JP6901080B2 - Direct carbon fuel cell - Google Patents

Description

The present invention relates directly to a carbon fuel cell.

Known conventional fuel cells include phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC), solid oxide fuel cells (SOFC), and polymer electrolyte fuel cells (PEFC). There is.
Further, while molten carbonate fuel cells and solid oxide fuel cells use gaseous fuel, direct carbon fuel cells (DCFC) using solid carbon fuel have also been proposed.
Non-Patent Documents 1 and 2 describe an example of a direct carbon fuel cell in which molten carbonate is used as an electrolyte and carbon powder as a fuel is dispersed in the electrolyte.
The electrochemical reaction of the direct carbon fuel cell is as follows.
The _{cathode: O 2 + 2CO 2 + 4e} - → 2CO 3 2-
Anode: C + 2CO ₃ ^2- → 3CO ₂ + 4e ⁻
Summary reaction: C + O ₂ → CO ₂
The reaction at the anode occurs at the three-phase interface between the anode, the solid carbon fuel and the molten carbonate, which is the electrolyte.

Xiang Li, 4 outsiders, "Evaluation of coals as fuels for direct carbon fuel cells", Journal of Power Fuel cells, 195 (2010), p. 4051-4058 Koyo Yamamura, 3 outsiders, "No. 56 Effect of Direct Carbon Fuel Cell (DCFC) Components on Power Generation Characteristics", Proceedings of the Coal Science Conference, Japan Energy Society, October 2015, No. Volume 52, p. 112-113

However, the power generation performance of the direct carbon fuel cell disclosed in Non-Patent Documents 1 and 2 is not sufficient, and improvement is required.
An object of the present invention is to provide a direct carbon fuel cell having excellent power generation performance.

The present invention has the following aspects.
[1] An electrolyte composed of molten carbonate, a cathode in contact with the electrolyte, oxygen and carbon dioxide, an anode in contact with the electrolyte, granular solid carbon existing in the electrolyte in the vicinity of the anode, the anode and the above. It has a partition wall that is provided between the anode and the anode and allows the electrolyte to pass through and does not allow the solid carbon to pass through, and at least a part of the anode is provided with a through hole that does not allow the solid carbon to pass through. A direct carbon fuel cell characterized in that the solid carbon is pressure-welded to the opening surface of the hole.
[2] The direct carbon fuel cell according to [1], wherein the solid carbon is sandwiched between the opening surface of the through hole and a part of the partition wall.
[3] The direct carbon fuel cell according to [1] or [2], which has a pressurizing means for pressurizing the opening surface of the through hole and the solid carbon in a close contact direction.
[4] The average particle size of the solid carbon is 20 to 2000 μm, and the diameter of the through hole on the opening surface of the through hole is 1/3 or less of the average particle size of the solid carbon [1]. ~ [3] Direct carbon fuel cell.

The direct carbon fuel cell of the present invention is excellent in power generation performance.

It is sectional drawing which shows one Embodiment of the direct carbon fuel cell of this invention. It is a top view which shows an example of the anode 2 in FIG. It is explanatory drawing which shows typically the neighborhood of the anode 2 at the time of power generation. It is a graph which shows the measurement result of an Example. It is a graph which shows the measurement result of the comparative example.

FIG. 1 is a cross-sectional view showing an embodiment of a direct carbon fuel cell of the present invention.
The direct carbon fuel cell of the present embodiment has a molten carbonate (electrolyte) 3, a cathode 1 in which a part thereof is immersed in the molten carbonate 3, and an anode 2 in which the whole is immersed in the molten carbonate 3. And the granular solid carbon (fuel) 4 present in the molten carbonate 3 in the vicinity of the anode 2. The molten carbonate 3 is housed in an electric furnace 5c provided at the bottom of the reactor 5.
In this embodiment, a reference electrode 20 is provided to directly measure the power generation characteristics of the carbon fuel cell, and the cathode 1, anode 2 and reference electrode 20 are connected to a potentiostat / galvanostat (not shown). Reference numerals 11 and 21 indicate wiring. The wiring of the anode 2 is not shown.
A porous partition wall 6 is provided between the cathode 1 and the reference electrode 20 and the anode 2 so that the molten carbonate 3 and the solid carbon 4 do not pass through. The partition wall 6 has a bottomed cylindrical shape, and the bottom portion of the partition wall 6 is immersed in the molten carbonate 3.

In the present embodiment, the anode 2 is a disk-shaped member provided with a through hole 2a penetrating from the front surface to the back surface, and is fitted inside the partition wall 6.
The solid carbon 4 is filled between the anode 2 and the bottom surface of the partition wall 6. The anode 2 is pressurized by the pressurizing means 7 in the direction toward the bottom surface of the partition wall 6, whereby the solid carbon 4 is pressed against the lower surface (opening surface of the through hole) of the anode 2. That is, the pressurizing means 7 applies pressure in the direction in which the lower surface of the anode 2 and the solid carbon 4 are in close contact with each other, and the solid carbon 4 is sandwiched between the lower surface of the anode 2 and the bottom surface of the partition wall 6. Reference numeral 8 is a measuring instrument for measuring the pressing force of the pressurizing means 7.

In the present embodiment, the cathode 1 and the reference electrode 20 are housed in the ventilation tubes 12 and 22, respectively. One end of the ventilation pipes 12 and 22 is immersed in the molten carbonate 3 and the other end is open to the outside of the reactor 5. A mixed gas containing _{O 2} and CO _{2 is provided around the cathode 1 and the reference electrode 20.} Is to be supplied. Reference numerals 13 and 23 indicate an air supply pipe for supplying the mixed gas, and reference numerals 14 and 24 indicate an exhaust pipe for exhausting the gas in the ventilation pipes 12 and 22 to the outside of the reactor 5. That is, the cathode 1 and the reference electrode 20 are in contact with the molten carbonate 3, oxygen and carbon dioxide.
An inert gas supply port 5a and an exhaust gas port 5b are provided above the reactor 5 so that the reactor 5 is filled with the inert gas. _{Reference numeral 9 is an exhaust pipe that exhausts CO 2} generated at the anode 2 during power generation to the outside of the reactor 5.

As the molten carbonate ₃ , one or more selected from the group consisting of _{Li 2} CO ₃ , Na ₂ CO 3, and K ₂ CO _{3 is preferable.}
In terms of freezing point depression, a ternary mixture of _{Li 2} CO ₃ and Na ₂ CO ₃ and K ₂ CO _{3 or a binary} mixture of Li 2 CO ₃ and K ₂ CO ₃ or Li ₂ CO ₃ and Na ₂ CO _3. The mixture is more preferred.

As the granular solid carbon 4, activated carbon, coal or a carbide derived from biomass is preferable. Activated carbon is preferable because it does not contain impurities (ash).
The average particle size of the solid carbon 4 is preferably 20 to 2000 μm.
Here, the average particle size of the granular solid carbon is the mass average diameter by the sieving method.

Examples of the material of the anode 2 include gold (Au) and nickel (Ni). Nickel is preferable in terms of catalytic activity and cost.
Examples of the material of the cathode 1 include gold (Au), Li-doped nickel oxide, and the like.
The partition wall 6 is preferably formed of a material having pores that allow the molten carbonate 3 to pass through but not the solid carbon 4 to pass through. For example, a porous material having an average pore diameter of 100 to 500 nm is preferable. Here, the method for measuring the average pore diameter is the mercury press-fitting method.
As the material of the partition wall 6, porous alumina is preferable from the viewpoint of stability against molten salt.

As the inert gas to be filled in the reactor 5, argon (Ar), nitrogen (N ₂ ) or the like can be used.
_{As the mixed gas containing O 2} and CO ₂ supplied around the cathode 1 and the reference electrode 20, a mixed gas of O ₂ and CO ₂ or CO ₂ and air can be used.

FIG. 2 is a plan view of the anode 2, and FIG. 3 is an explanatory view schematically showing the vicinity of the anode 2 during power generation. Reference numeral 32 in FIG. 3 indicates a three-phase interface (reaction site) of carbon / electrode / molten carbonate.
In the present embodiment, the anode 2 has a plate shape, is arranged so that the thickness direction thereof is vertical, and the granular solid carbon 4 is pressed against the lower surface of the anode 2 by the pressurizing means 7. By pressing the solid carbon 4 to the anode 2 in this way, the anode 2 and the solid carbon 4 can be brought into contact with each other more reliably.
Further, as shown in FIG. 2, the anode 2 of the present embodiment is provided with through holes 2a that penetrate from the front surface to the back surface concentrically. The arrangement of the through hole 2a is not limited to this, and can be changed as appropriate.
_{During power generation, CO 2} gas (bubbles), which is a product, is generated from the three-phase interface (reaction site) 32 between the lower surface of the anode 2 and the solid carbon 4 and the molten carbonate 3 _{(C + 2CO 3} ^2- → 3CO ₂ + 4e). ^- ). In the conventional direct carbon fuel cell, since the anode is inserted into the slurry of solid carbon and molten salt to generate electricity, bubbles cover the anode during continuous power generation, and the continuous formation of reaction sites is suppressed. .. In the present embodiment, as shown in FIG. 3, in order to generate electricity in a state where the anode 2 having the through hole 2a is pressed from above with respect to the slurry of the solid carbon 4 and the molten carbonate 3, the bubbles 31 which are products are generated. Is discharged above the anode 2 through the through hole 2a. As a result, the formation of a three-phase interface (reaction site) is promoted in the lower part of the anode 2, and the power generation performance is improved.
On the other hand, if the area of the lower surface of the anode 2 (the opening surface of the through hole 2a) is significantly reduced by providing the through hole 2a in the anode 2, the power generation performance is rather deteriorated. It is preferable to set the size and number of 2a.

The diameter of the through hole 2a on the lower surface of the anode 2 (the opening surface of the through hole 2a) is smaller than the particle size of the solid carbon 4, and the solid carbon 4 does not pass through the through hole 2a. The diameter of the through hole 2a is preferably 1/3 or less of the average particle size of the solid carbon 4.
The distance from one opening of the through hole 2a to the other opening (thickness of the anode 2 in this embodiment) is preferably 0.05 to 1 mm.

In the present embodiment, the particle size of the solid carbon 4 and the solid carbon 4 existing in the compartment where the solid carbon 4 is sandwiched, that is, the compartment surrounded by the lower surface of the anode 2, the bottom surface of the partition 6 and the inner wall of the partition 6. When the total volume is constant, the number of contact sites between the anode 2 and the solid carbon 4 can be increased by increasing the pressing force of the pressurizing means 7. When the solid carbon 4 is in the most densely packed state in the compartment, the power generation performance does not change even if the pressing force is further increased.
Further, the number of the contact sites can be increased by reducing the particle size of the solid carbon 4 and increasing the number of the solid carbon 4.

For example, the pressing force of the pressurizing means 7 is preferably ^{30 to 100 kN / m 2 per unit area of the lower surface of the anode 2.}
The ratio of the solid carbon 4 to the total mass of the section in which the solid carbon 4 is sandwiched (the total of the molten carbonate 3 and the solid carbon 4 inside the partition wall 6) (hereinafter referred to as the solid carbon filling rate). Is preferably 0.5 to 2.0% by mass.

<Modification example>
In the present embodiment, the plate-shaped anode is arranged so that its surface is in the horizontal direction, and solid carbon is pressed against the lower surface of the anode, but bubbles that hinder the formation of the three-phase interface pass through the holes of the anode. The structure may be any structure to be removed, the surface of the anode may be inclined, and the solid carbon may not be pressure-welded to the lower surface of the anode. It is preferable that the reaction site is present on the lower surface of the anode in that bubbles generated at the reaction site of the anode can easily escape.
In the present embodiment, the solid carbon is sandwiched between the lower surface of the anode and the bottom surface of the partition wall, but the configuration is not limited to this as long as the solid carbon can be pressure-welded to the anode. Further, the shape of the anode does not have to be plate-shaped, and may be any shape as long as it can form a through hole for allowing air bubbles to escape. For example, the inside of the porous electrode may be filled with solid carbon.
The solid carbon 4 may be present only inside the partition wall 6 and may be partially present in the molten carbonate above the anode as well as below the anode.
In the present embodiment, the pressurizing means is provided separately from the anode, but solid carbon may be pressure-welded to the anode without using the pressurizing means.

Further, since the contact between the anode and the solid carbon can be more reliably performed by pressure-welding the solid carbon to the anode, the structure may be such that the granular solid carbon is pressure-welded to the anode having no through hole. According to this configuration, it is possible to improve the power generation performance as compared with the case where granular solid carbon is dispersed in the molten carbonate in the vicinity of the anode having no through hole as in Comparative Example 1 described later. it can. Further, the power generation performance can be further improved by providing a gap in the vicinity of the anode through which bubbles generated at the three-phase interface (reaction site) escape.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.
(Example 1)
A direct carbon fuel cell having the configuration shown in FIG. 1 was prepared, and the change over time of the voltage was measured in the constant current mode. The results are shown in FIG.
The material of the anode 2 was gold (Au). The shape of the anode 2 was a disk shape having a diameter of 14.5 mm and a thickness of 0.5 mm, and 25 holes having a diameter of 0.5 mm were provided concentrically as shown in FIG. The cathode 1 and the reference electrode 20 are made of gold (Au). As the partition wall 6, a porous alumina tube (inner diameter of 15 mm) having an average pore diameter of 120 nm was used. The material of the reactor 5 was quartz.
As the molten carbonate 3, a mixture of 16.9 mol% of _{Li 2} CO ₃ , 25.0 mol% of _{Na 2} CO ₃ , and 58.1 mol% of _{K 2} CO _{3 was used.} The wall surface temperature of the electric furnace 5c was set to 800 ° C. As the solid carbon 4, activated carbon having an average particle size of 1.0 mm was used, and the solid carbon 4 was set to 1% by mass with respect to the molten carbonate 3 in the electric furnace 5c.
Ar gas was supplied into the reactor 5, and the molten carbonate 3 in the partition wall 6 was stirred by bubbling with Ar gas. _{A mixture gas having an O 2} / CO ₂ (volume ratio) of 1/2 was supplied around the cathode 1 and the reference electrode 20.
The pressure applied by the pressurizing means 7 was 70 kN / m ² per unit area of the lower surface of the anode 2. Under pressure, the volume of the section surrounded by the lower surface of the anode 2, the bottom surface of the partition wall 6, and the inner wall of the partition wall 6 is 880 mm ³ , and the filling rate of solid carbon 4 in the section is 1.0% by mass. Met.

(Comparative Example 1)
In the configuration shown in FIG. 1, the pressurizing means 7 was not provided, and an air supply pipe (not shown) was provided so that the molten carbonate 3 in the partition wall 6 could be agitated by bubbling with an inert gas. The anode 2 was a plate-shaped gold (Au) electrode. The average particle size of the solid carbon was 17 μm. The solid carbon and the molten carbonate 3 in the partition wall 6 were stirred by bubbling with Ar gas in the same manner as in Example 1 to promote the contact of the solid carbon with the anode 2.
In the same manner as in Example 1, the change over time of the voltage was measured in the constant current mode. The results are shown in FIG.

As shown in the results of FIGS. 4 and 5, the power generation characteristics of Example 1 were improved as compared with Comparative Example 1. Specifically, in Comparative Example 1, it took about 5 minutes for the voltage to reach a steady state, and the voltage fluctuated with time. On the other hand, in Example 1, the voltage quickly reached a steady state, no fluctuation was observed, and stable power generation could be realized. Further, in Comparative Example 1, it was difficult to generate power of ^{25 mA / cm 2} or more, but in Example, it was possible to generate power of ^{50 mA / cm 2 or more.}
The anode area in Example 1 was 1.0 cm ² , and the anode area in Comparative Example 1 was 1.6 cm ² , and the power generation characteristics were determined based on the anode area.

1 Cathode 2 Anode 2a Through hole 3 Molten carbonate (electrolyte)
4 Solid carbon (fuel)
5 Reactor 5a Inert gas supply port 5b Exhaust port 5c Electric furnace 6 Partition 7 Pressurizing means 8 Measuring instrument 9 Exhaust pipe 11 Wiring 12 Ventilation pipe 13 Air supply pipe 14 Exhaust pipe 20 Reference electrode 21 Wiring 22 Ventilation pipe 23 Air supply pipe 24 Exhaust pipe 31 Bubbles 32 Three-phase interface

Claims (4)

An electrolyte consisting of molten carbonate and
The cathode in contact with the electrolyte, oxygen and carbon dioxide,
The anode in contact with the electrolyte and
Granular solid carbon present in the electrolyte near the anode,
A partition wall provided between the anode and the cathode, which allows the electrolyte to pass through and does not allow the solid carbon to pass through .
It has a pressurizing means for pressurizing the anode in the direction toward the partition wall.
At least a part of the anode is provided with a through hole through which the solid carbon does not pass .
A direct carbon fuel cell, wherein the solid carbon is sandwiched between an opening surface of the through hole and a part of the partition wall, and the solid carbon is pressure-welded to the opening surface of the through hole. The direct carbon fuel cell according to claim 1, wherein the partition wall has a bottomed tubular shape, and the bottom portion of the partition wall is immersed in the electrolyte. The direct carbon fuel cell according to claim 2, wherein the anode is located inside the partition wall. Claims 1 to 3 wherein the average particle size of the solid carbon is 20 to 2000 μm, and the diameter of the through hole on the opening surface of the through hole is 1/3 or less of the average particle size of the solid carbon. The direct carbon fuel cell according to any one item.

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