Proton-conductive electrolytes with cross-linked copolymer additives for use in fuel cells

Abstract

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 are covalently bonded to each other or acid-base complexed to each other at the cross-linking functional groups of the copolymer; the copolymer comprises a polymerized network of a plurality of metal oxide monomers having cross-linking 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 with cross-linking 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 which are connected to the first metal and comprise a functional end group which contains nitrogen or oxygen and is characterized by a chemical structure which is selected from the group consisting of -NH2, =NH, -(aliphatic)-OH, -(aryl)-OH, where R is a hydrocarbon group; the metal oxide monomers with 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 which are bonded to the second metal and are selected from the group consisting of: i) proton-bearing functional groups comprising a functional end group selected from the group consisting of -PO3H2, -COOH, -SO3H, and -SO2NHSO2CF3, ii) functional groups forming a chelate complex with metal, which comprise a functional end group selected from the group consisting of phosphonic acid, bipyridine, phenanthroline and derivatives thereof, and iii) free radical scavenging functional groups having a functional end group selected from the group consisting of aminophenyl, hydroxyphenyl and derivatives thereof.

Description

Field of the invention

[0001] The invention relates to improved proton-conductive polymer electrolytes for use in polymer electrolyte fuel cells. More particularly, it relates to electrolytes comprising an ionomer crosslinked with a copolymer additive containing both crosslinking and other useful functional groups. Description of the related art

[0002] Solid polymer electrolyte fuel cells convert reactants, namely a fuel (such as hydrogen) and an oxidizer (such as oxygen or air), to generate 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 sandwiched between two electrodes is called a membrane-electrode assembly (MEA). The durability of the MEA is one of the most important issues for the development of fuel cell systems in either stationary or transportation applications. For mobile applications, the MEA is required to demonstrate a durability of approximately 6,000 hours.

[0003] The membrane serves as a separating element, preventing mixing of reaction gases, and as an electrolyte for transporting protons from the anode to the cathode. Perfluorosulfonic acid (PFSA) ionomers, such as Nafion®, are currently the preferred material and technology standard for membranes. Nafion® consists of a perfluorinated backbone bearing pendant vinyl ether side chains ending in SO3H.

[0004] 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 fuel cell failure due to mixing of reaction gases from the anode and cathode. The chemical degradation of PFSA membranes during fuel cell operation is thought to occur via the attack of hydroxyl (OH) or peroxyl (OOH) radical species on weak groups (such as a carboxylic acid group) in the ionomer molecular chain. These free radicals can be generated by the decomposition of hydrogen peroxide with impurities (such as Fe 2+ ) in a type of Fenton reaction. In fuel cells, hydrogen peroxide can be formed either on Pt supported on carbon black in the catalyst layers or during the oxygen reduction reaction.

[0005] The hydroxyl radical attacks the unstable end groups of the polymer and causes a coupling of chains and / or can also form a SO3 - group under dry conditions to cause chain scission. Both attacks degrade the membrane and may lead to membrane cracking, thinning, or pinhole formation. The rate of membrane degradation accelerates significantly with an increase in operating temperature and a decrease in the relative humidity (RH) of the gas at the inlet.

[0006] Numerous modifications and / or additives to the membrane electrolyte have been studied for the purpose of improving membrane performance and / or durability. For example, with regard to durability, US 2008 / 0 152 986 A1 discloses an impregnated, cross-linked or non-cross-linked basic polymer (e.g., poly(benzimidazole)) membrane, which was prepared here with an acidic dopant (e.g., a phosphoric acid or an organic phosphonic acid) to obtain a polymer electrolyte membrane. Such membranes have good dimensional stability, but they generally do not have good conductivity at low temperatures (<100 °C). Furthermore, the acidic dopant is leached from the membrane over time during fuel cell operation.

[0007] In US 2006 / 0 199 062 A1, a polymer blend of a perfluorocarbonsulfonic acid resin and a polyazole-based compound or polymer was used as the proton-conducting electrolyte. The strong interaction between the two reduces hydrogen transfer and improves membrane durability. However, this interaction also dramatically reduces proton conductivity and, consequently, the performance of the membrane electrolyte.

[0008] Crosslinking electrolyte membranes generally improves durability. In US Pat. No. 6,733,914 B1, a crosslinked proton exchange membrane was prepared by using ammonia water to treat a precursor Nafion® film. The crosslinking groups in this member were sulfonamide. US Pat. No. 2002 / 0091201 A1 discloses a general technique for creating crosslinks in perfluorinated polymers, in which the crosslinks or bonds exist between sulfonyl groups attached to adjacent polymeric chains. However, such crosslinking reduces the number of sulfonic acid groups in the membrane, and while thus improving durability, it also results in lower proton conductivity and membrane performance.

[0009] US 2010 / 0 040 927 A1 discloses a process for producing a grafted polymer electrolyte film for a fuel cell. This grafted electrolyte is heterogeneous and has a silane crosslinked structure between the grafted molecular chains. However, its vinyl structure has proven unstable in the fuel cell environment.

[0010] Furthermore, WO 2005 / 027240 A2 describes the preparation of phosphonic acid-grafted inorganic-organic hybrid polymers with a metal oxide backbone. These polymers can be used directly as proton-conducting electrolyte membranes in fuel cells. The phosphonic acid groups allow proton conduction through the membrane at low RH. Composite materials comprising these polymers and other basic polymers are also recommended.

[0011] US 2007 / 0 154 764 A1 discloses electrolyte additives comprising hygroscopic particles formed from metal oxide, such as silica or zirconia, heteropolyacids, silica phosphonate, etc., to increase water retention and thereby improve the performance of the MEA under low RH conditions.

[0012] WO 2005 / 036687 A2 discloses a water-insoluble additive comprising a cross-linked metal oxide matrix containing phosphonic acid groups covalently bonded to the matrix via cross-linkers. The additive can then be homogeneously distributed over a proton-conductive membrane and improve the membrane's conductivity for ions at high temperatures (> 100 °C).

[0013] US 2006 / 0 141 313 A1 discloses particles comprising a metal-oxygen crosslinked structure as an additive for a proton-conducting membrane. The particles have an acid group, such as a sulfonic acid group, integrated into their surface. However, these and many other prior art additives are susceptible to leaching from the membrane during fuel cell operation, either because they are water-soluble or because they lack covalent bonding or strong interaction with the base polymer.

[0014] Many different functional groups can also be incorporated into proton-conductive electrolytes for various reasons. US 2004 / 0 043 283 A1 discloses the incorporation of metallic elements or compositions containing metallic elements or metal alloys that act as free radical scavengers or as catalysts for the decomposition of hydrogen peroxide. US 2006 / 0 046 120 A1 discloses the use of phenol-type antioxidants, where the antioxidant can be a small molecule or a polymer. And US 6 607 856 B2 discloses a solid polymer electrolyte exhibiting 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 moiety. The chelate complexing agents reduce the formation of free radicals.However, the presence of such additives in the MEA can result in reduced performance of the fuel cell.

[0015] US 7 309 523 B2 relates to a hybrid material, its use and its production process, wherein the hybrid material comprises a polymer with acid groups, and the inorganic part of the hybrid material consists of the combination of at least two metal oxide components, at least one of which comprises a functional group that enables an interaction and a spatial relationship with the acid groups of the polymer.

[0016] Further prior art is disclosed in the documents US 6 949 616 B2, US 2003 / 0 032 739 A1 and US 7 064 226 B2.

[0017] Indeed, all of the aforementioned recommended modifications and additives may suffer from one or more of the following problems: reduced proton conductivity of the electrolyte, insufficient membrane durability, or additives that leach from the electrolyte over time during fuel cell operation. 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

[0018] Proton-conductive electrolytes with improved durability and other desirable properties 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 that can impart additional desirable properties to the electrolyte. The other functional groups include proton carriers, metal chelating groups, and radical scavengers. The proton-conductive ionomer and the copolymer are bonded together at the crosslinking functional groups of the copolymer. This crosslinking, or bonding, imparts durability and generally improved mechanical properties, while also serving to more reliably attach the other functional groups, thus preventing them from leaching over time.The electrolyte may contain other polymers or ionomers, but basic ionomers may desirably be excluded.

[0019] Specifically, the copolymer additives 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 a random sequence. The polymerized network is characterized by an alternating sequence of oxygen bonds and metal bonds.

[0020] The metal oxide monomers with 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 which are bonded to the first metal and have a functional end group containing nitrogen or oxygen and are characterized by a chemical structure selected from the group consisting of: -NH2, =NH, -(aliphatic)-OH or -(aryl)-OH, and where R is a hydrocarbon group.

[0021] The metal oxide monomers with 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 which are bonded to the second metal and are selected from the group consisting of: i) proton-bearing functional groups comprising a functional end group selected from the group consisting of -PO3H2, -COOH, -SO3H and -SO2NHSO2CF3, ii) functional groups which form a chelate complex with metal and which have a functional end group selected from the group consisting of phosphonic acid, bipyridine, phenanthroline and the like and derivatives thereof, and iii) free radical scavenging functional groups having a functional end group selected from the group consisting of aminophenyl, hydroxyphenyl and the like and derivatives thereof.

[0022] The first and second metals in the two types of monomers may conveniently be the same metal, and in particular may be Si. Such copolymers therefore have a silicon-oxygen backbone.

[0023] 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 CH2, O, NH, or aryl groups in a random sequence. As demonstrated in the examples below, the crosslinking functional groups may in particular be -(CH2)2-NH2, -phenyl-NH2, or -(CH2)3-(1H-benzimidazol-2-yl).

[0024] The other functional groups may be proton-bearing functional groups having an end group selected from the group consisting of PO3H2, -COOH, -SO3H2, and -SO2NHSO2CF3. These proton-bearing functional groups may have a chemical structure of the form -Y-(end group), where Y is a linear chain comprising a number of CH2, CF2, or aryl groups in random sequence. As again demonstrated in the examples below, the proton-bearing functional groups may in particular be -(CH2)2-PO3H2. Furthermore, the ratio of crosslinking functional groups to proton-bearing functional groups in the copolymer may be from about 1:9 to 3:7.

[0025] More than one type of functional group can be employed in the copolymer additives. For example, the polymerized network can comprise at least two different metal oxide monomers with different functional groups, such as both a plurality of metal oxide monomers with proton-bearing functional groups and a plurality of metal oxide monomers with free radical scavenging functional groups. Exemplary free radical scavenging functional groups are -3-nitro-4-aminophenyl.

[0026] The proton-conductive base ionomer may comprise sulfonic acid groups and, in particular, may be a perfluorosulfonic acid ionomer. An effective amount of the copolymer additive in the ionomer may be from about 5% to 10% by weight of the electrolyte.

[0027] 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 can even be improved under certain operating conditions, such as when operating at temperatures greater than 95°C and relative humidity below 50% RH.

[0028] The electrolytes can be prepared by mixing a quantity of the copolymer with a quantity of the proton-conducting ionomer, and then heating the mixture so that the copolymer bonds with the proton-conducting ionomer. Several different sequences could be used in the preparation.

[0029] One possible approach involves preparing 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 then heating the mixture.

[0030] Another possible approach involves adding the metal oxide monomer with crosslinking functional groups and the metal oxide monomer with other functional groups to a dispersion comprising the proton-conductive ionomer, 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 then heating the mixture.

[0031] Yet another approach involves preparing the copolymer, adding the copolymer to a dispersion comprising a precursor for the proton-conductive ionomer, removing solvent from the dispersion to provide a solid mixture of the copolymer and the precursor, heating the mixture, and then converting the precursor to the proton-conductive ionomer.

[0032] The copolymer can 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) for a period of time (for example, greater than or about 3 days), and thus forming the copolymer in solution.

[0033] In the process, the metal oxide monomers with crosslinking functional groups can be prepared by hydrolyzing non-hydrolyzed metal oxide monomers with crosslinking functional groups. Exemplary non-hydrolyzed metal oxide monomers with crosslinking functional groups include 3-aminopropyltrimethoxysilane, aminophenyltrimethoxysilane, or 3-(1H-benzimidazol-2-yl)propyltrimethoxysilane.

[0034] Furthermore, metal oxide monomers with other functional groups can be prepared by hydrolyzing non-hydrolyzed metal oxide monomers with other functional groups. Examples of non-hydrolyzed metal oxide monomers with other functional groups include (2-diethylphosphatoethyl)triethoxysilane and 3-nitro-4-aminophenyltriethoxysilane.

[0035] With regard to both types of metal oxide monomers, the hydrolyzing step and the preparation of the copolymer can be carried out in the same solution.

[0036] The invention encompasses proton-conductive polymer electrolytes, fuel cells comprising such electrolytes (such as in the membrane or catalyst layers), and methods for producing such composite electrolytes and fuel cells.

[0037] These and other aspects of the invention will become apparent with reference to the attached figures and the following detailed description. Short description of the drawings: Fig. Figure 1 shows the chemical structure of a copolymer with an exemplary metal oxide network and with general functional groups. Fig. 2a and Fig.2b show exemplary proton-conductive electrolytes comprising a persulfonic acid ionomer cross-linked with a copolymer having a silicon oxide network and cross-linking and proton-bearing functional groups. Fig. 2a shows an example of a covalently bonded electrolyte and Fig. Figure 2b shows an example of an acid-base bonded electrolyte. Fig. Figure 3 shows a plot of open circuit voltage and fluoride release rate versus time for stacks studied in the examples. Detailed description

[0038] Photon-conductive electrolytes with improved durability and other desirable characteristics 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 useful for various other purposes. The proton-conductive ionomer and the copolymer are linked together at the crosslinking functional groups of the copolymer.

[0039] As used herein, "proton-conductive ionomer" refers to acidic ionomers characterized by significant proton conduction capability (and thus excludes basic ionomers). And with regard to the copolymer and ionomer being "linked together," this means that the two are either covalently bonded or acid-base complexed.

[0040] The chemical structure of a suitable copolymer is shown in Fig. 1. A portion of the entire 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 metal oxide monomer includes cross-linking functional groups, which in Fig. 1 are illustrated by R1. The other type of metal oxide monomer comprises other functional groups which are Fig. 1 are illustrated by R2 and R2. The metals in these two types of monomers are in Fig.1 by M1 and M2, respectively. The two types of monomers can occur in the copolymer in random or block sequence, and thus numerous configurations of the network are possible (including variations in chain length, branches, etc.). Therefore, in Fig. Figure 2 shows only an exemplary, partial structure for the network. Those of ordinary skill in the art will recognize that numerous variations in the network design are possible.

[0041] The metals in both types of monomers are bonded to at least two oxygen atoms and are thus able to form an extensive polymeric network. Although this is not Fig. As shown in Figure 1, each of the monomers forming the copolymer may contain more than two oxygen atoms. The metals (M1, M2) in both types of monomers may be selected from the group consisting of Si, Ti, Zr, Ce, Ta, and Cr.

[0042] The crosslinking functional groups R1 in the metal oxide monomers with crosslinking functional groups are linked to the metals M1. The various crosslinking functional groups that can be considered here comprise a functional end group containing nitrogen or oxygen and are characterized by a chemical structure selected from the group consisting of -NH2, =NH, -(aliphatic)-OH, or -(aryl)-OH, where R is a hydrocarbon group. The crosslinking functional groups can further have a chemical structure of the form -X-(end group), where X is a linear chain comprising a number of CH2, O, NH, or aryl groups in random sequence. The crosslinking functional groups -(CH2)2-NH2, -phenyl-NH2, and -(CH2)3-(1H-benzimidazol-2-yl) have been found to be suitable in the following examples.Because the copolymer can combine with the proton-conductive ionomer either through covalent bonds or acid-base complexation, the crosslinking functional groups include groups such as amino, hydroxy, pyridine, imidazole, and benzimidazole.

[0043] The functional groups R2 and R2' in the metal oxide monomers with other functional groups are linked to the metals M2. The other functional groups that can be considered here fall into one of three different types: proton carriers, metal chelating groups, and / or free radical scavenging functional groups.

[0044] Suitable proton-bearing functional groups comprise an end group selected from the group consisting of -PO3H2, -COOH, -SO3H, and -SO2NHSO2CF3. 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 CH2, CF2, or aryl groups in random sequence. (A CF2-perfluoro structure may be preferred because the strong electron-withdrawing power of the perfluoro moieties increases the acidity of the proton-bearing end group, thereby increasing the proton conductivity of the final electrolyte.) The proton-bearing functional group -(CH2)2-PO3H2 was successfully used in the following examples. Sulfonic acid functional groups are generally expected to provide higher proton conductivity than phosphonic acid or carboxylic acid functional groups.However, a phosphonic acid functional group also serves as a good chelating agent for metal ions and can serve more than one purpose.

[0045] Suitable functional groups that form a chelate complex with metals include a functional end group selected from the group consisting of phosphonic acid, bipyridine, phenanthroline and the like and derivatives thereof.

[0046] Suitable free radical scavenging functional groups include a functional end group selected from the group consisting of aminophenyl, hydroxyphenyl, and the like, and derivatives thereof. The 3-nitro-4-aminophenyl functional group was found to be suitable in the following examples.

[0047] More than one type of other functional groups (i.e., where R2 is not the same as R2' in Fig.1) can be used in the copolymer additives. For example, the polymerized network can comprise at least two different metal oxide monomers with different functional groups, such as both a plurality of metal oxide monomers with proton-bearing functional groups and a plurality of metal oxide monomers with free radical scavenging functional groups.

[0048] Also, although the two metals M1 and M2 may be different in different monomers, it may be advantageous for them to be the same. For example, Si is a preferred metal for both types of monomer.

[0049] Along with the numerous choices for the types of functional groups in the copolymer, the relative amounts of these different functional groups can also be varied widely according to the desired properties.

[0050] A proton-conductive electrolyte according to the invention comprises a proton-conductive ionomer and the copolymer described above. Part of the structure of the electrolyte is exemplified in the Fig. 2a and Fig. 2b. The exemplary polymer electrolyte of Fig. 2a comprises a perfluorosulfonic acid ionomer 1, which is cross-linked to the copolymer 2 via covalent bonds at sites 3. Here, the copolymer 2 is a silicon oxide network (with M1=M2 = Si) and has cross-linking functional groups (R1) comprising -NH functional end groups and -(CH2)2-PO3H2 proton-bearing functional groups (R2). The exemplary polymer electrolyte of Fig.2b comprises a perfluorosulfonic acid ionomer 1, which is cross-linked to the copolymer 2 via acid-base bonds at sites 3. Here, the copolymer 2 is a silicon oxide network (with M1 = M2 = Si) containing 3-(1H-benzimidazol-2yl)propyl cross-linking functional groups (R1) and -(CH2)2-PO3H2 proton-bearing functional groups (R2).

[0051] The amount of copolymer additive to be used in the electrolyte depends on several factors. Preferably, a minimal amount of the additive is used to achieve the desired results. A typical range might be from about 5 to 10 weight percent, although amounts outside this range are certainly contemplated.

[0052] When used as an electrolyte in solid polymer electrolyte fuel cells, the electrolyte according to the invention provides improved durability over the ionomer alone, as well as other advantages. Although primarily intended for use as the membrane electrolyte in such fuel cells, the electrolyte according to the invention may also be considered for other uses, for example, in a catalyst layer for either the cathode or the anode, or in a coating in the gas diffusion layers or electrodes.

[0053] The electrolyte according to the invention can be prepared in several ways. A general method involves mixing an amount of the copolymer with an amount of the proton-conductive ionomer and then heating the mixture such that the copolymer bonds with the proton-conductive ionomer. One approach to accomplishing this involves 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.

[0054] Another possible approach involves preparing the copolymer in situ in a dispersion containing the proton-conductive ionomer. For example, the metal oxide monomers with cross-linking functional groups and the metal oxide monomers with other functional groups can be added to a dispersion containing the proton-conductive ionomer. The solvent is then removed from the dispersion, providing a solid mixture of the copolymer and the proton-conductive ionomer. The solid mixture is then heated to complete the preparation.

[0055] Yet another approach involves first preparing the copolymer, adding the copolymer to a dispersion comprising a proton-conductive ionomer precursor, and removing solvent from the solution, thus providing a solid mixture of the copolymer and the precursor. The mixture is then heated to complete the crosslinking reaction, followed by an acid treatment to convert the precursor to the proton-conductive ionomer.

[0056] 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 can 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 for a period of time to form the copolymer in the solution.

[0057] The metal oxide monomers with crosslinking functional groups used in such a process can be prepared by hydrolyzing non-hydrolyzed metal oxide monomers with crosslinking functional groups. Exemplary non-hydrolyzed metal oxide monomers with crosslinking functional groups include 3-aminopropyltrimethoxysilane, aminophenyltrimethoxysilane, or 3-(1H-benzimidazol-2yl)propyltrimethoxysilane.

[0058] Furthermore, metal oxide monomers with other functional groups can be prepared by hydrolyzing non-hydrolyzed metal oxide monomers with other functional groups. Examples of non-hydrolyzed metal oxide monomers with other functional groups include (2-diethylphosphatoethyl)triethoxysilane and 3-nitro-4-aminophenyltriethoxysilane.

[0059] With regard to both types of metal oxide monomers, the hydrolyzing step and the preparation of the copolymer can be carried out in the same solution.

[0060] Fuel cells comprising the prepared electrolyte can be manufactured in a conventional manner. For example, a dispersion / solution comprising the prepared electrolyte can be used to cast a membrane electrolyte, prepare catalyst layers, or otherwise be integrated into the membrane electrode assembly as desired. In particular, a membrane can be cast directly from the dispersion / solution mixture. In another embodiment, a catalyst can be mixed with the dispersion / solution of the ionomer containing the additive to produce an ink, and then the ink can be applied to a membrane to produce a catalyst-coated membrane.

[0061] 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 can be fairly easy to synthesize. The metal oxide backbone in the copolymer can improve the thermal stability of the membrane electrolyte and help retain water therein at high temperatures, thus improving durability and performance under high-temperature, low-humidity conditions. The crosslinking between the copolymer additive and the base ionomer prevents the copolymer additive from leaching during operation and improves the dimensional stability of the membrane electrolyte.Due to the presence of proton-bearing functional groups, protons can be released from the anion itself, even without facilitation by water molecules, thus improving the performance of the membrane electrolyte even under dry conditions. Due to the presence of groups that form a chelate complex with a metal ion, metal ions in the membrane electrolyte can be inactivated, which subsequently reduces or prevents the formation of free radicals. Due to the presence of free radical scavenging functional groups, free radicals can be recaptured, thus improving the durability of the membrane.

[0062] The following examples illustrate the invention but should not be construed as limiting in any way. Examples

[0063] Several different improved additives were prepared and incorporated into improved polymer membrane samples as described below. Additionally, two conventional polymer membrane samples were prepared for comparison purposes.

[0064] The improved additives were generally prepared by first preparing suitable crosslinking monomers and proton-bearing monomers. The copolymer additives were then typically prepared by polymerizing a plurality of both monomer types from a suitable mixture. However, in one case, the synthesis of the crosslinked monomer and polymerization were carried out simultaneously. Finally, membrane samples were prepared by adding a desired amount of the additive to a dispersion of a perfluorosulfonic acid (PFSA) ionomer with equivalent weights (EW) of either 830 or 950, mixing with stirring overnight, outgassing the solution, and pouring samples onto a glass plate. After evaporating the solvent at room temperature for 2 hours, the resulting membrane samples were annealed at 150 °C for 1 hour.

[0065] The manufacturing details, specific to each example, were as follows: Membranes according to the invention Examples 1a, 1b, 1c and 1d Preparation of a silicon oxide monomer with a proton-bearing functional group:

[0066] 48 g of (2-diethylphosphatoethyl)triethoxysilane (EPETES) was hydrolyzed in 250 mL of 37% hydrochloric acid in a three-necked flask equipped with a condenser while bubbling nitrogen into the solution. The solution was heated to 85 °C and maintained there for 24 hours with continuous stirring. The product was then cooled to 50 °C, and the hydrochloric acid was 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 is shown in Equation 1 below: Preparation of a silicon oxide monomer with a crosslinking functional group:

[0067] 0.92 g (5.13 mmol) of 3-aminopropyltrimethoxysilane (APMS) was hydrolyzed in a 25 g alcohol solution containing 0.1 ml of 2M hydrochloric acid and 2.77 g of water at 50 °C for 24 hours in a round-bottom flask. The monomer was left in solution to produce the copolymer additive below. The hydrolysis reaction is shown in Equation 2 below: Preparation of the first silica copolymer additive with N:P of 1:9:

[0068] 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 consequently, the N:P (nitrogen:phosphorus) ratio was 1:9. The reaction here is given by Equation 3 below: Preparation of the second silica copolymer additive with N:P of 3:7:

[0069] A hydrolyzed APMS solution was prepared in a similar manner to the above and comprised 2.04 g of APMS in 40 g of alcohol solution containing 0.2 ml of 2M HCl and 5 g of water. 47 g of this hydrolyzed APMS solution was then added to 26.6 mmol 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.

[0070] This time the molar ratio of APMS to EPETES was 3:7 and consequently the N:P (nitrogen:phosphorus) ratio was also 3:7.

[0071] Composite membrane samples were then prepared with each additive as generally described above. Two different amounts were used, and ionomer dispersions with two different EWs of 830 and 950 were used. The composite membrane samples were: Inventive membrane 1a: 10 wt% of the first silica copolymer additive with N:P of 1:9 in a base PFSA ionomer with an EW of 950. Inventive membrane 1b: 10 wt% of the second silica copolymer additive with N:P of 3:7 in a base PFSA ionomer with an EW of 950. Inventive membrane 1c: 5 wt% of the first silica copolymer additive with N:P of 1:9 in a base PFSA ionomer with an EW of 950. Inventive membrane 1d: 10 wt% of the first silica copolymer additive with N:P of 1:9 in a base PFSA ionomer with an EW of 830. Inventive Membrane Example 2Preparation of the silicon oxide monomer with proton-bearing functional group:

[0072] Hydrolyzed EPETES was again prepared as described in Example 1 above. Preparation of the silicon oxide monomer with crosslinking functional group:

[0073] 0.79 g (3.70 mmol) of P-aminophenyltrimethoxysilane (APS) was hydrolyzed in a round-bottom flask in 80 mL of 2M HCl at 50 °C for 24 hours. Again, the monomer was left in solution to produce the polymer additive below. The hydrolysis reaction is shown in Equation 4 below: Preparation of the silicon oxide copolymer additive:

[0074] 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 is given by Equation 5:

[0075] Composite membrane samples were then prepared with this additive as generally described above and are designated Inventive Membrane 2. All had the same composition of 10 weight percent of the silica copolymer additive with N:P of 1:9 in a base PFSA ionomer with an EW of 950. Inventive Membrane Example 3Preparation of the silicon oxide monomer with proton-bearing functional group:

[0076] Hydrolyzed EPETES was again prepared as described in Example 1 above. Production of silicon oxide monomers with cross-linking functional groups and with radical scavenger groups for free radicals:

[0077] 3-Aminopropyl-trimethoxysilane (APMS) and 3-nitro-4-amino-phenyl-triethoxysilane (NPS) were obtained from a chemical supplier. Preparation of the silicon oxide copolymer additive:

[0078] 0.5 g (2.788 mmol) of 3-aminopropyltrimethoxysilane (APMS), 0.79 g (2.788 mmol) of 3-nitro-4-aminophenyltriethoxysilane (NPS), and 4.20 g (22.3 mmol) of hydrolyzed EPETES were placed in a round-bottom flask and stirred using a magnetic stir bar. 1.505 g of water and 100 g of alcohol were added with stirring, followed by 0.1 mL of 2M HCl and allowed to react for 3 days at 50 °C. 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 is given in Equation 6:

[0079] Composite membrane samples were then prepared with this additive as generally described above and are designated Inventive Membrane 3. All had the same composition with 10 wt. % of the silica copolymer additive with an APMS:NPS:EPETES ratio of 1:1:8 in a base PFSA ionomer with an EW of 950. Inventive Membrane Example 4Preparation of the silicon oxide monomer with proton-bearing functional group:

[0080] Hydrolyzed EPETES was again prepared as described in Example 1 above. Combined hydrolysis of the silicon oxide monomer with crosslinking functional group and preparation of the silicon oxide copolymer additive:

[0081] 0.45 g (1.78 mmol) of 3-(1H-benzimidazol-2-yl)propyltrimethoxysilane (BIMS) and 3.01 g (16.0 mmol) of hydrolyzed EPETES were placed in a round-bottom flask and stirred using a magnetic 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, followed by 0.1 mL of 2M HCl 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 given by Equations 7 and 8 below, respectively:

[0082] Composite membrane samples were then prepared with this additive as generally described above and are designated Inventive Membrane 4. All had the same composition with 10 weight percent of the silica copolymer additive with an N:P of 1:9 in a base PFSA ionomer with an EW of 950. Comparative membrane examples PFSA 830EW, PFSA 950EW and NRE211

[0083] For comparison purposes, conventional membranes without additives were cast from a dispersion of a persulfonic acid (PFSA) ionomer with 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, namely a DuPont™ Nafion® PFSA NRE211 membrane, was also obtained, which is referred to as NRE211.

[0084] 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

[0085] 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 tested samples. Table 1 Membrane sample N:P ratio Tg (°C) PFSA 950EW N / A 93 Inventive membrane 1a (with 10% additive) 1:9 130 Inventive membrane 1b (with 10% additive) 3:7 169

[0086] As can be seen from Table 1, the thermal stability of the composite membranes according to the invention is significantly improved compared to that of the conventional PFSA 950EW membrane. The Tg values of the former are significantly higher than the latter. Furthermore, the membrane sample comprising 10% of the additive according to Inventive Example 1b has a larger amount of crosslinking functional groups (higher N:P ratio) and, as a result, exhibits a significantly higher Tg than the membrane group with the same amount of the additive according to Example 1a. Membrane stability in dimethylacetamide (DMAc)

[0087] 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 an inventive membrane sample comprising 10 wt. percent of the additive according to Inventive Example 1b were placed in separate vials containing 25 ml of DMAc at 50°C. After 3 days, the NRE211 sample had broken into small pieces, and after 15 days, it had completely dissolved. After 3 days, the inventive membrane sample had swollen but remained otherwise unchanged after 15 days.

[0088] The cross-linked membrane sample of the invention clearly showed increased solvent resistance to that of the comparative NRE211 membrane in DMAc solvent. Membrane conductivity

[0089] The in-plane proton conductivity of the inventive sample, which comprised 5% of the additive according to Inventive Example 1c, was determined together with the comparative PFSA 950EW membrane by testing the AC impedance of samples using a four-probe technique and a Solarton FRA 1260 frequency response analyzer. The sampling frequencies ranged from 10 MHz to 100 Hz, and the samples were held under test conditions for 6 hours to achieve equilibrium before measurements were taken. Measurements were performed 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 Inventive membrane 1c (with 5% additive) 0,0049 0,021

[0090] 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 assemblies (MEA)

[0091] Experimental fuel cells were fabricated using some of the above inventive membrane samples 1a and 1d to compare their performance with that of conventional membranes comprising the same base ionomer. Individual MEAs were fabricated by sandwiching the appropriate membrane samples between the cathodic and anodic electrodes. The cathode and anode had Pt loadings of 0.7 mg / cm 2 and correspondingly 0.3 mg / cm 2 The performance evaluation was performed using a single cell stack with 50 cm 2 active device area.

[0092] The performance was evaluated by obtaining polarization curves (voltage versus current) at different 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 at gas stoichiometries of 9 and 12, respectively. Table 3 shows the voltage obtained at 1 A / cm 2 was received in every case. Table 3 Membrane sample Voltage at 95 °C & 95% RH Voltage at 95 °C & 50% RH Voltage at 95 °C & 35% RH Voltage at 120 °C & 50% RH Voltage at 120 °C & 35% RH PFSA 950EW 0,702 0,604 0,538 0,551 0,420 Inventive membrane 1a (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 Inventive membrane 1d (10% additive & PFSA 830EW) 0,684 0,615 0,555 0,582 0,499

[0093] From Table 3, it can be seen that the effect of the additive on membrane performance is a function of temperature and RH. The performance of the membranes of the invention is competitive with, and under certain conditions, improved over, that of conventional membranes. Generally, for a given base ionomer, the additive improves performance more the lower the RH and the higher the temperature (e.g., 35% RH and 120°C). Therefore, such additives are well suited to improving performance in fuel cells operating at relatively high temperatures and / or low RH. Durability of membrane electrode assemblies

[0094] The relative durability of MEAs can be evaluated by operating fuel cells fabricated with experimental MEAs in an open-circuit state to accelerate the chemical degradation of the membrane therein. The rate of degradation at open-circuit voltage (OCV) can be indicative of the chemical stability of the membrane. MEAs fabricated with the inventive membrane sample 1a and the comparative membrane PFSA 950EW were tested and compared. Here, stacks of three cells were fabricated using the same procedure and equipment as in the previous example.

[0095] The experimental 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, respectively. The OCV of each cell in the stack was monitored over time. The experiment was terminated when the OCV in any of the three cells in the stack reached 0.75 V. In addition, the amount of fluoride released as a result of membrane degradation was determined over time (i.e., the fluoride release rate) by measuring the fluoride ion found in the water at both the cathode and anode outlets.

[0096] Fig.Figure 3 shows plots of OCV and fluoride release rate versus time for the stacks tested. The OCV decay rate of the stack prepared with the inventive membranes 1a was 0.0008 V / h, while that of the stack prepared with the comparative PFSA 950EW membranes was 0.0015 V / h. The former failed at 113 h, while the latter failed at 66 h. Furthermore, the fluoride release rate for the stack prepared with the inventive membranes 1a was much lower than that of the stack prepared with the comparative PFSA 950EW membranes.

[0097] The stacks with the membrane according to the invention showed a higher durability compared to that of the conventional stack.

[0098] While specific 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, since modifications may be made by those skilled in the art without departing from the spirit and scope of the present disclosure, particularly in light of the above teachings. Such modifications are to be considered within the scope and spirit of the following claims.

Claims

[1] 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 are covalently bonded to each other or acid-base complexed to each other at the cross-linking functional groups of the copolymer; the copolymer comprises a polymerized network of a plurality of metal oxide monomers having cross-linking 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 with cross-linking 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 which are connected to the first metal and comprise a functional end group which contains nitrogen or oxygen and is characterized by a chemical structure which is selected from the group consisting of -NH2, =NH, -(aliphatic)-OH, -(aryl)-OH, where R is a hydrocarbon group; the metal oxide monomers with 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 which are bonded to the second metal and are selected from the group consisting of: i) proton-bearing functional groups comprising a functional end group selected from the group consisting of -PO3H2, -COOH, -SO3H, and -SO2NHSO2CF3, ii) functional groups forming a chelate complex with metal, which comprise a functional end group selected from the group consisting of phosphonic acid, bipyridine, phenanthroline and derivatives thereof, and iii) free radical scavenging functional groups having a functional end group selected from the group consisting of aminophenyl, hydroxyphenyl and derivatives thereof. [2] The electrolyte of claim 1, wherein the first and second metals are the same. [3] The electrolyte of claim 2, wherein the first and second metals are silicon. [4] The 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 comprising a number of CH2, O, NH or aryl groups in random sequence. [5] The electrolyte according to claim 4, wherein the crosslinking functional groups are -(CH2)3-NH2, -phenyl-NH2 or -(CH2)3-(1H-benzimidazol-2-yl). [6] The electrolyte according to claim 1, wherein the other functional groups are proton-bearing functional groups having an end group selected from the group consisting of -PO3H2, -COOH, -SO3H and -SO2NHSO2CF3. [7] Electrolyte according to claim 6, wherein the proton-bearing functional groups have a chemical structure of the form -Y-(end group), where Y is a linear chain comprising a number of CH2, CF2, or aryl groups in random sequence. [8] Electrolyte according to claim 7, wherein the proton-bearing functional groups are -(CH2)2-PO3H2. [9] The electrolyte according to claim 7, wherein the crosslinking functional groups are -(CH2)3-NH2, -phenyl-NH2 or -(CH2)3-(1H-benzimidazol-2-yl) and the proton-bearing functional groups are -(CH2)2-PO3H2. [10] The electrolyte of claim 9, wherein the ratio of crosslinking functional groups to proton-bearing functional groups in the copolymer is from about 1:9 to 3:

7. [11] The electrolyte of claim 1, wherein the polymerized network comprises at least two different metal oxide monomers having different functional groups. [12] 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 metal oxide monomers having free radical scavenging functional groups. [13] The electrolyte of claim 12, wherein the free radical scavenging functional groups are -3-nitro-4-amino-phenyl. [14] The electrolyte of claim 1, wherein the proton-conductive ionomer comprises sulfonic acid groups. [15] The electrolyte of claim 14, wherein the proton-conductive ionomer is a perfluorosulfonic acid ionomer. [16] The electrolyte of claim 15, wherein the amount of the copolymer is from about 5% to 10% by weight of the electrolyte. [17] 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. [18] A polymer electrolyte fuel cell comprising an electrolyte according to claim 1. [19] The polymer electrolyte fuel cell of claim 18, wherein the fuel cell is configured to operate at temperatures greater than 95°C and relative humidity of less than 50% RH. [20] A method for producing the proton-conductive polymer electrolyte according to 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 bonds to the proton-conductive ionomer. [21] A method according to claim 20, which comprises: Preparing 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. [22] A method according to 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. [23] A method according to claim 20, which comprises: Preparing the copolymer; Adding the copolymer to a dispersion comprising a precursor for the proton-conductive ionomer; Removing solvent from the dispersion and thus providing a solid mixture of the copolymer and the precursor; Heating the mixture; and Converting the precursor into the proton-conducting ionomer. [24] The process of claim 20, wherein the copolymer is prepared 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 and thus producing the copolymer in solution. [25] The process of claim 24, which comprises preparing the metal oxide monomers having crosslinking functional groups by hydrolyzing non-hydrolyzed metal oxide monomers having crosslinking functional groups. [26] The process of claim 25, wherein the non-hydrolyzed metal oxide monomers having crosslinking functional groups are 3-aminopropyltrimethoxysilane, aminophenyltrimethoxysilane or 3-(1H-benzimidazol-2-yl)propyltrimethoxysilane. [27] The process of claim 24, which comprises preparing the metal oxide monomers having other functional groups by hydrolyzing non-hydrolyzed metal oxide monomers having other functional groups. [28] The process of claim 27, wherein the non-hydrolyzed metal oxide monomers having other functional groups are (2-diethylphosphatoethyl)triethoxysilane. [29] The process of claim 25, wherein the hydrolyzing and the preparation of the copolymer are carried out in the same solution. [30] The process of claim 27, wherein the hydrolyzing and the preparation of the copolymer are carried out in the same solution. [31] The method of claim 20, wherein the solution comprises at least two different metal oxide monomers having different functional groups. [32] The method of claim 31, wherein the solution comprises metal oxide monomers having free radical scavenging functional groups, and the free radical scavenging functional groups are hydrolyzed 3-amino-4-nitrophenyl triethoxysilane.

Images