JP2008041291A - Fuel electrode catalyst, membrane electrode assembly, and fuel cell - Google Patents

Abstract

To provide a fuel cell with higher power density and lower cost than conventional ones.
A fuel electrode catalyst according to the present invention is a fuel electrode catalyst for use in a fuel cell comprising a fuel electrode and an oxygen electrode, the catalyst being 1 to 80 at. Pd—Ni binary fine particles containing% Ni and the balance being substantially Pd are supported on conductive carbon. Thereby, the oxidation activity of the fuel is improved as compared with the pure Pd catalyst. As a result, the power density is improved. The fuel electrode catalyst is suitable as an oxidation catalyst for a fuel cell using a carboxyl group-containing organic acid, particularly formic acid as a fuel.
[Selection] Figure 1

Description

  The present invention relates to a fuel electrode catalyst, a membrane electrode assembly including the same, and a fuel cell.

  In recent years, research and development on power generation devices that can extract electrical energy from hydrogen energy, such as hydrogen fuel cells, has become active. Hydrogen is obtained by decomposing water and is not only inexhaustible on the earth, but also contains a large amount of chemical energy per substance, and when used as an energy source, harmful substances and global warming gases are used. It has the advantage that it does not occur.

  Research on fuel cells using methanol instead of hydrogen gas is also actively conducted. A direct methanol fuel cell (DMFC) that uses methanol, a liquid fuel, is a low-cost power source for household and industrial use because it is an inexpensive fuel in addition to ease of handling fuel. Suitable as The theoretical output voltage of the methanol-oxygen fuel cell is almost the same as that of the hydrogen-oxygen fuel cell, and is about 1.2 V at 25 ° C. DMFC has a theoretical volume energy density about 10 times higher than that of a lithium ion battery, and is expected as a next-generation battery. In the center of the DMFC, there is a proton conductive film, and anode and cathode catalyst layers are arranged on both sides thereof. Methanol and water are supplied to the anode and oxygen is supplied to the cathode. Platinum ruthenium (PtRu) is used for the anode catalyst, and platinum (Pt) catalyst is used for the cathode catalyst.

The reaction mechanism at the anode will be described below. First, according to the formula (1), CO is generated during the methanol oxidation process and chemisorbed on the Pt catalyst. This is Pt catalyst poisoning by CO. On the other hand, water adsorbs on Ru, and generates a hydroxyl group according to the formula (2). The hydroxyl group bonded to Ru attacks CO chemisorbed on Pt according to the formula (3) and oxidizes it to CO ₂ .
CH ₃ OH + Pt = Pt—CO + 4H ⁺ + 4e ⁻ (1)
Ru + H ₂ O = Ru—OH + H ⁺ + e ⁻ (2)
Pt—CO + Ru—OH = Pt + Ru + CO ₂ + H ⁺ + e ⁻ (3)
Ru has no catalytic action to oxidize methanol, and Ru is a promoter that reduces Pt poisoning. From the above reaction mechanism, it is ideal that the Pt and Ru atoms exist close to each other, and it is known that the composition of Pt ₅₀ Ru ₅₀ is the most active.

  As described above, the DMFC thermodynamically generated battery voltage is about 1.2 V, but this voltage cannot be obtained in an actual battery. The main causes are the following two points. First, the activation energy of the methanol oxidation reaction is still large even when the PtRu catalyst is used, and the anodic polarization is large. Second, methanol passes through the proton conductive film and reaches the cathode, where it is directly oxidized at the cathode. The permeation of methanol is called methanol crossover and is a big problem.

Further, Pt used for fuel cells is a very expensive noble metal. Pt used as a catalyst is synthesized by finally reducing a compound in an oxidized state. The cheapest compound as a source of Pt is hexachloroplatinic acid, and the reagent level price is about 2750 yen / g as of July 2006. The reagent cost required to synthesize 1 g of metal Pt catalyst from this reagent is 7320 yen. The amount of electrode catalyst used in DMFC is around 50 g / m ^{2 for} both electrodes, and the general power density at a battery voltage of 0.4 V is 200 W / m ² . From this, the amount of catalyst necessary for supplying the electric power 1 W required for the mobile phone is 0.5 g in terms of metal Pt, and the reagent cost is 3660 yen. With such a high cost, it is difficult to spread portable devices equipped with DMFC.

Under such circumstances, a new type of fuel cell was reported in 2004 by the University of Illinois (Non-Patent Document 1). It is a direct formic acid fuel cell (DFAFC) that uses formic acid as the anode fuel and oxygen as the cathode fuel. The merit of this fuel cell is that if Pd is selected as a catalyst, when formic acid is oxidized to carbon dioxide, carbon monoxide is not generated in the reaction process unlike methanol oxidation, and formic acid is proton conductive. The point is that it hardly penetrates the membrane. The formic acid oxidation reaction is shown in Formula (4).
HCOOH = CO ₂ + 2H ⁺ + 2e ⁻ (4)
This reaction is a direct deprotonation reaction from formic acid. Therefore, carbon monoxide (CO) is not generated in the elementary reaction process, and there is no problem of poisoning the catalyst. Further, since formic acid is an organic acid unlike alcohol, it is ionized in an aqueous solution to produce formate ion (HCOO ⁻ ). The formate ion is an anion, and an electrostatic repulsive force is generated between the sulfonate anion in the Nafion film of DuPont, which is a typical proton conductive film. This repulsive force makes it difficult for formate ions to permeate through the proton conductive film, and the amount of crossover is extremely small compared to DMFC methanol. Because of this feature, DFAFC achieves a power density more than twice that of DMFC in a passive state (a type that does not use any pump for supplying fuel).

Pd is used for the anode catalyst of DFAFC. As of July 2006, the price of Pd ingot is about 1/4 of the price of Pt ingot, and the cost of catalyst material can be greatly reduced.
B. Adams, 6 others, “Formic acid fuel cells: New possibilities for portable power”, Proceedings of the 2004 Fuel Cell Seminar, Power Sources Conference, November 2004 Kobayashi, 4 others, “Nonlocal-density-functional bond-energy calculations of cage-shaped carbon fullerenes: C32 and C60”, Physical Review B, June 1992, Vol. 45, p. 13690-13893 Kobayashi, two others, “Efficient, direct self-consistent-field method in density-functional theory”, Physical Review A, March 1996, Vol. 53, p. 1903-1906

  As described above, DFAFC has a higher power density and lower catalyst material cost than DMFC, but in order to spread as a fuel cell, further improvement in power density and cost reduction are desired.

  The present invention has been made in view of the above, and an object thereof is to provide a fuel cell with higher power density and lower cost.

  The fuel electrode catalyst according to the first aspect of the present invention is a fuel electrode catalyst for use in a fuel cell including a fuel electrode and an oxygen electrode, and is 1 to 80 at. % Pd-Ni binary fine particles containing Ni and the balance being substantially Pd. Thereby, the oxidation activity of the fuel is improved as compared with the pure Pd catalyst, and as a result, the output density is improved.

  A fuel electrode catalyst according to a second aspect of the present invention is the fuel electrode catalyst described above, wherein the Pd—Ni binary fine particles are supported on conductive carbon. Thereby, the catalyst can be made into ultrafine particles, and as a result, the output density is improved.

  The fuel electrode catalyst according to a third aspect of the present invention is characterized in that, in the above fuel electrode catalyst, the fuel of the fuel electrode is an organic acid having a carboxyl group. The fuel electrode catalyst according to the present invention is suitable as an oxidation catalyst for an organic acid having a carboxyl group.

  A fuel electrode catalyst according to a fourth aspect of the present invention is the fuel electrode catalyst described above, wherein the fuel of the fuel electrode is formic acid. The fuel electrode catalyst according to the present invention is particularly suitable as a formic acid oxidation catalyst.

  The fuel electrode catalyst according to the fifth aspect of the present invention is the above fuel electrode catalyst, wherein the oxygen electrode catalyst provided to the oxygen electrode is 1 to 50 at. The Pt—P binary fine particles containing% P and the balance being substantially Pt are those supported on conductive carbon. Thereby, the output density as the whole fuel cell can be further improved.

  A membrane electrode assembly according to a sixth aspect of the present invention includes a fuel electrode catalyst layer having the above fuel electrode catalyst, an oxygen electrode catalyst layer, and a solid polymer electrolyte membrane interposed between both catalyst layers. It will be.

  A fuel cell according to a seventh aspect of the present invention includes the membrane electrode assembly described above.

  According to the present invention, a fuel cell with higher power density and lower cost can be provided.

  The inventors have used formic acid having a carboxyl group by using a PdNi catalyst in which Ni is added to Pd instead of pure Pd as a fuel electrode catalyst by a first-principles molecular orbital calculation method using density functional theory. It has been found that the oxidation activity of is increased. When Ni, which has a high hydrogen storage capacity like Pd, is added to Pd, it is presumed that the deprotonation reaction from formic acid is promoted by its synergistic effect, and the oxidation activity of formic acid is improved. This will be described in detail below.

  Regarding the oxidative decomposition reaction of the fuel on the catalyst surface, each elementary reaction can be examined by the first principle molecular orbital calculation method using density functional theory. The methods and analysis examples used in the present invention are shown in Non-Patent Documents 1 and 2 above.

When the decomposition reaction of formic acid on the Pd (111) surface was examined, it was found that the following series of elementary reaction pathways were the main ones.
HCOOH → HCOO ⁻ + H ⁺ (5)
HCOO ⁻ → H ⁺ + OCO ²⁻ (6)
OCO ²⁻ → CO ₂ ↑ + 2e ⁻ (7)
The decomposed H diffuses into the electrolyte membrane, and CO ₂ is vaporized. The adsorption structure of HCOO takes the form in which two O atoms are adsorbed immediately above the two Pd atoms on the Pd (111) surface, and is the most stable adsorption structure among decomposition products in which H1 atoms are desorbed from formic acid. I found out. For this reason, it has been found that the reaction formulas (5) to (7) are the main reaction paths. The rate-limiting reaction in these series of elementary reactions is (6), and it was found that an activation energy of 1.0 eV is necessary. H vibrates at a higher speed than C and O, and has a large zero-point vibration energy, and thus easily causes a desorption reaction. This can be understood as the reason why the output of the direct formic acid fuel cell is high.

  In this analysis, a part of the surface of the PdNi catalyst was extracted and used as an atomic group model, and the reaction (6) was analyzed and the activation energy was examined. If the activation energy on PdNi is smaller than the Pd (111) plane, it indicates that the catalytic activity is high. When the activation energy is 0.1 eV lower in the same reaction, the increase in the reaction rate at 300 K (27 ° C.) is estimated as exp [0.1 eV / kT] ≈48 times from the Arrhenius equation. Therefore, it can be considered that the difference in activation energy of 0.1 eV is sufficiently large. FIG. 1 (a) shows the HCOO adsorption structure, FIG. 1 (b) shows the intermediate structure of the desorption reaction, and FIG. 1 (c) shows the result of analyzing the structure after H desorption. The intermediate structure of the elimination reaction is a structure having an energy maximum in the H elimination reaction direction and an energy minimum in the atomic displacement in the other direction. The energy required to transform from the adsorption structure of HCOO to the intermediate structure of the elimination reaction was the activation energy ΔE, and was calculated to be 0.8 eV for the PdNi catalyst. Further, the energy decreased by 1.1 eV from the intermediate structure to the structure after H desorption.

  As a comparison, a part of the surface of the Pd catalyst was taken out and used as an atomic group model, and the reaction (6) was analyzed and the activation energy was examined. FIG. 2 (a) shows the HCOO adsorption structure, FIG. 2 (b) shows the intermediate structure of the desorption reaction, and FIG. 2 (c) shows the result of analyzing the structure after H desorption. The intermediate structure of the elimination reaction is a structure having an energy maximum in the H elimination reaction direction and an energy minimum in the atomic displacement in the other direction. The energy required to transform from the HCOO adsorption structure to the intermediate structure of the elimination reaction was the activation energy ΔE, and 1.0 eV for the Pd catalyst. Further, the energy decreased by 1.0 eV from the intermediate structure to the structure after H desorption.

  From the above two results, the activation energy ΔE required to transform from the adsorption structure of HCOO to the intermediate structure of the elimination reaction is 0.8 eV for the PdNi catalyst, 1.0 eV for the Pd catalyst, and 0 for PdNi. Low eV activation energy was shown. This indicates that the PdNi catalyst obtained by adding Ni to Pd has higher formic acid oxidation activity than the Pd catalyst.

  Embodiments of the present invention will be described below. However, the present invention is not limited to the following embodiment. Further, in order to clarify the explanation, the following description and drawings are appropriately omitted and simplified.

Embodiment 1 of the Invention
FIG. 3 shows a typical configuration example of the fuel cell according to the present invention. The fuel cell 40 includes a solid polymer electrolyte membrane 41, an air introduction hole 42, an oxygen electrode side diffusion layer 43, an oxygen electrode side current collector 44, an oxygen electrode catalyst layer 45, a fuel electrode catalyst layer 46, and a fuel electrode side current collector. 47, a fuel electrode side diffusion layer 48, and a fuel introduction hole 49. Here, the solid polymer electrolyte membrane 41, the oxygen electrode catalyst layer 45, and the fuel electrode catalyst layer 46 constitute a membrane electrode assembly.

  The solid polymer electrolyte membrane 41 has a function of transporting protons generated at the fuel electrode to the oxygen electrode side and a function as a separator for preventing a short circuit between the fuel electrode and the oxygen electrode. Specifically, Nafion 112 manufactured by DuPont can be used.

  The oxygen electrode side current collector 44 has a function as a structure for taking in air (oxygen) through the air introduction hole 42 and a current collecting function. As the cathode fuel, oxygen in the atmosphere can be used. The oxygen taken in from the oxygen electrode side current collector 44 is guided to the oxygen electrode catalyst layer 45 through the oxygen electrode side diffusion layer 43. The oxygen electrode catalyst layer 45 is reduced by electrons generated at the fuel electrode and simultaneously reacts with protons generated at the fuel electrode to generate water.

  The oxygen electrode catalyst used for the oxygen electrode catalyst layer 45 is preferably a Pt or PtP catalyst. When P is added to Pt, when Pt catalyst particles are precipitated on the carbon support, P acts from the inside and outside of the particles, the precipitated Pt catalyst particles are refined to increase the specific surface area of the catalyst, and the oxygen reduction activity Will improve. If a PtP oxygen electrode catalyst is used in combination, the fuel cell characteristics can be further enhanced. In this case, the P content is 1 to 50 at. % Is preferred. P content is 1 at. If it is less than%, the particle size of the Pt catalyst cannot be made sufficiently fine and the catalytic activity cannot be sufficiently increased. On the other hand, the P content is 50 at. When it exceeds%, the Pt content is lowered and the catalytic activity is lowered.

  The fuel electrode side current collector 47 has a function as a structure for taking in fuel through the fuel introduction hole 49 and a current collecting function. As the anode fuel, an organic acid having a carboxyl group can be used, and formic acid is particularly preferable. Specifically, formic acid having a concentration of about 10 mol / l is preferable. The fuel supplied from the fuel electrode side current collector 47 is guided to the fuel electrode catalyst layer 46 through the fuel electrode side diffusion layer 48. In the fuel electrode catalyst layer 46, the fuel is oxidized to release electrons and protons. The electrons and protons move to the oxygen electrode side through the solid polymer electrolyte membrane 41. Electricity is generated by the oxidation reaction of the fuel and the reduction reaction of oxygen.

  The fuel electrode fuel electrode catalyst according to the present invention used for the fuel electrode catalyst layer 46 is composed of binary fine particles represented by the general formula PdNi supported on a carbon carrier. The Ni content is 1 to 80 at. % Is preferred. The Ni content is 1 at. If it is less than%, the activity of the Pd catalyst is not sufficiently increased. On the other hand, the Ni content is 80 at. When it exceeds%, the content of Pd is lowered and the catalytic activity is lowered.

  The particle size of the PdNi catalyst is preferably 20 nm or less. When the catalyst particle size is larger than 20 nm, the specific surface area of the catalyst is reduced and sufficient catalytic activity cannot be obtained. The lower limit of the particle size is not particularly limited, but if it is less than 1 nm, the electrochemical dissolution of the catalyst becomes significant, causing a problem in the long-term durability of the catalyst.

In the present invention, as the catalyst carrier, the specific surface area is preferable to use a conductive carbon 20~800m ² / g, it is more preferable to use a conductive carbon 20 to 300 m ² / g. In this specific surface area range, the porosity of the carbon support is low, and there are few micropores present in the carbon support.

  Acetylene black (AB), multi-wall carbon nanotubes (MWCNT), etc. are non-porous carbon carriers having no micropores. If a low porous or non-porous carbon support is used, the catalyst particles buried in the micropores existing in the support are reduced, and more catalyst particles can be deposited on the carbon support surface. If the catalyst particles are present on the surface of the carbon support, the probability that the fuel and the proton conductive polymer come into contact with each other increases, and the catalyst utilization rate can be improved.

  In particular, MWCNT has a lower density than the conventionally used carbon black. For this reason, when a catalyst is made into a paste with Nafion resin and made into an electrode catalyst film by a press, there are more physical voids in the electrode catalyst film than in a conventional carbon black carrier. Thus, sufficient fuel diffusion on the catalyst is ensured. Moreover, the diffusibility of the water produced | generated on the oxygen electrode catalyst improves, and produced water can move to the electrode catalyst membrane surface rapidly, and a battery characteristic improves. Furthermore, MWCNT has a lower specific resistance than carbon black that has been conventionally used. For this reason, IR loss can be suppressed and a decrease in battery voltage can be suppressed, which is preferable.

  Next, the method for producing the catalyst according to the present invention will be described. As a method for synthesizing a PdNi catalyst that is a fuel electrode catalyst, for example, an alcohol reduction method, an ultrasonic reduction method, an electron beam irradiation reduction method, a radiation irradiation reduction method, an impregnation method, and the like can be used. It is not something.

  As the Pd supply source, for example, palladium (II) acetate, acetylacetonato palladium (II), sodium palladium (II) chloride, palladium (II) nitrate, palladium (II) sulfate and the like can be used. These palladium compounds may be used alone or in combination of two or more.

  Moreover, the Nafion membrane of DuPont which is a typical proton conductive film used for a fuel cell is perfluoroalkyl sulfonic acid, and shows extremely strong acidity. Therefore, transition metals such as Fe, Co, and Ni are ionized and dissolved. The dissolved transition metal ions are ion-exchanged with protons in the Nafion membrane, resulting in a decrease in proton conductivity, which causes deterioration of battery characteristics. This dissolution phenomenon also becomes a problem in the PdNi catalyst of the present invention. As a means for solving this problem, it is effective to remove Ni present on the surface of the PdNi catalyst by immersing the catalyst in an acid such as sulfuric acid, nitric acid and hydrochloric acid after the synthesis of the catalyst.

Specifically, for example, the carbon-supported PdNi catalyst according to the present invention can be obtained by the following method.
Acetylacetonato palladium (II) 2.82 mmol and nickel nitrate hexahydrate 2.82 mmol was dissolved in ethylene glycol 100ml respectively, conductive carbon (Cabot (Cabot) manufactured by Vulcan XC-72R, specific surface area 254m ² / G) Add to a 200 ml ethylene glycol solution with 0.5 g dispersed. The solution is refluxed for 4 hours while stirring at 200 ° C. in a nitrogen gas atmosphere to deposit and support the PdNi catalyst on the conductive carbon. After completion of the reaction, the catalyst is immersed and stirred in a 1 mol / l sulfuric acid aqueous solution at 50 ° C. for 72 hours to dissolve and remove Ni present on the catalyst surface. Thereafter, the catalyst is filtered off, washed with pure water and dried to obtain the carbon-supported PdNi catalyst according to the present invention. In place of nickel nitrate hexahydrate, the same amount of nickel oxalate may be used.

  On the other hand, a method for producing a PtP catalyst used for the oxygen electrode will be described. P-containing compounds to be used are preferably phosphorous acid, phosphites (including both normal salts and acidic salts), hypophosphorous acid, and hypophosphites. The salt is preferably an alkali metal salt (for example, sodium phosphite, sodium hydrogen phosphite, sodium hypophosphite, etc.) or an ammonium salt (ammonium phosphite, ammonium hydrogen phosphite, ammonium hypophosphite, etc.). The oxidation number of P in phosphoric acid or phosphate is +5. The + 5-valent oxidation number is the highest oxidation number of P, and the electron arrangement is the same noble gas electron arrangement as Ne. For this reason, P atoms in phosphoric acid and phosphates are not suitable because they are chemically stabilized and do not serve as a P supply source. For the synthesis of PtP catalysts, compounds containing P with an oxidation number of less than +5 must be used.

Specifically, for example, 2.56 mmol of bis (acetylacetonato) platinum (II) and 1.28 mmol of sodium hypophosphite are dissolved in 100 ml of ethylene glycol, respectively, and conductive carbon (Vulcan XC-72R manufactured by Cabot Corporation, Specific surface area 254 m ^{2 /} g) is added to 200 ml ethylene glycol solution in which 0.5 g is dispersed. The solution is refluxed for 4 hours while stirring at 200 ° C. in a nitrogen gas atmosphere to deposit and support the PtP catalyst on the conductive carbon. After completion of the reaction, the carbon-supported PtP catalyst can be obtained by filtration, washing and drying.

  The PdNi catalyst according to the present invention can be used particularly for a fuel electrode of a fuel cell using an organic acid having a carboxyl group as an anode fuel and oxygen as a cathode fuel. Moreover, it can be used for the fuel electrode catalyst layer in the membrane electrode assembly of this fuel cell.

It is a figure which shows the atomic arrangement | positioning of a part of PdNi surface used in order to calculate the decomposition reaction of formic acid by the PdNi catalyst based on this invention, and the structure of adsorption | suction of HCOO, an intermediate | middle, and decomposition | disassembly. It is a figure which shows the atomic arrangement | positioning of a part of Pd surface used in order to calculate the decomposition reaction of formic acid by a Pd catalyst, and the structure of adsorption | suction of HCOO, intermediate | middle, and decomposition | disassembly. 1 is a schematic cross-sectional view showing a fuel cell according to the present invention.

Explanation of symbols

40 Fuel Cell 41 Solid Polymer Electrolyte Membrane 42 Air Introduction Hole 43 Oxygen Electrode Side Diffusion Layer 44 Oxygen Electrode Side Current Collector 45 Oxygen Electrode PtP Catalyst Layer 46 Fuel Electrode PdNi Catalyst Layer 47 Fuel Electrode Side Current Collector 48 Fuel Electrode Side Diffusion Layer 49 Fuel introduction hole

Claims (7)

A fuel electrode catalyst for use in a fuel cell having a fuel electrode and an oxygen electrode,
1-80 at. A fuel electrode catalyst comprising Pd—Ni binary fine particles containing% Ni and the balance being substantially Pd.   A fuel electrode catalyst, wherein the Pd-Ni binary fine particles are supported on conductive carbon.   The fuel electrode catalyst according to claim 1 or 2, wherein the fuel of the fuel electrode is an organic acid having a carboxyl group.   The fuel electrode catalyst according to claim 3, wherein the organic acid having a carboxyl group is formic acid.   The oxygen electrode catalyst used for the oxygen electrode is 1 to 50 at. 5. The Pt—P binary fine particles containing% P and the balance being substantially Pt are those supported on conductive carbon. 6. The fuel electrode catalyst as described.   A membrane electrode assembly comprising a fuel electrode catalyst layer comprising the fuel electrode catalyst according to claim 1, an oxygen electrode catalyst layer, and a solid polymer electrolyte membrane interposed between both catalyst layers.   A fuel cell comprising the membrane electrode assembly according to claim 5.

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