KR100964823B1 - Polymer electrolyte fuel cells with high freeze/thaw durability - Google Patents

Abstract

PURPOSE: A polymer electrolyte fuel cell is provided to suppress separation between electrolyte and electrode or electrode and gas diffusion layer and to ensure high durability in a freezing condition. CONSTITUTION: A polymer electrolyte fuel cells having durability comprises: a perfluoro sulfonic acid elelctrolyte membrane and a gas diffusion layer with both sides of a membrane-electrode assembly. The polymer electrolyte membrane is perfluorinated sulfonic acid nafion membrane, flemion membrane, asiplex membrane, or Dow XUS membrane.

Description

Polymer Electrolyte Fuel Cells with High Freeze / Thaw Durability}

The present invention relates to a polymer electrolyte membrane fuel cell with excellent freeze durability, and relates to an optimal gas diffusion layer capable of suppressing interlayer peeling, which is a problem of lowering freeze durability. More specifically, the polymer electrolyte membrane fuel cell comprising a perfluorosulfonic acid-based polymer electrolyte membrane; and a gas diffusion layer having a bending stiffness of less than 1 g · cm or 10 g · cm or more; will be.

A fuel cell is a device that generates electricity by generating water by reacting hydrogen and oxygen electrochemically.It is more efficient than other types of generators, has a higher current density and power density, shorter startup time and faster load change. Due to the advantage of having a response characteristic, it can be applied to various fields such as a power source of a pollution-free vehicle, self-generation, mobile and military power.

Depending on the type of electrolyte used, polymer electrolyte membrane fuel cells (PEMFC), phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC), solids Solid oxide fuel cell (SOFC) can be classified into PEMFC, which can be operated from room temperature to 200 ° C, which is a low operating temperature of the fuel cell. There is no risk of corrosion or evaporation due to this, and since a very high current density per unit area can be obtained, it can be applied as various power sources for automobiles, homes, portable devices, and the like.

Here, the polymer electrolyte membrane is one of the most important key components to determine the performance and price of the polymer electrolyte fuel cell, and the commercially recognized polymer electrolyte membrane is a perfluoro sulfonic acid-based ion exchange membrane. It is coming true. For example, Nafion (trade name manufactured by DuPont) membrane, Premion membrane (Flemion, trade name manufactured by Asahi Glass) membrane, Asiplex (trade name manufactured by Asahi Chemical) membrane and Dow XUS (Dow XUS, Dow Chemical Co., Ltd. brand name film | membrane, etc. are used.

On the other hand, an electrode assembly consisting of the polymer electrolyte membrane and the electrode catalyst layer (membrane-electrode assembly, 'MEA'); and a gas diffusion layer mounted adjacent to the MEA; And a PEMFC having an outermost side and a unit cell of a fuel cell including a separator plate formed with a flow path of a reactor body to generate electricity through the reaction of the reactor body, and to simultaneously recover the water generated by the byproduct from the outside. However, if the operation is stopped some of the water will remain in the unit cell.

At a temperature below 100 ° C., which is a general operation temperature of a polymer electrolyte fuel cell, water management in a direction of minimizing performance degradation due to cell drying and flooding has been considered important. However, for full-scale commercialization of the fuel cell system, it is essential to secure durability in the freezing conditions of winter, and water in the unit cell at sub-zero temperatures may cause problems with cold start-up performance degradation or freezing at low temperature of the fuel cell system. May cause freeze / thaw durability problems.

1 is a graph illustrating the results of freezing / thawing durability of a fuel cell using a Nafion membrane among the perfluorinated sulfonic acid-based ion exchange membranes. As shown in FIG. 1, a polymer electrolyte membrane fuel cell is repeatedly fueled as a freezing / thawing cycle is repeated. There is a problem that the performance of the battery is reduced. In order to investigate the main causes of such performance degradation, various experiments and evaluations on the major factors affecting performance were conducted.

FIG. 2 is a graph showing the change in catalytic activity area according to the freeze / thaw cycle. As a result of 50 cycles, regardless of the type of gas diffusion layer, the result shows the same change in catalytic activity area. Cannot be seen as an argument.

Similarly, referring to FIG. 3 as a graph showing the change of H ₂ cross-over according to the freezing / thawing cycle, the electrolyte membrane is not physically damaged by the cycle and it is not directly related to the performance degradation due to the freezing / thawing cycle. It was.

On the other hand, as a result of examining the resistance change of the cell according to the freeze / thaw cycle, as the temperature is lowered as shown in Figure 4 when the resistance of the cell increases and the freeze / thaw cycle is performed more than 50 times, the cell resistance of the initial 500 to 1000 or more It can be seen that the value increases a lot, so that the interfacial resistance between the electrolyte membrane and the electrode or the electrode and the gas diffusion layer is increased, which can be seen as the main cause of the performance degradation due to the freeze / thaw cycle. In particular, there is no significant change in the case of the resistance value (R _c , Charge Transfer Resistances) related to the electrode reaction as shown in FIG. 5, but in the case of the contact resistance (R _ohm , Contact Resistances), the number of cycles increases. It can be seen that poor contact has an effect on performance degradation.

In more detail, the SEM image of the membrane electrode assembly after the freeze / thaw cycle is performed through FIG. 6, and it can be seen that delamination occurs and in particular, the catalyst layer under the reaction gas channel formed on the outermost separator is severely damaged. have.

The present invention is to propose a gas diffusion layer that can ensure the freeze durability of the polymer electrolyte membrane fuel cell as described above.

In more detail, the main performance deterioration of the freeze / thaw cycle is due to an increase in resistance due to peeling between the electrolyte membrane and the electrode or the electrode and the gas diffusion layer, thereby presenting the bending strength of the gas diffusion layer to improve the durability under freezing conditions. An excellent polymer electrolyte membrane fuel cell is proposed.

The polymer electrolyte membrane fuel cell having excellent freeze thaw durability according to the present invention includes a perfluorosulfonic acid-based polymer electrolyte membrane forming a membrane electrode assembly (MEA) and bending membranes (Bending Stiffness) provided on both sides of the membrane electrode assembly (MEA). It includes less than 1 g · cm or more than 10 g · cm Gas Diffusion layer (Gas Diffusion layer).

On the other hand, the polymer electrolyte membrane is characterized in that the perfluorinated sulfonic acid-based Nafion membrane, premium membrane, asiprex membrane, Dow Caical membrane.

By suggesting the role of the gas diffusion layer and the deformation of the electrolyte membrane in the freezing / thawing conditions, it significantly improves the durability of the fuel cell upon freezing. The above object can be achieved by easily setting the bending strength of the gas diffusion layer, and no additional configuration or design change is necessary for this purpose.

Hereinafter, a polymer electrolyte membrane fuel cell having excellent freeze thaw durability of the present invention will be described in detail with reference to the accompanying drawings.

The polymer electrolyte membrane fuel cell according to the present invention includes a perfluorosulfonic acid-based polymer electrolyte membrane constituting a membrane electrode assembly (MEA) and the membrane electrode assembly (MEA) on both sides, and a bending stiffness of 1 g · cm And a gas diffusion layer that is less than 10 g · cm or more.

In the case of the perfluorinated sulfonic acid-based polymer electrolyte membrane, when the water is contained, physical shape change due to the volume expansion occurs. Depending on the type of membrane, the strain may be about 15 to 20%, and water condensed in the pores formed during the deformation may exist. If the water becomes ice under sub-zero conditions, its volume increases by about 8%. This deformation causes the membrane and the electrode layer to peel off. On the other hand, the effect of the deformation of the polymer electrolyte membrane is negligible by the physical support force on the outside of the reaction gas flow path provided in the separator, that is, Rib, but physically like a rib on the reaction gas flow path, that is, the Channel. Since there is no support, more serious deformation occurs, thereby accelerating the peeling of the electrocatalyst layer and causing cracks. In addition, ice present between the cracks of the electrode catalyst layer or the gap with the electrolyte membrane accelerates physical damage of the fuel cell. As the empty space increases, the physical damage caused by ice becomes more severe at low temperatures.

In order to reduce the influence of the deformation due to the water content of the polymer electrolyte membrane as described above, the polymer electrolyte membrane fuel cell according to the present invention has a bending stiffness of less than 1 g · cm or 10 g · cm or more. ).

In the present invention, the gas diffusion layer serves to complement the physical bearing capacity on both sides of the MEA. The gas diffusion layer having different bending strengths is applied to the polymer electrolyte membrane fuel cell under the same conditions to perform the freeze / thaw cycle test. 7 is shown. Representative gas diffusion layers used in freeze / thaw cycle tests are shown in Table 1 below.

Gas Diffusion Layer Type
Electrical resistance ^{1 )}
[mΩ cm ² ]
PTFE content ^{2 )}
[wt%]
Existence of MPL ^{3 )}
Porosity
[%]
Bending stiffness
[g / cm] Category (b)
10.89
5
Yes
80
0.4 Category (c)
10.27
5
Yes
84
78.4 Category (d)
11.18
5
Yes
80
5.4
1) 2-point measurement at 10 kg _f / cm ² of gold-coated plate compression.
2) PTFE (polytetrafluoroethylene) contents of diffusion media.
3) Microporous layer (MPL).

The bending strength of each gas diffusion layer was measured using a V-5 Model 150-E stiffness tester (Taber Industries) to give a bending angle of 15 ° to each gas diffusion layer specimen of 3.8 cm × 7 cm.

Here, the freeze / thaw cycle test was a cycle that repeatedly experienced a temperature range of −30 to 70 ^° C. and was performed under the conditions shown in Table 2 below, and the cycle is shown in FIG. 8.

Operation temp of Cell [℃]

70
Maintaining freeze temp. of cell [℃]

-30
Relative humidity [%]

80

cooling rate [℃ / min]

≥1

Maintaining time at freeze temp. [Min]
30


Gas utilization
Anode [%]
75
Cathode [%]
50

Referring to FIG. 7, which is a graph showing the measurement results, when the gas diffusion layer having a bending strength of less than 1 g · cm or 10 g · cm or more is applied, it can be seen that the performance decrease is remarkably reduced. This does not simply impose physical support by increasing the bending strength of the gas diffusion layer, but rather means that a gas diffusion layer below a certain bending strength may exhibit better freeze / thaw durability.

This can be explained with reference to FIG. 9. 9 is a block diagram of a cross section of a fuel cell, (a) is a general pre-freeze / thaw test state, (b) is a bending strength of less than 1 g · cm, and (c) is a bending strength of 10 g · cm or more (d ) Is 1 g · cm or more and less than 10 g · cm.

When the bending strength is 10 g · cm or more, (c) prevents the MEA from being physically supported by the bending strength of the gas diffusion layer itself in the portion above the reaction gas channel despite deformation of the electrolyte membrane during freezing, and prevents peeling and breakage from occurring. do.

On the other hand, when the bending strength is less than 1 g · cm (b), the flexible gas diffusion layer flexibly corresponds to the deformation of the electrolyte membrane and the deformation occurs together in the channel space, thereby preventing the empty space between layers due to the deformation of the electrolyte membrane. do. Therefore, the ice produced during freezing also reduces the risk of breakage to a relatively small size.

Wherein (b) increases the freeze durability by physically inhibiting the deformation, (c) corresponds to the gas diffusion layer to increase the freeze durability by flexibly corresponding to the deformation at all. This point can be easily seen by examining the gas diffusion layer having a bending strength of 1 g · cm or more and less than 10 g · cm (d) outside of the ranges (b) and (c). In the case of (d), the physical impact is transmitted as the electrolyte membrane is deformed, and further, each layer is separated and a large space is created between the layers by not following the deformation. Between these spaces, a large size of ice is produced at a low temperature, thereby deteriorating durability.

On the other hand, in the polymer electrolyte membrane fuel cell according to the present invention, the gas diffusion layer preferably has a thermal conductivity of 0.6 W / mK or more and 2 W / mK or less. In securing freeze durability, it is important that the gas diffusion layer not only accommodates physical deformation upon freezing but also has the necessary thermal conductivity in returning to the thawed state.

In the present invention, the thermal conductivity of the gas diffusion layer suggests that the thermal conductivity of thawed water is 0.6 W / mK or more and the thermal conductivity of frozen ice is 2 W / mK or less. When the transition from the frozen state to the thawed state has a lower thermal conductivity than the ice, the ice is thawed more quickly and smoothly, thereby reducing the performance degradation due to cold start-up. On the other hand, the reaction heat generated in normal operation will be conducted to the whole cell and the outside through the gas diffusion layer having a higher thermal conductivity than water.

The present invention can be applied to any type of fuel cell using a perfluorinated sulfonic acid series polymer electrolyte membrane containing water. For example, the polymer electrolyte membrane may be a perfluorinated sulfonic acid-based nafion membrane, a premium membrane, an asiprex membrane, or a Dow Caical membrane.

The present invention is not limited to the above-described embodiments, and the scope of application is not limited, and various modifications can be made without departing from the gist of the present invention as claimed in the claims.

1 shows freezing / thawing durability test results of a fuel cell using a Nafion membrane.

2 is a graph of the change in catalytic activity area

3 is a graph of H ₂ Cross-over change

4 is a graph of resistance change of a cell

5 is a graph of charge transfer resistance and contact resistance change of a cell;

6 is an SEM image of the membrane electrode assembly after freeze / thaw cycles.

7 is a graph of degradation in performance of gas diffusion layers having different bending strengths.

8 is a freeze / thaw test cycle

9 is a block diagram of a cross section of a fuel cell;

10: Separator

11: channel 12: rib

20 gas diffusion layer

30 electrode

40: electrolyte membrane

Claims (3)

Perfluorosulfonic acid-based polymer electrolyte membrane forming a membrane electrode assembly (MEA); and A polymer electrolyte membrane fuel cell having excellent freeze thaw durability, which is provided on both sides of the membrane electrode assembly (MEA) and includes a gas diffusion layer having a bending stiffness of less than 1 g · cm or 10 g · cm or more. . delete The method of claim 1, The polymer electrolyte membrane is a polymer electrolyte membrane fuel cell having excellent freeze thawing durability, characterized in that the perfluorinated sulfonic acid-based Nafion membrane, premium membrane, asiprex membrane, Dow Caical membrane.

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