WO2006019473A2 - Enhanced stability bipolar plate - Google Patents

Enhanced stability bipolar plate Download PDF

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Publication number
WO2006019473A2
WO2006019473A2 PCT/US2005/020381 US2005020381W WO2006019473A2 WO 2006019473 A2 WO2006019473 A2 WO 2006019473A2 US 2005020381 W US2005020381 W US 2005020381W WO 2006019473 A2 WO2006019473 A2 WO 2006019473A2
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WO
WIPO (PCT)
Prior art keywords
corrosion
bipolar plate
resistant coating
stainless steel
alloy
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/US2005/020381
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French (fr)
Other versions
WO2006019473A3 (en
Inventor
Mahmoud H. Abd Elhamid
Richard H. Blunk
Gayatri Vyas
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Motors Liquidation Co
Original Assignee
General Motors Corp
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Publication date
Application filed by General Motors Corp filed Critical General Motors Corp
Priority to DE112005001704T priority Critical patent/DE112005001704T5/en
Priority to CN2005800308261A priority patent/CN101019257B/en
Priority to JP2007522502A priority patent/JP2008507824A/en
Publication of WO2006019473A2 publication Critical patent/WO2006019473A2/en
Anticipated expiration legal-status Critical
Publication of WO2006019473A3 publication Critical patent/WO2006019473A3/en
Ceased legal-status Critical Current

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
    • C23C14/06—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material characterised by the coating material
    • C23C14/14—Metallic material, boron or silicon
    • C23C14/16—Metallic material, boron or silicon on metallic substrates or on substrates of boron or silicon
    • C23C14/165—Metallic material, boron or silicon on metallic substrates or on substrates of boron or silicon by cathodic sputtering
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
    • B32B15/015—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium the said other metal being copper or nickel or an alloy thereof
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/018—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of a noble metal or a noble metal alloy
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
    • C23C14/58—After-treatment
    • C23C14/584—Non-reactive treatment
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/02—Details
    • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
    • H01M8/0204—Non-porous and characterised by the material
    • H01M8/0206—Metals or alloys
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/02—Details
    • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
    • H01M8/0204—Non-porous and characterised by the material
    • H01M8/0206—Metals or alloys
    • H01M8/0208—Alloys
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/02—Details
    • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
    • H01M8/0204—Non-porous and characterised by the material
    • H01M8/0206—Metals or alloys
    • H01M8/0208—Alloys
    • H01M8/021—Alloys based on iron
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/02—Details
    • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
    • H01M8/0204—Non-porous and characterised by the material
    • H01M8/0223—Composites
    • H01M8/0228—Composites in the form of layered or coated products
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/50—Fuel cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T428/00—Stock material or miscellaneous articles
    • Y10T428/12—All metal or with adjacent metals
    • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
    • Y10T428/12771—Transition metal-base component
    • Y10T428/12861—Group VIII or IB metal-base component
    • Y10T428/12951—Fe-base component

Definitions

  • the present invention relates to fuel cells, which generate electricity to power vehicles or other machinery. More particularly, the present invention relates to a bipolar plate, which is coated with a corrosion-resistant coating of a high-grade stainless steel or alloy to impart fluoride resistance to the bipolar plate when used in a polyelectrolyte membrane ( " PEM) fuel cell " .
  • PEM polyelectrolyte membrane
  • Fuel cell technology is a relatively recent development in the automotive industry. It has been found that fuel cell power plants are capable of achieving efficiencies as high as 55%. Furthermore, fuel cell power plants emit only heat and water as by-products.
  • Fuel cells include three components: a cathode, an anode and an electrolyte which is sandwiched between the cathode and the anode and passes only protons. Each electrode is coated on one side by a catalyst.
  • the catalyst on the anode splits hydrogen into electrons and protons. The electrons are distributed as electric current from the anode, through a drive motor and then to the cathode, whereas the protons migrate from the anode, through the electrolyte to the cathode.
  • the catalyst on the cathode combines the protons with electrons returning from the drive motor and oxygen from the air to form water. Individual fuel cells can be stacked together in series to generate increasingly larger quantities of electricity.
  • a polymer electrode membrane serves as the electrolyte between a cathode and an anode.
  • the polymer electrode membrane currently being used in fuel cell applications requires a certain level of humidity to facilitate conductivity of the membrane. Therefore, maintaining the proper level of humidity in the membrane, through humidity/water management, is very important for the proper functioning of the fuel cell. Irreversible damage to the fuel cell will occur if the membrane dries out.
  • PEM fuel cell multiple fuel cells are frequently stacked in series to form a fuel cell stack.
  • a flow field plate serves as the anode for one fuel cell while the opposite side of the flow field plate serves as the cathode for an adjacent fuel cell. Because each flow field plate serves as both an anode and a cathode, the flow field plate is also known as a bipolar plate. Monopolar plates, such as anode coolant flow field plates, may be provided in the fuel cell stack. One side of the anode coolant flow field plate- serves- as-an ⁇ anode flowfield-prate. The opposite side of the anode coolant flow field plate serves as a cathode coolant flow field plate. Coolant channels of the anode coolant flow field plate and of the cathode coolant flow field plate may be combined to form collective coolant channels for cooling the fuel cell stack.
  • Bipolar plates for PEM fuel cells must be electrochemically stable, electrically conductive and inexpensive.
  • the corrosion of metallic bipolar plates in the fuel cell environment accelerates the corrosion process through degradation of the membrane.
  • the degradation products of the membrane include hydrogen fluoride (HF) , which accelerates the corrosion process, causing the corrosion process to become autocatalytic in nature.
  • HF hydrogen fluoride
  • 316L and other lower grades of stainless steels have been used as inexpensive bipolar plate materials.
  • US20030228512 Al discloses a method of improving the contact resistance of the surface of a stainless steel substrate while maintaining optimum corrosion resistance of the substrate by depositing a gold coating on the substrate.
  • US20040091768 Al discloses a method of increasing the corrosion resistance of a substrate by providing a polymeric conductive coating on the substrate.
  • U.S. Pat. No. 6,372,376 Bl discloses a method of increasing the corrosion resistance of a substrate by providing an electrically-conductive, corrosion-resistant polymer containing a plurality of electrically conductive, corrosion-resistant filler particles on the substrate.
  • the present invention is generally directed to a novel bipolar plate which is characterized by enhanced stability and resistance to fluoride corrosion in a fuel cell.
  • the bipolar plate of the present invention includes a stainless steel bipolar plate substrate which is typically a low-grade stainless steel, such as 316L, for example, and a corrosion-resistant coating, which is a higher-grade stainless steel or alloy, provided on the bipolar plate substrate.
  • the corrosion-resistant coating may be a high-grade stainless steel such as 904L or AL6XN stainless steel, for example.
  • the corrosion-resistant coating may be an alloy such as C-276, 254SMO, Carp-20, niobium and its alloys, tantalum and its alloys, or molybdenum alloys. Accordingly, the stainless steel or alloy coating renders the lower-grade stainless steeT bipolar " plate " substrate " sub ' stantially resistant to fluoride ions in the fuel cell environment, thus substantially prolonging the lifetime of the bipolar plate.
  • the present invention is further directed to a novel method for enhancing the corrosion resistance of a bipolar plate.
  • the method includes providing a bipolar plate substrate, which is typically a lower-grade stainless steel such as 316L or 304L stainless steel, for example, and providing a corrosion-resistant coating on the bipolar plate substrate.
  • the corrosion-resistant coating may be a high-grade stainless steel such as 904L or AL6XN stainless steel, for example.
  • the corrosion-resistant coating may be an alloy such as C-276, 254SMO, Carp-20, niobium and its alloys or tantalum and its alloys or molybdenum alloys, for example.
  • Figure 1 is a cross-section of a portion of a bipolar plate according to the present invention.
  • Figure 2 is a flow diagram illustrating sequential process steps carried out according to a method of fabricating the bipolar plate of the present invention
  • Figure 3 is a bar graph which compares the corrosion rates (plotted along the Y-axis) of various stainless steel alloys (plotted along the X-axis) ;
  • Figure 8 is a graph on which is plotted polarization curves obtained on 316L stainless steel and Nb in de-aerated 3.5% NaCl solution at a scan rate of ImV/s and at room temperature.
  • a bipolar plate substrate which is a low-grade stainless steel such as 316L stainless steel, is sputter-coated with a higher grade of stainless steel to fabricate a bipolar plate having an improved corrosion resistance to hydrogen fluoride.
  • the cathode side of the bipolar plate substrate is coated with a corrosion-resistant coating to increase the resistance of the cathode side of the bipolar plate to corrosion by chloride during operation of a fuel cell. Sputtering the surface of a 316L stainless steel bipolar plate substrate with a thin
  • corrosion-resistant coating of the higher-grade 904-L or AL6XN stainless steel for example, significantly lowers the corrosion rate of the bipolar plate in a fluoride ion environment while not adding significant cost to the bipolar plate.
  • Alloys which may be coated on the bipolar plate substrate as the corrosion-resistant coating include C-276, 254SMO, Carp-20, niobium and its alloys, molybdenum alloys or tantalum and its alloys, for example.
  • the contact resistance of the sputtered stainless steel coating can be minimized by providing a cover layer on the stainless steel coating.
  • the cover layer may be a thin layer ( ⁇ 10 run) of gold (Au) or an organic coating, for example.
  • cover layer examples include platinum and its alloys, rhodium, ruthenium and its alloys, and palladium and its alloys. Coating a lower-grade stainless steel or material with a higher- grade stainless steel or material according to the invention is cost-effective, since fabricating a bipolar plate using a higher-grade stainless steel or material is cost-prohibitive.
  • FIG. 1 is a cross-sectional view of an enhanced stability bipolar plate 10, hereinafter bipolar plate, according to the present invention.
  • the bipolar plate.10 includes a bipolar plate substrate 12 which is typically a low-grade stainless steel such as 316L stainless steel, for example.
  • the corrosion-resistant coating 14 preferably has a thickness of typically about 0.1-2 ⁇ m. Suitable grades of stainless steel for the corrosion-resistant coating 14 include 904L ss and AL6XN ss, for example.
  • the corrosion-resistant coating 14 may be an alloy such as C-276, 254SMO, Carp-20 or niobium and its alloys, for example.
  • the thickness of the cover layer 16 is preferably less than typically about 10 nm in the case of gold coatings and typically about 10-28 micrometers in the case of polymeric coatings.
  • FIG. 2 is a flow diagram which illustrates sequential process steps carried out for fabrication of an enhanced stability bipolar plate according to the present invention.
  • a bipolar plate substrate is provided.
  • the bipolar plate substrate is typically a low- grade stainless steel such as 316L, for example, and has the suitable size and configuration for use in a fuel cell stack of a fuel cell.
  • a corrosion-resistant coating is provided on the outer surface of the bipolar plate substrate.
  • the thickness of the corrosion-resistant coating is preferably in the range of typically about 0.1- 2 ⁇ m.
  • the corrosion-resistant coating may be a higher- grade stainless steel including, for example, 904L or AL6XN stainless steel.
  • the corrosion- resistant coating may be a corrosion-resistant alloy such as C-276, 254SMO, Carp-20 or niobium and its alloys, for example.
  • the corrosion-resistant layer is typically- sputtered onto the exterior surface of the bipolar plate substrate using a conventional magnetron sputtering process.
  • a cover layer is deposited on the corrosion-resistant coating to minimize the contact resistance of the corrosion-resistant layer.
  • the cover layer preferably has a thickness of less than typically about 10 run.
  • the cover layer may be, for example, gold (Au) or an organic material, for example.
  • Table (I) below presents the actual corrosion rates and the normalized corrosion rates far various grades of stainless steel, alloys and titanium.
  • the corrosion rate and normalized corrosion rate of each sample was obtained by soaking the sample in an etching solution (IM H 2 SO 4 + 0.IM HF) for an hour at 80 degrees C.
  • Table (I) above shows that the higher the grade of stainless steel/alloy, the better the corrosion resistance of the stainless steel/alloy in a sulfuric acid/hydrogen fluoride mixture.
  • the corrosion resistance of titanium in the same solution is comparable to that of 304L stainless steel.
  • the bar graph of Figure 3 prevents a visual comparison of the corrosion rates of various grades of stainless steel and various alloys.
  • the composition of the sputtered AL6XN alloy was almost identical to the source AL6XN alloy.
  • the same phase (as the target) of a costly high grade, high corrosion-resistant, stainless steel can be sputtered as a very thin layer on to a low grade, less expensive, lower corrosion-resistant stainless steel.
  • sputtering also avoids the environmental problems associated with plating. Sputtering also provides for superior adhesion of the corrosion-resistant coating to the bipolar plate substrate. The adhesion is particularly superior when high grade stainless steel is sputtered onto a lower grade stainless steel bipolar plate substrate.
  • the graph of Figure 6 shows a comparison between the 316L stainless steel and the sputtered samples. According to the graph, sputtering of the AL6XN alloy on the stainless steel significantly enhances the corrosion properties of the substrate. A thicker coating is desired to increase the lifetime of the sputtered samples.
  • Coatings from the group of niobium and its alloys, tantalum and its alloys and molybdenum alloys are well known to those skilled in the art to have excellent corrosion resistances in aggressive environments. These coatings can be sputtered on top of lower grades of stainless steel to provide the required electrochemical stability in the fuel cells environment.
  • Figure 8 shows two potentiodynamic polarization curves obtained on 316L stainless steel and on niobium in 3.5% NaCl solution.
  • the pitting potential for 316L is around 0.4 V (Ag/AgCl) , while a much higher potential is seen for the niobium sample. Severe pitting was seen on the 316L stainless steel sample after the experiment, while no such pitting was observed on the niobium sample after the experiment.
  • niobium a “ s " a coating, combines both fluoride corrosion-resistance and chloride corrosion-resistance, properties which are required for bipolar plates since fluoride ions are released during degradation of the membrane and chloride ions are present in the humidified air stream in the fuel cell stack.

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Abstract

An enhanced stability and inexpensive bipolar plate ( 10) for a fuel cell is disclosed. The enhanced stability bipolar plate ( 10) includes a bipolar plate substrate (12) and a corrosion-resistant coating (14) provided on the bipolar plate substrate (12). A method for enhancing corrosion resistance of a bipolar plate (10) is also disclosed.

Description

ENHANCED STABILITY BIPOLAR PLATE
Field of the Invention
[001] The present invention relates to fuel cells, which generate electricity to power vehicles or other machinery. More particularly, the present invention relates to a bipolar plate, which is coated with a corrosion-resistant coating of a high-grade stainless steel or alloy to impart fluoride resistance to the bipolar plate when used in a polyelectrolyte membrane ("PEM) fuel cell".
Background of the Invention
[002] Fuel cell technology is a relatively recent development in the automotive industry. It has been found that fuel cell power plants are capable of achieving efficiencies as high as 55%. Furthermore, fuel cell power plants emit only heat and water as by-products.
[003] Fuel cells include three components: a cathode, an anode and an electrolyte which is sandwiched between the cathode and the anode and passes only protons. Each electrode is coated on one side by a catalyst. In operation, the catalyst on the anode splits hydrogen into electrons and protons. The electrons are distributed as electric current from the anode, through a drive motor and then to the cathode, whereas the protons migrate from the anode, through the electrolyte to the cathode. The catalyst on the cathode combines the protons with electrons returning from the drive motor and oxygen from the air to form water. Individual fuel cells can be stacked together in series to generate increasingly larger quantities of electricity.
[004] In a Polymer-Electrolyte-Membrane (PEM) fuel cell, a polymer electrode membrane serves as the electrolyte between a cathode and an anode. The polymer electrode membrane currently being used in fuel cell applications requires a certain level of humidity to facilitate conductivity of the membrane. Therefore, maintaining the proper level of humidity in the membrane, through humidity/water management, is very important for the proper functioning of the fuel cell. Irreversible damage to the fuel cell will occur if the membrane dries out. [005] In the PEM fuel cell, multiple fuel cells are frequently stacked in series to form a fuel cell stack. In the fuel cell stack, one side of a flow field plate serves as the anode for one fuel cell while the opposite side of the flow field plate serves as the cathode for an adjacent fuel cell. Because each flow field plate serves as both an anode and a cathode, the flow field plate is also known as a bipolar plate. Monopolar plates, such as anode coolant flow field plates, may be provided in the fuel cell stack. One side of the anode coolant flow field plate- serves- as-an~ anode flowfield-prate. The opposite side of the anode coolant flow field plate serves as a cathode coolant flow field plate. Coolant channels of the anode coolant flow field plate and of the cathode coolant flow field plate may be combined to form collective coolant channels for cooling the fuel cell stack.
[006] Bipolar plates for PEM fuel cells must be electrochemically stable, electrically conductive and inexpensive. The corrosion of metallic bipolar plates in the fuel cell environment accelerates the corrosion process through degradation of the membrane. The degradation products of the membrane include hydrogen fluoride (HF) , which accelerates the corrosion process, causing the corrosion process to become autocatalytic in nature. 316L and other lower grades of stainless steels have been used as inexpensive bipolar plate materials.
[007] While 316L stainless steel exhibits a fair corrosion resistance to fluoride ions, the corrosion rate increases with the increase in the fluoride ion leach out rate. This problem can be mitigated somewhat by removing the hydrogen fluoride from the fuel cell environment or by ~usrng"high"er~ grades" of" stainless" steel which are more resistant to corrosion by fluoride ions than 316L stainless steel. The use of higher grades of stainless steel for the bipolar plate tends to increase the cost of the bipolar plate.
[008] Various methods are known for increasing the corrosion resistance of a corrosion-susceptible substrate. For example, US20030228512 Al discloses a method of improving the contact resistance of the surface of a stainless steel substrate while maintaining optimum corrosion resistance of the substrate by depositing a gold coating on the substrate. US20040091768 Al discloses a method of increasing the corrosion resistance of a substrate by providing a polymeric conductive coating on the substrate. U.S. Pat. No. 6,372,376 Bl discloses a method of increasing the corrosion resistance of a substrate by providing an electrically-conductive, corrosion-resistant polymer containing a plurality of electrically conductive, corrosion-resistant filler particles on the substrate.
[009] It has been found that coating the surface of a lower "grade"'s~Cai.nless"steeT"T5ϊpoTar~ plate," such as a 316L or 304L stainless steel bipolar plate, for example, with a thin layer of high-grade stainless steel or alloy imparts a high degree of fluoride ion corrosion resistance to the bipolar plate while maintaining the cost of the bipolar plate within acceptable levels.
Summary of the Invention
[0010] The present invention is generally directed to a novel bipolar plate which is characterized by enhanced stability and resistance to fluoride corrosion in a fuel cell. The bipolar plate of the present invention includes a stainless steel bipolar plate substrate which is typically a low-grade stainless steel, such as 316L, for example, and a corrosion-resistant coating, which is a higher-grade stainless steel or alloy, provided on the bipolar plate substrate. The corrosion-resistant coating may be a high-grade stainless steel such as 904L or AL6XN stainless steel, for example. Alternatively, the corrosion-resistant coating may be an alloy such as C-276, 254SMO, Carp-20, niobium and its alloys, tantalum and its alloys, or molybdenum alloys. Accordingly, the stainless steel or alloy coating renders the lower-grade stainless steeT bipolar"plate" substrate" sub'stantially resistant to fluoride ions in the fuel cell environment, thus substantially prolonging the lifetime of the bipolar plate.
[0011] The present invention is further directed to a novel method for enhancing the corrosion resistance of a bipolar plate. The method includes providing a bipolar plate substrate, which is typically a lower-grade stainless steel such as 316L or 304L stainless steel, for example, and providing a corrosion-resistant coating on the bipolar plate substrate. The corrosion-resistant coating may be a high-grade stainless steel such as 904L or AL6XN stainless steel, for example. Alternatively, the corrosion-resistant coating may be an alloy such as C-276, 254SMO, Carp-20, niobium and its alloys or tantalum and its alloys or molybdenum alloys, for example.
Brief Description of the Drawings
[0012] The invention will now be described, by way of example, with reference to the accompanying drawings, in which:
[0013] Figure 1 is a cross-section of a portion of a bipolar plate according to the present invention;
[0014] Figure 2 is a flow diagram illustrating sequential process steps carried out according to a method of fabricating the bipolar plate of the present invention;
[0015] Figure 3 is a bar graph which compares the corrosion rates (plotted along the Y-axis) of various stainless steel alloys (plotted along the X-axis) ; [0016] Figure 4 is a graph on which is plotted potβntiodynamic polarization curves obtained on 316L and AL6XN samples in a de-aerated 50 ppm HF solution of pH=3 and at 80 degrees C (scan rate 0.1 mV/S) ;
[0017] Figure 5 is a graph on which is plotted polarization curves for a sputtered AL6XN alloy coating on 316L stainless steel in a de-aerated solution of pH=3 and at different fluoride ion concentrations at 80 degrees C;
[0018] Figure 6 is a graph which compares the polarization curves"""b"f" Figure 4 with the"polarization curve obtained on 316L stainless steel at 50 ppm HF solution of pH=3 and at 80 degrees C;
[0019] Figure 7 is a graph on which is plotted potentiodynamic polarization curves obtained on 316L stainless steel and Nb in a de-aerated HF solution of pH=3 at 80 degrees C; and
[0020] Figure 8 is a graph on which is plotted polarization curves obtained on 316L stainless steel and Nb in de-aerated 3.5% NaCl solution at a scan rate of ImV/s and at room temperature. Detailed Description of the Invention
[0021] According to the present invention, a bipolar plate substrate, which is a low-grade stainless steel such as 316L stainless steel, is sputter-coated with a higher grade of stainless steel to fabricate a bipolar plate having an improved corrosion resistance to hydrogen fluoride. Preferably, the cathode side of the bipolar plate substrate is coated with a corrosion-resistant coating to increase the resistance of the cathode side of the bipolar plate to corrosion by chloride during operation of a fuel cell. Sputtering the surface of a 316L stainless steel bipolar plate substrate with a thin
(0.1-2μm) corrosion-resistant coating of the higher-grade 904-L or AL6XN stainless steel, for example, significantly lowers the corrosion rate of the bipolar plate in a fluoride ion environment while not adding significant cost to the bipolar plate. Alloys which may be coated on the bipolar plate substrate as the corrosion-resistant coating include C-276, 254SMO, Carp-20, niobium and its alloys, molybdenum alloys or tantalum and its alloys, for example. The contact resistance of the sputtered stainless steel coating can be minimized by providing a cover layer on the stainless steel coating. The cover layer may be a thin layer (<10 run) of gold (Au) or an organic coating, for example. Other suitable materials for the cover layer include platinum and its alloys, rhodium, ruthenium and its alloys, and palladium and its alloys. Coating a lower-grade stainless steel or material with a higher- grade stainless steel or material according to the invention is cost-effective, since fabricating a bipolar plate using a higher-grade stainless steel or material is cost-prohibitive.
[0022] Figure 1 is a cross-sectional view of an enhanced stability bipolar plate 10, hereinafter bipolar plate, according to the present invention. The bipolar plate.10 includes a bipolar plate substrate 12 which is typically a low-grade stainless steel such as 316L stainless steel, for example. A corrosion-resistant coating 14, which may be a grade of stainless steel higher than that of the bipolar plate substrate 12, is provided on the outer surface 13 of the bipolar plate substrate 12. The corrosion-resistant coating 14 preferably has a thickness of typically about 0.1-2μm. Suitable grades of stainless steel for the corrosion-resistant coating 14 include 904L ss and AL6XN ss, for example. Alternatively, the corrosion-resistant coating 14 may be an alloy such as C-276, 254SMO, Carp-20 or niobium and its alloys, for example. A cover layer 16, which may be gold (Au) or an organic material, for example, is typically provided on the corrosion-resistant coating 14 to keep the contact resistance of the corrosion-resistant coating 14 low. The thickness of the cover layer 16 is preferably less than typically about 10 nm in the case of gold coatings and typically about 10-28 micrometers in the case of polymeric coatings.
[0023] Figure 2 is a flow diagram which illustrates sequential process steps carried out for fabrication of an enhanced stability bipolar plate according to the present invention. In step 1, a bipolar plate substrate is provided. The bipolar plate substrate is typically a low- grade stainless steel such as 316L, for example, and has the suitable size and configuration for use in a fuel cell stack of a fuel cell.
[0024] In step 2, a corrosion-resistant coating is provided on the outer surface of the bipolar plate substrate. The thickness of the corrosion-resistant coating is preferably in the range of typically about 0.1- 2μm. The corrosion-resistant coating may be a higher- grade stainless steel including, for example, 904L or AL6XN stainless steel. Alternatively, the corrosion- resistant coating may be a corrosion-resistant alloy such as C-276, 254SMO, Carp-20 or niobium and its alloys, for example. The corrosion-resistant layer is typically- sputtered onto the exterior surface of the bipolar plate substrate using a conventional magnetron sputtering process.
[0025] In step 3, a cover layer is deposited on the corrosion-resistant coating to minimize the contact resistance of the corrosion-resistant layer. The cover layer preferably has a thickness of less than typically about 10 run. The cover layer may be, for example, gold (Au) or an organic material, for example.
[0026] Table (I) below presents the actual corrosion rates and the normalized corrosion rates far various grades of stainless steel, alloys and titanium. The corrosion rate and normalized corrosion rate of each sample was obtained by soaking the sample in an etching solution (IM H2SO4 + 0.IM HF) for an hour at 80 degrees C.
Table I
Figure imgf000014_0001
[0027] Table (I) above shows that the higher the grade of stainless steel/alloy, the better the corrosion resistance of the stainless steel/alloy in a sulfuric acid/hydrogen fluoride mixture. The corrosion resistance of titanium in the same solution is comparable to that of 304L stainless steel. The bar graph of Figure 3 prevents a visual comparison of the corrosion rates of various grades of stainless steel and various alloys.
[0028] The invention will be better understood by reference to the following example.
EXAMPLE-AL6XN COATING ON STAINLESS STEEL
[0029] One (1) micron of an AL6XN stainless steel alloy coating was sputtered on a 316L stainless steel sample using magnetron sputtering. The composition of the coating was analyzed using Electron Probe Microanalysis (EPM) , the results of which are presented in Table (II) below.
Figure imgf000015_0001
[0030] From a consideration of Table (II) , it can be seen that the composition of the sputtered AL6XN alloy was almost identical to the source AL6XN alloy. As such, in one embodiment of the invention, the same phase (as the target) of a costly high grade, high corrosion-resistant, stainless steel can be sputtered as a very thin layer on to a low grade, less expensive, lower corrosion-resistant stainless steel. In addition to allowing the same phase to be deposited, sputtering also avoids the environmental problems associated with plating. Sputtering also provides for superior adhesion of the corrosion-resistant coating to the bipolar plate substrate. The adhesion is particularly superior when high grade stainless steel is sputtered onto a lower grade stainless steel bipolar plate substrate.
"["0"03I] 'me graph of"Figure "4"shows the corrosion behavior of the bulk AL6XN alloy as compared to that of the 316L stainless steel in de-aerated HF solutions of pH=3 and at 80 degrees C. The graph indicates that with regard to corrosion characteristics, the AL6XN alloy is vastly superior to the 316L stainless steel in the same potential domain.
[0032] The corrosion behavior of the sputtered AL6XN alloys on 316L stainless steel samples is shown in the graph of Figure 5. Figure 5 shows that the sputtered alloys show a similar behavior to that of the bulk alloy at 50 ppm, HF. The corrosion rates of the samples decrease with the decrease in the HF concentration.
[0033] The graph of Figure 6 shows a comparison between the 316L stainless steel and the sputtered samples. According to the graph, sputtering of the AL6XN alloy on the stainless steel significantly enhances the corrosion properties of the substrate. A thicker coating is desired to increase the lifetime of the sputtered samples.
[0034] Coatings from the group of niobium and its alloys, tantalum and its alloys and molybdenum alloys are well known to those skilled in the art to have excellent corrosion resistances in aggressive environments. These coatings can be sputtered on top of lower grades of stainless steel to provide the required electrochemical stability in the fuel cells environment.
[0035] The graph of Figure 7 shows potentiodynamic polarization curves obtained on niobium and 316L stainless steel in a de-aerated solution of 50 ppm HF of pH=3 at 80 degrees C. Under the same potential conditions, niobium exhibits a much lower current than does 316L stainless steel. Niobium also exhibits an excellent chloride-pitting corrosion resistance.
[0036] Figure 8 shows two potentiodynamic polarization curves obtained on 316L stainless steel and on niobium in 3.5% NaCl solution. The pitting potential for 316L is around 0.4 V (Ag/AgCl) , while a much higher potential is seen for the niobium sample. Severe pitting was seen on the 316L stainless steel sample after the experiment, while no such pitting was observed on the niobium sample after the experiment. These data indicate that niobium, a"s "a coating, combines both fluoride corrosion-resistance and chloride corrosion-resistance, properties which are required for bipolar plates since fluoride ions are released during degradation of the membrane and chloride ions are present in the humidified air stream in the fuel cell stack.
[0037] While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.

Claims

1. An enhanced stability bipolar plate for a fuel cell, comprising: a bipolar plate substrate; and a corrosion-resistant coating sputtered on said bipolar plate substrate.
2. The enhanced stability bipolar plate of claim 1 wherein said bipolar plate substrate comprises 304, 316L or other low corrosion resistance grades stainless steel.
3. The enhanced stability bipolar plate of claim 1 wherein said corrosion-resistant coating comprises 904L grade stainless steel.
4. The enhanced stability bipolar plate of claim 1 wherein said corrosion-resistant coating comprises C-276 alloy.
5. The enhanced stability bipolar plate of claim 1 wherein said corrosion-resistant coating comprises 254SMO alloy.
6. The enhanced stability bipolar plate of claim 1 wherein said corrosion-resistant coating comprises Carp-20 alloy.
7. The enhanced stability bipolar plate of claim 1 wherein said corrosion-resistant coating comprises niobium.
8. The enhanced stability bipolar plate of claim 1 wherein said corrosion-resistant coating comprises AL6XN grade stainless "steel.
9. An enhanced stability bipolar plate for a fuel cell, comprising: a stainless steel bipolar plate substrate; a corrosion-resistant coating provided on said bipolar plate substrate, said corrosion-resistant coating having a corrosion resistance higher than a corrosion resistance of said bipolar plate substrate; and a cover layer provided on said corrosion- resistant coating for minimizing contact resistance of said corrosion-resistant coating.
10. The enhanced stability bipolar plate of claim 9 wherein said bipolar plate substrate comprises 304L or 316L grade stainless steel.
11. The enhanced stability bipolar plate of claim 9 wherein said corrosion-resistant coating comprises at least one of 904-L grade stainless steel and AL6XN grade stainless steel.
12. The enhanced stability bipolar plate of claim 9 "wEerein" saTd "cόrro'siδh-resTstantT coating comprises "at least one of C-276 alloy, 254SMO alloy, Carp-20 alloy, niobium and a niobium alloy.
13. The enhanced stability bipolar plate of claim 9 wherein said cover layer comprises at least one of gold, an organic material, platinum, platinum alloys, rhodium, ruthenium, ruthenium alloys, palladium and palladium alloys.
14. The enhanced stability bipolar plate of claim 13 wherein said bipolar plate substrate comprises 304L or 316L grade stainless steel.
15. The enhanced stability bipolar plate of claim 14 wherein said corrosion-resistant coating comprises at least one of 904-L grade stainless steel and AL6XN grade stainless steel.
16. The enhanced stability bipolar plate of claim 14 wherein said corrosion-resistant coating comprises at least""one"of"C-2T6 "alloy,~~254SMCTaIlOy, Carp-20" alloy, niobium and a niobium alloy, tantalum and its alloys and molybdenum alloys.
17. A method for enhancing corrosion resistance of a bipolar plate, comprising: providing a bipolar plate substrate; and providing a corrosion-resistant coating on said bipolar plate substrate.
18. The method of claim 17 wherein said bipolar plate substrate comprises 316L grade stainless steel.
19. The method of claim 17 wherein said corrosion- resistant coating comprises at least one of 904-L grade stainless steel and AL6XN grade stainless steel.
20. The method of claim 17 wherein said corrosion- resistant coating comprises at least one of C-276 alloy, 254SMO alloy, Carp-20 alloy, niobium and a niobium alloy, tantalum and its alloys and molybdenum alloys.
21. The method of claim 17 further comprising providing a cover layer on said corrosion-resistant layer for minimizing contact resistance of said corrosion- resistant layer.
22. The method of claim 21 wherein said cover layer comprises at least one of gold, an organic material, platinum, platinum alloys, rhodium, ruthenium, ruthenium alloys, palladium and palladium alloys.
23. The method of claim 21 wherein said cover layer comprises a sputtered layer.
24. The method of claim 21 wherein said bipolar plate substrate comprises 316L grade stainless steel.
25. The method of claim 24 wherein said corrosion- resistant coating comprises at least one of 904-L grade stainless steel and AL6XN grade stainless steel.
26. The method of claim 24 wherein said corrosion- resistant coating comprises at least of one of C-276 alloy, 254SMO alloy, Carp-20 alloy, niobium and a niobium alloy.
27. The method of claim 17 wherein said providing a corrosion-resistant coating on said bipolar plate substrate comprises sputtering said corrosion-resistant coating on said bipolar plate using a physical vapor deposition process.
28. A method for enhancing chloride-corrosion resistance of a cathode side of a bipolar plate in a fuel cell, comprising: providing a bipolar plate substrate having a cathode side; and providing a corrosion-resistant coating on said cathode side of said bipolar plate substrate.
29. The method of claim 28 wherein said bipolar plate substrate comprises 316L grade stainless steel.
30. The method of claim 29 wherein said corrosion- resistant coating comprises at least one of 904-L grade stainless steel and AL6XN grade stainless steel.
31. The method of claim 29 wherein said corrosion- resistant coating "comprises at least one of C-276 alloy, 254SMO alloy, Carp-20 alloy, niobium and a niobium alloy, tantalum and its alloys and molybdenum alloys.
32. The method of claim 29 further comprising providing a cover layer on said corrosion-resistant layer for minimizing contact resistance of said corrosion- resistant layer.
33. The method of claim 32 wherein said cover layer comprises at least one of gold, an organic material, platinum, platinum alloys, rhodium, ruthenium, ruthenium alloys, palladium and palladium alloys.
34. A method for enhancing corrosion resistance of a bipolar plate, comprising: providing a bipolar plate substrate, the bipolar substrate have a first corrosion resistance; and sputtering a corrosion-resistant coating on said bipolar plate substrate, the corrosion-resistant coating have a second corrosion resistance, and wherein the second corrosion resistance is greater than the first corrosion resistance.
35. The method as set forth in claim 34 wherein the bipolar substrate comprises a metal.
36. The method as set forth in claim 34 wherein the bipolar plate substrate comprises a material comprising a first atomic element, and the corrosion-resistant coating comprising the first atomic element.
37. The method as set forth in claim 35 wherein the bipolar plate substrate comprises a material comprising first, second and third atomic elements, and the corrosion-resistant coating comprising the first, second and third atomic elements.
38. The method as set forth in claim 34 wherein the sputtering comprises physic vaporization of a target comprising a material having a first elemental composition and wherein the corrosive-resistant coating has substantially the same first elemental composition.
39. The method as set forth in claim 34 wherein the bipolar "plate substrate comprises a first" grade" of stainless steel and the corrosion-resistant coating comprises a second grade of stainless steel.
40. The method of claim 34 wherein said bipolar plate substrate comprises 316L grade stainless steel.
Al.' The method of claim 34 wherein said corrosion- resistant coating comprises at least one of 904-L grade stainless steel and AL6XN grade stainless steel.
42. The method of claim 34 wherein said corrosion- resistant coating comprises at least one of C-276 alloy, 254SMO alloy, Carp-20 alloy, niobium and a niobium alloy, tantalum and its alloys and molybdenum alloys.
43. The method of claim 34 further comprising providing a cover layer on said corrosion-resistant layer for minimizing contact resistance of said corrosion- resistant layer.
44. The method of claim 43 wherein said cover layer comprises at least one of gold, an organic"material, platinum, platinum alloys, rhodium, ruthenium, ruthenium alloys, palladium and palladium alloys.
45. The method of claim 43 wherein said cover layer comprises a sputtered layer.
46. The method of claim 43 wherein said bipolar plate substrate comprises 316L grade stainless steel.
47 . The method of claim 44 wherein said corrosion- resistsnt coating comprises at least one of 904-L grade stainless steel and AL6XN grade stainless steel .
48 . The method of claim 44 wherein said corrosion- resistanfc coating comprises at least of one of C-276 alloy, 254SMO alloy, Carp-20 alloy, niobium and a niobium alloy.
49 . The method as set forth in claim 34 wherein the thicknessof the corrosion-resistant coating ranges from 0 . 1-2 μm.
50. The method as set forth in claim 34 wherein the thickness of the corrosion-resistant coating ranges from 0.1-10 μm.
51. The method as set forth in claim 34 wherein the bipolar plate substrate comprises a first grade of stainless steel and the corrosion-resistant coating comprises a second grade of stainless steel, and wherein the thickness of the corrosion-resistant coating ranges from 0.1'-2 μm.
29
&i
52. The method of claim 43 wherein said bipolar plate substrate comprises 304L grade stainless steel.
53.' The method as set forth in claim34 wherein the bipolar plate substrate comprises at least one of a ferrite and austenitic material.
54. The method as set forth in claim 34 wherein the bipolar plate substrata comprises at least one of a low grade ferrite material and a low grade austenitic material.
55. The method as set forth in claim 34 wherein both the bipolar plate substrate and the corrosion-resistant coating each comprises chromium, and wherein the elemental weight percent of chromium in the corrosion resistant coating is greater than the elemental weight percent of chromium in the bipolar plate substrate.
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