US20180274112A1

US20180274112A1 - Membrane electrode assembly for electrochemical cell

Info

Publication number: US20180274112A1
Application number: US15/471,206
Authority: US
Inventors: Sang Bong Moon; Yun Ki CHOI; Chang Hyun Han; Su Hyun IM; Dae Jin YOON; Chang Hwan Moon
Original assignee: Elchemtech Co Ltd
Current assignee: Elchemtech Co Ltd
Priority date: 2017-03-23
Filing date: 2017-03-28
Publication date: 2018-09-27
Also published as: KR101931504B1; KR20180107849A

Abstract

Disclosed is a membrane electrode assembly that includes a polymer electrolyte membrane, a first electrochemical reaction layer formed on one side of the polymer electrolyte membrane to allow an oxidation reaction to occur thereon, a first electron-conductive layer formed between the polymer electrolyte membrane and the first electrochemical reaction layer, a second electrochemical reaction layer formed on a remaining side of the polymer electrolyte membrane to allow a reduction reaction to occur thereon, and a second electron-conductive layer formed between the polymer electrolyte membrane and the second electrochemical reaction layer. The first electron-conductive layer and the second electron-conductive layer include a porous metal.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a membrane electrode assembly for an electrochemical cell, and more particularly to a membrane electrode assembly for an electrochemical cell including a separate electron conduction layer for an electron movement path between electrode catalysts therein.

2. Description of the Related Art

Generally, an electrochemical cell is an energy converter that uses electric energy or generates electric energy, and is classified into an electrolytic cell and a fuel cell. In order to put the electrochemical cell to practical use, the fuel cell needs to have improved output density (reduced electric energy consumption in the case of water electrolysis), improved durability, and a low price.
FIGS. 1 to 4 show a unit structure of a typical electrochemical cell, an electrochemical stack structure, and a system structure.
FIG. 1 is a view showing the concept of a membrane electrode assembly 100 which constitutes a portion of a typical electrolytic cell, which electrochemically decomposes water to generate hydrogen and oxygen gases. FIG. 1 shows the thickness of the layer of each of the elements at a lower part thereof.
The electrochemical cell for electrolysis, which electrolyzes water (H₂O) to generate oxygen gas (O₂) and hydrogen gas (H₂), includes a first electrochemical reaction layer 104, a second electrochemical reaction layer 108, a membrane 106, a first diffusion layer 102, and a second diffusion layer 110. The first electrochemical reaction layer 104 includes a first electrochemical catalyst 112 and a first carrier 114, and the second electrochemical reaction layer 108 includes a second electrochemical catalyst 116 and a second carrier 118.
The first diffusion layer 102 and the second diffusion layer 110 help to move electrons, reactants, or products to or from the first and the second electrochemical catalysts 112 and 116. The first and the second electrochemical catalysts 112 and 116 are the most important materials that are used to perform electrolysis or generate electric energy, and the first and the second carriers 114 and 118 function to support the first and the second electrochemical catalysts 112 and 116 and provide an electron movement path.
The first and the second electrochemical catalysts 112 and 116 are mixed with the first and the second carriers 114 and 118, a binder, and a solvent to form a slurry or paste, which is then applied on the membrane 106 or on the first and the second diffusion layers 102 and 110 to form the first and the second electrochemical reaction layers 104 and 108. The manufactured assembly of “electrochemical reaction layers 104 and 108-membrane 106” or “electrochemical reaction layers 104 and 108-membrane 106- diffusion layers 102 and 110” is called a membrane electrode assembly (hereinafter, referred to as “MEA”).
The interval between the first electrochemical reaction layer 104 and the second electrochemical reaction layer 108, which are formed in the MEA, has a physical thickness value of the membrane. Bubbles are not present in the first electrochemical reaction layer 104 or in the second electrochemical reaction layer 108, thereby making it possible to perform high-current operation at low voltages. Further, since the conductivity of an electrolyte solution is not used, unlike in an alkali electrolytic cell, water, which is a raw material, may be used while ensuring high purity, and accordingly, high-purity hydrogen and oxygen may be obtained.
The process of electrolyzing water will be described below using the constitution shown in FIG. 1. The place at which an oxidation reaction occurs is considered the first electrochemical reaction layer 104, and the place at which a reduction reaction occurs is considered the second electrochemical reaction layer 108. The oxidation and reduction reactions occur simultaneously.
First, when water (H₂O) is supplied through the first diffusion layer 102 to the first electrochemical reaction layer 104, the water is decomposed into oxygen gas (O₂), electrons (e⁻) , and hydrogen ions (H⁺) (protons) at the first electrochemical catalyst 112 (also called an oxidation catalyst, an anode active material, or an oxygen-gas generating electrode), as shown in the following Reaction Scheme 1. The oxygen gas (O₂) is discharged to the outside of the electrolytic cell via diffusion, and the hydrogen ions (H⁺) are moved through the membrane 106 to the second electrochemical catalyst 116 (also called a reduction catalyst, a cathode active material, or a hydrogen-gas generating electrode) by an electric field. The electrons (e⁻), which are generated due to the aforementioned reaction, are moved from the first electrochemical catalyst 112 through the first diffusion layer 102 and an external circuit (not shown) to the second diffusion layer 110 and the second electrochemical catalyst 116.
Meanwhile, the hydrogen ions (H⁺) and the electrons (e⁻), which are moved from the first electrochemical catalyst 112, react at the second electrochemical catalyst 116 to generate hydrogen gas (H₂), as shown in Reaction Scheme 2. In addition, a portion of the water supplied to the first electrochemical reaction layer 104 is moved to the second electrochemical reaction layer 108 by an electric field to thus be discharged together with the hydrogen gas (H₂) to the outside of the electrolytic cell.
The electrochemical reactions, which occur at the first electrochemical catalyst 112 and the second electrochemical catalyst 116, are shown in the following Reaction Schemes 1 and 2. The overall reaction at the first electrochemical catalyst 112 and the second electrochemical catalyst 116 is shown in Reaction Scheme 3.
2H₂O→4H⁺+4e ⁻+O₂(Anode) [Reaction Scheme 1]
4H⁺+4e ⁻→2H₂(Cathode) [Reaction Scheme 2]
2H₂O→O₂+2H₂ [Reaction Scheme 3]
Meanwhile, a reverse reaction of electrolysis of water occurs in the fuel cell, and will be described below (See Reaction Schemes 4 to 6).
First, hydrogen gas is introduced into a first electrochemical reaction layer 104, and oxygen gas is supplied to a second electrochemical reaction layer 108. The hydrogen gas is then converted into hydrogen ions (proton) and electrons via an electrochemical reaction at a first electrochemical catalyst 112, and the electrons are moved through an external load, which is electrically connected to the fuel cell, and the protons are moved through a membrane to a second electrochemical catalyst 116. The protons and the electrons, which are generated at and moved from the first electrochemical catalyst 112, react with oxygen gas, which is supplied from the outside, at the second electrochemical catalyst 116 to generate water, energy, and heat.
2H₂→4H⁺+4e ⁺(Anode) [Reaction Scheme 4]
4H⁺+O₂+4e ⁻→2H₂O (Cathode) [Reaction Scheme 5]
O₂+2H₂→2H₂O [Reaction Scheme 6]
The following description pertains mainly to water electrolysis for the electrolysis of water, but is applicable not only to water electrolysis but also to fuel cells.
FIG. 2 is a view showing the structure of a typical electrochemical cell which includes the MEA of FIG. 1 to electrolyze water. An electrochemical cell 200, like that shown in FIG. 2, includes a first end plate 202, a first insulating plate 204, a first current application plate 206, a first electrochemical reaction chamber frame 208, a first electrochemical reaction chamber 210, the MEA 100 of FIG. 1, a second electrochemical reaction chamber 212, a second electrochemical reaction chamber frame 214, a second current application plate 216, a second insulating plate 218, and a second end plate 220. A direct current power supply is used as a power converter 224, which applies current to the electrochemical cell.
The first end plate 202 and the second end plate 220 have bolt/nut fastening holes (not shown) for assembling the unit electrochemical cells, and provide paths (not shown) through which reactants and products are moved. The first insulating plate 204 and the second insulating plate 218 provide an electric insulation function between the first end plate 202 and the first current application plate 206 and between the second end plate 220 and the second current application plate 216. The first current application plate 206 and the second current application plate 216 are connected to the power converter 224 to apply required current to the electrochemical cell 200.
Meanwhile, when the first electrochemical catalyst 112 is positioned in the first electrochemical reaction chamber 210 to allow an oxidation reaction to occur, the first electrochemical reaction chamber 210 becomes a space through which water, as the reactant, and oxygen, as the product, are moved. The second electrochemical reaction chamber 212, which is positioned at the opposite side of the first electrochemical reaction chamber 210 while the membrane 106 is interposed between the first and the second electrochemical reaction chambers, provides a space through which hydrogen, which is generated in the reduction reaction, and water, which is moved from the first electrochemical reaction chamber 210, are moved.
The first electrochemical reaction chamber 210 is isolated from the outside by the first electrochemical reaction chamber frame 208, and the second electrochemical reaction chamber 212 is isolated from the outside by the second electrochemical reaction chamber frame 214. In addition, a gasket (or packing) 222 is provided between the MEA 100 and the first electrochemical reaction chamber frame 208 and between the MEA 100 and the second electrochemical reaction chamber frame 214 in order to prevent the reactants and the products from leaking to the outside.
Among the elements constituting the electrochemical cell 200, the first electrochemical reaction chamber frame 208, the second electrochemical reaction chamber frame 214, and the gasket 222 have predetermined holes, through which the reactants or the products are easily supplied to and discharged from the electrochemical cell. The first electrochemical reaction chamber frame 208 and the second electrochemical reaction chamber frame 214 have fluid paths (represented by the dotted line in (A) of FIG. 2).
Meanwhile, another electrochemical cell 200 may have a pressure pad (not shown, refer to 304 of FIG. 3) between the second electrochemical reaction chamber frame 214 and the second current application plate 216 so as to maintain the balance of the electrochemical cell 200.
FIG. 3 is a view showing the concept of a known electrochemical stack. A plurality of unit electrochemical cells is required in order to obtain a desired amount of products during an electrolysis reaction, and an assembly of the two or more layered electrochemical cells is called an electrochemical stack.
When the electrochemical cells are layered in order to constitute the electrochemical stack 300 shown in FIG. 3, unit electrochemical cells are repeatedly disposed in a desired number between the basic electrochemical cells 200. A pressure pad 304 is interposed between the unit electrochemical cells in order to press the elements to each other. Bolts 306 are fastened with nuts 310 through holes, which are formed through edges of the first and the second end plates 202 and 220, in order to assemble the unit electrochemical cells in the electrochemical stack.
FIG. 4 is a view showing a system for electrolyzing water using the electrolysis stack that is the same as the electrochemical stack of FIG. 3 in order to produce hydrogen. A hydrogen-generating system 400, as shown in FIG. 4, includes an electrolysis stack 420, a water-treating unit for treating water, which is supplied to the electrolysis stack 420, and a gas-treating unit for purifying hydrogen gas, which is generated from the electrolysis stack 420, and controlling pressure.
Pure water of 1 Mega ohm cm or more is used as water, which is a raw material used in the electrolysis stack 420. An automatic valve 402, which is provided in a pure water-supplying line s1, is adjusted to supply pure water, and the automatic valve 402 is controlled using a level sensor 405, which is used to sense the level, in an oxygen-water separation bath 404 (dotted line e2). Water is supplied from the oxygen-water separation bath 404 to the electrolysis stack 420 using a circulation pump 406, which is provided in a circulation pipe s2, joins water circulating through a circulation line s9 from a hydrogen-water separation bath 424, and then passes through a pipe in which a heat exchanger 408, a water-quality sensor 410, and an ion-exchanging filter 412 are provided. The water is then supplied to a first electrochemical reaction chamber 414 (the place in which an oxidation reaction occurs) of the electrolysis stack 420. Meanwhile, when direct current is supplied from a power converter 440 through a wire e1 to the electrolysis stack 420, the water undergoes a decomposition reaction.
Oxygen, which is generated from the first electrochemical reaction chamber 414, and unreacted water are moved through a discharge pipe s4 to the oxygen-water separation bath 404, and a temperature sensor 416 is provided in the discharge pipe s4 to sense the temperature. Oxygen, which is separated in the oxygen-water separation bath 404, is discharged through an oxygen-discharge pipe s5 to the outside, and the water is subjected to a re-circulation process.
The hydrogen gas, which is generated from the second electrochemical reaction chamber 422, entails water, and is moved through a discharge pipe s6 to the hydrogen-water separation bath 424 so as to be separated from water. A level sensor 426, which is used to sense the level, is provided in the hydrogen-water separation bath 424 so as to adjust the level. When the level of the hydrogen-water separation bath 424 is a predetermined value or more, an automatic valve 428 is opened (electric signal of e3) to supply the water through the circulation line s9 to the circulation pipe s2.
Meanwhile, the hydrogen gas, which is separated in the hydrogen-water separation bath 424, is supplied through a gas pipe s7 to a hydrogen-gas purifier 430 to thus remove moisture from hydrogen. Typically, a bed, which is filled with a moisture absorbent, is applied to the hydrogen gas purifier 430. The hydrogen that passes through the hydrogen gas purifier 430 is supplied through a high-purity hydrogen gas pipe s8 to a field requiring hydrogen. A pressure control valve 434 is provided in the high-purity hydrogen gas pipe s8 to control the pressure of the hydrogen gas generated from the electrolysis stack 420. Pressure sensors 432 and 438 are provided in the front and the rear of the pressure control valve 434 to measure pressure, and a check valve 436 is provided to maintain the flow of gas in a predetermined direction.
In this water electrolysis system, a reduction in electric energy consumption (in the case of a fuel cell, an increase in output density), improvement in durability, and cost reduction are required in order to realize practical use of an electrochemical cell. In this regard, in the case of the structure of the conventional MEA 100, since the structure for delivering electrons between the electrode catalysts in the MEA is not developed, the electrochemical activity is low and it is difficult to realize performance at high current density.
FIG. 5 shows photographs of the catalyst distribution on the surface of a typical MEA (Comparative Example 1, to be described later) manufactured using a conventional method. As seen from FIG. 5, the electrochemical reaction layer is locally formed, causing a lack of electron delivery layers (black portions in the photographs). Accordingly, there is a problem in that a large amount of catalyst is consumed in order to maintain the electrochemical performance.

CITATION LIST

Patent Document

Korean Patent No. 10-1357146
Korean Patent Application Publication No. 10-2008-0032962

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made keeping in mind the above problems occurring in the related art, and an object of the present invention is to provide a membrane electrode assembly (MEA) for an electrochemical cell including a separate electron conduction layer for an electron movement path between electrode catalysts therein. In the MEA, it is possible to enable high current density operation, reduce electric energy consumption even using typical current density, improve durability, and reduce manufacturing costs.
In order to accomplish the above object, the present invention provides a membrane electrode assembly for an electrochemical cell that includes a polymer electrolyte membrane, a first electrochemical reaction layer formed on one side of the polymer electrolyte membrane to allow an oxidation reaction to occur thereon, a first electron-conductive layer formed between the polymer electrolyte membrane and the first electrochemical reaction layer, a second electrochemical reaction layer formed on a remaining side of the polymer electrolyte membrane to allow a reduction reaction to occur thereon, and a second electron-conductive layer formed between the polymer electrolyte membrane and the second electrochemical reaction layer. The first electron-conductive layer and the second electron-conductive layer include a porous metal.
Further, according to the present invention, the porous metal may further include a coating layer including a platinum group applied on a surface thereof.
Further, according to the present invention, the first electron-conductive layer and the second electron-conductive layer may each have a thickness of 0.1 to 1 mm.
Further, according to the present invention, the first electron-conductive layer and the second electron-conductive layer may include any one among platinum, palladium, rhodium, iridium, ruthenium, osmium, carbon, gold, tantalum, tin, indium, nickel, tungsten, manganese, and titanium, or a complex including two or more thereof.
Further, according to the present invention, the first electrochemical reaction layer and the second electrochemical reaction layer may each include an electrochemical catalyst (or the electrochemical catalyst on a carrier), an ionic conductor, and a binder.
Further, according to the present invention, the electrochemical catalyst may include any one among platinum group elements including platinum, palladium, ruthenium, iridium, rhodium, and osmium, any one metal among iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum, or alloys, oxides, or double oxides thereof.
Further, according to the present invention, elements constituting the membrane electrode assembly may include the polymer electrolyte membrane, the first electron-conductive layer, the second electron-conductive layer, the first electrochemical reaction layer, and the second electrochemical reaction layer in descending order of size.
According to the present invention, for an electron movement path, electrons are sequentially moved through an anode catalyst, an electron-conductive layer on a membrane, an external circuit, the electron-conductive layer on the membrane, and a cathode catalyst. Therefore, the electron movement path is short compared to a known electrochemical cell having an electron movement path, which is formed so that the electrons are sequentially moved through an anode catalyst, an anode-chamber diffusion layer, an anode-chamber current application plate, an external circuit, a pressure pad, a cathode-chamber current application plate, a cathode-chamber diffusion layer, and a cathode catalyst. Accordingly, the current density-voltage characteristics of the electrochemical cell are excellent, thereby reducing energy consumption during electrolysis.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a view showing the concept of an MEA, which is a portion of a typical electrolytic cell that electrochemically decomposes water to produce hydrogen and oxygen gases;

FIG. 2 is a view showing the structure of a typical electrochemical cell which includes the MEA of FIG. 1 to electrolyze water;

FIG. 3 is a view showing the concept of a known electrochemical stack;

FIG. 4 is a view showing a system for electrolyzing water using the electrochemical stack of FIG. 3 to produce hydrogen;

FIG. 5 shows photographs of the catalyst distribution on the surface of a typical MEA manufactured using a conventional method;

FIG. 6 is a view showing an MEA according to an embodiment of the present invention; and

FIG. 7 is a comparative graph showing the performance of Example 1 of the present invention and Comparative Example 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the appended drawings so as to easily perform the present invention by the person having ordinary skill in the related art. However, descriptions of known techniques, even if they are pertinent to the present invention, are considered unnecessary and may be omitted insofar as they would make the characteristics of the invention unclear. Furthermore, the same or similar portions are represented using the same reference numeral in the drawings.
FIG. 6 is a view showing an MEA according to an embodiment of the present invention. As shown in FIG. 6, an MEA 500 according to the embodiment of the present invention includes a first electron-conductive layer 506, a first electrochemical reaction layer 504, a membrane 502, a second electron-conductive layer 518, and a second electrochemical reaction layer 508. The first electron-conductive layer 506 and the first electrochemical reaction layer 504 are sequentially formed on one side of the membrane 502, and the second electron-conductive layer 518 and the second electrochemical reaction layer 508 are sequentially formed on the remaining side of the membrane 502. Meanwhile, the MEA 500 may or may not include a diffusion layer.
An electrolysis reaction of water in the MEA 500 of the present embodiment will be described below. The description will be given on the assumption that an oxidation reaction (oxygen generation reaction) occurs at a first electrochemical catalyst and a reduction reaction (hydrogen generation reaction) occurs at a second electrochemical catalyst.
First, when water (H₂O) is supplied to a first electrochemical catalyst 510 (oxidation catalyst, oxygen catalyst), water is decomposed into oxygen gas (O₂), electrons (e⁻), and hydrogen ions (H⁺) (protons). A portion of the water (H₂O) is discharged to the outside together with the oxygen gas (O₂), and the hydrogen ions (H⁺), which are obtained due to decomposition, are moved through the membrane 502 to a second electrochemical catalyst 516 (reduction electrode, hydrogen electrode).
In addition, the decomposed electrons are moved to an external circuit (not shown) via the first electron-conductive layer 506 formed on the membrane 502 and the first electrochemical catalyst 510 (oxidation catalyst, oxygen catalyst). Meanwhile, the electrons (e⁻) moved along the external circuit (not shown) for connecting the first electron-conductive layers 506 are moved through the second electron-conductive layer 518. The electrons moved to the second electron-conductive layer 518 reach the first electrochemical catalyst 510. The electrons that are moved react with hydrogen ions (H⁺) in the second electrochemical reaction layer 508 to generate hydrogen gas. In addition, water (H₂O) which has passed through the membrane 502 in conjunction with the hydrogen ions (H⁺) is discharged to the outside of the electrolytic cell together with the hydrogen gas. The electrochemical reaction that occurs under the first and the second electrochemical catalysts 510 and 516 is shown in the aforementioned Reaction Schemes 1 and 2.
Any membrane may be used as the membrane 502 of the present embodiment as long as the membrane has hydrogen ion (proton) conductivity, and a fluorine-based polymer electrolyte and a hydrocarbon-based polymer electrolyte may be used. Examples of the fluorine-based polymer membrane may include Nafion (Registered trademark), manufactured by the DuPont Company, Flemion (Registered trademark), manufactured by Asahi Glass Co., Ltd., Aciplex (Registered trademark) manufactured by Asahi Kasei Corporation, and Gore Select (Registered trademark) manufactured by Gore & Associates, Inc. Examples of the hydrocarbon-based polymer membrane may include an electrolyte membrane such as sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, and sulfonated polyphenylene. Among the aforementioned examples, it is preferable to use a Nafion (Registered trademark)-based material, which is manufactured by the DuPont Company, as the polymer membrane.
The first and the second electron- conductive layers 506 and 518 of the present embodiment are formed on either side of the membrane 502, and function to conduct electrons. The first and the second electron- conductive layers 506 and 518 formed on the membrane 502 are 0.1 to 1 mm and preferably 0.1 to 0.5 mm in thickness. This is because when the thickness of the first and second electron- conductive layers 506 and 518 is 0.1 mm or less, it is difficult to form the electrochemical reaction layer on the electron-conductive layer, and when the thickness of the electron-conductive layer is 1 mm or more, the excessive formation of the electrochemical reaction layer interferes with the movement of the proton, thereby lowering the ionic conductivity. The material of the first and second electron- conductive layers 506 and 518 may be a metal having excellent conductivity such as platinum, palladium, rhodium, iridium, ruthenium, osmium, carbon, gold, tantalum, tin, indium, nickel, tungsten, manganese, and titanium. It is preferable that a platinum group coating be applied on titanium from the viewpoint of chemical resistance.
Meanwhile, the first and second electron- conductive layers 506 and 518 may include a metal or may include a coating layer including a platinum group applied on the metal. Meanwhile, a material having excellent conductivity may be formed on the metal by the principle of electroless plating of a process of precipitating a metal precursor with a reducing agent, or by thermal decomposition of the metal precursor.
The first and the second electrochemical reaction layers 504 and 508 of the present embodiment are formed on either side of the membrane 502 having the first and the second electron- conductive layers 506 and 518. The first and second electrochemical reaction layers 504 and 508 may include an electrochemical catalyst (or the electrochemical catalyst on a carrier), an ionic conductor, and a binder. The catalyst ink for the first electrochemical reaction layer 504 includes at least a first electrochemical catalyst 510, a carrier 512 (may not be included), a polymer electrolyte, and a solvent, and the catalyst ink for the second electrochemical reaction layer 508 includes at least a second electrochemical catalyst 516, a carrier 514 (may not be included), a polymer electrolyte, and a solvent.
Examples of the polymer electrolyte included in the catalyst ink of the present embodiment may include a fluorine-based polymer electrolyte and a hydrocarbon-based polymer electrolyte exhibiting proton conductivity. In addition, examples of the fluorine-based polymer electrolyte may include a Nafion (Registered trademark)-based material manufactured by the DuPont Company. Examples of the hydrocarbon-based polymer electrolyte may include an electrolyte such as sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, and sulfonated polyphenylene. Considering the adhesion between the first and second electrochemical reaction layers 504 and 508 and the first and second electron conductive layers 506 and 518, it is preferable to use the same material as the membrane 502, that is, the Nafion ionomer.
Examples of the first and second electrochemical catalysts 510 and 516 used in the first electrochemical reaction layer 504 and the second electrochemical reaction layer 508 in the present embodiment may include platinum group elements including platinum, palladium, ruthenium, iridium, rhodium, and osmium, metal such as iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum, or alloys, oxides, or double oxides thereof. It is preferable to use one or more metals, which are selected from platinum, palladium, rhodium, ruthenium, and iridium, or oxides thereof in order to ensure excellent electrode reactivity and effectively perform a stable electrode reaction over a long period of time.
In the present embodiment, when the particle diameter of the first and the second electrochemical catalysts 510 and 516 is very large, the activity of the catalyst is reduced, and when the particle diameter is very small, the stability of the catalyst is reduced. Accordingly, the particle diameter is preferably 0.5 to 20 nm, and more preferably 1 to 5 nm.
Meanwhile, carriers 512 and 514 for carrying the catalyst include powder that exhibits electron conductivity, and titanium oxide or carbon particles may be used. The carrier is provided in a fine particle form and exhibits conductivity, and any carrier may be used as long as the carrier does not intrude the catalyst. However, it is preferable to use titanium oxides, carbon black, graphites, black lead, activated carbon, carbon fibers, carbon nanotubes, or fullerene.
In addition, when the particle diameter of the carriers 512 and 514 is very small, it is difficult to form an electron-conductive path, and when the particle diameter is very large, the diffusion of gas into the electrode catalyst layer formed on the carrier is reduced, or the availability of the catalyst is reduced. Accordingly, the particle diameter is preferably 10 to 1,000 nm, and more preferably 10 to 100 nm.
In the present embodiment, the thickness of the first and second electrochemical reaction layers 504 and 508 is the same as or larger than that of the first and second electron- conductive layers 506 and 518, and is preferably 0.1 mm or less.
Meanwhile, the size of each component constituting the MEA 500 of the present embodiment is as follows. The size Dc of the membrane 502 is larger than that of first and second electrochemical reaction chamber frames 208 and 214 in FIG. 2, and the size Db of the first and second electron- conductive layers 506 and 518 is smaller than that of the first and second electrochemical reaction chamber frames 208 and 214 in FIG. 2. The size Da of the first and second electrochemical reaction layers 504 and 508 is preferably the same as the internal area of first and second electrochemical reaction chambers 210 and 212 in FIG. 2. Further, the components constituting the MEA 500 preferably include the membrane 502, the first electron-conductive layer 506, the second electron-conductive layer 518, the first electrochemical reaction layer 504, and the second electrochemical reaction layer 508 in descending order of size.
A method of manufacturing an MEA according to the embodiment of the present invention will be described hereinafter.
Process 1: Pre-Treatment Process of the Membrane 502
Pre-treatment of the membrane 502 is a process which includes roughening the surface of the membrane using a mechanical process, and physically and chemically treating organic and inorganic impurities present in the membrane 502. The process will be described in detail below.
Process 2: Process of Forming the First and the Second Electron- Conductive Layers 506 and 518
The first and second electron- conductive layers 506 and 518 are integrated with the membrane 502 obtained during process 1. A metal precursor solution having electron conductivity (for example, copper, platinum, silver, gold, etc.) and a reducing agent are added to a porous metal to form a metal thin film layer having electron conductivity on the porous metal, thereby forming the first and second electron- conductive layers 506 and 518. The thickness of the metal thin film layer is obtained by repeating a deposition reduction process. The detailed procedure thereof will be described later. The procedure is constituted based on the principle of electroless plating, but the constitution is not limited thereto. The metal precursor may be thermally decomposed, thus forming the metal thin film layer on the porous metal. Meanwhile, the first and second electron- conductive layers 506 and 518 that are manufactured in the above-described manner may be simply integrated with the membrane 502 by thermal pressing using a hot press.
Process 3: Process of Forming the First and the Second Electrochemical Reaction Layers 504 and 508
Process 3 is a process of forming the first and the second electrochemical reaction layers 504 and 508 on the membrane 502 which has the first and the second electron- conductive layers 506 and 518 obtained during process 2. Process 3 includes a catalyst synthesis process, a catalyst ink manufacturing process, a catalyst ink transfer process, and a thermal-pressing process. The catalyst synthesis process includes forming mixture oxides, using a reaction of a desired catalyst precursor and an oxidant, and drying the mixture oxides to obtain an electrochemical catalyst having a powder structure. The catalyst ink manufacturing process includes mixing the electrochemical catalyst, which is synthesized during the catalyst synthesis process, a particulate material, a dispersant, and a binder made of the same material as the membrane 502 to manufacture the catalyst ink. Further, the catalyst ink transfer process includes transferring the catalyst ink, which is manufactured during the catalyst ink manufacturing process, onto a Teflon sheet using a spray and then drying the catalyst ink. The thermal-pressing process includes attaching the Teflon sheet, which is obtained during the catalyst ink transfer process, to both sides of the membrane 502 having the first and the second electron- conductive layers 506 and 518, which are formed during process 2, and then thermal-pressing the Teflon sheet using a hot press. The processes will be described in detail below.
Hereinafter, the method of manufacturing the membrane electrode assembly according to Examples of the aforementioned embodiment and Comparative Examples will be specifically described, and experimental results will be given. However, the present invention is not limited to the following Examples.

EXAMPLE 1

1. Manufacture of the MEA 500
(1) Process 1: Pre-Treatment Process of the Membrane 502
Both surfaces of the membrane 502 (Nafion 117) were scratched in four directions using sandpaper (Emery sand paper 1100CW), and then subjected to the swelling process in pure water at 90° C. Impurities were removed from the membrane, which was subjected to the swelling process, using ultrasonic wave treatment in pure water, and the membrane was treated in 3% hydrogen peroxide (H₂O₂) for 30 min and in 0.5 to 1M sulfuric acid (H₂SO₄) at 90° C. for 30 min, and then subjected to the aforementioned pure water process again.
(2) Process 2: Process of Forming First and Second Electron- Conductive Layers 506 and 518
A titanium porous metal (thickness 0.2 mm, porosity 60%) was deposited in a chloroplatinate ((NH₃)₄PtCl₂.H₂O) precursor solution for 5 hours. After the deposition process, a NaBH₄solution was added dropwise every 20 minutes for 2 hours in order to reduce the metal precursor. After the reduction was finished, platinum-plated titanium porous metal was obtained. In order to obtain a plated layer having a desired thickness, the impregnation reduction process was repeated as many times as desired, thus forming the first and second electron- conductive layers 506 and 518. The first and second electron- conductive layers 506 and 518, which were manufactured in the above-described manner, were then integrated with both sides of the membrane 502. The membrane 502 and the first and second electron- conductive layers 506 and 518 were integrated by being thermally pressed using a hot press.
(3) Process 3: Process of Forming First and Second Electrochemical Reaction Layers 504 and 508
During the process of forming the first and the second electrochemical reaction layers 504 and 508, the first and the second electrochemical reaction layers 504 and 508 were formed on the membrane 502 having the first and the second electron- conductive layers 506 and 518. The process of forming the first and the second electrochemical reaction layers 504 and 508 included a process of synthesizing first and second electrochemical catalysts, a process of manufacturing ink for the first and the second electrochemical catalysts, a process of transferring ink for the first and the second electrochemical catalysts, and a thermal-pressing process.
(3-1) Process 3-1: Process of Synthesizing the First and the Second Electrochemical Catalysts
(3-1-1) Synthesis of the First Electrochemical Catalyst
An oxidized iridium-ruthenium mixture catalyst was manufactured using a reaction of iridium chlorides (IrCl₃.xH₂O) and ruthenium chlorides (RuCl₃.xH₂O) in a sodium nitrate solution. In addition, iridium chlorides and ruthenium chlorides were agitated in the solution having sodium nitrates dissolved therein for about 2 hours to be uniformly dissolved. The manufactured mixture catalyst solution was heated to 100° C. to vaporize distilled water for 1 hour to thus perform concentration, and the concentrate was sintered in an electric furnace at 475° C. for 1 hour and then slowly cooled. Subsequently, the resulting material was washed with 9 L of distilled water and filtered in order to remove generated sodium chlorides. The obtained solid was dried at 80° C. for 12 hours to manufacture a final iridium-ruthenium electrochemical mixture catalyst.
(3-1-2) Synthesis of the Second Electrochemical Catalyst
Commercially available Pt/C (Premetek Inc., amount of carried platinum of 30%) was used as the second electrochemical catalyst 516.
(3-2) Process 3-2: Process of Manufacturing Ink for the First and the Second Electrochemical Catalysts
(3-2-1) Manufacture of Ink for the First Electrochemical Catalyst
The oxidized iridium-ruthenium catalyst, which was manufactured during process 3-1, nano-sized titanium dioxides as the carrier, and the Nafion solution as the binder were used, and the used catalyst and Nafion ionomers were mixed in an isopropyl alcohol solvent at a ratio of 1:3.5 based on the weight of the solid. Agitation for 1 hour and ultrasonic wave treatment for 1 hour were alternately performed twice in order to disperse the catalyst.
(3-2-2) Manufacture of Ink for the Second Electrochemical Catalyst
Pt/C (Premetek Inc., amount of carried platinum of 30%) was used as the second electrochemical catalyst, and the Nafion solution (a registered product from DuPont) was used as the binder. The used catalyst and Nafion solution were mixed in an isopropyl alcohol solvent at a ratio of 1:7.5 based on the weight of the solid. Agitation for 1 hour and ultrasonic wave treatment for 1 hour were alternately performed twice in order to disperse the catalyst.
(3-3) Process 3-3: Process of Transferring the First and the Second Electrochemical Catalysts
(3-3-1) Transferring of the First Electrochemical Reaction Layer 504
The polytetrafluoroethylene (PTFE) sheet was used as the transfer sheet. The ink for the first electrochemical catalyst, which was obtained during process 3-2, was moved to a syringe for electrospraying only. The catalyst ink was transferred onto the base material and then dried in the atmosphere at 90° C. for 30 min to manufacture an electrochemical catalyst layer. The amount of carried oxide catalyst was adjusted to about 4 mg/cm²to set the thickness of the first electrochemical reaction layer 504.
(3-3-2) Transferring of the Second Electrochemical Reaction Layer 518
The ink for the second electrochemical catalyst 516, which was obtained during process 3-2, was moved to a syringe for electrospraying only. The catalyst ink was transferred onto the carbon sheet and then dried in the atmosphere at 90° C. for 30 min to manufacture an electrochemical catalyst layer. The amount of the carried oxide catalyst was adjusted to about 1 mg/cm²to set the thickness of the second electrochemical reaction layer 508.
(3-4) Process 3-4: Thermal-Pressing Process
(3-4-1) Formation of the First Electrochemical Reaction Layer 504
The first electrochemical catalyst, which was obtained during process 3-3 and loaded on the Teflon sheet, was thermal-pressed twice on the membrane 502, which was obtained during process 2, under a condition of 120° C. and pressure of 10 MPa for 3 min. The Teflon sheet was removed to transfer the catalyst.
(3-4-2) Formation of the Second Electrochemical Reaction Layer 508
The carbon sheet, on which the manufactured second electrochemical catalyst 516 was loaded, was thermal-pressed under a condition of 120° C. and pressure of 10 MPa for 2 min on the opposite surface of the membrane 502, with which the manufactured first electrochemical reaction layer 504 was combined, to obtain the MEA 500 shown in FIG. 6.
2. Electrochemical Cell for Evaluation and Evaluation System
The MEA of Example 1 had an electrochemically active area of 314 cm²(Da=20 cm), an electron-conductive layer thickness of 0.5 mm, Db of 21 cm, an area of 346 cm², and a membrane size Dc of 25 cm. Titanium fibers were layered on the first electrochemical reaction layer 504, and carbon fibers having high diffusibility were layered on the second electrochemical reaction layer 508 to perform evaluation. A cell for evaluation, shown in FIG. 2, and a water electrolysis system, shown in FIG. 4, were actually manufactured to perform evaluation.
The temperature of the cell was maintained at 80° C. (a temperature sensor 416 in FIG. 4), and the current-voltage measurement of the cell for evaluation was performed. Meanwhile, the discharge pressure of hydrogen (controlled using s8 and 434 in FIG. 4) was maintained at about 10 bar.
3. Measurement Result
From FIG. 7, it can be seen that the MEA manufactured in Example 1 has a small voltage change, that is, a small slope, depending on current density even when the current density is increased. FIG. 7 is a comparative graph showing the performance of Example 1 of the present invention and Comparative Example 1, and shows the performance of the current density depending on voltage.

COMPARATIVE EXAMPLE 1

1. Manufacture of the MEA (Manufacture of the MEA According to a Known Method)
The pre-treatment process of the membrane and the processes of forming the first and the second electrochemical reaction layers were performed using the same procedure and conditions as Example 1, and the processes of forming the first and the second electron-conductive layers were not performed in order to compare Comparative Example 1 and Example 1.
FIG. 5 shows photographs of the catalyst distribution on the surface of a typical MEA that is manufactured using a conventional method and is used as Comparative Example 1, and also shows the concentration distribution for each noble metal. It can be seen that the catalyst on the surface is not uniformly distributed, as indicated by the arrows in the photographs.
2. Electrochemical Cell for Evaluation and Evaluation System
The same evaluation was performed in the electrochemical cell and evaluation system to which the MEA (electrochemically active area of 314 cm²) of Comparative Example 1 was applied, like in Example 1.
3. Measurement Result
From FIG. 7, it can be seen that the voltage slope is significantly increased as the current density is increased in Comparative Example 1.
[Evaluation of Example 1 and Comparative Example 1]
FIG. 7 shows the current density-voltage characteristic of the MEAs of Example 1 and Comparative Example 1, region (1) shows the high and low performance, depending on the electrochemical catalyst, and region (2) is a high current density region. The constitutions of the electrochemical catalyst of Example 1 and the electrochemical catalyst of Comparative Example 1 are the same. Accordingly, from FIG. 7, it can be seen that the MEA of Example 1 and the MEA of Comparative Example 1 have similar performance in region (1). However, it can be seen that since the membrane having the electron-conductive layer of Example 1 has excellent electron conductivity to the electrochemical reaction layer in the MEA, the membrane exhibits the low-voltage characteristic in region (2), which is the high-density current region, compared to the membrane of Comparative Example 1.
As described above, the present invention has a better electron movement path than the conventional electrochemical cell, and accordingly the current density-voltage characteristic of the electrochemical cell is excellent and the energy consumption required for electrolysis is reduced. In other words, in the related art, electrons are sequentially moved through an anode catalyst, an anode-chamber diffusion layer, an anode-chamber current application plate, an external circuit, a pressure pad, a cathode-chamber current application plate, a cathode-chamber diffusion layer, and a cathode catalyst to thus form the electron movement path. However, according to the present invention, the electrons are sequentially moved through an anode catalyst, an electron-conductive layer on a membrane, an external circuit, the electron-conductive layer on the membrane, and a cathode catalyst to thus form the electron movement path. Therefore, the electrochemical cell of the present invention has an electron movement path that is shorter than that of the known electrochemical cell, and accordingly, the current density-voltage characteristics of the electrochemical cell may be excellent due to the short electron movement path, thereby reducing energy consumption during electrolysis.
Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

What is claimed is:

1. A membrane electrode assembly for an electrochemical cell comprising:

a polymer electrolyte membrane;

a first electrochemical reaction layer formed on one side of the polymer electrolyte membrane to allow an oxidation reaction to occur thereon;

a first electron-conductive layer formed between the polymer electrolyte membrane and the first electrochemical reaction layer;

a second electrochemical reaction layer formed on a remaining side of the polymer electrolyte membrane to allow a reduction reaction to occur thereon; and

a second electron-conductive layer formed between the polymer electrolyte membrane and the second electrochemical reaction layer,

wherein the first electron-conductive layer and the second electron-conductive layer include a porous metal.

2. The membrane electrode assembly of claim 1, wherein the porous metal further includes a coating layer including a platinum group applied on a surface thereof.

3. The membrane electrode assembly of claim 1, wherein the first electron-conductive layer and the second electron-conductive layer each have a thickness of 0.1 to 1 mm.

4. The membrane electrode assembly of claim 1, wherein the first electron-conductive layer and the second electron-conductive layer include any one among platinum, palladium, rhodium, iridium, ruthenium, osmium, carbon, gold, tantalum, tin, indium, nickel, tungsten, manganese, and titanium, or a complex including two or more thereof.

5. The membrane electrode assembly of claim 1, wherein the first electrochemical reaction layer and the second electrochemical reaction layer each includes an electrochemical catalyst (or the electrochemical catalyst on a carrier), an ionic conductor, and a binder.

6. The membrane electrode assembly of claim 5, wherein the electrochemical catalyst includes any one among platinum group elements including platinum, palladium, ruthenium, iridium, rhodium, and osmium, any one metal among iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum, or alloys, oxides, or double oxides thereof.

7. The membrane electrode assembly of claim 1, wherein elements constituting the membrane electrode assembly include the polymer electrolyte membrane, the first electron-conductive layer, the second electron-conductive layer, the first electrochemical reaction layer, and the second electrochemical reaction layer in descending order of size.