JPH1032010A - Phosphoric acid fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To retain phosphoric acid in a matrix even if the amount of phosphoric acid is decreased, retard decrease in cell characteristics caused by cross leak, and stably operate a fuel cell for a long time. SOLUTION: In a unit cell constituted by interposing a matrix 1 containing phosphoric acid between a fuel electrode 4A comprising a fuel electrode catalyst layer 2A, a carbon paper sheet 21 supporting the catalyst layer 2A, and a reservoir plate with rib 22 and an air electrode 7A comprising an air electrode catalyst layer 5, a carbon paper sheet 23 supporting the catalyst layer 5, and a reservoir plate with rib 24, the fuel electrode catalyst layer 2A is formed with a platinum-cobalt alloy catalyst.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a phosphoric acid fuel cell using phosphoric acid as an electrolyte, and more particularly to a catalyst used for a fuel electrode.

[0002]

BACKGROUND OF THE INVENTION Fuel cell high efficiency, since almost no generation of pollution-causing substances, such as NO _X, a large gas as a future oil alternative energy. Above all, the phosphoric acid type fuel cell using phosphoric acid as the electrolyte is at a stage near the most practical use. FIG. 3 is a perspective view schematically showing a configuration of a single cell which is a basic structural unit of the phosphoric acid type fuel cell. As can be seen, the single cell is a matrix 1 containing phosphoric acid.
Is sandwiched between a fuel electrode 4 and an air electrode 7, and the fuel electrode 4 serving as an anode is provided with a fuel electrode catalyst layer 2 on a fuel electrode base material 3,
The air electrode 7 serving as a cathode is formed by binding the air electrode catalyst layer 5 to the air electrode base material 6. Also, the fuel electrode base material 3
The air electrode base material 6 is provided with a fuel gas flow groove 8 for gas supply and an air flow groove 9. The main component of the matrix 1 is silicon carbide (SiC), and the main components of the fuel electrode base material 3 and the air electrode base material 6 are graphite, all of which are porous materials. The thickness of the formed single cell is several mm, and the length of one side is 600 to 1000 mm. In this configuration, when a fuel gas containing hydrogen is supplied to the fuel gas flow groove 8 and air is supplied to the air flow groove 9, the supplied gas permeates by diffusion to reach the catalyst layer of each electrode, and the catalyst and the electrolytic solution. The following reaction occurs at the interface with the liquid, and current flows to the external circuit.

[0003]

## STR1 ## ^{_{anode; H 2 → 2H + + 2e}} - (1) a ^{cathode; 2H + + (1/2) O} 2 + 2e - → H 2 O (2) the output voltage of the single cell of the phosphoric acid fuel cell 0.6 Since the voltage is as low as about V, a large number of single cells are usually stacked and connected in series to increase the output voltage. FIG. 4 is a schematic view showing a laminated structure, in which the single cells 10 are laminated with a gas-impermeable separator 11 interposed therebetween, and a cooling plate 12 is inserted every four to five single cells 10 laminated. By passing cooling water through the inside, heat generated due to power generation is removed and used at a predetermined temperature.

The present inventors have studied the transfer of phosphoric acid inside a phosphoric acid fuel cell (Tanuma, Nishihara; 36th Battery Symposium Abstracts, 213 (1995), Tanuma,
Nishihara: Abstracts of 1995 Autumn Meeting of Electrochemistry, 197 (199
5), R. Tanuma, H. Nishihara; Proceedings of the 2nd
International Fuel Cell Conference, 417 (1996)
), Revealed that the phosphoric acid electrolyte moves from the air electrode to the fuel electrode, and that the driving force is generated by electroosmosis in the catalyst layers of the air electrode and the fuel electrode.

In these studies, a platinum catalyst using acetylene black as a carrier is used as a catalyst for a fuel electrode, and a platinum catalyst using acetylene black as a carrier or a platinum-based alloy using furnace black as a catalyst for an air electrode. A catalyst is used. Such a structure of the catalyst is a structure widely used so far. In the case of phosphoric acid type fuel cells, in general, the rate of the reaction of the air electrode ((2) in the above formula) is small and the cell reaction is governed. Therefore, various alloy catalysts have been studied to increase the catalytic activity of the air electrode. I have. On the other hand, the reaction of the fuel electrode ((1) in the above formula) proceeds much more easily than the reaction of the air electrode, and therefore, platinum alone is used as a catalyst for the fuel electrode.

[0006]

In order to put a phosphoric acid fuel cell into practical use, it is necessary to guarantee a life of 40,000 to 50,000 hours, but this is a major obstacle to achieving this goal. The problem is that phosphoric acid in the battery is reduced by evaporation and scattering. The conventional phosphoric acid fuel cell attempts to solve this problem by replenishing phosphoric acid from the outside. However, there is no means for accurately measuring the amount of phosphoric acid in the cell, and as shown in FIG. In the configuration in which the plurality of single cells 10 exist between the cooling plates 12, the temperature of the cell located in the center between the cooling plates 12 is high, and the temperature of the cells adjacent to the cooling plate 12 is low. Since the rate of decrease also varies depending on the cell, it has been virtually impossible to grasp the amount of phosphoric acid scattered in each cell and replenish phosphoric acid without excess or deficiency for a long period of time. Therefore, as the amount of phosphoric acid in the battery becomes insufficient, the gas penetrates (cross leaks) through the matrix, causing a situation in which the cell voltage decreases. As the amount of phosphoric acid decreases, the decrease in cell voltage tends to accelerate, which is a major factor in shortening the battery life.

An object of the present invention is to maintain phosphoric acid in a matrix even when the amount of phosphoric acid in the entire cell decreases as described above, to suppress a decrease in cell voltage due to cross leak, and to operate stably for a long period of time. To provide a phosphoric acid type fuel cell that can be used.

[0008]

In order to achieve the above object, according to the present invention, there is provided a fuel electrode comprising a matrix containing a phosphoric acid electrolyte, and a catalyst layer sandwiched between the matrix and a catalyst layer bound to a surface in contact with the matrix. And a phosphoric acid fuel cell having an air electrode, wherein the catalyst layer of the fuel electrode comprises an alloy catalyst composed of platinum and another transition metal, for example, an alloy catalyst composed of platinum and cobalt, or composed of platinum, cobalt and iron. An alloy catalyst is used.

In the conventional phosphoric acid type fuel cell, the driving force applied to the phosphoric acid at the fuel electrode acts in a direction to attract the phosphoric acid from the matrix to the fuel electrode side, and the phosphoric acid moves to the fuel electrode and is moved to the fuel electrode. It is thought that phosphoric acid is deficient and cross leak occurs. Therefore, if the driving force to the phosphoric acid at the fuel electrode acts in the opposite direction to the above, that is, in the direction of driving from the fuel electrode to the matrix, deficiency of phosphoric acid in the matrix is suppressed, and the battery performance due to cross leak is reduced. Is significantly suppressed.

FIG. 5 is an exploded cross-sectional view showing the configuration of a small phosphoric acid fuel cell used by the present inventors in an experiment for examining the transfer of phosphoric acid. 11 is a matrix, 15 is an air electrode catalyst layer formed by forming a platinum catalyst using acetylene black as a carrier into a film using polytetrafluoroethylene as a binder, and 16 is carbon paper which binds and supports the air electrode catalyst layer 15 The catalyst layer base material, 17 is an air passage 9
An air electrode base material, 12 is a fuel electrode catalyst layer formed by forming a platinum-cobalt alloy catalyst using furnace black as a carrier into a film using polytetrafluoroethylene as a binder,
Reference numeral 13 denotes a catalyst layer base material made of carbon paper that binds and supports the fuel electrode catalyst layer 12, and 14 denotes a fuel electrode base material having a fuel gas passage 8. Reference numeral 18 denotes a carbon plate disposed in contact with the matrix 11, and has a hole so that the matrix 11 and the fuel electrode catalyst layer 12 come into contact with each other. Reference numeral 19 denotes a fluororesin sheet inserted to prevent gas cross leak. In this configuration, for example, when a driving force in the fuel electrode direction is applied to phosphoric acid in the air electrode catalyst layer 15, the carbon plate 18 impregnation rate of phosphoric acid becomes larger than the impregnation rate of the catalyst layer base material 16 on the air electrode side. Then, a difference occurs between the water absorption pressures of the two. Therefore, the driving force of the air electrode catalyst layer 15 can be estimated by obtaining the difference in water absorption pressure between the carbon plate 18 and the catalyst layer base material 16 from the impregnation ratio in the equilibrium state. Similarly, the driving force of the fuel electrode catalyst layer 12 can be evaluated from the difference in water absorption pressure between the catalyst layer base material 13 on the fuel electrode side and the carbon plate 18.

FIG. 6 is a characteristic diagram in which the driving force acting on phosphoric acid at the fuel electrode and the air electrode measured by the above method is plotted against the current density. In calculating the water absorption pressure when calculating the driving force, the surface tension was 7.275 × 10 ^-2 [N /
m] and the contact angle was 57 °. The vertical axis in this figure represents the driving force [
kPa], and the direction from the air electrode to the fuel electrode is defined as the positive direction. As can be seen from the figure, the driving force generated at the air electrode is positive and the driving force generated at the fuel electrode is negative, and it can be seen that both phosphoric acids at both electrodes are receiving the driving force in the direction of the matrix.

As shown in the above experimental results, if the catalyst layer of the fuel electrode is formed using a platinum-cobalt alloy catalyst, phosphoric acid at the fuel electrode generates a driving force in the matrix direction. Even if the amount of phosphoric acid decreases, phosphoric acid is retained in the matrix, and cross leak is suppressed. If the catalyst layer of the air electrode is formed using a platinum catalyst or a platinum-based alloy catalyst as in the past, phosphoric acid generates a driving force in the matrix direction even at the air electrode. Will be retained in the matrix.

In the small phosphoric acid type fuel cell shown in FIG. 5, a platinum-cobalt-iron alloy catalyst is used for the anode catalyst layer 12 instead of the platinum-cobalt alloy catalyst. It has been recognized that phosphoric acid generates a driving force in the direction of the matrix, and platinum-cobalt-
Even if an iron alloy catalyst is used, as in the case of the platinum-cobalt alloy catalyst, even if the phosphoric acid in the cell decreases, phosphoric acid is retained in the matrix, and cross leak is prevented.

[0014]

FIG. 1 is an exploded perspective view schematically showing the structure of a single cell of a first embodiment of a phosphoric acid fuel cell according to the present invention. The single cell of this configuration includes the matrix 1 containing phosphoric acid, the fuel electrode 4A, and the air electrode 7A. The fuel electrode 4A comprises a fuel electrode catalyst layer 2A, carbon paper 21 as a base material thereof, and a ribbed reservoir plate 22 for storing replenishing phosphoric acid.
Consists of a cathode catalyst layer 5, a carbon paper 23 as a base material thereof, and a reservoir plate 24 with ribs for storing replenishing phosphoric acid. In the ribbed reservoir plates 22 and 23, fuel gas flow grooves 8 and air flow grooves 9 are formed in the gaps between the ribs. As the air electrode catalyst layer 5, similarly to the conventional air electrode catalyst layer, a platinum catalyst supported on acetylene black and formed into a film using polytetrafluoroethylene as a binder is used. On the other hand, the anode catalyst layer 2A includes
A platinum-cobalt alloy catalyst supported on furnace black and formed into a film using polytetrafluoroethylene as a binder is used.

In the fuel electrode catalyst layer 2A of this configuration, as shown in the experiment of the small-sized phosphoric acid type fuel cell in FIG. Even if the amount of acid decreases, it acts to hold phosphoric acid inside the matrix 1. Further, in the air electrode catalyst layer 5, a driving force is generated for phosphoric acid in the direction of the matrix 1 in the same manner as in the prior art, and acts to hold phosphoric acid in the matrix 1. Therefore, if the single cell is configured as in this configuration, the leakage of phosphoric acid from the matrix 1 is suppressed, and phosphoric acid is retained in the matrix even if the amount of phosphoric acid in the entire cell decreases. Therefore,
A decrease in cell voltage due to cross leak is suppressed, and stable operation can be performed for a long time.

FIG. 2 is an exploded perspective view schematically showing the structure of a single cell of a second embodiment of the phosphoric acid type fuel cell according to the present invention. The difference between the present configuration and the configuration of the first embodiment shown in FIG. 1 is that, instead of the platinum-cobalt alloy catalyst, a ternary alloy catalyst composed of platinum-cobalt-iron is used, and this is supported on acetylene black, The fuel electrode 4B is constituted by using the fuel electrode catalyst layer 2B formed in a film shape using polytetrafluoroethylene as a binder. As described above, in the fuel electrode catalyst layer using the platinum-cobalt-iron alloy catalyst, similarly to the fuel electrode catalyst layer using the platinum-cobalt alloy catalyst, phosphoric acid is directed toward the matrix 1 in the direction of the matrix 1. Since a driving force is generated to act to hold phosphoric acid inside the matrix 1, the same effect as in the first embodiment can be obtained.

[0017]

As described above, according to the present invention, according to the present invention, there is provided a matrix comprising a phosphoric acid electrolyte, and a phosphor having a fuel electrode and an air electrode sandwiching the matrix and having a catalyst layer bound to a surface in contact with the matrix. In an acid fuel cell, an alloy catalyst composed of platinum and another transition metal is used for a catalyst layer of an anode,
For example, since an alloy catalyst composed of platinum and cobalt, or an alloy catalyst composed of platinum, cobalt and iron was used, in both the air electrode and the fuel electrode, phosphoric acid tends to move in the matrix direction. Phosphoric acid is retained in the matrix even if the amount of phosphoric acid decreases in the entire cell. Therefore, the cell voltage hardly decreases due to the cross leak, and a long-life phosphoric acid fuel cell can be obtained.

[Brief description of the drawings]

FIG. 1 is an exploded perspective view schematically illustrating a configuration of a single cell of a first embodiment of a phosphoric acid fuel cell according to the present invention.

FIG. 2 is an exploded perspective view schematically showing a configuration of a single cell of a second embodiment of the phosphoric acid fuel cell according to the present invention.

FIG. 3 is a perspective view schematically showing a configuration of a single cell which is a basic structural unit of the phosphoric acid type fuel cell.

FIG. 4 is a schematic view showing a stacked structure of a phosphoric acid type fuel cell.

FIG. 5 is an exploded cross-sectional view showing a configuration of a small phosphoric acid fuel cell used in an experiment.

FIG. 6 is a characteristic diagram in which driving force acting on phosphoric acid measured using a small phosphoric acid type fuel cell is plotted against current density.

[Explanation of symbols]

 Reference Signs List 1 matrix 2A fuel electrode catalyst layer (platinum-cobalt alloy catalyst) 2B fuel electrode catalyst layer (platinum-cobalt-iron alloy catalyst) 4A fuel electrode 4B fuel electrode 5 air electrode catalyst layer 7A air electrode 8 fuel gas flow groove 9 air Flow groove 21 Carbon paper 22 Reservoir plate with ribs 23 Carbon paper 24 Reservoir plate with ribs

[Procedure amendment]

[Submission date] November 6, 1996

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0002

[Correction method] Change

[Correction contents]

[0002]

2. Description of the Related Art Fuel cells have high efficiency and hardly generate pollutants such as NOx, so that they are expected to be used as an alternative to petroleum in the future. Above all, the phosphoric acid type fuel cell using phosphoric acid as the electrolyte is at a stage near the most practical use. FIG. 3 is a perspective view schematically showing a configuration of a single cell which is a basic structural unit of the phosphoric acid type fuel cell. As can be seen, the single cell is a matrix 1 containing phosphoric acid.
Is sandwiched between a fuel electrode 4 and an air electrode 7, and the fuel electrode 4 serving as an anode is provided with a fuel electrode catalyst layer 2 on a fuel electrode base material 3,
The air electrode 7 serving as a cathode is formed by binding the air electrode catalyst layer 5 to the air electrode base material 6. Also, the fuel electrode base material 3
The air electrode base material 6 is provided with a fuel gas flow groove 8 for gas supply and an air flow groove 9. The main component of the matrix 1 is silicon carbide (SiC), and the main components of the fuel electrode base material 3 and the air electrode base material 6 are graphite, all of which are porous materials. The thickness of the formed single cell is several mm, and the length of one side is 600 to 1000 mm. In this configuration, when a fuel gas containing hydrogen is supplied to the fuel gas flow groove 8 and air is supplied to the air flow groove 9, the supplied gas permeates by diffusion to reach the catalyst layer of each electrode, and the catalyst and the electrolytic solution. The following reaction occurs at the interface with the liquid, and current flows to the external circuit.

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0005

[Correction method] Change

[Correction contents]

In these studies, a platinum catalyst using acetylene black as a carrier is used as a catalyst for a fuel electrode, and a platinum catalyst using acetylene black as a carrier or a platinum-based alloy using furnace black as a catalyst for an air electrode. A catalyst is used. Such a structure of the catalyst is a structure widely used so far. In the phosphoric acid type fuel cell, generally, the rate of the reaction of the air electrode ((2) in the above formula) is small and the rate of the cell reaction is limited. Therefore , various alloy catalysts are being studied to increase the catalytic activity of the air electrode. . On the other hand, the reaction of the fuel electrode ((1) in the above formula) proceeds much more easily than the reaction of the air electrode, and therefore, platinum alone is used as a catalyst for the fuel electrode.

Claims (3)

[Claims] 1. A phosphoric acid type fuel cell comprising a matrix containing a phosphoric acid electrolyte, a fuel electrode having a catalyst layer bound to a surface in contact with the matrix, and an air electrode, wherein a catalyst layer of the fuel electrode is provided. A phosphoric acid-type fuel cell characterized by using an alloy catalyst comprising platinum and another transition metal. 2. The phosphoric acid fuel cell according to claim 1, wherein said alloy catalyst is an alloy catalyst comprising platinum and cobalt. 3. The phosphoric acid fuel cell according to claim 1, wherein the alloy catalyst is an alloy catalyst comprising platinum, cobalt and iron.