WO2004038842A2

WO2004038842A2 - FUEL CELL HAVING TiAlNO DEPOSITED AS A PROTECTIVE LAYER ON METALLIC SURFACES

Info

Publication number: WO2004038842A2
Application number: PCT/US2003/033223
Authority: WO
Inventors: Hyunchul C. Kim; Terry L. Alford
Original assignee: Arizona Board Of Regents, Acting For And On Behalf Of Arizona State University
Priority date: 2002-10-22
Filing date: 2003-10-20
Publication date: 2004-05-06
Also published as: WO2004038842A3; AU2003286517A1

Abstract

A molten carbonate fuel cell (10) generates electricity based on an electrochemical reaction. The carbonate fuel cell has first and second separator plates (20, 22) made with stainless-steel. Electroplates (24, 26) are mounted to the separator plates. An electrolyte (40) is disposed between the electroplates. The electrolyte is corrosive at high temperatures. A protective layer is formed on the separator plates and the electroplates. The protective layer is formed using RF reactive sputtering chamber. The protective layer comprises titanium, aluminum, nitrogen, and oxygen with a composition ratio of about 1:1.4:3.0:1.0, respectively. The protective layer is corrosion resistant at high temperature and has good adhesion to stainless-steel and other metals.

Description

Fuel Cell having TiAlNO Deposited as a Protective Layer on Metallic Surfaces

Claim to Domestic Priority

[00001] The present non-provisional patent application claims priority to provisional application serial no. 60/420,358, entitled "Use of TiAlxNyOz Deposited by Reactive Sputtering as a Coating Material on Stainless Steel for Fuel Cells," filed on October 22, 2002, by Hyunchul Kim et al .

Cross Reference to Related Patent Application (s)

[00002] The present non-provisional patent application is related to a non-provisional application entitled "Apparatus and Method of Using Thin Film Material as Diffusion Barrier for Metallization," filed on September 24, 2003, Attorney Docket No. 130588.91477, by Hyunchul Kim et al .

Field of the Invention

[00003] The present invention relates in general to carbonate fuel cells and, more particularly, to a TiAlNO protective layer deposited on the metallic surfaces of carbonate fuel cells.

Background of the Invention

[00004] Portable electric power generators are used in many locations and applications. Portable electric generators generate electricity for buildings, equipment, lighting, refrigeration units, monitors, computers, security sensors, heating, and air conditioning. Typically applications include power distribution centers, hospitals needing back-up electric power supply, emergency agencies, military, critical government facilities, crisis centers, and remote settlements and settings. Portable power generators also find uses in houseboats, recreational vehicles, residential and business back-up power, construction sites, entertainment venues, and sporting events.

[00005] Gasoline powered generators are commonly used in portable power generator applications. In some portable generator applications, e.g., houseboats and recreational vehicles, severe injuries and even death have occurred from exposure to the gasoline engine exhaust. Moreover, gasoline engines are known to create noise and air pollution. Solar powered generators are less developed, more expensive, and have limited power generation capacity. In solar powered generators, electric energy must be stored in a battery for use during non-daylight hours. Other types of fuel sources, such as natural gas and propane, have been tried for portable electric generators but received limited acceptance. [00006] Another type of electric power generator is based on carbonate fuel cells (CFC) . CFCs are known for high power applications such as utility power distribution centers, on the order of 250 kilowatts. CFCs convert fuel, such as coal- derived gas, liquid petroleum gas (LPG) , methanol, and natural gas, to electricity through an electrochemical reaction involving hydrogen and oxygen interaction with carbonate ions. [00007] The typical CFC includes an electrolyte contained between an anode electroplate and a cathode electroplate. The electrolyte is a carbonate salt mixture. The electroplates are made with nickel or nickel alloy and are mounted to or supported by separator plates. The separator plates are made with stainless-steel having nickel or nickel-copper alloy coating on the anode side and iron or nickel-based alloy on the cathode side. The separator plates provide a conduit for the fuel gases on the anode side and oxidant gas on the cathode side. [00008] The electrolyte operates at a high temperature, on the order of 600-700°C, to maintain the carbonate salt mixture in a molten state, which is necessary for an efficient chemical reaction to generate free electrons. The high temperature in combination with the electrolyte and reaction gases creates a corrosive environment, which cause the electroplates and separator plates to corrode and fail. One factor limiting the economic feasibility and commercial acceptance and viability of CFCs is the maintenance issues and limited operating life caused by the corrosive environment.

Summary of the Invention

[00009] In one embodiment, the present invention is a carbonate fuel cell comprising first and second separator plates made with stainless-steel. First and second electroplates are supported by the first and second separator plates, respectively. An electrolyte is disposed between the first and second electroplates. A protective layer is formed on the separator plates. The protective layer comprises titanium, aluminum, nitrogen, and oxygen.

[00010] A method of making an electrical generating fuel cell comprising the steps of providing first and second separator plates, providing first and second electroplates supported by the first and second separator plates, respectively, disposing an electrolyte between the first and second electroplates, and forming a protective layer on a metallic surface within the electrical generating fuel cell which is subject to corrosion. The protective layer comprises titanium, aluminum, nitrogen, and oxygen.

Brief Description of the Drawings

[00011] FIG. 1 illustrates a molten carbonate fuel cell; and FIG. 2 illustrates a reactive sputtering chamber for depositing TiAlNO thin film on the surface of a metallic plate.

Detailed Description of the Drawings

[00012] Referring to FIG. 1, a carbonate fuel cell (CFC) 10 is shown. In one embodiment, CFC 10 is referred to as a molten carbonate fuel cell. CFC 10 is an energy conversion device that consumes fuel and oxygen and generates electric power at terminals 12 and 14. An electrical load 16 is connected across terminals 12 and 14. CFC 10 has applications in electric power distribution centers and electric generators. When used as a portable generator, CFC 10 can generate electricity for buildings, equipment, lighting, refrigeration units, monitors, computers, security sensors, heating, and air conditioning.

Other applications include hospitals needing back-up electric power supply, emergency agencies, military, critical government facilities, crisis centers, automotive power plants, residential and business back-up power, and remote settlements and settings.

[00013] CFC 10 is an attractive design choice for electrical generators, including portable applications, because it does not rely on combustion to convert fuel to electrical energy. The energy stored in the fuel is converted directly into DC electricity through an electrochemical process. The fuel is catalytically reacted, i.e., electrons are removed from the fuel elements, in the fuel cell to create an electric current. The fuel cells include an electrolyte material disposed between two porous electroplates functioning as the anode and cathode. The fuel passes over the anode where it catalytically splits into ions and electrons. Oxygen passes over the cathode. The free electrons create an electrical current which flows to the electric load. The ions move through the electrolyte toward the oppositely charged cathode electroplate. At the cathode, ions combine to create by-products, primarily water and C0₂. CFC 10 is safe and environmental friendly in the generation of electricity.

[00014] CFC 10 includes separator plates 20 and 22. Separator plates 20 and 22 are made with 306 or 316 stainless- steel or nickel alloy. Multiple CFCs can be stacked to increase the total power generation capacity. Separator plates 20 and 22 provide separation and support for stacked CFCs and further provide mounting support structure for electroplate 24 and electroplate 26, respectively. Electroplate 24 is the negatively charge anode and electroplate 26 is the positively charged cathode of CFC 10. Electroplates 24 and 26 should have low resistance and high conductivity and can be made from nickel, copper, silver, iron, chrome, or gold. [00015] Separator plate 20 also includes a conduit 28 to receive a fuel gas. The fuel gas can be based on hydrogen, methanol, coal-derived gases, liquid petroleum gas (LPG) , or natural gas. The exhaust gas from the depleted fuel and product gases are vented through conduct 30. Separator plate 22 includes a conduit 32 to receive oxidant gas and a conduit 34 to exhaust the depleted oxidant gas. The fuel gas and oxidant gas are used in the carbonate chemical reaction. A wet seal is formed around the perimeter where electroplate 24 is mounted to or supported by separator plate 20. Likewise, a wet seal is formed around the perimeter where electroplate 26 is mounted to or supported by separator plate 22. An electrolyte 40 is suspended in a porous, insulating, and chemically inert ceramic (LiA10₂) matrix 42 disposed between electroplates 24 and 26. Electrolyte 40 is a eutectic solution of lithium carbonate (Li₂C0₃) , potassium carbonate (K₂C0₃) , or sodium carbonate

(Na₂C0₃) , forming a carbonate salt solution. The electrolyte is heated to a high temperature in the range of 600-700°C and maintained in a molten (liquid) state to make a good ionic conductor. [00016] The fuel gas is introduced into conduit 28 of separator plate 20. An oxidant gas is introduced into conduit 32 of separator plate 22. In some embodiments of CFC 10, conduits 28 and 30 in separator plate 20 are routed normal with respect to conduits 32 and 34 in separator plate 22.

Accordingly, the fuel gas flow is normal to the oxidant gas flow. The fuel and oxidant gases flow past the surface of the anode and cathode opposite the electrolyte and generate electrical energy by the electrochemical oxidation of hydrogen and the electrochemical reduction of oxygen. The introduction of the fuel gas, in combination with the oxidant gas, produces an electrochemical reaction in electrolyte 40 as follows:

Anode reaction : H₂ + C0₃ ^2" => H₂0 + C0₂ + 2e^"

Cathode reaction : 0₂ + 2C0₂ + 4e^~ => 2C0₃ ^2~

[00017] Notice that the anode process involves an electrochemical reaction between hydrogen and carbonate ions from the electrolyte and produces water and carbon dioxide while releasing free electrons. The cathode process involves an electrochemical reaction between oxygen and carbon dioxide from the oxidant gas stream with electrons from the cathode to produce carbonate ions which enter electrolyte 40. [00018] When electrolyte 40 is heated to a molten state, about 650°C, the carbonate ions become conductive and flow from the cathode to the anode. At the anode, hydrogen reacts with the carbonate ions to produce water, carbon dioxide, and electrons. The electrons travel through terminal 12, load 16, and return by way of terminal 14 to the cathode. At the cathode, oxygen and carbon dioxide recycled from the anode react with the electrons to form C0₃ ions that replenish electrolyte 40. This electrochemical process transfers electrical energy from CFC 10 to electrical load 16. CFC 10 can reach fuel-to-electricity generation efficiencies approaching 60%.

[00019] In its molten state at high temperatures, electrolyte 40 and the reaction gases are highly corrosive to the metallic surfaces of separator plates 20 and 22 and electroplates 24 and 26. Within CFC 10, separator plates 20 and 22 and electroplates 24 and 26 are exposed to or subject to the corrosive nature of electrolyte 40 and the reaction gases. Without some form of protection, separator plates 20 and 22 and electroplates 24 and 26 can corrode and require maintenance, or fail to operate, in less than 10000 hours of operation. Corrosion is a primary factor limiting the practical operating life of CFC 10. In order to make CFC 10 economically efficient and commercially viable as an electrical power generator, it is important to extend the operating life and otherwise keep the maintenance costs of CFC 10 within reason. [00020] In addition, many types of metals have been considered and used for the components of CFC 10. Some metals such as silver and copper are more conductive. Other metals such as iron and nickel are less costly. Stainless-steel has corrosion resistant properties. The present invention, which involves forming a protective layer around certain metallic components of CFC 10, gives the designer freedom to choose metals based on other attributes such as cost and conductivity. The protective layer is effective in the corrosive environment and provides good adhesion to the metallic surfaces. [00021] Accordingly, an anti-corrosive protective layer is deposited or formed on the metallic surfaces of separator plates 20 and 22 and electroplates 24 and 26. The protective layer acts as a diffusion barrier to prevent corrosive gases from penetrating to the metallic surfaces. The protective layer also has a high temperature property. The protective layer increases the operating life of separator plates 20 and 22 and electroplates 24 and 26 to a mean time between maintenance/failure in excess of 40000 hours. [00022] The protective layer is implemented as a thin film material comprising titanium (Ti) , aluminum (Al) , nitrogen (N) , and oxygen (0) . In one embodiment, the thin film material has a composition ratio given as Ti_wAl_xN_yO_z, where w=l, x=1.4±0.5, y=3.0+0.3, and z=l.l±0.2. Other composition ratios of TiAlNO may also be used for the protective layer. The TiAlNO thin film deposited or formed on the metallic surfaces of separator plates 20 and 22 and electroplates 24 and 26 is between 90-200 nm in thickness.

[00023] The formation of the TiAlNO thin film protective layer on metal surfaces within CFC 10 is described using FIG. 2. A metallic plate 50 is placed in RF reactive sputtering chamber 52. Metallic plate 50 represents separator plates 20 and 22, electroplates 24 and 26, or other corrosion sensitive metallic components of CFC 10, before final assembly of the fuel cell. Metallic plate 50 can be made from stainless-steel, nickel, copper, silver, iron, chrome, or gold. A positive DC voltage is applied to the platter supporting metallic plate 50. One or more intermetallic TiAl solid disk sputtering targets 54 are mounted to a platter and suspended in chamber 52. The sputtering target 54 contains 40% Ti and 60% Al with 99.95% purity. Each sputtering target 54 is about 5 centimeters (cm) in diameter. An AC voltage in series with a negative DC voltage is applied to the platter supporting sputtering targets 54. A vacuum of about 1.3 x 10^~6 Pa is drawn on chamber 52 by vacuum pump 56.

[00024] Gases are introduced into chamber 52 from gas supply 58. Nitrogen gas having 99.999% purity and 10 standard cubic centimeter/meter (seem) flow rate is introduced into chamber 52. In addition, Argon (Ar) at a pressure of 0.8 Pascals (Pa) and 99.999% purity, and a combination of less than 10 parts per million (ppm) of 0₂, CO, C0₂, H₂0, and CH₄, are introduced into chamber 52 from gas supply 58. The introduced gases are mixed with the residual oxygen in chamber 52. The collision of argon atoms in chamber 52 creates positively charged ions (plasma) which are attracted to and collide with negatively biased sputtering targets 54. Molecules and particles of the TiAl sputtering target are dislodged or eroded from the impact and fall through the nitrogen and oxygen gases in chamber 52 toward metallic plate 50. The AC signal keeps the smaller electrons oscillating in the middle of chamber 52 to induce more collisions. The nitrogen and oxygen ions react with the TiAl to form Ti_wAl_xN_yO_z in the ratios noted above. The RF power, bias, pressure, substrate temperature, gas flows, and gas ratios in chamber 52 can be controlled to alter the TiAlNO ratios . [00025] With the temperature at about 400°C, the RF power about 300 watts, and deposition rate of about 0.1 nm/sec, a TiAlNO thin film layer of about 140 nm in thickness is deposited or formed on metallic plate 50. The base pressure and operation pressure are kept about 6.65 x 10^~5 Pa and 6.65 x 10^-4 Pa, respectively. The TiAlNO thin film is formed on metallic plate 50 to provide an anti-corrosive protective layer and diffusion barrier. The TiAlNO thin film protective layer has excellent adhesion to stainless steel and other metals. [00026] Rutherford backscattering spectrometry (RBS) analysis shows that TiAlNO thin film protective layer formed by reactive sputtering method as explained above has good thermal stability up to 800°C. The TiAlNO thin film formed by the aforedescribed deposition process can be utilized as a protective layer on metallic surfaces, including separator plates 20 and 22 and electroplates 24 and 26, in CFC 10 and other fuel cell applications because it exhibits corrosion resistance, high temperature properties, and excellent adhesion to stainless- steel and other metals. The protective layer provides the needed corrosion barrier and allows the metal components for CFC 10 to be selected based on other properties such as cost and conductivity.

[00027] A person skilled in the art will recognize that changes can be made in form and detail, and equivalents may be substituted for elements of the invention without departing from the scope and spirit of the invention. The present description is therefore considered in all respects to be illustrative and not restrictive, the scope of the invention being determined by the following claims and their equivalents as supported by the above disclosure and drawings.

Claims

ClaimsWhat is claimed is:

1. A carbonate fuel cell, comprising: first and second separator plates made with stainless- steel; first and second electroplates supported by the first and second separator plates, respectively; an electrolyte disposed between the first and second electroplates; and a protective layer formed on the separator plates, wherein the protective layer comprises titanium, aluminum, nitrogen, and oxygen.

2. The carbonate fuel cell of claim 1 wherein a composition ratio of the protective layer is about 1:1.4:3.0:1.0 for titanium, aluminum, nitrogen, and oxygen, respectively.

3. The carbonate fuel cell of claim 1 wherein the protective layer is formed on the first and second electroplates.

4. The carbonate fuel cell of claim 3 wherein the first and second electroplates are made with nickel.

5. The carbonate fuel cell of claim 1 wherein the electrolyte is maintained at a high temperature above 600 degree Celsius.

6. The carbonate fuel cell of claim 5 wherein the protective layer is corrosion resistant at the high temperature.

7. An electrical generating fuel cell, comprising: first and second metallic separator plates; first and second electroplates supported by the first and second metallic separator plates, respectively; an electrolyte disposed between the first and second electroplates; and a protective layer formed on the metallic separator plates, wherein the protective layer comprises titanium, aluminum, nitrogen, and oxygen.

8. The electrical generating fuel cell of claim 7 wherein a composition ratio of the protective layer is about 1:1.4:3.0:1.0 for titanium, aluminum, nitrogen, and oxygen, respectively.

9. The electrical generating fuel cell of claim 7 wherein the protective layer is formed on a metallic surface of the first and second electroplates.

10. The electrical generating fuel cell of claim 9 wherein the first and second electroplates are made with nickel.

11. The electrical generating fuel cell of claim 7 wherein the electrolyte is maintained at a high temperature above 600 degree Celsius.

12. The electrical generating fuel cell of claim 11 wherein the protective layer is corrosion resistant at the high temperature.

13. An electrical generating fuel cell, comprising: first and second separator plates having a metallic surface; first and second electroplates having a metallic surface and supported by the first and second separator plates, respectively; an electrolyte disposed between the first and second electroplates; and a protective layer formed on a metallic surface within the electrical generating fuel cell which is subject to corrosion, wherein the protective layer comprises titanium, aluminum, nitrogen, and oxygen.

14. The electrical generating fuel cell of claim 13 wherein the protective layer is formed on the metallic surface of the first and second separator plates.

15. The electrical generating fuel cell of claim 13 wherein the protective layer is formed on the metallic surface of the first and second electroplates.

16. The electrical generating fuel cell of claim 13 wherein a composition ratio of the protective layer is about 1:1.4:3.0:1.0 for titanium, aluminum, nitrogen, and oxygen, respectively.

17. The electrical generating fuel cell of claim 13 wherein the electrolyte is maintained at a high temperature above 600 degree Celsius.

18. The electrical generating fuel cell of claim 13 wherein the protective layer is corrosion resistant.

19. A method of making an electrical generating fuel cell, comprising: providing first and second separator plates; providing first and second electroplates supported by the first and second separator plates, respectively; disposing an electrolyte between the first and second electroplates; and forming a protective layer on a metallic surface within the electrical generating fuel cell which is subject to corrosion, wherein the protective layer comprises titanium, aluminum, nitrogen, and oxygen.

20. The method of claim 19 wherein a composition ratio of the protective layer is about 1:1.4:3.0:1.0 for titanium, aluminum, nitrogen, and oxygen, respectively.

21. The method of claim 19 further including the step of forming the protective layer on a metallic surface of the first and second separator plates.

22. The method of claim 21 wherein the first and second separator plates are made with stainless-steel.

23. The method of claim 19 further including the step of forming the protective layer on a metallic surface of the first and second electroplates.

24. A method of forming a protective layer on a metallic surface, comprising: placing a metal plate in a reactive sputtering chamber; placing a titanium aluminum sputtering target in the chamber; drawing a vacuum on the chamber; introducing nitrogen and oxygen gases into the chamber; dislodging particles from the titanium aluminum sputtering target; reacting the particles with the nitrogen and oxygen gases within the chamber; and forming the protective layer on the metal plate, wherein the protective layer comprises titanium, aluminum, nitrogen, and oxygen.

25. The method of claim 24 wherein a composition ratio of the protective layer is about 1:1.4:3.0:1.0 for titanium, aluminum, nitrogen, and oxygen, respectively.

26. The method of claim 24 wherein the metal plate is stainless-steel.

27. The method of claim 24 wherein the metal plate is used to form a separator plate within a carbonate fuel cell.

28. The method of claim 24 wherein the metal plate is used to form an electroplate within a carbonate fuel cell.