JPH0414466B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to bromine-zinc batteries, and in particular, by applying a specific material to the surface of the carbon plastic electrode used in zinc-bromine batteries, the abnormal rise in activation overvoltage at the end of discharge can be suppressed and the discharge level can be increased. The purpose is to prevent a sudden decline in Plastic electrodes used as electrode materials for bromine-zinc secondary batteries are practical because they are cheaper and lighter than metal electrodes or carbon electrodes. However, the characteristics exhibited by this plastic electrode are such that the specific resistance of the electrode itself is high, and therefore, unless some kind of surface treatment is performed, it is not possible to approach the performance exhibited by metal electrodes or carbon electrodes. Under these circumstances, the effective surface area necessary for electrochemical reaction with the active material on the surface of the plastic electrode is increased, and the electrode potential is increased, resulting in a reduction in the voltage efficiency and coulomb of a battery incorporating this electrode. As a result of our study to significantly improve efficiency,
This led to the creation of extremely efficient plastic electrodes. That is, in carrying out conductivity improvement treatment on the surface of a plastic electrode, the present invention involves further thermocompression bonding a carbon fiber sheet to the surface of the plastic electrode on which a conductive fluororesin sheet has been thermocompression bonded in advance, or The present invention relates to a plastic electrode characterized in that a fluorocarbon resin sheet and a carbon fiber sheet are bonded together by thermocompression. Carbon fiber sheets refer to conductive carbon fibers that maintain the form of paper or fabric such as felt, knit, or cloth.
This form is easy to handle when forming a layer on the surface of the plastic electrode. When this carbon fiber sheet is used as a cloth, a commercially available carbon fiber sheet with a thickness of about 1 to 3 mm is used, and when it is applied to the surface of a plastic electrode, it is ^about 40 to 95 g/m2. It is preferable to provide the layers as follows. The electrode has a layer of carbon fiber sheet and conductive fluororesin sheet on the surface of the plastic electrode.
It increases the effective action surface area for the active material, improves the electrode potential characteristics on the bromine side, and has the effect of sufficiently increasing the voltage efficiency and Coulombic efficiency when viewed as a whole battery, and the bromine concentration in the electrolyte is extremely low (for example, about 1.0 A stable discharge level can be obtained even when the current density is high (in the region of mol/or lower). The present invention will be further explained below with reference to Examples. When thermocompression-bonding a conductive fluororesin sheet onto the surface of a plastic electrode, at the same time, a double-backed plastic electrode (hereinafter referred to as WB electrode) was formed by overlapping knitted carbon fibers. Apart from this, there are also electrodes obtained by thermocompression bonding only conductive fluororesin sheets without using carbon fibers (hereinafter referred to as FB electrodes) and electrodes obtained by thermocompression bonding only knitted carbon fibers to plastic electrodes (hereinafter referred to as FB electrodes). , CB electrode),
The values of specific resistance ρ (Ω·cm), + point average surface roughness R _z (μm), and specific surface area S (m ² /g) by Bett method were measured, and the results shown in the table below were obtained.

[Table] From the results in the table, the WB electrode shows a slight increase in the specific resistance value due to the influence of contact resistance when compared to the FB electrode, but the + point average surface roughness R _z and specific surface area S
It can be seen that the values of are all improved, and a significant improvement in the electrode reaction area has been achieved. In addition, in comparison with conventional CB electrodes, the values of + point average surface roughness _Rz and specific surface area S were similarly improved, indicating that the carbon fiber layer on the surface of the plastic electrode was sufficiently effective to achieve the desired effect. You can see that it is heat-pressed to make it possible. When the discharge level characteristics E(V) at the positive electrode were determined for the three types of electrodes mentioned above, the behavior shown in FIG. 1 was obtained. In addition, the electrolyte at this time is
3mol/ZnBr ₂ +Br ₂ , current density 20mA/cm ² ,
Measurements were made using an Ag-Agcl electrode in an atmosphere at 25°C. As is clear from the figure, in the early stage of discharge, 3
Both electrodes show similar potentials, but when the bromine concentration at the end of discharge is low, the FB electrode shows a rapid drop, while the CB electrode also shows a decrease, albeit to a somewhat lesser degree. is recognized. On the other hand, in the case of the WB electrode, it is clear that a high discharge potential is maintained over the entire range of bromine concentration (Br ₂ (mol/)), indicating that the double-backed plastic electrode is effective. It turns out that this is remarkable. In addition to this, a conventional plastic electrode is used as the negative electrode.
A zinc-bromine battery was constructed using each of the above three types of plastic electrodes as the positive electrode, and charged and discharged under the conditions of a distance between the electrodes of 1 mm, a depth of charge of 80%, and a current density of 20 mA/cm ² to determine the voltage efficiency, When the coulomb efficiency and energy efficiency were determined, the results shown in the following table were obtained. The table results show that the double-packed plastic electrode outperforms the other two in both efficiencies.

【table】 [Brief explanation of drawings]

FIG. 1 is a graph showing the results of tracing the discharge level behavior when each electrode is used.

Claims (1)

[Claims] 1. A plastic electrode for a zinc-bromine battery in which several conductive layers are formed on the surface of the plastic electrode, wherein the conductive layer is a conductive fluororesin sheet superimposed on the surface of the plastic electrode. and a carbon fiber sheet superimposed on the surface of the fluororesin sheet, which are integrally bonded by thermocompression.A plastic electrode for a zinc-bromine battery.