KR102123930B1 - Activation method of membrane electrode assembly - Google Patents

Abstract

The present invention is a membrane electrode comprising the step of setting a constant resistance value and performing a constant resistance mode (CR Mode) repeating a cycle between the first voltage and the second voltage while turning on and off the gas. Provided is a method for activating a conjugate.

Description

Activation method of membrane electrode assembly

The present invention relates to a method for activating a membrane electrode assembly, and more particularly, to a method for activating a membrane electrode assembly using a constant resistance mode (CR Mode).

In general, a fuel cell is a device that generates electrical energy by reacting hydrogen and oxygen, and includes a membrane electrode assembly (MEA).

As shown in FIG. 2, the membrane electrode assembly has an electrolyte membrane 10 through which hydrogen ions (H ⁺ ) are transferred, and an anode 12 through which hydrogen is supplied to both sides of the electrolyte membrane 10. And a cathode 14 through which air is supplied, a structure in which a gas diffusion layer 16 is disposed outside the anode 12 and the cathode 14 including a catalyst layer, and the membrane electrode assembly A fuel cell stack is a stack of separator plates sequentially.

Looking at the electric generation principle of the fuel cell stack with reference to FIG. 2, hydrogen as a fuel is supplied to the anode 12, air as an oxidant is supplied to the anode 14, and hydrogen supplied from the anode 12 is its catalyst layer. It is separated into hydrogen ions and electrons by an oxidation reaction, and the generated hydrogen ions are supplied to the cathode 14 through the polymer electrolyte membrane 10 and electrons through an external circuit, and thus the oxygen supplied from the cathode 14 And electrons meet to generate oxygen ions by the catalytic layer reduction reaction, and hydrogen ions and oxygen ions are combined to generate electricity through the principle that water is generated.

In the fuel cell stack having the above-described configuration and the principle of generating electricity, the activity is reduced in the electrochemical reaction during initial operation after being fabricated and manufactured as a fuel cell stack, so as to secure the normal initial performance as much as possible after assembly of the fuel cell stack In order to do this, a process called activation is performed.

The purpose of fuel cell activation, also called pre-conditioning or break-in, is to remove residual impurities introduced during membrane electrode assembly and stack manufacturing, and catalytic metal reaction sites that cannot participate in the reaction. , Activation, securing the passage to the catalyst of the reactant, and securing of a hydrogen ion channel through sufficient hydration of the electrolyte contained in the electrolyte membrane and electrode.

Specific activation items for fuel cell activation can be divided into catalytic reaction promotion, membrane hydration, electrical contact surface formation, and three-phase interface formation.

The catalytic reaction is to reduce the oxidized platinum catalyst (PtxOy → Pt metal), and there is a reduction method using a voltage scan method (CV scan) and a method of reducing the catalyst by exposing it to hydrogen gas.

Membrane hydration is to improve the conductivity of hydrogen ions, and water molecules are present in the pores of the membrane, so that hydrogen ion migration can be smoothly performed.

The formation of the electrical contact surface means reducing the electrical contact resistance at the air electrode and the fuel electrode, that is, the interface between each electrode and the gas diffusion layer, or the interface between the gas diffusion layer and the membrane.

The formation of a triple phase boundary (TPB) means that an electrochemical reaction is promoted by forming an interface between an electrolyte, an electrode catalyst, and a reaction gas.

Among the conventional methods for activating the fuel cell, there is an activating method by a constant voltage and cycle operation, which is generally performed. In this case, the activation time is long, the amount of hydrogen is used, and the equipment for activation is complicated. There are disadvantages.

As a conventional technique for overcoming the disadvantages of such a general activation method, a voltage applied reactive gas control method is disclosed in US Patent No. 7,078,118, and a constant current mode operation and activation process is disclosed in Japanese Patent No. 2004-349050. , US Patent No. 6,896,982 discloses an activation method for reducing the cathode side catalyst by exposure to hydrogen gas, US Patent No. 5,601,936 discloses a battery-powered activation method, and US Patent No. 6,576,356 A method of activation using hydration is disclosed.

However, these prior arts have the following disadvantages.

In the case of controlling the voltage applied reaction gas, since the gas must be changed from air to nitrogen, there are disadvantages in that the complexity of the process and the need for an additional device and supply of nitrogen gas are required to supply additional nitrogen.

In the case of the constant current mode operation and resurrection treatment technique, there is also a disadvantage that additional gas such as nitrogen and device/piping are required.

The method of activating by reducing the catalyst on the cathode side by exposure to hydrogen gas does not completely remove the hydrogen gas on the cathode side, but there is a risk of damaging the catalyst when supplying air. To completely remove the residual hydrogen gas, nitrogen and nitrogen The disadvantage is that purging with the same inert gas is necessary.

The method of activating the voltage application using the battery has a disadvantage in that the system is complicated because the electronic load must also have a galvanic cell & capacitor along with a separate battery.

The activation method using membrane hydration has the disadvantage of using an inert gas such as nitrogen instead of air, and an additional activation process after completion of the hydration process. Therefore, the system takes a long time to be activated with the complexity. The problem follows.

An object of the present invention is to provide a method for activating a membrane electrode assembly that can be activated to a low voltage without raising a current and solves the problem that hydrogen is used in excess, thereby reducing cost.

In order to achieve the above object, the present invention sets a constant resistance value and a constant resistance mode (CR mode) repeating cycles between the first voltage and the second voltage while turning on and off the gas. It provides a method of activating a membrane electrode assembly comprising the step of performing.

In the present invention, the resistance may be 1 to 20 Ω, the gas may be one or more of hydrogen and air, the gas input time may be 1 to 40 seconds, the gas removal time may be 1 to 40 seconds, and the number of cycle repetitions is It can be three or more times.

In the present invention, the first voltage may be an open circuit voltage (OCV), the second voltage may be lower than the open circuit voltage, but may be at least 0.4 V or more, and may be increased to an open circuit voltage (OCV) when gas is supplied, and gas When removing, it may drop to the second voltage, which is the lowest voltage.

A method of activating a membrane electrode assembly according to a preferred embodiment of the present invention includes a first step of supplying nitrogen; A second step of measuring the open circuit voltage (OCV); A third step of performing a constant current mode (CC Mode); A fourth step of measuring the open circuit voltage (OCV); A fifth step of performing the constant resistance mode (CR Mode); A sixth step of measuring the open circuit voltage (OCV); A seventh step of performing a constant current mode (CC Mode); And an eighth step of supplying nitrogen.

In the present invention, the first step may be performed for 0.5 to 3 hours under conditions of humidification and temperature of 50 to 90°C, and the second step may be performed for 0.5 to 3 minutes based on the current density of 250±100 mA/cm 2. (First voltage measurement), the third step can be performed for 0.1 to 3 hours at a current density of 250±100 mA/cm 2 and a third voltage between the first and second voltages, and the fourth and sixth steps The step can be performed for 0.1 to 10 seconds based on the current density of 250±100 mA/cm 2 (first voltage measurement), and the seventh step is the current density of 250±100 mA/cm 2, the third voltage, anode: hydrogen 100 %, cathode: step 7-1 performed for 0.5 to 3 hours at 100% air; And current density 250±100 mA/cm 2, fourth voltage between the second voltage and the third voltage, anode: reformed gas (H ₂ 75±5%, CH ₄ 1.5±0.5%, CO ₂ 20±5%, N ₂ 3.5±1%), cathode: may include steps 7-2 performed for 0.5 to 3 hours at 100% air, and step 8 is 0.5 to 3 hours while removing the remaining gas under the temperature drop condition. While you can.

The method according to the present invention comprises the steps of lowering the voltage from the first voltage to the third voltage under the condition of a current ramp of 0.05 to 0.5 A/sec between the second and third stages; Increasing the voltage from the third voltage to the first voltage under the condition of the current ramp 0.05 to 0.5 A/sec between the third and fourth stages; Lowering the voltage from the first voltage to the third voltage under the conditions of the current ramp 0.05 to 0.5 A/sec between the sixth and seventh stages; And lowering the voltage from the third voltage to the fourth voltage under the condition of the current ramp 0.05 to 0.5 A/sec between steps 7-1 and 7-2.

According to the method for activating the membrane electrode assembly of the present invention, by using a constant resistance mode (CR Mode), it is possible to activate a low voltage without raising the current, and it is possible to reduce the cost by solving the problem that hydrogen is used in excess.

1 is a graph showing the voltage change at each step of the method for activating a membrane electrode assembly according to the present invention.
2 is a schematic diagram illustrating a fuel cell and its operating principle.

Hereinafter, the present invention will be described in detail.

The method for activating a membrane electrode assembly according to the present invention sets a constant resistance value and repeats a cycle between a first voltage and a second voltage while turning on and off gas (CR). Mode).

In the constant resistance mode, the resistance may be 1 to 20 MPa, preferably 5 to 15 MPa, more preferably 8 to 10 MPa. If the resistance is too small, the voltage may drop too much. If the resistance is too large, the flow rate may drop and the voltage may drop less. The resistance value may vary depending on the evaluation conditions.

In the constant resistance mode, the gas can be one or more of hydrogen and air. For example, hydrogen may be added or removed from the anode, and air may be introduced or removed from the cathode.

In the constant resistance mode, the gas input time may be 1 to 40 seconds, preferably 10 to 30 seconds, the gas removal time may be 1 to 40 seconds, preferably 10 to 30 seconds, and the number of cycle repetitions is 3 or more times, Preferably it may be 5 to 20 times. If the gas input and removal time is too short, the desired voltage cannot be reached. If the gas input and removal time is too long, the voltage may drop too much. Gas input and removal times can vary depending on the evaluation conditions. In addition, if the number of cycle repetitions is too small, activation may not be performed completely, and if the number of cycle repetitions is too large, a cost problem may occur. The cycle can be repeated until the current no longer increases.

In the constant resistance mode, the first voltage may be an open circuit voltage (OCV, a state in which no current flows), and the second voltage may be a minimum voltage lower than the open circuit voltage but at least 0.4 V or more, and an open circuit voltage when gas is supplied. It can be increased to (OCV), it can be lowered to the second voltage, the lowest voltage when removing gas. At this time, even in one cell, the voltage should not fall below 0.4 V. Accordingly, the time can be changed. The open circuit voltage (OCV) can be, for example, 0.9 to 1.2 V. The upper limit of the second voltage may be, for example, 0.6 V, 0.5 V, or 0.4 V.

A method of activating a membrane electrode assembly according to a preferred embodiment of the present invention includes a first step of supplying nitrogen; A second step of measuring the open circuit voltage (OCV); A third step of performing a constant current (CC) mode; A fourth step of measuring the open circuit voltage (OCV); A fifth step of performing the constant resistance mode (CR Mode); A sixth step of measuring the open circuit voltage (OCV); A seventh step of performing a constant current mode (CC Mode); And an eighth step of supplying nitrogen.

1 is a graph showing the voltage change at each step of the method for activating a membrane electrode assembly according to the present invention, wherein the x-axis is a time axis, and each step number is described, and the y-axis is a voltage axis. Steps 1 to 8 may be sequentially performed.

The first step can be carried out for 0.5 to 3 hours, preferably 0.5 to 1.5 hours under humidifying and temperature conditions of 50 to 90°C, preferably 60 to 80°C. Humidification conditions may be about 100% humidity (99% or more). If the humidity is too low, the electrolyte is not fully hydrated, and thus the hydrogen ion conductivity of the electrolyte decreases, and the rate of reaction of water formation through the oxygen reduction reaction of the catalyst may be slowed down. If the humidity is too high, water may condense between the gas diffusion layer (GDL) and the channel and gas may not diffuse in the catalyst layer. The temperature can be raised to a temperature of 50 to 90°C, preferably to a temperature of 60 to 80°C. If the temperature is too low, the reaction yield may drop, and if the temperature is too high, the membrane may be damaged. If the time is too short, the humidification may not be complete, and if the time is too long, a cost problem may occur. Nitrogen may be supplied to the anode and the cathode, respectively, and the supply flow rate may be 10±5 LPM (Liter Per Minute). If the supply flow rate is too low, humidification takes a long time, and if the supply flow rate is too high, the film may tear.

The second step may be performed for 0.5 to 3 minutes, preferably 0.5 to 1.5 minutes, based on the current density of 250±100 mA/cm 2 (measured OCV as the first voltage). If the time is too short, the current suddenly increases and local starvation may occur. If the time is long, there is no particular problem, but the above range is preferred. The open circuit voltage (OCV) can be measured using a voltmeter or the like.

Between the second and third steps, the step of lowering the voltage under a condition of a current ramp of 0.05 to 0.5 A/sec, preferably 0.05 to 0.2 A/sec, may be further included. This step is a step of raising the current (lowering the voltage) in the OCV state (current is not flowing), to prevent damage to the MEA due to a sudden voltage change. If the current lamp is too fast, a sudden current rise can cause MEA damage (by Starvation). In this step, the first voltage to the third voltage may be lowered, and the third voltage may be a voltage between the first voltage (OCV) and the second voltage, for example, 0.6 to 1.1 V.

The third step may be performed at a constant current (current density 250±100 mA/cm 2) and a constant voltage (third voltage) for 0.1 to 3 hours, preferably 0.5 to 1.5 hours. If the current density is too small, it is not possible to check whether it is activated at the desired current. If the current density is too large, a sudden load can cause MEA damage. Although time does not matter in particular, the said range is preferable.

Between the third and fourth steps, the step of increasing the voltage under the conditions of the current lamp 0.05 to 0.5 A/sec, preferably 0.05 to 0.2 A/sec may be further included. This step is a step of increasing the voltage from the third voltage to the first voltage (OCV) again, to prevent damage to the MEA due to a sudden voltage change. If the current lamp is too fast, a sudden current rise can cause MEA damage.

Steps 4 and 6 can be performed for 0.1 to 10 seconds, preferably 1 to 3 seconds, based on the current density of 250±100 mA/cm 2 (measured OCV as the first voltage). If the time is too short, it can be measured in the state where hydrogen and air are not completely supplied after CR mode, so it can be measured lower than the actual OCV value. If the time is too long, oxygen and hydroxyl groups may be adsorbed due to water present on the surface of the platinum catalyst, thereby degrading the performance of the ORR reaction. The open circuit voltage (OCV) can be measured using a voltmeter or the like.

The conditions of the fifth step are as described above.

The third to sixth steps may be repeated a plurality of times, for example, 5 to 20 cycles, until the voltage is constant. At this time, the voltage should be maintained over 0.4 V. If the number of cycle repetitions is too small, it may not be activated, and if the number of cycle repetitions is too large, the durability of the MEA may be affected.

In steps 2 to 6, hydrogen may be supplied to the anode, and air may be supplied to the cathode. In the second to sixth steps, each gas may be input in excess, and the gas flow rate may vary depending on the number of cells and the current density. If the gas flow rate is too low, the MEA may be damaged due to lack of fuel (by Starvation), and if the gas flow rate is too high, the membrane may be damaged by differential pressure. In the second to sixth steps, the current density may be 200 to 400 mA/cm 2, preferably 250 to 350 mA/cm 2.

Between the sixth and seventh steps, the step of lowering the voltage under the conditions of the current ramp 0.05 to 0.5 A/sec, preferably 0.05 to 0.2 A/sec may be further included. This step is a step of raising the current (lowering the voltage) in the OCV state, to prevent damage to the MEA due to a sudden voltage change. If the current lamp is too fast, a sudden current rise can cause MEA damage. In this step, the first voltage to the third voltage may be lowered.

The seventh stage can be divided into two stages, that is, the seventh stage is a constant current (current density 250±100 mA/cm 2 ), a constant voltage (third voltage), an anode: 100% hydrogen (only 100 hydrogen without other gases) % Means), cathode: a step 7-1 performed for 0.5 to 3 hours, preferably 0.5 to 1.5 hours under conditions of 100% air (only 100% of air without other gases); And constant current (current density 250±100 mA/cm 2 ), constant voltage (fourth voltage), anode: reformed gas (H ₂ 75±5%, CH ₄ 1.5±0.5%, CO ₂ 20±5 %, N ₂ 3.5±1%), Cathode: In the condition of 100% of air, step 7-2 may be performed for 0.5 to 3 hours, preferably 0.5 to 1.5 hours. In each step, if the current density is too small, it is not possible to check if it is activated at the desired current. If the current density is too large, MEA damage may occur due to a sudden load. Although it is largely independent of time, stabilization occurs after a certain period of time, and the above range is preferred, without measuring too short to see the value at that time. In the seventh step, the voltage can be measured.

Between the 7-1 step and the 7-2 step may further include the step of lowering the voltage under the conditions of the current lamp 0.05 to 0.5 A/sec, preferably 0.05 to 0.2 A/sec. This step is a step of further lowering the voltage from the third voltage to the fourth voltage, to prevent damage to the MEA due to a sudden voltage change. If the current lamp is too fast, a sudden current rise can cause MEA damage. In this step, the third voltage to the fourth voltage may be lowered. The fourth voltage may be a voltage between the second voltage and the third voltage, for example, 0.5 to 1 V.

The eighth step may be performed for 0.5 to 3 hours while removing the remaining gas under the temperature drop condition. The temperature can drop to room temperature. If the time is too short, the OCV state may be maintained by the remaining hydrogen and oxygen, and if the time is too long, the film may dry out and the film may be damaged. Nitrogen may be supplied to the anode and the cathode, respectively, and the supply flow rate may be 10±5 LPM (Liter Per Minute). If the feed flow rate is too small, residual gas may not be completely removed, and if the feed flow rate is too high, the film may be torn by differential pressure.

Table 1 compares the MEA activation method and the existing method of the polymer electrolyte fuel cell using the CR activation method according to the present invention.

Existing MEA activation method MEA activation method of the present invention Assessment Methods CC (Constant Current) mode CR(Constant Resistance) mode Activation
protocol OCV→0.6V→0.4V cycle repeated while changing the current value Set resistance value and repeat OCV→0.4V cycle while turning flow on/off pros and cons
compare Since the evaluation is performed in the high current section, an excessive amount of hydrogen/air flow rate is required → As an evaluation method suitable for transportation, it is inefficient for MEA for buildings It is possible to activate to a low voltage without raising the current, and it is possible to reduce the cost by solving the problem of excessive use of hydrogen.

Table 2 illustrates a specific step-by-step embodiment of the MEA activation method according to the present invention, wherein the current density is based on 250 mA/cm 2. In Table 2, Ramp means the lamp in the intermediate stage between the second and third stages, between the sixth and seventh stages, and between the seventh and seventh stages.

Step Process Condition time flux
(Anode/Cathode) Remark One N ₂ supply Temperature: 70℃ 1hr Humidification
Temperature rise 2 OCV measurement 1min Excess flow
(@300 mA/㎠) Based on 250 mA/㎠ 3 CC mode Ramp: 0.1 A/sec 10min maintenance 3-6 step
10 cycle repeat
(Until voltage becomes constant)
*Voltage maintained at 0.4 V or higher 4 OCV measurement 2sec 5 CR mode Resistance: X Ω -X sec:
Flow input
-X sec:
Flow removal 6 OCV measurement 2sec 7-1 CC mode -Current density:
250 mA/㎠
-Ramp: 0.1 A/sec 1hr measurement -Anode: H ₂ 100%
-Cathode: Air 100% Voltage measurement 7-2 CC mode -Current density:
250 mA/㎠
-Ramp: 0.1 A/sec 1hr measurement -Anode:
Reformated gas
-Cathode: Air 100% 8 N ₂ supply 1hr Temperature drop
Removal of residual gas

Table 3 compares the current density and hydrogen consumption of the polymer electrolyte fuel cell MEA activation method and the conventional method using the CR activation method according to the present invention.

Existing MEA activation method MEA activation method of the present invention Assessment Methods CC mode CR mode Activation
protocol OCV→0.6V→0.4V cycle repeated while changing the current value Set resistance value and repeat OCV→0.4V cycle while turning flow on/off Current density 1600~2500 mA/㎠ About 250 mA/㎠ Hydrogen consumption 200 liters 100 liters

According to Table 3, by activating MEA using a constant resistance mode (CR Mode) according to the present invention, it is possible to activate to a low voltage without raising the current, and it is possible to reduce cost by using less hydrogen than before. In the present invention, the current density varies depending on the situation, but is 250 to 500 mA/cm 2, preferably about 250 mA/cm 2, which is significantly smaller than the conventional method. In addition, in the present invention, the amount of hydrogen used depends on the number of cells and the current density, but is significantly less than the conventional method.

Claims (5)

A first step of supplying nitrogen;
A second step of measuring the open circuit voltage (OCV);
A third step of performing a constant current mode (CC Mode);
A fourth step of measuring the open circuit voltage (OCV);
A fifth step of performing a constant resistance mode (CR Mode);
A sixth step of measuring the open circuit voltage (OCV);
A seventh step of performing a constant current mode (CC Mode); And
Including the eighth step of supplying nitrogen,
The fifth step is a membrane electrode which is a step of performing a constant resistance mode (CR Mode) in which a cycle between the first voltage and the second voltage is repeated while setting a constant resistance value and turning on and off gas. How to activate the conjugate. According to claim 1,
The first voltage is an open circuit voltage (OCV), and the second voltage is lower than the open circuit voltage, but is at least 0.4 V or more, and when the gas is supplied, the voltage rises to the open circuit voltage (OCV), and the second lowest voltage when gas is removed Method of activating a membrane electrode assembly, characterized in that the voltage is lowered. delete According to claim 1,
In the second, fourth and sixth stages, the first voltage is measured,
The third step is performed at a third voltage between the first voltage and the second voltage,
The seventh step is a seventh step performed at a third voltage; And a 7-2 step performed at a fourth voltage between the second voltage and the third voltage. The method of claim 4,
Lowering the voltage between the first and third voltages between the second and third steps;
Increasing the voltage from the third voltage to the first voltage between the third and fourth steps;
Lowering the voltage from the first voltage to the third voltage between steps 6 and 7-1; And
A method of activating a membrane electrode assembly further comprising lowering the voltage from the third voltage to the fourth voltage between steps 7-1 and 7-2.

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