Membrane electrode assembly with fluoro alkyl compound additive
11431014 · 2022-08-30
Assignee
Inventors
Cpc classification
H01M8/102
ELECTRICITY
Y02E60/50
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
International classification
H01M4/86
ELECTRICITY
Abstract
A membrane electrode assembly comprises an anode electrode comprising an anode catalyst layer and an anode gas diffusion layer, a cathode electrode comprising a cathode catalyst layer and a cathode gas diffusion layer, a polymer electrolyte membrane interposed between the anode catalyst layer and the cathode catalyst layer, and a layer comprising a fluoroalkyl-phosphonic acid compound between at least one of the anode gas diffusion layer and the anode catalyst layer, the anode catalyst layer and the polymer electrolyte membrane, the polymer electrolyte membrane and the cathode catalyst layer, and the cathode catalyst layer and the cathode gas diffusion layer.
Claims
1. A membrane electrode assembly comprising: an anode electrode comprising an anode catalyst layer and an anode gas diffusion layer; a cathode electrode comprising a cathode catalyst layer and a cathode gas diffusion layer; a polymer electrolyte membrane interposed between the anode catalyst layer and the cathode catalyst layer; and a layer comprising a fluoroalkyl-phosphonic acid compound between at least one of the anode gas diffusion layer and the anode catalyst layer, the anode catalyst layer and the polymer electrolyte membrane, the polymer electrolyte membrane and the cathode catalyst layer, and the cathode catalyst layer and the cathode gas diffusion layer; wherein the fluoroalkyl-phosphonic acid compound has the following structure: ##STR00010## wherein: m is an integer from 1 to 12; and n is an integer from 0 to 6; wherein the fluoroalkyl-phosphonic acid compound has a molecular weight of at least 200 g/mol.
2. The membrane electrode assembly of claim 1, wherein m is 5-7 and n is 1-2.
3. The membrane electrode assembly of claim 1, wherein the fluoroalkyl-phosphonic acid compound has the following structure: ##STR00011##
4. The membrane electrode assembly of claim 1, wherein the fluoroalkyl-phosphonic acid compound has the following structure: ##STR00012##
5. The membrane electrode assembly of claim 1, wherein the layer comprising the fluoroalkyl-phosphonic acid compound comprises about 1 microgram/cm.sup.2 to about 100 microgram/cm.sup.2 of the fluoroalkyl-phosphonic acid compound.
6. The membrane electrode assembly of claim 1, wherein the layer comprising the fluoroalkyl-phosphonic acid compound comprises about 3 microgram/cm.sup.2 to about 50 microgram/cm.sup.2 of the fluoroalkyl-phosphonic acid compound.
7. The membrane electrode assembly of claim 1, wherein the layer comprising the fluoroalkyl-phosphonic acid compound is between the anode catalyst layer and the polymer electrolyte membrane.
8. The membrane electrode assembly of claim 1, wherein the layer comprising the fluoroalkyl-phosphonic acid compound is between the anode gas diffusion layer and the anode catalyst layer.
9. The membrane electrode assembly of claim 1, wherein the layer comprising the fluoroalkyl-phosphonic acid compound is between the polymer electrolyte membrane and the cathode catalyst layer.
10. The membrane electrode assembly of claim 1, wherein the layer comprising a fluoro phosphonic the fluoroalkyl-phosphonic acid compound is between the cathode catalyst layer and the cathode gas diffusion layer.
11. A method of making a membrane electrode assembly comprising: mixing a fluoroalkyl-phosphonic acid compound with a solvent, an ionomer, conductive particles, or combinations thereof to create a mixture; and applying the mixture to one or more of the following: an anode catalyst layer; an anode gas diffusion layer; a polymer electrolyte membrane; a cathode catalyst layer; or a cathode gas diffusion layer; wherein the fluoroalkyl-phosphonic acid compound has the following structure: ##STR00013## wherein: m is an integer from 1 to 12; and n is an integer from 0 to 6; wherein the fluoroalkyl-phosphonic acid compound has a molecular weight of at least 200 g/mol.
12. The method of claim 11, wherein m is 5-7 and n is 1-2.
13. The method of claim 11, wherein the fluoroalkyl-phosphonic acid compound has the following structure: ##STR00014##
14. The method of claim 11, wherein the fluoroalkyl-phosphonic acid compound has the following structure: ##STR00015##
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1)
(2)
DETAILED DESCRIPTION
(3) In the following description, certain specific details are set forth in order to provide a thorough understanding of the various embodiments of the disclosure. However, one skilled in the art will understand that embodiments of the disclosure may be practiced without these details. In other instances, well-known structures associated with fuel cells, fuel cell stacks, batteries and fuel cell systems have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the disclosure.
(4) Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, that is as “including, but not limited to.”
(5) “Alkyl” refers to a straight or branched hydrocarbon chain group consisting solely of carbon and hydrogen atoms, which is saturated or unsaturated (i.e., contains one or more double and/or triple bonds), having from one to twelve carbon atoms (C.sub.1-C.sub.12 alkyl), one to eight carbon atoms (C.sub.1-C.sub.8 alkyl) or one to six carbon atoms (C.sub.1-C.sub.6 alkyl), and which is attached to the rest of the molecule by a single bond, e.g., methyl, ethyl, n-propyl, 1-methylethyl (iso-propyl), n-butyl, n-pentyl, 1,1-dimethylethyl (t-butyl), 3-methylhexyl, 2-methylhexyl, ethenyl, prop-1-enyl, but-1-enyl, pent-1-enyl, penta-1,4-dienyl, ethynyl, propynyl, butynyl, pentynyl, hexynyl, and the like. Unless stated otherwise specifically in the specification, an alkyl group may be optionally substituted.
(6) “Fluoroalkyl” refers to a straight or branched fluorocarbon chain group consisting solely of carbon and fluorine atoms, which is saturated or unsaturated (i.e., contains one or more double and/or triple bonds), having from one to twelve carbon atoms (C.sub.1-C.sub.12 fluoroalkyl), one to eight carbon atoms (C.sub.1-C.sub.8 fluoroalkyl) or one to six carbon atoms (C.sub.1-C.sub.6 fluoroalkyl), and which is attached to the rest of the molecule by a single bond. Unless stated otherwise specifically in the specification, a fluoroalkyl group may be optionally substituted.
(7) “Alkylene” or “alkylene chain” refers to a straight or branched divalent hydrocarbon chain linking the rest of the molecule to a substituent group, consisting solely of carbon and hydrogen, which is saturated or unsaturated (i.e., contains one or more double and/or triple bonds), and having from one to twelve carbon atoms, e.g., methylene, ethylene, propylene, n-butylene, ethenylene, propenylene, n-butenylene, propynylene, n-butynylene, and the like. The alkylene chain is attached to the rest of the molecule through a single or double bond and to the substituent group through a single or double bond. The points of attachment of the alkylene chain to the rest of the molecule and to the substituent group can be through one carbon or any two carbons within the chain. Unless stated otherwise specifically in the specification, an alkylene chain may be optionally substituted.
(8) A “corrosion resistant support material” is at least as resistant to oxidative corrosion as Shawinigan acetylene black (Chevron Chemical Company, TX, USA; e.g., as shown in Modern Aspects of Electrochemistry, Number 38, edited by B. E. Conway (2006)).
(9) “Fluoro-phosphonic acid compound” refers to a compound comprising a phosphonic acid moiety (i.e., —P(═O)(OH).sub.2) and at least one fluoro substituent (i.e., —F). A fluoro-phosphonic acid compound may further comprise additional components, for example, aryl (e.g., phenyl), alkyl (or alkylene) and the like. Generally, the fluoro substituent may be attached to an additional component, for example as a substituent on a phenyl or alkyl.
(10) “Fluoroalkyl-phosphonic acid compound” refers to a compound comprising a phosphonic acid moiety (i.e., —P(═O)(OH).sub.2), and a fluoroalkyl moiety (i.e., a branched or straight fluorocarbon chain), for example, —(CF.sub.2).sub.nCF.sub.3 wherein n ranges from 0 to about 12. The fluoroalkyl moiety may be branched and a fluoroalkyl-phosphonic acid compound may further comprise additional components, for example, aryl (e.g., phenyl), alkyl (or alkylene) and the like.
(11) As shown in
(12) As discussed in the foregoing, hydrophobic materials are desirable for water management, particularly for promoting water electrolysis in the anode for voltage reversal mitigation reasons. However, hydrophobic materials typically are difficult to incorporate into the catalyst layer, particularly those that require high sintering temperatures. Furthermore, hydrophobic additives also have a tendency to agglomerate, thereby requiring high shear mixing, which makes them difficult to process with the catalyst into a catalyst ink. In addition, hydrophobic additives are difficult to wet and keep well dispersed in the catalyst ink, which makes it challenging to apply the catalyst ink evenly or uniformly to any of the gas diffusion layer, catalyst decal film and polymer electrolyte membrane.
(13) The inventors surprisingly discovered that by including a layer comprising a fluoro-phosphonic acid compound between the anode catalyst layer and the polymer electrolyte membrane, cell reversal tolerance was improved without affecting performance even though the fluoro-phosphonic acid compound was not mixed into the anode catalyst layer. Without being bound by theory, it is believed that the phosphonic acid groups help with water retention while the fluoroalkyl groups help with the removal of oxygen, which is a by-product of the water electrolysis reaction, away from the reaction sites of the water electrolysis catalyst.
(14) In some embodiments, the average molecular weight of the fluoro-phosphonic acid compound ranges from about 200 to 1200 g/mol. In specific embodiments, the average molecular weight of the fluoro-phosphonic acid compound ranges from about 300 to 1000 g/mol. In some embodiments, the average molecular weight of the fluoro-phosphonic acid compound ranges from about 250 to 1100 g/mol, from about 350 to 900 g/mol, from about 350 to 800 g/mol, from about 350 to 700 g/mol, from about 400 to 650 g/mol, or from about 400 to 600 g/mol.
(15) In certain embodiments, the fluoro-phosphonic acid compound has a molecular weight of at least 200 g/mol. In some embodiments, the fluoro-phosphonic acid compound has a molecular weight of at least 150 g/mol. In certain embodiments, the fluoro-phosphonic acid compound has a molecular weight of at least 100 g/mol.
(16) The loading of the fluoro-phosphonic acid compound in the layer may range from about 1 microgram/cm.sup.2 to about 100 microgram/cm.sup.2. In some embodiments, the loading of the fluoro-phosphonic acid compound in the layer ranges from about 1.5 microgram/cm.sup.2 to about 90 microgram/cm.sup.2, from about 1.5 microgram/cm.sup.2 to about 80 microgram/cm.sup.2, from about 2 microgram/cm.sup.2 to about 70 microgram/cm.sup.2, from about 2 microgram/cm.sup.2 to about 65 microgram/cm.sup.2, or from about 2.5 microgram/cm.sup.2 to about 60 microgram/cm.sup.2. In some embodiments, the loading may range from about 3 microgram/cm.sup.2 to about 50 microgram/cm.sup.2. One skilled in the art will readily be able to determine a suitable loading for a given membrane electrode assembly design and fuel cell application.
(17) In one embodiment, the fluoro-phosphonic acid compound is a fluoroalkyl-phosphonic acid compound. In certain embodiments, the fluoroalkyl-phosphonic acid compound has an alkyl component and a perfluoro-alkyl component. For example, in some embodiments, the fluoro-phosphonic acid compound has the following structure:
(18) ##STR00003##
wherein:
(19) m is an integer from 1 to 12; and
(20) n is an integer from 0 to 6.
(21) In some embodiments, m is 3-10. In certain embodiments, m is 4-9. In some specific embodiments, m is 5-7. In some embodiments, m is 5. In some embodiments, m is 6. In some embodiments, m is 7. In some embodiments, m is 9.
(22) In some embodiments, n is 1-6. In certain embodiments, n is 1-4. In some specific embodiments, n is 1-2. In some embodiments, n is 2. In some embodiments, n is 0.
(23) In some embodiments, m is 3-10 and n is 1-4. In certain embodiments, m is 4-9 and n is 1-4. In some embodiments, m is 4-9 and n is 1-2. In some embodiments, m is 5-7 and n is 1-2. In some specific embodiments, m is 5 and n is 2. In other embodiments, m is 7 and n is 2. In still other embodiments, m is 9 and n is 0.
(24) In specific embodiments, the fluoro-phosphonic acid compound is a fluoroalkyl-phosphonic acid compound, such as 2-perfluorohexyl ethyl phosphonic acid and 1H, 1H, 2H, 2H-heptadecafluorodec-1-yl phosphonic acid (C.sub.10—PFPA). That is, in some embodiments, the fluoro-phosphonic acid compound has one of the following structures:
(25) ##STR00004##
(26) In more specific embodiments, the fluoro-phosphonic acid compound has the following structure:
(27) ##STR00005##
(28) In other specific embodiments, the fluoro-phosphonic acid compound has the following structure:
(29) ##STR00006##
(30) Although the compounds of the disclosure depicted as fully protonated (i.e., —P(═O)(OH).sub.2), one of skill in the art will understand that the protonation state of the compound will vary depending on pH and the deprotonated forms (i.e., —P(═O)(O.sup.−).sub.2 and —P(═O)(O.sup.−)(OH)) are included in the foregoing embodiments. The anode and/or cathode catalyst layers may include at least one noble metal and a binder, such as an ionomer and/or hydrophobic agent. The noble metal may comprise a precious metal, such as platinum or an alloy of platinum. In embodiments where a platinum alloy catalyst is employed, the alloy may include another noble metal, such as gold, ruthenium, iridium, osmium, palladium, silver; and compounds, alloys, solid solutions, and mixtures thereof. While noble metals are presently described, it is expected that non-noble metals, such as such as cobalt, iron, molybdenum, nickel, tantalum, tin, tungsten; and compounds, alloys, solid solutions, and mixtures thereof, can also be used in place of, or in addition to, the noble metal catalyst in the anode and/or cathode catalyst layers.
(31) The noble metal catalyst may either be unsupported or supported in dispersed form on a suitable electrically conducting particulate support. In some embodiments, the support used is itself tolerant to voltage reversal. Thus, it is desirable to consider using supports that are more corrosion resistant.
(32) The corrosion resistant support material may comprise carbon, if desired. High surface area carbons, such as acetylene black or furnace black, are commonly used as supports for such catalysts. Generally, the corrosion resistance of a carbon support material is related to its graphitic nature: the more graphitic the carbon support, the more corrosion resistant it is. Graphitized carbon BA (TKK, Tokyo, JP) has a similar BET surface area to Shawinigan acetylene carbon and is a suitable carbon support material in some embodiments. In other embodiments suitable carbon support materials may include nitrogen-, boron-, sulfur-, and/or phosphorous-doped carbons. In some embodiments, the carbon support material comprises carbon nanofibres, carbon nanotubes, carbon nanohorns, graphenes, aerogels and combinations thereof. Instead of carbon, carbides or electrically conductive metal oxides may be considered as a suitable high surface area support for the corrosion resistant support material. For instance, in some embodiments, the corrosion resistant support material comprises tantalum oxide, titanium oxide, niobium oxide or combinations thereof. In this regard, other valve metal oxides might be considered as well if they have acceptable electronic conductivity when acting as catalyst supports.
(33) The layer comprising a fluoro-phosphonic acid compound may be dispersed or dissolved in a solvent and then applied by various methods. Such methods include, but are not limited to, spraying, dip-coating, wire-wound coating, screen-printing, microgravure coating, atomic layer deposition, and the like on any one of the gas diffusion layers, the catalyst layers, and the polymer electrolyte membrane. Alternatively, fluoro-phosphonic acid compound may be incorporated by soaking the gas diffusion layer (“GDL”) and or/catalyst layer. In some embodiments, the fluoro-phosphonic acid compound may be mixed with another ionomer, such as a sulfonic acid ionomer, and/or conductive particles, such as carbon, prior to application.
(34) As previously mentioned, the anode and cathode catalyst layers may be applied to a GDL to form anode and cathode electrodes, or to a decal transfer sheet which is then decal transferred to a surface of the GDL or polymer electrolyte, or applied directly to the surface of the polymer electrolyte to form a catalyst-coated membrane (“CCM”). The electrodes or CCM can then be bonded under heat and/or pressure with other components to form an MEA. Alternatively, the application of the catalyst layer on the desired substrate may occur at the same time the remaining MEA components are bonded together.
(35) The present catalyst layers may be applied according to known methods. For example, the catalyst may be applied as a catalyst ink or slurry, or as a dry mixture. Catalyst inks may be applied using a variety of suitable techniques (e.g., hand and machine methods, including hand brushing, notch bar coating, fluid bearing die coating, wire-wound rod coating, fluid bearing coating, slot-fed knife coating, three-roll coating, screen-printing, ultrasonic spray, inkjet, and decal transfer) to the surface of the polymer electrolyte or GDL. Examples of dry deposition methods include spraying, vacuum deposition, and electrostatic powder deposition techniques.
(36) Catalyst inks typically incorporate the catalysts and binder in a solvent/dispersant to form a solution, dispersion or colloidal mixture. Suitable solvents/dispersants include water, organic solvents such as alcohols (e.g., methanol, ethanol, isopropyl alcohol, etc.) and polar aprotic solvents (e.g., N-methylpyrrolidinone, dimethylsulfoxide, dimethylformamide and N,N-dimethylacetamide), and mixtures thereof. Depending on the amount of water, one can distinguish water-based inks, wherein water forms the major part of the solvents used, from inks wherein organic solvents form the major part. Catalyst inks may further include surfactants and/or pore forming agents, if desired. Suitable pore formers include methyl cellulose; sublimating pore-forming agents such as durene, camphene, camphor and naphthalene; and pore-forming solvents that are immiscible with the catalyst ink solvent/dispersant, such as n-butyl acetate in polar aprotic solvent/dispersant systems.
(37) Accordingly, one embodiment provides a method of making a membrane electrode assembly comprising mixing a fluoro-phosphonic acid compound with a solvent, an ionomer, conductive particles, or combinations thereof to create a mixture, and applying the mixture to one or more of the following, an anode catalyst layer, an anode gas diffusion layer, a polymer electrolyte membrane, a cathode catalyst layer, or a cathode gas diffusion layer.
(38) In some of the foregoing embodiment, the fluoro-phosphonic acid compound is a fluoroalkyl-phosphonic acid compound. In more specific embodiments, the fluoro-phosphonic acid compound has the following structure:
(39) ##STR00007##
wherein:
(40) m is an integer from 1 to 12; and
(41) n is an integer from 0 to 6.
(42) For example, in some embodiments, m is 5-7 and n is 1-2.
(43) In some embodiments, the fluoro-phosphonic acid compound has the following structure:
(44) ##STR00008##
(45) In other embodiments, the fluoro-phosphonic acid compound has the following structure:
(46) ##STR00009##
(47) In certain specific embodiments, the fluoro-phosphonic acid compound has a molecular weight of at least 200 g/mol.
(48) The selection of additional components for the catalyst mixture and the choice of application method and GDL to which it is applied are not essential to the present disclosure, and will depend on the physical characteristics of the mixture and the substrate to which it will be applied, the application method and desired structure of the catalyst layer. Persons of ordinary skill in the art can readily select suitable catalyst mixtures and application methods for a given application.
EXAMPLES
(49) The MEAs were made with the following electrode structures as listed in Table 1, with the CCM sandwiched between two AvCarb® GDLs (AvCarb Materials Solutions, Lowell, Mass.). The active area of each of the MEAs was 45 cm.sup.2.
(50) TABLE-US-00001 TABLE 1 Anode and cathode catalyst structures for MEAs MEA Example Anode Cathode Compar- 50% Pt supported on graphitized 50% Pt supported on ative carbon black at a catalyst loading graphitized carbon MEA 1 of ~0.1 mg Pt/cm.sup.2; black at ~0.4 mg Pt/cm.sup.2 ~0.0067 g/m.sup.2 IrO.sub.2/TiO.sub.2, Ionomer (23%): Ionomer (23%): Nafion ® Nafion ® Example 50% Pt supported on graphitized 50% Pt supported on MEA 1 carbon black at a catalyst loading graphitized carbon of ~0.1 mg Pt/cm.sup.2; black at ~0.4 mg Pt/cm.sup.2 ~0.0067 g/m.sup.2 IrO.sub.2/TiO.sub.2, Ionomer (23%): C.sub.10-PFPA loading of 0.02 mg/cm.sup.2 Nafion ® Ionomer (23%): Nafion ®
(51) For Example MEA 1, the C.sub.10-PFPA was mixed with isopropanol and then sprayed onto the anode catalyst layer at 75° C. After spraying, the anode catalyst layer with a thin layer of C.sub.10-PFPA was decal transferred to a surface of a Nafion® membrane such that the C.sub.10-PFPA layer was between the anode catalyst layer and the membrane.
(52) The MEAs were then tested in a Ballard Standard Test Cell (STC) test fixture with graphite plates. The fuel cells were first conditioned overnight under the following conditions at 1.3 A/cm.sup.2:
(53) TABLE-US-00002 TABLE 2 Conditioning parameters Temperature 75° C. (coolant) Inlet Dew Point 75° C. (fuel and oxidant) Fuel 100% hydrogen Oxidant air Reactant inlet pressure 5 psig (fuel and oxidant) Reactant flow 4.5 (fuel), 9.0 (oxidant) slpm
Cell Reversal Testing
(54) The fuel cells were conditioned overnight at 1.3 A/cm.sup.2 at the conditions listed in Table 2. It was shown that both there were no performance difference between the two fuel cells. Therefore, the C.sub.10-PFPA did not have a negative effect on performance.
(55) The fuel supply was then switched to humidified nitrogen and the cell was supplied with 200 mA/cm.sup.2 of current through an external power supply under current control mode. The cell reversal tolerance time was monitored until the cell voltage reached −2.0 V. It was shown that Comparative MEA 1 had a cell reversal tolerance time of about 50 minutes while Example MEA 1 with the C.sub.10-PFPA layer had a cell reversal tolerance time of over 70 minutes.
(56) While the fluoro-phosphonic acid compound has been described for use in the anode, it is expected that the fluoro-phosphonic acid is also beneficial for the cathode, either between the cathode catalyst layer and the polymer electrolyte membrane and/or between the cathode catalyst layer and the cathode gas diffusion layer. One skilled in the art can readily determine a desirable amount of fluoro-phosphonic acid compound in one or both of those layers for a given MEA design and fuel cell application.
(57) While the present electrodes have been described for use in PEM fuel cells, it is anticipated that they may be useful in other fuel cells having an operating temperature below about 250° C. They are particularly suited for acid electrolyte fuel cells, including phosphoric acid, PEM and liquid feed fuel cells. It is also contemplated that this treatment may also be useful for other metal oxides comprising ruthenium.
(58) All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference in their entirety. This application also claims the benefit of U.S. Provisional Patent Application No. 62/537,365, filed Jul. 26, 2017, and is incorporated herein by reference in its entirety.
(59) While particular elements, embodiments and applications of the present disclosure have been shown and described, it will be understood, of course, that the disclosure is not limited thereto since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings. It is therefore contemplated by the appended claims to cover such modifications that incorporate those features coming within the scope of the present disclosure.