KR100918867B1 - Membrane-electrode assemblies comprising proton conducting electrolyte and ionomer binder based on ionic liquid impregnated polymer, method of manufacturing thereof, and fuel cells using the same - Google Patents

Abstract

The present invention comprises the steps of preparing a polymer copolymer containing an ionic liquid; Preparing an electrolyte membrane and an ionomer binder using an ionic liquid-containing polymer copolymer; Preparing a catalyst slurry and an electrode; And a membrane-electrode assembly manufacturing method, a membrane-electrode assembly manufactured according to the above, and a method of manufacturing a membrane-electrode assembly comprising the steps of heating and compressing the electrolyte membrane and the gas diffusion material coated with the electrode.

The polymer electrolyte membrane and the ionomer binder including the ionic liquid according to the present invention can be manufactured in a thermally stable and flexible film form up to 230 to 320 ° C., which can be prepared as a membrane-electrode assembly and finally used in PEMFCs.

Since the conductivity of the polymer electrolyte and the ionomer binder according to the present invention can be adjusted according to the concentration of the ionic liquid, the tritic acid and the plasticizer, it is possible to maximize the performance of the membrane-electrode assembly and the fuel cell.

Description

Membrane-electrode assemblies comprising proton conducting electrolyte and ionomer binder based on ionic liquid impregnated polymer, method of manufacturing etc, and fuel cells using the same}

The present invention relates to a proton conductive electrolyte and a fuel cell including the same.

A fuel cell is an apparatus for producing electrical energy by electrochemically reacting fuel and oxygen. Depending on the type of electrolyte used, a fuel cell may include a proton exchange membrane fuel cell (PEMFC), a direct methanol fuel cell (DMFC), and a phosphate fuel cell ( It is divided into PAFC (phosphoric acid fuel cell), molten carbonate fuel cell (MCFC) and solid oxide fuel cell (SOFC).

PEMFC is a fuel cell using a proton conductive polymer electrolyte membrane as an electrolyte, and typically includes an anode (fuel electrode), a cathode (oxidant electrode), and a polymer electrolyte membrane disposed between the anode and the cathode.

As a fuel supplied to the anode of PEMFC, hydrogen, a hydrogen containing gas, etc. are used normally. The oxidant supplied to the cathode of the PEMFC is typically oxygen, oxygen containing, gas or air.

At the anode of the PEMFC, the fuel is oxidized to produce hydrogen ions and electrons. Hydrogen ions are transferred to the cathode through the electrolyte membrane, and electrons are transferred to the external circuit (load) through the conductor (or current collector). In the cathode of the PEMFC, water is generated by combining hydrogen ions transferred through the electrolyte membrane, electrons transferred from an external circuit through a conductor (or current collector), and oxygen. At this time, the movement of electrons via the anode, the external circuit and the cathode is power.

In membrane-electrode assemblies for PEMFCs, polymer electrolyte membranes serve as ion conductors for the migration of hydrogen ions from anode to cathode, as well as separators and electronic insulators that block mechanical contact between the anode and the cathode. It also acts as an electron-insulator. In addition, the ionomer provider for the PEMFC is contained in the electrode for the effect of improving the adhesion compatibility between the electrolyte membrane and the electrode, maintaining the binding between the electrode catalyst particles and forming a three-dimensional structure to make a three-phase interface, The same kind of thing can be used to maximize this effect.

As the material of the polymer electrolyte membrane, a polymer electrolyte such as a sulfonate and a fluorinated polymer having a main chain composed of fluorinated alkylene and a side chain composed of fluorinated vinyl ether having a sulfonic acid group at its terminal has been used (for example, Nafion: trademark of Dupont). Such a polymer electrolyte membrane exhibits excellent ion conductivity by impregnating an appropriate amount of water.

However, in PEMFC employing a polymer electrolyte membrane, when the protons generated at the anode move to the cathode, they are accompanied by water by osmotic drag, so that the anode side of the polymer electrolyte membrane is dried, thereby proton conductivity of the polymer electrolyte membrane Decreases rapidly and, in severe cases, the PEMFC becomes inoperable. In addition, when the operating temperature of the PEMFC is a high temperature of about 80 to 100 ° C. or more, the drying of the polymer electrolyte membrane is intensified due to the evaporation of water from the polymer electrolyte membrane, and thus the proton conductivity of the polymer electrolyte membrane is lowered more drastically. In other words, the performance of the membrane-electrode assembly employing the same rapidly decreases.

Ionic liquids are used in various fields because of their high ionic conductivity, the presence of a wide temperature range, environmental friendliness, a wide range of electrochemical stability, and excellent thermal stability. In particular, it has been proposed as a fuel cell electrolyte due to high ionic conductivity. In other words, the ionic liquid solvates naked protons to act as a carrier.

However, in use, the liquid state has a problem of water leakage and unsuitable for carrying and miniaturization. As a fuel cell electrolyte, a membrane form is preferable. The electrolyte in the form of a membrane can be obtained by incorporating heterogeneous ionic liquids into the polymer. However, this results in degraded ionic conductivity.

In the polymer electrolyte fuel cell, much research has been conducted on the technique of containing an ionic liquid in Nafion, which is a commercially available electrolyte membrane. The ionic liquid exhibits a property that the ionic conductivity is maintained even after several heating and cooling cycles without degrading the physical properties of the electrolyte membrane at the temperature rise. Ionic conductivity depends on the equilibrium weight of the membrane, and ionic conductivity of at least 0.1 S cm ⁻¹ at 180 ° C. has been reported for Nafion containing 1-butyl-3-methylimidazolium triromethanesulfonate ionic liquid have.

It has been reported that protic ionic liquids obtained by a combination of Bronsted bases and superacids conduct protons in an anhydrous environment. In addition, fuel cell electrolytes have been proposed in a humidified environment at elevated temperatures due to the transfer of protons between ammonium cations in room temperature ionic liquids obtained by various quaternary amines. Ion gels based on Bronsted acid-base ionic liquids have also been used as proton conductive non-aqueous electrolytes in fuel cells with a maximum current density of 3.14 mA cm ⁻² at 130 ° C.

Ionic liquids at room temperature, such as 1-butyl-3-methylimidazolium tetrafluoroborate / hexafluorophosphate, have been used in alkaline fuel cells, but there is a problem that efficiency decreases with increasing temperature. Although fluorinated anion (HF) _n F ^- has been reported to be electrochemically active, its use is limited by the release of HF as the temperature rises.

US 2004/0038105 claims a proton conductive solid ceramic membrane. However, there is a problem that the solid ceramic film is not flexible. Thus, ultimately, it becomes impossible to manufacture the membrane-electrode assembly used in the fuel cell.

The inventors have studied the synthesis of ionic liquids containing 2,3-dimethyl-1-octylimidazolium cation (DMOIm ⁺ ) and anions and polymer electrolytes comprising other polymers. The copolymer polymer PVdF-HFP showed the highest proton conductivity here. Thus, the search continued to find other ionic liquids for the polymers.

It is an object of the present invention to provide a membrane-electrode assembly comprising a ionic liquid-containing polymer electrolyte and ionomer provider that is thermally stable up to a temperature of 230 to 320 ° C. and can be used in a humidified environment.

In addition, in the present invention, in order to improve the proton conductivity of the electrolyte in the membrane-electrode assembly, a means for providing free ions and preventing aggregation of ions is used.

The polymer electrolyte may be manufactured in the form of a flexible film (membrane) and employed in a membrane-electrode assembly to be used in a fuel cell.

The present invention relates to a membrane-electrode assembly comprising a polymer electrolyte and an ionomer binder used as a proton conductor and a fuel cell using the same.

Specifically, the present invention includes a polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP) copolymer containing 2,3-dimethyl-1-octylimidazolium trifluoromethane sulfonate (DMOImTf) Provided is a membrane-electrode assembly. The copolymer may be prepared in the form of a membrane to be prepared in the form of an electrolyte and a solution of a polymer fuel cell and used as an ionomer binder.

Method for producing a membrane-electrode assembly according to the present invention comprises the steps of 1) preparing a polymer copolymer comprising an ionic liquid; 2) preparing an electrolyte membrane and an ionomer binder using an ionic liquid-containing polymer copolymer; 3) preparing a catalyst slurry and an electrode; And 4) preparing a membrane-electrode assembly including the electrolyte membrane and the ionomer binder.

The fuel cell according to the present invention is manufactured according to the method for producing a membrane-electrode assembly, and includes a membrane-electrode assembly having a polymer electrolyte membrane containing an ionic liquid and an ionomer binder.

In addition, the fuel cell according to the present invention includes a membrane-electrode assembly, and supplies one or more fuels selected from hydrogen-containing gas series of hydrogen, reformed gas, and mixed hydrogen gas to the anode, and supplies oxygen, air, and mixed oxygen gas. It is characterized by supplying at least one oxidant selected from the oxygen-containing gas series group to the air electrode.

Hereinafter, each step of the method for manufacturing a membrane-electrode assembly according to the present invention will be described in detail.

1) preparing a polymer copolymer containing an ionic liquid

The DMOImTf is an ionic liquid at room temperature. As the temperature is increased (from 5 ° C. to 80 ° C.), the viscosity decreases while the conductivity increases. The ionic liquid is preferably contained in the polymer copolymer in the range of 10 to 80 parts by weight based on 100 parts by weight of PVdF-HFP. The conductivity of the polymer electrolyte depends on the concentration of the ionic liquid. Most preferably, in the present invention, DMOImTf is used at the same weight as PVdF-HFP.

The polymer electrolyte of the present invention may further include triflic acid (HCF ₃ SO ₃ ). The triflic acid increases the conductivity of the electrolyte until it is included in 3M concentration with respect to the ionic liquid DMOImTf. This raises the conductivity by providing more free ions (H ⁺ and CF ₃ SO ₃ ⁻ ) to the conductivity of the reduced ionic liquid due to the polymer.

The polymer electrolyte of the present invention may further include a plasticizer. Ionic liquids generally consist of ions and aggregates of ions. If triic acid is added here, more ions will be present in the polymer electrolyte. As a result, one ion is surrounded by ions of opposite charge to form an ion cluster. This does not contribute to conductivity. Plasticizers are those that break down ion clusters to improve conductivity. As the plasticizer, one having a high dielectric constant (64.4) such as propylene carbonate (PC) is used.

However, the effect of the plasticizer is relatively small compared to the effect that the ion cluster is self-decomposing as the temperature increases, the effect of the plasticizer on the conductivity decreases as the temperature increases. In addition, the plasticizer is preferably included in 21% by weight or less of the total weight of the copolymer. This is to prevent deterioration of the physical properties of the electrolyte membrane due to the addition of the plasticizer.

The polymer electrolyte of the present invention is thermally stable up to a temperature of 320 ° C or lower.

However, the addition of triflic acid reduces the thermal stability. Specifically, it was observed that pyrolysis starts at 230 ° C. when 0.5 M of tritic acid was included.

2) preparing an electrolyte membrane and an ionomer binder using an ionic liquid-containing polymer copolymer

The polymer copolymer containing the ionic liquid is prepared using a solvent acetonitrile to prepare a polymer solution having 5 to 30 parts by weight of polymer solids based on 100 parts by weight of the polymer solution. The polymer solution is cast on a polypropylene support, and then an electrolyte membrane is formed by solution extrusion at room temperature. An ionomer binder having 1 to 10 parts by weight of polymer solids is prepared based on 100 parts by weight of the polymer solution using the polymer solution preparation method.

3) preparing catalyst slurry and electrode

After distilled water was wetted onto the catalyst, the ionomer binder solution prepared in step 2) was mixed, stirred for 10 minutes, mixed and stirred for 1 hour with ultrasonic waves, and further stirred for 3 days to prepare a catalyst slurry. .

The slurry is prepared by adding 1 to 40 parts by weight of the ionomer binder solution and 100 to 1000 parts by weight of isopropyl alcohol with respect to 100 parts by weight of the catalyst. In the above catalyst, any one or more selected from Pt / C and Pt-Ru / C may be used.

The prepared catalyst slurry is applied to the surface of the PTFE treated gas diffusion layer containing 1 to 30 parts by weight of polytetrafluoroethylene (PTFE) based on 100 parts by weight of the gas diffusion material. Such catalyst slurry is applied to the surface of the gas diffusion material by spraying or using a coating machine to prepare an electrode in which the ionomer slurry is applied to the surface of the electrolyte membrane.

4) preparing a membrane-electrode assembly comprising the electrolyte membrane and the ionomer binder

Wherein 2) the in the electrolyte membrane on both the surfaces prepared in Step 3) is a gas diffusion material produced by the electrodes are applied the electrode part of the step and the adhesive to be positioned in the electrolyte membrane surface, by using a heat press of 20 to 60 ^o C After heat-pressing at a temperature and a pressure of 0.1 to 100 kgf / cm ² for 0.01 to 0.5 hours, a membrane-electrode assembly may be manufactured by firmly attaching an electrolyte membrane and a gas diffusion material coated with electrodes.

The polymer electrolyte and ionomer binder comprising the ionic liquid according to the invention are thermally stable up to 230 to 320 ° C. In addition, it can be manufactured in the form of a flexible film can be produced as a membrane-electrode assembly can be finally used in PEMFCs.

Since the conductivity of the polymer electrolyte and the ionomer binder according to the present invention can be adjusted according to the concentration of the ionic liquid, the tritic acid and the plasticizer, it is possible to maximize the performance of the membrane-electrode assembly and the fuel cell.

FIG. 1 shows the conductivity of DMOImTf and PVdF-HFP-containing polymer electrolytes according to the concentration of triflic acid.

2 shows the dependence of the ionic liquid on the conductivity of the polymer electrolyte.

3 shows the effect of a plasticizer on the conductivity of the polymer electrolyte.

4A to 4C show narrowing peaks of ¹ H and ¹⁹ F NMR of the ionic liquid and the polymer electrolyte membrane.

5A and 5B show DSC-TGA-DTG measurement results for the polymer electrolyte membrane.

6 shows changes in cell voltage and voltage density with respect to the current density of a unit fuel cell in a humidified environment.

Example 1

1-bromooctane (Merck,> 98%) and 1,2-dimethylimidazole (Merck,> 98%) were reacted to give 2,3-dimethyl-1-octylimidazolium bromide, followed by lithium tri An ionic liquid 2,3-dimethyl-1-octylimidazolium trichloromethane sulfonate (DMOImTf) was obtained by anion exchange reaction using a plate (Aldrich, 96%).

Residual water was removed by azotropic distillation using benzene, and residual benzene was removed under reduced pressure.

The ionic liquid DMOImTf was impregnated in PVdF-HFP (Fluka), and an electrolyte membrane was formed by solution extrusion using acetonitrile (SRL, 99.5%) as a solvent.

Examples 2-5

An electrolyte membrane was formed in the same manner as in Example 1 except that HCF ₃ SO ₃ (Aldrich, 98%) was additionally included in the ionic liquid DMOImTf at concentrations of 0.5M, 1.0M, 2M, and 3M, respectively.

Example 6

An electrolyte membrane was formed in the same manner as in Example 4 except that the polypropylene carbonate was further included in the polymer electrolyte in an amount of 21% by weight.

Experimental Example

(1) conductivity and dielectric constant

 Hioki 3532-50 LCR HiTester and HP4284A precision LCR meter were used to measure the conductivity and dielectric constant of the polymer electrolyte membranes obtained in Examples 1 to 3.

Figure 1 is a graph showing the conductivity of the polymer electrolyte of Examples 1 to 5 measured. In other words, it can be seen that the conductivity of the electrolyte increases as the concentration of triflic acid in the ionic liquid increases.

2 shows the conductivity in the electrolytes of Examples 1 and 3 compared to that of the ionic liquid itself. It can be reconfirmed that the conductivity of the electrolyte membrane including DMOImTf and PVdF-HFP increases with the concentration of triflic acid.

3 is a measurement of the conductivity of the polymer electrolyte of Example 4 and Example 6. As temperature increases, the effect of the addition of plasticizer on the conductivity decreases.

(2) NMR

¹ H and ¹⁹ F solid NMR were taken to measure the mobility of protons and anions in the polymer electrolyte membrane. Images were taken at Larmor frequencies 271.25 and 255.20 MHz in the temperature range of 100-350K.

Broad peaks were observed in the NMR spectrum at 100-180K. This is a characteristic of a solid lattice in which ions do not diffuse. However, it was observed that as the temperature rose, the characteristics of the flow grating appeared and the peak narrowed. This is a phenomenon which occurs in the vicinity of the glass transition temperature T _g and the melting point of the polymer film.

4A shows the narrowing of the ¹ H and ¹⁹ F NMR peaks of the ionic liquid DMOImTf. Peak narrowness was observed at ¹ H NMR at 140 K, and peak narrowing was observed at 180 K in ¹⁹ F NMR. At 280K, the broad peaks in both spectra disappear, leaving only a narrow peak. The peak narrowing at 140K and 180K occurs near the glass transition temperature of the electrolyte, and the phenomenon at 280K occurs near the melting point of the electrolyte.

4B shows the narrowing of the ¹ H and ¹⁹ F NMR peaks of the polymer electrolyte of Example 1. FIG. Peak narrowing was seen at 220K. In other words, the addition of an ionic liquid to the polymer resulted in an improvement in the glass transition temperature.

4C shows the narrowing of the ¹ H and ¹⁹ F NMR peaks of the polymer electrolyte of Example 2. FIG. The peak narrowing phenomenon at 160K is that tritric acid acts as a plasticizer, lowering the glass conduction temperature of the film and causing peak narrowing at relatively low temperatures.

(3) thermal stability

DSC-TGA-DTG was simultaneously measured using the Perkin Elmer System for thermal stability testing of the polymer electrolyte membrane.

In FIG. 5A, the weight loss of the electrolyte membrane of Example 1 was observed at 323 ° C. However, in FIG. 5B, it was observed that the electrolyte membrane of Example 2 started to lose weight at 231 ° C.

(4) Fabrication and performance test of fuel cell

On both sides of the polymer electrolyte membrane and the polymer electrolyte membrane of Example 2, a platinum catalyst layer (manufactured by applying a catalyst ink on a toray carbon paper, a platinum support amount of 0.926 mg cm ^-2 ) was hot pressed to prepare a membrane-electrode assembly (MEA). It was.

The fuel cell was mounted in a fuel cell test apparatus having an active area of 10 cm ² . The test device is a fuel source, a temperature control unit and a fuel supply control device.

6 shows the relationship between the current density, the cell voltage and the power density of a unit fuel cell at 100 ° C. This is measured in a humidified environment and is low compared to fully hydrated Nafion. However, the values are similar to those of similar membranes containing ionic liquids. Moreover, the fuel cell test apparatus is optimally designed for fully hydrated Nafion membranes.

Claims (19)

In the method of manufacturing the membrane-electrode assembly, 1) 10 to 2,3-dimethyl-1-octylimidazolium trifluoromethane sulfonate (DMOImTf), an ionic liquid, based on 100 parts by weight of polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP) Polyvinylidene fluoride-hexafluoropropylene containing 80 parts by weight, containing tricarboxylic acid (HCF ₃ SO ₃ ) at a concentration of 3M or less, and a plasticizer of 21% by weight or less of the total weight of the copolymer (PVdF- Preparing a HFP) copolymer; 2) 5 to 30 parts by weight of the polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP) copolymer is dissolved in 100 parts by weight of a polymer solution in which acetonitrile is a solvent to prepare an electrolyte membrane, and the polyvinylidene fluoride- Preparing an ionomer binder by dissolving 1 to 10 parts by weight of a hexafluoropropylene (PVdF-HFP) copolymer in 100 parts by weight of a polymer solution in which acetonitrile is a solvent; 3) adding 1 to 40 parts by weight of the ionomer binder and 100 to 1000 parts by weight of isopropyl alcohol, and mixing and stirring the mixture, followed by mixing and stirring with ultrasonic waves to prepare catalyst slurry and electrodes; And 4) a membrane-electrode assembly manufacturing method comprising the step of heat-compressing the electrolyte membrane, and the gas diffusion material to which the electrode is applied. delete delete delete delete delete delete delete delete delete delete delete delete The method of claim 1, Wherein said catalyst is selected from at least Pt / C or Pt-Ru / C. The method of claim 1, The gas diffusion material coated with the electrode is prepared by applying the catalyst slurry on the surface of the PTFE treated gas diffusion layer containing 1 to 30 parts by weight of polytetrafluoroethylene (PTFE) based on 100 parts by weight of the gas diffusion material. -Electrode assembly manufacturing method. The method of claim 15, The method of manufacturing a membrane-electrode assembly according to claim 1, wherein a gas diffusion material coated with the electrode is placed on both surfaces of the electrolyte membrane so that the electrode is adhered to the surface of the electrolyte membrane. A membrane-electrode assembly having a polymer electrolyte membrane containing an ionic liquid and an ionomer binder prepared by the method of claim 1. A fuel cell comprising a polymer electrolyte membrane containing an ionic liquid prepared by the method of claim 1 and a membrane-electrode assembly having an ionomer binder. The method of claim 18, The fuel cell is: A fuel electrode for supplying at least one fuel selected from a hydrogen-containing gas series group of hydrogen, reformed gas, and mixed hydrogen gas; And And a cathode for supplying at least one oxidant selected from an oxygen-containing gas series group of oxygen, air, and mixed oxygen gas.