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WO2012116782A1 - Électrolytes conducteurs de protons renfermant des additifs copolymères réticulés destinés à être utilisés dans des piles à combustible - Google Patents

Électrolytes conducteurs de protons renfermant des additifs copolymères réticulés destinés à être utilisés dans des piles à combustible Download PDF

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Publication number
WO2012116782A1
WO2012116782A1 PCT/EP2012/000663 EP2012000663W WO2012116782A1 WO 2012116782 A1 WO2012116782 A1 WO 2012116782A1 EP 2012000663 W EP2012000663 W EP 2012000663W WO 2012116782 A1 WO2012116782 A1 WO 2012116782A1
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WO
WIPO (PCT)
Prior art keywords
functional groups
copolymer
electrolyte
cross
metal oxide
Prior art date
Application number
PCT/EP2012/000663
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English (en)
Inventor
Keping Wang
Jing Li
Yunsong Yang
Original Assignee
Daimler Ag
Ford Motor Company
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from US13/039,470 external-priority patent/US8710108B2/en
Priority claimed from US13/039,432 external-priority patent/US8709677B2/en
Application filed by Daimler Ag, Ford Motor Company filed Critical Daimler Ag
Priority to DE112012001070T priority Critical patent/DE112012001070T5/de
Publication of WO2012116782A1 publication Critical patent/WO2012116782A1/fr

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
    • H01M8/1018—Polymeric electrolyte materials
    • H01M8/102—Polymeric electrolyte materials characterised by the chemical structure of the main chain of the ion-conducting polymer
    • H01M8/1037—Polymeric electrolyte materials characterised by the chemical structure of the main chain of the ion-conducting polymer having silicon, e.g. sulfonated crosslinked polydimethylsiloxanes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
    • H01M8/1018—Polymeric electrolyte materials
    • H01M8/102—Polymeric electrolyte materials characterised by the chemical structure of the main chain of the ion-conducting polymer
    • H01M8/1023—Polymeric electrolyte materials characterised by the chemical structure of the main chain of the ion-conducting polymer having only carbon, e.g. polyarylenes, polystyrenes or polybutadiene-styrenes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
    • H01M8/1018—Polymeric electrolyte materials
    • H01M8/1039—Polymeric electrolyte materials halogenated, e.g. sulfonated polyvinylidene fluorides
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
    • H01M8/1018—Polymeric electrolyte materials
    • H01M8/1041—Polymer electrolyte composites, mixtures or blends
    • H01M8/1044—Mixtures of polymers, of which at least one is ionically conductive
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
    • H01M8/1018—Polymeric electrolyte materials
    • H01M8/1069—Polymeric electrolyte materials characterised by the manufacturing processes
    • H01M8/1072—Polymeric electrolyte materials characterised by the manufacturing processes by chemical reactions, e.g. insitu polymerisation or insitu crosslinking
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
    • H01M8/1018—Polymeric electrolyte materials
    • H01M8/1069—Polymeric electrolyte materials characterised by the manufacturing processes
    • H01M8/1072—Polymeric electrolyte materials characterised by the manufacturing processes by chemical reactions, e.g. insitu polymerisation or insitu crosslinking
    • H01M8/1074—Sol-gel processes
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/50—Fuel cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • This invention relates to improved proton conducting polymer electrolytes for use in polymer electrolyte fuel cells. More specifically, it relates to electrolytes comprising an ionomer cross-linked with a copolymer additive comprising both cross-linking and other useful functional groups.
  • Solid polymer electrolyte fuel cells convert reactants, namely fuel (such as hydrogen) and oxidant (such as oxygen or air), to generate electric power.
  • fuel cells generally employ a proton conducting polymer membrane electrolyte between two electrodes, namely a cathode and an anode.
  • a structure comprising a proton conducting polymer membrane sandwiched between two electrodes is known as a membrane electrode assembly (MEA).
  • MEA durability is one of the most important issues for the development of fuel cell systems in either stationary or transportation applications. For automotive applications, an MEA is required to demonstrate durability of about 6,000 hours.
  • the membrane serves as a separator to prevent mixing of reactant gases and as an electrolyte for transporting protons from anode to cathode.
  • Perfluorosulfonic acid (PFSA) ionomer e.g., Nafion®
  • Nafion® has been the material of choice and the technology standard for membranes to date.
  • Nafion® consists of a perfluorinated backbone that bears pendent vinyl ether side chains, terminating with S0 3 H.
  • PFSA membranes failure of the membrane as an electrolyte will result in decreased performance due to increased ionic resistance, and failure of the membrane as a separator will result in fuel cell failure due to mixing of anode and cathode reactant gases.
  • the chemical degradation of PFSA membranes during fuel cell operation is proposed to proceed via the attack of hydroxyl ( ⁇ ) or peroxyl ( ⁇ ) radical species on weak groups (such as a carboxylic acid group) on the ionomer molecular chain.
  • the free radicals may be generated by the decomposition of hydrogen peroxide with impurities (such as Fe 2+ ) in a Fenton type reaction.
  • impurities such as Fe 2+
  • hydrogen peroxide can be formed either at Pt supported on carbon black in the catalyst layers or during the oxygen reduction reaction.
  • the hydroxyl radical attacks the polymer unstable end groups to cause chain zipping and/or could also attack an S0 3 " group under dry conditions to cause polymer chain scission. Both attacks degrade the membrane and eventually lead to membrane cracking, thinning or forming of pinholes. The membrane degradation rate is accelerated
  • US2008/0152986 discloses an impregnated cross- linked or non-cross-linked basic polymer (e.g. poly(benzimidazole)) membrane prepared with an acidic dopant (e.g. a phosphoric acid or an organic phosphonic acid) in order to obtain a polymer electrolyte membrane.
  • an acidic dopant e.g. a phosphoric acid or an organic phosphonic acid
  • Such membranes have good dimensional stability but generally do not have good conductivity at low temperature ( ⁇ 100 °C).
  • the acid dopant leaches out of the membrane with time during fuel cell operation.
  • Cross-linking the electrolyte membranes generally improves durability.
  • a cross-linked proton exchange membrane was prepared using aqueous ammonia to treat Nafion® precursor film.
  • the cross-linking groups in this member were sulfonamide.
  • US2002/0091201 discloses a general technique to create cross-links in perfluorinated polymers in which the cross-links or bonds are between sulfonyl groups attached to adjacent polymeric chains.
  • cross-linking reduces the number of sulfonic acid groups in the membrane, and thus while it improves durability, it also results in lower proton conductivity and performance of the membrane as well.
  • US2010/0040927 a process was disclosed for producing a graft polymer electrolyte film for a fuel cell. This graft electrolyte is heterogeneous and has a silane-cross-linked structure between the graft molecule chains. However, its vinyl structure has been proved to be unstable in the fuel cell environment.
  • WO2005/027240 describes the preparation of phosphonic acid grafted hybrid inorganic-organic polymers with a metal oxide backbone.
  • the polymers can be used directly as proton conducting electrolyte membranes in fuel cells.
  • the phosphonic acid groups allow proton conduction through the membrane under low RH.
  • Composites comprising these polymers and other basic polymers are also suggested therein.
  • US2007/0154764 discloses electrolyte additives including hygroscopic particles made of metal oxide, such as silica or zirconium dioxide, heteropoly acids, phosphonate silica, etc. in order to increase water retention and thereby improve MEA performance under low RH conditions.
  • WO2005/036687 discloses a water insoluble additive comprising a metal oxide cross- linked matrix having phosphonic acid groups covalently attached to the matrix through linkers.
  • the additive can then be homogeneously dispersed throughout a proton conducting membrane and improve ionic conductivity of the membrane at high
  • US2006/0141313 discloses particles comprising a metal-oxygen cross-linked structure as an additive for a proton conducting membrane.
  • the particles have an acid group such as a sulfonic acid group incorporated in the surface thereof.
  • these and many other additives in the prior art are prone to being leached out from the membrane during fuel cell operation, either because they are water soluble or because they have no covalent bonding or no strong interaction with the host polymer.
  • US2004/0043283 discloses incorporating metal elements or compositions containing metal elements or metal alloys that act as free radical scavengers or hydrogen peroxide decomposition catalysts.
  • US2006/0046120 discloses the use of phenol type antioxidants where the antioxidant can be a small molecule or a polymer.
  • US6607856 discloses a solid polymer electrolyte having high-durability and oxidation resistance which is prepared by introducing a chelate group and an electrolyte group into a polymer electrolyte material having a hydrocarbon part. The chelating agents reduce the formation of free radicals. The presence of such additives in the MEA may however result in reduced performance of the fuel cell.
  • Proton conducting electrolytes with improved durability and other desirable features can be obtained by cross-linking certain copolymer additives to suitable host proton conducting ionomers.
  • the copolymer additives comprise both cross-linking functional groups and other functional groups that can provide additional desired properties to the electrolyte.
  • the other functional groups include proton carriers, metal chelating groups, and radical scavengers.
  • the proton conducting ionomer and the copolymer are bonded together at the cross-linking functional groups of the copolymer. This cross-linking or bonding provides durability and improved mechanical properties in general, while also serving to more reliably attach the other functional groups and thereby prevent them from leaching out over time.
  • the electrolyte may contain other polymers or ionomers, but basic ionomers may desirably be excluded.
  • the copolymer additives specifically comprise a polymerized network of a plurality of metal oxide monomers with cross-linking functional groups and a plurality of metal oxide monomers with other functional groups in random sequence.
  • the polymerized network is characterized by an alternating series of oxygen and metal bonds.
  • R is a hydrocarbon group
  • the metal oxide monomers with other functional groups comprise: a second metal bonded to at least two oxygen atoms and selected from the group consisting of Si, Ti, Zr, Ce, Ta, and Cr, and other functional groups bonded to the second metal and selected from the group consisting of: i) proton carrier functional groups comprising a functional end group selected from the group consisting of -P0 3 H 2 , -COOH, -S0 3 H, and -SO 2 NHSO 2 CF 3 , ii) metal chelating functional groups comprising a functional end group selected from the group consisting of phosphonic acid, bipyridine, phenanthroline, and the like, and derivatives thereof, and iii) free radical scavenger functional groups comprising a functional end group selected from the group consisting of aminophenyl, hydroxyphenyl, and the like, and derivatives thereof.
  • the first and second metals in the two types of monomers can conveniently be the same metal, and particularly can be Si. Such copolymers therefore have a silicon-oxygen backbone.
  • the cross-linking functional groups can further have a chemical structure of the form -X-(end group) wherein X is a linear chain comprising a number of CH 2 , O, NH, or aryl groups in random sequence.
  • the cross-linking functional groups can particularly be -(CH 2 )2-NH 2 , -phenyl-NH 2> or -(CH 2 ) 3 -(1 H- benzimidazol-2-yl).
  • the other functional groups can be proton carrier functional groups comprising an end group selected from the group consisting of -P0 3 H 2 , -COOH, -S0 3 H, and
  • proton carrier functional groups can have a chemical structure of the form -Y-(end group) wherein Y is a linear chain comprising a number of CH 2 , CF 2 , or aryl groups in random sequence.
  • the proton carrier functional groups can particularly be -(CH 2 ) 2 -P0 3 H 2 .
  • the ratio of cross linking functional groups to proton carrier functional groups in the copolymer can be from about 1 :9 to 3:7.
  • the polymerized network can comprise at least two different metal oxide monomers with other functional groups, such as both a plurality of metal oxide monomers with proton carrier functional groups and a plurality of metal oxide monomers with free radical scavenger functional groups.
  • exemplary free radical scavenger functional groups are -3-nitro-4-aminophenyl.
  • the host proton conducting ionomer can comprise sulfonic acid groups, and in particular can be a perfluorosulfonic acid ionomer.
  • An effective amount of copolymer additive in the ionomer may be from about 5% to 10% by weight of the electrolyte.
  • the electrolytes of the invention are suitable for use in solid polymer electrolyte fuel cells. Along with improved durability, mechanical and chemical properties, fuel cell performance can even be improved under certain operating conditions, such as when operating at temperatures greater than 95 °C and at relative humidity less than 50% RH.
  • the electrolytes may be made by mixing an amount of the copolymer with an amount of the proton conducting ionomer, and then heating the mixture such that the copolymer bonds to the proton conducting ionomer.
  • sequences may be used in the preparation.
  • One possible approach comprises preparing the copolymer, adding the copolymer to a dispersion comprising the proton conducting ionomer, removing solvent from the dispersion thereby providing a solid mixture of the copolymer and the proton conducting ionomer, and then heating the mixture.
  • Another possible approach comprises adding the metal oxide monomers with cross- linking functional groups and the metal oxide monomers with other functional groups to a dispersion comprising the proton conducting ionomer thereby making the copolymer in situ in the dispersion, removing solvent from the dispersion thereby providing a solid mixture of the copolymer and the proton conducting ionomer, and then heating the mixture.
  • Yet another approach comprises preparing the copolymer, adding the copolymer to a dispersion comprising a precursor for the proton conducting ionomer, removing solvent from the dispersion thereby providing a solid mixture of the copolymer and the precursor, heating the mixture, and then converting the precursor to the proton conducting ionomer.
  • the copolymer may be prepared by preparing a solution comprising the metal oxide monomers with cross-linking functional groups and the metal oxide monomers with other functional groups, and heating the solution to a reaction temperature (e.g. greater than or about 50 °C) for a period of time (e.g. greater than or about 3 days) thereby forming the copolymer in solution.
  • a reaction temperature e.g. greater than or about 50 °C
  • a period of time e.g. greater than or about 3 days
  • the metal oxide monomers with cross-linking functional groups may be prepared by hydrolyzing unhydrolyzed metal oxide monomers with cross-linking functional groups.
  • exemplary unhydrolyzed metal oxide monomers with cross-linking functional groups include 3-aminopropyl-trimethoxysilane, aminophenyl-trimethoxysilane, or 3-(1 H- benzimidazol-2-yl)propyl-trimethoxysilane.
  • metal oxide monomers with other functional groups can be prepared by hydrolyzing unhydrolyzed metal oxide monomers with other functional groups.
  • exemplary unhydrolyzed metal oxide monomers with other functional groups include (2- diethylphosphatoethyl)triethoxysilane and 3-nitro-4-amino-phenyltriethoxysilane.
  • the hydrolyzing step and the making of the copolymer can be performed in the same solution.
  • the invention includes proton conducting polymer electrolytes, fuel cells comprising such electrolytes (such as in the membrane or catalyst layers), and methods of preparing such composite electrolytes and fuel cells.
  • Fig. 1 depicts the chemical structure of a copolymer with an exemplary metal oxide network and with general functional groups.
  • Figs. 2a and 2b show exemplary proton conducting electrolytes comprising a
  • Fig. 2a shows an example of a covalently bonded electrolyte
  • Fig. 2b shows an example of an acid-base bonded electrolyte.
  • Fig. 3 shows a plot of the open circuit voltage and fluoride release rate versus time for the tested stacks in the Examples.
  • Proton conducting electrolytes with improved durability and other desirable features can be obtained by bonding certain copolymer additives to suitable host proton conducting ionomers.
  • the copolymer additives comprise both cross-linking functional groups and other functional groups that are useful for various other purposes.
  • the proton conducting ionomer and the copolymer are bonded together at the cross-linking functional groups of the copolymer.
  • proton conducting ionomer refers to acidic ionomers characterized by a significant capability for proton conduction (and thus does not include basic ionomers). And with regards to the copolymer and the ionomer being "bonded together", this means the two are either covalently bonded or acid-base complexed together.
  • the chemical structure of a suitable copolymer is exemplified in Figure 1.
  • a portion of the overall structure is shown and comprises an exemplary metal oxide network and general functional groups.
  • the metal oxide network is generally a polymerized network of two types of metal oxide monomers and is characterized by an alternating series of oxygen and metal bonds.
  • One type of metal oxide monomer comprises cross-linking functional groups, represented by in Figure 1.
  • the other type of metal oxide monomer comprises other functional groups, represented by R 2 and R 2 ' in Figure 1.
  • the metals in these two types of monomers are represented by and M 2 respectively in Figure 1.
  • the two types of monomers can appear in random or block sequence in the copolymer and thus numerous configurations are possible for the network (including variations in chain length, branches, etc.).
  • Thus only an exemplary, partial structure for the network is shown in Figure 1. Those of ordinary skill in the art will appreciate the numerous variations possible for the configuration for the network.
  • the metals in both types of monomer are bonded to at least two oxygen atoms and thus are capable of forming a large polymeric network.
  • either of the monomers making up the copolymer may comprise more than two oxygen atoms.
  • the metals (M ⁇ M 2 ) in both types of monomer may be selected from the group consisting of Si, Ti, Zr, Ce, Ta, and Cr.
  • cross linking functional groups in the metal oxide monomers with cross-linking functional groups are bonded to the metals
  • R is a hydrocarbon group.
  • the cross-linking functional groups can further have a chemical structure of the form -X-(end group) wherein X is a linear chain comprising a number of CH 2 , O, NH, or aryl groups in random sequence.
  • the cross-linking functional groups -(CH 2 )2-NH 2 , -phenyl-NH 2 , and -(CH 2 ) 3 -(1 H-benzimidazol-2-yl) have been demonstrated as suitable in the Examples to follow.
  • the cross-linking functional groups include groups such as amino, hydroxy, pyridine, imidazole, and benzimidazole.
  • the functional groups R 2 and R 2 ' in the metal oxide monomers with other functional groups are bonded to the metals M 2 .
  • the other functional groups which may be considered here fall into one of three different types: proton carrier, metal chelating, and/or free radical scavenger functional groups.
  • Suitable proton carrier functional groups comprise an end group selected from the group consisting of -P0 3 H 2 , -COOH, -S0 3 H, and -S0 2 NHS0 2 CF 3 .
  • These proton carrier functional groups can have a chemical structure of the form -Y-(end group) in which Y is a linear chain comprising a number of CH 2 , CF 2 , or aryl groups in random sequence.
  • a CF 2 perfluoro structure may be preferred since the strong electron withdrawing power of the perfluoro units increase the acidity of the proton carrier end group, thereby increasing proton conductivity of final electrolyte.
  • the proton carrier functional group may be preferred since the strong electron withdrawing power of the perfluoro units increase the acidity of the proton carrier end group, thereby increasing proton conductivity of final electrolyte.
  • sulfonic acid functional groups are expected to provide higher proton conductivity than phosphonic acid or carboxylic acid functional groups.
  • a phosphonic acid functional group also serves as a good metal ion chelator and thus can serve more than one purpose here.
  • Suitable metal chelating functional groups comprise a functional end group selected from the group consisting of phosphonic acid, bipyridine, phenanthroline, and the like, and derivatives thereof.
  • Suitable free radical scavenger functional groups comprise a functional end group selected from the group consisting of aminophenyl, hydroxyphenyl, and the like, and derivatives thereof.
  • the free radical scavenger functional group 3-nitro-4-aminophenyl was found to be suitable in the Examples to follow.
  • the polymerized network can comprise at least two different metal oxide monomers with other functional groups, such as both a plurality of metal oxide monomers with proton carrier functional groups and a plurality of metal oxide monomers with free radical scavenger functional groups.
  • the two metals M 2 in the different monomers may be different, it can be advantageous for them to be the same.
  • Si for instance is a preferred metal for both types of monomer.
  • the relative amounts of these various functional groups may also be varied widely in accordance with the properties desired.
  • a proton conducting electrolyte of the invention comprises a proton conducting ionomer and the copolymer described above.
  • a portion of the electrolyte structure is exemplified in Figures 2a and 2b.
  • the exemplary polymer electrolyte of Figure 2a comprises perfluorosulfonic acid ionomer 1 cross-linked via covalent bonding at locations 3 to copolymer 2.
  • the exemplary polymer electrolyte of Figure 2b comprises perfluorosulfonic acid ionomer 1 cross-linked via acid-base bonding at locations 3 to copolymer 2.
  • the amount of copolymer additive to be used in the electrolyte will depend on several factors. Preferably, a minimal amount of additive is used to obtain the desired results. A common range might be from about 5-10% by weight although amounts outside this range can certainly be considered.
  • the electrolyte of the invention When used as an electrolyte in solid polymer electrolyte fuel cells, the electrolyte of the invention provides for improved durability over the ionomer alone and also other benefits. While primarily intended for use as the membrane electrolyte in such fuel cells, the inventive electrolyte may also be considered for use elsewhere, for instance in a catalyst layer for either the cathode or anode, or in a coating in the gas diffusion layers or electrodes.
  • the electrolyte of the invention can be prepared in various ways.
  • a general method involves mixing an amount of the copolymer with an amount of the proton conducting ionomer, and then heating the mixture such that the copolymer bonds to the proton conducting ionomer.
  • An approach for accomplishing this comprises preparing the copolymer, adding the copolymer to a dispersion comprising the proton conducting ionomer, and removing solvent from the dispersion to provide a solid mixture of the copolymer and the proton conducting ionomer. The solid mixture is then heated to complete the preparation.
  • Another possible approach involves making the copolymer in situ in a dispersion with the proton conducting ionomer.
  • the metal oxide monomers with cross- linking functional groups and the metal oxide monomers with other functional groups can be added to a dispersion comprising the proton conducting ionomer.
  • the solvent is then removed from the dispersion thereby providing a solid mixture of the copolymer and the proton conducting ionomer. And, the solid mixture is then heated completing the preparation.
  • Yet another approach comprises preparing the copolymer first, adding the copolymer to a dispersion comprising a precursor for the proton conducting ionomer, and removing solvent from the dispersion thereby providing a solid mixture of the copolymer and the precursor. This mixture is then heated to complete the cross-linking reaction and then followed by acid treatment to convert the precursor to the proton conducting ionomer.
  • the copolymer additives are therefore preferably water, alcohol, or acid soluble. Certain desirable copolymers may be commercially obtained.
  • a desired copolymer may be prepared by preparing a solution comprising appropriate metal oxide monomers with cross-linking functional groups and appropriate metal oxide monomers with other functional groups, and heating this solution to a reaction
  • the metal oxide monomers with cross-linking functional groups used in such a method may be prepared by hydrolyzing unhydrolyzed metal oxide monomers with cross-linking functional groups.
  • exemplary unhydrolyzed metal oxide monomers with cross-linking functional groups include 3-aminopropyl-trimethoxysilane, aminophenyltrimethoxysilane, or 3-(1 H-benzimidazol-2yl)propyl-trimethoxysilane.
  • metal oxide monomers with other functional groups can be prepared by hydrolyzing unhydrolyzed metal oxide monomers with other functional groups.
  • exemplary unhydrolyzed metal oxide monomers with other functional groups include (2- diethylphosphatoethyl)triethoxysilane and 3-nitro-4-amino-phenyl-triethoxysilane.
  • the hydrolyzing step and the making of the copolymer can be performed in the same solution.
  • Fuel cells comprising the prepared electrolyte can be made in a conventional manner.
  • a dispersion/solution comprising the prepared electrolyte can be used to cast membrane electrolyte, to prepare catalyst layers, or otherwise be incorporated into membrane electrode assemblies as desired.
  • a membrane can be cast directly from the mixture dispersion/solution.
  • a catalyst can be mixed with the ionomer dispersion/solution containing the additive to make an ink, and then the ink can be coated onto a membrane to make a catalyst coated membrane.
  • the electrolytes of the invention offer many potential advantages in fuel cells depending on the copolymers employed and the functional groups involved.
  • the copolymer additives and the electrolytes comprising them can be quite facile to synthesize.
  • the metal-oxide backbone in the copolymer can improve thermal stability of the membrane electrolyte, and help retain water therein at high temperature, thus improving durability and also performance at high temperature and low humidity conditions.
  • the cross-linking between the copolymer additive and the host ionomer prevents the copolymer additive from being leached out during operation and improves the dimensional stability of the membrane electrolyte.
  • the protons can be self-dissociated from the anion, even without water molecule facilitation, such that the performance of the membrane electrolyte even under dry condition can be improved.
  • metal ion chelating groups present, metal ions in the membrane electrolyte can be inactivated, consequently reducing or inhibiting free radical formation therein.
  • free radical scavenger functional groups present, free radicals can be trapped again improving membrane durability.
  • the improved additives were generally prepared by first making appropriate cross-linking monomers and proton carrier monomers.
  • the copolymer additives were then usually made by polymerizing a plurality of both monomer types together from a suitable mixture. In one case however, cross-linking monomer synthesis and polymerization was done concurrently.
  • membrane samples were prepared by adding a desired amount of additive to dispersions of perfluorosulfonic acid (PFSA) ionomer with equivalent weights (EW) of either 830 or 950, mixing while stirring overnight, degassing the solution, and casting samples onto a glass plate. After evaporating off the solvent at room temperature for 2 hours, the obtained membrane samples were annealed at 150 °C for 1 hour.
  • PFSA perfluorosulfonic acid
  • EW equivalent weights
  • EPETES (2-diethylphosphatoethyl)triethoxysilane
  • Equation 2 OCH 3 OH
  • a hydrolyzed APMS solution was prepared in a like manner to the above and comprised 2.04 g APMS in 40 g of alcohol solution comprising 0.2 ml of 2M HCI and 5 g of water. 47 g of this hydrolyzed APMS solution was then added to 26.6 mmol of the above 25% hydrolysed EPETES alcohol solution and allowed to react at 50 °C for 3 days. Again, the solution was then filtered and washed to provide a white, water insoluble solid powder. This time, the molar ratio of APMS to EPETES was 3:7 and hence the N:P
  • Composite membrane samples were then prepared with each additive as described generally above. Two different amounts were used and ionomer dispersions having two different EW of 830 and 950 were used. The composite membrane samples were:
  • Inventive membrane 1a 10% by weight of the first silicon oxide copolymer additive having N:P of 1 :9 in PFSA ionomer host with EW of 950.
  • Inventive membrane 1 b 10% by weight of the second silicon oxide copolymer additive having N:P of 3:7 in PFSA ionomer host with EW of 950.
  • Inventive membrane 1c 5% by weight of the first silicon oxide copolymer additive having N:P of 1 :9 in PFSA ionomer host with EW of 950.
  • Inventive membrane 1d 10% by weight of the first silicon oxide copolymer additive having N:P of 1 :9 in PFSA ionomer host with EW of 830.
  • APMS 3-aminopropyl-trimethoxysilane
  • NPS 3-nitro-4-amino-phenyl-triethoxysilane
  • Inventive membrane 4 Composite membrane samples were then prepared with this additive as described generally above and are denoted as Inventive membrane 4. All had the same composition of 10% by weight of the silicon oxide copolymer additive having a N:P of 1 :9 in PFSA ionomer host with EW of 950.
  • PFSA perfluorosulfonic acid
  • NRE21 DuPontTM Nafion® PFSA NRE21 1 membrane
  • Tg Glass transition temperatures of Inventive membranes 1 a, 1 b and comparative PFSA 950EW were determined from dynamic mechanical analysis (DMA) measurements carried out using a DMA 800. Table 1 compares the Tg values for the samples studied. Table 1
  • the thermal stability of the Inventive composite membranes are markedly improved compared to that of the conventional PFSA 950EW membrane.
  • the Tg values of the former are substantially higher than the latter.
  • the membrane sample comprising 10% of the Inventive Example 1 b additive has a greater amount of the cross-linking functional groups (greater N:P ratio) and as a result shows a substantially higher Tg than the membrane sample with the same amount of the Inventive Example 1a additive.
  • the extent to which membrane samples are cross-linked can be determined qualitatively using a simple solubility test in DMAc solvent.
  • 2 square centimeter pieces of a comparative NRE211 sample and an inventive membrane sample comprising 10% by weight of Inventive Example 1 b additive were placed in separate vials containing 25 ml DMAc at 50 °C. After 3 days, the NRE211 sample had broken up into small pieces and after 15 days, it had dissolved completely. After 3 days, the inventive membrane sample had swelled but otherwise remained unchanged after 15 days.
  • the inventive cross-linked membrane sample clearly showed increased solvent resistance to that of the comparative NRE211 membrane in DMAc solvent.
  • the in-plane proton conductivity of the inventive sample comprising 5% of the Inventive Example 1c additive along with a comparative PFSA 950EW membrane were determined by checking the AC impedance of samples using a four-probe technique and a Solarton FRA 1260 frequency response analyzer.
  • the scanning frequencies ranged from 10 MHz to 100 Hz and the samples were held at the test conditions for 6 h in order to reach equilibrium before measurements were made. Measurements were made at 80 °C and two different relative humidities (RH) of 30% and 50%. The results are summarized in Table 2.
  • the conductivity of the inventive sample was similar to that of the comparative PFSA 950EW sample at 50% RH and better at 30% RH.
  • Test fuel cells were assembled with some of the preceding Inventive membrane samples 1a and 1d in order to compare their performance to that of the conventional membranes comprising the same base ionomer.
  • Individual MEAs were made by bonding the appropriate membrane sample between cathode and anode electrodes.
  • the cathode and anode had Pt loadings of 0.7 mg/cm 2 and 0.3 mg/cm 2 respectively.
  • Performance evaluation was carried out using a single cell stack with 50 cm 2 of active area hardware.
  • Table 3 shows the voltage obtained at 1 A/cm 2 in each case.
  • the relative durability of MEAs can be evaluated by setting fuel cells made with test MEAs in an open circuit condition in order to accelerate chemical decay of the membrane therein.
  • the rate of decay at open circuit voltage (OCV) can be indicative of membrane chemical stability.
  • MEAs made with Inventive membrane sample 1a and Comparative membrane PFSA 950EW were tested and compared.
  • 3-cell stacks were made using the same method and hardware as in the preceding example.
  • test stacks were evaluated under OCV conditions at 30% relative humidity (RH) and 95 °C.
  • the supplied gas flow-rates were 3.5 and 11 slpm for hydrogen and air
  • the OCV of each cell in the stack was monitored over time. Testing was stopped when the OCV in any one of the 3 cells in the stack reached 0.75 V.
  • the amount of fluoride released as a result of decomposition of the membrane was determined over time (i.e. the fluoride release rate) by measuring the fluoride ion found in both the cathode and anode outlet water.
  • FIG. 3 shows plots of OCV and fluoride release rate versus time for the tested stacks.
  • the OCV decay rate of the stack made with Inventive membranes 1a was 0.0008 V/h, while that of the stack made with Comparative membranes PFSA 950EW was 0.0015 V/h.
  • the former failed at 113 h, while the latter failed at 66 h.
  • the fluoride release rate for the stack made with Inventive membranes 1a was much lower than that of the stack made with Comparative membranes PFSA 950EW.
  • the stacks made with the Inventive membrane showed superior durability compared to that of the conventional stack.

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Abstract

L'invention porte sur un électrolyte polymère conducteur de protons comprenant un ionomère conducteur de protons réticulé avec une certaine quantité d'un additif copolymère comprenant des groupes fonctionnels de réticulation et d'autres groupes fonctionnels (par exemple des porteurs de protons, des agents chélateurs, des désactivateurs de radicaux) qui présente une durabilité améliorée par rapport à l'ionomère seul et qui permet une incorporation plus stable de ces autres groupes fonctionnels. L'additif copolymère comprend au moins deux types de monomères oxydes métalliques, l'un ayant des groupes fonctionnels de réticulation et l'autre les autres groupes fonctionnels.
PCT/EP2012/000663 2011-03-03 2012-02-15 Électrolytes conducteurs de protons renfermant des additifs copolymères réticulés destinés à être utilisés dans des piles à combustible WO2012116782A1 (fr)

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US13/039,432 2011-03-03
US13/039,470 US8710108B2 (en) 2011-03-03 2011-03-03 Proton conducting electrolytes with cross-linked copolymer additives for use in fuel cells
US13/039,470 2011-03-03
US13/039,432 US8709677B2 (en) 2011-03-03 2011-03-03 Proton conducting electrolytes with cross-linked copolymer additives for use in fuel cells

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Cited By (1)

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DE102014208547A1 (de) * 2014-05-07 2015-11-12 Volkswagen Ag Membran-Elektroden-Einheit mit funktionalisiertem Trägermaterial sowie Brennstoffzelle mit einer solchen

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