US20100206722A1

US20100206722A1 - Electrolysis apparatus

Info

Publication number: US20100206722A1
Application number: US12/707,309
Authority: US
Inventors: Eiji HARYU; Masanori Okabe
Original assignee: Honda Motor Co Ltd
Current assignee: Honda Motor Co Ltd
Priority date: 2009-02-18
Filing date: 2010-02-17
Publication date: 2010-08-19
Also published as: JP2010189710A; JP5054049B2

Abstract

A unit cell of an electrolysis apparatus includes a membrane electrode assembly sandwiched between an anode separator and a cathode separator. The membrane electrode assembly includes a solid polymer electrolyte membrane, an anode current collector disposed on one side of the solid polymer electrolyte membrane and held against the anode separator, and a cathode current collector disposed on the other side of the solid polymer electrolyte membrane and held against the cathode separator. A protective sheet having a number of through holes defined therein is interposed between the anode current collector and the solid polymer electrolyte membrane.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from Patent Application No. 2009-035354 filed on Feb. 18, 2009, in the Japan Patent Office, of which the contents are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to an electrolysis apparatus comprising an electrolyte membrane, a pair of current collectors disposed respectively on the opposite sides of the electrolyte membrane, and a pair of separators stacked respectively on the current collectors.
2. Description of the Related Art
Solid polymer electrolyte fuel cells generate DC electric energy when anodes thereof are supplied with a fuel gas, i.e., a gas mainly containing hydrogen, e.g., a hydrogen gas, and cathodes thereof are supplied with an oxygen-containing gas, e.g., air.
Generally, water electrolysis apparatus are used to generate a hydrogen gas for use as a fuel gas for such solid polymer electrolyte fuel cells. The water electrolysis apparatus employ a solid polymer electrolyte membrane for decomposing water to generate hydrogen (and oxygen). Electrode catalyst layers are disposed on the respective sides of the solid polymer electrolyte membrane, making up a membrane electrode assembly. Current collectors are disposed on the respective sides of the membrane electrode assembly, making up a unit. The unit is essentially similar in structure to the fuel cells described above.
A plurality of such units are stacked, and a voltage is applied across the stack while water is supplied to the current collectors on the anode side. On the anodes of the membrane electrode assemblies, the water is decomposed to produce hydrogen ions (protons). The hydrogen ions move through the solid polymer electrolyte membranes to the cathodes, where the hydrogen ions combine with electrons to generate hydrogen. On the anodes, oxygen generated together with hydrogen is discharged with excess water from the units.
A current collector disclosed in Japanese Laid-Open Patent Publication No. 2001-279481, for example, is known in the art for use in such a water electrolysis apparatus. As shown in FIG. 9 of the accompanying drawings, the disclosed current collector is a dual-structure current collector 3 comprising a sintered powder part 1 and a sintered fiber part 2 which are integrally joined to each other.
The sintered powder part 1 is made of a sintered powder of titanium, and the sintered fiber part 2 is in the form of a sintered sheet of titanium fiber. The dual-structure current collector 3 is incorporated in the electrolysis cells of the hydrogen oxygen generating apparatus with the sintered powder part 1 being held in pressed contact with a solid electrolyte membrane 4.
However, since the sintered powder part 1 is produced by sintering the powder of titanium, the sintered powder part 1 tends to have differently sized pores due to the grain aggregation, resulting in a wide distribution of pore diameters. If the dual-structure current collector 3 is applied to a high-pressure water electrolysis apparatus for generating high-pressure hydrogen, then the solid electrolyte membrane 4 is liable to become damaged when it is pressed against the sintered powder part 1 due to a pressure difference between the anode and the cathode.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an electrolysis apparatus which includes a simple structure for preventing an electrolyte membrane from being damaged.
An electrolysis apparatus according to the present invention includes an electrolyte membrane, a pair of current collectors disposed respectively on opposite sides of the electrolyte membrane, a pair of separators stacked respectively on the current collectors, and a protective sheet interposed between the electrolyte membrane and one of the current collectors, the protective sheet having a plurality of through holes defined therein.
According to the present invention, since the protective sheet is interposed between the electrolyte membrane and the one of the current collectors, the electrolyte membrane is kept out of direct contact with the one of the current collectors. The through holes defined in the protective sheet have their diameters easily controllable. By the simple structure, damage to the electrolyte membrane is prevented when the electrolyte membrane is brought into contact with the protective sheet.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an electrolysis apparatus according to an embodiment of the present invention;

FIG. 2 is a side elevational view, partly in cross section, of the electrolysis apparatus shown in FIG. 1;

FIG. 3 is an exploded perspective view of a unit cell of the electrolysis apparatus;

FIG. 4 is a fragmentary cross-sectional view of the unit cell;

FIG. 5 is a diagram showing the widths of pore diameter distributions of a protective sheet and an anode current collector;

FIG. 6 is a fragmentary cross-sectional view of a membrane retaining pressure inspecting apparatus;

FIG. 7 is a diagram showing measured membrane holding pressures according to the related art;

FIG. 8 is a diagram showing measured membrane retaining pressures according to the present embodiment; and

FIG. 9 is a fragmentary cross-sectional view of a current collector disclosed in Japanese Laid-Open Patent Publication No. 2001-279481.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIGS. 1 and 2, a water electrolysis apparatus (electrolysis apparatus) 10 according to an embodiment of the present invention serves as a high-pressure hydrogen manufacturing apparatus, and includes a stack assembly 14 comprising a plurality of unit cells 12 stacked in a vertical direction indicated by the arrow A. The unit cells 12 may be stacked in a horizontal direction indicated by the arrow B. The water electrolysis apparatus 10 also includes a terminal plate 16 a, an insulating plate 18 a, and an end plate 20 a which are mounted on an upper end of the stack assembly 14 upwardly in the order named, and a terminal plate 16 b, an insulating plate 18 b, and an end plate 20 b which are mounted on a lower end of the stack assembly 14 downwardly in the order named. The unit cells 12, the terminal plates 16 a, 16 b, the insulating plates 18 a, 18 b, and the end plates 20 a, 20 b are of a disk shape.
The stack assembly 14, the terminal plates 16 a, 16 b, and the insulating plates 18 a, 18 b are fastened integrally together by the end plates 20 a, 20 b that are interconnected by a plurality of tie rods 22 extending in the directions indicated by the arrow A between the end plates 20 a, 20 b. Alternatively, the stack assembly 14, the terminal plates 16 a, 16 b, and the insulating plates 18 a, 18 b may be integrally held together in a box-like casing, not shown, which includes the end plates 20 a, 20 b as end walls. The electrolysis apparatus 10 is illustrated as being of a substantially cylindrical shape. However, the electrolysis apparatus 10 may be of any of various other shapes such as a cubic shape.
As shown in FIG. 1, terminals 24 a, 24 b project radially outwardly from respective side edges of the terminal plates 16 a, 16 b. The terminals 24 a, 24 b are electrically connected to a power supply 28 by electric wires 26 a, 26 b, respectively. The terminal 24 a, which is an anode terminal, is connected to the positive terminal of the power supply 28, and the terminal 24 b, which is a cathode terminal, is connected to the negative terminal of the power supply 28.
As shown in FIGS. 2 and 3, each of the unit cells 12 comprises a disk-shaped membrane electrode assembly 32, and an anode separator 34 and a cathode separator 36 which sandwich the membrane electrode assembly 32 therebetween. Each of the anode separator 34 and the cathode separator 36 is of a disk shape and is in the form of a carbon plate, or in the form of a metal plate such as a steel plate, a stainless steel plate, a titanium plate, an aluminum plate, or a plated steel plate. Alternatively, each of the separators 34, 36 is formed by performing anti-corrosion treatment on the surface of such a metal plate and thereafter pressing the metal plate into shape, or by cutting the metal plate into shape and thereafter performing anti-corrosion treatment on the surface of the cut metal plate.
The membrane electrode assembly 32 has a solid polymer electrolyte membrane 38 comprising a thin membrane of perfluorosulfonic acid which is impregnated with water, and an anode current collector 40 and a cathode current collector 42 which are disposed respectively on the opposite surfaces of the solid polymer electrolyte membrane 38.
An anode catalyst layer 40 a and a cathode catalyst layer 42 a are formed on the opposite surfaces of the solid polymer electrolyte membrane 38, respectively. The anode catalyst layer 40 a is made of a Ru (ruthenium)-based catalyst, for example, and the cathode catalyst layer 42 a is made of a platinum catalyst, for example.
Each of the anode current collector 40 and the cathode current collector 42 is made of a sintered spherical atomized titanium powder (porous electrically conductive material), and has a smooth surface area which is etched after it is cut to shape. Each of the anode current collector 40 and the cathode current collector 42 has a porosity in the range of 10% to 50%, or more preferably in the range from 20% to 40%.
As shown in FIGS. 3 and 4, a protective sheet 44 having a number of through holes 44 a defined therein is interposed between the solid polymer electrolyte membrane 38 and the anode current collector 40. The protective sheet 44 comprises a sheet of titanium, for example, having a thickness in the range from 20 μm to 500 μm. The sheet of titanium has a surface roughness which is equal to or smaller than 6.3 μm, preferably equal to or smaller than 3.2 μm. The sheet of titanium is preferably formed by cold rolling.
The through holes 44 a have a diameter distribution whose width is smaller than the width of a diameter distribution of pores of the anode current collector 40. Specifically, the through holes 44 a have inside diameters in the range from 30 μm to 200 μm, and the opening diameters of the through holes 44 a are kept within a range of ±20 μm with respect to a certain value. The anode current collector 40 has a grain diameter in the range from 45 μm to 150 μm. The through holes 44 a are formed by etching, drilling, electric discharge machining, electron beam, laser beam, pressing, or the like.
The through holes 44 a are typically circular in shape. However, the through holes 44 a are not limited to a circular shape, but may be of any of various shapes insofar as they do not cause damage to the solid polymer electrolyte membrane 38, i.e., any shape free of sharp edges, such as an elliptical shape.
As shown in FIG. 3, each of the unit cells 12 has, in an outer circumferential edge portion thereof, a water supply passage 46 for supplying water (pure water), a discharge passage 48 for discharging oxygen generated by a reaction in the unit cells 12 and used water, and a hydrogen passage 50 for passing therethrough hydrogen generated by the reaction. The water supply passages 46 defined in the respective unit cells 12 communicate with each other in the stacking directions indicated by the arrow A. The discharge passages 48 defined in the respective unit cells 12 communicate with each other in the stacking directions indicated by the arrow A. The hydrogen passages 50 defined in the respective unit cells 12 communicate with each other in the stacking directions indicated by the arrow A.
The anode separator 34 has a supply channel 52 a held in fluid communication with the water supply passage 46 and a discharge channel 52 b in fluid communication with the discharge passage 48. The supply channel 52 a and the discharge channel 52 b are defined in a surface 34 a of the anode separator 34 which faces the membrane electrode assembly 32. The anode separator 34 also has a first flow field 54 defined in the surface 34 a and held in fluid communication with the supply channel 52 a and the discharge channel 52 b. The first flow field 54 extends within a range corresponding to the surface area of the anode current collector 40, and comprises a plurality of fluid passage grooves, a plurality of embossed ridges, or the like (see FIGS. 2 and 3).
As shown in FIG. 3, the cathode separator 36 has a discharge channel 56 held in fluid communication with the hydrogen passage 50. The discharge channel 56 is defined in a surface 36 a of the cathode separator 36 which faces the membrane electrode assembly 32. The cathode separator 36 also has, in the surface 36 a, a second flow field 58 held in fluid communication with the discharge channel 56. The second flow field 58 extends within a range corresponding to the surface area of the cathode current collector 42, and comprises a plurality of fluid passage grooves, a plurality of embossed ridges, or the like (see FIGS. 2 and 3).
Seal members 60 a, 60 b are integrally combined with respective outer circumferential edge portions of the anode separator 34 and the cathode separator 36. The seal members 60 a, 60 b are made of a seal material, a cushion material, or a gasket material such as EPDM, NBR, fluororubber, silicone rubber, fluorosilicone rubber, butyl rubber, natural rubber, styrene rubber, chloroprene, acrylic rubber, or the like.
As shown in FIG. 1, pipes 62 a, 62 b, 62 c are connected to the end plate 20 a in fluid communication with the water supply passages 46, the discharge passages 48, and the hydrogen passages 50, respectively. A back pressure valve or a solenoid-operated valve, not shown, is connected to the pipe 62 c for maintaining the pressure of hydrogen generated in the hydrogen passages 50 at a high pressure level.
Operation of the water electrolysis apparatus 10 will be described below.
As shown in FIG. 1, water is supplied from the pipe 62 a to the water supply passages 46 in the water electrolysis apparatus 10, and a voltage is applied between the terminals 24 a, 24 b of the terminal plates 16 a, 16 b by the power supply 28. As shown in FIG. 3, in each of the unit cells 12, the water is supplied from the water supply passage 46 into the first flow field 54 of the anode separator 34 and moves along the anode current collector 40.
The water is electrolyzed by the anode catalyst layer 40 a, generating hydrogen ions, electrons, and oxygen. The hydrogen ions generated by the anodic reaction move through the solid polymer electrolyte membrane 38 to the cathode catalyst layer 42 a where they combine with the electrons to produce hydrogen.
The produced hydrogen flows along the second flow field 58 that is defined between the cathode separator 36 and the cathode current collector 42. The hydrogen is kept under a pressure higher than the pressure in the water supply passage 46, and flows through the hydrogen passage 50. Thus, the hydrogen is extracted from the water electrolysis apparatus 10. The oxygen generated by the anodic reaction and the water that has been used flow in the first flow field 54 and then flow through the discharge passage 48 for being discharged from the water electrolysis apparatus 10. The pressure in the second flow field 58 is higher than the pressure in the first flow field 54.
According to the present embodiment, as shown in FIG. 4, the protective sheet 44 with the through holes 44 a defined therein is interposed between the solid polymer electrolyte membrane 38 and the anode current collector 40. Therefore, when the solid polymer electrolyte membrane 38 is pressed toward the anode current collector 40 under the pressure difference between the second flow field 58 in which a high-pressure hydrogen gas is generated and the first flow field 54 in which water and oxygen flow under normal pressure, the solid polymer electrolyte membrane 38 is brought into contact with the protective sheet 44 and hence is kept out of direct contact with the anode current collector 40.
The through holes 44 a defined in the protective sheet 44 have their opening diameters easily controllable. The opening diameters of the through holes 44 a can be kept within a small range with respect to a certain value, e.g., a range of ±20 μm, so that the width of the opening diameter distribution of the through holes 44 a can be significantly smaller than the width of the diameter distribution of pores of the anode current collector 40, as shown in FIG. 5.
A membrane holding pressure inspection apparatus 70 shown in FIG. 6 was used to conduct a test for inspecting samples of two structures, i.e., a structure according to the prior art wherein the anode current collector 40 and the solid polymer electrolyte membrane 38 are held in direct contact with each other and a structure according to the present embodiment wherein the protective sheet 44 is interposed between the anode current collector 40 and the solid polymer electrolyte membrane 38. In the test, the pressure of a nitrogen gas was applied to the solid polymer electrolyte membrane 38 of each of the samples of the structures.
Gas pressures applied when the solid polymer electrolyte membranes 38 of the samples of the structures were broken were detected as membrane holding pressures. The detected gas pressures are shown in FIGS. 7 and 8.
FIG. 7 shows the detected gas pressures of the samples of the prior-art structure which was free of the protective sheet 44. It can be seen from FIG. 7 that the detected gas pressures, i.e., the membrane holding pressures, varied widely from one anode current collector 40 to another in the prior-art samples. FIG. 8 shows the detected gas pressures of the samples of the structure according to the present embodiment, which included the protective sheet 44. It can be understood from FIG. 8 that the membrane holding pressures did not greatly vary from one anode current collector 40 to another, and were much higher than the membrane holding pressures of the samples of the structure according to the prior art.
According to the present embodiment, damage to the solid polymer electrolyte membrane 38 of the electrolysis apparatus 10 for generating high-pressure hydrogen is prevented as far as possible, by a simple arrangement which includes the protective sheet 44. The electrolysis apparatus 10 can thus electrolyze water economically and efficiently.
Although a certain preferred embodiment of the present invention has been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.

Claims

1. An electrolysis apparatus comprising:

an electrolyte membrane;

a pair of current collectors disposed respectively on opposite sides of the electrolyte membrane;

a pair of separators stacked respectively on the current collectors; and

a protective sheet interposed between the electrolyte membrane and one of the current collectors, the protective sheet having a plurality of through holes defined therein.

2. An electrolysis apparatus according to claim 1, wherein the one of the current collector comprises a porous electrically conductive material, and the through holes defined in the protective sheet have a diameter distribution whose width is smaller than a width of a diameter distribution of pores of the porous electrically conductive material.

3. An electrolysis apparatus according to claim 1, wherein the current collectors comprise an anode current collector for electrolyzing water to generate oxygen and a cathode current collector for generating hydrogen having a pressure higher than the oxygen generated by the anode current collector, and the protective sheet is interposed between the anode current collector and the electrolyte membrane.

4. An electrolysis apparatus according to claim 1, wherein the protective sheet comprises a sheet of titanium.