JP6907923B2 - Aging method for polymer electrolyte fuel cells - Google Patents

Description

The present invention relates to a method for aging a polymer electrolyte fuel cell having a membrane electrode assembly in which a pair of electrodes are arranged on both sides of an electrolyte membrane and a separator.

A fuel cell supplies electrical energy (electromotive force) by supplying a fuel gas (for example, hydrogen) and an oxidizing agent gas (for example, air), which are reaction gases, to an anode side electrode and a cathode side electrode for an electrochemical reaction. It is a system to get.

For example, a cell of a solid polymer fuel cell (sometimes referred to as a fuel cell or a single cell) has an ion-permeable electrolyte membrane, an anode-side catalyst layer (electrode layer) sandwiching the electrolyte membrane, and a cathode-side catalyst. A membrane electrode assembly (MEA) composed of a layer (electrode layer) is provided. Gas diffusion layers (GDL) are formed on both sides of the MEA to provide fuel gas or oxidant gas and to collect electricity generated by an electrochemical reaction. MEA in which GDL is arranged on both sides is called MEGA (Membrane Electrode & Gas Diffusion Layer Assembly), and MEGA is sandwiched by a pair of separators. Here, MEGA is the power generation unit of the fuel cell, and when there is no gas diffusion layer, MEA is the power generation unit of the fuel cell.

A fuel cell (sometimes referred to as a fuel cell stack) is used by stacking a plurality of cells having the above-described configuration on top of each other and mounting them on a vehicle such as an automobile, for example.

In this type of polymer electrolyte fuel cell, the water content of the electrolyte membrane immediately after assembly is not sufficient (specifically, because the electrolyte membrane is not sufficiently dried and the proton conduction path is not sufficiently formed), so the initial power generation performance is poor. It's getting low. Therefore, in order to bring out the desired power generation performance after assembling the fuel cell, a preliminary operation (break-in operation) called an aging operation (simply also referred to as aging) of the fuel cell is usually performed. In this aging operation, the performance of the cell can exhibit the desired capacity by preliminarily generating electricity after assembling the fuel cell. In addition, not only after manufacturing, but also when regenerating power (at the time of restarting) after suspending the fuel cell (particularly after suspending for a long period of time), or when power generation for a long period of time causes output characteristics such as electromotive force. Even when the value is lowered, the output characteristics of the fuel cell may be restored by performing the aging operation described above.

As a method for shortening the aging time in the aging (break-in operation) of such a polymer electrolyte fuel cell, methods such as periodic fluctuation of the load current, humidification of the reaction gas, and potential fluctuation based on the cyclic voltamograph are used. Has been taken. However, these conventional aging methods still require a long time for aging, which poses a problem in improving the productivity of fuel cells.

By the way, in aging, it is necessary to carry out three processes of wetting the electrolyte membrane (formation of proton path), securing the surface area of the catalyst (removing the oxide film), and securing the surface area of the catalyst (removing organic contamination) (FIG. 6 (A)). reference). It is necessary to establish all three points at the same time, but it takes a long time to secure the surface area of the catalyst (remove organic contamination).

"Organic contamination" here refers to the case where a solvent is used when applying the catalyst to the electrolyte membrane or an adhesive is used in the cell formation process of the fuel cell based on the results of liquid chromatography-mass spectrometry (LC-MS). In addition, it has been confirmed that the adhesive often volatilizes and adheres during heat bonding or curing by UV irradiation (see FIG. 6 (B)).

In order to carry out the above three points at the same time, it is necessary to generate electricity at a low potential using a humidified fuel (humidified reaction gas). In particular, in order to secure the surface area of the catalyst (removal of organic contamination), which requires time, that is, to decompose and clean the organic contamination adhering in the cell manufacturing process, it is necessary to keep the cell voltage during power generation below a predetermined value.

In response to such a request, for example, Patent Document 1 below proposes a technique for maintaining the cell voltage during aging at a predetermined value (0.3 V) or less by adjusting the load current value.

Japanese Unexamined Patent Publication No. 2016-035910

However, since the cell performance gradually recovers during the aging process, it is difficult to maintain the cell voltage below a predetermined value only by the same method. Further, if the load current value is increased too much, the amount of heat generated increases and the amount of liquid water decreases, which may hinder the "wetting" required for promoting aging.

The present invention has been made in view of the above problems, and an object of the present invention is a polymer electrolyte fuel cell capable of maintaining a low cell voltage during aging while suppressing cell damage. Is to provide a method of aging.

In order to solve the above problems, the polymer electrolyte fuel cell aging method according to the present invention comprises a polymer electrolyte fuel cell having a membrane electrode assembly in which a pair of electrodes are arranged on both sides of an electrolyte membrane and a separator. A method for aging a polymer electrolyte fuel cell for aging, wherein the load current value is raised to a predetermined value, and after the first step, the stoichiometric ratio of the cathode fuel is changed to a predetermined ratio. It is characterized by including two steps and, after the second step, a third step of increasing the amount of the inert gas mixed with the cathode fuel to a predetermined amount.

According to the present invention, drying is prevented by first adjusting the load current value, and the inert gas is adjusted by adjusting the stoichiometric ratio before adjusting the inert gas mixing ratio, which tends to cause a negative voltage due to unstable voltage. Since the fluctuation range of the voltage at the time of adjusting the mixing ratio can be narrowed, it is possible to maintain the cell voltage during aging at a low voltage while suppressing cell damage.

It is a figure which shows an example of the solid polymer type fuel cell (fuel cell stack) to which the aging method of the solid polymer type combustion battery by this invention is applied, (A) is the cross-sectional view of the main part, (B) is the whole. It is an external view. It is a flowchart explaining the aging method of the polymer electrolyte fuel cell according to this invention. It is a time chart which shows the relationship between the cell voltage and the oxidant gas supply amount in the power generation pattern at the time of aging of this embodiment. It is a time chart which shows the relationship between the cell voltage and the nitrogen mixture amount in the power generation pattern at the time of aging of this embodiment. It is a time chart which shows the cell voltage in the power generation pattern at the time of the conventional aging. (A) is a schematic diagram showing an outline of a cell configuration, and (B) is a diagram for explaining a cell formation step and an organic contamination generation step.

Hereinafter, the configuration of the present invention will be described in detail based on an example of an embodiment shown in the drawings. In the following, as an example, a case where the present invention is applied to a fuel cell mounted on a fuel cell vehicle or a fuel cell system including the present invention will be described as an example, but the scope of application is not limited to such an example. ..

[Structure of polymer electrolyte fuel cell (fuel cell stack)]
FIG. 1 is a diagram showing an example of a polymer electrolyte fuel cell (fuel cell stack) to which the aging method for a polymer electrolyte fuel cell according to the present invention is applied, and FIG. 1 (A) is a cross-sectional view of a main part. , FIG. 1 (B) is an overall external view.

As shown in FIG. 1A, a plurality of cells (cell cells) 1 which are basic units are stacked on the polymer electrolyte fuel cell (fuel cell stack) 10. Each cell 1 is a solid height that generates an electromotive force by an electrochemical reaction between an oxidant gas (for example, air) (sometimes called a cathode fuel) and a fuel gas (for example, hydrogen) (sometimes called an anode fuel). It is a molecular fuel cell. The cell 1 includes a MEGA 2 and a separator (fuel cell separator) 3 that contacts the MEGA 2 so as to partition the MEGA 2. In this embodiment, MEGA2 is sandwiched by a pair of separators 3 and 3.

MEGA2 is a combination of a membrane electrode assembly (MEA) 4 and gas diffusion layers 7 and 7 arranged on both sides thereof. The membrane electrode assembly 4 is composed of an electrolyte membrane 5 and a pair of electrodes 6 and 6 bonded so as to sandwich the electrolyte membrane 5. The electrolyte membrane 5 is made of a proton-conducting ion exchange membrane made of a solid polymer material, and the electrode 6 is made of, for example, a porous carbon material carrying a catalyst such as platinum. The electrode 6 arranged on one side of the electrolyte membrane 5 serves as an anode, and the electrode 6 on the other side serves as a cathode. The gas diffusion layer 7 is formed of a gas-permeable conductive member such as a carbon porous body such as carbon paper or carbon cloth, or a metal porous body such as a metal mesh or foamed metal.

In the present embodiment, the MEGA 2 is the power generation unit of the polymer electrolyte fuel cell 10, and the separator 3 is in contact with the gas diffusion layer 7 of the MEGA 2. When the gas diffusion layer 7 is omitted, the membrane electrode assembly 4 is the power generation unit, and in this case, the separator 3 is in contact with the membrane electrode assembly 4. Therefore, the power generation unit of the polymer electrolyte fuel cell 10 includes the membrane electrode assembly 4, and comes into contact with the separator 3.

The separator 3 is a plate-shaped member whose base material is a metal having excellent conductivity and gas impermeableness, and one side thereof is in contact with the gas diffusion layer 7 of MEGA2, and the other side is adjacent to another separator. It is in contact with the other surface side of 3.

In the present embodiment, each separator 3 is formed in a wavy or uneven shape. The shape of the separator 3 is an isosceles trapezoid in the shape of the wave, the top of the wave is flat, and both ends of the top are angular at equal angles. That is, each separator 3 has substantially the same shape when viewed from the front side and the back side. The top of the separator 3 is in surface contact with one gas diffusion layer 7 of MEGA2, and the top of the separator 3 is in surface contact with the other gas diffusion layer 7 of MEGA2.

The gas flow path 21 defined between the gas diffusion layer 7 on the one electrode (that is, the anode) 6 side and the separator 3 is a flow path through which the fuel gas flows, and the other electrode (that is, the cathode) 6 side. The gas flow path 22 defined between the gas diffusion layer 7 and the separator 3 is a flow path through which the oxidizing agent gas flows. When the fuel gas is supplied to one of the gas flow paths 21 facing each other via the cell 1 and the oxidant gas is supplied to the gas flow path 22, an electrochemical reaction occurs in the cell 1 to generate an electromotive force.

Further, one cell 1 and another cell 1 adjacent thereto are arranged so that the electrode 6 serving as an anode and the electrode 6 serving as a cathode face each other. Further, a top of the back side of the separator 3 arranged along the electrode 6 which is the anode of a certain cell 1 and a top of the back side of the separator 3 arranged along the electrode 6 which is the cathode of another cell 1. Are in surface contact with each other. Water as a refrigerant for cooling the cell 1 flows through the space 23 defined between the separators 3 and 3 which are in surface contact with each other between the two adjacent cells 1.

Further, a gasket (not shown) as a sealing member for sealing a fuel gas (for example, hydrogen) or an oxidant gas (for example, air) or cooling water is held between the ends of two adjacent cells 1. Has been done.

Further, as shown in FIG. 1 (B), the solid polymer type fuel cell 10 described above includes a cell monitor cable (not shown) and electrodes (not shown) for inspecting and monitoring the voltage (cell voltage) between each cell. Gas pipe 11 for supplying / discharging fuel gas to the gas flow path 21 on the anode) 6 side, gas pipe 12 for supplying / discharging oxidizing agent gas to the gas flow path 22 on the electrode (cathode) 6 side, space A cooling water pipe 13 or the like for supplying / discharging water (cooling water) as a refrigerant is connected to the 23.

[Aging method for polymer electrolyte fuel cell (fuel cell stack)]
In the polymer electrolyte fuel cell 10 having the above-described configuration, in order to stabilize the power generation performance (output characteristics), for example, after assembling the polymer electrolyte fuel cell 10, aging (break-in operation) is performed. Will be.

Here, as described above, in order to carry out the above-mentioned aging (wetting of the electrolyte membrane, power generation for the purpose of securing the catalyst surface area) in a short time, organic contamination adhering in the cell manufacturing process is decomposed and washed. In order to do so, it is necessary to keep the cell voltage during power generation below a predetermined value (for example, 0.3V), but since the cell voltage gradually rises as aging progresses, the cell voltage can be kept below the predetermined value only by the same method. Is difficult to maintain.

Therefore, in the present embodiment, the cell voltage during power generation (aging power generation) is always maintained at a low voltage by sequentially adding measures to reduce (decrease) the cell voltage. Specifically, a step of increasing the discharge current density (load current value) (first step), a step of reducing the amount of cathode fuel (oxidant gas) supplied to the electrode (cathode) 6 (in other words, a step of lowering the oxygen partial pressure). ) (Second step), and the step (third step) of mixing the inert gas (for example, nitrogen N _{2) with the electrode (cathode) 6 is carried out in this order.} As a guideline for step switching, for example, the cell voltage can be set to exceed 0.3 V.

Hereinafter, the aging method of the present embodiment described above will be specifically described with reference to FIGS. 2 to 4.

The aging method of the present embodiment is effective for a polymer electrolyte fuel cell, and is particularly effective when an adhesive is used for joining an electrolyte membrane or a separator as a cell configuration (FIG. 6B). reference).

As a premise, a humidifying fuel (humidified reaction gas) is used to promote the wetting of the electrolyte membrane during power generation.

FIG. 2 is a flowchart illustrating a method of aging the polymer electrolyte fuel cell according to the present invention.

First, in the aging method of the present embodiment, in order to promote the reaction of the reaction gas on the catalyst surface, the temperature of the cooling water supplied from the cooling water pipe 13 to the cell 1 is raised as compared with the normal operation in the vehicle (step S11). For example, when the normal cooling water temperature is 55 ° C, the cooling water temperature during aging is raised to about 70 ° C.

This increases the reaction probability in cell 1.

Next, in order to increase the reaction amount, the discharge current density is increased from, for example, the conventional 2.2 A / cm ² to 2.8 A / cm ² (step S12). However, at this time, if the discharge current density is increased instantaneously, the voltage may drop due to insufficient distribution of the reaction gas, and the equipment may stop. Therefore, 2.2 A / cm ² is set a predetermined number of times (for example, 80 seconds x 3). Time) After implementation, increase the load to ^{2.8 A / cm 2.}

This promotes wetting and cleaning of the electrolyte membrane due to the increase in generated water.

Next, when the discharge is continued, the catalyst surface is gradually covered with the oxide film, so that the discharge and the undischarged (open circuit state) are repeated. For example, the discharge is repeated for 80 seconds and the open circuit is repeated for 10 seconds (see FIG. 3). Furthermore, in order to remove organic contamination on the catalyst surface, it is effective to keep the cell voltage during power generation at 0.3 V or less. Therefore, in order to suppress an increase in the cell voltage as the aging progresses, the amount of cathode fuel (oxidizing agent gas) supplied to the electrode (cathode) 6 is reduced (step S13). For example, the cathode fuel of the electrode (cathode) 6 so that the stoichiometric ratio (sometimes referred to as the oxidant gas stoichiometric ratio or the positive electrode fuel stoichiometric ratio) St changes from 1.3 to 1.2 to 1.13 at a time ratio of 1: 1: 1. The amount of (oxidizing agent gas) supplied is gradually (stepwise) reduced (see FIG. 3).

As a result, the cell potential is maintained at 0.3 V or less, and organic contamination decomposition is promoted.

At this time, in general, in order to prevent carbon oxidation (destruction of the electrolyte membrane) due to lack of fuel gas, the lower limit of the cell voltage is set to stop at about 0.1 V, but in the present embodiment, the electrode (anode) 6 By keeping the supply amount of fuel gas constant, a voltage drop of up to -0.2V is allowed under the condition that carbon oxidation does not occur.

On the other hand, at the stage where the stoichiometric ratio St is 1.13, the amount of the cathode fuel (oxidizing agent gas) is reduced, so that the generated water is likely to be clogged. The clogging with water deteriorates the distribution of the reaction gas on the electrode surface, and the cell voltage may drop below -0.2V to a level where power generation cannot be continued.

In order to prevent this, an inert gas (for example, nitrogen N ₂ ) is mixed with the cathode fuel (oxidizing agent gas) to improve the drainage of water in the cell 1 (step S14). For example, when air Air is used as the oxidant gas and nitrogen N ₂ is used as the inert gas, the mixing ratio is Air: N ₂ = 3: 2 (see FIGS. 3 and 4). Here, for example, as shown in FIG. 4, the mixing ratio of _{air Air and nitrogen N 2 (similar to the stoichiometric ratio St described above) is set to Air: N 2} = 2: 1 → 3: 2, etc. May be changed.

As a result, the cell potential is maintained at 0.3 V or less, organic contamination decomposition is promoted, and excess generated water is drained to the outside of the cell 1.

The above-mentioned aging method can be executed by using, for example, a control device (not shown) configured as a logic circuit centered on a microcomputer.

As described above, in the present embodiment, the discharge current during power generation is increased, the amount of cathode fuel (oxidant gas) supplied is reduced (that is, the stoichiometric ratio St of the cathode fuel is decreased), and the cathode fuel is mixed with the inert gas. By performing multiple processes to lower the cell voltage, such as lowering the oxygen partial pressure, in a predetermined order, the cell voltage during power generation (aging power generation) is always maintained at a low voltage (for example, 0.3 V or less). , The aging (activation) of the solid polymer fuel cell 10 can be carried out in a short time.

Hydrogen as a fuel gas and air as an oxidant are humidified at a dew point of 45/85 ° C., and as described above, the cooling water temperature in step S11 is set to 70 ° C., and the discharge current density in step S12 is 2.2 A / cm. ^{Increase to 2} → 2.8 A / cm ² , change the stoichiometric ratio St in step S13 from 1.3 → 1.2 → 1.13, and set the mixing ratio in step S14 to Air: N ₂ = 3: 2, within about 30 minutes. It has been confirmed by the present inventors that aging can be completed in a short time (see FIG. 3).

On the other hand, hydrogen as a fuel gas and air as an oxidant are humidified at a dew point of 45/60 ° C., the cooling water temperature in step S11 is set to the normal 55 ° C., and the discharge current density in step S12 is constant ^{at 2.2 A / cm 2.} In the conventional aging (power generation) pattern in which the stoichiometric ratio St in step S13 is constant at 1.5 and the inert gas mixing operation in step S14 is eliminated, the cell voltage cannot be maintained at a low voltage, and 0.3V. It has been confirmed that it takes about 130 minutes to complete the aging (see FIG. 5).

Furthermore, if the order of each of the above steps is changed, the cell voltage drops sharply, and an interlock with a voltage drop abnormality is generally set near 0V, which leads to equipment protection stoppage and power generation cannot be continued. It has been confirmed by the person.

As described above, in the aging method of the present embodiment, drying is prevented by first adjusting the load current value, and the stoichiometric ratio is set before adjusting the inert gas mixing ratio, which tends to cause a negative voltage due to unstable voltage. By adjusting the above, the fluctuation range of the voltage when adjusting the inert gas mixing ratio can be narrowed, so that it is possible to maintain the cell voltage during aging at a low voltage while suppressing damage to the cell 1. Therefore, aging can be carried out in a short time, and productivity can be improved.

Although the embodiment of the present invention has been described in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and there are design changes and the like within a range that does not deviate from the gist of the present invention. Also, they are included in the present invention.

1 ... cell (fuel cell), 2 ... MEGA, 3 ... separator, 4 ... membrane electrode assembly (MEA), 5 ... electrolyte membrane, 6 ... electrode (anode, cathode), 7 ... gas diffusion layer, 10 ... solid Polymer fuel cell (fuel cell stack), 11, 12 ... gas piping, 13 ... cooling water piping, 21, 22 ... gas flow path, 23 ... space through which water flows

Claims (1)

An aging method for a polymer electrolyte fuel cell for aging a polymer electrolyte fuel cell having a membrane electrode assembly and a separator in which a pair of electrodes are arranged on both sides of an electrolyte membrane.
The first step to raise the load current value to a predetermined value,
After the first step, a second step of changing the stoichiometric ratio of the cathode fuel to a predetermined ratio, and
A method for aging a polymer electrolyte fuel cell, comprising: after the second step, a third step of increasing the amount of the inert gas mixed with the cathode fuel to a predetermined amount.

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