JP2017112036A - Fuel battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel battery capable of solving problems that a fuel battery is inferior in followability to load fluctuations, a secondary battery can store electricity, but has a limit to electric capacity for that can be stored in the battery and furthermore a conventional fuel battery does not have a high conversion efficiency from hydrogen to electric energy.SOLUTION: A fuel battery includes a positive electrode containing nickel hydroxide, a negative electrode containing a hydrogen storage alloy, and a third electrode containing manganese dioxide, and a porous metal plate is disposed between a first chamber housing the positive electrode, the negative electrode, and first electrolytic solution, and a second chamber housing the third electrode and second electrolytic solution. By setting the pH value of the first electrolytic solution to a higher value than the pH value of the second electrolytic solution, the positive electrode is charged by the third electrode, the negative electrode is charged by the hydrogen gas, and the conversion efficiency of hydrogen gas to electric energy is improved, and power generation is performed by using battery reaction, thereby providing a fuel battery having excellent load followability.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a fuel cell that can be used by converting chemical energy into electrical energy and that operates as a secondary battery.

  Secondary batteries and fuel cells are highly efficient and clean energy sources. In recent years, the development of electric vehicles, fuel cell vehicles, and trains that use secondary batteries and fuel cells as power sources has progressed worldwide.

  Fuel cells are attracting attention as power sources with high energy conversion efficiency and low environmental impact. Although a fuel cell cannot store electricity, it is possible to construct a kind of power storage system by combining a fuel cell and a hydrogen production apparatus by electrolysis of water. Such a power storage system is called a reversible fuel cell (see Patent Document 1 and Patent Document 2).

  On the other hand, the secondary battery is used as a power source for electric and electronic devices that require large current discharge such as electric tools. In particular, recently, nickel-hydrogen secondary batteries and lithium-ion secondary batteries have attracted attention as batteries for hybrid vehicles driven by an engine and a secondary battery.

  A normal secondary battery is charged by receiving supply of electric energy and can store electricity. Patent Document 3 discloses a secondary battery that can be charged using a gas. Patent Document 4 discloses a new type of fuel cell that combines a secondary battery and a fuel cell that uses manganese hydroxide as a positive electrode active material and a hydrogen storage alloy as a negative electrode active material.

JP 2002-348694 A JP 2005-65398 A JP 2010-15729 A

  The secondary battery can store electricity. In addition, the output characteristics are excellent and the load followability is good. However, since the amount of the active material of the negative electrode and the positive electrode depends on the volume of the battery, there is a limit to the electric capacity that can be stored in the battery, so it is difficult to significantly increase the energy density of the secondary battery. .

  On the other hand, the fuel cell generates power (discharges) using hydrogen gas and oxygen gas supplied from the outside. For this reason, the fuel cell does not have a problem related to the limit of energy density, which the secondary battery has. However, since the fuel cell has poor output characteristics and poor followability with respect to load fluctuations during discharge, it is difficult to apply the fuel cell to applications with large load fluctuations. Usually, a fuel cell is often used in combination with a power storage device such as a secondary battery or a capacitor. Furthermore, the fuel cell has a structural feature that the reaction space is small and the reaction speed is slow because the line where the solid, gas, and liquid simultaneously contact each other is used as a reaction space. It has problems such as requiring an expensive catalyst such as platinum.

  A conventional reversible fuel cell generates electricity from hydrogen and oxygen, but its voltage is about 1.0 V, which is lower than the theoretical voltage of 1.23 V. The reason why the voltage is low is considered to be that a part of the energy of hydrogen is converted into heat during the discharge process for some reason. That is, there is room for improvement in the energy conversion efficiency from hydrogen to electricity.

  When taking out electric power using a chemical reaction, if the energy obtained from the chemical substance used is ΔH, the amount of electricity that can be taken out is ΔG, and the generated heat is TΔS, then the relationship ΔH = ΔG + TΔS holds. ΔG is called free energy and is energy that can be effectively extracted as work, and is also called exergy. On the other hand, TΔS is heat generated in response to the reaction, and is expressed by a product of entropy change and temperature, and is called anergy. The ability to extract effective energy (ΔG / ΔH) is referred to as theoretical energy conversion efficiency.

  In the reaction (H2 + 1 / 2O2 → H2O) in the fuel cell, the enthalpy change ΔH = 285.83 [kJ / mol] and the free energy change ΔG = 237.13 [kJ / mol], so the theoretical conversion efficiency ΔH / ΔG = 83%. . When hydrogen is converted into electric energy using a fuel cell, 17% of chemical energy ΔH obtained from hydrogen becomes heat (TΔS), and ΔG is energy that can be extracted as electric power. It is required to reduce the rate at which ΔG is heated and convert the energy (ΔH) obtained from hydrogen into electrical energy as much as possible.

  A battery using manganese dioxide for the positive electrode has characteristics that the pH value of the electrolytic solution is high, so that the discharge potential is as low as 1.0 V, and the electronic conductivity of manganese dioxide is low. For this reason, the battery using manganese dioxide has the subject that it cannot take out electricity with high output and is inferior to an output characteristic.

  The present invention has been made in consideration of such points, and an object of the present invention is to provide a fuel cell having high energy conversion efficiency, high energy density, and excellent output characteristics.

  To achieve the above object, a fuel cell according to the present invention includes a negative electrode including a hydrogen storage alloy, a positive electrode including nickel hydroxide or silver oxide, a third electrode that can be reduced with oxygen, the positive electrode, Conductivity interposed between the first chamber containing the negative electrode and the first electrolyte, the second chamber containing the third electrode and the second electrolyte, and the first chamber and the second chamber. And an electrolytic solution separating means having ion permeability, and the pH value of the first electrolytic solution is larger than the pH value of the second electrolytic solution.

In this configuration, the positive electrode can be charged by the third electrode by making the pH value of the first electrolytic solution larger than the pH value of the second electrolytic solution.
The negative electrode and the positive electrode are arranged to face each other with a separator interposed therebetween, and the third electrode and the positive electrode are arranged to face each other via an electrolytic solution separating means. The fuel cell according to the present invention is a fuel cell that can be charged by electricity and can also be charged by fuel gas and can be charged.

  In the fuel cell according to the present invention, the third electrode contains manganese dioxide. In this configuration, by using manganese dioxide for the third electrode, a fuel cell can be configured at low cost.

  In the fuel cell according to the present invention, one surface of the negative electrode is in contact with hydrogen gas, the other surface is in contact with the first electrolytic solution, and one surface of the positive electrode is in contact with the first electrolytic solution. The other surface is in contact with the electrolytic solution separating means, one surface of the third electrode is in contact with the electrolytic solution separating means, and the other surface is in contact with the second electrolytic solution. In contact.

  In the fuel cell according to the present invention, the pH value of the first electrolyte solution is 12 to 14, the pH value of the second electrolyte solution is 6 to 8.4, and the pH value of the first electrolyte solution is And the pH value of the second electrolyte solution is 5.6 or more.

  In the fuel cell according to the present invention, the electrolytic solution separating means is made of an ion exchange resin. In the fuel cell according to the present invention, the electrolytic solution separating means is a porous metal plate.

  In the fuel cell according to the present invention, a hydrogen gas supply source is connected to the first chamber. In the fuel cell according to the present invention, an oxygen gas supply source is connected to the second chamber.

  In the fuel cell according to the present invention, the third electrode is charged by oxygen dissolved in the second electrolytic solution, and the positive electrode is charged by the third electrode. In the fuel cell according to the present invention, a supply source of an electrolytic solution in which oxygen is dissolved is connected to the second chamber. The fuel cell according to the present invention generates electric power by discharging between the positive electrode and the negative electrode.

  The battery assembly according to the present invention includes: the negative electrode; a separator interposed between the negative electrode and the positive electrode; the electrolyte separating means interposed between the positive electrode and the third electrode; and the positive electrode. In the laminated electrode group, a metal bipolar partition is arranged between the electrode groups. In this configuration, a separator that allows ions to pass but not electrons is interposed between the negative electrode and the positive electrode, and the negative electrode and the positive electrode are insulated.

  As described above, according to the fuel cell of the present invention, it is possible to charge by using the chemical energy of hydrogen gas and oxygen gas and convert it into electric energy for use. For this reason, in the fuel cell of the present invention, the electrical energy that can be taken out does not depend on the amount of the active material contained in the electrode material, so that it is possible to increase the energy density. Moreover, since electricity can be taken out by the reaction of the secondary battery, the fuel cell has excellent output characteristics.

  In an existing fuel cell, an energy conversion efficiency of 83% is theoretically about 30 to 40% in actual operation. This is presumably because the overvoltage increases due to the three-phase interface reaction. In particular, it is considered that the influence of the overvoltage of the oxygen reduction reaction at the positive electrode is large. As means for realizing high-efficiency energy conversion, the inventors propose (1) charging the negative electrode using hydrogen and (2) charging the manganese dioxide electrode (third electrode) using oxygen. . As a result, the overvoltage on the positive electrode side can be reduced, and the power generation voltage 0.7 to 0.8 V of the existing fuel cell can be increased to 1.3 V of the nickel metal hydride battery, so that the energy conversion efficiency can be improved. it can.

  Regarding the output characteristics, the inventors also (1) charged the nickel hydroxide electrode (positive electrode) with the manganese dioxide electrode (third electrode), and (2) generated power between the hydrogen storage alloy negative electrode and the nickel hydroxide positive electrode. Suggest to do. This makes it possible for the reaction to proceed at the two-phase interface instead of the three-phase interface reaction as in the conventional fuel cell, and to reduce the reaction during power generation, instead of manganese dioxide. Output characteristics are greatly improved by using excellent nickel oxyhydroxide.

  The fuel cell of the present invention is characterized by high energy density, excellent output characteristics, and high energy conversion efficiency.

It is drawing for demonstrating the principle of the fuel cell of this invention. It is drawing which shows the structure of the fuel cell of this invention typically. 3 is a drawing schematically showing a structure of a first modification of the fuel cell of FIG. 2. It is drawing which shows typically the structure of the 2nd modification of the fuel cell of FIG. It is drawing which shows typically the structure of the 3rd modification of the fuel cell of FIG. It is drawing explaining the outline | summary of an experimental apparatus. It is a graph which shows an experimental result.

  Hereinafter, embodiments according to the present invention will be described with reference to the drawings. However, the present invention is not limited to these embodiments, and various other modifications are possible.

Prior to detailed description of the fuel cell according to the first embodiment, electrodes that are main components will be described, and then details of the fuel cell will be described.
<Positive electrode material>
As the positive electrode active material, nickel hydroxide is preferable in terms of price, but silver oxide may be used. Moreover, since it is easy to achieve high capacity | capacitance, a positive electrode active material has a large bulk density, for example, a spherical thing is preferable.

<Hydrogen storage alloy>
The hydrogen storage alloy contained in the negative electrode material is not particularly limited as long as it can store and release hydrogen. For example, AB5 type that is a rare earth alloy, AB2 type that is a Laves phase alloy, AB type that is a titanium-zirconium alloy, and A2B type that is a magnesium alloy can be used.

  Among these, from the viewpoints of hydrogen storage capacity, charge / discharge characteristics, self-discharge characteristics, and cycle life characteristics, it is preferable to be an AB5 type rare earth-nickel alloy containing a MmNiCoMnAl misch metal.

<Third electrode material>
The third electrode needs to be a substance capable of oxygen reduction. It needs to be charged by contacting oxygen gas or charged by contacting an aqueous solution in which oxygen is dissolved. Examples of such substances include manganese dioxide, platinum, palladium, gold, and silver. Silver oxide can also be used. Among these, manganese dioxide is the most appropriate considering the price and environmental burden. Manganese dioxide has abundant reserves and is 1/5 to 1/10 the price of nickel hydroxide. Manganese dioxide and silver oxide act as a catalyst in the reaction of the third electrode.

<Binder>
Examples of the binder include polyacrylic acid soda, methyl cellulose, carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), polyvinyl butyral (PVB), ethylene-vinyl alcohol, ethylene vinyl acetate copolymer (EVA), polyethylene ( It is conceivable to use PE), polypropylene (PP), and styrene-ethylene-butylene-styrene copolymer (SEBS).
When the total of the electrode materials such as the active material, the binder and the conductive auxiliary agent is 100% by weight, the weight ratio of the binder mixed in each electrode is 0.1 to 10% by weight. It is preferable.

<Conductive aid>
The conductive auxiliary agent may be a conductive powder. The conductive aid may be a carbon powder such as graphite powder, acetylene black, and ketjen black. When the total of the electrode materials such as the active material, the binder and the conductive auxiliary agent is 100% by weight, the weight ratio of the conductive auxiliary agent mixed in each electrode is in the range of 0.1 to 10% by weight. It is preferable.

[Positive electrode]
Nickel hydroxide was used as the active material, carbon black was used as the conductive additive, and ethylene vinyl acetate copolymer (EVA) was used as the binder, and the blending ratio was 100: 5: 5. A powdered positive electrode material was mixed and kneaded into a paste, and this paste was applied or filled on both sides of the current collector. After drying, the current collector was rolled with a roller press to produce a positive electrode. Note that. Although nickel foam having a thickness of 250 μm was used as the current collector, nickel metal or platinum metal may be used, and the shape may be three-dimensional or plate-like, and the electrodes may be formed on both sides.

[Negative electrode]
A hydrogen storage alloy was used as the active material, carbon black was used as the conductive additive, and ethylene vinyl acetate copolymer (EVA) was used as the binder, and the blending ratio was 100: 15: 10. Powdered negative electrode materials were mixed and kneaded into a paste, and this paste was applied or filled on both sides of the current collector. After drying, the current collector was rolled with a roller press to produce a negative electrode. The electrode capacity was 48 mAh. Although nickel foam is used as the current collector, it may be nickel metal or platinum metal, the shape may be three-dimensional or plate-like, and the electrodes may be formed on both sides.

[Third electrode]
Manganese dioxide was used as the active material, carbon black was used as the conductive assistant, and ethylene vinyl acetate copolymer (EVA) was used as the binder, and the blending ratio was 100: 5: 5. The powdered third electrode material was mixed and kneaded into a paste, and the paste was filled into a nickel foam current collector. After drying, the current collector was rolled with a roller press to produce a third electrode. . Although nickel foam is used as the current collector, it may be nickel metal or platinum metal, the shape may be foil or plate, and the electrodes may be formed on both sides.

[Electrolyte separation means]
The electrolytic solution separating means used in the present invention needs to be a substance that does not pass or is difficult to pass through the electrolytic solution. This is because if two types of electrolytes are mixed, the pH value of the electrolyte may change and performance may not be exhibited. In addition, the electrolytic solution separating means needs to have conductivity and ion (H ⁺ , OH ⁻ ) permeability due to its character. Specifically, an ion exchange membrane and a porous metal plate having conductivity are exemplified. The porous metal plate may be, for example, a three-dimensional substrate such as a foamed nickel substrate, a net-like sintered fiber substrate plated with nickel, or a felt-plated substrate that is a nickel-plated nonwoven fabric. And it is preferable that thickness is the range of 5 micrometers-1 mm. If it is too thin, mechanical strength becomes a problem, and if it is too thick, volume efficiency as a battery is deteriorated, so about 20 μm is desirable.

[Electrolyte]
The type of the electrolyte used in the present invention is not particularly limited as long as it is an alkaline aqueous solution usually used in water electrolysis. For example, potassium hydroxide (KOH), lithium hydroxide (LiOH), sodium hydroxide (NaOH) ) Or the like in which one or more alkali substances are dissolved in water is suitable. From the viewpoint of battery output characteristics, the electrolytic solution is preferably an aqueous potassium hydroxide solution.

[Separator]
The separator used in the present invention is preferably a separator that does not allow electrons to pass through and allows protons to pass therethrough, but prevents gas from passing through. Examples of the separator include microporous membranes, woven fabrics, non-woven fabrics, and green compacts. Among these, non-woven fabrics are preferable from the viewpoint of output characteristics and production cost. The material of the separator is not particularly limited, but is preferably a separator having alkali resistance, oxidation resistance, and reduction resistance. Examples thereof include materials such as polytetrafluoroethylene (PTFE), polyimide (PI), polyamide, polyamideimide, aramid, polyethylene, and polypropylene.
The thickness is preferably in the range of 5 to 500 μm, and more preferably in the range of 20 to 250 μm. If the thickness is less than 5 μm, the possibility of a rare short increases, and if it is 500 μm or more, the electrical resistance increases and the heat loss increases. In the examples, a polypropylene nonwoven fabric having a thickness of 120 μm was employed.

  The fuel cell according to the present invention incorporates a reaction mechanism of a secondary battery into a fuel cell that generates electric power by converting chemical energy of hydrogen and oxygen into electric energy. Hereinafter, the operation principle of such a fuel cell will be described with reference to FIG. 1, and the main components of the fuel cell will be described with reference to FIG.

  In FIG. 1, the fuel cell of the present invention includes a positive electrode 1, a negative electrode 2, a first electrolyte solution 5, a first chamber 8 that houses a separator 4 interposed between the positive electrode 1 and the negative electrode 2, and a third electrode 3. And a second chamber 9 in which the second electrolyte 6 is stored, and an electrolyte separating means 7 interposed between the first chamber 8 and the second chamber 9. The positive electrode 1 and the negative electrode 2 are disposed so as to face each other via the separator 4, and the first chamber 8 and the second chamber 9 are disposed so as to face each other via the electrolytic solution separating means 7. ing.

  The first electrolytic solution 5 is an aqueous solution of 6M potassium hydroxide (KOH). At this time, the pH value of the first electrolytic solution 5 is about 14. The second electrolytic solution 6 is an aqueous solution of potassium hydroxide (KOH) having a pH value of about 8.

  The separator 4 interposed between the positive electrode 1 and the negative electrode 2 serves to hold the electrolyte solution 5 while preventing the positive electrode 1 and the negative electrode 2 from being electrically short-circuited while allowing permeation of ions.

One surface of the positive electrode 1 disposed in the first chamber 8 is immersed in the first electrolytic solution 5, and the other surface of the positive electrode 1 is in contact with the electrolytic solution separating means 7 over almost the entire surface. And the electrolytic solution separating means 7 are in an electrically connected state.
Moreover, one surface of the negative electrode 2 faces hydrogen gas, and the negative electrode 2 can be chemically charged with hydrogen gas. The other surface of the negative electrode 2 is immersed in the first electrolytic solution 5.
Furthermore, one surface of the third electrode 3 disposed in the second chamber 9 is almost entirely in contact with the electrolytic solution separating means 7, and the third electrode 3 and the electrolytic solution separating means 7 are in an electrically connected state. It is in. The other surface of the third electrode 3 is immersed in the second electrolytic solution 6, and can be chemically charged by oxygen dissolved in the second electrolytic solution 6.

  In FIG. 2, the fuel cell 20 includes a first chamber 18 in which the positive electrode 11, the negative electrode 12, the first electrolyte solution 15 and the separator 14 are stored, and a second chamber in which the third electrode 13 and the second electrolyte solution 16 are stored. 19 and the electrolytic solution separating means 17 interposed between the first chamber 18 and the second chamber 19 are main components. Note that description of items that are the same as those in FIG. 1 is omitted.

The first chamber 18 is divided by a partition wall 32 into a hydrogen gas chamber 27 that holds hydrogen gas and a region that holds the first electrolyte solution 15. One surface of the negative electrode 12 faces the hydrogen gas chamber 27, and the other surface of the negative electrode 12 is immersed in the first electrolytic solution 15. The hydrogen gas chamber 27 is provided with a hydrogen circulation port 22 communicating with a hydrogen gas supply source 23 provided outside, so that the negative electrode 12 can be charged by receiving the supply of hydrogen gas.
One surface of the positive electrode 11 is immersed in the first electrolytic solution 5, and the other surface of the positive electrode 11 is almost entirely in contact with the electrolytic solution separating means 17.

  An oxygen gas chamber 28 is provided in the lower portion of the second chamber 19. Oxygen gas is bubbled into the second electrolytic solution 16 from an oxygen gas ejection hole 33 provided in the upper part of the oxygen gas chamber 28. The oxygen gas chamber 28 is provided with an oxygen circulation port 24 communicating with an oxygen gas supply source 25 provided outside, and oxygen gas is supplied to the oxygen gas chamber 28. The third electrode 13 is immersed in the second electrolytic solution 16, and one surface of the third electrode 13 is almost entirely in contact with the electrolytic solution separating means 17. A part of the bubbled oxygen gas directly reaches the electrode surface of the third electrode 13, and the remaining part is dissolved in the second electrolytic solution 16. The third electrode 13 is charged with oxygen dissolved in the second electrolytic solution 16 or with oxygen gas that is in direct contact with the electrode surface.

The electrolytic solution separating means 17 provided between the first chamber 18 and the second chamber 19 prevents the first electrolytic solution 15 and the second electrolytic solution 16 from being mixed, and between the positive electrode 11 and the third electrode 13. Allows circulation of ions (H ⁺ , OH ⁻ ). Since the electrolytic solution separating means 17 has conductivity, electrons can move between the positive electrode 11 and the third electrode 13. A seal 31 is provided between the electrolytic solution separating means 17 and the first chamber 18 and the second chamber 19 to prevent the electrolytic solution from flowing out.

  Each of the positive electrode 11 and the negative electrode 12 has a terminal (not shown) for connection with an external device at the end. Electric power can be taken out from the connection terminal. Alternatively, it is possible to electrically charge the positive electrode 11 and the negative electrode 12 by connecting a DC power source (not shown) to the connection terminal.

  Next, a first modification of the first embodiment will be described with reference to FIG. The second chamber 19 is divided by the partition wall 34 into an oxygen gas chamber 28 that holds oxygen gas and a region that holds the second electrolyte solution 16. One surface of the third electrode 13 accommodated in the second chamber 19 is in contact with the electrolytic solution separating means 17 over almost the entire surface. The other surface of the third electrode 13 faces the oxygen gas chamber 28. The oxygen gas chamber 28 is provided with an oxygen circulation port 24 communicating with an oxygen gas supply source 25 provided outside, and oxygen gas is supplied to the oxygen gas chamber 28. The third electrode 13 is charged with oxygen gas in the oxygen gas chamber 28.

  A second modification of the first embodiment will be described with reference to FIG. A description of parts common to the above description is omitted. The fuel cell 40 includes the first sealed container 38, the second sealed container 39, and the electrolytic solution separating means 17 as main components. The internal configuration of the first sealed container 38 is the same as that of the first chamber 18 of FIG. The first sealed container 38 and the second sealed container 39 are sealed containers having pressure resistance. The inside of the second sealed container 39 is filled with the second electrolytic solution 36 in which oxygen is dissolved. One surface of the third electrode 13 is in contact with the electrolytic solution separating means 17 over almost the entire surface. The third electrode 13 is immersed in the second electrolytic solution 36 in which oxygen is dissolved, and the third electrode 13 is charged by the electrolytic solution-dissolved oxygen. The second electrolytic solution 36 flows out of the electrolytic solution circulation port 35 a provided at the bottom of the second sealed container, is pressurized by the transport pump 42, and flows into the electrolytic solution tank 41. The electrolyte tank 41 can be supplied with high-pressure oxygen from an oxygen source (not shown), so that more oxygen is dissolved in the electrolyte by increasing the electrolyte pressure. The oxygen-rich electrolyte is supplied to the second sealed container 39 from the electrolyte inlet 35b.

  A third modification of the first embodiment will be described with reference to FIG. The fuel cell pack 50 includes a negative electrode 52, a separator 54 interposed between the negative electrode 52 and the positive electrode 51, a positive electrode 51, an electrolyte separation means 57 interposed between the positive electrode 51 and the third electrode 53, In the electrode group in which the three electrodes 53 are stacked in this order, the battery assembly is configured by arranging a metal bipolar partition wall 55 between the electrode groups. The bipolar partition wall 55 is a nickel-plated steel plate and plays a role of blocking electricity and electrolyte while passing electricity. The fuel cell pack 50 has a positive electrode current collector 48 and a negative electrode current collector 49 at both ends thereof.

  The fuel cell pack 50 is equipped with an oxygen gas header 58 for supplying oxygen gas, and oxygen gas is supplied to the electrode surface of the positive electrode 51. Further, a hydrogen gas header 59 for supplying hydrogen gas is provided, and hydrogen gas is supplied to the electrode surface of the negative electrode 52. Further, the fuel cell pack 50 is equipped with an electrolyte supply header (not shown), and the separator 54 is supplied with an electrolyte in which oxygen is dissolved.

  Since the fuel cell pack 50 has a laminated structure, its output voltage is obtained by connecting the electrode groups 56 in series. The output voltage can be easily changed by adjusting the number of electrode groups.

As a method for improving the energy conversion efficiency, the inventors devised to lower the pH of the electrolytic solution (electrolytic solution 6) on the manganese dioxide side. The pH of the 6M aqueous KOH solution is about 14. In this electrolytic solution (pH = 14), the oxidation-reduction potential (E) of manganese dioxide is 0.15 V vs. SHE. In the case of 25 ° C., according to the Nernst equation, the electrode potential is determined thermodynamically as follows.
E = E ⁰ −0.0591 × pH (where E ^{0 is the} standard redox potential)
Therefore, the oxidation-reduction potential of the manganese dioxide electrode can be increased by lowering the pH value of the electrolyte solution on the manganese dioxide electrode side. For example, when pH = 8, the oxidation-reduction potential (E) of the manganese dioxide electrode is increased by 0.355V to 0.505V vs SHE according to the above formula.

As the active material, each of the fuel cells 20 uses a hydrogen storage alloy for the negative electrode, nickel hydroxide for the positive electrode, and manganese dioxide for the oxygen reduction electrode. As the electrolytic solution, a first electrolytic solution 5 having a pH of 14 and a second electrolytic solution 6 having a pH of 7 to 8 are used. At this time, the reaction in the fuel cell will be described in the following three cases.
(1) Charging the third electrode with oxygen The third electrode is charged by supplying oxygen to the third electrode.
4MnOOH + O ₂ → 4MnO ₂ + 2H ₂ O (1)

(2) Charging the positive electrode with the third electrode (second electrolyte; pH 8)
The reaction in manganese dioxide (formula (2)) occurs at pH 8 with a potential of 0.505 V vs SHE.
_{MnO 2 + H 2 O + e} - → MnOOH + OH - (2)
On the other hand, the charge reaction of nickel hydroxide (formula (3)) occurs at a pH of 14 and a potential of 0.480 V vs. SHE.
^{_{Ni (OH) 2 + OH -}} → NiOOH + H 2 O + e - (3)
Therefore, the manganese dioxide discharge reaction potential becomes higher than the charge reaction potential of nickel hydroxide due to the pH difference of the electrolytic solution, the following reaction (formula (4)) occurs, and the positive electrode is charged.
MnO ₂ + Ni (OH) ₂ → MnOOH + NiOOH (4)
Moreover, by supplying oxygen to the third electrode, the third electrode is charged and returned to manganese dioxide.
4MnOOH + O ₂ → 4MnO ₂ + 2H ₂ O (1)

(3) Discharge between negative electrode and positive electrode (first electrolyte solution; pH 14)
The hydrogen storage alloy of the negative electrode is oxidized by hydrogen by an electrochemical reaction and becomes charged. At this time, the electrode potential of the negative electrode is −0.828 V vs. SHE at pH 14.
2M + H ₂ → 2MH (5)
^{MH + OH - → M + H} 2 O + e - (6)
Power can be generated by combining the reverse reaction of equation (3) and the reaction of equation (6).
MH + NiOOH → M + Ni (OH) ₂ (7)
As a result, the fuel cell can generate power at 1.308V.

  Unlike the conventional secondary battery, the electric capacity of the fuel cell of the present invention is not limited by the amount of the active material, so that the energy density can be significantly improved (for example, several tens of times).

  Furthermore, as described above, when the fuel cell of the present invention is discharged, electric energy is output based on the cell reaction. For this reason, as compared with the conventional fuel cell, the followability to the load and the power are greatly improved.

(Experimental result)
FIG. 6 shows an outline of the experimental apparatus. The experimental apparatus is composed of three beakers A, B and C, a data logger (not shown), wiring for connecting them, and a switch SW. In the beaker A, a nickel hydroxide electrode a in a discharged state is immersed in a 6M KOH aqueous solution (pH 14), and in the beaker B, a reference electrode b (Ag / AgCl) is immersed in a saturated KCl aqueous solution. In FIG. 2, the charged manganese dioxide electrodes c are respectively immersed in a diluted KOH aqueous solution (pH about 8). Each beaker is connected by a KCl salt bridge (not shown).

FIG. 7 shows the experimental results of measuring the potential difference between the electrodes using a data logger. The vertical axis indicates the potential of the electrode a and the electrode c based on the reference electrode in V units, and the horizontal axis indicates the time in seconds. In the graph, the solid line is a graph showing the potential change of the manganese dioxide electrode c, and the broken line is a graph showing the potential change of the nickel hydroxide electrode a. Where the solid line and the broken line overlap, a solid line is displayed.
In the figure, the section indicated by (1) is when the electrode a and the electrode c are not connected, and the section indicated by (2) is when the electrode a and the electrode c are connected. The connection between the electrode a and the electrode c was controlled by the switch SW.
From the graph, it can be seen that in section (1), the manganese dioxide electrode is about 0.13 V higher in potential than the nickel hydroxide electrode. In section (2), the potential difference between the manganese dioxide electrode and the nickel hydroxide electrode disappears, but it can be seen that the potential gradually increases after connection. When the connection was released, the potential of the nickel hydroxide electrode increased compared to before connection. Thereby, it turned out that a nickel hydroxide electrode (aqueous solution pH14) can be charged with a manganese dioxide electrode (aqueous solution pH8).

  The fuel cell of the present invention can be suitably used as an industrial and consumer DC power supply.

DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 3rd electrode 4 Separator 5 1st electrolyte solution 6 2nd electrolyte solution 7 Electrolyte separation means 8 1st chamber 9 2nd chamber 11 Positive electrode 12 Negative electrode 13 3rd electrode 14 Separator 15 1st electrolyte solution 16 1st 2 Electrolyte 17 Electrolyte separator 18 First chamber 19 Second chamber 20 Fuel cell 22 Hydrogen flow port 23 Hydrogen gas supply source 24 Oxygen flow port 25 Oxygen gas supply source 27 Hydrogen gas chamber 28 Oxygen gas chamber 32 Partition wall 33 Oxygen Gas outlet 35 Electrolyte flow port 36 Second electrolyte 38 First airtight container 39 Second airtight container 40 Fuel cell 41 Electrolyte tank 42 Transport pump 48 Positive electrode current collector 49 Negative electrode current collector 50 Fuel cell pack 51 Positive electrode 52 Negative electrode 53 Third electrode 54 Separator 55 Bipolar partition wall 57 Electrolyte separation means 58 Oxygen gas header 59 Hydrogen gas header

JP 2002-348694 A JP 2005-65398 A JP 2010-15729 A JP 2010-15783 A

Claims (12)

A negative electrode containing a hydrogen storage alloy;
A positive electrode comprising nickel hydroxide or silver oxide;
A third electrode that can be reduced with oxygen;
A first chamber containing the positive electrode, the negative electrode, and a first electrolyte;
A second chamber for accommodating the third electrode and the second electrolyte;
An electrolyte separating means having conductivity and ion permeability interposed between the first chamber and the second chamber;
A fuel cell in which the pH value of the first electrolyte solution is larger than the pH value of the second electrolyte solution.   The fuel cell according to claim 1, wherein the third electrode contains manganese dioxide. One surface of the negative electrode is in contact with hydrogen gas, the other surface is in contact with the first electrolyte,
One surface of the positive electrode is in contact with the first electrolytic solution, and the other surface is in contact with the electrolytic solution separating means;
3. The fuel cell according to claim 1, wherein one surface of the third electrode is in contact with the electrolytic solution separating unit, and the other surface is in contact with the second electrolytic solution. 4. .   The pH value of the first electrolyte solution is 12 to 14, the pH value of the second electrolyte solution is 6 to 8.4, and the pH value of the first electrolyte solution and the pH value of the second electrolyte solution The fuel cell according to claim 1, wherein a difference from the value is 5.6 or more.   The fuel cell according to claim 1, wherein the electrolytic solution separating unit is made of an ion exchange resin.   The fuel cell according to claim 1, wherein the electrolytic solution separating means is a porous metal plate.   The fuel cell according to claim 1, wherein a hydrogen gas supply source is connected to the first chamber.   The fuel cell according to claim 1, wherein an oxygen gas supply source is connected to the second chamber.   3. The fuel cell according to claim 1, wherein the third electrode is charged with oxygen dissolved in the second electrolyte solution, and the positive electrode is charged with the third electrode. 4.   The fuel cell according to claim 9, wherein a supply source of an electrolytic solution in which oxygen is dissolved is connected to the second chamber.   The fuel cell according to claim 9, wherein power generation is performed by discharging between the positive electrode and the negative electrode. In the electrode group in which the negative electrode, the separator interposed between the negative electrode and the positive electrode, the electrolyte solution separating means interposed between the positive electrode and the third electrode, and the positive electrode are laminated, An assembled battery in which metal bipolar barriers are arranged between electrode groups.

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