JP4998984B2 - Electrode active material and positive electrode oxygen reduction electrode using the same - Google Patents

Description

  The present invention relates to an oxygen reduction electrode used in a polymer electrolyte fuel cell.

Solid polymer electrolyte (PEFC) fuel cells operate at a low temperature of 100 ° C or lower, which has the advantage that plastics and inexpensive metals can be used in the equipment, and are being developed for portable applications, mobile power supplies, and distributed power supplies. . This type of fuel cell has a structure in which a proton conductive polymer electrolyte membrane is sandwiched between a negative electrode and a positive electrode, hydrogen or a hydrogen-containing fuel (such as an aqueous methanol solution) is supplied to the negative electrode, and air is supplied to the positive electrode. Yes. Then, hydrogen (fuel) is oxidized at the negative electrode, and oxygen is reduced at the positive electrode, and electric energy is extracted outside.
Solid polymer fuel cells are divided into a hydrogen type that supplies hydrogen gas to the negative electrode and a direct type that uses liquid fuel such as methanol as the direct fuel. The latter is characterized by a simple structure without the need for a hydrogen gas cylinder. is there.

  Conventionally, as the positive electrode, a material in which platinum is supported on carbon black is generally used, but it has been studied to use a metal compound as an electrode active material without using expensive platinum. As such metal compounds, metal oxides, oxynitrides, and nitrides have been proposed (see, for example, Non-Patent Documents 1 and 2).

A. Ishihara, et.al., Electrochem. Soild-State Lett., 8, A201 (2005) Y. Liu, et.al., Electrochem. Solid-State Lett., A400 (2005)

However, metal oxides, oxynitrides, and nitrides still have insufficient catalytic ability as active materials used for positive electrode oxygen reduction electrodes of polymer electrolyte fuel cells.
Accordingly, an object of the present invention is to provide an electrode active material capable of improving the oxygen reduction catalytic ability of a polymer electrolyte fuel cell, and an oxygen reduction electrode for a positive electrode using the same.

The electrode active material of the present invention is an electrode active material for an oxygen reduction electrode used as a positive electrode of a polymer electrolyte fuel cell, and is made of a carbonitride of Ta, Cr or Zr .

Before SL element is Ta, it is preferred that the reciprocal of the full width at half maximum of the peak of 35 ° near obtained by X-ray powder diffractometry is 0.4.

  The oxygen reduction electrode for positive electrode of the present invention carries the electrode active material.

  According to the present invention, it exhibits high performance as a non-platinum oxygen reduction catalyst for an oxygen reduction electrode for a polymer electrolyte fuel cell.

  Hereinafter, embodiments of the present invention will be described. In addition, the potential used in the following description and chart is based on the reversible hydrogen electrode potential reference, and this is indicated as RHE as necessary.

<Direct fuel cell>
The electrode active material of the present invention is supported on an oxygen reduction electrode used as a positive electrode (air electrode) of a polymer electrolyte fuel cell. A polymer electrolyte fuel cell sandwiches an electrolyte membrane between a positive electrode and a negative electrode, supplies hydrogen or a hydrogen-containing fuel (such as an aqueous methanol solution) from the outside of the negative electrode, and supplies an oxygen-containing gas (usually air) from the outside of the positive electrode Then, the electric energy is taken out to the outside.
The negative electrode and the positive electrode are usually formed by applying an electrode active material as a catalyst on the surface of a porous electrode substrate.
As said fuel, what contains a carbon atom and a hydrogen atom in chemical structures, such as alcohol and ether, can be used, for example. Specific examples of the fuel include methanol, ethanol, glycol, acetal, dimethyl ether, etc. However, it is effective for improving the energy conversion efficiency of the fuel cell, particularly when methanol is used as the fuel having a small activation energy for the oxidation reaction. is there.

<Electrode active material>
The electrode active material of the present invention comprises a carbonitride of one or more elements selected from the group consisting of Group 5 elements excluding V, Group 4 elements excluding Ti, and Group 6 elements. As a result of extensive studies by the present inventors, it has been found that the oxygen reduction catalytic ability of carbonitrides of the above elements is higher than that of other compounds (for example, carbides).
Examples of the Group 5 element include Nb and Ta, examples of the Group 4 element include Zr and Hf, and examples of the Group 6 element include Cr, Mo, and W.
The reason for removing V and Ti is that it dissolves in an acidic electrolyte and is unstable.
In the present invention, TaCN includes an amorphous substance of Ta, C, and N and a crystal having a predetermined atomic ratio. The same applies to other compounds.

<Oxygen reduction electrode>
The oxygen reduction electrode in which the electrode active material is supported on the surface of the electrode base material, for example, supports the powder having the above composition on the surface of the electrode base material, and forms a film having the above composition on the surface of the electrode base material by vapor deposition or sputtering. By doing so, it can be manufactured.
As the electrode base material, for example, carbon powder and conductive oxide powder can be used.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples.
(Production of electrodes carrying TaCN, CrCN and ZrCN)
A TaCN supported electrode was prepared by sputtering. TaC was used as a target, and the atmosphere was set to an Ar flow rate of 5 sccm and a nitrogen flow rate of 25 sccm so that the total pressure was 0.47 Pa. In this atmosphere, columnar glassy carbon having a diameter of 5.2 mm was used as an electrode base material, and a substrate temperature during film formation was heated to 800 ° C. using a halogen lamp. Sputtering was performed on the bottom surface of the base material to form a film containing nitrogen. The film thickness of the film was measured using a quartz oscillator type film thickness meter, and the film thickness was controlled to 10 nm. For CrCN and ZrCN, electrodes were prepared in the same manner as in TaCN except that the targets were changed to CrC and ZrC, respectively.

(Characterization of electrode active material)
The XRD (X-ray diffraction) pattern of the crystal structure of the above TaCN thin film is shown in FIG. When the electrode substrate was sputtered by heating to 800 ° C., peaks derived from TaC and TaN (diffraction angle of about 35 °) were observed, confirming that a Ta—CN compound (partially crystallized) was generated. . This is considered to have appeared as a single peak due to the solid solution. Similarly, the XRD pattern of the crystal structure of the CrCN thin film is shown in FIG. When the electrode substrate was sputtered by heating to 800 ° C., a peak derived from Cr ₃ C ₂ , CrN, Cr ₂ N _0.29 C _0.61 was observed, and a Cr—CN compound (partially crystallized) was generated. confirmed.
The electrodes using other compounds as electrode active materials were similarly characterized by XRD measurement.
Table 1 shows the TaCN component composition when the electrode substrate temperature during sputtering is changed in the sputtering method. The composition was determined by RBS (Rutherford backscattering analysis).
In Table 1, the oxygen in the component seems to be due to H ₂ O or O ₂ remaining in the sputtering chamber being mixed into the compound.

(Evaluation of catalytic performance of oxygen reduction electrode)
Each oxygen reduction electrode produced by the method described above was immersed in a 0.1 mol · dm ⁻³ sulfuric acid solution, and the oxygen reduction current density at 0.4 V was measured in an atmosphere of 30 ° C., atmospheric pressure, nitrogen, and oxygen. A reversible hydrogen electrode in the same concentration sulfuric acid solution was used as a reference electrode. The display of current density was per geometric area.

<Comparison of oxygen reduction catalytic ability>
FIG. 3 shows a case where TaCN (Ta carbonitride), CrCN and ZrCN, which are one of the electrode active materials of the present invention, are used as an electrode active material, and TaC, CrC and ZrC which are other compounds as electrode active materials. The comparison of the catalytic ability of the oxygen reduction electrode when used as a substance is shown. The electrodes carrying TaCN, CrCN and ZrCN were prepared in the same manner as described above.
(Production of electrodes carrying TaC, CrC and ZrC)
Electrodes carrying TaC, CrC and ZrC were prepared in the same manner as electrodes carrying TaCN, CrCN and ZrCN, except that sputtering was performed using TaC, CrC and ZrC as targets, respectively, and the sputtering atmosphere was only Ar. did. About these electrodes, the XRD measurement was performed similarly to the above, and it confirmed that TaC had arisen.

(Evaluation of catalytic performance of oxygen reduction electrode)
The oxygen reduction current-potential curve when the potential was changed was measured by the same method as described above.
(result)
As is apparent from FIG. 3, in the case of the electrode using TaC, CrC and ZrC, an oxygen reduction current was observed from around 0.5 V, which indicates that the oxygen reduction catalytic ability is very low. On the other hand, in the case of an electrode carrying TaCN, CrCN and ZrCN as an electrode active material, it was found that oxygen reduction started from 0.75 V and the oxygen reduction current was large, and the oxygen reduction catalytic ability was excellent.
From this, it was found that it is important to include N in the compound which is an electrode active material.

<Evaluation of electrochemical stability of oxygen reduction electrode>
The current-potential curve (cyclic voltammogram: CV) of the electrode using the above TaCN, CrCN and ZrCN was measured. CV was performed by scanning at 50 mV / s at a potential between 0.05 V and 1.0 V in a nitrogen atmosphere at 30 ° C. As an electrolyte, a 0.1 mol / L sulfuric acid aqueous solution was used.
The results are shown in FIG. Although the shape of CV shows a typical charge / discharge current of a capacitor, the shape of CV did not change in each electrode, and oxidation current and reduction current based on reaction were not observed. Therefore, each electrode was found to be extremely stable in sulfuric acid (acid electrolyte) in the range of 0.05 to 1.0 V.

<Preparation and characterization of TaCN electrode by powder method>
Ta ₂ O ₅ powder and carbon powder were uniformly mixed and heat-treated at 1600 ° C. in a nitrogen atmosphere to prepare TaC _x N _y powder. The composition (x, y) was adjusted by the amount of carbon mixed. Using this as a starting material, an electrophoretic electrodeposition method was used, followed by further heat treatment to produce an electrode. Electrodeposition bath for use in electrophoretic deposition method of TaC _x N _y powder 100mg 0.2gdm ^-3 iodine in 50 mL - was assumed dispersed in acetone. In this bath, a 100 V DC voltage was applied for 60 seconds between the anode and the cathode, and electrodeposited onto a 10 × 10 mm Ti substrate on the cathode side. Next, the substrate was subjected to heat treatment for 1 hour in a temperature range of 600 ° C. to 1400 ° C. in an N ₂ / H ₂ (2%) atmosphere containing a trace amount of oxygen.
(characterization)
FIG. 5 shows an XRD pattern of a TaCN electrode that was not heat-treated after electrodeposition and was produced at a heat treatment temperature of 600 ° C. to 1400 ° C. The X-ray intensity was normalized by the strongest peak intensity of each diffraction line. In the case of no heat treatment and heat treatment temperatures of 600 ° C. and 800 ° C., only a peak corresponding to TaC _0.56 N _0.44 (Cubic) was observed. On the other hand, when the heat treatment temperature was 1000 ° C. to 1400 ° C., TaC _0.56 N _0.44 and Ta ₂ O ₅ (Orthorhombic) were observed. In addition, the TaC _0.56 N _0.44 peak decreased relatively and the Ta ₂ O ₅ peak increased as the heat treatment temperature increased. From this, it is considered that the oxidation gradually progressed due to the influence of a trace amount of oxygen in the heat treatment atmosphere as the heat treatment temperature increased.

<Relationship between crystallinity and catalytic ability of TaCN>
The TaCN crystallinity was determined from the XRD patterns shown in FIGS. 1 and 5 in the TaCN electrodes prepared by the sputtering method and the powder method, respectively. For the crystallinity, the reciprocal of the full width at half maximum (FWHM ⁻¹ ) of the peak at a diffraction angle of 35 ° was used. It shows that crystallization is progressing, so that FWHM- ¹ is large.
Next, at each temperature shown in FIGS. 1 and 5 (in the case of sputtering, the electrode substrate temperature during sputtering, and in the case of powder, the heat treatment temperature), the oxygen reduction current density at 0.4 V in the same manner as described above. Was measured. And the relationship between FWHM- ¹ and oxygen reduction current density in each temperature was plotted in the following FIG.

FIG. 6 shows the relationship between the FWHM- ¹ obtained for the TaCN electrode by the sputtering method and the powder method and the oxygen reduction current density. As is clear from this figure, it was found that the higher the crystallinity of TaCN (the higher the FWHM- ¹ ), the higher the oxygen reduction current density and the better the catalytic ability.
Next, a preferable threshold value (minimum value) of FWHM ⁻¹ for exhibiting the catalytic ability required for an actual fuel cell is calculated. The cell voltage of an actual portable fuel cell is 0.3V, but if the voltage loss of the fuel electrode is estimated to be 0.1V, the reaction proceeds at a voltage of 0.4V at the air electrode as a reversible hydrogen electrode reference. Therefore, the value of FIG. 6 measured at 0.4 V can be referred to as it is.

Here, the output (current density) required for the portable fuel cell is 100 mA / cm ^{2 on the} basis of the cell geometric area. The cell geometric area is the apparent geometric area of the membrane-electrode assembly of the actual battery. The geometric area of the substrate when the fuel cell is actually produced and the catalyst powder is applied to the substrate to form an electrode. That is.
A platinum catalyst used in a general fuel cell is supported on the order of 1 mg / cm ^{2 on the} basis of the cell geometric area. Since platinum is a fine particle with an average particle diameter of 2 to 3 nm and the surface area is increased, the actual electrode surface area of the platinum catalyst is approximately 500 cm ² (actual surface area) / 1 cm ² (cell geometric area) relative to the cell geometric area. There is a relationship (ratio 500).

Based on the above, the actual surface area based output (current density, the value on the vertical axis in FIG. 6) when the electrode active material of the present invention is used as an alternative to platinum is obtained. Since the electrode active material of the present invention is cheaper than platinum, the amount supported on the electrode can be increased 10 times or more than platinum. Now, in the electrode active material of the present invention, assuming that the ratio of the actual surface area to the cell geometric area is about the same as that of the platinum catalyst (= 500), the actual surface area of the electrode active material of the present invention is 5000 cm ² (actual surface area) / 1 cm. ² (cell geometric area). On the other hand, as described above, the output (current density) based on the cell geometric area required for an actual fuel cell is 100 mA / cm ² .
Therefore, when using the electrode active material of the present invention, the output (current density) based on the actual surface area is
{100mA / cm ² (cell geometric area)} / {5000cm ² (actual surface area) / 1cm ² (cell geometric area)}
= 20 μA / cm ² (actual surface area).

Here, the value of the oxygen reduction current density on the vertical axis in FIG. 6 is based on a second geometric area different from the cell geometric area of the membrane-electrode assembly of the actual battery described above. The second geometric area is the surface area of the substrate (glassy carbon) used in the experiment of FIG.
Therefore, in the electrode active material of the present invention, the ratio of the actual surface area to the second geometric area is required, but as shown in this example, the thin film electrode produced by the sputtering method has few surface irregularities, The second geometric area and the actual surface area are approximately equal. Therefore, in FIG. 6, if an oxygen reduction current of ²⁰ μA / cm ² or more can be secured, the performance required for the portable fuel cell is satisfied, and the FWHM ^{−1 at} this time is 0.4 or more. Therefore, FWHM ⁻¹ is preferably 0.4 or more.

<Relationship between TaCN composition and catalytic activity>
A TaCN electrode was produced by the above powder method. The composition (x, y) of TaC _x N _y powder was adjusted according to the amount of carbon mixed during powder preparation. For the obtained TaCN electrode, the oxygen reduction current density at 0.4 V was measured in the same manner as in the case of Comparative Example 1 for oxygen reduction catalytic ability.
The obtained results are shown in FIG. FIG. 7 shows that the oxygen reduction current density is high at a heat treatment temperature of 1000 ° C. when the TaCN composition is TaC _0.99 N _0.01 , TaC _0.58 N _0.42 , or TaC _0.28 N _0.72 . On the other hand, in the case of an electrode using Ta ₂ O ₅ , almost no oxygen reduction current was generated.
As a result, the composition of TaCN is high (for example, 0.99) and low (for example, 0.28), which has a high catalytic ability, and the composition of TaCN can be set in a wide range. Productivity is also improved.

<Influence on oxygen reduction reaction by methanol>
In the direct fuel cell, liquid fuel typified by methanol passes through the electrolyte membrane from the fuel electrode and reacts on the catalyst of the air electrode. For this reason, when the air electrode reacts with methanol, the potential decreases, and as a result, the battery voltage decreases.
Therefore, it was measured whether the oxygen reduction reaction at the TaCN electrode was affected by the presence or absence of methanol. In the measurement, the oxygen reduction current-potential curve when the potential was changed was measured by the same method as described above. When investigating the influence of methanol, 0.1 mol / L of methanol was added to the electrolyte.
The obtained current-potential curve is shown in FIG. It can be seen that the oxygen reduction reaction by the TaCN electrode is not affected by methanol at all, and from this, the electrode active material according to the present invention is expected to have high practicality as an air electrode catalyst of a direct fuel cell.

To summarize the results of the above examples,
1) The electrode carrying TaCN, CrCN and ZrCN was superior in oxygen reduction catalytic ability as compared with the electrode using other compounds.
2) It was found that the electrode carrying TaCN, CrCN and ZrCN is extremely stable in sulfuric acid (acidic electrolyte).
3) The higher the crystallinity of TaCN, the higher the oxygen reduction catalytic ability.
4) The oxygen reduction catalytic ability was maintained over a wide range of TaCN composition.
5) The TaCN electrode was not affected by the oxygen reduction reaction by methanol.

It is a figure which shows the XRD (X-ray diffraction) pattern of the crystal structure of a TaCN thin film. It is a figure which shows the XRD pattern of the crystal structure of a CrCN thin film. It is another figure which shows the comparison of the catalytic ability of an oxygen reduction electrode. It is a figure which shows the cyclic voltammogram of an oxygen reduction electrode. It is a figure which shows the XRD pattern of the TaCN electrode by a powder method. It is a figure which shows the relationship between FWHM- ¹ obtained about the TaCN electrode by the sputtering method and the powder method, and oxygen reduction current density. FIG. 5 is a diagram showing the relationship between TaCN composition and catalytic ability. It is a figure which shows the difference of the electric current-potential curve of the oxygen reduction reaction by a TaCN electrode by the presence or absence of methanol.

Claims (3)

An electrode active material for an oxygen reduction electrode used as a positive electrode of a polymer electrolyte fuel cell,
An electrode active material comprising a carbonitride of Ta, Cr or Zr . The electrode active material according to claim 1, wherein the element is Ta, and the reciprocal of the full width at half maximum of a peak near 35 ° obtained by X-ray powder diffraction is 0.4. The oxygen reduction electrode for positive electrodes formed by carrying the electrode active material according to claim 1 or 2.