DE112012001070T5 - Proton-conducting electrolytes with cross-linked copolymer additives for use in fuel cells - Google Patents

Abstract

A proton conductive polymer electrolyte comprising a proton conductive ionomer crosslinked with an amount of a copolymer additive comprising crosslinking functional groups and other functional groups (e.g., proton supports, chelating chelants, radical scavengers) exhibits improved durability versus the ionomer alone and provides a more stable inclusion of these other functional groups. The copolymer additive comprises at least two types of metal oxide monomers, one of which has crosslinking functional groups and the other has other functional groups.

Description

Field of the invention

The invention relates to improved proton conductive polymer electrolytes for use in polymer electrolyte fuel cells. In particular, it relates to electrolytes comprising an ionomer crosslinked with a copolymer additive comprising both crosslinking and other useful functional groups.

Description of the Related Art

Solid polymer electrolyte fuel cells convert reactants, namely a fuel (such as hydrogen) and an oxidant (such as oxygen or air) to produce electrical power. Such fuel cells generally use a proton-conductive polymer electrolyte membrane between two electrodes, namely a cathode and an anode. A structure comprising a proton conductive polymer membrane interposed between two electrodes is referred to as a membrane electrode assembly (MEA). The durability of the MEA is one of the most important topics for the development of fuel cell systems in either stationary or traffic related applications. For mobile applications it is required that the MEA prove a durability of approximately 6000 hours.

The membrane serves as a separator, which prevents mixing of reaction gases, and as an electrolyte for the transport of protons from the anode to the cathode. Perfluorosulfonic acid (PFSA) ionomers, such as Nafion ^®, are currently the preferred material and the technology standard for membranes. Nafion ^® consists of a perfluorinated backbone bearing pendant vinyl ether side chains ending in SO ₃ H.

Failure of the membrane as an electrolyte results in reduced performance due to increased ionic resistance, and failure of the membrane as a separator results in failure of the fuel cell due to mixing reaction gases of the anode and the cathode. The chemical degradation of PFSA membranes during fuel cell operation is said to occur via the attack of hydroxyl (· OH) or peroxyl (· OOH) radical species on weak groups (such as a carboxylic acid group) in the molecular chain of the ionomer. The free radicals can be generated by the decomposition of hydrogen peroxide with impurities (such as Fe ²⁺ ) in a kind of Fenton reaction. In fuel cells, hydrogen peroxide can be formed either on carbon black-supported Pt in the catalyst layers or during the reduction reaction of the oxygen. The hydroxyl radical attacks the unstable end groups of the polymer and causes coupling of chains and / or can also attack an SO ₃ ^- group under dry conditions to effect chain scission. Both attacks degrade the membrane and possibly lead to cracking in the membrane, thinning or the formation of pinholes. The degradation rate of the membrane is significantly accelerated with an increase in operating temperature and a decrease in the relative humidity (RH) of the gas at the inlet.

Numerous modifications and / or additives for the membrane electrolyte have been studied for purposes of improving the performance and / or durability of the membrane. For example, the US 2008/0152986 in terms of durability, an impregnated, crosslinked or uncrosslinked basic polymer (for example, poly (benzimidazole)) membrane prepared here with an acidic dopant (for example, a phosphoric acid or an organic phosphonic acid) to form a polymer electrolyte To receive membrane. Such membranes have good dimensional stability but generally do not have good low temperature conductivity (<100 ° C). In addition, the acidic dopant is washed out of the membrane over time during operation of the fuel cell.

In the US 2006/0199062 For example, a polymer mixture of a perfluorocarbonsulfonic acid resin and a polyazole-based compound or a polymer has been used as the proton-conductive electrolyte. The strong interaction between the two reduces the transition of hydrogen and improves the durability of the membrane. Nonetheless, this interaction also dramatically reduces proton conductivity and, consequently, the performance of the membrane electrolyte.

Crosslinking of the electrolyte membranes generally improves durability. In the US 6733914 a crosslinked proton exchange membrane was prepared by adding ammonia water was used to treat a precursor in the form of a Nafion ^® film. The cross-linking groups in this member were sulfonamide. The US 2002/0091201 discloses a general technique to To provide crosslinks in perfluorinated polymers in which the crosslinks or bonds exist between sulfonyl groups attached to adjacent polymeric chains. However, such cross-linking reduces the number of sulfonic acid groups in the membrane, and while thus improving durability, it also causes less proton conductivity and also membrane performance.

In the US 2010/0040927 A process for the production of a grafted polymer electrolyte film for a fuel cell has been disclosed. This grafted electrolyte is heterogeneous and has a silane cross-linked structure between the grafted molecular chains. However, its vinyl structure has been found to be unstable in the fuel cell environment.

Furthermore, the describes WO 2005/027240 the preparation of phosphonic acid-grafted inorganic-organic hybrid polymers with a metal oxide backbone. The polymers can be used directly as proton-conductive electrolyte membranes in fuel cells. The phosphonic acid groups allow protons to pass through the membrane at low RH. Composite materials comprising these polymers and other basic polymers are also recommended therein.

The US 2007/0154764 discloses electrolyte additives comprising hygroscopic particles formed from metal oxide, such as silica or zirconia, heteropolyacids, silica gel phosphonate, etc., to increase water retention and thereby improve the performance of the MEA under low RH conditions.

The WO 2005/036687 discloses a water-insoluble additive comprising a crosslinked metal oxide matrix having phosphonic acid groups covalently bonded to the matrix via crosslinkers. The additive can then be distributed homogeneously over a proton-conductive membrane and improve the conductivity of the membrane for ions at high temperatures (> 100 ° C).

The US 2006/0141313 discloses particles comprising a metal-oxygen cross-linked structure as an additive for a proton conductive membrane. The particles have an acid group such as a sulfonic acid group integrated in the surface thereof. However, these and many other additives in the prior art are susceptible to being washed out of the membrane during operation of the fuel cell, either because they are water-soluble or because they have no covalent bond or strong interaction with the base polymer.

Many different functional groups can also be integrated into the proton-conducting electrolytes for various reasons. The US 2004/0043283 discloses the incorporation of metallic elements or compositions containing metallic elements or metal alloys which function as free radical scavengers or as catalysts for the decomposition of hydrogen peroxide. The US 2006/0046120 discloses the use of phenolic antioxidants wherein the antioxidant may be a small molecule or a polymer. And the US 6607856 discloses a solid-polymer electrolyte having high durability and resistance to oxidation, which is prepared by introducing a chelate group and an electrolyte group into a polymer electrolyte material having a hydrocarbon portion. The chelate complexing agents reduce the formation of free radicals. However, the presence of such additives in the MEA can result in reduced fuel cell performance.

In fact, all of the aforementioned recommended changes and additives may suffer from one or more of the following problems: decreased proton conductivity of the electrolyte, insufficient durability of the membrane, or additives that are washed out of the electrolyte over time during operation of the fuel cell. Accordingly, there remains a need for improved electrolytes for MEAs in polymer electrolyte fuel cells. The present invention fulfills this need and provides other related advantages.

Summary

Proton conductive electrolytes having improved durability and other desirable features can be obtained by crosslinking certain copolymer additives with suitable proton conductive base ionomers. The copolymer additives include both crosslinking functional groups and other functional groups which can impart additional desirable properties to the electrolyte. The other functional groups include proton carriers, chelated metal-forming groups, and radical scavengers for radicals. The proton conductive ionomer and the Copolymer are bonded to each other at the crosslinking functional groups of the copolymer. This crosslinking or bonding provides durability and generally improved mechanical properties, while also serving to more reliably attach the other functional groups so as to prevent them from being washed 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 having crosslinking functional groups and a plurality of metal oxide monomers having other functional groups in a random sequence. The polymerized network is characterized by an alternating sequence of oxygen bonds and metal bonds.

wobei R eine Kohlenwasserstoffgruppe ist, besteht.The metal oxide monomers having crosslinking functional groups include:
a first metal bonded to at least two oxygen atoms and selected from the group consisting of Si, Ti, Zr, Ce, Ta and Cr, and
crosslinking functional groups bonded to the first metal and having a functional end group containing nitrogen or oxygen and characterized by a chemical structure selected from the group consisting of: -NH ₂ , = NH, - (aliphatic ) -OH or - (aryl) -OH,

where R is a hydrocarbon group.

• i) Protonen tragenden funktionellen Gruppen, welche eine funktionelle Endgruppe umfassen, welche aus der Gruppe ausgewählt ist, welche aus -PO₃H₂, -COOH, -SO₃H und -SO₂NHSO₂CF₃ besteht,
• ii) mit Metall einen Chelat-Komplex bildenden funktionellen Gruppen, welche eine funktionelle Endgruppe aufweisen, welche aus der Gruppe ausgewählt ist, welche aus Phosphonsäure, Bipyridin, Phenanthrolin und dergleichen und Derivaten davon besteht, und
• iii) funktionellen Radikalfängergruppen für freie Radikale, welche eine funktionelle Endgruppe aufweisen, welche aus der Gruppe ausgewählt ist, welche aus Aminophenyl, Hydroxyphenyl und dergleichen und Derivaten davon besteht,

besteht.The metal oxide monomers having other functional groups include:
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 connected to the second metal and selected from the group consisting of:

• i) proton-bearing functional groups comprising a functional end group selected from the group consisting of -PO ₃ H ₂ , -COOH, -SO ₃ H and -SO ₂ NHSO ₂ CF ₃ ,
• ii) metal-chelating complex-forming functional groups having a functional end group selected from the group consisting of phosphonic acid, bipyridine, phenanthroline and the like and derivatives thereof, and
• iii) free radical functional radical scavenger groups having a functional end group selected from the group consisting of aminophenyl, hydroxyphenyl and the like and derivatives thereof,

consists.

The first and second metals in the two types of monomers may suitably be the same metal and may be Si in particular. Such copolymers therefore have a silicon-oxygen backbone.

The crosslinking functional groups may further have a chemical structure of the form -X- (end group), where X is a linear chain having a number of CH ₂ , O, NH or aryl groups in random order. As shown in the examples below, the crosslinking functional groups may in particular be - (CH ₂ ) ₂ -NH ₂ , -phenyl-NH ₂ or - (CH ₂ ) ₃ - (1H-benzimidazol-2-yl).

The other functional groups may be proton-bearing functional groups having an end group selected from the group consisting of PO ₃ H ₂ , -COOH, -SO ₃ H ₂ and -SO ₂ NHSO ₂ CF ₃ . These proton-bearing functional groups can have a chemical structure Y is a linear chain comprising a number of CH ₂ , CF ₂ or aryl groups in random order. Again, as shown in the examples below, the proton-bearing functional groups may be, in particular, - (CH ₂ ) ₂ -PO ₃ H ₂ . Furthermore, the ratio of crosslinking functional groups to proton-bearing functional groups in the copolymer can be from about 1: 9 to 3: 7.

More than one kind of functional groups can be used in the copolymer additives. For example, the polymerized network may comprise at least two different metal oxide monomers having other functional groups, such as both a plurality of metal oxide monomers having proton-bearing functional groups and a plurality of free-radical functional radical scavenger metal oxide monomers. Exemplary functional radical scavenger groups for free radicals are -3-nitro-4-aminophenyl.

The proton conductive base ionomer may comprise sulfonic acid groups and, in particular, be a perfluorosulfonic acid ionomer. An effective amount of the copolymer additive in the ionomer may be from about 5 percent to 10 percent by weight of the electrolyte.

The electrolytes according to the invention are suitable for use in solid polymer electrolyte fuel cells. Along with improved durability and improved mechanical and chemical properties, the performance of the fuel cell may even be improved under certain operating conditions, such as when operating at temperatures greater than 95 ° C and a relative humidity of less than 50% RH.

The electrolytes may be prepared by mixing an amount of the copolymer with an amount of the proton conductive ionomer and then heating the mixture such that the copolymer combines with the proton conductive ionomer. Several different sequences could be used in the manufacture.

One possible approach involves making the copolymer, adding the copolymer to a dispersion comprising the proton conductive ionomer, removing solvent from the dispersion, and thus providing a solid mixture of the copolymer and the proton conductive ionomer, and then heating the mixture.

Another possible approach involves adding the metal oxide monomer having crosslinking functional groups and the metal oxide monomer having other functional groups to a dispersion comprising the proton conductive ionomer, thereby preparing the copolymer in situ in the dispersion, removing solvent from the dispersion, and thus providing a solid mixture of the copolymer and the proton conductive ionomer and then heating the mixture.

Yet another approach involves making the copolymer, adding the copolymer to a dispersion comprising a precursor to the proton conductive ionomer, removing solvent from the dispersion, and thus providing a solid mixture of the copolymer and the precursor, heating the Mixing and then converting the precursor into the proton conductive ionomer.

The copolymer may be prepared by preparing a solution comprising the metal oxide monomers having crosslinking functional groups and the metal oxide monomers having other functional groups, heating the solution to a reaction temperature (for example, greater than or about 50 ° C). during a period of time (for example, more than or about 3 days) and so forming the copolymer in solution.

In the process, the metal oxide monomers having crosslinking functional groups can be prepared by hydrolyzing unhydrolyzed metal oxide monomers having crosslinking functional groups. Exemplary unhydrolyzed metal oxide monomers having crosslinking functional groups include 3-aminopropyltrimethoxysilane, aminophenyltrimethoxysilane or 3- (1H-benzimidazol-2-yl) propyltrimethoxysilane.

Further, the metal oxide monomers having other functional groups can be prepared by hydrolyzing unhydrolyzed metal oxide monomers having other functional groups. Exemplary non-hydrolyzed metal oxide monomers having other functional groups include (2-diethylphosphatoethyl) triethoxysilane and 3-nitro-4-amino-phenyltriethoxysilane.

With regard to both kinds of metal oxide monomers, the step of hydrolyzing and producing the copolymer can be carried out in the same solution.

The invention includes proton conductive polymer electrolytes, fuel cells comprising such electrolytes (such as in the membrane or catalyst layers), and methods of making such composite electrolytes and fuel cells.

These and other aspects of the invention will become apparent with reference to the attached figures and the following detailed description.

Brief description of the drawings:

1 discloses the chemical structure of a copolymer having an exemplary metal oxide network and having general functional groups.

2a and 2 B show exemplary proton conductive electrolytes comprising a persulfonic acid ionomer crosslinked with a copolymer having a silica network and with crosslinking and proton bearing functional groups. 2a shows an example of a covalently bonded electrolyte and 2 B shows an example of an acid-base bound electrolyte.

3 Figure 11 is a plot of open circuit voltage and fluoride release rate versus time for stacks tested in the examples.

Detailed description

Photonic conductive electrolytes with improved durability and other desirable features can be obtained by combining certain copolymer additives with suitable proton conductive base ionomers. The copolymer additives include both crosslinking functional groups and other functional groups which are useful for various other purposes. The proton conductive ionomer and the copolymer are linked together at the crosslinking functional groups of the copolymer.

As used herein, "proton-conductive ionomer" refers to acidic ionomers characterized by significant proton conduction capability (and thus not including basic ionomers). And with regard to the copolymer and the ionomer, which are "bonded together", this means that the two are either covalently bonded or acid-base complexed together.

The chemical structure of a suitable copolymer is exemplified in 1 shown. Part of the overall structure is shown and includes 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 sequence of oxygen bonds and metal bonds. One type of the metal oxide monomer includes crosslinking functional groups which are described in U.S. Pat 1 are illustrated by R ₁ . The other type of metal oxide monomer includes other functional groups which are known in the art 1 are illustrated by R ₂ and R ₂ '. The metals in these two types of monomers are in 1 each illustrated by M ₁ and M ₂ . The two types of monomers can occur in the copolymer in random order or block order, and thus numerous designs of the network are possible (which include variations in chain length, branches, etc.). Therefore, in 2 only an exemplary partial structure for the network is shown. One of ordinary skill in the art will recognize that numerous variations in design are possible for the network.

The metals in both types of monomers are linked to at least two oxygen atoms and thus capable of forming a broad polymeric network. Although not in 1 As shown, each of the monomers forming the copolymer may comprise more than two oxygen atoms. The metals (M ₁ , M ₂ ) in both types of the monomers may be selected from the group consisting of Si, Ti, Zr, Ce, Ta and Cr.

wobei R eine Kohlenwasserstoffgruppe ist, besteht. Die quervernetzenden funktionellen Gruppen können des Weiteren eine chemische Struktur der Form -X-(Endgruppe) aufweisen, wobei X eine lineare Kette ist, welche eine Anzahl an CH₂-, O-, NH- oder Aryl-Gruppen in zufälliger Abfolge umfasst. Die quervernetzenden funktionellen Gruppen -(CH₂)₂-NH₂, -Phenyl-NH₂ und -(CH₂)₃-(1H-Benzimidazol-2-yl) haben sich in den folgenden Beispielen als geeignet erwiesen. Weil das Copolymer sich mit dem protonenleitfähigen Ionomer entweder über kovalente Bindungen oder Säure-Base-Komplexierung verbinden kann, umfassen die quervernetzenden funktionellen Gruppen Gruppen wie etwa Amino-, Hydroxy-, Pyridin, Imidazol und Benzimidazol. The crosslinking functional groups R ₁ in the metal oxide monomers having crosslinking functional groups are bonded to the metals M ₁ . The various cross-linking functional groups which may be considered herein include a functional end group containing nitrogen or oxygen and are characterized by a chemical structure selected from the group consisting of -NH ₂ , = NH, - (aliphatic) -OH or - (aryl) -OH,

where R is a hydrocarbon group. The crosslinking functional groups may further have a chemical structure of the form -X- (end group), where X is a linear chain comprising a number of CH ₂ , O, NH or aryl groups in random order. The crosslinking functional groups - (CH ₂ ) ₂ -NH ₂ , -phenyl-NH ₂ and - (CH ₂ ) ₃ - (1H-benzimidazol-2-yl) have been found to be useful in the following examples. Because the copolymer can combine with the proton conductive ionomer through either covalent bonding or acid-base complexation, the crosslinking 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 having other functional groups are linked to the metals M ₂ . The other functional groups that can be considered here fall into one of three different types: proton carriers, chelated metal-forming and / or functional free-radical scavenger groups.

Suitable proton-bearing functional groups include an end group selected from the group consisting of -PO ₃ H ₂ , -COOH, -SO ₃ H, and -SO ₂ NHSO ₂ CF ₃ . These proton-bearing functional groups may 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 order. (A CF ₂ perfluoro structure may be preferred because the strong electron withdrawing power of the perfluoro units increases the acidity of the proton-bearing end group and thereby increases the proton conductivity of the final electrolyte.) The Proton-bearing Functional Group - (CH ₂ ) ₂ - PO ₃ H ₂ was successfully used in the following examples. Generally, it is expected that sulfonic acid functional groups bring about higher proton conductivity than phosphonic acid functional groups or carboxylic acid groups. However, a functional phosphonic acid group also serves as a good chelating agent for metal ions and can serve more than one purpose.

Suitable chelated metal-forming functional groups include a functional end group selected from the group consisting of phosphonic acid, bipyridine, phenanthroline and the like and derivatives thereof.

Suitable free radical functional scavenger groups include a functional end group selected from the group consisting of aminophenyl, hydroxyphenyl and the like and derivatives thereof. The functional radical scavenger group 3-nitro-4-aminophenyl was found suitable in the following examples.

More than one kind of other functional groups (that is, where R _{2 is} not the same as R ₂ 'in 1 ) can be used in the copolymer additives. For example, the polymerized network may comprise at least two different metal oxide monomers having other functional groups, such as both a plurality of metal oxide monomers having proton-bearing functional groups and a plurality of metal radical monomers having free radical functional scavenging groups.

Also, although the two metals M ₁ , M ₂ may be different in the different monomers, it may be advantageous for them to be the same. For example, Si is a preferred metal for both types of monomer.

Along with the numerous choices for the types of functional groups in the copolymer, the relative amounts of these various functional groups can also be varied widely according to the desired properties.

A proton-conductive electrolyte according to the invention comprises a proton-conductive ionomer and the above-described copolymer. Part of the structure of the electrolyte is exemplary in FIGS 2a and 2 B shown. The exemplary polymer electrolyte of 2a includes a perfluorosulfonic acid ionomer 1 , which via covalent bonds in places 3 with the copolymer 2 is cross-linked. Here, the copolymer 2 a silica network (where M ₁ = M ₂ = Si) and has cross-linking functional groups (R ₁ ) containing functional -NH end groups and - (CH ₂ ) ₂ -PO ₃ H ₂ proton-bearing functional groups (R ₂ ) include. The exemplary polymer electrolyte of 2 B includes a perfluorosulfonic acid ionomer 1 , which has acid-base bonds in places 3 with the copolymer 2 is cross-linked. Here, the copolymer 2 a silica network (with M ₁ = M ₂ = Si), which contains 3- (1H-benzimidazol-2yl) propyl cross-linking functional groups (R ₁ ) and - (CH ₂ ) ₂ -PO ₃ H ₂ proton-bearing functional groups (R ₂ ).

The amount of copolymer additive to be used in the electrolyte depends on several factors. Preferably, a minimum amount of the additive is used to obtain the desired results. A typical range could be from about 5 to 10 weight percent, although amounts outside this range can certainly be considered.

When used as an electrolyte in solid polymer electrolyte fuel cells, the electrolyte according to the invention provides improved durability over the ionomer alone and also for other benefits. Although intended primarily for use as the membrane electrolyte in such fuel cells, the electrolyte of the invention may also be considered for alternative use, for example in a catalyst layer for either the cathode or the anode or in a coating in FIG the gas diffusion layers or electrodes.

The electrolyte according to the invention can be produced in various ways. One general method involves blending an amount of the copolymer with an amount of the proton conductive ionomer and then heating the blend such that the copolymer combines with the proton conductive ionomer. One approach to accomplish this includes preparing the copolymer, adding the copolymer to a dispersion containing the proton conductive ionomer, and removing solvent from the dispersion to provide a solid mixture of the copolymer and the proton conductive 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 conductive ionomer. Here, for example, the metal oxide monomers having crosslinking functional groups and the metal oxide monomers having other functional groups may be added to a dispersion comprising the proton conductive ionomer. The solvent is then removed from the dispersion to provide a solid mixture of the copolymer and the proton conductive ionomer. And the solid mixture is then heated to complete the production.

Yet another approach involves first preparing the copolymer, adding the copolymer to a dispersion comprising a precursor of the proton conductive ionomer, and removing solvent from the solution to provide a solid mixture of the copolymer and the precursor. The mixture is then heated to complete the cross-linking reaction and then followed by acid treatment to convert the precursor to the proton conductive ionomer.

In the above, therefore, the copolymer additives are preferably soluble in water, alcohol or acid. Certain desirable copolymers can be obtained commercially. Alternatively, a desired copolymer may be prepared by preparing a solution comprising suitable metal oxide monomers having crosslinking functional groups and suitable metal oxide monomers having other functional groups, and heating this solution to a reaction temperature over a period of time to form the copolymer in to form the solution.

The metal oxide monomers having crosslinking functional groups used in such a process can be prepared by hydrolyzing unhydrolyzed metal oxide monomers having crosslinking functional groups. Exemplary unhydrolyzed metal oxide monomers having crosslinking functional groups include 3-aminopropyltrimethoxysilane, aminophenyltrimethoxysilane or 3- (1H-benzimidazol-2yl) propyltrimethoxysilane.

Furthermore, the metal oxide monomers having other functional groups can be prepared by hydrolyzing unhydrolyzed metal oxide monomers having other functional groups. Exemplary non-hydrolyzed metal oxide monomers having other functional groups include (2-diethylphosphatoethyl) triethoxysilane and 3-nitro-4-amino-phenyl-triethoxysilane.

With regard to both kinds of the metal oxide monomers, the step of hydrolyzing and the production of the copolymer can be carried out in the same solution.

Fuel cells comprising the produced electrolyte can be prepared in a conventional manner. For example, a dispersion / solution comprising the produced electrolyte can be used to cast a membrane electrolyte, make catalyst layers, or otherwise be integrated into the membrane-electrode assembly as desired. In particular, a membrane can be poured directly from the mixture of the dispersion / solution. Alternatively, a catalyst may be mixed with the dispersion / solution of the ionomer containing the additive to make an ink, and then the ink may be applied to a membrane to produce a catalyst coated membrane.

The electrolytes according to the invention offer many potential advantages in fuel cells depending on the copolymers used and the functional groups involved. The copolymer additives and the electrolytes comprising them may be quite easy to synthesize. The metal oxide backbone in the copolymer can improve the thermal stability of the membrane electrolyte and help retain therein high temperature water, thus improving durability and also performance under high temperature and low humidity conditions. Crosslinking between the copolymer additive and the base ionomer prevents the copolymer additive from being washed out during operation and improves the dimensional stability of the membrane electrolyte. Due to the presence of the proton-bearing functional groups, the protons can be cleaved off the anion itself, even without water molecule facilitation, so that the performance of the membrane electrolyte can be improved even under dry conditions. Due to the presence of metal ion chelating complex-forming groups, metal ions in the membrane electrolyte can be inactivated, thus reducing or preventing the formation of free radicals therein. Due to the presence of free-radical functional scavenging groups, free radicals can be recaptured, which improves the durability of the membrane.

The following examples illustrate the invention, but should not be construed as limiting in any way.

examples

Several different improved additives were prepared and incorporated into improved polymer membrane samples as described below. In addition, two conventional polymer membrane samples were prepared for comparison.

The improved additives were generally prepared by first preparing suitable crosslinking monomers and proton-bearing monomers. The copolymer additives were then usually prepared by polymerizing a majority of both types of monomers from a suitable mixture. In one case, however, the synthesis of the crosslinked monomer and the polymerization were carried out simultaneously. Finally, membrane samples were prepared by adding a desired amount of the additive to a dispersion of perfluorosulfonic acid (PFSA) ionomer of equivalent weights (EW) of either 830 or 950, mixing with stirring overnight, outgassing the solution, and pouring samples onto a glass plate. After evaporation of the solvent at room temperature for 2 hours, the resulting membrane samples were annealed at 150 ° C for 1 hour.

The details of the preparation, which are specific to each example, were as follows:

Membranes of the Invention Examples 1a, 1b, 1c and 1d

Preparation of a Silicon Oxide Monomer Having a Proton-bearing Functional Group

48 g of (2-diethylphosphatoethyl) triethoxysilane (EPETES) were hydrolyzed in a three-necked flask equipped with a condenser in 250 ml of 37% hydrochloric acid while bubbling nitrogen into the solution. The solution was heated to 85 ° C and kept there with continuous stirring for 24 hours. The product was then cooled to 50 ° C and the hydrochloric acid removed under reduced pressure. A light yellow, viscous product was obtained with a yield of 95%. 5 g of this product was then added to 15 g of alcohol to obtain a 25 wt% hydrolyzed EPETES solution. The hydrolysis reaction herein is shown in Equation 1 below: Equation 1:

Preparation of a Silica Monomer Having Crosslinking Functional Group:

0.92 g (5.13 mmol) of 3-aminopropyltrimethoxysilane (APMS) was hydrolyzed at 50 ° C. for 24 hours in a round bottom flask in 25 g of alcohol solution containing 0.1 ml of 2 M hydrochloric acid and 2.77 g of water includes. The monomer was left in solution to produce the copolymer additive below. The hydrolysis reaction here is shown in Equation 2 below: Equation 2:

Preparation of the first silica copolymer additive with N: P of 1: 9:

34 g (45.2 mmol) of the above 25% hydrolyzed EPETES alcohol solution was added to the above hydrolyzed APMS solution and allowed to react at 50 ° C for 3 days. The solution was then filtered and washed to provide a white, water-insoluble solid powder. The molar ratio of APMS to EPETES was 1: 9, and thus the N: P (nitrogen: phosphorus) ratio was 1: 9. The reaction hereby is given by Equation 3 below: Equation 3:

Preparation of the Second Silica Copolymer Additive with N: P of 3: 7:

A hydrolyzed APMS solution was prepared in a similar manner to that above and included 2.04 g of APMS in 40 g of alcohol solution comprising 0.2 ml of 2 M HCl and 5 g of water. 47g of this hydrolyzed APMS solution was then added to 26.6mmol of the above 25% hydrolyzed EPETES alcohol solution and allowed to react at 50 ° C for 3 days. Again, the solution was filtered and washed to provide a white, water-insoluble solid powder.

This time, the molar ratio of APMS to EPETES was 3: 7, and thus the N: P (nitrogen: phosphorus) ratio was also 3: 7.

Composite membrane samples were then made with each additive as generally described above. Two different amounts were used and ionomer dispersions, which had two different EW's of 830 and 950, were used. The composite membrane samples were:
Invention Membrane 1a: 10% by weight of the first N: P silica additive copolymer of 1: 9 in a base PFSA ionomer with an EW of 950.
Invention Membrane 1b: 10 weight percent of the second N: P silica additive copolymer of 3: 7 in a base PFSA ionomer with an EW of 950.
Invention Membrane 1c: 5 weight percent of the first N: P silica additive copolymer of 1: 9 in a base PFSA ionomer with an EW of 950.
Invention Membrane 1d: 10 weight percent of the first N: P silica additive copolymer of 1: 9 in a base PFSA ionomer with an EW of 830.

Membrane of the invention Example 2

Preparation of the silicon oxide monomer with proton-bearing functional group:

Hydrolyzed EPETES was again prepared as described in Example 1 above.

Preparation of the Crosslinking Functional Group Silica Monomer:

0.79 g (3.70 mmol) of P-aminophenyltrimethoxysilane (APS) was hydrolyzed in a round bottom flask in 80 ml of 2 M HCl at 50 ° C for 24 hours. Again, here the monomer was left in solution to produce the polymer additive below. The hydrolysis reaction here is shown in Equation 4 below: Equation 4:

Preparation of the Silica Copolymer Additive:

25.10 g (33.3 mmol) of the 25% hydrolyzed EPETES solution was added to the above hydrolyzed APS solution and allowed to react at 50 ° C for 3 days. The molar ratio of APS to EPETES and thus the N: P ratio was 1: 9. The hydrochloric acid was removed under reduced pressure to provide a light pink powder. The reaction hereby is given by Equation 5: Equation 5:

Composite membrane samples were then made with this additive as generally described above and are designated Membrane 2 of the invention. All had the same composition of 10 weight percent of the N: P silica additive at 1: 9 in a base PFSA ionomer with an EW of 950.

Membrane of the invention Example 3

Preparation of the silicon oxide monomer with proton-bearing functional group:

Hydrolyzed EPETES was again made as described in Example 1 above.

Preparation of Silica Monomers Having Crosslinking Functional Groups and Free Radical Scavenger Groups:

3-Aminopropyl-trimethoxysilane (APMS) and 3-nitro-4Amino-phenyl-triethoxysilane (NPS) were obtained from a chemical supplier.

Preparation of the Silica Copolymer Additive:

0.5 g (2.788 mmol) of 3-aminopropyltrimethoxysilane (APMS), 0.79 g (2.788 mmol) of 3-nitro-4-amino-phenyl-triethoxysilane (NPS) and 4.20 g (22.3 mmol) hydrolyzed EPETES were placed in a round bottom flask and stirred using a magnetic stir bar. 1.550 g of water and 100 g of alcohol were added with stirring, and then 0.1 ml of 2 M HCl was added and allowed to react at 50 ° C for 3 days. The molar ratio of APMS to NPS to EPETES was 1: 1: 8. The solution was then filtered and washed to provide a white, water-insoluble, solid powder. The reaction here is given in Equation 6: Equation 6:

Composite membrane samples were then made with this additive as generally described above and are referred to as inventive membrane 3. All had the same composition with 10 weight percent of the silica copolymer additive having an APMS: NPS: EPETES ratio of 1: 1: 8 in a base PFSA ionomer with an EW of 950.

Membrane of the invention Example 4

Preparation of the silicon oxide monomer with proton-bearing functional group:

Hydrolyzed EPETES was again prepared as described in Example 1 above.

Combined Hydrolysis of the Crosslinking Functional Group Silica Monomer and Preparation of the Silica Copolymer Additive

Gleichung 8:

0.45 g (1.78 mmol) of 3- (1H-benzimidazol-2-yl) propyl-trimethoxysilane (BIMS) and 3.01 g (16.0 mmol) of hydrolyzed EPETES were placed in a round bottom flask and heated using a magnetic Stirred stir bar. The molar ratio of BIMS to EPETES and thus the N: P ratio was 1: 9. 0.96 g of water and 100 g of alcohol were added with stirring and then 0.1 ml of 2 M HCl was added and allowed to react at 50 ° C for 3 days. The solution was then filtered and washed to obtain a white, water-insoluble, solid powder. Here, the reactions of BIMS hydrolysis and polymerization occur simultaneously, and they are correspondingly given by Equations 7 and 8 below: Equation 7:

Equation 8:

Composite membrane samples were then made with this additive as generally described above and are designated Membrane 4 of the invention. All had the same composition with 10 weight percent of the 1: 9 N: P silica additive in a base PFSA ionomer with an EW of 950.

Comparative membranes Examples PFSA 830EW, PFSA 950EW and NRE211

For purposes of comparison, conventional membranes without additives were cast from a dispersion of a persulfonic acid (PFSA) ionomer having equivalent weights of either EW 830 or EW 950 (hereinafter referred to as PFSA 830EW and PFSA 950EW, respectively). In addition, a commercially available polymer membrane was obtained, namely, which is referred to as a NRE211 ^TM DuPont Nafion ^® PFSA NRE211 membrane.

The above inventive composite membrane samples and comparative membrane samples were then evaluated and compared in various ways as summarized below.

Glass transition temperatures (Tg) of the membranes

Glass transition temperatures (Tg) of the inventive membranes 1a, 1b and the comparative PFSA 950EW were determined by dynamic mechanical analysis (DMA) measurements performed using a DMA 800. Table 1 compares the Tg values for the samples tested. Table 1 membrane sample N: P ratio Tg (° C) PFSA 950EW N / A 93 Membrane 1a according to the invention (with 10% additive) 1: 9 130 Membrane 1b according to the invention (with 10% additive) 3: 7 169

As can be seen from Table 1, the thermal stability of the composite membranes of the invention is significantly improved compared to that of the conventional PFSA 950EW membrane. The Tg values of the former are much higher than the latter. Further, the membrane sample comprising 10% of the additive according to Example 1b of the present invention has a larger amount of the crosslinking functional groups (larger N: P ratio) and, as a result, exhibits a substantially higher Tg than the membrane group having the same amount of Additive according to Example 1a.

Membrane stability in dimethvlacetamide (DMAc)

The extent to which membrane samples are cross-linked can be qualitatively determined using a simple solubility test in a DMAc solvent. Two square centimeter pieces of a comparative NRE211 sample and a membrane sample of the invention comprising 10% by weight of the additive of Inventive Example 1b were placed in separate vials containing 25 ml of DMAc at 50 ° C. After 3 days, the NRE211 sample was broken into small pieces, and after 15 days it had completely dissolved. After 3 days, the membrane sample of the invention swelled but otherwise remained unchanged after 15 days.

The cross-linked membrane sample of the present invention clearly showed increased resistance to solvents to that of the comparative NRE211 membrane in DMAc solvent.

membrane conductivity

The proton conductivity in the plane of the inventive sample comprising 5% of the additive according to Inventive Example 1c was determined together with the comparative PFSA 950EW membrane by measuring the AC impedance of samples using a four-probe technique and a solar clay FRA 1260 frequency response analyzer was tested. The sampling frequencies ranged from 10 MHz to 100 Hz and the samples were held under test conditions for 6 hours to reach equilibrium before the 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. Table 2 membrane sample Conductivity at 80 ° C 30% RH (S / cm) Conductivity at 80 ° C 50% RH (S / cm) PFSA 950EW without additive 0.0040 0.019 Invention membrane 1c (with 5% additive) 0.0049 0,021

The conductivity of the inventive sample was similar to that of the comparative PFSA 950EW sample at 50% RH and better at 30% RH.

Performance of Membrane Electrode Arrangements (MEA)

Experimental fuel cells were prepared with some of the above inventive membrane samples 1a and 1d to compare their performance with that of the conventional membranes comprising the same basic ionomer. Individual MEAs were prepared by determining the appropriate membrane samples between cathodic and anodic electrodes. The cathode and anode had Pt loadings of 0.7 mg / cm ² and 0.3 mg / cm ^2, respectively. Performance evaluation was performed using a single cell stack with 50 cm ² active device area.

Performance was evaluated by obtaining polarization curves (voltage vs. current) at various relative humidities of the reaction gases at the inlet (35, 50 and 95%) and temperatures (95 ° C and 120 ° C). In all cases, the experiments were carried out using hydrogen at the anode, air at the cathode and gas stoichiometries of 9 and 12, respectively. Table 3 shows the voltage obtained at 1 A / cm ² in each case. Table 3 membrane sample Voltage at 95 ° C & 95% RH Tension at 95 ° C & 50% RH Tension at 95 ° C & 35% RH Stress at 120 ° C & 50% RH Stress at 120 ° C & 35% RH PFSA 950EW 0.702 0.604 0,538 0.551 0,420 Membrane 1a according to the invention (10% additive & PFSA 950EW) 0.681 0.609 0.547 0,577 0.485 PFSA 830EW 0.692 0.623 0.554 0.588 0.481 Invention membrane 1d (10% additive & PFSA 830EW) 0.684 0.615 0,555 0.582 0.499

It can be seen from Table 3 that the effect of the additive on the performance of the membrane is a function of temperature and RH. The performance of the membranes of the invention is competitive with and, under certain conditions, an improvement over that of conventional membranes. And in general, for a given base ionomer, the lower the RH and the higher the temperature (eg, 35% RH and 120 ° C), the more the additive will improve performance. Therefore, such additives are well suited to improve performance in fuel cells operating at relatively high temperatures and / or low RH.

Durability of the membrane electrode assemblies

The relative durability of the MEAs can be evaluated by using fuel cells made with experimental MEAs in an idle state to accelerate the chemical breakdown of the membrane therein. The rate of decay at open circuit voltage (OCV) may be indicative of the chemical stability of the membrane. MEAs prepared with the inventive membrane sample 1a and the comparative membrane PFSA 950EW were tested and compared. Here, 3-cell stacks were made using the same method and equipment as in the previous example.

The test piles were evaluated under OCV conditions at 30% relative humidity (RH) and 95 ° C. The gas flows supplied were 3.5 and correspondingly 11 slpm for hydrogen and air. The OCV of each cell in the stack was monitored over time. The experiment was terminated when the OCV in one of the 3 cells in the stack reached 0.75V. In addition, the amount of fluoride released as a result of membrane degradation was determined over time (ie, the fluoride release rate) by measuring the fluoride ion found in both the water at the outlet of the cathode and the anode.

3 shows plots of OCV and fluoride release rate versus time for the stacks examined. The OCV decomposition rate of the stack made with the inventive membranes 1a was 0.0008 V / h, while that of the stack made with the comparative membranes PFSA 950EW was 0.0015 V / h. The former failed at 113 h while the latter failed at 66 h. And the fluoride release rate for the stack made with the inventive membranes 1a was much lower than that of the stack made with the comparative membranes PFSA 950EW.

The stacks with the membrane according to the invention showed a higher durability compared to that of the conventional stack.

All of the above-mentioned US patents, publications of US patent applications, US patent applications, foreign patent applications and non-patent literature referenced in this specification are hereby incorporated by reference in their entireties.

While particular elements, embodiments and applications of the present invention have been shown and described, it is to be understood that the invention is not limited thereto, as 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 art preceding teachings. Such changes are to be considered within the scope and scope of the following claims.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 2008/0152986 [0005]
• US 2006/0199062 [0006]
• US 6733914 [0007]
• US 2002/0091201 [0007]
• US 2010/0040927 [0008]
• WO 2005/027240 [0009]
• US 2007/0154764 [0010]
• WO 2005/036687 [0011]
• US 2006/0141313 [0012]
• US 2004/0043283 [0013]
• US 2006/0046120 [0013]
• US 6607856 [0013]

Claims (32)

A proton conductive polymer electrolyte comprising a proton conductive ionomer and an amount of a copolymer comprising crosslinking functional groups and other functional groups, wherein: the proton conductive ionomer and the copolymer at the crosslinking functional groups of the copolymer are covalently bonded together or acid-base complexed together ; the copolymer comprises a polymerized network of a plurality of metal oxide monomers having crosslinking functional groups and a plurality of metal oxide monomers having other functional groups in random or block sequence, and wherein the polymerized network is characterized by an alternating sequence of oxygen bonds and metal bonds; the metal oxide monomers having crosslinking functional groups include: a first metal bonded to at least two oxygen atoms and selected from the group consisting of Si, Ti, Zr, Ce, Ta and Cr; and crosslinking functional groups linked to the first metal and comprising a functional end group containing hydrogen and oxygen and characterized by a chemical structure selected from the group consisting of -NH 2, = NH, - (aliphatic) -OH, - (aryl) -OH,

wherein R is a hydrocarbon group; the metal oxide monomers having other functional groups include: 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 attached to the second metal selected from the group consisting of: i) proton-bearing functional groups comprising a functional end group selected from the group consisting of -PO ₃ H ₂ , -COOH, -SO ₃ H, and -SO ₂ NHSO ₂ CF ₃ , ii) metal having a chelate complex-forming functional group comprising a functional end group selected from the group consisting of phosphonic acid, bipyridine, Phenanthroline and derivatives thereof, and iii) free radical functional radical scavengers having a functional end group selected from the group consisting of aminophenyl, hydroxyphenyl and derivatives thereof. The electrolyte of claim 1, wherein the first and second metals are the same. The electrolyte of claim 2, wherein the first and second metals are silicon. An electrolyte according to claim 1, wherein the crosslinking functional groups have a chemical structure of the form -X- (end group), where X is a linear chain containing a number of CH ₂ , O, NH or aryl groups in random order includes. An electrolyte according to claim 4, wherein the cross-linking functional groups are - (CH ₂ ) ₃ -NH ₂ , -phenyl-NH ₂ or - (CH ₂ ) ₃ - (1H-benzimidazol-2-yl). The electrolyte of claim 1, wherein the other functional groups are proton-bearing functional groups having an end group selected from the group consisting of -PO ₃ H ₂ , -COOH, -SO ₃ H, and -SO ₂ NHSO ₂ CF. ₃ exists. The electrolyte of claim 6, wherein the proton-bearing functional groups have a chemical structure of the form -Y- (end group), where Y is a linear chain containing a number of CH ₂ , CF ₂ , or aryl groups in random order includes. An electrolyte according to claim 7, wherein the proton-bearing functional groups are - (CH ₂ ) ₂ -PO ₃ H ₂ . An electrolyte according to claim 7, wherein the crosslinking functional groups are - (CH ₂ ) ₃ -NH ₂ , -phenyl-NH ₂ or - (CH ₂ ) ₃ - (1H-benzimidazol-2-yl) and the proton-bearing functional groups - (CH ₂ ) ₂ -PO ₃ H ₂ . The electrolyte of claim 9, wherein the ratio of the crosslinking functional groups to the proton-bearing functional groups in the copolymer is from about 1: 9 to 3: 7. The electrolyte of claim 1, wherein the polymerized network comprises at least two different metal oxide monomers having other functional groups. The electrolyte of claim 11, wherein the polymerized network comprises a plurality of metal oxide monomers having proton bearing functional groups and a plurality of free radical functional radical scavenger metal oxide monomers. An electrolyte according to claim 12, wherein the free-radical functional scavenger groups are 3-nitro-4-amino-phenyl. The electrolyte of claim 1, wherein the proton conductive ionomer comprises sulfonic acid groups. The electrolyte of claim 14, wherein the proton conductive ionomer is a perfluorosulfonic acid ionomer. An electrolyte according to claim 15, wherein the amount of the copolymer is from about 5% to 10% by weight of the electrolyte. The electrolyte of claim 1 which comprises the proton conductive ionomer and the amount of a copolymer comprising crosslinking functional groups and other functional groups excluding a basic ionomer. A polymer electrolyte fuel cell comprising an electrolyte according to claim 1. The polymer electrolyte fuel cell of claim 18, wherein the fuel cell is configured for operation at temperatures greater than 95 ° C and relative humidity less than 50% RH. A method for producing the proton conductive polymer electrolyte of claim 1, which comprises: Mixing an amount of the copolymer with an amount of the proton conductive ionomer; and heating the mixture such that the copolymer binds to the proton conductive ionomer. The method of claim 20, which comprises: Producing the copolymer; Adding the copolymer to a dispersion comprising the proton conductive ionomer; Removing solvent from the dispersion to provide a solid mixture of the copolymer and the proton conductive ionomer; and heating the mixture. The method of claim 20, which comprises: Adding the metal oxide monomers having crosslinking functional groups and the metal oxide monomers having other functional groups to the dispersion comprising the proton conductive ionomer and thus preparing the copolymer in situ in the dispersion; Removing solvent from the dispersion to provide a solid mixture of the copolymer and the proton conductive ionomer; and Heating the mixture. The method of claim 20, which comprises: preparing the copolymer; Adding the copolymer to a dispersion comprising a proton conductive ionomer precursor; Removing solvent from the dispersion to provide a solid mixture of the copolymer and the precursor; Heating the mixture; and converting the precursor into the proton conductive ionomer. The method of claim 20, wherein the copolymer is made by: Preparing a solution comprising the metal oxide monomers having crosslinking functional groups and the metal oxide monomers having other functional groups; and Heating the solution to a reaction temperature for a period of time to produce the copolymer in solution. The process of claim 24, which comprises preparing the metal oxide monomers having crosslinking functional groups by hydrolyzing unhydrolyzed metal oxide monomers having crosslinking functional groups. A process according to claim 25, wherein the unhydrolyzed metal oxide monomers having crosslinking functional groups are 3-aminopropyltrimethoxysilane, aminophenyltrimethoxysilane or 3- (1H-benzimidazol-2-yl) propyltrimethoxysilane. The process of claim 24 which comprises preparing the metal oxide monomers with other functional groups by hydrolyzing unhydrolyzed metal oxide monomers having other functional groups. The process of claim 27, wherein the non-hydrolyzed metal oxide monomers having other functional groups are (2-diethylphosphatoethyl) triethoxysilane. The method of claim 25, wherein the hydrolyzing and the production of the copolymer are carried out in the same solution. The method of claim 27, wherein the hydrolyzing and the production of the copolymer are carried out in the same solution. The method of claim 20, wherein the solution comprises at least two different metal oxide monomers having other functional groups. The method of claim 31, wherein the solution comprises free radical radical scavenger metal oxide monomers and the free radical functional scavenging groups are hydrolyzed 3-amino-4-nitrophenyl triethoxysilane.

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