POLYMER ELECTROLYTE MEMBRANE FUEL CELL INCLUDING TUNGSTEN OXIDE-COATED PART AND METHOD FOR MANUFACTURING THE SAME

Abstract

The present disclosure relates to a polymer electrolyte membrane fuel cell including a tungsten oxide-coated component, where the polymer electrolyte membrane fuel cell includes a unit cell constituted by a membrane-electrode assembly (MEA), in which an electrolyte membrane and a catalyst layer are integrally combined, a gas diffusion layer, and a bipolar plate, and tungsten oxide is coated on a surface of at least one of the membrane-electrode assembly, the gas diffusion layer, or the bipolar plate, and a method for manufacturing the same. According to the present disclosure, catalyst and cell durability can be enhanced by reducing a carbon oxidation reaction through prevention of the occurrence of high voltage under SU/SD (start-up/shut-down) conditions, and the performance can be maintained due to the absence of current density reduction even under SU/SD conditions.

Claims

1. A polymer electrolyte membrane fuel cell comprising: a unit cell including a membrane-electrode assembly (MEA) wherein an electrolyte membrane and a catalyst layer are integrally combined; a gas diffusion layer, and a bipolar plate, wherein a tungsten oxide layer is coated on a surface of at least one of the MEA, the gas diffusion layer, or the bipolar plate constituting the unit cell.

2. The polymer electrolyte membrane fuel cell of claim 1, wherein the tungsten oxide layer is deposited and coated with a thickness of 450 to 550 nm.

3. The polymer electrolyte membrane fuel cell of claim 2, wherein the deposition is performed by physical vapor deposition.

4. The polymer electrolyte membrane fuel cell of claim 3, wherein the physical vapor deposition is performed by sputtering or electron beam vapor deposition.

5. The polymer electrolyte membrane fuel cell of claim 1, wherein the tungsten oxide layer is coated on a surface of the MEA.

6. The polymer electrolyte membrane fuel cell of claim 1, wherein the tungsten oxide layer is coated on a surface of the membrane-electrode assembly using a decal technique.

7. The polymer electrolyte membrane fuel cell of claim 6, wherein the tungsten oxide layer is formed by: forming a tungsten oxide layer and a catalyst layer on a first imide film; stacking the first imide film, with the catalyst layer and the tungsten oxide layer formed thereon, onto a surface of the polymer electrolyte membrane; compressing the first imide film with a heating press, and then removing the first imide film.

8. A method for manufacturing a polymer electrolyte membrane fuel cell comprising a tungsten oxide-coated component, the method comprising: forming a membrane-electrode assembly by forming a catalyst layer on each of a front surface and a back surface of a polymer electrolyte membrane; and assembling a unit cell by sequentially stacking the membrane-electrode assembly, the gas diffusion layer, and the bipolar plate, wherein tungsten oxide is coated on a surface of at least one of the membrane-electrode assembly, the gas diffusion layer, or the bipolar plate constituting the unit cell.

9. The method of claim 8, wherein the forming of the membrane-electrode assembly comprises: applying a catalyst by applying a catalyst slurry to a surface of an imide film and drying it; forming a catalyst layer by attaching and stacking the catalyst-applied imide film to both the front surface and back surface of the polymer electrolyte membrane, applying pressure and heat from outside to transfer the catalyst to the front surface and back surface of the polymer electrolyte membrane, forming a catalyst layer respectively, and then removing the imide film.

10. The method of claim 8, further comprising coating tungsten oxide on the membrane-electrode assembly, the gas diffusion layer, or the bipolar plate by physical vapor deposition.

11. The method of claim 10, wherein the physical vapor deposition is either sputtering or electron beam vapor deposition.

12. The method of claim 8, wherein the forming of the membrane-electrode assembly comprises: forming a tungsten oxide layer on a first imide film; forming a catalyst layer on the tungsten oxide layer; stacking the first imide film on which the catalyst layer and the tungsten oxide layer are formed on the surface of the polymer electrolyte membrane; and compressing the first imide film with a heating press and then removing the first imide film.

13. The method of claim 12, wherein the forming of the tungsten oxide layer comprises dropping a tungsten slurry on the first imide film, uniformly applying the tungsten slurry with a blade and drying the tungsten slurry, and wherein the forming of the catalyst layer comprises dropping a catalyst slurry on the tungsten oxide layer, uniformly applying the catalyst slurry with a blade, and drying the catalyst slurry.

14. The method of claim 12, wherein the stacking further comprises forming a catalyst layer on a second imide film and then stacking the second imide film on which the catalyst layer is formed on the back surface of the polymer electrolyte membrane.

15. The method of claim 14, wherein the removing of the imide film comprises removing the first imide film and the second imide film with a heating press after sequentially stacking the second imide film, the catalyst layer, the polymer electrolyte membrane catalyst layer, the tungsten oxide layer, and the first imide film.

Description

DESCRIPTION OF DRAWINGS

[0019] FIG. 1 shows the overall manufacturing process of a method for manufacturing a polymer electrolyte membrane fuel cell, including a tungsten oxide-coated part according to the present disclosure.

[0020] FIG. 2 compares surface states before and after depositing and coating tungsten oxide on the surface of each part using sputtering.

[0021] FIGS. 3A, 3B, and 3C are diagrams showing the structure of a polymer electrolyte membrane fuel cell and the deposition position of tungsten oxide.

[0022] FIGS. 4A, 4B, 4C and 4D show the results of an experiment comparing the performance of a fuel cell, in which tungsten oxide is coated on each of a bipolar plate (BP), a gas diffusion layer (GDL), and a membrane-electrode assembly (MEA) constituting the polymer electrolyte membrane fuel cell, with a fuel cell that uses a conventional platinum (Pt/C) catalyst without coating the constituent parts with tungsten oxide, under SU/SD conditions.

[0023] FIGS. 5A, 5B, 5C and 5D show the results of an experiment as to whether a high voltage of 1.5 V, which instantaneously causes a carbon oxidation reaction in a cell under SU/SD conditions, is applied through a dynamic hydrogen electrode (DHE) experiment for fuel cells manufactured according to Comparative Examples and Examples.

[0024] FIGS. 6A, 6B, 6C and 6D feature cross-sectional SEM images showing the thickness of cathode catalyst layers measured before and after a SU/SD protocol test for fuel cells manufactured according to the Comparative Example and the Examples.

[0025] FIG. 7 is a flowchart illustrating a method of coating a tungsten oxide layer by applying a decal technique in a step of forming a membrane-electrode assembly according to an exemplary embodiment of the present disclosure.

[0026] FIG. 8A is a graph showing polarization curves of a polymer electrolyte membrane fuel cell before and after the SU/SD protocol test for a sample made by the decal technique, FIG. 8B is a graph showing direct measurement of cell and electrode potential with DHE, and FIG. 8C features cross-sectional SAM images of the MEA cathode before and after the SU/SD protocol test.

BEST MODE

[0027] Hereinafter, an exemplary embodiment of the present disclosure will be described in detail with reference to the accompanying drawings.

[0028] FIG. 1 shows an overall manufacturing process of a method for manufacturing a polymer electrolyte membrane fuel cell that includes a tungsten oxide-coated component according to the present disclosure.

[0029] The membrane-electrode assembly forming step (S100) is a process of forming a membrane-electrode assembly by forming a catalyst layer on both the front and back surfaces of a polymer electrolyte membrane. Specifically, the membrane-electrode assembly forming step includes a catalyst applying step (S110), where a catalyst slurry is applied to the surface of an imide film and dried, and a catalyst layer forming step (S120), where the catalyst-applied imide film is attached to both the front and back surfaces of the polymer electrolyte membrane, stacked, and then subjected to external pressure and heat to transfer the catalyst to the front and back surfaces of the polymer electrolyte membrane, forming a catalyst layer on each surface, followed by the removal of the imide film.

[0030] In this embodiment, platinum (Pt/C) was chosen as the catalyst to form the membrane-electrode assembly (MEA). A 40 wt. % commercial platinum (Pt/C) catalyst was loaded onto the anode at 0.1 mg/cm.sup.2 and onto the cathode at 0.4 mg/cm.sup.2 using a decal method. Nafion 211 was used as the material for the polymer electrolyte membrane in the MEA.

[0031] The membrane-electrode assembly coating step (S200) involves depositing and coating tungsten oxide onto the surface of the formed MEA using physical vapor deposition (PVD). PVD includes sputtering and electron beam vacuum deposition methods. In this embodiment, tungsten oxide is deposited onto the surface of the MEA to a thickness of, for example, about 500 nm using the sputtering method with a sputter.

[0032] The gas diffusion layer coating step (S300) and the bipolar plate coating step (S400) can be performed simultaneously with the MEA coating step (S200) or separately before or after it. In this embodiment, the gas diffusion layer and bipolar plate were placed in the sputter chamber together with the MEA, and tungsten oxide was deposited and coated onto each surface to a thickness of about 500 nm using the sputtering method. The bipolar plate (BP) used in this case was made of graphite material with no surface coating and featured a serpentine-type flow path with a depth and width of 1 mm.

[0033] The gas diffusion layer (GDL) was made of a carbon material with a porous structure, and its surface was waterproofed using 20% PTFE.

[0034] However, the MEA coating step (S200), gas diffusion layer coating step (S300), and bipolar plate coating step (S400) are optional. Only one of the three steps can be performed to coat tungsten oxide onto a surface of either the MEA, gas diffusion layer, or bipolar plate, and a tungsten oxide layer can also be coated by applying, for example, a decal method in the MEA forming step (S100).

[0035] Then, in the fuel cell assembly step (S500), a polymer electrolyte membrane fuel cell including a unit cell is assembled, where the unicell includes at least one of the MEA, gas diffusion layer, or bipolar plate, each coated with tungsten oxide.

[0036] FIG. 2 compares surface states before and after depositing and coating tungsten oxide on a surface of each part by sputtering. FIG. 3 is a view showing a structure of a polymer electrolyte membrane fuel cell and a deposition position of tungsten oxide.

[0037] In FIG. 2, the states before and after depositing tungsten oxide on the surface of each component are shown and compared in the upper and lower parts. ‘a’ and ‘d’ compare and illustrate the states before and after forming a coating layer by depositing tungsten oxide on the surface of the bipolar plate, ‘b’ and ‘e’ compare and illustrate the states before and after forming a coating layer by depositing tungsten oxide on the surface of the gas diffusion layer, and ‘c’ and ‘f’ compare and illustrate the states before and after forming a coating layer by depositing tungsten oxide on the surface of the membrane-electrode assembly (MEA).

[0038] More specifically, a, b, and c in FIG. 2 represent the bipolar plate (BP), the gas diffusion layer (GDL), and the membrane-electrode assembly (MEA) before tungsten oxide deposition, respectively, while and d, e, and f in FIG. 2 represent the surfaces of the bipolar plate (BP), the gas diffusion layer (GDL), and the membrane-electrode assembly (MEA) after tungsten oxide deposition, respectively.

[0039] FIG. 3 is a diagram showing the structure of a polymer electrolyte membrane fuel cell and the deposition position of tungsten oxide. Referring to FIG. 3, the polymer electrolyte membrane fuel cell includes a first current collector (310), a first bipolar plate (BP, 320) containing an oxidation electrode flow path, a first gas diffusion layer (GDL, 330), a first gasket (340), a membrane-electrode assembly (MEA, 350), a second gas diffusion layer (360), a second gasket (370), a second bipolar plate (380) containing a reduction electrode flow path, and a second current collector. In this case, the first bipolar plate (BP, 320) containing an oxidation electrode flow path, the first gas diffusion layer (GDL, 330), the membrane-electrode assembly (MEA, 350), the second gas diffusion layer (360), and the second bipolar plate (380) containing a reduction electrode flow path are sequentially stacked.

[0040] In this case, tungsten oxide can be deposited on the oxidation electrode flow path of the first bipolar plate (320), as shown in FIG. 3a. Alternatively, tungsten oxide can be deposited on either the first gas diffusion layer (330) side or the membrane-electrode assembly (MEA) side between the first gas diffusion layer (330) and the membrane-electrode assembly (350), as shown in FIG. 3b. In this case, FIG. 3a corresponds to d in FIG. 2, FIG. 3b corresponds to g in FIG. 2, and FIG. 3c corresponds to fin FIG. 2.

[0041] When comparing images before and after depositing tungsten oxide on the surface of each component to form a coating layer, it is easy to visually confirm that the color has changed due to the formation of the tungsten oxide layer.

[0042] FIG. 4 shows the results of an experiment comparing the performance of fuel cells under SU/SD conditions. One fuel cell has tungsten oxide coated on each of the bipolar plate (BP), gas diffusion layer (GDL), and membrane-electrode assembly (MEA) that include the polymer electrolyte membrane fuel cell, while the other fuel cell uses a conventional platinum (Pt/C) catalyst without coating tungsten oxide on the constituent parts.

[0043] A polymer electrolyte membrane fuel cell includes parts without tungsten oxide deposition and coating was constructed with an oxidation electrode (anode), an uncoated graphite bipolar plate, a gas diffusion layer with a PTFE (Poly Tetra Fluoro Ethylene) treated porous carbon surface, a membrane-electrode assembly (MEA) with a platinum (Pt/C) catalyst attached to both the front and back surfaces of the polymer electrolyte membrane, and a reduction electrode (cathode).

[0044] Compared to the conventional polymer electrolyte membrane fuel cell (Comparative Example, a), the performances of fuel cells with tungsten oxide deposited and coated to a thickness of 500 nm on the surface of the bipolar plate (Example 1, b), the gas diffusion layer (Example 2, c), and the membrane-electrode assembly (MEA) (Example 3, d) were mutually compared by the current density at about 0.6 V.

[0045] In this experiment, the basic performance was measured under conditions of a cell temperature of 70° C. and a relative humidity of 100%. The SU/SD protocol test involved evaluating the fuel cell's performance after 100 cycles, where one cycle includes allowing hydrogen to flow through the oxidation electrode and oxygen through the reduction electrode under normal operating conditions, and under SU/SD conditions allowing oxygen to flow through the oxidation electrode until the open circuit voltage (OCV) dropped to 0.2 V.

[0046] Referring to FIG. 4a, in the SU/SD protocol test, the conventional fuel cell using only a platinum catalyst according to the Comparative Example showed a current density of 0.95 A/cm.sup.2 at the beginning of the test when maintaining 0.6V but decreased to 0.25 A/cm.sup.2 after 100 cycles. In contrast, FIG. 4b shows that for the fuel cell of Example 1 under the same operating conditions, the decrease in current density after 100 cycles was small and the performance of the cell itself did not deteriorate, but the initial performance itself was not high due to a large electrical resistance. According to FIG. 4c, the fuel cell of Example 2 showed a small decrease in current density even after 100 cycles, and the performance reduction was less than that of the Comparative Example, but not significantly comparable.

[0047] On the other hand, as shown in FIG. 4d, when compared with the Comparative Examples and Examples, the fuel cell of Example 3, i.e., the fuel cell with tungsten oxide deposited and coated on the surface of the membrane-electrode assembly (MEA), exhibited almost no decrease in current density even after 100 cycles in the SU/SD protocol test, confirming that no performance degradation occurred.

[0048] FIG. 5 shows the results of an experiment conducted to determine whether a high voltage of 1.5 V, which instantaneously causes a carbon oxidation reaction in a cell under SU/SD conditions, is applied through a Dynamic Hydrogen Electrode (DHE) experiment for fuel cells manufactured according to the Comparative Example and the Examples.

[0049] This experiment involves observing the potential profile for 100 seconds. The objective is to determine whether a high voltage of 1.5 V, which triggers a carbon oxidation reaction in the fuel cell when the experiment starts under normal operating conditions and switches to SU/SD conditions after 30 seconds, subsequently degrading the durability of the platinum (Pt/C) catalyst, is generated in the cell.

[0050] FIG. 5a shows the results of an experiment on a conventional fuel cell using a platinum catalyst manufactured according to the Comparative Example, and FIGS. 5b to 5d show the results of experiments performed under the same conditions and methods on fuel cells manufactured according to Examples 1 to 3, respectively.

[0051] Based on the experimental results, it was confirmed that the voltage in the cell rapidly increased up to 1.5 V in the case of the conventional fuel cell according to the Comparative Example, whereas in the case of the fuel cells manufactured according to Examples 1 to 3, the voltage did not rise rapidly up to 1.5 V in the cell itself even when the same experiment was performed.

[0052] Therefore, it can be confirmed that in the case of the polymer electrolyte membrane fuel cell according to the present disclosure, even when the condition is suddenly switched from normal operating conditions to SU/SD conditions, the voltage inside the cell does not rapidly increase, preventing the occurrence of carbon oxidation reactions that reduce the durability of the catalyst and maintaining the durability of the fuel cell.

[0053] Furthermore, FIG. 6a shows a cross-sectional SEM image of the results of the experiment on the conventional fuel cell using the platinum catalyst manufactured according to the Comparative Example, and FIGS. 6b to 6d show cross-sectional SEM images of the results of the experiment performed under the same conditions and methods on the fuel cells manufactured according to Examples 1 to 3, respectively.

[0054] In this case, it can be inferred that the greater the reduction in thickness in the SEM image is, the more severe the carbon corrosion is. Referring to FIG. 6, the pristine MEA of the Comparative Example showed a thickness reduction of about 41% (8.75 μm.fwdarw.5.15 μm), the WO.sub.3-deposited BP of Example 1 showed a thickness reduction of about 10% (9.01 μm.fwdarw.8.09 rim), the WO.sub.3-deposited GDL of Example 2 showed a thickness reduction of about 23% (8.52 μm.fwdarw.6.53 μm), and the WO.sub.3-deposited MEA of Example 3 showed a low thickness reduction rate of about 9% (8.46 μm.fwdarw.7.67 μm).

[0055] According to another embodiment of the present disclosure, WO.sub.3 can be coated using the decal technique in addition to the PVD method. In this case, the WO.sub.3 layer is preferably coated on the anode side of the MEA, and the WO.sub.3 layer is stacked on the polymer electrolyte membrane (e.g., Nafion membrane) together with a catalyst layer to prepare an MEA. That is, when using the decal technique, the catalyst layer and the WO.sub.3 layer are formed on the polymer electrolyte membrane in the step of combining the MEA.

[0056] More specifically, referring to FIG. 7, first, before coating a catalyst slurry, a tungsten oxide (WO.sub.3) slurry dropped on the imide film is coated to a uniform thickness using a doctor blade, and is then sufficiently dried (S210).

[0057] Next, after dropping a catalyst slurry on the WO.sub.3-coated imide film, i.e., on the tungsten oxide layer, the catalyst slurry is coated to a uniform thickness using the doctor blade and is then sufficiently dried (S220).

[0058] Following this, the imide film with the catalyst layer and WO.sub.3 layer formed on it is cut to an appropriate size and laminated onto a Nafion membrane for forming an MEA. At this stage, the catalyst layer is stacked in such a way that it comes into contact with the Nafion membrane and is stacked on the anode-side surface of the Nafion membrane. At this point, an imide film with only a uniformly formed catalyst layer may be laminated onto the cathode side (back surface) of the Nafion membrane.

[0059] Finally, a laminate in which the imide film, catalyst layer, Nafion membrane, catalyst layer, WO.sub.3 layer, and imide film are sequentially stacked (in the cathode.fwdarw.anode direction) is compressed using a heating press while adjusting pressure and temperature(S230). Then, the imide films on both sides are peeled off, leaving the catalyst layer (cathode side) and the catalyst layer+WO.sub.3 layer (anode side) on both surfaces of the membrane (S240).

[0060] Referring to FIGS. 8a and 8b, in particular, the DHE experiment showed a lower potential jump peak (a level with almost no potential jump) compared to that of Example 3. In addition, referring to FIG. 8c, it can be seen that the WO.sub.3-deposited MEA coated using the decal technique, as described above, showed a low thickness reduction rate of about 8.3% (8.31 μm.fwdarw.7.62 μm). As a result, the WO.sub.3-deposited MEA coated by the decal technique demonstrated better performance (retention rate) than the MEA deposited with WO.sub.3 by PVD (WO.sub.3-deposited MEA).

[0061] Although the preferred embodiments of the present disclosure have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, changes, and substitutions are possible without departing from the scope and spirit of the invention as disclosed in the accompanying claims. Therefore, the embodiments and accompanying drawings of the present disclosure are presented only for illustrative purposes, not for limiting the technical concept of the present disclosure, and the scope of the technical concept of the present disclosure is not limited by the embodiments and accompanying drawings. The scope of the present disclosure should be construed according to the following claims, and all technical ideas within the equivalent range should be interpreted as being included in the scope of the present disclosure.