JPS609596B2 - Electrode for methanol electrolysis - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION 1. Technical Field The present invention relates to an electrode for methanol electrolytic oxidation.

More specifically, the present invention relates to an electrode for methanol electrolytic oxidation, which is formed by forming a coating layer having methanol electrolytic oxidation catalytic activity on a conductive base material. B. Conventional technology Starting from the depletion of petroleum resources, the effective use of energy has become
It is becoming an urgent issue for human society, and the use of methanol as an energy source has attracted attention in recent years.

This is because methanol is expected to be available in large quantities and at low cost from coal, natural gas, biomass, etc. in the future. One way to utilize methanol as an energy source is to convert the chemical energy of methanol into electrical energy using an electrochemical reactor.

In this case, to convert the chemical energy of methanol into electrical energy, an anode with catalytic activity is used.
An electrochemical reactor may be configured in which the methanol electrolytic oxidation reaction is an anode reaction.

At this time, it can be used as electrical energy of the highest quality, and power can be saved.
In addition, if the chemical energy of methanol is efficiently converted to electrical energy, the available energy associated with the 6-electron oxidation reaction of methanol to carbon dioxide gas, in terms of electrical energy, is theoretically equivalent to 1 ton of methanol. 50
00KWh or more, making it possible to use energy effectively. Furthermore, if chemical energy is converted to electrical energy via an electrochemical IJ factor, unlike the current power generation method performed by a heat engine, it is essentially free from the limitations of the Carnot cycle. Therefore, high thermal efficiency can be expected. To explain in more detail an example of configuring an electrochemical IJ factor and utilizing the chemical energy of methanol, one example is ``Utilization in electrowinning from a sulfuric acid-zinc sulfate solution.'' be able to.

In this case, in the current electrolytic method, the anodic reaction is 40 to 6
This is an oxygen evolution reaction that is carried out at 00C, and the standard electrode potential for this reaction is 1.20 at 60oo, based on the hydrogen electrode.
It is 0V. On the other hand, when methanol is dissolved in an electrolytic solution and an anode having catalytic activity is used, the anode reaction can be replaced from an oxygen generation reaction to an oxidation reaction of methanol to carbon dioxide gas. And this C0
The oxidation reaction to 2 was carried out at 6,000 ℃ and 0.
034V. That is, by dissolving methanol in the electrolytic solution, using a predetermined anode, and incorporating an electrochemical reactor into the electrolytic cell itself, the electrolytic cell voltage can, in principle, be reduced by the difference between the two. It is expected that it will be possible to devalue 1.166V.

At this time, as an electrolytic energy source, it is possible to theoretically save 5852 KWh of electrical energy per ton of methanol. Furthermore, when a methanol-oxygen fuel cell is configured using a catalytically active anode as an electrochemical reactor, it is also useful as a power generation device, and, along with other hydrogen-oxygen fuel cells, it will be useful in the future. It is also promising as a power generation device. By the way, platinum group elements such as platinum, palladium, ruthenium, rhodium, and iridium are conventionally known as substances having electrode reaction catalytic activity, and electrodes made of these as a coating layer on a base material can be used in various electrolytic reactions. Used as a reaction electrode.

However, as an anode for methanol electrolytic oxidation as described above, remarkable methanol electrocatalytic activity is exhibited only when platinum is used as a coating layer, and other materials such as palladium, rhodium, etc. do not exhibit methanol electrocatalytic activity.

However, even when using an anode with a platinum coating layer,
Although the electrolytic oxidation catalyst activity is sufficiently high at the beginning of the electrolytic oxidation of methanol, as the electrolysis time progresses,
This results in a significant decrease in catalytic activity.

For this reason, it becomes difficult to maintain a desired fog scale current density throughout the predetermined electrolysis time, resulting in a major practical inconvenience. This decrease in catalytic activity over time of electrolysis is due to the adsorption and deposition of oxidation intermediate products on the electrode surface, which are thought to be generated during the process in which methanol reaches carbon dioxide, the final product of the electrolytic oxidation reaction. It is assumed that this is due to the catalyst activity being poisoned.

Under these circumstances, in order to put into practical use the technology that converts the chemical energy of methanol into electrical energy, it is necessary to use a coating layer-forming material that has high methanol oxidation catalytic activity, and that has a high level of methanol oxidation catalytic activity. It is desired to develop a catalyst material that does not cause a decrease in catalytic activity.

m. Purpose of the Invention The present invention has been developed in view of the above circumstances, and has a high methanol electrolytic oxidation catalyst activity, and is capable of significantly reducing catalyst activity during electrolysis over a long period of time required for practical use. The main purpose is to provide an electrode for methanol electrolysis with a small amount of use.

The present inventors have conducted intensive research for such purposes,
The methanol electrocatalytic activity and poisoning properties of various materials, especially compositions of platinum and other metals, were measured, and as a result, the present invention was developed.

That is, the present invention provides an electrode for methanol electrolysis in which a coating layer having methanol electrolytic oxidation catalyst activity is formed on a conductive base material, wherein the coating layer has a content of 5 mol% to 20 mol%.
An electrode for methanol electrolysis characterized by being formed from a platinum-rhodium composition containing rhodium.

According to the present invention, by using a platinum-rhodium composition in a predetermined quantitative ratio, high catalytic activity and strong resistance to poisoning of the catalytic activity over time of electrolysis are exhibited. Such an effect is not achieved when is replaced with another platinum group element, such as ruthenium. This fact will become clear from the examples described later.
N. Specific Configuration of the Invention The specific configuration of the present invention will be described in detail below.

The coating layer in the present invention is formed from a platinum-rhodium composition containing rhodium in an amount of 5 mol% to 20 mol%, more preferably 8 mol% to 12 mol%.

In this case, if the amount of rhodium is less than 5 mol % and greater than 20 mol %, the resistance to poisoning will be weakened and the electrolytic current density will deteriorate significantly over time. The coating layer may be formed essentially only from the platinum-rhodium composition.

The platinum-rhodium composition forming this coating layer is usually formed on the conductive substrate described below in a state in which platinum and rhodium are substantially absent as simple substances in the composition. The thickness of the coating layer is not particularly limited, but is generally about 0.2 to 20 mm thick, particularly about 0.5 to 5 mm thick. On the other hand, there is no particular restriction on the material of the conductive base material serving as the core material, and various metals can be used.

However, when used as an anode in an acidic electrolyte, such as when used as an anode in the electrowinning of zinc in a sulfuric acid bath, the anode must have sufficient corrosion resistance as an insoluble anode. It is preferable to use a so-called valve metal as the conductive base material, which is known to form a dense corrosion-resistant film on its surface under certain conditions.

Typical examples of such valve metals include titanium and titanium alloys, but tantalum, zirconium, niobium, and alloys thereof can also be used. Further, in the present invention, the platinum-rhodium composition coating layer may be made of, for example, a caustic potassium aqueous solution.
Even in an alkali-poor electrolyte, it has extremely high methanol electrooxidation catalytic activity similar to that in an acidic electrolyte, and has strong resistance to poisoning of the catalytic activity over time during electrolysis.

For example, using a concentrated caustic potassium aqueous solution as the electrolyte,
When used as an anode for methanol electrolysis in a methanol-air fuel cell, superior discharge characteristics can be obtained compared to the use of conventional electrodes, and it is expected to make a significant contribution to practical application. When used in such an alkaline electrolytic solution, the oxide film of a valve metal such as titanium as a conductive base material has a high solubility in the electrolytic solution, making it impractical for practical use. It is preferable to use a metal that forms an infusible film with respect to an alkaline electrolyte, such as nickel or the like.

As described above, the shape and dimensions of the electrode of the present invention, which has a coating layer made of a predetermined platinum-rhodium composition on a conductive base material, are not limited and may be determined according to the intended use.

In manufacturing such an electrode for methanol electrolysis of the present invention, various known methods can be used to form a coating layer on the conductive base material.

Among these various methods, the coating layer is preferably formed by a so-called pyrolysis method.

That is, for example, first, butanol is used as a solvent, and a compound that becomes platinum metal by thermal decomposition, such as chloroplatinic acid (Nippon 2FtC16/Hoso 20), and a compound that turns into rhodium metal by thermal decomposition, such as rhodium chloride, are added. A coating solution is prepared by dissolving a predetermined amount of each of (RhC13, 4 days 20).

Next, this is applied to the surface of the conductive substrate, for example, by brush coating, and after drying, heat treatment is performed to bake a coating layer of the platinum-rhodium composition onto the substrate as a thermal decomposition reaction product of the coating liquid components. be able to. According to such a method, an electrode for methanol electrolytic oxidation having extremely excellent performance can be obtained, as shown in Examples below. In addition, when manufacturing the electrode of the present invention by such a pyrolysis method, in order to make the electrode sufficiently robust for practical use and capable of maintaining stable performance over a long period of use, the above-mentioned coating method is necessary. It is more preferable to repeat the heat treatment step multiple times, and in particular, sufficiently favorable results have been obtained when this step is repeated 5 to 10 times.

Baking can be performed by heating at an optimum temperature of 200 to 80,000 for 5 to 10 minutes in a gas atmosphere with an oxygen partial pressure in the range of 0.002 to 0.5 atm.

In this case, the heating may be carried out in a reducing or non-oxidizing atmosphere, or may be carried out in air, with a reducing substance contained in the coating liquid if necessary. Furthermore, as a solvent for the coating solution,
Water, ethanol, butanol, etc. are preferable, and the concentration of the liquid is 0.01 to 10 μl/h in terms of metal, especially considering viscosity, ease of application, thickness of coating, etc., especially in terms of total metal. So, 0.
It is preferable to set it as 02-2 evening/scream.

Further, the coating liquid may contain reducing substances such as lavender oil and turpentine oil. Note that the conductive base material may be subjected to a pretreatment such as a surface purification treatment or a surface-blocking treatment in advance, and then a coating layer may be formed thereon as described above.

V. Specific Effects of the Invention The electrode for methanol electrolysis of the present invention is useful as an anode for electrolytic oxidation of methanol.

That is, even if the electrolytic solution is acidic or alkaline, it exhibits high methanol electrooxidation catalytic activity and can obtain a high current density. In addition, poisoning of the catalyst, which is poor over time during electrolysis, is extremely small, and it exhibits a high current density during a period sufficient for practical use. For this reason, as mentioned above, in the electrowinning of zinc, several moles of methanol are added to the sulfuric acid-zinc sulfate electrolyte, and electrolysis is carried out at a rate of several A/dm2 at 40 to 600 C, for example. It exhibits extremely good characteristics as a node and as an anode in the aforementioned methanol-oxygen fuel cells, etc., and contributes greatly to the effective use of energy.

EXAMPLES Hereinafter, the present invention will be explained in more detail by presenting examples of the present invention.

Example Chloroplatinic acid (2PtC16, Hoso 20) and odium chloride (RhC13, 4D20) were dissolved in butanol, and the total metal content was 0.1/1' in metal terms.
The preparation composition was 90 mol% platinum and 10 mol% rhodium, and a mixture of turpentine oil and lavender oil (turpentine oil: lavender oil 2:8) in an amount of 1.3 times the volume of butanol was added to form a coating solution. (hereinafter referred to as liquid A).

Separately, a solution with a total metal content of 95 mol% platinum and 5 mol% rhodium (hereinafter referred to as "Liquid B") with a total metal content of 0.1 min/1 min (hereinafter referred to as "Liquid B"): Content is 0.1
A liquid containing 80 mol% of platinum and 20 mol% of rhodium (hereinafter referred to as liquid C) was prepared on 1/2.

Furthermore, for comparison, a total metal content of 70 mol% platinum and 30 mol% rhodium (hereinafter referred to as liquid D) was prepared in the same manner as the liquid A, with a total metal content of 0.1 m/1; metal content 0.1 evening/
No. 1' monoplatinum version (hereinafter referred to as liquid E); and rhodium chloride was replaced with ruthenium chloride (RuC13/Chihichi 20), and the total metal content was 0.
．． 1 evening/1 [, platinum is 90 mol%, ruthenium is 1
A solution containing 0 mol % (hereinafter referred to as solution F) was prepared.

Next, each of these coating solutions A to F was degreased with a commercially available trichlene degreasing solution, and then diluted with a boiling 10% oxalic acid aqueous solution for 30 min.
The surface was treated for 0 minutes.

It was applied with a brush to a titanium wire base material (2 bars ◇), dried and fired. Coating and baking were carried out in the same manner 10 times, and heat treatment was carried out in air at 400° C. for 10 minutes each time. In this way, six types of electrodes A to F were created using a total of six types of coating liquids A to F.

The coating layer thickness of these six types of electrodes was 2 mm.
Furthermore, when X-ray diffraction was performed on the coating layer applied to each electrode A to F, no peaks attributed to Rh or Ru alone were observed, and peaks attributed to Pt were not observed due to Rh or Ru.
from now on, Rh or Ru will become Pt.
It was surmised that something had fallen inside.

Furthermore, when each coating layer was subjected to crab light X-ray spectroscopy, it was confirmed that each coating layer had a composition that matched the charged composition within experimental error. Next, as shown in FIG. 1, a Teflon heat shrink tube 3 is placed over each of these electrodes A to F.
The same predetermined area of the coating layer 1 was exposed to form two samples for each anode. Next, using each of these anodes 2, the catalytic activity of each electrode A to F is determined using an apparatus as shown in FIG.
Its poison resistance was evaluated.

That is, as an electrolytic solution, 1 mol/its ratio S04 and 1
Using an aqueous solution containing mol/cumethanol, an anode 2 and a cathode 5 consisting of a platinum wire electrode were arranged as shown.

Further, the electrolytic solution was kept at 25° C. in a constant temperature bath 7 while monitoring the solution temperature with a thermometer 9. Furthermore, N2 bubbles were introduced into the electrolytic solution to eliminate dissolved oxygen in the solution. On the other hand, the electrolytic solution was connected via a bridge 6 to a hydrogen electrode 4 placed in a 25o0, 1 mol/so star S04 aqueous solution. In addition, in the figure, 8 indicates a stirrer. Using such a device, for each electrode A to F, the Fnode potential was set to 0.0 with respect to the hydrogen electrode. It was set up as a fence, and the change in electrolytic current density over time after the start of electrolysis was measured.

The results are shown in Figure 2. In the figure, symbols A to F indicate the electrode numbers used. As is clear from Figure 2, Rh5
Electrode A using ~20 mol% platinum-rhodium composition~
It can be seen that electrode C has better poisoning resistance and less deterioration of electrolytic current density over time than electrode E having a platinum coating layer.

In particular, electrode A using a platinum-rhodium composition containing 8 to 12 mol % Rh has significantly high poisoning resistance. It is also seen that in electrode C containing more than 20 mol % of Rh, the Rh toxicity is significant.

Furthermore, from the results of electrode F, it can be seen that when Rh is replaced with Ru, the poisoning is significant. On the other hand, separately from the above, using the electrode A and comparative electrodes E and F, and using them as anodes, methanol was dissolved in an electrolytic solution under conditions similar to the actual operating conditions for zinc electrowinning, and the present invention was carried out. We further confirmed the effect of

That is, using 5 mol of methanol dissolved in 1 mol of sulfuric acid aqueous solution as the electrolytic solution, at a liquid temperature of 60 oo,
The initial anode potential was set to 0.6V (based on hydrogen electrode),
Electrolysis was performed while maintaining an electrolytic current density of 1 vessel/dm2.

As a result, in electrode A using the Rhlo mol% platinum-rhodium composition of the present invention, the anode potential after 10 hours after electrolysis was 0.68 V based on the hydrogen electrode, which is a sufficiently practical overvoltage. showed only an increase in On the other hand, "Electrode E using platinum and Electrode F using platinum-ruthenium composition were used for 5 hours and 4 hours after the start of electrolysis, respectively.
After time in the field, the anode potential rose to about 2V, which was confirmed to be unsuitable for long-term electrolysis in practical terms.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view for explaining an apparatus used to confirm the effects of the present invention in an example of the present invention, and FIG. 2 is a cross-sectional view for explaining the effects of the present invention. FIG. 2 is a green chart showing changes over time in electrolytic current density measured using the device shown in the figure. 1...Coating layer. Figure 1 Figure 2

Claims (1)

[Claims] 1. An electrode for methanol electrolysis in which a coating layer having methanol electrooxidation catalyst activity is formed on a conductive base material, wherein the coating layer is formed from a platinum-rhodium composition containing 5 mol% to 20 mol% rhodium. An electrode for methanol electrolysis characterized by: