Patent Application
20140099571
The subject matter of this disclosure relates to catalyst precursors, catalysts, and methods of producing these catalyst precursors and catalysts. More specifically, the present invention is concerned with non-precious metal catalysts. Such materials can be used for oxygen reduction reactions in fuel cells, including acid or alkaline polymer electrolyte membrane fuel cells, microbial fuel cells and metal-air batteries.
Polymer electrolyte membrane fuel cells (PEMFC's) are one of today's promising power generation alternatives to the internal combustion engine and rechargeable batteries. Their advantages include zero point-of-use emissions, which is especially attractive for automotive propulsion. Moreover, unlike rechargeable batteries, PEM fuel cell systems allow vehicles to be refueled quickly and offer driving ranges comparable to conventional gasoline engine vehicles. While this technology has matured significantly over the past decades, the high cost of PEM fuel cell systems is still a major impediment for their widespread commercial use, particularly for automotive propulsion.
Due to the rather low operating temperature of PEMFCs (ca. 80° C.), catalysts play an essential role in boosting the reaction kinetics to produce the desired high power densities. It is widely accepted that the platinum-based catalysts, used in the cathode in current PEM fuel cells are one of the key contributors to the high cost of these systems. According to numerous studies on mass production cost of PEMFC systems submitted to the U.S. DOE Annual Merit Review and independent studies over the last 5 years, Pt-based electrodes alone account for roughly half the cost of the PEMFC stack. The two approaches for addressing this issue are to either lower platinum loading while maintaining high power and durability performance or replace platinum-based electrocatalysts altogether with a well-performing lower-cost alternative, such as non-precious transition metal-based electrocatalysts, for example.
Recent efforts in PEM fuel cell research have been in the development of low-cost non-precious metal catalysts (NPMC). In order to obtain a non-precious metal catalyst for oxygen reduction in fuel cells, a carbon support (that is not thermally decomposable in an inert atmosphere), a nitrogen source and a non-precious metal precursor (NPMP) are typically used.
One approach in the synthesis of NPMCs has been to use a nitrogen source such a ammonia, an organic compound, an iron- or cobalt-based compound and a carbon support (that is not thermally decomposable in an inert atmosphere). The catalysts are obtained by impregnation of a porous carbon black support (that is not thermally decomposable in an inert atmosphere) with an iron precursor like iron(II) acetate (FeAc) and a nitrogen source, followed by pyrolysis, in an inert or reactive atmosphere. Major improvements to the overall performance of these low-cost non-precious metal catalysts (NPMC) for the oxygen reduction reaction in polymer electrolyte membrane fuel cells are necessary if they are to replace the expensive platinum-based catalysts currently used. One of the toughest challenges for NPMCs is achieving higher power density at efficient fuel cell voltages, such as 0.6V or higher, for example.
Catalysts made using thermally decomposable porous support (TDPS) and method of their manufacture are described. Catalyst precursor compositions and their manufacture are also described.
In one aspect, a catalyst precursor includes a thermally decomposable porous support; and organic coating/filling compound; a non-precious metal precursor, wherein the organic coating/filling compound and the non-precious metal catalyst precursor coat and/or fill the pores of the thermally decomposable porous support.
In one or more embodiments, the at least one of the thermally decomposable porous support, the non-precious metal precursor or the organic coating/filling compound includes nitrogen.
In any of the preceding embodiments, the thermally decomposable porous support is microporous and is one or more supports selected from the group consisting of metal-organic-frameworks, covalent-organic-frameworks, polymer-organic-frameworks, microporous organic polymers, polymers of intrinsic microporosity and a microporous polymers.
In any of the preceding embodiments, the metal organic framework includes a zeolitic imidazolate framework, and for example, the zeolitic imidazolate framework includes ZIF-8.
In any of the preceding embodiments, the metal of the zeolitic imidazolate framework includes zinc.
In any of the preceding embodiments, the thermally decomposable porous support includes a metal organic framework and the metal is one or more selected from the group consisting of zinc, cobalt, manganese, magnesium, iron, copper, aluminum and chromium.
In any of the preceding embodiments, the thermally decomposable porous support has a total surface area of greater than 500 m2/g, or the thermally decomposable porous support has a total surface area of greater than 1000 m2/g, or the thermally decomposable porous support has a total surface area of greater than 1500 m2/g.
In any of the preceding embodiments, the thermally decomposable porous support is one that loses between 20% and 90% of its mass in an inert atmosphere at a temperature in the range of 100° C. to 1200° C., or the thermally decomposable porous support is one that loses at least 50% of its mass in an inert atmosphere at a temperature in the range of 100° C. to 1200° C.
In any of the preceding embodiments, the non-precious metal precursor is a precursor of iron or cobalt.
In any of the preceding embodiments, the catalyst has an iron loading of about 0.2 wt % to about 5 wt % or more based on the total weight of the catalyst precursor, or an iron loading of about 1-2 wt % based on the total weight of the catalyst precursor.
In any of the preceding embodiments, the non-precious metal precursor is a salt of a non-precious metal or an organometallic complex of a non-precious metal, and for example, the non-precious metal precursor is Fe(II) acetate.
In any of the preceding embodiments, the non-precious metal precursor and the organic coating/filling compound are the same molecule.
In any of the preceding embodiments, the organic coating/filling compound includes a poly-aromatic structure.
In any of the preceding embodiments, the organic coating/filling compound is selected from the group consisting of perylene-tetracarboxylic-dianhydride, 1,10-phenanthroline, perylene tetracarboxylic-diimide, and polypyrrole or polyaniline and mixtures thereof.
In any of the preceding embodiments, the mass ratio of organic coating/filling compound to thermally decomposable porous support is about 95:5 to about 5:95.
In another aspect, a catalyst is prepared by pyrolysing the catalyst precursor as described above, wherein the catalyst precursor has been pyrolysed so that the micropore surface area of the catalyst is substantially larger than the micropore surface area of catalyst precursor, with the proviso that the pyrolysis is performed in the presence of a gas that is a nitrogen precursor when the thermally decomposable porous support, the non-precious metal precursor and the organic coating/filling compound are not nitrogen precursors.
In one or more embodiments, the pyrolysis temperature is between 300° C. and 1200° C., or the pyrolysis temperature is at least 700° C.
In any of the preceding embodiments, the mass loss during pyrolysis is greater than 50%, or the mass loss during pyrolysis is greater than 80%.
In another aspect, a catalyst includes a microporous carbon support and having a carbon content of at least 80 wt % and a total surface area of at least 500 m2/g; and a non-precious metal at a loading of at least 0.2 wt %, wherein the non-precious metal-ion is in contact with the microporous support through a pyridinic or pyrrolic-type structure forming the catalytic sites, wherein the catalyst when incorporated into a membrane electrode assembly demonstrates a volumetric activity of greater than 100 A/cm3 at an iR-free cell voltage of 0.8V.
In any of the preceding embodiments, the catalyst has a nitrogen content of about 0.5 wt % or more based on the total weight of the catalyst.
In any of the preceding embodiments, the catalyst is an oxygen reduction catalyst, a catalyst for the electroreduction of hydrogen peroxide, a catalyst for the disproportionation of hydrogen peroxide or a catalyst for the reduction of CO2.
In another aspect, a method for producing a catalyst precursor, the method includes providing one or more thermally decomposable porous supports; one or more non-precious metal precursors; and optionally one or more organic coating/filling compounds; and coating and/or filling the micropores of the thermally decomposable porous support with the optional organic coating/filling compound and the non-precious metal precursor so that the surface area of the catalyst precursor is substantially smaller than the surface area of the thermally decomposable porous support when the organic coating/filling compound and the non-precious metal precursor are absent.
In one or more embodiments, the method further provides pyrolysing the catalyst precursor so that the micropore surface area of the catalyst is substantially larger than the micropore surface area of the catalyst precursor, with the proviso that the pyrolysis is performed in the presence of a gas that is a nitrogen precursor when the thermally decomposable porous support, the non-precious metal precursor and the organic coating/filling compound are not nitrogen precursors.
In any of the preceding embodiments, the method has a pyrolysis temperature between 300° C. and 1200° C., or the pyrolysis temperature is at least 700° C.
In any of the preceding embodiments, the method has a mass loss during pyrolysis is greater than 50%, or the mass loss during pyrolysis is greater than 80%.
In any of the preceding embodiments, the thermally decomposable porous support loses between 20% and 90% of its mass during pyrolysis.
In any of the preceding embodiments, the non-precious metal precursor includes iron or cobalt.
The invention is described with reference to the following drawings, which are presented by way of illustration only and are not intended to be limiting of the invention.
As used herein a “catalyst” means a substance that initiates or facilitates a chemical or electrochemical reaction; a substance that boosts the kinetics of a given reaction.
A “catalyst precursor” is a substance from which a catalyst can be produced under appropriate processing conditions.
“Pyrolysis” means the transformation of a substance into one or more other substances by heat in the presence or absence of a gas (vacuum). Pyrolysis can occur in an inert gas (Ar or N2 for example) or a reactive gas (NH3, O2, air, CO2 or H2 for example). Pyrolysis of organic substances produces gas and/or liquid products and leaves a solid residue richer in carbon content.
M/N/C-catalysts are electrocatalysts that include carbon (C), nitrogen (N) and a non-precious metal (M) that forms the center of the molecular catalytic site.
As used herein, a “non-precious metal” is a metal other than a precious metal. Precious metals are usually considered by the persons of skill in the art to be ruthenium, rhodium, palladium, osmium, iridium, platinum, and gold.
Porous materials are classified into several kinds by their size. According to IUPAC notation (see J. Rouquerol et al., Pure & Appl. Chem, 66 (1994) 1739-1758), microporous materials have pore diameters (or widths) of less than 2 nm, mesoporous materials have pore diameters (or widths) between 2 nm and 50 nm and macroporous materials have pore diameters (or widths) of greater than 50 nm.
In order for non-precious metal catalysts to compete with Pt-based catalysts for the oxygen reduction reaction in PEM fuel cells, they desirably possess one or more of the following three characteristics; (i) high volumetric activity, (ii) excellent mass transport properties and, (iii) high durability. Characteristics (i) and (ii) are relevant for achieving high power density.
The electrocatalyst suitable for use in oxygen reduction reactions contains a large number of catalytic sites on a microporous carbon support. The catalysts thus contain a high density of active sites. There may be different kinds of active sites in the same catalyst but all active sites for oxygen reduction are believed (without being bound by such interpretation) to include a carbon poly-aromatic structure, at least one non-precious metal ion and at least four nitrogen atoms. Without being bound by theory, it is believed that the nitrogen atoms are bound to the carbon atoms and/or to the metal ion(s), resulting in pyridinic-type or pyrrolic-type N atoms. It is also believed that the center of each active site is somewhat similar to the center of porphyrin or phthalocyanine molecules, for which all nitrogen atoms are of the pyrrolic-type. Finally, it is believed that the active sites have an electronic contact with the walls of the micropores. Such catalysts are referred to as a type of “M/N/C catalysts.”
In one or more embodiments, the microporous support is a support comprising micropores. For example, a microporous support may have a micropore surface area of more than about 100 m2/g. Herein, “micropores” refer to pores having a size of less than or equal to 2 nm (<2 nm). Most microporous supports usually also comprise mesopores (between 2 and 50 nm in size) and macropores (having a size>50 nm) and a total surface area of greater than 100 m2/g. As such, microporous supports have a “total” surface area, which is provided by the micropores, the mesopores and the macropores. As used herein the “micropore surface area” of a substance is the surface area of this substance provided by its micropores. The “total” surface area, e.g., micropore surface area, mesopore surface area and macropores surface area, can be determined by methods well known in the art. For example, by measuring the N2-adsorption isotherm and analyzing it with the Brunauer Emett Teller (BET) equation and by applying quenched solid density functional theory using a slit-pore model (Quantachrome software) to determine pore size distribution.
In one or more embodiments, the microporous support is a highly microporous support. For example, a “highly microporous support” may be a microporous support having a micropore surface area of more than about 500 m2 g, more than 750 m2/g, more than 1000 m2/g; more than 1100 m2/g and up to 2000 m2/g. M/N/C-catalysts have been prepared using carbon black supports (that are not thermally decomposable in an inert atmosphere). Carbon black based M/N/C-catalysts (such as those made with Black Pearls 2000) have a lower total surface area (750 m2/g vs. 1000 m2/g or higher) than the electrocatalysts described herein.
In one or more embodiments, the catalyst comprises up to about 10 wt % of the non-precious metal based on the total weight of the catalyst. In one or more embodiments, the catalyst has an iron loading of between about 0.2 wt % and about 5.0 wt %, or about 0.2 wt % or 1.0 or 5.0 or more based on the total weight of the M/N/C catalyst. In more specific embodiments, the M/N/C catalyst has an iron loading of about 3 wt % based on the total weight of the catalyst.
As noted above, the M/N/C catalyst includes a non-precious metal precursor (NPMP). It is to be understood that a mixture of non-precious metal can be used. Examples of the non-precious metals include metals having atomic numbers between 22 and 32, between 40 and 50 or between 72 and 82, with the exclusion of atomic numbers 44-47 and 75-79. In one or more embodiments, the non-precious metal is iron, cobalt, copper, chromium, manganese or nickel. In one or more embodiments, the non-precious metal is iron or cobalt.
The M/N/C catalyst may comprise between about 0.5 to about 10.0 wt % nitrogen based on the total weight of the catalyst. In embodiments, the catalyst has a nitrogen content, as provided by the nitrogen precursor, of about 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, or 9.0 wt % or more based on the total weight of the catalyst. This nitrogen content may be measured by methods known in the art, for example, x-ray photoelectron spectroscopy.
The disclosed M/N/C-catalysts show a catalytic activity in a fuel cell that is two or three times higher than that of the catalysts prepared using carbon black supports (that are not thermally decomposable in an inert atmosphere). With 276 A/cm3 (in at least one tested measured example), they surpass the target of 130 A/cm3 of volumetric activity set for year 2010 by the U.S. Department of Energy (U.S. DOE) and come much closer to the target of 300 A/cm3 set for year 2015. Moreover, and therein resides the main advantage of these novel non-precious-metal catalysts, the performance of these catalysts has been greatly improved at high current density, unlike the behavior observed for the catalysts prepared using carbon black supports (that are not thermally decomposable in an inert atmosphere). Stated differently, the power density has been increased substantially.
A precursor to the electrocatalyst and a method for its manufacture is also described. A catalyst precursor includes (i) a thermally decomposable porous support (TDPS); (ii) a non-precious metal precursor (NPMP); and (iii) an organic coating/filling compound (OCFC), optionally containing nitrogen, that coats and/or fills the pores of the TDPS.
A TDPS suitable for use in a catalyst precursor at least contains micropores, but may contain other pore sizes as well. A feature of the TDPS is that it contains a structure (framework or other) at the outset that is porous, but which can thermally decompose (with concomitant loss of mass) to provide a porous structure of enhanced carbon content. By way of example, the TDPS can have a mass loss of at least 20% and up to 90% in an inert atmosphere at temperatures between 300° C. and 1200° C. A TDPS can be pyrolyzed to a high carbon content structure of greater than about 60% by weight, or greater than 70% or 80% or 85% by weight carbon. In contrast, typical carbon blacks can have a maximum mass loss of up to about 5% in an inert atmosphere at temperatures between 300° C. and 1200° C. Significantly, the carbon black does not undergo significant, if any, changes or decomposition when heated up to temperatures of 1200° C. in an inert atmosphere. The mass loss experienced by typical carbon blacks is principally related to the removal of adsorbed water and small organic molecules. In comparison, significant mass loss in preparing the catalyst according to one or more embodiments causes changes in the catalyst precursor that can increase catalytic site density and create a catalyst morphology that provides better mass transport properties. Exemplary TDPSs include metal-organic-frameworks (MOFs), covalent-organic-frameworks (COFs), porous polymers or polymers of intrinsic microporosity (PIMs), hypercrosslinked polymers (HCP) or others. MOFs and COFs are sometimes referred to as coordination polymers or polymer-organic-frameworks (POFs).
Metal-organic frameworks (MOFs) are materials in which metal-to-organic ligand interactions yield porous coordination networks with record-setting surface areas surpassing activated carbons and zeolites. A characteristic of metal-organic frameworks (MOFs) is their high porosity (fraction of void volume to total volume) and high specific surface area. Typical total surface areas can range from 100 m2/g to 5000 m2/g. However, recent literature has reported surface areas of over 10000 m2/g. MOFs form three-dimensional crystal structures that support well-defined pores with internal diameters ranging from 0.1 to several nanometers. MOFs based on zinc, cobalt, manganese, magnesium, iron, copper, aluminum and chromium are known and can be used as TDPSs. Zeolitic imidazolate frameworks (ZIFs) prepared by copolymerization of either Zn(II) or Co(II) with imidazolate-type links are examples of suitable MOFs. The ZIF crystal structures are based on aluminosilicate zeolites, in which the tetrahedral Si(Al) and the bridging O are replaced with transition metal ion and imidazolate link, respectively. Exemplary MOFs include zinc imidazolate frameworks sold by BASF under the trade name Basolite, ZIF-1 to ZIF-12 and others (zinc- and cobalt-based MOF), and magnesium formate frameworks sold by Sigma Aldrich under the trade name Basosiv.
ZIF-derived catalysts resulted in significantly improved power performance. An exemplary ZIF has the trademark name Basolite™ Z1200 from BASF, with chemical formula ZnN4CsH12, that is commonly referred to in the literature as ZIF-8. ZIF-8 has a high BET surface area (1800 m2 g−1, and is almost entirely microporous), a pore size of 11.6 angstroms with openings to these pores of only 3.4 angstroms. Zinc, the metal in ZIF-8, is conveniently removed when the catalyst precursor containing ZIF-8 is heat treated at a temperature of about 850° C. or higher and thereby eliminates a processing step. Other characteristics of this TDPS are its (i) low carbon content (42 wt %), (ii) electrically insulating character and (iii) decomposition temperature (500-600° C.). Using ZIF as a thermally decomposable porous support (TDPS) with a non-precious metal precursor (NPMP) and an organic coating/filling compound (OCFC) in the catalyst precursor allows the catalyst to overcome the mass-transport limitations previously experienced with M/N/C-catalysts made using porous supports that do not thermally decompose in an inert atmosphere, such as carbon blacks, furnace blacks, activated carbons, ordered mesoporous carbons, graphite, etc.
Covalent organic frameworks (COFs) are another class of porous polymeric materials, consisting of porous, crystalline, covalent bonds that usually have rigid structures, exceptional thermal stabilities (to temperatures up to 600°), and low densities. COFs are porous, and crystalline, and made entirely from light elements (e.g., H, B, C, N, and O) that are known to form strong covalent bonds. They exhibit permanent porosity with specific surface areas surpassing those of well-known zeolites and porous silicates. Typical surface areas are greater than 200 m2/g and typically between about 500 m2/g and 5000 m2/g. A well-known COF synthesis route is a boron condensation reaction, which is a molecular dehydration reaction between boronic acids. In the case of COF-1, three boronic acid molecules converge to form a planar six-membered B3O3 (boroxine) ring with the elimination of three water molecules. Another class of high performance polymer frameworks with regular porosity and high surface area is based on triazine materials which can be achieved by dynamic trimerization reaction of aromatic nitriles in ionothermal conditions (molten zinc chloride at high temperature (400°)). CTF-1 is a good example of this chemistry. Another class of COFs can be obtained by imine condensation of aniline with benzaldehyde that results in imine bond formation with elimination of water. COF-300 is an example of this type of COF.
A microporous polymer is an organic polymer material containing pores with diameters less than 2 nm, which can be used as the thermally decomposable porous structure. Such polymers may include polymers of intrinsic microporosity (PIMs) or hypercrosslinked polymers (HCPs). Some examples may be found in the following paper and in the references within, which is incorporated in its entirety by reference. (M. G. Schwab, J. Am. Chem. Soc., 2009, 131, 7216). Microporous polymers can vary in degree of order and can be amorphous. Microporous polymers typically have a surface area of about 1000-2000 m2/g.
As noted above, the catalyst precursors include a non-precious metal precursor (NPMP). A mixture of NPMPs can be used. Any NPMP known to the skilled person to be useful in catalysts of the prior art (e.g., those produced by adsorption or impregnation) may be used.
Examples of non-precious metals include metals having atomic numbers between 22 and 32, between 40 and 50 or between 72 and 82, with the exclusion of atomic numbers 44-47 and 75-79. In one or more embodiments, the non-precious metal is iron, cobalt, copper, chromium, manganese or nickel. In one or more embodiments, the non-precious metal is iron or cobalt.
In one or more embodiments, the NPMP (iron(II) acetate) makes up about 3 wt % of the catalyst precursor and typically is in the range of 0.6 and 6.0 wt %. Depending on the amount of non-precious metal present in the NPMP, the catalyst precursor comprises between about 0.05 and about 5.0 wt % of the non-precious metal. In one or more embodiments, the catalyst precursor has a non-precious metal content, as provided by the NPMP, of about 0.2, 0.5, 1.0, 2.5, 3.0, 3.5, 4.0, or 4.5 wt % or more. In one or more embodiments, the catalyst precursor has an iron loading of about 0.2 wt % or more based on the total weight of the catalyst precursor. In more specific embodiments, the catalyst precursor has an iron loading of about 1 wt % based on the total weight of the catalyst precursor. Note that the wt % of the non-precious metal is lower in the catalyst precursor than in the final catalyst due to weight loss of the volatile compounds during heat treatment.
As used herein, a “non-precious metal precursor” or NPMP is a molecule that provides a non-precious metal ion to the catalyst during pyrolysis. A NPMP may contain only one non-precious metal ion or a mixture of several non-precious metal ions. As noted above, the active sites of the catalyst comprise at least one non-precious metal ion.
The NPMP may be organometallic or inorganic. The NPMP may be a salt of the non-precious metal or an organometallic complex of the non-precious metal. Non-limiting examples of NPMPs include the following broad classes (with more specific examples in each class given between parentheses): metal acetates and acetylacetonates (Fe(II) acetylacetonate, iron acetate, cobalt acetate, copper acetate, chromium acetate, manganese acetate, nickel acetate); metal sulfates (Fe(II) sulfate); metal chlorides (Fe(II) chloride); metal nitrates (Fe(II) nitrate); metal oxalates (Fe(II) oxalate); metal citrates (Fe(II) citrate); Fe(II) ethylene diammonium sulfate; metal porphyrins (Fe tetramethoxyphenylporphyrin, Fe 4-hydroxy-phenyl porphyrin, mesotetra-phenyl Fe porphyrin, octaethyl Fe porphyrin, Fe pentafluorophenyl porphyrin); metallocenes (ferrocene, cobaltocene); metal-phthalocyanines (cobalt phthalocyanine, iron phthalocyanine); tetra-aza-annulenes (cobalt tetra-aza-annulene); metal oxides; metal nitrides; metal carbides; metal hydroxides (iron hydroxide); metal sputtered over the microporous support; and mixtures of the above.
Other non-limiting examples of NPMPs include cobalt porphyrins, such as Co tetramethoxyphenylporphyrin (TMPP); Fe tetramethoxyphenylporphyrin (TMPP) on pyrolysed perylene tetracarboxylicdianhydride (PTCDA); Fe phthalocyanines; (K3Fe(CN)6); Fe and Co tetraphenylporphyrin; Co phthalocyanines; Mo tetraphenylporphyrin; metal/poly-o-phenylenediamine on carbon black; metal porphyrin; molybdenum nitride; cobalt ethylene diamine; hexacyanometallates; pyrrol, polyacrylonitrile and cobalt; cobalt tetraazaannulene; and cobalt organic complexes.
In one or more embodiments, the NPMPs may also be a nitrogen precursor.
As noted above, the catalyst precursors include an organic compound, which is referred to as an organic coating/filling compound (OCFC). Other components in the catalyst precursor may also be organic. A mixture of OCFCs also can be used. The NPMP may be the OCFC, e.g., the NPMP and the OCFC may be the same molecule (in which case the molecule can be an organometallic molecule).
OCFCs (nitrogen-containing or not) are used to coat and/or fill the pores of TDPS particles. The OCFC is carbon-based (e.g., organic) so that it can react with the TDPS to become the building blocks of catalytic sites. As will be appreciated by the person of skill in the art, the exact nature of the OCFC has therefore little importance to the present catalysts as long as the OCFC fulfills the above-noted requirements and roles.
In one or more embodiments, the OCFC may comprise a poly-aromatic structure, i.e., a structure made of rings (formed by a series of connected carbon atoms), preferably aryl rings such as C6 rings, for example benzene. These rings may more easily construct active sites and extend the graphitic platelets (if present) that are found on the edge of the graphitic crystallites (if present) within the microporous carbon support formed during pyrolysis to provide the desired carbon poly-aromatic structure in the micropores of the catalyst.
Different types of OCFC may be used. A first type comprises molecules that contain carbon, but that do not contain nitrogen atoms. Non-limiting examples of classes of such OCFCs include polycyclic aromatic hydrocarbons or their derivatives. Non-limiting examples of OCFCs in these classes include perylene and perylene tetracarboxylic dianhydride.
A second type of OCFC comprises molecules that contain both carbon and nitrogen atoms in their structure. Non-limiting examples of classes of such OCFCs include phenanthrolines, melamine and cyanuric acid.
A further type of OCFC comprises molecules that contain carbon, nitrogen atoms and at least one metal atom in their molecular structure. Non-limiting examples of classes of such OCFCs include metal-phenanthroline complexes, metal-phthalocyanines, and metal-porphyrins.
The OCFC may be any combination of OCFCs from the first, second and/or third above-described types of OCFCs.
In one or more embodiments, the OCFC may be a nitrogen precursor. Non-limiting examples of OCFC that also are nitrogen precursors include the following broad classes (with specific examples given between parenthesis): phenanthrolines (1,10-phenanthroline, bathophenanthroline disulfonic acid disodium salt hydrate, 4,7-diphenyl and 5,6-dimethyl phenanthroline, 4-aminophenanthroline); phthalocyanines; porphyrines; pyrazines (tetra 2 pyridinyl pyrazine, dihydropyridylpyridazine); phthalonitriles (4-amino-phthalonitrile); pyridines (2,2′:6′,2″-terpyridine, 4′-(4-methylphenyl)-2,2′:6′,2″-terpyridine, 6,6″-dibromo-2,2′:6′,2″-terpyridine, 6″-dibromo-2,2′:6′,2″-terpyridine, aminopyridines); melamines; tetra-aza-annulenes; hexaazatriphenylene; tetracarbonitrile; benzene-1,2,4,5-tetracarbonitrile; 6-pyridin-2-yl-[1,3,5]triazine-2,4-diamine; amino-acids; polypyrrole; and polyaniline.
Non-limiting examples of OCFCs that do not contain nitrogen atoms and are thus not nitrogen precursors include the following broad classes (with specific examples given between parenthesis): perylenes (perylene-tetracarboxylic-dianhydride (PTCDA)); cyclohexane; benzene; toluene; pentacene; coronene; graphite transformed into disordered carbon of size<2 nm by ballmilling; polycyclic aromatics (including perylene, pentacene, coronene, etc.); and coal tar or petroleum pitch (these are raw materials for a commercial process for carbon fiber production and are high in polycyclic aromatics).
In one or more embodiments, the OCFC is perylene tetracarboxylic-dianhydride, 1,10-phenanthroline, perylene tetracarboxylic diimide, or polyacrylonitrile or mixtures thereof. Some examples are nitrogen-containing, such as 1,10-phenanthroline, tetra-cyanobenzene; some examples are non-nitrogen containing such as PTCDA, carbon black, graphite; conductive polymers: polypyrrole, polyaniline; phenolic resin, and ionic liquids (1-ethyl-3-methylimidazolium dicyanamide).
To form the catalyst precursor, OCFCs (nitrogen-containing or not) and a NPMP that is a source of a non-precious metal catalyst are mixed to coat and/or fill the pores of the TDPS using some form of mechanical mixing. While mixing may result in the filling of some or all of the pores in the TDPS, it is not absolutely necessary to fill the micropores. Coating part of the TDPS is sufficient. During the pyrolysis of the catalyst precursor, the TDPS, the OCFC and the NPMP decompose (mass loss>60%) to form a unique structure that contains a very high density of catalytic sites inside its micropores and excellent mass transport properties.
The mass ratio of OCFC to TDPS prior to heat treatment can range from 95:5 OCFC:TDPS to about 5:95 OCFC:TDPS by weight. In one or more embodiments, the load of OCFC is 50% or less by weight. In one or more embodiments, the load of OCFC is about 10 wt % to about 40 wt %. In one or more embodiments, the mass ratio of OCFC to TDPS is about 40:60, about 30:70, about 25:75: about 20:80 or 15:85.
Three components, namely at least one thermally decomposable porous support (TDPS), at least one non-precious metal precursor (NPMP) and optional (none, one or more) organic coating/filling compound (OCFC) are mixed together by planetary ball milling or other mixing techniques to obtain a catalyst precursor that is subsequently pyrolyzed in an inert atmosphere such as nitrogen or argon or in a reactive atmosphere such as, NH %, CO2 or H2, depending on the choice of the NPMP and/or of the OCFC used. When argon is chosen as a gas during the pyrolysis, it is possible to further improve the catalyst by subjecting it to a second pyrolysis in a reactive gas, such as NH3 or CO2, for example. Alternatively, a second treatment other than pyrolysis may be used to achieve a similar effect, such as known methods used to produce activated carbons, for example. See, e.g., Marsh, A & Rodríguez-Reinoso, F (2006). Activation Processes: Thermal or Physical. Activated Carbon (pp. 243-321). Oxford, UK: Elsevier Science] [Marsh, A & Rodríguez-Reinoso, F (2006). Activation Processes: Chemical. Activated Carbon (pp. 322-365). Oxford, UK: Elsevier Science from supplemental material, which is incorporated in its entirety by reference. In some embodiments, the catalyst precursors are subjected to two consecutive pyrolyses: the first in Ar and the second in NH3. Mixing these three components can also be done by a wet impregnation method.
The TDPS decomposes during the pyrolysis and forms carbon structures that have significantly improved mass transport properties. While not being bound by any particular theory or mode of operation, the improved mass transport properties (over prior M/N/C-catalysts prepared using porous carbon supports that do not thermally decompose in an inert atmosphere, such as carbon black) are observed due to the significant change in carbon structure arising from the decomposition of the catalyst precursor during pyrolysis. When a porous carbon support that does not thermally decompose in an inert atmosphere, such as a microporous carbon black and in particular Black Pearls 2000 which inherently has poor mass transport properties, is used as the microporous support in the catalyst precursor, it remains in large part intact after the pyrolysis and therefore does not result in improved mass transport properties in the catalyst. In contrast, TDPSs undergo a significant mass loss (e.g., greater than about 60% wt) that also gives rise to a significant rearrangement of the structure which forms a microporous and mostly carbonaceous support with improved mass transport performance. In some embodiments, the overall catalyst precursor experiences a mass loss of greater than about 80% by weight, or 75% or 70% of 65% or 60% as compared to the starting mass. In addition, for the resulting catalysts, the decomposition and gasification of the TDPS and the OCFC simultaneously enables results in an even higher concentration of active sites in the catalyst, compared to the M/N/C-catalyst prepared using porous carbon supports that do not thermally decompose in an inert atmosphere. The final result is a non-precious metal catalyst with unprecedented power performance in PEM fuel cell.
The mass loss experienced by the catalyst precursor based on TDPSs decomposition during pyrolysis is very different from that of catalyst precursors based on porous carbon supports that do not thermally decompose in an inert atmosphere, such as carbon blacks and others. For porous carbon supports that do not thermally decompose in an inert atmosphere, the mass loss of the catalyst precursor during pyrolysis to obtain optimal activity was substantially the same as the mass fraction of the OCFC in the catalyst precursor, e.g., the mass loss of the catalyst precursor was due almost exclusively to the decomposition of the OCFC and mass loss of the porous carbon support during pyrolysis was insignificant. Here, for the catalysts based on TDPSs the mass loss leading to the optimal catalytic activity and mass transport properties is far more than simply the mass fraction of OCFC in the catalyst precursor. For example, for a catalyst precursor containing about 20 wt % OCFC, the optimal mass loss during pyrolysis was about 85%. This is the result of the combined decomposition of the TDPS, the OCFC and the NPMP.
When the TDPS, the NPMP and the OCFC are not nitrogen precursors, the necessary nitrogen atoms are provided by a gas used during pyrolysis. Therefore in that case, the gas itself is a nitrogen precursor. The OCFC, the TDPS and the NPMP in the catalyst precursor are believed to react as a whole during pyrolysis to produce the desired catalytic sites in the catalyst. This creation of catalytic sites is different than that in catalysts based on porous carbon supports that do not thermally decompose in an inert atmosphere, such as carbon blacks and others, in that the carbon support and the catalytic sites in the catalyst are formed during the pyrolysis, while the carbon support in catalysts made using porous carbon supports that do not thermally decompose in an inert atmosphere, such as carbon blacks and others, is present in the catalyst precursor and remains throughout the pyrolysis and the final catalyst.
This process has also caused the NPMP and the nitrogen precursor (be it the TDPS, the NPMP, the OCFC or the gas used for pyrolysis) to react and give up some or all of their non-precious metal and nitrogen atoms to the catalytic sites. The active catalytic sites are thus formed from the carbon from the TDPS and/or the OCFC and/or the NPMP, the nitrogen from the TDPS and/or the OCFC and/or the NPMP and/or the pyrolysis gas, and the non-precious metal from the NPMP.
In one or more embodiment, the nitrogen precursor and the NPMP decompose during pyrolysis. The order of decomposition for the TDPS, the NPMP and the OCFC depends on their respective decomposition temperatures and will be different for each combination. In one or more embodiments, that TDPS decomposes last. For example, in one particular catalyst precursor, the NPMP (iron(II) acetate) decomposes first at ca. 190-200° C., followed by the OCFC (1,10-phenanthroline) at ca. 200-300° C. and finally the TDPS (ZIF-8) at ca. 500-600° C. In addition, during decomposition, the TDPS loses mass, gives off gases and/or liquid products and ultimately becomes a highly microporous carbonaceous support as the temperature increases. Therefore, the micropore surface area of the catalyst becomes substantially larger than the catalyst precursor during pyrolysis. In other words, the micropore surface area of the catalyst as described here is substantially larger than the micropore and in particular surface area of the catalyst precursor, which originally had a substantially lower surface area than the TDPS when the OCFC and the NPMP are absent. In the extreme, the micropore surface area of the catalyst may be almost as high as, as high as or higher than the micropore surface area of the TDPS when OCFC and the NPMP were absent.
The non-precious metal content of the catalyst after pyrolysis may be measured by methods known in the art, for example neutron activation analysis.
The catalyst may comprise between about 0.5 to about 10.0 wt % of the nitrogen based on the total weight of the catalyst. In one or more embodiments, the catalyst has a nitrogen content, as provided by the nitrogen precursor, of about 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, or 9.0 wt % or more based on the total weight of the catalyst. This nitrogen content may be measured by methods known in the art, for example, x-ray photoelectron spectroscopy.
If the TDPS is carbon-based, the carbon content in the catalyst is usually about 80 wt % or more based on the total weight of the catalyst. The catalyst may comprise between about 80 and about 99.9 wt % of carbon. It is to be noted that carbon usually comprises some oxygen (usually between 0.5 and 5% wt). If the TDPS has a low carbon content, the carbon content of the catalyst may be lower since the carbon content will be provided primarily only by the OCFC (and optionally the NPMP) used to coat and/or fill the pore of the TDPS.
Catalysts prepared from TDPS-based catalyst precursors enjoy certain advantages. Non-limiting examples include higher catalytic site density, improved mass transport properties and improved operation in fuel cells. First, a higher catalytic site density is achieved, largely due to the high mass loss experienced by the catalyst precursor and in particular the TDPS during the pyrolysis. Secondly, the catalyst has much better mass transport properties, due to its higher permeability to gases and water. Thirdly, the catalyst produced after only one pyrolysis in an inert atmosphere exhibits improved stability in fuel cell operation.
The catalyst precursor may be prepared by the mixing of these three components e.g., the TDPS, the NPMP and the OCFC. At a minimum the catalyst precursor must contain at least one TDPS and at least one NPMP. The catalyst precursor may contain a mixture of two or more species selected from each component category. For example, a catalyst precursor may contain a mixture of MOFs (a TDPS), phenanthroline and PTCDA (OCFCs), iron (II) acetate and cobalt porphyrin (NPMPs).
In one or more embodiments, the catalyst is an oxygen reduction catalyst, a catalyst for the disproportionation of hydrogen peroxide or a catalyst for the reduction of CO2. H2O2 disproportionation reaction has been measured on a catalyst prepared as described herein. The present catalysts can be useful for the disproportionation of hydrogen peroxide and the reduction of CO2 because it is known for non-precious metal catalysts obtained from heat treatment or without heat treatment (metal-N4 molecules as phthalocyanines) that the activity for the O2 electro-reduction reaction and for the chemical disproportionation of H2O2 follow the same trend, i.e. if a catalyst shows high activity for one reaction, it will show high activity for the other reaction as well. Further, it is also known that electroreduction of CO2 is catalyzed by metal macrocycles in which a metal ion is coordinated to 4 nitrogen atoms located in a polyaromatic frame, a structure similar to that proposed for the present catalytic sites used for the reduction of oxygen.
In more specific embodiments, the catalyst is an oxygen reduction catalyst. Such a catalyst will be useful at the cathode of various low temperature fuel cells, including principally polymer electrolyte membrane (PEM) such as H2/O2 polymer electrolyte membrane fuel cells, direct alcohol fuel cells, direct formic acid fuel cells and even alkaline fuel cells or microbial fuel cells. Such a catalyst may be useful at the cathode of various primary and secondary metal-air batteries, including zinc-air batteries. In one or more embodiments, the catalyst can serve as a support for precious metal catalyst that are used as conventional catalyst in oxygen reduction reactions.
The M/N/C catalysts may be used at the cathode of PEM, alkaline or microbial fuel cells and in metal-air batteries. With a cathode catalyst loading of 4 mg/cm2, power densities comparable to those of Pt-based cathodes are achievable with this catalyst.
The method of making the catalyst precursor is described with reference to
Mechanical mixing refers to mixing involving a milling, grinding or pulverizing system like a planetary ballmiller, high energy ballmiller (sometimes called a shaker mill) or other types such as sonic mixing, freeze mixing, etc. Examples of such methods include any form of ballmilling or reactive ballmilling, including but not limited to planetary ballmilling, and resonant acoustic mixing. Planetary ballmilling is a low-energy material processing technique involving a container with grinding media that rotates in a planet-like motion. It uses both friction and impact effects to thoroughly mix all components and to coat and/or fill the micropores of the TDPS with a mixture of NPMP and OCFC, while leaving some or all of the microstructure of the TDPS relatively unaffected. Resonant acoustic mixing is a method that uses low-frequency high-intensity sound energy for mixing. It may be carried out with or without grinding media. Other forms of mixing are contemplated, so long as it accomplishes the objective of the mixing step which is to form intimate mixture of all the components while dispersing the NPMP and OCFC and coating and/or filling the pores of the microporous support, which is a TDPS. The mechanical mixing may be performed on dry powder mixtures of NPMP, the OCFC and the TDPS (step 110). Alternatively, mechanical mixing may be performed in wet conditions with the NPMP and the OCFC in solution and the TDPS in suspension in this solution as in step 120. In the latter case, the mixture is then dried in step 125 to provide the catalyst precursor 160. In another embodiment, the NPMP and the OCFC in solution and the TDPS in suspension are combined in a solvent (step 130) and then dried (step 140) before being mechanically dry mixed in step 110.
Optionally, prior to becoming a catalyst precursor after mechanical mixing (step 110) or after drying (step 125), a low-temperature treatment (step 150) may be performed. The purpose of the low-temperature treatment is to (i) liquefy the OCFC and/or the NPMP in order to enhance their penetration into the pores TDPS and/or to better disperse the OCFC and/or the NPMP throughout the overall mixture, (ii) to polymerize the OCFC and/or the NPMP, (iii) to complex the OCFC and the NPMP or (iv) to chemically bind the OCFC and/or the NPMP to the TDPS. In one or more embodiments, the reaction time and temperature required for the low-temperature treatment will be easily determined by the person of skill in the art. In one or more embodiments, the low-temperature treatment may be performed at temperatures ranging from about 50° C. to about 500° C.
In one or more embodiments, the micropores of the TDPS are coated and/or filled with the OCFC and the NPMP by mixing, e.g., ballmilling or by resonant acoustic mixing with or without grinding media. In more specific embodiments, the ballmilling is planetary ballmilling.
The NPMP and the OCFC may be introduced into a mixer either together or separately to coat and/or fill the micropores of the TDPS. One or more NPMP, OCFC and/or TDPS can be used in the preparation of the catalyst precursor.
A method of producing a catalyst is described with reference to
For either the Single Step Pyrolysis or the Multi-Step Pyrolysis the atmosphere in which the pyrolysis is performed may be a nitrogen-containing reactive gas or vapor, non-limiting examples of which being NH3, HCN, and CH3CN; a non-nitrogen-containing reactive gas or vapor, non-limiting examples of which being CO2, H2O, and air; an inert gas or vapor, non-limiting examples of which being N2 and Ar; or any mixture of a nitrogen-containing or non-nitrogen containing reactive and an inert gas or vapor.
As used herein a nitrogen-containing reactive gas or vapor is a nitrogen-containing gas or vapor that will react during pyrolysis to provide a nitrogen atom to the catalyst and may also create porosity depending on the gas used. As used herein a non-nitrogen-containing reactive gas or vapor is a non-nitrogen-containing gas or vapor that will create porosity only. As used herein, an inert gas is a gas that will not react with the catalyst precursor/catalyst at the pyrolysis temperature.
The Single Step Pyrolysis involves only one heat treatment step (step 210). If the catalyst precursor (step 200) is nitrogen-containing the pyrolysis may be performed in an inert gas or vapor, a nitrogen-containing reactive gas or vapor or a non-nitrogen-containing gas or vapor. If the catalyst precursor (step 200) does not contain nitrogen the pyrolysis must be performed in a nitrogen-containing reactive gas or vapor.
The Multi-Step Pyrolysis involves two or more cycles of (heat treatment)/(optional particle refinement)/(optional metal leaching). If the catalyst precursor (step 200) is nitrogen-containing, the heat treatment step (step 220) in any cycle within the Multi-Step Pyrolysis may be performed in an inert gas or vapor, a nitrogen-containing reactive gas or vapor or a non-nitrogen-containing gas or vapor. If the catalyst precursor (step 200) is not nitrogen-containing, at least one heat treatment step (step 220) among the two or more cycles within the Multi-Step Pyrolysis must be performed in a nitrogen-containing reactive gas or vapor. Each cycle within the Multi-Step Pyrolysis may contain an optional particle refinement step and/or metal leaching step (step 230).
In steps 210 or 220, the catalyst precursor is heated at temperatures sufficient to pyrolyse the catalyst precursor. The catalyst precursor may be heated to a set temperature using a ramp up with or without intermediate plateaus, or it may be inserted directly into the furnace heating zone at the set temperature. In one or more embodiments, the reaction time and temperature required for the pyrolysis will be easily determined by the person of skill in the art. In one or more embodiments, the pyrolysis may be performed at temperatures ranging from about 300 to about 1200° C. In some embodiments, the pyrolysis is performed at a temperature greater than about 700° C.
After the pyrolysing heat treatment(s) from either the Single Step Pyrolysis (step 210 or the Multi-Step Pyrolysis (two or more cycles of step 220 and optional step 230), the catalyst 250 is obtained.
The Optional Post Treatment (step 240) may be performed to modify or enhance the properties of the catalyst 250. These Optional Post Treatments may involve particle refinement and/or metal leaching and/or a post-heat treatment.
Particle refinement refers to the process by which particle size in a material is reduced. Some non-exhaustive examples of methods used to perform particle refinement include ballmilling and grinding. Ballmilling may be either high-energy (shaker or vibratory mill for example) or low-energy (planetary ball mill or attritor mill for example). Resonant acoustic mixing with grinding media may also be used for particle refinement. Grinding may be performed using a mortar and pestle, or any type of grinding mill that serves to produce fine powders. In step 230 of the Multi-Step Pyrolysis the purpose of particle refinement is to obtain a finer powder for pyrolysis to maximize the reactivity of the powder with the pyrolysis gas. In step 240 of the Optional Post Treatment the purpose of particle refinement is to obtain a finer powder with higher activity and that will produce a smoother and more homogeneous catalyst ink for better performance.
Metal leaching refers to the process by which metal impurities are removed from a material. Some examples of methods used to perform metal leaching are acid-washing and base-washing. Acid-washing may be performed using acid solutions (pH0-pH4, for example) using acids such as H2SO4 or HCl, for example. Base-washing may be performed using basic solutions (pH10-pH14, for example) using bases such as KOH or NaOH, for example. Metal leaching may be performed any number of times to achieve the desired result. In particular, when the TDPS is a MOF, for example, it is possible that pyrolysis will leave traces of metals from the source TDPS. This can especially be the case in instances where the MOF includes non-volatile (at the pyrolysis temperature) metals such as cobalt and manganese. In step 230 of the Multi-Step Pyrolysis the purpose of metal leaching is to remove metal impurities originating from the TDPS and/or excess and inactive non-precious metal originating from the NPMP. In step 240 of the Optional Post Treatment the purpose of metal leaching is to remove all excess and inactive metal impurities and unstable catalytic sites.
A post-heat treatment refers to the process by which the catalyst powder undergoes a thermal treatment in an inert or reactive gas or vapor to remove any traces of acid residues in the catalyst powder and/or change the surface functionalities on the surface of the catalyst powder in order to create more or less hydrophilicity or hydrophobicity. In one or more embodiments, the reaction time and temperature required for the post-heat treatment will be easily determined by the person of skill in the art. In one or more embodiments, the post-heat treatment may be performed at temperatures ranging from about 300 to about 1200° C. In one or more embodiments, the post-heat treatment is performed at a temperature greater than about 500° C. In some embodiments, the post-heat treatment gas is H2.
An advantage of processes using zinc-based MOFs, such as ZIF-8 for example, is that zinc is conveniently removed as a volatile compound during heat treatment of the catalyst precursor containing the zinc-based MOF at a temperature of about 850° C. or higher, depending on the MOF. Samples prepared using ZIF microporous structures, and ZIF-8 in particular contained about 0.5 wt % zinc after pyrolysis. Acid-washing reduced that number down to 0 wt %.
For use in a fuel cell, the catalyst is processed in order to form part of the cathode of the fuel cell. This is typically accomplished by thoroughly mixing the catalyst and an ionomer like Nafion®. The ionomer-to-catalyst mass ratio has to be adjusted and depends on the catalyst, but can be easily determined by the person of skill in the art. The optimal ionomer-to-catalyst mass ratio may range between about 0.5 and about 4. The current density of the fuel cell may be increased by increasing the loading of the catalyst. Therefore, the loading of present catalysts may be increased as long as mass transport losses are acceptable.
If the electronic conductive properties of the obtained catalysts are not sufficient for optimal performance in fuel cell, a given ratio of a conductive powder, e.g., carbon black or any electronic conductive powder that does not corrode in acid medium (for all PEM fuel cells) or alkaline medium (for alkaline fuel cell), may be added.
Other objects, advantages and features will become more apparent upon reading of the following non-restrictive description of specific embodiments thereof, given by way of example only with reference to the accompanying drawings.
A comparative electrocatalyst was prepared as described in Lefevre et al. [Science 324 71 (2009)]. Briefly, a mixture of carbon support (Black Pearls 2000), organic compound (1,10-phenanthroline) and iron precursor (ferrous acetate) having a carbon support/organic compound mass ratio of 50/50 and an iron content of 1 wt % was ball milled to form a catalyst precursor. The ball milled mixture was first pyrolysed in argon gas at 1050° C. for 60 minutes, then in ammonia at 950° C. for a time corresponding to a combined mass loss of ca. 50% for both pyrolyses. The resulting powder was the catalyst.
Briefly, a mixture of thermally decomposable porous support (TDPS), ZIF-8, an organic coating/filling compound (OCFC), 1,10-phenanthroline, and a non-precious metal precursor (ferrous acetate) having a TDPS/OCFC mass ratio of 80/20 and an iron content of 1 wt % was ball milled to form a catalyst precursor. The ball milled mixture was first pyrolysed in argon gas at 1050° C. for 60 minutes, then in ammonia at 950° C. for a time corresponding to a combined mass loss of ca. 87% for both pyrolyses. The resulting powder was the catalyst.
The performance of a cathode catalyst may be assessed by conducting a fuel cell test using a test fuel cell. The test fuel cell used to assess catalysts in embodiments of this invention was a single-MEA test fuel cell. It consisted of a metal end plate, current collector and a graphite gas flow field plate for both anode and cathode sides. It has of an input and output for anode and cathode gases. It includes a means of fastening the test fuel cell tightly together, either using bolts and nuts or using bolts that may be screwed directly into threaded holes in one of the end plates. To assemble a test fuel cell, a membrane electrode assembly (MEA) is placed between the anode and cathode gas flow field plates so that the anode and cathode of the MEA is well positioned and aligned with the gas flow channels of the graphite gas flow field plates. In addition, Teflon gaskets (which also act as spacers) having a cut-out exactly matching and aligning with the shape and size of the anode and cathode of the MEA are placed on either side of the MEA. These gaskets serve to prevent any gas leakage on either side of the fuel cell once it is tightly fastened together, while at the same time controlling the compression exerted directly on the active area of the MEA, i.e. the area defined by the electrodes, to allow a good balance between electrical contact and permeability to gases. The MEA is prepared by hot-pressing an anode and cathode gas diffusion electrode (GDE), consisting of a gas diffusion layer (GDL) coated with a catalyst ink and dried, to either side of a proton exchange membrane (PEM). The catalyst ink is prepared by mixing the catalyst with an ionomer solution and solvents. The catalyst ink may be applied to the GDL using one of many methods, such spray coating, the doctor blade method or simply dropping the ink directly over the GDL and letting it dry, as is the case for embodiments of this invention.
The polarization and power density curves of a membrane electrode assembly (MEA) having a cathode made with a catalyst prepared as in Example 2 (stars), a MEA having a cathode made with a catalyst as prepared in Example 1 (previously most active iron-based catalyst) (circles), and a commercial Pt-based MEA (Gore 5510 PRIMEA, squares) are shown in
To help in the understanding of the factors contributing to improved power performance measured in PEMFC, a number of chemical and physical characterizations were conducted on selected catalysts, which included nitrogen adsorption/desorption isotherms (BET surface area and porosimetry using QSDFT), x-ray photoelectron spectroscopy (XPS), neutron activation analysis (NAA), thermo-gravimetric analysis (TGA), powder x-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The effect of (i) a single pyrolysis in argon and (ii) two pyrolyses; the first in argon and the second in ammonia on a catalyst precursor having a ZIF-8/1,10-phenanthroline/Fe mass ratio of 80/20/1 were compared.
Initial study was conducted to evaluate the effect of different pyrolysis temperatures on the catalyst. The Tafel plots, XRD, XPS N1s narrow scan and TEM images for catalyst precursors heated in argon at 400° C. (A), 700° C. (B), 850° C. (C) and 1050° C. (D) for one hour are shown on
Physical and chemical characterization of the catalyst precursor (i.e. the ball milled mixture prior to pyrolysis) and the four catalysts pyrolysed at four different temperatures in Ar (
Next, each of the four catalysts shown in
SEM images obtained for the previously most active iron-based catalyst prepared using a carbon black support (
From TEM images one can observe differences in carbon structure, as shown in
Investigation of the effect of the ZIF-8/1,10-phenanthroline mass ratio (90/10, 80/20, 75/25, 50/50, 75/25) in the catalyst precursor while maintaining the nominal iron content to 1 wt % is shown in
Then, using the latter mass ratio, the effect of nominal iron content (0.5, 1.0 and 1.5 wt %) in the catalyst precursor was investigated. The catalytic activities of catalysts made with 0.5 and 1.5 wt % were lower than those made with 1 wt %, although the current densities at 0.6V iR-free were comparable. Hence, the optimal nominal iron content was found to be 1 wt % for this system. Measurements using NAA of the catalyst (ZIF-8/1,10-phenanthroline/Fe mass ratio of 80/20/1, 1st pyrolysis in Ar at 1050° C. for 60 minutes followed by a 2nd pyrolysis in NH3 at 950° C. for 15 minutes) reveal a catalyst bulk iron content of approximately 3 wt %. When iron was completely omitted from the catalyst precursor the catalytic activity was more than three orders of magnitude lower than the latter, emphasizing the importance of iron.
One comparative parameter that is often reported for NPMCs is the volumetric activity for ORR in terms of Acm−3cathode.
Nafion® NRE211 membranes (˜25 nm thick) were used for fuel cell testing performed with the aim of demonstrating maximum power density at practical fuel cell voltage without iR (voltage drop related to ohmic resistance) correction, by optimizing catalyst loading. Using the latter membranes minimizes protonic resistance and better reflects what is actually used in prototype and commercial H2/Air fuel cells. Catalyst loadings of roughly 1, 2, 3, 4 and 5 mg cm−2 were tested. Catalyst loading of approximately 4 mg cm−2 (curve 4 of
Lastly, 100-hour durability tests in H2/O2 and H2/Air at 0.5 V cell voltage using Nafion® N117 membranes were performed using two catalysts: (i) a catalyst having undergone 2 pyrolyses (ZIF-8/1,10-phenanthroline/Fe mass ratio of 80/20/1, 1st pyrolysis in Ar at 1050° C. for 60 minutes followed by a 2nd pyrolysis in NH3 at 950° C. for 15 minutes) and (ii) a catalyst having undergone only one pyrolysis (ZIF-8/1,10-phenanthroline/Fe mass ratio of 80/20/1, pyrolysis in Ar at 1050° C. for 60 minutes only). The durability test results in H2/O2, presented in
In summary, iron-based cathodes catalysts for polymer electrolyte membrane fuel cells (PEMFCs) were prepared using a mixture of (i) a thermally decomposable porous support (ZIF-8, a metal-organic framework), (ii) an organic coating/filling compound (1,10-phenanthroline, a small nitrogen-containing organic molecule), and (iii) a non-precious metal precursor (ferrous acetate, an iron compound). The mixture was ball milled, then pyrolysed twice, first in argon at 1050° C., then in ammonia at 950° C. A PEMFC cathode made with the best catalyst in this work produces high power density comparable to commercial platinum-based cathodes at efficient fuel cell voltages (above 0.6 V), with a peak power density of 0.91 watts per square centimeter of cathode. Its volumetric activity of 276 amps per cubic centimeter of cathode is the highest ever reported to date for a non-precious metal catalyst for oxygen reduction in PEMFC and is within grasp of the U.S. DOE's 2015 target of 300 amps per cubic centimeter of cathode.
M/N/C Catalysts were prepared using a variety of different OCFC's and NPMP's and using a range of processing conditions.
Examples of catalysts were made using the methods described in
This application is continuation application of co-pending International Application No. PCT/2012/000371 filed Feb. 8, 2012, which claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No. 61/440,663, filed on Feb. 8, 2011, entitled CATALYSTS MADE USING THERMALLY DECOMPOSABLE POROUS SUPPORTS, which are incorporated by reference herein in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61440663 | Feb 2011 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/IB2012/000371 | Feb 2012 | US |
| Child | 13958301 | US |
