KR101371163B1 - Electrode coated by diamond like carbon for redox flow battery - Google Patents

Abstract

The present invention relates to an electrode of a redox flow battery equipped with a DLC coated film, and to realize an electrode having a thinner thickness by coating an amorphous solid carbon on the electrode used in the redox flow battery. And, it is possible to implement an electrode that can increase the efficiency of the redox flow battery.
DLC coating electrode for redox flow battery according to the present invention is preferably coated with DLC on both sides of the electrode made of copper or aluminum.
As a method for coating the DLC, any one selected from plasma enhanced chemical vapor deposition, ion beam sputtering, ion beam deposition, and laser ablation may be used.
Electrode coated with conductive diamond for redox flow battery according to the present invention has the advantage that there is no limitation to the object of the film that can be deposited because the above-described method can be used as a conductive diamond coating method.

Description

DLC coating electrode for redox flow battery {ELECTRODE COATED BY DIAMOND LIKE CARBON FOR REDOX FLOW BATTERY}

The present invention relates to an electrode for a redox flow battery, and more particularly, to a DLC coated electrode for a redox flow battery using a diamond like carbon (DLC) coated metal instead of graphite as an electrode of a redox flow battery.

With the development of transportation and communication, energy demand is increasing worldwide. Due to this increase in energy demand, the continuous use of fossil fuels results in the continuous emission of carbon dioxide, resulting in environmental pollution.

Carbon dioxide emitted as such is a greenhouse gas, and is cited as the main cause of global warming. Humans are trying to develop various kinds of energy such as solar energy, wind power, and fuel cell to suppress the above-mentioned emission of greenhouse gas.

Renewable energy such as solar power and wind power is greatly affected by the location environment and natural conditions, so the output fluctuates so that it is not easy to continuously supply, and there is a time difference between the time of energy production and the demand time.

MW class large capacity power storage batteries include lead acid batteries, NaS batteries, super capacitors, lithium secondary batteries and redox flow batteries (RFB).

A secondary battery is a term for a battery that can be recycled by repeatedly charging and discharging, not a battery that is finished with a single use. Redox batteries are also a secondary battery.

Unlike conventional secondary batteries, a redox battery is a system in which an active material in an electrolyte is oxidized, reduced, charged and discharged, and is an electrochemical storage device that directly stores chemical energy of an electrolyte as electrical energy.

The basic structure of the redox battery is shown in FIG. 1, and the components of the redox battery include a positive / negative electrolyte tank 11a and 11b in which active materials having different oxidation states are stored, and a pump for circulating the active material during charging and discharging. Positive and negative electrodes 14a and 14b separated into 12a and 12b, positive electrode cell 13a, negative electrode cell 13b and ion exchange membrane (membrane) 15.

The ion exchange membrane (membrane) (15) is used to separate the ions in the redox reaction (redox reaction refers to the reduction and oxidation reaction occurring in the anode cell 13a and the cathode cell 13b) during charging and discharging It is located between the electrodes 14a and 14b.

In this case, an electrolyte prepared by dissolving transition metals such as V, Fe, Cr, Cu, Ti, Mn, and Sn in a strong acid solution is used.

The electrolyte is not stored in the electrodes 14a, 14b in the positive cell 13a and the negative cell 13b, and is stored in the liquid state in the external electrolyte tanks 11a, 11b, and the pump 12a during the charging / discharging process. 12b is supplied into the positive cell 13a and the negative cell 13b.

In addition, the electrodes 14a and 14b are inactive electrodes, and the electrode itself reacts between the surface of the electrode and the electrolyte without a chemical reaction, and thus has a long life, and thus distinguishes it from a conventional battery. The electrode commonly used in redox cells is a graphite electrode.

In addition, since the redox battery has a structure capable of separating the battery stack (output: the positive cell 13a and the negative cell 13b) and the positive / negative electrolyte tank (capacity), the output and capacity can be freely designed. There are few restrictions on the installation location.

However, since the redox battery has a structure in which graphite is bonded to PVC (Poly Vinyl Chloride) as an electrode in the positive cell 13a and the negative cell 13b, the total thickness of the positive cell 13a and the negative cell 13b is 10-. It is about 20mm. In addition, if the surface area of the electrode is designed to increase the output, the thickness of the electrode becomes thicker.

In addition, the thicker the electrode, the greater the electrical resistance, resulting in a decrease in output per unit area. Thus, there is a limit in the output power that can be produced under the combination of graphite and PVC, and there is a problem in that the application range to which the redox battery can be applied is limited as the output is limited.

Patent Registration 10-698804 (Manufacturing method of electrode for electrochemical capacitor, manufacturing method of electrochemical capacitor and porous attached particle with solvent used in them)

The present invention has been made to solve the problems described above, for the redox flow battery that can be applied to a thin and light electrode that can increase the output compared to the cell area of the vanadium redox flow battery, and can reduce the weight of the entire battery To provide an electrode.

The present invention is to provide a DLC coating electrode for a redox flow battery, characterized in that the metal and the metal are centered, and a DLC coated film is bonded to both surfaces of the metal.

The metal used in the electrode for redox flow battery according to the present invention may be titanium, aluminum or copper.

The DLC film used for the redox flow battery electrode according to the present invention may be a solid carbon film in an amorphous state.

DLC film according to the present invention may be a mole fraction of 20 to 60 mol% of hydrogen.

Carbon in the DLC film according to the present invention may be synthesized by any one or more methods selected from plasma enhanced chemical vapor deposition, ion beam sputtering, ion beam deposition, and laser ablation.

The deposition of carbon in the DLC film may be deposited at 20 ~ 200 ℃.

The DLC coated electrode for redox flow battery according to the present invention is a DLC coated electrode formed at a low processing temperature, so there is no restriction on the target material to which carbon can be attached.

In addition, the DLC coated electrode is more durable than the graphite film electrode, and the structure of the entire cell can be designed thinner, thereby reducing the electric resistance by reducing the gap between the electrodes, thereby increasing the charge and discharge output per unit area of the cell.

In addition, as the surface deposition method, a plasma enhanced chemical vapor deposition method or a method such as ion sputtering is used, it has the advantage that can realize a very smooth surface with low electrode surface roughness.

1 is a view showing the basic structure of a typical redox battery.
2 is a view showing that carbon may have various forms depending on the bonding structure of carbon.
3 is a graph showing density of states according to energy.
Figure 4 is a schematic diagram showing the structure of a cell for a redox flow battery according to the present invention.
5 is a schematic view showing the structure of a DLC coated electrode for a redox flow battery according to the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, it should be understood that the following embodiments are provided so that those skilled in the art will be able to fully understand the present invention, and that various modifications may be made without departing from the scope of the present invention. It is not. Wherein like reference numerals refer to like elements throughout.

2 is a view showing that carbon may have various forms depending on the bonding structure of carbon.

Referring to FIG. 2, carbon has a tetrahedral structure in which carbon-to-carbon bonds form a diamond structure in which an angle from the center is 109.5 °, or three carbon atoms exist in the same plane and one carbon Atoms can form graphite in hexagonal closed packing structure, which is a structure located above the center of gravity of the triangle formed by three carbon atoms.

Carbon used as the coating material in the present invention refers to a solid carbon film (Diamond Like Carbon; DLC) in the amorphous state. The solid carbon film in the above-described amorphous state is a state in which sp1 hybrid bonds, sp2 hybrid bonds, and sp3 hybrid bonds are mixed.

Electrode for redox flow battery according to the present invention is a form in which the metal is coated using DLC and the sp1, sp2 and sp3 bonds are mixed and the mole% of hydrogen between the above-mentioned bonds is 20 ~ 60 mol% It is preferable.

The amorphous solid carbon film (DLC) is also called hydrated amorphous carbon, i-carbon, or tetrahedral amorphous carbon.

As described above, the physical properties of the DLC have a density of 1.8 to 3.6 g / cm 3, and the number of moles per unit volume is 0.2 to 0.3 (mol / cm 3). In addition, the compressive strength is 2000 ~ 8000 (kgf / ㎠) and the friction coefficient (friction coefficient) is 0.03 ~ 0.2.

In addition, as an optical property, since the refractive index has a range of 1.8 to 2.6, it is known to have a refractive index close to quartz 2.4.

The electrical resistivity of the solid carbon film in the amorphous state is 1013 to 1016 (Ωcm), and has a high resistivity. Therefore, no current flows to the surface on which the DLC is coated.

In addition, when looking at the internal structure of the material consisting of DLC, it can be described by the energy band diagram as shown in FIG.

3 is a graph showing density of states according to energy.

Referring to FIG. 3, in the case of DLC (solid carbon film in an amorphous state), the conductive band and the valence band do not overlap, and thus free electrons do not flow. Therefore, the insulator state, but when the conductive band existing above the Fermi level (E _f ) and the valence band existing below the Fermi level overlap due to external factors such as temperature increases, DLC In addition, there is the potential to become a conductive material.

According to the model as shown in FIG. 3, if the electrons near the Fermi energy or the conductive band are locally discontinuously hopping, DLC may also flow.

Equation 1 shows the electrical conduction of a material having a structure of DLC.

In Equation 1 p represents an electron density.

Thus, as the temperature increases, the number of electron densities that can be hopped also increases.

The state described above may be referred to as a locally conductive state, and when the local density of state overlaps, the DLC exhibits the same electrical conductivity as the metal.

However, in the case of the DLC as described above, since it contains a high mole fraction of hydrogen, it shows a high specific resistance.

Hereinafter, the structure of a redox flow battery cell including a DLC coated electrode will be described.

Figure 4 is a schematic diagram showing the structure of a cell for a redox flow battery according to the present invention.

Referring to FIG. 4, the conductive diamond coated electrode for the redox flow battery includes a positive electrode 14a and a negative electrode 14b, and the positive electrode 14a and the negative electrode 14b are bonded to each other with an ion exchange membrane 15 interposed therebetween. It is in the form that it is. The carbon felt 25 is disposed between the positive electrode 14a and the ion exchange membrane 15, and between the negative electrode 14b and the ion exchange membrane 15.

According to the DLC coated electrode according to the present invention, the anode and the cathode are preferably made of titanium.

Alternatively, the cathode of the DLC coated electrode may be made of copper, and the cathode may be made of aluminum. When the positive electrode is made of copper and the negative electrode is made of aluminum, oxidation may occur, so it is preferable to substitute titanium.

Another major component in the redox flow battery as described above may be referred to as the electrolyte and the ion exchange membrane (15). In particular, the ion exchange membrane 15 should have excellent ion permeability and low electrical resistance.

A redox flow battery is a battery composed of a cathode electrolyte and a cathode electrolyte in which the number of oxides is different in a solution state in which a material showing conductivity at an electrode is not solid, as in a conventional battery. The electromotive force of the redox flow cell is determined by the difference of the standard electrode potential E ⁰ of the redox couple (anode cell and cathode cell) constituting the cathode electrolyte and the cathode electrolyte.

In the redox battery, the ion exchange membrane 15 is a component that determines battery life and price. Examples of the ion exchange membrane 15 that can be used as the ion exchange membrane 15 include a Nafion membrane and a cellulose acetate film.

The redox battery is a secondary battery that circulates ions having variable valencies in both half cells separated by the ion exchange membrane 15 and charges and discharges the ions using the difference in electromotive force when the ions are oxidized and reduced. Therefore, the voltage of 1.2V to 1.5V is connected to each cell in series according to the degree of stacking of unit cells, so the overall voltage can be adjusted.

In addition, the use of DLC-coated electrodes does not require the use of thick PVC for structural design, and the thickness can be made thinner than the conventional graphite film as an electrode, which is lighter and less bulky than other batteries.

The ions used as electrolytes of the redox battery usually use elements of transition metals which may have various valences. In the case of using a different type of metal in each half cell, a mixture of transition metal ions through the ion exchange membrane 15 is required. This causes a decrease in capacity of the battery.

However, in the redox battery using vanadium metal as an electrolyte as in the present invention, there is no decrease in capacity of the battery due to mixing through the ion exchange membrane 15 by using the same electrolyte (vanadium sulfate) in both half cells. However, when the Nafion membrane is used, a transfer phenomenon occurs between the positive electrode 14a and the negative electrode 14b of vanadium ions through the Nafion membrane, so that the efficiency decreases due to self discharge. Because of the characteristics of the redox flow battery using the vanadium metal as described above, it may be an important problem to increase the selectivity of the ion exchange membrane (15). The selectivity of the ion exchange membrane 15 refers to a property that can transmit only specific ions.

For example, when V ^{4 +} or V ^{5 +} ions pass through the ion exchange membrane 15, a phenomenon in which V ^{5 +} ions are accepted and reduced by electrons is generated at the cathode, which is called self discharge.

In consideration of the above characteristics, the ion exchange membrane 15 in the redox battery should have high selectivity for ions, low electrical resistance, small diffusion coefficient of solute and solvent, and chemical stability. In addition, the mechanical strength must be excellent and the price must be low.

In the redox flow battery, the vanadium battery has a high acid resistance, oxidation resistance, and selective permeability because it uses a mixture of a transition metal element (V) and a strong acid (H ₂ SO ₄ ) as an electrolyte. However, when the Nafion membrane is used as an ion exchange membrane, the energy efficiency is lowered due to the improvement of the permeability of vanadium ions, and in the case of the CMV membrane, the lifespan characteristics are deteriorated.

As described above, since the redox flow battery is characterized in that the electrodes 14a and 14b are separated from the electrolyte material, the electrolytes of both electrodes 14a and 14b are separated and stored in the positive / negative electrolyte storage tanks 11a and 11b. . Therefore, the battery output can be controlled by the inflow flow rate. In addition, the redox flow battery is easy to maintain and manage the battery, it is possible to estimate the amount of power storage only by changing the capacity of the positive / negative electrolyte storage tank (11a, 11b).

Looking at the characteristics of the ions used as electrolytes, the V ^{5 +} ion solution is stable at -5 ° C for up to 12 months in vanadium ions and does not recrystallize. However, the V ^{2 +} solution is oxidized in the presence of air.

As described above, the decrease in current efficiency in the redox flow battery according to the present invention is caused by the self discharge on the permeate side due to the permeation of the ion exchange membrane 15 of each vanadium ion.

There are various kinds of ion migration phenomena that can occur in the ion exchange membrane (15), but the most representative ones are migration of the hydrogen cation by the concentration difference and migration by the potential difference.

Referring to the movement of ions due to the concentration difference, since there is a basic condition that maintains electrical neutrality when ions diffuse through the ion exchange membrane 15, only the transferred hydrogen cations cannot be independently evaluated. If only one ion moves alone, a space charge is formed therein and the conditions of electrical neutrality are destroyed, so that when cations and anions in the electrolyte solution diffuse through the ion exchange membrane 15, they always move in an equivalent amount. In order for diffusion of positive and negative ions to occur through the ion exchange membrane 15, generally there should be a concentration difference on both sides of the ion exchange membrane 15 and a concentration gradient must exist inside the ion exchange membrane 15. In addition, a diffusion boundary layer is formed at the boundary between the electrolyte solution and the ion exchange membrane, and the diffusion of the solvent as well as the diffusion of the solute should be considered.

In the case of ion transfer due to the potential difference, the charge is mainly carried by the cation, and the cation transport water in the membrane becomes 1.

When the vanadium-based redox battery is operated, most of the voltage loss is due to the resistance of the ion exchange membrane 15. Since the electrical resistance of the ion exchange membrane 15 is the biggest factor that hinders the flow of charge through the ion exchange membrane 15, the larger the resistance of the ion exchange membrane 15, the more difficult the hydrogen ion, the carrier of charge, to pass through the membrane. Therefore, as the ion exchange membrane (15) having a low electrical resistance is used, the voltage difference during charge and discharge of the redox flow cell becomes larger and the voltage efficiency becomes larger.

The amount of vanadium ions that pass through the ion exchange membrane 15 is closely related to the current efficiency of the battery. In the vanadium-based redox flow battery, the electric power is stored or generated in each of the half battery cells 13a and 13b through the redox reactions of Formulas 1 and 2, respectively. Formulas 1 and 2 show reactions at the positive electrode 14a and the negative electrode 14b during charging.

In FIG. 4, the film used for the redox flow battery is preferably a DLC coated film. In the case of a redox flow battery using a strong acid as an electrolyte, a carbon felt 25 is used as the electrodes 14a and 14b, and the carbon fibers forming the carbon felt 25 have different characteristics from those of bulk carbon or graphite. As a material, the microstructure of the carbon felt 25 is greatly changed depending on the precursor and the manufacturing conditions. Rayon, polyacrylonitrile (PAN), pitch, and the like are used as starting materials most commonly used in the production of the carbon felt 25.

The carbon felt 25 expands the electrode surface area through chemical, electrochemical and thermal activation, and when oxygen functional groups are introduced to the surface of the carbon felt 25, oxygen atoms move to smoothly oxidize / reduce reactions in the electrolyte. It serves to make it happen.

A conductive DLC coating or film is fused to an electrode other than a carbon felt (strictly current collector).

5 is a schematic view showing the structure of a conductive diamond coating electrode for a redox flow battery according to the present invention.

Referring to FIG. 5, the electrode is made of copper 14a (used as an anode) or aluminum 14b (used as a cathode). It is preferable that DLC-coated layers 20 are formed on both surfaces of the electrodes 14a and 14b.

Carbon in the DLC-coated film 20 is any one selected from plasma enhanced chemical vapor deposition (Plasma Enhanced Chemical Vapor Deposition), ion sputtering, ion beam deposition method and laser ablation method (laser ablation) Coated is preferred.

Plasma Enhanced Chemical Vapor Deposition (CVD) is a type of chemical vapor deposition, which is a process in which a gaseous reactant is reacted to cover a desired material on a silicon wafer. However, the more semiconductors are produced at lower process temperatures, the fewer defects there are.

In the case of plasma enhanced chemical vapor deposition, a chemical reaction occurs at a low temperature by increasing a chemical activity by passing a reactant gas through an argon plasma.

In the case of the plasma enhanced chemical vapor deposition method, the productivity is high, a smooth surface state can be obtained on the film, and the thermal stability is excellent. However, when applied to the object having a complex shape there is a disadvantage that it is difficult to coat uniformly.

Another coating method is an ion beam sputtering method. Ion beam sputtering is the process of bombarding a surface by particles with energy, causing material to be released and released from the solid surface by the momentum exchange at this time. Therefore, in the target material (diode state) facing the object to be treated, a heavy inert gas, argon gas, forms a plasma by glow discharge, and causes an ion bombardment in which argon ions collide with the surface of the target material, which is a cathode. Will be emitted. Such a method is not a chemical thermal reaction process, there is an advantage that can be used as a target material of any material as a method of making a vapor phase by a mechanical process. Generally, DC power is used, but in the case of non-conductive target material, sputtering can be performed by using RF potential, which is an AC process. However, there is a disadvantage in that the adhesiveness is not good, and the film is also poor in quality.

It is preferable to use in combination with another vapor deposition method.

Ion beam deposition is a method that is useful for making high purity thin films with low defects by direct deposition by relatively low energy metal ions that do not damage the substrate at relatively low substrate temperatures.

It is thought that the research on ion beam deposition method has been done for a long time, but when the full-fledged study was started, it was found that DLC (diamond like carbon) film which was optically transparent and chemically stable at room temperature was obtained by irradiating carbon ion beam on the silicon substrate in vacuum. Can be.

The ion beam deposition method has an advantage of eliminating the influence of impurities such as Cl by not using a reactive gas such as CCl ₄ gas used for other deposition production.

Laser ablation is also referred to as pulsed laser deposition. The laser ablation method may be referred to as a process of forming a plum through an ablation phenomenon of a carbon target. Pulsed laser deposition is a form of physical vapor deposition that produces thin films. Plasma is emitted from a short wavelength (high energy) pulse laser focused on a lens by placing the target of the material in a vacuum chamber and then crystallized by buried in a hot substrate facing the target. The PLD method has a property of easily moving the composition of the target material, and is fast and relatively harmless, but has a disadvantage in that the size of the thin film is limited.

In the process of depositing the carbon particles, the ion beam deposition method and the plasma enhanced chemical vapor deposition method are preferably performed at a temperature of 25 to 200 ° C.

The reason why the deposition is possible at such a low temperature is that the ion beam deposition method and the plasma enhanced chemical vapor deposition method use a process of ionizing a gas.

It is preferable that the temperature is lower than 200 ° C because gas is difficult to ionize at a temperature lower than 25 ° C, and deformation of an organic material that is a target of the target may be caused at a temperature higher than 200 ° C. In the case of the ion beam deposition method and the PECVD method as described above, since the deposition is possible at a low temperature, it is possible to coat the DLC without limiting the target material.

The DLC may be referred to as a conductive diamond, but is not limited to the above-described process as long as the conductive diamond may be coated.

As mentioned above, although preferred embodiment of this invention was described in detail, this invention is not limited to the said embodiment, A various deformation | transformation by a person of ordinary skill in the art within the scope of the technical idea of this invention is carried out. This is possible.

10: redox battery 11a: positive electrolyte tank
11b: negative electrolyte tank 13a: positive electrode cell
13b: cathode cell 14a: anode
14b: cathode 15: ion exchange membrane
20: conductive diamond coated or conductive diamond coated film (DLC coated film), 25: carbon felt

Claims (7)

Metal and
DLC coating electrode for a redox flow battery comprising a film 20, the center of the metal, the DLC coated on both sides of the metal. The method of claim 1,
DLC coating electrode for a redox flow battery, characterized in that the metal is titanium, aluminum or copper. The method according to claim 1 or 2
One of the DLC coating electrodes for the redox flow battery forms an anode 14a, and the other forms an anode 14b, and an ion exchange membrane 15 is disposed between the positive electrodes 14a and 14b. DLC coated electrode for redox flow battery. The method of claim 1,
DLC coated film 20 used for the redox flow battery electrode is a DLC coating electrode for a redox flow battery, characterized in that the amorphous carbon film. The method according to any one of claims 1, 2 and 4,
The DLC coated film 20 is a DLC coating electrode for a redox flow battery, characterized in that the mole fraction of hydrogen 20 to 60 mol%. The method of claim 1,
The carbon in the DLC coated film 20 is formed by any one or more methods selected from plasma enhanced chemical vapor deposition, ion sputtering, ion beam deposition, and laser ablation. DLC coated electrode for redox flow battery, characterized in that the coating. The method according to claim 6,
The deposition of carbon by plasma enhanced chemical vapor deposition or ion beam deposition in the coating process for carbon, DLC coating electrode for redox flow battery, characterized in that made at 25 ~ 200 ℃.

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