CN1870339A

CN1870339A - Fuel cell system comprising vapor-phase fuel supplying system

Info

Publication number: CN1870339A
Application number: CNA2006100845658A
Authority: CN
Inventors: 金惠庆; 吴柾旻; 李在镛; 张赫
Original assignee: Samsung SDI Co Ltd
Current assignee: Samsung SDI Co Ltd
Priority date: 2005-05-25
Filing date: 2006-05-25
Publication date: 2006-11-29
Anticipated expiration: 2026-05-25
Also published as: KR20060121682A; KR100718114B1; CN100463277C

Abstract

A fuel cell system is provided with a first separation layer and a buffer solution layer disposed between a liquid-phase fuel storage layer and an anode of a membrane electrode assembly. A vapor-phase fuel is transferred to the buffer solution layer through the first separation layer, condensed, and diluted to produce a liquid-phase fuel with a low concentration in the buffer solution layer, and the low concentration liquid-phase fuel is supplied to the membrane electrode assembly. A second separation layer may be interposed between the first separation layer and the fuel storage layer. Fuel is supplied by a passive supplying method so that the system can be small with a high efficiency and unnecessary power consumption can be prevented. The fuel cell system can be operated regardless of orientation.

Description

Fuel cell system including vapor phase fuel supply apparatus

Technical Field

The present invention relates to a fuel cell system, and more particularly, to a fuel cell system having high energy density due to high system efficiency and being capable of being fabricated in a small size to be suitable as a small compact power source.

This application claims priority to korean patent applications serial nos. 10-2005-0044252 and 10-2006-0041965 filed on 25.2005 and 10.2006 with 10.10.2006 with the korean intellectual property office, the contents of which are incorporated herein by reference in their entirety.

Background

A fuel cell is a system for generating energy in which chemical reaction between hydrogen and oxygen or between hydrogen and oxygen contained in a hydrocarbon-based material such as methanol, ethanol, or natural gas is directly converted into electric energy. Fuel cells may be classified into phosphoric acid type fuel cells, molten carbonate type fuel cells, solid oxide type fuel cells, polymer electrolyte membrane fuel cells, alkaline fuel cells, and the like, according to the electrolyte used therein. These fuel cells operate on the same principle but with different fuels, different operating temperatures, different catalysts and different electrolytes.

Among these fuel cells, a Polymer Electrolyte Membrane Fuel Cell (PEMFC) has better output performance, lower operating temperature, shorter start-up time, and faster response than other fuel cells. Because of these advantages, PEMFCs have a wider range of applications, including portable power sources for automobiles, independent power sources for homes and public buildings, and small power sources for electronic devices.

Among PEMFCs, there is a Direct Methanol Fuel Cell (DMFC) that uses an aqueous methanol solution as a fuel. DMFC can operate at room temperature and is easy to miniaturize and seal, so it can be used as a power source for various application ranges, for example, as a power source for cleaning electric vehicles, home power generation systems, mobile communication equipment, medical equipment, military equipment, space business equipment, portable electronic equipment, and the like.

The electrical power generated by a DMFC depends on the rate at which reactions occur in the anode and cathode. More specifically, at the anode, 1 mole of methanol reacts with 1 mole of water, and the methanol is oxidized and generates carbon dioxide and 6 electrons, as shown in equation 1.

[ reaction formula 1]

Since the stoichiometric ratio of methanol to water is 1: 1 in the anode reaction shown in equation 1, the anode reaction shown in equation 1 requires constant supply of reactants in a suitable ratio. In practice, to fully oxidize methanol, water is supplied in a larger amount than according to the methanol to water stoichiometric ratio (1: 1). If methanol is not completely oxidized, the reaction shown in reaction formula 2 or reaction formula 3 occurs, and the power generation efficiency is lowered.

[ reaction formula 2]

[ reaction formula 3]

Methods of supplying fuel to the above DMFC include an active supplying method and a passive supplying method. The active supply method requires an external supply unit that delivers fuel under pressure. From another point of view, according to the passive supply method, the fuel is supplied spontaneously (volumtarily) without providing such a pressure delivery device.

According to the active supply method, the concentration of the reaction fluid supplied to the anode is suitably maintained by supplying pure methanol or high-concentration methanol to a circulation loop (recirculation loop) that collects water produced by the cathode reaction and supplies the collected water to the anode. The advantage of active supply is that the energy density of the whole system can be increased by using methanol cartridges (cartridges). On the other hand, the active supply method has disadvantages in that the system is complicated, additional equipment is required to cause an increase in size, and power loss is caused as the external supply equipment consumes its operating energy. More specifically, the active supply method is not suitable for the present trend that the size of the power supply is also required to be small due to the size reduction of the device.

From another point of view, the passive supply method can achieve the same object by selecting suitable elements and suitable structures. The greatest advantage of the passive supply method is the simplicity of the system. Of course, the fuel cartridge contains water and methanol, which results in an increase in volume.

This problem can be solved by supplying the water produced at the cathode by a passive supply method, which is disclosed in us 2004-209136. That is, a hydrophobic microporous layer is formed in a cathode of a membrane electrode assembly, and water generated in the cathode is transported to an anode by hydrostatic pressure thereof.

However, in such a system, a unit cell (unit cell) formed of multiple layers is easily broken by hydrostatic pressure. Further, U.S. patent No. 2004-209136 gives no teaching to stably supply and dilute methanol used as fuel.

Disclosure of Invention

An object of the present invention is to provide a fuel cell system which has high energy density due to high system efficiency and can be made small-sized to be suitable as a small and compact power source.

According to an aspect of the present invention, there is provided a fuel cell system including: a membrane electrode assembly comprising a cathode, a proton-conducting membrane, and an anode; a buffer solution layer facing the surface of the anode and containing liquid-phase water and vapor-phase fuel generated in the cathode; a first separation layer facing a surface of the buffer solution layer; and a fuel storage layer facing a surface of the first separator layer.

The buffer solution layer can ensure that methanol of a predetermined concentration is supplied by mixing liquid-phase water and vapor-phase fuel generated in the cathode. The concentration of methanol can be controlled according to the required output power.

The buffer solution layer may be spaced apart from the first separation layer. Further, the first separation layer may be spaced apart from the fuel storage layer.

The buffer solution layer may contain a porous medium and a buffer solution uniformly dispersed in the porous medium.

The pore diameter distribution of the porous medium may be bimodal. The first peak of the bimodal distribution lies between 1nm and 10 μm, while the second peak of the bimodal distribution lies between 10 μm and 10 mm.

The porous medium may be hydrophilic.

The first separation layer may have a pore diameter of 0.001 μm to 50 μm. The thickness of the first isolation layer may be in a range of 1 μm to 500 μm. Further, the first separation layer may be separated from the buffer solution layer.

The first separation layer is made of a material that allows vapor phase methanol to pass through faster than water. The diffusion coefficient of methanol relative to the first separation layer is at least three times the diffusion coefficient of water relative to the first separation layer. The first separation layer may be a laminate comprising at least two layers having different porosities and pore sizes or gas permeabilities. The laminate layers may be separated.

The fuel may be methanol, and the concentration of methanol may be in the range of 0.5 to 5M.

Drawings

The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings. In the drawings:

fig. 1 is a schematic diagram of important elements of a fuel cell system according to an embodiment of the present invention;

FIGS. 2A to 2C are schematic views of a channel structure formed in a porous medium according to an embodiment of the present invention;

FIG. 3 is a schematic view of a concavo-convex structure formed in a porous medium according to an embodiment of the present invention;

fig. 4 is an exploded cross-sectional view of a fuel cell system of an embodiment of the present invention;

fig. 5 is a cross-sectional view of a fuel cell system of an embodiment of the invention;

fig. 6 is a cross-sectional view of a fuel cell system of another embodiment of the present invention;

fig. 7 is a cross-sectional view of a fuel cell system including a current collector according to another embodiment of the present invention;

fig. 8 is a graph showing the results of performance tests of the fuel cell systems of examples 1 to 3 and comparative example.

Detailed Description

The present invention will now be described more fully with reference to the accompanying drawings.

The fuel supply apparatus of an embodiment of the present invention includes a membrane electrode assembly 10, a buffer solution layer 21, a first separation layer 30, and a fuel storage layer 41 (see fig. 1). The buffer solution layer 21 faces the anode surface of the membrane electrode assembly 10. The first separation layer 30 faces the surface of the buffer solution layer 21. The fuel storage layer 41 faces the surface of the first separation layer 30.

The fuel is stored in the fuel storage layer 41 in a liquid state. When the fuel supply apparatus is operated, the liquid-phase fuel is vaporized and diffused to the first separation layer 30. The fuel storage layer 41 may be physically separated from the first separation layer 30 to prevent the liquid phase fuel from flowing into the buffer solution layer 21 through the first separation layer 30.

That is, the liquid-phase fuel stored in the fuel storage layer 41 is converted into the vapor-phase fuel, and then, the vapor-phase fuel reaches the buffer solution layer 21 through the first separation layer 30.

When the liquid-phase fuel flows into the buffer solution layer 21, the fuel is excessively supplied to the buffer solution layer 21. Such an excessive supply is undesirable because the high concentration of fuel causes undesirable consequences such as rapid poisoning of the catalyst and methanol crossover.

The fuel storage layer 41 itself may be a liquid-phase fuel having a free surface, or may be formed so that the liquid-phase fuel is uniformly dispersed in the porous medium. Of course, the fuel storage layer 41 is not limited thereto. If the liquid phase fuel is uniformly dispersed in the porous medium, the fuel can be stably supplied regardless of the orientation of the entire system.

Further, the fuel storage layer 41 may be accommodated in the fuel cartridge 40 for easy disposal. In this case, the upper portion of the fuel cartridge 40 faces the surface of the first insulation layer 30, and the fuel cartridge 40 may have an opening 42 through which the vaporized fuel passes.

Additionally, a liquid transfer medium may also be formed between the fuel cartridge 40 and the first barrier layer 30. The liquid transfer medium may be any material that allows liquid to pass through faster than gas. The liquid transfer medium may be formed in a thin membrane (membrane) for use.

The fuel storage layer 41 may contain a porous medium, such as a foam material, to allow stable storage of the liquid phase fuel regardless of the orientation of the overall system. In this case, the fuel is not limited and may be any material that reacts with water at the anode to generate electrons and protons. The fuel may be, but is not limited to, hydrogen, methanol, ethanol, other hydrocarbon materials or mixtures of these materials, or aqueous solutions of these materials. If the fuel cell system of this embodiment of the present invention is a Direct Methanol Fuel Cell (DMFC), the fuel may be pure methanol or a high concentration methanol aqueous solution. The high-concentration aqueous methanol solution refers to an aqueous methanol solution having a concentration of 5M or more.

The first separation layer 30 delivers the vapor phase fuel to the buffer solution layer 21 and prevents the water in the buffer solution layer 21 from flowing into the fuel storage layer 41. The material forming the first separation layer 30 is not limited, and may be any material that allows fuel to pass through faster than water. The material forming the first separation layer 30 may be a microporous element. For example, the diffusion coefficient of methanol relative to first separation layer 30 is at least three times the diffusion coefficient of water relative to first separation layer 30. If the diffusion coefficient of methanol with respect to the first separation layer 30 is less than three times the diffusion coefficient of water with respect to the first separation layer 30, the selectivity of methanol with respect to water is insufficient, and water flows into the fuel storage layer 41.

The first separation layer 30 may be a porous layer made of Nafion 112, 115, or 117 or Teflon, but is not limited thereto.

The pore diameter of the first separation layer 30 may be in the range of 0.001 μm to 50 μm. The pore diameter distribution of the first separator layer 30 may be bimodal. For example, a first peak of a bimodal distribution may lie between 0.001 μm and 0.05 μm, and a second peak of a bimodal distribution may lie between 1 μm and 50 μm. Alternatively, the first separation layer 30 may have a stacked structure of a first layer having an average pore size of 0.001 μm to 0.05 μm and a second layer having an average pore size of 1 μm to 50 μm.

If the average diameter of the pores of the first separation layer 30 is less than 0.001 μm, it is difficult for the fuel to pass through the first separation layer 30. If the average diameter of the pores of the first separation layer 30 is greater than 50 μm, the fuel at a high concentration is rapidly diffused, thereby decreasing the efficiency of the electrode, and water easily flows into the fuel storage layer 41. It is thus difficult to maintain the fuel concentration in the buffer solution layer 21 at a low level.

The thickness of the first isolation layer 30 may be in the range of 1 to 500 μm. If the thickness of the first separation layer 30 is less than 1 μm, the first separation layer 30 is easily broken, and thus careful handling thereof is required. If the thickness of the first separation layer 30 is more than 500 μm, it is difficult to transport fuel, thereby degrading the performance of the fuel cell system.

As described above, the first separation layer 30 may be separated from the buffer solution layer 21. When the first separation layer 30 is separated from the buffer solution layer 21, the liquid-phase water contained in the buffer solution layer 21 can be prevented from being transferred into the fuel storage layer 41 through the first separation layer 30. When the liquid-phase water reaches the fuel storage layer 41, the concentration of the fuel is reduced, and thus, the balance of the fuel supply is broken, and therefore, the entire system may become unstable.

Typically, methanol used as fuel for the DMFC is diluted before being supplied to the membrane electrode assembly. When methanol is supplied at a high concentration, the catalyst is poisoned quickly due to the lack of water that can prevent the catalyst on the electrode from absorbing CO generated during the oxidation of methanol. In addition, when high-concentration methanol is supplied, unreacted methanol passes through the membrane of the membrane electrode assembly, causing a decrease in the efficiency of the entire fuel cell system and poisoning the catalyst of the cathode.

In order to supply the low concentration fuel aqueous solution required according to the above reasons, the buffer solution layer 21 is formed. The buffer solution layer 21 converts the vapor-phase fuel, which is delivered through the first separation layer 30, into a liquid phase to prepare a fuel mixture of a low concentration.

The vapor phase fuel is converted into the liquid phase fuel by causing the vapor phase fuel entering the buffer solution layer 21 to diffusively collide with the liquid surface of the buffer solution layer 21. That is, the buffer solution layer 21 may contain vapor phase fuel and liquid phase water generated in the cathode.

When starting up the fuel cell system, the buffer solution layer 21 may be pure water or a low-concentration fuel mixture. In this case, however, it takes a long time for the fuel to diffuse to the membrane electrode assembly 10. As a result, during the start-up operation, a low-concentration fuel mixture may be used instead of pure water.

The buffer solution layer 21 itself may be a liquid having a free surface, or may be formed as a liquid fuel aqueous solution uniformly dispersed in a porous medium. Of course, the buffer solution layer 21 is not limited thereto. If the liquid-phase fuel solution is uniformly dispersed in the porous medium, the fuel can be stably supplied regardless of the orientation of the entire system.

In addition, the buffer solution layer 21 may be accommodated in the buffer solution cartridge 20 for easy disposal. In this case, the upper portion of the buffer solution cartridge 20 may be completely opened such that the contained buffer solution layer 21 is sufficiently in contact with the anode surface, and the lower portion of the buffer solution cartridge 20 may have an opening 22 such that the fuel having passed through the first separation layer 30 is smoothly transferred to the buffer solution layer 21. In addition, the buffer solution cartridge 20 may have CO₂ An exhaust hole 23 for smoothly discharging CO generated by the reaction on the anode₂。

The porous medium may be hydrophilic. Since the buffer solution layer 21 serves as both the fuel supply and the CO₂The pore size distribution of the vent hole 23, the porous medium may be broad (broad) or bimodal.

That is, although it is preferable that the small hole is supplied with the fuel aqueous solution due to the large capillary pressure, it is very importantDifficult to discharge CO through evenly distributed small holes₂. That is, since the small pores have a high capillary pressure, the fuel aqueous solution is supplied mainly through the small pores, and since the large pores have a lower capillary pressure than the small pores, CO is discharged through the large pores₂. In view of these properties of the small and large pores, the first peak of the bimodal distribution lies between 1nm and 10 μm, while the second peak of the bimodal distribution lies between 10 μm and 10 mm.

The porous medium may be one of an inorganic oxide material, a polymer material, and a composition (compound) thereof.

The inorganic oxide material may be silicon dioxide (SiO)₂) Alumina (Al)₂O₃) Titanium dioxide (TiO)₂) Zirconium oxide (ZrO)₂) And combinations thereof, but are not limited thereto.

The polymer material may be a polymer resin containing hydroxyl, carboxyl, amine or sulfo groups; polyvinyl alcohol-based polymer resins; a cellulose-based polymeric resin; a polyvinylamine based polymeric resin; polyethylene oxide-based polymer resins; polyethylene glycol-based polymeric resins; nylon-based polymeric resins; polyacrylate-based polymer resins; polyester-based polymeric resins; a polyvinyl pyrrolidone-based polymer resin; a polymer resin based on ethylene vinyl acetate; a polyethylene-based polymeric resin; polystyrene-based polymeric resins; fluorine-based polymer resin (fluororesin-based polymer resin); a polypropylene-based polymeric resin; polymethyl methacrylate-based polymer resins; polyimide-based polymeric resins; polyamide-based polymeric resins; a polymeric resin based on polyethylene terephthalate; and combinations thereof, but is not limited thereto.

The pores of the porous medium may have an average diameter of 0.01 to 10 μm. If the average diameter is less than 0.01. mu.m, methanol cannot sufficiently diffuse, and thus may cause deterioration in fuel cell performance. If the average diameter is larger than 10 μm, a balance between supply and consumption of methanol cannot be maintained.

By adjusting the mobility of methanol to 0.8X 10^-6g/cm²Sec to 4X 10^-6g/cm²Sec can adjust the porosity and curvature of the porous media. Here, the curvature represents the degree of bending or twisting in the pore, and may be calculated by dividing the actual distance of molecular motion between two random points by the straight-line distance between the two points. That is, if the curvature is one, the hole is straight, and the larger the curvature, the more curved the hole.

The porous medium has a thickness of 0.01mm to 10 mm. If the thickness is less than 0.01mm, it is not easy to handle the porous medium because of its low mechanical strength. If the thickness is more than 10mm, the volume of the fuel cell system is too large.

The porous medium has CO in a surface in contact with the surface of the anode₂A discharge passage. The channels may have various shapes and structures according to the size of the fuel cell system, without being particularly limited thereto. For example, the channels may have the shape shown in FIGS. 2A-2C, although not limited toThis is done.

Further, the porous medium may have a pattern in a surface in contact with the surface of the first insulating layer. The pattern can enlarge the effective surface area through which the fuel from the buffer solution layer flows, and shorten the path for supplying the fuel. The pattern may have a structure in which islands are formed on the surface of the porous medium or a concavo-convex structure in which concave portions are partially formed. For example, the pattern may have the shape shown in fig. 3, but is not limited thereto.

The working principle of the buffer solution layer 21 will be described below.

When the methanol concentration in the buffer solution layer 21 is low, the amount of methanol supplied to the buffer solution layer 21 through the first separation layer 30 is larger than the amount of methanol delivered from the buffer solution layer 21 to the membrane electrode assembly 10. As a result, the methanol concentration in the buffer solution layer 21 increases. Stated another way, when the methanol concentration in the buffer solution layer 21 is high, the amount of methanol delivered from the buffer solution layer 21 to the membrane electrode assembly 10 is larger than the amount of methanol supplied to the buffer solution layer 21 through the first separation layer 30 due to the active reaction of the membrane electrode assembly 10. As a result, the methanol concentration in the buffer solution layer 21 decreases. By such self-regulation (self regulation) as described above, the methanol concentration in the buffer solution layer 21 is maintained in a stable state. That is, liquid-phase water and vapor-phase fuel generated at the cathode are mixed so that methanol has a predetermined concentration.

In the low concentration fuel solution, the concentration of the fuel may be in the range of 0.5 to 5.0M in a normal state. If the concentration of the fuel is less than 0.5M, it takes a long time for the mea 10 to generate a predetermined level or more of electric power by the self-conditioning as described above. If the concentration of the fuel is greater than 5.0M, the performance of the fuel cell system is degraded. That is, such a high concentration fuel brings about undesirable effects such as rapid poisoning of the catalyst and methanol crossover.

The fuel cell may initially operate using only the water supplied to the buffer solution layer 21. However, in order to maintain a constant methanol concentration in the buffer solution layer 21, as much water is supplied to the buffer solution layer 21 as is consumed in the reaction occurring at the anode. The method of supplying water to the buffer solution layer 21 is not limited, and any conventional method known in the art may be employed. For example, water generated at the cathode is collected and circulated to the buffer solution layer 21 through a water circulation passage provided outside the electrode. Alternatively, a hydrophobic film is formed at the cathode and water generated in the cathode is diffused toward the anode through the electrolyte membrane by means of hydrostatic pressure generated by the water generated in the cathode accumulated in the cathode.

By the above method, the fuel in the buffer solution layer 21 can be controlled. More specifically, the concentration of methanol may be adjusted to a predetermined level with liquid-phase water generated in the cathode. Furthermore, the concentration of the fuel can be adjusted by controlling the amount of water according to the output power and the requirements of the external load circuit.

The aqueous fuel solution in the buffer solution layer 21 is transported by capillary pressure to the anode of the membrane electrode assembly 10 in close contact with the buffer solution layer 21. To facilitate uniform supply of fuel, a liquid transfer medium may also be formed between the buffer solution layer 21 and the anode. The liquid transport medium may be any medium that allows a liquid to pass through faster than a gas, but is not limited thereto. The liquid transfer medium may be formed in a thin membrane.

The membrane electrode assembly 10 includes a cathode, an anode, and a proton conductive membrane disposed therebetween. The material, shape and production method of the proton-conducting membrane and the electrodes (cathode and anode) are not limited, and any method known in the art may be used. The aqueous fuel solution that has been delivered from the buffer solution layer 21 comes into contact with the catalyst of the anode and generates electric energy through the chemical reaction shown in reaction formula 1.

A fuel cell system according to an embodiment of the present invention will be described with reference to fig. 4 and 5. The size of the fuel cell system shown in fig. 1 to 7 is exaggerated for quick and clear understanding.

Referring to FIG. 4, a fuel storage layer 41 may be placed in the fuel cartridge 40.

Referring to fig. 5, the fuel cartridge 40 may have an upper cover 43 at an upper portion thereof to separate the fuel storage layer 41 from the first barrier layer 30. The upper cap 43 may have an opening 42 to allow the delivery of vaporized fuel. The upper cover 43 having the openings 42 may be a flat plate having openings or meshes, which substantially physically separates the fuel storage layer 41 from the first separation layer 30. Of course, the upper cap 43 may have any structure to deliver the vaporized fuel to the first separation layer 30, without being limited thereto.

The first barrier layer 30 may contact and be attached to the upper lid of the fuel cartridge 40 in which the opening 42 is formed. The buffer solution cartridge 20 may contact and be attached to an upper surface of the first barrier layer 30 that contacts the fuel cartridge 40.

Further, as shown in fig. 4, the buffer solution cartridge 20 may contain a buffer solution layer 21. The lower portion of the buffer solution cartridge 20 may separate the buffer solution layer 21 from the first separation layer 30. In this case, the lower portion of the buffer solution cartridge 20 may have an opening 22 to deliver the fuel that has passed through the first barrier layer 30. The lower portion of the buffer solution cartridge 20 having the opening 22 may be a flat plate having holes or meshes that substantially physically separate the buffer solution layer 21 from the first separation layer 30. The lower portion of the buffer solution cartridge 20 may have any structure that allows the fuel that has passed through the first separation layer 30 to move to the buffer solution layer 21, without being limited thereto.

As shown in fig. 5, the buffer solution layer 21 may face the membrane electrode assembly 10, specifically, the anode of the membrane electrode assembly 10. The cathode of the membrane electrode assembly 10 may be protected using a flat plate having an air supply port.

The layers of the above combination may be bonded together using suitable bonding means.

A fuel cell system according to another embodiment of the present invention will be described with reference to fig. 6.

The fuel cartridge 40 and the first barrier layer 30 may be the same as in the previous embodiment. A second barrier layer 50 may also be formed between the fuel cartridge 40 and the first barrier layer 30. The second barrier layer 50 maintains the shape of the first barrier layer 30 and controls the rate of fuel supply together with the first barrier layer 30.

The material forming the second isolation layer 50 is not limited, and may have pores with a diameter of 1 μm to 10 μm. If the average pore diameter of the second separator 50 is less than 1 μm, the vaporized fuel hardly passes through the second separator 50. On the other hand, if the average pore diameter of the second separation layer 50 is greater than 10 μm, it may be difficult to control the supply rate of the fuel.

The buffer solution cartridge 20 may be the same as in the previous embodiment, and may additionally include CO₂And an exhaust hole 23.

Can convert CO into₂The exhaust hole 23 is formed at the side of the buffer solution cartridge 20, and if necessary, a plurality of CO may be formed₂And an exhaust hole 23. CO 2₂The discharge hole 23 may be formed only on one side portion or on a plurality of side portions. Can convert CO into₂The diameter of the exhaust hole 23 is formed so that the gas phase CO can be discharged₂While the liquid phase buffer solution does not leak. CO 2₂The vent holes 23 may have a diameter of 0.01 to 0.5 mm. CO with a diameter of less than 0.01mm₂Exhaust vent 23It is difficult to prepare. If CO is present₂When the diameter of the vent hole 23 is larger than 0.5mm, the liquid-phase buffer solution may leak.

In the fuel cell system of this embodiment of the invention, the current collector may be formed on the surface of each electrode that is not in contact with the proton-conductive membrane of the conventional structure. Or referring to fig. 7 showing a fuel cell system of another embodiment of the present invention,

current collectors

12a and 12b may be disposed between the proton-conducting membrane 11 and the cathode 14 and between the proton-conducting membrane 11 and the anode 13, respectively. The

current collectors

12a and 12b collect the current generated in the electrodes and transmit the collected current to an external circuit. The material forming the current collector is not limited and may be any material that conducts current and resists corrosion.

The

current collectors

12a and 12b may be disposed between the proton-conducting membrane and the electrode because the most active electrochemical reaction that generates current occurs between the proton-conducting membrane 11 and the electrode due to the large concentration of the reactant, and thus, the generated current may be most efficiently collected.

Further, the high concentration methanol may be diffused in the liquid phase into the fuel storage layer 41 and the second separation layer 50 using a porous medium in advance.

The fuel cell system of this embodiment of the present invention is supplied with fuel using a passive supply method, and thus, the system is small and does not consume unnecessary electric power. As a result, the efficiency of the entire system is high. In addition, the use of pure fuel or highly concentrated aqueous fuel solution allows the energy density of the entire system to be high. Therefore, the fuel cell can be used as a small and compact power supply source. Further, the liquid phase fuel is provided in a vapor form, and therefore, the fuel can be supplied regardless of the orientation of the entire system. Therefore, the present fuel cell system can be used in a portable power source.

The present invention is described in more detail below with reference to examples. These examples are intended to be illustrative only and are not intended to limit the scope of the present invention.

Example 1

Porous foam with uniformly dispersed pure methanol was placed in the fuel canister, and the resulting fuel canister was sealed with an upper lid. In this case, Foamex is used^®As a porous foam, and an acrylic plate having a linear opening of 0.8mm width was used as an upper cover.

Then, a second separator plate having pores with a diameter of 200nm was placed on the sealed fuel cartridge, and then a first separator layer formed of Nafion 117 was placed thereon.

Next, a buffer solution cartridge containing a buffer solution is placed on the first separator. In this case, a 1M aqueous methanol solution was used as a buffer solution, and the buffer solution was uniformly dispersed on the carbon cloth. The resulting carbon cloth was placed in a buffer solution cartridge. Four 0.4mm diameter CO₂The vent holes are formed in four side portions of the buffer solution cartridge, respectively. Further, a linear opening 0.8mm wide is formed in the lower portion of the buffer solution cylinder to allow the vaporized fuel to pass therethrough.

A membrane electrode assembly prepared using a conventional method known in the art is placed on the multi-layer stack of the fuel cartridge, the second separator/first separator layer, and the buffer solution cartridge prepared as described above. The proton conductive membrane of the membrane electrode assembly was formed of Nafion 117 using a Pt/Ru alloy catalyst as the anode catalyst and a Pt/Al alloy catalyst as the cathode catalyst.

The membrane electrode assembly deposited above was covered with an acryl plate having a circular opening of 5mm diameter to protect the membrane electrode assembly and smoothly supply air.

The electric power density of the fuel cell prepared in the above manner was measured with respect to time, and the results thereof are shown in fig. 8.

Example 2

A fuel cell was prepared in the same manner as in example 1, except that a 3M aqueous methanol solution was used as a buffer solution.

Example 3

Except that Nafion 112 and a film having a thickness of 45 μm and a density of 5500g/m were used by lamination²A laminate formed of a Teflon porous layer having a gas permeability of/24 hr was used as the outside of the first separator, and a fuel cell was prepared in the same manner as in example 1.

Comparative example

A fuel cell system was prepared such that the anode of the membrane electrode assembly used in example 1 was directly contacted with an aqueous methanol solution.

First, the anode of the membrane electrode assembly was oriented upward, and the cathode was covered with an acryl plate having a circular opening of 5mm diameter to protect the membrane electrode assembly and smoothly supply air. Then, the fuel cartridge containing 3M methanol aqueous solution was brought into direct contact with the upper portion of the anode, so that methanol aqueous solution was directly supplied to the anode. In this case, the fuel cartridge has no upper lid.

The electric power density of the fuel cell prepared in the above manner was measured as a function of time, and the results thereof are shown in fig. 8.

Referring to fig. 8, during initial operation, the fuel cell prepared according to the comparative example exhibited higher electric power density than the fuel cells prepared according to examples 1 to 3. However, the electric power density of the fuel cell prepared according to the comparative example was significantly reduced with time. For example, when the fuel cell is operated for 1 hour or less than 1 hour, the electric power density of the fuel cell is significantly reduced.

That is, during the initial operation, methanol is smoothly supplied at a high concentration, so that the reaction can be rapidly performed. Accordingly, as the temperature of the membrane electrode assembly increases, the electric power density also increases. However, the electric power density of the fuel cell suddenly decreases due to the poisoning of the membrane electrode assembly, and the water concentration required for the anode reaction decreases due to the permeation of methanol, resulting in a decrease in efficiency.

From another point of view, although the fuel cells prepared according to examples 1 to 3 had lower electric power densities than those of the fuel cells prepared according to comparative examples during the initial operation, they had constant electric power densities in long-term use. When the system is started, the temperature of the membrane electrode assembly is increased along with the progress of the reaction, the reaction speed is increased, and the electric power density is also increased. The supply of methanol is then regulated by itself, as the heat generated by the exothermic reaction balances the cooling rate relative to the ambient environment. As a result, a constant electric power density can be obtained.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. A fuel cell system comprising:

a membrane electrode assembly comprising a cathode, a proton-conducting membrane, and an anode;

a buffer solution layer facing a surface of the anode and containing liquid-phase water and vapor-phase fuel generated in the cathode;

a first separation layer facing a surface of the buffer solution layer; and

a fuel storage layer facing a surface of the first barrier layer.

2. The fuel cell system according to claim 1, wherein a concentration of the fuel contained in the buffer solution layer is controllable.

3. The fuel cell system according to claim 1, wherein an amount of liquid-phase water contained in the buffer solution layer is controlled according to a required electric power.

4. The fuel cell system of claim 1, wherein the buffer solution layer is spaced apart from the first isolation layer.

5. The fuel cell system of claim 1, wherein the first isolation layer is spaced from the fuel storage layer.

6. The fuel cell system according to claim 1, wherein the buffer solution layer contains a porous medium and a buffer solution uniformly distributed in the porous medium.

7. The fuel cell system of claim 6, wherein the diameter distribution of the pores of the porous medium is bimodal.

8. The fuel cell system of claim 7, wherein a first peak of the bimodal distribution is between 1nm and 10 μm and a second peak of the bimodal distribution is between 10 μm and 10 mm.

9. The fuel cell system of claim 6, wherein the porous media is hydrophilic.

10. The fuel cell system of claim 6, wherein the porous medium is selected from the group consisting of inorganic oxide materials, polymeric materials, and combinations thereof.

11. The fuel cell system of claim 10, wherein the inorganic oxide material is selected from the group consisting of silicon dioxide (SiO)₂) Alumina (Al)₂O₃) Titanium dioxide (TiO)₂) Zirconium oxide (ZrO)₂) And combinations thereof.

12. The fuel cell system according to claim 10, wherein the polymer material is selected from the group consisting of a polymer resin containing a hydroxyl group, a carboxyl group, an amine group, or a sulfo group; polyvinyl alcohol-based polymer resins; a cellulose-based polymeric resin; a polyvinylamine based polymeric resin; polyethylene oxide-based polymer resins; polyethylene glycol-based polymeric resins; nylon-based polymeric resins; polyacrylate-based polymer resins; polyester-based polymeric resins; a polyvinyl pyrrolidone-based polymer resin; a polymer resin based on ethylene vinyl acetate; a polyethylene-based polymeric resin; polystyrene-based polymeric resins; a fluororesin-based polymer resin; a polypropylene-based polymeric resin; polymethyl methacrylate-based polymer resins; polyimide-based polymeric resins; polyamide-based polymeric resins; a polymeric resin based on polyethylene terephthalate; and combinations thereof.

13. The fuel cell system according to claim 6, wherein the average diameter of the pores of the porous medium is 0.01 μm to 10 μm.

14. The fuel cell system according to claim 6, wherein the mobility of methanol is adjusted to 0.8 x 10^-6g/cm²Sec to 4X 10^-6g/cm²Sec adjusts the porosity and curvature of the porous media.

15. The fuel cell system of claim 6, wherein the porous medium has a thickness of 0.01mm to 10 mm.

16. The fuel cell system of claim 6, wherein the porous medium has CO in a surface in contact with the surface of the anode₂A discharge passage.

17. The fuel cell system of claim 6, wherein the porous medium has a pattern in a surface in contact with the surface of the first separator layer.

18. The fuel cell system according to claim 1, wherein the buffer solution layer is accommodated in a buffer solution cartridge;

the upper portion of the buffer solution cartridge is completely open, allowing the entire buffer solution layer to contact the surface of the anode; and

a lower portion of the buffer solution cartridge faces the surface of the first barrier layer.

19. The fuel cell system of claim 18, further comprising a liquid transfer medium disposed between the buffer solution layer and the anode.

20. The fuel cell system of claim 18, wherein the buffer solution cartridge has CO₂And (4) exhausting holes.

21. The fuel cell system according to claim 18, wherein an opening is formed in a lower portion of the buffer solution cartridge.

22. The fuel cell system according to claim 1, wherein the diameter of the pores of the first separation layer is 0.001 μm to 50 μm.

23. The fuel cell system of claim 22, wherein the diameter distribution of the pores of the first separator layer is bimodal, a first peak of the bimodal distribution being between 0.001 μm and 0.05 μm, and a second peak of the bimodal distribution being between 1 μm and 50 μm.

24. The fuel cell system according to claim 1, wherein a thickness of the first separation layer is in a range of 1 to 500 μm.

25. The fuel cell system of claim 1, wherein the first barrier layer passes methanol faster than water and passes gas faster than liquid.

26. The fuel cell system according to claim 1, wherein a diffusion coefficient of methanol with respect to the first separation layer is at least three times a diffusion coefficient of water with respect to the first separation layer.

27. The fuel cell system according to claim 1, wherein the first separation layer is a laminate including at least two layers having different porosities and gas permeabilities.

28. The fuel cell system of claim 1, wherein the fuel storage layer comprises a porous medium and a fuel uniformly dispersed in the porous medium.

29. The fuel cell system according to claim 1, wherein the fuel storage layer is accommodated in a fuel cartridge facing a surface of the first separator.

30. The fuel cell system of claim 29, further comprising a liquid transfer medium disposed between the fuel cartridge and the first barrier layer.

31. The fuel cell system of claim 29, wherein the fuel cartridge has an opening in a surface in contact with the surface of the first barrier layer.

32. The fuel cell system of claim 1, further comprising a second separator layer disposed between the first separator layer and the fuel storage layer.

33. The fuel cell system of claim 1, wherein the fuel is methanol.

34. The fuel cell system of claim 33, wherein the methanol concentration in the buffer solution layer is in the range of 0.5 to 5M.

35. The fuel cell system according to claim 33, wherein the fuel contained in the fuel storage layer is pure methanol or high-concentration methanol having a concentration of 5M or more.

36. The fuel cell system according to claim 1, wherein a cathode current collector is formed between the cathode and the proton-conductive membrane, and an anode current collector is formed between the anode and the proton-conductive membrane.