Electrode for redox flow battery and method for manufacturing the same

Abstract

Provided is an electrode for a redox flow battery, the electrode being configured from a plate-form carbon electrode material in which uniform communication macro holes are formed in three-dimensional mesh form and there is no contact interface between carbon particles, and moreover being such that the average macro hole diameter of the carbon electrode material is within the range of 6-35 µm, the surface interval of a graphite crystallite (002) surface in the carbon electrode material is within the range of 0.33-0.40 nm, and the c-axis-direction crystallite size of the graphite crystallite is within the range of 0.9-8.5 nm.

Description

[Technical Field]

[0001] The present invention relates to an electrode for a redox flow battery, which is a type of fluid flow storage battery, and a manufacturing method thereof. In particular, the present invention relates to an electrode for a redox flow battery that has a high maximum power density and low pressure loss when an electrolyte is pumped, and a manufacturing method thereof. In this specification, "activation treatment" refers to a treatment in which a carbonized plate is heated to a predetermined temperature and then an activation gas is supplied to the carbonized plate to form micropores in the carbonized plate, making the carbonized plate porous and increasing the active surface area. "Air oxidation treatment" refers to a treatment in which a carbonized plate is heated to a predetermined temperature in air to form micropores in the carbonized plate, making the carbonized plate porous, increasing the active surface area, and introducing oxygen functional groups that serve as reaction active sites onto the surface of the carbonized plate. This international application claims priority based on Japanese Patent Application No. 2019-213813, filed on November 27, 2019, the entire contents of which are incorporated herein by reference. [Background technology]

[0002] In recent years, vanadium redox flow batteries (VRFBs, hereafter simply referred to as redox flow batteries) have attracted attention as energy storage batteries among fluid flow-type storage batteries. Redox flow batteries use a pump to deliver an electrolyte containing an active material to electrodes. They adjust for fluctuations in power generated by renewable energy sources such as solar and wind power, and store this power. Redox flow batteries consist of an electrolytic cell separated into a positive electrode chamber and a negative electrode chamber by a hydrogen ion-permeable diaphragm; a positive electrode tank for storing a positive electrode electrolyte; a negative electrode tank for storing a negative electrode electrolyte; and a pump for circulating the electrolyte between the tank and the electrolytic cell. Charging and discharging are performed by circulating the positive electrode electrolyte between the positive electrode tank and the positive electrode chamber, and the negative electrode electrolyte between the negative electrode tank and the negative electrode chamber, causing oxidation-reduction reactions to occur on the electrodes installed in the positive electrode chamber and the negative electrode chamber.

[0003] Conventionally, carbon fiber aggregates made of carbon felt, carbon paper, carbon cloth, etc. have been used as electrodes in redox flow batteries (see, for example, Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2017-10809 A (Claim 8) [Patent Document 2] JP 2018-147595 A (Claim 1, paragraph

[0002] ) Summary of the Invention [Problem to be solved by the invention]

[0005] Redox flow batteries, which are used to store renewable energy such as solar and wind power, require electrodes that enable large current charging and discharging in response to short-cycle fluctuations in renewable energy output due to weather. However, the electrodes for redox flow batteries described in Patent Documents 1 and 2 are carbon fiber assemblies made of a fiber laminate material in which carbon fibers are folded over each other, resulting in high pressure loss (fluid resistance) when delivering electrolyte and low activity of the electrode material. As a result, there are limits to improving the current output at the sending end of the redox flow battery, i.e., the maximum power density of the battery, and there are still issues to be resolved regarding electrodes for redox flow batteries.

[0006] An object of the present invention is to provide an electrode for a redox flow battery that has a high maximum power density and a low pressure loss when an electrolyte is fed, and a method for producing the electrode. [Means for solving the problem]

[0007] A first aspect of the present invention is an electrode for a redox flow battery, which is constructed by stacking one or more plate-shaped carbon electrode materials, each of which has uniform, interconnected macropores formed in a three-dimensional network pattern and no contact interfaces between carbon particles, characterized in that the carbon electrode materials have an average macropore diameter in the range of 6 μm to 35 μm, the interplanar spacing of the (002) planes of graphite crystallites in the carbon electrode material is in the range of 0.33 nm to 0.40 nm, the crystallite size in the c-axis direction of the graphite crystallites is in the range of 0.9 nm to 8.5 nm, and the thickness of the electrode is in the range of 0.4 mm to 0.8 mm.

[0008] A second aspect of the present invention is an invention based on the first aspect, wherein the carbonized product has a BET specific surface area of ​​100 m2 or less as measured by a nitrogen adsorption method at 77 K. 2 / g~1500m 2 / g, the micropore volume of the carbonized material is in the range of 0.05 ml / g to 0.70 ml / g, the interplanar spacing of the (002) planes of graphite crystallites in the carbonized material is in the range of 0.33 nm to 0.40 nm, and the crystallite size in the c-axis direction of the graphite crystallites is in the range of 0.9 nm to 8.5 nm.

[0009] A third aspect of the present invention is a method for producing an electrode for a redox flow battery, comprising the steps of: cutting a block of porous phenolic resin, the block having interconnected uniform macropores formed in a three-dimensional network and having an average macropore diameter in the range of 4 μm to 70 μm, into a plate; heating the cut-out plate in an inert gas atmosphere from room temperature to a range of 800°C to 1000°C and maintaining it at the elevated temperature in the inert gas atmosphere to thereby obtain a carbonized plate; heating the carbonized plate from room temperature to a range of 1100°C to 2500°C and maintaining it at the elevated temperature in the inert gas atmosphere to thereby perform a high-temperature heat treatment; and heating the high-temperature heat-treated carbonized plate in air from room temperature to a range of 350°C to 600°C and maintaining it at the elevated temperature in the air to thereby obtain a carbon plate electrode material.

[0010] A fourth aspect of the present invention is a method for producing an electrode for a redox flow battery, comprising the steps of: cutting a block of porous phenolic resin, the block having interconnected uniform macropores formed in a three-dimensional network and having an average macropore diameter in the range of 4 μm to 70 μm, into a plate; heating the cut-out plate in an inert gas atmosphere from room temperature to a range of 800°C to 1000°C and maintaining it at the elevated temperature in an inert gas atmosphere to obtain a carbonized plate; heating the carbonized plate in an inert gas atmosphere from room temperature to a range of 1100°C to 2500°C and maintaining it at the elevated temperature in an inert gas atmosphere to perform a high-temperature heat treatment; and activating the high-temperature heat-treated carbonized plate so as to obtain a carbon plate electrode material with an activation yield in the range of 50% to 90%.

[0011] A fifth aspect of the present invention is an invention based on the fourth aspect, in which the activation treatment is carried out by raising the temperature of the high-temperature heat-treated carbonized plate in an inert gas atmosphere from room temperature to a temperature range of 800°C to 1000°C, and maintaining the temperature at that elevated temperature in a carbon dioxide gas flow.

[0012] A sixth aspect of the present invention is a redox flow battery using the electrode of the first or second aspect. [Effects of the Invention]

[0013] A redox flow battery electrode according to a first aspect of the present invention is comprised of one or more plate-shaped carbon electrode materials having uniform, interconnected macropores formed in a three-dimensional network and no contact interfaces between carbon particles, wherein the carbon electrode materials have an average macropore diameter in the range of 6 μm to 35 μm, a lattice spacing of the (002) planes of graphite crystallites in the carbon electrode material in the range of 0.33 nm to 0.40 nm, a crystallite size in the c-axis direction of the graphite crystallites in the range of 0.9 nm to 8.5 nm, and a thickness of the electrode in the range of 0.4 mm to 0.8 mm. Therefore, compared with electrodes comprised of conventional fiber laminated materials in which carbon fibers are folded over one another, such as carbon felt, carbon paper, and carbon cloth, the electrode has properties suitable for a redox flow battery electrode, such as a higher maximum power density and a lower pressure loss when an electrolyte solution is pumped through the battery.

[0014] A redox flow battery electrode according to a second aspect of the present invention is the redox flow battery electrode according to the first aspect, wherein the carbonized material has a BET specific surface area of ​​100 m as measured by a nitrogen adsorption method at 77 K. 2 / g~1500m 2 / g, the micropore volume of the carbon electrode material is in the range of 0.05 ml / g to 0.70 ml / g, the interplanar spacing of the (002) planes of the graphite crystallites in the carbon electrode material is in the range of 0.33 nm to 0.40 nm, and the crystallite size in the C-axis direction of the graphite crystallites is in the range of 0.9 nm to 8.5 nm. Therefore, compared with electrodes made of conventional fiber laminate materials in which carbon fibers are folded over, such as carbon felt, carbon paper, and carbon cloth, the carbon electrode material has properties suitable for use as an electrode for a redox flow battery, such as a higher maximum power density of the battery and a lower pressure loss when the electrolyte is pumped through the battery.

[0015] In a method according to a third aspect of the present invention, a block of porous phenolic resin having a three-dimensional network of interconnected, uniform macropores with an average macropore diameter in the range of 4 μm to 70 μm is cut into a plate, and the cut plate is heated in an inert gas atmosphere from room temperature to a range of 800°C to 1000°C and maintained at the elevated temperature to obtain a carbonized plate. The carbonized plate is then heated in an inert gas atmosphere from room temperature to a range of 1100°C to 2500°C and maintained at the elevated temperature to perform a high-temperature heat treatment. The high-temperature heat-treated carbonized plate is then heated in air from room temperature to a range of 350°C to 600°C and maintained at the elevated temperature to perform an air oxidation treatment. Thus, the plate cut from the block of porous phenolic resin, while maintaining the three-dimensional network of interconnected, uniform macropores, undergoes the carbonization, high-temperature heat treatment, and air oxidation treatment to become the final carbonized plate electrode. The high-temperature heat treatment and air oxidation treatment of the carbon platelets increase the crystallinity of the carbon matrix, resulting in a predetermined size for the interplanar spacing of the graphite crystallite (002) planes and an increase in the BET specific surface area of ​​the carbon platelets. As a result, this manufacturing method gives the carbon platelets a highly active material structure, making it possible to produce an electrode that can appropriately handle short-cycle, large-current charging and discharging of redox flow batteries and enable high battery output.

[0016] In a method according to a fourth aspect of the present invention, a block of porous phenolic resin having a three-dimensional network of interconnected, uniform macropores with an average macropore diameter in the range of 4 μm to 70 μm is cut into a plate, and the cut plate is heated in an inert gas atmosphere from room temperature to a range of 800°C to 1000°C and maintained at the elevated temperature to obtain a carbonized plate. The carbonized plate is then heated in an inert gas atmosphere from room temperature to a range of 1100°C to 2500°C and maintained at the elevated temperature to be subjected to a high-temperature heat treatment, and the high-temperature heat-treated carbonized plate is then activated so that the activation yield is in the range of 50% to 90%. Here, the "activation yield" is the rate of change in sample mass due to activation treatment, as expressed by the following formula: Activation yield (%) = (mass of sample after activation / mass of sample before activation) × 100% Therefore, plates cut from a block of porous phenolic resin undergo carbonization, high-temperature heat treatment, and activation treatment while maintaining the structure of interconnected, uniform macropores formed in a three-dimensional network. They become the final carbonized plate electrode. The high-temperature heat treatment of the carbonized plate adjusts the interplanar spacing of the graphite crystallite (002) planes to a predetermined size. The activation treatment primarily forms micropores in the carbon material, further increasing the BET specific surface area and reactive surface area of ​​the carbonized plate. This manufacturing method thereby gives the carbonized plate a more active material structure, reduces the electrode's reactive resistance, and enables the production of electrodes that can adequately handle short-cycle, high-current charging and discharging of redox flow batteries and enable high battery output.

[0017] In the method of the fifth aspect of the present invention, the activation treatment is carried out by heating the high-temperature heat-treated carbonized plate in an inert gas atmosphere from room temperature to a range of 800°C to 1000°C, and then maintaining the plate at the elevated temperature in a carbon dioxide gas flow. Therefore, when carbon dioxide activation treatment is carried out, micropores are formed. This manufacturing method has the advantage that the BET specific surface area is increased by the micropores.

[0018] The redox flow battery of the sixth aspect of the present invention uses the electrode of either the first or second aspect, and therefore has the characteristics of a higher reaction current density and a higher maximum battery power density than redox flow batteries using electrodes made of a conventional fiber laminate material in which carbon fibers are folded over each other, such as carbon felt, carbon paper, or carbon cloth. [Brief explanation of the drawings]

[0019] [Figure 1] 1A and 1B are scanning electron microscope (SEM) photographs of the carbonized platelet of Example 1. Fig. 1(a) is a photograph at a magnification of 5000 times, Fig. 1(b) is a photograph at a magnification of 2000 times, and Fig. 1(c) is a photograph at a magnification of 500 times. [Figure 2]FIG. 2 is an SEM photograph of the porous phenolic resin of Example 1. [Figure 3] FIG. 1 is an assembled perspective view of a cell using carbonized plate-like materials as electrodes. [Figure 4] FIG. 1 is a schematic diagram of a current-voltage (IV) measurement test device. [Figure 5] 1 is a graph showing current-voltage curves and power curves of batteries using the carbonized plate materials of Examples 2 and 6 and the carbon papers of Comparative Examples 3 to 5. FIG. [Figure 6] FIG. 2 is a graph showing current-voltage curves and power curves of batteries using the carbonized material plates of Examples 1, 2, and 10 and Comparative Example 1. [Figure 7] FIG. 1 is a graph showing current-voltage curves and power curves of batteries using the carbonized material plates of Examples 3, 4, and 6. [Figure 8] 1 is a graph showing current-voltage curves and power curves of batteries using the carbonized material plates of Examples 1, 5, and 9 and Comparative Example 2. FIG. [Figure 9] FIG. 2 is a graph showing current-voltage curves and power curves of batteries using the carbonized material plates of Example 1, Example 7, Comparative Example 6, and Comparative Example 7. [Figure 10] FIG. 10 is a graph showing current-voltage curves and power curves of batteries using the carbonized platelets of Examples 4 and 8. [Figure 11] FIG. 2 is a graph showing current-voltage curves and power curves of batteries using the carbonized platelets of Examples 1, 11, and 12. [Figure 12] FIG. 10 is a diagram showing nitrogen adsorption / desorption isotherms of carbonized plate-like materials according to different treatment methods. [Figure 13] FIG. 10 is a diagram showing nitrogen adsorption / desorption isotherms of carbonized plate-like materials according to different treatment methods. [Figure 14] FIG. 10 is a diagram showing nitrogen adsorption / desorption isotherms of carbonized plate-like materials according to different treatment methods. DETAILED DESCRIPTION OF THE INVENTION

[0020] Next, a method for producing an electrode for a redox flow battery of the present invention will be described in two separate embodiments: a first embodiment and a second embodiment. The first embodiment and the second embodiment differ in whether the treatment step after the high-temperature heat treatment is an air oxidation treatment or a carbon dioxide activation treatment.

[0021] First Embodiment [Method for manufacturing electrodes for redox flow batteries] In the method for manufacturing a redox flow battery electrode according to the first embodiment, a block of porous phenolic resin having interconnected macropores formed in a three-dimensional network with an average macropore diameter in the range of 4 μm to 70 μm is cut into a plate. The cut-out plate is heated in an inert gas atmosphere from room temperature to a range of 800°C to 1000°C and maintained at the elevated temperature in the inert gas atmosphere to obtain a carbonized plate. The carbonized plate is then heated in an inert gas atmosphere from room temperature to a range of 1100°C to 2500°C and maintained at the elevated temperature in the inert gas atmosphere to perform a high-temperature heat treatment. The high-temperature heat-treated carbonized plate is then heated in air from room temperature to a range of 350°C to 600°C and maintained at the elevated temperature in the air to perform an air oxidation treatment.

[0022] Next, each step of the manufacturing method of the first embodiment will be described in detail.

[0023] (a) Manufacturing of porous phenolic resin blocks and cutting the blocks into plates (a-1) Production of porous phenolic resin blocks Porous phenolic resin blocks are manufactured by the following method. First, phenolic resin and polyvinyl alcohol (PVA) are mixed. To mix the phenolic resin and PVA uniformly, it is preferable to disperse the liquid phenolic resin in water, dissolve the PVA in the water as a dispersion medium, and then stir and mix the two. During this mixing, it is preferable to add a pore-forming agent such as rice starch, wheat starch, corn starch, or potato starch, and a crosslinking agent such as an aqueous formaldehyde solution, butyraldehyde, or glutaraldehyde. It is also preferable to add a catalyst such as maleic acid, hydrochloric acid, or sulfuric acid to harden the mixture.

[0024] Next, water is added to this mixture and mixed, resulting in a reaction liquid, which is poured into a synthetic resin block mold and heated to react for a predetermined time. The resulting reaction product is removed from the mold, washed with water to remove the pore-generating agent and any unreacted materials, and then dried. This manufacturing method yields a porous phenolic resin block in which interconnected macropores with an average macropore diameter ranging from 4 μm to 120 μm are formed in a three-dimensional network. In the manufacturing method for the phenolic resin block of this embodiment, the pore-generating agent is uniformly mixed into the phenolic resin, and the interconnected fine and uniform pore diameter can be adjusted to the desired size by selecting the type, amount, and temperature of the pore-generating agent. The average pore diameter of the interconnected macropores of the precursor formed in a three-dimensional network, ranging from 4 to 70 μm, is determined taking into consideration the pressure loss (fluid resistance) when delivering the electrolyte when used as an electrode, the transport of the active material, the reaction specific surface area, and the reaction resistance. If the average pore size of the interconnected macropores of the precursor is less than the lower limit, the pressure loss during electrolyte transport cannot be reduced when the precursor is used as an electrode, and transport of the active material is not promoted. If the average pore size exceeds the upper limit, the reaction specific surface area decreases and the reaction resistance increases. The average macropore size is measured using a mercury porosimeter.

[0025] (a-2) Cutting the porous phenolic resin block into plates There are no particular restrictions on the size of the plates cut from the porous phenolic resin block. For example, a rectangular parallelepiped with a length of 30 mm and a width of 50 mm can be cut using a diamond saw. After cutting into the rectangular parallelepiped, the block can be cut into plates with a thickness of 0.5 mm to 1.0 mm using a diamond saw. The shape, dimensions, and thickness of the cut plates are determined based on the shape and size of the redox flow battery electrode. The thickness of the redox flow battery electrode is preferably 0.4 mm to 0.8 mm from the standpoints of reaction area, pressure loss, ohmic resistance, and cost. A thin electrode results in low maximum output, while an increased electrode thickness increases ohmic resistance, making it uneconomical from a material cost perspective. Taking into account the shrinkage rate, including carbonization, a plate of a thickness appropriate for the electrode is cut from the block.

[0026] (b) Manufacturing process of carbonized plate Next, the cut-out plate-shaped piece made of porous phenolic resin is placed in a heat treatment furnace. A horizontal tubular electric furnace is preferably used as the heat treatment furnace. Subsequently, the furnace is filled with an inert gas atmosphere, and the temperature is increased from room temperature to 800°C to 1000°C, preferably 800°C to 900°C, and the heat treatment is performed by maintaining the temperature in the inert gas atmosphere. After the heat treatment, the electric furnace is preferably slowly cooled to room temperature. The temperature increase rate is preferably 5°C / min to 20°C / min, and the time for maintaining the temperature is preferably 0.5 to 2 hours. By performing the heat treatment under the above conditions, the plate-shaped piece cut out from the block is carbonized to obtain a carbonized plate. The inert gas used is nitrogen, argon, helium, or another gas.

[0027] (c) High-temperature heat treatment process of the carbonized plate Next, the carbonized plate is subjected to high-temperature heat treatment. This high-temperature heat treatment is preferably performed using the same heat treatment furnace as used in the carbonization treatment described above, and a horizontal tubular electric furnace is preferably used as the heat treatment furnace. After placing the carbonized plate into the furnace, the furnace is filled with an inert gas atmosphere, and the temperature is increased from room temperature to 1100°C to 2500°C, preferably 1200°C to 2200°C. The high-temperature heat treatment is then performed by maintaining the temperature in the inert gas atmosphere. After the high-temperature heat treatment, the electric furnace is preferably slowly cooled to room temperature. The temperature increase rate during the high-temperature heat treatment is preferably 5°C / min to 20°C / min, and the time for maintaining the temperature is preferably 0.5 hours to 2 hours. By performing the high-temperature heat treatment under the above conditions, impurities in the carbonized material are removed and the crystallinity of the carbon matrix is ​​increased. The inert gas used is nitrogen, argon, helium, or other gases.

[0028] The reason why the temperature to be raised for the high-temperature heat treatment is specified within the above range is that if the temperature is lower than the lower limit, the crystallinity of the carbon matrix does not improve sufficiently, and if the temperature is higher than the upper limit, the BET specific surface area of ​​the carbonized platelets is not easily increased in the air oxidation treatment in the subsequent step. Also, the reason why the temperature rise rate for the high-temperature heat treatment is specified within the above range is that if the temperature is lower than the lower limit, the time required for the high-temperature heat treatment is likely to be long, and if the temperature is higher than the upper limit, the crystallinity of the carbon matrix is ​​not easily improved sufficiently.

[0029] (d) Air oxidation treatment process of the high-temperature heat-treated carbonized plate The air oxidation treatment is performed by placing the high-temperature heat-treated carbonized plate in a muffle furnace. The high-temperature heat-treated carbonized plate is placed in the muffle furnace in air, and the muffle furnace is heated from room temperature to 350°C to 600°C, preferably 400°C to 500°C, and maintained at the elevated temperature. The carbonized plate is preferably maintained in air at the elevated temperature for 1 hour to 24 hours so that the mass loss rate of the carbonized plate is 1.0% to 25.0%, preferably 1.8% to 20.8%. By performing the air oxidation treatment under the above conditions, micropores are formed in the carbonized plate, making it porous, and the BET specific surface area is increased. Oxygen functional groups are introduced onto the surface of the carbonized plate, creating reactive sites. This allows the manufacturing method of the first embodiment to provide a highly active material structure for the carbonized plate, enabling the production of an electrode that can adequately handle short-cycle, high-current charging and discharging of redox flow batteries and enable high battery output.

[0030] The temperature to which the carbonized platelets are heated for air oxidation treatment is specified within the above range because, below the lower limit, oxidation of the carbonized platelets is insufficient, while above the upper limit, the mass loss rate increases dramatically, resulting in problems such as excessive oxidation that makes it impossible to maintain the shape and thermal decomposition of oxygen on the surface. The mass loss rate is specified within the above range because, below the lower limit, it is difficult to obtain an electrode with a sufficient BET specific surface area, while above the upper limit, there is a risk of excessive oxidation that makes it impossible to maintain the shape.

[0031] <Second embodiment> [Method for manufacturing electrodes for redox flow batteries] In the manufacturing method of the electrode for a redox flow battery of the second embodiment, (a) the process of manufacturing a block of porous phenolic resin and cutting the block into plates, (b) the process of manufacturing a carbonized plate, and (c) the high-temperature heat treatment process of the carbonized plate are the same as those of the manufacturing method of the first embodiment.

[0032] In the method for manufacturing a redox flow battery according to the second embodiment, the step following the (c) high-temperature heat treatment step of the carbonized plate is a carbon dioxide activation treatment step.

[0033] (e) Carbon dioxide activation treatment process for the high-temperature heat-treated carbonized plate The carbon dioxide activation treatment is carried out by placing the high-temperature heat-treated carbonized plate in a horizontal tubular furnace. The high-temperature heat-treated carbonized plate is heated to a temperature of 800°C to 1000°C in an inert gas atmosphere in the horizontal tubular electric furnace. Next, the introduction of the inert gas is stopped and carbon dioxide gas is introduced. The temperature is maintained under carbon dioxide gas flow so that the activation yield is preferably 50% to 90%, more preferably 55% to 85%. The maintenance time is preferably 0.5 to 12 hours, more preferably 1 to 10 hours.

[0034] In the carbon dioxide activation treatment, the reaction shown in the following formula occurs, forming pores in the carbon matrix. C+CO2→2CO↑ That is, the carbon dioxide activation treatment following the high-temperature heat treatment of the carbonized plate forms micropores in the carbon on the surface of the carbon material, further increasing the BET specific surface area of ​​the carbonized plate and further increasing the reactive surface area. Furthermore, performing the activation treatment in a carbon dioxide gas atmosphere facilitates the development of micropores. Note that the micropores referred to here are in the range of less than 2 nm. As a result, the manufacturing method of the second embodiment makes the carbonized plate have a more active material structure, reduces the reaction resistance of the electrode, and enables the production of an electrode that can appropriately handle short-cycle, large-current charging and discharging of a redox flow battery and enables high battery output.

[0035] [Electrodes for redox flow batteries] The redox flow battery electrodes produced by the methods of the first and second embodiments are composed of a plate-shaped carbonized material. This carbonized material has uniform interconnected macropores formed in a three-dimensional network, and no contact interfaces between carbon particles. In other words, the carbonized material plate is made of seamless carbon in which no contact interfaces between carbon particles exist, and has a three-dimensional network structure of homogeneous and uniform interconnected macropores in the thickness direction and in-plane direction.

[0036] Regarding the interconnected pores, the average macropore diameter of the carbonized plate is in the range of 6 μm to 35 μm, preferably 6 μm to 25 μm. If the average macropore diameter is less than 6 μm, when the carbonized plate is used as an electrode, the pressure loss when the electrolyte is pumped using a power pump cannot be reduced, and the transport of the active material is not promoted. If the average macropore diameter exceeds 35 μm, when the carbonized plate is used as an electrode, the reaction specific surface area decreases and the reaction resistance increases. The average pore diameter of the carbonized plate is measured using a mercury porosimeter. The BET specific surface area of ​​the carbonized plate is determined using a BELSORP28A gas analyzer manufactured by Microtrac BEL Corp. by measuring the amount of nitrogen adsorption at each time while varying the relative pressure at a temperature of 77 K, and then using the nitrogen adsorption / desorption isotherm obtained by the measurement according to the BET equation.

[0037] In addition to the above, the BET specific surface area of ​​the carbonized plate is 100m 2 / g~1500m 2 / g range, preferably 600m 2 / g~1500m 2 / g. BET specific surface area is in the range of 100m 2 If the specific surface area is less than 1 / g, when the electrode is formed, the BET specific surface area is insufficient and a sufficient current output cannot be obtained.

[0038] Furthermore, the micropore volume is in the range of 0.05 ml / g to 0.70 ml / g, preferably in the range of 0.2 ml / g to 0.40 ml / g. The reason for specifying the micropore volume within the above range is that if the micropore volume is below the lower limit, sufficient capacity cannot be ensured, and if the micropore volume is above the upper limit, there is a problem of a decrease in the electrode bulk density. The micropore volume was determined by the Dubinin-Radushkevich (DR) method.

[0039] The interplanar spacing of the (002) planes of the graphite crystallites in the carbonized plate is in the range of 0.33 nm to 0.40 nm, preferably 0.34 nm to 0.39 nm. If this interplanar spacing exceeds the upper limit, the crystallinity will be insufficient and the electrical conductivity will be poor. It is known that the (002) interplanar spacing of graphite with a sufficiently high crystallinity is 0.3354 nm, and no carbon material has an interplanar spacing of the (002) planes less than the lower limit. Furthermore, the crystallite size in the c-axis direction of the graphite crystallites in the carbonized plate is in the range of 0.9 nm to 8.5 nm. If this crystallite size is less than the lower limit, the crystallinity of the carbon matrix will be insufficient. If it exceeds the upper limit, the effects of the air oxidation treatment or activation treatment will be difficult to obtain. If the interplanar spacing of the (002) planes of the graphite crystallites and the crystallite size in the c-axis direction of the graphite crystallites are within the above ranges, sufficient crystallinity will be obtained, the electrode will have high strength, and carbon particles of the carbonized plate will be less likely to leak into the electrolyte. The interplanar spacing of the (002) planes of graphite crystallites in the plate-shaped carbonized material is determined from the spacing of the (002) planes in the diffraction pattern obtained by X-ray diffraction (XRD) measurement. The crystallite size in the c-axis direction of the graphite crystallites of the plate-shaped carbonized material is determined from X-ray diffraction data using the Scherrer equation: D = Kλ / β cosθ. Here, D is the crystallite diameter (nm), λ is the wavelength of the X-ray tube (1.5418 Å for Cu-Kα radiation), β is the broadening of the diffracted X-rays by the crystallites, θ is the diffraction angle (rad) for the (002) plane, and K is the Scherrer constant, which is set to 0.9. For XRD measurement, a powder X-ray diffractometer (Rigaku Rint2100) using a Ni filter and CuKα radiation is used.

[0040] The redox flow battery electrodes of the first and second embodiments, which have the above-described properties, can reduce pressure loss during electrolyte transport and promote the transport of active materials. They also feature a large BET specific surface area and low reaction resistance. This provides high activity during battery charging and discharging, improving the current output at the sending end of the redox flow battery, i.e., the maximum power density of the battery. [Example]

[0041] Next, examples of the present invention will be described in detail together with comparative examples.

[0042] Example 1 First, phenolic resin and PVA were mixed at a solids ratio (phenolic resin:PVA) of 3:1 and a total solids mass of 30 w / v% was prepared as an aqueous solution. Next, 4 w / v% rice starch was added to this aqueous solution and thoroughly mixed. Subsequently, 5 w / v% of a 37 wt% aqueous formaldehyde solution was added as a crosslinker and mixed. Subsequently, 7 w / v% maleic acid was added as a curing catalyst, and water was added to the specified volume and mixed uniformly to obtain a reaction solution. The resulting reaction solution was poured into a mold and reacted at 60°C for 20 hours. The resulting reaction product was removed from the mold, washed with water to remove the starch, and then dried. This process yielded a porous phenolic resin block with a porosity of 75% and an average macropore diameter of 27 μm, forming a three-dimensional network of interconnected macropores.

[0043] This porous phenolic resin block was cut using a diamond saw to obtain a plate-like object measuring 26 mm in length, 26 mm in width, and 0.53 mm in thickness. This plate-like object was heated from room temperature to 800°C at a heating rate of 5°C / min in a nitrogen gas atmosphere and then held at 800°C for 1 hour in a nitrogen atmosphere to produce a carbonized plate. Next, this carbonized plate was heated from room temperature to 1500°C at a heating rate of 5°C / min in an argon (Ar) atmosphere, and then held at 1500°C for 1 hour in an argon atmosphere to produce a carbonized plate-like object measuring 18.4 mm in length, 18.4 mm in width, and 0.43 mm in thickness. Finally, it was subjected to air oxidation treatment at 420°C for 3 hours in a muffle furnace to obtain a carbonized plate-like object measuring 18 mm in length, 18 mm in width, and 0.41 mm in thickness. The thickness of the carbonized plate-like object was measured using a micrometer.

[0044] Table 1 below shows the production conditions for the carbonized plates finally obtained in Example 1, Examples 2 to 12, and Comparative Examples 1 to 7 described below: (i) the average macropore diameter, thickness, and number of layers of the precursor, and (ii) the atmosphere, temperature, and time for the carbonization treatment, high-temperature heat treatment, air oxidation treatment, and carbon dioxide activation treatment.

[0045] [Table 1]

[0046] <Examples 2 to 12 and Comparative Examples 1 to 2 and 6 to 7> As shown in Table 1, (i) the average macropore diameter, thickness, and number of layers of the carbon material before the carbonization treatment, and (ii) the atmosphere, temperature, and time of each of the carbonization treatment, high-temperature heat treatment, air oxidation treatment, and carbon dioxide activation treatment were the same as those in Example 1, and the final carbonized plate-like material of Examples 2 to 12 and Comparative Examples 1 to 2 and 6 to 7 was produced by changing these settings. The same procedures as in Example 1 were performed except that Example 3 used a carbon material before the high-temperature heat treatment with a thickness of 0.79 mm, and Example 8 used a carbon material before the high-temperature heat treatment with a thickness of 0.78 mm. Examples 1 to 4, 6 to 8, and 10 to 12 and Comparative Examples 6 to 7 are examples in which air oxidation treatment was performed as the final treatment, and Examples 5 and 9 are examples in which carbon dioxide activation treatment was performed as the final treatment. Example 6 is a laminate of two carbonized plate-like material sheets produced in Example 2. Comparative Example 1 is an example in which only carbonization treatment and air oxidation were performed.

[0047] <Comparative Example 1> Comparative Example 1 was produced under the same conditions as in Example 6, except that the high-temperature heat treatment was not carried out and the air oxidation treatment was carried out at 400° C. for 1 hour.

[0048] <Comparative Example 2> Comparative Example 2 was prepared under the same conditions as in Example 1, except that the air oxidation treatment was not carried out.

[0049] <Comparative Example 3> The carbon material used was carbon paper (manufactured by SGL Carbon Japan, product name: SGL-10AA) with a thickness of 0.317 mm. This carbon paper was activated (air oxidation) by heat treatment at 400°C for 24 hours in a nitrogen gas atmosphere. Five sheets of this activated carbon paper were stacked to obtain a carbonized plate.

[0050] <Comparative Example 4> The carbon material used was a 0.178 mm thick carbon cloth (manufactured by ElectroChem, product name: EC-CC1-060). This carbon cloth was activated (air oxidation) by heat treatment at 650°C for 3 hours in a nitrogen gas atmosphere. Three sheets of this activated carbon cloth were stacked to obtain a carbonized plate.

[0051] <Comparative Example 5> The carbon material used was 0.174 mm thick carbon paper (manufactured by Toray Industries, Inc., product name: TGP-H-60). This carbon paper was activated (air oxidation) by heat treatment at 630°C for 3 hours in a nitrogen gas atmosphere. Three sheets of this activated carbon paper were stacked to obtain a carbonized plate.

[0052] <Comparative testing and evaluation> The physical properties of the carbonized plates finally obtained in Examples 1 to 12 and Comparative Examples 1 to 7 as electrodes for redox flow batteries were measured. The average macropore diameter, thickness, micropore volume, BET specific surface area, interplanar spacing of the (002) plane, and crystallite diameter were measured using the methods described above. The uniformity and interconnection of the macropores of the carbonized plates, the pressure loss when the electrolyte was pumped using a power pump, and the maximum power density were measured using the following methods. These results are shown in Table 2 below.

[0053] [Table 2]

[0054] (Uniformity of macropores in carbonized plates and interconnectedness of macropores) The surface of the carbonized platelets was observed using a scanning electron microscope (SEM) (JSM-6700F manufactured by JEOL). Uniform macropores were marked as "present," and non-uniform macropores were marked as "absent." The same microscope was also used to check whether the macropores in the carbonized platelets were interconnected. Interconnected macropores were marked as "present," and non-interconnected macropores were marked as "absent."

[0055] FIG. 1 shows scanning electron microscope (SEM) photographs of the carbonized plate of Example 1. FIG. 1(a) is a view at a magnification of 5000x, FIG. 1(b) is a view at a magnification of 2000x, and FIG. 1(c) is a view at a magnification of 500x. These SEM photographs confirmed that the macropores in the thickness direction and in-plane direction of the carbonized plate were uniform in shape. FIG. 2 shows an SEM photograph of the porous phenolic resin of Example 1. These photographs confirmed that the carbonized plate had uniform interconnected macropores.

[0056] Next, the pressure drop and maximum power density of the carbonized plate as an electrode for a redox flow battery were measured using a redox flow battery system and an electrochemical measurement system shown in Figure 4 after assembling a cell using the carbonized plate as an electrode, as shown in Figure 3.

[0057] (Fabrication of a single redox flow battery) As shown in Figure 3, a single redox flow battery cell 10 is composed of, from the outside in, a carbon block 2 serving as a current collector with a comb-shaped flow channel 1, a gasket 3, a plate-shaped carbon electrode material 4, and a diaphragm (Nafion 117 manufactured by DuPont) 5. The thickness of the plate-shaped carbon electrode material 4 was adjusted with a gasket so that it was 75% of its original thickness, and the tightening torque was 1 Nm. The plate-shaped carbon electrode material 4 had an electrode area of ​​3.24 cm 2 It was.

[0058] (Preparation of Electrolyte) The electrolyte for IV measurement contains 1.0M vanadium ions (V ion ) + 3.0M H2SO4 solution was used. This electrolyte was prepared by mixing 354.12g of concentrated sulfuric acid (95%) with distilled water to prepare 1.0L of 3.43M H2SO4 aqueous solution, and then adding 1.7V to 41mL of this solution. ion 59 mL of 1 M V(V) + 3.0 M H2SO4 solution (manufactured by LE Systems) was added to prepare the cathode electrolyte, and 100 mL of 1 M V(II) + 3 M H2SO4 solution was used as the anode electrolyte.

[0059] (Electrolysis of electrolyte) The redox flow battery system 20 shown in FIG. 4 includes a single redox flow battery cell 10, a positive electrode electrolyte tank 11 and a negative electrode electrolyte tank 12 for the electrode cell, containers 13 and 14 containing nitrogen (N) gas, bubblers 15 and 16, and pumps 17 and 18. 100 mL of the positive electrode electrolyte and negative electrode electrolyte were placed in the electrolyte tanks 11 and 12, respectively. To remove O from the electrode cell and electrolyte, humidified N gas was steadily flowed from the containers 13 and 14 through the bubblers 15 and 16 at a flow rate of 20 mL / min into the electrolyte in the tanks 11 and 12. The electrolyte in the tanks 11 and 12 was circulated through the single redox flow battery cell 10 by pumps 17 and 18 at the volume shown in Table 2. The negative and positive electrodes of the single redox flow battery cell 10 were connected to a charge / discharge test device (PFX2011, manufactured by Kikusui Electronics Co., Ltd., not shown). 200mA / cm for a single redox flow battery cell 2 A constant current of 1.8 V was applied, and constant current charging was performed until the voltage exceeded 1.8 V, and then constant potential charging was performed at 1.8 V until the current was 20 mA or less (state of charge = SOC 99%), and the electrolyte was electrolyzed.

[0060] (Measurement of current and voltage (IV)) The IV measurement of the redox flow battery system through which the charged electrolyte was circulated was carried out using an electrochemical measurement system (HZ-5000 manufactured by Hokuto Denko Corporation). The voltage was decreased from the open circuit voltage (OCV) at a constant rate (2 mV / s) and the current value at that time was measured. At the same time, the liquid pressure at the cell inlet was measured using a pressure transmitter. The cell outlet was at atmospheric pressure, and the pressure loss was measured from the difference between the cell inlet and outlet pressures.

[0061] (Maximum battery power density) The maximum power density of the battery was determined from the current and voltage values ​​obtained by measuring the current-voltage (IV). Specifically, the maximum power density was determined from the peak value of the power curve shown alongside the current-voltage curve of the battery. The maximum power density was determined from the current-voltage curves and power curves of the batteries using the plate-like carbon electrode materials of Examples 2 and 6 and the batteries using the carbon paper or carbon cloth of Comparative Examples 3 to 5 shown in Figure 5. Similarly, the maximum power density of the batteries using the plate-shaped carbon electrode materials of Example 1, Example 2, Example 10, and Comparative Example 1 was determined from FIG. 6 , the maximum power density of the batteries using the plate-shaped carbon electrode materials of Example 3, Example 4, and Example 6 was determined from FIG. 7 , the maximum power density of the batteries using the plate-shaped carbon electrode materials of Example 1, Example 5, Example 9, and Comparative Example 2 was determined from FIG. 8 , the maximum power density of the batteries using the plate-shaped carbon electrode materials of Example 1, Example 7, Comparative Example 6, and Comparative Example 7 was determined from FIG. 9 , the maximum power density of the batteries using the plate-shaped carbon electrode materials of Example 4 and Example 8 was determined from FIG. 10 , and the maximum power density of the batteries using the plate-shaped carbon electrode materials of Example 1, Example 11, and Example 12 was determined from FIG. 11 .

[0062] As is clear from Table 2, the plate-shaped carbon electrode material of Comparative Example 1 was subjected to only the carbonization treatment and the air oxidation treatment, and was not subjected to the high-temperature heat treatment, so the maximum power density was 0.27 / cm 2 In addition, the plate-shaped carbon electrode material of Comparative Example 2 was subjected to only the carbonization treatment and the high-temperature heat treatment, and was not subjected to the air oxidation treatment, so the maximum power density was 0.50 / cm 2 It was low.

[0063] Furthermore, since the carbon paper or carbon cloth in Comparative Examples 3 to 5 was made of a laminated material in which carbon fiber or carbon cloth was folded over, the pressure loss during electrolyte delivery was low at 15 kPa, 7 kPa, and 11 kPa, respectively. However, the maximum power density of Comparative Examples 3 to 5 was 0.36 W / cm 2 ~0.59W / cm 2 This shows that the plate-shaped carbon electrode materials of Comparative Examples 1 and 2 and the carbon papers or carbon cloths of Comparative Examples 3 to 5 cannot simultaneously satisfy the two requirements for electrodes for redox flow batteries, namely, low pressure loss and high maximum power density, and therefore cannot be used as electrodes for redox flow batteries.

[0064] Furthermore, the plate-shaped carbon electrode material of Comparative Example 6 has a small macropore diameter, so the maximum power density is 0.87 W / cm 2 However, the pressure loss was as large as 83.3 kPa. In addition, the plate-shaped carbon electrode material of Comparative Example 7 had a large macropore diameter, so the maximum power density was 0.53 W / cm 2 The pressure loss was as low as 4.5 kPa.

[0065] In contrast, as is clear from Table 2, the plate-shaped carbon electrode materials of Examples 1 to 12 had the characteristics of the first aspect of the present invention and were produced under the conditions of the second or third aspect, and therefore the pressure loss during electrolyte delivery was small, ranging from 7 kPa to 30 kPa, and the maximum power density was 0.63 W / cm. 2 ~0.88W / cm 2 Therefore, it was found that the plate-shaped carbon electrode materials of Examples 1 to 12 had small pressure loss and high maximum power density, and were suitable as electrodes for redox flow batteries.

[0066] In the evaluation of the Examples and Comparative Examples, the BET specific surface area was compared depending on the treatment method. The BET specific surface area of ​​the plate-shaped carbonized material of Comparative Example 2, which was subjected to only carbonization treatment and high-temperature heat treatment, was 12 m 2 / g, and the BET specific surface area of ​​the carbonized plate material of Comparative Example 1 that was subjected to carbonization treatment but not high-temperature heat treatment was 620 m 2 / g, whereas the BET specific surface area of ​​the plate-shaped carbon electrode material of Example 5 was 855 m 2 / g, and the BET specific surface area of ​​Example 1 is 640 m 2 / g, and the BET specific surface area of ​​Example 4 is 640 m 2 / g, and the BET specific surface area of ​​Example 7 is 740 m 2 / g, and the BET specific surface area of ​​Example 9 is 850 m 2 / g, and the BET specific surface area of ​​Example 10 is 820 m 2 / g.

[0067] The reason why the BET specific surface area of ​​Examples 5 and 9 is increased is that the nitrogen Absorption / desorptionThe difference in the amount of nitrogen adsorption in the adsorption isotherms can be explained as follows. As shown in Fig. 12, at the time of carbonization treatment at 800°C, the carbon is not sufficiently crystallized and is irregularly stacked, resulting in a relatively large BET specific surface area. However, by subsequently performing a high-temperature heat treatment at 1500°C, the crystallinity improves and the BET specific surface area decreases once. In this state, further carbon dioxide activation treatment as in Examples 5 and 9 is performed, which is thought to develop micropores in the carbonized material and increase the BET specific surface area. The reason for the increase in the BET specific surface area in Examples 1, 4, 7, and 10 is due to the nitrogen adsorption isotherms shown in Figs. 13 and 14. Absorption / desorption The difference in the amount of nitrogen adsorption in the adsorption isotherms can be explained as follows. As shown in Fig. 14, at the time of carbonization treatment at 800°C, the carbon is not sufficiently crystallized and is irregularly layered, resulting in a relatively large BET specific surface area. However, subsequent high-temperature heat treatment at 1500°C improves the crystallinity and reduces the surface area once. It is believed that further air oxidation treatment in Examples 1 to 4, 6 to 8, and 10 to 12 in this state develops micropores in the carbonized material, increasing the BET specific surface area. [Industrial Applicability]

[0068] The plate-shaped carbon electrode material of the present invention is used as an electrode for a redox flow battery.

Claims

1. An electrode for a redox flow battery, which is constructed by stacking one or more plate-shaped carbon electrode materials, the plate-shaped carbon electrode materials having uniform interconnected macropores formed in a three-dimensional network and no contact interface between carbon particles, the average macropore diameter of the carbon electrode material is in the range of 6 μm to 35 μm; the interplanar spacing of the (002) planes of the graphite crystallites in the carbon electrode material is in the range of 0.33 nm to 0.40 nm, and the crystallite size in the c-axis direction of the graphite crystallites is in the range of 0.9 nm to 8.5 nm; The thickness of the electrode is in the range of 0.4 mm to 0.8 mm, the carbon electrode material has a BET specific surface area measured by a nitrogen adsorption method at 77K in the range of 100 m 2 / g to 1500 m 2 / g; The micropore volume of the carbon electrode material is in the range of 0.05 ml / g to 0.70 ml / g; Electrodes for redox flow batteries.

2. a step of cutting a block of porous phenolic resin having interconnected uniform macropores formed in a three-dimensional network with an average macropore diameter in the range of 4 μm to 70 μm into a plate-shaped body; a step of heating the cut-out plate-like body from room temperature to a range of 800°C to 1000°C in an inert gas atmosphere and maintaining the temperature at the elevated temperature in an inert gas atmosphere to carbonize the cut-out plate-like body, thereby obtaining a carbonized plate-like body; a step of heating the carbonized platelets from room temperature to a temperature in the range of 1100°C to 2500°C and maintaining the temperature at the elevated temperature in an inert gas atmosphere to perform a high-temperature heat treatment; a step of heating the high-temperature heat-treated carbonized plate in air from room temperature to a temperature in the range of 350°C to 600°C, and maintaining the plate at the elevated temperature in air to perform an air oxidation treatment, thereby obtaining a carbon plate electrode material; A method for producing an electrode for a redox flow battery, comprising:

3. a step of cutting a block of porous phenolic resin having interconnected uniform macropores formed in a three-dimensional network with an average macropore diameter in the range of 4 μm to 70 μm into a plate-shaped body; a step of heating the cut-out plate-like body from room temperature to a range of 800°C to 1000°C in an inert gas atmosphere and maintaining the temperature at the elevated temperature in an inert gas atmosphere to carbonize the cut-out plate-like body, thereby obtaining a carbonized plate-like body; a step of heating the carbonized platelets from room temperature to a temperature in the range of 1100°C to 2500°C and maintaining the temperature at the elevated temperature in an inert gas atmosphere to perform a high-temperature heat treatment; a step of activating the high-temperature heat-treated carbonized plate material to obtain a carbon plate electrode material so that the activation yield is in the range of 50% to 90%; A method for producing an electrode for a redox flow battery, comprising:

4. 4. The method for producing an electrode for a redox flow battery according to claim 3, wherein the activation treatment is carried out by raising the temperature of the high-temperature heat-treated carbonized material plate from room temperature to 800°C to 1000°C in an inert gas atmosphere and maintaining the temperature at that elevated temperature in a carbon dioxide gas flow.

5. A redox flow battery using the electrode according to claim 1.

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