JPH0713897B2 - Surface activated amorphous alloy for fuel cell electrodes - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an electrode material for a fuel cell, and more specifically to an electrode material for a fuel cell having excellent anodizing characteristics for a C ₁ compound (methanol, formaldehyde and formic acid) fuel. Regarding

(Prior Art) Conventionally, as a fuel cell electrode material, generally, a material in which a platinum group element is supported on a carbonaceous base material is often used. The use of expensive precious metal elements is indispensable for fuel cell electrodes, but with this type of conventional electrode, the electrocatalytic activity is unstable and the characteristics deteriorate over time, so the activity should be kept above a certain value. The amount of platinum group element used must be increased in order to maintain it, and this improvement has been desired.

(Object of the Invention) In view of the above, it is an object of the present invention to provide a fuel cell electrode material that is inexpensive, has excellent anodizing characteristics, and is stable.

(Structure and Effect of the Invention) The present invention is an amorphous alloy composed of 10 to 50 atomic% of one or more platinum group elements and 50 to 90 atomic% of zirconium, and has a porous active surface. A surface-activated amorphous alloy for fuel cell electrodes (first invention) and zirconium in an amount of 50 to 89 atomic%;
Amorphous alloy gold containing 20 atomic% or less of tin, niobium, and molybdenum of 1 or 2 or more and 10 to 50 atomic% of one or more of platinum group elements and having a porous surface. The object is achieved by providing a surface activated amorphous alloy for fuel cell electrodes (second invention) in which an active layer is formed.

In the present invention, the platinum group element contained in the electrode alloy is 10 to 50
The range is atomic%. In the present invention, even if a large amount of platinum group element exceeding 50 atomic% is contained, the effect of improving the characteristics is not improved, but the cost is increased.
It was set to 50 atom%. Also, the platinum group element content is 10 atomic%
If the amount is less than this, sufficient anodic oxidation properties cannot be obtained, so the lower limit of the content is made 10 atom%.

The use of zirconium is effective as an alloy element other than the platinum group element. When the content of zirconium is less than 50 atomic%, the amount of platinum group element used becomes large and expensive, and it becomes unsuitable for porosity described later, which is not preferable,
Further, if zirconium is contained in a large amount exceeding 90 atomic%, the platinum group element content becomes insufficient and sufficient characteristics cannot be obtained. Therefore, the content of zirconium is set in the range of 50 to 90 atomic%.

The fuel cell electrode alloy of the present invention is an amorphous alloy having the above composition, and has a porous active layer formed on its surface by activation treatment.

Amorphous alloys are generally obtained in the form of thin ribbons (thickness of about 50 μm or less) or fine particles. Here, the ribbon-shaped amorphous alloy is easily manufactured according to a conventionally known method using a single roll or twin roll type quenching device, and the fine grained amorphous alloy powder is a conventional cavitation method or a high frequency sputtering method. It can be easily manufactured by a known method.

The amorphous alloy is obtained by ultra-quenching an alloy in a molten state and has a structure similar to a liquid. Therefore, even a complex alloy composition containing various alloy elements can be obtained in the form of a chemically uniform solid solution. Therefore, the amorphous alloy has an advantage that it can be prepared as having a unique surface chemical property which is not present in the crystalline alloy and a useful catalytic activity resulting therefrom.

In the present invention, an amorphous alloy is used as the fuel cell electrode alloy, and the amorphous alloy is subjected to activation treatment to form a porous active layer on the surface of the amorphous alloy.

Here, the activation treatment is preferably performed by immersing the amorphous alloy in an aqueous solution containing hydrofluoric acid. By this activation treatment, a porous active layer having an extremely large specific surface area is formed on the surface of the amorphous alloy, and the anodic oxidation characteristics as a fuel cell electrode are markedly improved, and a series of C ₁ compounds (methanol, formaldehyde and Formic acid) An electrode having excellent properties for a fuel cell can be obtained.

As described above, the platinum group element is an element that is indispensable for obtaining good anodic oxidation characteristics, and it is necessary to contain 10 atom% or more. However, in the platinum group element, platinum is particularly effective. is there. Therefore, it cannot be said that platinum is entirely used for the required content of the platinum group element. Further, in the present invention, the content of the platinum group element is 10 to 50 atomic%, but 10 to 90 atomic% of the platinum group element content is platinum, and the remaining 10 to 90 atomic% is palladium, ruthenium,
It may also be composed of one or more of rhodium, osmium and iridium.

Further, in the present invention, the addition of a small amount of tin can significantly improve the anodic oxidation characteristics of the electrode. However, if a large amount of tin is added in excess of 20 atom%, the catalyst activity will be rather deteriorated. Therefore, the amount of tin added is preferably 20 atom% or less.

It is also effective to add niobium or molybdenum instead of or in addition to the above tin addition.
However, it is desirable that the total of these additional elements is also within the range of 20 atomic% or less.

(Examples) Pt ₂₁ Zr ₇₉ and (Pt _0.21 Zr _0.79 ) ₉₁ Sn ₉ [Hereinafter, the numbers indicate atomic%, (Pt _α Zr _β ) _γ Sn _δ is Pt _{α × γ} Zr _{β × γ} Sn
Indicates _δ . ] Amorphous alloy ribbons (thickness 20
μm) and powder (average particle size of about 5 μm) were produced by the conventional single roll method and sputtering method, respectively.

The obtained ribbon and powder exhibited metallic luster and black color, respectively, and the electrocatalytic activity (anodizing property) for a series of C ₁ compound fuels was extremely low in this state. Next, the ribbon and the powder made of these amorphous alloys were subjected to a treatment of being immersed in a 5% hydrofluoric acid solution for 3 to 5 minutes. By this hydrofluoric acid treatment, the surface of the amorphous alloy was activated and its anodic oxidation characteristics were dramatically improved.

FIG. 1 shows an amorphous alloy (ribbon) which has been subjected to the above treatment.
1M CH ₃ OH containing constant current when methanol anodic oxidation in an aqueous solution of sulfuric acid (0.5M) - shows the potential curve. For comparison, the above amorphous alloy is shown in FIG.
The steady-state current-potential curve of the crystalline alloy which was annealed for 0 minutes, crystallized, and then hydrofluoric acid-treated in the same manner as above was also shown.

From FIG. 1, it is understood that the alloy of the present invention, which is an amorphous alloy, has a significantly improved electrode activity as compared with the crystalline alloy. Figure 2 shows Pt ₂₁ Zr ₇₉ and (Pt _0.21 Zr _0.79 ).
An anode in an aqueous solution of sulfuric acid containing a C ₁ compound fuel of the activated amorphous alloy of the present invention obtained by subjecting an amorphous alloy (ribbon) having a composition of ₉₁ Sn ₉ to a surface activation treatment by hydrofluoric acid treatment as described above. The stationary current-potential curve at the time of oxidation is shown.

The roughness factor of the above amorphous alloy was 500 to 710 due to surface activation with a hydrofluoric acid solution.

It can be understood from FIG. 2 that the activated amorphous alloy of the present invention, by containing tin, has an anodizing property improved by one to two orders of magnitude as compared with the case where tin is not added.

As described above, the activated amorphous alloy for a fuel cell electrode of the present invention is a ribbon-shaped or powder-shaped amorphous alloy having a composition in which a platinum group element and zirconium or tin is further added. And the surface of the amorphous alloy is activated by being treated in an aqueous solution containing hydrofluoric acid.

Fig. 3 shows the XPS of the surface of an amorphous alloy composed of Pt ₂₁ Zr ₇₁ .
(X-ray photoelectron spectroscopy) spectra, and FIGS. 3 (a) and 3 (b) show XPS spectra in a state before hydrofluoric acid treatment and a state after hydrofluoric acid treatment, respectively.

From Fig. 3, the XPS spectrum of the amorphous alloy before hydrofluoric acid treatment shows that the peak intensity of Zr is much larger than that of Pt, and
It is understood that Zr exists on the surface in the form of ZrO ₂ (Fig. 3 (a)).

On the other hand, the XPS spectrum of the amorphous alloy after hydrofluoric acid treatment has a small Zr peak intensity, which is contrary to that before the treatment.
The peak intensity of Pt is remarkably increased (Fig. 3 (b)).

As is clear from this, when the Pt-Zr-based amorphous alloy is treated with hydrofluoric acid, the surface of the alloy is remarkably rich in Pt.

Further, it has been clarified from the surface area measurement result by the BET method that the hydrofluoric acid treatment makes the surface of the amorphous alloy more porous. In FIG. 1 showing the comparison of the electrode activity after the Pt-Zr type amorphous alloy and the crystallized Pt-Zr type crystalline alloy were subjected to hydrofluoric acid treatment, respectively. Those treated with hydrofluoric acid showed significantly higher electrode activity than crystalline ones, but it is considered that the difference in these activities is mainly due to the difference in roughness factor of the electrode (sample). Table 1 shows typical Pt-Zr based amorphous alloys, Pt-Zr based crystalline alloys (both treated with hydrofluoric acid) and platinum black specific surface area (BET Surface area), and the corresponding values.
Equivalent particle size of Pt
Is shown.

The Pt content (Pt loading) is based on the apparent surface area of the alloy ribbon.

As is clear from Table 1, the specific surface area of the activated amorphous alloy of the present invention is much larger than the specific surface area of the crystalline alloy of the same composition or platinum black, and its value is the same as that of Raney platinum. It goes beyond that. Although Table 1 shows the values obtained for the ribbon-shaped amorphous alloy, even better results are obtained in the case of the amorphous alloy powder.

As described above, the activated amorphous alloy for fuel cell electrodes of the present invention has a Pt-rich porous layer on its surface and has a remarkably large specific surface area as compared with the conventional crystalline alloy, C ₁
Good and stable electrode characteristics for compound fuels,
It is effective in improving the characteristics of fuel cells and has great industrial value.

[Brief description of drawings]

FIG. 1 is a steady-state current-potential characteristic curve at the time of methanol anodic oxidation of the activated amorphous alloy and the crystalline alloy of the embodiment of the present invention, and FIG. 2 is the Pt-Zr type amorphous alloy of the present invention. Steady state current-potential characteristic curve of Pt-Zr type amorphous alloy with tin added in sulfuric acid aqueous solution containing 1M C ₁ compound, Fig. 3 shows hydrofluoric acid treatment of Pt-Zr type amorphous alloy. Shows changes in surface condition
In XPS spectrum, FIG. 3 (a) shows the state before hydrofluoric acid treatment, and FIG. 3 (b) shows the state after hydrofluoric acid treatment.

Claims (4)

[Claims] 1. A surface-activated amorphous alloy for a fuel cell electrode, which has a porous layer rich in platinum group elements on the surface, comprising 50 to 90 atomic% zirconium and the balance of one or more platinum group elements. 2. A surface activated amorphous alloy for fuel cell electrodes according to claim 1, wherein the platinum group element is Pt. 3. Platinum containing 10 to 90 atom% of the platinum group element,
2. The surface-activated amorphous alloy for fuel cell electrodes according to claim 1, wherein the balance is one or more of ruthenium, rhodium, palladium, osmium and iridium. 4. A fuel cell electrode comprising 50 to 89 atomic% and 20 atomic% or less zirconium of one or more kinds of tin, niobium and molybdenum, and the remaining 10 atomic% or more of one or more platinum group elements. Surface activated amorphous alloy for