JP3037344B2 - Electrode for direct methanol fuel cell and method for producing the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an electrode for a direct methanol fuel cell, which has greatly enhanced electrocatalytic activity.

[Prior art] Conventionally, as an electrode for a direct fuel cell using methanol or a series thereof as a fuel, an electrode obtained by supporting platinum on a carbon-based electrode substrate which has been subjected to an appropriate treatment, or further comprising the platinum Electrodes surface-modified with dissimilar metals, especially ultra-trace tin ｛MMP Janssen and J. Moolhuysen “Electrochi
mica Acta " 21 , 861 (1976), M. Watanabe and S. Motoo
"J. Electroanal: Chem., 60 , 259 (1975)} is known.

However, the catalytic activity of the electrode is still extremely low, and the activity is significantly reduced with time. The output voltage when used as a fuel cell electrode is much lower than the theoretically predicted value. Then, the energy conversion efficiency of the fuel cell is low.

[Problems to be solved by the invention]

Conventional electrodes are not sufficient for practical use of direct methanol fuel cells and the like, and therefore, the present invention aims to develop a more active electrode catalyst.

[Means for solving the problem]

The direct methanol fuel cell electrode of the present invention, a platinum electrode surface formed by supporting platinum on a carbon-based electrode substrate,
Further, a metal component is provided with a conductive metal oxide film of iron, tin, vanadium, molybdenum, lanthanum, cadmium, indium, lead, gallium, thallium, antimony, or a conductive mixed metal oxide film. .

 Hereinafter, the present invention will be described.

In the present invention, the platinum electrode coated with the conductive metal oxide is obtained by supporting platinum on a carbon-based electrode substrate by means known in this field.

The metal oxide film provided on the platinum electrode is formed by dropping an alkali solution into an aqueous solution of a metal compound such as a metal oxide or chloride of the metal to form a hydroxide. And heat-treated in the air to form a metal oxide.

The metal component in the metal compound can be iron, tin, vanadium, molybdenum, lanthanum, cadmium, indium, lead, gallium, thallium, antimony, etc., with indium, lead, tin and antimony being particularly preferred.

Further, in order to provide a mixed metal oxide containing a plurality of metals on the electrode surface, the mixed metal oxide can be formed in the same manner as described above by dropping an alkali solution into a mixed aqueous solution of the above metal compounds. In things
Indium oxide-lead oxide, indium oxide-gallium oxide, indium oxide-thallium oxide, indium oxide-
Lanthanum oxide, tin oxide-antimony oxide.

These metal oxide films have a thickness of 0.01 μm after heat treatment.
m to 100 μm. The metal oxide film of the present invention is not ultra-trace as described above, but needs to be clearly in the form of an oxide, and it is necessary that the oxide has electrical conductivity. The heat treatment temperature is 300
℃ ~ 700 ℃, preferably 350 ℃ ~ 650 ℃, heat treatment time is 1
0 to 60 minutes is recommended.

In addition, the direct methanol fuel cell electrode of the present invention,
By forming a conductive metal oxide layer containing a carbon monoxide oxidation catalyst, particularly ultrafine gold particles, on a conductive metal oxide provided on the surface of a platinum electrode, an electrode with excellent CO oxidizability can be obtained. It is. Also, platinum ultrafine particles may be used instead of gold ultrafine particles.

Such an electrode can be produced by further mixing chloroauric acid with the above-mentioned metal compound alone or in a mixture, treating the mixture with an alkali in the same manner as described above, coating the precipitate on the electrode, and performing a heat treatment.

The relative amounts of the metal oxide and gold are in a weight ratio in the form of a metal salt and chloroauric acid, preferably 1-1 and 100-1;
Especially, it is good to make it 10: 1.

[Action]

In an electrode for a direct methanol fuel cell, it is considered necessary to hold an appropriate oxygen species such as lattice oxygen of a metal oxide or an adsorbed oxygen atom on the electrode surface, thereby promoting the oxidation reaction. . Conventionally, a small amount of a different metal (for example, tin) is used for the surface modification of platinum, and it is understood that it plays a certain role. However, there is a problem in the stability of the different metal itself on the platinum surface. This causes instability of the electrode activity.

Therefore, in the present invention, it has been found that an electrode having stable electrode activity can be obtained by forming an appropriately selected conductive metal oxide film on the surface of a gold electrode or a platinum electrode.

That is, the direct methanol fuel cell needs to supply oxygen necessary to oxidize methanol smoothly, but the metal oxide on the electrode temporarily retains oxygen on the electrode surface, that is, oxygen. It is considered to function as a transit point for supply. In general, the electrical conductivity of metal oxides is low. In particular, in the present invention, a composite oxide having electrical conductivity sufficient for operation as a battery system is used, and furthermore, the stability of the bond between the metal oxide and the electrode is increased. Further, they have found that an electrode having high catalytic activity can be prepared by forming a film of a metal compound on the electrode surface and then fixing the film by heat treatment.

Also, it is known that CO (carbon monoxide) -like substance accumulates on the electrode surface as an intermediate product of the reaction during methanol oxidation and inhibits the reaction itself.
An electrode capable of oxidizing and removing a CO (carbon monoxide) -like substance from the electrode surface and improving the fuel use efficiency by complete oxidation can be manufactured. Conventionally CO
The need to remove (carbon monoxide) analogs is recognized, for example, platinum electrodes surface modified with dissimilar metals,
Alternatively, a homogeneous liquid phase system, Redox comparable, etc.
The electrode potential required for the oxidative removal of O (carbon monoxide) is located much more positive than the potential at which methanol oxidation is performed. Therefore, even if CO-like substances are removed at such a potential, methanol-based fuel can be removed. There is a problem that the fuel cell used is not practical and does not contribute to the improvement of energy conversion efficiency.

The catalyst produced by the present invention can realize complete oxidation at a much lower potential by using a CO oxidation catalyst having extremely high catalytic activity for oxygen oxidation of CO, for example, ultrafine gold or ultrafine platinum. It is. A catalyst using ultrafine gold in particular as a CO oxidation catalyst, unlike other CO oxidation catalysts,
It exhibits high activity in a low temperature range of low temperature and ordinary temperature, and further has the feature that not only the activity is not lost due to the presence of water vapor but also the activity is enhanced, and high activity in an electrode system is obtained.

 The method for manufacturing an electrode of the present invention will be described.

(1) First, after dissolving a metal nitrate or chloride in an appropriate amount of water, and further mixing an aqueous chloroauric acid solution as needed, a sodium hydroxide solution is gradually added dropwise to precipitate as a hydroxide.

(2) The obtained gel precipitate is thinly applied to an electrode surface of platinum or the like which has been kept clean in advance, and then kept at about 60 ° C. on a porcelain heater, and gently taking care not to boil. dry.

(3) Further firing in an electric furnace, the temperature of which is, for example,
It is in the range of 300-600 ° C.

(4) The test electrode fabricated in this manner was attached to the tip of a tantalum wire sealed in a glass tube and introduced, and held in an electrolytic cell. Before the measurement, these electrodes were partially reduced by performing negative polarization in an electrolyte solution in order to bring the electrodes into a reduced state to a certain degree in advance.

The conditions of the negative polarization vary depending on the oxide system.For example, for an indium oxide-supported electrode, -0.1 V for 10 minutes, and for a lead oxide or an indium oxide-lead oxide mixed-support electrode system, -0.3 V, 30 minutes.
[The potential is the reversible hydrogen electrode potential (RH
E) Criteria].

Hereinafter, embodiments of the present invention will be described with reference to the drawings. [Example 1] A sodium hydroxide solution is gradually dropped into an aqueous solution of indium nitrate to precipitate gelled indium hydroxide. The hydroxide is applied to the platinum surface of the carbon electrode substrate on which platinum has been kept clean beforehand, and then kept at about 60 ° C. on a magnetic heater and gently dried while being careful not to boil. . Further, heat treatment was performed at 600 ° C. for 60 minutes in an electric furnace to obtain an electrode of the present invention having an oxide film having a thickness of about 10 μm.

[Examples 2 and 3] In Example 1, lead nitrate was used instead of indium nitrate.
Electrodes were obtained in the same manner as in Example 1 using tin chloride.

[Example 4] In Example 1, instead of the indium nitrate aqueous solution,
Mixed solution of indium nitrate and lead nitrate (1: 1 weight ratio)
And an electrode was obtained in the same manner as in Example 1.

[Example 5] Instead of the indium nitrate aqueous solution in Example 1, a mixed solution was used in which a chloroauric acid aqueous solution was added to a lanthanum nitrate aqueous solution and chloroauric acid was added in a weight ratio of 10% to lanthanum nitrate. An electrode was obtained in the same manner as in Example 1 except that a carbon electrode substrate supporting gold was used instead of the carbon electrode substrate supporting platinum.

[Examples 6 to 8] Instead of lanthanum nitrate in Example 5, iron nitrate (II
An electrode was obtained in the same manner as in Example 5, except that I), vanadium chloride (V) was used, and a carbon electrode substrate supporting platinum was used.

Table 1 below shows the electrode potential at no load (referred to as resting potential) of each electrode manufactured in the above example, which was 0.05M.
It is a result measured in a solution of H ₂ SO ₄ + 1M CH ₃ OH at 25 ° C.

Conventionally, an electrode for methanol oxidation has insufficient electrode catalytic activity, so that a stable methanol oxidation-reduction potential has not been obtained, and the resting potential has only a much larger positive value. Thus, one measure of whether an electrode has high activity can be how negative its resting potential is located. That is, the goal is to make the resting potential as close as possible to the theoretical equilibrium potential of the methanol oxidation reaction (about 0.05 V in an acidic solution, based on RHE).

As can be seen from the above table, the platinum electrode coated with indium oxide, or lead oxide, or a mixed system of indium oxide and lead oxide is far superior to the others.

Example 9 Instead of the aqueous solution of indium nitrate in Example 1, an aqueous solution of lead nitrate and an aqueous solution of chloroauric acid were added to an aqueous solution of indium nitrate, and lead nitrate and chloroauric acid were added in a weight ratio of 1: 2: 0.6 to indium nitrate. An electrode was obtained in the same manner as in Example 1 except that a mixed solution added so as to be as follows was used.

 The static potential of this electrode was as excellent as 0.105V.

In addition, FIG. 1 and FIG. 2 show measurement results of so-called polarization characteristics of this electrode, which indicate the magnitude of the steady and unsteady methanol oxidation rate (indicated by the current).

FIG. 1 (voltammogram) shows the change in the unsteady current value (value per true unit surface area, 20 ° C.) when the electrode potential was swept in the positive and negative directions. In the solution containing methanol (0.05MH ₂ SO ₄ + 1M CH ₃ OH, solid line) compared to the medium (0.05MH ₂ SO ₄ , dotted line)
Then a large current is flowing. The spread of the upper and lower curves is due to the capacitance component of the electrode, and when the line A (dashed-dotted line) is removed, the oxidation current is already observed at 0.2 V (RHE). This indicates that methanol can be oxidized at a much lower potential than the conventional SnO ₂ -modified platinum electrode shown by the B line (two-dot chain line).

Next, FIG. 2 (Tafel line) shows the steady-state current value (20 units per true unit surface area) observed with the electrode according to the present invention.
(° C.) to explain the steady-state polarization characteristics. In the figure, Janssen (23 ° C.) and Motoo et al.
C) are also shown for comparison. (H ₂ SO ₄ + 1M CH
_{In a 3} OH solution).

Compared with the conventional electrode, the electrode according to the present invention is required to maintain the same current (eg, 10 ⁻⁵ A · cm ⁻² ) at a current value of 5 times or more at the same potential (eg, 0.4 V, RHE). In terms of potential, it shows a high activity of a potential lower than 0.1 to 0.2 V. This means that when a methanol fuel cell is configured, the output voltage is 0.5 to 0.6 V, which is about 0.4 V, which is the conventional value, which results in an improvement in energy conversion efficiency of 20 to 30%.

Next, referring to FIG. 3, the characteristics of the catalytic activity of the various electrodes over time will be described. In the figure, a comparison was also made with previously reported platinum and tin-modified platinum electrodes. (All solutions are 0.05 MH ₂ SO ₄ +1 M CH ₃ OH at 20 ° C.). In addition, In ₂ O ₃ -PbO
-Au / Pt is shown on the right scale, and others are shown on the left scale.

As is clear from FIG. 3, the In ₂ O ₃ —PbO-modified platinum electrode of the present invention (the one prepared in Example 4 so that the weight ratio of lead nitrate to indium nitrate was 1: 2) was obtained. It is extremely excellent in its catalytic activity over time, and the In ₂ O ₃ -PbO-Au modified platinum electrode prepared in Example 9 shows higher activity and the same degree of durability. You can see that.

FIG. 4 is a graph for explaining the time-dependent characteristics of the catalytic activity of the electrode of Example 9 at a low potential at 20 ° C., 40 ° C. and 60 ° C. (All solutions are 0.05MH ₂ SO ₄
+ 1M CH ₃ OH) As can be seen in FIG.
_{The 2} O ₃ -PbO-Au modified platinum electrode maintains a significantly higher activity even at 300 mV (RHE) {FIG. 4 (a)} or 200 mV (RHE) {FIG. 4 (b)}.

Next, after the electrode prepared in Example 9 was used for the methanol oxidation reaction, the surface composition was measured by Auger electron spectroscopy. The results are shown in Table 2 below.

According to this table, the surface concentration of In was extremely low from the viewpoint of the charged composition, indicating that In was lost from the surface by the heat treatment. That is, it can be seen that the heat treatment temperature is not only a stabilization of the metal oxide, but also an important factor in defining the surface concentration of each component and determining the electrocatalytic activity.

Further, the fact that the amount of carbon on the surface is small indicates that accumulation of the CO-like substance is extremely unlikely to occur on this electrode. Furthermore, the underlying platinum is scarcely present on the surface, and most of the underlying platinum is covered with a metal oxide. It can be seen that this high activity is realized with only a small amount of platinum.

FIG. 5 shows the results obtained by a scanning electron microscope (SEM, magnification: 10 ⁴ ).
14 is a photograph instead of a drawing, showing a crystal structure of an electrode surface after being used as an electrode prepared in Example 9. The surface includes large crystals of orthorhombic system, and clumps of circular or irregular small particles. Characteristics such as,, fine particles in the flat part, and black grooves running vertically and horizontally on the flat part are found.

FIG. 6 is an EPMA image of the same electrode surface. FIG. 6 (a) shows an image relating to platinum, and FIG. 6 (b) shows an image relating to lead.
FIG. 6 (c) shows an image of gold. Again, platinum is distributed in flat areas, while lead is more distributed in large orthorhombic crystal parts. Gold is highly localized, distributed in small clusters, and is often distributed in small spherical particles seen in SEM images.

Despite the substantial contribution of platinum to the electrocatalytic activity, its abundance on the electrode surface is very low, ~ 1%.
According to the EPMA measurement, the platinum is largely distributed in the flat part and the groove part, and since this part seems to have a thin layer of the supporting material, most of the observed characteristic X-rays of platinum are Probably from the place.

From this, it was found that, although the electrode made in the present invention had a significantly higher activity, its activity was derived from only a part of platinum.

〔The invention's effect〕

The direct-type methanol fuel cell electrode of the present invention has a high catalytic activity by modifying the platinum surface with a metal oxide or a composite metal oxide and providing a transit point for the oxidation required for the oxidation of fuel such as methanol. The electrode has a carbon monoxide oxidation catalyst, especially ultrafine gold in the metal oxide or composite metal oxide to form the electrode, thereby removing CO species generated on the surface of the electrode catalyst by oxidation. Thus, an electrode having high catalytic activity can be obtained, which can greatly improve the energy conversion efficiency of a methanol fuel cell.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining a change in an unsteady current when the electrode potential is swept in the positive or negative direction using the electrode of the present invention, and FIG. FIG. 3 is a diagram for explaining the temporal stability of the methanol oxidation current measured on the electrode of the present invention,
FIG. 4 is a view for explaining the temporal stability of methanol oxidation current measured at each temperature at a lower potential in the same manner as in FIG. 3, and FIG. 5 is a scanning electron microscope (SEM, magnification 10 ^4). ) Shows the particle structure of the electrode surface after use, and shows a photograph instead of a drawing, and FIG.
It is a PMA image and is a photograph in place of a drawing, showing a particle structure on an electrode surface after use.

Claims (4)

(57) [Claims] 1. A platinum electrode surface formed by supporting platinum on a carbon-based electrode substrate, wherein a metal component further comprises iron, tin, vanadium, molybdenum, lanthanum, cadmium, indium,
An electrode for a direct methanol fuel cell, comprising a conductive metal oxide film of lead, gallium, thallium, and antimony, or a conductive mixed metal oxide film. 2. A platinum electrode surface formed by supporting platinum on a carbon-based electrode substrate, and further comprising metal components such as iron, tin, vanadium, molybdenum, lanthanum, cadmium, indium, and the like.
An electrode for a direct methanol fuel cell, comprising a coating containing a conductive metal oxide such as lead, gallium, thallium, and antimony, or a conductive mixed metal oxide, and a carbon monoxide oxidation catalyst. 3. The direct methanol fuel cell electrode according to claim 2, wherein the carbon monoxide oxidation catalyst is ultrafine gold particles or ultrafine platinum particles. 4. A platinum electrode surface formed by supporting platinum on a carbon-based electrode substrate, wherein the metal component is iron, tin, vanadium, molybdenum, lanthanum, cadmium, indium, lead, gallium, thallium, and antimony. A direct methanol fuel cell, wherein a metal hydroxide or a mixed metal hydroxide is coated and then heat-treated to form a conductive metal oxide film or a conductive mixed metal oxide film. Manufacturing method of electrode.