Patent Application
20080199754
The present invention relates to a method for preparing a membrane to be assembled in a membrane electrode assembly. Further, the present invention relates to a membrane electrode assembly.
Solid polymer electrolytes are used in electrochemical cells to exchange ions between anode and cathode and as separator for electrons and reactants between anode and cathode. The use of an ion exchange membrane as electrolyte was first described around 40 years ago by Grubb (General Electric, USA), i.e. in the U.S. Pat. No. 2,913,511. Solid polymer electrolytes find widespread application in a range of electrochemical devices, such as electrolyzers, supercapacitors, ozone generators, fuel cells, etc.
The use of an ion exchange membrane is particularly interesting in a fuel cell, because it leads to simplification of the cell construction and system design, since the use of corrosive liquid electrolytes can be avoided. Proton exchange membranes (PEMs) find application in polymer electrolyte fuel cell (PEFC) stacks. These materials contain acid groups that are attached to the macromolecules of the polymer. The dissociation of the acid leads to the formation of mobile protons and fixed anions.
Predominantly, perfluorinated membrane materials, such as Nafion® (DuPont, USA), Flemion® (Asahi Glass, Japan), Aciplex® (Asahi Kasei, Japan), are used in PEFCS. The elaborate fabrication process, however, renders these membranes expensive components. Alternative, cost-effective membranes and processes have therefore a considerable relevance to enable the development and fabrication of cost-competitive materials for fuel cells. One such attractive method is the pre-irradiation induced graft copolymerization, whereby a preformed commodity polymer film is modified to introduce desirable functionality, such as proton conductivity. The method has been employed by a range of companies and groups from academia for fuel cell application. The process consists of the irradiation of the base polymer film to generate radicals, followed by a grafting step, whereby the activated film is brought into contact with a solution containing monomers, which results in a radical polymerization reaction and hence the growth of polymer side chains attached to the polymer chains of the base film. Subsequent reaction steps may follow to introduce proton conductivity. A popular monomer for radiation grafting is styrene, because it shows fast radical polymerization and, hence, practical degrees of grafting are obtained within a suitably short time of a few hours. The grafted polystyrene is subsequently sulfonated to introduce sulfonic acid sites to the styrene rings.
Sulfonated polystyrene has been used as proton exchange membrane material in fuel cells. It has been recognized early, however, that the aggressive conditions within the fuel cell, i.e. the reductive as well as oxidative conditions, and peroxide intermediates, impose considerable chemical stress onto the membrane material. Polystyrene is particularly prone to chemical attack in an environment of peroxide and radical intermediates due to the susceptibility of the α-hydrogen position towards oxidative attack.
Therefore, the use of an alternative grafting monomer with higher intrinsic chemical stability is of advantage. The grafting monomer is from the wide range of radically copolymerizable monomers. Non-limiting examples of monomers include acrylic acid, methacrylic acid, maleic anhydride, maleimide, N-phenylmaleimide, acrylates, methacrylates; vinyl sulfonic acid, vinyl phosphonic acid; α-methylstyrene, α-fluorostyrene, α,β,β-trifluorostyrene, trifluoro-α-methylstyrene; 2-acrylamido-2-methyl-1-propanesulfonic acid, 2-acrylamido-1-ethanesulfonic acid. The monomers may already carry cation exchange functionality (e.g. vinylsulfonic acid), or it may be introduced in a subsequent step (e.g. sulfonation of styrene units). The radical induced graft copolymerization kinetics of these monomers may be poor. In this case, only small graft levels are obtained or long grafting times have to be used. In order to improve grafting kinetics, an additional monomer, i.e. a co-monomer, can be used to obtain practical graft levels within reasonable grafting times. The selection of base monomer M1 and co-monomer M2 is such that hetero-polymerization, i.e. the formation of -M1-M2-M1-M2-sequences, is kinetically favored, leading to overall faster incorporation of M1 into the graft polymer compared to when the base monomer M1 alone. The co-monomer may but need not contribute to the cation exchange functionality, either directly or after post-treatment. Therefore, the co-monomer can be any monomer amenable to radical copolymerization, such as vinyl chloride, vinyl fluoride, vinylidene fluoride, tetrafluoroethylene, hexafluoropropylene; vinyl ethers, fluorinated vinyl ethers, vinyl esters, fluorinated vinyl esters; acrylamide, acrylonitrile, methacrylonitrile; N-vinylpyrrolidone, or any of the monomers listed above as base monomers.
In addition to base monomer and co-monomer, a third monomer acting as crosslinker may be added to the grafting solution. Crosslinkers have two or more double bonds to provide the necessary links between different polymer chains, such as divinylbenzene, bis(vinyl phenyl)ethane, diisopropenylbenzene, triallylcyanurate, N,N′-methylene-bis-acrylamide, diallylmaleinate.
In view of the degradation mechanism of styrene grafted and sulfonated membranes in the polymer electrolyte fuel cell, as mentioned earlier, styrene derived monomers with protected α-position are regarded as promising candidates for obtaining grafted membranes with intrinsically higher chemical stability. In the prior art, α,β,γ-trifluorostyrene (TFS) and α-methylstyrene (AMS) have been suggested. Being a fluorinated compound, TFS has the drawback of being significantly more expensive than styrene or AMS. In addition, grafting kinetics are poor, sulfonation is difficult, and the resulting mechanical properties of the membranes mediocre. The drawback of AMS, on the other hand, is the poor radical polymerization kinetics, low ceiling temperature of 61 C, the tendency for adverse chain transfer reactions to occur, and the concomitant poor grafting yield, which has been confirmed in our own experiments. It has been shown, however, that AMS grafted films can be obtained if acrylonitrile (AN) is used as co-monomer, because it stabilizes the terminal radical on the propagating graft polymer chain. It was shown that the graft copolymerization of AMS:AN mixtures, followed by sulfonation, yields proton exchange membranes of higher chemical stability compared to pure styrene grafted or styrene-AN grafted membranes, using FEP and ETFE as base polymer. Fuel cell experiments, however, have not been reported and the electrochemical characteristics of these membranes in a realistic environment remain to be established.
It is therefore the aim of the present invention to provide a method for preparing a membrane to be assembled in a membrane electrode assembly and a membrane electrode assembly which both have significant mechanical stability and appropriate fuel cell characteristics.
These objectives are achieved according to the present invention by a method for preparing a membrane to be assembled in a membrane electrode assembly, such as a polymer electrolyte membrane fuel cell, comprising the steps of:
With respect to the membrane electrode assembly these objects are achieved according to the present invention by a membrane electrode assembly, comprising a polymer electrolyte layer which is sandwiched between a cathode layer and an anode layer, whereby said polymer electrolyte layer is a graft copolymer membrane which comprise α-methylstyrene (AMS) and methacrylonitrile (MAN) as co-monomer. Other possible monomer combinations may be derivatives of α-methylstyrene, such as sodium α-methylstyrene sulfonate, methyl-α-methylstyrene, methoxy-α-methylstyrene. Base polymers may be selected from the range of fluorinated, partially fluorinated or non-fluorinated films, including polytetrafluoroethylene, poly(tetrafluoroethylene-co-perfluoropropyl vinyl ether), poly(tetrafluoroethylene-co-hexafluoropropylene), poly(vinylidene fluoride), poly(vinylidene fluoride-co-hexafluoropropylene), poly(ethylene-alt-tetrafluoroethylene), polyvinylfluoride, polyethylene, polypropylene.
In a preferred embodiment of the present invention the molar ratio of α-methylstyrene/methacrylonitrile may be in the range of 50/50 to 90/10, preferably in the range of 60/40 to 80/20. The monomer mixture may comprise additional monomers to obtain specific added membrane functionality, e.g. crosslinking. In a preferred embodiment for a crosslinked membrane, the molar fraction of crosslinker with respect to total monomer content may be in the range of 1 to 20%, preferably in the range of 5 to 10%. The neat monomer mixture may be used for the grafting reaction, or a solvent or solvent mixture, such as isopropanol and water, may be added to the monomer mixture.
In summarizing the invention it was found that for the radiation induced graft polymerization of AMS onto base films such as FEP and ETFE, methacrylonitrile (MAN) is a suitable co-monomer to circumvent the poor polymerization kinetics of AMS alone and yield practical graft levels within reasonable grafting times. Following experimental observations were made:
These observation show a number of significant advantages over the teachings derived from the prior art:
The following figures and tables are used to introduce preferred embodiments of the present invention. Table 1 shows the properties of radiation grafted and sulfonated membranes based on FEP-25.
Membrane 1 is a standard styrene grafted and divinylbenzene (DVB) crosslinked membrane which is used in H2/air fuel cells. Membrane 2 is a comparison example without DVB crosslinker. This type of membrane is very unstable in the fuel cell and leads to rapid failure of the membrane electrode assembly (cf. Table 2). Membranes 3, 4 and 5 are of the inventive type, using AMS/MAN (membrane 3), AMS/MAN/DVB (membrane 4), and AMS/MAN/DIPB (membrane 5) as monomers, respectively. It is shown that ion exchange capacity and conductivity similar to the styrene grafted membranes and a commercial membrane (Nafion® 112) are obtained.
Table 2 shows the life time of uncrosslinked radiation grafted and sulfonated membranes based on FEP-25 in the single cell.
Pure styrene grafted membranes show inferior temperature stability and life time in the single cell compared to membranes of the inventive type.
The single cell polarization curve (
Compared to the styrene grafted and DVB crosslinked membrane, the AMS/MAN grafted membrane shows similar single cell performance (
The following section explains two embodiments of the method to generate a radiation grafted fuel cell membrane with the afore-mentioned enhanced properties. Thereby, the synthesis of AMS/MAN and AMS/MAN/DVB membranes for PEFC contains the two known steps of grafting and sulfonation.
An FEP film of 25 μm thickness was irradiated with an electron beam in air atmosphere with dose of 25 kGy. Piece of 1 g of preirradiated FEP film was placed in a trap-type reaction tube, equipped with two stopcocks. The tube was filled with a 60 ml of reaction mixture prepared by mixing: 12.7 cm3 of AMS, 5.3 cm3 of MAN, 12 cm3 of water and 30 cm3 of isopropanol. The tube was closed. Nitrogen was passed through at a flow rate 12 Nl/h, by the valve connected with the bottom tube. After one hour of purging the tube was sealed and transferred to a water bath. The temperature of the water bath was maintained at 60° C. After 22 hrs of reaction, the solution was removed from the tube, and the tube with the product was washed three times with acetone (60 ml each washing). The product was removed from the tube and left for drying in a vacuum oven at 50° C. for 3 hrs.
The procedure was identical to the one described in the previous section, the only difference being the addition of 0.5 vol-% (0.3 cm3) DVB to the grafting solution.
650 cm3 of dry methylene chloride and 30 cm3 of chlorosulfonic acid were placed in a beaker shaped glass vessel, equipped with magnetic stirrer, gas vent and cap. 5 g (about five sheets) of FEP-g-(AMS-co-MAN) film were placed in the reaction vessel. The mixture was stirred for 6 hrs. After 6 hrs product was transferred to a beaker filled with water. After 12 hrs the product was placed in a beaker with 500 cm3 of aqueous solution of sodium hydroxide (4 g/dm3) and stirred for 6 hrs. Subsequently, the product was washed with water and treated with 500 cm3 of 2M H2SO4 for 6 hrs. To remove sulfuric acid, the product was treated in water at 80° C. for 6 hrs. The water was changed until pH was neutral.
MAN—methacrylonitrile
AMS—alpha methylstyrene
FEP—poly(tetrafluoroethylene-co-hexafluoropropylene)
Nl/h—normal liter per hour
FEP-g-(AMS-co-MAN)—FEP grafted with AMS and MAN
kGy—kilo Grey
Monomers were used as received (MAN Aldrich 19541-3, AMS Aldrich M8, 090-3). (DVB) Water was demineralized using a Serapur Pro 90CN system (conductivity <0.5 μS/cm). Chlorosulfonic acid was pure grade, purchased from Fluka (26388). Sodium hydroxide, sulfuric acid and isopropanol were analytical grade. FEP film was purchased from DuPont, and was stored between irradiation and grafting at −80° C. for ca. 2 months.
| Number | Date | Country | Kind |
|---|---|---|---|
| 05002875.2 | Feb 2005 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP06/00752 | 1/28/2006 | WO | 00 | 3/4/2008 |
