JP6137011B2 - Method for producing fuel cell catalyst - Google Patents

Description

  The present invention relates to a method capable of producing a fuel cell catalyst having a desired composition.

  Various metal catalysts including platinum are used for electrodes of fuel cells. Non-Patent Document 1 discloses considerations regarding the stability and activity of platinum alloy nanocatalysts for oxygen reduction.

Seung, J. et al. H. et al. , J .; Am. Chem. Soc. 2012, 134, 19508

Since the platinum yttrium alloy disclosed in Non-Patent Document 1 is manufactured by a sputtering method, it is difficult to prepare a target composition ratio, and it is considered that there is a problem in mass productivity and production cost.
The present invention has been accomplished in view of the above circumstances, and an object thereof is to provide a method for producing a fuel cell catalyst having a desired composition.

  The method for producing a fuel cell catalyst of the present invention is a method for producing a catalyst for a fuel cell containing platinum and yttrium, wherein the conductive support, the platinum-containing solution, and the yttrium-containing solution are mixed in an aqueous solution, A catalyst precursor comprising platinum obtained by reducing a platinum component in the mixture by adding a reducing agent under basic conditions to the mixture obtained by the mixing step, and an yttrium compound derived from the yttrium component in the mixture And a calcination step of calcination of the catalyst precursor obtained by the reduction step in a hydrogen atmosphere and under a temperature condition of 600 ° C. or higher and 1,000 ° C. or lower.

  According to the present invention, by using a hydrogen atmosphere in the firing step, the platinum component and the yttrium component that are raw materials are both reduced to a metal state (that is, zero-valent platinum and zero-valent yttrium) and subjected to alloying. As a result, a fuel cell catalyst having a desired composition ratio can be produced.

2 is an XRD spectrum after firing in Example 1. FIG. FIG. 3 is a diagram showing XRD spectra before and after firing in Example 1 side by side. 2 is a STEM image after the firing step of Example 1. FIG. It is the graph which showed the composition ratio of platinum and yttrium in arbitrary 9 points | pieces in Fig.3 (a)-(c). It is a STEM image after the reduction process of Example 1 and Comparative Example 2 and before the firing process. It is the graph which showed the composition ratio of platinum and yttrium in arbitrary 10 points | pieces in Fig.5 (a)-(c). 6 is a STEM image after a firing step of Comparative Example 2. It is the graph which showed the composition ratio of platinum and yttrium in arbitrary 8 points | pieces in Fig.7 (a)-(c).

  The method for producing a fuel cell catalyst of the present invention is a method for producing a catalyst for a fuel cell containing platinum and yttrium, wherein the conductive support, the platinum-containing solution, and the yttrium-containing solution are mixed in an aqueous solution, A catalyst precursor comprising platinum obtained by reducing a platinum component in the mixture by adding a reducing agent under basic conditions to the mixture obtained by the mixing step, and an yttrium compound derived from the yttrium component in the mixture And a calcination step of calcination of the catalyst precursor obtained by the reduction step in a hydrogen atmosphere and under a temperature condition of 600 ° C. or higher and 1,000 ° C. or lower.

The present invention includes (1) a mixing step, (2) a reduction step, and (3) a firing step. The present invention is not necessarily limited to only the above three steps, and may include, for example, a filtration and washing step, a drying step, a pulverizing step and the like as described later in addition to the above three steps.
Hereinafter, the steps (1) to (3) and other steps will be described in order.

1. Mixing step This step is a step of mixing the conductive carrier, the platinum-containing solution, and the yttrium-containing solution in an aqueous solution.
The production method of the present invention applies a liquid phase method (solution method). In this step, a raw material mixture that can be used in the liquid phase method is prepared.
The platinum-containing solution that can be used in this step is not particularly limited as long as it is a solution that mainly contains at least a platinum component of platinum compounds such as platinum ions or platinum complexes as active species. Although the solvent in a platinum containing solution is not specifically limited, For example, water and alcohol are mentioned.
The yttrium-containing solution that can be used in this step is not particularly limited as long as it is a solution that mainly contains at least one yttrium component of an yttrium ion or an yttrium compound such as an yttrium complex as an active species. The solvent in the yttrium-containing solution is not particularly limited, and examples thereof include water and alcohol.

The conductive carrier that can be used in this step is preferably a carbon carrier. Examples of the carbon carrier include carbon black, graphite carbon, CNT, CNF, highly crystalline carbon, and acetylene black.
The aqueous solution that can be used in this step is preferably an aqueous alcohol solution.
The order in which the conductive support, the platinum-containing solution, and the yttrium-containing solution are mixed is not particularly limited.
Further, the mixing ratio of the conductive support, the platinum-containing solution, and the yttrium-containing solution can be adjusted according to the composition ratio of the target fuel cell catalyst. From the phase diagram of the PtY mixture (for example, the Pt-Y phase diagram published in the NIMS database, etc.), it has been confirmed that there is no hindrance to compounding even if the composition ratio of Pt and Y is arbitrarily set.

Hereinafter, an embodiment of this process will be described. In addition, this process is not necessarily limited only to the following embodiment.
First, a dispersion medium such as distilled water and a conductive carrier are mixed, and the mixture is stirred. A platinum-containing solution and an yttrium-containing solution are further added to the obtained mixture to prepare a raw material mixture.

2. Reduction Step This step is a step of generating a catalyst precursor containing platinum and an yttrium compound by adding a reducing agent to the mixture obtained by the above mixing step under basic conditions. Here, platinum is obtained by reducing the platinum component in the mixture obtained in the mixing step. In addition, the yttrium compound here is derived from the yttrium component in the mixture.
In this step, one of the main features is that the platinum component is reduced to platinum and a yttrium compound is generated under basic conditions.
As can be seen from the Pourbaix Diagram relating to yttrium, yttrium has a low oxidation-reduction potential under acidic conditions and neutral conditions, so it is difficult to precipitate as a solid (that is, easily ionized). However, yttrium is known to precipitate as Y (OH) ₃ under basic conditions.
Therefore, in this step, the platinum component (platinum ion or platinum compound) in the platinum-containing solution is reduced to platinum metal by a reducing agent, and yttrium is precipitated as a hydroxide under basic conditions. A mixture of Pt) and yttrium hydroxide (Y (OH) ₃ ) (ie catalyst precursor) is obtained. In the subsequent calcination step, the target fuel cell catalyst is obtained by thermally reducing yttrium hydroxide in the catalyst precursor to yttrium and further combining platinum and yttrium.

The reducing agent preferably has a basicity as well as having a reducing power capable of reducing platinum ions or platinum compounds to platinum metal. Examples of the reducing agent having sufficiently strong reducing power and basicity include NaBH ₄ .
In addition, in order to ensure the basic conditions in the reaction mixture, a base may be added together with the reducing agent.

Hereinafter, an embodiment of this process will be described. In addition, this process is not necessarily limited only to the following embodiment.
First, a reducing agent is mixed with the mixture obtained by the mixing step and stirred. Since the reducing agent also serves as a base, platinum metal is generated from the platinum component, and yttrium hydroxide (Y (OH) ₃ ) is generated from the yttrium component.

You may perform a filtration and washing | cleaning process, a drying process, and a grinding | pulverization process with respect to a catalyst precursor after a reduction process and before a baking process.
Filtration and washing of the catalyst precursor is not particularly limited as long as it is a method that can remove impurities without impairing the composition of the obtained catalyst precursor. Examples of filtration and washing include a method of suction filtration using water or the like.
The drying of the catalyst precursor is not particularly limited as long as the method can remove the solvent and the like. As an example of the drying, a method of drying for 1 to 12 hours at a temperature of 60 to 80 ° C. in a drying furnace may be mentioned.
The pulverization of the catalyst precursor is not particularly limited as long as it is a method capable of pulverizing a solid lump so that the catalyst precursor is uniformly fired in a firing step described later. As an example of the pulverization, an example in which a mortar filer is used.

3. Calcination step This step is a step of calcination of the catalyst precursor obtained in the reduction step under a hydrogen atmosphere and under a temperature condition of 600 ° C or higher and 1,000 ° C or lower. Through this step, a yttrium compound (mainly yttrium hydroxide) is thermally reduced to yttrium, and platinum and yttrium are combined to obtain a fuel cell catalyst having a target composition.
In this step, one of the main features is firing in a hydrogen atmosphere. By applying a high temperature under a hydrogen atmosphere, platinum metal and zero-valent yttrium can be dissolved in each other, and a homogeneous platinum-yttrium complex can be obtained.
The firing temperature is usually from 600 ° C. to 1,000 ° C., preferably from 650 ° C. to 900 ° C., and more preferably from 700 ° C. to 800 ° C. When the calcination temperature is less than 600 ° C., the calcination does not proceed sufficiently and it is difficult to produce a homogeneous platinum-yttrium composite. When the calcination temperature exceeds 1,000 ° C., as the calcination proceeds excessively, a side reaction occurs and impurities are generated in the obtained fuel cell catalyst.

Hereinafter, an embodiment of this process will be described. In addition, this process is not necessarily limited only to the following embodiment.
First, the catalyst precursor powder that has been filtered, washed, dried, and pulverized is calcined under a hydrogen atmosphere and at a temperature of 600 ° C. to 1,000 ° C., so that yttrium hydroxide (Y (OH) ₃ ) Is reduced to yttrium (Y), and Y after reduction and Pt are combined.

Rare earths such as yttrium are difficult to form composite particles with metals due to their low reduction potential. Therefore, only a composite example of a composite of metal and yttrium is known using a sputtering method. Therefore, the conventional synthesis method of the composite of metal and yttrium has a problem in mass productivity and production cost.
However, in the production method according to the present invention, yttrium is converted to yttrium hydroxide in the step of reducing platinum, yttrium is generated by thermal reduction in the subsequent firing step, and platinum and yttrium are dissolved in a hydrogen atmosphere. By doing so, the catalyst for fuel cells containing a platinum-yttrium complex can be manufactured efficiently.
Further, the obtained fuel cell catalyst has higher activity and higher durability than the conventional platinum-supported carbon, as shown in Examples described later. This is considered to be due to the fact that the lattice constant of platinum was increased by complexing with yttrium, and that the elution resistance of platinum was improved by the influence of yttrium oxide contained in the fuel cell catalyst.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples.

1. Production of catalyst for fuel cell [Example 1]
1-1. Mixing Step First, 175 g of distilled water, 6.66 g of ethanol, and 1.20 g of carbon black (trade name: Ketjenblack) are mixed and stirred, and then 3.49 g of platinum-containing solution and 0.1963 g of yttrium-containing solution are further added thereto. In addition, the mixture was prepared by stirring.

1-2. Reduction step Next, 700 g of distilled water and 0.348 g of NaBH ₄ were mixed to prepare a reducing agent.
The reducing agent was mixed with the mixture obtained in the mixing step and stirred for 30 minutes. By this operation, the platinum component in the mixture is reduced to zero-valent platinum, and NaBH ₄ is also a basic compound, so that yttrium hydroxide (Y (OH) ₃ ) is generated from the yttrium component in the mixture. The
After 30 minutes, the filtrate obtained by filtration was washed with distilled water at 60 ° C. and then dried in a drying furnace under a temperature condition of 80 ° C. to remove water and ethanol in the mixture. The obtained dry solid was crushed with a mortar to obtain a catalyst precursor.

1-3. Firing step The catalyst precursor obtained in the reducing step is calcined under a hydrogen atmosphere and at a temperature of 700 ° C. to reduce yttrium hydroxide (Y (OH) ₃ ) to yttrium (Y). The reduced Y and Pt were combined to produce a fuel cell catalyst of Example 1.

[Comparative Example 1]
For the fuel cell of Comparative Example 1 as in Example 1, except that the yttrium-containing solution was not used in the mixing step of Example 1 and that an argon atmosphere was used instead of the hydrogen atmosphere in the firing step. A catalyst was prepared.

[Comparative Example 2]
A fuel cell catalyst of Comparative Example 2 was produced in the same manner as in Example 1 except that an argon atmosphere was used instead of the hydrogen atmosphere in the firing step of Example 1.

2. 2. Structural analysis of catalyst for fuel cell 2-1. XRD Analysis Regarding the fuel cell catalysts of Example 1, Comparative Example 1, and Comparative Example 2, the chemical structures before and after calcination were examined by X-ray diffraction (XRD).
Table 1 below is a table summarizing the average particle sizes before and after firing determined by XRD together with the composition and firing atmosphere.

  From Table 1 above, Comparative Example 1 (composition: Pt / C) had an average particle size increased by 1.8 times by firing, whereas Example 1 and Comparative Example 2 (composition: PtY / C) The increase in the average particle size due to firing was about 1.5 times. Moreover, the average particle diameter of the raw material is smaller in PtY / C than in Pt / C. Therefore, it can be seen that Example 1 and Comparative Example 2 containing yttrium can maintain a finer particle size after firing than Comparative Example 1 containing no yttrium.

FIG. 1 is an XRD spectrum after firing in Example 1, and FIG. 2 is a diagram showing XRD spectra before and after firing in Example 1 side by side. FIG. 2 shows the XRD spectrum after firing in the upper stage, and the XRD spectrum before firing in the lower stage. The arrows in Figures 1 and 2 show a diffraction peak of Y ₂ O _3.
As can be seen from FIGS. 1 and 2, it was revealed that Y ₂ O ₃ (2θ = 29 °) was also generated by the firing.

  Table 2 below is a table summarizing the peak position (2θ) derived from the Pt [111] plane in the fuel cell catalyst of Example 1 and platinum. In Table 2 below, 2θ of Example 1 is obtained from FIG.

  From Table 2, it can be seen that 2θ of PtY in Example 1 is shifted to a lower angle side than Pt. In view of the fact that the atomic radius of platinum is 1.30 mm while the atomic radius of yttrium is 1.62 mm, the shift to the low angle side in Example 1 is caused by yttrium having a larger atomic radius than platinum. It is thought that it is derived from the fact that it entered the platinum particles and the lattice constant increased.

2-2. ICP-AES Analysis With respect to the fuel cell catalyst of Example 1, the composition ratio and the loading density of platinum and yttrium were measured by inductively coupled plasma atomic emission spectrometry (ICP-AES). Table 3 below is a table summarizing the predicted values from the raw material mixing ratio and the measured values by ICP-AES analysis for the composition ratio and the loading density of platinum and yttrium.

  From Table 3 above, regarding the difference between the predicted value and the measured value, the platinum composition ratio was 2%, the yttrium composition ratio was 6%, the platinum support density was 10%, and the yttrium support density was 19%. Therefore, it can be seen that the platinum-yttrium composite particles almost as aimed could be manufactured as compared with the conventional case.

2-3. STEM Analysis Regarding the fuel cell catalysts of Example 1 and Comparative Example 2, the chemical structures before and after calcination were observed with a scanning transmission electron microscope (STEM).

5A to 5C are STEM images after the reduction process of Example 1 and Comparative Example 2 and before the firing process. FIG. 6 is a graph showing the composition ratio of platinum and yttrium at arbitrary 10 points (10-1 to 10-10) in FIGS. In FIG. 6, the gray area indicates the composition occupied by platinum, and the black area indicates the composition occupied by yttrium. Moreover, the thick broken line in FIG. 6 shows the target composition ratio (Pt: Y = 75 atom%: 25 atom%).
In FIGS. 5A to 5C, in addition to fine particles of 2 to 3 nm, coarse particles on the order of microns can be confirmed. The coarse particles are presumed to be yttria or yttrium hydroxide. It is considered that the target platinum yttrium composite particles are not sufficiently formed at the stage after reduction and before firing. Further, as shown in FIG. 6, in the mixture before firing, the range of the composition ratio of platinum and yttrium is as wide as Pt: Y = 100 atom%: 0 atom% to 12 atom%: 88 atom%, and the composition ratio varies within the mixture. Further, it can be seen that it is far from the target composition ratio (Pt: Y = 75 atom%: 25 atom%).

7A to 7C are STEM images after the firing step of Comparative Example 2. FIG. FIG. 8 is a graph showing the composition ratio of platinum and yttrium at any eight points (11-1 to 11-8) in FIGS. In FIG. 8, the gray area, the black area, and the thick broken line are all the same as in FIG.
7A to 7C, it can be confirmed that coarse particles of micron order still remain in addition to the fine particles of 2 to 3 nm. The coarse particles are presumed to be yttria or yttrium hydroxide, similar to those observed in FIGS. Further, from FIG. 8, even in the mixture after firing, the range of the composition ratio of platinum and yttrium varies from Pt: Y = 95 atom%: 5 atom% to 1 atom%: 99 atom%, and the composition ratio is still uneven in the mixture. Further, it can be seen that the composition ratio is further away from the target composition ratio (Pt: Y = 75 atom%: 25 atom%).

3A to 3C are STEM images after the firing step of Example 1. FIG. FIG. 4 is a graph showing the composition ratio of platinum and yttrium at arbitrary nine points (1 to 9) in FIGS. The regions shown in gray in FIG. 4, the regions shown in black, and the thick broken lines are all the same as in FIG. 6.
In FIGS. 3A to 3C, although 2 to 3 nm fine particles are observed, coarse particles on the order of microns are not observed. Therefore, it is considered that the yttria or yttrium hydroxide as seen in FIGS. 5A to 5C and FIGS. 7A to 7C has disappeared. In addition, as shown in FIG. 4, in the mixture after firing, the range of the composition ratio of platinum and yttrium is as narrow as Pt: Y = 85 atom%: 15 atom% to 55 atom%: 45 atom%, and the target composition ratio ( Pt: Y = 75 atom%: 25 atom%).

3. Activity Evaluation of Fuel Cell Catalysts The catalyst activity of the fuel cell catalysts of Example 1 and Comparative Example 2 was examined by a rotating disk electrode method (RDE).

A rotating disk electrode (RDE) for measurement was prepared. First, a fuel cell catalyst, water, and ethanol were mixed, and the fuel cell catalyst was dispersed in a dispersion medium using an ultrasonic disperser. Next, 20 μL of the dispersion obtained on the glassy carbon RDE was dropped with a microsyringe, and then the RDE was dried at room temperature. 5 μL of Nafion (trade name) solution was dropped into the RDE after drying, and the RDE was further dried at room temperature, and this was used for evaluation of catalyst activity.
The outline of the measuring apparatus is as follows.
Rotating disk electrode device: 1470E manufactured by Toyo Technica
Reference electrode: Hydrogen electrode (RHE)
Working electrode: RDE after the above adjustment
Counter electrode: platinum electrode Electrolyte: 0.1 M HClO ₄ aqueous solution

  The measurement procedure is as follows. First, after adjusting the temperature in the measuring apparatus, a cleaning process was performed to remove impurities on the surface of the working electrode. Next, cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were performed. Each measurement condition is as shown in Table 4 below.

CV: platinum electrochemical surface area than the power supply amount of 0.05~0.4V ^(vs.RHE): was measured (ECSA m 2 / g-Pt ).
LSV: Activity evaluation for oxygen reduction reaction (ORR) was performed.

The activation dominant current i _k (mA) indicating catalytic activity was determined using the Koutecky-Levic equation (1).
1 / i = 1 / i _k + 1 / i Formula _L (1)
(In the above formula (1), i _L represents the diffusion limit current value (mA), and i represents the current value (mA) at 0.9 V (vs. RHE)).
The relationship between 1 / i and ω ^−1/2 (ω is the electrode rotation speed) at 0.9 V (vs. RHE) is plotted, and ω ^−1/2 → 0 (ω → ∞) of the obtained graph. was asked to _{i k} than sections. The _{i k} obtained by dividing the amount of catalyst coated onto RDE (g), was calculated catalyst activity per unit mass (A / g-Pt).

The same catalytic activity evaluation was performed on the fuel cell catalyst after the durability test. Here, the durability test is a test in which the potential of the fuel cell catalyst of Example 1 and Comparative Example 1 is repeatedly swept at a sweep rate of 200 mV / sec within a range of 0.4 to 1.2 V (vs. RHE). Point to.
Table 5 below is a table summarizing the catalyst activity (A / g-Pt) and the activity retention rate (%) after the initial and durability tests, together with the composition and the firing atmosphere. Here, the initial catalytic activity refers to the catalytic activity measured without going through a durability test. The activity maintenance rate is a value (%) obtained by dividing the catalyst activity after the durability test by the initial catalyst activity and multiplying by 100.

From Table 5 above, it can be seen that the initial catalytic activity of Example 1 is 2.2 times that of Comparative Example 1, and the catalytic activity after the durability test is 3.3 times that of Comparative Example 1. . From this result and the result of the structural analysis, it is considered that high activity was obtained because platinum was evenly complexed with yttrium and the lattice constant of platinum was increased.
From Table 5 above, the activity maintenance rate of Example 1 is 10% higher than that of Comparative Example 1. From this result, probably, in the fuel cell catalyst of Example 1, the elution resistance and the like of platinum was improved by the influence of Y ₂ O ₃ observed in the XRD spectrum of FIG. It is thought that it improved.

Claims (1)

A method for producing a catalyst for a fuel cell containing platinum and yttrium,
A mixing step of mixing a conductive carrier, a platinum-containing solution, and an yttrium-containing solution in an aqueous solution;
By adding a reducing agent under basic conditions to the mixture obtained by the mixing step, platinum obtained by reducing the platinum component in the mixture and yttrium hydroxide derived from the yttrium component in the mixture are included. A reduction step to produce a catalyst precursor, and
A method for producing a fuel cell catalyst, comprising a calcining step of calcining the catalyst precursor obtained by the reduction step under a hydrogen atmosphere and under a temperature condition of 600 ° C or higher and 1,000 ° C or lower.