JPH0512425B2 - - Google Patents

Description

[Detailed description of the invention]

Industrial Application Field The present invention is suitable as a fuel electrode for a fuel cell using methanol fuel such as methanol, formaldehyde, formic acid, etc., which contains at least a portion of an amorphous phase, and which has a surface active surface. The present invention relates to an inexpensive amorphous alloy that has been subjected to chemical treatment and has high electrode activity and excellent corrosion resistance. Conventional technology Electrodes for methanol-based fuel cells have been known in which fine platinum powder is coated on a wire mesh or porous carbon plate, but since a large amount of expensive platinum is used, it is not expensive. Unavoidably, there are also problems such as a large decrease in activity during use. On the other hand, amorphous alloys have been attracting attention in recent years.
Conventionally known conventional alloys are crystallized in the solid state, but in the process of solid formation, the atomic arrangement has a long periodicity, such as by limiting the alloy composition and solidifying it by ultra-rapid cooling from the molten state. By applying a method that does not provide regularity, an amorphous structure similar to a liquid can be obtained without having a crystalline structure. Such alloys are called amorphous alloys, and most of them are homogeneous single-phase alloys of supersaturated solid solutions, and have significantly higher strength than conventional ordinary practical metals, and have a composition of It exhibits various unique properties, including abnormally high corrosion resistance depending on the situation. Furthermore, even if the composition does not allow the entire alloy to be in an amorphous phase, the alloy produced by applying a normal amorphous alloy production method is a supersaturated solid solution with an expanded solid solubility limit. It has excellent properties similar to those of amorphous alloys. The present inventors have already taken advantage of the excellent properties of such amorphous alloys to create a fuel electrode for methanol fuel cells, which is made by surface-activating an amorphous alloy mainly composed of palladium. A surface-activated amorphous alloy has been obtained (Japanese Patent Application No. 12561/1983), but it is still expensive. Furthermore, in order to obtain an electrode that generates oxygen and chlorine by electrolyzing an aqueous solution, the present inventors conducted research using an amorphous alloy, and found that it contains only a small amount of platinum group elements responsible for electrode activity. A corrosive solution that selectively dissolves alloying elements other than platinum group metals from the alloy surface, taking advantage of the fact that these alloys are alloys in which platinum group metals are uniformly dispersed, even if they are amorphous alloys or supersaturated solid solution alloys. It has been discovered that a highly active electrode for solution electrolysis can be obtained by surface activation treatment by immersion in water (Japanese Patent Application No. 60-169764,
(Japanese Patent Application No. 60-169765, Japanese Patent Application No. 169766, and Japanese Patent Application No. 169767). Problems to be Solved by the Invention Therefore, the present inventors have developed a method that has high electrocatalytic activity for electrochemical oxidation of methanol-based fuels while reducing the amount of expensive platinum used, and which can be used under electrolytic conditions. With the aim of obtaining an amorphous alloy for methanol fuel cell electrodes with high corrosion resistance,
This led to the invention of the present invention. Means for Solving the Problems The inventors of the present invention have conducted intensive research to obtain a fuel electrode for fuel cells by utilizing the characteristics of the amorphous alloy described above, and as a result, in principle, ( a) an amorphous alloy consisting of at least one selected from Ni and Co; (b) at least one selected from Ti and Zr and/or at least one selected from Nb and Ta; Adding Pt, which is active in the electrolytic oxidation of methanol, to a supersaturated solid solution alloy containing a solid phase, and further adding various elements that assist the action of Pt as necessary, and subjecting the alloy to the above-mentioned surface activation treatment. It has been found that it is possible to obtain an inexpensive fuel electrode for fuel cells that has higher activity than platinum black and has high corrosion resistance in a sulfuric acid acidic methanol fuel solution. That is, the first surface-activated amorphous alloy for methanol-based fuel cell electrodes of the present invention contains 0.5 to 20 atomic % of pt, 20 to 80 atomic % of one or both of Ti and Zr,
The remainder is essentially 10 atom% of one or both of Ni and Co.
It consists of the above. The second is 0.5 to 20 at% of Pt, 20 to 80 at% of one or both of Ti and Zr, Ru, Rh, Pd, Ir, Tl,
One or more selected from the group consisting of Si, Ge, Sn, Pb and Bi, up to 10 atomic % (however, when Pt is 10 atomic % or less, the atomic % is the same amount or less as Pt), and the remainder is substantially Consists of 10 atomic % or more of one or both of Ni and Co. The third is 0.5 to 20 at% of Pt, 20 to 70 at% of one or both of Nb and Ta, and the balance is essentially Ni and Co.
Consisting of one or two types. The fourth is pt0.5-20 at%, 20-70 at% of one or both of Nb and Ta, Ru, Rh, Pd, Ir, Tl,
One or more selected from the group consisting of Si, Ge, Sn, Pb and Bi, up to 10 atomic % (however, when Pt is 10 atomic % or less, the atomic % is the same amount or less as Pt), and the remainder is substantially Consists of 10 atomic % or more of one or both of Ni and Co. Fifth, 0.5 to 20 at% of Pt, 70 at% or less of one or two of Nb and Ta, and one or two of Ti and Zr.
20 to 80 atomic% in total with seeds (1% of the above Nb and Ta)
Contains a quantity of a species or two species. ), the remainder substantially Ni and
Consists of 10 atomic % or more of one or two types of Co. The sixth is 0.5 to 20 at% of Pt, 70 at% or less of one or two of Nb and Ta, and one or two of Ti and Zr.
20 to 80 atomic% in total with seeds (1% of the above Nb and Ta)
Contains a quantity of a species or two species. ), Ru, Rh, Pd, Ir,
One or more selected from the group consisting of Tl, Si, Ge, Sn, Pb, and Bi and up to 10 atomic % (however,
When the amount of Pt is 10 atomic % or less, the amount in atomic % is the same as that of Pt), and the remainder substantially consists of 10 atomic % or more of one or both of Ni and Co. Table 1 shows the constituent elements and content rates of these first to sixth inventions. Various conventionally known methods for manufacturing amorphous alloys include melting and ultra-rapidly solidifying an alloy having the above composition, or performing sputter deposition using a mixture having the same average composition as above as a target.

The amorphous alloy obtained by [Table] is a single-phase alloy in which the above elements are uniformly dissolved in solid solution. Similarly, a supersaturated solid solution alloy containing at least a portion of an amorphous phase is also an alloy in which the distribution of the above elements is extremely uniform.
Originally, in order to obtain an alloy with selective electrocatalytic activity for a specific electrochemical reaction and high corrosion resistance that can withstand the reaction conditions, it is necessary to add a certain amount of effective elements. However, in the case of crystalline alloys manufactured by conventional methods, when a large number of different elements are added, a multiphase structure with different chemical properties is often formed, and the expected electrode activity may not be obtained. Moreover, its corrosion resistance and mechanical strength are also inferior. In contrast, the amorphous alloy of the present invention, or the supersaturated solid solution alloy containing at least a portion of the amorphous phase, is produced by amorphous alloy production that does not allow the constituent elements to localize. Therefore, it has extremely high compositional uniformity and has the necessary electrocatalytic activity and corrosion resistance. Next, the reason for limiting the chemical components in the alloy of the present invention will be explained. Pt is an essential element responsible for the electrochemical oxidation activity of methanol fuel cells, and if it is less than 0.5 atom%, sufficient activity cannot be obtained, whereas if it is added in excess of 20 atom%, the electrode activity is There is no significant improvement,
Moreover, the electrodes become expensive. Therefore, in the present invention, Pt is contained in a range of 0.5 to 20 atomic %. Any one or more elements selected from the group consisting of Ti, Zr, Nb and Ta are Ni and Co.
When coexisting with one or both of these, an amorphous alloy can be obtained by applying the various amorphous alloy manufacturing methods described above to this. When the element coexisting with 10 atomic % or more of one or two of Ni and Co is one or two of Ti and Zr (the first and second alloys), one or two of Ti and Zr An amorphous structure can be easily obtained when the species is 20-80 at%. Similarly, 1 of Ni and Co
When the element coexisting with the species or two species is one or two of Nb and Ta (the third and fourth alloys)
When one or both of Nb and Ta are present in an amount of 20 to 70 atomic %, an amorphous structure can be easily obtained. One or two of Ti and Zr and one of Nb and Ta
The species or two species are both Ni and Co10
When Nb and Ta coexist at 70 atomic % or more (in the fifth and sixth alloys), one or both of Nb and Ta are present at 70 atomic %.
and one or two of Ti and Zr (the above Nb
and Ta. ) is 20 to 80 atomic %, an amorphous structure can be easily obtained. In the present invention, the alloy contains 10 atomic % or more of one or two of Ni and Co, one or two of Ti and Zr, and one or two of Nb and Ta, as well as Ru,
It can contain at least one member selected from the group consisting of Rh, Pd, Ir, Tl, Si, Ge, Sn, Pb, and Bi. (The second, fourth and sixth alloys). Ru, Rh, Pd, Ir, Tl, Si, Ge, Sn, Pb and
Bi helps the effect of Pt and improves its activity, but
Addition of large amounts is not effective. Therefore, in the present invention, the total amount of one or more of these elements added is 10 atomic % or less, and in particular, if the amount of Pt is
When it is 10 atomic % or less, it is necessary that the atomic % is equal to or less than the amount of Pt. As mentioned above, one or both of Ni and Co are
It is an element for forming an amorphous structure in coexistence with one or more of Ti, Zr, Nb, and Ta.
Although replacing Ni and Co with Pt is also effective for forming an amorphous structure,
When the entire amount of is replaced with Pt, it becomes difficult to form an amorphous structure. Therefore, in the present invention,
The amount of one or both of Ni and Co added is 10 atomic%
It is necessary to do the above. Even if the alloy according to the present invention contains one or more of V, Mo, and W in an amount of 3 atomic % or less, it also contains one or more of Cr and Fe in an amount of 10 atomic %.
Even if it is contained within the following range, it will not impede the function as an electrode for methanol fuel cells. Metalloid elements such as B, C, and P are conventionally known to be effective in forming an amorphous structure, but in the present invention, when adding a large amount of these elements, the electrode activity decreases. However, these elements are effective in forming an amorphous structure when added in amounts up to about 7 atomic percent, but do not have a detrimental effect on electrode activity. Addition of up to 7 atomic % of metal elements is permitted. Next, in order to have sufficient catalytic activity as a fuel cell electrode, it is necessary to increase the electrochemically effective surface and act as an active site for electrode reactions.
It is necessary to collect Pt on the alloy surface. For this purpose, in the present invention, an amorphous alloy or a supersaturated solid solution alloy containing at least a portion of an amorphous phase is subjected to a surface activation treatment by immersing it in hydrofluoric acid. The concentration and temperature of hydrofluoric acid are appropriately selected depending on the composition of the target amorphous alloy or the supersaturated solid solution alloy containing at least a portion of the amorphous phase. When an amorphous alloy of the present invention or a supersaturated solid solution alloy containing at least a portion of an amorphous phase is immersed in hydrofluoric acid, such alloys have a high degree of homogeneity, resulting in a uniform distribution of Pt or other platinum group elements. As a result, elements such as Ni, Co, Ti, Zr, Nb, and Ta, which are more base than platinum group elements, are selectively dissolved from the alloy surface, and the alloy surface becomes finer. , Pt has a black color and is responsible for electrode activity.
And supporting elements, including other platinum group elements, are concentrated on the alloy surface. Therefore, the surface activation treatment may be completed when the surface becomes blackish. In addition, in the case of a crystalline alloy manufactured by a normal method, even if its average composition is the same as the composition specified in the present invention, it has a multiphase structure and the alloying elements are localized. Therefore, even if the above surface activation treatment is performed, hydrogen generation is not observed.
Selective dissolution of elements such as Co, Ti, Zr, Nb, and Ta is also difficult to occur, so surface activity is not improved. Additionally, heterogeneous crystalline alloys produced by conventional methods do not withstand acids, including methanol-based fuels, due to their low corrosion resistance. However, since the amorphous alloy according to the present invention or the supersaturated solid solution alloy containing at least a portion of the amorphous phase has a uniform distribution of component elements, it can be dissolved in hydrofluoric acid by the above surface activation treatment. Ni, Co,
Elements such as Ti, Zr, Nb, Ta, etc. are uniformly dissolved, and the effective surface area is significantly increased. Pt, which is responsible for electrode activity, and elements that help Pt are concentrated on the surface, allowing for sufficient activation. . In this way, the alloy according to the present invention exhibits outstanding electrode activity and corrosion resistance as an electrode for methanol fuel cells. Amorphous alloys and supersaturated solid solution alloys containing at least a partially amorphous phase according to the present invention can be prepared by ultra-rapid solidification of solution alloys by various methods widely used to produce amorphous alloys. It can be manufactured by An example of an apparatus for producing an amorphous alloy of the present invention and a supersaturated solid solution alloy containing at least a portion of an amorphous phase is shown in FIG. The device is placed in a vacuum vessel, indicated by dashed lines. A nozzle 3 is attached to the lower end of a vertical quartz tube 2, and a roll 7 rotated at high speed is disposed below the nozzle. This roll is driven by a motor 6. Further, a heater 5 for heating and melting the raw material alloy 4 is arranged around the quartz tube, and at the upper end of the quartz tube, an inert gas is provided for blowing out the raw material alloy 4 and the molten raw material alloy. An entrance 1 is provided. In order to produce the amorphous alloy of the present invention or a supersaturated solid solution alloy containing at least a portion of an amorphous phase using this apparatus, first, the pressure inside the apparatus is reduced to about 10 ^-5 Torr, and then the pressure is 1 inert gas
Introduce it into the device to about atmospheric pressure. In this inert gas atmosphere, the raw material alloy of a predetermined composition in the quartz tube is heated and melted using a heater. This heated and molten alloy is blown out as a jet from a nozzle with an inert gas of about 0.4 to 2 kg/ ^cm2 , and
When the molten alloy collides with the outer surface of a roll rotating at a high speed of about 10,000 rpm, the heat of the molten alloy is rapidly removed by the roll, resulting in a ribbon-shaped amorphous alloy or a supersaturated alloy containing at least a portion of the amorphous phase. A solid solution alloy can be obtained. The alloy thus obtained has a thickness of, for example, 0.005 to 0.1 mm and a width of 0.5 to 10 mm.
mm, and the length is several mm to several tens of meters. Examples The present invention will be described below based on examples. Example 1 Commercially available metals were mixed at a predetermined ratio and heated and melted in a high frequency furnace in an Ar atmosphere to obtain a raw material alloy. Also
Among Ti, Zr, Nb, and Ta, alloys not containing Nb and Ta were melted in an argon arc to obtain raw material alloys. These raw material alloys were remelted and rapidly cooled using the apparatus shown in FIG. 1 to produce an amorphous alloy and a supersaturated solid solution alloy partially containing an amorphous phase. The chemical compositions of these alloys are shown in Table 1. The formation of an amorphous phase was confirmed by X-ray diffraction, and many alloys showed a halo pattern characteristic of amorphous alloys. However, in some halo patterns, sharp diffraction lines based on the crystalline phase are superimposed, indicating a mixed structure of amorphous and crystalline phases, but since the amorphous phase exists, these alloys are not supersaturated solid solutions. It turns out that there is something. To confirm that these alloys have sufficient corrosion resistance in a sulfuric acid acidic environment that electrochemically oxidizes methanol-based fuel, these alloys were polished in cyclohexane to No. 1000 SiC paper and then polished in a 0.5M sulfuric acid solution. The anodic polarization curves were measured in medium and _{0.5MH2SO4-1MCH3OH solutions .} An example is shown in FIG. The current in the negative potential region observed in 0.5M sulfuric acid is the negative current consumed for hydrogen generation with its sign changed. In 0.5MH ₂ SO ₄ , no current of 10 ⁻² Am ⁻² or more was observed over a wide potential range, indicating that this alloy has extremely high corrosion resistance. Note that the increase in current observed from around 1.1V (SCE) is based on oxygen generation. 0.4V in 0.5MH ₂ SO ₄ −1M CH ₃ OH solution
An anode current on the order of 10 ^-2 Am ^-2 is observed from around (SCE). This is the current for methanol oxidation. However, at methanol oxidation currents of this order, the activity is too low and it cannot be used as an electrode material. In order to improve the electrode activity, a surface activation treatment was performed by immersing the electrode in a commercially available 46% HF solution for several minutes.
In addition, in the case of alloys that do not contain Nb or Ta among Ti, Zr, Nb, and Ta, the time required for activation with 46% HF solution was short and control was difficult, so 4.6% HF solution was used.
Surface activation treatment was performed by immersing it in a solution for several minutes. The anodic polarization curve of the surface activated amorphous alloy measured in a 0.5MH ₂ SO ₄ -1M CH ₃ OH solution is shown in FIG. 3 in comparison with the anodic polarization curve of platinum black.
Since the amorphous alloy of the present invention has higher activity for methanol oxidation than platinum black, a current for methanol oxidation is observed from a lower potential than platinum black, and at the same potential, the non-crystalline alloy of the present invention The crystalline alloy exhibits higher current densities for methanol oxidation from lower potentials than the comparative platinum black. Table 3 shows 0.5MH ₂ for surface activated amorphous alloys and supersaturated solid solution alloys containing some amorphous phase.
The current density observed during potentiostatic polarization in SO ₄ -1M CH ₃ OH solution is shown in comparison with the current density observed in platinum black and smooth platinum. The current for both alloys is greater than or approximately the same magnitude as platinum black. Therefore, these surface-activated alloys have similar or better electrocatalytic activity than platinum black for the electrochemical oxidation of methanol, despite their extremely low platinum content. . Example 2 Example 1 was applied to an amorphous alloy produced in the same manner as in Example 1 and a supersaturated solid solution alloy containing a partially amorphous phase.
The surface activation treatment was performed as described in , and the catalytic activity for electrochemical oxidation of formaldebide was investigated. Table 4 shows 0.5MH ₂ for surface activated amorphous alloys and supersaturated solid solution alloys containing some amorphous phase.
The current density observed during potentiostatic polarization in SO ₄ -1M HCHO solution is shown in comparison with the current density observed in platinum black and smooth platinum. The current density of both alloys is higher than that of platinum black. These surface-activated alloys therefore have better electrocatalytic activity than platinum black for the electrochemical oxidation of formaldebide, despite their extremely low platinum content. Example 3 Example 1 was applied to an amorphous alloy produced in the same manner as in Example 1 and a supersaturated solid solution alloy containing a partially amorphous phase.
Surface activation treatment was performed as described in , and the catalytic activity for electrochemical oxidation of formic acid was investigated. Table 5 shows 0.5MH ₂ for surface-activated amorphous alloys and supersaturated solid solution alloys containing some amorphous phase.
The current density observed during potentiostatic polarization in SO ₄ -1M HCOOH solution is shown in comparison with the current density observed in platinum black and smooth platinum. The current density of both alloys is higher than that of platinum black. These surface-activated alloys therefore have better electrocatalytic properties than platinum black for the electrochemical oxidation of formic acid, despite their extremely low platinum content.

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[Table] Effects of the Invention As detailed above, the surface-activated amorphous alloy for methanol fuel cells of the present invention can be used in methanol fuel cells even though the amount of expensive platinum added is extremely low. It has extremely high electrocatalytic activity for electrochemical oxidation of , and has high corrosion resistance under electrolytic conditions. Moreover, such an alloy according to the present invention can be manufactured by any of the already widely known techniques for manufacturing amorphous alloys, and therefore does not require any special equipment.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view showing an example of an apparatus for producing the alloy of the present invention, and Figure 2 is _{a cross- sectional} view showing an example of an apparatus for producing the alloy of the present invention.
Polarization curve of amorphous Ni−40Nb−3Pt alloy measured in 0.5MH ₂ SO ₄ −1M CH ₃ OH, Figure 3 shows amorphous Ni−40Nb− subjected to surface activation treatment.
Using 2.7Pt−0.3Ru alloy and platinum black for comparison as samples, 0.5MH ₂ SO ₄ −1MCH ₃
This is a polarization curve measured in OH.

Claims (1)

[Claims] 1. 0.5 to 20 atomic % of Pt, one or two of Ti and Zr.
1. A surface-activated amorphous alloy for methanol-based fuel cell electrodes, characterized in that it comprises 20 to 80 atomic %, and the remainder substantially 10 atomic % or more of one or both of Ni and Co. 2 Pt0.5-20 at%, one or two of Ti and Zr
20-80 atomic%, Ru, Rh, Pd, Ir, Tl, Si, Ge,
One or more selected from the group consisting of Sn, Pb and Bi, up to 10 atomic % (however, when Pt is 10 atomic % or less, the atomic % is the same amount or less as Pt), and the remainder is substantially Ni and Co. A surface-activated amorphous alloy for a methanol-based fuel cell electrode, characterized by comprising 10 atomic % or more of one or two types. 3 Pt0.5-20 atomic%, one or two of Nb and Ta
1. A surface-activated amorphous alloy for methanol-based fuel cell electrodes, characterized in that the surface-activated amorphous alloy is comprised of 20 to 70 atomic % of seeds, and the remainder substantially of one or both of Ni and Co. 4 Pt0.5-20 atomic%, one or two of Nb and Ta
Species 20-70 atomic%, Ru, Rh, Pd, Ir, Tl, Si,
One or more selected from the group consisting of Ge, Sn, Pb and Bi, up to 10 atomic % (However, Pt10 atomic %
In the following cases, the surface-activated amorphous material for methanol-based fuel cell electrodes is characterized in that the remainder consists essentially of at least 10 at % of one or both of Ni and Co. quality alloy. 5 Pt0.5-20 atomic%, one or two of Nb and Ta
A total amount of 70 at % or less of species and one or two types of Ti and Zr is 20 to 80 at % (including the amount of one or two of the above Nb and Ta), the balance is substantially one of Ni and Co or a surface-activated amorphous alloy for methanol-based fuel cell electrodes, characterized by comprising 10 atomic % or more of two types. 6 Pt0.5 to 20 atomic%, one or two of Nb and Ta
A total amount of 70 at % or less of species and one or two of Ti and Zr (including the amount of one or two of the above Nb and Ta), Ru, Rh, Pd, Ir, Tl, Si,
One or more selected from the group consisting of Ge, Sn, Pb and Bi, up to 10 atomic % (However, Pt10 atomic %
In the following cases, the surface-activated amorphous material for methanol-based fuel cell electrodes is characterized in that the remainder consists essentially of at least 10 at % of one or both of Ni and Co. quality alloy.