KR20100133698A - Method for testing electrolyte membrane endurance of fuel cell - Google Patents

Abstract

PURPOSE: A method for testing electrolyte membrane endurance of fuel cell is provided to form pinhole and cracks on a film in a short time, and to accurately perform the lifetime of an electrolyte film. CONSTITUTION: A method for testing electrolyte membrane endurance of fuel cell comprises the steps of: performing electrochemical deterioration acceleration operation to create cracks or pin holes on a weak portion of an electrolyte film; accelerating generation of pinhole while inducing contraction and expansion of the electrolyte film by repeating humidification/drying cycle; confirming the formation of pinhole on the electrolyte film; and predicting a lifetime while evaluating durability.

Description

Durability Evaluation Method for Predicting Electrolyte Membrane Lifetime for Fuel Cell {Method for testing electrolyte membrane endurance of fuel cell}

The present invention relates to a durability evaluation method for predicting the life of an electrolyte membrane for a fuel cell. More specifically, the electrochemical deterioration process according to the electrochemical deterioration acceleration operation and the mechanical strength deterioration process according to a humidification / drying cycle are performed. The present invention relates to a method for evaluating the durability of a fuel cell electrolyte membrane for accurately predicting the durability for predicting the lifetime of an electrolyte membrane while promoting pinhole formation at a weak site.

In general, a fuel cell is a device for generating electrical energy by reacting hydrogen (H ₂ ) with oxygen (O ₂ ), and includes a membrane electrode assembly (MEA), which is a membrane electrode assembly. An electrolyte membrane to which hydrogen ions (H +) are delivered, a fuel anode (laminate) formed on one side of the electrolyte membrane to supply hydrogen (H ₂ ) as a fuel, and the other side of the electrolyte membrane to supply air (oxygen) It is configured to include a cathode (cathode), etc. which are laminated on the, and the membrane-electrode assembly and the separator is sequentially stacked is called a fuel cell stack.

In the case of a polymer electrolyte membrane fuel cell (PEMFC) for constructing such a stack, the biggest obstacle to be commercialized is high price and short lifespan.

To this end, the durability evaluation of the polymer electrolyte membrane is essential for long-term operation, it is particularly important to evaluate the defective rate in the durability of the newly developed electrolyte membrane and the durability of the electrolyte membrane purchased in large quantities.

However, deterioration studies of the polymer electrolyte membrane have been progressed a lot, but no measurement method that has been accepted as a method for evaluating the durability of the electrolyte membrane has not been reported yet.

When there is a part that is weakly electrochemically due to impurity such as carboxyl group due to a mistake in the manufacturing process of electrolyte membrane or carelessness, or there is a part that has a weak mechanical strength due to a thin thickness or a crack such as a crack compared to other parts, Deterioration in the area is intensified, which in turn shortens the life of the electrolyte membrane.

However, it is difficult to determine where the electrochemically weak site or the weak mechanical strength exists in the electrolyte membrane before the actual fuel cell is operated.

In particular, since durability evaluation should be performed within a short time, membrane fluorine outflow rate (FER) is measured under accelerated membrane degradation operation conditions. Even if it occurs, it hardly affects the change in the total value, and thus, there is a problem in which the defective part of the electrolyte membrane cannot be determined at the measured membrane fluorine outflow rate.

The present invention has been made in view of the above-described conventional problems, and accelerates electrochemical degradation (degradation due to radicals and hydrogen peroxide) to defective areas of the electrolyte membrane, and then, in an iterative humidification / drying cycle for mechanical strength degradation. By inducing the contraction / expansion of the electrolyte membrane, pinholes are generated in the defective areas (weak areas) of the electrolyte membrane, and then the presence of the defective sites in the electrolyte membrane is accurately determined by measuring the increase in hydrogen permeability or decrease in OCV due to the pinhole generation. It is an object of the present invention to provide a method for evaluating the durability of an electrolyte membrane for a fuel cell, which can predict the durability and at the same time evaluate the durability of the electrolyte membrane through a procedure such as comparing with a reference electrolyte membrane in which the operation is performed.

The present invention for achieving the above object comprises the steps of performing an electrochemical deterioration accelerated operation so that scratches or pinholes are generated in the weak portion of the electrolyte membrane; Repeating a predetermined humidification / drying cycle to induce contraction and expansion of the electrolyte membrane and at the same time to accelerate pinhole generation; Measuring hydrogen permeation current to the electrolyte membrane to accurately confirm that pinholes are formed in the electrolyte membrane; Comparing the electrolyte membrane confirmed to have produced pinholes with the electrolyte membrane serving as an evaluation criterion to evaluate durability and predict a lifespan; It provides a durability evaluation method for predicting the life of the electrolyte membrane for a fuel cell comprising a.

In a preferred embodiment, the pinhole is formed in the electrolyte membrane can be confirmed by measuring the OCV for the fuel cell, characterized in that it is determined that the pinhole is formed in the electrolyte membrane when the measured OCV is gradually reduced.

In particular, the step of performing the electrochemical degradation acceleration operation is the fuel cell temperature of 90 ℃ or more, supplying hydrogen of the relative humidity (RH) of 0% to the anode (anode), relative humidity (to the cathode) RH) characterized in that the fuel cell is operated for 10 hours or 20 hours under the conditions of supplying 50% or more oxygen or air.

In the predetermined humidification / drying cycle, the temperature of the fuel cell is 65-70 ° C., nitrogen is supplied to the anode and the cathode at 100% relative humidity (RH) for 10 to 20 minutes to expand the electrolyte membrane, and then 0 It is characterized by consisting of 5 to 10 times the process of shrinking the electrolyte membrane by introducing nitrogen of the relative humidity to the fuel electrode and the air electrode for 20 to 60 minutes.

In addition, if there is no difference in the hydrogen permeation current measurement value or the OCV measurement value for the electrolyte membrane compared with the value before repeating the humidification / drying cycle, the electrochemical degradation acceleration operation and the predetermined humidification / The drying cycle is further repeated.

According to the present invention, the electrochemical deterioration due to the electrochemical deterioration accelerated operation and the mechanical strength deterioration due to the humidification / drying cycle are performed in parallel to provide the following advantages.

(1) Pinholes, cracks, and the like can be formed on the film in a short time.

Even if the electrochemical deterioration process is operated for 200 hours or more under severe conditions (OCV, low-humidity, high temperature), pinhole formation is not good, and pinhole formation is not good even after repeating the humidification / drying process for tens of thousands of times to weaken the mechanical strength of the electrolyte membrane. Although it was impossible to predict the lifetime of the electrolyte membrane, in the present invention, pinholes and the like can be easily formed in a weak portion of the electrolyte membrane within 100 hours as the electrochemical degradation and the mechanical strength degradation are performed together, thereby accurately predicting the lifetime of the electrolyte membrane. It can be carried out.

(2) It is possible to discriminate between electrochemically weak and mechanically weak areas.

It is not easy to determine whether there is an electrochemically weak part only by the electrochemical degradation acceleration test method, and it is not easy to determine whether there is a weak mechanical strength part by pinholes or the like by only measuring the mechanical strength.

However, according to the method of the present invention, there is an advantage in that it is possible to simultaneously determine whether two kinds of weak areas exist.

That is, when the pinhole is initially formed in the electrolyte membrane through the electrochemical degradation acceleration and the mechanical strength degradation acceleration of the present method, it is possible to determine whether it is formed electrochemically weak or mechanical strength is weak.

In other words, when pinholes are formed in the electrolyte membrane through the electrochemical degradation acceleration and the mechanical strength degradation acceleration of the present method, it is the starting point of pinhole formation that the defects formed in the electrolyte membrane due to electrochemical weakness are enlarged by contraction / expansion. If the thickness of the electrolyte membrane is thinner than other parts, or if there are very small cracks or fine pinholes, whether the gas crossover is good is the cause of larger pinholes, or the rate of electrochemical degradation due to radical and hydrogen peroxide increases. Accurate analysis of the cause of the formation of larger pinholes can confirm the presence of vulnerable sites in the electrolyte membrane.

(3) Since the lifetime of the polymer electrolyte membrane is determined not by the whole but by any one part of the electrolyte membrane, the weak part of the electrolyte membrane can be found to determine how weak it is.

In general, there are methods for measuring the degree of degradation of the polymer electrolyte membrane, such as fluorine outflow rate, hydrogen permeability, impedance, OCV measurement, etc., but fluorine outflow rate and impedance measurement method for the entire membrane is weak in one part of the electrolyte membrane. If the presence of a weak site, the presence of the weak site can not be determined, and the hydrogen permeability and OCV measurement method is difficult to determine the presence of a flaw or pinhole, such as the small change in the measurement when the flaw in any part of the electrolyte membrane have.

However, according to the present invention, it is possible to clearly confirm the presence of a weak portion of the electrolyte membrane by increasing the defects on the electrolyte membrane, small pinholes, cracks, and the like, so that the change also occurs in the hydrogen permeability and OCV.

4) Can be measured by simple equipment and measuring devices.

Since the deterioration degree can be measured by the OCV measurement after the accelerated operation of membrane deterioration in the fuel cell station, the special equipment or analysis device such as potentiostat or impedance analyzer has been used. Electrolyte membrane life prediction method of the present invention has the advantage that can be carried out even without the specific equipment.

Hereinafter, the present invention will be described in more detail.

The electrolyte membrane life prediction method of the fuel cell stack according to the present invention scratches a weak portion of the electrolyte membrane by an electrochemical degradation acceleration operation, and then expands the scratched portion by repeating contraction / expansion by a humidification / drying cycle. It is characterized by the fact that it is repeated.

That is, by combining electrochemical deterioration due to accelerated electrochemical deterioration operation and mechanical strength deterioration due to humidification / drying cycles, pinholes can be formed quickly in weak areas of the electrolyte membrane, thereby easily predicting the lifetime of the electrolyte membrane. It is characterized by.

In view of the fact that a fuel cell having a polymer electrolyte membrane, in particular, a fuel cell for transport that requires frequent on / off and cold start, the electrochemical degradation process and the mechanical strength degradation process are often performed in parallel. By applying the electrolyte membrane life prediction method of the present invention, it is possible to accurately and precisely evaluate the durability for the lifetime prediction of the electrolyte membrane.

According to the present invention, after scratching a weak portion of the electrolyte membrane by an accelerated electrochemical degradation operation, by repeatedly shrinking / expanding the scratched portion for deterioration of mechanical strength according to the humidification / drying cycle, the damage of the scratch becomes severe. Pinholes are formed, resulting in increased hydrogen permeability.

Thus, by measuring the hydrogen crossover current for the electrolyte membrane using a potentiostat, it is possible to more accurately confirm that the flaw or the pinhole is formed on the electrolyte membrane.

However, although it is not as accurate as hydrogen permeation current, it is possible to confirm the formation of a flaw or pinhole on the electrolyte membrane by simply measuring the OCV and confirming the OCV reduction.

In this case, the fuel cell cell for durability evaluation may use a normal operation cell, but in order to shorten the durability evaluation time, when a cell having a width of the separator plate is 2 to 5 times wider than the operation cell, a pinhole formation process is performed. This is faster because there is an advantage.

The conditions for accelerating the electrochemical degradation are the fuel cell temperature of 90 ℃ or more, supplying hydrogen of 0% relative humidity (RH) to the anode (anode), 50% relative humidity (RH) to the cathode (cathode) The above-mentioned oxygen or air is supplied, and when oxygen is supplied, the deterioration rate becomes faster and the durability evaluation time is shortened.

Under these conditions, after operating the fuel cell for 10 hours or 20 hours, hydrogen permeation current or OCV is measured.

Humidification / drying cycle for deterioration of mechanical strength is to make the temperature of fuel cell at 65 ~ 70 ℃, and supply electrolyte to both electrodes (fuel electrode and air electrode) at 100% relative humidity (RH) for 10-20 minutes to expand electrolyte membrane. Then, the process of shrinking the electrolyte membrane by introducing nitrogen of 0% relative humidity to both electrodes for 20 to 60 minutes is repeated 5 to 10 times.

Subsequently, after electrode activation, the hydrogen permeation current is measured or the OCV is measured and compared with the value before repeating the humidification / drying cycle, and if the difference is small, the humidification / The drying process is repeated one more set to measure the hydrogen permeation current and OCV. After checking the difference, if there is no big difference, it is repeated until there is a difference.

The membrane durability is then evaluated by comparing the sum of total electrochemical degradation time and the number of humidification / dry cycles for mechanical strength degradation.

In this way, the long-term operation under normal conditions confirms the endurance life of the polymer electrolyte membrane, sets the electrolyte membrane as an evaluation criterion, measures the durability of the electrolyte membrane under various operating conditions, and compares the result with the reference electrolyte membrane, thereby increasing the lifetime of the membrane. Can be expected.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the following Examples.

Example 1

In order to predict the durability and lifespan of the electrolyte membrane, as shown in FIG. 1, equipment for measuring various evaluation items of the fuel cell was used, and the electrochemical deterioration due to the electrochemical deterioration acceleration operation and the humidification / drying cycle The mechanical strength deterioration was performed in parallel to predict the durability and lifespan of the electrolyte membrane as follows.

Briefly describing the equipment shown in FIG. 1, the unit cell mounting portion 10, the potentiostat 20, and the electronic load 30 are disposed at an upper portion thereof, and at the bottom thereof, hydrogen, oxygen, and nitrogen are disposed. Various controllers are connected to the gas supply tank to control the temperature and the amount of each gas supplied to the unit cell mounting unit 10.

For the experiment according to Example 1, first, a unit cell having a predetermined size electrode and an electrolyte membrane in the unit cell mounting portion 10 of the equipment, that is, an electrode membrane assembly (MEA) of A company is fastened together with a Teflon gasket.

The temperature of the cell is 70 ° C, the temperature of the anode and cathode humidifying water is 70 ° C, air (about 300ml / min) for the cathode, and hydrogen (about 100ml / min) for the anode, respectively. It was supplied, and activated for 24 hours at a constant current, the first hydrogen permeability was measured.

In this case, the hydrogen permeability is an electrochemical method in the cell at 70 ° C., 100% relative humidity (RH) of nitrogen (200 ml / min) and 100% relative humidity (RH) of hydrogen (about 40 ml / min) to the air electrode and the fuel electrode, respectively. Was supplied and measured through a current value exhibited by hydrogen crossover through the electrolyte membrane.

That is, when a voltage is applied to the potentiostat (Solatron), the crossovered hydrogen is oxidized at the cathode to give electrons. By measuring the amount of electrons, the amount of hydrogen that has passed through the electrolyte membrane can be known. As the voltage was increased to the voltage at which the limit current density appeared, the current value according to the hydrogen crossover was measured, and the normal electrolyte membrane and the degraded electrolyte membrane were compared.

On the other hand, after measuring the hydrogen permeability as described above, as an OCV condition for electrochemical deterioration of the electrolyte membrane, the cell temperature is 90 ℃, humidified hydrogen is introduced into the fuel electrode at about 100ml / min, the relative humidity (RH) 65% to the cathode After operating for 20 hours while introducing oxygen at about 150 ml / min, the hydrogen permeation current was measured by applying a voltage to the potentiostat as described above.

In addition, after 20 hours of electrochemical deterioration process, in order to deteriorate the mechanical strength of the electrolyte membrane, the cell temperature is set to 70 ℃, relative humidity (RH) 100% nitrogen (N ₂ ) to the anode (air electrode and fuel electrode) Was infused at about 400 ml / min for 20 minutes to humidify the electrolyte membrane.

After subsequent humidification, the cell was cooled to 30 DEG C while supplying unhumidified (RH 0%) nitrogen at about 400 ml / min to the positive electrode for 40 minutes to dry the electrolyte membrane.

After repeating the humidification / drying process 8 times, 16 times, and 24 times, hydrogen permeation current was measured, respectively.

As a result of the measurement, as shown in the attached hydrogen permeation current graph of FIG. 2, the hydrogen permeation current after 20 hours of electrochemical degradation and after 8, 16, and 24 humidification / drying repetitions was almost equal to the initial hydrogen permeation current. It was like

Thus, after further 20 hours of electrochemical deterioration, a total of 40 hours of electrochemical deterioration was performed, and the hydrogen permeation currents were measured after 8 times of humidification / drying, respectively.

In addition, the hydrogen permeation current was measured after further 26 hours of electrochemical deterioration and after a total of 66 hours of electrochemical deterioration and further 8 times and 16 times of humidification / drying repetition. The transmission current began to increase.

That is, after 66 hours of electrochemical deterioration, the electrolyte membrane is scratched, and when the expansion / contraction by the humidification / drying cycle is repeated, it is determined that a pinhole is formed and the hydrogen permeability is rapidly increased.

Therefore, the pinhole was formed by the indicator method (Patent Application No. 10-2008-0032963) and the location was indicated by SEM photographs. As a result, the surface of the electrolyte membrane was torn off as shown in FIG. It can be seen that the smaller pinholes are formed as shown in FIG.

Example 2

In order to compare the polymer membrane durability of the company B MEA, the experiment was performed in the same manner as in Example 1, and the results are as shown in the graph of FIG.

That is, the hydrogen permeability current after 20 hours of electrochemical deterioration described in Example 1 and after repeated 8 times and 16 times of humidification / drying cycles is hardly different from the hydrogen permeation current measured first, and further 20 hours of electrical When the hydrogen permeation current was further measured after 40 hours of electrochemical degradation and after 8 and 16 humidification / drying cycles, the increase in hydrogen permeation current was observed.

Accordingly, the electrode membrane assembly (MEA) having the electrolyte B company membrane increased the hydrogen permeation current after 40 hours of electrochemical degradation, deteriorating in 26 hours shorter than the electrolyte membrane of the company A described in Example 1, As a result, it can be determined that the electrochemical durability of the electrode membrane assembly of the B company is about 60% of the electrode membrane assembly of the A company.

When the electrolyte membrane is expanded and contracted through a humidification / drying cycle for mechanical strength deterioration in the state where the electrolyte membrane is flawed through electrochemical degradation (66 hours for A company and 40 hours for B company), the hydrogen permeation current increase ratio (pinhole) When the mechanical strength was roughly calculated, the increase in hydrogen permeation current of the electrode membrane assembly of Company B was 8 times and 10 times greater than that of Company A, respectively, after 8 and 16 times of humidification / drying. It is possible to determine that 8 to 10 times the pinhole formation is good, and that the mechanical strength is weak.

As a result, it was confirmed through Example 1 and Example 2 that the electrode membrane assembly MEA of Company A was superior in electrochemical and mechanical strength than the electrode membrane assembly MEA of Company B.

Meanwhile, in Examples 1 and 2, the hydrogen permeability current can be measured by a simpler method as follows without using an expensive potentiostat.

That is, as hydrogen permeability increases, OCV decreases so that durability of the electrolyte membrane can be evaluated by measuring OCV by electronic load.

The relationship between hydrogen permeation current and OCV can be obtained theoretically by the Nernst equation, but since it is different from the actual one, the relationship between hydrogen permeation current and OCV can be obtained by experiment.

In other words, artificially made two to ten pinholes of 50 to 70 μm in the electrode membrane assembly (MEA) of Company A used in Example 1, and fastened them with a fuel cell, respectively, and proceeded with the normal activation procedure to perform OCV and hydrogen. The transmission current was measured, and as shown in the graph of FIG. 5 showing the measurement result, it was found that the OCV decreased as the hydrogen permeation current increased, which means that the OCV decreases due to deterioration of the electrolyte membrane. Can be easily adopted as a predictable method.

1 is a schematic diagram showing equipment for implementing a method for evaluating durability for predicting electrolyte membrane life for fuel cells according to the present invention;

2 is a graph showing a hydrogen permeation current as a result of an embodiment of the durability evaluation for the prediction of the life of the electrolyte membrane for fuel cells according to the present invention;

3a and 3b is a result of an embodiment of the durability evaluation for predicting the life of the electrolyte membrane for fuel cells according to the present invention, SEM pictures of the pinholes,

4 is a graph showing a hydrogen permeation current as a result of another embodiment of durability evaluation for predicting the life of an electrolyte membrane for a fuel cell according to the present invention;

Figure 5 is a graph showing the results of measuring the OCV to determine the pinhole is formed during the durability evaluation for the electrolyte membrane life prediction of the fuel cell according to the present invention.

Claims (5)

Performing an electrochemical deterioration accelerated operation to generate a scratch or pinhole at a weak portion of the electrolyte membrane; Repeating a predetermined humidification / drying cycle for mechanical strength deterioration to induce contraction and expansion of the electrolyte membrane and at the same time to accelerate pinhole generation; Measuring a hydrogen permeation current with respect to the electrolyte membrane to confirm that pinholes are formed in the electrolyte membrane; Comparing the electrolyte membrane confirmed to have produced pinholes with the electrolyte membrane serving as an evaluation criterion to evaluate durability and predict a lifespan; Durability evaluation method for fuel cell electrolyte membrane life prediction comprising a. The method according to claim 1, The pinhole is formed in the electrolyte membrane can be confirmed by measuring the OCV for the fuel cell, the durability for fuel cell electrolyte membrane life prediction, characterized in that it is determined that the pinhole is formed in the electrolyte membrane when the measured OCV is gradually reduced Assessment Methods. The method according to claim 1, In the step of performing the electrochemical degradation acceleration operation, the fuel cell temperature is 90 ° C. or higher, the hydrogen is supplied with a relative humidity (RH) of 0% to the anode, and the relative humidity (RH) is applied to the cathode. A method for evaluating the durability of a fuel cell electrolyte membrane for a fuel cell, characterized in that the fuel cell is operated for 10 hours or 20 hours under a condition of supplying 50% or more of oxygen or air. The method according to claim 1, In the predetermined humidification / drying cycle, the temperature of the fuel cell is 65-70 ° C, nitrogen is supplied to the fuel electrode and the air electrode at 100% relative humidity (RH) for 10 to 20 minutes to expand the electrolyte membrane, and then 0% relative. A method for evaluating the durability of an electrolyte membrane for a fuel cell, comprising: repeating a process of shrinking an electrolyte membrane by introducing nitrogen of humidity to a fuel electrode and an air electrode for 20 to 60 minutes for 5 to 10 times. The method according to claim 1 or 2, If the change in the hydrogen permeation current measurement value or the OCV measurement value for the electrolyte membrane is not different from the value before repeating the humidification / drying cycle, the electrochemical degradation acceleration operation and the predetermined humidification / drying cycle until the difference is large. Endurance evaluation method for predicting the life of the electrolyte membrane for a fuel cell, characterized in that the further proceeds.

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