High temperature proton exchange composite membrane
Technical Field
The invention relates to a high-temperature proton exchange membrane, in particular to a composite membrane consisting of a fluorine-containing ionic polymer and a porous membrane.
Background
The proton exchange membrane fuel cell is a power generation device which directly converts chemical energy into electric energy in an electrochemical mode, and is considered as a first choice clean and efficient power generation technology in the 21 st century. Proton Exchange Membranes (PEM) are key materials for Proton Exchange Membrane Fuel Cells (PEMFCs).
The proton exchange membrane of the perfluorosulfonic acid used at present has good proton conductivity and chemical stability at lower temperature (80 ℃) and higher humidity. However, they also have many disadvantages such as poor dimensional stability, low mechanical strength, poor chemical stability, and the like. The water absorption rate and the size expansion caused by water absorption of the membrane are different under different humidity, and when the membrane is changed under different working conditions, the size of the membrane is changed accordingly. Such repetition eventually leads to mechanical breakage of the proton exchange membrane. In addition, the positive electrode reaction of the fuel cell often generates a large amount of strongly oxidizing substances such as hydroxyl radicals and hydrogen peroxide, which attack non-fluorine groups on the film-forming resin molecules, resulting in chemical degradation and breakage of the film, resulting in foaming. Finally, when the operating temperature of the perfluorosulfonic acid exchange membrane is higher than 90 ℃, the proton conductivity of the membrane is drastically reduced due to rapid water loss of the membrane, thereby greatly reducing the efficiency of the fuel cell. But the high operating temperature can greatly improve the carbon monoxide resistance of the fuel cell catalyst. In addition, the existing perfluorosulfonic acid membranes have certain hydrogen or methanol permeability, and particularly in direct methanol fuel cells, the methanol permeability is very high, which is a fatal problem. Therefore, it is a major problem in the fuel cell industry to improve the strength, dimensional stability, and proton conduction efficiency at high temperature of the perfluorosulfonic acid proton exchange membrane and to reduce the permeability of the working medium.
Several approaches have been proposed to address these problems. For example, JP-B-5-75835 uses perfluorosulfonic acid resin to impregnate a porous medium made of Polytetrafluoroethylene (PTFE) to reinforce the strength of the membrane. However, such porous media of PTFE have not been able to solve the above problems because the PTFE material is relatively soft and the reinforcing effect is insufficient. The Gore-Select series composite membrane liquid developed by Gore company adopts a method of filling Nafion ion conductive liquid with porous Teflon (US5547551, US 56565041, US5599614), and the membrane has high proton conductivity and larger dimensional stability, but the Teflon creeps greatly at high temperature, so that the performance is reduced. JP-B-7-68377 also proposes a method of filling a porous medium made of polyolefin with a proton exchange resin, but has insufficient chemical durability and thus has a problem in long-term stability. And because of the addition of the porous medium without proton conductivity, the proton conduction path is reduced, and the proton exchange capacity of the membrane is reduced.
Further, Japanese patent JP-A-6-231779 proposes another reinforcing method using fluororesin fibers. Which employs an ion exchange membrane reinforced with a fibril form of fluorocarbon polymer reinforcement. However, this method necessitates the addition of a relatively large amount of reinforcing material, and in this case, the processing of the film tends to be difficult, and an increase in the film resistance is likely to occur.
While european patent EP0875524B1 discloses a technique for reinforcing nafion membranes with glass fiber membranes prepared by a glass fiber nonwoven technique, in which oxides such as silica are also mentioned. However, the non-woven glass fiber cloth in the patent is a necessary substrate, which greatly limits the range of applications of the reinforcement.
U.S. Pat. No. 6,6692858 discloses the art of polytetrafluoroethylene fiber reinforced perfluorosulfonic acid resins. In this technique, a perfluorosulfonyl fluoride resin and a polytetrafluoroethylene fiber are mixed, extruded, and transformed to prepare a fiber-reinforced perfluorosulfonic acid resin. This method does not allow continuous production due to the time consuming transformation process.
The US RE37701 describes the invention of compounding ionic polymers with expanded tetrafluoroethylene membranes into composite membranes, but the expanded tetrafluoroethylene membranes used by them have a thickness greater than 20 μm and cannot be adapted to the trend of modern fuel cell development and to thinner and stronger membranes. Therefore, the method can not adapt to the use condition of the high-temperature proton exchange membrane fuel cell.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a high-temperature proton exchange composite membrane and a preparation method thereof.
The technical scheme of the invention is as follows:
a proton exchange composite membrane uses a fluorine-containing polymer film with a microporous structure as a carrier, and full fluororesin with an ion exchange function is filled in micropores of a microporous membrane and covers the surface of the microporous membrane, and the thickness of the composite membrane is 5-15 microns.
The fluoropolymer membrane with the microporous structure is a Polytetrafluoroethylene (PTFE) porous membrane, a polyvinylidene fluoride porous membrane (PVDF), a polychlorotrifluoroethylene porous membrane and a polytetrafluoroethylene-ethylene (ETFE) porous membrane.
The thickness of the fluoropolymer film with the microporous structure is 1-20 micrometers, the porosity is 60-97%, and the pore diameter is 0.2-5 micrometers.
The preferable thickness of the fluoropolymer film carrier with the microporous structure is 5-20 micrometers, the more preferable thickness is 5-15 micrometers, and the most preferable thickness is 8-12 micrometers; the preferable porosity is 80-95%; the preferred pore size is 0.5 to 4 microns, and the more preferred pore size is 1 to 3 microns.
Preferably, the thickness of the fluoropolymer film carrier with the microporous structure is 5-15 micrometers, the void ratio is 80-95%, and the pore diameter is 1-3 micrometers.
The fluoropolymer film with the microporous structure can be a unidirectional stretching film or a bidirectional stretching film.
The perfluoro resin with ion exchange function is selected from one or the combination of the following: perfluorosulfonic acid resin, perfluorocarboxylic acid resin, or perfluorophosphoric acid resin. Wherein,
the perfluorosulfonic acid resin has the following general formula:
wherein x is 3 to 15, n is 0 to 2, p is 2 to 5, and the ion exchange capacity is 0.90 to 1.60 mmol/g. The molecular weight is 10-60 ten thousand, preferably 15-30 ten thousand.
The perfluorocarboxylic acid resin has the following structural unit:
in the formula (II), c is 0 or 1, d is 1-5, a is 3-11, b is 1-3, and the ion exchange capacity is 0.85-1.50 mmol/g. The molecular weight is 10-60 ten thousand, preferably 15-30 ten thousand.
The perfluoro phosphoric acid resin has the following structural units:
in the formula (III), e is 3-20, f is 1, and the ion exchange capacity is 0.80-2.5 mmol/g. The molecular weight is 10-60 ten thousand, preferably 15-30 ten thousand.
The perfluorosulfonic acid resin, the perfluorocarboxylic acid resin or the perfluorophosphoric acid resin is prepared by a known method.
The preparation method of the proton exchange composite membrane comprises the following steps:
(1) the full fluorine resin with ion exchange function or the precursor thereof is compounded with the fluorine-containing polymer film with a microporous structure into a film by extrusion, hot pressing, solution casting, tape casting, screen printing, spraying or dipping processes;
(2) and (2) carrying out heat treatment on the membrane prepared in the step (1) at 30-250 ℃ to obtain the high-temperature proton exchange composite membrane.
The extrusion, hot pressing, solution casting, screen printing, spraying or dipping processes in step (1) above can be according to the state of the art. Wherein,
when the process of solution casting, tape casting, screen printing, spraying or dipping is used, the perfluorinated resin is dissolved in a solvent to prepare a perfluorinated resin solution, and the used solvent is selected from one or the combination of the following solvents: one or more of dimethylformamide, dimethylacetamide, methylformamide, dimethyl sulfoxide, N-methylpyrrolidone, hexamethyl ammonium phosphate, acetone, water, ethanol, methanol, propanol, isopropanol, ethylene glycol or glycerol. The solvent used is preferably an alcohol-water solution, the ratio of alcohol to water being chosen as is conventional in the art, and the preferred ratio of alcohol to water in the present invention is 1: 1 by volume.
The solid content of the perfluorinated resin solution is 1-80%, the weight ratio and the viscosity are 1-4000 cP, and 20-2000 cP is preferred.
Preferably, the heat treatment time of the film in the step (2) is 10-600 minutes, and preferably 20-120 min.
The proton exchange composite membrane after heat treatment needs post treatment according to the form of the membrane forming substance and the kind of solvent of the membrane forming solution, and the post treatment method is the prior art in the field, for example, boiling in 5% sulfuric acid for 30-60 minutes, boiling in distilled water for 30 minutes to swell the membrane, hydrolyzing in 10% KOH solution for 6-8 hours, and then placing in 5% H solution2SO4And (5) neutralizing for 1 h.
The more important detailed preparation is illustrated in the examples, which are not intended to limit the invention in any way according to the prior art.
The invention has the following excellent effects:
according to the invention, the film thickness of the composite membrane is less than 20 microns by compounding the fluorine-containing polymer porous membrane of 1-20 microns with the perfluorinated ionic polymer, the conductivity of the composite membrane is superior to that of the composite membrane with large thickness due to the small thickness of the ionic membrane, the influence on the performance of the fuel cell is unexpected, and the internal resistance of the cell is greatly reduced due to the reduction of the thickness, so that the output power of the cell is improved. It is particularly surprising that such thin composite membranes are comparable membranes that perform well above 20 microns at high temperatures > 95 ℃ and low humidity (30% RH) cells. The possible reason is that when the membrane becomes very thin, water generated at the positive electrode of the cell, i.e., the oxygen and hydrogen ion reaction electrode, can easily migrate backwards to another stage, and thus the membrane is humidified during the reverse migration, thereby improving the conductivity of the membrane and the output performance of the cell virtually.
Drawings
Fig. 1 is a plot of the output performance (95 degrees, 30% RH) of the single cells of example 3 and comparative example, with voltage on the ordinate, hydrogen permeation current on the abscissa, 10 micron composite membrane on the top line, and 25 micron composite membrane on the bottom line.
Detailed Description
The present invention will be further described with reference to examples and comparative examples. The concentration of all the perfluoro resin solutions in the examples is mass percent.
Example 1
Soaking a 5-micron polytetrafluoroethylene membrane (with the porosity of 80 percent and the pore diameter of 0.5-3 microns) in 5 percent perfluorosulfonic acid resin propanol-water solution, wherein the structural formula of the perfluorosulfonic acid resin is shown in the specification
n is 1, p is 2, ion exchange capacity is 0.97mmol/g, molecular weight is 19 ten thousand.
The wet film samples were then dried in an oven at 140 ℃ for 30 seconds. This process step can be repeated more than 2 times in order to completely occlude the pores in the membrane. And finally, treating the composite membrane for 30 minutes at 190 ℃ to obtain a composite membrane with the thickness of 8 microns, and then boiling the composite membrane in 5% sulfuric acid for 30 minutes at normal pressure to obtain the 5 micron composite membrane.
Example 2
A biaxially oriented polytetrafluoroethylene film (porosity 89%, pore diameter 0.2-2 microns) 10 microns thick was fixed on a flat plate. And (3) coating a 15% perfluorosulfonic acid resin isopropanol-water solution on one side of the polytetrafluoroethylene membrane to completely fill the pore volume in the membrane with the perfluorosulfonic acid resin solution so as to completely block all pores. Wherein the structural formula of the perfluorinated sulfonic acid resin is shown in the specification
n is 1, p is 2, and the ion exchange capacity is 1.05mmol/g molecular weight is 21 ten thousand.
The coated film samples were dried in a drying oven at 100 ℃ for 15 minutes. The coating and drying process can be repeated more than 2 times in order to completely occlude the pores in the membrane. The drying temperature was then raised to 210 ℃ and dried for 5 minutes. Then boiling the mixture in 5 percent sulfuric acid for 60 minutes under normal pressure to obtain the 12 micron composite membrane.
Example 3
A biaxially oriented polyvinylidene fluoride film (porosity 75%, pore diameter 5 μm) 8 μm thick was fixed on a flat plate. Coating 24% perfluorosulfonic acid resin isopropanol-propanol-water solution on two sides of polyvinylidene fluoride membrane by screen printing, wherein the structural formula of the perfluorosulfonic acid resin is shown in the specification
n is 0, p is 2, ion exchange capacity is 1.35mmol/g, molecular weight is 24 ten thousand.
The film was then dried in an oven at 160 ℃ for 40 minutes. Then, the dried film after drying treatment was taken out and boiled in distilled water for 30 minutes to swell the film to obtain a 10-micron composite film.
Example 4
A biaxially oriented polytetrafluoroethylene film (porosity 85%, pore diameter 0.2-3 microns) with the thickness of 12 microns is fixed on a flat plate. Compounding perfluoro sulfonic acid resin precursor with polyvinylidene fluoride membrane by hot pressing, hydrolyzing the composite membrane in 10% KOH solution for 8H, and placing in 5% H2SO4Obtaining the 12-micron composite membrane in 1 h. Wherein the structural formula of the perfluorinated sulfonic acid resin is as follows:
n is 0, p is 2, ion exchange capacity is 1.50mmol/g, molecular weight is 26 ten thousand.
Example 5
A12 μm thick biaxially oriented polyvinylidene fluoride membrane (porosity 75%, pore size 5 μm) was fixed to a flat plate. Coating 24% ethanol-water solution of perfluorosulfonic acid resin on both sides of the polyvinylidene fluoride membrane by spraying, wherein the structural formula of the perfluorosulfonic acid resin is shown in the specification
n is 0, p is 4, ion exchange capacity is 1.20mmol/g, molecular weight is 17 ten thousand. The film was then dried in an oven at 160 ℃ for 1 minute. Then, the dried film after drying treatment was taken out and boiled in distilled water for 30 minutes to obtain a 12-micron composite film.
Example 6
A20 μm thick uniaxially stretched ETFE membrane (porosity 60%, pore size 1 μm) was fixed on a flat plate. Coating 36% of perfluorosulfonic acid resin DMF solution on an ETFE membrane in a tape casting manner, wherein the structural formula of the perfluorosulfonic acid resin is shown in the specification
n is 1, p is 3, ion exchange capacity is 1.10mmol/g, molecular weight is 21 ten thousand. The film was then dried in an oven at 250 ℃ for 30 minutes. Then, the dried film after drying was taken out and boiled in distilled water for 30 minutes to obtain a 20-micron film.
Example 7
A15 micron thick uniaxially stretched PVDF membrane (porosity 85%, pore size 2 micron) was fixed on a flat plate. Coating 10% ethanol solution of perfluorocarboxylic acid resin on a PVDF membrane in a tape casting manner, wherein the structural formula of the perfluorocarboxylic acid resin is shown in the specification
c is 1, d is 2, ion exchange capacity is 1.00mmol/g, molecular weight is 22 ten thousand. The film was then dried in an oven at 120 ℃ for 10 minutes. Then, the dried film after drying treatment was taken out and boiled in distilled water for 30 minutes to obtain a 16-micron composite film.
Example 8
A biaxially oriented polytetrafluoroethylene film (porosity 85%, pore diameter 3 μm) having a thickness of 15 μm was fixed on a flat plate. Coating 5% perfluorocarboxylic acid resin NMP (N-methyl pyrrolidone) alcohol solution on a biaxially oriented polytetrafluoroethylene membrane in a casting mode, wherein the perfluorocarboxylic acid resin has a structural formula shown in the specification
c is 1, d is 3, ion exchange capacity is 1.15mmol/g, molecular weight is 18 ten thousand. The film was then dried in an oven at 100 ℃ for 5 minutes. Then, the dried film after drying treatment was taken out and boiled in distilled water for 30 minutes to obtain a 15-micron composite film.
Example 9
A biaxially oriented polytetrafluoroethylene film (porosity 75%, pore size 0.4 μm) 14 μm thick was fixed to a flat plate. And compounding a precursor of the perfluorinated phosphoric acid resin with a biaxially oriented polytetrafluoroethylene membrane in a hot pressing mode, and then placing the membrane in 10% sulfuric acid for treatment for 5 hours to obtain the acid type 14-micron composite membrane. Wherein the structural formula of the precursor of the perfluoro phosphoric acid resin is shown as
The ion exchange capacity is 1.70mmol/g, and the molecular weight is 25 ten thousand.
Comparative example:
a biaxially oriented polyvinylidene fluoride film (porosity 75%, pore diameter 5 μm) 25 μm thick was fixed on a flat plate. Coating 24% perfluorosulfonic acid resin isopropanol-propanol-water solution on two sides of polyvinylidene fluoride membrane by screen printing, wherein the structural formula of the perfluorosulfonic acid resin is shown in the specification
n is 0, p is 2, ion exchange capacity is 1.35mmol/g, molecular weight is 23 ten thousand.
The film was then dried in an oven at 160 ℃ for 20 minutes. Then, the dried film after drying treatment was taken out and boiled in distilled water for 30 minutes to swell the film to obtain a 25-micron composite film.
Measurement of cell Performance:
the membrane electrode assembly was prepared as follows:
(1) preparation of gas diffusion layer: the carbon paper was subjected to hydrophobic treatment by dipping it into 25 wt% PTFE emulsion. Then the carbon paper soaked with PTFE is placed in a muffle furnace at 340 ℃ for roasting, so that the surfactant soaked in the carbon paper is removed, and a good hydrophobic effect is achieved. Mixing a certain amount of carbon powder, PTFE and a proper amount of isopropanol aqueous solution, oscillating for 15min by ultrasonic waves, coating the mixture on carbon paper by a brush coating process, and respectively baking the coated carbon paper for 30min at 340 ℃ to obtain the gas diffusion layer.
(2) Preparation of membrane electrode MEA: the Pt loading in the catalyst layer is 0.3mg/cm2Mixing a certain amount of 40% Pt/C (JM company) electrocatalyst, deionized water and isopropanol, and ultrasonically oscillating for 15 min; then adding a certain amount of 5% Nafion solution (Dupont company), continuing ultrasonic oscillation for 15min, and after the ultrasonic oscillation is performed to form ink shape, uniformly spraying the ink on the composite membrane of the invention to obtain the membrane electrode MEA.
Two gas diffusion layers were placed on both sides of the MEA at a pressure of 3MPa to obtain a single cell, which was used for testing on a cell station apparatus.
The properties of the various films were measured as described above and the results are shown in Table 1. As can be seen from Table 1, the composite membrane of the invention has the performances of conductivity at 95 ℃, tensile strength, hydrogen permeation current and the like which are superior to or equivalent to those of the composite ionic membrane with the common thickness. But the output performance of a single cell at 95 ℃ is superior to that of the similar composite membrane with the thickness of more than 20 microns.
TABLE 1 various film Properties