Photocatalytic methods for preparation of electrocatalyst materials

Statement of Government Involvement The present invention is based on CRADA (SC04 / 01686) between Toyota Technical Center, USA Inc. and Sandia National Laboratories working for the United States Department of Energy. The government reserves certain rights in the present invention.

References to Related Applications This application claims priority to US Patent Provisional Application No. 60 / 643,283 filed on January 12, 2005, the entire contents of which are incorporated herein by reference.

The present invention relates to a catalyst, specifically an electrode catalyst containing metal particles carried on a conductive supporting material.

Extensive research has been conducted on fuel cells for a huge number of applications, including those for automobiles. An important component of a fuel cell is the electrode catalyst, whose properties will affect both the performance and cost of the fuel cell. A typical electrode catalyst is formed from platinum particles on a carbon carrier. There is often the problem that platinum particles on the carbon carrier agglomerate under fuel cell conditions, which reduces the efficiency of the electrode catalyst and results in more platinum than would otherwise be required. You will need it. The utilization of the catalyst is also reduced by the presence of large particles. Its cost has risen significantly due to the tremendous demand for platinum. If the amount of platinum used in fuel cells is reduced, it will greatly contribute to the industrialization of this technology.

<p> In a typical polymer electrolyte membrane (PEM) fuel cell (FC), the PEM is sandwiched between two electrodes, an anode and a cathode. A fuel cell comprises a supply of fuel such as hydrogen gas to the anode, which converts hydrogen into hydrogen ions (protons) and electrons. Oxygen is supplied to the cathode, and oxygen, hydrogen ions conducting in the PEM, and electrons conducting in the external circuit combine to form water. An electrode catalyst is used to promote such an electrode reaction. In order for a fuel cell to perform better, the catalytically active material (platinum) must be in contact with an electron-conducting material such as carbon black, which conducts electrons, and a proton conductor (PEM), which conducts protons. It doesn't become. However, in a conventional PEM fuel cell, when platinum is present in the pores of carbon black, contact with the PEM may be lost, resulting in a decrease in efficiency.</p>

<p> The present invention relates to a method for preparing metal particles on a supporting material, such as platinum-containing nanoparticles on a carbon carrier. The present invention also relates to electrode catalysts using such materials, such as catalytically active metal particles on supported materials, and other application examples of such electrode catalysts. As a specific example, the present invention relates to an improved electrode catalyst for polymer electrolyte membrane fuel cells (PEM-FC (s)).</p><p> In an improved method of producing an electrode catalyst, the photocatalyst is used as a seed for the formation of metal particles such as platinum-containing nanoparticles on the surface of a supporting material such as carbon. Since the photocatalyst can be easily and sufficiently dispersed over the entire surface of the supported material, unlike the conventional method, the metal particles are also sufficiently dispersed. The presence of a large number of well-dispersed photocatalytic sites on the carrier material is also the source of a uniform particle size distribution, unlike conventional methods. In addition, all unwanted organic components of the photocatalyst can be converted to additional carbon-supporting materials by forming metal particles and then pyrolyzing.</p><p> The electrode catalyst according to the present invention includes metal particles in which one or more metal species are present in the particles, such as platinum, platinum alloys such as platinum-iron alloys. By using a mixture of metal precursor compounds, alloy particles such as ferroalloy particles can be formed on the supporting material.</p><p> The method for producing an electrode catalyst includes a step of preparing a mixture containing a photocatalyst, a reducing agent and a metal compound, and a step of reducing the metal compound to form metal particles by irradiating the mixture. The metal particles form an electrode catalyst by being supported on a supporting material. Since the supporting material is conductive, it is a good conductor of electrons. An example of a supporting material is a carbon-containing material such as carbon black. The photocatalyst may be an organometallic compound such as tin porphyrin or metal porphyrin which is an antimony porphyrin. The reducing agent may be, for example, ascorbic acid and hydroquinone. Irradiation can include exposure to visible and / or ultraviolet light.</p><p> The mixture comprises at least one metal compound, such as a platinum compound, an iron compound or a combination thereof. By using a mixture of metal compounds, alloy particles such as platinum alloy particles such as iron-platinum alloy particles can be formed. The metal particles may be nanoparticles and have a median diameter of less than 100 nm, such as 1-10 nm.</p><p> The supporting material may be present in the irradiation mixture or may be combined later. In one method, the photocatalyst is dispersed on the surface of the supporting material, and then the photocatalyst-coated supporting material is combined with a reducing agent and a metal compound to form a mixture. The photocatalyst can be dispersed over the entire surface of the supported material by dissolving (or suspending) the photocatalyst and the supporting material in a common solvent and then evaporating the solvent. Irradiation may occur during and / or after the coalescence operation. In another approach, the metal particles are formed during irradiation with a mixture that does not contain the supporting material, and the supporting material and the metal particles are later combined. For example, a suspension of supporting material (such as a carbon suspension) can be combined with a suspension of metal particles. The suspension of metal particles may be a mixture after irradiation or may be subjected to additional filtration.</p><p> An improved method for making an electrode catalyst comprises the step of forming metal particles on a supported material having a surface photocatalyst dispersed on its surface by photoreducing a metal compound. By using an additional pyrolysis step, any existing organic component (such as a photocatalyst) can be converted to an additional carbon-supported material.</p>

Examples of the present invention include an electrode catalyst having metal particles supported by a conductive supporting material, and a method for producing such an electrode catalyst.

The modified electrode catalyst can be made using the step of producing metal particles via reduction of the metal precursor compound by irradiating a mixture containing one or more metal precursor compounds, a photocatalyst and a reducing agent. A photocatalyst, such as an organometallic photocatalyst, can be dispersed over the surface of the supporting material (or otherwise dispersed through a mixture that does not contain the supporting material), and the photocatalyst can form well-dispersed metal particles. Can be used as a seed for. In the case of an electrode catalyst containing platinum-containing particles on a carbon carrier, the utilization rate of platinum is increased due to the present invention, and the amount of platinum required for fuel cell applications can be reduced. The formation of particles in the fine pores of the carbon carrier can also be prevented.

Since the photocatalyst can be sufficiently dispersed over the entire surface of the supported material, the produced metal particles are also sufficiently dispersed. Metal particles also have a more uniform and controlled size compared to previous methods.

The metal particles may be substantially a single metal species, such as platinum particles, or may include multiple metal species, such as in the form of alloy particles. The metal particles may be platinum-containing particles (such as substantially pure platinum or a platinum alloy such as a ferroalloy) or other species that are catalytically active materials. The particles may be nanoparticles with dimensions less than 1 micron or may be substantially spherical.

Examples of the electrode catalyst according to the present invention include nanostructured metal particles having a diameter of about 1 nanometer to about 10 nanometers supported on the surface of the supported material. By using, for example, thermal decomposition under ammonia or other inert atmosphere, organic compounds can be converted into parts of the supporting material, such as carbon formed by thermal decomposition. This can improve the thermal stability of the particles and prevent agglomeration. Some applications may not require a pyrolysis step.

In the conventional platinum-carbon electrode catalyst, a large number of platinum particles do not contribute to the reaction of the electrode layer. As a result, the utilization rate of platinum is low. In addition, the electrolyte cannot effectively penetrate into the fine pores of carbon in which a large number of platinum particles are present. The catalyst is inert if it does not come into contact with the electrolyte. Such waste of platinum is a cause of reduced proton formation under the conditions of PEM fuel cells, and is a cause of low platinum utilization.

FIG. 1A illustrates a conventional structure comprising a carbon-supporting material 10 having platinum particles such as 12 dispersed over the entire carbon surface. The presence of large particles such as 14 is one reason for the low utilization of platinum, another cause is the presence of particles such as 16 in the narrow pores of the carbon-supporting material, in which the electrolyte is , There is a high possibility that there is no contact in the PEM fuel cell. If not in contact with the electrolyte, the catalyst will be substantially inactive and the utilization of the catalyst will be reduced.

FIG. 1B is a schematic view of an electrode catalyst formed by using the method according to the present invention. The carbon particles 20 support platinum nanoparticles (or other metal or alloy particles) 22 that are sufficiently dispersed over the entire surface of the carbon-supporting material. The organometallic photocatalyst 24 (shown as a non-circular patch on the surface of the carrier material) may be pre-absorbed onto the surface of the carbon carrier, and the photocatalyst then contains the metal precursor compound and reducing agent under irradiation. It may be used as a seed for the formation of metal particles from the mixture. By using the method according to the invention, the metal particles are better dispersed over the surface of the carrier material and have a more uniform size distribution.

FIG. 1C illustrates how the organic precursor is converted to a new carbon-supported material, for example, at 34 on the surface of the carbon carrier 30, there is a metal cluster 32 supported on it.

<u style="single"> photocatalyst</u> Possible photocatalysts include one or more porphyrins (such as metal porphyrins and free base porphyrins), or other organometallic materials. It is preferred that the photocatalyst produce a long-lived excited triplet state upon exposure to visible or ultraviolet light and an interaction with a reducing agent to produce a radical anion or cation.

Platinum particles are produced by reducing a platinum precursor compound in the presence of a photocatalyst such as tin porphyrin under irradiation with ultraviolet or visible light.

Photocatalytic methods for forming platinum nanostructures are YJ Song, Y. Yang, C. Medforth, E. Pereira, AK Singh, H. Xu, Y. Jiang, CJ Brinker, F. van Swol, JA. It is described in detail in Shelnutt, "Controlled Synthesis of 2-D and 3-D Platinum Particles Using Porphyrin Photocatalysts", Journal of the American Chemical Society, 126 (2), 635-645, 2004. It is described in more detail in US Pat. No. 6,627,048 by Shelnutt et al. And No. 4,568,435 of Shelnutt. As explained there, by using tin porphyrins and antimony porphyrins, metal ions were reduced to metals via photoinduced reactions. Metal precursors include metal compounds including metal nitrates, metal chloride salts, uranium, mercury, copper, lead, gold, silver or platinum compounds. Examples of the reducing agent included ethylenediaminetetraacetic acid, triethylamine, triethanolamine and sodium nitrite. The examples described herein can be used in embodiments of the present invention.

The photocatalytic method produces platinum particles with well-controlled size and uniformity.

Photocatalysts can provide an accurate method for producing large initial concentrations of seeds by acting as an initiator for the growth of small metal particles. This allows for well-controlled size and uniformity of metal particles such as platinum particles. In some embodiments of the present invention, the photocatalyst can be dispersed throughout the conductive carrier material and can be used to form metal particles on the carrier material by photocatalytic action.

<u style="single"> Irradiation</u> Irradiation steps include exposure to visible and / or ultraviolet light, or other radiation. The irradiated mixture can be agitated while irradiating the mixture components. Irradiation light may be introduced into the entire mixture by using optical components such as fibers. The mixture may further comprise a fluorescent substance or other light emitting device that emits light when stimulated by an external energy source.

<u style="single"> Reducing agent</u> Possible reducing agents include hydroquinone, ascorbic acid (such as all salts such as sodium ascorbate), and other reducing agents well known in the art.

<u style="single"> Metal precursor</u> Metal precursors are metal compounds that are reduced by a photocatalyst and include the following metal atoms: Pt, Sn, Fe, Co, Cr, Ni, Nb, V, Mo, Mn, Pd, Ru, Zr, Ir, Rh or Can contain one or more of Vs. Alloy particles can be formed by using the corresponding mixture of metal precursors.

For example, the cobalt compound and the platinum compound can form platinum-cobalt alloy particles by reducing both of them with a catalyst, and the alloy composition can be controlled by the ratio of the metal precursor compound. In a representative example, the metal particles may include an alloy of platinum with at least one metal from the group consisting of Fe, Co, Pd and Ni.

<u style="single"> Supporting material</u> The supporting material may be carbon or another conductive material such as an electron conductive polymer which is a polymer having a delocalized electron main chain. The supporting material may be particles, sheets, woven fabrics, fibrous or any other form desired. Supporting materials such as carbon black and other forms of carbon may be commercial products, but may be further treated with ammonia or other gases prior to use.

Examples of the present invention include two methods for arranging metal particles on the surface of a supporting material, the first is an in situ method for preparing metal particles in the presence of the supporting material, and the second is a metal. It is an "independent" method in which the particles are prepared independently of the carrier material and then adsorbed onto the carrier material.

<u style="single"> in situ preparation</u> The method of in situ preparation will be described below as an example, in which the photocatalyst is present on the surface of the carrier material and the metal particles are generated in situ on the surface of the carrier material.

In the first method, the photocatalytic molecules are dispersed on the surface of the carbon carrier. Under visible / ultraviolet light, the carbon carrier is mixed with the metal precursor and reducing agent in solution. Alternatively, the metal precursor may be added in an appropriate manner. The resulting suspension is filtered to obtain a solid material, which is then dried to obtain the final product.

In the second in situ preparation method, the molecules of the photocatalytic material are dispersed on the surface of the carbon carrier. Under visible / ultraviolet light, the carbon carrier is mixed with the metal precursor and reducing agent in solution. Alternatively, the metal precursor is added dropwise. The solid material is obtained by filtering the suspension. The solid material is then dispersed in a solution, such as chloroform, to dissolve organic components such as porphyrins. A second solid material is obtained by filtering the suspension again and then the final product is obtained by drying the second solid.

In a further in situ preparation, the solid material is obtained by filtering the suspension as in the first method above. Any residual organic material is then converted to an inorganic material by heating the solid material under ammonia or other gas in a pyrolysis method. For example, organic components (eg, of photocatalysts) can be converted to carbon by thermal decomposition, which carbon is added to the supporting material.

In the technique of in situ preparation, the photocatalyst is well dispersed on the surface of the supporting material. The supporting material may be carbon black. An impregnation method may be used. In that case, the carbon carrier can be exposed to visible / ultraviolet light, as well as a solution containing at least one metal precursor and at least one reducing agent.

Solvents and impurities can be eliminated by further processing the final product. By controlling the metal concentration, reaction time and temperature, nano-sized metal or alloy particles well dispersed on the carbon-supporting material can be obtained. Additional parameters that can be adjusted include photon absorption efficiency and the dispersity and amount of photocatalyst.

<u style="single"> "Independent" preparation method</u> Metal particles can be formed on a photocatalyst in a mixture containing one or more metal precursors, reducing agents and photocatalysts under visible / ultraviolet light. The metal particles are formed prior to introducing the carrier material into the mixture. The formed metal particles are then dispersed on the surface of the carbon carrier. As mentioned above, the solvent and other impurities can be eliminated by further treating the final product. By controlling parameters such as the concentration of the photocatalyst, the concentration of the metal precursor, the concentration of the reducing agent, the irradiation time, and the temperature, nano-sized metal or alloy particles that are sufficiently dispersed can be obtained.

In one approach, the following: prepare a mixture (eg, suspension or solution) containing one or more of each of a metal compound (a metal precursor that is subsequently reduced to a metal), a reducing agent (reducing agent) and a photocatalyst. To do. Irradiate the mixture and expose it to visible and / or ultraviolet light. The metal precursor can be added dropwise. The metal particles are formed by reducing the metal compound, and the obtained suspension of the metal particles is mixed with a second suspension containing the supporting material to contain the supporting material, the metal particles and the photocatalyst. Form a suspension. The second suspension may be a suspension of carbon particles. The solid material is then obtained by filtering the combined suspension. The carrier material becomes a carrier for the metal particles and then the solid is dried to obtain the final product. Any residual organic material can be converted to carbon by using pyrolysis.

Next, the second method will be described. Suspensions / solutions are formed by mixing metal precursors, reducing agents and photocatalysts under visible / ultraviolet light. The metal precursor can be added dropwise. A new suspension is formed by mixing this suspension with a second suspension containing a carbon carrier. The solid material is then obtained by filtering the new suspension. So far, this method is similar to the first independent law. The solid material is then dispersed in a solution such as chloroform to dissolve the organic compound such as porphyrin. The suspension is then filtered to obtain a new solid material. The final product is then obtained by drying the new solid material.

Next, the third method will be described. This preparation method is similar to the first method described above. However, the solid material obtained by filtering the suspension is subjected to a heat treatment such as thermal decomposition under ammonia or other gas to form the final product. The final product is a completely inorganic material with the organic component converted to carbon.

<u style="single"> Thermal decomposition</u> The electrode catalyst can include an organic-inorganic hybrid material such as a photocatalyst used for seed formation for forming metal particles, and can be used in a fuel cell as long as the presence of the organic compound does not deteriorate the performance of the fuel cell. However, the organic component can be converted to carbon by using further heat treatments such as pyrolysis, which is effective as an additional carrier material for the metal particles. In this thermal method, sites of catalyst structure are formed on the surface of carbon as illustrated below.<chemistry num="1"><img file="JP2008527673A_D0001.tif" /></chemistry>

Scheme I Scheme I above illustrates the formation of iron-nitrogen centers formed on carbon-supported materials using the pyrolysis method. These metal sites are catalytically active for oxygen reduction reactions (ORRs). This means, for example, Anders Widelov, "Pyrolysis of iron and cobalt porphyrins sublimated onto the surface of carbon black as a method to prepare catalysts for 0.₂ "Reduction", Electrochimica Acta, 2493-2502, 38 (17) (1993), and Lefevre, M., Dodelet, JP, Bertrand, P., "O-2 Reduction in PEM Fuel Cells: Activity and Active Site Structural Information for Catalysts Obtained by the Pyrolysis at High Temperature of Fe Precursors , Journal of Physical Chemistry, B104, 11238-11247 (2000).

The active sites of the catalytic structure can be formed by pyrolyzing one or more metal organometallic compounds, such as metallic porphyrins, on a carbon carrier or other supporting material. Scheme I shows only the iron (Fe) present, but Pt, Sn, Fe, Co, Cr, Ni, Nb, V, Mo, Mn, Pd, Ru, Zr, Ir, Rh, or V, etc. One or more metal species of can be used.

Therefore, metallic porphyrins such as tin porphyrins can be used as photocatalysts and then catalytic sites can be formed on the surface of the supported material by performing heat treatments such as thermal decomposition. Such catalytic sites can be used, for example, for oxygen reduction in fuel cell applications. Alternatively, metal porphyrins or homo or hetero-aggregates thereof can be used primarily as photocatalysts, while other porphyrins can be used primarily to form metal macrocycles on the surface of the supporting material.

Pyrolysis can be performed in an atmosphere such as ammonia or nitrogen. Other metal particles can deposit on the surface of the carrier either before or after the pyrolysis step. One or more species of metallic porphyrins can be used, for example, for photocatalytic-based formation of metal particles and / or production of catalysts for oxygen reduction. Unless otherwise indicated, the term porphyrin herein includes not only the organic moiety of 20 carbons, but also porphyrin derivatives and related macrocycles.<u style="single">Example</u>

In the following examples, platinum particles (dendrites) were deposited on carbon black particles. However, common techniques can be applied to other metal particles on other supporting materials.

<u style="single"> Accumulation of SnOEP on carbon black</u> Unlike chemical and photochemical reductions, a single photocatalyst can repeatedly reduce a large number of metal cations to metal atoms. Nanostructures can be formed by depositing the metal atoms in the vicinity of the photocatalyst. Therefore, the first problem for growing well-dispersed platinum nanoparticles on carbon black using the in situ photocatalytic method is how the photocatalytic molecule (in this example, SnOEP, or Sn (IV)). Octaethylporphyrin dichloride) is evenly distributed on the carbon black.

The carbon black used was hydrophobic and was difficult to suspend in water even after 5 minutes of gentle sonication. However, carbon black was easily suspended in chloroform. The dissolved SnOEP molecules are deposited on the carbon black by dissolving both carbon black and SnOEP in chloroform and then slowly evaporating the solvent, provided that SnOEP is also hydrophobic and can be easily dissolved in chloroform. did.

Using this evaporation method, SnOEP was successfully deposited on carbon black. The method can be used immediately to grow platinum nanostructures in situ. The distribution of platinum nanoparticles on carbon black discussed in Example 2 below (eg, FIG. 3A) suggests that SnOEP is well dispersed on carbon black.

<u style="single"> Growth of Pt particles on carbon black</u> By using the photocatalytic method, the size and uniformity of platinum nanoparticles grown on the surfactant by reducing the platinum complex with ascorbic acid under irradiation with tungsten light are controlled. In this example, platinum nanostructures were grown on carbon black, but a photocatalytic method was used to control their size and uniformity.

An aqueous mixture (20 mL) containing SnOEP (0.4 mg) (prepared by evaporation), platinum complex (10 mM), and ascorbic acid (75 mM) deposited on carbon black (10 mg) was irradiated for 80 minutes with stirring and became black. A slurry was produced. When the stirring was stopped, a black precipitate sank to the bottom of the reaction vessel, and the supernatant was colorless and transparent. The original color of the platinum complex is yellow, suggesting that the reaction is complete. The colorless supernatant also indicates that the majority of nanostructured Pt is associated with carbon black.

Figures 2A and 2B are low (a) and high (b) magnification TEM images of platinum nanoparticles grown on carbon black (20 mL aqueous reaction mixture in a 25 mL glass reaction vessel, 10 mg carbon black, SnOEP0). .4 mg i.e. [SnOEP] = 27.7 μM, [K₂PtCl₄] = 10mM, [AA] = 75mM).

Spherical platinum dendrites were observed and all platinum dendrites were attached to carbon black as shown in FIGS. 2A and 2B. The shape of the Pt nanoparticles matches the curvature of the carbon surface, indicating that the nanoparticles grew on the carbon without adhering to the carbon after formation. There may be a chemical link between Pt and carbon, which will improve the stability of the electrode catalyst and will be desirable for fuel cell applications.

The product contains nanoparticles of varying sizes, from particles of 2-3 nm (particle embryos) to large dendrites up to 70 nm in diameter (Figures 2A and 2B). The wide distribution of magnitude suggests that the photocatalytic reaction does not work as well as the independent approach in terms of controlling magnitude and uniformity. Under the conditions used, it is most likely that carbon black absorbs most of the light and tin (IV) porphyrins are not effectively sensitive to the Pt growth reaction.

Figures 3A and 3B are low (A) and high (B) magnification TEM images of platinum nanoparticles grown on carbon black (20 mL aqueous reaction mixture in a 25 mL glass reaction vessel, 10 mg carbon black, SnOEP0). .4 mg i.e. [SnOEP] = 27.7 μM, [K₂PtCl₄] = 2mM, [AA] = 75mM). At low concentrations of platinum complexes corresponding to 45% by weight of platinum on carbon, the nanoparticles have smaller average diameters but still exhibit a wide size distribution as shown in Figures 3A and 3B.

In this method for growing platinum nanoparticles on carbon black, all the prepared platinum was associated with carbon black and well dispersed.

<u style="single"> Effect of titration on Pt particle formation</u> Due to the two-step titration of the platinum complex, the size of the Pt particles on the carbon black was reduced. By adding the Pt complex to the reaction vessel in two steps in order to reduce the average size, a platinum complex raw material solution capable of supporting 45% by weight of Pt on carbon was reduced on carbon black.

CB10.2mg, 20mM K₂PtCl₄1.1 mL and 8.9 mL of water were added to a 25 mL glass vial and sonicated for 5 minutes. Then 10 mL of 0.15M AA was added and the reaction mixture was irradiated for 30 minutes with stirring. Then, the obtained black slurry was centrifuged to remove the supernatant. The black precipitate was washed 5 times with 20 mL of nanopure water each time. The cleaned precipitate was oven dried overnight at 65 ° C. The powder was then added to a clean glass vial. 20mM K₂PtCl₄1.1 mL and 8.9 mL of water were added and the mixture was sonicated for 5 minutes. 10 mL of 0.15M AA was added, the mixture was stirred and irradiated for 30 minutes.

FIG. 4A shows a TEM image of platinum nanoparticles synthesized using the titration method (25 mL glass vial, carbon black 10.2 mg, [SnOEP] = 0 μM, [K].₂PtCl₄]_all= 2 mM, [AA] = 75 mM), Figure 4B shows a TEM image of platinum nanoparticles prepared using previous methods (25 mL glass vial, carbon black 11 mg, [K].₂PtCl₄] = 2mM, [AA] = 75mM).

The average size of Pt particles prepared using the titration method (Fig. 4A) appears to be smaller than that of particles prepared using previous methods (Fig. 4B).

The titration method may be regarded as a seed formation method. That is, the first addition of the metal source forms platinum seeds, and then the addition of the second half of the platinum complex causes the seeds to grow autocatalytically into larger nanostructures. The titration method may reduce the size due to one or more of the following factors: low concentrations of platinum complex promote the formation of more nucleation sites and at the same time each nuclei. The autocatalytic growth of the production site may be suppressed; at the nucleation site formed during the first titration, Cl may be important for the autocatalytic growth of the particles.^-Some autocatalytic properties may be lost due to the removal of species such as and oxidized AA during the cleaning process; or the autocatalytic properties of the particles on carbon black are the autocatalytic properties of the non-supported particles Not better than the traits. However, the exact mechanism is not clear.

<u style="single"> Effect of different reducing agents on the synthesis of platinum nanoparticles on carbon black</u> In the examples so far, platinum nanostructures have been synthesized on carbon by using ascorbic acid as a reducing agent. In this experiment, the synthetic system was extended with two alternative reducing agents (NaBH).₄And NH₂NH₂) Is used. Unlike ascorbic acid, such a strong reducing agent quickly reduces the Pt complex, and the reaction is too rapid to control the photocatalyst, so that the particles grow non-dendritic and the particle size becomes uniform. May be inferior.

Carbon black 10mg and 2m M K₂PtCl₄After adding 10 mL of the aged aqueous solution to a 25 mL glass vial, carbon black was suspended in the mixture by sonication for 5 minutes. Addition of 10 mL of 150 mM ascorbic acid, sodium borohydride or hydrazine produced a black slurry by stirring under irradiation for 30 minutes. After stopping the stirring, a black precipitate and a colorless supernatant were observed, indicating that the reduction was complete.

When 75 mM ascorbic acid was used as the reducing agent, most platinum nanoparticles grew on carbon black as shown in Figure 5A. FIG. 5B more clearly shows that the particles have a particle size distribution containing 2-3 nm particles and larger particles up to 35 nm in diameter. Since the same concentration of sodium borohydride was used in the synthesis, almost all platinum was reduced on carbon as well. However, the platinum nanoparticles produced are not Pt dendrites, have a less regular shape, and have a wider particle size distribution of 2-50 nm (Figures 5C and 5D). When hydrazine was used as the reducing agent, only large platinum nanospheres with a diameter of about 500 nm were obtained, as shown in FIGS. 5E and 5F. It is unclear whether the nanospheres grew on carbon or not.

Considering the control of the size and shape of the nanostructured platinum, a photocatalytic reaction system of ascorbic acid, potassium tetrachloride and carbon black is preferable. Ascorbic acid plays a role in the formation of platinum nanoparticles, including promoting the growth of dendrites through weak surface interactions with platinum. The structure of ascorbic acid does not contain strong coordination atoms such as nitrogen atoms in hydrazine.

In summary, FIGS. 5A-5F show TEM images of platinum nanostructures synthesized on carbon black using various reducing agents. (A) and (B) are for ascorbic acid (25 mL glass vial, carbon black 11 mg, [K]₂PtCl₄] = 2mM, [AA] = 75mM); (C) and (D) are NaBH₄In the case of (25 mL glass vial, carbon black 11 mg, [K₂PtCl₄] = 2mM, [NaBH₄] = 75mM); (E) and (F) are NH₂-NH₂In the case of (25 mL glass vial, carbon black 9 mg, [K₂PtCl₄] = 2mM, [N₂H₄] = 75mM). Although the platinum complex can be successfully reduced by using all such reducing agents, ascorbic acid appears to be the best in terms of particle size and shape control, especially with the use of photocatalysts.

<u style="single"> Effect of surfactant on fuel cell performance of spherical platinum nanoparticles grown on carbon black</u> High concentrations of SnOEP dissolved in SDS micelles were found to promote better reduction of platinum nanoparticles size due to better light absorption due to the large amount of SnOEP on the surface. In other words, the use of surfactants provides a promising way to overcome the light absorption problem caused by the presence of carbon black. However, it is not yet clear whether the presence of SDS has any effect on the fuel cell performance of the prepared platinum particles. In the following, we will focus on the effect of SDS on fuel cell performance.

100 mg of carbon black was added to 100 mL of 0.15 M aqueous ascorbic acid solution, and the mixture was sonicated for 10 minutes to suspend the carbon black. Then 20 mM K₂PtCl₄22 mL of the aged aqueous solution and 78 mL of water were added. The mixture was stirred and irradiated with two light sources for 1 hour. After stopping the stirring, a colorless supernatant and a black precipitate at the bottom of the reaction vessel were observed. The supernatant was removed mechanically. The remaining black slurry was then transferred to two 45 mL centrifuge tubes. After adding 50 mL of nanopure water to each tube, the product was precipitated by overflow stirring at 3500 RPM for 10 minutes and centrifugation at 3500 RPM for 2 minutes. Again, 45 mL of the supernatant was removed. This purification method was repeated 5 times for each tube. The obtained product was then dried overnight by leaving it in an oven at 65 ° C. The total weight of the dried product was 0.186 g.

The membrane electrode assembly (MEA) was prepared using the above catalyst, 45 wt% platinum nanoparticles grown on carbon black, with or without SDS added to the electrode ink later. This procedure gives Pt particles of the same type and concentration on the surface and determines the effect of the surfactant applied after the particles have grown.

The electrode ink on one side of the electrode is catalyst 16.7 mg (45 wt% platinum nanostructures on carbon black); Nafion solution 133 mg (Aldrich, 5 wt% Nafion); 18 MΩ water 600 mg; reagent grade isopropyl alcohol 500 mg; and none. Consists of SDS 0 or 13.8 mg (40 mM) for each of the SDS and SDS-containing electrodes.

The electrode ink was sonicated for 20-30 seconds to disperse the catalyst particles, then the ink was brushed onto a Nafion 112 film and held flat on a vacuum hot plate at 60 ° C. Approximately 1 mg / cm on one side of the electrode, taking into account the amount wasted as a result of this method²Platinum was supported. The other side of the electrode was also applied using the same formulation and procedure. After drying, two MEAs (with and without SDS) were placed in standard fuel cell testing hardware (Fuel Cell Technologies) with SGL 21BC carbon paper as a gas diffusion layer and substrate layer.

MEA was hydrated under hydrogen (anode) and nitrogen (cathode) at 100% RH and 25 ° C for 2 hours. By drawing a cyclic voltammogram, qualitative measurements of the electrochemically active surface area (ECA) of both SDS-free and SDS-containing electrodes were obtained.

6 and 7 show the initial CV of each of the SDS-free and SDS electrodes. FIG. 6 shows the CV curve of the no SDS Pt electrode before and after the break-in test and the performance test, and FIG. 7 shows the CV curve of the 40 mM SDS Pt electrode before and after the break-in test and the performance test. Comparing the two curves, the peak current of the SDS electrode is much smaller than that of the no SDS electrode, ie 0.03 vs. 0.1A. After the first CV at 25 ° C, each battery was heated to 80 ° C at 100% RH.

For the SDS-free battery, the break-in operation of this battery was measured by pulling at a voltage of 0.5 V and recording the current over time. FIG. 8 shows a break-in curve at 0.5 V for a SDS-free battery. The current increased first and then slowly decreased throughout the course of the test. This slow reduction is common to brush-coated electrodes that are not heat treated. For platinum particles grown on carbon black, the performance of the electrodes can be stabilized by performing heat treatment. In any case, the performance of this battery, about 1300mA / cm²Is reasonably good for this non-optimized MEA preparation.

FIG. 9 shows a break-in curve at an applied potential of 0.1 V for the SDS-containing electrode. The performance of this electrode was so low that a low potential was required in this case. In fact, at the start of the break-in, the open circuit voltage of this battery was just over 0.5V. If the battery breaks in at a voltage higher than 0.1V, no current will be produced.

As can be seen in Figure 9, the current density at 0.1 V increased slowly over the course of the 21-hour test. This is likely to indicate that the SDS-containing electrode cleaned itself during the operation of the fuel cell. The mode of cleanup remains unknown, but it is possible that the SDS slowly oxidized over time at the platinum reaction site.

Figure 10 shows the performance of each battery after the break-in period. It was clear that SDS-containing batteries performed much worse than non-SDS batteries. This was suggested by their slow break-in behavior.

Cyclic voltammetry was performed after break-in and performance testing of each battery. For SDS-free batteries, the ECA decreased after break-in and performance testing, reflecting the lower peak current in the low voltage region of the CV. The slight decrease in performance of the battery during break-in is consistent with this observation. Qualitatively, the ECA of SDS-containing batteries increased after break-in and performance testing periods (higher peak current). This result would be expected if there were methods of cleaning the SDS-containing electrodes during the break-in period. These results indicate that the 40 mM SDS in the electrode ink had a destructive effect on battery performance. The performance of the SDS-containing battery was an indication of platinum surface contamination, probably due to the SDS surfactant. Cleaning of a portion of the catalyst surface was performed during break-in and performance tests, evidence of increased ECA during the test period. However, the performance of the SDS-containing battery did not approach the performance of the SDS-free battery. The ECA of SDS-free batteries decreased slightly during the test, which is normal for non-heat treated MEAs.

<u style="single"> Study of Pt-C interface interaction</u> Spherical platinum particles grew on carbon black. A typical TEM image shows that the particles match the curvature of the supported carbon black (Fig. 11A), indicating the formation of chemical bonds at the Pt-C interface. The chemical bond between the nanostructured platinum and its supported carbon black can be important in fuel cell applications in that the bond delays the degradation of the electrode catalyst and prolongs its durability in fuel cell applications. Further research was conducted on the Pt-C interface.

Platinum particles grown on carbon black were separated by centrifugation at 3000 rpm for 2 minutes, then the supernatant was removed, fresh nanopure water was added, and the product was resuspended by gentle sonication. This procedure was repeated 3 times to ensure that all by-products were removed. Prior to XPS measurements, the cleaning material was oven dried overnight at 65 ° C.

Figure 11B shows a double peak with a low energy band at 71.9 eV (4f7 / 2) and a high energy band 3.3 eV high in the center (4f5 / 2). The two wide bands could be curve approximated to two pairs of Pt4f peaks at 72.0, 75.4, and 73.6, 77.5 eV, respectively. They all have spin-orbit splits in the 4f7 / 2 and 4f5 / 2 states. Therefore, there are two different Pt oxidation states, Pt (0) and Pt (&gt; 0), each contributing to the two peaks in the generated XPS spectrum. This seems to be able to show a chemical bond at the Pt-C interface, but it may also be derived from a Pt-O bond.

Figure 12A is an HRTEM image showing that the atomic fringes of platinum have just begun to form on the carbon surface, and there is likely to be some difference between the Pt-C interface and the bulk platinum crystals within the fringes. is there. FIG. 12B shows an STEM image in which brightness is related to electron density. For selected squares at the Pt-C interface, the corresponding density profile clearly shows the density transition at the Pt-C interface rather than a sudden jump. This indicates that a substance or compound having an electron density higher than that of carbon and smaller than that of pure platinum is present at the interface. However, such results may also result from the formation of Pt-O bonds.

Taken together, these results suggest the presence of chemical bonds at the Pt-C interface.

<u style="single"> Other Examples and Alternatives</u> Other preparation examples may include exposing carbon carriers, radical anions and metal precursors in the absence of a photocatalyst. The electrode catalyst prepared according to the present invention may be an organic-inorganic hybrid material or an inorganic material. In the former case, the organic component would have no effect on electron conductivity, proton passage, or absorption of gas molecules on the catalyst, and gas diffusion. The electrode catalyst produced by the method according to the present invention may also contain carbon materials formed from organic components via thermal decomposition.

In other examples, no photocatalyst is used. For example, the preparation may include a carbon-supporting material, one or more radical anions, and one or more metal precursors without the use of photocatalysts. Seeds for the growth of metal particles on the surface can be formed by dispersing other reduction initiators, such as other catalysts, over the surface of the carrier material.

The described examples include electrode catalysts for fuel cells. However, the present invention also relates to other devices containing metal particles formed on the supporting material. For example, optical, electronic and photonic devices can be made using similar methods.

The present invention is not limited to the above exemplary examples. The examples are not intended to limit the scope of the invention. The methods, devices, compositions and the like described herein are exemplary and are not intended to limit the scope of the invention. Those skilled in the art would implement those variations and other uses. The scope of the present invention is defined by the scope of claims.

The patents, patent applications or publications referred to herein are, by reference, to the same extent as expressly incorporated herein by reference, specifically and individually with respect to the individual documents. Incorporated herein. In particular, US Patent Provisional Application No. 60 / 643,283, filed January 12, 2005, is incorporated herein by reference.

Since the present invention has been described, the claims are claimed.

<figref num="1">FIG. 1A-1C is a schematic diagram illustrating metal particles on carbon particles, a conventional electrode catalyst in which the utilization rate of platinum is reduced due to insufficient control of particle size and dispersion over the entire surface (Fig. 1A). ), The electrode catalyst according to the present invention (FIG. 1B) having metal particles sufficiently dispersed over the entire surface of the carbon carrier, and the thermal decomposition method (FIG. 1C).</figref><figref num="2">2A and 2B are diagrams showing TEM images of platinum nanoparticles grown on carbon black.</figref><figref num="3">Figures 3A and 3B show TEM images of platinum nanoparticles grown on carbon black using a lower concentration of Pt complex.</figref><figref num="4">FIGS. 4A and 4B are diagrams showing TEM images of platinum nanoparticles synthesized by the titration method and the non-titration method, respectively.</figref><figref num="5">Figures 5A-5F show TEM images of platinum nanostructures synthesized on carbon black using various reducing agents.</figref><figref num="6">It is a figure which shows the CV curve of the SDS-free Pt electrode.</figref><figref num="7">It is a figure which shows the CV curve of the Pt electrode containing 40 mM SDS.</figref><figref num="8">It is a figure which shows the break-in curve at 0.5V in a non-SDS battery.</figref><figref num="9">It is a figure which shows the break-in curve at 0.1V in the SDS-containing battery.</figref><figref num="10">It is a figure which shows the performance curve of the SDS and no SDS battery after a break-in period.</figref><figref num="11">11A and 11B are diagrams showing a TEM image and a Pt4f X-ray photoelectron spectrum of platinum nanoparticles grown on carbon black, respectively.</figref><figref num="12">12A-12C are high-resolution TEM images, scanning TEM images, and interfacial electron density profiles of platinum nanoparticles grown on carbon black, respectively.</figref>