Encapsulated transition metal oxide nanorods for durable air cathodes

Abstract

The present invention relates to a material comprising a plurality of nanorods encapsulated within an open hollow carbon nanostructure, wherein the plurality of nanorods consists of a) a transition metal oxide of formula AzM '2yMn1-xO2 (A), or optionally, b) a transition metal oxide of formula M' 3 m / nM2-mO3 (B), as defined herein, where the amount of the transition metal oxide of formula (A) or formula (B) is from 20 wt% to 60 wt% with respect to the total material weight; the volume of the nanorods encapsulated within the hollow carbon nanostructure is equal to or less than 50% relative to the total cavity volume of the hollow carbon nanostructure, in particular wherein the hollow carbon nanostructure is tubular, the average inner diameter of which is at least twice the average thickness of the nanorods. The invention also relates to a method for preparing the material, to a precursor material RtM ''3-tO4 (C) for obtaining the material as defined herein, and to the use of the material as an electrocatalyst in various applications.

Description

[0001] This application claims priority to European patent application EP23382707.0, filed on July 11, 2023. Technical Field

[0002] This disclosure relates to the field of nanomaterials. In particular, the present invention relates to a hybrid material comprising transition metal oxide nanorods (e.g., manganese oxide nanorods) encapsulated within an open hollow carbon nanostructure. This disclosure also relates to methods for preparing the hybrid material and its use as an electrocatalyst in the fields of energy storage and energy conversion. Background Technology

[0003] Rechargeable zinc-air batteries offer a promising alternative for future sustainable energy storage due to their high theoretical energy density, use of abundant and environmentally friendly materials, and safety. However, the economic potential and efficiency of these batteries are currently far from satisfactory. Limitations on achieving high reversibility and long lifespan are related to the zinc anode and the bifunctional air electrode (cathode). Several methods exist to significantly improve the reversibility of the zinc anode. However, the lack of methods to address the limitations of the cathode severely hinders the practical application and commercialization of rechargeable zinc-air batteries.

[0004] Rapid degradation of cathode materials (e.g., loss of catalyst active surface area) leads to a rapid decrease in (storage) capacity. On the other hand, the efficiency and power density of rechargeable zinc-air batteries are severely limited by the slow kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) occurring on the air electrode. Although ORR is thermodynamically favorable, both ORR and OER are kinetically hindered. Therefore, materials that are not only durable but also catalytically active for both reactions (bifunctional) are needed.

[0005] Furthermore, the poor accessibility of reactants (gas molecules and ions) to catalytically active sites further reduces the activity of electrocatalyst materials. Currently, there is a lack of methods for large-scale, low-cost production of highly durable catalysts that can precisely control porosity while ensuring efficient mass transport and electron transfer pathways, which hinders the large-scale application of zinc-air batteries.

[0006] Materials based on platinum, palladium, or other noble metals and their oxides (e.g., iridium oxide) have been successfully used as electrocatalysts for ORR or OER. However, their limited availability and high cost have prompted researchers to search for alternative electrocatalysts. In this context, transition metal oxides, particularly manganese and iron oxides, have attracted considerable attention due to their high specific capacitance, abundance, low cost, versatility, and non-toxicity. Unfortunately, the applicability of these transition metal oxides is limited by their dissolution during electrochemical processes occurring on electrode surfaces. As mentioned above, this electrode degradation is a major technical obstacle in energy storage and conversion applications, as the loss of active material means a loss of activity, and therefore a reduction in the efficiency of the processes involved.

[0007] Therefore, efforts have been made to develop nanomaterials based on transition metal oxides to maximize electrocatalytic activity while improving conductivity and maintaining accessible surface area. In the field of nanomaterial development, a close relationship has been demonstrated between the structure (morphology) of a material and its properties. This is because certain crystal facets are exposed more in some morphologies than in others. Despite these advances, the considerable effort to synthesize these active materials is often overshadowed by their still relatively low stability and durability, resulting in the loss of much of their initial catalytic activity upon use. One strategy used to reduce the dissolution of metal oxides is to provide coatings on the oxide surface. However, the increased stability of these coated materials usually comes at the cost of their activity; furthermore, the synthetic processes for preparing these coated materials are complex.

[0008] Therefore, there is still a need to develop active and stable materials that can be effectively used as electrocatalysts on an industrial scale, for example in zinc-air batteries. Summary of the Invention

[0009] The inventors have developed a novel hybrid material comprising transition metal oxide nanorods, specifically manganese (IV), cobalt (III), nickel (III), and iron (III), encapsulated within an open-ended hollow carbon nanostructure. The hybrid oxide material may optionally be doped with other metals, also referred to herein as rA. z M' 2y Mn 1-x O2@CN and r-M'' 3m / n M 2-m O3@CN, where A is a monovalent metal, M, M' and M'' are transition metals as defined in this paper, and CN is an open carbon nanostructure.

[0010] These materials can be used as electrode materials in electrochemical technologies for energy conversion and storage, especially in metal-air batteries (such as zinc-air batteries). They can act as electrocatalysts in bifunctional air electrodes to improve durability and reversibility during charge and discharge processes.

[0011] The inventors discovered that these transition metal oxide particles in the form of nanorods (especially manganese oxide (IV), cobalt oxide (III), nickel oxide (III) and iron oxide (III) particles and their doping forms) create a very stable structure. On the one hand, they have a low tendency to aggregate, which can maintain a constant active surface area. On the other hand, they have low solubility, which keeps the crystal face active.

[0012] One-dimensional morphology (i.e., nanorods) has a large contact area with the substrate, which promotes its electrocatalytic ability, such as in the water oxidation process that occurs in zinc-air batteries. Figure 16 Compared to other forms, it is better able to withstand structural changes associated with ion insertion or extraction (which means improved cycling performance). These two properties translate into improvements in electrochemical parameters, including improvements in reversibility and the ability to withstand high charge and discharge currents.

[0013] Furthermore, encapsulating transition metal oxide nanorods (especially manganese oxide (IV), cobalt oxide (III), nickel oxide (III), and iron oxide (III) nanorods and their doped forms) within open hollow carbon nanostructures that act as nanocontainers can improve the inherent low conductivity of metal oxides because strong interactions are generated between the nanoparticles and the carbon surface.

[0014] Unbound by theory, this is because the nanorods are encapsulated in a limited space, restricting the Mn... 2+ Co 2+ Ni 2+ or Fe 2+ Diffusion into the electrolyte, when a positive potential is reached, is favored for recapture as an oxide (e.g., MnO2 when the transition metal is manganese). Therefore, since the transition metal oxide nanorods are encapsulated within an open hollow carbon nanostructure, the loss of active material at the electrode due to dissolution, a direct cause of the low stability of metal oxides, is minimized. This is illustrated in the examples of the precursor s-Mn3O4@CNF, see [link to example]. Figure 12 ab. In contrast, similar materials with nanoparticles (s-Mn3O4@CNF) deposited on the exterior of nanofibers were found to have poor stability. Figure 13 ).

[0015] Unlike coating materials, where increased stability typically comes at the expense of activity, the hybrid materials of this invention do not inhibit dissolution but only reduce Mn. 2+ Co 2+ Ni 2+ or Fe 2+ Diffusion of cations into the electrolyte. In this invention, as shown in the examples, encapsulation does not inhibit the inherent reactivity of the nanoparticles. Figure 14 This means that the dissolution process continues.

[0016] The materials of the present invention can be conveniently obtained from precursor materials by electrosynthesis, the precursor materials comprising spherical nanoparticles composed of transition metal (II,III) oxides (particularly manganese (II,III) oxides), the spherical nanoparticles being encapsulated in open hollow carbon nanostructures, also referred to herein as sR. t M''' 3-t O4@CN, where R and M''' are transition metals as defined herein; CN denotes an open carbon nanostructure.

[0017] Electrochemical (or electrosynthetic) processes have proven to be widely applicable, replacing hydrothermal processes that require large amounts of solvents, high temperatures, or high pressures. The inventors have discovered that by controlling electrochemical parameters, such as scanning potential, current density, or electrolyte composition, desired nanorod-like material morphologies can be obtained. Figure 2 ).

[0018] Conversely, precursor materials (SR) t M''' 3-t The synthesis of O4@CN is very simple and rapid, requiring only one step and typically taking about 1-3 hours. These characteristics facilitate the implementation of this method on an industrial scale. Furthermore, the method is cost-effective because it can use only inexpensive metals such as manganese or iron salts, and the reaction temperature is relatively low (typically around 90°C). This allows the use of low-melting-point organic solvents (such as xylene) instead of other common solvents with higher melting points used in the synthesis of metal oxide nanoparticles, such as diphenyl ether, 1-octadecene, or trioctadecene (i.e., 260°C, 317°C, and 365°C, respectively).

[0019] Furthermore, the preparation method of the material of the present invention has wide applications because it allows other transition metals to be doped into the crystal structure of first transition metal oxides (especially manganese oxide, cobalt oxide, nickel oxide, and iron oxide), typically with low doping levels, replacing Mn or M atoms with other cations to obtain doped materials. As shown in the following examples, doped materials can be obtained by simply replacing the initial manganese salt used to prepare the precursor for encapsulating spherical nanoparticles with other transition metal salts (e.g., cobalt salts, nickel salts, or iron salts). Figure 3-10 The applicability difference depends on its responsiveness. Figure 11 Doped nanorods formed inside nanofibers after electrosynthesis ( ). Figure 18-20 ), thus possessing highly stable and active materials ( Figure 17 This allows for the generation of a family of one-dimensional polymetallic nanostructures within carbon nanofibers, which are applicable in various fields.

[0020] Therefore, a first aspect of the present invention relates to a material comprising a plurality of nanorods encapsulated within an open hollow carbon nanostructure, wherein the plurality of nanorods are composed of any of the following: a) Transition metal oxides of formula (A) A z M' 2y Mn 1-x O2 (A) Alternatively, b) Transition metal oxides of formula (B) M” 3m / n M 2-m O3 (B) in: In formula (A), A is a monovalent metal, and M' is a divalent transition metal other than Mn; x equals (z / 4+y), 0≤y<1, 0≤z<4; manganese can be in the oxidation state of (IV) or a mixture of (IV) and (III), the latter case has oxygen vacancies; In formula (B), M is a transition metal selected from iron, cobalt and nickel in the trivalent oxidation state; M'' is a transition metal with the same or different oxidation state n as M, where the value of n is 2 to 6, and 0 ≤ m < 2; The amount of transition metal oxide of formula (A) or formula (B) is 20% to 60% by weight relative to the total material weight; and The volume of the nanorods encapsulated within the hollow tubular carbon nanostructure is equal to or less than 50% of the total cavity volume of the hollow carbon nanostructure. Specifically, the hollow carbon nanostructure is tubular, and the average inner diameter of the hollow tubular carbon nanostructure is at least twice the average thickness of the nanorods.

[0021] A second aspect of the invention relates to an electrochemical method for preparing a material comprising a plurality of nanorods composed of transition metal oxides as defined herein, the method comprising the following steps: i) A modified working electrode is provided by coating a precursor material onto a conductive surface, the precursor material comprising a plurality of spherical nanoparticles composed of transition metal (II,III) oxides of formula (C). R t M''' 3-t O4 (C) The spherical nanoparticles are encapsulated within an open, hollow tubular carbon nanostructure, wherein: M''' is a transition metal selected from the group consisting of manganese, iron, cobalt, and nickel in divalent and trivalent oxidation states; R is a transition metal in a divalent oxidation state other than M'''; t is a value from 0 to less than 1; The amount of transition metal oxide of formula (C) is 20% to 60% by weight relative to the total material weight; and The volume of the spherical nanoparticles encapsulated within the hollow tubular carbon nanostructure is equal to or less than 50% of the total cavity volume of the hollow tubular carbon nanostructure; in particular, the hollow carbon nanostructure is tubular, and the average inner diameter of the hollow tubular carbon nanostructure is at least twice the average thickness of the nanorod. ii) Electrosynthesizing an aqueous solution containing an electrolyte using a counter electrode, a reference electrode, and the modified working electrode obtained in step i), said electrolyte comprising a halide salt selected from alkali metals or alkaline earth metals, or hydroxides of alkali metals or alkaline earth metals: (iia) Applying 5000 or fewer constant current cycles in the range of -0.2V to 0.8V at a current density equal to or greater than 0.5A per gram of precursor material, or alternatively, iib) applies 5000 or fewer potentiodynamic cycles in the range of 1V to 1.7V at a scan rate equal to or greater than 40mV / s.

[0022] In the above method, when M''' in the precursor material of formula (C) is manganese, a material containing multiple nanorods encapsulated within an open hollow carbon nanostructure is obtained, wherein the multiple nanorods are composed of transition metal oxides of formula (A); when M''' in the precursor material of formula (C) is a transition metal selected from the group consisting of iron, cobalt and nickel, a material containing multiple nanorods encapsulated within an open hollow carbon nanostructure is obtained, wherein the multiple nanorods are composed of transition metal oxides of formula (B).

[0023] A third aspect of the present invention relates to a precursor material comprising a plurality of spherical nanoparticles composed of transition metal (II,III) oxides of formula (C). R t M''' 3-t O4 (C) The spherical nanoparticles are encapsulated within an open, hollow tubular carbon nanostructure, wherein: M''' is a transition metal selected from the group consisting of manganese, iron, cobalt, and nickel in divalent and trivalent oxidation states; R is a transition metal in a divalent oxidation state other than M'''; The value of t is 0 to less than 1; The amount of transition metal oxide of formula (C) is 20% to 60% by weight relative to the total material weight; and The volume of the spherical nanoparticles encapsulated within the hollow tubular carbon nanostructure is equal to or less than 50% of the total cavity volume of the hollow tubular carbon nanostructure. Specifically, the hollow carbon nanostructure is tubular, and its average inner diameter is at least twice the average diameter of the spherical nanoparticles.

[0024] A fourth aspect of the invention relates to a method for preparing a precursor material as defined herein, the method comprising the steps of: a) In the presence of a surfactant in a suitable solvent, a mixture comprising a transition metal (II) salt and optionally a second salt, the second salt being a salt of a transition metal having a divalent oxidation state other than a first transition metal, wherein the metal of the first transition metal (II) salt is selected from manganese, iron and cobalt, and the metal of the second salt is different from the metal of the first salt; b) Add the mixture from step a) to a suspension of open hollow tubular carbon nanostructures in a suitable solvent; c) Heating the suspension to a temperature of 80°C to 110°C; and d) Add water to the suspension from step c) and heat to a temperature of 80°C to 110°C to obtain the precursor material of formula (C); Wherein, when step a) is carried out in the absence of another transition metal salt, the material of formula (C) is obtained, where t is 0; when step a) is carried out in the presence of a second salt of a transition metal, the precursor material of formula (C) is obtained, where t is not 0, and R is a transition metal in the divalent oxidation state other than M'''.

[0025] The fifth aspect of the invention relates to the use of materials comprising a plurality of nanorods as defined above as electrocatalysts. Attached Figure Description

[0026] Figure 1 The bright-field transmission electron microscopy (TEM) image (a), dark-field scanning mode (STEM) image (b), and energy-dispersive X-ray (EDX) elemental mapping images of oxygen (c), carbon (d), and manganese (e) are shown. Scale bars: (a) 100 nm, (be) 20 nm.

[0027] Figure 2 Bright-field TEM image (a), dark-field STEM image (b), and EDX elemental mapping images and EDX spectra (f) of manganese oxide nanorods (r-MnO2@CNF) within carbon nanofibers are shown (C: count, E: energy, the presence of Cu is due to the TEM grid made using Cu). The scale bar for all images is 100 nm. Inset in (a): Example of a rod with crystal planes (interplanar spacing (d=0.34 nm)) associated with the MnO2 crystal structure (pyrolusite type).

[0028] Figure 3 Raman spectra of (a) undoped manganese oxide nanoparticles (s-Mn3O4@CNF) in carbon nanofibers; (b) cobalt-doped manganese oxide nanoparticles (s-Co / Mn3O4@CNF) in carbon nanofibers; (c) nickel-doped manganese oxide nanoparticles (s-Ni / Mn3O4@CNF) in carbon nanofibers; (d) iron-doped manganese oxide nanoparticles (s-Fe / Mn3O4@CNF) in carbon nanofibers; (e) undoped cobalt oxide nanoparticles (s-Co3O4@CNF) in carbon nanofibers; and (f) undoped iron oxide nanoparticles (s-Fe3O4@CNF) in carbon nanofibers are shown. C: count, Rs: Raman shift.

[0029] Figure 4X-ray powder diffraction patterns are shown for (a) undoped manganese oxide nanoparticles (s-Mn3O4@CNF) in carbon nanofibers; (b) cobalt-doped manganese oxide nanoparticles (s-Co / Mn3O4@CNF) in carbon nanofibers; (c) nickel-doped manganese oxide nanoparticles (s-Ni / Mn3O4@CNF) in carbon nanofibers; (d) iron-doped manganese oxide nanoparticles (s-Fe / Mn3O4@CNF) in carbon nanofibers; (e) undoped cobalt oxide nanoparticles (s-Co3O4@CNF) in carbon nanofibers; and (f) undoped iron oxide nanoparticles (s-Fe3O4@CNF) in carbon nanofibers. I: Intensity.

[0030] Figure 5 Thermogravimetric analysis (TGA) measurements of (a) undoped manganese oxide nanoparticles (s-Mn3O4@CNF) in carbon nanofibers; (b) cobalt-doped manganese oxide nanoparticles (s-Co / Mn3O4@CNF) in carbon nanofibers; (c) nickel-doped manganese oxide nanoparticles (s-Ni / Mn3O4@CNF) in carbon nanofibers; (d) iron-doped manganese oxide nanoparticles (s-Fe / Mn3O4@CNF) in carbon nanofibers; (e) undoped cobalt oxide nanoparticles (s-Co3O4@CNF) in carbon nanofibers; and (f) undoped iron oxide nanoparticles (s-Fe3O4@CNF) in carbon nanofibers are shown. W: weight, T: temperature, L: percentage of metal oxide remaining after annealing at 1000 °C.

[0031] Figure 6 The particle size distribution histograms of (a) undoped manganese oxide nanoparticles (s-Mn3O4@CNF) in carbon nanofibers; (b) cobalt-doped manganese oxide nanoparticles (s-Co / Mn3O4@CNF) in carbon nanofibers; (c) nickel-doped manganese oxide nanoparticles (s-Ni / Mn3O4@CNF) in carbon nanofibers; (d) iron-doped manganese oxide nanoparticles (s-Fe / Mn3O4@CNF) in carbon nanofibers; (e) undoped cobalt oxide nanoparticles (s-Co3O4@CNF) in carbon nanofibers; and (f) undoped iron oxide nanoparticles (s-Fe3O4@CNF) in carbon nanofibers are shown. C: count, D: nanoparticle diameter.

[0032] Figure 7 Bright-field TEM image (a) of cobalt-doped manganese oxide nanoparticles (s-Co / Mn3O4@CNF) in carbon nanofibers is shown; dark-field EDX-STEM elemental mapping images and EDX spectra (f) of manganese (b), carbon (c), oxygen (d), and cobalt (e) are shown (C: count, E: energy, the presence of Cu is due to the TEM grid made using Cu). The scale bar for all images is 100 nm.

[0033] Figure 8 Bright-field TEM image (a) of nickel-doped manganese oxide nanoparticles (s-Ni / Mn3O4@CNF) in carbon nanofibers; dark-field EDX-STEM elemental mapping images and EDX spectra (f) of manganese (b), carbon (c), oxygen (d), and nickel (e) are shown (C: count, E: energy, the presence of Cu is due to the TEM grid made using Cu). The scale bar for all images is 100 nm.

[0034] Figure 9 Bright-field TEM image (a) of iron-doped manganese oxide nanoparticles (s-Fe / Mn3O4@CNF) in carbon nanofibers; dark-field EDX-STEM elemental mapping images and EDX spectra (f) of manganese (b), carbon (c), oxygen (d), and iron (e) are shown (C: count, E: energy; the presence of Cu is due to the TEM grid made using Cu). The scale bar for all images is 50 nm.

[0035] Figure 10 Cyclic voltammetry curves of manganese oxide nanoparticles in carbon nanofibers, measured at a scan rate of 50 mV / s, are shown in nitrogen-saturated 1 M KOH solution, where (a) is undoped (s-Mn3O4@CNF) and doped (b) cobalt (s-Co / Mn3O4@CNF), (c) nickel (s-Ni / Mn3O4@CNF) and (d) iron (s-Fe / Mn3O4@CNF). C: current density, P: potential.

[0036] Figure 11 Linear sweep voltammetry curves of oxygen evolution reaction (OER, 1 V to 1.8 V) and oxygen reduction reaction (ORR, 1 V to 0.3 V), measured at a scan rate of 10 mV / s, are shown for undoped manganese oxide nanoparticles (s-Mn3O4@CNF) and manganese oxide nanoparticles doped with cobalt (s-Co / Mn3O4@CNF), nickel (s-Ni / Mn3O4@CNF), and iron (s-Fe / Mn3O4@CNF) in oxygen-saturated 1 M KOH solution. C: current density, P: potential (RHE: reversible hydrogen electrode).

[0037] Figure 12 Bright-field TEM images of unwashed carbon nanofibers (s-Mn3O4@CNF) containing manganese oxide nanoparticles outside a nanocontainer, in a nitrogen-saturated 2M KCl solution, are shown in (a) before cycling and (b) after 500 cycles (at a scan rate of 50 mV / s between -0.2 V and 0.8 V). Scale bar is 100 nm.

[0038] Figure 13Cyclic voltammetry (n = number of cycles) is shown for unwashed carbon nanofibers (s-Mn3O4@CNF) containing manganese oxide nanoparticles on a nanocontainer in a nitrogen-saturated 2M KCl solution. C: current density, P: potential.

[0039] Figure 14 The analysis of bright-field (a) and dark-field (b) TEM images of manganese oxide nanoparticles (s-Mn3O4@CNF) in carbon nanofibers after several charge-discharge cycles (1500 cycles) at 1 A / g in a nitrogen-saturated 2M KCl solution, and dark-field EDX-STEM elemental mapping images and EDX spectra (f) of carbon (c), oxygen (d), and manganese (e) are shown (C: count, E: energy; the presence of Cu is due to the TEM grid made using Cu). The scale bar for all images is 50 nm.

[0040] Figure 15 The following figures illustrate the effects of constant current charge-discharge cycling at 1 A / g on (a) manganese oxide nanoparticles in carbon nanofibers before (s-Mn3O4@CNF, solid line) and after (r-MnO2@CNF, dashed line) electrosynthesis in a nitrogen-saturated 2M KCl solution, comprising 2500 cycles. The black solid line represents carbon nanofibers without manganese oxide nanoparticles. (b) Capacitance retention (%) varies with the number of cycles. The significant increase in capacitance retention (more than 4-fold) is associated with the formation of nanorods within the carbon nanofibers. P: potential, T: time, CR: capacitance retention, C: number of cycles.

[0041] Figure 16 The following figures show (a) linear sweep voltammetry and (b) Tafel curves of manganese oxide nanoparticles on carbon nanofibers before (s-Mn3O4@CNF, solid line) and after (r-MnO2@CNF, dashed line) electrosynthesis (2500 cycles in 2M KCl) in oxygen-saturated 1M KOH solution. C: current density, P: potential.

[0042] Figure 17 Linear sweep voltammetric curves of manganese oxide nanoparticles on carbon nanofibers doped with (a) cobalt, (b) nickel, and (c) iron are shown before (solid line) and after (dashed line) electrosynthesis (2500 cycles in 2M KCl or 1M KOH) in an oxygen-saturated 1M KOH solution (2500 cycles). C: current density, P: potential.

[0043] Figure 18Bright-field TEM image (a) of cobalt-doped manganese oxide nanorods (r-KCo / MnO2@CNF) in carbon nanofibers is shown; dark-field EDX-STEM elemental mapping images and EDX spectra (f) of manganese (b), carbon (c), oxygen (d), and cobalt (e) are shown (C: count, E: energy, the presence of Cu is due to the TEM grid made using Cu). Inset in (a): Having the same characteristics as Formula A z M' 2y Mn 1-x An example of a bar-shaped MnO2 crystal structure (hollandite-type, illustration) with respect to the crystal planes (interplanar spacing (d=0.49nm)). All images are at 50nm scale.

[0044] Figure 19 Bright-field TEM image (a) of nickel-doped manganese oxide nanorods (r-KNi / MnO2@CNF) in carbon nanofibers is shown; dark-field EDX-STEM elemental mapping images and EDX spectra (f) of manganese (b), carbon (c), oxygen (d), and nickel (e) are shown (C: count, E: energy, the presence of Cu is due to the TEM grating made using Cu). Inset in (a): Having the same structure as formula M' 2y Mn 1-x An embodiment of a rod with an associated crystal plane (interplanar spacing d = 0.24 nm) of the MnO2 crystal structure of O2 (pyrolusite type, illustration). All images are scaled at 50 nm.

[0045] Figure 20 Bright-field TEM image (a) of iron-doped manganese oxide nanorods (r-KFe / MnO2@CNF) in carbon nanofibers is shown; dark-field EDX-STEM elemental mapping images and EDX spectra (f) of manganese (b), carbon (c), oxygen (d), and iron (e) are shown (C: count, E: energy, the presence of Cu is due to the TEM grid made using Cu). Inset in (a): [Image showing a manganese oxide nanorod with the formula AzM'] 2y Mn 1-x An example of the interplanar spacing (d = 0.49 nm) associated with the O2-intercalated potassium-ion MnO2 crystal structure (barium pyrolusite type, illustration). All images are scaled at 50 nm.

[0046] Figure 21 The changes in the amount of cobalt-doped Mn3O4 nanoparticles (s-Co / Mn3O4@CNF) in carbon nanofibers are shown when 5% mmol, 10% mmol, 15% mmol and 20% mmol of manganese(II) acetate are replaced by cobalt(II) acetate.

[0047] Figure 22Bright-field (a) and dark-field (b) TEM images of iron oxide nanorods (r-Fe2O3@CNF) in carbon nanofibers are shown, along with dark-field EDX-STEM elemental mapping images and EDX spectra (f) of carbon (c), oxygen (d), and iron (e) (C: count, E: energy; the presence of Cu is due to the TEM grid made using Cu). The scale bar for all images is 50 nm. Detailed Implementation

[0048] Unless otherwise stated, all terms used in this application shall be understood in their ordinary sense in the art. Other more specific definitions of terms used in this application are set forth below, and unless otherwise expressly provided to provide a broader definition, these definitions are intended to be consistent throughout the specification and claims.

[0049] As used in this article, the terms “about” or “around” refer to a numerical range of ±10% of a specific value. For example, the expression “about 10” or “around 10” includes ±10% of 10, that is, 9 to 11.

[0050] For the purposes of this invention, any given range includes both the lower and upper limits of that range. Unless otherwise expressly stated, given ranges such as weight, temperature, and time should be considered approximate values.

[0051] As used in this paper, the term "material" refers to a hybrid material composed of at least two different materials: nanoparticles of transition metal oxides of formula (A), (B) or (C), and open hollow carbon nanostructures that encapsulate them.

[0052] The term "precursor material" refers to spherical nanoparticles (SRs) composed of transition metal (II,III) oxides (especially manganese oxide (II,III), iron oxide (II,III), or cobalt oxide (II,III)) encapsulated within open hollow carbon nanostructures. t M''' 3-t O4@CN). The abbreviation CN used in this article refers to open hollow carbon nanostructures.

[0053] Unless otherwise stated, the term “nanoparticle” as used herein may generally refer to nanorods and nanospheres as defined herein.

[0054] The term “multiple” as used in this article refers to a group of nanoparticles, that is, a collection of two or more nanoparticles.

[0055] The terms “encapsulation” and “confinement” are used interchangeably, referring to nanoparticles located within an open hollow carbon nanostructure; therefore, the nanoparticles are not located on the outer surface of the hollow carbon nanostructure.

[0056] The term "room temperature" refers to the ambient temperature without heating or cooling, typically between 20 and 25°C.

[0057] The term "obtained by means of" is used herein to define the material or precursor material of the present invention by means of its preparation method, and refers to a product that can be obtained by the preparation method disclosed herein. For the purposes of this invention, the terms "available," "obtained," and similar equivalent expressions are used interchangeably, and in any case, the term "available" includes "obtained."

[0058] As described above, a first aspect of the present invention relates to a material comprising a plurality of nanorods encapsulated within an open hollow carbon nanostructure, wherein the plurality of nanorods are composed of any of the following: a) Transition metal oxides of formula (A) A z M' 2y Mn 1-x O2 (A) Alternatively, b) Transition metal oxides of formula (B) M” 3m / n M 2-m O3 (B) in: In formula (A), A is a monovalent metal, and M' is a divalent oxidation state of a transition metal other than Mn; x equals (z / 4+y), where 0≤y<1 and 0≤z<4; manganese can be in oxidation state (IV) or a mixture of (IV) and (III), the latter having oxygen vacancies; In formula (B), M is a transition metal selected from iron, cobalt and nickel in the trivalent oxidation state; M'' is a transition metal with the same or different oxidation state n as M, where the value of n is 2 to 6, and 0 ≤ m < 2; The amount of transition metal oxide of formula (A) or formula (B) is 20% to 60% by weight relative to the total material weight; and The volume of the nanorods encapsulated within the hollow tubular carbon nanostructure is equal to or less than 50% of the total cavity volume of the hollow carbon nanostructure. Specifically, the hollow carbon nanostructure is tubular, and the average inner diameter of the hollow tubular carbon nanostructure is at least twice the average thickness of the nanorods.

[0059] In one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the present invention relates to a material comprising a plurality of nanorods encapsulated within an open hollow carbon nanostructure, wherein the plurality of nanorods are derived from formula A. z M' 2y Mn1-x O2(A) is composed of transition metal (IV) oxides.

[0060] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the present invention relates to a material comprising a plurality of nanorods encapsulated within an open hollow carbon nanostructure, wherein the plurality of nanorods are of formula M'' 3m / n M 2-m O3(B) is composed of a transition metal (III) oxide. In another more specific embodiment, M in the transition metal (III) oxide of formula (B) is iron. In another more specific embodiment, M in the transition metal (III) oxide of formula (B) is cobalt. In another more specific embodiment, M in the transition metal (III) oxide of formula (B) is nickel.

[0061] According to another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the content of transition metal oxides in the material is 20% to 60%, more particularly 25% to 55%, and even more particularly 30% to 40% relative to the total material weight. The amount of transition metal oxides of formula (A) or formula (B) can be determined by thermogravimetric analysis (TGA).

[0062] The transition metal oxides of formula (A), (B), or (C) can be undoped or doped. For the purposes of this invention, the term "undoped" means that the transition metal oxide does not contain other metals as dopants. The term "doped" as used herein means that the metal oxide contains impurities (dopants) from different metals and ultimately exhibits modified properties.

[0063] Non-limiting examples of transition metals that can be used in this invention include manganese, iron, cobalt, nickel, copper, chromium, molybdenum, titanium, vanadium, zinc, etc.

[0064] Non-limiting examples of alkali metals that can be used in this invention include sodium, potassium, cesium, etc.

[0065] According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the transition metal oxide of formula (A) is undoped, i.e., y and z are 0. In this case, the transition metal oxide of formula (A) has the formula MnO2.

[0066] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the transition metal oxide is doped, i.e., in the transition metal (IV) oxide of formula (A), at least one of y or z is not 0, and therefore x is not 0. More specifically, A is selected from the group consisting of sodium, potassium, and cesium, and M' is selected from the group consisting of manganese, iron, cobalt, nickel, copper, chromium, molybdenum, titanium, vanadium, and zinc. Even more particularly, A is potassium, and M' is selected from the group consisting of iron, cobalt, and nickel. In a more specific embodiment, y is 0 and z is not 0. In another more specific embodiment, y is not 0, and z is 0. In yet another more specific embodiment, y and z are each independently not 0.

[0067] When z is 0, a pyrolusite-type structure is obtained (for example, see...). Figure 1 and Figure 19 When z is not 0, hollandite-type structures are obtained (see, for example, see...). Figure 18 and Figure 20 ).

[0068] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the transition metal oxide is doped, i.e., in the transition metal (IV) oxide of formula (A), the value of x is greater than 0 and less than 1. In a more specific embodiment, x is from 0.01 to 0.4, or even more specifically, x is from 0.05 to 0.09. Even more specifically, x is about 0.01, about 0.02, about 0.03, about 0.05, about 0.07, about 0.09, about 0.1, about 0.15, about 0.20, about 0.25, about 0.3, about 0.35, or about 0.4.

[0069] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the present invention relates to a material comprising a plurality of nanorods encapsulated within an open hollow carbon nanostructure, wherein the plurality of nanorods are composed of a mixture of one or more transition metal oxides of formula (A). More specifically, the plurality of nanorods are composed of a mixture of metal oxides of formula (A1). M' 2y Mn 1-x’ O2 (A1), Metal oxides of formula (A2), A z’ M' 2y Mn 1-x’’ O2 (A2) In this context, M' represents a divalent oxidation state of a transition metal other than Mn; x' equals y and x'' = (z' / 4 + y), where 0 ≤ y < 1 and 0 < z' < 4; A represents a monovalent metal; manganese can be in oxidation state (IV) or a mixture of (IV) and (III), the latter case having oxygen vacancies. When z is 0, the metal oxide of formula (A1) corresponds to the metal oxide of formula (A); when z is not 0, the metal oxide of formula (A2) corresponds to the metal oxide of formula (A).

[0070] In a more specific embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the weight ratio of the metal oxide of formula (A1) to the metal oxide of formula (A2) is 100:0 to 50:50, more particularly 80:20 to 60:40, when y=0.

[0071] In another, more specific embodiment, when y=0, the weight ratio of the metal oxide of formula (A1) to the metal oxide of formula (A2) is about 100:0, about 90:10, about 85:15, about 80:20, about 75:25, about 70:30, about 65:35, about 60:40, about 55:45, or about 50:50.

[0072] In a more specific embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the weight ratio of the metal oxide of formula (A1) to the metal oxide of formula (A2) is 10:90 to 50:50, more particularly 20:80 to 40:60, when y is not 0. In another more specific embodiment, when y is not 0, the weight ratio of the metal oxide of formula (A1) to the metal oxide of formula (A2) is about 10:90, about 15:85, about 20:80, about 25:75, about 30:70, about 33:67, about 35:65, about 40:60, about 45:55, or about 50:50.

[0073] According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the transition metal (III) oxide of formula (B) is undoped, i.e., m is 0.

[0074] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the transition metal (III) oxide is doped, i.e., in the transition metal (III) oxide of formula (B), the value of m is greater than 0 and less than 2, and M'' is a transition metal with an oxidation state of 2 to 6 that is the same as or different from M. More particularly, M'' is a transition metal selected from the group consisting of iron, cobalt, nickel, manganese, chromium, molybdenum, titanium, vanadium, and mixtures thereof; even more particularly, M'' is selected from the group consisting of chromium and molybdenum. In a more specific embodiment, m is 0.01 to 0.4, even more particularly, m is 0.05 to 0.09. Even more particularly, m is about 0.01, about 0.02, about 0.03, about 0.05, about 0.07, about 0.09, about 0.1, about 0.15, about 0.20, about 0.25, about 0.3, about 0.35, or about 0.4.

[0075] In another embodiment, M'' may be a transition metal other than M, in the transition metal oxide of formula (B), optionally combined with one or more features of the various embodiments described above or below throughout the specification.

[0076] In another embodiment, M'' may be combined with one or more features of the various embodiments described above or below throughout the specification, wherein in the transition metal oxide of formula (B), M'' is the same transition metal as M.

[0077] In another embodiment, the value of n in the transition metal oxide of formula (B) is 2 to 6, and more particularly, n is selected from the group consisting of metals with oxidation states of 2, 3, 4, 5 and 6, optionally combined with one or more features of the various embodiments described above or below throughout the specification.

[0078] The transition metal (IV) oxide of formula (A) or the transition metal (III) oxide of formula (B) are in the form of nanorods. For the purposes of this invention, the terms "nanororod" or "nanowire" are used interchangeably and refer to nanoparticles having a narrow dimension (i.e., thickness) and a long dimension (i.e., length), wherein the ratio between the long dimension and the narrow dimension (i.e., aspect ratio) is equal to or greater than 2. The narrow dimension or diameter corresponds to the shortest dimension or cross-sectional thickness of the nanorod, which is substantially constant along the length direction for a given nanorod. The long dimension or length corresponds to the longest dimension of the nanorod, which is generally orthogonal to the diameter of the nanorod. The diameter and length of the nanorod can be measured by methods known in the art, such as transmission electron microscopy (TEM). Typically, average thickness and average length values ​​are given. The term "average thickness" refers to the average value taken from at least 20 different diameter measurements. The term "average length" refers to the average value taken from at least 20 different length measurements.

[0079] According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the average thickness of the nanorod is at least 5 nm, more specifically 5 nm to 20 nm, even more specifically 8 to 16 nm, even more specifically about 5 nm, about 5.5 nm, about 6 nm, about 6.5 nm, about 7 nm, about 7.5 nm, about 8 nm, about 8.5 nm, about 9 nm, about 9.5 nm, about 10 nm, about 10.5 nm, about 11 nm, about 11.5 nm, about 12 nm, about 12.5 nm, about 13 nm, about 13.5 nm, about 14 nm, about 15 nm, about 16 nm, about 17 nm, about 18 nm, about 19 nm, or about 20 nm.

[0080] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the average length of the nanorod is at least 16 nm, more specifically from 16 nm to 180 nm, and more specifically from 50 nm to 150 nm.

[0081] In another embodiment, the nanorods may optionally be combined with one or more features of the various embodiments described above or below throughout the specification, wherein the aspect ratio of the nanorods is 2 to 15, more specifically 3 to 10.

[0082] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the nanorods have a diameter equal to or greater than 280 nm. 2 More specifically, equal to or less than 14500nm 2 Surface area.

[0083] The materials of this invention also include open hollow carbon nanostructures. As used herein, the term "hollow carbon nanostructure" refers to a three-dimensional structure with an open interior, such as a tubular, spherical, or polyhedral structure with an open interior, wherein one or more ends, particularly both ends, of the hollow cavity are open. In this way, electrolyte liquids can easily flow within the carbon nanostructure.

[0084] According to one embodiment, the hollow carbon nanostructure may optionally be a hollow tubular carbon nanostructure in combination with one or more features of the various embodiments described above or below throughout the specification. The term "hollow tubular carbon nanostructure" as used herein refers to a tubular carbon material having continuous closed walls along its length, wherein the tube is open at two opposite ends. In particular, the hollow tubular carbon nanostructures involved in this invention have a circular cross-section.

[0085] In one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the amount of open hollow carbon nanostructures in the material is 40% to 80% by weight, more specifically 45% to 75% by weight, or even more specifically 60% to 70% by weight, relative to the total material weight.

[0086] In another embodiment, the open hollow carbon nanostructure may optionally be combined with one or more features of the various embodiments described above or below throughout the specification, selected from carbon nanotubes, graphitized carbon nanotubes, heteroatom-doped carbon nanotubes, carbon nanofibers, graphitized carbon nanofibers, and heteroatom-doped carbon nanofibers. More specifically, the hollow carbon nanostructure is a hollow tubular carbon nanostructure selected from carbon nanotubes and carbon nanofibers, or even more specifically, carbon nanofibers.

[0087] For the purposes of this invention, the term "heteroatom-doped" or "heteroatom-doped" refers to heteroatoms embedded in and / or incorporated into carbon nanostructures. Heteroatoms include, but are not limited to, nitrogen, sulfur, phosphorus, boron, silicon, and aluminum.

[0088] The term "open hollow carbon nanotubes," also referred to as CNTs in this article, refers to hollow materials composed of graphene sheets rolled into cylindrical shapes, in which the graphene layers form multiple concentric cylinders along the tube axis.

[0089] The term "hollow carbon nanofiber," also referred to as CNF in this document, is composed of graphene sheets tilted at an angle relative to the main axis to form stacked nanocones. CNFs differ from carbon nanotubes in that, in CNTs, this angle is zero.

[0090] CNTs and CNFs can be prepared by methods known in the art, such as chemical vapor deposition (CVD) of hydrocarbons. They are also commercially available. Graphitized carbon nanofibers can be prepared, for example, by heating carbon nanofibers under suitable conditions, such as heating at about 1000°C in an inert (argon) atmosphere.

[0091] The diameter and length of CNTs and CNFs can be measured using methods known in the art, such as transmission electron microscopy (TEM). Typically, values ​​for the average inner diameter and average outer diameter, as well as the average length, are given. The average inner diameter is smaller than the average outer diameter. The term "average diameter" refers to the average of at least three different diameter measurements. The term "average length" refers to the average of at least three different length measurements.

[0092] In one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the hollow tubular carbon nanostructure has an average inner diameter of 20 nm to 180 nm, or 30 nm to 180 nm, or 40 nm to 100 nm, more specifically 60 nm to 80 nm, or even more specifically about 70 ± 20 nm.

[0093] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the hollow tubular carbon nanostructure has an average outer diameter of 40 nm to 200 nm, or 60 nm to 140 nm, more specifically 80 nm to 130 nm, or even more specifically about 110 nm ± 30 nm.

[0094] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the length of the hollow tubular carbon nanostructure is equal to or less than 10 μm, more specifically from 1 μm to 10 μm, or even more specifically from 1 μm to 2 μm.

[0095] In one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the hollow tubular carbon nanostructure has i) an average inner diameter of 20 nm to 180 nm, or 30 nm to 180 nm, or 40 nm to 100 nm, more specifically 60 nm to 80 nm, or even more specifically about 70 ± 20 nm; ii) an average outer diameter of 40 nm to 200 nm, more specifically 80 nm to 130 nm, or 60 nm to 140 nm, or even more specifically about 110 nm ± 30 nm, provided that the average inner diameter is less than the average outer diameter; and iii) a length equal to or less than 10 μm, more specifically 1 μm to 10 μm, or even more specifically 1 μm to 2 μm.

[0096] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the present invention relates to a material comprising a plurality of nanorods composed of transition metal oxides, the nanorods being encapsulated within an open hollow carbon nanostructure as defined herein.

[0097] As described above, in the material of the present invention, multiple nanorods composed of transition metal oxides are encapsulated within an open hollow carbon nanostructure. Typically, the average thickness of the nanorods is much smaller than the internal space of the hollow carbon nanostructure; that is, the nanorods do not occupy the entire cavity of the carbon nanostructure. In particular, when the hollow carbon nanostructure is a hollow tubular carbon nanostructure, the average thickness of the nanorods is much smaller than their inner diameter.

[0098] In the material of this invention, the cavity of the hollow carbon nanostructure is only partially filled with multiple nanorods composed of transition metal (IV) oxides. Specifically, the volume of the nanorods encapsulated within the hollow carbon nanostructure is equal to or less than 50% relative to the total cavity volume of the hollow carbon nanostructure.

[0099] In one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the volume of the nanorods encapsulated within the hollow carbon nanostructure is equal to or less than 45%, equal to or less than 40%, equal to or less than 35%, equal to or less than 30%, equal to or less than 25%, equal to or less than 20%, equal to or less than 15%, equal to or less than 10%, equal to or less than 5%, equal to or less than 4%, equal to or less than 3%, equal to or less than 2%, or equal to or less than 1%, relative to the total cavity volume of the hollow carbon nanostructure.

[0100] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the volume of the nanorod encapsulated within the hollow carbon nanostructure is equal to or less than 5% relative to the total cavity volume of the rods with a diameter in the range of 8 nm to 14 nm encapsulated within the hollow tubular carbon nanostructure with an inner diameter of about 70 nm.

[0101] In another embodiment, the plurality of nanorods may optionally be combined with one or more features of the various embodiments described above or below throughout the specification, wherein the plurality of nanorods do not obstruct the internal space of the hollow tubular carbon nanostructure.

[0102] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the average inner diameter of the hollow carbon nanostructure is at least twice the average thickness of the nanorod.

[0103] Because the filling volume is equal to or less than 50%, and / or the average inner diameter of the hollow carbon nanostructure is at least twice the average thickness of the nanorod, all the rods can contact the electrolyte, which translates into a larger surface area that facilitates the flow / diffusion of reactants / products.

[0104] In one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the average inner diameter of the hollow carbon nanostructure is at least twice the average thickness of the nanorod, more specifically 2 to 20 times, even more specifically 3 to 10 times, even more specifically about 2 times, about 3 times, about 4 times, about 5 times, about 6 times, about 7 times, about 8 times, about 9 times, about 10 times, about 11 times, about 12 times, about 13 times, about 14 times, about 15 times, about 16 times, about 17 times, about 18 times, about 19 times, about 20 times.

[0105] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, a plurality of nanorods composed of transition metal oxides are disposed on the inner surface of a hollow carbon nanostructure. Thus, in this embodiment, the central portion of the cavity of the hollow carbon nanostructure is empty.

[0106] Another advantage of placing nanorods on the inner surface of carbon nanostructures is the establishment of a close contact between the metal oxide nanoparticles and the carrier (carbon nanostructure), which benefits from the high electrical conductivity of carbon, thereby improving electrical conduction. This close contact is a result of the nanorod fabrication process, namely, an in-situ transformation of the precursor triggered by electrochemical processes. Therefore, in one embodiment, the metal oxide nanoparticles are in close contact with the hollow carbon nanostructure, optionally in combination with one or more features of the various embodiments described above or below throughout the specification.

[0107] As described above, the materials of the present invention are prepared by an electrochemical method, which has advantages over conventional hydrothermal processes. Therefore, according to a second aspect, the present invention relates to an electrochemical method for preparing materials as defined herein, the method comprising the following steps: i) A modified working electrode is provided by coating a precursor material onto a conductive surface, the precursor material comprising a plurality of spherical nanoparticles formed by R as defined herein. t M''' 3-t The spherical nanoparticles are composed of transition metal (II,III) oxides of O4(C) and are encapsulated within an open hollow carbon nanostructure. ii) Electrosynthesizing an aqueous solution containing an electrolyte using a counter electrode, a reference electrode, and the modified working electrode obtained in step i), wherein the electrolyte is selected from the group consisting of alkali metal or alkaline earth metal halide salts, or hydroxides of alkali metals or alkaline earth metals. iia) Applying 5000 or fewer constant current cycles in the range of -0.2V to 0.8V at a current density equal to or greater than 0.5A per gram of precursor material, or alternatively, (iib) Apply 5000 or fewer potentiodynamic cycles in the range of 1V to 1.7V at a scan rate equal to or greater than 40mV / s. When M''' in the precursor material of formula (C) is manganese, a material containing multiple nanorods encapsulated within an open hollow carbon nanostructure is obtained, wherein the multiple nanorods are composed of transition metal oxides of formula (A); when M''' in the precursor material of formula (C) is a transition metal selected from the group consisting of iron, cobalt and nickel, a material containing multiple nanorods encapsulated within an open hollow carbon nanostructure is obtained, wherein the multiple nanorods are composed of transition metal oxides of formula (B).

[0108] For the purposes of this invention, the term "working electrode" refers to an electrode in an electrochemical cell that undergoes a targeted electrochemical transformation (oxidation in this example) when electron exchange occurs between the electrode and the electrolyte. The working electrode may be a carbon-based electrode.

[0109] As used in this article, the term "counter electrode" refers to the electrode in an electrochemical cell configured to balance the current flowing through the working electrode; that is, the charge flowing to the counter electrode must have the opposite sign to the charge flowing to the working electrode. The counter electrode can be a platinum (Pt) electrode or a carbon-based electrode.

[0110] Non-limiting examples of carbon-based electrodes include electrodes made of glassy carbon, pyrolytic carbon films, graphene, carbon black, carbon particles, carbon nanotubes, graphite, etc.

[0111] As used herein, the term "reference electrode" refers to a non-polarized electrode with a known and highly repeatable potential. A reference electrode provides a stable reference point for measuring the voltage of the working electrode. Specifically, a reference electrode may include an Ag / AgCl electrode or a reversible hydrogen electrode (RHE).

[0112] According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the working electrode is a carbon-based electrode, more specifically a glassy carbon electrode.

[0113] In another embodiment, the counter electrode may be selected from platinum electrodes and carbon-based electrodes, more specifically platinum electrodes, in combination with one or more features of the various embodiments described above or below throughout the specification.

[0114] In another embodiment, the reference electrode may be selected from Ag / AgCl electrodes and reversible hydrogen electrodes (RHE), optionally in combination with one or more features of the various embodiments described above or below throughout the specification.

[0115] In step i) of the electrochemical method, the conductive surface can be a glassy carbon electrode, foam, cloth, buckypaper, or indium tin oxide (ITO).

[0116] According to one embodiment, it may optionally be combined with one or more features of the various embodiments described above or below throughout the specification, per 1cm 2 The working electrode is coated with 14 μg of precursor material.

[0117] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, in step i), a suspension of the precursor material formed in a suitable solvent is used, such as hexane, ethanol, methanol, isopropanol, deionized water, dimethylformamide (DMF), pentane, etc.

[0118] Electrochemical processes are typically carried out at room temperature in the presence of an electrolyte, which may be a halide salt of an alkali metal or an alkaline earth metal, or a hydroxide of an alkali metal or an alkaline earth metal.

[0119] Non-limiting examples of alkali metal or alkaline earth metal halide salts that may be used include sodium chloride, potassium chloride or lithium chloride, calcium chloride or magnesium chloride. Non-limiting examples of alkali metal or alkaline earth metal hydroxides that may be used include sodium hydroxide, potassium hydroxide or lithium hydroxide, calcium hydroxide or magnesium hydroxide.

[0120] According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the electrolyte is a chloride salt of an alkali metal or alkaline earth metal, particularly potassium chloride, with a concentration equal to or greater than 1 M, more specifically 1 M to 3 M, more specifically 1.5 M to 2.5 M, or even more specifically about 2 M.

[0121] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the electrolyte is an alkali metal or alkaline earth metal hydroxide, particularly KOH, with a concentration equal to or greater than 0.1 M, more specifically 0.1 to 2 M, more specifically 0.5 to 1.5 M, and even more particularly about 1 M.

[0122] In the electrochemical method of the present invention, formula A can be formed. z M' 2y Mn 1-x O2(A) is a transition metal oxide, where A is an alkali metal or alkaline earth metal ion of a halide or hydroxide salt, and z is not 0.

[0123] According to one embodiment, it may optionally be combined with one or more features of the various embodiments described above or below throughout the specification, when using formula R t M''' 3-tWhen the method is performed on spherical nanoparticles composed of transition metal (II,III) oxides of formula (C) O4(C), where t is 0 and M''' is Mn, i.e., the transition metal (II,III) oxide of formula (C) has the formula Mn3O4, and an alkali metal halide salt or an alkali metal hydroxide is used as the electrolyte, the obtained nanorods are composed of a mixture of one or more transition metal oxides of formula (A). More specifically, multiple nanorods are composed of M' 2y Mn 1-x’ Metal oxides of O2 (Al) and formula A z’ M' 2y Mn 1-x’’ The nanorods are composed of a mixture of metal oxides of formula MnO2(A2), where x'=y and x''=(z' / 4+y), where y=0 and 0<z'<4, and A is a monovalent metal; and manganese can be in oxidation state (IV) or a mixture of (IV) and (III), the latter having oxygen vacancies; that is, multiple nanorods are composed of metal oxides of formula MnO2(A1) and formula A z’ Mn 1-x’’ The composition of O2 (A2) is a mixture of metal oxides, where x'' = z' / 4 and 0 < z' < 4.

[0124] In another embodiment, it may optionally be combined with one or more features of the various embodiments described above or below throughout the specification, when using formula R t M''' 3-t When the method is performed on spherical nanoparticles composed of transition metal (II,III) oxides of O4(C), where t is not 0 and M''' is Mn, that is, the transition metal (II,III) oxide of formula (C) has the formula R t Mn 3-t O4, and alkali metal halide salts or alkali metal hydroxides are used as electrolytes, the resulting nanorods consist of a mixture of one or more transition metal oxides of formula (A). More specifically, multiple nanorods are composed of formula M' 2y Mn 1-x’ Metal oxides of O2 (Al) and formula A z’ M' 2y Mn 1-x’’ O2 (A2) is a mixture of metal oxides, wherein M' is a divalent transition metal other than Mn; x' equals y and x'' = (z' / 4 + y), where 0 < y < 1 and 0 < z' < 4, and A is a monovalent metal; and manganese can be in the (IV) oxidation state or a mixture of (IV) and (III), the latter having oxygen vacancies.

[0125] Electrosynthesis can be performed in two different ways: by iia) applying 1,000 to 5,000 constant current cycles in the range of -0.2V to 0.8V at a current density of 0.5 A to 5A per gram of precursor material; or alternatively, by iib) applying 1,000 to 5,000 potentiodynamic cycles in the range of 1V to 1.7V at a scan rate of 40mV / s to 400mV / s.

[0126] Electrosynthesis can be performed in two different ways: by applying a constant current cycle (iia), where the current remains constant, or by applying a kinetic potential cycle (iib), where the potential changes at a constant rate (potential scan rate) by changing the current.

[0127] According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the method includes applying 1,000 to 5,000 constant current cycles, more specifically 1,500 to 3,500 cycles, or even more specifically about 2,500 constant current cycles.

[0128] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, in step iia) of the method, the current density per gram of precursor material is 0.5 to 5, more specifically 0.7 to 1.3, or even more specifically about 1 A.

[0129] According to another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the method includes applying 1,000 to 5,000 kinetic energy cycles, more specifically 1,500 to 3,500 cycles, or even more specifically about 2,500 kinetic energy cycles.

[0130] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, in step iia) of the method, the current density is 40 mV / s to 400 mV / s, more specifically 100 to 300 mV / s, or even more specifically about 200 mV / s.

[0131] A material comprising a plurality of nanorods encapsulated within an open hollow carbon nanostructure also constitutes part of this invention, wherein the plurality of nanorods are composed of any of the following: a) As defined in this paper, Equation A z M' 2y Mn 1-x O2(A) transition metal oxides; or alternatively, b) As defined in this paper, M'' 3m / n M 2-mO3(B) transition metal oxides; The material can be obtained by a method including the following steps: i) A modified working electrode is provided by coating a precursor material onto a conductive surface, the precursor material comprising R as defined herein. t M''' 3-t Multiple spherical nanoparticles composed of O4(C) transition metal (II,III) oxides are encapsulated within an open hollow carbon nanostructure. ii) Using a counter electrode, a reference electrode, and the modified working electrode obtained in step i), an aqueous solution containing an electrolyte selected from the group consisting of alkali metal or alkaline earth metal halide salts, or alkali metal or alkaline earth metal hydroxides, is electrosynthesized by the following method: (iia) Applying 5000 or fewer constant current cycles in the range of -0.2V to 0.8V at a current density equal to or greater than 0.5A per gram of precursor material, or alternatively, (iib) Apply 5000 or fewer potentiodynamic cycles in the range of 1V to 1.7V at a scan rate equal to or greater than 40mV / s. When M''' in the precursor material of formula (C) is manganese, a material containing multiple nanorods encapsulated within an open hollow carbon nanostructure is obtained, wherein the multiple nanorods are composed of a transition metal oxide of formula (A); when M''' in the precursor material of formula (C) is a transition metal selected from the group consisting of iron, cobalt and nickel, a material containing multiple nanorods encapsulated within an open hollow carbon nanostructure is obtained, wherein the multiple nanorods are composed of a transition metal oxide of formula (B).

[0132] All embodiments of the electrochemical methods mentioned herein for preparing the materials of the present invention are also applicable to the materials obtained by such methods.

[0133] A third aspect of the invention relates to a precursor material that can be used to obtain the material of the invention. The precursor material comprises a plurality of spherical nanoparticles composed of transition metal (II,III) oxides of formula (C). R t M''' 3-t O4 (C) The spherical nanoparticles are encapsulated within an open, hollow tubular carbon nanostructure, wherein: M''' is a transition metal selected from the group consisting of manganese, iron, cobalt, and nickel in divalent and trivalent oxidation states; R is a transition metal in a divalent oxidation state other than M'''; t is a value from 0 to less than 1; The amount of transition metal oxide of formula (C) is 20% to 60% by weight relative to the total material weight; and The volume of the spherical nanoparticles encapsulated within the hollow tubular carbon nanostructure is equal to or less than 50% of the total cavity volume of the hollow carbon nanostructure. Specifically, the hollow carbon nanostructure is tubular, and the average inner diameter of the hollow tubular carbon nanostructure is at least twice the average diameter of the spherical nanoparticles.

[0134] In one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the present invention relates to a precursor material comprising a plurality of spherical nanoparticles composed of transition metal (II,III) oxides of formula (C), the spherical nanoparticles being encapsulated within an open hollow tubular carbon nanostructure as defined herein.

[0135] In another embodiment, M''' is manganese, optionally combined with one or more features of the various embodiments described above or below throughout the specification, in the transition metal (II,III) oxide of formula (C).

[0136] In another embodiment, M''' is iron in the transition metal (II,III) oxide of formula (C), optionally combined with one or more features of the various embodiments described above or below throughout the specification.

[0137] In another embodiment, M''' is cobalt in the transition metal (II,III) oxide of formula (C), optionally combined with one or more features of the various embodiments described above or below throughout the specification.

[0138] In another embodiment, M''' is nickel in the transition metal (II,III) oxide of formula (C), optionally combined with one or more features of the various embodiments described above or below throughout the specification.

[0139] Regarding the transition metal (II,III) oxides of formula (C), the precursor material also includes undoped and doped materials. According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout this specification, the transition metal (II,III) oxide of formula (C) is undoped, i.e., t is 0.

[0140] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the transition metal (II,III) oxide is doped, i.e., in the transition metal (II,III) oxide of formula (C), the value of t is greater than 0 and less than 1, and R is a transition metal having a divalent oxidation state other than M'''. More specifically, R is selected from the group consisting of manganese, iron, cobalt, nickel, copper, chromium, molybdenum, titanium, vanadium, and zinc. Even more specifically, R is selected from the group consisting of iron, cobalt, and nickel. In a more specific embodiment, t is from 0.01 to 0.4, even more specifically, t is from 0.05 to 0.09. Even more specifically, t is about 0.01, about 0.02, about 0.03, about 0.05, about 0.07, about 0.09, about 0.1, about 0.15, about 0.20, about 0.25, about 0.3, about 0.35, or about 0.4.

[0141] All the embodiments described above concerning the hollow carbon nanostructure of the material of the present invention are also applicable to precursor materials.

[0142] The precursor material comprises spherical nanoparticles. For the purposes of this invention, the term "spherical" nanoparticle refers to any shape that is perfectly spherical or nearly spherical. Spherical nanoparticles are characterized by their diameter, which is the length from one end of the surface to the other through the midpoint of the nanoparticle. The diameter can be measured by methods known in the art, such as transmission electron microscopy (TEM). The term "average diameter" refers to the average value typically taken from 80 different diameter measurements.

[0143] According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the spherical nanoparticles have an average diameter of 2 nm to 20 nm, more specifically 7 nm to 15 nm, and even more particularly about 10 ± 2 nm. In another embodiment, the spherical nanoparticles have an average diameter of about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, about 10 nm, about 11 nm, about 12 nm, about 13 nm, about 14 nm, about 15 nm, about 16 nm, about 17 nm, about 18 nm, about 19 nm, or about 20 nm.

[0144] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the spherical nanoparticles have a size of 50 nm. 2 Up to 5000nm 2 The surface area, more specifically 600 nm 2 Up to 2800nm 2 Even more specifically, 800nm 2 Up to 1800nm2 .

[0145] The precursor material can be prepared by a method as defined in the fourth aspect, the method comprising the following steps: a) In the presence of a surfactant in a suitable solvent, a mixture of a transition metal (II) salt and optionally a second salt is provided, the second salt being a salt of a transition metal having a divalent oxidation state other than the first transition metal, wherein the metal of the first transition metal (II) salt is selected from manganese, iron and cobalt, and the metal of the second salt is different from the metal of the first salt. b) Add the mixture from step a) to a suspension of hollow carbon nanostructures in a suitable solvent; c) Heating the suspension to a temperature of 80 to 110°C; and d) Add water to the suspension from step c) and heat to a temperature of 80°C to 110°C to obtain the precursor material of formula (C); Wherein, when step a) is carried out in the absence of another transition metal salt, the material of formula (C) is obtained, where t is 0; when step a) is carried out in the presence of a second salt of a transition metal, the precursor material of formula (C) is obtained, where t is not 0, and R is a transition metal in the divalent oxidation state other than M'''.

[0146] The advantage of the above method is that it can be used under environmental conditions (room temperature and atmospheric pressure (approximately 10¹³.25)). hPa The method can be performed in the presence of water.

[0147] Step (a) is carried out in the presence of a surfactant. Non-limiting examples of surfactants that may be used include acids (e.g., (C1-C5)). 20 Alkyl carboxylic acids, such as oleic acid, lauric acid, stearic acid, myristic acid, and hexadecanoic acid, (C1-C2) 20 Alkylamines (such as oleylamine, laurylamine, hexadecylamine, trioctylamine and dioctylamine) or mixtures thereof.

[0148] In one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the molar ratio of metal to surfactant is 1:8 to 1:14, particularly 1:10 to 1:12.

[0149] For the purposes of this invention, the term "(C1-C)" is used. n "alkyl" refers to a straight-chain or branched saturated hydrocarbon group having 1 to n carbon atoms. Examples include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl or tert-butyl, n-pentyl or n-hexyl, etc.

[0150] According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the surfactant is (C1-C 20 Alkylamines, more specifically selected from the group consisting of oleylamine, laurylamine, hexadecylamine, trioctylamine, dioctylamine, and mixtures thereof.

[0151] Non-limiting examples of salts that may be used in step a) include nitrates, sulfates, chlorides, or acetates. When a second salt is present, the anions of the salts may be the same or different.

[0152] According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the transition metal (II) salt of step (a) is selected from the group consisting of nitrates, sulfates, chlorides and acetates, more specifically acetates.

[0153] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the salt of the second transition metal having a divalent oxidation state is selected from the group consisting of nitrates, sulfates, manganese salts, chloride salts and acetates, more specifically acetates.

[0154] Step a) is typically carried out at room temperature and in a suitable solvent. Non-limiting examples of solvents that may be used include (C6) aromatic solvents such as xylene, benzene, toluene, fluorinated benzene, chlorobenzene, iodobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene, 1,2,3-triiodobenzene, 1,2 4-Triiodobenzene, fluorotoluene, 1,2-difluorotoluene, 1,3-difluorotoluene, 1,4-difluorotoluene, 1,2,3-trifluorotoluene, 1,2,4-trifluorotoluene, chlorotoluene, 1,2-dichlorotoluene, 1,3-dichlorotoluene, 1,4-dichlorotoluene, 1,2,3-trichlorotoluene, 1,2,4-trichlorotoluene, iodotoluene, 1,2-diiodotoluene, 1,3-diiodotoluene, 1,4-diiodotoluene, 1,2,3-triiodotoluene, 1,2,4-triiodotoluene, or mixtures thereof.

[0155] In one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the solvent used in step a) is a (C6) aromatic solvent, more specifically xylene.

[0156] The solvent used in step b) may be the same as or different from the solvent used in step (a).

[0157] In one embodiment, the solvent used in step b) may be combined with one or more features of the various embodiments described above or below throughout the specification, and the solvent used in step a) may be the same as the solvent used in step a).

[0158] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the solvent used in step b) is a (C6) aromatic solvent, more specifically xylene.

[0159] Step c) involves heating the solution obtained in step b) to a temperature of 80 to 110°C to obtain the material of formula (C). A convenient temperature ramp can be used, for example, a temperature ramp of 1°C / min to 10°C / min, more specifically about 5°C / min.

[0160] In one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the temperature used in step c) is 80 to 110°C, more specifically about 90°C.

[0161] After step c), water may optionally be added at the same temperature as in step c), particularly under vigorous stirring.

[0162] The precursor material can be separated by centrifugation (e.g., at 6000 rpm to 10000 rpm for 5 to 15 minutes), particularly at room temperature, and then filtered, for example by using a polytetrafluoroethylene (PTFE) membrane filter (pore size 0.45 μm, diameter 47 mm), which can be washed with one or more solvents, such as hexane, ethanol, acetone, or mixtures thereof.

[0163] Optionally, the obtained precursor material can be resuspended and sonicated in a suitable solvent (such as hexane) for a period of time (e.g., 10 to 30 minutes) and then filtered again to remove nanoparticles from the outer surface of the hollow carbon nanostructure.

[0164] Optionally, in the obtained precursor material, the organic coating (i.e., surfactant) can be removed from the surface of the nanoparticles by heating at a temperature of at least 300°C.

[0165] In one embodiment, the solvent used in step b) may optionally be combined with one or more features of the various embodiments described above or below throughout the specification, and the solvent used in step a) may be the same as the solvent used in step a).

[0166] According to another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the present invention relates to a method for preparing a precursor material as defined above, the method comprising the steps of: a) In the presence of a surfactant in a suitable solvent, a mixture comprising a transition metal (II) salt and optionally a second salt, the second salt being a salt of a transition metal having a divalent oxidation state other than a first transition metal, wherein the metal of the first transition metal (II) salt is selected from manganese, iron and cobalt, and the metal of the second salt is different from the metal of the first salt; b) Add the mixture from step a) to a suspension of hollow carbon nanostructures in a suitable solvent; c) Heating the suspension to a temperature of 80 to 110°C; d) Add water to the suspension from step c) and heat to a temperature of 80°C to 110°C to obtain the precursor material of formula (C); e) Optionally, the material obtained in step c) can be separated by centrifugation, specifically by centrifugation at 6000 rpm to 10000 rpm for 5 to 15 minutes, particularly at room temperature, and then filtered; f) Optionally, the material separated in step d) is washed with one or more solvents, specifically selected from the group consisting of hexane, ethanol, acetone, and mixtures thereof; g) Optionally, the obtained precursor material is resuspended in a suitable solvent and sonicated for a period of 10 to 30 minutes, followed by filtration of the precursor material to remove nanoparticles from the outer surface of the hollow carbon nanostructure; and h) Optionally, the organic coating (i.e., surfactant) can be removed from the surface of the nanoparticles by heating at a temperature of at least 300°C.

[0167] Precursor materials comprising multiple spherical nanoparticles are also part of this invention, wherein the spherical nanoparticles are defined herein as R. t M''' 3-t The spherical nanoparticles are composed of transition metal (II,III) oxides of O4 (C) and are encapsulated within an open hollow carbon nanostructure. The precursor material can be obtained by a method including the following steps: a) In the presence of a surfactant in a suitable solvent, a mixture comprising a transition metal (II) salt and optionally a second salt, the second salt being a salt of a transition metal having a divalent oxidation state other than a first transition metal, wherein the metal of the first transition metal (II) salt is selected from manganese, iron and cobalt, and the metal of the second salt is different from the metal of the first salt; b) Add the mixture from step a) to a suspension of open hollow carbon nanostructures in a suitable solvent; c) Heating the suspension to a temperature of 80 to 110°C; and d) Add water to the suspension from step c) and heat to a temperature of 80°C to 110°C to obtain the precursor material of formula (C); Wherein, when step a) is carried out in the absence of another transition metal salt, the material of formula (C) is obtained, where t is 0; when step a) is carried out in the presence of a second salt of a transition metal, the precursor material of formula (C) is obtained, where t is not 0, and R is a transition metal in the divalent oxidation state other than M'''.

[0168] All embodiments described herein relating to the electrochemical method for preparing the precursor materials of the present invention are also applicable to precursor materials that can be obtained by this method.

[0169] According to another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the present invention relates to a method for preparing a material comprising a plurality of nanorods composed of transition metal oxides as defined above, the method comprising the following steps: a) In the presence of a surfactant in a suitable solvent, a mixture comprising a transition metal (II) salt and optionally a second salt, the second salt being a salt of a transition metal having a divalent oxidation state other than a first transition metal, wherein the metal of the first transition metal (II) salt is selected from manganese, iron and cobalt, and the metal of the second salt is different from the metal of the first salt; b) Add the mixture from step a) to a suspension of open hollow carbon nanostructures in a suitable solvent; c) Heating the suspension to a temperature of 80 to 110°C; and d) Add water to the suspension from step c) and heat to a temperature of 80°C to 110°C to obtain the precursor material of formula (C); Wherein, when step a) is carried out in the absence of another transition metal salt, the material of formula (C) is obtained, where t is 0; when step a) is carried out in the presence of a second salt of a transition metal, the precursor material of formula (C) is obtained, where t is not 0, and R is a transition metal in the divalent oxidation state other than M'''. i) Provide a modified working electrode by coating a conductive surface with the precursor material obtained in step c); ii) Electrosynthesizing an aqueous solution containing an electrolyte using a counter electrode, a reference electrode, and the modified working electrode obtained in step i); the electrolyte comprising the group consisting of chloride salts of alkali metals or alkaline earth metals at a concentration equal to or greater than 1 M, and hydroxides of alkali metals or alkaline earth metals at a concentration equal to or greater than 0.1 M. iia) Applying 5000 or fewer constant current cycles in the range of -0.2V to 0.8V at a current density equal to or greater than 0.5A per gram of precursor material, or alternatively, (iib) Apply 5000 or fewer potentiodynamic cycles in the range of 1V to 1.7V at a scan rate equal to or greater than 40mV / s. When M''' in the precursor material of formula (C) is manganese, a material containing multiple nanorods encapsulated within an open hollow carbon nanostructure is obtained, wherein the multiple nanorods are composed of transition metal oxides of formula (A); when M''' in the precursor material of formula (C) is a transition metal selected from the group consisting of iron, cobalt and nickel, a material containing multiple nanorods encapsulated within an open hollow carbon nanostructure is obtained, wherein the multiple nanorods are composed of transition metal oxides of formula (B).

[0170] The materials defined above also form part of this invention, and said materials can be obtained by means of steps a), b), c), i), ii), and iii) as defined herein.

[0171] As described above, the materials of the present invention are used not only as electrocatalysts in the field of battery energy storage, but also as cathodes in fuel cells and anodes in water splitting in the field of energy conversion, which respectively involve the reduction reaction of oxygen to water and the oxidation reaction of water to oxygen.

[0172] Therefore, another aspect of the present invention relates to the use of materials as defined above, comprising a plurality of nanorods composed of transition metal oxides, as electrocatalysts. According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout the specification, the present invention relates to the use of materials as defined above as electrocatalysts, as cathodes in fuel cells, or as anodes in water splitting.

[0173] Throughout the specification and claims, the word "comprising" and its variations are not intended to exclude other technical features, additives, components, or steps. Furthermore, the word "comprising" covers situations where it "consists of...". Other objects, advantages, and features of the invention will be understood by those skilled in the art upon reading the specification, or may be learned by practicing the invention. The following embodiments and figures are provided by way of illustration and are not intended to limit the invention. Moreover, the invention covers all possible combinations of the specific and preferred embodiments described herein.

[0174] Example Materials: Carbon nanofibers (CNFs) prepared by chemical vapor deposition were purchased from Pyrograph Products (outer diameter: 110 ± 30 nm, inner diameter: 70 ± 20 nm). Both pristine and milled carbon nanofibers were used in this work, resulting in nanofiber lengths ranging from 10 (pristine) μm to 2 (milled) μm.

[0175] Grinded carbon nanofibers were prepared by mechanical ball milling using a Retsch MM400 ball mill (600 rpm). In a typical experiment, 50 mg of CNF was placed in a 5 mL stainless steel container with stainless steel balls (10 mm in diameter) and milled in air at 600 rpm for 180 minutes (yield 98%). No further purification steps were required. To produce graphitized carbon nanofibers, the ground carbon nanofibers (CNF, 100 mg) were placed in an alumina crucible and heated under an argon atmosphere. The sample was initially purged at room temperature for 30 minutes, heated to 1000 °C at a heating rate of 10 °C / min, held for 90 minutes, and then cooled under an argon atmosphere. All other reagents were purchased from Sigma-Aldrich and were ready for use without further purification.

[0176] In hollow carbon nanofibers (rA) z M' 2y Mn 1-x O2@CNF or r-M'' 3m / n M 2-m The synthesis of undoped or doped nanorods within O3@CNF is carried out in two steps: sR is synthesized within CNF. t M''' 3-t O4NP was then used for electrochemical synthesis.

[0177] TEM testing was performed using a JEOL JEM F200 microscope equipped with a cold field emission gun (Cold-FEG), operating at 200 kV and featuring ultra-high resolution pole pieces. TEM images were acquired using a Gatan OneView camera. Energy-dispersive X-ray spectroscopy (EDS) analysis was performed using a Centurio large-angle silicon drift detector (SDD), which collected data from 100 mm... 2X-rays were detected in the detection area. TEM samples were prepared by casting a suspension of carbonaceous material in hexane containing a few drops onto a porous carbon film fitted with a copper grid, followed by drying under a nitrogen atmosphere. Thermogravimetric analysis (TGA) was performed in air at a scan rate of 5 °C / min in the range of 25–1000 °C using a TGA / DSC 3+ Mettler Toledo instrument. Raman spectroscopy was performed using a RENISHAW Raman microscope with laser-ionized Ar (514 nm). Powder X-ray diffraction (XRD) patterns were recorded using a PANalytical diffractometer equipped with a Cu Kα source (λ = 1.5418 Å). Data were analyzed and processed using the X'Pert Data software package.

[0178] Example 1 In-situ synthesis of SR within CNF t M’’’ 3-t O 4 NP(sR) t M’’’ 3-t O 4 @CNF) In a typical experiment, a mixture of manganese(II) acetate (1.1 mmol, 190.3 mg) and oleylamine (12.2 mmol, 5.1 mL, metal (Mn) to surfactant ratio of 1:11.1) in xylene (7 mL) was added to a suspension of carbon nanofibers (CNF, 25 mg) that had been sonicated in xylene (10 mL) for 15 min. This was done to obtain Mn3O4 nanoparticles (sR) doped within CNF. t M''' 3-tO4@CNF), replacing 15% (mmol) of manganese acetate (II) with ferric acetate (II), cobalt acetate (II), or nickel acetate (II), respectively. Therefore, manganese acetate (II) (0.935 mmol) in xylene (7 mL) and optionally ferric acetate (II), cobalt acetate (II), or nickel acetate (II) (0.165 mmol), oleylamine (12.2 mmol, 5.1 mL) (metal (Mn+Fe or Mn+Co or Mn+Ni):surfactant ratio of 1:11.1) solution were added to a xylene (10 mL) CNF (25 mg) suspension. In all cases, the solution was heated to 90 °C in air at 5 °C / min. Deionized water (1 mL) was added to the dark suspension at 90 °C with vigorous stirring, and the resulting suspension was aged in air at 90 °C for 3 hours. The mixed materials (s-Mn3O4@CNF or sR when ferric acetate(II), cobalt acetate(II), or nickel acetate(II) were added) were separated by centrifugation at room temperature (8000 rpm, 10 min) and filtration using a PTFE (polytetrafluoroethylene) membrane filter (0.45 μm pore size, 47 mm diameter). t Mn 3-t O4@CNF was then thoroughly washed with hexane (50 mL), ethanol (50 mL), and finally acetone (25 mL) to obtain a dark black solid. The obtained solid was then resuspended in hexane (25 mL) and sonicated for 15 minutes, and filtered again to remove nanoparticles from the outer surface of the carbon nanofibers (CNF). The resulting black solid was heated in air in a furnace at 300 °C for 2 hours to remove the organic coating (i.e., surfactant) from the surface of the nanoparticles, yielding a dark black solid containing undoped (s-Mn3O4@CNF). Figure 1 ) or doped nanoparticles (sR) t Mn 3-t O4@CNF), cobalt-doped nanoparticles (s-Co / Mn3O4@CNF, Co 0.15 Mn 2.85 O4@CNF Figure 7 Nickel-doped nanoparticles (s-Ni / Mn3O4@CNF, Ni) 0.15 Mn 2.85 O4@CNF Figure 8 ) or iron-doped nanoparticles (s-Fe / Mn3O4@CNF, Fe 0.15 Mn 2.85 O4@CNF Figure 9 ).

[0179] Figure 3The Raman spectra of s-Mn3O4@CNF (a), s-Co / Mn3O4@CNF (b), s-Ni / Mn3O4@CNF (c), and s-Fe / Mn3O4@CNF (d) are shown. In all cases, a band associated with Mn3O4 (i.e., 660 cm⁻¹) was observed. -1 This confirms that exchanging Mn atoms with Co, Ni, or Fe (i.e., doping) does not affect the crystal structure.

[0180] Figure 4 The X-ray powder diffraction patterns of s-Mn3O4@CNF (a), s-Co / Mn3O4@CNF (b), s-Ni / Mn3O4@CNF (c), and s-Fe / Mn3O4@CNF (d) are shown. In all cases, a peak associated with Mn3O4 was observed (i.e., the strongest near 36.2°), confirming that the exchange of Mn atoms with Co, Ni, or Fe (i.e., doping) does not affect the crystal structure. The peak at 26.4° corresponds to carbon nanofibers.

[0181] Figure 5 The TGA measurements for (a) s-Mn3O4@CNF, (b) s-Co / Mn3O4@CNF, (c) s-Ni / Mn3O4@CNF, and (d) s-Fe / Mn3O4@CNF are shown. It can be seen that the gradual oxidation of the carbon nanofiber structure in all materials occurs in the same region (i.e., 400–600 °C), while the content of metal oxides in the mixture (L) after annealing at 1000 °C varies between 13% and 30%.

[0182] Figure 6 Histograms of nanoparticle size distributions are shown for (a) s-Mn3O4@CNF, (b) s-Co / Mn3O4@CNF, (c) s-Ni / Mn3O4@CNF, and (d) s-Fe / Mn3O4@CNF. It can be seen that doping Mn3O4 with Ni or Fe has less impact on nanoparticle size than doping with Co atoms. Furthermore, in Figure 10 In this context, it can be understood that incorporating cobalt, nickel, or iron atoms into the crystal structure of manganese oxide alters the oxidation and reduction potentials of manganese.

[0183] Additional experiments demonstrated the scalability of the synthesis process by increasing the amount of reagents (e.g., tenfold).

[0184] Additional experiments were conducted by replacing 5% mmol, 10% mmol, 15% mmol, and 20% mmol of manganese(II) acetate with cobalt(II) acetate (t ranges from 0.05 to 0.20) to obtain Mn3O4-doped nanoparticles (sR) within carbon nanofibers. t Mn 3-tO4@CNF). Similar materials were found to be achievable. However, doping exceeding 15% reduces the loading of metal oxides in the mixed material. Figure 21 ).

[0185] Example 2 In-situ synthesis of SR within graphitized carbon nanostructures t Mn 3-t O 4 NP Using a procedure similar to that in Example 1, but replacing the original carbon nanofibers with graphitized carbon nanofibers, sR within the graphitized carbon nanostructure was obtained. t Mn 3-t O4NP. The obtained material was found to be similar to that of Example 1 (based on powder X-ray diffraction, thermogravimetric analysis, Raman spectroscopy, and transmission electron microscopy characterization, the composition and morphology of the NPs remained unchanged). More importantly, compared to the material of Example 1, the electrochemical impedance spectroscopy of the obtained material showed a slight increase in the resistance between the electrode surface and the electrolyte from 10 Ω to 22 Ω (referred to as solution resistance). This increase confirms the increased hydrophobicity of the material when graphitized carbon nanofibers are used, without affecting the electrocatalytic performance of the NPs.

[0186] Example 3 In-situ synthesis of s-Co within CNF 3 O 4 NP(s-Co) 3 O 4 @CNF) Following the same procedure as described in Example 1, but using cobalt(II) acetate instead of manganese(II) acetate, s-Co3O4 nanoparticles in CNF were obtained.

[0187] Figure 3 (e) The Raman spectrum of s-Co3O4@CNF is shown. At 685.3 cm⁻¹ -1 A band associated with Co3O4 was observed at the location.

[0188] Figure 4 (e) shows the X-ray powder diffraction pattern of s-Co3O4@CNF. Co3O4-related peaks were observed at 36.9°, 31.3°, 59.5° and 44.9°.

[0189] Figure 5 (e) shows the TGA plot of s-Co3O4@CNF. The gradual oxidation of the CNF structure occurred between 350 °C and 600 °C, and the content (L, loading) of the metal oxide in the mixture was 67%.

[0190] Figure 6(d) shows the histogram of the nanoparticle size distribution of s-Co3O4@CNF, indicating an average size of 15±5 nm.

[0191] Example 4 In-situ synthesis of s-Fe within CNF 3 O 4 NP(s-Fe) 3 O 4 @CNF) Following the same procedure as described in Example 1, but using ferric acetate (II) instead of manganese acetate (II), s-Fe3O4 nanoparticles in CNF were obtained.

[0192] Figure 3 (f) shows the Raman spectrum of s-Fe3O4@CNF. At 221 cm⁻¹ -1 286cm -1 and 397cm -1 A band associated with Fe3O4 was observed at the location.

[0193] Figure 4 (f) shows the X-ray powder diffraction pattern of s-Fe3O4@CNF. The strongest peak associated with Fe3O4 was observed at 35.7°.

[0194] Figure 5 (f) shows the TGA diagram of s-Fe3O4@CNF. The gradual oxidation of the CNF structure occurred between 400 °C and 650 °C, and the content (L, loading) of the metal oxide in the mixture was 40%.

[0195] Figure 6 (f) shows the histogram of the nanoparticle size distribution of s-Fe3O4@CNF, indicating an average size of 7±2 nm.

[0196] Additional experiments were conducted to obtain s-Fe3O4@CNF by adjusting the metal / oleoamine ratio to 1:0.9 and 1:0.75. It was found that reducing the amount of surfactant did not significantly increase the size of the nanoparticles.

[0197] Example 5 r-A z M’ 2y Mn 1-x O 2 @CNF and r-M'' 3m / n M 2-m O 3 @CNF's Electrosynthesis All electrochemical tests were performed at room temperature using a standard three-electrode cell on an electrochemical station (Autolab PGSTAT302N), while electrochemical synthesis was performed using an electrochemical station (Autolab 201A). Ag / AgCl and a reversible hydrogen electrode (RHE) were used as reference electrodes, and carbon rods were used as counter electrodes. A sample-coated glassy carbon electrode (GCE) and a rotating ring disk electrode (RRDE) were used as working electrodes. 5 μL of catalyst ink (s-Mn3O4@CNF, Fe3O4@CNF, or sR) was used as the catalyst. 0.15 Mn 2.85 Modified working electrodes were prepared by dropping O4@CNF (where R = Co, Ni, or Fe) onto a GCE (3 mm diameter) or by dropping 10 μL of catalyst ink onto an RRDE (5 mm diameter) and allowing it to air dry. This process was repeated until the catalyst mass loading reached 0.1 mg / cm² on the RRDE and 0.01 mg / cm² on the GCE. Before use, the GCE and RRDE were mechanically polished with an aqueous slurry of alumina powder (0.05 μm), rinsed with ultrapure water and acetone, and dried under nitrogen. Catalyst ink for coating the RRDE was prepared by ultrasonically dispersing the catalyst (2 mg) in a mixed solution of ethanol (980 μL) and Nafion solution (20 μL, 0.5 wt%). Catalyst ink for coating the GCE was prepared by ultrasonically dispersing the catalyst (1 mg) in hexane (1 mL). Cyclic voltammetry (CV) measurements were performed in nitrogen-saturated 2M KCl at a scan rate of 50 mV / s. Linear sweep voltammetry (LSV) measurements of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) were performed in oxygen-saturated 1M KOH at a scan rate of 10 mV / s and a rotational speed of 1600 rpm. Several galvanostatic or potentiodynamic cycles (i.e., 2500 cycles) were performed in KCl (2M) at high current densities (i.e., 1 A / g) or high scan rates (i.e., 200 mV / s) in the range of -0.2 V to 0.8 V (relative to Ag / AgCl) or in KOH (1M) in the range of 1 V to 1.7 V (relative to RHE). Under these conditions, undoped (rK / MnO2@CNF) was obtained based on a dissolution-precipitation mechanism. Figure 2 ; and r-Fe2O3@CNF, Figure 22 ) or doped with cobalt (r-KCo / MnO2@CNF, Figure 18 ), nickel (r-KNi / MnO2@CNF, Figure 19 ) or iron (r-KFe / MnO2@CNF, Figure 20 Nanorods.

[0198] To confirm the morphological transformation, s-Mn3O4@CNF was imaged by transmission electron microscopy (TEM) after multiple charge-discharge cycles in 2M potassium chloride solution. Figure 14 It can be seen that electrochemical treatment induces the aggregation of nanoparticles (see...). Figure 1 (Comparison before intermediate treatment), this is the initial stage of achieving a complete morphological transformation. This shows that although confinement provides high stability, it does not suppress the inherent reactivity of nanoparticles.

[0199] Furthermore, constant current charge-discharge cycle tests were performed on manganese oxide nanoparticles in carbon nanofibers before (s-Mn3O4@CNF, solid line) and after (r-MnO2@CNF, dashed line) 2500 cycles of electrosynthesis in 2M KCl. Figure 15 (a) The solid black line is carbon nanofibers without manganese oxide), and the change in capacitance with stability cycling ( Figure 15 (b) shows a significant (more than 4 times) increase in capacitance, which is related to the formation of nanorods within the carbon nanofibers, meaning that the nanorods have a larger surface area than spherical nanoparticles.

[0200] At room temperature, in an oxygen-saturated KOH (1M) solution, linear sweep voltammetry (LSV) was used to measure the voltammetry results. Figure 11 The electrocatalytic performance of manganese oxide nanoparticles in undoped (s-Mn3O4@CNF) and cobalt-doped (s-Co / Mn3O4@CNF), nickel-doped (s-Ni / Mn3O4@CNF), and iron-doped (s-Fe / Mn3O4@CNF) carbon nanofibers was evaluated. It was observed that incorporating cobalt, nickel, and iron atoms into the Mn3O4 crystal structure increased the catalytic activity for oxygen reduction (ORR, in the potential range less than 1 V, inset) and water oxidation (OER, in the potential range greater than 1.4 V).

[0201] The electrocatalytic activity of the nanomaterials of the present invention was further measured by linear sweep voltammetry in an oxygen-saturated alkaline solution of 1M KOH before electrosynthesis (s-Mn3O4@CNF, solid line) and after 2500 cycles in 2M KCl solution (r-MnO2@CNF, dashed line). Figure 16 It is clear that the morphological changes enhance the catalytic activity of water oxidation (OER, reduced overpotential) and improve its kinetics (based on the lower Tafel slope).

[0202] For manganese oxide nanoparticles on carbon nanofibers doped with (a) cobalt, (b) nickel or (c) iron ( Figure 17It is clear that the electrocatalytic activity remains high even after several cycles (e.g., 2500 cycles), and the ability to oxidize water (OER, in the potential range greater than 1.4V) is significantly improved in the case of s-Ni / Mn3O4@CNF.

[0203] Finally, to test the stability of the confined manganese oxide nanoparticles, the manganese oxide nanoparticles (s-Mn3O4) deposited on the outside and inside of the nanofibers were evaluated. Figure 12 (a) A material containing nanoparticles deposited on the exterior of nanofibers was synthesized using the same method as s-Mn3O4@CNF, but without a purification step, thus leaving nanoparticles on the exterior of the nanofibers. The results showed that this material with nanoparticles deposited on the exterior of the nanofibers had low stability; the current decreased significantly after only 500 cycles during electrosynthesis in KCl solution. Figure 13 This may be related to the loss of material (nanoparticles attached to the outside of the nanofibers). TEM images show that only the nanoparticles loaded on the outside of the nanofibers dissolved. Figure 12 (b) confirms that the encapsulation of nanoparticles restricts their dissolution.

[0204] For the sake of completeness, various aspects of the present invention are set forth in the following numbered clauses: Clause 1. A material comprising a plurality of nanorods encapsulated within an open hollow carbon nanostructure, wherein the plurality of nanorods are composed of any of the following: a) Transition metal oxides of formula (A) A z M' 2y Mn 1-x O2 (A) Alternatively, b) Transition metal oxides of formula (B) M” 3m / n M 2-m O3 (B) in: In formula (A), A is a monovalent metal, and M' is a divalent oxidation state of a transition metal other than Mn; x equals (z / 4+y), where 0≤y<1 and 0≤z<4; manganese can be in oxidation state (IV) or a mixture of (IV) and (III), the latter having oxygen vacancies; In formula (B), M is a transition metal selected from iron, cobalt and nickel in the trivalent oxidation state; M'' is a transition metal with the same or different oxidation state n as M, where the value of n is 2 to 6, and 0 ≤ m < 2; The amount of transition metal oxide of formula (A) or formula (B) is 20% to 60% by weight relative to the total material weight. The volume of the nanorods encapsulated within the hollow carbon nanostructure is equal to or less than 50% of the total cavity volume of the hollow carbon nanostructure.

[0205] Clause 2. The material according to Clause 1, wherein in the transition metal oxide of formula (A), y and z are 0, and in the transition metal oxide of formula (B), m is 0.

[0206] Clause 3. The material according to Clause 1, wherein, in the transition metal oxide of formula (A), at least one of y or z is not 0, and in the transition metal oxide of formula (B), m is not 0.

[0207] Clause 4. The material according to any one of Clauses 1-3, wherein the plurality of nanorods are composed of Formula A z M' 2y Mn 1- x O2(A) is composed of transition metal (IV) oxides.

[0208] Clause 5. The material according to any one of Clauses 1-3, wherein the plurality of nanorods are composed of formula M” 3m / n M 2-m O3(B) is composed of transition metal (III) oxides.

[0209] Clause 6. The material described in Clause 5, wherein M is iron.

[0210] Clause 7. The material described in Clause 5, wherein M is cobalt.

[0211] Clause 8. The material described in Clause 5, wherein M is nickel.

[0212] Clause 9. The material according to any one of Clauses 1-8, wherein the nanorod has an average thickness of 5 nm to 20 nm and an average length of 16 nm to 180 nm.

[0213] Clause 10. The material according to any one of Clauses 1-9, wherein the nanorod has a diameter equal to or greater than 280 nm. 2 Surface area.

[0214] Clause 11. The material according to any one of Clauses 1-10, wherein, for the total weight of the material, the amount of hollow carbon nanostructures in the material is 40% to 80% by weight.

[0215] Clause 12. The material according to any one of Clauses 1-11, wherein the hollow carbon nanostructure is selected from carbon nanotubes, graphitized carbon nanotubes, heteroatom-doped carbon nanotubes, carbon nanofibers, graphitized carbon nanofibers, and heteroatom-doped carbon nanofibers.

[0216] Clause 13. The material according to any one of Clauses 1-12, wherein the hollow carbon nanostructure is a hollow tubular carbon nanostructure, particularly a carbon nanotube.

[0217] Clause 14. The material according to Clause 13, wherein the average inner diameter of the hollow tubular carbon nanostructure is at least twice the average thickness of the nanorod, more specifically 2 to 20 times.

[0218] Clause 15. The material according to any one of Clauses 13 or 14, wherein the hollow tubular carbon nanostructure has an average inner diameter of 20 nm to 180 nm, or 30 nm to 180 nm, or 40 nm to 100 nm, an average outer diameter of 40 nm to 200 nm, or 60 nm to 140 nm, and an average length equal to or less than 10 μm.

[0219] Clause 16. The material according to any one of Clauses 1-15, wherein the volume of the nanorod encapsulated within the hollow carbon nanostructure is equal to or less than 10% relative to the total cavity volume of the hollow carbon nanostructure.

[0220] Clause 17. An electrochemical method for preparing materials as defined in Clause 1, said method comprising the following steps: i) A modified working electrode is provided by coating a precursor material onto a conductive surface, the precursor material comprising a plurality of spherical nanoparticles composed of transition metal (II,III) oxides of formula (C). R t M''' 3-t O4 (C) The spherical nanoparticles are encapsulated within an open hollow carbon nanostructure, wherein: M''' is a transition metal selected from the group consisting of manganese, iron, cobalt, and nickel, in both divalent and trivalent oxidation states; R is a transition metal in a divalent oxidation state other than M'''; t is a value from 0 to less than 1; The amount of transition metal oxide in formula (C) is 20% to 60% by weight relative to the total material weight. The volume of spherical nanoparticles encapsulated within the hollow carbon nanostructure is equal to or less than 50% of the total cavity volume of the hollow carbon nanostructure. ii) Electrosynthesis of an aqueous solution containing an electrolyte, using a counter electrode, a reference electrode, and the modified working electrode obtained in step i), wherein the electrolyte is selected from the group consisting of alkali metal or alkaline earth metal halide salts, or hydroxides of alkali metals or alkaline earth metals. iia) Applying 5000 or fewer constant current cycles in the range of -0.2V to 0.8V at a current density equal to or greater than 0.5A / g of precursor material, or alternatively, (iib) Apply 5000 or fewer potentiodynamic cycles in the range of 1V to 1.7V at a scan rate equal to or greater than 40mV / s.

[0221] Clause 18. The electrochemical method according to Clause 17, wherein, in the precursor material of formula (C), the average inner diameter of the hollow tubular carbon nanostructure is at least twice the average diameter of the spherical nanoparticles, more specifically, from 2 to 20 times.

[0222] Clause 19. A precursor material comprising a plurality of spherical nanoparticles composed of transition metal (II,III) oxides of formula (C). R t M''' 3-t O4 (C) The spherical nanoparticles are encapsulated within an open hollow carbon nanostructure, wherein: M''' is a transition metal selected from the group consisting of manganese, iron, cobalt, and nickel in divalent and trivalent oxidation states; R is a transition metal in a divalent oxidation state other than M'''; t is a value from 0 to less than 1; The amount of transition metal oxide of formula (C) is 20% to 60% by weight relative to the total material weight; and The volume of the spherical nanoparticles encapsulated within the hollow carbon nanostructure is equal to or less than 50% of the total cavity volume of the hollow carbon nanostructure.

[0223] Clause 20. The precursor material as described in Clause 19, wherein M''' is manganese.

[0224] Clause 21. The precursor material as described in Clause 19, wherein M''' is iron.

[0225] Clause 22. The precursor material as described in Clause 19, wherein M''' is cobalt.

[0226] Clause 23. The precursor material as described in Clause 19, wherein M''' is nickel.

[0227] Clause 24. A precursor material according to any one of Clauses 19-23, wherein the spherical nanoparticles have an average diameter of 2 nm to 20 nm.

[0228] Clause 25. The precursor material according to any one of Clauses 19-24, wherein the spherical nanoparticles have a diameter of 50 nm. 2 Up to 5000nm 2 Surface area.

[0229] Clause 26. A precursor material according to any one of Clauses 19-25, wherein the amount of hollow carbon nanostructures in the material is 40% to 80% by weight relative to the total material weight.

[0230] Clause 27. The precursor material according to any one of Clauses 19-26, wherein the hollow carbon nanostructure is selected from carbon nanotubes, graphitized carbon nanotubes, heteroatom-doped carbon nanotubes, carbon nanofibers, graphitized carbon nanofibers, and heteroatom-doped carbon nanofibers.

[0231] Clause 28. A precursor material according to any one of Clauses 19-27, wherein the hollow carbon nanostructure is a hollow tubular carbon nanostructure, particularly carbon nanotubes.

[0232] Clause 29. The precursor material according to Clause 28, wherein the average inner diameter of the hollow tubular carbon nanostructure is at least twice the average diameter of the spherical nanoparticles, and more specifically, 2 to 20 times.

[0233] Clause 30. A precursor material according to any one of Clauses 28 or 29, wherein the hollow tubular carbon nanostructure has an average inner diameter of 20 nm to 180 nm, or 30 nm to 180 nm, or 40 nm to 100 nm, an average outer diameter of 40 nm to 200 nm, or 60 nm to 140 nm, and an average length equal to or less than 10 μm.

[0234] Clause 31. The precursor material according to any one of Clauses 19-30, wherein the volume of the spherical nanoparticles encapsulated within the hollow carbon nanostructure is equal to or less than 10% relative to the total cavity volume of the hollow tubular carbon nanostructure.

[0235] Clause 32. A method for preparing a precursor material as defined in Clause 19, the method comprising the steps of: a) In the presence of a surfactant in a suitable solvent, a mixture comprising a transition metal (II) salt and optionally a second salt, the second salt being a salt of a transition metal having a divalent oxidation state other than a first transition metal, wherein the metal of the first transition metal (II) salt is selected from manganese, iron and cobalt, and the metal of the second salt is different from the metal of the first salt; b) Add the mixture from step a) to a suspension of open hollow carbon nanostructures in a suitable solvent; c) Heating the suspension to a temperature of 80°C to 110°C to obtain the precursor material of formula (C); Wherein, when step a) is carried out in the absence of another transition metal salt, the material of formula (C) is obtained, where t is 0; when step a) is carried out in the presence of a second salt of a transition metal, the precursor material of formula (C) is obtained, where t is not 0, and R is a transition metal in the divalent oxidation state other than M'''.

[0236] Clause 33. The method for preparing precursor materials according to Clause 32, wherein, after heating the suspension to a temperature of 80°C to 110°C in step c), step d) is performed by adding water to the suspension in step c) and heating it to a temperature of 80°C to 110°C to obtain the precursor material of formula (C).

[0237] Clause 34. Use of the material as an electrocatalyst as defined in any of Clauses 1-17.

[0238] Clause 35. The electrocatalyst is used as a cathode in a fuel cell or as an anode in water splitting, according to the uses described in Clause 34.

Claims

1. A material comprising a plurality of nanorods encapsulated within an open, hollow tubular carbon nanostructure, wherein, The plurality of nanorods are composed of any of the following composition: a) Transition metal oxides of formula (A) A z M’ 2y Mn 1-x O2 （A） Alternatively, b) Transition metal oxides of formula (B) M” 3m / n M 2-m O3 （B） in: In formula (A), A is a monovalent metal, and M' is a divalent transition metal other than Mn; x is equal to (z / 4+y), where 0≤y<1 and 0≤z<4; manganese can be in the (IV) oxidation state or a mixture of (IV) and (III), the latter of which has oxygen vacancies; In formula (B), M is a transition metal selected from iron, cobalt and nickel in the trivalent oxidation state; M'' is a transition metal with the same or different oxidation state n as M, where the value of n is 2 to 6, and 0 ≤ m < 2; The amount of the transition metal oxide of formula (A) or formula (B) is 20% to 60% by weight relative to the total material weight. The volume of the nanorods encapsulated within the hollow tubular carbon nanostructure is equal to or less than 50% of the total cavity volume of the hollow tubular carbon nanostructure; and The average inner diameter of the hollow tubular carbon nanostructure is at least twice the average thickness of the nanorod.

2. The material according to claim 1, wherein, In the transition metal oxide of formula (A), y and z are 0, and in the transition metal oxide of formula (B), m is 0.

3. The material according to claim 1, wherein, In the transition metal oxide of formula (A), at least one of y or z is not 0, and in the transition metal oxide of formula (B), m is not 0.

4. The material according to any one of claims 1-3, wherein, The nanorods have an average thickness of 5 nm to 20 nm and an average length of 16 nm to 180 nm.

5. The material according to any one of claims 1-4, wherein, The nanorods have a size equal to or greater than 280 nm. 2 Surface area.

6. The material according to any one of claims 1-5, wherein, The amount of the hollow tubular carbon nanostructures in the material is 40% to 80% by weight relative to the total material weight.

7. The material according to any one of claims 1-6, wherein, The hollow tubular carbon nanostructure is selected from carbon nanotubes, graphitized carbon nanotubes, heteroatom-doped carbon nanotubes, carbon nanofibers, graphitized carbon nanofibers, and heteroatom-doped carbon nanofibers.

8. The material according to any one of claims 1-7, wherein, The hollow tubular carbon nanostructure is a carbon nanotube.

9. The material according to claim 8, wherein, The hollow tubular carbon nanostructure has an average inner diameter of 20 nm to 180 nm, an average outer diameter of 40 nm to 200 nm, and an average length equal to or less than 10 μm.

10. The material according to any one of claims 1-9, wherein, The volume of the nanorods encapsulated within the hollow tubular carbon nanostructure is equal to or less than 10% of the total cavity volume of the hollow tubular carbon nanostructure.

11. An electrochemical method for preparing a material as defined in any one of claims 1-10, the method comprising the following steps: i) A modified working electrode is provided by coating a precursor material onto a conductive surface, the precursor material comprising a plurality of spherical nanoparticles composed of transition metal (II,III) oxides of formula (C). R t M''' 3-t O4 （C） The spherical nanoparticles are encapsulated within an open, hollow tubular carbon nanostructure, wherein: M''' is a transition metal selected from the group consisting of manganese, iron, cobalt, and nickel in divalent and trivalent oxidation states; R is a transition metal in a divalent oxidation state other than M'''; t is a value from 0 to less than 1; The amount of transition metal oxide in formula (C) is 20% to 60% by weight relative to the total material weight. The volume of the spherical nanoparticles encapsulated within the hollow tubular carbon nanostructure is equal to or less than 50% of the total cavity volume of the hollow tubular carbon nanostructure; and The average inner diameter of the hollow tubular carbon nanostructure is at least twice the average thickness of the nanorod; ii) Electrosynthesizing an aqueous solution containing an electrolyte using a counter electrode, a reference electrode, and the modified working electrode obtained in step i), wherein the electrolyte is selected from the group consisting of alkali metal or alkaline earth metal halide salts, or alkali metal or alkaline earth metal hydroxides: iia) Applying 5000 or fewer constant current cycles in the range of -0.2V to 0.8V at a current density equal to or greater than 0.5A per gram of precursor material, or alternatively, (iib) Apply 5000 or fewer potentiodynamic cycles in the range of 1V to 1.7V at a scan rate equal to or greater than 40mV / s. When M''' in the precursor material of formula (C) is manganese, a material containing multiple nanorods encapsulated within an open hollow carbon nanostructure is obtained, wherein the multiple nanorods are composed of transition metal oxides of formula (A); when M''' in the precursor material of formula (C) is a transition metal selected from the group consisting of iron, cobalt and nickel, a material containing multiple nanorods encapsulated within an open hollow carbon nanostructure is obtained, wherein the multiple nanorods are composed of transition metal oxides of formula (B).

12. A precursor material comprising a plurality of spherical nanoparticles composed of transition metal (II,III) oxides of formula (C), R t M''' 3-t O4 （C） The spherical nanoparticles are encapsulated within an open, hollow tubular carbon nanostructure, wherein: M''' is a transition metal selected from the group consisting of manganese, iron, cobalt, and nickel in divalent and trivalent oxidation states; R is a transition metal in a divalent oxidation state other than M'''; t is a value from 0 to less than 1; The amount of transition metal oxide in formula (C) is 20% to 60% by weight relative to the total material weight. The volume of the spherical nanoparticles encapsulated within the hollow tubular carbon nanostructure is equal to or less than 50% of the total cavity volume of the hollow tubular carbon nanostructure; and The average inner diameter of the hollow tubular carbon nanostructure is at least twice the average diameter of the spherical nanoparticles.

13. A method for preparing the precursor material as described in claim 12, the method comprising the following steps: a) In the presence of a surfactant in a suitable solvent, a mixture comprising a transition metal (II) salt and optionally a second salt, the second salt being a salt of a transition metal having a divalent oxidation state other than a first transition metal, wherein the metal of the first transition metal (II) salt is selected from manganese, iron and cobalt, and the metal of the second salt is different from the metal of the first salt; b) Add the mixture from step a) to a suspension of open hollow tubular carbon nanostructures in a suitable solvent; c) Heating the suspension to a temperature of 80 to 110°C; and d) Add water to the suspension from step c) and heat to a temperature of 80°C to 110°C to obtain the precursor material of formula (C); Wherein, when step a) is carried out in the absence of another transition metal salt, the material of formula (C) is obtained, where t is 0; when step a) is carried out in the presence of a second salt of a transition metal, the precursor material of formula (C) is obtained, where t is not 0, and R is a transition metal in the divalent oxidation state other than M'''.

14. Use of the material as defined in any one of claims 1-10 as an electrocatalyst.

15. The use according to claim 14, wherein the electrocatalyst is used as a cathode in a fuel cell or as an anode in water splitting.

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