Carbon-supported electrodes

Abstract

The disclosed technology relates to compositions for mitigating corrosion of carbon supports in catalyst layers of membrane electrode assemblies by including a reversible organic inhibitor within the layers of the membrane electrode assemblies.

Description

[Technical field]

[0001] The disclosed technology relates to compositions for mitigating corrosion of carbon supports in catalyst layers of membrane electrode assemblies by including a reversible organic inhibitor within the layers of the membrane electrode assemblies. [Background technology]

[0002] Proton exchange membrane fuel cells (PEMFCs) are a promising technology to enable drivetrain diversification for heavy vehicle applications and provide a clean fuel alternative to traditional combustion engines. Catalytic decomposition is one of the main factors limiting fuel cells from reaching their ultimate goal of 8,000 hours (150,000 miles) of operation for light vehicles or 25,000 hours (1,000,000 miles) for heavy vehicles with less than 10% performance loss set by the Department of Energy (DOE).

[0003] Fuel cell catalyst layers (whether PEMFC, AFC, DMFC / DEFC) are known to degrade through corrosion of the carbon support within the layer. The carbon support is a large area conductive material that acts as a mechanical support for the active catalyst and also provides electrical conductivity for the transport of electrons. Carbon corrosion in the catalyst leads to destruction of catalyst adhesion, collapse of electrode pore structure, loss of hydrophobic properties, and in the case of Pt-containing catalysts, an increase in Pt particle size, all of which negatively impact the efficiency of the fuel cell system.

[0004] The main mechanism by which carbon supports are decomposed in PEMFCs is electrochemical oxidation (C+2H2O→CO2+4H ++4e-). This oxidation reaction begins to occur at 0.207 V vs. the RHE (for purposes of this specification, the "reversible hydrogen electrode" measured at 0.1 molar concentration of free protons in an aqueous electrolyte). However, when localized regions are temporarily starved of hydrogen (also known as "fuel starvation"), or when a hydrogen-oxygen (or air) gas front moves through the cell (frequently encountered during start-up and shutdown), this can lead to interfacial potentials of up to 1.44 V vs. the RHE, or greater when the rate of the carbon oxidation reaction is sufficient to consume all of the carbon support within a few hours.

[0005] During a start-up-shutdown event, when a hydrogen-oxygen gas front is allowed to travel through what would be the anode side of the cell during normal operation, a "reverse current mechanism" can ensue in which the cathode experiences an oxidizing potential during normal operation. During this period of high electrode potential on what would normally be the cathode, carbon on the cathode may corrode while protons are shuttled toward the anode, opposite to the normal fuel cell operating current flow. Once hydrogen fuel reaches continuity across the anode, the current and ion flux return to the normal mechanism and the cathode operates normally (i.e., electrochemically reducing oxygen).

[0006] During a hydrogen fuel starvation event, when oxygen is still present on what would normally be the cathode side of the cell, but there is insufficient hydrogen supply to what would normally be the anode side of the cell, the anode potential can increase until a sufficient potential is reached to corrode carbon at a significant rate.

[0007] Several materials and engineering mitigation strategies have been investigated or implemented to extend the durability of carbon supports, such as the use of highly graphitized carbon, but their lower platinum utilization prevents widespread use. Other investigated strategies include voltage limiting through optimization of cathode outlet size and stack shunting.

[0008] None of the above have proven to be a sufficient solution for mitigating carbon corrosion, especially during start-up and shut-down periods of operation.

[0009] In addition to catalyst decomposition, membrane decomposition is another important factor limiting fuel cells from reaching the ultimate goals set by the Department of Energy (DOE).

[0010] The lifetime requirements and high demands imposed on PEMFCs lead to premature failure of a critical PEMFC component, the polymer electrolyte membrane (PEM). Over time, free radical species and peroxides produced during normal operation of a PEMFC chemically react with the PEM, impairing system performance by reducing its mechanical integrity and proton conductivity.

[0011] Current strategies to address this problem utilize some combination of end-group fluorination of perfluorosulfonic acid (PFSA) polymers and the use of suspended metal antioxidants (Ce or Mn). Unfortunately, unbound metal antioxidants cause an undesirable decrease in proton conductivity and leaching into other layers of the fuel cell, exposing the membrane to chemical degradation and catalyst poisoning. These same problems can occur in anion exchange membranes. Alternative strategies attempt to immobilize the antioxidants by covalently bonding them to the polymers used as ion exchange membranes, but this approach requires chemical transformations and often involves additional linking groups. Furthermore, by modifying the polymer from which the ion exchange membrane is cast, such approaches alter the mechanical and electrical properties of the resulting membrane. Finally, the incorporation of some antioxidants into a PEMFC can harm the proton conductivity and voltage produced by the PEMFC, but current approaches fail to identify criteria by which antioxidants with desirable and undesirable properties can be differentiated.

[0012] Thus, a need exists for new strategies to extend the durability of carbon supports in fuel cell electrodes and to extend the life of PEMs against chemical degradation. Summary of the Invention [Means for solving the problem]

[0013] The disclosed technology solves the problems of carbon support decomposition in the electrodes and chemical degradation of the membrane by employing redox active molecules in at least one layer of the membrane electrode assembly composition.

[0014] Thus, in one aspect, the present technology provides a composition comprising a carbonaceous compound and a reversible organic inhibitor, the composition may be a fuel cell carbon support layer, a microporous layer, or a catalyst layer composition.

[0015] As a catalyst layer, the composition can further comprise a catalyst, as well as an ionomer and an optional non-ionically conductive material.

[0016] In one embodiment, the reversible inhibitor is immobilized on the carbonaceous compound or in the membrane layer.

[0017] In another aspect of the invention, the present technology encompasses a composition comprising an ionomer and a reversible organic inhibitor. In one embodiment, the reversible organic inhibitor can be immobilized on the ionomer.

[0018] The ionomer composition may be mixed with a carbonaceous compound, a catalyst, and an optional non-ionically conductive material to prepare a catalyst layer for a fuel cell.

[0019] Any of the compositions can also be delivered as an "ink" diluted in a solvent to allow the composition to be coated in place.

[0020] Also encompassed by the present technology is a catalyst coated membrane ("CCM") that includes (i) an electrolyte membrane catalyst layer, and (ii) a catalyst layer that includes a reversible organic inhibitor.

[0021] Another aspect of the present technology is a gas diffusion layer ("GDL") for a fuel cell having (iii) a microporous layer, and (iv) a carbon substrate, where either or both of the layers contain a reversible organic inhibitor.

[0022] A further aspect of the present technology includes a gas diffusion electrode ("GDE") for a fuel cell having (ii) a catalyst layer, (iii) a microporous layer, and (iv) a carbon substrate, at least one of the layers having a reversible organic inhibitor.

[0023] A still further aspect of the present technology is a fuel cell having (i) an electrolyte membrane, (ii) a catalyst layer, (iii) any other layers in electrical contact with the catalyst layer, including but not limited to a microporous layer, and (iv) a reversible inhibitor in a carbon substrate, wherein at least one layer comprises a reversible organic inhibitor.

[0024] The present technology also encompasses a method of preventing corrosion to carbonaceous compounds in a catalyst layer of a fuel cell by including a reversible organic inhibitor in at least one layer of the fuel cell and operating the fuel cell.

[0025] The present technology also includes a method of preventing degradation of a fuel cell membrane by chemical attack by oxidants such as hydrogen peroxide or radical species by including a reversible organic inhibitor in at least one layer of the fuel cell and operating the fuel cell.

[0026] Both the electrochemical carbon corrosion and membrane chemical oxidation events described above are hereinafter referred to as "oxidative degradation events."

[0027] The disclosed technology provides compositions and methods for mitigating corrosion of carbon supports in catalyst layers of membrane electrode assemblies, and for mitigating degradation of membrane layers by including reversible organic inhibitors within the layers of membrane electrode assemblies, enabling improved fuel cells. [Brief description of the drawings]

[0028] [Figure 1] CV of 3,4-dihydroxybenzoic acid at the fuel cell cathode. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Various preferred features and embodiments are described below by way of non-limiting example.

[0030] Unless otherwise stated, all parts levels of components are based on 100 parts by weight of carbonaceous compound, membrane ionomer, or combination thereof, as determined by the context of this disclosure, abbreviated as "phr."

[0031] A fuel cell membrane electrode assembly (MEA) generally includes several layers, including but not limited to a carbon substrate layer, a microporous layer, and a catalyst layer, all surrounding a membrane layer. As used herein, the term "MEA" refers to an assembly that includes an ion-conducting polymer membrane layer surrounded by a catalyst layer, which in turn is surrounded by a microporous layer, and finally by a carbon substrate layer.

[0032] A "microporous layer", which may also be referred to herein as a "microporous layer, MPL", is a porous layer containing carbon and a polymer (typically PTFE) coated directly onto a carbon substrate.

[0033] The "carbon substrate" is the layer that serves to support the MEA.

[0034] The combination of the carbon substrate and MPL is referred to herein as the "gas diffusion layer" or "GDL."

[0035] The catalyst layer can be coated either on the carbon substrate, or on the GDL where the MPL is employed, or on the membrane layer. When the catalyst layer is coated on the carbon substrate or on the GDL, it is referred to herein as a "gas diffusion electrode" or "GDE." When the catalyst layer is coated on the membrane, it is referred to herein as a "catalyst coated membrane" or "CCM."

[0036] The membrane layer consists of a solid, ionically conductive medium, often polymeric in nature, that serves multiple functions including transport of ions and separation of the anode and cathode.

[0037] All of the above nomenclature conventions are common in the art and literature and will be familiar to those of skill in the art.

[0038] Each of the carbon matrix layer, the microporous layer, and the catalyst layer contains an electrically conductive carbonaceous compound, also referred to simply as a carbonaceous compound, that is capable of conducting electricity throughout all of the layers of the GDE on each side of the membrane, because each of the layers are sandwiched together and in contact.

[0039] The carbonaceous compound can include, for example, conductive carbon black, such as "acetylene black" or "furnace black," or any commercial grade conductive carbon black, with acetylene black being excellent for producing conductive blends.

[0040] Graphite is also a well-known carbonaceous compound and may be employed in the present technology in any of its various forms, including natural or synthetic, crystalline or amorphous, so long as the graphite is electrically conductive.

[0041] Carbonaceous compounds can also include conductive carbon fibers, fullerenes, and carbon nanotubes.

[0042] The type of carbonaceous compound will vary depending on the layer in which the support material is contained, for example, whether the carbonaceous compound is part of a carbon matrix or is a host for a catalyst.

[0043] The carbon substrate can include carbonaceous compounds in the form of woven carbon fibers, carbon fiber mats, or carbon felts, which can include, for example, Toray carbon paper, Freudenberg carbon paper, and SGL carbon paper from Sigracet. As discussed further below, the carbon substrate can be prepared with the reversible organic inhibitor as an essential component, or the carbon substrate can be purchased commercially and the reversible organic inhibitor can be coated thereon.

[0044] The microporous layer may comprise a carbonaceous compound and / or graphitic carbon, which is a high surface area as opposed to a carbon substrate. The MPL may also contain non-carbonaceous materials, which may include PTFE or other additives, with individual component ranges of 2-200 phr. As discussed further below, the microporous layer may be prepared with the reversible organic inhibitor as an essential component, or the microporous layer may be purchased commercially and the reversible organic inhibitor may be coated thereon.

[0045] The catalyst layer can include, for example, a carbonaceous compound that is also a high surface area carbon, which can include, for example, a carbon black material such as Vulcan carbon black or Ketjen black. Again, as discussed further below, the catalyst layer can be prepared with the reversible organic inhibitor as an essential component, or the catalyst layer can be purchased commercially and the reversible organic inhibitor can be coated thereon.

[0046] The reversible redox inhibitor may be incorporated into the membrane layer by any means, including, but not limited to, addition to the ionomer dispersion prior to membrane casting or absorption into a prefabricated membrane. The additive may be non-covalently or chemically bound to the membrane ionomer via synthetic modifications resulting in ionic or covalent interactions.

[0047] The present technology provides compositions that enable improved carbon-supported electrodes or polymer electrolyte membranes by incorporating reversible organic inhibitors into either the carbonaceous compound or MEA layers that oxidize and act as sacrificial materials to prevent fuel cell oxidative degradation events. The oxidation of the reversible organic inhibitors can be driven by an electric field or a chemical reaction with an oxidant.

[0048] A reversible organic inhibitor is a compound that can mitigate carbon corrosion based on its reversible electrochemical redox potential. The reversible electrochemical redox potential of the inhibitor allows the inhibitor to be oxidized at potentials below the potential at which carbon oxidation becomes significantly detrimental (i.e., 1.2 V vs. RHE) but above the practical operating potential of the fuel cell electrode under normal operating conditions (e.g., −0.2 to 1.0 V vs. RHE). When the reversible organic inhibitor is in electrical contact with carbonaceous compounds in the cathode catalyst layer and the local interfacial potential reaches a value that matches or exceeds the oxidation potential of the reversible organic inhibitor (typically during fuel cell start-up / shutdown or fuel starvation events), the reversible organic inhibitor is oxidized in preference to the carbonaceous compounds, thereby preserving the integrity of the catalyst layer.

[0049] Reversible organic inhibitors are compounds that can mitigate deleterious oxidation of membrane materials by chemical oxidants, such as oxidative attack by peroxide radicals that can be formed during operation, especially during operation at low overpotentials (i.e., near open circuit). When chemical oxidants are formed, the reversible organic inhibitors can be chemically oxidized instead of the membrane materials.

[0050] In other words, during an oxidation event, the reversible organic inhibitors are oxidized to redox active species instead of detrimentally oxidizing the carbonaceous compounds or the membrane material.

[0051] The reversible organic inhibitor can have a redox potential while in an aqueous environment in the range of 0.5 to 1.4 V vs. RHE. Examples of reversible organic inhibitors are compounds that have a redox potential in an aqueous environment in the range of 0.6 to 1.3 V vs. RHE, or even 0.7 or 0.8 to 1.2 V vs. RHE.

[0052] The reversible organic inhibitor must also not poison the active catalyst, ie, reduce the activity of the catalyst towards oxygen reduction to a detrimental effect.

[0053] The oxidation (chemical or electrochemical) of the reversible organic inhibitor must also have facile kinetics so that the oxidation of the molecule occurs at a significant rate (i.e., is not limited by overpotential) prior to the fuel cell oxidation event (carbon corrosion or membrane) and its subsequent reduction to its original state occurs before further subsequent oxidation is required. Upon return to normal operation and normal current and electrode potential, the redox active species is then reduced back to its state before the oxidation event, regenerating the protective reversible organic inhibitor to be oxidized again during the subsequent oxidation event.

[0054] In addition, the reversible organic inhibitor must be in contact with the conductive carbon material to be electrochemically reduced and regenerate the protective state of the reversible redox inhibitor. For clarity, the reversible inhibitor can be included in any of the carbon-containing layers, i.e., the carbon matrix layer, the microporous layer, and / or the catalyst layer or membrane, along with the carbonaceous compound. Furthermore, each layer can contain one reversible organic inhibitor or a mixture of two or more reversible organic inhibitors.

[0055] Examples of currently known reversible organic inhibitors that meet the aforementioned criteria include many of the hydroquinones / quinones, such as potassium 1,4-hydroquinone sulfonate, methyl 2,5-dihydroxybenzoate, 2,5-dihydroxybenzoic acid, 2,5-dihydroxybenzoic acid, 2,5-dimethoxybenzonitrile, 3,6-dihydroxyphthalonitrile, 3,4-dihydroxybenzoic acid, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone.

[0056] One of ordinary skill in the art, following the techniques described herein, could readily test for compounds that constitute reversible organic inhibitors based on the foregoing criteria, and all such reversible organic inhibitors are contemplated under the present disclosure.

[0057] One method of assessing redox potential is to first prepare an ink containing a mixture of carbon or platinum carbon, a proton conducting ionomer such as perfluorosulfonic acid ionomer, a reversible redox inhibitor, and a carrier solvent such as water and isopropanol. The ink is homogenized using techniques such as high shear mixing, sonication, or ball milling. The resulting ink is then deposited onto a carbon electrode and the carrier solvent is evaporated. The preparation and formation of these inks and the subsequently formed electrodes are generally well described in the art. The resulting electrodes can then be used in a three-electrode cell and cycled through various potential windows in an acidic aqueous electrolyte via conventional electrochemical techniques such as cyclic voltammetry to obtain redox profiles. Cyclic voltammograms can also be used, along with other electrochemical techniques such as electrochemical impedance spectroscopy, to infer the kinetic rates at which redox events occur. From these cyclic voltammetry experiments, the redox activity of each molecule was observed at 100 mV / s and was performed from -0.28 to 1.0 V vs. Ag / AgCl. o The measured potential of the combined reduction and oxidation reaction, described as , is calculated as the average of the peak oxidation and peak reduction currents observed during cyclic voltammetry performed at a scan rate of 100 mV / s during the third cycle.

[0058] In one embodiment, the present technology provides a composition containing a carbonaceous compound and a reversible organic inhibitor. The carbonaceous compound and the reversible organic inhibitor may simply be mixed together and physically contacted, or the reversible organic inhibitor may be immobilized on the carbonaceous compound. Immobilization may be by covalent attachment of the reversible organic inhibitor to the carbonaceous compound, for example, by methods known in the art for covalently attaching organic compositions to carbon compositions. Such covalent attachment techniques include, for example, carbon functionalization via grafting of aryl radicals formed from decomposition of aryl diazonium salts or aryl iodonium salts. Another option is pre-functionalization of the carbonaceous material via incorporation of oxygen, nitrogen, sulfur, or other atoms via an oxidation process, and further covalent bond-forming reaction of the oxidized carbonaceous material and the reversible inhibitor.

[0059] The amount of reversible organic inhibitor, when present in a particular layer, may range from 1 phr to 50 phr based on the total carbon content of the particular layer at hand, or from 2 phr to 40 phr based on the total carbon content of the particular layer at hand, or from 3 phr to 37 phr based on the total carbon content of the particular layer at hand, or from 4 phr to 35 phr based on the total carbon content of the particular layer at hand, or from 5 phr to 34 phr based on the total carbon content of the particular layer at hand.

[0060] Each of the MEA layers needs to be deposited in some manner to form a layer. To allow the layer to be of a consistency that allows coating, a solvent is added to the mixture of carbonaceous compound and reversible organic inhibitor to obtain an "ink". In the simplest embodiment, the layer can be deposited as an "ink", which is used herein to mean a flowable precursor to the final deposited layer depending on the coating technique employed. The ink can contain all the components of the electrode. For example, the ink can contain the carbonaceous compound, ionomer, additional types of binder (e.g., polymeric material, reversible organic inhibitor, and solvent. This ink is deposited and the solvent is evaporated leaving a selected layer of the carbonaceous compound, ionomer, and organic inhibitor.

[0061] For example, as discussed above, a microporous layer may be coated onto a carbon substrate layer. The MPL ink is coated onto the carbon substrate and the solvent is evaporated, leaving behind the MPL carbonaceous compound and inhibitor.

[0062] Similar to MPL, the catalyst layer can be applied as an ink, where again the ink can be coated onto a suitable substrate (e.g., either a GDL or a membrane) and the solvent allowed to evaporate, leaving behind the catalyst layer.

[0063] Solvents suitable for use in the "ink" can include, for example, 1-propanol, 2-propanol, water, or any combination of other solvents suitable for dispersion of catalytic materials, which will also be appropriate for the coating method selected and will be readily discernible to one of ordinary skill in the art.

[0064] Each layer may also include other additives, for example the MPL may include a binder such as PTFE, as previously referenced.

[0065] The catalyst layer may also include a metal or non-metal catalyst that facilitates the electrochemical reactions of the fuel cell. The metal catalyst may be a noble metal or a transition metal, or an alloy of any of them. Examples of such metals include, for example, ruthenium, rhodium, palladium, osmium, iridium, platinum, gold, silver, copper, rhenium, mercury, iron, cobalt, and nickel. In one embodiment, the metal catalyst is platinum or a platinum alloy.

[0066] The catalyst layer may also include an ionomeric polymer binder. An ionomer is a polymer that can transport protons (i.e., H+) to and from reaction sites in a fuel cell and can help distribute electrode components. By "reaction sites" is meant sites in the electrode layer that can transport electrons (e.g., via conductive carbon), protons (e.g., via ion-conducting polymers), and reactant gases all to and from the active catalyst. Any proton-conducting polymer can be employed as an ionomer. An example of a proton-conducting polymer that is commonly employed as an ionomer and is suitable in the present invention is a sulfonic acid polymer. Sulfonic acid polymers have been widely discussed in the literature and are not particularly limited here. Examples of sulfonic acid polymers include any sulfonate ion-exchange polymer, i.e., a polymer that contains sulfonic acid moieties. The sulfonic acid polymers may include, by way of example and without limitation, perfluorosulfonic acid polymers, sulfonated poly(benzimidazole) polymers, sulfonated poly(arylene ether) polymers, sulfonated poly(ether ether ketone) polymers, sulfonated polyvinyl chloride, 2-acrylamido-2-methylpropane sulfonic acid (AMPS), and poly(styrene sulfonate) (block co)polymers, although the sulfonic acid may be any other sulfonic acid polymer now known or developed in the future. Other examples of ionomers may include sulfonated polybenzimidazole polymers, carboxylic acid polymers, phosphonic acid polymers, polymers doped with phosphoric acid, and the like. The ionomeric polymer binder may be, without limitation, any proton conducting polymer now known or developed in the future.

[0067] The catalyst layer may also include a non-ionically conductive material to help maintain the integrity of the catalyst layer. Examples of non-ionically conductive materials include polymers such as polyvinyl alcohol, polyacrylates, polymethacrylates, functionalized polyethylene oxides, functionalized polypropylene oxides, thermoplastic polyurethanes, etc. Again, the non-ionically conductive material is not limited and may be any material now known or later developed to help maintain the integrity of the catalyst layer.

[0068] There are alternative embodiments, particularly with respect to the catalyst, of first preparing the carbonaceous compound and the reversible organic inhibitor. In alternative embodiments, the reversible organic inhibitor can be immobilized by functionalizing it onto an ionomer and subsequently mixing the functionalized ionomer with the carbonaceous compound, the catalyst, and optionally the non-ionically conductive material. The entire mixture can in turn be mixed with a solvent to prepare the ink.

[0069] In one embodiment, the present technology provides an ionomer additive composition. The ionomer additive composition can include an ionomer, an optional non-ionically conductive material, a reversible organic inhibitor, and a solvent.

[0070] The ionomeric additive composition may be mixed with a catalyst deposition composition on a carbonaceous compound to prepare a catalyst ink. For catalyst deposition compositions on carbonaceous compounds, often a reducing agent is employed to reduce an acidic solution of the catalyst onto the carbonaceous compound. However, it is well known in the art that catalysts can be deposited on carbonaceous compounds, and such deposition processes are not within the scope of the present technology; suffice it to say that any method for depositing a catalyst on a carbonaceous compound may be included herein.

[0071] The ionomer additive composition and the catalyst deposition composition on the carbonaceous compound can be mixed together to form a catalyst ink. Examples of mixing can be any combination of physical stirring, high shear mixing, sonication, or any other type of mixing. The catalyst ink then prepared contains (a) the carbonaceous compound, (b) a metal or non-metal catalyst, (c) a reversible organic inhibitor, (d) an ionomer, (e) an optional non-ion conductive material, all in (f) a solvent. The concentrations of the aforementioned components depend on the concentrations desired in the final electrode, as described above. In general, the catalyst can be present at about 5 to about 150 phr, or about 20 to about 130 phr, or about 40 to about 110 phr, or even about 60 to about 90 phr.

[0072] The ink composition may consist of 40-99.9% solvent and 0.1-60% solids, the solids consisting of a mixture of catalyst, ionomer, and reversible redox inhibitor on a carbonaceous compound. The carbonaceous material in the ink may range from 25-70% by weight of the total solids in either the catalyst-containing ink or the MPL ink. The ionically conductive polymer in the catalyst ink may range from 20-60% by weight of the total solids. The catalyst for performing ORR in the catalyst ink may range from 1-40% by weight of the total solids. The non-ionically conductive binder in the MPL ink may range from 20-70% by weight of the total solids. The polymeric additive may range from 0-25% by weight of the total solids in either the MPL or the catalyst ink, in addition to the non-ionically conductive material in the MPL or the ionically conductive material in the catalyst ink. Finally, the reversible organic carbon corrosion inhibitor may be present in the range of 1-25% by weight of the total solids in either the MPL or the catalyst ink.

[0073] Carbon-supported electrodes can be prepared from the catalyst ink by known methods such as known coating or printing techniques, for example, spray coating, decal coating, screen printing, inkjet printing, or other methods such as roll-to-roll transfer, doctor blade, colander rolling, or any other known method.

[0074] Also encompassed in the present technology is a fuel cell that includes an electrolyte membrane interposed or sandwiched between two catalyst layers to form a catalyst coated membrane (or CCM), which act as the anode and cathode during operation of the fuel cell. The fuel cell may further include a CCM interposed or again sandwiched between two gas diffusion (or GDL) layers, each of which includes a microporous layer coated on a carbon substrate.

[0075] The present technology also enables a method of preventing carbon corrosion in carbon-supported catalyst layers in a fuel cell by including a reversible organic inhibitor in at least one of the carbon-containing layers of the fuel cell and operating the fuel cell.

[0076] The disclosed technology solves the problem of carbon support decomposition in the electrodes by employing redox active molecules within at least one layer of the membrane electrode assembly composition.

[0077] The amount of each chemical component listed is exclusive of any solvent or diluent oil that may be customarily present in commercially available materials, i.e., expressed on an active chemical basis, unless otherwise indicated. However, unless otherwise indicated, each chemical or composition referred to herein should be construed as being a commercial grade material that may contain isomers, by-products, derivatives, and other such materials normally understood to be present in commercial grades.

[0078] It is known that some of the materials described above may interact in the final formulation, so that the components of the final formulation may differ from those originally added. For example, metal ions may migrate to other acidic or anionic sites of other molecules. The products formed thereby may not be easily explained, including the products formed when employing the composition of the present invention in its intended use. Nevertheless, all such modifications and reaction products are included within the scope of the present invention. The present invention encompasses the composition prepared by mixing the components described above. EXAMPLES

[0079] The redox potential and activity of candidate molecules were measured by including the molecules in an electrode ink as described above, which was then tested on a rotating disk electrode in a three-electrode cell configuration similar to that found in the published literature. The inks utilized in these tests to specifically probe redox activity consisted of 7.6 mg of XC-72 carbon, 40 μL of Nafion D2020 solution, 2.6 mL of isopropanol, and 7.4 mL of deionized water, with 1.5 mg of molecules tested. The inks were dispersed by sonication in an ice bath for 16 minutes. After sonication, 10 μL of the resulting ink was dispersed in a 0.196 cm Pine Research 1000 sapphire crystals. 2 The ink was deposited on the tip of a glassy carbon electrode. The glassy carbon electrode was polished with 0.05 μm alumina polishing media and dried under nitrogen prior to ink deposition. After the ink was deposited on the surface of the glassy carbon electrode, the ink was dried by spinning the electrode at 700 RPM. Once the ink was dry, the electrode was submerged in a glass cell filled with 0.1 molar perchloric acid in deionized water. Nitrogen gas was bubbled through the solution for 30 minutes while the glass carbon electrode with the dried ink was rotated at 1600 RPM. The nitrogen gas supply was then changed to provide nitrogen to the headspace within the cell. In this three-electrode cell configuration, the counter electrode is a platinum wire coil supplied by Pine Research and the reference electrode is an Ag / AgCl electrode equilibrated in 3 molar sodium chloride provided by BASi. The reference electrode was placed in a fritted reservoir with a frit in the main solution volume and the reservoir was filled with 0.1 M perchloric acid. Subsequent electrochemical tests were carried out using an active nitrogen blanket and a glassy carbon electrode rotating at 1600 RRM.

[0080] Multiple cyclic voltammetry tests were performed on each of the inks over a potential range of -0.28 to +1.0 volts versus an Ag / AgCl electrode. Each cyclic voltammetry test was IR corrected to 90% of the actual impedance measured at 60-90 kHz. From these cyclic voltammetry experiments, the redox activity of each molecule was observed at 100 mV / s. The table below shows the E of the molecules, calculated as the average of the peak currents observed during cyclic voltammetry performed at a scan rate of 100 mV / s during the third cycle. o The measured potentials of the combined reduction and oxidation reactions are described as E o Values ​​are those of the observed redox pairs of interest from these experiments. E o Although only one of the values ​​is reported in the tables below, there may be other redox couples not reported here for each molecule, for example if the redox couple had a relatively small peak current (indicating lower activity) or was outside the window of interest for these applications. The potentials reported in the tables are relative to the reversible hydrogen electrode potential. For these purposes, the Ag / AgCl reference electrode is assumed to be +0.28 V positive of the RHE electrode.

[0081] [Table 1]

[0082] For the embodiment where the reversible organic inhibitor was added to the catalyst layer, the catalyst ink was prepared from Pt / C fuel cell catalyst, ionomer, isopropanol, and water; and, in the case of the cathode ink, the reversible organic inhibitor. The ink was then coated onto a gas diffusion substrate to prepare the gas diffusion electrode. A commercial ionomer membrane was then sandwiched between the anode gas diffusion electrode and the cathode gas diffusion electrode using a hot press process. When only carbon was used instead of Pt / C, the redox profile of the reversible organic inhibitor could be clearly seen in the fuel cell CV. Figure 1 shows the observed reversible redox potential of 3,4-dihydroxybenzoic acid from the fuel cell assembly, which is consistent with the redox potential measured using the RDE bench test performed in the above example. The redox potential of 3,4-dihydroxybenzoic acid from the fuel cell assembly is also presented in Table 1 below. This data confirms that reversible organic inhibitors have a redox potential at or near the OCV in an operating fuel cell and can cycle the oxidation state in the fuel cell, suggesting that a regeneration mechanism is possible. A further implication of redox activity in a fuel cell is the ability to change the operating potential of the cathode, with potential negative consequences if the redox potential of the reversible redox inhibitor is not near the OCV for oxygen reduction.

[0083] [Table 2]

[0084] For embodiments in which a reversible organic inhibitor was added to the fuel cell in the membrane layer, the ionomer was dispersed in a solution of isopropanol and water, and the desired reversible organic inhibitor was added to the solution. The solution was then cast onto a flat surface. The solvent was allowed to evaporate, leaving a film having a thickness of 15-20 microns. In many embodiments, a 5-10 micron layer of ePTFE was used to support the membrane.

[0085] Membrane electrode assemblies were fabricated from the cast membranes. Each membrane was placed between two protective gaskets, each 1 mil, such that 50 square centimeters of the membrane (the "active area") was exposed while the outer edges of the membrane were covered by the protective gasket. On one side of the membrane, the active area was placed in contact with the anode, while on the other side of the membrane, the active area was placed in contact with the cathode. In addition to contacting the entirety of the active area on that side of the membrane, each electrode also covered a portion of the protective gasket, overlapping by 1.85 mm. The remaining portion of the protective gasket was covered by a conventional gasket. Each electrode, in turn, was covered by a gas diffusion layer bounded on its outer edges by a conventional gasket. Platinum-carbon catalyst was present on both electrodes with a loading of 0.1-0.3 mg of platinum per cubic centimeter.

[0086] Membrane electrode assemblies made from each membrane were subjected to an open circuit voltage (OCV) accelerated stress test to measure the maximum operating voltage of the assemblies in the absence of electrical current and in the presence of significant concentrations of hydrogen peroxide and its radical decomposition products. An initial test was performed at 25°C to confirm normal hydrogen crossover, OCV, and cyclic voltammetry (CV) readings. Beginning-of-life (BOL) characterization was then performed to measure hydrogen crossover, CV, and VI using high frequency resistance (HFR) / electrochemical impedance spectroscopy (EIS) at 95% relative humidity (RH), 150 kPa, and 80°C. Hydrogen crossover, CV, and VI were then measured at 0.2 A / cm. 2 Beginning-of-test (BOT) measurements of EIS and OCV were performed. The OCV conditions were 30% RH, 90°C, 150 kPa, and 700 / 1750 sccm H2 / air. After the BOT characterization, a chemical degradation test was performed. The OCV was measured under the same conditions as the BOT and was 0.2 A / cm 2EIS at 0.25 V was performed every 24 hours. The chemical degradation test continued until a failure event occurred, such as pinhole formation, characterized by a sudden drop in OCV, or until the OCV reading fell below 0.8 V. The test was considered successful if the OCV was maintained for at least 500 hours, a benchmark defined by the Department of Energy. Next, end-of-test (EOT) characterization was performed. This test included OCV, EIS, and hydrogen crossover under the same conditions as the BOT characterization. Finally, polarization curves after cycling and conditioning were obtained. Hydrogen crossover, CV, and VI were measured in the same manner as the BOL test. The data reported in Table 3 correspond to membranes initially with thicknesses of 15-20 μm. All of the membranes have an ePTFE support with a thickness of 5 μm.

[0087] The OCV test results show that only those membrane electrode assemblies containing the redox regenerative additive, which have a redox potential greater than 0.9 V, maintain their voltage for at least 500 hours.

[0088] [Table 3]

[0089] Effluent collection was performed to analyze the content of the wastewater discharged by the fuel cell. The addition of the reversible organic inhibitor 3,4-dihydroxybenzoic acid to the ionomer reduced the concentration of fluoride ions, a decomposition product of perfluorinated ionomer, present in the effluent water.

[0090] [Table 4]

[0091] [Table 5]

[0092] Except in the examples or where otherwise expressly indicated, all quantities in this description specifying amounts of materials, reaction conditions, molecular weights, numbers of carbon atoms, and the like, should be understood as being modified by the word "about". It should be understood that the upper and lower limits of amounts, ranges, and ratios described herein may be independently combined. Similarly, the ranges and amounts for each element of the invention may be used together with ranges or amounts for any of the other elements.

[0093] As used herein, the transitional term "comprising," which is synonymous with "including," "containing," or "characterized by," is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. However, in each occurrence of "comprising" herein, the term is also intended to encompass, as alternative embodiments, the phrases "consisting essentially of" and "consisting of," where "consisting" excludes any unspecified element or step, and "consisting essentially of" permits the inclusion of additional, unrecited elements or steps that do not materially affect the essential or basic and novel characteristics of the composition or method under consideration.

[0094] A composition comprising: (a)(i) a carbonaceous compound; (a)(ii) an ionomer; or (a)(iii) a mixture of (a)(i) and (a)(ii); and (b) a reversible organic inhibitor.

[0095] A composition comprising: (a) an ionomer; and (b) a reversible organic inhibitor.

[0096] A composition comprising: (a) a carbonaceous compound; and (b) a reversible organic inhibitor.

[0097] A composition comprising: (a) a carbonaceous compound; and (b) a reversible organic inhibitor; and a solvent.

[0098] A composition comprising: (a) a carbonaceous compound; and (b) a reversible organic inhibitor; and a porous binder.

[0099] A composition comprising: (a) a carbonaceous compound; and (b) a reversible organic inhibitor, a porous binder, and a solvent.

[0100] A composition comprising: (a) a carbonaceous compound; (b) a reversible organic inhibitor; (c) a catalyst; and (d) an ionomer.

[0101] A composition comprising: (a) a carbonaceous compound; (b) a reversible organic inhibitor; (c) a catalyst; and (d) an ionomer and a solvent.

[0102] A composition comprising: (a) a carbonaceous compound; (b) a reversible organic inhibitor; (c) a catalyst; (d) an ionomer; and (e) a non-ionically conductive material.

[0103] A composition comprising: (a) a carbonaceous compound; (b) a reversible organic inhibitor; (c) a catalyst; (d) an ionomer; and (e) a non-ionically conductive material; and a solvent.

[0104] A composition comprising an ionomer and a reversible organic inhibitor.

[0105] A composition comprising an ionomer, a reversible organic inhibitor, and a solvent.

[0106] The composition of the preceding sentence, wherein the reversible organic inhibitor is immobilized on an ionomer.

[0107] The composition of any preceding sentence, wherein the reversible organic inhibitor is immobilized on a carbonaceous compound.

[0108] A catalyst coated membrane ("CCM") comprising: (i) an electrolyte membrane; and (ii) a catalyst layer, the catalyst layer comprising a carbonaceous compound and a reversible organic inhibitor.

[0109] A gas diffusion layer ("GDL") comprising: (iii) a microporous layer comprising a carbonaceous compound and a porous binder; and (iv) a carbon substrate, wherein either or both of elements (iii) and (iv) comprise a reversible organic inhibitor.

[0110] The GDL of the preceding paragraph, wherein the microporous layer comprises a reversible organic inhibitor.The GDL of the preceding paragraph, wherein the carbon substrate comprises a reversible organic inhibitor.

[0111] A gas diffusion electrode ("GDE") comprising: (ii) a catalyst layer comprising a carbonaceous compound; (iii) a microporous layer comprising a carbonaceous compound and a porous binder; and (iv) a carbon substrate, wherein at least one of elements (ii), (iii), and (iv) comprises a reversible organic inhibitor.

[0112] The GDE of the preceding paragraph, wherein the catalyst layer comprises a reversible organic inhibitor.The GDE of the preceding paragraph, wherein the microporous layer comprises a reversible organic inhibitor.The GDE of the preceding paragraph, wherein the carbon substrate comprises a reversible organic inhibitor.

[0113] 1. A fuel cell comprising: (i) an electrolyte membrane; (ii) a catalyst layer; (iii) a microporous layer; and (iv) a carbon substrate, wherein at least one of elements (i), (ii), (iii), and (iv) comprises a reversible organic inhibitor.

[0114] A method for inhibiting corrosion to carbonaceous compounds in a catalyst layer of a fuel cell, the method comprising: including a reversible organic inhibitor in at least one layer of the fuel cell; and operating the fuel cell.

[0115] While certain representative embodiments and details have been shown for the purpose of illustrating the present invention, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the scope of the invention, which in this regard should be limited only by the scope of the claims that follow.

Claims

1. 1. A composition comprising a mixture of (a)(i) a carbonaceous compound, (a)(ii) an ion-conducting polymer, and (b) a reversible organic inhibitor, the reversible organic inhibitor being regenerated by electrochemical reduction and having a reversible oxidation / reduction potential at a carbon electrode in the range of 0.5 to 1.4 V vs. RHE.

2. The composition of claim 1 , wherein the ionomer comprises a polymer containing sulfonic acid moieties.

3. The composition of claim 1 , wherein the ionomer comprises a perfluorosulfonic acid polymer.

4. 4. The composition of claim 2, comprising 0.2 to 25 mole percent of said reversible organic inhibitor relative to said sulfonic acid moieties of said ionomer.

5. 4. The composition of claim 2, comprising 0.2 to 10 mole percent of said reversible organic inhibitor relative to said sulfonic acid moieties of said ionomer.

6. The composition of claim 1, comprising 1 to 50 parts by weight of the reversible organic inhibitor per 100 parts by weight of the carbonaceous compound.

7. 10. The composition of claim 1, comprising 5 to 34 parts of said reversible organic inhibitor per 100 parts by weight of said carbonaceous compound.

8. The composition of claim 1 further comprising a catalyst.

9. The composition of claim 1 further comprising a non-ionically conductive material.

10. The composition of claim 1 further comprising a solvent.

11. The composition of claim 8 wherein the catalyst comprises a precious metal.

12. The composition of claim 8 wherein the catalyst comprises platinum.

13. The composition of claim 8 , wherein the catalyst comprises a transition metal.

14. The composition of claim 8 wherein the catalyst comprises iron.

15. The composition of claim 8 wherein the catalyst comprises nickel.

16. 9. The composition of claim 8, wherein the catalyst comprises any of the foregoing alloys.

17. The composition of claim 8 , wherein the catalyst comprises a non-metallic catalyst.

18. 10. The composition of claim 1, wherein the reversible organic inhibitor has a reversible oxidation / reduction potential in the range of 0.9 to 1.2 V on a carbon electrode vs. RHE.

19. The composition of claim 1 , wherein the reversible inhibitor comprises a hydroquinone or a quinone.

20. 10. The composition of claim 1, wherein the reversible organic inhibitor comprises 1,2-dihydroxybenzene-3,4-disulfonic acid sodium salt, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone, 2,3-dicyanohydroquinone, or 3,4-dihydroxybenzoic acid.

21. A catalyst coated membrane ("CCM") comprising: (i) an electrolyte membrane; and (ii) a catalyst layer, wherein the catalyst layer comprises a carbonaceous compound and a reversible organic inhibitor.

22. A gas diffusion layer ("GDL") comprising: (iii) a microporous layer comprising a carbonaceous compound and a porous binder; and (iv) a carbon substrate, wherein either or both of elements (iii) and (iv) comprise a reversible organic inhibitor.

23. A gas diffusion electrode ("GDE") comprising at least one of: (ii) a catalyst layer comprising a carbonaceous compound; (iii) a microporous layer comprising a carbonaceous compound and a porous binder; and (iv) a carbon substrate, wherein at least one of elements (ii), (iii), and (iv) comprises a reversible organic inhibitor.

24. A method for inhibiting corrosion to carbonaceous compounds in a catalyst layer of a fuel cell, the method comprising including a reversible organic inhibitor in at least one layer of the fuel cell and operating the fuel cell.

Images