KR102142879B1 - Membrane electrode assembly for proton exchange membrane fuel cell and manufacturing method of catalyst for proton exchange membrane fuel cell - Google Patents

Abstract

The present invention relates to a membrane electrode assembly for a proton exchange membrane fuel cell and a method for producing a catalyst for a proton exchange membrane fuel cell.
A membrane electrode assembly for preventing fuel in a proton exchange membrane according to an embodiment of the present invention includes: a cathode and an anode positioned to face each other; And a polymer electrolyte membrane positioned between the cathode and the anode, wherein the cathode includes a platinum/carbon nanotube catalyst in which platinum (Pt) is supported on a carbon nanotube (CNT) support, and the carbon nanotube is C It may contain an oxygen functional group of -OH (hydroxy).

Description

Membrane electrode assembly for a proton exchange membrane fuel cell and a method of manufacturing a catalyst for a proton exchange membrane fuel cell {MEMBRANE ELECTRODE ASSEMBLY FOR PROTON EXCHANGE MEMBRANE FUEL CELL AND MANUFACTURING METHOD OF CATALYST FOR PROTON EXCHANGE MEMBRANE FUEL CELL}

The present invention relates to a membrane electrode assembly for a proton exchange membrane fuel cell and a method for producing a catalyst for a proton exchange membrane fuel cell.

Since carbon nanotubes have excellent electrical conductivity, specific surface area, and hydrogen storage properties, their use as a catalyst support is promising, and in particular, use as a fuel cell electrode is preferable. Carbon nanotubes have the advantage of preventing agglomeration of particles when carrying metal particles due to their unique surface structure.

Platinum catalysts, which are noble metals, are widely used in various hydrogenation reactions or reforming reactions, and have the advantage of showing superior activity compared to other metal catalysts.

The proton exchange membrane fuel cell (PEMFC) is compact, lightweight, clean, and has a high power density, and thus has attracted considerable interest as a power source for pollution-free vehicles.

Oxygen Reduction Reaction (ORR) at the cathode of PEMFC plays an important role in determining the performance of fuel cells, and the durable ORR catalyst that can withstand more than 5000 hours of operation is a commercial choice for fuel cells. Essential to

Platinum (Pt) is regarded as a standard ORR catalyst in a proton exchange membrane fuel cell (PEMFC) because of its high catalytic activity, and the catalyst is generally prepared by loading platinum on a carbon black support.

However, the Pt/CB (carbon black) catalyst has low long-term durability, high overvoltage loss, and accelerates catalyst deterioration in transient conditions such as start/stop and cell reversal processes.

During fuel cell operation, fuel shortage can be caused by a number of factors such as water flooding in the fuel cell, ice formation in winter, and abnormal operation of the supply of reactive gas (abnormal reaction gas supply).

Insufficient hydrogen (H ₂ ) supply from the anode causes the CR (Cell Reversal) state of the proton exchange membrane fuel cell (PEMFC), and the Pt/CB catalyst support is the current of the membrane electrode assembly for a proton exchange membrane fuel cell. Reacts quickly with water to keep it.

Carbon Oxidation Reaction (COR) is an unfavorable proton exchange membrane fuel cell such as disconnection of an electrical network, Pt separation, and Pt particle growth by reducing the amount of the CB support in the cathode of the membrane electrode assembly for the proton exchange membrane fuel cell. PEMFC) There is a problem that leads to operating conditions.

When such a CR (Cell Reversal) phenomenon is repeated, the fuel cell performance is characterized by an irreversible deterioration. In this connection, the use of highly graphitized carbon as a support for a Pt catalyst has been extensively studied to prevent carbon oxidation (COR) reactions.

Carbon nanotubes (CNTs) have many desirable properties as a carbon support for a proton exchange membrane fuel cell, including a large surface area, excellent electrochemical durability, high electrical conductivity, and excellent mechanical strength.

Recently, CNT has been regarded as a promising Pt catalyst support for preventing COR (carbon oxidation reaction) of ORR (oxygen reduction reaction) electrodes.

According to the prior art (Durability investigation of carbon nanotube as catalyst support for proton exchange membrane fuel cell. J Power Sources 158, 154-159 (2006).), it has been demonstrated that the CNT support can inhibit the sintering of Pt particles, Several studies have reported that Pt usage is reduced and battery performance is improved due to the CNT-supported Pt catalyst.

Advances in Polymer Technology, 18. Feb. 2016(Preparation And Performance of PVC/CNT nanocomposite) Durability investigation of carbon nanotube as catalyst support for proton exchange membrane fuel cell. J Power Sources 158, 154-159 (2006).

The present invention was conceived to solve the above problems, and as a functional group for bonding Pt to carbon nanotubes in a membrane electrode assembly for a proton exchange membrane fuel cell (PEMFC), Pt/CNT electrical network and ECSA (electrochemical active surface), which are their own properties, even after 100 CR conditions by adding oxygen functional groups (reversing the total voltage of the proton exchange membrane fuel cell by reducing the supply flow rate of hydrogen or fuel) Area) to provide a membrane electrode assembly for a proton exchange membrane fuel cell (PEMFC).

In addition, in the present invention, carbon nanotubes (CNTs) have high crystallinity and low defect density, so that they may not sufficiently provide active sites for carbon oxidation reactions, and have large defects. It is to provide a method of manufacturing a catalyst for a proton exchange membrane fuel cell that can effectively lower the oxidation rate of CNT because it is present at the edge of a tube.

Meanwhile, other objects not specified of the present invention will be additionally considered within a range that can be easily deduced from the following detailed description and effects thereof.

In order to achieve such an object, a membrane electrode assembly for a proton exchange membrane fuel blocking according to an embodiment of the present invention includes: a cathode and an anode positioned to face each other; And a polymer electrolyte membrane positioned between the cathode and the anode, wherein the cathode includes a platinum/carbon nanotube catalyst in which platinum (Pt) is supported on a carbon nanotube (CNT) support, and the carbon nanotube is C It may contain an oxygen functional group of -OH (hydroxy).

In one embodiment of the present invention, the carbon nanotubes in the membrane electrode assembly for preventing fuel in the proton exchange membrane may further include C-O (acyl) and C=O (carbonyl).

In one embodiment of the present invention, the carbon nanotubes in the membrane electrode assembly may have an oxygen mole ratio of 15 to 35%.

In an embodiment of the present invention, the ratio of C-OH among the oxygen functional groups included in the carbon nanotubes among the membrane electrode assemblies may be the highest.

In one embodiment of the present invention, before and after the acid treatment, the ID/IG ratio on the Raman spectrum may be constant.

The membrane electrode assembly may have a cell polarization voltage reduction of 5% or less after 100 reverse voltage cycles.

The membrane electrode assembly may have an electrochemical surface area reduction of 10% or less after 100 reverse voltage cycles.

The membrane electrode assembly may have an increase in ohmic resistance and polarization resistance of 5% or less after 100 reverse voltage cycles.

In the method for producing a catalyst for a proton exchange membrane fuel cell according to an embodiment of the present invention, the step of attaching an oxygen functional group to a carbon nanotube (CNT) (s10); and dispersing the carbon nanotubes with the functional group attached in a liquid reducing agent. And dissolving a platinum precursor in a solvent to prepare a platinum (Pt) solution (s30); And mixing and reacting the platinum solution with the liquid reducing agent in which the carbon nanotubes are dispersed (s40) to prepare a platinum/carbon nanotube catalyst (s40), wherein the oxygen functional group is CO(acyl), C=O( carbonyl) and C-OH (hydroxy).

The solvent may be distilled water. The liquid reducing agent may be ethylene glycol.

In one embodiment of the present invention, the step of attaching the oxygen functional group to the carbon nanotubes (CNT) is performed by treating the carbon nanotubes (CNT) with a mixture of sulfuric acid (H ₂ SO ₄ ) and nitric acid (HNO ₃ ). Can be.

In an embodiment of the present invention, the acid treatment may be performed for 2 to 4 hours.

In an embodiment of the present invention, the concentration of sulfuric acid may be 3 to 5 M, and the concentration of nitric acid may be 3 to 5 M.

According to the membrane electrode assembly for a proton exchange membrane fuel cell according to an exemplary embodiment of the present invention, Pt nanoparticles coated on a conventional carbon support are easily caused by carbon oxidation (CO), which may occur during cell reversal (CR). It is possible to minimize problems that may occur in the carbon assembly that may be separated and agglomerated. In addition, when producing a durable ORR (Oxygen Reduction Reaction) catalyst under CR conditions, it can be seen that CNT can be used as a support for the Pt catalyst.

This is due to the high crystallinity, low defect density, and high aspect ratio of carbon nanotubes, which inhibits oxidation of the CNT support, resulting in cell voltage and electrochemical surface area (ECSA) even after 100 CRs. And it can be said that this is because the electrical conductivity was kept constant.

As such, the durability of the platinum (Pt) catalyst bonded to the carbon nanotubes shown through the present specification can be confirmed by a transmission electron microscope and X-ray Diffraction (XRD) analysis that can identify the particle properties of the sample. These results are expected to provide useful insights not only for electrochemical applications, but also for future research designs for other types of fuel cells.

The above-described effects of the present invention according to the technical idea of the present invention have been exemplarily described, and the scope of the present invention is not limited by these effects.

1 is a view showing the configuration of a membrane electrode assembly for a proton exchange membrane fuel cell according to an embodiment of the present invention.
2 is an XPS (X-Ray Photo Spectroscopy) graph showing the bonding state of oxygen functional groups according to the acid treatment time.
3 is raman data showing a rate of change of ID/IG of carbon nanotubes according to an embodiment of the present invention.
4 is a photograph showing the adsorption state of Pt nanoparticles according to the acid treatment on the CNT surface according to an embodiment of the present invention.
5 and 6 are transmission electron micrographs at low and high magnifications of carbon nanotubes to which Pt particles are adsorbed, respectively.
7(a), (b), (c), and (d) show the anode of (a) MEA-a in the membrane electrode assembly 100 for a proton exchange membrane fuel cell measured at a scan rate of 30 mV/s, ( b) The cathode, (c) the anode of the MEA-b and (d) the cathode is a graph showing the CV after inducing the prepared state (black) and the CR state 100 times (red).
FIG. 8 is a graph showing changes in cell voltage after performing the CR state of the membrane electrode assembly (MEA-a) and MEA-b for a proton exchange membrane fuel cell 100 times according to an embodiment of the present invention. to be.
9(a), (b), (c), and (d) show the anode of (a) MEA-a in the membrane electrode assembly 100 for a proton exchange membrane fuel cell measured at a scan speed of 30 mV/s, ( b) The cathode, (c) the anode of the MEA-b and (d) the cathode is a graph showing the CV of the prepared state (black) and the CV after inducing the CR state 100 times (red)
Fig. 10 is a diagram showing Nyquist plots before (black) and after (red) CR of MEA.
11 (a) and (b) are XRD graphs of (a) an anode 120, a cathode of MEA-a and an anode of MEA-b according to an embodiment of the present invention, (b ) Is an XRD graph of the MEA-b cathode.

Hereinafter, the present invention will be described in detail. The following examples and drawings are provided as examples to sufficiently convey the spirit of the present invention to those skilled in the art. In addition, unless there are other definitions in the technical terms and scientific terms used in the present invention, the present invention has the meaning commonly understood by those of ordinary skill in the art to which this invention belongs, and the present invention in the following description and accompanying drawings Descriptions of known functions and configurations that may unnecessarily obscure the gist of are omitted.

1 is a view showing the configuration of a membrane electrode assembly for a proton exchange membrane fuel cell according to an embodiment of the present invention.

Referring to FIG. 1, the membrane electrode assembly 100 for a proton exchange membrane fuel cell according to an embodiment of the present invention may include a first gas diffusion layer 110 into which hydrogen, which can be referred to as a fuel gas, can be introduced, Hydrogen (H ₂ ) gas may be introduced through the first gas diffusion layer 110.

When water is electrolyzed, it is broken down into hydrogen and oxygen. On the contrary, hydrogen and oxygen are combined to make water, and the energy generated at this time can be converted into electricity. Fuel cells basically use this principle.

In this way, fuel cells are similar to batteries in that they generate electricity through chemical reactions, but fuel cells do not require charging, unlike batteries, because they receive hydrogen and oxygen, which are reactive substances, and do not require electricity as long as fuel is supplied. It has a feature that can cause In addition, since electricity is generated without the combustion reaction of fuel, there is no emission of toxic pollutants such as sulfur and nitrogen oxides, unlike conventional internal combustion engines. It can be said to be eco-friendly because it can dramatically reduce carbon dioxide emissions.

As described above, hydrogen (H ₂ ) molecules enter the anode (cathode side, 120) to generate hydrogen ions, and the thus produced hydrogen ions may be transferred to the cathode through the polymer electrolyte membrane 130, which is an electrolyte. In addition, oxygen molecules (O ₂ ) are introduced through the second gas diffusion layer 150 from the cathode (anode side, 140), which is the reduction electrode, and the introduced oxygen (O ₂ ) is reduced (Oxygen Reduction Reaction; ORR) polymer. It is configured to meet with hydrogen ions introduced through the electrolyte membrane 130 to form water (H ₂ O).

Of course, although not shown, the electrons (e-) generated at this time are configured to transmit electricity to an external circuit and to be transferred to the cathode 140 to discharge water to the outside.

Oxygen reduction reaction (ORR) in the membrane electrode assembly 100 for a proton exchange membrane fuel cell plays an important role in determining the performance of a fuel cell, and a durable ORR catalyst (a cathode 140 in an embodiment of the present invention) It can be said that it plays this role).

In the same principle, insufficient hydrogen (H ₂ ) supply at the anode 120 can lead to a cell reversal (CR) of a proton exchange membrane fuel cell. At this time, a phenomenon of rapidly reacting with water occurs between platinum (Pt) and carbon black, which is an anode, which is a support, in order to maintain the flow of current flowing through the proton exchange membrane membrane electrode assembly 100.

The same phenomenon occurs not only in the anode 120 but also in the cathode 140. Because of this, carbon oxidation reaction of the cathode 140 occurs, and if the carbon support of the cathode 140 is carbon black, the amount of such carbon black, that is, the carbon support, is greatly reduced to disconnect the electrical network. (short circuit), separation of Pt, which is a catalyst particle, and growth of Pt particles may lead to poor operating conditions of the proton exchange membrane fuel cell.

When CR, which is a transition condition, is repeated as described above, the performance of the proton exchange membrane fuel cell may be degraded to such an extent that it cannot be recovered.

Efforts to improve such problems have been continued even in conventional studies.

For example, it has been demonstrated that CNT (carbon nanotube) support can prevent electrochemical recrystallization of Pt particles, and in several studies, the performance of proton exchange membrane fuel cells as the amount of Pt used decreases due to the use of Pt, which is a CNT support catalyst. Reported getting better.

In the proton exchange membrane fuel cell according to an embodiment of the present invention, a proton exchange membrane having excellent durability is achieved by making a CR (Cell Reversal) state by reducing the amount of fuel and using a CNT support in which Pt catalyst nanoparticles are evenly dispersed. A catalyst used in the membrane electrode assembly 100 for a fuel cell may be provided.

(Example 1)

In order to manufacture the membrane electrode assembly 100 for a proton exchange membrane fuel cell having excellent durability, the membrane electrode assembly 100 for a proton exchange membrane fuel cell may be manufactured by the following process.

First, a process (s10) of treatment with a mixture of sulfuric acid (H ₂ SO ₄ ) and nitric acid (HNO ₃ ) for multi-wall carbon nanotubes may be performed.

Through such a process, a process of attaching an oxygen functional group to the carbon nanotube can be performed.

The concentration of sulfuric acid and nitric acid is preferably 3 to 5 M, and in Examples, sulfuric acid and nitric acid of 4 M concentration were used, and the volume ratio of sulfuric acid and nitric acid was 1:1.

By performing the acid treatment as described above, a functional group (an oxygen functional group may play this role in an embodiment of the present invention) is generated so that platinum can be well formed on the surface of the carbon nanotube support. It goes without saying that single-walled carbon nanotubes can be used together with multi-walled carbon nanotubes as such carbon nanotubes.

In addition, as the acid solution, not only sulfuric acid or nitric acid, but also a strong acid such as hydrochloric acid may be used. Thus acid washed The (acid-treated) carbon nanotubes may be subjected to a cleaning process. In the present invention, after filtering CNTs with a vacuum filter, sulfuric acid and nitric acid were removed by washing with DI water 2-3 times, and dried in a vacuum oven at 60° C. for 4 hours.

Such an acid treatment process was performed to determine an appropriate execution time through the following experimental process.

That is, oxygen functional groups C-O(asyl), C=O(carbonyl), and C-O-H(hydroxy), which are oxygen functional groups according to acid treatment using nitric acid and sulfuric acid, may be included.

2 is an X-Ray Photo Spectroscopy (XPS) spectrum showing the bonding state of oxygen functional groups according to the acid treatment time. The increase in oxygen functional groups on the CNT surface can be confirmed by XPS.

Referring to FIG. 2, as the acid treatment time increases, the oxygen functional group tends to increase, and it can be seen that the oxygen functional group is optimally included in the third treatment time.

In other words, it can be seen that the oxygen functional group is reduced when it exceeds 3.5 hours or less than 2.5 hours. Table 1 shows the mol% of oxygen (O) according to the acid treatment time obtained from the XPS specification of FIG. 2.

Table showing the mole fraction of oxygen functional groups according to the acid treatment time Acid treatment time C amount (molar fraction) O quantity (molar fraction) 0h 98% 2% 1h 94% 6% 2h 85% 15% 3h 71% 29% 4h 87% 13% 5h 92% 8% 6h 94% 6%

As the acid treatment time increases, the oxygen functional group on the surface of the CNT, which is the carbon support, increases, and it can be seen that the acid treatment has the highest oxygen mole fraction after 3 hours of acid treatment.

In the case of acid treatment for 2 to 4 hours, the oxygen mole fraction on the surface of the CNT becomes 10% or more.

In the present invention, the C-OH ratio is the highest among C-OH, C=O and C-O in the binding form of O1s. Among the oxygen functional groups, CNTs become hydrophilic due to C-OH, and Pt catalyst particles are made by reacting most actively with the platinum aqueous solution in the presence of this functional group.

The change pattern of the oxygen functional group may vary depending on the ratio and concentration of sulfuric acid/nitric acid.

In addition, as shown in FIG. 3, it can be confirmed from Raman analysis data that the ID/IG (structure of CNT) value does not change significantly as the acid treatment time increases.

4 is a raman spectrum showing a rate of change of ID/IG of a carbon nanotube according to an embodiment of the present invention.

In the carbon nanotubes, the ID/IG ratio represents a relative ratio to the intensity of the D band and G band in the Raman spectrum of the carbon nanotube.

The degree of disorder or defect of carbon nanotubes can be evaluated by the intensity ratio (ID/IG) of the D-band and G-band. If this ratio is high, it can be evaluated as having many disorders or defects. If this ratio is low, the carbon It can be evaluated that there are few defects in the nanotube and the degree of crystallinity is high. The defect referred to herein is an imperfect part of the arrangement of carbon nanotubes, such as a lattice defect, caused by intrusion of unnecessary atoms as impurities into the bonds between carbons constituting the carbon nanotubes, lack of necessary carbon atoms, or misalignment. (lattice defect).

Referring to FIG. 3, there is no change in the ID/IG ratio, but since the oxygen functional group is increased, the existing disordered carbon (d band) is changed to a disordered carbon (D band) including an oxygen functional group. Means.

Therefore, it can be confirmed that the acid treatment process is formed by replacing the existing defect position with an oxygen functional group without damaging the lattice structure of the carbon nanotube.

For this reason, it can be seen that when used as a membrane electrode assembly of a fuel cell, it can be used as a catalyst support through the attachment of platinum (Pt) while maintaining constant electrical conductivity of CNTs.

For the carbon nanotubes thus cleaned, a process of attaching platinum particles, which is metal nanoparticles, may be performed (s10). Platinum may be used as a catalyst for the oxidation and reduction reactions of the proton exchange membrane fuel cell. Examples of methods for synthesizing a catalyst such as platinum include impregnation, solver thermal, sputtering, nanocapsule, and polyol methods.

Among them, the polyol method, one of the most popular synthetic methods, can properly control the size of metal catalyst particles, and the arrangement of catalyst particles is fixed without the use of a surface active agent, a surfactant, or a stabilizer. There is an advantage that you can do it. In order to effectively induce the oxidation of hydrogen and the reduction of oxygen in the oxidation and reduction reactions of the proton exchange membrane fuel cell, platinum particles, which are the mediators for this, must be uniformly dispersed on the carbon support to ensure smooth operation of the proton exchange membrane fuel cell In other words, in the polyol process using ethylene glycol (C ₂ H ₄ (OH) ₂ , ethylene glycol), the metal precursor ions in the metal precursor (H ₂ PtCl ₆ -6H ₂ O) are reduced to a metal colloid, Ethylene glycol is oxidized to glycolic acid. Glycolic acid is deprotonated to a glycol anion in a basic solvent and is adsorbed on the metal surface, thereby playing a role as a catalyst stabilizer and a surface active agent.

Looking at this in more detail, after stirring CNT, 4M HNO ₃ , 4MH ₂ SO ₄ , it is reacted for 0-6 hours. At this time, it is preferable that the molar ratio is 1:1.

At this time, 3 hours is most appropriate as shown in FIG. 2 or 3 in order to obtain an optimal result.

After filtering CNTs with a vacuum filter, sulfuric acid and nitric acid are removed by washing 2-3 times with distilled water (DI water). A Pt solution was prepared by dissolving H ₂ PtCl ₆ -6H ₂ O in 20 ml of distilled water so that the weight ratio of Pt to CNT was 20 wt% (for 20 wt% Pt/CNT).

CNTs with functional groups are added to 200 ml of ethylene glycol, and then CNTs are dispersed through ultrasonic dispersion treatment.

Adjust the pH to 10 using 1M NAOH solution, slowly mix the Pt solution while stirring, and react for 1 hour to prepare a Pt/CNT catalyst.

After the reaction is completed, the Pt/CNT catalyst is dried in a vacuum oven for 4 hours.

Because of the role of ethylene glycol, the spacing of platinum (Pt) particles as metal particles in the proton exchange membrane fuel cell membrane electrode assembly 100 according to an embodiment of the present invention is maintained, and platinum as metal particles according to the concentration of glycol anions It may be possible to control the size of the particles.

In addition, since the glycol anion concentration increases as the pH increases, the spacing and size of the platinum particles, which are metal particles, can be controlled in a desired direction through a method of adjusting the pH.

4 is a photograph showing the adsorption state of Pt nanoparticles according to the acid treatment on the CNT surface according to an embodiment of the present invention.

Referring to FIG. 4, it is important to have high density and dispersibility in the catalyst support because the density of Pt nanoparticles on the CNT surface is proportional to the amount of oxygen reduction reaction (ORR). The density of oxygen functional groups increased with the acid treatment time directly affects the formation of Pt nanoparticles, and the more functional groups, the higher the dispersion and higher density Pt/CNT electrodes can be manufactured.

A catalyst slurry was prepared by dispersing the thus prepared carbon support (carbon black or carbon nanotube) on which platinum nanoparticles were formed in isopropyl alcohol and nafion solution (5wt%, Dupont.Co). , The catalyst slurry may be directly sprayed on a polymer electrolyte membrane (s30, s40).

Here, MEA-a may use Pt/CB (a platinum catalyst attached to carbon black) as the anode 120 and the cathode 140.

In addition, MEA-b may use Pt/CB (a platinum catalyst attached to carbon black) for the anode 120, and Pt/CNT (a platinum catalyst attached to carbon nanotube) for the cathode 140.

Details of this are shown in Table 2 shown below.

Composition of anode and cathode in Membrane electrode assembly MEA Catalyst Pt loading
(mg/cm ² ) Catalyst: Nafion
(weight ratio) Anode Cathode Anode Cathode Anode Cathode MEA-a Pt/CB Pt/CB 0.4 0.4 3: 1 3: 1 MEA-b Pt/CB Pt/CNT

Here, as shown in FIG. 1, by combining the first and second gas diffusion layers 110 and 150, a membrane electrode assembly for a proton exchange membrane fuel cell may be manufactured (s40).

The membrane electrode assembly 100 for a proton exchange membrane fuel cell (PEMFC) manufactured as described above operates by supplying hydrogen (H ₂ ) and air, which are fuel gases, through the _first and second gas diffusion layers 110 and 150, respectively. Can be done.

At this time, the normal operation of the proton exchange membrane fuel cell is that after humidification at 100% relative humidity, the anode 120 and the cathode 140 have a stoichiometric ratio (SR) of 1.5 and 2.0. Can supply (anode SR =1.5, cathode SR =2.0)

The evaluation of the operation of the electrode assembly 100 for a proton exchange membrane fuel cell may be performed in a single cell state.

Here, the single cell refers to a cell in which only one anode 120 and one cathode 140 are coupled to one polymer electrolyte membrane 130.

The electrode assembly 100 for the proton exchange membrane fuel cell is stabilized for 24 hours, and the initial performance of the proton exchange membrane fuel cell (PEMFC) is a polarization curve, electrochemical impedance spectroscopy (EIS), and circulating voltage. It can be measured by the amperometric method (at this time, the potential range is 0.08~1.2V, and the scan speed can be performed at 30mV/s).

Electrochemical impedance spectroscopy (EIS) was performed in the range of 10kHz-0.1Hz with a current density of 400㎃/㎠, and CV (Cyclic voltammetry) measurements were performed in the potential range of 0.08~1.2V with a scanning speed of 30mV/s. .

Cyclic voltammetry is one of the methods that can directly grasp what kind of reaction occurs on the electrode surface, and is widely used. More specifically, the method of recording the current flowing when the potential is changed in proportion to time as a current-potential curve is called the potential sweep method, and the case of repeatedly scanning this several times is called the cyclic voltage current method.

Cyclic voltammetry is the electricity generated on the electrode surface or near the electrode surface by applying a voltage to a working electrode (cathode, 140) as a circulating potential in the presence of a species capable of oxidation/reduction reaction in the electrolyzer, and responding to the current. It is a method by which the thermodynamic and kinetic parameters of a chemical reaction can be determined. The current potential curve obtained by such a cyclic voltammetry is called a cyclic voltammetry curve, and in the fields of inorganic chemistry, organic chemistry, polymer chemistry, and biochemistry, the type of redox pair, redox potential, equilibrium constant, concentration, electrode reaction. It is used to find the number of electrons involved in, the rate constant of the chemical reaction, the adsorption phenomenon, and the kinetic parameters of the electron transfer reaction.

The CV curve consists of a total of four sections, a section for increasing the cathode current (step 1), a section for reducing the cathode current (step2), a section for increasing the anode current (step 3), and a section for reducing the anode current ( step4) corresponds to that.

In an embodiment of the present invention, hydrogen (H ₂ ) is supplied to the anode 120, which is a reference electrode, at 20 sccm (standard cubic centimeter per minute), and nitrogen (N ₂ ) is supplied to the working electrode 140. The experiment can be performed under the condition of supplying 100 sccm.

In order to induce a CR (Cell Reversal) state, the proton exchange membrane fuel cell is obtained by lowering the stoichiometric ratio of hydrogen (H ₂ ) of the anode 120, which is the cathode, from 1.5 to 1.0 while maintaining a current density of 400 mA/㎠. The potential state of is changed to the (-) state.

When the potential of the proton exchange membrane fuel cell reaches -1.0V, it returns to the open circuit state, and the amount of H2 and air supplied to the cathode is converted into a stoichiometric ratio of the anode 120 and the cathode 140, which are normal operating conditions. ratio, SR) Hydrogen and air are supplied at a ratio of 1.5 and 2.0. After that, 100 times the operation of changing the hydrogen supply condition of the cathode to induce the CR state from 1.5 to 1.0 was performed.

Before performing the operation to induce the CR state as described above, the initial performance of the proton exchange membrane fuel cell was evaluated using various methods.

And after performing the operation to induce the CR state 100 times, the performance was evaluated again.

5 and 6 are transmission electron micrographs at low and high magnifications of carbon nanotubes to which Pt particles are adsorbed, respectively.

5 and 6, it can be seen that platinum (Pt) particles are well dispersed on the carbon nanotubes. At this time, it was confirmed that the average diameter of the carbon nanotubes was 18 mm and the average diameter of the platinum particles was 6.3 nm.

The platinum nanoparticles are uniformly dispersed after the carbon nanotubes are acid-treated with nitric acid and sulfuric acid, and then -CO, -C=O, and -C-OH are attached to platinum using ethylene glycol. It is believed that this is because the precursor of H ₂ PtCl ₆ -6H ₂ O is well reduced and attached.

At this time, it was confirmed that the bonding strength of platinum and carbon nanotubes was strong enough to withstand ultrasonic treatment for several hours.

7(a), (b), (c), and (d) are (a) the anode of MEA-a in the membrane electrode assembly 100 for a proton exchange membrane fuel cell measured at a scan speed of 30 mV/s, ( b) a cathode, (c) the anode of MEA-b and (d) a graph showing the CV after inducing the prepared state (black) and the CR state once (red) for the cathode.

Referring to Figure 7 (a), (b), (c), (d), while the carbon support CB or carbon nanotubes (CNT) are electrochemically oxidized, incomplete oxidation of carbon is (Scheme 2) CO can be produced by

[Scheme 2]

_{C + H 2 O → CO +} 2H + + 2e -

The generation of carbon monoxide (CO) is observed in the anodes of MEA-a and MEA-b (Fig. 4 (a) and (c), respectively) compared to the cathode of MEA-b (Fig. 4 (d)). It can be (indicated by red circles and A in Fig. 4 (a), (b), (c)).

This difference is due to defects and chemical functional groups on the surface of carbon black, which is a carbon support of a membrane electrode assembly (MEA) for a proton exchange fuel cell.

Such a defect provides an active site that reacts with water (H ₂ O) to form CO ₂ or CO, and the COR (carbon oxidation reaction) induced during the transition state of CR is a carbon support (carbon black or carbon Nanotubes) can further accelerate the aggregation and sequestration of Pt particles.

As a result, the Pt particles formed on the carbon black significantly decrease the electrochemical surface area, which leads to rapid deterioration of battery performance.

Even from the CV (circulating voltage current) measurement results, oxidation occurred very slowly in the carbon nanotubes mounted on the cathode 140 of the membrane electrode assembly (MEB) for a proton exchange membrane fuel cell. It can be seen that the surface area did not change after inducing the CR state.

Judging from this, it can be seen that the carbon nanotubes have high crystallinity and low defect density, and thus do not provide active sites for carbon oxidation (COR).

That is, in the case of CNTs, since defects exist at the edge of a tube having a very large aspect ratio, oxidation of CNTs can be effectively suppressed. From this, the high oxidation resistance of CNTs can suppress oxidation of the cathode 140 even when the CR state is induced during the operation of the proton exchange membrane fuel cell electrode assembly (MEA-b) according to an embodiment of the present invention. .

FIG. 8 is a graph showing changes in cell voltage after performing the CR state of the membrane electrode assembly (MEA-a) and MEA-b for a proton exchange membrane fuel cell 100 times according to an embodiment of the present invention. to be.

Referring to FIG. 8, it can be seen that the polarization curves of the membrane electrode assemblies of MEA-a and MEA-b are shown to determine whether the carbon support is oxidized after inducing the CR state 100 times. have.

As shown in FIG. 8, it can be seen that the performance of the electrode assembly was significantly reduced in MEA-a through the process of inducing the CR state 100 times. This is presumed to be due to the oxidation of carbon black that can serve as a catalyst caused by insufficient hydrogen supplied.

In contrast, in the membrane electrode assembly for a proton exchange membrane fuel cell, MEA-b was able to maintain its initial performance even after inducing a CR state of 100 times.

These results indicate that carbon nanotubes have electrochemical durability because they do not have defects due to structural characteristics.

9(a), (b), (c), and (d) are (a) the anode of MEA-a in the membrane electrode assembly 100 for a proton exchange membrane fuel cell measured at a scan speed of 30 mV/s, ( b) A graph showing the CV of the cathode, (c) the anode of MEA-b and (d) the cathode of the prepared state (black) and the CV after inducing the CR state 100 times (red)

Referring to FIG. 9, similar to FIG. 7, it can be seen that the cathode of MEA-b shows the same aspect as the initial CV curve even after CR 100 times.

Table 3 is a table showing the results of ECSA loss, Pt particle size change, cell potential, electrical resistance, and polarization resistance when CR induction was not performed.

Comparison Table of Physical Properties of Membrane Electrode Assembly for Proton Exchange Membrane Fuel Cell MEA Electrode
(catalyst) ECSA loss
(%) Pt size
(nm) Cell potential
at 480 mA/cm ² (V) Ohmic
R (mΩ) Polarization
R (mΩ) Before After Before After Loss (%) Before After Before After MEA-a Anode
(Pt/CB) 61.6 2.7 6.7 0.511 0.369 27.8 21 26 49 55 Cathode
(Pt/CB) 41.4 2.7 5.6 MEA-b Anode
(Pt/CB) 64.4 2.7 7.2 0.495 0.487 1.6 21 21 49 51 Cathode
(Pt/CNT) 6.2 6.3 6.3

Referring to Table 3, ECSA decreased in the case of MEA-a (Pt/CB).

On the other hand, the ECSA of the cathode 140 of MEA-b hardly changed.

In the amount of change in cell potential, the small change in MEA-b is estimated to be due to the stability of the binding of CNTs to Pt in the cathode 140 in the electrode assembly for a proton exchange membrane fuel cell.

The hydrogen oxidation reaction (HOR) is generally much faster than the oxygen reduction reaction (ORR), and the oxygen reduction reaction is a rate limiting step (also known as the rate-limiting step) for the performance of a proton exchange membrane fuel cell (PEMFC), which is greater for cell performance. affect.

Therefore, as shown in Table 3, even if the performance of the hydrogen oxidation reaction (HOR) is deteriorated, the carbon nanotubes 140 bonded with platinum, which can serve as a catalyst for the oxygen reduction reaction (ORR), are stable. It can be seen that the performance of b was maintained at its initial state.

In the MEA-b membrane electrode assembly 100, after 100 reverse voltage cycles, the cell polarization voltage (cell potential) changes from 0.495V to 0.487V, so that the voltage decrease is less than 5%, and the electrochemical surface area (ECSA) decreases (loss). Is 6.2%, less than 10%, and the increase in ohmic resistance (21→21 mΩ) and polarization resistance (49→51 mΩ) is less than 5%, showing superior performance compared to MEA-a membrane electrode assembly.

As described above, the electrical characteristics of the membrane electrode assembly (MEA) for a proton exchange membrane fuel cell are changed when the catalyst layer undergoes carbon oxidation.

In order to check the electrical characteristics of the membrane electrode assembly (MEA) for a proton exchange membrane fuel cell under transient conditions such as CR or start/stop, the impedance was measured by EIS under the condition of a current density of 400 mA/㎠.

10(a) and (b) are diagrams showing Nyquist plots before (black) and after (red) CR of MEA.

10(a) and (b), a high frequency arc represents the total resistance (sum of ohmic resistance and polarization resistance) of a proton exchange membrane fuel cell (PEMFC), and a medium frequency arc arc) indicates that it is caused by activation polarization and proton transfer. At this time, the current density was 400 mA/cm 2.

Therefore, the increase in both ohmic resistance and polarization resistance in the membrane electrode assembly (MEA-a) for a proton exchange membrane fuel cell in the transition state of the CR state is believed to be due to the poor electrical contact between the CB support and the Pt nanoparticles. (See Fig. 8(a)).

Compared with this MEA-a, MEA-b maintained the initial resistance as shown in FIG. 8(b) (see FIG. 10(b)).

In MEA-a and MEA-b, the analysis of the structure of Pt particles after 100 times of performing the CR state can also be confirmed by XRD (X-Ray Diffraction).

11 (a) and (b) are XRD graphs of (a) an anode of MEA-a (anode, 120), a cathode (cathode, 140), and an anode of MEA-b according to an embodiment of the present invention, ( b) is an XRD graph of the MEA-b cathode (cathode, 140). Here, a) XRD pattern CR 100 times of as-prepared Pt/C (black), the anode of MEA-a (green), the cathode of MEA-a (blue) and the anode of MEA-b (red). (b) XRD patterns of the as-prepared Pt/CNT (black) and the cathode (red) of MEA-b after 100 times of CR are shown.

및 3.456

이었다.Referring to FIG. 11, the XRD peak near 2θ = 26° represents the (0002) crystal plane of CB and CNT (JCPDS card no: 23-0604). (002) XRD peaks at 2θ = 25.04° (Pt/CB) and 2θ = 25.76° (Pt/CNT) are 3.553, respectively.

And 3.456

Was.

This result confirms that the crystal planes in CNT are more densely stacked than the crystal planes of CB because of their high crystallinity.

After 100 CR cycles, the strength of the (002) side of Pt/CB (anode and cathode) was significantly reduced due to the carbon oxidation reaction (COR) when compared to the as-prepared (if not subjected to CR).

Compared with Pt/CB, the (002) peak intensity of Pt/CNT was similar to the (002) peak intensity after as-prepared Pt/CNT and CR 100 times (Fig. 9(b)).

After 100 CR, the Pt peak in Pt/CB increased significantly, indicating that the Pt particles had a regular arrangement.

As described in Example 1 of the present invention, the Pt particles chemically synthesized through ethylene glycol are grown through electrochemical recrystallization after repetition for the transition state of CR to form Pt nanoparticles having well-arranged large-sized particles. Can be generated.

As described above, in the present invention, it has been described by specific matters and limited embodiments and drawings, which are provided to help the overall understanding of the present invention, but the present invention is not limited to the above embodiments, and the present invention Various modifications and variations are possible from those who have ordinary knowledge in the field to which they belong.

Accordingly, the spirit of the present invention is limited to the described embodiments and should not be defined, and all things having equivalent or equivalent modifications to the claims as well as the claims to be described later fall within the scope of the spirit of the present invention. .

Claims (12)

In the membrane electrode assembly for a proton exchange membrane fuel cell,
A cathode and an anode positioned to face each other; And
Including a polymer electrolyte membrane positioned between the cathode and the anode,
The cathode includes a platinum/carbon nanotube catalyst in which platinum (Pt) is supported on carbon nanotubes,
Carbon nanotubes (CNT) are prepared by treating a mixture of 4M sulfuric acid ((H ₂ SO ₄ ) and 4M nitric acid (HNO ₃ ) 1:1 for 3 hours,
The carbon nanotubes contain oxygen functional groups of C-OH (hydroxy), -CO (acyl) and -C=O (carbonyl), and the ratio of C-OH is the highest among the oxygen functional groups,
After 100 reverse voltage cycles, the electrochemical surface area (ECSA) is reduced by 10% or less.
Membrane electrode assembly for proton exchange membrane fuel cells. delete delete delete The method of claim 1,
The carbon nanotubes in the membrane electrode assembly,
Before and after acid treatment, characterized in that the ID / IG ratio on the Raman spectrum is constant
Membrane electrode assembly for proton exchange membrane fuel cells. The method of claim 1,
The membrane electrode assembly, characterized in that the cell polarization voltage (cell potential) decreases by 5% or less after 100 reverse voltage cycles.
Membrane electrode assembly for proton exchange membrane fuel cells. delete The method of claim 1,
The membrane electrode assembly is characterized in that, after 100 reverse voltage cycles, an increase in ohmic resistance and polarization resistance is 5% or less.
Membrane electrode assembly for proton exchange membrane fuel cells. In the method for producing a catalyst for a proton exchange membrane fuel cell,
(a) attaching an oxygen functional group to the carbon nanotube (s10);
(b) dispersing the carbon nanotubes to which the oxygen functional group is attached in a liquid reducing agent (s20);
(c) preparing a platinum (Pt) solution by dissolving a platinum precursor in a solvent (s30);
(d) mixing the platinum solution with the liquid reducing agent in which the carbon nanotubes are dispersed, and reacting to prepare a platinum/carbon nanotube catalyst (s40),
The oxygen functional group is C-OH (hydroxy), -CO (acyl) and -C=O (carbonyl),
Attaching the oxygen functional group to the carbon nanotube (s10),
Carbon nanotubes (CNT) were treated with a mixture of 4M sulfuric acid ((H ₂ SO ₄ ) and 4M nitric acid (HNO ₃ ) 1:1 for 3 hours,
A membrane electrode assembly to which a catalyst for a proton exchange membrane fuel cell is applied has an electrochemical surface area (ECSA) reduction of 10% or less after 100 reverse voltage cycles.A method for producing a catalyst for a proton exchange membrane fuel cell.
delete delete delete

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