WO2008062278A1

WO2008062278A1 - Electrolyte membrane forming method and fuel cell manufacturing method

Info

Publication number: WO2008062278A1
Application number: PCT/IB2007/003563
Authority: WO
Inventors: Satoshi Aoyama; Ryoko Kanda
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2006-11-24
Filing date: 2007-11-20
Publication date: 2008-05-29
Also published as: JP2008130514A

Abstract

An electrolyte membrane forming method includes a process of forming a proton-conducting metal-oxide electrolyte membrane (20) on a hydrogen-permeable hydrogen separation member (10) in an oxygen atmosphere greater than 0.0001 Torr but less than 0.1 Torr. In this case, the membrane density of the electrolyte membrane that is formed increases. Accordingly, the proton conductivity of the electrolyte membrane improves which in turn improves the power generating performance of the fuel cell.

Description

ELECTROLYTE MEMBRANE FORMING METHOD AND FUEL CELL MANUFACTURING METHOD

FIELD OF THE INVENTION

[0001] The invention relates to an electrolyte membrane forming method and a fuel cell manufacturing method.

BACKGROUND OF THE INVENTION

[0002] Fuel cells are apparatuses that generate electrical energy typically using hydrogen and oxygen. Fuel cells are very environmentally friendly and extremely energy efficient, which is why they are being widely developed as future energy supply systems. [0003] Among the various type of fuel cells that exist, those that use solid electrolytes include polymer electrolyte membrane fuel cells, solid oxide fuel cells, and hydrogen separation membrane fuel cells. A hydrogen separation membrane fuel cell is a fuel cell provided with an elaborate hydrogen separation membrane. This elaborate hydrogen separation membrane is a layer formed of hydrogen-permeable metal and also functions as an anode. A hydrogen separation membrane fuel cell has a structure in which a proton-conducting electrolyte is laminated onto this hydrogen separation membrane. Hydrogen supplied to the hydrogen separation membrane is converted into protons which move through the proton-conducting electrolyte and combine with oxygen at a cathode, thus generating electricity. This electrolyte is formed on the hydrogen separation membrane through a membrane forming process, for example (see Japanese Patent Application Publication No. 2006-32192 (JP-A-2006-32192), for example).

[0004] However, the proton conductivity of the electrolyte membrane changes depending on the vacuum of the atmosphere in the process of forming the electrolyte membrane. As a result, the power generating performance of the hydrogen separation membrane fuel cell varies.

DISCLOSURE OF THE INVENTION

[0005] This invention thus provides a method for forming an electrolyte membrane having high proton conductivity, and a fuel cell manufacturing method.

[0006] A first aspect of the invention relates to an electrolyte membrane forming method that includes a process of forming a proton-conducting metal-oxide electrolyte membrane on a hydrogen-permeable hydrogen separation membrane in an oxygen atmosphere greater than 0.0001 Torr but less than 0.1 Torr. In this forming process, an electrolyte membrane is formed in an oxygen atmosphere greater than 0.0001 Torr but less than 0.1 Torr. In this case, the membrane density of the electrolyte membrane that is formed increases, thus improving the proton conductivity of the electrolyte membrane. [0007] The electrolyte membrane may be a perovskite electrolyte membrane.

Also, the electrolyte membrane may be made of SrZr₍i -_X)In_xO₃. In this case, x is a value greater than 0 but less than one. Also, the electrolyte membrane may be made of SrZr₀₈In₀₂O₃. Further, in the forming process, the electrolyte membrane may be formed by a PLD method. In this case, the composition of the electrolyte membrane is easier to adjust.

[0008] A second aspect of the invention relates to an electrolyte membrane forming method that includes the process of forming a proton-conducting metal-oxide electrolyte membrane on a hydrogen-permeable hydrogen separation membrane. In this forming process, an oxygen partial pressure when the electrolyte membrane is formed on the hydrogen separation membrane may be set to a value i) at which the formed electrolyte membrane has substantially the theoretical density of an electrolyte of which the electrolyte membrane is formed, and ii) at which a component of which the electrolyte membrane is formed is sufficiently oxidized. In this case, the proton conductivity of the electrolyte membrane improves. [0009] A third aspect of the invention relates to a fuel cell manufacturing method that includes the process of forming a cathode on an electrolyte membrane formed by the method described above. The fuel cell manufacturing method according to the invention improves the proton conductivity of the electrolyte membrane. As a result, the power generating performance of the fuel cell according to the invention is improved.

[0010] According to the invention, the proton conductivity of the electrolyte membrane which is formed is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The foregoing and further objects, features and advantages of the invention will become apparent from the following description of exemplary embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein:

FIGS. IA to 1C are views illustrating an electrolyte membrane forming method and a fuel cell manufacturing method according to an example embodiment of the invention;

FIG. 2 is a graph showing the relationship between vacuum and current density in the forming process; and

FIG 3 is a graph showing the relationship between vacuum and membrane density in the forming process.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0012] FIGS. IA to 1C are views illustrating a method for forming an electrolyte membrane 20 and a manufacturing method of a fuel cell 100 according to an example embodiment of the invention. First, as shown in FIG. IA, a hydrogen separation membrane 10 is prepared. This hydrogen separation membrane 10 functions both as an anode to which fuel gas is supplied, and as a base which supports and reinforces an electrolyte membrane 20 which will be described later.

[0013] The hydrogen separation membrane 10 is formed of a hydrogen permeable metal layer. The material of which the hydrogen separation membrane 10 is formed is not particularly limited as long as it is hydrogen permeable and conductive. For example, the hydrogen separation membrane 10 may be made of a metal such as Pd (palladium), V (vanadium), Ta (tantalum), or Nb (niobium), or an alloy of any of these. Also, the hydrogen separation membrane 10 may also be a structure in which a membrane of palladium or a palladium alloy or the like that can separate hydrogen is formed on the surface, of the two surfaces of the hydrogen permeable metal layer, on the side on which the electrolyte membrane 20, which will be described later, is formed. The thickness of the hydrogen separation membrane 10 is approximately 5 μm to 100 μm, inclusive, for example. The hydrogen separation membrane 10 may be a self-supported membrane or it may be supported by a porous base metal. [0014] Next, as shown in FIG IB, the electrolyte membrane 20 is formed on the hydrogen separation membrane 10. The electrolyte membrane 20 is formed of a proton-conducting metal-oxide electrolyte. The electrolyte of which this electrolyte membrane 20 is formed is not particularly limited as long as it is a proton-conducting metal oxide. For example, the electrolyte membrane 20 may be a perovskite electrolyte (such as SrZrInO₃), a pyrochlore electrolyte (such as Ln₂Zr₂O₇ (Ln : La (lanthanum), Nd (neodymium), Sm (samarium), etc.), a monazite rare-earth orthophosphate electrolyte (LnPO₄ (Ln : La, Pr (praseodymium), Nd, Sm, etc.)), a xenotime-type rare-earth orthophosphate electrolyte (LnPO₄ (Ln : La, Pr, Nd, Sm, etc.)), a rare-earth metaphosphate electrolyte (LnP₃O₉ (Ln : La, Pr, Nd, Sm, etc.)), or a rare-earth oxy-phosphate electrolyte (Ln₇P₃O₁₈ (Ln : La, Pr, Nd, Sm, etc.)) or the like.

[0015] In the forming process shown in FIG. IB, the electrolyte membrane 20 is formed in an oxygen atmosphere greater than 0.0001 Torr but less than 0.1 Torr. Here, 1 Torr is 1/760 atmospheric pressure. The oxygen atmosphere is obtained by adjusting the pressure inside a vacuum chamber using a vacuum pump with the flowrate of the oxygen introduced into the vacuum chamber and the discharge ability of the vacuum pump (e.g., the open/close amount of the vacuum pump line), for example. Any one of various methods, such as a PLD method (pulse laser deposition), a spatter method, or an ion plating method, may be used to form the electrolyte membrane 20. Incidentally, the composition of the electrolyte membrane 20 can easily be adjusted using the PLD method so this method is preferable.

[0016] When the electrolyte membrane 20 is made of SrZrO₃, the substrate temperature during formation of the electrolyte membrane 20 is set within a range of 500⁰C to 800⁰C, inclusive, because SrZrO₃ will include amorphous matter, not crystal, below 500⁰C. Amorphous matter is unstable so it crystallizes at a predetermined temperature (such as 400⁰C and above) while the fuel cell 100 is operating, and as a result, the electrolyte membrane 20 becomes susceptible to cracking. On the other hand, when the temperature during formation of the electrolyte membrane 20 is above 800⁰C, problems occur due to thermal deformation of the hydrogen separation membrane 10 (such as Pd) or stress deformation resulting from a difference in the coefficient of thermal expansion between the hydrogen separation membrane 10 and the electrolyte membrane 20.

[0017] Next, as shown in FIG. 1C, a cathode 30 is formed on the electrolyte membrane 20. The cathode 30 is an electrode to which oxidant gas is supplied and is made of La_{O 6}Sr_{O 4}CoO₃, for example. The cathode 30 is formed by the screen printing method, for example. The completed fuel cell 100 is obtained according to the foregoing process.

[0018] In this example embodiment, the membrane density of the electrolyte membrane 20 is increased by having the oxygen partial pressure of the oxygen atmosphere in the forming process of the electrolyte membrane 20 within the range described above. As a result, the proton conductivity of the electrolyte membrane 20 improves. Incidentally, the oxygen partial pressure is preferably between 0.001 Torr and 0.05 Torr, inclusive.

[0019] Continuing on, the operation of the fuel cell 100 will now be described. First, fuel gas containing hydrogen is supplied to the hydrogen separation membrane 10. The hydrogen in the fuel gas reaches the electrolyte membrane 20 by passing through the hydrogen separation membrane 10. The hydrogen that has reached the electrolyte membrane 20 is then separated into protons and electrons. The protons are conducted through the electrolyte membrane 20 after which they reach the cathode 30. Meanwhile, oxidant gas containing oxygen is supplied to the cathode 30. At the cathode 30, water is produced and power is generated from the oxygen in the oxidant gas combining with the protons that have reached the cathode 30. It is through this operation that the fuel cell 100 generates power. Accordingly, improving the proton conductivity of the electrolyte membrane 20 in turn results in greater power generating performance of the fuel cell 100.

[0020] Incidentally, in this example embodiment, the oxygen partial pressure in the forming process shown in FIG IB is set to a value that is greater than 0.0001 Torr but less than 0.1 Torr. However, it may also be set to a value i) at which the formed electrolyte membrane 20 has substantially the theoretical density of the electrolyte of which the electrolyte membrane 20 is formed, and ii) at which the component of which the electrolyte membrane 20 is formed is sufficiently oxidized.

[0021] Hereinafter, formation of the electrolyte membrane 20 according to the foregoing example embodiment, as well as the characteristics thereof, will be described. [0022] (Example 1) In example 1, a palladium substrate 80 μm thick was used as the hydrogen separation membrane 10. Next, the hydrogen separation membrane 10 was placed in a vacuum chamber. Then the atmosphere inside the vacuum chamber was adjusted to an oxygen atmosphere of 0.01 Torr by supplying pure oxygen into the vacuum chamber while extracting the air from inside the vacuum chamber using a vacuum pump. Next, an electrolyte membrane 20 two μm thick made of SrZr_{0 8}In₀₂O₃ was formed on the hydrogen separation membrane 10 by the PLD method. Then a cathode 30 made of La₀₆Sr₀₄CoO₃ was formed on the electrolyte membrane 20, thus completing the fuel cell 100.

[0023] (Example 2) In example 2, an electrolyte membrane 20 was formed by adjusting the atmosphere inside the vacuum chamber to an oxygen atmosphere of 0.001 Torr. All other conditions were the same as those in example 1.

[0024] (Comparative example 1) In comparative example 1, an electrolyte membrane 20 was formed by adjusting the atmosphere inside the vacuum chamber to an oxygen atmosphere of 0.1 Torr. All other conditions were the same as those in example 1.

[0025] (Comparative example 2) In comparative example 2, an electrolyte membrane 20 was formed by adjusting the atmosphere inside the vacuum chamber to an oxygen atmosphere of 0.0001 Torr. All other conditions were the same as those in example 1. Incidentally, a plurality of fuel cells 100 according to examples 1 and 2 and comparative example 2 were made.

[0026] (Analysis 1) The proton conductivity of the electrolyte membranes 20 according to the examples 1 and 2 and comparative examples 1 and 2 was examined. Each fuel cell 100 was made to generate power by supplying pure hydrogen gas to the hydrogen separation membrane 10 and 40⁰C humidified air to the cathode 30 under temperature conditions of 400⁰C. The current density in each case is shown in FIG 2. The vertical axis in FIG 2 represents the current density (A / cm²) at a power generating voltage of 0.5 V. The horizontal axis in FIG 2 represents the oxygen partial pressure of the oxygen atmosphere in the forming process of the electrolyte membrane 20.

[0027] As shown in FIG 2, the current densities of the electrolyte membranes 20 in examples 1 and 2 are much greater than the current densities of the electrolyte membranes 20 in comparative examples 1 and 2. This is thought to be because the oxygen partial pressure in the forming process of those electrolyte membranes 20 was optimized.

[0028] (Analysis 2) Next, the membrane density of each electrolyte membrane 20 was measured. The membrane densities were obtained by measuring the weight, thickness, and area of the electrolyte membranes 20 that were formed. FIG 3 shows the results. The vertical axis in FIG 3 represents the membrane density of the electrolyte membranes 20 and the horizontal axis in FIG. 3 represents the oxygen partial pressure of the oxygen atmosphere in the forming process of the electrolyte membranes 20.

[0029] As shown in FIG 3, the membrane density changes depending on the oxygen partial pressure. Therefore, the membrane density of the electrolyte membrane 20 relies on the oxygen partial pressure in the forming process of the electrolyte membrane 20. The membrane densities of the electrolyte membranes 20 according to examples 1 and 2 are greater than the membrane density of the electrolyte membrane 20 according to comparative example 1. Also, the membrane densities of the electrolyte membranes 20 according to examples 1 and 2 are values near the theoretical density. From the above, we discovered that the proton conductivity of the electrolyte membrane 20 improves by adjusting the atmosphere in the forming process of the electrolyte membrane 20 to an oxygen atmosphere greater than 0.0001 Torr but less than 0.1 Torn Incidentally, the reason why sufficient current density is unable to be obtained in comparative example 2 even though the membrane density is the theoretical density is thought to be because the electrolyte membrane is formed in insufficient oxidizing conditions.

[0030] While the invention has been described with reference to example embodiments thereof, it is to be understood that the invention is not limited to the example embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the example embodiments are shown in various combinations and configurations, which are exemplary, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.

Claims

CLAIMS:

1. An electrolyte membrane forming method comprising: forming a proton-conducting metal-oxide electrolyte membrane on a hydrogen-permeable hydrogen separation membrane in an oxygen atmosphere greater than 0.0001 Torr but less than 0.1 Torr.

2. The forming method according to claim 1, wherein the electrolyte membrane is a perovskite electrolyte membrane.

3. The forming method according to claim 2, wherein the electrolyte membrane is made Of SrZr(J-X)In_xO₃.

4. The forming method according to claim 2, wherein the electrolyte membrane is made of SrZr_{0 8}In₀₂O₃.

5. The forming method according to any one of claims 1 to 4, wherein the electrolyte membrane is formed by a PLD method.

6. An electrolyte membrane forming method comprising: forming a proton-conducting metal-oxide electrolyte membrane on a hydrogen-permeable hydrogen separation membrane, wherein an oxygen partial pressure when the electrolyte membrane is formed on the hydrogen separation membrane is set to a value i) at which the formed electrolyte membrane has substantially the theoretical density of an electrolyte of which the electrolyte membrane is formed, and ii) at which a component of which the electrolyte membrane is formed is sufficiently oxidized.

7. A fuel cell manufacturing method comprising: forming a cathode on an electrolyte membrane formed by the electrolyte membrane forming method according to any one of claims 1 to 6.