Novel catalyst for oxygen reduction reaction in fuel cells   

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application Ser. No. 12/583,532, filed on Aug. 21, 2009, which claims the benefit of U.S. Provisional Patent Application No. 61/090,780, filed Aug. 21, 2008. The entire disclosure of each of the above applications is incorporated herein by reference.

The present technology relates to methods for producing improved metal, nitrogen, and carbon containing catalysts effective for the reduction of oxygen in low temperature fuel cells and other electrochemical reactions and cathode catalysts produced by these methods.

There is an increasing interest to replace platinum (Pt) based electro-catalysts with cost-effective non-noble catalysts for the oxygen reduction reaction (“ORR”) in low-temperature fuel cells, such as Polymer Electrolyte Fuel Cells (PEFCs) and Direct Methanol Fuel Cells (“DMFCs”) etc. Non-noble metal catalysts based on iron (Fe) and cobalt (Co) ions are among the possible candidates for replacement of Pt based catalyst metals for ORR. These catalysts are active towards ORR and exhibit selectivity towards ORR in the presence of a fuel, thereby increasing the volumetric energy density of a DMFC.

Others in the field have discovered the catalytic nature of nitrogen-doped carbon materials, and subsequently various non-precious metal catalysts were produced by pyrolyzing materials such as metal-N4 macrocyles adsorbed on carbon black in an inert atmosphere. Others have demonstrated an active catalyst for ORR by pyrolyzing a metal precursor (cobalt acetate), carbon black and a nitrogen precursor such as polyacrylonitrile in inert atmosphere. Following this approach, many methods have been developed to prepare non-noble metal catalysts, including these steps: (a) heat-treating carbon-supported organometallic complexes by pyrolyzing a metal source with carbon source in ammonia or acetonitrile atmosphere, (b) cosputtering cobalt or iron and carbon in a nitrogen atmosphere with or without subsequent heat-treatment, and (c) mixing nitrogen-containing ligands with cobalt oxide solution which are subsequently entrapped in polypyrrole matrix supported on carbon.

Recently, investigators have ball-milled highly-ordered synthetic graphite for use as a carbon support as it contains low levels of iron as impurities and low surface area (3.5 m2/g), Pyrolysis of the milled material with an iron source in ammonia environment produced catalysts with nitrogen content as high as 4 atom %. These nitrogen containing catalysts demonstrate that the catalytic activity increases as a result of decreasing metal crystallite size, increasing degree of disorder, nitrogen content, and microporous (<22 Å) specific surface area. Others in the field have also suggested that active sites containing pyridinic nitrogen can be responsible for the catalytic activity for ORR and reported low levels of H2O2 production while reducing oxygen in an acidic medium.

U.S. Patent Application Publication No. 2007/0248752, O\'Brien et al., published Oct. 25, 2007, discloses making an oxygen-reducing catalyst layer. The catalyst layer is prepared by physical vapor depositing (PVD) a transition metal onto a carbon support under a reduced pressure (e.g. about 1×10−5 Ton or less). After a film of catalyst metal has been applied to a substrate, the resulting coated substrate is thermally treated either separately or as part of the PVD step. The thermal treatment and/or PVD treatment can be performed under a nitrogen gas environment to provide a source of nitrogen to the catalyst. The thermal treatment can comprise heating the coated substrate for 15 minutes or so at temperatures of at least 600-900° C. However, the deposition of the nitrogen source is not readily controllable. Moreover, it is believed that a greater amount of nitrogen can be incorporated into a high-surface area support by introducing nitrogen at higher activity (for example, higher partial pressure) in the presence of the carbon support in contrast to the use of gaseous nitrogen at reduced pressure.

As such there is a need for alternative methods for producing alternative catalyst materials having improved catalytic activity. There is also a need for methods to increase the availability of nitrogen target sites on catalytic supports for oxygen reduction reactions and provide enhanced stability of these alternative catalysts when used in acidic fuel cell environments.

SUMMARY

The present technology provides methods for making non-precious metal electrochemical cathode catalysts for the reduction of molecular oxygen, for example, in a fuel cell. In addition, the present technology provides for a method to control the anchoring of a nitrogen containing compound on a high surface area carbon surface, which actively contributes to the catalytic activity of the cathode catalyst over preexisting methods of depositing nitrogen, thereby effectively increasing the catalytic activity per unit mass of catalytic material on a substrate. The cathode catalyst material produced in accordance with the present technology lowers the cost for producing the catalyst material and follows a simple synthesis method compared to platinum/carbon catalysts and other non-precious metal catalysts conventionally used in fuel cell designs.

In one aspect, the present technology provides a method for making a carbon-metal-nitrogen oxygen reducing cathode catalyst, the method comprising: (a) mixing a carbon source with a transition metal precursor to form a metal precursor loaded carbon substrate, preferably wherein the substrate is substantially free of precious metals; (b) adding a nitrogen precursor compound to the metal precursor loaded carbon substrate to form a carbon supported metal-nitrogen complex precursor; and (c) pyrolyzing the carbon-metal-nitrogen precursor at an elevated pressure to form an oxygen reducing cathode catalyst.

In another aspect, the present technology provides for a method for making a membrane electrode assembly for a fuel cell, the membrane electrode assembly comprising: (a) an ionomeric membrane; (b) an anode catalyst disposed on a first surface of the ionomeric membrane; and (c) a cathode catalyst disposed on a second surface of the ionomeric membrane wherein the cathode catalyst is synthesized by: (i) mixing a carbon source with a transition metal precursor to form a metal precursor loaded carbon substrate, preferably wherein the substrate is free of precious metals; (ii) adding a nitrogen precursor compound to the metal precursor loaded carbon substrate to form a carbon supported metal-nitrogen complex precursor; and (iii) pyrolyzing the carbon supported metal-nitrogen complex precursor at an elevated pressure to form an oxygen reducing cathode catalyst.

Still further, the present technology provides a method for making a cathode catalyst coated diffusion layer for a fuel cell, the method comprising: (a) providing a gas diffusion layer; and (b) applying a cathode catalyst on at least a portion of the gas diffusion layer, wherein the cathode catalyst is synthesized by: (i) mixing a carbon source with a transition metal precursor to form a metal precursor loaded carbon substrate, preferably wherein the substrate is free of precious metals; (ii) adding a nitrogen precursor compound to the metal precursor loaded carbon substrate to form a carbon-metal-nitrogen precursor; and (iii) pyrolyzing the carbon-metal-nitrogen precursor at a pressure ranging from about 2 bar to about 100 bar, thereby forming an oxygen reducing cathode catalyst.

In yet another aspect, the present technology provides method for making an oxygen reducing cathode catalyst, the method comprising: (a) mixing a carbon source with a transition metal precursor to form a metal precursor loaded carbon substrate substantially free of precious metals; (b) adding a nitrogen precursor compound having a N:C ratio of at least about 1:1 to the metal precursor loaded carbon substrate to form a carbon-metal-nitrogen precursor; and (c) pyrolyzing the carbon-metal-nitrogen precursor at an elevated pressure ranging from about 2 bar to about 100 bar, thereby forming the oxygen reducing cathode catalyst.

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawings will be provided by the U.S. Patent and Trademark Office upon request and payment of the necessary fee.

FIG. 1 is a flow diagram of a method for making an oxygen reduction cathode catalyst in accordance with the methods of the present technology.

FIG. 2 shows the polarization curves obtained for various catalysts synthesized at 40° C. in 1N aqueous sulfuric acid, showing effect of heat-treatment temperature on the catalytic activity towards ORR.

FIG. 3 shows the plot of current density as a function of nominal nitrogen content observed at three different potentials.

FIG. 4 shows a Koutecky-Levich analysis performed on catalysts loaded with 10.3% nitrogen.

FIG. 5A shows a polarization curve obtained from rotating ring disc electrode (“RRDE”) measurements at 40° C. in 1N aqueous sulfuric acid; FIG. 5B shows a disk potential dependent H2O2 production curve for the optimized catalyst of the present technology having a nominal nitrogen of 10.3% coated on a rotating ring disc electrode (“RRDE”).

FIG. 6A shows the surface area distribution of various catalysts with different nominal nitrogen % content obtained from BJH desorption employing Halsey-Faas correction; FIG. 6B depicts a calculated BET area for various catalysts with different nominal nitrogen content (%).

FIGS. 7A-7D relate to oxygen reduction current density at thin-film rotating disk electrodes with MNC catalysts of varying precursor N:C ratios. FIG. 7A depicts pseudo-steady state polarization; FIG. 7B depicts iR- and mass transfer corrected Tafel curves; FIG. 7C depicts kinetic current density at 0.8 V/RHE as a function of precursor N:C with the conditions: O2-saturated, 1N aqueous sulfuric acid, 40° C. Scan rate 0.5 mV s−1, 1200 rpm, nominal 6.3 wt % nitrogen loading; FIG. 7D depicts observed BET surface area and bulk nitrogen obtained through CHN analysis.

FIGS. 8A-8D relate to performance of fuel cell membrane-electrode assemblies employing Fe-NC oxygen reduction catalysts. FIGS. 8A and 8B depict polarization curves for Fe-bipyridine and Fe-melamine based catalysts, in comparison to a commercial Pt-catalyzed MEA (solid and dotted lines indicate forward and reverse scans respectively); FIGS. 8C and 8D depict durability of Fe-melamine and Fe-bipyridine based MEA fuel cells at 0.5 V/RHE; Conditions: H2—O2 feeds (pO2=pH2=1.5 bar, 80° C., 100% RH), MNC catalyst loading 1.3 mg cm−2.

It should be noted that the figures set forth herein are intended to exemplify the general characteristics of an apparatus, materials and methods among those of the present technology, for the purpose of the description of such embodiments herein. These figures may not precisely reflect the characteristics of any given embodiment, and are not necessarily intended to define or limit specific embodiments within the scope of this technology.

DETAILED DESCRIPTION

The following description of technology is merely exemplary in nature of the subject matter, manufacture and use of one or more inventions, and is not intended to limit the scope, application, or uses of any specific invention claimed in this application or in such other applications as may be filed claiming priority to this application, or patents issuing therefrom. A non-limiting discussion of terms and phrases intended to aid understanding of the present technology is provided at the end of this Detailed Description.

In accordance with the various embodiments of the present technology, it has been discovered that an effective way of preparing a cathode-catalyst which, is used in low temperature fuel cells and other electrochemical cell applications can be achieved by pyrolyzing the catalyst components under high temperature in a closed vessel. The vessel can be pressurized due to the catalyst-components (autogenic pressure) to deliver a finite and specific amount of nitrogen content to the catalyst and more particularly, to control the ratio of nitrogen to carbon. In various embodiments, methods produce cathode catalysts with higher yields without gasifying carbon precursor. Having processes capable of producing a more efficient and non-gasified catalysts directly translates to a higher catalytic activity per unit of mass, volume and a lower cost per unit catalytic activity.

Although the present technology is not limited to or dependent on a particular theory, it is believed that the transition metal/nitrogen component on the carbon support promotes the reduction of molecular oxygen to water.

FIG. 1 represents a flow diagram of method 100 for making a cathode catalyst for the reduction of oxygen typically found in a fuel cell and includes steps 110-140. Method 100 initially involves generating a carbon-metal substrate (step 110). Step 110 generally involves mixing a carbon support and a transition metal precursor. In some embodiments, the carbon support and transition metal precursor are mixed in the presence of a chemically compatible solvent, for example, a small chain alcohol and water. The alcohol can be any C1-C6 alcohol, for example, ethanol, isopropyl alcohol, n-propyl alcohol and butanol which are readily or moderately miscible with water, the carbon support and the transition metal. The dispersion can be stirred for one to six hours to have the transition metal precursor deposited to the carbon support.

In some embodiments, the amount of transition metal added to the carbon support (as a percentage of dry weight of the two components) can range from about 0.01% to about 30% by weight. In various aspects, the amount of metal ranges from about 0.1% by weight to about 10% by weight, or from about 0.5% to about 10% by weight, or from about 0.75% to about 10% by weight, or from about 1% to about 10% by weight, or from about 2% to about 10% by weight, or from about 5% to about 10% by weight, or from about 0.1% to about 8% by weight, or from about 0.1% to about 6% by weight, or from about 0.1% to about 5% by weight, or from about 0.1% to about 3% by weight, or from about 0.1% to about 2% by weight.

The carbon support can include any activated or non-activated carbon material, generally having a high surface area. In some embodiments, the carbon support can include one or more of the following illustrative examples of carbon supports including: carbon powder, carbon black, acetylene black, activated carbon, carbon fiber, fullerene, nano-carbon or combinations thereof. Specific examples of carbon supports among those useful in the present technology include Norit® SX Ultra (Marshall, Tex., USA), Ketjenblack® (600J and 300J, Akzo-Nobel Polymer Chemicals, Chicago, Ill., USA), C55 carbon particles (Chevron Phillips Chemical Company, Texas), Black Pearls® (Cabot Corporation, Boston, Mass., USA), Printex® XE (Degussa Engineered Carbons, Parsipanny, N.J., USA), pyrrole black, activated charcoal, graphitic powder, Vulcan® XC72 (Cabot Corporation, Boston, Mass., USA) and pyrolyzed form of perylene tetracarboxylic anhydride (PTCDA), polyacrylonitrile (PAN), and combinations thereof.

Ketjenblack® is an electroconductive carbon black, in pellet form, having a pore volume of from about 300 ml/100 g to 520 ml/100 g (e.g., 310-345 ml/100 g, and 480-510 ml/100 g) with fines (<125 micron) of less than about 7%, a pH of 8-10, and an apparent bulk density of from about 100 to about 150 kg/m3 (e.g., 125-145 kg/m3, and 100-120 kg/m3). Black Pearls® engineered pigment black has an OAN of 65 cc/100 g, a 325 mesh residue of less than 200 ppm, and a density of 430 kg/m3. C55 carbon black consists of acetylene black carbon particles, 99.99% purity, having a surface area of 82 m2/g, and is commercially available under the trade designation “Shawinigan Black, Grade C55.” Norit® SX Ultra is an acid washed, steam activated carbon having a surface area (BET), of about 1200 m2/g, an apparent density, tamped, of 0.32 g/mL, a particle size distribution of d10 of 5 μm, d50 of 25 μm, d90 of 100 μm, and a pH of about 7. Printex® carbon black has a CTAB surface area of 600 m2/g, an OAN of 380 m/100 g, a COAN of 370 ml/100 g, a sieve residue, 325 mesh of 20 ppm, and a pour density of 130 g/dm3. Vulcan® is a conductive carbon black pellet or powder having an OAN of about 174 cc/100 g, surface area of 210 m2/g, 325 mesh residue of less than 25 ppm, and density of about 264 kg/m3.

Oxidized carbon supports (oxidized for example by HNO3/H2SO4) and other carbides, nitrides and silicides of metals, for example, titanium carbide (TiC), tungsten carbide (WC), titanium nickel carbide (TiN) and silicon carbide (SiC) can all be used as a carbon support in the present methods. The nano-carbon supports can include carbon nanotubes, carbon nanofibers, carbon nanowires, carbon nanohorns and carbon nanorings.

The transition metal plays an important role in the catalytic activity of the present catalysts. The transition metal, and hence the resulting substrate and catalyst containing the transition metal, is preferably substantially free of all precious metals, such as ruthenium, rhodium, palladium, osmium, iridium, platinum, gold, and silver. As used herein, the term substantially free of precious metals means that precious metals are not intended to be included with the transition metal, and the transition metal precursor is either free of precious metals, or the presence of precious metal is a negligible amount, for example, less than 0.1% by weight. Precious materials have high material costs, and are required in large amounts to achieve desirable operation voltages and currents. In comparison, examples of suitable transition metals for the transition metal target include iron, cobalt, nickel, chromium, cerium, zinc, zirconium, molybdenum and manganese. These suitable transition materials are less expensive than precious metals, thereby reducing material costs during manufacturing. In addition to the transition metals in ionic form, the present methods also contemplate the use of these transition metals in the form of transition metal macrocycles, transition metal salts and combinations thereof.

The transition metal macrocycle comprises a large, generally ring or crown-like molecule such as a phthalocyanine, having a metal atom retained in its central portion, generally by co-ordinatively bonding with one or more of nitrogen, oxygen and/or other atoms having an unshared pair of electrons, or delocalized electrons, as for example in a bond. Other examples of macrocycles include metallocenes, porphyrins, chlorophyll derivatives of imidazoles or pyrroles and the like. While a variety of transition metals may be employed in the practice of the present technology, some particularly preferred transition metals include iron, cobalt, nickel, chromium, cerium, zinc, zirconium, molybdenum and manganese.

Dispersion of the transition metal containing macrocycles may be accomplished by dissolving the macrocyclic compound in a solvent, dispersing the carbon support material into the solvent, and evaporating the solvent to provide a support material having the transition metal macrocyclic compound adsorbed onto the carbon support. In other embodiments, the adsorption may be accomplished by ball milling the materials together or by evaporating the macrocyclic compound onto the support substrate provided that the macrocyclic compound has sufficient volatility.

In some embodiments, the transition metal macrocycles can include one or more of transition metal organometallic derivative complexes. Such organometallic macrocycles can include for example cobalt pthalocyanine, iron pthalocyanine, iron and cobalt naphthalocyanine, cobalt tetraazannulene, iron tetramethoxy phenyl porpyrin chloride, tetracarboxylic cobalt, iron pthalocyanine, tetramethoxy phenyl porpyrin chloride, cobalt salen-N,N′-bissalicylidine, ethylenediaminocobalt, cobalt-anten-O-amino, ferrocene, benzaldehyde, dimethylglyoxime, ethylenediamino cobalt and iron phenanthroline.

In some embodiments, the transition metal precursor may be a transition metal salt. In various embodiments, the transition metal salt is a combination of a transition metal cation, for example: Fe2+, Fe3+, Co2+, Co3+, Cr2+, Cr3+, Cr6+, Mn2+, Mn3+, Mn4+, Mn7+, Zn2+, Ni2+ and Ni3+ paired with a common anion species, for example, acetate, formate, nitrate, chloride, sulfate, oxy-chloride and phosphate. In some embodiments, preferred transition metal salts can include, for example, ferrous and ferric salts with one or more of acetate, formate, chloride, sulfate, oxy-chloride, phosphate anions; cobaltous and cobaltic salts with one or more of chloride, acetate, nitrate, sulfate anions; chromium acetate; cerium acetate; zinc chloride and zirconium acetate.

As shown in FIG. 1, step 120 provides for the addition or blending of a nitrogen precursor compound with the metal precursor loaded carbon substrate to form a carbon-metal-nitrogen precursor. The amount of the nitrogen precursor added to the carbon-metal substrate prior to pyrolysis can vary depending on the specific application requiring the cathode catalysts of the present technology. In some embodiments, the metal precursor loaded carbon-metal substrate in the form of powder or granular particles can be admixed with the nitrogen precursor in any suitable mixing vessel. In some embodiments, the carbon-metal substrate and the nitrogen precursor can be mixed together in a mortar with a pestle.

Typically, for fuel cell cathode catalysis (both hydrogen and methanol fuel cells), a mixture of metal precursor loaded carbon substrate and a nitrogen precursor in a range of from about 0.1 to about 40 nominal weight % can be used to prepare the cathode catalysts of the present technology. In some embodiments, the carbon-metal-nitrogen precursor can contain an amount of nitrogen precursor (weight % nominal nitrogen) ranging from 0.1% to about 18%, or from about 1% to about 15%, or from about 1% to about 12%, or from about 1% to about 9%, or from about 1.5% to about 15%, or from about 6% to about 15%, or from about 9% to about 15%, or from about 10% to about 15%, or from about 12% to about 15%. In some embodiments the carbon-metal-nitrogen precursor can contain an amount of nitrogen precursor (weight % nominal nitrogen content) ranging from 1% to about 15%, more preferably from 1.5 to about 12% by weight.

In various embodiments of the present technology, a nitrogen containing precursor compound is added to the metal precursor loaded carbon substrate as shown in method step 120. Without wishing to be bound to any specific theory, it is believed that the after pyrolysis metal-NxCy type of catalytic sites are formed possessing high catalytic activity for oxygen reduction as well as enhanced resistance to methanol poisoning while reducing oxygen. (Gupta, S. et al., J. Appl. Electrochem. (1998), 28:673-682).

In some embodiments, the nitrogen precursor is one or more heterocyclic nitrogen containing organic aromatic compounds and polymers comprising heterocyclic nitrogen containing organic aromatic compounds, including, for example, porphryins, pyridines, pyrimidines, quinolines, aromatic amines and polymers of pyrrole and aniline.

In some embodiments, the nitrogen precursor can include one or more nitrogen containing precursor molecules, for example, porphryins, pyridines, pyrimidines, aromatic amines, amines, melamine, urea and urea derivatives, poly(quinoxaline), nitroaniline, 1,10 phenanthroline, pthalocyanine, pyridine, bipyridine, polyaniline, pyrrole, polyvinyl pyridine, Pyridine based ligands-1,6 bis(4′-pyridine)-2,5-diazahexane (BPDH), Bipyridine based ligands e.g. 4,4′ bipyridine, terpyridine ligands: 4′ phenyl 2,2′-6′,2′-terpyridine, 2-2″ bipyrimidine, 4-7 phenanthroline dipyrido[3,2,2′3′ phenazine], 3-nitrophalimide, p-phenylazophenol, 6-quionoline carboxylic acid, 6-nitrobenzimidazole, 5-amino 6-nitro quinoline, 2,3 naphthalocyanine, 4,4′-azoxydibenzoic acid, 2 amino 5-nitro pyrimidine, hematin, 4,4′ azo-bis[cyanovaleric acid], heamotoporpyrin dihydrochloride, 4,4′ nitrophenyl azo catechol 4,6 dihydroxy pyrimidine, nitrophenyl, benzylamine, 1,6 phenylendiamine, tetracyanoquinodimethane, propylene di-amine, ethylene diamine, urea, selenourea, thiourea, dimethylformamide, ammonia and acetonitrile.

It has been found that the nitrogen/carbon (N:C) ratio may demonstrate an important property of nitrogen precursors for metal-nitrogen-carbon catalysts. As described in more detail in Example 3, below, increasing the N:C ratio of the nitrogen precursor may increase the accessible active site density by reducing carbon deposition in the pores of the carbon support during pyrolysis. For example, carbon deposition from various organic precursors post pyrolysis may lead to pore blockages and decreased oxygen reduction activity. As such, in certain aspects of the present technology, the nitrogen precursor compound used with the present disclosure may have a N:C ratio of 1:1, or greater. In other aspects, the nitrogen precursor compound may have a N:C ratio of at least about 2:1, or greater.

In still other aspects, the nitrogen precursor compound may be provided such that it is free of carbon, or its decomposition components are substantially free of carbon. As used herein, the term substantially free of carbon means that once the nitrogen precursor is subject to pyrolysis, the transformed product does not end up containing anything more than a negligible amount of carbon containing components that could potentially form deposits or lead to pore blockage. In certain embodiments, the pyrolyzed nitrogen precursor may contain less than 3 percent by weight of carbon containing components based on the total weight of the nitrogen precursor; for example, less than about 1 percent, less than about 0.5 percent, or less than about 0.1 percent by weight. For example, the nitrogen precursor compound itself may be free of carbon, in the case of ammonium hydroxide, or the precursor compound may undergo a decomposition reaction during pyrolysis that forms ammonia as a nitrogen precursor, along with water or carbon dioxide as a by-product, such that the nitrogen precursor does not provide any additional carbon material. In certain instances, it should be understood that the decomposition reaction may not provide for 100% conversion of all of the carbon containing compounds, thus there may be trace amounts of organic carbon containing compounds in the nitrogen precursor component. Table 1, below, provides decomposition reactions for non-limiting exemplary nitrogen precursor compounds that are either initially free of carbon, or that decompose and release the carbon as gaseous carbon dioxide, leaving nitrogen containing compounds free of carbon, such as ammonia. Numerous ammonia generating nitrogen precursors are known to those skilled in the art, and it has been found that these precursors may assist in minimizing any unwanted carbon deposits. Additional non-limiting exemplary nitrogen precursor compounds may include amides; carbamates; carbodimides; and thiocarbamides; as well as various ammonium salts, including those of acetate, carbonate, bicarbonate, sulfate, chloride, bisulfate, iodide, and the like.

TABLE 1 Exemplary Carbon-free Nitrogen Precursors. Precursor Decomposition reactions Ammonium hydroxide NH4OH → NH3 + H2O Urea (+ water) NH2CONH2 → NH3 + HCNO HCNO + H2O → NH3 + CO2 Ammonium carbamate NH4COONH2 → 2NH3 +CO2

Once the carbon-metal-nitrogen precursor has been prepared, the next step in the synthesis of the present cathode catalysts is to pyrolyze the carbon-metal-nitrogen precursor in a closed vessel, in which reactions occur at its autogenic pressure. The autogenic pressure may be based on the nitrogen precursor evaporation, as the decomposition reactions typically increase nitrogen activity and mobility.

As shown in FIG. 1, method step 130, the carbon-metal-nitrogen precursor is placed in a pressure resistant vessel capable of sustaining the interior of the vessel with a reducing or neutral (inert) gaseous environment. Placing carbon-metal-nitrogen precursor in a vessel capable of withstanding both elevated temperatures and internal pressures, sealing of the vessel (with the precursor compounds inside) and heating of that vessel to elevated temperature, where the elevated internal pressure results from the existence of a gaseous phase for some or all of the resulting chemical constituents. While the inner vessel wall can in principle react to some extent (catalytically or non-catalytically) with the confined chemical species, and permit some degree of diffusion of atoms or molecules into the vessel from the interior of the vessel, the vessel must limit such processes to the extent of maintaining a substantial portion of the initial atoms in the vessel (as opposed to permitting substantial diffusion of atoms and/or molecules into and/or through the containment vessel or forming compounds with the inner vessel wall material and thus not being further available for reaction). The vessel materials, apart from the above general requirements, can in principle vary widely. A thick-walled quartz vessel was found to possess the necessary mechanical strength at high temperature and pressure, minimize chemical reactions with the reactants and minimize diffusion of the reactants into the vessel wall. However, other containment materials could be used for this purpose. In some embodiments, the vessel may be made from any industrial metal, for example heat and pressure resistant stainless steel. In some embodiments, the vessel can be made from quartz commonly used to digest or pyrolyze organic matter. Alternatively, any industrial vessel capable of passing a reducing or neutral gas into a chamber and capable of operating at an internal pressure of at least 2 bar can be used.

In method step 130, the carbon support is pyrolyzed (i.e., heated) at a temperature preferably in the range of from about 500° C. to about 1,200° C., and more preferably from about 600° C. to about 1,000° C. The pyrolysis step may be accomplished, for example, using a rotary kiln, a fluidized bed reactor, or a conventional furnace. The contents of the vessel can be then thermally treated by placing the vessel in a furnace or other heating apparatus capable of thermally treating the contents of the pressurized vessel to at least 1,000° C. This is accomplished by thermally treating the precursor material under elevated pressure, for example, pyrolizing the carbon-metal-nitrogen precursor in a vessel with an internal pressure of about 2 bar to about 100 bar.

A typical pyrolysis process 130 can employ a thermal treatment schedule, for example, the carbon-metal-nitrogen precursor material can be heated from a starting temperature of 5° C. over a period of 15 minutes to a temperature of 150° C. and held at that temperature for 20 minutes. Thereafter, the temperature can be raised over a period of 30 minutes to a pyrolysis temperature in the range of 600-900° C., and held at that temperature for approximately 30-360 minutes.

Thereafter, the pyrolyzed material is rapidly cooled to room temperature. The cooling can be facilitated by opening the furnace or microwave device while maintaining the flow of reducing gas over the material. The contemplated pyrolysis vessel enables the pyrolysis of the carbon-metal-nitrogen precursor to yield a carbon nano-structure, for example, porous carbon nanotubes containing disordered surfaces and coated with nitrogen precursor and the transition metal.

Other embodiments may require or preferentially use a more automated form of substrate pyrolysis under elevated pressure. For example, a continuous flow spray pyrolizer (SP) injects a spray of carbon-metal-nitrogen precursor into a connected furnace under elevated pressure. The droplets are atomized from the starting precursor solution with an atomizer and the droplets are then placed in a furnace. A variety of activities may occur inside the furnace during formation of the final product including evaporation of the solvent, diffusion of solutes, drying, precipitation, reaction between the precursor and surrounding gas, pyrolysis and sintering.

Once the carbon-metal-nitrogen precursor has been pyrolized in accordance with step 130, a carbon-metal-nitrogen catalyst, which can effectively reduce oxygen, is obtained. Although, the present catalyst will find primary use in low temperature fuel cells, as the cathode catalyst in membrane electrode assemblies for oxygen reduction reactions, the present cathode catalyst also finds utility in batteries and in electrochemical sensors.

Several electrochemical catalytic applications can be envisioned for the present cathode carbon-metal-nitrogen catalysts of the present technology. Returning back to FIG. 1, step 140, the catalyst powder or carbon-metal-nitrogen cathode catalyst is taken from the reaction vessel and can be admixed with an ionomeric substrate, for example, Nafion® (E. I. du Pont de Nemours, Wilmington, Del., USA) to form a catalytic ink. The catalytic ink can be applied to any variety of solid supports, including, for example, any well known cathode material used in fuel cell manufacture. In some embodiments, the catalytic ink comprising the catalyst powder or carbon-metal-nitrogen cathode catalyst is deposited on an electrolyte membrane to form a membrane electrode assembly for use in a hydrogen or methanol fuel cell.

As described above, the catalysts produced using the methods described herein have particular efficacy in polymer electrolyte fuel cells requiring oxygen reduction reactions to generate electric current. As such, the present methods can be employed to produce cathode catalysts that can be used in direct methanol fuel cells, conventional hydrogen fuel cells and other electrochemical applications requiring a oxygen reducing cathode catalyst. In some embodiments, these carbon-metal-nitrogen cathode catalysts find particular utility in membrane electrode assemblies that can be used in the aforementioned methanol and hydrogen fuel cells. Essentially, a membrane electrode fuel cell comprises an electrolyte membrane disposed between a pair catalyst layers, i.e. an anode and cathode catalyst layer. The respective sides of the electrolyte membrane are referred to as the anode surface and the cathode surface. In a typical proton exchange membrane fuel cell, (“PEM fuel cell”) hydrogen fuel is introduced into the anode portion where the hydrogen reacts and separates into protons and electrons. The electrolyte membrane transports the protons to the cathode portion, while allowing a current of electrons to flow through an external circuit to the cathode portion to provide power. Oxygen is introduced into the cathode portion and reacts with the protons and electrons to form water and heat. The reduction of the oxygen at the cathode is catalyzed by the catalysts produced by the methods described herein.

In one embodiment, ionomeric membrane is any commercially available electrolyte membrane, for example, Nafion® (poly (perfluorosulphonic acid), also commercially available as Aciplex® or Flemion®). Other ionomeric membrane materials known in the art, such as sulfonated styrene-ethylene-butylene-styrene; polystyrene-graft-poly(styrene sulfonic acid); poly(vinylidene fluoride)-graft-poly(styrene sulfonic acid); poly(arylene ether), such as poly(arylene ether ether ketone) and poly(arylene ether sulfone); polybenzimidazole; polyphosphazene, such as poly [(3-methylphenooxy) (phenoxy) phosphazene] and poly [bis(3-methylphenoxy) phosphazene]; and combinations thereof, may also be used. An anode catalyst comprises at least one metal. The at least one metal can include platinum, ruthenium, palladium, and combinations thereof, that are known and used in the art as fuel cell anode materials. The anode catalyst is typically deposited on ionomeric membrane by preparing a catalyst ink containing the at least one metal and applying the ink to one side of the ionomeric membrane. The anode catalyst can comprise a mixture of platinum and ruthenium, such as, for example, platinum-ruthenium black. The cathode catalyst of the present technology can similarly be applied to the other side of the ionomeric membrane. In some embodiments the cathode catalyst to be applied to the other side can include the oxygen reducing cathode catalyst of the present technology mixed with one or more recast ionomers. The recast ionomer can be an ionic conductor including, for example, poly(perfluorosulphonic acid), such as Nafion®, Aciplex®, or Flemion®; sulfonated styrene-ethylene-butylene-styrene; polystyrene-graft-poly(styrene sulfonic acid); poly(vinylidene fluoride)-graft-poly(styrene sulfonic acid); poly(arylene ether), such as poly(arylene ether ether ketone) and poly(arylene ether sulfone); polybenzimidazole; polyphosphazene, such as poly [(3-methylphenooxy) (phenoxy) phosphazene] and poly [bis(3-methylphenoxy) phosphazene]; and combinations thereof. In a preferred embodiment, the recast ionomer is Nafion®.

Yet, in other aspects of the present technology, the cathode catalyst may be coated on a gas diffusion media, or gas diffusion layer for use in an electrochemical cell. Thus, the present technology also relates to methods for making a cathode catalyst coated diffusion layer for a fuel cell. In various aspects, the method includes providing a gas diffusion layer, which may comprise a typical carbon fiber or carbon paper substrate as is generally known in the art to allow for gas and water transport. The method includes applying a cathode catalyst on at least a portion of the gas diffusion layer. As described in detail above, the cathode catalyst may be synthesized by: (i) mixing a carbon source with a transition metal precursor to form a metal precursor loaded carbon substrate, wherein the substrate is free of precious metals; (ii) adding a nitrogen precursor compound to the metal precursor loaded carbon substrate to form a carbon-metal-nitrogen precursor; and (iii) pyrolyzing the carbon-metal-nitrogen precursor at a pressure ranging from about 2 bar to about 100 bar, thereby forming an oxygen reducing cathode catalyst. The cathode catalyst may also be applied to the gas diffusion layer using a catalyst ink, as detailed in Example 3, below.

The following examples illustrate the various features and advantages of the technology and are not intended to limit the technology thereto. While the examples refer to Ketjenblack® 600JD, iron (II) acetate and 2,2′ bipyridine, etc., it is understood that these materials represent one embodiment and that other embodiments describing different carbon supports, transition metals and nitrogen precursors described herein can be used.

Nafion® solution (1100 EW, 5 wt. %) were purchased from Alfa Aesar, (Ward Hill, Mass., USA). A 5 mm glassy carbon rotating disk electrode (“RDE”) was purchased from Pine Instruments (Grove City, Pa., USA). Ketjenblack® 600JD (Akzo-Nobel Polymer Chemicals, Chicago, Ill., USA) (CAS No. 1333-86-4) is used as carbon support, which is dispersed in 95% ethanol. To this solution, Iron (II) acetate corresponding to 1 wt. % of iron on carbon is added and the slurry is kept stirring for about 6 hr. After the solvent is evaporated and a dry composite powder is obtained, 55 mg of the composite material thus obtained is ground with varying amounts of 2,2′ bipyridine ranging from 35 to 85 mg and the powder is subsequently charged into a stainless steel bomb that has a volume of 1.7 ml. The pyrolysis vessel (bomb) can be a closed vessel made from steel, ceramics or quartz. The material was charged into the bomb in an inert atmosphere. At around 273° C., bipyridine decomposes and increases the pressure inside the bomb and this in turn fixes nitrogen in the catalyst. Final chemical analyses of the resulting carbon-metal-nitrogen cathode catalysts having different quantities of pyridine nitrogen were determined using CHN analysis and provided in Table 2.

TABLE 2 Chemical data on the finalized carbon-metal-nitrogen cathode catalyst. Nominal Carbon, Hydrogen and Nitrogen Nitrogen content data content Final Weight (mg) derived from CHN analysis (%) Sample 1 Sample 2 C H N

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