JP6859731B2 - Fuel cell cathode catalyst - Google Patents

Description

The present invention relates to a fuel cell cathode catalyst, and more particularly to a fuel cell cathode catalyst containing platinum bronze containing a metal element M.

For the catalyst layer of a polymer electrolyte fuel cell, a mixture of a carrier (electrode catalyst) carrying catalyst particles and a solid polymer electrolyte (catalyst layer ionoma) is usually used. Further, platinum or a platinum alloy is usually used as the catalyst particles.
Generally, the larger the amount of catalyst particles supported, the better the performance of the electrode catalyst. However, since platinum or a platinum alloy is expensive, the cost of the fuel cell increases as the amount of platinum used increases. In order to reduce the cost of the fuel cell, it is necessary to reduce the amount of platinum contained in the entire catalyst layer. Further, in order to reduce the amount of platinum used in the fuel cell, an electrode catalyst that exhibits high performance with a small amount of platinum is required.

Therefore, in order to solve this problem, various proposals have been made conventionally.
For example, Non-Patent Document 1 describes a _{method for synthesizing platinum metal oxides of M x} Pt ₃ O ₄ type (M = Li, Na, Mg, Ca, Zn, Cd, Co, and Ni). It is disclosed.
According to the same document, Li _x Pt ₃ O ₄ , Co _x Pt ₃ O ₄ , and Ni _x Pt ₃ O ₄ have high oxygen reduction activity and voltanmetric properties equivalent to those of the _{Pt 3} O _{4 electrode.} Is described.

In addition, Non-Patent Document 2 describes a _{method for synthesizing nickel-platinum oxide bronze (Ni x} Pt ₃ O ₄ , x = 0.4 to 0.5) and its physical properties (cyclic voltammetry, X-ray diffraction, X-ray). Photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS)) are disclosed.
The document describes that nickel-platinum oxide bronze can be used as a cathode catalyst because it has low solubility in thermal acid and exhibits catalytic activity close to that of metallic platinum.

In a fuel cell, platinum is generally used as the catalyst for the cathode. Further, platinum is generally used in a state of being supported on the surface of a carbon carrier. However, since platinum is expensive, it is required to reduce the amount of platinum used in order to reduce the cost of the fuel cell.

On the other hand, Non-Patent Documents 1 and 2 describe that platinum bronze can be used as a catalyst for a cathode. However, in these documents, the oxygen reduction and water electrolysis performance of only the platinum bronze surface (that is, the oxide surface) is evaluated (that is, the evaluation at a potential higher than 0.6 V in which metal platinum is not produced). However, the activity of the metallized surface has not been evaluated. Moreover, the oxygen reduction activity of the platinum bronze surface (oxide surface) is lower than that of platinum. Therefore, when platinum bronze is used as a catalyst in a device such as an in-vehicle device that requires high output, a large amount of catalyst is required. Furthermore, Non-Patent Documents 1 and 2 do not evaluate the durability of platinum bronze.

R.D. Shannon et al., Inorg. Chem., 21, 3372 (1982) D.A.Tryk et al., Proceeding of the Symposium on Electrode Materials and Processed for Energy Conversion and Strage, 408-417 (1994)

An object to be solved by the present invention is to provide a novel fuel cell cathode catalyst containing platinum bronze.
Another problem to be solved by the present invention is to provide a novel catalyst for a fuel cell cathode that exhibits high durability even in a cathode environment.

In order to solve the above problems, it is a gist that the fuel cell cathode catalyst according to the present invention has the following configurations.
(1) The fuel cell cathode catalyst contains platinum bronze containing a metal element M.
(2) The metal element M is Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge. , Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Hf, Ta, W, Re, Os, Ir, Au, Tl, Pd, Bi, and lanthanoids. Consists of any one or more elements selected from.
(3) A part of the surface of the platinum bronze is made of a metal platinum layer.

When platinum bronze is used in a cathode environment, the surface of the platinum bronze remains an oxide, and a part of the platinum bronze surface does not become metal platinum. However, when the platinum bronze is subjected to a predetermined treatment, a part of the platinum bronze surface becomes metal platinum. The specific activity (oxygen reduction activity) of platinum bronze whose surface is partially metal-platinated shows almost the same value as platinum-supported carbon. Therefore, when this is used as a catalyst for a fuel cell cathode, oxygen reduction activity equal to or higher than that of platinum-supported carbon can be obtained. In addition, platinum bronze has a lower elution rate of platinum in a cathode environment and higher durability than platinum.
Further, when platinum bronze having a part of the surface plated with metal is used as a catalyst, the amount of platinum used can be reduced as compared with the case where metal platinum is used as a catalyst.

It is a figure which shows the crystal structure of platinum bronze. It is an XRD pattern of the sample after the heat treatment (Co _x Pt ₃ O ₄ + Pt) and the raw material mixture before the heat treatment _{(Pt O 2 + Co nitrate).} 3 is an SEM image of the sample after the heat treatment (FIG. 3 (A)) and the raw material mixture before the heat treatment (FIG. 3 (B)). It is an XRD pattern of a sample before and after aqua regia treatment.

Co _x Pt ₃ O ₄ (FIG. 5 (A)), and Pt / Vulcan (registered trademark) is a cyclic voltammogram at conditioning (Fig. 5 (B)). Co _x Pt ₃ O _4, and is a cyclic voltammogram of an oxygen atmosphere of Pt / Vulcan (registered trademark). Co _x Pt ₃ O _4, and Pt / Vulcan electrochemically effective specific surface area of (R) (ECSA), by weight active, and specific activity. Pt / Vulcan® and M _x Pt ₃ O ₄ (M = Co, Ce, Li, Mn, Cu, In, Na, Ca) oxygen reduction activity.

Is an XRD pattern of Co _x Pt ₃ O ₄ / GC after electrochemical evaluation. STEM images of Co _x Pt ₃ O ₄ after the electrochemical evaluation (FIG. 10 (A): Higher magnification: low magnification image, FIG. 10 (B)) is. Is an XPS spectrum of the electrochemical evaluation before and after Co _x Pt ₃ O _4. It is a schematic diagram for demonstrating the contribution to the signal of the XPS spectrum of the atom on the surface and the bulk layer. It is a figure which shows the relationship between the thickness (Δx / λ') of metal Pt, and ΔS'/ S _Pt.

Co _x Pt ₃ O _4, and illustrates a platinum dissolution rate during the durability test of the platinum black. Is a cyclic voltammogram at conditioning Co _x Pt ₃ O ₄ treated with 80 wt% ethanol. A comparison of processing different Co _x Pt ₃ electrochemically effective specific surface area of the O ₄ of the process (ECSA). A comparison of a different Co _x Pt ₃ platinum coverage O ₄ of processing method (ECSA / BET specific surface area).

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Fuel cell cathode catalyst]
The fuel cell cathode catalyst according to the present invention has the following configurations.
(1) The fuel cell cathode catalyst contains platinum bronze containing a metal element M.
(2) The metal element M is Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge. , Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Hf, Ta, W, Re, Os, Ir, Au, Tl, Pd, Bi, and lanthanoids. Consists of any one or more elements selected from.
(3) A part of the surface of the platinum bronze is made of a metal platinum layer.

[1.1. Platinum bronze]
FIG. 1 shows the crystal structure of platinum bronze. "Platinum bronze" refers to a composite metal oxide containing platinum, oxygen, and ions of the metal element M as a third component. The composition formula of platinum bronze is generally represented by _{M x} Pt ₃ O _4. The space group of platinum bronze is Pm3n, and the platinum atom is located at 6c site, the oxygen atom is located at 8e site, and the metal element M is located at 2a site. The amount x of the metal element M differs depending on the synthesis conditions (metal charge amount, synthesis temperature, etc.). x is usually 0 <x ≦ 1.

The fuel cell cathode catalyst according to the present invention may consist of only platinum bronze, or may have platinum bronze supported on the surface of an appropriate carrier (for example, carbon carrier, conductive oxide carrier, metal carrier). good. Furthermore, platinum bronze may be used alone or in combination with other catalysts.

[1.2. Metal element M]
The metal element M is
(A) Li, Na, K, Rb, Cs,
(B) Mg, Ca, Sr, Ba,
(C) Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge,
(D) Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb,
(E) Consists of Hf, Ta, W, Re, Os, Ir, Au, Tl, Pd, Bi, or lanthanoids. The platinum bronze may contain any one of these metal elements M, or may contain two or more of them.

Among these, the metal element M is preferably any one or more elements selected from the group consisting of Co, Ce, Li, Mn, Cu, In, Na, and Ca. All of the platinum bronze containing these metal elements M show relatively high oxygen reduction activity and also have high durability in a cathode environment.

[1.3. Surface condition of platinum bronze]
The surface of platinum bronze immediately after synthesis is considered to be in an oxide state. However, when the platinum bronze is treated under predetermined conditions, the platinum bronze is reduced and a part of the surface becomes a metal platinum layer. However, although the details of the reason are unknown, the entire surface of the platinum bronze does not become a metal platinum layer even after long-term treatment.
The ratio of the area of the metal platinum layer to the surface area of the platinum bronze varies depending on the type of the metal element M and the treatment conditions. If the coverage of the metal platinum layer is defined as = electrochemical effective surface area (ECSA) ÷ BET specific surface area × 100 (%), the coverage is 10 to 40% or more.

The thickness of the metal platinum layer formed by reducing the surface of platinum bronze is extremely thin. Therefore, when X-ray diffraction measurement is performed on platinum bronze, no peak derived from platinum is observed.
On the other hand, the thickness of the metal platinum layer can be estimated by performing XPS measurement. Using the method described below, the thickness of the metal platinum layer determined from XPS measurement is estimated to be 1.6 nm or less.

[1.4. Use]
The catalyst according to the present invention can be used as a cathode catalyst for various fuel cells. Examples of the fuel cell to which the present invention can be applied include a polymer electrolyte fuel cell, an alkaline fuel cell, and a phosphoric acid fuel cell.

[2. Manufacturing method of catalyst for fuel cell cathode]
[2.1. Manufacture of platinum bronze]
Platinum bronze
(A) A predetermined amount of the raw material of the metal element M is added to the platinum source, and the raw material is added.
(B) The raw material mixture is subjected to a solid phase reaction under predetermined conditions.
(C) If necessary, remove the by-produced metal platinum from the obtained reactant, and remove it.
(D) Obtained by reducing the surface of platinum bronze.

[2.1.1. Blending process]
First, a predetermined amount of a raw material of the metal element M (hereinafter, also referred to as “metal source”) is added to the platinum source (blending step).
The platinum source is not particularly limited as long as platinum bronze can be synthesized, and examples of the platinum source include, for example.
(A) Platinum oxide (PtO ₂ ),
(B) Platinum nitrate, chloro complex, ammine salt, hydroxy complex and the like.
The metal source is not particularly limited as long as platinum bronze can be synthesized. As a metal source, for example
(A) Salts with inorganic anions such as nitrates, fluoride salts, chloride salts, bromide salts, iodide salts, carbonates, perchlorates, phosphates, sulfates, borates, etc.
(B) Salts with organic anions such as acetates, oxalates, citrates,
and so on.
For the amount of the metal source added, the optimum amount to be added is selected according to the purpose. In general, the larger the amount of the metal source added, the more platinum bronze having a large x can be obtained.

[2.1.2. Reaction process]
Next, the raw material mixture is subjected to a solid phase reaction under predetermined conditions (reaction step). As the reaction conditions, the optimum conditions are selected according to the composition of platinum bronze and the type of raw material. The reaction temperature is usually 650 to 700 ° C., although it depends on the composition of platinum bronze and the like. The reaction time is usually about several minutes to several hours. When the raw material mixture is heat-treated under predetermined conditions, platinum bronze is produced and at the same time, metal platinum is produced as a by-product.

[2.1.3. Purification process]
Next, if necessary, the metal platinum by-product is removed from the obtained reactant (purification step). The removal of metallic platinum is preferably carried out by aqua regia treatment. When aqua regia treatment is performed under predetermined conditions, platinum bronze can be isolated from the reaction product.

[2.1.4. Reduction process]
Next, the platinum bronze surface is reduced and a part of the surface is metal platinumized. The reduction treatment method is not particularly limited, and various methods can be used depending on the purpose.
Examples of the reduction treatment method include a potential cycle method and a reducing agent method. Any one of these methods may be used, or two or more of these methods may be used in combination. In particular, when the potential cycle and the reducing agent method are used in combination, higher ECSA can be obtained as compared with the case where only the potential cycle method is used.

[A. Potential cycle method]
The "potential cycle method" refers to a method of imparting a potential cycle to platinum bronze in an atmosphere of an inert gas (for example, Ar). As the treatment conditions, the optimum conditions can be selected according to the composition of the platinum bronze.

For example, the voltage range at the time of applying the potential cycle may be a range in which the reduction treatment of the platinum bronze surface proceeds. The voltage range is usually preferably 0.05 to 1.2 V.
The scanning speed at the time of applying the potential cycle is not particularly limited, and the optimum speed can be selected according to the purpose.

In general, as the number of potential cycles increases, the reduction of the platinum bronze surface progresses. In order to obtain high oxygen reduction activity, the number of potential cycles is preferably 50 or more. The number of potential cycles is more preferably 100 cycles or more, still more preferably 300 cycles or more.
On the other hand, even if the potential cycle is applied more than necessary, there is no difference in the effect and there is no actual benefit. Therefore, it is preferable to select the optimum number of potential cycles according to the purpose.

[B. Reducing agent method]
The "reducing agent method" refers to a method of reducing the surface of platinum bronze with a reducing agent having an oxidation-reduction potential lower than that of platinum. Examples of such reducing agents include alcohols such as ethanol, hydrogen gas, and hydrazine. It is preferable to select the optimum treatment conditions according to the composition of platinum bronze and the type of reducing agent.
In general, the higher the reduction temperature and / or the longer the treatment time with the reducing agent, the more the reduction of the platinum bronze surface progresses. On the other hand, treatment at a higher temperature than necessary and / or long-term treatment has no difference in effect and has no actual benefit.

[2.2. Manufacture of catalysts for fuel cell cathodes]
The obtained platinum bronze may be used as it is as a catalyst, or may be used in a state of being supported on the surface of an appropriate carrier.
The catalyst in which platinum bronze is supported on the surface of the carrier is obtained by dispersing the platinum bronze powder and the carrier in a suitable solvent and removing the solvent.

[3. Action]
When platinum bronze is used in a cathode environment, the surface of the platinum bronze remains an oxide, and a part of the platinum bronze surface does not become metal platinum. However, when the platinum bronze is subjected to a predetermined treatment, a part of the platinum bronze surface becomes metal platinum. The specific activity (oxygen reduction activity) of platinum bronze whose surface is partially metal-platinated shows almost the same value as platinum-supported carbon. Therefore, when this is used as a catalyst for a fuel cell cathode, oxygen reduction activity equal to or higher than that of platinum-supported carbon can be obtained.

Moreover, since platinum bronze is insoluble in aqua regia, it does not elute strong acid conditions and does not deteriorate even when exposed to a high potential. Moreover, since it has oxygen-evolving activity, it does not corrode the carbon carrier even at a high potential. Therefore, the durability as a catalyst is also high.
Further, when platinum bronze having a part of the surface plated with metal is used as a catalyst, the amount of platinum used can be reduced as compared with the case where metal platinum is used as a catalyst.

Platinum bronze exhibits oxygen reduction activity regardless of the type of metal element M. It is considered that this is because the oxygen reduction activity is derived from the metal platinum produced by the metalization treatment and does not depend on the metal of the third component.

(Examples 1 to 10, Comparative Examples 1 to 2)
[1. Preparation of sample]
Platinum bronze was synthesized by the method described below with reference to Non-Patent Document 1. Platinum oxide (PtO ₂ ) and metal nitrate were weighed in a molar ratio of 3: 1 and mixed in a mortar. The metal _{nitrate, Co (NO 3) 2 ·} 6H 2 O ( Example _{1), Ce (NO 3)} 3 · 6H 2 O ( Example 2),
_{Ca (NO 3) 2 · 4H} 2 O ( Example 3), LiNO ₃ (Example 4),
NaNO ₃ (Example _{5), Bi (NO 3)} 3 · 5H 2 O ( Example 6),
AgNO ₃ (Example _{7), Cu (NO 3)} 2 · 3H 2 O ( Example 8),
_{Mn (NO 3) 2 · 6H} 2 O ( Example 9), or _{In (NO 3) 3 · 5H} 2 O ( Example 10)
Was used.

The mixture was then heat treated at 650 ° C. for 5 hours under air aeration (1 L / min). Further, the obtained reaction product (mixture of platinum bronze and metal platinum) was immersed in hot aqua regia for 30 minutes, and the supernatant liquid was removed to remove the metal platinum.
For comparison, Pt / Vulcan® (CO pulse specific surface area: 158 m ² / g) (Comparative Example 1) as a representative example of platinum-supported carbon, and platinum black having a particle size similar to that of _{Co x} Pt ₃ O ₄ (BET specific surface area: 12.7 m ² / g, particle size: 22.0 nm) (Comparative Example 2) was used.

[2. Evaluation]
[2.1. XRD measurement and SEM observation of platinum bronze immediately after synthesis]
2 shows an XRD pattern of the sample after the heat treatment _{(Co x Pt 3 O 4 +} Pt) and the heat treatment prior to the raw material mixture (PtO ₂ + Co nitrate). Further, FIG. 3 shows an SEM image of the sample after the heat treatment (FIG. 3 (A)) and the raw material mixture before the heat treatment (FIG. 3 (B)). From FIGS. 2 and 3,
(A) A fine reactant can be obtained from a coarse raw material mixture, and
(B) reaction, platinum bronze (Co _x Pt ₃ O ₄₎ and consist of a mixture of the metal platinum,
I understand.
FIG. 4 shows the XRD pattern of the sample before and after the aqua regia treatment. From FIG. 4, it can be confirmed that the metal platinum was removed by the aqua regia treatment and the platinum bronze was isolated.

[2.2. BET specific surface area]
For each platinum bronze, the BET specific surface area was determined by nitrogen adsorption measurement. The results are shown in Table 1. All of the obtained platinum bronze had a high specific surface area.

[2.3. Evaluation of oxygen reduction performance]
After the platinum bronze powder was dispersed in acetone, it was applied to the surface of a glassy carbon (GC) electrode and dried. Using this as the working electrode, electrochemical measurement was performed to evaluate the oxygen reduction activity (ORR). The reference electrode was a reversible hydrogen electrode (RHE), the counter electrode was a gold mesh, the electrolytic solution was 0.1 M perchloric acid, and the solution temperature was 30 ° C. The effective surface area of platinum was determined from the average value of the electric amount of the adsorption wave and the electric amount of the desorption wave of hydrogen by a cyclic voltammogram in an inert atmosphere. The oxygen reduction activity was evaluated in an oxygen atmosphere while rotating the electrode at 400 rpm, and the current value at 0.9 V was normalized by the platinum effective surface area as an index.

Figure 5 shows the Co _x Pt ₃ O ₄ (FIG. 5 (A)), and Pt / Vulcan (TM) cyclic voltammograms when conditioning (Fig. 5 (B)). Conditioning by potential cycles (0.05~1.2V, 500mV / sec, Ar atmosphere) was carried out, Co _x Pt ₃ In O _4, the same shape as the metal platinum Repeating potential cycles (hydrogen desorption wave and It changed to a cyclic voltammogram (where the redox wave of platinum can be seen). This indicates that metal platinum was formed on the platinum bronze surface.

Figure 6 shows the cyclic voltammogram of an oxygen atmosphere of Co _x Pt ₃ O _4, and Pt / Vulcan (registered trademark). ORR activity of conditioning before the Co _x Pt ₃ O ₄ is small, the voltammogram after conditioning became similar to that of Pt / Vulcan (registered trademark).

Figure 7 shows a Co _x Pt ₃ O _4, and Pt / Vulcan ECSA (registered trademark), the weight activity, and specific activity. From FIG 7, Co _x Pt ₃ O _4, because the particle size of the catalyst is large, the ECSA and weight activity Pt / Vulcan (registered trademark) smaller than the specific activity is found to be equivalent.

FIG. 8 shows the oxygen reduction activity of Pt / Vulcan® and M _x Pt ₃ O ₄ (M = Co, Ce, Li, Mn, Cu, In, Na, Ca). From FIG. 8, it can be seen that the oxygen reduction activity per platinum area is 100 μA / cm ^{2 or more for all platinum bronze.}

[2.4. XRD measurement, TEM observation and XPS measurement of platinum bronze after electrochemical evaluation]
The sample after the electrochemical evaluation was evaluated by XRD measurement, TEM observation, and XPS measurement.
Figure 9 shows the XRD pattern of Co _x Pt ₃ O ₄ / GC after electrochemical evaluation. Incidentally, in FIG. 9, GC, Pt _metal (simulation patterns), and showed XRD patterns together of Co _x Pt ₃ O ₄ (powder). As shown in FIG. 9, in the XRD pattern after electrochemical evaluation _{Co x Pt 3 O 4 / GC} , the peak of Co _x Pt ₃ O ₄ is seen, the peak of metal platinum can not be seen. Therefore, it is presumed that due to the potential cycle, the bulk phase remained platinum bronze, and only the surface layer became metal platinum having particles or a thin layer that could not be detected by XRD measurement. This can also be confirmed from the following STEM image.

Figure 10, STEM images after electrochemical evaluation Co _x Pt ₃ O ₄ (FIG. 10 (A): Higher magnification: low magnification image, FIG. 10 (B)) shows a. FIG. 10B is an enlarged view of the area surrounded by the broken line in FIG. 10A. From FIG. 10, it can be seen that there are particulate or island-shaped metal platinum on the surface of the platinum bronze.
Figure 11 shows the XPS spectrum of the electrochemical evaluation before and after Co _x Pt ₃ O _4. From FIG. 11, the thickness of the platinum layer is estimated to be 1.6 nm or less. The details of the calculation will be described later.

[2.5. Calculation of the thickness of the metal platinum layer on the platinum bronze surface]
FIG. 12 shows a schematic diagram for explaining the contribution of the atoms on the surface and bulk layer to the signal in the XPS spectrum. In general, the contribution of atoms on the surface and bulk layer to the signal of XPS is exponentially attenuated, as shown in FIG. 12 (A) (references 1 and 2).
[Reference 1] Carley, AF; Robert, MW Proc. R. Soc. London, Ser. A, 363, 403-424 (1978)
[Reference 2] Tanuma, S .; Powell, CJ; Penn, DR Surf. Interface Anal. 11, 577-589 (1988)

Here, the distance from the surface where the strength is 1 / e can be approximated by the product λ'(= λ × cosφ) of the inelastic mean free path (IMFP, hereinafter referred to as “λ”) and cosφ. φ is the detection angle, which is 45 °.
Consider the change in the intensity _{of the O 1s} peak when the portion of the platinum bronze surface thickness Δx is metal platinumized. In this case, since the oxygen atom in the region of Δx is lost, the O _1s signal intensity is reduced by the area of the hatched portion in the upper figure of FIG. 12 (B). The rate of decrease is expressed by the following equation (1).

The above is a consideration when metal platinum is formed in all regions of the platinum bronze surface. In the case of Co _x Pt ₃ O ₄ , the BET specific surface area: 18.8 m ² / g, whereas the effective surface area (ECSA) of metal platinum determined by electrochemical measurement is 3.7 m ² / g. Therefore, it is considered that 20% of the platinum bronze surface was metal platinumized. Therefore, the actual _{reduction rate of the O 1s} signal is expressed by the equation (2).

FIG. 13 shows the relationship between the thickness (Δx / λ') of the metal Pt and ΔS' / S _{Pt estimated from the equation (2).} Here, the horizontal axis (thickness scale) is expressed as a ratio to λ'. No literature value has been found for the platinum bronze IMFP, but the largest value among the platinum group oxide IMPVs for which the literature value was obtained (λ = 2.3 nm for IrO ₂ , @ electron energy 958 eV (X-ray source: In the case of AlKα (1486.6 eV))), λ'= 1.6 nm can be obtained. As can be seen from FIG. 13, assuming that Δx = λ', _{the reduction rate of the O 1s} signal is 0.12 to 0.13.

Table 2 shows the changes in _{O 1s} signal intensity before and after metal platinum formation obtained in this experiment. In Table 2, the signal intensity of Pt, in which the number of atoms per weight of platinum bronze does not change before and after metallization, is standardized. As estimated above, if a 1.6 nm metal platinum layer were formed, the O _1s signal intensity should decrease by 10% or more, but in this experiment, the O _1s signal intensity was substantially reduced. Has not decreased. Therefore, it is suggested that the formed metal platinum layer is much thinner than 1.6 nm (at most, the thickness is about 1 ML). This result is also consistent with the XRD result.

[2.6. Evaluation of durability]
After the platinum bronze powder was dispersed in acetone, it was applied to the surface of a glassy carbon (GC) electrode and dried. With this as the working electrode, a potential fluctuation endurance test was carried out in the liquid phase under fuel cell operating conditions (0.4 to 1.0 V). The reference electrode was a reversible hydrogen electrode (RHE), the counter electrode was a gold mesh (isolated by a salt bridge), the electrolytic solution was 0.1 M perchloric acid, and the liquid temperature was adjusted to 60 ° C. Further, in order to accelerate the deterioration, chloride ions were added so that the concentration in the electrolytic solution was 50 ppm. Platinum black (Comparative Example 2) was used as a comparative sample. Table 3 shows the amount of each catalyst supported on the GC.

In an inert atmosphere (degassed with Ar gas), while rotating the electrode at 400 rpm, the upper limit was 1.0 V and the lower limit was 0.4 V, and a potential fluctuation cycle of holding each for 3 seconds was carried out 10,000 times. The platinum concentration in the electrolytic solution after 0, 100, 200, 1000, 2000 and 10000 potential fluctuations was quantified by the stripping voltammetry method (References 3 and 4).
[Reference 3] J. Wang, et al., J. Electroanal. Chem., 237, 281 (1987)
[Reference 4] H. Obata, et al., Anal. Chim. Acta, 580, 32 (2006)

Figure 14 shows a Co _x Pt ₃ O _4, and platinum dissolution rate during the durability test of the platinum black. Than 14, platinum elution amount of Co _x Pt ₃ O ₄ after 10000 cycles is smaller than that of platinum black (about 1/10), it can be seen that high durability in a fuel cell operating conditions.

[2.7. Metal platinumization of platinum bronze surface by methods other than potential cycle]
The Co _x Pt ₃ O ₄ powder was dispersed in 80 wt% ethanol solution and allowed to stand for 3 days. Then, the powder was applied to the surface of the glassy carbon (GC) electrode and dried. Using this as the working electrode, electrochemical measurement was performed to evaluate ECSA. The reference electrode was a reversible hydrogen electrode (RHE), the counter electrode was a gold mesh, the electrolytic solution was 0.1 M perchloric acid, and the solution temperature was 30 ° C. Under these conditions, conditioning by potential cycle (0.05 to 1.2 V, 500 mV / sec, 50 cycles, Ar atmosphere) was performed. The effective surface area of platinum was determined from the average value of the amount of electricity of the adsorption wave and the desorption wave of hydrogen from the cyclic voltammogram in an inert atmosphere.

Figure 15 shows the cyclic voltammograms when the conditioning of Co _x Pt ₃ O ₄ treated with 80 wt% ethanol. From FIG. 15, it can be seen that when the powder is dispersed in 80% ethanol, the cyclic voltammogram of the powder shows a shape similar to that of metal platinum from the beginning of the potential cycle. This result indicates that it is possible to metallize the platinum bronze surface with ethanol as well. In addition to ethanol, any reducing agent having a redox potential lower than that of platinum, such as hydrogen gas, alcohols, and hydrazine, can be used.

Figure 16 shows a comparison of different Co _x Pt ₃ electrochemically effective specific surface area of the O ₄ of processing method (ECSA). From FIG. 16, it can be seen that the powder treated with ethanol and treated with potential cycle has a higher ECSA than the powder treated with only potential cycle treatment.
Figure 17 shows a comparison of different platinum coverage of Co _x Pt ₃ O ₄ with processing method (ECSA / BET specific surface area). From FIG. 17, it can be seen that the powder treated with ethanol and treated with potential cycle has a higher coverage of metal platinum on the surface of platinum bronze than the powder treated with only potential cycle treatment. The coverage of metal platinum can be controlled by the degree of reduction treatment (conditions such as the type of reducing agent, treatment time, and temperature).

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the present invention.

The fuel cell cathode catalyst according to the present invention can be used as a cathode catalyst for various fuel cells.

Claims (3)

A catalyst for a fuel cell cathode having the following configuration.
(1) The fuel cell cathode catalyst contains platinum bronze containing a metal element M.
(2) The metal element M is Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge. , Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Hf, Ta, W, Re, Os, Ir, Au, Tl, Pd, Bi, and lanthanoids. Consists of any one or more elements selected from.
(3) A part of the surface of the platinum bronze is made of a metal platinum layer.
(4) The coverage of the metal platinum layer (= electrochemical effective surface area (ECSA) ÷ BET specific surface area × 100 (%)) is 10% or more and less than 100%. The platinum bronze
(A) When the X-ray diffraction measurement was performed, there was no peak derived from platinum, and there was no peak.
(B) The catalyst for a fuel cell cathode according to claim 1, wherein the thickness of the metal platinum layer obtained from XPS measurement is 1.6 nm or less. The catalyst for a fuel cell cathode according to claim 1 or 2, wherein the metal element M is any one or more elements selected from the group consisting of Co, Ce, Li, Mn, Cu, In, Na, and Ca.

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