JPH0121592B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a surface-activated amorphous alloy suitable as a fuel electrode material for fuel cells using methanol-based fuels such as methyl alcohol, formaldehyde, and formic acid. Conventionally, platinum black and the like have been used as electrodes in methanol-based fuel cells, but they have drawbacks such as a large decrease in activity during use. Usually, alloys are crystallized in the solid state, but
When the alloy composition is limited and the alloy is rapidly solidified, an amorphous structure similar to that of a liquid without a crystalline structure is obtained even in the solid state, and such an alloy is called an amorphous alloy.
This amorphous alloy has significantly higher strength than conventional practical metals, and exhibits various properties depending on its composition. In addition, by diffusing and penetrating Zn etc. into the surface layer of the metal,
The process of leaching out this material and promoting the special properties of the metal is called surface activation treatment. The present inventors have discovered that when such an amorphous alloy is used as a fuel electrode of a fuel cell, electric power can be extracted effectively and stably, and the present invention has been achieved. The present invention provides an amorphous alloy that has excellent performance as a fuel electrode for fuel cells, such as being able to effectively extract electric power when used as a fuel electrode for fuel cells that use methanol-based fuels such as methyl alcohol, formaldehyde, and formic acid. The purpose is to provide. The present invention consists of the following two inventions. 1 A surface-activated amorphous alloy for methanol fuel cell fuel electrodes containing 4-35 atomic % of any one or more of P, Si and B, with the balance being palladium 2 Any 1 of P, Si and B (1) One or more of Ni, Ru, Rh, and Ir at 65% or less (2) One or more of Fe, Co, Pt, and Sn40 Atomic % or less (3) Any one of Ti, Zr, Cu, Au, and Ag
Contains 65 at% or less in total of one or more groups selected from species or groups of two or more and up to 25 at%, and contains Pd as the substantial balance of at least 15 at%, with a total of 100 atoms Surface-activated amorphous alloy for methanol-based fuel cell fuel electrodes In the present invention, the amorphous alloy obtained by ultra-rapidly solidifying the molten alloy having the above composition is a monomer in which each of the above elements is uniformly dissolved in solid solution. It is a phase alloy. Originally, in order to impart selective activity to a specific chemical reaction to a metal electrode, it is necessary to make it contain a necessary amount of effective elements. However, in crystalline metals, when a large amount of various alloying elements are added, a multiphase structure with different chemical properties is often formed, and for this reason, it is often difficult to obtain mechanical strength. In contrast, the amorphous alloy of the present invention has an amorphous structure created by ultra-rapid cooling from a liquid state, so it is always a uniform single-phase solid solution, and has excellent mechanical properties and corrosion resistance. Shows stable and uniform electrode characteristics. Furthermore, in order to increase the activity as a fuel electrode of a fuel cell, it is necessary to perform a surface activation treatment such as diffusing and permeating Zn into the alloy surface layer and then permeating this into an alkaline solution. In crystalline metals, diffusion and infiltration of Zn and other substances mainly occurs at grain boundaries, so if Zn and other substances are subsequently leached, the crystal grains will fall off from the metal surface and the metal will become brittle.
Surface activation is often not effective. On the other hand, since the amorphous alloy of the present invention is not crystalline, it naturally does not have grain boundaries and does not suffer from embrittlement due to preferential diffusion and penetration of Zn and the like into the grain boundaries. Due to the inherently fast diffusion rate of Zn, etc., Zn can diffuse into the entire surface layer of the alloy, and then be leached out, thereby sufficiently activating the surface, even during relatively low-temperature processing. can be done. This is the reason why the surface-activated amorphous alloy of the present invention has excellent properties as a fuel electrode material for methanol fuel cells. Incidentally, the diffusion and penetration of Zn and the like can be achieved, for example, by heat treating the alloy in Zn powder, or by performing vacuum heat treatment after galvanizing the alloy. In this case, the fact that the heat treatment temperature is high and the amorphous alloy crystallizes does not pose any particular problem in activating the surface. However, since the progress of crystallization may cause the alloy to become brittle, it is desirable to avoid the progress of crystallization. Next, a method for manufacturing the amorphous alloy of the present invention will be explained. An amorphous alloy can be produced by ultra-quenching an alloy melt field having the composition of the present invention from a molten state at a cooling rate of about 10 ⁴ C/sec or more. If the cooling rate is slower than about 10 ⁴ C/sec, complete amorphization cannot be achieved. Therefore, if such ultra-rapid cooling can be achieved, it is theoretically possible to produce the amorphous alloy of the present invention using any type of equipment. As an example, an apparatus for producing the amorphous alloy of the present invention is shown in FIG. In the figure, reference numeral 2 denotes a quartz tube with a nozzle 3 vertically disposed at its lower end.
Additionally, an inert gas can be introduced to prevent oxidation of the raw material. A heating furnace 5 is provided around the quartz tube 2 to heat the sample 4. A high speed rotating roll 7 is provided vertically below the nozzle 3 and is rotated by a motor 6. To produce an amorphous alloy, a raw material 4 of a predetermined composition is heated and melted in a heating furnace 5 in an inert gas atmosphere in a quartz tube 2, and the molten metal is heated by a motor 6 for 1000 to 10000 r. When the pressurized inert gas is injected onto the outer peripheral surface of the roll 7 which is rotating at high speed at pm, for example, a thickness of 0.1
The amorphous alloy of the present invention can be obtained as a long thin plate with a width of 10 mm, a width of 10 mm, and a length of several meters. The amorphous alloy of the present invention produced by the above method has a Vickers hardness of about 400 to 600 and a tensile strength of about 120 to 200 Kg/mm ² , and can be fully contact bent or cold rolled. (more than 50%) Possesses excellent mechanical properties unique to amorphous alloys. Next, details of the alloy of the present invention will be explained. The conditions that the fuel electrode of a fuel cell must have are that it has a high electrode catalytic ability for the electrochemical oxidation reaction of the fuel and is stable for a long time.Under these electrode reaction conditions, it must have high corrosion resistance and sufficient mechanical strength. It is about having strength. The amorphous structure of the alloy makes it possible to prepare alloys with complex compositions as single-phase solid solutions, and also facilitates surface activation, resulting in high and stable electrocatalytic ability, high corrosion resistance, and This is essential in order to have excellent mechanical properties. For this amorphous alloy, it has been found that an alloy that has the objective of the present invention of stable and high electrocatalytic ability, high corrosion resistance, and excellent mechanical properties has the composition described in the present invention. . Examples are summarized in Table 1. The surface-activated amorphous alloy of the present invention has much higher electrocatalytic ability as a fuel electrode for methanol fuel cells than surface-activated palladium, and many have better electrocatalytic ability than platinum black. . Furthermore, while the electrocatalytic ability of materials such as platinum black deteriorates considerably over time, the electrocatalytic ability of the surface-activated amorphous alloy of the present invention hardly changes even after long-term use, and is extremely It is stable. Therefore, the surface-activated amorphous alloy of the present invention has excellent properties as a fuel electrode for methanol fuel cells. Next, the reason for limiting the component composition in the present invention will be described. P, Si and B are elements necessary to obtain an amorphous structure. However, if the total content of one or more of P, Si and B is less than 4 at % or more than 35 at %, it becomes difficult to obtain an amorphous structure. Therefore, the total amount of one or more of P, Si, and B must be in the range of 4-35 at%, and in particular, an amorphous structure is easily formed when the content is 16-25 at%. can get. Pd is the basic metal of the amorphous alloy of the present invention, and is an effective element for amorphization and electrochemical oxidation reaction of methanol fuel. Therefore, in the second aspect of the present invention, it is necessary to contain Pd in an amount of 15 atomic % or more. Ni, Ru, Rh, and Ir are effective elements that facilitate amorphization and increase the electrocatalytic ability when added in appropriate amounts; however, when added in large amounts, they reduce the electrocatalytic ability. Therefore, in the second aspect of the present invention, it is necessary to keep the total amount of one or more of Ni, Ru, Rh, and Ir to 65 atomic % or less. Both Pt and Sn are elements that enhance the electrode catalytic ability, but when added in large amounts, the crystallization temperature of the amorphous alloy decreases, making it more likely to become brittle during surface activation treatment. Therefore, in Section 2 of the present invention,
The sum of one or two of Pt and Sn
It is necessary to keep it below 40 atomic percent. Fe and
Both Co and Co are effective elements that promote amorphization.
However, adding a large amount will reduce the electrode catalytic ability.
In item 2 of the present invention, the total content of one or both of Fe and Co must be kept at 40 atomic % or less. Ti, Zr, Cu, Au and Ag are all effective elements for improving the corrosion resistance of amorphous alloys in acid and alkaline solutions, but when added in large amounts,
Since the electrode catalytic ability decreases, in the second aspect of the present invention, it is necessary to keep the total amount of one or more of Ti, Zr, Cu, Au, and Ag at 25 atomic % or less. In addition, elements such as C, N, O, Al, S, and Ge all stabilize the amorphous structure, and adding less than 5% will not impede the purpose of the present invention. do not have. Next, the present invention will be further explained by examples. Example 1 A raw material alloy having a predetermined composition was heated and melted using the method described above, and then ultra-quenched to obtain an amorphous alloy thin plate having a thickness of 0.02-0.05 mm, a width of 1-5 mm, and a length of about 10 m. Cut out a predetermined length from these amorphous alloy thin plates, and
At a constant current density of 20 mA/cm ² in an aqueous solution of 400 g/ZnSO ₄ 7H ₂ O and 70 g/Na ₂ SO ₄ at 30°C.
Zn plating was applied. Then heat these to 200-300℃
After vacuum heat treatment for 30 minutes to diffuse and infiltrate Zn, Zn was leached in a 6M KOH aqueous solution at 80°C to obtain a sample electrode. Using sample electrodes made in this way, samples containing various concentrations of methyl alcohol were used.
The anodic polarization curve was determined by potentiodynamic method at a potential transfer rate of 45 mV/min in a 1M KOH aqueous solution. 1
As an example, Table 2 shows the temperature at 30℃ containing 1MCH ₃ OH.
Potentials indicating a current density of 1 mA/cm ² in a 1M KOH solution were determined from potentiodynamic polarization curves and are summarized. All of the potentials have negative values, but the larger the absolute value, the more effective the potential is as a result of being able to obtain a larger voltage as a heating cell. The surface-activated amorphous alloy of the present invention is much more active than surface-activated Pd under the same conditions. Moreover, most of the alloys of the present invention have high electrode soot contact ability even when compared to platinum black. The remaining ones also have electrocatalytic abilities comparable to or slightly inferior to platinum black. Furthermore, the surface-activated _amorphous alloy of the present invention is ^extremely stable during the oxidation reaction of methyl alcohol. There is almost no change in electrode potential even when alcohol is oxidized. On the other hand, when platinum black and surface-activated palladium continue to oxidize methyl alcohol at the same current density for 10 hours, the voltage is 0.063V and
The absolute value of the 0.050V potential decreases, and the voltage of the fuel cell decreases accordingly. Example 2 Using a sample electrode prepared in the same manner as in Example 1, a potentiodynamic anodic polarization curve was determined in a 1M KOH solution containing 1MHCOONa at 30°C. Potentials representing a current density of 1 mA/cm ² determined from these potentiodynamic polarization curves are summarized in Table 3. The reason why the potentials in Table 3 are all negative values compared to Table 2 is HCOONa.
This is because the equilibrium potential for oxidation of CH 3 OH is more negative than that of CH ₃ OH. The surface activated amorphous alloy of the present invention is
It has a much higher electrode soot ability than Pd whose surface has been activated under the same conditions for the oxidation of HCOONa.
Moreover, when compared with platinum black, the electrode catalytic ability is superior or equivalent. Example 3 Using a sample electrode prepared in the same manner as in Example 1, a potentiodynamic anodic polarization curve was measured in a 1M KOH aqueous solution containing 1MHCHO at 30°C. Table 4 summarizes the potentials representing a current density of 1 mA/cm ² determined from these potentiodynamic polarization curves. The reason why the potentials in Table 4 are all negative values compared to Tables 2 and 3 is that
The equilibrium potential for oxidation of HCHO is CH ₃ OH and
This is because it is more negative than HCOONa. The surface activated amorphous alloy of the present invention is
It has a much higher electrocatalytic ability for the oxidation of HCHO than Pd whose surface is activated under the same conditions. Most of them are superior to platinum black, and the rest have electrocatalytic ability that rivals platinum black.

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【table】 [Brief explanation of drawings]

FIG. 1 is a schematic diagram showing an example of an apparatus for producing the amorphous alloy of the present invention. 1... Raw material inlet, 2... Quartz tube, 3... Nozzle section, 4... Raw material, 5... Heating furnace, 6... Motor, 7... High speed rotating roll.

Claims (1)

[Scope of Claims] 1. A surface-activated amorphous alloy for a fuel electrode of a methanol-based fuel cell, comprising 4 to 35 atomic % of one or more of P, Si, and B, and the balance being palladium. 2 Contains 4-35 atomic % of one or more of P, Si, and B, and (1) 65 atomic % or less of one or more of Ni, Ru, Rh, and Ir (2) Fe, Co , one or more of Pt and Sn, or more than 40 atomic % (3) One group selected from the group consisting of one or more of Ti, Zr, Cu, Au, and Ag, or more than 25 atomic % of Ti, Zr, Cu, Au, and Ag or contains two or more groups in a total amount of 65 at% or more, with the substantial balance being 15
A surface-activated amorphous alloy for methanol-based fuel cell fuel electrodes containing Pd at % or more, with a total content of 100 atomic%.