KR100195080B1 - Electrolyte matrix for phosphoric acid type fuel cell - Google Patents

Abstract

Disclosed is a matrix using SiC whisker crystals and metal oxides as electrolyte holding agents of an electrolyte matrix for phosphoric acid fuel cells. In other words, the electrolyte matrix for a phosphate fuel cell of the present invention is characterized by impregnating a phosphate electrolyte into a matrix plate composed of silicon carbide (SiC) whisker crystals, a metal oxide, and a hydrophobic binder. In addition to excellent heat resistance and excellent electrical insulation, the wettability and retention of phosphoric acid are improved, thereby improving the performance and long life characteristics of the fuel cell employing the same.

Description

Electrolyte Matrix for Phosphoric Acid Fuel Cell

1 is a schematic cross-sectional view illustrating the operation principle of a conventional phosphoric acid fuel cell.

* Explanation of symbols for main parts of the drawings

1 matrix 2 anode

3: cathode 4: separator

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrolyte matrix for phosphoric acid fuel cells, and more particularly, to an electrolyte matrix for phosphoric acid fuel cells in which whisker crystal silicon carbide and metal oxide are employed as an electrolyte retention agent, thereby increasing affinity and retention for phosphoric acid. .

A fuel cell is a new power generation system that converts energy generated by reacting fuel gas and oxidant gas directly into electric energy. Since electrical energy is generated by electrochemical reactions, not by combustion, etc., there are advantages of low generation of environmental pollutants, no noise, and high thermal efficiency, while evaporation of electrolytes and deterioration of materials due to operation at high temperatures. It has the same disadvantage. Such fuel cells include power generation facilities, power supplies for aerospace stations, power supplies for unmanned facilities at sea or coast, power supplies for fixed or mobile radios, power supplies for automobiles, power supplies for home appliances, and power supplies for leisure. We are interested in reviewing.

Dividing the fuel cell, the molten carbonate electrolyte fuel cell operating at a high temperature (about 500 to 700 ℃). Phosphate electrolyte fuel cells operating near 200 ° C, alkali electrolyte fuel cells operating at room temperature up to about 100 ° C or lower, and solid electrolyte fuel cells operating at high temperature of 1000 ° C or higher.

The phosphoric acid fuel cell is a matrix containing an electrolyte (hydrogen electrode) and a cathode (oxygen electrode) and containing an electrolyte therebetween. The positive electrode and the negative electrode are coated on a carbon substrate having a large surface area with Teflon (polytetrafluoroethylene: PTFE), which is a platinum catalyst as a binder and a wet sealant, as an active electrode, and the electrolyte matrix is formed of a phenol resin or a silicon carbide (silicon carbide). Concentrated phosphoric acid is retained in the matrix formed of phenol fibers combined with SiC) as an electrolyte providing a hydrogen cation transport medium.

1 is a schematic cross-sectional view illustrating the operation principle of a conventional phosphoric acid fuel cell. The unit cell is composed of a matrix (1) made of a porous insulating material containing phosphoric acid as an electrolyte, and a cathode (2) and a cathode (3), which are gas diffusion electrodes having a platinum-based catalyst coated on a carbon substrate by a hydrophobic material such as PTFE. The unit cells are stacked in plural through the separator 4 to form a battery. Hydrogen is supplied to the anode 2 and passed through the gas diffusion electrode to be separated into hydrogen cations and electrons by a platinum-based catalyst, and the hydrogen cations move through the electrolyte to the cathode 3 to which oxygen is supplied. In the cathode 3, which is also a gas diffusion electrode, electrons generated from the anode are transferred and oxygen is supplied through an external circuit, and hydrogen cation and oxygen react electrochemically to generate water. At this time, electrons flow to an external wire connecting the anode 2 and the cathode 3 to generate electricity.

In order to maintain a stable output for a long time, a phosphoric acid fuel cell operating with the above structure must stably retain phosphoric acid used as an electrolyte in a matrix so that the phosphoric acid can come into contact with the electrode catalyst layer to sustain an electrode reaction. Therefore, the electrolyte matrix should not be corroded to phosphoric acid at operating temperature, and should have good affinity and retention ability with phosphoric acid in order to have good ion conductivity and facilitate the movement of phosphoric acid within the porous structure of the matrix. It must also have a sufficiently high bubble pressure to prevent cross-over phenomena of the reactant gas and must be electrically insulated to prevent shorts between the positive and negative electrodes.

Conventionally, organic fibers such as phenol resins, materials obtained by combining silicon carbide (silicon carbide (SiC)) powder with a hydrophobic binder, or the like have been used as a matrix to hold an electrolyte in a phosphoric acid fuel cell. According to U.S. Patent No. 4,017,664, the problem of the phenolic resin matrix is that the reaction occurs between phosphoric acid and organic substances after a long time at a temperature higher than about 120 ℃, as a result of adsorption to the electrode catalyst to produce molecules that become catalyst poisons This will degrade electrode performance. On the other hand, using a matrix made from SiC particles and a hydrophobic binder, it is described that the above problems caused by the use of a phenolic resin matrix can be avoided. However, the matrix thus produced has the drawback that it does not hold the phosphate electrolyte completely satisfactorily.

U.S. Patent No. 4,493,879 discloses a fuel cell having a matrix having a superior phosphoric acid capacity than that made of silicon carbide. Here, the matrix for retaining the phosphate electrolyte contains at least one metal oxide and a binder which are electronically insulating and insoluble in phosphoric acid. Examples of the metal corals include SiO ₂ , ZrO ₂ , TiO ₂ , Al ₂ O ₃ , SnO ₂ , Examples of complex oxides containing selenium, phosphoric acid, or silicon in zirconium, such as Ta ₂ O ₅ , include fluoropolymers such as polytetrafluoroethylene, fluorinated ethylene-propylene copolymers, or phenol resins. It is described that this can be used.

However, the matrix produced using polytetrafluoroethylene (PTFE) as a binder as described above depends on the content of PFTE, and the addition of a small amount of hydrophobic PTFE leads to a weak bonding force resulting in a decrease in physical strength of the matrix. When the excess amount is added, the matrix becomes hydrophobic and the affinity with phosphoric acid is reduced, thereby degrading fuel cell performance.

Therefore, in order to improve the above problems, there is a need for an electrolyte matrix for a phosphate fuel cell having high chemical resistance, high heat resistance, and excellent affinity and retention for an electrolyte.

The electrolyte matrix for a phosphate fuel cell of the present invention for achieving the above object is characterized in that the phosphate electrolyte is impregnated into a matrix plate composed mainly of silicon carbide (SiC) whisker crystals, metal oxides and hydrophobic binders.

The present invention uses the silicon carbide and the metal oxide of the whisker crystal together with a binder to prepare a matrix plate to hold the phosphate electrolyte therein, thereby improving the affinity and retention of phosphoric acid while improving the physical and chemical strength of the matrix The castle was improved at the same time.

A whisker crystal refers to a thin, long needle-like single crystal that is several micrometers in size and several hundred micrometers in length. It is an ideal near perfect crystal with almost no structural defects, and is close to ideal strength in terms of mechanical strength. Intrinsic whiskers in which the same material as the material is naturally grown from the solid surface according to the manufacturing method, and non-intrinsic (growth accompanied by phase changes such as condensation of vapor phase, precipitation from solution, reduction of compounds, or pyrolysis). There are two kinds of whiskers. It can be obtained under controlled conditions from minerals and ceramics such as aluminum oxide, beryllium oxide, magnesium oxide, aluminum nitride, silicon nitride, silicon carbide, graphite and the like.

In the present invention, a whisker crystal having the above characteristics is preferably manufactured by using a silicon carbide whisker having an average length of 5 to 15 μm, thereby preparing a matrix for phosphoric acid electrolyte, thereby having a strong chemical resistance and thermal resistance to phosphoric acid as an electrolyte, and operating temperature. Chemically stabilized matrices can be prepared in.

In addition, it is preferable to use a metal oxide mixed with the silicon carbide whisker that is electrically insulating and capable of forming phosphate by reacting with phosphoric acid without being dissolved in phosphoric acid. Metal oxides capable of reacting with phosphoric acid to form phosphates are converted to phosphates in part or in whole while retaining the phosphate electrolyte in the matrix, which can further enhance phosphate retention because phosphates wet very well with phosphoric acid. This metal oxide with the phosphate can be formed include, for example, and the like _{SiO 2, ZrO 2, TiO 2} , Al 2 O 3, SnO 2, and Ta ₂ O _5.

Hereinafter, a method of manufacturing an electrolyte matrix for a phosphoric acid fuel cell of the present invention will be described in detail.

Polytetrafluoroethylene (PTFE), which is a binder, is mixed with a metal oxide such as SiO ₂ , ZrO ₂ , TiO ₂ , Al ₂ O ₃ , SnO ₂ , or Ta ₂ O ₅ , followed by mixing.

Silicon carbide whisker crystals are heated in an air atmosphere at about 800 ~ 900 ℃ for about 1 ~ 1.5 hours to remove impurities such as organic matter and water. A mixture of PTFE and a solvent is added to the heat treated silicon carbide whisker and mixed thoroughly by stirring.

The suspension of the metal oxide and the suspension of the silicon carbide whisker are thoroughly mixed using a stirrer or the like. The mixture of the fully mixed metal oxide and silicon carbide whisker is dried at a temperature of about 100 to 150 ° C. for about 5 hours to remove moisture. The dried mixture is molded into a plate having a thickness of about 0.2 to 0.4 mm using a roller.

The molding is impregnated in 105% phosphoric acid for about 1-1.5 hours. The matrix impregnated with phosphate electrolyte is heat treated at a temperature of about 300 to 500 ° C. for about 30 to 60 minutes to strengthen the structure, and then gradually cooled to prepare an electrolyte-matrix impregnated with phosphate electrolyte.

Hereinafter, the present invention will be described in more detail with reference to the following examples. However, the following examples are only examples of the present invention, and the present invention is not limited thereto.

EXAMPLE

After adding PTFE to pure water and mixing, SiO ₂ was added thereto and stirred to disperse, and the mixing ratio of SiO ₂ to PTFE was 9: 1 by weight.

SiC whisker crystals are heated in an air atmosphere at about 800 to 900 ° C. for about 1 to 1.5 hours to remove impurities such as organic matter and water. After PTFE was added to pure water and mixed, the heat-treated SiC whisker crystals were added and dispersed by ultrasonic stirring. The mixing ratio of SiC whisker and PTFE was set to 8: 2 by weight.

After mixing the same amount of the SiO ₂ dispersion and the dispersion of SiC whisker with each other in the same amount and stirred thoroughly for about 1 to 1.5 hours at 1500 ~ 3000rpm using a stirrer to completely mix. The mixture is dried for about 5 hours at a temperature of about 100 ~ 150 ℃ to remove moisture. The dried mixture is molded into a plate of about 0.2 to 0.4 mm thick using a roller.

The molding is impregnated in 105% phosphoric acid for about 1-1.5 hours. The matrix impregnated with the phosphate electrolyte is heat treated at a temperature of about 300 to 350 ° C. for about 30 to 60 minutes to strengthen the structure, and then gradually cooled to prepare an electrolyte-matrix impregnated with the phosphate electrolyte of the present invention.

As described above, the electrolyte matrix for a phosphate fuel cell of the present invention in which a phosphate electrolyte is impregnated into a matrix plate including silicon carbide (SiC) whisker crystal, a metal oxide, and a hydrophobic binder as a main component, has a high chemical resistance and a high heat resistance to the phosphate electrolyte. In addition to excellent electrical insulation, the wettability and retention of phosphoric acid are also improved, thereby improving performance and long life characteristics of the fuel cell employing the same.

Claims (3)

An electrolyte matrix for a phosphate fuel cell, wherein a phosphate electrolyte is impregnated into a matrix plate composed of silicon carbide (SiC) whisker crystals, metal oxides, and hydrophobic binders. The electrolyte matrix for a phosphate fuel cell according to claim 1, wherein the silicon carbide whisker crystal is a crystal having an average length of 5 to 15 µm. The phosphoric acid of claim 1 or 2, wherein the metal oxide is at least one selected from the group consisting of SiO ₂ , ZrO ₂ , TiO ₂ , Al ₂ O ₃ , SnO ₂ , and Ta ₂ O ₅ . Electrolyte matrix for fuel cells.