, which is electrically non-conductive.
Table I shows the total resistance for a bipolar plate coated with a ruthenium oxide/titanium oxide layer in the manner as discussed above at different compression pressures, and Table II shows contact resistances for a bipolar plate including a titanium oxide layer on a stainless bipolar plate without the titanium oxide/ruthenium oxide layer with the same compression pressures. The contact resistance advantages of the ruthenium oxide are apparent.
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.
