JPH0711963B2 - Carbon-based electrode material for flow-through electrolyzer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of use] The present invention relates to an electrode material used by immersing it in an electrolytic cell of a redox flow secondary battery, and more specifically, it has excellent current efficiency and voltage efficiency and deterioration of performance. The present invention relates to a carbon-based electrode material having a low content.

[Prior Art] Since the energy crisis of 1973, the energy problem has been widely recognized by various layers, and at the same time as the development of a new energy source, the energy generated by the energy conversion, storage, transportation, and use that effectively uses the generated energy Development of the system including is becoming important.

For example, in large-scale power generation such as nuclear power and coal-fired thermal power, which is expected to occupy a large proportion in the future power source composition, it is said that it is necessary to maintain a constant output and perform steady power generation in order to maintain high power generation efficiency. However, due to the large difference in power demand between day and night, the output has been forced to fluctuate, and the current annual operating rate of major power generation facilities continues to fall below 60%. Therefore, there is an increasing demand for an electric power storage technology capable of appropriately storing the surplus power at night and discharging the surplus power when the demand increases during the daytime (load leveling).

Various methods such as pumped storage power generation, new secondary batteries, flywheels, compressed air, superconductivity, etc. are being studied as methods for storing electric power. Among them, the practical utilization rate of pumped storage power generation is high, but this is also the power transmission loss. It is not in a satisfactory state due to difficulties such as large size and restrictions on location. Under these circumstances, electrochemical operation using a new battery is being considered as a promising method, and for the time being, it is considered to be the most viable method to replace pumped storage power generation as a solution system including transportation. Also new type 2
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 batteries for electric vehicles. Lead-acid batteries, sodium-sulfur batteries, lithium-iron sulfide batteries, metal-halogen batteries, redox flow batteries, etc. are currently being developed as secondary batteries applicable to the above purpose.

Among them, the redox flow secondary battery has the following features,
Development is proceeding rapidly in the United States and Japan. That is, in the battery, the flow-through type electrolytic cell for changing the electrochemical energy at the time of charging and discharging and the tank for storing the redox aqueous solution as the active material are completely separated. It is possible to change the amount, so it is suitable for long-term, large-capacity power storage, it is easy to adjust the battery output because it is a liquid circulation type, there is almost no self-discharge when the battery is stopped, wind power It has excellent features such as being suitable for backing up natural energy generation such as solar power generation.

[Problems to be Solved by the Invention] However, in order to put the battery into practical use, there are inherent problems to be solved, like other new secondary batteries. That is, the most developed redox flow secondary battery at present is an iron-chromium redox flow secondary battery (hereinafter referred to as Fe-Cr battery) that uses an iron chloride aqueous solution as a positive electrode active material and a chromium chloride aqueous solution as a negative electrode active material. (Abbreviated), and an ordinary carbon fiber assembly having chemical resistance and conductivity is used as the electrode material of the battery. However, the redox reaction of iron ions at the positive electrode using the carbon fiber aggregate in the battery is not so problematic because the reaction rate is relatively fast during charging / discharging and side reactions do not occur, but it is more complex than iron ions. Redox reaction of negative electrode of chromium complex ion (containing chloroaco complex in aqueous solution) containing nickel is slow, that is, the electric conductivity of the battery is low, and hydrogen is generated during charging, and the current efficiency of the battery is likely to decrease. Is the first problem. At the same time, the second problem is the charge / discharge life characteristics of the current, voltage, and energy efficiencies, and the rate of change (decrease rate) of these efficiencies is large with the progress of charge / discharge cycles. It's a problem.

As described above, the conventional Fe-Cr battery has the above-mentioned problems inherent therein, and such a point is greatly related to the selection of the electrode material. That is, since the electrochemical reaction during charging / discharging proceeds on the carbon fiber surface of the electrode material, the generation amount of hydrogen gas during charging is suppressed, and the current efficiency and conductivity (related to the speed of the redox reaction)
It is important to select an electrode material that can improve the performance and an electrode material that can stably exhibit such performance regardless of the progress of charge / discharge cycles.

The present inventors have arrived at the present invention as a result of earnest studies on the electrode material of the Fe-Cr battery that improves the total energy efficiency of the battery and improves the cycle life in view of the above circumstances.

[Means for Solving Problems] That is, the carbon-based electrode material for an electrolytic cell according to the present invention has a quasi-graphite microcrystal structure with a <002> interplanar spacing of 3.70Å or less determined by wide-angle X-ray analysis. , The number of bonding atoms on the surface of the carbon-based material determined by ESCA surface analysis is at least 3% of the number of carbon atoms, and the point is that boron is contained.

[Operation] That is, by using the above-mentioned electrode material, not only the current efficiency and electric conductivity which are the characteristic values of the battery are improved, but also the cycle life is improved at the same time. In other words, by using a carbon-based material having a pseudo-graphite microcrystalline structure with a <002> interplanar spacing of 3.70Å or less obtained by X-ray wide-angle analysis (the analysis method will be described later) as described above as an electrode material, The amount of hydrogen generated in the negative electrode during charging is suppressed, and the current efficiency can be significantly increased. When a carbon-based material having a <002> plane spacing of more than 3.70Å and low crystallinity is used, the amount of hydrogen generated in the negative electrode during charging is large, and the current efficiency cannot be increased. Although it is not clear why the current efficiency is improved by using a carbon-based material having a specific crystal structure with high crystallinity as an electrode material, the hydrogen overvoltage increases as the crystal structure develops, and the chromium overcharge occurs during charging. It is considered that the reduction of the complex ions occurs selectively, and thus the current efficiency increases.

On the other hand, ESCA surface analysis as described above (analysis method will be described later)
The ratio of the number of bonded oxygen atoms on the carbon material surface to the number of carbon atoms (obtained as% or less O / C ratio) obtained from the above is 3% or more. It can be significantly increased. When a carbon-based material having a low oxygen concentration with an O / C ratio of less than 3% on the surface of the material by ESCA analysis is used, the electrode reaction rate during discharge is small and the conductivity cannot be increased. It is not clear why the electric conductivity, in other words, the voltage efficiency can be improved by using a carbon-based material having a large number of oxygen atoms bonded to the surface of the material as an electrode material. It is considered that the oxygen atoms on the surface are effectively acting on the elimination of oxygen, the complex exchange reaction, and the like.

The carbon-based material having an increased concentration of surface oxygen atoms can be obtained by dry-oxidizing the carbon-based material having the internal crystal structure described above. This is, for example, in a weight yield of 65 ~ under an oxygen atmosphere having an oxygen partial pressure of 1 x 10 ^-2 torr or more.
It is implemented so that it is within the range of 99%. Processing temperature is usually 400
C. or higher is preferable. At a low temperature (for example, 200 to 300 ° C.), the carbonaceous material to be treated loses its reactivity and the effect of oxidation cannot be obtained. When the oxidation treatment is performed wet, the formation of intercalation compounds,
It should be avoided because there are many problems such as generation of harmful gas during processing. Further, this dry oxidation treatment may be a one-step system, or may be a two-step system at different temperatures.

By performing the dry oxidation treatment as described above, it is possible to expose more edges of the surface of the pseudo-graphite crystallite perpendicular to the C-axis to the material surface, and to form oxygen atoms effectively at this edge in the electrochemical reaction. be able to. This oxygen atom is generated as a carboxyl group, a phenolic hydroxyl group, a carbonyl group, a quinone group, a lactone group, or a free radical-like oxide, and these reactive groups greatly contribute to the electrode reaction, so that the conductivity (voltage efficiency) Can be increased.

With the above configuration, 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.

However, when the efficiency of each charge / discharge cycle of the battery using the electrode material having the above-mentioned structure is changed (the measuring method will be described later), the current efficiency hardly decreases, but the voltage efficiency gradually decreases. I understood.

We also arrived at the present invention as a result of earnest research on this second problem. That is, it has been found that by incorporating boron into the carbon-based material having the internal and surface structures as described above, a practically useful electrode material with very little change over time in the cycle can be obtained. The reason why the voltage efficiency is stabilized by including boron is not yet clear, but it is considered that boron influences the formation or protection of more stable surface oxygen groups in the redox system.

There is no particular limitation on the method of containing boron in the carbon-based material, but it is also possible to impregnate an inorganic or organic boron compound to the raw material before carbonization, or to impregnate the boron compound to what was once low-temperature carbonized, Further, high temperature treatment may be performed.

Examples of the inorganic boron compound include boric acid and borate, and examples of the organic boron compound include organic borate such as triethyl borate, tributyl borate, tripropyl borate and triphenyl borate. Further, it is preferable that the boron compound does not contain a metal element. When the metal element remains, the temperature dependence of the current efficiency increases, which is not preferable. It is considered that these compounds are decomposed by carbon after the attachment and remain in the carbon as boron. In addition, an ion implantation method, which is used for modifying various materials in recent years, may be used, or deposition may be performed by vapor deposition or sputtering. The impregnation by these methods is preferably carried out before the final carbonization. The amount of impregnation is 0.01 to 50 with respect to carbon as boron.
It is desirable to prepare it so that it exists in a weight percentage. It is more preferably 0.05 to 10% by weight. Regarding the distribution of boron in the carbonaceous material, it may be localized on the surface or the like or may be uniformly distributed over the entire surface.

As is apparent, all carbon-capable raw materials can be applied as the raw material of the carbon-based material of the present invention. For example, pitch from coal / petroleum, phenol-based, acrylic-based,
Cellulosic raw materials and the like can be mentioned. Furthermore, as the constituent structure of the carbon-based material, spun yarn, non-woven fabric, woven fabric,
A carbonaceous fiber aggregate composed of a knitted fabric or a hybrid structure thereof,
Examples thereof include a porous carbon body, a carbon-carbon composite, and a particulate carbon material, which are not particularly limited.

In the present invention, the carbon material specified for the <002> interplanar spacing and the O / C ratio was made to contain boron, but the current efficiency can be improved even when boron is contained in an ordinary carbon material which has not been prepared. Also, it is possible to obtain an effect peculiar to the addition of boron that the cycle change of the initial conductivity is reduced.

In addition, it is needless to say that the electrode according to the present invention can be effectively used for an electrode used for electrolytic synthesis, a zinc-chlorine secondary battery, a zinc-bromine secondary battery and the like.

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

<002> Surface spacing: d ₀₀₂ Carbon fiber knitted fabric was pulverized in an agate mortar, and about 5-10% by weight of high purity silicon powder for X-ray standard was added as an internal standard substance to the sample and mixed to form a sample cell. First, a wide-angle X-ray diffraction curve is measured by a transmission diffractometer method using CuKα rays as a radiation source.

For the correction of the curve, the so-called Lorentz, 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 from the baseline is plotted again 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 intensity curve, correct the angle of the midpoint with the internal standard, and turn this. It is twice the bending angle, and the <002> plane spacing is calculated from the wavelength λ of CuKα by the Bragg equation below.

λ: 1.5418Å θ: Diffraction angle Current efficiency A small flow-through type electrolytic cell shown in Fig. 1 is made, and charging / discharging is repeated at various constant current densities to test the electrode performance. 4N hydrochloric acid aqueous solution with ferric chloride and ferric chloride concentrations of 1M / l each was used for the positive electrode, and 4N hydrochloric acid aqueous solution with ferric chloride concentration of 1M / l was prepared for the negative electrode. In addition, although the shape of the electrode material is a rectangular plate in the figure, a triangular shape may be used to correspond to the shape of the electrolytic cell.
It may have a plate shape such as a circular shape, a spherical shape, a linear shape, or another irregular shape.

The amount of the positive electrode liquid was made to be a large excess with respect to the amount of the negative electrode liquid, so that the negative electrode characteristics could be mainly studied. 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 , but the same value was used during charging and discharging. In the one-cycle test that starts with charging and ends with discharging, the amount of electricity required for charging is Q ₁ coulomb, the amount of electricity extracted by constant current discharge up to 0.2 V and the subsequent constant potential discharge at 0.8 V is Q _{2 respectively.} , Q ₃ coulomb, and calculate the current efficiency by the following formula.

If a reaction other than the reduction of Cr ³⁺ to Cr ²⁺ , such as a side reaction such as reduction of H ⁺ , occurs during charging, the amount of electricity that can be taken out will decrease, and the current efficiency will decrease.

Cell conductivity The ratio of the quantity of electricity taken out during 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, the higher the discharge potential at high current density, the higher the voltage efficiency of the cell, and it can be judged that this is an excellent electrode.

After completing the measurement of cycle time change, and then using the same cell,
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 has elapsed, discharge to 0.2 V and discharge at a constant voltage of 0.8 V, and then measure the data. The tests of and were carried out at 40 ° C.

Measurement of C / O ratio by ESCA The device used for measuring the O / C ratio by X-ray photoelectron spectroscopy, which is abbreviated as ESPC or XPC, was Shimadzu ESCA 750, and ESCAPAC 760 was used for analysis.

Each sample was molded into a diameter of 6 mm, and was attached to a heating type sample stand with a conductive paste for analysis. 120 ° C before measurement
And vacuum degassed for 3 hours or more. A MgKα ray (1253.6 eV) was used as the radiation source, and the vacuum degree inside the apparatus was set to 10 ^-7 torr.

Measurement is performed for Cl _s and Ol _s peaks, and each peak is ESCAPed.
Perform a correction analysis using AC 760 (based on the correction method by JHS cofieid) to obtain each peak area. The area obtained is Cl _s
Is obtained by multiplying 1.00, Ol _s by the 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.

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 monofilament regenerated cellulose fiber which was sufficiently desulfurized, bleached, washed with water and dried. This knitted fabric had a basis weight of 424 g / m ² , an apparent density of 0.3709 g / cm ² , and a thickness of 1.2 mm. After scouring and drying this knitted fabric, the temperature was raised to 270 ° C. at a heating rate of 1 degree per minute in inert gas, then to 850 ° C. at a heating rate of 400 degree per hour, and after holding for 30 minutes It was cooled and further oxidized at 400 ° C. in air to obtain a carbon fiber cloth A. The X-ray analysis result for the fabric A was d ₀₀₂ = 3.85Å, and O / C was 11.3%. The initial current efficiency of the battery using the cloth A was 74%, and the electric conductivity thereof was 0.12 cm ^-2 . The fabric A had a basis weight of 265 g / m ² and a density of 0.26 g / cc.

Comparative Example 2 The maximum carbonization temperature of Fabric A was 850 ° C., but this temperature was 2
It was cooled to 400 ° C. to obtain a fabric B. Cloth B at 650 ° C
Fabric C was obtained by treating in air for 15 minutes. Cloth B has d ₀₀₂ of 3.
51Å, O / C is 2.1%, d ₀₀₂ of Fabric C is 3.50Å, O / C is 1
It was 0.1%. The initial current efficiency of the battery using fabric B is 9
3%, the initial conductivity was 0.24Scm ^-2 . The initial current efficiency of Fabric C was 98%, and the conductivity was 0.65 Scm ^-2 , but 100
Current efficiency after cycle is 98%, conductivity is 0.40Scm ^-2
Met.

Example The knitted fabric knitted in Comparative Example 1, scoured and dried, was dipped in a 5.0 wt% aqueous solution of boric acid, dehydrated, impregnated with 8.1% boric acid (1.4 wt% as boron), and dried to obtain a fabric. Comparative Example 1
Flame resistance and carbonization up to 2200 ° C. were carried out in the same manner as in the above, followed by cooling and treatment in air at 650 ° C. for 15 minutes to obtain a fabric D. The current efficiency of Fabric D was 98% at the initial stage and after 100 cycles. The initial conductivity was 0.66 Scm ^-2 , which was 0.57 Scm ^-2 after 100 cycles, which was a great improvement compared to Fabric C, and the rate of change after 100 cycles was even smaller.

Fabric D had d ₀₀₂ of 3.49Å and O / C of 9.7%.

[Advantages of the Invention] The present invention is configured as described above, and it is possible to suppress the amount of hydrogen gas generated during charging and significantly increase the current efficiency and the cell conductivity. In addition, it was possible to reduce the change with time of the cycle, and it was possible to bring about great industrial utility.

[Brief description of drawings]

FIG. 1 is an explanatory view showing an experimental device 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)

[Claims] 1. A <002> plane spacing obtained by X-ray wide-angle analysis is
It has a pseudo-graphite crystallite structure of 3.70 Å or less, the number of bound oxygen on the surface of the carbon-based material determined by ESCA surface analysis is at least 3% of the number of carbon atoms, and it contains boron. Carbon-based electrode material for electrolytic cells.