Carbon Aerogel Supported Metal Catalysts and Solid Electrolyte Composite

[0001] This application claims the benefit of U.S. Provisional Application No.

60/670,214, filed on April 11, 2005, U.S. Provisional Application No.60/670,215, filed on April 11, 2005, and U.S. Provisional Application No. 60/670,216, filed on April 11, 2005, the contents of which are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

[0002] The present invention relates to carbon aerogel supported catalyst and solid electrolyte composites for use in electrochemical reactions.

BACKGROUND OF THE INVENTION

[0003] Aerogels are porous materials that are produced by polycondensation reactions known as the "sol-gel processes." A common feature among aerogels is their small inter-connected pores. The aerogel chemical composition, microstructure and physical properties can be controlled at the nanometer scale by sol-gel processing. There are three major types of aerogels; inorganic, organic, and carbon aerogels. Inorganic aerogels can be obtained by supercritical drying of highly cross-linked and transparent hydro-gels synthesized by polycondensation of metal alkoxides. Silica aerogels are the most well known inorganic aerogels. Organic aerogels can be synthesized by supercritical drying of the gels obtained by the sol-gel polycondensation reaction of monomers such as, for example, resorcinol and formaldehyde in aqueous solutions. Carbon aerogels can be obtained by pyrolizing the organic aerogels at elevated temperature. [0004] Additives can be incorporated into aerogels to enhance the properties of pure aerogels or to impart additional desirable properties depending on the application. For example, catalysts can be incorporated into the aerogel. In general, additives are typically incorporated into aerogels using two different methods. The first one involves

adding the additive to the sol prior to polymerization and the second method involves contacting the produced aerogel with a liquid or gaseous stream containing the additive. [0005] The catalytic activity of metal catalysts depends on the surface area of metal crystals accessible to the reactants. In general, there are two ways to increase the active metal surface area. One is to reduce the metal particle size and the other is to increase the loading amount of supported metal particles. Unfortunately, increasing the loading amount often results in agglomeration of metal particles, which reduces the active metal surface area. Thus, practically, the former method is the better solution, especially when expensive metal is employed since this method results in effective utilization of a given amount of metals, and accordingly a reduction in cost.

[0006] Theoretically, highly dispersed metal particles can be prepared as long as the nuclei formation rate is faster than the nuclei growth rate. In practice, however, the dispersion is dependant on the support surface area and once the process hits equilibrium, the metal crystals are prone to agglomeration. Synthesis of highly dispersed supported metal catalyst with uniform nanoparticle size still remains a challenge, especially for high metal loading.

[0007] Tremendous effort has been made to achieve highly dispersed metal particles. As stated previously, there are two general techniques for preparing carbon aerogel supported metal catalysts, including adding the metal precursor to the sol prior to polymerization and contacting the produced aerogel with a liquid or gaseous stream containing the metal. Adding the metal precursor to the sol prior to polymerization results in formation of large metallic particles due to high pyrolysis temperature during the carbonizing process. A carbon aerogel supported metal catalyst, therefore, is generally prepared using the second technique, for example, using an impregnation method or a colloidal method. The impregnation method usually generates large metal particles and

non-uniform nanoparticles when the metal loading is high. The colloidal method involves the formation of metal oxide colloid by the reaction of metal salt with a reducing agent, deposition of metal oxide onto a support, elimination of the ion by repeatedly rinsing and boiling in deionized (DI) water, drying, and reduction. The complexity of this method and the difficulty in obtaining metal particles smaller than 2 nm in diameter with high metal loading hinders its utilization. Other existing methods, including electrochemical deposition, incipient-wetness, chemical vapor deposition, and the like, also have difficulty controlling of the characteristics and dispersion of the metal particles. Frequently, inconsistent metal particle sizes and broad particle size distributions are formed. [0008] Efforts have been directed to the development of carbon aerogel supported catalysts for use in fuel cells, e.g., proton exchange membrane ("PEM") fuel cells, direct methanol fuel cells (DMFC), electrocapacitive deionization devices, and the like. [0009] The most common method for fabricating a PEM fuel cell and DMFC catalyst layer is to mix a platinum catalyst with a polymer electrolyte, for example, Nafϊon ^® ionomer in a solvent or dispersing agent, and apply this paste to the membrane. Ideally, all of the catalyst contained within a fuel cell would be active for the reactions. However, up to 90% of the catalyst atoms in such electrodes may be inactive. Catalysts, in general, and especially precious metal catalysts used in PEM fuel cells and DMFC, are expensive. Therefore, one of the challenges facing fuel cells, especially PEM fuel cells and DMFC, is to improve the utilization of catalyst within the catalyst layer which should ultimately allow a reduction in the catalyst loading at a given fuel cell performance. [0010] Increasing the amount of polymer electrolyte throughout the catalyst might increase the catalyst utilization by increasing the area of the so-called three-phase reaction zone. However, too much polymer content may restrict gas accessibility to the catalyst.

[0011] Other techniques have been investigated to load catalyst onto fuel cell supports. A pulse electrodeposition technique was developed, by which small catalyst particles were deposited in a preformed electrode. Other similar methods, such as electroreducing catalyst precursor solution onto glassy carbon substrate or at the carbon/NAFION ^® interface, have been developed. Another technique is to dip carbonized

polyacrylontrile foam into H ₂ PtCl ₆ solution and reduce the platinum ion to platinum using a NaBH ₄ chemical technique. Fuel cell performance obtained by brushing or by spraying a catalyst ink made by mixing catalyst particles with electrolyte solution to the diffusion layer have also been reported. Another technique involves sputter deposition of catalyst on the front surface of the electrode. However, all these methods face the same low catalyst utilization problem as in the case of conventional electrode fabrication. [0012] Therefore, there remains a need in the art for a carbon aerogel supported catalyst and electrolyte composite having high surface area and high catalyst utilization. In fact, there is a desire for a carbon aerogel supported catalyst and electrolyte composite having exceptionally high surface area and exceptionally high catalyst utilization.

SUMMARY OF THE INVENTION

[0013] In one embodiment of the present invention a carbon aerogel supported catalyst and electrolyte composite includes catalyst metal particles having an average metal particle size of 2.5 nm or less, more typically 2 nm or less, and most typically 1.5 nm or less, supported on a carbon aerogel. In one aspect, the weight of the catalyst metal particles in the composite is 20 wt% or more of the weight of the carbon aerogel supported catalyst (excluding the electrolyte weight). In another aspect, the catalyst utilization (i.e., percentage of metal particles that can be accessed by reactants for catalysis in electrochemical reactions) in the carbon aerogel supported catalyst and electrolyte composite may be as high as 80-100%. Suitable metals include iron, cobalt, magnesium,

molybdenum, nickel, titanium, tungsten, chromium, copper, platinum, osmium, gold, silver, rhodium, ruthenium, palladium, iridium, or the like, or combinations (including alloys) comprising at least one of the foregoing metals and the alloy thereof. Suitable electrolytes include polymer electrolytes including, as a non-limiting example, ion exchange resins.

[0014] In one embodiment of the present invention, there is provided a process of preparing a supported metal catalyst which comprises creating a solution of a selected metal precursor, such as a metal salt or complex, and a support, and reducing the metal precursor to a metallic state either by thermal reduction or hydrogen reduction or combination thereof at proper conditions. Preferably, the reduction temperature is not higher than 220 ⁰ C, more preferably not higher than 150 ⁰ C.

[0015] Further in accordance with one aspect of this invention, there is provided a process of preparing a supported metal catalyst which comprises contacting a support with a metal precursor dissolved in a supercritical fluid and reducing the metal precursor to a metallic state either by thermal reduction or hydrogen reduction at proper conditions. Preferably, the reduction temperature is not higher than 220 ⁰ C, more preferably not higher than 150 ⁰ C. Further in accordance with an aspect of the present invention, an electrolyte is deposited on the supported catalyst by dipping the supported catalyst in a solution within which a solid electrolyte is dispersed or dissolved until attaining the isothermal equilibrium of the electrolyte in the solution inside and outside of the pore structure of such supported catalyst.

[0016] Further in accordance with an embodiment of the present invention, an electric pulse may be applied to a carbon aerogel support, catalyst and solid electrolyte solution to create a carbon aerogel supported catalyst and electrolyte composite. In one aspect, the electric pulse is low voltage pulse. The low voltage pulse may assist in the

deposition of ionic conducting solid electrolyte to the internal surface of the carbon aerogel support thereby creating three phase contact. In another aspect, the pulsed electrical field may cycle between zero and a positive charge.

[0017] Further in accordance with another embodiment of the present invention, a static electricity field may be applied to a carbon aerogel support, catalyst and solid electrolyte solution to create a carbon aerogel supported catalyst and solid electrolyte composite. In one aspect, the static electricity field may be a high voltage field. The high voltage static electricity field may orient and/or force an ionic conducting portion of the electrolyte close to (thereby possibly facilitating binding with) the facades of metal catalyst atoms in the crystals.

[0018] The PEM fuel cell catalyst layer prepared by using such carbon aerogel supported catalyst and electrolyte composite made by applying an electric field has shown exceptional electrochemical surface area in the membrane electrode assembly (MEA).

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 is a graph of cyclic voltammetry measurements as described in

Example 7.

[0020] FIG. 2 is a graph of Tagel slopes as described in Example 9.

[0021] FIG. 3 is a graph of mass and specific activity of a CASPC catalyst as described in Example 9.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] Catalyst support structures may be synthesized using any of a variety of techniques. As used herein, the term "aerogel" is intended to indicate all aerogel forms including, but not limited to, particularly carbon aerogels, and xerogels (gels formed when hydro-gels are air dried as opposed to supercritically dried). The support employed is not especially restricted and a conventional inorganic oxide support such as silica or, for

example, a carbon support may be employed. Most typically, an aerogel such as carbon aerogel may be employed.

[0023] Aerogels are porous materials that are produced by polycondensation reactions known as the "sol-gel processes." A common feature among aerogels is then- small inter-connected pores. The aerogel chemical composition, microstructure and physical properties can be controlled at the nanometer scale due to sol-gel processing. The three major types of aerogels are inorganic, organic, and carbon aerogels. Inorganic aerogels can be obtained by supercritical drying of highly cross-linked and transparent hydro-gels synthesized by polycondensation of metal alkoxides. Silica aerogels are the most well known inorganic aerogels. Organic aerogels may be produced by the reaction of any one or a combination of various monomers in an appropriate ratio with formaldehyde, furfural, or the like in the presence of a catalyst via a polymerization reaction (e.g., a polycondensation reaction). The monomer(s) is preferably a polyhydroxybenzene compound, exemplary embodiments of which include, but are not limited to, resorcinol, phenol, catechol, chloroglucinal, combinations thereof, and the like. Reaction of such monomers with formaldehyde or furfural generally produces, for example, resorcmol-furfural, resorcinol-formaldehyde, phenol-resorcinol-formaldehyde, catechol-formaldehyde, chloroglucinol-formaldehyde, or the like.

[0024] In one exemplary polymerization reaction to form an organic aerogel the reactants (i.e., the monomers) are mixed with the catalyst to produce the aerogel in the form of a monolithic gel, which is then dried by solvent exchange and extraction. The resulting organic aerogel is then pyrolized in an inert atmosphere (e.g., nitrogen) to form the carbon aerogel. More specifically, the polymerization reaction is a sol-gel polymerization of multifunctional organic monomers in a solvent (e.g., water). The sol-

gel polymerization leads to the formation of highly cross-linked, transparent or translucent gels referred to hereinafter as "hydro-gel sols").

[0025] In a preferred sol-gel polymerization, one mole of resorcinol (1,3- dihydroxybenzene) condenses in the presence of a basic catalyst with two moles of formaldehyde. A mildly basic sol-gel catalyst (e.g., sodium carbonate) is preferred. Resorcinol is a trifunctional monomer capable of receiving formaldehyde molecules in the 2-, 4-, and/or 6-positions on its ring. The substituted resorcinol rings condense with each other to form nanometer-sized clusters in solution. Eventually, the clusters crosslink through their surface groups (e.g., -CH ₂ OH) to form the hydro-gel sol. [0026] The size of the clusters can be regulated by the concentration of the sol-gel catalyst in the resorcinol-formaldehyde (RF) mixture. More specifically, the mole ratio of resorcinol (R) to catalyst (C) (R/C) controls the surface area and electrochemical properties of the resulting gel. Preferably, the R/C ratio is about 50 to about 300. Other commonly referenced ratios include resorcinol (R) to formaldehyde (F) (BJF) and resorcinol (R) to water (W) (R/W). Typically, the R/F and R/W molar ratios are each about 0.01 to about 10.

[0027] The hydro-gel sol is then cured for a time and temperature sufficient to stabilize the aerogel structure and form a cured hydro-gel. Curing times range from about 2 hours to about 5 days or more. Curing temperatures range from about 25 degrees centigrade (C) to about 150 degrees C. Pressures greater than 1 atmosphere (atm) can be used to decrease the curing time. After curing, RF aerogels may be translucent and dark red or black in color or substantially transparent.

[0028] The next step in organic aerogel preparation is to dry the cured hydro-gel.

If the polymerization solvent is removed from the gel by simple evaporation, large capillary forces are exerted on the pores, thereby forming a collapsed structure, i.e., a

xerogel. In order to preserve the gel skeleton and minimize shrinkage, it is preferable to perform the drying step under supercritical conditions (described hereinafter). Other drying steps may also be conducted, if desired, usually before the supercritical extraction step. For example a solvent exchange may be conducted in which the cured hydro-gel is contacted with an exchange solvent, e.g., acetone, prior to subjecting the cured hydro-gel to supercritical extraction to form the dried aerogel.

[0029] Supercritical extraction may be performed with a supercritical fluid, such as liquid carbon dioxide. Also, as an alternative or in addition to the exchange step, surfactants may be used to remove water from the cured hydro-gel. The highly porous material obtained from this removal operation is the organic aerogel. By appropriate adjustment of drying conditions, a hybrid structure having characteristics of both a xerogel and an aerogel may be produced. For example, such a hybrid may be produced as a result of a partial evaporation of the gel solvent under conditions promoting xerogel formation followed by evaporation of the remaining solvent under conditions promoting aerogel formation. The resulting hybrid structure would then be dried under supercritical conditions and pyrolized. Preparation of other xerogel-aerogel hybrids may be produced by first evaporating under conditions promoting aerogel formation and completing the evaporation under xerogel-promoting conditions.

[0030] As used herein, a "supercritical fluid" (also referred to in the art as

"supercritical solution" or "supercritical solvent") is one in which the temperature and pressure of the fluid are greater than the respective critical temperature and pressure of the fluid. A supercritical condition for a particular fluid refers to a condition in which the temperature and pressure are both respectively greater than the critical temperature and critical pressure of the particular fluid. A "near-supercritical fluid" is one in which the reduced temperature (actual temperature measured in Kelvin divided by the critical

temperature of the solution (or solvent) measured in Kelvin) and reduced pressure (actual pressure divided by critical pressure of the fluid) of the fluid are both greater than 0.8 but the fluid is not a supercritical fluid. A near-supercritical condition for a particular fluid refers to a condition in which the reduced temperature and reduced pressure are both respectively greater 0.8 but the condition is not supercritical. Under ambient conditions, the fluid can be a gas or a liquid. The term fluid is also meant to include a mixture of two or more different individual fluid. As used herein, the term "supercritical fluid" and "supercritical conditions" are intended to include near-supercritical fluids and near- supercritical conditions respectively.

[0031] The temperature and pressure of the extraction process depend on the choice of supercritical fluid. Generally, the temperature is less than about 350 degrees C and often less than 100 degrees C, while the pressure is about 50 arm to about 500 arm. [0032] Suitable solvents for use as supercritical fluids include, as non-limiting examples, carbon dioxide, ethane, propane, butane, pentane, dimethyl ether, ethanol, water and mixtures thereof. Carbon dioxide is a preferred supercritical fluid for use in accordance with the present invention. For example, at 333 Kelvin (K) and 150 arm, the density Of CO ₂ is 0.60 grams per cubic centimeter (g/cm3); therefore, with respect to CO2, the reduced temperature is 1.09, the reduced pressure is 2.06, and the reduced density is 1.28. Carbon dioxide is a particularly good choice of supercritical fluid. Its critical temperature (31.1 degrees C) is close to ambient temperature and thus allows the use of moderate process temperatures (less than about 80 degrees C). The time required for supercritical drying depends on the thickness of the gel.

[0033] In cases where the cured hydro-gels are of sufficiently high density, such as greater than about 40 weight percent (wt %) solids, the pore network may have sufficient inherent strength to withstand the drying process without resort to supercritical drying

conditions. Thus, carbon dioxide may be bled from the vessel under non-supercritical conditions. Non-supercritical drying is particularly attractive because of its reduced processing time. To maximize cross-linking and further increase the density of the gels, the cured hydro-gel may be subjected to a cure cycle.

[0034] Following the solvent exchange/extraction step and any cure cycle, the organic aerogel may be pyrolized at elevated temperatures of about 400 degrees C to about 2,000 degrees C in an inert atmosphere of nitrogen, argon, neon, helium, or any combination of the foregoing gases to form a pyrolized aerogel, e.g., a carbon aerogel. The pyrolysis temperatures can alter the surface area and structure of the pyrolized aerogel. In particular, higher surface areas are achieved at lower temperatures. The resulting aerogels, independent of the procedure by which they are pyrolized, are black and not transparent due to the visible absorption properties of the carbon matrix. [0035] The aerogels of the present invention typically have a surface area of from about 400 to about 2,000 meters squared per gram ("m2/g"), a pore volume of about 0.5 cubic centimeters per gram ("cm ³ /g")to about 10 cm3/g, and a density of about 0.01 g/cm3 to about 2.0 g/cm3. Such properties can be readily determined by those skilled in the art. For example, surface area and pore volume can be determined by the BET method (the Brunauer, Emmett, and Teller method), and density can be determined by using a pycnometer.

[0036] Any of a variety of carbon aerogels may be used for making a carbon aerogel supported catalyst and solid electrolyte composite. Carbon aerogels having high surface areas and relatively large pore sizes may be preferred for this application. By way of a non-limiting example, it may be advantageous to use a carbon aerogel having a surface area larger than 600 m ² /g and an average pore size larger than 15 nm, which is approximately related to the electrolyte cluster size.

[0037] The catalyst used for making carbon aerogel supported catalyst and solid electrolyte composite may be employed without limitation. In one embodiment, the catalyst is a catalytically active metal, although it is not limited to any particular metals. Suitable metals include iron, cobalt, magnesium, molybdenum, nickel, titanium, tungsten, chromium, copper, platinum, osmium, gold, silver, rhodium, ruthenium, palladium, iridium, or the like, or combinations (including alloys) including at least one of the foregoing metals. For PEM fuel cells, as a non-limiting example, the catalyst may be a platinum/platinum alloy catalyst.

[0038] A metal precursor may be employed to load the aerogel support structure with catalyst. Metal precursors may include, but are not limited to, metal salts and metal complexes. Examples of useful metal complex compounds include beta-diketonates (e.g., Cu(hfac)2 or Pd(hfac) ₂ , where "hfac" is an abbreviation for 1,1,1, 5,5, 5- hexafluoroacetylacetonate), acetoacetonates, tetramethylethyl diamines, beta-ketoiminates, dialkylmides, alkyls (e.g., Zn(ethyl) ₂ or dimethyl(cyclooctadiene)platinum(II) ("CODPtMe ₂ "), allyls (e.g. bis(allyl)zinc or W(π5 -ally I) ₄ ), dienes (e.g., CODPtMe ₂ ), or metallocenes (e.g., Ti(π5-C ₅ H ₅ ) ₂ or Ni(π5-C ₅ H ₅ ) ₂ ). Specific organometallic compounds include dimethyl(cyclooctadiene)platinum(II), tetraamine platinum (II) chloride, platinum(II)hexafluoroacetylacetone, (trimethyl) methylcyclopentadienylplatinum^V), bis(cyclopentadienyl)ruthenium, bis(ethylcyclopentadienyl)ruthenium(II), bis(pentamethylcyclodienyl)ruthenium, (methylcyclopentadienyl)( 1 ,5- cyclooctadiene)iridium(I), and combinations comprising at least one of the foregoing organometallic compounds. Other useful organometallic compounds may be readily determined by those skilled in the art. In one specific embodiment, the organometallic compound may be CODPtMe ₂ , which has a thermal decomposition temperature of about 150°C. One of the advantages of low thermal decomposition temperature is that it is

possible to get small crystal size, because metal crystals tend to sinter at high temperatures during the process of thermal reduction.

[0039] In one embodiment of the present invention, a carbon aerogel supported catalyst is provided. Any of various methods for forming the carbon aerogel supported catalyst, such as a colloid method, may be used. As a non-limiting example, a process of preparing a carbon aerogel supported metal catalyst may include creating a solution of a selected metal precursor, such as metal salt or metal complex, and a carbon aerogel, and reducing the metal precursor to a metallic state either by thermal reduction or hydrogen reduction or combination thereof at proper conditions. Specifically, a process of preparing a carbon aerogel supported metal catalyst may include contacting a carbon aerogel with a metal precursor dissolved in a supercritical fluid, and then reducing the metal precursor to a metallic state either by thermal reduction or hydrogen reduction at proper conditions. [0040] Furthermore, it is not necessary to include a reaction reagent to promote the deposition of the metallic compound onto the surface of the support, such as is required in chemical vapor deposition or chemical fluid deposition processes, e.g., H ₂ , H ₂ S, O ₂ or N ₂ O. Preferably, the metallic compound is deposited in the substantial absence a reaction reagent (of the metallic compound). Preferably, the supercritical fluid containing the metallic compound comprises less than about 5 wt. %, more preferably less than about lwt. %, and most preferably less than about 0. 1 wt. % of a reaction reagent, based on the total weight of the supercritical fluid, reaction reagent and metallic compound. Preferably, there is no chemical change to the metallic compound during the supercritical deposition of the metallic compound onto the support. As described hereinafter, when a chemical change is desired, e.g., reduction with hydrogen, it is not conducted until after the metallic compound is deposited onto the support.

[0041] In one embodiment of the present invention, the reduction temperature is not higher than 220 ⁰ C ₅ and more preferably is not higher than 150 ⁰ C. [0042] In one aspect, the method for preparing a carbon aerogel supported catalyst avoids the use of protecting agents such as surfactants, polymers or organic ligands. The use of protecting agents typically leads to poor activity, due to the occupation of catalytic sites by those agents and several conditions to remove those agents. [0043] In accordance with one embodiment of the present invention, the metallic particles have an average particle size of about 2.5 nm or less, preferably about 1 nm to about 2 nm, and more preferably about 1 nm. As used herein, the term "average particle size" is intended to indicate the average diameter (also referred to in the art as "effective diameter"). A preferred technique for measuring the average particle size is to measure the diameter of a representative number of particles from an electron micrograph, e.g., from a transmission electron microscope ("TEM") and calculate an average. Another method is hydrogen or carbon dioxide chemisorption where the total metal surface area is measured. This information can then be used to calculate the average particle diameter. [0044] Moreover, the metallic particles of the catalyst obtained may have a very narrow particle size distribution. For example, when the metallic particles have an average particle size of about 3 nm, less than about 20% of the metallic particles have a particle size of about 4 nm or greater and less than about 20% of the metallic particles have a particle size of about 2 nm or less. When the metallic particles have an average particle size of about 2 nm, less than about 20% of the metallic particles have a particle size of about 3 nm or greater and less than about 20% of the metallic particles have a particle size of about 1 nm or less. Preferably, less than about 20% of the metallic particles have a particle size of about 3 nm or greater and less than about 20% of the particles have a particle size of less than about 1 nm. More preferably, when the metallic

particles have an average particle size of about 1 nm, less than about 20% of the metallic particles have a particle size of about 2 nm or greater and less than about 20% of the metallic particles have a particle size of less than about 1 nm, based on the number of metallic particles. The particle size distribution can readily be determined by generating a histogram of the particle sizes from TEM micrographs.

[0045] In one aspect of making an aerogel supported metal catalyst, the manufacturing process comprises contacting an aerogel with a supercritical fluid comprising a metallic compound. The concentration of the metallic compound should be sufficient to provide the desired amount of the metallic particle dispersed within the aerogel.

[0046] In one preferred aspect of the invention, there is provided a process for making an aerogel based catalyst comprising polymerizing at least two monomers in a liquid medium to form a polymerization product comprising a hydro-gel sol and the liquid medium, curing the hydro-gel sol to form a cured hydro-gel, removing at least a portion of the liquid medium from the cured hydro-gel to form an organic aerogel, pyrolizing the organic aerogel to form a pyrolized aerogel, and contacting the pyrolized aerogel with a supercritical fluid comprising a metallic compound to form an aerogel based metal catalyst. The as-produced form of the supported catalyst is not critical to the present invention. The various forms include particles, pellets, films, coatings, fibers, and the like. [0047] Likewise, the compositions of the present invention can have a variety of end uses such as, for example, use in fuel cell electrodes, use as catalysts for chemical reactions, e.g., hydrogenation or dehydrogenation, oxidation, isomerization, reforming, hydrocracking, polymerization, etc. Use of the compositions of the present invention as fuel cell electrodes, e.g., PEM electrodes, is especially preferred, as is described below.

[0048] The invention is hereafter described with respect to the following examples, which are not intended to limit the scope of the claims. Example 1: Synthesis of RF-aerogel

[0049] RF aerogels were synthesized by the reaction of resorcinol with formaldehyde. For each run, 7.76 g of resorcinol was dissolved in a sodium carbonate solution (37 milligrams (mg) of sodium carbonate in 89.05 g of de-ionized water). Subsequently, 11.45 g of formaldehyde was added into the solution. The solution was then sealed in a glass vessel and kept one day at room temperature, one day at 50 degrees C in an oven, and three days at 87 degrees C in an oven. At the end of the first day (at room temperature), the solution gelled and had a yellow-orange color. The gel progressively became darker during the curing period in the oven and ultimately achieved a dark red color. At the end of the three day 87 degrees C period, the resulting gelled monolith was taken out and immersed in acetone for 5 days. Each day the old acetone was replaced with fresh acetone.

[0050] Acetone in the aerogel was supercritically extracted from the monolith with carbon dioxide by using a modified ISCO supercritical fluid extractor (SFX 220, internal volume of 2 X 50 cc). The monolith was placed in an extraction chamber, and the chamber was filled with acetone. The chamber was then heated to 46 degrees C. Subsequently, an extraction program (6 liters of CO ₂ at 3000 psi) was set up to conduct extraction. At the end of the extraction, the chamber was slowly depressurized at 46 degrees C. Once the depressurization was complete, the chamber was opened and the monolith removed as RF aerogel. Its weight and dimensions were recorded. Example 2: Synthesis of carbon aerogel

[0051] A carbon aerogel was synthesized by the pyrolysis of an RF aerogel in a tube furnace (Mellen, MTl 3 -3X12- IZ) under an inert nitrogen atmosphere. The RF

aerogel was placed in a pure alumina tube, which was placed in the furnace. One end of the tube was connected to a nitrogen cylinder and the other end was connected to the outlet of a fume hood. Nitrogen was flowed into the tube at 100 ml/min. and monitored using a flowmeter. The pyrolysis temperature profile was edited to process the pyrolysis. Specifically, the carbon aerogel was heated in the furnace at a rate of approximately 15 centigrade degrees per minute to result in a final temperature of 1,000 degrees C, and the temperature of the furnace was maintained at 1,000 degrees C for an additional six hours. The furnace was then cooled to room temperature with flowing nitrogen. The material removed as carbon aerogel from the tube was black in color and its weight and dimensions were recorded.

Example 3: Deposition of platinum precursor onto carbon aerogel [0052] The platinum was first supercritically deposited onto the carbon aerogel in the form of a platinum precursor, dimethyl(cyclooctadiene)platinum (II) using supercritical deposition. For 20 wt. % Pt catalyst, 7.36 g of carbon aerogel and 3.46 g of platinum precursor were placed in the vessel together with a stir bar. The vessel was sealed and placed on a magnetic stirrer. A selected amount of carbon dioxide was charged to vessel using a syringe pump (ISCO, 260D). The vessel was then heated to 80 degrees C and adjusted carbon dioxide to 4,000 psig. The conditions were maintained for 24 hours with stirring. The temperature of the vessel was then reduced to 60 degrees C and carbon dioxide was vented out. The actual deposition amount was determined based on the weight difference before and after deposition.

[0053] For 30 wt. % Pt catalyst, the deposition procedure was the same as that described above, except the amount of carbon aerogel and platinum precursor. Carbon aerogel in the amount of 7.63 g as well as 6.14 g of platinum precursor were placed in the vessel and the deposited platinum content was found to be 30 wt. %.

Example 4: Thermal reduction of carbon aerogel supported catalyst precursor [0054] The formed 20wt % Pt carbon aerogel supported catalyst precursor was subsequently placed in a furnace tube. One end of the tube was connected to a nitrogen cylinder and the other end was the outlet to the fume hood. The flow rate of the nitrogen was controlled using a flow meter, which was adjusted to 200 ml/min. The temperature profile was edited to process the reduction. Specifically, the heating rate was approximately 6 centigrade degrees per minute, and the final temperature was 220 degrees C. The furnace was kept at this temperature for another 15 minutes. The oven was then cooled to room temperature with flowing nitrogen. The material was removed from the tube as carbon aerogel supported platinum catalysts.

Comparative Example 5 : Thermal reduction of carbon aerogel supported catalyst precursor [0055] The formed 20wt % R carbon aerogel supported catalyst precursor was reduced as the method described in Example 4 except the final temperature was 300 degree C and the furnace was kept at this temperature for 120 minutes. Example 6: Hydrogen reduction of carbon aerogel supported catalyst precursor [0056] The formed 30 wt% R carbon aerogel supported catalyst precursor was placed in a furnace tube. One end of the tube was connected to a hydrogen cylinder or hydrogen/nitrogen mixture and the other end was the outlet to the fume hood. The flow rate of the gas was controlled using a flow meter, which was adjusted to 200 ml/min. The temperature profile was edited to process the reduction. Specifically, the heating rate was approximately 5 centigrade degrees per minute, and the final temperature was 100 degrees C. The furnace was kept at this temperature for another 60 minutes. The oven was then cooled to room temperature. The material was removed from the tube as carbon aerogel supported platinum catalysts.

Comparative Example 7: Thermal reduction of carbon aerogel supported catalyst precursor [0057] The formed 30wt % Pt carbon aerogel supported catalyst precursor was reduced as the method described in Example 4.

Example 8: Effect of Reduction Conditions on Platinum Nanoparticle Size [0058] The chemisorption measurements were carried out with a Sorptomatic

1990 instrument. The chemical surface area of carbon aerogel supported catalyst and the Pt particle size were determined by the standard hydrogen or carbon monoxide chemisorption measurements. Typically, the sample was pretreated with hydrogen at 100 degrees C for 1 hour, and then degassed at 100 degrees C for 1 hour. The extract amount of sample was determined based on the weight difference between the empty outgassed sample burette and the sample burette containing outgassed sample. Subsequently, the sample was subjected to chemisorption with either hydrogen or carbon monoxide. [0059] The Pt particle size was further confirmed with transmission electron microscope (TEM) analysis. The samples for TEM were prepared by crushing gently with a mortar and pestle and were then suspended in a volatile solvent. A drop of the resulting suspension was deposited onto a carbon coated TEM grid. Table 1, below, shows the

effect of reduction on Pt particle size and surface area.

[0060] In a further embodiment of the present invention, the aerogel supported catalyst, as described above, is combined with an electrolyte. Although, the electrolyte may be any suitable electrolyte, in a preferred embodiment the electrolytes include solid electrolytes or polymer electrolytes, including, for example, ion exchange resins. These resins include ionic groups in their polymeric structure, one ionic component of which is fixed or retained by the polymeric matrix and at least one other ionic component being a mobile replaceable ion electrostatically associated with the fixed component. The ability of the mobile ion to be replaced under appropriate conditions with other ions imparts ion exchange characteristics to these materials. The ion exchange resins can be prepared by polymerizing a mixture of components, one of which contains an ionic constituent. One broad class of cation exchange, proton conductive resins is the so-called sulfonic acid cation exchange resin having hydrated sulfonic acid radicals attached to the polymer backbone by sulfonation. A specific type is a perfluorinated sulfonic acid polymer electrolyte, an example of which is a commercial sulfonated perfluorocarbon, in the form of a 5 wt.% solution in water sold by EJ. Dupont de Nemours & Co. under the trade designation NAFION ^® .

[0061] The amount of electrolyte associated with the catalyst will depend on a number of factors including the type of electrolyte, the nature of the catalyst, and other considerations. The percentage of electrolyte in the catalyst layer may be optimized so that the electrolyte amount is high enough to form a three-phase zone effectively. In one embodiment, the weight of electrolyte in the composite solid is at least 5 wt% of the total weight of the composite.

[0062] In one embodiment, a static electricity field is applied to the mixture of supported catalyst and the electrolyte solution. Prior to applying the static electricity field

to the mixture, the supported catalyst was loaded with electrolyte. This can be done by dipping the supported catalyst into the electrolyte solution or any other suitable methods. It is preferable that the electrolyte penetrates into the catalyst support microstructures, which can be done, for example, by soaking for a long time and/or by sonication, or any other suitable methods. Thus, catalyst particles residing within the pores of an aerogel support may be in contact with solid electrolyte that has also penetrated the pores of the aerogel.

[0063] The particular apparatus for applying static electricity to the catalyst solution is not especially restricted. Any suitable design can be used without limitation. For example, a container of dielectric nature, such as glass or plastic, may be filled with electrolyte solution. Subsequently, supported catalyst may be placed in the solution and subject to agitation. A probe with positive charge may be placed in contact with the conductive catalyst, and the outside of the dielectric container may be charged negatively to form a static electrical field.

[0064] The static electrical field may be applied after a certain amount of electrolyte is deposited into catalyst supports' microstructure, but before drying out the solvent, i.e. such as de-ionized water and iso-propanol in the case where NAFION ^® is used as the electrolyte. The optimum time period and voltage of the static electrical field to which the catalyst and electrolyte solution mixture are exposed depend on a number of factors including the type of electrolyte, the nature of the catalyst, and like considerations. [0065] In one embodiment, after applying the static electrical field across the catalyst and electrolyte solution mixture, the solvent may be removed, optionally first with an absorbent product, followed by evaporative drying (e.g., in a vacuum oven). In a preferred aspect, drying temperatures are kept below the electrolyte decomposition temperature. In another preferred aspect, drying temperatures are as high as or higher than

any subsequent hot press temperature. Preferably, the temperature should not result in decomposition of the catalyst or electrolyte. Suitable drying times may vary greatly, which again depends on a number of factors including the type of electrolyte, solvent, the nature of the catalyst, and like considerations.

[0066] Additional electrolyte may be added after drying to optimize its content.

The resulting material may be further subjected to grinding to microsize. This may be desirable in the case of PEM fuel cell and DMFC applications. The apparatus for the microsizing step is not critical to the present invention, but the resulting material should retain its original microstructures.

[0067] The electrode assembly may be formed by those skilled in the art. For example, the membrane electrode assembly (MEA) in PEM fuel cells and DMFC are preferably formed by depositing catalyst ink onto electrolyte membrane by means of printing, pasting, spraying, dipping, and the like. The thickness to which the catalyst ink is deposited is preferably less than 100 micrometers, more preferably less than 10 micrometers and even more preferably less than about 5 micrometers. [0068] The catalyst utilization measurement is not restricted in the present invention. In the case of PEM fuel cells, the chemical surface area (CSA) of the fresh catalyst (prior to dipping into the electrolyte solution) may be obtained first by means of those skilled in the art such as by a chemisorption approach. The available active surface area for electrochemical reactions (also termed as electrochemical surface area, ESA) may be measured by those skilled in the art, for example by an in-situ cyclic voltammetry technique. The catalyst utilization may be calculated by the percentage of the ESA with respect to the chemical surface area of the catalyst.

[0069] The following examples describe the synthesis of a carbon aerogel supported platinum catalyst (CASPC) and solid electrolyte composite.

Example 9: Synthesis of carbon support

[0070] Carbon supports were synthesized by the reaction of resorcinol with formaldehyde. For each run, 7.76 g of resorcinol was dissolved in a sodium carbonate solution (37 milligrams (mg) of sodium carbonate in 89.05 g of de-ionized water). Subsequently, 11.45 g of formaldehyde was added into the solution. The solution was then sealed in a glass vessel and kept one day at room temperature, one day at 50 degrees C in an oven, and three days at 87 degrees C in an oven. At the end of the first day (at room temperature), the solution gelled and had a yellow-orange color. The gel progressively became darker during the curing period in the oven and ultimately achieved a dark red color. At the end of the three day 87 degrees C period, the resulting gelled monolith was taken out and immersed in acetone for 5 days. Each day the old acetone was replaced with fresh acetone.

[0071] Acetone in the resulting material was supercritically extracted from the monolith with carbon dioxide by using a modified ISCO supercritical fluid extractor (SFX 220, internal volume of 2 X 50 cc). The monolith was placed in an extraction chamber, and the chamber was filled with acetone. The chamber was then heated to 46 degrees C. Subsequently, an extraction program (6 liters of CO ₂ at 3000 psi) was set up to conduct extraction. At the end of the extraction, the chamber was slowly depressurized at 46 degrees C. Once the depressurization was complete, the chamber was opened and its weight and dimensions were recorded.

[0072] Subsequently, the resulting material was subject to pyrolysis in a tube furnace (Mellen, MT13-3X12-1Z) under an inert nitrogen atmosphere. The material was placed in a pure alumina tube, which was placed in the furnace. One end of the tube was connected to a nitrogen cylinder and the other end was connected to the outlet of a fume hood. Nitrogen was flowed into the tube at 100 ml/min. and monitored using a flowmeter.

The pyrolysis temperature profile was edited to process the pyrolysis. Specifically, it was heated in the furnace at a rate of approximately 15 centigrade degrees per minute to result in a final temperature of 1,000 degrees C, and the temperature of the furnace was maintained at 1,000 degrees C for an additional six hours. The furnace was then cooled to room temperature with flowing nitrogen. The material removed from the tube was black in color and its weight and dimensions were recorded. Example 10: Preparation of platinum catalyst supported onto carbon [0073] Platinum was supercritically deposited onto the carbon in the form of a platinum precursor, dimethyl(cyclooctadiene)platinum (II) using supercritical deposition. For 30 wt. % Pt catalyst, 7.63 g of carbon aerogel and 6.14 g of platinum precursor were placed in the vessel together with a stir bar. The vessel was sealed and placed on a magnetic stirrer. A selected amount of carbon dioxide was charged to vessel using a syringe pump (ISCO, 260D). The vessel was then heated to 80 degrees C and adjusted carbon dioxide to 4,000 psig. The conditions were maintained for 24 hours with stirring. The temperature of the vessel was then reduced to 60 degrees C and the carbon dioxide was vented out. The actual deposition amount of the platinum was determined based on the weight difference before and after deposition.

[0074] The formed 30 wt% Pt carbon supported catalyst precursor was placed in a furnace tube. One end of the tube was connected to a hydrogen cylinder or hydrogen/nitrogen mixture and the other end was connected to the outlet to the fume hood. The flow rate of the gas was controlled using a flow meter, which was adjusted to 200 ml/min. The temperature profile was edited to process the reduction. Specifically, the heating rate was approximately 5 centigrade degrees per minute, and the final temperature was 100 degrees C. The furnace was kept at this temperature for another 60 minutes. The

oven was then cooled to room temperature. The material was removed from the tube as carbon supported platinum catalysts. Example 11 : Chemical surface area measurement

[0075] Chemisorption measurements were carried out with a Sorptomatic 1990 instrument. The chemical surface area of carbon supported platinum catalyst was determined by the standard hydrogen chemisorption measurements. Typically, the sample was pretreated with hydrogen at 100 degrees C for 1 hour, and then degassed at 100 degrees C for 1 hour. The extract amount of sample was determined based on the weight difference between the empty outgassed sample burette and the sample burette containing outgassed sample. Subsequently, the sample was subjected to chemisorption with hydrogen. The result shows that the chemical surface area (CSA) of prepared 30 wt% Pt catalyst is 170 m ² /g Pt.

Example 12: Impregnation of electrolyte into carbon supported catalyst [0076] Five (5) grams of the carbon supported catalyst were placed in a glass jar

containing 1% NAFION ^® solution, which was made by dilution of 5% NAFION" solution

purchased from DuPont. The mixture was sonicated for 15 minutes and was kept at room temperature for 2 to 5 days. The weight increase of the catalyst solid measured after drying the catalyst solid was generally consistent with the pore void and the concentration of NAFION ^® in the solution that occupied the pore volume. In other words, the pore volume of 90% catalyst had about 5 to 9 wt% NAFION ^® against carbon weight. The proportion of NAFION ^® changes according to additional platinum and/or other metal loading weight. Similarly, increasing the concentration of NAFION ^® seems to increase the NAFION ^® weight deposited. With higher concentrations, a film-like micelle may develop on the surface of the carbon aerogel monolith, and the weight increase may not be linear.

Example 13: Application of static electrical field in the catalyst-electrolyte composite [0077] Subsequently, a stand steel probe that would be charged positively was placed into the solution mixture and contacted with solid catalyst. The outside of the glass jar was wrapped by aluminum foil and a needle probe was attached with the aluminum foil, which would be charged negatively. A static electrical field was then applied at a voltage of 10,000V and kept for a period of 30 minutes. Example 14: Preparation of membrane electrode assembly

[0078] The resulting catalyst-electrolyte composite was then taken out and dried under air at room temperature. The amount of electrolyte deposition was then calculated by the weight difference before and after electrolyte deposition. Additional 5% NAFION ^® solution was added to the solid in order to prepare 35 wt % NAFION ^® with respect to the catalyst-electrolyte composite. The mixture was then homogenized with a ball mill and a homogenizer. The catalyst ink solution was then dried and the final viscosity was further adjusted by addition of propanediol. The ink was screen-printed using the Systematic Automation Model 810 Series Screen Printer. To form MEA, the appropriate decals were- placed on the sides of a NAFION ^® 112 membrane. After drying and hot pressing at 70 kg/cm ² for 5 min. with the NAFION ^® 112 membrane, the Teflon films were peeled off from the cathode and anode sides of the MEAs. Further experiments were employed in the ElectroChem Inc. hardware with 6.25 cm ² active areas. The catalyst loading was found to be 0.1 mg Pt/ cm ² .

Example 15; Cyclic voltammetry measurement

[0079] Platinum surface areas in contact with NAFION ^® and available for the electrochemical reactions were determined using cyclic voltammetry (CV) technique in four-point probe configuration. In the measurements of CV curves, extra-pure nitrogen was supplied to the cathode side as a working electrode and hydrogen to the anode as a

counter electrode at 200 cc/min flow rate. The CV curves were obtained in the range of

0.01-0.8 V and a scan rate of 20 mV/sec at room temperature using Princeton potentiostat

Model 273. The ESA was found to be 300 nϊVgPt.

Example 16; Calculation of catalyst utilization

[0080] The catalyst utilization was calculated by the following formula:

Catalyst utilization % = ESA/CSA * 100

It was found that the catalyst utilization by this method was very high (176%), which is believed to be due to very high contact between catalyst and NAFI ON ^® electrolyte. Example 17: PEM fuel cell performance tests

[0081] The fuel cell performance was evaluated using a Teledyne test station, which is equipped with humidification chambers, mass flow and temperature controllers, and a 50 Amp load box from Scribner. In order to avoid condensation of water inside the cell, the line temperatures from the cathode and anode side were always 1O ⁰ C higher than the cell temperature. The test conditions were:

Flow rate: Anode 288 cc/min; Cathode 866 cc/min

Cell size: 6.5 cm2

Cell Temperature: 80 degree C with 100% humidity (90 degrees at port)

Operating pressure: 1 Bar at Gauge, (2 Bar abs.)

NAFION ^® 112 membrane

Anode: 0.2mgPt/cm2 TKK Pt/Ru catalyst

Cathode: 0. lmg Pt/cm2 CASPC 30wt% Pt on 22nm pore size carbon aerogel Example 18: Capacitive pulse deposition of solid electrolyte into the catalyst [0082] In an alternative embodiment, while depositing NAFION ^® solid electrolyte into the carbon aerogel supported catalyst by soaking, the carbon aerogel monolithic support can be pulse charged positively by a capacitive electrolyte deposition method.

This capacitive pulsing may be used to agitate the electrolyte solution. In a preferred embodiment, capacitive pulsing may occur before application of a static electrical field as described above.

[0083] For this purpose, and as a non-limiting example, a small pulse circuit was built. The pulse circuit received an input of steady DC voltage and used an asynchronous multi-vibrator to drive a MOSFET switch off and on. The switch is on the low side of the load (electrodes in solution) and causes the potential between the electrodes to pulse. The pulse was a square wave with a frequency of roughly 50Hz and a positive duty cycle of 50% (the duty cycle can be varied if necessary). The rise and fall time of the pulse was in microseconds. The amplitude of the pulse was adjustable using the lab power supply up to 30 VDC. The electric field, in one embodiment, is pulsed between zero and a positive value. As a non-limiting example, a pulse amplitude varying from zero to 5 or 6 volts could be employed. The pulse circuit was rated for the voltage current available from the power supply. Other pulse circuits having other potentials, wave shapes, periods, duty cycles, amplitudes, and rise and fall times may be suitable.

[0084] The positive electrode may be applied to the solid electron conducting carbon aerogel support. The negative electrode may be placed either inside the solution, but not contacting the carbon aerogel support, or alternatively, placed on the aluminum foil wrapped over the outside of non-conductive container, such as glass jar. [0085] Although this invention has been shown and described with respect to the detailed embodiments thereof, it will be understood by those of skill in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In particular, although examples of the invention were described above, these examples are not to be construed as restricting the scope of the invention. In addition, modifications may be made to adapt a particular

situation or material to the teaching of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed in the above detailed description, but that the invention will include all embodiments falling within the scope of the appended claims.