JP4426205B2 - Fuel cell - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a fuel cell.
[0002]
[Prior art]
[Patent Document 1]
JP 2002-305001 A
[Non-Patent Document 1]
“Website“ Fuel Cell Vehicle Delivery Ceremony ”(December 2, 2002) http://www.kantei.go.jp/jp/koizumiphoto/2002/12/02nenryo.html”
[Non-Patent Document 2]
“Paper“ Annealing effect on C-doped InGaAs grown by metalorganic chemical vapor deposition, ”N. Watanabe, et al., J. Crystal Growth Vol. 195 (1998), pp. 48-53”
[Non-Patent Document 3]
“Paper“ Thermal Annealing Effects on P-Type Mg-Doped GaN Films, ”S. Nakamura, et al., Jpn. J. Appl. Phys. Vol. 31 (1992), pp. L139-L142”
[Non-Patent Document 4]
“Paper“ Highly resistive CH-doped GaN grown by plasma-assisted metalorganic chemical vapor deposition, ”M. Sato, Appl. Phys. Lett. Vol. 68 (1996), p. 935”
A fuel cell is attracting attention as a component of an energy-saving power generation system because high power generation efficiency is obtained and exhaust energy is used for air conditioning, hot water supply, and the like, so that overall energy use efficiency is increased. When hydrogen gas is used as a fuel gas, the fuel cell generates energy by reacting hydrogen with oxygen in the air, so that only water is discharged. Production of harmful gases such as nitrogen oxides is extremely low. Such environmentally friendly characteristics are also highly evaluated.
[0003]
Unlike a system in which a fuel cell is driven by an engine or turbine, the fuel cell does not have a portion that rotates at high speed. Therefore, there is almost no noise and vibration associated with operation, and there are few restrictions on installation.
[0004]
The fields to which fuel cells can be applied are wide. Considering a method that uses both heat and electricity, such as installing in a facility such as a hotel or hospital that requires many heat sources for air conditioning or hot water supply, or building a plant that supplies hot water and electricity to newly developed areas It has been implemented. It is expected that if the price of fuel cells is reduced, it will become popular in ordinary households and will be used for power generation, hot water supply and heating.
[0005]
Automobiles using fuel cells have been developed because of the high fuel use efficiency and low environmental impact of exhaust gas. As described in Non-Patent Document 1, two fuel cell vehicles were leased and sold in the country in December 2002, indicating a great progress toward practical application.
[0006]
Application as a power source for mobile terminals such as mobile phones and notebook computers is also expected. Compared to a rechargeable battery such as a lithium ion battery, there is a possibility that the amount of power generation per volume can be increased, and there is an advantage that the time required for conventional charging can be shortened to a short time for refueling. As a power source of the portable terminal, it is essential that the portable terminal is small and operates at a low temperature.
[0007]
The principle of power generation of a fuel cell is based on the reverse reaction of water electrolysis when hydrogen is used as fuel.
[0008]
FIG. 1 shows a schematic diagram of a single cell of a fuel cell. In the figure, 1 is a cathode, 2 is an electrolyte layer, 3 is an anode, 4 is fuel gas, 5 is air, 6 is a fuel gas supply port, 7 is an air supply port, 8 is an exhaust gas discharge port, 9 is a power load, Reference numeral 10 denotes a voltmeter.
[0009]
Here, the fuel gas 4 is hydrogen gas (H ₂ The operation principle of the fuel cell will be described. The fuel gas 4 (hydrogen) supplied from the fuel gas supply port 6 is decomposed into protons and electrons at the cathode 1 (also referred to as a hydrogen electrode or a fuel electrode), and electrons (e in FIG. ⁻ Is passed to the conductive part of the cathode 1. Protons generated at the cathode 1 (in the figure, H ⁺ ) Diffuses in the electrolyte layer 2 and reaches the anode 3 (also referred to as an air electrode). The protons that have reached the anode 3 receive the electrons that have reached the anode 3 from the cathode 1 through an external circuit, and oxygen (O) in the air 5 supplied from the air supply port 7. ₂ ) And water (H ₂ O). In this way, electrons flow from the cathode 1 to the anode 3 through the external circuit, and this cell functions as a battery. The water generated at the anode 3 is discharged together with the air 5 from the exhaust gas discharge port 8. The external circuit is provided with a power load 9 and the battery terminal voltage is measured by a voltmeter 10.
[0010]
Fuel cells are classified into solid polymer membrane type, phosphoric acid type, alkali type, molten salt type, etc., depending on the type of electrolyte. Since the operating temperature of the molten salt mold is as high as 700 ° C. or higher, no special catalyst is required for the decomposition of the fuel gas. Other fuel cells that can be operated at low temperatures require a catalyst to efficiently decompose hydrogen gas into protons and electrons.
[0011]
In the conventional method, platinum is used as a catalyst. Platinum is a noble metal and is very expensive. The price of platinum is not only a major hindrance to lowering the price of fuel cells, but there are also concerns that resources will be depleted as fuel cells become more widespread.
[0012]
In order to effectively use platinum, a material in which platinum fine particles are supported on a chemically stable conductive material such as graphite is used as an electrode. Attempts have been made to increase the specific surface area and reduce the amount of platinum used by making platinum fine particles and optimizing the loading method. Further, Patent Document 1 discloses a technique for reducing the amount of platinum used by using fine particles containing gold together with platinum as a catalyst. A search for an alternative to expensive platinum has also been made, but no effective catalyst has been reported.
[0013]
A porous material is used for the electrode in order to effectively support the platinum fine particles and to efficiently move protons generated by the decomposition of hydrogen gas on the platinum surface to the electrolyte. Therefore, the electrolyte layer is in partial contact with the fuel gas or air containing hydrogen. The electrolyte of an alkaline fuel cell has a problem that it rapidly deteriorates when it reacts with carbon dioxide or nitrogen oxides. Even in fuel cells using other electrolyte layers, it is a problem that salts that do not contribute to proton transfer are formed by the reaction between the impurity gas and the electrolyte.
[0014]
There are various methods for supplying hydrogen gas as fuel for fuel cells. The fuel cell vehicle described in Non-Patent Document 1 supplies hydrogen to the fuel cell from a high-pressure hydrogen gas cylinder. In the case of installing a fuel cell in a thermoelectric supply plant or the like, a method of obtaining hydrogen gas by reforming natural gas using a reformer is employed. Since the existing infrastructure for city gas supply can be used, the cost required for infrastructure development can be reduced. A method of modifying and using LPG that is easy to store has also been realized. In the case of a non-fixed type fuel cell such as a fuel cell vehicle, a method for obtaining hydrogen by reforming a liquid fuel such as methanol has been studied. A fuel cell system other than directly supplying hydrogen gas requires a fuel reforming mechanism, which requires considerable equipment costs.
[0015]
[Problems to be solved by the invention]
In a fuel cell that enables the use of exhaust heat in addition to increasing the efficiency of power generation and does not emit substances that pollute the environment, expensive platinum is used as a catalyst in the electrodes of conventional fuel cells that operate at a relatively low temperature. Therefore, the use of platinum is a major factor that hinders the price reduction of fuel cells.
[0016]
The present invention has been made in view of this problem, and a problem to be solved by the present invention is to provide a fuel cell that is configured by using an electrode that does not contain platinum and that can be reduced in size and can be reduced in size. .
[0017]
[Means for Solving the Problems]
In order to solve the above problems, in the present invention, as described in claim 1,
In a fuel cell having a cathode in contact with a fuel gas, an anode in contact with a gas containing oxygen gas, and an electrolyte connecting between the cathode and the anode, Using phosphoric acid as the electrolyte, The cathode comprises a III-V group compound semiconductor doped with a p-type impurity as a constituent element.
[0018]
In the present invention, as described in claim 2,
The III-V group compound semiconductor doped with the p-type impurity is grown on the GaAs substrate, doped with carbon as the p-type impurity, or doped with carbon as the p-type impurity grown on the AlGaAs or InP substrate. The fuel cell according to claim 1, wherein the fuel cell is InGaAs.
[0019]
In the present invention, as described in claim 3,
2. The III-V group compound semiconductor doped with p-type impurities is GaN, AlGaN or InGaN doped with magnesium, which is a p-type impurity, grown on a sapphire substrate or SiC substrate. The described fuel cell is constructed.
[0020]
In the present invention, as described in claim 4,
The III-V group compound semiconductor doped with the p-type impurity is grown on the GaAs substrate and then peeled off from the substrate. The p-type impurity-doped GaAs or AlGaAs layer is grown on the InP substrate. A p-type impurity-doped carbon-doped InGaAs layer or sapphire substrate or a SiC substrate peeled off from the substrate and then peeled off from the substrate, p-type magnesium-doped GaN layer, AlGaN layer or InGaN 2. The fuel cell according to claim 1, wherein the fuel cell is a layer.
[0021]
In the present invention, as described in claim 5,
2. The fuel cell according to claim 1, wherein the group III-V compound semiconductor doped with p-type impurities is a group III-V compound semiconductor doped with p-type impurities grown on porous ceramics. Configure.
[0022]
In the present invention, as described in claim 6,
2. The group III-V compound semiconductor doped with p-type impurities is a group III-V compound semiconductor doped with p-type impurities grown on a porous heat-resistant organic polymer material. The fuel cell described in 1 is configured.
[0023]
In the present invention, as described in claim 7,
The fuel cell according to claim 6, wherein the porous heat-resistant organic polymer material is porous polytetrafluoroethylene.
[0024]
In the present invention, as described in claim 8,
The group III-V compound semiconductor doped with the p-type impurity is GaAs, AlGaAs or InGaAs doped with carbon as a p-type impurity, or GaN, AlGaN or InGaN doped with magnesium as a p-type impurity. The fuel cell according to claim 5, 6 or 7 is configured.
[0025]
In the present invention, as described in claim 9,
9. The electrolyte according to claim 5, 6, 7 or 8, wherein the pores of the porous ceramic, the porous heat-resistant organic polymer material or the porous polytetrafluoroethylene are filled with an electrolyte. A fuel cell is configured.
[0026]
In the present invention, as described in claim 10,
Porous ceramics, porous heat-resistant organic polymer material or porous polytetrafluoroethylene in which the pores are filled with an electrolyte are in direct contact with the anode, and the electrolyte filled in the pores are in contact with the cathode. The fuel cell according to claim 9, wherein the fuel cell is connected to the anode.
[0027]
Moreover, in this invention, as described in Claim 11,
11. The fuel cell according to claim 1, wherein the anode includes a III-V group compound semiconductor doped with a p-type impurity.
[0028]
Moreover, in this invention, as described in Claim 12,
12. The fuel cell according to claim 1, wherein the fuel gas is hydrogen gas.
[0029]
Moreover, in this invention, as described in Claim 13,
The fuel cell according to any one of claims 1 to 11, wherein the fuel gas is a mixed gas containing at least methane, ethane, or propane and water vapor.
[0030]
In the present invention, as described in claim 14,
12. The fuel cell according to claim 1, wherein the fuel gas is a mixed gas of methanol vapor and water vapor.
[0031]
DETAILED DESCRIPTION OF THE INVENTION
The fuel cell according to the present invention is characterized in that a compound semiconductor doped with a p-type impurity is used as an electrode.
[0032]
At the cathode of a fuel cell that uses hydrogen as the fuel, hydrogen gas molecules are decomposed into hydrogen radicals, hydrogen radicals are dissociated into electrons and protons, electrons are transferred to the electrode material, and protons are transferred to the electrolyte continuously. It is necessary to make it happen quickly. At low temperatures, in the case of no catalyst, the decomposition rate of hydrogen gas molecules is small and radical recombination also occurs, so the above reaction rate is extremely slow.
[0033]
The present invention has been made based on the discovery that a compound semiconductor doped with a p-type impurity quickly absorbs hydrogen radicals generated by the decomposition of hydrogen gas molecules. Since recombination of hydrogen radicals is suppressed by rapid absorption of hydrogen radicals, a compound semiconductor doped with p-type impurities serves as a catalyst for hydrogen gas molecular decomposition. The absorbed hydrogen radical passes electrons to the p-type impurity atoms and becomes protons and exists in the semiconductor. If this proton is transmitted to the electrolyte on the surface opposite to the surface in contact with the hydrogen gas, the reaction proceeds continuously without saturation of the proton in the semiconductor. Further, electrons are conducted between p-type impurity atoms by the tunnel effect, and move to a terminal for taking out current. Based on such a principle, a compound semiconductor doped with a p-type impurity functions as an electrode of a fuel cell.
[0034]
The above findings were obtained in the course of studying the effects of hydrogen on semiconductors. In the field of manufacturing devices using compound semiconductors, the following knowledge has been obtained for arsenide semiconductors and nitride semiconductors.
[0035]
Hydrogen atoms that have penetrated into the compound semiconductor change the carrier concentration, which affects the electrical characteristics. Particularly in a p-type semiconductor, the influence of hydrogen atom permeation becomes significant. In a compound semiconductor with a high p-type impurity concentration, a hydrogen atom is taken in during crystal growth, and the hydrogen atom dissociates into a proton and an electron, and then passes the electron to the p-type impurity. In this way, the p-type impurity is electrically inactive and the intended electrical characteristics cannot be obtained. Such a phenomenon has started to be regarded as a problem in GaAs doped with carbon as a p-type impurity. As an electronic device that operates at a higher speed than GaAs, InGaAs using InP as a substrate has attracted attention. In the case of InGaAs doped with carbon, 90% of carbon impurities are inactivated by hydrogen. It is reported in Reference 2. In general, a compound semiconductor using carbon as a p-type impurity is easily affected by hydrogen, and hydrogen adversely affects characteristics and reliability of electronic devices.
[0036]
Devices using nitride semiconductors such as GaN such as blue light emitting diodes and ultraviolet lasers have been put into practical use. For practical use of a device using a nitride semiconductor, it was necessary to realize a high-concentration p-type semiconductor. As described in Non-Patent Document 3, a p-type semiconductor is realized by doping GaN with magnesium, which is a p-type impurity, and heat-treating in a nitrogen atmosphere, opening the way to practical use of devices. . It is reported that p-type impurities before heat treatment were inactivated with hydrogen. In the case of GaN, it is described in Non-Patent Document 4 that when carbon is doped, the resistance is increased with hydrogen and hydrogen is not desorbed by heat treatment.
[0037]
The use of a compound semiconductor doped with a p-type impurity as an electrode of a fuel cell has been made for the first time according to the present invention, thereby realizing a fuel cell that does not use a platinum catalyst.
[0038]
In principle, a compound semiconductor film doped with a p-type impurity does not transmit atoms other than hydrogen. Therefore, it is possible to completely prevent impurities contained in the hydrogen gas from coming into contact with the electrolyte and causing deterioration of the electrolyte. In this way, it is possible to operate the fuel cell while maintaining its performance for a long period of time.
[0039]
When applied to fuel cells that use natural gas or alcohol as fuel, compound semiconductors doped with p-type impurities quickly absorb only hydrogen radicals generated during the reforming reaction, and thus have the effect of promoting the reforming reaction. . Therefore, it does not require a reformer and operates efficiently even when fuel other than hydrogen is supplied. This advantage is also demonstrated by the present invention.
[0040]
【Example】
Hereinafter, embodiments of the present invention will be described by way of examples.
[0041]
The semiconductor doped with p-type impurities in the following examples was grown by a vapor phase growth method (MOCVD method) using an organic metal. Hydrogen gas was used as the carrier gas, and trimethylaluminum, trimethylgallium, and trimethylindium were used as the raw materials for the group III elements Al, Ga, and In, respectively. Arsine was used as a raw material for As, which is a group V element, and ammonia was used as a raw material for N in growing nitride. Carbon tetrabromide was used as a raw material for carbon to be a p-type impurity, and biscyclopentadienyl magnesium was used as a raw material for magnesium to be a p-type impurity. These raw materials are generally used when an electronic device is manufactured by the MOCVD method.
[0042]
Growth of carbon-doped GaAs, AlGaAs, and InGaAs was performed at a substrate temperature of 500 ° C., and growth of GaN, AlGaN, and InGaN doped with magnesium was performed at a substrate temperature of 900 ° C., respectively.
[0043]
Using a single cell of a fuel cell as schematically shown in FIG. 1 and using phosphoric acid as an electrolyte, the characteristics as a fuel cell were measured.
[0044]
In FIG. 1, reference numeral 1 denotes a cathode, and in the following examples, it is a cathode having a III-V group compound semiconductor doped with a p-type impurity as a constituent element. 2 is an electrolyte layer containing an electrolyte connecting the cathode 1 and the anode 3, 3 is an anode, 4 is fuel gas, 5 is air, 6 is a fuel gas supply port, 7 is an air supply port, and 8 is an exhaust gas exhaust. An outlet, 9 is a power load, and 10 is a voltmeter.
[0045]
Electrode area 1cm ² A voltage (measured by the voltmeter 10) when a constant current of 100 mA per unit is passed is defined as an electromotive force, and is described as an evaluation value of characteristics.
[0046]
In the following examples, the anode 3 is made of porous graphite unless otherwise specified.
[0047]
In the following examples, the fuel gas 4 is hydrogen gas having a purity of 99% or more unless otherwise specified.
[0048]
In all the embodiments, the cathode 1 is in contact with the fuel gas 4 and the anode 3 is in contact with the air 5 that is a gas containing oxygen gas.
[0049]
Example 1
On the GaAs substrate, carbon-doped GaAs and AlGaAs (the Al amount in the group III element is about 30%), which are III-V group compound semiconductors doped with p-type impurities, are epitaxially grown, and the cathode 1 of the fuel cell. It was made into the component of. The carbon impurity concentration is 8 × 10 for both GaAs and AlGaAs. ¹⁹ cm ^-3 The GaAs thickness was 2 μm and the AlGaAs thickness was 0.1 μm. After the epitaxial growth, the wafer is polished to a thickness of 0.2 mm, and a resist is applied to the entire surface of the epitaxial surface and the back surface of the substrate, and then circular holes with a diameter of 3 mm are opened at intervals of 5 mm only on the substrate side (opposite side of the epitaxial surface). The resist was removed. After this wafer is wet etched with a mixed solution of ammonia and hydrogen peroxide, all the resist is removed with an organic solvent, so that the surface of AlGaAs doped with carbon is exposed to the substrate side, and the epitaxial layer is a perforated GaAs substrate. The structure of the state currently hold | maintained was obtained. Carbon-doped AlGaAs also serves as a stopper to prevent etching from proceeding to the carbon-doped GaAs layer.
[0050]
This structure was used as the cathode 1 and attached so that the substrate side of the epitaxial layer was in contact with the fuel gas 4 and the epitaxial surface side was in contact with the electrolyte layer 2, and the operation as a fuel cell was confirmed. The electromotive force was 0.71 V when the operating temperature was 300 ° C., 0.73 V when the operating temperature was 350 ° C., and 0.74 V when the operating temperature was 400 ° C.
[0051]
There was no difference in electromotive force even when the GaAs thickness was 5 μm. By setting the hole diameter opened on the substrate side to 3.5 mm, the electromotive force was improved to 0.73 V when the operating temperature was 300 ° C. This indicates that the area where the semiconductor doped with carbon and the fuel gas 4 are in contact greatly affects the characteristics of the electrode.
[0052]
In the above fuel cell, GaAs and AlGaAs doped with carbon, which is a p-type impurity, can be used alone as the III-V group compound semiconductor doped with a p-type impurity.
[0053]
(Example 2)
The structure of the cathode 1 of the fuel cell is obtained by epitaxially growing, on an InP substrate, a group III-V compound semiconductor doped with p-type impurities, carbon-doped InGaAs (the amount of In in the group III element is about 50%). Element. Carbon impurity concentration is 8 × 10 ¹⁹ cm ^-3 The InGaAs thickness was 2 μm. A structure in which the surface of InGaAs doped with carbon was exposed to the InP substrate side was prepared in the same manner as described in Example 1, and this structure was used as the cathode 1. Hydrochloric acid was used for wet etching. Although hydrochloric acid dissolves the InP substrate, it does not dissolve InGaAs, so an etching stopper layer is not necessary.
[0054]
The produced cathode 1 was attached so that the substrate side was in contact with the fuel gas 4 and the epitaxial layer was in contact with the electrolyte layer 2 in the same manner as in Example 1, and the operation as a fuel cell was confirmed. The electromotive force was 0.73 V when the operating temperature was 300 ° C., 0.74 V when the operating temperature was 350 ° C., and 0.75 V when the operating temperature was 400 ° C.
[0055]
There was no difference in electromotive force even when the InGaAs thickness was 5 μm. By setting the hole diameter opened on the substrate side to 3.5 mm, the electromotive force was improved to 0.74 V when the operating temperature was 300 ° C.
[0056]
(Example 3)
Epitaxial growth of 2 μm of AlAs and 5 μm of GaAs doped with carbon on a GaAs substrate was carried out in pure water heated to 80 ° C. to dissolve AlAs, and the carbon-doped GaAs layer was peeled from the substrate. The carbon impurity concentration in GaAs is 8 × 10 ¹⁹ cm ^-3 It was. This carbon-doped GaAs layer (5 μm thin film) is not strong enough for practical use as a fuel cell electrode as it is, but by holding it with a net-like support or a lattice-like support, It can be a component of a practical fuel cell electrode. The electromotive force was measured using this 5 μm thin film as a component of the cathode 1 of the fuel cell. When the operating temperature was 300 ° C., 0.73 V was obtained as an electromotive force.
[0057]
The GaAs substrate after the AlAs is dissolved and the GaAs epitaxial layer is peeled off can be used for epitaxial growth again. An expensive substrate can be used repeatedly, which has the merit of reducing cost and can reduce the amount of toxic arsenic used in the fuel cell.
[0058]
In the fuel cell, the same result can be obtained by using a carbon-doped AlGaAs layer instead of the carbon-doped GaAs layer.
[0059]
Example 4
Before the step of dissolving AlAs described in Example 3, a 20 μm porous silica layer is formed on the carbon-doped GaAs by a CVD method using plasma, and then the AlAs is dissolved and peeled from the substrate. Thus, a carbon-doped GaAs layer reinforced with porous silica was obtained, and this was used as the cathode 1 of the fuel cell.
[0060]
When the cathode 1 was attached so that the carbon-doped GaAs layer was in contact with the electrolyte layer 2 and the operating temperature was 300 ° C., 0.73 V was obtained as an electromotive force.
[0061]
Even if a silicon nitride layer was formed instead of the silica layer, the effect of reinforcing the GaAs layer was obtained, and the same electromotive force was obtained.
[0062]
(Example 5)
On the InP substrate, 0.5 μm of AlAs and 5 μm of InGaAs doped with carbon were epitaxially grown, held in pure water heated to 80 ° C. to dissolve AlAs, and the carbon-doped InGaAs layer was peeled from the substrate. The carbon impurity concentration in InGaAs is 8 × 10 ¹⁹ cm ^-3 It was. This InGaAs layer (thin film) is not strong enough for practical use as an electrode of a fuel cell, but by holding it with a net-like support or a grid-like support, a practical fuel cell It can be a component of the electrode. The electromotive force was measured using this thin film as a constituent element of the cathode 1 of the fuel cell. When the operating temperature was 300 ° C., 0.74 V was obtained as an electromotive force.
[0063]
The InP substrate after the AlAs is dissolved and the InGaAs epitaxial layer is peeled off can be used again for epitaxial growth. An expensive substrate can be used repeatedly, which has the advantage of reducing costs.
[0064]
(Example 6)
Before the step of dissolving AlAs described in Example 5, a 20 μm silica layer is formed on the carbon-doped InGaAs by a CVD method using plasma, and then the AlAs is dissolved and peeled off from the substrate. A carbon-doped InGaAs layer reinforced with porous silica was obtained, and this was used as the cathode 1 of the fuel cell.
[0065]
When the cathode 1 was attached so that the InGaAs layer was in contact with the electrolyte layer 2 and the operating temperature was 300 ° C., an electromotive force of 0.74 V was obtained.
[0066]
Even if a silicon nitride layer was formed instead of the silica layer, the effect of reinforcing the InGaAs layer was obtained, and the same electromotive force was obtained.
[0067]
(Example 7)
Magnesium-doped GaN epitaxially grown on a sapphire substrate, AlGaN (the amount of Al in the group III element is about 10%), and InGaN (the amount of In in the group III element is about 10%) were used as the constituent elements of the cathode 1. Magnesium impurity concentration is 5 × 10 ¹⁸ cm ^-3 It was. The epitaxial film thickness was 5 μm. After epitaxial growth, a hole similar to that described in Example 1 was formed on the substrate side by reactive ion etching, and a fuel cell electrode was produced in which the surface of the magnesium-doped nitride semiconductor was exposed on the surface of the sapphire substrate side.
[0068]
Using this electrode as the cathode 1, the substrate side was in contact with the fuel gas 4 and the epitaxial layer was in contact with the electrolyte layer 2, and the operation as a fuel cell was confirmed. The electromotive force was 0.62V when the operating temperature was 300 ° C, 0.64V when the operating temperature was 350 ° C, 0.66V when the operating temperature was 400 ° C, and 0.68V when the operating temperature was 450 ° C. There was almost no difference due to differences in Group III elements.
[0069]
The same results were obtained when SiC was used as the substrate.
[0070]
(Example 8)
A ZnO film having a thickness of 2 μm is formed on a sapphire substrate by sputtering, and then a magnesium-doped GaN layer, an AlGaN layer (the Al amount in the group III element is about 10%), an InGaN layer (In in the group III element) The amount was about 10%) and 5 μm epitaxially grown. After the growth, the ZnO film was dissolved with hydrochloric acid, the nitride semiconductor thin film was peeled off from the substrate, and used as a component of the cathode 1 of the fuel cell.
[0071]
The electromotive force was 0.64 V when the operating temperature was 300 ° C, 0.66 V when the operating temperature was 350 ° C, 0.68 V when the operating temperature was 400 ° C, and 0.69 V when the operating temperature was 450 ° C. There was almost no difference due to differences in Group III elements.
[0072]
According to the method described in Example 7 and this example, a fuel cell can be manufactured without using any toxic arsenic.
[0073]
Also in this example, the reinforcement of the epitaxial film with silica or silicon nitride shown in Examples 4 and 6 is effective.
[0074]
The same results were obtained when SiC was used as the substrate.
[0075]
Example 9
Growth of carbon-doped GaAs and InGaAs (approximately 50% of the amount of In in group III elements) 12 on a ceramic plate 11 having a thickness of 1 mm and a typical pore diameter of 2 μm, 5 μm, 10 μm, and 20 μm. The electrode configured as described above was used as the cathode 1. The carbon impurity concentration when grown on a GaAs or InP substrate under the same growth conditions is 8 × 10 ¹⁹ cm ^-3 The film thickness was 5 μm. Since there is no clear crystal plane in ceramics, the semiconductor grown on the ceramic is polycrystalline or amorphous. FIG. 2 shows a schematic diagram of a cross section of a fuel cell using the cathode 1. In the figure, 11 is a ceramic plate, 12 is a compound semiconductor doped with p-type impurities, and 13 is a pore in the ceramic plate 11. The cathode 1 was attached so that the compound semiconductor 12 doped with a p-type impurity was in contact with the electrolyte layer 2.
[0076]
When the ceramic plate 11 having a pore diameter of 20 μm was used as the substrate, a large amount of the pores 13 were not filled with the semiconductor 12, and a phenomenon in which the fuel gas 4 and the electrolyte from the electrolyte layer 2 contacted each other through the pores 13 was observed. .
[0077]
When the pore diameter was 10 μm or less, the pore 13 was filled with the semiconductor 12, and the fuel gas 4 and the electrolyte from the electrolyte layer 2 were not in contact with each other, and the operation of the fuel cell was stable. The difference in characteristics due to the difference in pore diameter was small.
[0078]
When carbon-doped GaAs was used and the operating temperature was 300 ° C., an electromotive force of 0.71 V was obtained. In addition, when carbon-doped InGaAs was used and the operating temperature was 300 ° C., 0.72 V was obtained as an electromotive force.
[0079]
The electrode of this example was strong and withstood rapid temperature fluctuations.
[0080]
According to this embodiment, the amount of arsenic used for the electrode can be reduced. In addition, since there is no restriction on the shape of the electrode, it is possible to improve performance by devising to increase the effective film area by making it wavy.
[0081]
It should be noted that the same result can be obtained by using AlGaAs doped with carbon instead of the compound semiconductor in the present embodiment.
[0082]
(Example 10)
On the ceramic plate 11 described in Example 9, GaN, AlGaN (Al amount in Group III element is about 10%), and InGaN (In amount in Group III element is about 10%) are grown. A cathode 1 was obtained. The magnesium impurity concentration when grown on a sapphire substrate under the same growth conditions is 5 × 10 ¹⁸ cm ^-3 The film thickness was 5 μm. Similarly to Example 9, the cathode 1 was attached so that the compound semiconductor 12 doped with a p-type impurity was in contact with the electrolyte layer 2.
[0083]
When the ceramic plate 11 having a pore diameter of 20 μm was used as the substrate, a large amount of the pores 13 were not filled with the semiconductor 12, and a phenomenon in which the fuel gas 4 and the electrolyte from the electrolyte layer 2 contacted each other through the pores 13 was observed. .
[0084]
When the pore diameter was 10 μm or less, the pore 13 was filled with the semiconductor 12, and the fuel gas 4 and the electrolyte from the electrolyte layer 2 were not in contact with each other, and the operation of the fuel cell was stable. The difference in characteristics due to the difference in pore diameter was small.
[0085]
The electromotive force was 0.62 V when the operating temperature was 300 ° C. and 0.66 V when the operating temperature was 400 ° C. There was almost no difference due to the difference in the composition of group III elements.
[0086]
According to this embodiment, it is possible to produce a strong electrode that does not contain arsenic at all.
[0087]
(Example 11)
A porous polytetrafluoroethylene plate having a thickness of 1 mm and typical pore sizes of 2 μm, 5 μm, 10 μm, and 20 μm is used instead of the ceramic plate 11, and carbon-doped GaAs, InGaAs ( The amount of In occupying the group III element was grown to be about 50%, and the fuel cell cathode 1 was obtained. The carbon impurity concentration when grown on a GaAs or InP substrate under the same growth conditions is 8 × 10 ¹⁹ cm ^-3 The film thickness was 5 μm. The cathode 1 was attached so that the compound semiconductor 12 doped with a p-type impurity was in contact with the electrolyte layer 2.
[0088]
When the ceramic plate 11 having a pore diameter of 20 μm was used as the substrate, a large number of the pores 13 were not filled with the semiconductor, and a phenomenon in which the fuel gas 4 and the electrolyte from the electrolyte layer 2 contacted each other through the pores 13 was observed.
[0089]
When the pore diameter was 10 μm or less, the fuel gas 4 and the electrolyte from the electrolyte layer 2 were not in contact with each other, and the operation of the fuel cell was stable. The difference in characteristics due to the difference in pore diameter was small.
[0090]
When carbon-doped GaAs was used and the operating temperature was 300 ° C., an electromotive force of 0.71 V was obtained. In addition, when carbon-doped InGaAs was used and the operating temperature was 300 ° C., 0.72 V was obtained as an electromotive force.
[0091]
According to this embodiment, the amount of arsenic used for the electrode can be reduced. In addition, since there is no restriction on the shape of the electrode, it is possible to improve performance by devising to increase the effective film area by making it wavy.
[0092]
Similar results can be obtained by using carbon-doped AlGaAs or magnesium-doped GaN, AlGaN, or InGaN instead of the above-described compound semiconductor in this embodiment.
[0093]
Furthermore, the material to be the substrate is not limited to polytetrafluoroethylene as long as it is a heat-resistant organic polymer material that can withstand the temperature during semiconductor growth and is stable for a long time at the operating temperature of the fuel cell.
[0094]
(Example 12)
The electrolyte is filled in the pores 13 of the electrode substrate, that is, the ceramic plate 11 (or polytetrafluoroethylene plate) shown in Examples 9 to 11, so that the ceramic plate 11 (or polytetrafluoroethylene plate) is in contact with the electrolyte layer 2. Installed on. That is, in this case, the orientation of the surface of the ceramic plate 11 is opposite to that shown in FIG. 2 (the right-facing surface is facing left and the left-facing surface is facing right). The same applies to a polytetrafluoroethylene plate. In filling the electrolyte, a method was adopted in which the electrode (cathode 1) was placed in a vacuum vessel to remove gas components in the pores and phosphoric acid was injected therein.
[0095]
Since the area where the fuel gas 4 and the semiconductor are in contact with each other is increased, the performance of the fuel cell is improved, and an electromotive force of about 0.02 V is obtained from the electromotive forces shown in Examples 9 to 11, respectively.
[0096]
(Example 13)
The electrode (cathode 1) impregnated with the electrolyte (that is, the electrolyte 13 is filled in the pores 13) shown in Example 12 is attached so that the ceramic plate 11 or the polytetrafluoroethylene plate is in direct contact with the anode 3. A fuel cell with the above structure was fabricated. A schematic view of the cross section is shown in FIG. Similar to Example 12, the pores 13 are filled with an electrolyte to form pores 14 filled with the electrolyte. Since ceramics or polytetrafluoroethylene did not pass electrons, electrical insulation was maintained.
[0097]
In this example, an electromotive force equal to that shown in Example 12 was obtained.
[0098]
In the present embodiment, the electrolyte filled in the pores 13 of the ceramic plate 11 or the pores of the polytetrafluoroethylene plate serves as an electrolyte that connects the cathode 1 and the anode 3, so that There is no need to provide an electrolyte layer. The single cell of the fuel cell can be made thinner by the thickness of the electrolyte layer in the conventional method, and the size can be reduced.
[0099]
(Example 14)
A fuel cell was produced using the electrode having the same structure as that of the cathode 1 in Examples 9 to 13 instead of the anode having the porous graphite used in Examples 1 to 13 as a constituent element. In this embodiment, protons are transferred from the electrolyte to the semiconductor doped with p-type impurities at the anode 3. On the surface where the semiconductor doped with the p-type impurity contacts the air 5, protons, electrons supplied from the cathode 1 through the load 9, and oxygen in the air 5 react to form water.
[0100]
Compared to the case of using an anode having porous graphite as a constituent element, the area where air and a semiconductor doped with a p-type impurity are in contact with each other is reduced, so that the electromotive force is 0 respectively from the electromotive forces shown in Examples 1 to 13. The voltage dropped by about 0.05V.
[0101]
According to the present embodiment, the electrolyte does not come into contact with air, so that the electrolyte can be prevented from being deteriorated by the gas contained in the air.
[0102]
(Example 15)
The fuel gas 4 in Examples 1 to 14 is all hydrogen gas, but this hydrogen gas was changed to a mixed gas of methane and water vapor to operate the fuel cell. Methane reacts with water vapor and turns into hydrogen gas and carbon dioxide. The reaction is promoted because a semiconductor doped with a p-type impurity quickly absorbs hydrogen radicals which are intermediates of this reaction.
[0103]
As a result of measuring the electromotive force, a voltage substantially equal to the numerical values shown in Examples 1 to 14 was obtained.
[0104]
According to this embodiment, there is no need for a reformer that reforms methane into hydrogen gas, and the only mechanism required for fuel reforming is generation of water vapor and flow rate adjustment. For this reason, it is possible to simplify and reduce the price of a fuel cell system using natural gas mainly composed of methane gas as fuel.
[0105]
A similar electromotive force was obtained when ethane or propane was used instead of methane. In the same fuel cell system, LPG mainly composed of propane can be used as fuel.
[0106]
(Example 16)
The fuel gas 4 in Examples 1 to 14 is all hydrogen gas, but this hydrogen gas was changed to a mixed gas of methanol vapor and water vapor to operate the fuel cell. Methanol decomposes into hydrogen gas and carbon monoxide. Carbon monoxide reacts with water vapor to form hydrogen gas and carbon dioxide. Since the semiconductor doped with p-type impurities quickly absorbs hydrogen radicals that are intermediates of these reactions, the reaction is accelerated.
[0107]
As a result of measuring the electromotive force, a voltage substantially equal to the numerical values shown in Examples 1 to 14 was obtained.
[0108]
According to the present embodiment, a reformer that reforms methanol into hydrogen gas becomes unnecessary, and the fuel cell system that uses methanol as fuel can be simplified and reduced in price.
[0109]
As described above, according to the present invention, a fuel cell that does not use an expensive platinum catalyst can be realized by using an electrode having a compound semiconductor doped with a p-type impurity as a constituent element. According to the present invention, miniaturization of the fuel battery cell and simplification of the fuel reforming mechanism when using a fuel other than water tank gas are realized.
[0110]
【The invention's effect】
By implementing the present invention, it is possible to provide a fuel cell that is configured using an electrode that does not contain platinum and that is inexpensive and that can be miniaturized.
[Brief description of the drawings]
FIG. 1 is a schematic view of a fuel cell.
FIG. 2 is a schematic view of a cross section of a fuel cell using a cathode in which a compound semiconductor doped with a p-type impurity is grown on a porous ceramic plate according to the present invention.
FIG. 3 shows a fuel cell in which a compound semiconductor doped with a p-type impurity is grown on a porous ceramic plate according to the present invention, the pores of the ceramic are filled with an electrolyte, and the ceramic plate is directly in contact with the anode. It is a schematic diagram of the cross section of a cell.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Cathode, 2 ... Electrolyte layer, 3 ... Anode, 4 ... Fuel gas, 5 ... Air, 6 ... Fuel gas supply port, 7 ... Air supply port, 8 ... Exhaust gas discharge port, 9 ... Power load, 10 ... Voltage 11 ... ceramic plate, 12 ... compound semiconductor doped with p-type impurities, 13 ... pore, 14 ... pore filled with electrolyte.

Claims (14)

In a fuel cell having a cathode in contact with a fuel gas, an anode in contact with a gas containing oxygen gas, and an electrolyte connecting between the cathode and the anode, phosphoric acid is used as the electrolyte, and the cathode is a p-type impurity. A fuel cell comprising a group III-V compound semiconductor doped with bismuth. The III-V group compound semiconductor doped with the p-type impurity is grown on a GaAs substrate, doped with carbon as a p-type impurity, or doped with carbon as a p-type impurity grown on an AlGaAs or InP substrate. The fuel cell according to claim 1, wherein the fuel cell is InGaAs. 2. The III-V group compound semiconductor doped with p-type impurities is GaN, AlGaN or InGaN doped with magnesium, which is a p-type impurity, grown on a sapphire substrate or SiC substrate. The fuel cell as described. The III-V group compound semiconductor doped with the p-type impurity is grown on the GaAs substrate and then peeled off from the substrate. The p-type impurity-doped GaAs or AlGaAs layer is grown on the InP substrate. A p-type impurity-doped carbon-doped InGaAs layer or sapphire substrate or a SiC substrate peeled off from the substrate and then peeled off from the substrate, p-type magnesium-doped GaN layer, AlGaN layer or InGaN The fuel cell according to claim 1, wherein the fuel cell is a layer. 2. The fuel cell according to claim 1, wherein the group III-V compound semiconductor doped with p-type impurities is a group III-V compound semiconductor doped with p-type impurities grown on a porous ceramic. . 2. The III-V group compound semiconductor doped with a p-type impurity is a group III-V compound semiconductor doped with a p-type impurity grown on a porous heat-resistant organic polymer material. A fuel cell according to claim 1. The fuel cell according to claim 6, wherein the porous heat-resistant organic polymer material is porous polytetrafluoroethylene. The group III-V compound semiconductor doped with the p-type impurity is GaAs, AlGaAs or InGaAs doped with carbon as a p-type impurity, or GaN, AlGaN or InGaN doped with magnesium as a p-type impurity. The fuel cell according to claim 5, 6 or 7. 9. The electrolyte according to claim 5, 6, 7 or 8, wherein the pores of the porous ceramic, the porous heat-resistant organic polymer material or the porous polytetrafluoroethylene are filled with an electrolyte. Fuel cell. Porous ceramic, porous heat-resistant organic polymer material or porous polytetrafluoroethylene in which the pores are filled with an electrolyte are in direct contact with the anode, and the electrolyte filled in the pores are in contact with the cathode. The fuel cell according to claim 9, wherein the fuel cell is connected to the anode. 11. The fuel cell according to claim 1, wherein the anode comprises a III-V group compound semiconductor doped with a p-type impurity. 12. The fuel cell according to claim 1, wherein the fuel gas is hydrogen gas. The fuel cell according to any one of claims 1 to 11, wherein the fuel gas is a mixed gas containing at least methane, ethane or propane and water vapor. The fuel cell according to any one of claims 1 to 11, wherein the fuel gas is a mixed gas of methanol vapor and water vapor.

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