JPH0326507B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Technical Field of the Invention The present invention relates to an interelectrode pressure differential control device for controlling an interelectrode pressure differential between an oxidizer electrode and a fuel electrode of a fuel cell.

FIG. 1 is a perspective view showing the basic structure of a fuel cell. 1 is an electrolyte matrix in which a phosphoric acid electrolyte is held in a layer made of a nonwoven fabric of phenolic fibers or fine particles of silicon carbide. An oxidizer electrode 2a and a fuel electrode 2b are arranged to sandwich the electrolyte matrix 1 from both sides. These two electrodes are made of carbon paper or the like, have a catalyst such as platinum adhered to the surface in contact with the electrolyte matrix 1, and have a water-repellent treatment applied to the back surface.

A cell composed of the electrolyte matrix 1, the oxidizer electrode 2a, and the fuel electrode 2b is called a single cell and is usually manufactured in an integrated manner.

In order to form a fuel cell stack by stacking these single cells, carbon plates 3a and 3b are arranged so as to sandwich the single cells.

Gas channels 4a and 4b are formed in the carbon plates 3a and 3b for supplying an oxidizer and fuel to the oxidizer electrode 2a and fuel electrode 2b, respectively.

The electrode reaction of a fuel cell is a reaction in the coexistence of fuel gas (or oxidant gas), catalyst, and electrolyte.
In other words, it is an interfacial reaction in the coexistence of three phases: gas, solid, and liquid. That is, under normal operating conditions, H ₂ in the fuel flow flowing through the flow path 4b becomes H ⁺ on the surface of the fuel electrode 2b, which moves within the electrolyte matrix 1 to the oxidizer electrode 2a, and is transferred to the oxidizer electrode 2a. 2
O ₂ in the oxidant flow flowing in the flow path 4a on the surface of a
H ₂ O (water) is generated by reacting with O ^-- generated from

At this time, electrons (e ^- ) are emitted through the external circuit.
flows from the fuel electrode 2b to the oxidizer electrode 2a, and DC power is obtained. In order to maximize the performance of such a fuel cell, the three-phase interface must be maintained and controlled carefully. Controlling the differential pressure becomes an extremely important issue.

Moreover, the oxidizer electrode 2a and fuel electrode 2b that generally constitute a single cell are made of extremely brittle and easily damaged materials such as carbon paper, and it is necessary to maintain and control the three-phase interface during electrode reactions. Therefore, the interelectrode pressure difference between the oxidizer electrode 2a and the fuel electrode 2b must be controlled extremely carefully.

Furthermore, when the interelectrode pressure difference between the two electrodes increases, a crossover occurs in which fuel or oxidant leaks into another flow path, burns on the surface of the electrode, and consumes the fuel or oxidant. If this crossover occurs violently, there is a possibility that the electrode itself will burn, so from the viewpoint of maintaining battery performance, the occurrence of crossover must be prevented as much as possible.

Even if there is a small amount of crossover, there will be reactant gas that does not contribute to the cell reaction, which will reduce the fuel usage efficiency and oxidant usage efficiency, and the overall efficiency of the fuel cell will also decrease. do.

Therefore, in a fuel cell, it is necessary to improve the overall efficiency by preventing this crossover as much as possible and increasing the fuel usage efficiency and the oxidant usage efficiency.

(2) Prior Art FIG. 2 is a block diagram schematically showing a conventional control device for interelectrode pressure difference of a fuel cell.

A fuel cell having a stacked structure as described in FIG. 1 is housed in the storage container 5, and the surrounding area is filled with an inert gas 8.

Oxidizing gas is supplied to the oxidizing agent flow path 4a from the oxidizing agent supply path 9 and is discharged to the exhaust path 10.

Fuel gas is supplied to the fuel flow path 4b from the fuel supply path 11 and is discharged to the exhaust path 12.

The inert gas 8 in the storage container 5 is supplied from an inert gas supply flow path 17, and usually an inert gas such as argon or nitrogen is used.

The pressure of the oxidant gas flowing in the oxidant flow path 4a is measured by the differential pressure controller 13 based on the pressure of the inert gas 8.

Similarly, the pressure within the fuel flow path 4b is also measured by the differential pressure controller 14 with reference to the inert gas 8. Differential pressure controllers 13 and 14 control pressure regulating valves 15 and 16 provided in discharge passages 10 and 12, respectively, so as to achieve a preset pressure difference.

(3) Problems with the conventional technology In such a conventional interelectrode differential pressure control device, there is no circuit that can change the set values of the differential pressure controllers 13 and 14 according to the situation so as to eliminate crossover as much as possible. Since this is not provided, it is not possible to maintain optimal pressure control at all times in response to changes in the characteristics of the fuel cell.

As mentioned above, the basics of fuel cell pressure control is how to accurately maintain and control the three-phase interface, but from a macroscopic perspective, the fuel gas or oxidant gas is connected to the electrodes or electrolyte matrix. The purpose is to prevent it from penetrating through the inside and leaking out into other channels. Since the electrodes and electrolyte matrix have viscous resistance, even slight fluctuations in the pressure difference between the electrodes do not immediately affect crossover. However, the viscous resistance changes greatly depending on the temperature, and furthermore, the concentration of the electrolyte in the electrolyte matrix changes due to long-term operation, and the value of the viscous resistance may change over time. In order to carry out the control, it is necessary to control the interelectrode differential pressure so as to be able to respond to these changes.

(4) Summary of the Invention An object of the present invention is to provide a fuel cell interelectrode differential pressure control device that is effective in minimizing crossover in a fuel cell.

(5) Structure of the Invention In the present invention, in order to achieve the above object, the concentration of carbon dioxide in the discharge flow path is detected, and the interelectrode pressure difference is controlled according to this detected value.

(6) Embodiments of the invention Hereinafter, embodiments of the invention will be described in detail based on FIG. 3. In FIG. 3, the same parts as those shown in FIG. 2 are denoted by the same reference numerals. FIG. 3 is a configuration diagram of a fuel cell interelectrode differential pressure control device showing an embodiment of the present invention. A conventional device shown in FIG. 2 includes a carbon dioxide concentration meter 18 and a converter 1 that processes a signal from the carbon dioxide concentration meter 18 and converts it into a signal that changes the differential pressure setting value in the differential pressure controller 14.
9 is added.

Next, the operation of the embodiment shown in FIG. 3 will be explained. Under normal operating conditions, chemical reactions occur at the interface between the electrolyte matrix 1 and the oxidizer electrode 2a and at the interface between the electrolyte matrix 1 and the fuel electrode 2b. No carbon dioxide is released into the exhaust channel 10.

However, as mentioned above, when the pressure difference between the electrodes increases and crossover occurs, the oxidizer electrode 2a
Alternatively, a combustion reaction occurs on the fuel electrode 2b, reducing the catalytic activity.

Furthermore, when such a combustion reaction occurs, the carbon in the oxidizer electrode 2a or the fuel electrode 2b is burned, and as a result, carbon dioxide is generated and the oxidant flow path 4a is
Or it is released into the fuel flow path 4b.

Then, the carbon dioxide concentration meter 18 detects the amount of carbon dioxide in this flow path, and operates to change the differential pressure setting value in the differential pressure controller 14 via the converter 19. If the pressure difference between the fuel gas in the fuel flow path and the inert gas 8 in the storage container 5 changes in this way, the pressure difference between the oxidizer electrode 2a and the fuel electrode 2b changes accordingly. It's going to change.

When the detected value of the carbon dioxide concentration meter 18 is large, the converter 19 must be operated to reduce the pressure difference between the oxidizer electrode 2a and the fuel electrode 2b, thereby reducing crossover.

Note that when the pressure at the fuel electrode is greater than the pressure at the oxidizer electrode, the combustion reaction due to crossover occurs on the surface of the oxidizer electrode 2a, and in the opposite case, the combustion reaction occurs at the surface of the fuel electrode 2b. Occur.

Therefore, it is desirable that the carbon dioxide concentration meter 18 be arranged so that the carbon dioxide concentration can be detected no matter which electrode crossover occurs.

By the way, even in the case where the combustion reaction as described above is not involved, crossover detection is possible using the following principle. In other words, gas obtained from petroleum through processes such as steam reforming is generally used as fuel for fuel cells, and its composition is 80% hydrogen.
%, and carbon dioxide is about 20%. On the other hand, air is generally used as the oxidizing agent, and the carbon dioxide concentration in the air is extremely low, at 0.1% or less. Therefore, when the fuel penetrates the electrolyte matrix 1 and enters the oxidizer flow path 9 due to the occurrence of crossover, the carbon dioxide concentration in the oxidizer flow path 9 decreases because carbon dioxide does not react with the oxidizer. To increase. Therefore, by measuring the concentration of carbon dioxide in the oxidant discharge path 10 that discharges the oxidant from the oxidant flow path 9 with the carbon dioxide concentration meter 18, the degree of crossover can be determined. Conversely, when the oxidizer penetrates the matrix 1 and enters the fuel flow path 11, the carbon dioxide concentration in the fuel flow path 11 decreases. Therefore, the fuel flow path 11
The degree of crossover can also be determined by measuring the concentration of carbon dioxide in the fuel discharge path 12 from which fuel is discharged using a carbon dioxide concentration meter (not shown).

Therefore, in the embodiment shown in FIG. 3 when controlling the inter-electrode differential pressure via the converter 19, it is necessary to change the set value in the differential pressure controller 14 provided on the fuel electrode side. However, the same objective can also be achieved by controlling the set value in the differential pressure controller 13 provided on the oxidizer electrode side.

In the embodiment shown in FIG. 3, the carbon dioxide concentration meter 18 is arranged to detect the carbon dioxide concentration in the exhaust path 10, but this is because the crossover is from the fuel electrode 2b to the oxidizer electrode 2a. This is because it is assumed that the occurrence occurs in the direction.

(7) Effects of the Invention As explained above, in the present invention, when crossover occurs and a combustion reaction occurs on the oxidizer electrode or the fuel electrode, the carbon constituting the electrode burns, and as a result, carbon dioxide is released. The carbon dioxide concentration in the exhaust passage changes as carbon dioxide is generated and released into the exhaust passage, and the carbon dioxide in the fuel stream enters the oxidizer stream by crossover, causing a change in the carbon dioxide concentration in the exhaust passage. Focusing on changing the carbon dioxide concentration in the exhaust stream 10, we detected the carbon dioxide concentration in this exhaust path and controlled the interelectrode pressure between the oxidizer electrode and the fuel electrode. This has the excellent effect of minimizing crossover within the fuel cell.

Furthermore, since local heating due to crossover is less likely to occur, the life of the fuel cell is greatly extended.

Furthermore, there is the advantage that efficiency is improved because wasteful consumption of fuel and oxidizer due to the occurrence of crossover is eliminated.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing a schematic configuration of a fuel cell, FIG. 2 is a configuration diagram of a conventional interelectrode pressure differential control device for a fuel cell, and FIG. 3 is a configuration diagram showing an embodiment of the present invention. 1... Electrolyte matrix, 2a... Oxidizer electrode, 2b... Fuel electrode, 4a... Oxidizer channel, 4b
... Fuel flow path, 5 ... Storage container, 8 ... Inert gas, 9 ... Oxidizer supply path, 10 ... Oxidizer discharge path, 11 ... Fuel supply path, 12 ... Fuel discharge path,
13, 14... Differential pressure controller, 15, 16... Pressure control valve, 18... Carbon dioxide concentration meter, 19... Converter.

Claims (1)

[Scope of Claims] 1. At least one of the fuel flow path and the oxidizer flow path is a flow path to be controlled, and the pressure difference between the flow path to be controlled and the fuel cell storage container is controlled to a set value. a pressure control means; a carbon dioxide concentration detection means for detecting the carbon dioxide concentration in the detection target flow path, with at least one of the exhaust paths connected to the fuel flow path and the oxidizer flow path being a detection target flow path; An interelectrode differential pressure control device for a fuel cell, comprising: differential pressure set value control means for variably controlling the set value of the differential pressure control means to a value according to a detection signal from the carbon dioxide concentration detection means.