Electrodes and manufacturing methods for flow batteries

Abstract

The present invention relates to an electrode (1) for a flow battery (B) and a method for manufacturing the electrode (1), wherein the electrode (1) comprises a first portion (12) consisting of particles (11) of conductive material having a nanoscale size, wherein the first portion (12) is mesoporous and its porosity increases the number of redox reactions per unit time in the flow of the electrolyte solution 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; more specifically, the present invention relates to an electrode for a vanadium redox flow battery and a method for manufacturing the electrode, wherein the vanadium redox flow battery is a battery in which at least one of two electrolytes contains vanadium ions. Background Technology

[0002] As is well known, the maximum power that can be delivered by a flow battery is actually independent of the battery's capacity; in fact, the maximum power depends on the battery's characteristics (i.e., the size and chemophysical properties of the electrodes, the size and type of ion exchange membranes inserted between the two electrodes and the two electrolyte solutions in use, the number of electrodes per half-cell, etc.), while the capacity depends primarily on the type of electrolyte and redox material used and their total quantity stored in the tank. Therefore, it can be said that a flow battery allows the maximum power that can be delivered by the battery to be independent of its capacity.

[0003] This feature allows the flow battery to be optimally sized for specific applications, such as coupling the flow battery to power plants that use renewable energy sources (e.g., solar, wind, hydro, wave, etc.); in fact, renewable energy sources are characterized by a power-to-energy ratio that follows seasonal trends (especially solar and wind power plants), meaning that maximum power can be obtained in almost all seasons, but the energy generated each day varies depending on the time of year.

[0004] Currently, the energy density of flow batteries is lower than that of lithium-ion batteries, but the capacity of lithium-ion batteries is related to their maximum power and tends to decrease as the battery ages. Furthermore, the size of individual lithium battery components is determined by safety regulations and cooling requirements.

[0005] The article "GONZALEZ, Zoraida, et al. Carbon nanowalls as nanostructured electrode materials in vanadium redox flow batteries. Nano Energy, 2012, 1.6: 833-839" describes a solution that envisions generating carbon nanowalls (CNWs) on a gold electrode to increase the electrode's reactive surface; such electrodes are then used as the positive half-cell in vanadium redox flow batteries. This solution provides a more compact electrode, but it does not increase the kinetics of the redox reaction on the electrode's surface. Therefore, besides being economically disadvantageous due to the need for a gold electrode, this solution does not achieve any significant increase in the specific power of the vanadium redox flow battery. Summary of the Invention

[0006] The present invention aims to solve these and other problems by providing an electrode for flow batteries.

[0007] Furthermore, the present invention aims to address these and other problems by also providing a method for manufacturing electrodes for flow batteries.

[0008] The basic concept of this invention is to manufacture and use an electrode comprising a portion adapted to be placed in contact with an electrolyte solution of a flow battery, wherein said portion has a mesoporous structure composed of particles of conductive material having a nanoscale size (i.e., a structure containing pores having a diameter in the range of approximately 1 nanometer to 100 nanometers) to increase the kinetics of redox reactions in the flow of the electrolyte solution and / or the number of active sites per surface unit and / or the electrode area, so as to increase the current generated per projected surface unit of the electrode.

[0009] Using nanoparticles of conductive materials to create a mesoporous structure for electrodes in flow batteries yields the following technical effects: it allows electrolyte flow through the electrode while increasing the number of active sites per unit of electrode surface for electron exchange, increasing the electrode area in contact with the electrolyte, and increasing the kinetics of redox reactions in the flow (which occur on the electrode surface in contact with the electrolyte solution during normal charging / discharging of the flow battery), thereby advantageously generating electrocatalysis in the electrolyte flowing through the electrode according to the invention. In other words, the invention achieves the following technical effect: increasing the number of redox reactions per unit time in the electrolyte solution flow, and therefore increasing the current density per unit projected surface area of ​​the electrode.

[0010] This advantageously allows for an increase in power density in flow batteries; specifically, a preferred embodiment of the electrode according to the invention can generate a current per surface unit that is three times greater (efficiency exceeding 80%) than that generated by electrodes according to the prior art.

[0011] Furthermore, electrocatalysis produces a beneficial reduction in the electrode overpotential, thus making it possible to increase the current density of the battery; in fact, as a macroscopic effect, the reduced electrode overpotential reduces the battery's internal resistance (i.e., ohmic resistance and electrochemical resistance), and therefore reduces the battery's internal voltage drop. Indeed, experimental tests have shown that, when used as a negative electrode, the electrode according to the invention allows a single vanadium redox flow battery to operate at a charge / discharge current in the range of 1.0-1.8 volts that is at least twice as high as that of conventional batteries. This is indeed possible because the reduced overpotential advantageously makes it possible to reduce the amount of hydrogen and oxygen that can be generated when high currents are applied during the charging or discharging phases, thus allowing the battery to manage specific situations where it is necessary to absorb or transport high-intensity currents.

[0012] Therefore, the use of such flow batteries is superior to existing flow batteries, for example, in applications requiring the use of such batteries in large power plants that generate electricity from renewable energy sources, such as photovoltaic and wind power plants, where power generation follows highly variable trends. Furthermore, the electrodes according to the invention make it possible to reduce oxidation and / or the formation of hydrogen or oxygen on their surface, thus advantageously reducing degradation, allowing the flow battery performance to remain consistent for a greater number of charge / discharge cycles compared to other battery types.

[0013] Other advantageous features of the invention will be set forth in the appended claims. Attached Figure Description

[0014] These features and other advantages of the invention will become more apparent from the following description of preferred embodiments of the invention as illustrated in the accompanying drawings, which are provided only by way of non-limiting example, wherein:

[0015] - Figure 1 A simplified diagram illustrating the operation of a flow battery including electrodes according to the present invention is shown.

[0016] - Figure 2 a to Figure 2 c shows three images obtained by scanning electron microscopy, which represent the results that can be obtained by performing the manufacturing method according to the invention at different levels of radiation power;

[0017] - Figure 3 Show Figure 1 A schematic representation of the electrodes;

[0018] - Figure 4 Showing what can be used Figure 1 Two different types of flow distributors in the battery;

[0019] - Figure 5 Showing coupling to Figure 4 One of the flow distributors Figure 1 A schematic representation of the electrodes;

[0020] - Figure 6 Show Figure 1 A schematic cross-sectional representation of the electrode;

[0021] - Figure 7 Showing coupling to Figure 4 One of the flow distributors Figure 1 A schematic cross-sectional representation of the electrode;

[0022] - Figure 8 Showing Figure 1 A graph showing the trend of the area per unit mass and area per unit volume of the electrode particles as a function of the power of the radio frequency waves used during the manufacturing process of the electrode.

[0023] - Figure 9 Four images acquired by scanning electron microscopy are shown, with different magnification levels representing different portions of a preferred embodiment of the electrode according to the invention. Detailed Implementation

[0024] In this document, any reference to "embodiment" indicates that a particular configuration, structure, or feature is included in at least one embodiment of the invention. Therefore, it is clear that expressions such as "in an embodiment" in different parts of this document will not necessarily refer to the same embodiment. Furthermore, in one or more embodiments, any particular configuration, structure, or feature may be considered as a suitable combination. Therefore, the following references are for simplification purposes only and should not be construed as limiting the scope or extension of the various embodiments.

[0025] refer to Figure 1 The following describes a battery comprising at least one electrode 1 according to the invention; preferably, such electrode 1 has a planar shape.

[0026] It must be noted that, in this document, the term "electrode" refers to a component of the battery on which a redox reaction occurs on its surface during charging or discharging.

[0027] Battery B includes a negative electrode portion B1 and a positive electrode portion B2, wherein the negative electrode portion B1 includes a negative electrode half-cell S1, a first tank T1 and a first pump P1, and wherein the positive electrode portion B2 includes a positive electrode half-cell S2, a second tank T2 and a second pump P2.

[0028] The negative half-cell S1 is in fluid communication with the first tank T1 via the first delivery pipe D1 and the first return pipe R1, wherein the first tank T1, pipe D1, R1 and negative half-cell S1 contain a first electrolyte solution, and wherein the first pump P1 is preferably positioned along the first delivery pipe D1 such that the first pump P1 will generate a flow of the first electrolyte solution within the negative half-cell S1 when it is in operating conditions.

[0029] Similar to the negative half-cell S1, the positive half-cell S2 is in fluid communication with the second tank T2 via the second delivery pipe D2 and the second return pipe R2, wherein the second tank T2, pipe D2, R2 and positive half-cell S2 contain a second electrolyte solution, and wherein the second pump P2 is preferably positioned along the second delivery pipe D2 such that the second pump P2 will generate a flow of the second electrolyte solution within the positive half-cell S2 when it is in operating conditions.

[0030] Battery B2 also includes an ion exchange membrane (e.g., a membrane made of Nafion or another material), the ion exchange membrane having a first side and a second side, the first side and the second side being contacted by a first electrolyte solution contained in the negative electrode half-cell S1 and by a second electrolyte solution contained in the positive electrode half-cell S2, respectively.

[0031] Furthermore, at least one of the reaction half-cells S1 and S2 includes an electrode 1 according to the invention. In a preferred embodiment, the negative electrode half-cell S1 includes one such electrode 1, while the positive electrode half-cell S2 preferably includes a conventional electrode according to the prior art (e.g., a carbon electrode) or an electrode 1 according to the invention. At least one electrical load L and / or generator G may be connected to said electrode.

[0032] The first and second electrolyte solutions are preferably solutions containing vanadium ions. This advantageously allows the use of the same starting solution for both the negative electrode portion B1 and the positive electrode portion B2; such a starting solution is, for example, an aqueous solution of vanadium sulfate (brute formula VOSO4). It should be noted that vanadium actually has five oxidation states (+1, +2, +3, +4, +5), four of which (+2, +3, +4, +5) are effectively usable for electrochemical applications. This makes it possible to start from the said starting solution and use methods well known in the art to produce a first electrolyte solution (negative electrode portion B1) with vanadium having an oxidation state of +2 and / or +3 and a second electrolyte solution (positive electrode portion B2) with vanadium having an oxidation state of +4 and / or +5.

[0033] When battery B is under operating conditions and generator G is recharging battery B, vanadium in the first electrolyte solution (negative electrode portion B1) is reduced and switches from the +3 oxidation state to the +2 oxidation state by absorbing electrons, while vanadium in the second solution (positive electrode portion B2) is oxidized and switches from the +4 oxidation state (V₀) by generating electrons. 2+ Switching to the +5 oxidation state (VO2) + ).

[0034] When battery B is under operating conditions and the electrical load L discharges battery B, vanadium in the first electrolyte solution (negative electrode portion B1) is oxidized and switches from the +2 oxidation state to the +3 oxidation state by generating electrons, while vanadium in the second solution (positive electrode portion B2) is reduced and switches from the +5 oxidation state (VO2) by absorbing electrons. + Switching to the +4 oxidation state (VO) 2+ ).

[0035] Nevertheless, it is still possible to use dissimilar electrolyte solutions or solutions of other properties (aqueous or containing organic solvents and active substances, such as iodides, sulfides and bromides, alkali metals (lithium, sodium, etc.) or transition metals (iron, chromium, titanium, tin, zinc, cerium, manganese, etc.) or organic molecules and redox polymers (quinones, methyl viologen, ACA, ferrous cyanide, TEMPO, PANI, PNB-g-PTMA, polythiophene) or solid particles, without departing from the teachings of this invention.

[0036] The electrolyte solution used includes a solvent, which is preferably water. Organic solvents (e.g., acetonitrile, dimethyl sulfoxide, propylene carbonate, ethyl carbonate, dioxolane, etc.) or ionic liquids (e.g., 1-ethyl-3-methylimidazolium chloride (EMICl) / FeCl3 / FeCl2, tetrabutylammonium hexafluorophosphate (TEAPF6), 1-ethyl-3-methylimidazolium hexafluorophosphate (EMIPF6)) may also be used in combination with or as an alternative to water.

[0037] The electrode 1 according to the invention comprises a first portion having a mesoporous (physical) structure, i.e., a structure comprising pores having a diameter in the range of approximately 1 nanometer to 100 nanometers, wherein the first portion is adapted to be placed in contact with one of the electrolyte solutions of the battery B and is composed of particles of conductive material having a nanoscale size. The porosity of the first portion allows an electrolyte solution flow to diffuse therein and advantageously increases the number of redox reactions per unit time in the flow, wherein the flow is preferably generated by one of pumps P1 and P2.

[0038] In this way, it is advantageous to potentially increase the power density in flow batteries.

[0039] The first part of electrode 1 can form the entire body of such electrode 1, i.e., the mesoporous (first) part can be used as an independent electrode.

[0040] In a preferred embodiment of the present invention, the mesoporous part can be supported by other structural elements of battery B, for example, a commercially available carbon fiber electrode, or supported by other substrates, such as a gas diffusion layer (GDL, such as 29AA substrate), a membrane, a current collector, a foam and / or a metal / polymer grid, an electrospun material or other types of materials available on the market.

[0041] As will be further described below, a preferred embodiment of electrode 1 includes a second part, wherein the first part is constrained to the second part such that the second part serves as a support for the first part. In other words, the second part supports the first part.

[0042] Thus, advantageously, it is possible to increase the power and energy density in flow batteries already available on the market, because the overpotential of the electrode can be reduced by promoting the transfer of electrolyte towards the first part of the electrode and / or by increasing the reaction kinetics.

[0043] Particles of conductive materials having a nanoscale size can be zero-dimensional (0D nanoparticles) and / or two-dimensional (2D nanoclusters). It must be noted that it is also possible to employ a mixture of zero-dimensional and two-dimensional particles.

[0044] Preferably, such nanoparticles can be any conductive material, for example, a material belonging to any one of the following compound classes: carbon, metal, nitride, boride, carbide, oxide, chalcogenide, etc. More preferably, such nanoparticles can include carbon nitride (C3N4), carbon and nitrogen compounds (CN X , where 0 < x < 4 / 3), graphene, reduced graphene oxide, carbon nanoparticles, carbon nanotubes, fullerenes, titanium nitride (TiN), titanium oxynitride, titanium oxide (TiO x 0 < x < 2), molybdenum oxide (MoO x , where 0 < x < 3), tungsten oxide (WO x 0 < x < 3), tungsten oxynitride, tungsten nitride, tin oxide (SnO x 0 < x ≤ 2), indium and indium oxide, iridium oxide (IrO2), ruthenium and ruthenium oxide, bismuth and its oxides, borides, nitrides, carbides and chalcogenides, tellurium, manganese, niobium, yttrium, zirconium, hafnium, gallium, lead, lanthanum, cerium and / or other lanthanide elements, titanium, molybdenum, tungsten, iron, nickel, aluminum.

[0045] Moreover, referring to Figure 2In a preferred embodiment of the invention, the conductive particles are carbon nanoparticles; these carbon nanoparticles have an onion-like structure, i.e., the particles have a concentric layered structure 11. It has been unexpectedly observed that this type of structure promotes electrocatalysis in the electrolyte flow, thus advantageously increasing the power and energy density of battery B.

[0046] It has been observed that when the particles have a size (i.e., diameter) in the range of 1 nanometer to 50 nanometers, and even more preferably in the range of 4 nanometers to 5 nanometers, electrocatalysis is further improved (i.e., the number of redox reactions per unit time is further increased).

[0047] The particle assembly (e.g., obtained by means of a supersonic jet followed by heat treatment in a vacuum or controlled atmosphere) provides a certain degree of porosity within the first portion of electrode 1. The porosity generated by the particle assembly can be in the range of 1 nanometer to 50 nanometers, that is, the formed pores can have a diameter in the range of 1 nanometer to 50 nanometers.

[0048] More specifically, it must be pointed out that the average pore size can be defined as a function of the steric impedance of the active material used in the battery, where the term "active material" refers to ions that participate in the redox reactions that allow the storage and subsequent delivery of electrical energy. Specifically, when the electrolyte in use contains active materials, such as vanadium and / or other ionic substances, or active redox molecules with a molar mass of less than 1000 g / mol, such as quinone compounds (benzoquinone, anthraquinone, etc.), alkoxybenzene, derivatives of 2,2,6,6-tetramethyl-1-piperidinoxy (TEMPO), N-methylphthalimide, etc., the average pore size can be selected from 1 nanometer to 10 nanometers.

[0049] If redox substances with high molecular weight are used, specifically polymers (e.g., derivatives of polyaniline, derivatives of poly(vinyl-benzyl ethyl viologen), derivatives of the so-called "bottle brush" polymer class, derivatives of 2,2,6,6-tetramethyl-1-piperidinoxy (TEMPO), polythiophene, water-soluble polymers such as polymethacrylate and polystyrene, boron-dipyrrole methylene, etc.), it will be possible to use pores with an average diameter of 10 nm to 100 nm.

[0050] The pore size can also be adapted to the viscosity of the electrolyte; for example, pores with an average diameter of 1 to 10 nanometers are more suitable for low-viscosity electrolyte solutions, while pores with an average diameter of 10 to 100 nanometers are more suitable for high-viscosity electrolyte solutions.

[0051] In addition, the pore size can be selected to match the steric hindrance of the active material and the viscosity of the electrolyte.

[0052] By using the porosity and active elements specified above, it is possible to increase redox reactions. Therefore, advantageously, it is possible to increase the power and energy density of commercially available flow batteries.

[0053] Moreover, reference Figure 3 The following describes a possible embodiment of electrode 1. Electrode 1 includes a first portion 12 and a second portion 13, wherein the first portion is mainly composed of carbon particles 11 having a concentric layered structure, and wherein the second portion 13 is preferably a carbon electrode according to the prior art.

[0054] It must be noted that, for illustrative reasons, the second part is schematically represented as a planar layer in order to better highlight the porosity aspect of the first part 12 graphically, without causing any complication due to the (fibrous) structure of the carbon fiber that preferably constitutes the second part 13 of the electrode 1.

[0055] The thickness of the first portion 12 is in the range of 0.01 μm to 1000 μm, preferably 0.1 μm to 50 μm. Even more preferably, the thickness of the first portion of the electrode is in the range of 0.5 μm to 10 μm. According to a preferred embodiment, the thickness of the first portion 12 of the electrode is in the range of 1 μm to 6 μm.

[0056] Regardless of the initial active material used, the internal porosity can vary along the depth D of the first portion 12 of the electrode 1, up to 10% to 90% of the total thickness of the first portion.

[0057] More specifically, the porosity varies along the depth D of the first portion 12, and preferably, the porosity varies according to a gradient of 0% to 80% per 100 nanometers. This gradient allows for increased redox reactions in those regions of the first portion 12 of electrode 1 closer to the second portion 13 of electrode 1, where the concentration of ions that can be oxidized or reduced is lower, and therefore the porosity must be lower; such regions are located near the second portion 13 of electrode 1 because in such regions the electrolyte has encountered the outermost particles of the first portion 12 of electrode 1 and has therefore undergone a reduction in the concentration of ions ready for oxidation or reduction.

[0058] This makes it possible to reduce the potential difference between the surface of the first portion 12 of electrode 1 that is further away from the second portion 13 and the surface of the first portion 12 that is closer to the second portion 13, thus advantageously minimizing the intensity of the current within electrode 1, which reduces the efficiency of electrode 1 by reducing the maximum transferable current.

[0059] Furthermore, the presence of this gradient promotes electrolyte flow by allowing the electrolyte to fully permeate the first portion 12 of electrode 1. This advantageously permits the use of electrodes with a large contact area with the electrolyte, while ensuring continuous electrolyte renewal.

[0060] In this way, it is advantageous to potentially increase the power and energy density in flow batteries.

[0061] As an alternative to or in combination with the above, the porosity and thickness of the first part 12 may vary along the length L and / or depth D of the electrode 1.

[0062] Specifically, the porosity may vary along the length L of the first portion, preferably as a percentage value of 0% to 80% of the predefined porosity value per millimeter of the length L of the electrode 1, and / or the porosity may vary along the depth D of the electrode 1, preferably as a percentage value within the range of 0% to 80% of the predefined porosity value per micrometer of the depth D of the electrode 1.

[0063] Moreover, this gradient is useful to minimize the overpotential of redox reactions in those regions of the first portion 12 of electrode 1 where the concentration of ions that can be oxidized or reduced is low, thus requiring a smaller porosity and therefore a larger active surface, i.e. a larger number of active sites; such regions are located in the last region of electrode 1 to be contacted by the electrolyte flow, because in such regions the electrolyte has already encountered most of the first portion 12 and has therefore undergone a reduction in the concentration of ions ready for oxidation or reduction.

[0064] In this way, it is advantageous to potentially increase the power and energy density in flow batteries.

[0065] The first part 12 of electrode 1 has at least 500m 2 / g, preferably at least 600m 2 / g, and even more preferably 610m 2 Specific surface area per g (preferably measured according to the BET method), and at least 20 μm -1 Preferably at least 200 μm -1 And even more preferably 285μm -1 The roughness factor (i.e., the area per unit volume).

[0066] Just like particle size and porosity, the roughness factor can be non-uniform across the thickness of the first portion 12 of electrode 1. According to a preferred embodiment, the roughness factor of the first portion 12 can be gradient from 20 μm... -1 Change to 500μm -1This makes it possible to maintain uniform and / or constant redox reactions in different zones of the first part 12 of electrode 1 when the concentration of ions ready for oxidation or reduction changes.

[0067] In this way, it is advantageous to potentially increase the power and energy density in flow batteries.

[0068] As previously described, the carbon particles constituting the first portion 12 of electrode 1 have a concentric layered structure, with a diameter preferably ranging from 2 nm to 100 nm, more preferably from 3 nm to 7 nm. Additionally, the first portion 12 of electrode 1 has a thickness preferably ranging from 0.1 μm to 100 μm, more preferably from 1 μm to 10 μm. Along this thickness, the porosity between the aggregated particles ranges from 1 nm to 100 nm. The first portion 12 is supported by a second portion, which preferably comprises a commercially available electrode made of carbon fiber.

[0069] Moreover, reference Figure 4 According to the invention, the electrode 1 is preferably coupled to a flow distributor, for example, a serpentine flow distributor 21 or an interdigitated flow distributor 22.

[0070] After the electrolyte solution has flowed out of one of tanks T1 or T2 under the action of one of pumps P1 or P2, flow distributors 21 and 22 allow the electrolyte flow to be distributed on electrode 1. It must be noted that the distribution of the electrolyte solution flow on electrode 1 requires that the electrolyte solution diffuse through most of the porous volume of electrode 1.

[0071] When electrode 1 is used as the negative electrode in a flow battery in which the electrolyte contains vanadium ions, the achievable power density is strictly related to a combination of factors such as electrode thickness and the type of flow distributor coupled to the electrode.

[0072] When using the serpentine flow distributor 21, the thickness of the first portion 12 of the electrode 1 is preferably in the range of 0.5 μm to 5 μm, more preferably 1 μm.

[0073] When using the interdigitated flow distributor 22, the thickness of the first portion 12 of the electrode 1 is preferably in the range of 2 μm to 10 μm, more preferably 4 μm.

[0074] Furthermore, when electrode 1 is used as the positive electrode in a flow battery in which the electrolyte contains vanadium ions, the achievable power density is strictly related to the combination of electrode thickness and the type of flow distributor coupled to the electrode. It must also be noted that the best results in electrocatalysis are obtained on a smaller scale with the same combination of thickness and flow distributor as described above.

[0075] Electrode 1 according to the present invention has been shown in more than 10 5 Conductivity in S·m.

[0076] refer to Figure 5 The following section will describe how the electrolyte solution flows in electrode 1 under the action of the interdigitated flow distributor 22.

[0077] As is well known, the interdigitated flow distributor 22 includes at least one grid of delivery conduits 221 and a grid of discharge conduits 222, which lead to the outside (i.e., to electrode 1 under operating conditions) but are not directly connected to each other. With this configuration of the distributor, it is advantageously possible to generate at least one electrolyte flow F within almost the entire electrode 1 when the distributor 22 is coupled to said electrode 1. This advantageously increases the number of redox reactions per unit time within electrode 1 as the current circulates across electrode 1 of the battery B.

[0078] In this way, it is advantageous to potentially increase the power and energy density in flow batteries.

[0079] Moreover, reference Figure 6 and Figure 7 The preferred embodiment of electrode 1 will be described below, wherein the second part 13 of electrode 1 is composed of carbon fiber electrode, that is, the second part 13 includes a plurality of carbon filaments.

[0080] To better describe the technical characteristics associated with carbon fiber, it must be pointed out that, Figure 6 and Figure 7 Showing a cross-sectional view of carbon fiber filaments, but not representing... Figure 3 and Figure 5 The gradient is highlighted in the image (for clarity).

[0081] At least one of the filaments constituting the second portion 13 of the electrode is coated with a layer of particles 11, said particles being the same as those constituting the first portion 12 of the electrode. In other words, the second portion 13 comprises a plurality of carbon filaments, and wherein the first portion (12) is at least partially coated with a layer of said particles 11 on at least one of the filaments.

[0082] This makes it possible to reduce the overpotential of electrode 1 while improving the mechanical strength of the electrode in order to increase the current and / or energy density of the flow battery.

[0083] Furthermore, the layer of the first part 12 has a thickness S that can vary depending on the position of the filament in the electrode 1.

[0084] More specifically, the higher the concentration of active material in the electrolyte flow, the smaller the thickness S; conversely, the lower the concentration of active material in the electrolyte flow, the larger the thickness S. In other words, the thickness S of the particle 11 layer varies along the depth D and / or length L of the electrode 1.

[0085] The thickness S of the first portion 12 surrounding the second portion 13 (i.e., the filament) of electrode 1 preferably varies in the amount from 10 nanometers to 1000 nanometers per millimeter of length L and / or in the amount from 10 nanometers to 1000 nanometers per micrometer of thickness D of electrode 1.

[0086] Furthermore, the thickness S of the first part 12 can vary around the same filament, for example, according to Figure 5 The gradient is highlighted in the image.

[0087] The potential difference in the electrodes is thus reduced, thereby advantageously increasing the current that electrode 1 can absorb or supply to the electrolyte, and thus advantageously increasing the power and energy density in the flow cell.

[0088] Electrode 1, as described in this paper, can be fabricated using various manufacturing methods. Generally, any film growth method can be employed, which allows for the controlled deposition of nanoparticles on a surface using controlled kinetic energy.

[0089] More specifically, such electrodes 1 can be manufactured by depositing nanoparticles using a plasma source, sputtering system, pulsed laser deposition (PLD) system, plasma-enhanced chemical vapor deposition (PECVD) system, atmospheric spraying, etc.

[0090] In other words, a method for manufacturing the electrode 1 according to the present invention includes the following stages:

[0091] a. The synthesis stage, in which multiple particles of conductive material with nanoscale dimensions are synthesized;

[0092] b. Deposition stage, wherein the plurality of particles are used to form a first portion of the electrode 1.

[0093] Electrode 1, as described above, can be manufactured by generating a plasma-assisted supersonic particle source, as described by the University of Milano Bicoccca in International Patent Application Publication WO 2011 / 064392 A2. Such a method envisions the use of an apparatus capable of generating an aerosol of nanoparticles, preferably by means of reactive plasma, in a first chamber separated from a second chamber by a sufficiently small orifice, such that a supersonic gas jet containing suspended particles is generated by maintaining the chambers at different pressures. To maintain the pressure difference between the chambers, a pumping system is connected to one of the chambers. The chamber maintained at the higher pressure (referred to as the "synthesis chamber") comprises a pair of electrodes supplied by a signal generator (also referred to as the "RF signal generator"), thus effectively generating a confined space in which an electromagnetic field, preferably generated by the signal generator, exists; a gas mixture comprising at least one carbon-containing gas, such as acetylene (C2H2), is injected between the two electrodes (i.e., within the confined space). More specifically, the mixture preferably consists of 99.63% argon (Ar) and 0.38% acetylene (C2H2). An RF signal generator is configured to generate a signal preferably with a frequency of 13.56 MHz and a power of at least 20 watts, preferably 120 watts. This causes acetylene molecules flowing across the electrodes to dissociate into free radicals, which, in an inert argon atmosphere (i.e., with a negligible amount of oxygen per unit time), will begin to polymerize into clusters of several atoms, and thus into nanoparticles. The synthesized particles are then collected by a supersonic jet and accelerated in a lower-pressure chamber (also referred to as the “impact chamber”). In operation, the pressure in the synthesis chamber is preferably maintained at 130 Pascals, while the pressure in the impact chamber is preferably maintained at 2 Pascals.

[0094] As an alternative to the RF signal generator and the two electrodes that generate reactive plasma when supplied by the generator, it is possible to use other heating systems, such as heated filaments (e.g., heated by the Joule effect) or high-power lamps.

[0095] Depending on the power generated during the synthesis of carbon nanoparticles, the latter can be graphitized or hydrogenated. In the latter case, such materials must undergo high-temperature heat treatment (i.e., above 700°C, preferably above 1000°C) to obtain fully graphitized carbon.

[0096] The deposition can occur on a sacrificial substrate (i.e., the substrate that will subsequently be removed and therefore will not be part of the finished electrode 1), or on a commercial electrode, preferably a carbon fiber electrode, for example. It occurs on type 29AA, and more preferably on both sides of the electrode, in order to advantageously obtain maximum cladding coverage.

[0097] If deposition occurs on a carbon fiber electrode (i.e., on carbon filaments), during the deposition phase, the electrode (i.e., the second portion 13) can rotate and / or translate at a speed determined based on the thickness S of the first portion 12 to be obtained. In other words, during the deposition phase, the first portion 12 is deposited on the second portion 13, which comprises multiple carbon filaments, such that the first portion 12 forms a layer on at least one of the carbon filaments, and the second portion 13 rotates and / or translates at a speed determined based on the desired thickness of the layer.

[0098] In this way, by changing the speed, it is possible to obtain one or more of the gradients described above in order to reduce the potential difference in electrode 1 and thus increase the current and / or energy density of the flow cell.

[0099] It should be noted that the carbon fibers constituting the electrode advantageously allow the jet of nanoparticles to completely penetrate the bulk of the second section 13, thus permitting full functionalization of the filaments. Furthermore, the jet of nanoparticles can be deposited on both sides of the initial electrode, either by rotating the electrode inside the impact chamber after the first surface has been functionalized, allowing all the filaments to be coated in a more uniform or less uniform manner, or by using two sources on both sides of the electrode. It is also possible to deposit a larger thickness on one side, thereby obtaining one or more of the gradients described previously.

[0100] As described above, the power of the generated RF signal is preferably in the range of 20 watts to 1000 watts, and more preferably 120 watts, while its frequency is preferably 13.56 MHz. More generally, the power of the RF signal per surface unit of electrode 1 is preferably 0.1 watts / cm². 2 Up to 30 watts / cm 2 More preferably 1.6 watts / cm 2 Up to 8 watts / cm 2 Within the range.

[0101] This makes it possible to maximize the roughness (i.e., area per unit volume) of the nanoparticle coating. Figure 2 (a) to Figure 2 This effect can be understood from the image shown in (c), which illustrates the first part of electrode 1 (i.e., the concentric layered particles 11 constituting it) generated by using power levels of 20W, 70W and 120W on a standard-sized electrode. Figure 8The trends of roughness factor and BET specific surface area are shown for different RF power levels. It can be noted that higher RF power produces an increased roughness factor and a decreased BET specific surface area, and both the roughness factor and BET specific surface area are strictly correlated with the delivered RF power.

[0102] In other words, during the synthesis phase, the power of the radio frequency signal output by the RF signal generator is determined based on the roughness factor that describes the (desired) roughness of the first part 12 of electrode 1.

[0103] This allows for control of the roughness factor to reduce the potential difference in the electrodes and thus increase the current and / or energy density of the flow cell.

[0104] As a supplement or alternative to the above, during the deposition stage, it is possible to change and / or control the roughness factor of the active area density per volume of the first portion 12 of the descriptive electrode 1 (i.e., the clustered nanoparticles) by causing the flow of particles to impinge on the first portion 12 of the descriptive electrode 1 at a velocity determined based on the desired roughness factor of the desired active area density per volume unit of the first portion 12 of the descriptive electrode 1. The velocity can be determined and / or controlled by changing the temperature of the synthesis chamber and / or the electrodes 1 positioned in the impingement chamber and / or by changing the pressure difference between the chambers and / or the distance between the surface of the electrode 1 on which deposition occurs and the distance between the nanoparticles and the nozzle through which they exit.

[0105] By controlling the roughness factor, it is possible to reduce the potential difference in the electrodes and thus increase the current and / or energy density of the flow cell.

[0106] Figure 9 Electrode 1 is shown (at different magnification levels), the electrode comprising a 285 μm electrode deposited on a second portion 13 consisting of 29AA electrodes serving as a support substrate. -1 The first part of the roughness factor. This electrode was tested under actual operating conditions in a flow battery using vanadium as the active element.

[0107] These tests highlight that the electrode according to the invention, with a thickness of 4 μm and coupled to the interdigitated flow distributor, provides the highest current value per surface unit in contact with the electrolyte solution, while the electrode according to the invention, with a thickness of 1 μm and coupled to the serpentine flow distributor, provides the highest current value per surface unit in contact with the electrolyte solution.

[0108] More specifically, a test cell is considered, said cell having: an electrode 1 according to the invention, as the negative electrode, coupled to an interdigitated flow distributor 22, wherein the electrode has a first portion 12 with a thickness of 4 μm; and an electrode according to the prior art, as the positive electrode, coupled to a second interdigitated flow distributor 22. By performing charge / discharge cycles, it is possible to observe that the electrode 1 coupled to the interdigitated flow distributor 22 reduces the overpotential compared to an untreated electrode. Thus, at equal discharge currents, the coupling of the electrode 1 according to the invention to the interdigitated flow distributor 22 increases the efficiency and extends the depth of discharge, thus increasing the (usable) capacity of the cell B.

[0109] After calculating the high-frequency resistance (HFR) parameter by measuring the electrochemical impedance spectroscopy (EIS), it is possible to subtract the contribution of the ohmic losses of the ion-exchange membrane to perform a so-called IR correction. By this correction, it is possible to observe that the electrode 1 with a contact area with the electrolyte of 25 cm 2 reaches an extremely high efficiency value of over 80% in the presence of a discharge current of 300 mA / cm 2 per surface unit and in the presence of an electrolyte flow with a volumetric flow rate of 100 ml per minute. It should be noted that, under the same operating conditions, an electrode according to the prior art, i.e., without a nanoparticle layer and having the same (macroscopic) area (25 cm 2 ) in contact with the electrolyte, reaches a discharge current value of 100 mA / cm 2 per surface unit, where the efficiency value exceeds 80%. Thus, it is possible to triple the power density of a vanadium redox flow battery by coupling the interdigitated flow distributor 22 to the electrode 1 according to the invention instead of an electrode according to the prior art.

[0110] In combination with the above, the electrode 1 can be coated with a nano-layer (0.1 - 50 nm thick) optimized for a specific function, e.g., reducing the formation of hydrogen on the negative electrode and / or increasing the high-voltage stability of the positive electrode. Generally, the coating can be made of materials that are stable under the pH and voltage conditions of the positive or negative electrode, have high catalytic activity for the vanadium redox reaction, and are inactive in terms of hydrogen evolution, such materials as graphene, reduced graphene oxide, titanium nitride (TiN), titanium oxynitride, titanium oxide (TiO x 0 < x < 2), molybdenum oxide (MoO x , where 0 < x < 3), tungsten oxide (WO x 0 < x < 3), tungsten oxynitride, tungsten nitride, tin oxide (SnO x(0 < x ≤ 2), indium and indium oxide, bismuth, tellurium, manganese, niobium, yttrium, zirconium, hafnium, gallium, lead, lanthanum, cerium and / or other lanthanide elements, titanium, molybdenum, tungsten, iron, aluminum, silicon, germanium, boron, silver and their oxides, borides, nitrides, carbides and chalcogenides; other possible functionalizations can be obtained by using anionic groups such as O, N, F, P, S, Cl, Se, Br, I and their compounds (e.g., sulfur and oxygen, nitrogen and oxygen, phosphorus and oxygen or chlorine and oxygen compounds) and organic functional molecules. Such a cladding is chosen so as to reduce the degradation of electrode 1 and / or its reactivity towards the water oxidation reaction, thus resulting in the production of O2 or its reduction along with the production of H2. In addition, the cladding is optimized such that it is possible to reduce the minimum potential on the negative electrode while increasing the maximum potential on the positive electrode, thus extending the maximum voltage range within which the cell can operate. This advantageously permits the delivery or absorption of a higher current. Thus, it is possible to increase the service life of cell B, thereby providing more efficient peak shaving, load transfer and market arbitrage (i.e., storing the large amount of electrical energy when it can be obtained at a low cost in order to resell the large amount of electrical energy at a higher price later).

[0111] Of course, the examples described so far are subject to many variations.

[0112] Some of the possible variations of the present invention have been described above, but it will be obvious to those skilled in the art that other embodiments can also be implemented in practice, in which several elements can be replaced by other technically equivalent elements. Therefore, the present invention is not limited to the illustrative examples described above, but can be subject to various modifications, improvements, equivalent parts and element replacements without departing from the basic inventive concept as specified in the appended claims.

Claims

1. An electrode (1) for a flow battery (B) comprising a first portion (12) capable of being placed in contact with an electrolyte solution of said battery (B), wherein said first portion (12) is mesoporous and composed of particles (11) of conductive material having a nanoscale size. Its features are, The electrode further includes: The second part (13) supports the first part (12) and comprises a plurality of carbon filaments, wherein the first part (12) is at least partially coated with a layer of particles (11) having a thickness (S) varying along the depth (D) and / or length (L) of the electrode (1), and / or The first portion (12) has a porosity that varies along the depth (D) and / or length (L) of the electrode (1).

2. The electrode (1) according to claim 1, characterized in that, The first part (12) is formed of carbon particles (11), wherein the particles (11) have a concentric layered structure.

3. The electrode (1) according to claim 2, characterized in that, The particle (11) has a size in the range of 1 nanometer to 100 nanometers.

4. The electrode (1) according to claim 3, characterized in that, The particle (11) has a size in the range of 3 nanometers to 7 nanometers.

5. The electrode (1) according to any one of claims 1 to 4, characterized in that, The first part (12) includes pores having an average size in the range of 2 nanometers to 50 nanometers.

6. The electrode (1) according to any one of claims 1 to 5, characterized in that, The first portion (12) has a thickness in the range of 0.5 μm to 10 μm.

7. The electrode (1) according to any one of claims 1 to 6, characterized in that, The first portion (12) of the electrode (1) has at least 500m 2 Specific surface area per g and at least 20 μm -1 The area per unit volume.

8. A flow battery (B) comprising at least one electrode (1) according to any one of claims 1 to 7.

9. The battery (B) according to claim 8, characterized in that, The electrode (1) is configured as the negative electrode of the battery (B).

10. The battery (B) according to claim 8 or 9, comprising a serpentine flow distributor (21) coupled to the electrode (1), wherein the first portion (12) of the electrode (1) has a thickness in the range of 0.5 μm to 5 μm.

11. The battery (B) according to claim 8 or 9, comprising an interdigitated flow distributor (22) coupled to the electrode (1), wherein the first portion (12) of the electrode (1) has a thickness in the range of 2 μm to 10 μm.

12. A method for manufacturing an electrode (1) according to any one of claims 1 to 7, comprising: - Synthesis stage, in which multiple particles of conductive material with nanoscale dimensions are synthesized. - Deposition stage, wherein the plurality of particles are used to form the first part (12) of the electrode (1).

13. The method according to claim 12, characterized in that, During the synthesis phase, the plurality of particles are synthesized by causing a gas mixture to flow through a confined space in which an electromagnetic field exists, and the gas mixture includes at least one carbon-containing gas.

14. The method according to claim 12 or 13, characterized in that, During the deposition stage, the flow of particles is caused to impact the first portion (12) of the electrode (1) at a velocity determined based on a roughness factor describing the roughness of the first portion (12) of the electrode (1).

15. The method according to claim 12 or 14, characterized in that, During the deposition phase, the first portion (12) is deposited on a second portion (13) comprising a plurality of carbon filaments, such that the first portion (12) forms a layer on at least one of the carbon filaments, and wherein the second portion (13) rotates and / or translates at a speed determined based on the desired thickness of the layer.

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