JPS6232576B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a fuel cell, and particularly to a fuel cell having a pair of gas diffusion electrodes and a matrix that holds phosphoric acid as an electrolyte (hereinafter referred to as a phosphoric acid electrolyte holding matrix). Figure 1 shows the basic structure of a fuel cell that uses hydrogen as the fuel, oxygen in the air as the oxidizing agent, and phosphoric acid as the electrolyte. 1 is the anode (gas diffusion electrode), 2 is the cathode (gas diffusion electrode). 3 indicates a matrix for holding a phosphoric acid electrolyte, and the anode 1 is in contact with hydrogen and the cathode 2 is in contact with air. Traditionally, phosphate electrolyte retention matrices include
Fiber cloth made of phenolic resin or a mixture of silicon carbide powder and polytetrafluoroethylene, an organic binder, were used. When using phenolic resin fiber cloth as a matrix material, if the operating temperature of the fuel cell exceeds 130℃,
The phenolic resin gradually carbonizes, making long-term operation impossible. To eliminate this problem,
It is possible to lower the operating temperature of the battery, but
In this case, a new problem arises in that the output of the battery is significantly reduced. Furthermore, when using a mixture of silicon carbide powder and polytetrafluoroethylene, which is an organic binder, as the matrix material, there is an advantage that the operating temperature of the fuel cell can be raised to about 190 to 200 degrees Celsius. Organic binders are generally water repellent and, inter alia,
Fluorine resins such as polytetrafluoroethylene have a drawback of being highly water repellent and lacking in affinity with phosphoric acid. As a result, the ability to retain phosphoric acid in the electrolyte retention matrix (phosphate retention capacity)
is small, and as a result, there was a problem that the performance of the fuel cell deteriorated. The present invention eliminates these problems and makes it possible to provide a fuel cell having an electrolyte retention matrix that is stable against high-temperature phosphoric acid and has a large phosphoric acid retention capacity, and which includes a pair of gas diffusion electrodes and , a fuel cell having a matrix holding phosphoric acid as an electrolyte, characterized in that the matrix is made of a porous body consisting of solid particles that do not react with phosphoric acid and a metal phosphate binder. The conditions that a phosphoric acid electrolyte holding matrix must meet are: (1) it must be stable to phosphoric acid at high temperatures (190 to 200°C), (2) it must be an insulator with no electronic conductivity, and (3) it must be an insulating material with no electronic conductivity. (4) It has a high affinity with acids and a large retention capacity, and (4) it has a large pore volume that can contain phosphoric acid. The present invention is the result of various studies on matrices for retaining phosphoric acid electrolytes that satisfy these conditions. It uses various inorganic substances such as chemical compounds. For example, oxides include composite oxides containing zirconium as a main component, such as zircon and tantalum pentoxide, carbides include silicon carbide and tantalum carbide, and tides include boron nitride. However, when using silicon carbide,
Above all, good results are obtained. Further, as a result of studying binders for fixing these powders, it became clear that various phosphates are suitable as binders. especially,
Phosphates of zirconium, tin, titanium, silicon, and aluminum are preferred; these substances have a high affinity with phosphoric acid and are highly effective as binders. These phosphates are prepared in advance.
For example, it is mixed into silicon carbide, or mixed with silicon carbide in the form of an oxide or salt solution, and then phosphoric acid is added and reacted to form a phosphate. In this invention, the latter method is particularly preferable, and has a large effect as a binder. Examples will be described below. Example (1) An aqueous solution of a chloride or hydroxide of zirconium, tin, titanium, silicon or aluminum was added to and mixed with silicon carbide powder having an average particle size of 0.3 μm, and phosphoric acid was further added thereto. The mixing ratio of silicon carbide and zirconium, tin, titanium, silicon, or aluminum salt solution is
70% by weight, and the phosphate produced by the reaction of the metal salt and phosphoric acid mentioned above should be 30% by weight, and the amount of phosphoric acid is in addition to the amount required to make the phosphate.
Additionally, 60% by weight was added to silicon carbide. Then, these mixtures were heated at 200°C for 15 hours to complete the reaction and remove moisture, yielding highly viscous granules. This highly viscous material is applied onto the gas diffusion electrode to form an electrolyte retention matrix, but this matrix contains free phosphoric acid in an amount of 60% by weight based on the aforementioned silicon carbide. , this free phosphoric acid will act as an electrolyte in the fuel cell. Table 1 shows the electrolyte retention matrix obtained in this example and the conventional method, that is, mixing 5% by weight of polytetrafluoroethylene with silicon carbide, and then adding 60% by weight of phosphoric acid.

【table】

[Table] Comparison of viscosity of phosphoric acid electrolyte retention matrices with % added. The viscosity was measured by applying a load of 50 kg to 5 g of a phosphoric acid electrolyte holding matrix on a flat plate, and examining the spread area. Table 1 shows that by using an inorganic binder, it is possible to obtain binding properties comparable to those of polytetrafluoroethylene. In the phosphoric acid electrolyte holding matrix of the example, one having a phosphoric acid holding power three times or more compared to the conventional matrix was obtained. Example (2) To 25 g of silicon carbide used in Example (1), 12.2 g of zirconium oxychloride (ZrOCl ₂ 8H ₂ O) was added to 50 g of water.
ml of the solution was mixed, 11 g of phosphoric acid was further added, and the mixture was heated at 200° C. for 15 hours to prepare an electrolyte retention matrix. This matrix was applied onto a known gas diffusion electrode to a thickness of 0.3 mm.
The other gas diffusion electrode was stacked and integrated to assemble a fuel cell. The gas diffusion electrode is made by coating carbon powder carrying a small amount of platinum on carbon paper. Figures 2 and 3 respectively show the current density-voltage characteristics of a fuel cell using the phosphoric acid electrolyte retention matrix of this example and the 200mA/cm ²
The change in battery voltage over time during continuous discharge at a current density of
This ^is shown in comparison with the characteristics of a fuel cell using polytetrafluoroethylene as a binder in silicon carbide. The discharge time (h) and the voltage (V) are plotted on the vertical axes of both figures, with A and C showing the example, and B and D showing the conventional example. As is clear from FIGS. 2 and 3, when the phosphoric acid electrolyte retention matrix of the example is used, there is no drop in voltage due to increase in current density or drop in voltage due to elapse of discharge time. This results in better performance compared to the conventional case. This performance improvement is based on the fact that the phosphate salt used as the binder has good affinity with phosphoric acid and has a large phosphoric acid retention ability. moreover,
Figures 2 and 3 show the results at an operating temperature of 190°C, and show that stable results can be obtained even at high temperatures when the phosphoric acid electrolyte holding matrix of the example is used. In addition, in Example (2), an example was shown in which a zirconium salt was used to generate the phosphate electrolyte holding matrix, but the same effect can be obtained by using a salt solution of tin, titanium, silicon, or aluminum. Can be done. As described above, the fuel cell of the present invention is stable against high-temperature phosphoric acid and has an electrolyte retention matrix that has a large phosphoric acid retention capacity, and has great industrial effects. It is what it is.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view showing the basic structure of a fuel cell having a phosphoric acid electrolyte holding matrix;
3 and 3 are characteristic diagrams showing the characteristics of an embodiment of the fuel cell of the present invention in comparison with the characteristics of a conventional fuel cell. 1... Anode, 2... Cathode, 3... Phosphoric acid electrolyte holding matrix.

Claims (1)

[Scope of Claims] 1. A fuel cell having a pair of gas diffusion electrodes and a matrix holding phosphoric acid as an electrolyte, wherein the matrix is a porous structure consisting of solid particles that do not react with phosphoric acid and a metal phosphate binder. A fuel cell characterized by consisting of a body. 2 The metal phosphate is zirconium, tin,
The fuel cell according to claim 1, wherein the fuel cell is a metal phosphate of at least one metal selected from the group consisting of titanium, silicon, and aluminum. 3. The fuel cell according to claim 1 or 2, wherein the solid particles are silicon carbide.