KR20110060035A - Method for accelerating activation of fuel cell - Google Patents

Abstract

PURPOSE: A method for accelerating the activation of a fuel cell is provided to reduce the activation process time of a fuel cell stack compared with existing constant current output method and pulse discharge method and to reduce hydrogen amount. CONSTITUTION: A method for accelerating the activation of a fuel cell comprises the steps of: discharging a fuel cell in order to perform the current output of set electric current density in a load state; and maintaining the fuel cell to a shutdown state by cutting off reactive gas supply. The step for maintaining the discharge state and a step for maintaining the shutdown state are repeated one time per a cycle.

Description

Method for accelerating activation of fuel cell}

The present invention relates to a fuel cell accelerated activation method, and more particularly, to a fuel cell accelerated activation method that can reduce the fuel cell stack activation process time and reduce the amount of hydrogen used to reduce fuel cell production costs.

Polymer Electrolyte Membrane Fuel Cells (PEMFC) have higher efficiency, higher current density and higher power density, shorter startup time and faster response to load changes than other types of fuel cells.

In particular, the polymer electrolyte membrane fuel cell is less sensitive to changes in the pressure of the reaction gas (hydrogen and oxygen in the air) and can produce a wide range of outputs. For this reason, it can be applied to various fields such as a power source of a pollution-free vehicle, power generation for self, mobile and military power.

A polymer electrolyte membrane fuel cell is a device that generates electricity by electrochemically reacting hydrogen and oxygen to generate water. The supplied hydrogen is separated into hydrogen ions and electrons in the catalyst of the anode, and the separated hydrogen ions are transferred through the electrolyte membrane. It goes to the cathode.

At this time, the oxygen in the air supplied to the cathode generates electrical energy by combining with the electrons entering the cathode through the external conductor to generate water, the theoretical potential is 1.23V, the reaction formula is shown in Scheme 1 below.

A negative electrode _{(Anode): H 2 → 2H} + + 2e -

Cathode _{(Cathode): 1/2 O 2 +} 2H + + 2e - → H 2 O

On the other hand, in order to obtain a potential required in an actual vehicle, unit cells must be stacked as required, and a stack of unit cells is called a stack (= fuel cell stack).

Each unit cell includes a membrane electrode assembly (MEA), in which the anode and air are supplied with hydrogen (H ₂ ) to both sides with an electrolyte membrane through which hydrogen ions (H ⁺ ) are transferred. A cathode supplied with (oxygen) is provided.

In addition, a gas diffusion layer is disposed outside the anode and the cathode including the catalyst layer, and the fuel cell stack sequentially stacks a membrane electrode assembly and a separator plate on which a reaction gas and a coolant flow path are formed.

In the fuel cell stack essentially including the above configuration, by flowing hydrogen containing fuel as an anode, oxygen or oxygen containing oxygen as a cathode to cause an electrochemical reaction, water by high efficiency electrical energy and reaction Will be generated.

That is, the electrochemical reaction by the reaction gas occurs in the catalyst layer inside the fuel cell, and the generated hydrogen ions are transferred through the electrolyte and the electrolyte membrane in the catalyst layer.

Since the hydrogen ions moving through the electrolyte or the electrolyte membrane move through the water present in the electrolyte membrane, the electrolyte and the electrolyte membrane in the catalyst layer must be sufficiently hydrated for the fuel cell to show better performance. The gas must reach the catalyst bed smoothly.

In the fuel cell stack, an electrode, such as a cathode and an anode, is a catalyst layer made by mixing a hydrogen ion carrier such as Nafion and a catalyst such as platinum, and has a problem in that its activity decreases in electrochemical reactions during initial operation after fuel cell production. The reason for this is as follows.

First, because the flow path of the reactants is blocked and can not reach the catalyst,

Second, because hydrogen ion carriers that form a three-phase interface like catalysts are not easily hydrolyzed at the beginning of operation,

Third, it is because the continuous mobility of hydrogen ions and electrons is not secured.

For this reason, in order to secure steady-state performance after fuel cell assembly, an activation process is required to secure a three-phase electrode reaction area, remove impurities from the polymer electrolyte membrane or electrode, and improve ion conductivity of the polymer electrolyte membrane. do.

1 is a view showing a three-phase structure of the electrolyte membrane in the wet state.

The purpose of fuel cell activation, also referred to as pre-conditioning or break-in, is to activate catalysts that do not participate in the reaction, and to fully hydrate the electrolyte contained in the electrolyte membrane and electrode to produce hydrogen ions. To secure a passageway.

Usually, the activation process performed to reach the highest performance after fuel cell assembly may take hours or days depending on the operating conditions, and the fuel cell may operate without reaching the best performance due to improper activation.

This improper activation procedure reduces the production speed in mass production of fuel cells, results in the use of large amounts of hydrogen, increases the stack cost, and maintains the low stack performance.

Although activation of fuel cells is carried out in various ways by fuel cell manufacturers, the main activation method is to operate for a long time under a constant voltage.

As an example, AISIN SEIKI Co. Ltd. Japanese Patent Application No. 2003-143126 discloses a method for activating a solid polymer fuel cell, which discloses a method of activating a fuel cell stack at a low voltage for a long time and activating it to a portion where the stack performance is no longer improved. In this case, however, the activation process is simple, but it takes a very long time to show the best performance of the fuel cell.

In addition, Korean Patent Application No. 2005-0120743 by the applicant of the present application discloses a method of activating a polymer electrolyte fuel cell applying a step voltage operation, and a high operating temperature (70 ° C.) by applying a voltage cycle to a stack. And a method of reducing the fuel cell activation time to around 3 hours by applying a voltage cycle of OCV (1 minute) to 0.4 V (5 minutes) at high relative humidity (RH 100%). There is a problem that the time efficiency decreases as the time becomes longer until the maximum power is output, and consumes excessive hydrogen and air during operation, which may adversely affect the fuel cell price competition in the future.

FIG. 2 is a view illustrating an example of an activation process according to the prior art, and describes an activation process using a pulse discharge method, which is a cycle operation method in which a fuel cell alternates between a load state and a no load state using a load device. It is for the drawing. Cell voltage distribution over time during the activation process.

As shown in the figure, the discharge is maintained for 3 minutes in a load state of high current density (current output per unit area of the electrode) having 1.2 or 1.4 A / cm2 as a reference output, and is maintained for 20 seconds under no-load OCV (Openn Circuit Voltage) state. In this case, a pulse discharge is performed. In this case, a process time of about 1 hour 30 minutes to 2 hours is required based on a fuel cell stack having 220 cells.

The activation process using the pulse discharge method has an advantage of increasing the activation speed, shortening the activation time and reducing hydrogen consumption as compared to the conventional constant current output method by changing the water flow in the fuel cell. Based on the stack consisting of 220 cells, the total time required for activation is about 107 minutes and the hydrogen consumption is 20.7㎥, which requires improvement to further reduce the process time, hydrogen consumption, and the corresponding cost.

Accordingly, the present invention has been invented in view of the above, and it is possible to reduce the fuel cell production cost by shortening the process time for activating the fuel cell stack and reducing the hydrogen consumption as compared with the conventional constant current output method and the pulse discharge method. Its purpose is to provide a fuel cell acceleration activation method.

In order to achieve the above object, the present invention is to maintain the discharge of the fuel cell to the current output of a predetermined current density in the load state, and the step of shutting off the supply of the reaction gas to maintain the fuel cell in the shutdown state Provided is a fuel cell acceleration activation method including a repetitive process.

In a preferred embodiment, the step of maintaining the discharge and maintaining the shutdown state is repeated once each repetition cycle.

In a preferred embodiment, it is characterized in that the step of maintaining the discharge in each of the repetition cycles is performed in a predetermined number of times and then the step of maintaining in the shutdown state is repeated.

Here, the step of maintaining the discharge is connected to the load device to maintain the load state in which the current output is generated from the fuel cell and then proceeds to the process of switching to the no-load state by disconnecting the load device, maintaining the shutdown state When the cell voltage rises to a predetermined level in the no-load state, the supply of the reaction gas is completely blocked, thereby shutting down the fuel cell operation.

Accordingly, in the fuel cell acceleration activation method of the present invention, the fuel cell is discharge-maintained so that a current output of a predetermined current density is achieved under a load state, and the fuel cell is shut down by shutting off the supply of the reaction gas. By repeating the process, the activation process time of the fuel cell stack can be shortened and the hydrogen consumption can be reduced compared to the conventional constant current output method and the pulse discharge method, thereby reducing the fuel cell production cost.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art may easily implement the present invention.

3 and 4 are diagrams illustrating a fuel cell acceleration activation method according to the present invention, which illustrates the pulse discharge method of the present invention to which a pulse discharge and a shutdown process for completely stopping supply of reaction gas are applied.

In order to achieve the steady-state performance after assembling the fuel cell stack, an activation process for securing the three-phase electrode reaction area, removing impurities from the polymer electrolyte membrane or the electrode, and improving ion conductivity of the polymer electrolyte membrane is required as described above. . Among the three internal structure changes through the activation process, the improvement of ion conductivity through the surface structure change inside the electrolyte membrane is an important activation acceleration factor.

Therefore, the present invention seeks to obtain an activation acceleration effect by effectively changing the surface structure of the electrolyte membrane through wetting of water to improve ion conductivity. In other words, by effectively improving the ion conductivity of the electrolyte membrane compared to the existing activation process to propose a method for dramatically shortening the process time required for the actual activation.

In the conventional activation process, a high current density (1.2 or 1.4 A / cm 2) discharge and a pulse discharge composed of OCV states are performed. In this case, a process time of about 1 hour 30 minutes to 2 hours based on a 220-cell fuel cell stack can be obtained. It takes In such an activation method through pulse discharge, not only the process time is still required but also the amount of hydrogen used is excessive.

Therefore, in the present invention, by improving the existing pulse discharge, instead of applying the OCV state maintenance at no load in the middle of the pulse discharge (shutdown) process to completely stop the supply of the reaction gas (air containing hydrogen and oxygen) Introduce.

That is, the acceleration activation method of the present invention repeatedly maintains the discharge of the fuel cell so that a current output of a predetermined current density is achieved under load, and the step of shutting off the supply of the reactive gas to keep the fuel cell in the shutdown state repeatedly. It involves the process of progressing.

At this time, the step of maintaining the discharge and the step of maintaining in the shutdown state can be performed once for each repetition cycle. Alternatively, the method may be repeatedly performed in a configuration in which the step of maintaining the discharge is performed once in a predetermined number of times after each of the repeated cycles.

Herein, the step of maintaining the discharge is performed by connecting the load device to maintain the load state in which the current output is generated from the fuel cell stack for a predetermined time, and then switching the load device to the no-load state.

Referring to FIG. 3, the cell voltage distribution over time during the activation process is shown, and discharge is maintained for 3 minutes in a load state of high current density (current output per unit area of the electrode) having 1.2 or 1.4 A / cm 2 as a reference output. An example of a pulse discharge in which a one-time three-minute pulse discharge and a one-time five-minute shutdown is repeatedly performed in such a manner that the rear shutdown process is maintained for five minutes is shown.

As a device for performing a pulse discharge type fuel cell activation process including a shutdown process as described above, an activation device having the same configuration as that of the related art may be used, and in terms of method, supply of reaction gas instead of an OCV state in the middle of pulse discharge. The only difference is in applying a method that maintains the shutdown state for a certain time, which completely shuts down the circuit.

For example, it may be configured by combining the basic components for operating the fuel cell stack, a hydrogen supply device for supplying hydrogen as fuel to the fuel cell stack (hydrogen storage unit such as hydrogen tank, high pressure / low pressure regulator, hydrogen supply Valves, etc.), an air supply device (including an air blower, a humidifier, etc.) for supplying air containing oxygen as an oxidant to the fuel cell stack, and selectively blocking the inlet and outlet of the air side (cathode) of the fuel cell stack And an air shutoff valve, a load device for controlling a current state of the fuel cell stack, a cooling device for cooling the fuel cell stack, a voltage detector for monitoring a cell voltage of the fuel cell stack, and the like.

Here, the load device is a component for controlling the current output state of the fuel cell stack, and selectively blocks the electric load connected to the current output terminal of the fuel cell stack and the current from the fuel cell stack to the electronic load. It may be configured to include a switch for opening and closing.

The cooling device is a component that cools the stack by circulating the coolant through the coolant flow path inside the fuel cell stack, and may include a coolant pump for circulating the coolant and a heat exchanger for cooling the coolant circulated in the stack. have.

The discharge is maintained for 3 minutes by connecting the load device to the load state where the current output is generated from the fuel cell stack, and then switching to the no load state by disconnecting the load device. When the cell voltage rises, the supply of the reaction gas is completely cut off, and the operation of the fuel cell is shut down for 5 minutes.

In the embodiment of FIG. 3, discharge and shutdown were repeatedly performed as described above to reduce the required time to 68 minutes and the hydrogen consumption to 6.6 m 3 until activation is completed.

Table 1 below shows the required time and hydrogen consumption in the one-time discharge and one-time shutdown application of FIG. 3 compared with the conventional pulse discharge method.

Referring to FIG. 4, a step of discharging output at a constant current density, a shutdown step of completely shutting off the reaction gas supply, a step of restarting the reaction gas supply and discharging again, and a shutdown step of completely shutting off the reaction gas supply again It is shown schematically the activation process of the present invention to proceed and increase the ion conductivity.

In the activation process of the present invention, when discharging output at a constant current density (1.2 A / ㎠), the water vapor state of the water, the formation of water in the vapor state, the electrolyte membrane movement of the water molecules for hydrogen ion movement (Electro-osmotic drag) Back diffusion of water by physical drag force such as concentration difference or pressure difference is performed.

In addition, according to the shutdown process inserted in the middle of the pulse discharge, the water in the vapor state can be easily condensed, and the water generated at the high current density can be easily diffused into the submicropore structure of the electrolyte membrane to improve the ion conductivity of the electrolyte membrane. do.

In particular, since the electrolyte membrane can be easily swelled through repeated high rate discharge and shutdown, the volume expansion of the electrolyte membrane favors the absorption of water in the membrane pores.

Under the existing OCV conditions, the flow of water in the membrane is limited, but in shutdown, the water generated from high-rate discharges or condensed water diffuses freely in more directions and wetting the pore structure of the membrane through various pathways It is easy to move.

Therefore, the surface structure of the membrane suitable for the movement of the hydrogen cation is formed by the infiltration by the expansion of the membrane, so that the activation acceleration effect and the performance improvement can be achieved.

Figure 5 shows several embodiments of the activation process of the present invention with a shutdown (pause), (b) is an activation process that repeats three three-minute pulse discharge and one minute shutdown, (c) is 3 The activation process is repeated with one minute pulse discharge and one 10 minute shutdown.

By introducing the shutdown process as described above, in the present invention, the hydrogen consumption was reduced to about 30% compared to the conventional pulse discharge activation process (applied to the OCV process of FIG. 2), and repeated shutdown once after three pulse discharges was performed. In the example in (b), the process time can be reduced to about 50% (107 minutes → 56 minutes).

The embodiments of the present invention have been described in detail above, but the scope of the present invention is not limited to the above-described embodiments, and various modifications of those skilled in the art using the basic concepts of the present invention defined in the following claims and Improved forms are also included in the scope of the present invention.

1 is a view showing a three-phase structure of the electrolyte membrane in the wet state.

2 is a view for explaining an example of the activation process according to the prior art.

3 and 4 illustrate a fuel cell acceleration activation method according to the present invention.

5 is a view showing various embodiments of the activation process of the present invention with a shutdown (pause).

Claims (7)

Activating fuel cell acceleration including repeatedly discharging and maintaining the fuel cell so that a current output of a predetermined current density is achieved under load, and maintaining the fuel cell in a shut down state by shutting off the supply of the reaction gas. Way. The method according to claim 1, And repeating the step of maintaining the discharge and maintaining the shutdown state once in each repetition cycle. The method according to claim 2, Activation of fuel cell acceleration, characterized by repeating a three-minute pulse discharge and a one-time five-minute shutdown in such a manner as to maintain three minutes of discharge at a current density of 1.2 or 1.4 A / cm 2 and then shut down for five minutes. Way. The method according to claim 2, Activation of fuel cell acceleration, characterized by repeating a three-minute pulse discharge and a one-time 10-minute shutdown in such a manner that the discharge is maintained for 3 minutes at a current density of 1.2 or 1.4 A / cm 2 and then shut down for 10 minutes. Way. The method according to claim 1, And performing the step of maintaining the discharge in a predetermined number of times after each step of maintaining the discharge for each repetitive cycle. The method according to claim 5, It is characterized by repeating three times of three minute pulse discharge and one time of one minute shutdown in such a manner that three times the step of maintaining the discharge for three minutes at a current density of 1.2 or 1.4 A / cm 2 is maintained for one minute. Fuel cell acceleration activation method. The method according to any one of claims 1 to 6, The step of maintaining the discharge proceeds to the process of switching to the no-load state by disconnecting the load device after maintaining the load state in which the current output is generated from the fuel cell by connecting the load device, The step of maintaining in the shutdown state is a fuel cell acceleration activation method characterized in that when the cell voltage rises to a certain level in the no-load state to shut off the supply of the reaction gas completely to shut down the fuel cell operation.

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