Electrode for Flow Battery and Manufacturing Method

Abstract

The present invention relates to an electrode (1) for a flow battery (B) and a method for manufacturing said electrode (1), wherein the electrode (1) comprises a first portion (12) consisting of nanometer-sized particles (11) of a conductive material, the first portion (12) being mesoporous, the porosity of the first portion (12) increasing the amount of redox reaction per unit time in the electrolyte of the battery (B).

Description

Technical Field

[0001] The present invention relates to an electrode for a flow battery and a method for manufacturing the electrode. Specifically, the present invention relates to an electrode for a vanadium flow battery, that is, a battery in which at least one of two electrolytes contains vanadium ions, and a method for manufacturing the electrode.

Background Art

[0002] As is well known, the maximum power that a flow battery can provide actually does not depend on the electrical capacity of the battery. In fact, the maximum power depends on the characteristics of a single cell (specifically, the dimensions and chemical-physical properties of the electrodes, the dimensions and types of the ion exchange membrane sandwiched between the two electrodes and the two electrolytes used, the number of electrodes per half-cell, etc.), while the electrical capacity mainly depends on the types of the electrolyte and redox species used and the total amount of the electrolyte and redox species stored in the tank. Therefore, it can be said that a flow battery can generate the maximum power that can be provided without depending on the electrical capacity of the battery.

[0003] Due to this particularity, the flow battery can be sized optimally for special applications, for example, for coupling to a power generation plant that utilizes a renewable energy source (such as a solar, wind, hydro, wave power, etc. power generation plant). In fact, renewable energy sources are characterized by a power-energy ratio, which follows seasonal trends (especially solar and wind power generation plants). That is, almost always the maximum power can be obtained in all seasons, but the energy generated in a day varies depending on the time of year.

[0004] Currently, the energy density of a flow battery is lower than that of a lithium-ion battery, but the capacity of a flow battery is correlated with the maximum power, and the capacity tends to decrease as the battery ages. Furthermore, the dimensions of the individual elements of a lithium battery are determined by safety regulations and cooling requirements.

[0005] "GONZALEZ, Zoraida, et al. Carbon nanowalls thin films as nanostructured electrode materials in vanadium redox flow batteries. Nano Energy, 2012, 1.6: 833-839." describes a solution for fabricating carbon nanowalls (CNW) on the surface of a gold electrode to expand the reaction surface of the gold electrode, and such an electrode is then used in the positive half-cell of a vanadium flow battery. Although a more compact electrode can be obtained with this solution, it does not increase the rate of the redox reaction on the surface of the electrode. Therefore, not only is it economically unfavorable due to the use of a gold electrode, but the above solution does not achieve any significant increase in the specific power of the vanadium flow battery.

Summary of the Invention

[0006] An object of the present invention is to solve the above and other problems by providing an electrode for a flow battery.

[0007] Another object of the present invention is to solve the above and other problems by providing a method for manufacturing an electrode for a flow battery.

[0008] The basic idea of the present invention is to manufacture and use an electrode having a portion arranged to contact the electrolyte of a flow battery, wherein the portion has a mesoporous structure (i.e., a structure having pores with a pore diameter in the range of about 1 to 100 nanometers) composed of particles of a nanometer-sized conductive material, thereby increasing the rate of the redox reaction in the flow of the electrolyte and / or increasing the number of active sites and / or the electrode area per unit surface area, and increasing the current generated per projected area of the electrode.

[0009] By using nanometer-sized particles of a conductive material to create a mesoporous structure of the electrode for a flow battery, an electrolyte can flow through the electrode. At the same time, the number of active sites where electrons are exchanged per unit surface area of the electrode can be increased, and the electrode area in contact with the electrolyte can be increased. This can raise the rate of the redox reaction in the above flow (the rate of the redox reaction occurring on the electrode surface in contact with the electrolyte during the normal charge and discharge process of the flow battery), and thereby advantageously achieve the technical effect of causing an electron catalysis phenomenon in the electrolyte flowing through the electrode of the present invention. In other words, the present invention has the technical effect of increasing the amount of the redox reaction per unit time in the flow of the electrolyte, thereby increasing the current density per unit projected area of the electrode.

[0010] Thereby, advantageously, the power density of the flow battery can be increased. In particular, for the electrode of a preferred embodiment of the present invention, the amount of current per unit surface area can be three times that of the current generated by the electrodes of the prior art (the efficiency is over 80%).

[0011] In addition, the above-mentioned electron catalysis also has the advantage of reducing the overpotential of the electrode, thereby increasing the current density of the battery. In fact, when the overpotential of the electrode is reduced, as a macroscopic effect, the internal resistance of the battery (i.e., not only the ohmic resistance but also the electrochemical resistance) is reduced, and consequently, the internal voltage drop of the battery is also reduced. As a result of actual experimental tests, when used as the negative electrode, the electrode of the present invention was found to be able to operate individual single cells of a vanadium flow battery in the range of 1.0 to 1.8 volts with a charge and discharge current at least twice that of the charge and discharge current of the batteries of the prior art. As a practical matter, the reason this can be achieved is that by reducing the overpotential, advantageously, the amounts of hydrogen and oxygen that can be generated when a high current is supplied during the charge and discharge phases can be reduced, thereby enabling the battery to handle special situations where it must absorb or release a high-intensity current.

[0012] Therefore, in applications such as those where it is necessary to use the above-described flow battery in a large-scale plant for generating electrical energy from a renewable energy source, such as a large-scale plant where power generation follows a highly variable trend, for example, a power generation plant such as solar power, wind power, etc., using the above-described flow battery is advantageous over prior art flow batteries.

[0013] Furthermore, the electrodes of the present invention reduce their oxidation and / or reduce the formation of hydrogen or oxygen on the surface of the electrodes, thereby reducing the deterioration of the electrodes. As a result, the performance of the flow battery can be maintained unchanged even after a large number of charge and discharge cycles compared to other types of batteries.

[0014] Other advantageous features of the present invention are described in the appended claims.

[0015] By referring to the following description of the preferred embodiments shown in the accompanying drawings, the above features and other advantages of the present invention will become more apparent, and such preferred embodiments are merely non-limiting examples.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Embodiments for Carrying Out the Invention

[0017] In this description, in all cases of "one embodiment", it means that a specific configuration, structure, or feature is included in at least one embodiment of the present invention. Therefore, although the expression such as "one embodiment" may be used in various places in this description, this does not necessarily refer to the same embodiment. Furthermore, all specific configurations, structures, or features can be combined with one or more embodiments if considered appropriate. Therefore, the above-mentioned references are used only for simplification hereinafter and do not limit the protection scope of various embodiments.

[0018] Hereinafter, a battery B including at least one electrode 1 of the present invention will be described with reference to FIG. 1. The electrode 1 is preferably planar.

[0019] In this description, the term "electrode" refers to a component of a battery in which an oxidation-reduction reaction occurs on the surface of the component during charging or discharging of the battery.

[0020] The battery B has a negative-side portion B1 and a positive-side portion B2. The negative-side portion B1 includes a negative half-cell S1, a first tank T1, and a first pump P1. The positive-side portion B2 includes a positive half-cell S2, a second tank T2, and a second pump P2.

[0021] The negative half-cell S1 is in fluid communication with a first tank T1 via a first forward conduit D1 and a first return conduit R1. A first electrolyte is contained in the above-described first tank T1, both conduits D1, R1, and the negative half-cell S1. Preferably, the first pump P1 is disposed on the first forward conduit D1 so as to cause a flow of the first electrolyte in the negative half-cell S1 when the first pump P1 is in an operating state.

[0022] Similarly to the negative half-cell S1, the positive half-cell S2 is in fluid communication with a second tank T2 via a second forward conduit D2 and a second return conduit R2. A second electrolyte is contained in the above-described second tank T2, both conduits D2, R2, and the positive half-cell S2. Preferably, the second pump P2 is disposed on the second forward conduit D2 so as to cause a flow of the second electrolyte in the positive half-cell S2 when the second pump P2 is in an operating state.

[0023] The battery B2 also includes an ion exchange membrane having a first surface and a second surface (for example, a membrane made of Nafion (registered trademark) or other materials). The first electrolyte contained in the negative half-cell S1 contacts the first surface, and the second electrolyte contained in the positive half-cell S2 contacts the second surface.

[0024] Furthermore, at least one of the reaction half-cells S1, S2 includes the electrode 1 of the present invention. In a preferred embodiment, the negative half-cell S1 includes one of the above-described electrodes 1, while the positive half-cell S2 preferably includes a conventional electrode of the prior art (for example, a carbon electrode or the like) or the electrode 1 of the present invention. At least one electrical load L and / or a generator G can be connected to the above-described electrode.

[0025] The first and second electrolytes are preferably solutions containing vanadium ions. Such a solution has the advantage that the same starting solution can be used for both the negative part B1 and the positive part B2. Such a starting solution is, for example, an aqueous solution of vanadyl sulfate (general formula VOSO4). In fact, vanadium element takes five oxidation states (+1, +2, +3, +4, +5), and it should be noted that four of them (+2, +3, +4, +5) can be effectively used for electrochemical applications. Thus, from the above starting solution, using well-known techniques of the prior art, a first electrolyte (negative part B1) in which vanadium takes the +2 and / or +3 oxidation states and a second electrolyte (positive part B2) in which vanadium takes the +4 and / or +5 oxidation states can be prepared.

[0026] When the battery B is in the operating state and the generator G is recharging the battery B, the vanadium (negative part B1) in the first electrolyte is reduced and switches from the +3 oxidation state to the +2 oxidation state by absorbing electrons. At the same time, the vanadium (positive part B2) in the second electrolyte is oxidized and switches from the +4 oxidation state (VO 2+ ) to the +5 oxidation state (VO2 + ) by releasing electrons.

[0027] When the battery B is in the operating state and the load L is discharging the battery B, the vanadium (negative part B1) in the first electrolyte is oxidized and switches from the +2 oxidation state to the +3 oxidation state by releasing electrons. At the same time, the vanadium (positive part B2) in the second electrolyte is reduced and switches from the +5 oxidation state (VO2 + ) to the +4 oxidation state (VO 2+ ) by absorbing electrons.

[0028] However, without departing from the teachings of the present invention, it is also possible to use electrolytes or solutions of different natures (including aqueous or organic solvents, containing active species such as iodides, sulfides, and bromides, alkali metals (such as lithium, sodium, etc.) or transition metals (such as iron, chromium, titanium, tin, zinc, cerium, manganese, etc.) or organic molecules and redox polymers (such as quinones, methyl viologen, ACA, ferricyanides, TEMPO, PANI, PNB-g-PTMA, polythiophene) or solid particles, etc.).

[0029] The electrolyte used is a solvent, which is preferably water. Together with water or in place of water, it is also possible to use organic solvents (such as acetonitrile, dimethyl sulfoxide, propylene carbonate, ethyl carbonate, dioxolane, etc.) or ionic liquids (such as 1-ethyl-3-methylimidazolium chloride (EMICl) / FeCl3 / FeCl2, tetrabutylammonium hexafluorophosphate (TEAPF6), 1-ethyl-3-methylimidazolium hexafluorophosphate (EMIPF6)).

[0030] The electrode 1 of the present invention comprises a first part having a mesoporous (physical) structure, that is, a structure having pores with a pore diameter in the range of about 1 to 100 nanometers. The first part is arranged to contact one of the two electrolytes of the battery B and is composed of particles of a nanometer-sized conductive material. The porosity of the first part is a rate that enables diffusion in the flow of the electrolyte, and advantageously, it is a rate that increases the amount per unit time of the redox reaction in the flow, where the flow is preferably generated by any one of the pumps P1, P2.

[0031] In this way, advantageously, the power density in the flow battery can be increased.

[0032] The first part of the electrode 1 can constitute the entire electrode 1. That is, the mesoporous (first) part can be used as an independent electrode.

[0033] In a preferred embodiment of the present invention, the above mesoporous portion can be supported by other structural elements of the battery B, such as commercially available carbon fiber electrodes or the like, or, for example, a gas diffusion layer (GDL such as Sigracet (registered trademark) 29AA substrate), a membrane, a current collector, a foam and / or a metal mesh and / or a polymer mesh, a material produced by an electrospinning method, or other substrates of other types of materials available on the market.

[0034] As will be described in detail below, the electrode 1 of the preferred embodiment includes a second portion, and the first portion is constrained by the second portion so that the second portion serves as a support portion for the first portion. In other words, the second portion supports the first portion.

[0035] Therefore, there is an advantage that the power density and energy density of the flow battery already on the market can be improved. This is because the movement of the electrolyte toward the first portion of the electrode is promoted and / or the reaction rate is increased, thereby reducing the overpotential of the electrode.

[0036] Particles of the conductive material having a nanometer size can be zero-dimensional (0D nanoparticles) and / or two-dimensional (2D nanoplatelets). It should also be noted that it is also possible to use a mixture of zero-dimensional particles and two-dimensional particles.

[0037] Preferably, the above-mentioned nanoparticles can be made of any conductive material, for example, a material belonging to any of the compound classes such as carbon, metal, nitride, boride, carbide, oxide, or chalcogenide. More preferably, the above-mentioned nanoparticles are carbon nitride (C3N4), carbon-nitrogen compound (CN where 0 < x < 4 / 3) x ), graphene, reduced graphene oxide, carbon nanoparticles, carbon nanotubes, fullerenes, titanium nitride (TiN), oxynitride titanium, titanium oxide (TiO x , 0 < x < 2), molybdenum oxide (MoO x, (0 < x < 3), tungsten oxide (WO x 0 < x < 3), tungsten oxynitride, tungsten nitride, tin oxide (SnO x , (0 < x ≤ 2), indium, indium oxide, iridium oxide (IrO2), ruthenium, ruthenium oxide, bismuth, bismuth oxides, borides, nitrides, carbides, chalcogenides, tellurium, manganese, niobium, yttrium, zirconium, hafnium, gallium, lead, lanthanum, cerium and / or other lanthanides, titanium, molybdenum, tungsten, iron, nickel, aluminum can be included.

[0038] Also, referring to FIG. 2, in a preferred embodiment of the present invention, the conductive particles are carbon nanoparticles, and the carbon nanoparticles have an onion-like structure. That is, the particles have a concentric layer structure 11. Surprisingly, such a structure promotes the electron catalyst in the flow of the electrolyte, thereby having the advantage of increasing the power density and energy density in the battery B.

[0039] When the particle size of the above particles is 1 to 50 nanometers, more preferably 4 to 5 nanometers, it was observed that the electron catalyst was further improved (that is, the amount per unit time of the redox reaction was further increased).

[0040] A certain porosity is provided in the first part of the electrode 1 by the aggregation of the particles (for example, those achieved by performing heat treatment in a vacuum atmosphere or a controlled atmosphere after performing a supersonic jet). The porosity obtained by this aggregation of particles can be in the range of 1 to 50 nanometers. That is, the pore diameter of the formed pores can be in the range of 1 to 50 nanometers.

[0041] More specifically, it should be noted that the average pore size can be determined depending on the steric hindrance of the active species used in the battery. Here, the term "active species" refers to ions involved in the redox reaction that can store and release electrical energy. In particular, when the electrolyte used contains active species such as vanadium and / or other ionic species or redox-active molecules with a molecular weight of less than 1,000 g / mol, such as quinone compounds (benzoquinone, anthraquinone, etc.), alkoxybenzene, derivatives of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), N-methylphthalimide, etc., the average pore size can be selected from 1 to 10 nanometers.

[0042] When using redox species with a large molecular weight, especially when using polymer particles (such as derivatives of polyaniline, derivatives of poly(vinylbenzylethylviologen), derivatives of the so-called "bottlebrush" polymer class, derivatives of 2,2,6,6-tetramethyl-1-piperidinoxy (TEMPO), polythiophene, water-soluble polymers such as polymethacrylate and polystyrene, boron dipyrromethene, etc.), pores with an average diameter of 10 to 100 nm can be used.

[0043] Also, the pore size can be adjusted according to the viscosity of the electrolyte. For example, for a low-viscosity electrolyte, pores with an average diameter of 1 to 10 nanometers are more suitable, while for a high-viscosity electrolyte, pores with an average diameter of 10 to 100 nanometers are more suitable.

[0044] Furthermore, the pore size can be selected to comply with the steric hindrance of the active species and the viscosity of the electrolyte.

[0045] By using the porosity and active elements as described above, the redox reaction can be improved. Therefore, there is an advantage that the power density and energy density of the flow battery already on the market can be improved.

[0046] The following describes a possible embodiment of electrode 1 with reference to FIG. 3. Electrode 1 has a first portion 12 and a second portion 13. The first portion mainly consists of carbon particles 11 having a concentric layer structure, and the second portion 13 is preferably a carbon electrode of the prior art.

[0047] For the purpose of explanation, the second portion is schematically shown as a planar layer to preferably eliminate all complexities based on the (fiber) structure of the carbon fibers constituting the second portion 13 of electrode 1 and to better emphasize graphically the porosity of the first portion 12.

[0048] The thickness of the first portion 12 ranges from 0.01 μm to 1000 μm, preferably from 0.1 μm to 50 μm. More preferably, the thickness of the first portion 12 of the electrode ranges from 0.5 μm to 10 μm. In a preferred embodiment, the thickness of the first portion 12 of the electrode ranges from 1 μm to 6 μm.

[0049] Regardless of the starting active species used, the internal porosity can vary in the direction along the depth D of the first portion 12 of electrode 1 to a depth of 10% to 90% of the total thickness of the first portion.

[0050] Specifically, the porosity varies in the direction along the depth of the first portion 12, preferably changing at a gradient of 0% to 80% every 100 nanometers. In the region of the first portion 12 of electrode 1 closer to the second portion 13 of electrode 1, the porosity must be lowered because the concentration of ions that can be oxidized or reduced is relatively low. However, such a region is located near the second portion 13 of electrode 1, and the above gradient can improve the oxidation-reduction reaction in this region. This is because in the region as described above, the electrolyte has already reached the outermost particles of the first portion 12 of electrode 1, and the concentration of ions that can be oxidized or reduced has already been reduced.

[0051] This can reduce the potential difference between the surface of the first portion 12 of the electrode 1 that is away from the second portion 13 and the surface of the first portion 12 that is closer to the second portion 13, thereby minimizing the intensity of the current within the electrode 1. The advantage is that the efficiency of the electrode 1 can be obtained by reducing the maximum transportable current.

[0052] Furthermore, due to the presence of the above-mentioned gradient, the electrolyte can fully penetrate the first portion 12 of the electrode 1, thereby promoting the electrolyte flow. This has the advantage that an electrode with a large contact area with the electrolyte can be used, and at the same time, it can ensure that the regeneration of the electrolyte is constant.

[0053] In this way, advantageously, the power density and energy density in the flow battery can be increased.

[0054] Instead of or in combination with the above configuration, the porosity and thickness of the first portion 12 can vary in the direction along the length L and / or depth D of the electrode 1.

[0055] In particular, the porosity can vary in the direction along the length L of the first portion, preferably by only a percentage value of 0% to 80% of a predetermined porosity value in millimeters of the length L of the electrode 1, and / or the porosity can vary in the direction along the depth D of the electrode 1, preferably by only a percentage value within the range of 0% to 80% of a predetermined porosity value in microns of the depth D of the electrode 1.

[0056] Also, the above-mentioned gradient is also useful for minimizing the overpotential of the redox reaction in the region of the first portion 12 of the electrode 1 where the concentration of oxidizable or reducible ions is relatively low, thereby reducing the required porosity and expanding the active surface area, that is, increasing the number of active sites. The above region is arranged in the region where the flow of the electrolyte in the electrode 1 finally contacts. This is because in such a region, the electrolyte already hits most of the first portion 12, and thus the concentration of oxidizable or reducible ions in the electrolyte has already decreased.

[0057] In this way, advantageously, the power density and energy density in the flow battery can be increased.

[0058] The specific surface area of the first portion 12 of the electrode 1 (preferably the specific surface area measured by the BET method) is at least 500 m 2 / g, preferably at least 600 m 2 / g, more preferably 610 m 2 / g, and the roughness coefficient (that is, the area per unit volume) is at least 20 μm -1 , preferably at least 200 μm -1 , more preferably 285 μm -1 is.

[0059] Similar to the particle size and porosity, the roughness coefficient can be made non-uniform over the entire thickness of the first portion 12 of the electrode 1. In a preferred embodiment, the roughness coefficient of the first portion 12 can vary with a gradient of 20 - 500 μm -1 . Thereby, even if the concentration of oxidizable or reducible ions changes in a plurality of different regions of the first portion 12 of the electrode 1, the redox reaction can be kept uniform and / or constant.

[0060] In this way, advantageously, the power density and energy density in the flow battery can be increased.

[0061] As described above, the carbon particles constituting the first portion 12 of the electrode 1 have a concentric layer structure, and their diameter is preferably in the range of 2 to 100 nanometers, more preferably in the range of 3 to 7 nanometers. Further, the thickness of the first portion 12 of the electrode 1 is preferably in the range of 0.1 to 100 μm, more preferably in the range of 1 to 10 μm. In the direction along such thickness, the porosity between the aggregated particles is in the range of 1 to 100 nm. The first portion 12 is supported by the second portion, and the second portion preferably comprises a commercially available electrode made of carbon fiber.

[0062] Also, referring to FIG. 4, the electrode 1 of the present invention is preferably coupled to a flow distributor, and this flow distributor is, for example, a serpentine flow distributor 21 or an interdigitated flow distributor 22, etc.

[0063] The flow distributors 21, 22 are capable of distributing the flow of the electrolytic solution at the electrode 1 after the electrolytic solution flows out from one of the tanks T1, T2 by the action of one of the pumps P1, P2. It should be noted that the distribution of the flow of the electrolytic solution at this electrode 1 necessarily involves diffusing the electrolytic solution through most of the porous volume of the electrode 1.

[0064] When the above electrode 1 is used as the negative electrode of a flow battery in which the electrolyte contains vanadium ions, the resulting power density is closely correlated with a combination of factors such as the thickness of the electrode and the type of flow distributor coupled to the electrode.

[0065] When using the serpentine flow distributor 21, the thickness of the first portion 12 of the electrode 1 is preferably 0.5 μm to 5 μm, more preferably 1 μm.

[0066] When using the interdigitated flow distributor 22, the thickness of the first portion 12 of the electrode 1 is preferably 2 μm to 10 μm, more preferably 4 μm.

[0067] When the above electrode 1 is used as the positive electrode of a flow battery in which the electrolyte contains vanadium ions, the resulting power density is strictly correlated with the combination of the thickness of the electrode and the type of flow distributor coupled to the electrode. Also, when reducing the scale, it should be noted that the best results in terms of the electron catalyst can be obtained with the same combination of thickness and flow distributor as above.

[0068] In the electrode 1 of the present invention, an electrical conductivity exceeding 10 5 S·m was observed.

[0069] Hereinafter, with reference to FIG. 5, how the electrolytic solution flows inside the electrode 1 under the action of the interdigital flow distributor 22 will be described.

[0070] As is well known, the interdigital flow distributor 22 includes at least one grid of the supply conduit 221 and a grid of the discharge conduit 222. These supply conduit 221 and discharge conduit 222 open outward (that is, open toward the electrode 1 in the operating state), but the supply conduit 221 and the discharge conduit 222 do not communicate directly with each other. Such a configuration of the distributor has the advantage that when the distributor 22 is coupled to the electrode 1, at least one flow F of the electrolyte can be generated in substantially all of the electrode 1. As a result, when a current is circulated between both ends of the electrode 1 of the battery B, there is an advantage that the amount of the redox reaction per unit time in the electrode 1 can be increased.

[0071] In this way, advantageously, the power density and the energy density in the flow battery can be increased.

[0072] Also, hereinafter, a preferred embodiment of the electrode 1 will be described with reference to FIGS. 6 and 7. In this embodiment, the second portion 13 of the electrode 1 is made of a carbon fiber electrode, that is, the second portion 13 includes a plurality of carbon fibers.

[0073] To better illustrate the technical features related to carbon fibers, Figures 6 and 7 show the cross-section of carbon fibers. It should be noted that the gradients highlighted in Figures 3 and 5 are not shown (for the sake of clarity).

[0074] At least one of the fibers constituting the second part 13 of the electrode is coated by a layer of particles 11 constituting the first part 12 of the electrode. In other words, the second part 13 has a plurality of carbon fibers, and the first part (12) at least partially coats at least one of the fibers by a layer of particles 11.

[0075] This can reduce the overpotential of the electrode 1 while improving the mechanical strength of the electrode, thereby increasing the current density and / or energy density of the flow battery.

[0076] Also, the thickness S of the layer of the first part 12 can vary according to the position of the fibers of the electrode 1.

[0077] Specifically, the higher the concentration of active species in the electrolyte flow, the thinner the thickness S; the lower the concentration of active species in the electrolyte flow, the thicker the thickness S. In other words, the thickness S of the layer of particles 11 changes in the direction along the depth D and / or length L of the electrode 1.

[0078] Preferably, the thickness S of the first part 12 surrounding the second part 13 (i.e., the fibers) of the electrode 1 changes by an amount in the range of 10 nanometers to 1000 nanometers per millimeter of the length L of the electrode 1, and / or changes by an amount in the range of 10 nanometers to 1000 nanometers per micron of the thickness D of the electrode 1.

[0079] Furthermore, the thickness S of the first part 12 surrounding the same fiber can also change, for example, with a gradient highlighted in Figure 5.

[0080] In this way, the potential difference at the electrode is reduced, and the advantage of increasing the current that electrode 1 can absorb or supply to the electrolyte is achieved. As a result, the advantage of increasing the power density and energy density of the flow battery is achieved.

[0081] Electrode 1 described in the specification of this application can be manufactured using various manufacturing methods. Generally, any film-forming method that can deposit nanoparticles on the surface while controlling the kinetic energy can be used.

[0082] Specifically, the above-mentioned electrode 1 can be fabricated by depositing nanoparticles using a plasma source, a sputtering system, a pulsed laser deposition (PLD) system, a plasma-enhanced chemical vapor deposition (PECVD) system, or an atmospheric pressure spray, etc.

[0083] That is, the method for manufacturing electrode 1 of the present invention has the following steps: a. A synthesis step of synthesizing a plurality of particles of a conductive material having a nanometer size; b. A deposition step of forming a first portion of electrode 1 using the plurality of particles.

[0084] In order to manufacture the above-mentioned electrode 1, a plasma-assisted supersonic particle source as described in International Publication No. WO 2011 / 064392, where the applicant is the University of Degree Study of Milano-Bicocca, can be fabricated. Such a method is assumed to use a device that can preferably generate an aerosol of nanoparticles in a first chamber separated from a second chamber by a sufficiently small orifice, and more preferably by using a reactive plasma or the like. By maintaining the above-mentioned plurality of chambers under different pressures respectively, a supersonic gas jet containing suspended particles is generated. To maintain the pressure difference between the two chambers, a pumping system is connected to one of the two chambers. The chamber maintained at a higher pressure (referred to as the "synthesis chamber") is equipped with a pair of electrodes. By being supplied with signals from a signal generator (also referred to as an "RF signal generator"), the synthesis chamber effectively forms a finite space with an electromagnetic field inside. This electromagnetic field is preferably generated by the signal generator. A gas mixture containing at least one carbon-containing gas, such as acetylene (C2H2), is injected between the two electrodes (i.e., inside the above-mentioned finite space). Specifically, the above-mentioned mixture is preferably composed of 99.63% argon (Ar) and 0.38% acetylene (C2H2). The RF signal generator is configured to generate a signal preferably having a frequency of 13.56 MHz and a power of at least 20 watts, preferably 120 watts. Due to this signal, acetylene molecules flow through the electrodes and dissociate radially. This radial polymerization starts in an inert argon atmosphere (i.e., an atmosphere where the amount of oxygen per unit time is negligible) and forms clusters of several atoms, thereby becoming nanoparticles. Subsequently, the particles synthesized as described above are collected by a supersonic jet and accelerated in a low-pressure chamber (also referred to as the "impact chamber"). During operation, the pressure in the synthesis chamber is preferably maintained at 130 pascals, and the pressure in the impact chamber is preferably maintained at 2 pascals.

[0085] As an alternative means to an RF signal generator and two electrodes that generate a reactive plasma when supplied by the generator, it is possible to use, for example, a thermal filament (e.g., one heated by the Joule effect) or other heating systems such as a high-power lamp.

[0086] Depending on the power for performing the synthesis process of carbon nanoparticles, the carbon nanoparticles can be graphitized or hydrogenated. In the latter case, in order to obtain completely graphitized carbon, it is necessary to subject the material to high-temperature heat treatment (i.e., above 700 °C, preferably above 1000 °C).

[0087] Deposition can be performed on a sacrificial substrate (i.e., a substrate that is removed after deposition and does not constitute the completed electrode 1) or on a commercially available electrode, which is preferably a carbon fiber electrode such as Sigracet® type 29AA. When deposition is performed on a commercially available electrode, it is more preferable to perform deposition on both sides of the electrode so that the coating range is maximized.

[0088] When deposition is performed on a carbon fiber electrode (i.e., carbon fiber), during the deposition process, the above electrode (i.e., the second portion 13) can be rotated and / or translated at a speed determined based on the thickness S of the first portion 12 to be formed. In other words, in the deposition process, the first portion 12 having carbon fiber is deposited on the second portion 13, the first portion 12 forms a layer on at least one of the carbon fibers, and the second portion 13 is rotated and / or translated at a speed determined based on the desired thickness of the layer.

[0089] In the above-described process, by changing the speed, one or more of the above gradients can be obtained, thereby reducing the potential difference in the electrode 1 and increasing the current density and / or energy density of the flow battery.

[0090] Note that the carbon fibers constituting the electrodes can completely cross the jet of nanoparticles throughout the second portion 13, thereby achieving complete functionalization of the fibers. Further, after functionalizing the first surface, the electrodes are rotated in the impact chamber so that all the fibers can be coated more or less uniformly, or two jet sources are used on both sides of the electrodes, whereby the jet of the above nanoparticles can be deposited on both sides of the starting electrode. Also, by increasing the thickness and depositing on one surface, it is possible to obtain one or more of the above gradients.

[0091] As already explained above, the power of the generated RF signal is preferably in the range of 20 to 1,000 watts, more preferably 120 watts, and the frequency of the RF signal is preferably 13.56 MHz. More generally, the power of the RF signal per unit surface area of the electrode 1 is preferably 0.1 to 30 watts / cm 2 and more preferably 1.6 to 8 watts / cm 2 of the range.

[0092] Thereby, the roughness of the coating of the nanometer particles (that is, the area per unit volume) can be maximized. Such an effect can be recognized from the images shown in FIGS. 2(a) to (c), in each of these images, the first portion of the electrode 1 (that is, the concentric layer particles 11 constituting the same) manufactured using power levels of 20 W, 70 W, and 120 W with standard-sized electrodes is shown. FIG. 8 shows the trends of the roughness coefficient and the BET specific surface area at a plurality of different RF power levels. It can be recognized that as the RF power increases, the roughness coefficient increases and the BET specific surface area decreases, and the roughness coefficient and the BET specific surface area are strictly correlated with the supplied RF power.

[0093] That is, in the synthesis process, the power of the high-frequency signal output from the RF signal generator is determined based on the roughness coefficient representing the (desired) roughness of the first portion 12 of the electrode 1.

[0094] In this way, the roughness coefficient can be controlled, the potential difference at the electrode can be reduced, and the current density and / or energy density of the flow battery can be increased.

[0095] In addition to or as an alternative to the above-described embodiment, in the deposition process, by causing a flow of particles to impinge on the first portion 12 of the electrode 1 at a speed determined based on a desired roughness coefficient representing the desired active region density per unit volume of the first portion 12 of the electrode 1, the roughness coefficient representing the active region density per unit volume of the first portion 12 of the electrode 1 (i.e., the aggregated nanoparticles) can be changed and / or controlled. The above speed can be determined and / or controlled by changing the temperature of the electrode 1 and / or the synthesis chamber placed in the impact chamber, and / or by changing the pressure difference between the two chambers, and / or by changing the distance between the surface of the deposition target electrode 1 and the nozzle from which the nanoparticles emerge, etc.

[0096] By controlling the roughness coefficient, the potential difference at the electrode can be reduced, and the current density and / or energy density of the flow battery can be increased.

[0097] FIG. 9 shows an electrode 1 having a first portion with a roughness coefficient of 285 μm -1 deposited on a second portion 13 made of a 29AA electrode used as a support substrate (at a plurality of different magnification levels). The above electrode was tested under actual operating conditions for use in a flow battery using vanadium as the active element.

[0098] This test revealed that by coupling an electrode of the present invention having a thickness of 4 μm to an interdigitated flow distributor, the current value per unit surface area in contact with the electrolyte becomes maximum, and by coupling an electrode of the present invention having a thickness of 1 μm to a serpentine flow distributor, the current value per unit surface area in contact with the electrolyte becomes maximum.

[0099] Specifically, as the negative electrode, the electrode 1 of the present invention having the first portion 12 with a thickness of 4 μm was coupled to the interdigital flow distributor 22, and as the positive electrode, an electrode of the prior art was coupled to the second interdigital flow distributor 22, and a single cell equipped therewith was tested. By performing charge and discharge cycles, it was observed that the one in which the electrode 1 was coupled to the interdigital flow distributor 22 could reduce the overpotential as compared with the untreated electrode. Therefore, when the discharge current is the same, the configuration in which the electrode 1 of the present invention and the interdigital flow distributor 22 are coupled can improve the efficiency and extend the discharge depth, thereby increasing the (useful) capacity of the battery B.

[0100] After calculating the high-frequency resistance (HFR) parameter by measuring electrochemical impedance spectroscopy (EIS), the contribution of the ohmic loss of the ion exchange membrane could be subtracted in order to perform so-called IR correction. By this correction, for the electrode 1 having a contact area with the electrolyte of 25 cm 2 which is coupled to the interdigital flow distributor 22, when the volume flow rate of the electrolyte flow is 100 ml / min, the discharge current per unit surface area reaches 300 mA / cm 2 and can reach a very high efficiency value of more than 80%. It should be noted here that for a prior art electrode having the same (macroscopic) area (25 cm 2 ) in contact with the electrolyte, that is, an electrode without a layer of nanoparticles, when under the same operating conditions, the discharge current value per unit surface area reaches 100 mA / cm 2 at an efficiency value of more than 80%. Therefore, by coupling the interdigital flow distributor 22 to the electrode 1 of the present invention instead of the prior art electrode, the power density of the vanadium flow battery can be tripled.

[0101] In combination with the above-described embodiments, a nanometer layer (0.1 to 50 nm) optimized to realize special functions such as, for example, a function of reducing the formation of hydrogen in the negative electrode and / or a function of improving the high-voltage stability of the positive electrode can be coated on the electrode 1. Generally, this coating can be made of a material that is stable under the pH and the voltage state of the positive or negative electrode, has high catalytic activity for the redox reaction of vanadium, and is inert with respect to hydrogen generation. Such materials can be, for example, graphene, reduced graphene oxide, titanium nitride (TiN), titanium oxynitride (TiO x , 0 < x < 2), molybdenum oxide (MoO x , 0 < x < 3), tungsten oxide (WO x 0 < x < 3), tungsten oxynitride, tungsten nitride, tin oxide (SnO x , 0 < x ≦ 2), indium, indium oxide, bismuth, tellurium, manganese, niobium, yttrium, zirconium, hafnium, gallium, lead, lanthanum, cerium and / or other lanthanides, titanium, molybdenum, tungsten, iron, aluminum, silicon, germanium, boron, silver, silver oxide, boride, nitride, carbide and chalcogenide, etc. Other possible functionalizations can also be realized by using anionic groups such as, for example, O, N, F, P, S, Cl, Se, Br, I and their compounds (for example, compounds of sulfur and oxygen, nitrogen and oxygen, phosphorus and oxygen, or chlorine and oxygen) and organic functional molecules. Such a coating is selected to reduce the deterioration of the electrode 1 and / or reduce the activity of the electrode 1 with respect to the hydroxide reaction, thereby causing the generation of O2 and the reduction of O2 accompanied by H2. Furthermore, by optimizing the coating, the minimum potential in the negative electrode can be reduced and the maximum potential in the positive electrode can be increased, thereby expanding the maximum voltage range in which the battery can operate. As a result, the advantage that a larger current can be supplied or absorbed is achieved. Therefore, the service life of the battery B can be extended, and more effective peak shaving, load shifting and market regulation (that is, storing a large amount of electrical energy when it is available at a low cost and reselling it at a higher price at a later time, etc.) can be provided.

[0102] Of course, it is also possible to implement many variations of the examples described above.

[0103] Some of the possible variations of the present invention have been described above. However, those skilled in the art can actually implement other embodiments and can replace some elements with other technically equivalent elements. Therefore, the present invention is not limited to the exemplary embodiments illustrated above, and can undergo various modifications, improvements, and substitutions of equivalent parts and elements without departing from the basic inventive concept specified in the appended claims.

Claims

1. An electrode (1) for a flow battery (B), comprising a first portion (12) that can be arranged to contact the electrolyte of the flow battery (B), wherein the first portion (12) is mesoporous and in the electrode (1) composed of particles (11) of a conductive material of nanometer size, the electrode (1) further comprises a second portion (13) that supports the first portion (12), the second portion (13) has a plurality of carbon fibers, and the first portion (12) at least partially covers at least one of the carbon fibers with a layer of the particles (11). The electrode (1) is characterized by this.

2. The electrode according to claim 1, wherein the thickness of the layer of the particles (11) varies in the direction along the depth (D) and / or length (L) of the electrode (1). The electrode according to claim 1.

3. The electrode according to claim 1 or 2, wherein the first portion (12) is formed by carbon particles (11) having a concentric layer structure. The electrode according to claim 1 or 2.

4. The electrode according to claim 3, wherein the particle size of the particles (11) ranges from 1 to 100 nanometers. The electrode according to claim 3.

5. The electrode according to claim 4, wherein the particle size of the particles (11) ranges from 3 to 7 nanometers. The electrode according to claim 4.

6. The electrode according to any one of claims 1 to 5, wherein the first portion (12) has pores with an average dimension in the range of 2 to 50 nanometers. The electrode according to any one of claims 1 to 5.

7. The electrode according to any one of claims 1 to 6, wherein the porosity of the first portion (12) varies in the direction along the depth (D) and / or length (L) of the electrode (1). The electrode according to any one of claims 1 to 6.

8. The electrode according to any one of claims 1 to 7, wherein the thickness of the first portion (12) ranges from 0.5 μm to 10 μm. The electrode according to any one of claims 1 to 7.

9. The specific surface area of the first portion (12) of the electrode (1) is at least 500 m ２ / g, and the area per unit volume is at least 20 μm －１ , provided that the specific surface area is measured by the BET method The electrode according to any one of claims 1 to 8.

10. A flow battery (B) comprising at least one electrode according to any one of claims 1 to 9.

11. The flow battery (B) according to claim 10, wherein the electrode (1) is configured to operate as the negative electrode of the flow battery (B). The flow battery (B) according to claim 10.

12. The flow battery (B) according to claim 10 or 11, comprising a serpentine flow distributor (21) coupled to the electrode (1), wherein the thickness of the first portion (12) of the electrode (1) ranges from 0.5 μm to 5 μm. The flow battery (B) according to claim 10 or 11.

13. The flow battery (B) according to claim 10 or 11, comprising an interdigitated flow distributor (22) coupled to the electrode (1). The thickness of the first portion (12) of the electrode (1) is in the range of 2 μm to 10 μm. The flow battery (B) according to claim 10 or 11.

14. A method for manufacturing the electrode (1) according to any one of claims 1 to 9, a synthesis step of synthesizing a plurality of particles of a nanometer-sized conductive material, a deposition step of forming a first portion (12) of the electrode (1) using the plurality of particles, characterized by comprising.

15. In the synthesis step, the plurality of particles are synthesized by flowing a gas mixture containing at least one carbon-containing gas in a finite space where an electromagnetic field exists. The method according to claim 14.

16. In the deposition step, to cause a flow of the particles to impact the first portion (12) of the electrode (1) at a speed determined based on a roughness coefficient representing the roughness of the first portion (12) of the electrode (1), the roughness coefficient is determined as the product of the density of the first portion (12) and the specific surface area of the first portion (12) measured by the BET method. The method according to claim 14 or 15.

17. In the deposition step, the first portion (12) is deposited on a second portion (13) containing a plurality of carbon fibers such that the first portion (12) forms a layer on the surface of at least one of the plurality of carbon fibers, rotating and / or translating the second portion (13) at a speed determined based on the desired thickness of the layer. The method according to any one of claims 14 to 16.

Images