CA2933122A1 - Method for making densely sulfonated poly(phenylene) copolymer electrolytes for fuel cells - Google Patents

Abstract

A method of making a desirable, densely sulfonated, proton conducting copolymer electrolyte is disclosed. The copolymer electrolyte comprises a densely sulfonated poly(phenylene) hydrophilic domain and a hydrophobic domain comprising a main chain comprising a plurality of bonded arylene groups wherein essentially all of the bonds in the main chain of the copolymer are carbon-carbon or, to a certain extent, carbon-sulfone bonds. In the method, the densely sulfonated poly(phenylene) hydrophilic domain is synthesized by direct polymerization of sulfonated monomers and preferably with precisely controlled position and number of sulfonic acid groups on the side chains.

Description

Docket No.:P83 151 8/C'A/1 METHOD FOR MAKING DENSELY SULFONATED POLY(PHENYLENE) COPOLYMER
ELECTROLYTES FOR FUEL CELLS
BACKGROUND
Field of the Invention This invention relates to methods for making hydrocarbon type proton conducting polymer electrolytes used in solid polymer electrolyte fuel cells. In particular, it relates to methods for making densely sulfonated poly(phenylene) copolymer electrolytes which comprise densely sulfonated hydrophilic domains that are synthesized by direct polymerization of sulfonated monomer.
Description of the Related Art Proton exchange membrane fuel cells (PEMFCs) convert reactants, namely fuel (such as hydrogen) and oxidant (such as oxygen or air), to generate electric power. PEMFCs generally employ a proton conducting polymer membrane electrolyte between two electrodes, namely a cathode and an anode. The structure comprising a proton conducting polymer membrane sandwiched between two electrodes is known as a membrane electrode assembly (MEA). The membrane electrolyte serves as a separator to prevent mixing of reactant gases as well as an electrolyte for transporting protons from anode to cathode.
Perfluorosulfonic acid (PFSA) ionomer, e.g., Nafion , has historically been the material of choice and the technology standard for both membranes and for ionomer employed in the catalyst layers of a MEA.
PFSA ionomer consists of a perfluorinated backbone that bears pendent vinyl ether side chains, terminating with SO3H groups. PFSA membranes show good operation under normal operating conditions, but they offer a poor permeance barrier to the hydrogen and oxygen reactants, which reduces the durability of a fuel cell stack and lowers fuel efficiency and the driving range of fuel cell vehicles.
Further, PFSA ionomer and membrane release fluorine compounds upon decomposition, which will cause catalyst dissolution and raise environmental concerns. When used in a catalyst layer, the strong acidity of PFSA ionomer can also accelerate degradation of the catalyst.
MEA durability and cost are crucial issues for the development and commercialization of fuel cell systems in either stationary or transportation applications. In automotive applications for instance, a MEA
may be required to demonstrate durability of about 6,000 hours. Being able to employ a low catalyst loading without compromising MEA performance and durability is very important for cost reduction of Docket No.:P83151 8/CA/1 fuel cells. Hydrocarbon polymer electrolytes (either as a hydrocarbon membrane or hydrocarbon ionomer in a catalyst layer, or the combination of hydrocarbon membrane with hydrocarbon ionomer in a catalyst layer) are expected to provide advantages over PFSA ionomers to lower catalyst loading in fuel cells for cost reduction due to reduction in or elimination of fluorine release from hydrocarbon polymer electrolytes.
Several classes of hydrocarbon polymers are under intense investigation. These include poly(ether arylenes), polyimides, polyphosphazenes, radiation-grafted polystyrene, organic-inorganic composites and hybrids, polystyrene di- and tri-block copolymers, and acid-complexes of basic polymers. However, most of these hydrocarbon membranes cannot meet the durability requirement of automotive fuel cells due to the presence of weak bonds in the ionomer chains. For instance, a-hydrogen of polystyrene is not stable under free radical attack, while ether bonds and polyimide structure are hydrolytically unstable (T.J.
Peckham et al, Proton Exchange Membranes, in Proton Exchange Membrane Fuel Cells: Materials Properties and Performance, Editors: David P. Wilkinson et al, CRC Press, 2009, 107-189).
Furthermore, many hydrocarbon membranes have insufficient performance under low relative humidity (RH) conditions (e.g. ¨30% or lower) for automotive fuel cells. Recently however, it was reported that hydrocarbon membranes with dense sulfonic acid groups exhibit higher performance than PFSA
membrane at high temperature (S. Tian et al, Macromolecules, 2009, 42, 1153-1160; K. Matsumoto et al, Macromolecules, 2009, 42, 1161-1166). While these hydrocarbon membranes with dense sulfonic acid groups were synthesized by condensation polymerization of dichlorodiphenylsulfone or difluorobenzophenone with dihydroxy monomers followed by post-sulfonation, limited durability of these membranes in fuel cells can be expected due to the existence of ether bonds in the ionomer chains.
In commonly owned US patent serial number 9,102,789 (titled "Sulfonated Poly(phenylene) Copolymer Electrolyte for Fuel Cells", improved ether bond free sulfonated poly(phenylene) copolymer electrolytes for fuel cells were disclosed. The chemical stability of ether bond free hydrocarbon membrane is expected to be significantly improved due to elimination of the weak ether bond from the hydrocarbon ionomer.
The hydrophilic domain of the claimed ionomers could be either densely sulfonated poly(phenylene) or sulfonated poly(benzophenone). Both types of ionomer membrane showed superior performance compared to NRE211 PFSA membrane, while ionomer with densely sulfonated poly(phenylene) hydrophilic domains had higher performance than that of ionomer with sulfonated poly(benzophenone) hydrophilic domains. lonomers with densely sulfonated poly(phenylene) hydrophilic domains were prepared by post-sulfonation of non-sulfonated polymer, in which non-sulfonated polymer was treated

2 Docket No.:13831518/CA/1 with strong acid (e.g. sulfuric acid, chlorosulfonic acid, etc.) with heating, to attach sulfonic acid groups on the side chains. The harsh conditions (high temperature ¨80 C and strong acid) in the post-sulfonation reaction can cause polymer degradation, and consequently decrease membrane mechanical properties and MEA durability in fuel cells (J.Y. Sanchez, Fuel Cells, 2005, 5(3), 344-354).
Furthermore, it is impossible to precisely control the position and number of sulfonic acid groups in a post sulfonation reaction, which can impact membrane morphology, proton conductivity, and performance in PEM fuel cells.
Recently, T. Skalski et al. explored the synthesis of sulfo-phenylated dienes monomers and polyphenylene homopolymer (J. Am. Chem. Soc. 2015, 137, 12223-12226).
There remains a continuing need to improve hydrocarbon ionomer electrolytes for solid polymer electrolyte fuel cells and, in particular, for electrolytes exhibiting both good performance and durability characteristics. This invention fulfills these needs and provides further related advantages.
SUMMARY
The present invention relates to a new method of making certain densely sulfonated poly(phenylene) proton conducting copolymer electrolytes described in the aforementioned US9102789. Such copolymer electrolytes provide improved durability and performance when used as ionomer and membrane electrolyte in fuel cells. In the method, telechelic sulfonated oligomer with reactive end groups are synthesized in a manner similar to that disclosed in the aforementioned Skalski et al. publication and are then further polymerized to prepare desirable ether bond free copolymer electrolytes.
Specifically, the inventive method can be used to make copolymer electrolyte comprising a proton conducting sulfonated poly(phenylene) hydrophilic domain and a hydrophobic domain comprising a main chain comprising a plurality of bonded arylene groups in which essentially all of the bonds in the main chain of the copolymer electrolyte are carbon-carbon or carbon-sulfone bonds.
In the method, the proton conducting, densely sulfonated poly(phenylene) hydrophilic domain is synthesized by direct polymerization of sulfonated monomers. In this way, the position and number of sulfonic acid groups in ionomer can be precisely controlled, and impact of post-sulfonation reaction on membrane durability and performance can be avoided.

3 Docket No.:P83151 8/CA/1 The method produces densely sulfonated poly(phenylene) proton conducting copolymer electrolyte for fuel cells that is characterized by competitive voltage versus current density and superior durability. The copolymer electrolyte can have different microstructures (i.e. the arrangement of monomer units along the polymer chain) depending on the polymerization route (e.g. random copolymer, sequenced copolymer or block copolymer).
The method can include polymerizing aryl monomers or oligomers in and between the hydrophilic and hydrophobic domains via aryl-aryl coupling. Essentially all of the bonds in the main chain of the synthesized copolymer electrolyte can be carbon-carbon bonds. Further, the chains formed can thus be essentially free of ether bonds, thereby producing an electrolyte with improved durability. Without ether bonds, the copolymer electrolyte is less susceptible to degradation when used in solid polymer fuel cells.
The produced copolymer electrolyte may be used anywhere that electrolyte is normally employed in a fuel cell. However, it is particularly useful for use as the membrane or ionomer in catalyst layer in a membrane electrode assembly for a solid polymer electrolyte fuel cell.
These and other aspects of the invention are evident upon reference to the attached Figures and following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
Figures 1 a and lb show the chemical structures of certain ether bond free copolymer electrolytes disclosed in US9102789 which can be prepared using the method of the invention.
Figure 2 reproduces the individual scheme showing the synthesis of sulfo-phenylated dienes and polyphenylene homopolymer disclosed in J. Am. Chem. Soc. 2015, 137, 12223-12226.
Figure 3a illustrates the synthesis steps used in the Examples to prepare an exemplary densely sulfonated poly(phenylene) copolymer electrolyte according to the method of the invention.
Figure 3b illustrates synthesis steps which can be expected to prepare a preferred densely sulfonated poly(phenylene) copolymer electrolyte according to the method of the invention.
DETAILED DESCRIPTION

4 Docket No.:P83 15 1 8/CA/1 The copolymer electrolytes of the present invention are densely sulfonated poly(phenylene) hydrocarbon copolymer electrolytes comprising a proton conducting sulfonated poly(phenylene) hydrophilic domain and a hydrophobic domain and in which the densely sulfonated poly(phenylene) hydrophilic domain is synthesized by direct polymerization of sulfonated monomers. The copolymer electrolyte can then be synthesized therefrom by aryl-aryl coupling polymerization. These copolymer electrolytes offer an advantage over prior art hydrocarbon ionomers in that: (1) densely sulfonated hydrophilic domains can be produced with precisely controlled position and number of sulfonic acid groups, which provide strong phase separation in the membrane electrolyte to form well connected proton conducting channels even under low relative humidity conditions, and thereby improve the fuel cell performance; (2) there is no post-sulfonation reaction of the copolymer, and therefore no polymer degradation occurs to decrease membrane mechanical properties; and (3) block copolymers can be synthesized by aryl-aryl coupling polymerization, in which there are no weak ether bonds in the chains formed and thus good durability characteristics are obtained. Further, the properties of the copolymer electrolyte can be further optimized by tuning the chemical structure of the hydrophilic domain and the hydrophobic domain (microstructure, length of hydrophilic block and hydrophobic block, etc).
Figures la and lb show certain prior art ether bond free sulfonated poly(phenylene) copolymer electrolytes disclosed in US9102789 that can be prepared using the method of the invention. Figures la and lb are copolymer electrolytes with densely sulfonated poly(phenylene) hydrophilic domains which, in the prior art, were synthesized by post-sulfonation methods. The copolymer electrolytes in Figures la and lb provide desirably competitive performance in fuel cells.
Figure 2 reproduces the individual scheme showing the prior art synthesis of sulfo-phenylated dienes and polyphenylene homopolymer disclosed in J. Am. Chem. Soc. 2015, 137, 12223-12226.
In a similar way to the method disclosed in Figure 2, telechelic sulfonated oligomer with densely sulfonated poly(phenylene) can be synthesized by direct polymerization of sulfonated monomer to produce suitable hydrophilic domain for the copolymer electrolyte. This is then followed by aryl-aryl coupling polymerization to prepare the copolymer electrolyte (designated as 17 in Figure 3a). Figure 3a illustrates synthesis steps for preparing a densely sulfonated poly(phenylene) copolymer electrolyte whose hydrophilic domains are obtained from sulfonated oligomer (designated as moiety 15 in Figure 3a) which is similar to that produced in Figure 2.

5 Docket No.:P831518/CA/1 In Figure 3a, the process steps involve the following: (a) KOH/Et0H, reflux;
(b) Me3SiOSO2C1, C2H4C12; (c) NEt3, n-BuOH; (d) PhNO2, 180 C, 12h; (e) bis-triphenylphosphine nickel dichloride, triphenylphosphine, activated zinc powder, sodium iodide, NMP, 80 C 24 h; and (f) 0.5M H2SO4 The molecular weight of sulfonated oligomer can be controlled by the molar ratios of moieties 9, 13 and 14. In the hydrophilic domain of Figure 3a, there are still two non-sulfonated benzene rings on the side chain. To further increase phase separation in the copolymer electrolyte, the hydrophilicity of the hydrophilic domain is increased. It is expected that this can be accomplished by using a stronger agent for sulfonation, such as chlorosulfonic acid or oleum, to prepare fully sulfonated monomer and ionomer as shown in Figure 3b (e.g. see US2697117). Thus in the process steps of Figure 3b, step (g) involving treating with oleum, 1,2-C2H4C12 is used instead of step (b). The improved copolymer electrolyte whose hydrophilic domain has increased hydrophilicity is designated as 20 in Figure 3b.
The copolymers produced essentially have only carbon-carbon or, to a certain extent, carbon-sulfone bonds in their main chains and have essentially no ether bonds in their chain network. While the majority of the bonds may be phenylene-phenylene, phenylene-carbon or carbon-carbon, the main chain may also contain sulfone groups in the alpha-position to phenylene/arylene groups.
Those sulfone groups improve stability of the inventive copolymer electrolyte by stabilizing the neighboring benzene rings due to their strong electron withdrawing property. Moreover, those sulfone groups themselves exhibit good resistance against chemical attacks.
The chemical structures of polymers which may be potentially suitable for use as the hydrophilic domain and the hydrophobic domain in the present copolymers are described in detail in the aforementioned US9102789. The produced copolymers comprise both a densely sulfonated poly(phenylene) hydrophilic domain and a hydrophobic domain, in which the hydrophilic domain provides proton conductivity, and the hydrophobic domain provides desirable mechanical properties and improves membrane durability.
US9102789 also discloses in detail how the copolymer electrolyte can be made as a random copolymer, a sequenced copolymer, or a block copolymer. Those skilled in the art will be aware of many detailed options available for preparing copolymers according to any of these schemes.
The following examples are illustrative of the invention but should not be construed as limiting in any way.

6 Docket No.:P831518/CA/1 EXAMPLES
The copolymer electrolyte of Figure 3a (i.e. 17) was synthesized in the following manner. First, sulfonated monomer 9 was synthesized as described in the aforementioned J. Am.
Chem. Soc. 2015, 137, 12223-12226.
To synthesize densely sulfonated oligomer with m=10, a 250 mL Schlenk flask was loaded with sulfonated monomer 9 (8.16 g), 1,4-diethynylbenzene (0.66 g), 4-chlorophenylacetylene (0.14 g), and nitrobenzene (70 mL). The resulting mixture was freeze-thaw degassed three times, before heating under nitrogen (1 atm) at 180 C for 12 hours. Periodically, carbon monoxide was vented to avoid over-pressurization of the reaction flask. Subsequently, an additional 0.10 g of 4-chlorophenylacetylene was added, and the mixture was stirred for an additional 60 minutes at 180 C
under nitrogen atmosphere. The reaction flask was then cooled to room temperature, and sulfonated oligomer was precipitated from ethyl acetate.
To prepare copolymer electrolyte 17 with IEC ¨2.10 mmol/g, a 100 mL Schlenk flask equipped with rubber septa cap were added bis-triphenylphosphine nickel dichloride (0.15 g), triphenylphosphine (1.5 g), activated zinc powder (1.5 g), and sodium iodide (0.08 g). All the chemicals were pre-dried under vacuum at 70 C for 4 h. Then, anhydrous N-Methyl-2-pyrrolidone (NMP, 5mL) was added via syringe.
The catalyst mixture was stirred at ambient temperature for 10 minutes. While maintaining the flask under argon atmosphere, monomer 2, 5-dichlorobenzophenone 16 and the hydrophilic oligomer from synthesis above 15 were loaded into a 50 mL round-bottom flask equipped with rubber septa cap in the following amounts: 1.05 g (4.18 mmol) of 16, 2.15 g of 15. To the mixture, 25mL of anhydrous NMP was added via syringe and the resulting solution was added into the Schlenk flask via syringe. The polymerization proceeded at 80 C for 24 h under argon atmosphere. The polymerization mixture was then cooled to room temperature and poured into 20% HCl/methanol solution.
While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art without departing from the spirit and scope of the present disclosure, particularly in light of the foregoing teachings.

7

Claims (5)

What is claimed is:

1. A method of making a proton conducting copolymer electrolyte, the copolymer electrolyte comprising a proton conducting sulfonated poly(phenylene) hydrophilic domain and a hydrophobic domain comprising a main chain comprising a plurality of bonded arylene groups wherein essentially all of the bonds in the main chain of the copolymer electrolyte are carbon-carbon or carbon-sulfone bonds, and the method comprising:
synthesizing the proton conducting sulfonated poly(phenylene) hydrophilic domain by direct polymerization of sulfonated monomers.

2. The method of claim 1 wherein essentially all of the bonds in the main chain of the copolymer electrolyte are carbon-carbon bonds.

3. The method of claim 1 wherein essentially none of the bonds in the chains of the copolymer electrolyte are ether bonds.

4. The method of claim 1 comprising polymerizing aryl monomers or oligomers in and between the hydrophilic and hydrophobic domains via aryl-aryl coupling.

5. The method of claim 1 wherein the copolymer electrolyte is a random, a sequenced, or a block copolymer.