JP2013178981A - Electrode catalyst for fuel cell, membrane electrode assembly and fuel cell system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a highly durable high activity electrode catalyst for oxygen reduction in which a very expensive platinum group metal of small reserves is not used, and to provide a membrane electrode assembly using the same, and a fuel cell system.SOLUTION: The electrode catalyst for fuel cell composed mainly of carbon containing any one or both of nitrogen and boron atom and iron, further contains at least one kind of element selected from a lanthanoid group. More specifically, the content of lanthanoid for the total of iron and lanthanoid contained in the catalyst is 20-60 at.%.

Description

  The present invention relates to a highly durable electrode catalyst for a fuel cell containing a lanthanoid as a main component, and a membrane using the fuel cell electrode catalyst. The present invention relates to an electrode assembly and a fuel cell system.

  In addition to the recent rise in crude oil prices, rapid economic development in China, India and other countries has led to global problems of fossil fuel depletion and carbon dioxide emissions. For this reason, research and development of fuel cells, lithium-ion batteries, biofuels, solar cells, etc. are being actively carried out for the de-oiling. Fuel cells that use solid polymer electrolyte membranes include direct methanol fuel cells (DMFC) that use methanol as the anode fuel, and polymer electrolyte fuel cells that use hydrogen gas as the anode fuel (DMFC). Polymer Electrolyte Fuel Cell (PEFC).

  DMFC and PEFC can operate at relatively low temperatures, and the power generation system is simple and can be downsized. Therefore, emergency power supplies for facilities, emergency power supplies for military and commercial mobile devices, laptop computers and portable music players Expected as a charger for mobile phones.

DMFC uses methanol as the fuel, and PEFC uses hydrogen as the fuel. The membrane electrode assembly composed of the anode catalyst layer / proton conductive film / cathode catalyst layer is sandwiched between the conductive gas diffusion layers, and the anode and The battery system is connected to an external circuit by a current collecting plate provided on the cathode electrode. When methanol, which is a liquid fuel, is supplied to the anode catalyst layer side of the DMFC, the methanol is oxidized and converted into carbon dioxide (CO ₂ ) by a chemical reaction represented by the following formula (1), and protons (H ⁺ ) and electrons ( e ^-) and is generated.
CH ₃ OH + H ₂ O → CO ₂ + 6H ⁺ + 6e ⁻ (1)

Protons and electrons generated by this reaction generate water (H ₂ O) by the reaction of the oxygen gas supplied to the cathode catalyst layer and the following formula (2).
O ₂ + 4H ⁺ + 4e ⁻ → 2H ₂ O (2)

Therefore, the reaction of the following formula (3) proceeds as a whole battery, and electrons generated at this time can be taken out by an external circuit to obtain electric energy.
CH ₃ OH + 3 / 2O ₂ → CO ₂ + 2H ₂ O (3)

  Currently, platinum-based catalysts are used as DMFC and PEFC electrodes as practical catalysts. However, since they are rare metals and very expensive, it is essential to reduce the amount of platinum used. It has become.

  As means for solving these problems, for example, attempts are made to improve the activity per gram of catalyst and reduce the amount of catalyst used by reducing the particle diameter of the platinum-based catalyst particles and increasing the specific surface area of the catalyst. ing.

  However, by reducing the catalyst particle diameter, the surface energy of the catalyst particles increases, the catalyst particles tend to aggregate and become coarser, and the performance deterioration is severe. In addition, since the catalyst particle diameter is small, there is a problem that the catalyst particles are easily eluted. These catalysts have high initial performance but poor durability. As a result, there is a problem that the cost reduction merit of the fuel cell is small and the desired cell performance cannot be obtained.

  For these reasons, development of an inexpensive catalyst that does not use platinum is required to reduce the cost of fuel cells. On the other hand, it is known that carbon-based catalysts doped with nitrogen atoms and / or boron atoms exhibit oxygen reduction activity. For example, Patent Document 1 discloses non-platinum-based cathode catalysts such as iron and cobalt. A fuel cell electrode based on a carbon-based catalyst containing a transition metal has been reported.

  Among these transition metals, it is known that a non-platinum-based catalyst containing iron exhibits the highest activity (Non-Patent Document 1).

JP 2004-362802 A JP 2008-282725 A

J. Phys. Chem. C (2009) 113, 15422-15432

However, in non-platinum-based carbon catalysts containing iron, hydrogen peroxide and iron generated during the oxygen reduction reaction generate radicals with very strong oxidizing power through the Fenton reaction (the following formulas (4) and (5)). By doing so, there is a problem that the catalyst and the membrane electrode assembly are deteriorated and the performance of the fuel cell system is lowered.
Fe ²⁺ + H ₂ O ₂ → Fe ³⁺ + OH ′ + OH ⁻ (4)
Fe ³⁺ + H ₂ O ₂ → Fe ²⁺ + OOH ′ + H ⁺ (5)

  In order to solve this problem, it is necessary to develop a carbon-based catalyst material that can maintain the oxygen reduction activity even if the iron content in the catalyst is reduced and can suppress the deterioration of the catalyst and the membrane electrode assembly. .

  The present invention has been made in view of such circumstances, and an object of the present invention is to suppress a decrease in oxygen reduction activity and to provide a carbon-based fuel cell electrode catalyst excellent in durability, and the fuel cell electrode catalyst. An object of the present invention is to provide a membrane electrode assembly and a fuel cell using the above.

  The electrode catalyst for a fuel cell of the present invention capable of achieving the above object includes at least one metal species selected from the lanthanoid group, and is based on carbon containing either or both of nitrogen and boron atoms. It is characterized by that.

  Furthermore, the membrane electrode assembly of the present invention is an membrane electrode assembly having an anode catalyst layer, a cathode catalyst layer, and a solid polymer electrolyte membrane disposed between the anode catalyst layer and the cathode catalyst layer. A catalyst based on carbon containing at least one metal species selected from a lanthanoid group and containing either or both of nitrogen and boron atoms, as the catalyst of the cathode catalyst layer, To do.

  Further, the present invention is characterized by a fuel cell system using the membrane electrode assembly.

  According to the present invention, there are provided a carbon-based electrode catalyst for a fuel cell that suppresses a decrease in oxygen reduction activity and is excellent in durability, and a membrane electrode assembly and a fuel cell using the fuel cell electrode catalyst. Can do.

The figure which showed the relationship between the composition ratio of iron and the maintenance rate of oxygen reduction activity. The figure which showed the relationship between the composition ratio of iron and the maintenance rate of oxygen reduction activity.

  As disclosed in Non-Patent Document 1, the oxygen reduction activity of the carbon-based catalyst containing iron is improved. However, in a carbon-based catalyst containing only iron, hydrogen peroxide and iron generated during the oxygen reduction reaction cause a Fenton reaction to generate radicals with strong oxidizing power. Accordingly, there is a problem that the solid polymer membrane constituting the electrode catalyst and the membrane electrode assembly is deteriorated by being oxidized, and the performance of the fuel cell system is lowered. On the other hand, as a result of intensive studies by the present inventors, at least selected from a lanthanoid group with respect to a carbon-based catalyst containing as a main component carbon containing iron or both of nitrogen and boron atoms and iron. It has been found that the durability of the electrode catalyst and membrane electrode assembly can be improved by including one type of metal species.

  In the fuel cell electrode catalyst of the present invention, in order to improve durability (oxygen reduction activity maintenance ratio), the content of lanthanoid relative to the total of iron and lanthanoid contained in the catalyst is preferably 20 at.% Or more. Further, from the viewpoint of maintaining the oxygen reduction activity at the same time, it is desirable to contain iron at a certain ratio, and the content of lanthanoid with respect to the total of iron and lanthanoid contained in the catalyst is in the range of 20 at.% To 60 at.%. It is preferable that If the content of lanthanoid relative to the total of iron and lanthanoid contained in the catalyst is less than 20 at.%, The oxygen reduction activity of the catalyst is maintained, but the catalyst is significantly deteriorated and the desired performance cannot be obtained. In addition, when the content of lanthanoid relative to the total of iron and lanthanoid contained in the catalyst is larger than 60 at.%, The oxygen reduction activity of the catalyst is decreased, and desired performance can be obtained. There are no prisoners.

  The fuel cell electrode catalyst of the present invention exhibits oxygen reduction activity and is used as a catalyst for a cathode catalyst layer of a fuel cell membrane electrode assembly.

  That is, the fuel cell membrane electrode assembly of the present invention uses the fuel cell electrode catalyst of the present invention as the catalyst of the cathode catalyst layer, and is a direct methanol fuel cell, a solid polymer using hydrogen as fuel. The present invention can be applied to a fuel cell such as a type fuel cell. Moreover, there is no restriction | limiting in particular about structures and structures other than the cathode catalyst layer of a fuel cell, The structure and structure conventionally known can be applied.

  Hereinafter, preferred embodiments of the present invention will be described in detail. However, the following examples do not limit the present invention.

Example 1
Iron acetate: 0.0015 mol and lanthanum nitrate: 0.0005 mol were added to a solution in which 0.5 g of a phenolic resin as a carbon-based catalyst precursor was dispersed in ethanol. Next, 0.006 mol of 1,10 phenanthroline was added and stirred. This mixed solution was dried under reduced pressure, 0.5 g of Ketjen Black EC300J as a carrier was added, and the mixture was uniformly mixed in a mortar. This mixture was uniformly placed in a quartz boat and put into a tubular electric furnace. Nitrogen gas was allowed to flow into this tubular electric furnace at a rate of 100 ml / min., And in an inert atmosphere, heat treatment was performed at a heat treatment temperature of 900 ° C. to obtain a desired carbon-based non-platinum catalyst.

(Example 2)
A carbon-based non-platinum catalyst was obtained in the same manner as in Example 1 except that the addition amounts of iron acetate and lanthanum nitrate in Example 1 were changed to 0.001 mol of iron acetate and 0.001 mol of lanthanum nitrate, respectively. It was.

(Example 3)
A carbon-based non-platinum catalyst was obtained in the same manner as in Example 1 except that the addition amounts of iron acetate and lanthanum nitrate in Example 1 were changed to 0.0005 mol of iron acetate and 0.0015 mol of lanthanum nitrate, respectively. It was.

Example 4
A carbon-based non-platinum catalyst was obtained in the same manner as in Example 1 except that the amount of lanthanum nitrate added in Example 1 was changed to 0.002 mol of lanthanum nitrate.

(Example 5)
A carbon-based non-platinum catalyst was obtained in the same manner as in Example 1 except that 0.0005 mol of praseodymium nitrate was added instead of 0.0005 mol of lanthanum nitrate in Example 1.

(Example 6)
The amount of iron acetate in Example 1 was changed to 0.001 mol, and carbon dioxide was added in the same manner as in Example 1 except that 0.001 mol of praseodymium nitrate was added instead of 0.0005 mol of lanthanum nitrate. A non-platinum catalyst was obtained.

(Example 7)
The amount of iron acetate in Example 1 was changed to 0.0005 mol, and carbon dioxide was added in the same manner as in Example 1 except that 0.0035 mol of praseodymium nitrate was added instead of 0.0005 mol of lanthanum nitrate. A non-platinum catalyst was obtained.

(Example 8)
A carbon-based non-platinum catalyst was obtained in the same manner as in Example 1 except that 0.0002 mol of praseodymium nitrate was added instead of 0.0015 mol of iron acetate and 0.0005 mol of lanthanum nitrate in Example 1. .

Example 9
A carbon-based non-platinum catalyst was obtained in the same manner as in Example 1 except that 0.0005 mol of gadolinium nitrate was added instead of 0.0005 mol of lanthanum nitrate in Example 1.

(Example 10)
The addition amount of iron acetate in Example 1 was changed to 0.001 mol, and instead of 0.0005 mol of lanthanum nitrate in Example 1, 0.001 mol of gadolinium nitrate was added, and the same as in Example 1. Thus, a carbon-based non-platinum catalyst was obtained.

(Example 11)
Example 1 is the same as Example 1 except that the addition amount of iron acetate in Example 1 is changed to 0.0005 mol, and 0.0035 mol of gadolinium nitrate is added instead of 0.0005 mol of lanthanum nitrate in Example 1. Thus, a carbon-based non-platinum catalyst was obtained.

(Example 12)
A carbon-based non-platinum catalyst was obtained in the same manner as in Example 1 except that 0.0002 mol of gadolinium nitrate was added instead of 0.0015 mol of iron acetate and 0.0005 mol of lanthanum nitrate in Example 1. .

(Comparative Example 1)
A carbon-based non-platinum catalyst was obtained in the same manner as in Example 1 except that the amount of iron acetate added in Example 1 was changed to 0.002 mol and lanthanum nitrate was not added.

(Evaluation of composition ratio of carbon-based non-platinum catalyst)
The composition ratio (at.%) Of the carbon-based non-platinum catalysts obtained in Examples 1 to 12 and Comparative Example 1 was evaluated using X-ray photoelectron spectroscopy (XPS). The measurement of XPS is performed using X-ray source: Monochrome Al (tube voltage 15 kV, tube current, 15 mA), lens condition: HYBRID (analysis area: 600 × 1000 μm mouth), resolution: Pass energy: 40, scanning speed: 20 eV / min ( (0.1 eV step), spectrum calibration: performed under calibration conditions with a carbon 1s peak.

(Evaluation of oxygen reduction activity and durability of carbon-based non-platinum catalysts)
Standard three-electrode cells were assembled for the carbon-based non-platinum catalysts obtained in Examples 1 to 12 and Comparative Example 1, and their oxygen reduction activity before and after the durability test was evaluated by the rotating disk electrode method.

  First, 20 μl of a carbon-based non-platinum catalyst dispersed in pure water is taken with a micropipette, applied onto a glassy carbon electrode for a rotating disk and dried, and then an ion conductive polymer dispersion (manufactured by Aldrich). A working electrode was obtained by applying 5 μl of “Nafion (registered trademark)” and drying it. Also, a platinum wire was used as the counter electrode, and a reversible hydrogen electrode (RHE) electrode was used as the reference electrode, thereby constituting a three-electrode cell.

  First, in order to evaluate the initial activity before the durability test, 10 potential cycles of 0 V to 1.2 V were performed in a 3-electrode cell in a 0.5 mmol / l sulfuric acid aqueous solution under nitrogen saturation to wash the catalyst surface. Next, while rotating the working electrode at an electrode rotation speed of 1600 rpm, a potential cycle from 0 V to 1.1 V was performed for one cycle, and the background current was measured. Thereafter, the nitrogen gas is changed from nitrogen to oxygen, and the working electrode is rotated at an electrode rotation speed of 1600 rpm while the oxygen is saturated, and a potential cycle from 0 V to 1.1 V is performed for one cycle, and the carbon-based non-platinum catalyst. The oxygen reduction current was measured. The background current was subtracted from the measured oxygen reduction current, and the current at 0.5 V was compared to evaluate the initial oxygen reduction activity before the durability test. Thereafter, 500 potential cycles from 0 V to 1.1 V were performed, and the same operation as the initial oxygen reduction activity was performed to evaluate the oxygen reduction activity after the durability test.

  For the carbon-based non-platinum catalysts obtained in Examples 1 to 12 and Comparative Example 1, the composition ratio (at.%), The oxygen reduction activity at the initial stage and after 500 cycles, and the activity maintenance of the oxygen reduction activity after 500 cycles The rates are shown in Table 1. In Table 1, the oxygen reduction activity was compared and evaluated with the oxygen reduction activity of Comparative Example 1 manufactured with an initial iron composition of 100% as 1.

  Further, based on the evaluation results shown in Table 1, FIG. 1 shows the relationship between the iron content (at.%) And the maintenance ratio of the oxygen reduction activity with respect to the total amount of lanthanoid and iron contained in the catalyst. Further, based on the evaluation results shown in Table 1, FIG. 2 shows the relationship between the iron content (at.%) And the oxygen reduction activity with respect to the total amount of lanthanoid and iron contained in the catalyst. Also in FIG. 1 and FIG. 2, the oxygen reduction activity is a comparison and evaluation of the oxygen reduction activity, assuming that the initial oxygen reduction activity of Comparative Example 1 which is an iron-containing carbon-based catalyst containing no lanthanoid is 1.

  From the results of Table 1, the activity retention rate of Comparative Example 1 in which lanthanoids lanthanum, praseodymium, and gadolinium were not added was 81%, whereas lanthanoids lanthanum, praseodymium, and gadolinium were added. 12, it can be seen that the activity maintenance ratio is improved from 81.2 to 100%. In addition, as shown in FIG. 1, the activity retention rate increases as the content of lanthanides, praseodymium and gadolinium increases with respect to the total amount of lanthanoid and iron contained in the catalyst. It can be seen that the durability is clearly improved as compared with Comparative Example 1 having a composition of 100 at. From these results, it is possible to dramatically improve the durability by adding at least one metal species selected from the lanthanoid group to the electrode catalyst. In particular, in order to improve durability, the content of lanthanoid is preferably 25 at.% Or more, more preferably 50 at.% Or more with respect to the total of lanthanoid and iron contained in the catalyst.

  Further, from the results of Table 1 and FIG. 2, the content of lanthanoids lanthanum, praseodymium, and gadolinium is 20 to 55 at.% Relative to the total of lanthanoid and iron contained in the catalyst with respect to Comparative Example 1. In Examples 1, 2, 5, 6, 9 and 10, which are within the range, the oxygen reduction activity is from 0.99 to a maximum of 1.15, and the iron content is equivalent to the oxygen reduction activity of 100 at. It can be seen that more oxygen reduction activity can be obtained.

  From the viewpoint of achieving both improvement in durability and maintenance of oxygen reduction activity, the content of lanthanoid relative to the total of lanthanoid and iron contained in the catalyst is 20 to 60 at.%, More preferably 25 to 55 at.%. Is preferred.

  In the present embodiment, an example in which lanthanum, praseodymium, and gadolinium are doped in the lanthanoid group is shown. However, the lanthanoid is not much changed in how electrons are packed in the 5d orbital 6s orbital. Since the properties of each element of lanthanoid are very similar, lanthanoid groups other than lanthanum, praseodymium and gadolinium are considered to have the same effect.

  According to this example, even if the ratio of iron contained in the catalyst is reduced, it is possible to provide a highly durable cathode fuel cell electrode catalyst while maintaining oxygen reduction activity. Thereby, the fuel cell which has the outstanding battery characteristic can be provided.

Claims (4)

In an electrode catalyst for a fuel cell mainly composed of carbon containing iron containing either or both of nitrogen and boron atoms,
A fuel cell electrode catalyst comprising at least one element selected from a lanthanoid group.   2. The fuel cell electrode catalyst according to claim 1, wherein the content of lanthanoid relative to the total of iron and lanthanoid contained in the catalyst is 20 at.% To 60 at.%.   The membrane electrode assembly in which a solid polymer electrolyte membrane is disposed between an anode catalyst layer and a cathode catalyst layer, wherein the catalyst contained in the cathode catalyst layer is the fuel cell electrode catalyst according to claim 1. A membrane electrode assembly.   A fuel cell comprising the membrane electrode assembly according to claim 3.