JP2535877B2 - Carbon electrode material for electrolytic cell - Google Patents

Description

TECHNICAL FIELD The present invention relates to an electrode material to be used by immersing it in an electrolytic cell, and more specifically, a carbon-based electrode having excellent current efficiency and voltage efficiency and little change in performance. It concerns materials.

(Prior Art) As fields in which an electrolytic cell is used, the electrolysis industry such as electroplating, salt electrolysis, and electrosynthesis of organic compounds and various batteries are typical. The electrode materials used in these electrolyzers are those in which the electrode itself, which is often found in batteries such as graphite batteries, undergoes an electrochemical reaction as the active material, and the reaction field that promotes the electrochemical reaction of the active material. There is something that does not change itself. The latter electrode material is mainly applied to the electrolytic industry and new secondary batteries. In this case, the characteristics of the electrode material are that side reactions other than the desired reaction do not occur (reaction selectivity), energy efficiency is high, deterioration due to repeated use is small (long life), and it depends on the system used. Chemical stability is required.
In particular, in a new type secondary battery that is being developed for power storage, it is essential to improve and improve the performance of the electrode material from the viewpoint of effective use of energy in the future.

As new type secondary batteries, sodium-sulfur batteries, lithium-iron sulfide batteries, metal-halogen batteries, redox flow type batteries and the like are currently under development. Further, these new secondary batteries are also expected to be used as backup devices for power generation using natural energy such as sunlight, wind power, and wave power, or as electric vehicle batteries.

However, in order to put these new secondary batteries into practical use,
There are inherent problems that need to be developed. That is, the performance such as energy efficiency is improved and the life is extended. For example, in a redox flow type secondary battery, an iron-chromium redox flow type secondary battery (hereinafter referred to as a Fe-Cr battery) using an iron chloride aqueous solution as a positive electrode active material and a chromium chloride aqueous solution as a negative electrode active material, which are currently most developed. For the electrode material (abbreviated), a normal carbon fiber aggregate having chemical resistance and electrical conductivity is used.

The redox reaction of iron ions in the positive electrode using the carbon fiber aggregate in the battery is not a serious problem because the reactivity is relatively fast during charging and discharging and no side reaction is generated. However, the redox reaction of the chromium complex ion including the complex exchange reaction in the negative electrode is slower than that of iron ion, that is, the conductivity of the battery is low, and hydrogen is generated during charging, and the current efficiency of the battery is likely to decrease. It is a problem listed in 1. At the same time, the second is that the rate at which the voltage and energy efficiencies change over the course of the cycle (rate of decrease) is large.
Has become a problem.

(Problems to be Solved by the Invention) In view of such circumstances, the present invention provides an electrode material for an electrolytic cell, which enhances the total energy efficiency of a battery and improves the cycle life.

(Means for Solving Problems) In the present invention, the <002> plane spacing obtained by X-ray wide-angle analysis is
Carbon having a pseudo-graphite crystal structure of 3.70 Å or less, wherein the ratio of bound oxygen atoms on the carbon surface is at least 3% or more with respect to carbon atoms, and aluminum, gallium, indium, thallium, and IVb group of the long periodic table Alternatively, it is a carbon-based electrode material for an electrolytic cell, which contains at least one element of the IVa group elements, and further contains a boron element in the above-mentioned material.

In the present invention, carbon having a pseudo-graphite crystal structure means X
3.3 when the <002> plane spacing obtained by line wide angle analysis is 3.70Å or more
A carbon material having a range of up to 54 Å (graphite structure), preferably 3.40 to 3.70 Å. By using the carbon material as an electrode material, side reactions such as hydrogen generation in the negative electrode during charging can be suppressed, and the current efficiency can be significantly increased. on the other hand,
When a carbon material with a <002> plane spacing of 3.70 Å or more is used, harmful side reactions such as hydrogen generation in the negative electrode during charging progress and current efficiency cannot be increased. Further, the surface oxidation treatment is easier in the pseudo graphite structure than in the complete graphite structure having high crystallinity. As the raw material of the carbon material of the present invention, all carbonizable raw materials can be applied, and examples thereof include pitch of coal and petroleum, phenol-based, acrylic-based, aromatic polyamide-based, cellulosic-based raw materials and the like. You can Furthermore, as the constituent structure of the carbon material, spun yarn,
Filament-concentrated yarn, non-woven fabric, woven fabric, knitted fabric or a carbonaceous fiber aggregate composed of a hybrid structure thereof, a porous carbon body,
A carbon-carbon composite, a particulate carbon material, and the like can be mentioned, and there is no particular limitation.

The bound oxygen ratio of the carbon material surface in the present invention means ESCA.
It means the amount of bound oxygen detected from the surface of the carbon material by surface analysis (the analysis method will be described later), and is expressed as the ratio of the number of bound oxygen atoms to the number of carbon atoms (%, hereinafter referred to as O / C ratio). By using a carbonaceous material having this value of 3% to 30%, preferably 5% to 15% for the electrode material, the electrode reaction rate, that is, the electric conductivity can be significantly increased. On the other hand, when using a low carbon material having an oxygen concentration of O / C ratio of less than 3% on the surface of the material by ESCA analysis, the electrode reaction rate of the charge / discharge battery is low and the electric conductivity cannot be increased. By using a carbon-based material having many oxygen atoms bonded to the surface of the material as an electrode material, the electric conductivity, in other words, the voltage efficiency can be improved. Such a carbon-based material having an increased concentration of surface oxygen atoms can be obtained by dry-oxidizing the above-mentioned carbon material having a pseudo-graphite crystal structure. For example, 1 × 10 ^-2 to
It is carried out in an oxygen atmosphere having an oxygen partial pressure of rr or more so that the weight yield is in the range of 65 to 99%. Generally, the treatment temperature is preferably 400 ° C. or higher. At a low temperature (for example, 200 to 300 ° C.), the carbon material to be treated loses its reactivity and the effect of oxidation cannot be obtained. Wet oxidation should be avoided because there are many problems such as formation of intercalation compounds and generation of harmful gas in the treated area. Further, this dry oxidation treatment may be a one-step method, or may be a two-step method at different temperatures.

With the above structure, an electrode material having high current efficiency and voltage efficiency can be obtained, the total energy efficiency of the battery can be significantly increased, and the first problem described above can be solved.

The elements contained in the electrode material in the present invention include aluminum, gallium, indium, thallium, and long-term periodic table.
It is at least one element selected from the group IVb and group IVa. Here, the group IVb element is silicon, germanium, tin, and lead, and the group IVa element is titanium, zirconium, and hafnium. Further, boron may be contained together with the above element. By using a carbon-based material having a pseudo-graphite crystal structure and a surface structure as described above as an electrode material, the total energy efficiency of the battery could be significantly increased. When implemented, it was found that the current efficiency hardly decreased with the lapse of charge / discharge cycles, but the voltage efficiency gradually decreased. On the other hand, it has been found that by incorporating the above elements into the carbon-based material, it is possible to obtain a practically useful electrode material in which the change in current and voltage efficiency with the passage of charge / discharge cycles is extremely small. That is, by incorporating the above element into the carbon-based material having the above-mentioned pseudo-graphite crystal structure and surface structure, the second problem can be solved in addition to the first problem.

There is no particular limitation on the method of containing the above element in the carbon-based material, but the raw material before carbonization may be mixed or impregnated with the above-mentioned elemental simple substance or an inorganic or organic compound, or once subjected to low temperature carbonization. The above or the compound may be attached to the above and further subjected to high temperature treatment.

Examples of inorganic compounds include aluminum oxide, gallium oxide, indium oxide, thallium oxide, silicon oxide, germanium oxide, tin oxide, lead oxide, titanium oxide, zirconium oxide, hafnium oxide, oxides such as boron oxide, and silica. Acids, titanic acid, aluminic acid, boric acid and their acid salts, and also aluminosilicate, complex salts such as borosilicate, as the organic compound, aluminum tri iso-propoxide, tetraethyl silicate,
Examples include various alkoxides such as tetra-n-butyl titanate and triethyl borate. Further, it is preferable that these compounds do not contain metal elements other than the above elements. When the metal element remains, the temperature dependence of the current efficiency increases, which is not preferable. In addition, an ion implantation method which has been used for modifying various materials in recent years may be used, or deposition may be performed by vapor deposition or sputtering. Addition by these methods is preferably carried out before the final carbonization. It is desirable that the addition amount is adjusted so that the relaxation of the added element finally exists in the range of 0.01 to 56 mol% with respect to carbon as an element standard. More preferably 0.05 to 12 with respect to carbon
mol%. When the element and boron are added together, the sum of boron and the additive element is in the above range with respect to carbon, and the molar ratio of boron and the additive element (boron / additive element) is 0.05 to 20.0. It is desirable to adjust the range.

In the present invention, it was decided to include boron in combination with the above-mentioned element or the above element in the carbon-based material having the <002> interplanar spacing and the O / C ratio specified. Even when these elements are contained, it is possible to obtain the effect of reducing the change over time in the charge / discharge cycle of the initial conductivity.

The electrolysis tank in the present invention is one in which an electrode material is immersed in a liquid electrolyte to cause an electrochemical reaction (oxidation, reduction) on the surface of the electrode material, which is used in an electrolysis process or a battery. It is a tank. That is, the electrode material according to the present invention,
It can be usefully used in various electrolytic processes and new secondary batteries such as sodium-sulfur batteries, zinc-bromine batteries, zinc-chlorine batteries and redox flow batteries. More preferably, it is most suitable for a redox flow secondary battery, especially an Fe-Cr battery.

Next, the <002> plane spacing, the battery efficiency / conductivity, and the method for measuring the cycle change amount of these adopted in the present invention will be described.

<002> Surface spacing: d002 The electrode material is crushed in an agate mortar to a particle size of about 10 μm, and about 5% by weight of high purity silicon powder for X-ray standard is added as an internal standard to the sample and mixed. Then, a wide angle X-ray diffraction curve is measured by a transmission diffractometer method using a CuKd ray as a radiation source.

For the correction of the curve, so-called Lorentz factor, polarization factor, absorption factor, atomic scattering factor, etc. are not corrected and the following simple method is used. That is, the baseline of the peak corresponding to <002> diffraction is drawn, and the actual intensity is plotted again from the baseline to obtain the <002> corrected intensity curve. Find the midpoint of the line segment where the line parallel to the angle axis drawn to the height of two-thirds of the peak height of this curve intersects the correction intensity curve, correct the angle of the midpoint with the internal standard, and Double the diffraction angle,
Based on the wavelength of CuKα and Bragg's equation below, <002>
Calculate the surface spacing.

λ: 1.5418 Å θ: Diffraction angle Current efficiency A small flow-through type electrolytic cell shown in Fig. 1 is made, and charging and discharging are repeated at various constant current densities to test electrode performance. As the positive electrode solution, 1M ferric chloride and ferric chloride 4N hydrochloric acid acidic aqueous solutions were used, respectively, and as the negative electrode solution, 1M ferric chloride 4N hydrochloric acid acidic aqueous solution was prepared. In FIG. 1, the shape of the electrode material is a rectangular plate shape, but it may be a triangular shape, a circular plate shape, a spherical shape, a linear shape, or any other irregular shape depending on the shape of the electrolytic cell.

The amount of the positive electrode solution was set to a large excess with respect to the amount of the negative electrode solution, so that the characteristics of the negative electrode could be examined mainly. The electrode area is 10 cm ² , and the liquid flow rate is about 5 ml / min. The current density was 40 mA / cm ² during charging and discharging. In a one-cycle test that starts with charging and ends with discharging, the amount of electricity required to charge up to 1.2V is Q ₁ coulomb, constant current discharge up to 0.2V, and then 0.8V.
Each amount of electricity taken out at a constant voltage discharge to the Q _2, Q ₃ coulombs, obtains the current efficiency by the following equation.

Reactions other than reduction of Cr ³⁺ to Cr ²⁺ during charging, eg H
^When side reactions such as reduction of ⁺ occur, the amount of electricity that can be taken out decreases and current efficiency decreases.

Cell conductivity The ratio of the quantity of electricity extracted by discharge to the theoretical quantity of electricity Qth required to completely reduce Cr ³⁺ in the negative electrode solution to Cr ²⁺ is defined as the charging rate, From the slope of the current-voltage curve when the charging rate is 50%, the cell resistance (Ωcm ² ) and its reciprocal cell conductivity (Sc
m ^-2 ). The higher the cell conductivity, the sooner the redox reaction of ions at the electrode occurs, and the higher the discharge voltage at high current density becomes. Therefore, the voltage efficiency of the cell is high, and it can be judged that the electrode is excellent.

After the measurement of the change with time of the charge / discharge cycle was completed, the same cell was used to
Charging / discharging is repeatedly performed at a cell voltage of 0.5 to 1.2 V at a constant current density of / cm ² . After the specified cycle, perform constant current discharge up to 0.2V and constant voltage discharge at 0.8V, and measure the data. The test was conducted at 40 ° C.

O / C ratio The O / C ratio of the carbon-based material surface is measured by X-ray photoelectron spectroscopy, which is abbreviated as ESCA or XPS. Shimadzu ESCA750 was used as the measuring device, and ESCA PAC760 was used for the analysis.

Each sample was cut into a diameter of 6 mm, attached to a heating type sample stand with a conductive paste, and the sample was vacuum degassed for 2 hours or more while heating at 120 ° C., and then the measurement was performed. For the radiation source
Using the MgKα line (1253.6 eV), the degree of vacuum in the device is 10 ^-7 tor
The surface of the sample was analyzed under the condition of r. The surface mentioned here means a range of depth from the outermost layer of the sample to several tens of liters (analysis area).

The measurement was performed for Cls and Ols peaks, and each peak was ESC
A PAC760 (based on the correction method by JHScofieid) is used for correction analysis, and each peak area is obtained. The area obtained is Cl
s is 1.00 and Ols is multiplied by a relative intensity of 2.85, and the surface (oxygen / carbon) atomic number ratio is directly calculated as a percentage from the area.

(Examples) The present invention will be described in detail below with reference to comparative examples and examples, but the present invention is not limited to these examples.

Example 1 A double-sided knitted fabric was knitted with a 14-gauge double-sided circular knitting machine using 20-count double yarn composed of 2.0-denier short-fiber regenerated cellulose fibers which had been sufficiently desulfurized, bleached, washed with water, and dried to fabric A. Got This knitted fabric has a basis weight of 424 g / m ² and 0.3709 g / cm ³
It had an apparent density of 1.2 mm and a thickness of 1.2 mm. This knitted fabric was scoured and dried, then immersed in a mixed solution of iso-propyl alcohol, ethyl alcohol and water (weight ratio 80: 40: 1.5) containing 9.6% by weight of aluminum triiso-propoxide, dehydrated and dried. Then, the fabric with 4.0% by weight of alumina gel (2.1% by weight of aluminum) impregnated therein is heated to 270 ° C. at a heating rate of 1 ° C./min in an inert gas (flame resistance treatment) and then 400 ° C./hour. At a heating rate of 2350
The temperature was raised to ℃, kept at this temperature for 30 minutes, carbonized, cooled, and treated in air at 650 ℃ for 15 minutes to obtain Fabric B. Fabric B
Current efficiency was 99% at the beginning and after 50 cycles. The initial conductivity is 0.68Scm ^-2, after 50 cycles was improved compared to 0.53Scm ^-2 next fabric F.

The d002 of Fabric B was 3.53Å, and the O / C ratio was 9.5%.

Example 2 The fabric A obtained in Example 1 was scoured and dried, immersed in a mixed solution of ethyl alcohol containing 12.1% by weight of tetraethyl orthosilicate, water and acetic acid (weight ratio 63: 90: 0.03), and then dehydrated. The cloth, which was dried and impregnated with 10.3% by weight of silica gel (4.8% by weight of silicon), was flame-proofed and carbonized at 2000 ° C. in the same manner as in Example 1, cooled, and treated in air at 650 ° C. for 15 minutes. To obtain a cloth C. The current efficiency of Fabric C was 99% at the beginning and after 50 cycles.

Initial conductivity is 0.70Scm ^-2 , 0.5 after 50 cycles
It was 5 Scm ^-2 , which was improved compared to Fabric F.

 The d002 of Fabric C was 3.60Å, and the O / C ratio was 11.0%.

In addition, a diffraction peak of Sic was recognized by X-ray diffraction, and a diffraction line of T component was also recognized from the (002) diffraction line of carbon (appeared near 2θ = 26.0 °, see Reference 1)).

Example 3 The fabric A knitted in Example 1 was scoured and dried to prepare a mixed suspension of retanol, water and acetic acid containing 12.3% by weight of tetra-n-butyl titanate (weight ratio 63: 180: 0.03). After soaking, dehydration and drying are performed to form titanium oxide gel 10.0
The fabric impregnated with 1% by weight (5.9% by weight of titanium) was subjected to flame resistance and carbonization at 2350 ° C. in the same manner as in Example 1 to be cooled, and then treated in air at 650 ° C. for 15 minutes to obtain a fabric D. The current efficiency of Fabric D was 98% at the beginning and after 50 cycles.

The initial conductivity is 0.65Scm ^-2, after 50 cycles, was improved compared with 0.49Scm ^-2 next fabric G.

The d002 of the cloth F was 3.55Å, and the O / C ratio was 9.5%.
Further, a TiC diffraction peak was confirmed by X-ray diffraction.

Example 4 The fabric A knitted in Example 1 was scoured and dried, and a mixed solution of ethyl alcohol and water containing 7.5 wt% tetraethyl orthosilicate and 5.1 wt% boric acid (weight ratio).
63: 180) and then dehydrated and dried, and a cloth impregnated with 15.0% by weight (3.5% by weight of silicon and 1.3% by weight of boron) as a mixture of silica gel and boric acid was subjected to flame resistance in the same manner as in Example 1. And was carbonized at 2350 ° C., cooled, and treated in air at 650 ° C. for 15 minutes to obtain a fabric E. The current efficiency of the fabric E was 98% at the initial stage and after 50 cycles. The initial conductivity was 0.71 Scm ^-2 , which was 0.66 Scm ^-2 after 50 cycles, which is an improvement over the fabric F.

The d002 of the fabric E was 3.48Å, and the O / C ratio was 9.3%.

Further, from the X-ray diffraction, diffraction lines corresponding to the T component and the G component were recognized from the (002) diffraction line of carbon (G component: graphite component, see Reference 1)).

Comparative Example 1 A double-sided knitted fabric was knitted by a 14-gauge double-sided circular knitting machine using 20-count double yarn composed of 2.0 denier short-fiber regenerated cellulose fibers obtained by sufficiently desulfurizing, bleaching, washing with water and drying, and a fabric A Got This knitted fabric has a basis weight of 424 g / m ² and 0.3709 g / c
It had an apparent density of m ² and a thickness of 1.2 mm. After the scouring and drying of this knitted fabric, the temperature is raised to 270 ° C. at a heating rate of 1 degree per minute in an inert gas, and then a heating rate of 850 ° C. at 400 degrees per hour
Up to 40 minutes, hold for 30 minutes, cool, and
Oxidation treatment was performed at 0 ° C. to obtain a carbon fiber cloth F. The X-ray analysis result for fabric A is d002 = 3.85Å, and ESCA O /
The C ratio was 11.3%. The initial current efficiency of the battery using the cloth F was 74%, and the electric conductivity thereof was 0.12 Scm ^-2 . The fabric F had a basis weight of 265 g / m ² and a density of 0.26 g / cc.

(Effects of the Invention) By using the electrode material of the present invention, in the field of utilizing various electrolytic cells, harmful side reactions can be suppressed to increase current efficiency, and cell conductivity can be increased, thus reducing energy consumption. The efficiency can be increased. Further, deterioration of the cycle with time could be reduced, and industrially great practicality could be brought about.

[Brief description of drawings]

FIG. 1 is a diagram illustrating an experimental apparatus for measuring the battery performance of the electrode material according to the present invention. 1 ... Graphite plate for collecting current 2 ... Spacer 3 ... Ion exchange membrane 4 ... Carbon fiber cloth (electrode) 5 ... Active material aqueous solution flow passage

Claims (1)

(57) [Claims] 1. A <002> plane spacing obtained by X-ray wide-angle analysis is
A carbon having a pseudo-graphite crystal structure of 3.70Å or less, wherein the ratio of bound oxygen atoms on the carbon surface is at least 3 with respect to carbon atoms.
% Or more and contains at least one element selected from the group consisting of aluminum, gallium, indium, thallium, and IVb group or IVa group element of the long periodic table, a carbon-based electrode material for an electrolytic cell.