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Improving Costs and Efficiency of PEM Fuel Cell Vehicles by Modifying the Surface of Stainless Steel Bipolar Plates

National Renewable Energy Laboratory

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Technology Marketing SummaryFuel cell vehicles have the potential to reduce our dependence on foreign oil and lower emissions. Running the vehicle’s motor on hydrogen rather than gasoline, fuel cell vehicles emit no greenhouse gases – only water and heat. However, fuel cell vehicles are currently too expensive to compete with conventional gasoline and diesel vehicles or even hybrids. Manufacturers must bring down production costs, especially the costs of the fuel cell stack and hydrogen storage, to make fuel cell vehicles price-competitive with conventional vehicles.

NREL scientists have addressed part of the costs associated with the fuel cell stack. A major contributor to this expense is the cost of the bipolar plates, which electrically connect the anode of cell to the cathode of the next to create voltage. The membrane electrode assembly (MEA) is sandwiched between two gas diffusion layers with these bipolar plates on either side. Improving the efficiency of these plates is a major breakthrough in cost reduction of the fuel cell module. DescriptionOther bipolar plate technologies must simultaneously address concerns of corrosiveness, electrical conductivity and price. Metallic alloys could be ideal for bipolar plates because they are low-cost, possess high thermal and electrical conductivities and can be made in thin sheets. However, metallic alloys are highly corrosive in the PEM fuel cell environment, causing problems in electrical resistance, thus deeming them impractical for bipolar plates. Carbon and graphite are sometimes used for bipolar plates but cause issues in performance due to low power density and are expensive to machine. Another PEM fuel cell using chromium balanced with a base metal proves to be very corrosion resistant and electrically conductive, however the use of it is too expensive to be used in widespread development of PEM fuel cells.

The present invention addresses the issues of cost, corrosion, and conductivity by making the bipolar plate out of stainless steel and treating it with a nitridation process wherein titanium and/or aluminum in the stainless steel are segregated to the surface layer, giving the surface layer a greater concentration of oxygen than nitrogen. The surface layer of the treated stainless steel is now chemically heterogeneous rather than uniform, with chromium, titanium, or aluminum. This precipitates the titanium nitride and aluminum oxide, forming a surface layer which has lower interfacial contact electrical resistance and increased corrosion resistance. Thus, increasing the efficiency and decreasing the costs of the bipolar plates.
BenefitsBy nitridating the surface, the oxide layer on the stainless steel alloys is modified, resulting in:
Decreased contact electrical resistance
Improved corrosion resistance

The nitridation process can be applied to inexpensive, commercially available stainless steel alloys.

Because the nitridation process makes the bipolar plates of PEM fuel cells more efficient, the commercialization of the PEM fuel cells for hydrogen vehicles is more feasible due to lower costs per unit of electricity.
Applications and IndustriesThe stainless steel alloys processed according to this invention are useful in bipolar plates of proton exchange membrane fuel cells for hydrogen vehicles, but could be used in other electrochemical energy conversion devices as well. Patents and Patent Applications
ID Number
Title and Abstract
Primary Lab
Date
Patent 7,247,403
Patent
7,247,403
Surface modified stainless steels for PEM fuel cell bipolar plates
A nitridation treated stainless steel article (such as a bipolar plate for a proton exchange membrane fuel cell) having lower interfacial contact electrical resistance and better corrosion resistance than an untreated stainless steel article is disclosed. The treated stainless steel article has a surface layer including nitrogen-modified chromium-base oxide and precipitates of chromium nitride formed during nitridation wherein oxygen is present in the surface layer at a greater concentration than nitrogen. The surface layer may further include precipitates of titanium nitride and/or aluminum oxide. The surface layer in the treated article is chemically heterogeneous surface rather than a uniform or semi-uniform surface layer exclusively rich in chromium, titanium or aluminum. The precipitates of titanium nitride and/or aluminum oxide are formed by the nitriding treatment wherein titanium and/or aluminum in the stainless steel are segregated to the surface layer in forms that exhibit a low contact resistance and good corrosion resistance.
Oak Ridge National Laboratory 07/24/2007
Issued
Technology Status
Technology IDDevelopment StageAvailabilityPublishedLast Updated
04-29PrototypeAvailable - available for licensing11/05/201011/05/2010

Contact NREL About This Technology

To: Eric Payne<Eric.Payne@nrel.gov>