JPH0831332B2 - Method of operating molten carbonate fuel cell - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application] The present invention relates to a method for operating a molten carbonate fuel cell.

[Conventional technology]

A device for electrochemically converting the chemical energy of fuel in a fuel cell directly into electrical energy,
Development is also in progress for large-scale power sources of high-temperature fuel cells that operate at high temperatures of 300 ° C or higher. Among the high temperature fuel cells, the cell structure of a molten carbonate fuel cell using molten carbonate as an electrolyte is such that a pair of electrodes are arranged on both sides of an electrolyte plate impregnated with molten carbonate, and the outside of each electrode. A separator having a mechanism for supplying a reaction gas to each electrode is disposed in the fuel cell.This fuel cell is usually composed of a plurality of stacks, the stack is composed of a plurality of blocks, and the block is composed of a plurality of cells. .

The operating principle of the molten carbonate fuel cell is that the fuel gas and molten carbonate electrolyte undergo an electrochemical reaction through the holes of the anode, the generated electrons are taken out to the outside and work as electric energy, and then return to the cathode, Reacting with an oxidant gas containing oxygen gas and carbon dioxide gas to generate carbonate ions,
This carbonate ion is returned to the electrolyte.

An electrode reaction of a fuel cell is generally a reaction that occurs at a three-phase interface of a gas, a liquid, and a solid of a reaction gas, an electrolyte, and an electrode catalyst, and the performance of the cell is determined by the balance of the three types. You have to control the bunras.

Further, after the operation of the fuel cell, the voltage of the cell decreases as the current density increases, and this phenomenon is called cell polarization.
The polarization of a battery is represented by the difference between the cell voltage of the battery during operation and the equilibrium voltage when the load current is 0, and the cell voltage during operation is called the polarization voltage of the battery.

The polarization of the battery is expressed as the sum of the polarization of each electrode.

Now, when the voltage of the fuel cell shows a low value at the time of starting the cell, or when it gradually decreases after starting, it is necessary to know which electrode of the cell has a large polarization in order to investigate the cause. Will be needed.

As a method for separating and measuring the polarization of each electrode, a reference electrode is attached to a small cell of a molten carbonate fuel cell as described in the document "Battery Debates, 26th Annual Meeting Summary" pages 77-80. A method has been proposed in which the anodic polarization and the cathodic polarization are separately separated and measured by incorporating them and applying the AC impedance method.

The fuel cells used in this measurement were LiAlO ₂ and Li / KCO ₃
An anode made of a Ni material impregnated with LiAlO ₂ and a cathode made of NiO are arranged on both sides of an electrolyte plate made of a mixture of On the outside, a cell is configured by arranging a separator having a reaction gas supply groove,
The reference electrode of the hole formed in the separator on the anode side of the cell is connected to and held by the electrolyte.

This reference electrode is made of LiAl at the bottom of the double alumina tube.
A filling layer of a mixture of O ₂ and Li / KCO ₃ is provided, a filling layer made of NiO as a reference electrode is connected to the filling layer, and a gold wire is connected to the filling layer to lead to the outside. ing.

The reaction gas, 80% H ₂ -CO ₂ gas at the anode side of the battery, 70% air -CO ₂ gas on the cathode side, the reference electrode 70
% Air-CO ₂ gas is used, and impedance measurement of each pole is performed using a potentiostat and a frequency response measuring instrument.
Measurements are made at equilibrium potential.

However, this measurement method is effective electrode area 10.5cm ²
Since it was carried out on a small cell, and its measurement operation is also complicated, applying this method as it is to a fuel cell in which a large number of large cells are stacked requires measurement on a large number of cells. Therefore, the measuring operation becomes very complicated, and the measuring device becomes large in scale, making it practically impossible.

Therefore, when the fuel cell voltage in which a large number of large-sized cells are stacked is started or after the start time and the cell voltage drops and the polarization shows a value larger than the target value, the polarization of both the anode and the cathode, or one of the electrodes is Since it was not possible to determine whether the value was greater than the target value, it was not possible to take appropriate corrective action, which led to a decrease in the performance of the fuel cell and eventually a decrease in its life.

In the above-mentioned measuring method, since LiAlO ₂ powder which is an impurity is used as the electrolyte of the fuel cell in the tube of the reference electrode, there is a problem that the performance of the fuel cell is deteriorated.

Further, as for the operation method of the fuel cell, see JP-A-60-10
566, JP-A-60-189177, and JP-A-61-24166 propose operation methods for extending the life of fuel cells, but they are not related to the present invention.

[Problems to be solved by the invention]

In the conventional method, the method of measuring the cell voltage of a battery by separating it into anodic polarization and cathodic polarization is performed on a single small cell, and its measurement operation is complicated and requires a large-scale precision instrument. Therefore, applying this method as it is to a fuel cell in which a large number of large cells are stacked requires measurement on a large number of cells, which makes the measurement operation very complicated and requires a large measuring device. This makes it practically impossible.

Therefore, when operating a fuel cell in which a large number of large cells are stacked, if the cell voltage of the cell shows a value lower than the target value, the polarization of both electrodes of that cell or one of the electrodes of the cell will reach the target value. Since it is not known whether it is larger, the cause of the decrease in the cell voltage cannot be determined, and the corrective action cannot be taken, resulting in deterioration of the fuel cell performance.

An object of the present invention is to solve the above problem and identify an electrode whose polarization is larger than a target value in a fuel cell in which a large number of large cells are stacked, when the cell voltage of the battery shows a value smaller than the target value. While providing a fuel cell that can
It is an object of the present invention to provide a method for operating a fuel cell, which is provided with effective means capable of returning an electrode whose polarization is larger than a target value to a normal state during operation of the fuel cell.

[Means for solving problems]

That is, the present invention is provided in at least one cell of a fuel cell so as to be in contact with the electrolyte while being insulated from anode and cathode electrodes, and measuring the cell voltage by separating it into anode polarization and cathode polarization. When the fuel cell is in operation, the cell voltage is separated into anode polarization and cathode polarization from the reference electrode terminal for measurement, and when the cell voltage is smaller than the target value, As a polarization of the electrode, a part or all of the reaction gas of the electrode having a larger value than the target value is temporarily cut off to operate the fuel cell to achieve the intended purpose.

[Action]

Since at least one cell of the fuel cell holds the reference electrode terminal, it becomes a cell in combination with one of the electrodes of the cell, and the polarization of that electrode depends on the voltage of this cell and the reference potential of the reference electrode terminal. Therefore, by separately measuring the cell voltage into anodic polarization and cathode polarization, and by measuring the cell voltage into anodic polarization and cathodic polarization during operation of the fuel cell, the voltage of the cell is Electrodes with a polarization larger than the target value can be identified when they show a value smaller than the target value.For electrodes with a polarization larger than the target value, if some or all of the reaction gas is temporarily cut off under a load, The polarization of the electrode changes significantly, the surface of the electrode becomes appropriately wet with the electrolyte, and the reaction field at the three-phase interface increases and activation proceeds. Is the cell voltage is restored small.

〔Example〕

According to the present invention, by holding the reference electrode terminal in at least one cell of each stack or each block of the fuel cell, it is possible to separate and measure the polarization of the electrode under load, so that the fuel cell can be measured. If the target battery voltage cannot be obtained at the start of operation of the cell or gradually decreases, the polarization of the electrode is separated and measured as schematically shown in FIG. 3 to measure the decrease in the cell voltage. It is possible to determine whether the increase in the polarization of is the anode, the cathode, or both.

FIG. 3 (A) shows the current-potential characteristics of the cell, the horizontal axis is the current density, the vertical axis is the cell voltage, and when the cell voltage is normal, the solid line shows the current-potential characteristics 1. However, when the battery is deteriorated, the current-potential characteristic 1'indicated by the broken line is obtained, and it is shown that the polarization of the cell is excessively increased by the amount indicated by the arrow and is deteriorated.

On the other hand, FIGS. 3 (B1) to (B3) show the current-potential characteristics of each electrode with respect to the current-potential characteristics of the cell shown in FIG. 3 (A).
The current density is plotted on the horizontal axis and the electrode potential is plotted on the vertical axis.

In Fig. 3 (B1) to (B3), when the cell voltage is normal, the polarization of the anode and cathode is represented by the current-potential characteristics 2 and 3, but the polarization of the anode and cathode after deterioration of the battery is respectively It is represented by current-potential characteristics 2'and 3 '.

The figure (B1) shows the current-potential characteristics when the cathode polarization is large, the figure (B2) shows the cases where both the cathode polarization and the anode polarization are large, and the figure (B3) shows the current-potential characteristics when the anode polarization is large. From these figures, when the battery performance deteriorates and the cell voltage decreases, it is possible to easily determine which electrode of the cell has the polarization larger than the target value.

On the other hand, regarding the performance of the electrode of the fuel cell, in addition to the catalytic activity due to the physical properties of the electrode material, the reaction field is the three-layer interface where the electrode material, the electrolyte and the reaction gas come into contact with each other. The existence probability of the range, that is, the so-called wet state is an important point, and in the same sense, the kind of gas of the adsorbed species at the active point is also important at the interface between the gas and the solid. That is, when the reaction gas is temporarily shut off under a load, the surface state of the electrode changes and the activation progresses as the polarization of the electrode changes significantly.

Here, a molten carbonate fuel cell will be taken as an example of the fuel cell, and its reaction will be considered.

The anode ^{_{reaction, H 2 + CO 3 2- →}} CO 2 + H 2 O + 2e - cathodic reaction is ^{_{O 2 + 2CO 2 + 4e -}} → 2CO 3 is ^2.

When the electrocatalyst M of each of the anode made of a metal and the cathode made of an oxide is involved in this, hydrogen gas is excluded from the reaction system in the anode and carbon dioxide gas is excluded from the reaction system in the cathode, and the transfer of electrons is considered, the following formula is established.

The anode reaction is M + CO ₃ ^2- → CO ₂ + MO + 2e ^−, or the M + CO ₃ ^2- → M ²⁺ + CO ₃ ^2- + 2e ⁻ cathode reaction is M + O ₂ + 4e ⁻ → M + 2O ²⁻ .

As a result, when hydrogen gas is temporarily cut off at the anode, either oxidation or elution of the electrode surface or oxidation and elution occurs, which makes it easier for the electrode surface to become wet and causes carbonate ions to move from the cathode side. Is considered to be increased and activated. Further, when the carbon dioxide gas is temporarily cut off at the cathode, it is considered that the reaction field increases and the activation progresses due to the oxidation of the electrode surface and the movement of carbonate ions on the electrode surface toward the anode.

 Details of embodiments of the present invention will be described with reference to the drawings.

First Example As shown in FIG. 4, the block of the molten carbonate fuel cell used in this example is an electrolyte 6 formed by plate-forming an electrolyte composed of a mixed carbonate of LiCO ₃ and K ₂ CO _3. And its electrolyte 6
A pair of electrodes arranged on both sides of the separator, a separator 5 arranged outside each of the electrodes and having a groove for supplying a reaction gas, and a separator attached to the separator 5 for extracting the voltage of each cell Three cells each composed of the voltage terminal 7 are laminated.

The separator 5 at the upper end of the block 4 is provided with a reference electrode hole 8 for allowing the reference electrode 10 shown in FIG. 1 or 5 to reach the electrolyte 6.

The structure of the reference electrode 10 shown in FIG. 1 is such that a reference electrode tube 12 of an outer cylinder having a hole at its tip is provided with a reference electrode tube 12 of small diameter having an open tip, and the reference electrode terminal is provided inside thereof.
11 are arranged. This reference electrode 10 is inserted into the reference electrode hole 8 and brought into contact with the electrolyte 6 of the cell, and the electrolyte 6 of the same material as the electrolyte 6 of the cell is attached to the tip of the reference electrode tube 12 of the outer cylinder.
Is filled in to connect with the electrolyte 6 of the cell, the reference electrode terminal 11 is embedded in the filled electrolyte 6, and a mixed gas of carbon dioxide gas and oxygen gas (2/1 molar ratio) is used as the reference electrode gas in the figure. The gas supplied from the inner reference electrode tube 12 as indicated by the arrow in FIG.
It goes through 12 and goes out of the reference electrode 10.

The structure of the reference electrode 10 shown in FIG. 5 is such that the reference electrode terminal 11 is provided inside the reference electrode tube 12 having holes at the tip and the side surface, and the tip of the reference electrode terminal 11 is the electrolyte of the cell. 6 is in contact with the reference electrode 6, and the reference electrode gas is supplied from the upper portion of the reference electrode tube 12 and discharged from the tube through the hole on the lower side surface.

If the reference electrode 10 of this structure is installed in each cell,
As shown in the figure, a reference electrode hole 8 is provided in the upper separator 5 for the upper cell, and the separator 5 and the electrolyte 6 provided in each cell are partially outwardly provided for the other cells. Each case is shifted to make a business trip, a reference electrode hole 8 is provided in the tripped part, and a reference electrode guide ring 13 is provided in the separator 5 at the upper end corresponding to each hole, as shown in FIG. , The vertical and horizontal ratios of the separator 5 and the electrolyte 6 are changed, and the reference electrode holes 8 penetrating until reaching the electrolyte 6 of each cell are provided side by side along the lengthwise end, and the electrolyte 6 is left only in that portion There are cases of cutting or providing a hole penetrating to a target cell.

Next, FIG. 6 shows an example in which the structure of the reference electrode 10 is simplified. This is a case where a material, such as a platinum wire, which itself is invariable in an electrochemically used region is embedded in the electrolyte 6 of the cell as the reference electrode terminal 11.

If the electrolyte 6 is formed by stacking tape-shaped ones formed by the tape casting method, it is possible only by sandwiching between them.

However, in this case, the stability of the reference electrode as the potential reference is low because the potential of the reference electrode due to the reaction gas having a constant composition is lost, and the relative potential of the electrode of each cell is shown in FIG. 1 or FIG. It is inevitable that it will be lower than in the case of the reference electrode.

Second Example FIG. 2 shows current-potential characteristics measured with a reference electrode terminal held in a small cell of a molten carbonate fuel cell. The horizontal axis represents the cell current density, and the vertical axis represents the cell voltage and the electrode potential.

In the figure, the plots with ○ are the current-potential characteristics 1 ', 1 "of the cell, the squares are the current-potential characteristics 2', 2" of the cathode, and the triangles are the current-potential characteristics of the anode. 3 ', 3 ".

The dashed lines in the figure indicate that this small cell is heated to 650 ° C,
The electric potential after driving for 5 hours is shown. The current-potential characteristic is the result of measurement by separating it into anodic polarization and cathodic polarization by the reference electrode terminal 11 of the method of FIG. 6 according to the present invention.

In this method, since the reaction gas is not used as the reference electrode gas, the open circuit potential value has little reproducibility, but it can be evaluated as the polarization value.

From this measurement result, it is understood that this small cell has a large anode polarization and a low cell performance.

By temporarily shutting off hydrogen gas, which is a reaction gas of the anode of this small cell, the current-potential characteristic of the cell is 1 '.
To 3 ″ from 3 ′ to 3 ″.

FIG. 9 shows how the electric potentials of the anode and the cell change when the hydrogen gas of the anode of this small cell is temporarily cut off.

The horizontal axis of this figure shows the elapsed time, and the vertical axis shows the polarization of the cell and the anode. The polarization of a cell is represented by the decrease in the voltage of the cell from the open circuit voltage of the cell (in this case 1.10V), that is, the parallel voltage, that is, the polarization voltage of the cell, that is, the cell voltage. It is represented by the amount of decrease from the equilibrium potential to the polarization potential of the anode.

Curve A shows the polarization of the cell and curve B shows the polarization of the anode. When the load current is 150 mA / cm ² , shutting off the hydrogen gas on the anode side changes the interfacial state of the anode significantly, and re-supply of hydrogen gas reduces the polarization of the anode and also reduces the polarization of the cell. Is recovering its performance.

Figure 10 shows the performance characteristics of the cell when the hydrogen gas was shut off several times and the elapsed operating time was about 220 hours.
The horizontal axis of this figure is the time after the start of resupply of the interrupted hydrogen gas, and the vertical axis is the cell voltage.

Curve A shows a load current of 150mA / cm when the cell is put into operation.
^In the state of ² , the hydrogen gas was shut off, and the time course of the cell voltage within 2 hours after re-supply was shown. Curve B continued to run this cell and shut off the hydrogen gas for the second time and re-supply at 70 hours of operation. The time-dependent change of the cell voltage within 5 hours after the start, the curve C shows the time-dependent change of the cell voltage within 9 hours after starting the hydrogen gas interruption for the third time and starting the resupply at the 90th time. D shows the change with time of the cell voltage within 12 hours after starting the resupply at 185 hours after performing the fourth hydrogen gas cutoff, and curve E shows the fifth hydrogen gas cutoff at 210 hours. It shows the change with time of the cell voltage within 19 hours after starting the supply.

As can be seen from this figure, the rate of decrease of the cell voltage after the re-supply of the interrupted hydrogen gas is started is reduced each time the hydrogen gas is interrupted, and the stabilization of the cell voltage is well represented. Stable performance is achieved by repeating the operation.

Although the cell of this embodiment has a large anode polarization, it may have a large cathode polarization, resulting in poor cell performance. In that case, contrary to the case of this cell, the cathode polarization is reduced and the cell performance is restored by cutting off the carbon dioxide gas, which is the reaction gas of the cathode, several times. In addition,
It is necessary to take appropriate measures for such performance recovery, and even if the carbon dioxide gas of the cathode is cut off when the anode polarization is large, not only the performance is not recovered, but also the performance of the cathode is deteriorated. High risk. In this sense as well, it can be said that it is important to separate the battery voltage into anode polarization and cathode polarization to understand the problem.

〔The invention's effect〕

According to the configuration of the present invention, even in a fuel cell in which a plurality of cells are stacked, at least one cell of the fuel cell holds the reference electrode terminal, so that the voltage of the cell is changed to anodic polarization and cathodic polarization. Since it can be measured separately,
If the cell voltage is smaller than the target value, it is precisely known which electrode is larger, and at the time of operation of the fuel cell in which at least one cell holds the reference electrode terminal, if the voltage of the cell is smaller than the target value, the electrode By temporarily blocking part or all of the reaction gas of the larger electrode as the polarization of the electrode, either oxidation or elution of the electrode surface or oxidation and elution occurs, and the electrode surface is easily wetted appropriately to Since the field is increased and activated, the polarization of the electrode is reduced, the voltage of the cell is restored, the normal operating condition is restored, and the battery life is extended.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view of a reference electrode according to the present invention, and FIG. 2 is a current-potential characteristic diagram of a battery showing reduction of anodic polarization and recovery of cell voltage due to interruption of hydrogen gas according to the present invention. FIG. 3 is a diagram showing a model according to factors of polarization separation of cell voltage and cell voltage drop of the battery according to the present invention, and FIG. 4 is a diagram showing a configuration of one block of the battery according to the present invention. 5 is a vertical sectional view of a reference electrode having another structure according to the present invention, and FIG. 6 is a vertical sectional view of a simple reference electrode terminal.
FIG. 7 is a diagram showing a block configuration when a reference electrode is used for each cell, FIG. 8 is a diagram showing another block configuration when a reference electrode is used for each cell, and FIG. FIG. 10 is a diagram showing changes with time in anodic polarization and cell polarization after shutting off hydrogen gas and starting re-supply, and FIG. 10 shows changes in cell voltage lowering rate by shutting off hydrogen gas and starting re-supply several times. FIG. 5 …… Separator, 6 …… Electrolyte, 11 …… Reference electrode terminal.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hideo Okada, 4026 Kuji Town, Hitachi City, Hitachi, Ibaraki Prefecture, Hitate Works, Ltd. (72) Inventor, Kazuo Iwamoto 4026, Kuji Town, Hitachi City, Ibaraki, Nitate Works, Ltd. Hitachi Research Laboratory (72) Inventor Koichi Miyoshi 4026 Kuji Town, Hitachi City, Hitachi, Ibaraki Prefecture Hitachi Research Institute, Ltd. (72) Inventor Yuichi Kamo 4026 Kuji Town, Hitachi City, Ibaraki Hitachi Research Institute, Ltd. (56) References JP-A-62-31957 (JP, A) JP-A-57-77955 (JP, A) JP-A-61-264683 (JP, A)

Claims (1)

[Claims] 1. A separator having a molten carbonate electrolyte, a pair of electrodes disposed on both sides of the electrolyte, and a separator disposed outside each of the pair of electrodes and having a mechanism for supplying a reaction gas. In a method for operating a molten carbonate fuel cell comprising a stack of a plurality of fuel cell units, each of which comprises a stack of a plurality of fuel cell units, and at least one cell of the fuel cell is in an insulated state from the electrode and is in contact with the electrolyte. A reference terminal for measuring the cell voltage by separating it into anodic polarization and cathodic polarization is provided, and the cell voltage is separated into anodic polarization and cathodic polarization from this reference electrode terminal during operation of the fuel cell. If the cell voltage is smaller than the target value, the polarization of the electrode of the cell may be partially or completely removed from the reaction gas of the electrode larger than the target value. The method of operating a molten carbonate fuel cell is characterized in that so as to operate the fuel cell in the blocking state.