JPH0449748B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention relates to a surface-treated electrode backed with a porous conductive sheet suitable for use as an electrode on the bromine active material side of a metal-bromine bipolar type stacked secondary battery with an electrolyte circulation type. The present invention relates to an excellent metal-bromine secondary battery that improves the efficiency of electrochemical reactions and stably maintains the efficiency by forming electrodes from a material that has a surface area. [Prior Art] Electrodes used in metal (e.g. Zn, Cd, Fe, Pb, etc.)-bromine batteries include conventional metal electrodes using noble metals such as platinum, and conductive materials such as plastics. Plastic electrodes kneaded with carbon or the like and having electrical conductivity, and carbon electrodes using carbon itself are used. First, metal electrodes are limited to noble metals, considering the corrosive nature of bromine generated during charging of this battery. When these metals are used for electrodes, the electrical resistance is extremely low and the voltage efficiency of the battery is also good. Furthermore, during discharge, the discharge time is long and the Coulombic efficiency is excellent. However, since precious metals are used, the cost is naturally high, so it is not suitable for practical use. Next, carbon electrodes also have the lowest electrical resistance value next to the metal electrodes mentioned above, but their mechanical strength is weak and cracks occur particularly due to impact resistance, which poses problems in their reliability. Furthermore, because carbon electrodes are generally porous, they are not suitable for bipolar series stacked battery systems in which the electrodes function as separators. Finally, plastic electrodes also have a satisfactory lifespan, but have a high electrical resistance, and in terms of Coulomb efficiency, the electrical resistance of the electrode surface is naturally high, so the electrode reaction resistance of the active material is also large, resulting in a problem that only low efficiency can be achieved. The present inventors previously proposed zinc-bromine as an electrode for the above-mentioned metal-bromine secondary battery in Japanese Patent Application No. 57-204460.
It has been found that a plastic electrode of a bromine battery, which is characterized by having a porous conductive sheet layer formed on its surface, improves the characteristics of the battery. [Purpose of the Invention] The present invention was developed because conventional metal, carbon, and plastic electrodes each had various problems in terms of economic efficiency, life efficiency, etc., and an electrode sufficiently applicable to metal (particularly zinc)-bromine batteries had not been found. In order to solve the conventional problems and obtain a practical electrode, we proposed a surface-treated electrode in which a conductive sheet is back-packed on the surface of a plastic electrode. The purpose is to obtain secondary batteries. [Summary of the Invention] The gist of the present invention is to provide an electrode formed by integrally molding a porous conductive plate-like body and a polyolefin resin plate-like body containing a conductive substance, the porous In a metal-bromine secondary battery in which the surface of the conductive plate is in contact with the electrolyte on the positive electrode side, the pore diameter of the porous conductive plate integrally formed with the electrode
This is a metal-bromine secondary battery characterized in that the amount of pores in the range of 3.0 nm to 8 μm is 0.1 cm ³ /g or more. Polyolefin is used as the material constituting the electrode substrate in the battery of the present invention because it is necessary to satisfy the condition that it is resistant to deterioration even if it is in contact with free bromine or ionic bromine for a long time. In addition, plastic electrodes have a density of, for example, 0.94g/
Electrodes (hereinafter referred to as CP) can be made by kneading 50 parts by weight of carbon black with 100 parts by weight of high-density polyethylene of cm ³ or more and press molding. The porous conductive plate-like material used in the present invention refers to a material that maintains the form of a felt-like, knit-like, or cloth-like fabric made of conductive carbon fibers made from polyacrylonitrile. The shape is advantageous for handling when forming a layer on the surface of a plastic electrode. In addition, this porous conductive plate-like material can be made by chemical treatment, etc.
It has micropores of 10 nm to several 100 nm, and its porosity is
Although it is made from carbon fibers or porous metal sheets with a carbon content of 2.0% or more, the present invention uses cross-shaped porous carbon fibers (hereinafter referred to as C-CC),
These are thermocompression bonded using a conventional method during frame molding of CP, and porous conductive sheet thermocompression bonded electrodes (hereinafter referred to as CC-
It's called CP. ) is preferably applied. As a result of research by the present inventor, the optimum specific surface area of C-CC-CP as a positive electrode for metal (Zn)-Br ₂ batteries is practically 100 m ² /g or more, preferably 300 m ³ /g.
As mentioned above, it was confirmed from the examples that the most preferable range is 500 to 600 m ² /g. In other words, metal-bromine batteries have a large amount of active material, bromine molecules (Br ₂ ), at the beginning of discharge, so there is no particular problem, but at the end of discharge, when the amount of active material, bromine molecules, decreases, the positive electrode Overvoltage (so-called concentration overvoltage) generated due to the delay in the movement of bromine molecules increases, resulting in a decrease in reactivity. Therefore, in order to provide an electrode that is highly reactive even at the end of discharge, it is necessary to provide an electrode that can trap bromine molecules well in the electrode area. Therefore, in the examples described later, metal-
100m ² /
g or more was confirmed. Therefore, in the case of other conductive sheets that can be buckled, as long as the specific resistance of the electrode obtained by buckling is 0.4Ωcm or less, the metal -
It can be said that it can be used as a positive electrode for Br ₂ batteries. Furthermore, as a porous conductive sheet, CC-
As a result of a comparative study of the specific surface area of CP and discharge level (Vm) characteristics depending on various current densities, it was found that as the porosity of C-CC differs, the specific surface area Ss of CC-CP also changes. The excellent ones show a large value of the specific surface area Ss, and it can be seen that there is some correlation between the discharge level characteristics and the electrode specific surface area. Next, carbon fibers made porous and thermocompressed onto the CP surface improve the discharge characteristics, but the pore volume (Po value) is 3.0 nm to 8 μm in pore diameter (D diameter). Must be 0.1 cm ³ /g or more within the range,
In particular, it has been clarified by the examples described below that the energy efficiency of a metal (for example, Zn)-bromine (Br ₂ ) battery of 70% can be satisfied when the density is 0.5 cm ³ /g or more.
In other words, it was shown that there is a correlation between the specific surface area of the surface of the porous conductive plate and the reaction activity at the end of discharge, but the amount of pores in the porous conductive plate (i.e., the diameter of the pores) distribution) and the discharge potential characteristics. Specifically, looking at the relationship between the pore size range and the discharge potential, there is a correlation between the distribution of pore diameters in the 3.0 nm to 8 μm range and the discharge potential, and an increase in the pore size within that range improves the bromine reaction. It was confirmed that it contributed the most. In addition, in order to obtain the best discharge level characteristics, within the above diameter range, up to 40mA/cm ² discharge is required.
It was confirmed that approximately 0.4 cm ³ /g or more is required, and next, in other porous conductive plate-like materials, those that are thermocompressed to CP as described above have a specific resistance of 0.4 Ωcm or less. Then, if the pore volume (Pv value) increases in the pore diameter (D diameter) range of 3.0 nm to 8 μm,
It can be said that the average potential is improved, making it effective as a positive electrode for metal- _Br2 batteries. Thus, in the D diameter range
This means that thermocompression electrodes can be selected based on the relationship between Pv value and Vm. Note that the specific surface area and pore content of an electrode integrally molded with a polyolefin resin plate are not the values of the porous conductive plate alone before integral molding, but the values of the porous conductive plate of the electrode after integral molding. These are the specific surface area and the amount of pores on the electrode surface on the side of the shaped body. The surface of the porous conductive plate that is in contact with the polyolefin resin plate does not directly contribute to the reaction with bromine, and the active surface may be destroyed or fall off due to mechanical action during integral molding. Therefore, in addition to the specific surface area of the electrode surface, the surface area and pore content of the porous conductive plate itself are described. The structure and effects of the present invention will be specifically explained below using Examples. [Embodiments of the invention] Example 1 High-density polyethylene 100 with a density of 0.94 g/cm ³ or more
Carbon plastic (hereinafter referred to as CP) made by kneading 50 parts of carbon black and press molding.
The specific surface area of the cross-shaped porous carbon fiber (hereinafter referred to as C-CC), which is a sheet of carbon fiber with micropores formed through chemical treatment, was determined by the BTE method by varying the size and amount of micropores. Ss ₀
Four types (A, B, C, D) of cross-form porous carbon fiber backing carbon plastics made by backing C-CC of different (m ² /g) onto CP by thermocompression bonding using the conventional framed electrode forming method. (hereinafter C-CC
−CP), the specific surface area Ss (m ² /
g) and then add 3 mol/g as the positive electrode electrolyte.
The average discharge potential Vm (V) (relative to the Ag-Agcl standard electrode) characteristics at various current densities were tested in ZnBr ₂ +Br ₂ . The results are shown in Table 1 below.

[Table] Table 1 shows the specific surface area of the above four types of C-CC,
The specific surface area of the same type of carbon plastic and their C-CC-
Current density 10, 20, 30 at CP Br ₂ 0.4-1.0mol/
Discharge average potential at 40 and 50mA/ ^cm2 discharge
It shows V _n10 , V _n20 , V _n30 , V _n40 , and V _n50 . From this Table 1, the larger the specific surface area of C-CC itself, the larger the specific surface area of the buckled C-CC-CP, and accordingly the higher the average discharge potential as the positive electrode of the Zn-Br ₂ battery. It is clear that there are The reason why the concentration of Br ₂ was set to 0.4 to 1.0 mol/ is that as the Br ₂ concentration decreases at the end of discharge, the discharge potential also tends to decrease, and normally the Br ₂ concentration is approximately
The results showed that the efficiency was good up to 1 mol/mol. This is because even if the Br ₂ concentration is 1 mol/or less, if the discharge potential is high, the effective discharge time will be longer and a highly efficient secondary battery can be obtained. Furthermore, FIG. 1 is a plot of the relationship between the specific surface area of C-CC-CP and the average potential Vm. The above tendency is clear from Figure 1, and the optimal specific surface area of C-CC-CP is 500 to 600 m ² /g (open potential is 0.83V), which is practically 100 m ² /g or more. preferably 300
m ² /g or more. In Figure 1 (Γ)
10mA/cm ² , (●) 20mA/cm ² , (△)
30mA/cm ² , (▲) 40mA/cm ² , (□)
Indicates 50mA/cm ² . Example 2 Next, the four types of C-CC-CP described in Example 1, A, B, C, and D, were used as the positive electrode (bromine electrode) of a Zn-Br ₂ battery, and an untreated CP electrode was used as the negative electrode (zinc electrode). Four types of Zn-Br ₂ batteries A, B,
C and D were constructed and battery characteristic tests were conducted. In the electrolyte, 0.5 mol/each of methyl ethyl morpholinium bromide and methyl ethyl pyrrolidinium bromide were added as bromine complexing agents to 3 mol/ZnBr ₂ in the electrolyte, and Sn ⁺⁺ was added to the negative electrode solution to enhance the dendrite suppression effect. (in the form of _SnCl2 ) 5×
An additional 10 ⁻⁴ mol/was added. Using the above electrolyte solution, the second battery was charged at 20 mA/cm ² for 3 hours, discharged to 0 V, and showed the voltage efficiency, coulomb efficiency, and energy efficiency (Veff, Ceff, and Eeff%, respectively) at that time. It is a table. From Table 2, it is clear that battery D, in which the D electrode with the largest specific surface area was the positive electrode, exhibited excellent efficiency. This is Br ₂
This seems to be caused by the size of the active sites on the extreme surface.

[Table] Example 3 The discharge potential curves of four types of C-CC-CP at 25° C. are shown in FIG. 2 with the same configuration as in Example 1 except that the Br ₂ concentration in the electrolyte was changed. As is clear from Figure 2, those with excellent discharge potential characteristics at 20 mA/cm ² have large values of porosity and specific surface area Ss, and there is some correlation between the discharge potential characteristics and the specific surface area of the electrode. It turns out that there is. Example 4 Four types of carbon fibers with different pore contents (hereinafter referred to as CC) were selected, and their porosity Pv values were measured.
The range was set at 8 μm because this value was approximately equal to the maximum pore diameter created by chemical treatment. Next, the four different CCs mentioned above were bonded to the CP by thermocompression using a conventional method, and the four types of CC-CP were CC A -CP, CC _A -CP,
CC _B -CP, CC _C -CP, and CC _D -CP, and the discharge potential characteristics of these electrodes were determined and summarized in Table 3.

〔Effect of the invention〕

The present invention is an electrode formed by integrally molding a porous conductive plate and a polyolefin resin plate containing a conductive substance, wherein the surface of the porous conductive plate of the electrode is used for positive electrode side electrolysis. Metal that comes into contact with liquid
In the bromine secondary battery, the integrally molded porous conductive plate of the electrode has a pore volume of 0.1 cm ³ /g or more with a pore diameter in the range of 3.0 nm/8 μm. For carbon plastic electrodes obtained by thermocompression bonding of porous conductive plates, by increasing the specific surface area of the electrode, the concentration overvoltage of bromine molecules can be reduced at the end of discharge when the amount of bromine molecules, which are active materials, decreases. It was found that by suppressing this, it was possible to obtain an electrode with high energy efficiency. Furthermore, the correlation between the amount of pores (Pv) in a specific pore diameter (D diameter) range of a carbon plastic electrode obtained by thermocompression bonding of a porous conductive plate and the average discharge potential Vm of the electrode was determined. As a result of considering the pore diameter
It has been found that by setting the amount of pores in the range of 3.0 nm to 8 μm to 0.1 cm ³ /g or more, the discharge potential value is improved and the reaction activity at the final stage of discharge is improved. Therefore, in a metal-bromine secondary battery in which the porous conductive plate-like body integrally formed with the electrode has a pore volume of 0.1 cm ³ /g or more with a pore diameter in the range of 3.0 nm to 8 μm, it is difficult to discharge Even in the final stage, discharge overvoltage can be minimized and a highly reliable metal-bromine secondary battery can be obtained.

[Brief explanation of the drawing]

FIGS. 1 to 3 are graphs showing the effects of the present invention.

Claims (1)

[Scope of Claims] 1. An electrode formed by integrally molding a porous conductive plate-like body and a polyolefin resin plate-like body containing a conductive substance, wherein the surface of the porous conductive plate-like body of the electrode is In a metal-bromine secondary battery that is in contact with a positive electrode side electrolyte, the pore volume of pores with a pore diameter in the range of 3.0 nm to 8 μm in the porous conductive plate integrally formed with the electrode is set to 0.1.
A metal-bromine secondary battery characterized by having a density of cm ³ /g or more. 2. The metal-bromine secondary battery according to claim 1, wherein the amount of pores is 0.5 cm ³ /g or more.