JPS60232669A - Electrode material for electrolytic bath - Google Patents

Abstract

PURPOSE:To restrict the amount of gaseous hydrogen generated at charging of an iron-chrome redoxflow secondary battery for improved current efficiency and conductivity by using an electrode material of a specified carbonaceous material. CONSTITUTION:There is used a carbonaceous material having a congulated graphite microcrystal structure whose <002> spacing measured by X-ray wide angle analysis is less than 3.70Angstrom on the average and in which the dimension of the crystal lattice is more than 9.0Angstrom on the average and having the total acid functional group rate more than 0.01meq/g. Using as electrode material such a carbonaceous material having a specified crystal structure with high crystallinity is effective to restrict the amount of gaseous hydrogen generated at charging of the iron-chrome redoxflow secondary battery and to remarkably improve current efficiency. Furthermore, using as electrode material the above-mentioned carbonaceous material having many acid functional groups combined remarkably improves electrode reaction rate and consequently conductivity.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a novel electrode material, and more particularly to an electrode for an electrolytic cell made of a carbonaceous material having a specific crystal structure and an amount of acidic functional groups. It is related to materials.

BACKGROUND OF THE INVENTION Since the energy crisis of 1973, energy problems have become widely recognized by all walks of life. Along with the development of new energy sources, it is also becoming important to develop systems that effectively utilize the energy generated, including energy conversion, storage, transportation, and utilization. Taking storage as an example, nuclear power and coal are expected to account for a large proportion of the power source mix in the future.

In large-scale power generation such as thermal power generation, it is necessary to maintain a constant output and generate electricity at a constant rate in order to maintain high efficiency. There is a growing demand for power storage technology that can accommodate (load leveling). Even now, the annual operating rate of major power generation facilities is below 60% and continues to decline.

In addition to pumped-storage power generation, which has been put into practical use but suffers from power transmission losses and is subject to restrictions on location, there are various energy storage methods such as new secondary batteries, Fufui wheels, compressed air, and superconductivity. methods are being considered, but electrochemical operation using a new type of battery is most likely, and for the time being, it is considered the most viable method to replace pumped storage power generation as a solution system that includes transportation. New again! The 1lu secondary battery uses sunlight.

It is also expected to be used as a backup device for power generation using natural energy such as wind and wave power, or as a battery for electric vehicles. As a secondary battery that can be used for memo purposes,
Lead-acid batteries, IJ sulfur batteries, lithium-iron sulfide batteries, metal-halogen batteries, redox flow batteries, etc. are currently being developed.

Among them, redox flow secondary batteries have the following characteristics:
Development is progressing rapidly in the United States and Japan. In this battery, the flow-through electrolytic cell that changes electrochemical energy during charging and discharging and the tank that stores the redox aqueous solution that is the active material are completely separated, so the amount of electricity stored can be increased simply by changing the tank capacity. 't - able to change, therefore for a long time.

Suitable for large-capacity power storage, easy to adjust battery output because it is a liquid flow type, and almost no self-discharge when the battery is stopped, making it useful as a backup for natural energy power generation such as m-power and solar power generation. It has excellent features such as being suitable.

However, like other new types of secondary batteries, there are inherent problems that must be solved in order to put this battery into practical use. In other words, among the redox flow secondary batteries, the one that is currently being developed is the iron-chromium redox flow secondary battery (hereinafter referred to as Fe-Cr battery), which uses an iron chloride aqueous solution as the positive electrode active material and a chromium chloride aqueous solution as the negative electrode active material. (abbreviated as ), and the electrode material of this battery uses a common carbon fiber aggregate that has chemical resistance and conductivity.

A problem to be solved in this battery is the redox reaction of chromium ions (which form a chloroaco complex in an aqueous solution) at the negative electrode. The redox reaction of iron ions at the positive electrode has a relatively fast reaction rate during charging and discharging, and side reactions do not occur, so it is not a big problem, but compared to iron ions, the redox reaction of chromium complex ions, including complex exchange reactions, is slower. Particular problems include the fact that the conductivity of the battery is low, and that hydrogen is generated during charging, which tends to reduce the current efficiency of the battery.

As described above, conventional Fe--Cr batteries have inherent problems as described above, but these points are also closely related to the selection of electrode materials. In other words, since the electrochemical reaction during charging and discharging proceeds on the surface of the carbon fiber, the amount of hydrogen gas generated during charging is suppressed,
It is important to select an electrode material that can increase current efficiency and conductivity (related to the speed of redox reaction).

Purpose of the Invention In view of the above circumstances, the present inventors have conducted intensive studies on electrode materials for Fe-Cr batteries that can improve the total energy efficiency of the battery, and as a result, have arrived at the present invention.

Components of the Invention: 1. Pseudographite microcrystals in which the <002> plane spacing determined by HX-ray wide-angle analysis is 3.70 A or less on average, and the crystallite size in the C-axis direction is on average 9.0λ or more. structure, and the total amount of acidic functional groups is at least 0.01 meq
/f is an electrode material made of a carbonaceous material.

By using such an electrode material, the current efficiency and conductivity, which are characteristic values of the battery, are improved. In other words, the (002) plane spacing determined by wide-angle X-ray analysis (the analysis method will be described later) is 3.70 A or less on average, and the crystallite size in the C-axis direction is on average 9. When a carbonaceous material having a pseudographite microcrystalline structure of 0 Å or more is used as an electrode material, the amount of hydrogen generated at the negative electrode during charging is suppressed.

It was possible to significantly increase current efficiency. <002> When using a carbonaceous material with low crystallinity in which the interplanar spacing exceeds 3.70 and the average crystallite size in the C-axis direction is less than 9.0λ, the amount of hydrogen generated at the negative electrode during charging is large. , the current efficiency cannot be increased.

Although it is not clear why the current efficiency is improved by using a carbonaceous material having a specific crystal structure with high crystallinity as an electrode material.

As the crystal structure develops, the hydrogen overvoltage increases.

During charging, reduction of chromium ions occurs selectively.

It is thought that this increases the current efficiency.

On the other hand, as mentioned above, the total amount of acidic functional groups is 0.01 meq/
By using the above carbonaceous material as an electrode material, it was possible to significantly increase the electrode reaction rate, that is, the electrical conductivity. When using a carbonaceous material having a small total acidic functional group amount of less than 0.01 meq72, the electrode reaction rate during discharge is low and the conductivity cannot be increased.

Although it is not clear why the electrical conductivity, or in other words, the voltage efficiency, can be improved by using a carbonaceous material with many acidic functional groups as an electrode material, it is possible to improve the conductivity, in other words, the voltage efficiency. Detachment from.

It is thought that the acidic functional groups on the surface work effectively in complex exchange reactions and the like.

A carbonaceous material with an increased concentration of acidic functional groups can be produced by subjecting a carbonaceous material having the above-mentioned internal crystal structure to a dry oxidation treatment.

This is carried out, for example, in an oxygen atmosphere having an oxygen partial pressure of I x 102 torr or more so that the weight yield is in the range of 65 to 99%. The treatment temperature is usually preferably 400°C or higher. Further, the processing time is preferably 1 second to 60 minutes. At low temperatures (for example, 200 to 300°C), the reactivity of the carbonaceous material to be treated decreases, so that the oxidation effect cannot be obtained. Wet oxidation treatment should be avoided because it causes many problems such as the formation of intercalation compounds and the generation of harmful gases during treatment.

By performing the dry oxidation treatment as described above, more edges of the plane perpendicular to the C axis of the pseudographite microcrystals can be exposed to the material surface, and acidic groups effective for electrochemical reactions are formed on these edges. can be done. Although carboxyl groups and phenolic hydroxyl groups are detected in the functional group measurement method (described later) in the present invention, these acidic groups greatly contribute to the electrode reaction, thereby increasing electrical conductivity (voltage efficiency).

The carbonaceous material according to the present invention is not particularly limited to carbon fibers, activated carbon fibers, aggregates of activated carbon, etc., but preferred examples include fabrics made of carbon fibers (woven fabrics, knitted fabrics, non-woven fabrics). can.

Note that the <002> plane spacing (do
o2), crystallite size in the C-axis direction C'r, c)s current efficiency, electrical conductivity, and total acidic functional group amount are measured by the following methods.

■ <002> Surface spacing: dooz Powder the carbon fiber woven fabric in an agate mortar, add approximately 15 weight of high purity silicon powder for X#i standard to the sample as an internal standard substance, mix, and pack into a sample cell. & CuKc
A wide-angle X-ray diffraction curve is measured using a transmission diffractometer method using t-rays as a radiation source.

To correct the curve, the following simple method is used without making corrections regarding so-called Lorentz, polarization factors, absorption factors, atomic scattering factors, etc. That is, draw the baseline of the peak corresponding to (002) diffraction.

Replot the real intensity from baseline (00
2) Obtain a corrected intensity curve. Find the midpoint of the line segment where a line parallel to the angular axis drawn at two-thirds the height of the peak height of this curve intersects the intensity curve.

Correct the angle of the midpoint using an internal standard, make it twice the diffraction angle, and calculate the <OO2> plane spacing from the wavelength λ of CuKα using the following Bragg equation.

dooz=--m-- 2sinθ λ: 1.5418 Person 02 Diffraction angle ■ Size of crystallite in C-axis direction Lc j In the corrected diffraction intensity curve obtained in the previous section, the so-called half-width at the position half the peak height Calculate the size of the crystallite in the C-axis direction using β.

There are various discussions regarding the shape factor K.

K = 0.90 was adopted. λ and θ have the same meaning as in the previous section.

■Cell current efficiency A small flow-through type electrolytic cell as shown in Figure 1 was made, and the electrode performance was tested by repeatedly charging and discharging at various constant current densities. The positive electrode contains ferrous chloride and ferric chloride concentrations each IM/! using an acidic aqueous solution of 4N hydrochloric acid.

A 4N acidic aqueous solution of hydrochloric acid having a concentration of nichrome chloride of IM/l was prepared for the negative electrode.

The amount of positive electrode liquid was set to be in large excess of the amount of negative electrode liquid, allowing the study to focus on the negative electrode characteristics. The electrode area was 10 d, and the liquid flow rate was 54 min. Current density is 20, 40, 60, 8
The test was conducted at 0.100 mA and 10 J, but the same value was used during charging and discharging. In a one-cycle test that starts with charging and ends with discharging, the amount of electricity required for charging f: Q + coulombs, constant current discharging to 0.2 V, and subsequent < 0
．． The amount of electricity taken out by constant potential discharge at 8 V is Q2.
, 03 coulombs, and calculate the current efficiency using a linear equation.

If a reaction other than the reduction of Cr3+ to Cr2+ occurs during charging, for example a side reaction such as reduction of H+, the amount of electricity that can be taken out decreases and the current efficiency decreases.

■ SEA/Electrical conductivity The charging rate is the ratio of the amount of electricity taken out mid-discharge to the theoretical amount of electricity Qth required to completely reduce Cra+ in the negative electrode liquid to Crz+. From the slope of the current/voltage curve at 50%, the cell resistance (Ω-) and its reciprocal cell conductivity (St
yn-2). The higher the cell conductivity, the more quickly the ion redox reaction occurs at the electrode, the higher the discharge potential at high current density, and the higher the voltage efficiency of the cell, making it an excellent electrode.

Note that the tests in ■ and ■ were conducted at around 25°C.

■ Total amount of acidic functional groups using a strong base Sodium hydroxide is used as a strong base. The carbon fiber fabric 2f is dried overnight in a vacuum dessicator with a width of about 100' and weighed accurately. The above-mentioned woven fabric 22 is placed in a dried Erlenmeyer flask.
(0.02N hydroxide) Add 35st of IJium aqueous solution using a whole pipette. This Erlenmeyer flask is shaken at 25°C for 2 hours or more. After shaking, filter and collect 5 portions of P solution using a whole pipette. Pour into an Erlenmeyer flask. This is titrated with a 0.02N aqueous hydrochloric acid solution. Perform the same operation without a sample to create a blank.

The total amount of acidic functional groups is calculated using the following formula.

Titration amount of DB Ni blank (1/) Ds: Titration amount of woven fabric (gl) G: Weight of woven fabric CP) Effects of the Invention As described above, the present invention has a unique graphite crystal structure that causes a drop in the negative electrode during charging.・The amount of hydrogen generated was suppressed, and the current efficiency was significantly increased. Furthermore, by setting the total amount of acidic functional groups within a predetermined range, the electrode reaction rate, that is, the electrical conductivity, can be significantly increased, thereby providing a redox flow type secondary battery electrode material with excellent practicality. This is what made it possible.

EXAMPLES The present invention will be explained in detail below using comparative examples and examples, but the present invention is not limited to these examples.

What a comparison.

1.5 denier regenerated cellulose fiber! The fabric made by spinning and making was brought to 270°C at a temperature increase rate of 50°C per hour, held for 1 hour to perform flame-retardant treatment, and then brought to 850°C at the same rate of 50°C per hour, After being held for a few minutes, it was cooled to obtain a carbon fiber woven fabric A. Woven fabric A was heated at 1400°C in an inert gas.

Reheat treatment at 2000°C was performed for 1 hour to obtain carbon fiber fabrics B and C. Table 1 shows the results of X-ray analysis 1 surface analysis and battery characteristic measurements of fabrics B and C.

Table 1 Not only Fabric B, which has low crystallinity and a small amount of acidic functional groups on one surface, but even Fabric C, which has high crystallinity, has poor cell characteristics when the amount of surface acidic functional groups is small.

Comparative Example 2゜The fabric Bi acid obtained in Comparative Example 1 was oxidized at 700°C for 3 minutes in an inert gas with an acid partial pressure of 00 torr, with a yield of 96%.
I got the fabric. Table 2 shows the surface analysis results of the fabric and the electrode performance.

Table 2.

Although cell properties are improved by increasing the amount of acidic functional groups through oxidation treatment, satisfactory values are still not achieved for fabrics with low crystallinity.

Example 1 Fabric Ct obtained in Comparative Example 1 Oxidation treatment was carried out in the same manner as in Comparative Example 2 to obtain Fabric E with a retention of 96%. Table 3 shows the surface analysis results and electrode performance of Fabric E.

Table 3 Fabric E with high crystallinity and increased amount of acidic functional groups
showed extremely excellent electrode performance.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram for measuring all cell characteristics of an electrode material according to the present invention. Graphite plate for current collection 2 Varnish spacer 3: Ion exchange membrane 4: Carbon fiber fabric (densei) 5: Active material aqueous solution flow path Patent applicant Toyobo Co., Ltd.

Claims (1)

[Claims] The average <002> plane spacing determined by X @ wide-angle analysis is 3.70.
λ or less, and the average size of crystallites in the C-axis direction is 9
．． An electrode material for an electrolytic cell, characterized in that it is made of a carbonaceous material having a pseudographite microcrystalline structure of OA or higher and having a total acidic functional group content of at least 0.01 meq/l.