JPH08148172A - Phosphoric acid fuel cell - Google Patents

Abstract

PURPOSE: To reduce a drop in power generation characteristic due to the excessive wetting of a cathode electrode solvent layer by providing a reducing liquid feed means for feeding and keeping a reducing liquid to and with a cathode. CONSTITUTION: When a cell output drop is observed, due to the excessive wetting of a cathode solvent layer during the operation of a phosphoric acid fuel cell, the operation is temporarily stopped at an elevated temperature. In this case, the circulation of nitrogen gases to a cathode 3b is started to purge residual oxygen therefrom. After a sufficient amount of nitrogen gases is supplied to the cathode 3b, formalin mists generated with a mist generator 23 are fed thereto, using the nitrogen gases as carrier gases, thereby reducing the cathode 3b. Also, after reducing the cathode 3b, the operation of the fuel cell is resumed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a phosphoric acid fuel cell in which phosphoric acid is held on a porous electrode substrate.

[0002]

2. Description of the Related Art Generally, a fuel cell is known as an apparatus for directly converting chemical energy of fuel into electric energy. In this fuel cell, usually, a pair of porous electrodes are arranged with an electrolyte layer sandwiched between them, a fuel gas such as hydrogen is brought into contact with the back surface of one electrode, and an oxidant gas such as oxygen is contacted with the back surface of the other electrode. The electrodes are brought into contact with each other, and the electrochemical reaction generated at this time is used to extract electric energy from between the electrodes. In the fuel cell configured as described above, as long as the fuel gas and the oxidant gas are supplied, electric energy can be extracted with high conversion efficiency.

A fuel cell using phosphoric acid as an electrolyte will be described below as a typical fuel cell. FIG. 8 shows a structural example of a unit cell in a ribbed electrode type fuel cell using phosphoric acid as an electrolyte based on the above principle. In FIG. 7, an electrolyte layer 1 is obtained by impregnating a matrix with phosphoric acid as an electrolyte, and an anode electrode 3a and a cathode electrode 3 made of a porous carbon material sandwiching the electrolyte layer 1 therebetween.
b are arranged respectively. Catalyst layers 2a and 2b are coated on the sides of the anode electrode 3a and the cathode electrode 3b which are in contact with the electrolyte layer 1, respectively. A plurality of ribs 4a and 4b are formed on the back side of each of the electrodes 3a and 3b, and flow grooves 5a and 5b for flowing the fuel gas and the oxidant gas are formed between the ribs 4a and 4b, respectively. There is.

Here, the flow groove 5a for flowing the fuel gas
And the flow grooves 5b for circulating the oxidant gas are arranged in directions orthogonal to each other, and a plurality of these flow grooves 5a, 5b are formed. The unit cell thus constructed is sandwiched between the separators 6 made of dense carbonaceous material to form a unit cell stack.

Further, as shown in FIG. 9, the unit cell laminated body has a collector plate 7, an insulating plate 8 and a tightening plate 9 at the upper and lower ends thereof.
The terminals 10 and 10 are attached to each other, and the terminals 10 are assembled by being tightened from above and below with an appropriate tightening pressure. And, the gasket 1 is provided on the side surface side of the unit cell laminate described above.
Fuel gas and oxidant gas through pipes 1 respectively
A pair of fuel gas supply manifold 12 and a fuel gas discharge manifold 13, a pair of oxidant gas supply manifold 14 and an oxidant gas discharge manifold 15 for supplying / discharging through 6 and 16, respectively,
A fuel cell is constructed by tightening and fixing with an appropriate pressure.

[0006]

However, the above-mentioned conventional fuel cell has the following problems to be solved. That is, since the power generation of the fuel cell is based on the electrochemical reaction of the raw material gas, three elements of the raw material gas, the electrolyte, and the catalyst efficiently cause the electrochemical reaction to obtain good power generation characteristics. It is necessary to form a field that functions like this in the catalyst layer.

In the case of a phosphoric acid fuel cell, the process of advancing in the catalyst layer by the three elements of the raw material gas, the electrolyte and the catalyst is as follows after the raw material gas is dissolved in phosphoric acid which is the electrolyte.
Since it diffuses into the platinum particles that are the catalyst and causes an electrochemical reaction on the surface of the platinum particles, the catalyst layer needs to be wet to some extent with phosphoric acid that is the electrolyte.

On the other hand, when the catalyst layer is excessively wetted by phosphoric acid, phosphoric acid becomes a factor that hinders the diffusion of the raw material gas to the surface of the platinum particles (substantially, the raw material gas diffuses in the phosphoric acid. Therefore, the source gas supply rate to the platinum particle surface is significantly slower than the electrochemical reaction rate on the platinum particle surface, which is a so-called diffusion-controlled state. Such a state slows down the entire power generation reaction of the fuel cell, resulting in deterioration of power generation characteristics. That is, it appears in the form of an increase in the amount of diffusion polarization, which is one of the factors that deteriorate the characteristics during actual fuel cell operation. Therefore, wetting of the catalyst layer by phosphoric acid must be maintained at an appropriate level.

However, since the electric potential is generated when the fuel cell is operated, the wetting of the catalyst layer progresses with the operation. Therefore, even if the wetting of the catalyst layer is at an appropriate level in the initial stage of operation, the state of overwetting will be accompanied by the operation. This tendency is more remarkable in the cathode electrode catalyst layer in the high potential state than in the anode electrode catalyst layer in the low potential state.

Therefore, in the conventional fuel cell, as a means for solving such a problem, by improving the water repellency of the cathode electrode catalyst layer, the cathode electrode catalyst layer becomes excessively wet. Have been trying to prevent. However, if the water repellency of the cathode electrode catalyst layer is increased, the initial output voltage of the fuel cell tends to decrease, so from the aspect of output efficiency that should be achieved in the initial operation of the fuel cell plant, improvement of water repellency is prevented. There were limits to the measures.

The present invention has been made in view of the above circumstances, and provides a phosphoric acid fuel cell capable of avoiding a decrease in cell output due to excessive wetting of the cathode electrode catalyst layer during operation. With the goal.

[0012]

In order to achieve the above-mentioned object, it is indispensable to understand the progress mechanism of wetting of the electrode catalyst layer in the phosphoric acid fuel cell. Therefore, as a means for providing the present invention, the present inventors conducted a test for searching a mechanism for progressing wetting of the electrode catalyst layer as follows, and obtained certain findings from the results.

(1) Consideration on Mechanism of Wetting Progression of Electrode Catalyst Layer by Electrochemical Measurement Cathode Electrocatalyst Prepared by Constant Potential (750 mV) Electrolysis in High Temperature 207 ° C., Concentrated (105%) Phosphoric Acid at Different Wetting Degrees For the layer samples, cyclic voltammogram measurements were performed in the double layer region in dilute (1M) sulfuric acid at room temperature. In these cyclic voltammograms, as shown in FIG. 1, a pair of redox peaks derived from the formation of the surface functional group was observed, and it was found that the quinone / hydroquinone surface functional group was generated from the peak potentials. I understand. As shown in FIG. 2, there is a correlation between the redox peak electricity and the degree of wetting of the cathode electrode catalyst layer. large.

This indicates that a cathode electrode catalyst layer having a high degree of wetting has a large amount of quinone / hydroquinone surface functional groups produced. On the other hand, the surface structure of a site having a quinone / hydroquinone-based surface functional group is a structure in which a basic skeleton of carbon is accompanied by a carbonyl group or a hydroxyl group, which is a hydrophilic functional group, as shown in chemical formula 1. The site where they can be formed in the catalyst layer is only on the carrier carbon carrying the platinum particles which are the catalyst.

[0015]

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From the above, it can be seen that in the cathode electrode catalyst layer having a high degree of wetting, a large amount of hydrophilic functional groups is produced on the surface of the carrier carbon. In other words, the hydrophilicity of the cathode electrode catalyst layer increases due to the formation of hydrophilic functional groups on the surface of the carrier carbon, and as a result, the wetting of the cathode electrode catalyst layer proceeds. Therefore, the wettability of the cathode electrode catalyst layer can be reduced by removing the hydrophilic functional group generated on the surface of the carrier carbon in the cathode electrode catalyst layer.

(2) Measures for reducing the wettability of the electrode catalyst layer As described above, by removing the hydrophilic functional group generated on the surface of the carrier carbon in the cathode electrode catalyst layer, the cathode electrode catalyst layer The degree of wetting can be reduced.
Therefore, as a measure for removing the hydrophilic functional group on the surface of the carrier carbon, that is, the carbonyl group or the hydroxyl group, treatment with a reducing substance can be considered. FIG. 5 shows the relationship between the treatment time and the residual rate of the hydrophilic functional groups when the cathode electrode catalyst layer is immersed in an aqueous solution of formaldehyde, which is a reducing substance, that is, formalin. As can be seen from FIG. 3, the residual rate of hydrophilic functional groups is reduced by the formalin treatment. The chemical reaction formula in that case is considered to be shown in Chemical formula 2.

[0018]

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From the above, by treating the cathode electrode catalyst layer with formalin, the hydrophilic functional groups formed on the surface of the carrier carbon can be removed. That is, the degree of wetting of the cathode electrode catalyst layer can be reduced. Further, it is considered that the same effect can be obtained when an aldehyde other than formaldehyde is used as the reducing substance.

The present invention has been completed based on the above findings. That is, in the phosphoric acid fuel cell according to the present invention, when a decrease in cell output characteristic due to excessive wetting of the cathode electrode catalyst layer is observed during operation, the fuel cell operation is temporarily stopped while the temperature is raised and It is characterized in that a reducing liquid for removing hydrophilic functional groups generated on the surface of the carrier carbon in the electrode catalyst layer is supplied and brought into contact with the cathode electrode.

[0021]

According to the first aspect of the phosphoric acid fuel cell having the above-mentioned structure, the hydrophilic property generated on the surface of the carrier carbon in the cathode electrode catalyst layer is obtained by supplying and contacting the reducing liquid with the cathode electrode. Since the functional group is removed, the wettability of the cathode electrode catalyst layer is reduced. Therefore,
It is possible to avoid deterioration of battery output characteristics due to excessive wetting of the cathode electrode catalyst layer.

In the second aspect of the present invention, the reducing liquid supply means is a mist generator for generating a mist of the reducing liquid. Therefore, the reducing liquid mist is generated on the carbon surface of the carrier in the cathode electrode catalyst layer. Hydrophilic functional groups are easily reduced and removed.

In the third aspect, the reducing liquid supply means comprises the ejector containing the reducing liquid and the thin tube attached to the ejector for ejecting the mist of the reducing liquid. Liquid can be surely supplied to the cathode electrode, and reliability can be improved.

In the fourth aspect, since the reducing liquid is an aldehyde solution, it is easy to handle.

[0025]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 4 is a schematic view showing an electrode in the first embodiment of the phosphoric acid fuel cell according to the present invention. The same parts as those of the conventional configuration will be described with the same reference numerals as those in FIG. The overall structure of the phosphoric acid fuel cell is shown in FIG.
In the same manner as described in 1., a unit cell is formed by arranging an anode electrode and a cathode electrode with an electrolyte layer impregnated with phosphoric acid as an electrolyte in a matrix, and a unit cell stack body in which a plurality of unit cells are stacked is used as a fuel. A manifold for supplying and discharging gas and oxidant gas is attached.

As shown in FIG. 4, a fuel gas supply line 21 for supplying a fuel gas such as hydrogen to the anode electrode 3a.
On the other hand, an oxidant gas supply line 22 for supplying an oxidant gas such as oxygen is connected to the cathode electrode 3b. The oxidant gas supply line 22 is connected to a mist supply line 24 for supplying mist from a mist generator 23 as a reducing liquid supply means for generating mist of formalin by a piezoelectric element.

In order to supply and contact the mist of formalin from the mist generator 23 to the cathode electrode 3b, nitrogen gas is passed through the oxidant gas supply line 22 to purge the oxygen content of the cathode electrode 3b, and then the mist is generated. Bowl 23
To generate a mist of formalin and send the mist to the oxidant gas supply line 22.

Next, the operation of this embodiment will be described.

First, when the phosphoric acid type fuel cell is operated, when the deterioration of the cell output characteristic due to excessive wetting of the cathode electrode catalyst layer is observed, the operation of the fuel cell is temporarily stopped while the temperature rises. At that time, the flow of nitrogen gas to the cathode electrode 3b is started, and the oxygen content remaining in the cathode electrode 3b is purged. After the nitrogen gas is sufficiently circulated to the cathode electrode 3b, the mist of formalin generated by the mist generator 23 is supplied to the cathode electrode 3b by using nitrogen gas as a carrier gas to reduce the cathode electrode 3b. After the reduction process is completed, the operation of the fuel cell is restarted.

As described above, according to this embodiment, by supplying and contacting the cathode electrode 3b with the mist of formalin,
Since the hydrophilic functional groups generated on the surface of the carrier carbon in the cathode electrode catalyst layer can be reduced and removed, the hydrophilicity of the cathode electrode catalyst layer is reduced, and as a result, the wettability of the cathode electrode catalyst layer is reduced and the cathode electrode It is possible to avoid deterioration of power generation characteristics due to excessive wetting of the catalyst layer.

Further, since the formalin mist is supplied to and brought into contact with the cathode electrode 3b from the mist generator 23, the hydrophilic functional groups generated on the surface of the carrier carbon in the cathode electrode catalyst layer are easily reduced and removed.

FIG. 5 is a graph diagram comparing the output characteristics of the fuel cell according to the first embodiment of the present invention and the conventional fuel cell. As is clear from FIG. 5, the fuel cell adopting the present embodiment shows an improvement in the output characteristics corresponding to the reduction amount of the diffusion polarization after the reduction treatment of the cathode electrode, and is different from the conventional fuel cell in which the reduction treatment is not performed. In comparison, the cell output characteristics are better.

FIG. 6 is a schematic view showing an electrode in the second embodiment of the phosphoric acid fuel cell according to the present invention. The same parts as those in the first embodiment will be described with the same reference numerals. In this embodiment, the cathode electrode 3b is directly connected to the mist supply line 24 from the mist generator 23 for generating the mist of formalin.

Therefore, in this embodiment, in the same operation as in the first embodiment, the mist supply line 24, which is independently provided, uses the nitrogen gas as the carrier gas to generate the formalin mist generated by the mist generator 23 as the cathode. It is supplied to the electrode 3b, and the cathode electrode 3b is reduced.

By such an operation, the hydrophilic functional groups formed on the surface of the carrier carbon in the cathode electrode catalyst layer are reduced and removed, and the hydrophilicity of the cathode electrode catalyst layer is reduced. The degree of wetting is reduced, and the deterioration of power generation characteristics due to excessive wetting of the cathode electrode catalyst layer can be reduced. The other structure and operation are the same as those of the first embodiment, and the description thereof will be omitted.

7 (A) and 7 (B) are a schematic plan view and a schematic sectional view showing an electrode in the third embodiment of the phosphoric acid fuel cell according to the present invention. The same parts as those in the conventional example and the first embodiment will be described with the same reference numerals.

In this embodiment, a plurality of ejectors 25 containing formalin solution 26 are arranged in the vicinity of the cathode electrode 3b, and each of these ejectors 25 has a thin pipe 27 made of a fluororesin attached thereto. 27 extends so as to be inserted into the gas flow groove 5b of the cathode electrode 3b.

Therefore, in this embodiment, in the same operation as in the first embodiment, instead of supplying the formalin mist generated by the piezoelectric element to the cathode electrode,
The thin tube 27 made of fluororesin attached to the ejector 25 containing the formalin solution 26 is attached to the cathode electrode 3b.
It is inserted in the gas circulation groove 5b. The formalin of the ejector 25 is jetted through the thin tube 27 made of fluororesin and injected into the cathode electrode 3b.

By such an operation, the hydrophilic functional groups formed on the surface of the carrier carbon in the cathode electrode catalyst layer are reduced and removed, and the hydrophilicity of the cathode electrode catalyst layer is reduced.
As a result, the degree of wetting of the cathode electrode catalyst layer is reduced, and the deterioration of power generation characteristics due to excessive wetting of the cathode electrode catalyst layer can be reduced.

Further, since the thin tube 27 made of fluororesin is inserted into the gas flow groove 5b of the cathode electrode 3b,
Formalin can be reliably supplied to the cathode electrode 3b, and the reliability can be improved. The other structure and operation are the same as those of the first embodiment, and the description thereof will be omitted.

The present invention is not limited to the above embodiments, but various modifications can be made. For example, although formalin was used as the reducing liquid in each of the above-described examples, an aqueous solution of acetaldehyde may be used instead of this, and in short, an aldehyde solution may be used. Therefore, by using the aldehyde solution as the reducing liquid, there is an effect that the handling is easy.

[0043]

As described above, according to the first aspect of the phosphoric acid fuel cell of the present invention, the cathode is provided with the reducing liquid supply means for supplying and contacting the reducing liquid. The degree of wetting of the electrode catalyst layer is reduced, and deterioration of power generation characteristics due to excessive wetting of the cathode electrode catalyst layer can be avoided.

According to the second aspect, since the reducing liquid supply means is a mist generator for generating mist of the reducing liquid, the reducing liquid mist is generated on the carbon surface of the carrier in the cathode electrode catalyst layer. The hydrophilic functional group is easily reduced and removed.

According to the third aspect, the reducing liquid supply means comprises the ejector containing the reducing liquid and the thin tube attached to the ejector for ejecting the mist of the reducing liquid. The reducing liquid can be reliably supplied to the cathode electrode, and the reliability can be improved.

According to the fourth aspect, since the reducing liquid is an aldehyde solution, it is easy to handle.

[Brief description of drawings]

FIG. 1 is a diagram showing a cyclic voltammogram measurement in a double layer region of a cathode electrode catalyst layer.

FIG. 2 is a graph showing the relationship between the redox peak electricity quantity and the degree of wetting of the cathode electrode catalyst layer.

FIG. 3 is a graph showing the relationship between the formalin treatment time and the residual rate of hydrophilic functional groups.

FIG. 4 is a schematic view showing an electrode in the first embodiment of the phosphoric acid fuel cell according to the present invention.

FIG. 5 is a graph showing the output characteristics of the fuel cell of the first embodiment and the conventional fuel cell in comparison.

FIG. 6 is a schematic view showing an electrode in a second embodiment of the phosphoric acid fuel cell according to the present invention.

7 (A) and 7 (B) are a schematic plan view and a schematic cross-sectional view showing an electrode in a third embodiment of the phosphoric acid fuel cell according to the present invention.

FIG. 8 is a perspective view showing a cross-sectional structure of a unit cell of a conventional fuel cell.

FIG. 9 is an exploded perspective view showing a conventional fuel cell.

[Explanation of symbols]

 3a Anode electrode 3b Cathode electrode 21 Fuel gas supply line 22 Oxidant gas supply line 23 Mist generator (reducing liquid supply means) 24 Mist supply line 25 Ejector 26 Formalin solution (reducing liquid) 27 Fluororesin thin tube

Claims (4)

[Claims] 1. A unit cell is formed by arranging an anode electrode and a cathode electrode with an electrolyte layer impregnated with phosphoric acid serving as an electrolyte sandwiched in a matrix to form a unit cell stack. A phosphoric acid fuel cell for supplying and discharging an oxidant gas, comprising a reducing liquid supply means for supplying and contacting a reducing liquid to the cathode electrode. 2. The phosphoric acid fuel cell according to claim 1, wherein the reducing liquid supply means is a mist generator that generates a mist of the reducing liquid. 3. The reducing liquid supply means comprises an ejector containing a reducing liquid and a thin tube attached to the ejector for ejecting the mist of the reducing liquid. The phosphoric acid fuel cell described. 4. The phosphoric acid fuel cell according to claim 1, wherein the reducing liquid is an aldehyde solution.