JP2000017470A - Hydrogen generator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the device for producing electrolytic hydrogen at low temps. with the consumption of power minimized. SOLUTION: In the hydrogen generator for electrolytically generating hydrogen, a gas diffusion electrode is used as the anode, a solid electrolyte membrane is interposed between the anode and cathode, a steam-methane gas mixture is supplied to the gas diffusion electrode as the anode, and hydrogen is obtained from the cathode side. A heat-resistant ion-exchange membrane is preferably used the solid electrolyte membrane. Further, the anode and cathode should be firmly attached to the solid electrolyte membrane, and the anode and solid electrolyte membrane can be integrally formed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydrogen generator for generating electrolytic hydrogen with low temperature and a simple structure in an energy-saving manner.

[0002]

2. Description of the Related Art The problem of preventing air pollution and the problem of reducing CO ₂ originating from environmental problems are related to the problem of driving an automobile as a means of transportation by driving an efficient electric motor from an internal combustion engine using fossil fuel. Changing to electric vehicles. Conventionally, this typical one is loaded with secondary batteries,
It is an electric vehicle that uses it as a fuel source. Such electric vehicles have the characteristic that they do not pollute the environment because they do not emit any exhaust gas while driving, but with the fact that the power source is power from the power plant and its energy efficiency is not very good, Being extremely heavy, and having the rechargeable battery, the electric vehicle itself would be heavier, requiring more power,
In addition, there is a drawback in that the traveling distance per charge is relatively small and less than the daily traveling distance, and there is also a problem in use because charging takes time.

Recently, a hybrid of an internal combustion engine and a battery has been developed, which has reduced the weight of the battery, eliminated the need for charging with external electric power, and improved the fuel efficiency by adding an internal combustion engine, while extending the mileage. Hybrid vehicles have begun to be used, but while reducing NO _{x in} exhaust gas and reducing CO ₂ emissions per mileage, the structure is complicated and the exhaust gas problem has been completely solved. Therefore, it is regarded as an intermediate stage toward higher efficiency through full electrification in the future.

It is said that the ultimate target is an electric vehicle that uses a fuel cell to generate electric power and emits an efficient electric motor using the electric power, and uses it as a power source. However, the fuel cell itself has been improved to a considerable extent, and the rest has come to cost reductions, but the supply of hydrogen as a fuel is a problem. That is, there are a method of supplying hydrogen from a fuel such as methanol by a reformer, and a method of supplying hydrogen at a hydrogen gas stand. Especially in the latter case, filling hydrogen in the tank of the car consumes a certain amount of hydrogen for cooling itself, so hydrogen is transported from the outside like a current gas station, and the stand is filled. If each vehicle is further filled with the same, there is a problem that it is considerably disadvantageous in terms of basic unit and cost. Therefore, solving the problem of hydrogen has been a major issue.

On the other hand, to gain reformer in a vehicle, it appears to be no seemingly problem, it can become its own large, from which it is generated in the CO ₂ which is exhaust gas, also when necessarily efficient Can not say,
It is said that the consumption of methanol and natural gas, which are hydrogen sources, increases due to the heating of the reformer itself and the reburning of the generated CO, which lowers the economic efficiency. There was a problem that it could not be done. At present, technology for holding hydrogen gas in automobiles is being established, and if hydrogen can be supplied on-site while making it at a stand, and if it can be supplied efficiently and inexpensively, some of these problems will be solved. In this case as well, highly pure electrolytic hydrogen is most desirable because it can be used as it is on the automobile side.

In recent years, high-purity hydrogen has been required for cooling bearings of generators and for manufacturing electronic materials. At present, hydrogen is transported in cylinders, but it is a batch operation. In addition, there were problems such as danger during transportation. In such a case, there has been a movement to partially introduce a conventional electrolytic hydrogen apparatus to use electrolytic hydrogen, but the electrolytic voltage is considerably high from 1.8 V to 2.4 V, which leaves a problem in economical efficiency. Economically in such places,
Moreover, the purity of hydrogen has been required to be equivalent to that of electrolytic hydrogen.

[0007]

In order to produce electrolytic hydrogen with extremely small electric power, a method for improving the performance of the electrolytic apparatus itself has been used. This can be seen as a national project in the so-called WE-NET and RITE projects, which use solid electrolyte type normal water electrolysis equipment and minimize the voltage loss of normal water electrolysis. It is the idea. However, under normal conditions, there is a problem that the voltage cannot be reduced below the theoretical decomposition voltage of 1.23 V, and hydrogen is economically produced using overseas hydropower, and it is used as a storage medium to reduce CO ₂ emissions. It is about to be used as a plan to reduce. Although it is more economical than the conventional electrolytic hydrogen generator, there is still a problem that the power consumption is large, and when the fuel cell is operated with this hydrogen, the generated power is less than half of the power consumption. .

On the other hand, it has been proposed to use a halogen reaction having a theoretical decomposition voltage lower than that of oxygen generation for the anodic reaction. This is disclosed in US Pat. No. 5,219,6.
As disclosed in Japanese Patent No. 71 and 5,443,804, electrolysis is performed using hydrogen halide as an electrolyte to obtain halogen and hydrogen in a cathode chamber. However, in the case of bromic acid, it becomes 1.04 V with respect to 1.24 V of water, and the anode overvoltage becomes almost zero when bromine is generated.
It is said that the voltage can be reduced to a degree. The produced bromine is reacted with natural gas or methanol to return to halogen acid, and is used again as an electrolytic raw material. This method is good for power reduction, but has the disadvantage that the structure is complicated and hydrogen production cannot be performed with sufficiently low power. In addition, the conversion to halogen acid requires a high temperature, and as a whole, it is complicated in terms of equipment and at the same time, considering its operation, it is difficult to become an on-site type generator. I was The present invention has been made in order to solve the above-mentioned problems, and an object of the present invention is to provide an apparatus for producing electrolytic hydrogen at a low temperature and with extremely low power consumption.

[0009]

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above-mentioned problems. As a result, in order to obtain electrolytic hydrogen with a small amount of electric power, methane and steam are used as raw material gases and anodes for electrolytic reformers are used. The present inventors have found that carbon dioxide gas can be obtained from the anode and high-purity electrolytic hydrogen can be obtained from the cathode by supplying the gas to the gas diffusion electrode described above. That is, the present invention has the following configuration.

(1) In a hydrogen generator for generating hydrogen by electrolysis, a gas diffusion electrode is used as an anode, a solid electrolyte membrane is interposed between the anode and the cathode, and steam and methane gas are supplied to the gas diffusion electrode of the anode. A hydrogen gas from the cathode side by supplying a mixed gas of the above. (2) The hydrogen generator according to (1), wherein an anode and a cathode are in close contact with the solid electrolyte membrane. (3) The hydrogen generator according to the above (1), wherein an ion exchange membrane is used as the solid electrolyte membrane. (4) The hydrogen generator according to (1), wherein the solid electrolyte membrane is obtained by introducing an ion exchange group into a fluororesin having a cyclic fluororesin as a skeleton. (5) The hydrogen generator according to (1), wherein the solid electrolyte membrane is formed by impregnating a heat-resistant porous substrate with a heat-resistant ion exchanger. (6) The hydrogen generator according to the above (1), wherein the electrolysis temperature is 100 ° C. or higher, and a heat retaining mechanism is provided together with a steam generating mechanism on the anode supply gas side.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. When hydrogen production by electrolysis is performed by water electrolysis, the reaction is anodic reaction H ₂ O → 2H ⁺⁺ O ₂ + 2e ⁻ (1.22 V) Cathodic reaction 2H ₂ O + 2e ⁻ → H ₂ + 2OH ⁻ (0.0V) And the theoretical decomposition voltage is 1.22V. In practice, an electrode overvoltage, an electric resistance and the like are added to this, and the electrolysis voltage becomes about 1.8 to 2 V.

On the other hand, when the decomposition of methane gas, which is a fuel cell reaction, is used for the anode reaction, the following is obtained. Anodic reaction CH ₄ + 2H ₂ O → CO ₂ + 8H ⁺ + 8e ⁻ (0.16V) Cathodic reaction 2H ₂ O + 2e ⁻ → H ² + 2OH ⁻ (0.0V) In other words, electrolytic hydrogen is theoretically obtained at 0.16V Will be. Moreover, the hydrogen reaction usually has a feature that the electrode overvoltage during electrolysis is extremely small because the diffusion rate is sufficiently high. That is, in the oxygen generation reaction, 400
Since the overvoltage of about mV becomes 100 mV or less, and the electric resistance is the same in any system, the methane reaction is used. .5V can be expected. However, there is a problem that side reactions are likely to occur in the decomposition reaction of methane.Through studies on these issues, it has been confirmed that hydrogen can be produced at a very low voltage and almost theoretically without side reactions. Found and led to the present invention.

[0013] That is, as a reaction related to this, a fuel cell has been conventionally considered, but it is a molten carbonate or a fuel for a solid electrolyte type fuel cell. An operation at about 1000 ° C. is performed, in which an electrochemical reaction occurs together with a chemical reaction, and because it is inevitably in a state easily reduced,
Side reactions, such as generation of carbon and CO, occurred. This is similar to the operation of a reformer by pyrolysis. However, it has been found that this type of side reaction is less likely to occur when operated at sufficient moisture and relatively low temperatures due to the proximity of the oxidizing atmosphere. However, it cannot be used in a reformer because the reaction slows down, but in the case of electrolysis, it has been found that electric energy works as a motive force, so that a suitable work can be performed. I found it.

In other words, in the apparatus of the present invention, methane is fed to the anode as fuel, and the electrode reaction is depolarized by oxidizing it, and hydrogen is generated from the cathode while keeping the anode potential extremely low. is there. The electrolysis temperature here must be a temperature at which water is stably mixed with methane gas as a gas, and furthermore, a temperature at which these are fed into the gas diffusion electrode as an anode supply gas to cause a reaction. It has been found that the temperature is appropriately lower than 100 ° C., and preferably 100 ° C. or higher in order to keep the gas phase stable. Therefore, a desirable temperature range is 100 ° C. to 500 ° C., and more preferably 100 ° C. to 300 ° C.
° C. This is sent to a gas diffusion anode that is in close contact with the electrolyte. The composition of the feed gas should be theoretically such that one molecule of methane has two molecules of water, that is, the volume ratio of the component gas should be 1: 2. Since it is better to contain water, the amount of water is preferably about 50% excess. As for the gas diffusion electrode,
Although there is no particular limitation other than heat resistance, a material in which an electrode material is supported on a generally used porous carbon material at 300 ° C or lower can be used, and a fluorinated graphite or the like can be used on a porous carbon sintered body at 300 ° C or higher. A substrate having adjusted water repellency can be used as a substrate, and a substrate carrying an electrode substance thereon can be used.

[0015] The electrode material is carried on this, but in view of the fact that CO is partially generated, it is preferable to use an alloy of platinum and ruthenium or a composite compound of platinum and ruthenium oxide. However, other electrode materials such as silver may be used as long as the reaction of the present purpose proceeds, and other electrode materials may be used. The electrode material may be supported by a usual method.For example, the electrode material or a precursor thereof is supported on the surface of carbon powder, and a treatment such as heating is applied thereto to form catalyst particles, which are then coated with fluorine on the electrode surface. A method of baking with a resin may be used, and an electrode body that does not support the electrode substance is produced, and then a precursor of the electrode substance, for example, a mixed aqueous solution of chloroplatinic acid and ruthenic acid, or a butyl alcohol solution as a coating liquid, After this is applied to the electrode surface, it is placed in a reducing atmosphere containing hydrogen at 200 to 350
By firing at ℃, an alloy of platinum and ruthenium can be formed on the electrode surface. However, when a fluorine resin is used as a water-repellent material or a binder for the electrode material, even PTFE having relatively high heat resistance starts to decompose at 350 ° C., so there is a limit to thermal conditions, and an oxidizing atmosphere such as an oxide is required. As described above, in the case of an electrode material to be used, particles supporting the electrode material can be prepared in advance, and the particles can be supported on the electrode surface by a fluororesin or the like.

Although the cathode is not limited, the surface area is as large as possible, and the liquid in the electrolytic portion has little, if any, conductivity. It is necessary to be. Of course, this is the same for the anode.
For this purpose, a certain degree of flexibility is required so that it can adhere to the ion exchange resin membrane which is a solid electrolyte. In order to obtain a lower electrolysis voltage, the hydrogen generation potential must be sufficiently low, that is, it must be an activated cathode.
For this reason, the cathode can be formed by kneading the carbon particles and the fluororesin to form a porous sheet, but carrying an active electrode material on the solid electrolyte side.
The sheet-like cathode material may include a net-like material made by knitting a metal wire as a core material.
By adding a metal core, the conductivity of the electrode itself is improved, and a uniform current distribution can be expected over the entire surface. This also applies to the case of the anode.

Therefore, the cathode can be made by the following method. That is, carbon black powder and PTF
A water suspension of the E resin is kneaded into a paste, which is uniformly spread on an aluminum foil. A sheet can be obtained by treating this with a hot press at a pressure of about 1 kg / cm ² to 5 kg / cm ² and a temperature of 200 ° C. to 300 ° C. The aluminum foil is removed by any method. If you want to put the heartwood, for example, spread the gold-plated copper mesh or nickel mesh,
The desired cathode material can be obtained by applying the above-mentioned paste on both surfaces and hot pressing similarly.

As the solid electrolyte membrane used between the anode and the cathode, the ion exchange membrane may be a commercially available fluorinated resin-based cation exchange membrane, for example, Nafion 117 or the like manufactured by DuPont. Since the heat-resistant temperature is at most 120 ° C to 130 ° C and the heat-resistant temperature is not enough, an ion exchange group is introduced into a more heat-resistant fluororesin having a cyclic structure of fluororesin having a different resin structure as a skeleton. It is desirable. Alternatively, a heat-resistant porous substrate impregnated with a heat-resistant ion exchanger such as Nafion or another ion-conductive substance acting as a solid electrolyte may be used. For example, a sheet of porous zirconium oxide is obtained by sintering at about 300 ° C. using a fibrous powder of zirconium oxide without a binder or, if necessary, using a fluororesin or the like as a binder. It can also be obtained by impregnating Nafion, an ion exchanger.

The electrode and such a solid electrolyte can be made in one piece. For example, in the above description, carbon was mainly used. However, a porous titanium powder called titanium sponge is volatilized by firing dextrin or the like on a surface using a knitted mesh made by knitting a titanium wire as a metal sintered body. The substance is applied on a substrate as a binder, sintered at 300 to 500 ° C. in a stream of hydrogen or argon to obtain a porous body, and an anode having an alloy of platinum and ruthenium formed on the surface by an arbitrary method is prepared. By baking a porous film of zirconium oxide, titanium oxide, or silicon oxide on one surface of the plate, and applying and impregnating an ion exchange resin from the baked surface, an anode and a solid electrolyte membrane can be integrated. . The cathode may also be formed so that it can be further integrated with this surface. It goes without saying that such a manufacturing method is not particularly limited as long as the object can be achieved. From the viewpoint of handling and upsizing, it is desirable that the electrode and the solid electrolyte are formed into a flexible sheet having physically sufficient strength.

The power supply means for these is not particularly limited, but it is desirable to use a heat-resistant and corrosion-resistant metal. For example, a conductive and stable conductive oxide such as ruthenium oxide may be provided on the surface of a titanium mesh. And the like are preferably used. The solid electrolyte, anode and cathode thus produced are placed in an electrolytic cell integrally or in close contact. To this anode side, natural gas or city gas from which impurities such as sulfur have been removed is supplied together with water vapor, and electrolysis is performed under the above conditions. In this way, electrolytic hydrogen can be obtained at extremely low voltages while minimizing CO and other side reactions. In addition, since CO tends to increase when water vapor is insufficient, it is necessary to adjust so that there is no shortage of water vapor.

[0021]

EXAMPLES The present invention will be described below with reference to examples, but it goes without saying that the present invention is not limited to only these examples.

Example 1 A knitted mesh made by knitting a titanium wire having a wire diameter of 0.1 mm was kneaded with a powder having an average particle diameter of 5 μm obtained by pulverizing titanium sponge and dextrin in an amount of 30% as a binder. Is applied to the entire surface so as to have an apparent thickness of 100 μm, and 400 ° C. in an argon stream containing 10% of hydrogen.
Sintered. As a result, a porous titanium plate in a state called loose sintering was obtained. The sintering time was 1 hour so that the carbon content of the dextrin was completely volatilized. The titanium thus prepared is then placed in an atmosphere consisting of 5% oxygen and then argon for 30 minutes 400 minutes.
By firing at ℃, a thin titanium oxide film was formed on the surface.

Using this as a base material, the metal is immersed in a solution in which chloroplatinic acid and ruthenic acid chloride are dissolved in butyl alcohol so that the ratio of platinum: ruthenium is 1: 1 as a metal, and the liquid is impregnated. After drying, it was baked at 300 ° C. for 15 minutes in an argon stream containing 10% of hydrogen to form a platinum / ruthenium alloy layer on the surface of the substrate. Further, this time, the same liquid was applied to one surface which functions more as an electrode, and was baked under the same conditions. The reason why the surface oxidation was performed in advance is that the sintered body of titanium sponge is very active, and there is a risk of ignition if it is used as it is. It was confirmed that the surface was oxidized under the conditions. In this way, an anode carrying a platinum / ruthenium alloy of 15 g / m ² as the whole electrode body was produced.

On the side of this anode that carries more platinum,
A mixture of a fibrous zirconium oxide and an aqueous suspension of PTFE resin in an amount of 10% thereof was applied. This is 300
Molding and sintering were performed by a hot press at ℃. Pressure is 1kg
/ Cm ² . Thus, a zirconium oxide porous body having an apparent thickness of 200 μm was formed. To make this part a solid electrolyte, a zirconium oxide layer was prepared by adding a 30% ethylene glycol as a thickening agent to a Nafion resin solution. This was heated at 180 ° C. to form a solid electrolyte membrane.

As the cathode, a material in which ruthenium oxide was thermally decomposed on the surface of a porous titanium material having an opening ratio of 85 to 90% and produced by sintering titanium long fibers was used. The production conditions are such that ruthenium oxide powder is suspended in ruthenic chlorate, applied to porous titanium as a base material, and thermally decomposed and baked at 350 ° C. in air. The cathode was attached to the electrolytic cell so as to be in close contact with the sheet in which the anode and the solid electrolyte were integrated. The collector was an expanded mesh made of titanium for both the cathode and the anode.

This electrolyzer was placed in a thermostat at 180 ° C., and electrolysis was performed while supplying a gas in which steam: methane = 3: 1 was supplied to the anode side. When the current density was set to 2 A / dm ² , the electrolysis voltage was 0.45 V. The anode compartment gas produced about 1000 ppm of CO, but consumed almost 20% more methane than theory. In addition, very little methane was found in the generated hydrogen. It seems that the gas separation by the solid electrolyte was insufficient. In addition, in the hydrogen generated on the cathode side, both CO and CO ₂ were 1 ppm or less, and showed a purity equivalent to ordinary electrolytic hydrogen. The current efficiency of hydrogen generation was 98% or more.

Example 2 DuPont was used as a cation exchange membrane as a solid electrolyte.
An anode and a cathode were provided so as to be in close contact with both surfaces by using Nafion 415 manufactured by the company. The anode has an apparent average particle size of 2
μm carbon black powder and 40% PTF
A water dispersion of E resin is kneaded into a sheet, and the pressure is 1 kg.
/ Cm ^{2 at} a temperature of 250 ° C. to produce a carbon sheet. A solution of chloroplatinic acid and ruthenic acid dissolved in butyl alcohol at a metal ratio of 1: 1 is applied to this carbon sheet, and heated and fired at 350 ° C. in an argon gas containing 10% hydrogen. To carry a platinum / ruthenium alloy. This operation was repeated three times so that platinum and ruthenium became 7 g / m ² . Thus, an electrode was formed. The platinum / ruthenium alloy supporting side was brought into close contact with the cation exchange membrane.

As the cathode, a sheet prepared by kneading graphite powder and a 20% PTFE resin thereof and hot pressing under the same conditions as the anode was used. As a cathode material, a butanol solution of chloroplatinic acid alone was prepared, and this was applied as a coating solution to one surface of the prepared sheet.
Baked in. This was repeated three times to carry 3 g / m ² of platinum. The cathode also had the platinum-supporting side in close contact with the ion exchange membrane. 1.5m thick as current collector
0.2m thick on the titanium expanded mesh surface
What welded the titanium thin mesh of m was used. The cathode, anode, and ion exchange membrane as a solid electrolyte were fixed at a pressure of 10 kg / cm ² . The electrolytic cell containing this product was placed in a thermostat at a temperature of 130 ° C., and a mixed gas of steam and methane was supplied as an anode gas.

In the cathode chamber, Nafion, which is a solid electrolyte, contains sufficient moisture to maintain conductivity.
Water vapor was supplied at the beginning of the electrolysis. The electrolysis conditions were such that the current density was 2 A / dm ² as in Example 1, but the electrolysis voltage at that time was 0.35 to 0.45 V. After the start of the electrolysis, the air supply was stopped, and the supply of water from the anode chamber side and the supply of water accompanying the movement of hydrogen ions were performed. Table 1 shows the composition of anode gas and cathode-generated hydrogen according to the composition of water vapor and methane on the anode side. By this, it was found that by supplying more water than the theoretical value of 2 to methane 1 which is the theoretical value, the reaction was almost as theoretically performed. In particular, the water content was increased to about 1.5 times the theoretical value. It turns out that it becomes optimal by doing.

[0030]

[Table 1]

[0031]

According to the present invention, the following effects can be obtained. (1) As a reformer of methane gas, electrolytic hydrogen was obtained by supplying only a small amount of electricity by electrolysis instead of the conventional high-temperature pyrolysis type. (2) Efficiency of consumption of methane itself is about 40% (it requires about 2.5 times the theoretical conversion amount of methane to hydrogen by using it as a heat source for self-heating in the pyrolysis method. ), But 80% or more by selecting the mixing ratio of steam and methane. Instead, the required electrolysis voltage was sufficiently low, and a large energy saving effect could be expected. (3) Even when this is used for power generation in combination with a fuel cell, a voltage of 0.5 V or more can be expected by subtracting power consumption, so that the conversion efficiency to electric power can be sufficiently high at 40% or more. all right. (4) The driving conditions were extremely mild, indicating that driving was easy. (5) Since the pressure and temperature are mild and the generated hydrogen is electrolytic hydrogen, it can be easily used as an on-site hydrogen generator. Also, from the purity of the generated hydrogen, hydrogen that requires almost no post-treatment is required. It turned out to be obtained. (6) It was found that it could be used without a booster because it was high-pressure hydrogen of several atmospheres. As described above, according to the present invention, the various effects described above are obtained, and a highly practical hydrogen generator for on-site use is obtained.

Claims (6)

[Claims] In a hydrogen generator for generating hydrogen by electrolysis, a gas diffusion electrode is used as an anode, a solid electrolyte membrane is interposed between the anode and the cathode, and steam and methane gas are supplied to the gas diffusion electrode of the anode. A hydrogen generator characterized in that a hydrogen gas is supplied from a cathode side by supplying a mixed gas. 2. The hydrogen generator according to claim 1, wherein an anode and a cathode are in close contact with the solid electrolyte membrane. 3. The hydrogen generator according to claim 1, wherein an ion exchange membrane is used as said solid electrolyte membrane. 4. The hydrogen generator according to claim 1, wherein the solid electrolyte membrane is obtained by introducing an ion exchange group into a fluororesin having a cyclic fluororesin as a skeleton. 5. The hydrogen generator according to claim 1, wherein the solid electrolyte membrane is formed by impregnating a heat-resistant porous substrate with a heat-resistant ion exchanger. 6. The hydrogen generator according to claim 1, wherein an electrolysis temperature is set to 100 ° C. or higher, and a heat retaining mechanism is provided on the anode supply gas side together with a steam generating mechanism.