WO2009054663A2

WO2009054663A2 - Fuel cell stack having current collector with elastic structure

Info

Publication number: WO2009054663A2
Application number: PCT/KR2008/006220
Authority: WO
Inventors: Yun Mi Kim; Nam Hyuk Kim; Dong Il Kim
Original assignee: Dongjin Semichem Co., Ltd.
Priority date: 2007-10-25
Filing date: 2008-10-21
Publication date: 2009-04-30
Also published as: WO2009054663A3; KR20090042000A

Abstract

Provided is a fuel cell stack having a current collector with an elastic structure. The fuel cell stack includes a pair of end plates disposed apart from each other to face each other, at least one current generator disposed between the pair of end plates to generate an electric current by means of electrochemical reaction between a reaction fuel and an oxidizer, and at least one current collector disposed between the each endplate and the current generator, and close to the current generator to collect the electric current generated by the current generator, wherein the at least one current collector has an elastic structure. Therefore, the fuel cell stack can provide increased clamping pressure and thus improved performance by having the elastic structure and can provide improved durability by having an appropriately adjusted effective area ratio.

Description

Description FUEL CELL STACK HAVING CURRENT COLLECTOR WITH

ELASTIC STRUCTURE

Technical Field

[1] An illustrative embodiment of the present invention generally relates to a fuel cell stack having a current collector, and more particularly, to a fuel cell stack having a current collector that increases the clamping pressure of the fuel cell stack and thus improves the performance of the fuel cell stack by having an elastic structure and that improves the durability of the fuel cell stack by having an appropriately adjusted effective area ratio. Background Art

[2] With the recent rapid increase in the use of portable electronic devices and wireless communication devices, much attention and research has been devoted to developing power-generation fuel cells as portable power sources and clean energy sources.

[3] A fuel cell is a new power-generation system that directly converts energy generated by electrochemical reaction between a fuel gas, e.g., hydrogen or methanol, and an oxidizer, e.g., oxygen or air, into electrical energy.

[4] Fuel cells may be classified according to the type of electrolyte used therein into phosphoric acid type fuel cells, molten carbonate type fuel cells, solid oxide type fuel cells, polymer electrolyte membrane fuel cells (PEMFCs), alkali type fuel cells, and the like. These fuel cells operate based on similar principles, but are different in terms of the types of fuels used therein, operating temperatures, catalysts, electrolytes, etc.

[5] Among these fuel cells, PEMFCs can be subdivided according to the type of fuel into hydrogen fuel cells using hydrogen as fuels and direct methanol fuel cells (DMFCs) using an aqueous methanol solution supplied to an anode.

[6] Hydrogen fuel cells and DMFCs can operate at room temperature and can be miniaturized, thus having very wide applications comprising pollution-free electric vehicles, home power-generation systems, and power sources for mobile communication equipment, medical appliances, military equipment, space industry equipment, portable electronic devices, and the like.

[7] In a monopolar-type fuel cell, which is a kind of PEMFC, a plurality of unit cells are disposed in parallel widthwise between a pair of end plates and the unit cells are connected in series. In particular, this fuel cell configuration is significantly smaller in terms of thickness and volume than other configurations. Moreover, a fuel cell employing both a PEMFC and a DMFC suitable for portable use under low- temperature and atmospheric pressure conditions has been proposed as the monopolar- type fuel cell.

[8] FIG. 1 is a cross-sectional view illustrating a unit cell of a monopolar-type fuel cell stack according to the prior art.

[9] Referring to FIG. 1, the unit cell of the conventional monopolar-type fuel cell stack includes a membrane electrode assembly (MEA) 10, an MEA protecting gasket 20, an anode fuel supply unit gasket 22, a cathode fuel supply unit gasket 24, an anode current collector 32, a cathode current collector 34, an anode end plate 42, and a cathode end plate 44.

[10] The MEA 10 has a structure in which an anode (not shown) and a cathode (not shown) are disposed close to each other with a polymer electrolyte membrane (not shown) interposed therebetween. In the MEA 10, an electric current is generated by electrochemical reaction between a reaction fuel and an oxidizer.

[11] The MEA protecting gasket 20 is disposed around the edge of the MEA 10, i.e., the edge of a polymer electrolyte membrane, in order to prevent a gas or a liquid used as a fuel from leaking outside through the polymer electrolyte membrane and to prevent damage to the electrodes of the MEA 10.

[12] The anode current collector 32 is disposed on a surface of the MEA 10 and the cathode current collector 34 is disposed on another surface of the MEA 10 so that the anode current collector 32 and the cathode current collector 34 collect the current generated in the MEA 10 and deliver the collected current to an external circuit.

[13] The anode fuel supply unit gasket 22 and the cathode fuel supply unit gasket 24 are disposed around the edge of the anode current collector 32 and the edge of the cathode current collector 34, respectively, in order to form a flow channel for facilitating injection and discharge of the fuel and to prevent the fuel from leaking outside. It is preferable that the anode and cathode fuel supply unit gaskets 22 and 24 are appropriately positioned so as not to cover through-holes (not shown) of the anode and cathode current collectors 32 and 34.

[14] The anode end plate 42 and the cathode end plate 44 are disposed on the anode fuel supply unit gasket 22 and the cathode fuel supply unit gasket 24, respectively.

[15] Fuel cell clamping screws 50 are engaged in such a way to penetrate edge portions of the anode and cathode end plates 42 and 44, thereby completing formation of the fuel cell stack and increasing the clamping pressure of the fuel cell stack.

[16] FIG. 2 illustrates the MEA 10, a current collector 30, and an end plate 40 of FIG. 1.

In this case, the current collector 30 may be the anode current collector 32 or the cathode current collector 34 illustrated in FIG. 1 and the end plate 40 may be the anode end plate 42 or the cathode end plate 44 illustrated in FIG. 1.

[17] Referring to FIG. 2, the longitudinal section and cross section of the current collector

30 are both stripe-shaped. In other words, the plane of the current collector 30 is completely flat. In the current collector 30 having the flat plane as illustrated in FIG. 2, a factor for increasing the clamping pressure cannot exist.

[18] Therefore, the clamping pressure of the monopolar- type fuel cell stack structured as described above can be generally increased by increasing the thickness and width of the end plate 40 and the size and number of the screw 50. When the clamping pressure is low, a loss of electricity may occur due to electric resistance generated between the MEA 10 and the current collector 30 and the liquid or the gas may leak outside.

[19] Such a monopolar-type PEMFC should be as small as possible because of its intended use as a portable type fuel cell. Thus, a balance should be struck between sufficiently small volume and high clamping pressure in order to maximize the performance of the monopolar-type PEMFC.

[20] As conventional approaches to increase the clamping pressure of a fuel cell, a pressurized area may be enlarged by increasing the size or number of screws and may be enlarged by increasing the thickness of an end plate. However, since these approaches increase the volume of a fuel cell when the fuel cell is used as a portable type fuel cell, they are not suitable for adoption.

[21] To reduce the entire volume of a fuel cell by reducing the width of the edge of the fuel cell, a technique using a bonding-type clamping scheme has been suggested in Korean Patent Publication No. 10-2006-0023501. However, this bonding-type clamping scheme cannot provide sufficiently high surface pressure, i.e., clamping pressure, to an MEA.

Disclosure of Invention Technical Problem

[22] An illustrative embodiment of the present invention provides a fuel cell stack with high clamping pressure and thus excellent performance by having a current collector with an elastic structure.

[23] Another illustrative embodiment of the present invention also provides a highly durable fuel cell stack by having a current collector with an appropriately adjusted effective area ratio. Technical Solution

[24] According to an aspect of the present invention, there is provided a fuel cell stack comprising a pair of end plates disposed apart from each other to face each other, at least one current generator disposed between the pair of end plates to generate an electric current by means of electrochemical reaction between a reaction fuel and an oxidizer, and at least one current collector disposed between the each endplate and the current generator, and close to the current generator to collect the electric current generated by the current generator, in which the current collector has an elastic structure. [25] According to an embodiment of the present invention, the current collector may has a curved shape and a convex portion of the current collector may be disposed to face the current generator. [26] According to another embodiment of the present invention, a cross-section of the current collector may has an arch shape, a cap shape, or a wave shape. [27] According to another embodiment of the present invention, a cross-section of the current collector may have a step difference and a protrusion portion of the current collector may be disposed to face the current generator. [28] According to another embodiment of the present invention, an effective area ratio of the current generator is in a range of 20 - 50%. [29] According to another embodiment of the present invention, the current collector may comprise a metal or a metal alloy which is electrochemically stable in a potential range of 0 - IV, for example, 0.1 - IV and has elasticity. [30] According to another embodiment of the present invention, the thickness of the current collector may be in a range of 0.1 - 2 mm. [31] According to another embodiment of the present invention, the current generator may include a membrane electrode assembly (MEA) which includes a polymer electrolyte membrane and an anode and a cathode that are disposed close to both sides of the polymer electrolyte membrane, respectively. [32] According to another embodiment of the present invention, the at least one current generator may be disposed in parallel in a width direction of the end plate and may be electrically connected to each other in series. [33] According to another embodiment of the present invention, the fuel cell stack may further include a bipolar plate disposed between the current generator and the current collector, and the current collector may be disposed close to the bipolar plate to collect the electric current generated by the current generator.

Description of Drawings [34] The above and other features and advantages of the present invention will become more apparent by describing in detail embodiments thereof with reference to the attached drawings in which: [35] FIG. 1 is a cross-sectional view illustrating a unit cell of a monopolar-type fuel cell stack according to the prior art; [36] FIG. 2 illustrates a membrane electrode assembly (MEA), a current collector, and an end plate of the monopolar-type fuel cell stack of FIG. 1 ;

[37] FIG. 3 illustrates a unit cell of a monopolar-type fuel cell stack according to an embodiment of the present invention; [38] FIGS. 4A through 4E are cross-sectional views illustrating current collectors that may be used in the monopolar-type fuel cell stack according to embodiments of the present invention;

[39] FIGS. 5 A through 5C are plan views illustrating cases where through-holes of various sizes and shapes are formed in the current collector illustrated in FIG. 3, according to embodiments of the present invention;

[40] FIGS. 6 A and 6B are graphs showing performance test results of unit cells of

Example 1 and Comparative example 1 ;

[41] FIG. 7 is a graph showing fuel persistent test results of the unit cells of Example 1 and Comparative example 1 ; and

[42] FIG. 8 is a graph showing durability test results of the unit cell of Example 1 with respect to effective area ratio. Mode for Invention

[43] Hereinafter, a fuel cell stack according to embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[44] FIG. 3 illustrates a unit cell of a monopolar-type fuel cell stack according to an embodiment of the present invention. The representation of the fuel cell stack in FIG. 3 corresponds to that of FIG. 2.

[45] Referring to FIG. 3, the monopolar-type fuel cell stack according to the present embodiment includes a membrane electrode assembly (MEA) 100, a current collector 130, and an end plate 140. Fig. 3 illustrates only a half part of the monopolar-type fuel cell stack according to the present embodiment. That is, although not illustrated in Fig. 3, there exist another end plate disposed apart from and to face the end plate 140 on one surface of the MEA 100 opposite to the end plate 140, and another current collector disposed between the end plate and the MEA 100.

[46] In the present embodiment, a MEA 100 and a current collector 130 having a sufficiently large area to cover the MEA 100 are disposed on the end plate 140. However, the present invention is not limited thereto and, although not illustrated, a plurality of MEAs and a plurality of current collectors each covering one of the MEAs are disposed in parallel on the end plate in the width direction of the end plate. The MEA 100, the current collector 130, and the end plate 140 are engaged by clamping screws 150. A fuel cell stack having such configuration is referred to as a monopolar-type fuel cell stack.

[47] The MEA 100 is a type of current generator for generating an electric current by electrochemical reaction between a reaction fuel and an oxidizer. The MEA 100 includes a polymer electrolyte membrane (not shown) and an anode (not shown) and a cathode (not shown) that are disposed close to both sides of the polymer electrolyte membrane, respectively. However, various kinds of current generators may also be used instead of the MEA 100. [48] The current collector 130 has an elastic structure. More specifically, in the present embodiment, the current collector 130 has a curved shape such that its cross-section is bent to have an arch shape and a convex portion C thereof faces the MEA 100, as illustrated in FIG. 3.

[49] The current collector 130 may be formed of a metal or a metal alloy having elasticity.

The current collector 130 may be disposed between the MEA 100 and the end plate 140. It is preferable that the metal or the metal alloy be platinum, titanium, gold, nickel, or an alloy thereof which is electrochemically stable in a potential range of 0 - IV, for example 0.1 - IV. In other words, since a reaction fuel (hydrogen) or an oxidizer (oxygen) is oxidized or deoxidized in a potential range of 0 - IV, the current collector 130 should be electrochemically stable in such a potential range.

[50] In the present embodiment, it is preferable that the thickness of the current collector

130 be in a range of 0.1 - 2 mm.

[51] In the fuel cell stack structured as described above, the clamping screws 150, when being tightened for stack clamping, push the end plate 140 toward the MEA 100 and the end plate 140 then pushes edge portions of the current collector 130 toward the MEA 100. If the clamping screws 150 are further continuously tightened, pressure applied to the edge portions of the current collector 130 increases and is delivered eventually to the convex portion C of the current collector 130, whereby the convex portion C is gradually straightened by an elastic force. As the convex portion C of the current collector 130 is straightened, significantly high pressure is applied to the portion of the MEA 100 which contacts the convex portion C. As a result, the clamping pressure of the fuel cell stack increases, thereby sufficiently lowering the electric resistance in the stack and thus providing a desired power density. By forming the current collector 130 having such an elastic structure, it is possible to improve the performance of the stack with a desired clamping pressure without requiring the end plate 140 to have a large thickness, and to have a relatively thin fuel cell stack.

[52] FIGS. 4A through E are cross-sectional views illustrating various forms of the current collector 130 that may be used in the monopolar- type fuel cell stack according to embodiments of the present invention.

[53] FIG. 4A illustrates a current collector having a cross-section in an arch shape, FIGS.

4B and 4C illustrate current collectors having cross-sections in a cap shape, and FIG. 4E illustrates a current collector having a cross-section in a wave shape. A current collector having a flat plane as illustrated in FIG. 4A can be easily bent for use without a special processing technique.

[54] The current collectors illustrated in FIGS. 4B, 4C, and 4E can be easily worked because their bent angles are not large in comparison to that of the current collector illustrated in FIG. 4A. In each of the current collectors illustrated in FIGS. 4B, 4C, and 4E, it is preferable that a thickness t_F of a flat portion F be in a range of 0.1 - 2 mm and a thickness t_c of a convex portion C be in a range of 0.01 - 1 mm. If the thicknesses t_F and tc are outside these ranges, it would not be possible to achieve desired clamping pressure or thinning. If the difference between the thickness t_F of the flat portion F and the thickness t_c of the convex portion C is outside a range that can be derived from those ranges, complete sealing would not be maintained during stack clamping, resulting in leakage of the reaction fuel.

[55] FIG. 4D illustrates a current collector having a cross-section with a step difference.

In this case, a protrusion portion P of the current collector is disposed to face an MEA. In this current collector, it is preferable that a thickness t_F of a flat portion F be in a range of 0.01 - 1 mm and a thickness t_P of the protrusion portion P be in a range of 0.1 - 2 mm. If the thicknesses t_F and t_P are outside these ranges, it would be difficult to obtain sufficiently high clamping pressure or achieve thinning. If a difference between the thickness t_F of the flat portion F and the thickness t_P of the protrusion portion P is outside a range that can be derived from those ranges, complete sealing would not be maintained during stack clamping, resulting in leakage of the reaction fuel.

[56] FIGS. 5A through 5C are plan views illustrating cases where through-holes of various sizes and shapes are formed in the current collector 130 illustrated in FIG. 3, according to embodiments of the present invention.

[57] FIG. 5 A illustrates a current collector 130-1 having circular through-holes hi formed therein, FIG. 5B illustrates a current collector 130-2 having small size square through- holes h₂ formed therein, and FIG. 5C illustrates a current collector 130-3 having large size square through-holes h₃ formed therein. The shapes of the through-holes hi, h₂, and h₃ are not important, but areas of the through-holes hi, h₂, and h₃ are important in terms of efficiency of supply of a fuel such as a reaction fuel.

[58] In FIGS. 5A, 5B, and 5C, area ratios of efficient fuel supply channels of the current collectors 130-1, 130-2, and 130-3, which will hereinafter be referred to as effective area ratios of a current generator, may change according to the total respective areas of the through-holes hi, h₂, and h₃.

[59] The effective area ratio can be calculated by:

[60] Effective area ratio (%) of current generator = (Total area of through-holes formed in current collector/Entire area of current generator) X 100 (1).

[61] In a current collector, there exist a substantial current collecting area contacting a current generator and an area of a fuel supply channel for supplying a fuel, i.e., an effective area.

[62] In order to guarantee efficient fuel supply and high durability of a fuel cell stack, optimal effective area ratio and current collecting area ratio should be provided.

[63] A relationship between the effective area ratio and the current collecting area ratio can be expressed as follows.

[64] Current collecting area ratio (%) = 100 - Effective area ratio (2)

[65] In the present embodiment, a current collector simultaneously executes the following

3 functions: a first function of collecting current as the original function of the current collector; a second function of increasing an clamping pressure during stack clamping by changing the shape of the current collector to minimize electric resistance generated by contact between a current generator, i.e., an MEA, and the current collector, thereby improving the performance of a fuel cell stack; and a third function of guaranteeing high durability of the fuel cell stack through efficient fuel supply.

[66] The fuel cell stack according to the present invention can be used in various applications such as hydrogen fuel cells (a type of PEMFCs), direct methanol fuel cells (DMFCs), phosphoric acid fuel cell (P AFC), and the like, and can be favorably applied particularly to room-temperature operating fuel cells employing the hydrogen fuel cells and the DMFCs.

[67] Although only a monopolar type fuel cell stack has been described illustratively as a fuel cell stack in the present embodiment, the present invention is not limited to a monopolar type fuel cell stack and technical features of the present invention can also be equally applied to a bipolar-type fuel cell stack. A bipolar-type fuel cell stack, although not illustrated, further includes a bipolar plate disposed between a current generator and a current collector, and the current collector is disposed close to the bipolar plate to collect current of the current generator.

[68] The present invention will now be described with reference to examples. However, these examples are for illustrative purposes only and are not intended to limit the scope of the invention.

[69] Examples

[70] Manufacturing Example 1 : Manufacturing of MEA

[71] An MEA was manufactured by using a catalyst coated gas diffusion layer (GDL)

(CCG) method as described below. In other words, a catalyst slurry containing a Pt catalyst supported on a carbon nano tube and a small amount of Nafion was applied onto a surface of a carbon paper SGLlOBC by using a spray method such that a Pt content was in a range of 0.4 mg/cm² - 0.8 mg/cm², thereby manufacturing an anode and a cathode, respectively. The anode and the cathode to which a catalyst layer obtained as described above was applied were disposed on one side and the other side of an electrolyte film formed of Nafion® polymer electrolyte 115, respectively, and then hot pressing was performed to obtain the MEA.

[72] Example 1 : Manufacturing of Unit Cell 1 of fuel cell stack

[73] For an end plate, a sheet of acryl resin having a thickness of 0.5 cm and a size of 6x6 cm² was used. In the end plate, a through-hole having a diameter of 2.5 mm was formed to facilitate injection and discharge of a fuel. For a current collector, an SUS 316L sheet having a thickness of 0.5 cm and a size of 6x6 cm² was used. The current collector was formed to have a cross-section in an arch shape, as illustrated in FIG. 4A. For a MEA, a MEA having a thickness of 800 μm and a size of 3x3 cm² which was manufactured according to Manufacturing Example 1 was used.

[74] In the current collector, a plurality of circular through-holes as illustrated in FIG. 5A were formed to adjust an effective area ratio to 36%. For gaskets used in fuel supply units and the MEA, red glass silicon having a thickness of 350 μm was used. Thus, the manufacture of a unit cell corresponding to that of FIG. 3, was completed. This will be referred to as Unit Cell 1.

[75] Example 2: Manufacturing of Unit Cell 2 of fuel cell stack

[76] A unit cell, Unit Cell 2, was manufactured in the same manner as Example 1, except that an effective area ratio was adjusted to 24%.

[77] Example 3: Manufacturing of Unit Cell 3 of fuel cell stack

[78] A unit cell, Unit Cell 3, was manufactured in the same manner as Example 1, the only difference being the shape of the through-holes and the effective area ratio. In a current collector of Unit Cell 3, a plurality of small size square through-holes as illustrated in FIG. 5B were formed to adjust the effective area ratio to 45%

[79] Example 4: Manufacturing of Unit Cell 4 of fuel cell stack

[80] A unit cell, Unit Cell 4, was manufactured in the same manner as Example 1, the only difference being the shape of the through-holes and the effective area ratio.

[81] In a current collector of Unit Cell 4, a plurality of small size square through-holes as illustrated in FIG. 5B were formed to adjust the effective area ratio to 55%.

[82] Example 5: Manufacturing of Unit Cell 5 of fuel cell stack

[83] A unit cell, Unit Cell 5, was manufactured in the same manner as Example 1, except that a plurality of large size square through-holes as illustrated in FIG. 5C were formed to adjust the effective area ratio to 72%.

[84] Comparative Example 1 : Manufacturing of Unit Cell 6 of fuel cell stack

[85] A unit cell, Unit Cell 6, was manufactured in the same manner as Example 1, except that a current collector having stripe-shaped cross-sections, as illustrated in FIG. 2, was used.

[86] Evaluation Test

[87] A performance test and a fuel persistent test were performed on Unit Cell 1 manufactured according to Example 1 and Unit Cell 6 manufactured according to Comparative Example 1. FIGS. 6 A and 6B are graphs showing the performance test results of Unit Cell 1 and Unit Cell 6. FIG. 7 is a graph showing the fuel persistent test results of Unit Cell 1 and Unit Cell 6. In addition, durability tests according to effective area ratio were conducted on Examples 1 - 5 and durability test results are shown in Table 1 and FIG. 8.

[88] <Performance Test>

[89] Performance test conditions were as follows: cell voltage with respect to current density was measured, with supplying a fuel to Unit Cell 1 of Example 1 and Unit Cell 6 of Comparative Example 1 at room temperature at speeds of 100 cc/min for H₂ and 100 cc/min for O₂. Performance test results are shown in FIG. 6 A.

[90] Referring to FIG. 6A, it can be seen that the degree of voltage drop with respect to an increase of current density is smaller in Example 1 than in Comparative Example 1. From this result, it can be seen that the performance of Unit Cell 1 of Example 1 is superior to that of Unit Cell 6 of Comparative Example 1.

[91] FIG. 6B shows power densities calculated by current densities and voltages of FIG.

6A.

[92] Referring to FIG. 6B, it can be seen that the degree of an increase in power density with respect to an increase in current density is larger in Example 1 than in Comparative Example 1. From this result, it can be confirmed that the performance of Unit Cell 1 of Example 1 is superior to that of Unit Cell 6 of Comparative Example 1.

[93] The above performance test results seem to be as a result of improvement in electric current characteristics due to the reduction of electric resistance in Example 1, when compared to Comparative Example 1, during the transfer of electricity from the MEA to the current collector.

[94] <Fuel persistent test with Fixed Effective Area Ratio>

[95] The fuel persistent test conditions were as follows. After a small amount of water was supplied to Unit Cell 1 of Example 1 and Unit Cell 6 of Comparative Example 1, the water was electrolyzed by applying a current of 300 rnA with the use of a solar cell or a power supply, and then hydrogen and oxygen generated by the electrolysis were collected as much as 15 cc and 7.5 cc, respectively. Next, the collected hydrogen and oxygen were supplied to Unit Cell 1 and Unit Cell 6 and a change of power over time was measured until a fuel, i.e., hydrogen and oxygen, was completely consumed. In this case, a voltage of Unit Cell 1 of Example 1 was set to 0.65 V and a voltage of Unit Cell 6 of Comparative Example 1 was set to 0.60V. Fuel persistent test results are shown in FIG. 7.

[96] Referring to FIG. 7, Example 1 with set voltage of 0.65V exhibits higher power than

Comparative Example 1 with set voltage of 0.60V, and after a 2-minute operation, power drop is less reduced in Example 1 than in Comparative Example 1. In other words, in Unit Cell 6 of Comparative Example 1, electric power is reduced by a large amount as electric resistance increases.

[97] In the present fuel persistent test, the performance of Unit Cell 1 of Example 1 is improved by about 41% when compared to the performance of Unit Cell 6 of Com- parative Example 1. This seems to be as a result of improvement in electric current characteristics due to the reduction of electric resistance in Example 1, when compared to Comparative Example 1, during the transfer of electricity from the MEA to the current collector.

[98] <Durability Test with Different Effective Area Ratios> [99] A durability test was conducted on Unit Cells 1 - 5 manufactured according to Examples 1 - 5 under the same conditions as those of the fuel persistent test. However, after the supplied fuel was completely consumed, a small amount of water was elec- trolyzed again and a fuel obtained by the electrolysis was supplied again to Unit Cells 1 - 5 by predetermined volumes(15 cc of hydrogen, 7.5 cc of oxygen) to generate electricity. In this way, the fuel persistent test was repeated several times. The number of possible test repetitions was measured under the condition that output power was maintained at a nearly constant value, and measurements thereof are shown in Table 1. This durability test is intended to observe a change of durability of a unit cell according to a change of an effective area ratio.

[100] Results of the durability test on Unit Cell 1 having an effective area ratio of 36% are shown in FIG. 8. Referring to FIG. 8, it can be seen that for Example 1, the fuel persistent test was repeated 46 times.

[101] [Table 1] [Table 1] [Table ]

[102] Referring to Table 1, for effective area ratios of 36%, 24%, 45%, 55%, and 72%, the numbers of possible test repetitions are 46, 32, 34, 22, and 6, respectively. In other words, from these results, it can be seen that durability is relatively good for an effective area ratio of 50% or less and durability degrades for an effective area ratio exceeding 50% due to an increase in electric resistance with the reduction of current collecting area ratio. Comparing Example 1 having an effective area ratio of 36% with Example 5 having an effective area ratio of 72%, durability of Example 1 is improved by 800% when compared to durability of Example 5. While the present invention has been particularly shown and described with reference to embodiments thereof, it will be understood by one of ordinary skill in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.

Claims

[1] L A fuel cell stack comprising: a pair of end plates disposed apart from each other to face each other; at least one current generator disposed between the pair of end plates to generate an electric current by means of electrochemical reaction between a reaction fuel and an oxidizer; and at least one current collector disposed between the each endplate and the current generator, and close to the current generator to collect the electric current generated by the current generator, wherein the current collector has an elastic structure. [2] 2. The fuel cell stack of claim 1, wherein the current collector has a curved shape and a convex portion of the current collector is disposed to face the current generator. [3] 3. The fuel cell stack of claim 2, wherein a cross-section of the current collector has an arch shape, a cap shape, or a wave shape. [4] 4. The fuel cell stack of claim 1, wherein a cross-section of the current collector has a step difference and a protrusion portion of the current collector is disposed to face the current generator. [5] 5. The fuel cell stack of claim 1, wherein an effective area ratio of the current generator is in a range of 20 - 50%. [6] 6. The fuel cell stack of claim 1, wherein the current collector comprises a metal or a metal alloy which is electrochemically stable in a potential range of 0 - IV and has elasticity. [7] 7. The fuel cell stack of claim 1, wherein the thickness of the current collector is in a range of 0.1 - 2 mm. [8] 8. The fuel cell stack of claim 1, wherein the current generator comprises a membrane electrode assembly (MEA) which comprises a polymer electrolyte membrane and an anode and a cathode that are disposed close to both sides of the polymer electrolyte membrane, respectively. [9] 9. The fuel cell stack of claim 1, wherein the at least one current generator are disposed in parallel in a width direction of the end plate and are electrically connected to each other in series.. [10] 10. The fuel cell stack of claim 1, further comprising a bipolar plate disposed between the current generator and the current collector, and the current collector is disposed close to the bipolar plate to collect the electric current generated by the current generator.