Encapsulated transition metal oxide nanorods for durable air cathodes

Abstract

It relates to a material comprising a plurality of nanorods encapsulated within open-ended hollow carbon nanostructures, wherein the plurality of nanorods is composed of either a) a transition metal oxide of the formula AzM'2 yMn1 -xO2 (A), or alternatively, b) a transition metal oxide of the formula M''3m / nM2-mO3 (B), as defined herein, wherein the transition metal oxide of the formula (A) or formula (B) is in an amount from 20 to 60% by weight with respect to the total material weight; and the volume of the nanorods encapsulated within hollow carbon nanostructures is equal to or less than 50% with respect of the total cavity volume of the hollow carbon nanostructures, in particular, wherein the hollow carbon nanostructures are tubular and their internal average diameter is at least 2 times the average thickness of the nanorods. It also relates to a process for preparing this material, to a precursor material RtM'''3-tO4 (C) as defined herein from which the material is obtained, and to the use of the material as electrocatalyst in different applications.

Description

[0001] Encapsulated transition metal oxide nanorods for durable air cathodes

[0002] This application claims the priority of the European Patent Application EP23382707.0 filed on 11.07.2023.

[0003] Technical Field

[0004] The present disclosure relates to the field of nanomaterials. In particular, it relates to a hybrid material which comprises transition metal oxide nanorods, such as manganeseoxide nanorods, which are encapsulated within open-ended hollow carbon nanostructures. It also relates to a process for the preparation of the hybrid materials and to their use as electrocatalysts both in the fields of energy storage and energy conversion.

[0005] Background Art

[0006] Electrically rechargeable zinc-air batteries offer a promising alternative for the future of sustainable energy storage thanks to their high theoretical energy density, the use of abundant and environmentally friendly materials, as well as their safety. However, the economic potential and efficiency of these batteries is currently far from being satisfactory. The limitations in achieving high reversibility and thus a long lifetime are related to both the zinc anode and the bifunctional air electrodes (cathode). There are methods that considerably improve the reversibility of the zinc anode. However, the lack of approaches that address the cathode limitations severely hampers the practical applications and commercialization of secondary zinc-air batteries at present.

[0007] The rapid degradation of the cathode materials (e.g., loss of active surface area of catalysts) leads to a rapid reduction of (storage) capacity. On the other hand, the efficiency and power density of rechargeable zinc-air batteries are severely restricted by the slow kinetics of the Oxygen Reduction Reaction (ORR) and Oxygen Evolution Reaction (OER) occurring at the air electrode. While ORR is thermodynamically favorable, both ORR and OER are kinetically hindered. Therefore, materials that are not only durable, but also catalytically active to both reactions (bifunctional) are required.

[0008] In addition, the poor accessibility of the reactants (gas molecules and ions) to the catalytically active centers further reduces the activity of the electrocatalyst materials. The lack of large-scale, low-cost production methods for highly durable catalysts that allow precise porosity control to simultaneously ensure efficient mass transport and electron transfer pathways is currently hindering the scale-up of zinc-air batteries for applications. Materials based on platinum, palladium, or other precious metals and their oxides (e.g., iridium oxide) have been successfully used as electrocatalysts for either the ORR or the OER. Nevertheless, their limited availability and high cost have urged researchers to look for alternative electrocatalysts. Within this framework, especially transition metal oxides, such as manganese and iron oxides, have attracted significant interest because of their high specific capacitance, abundance, low cost, great versatility, and nontoxicity. Unfortunately, the applicability of these transition metal oxides is limited by their dissolution during the electrochemical processes that take place on the electrode surface. As mentioned above, this electrode degradation is the main technological obstacle in energy storage and conversion applications since the loss of active material implies a loss of activity and, therefore, of the efficiency of the processes involved.

[0009] In view of this, efforts have been made to develop nanomaterials based on transition metal oxides to maximize electrocatalytic activity, while improving the electrical conductivity and preserving accessible surface area. In the field of nanomaterial development, it has been shown that there is an intimate relationship between the structure (morphology) of a material and its properties. This is because certain crystallographic planes are more exposed in a certain morphology than in others. Despite these advances, the great effort to synthesize these active materials is often overshadowed by their still low stability and durability, losing most of their initial catalytic capacity when used. One of the strategies which has been used to decrease the dissolution of the metal oxides has been to provide a coating on the surface of the oxides. However, in these coated materials the increased stability is usually at the expense of their activity and, besides, the synthesis for preparing these coated materials is complex.

[0010] Therefore, there is still a need to develop active and stable materials which can be effectively used as electrocatalysts at industrial scale, for example in zinc-air batteries.

[0011] Summary of Invention

[0012] The inventors have developed a new hybrid material comprising transition metal oxide nanorods, in particular of manganese (IV) oxide, cobalt (III) oxide, nickel (III) oxide, and iron (III) oxide, which are encapsulated inside an open-ended hollow carbon nanostructure. The hybrid oxide materials can be optionally doped with other metals and are herein also designated as r-AzM’2yMni-xO2@CN and r-M”3m / nM2-mC>3@CN, wherein A is a monovalent metal, M, M’, and M” are transition metals as defined herein, and CN represents the open-ended carbon nanostructure. These materials can be used as electrode material in the context of electrochemical technologies used in energy conversion and storage, and more particularly in metal-air (e.g., Zn-air) batteries, where they can act as electrocatalyst in the bifunctional air electrodes, increasing the durability and reversibility during the charging and discharging processes.

[0013] The inventors have found that the morphology of these transition metal oxide particles, in particular manganese (IV) oxide, cobalt (III) oxide, nickel (III) oxide, and iron (III) oxide particles and their doped forms, in the form of nanorods results in very stable structures, not only in terms of their low aggregation tendency, which allows maintaining constant their active surface area, but also in terms of low solubility, which enables preserving the crystallographic planes active.

[0014] The one-dimensional morphology (i.e. , nanorods) provides a large contact area with the substrate, which facilitates its electrocatalytic capacity, for example in the water oxidation process which takes place in zinc-air batteries (FIG. 16), and better withstands structural changes related to ion intercalation or deintercalation (which means an increase in cyclability) compared to other morphologies. These two characteristics translate into an improvement in electrochemical parameters in terms of reversibility and the ability to withstand high charge and discharge currents.

[0015] Furthermore, the confinement of the transition metal oxide nanorods, particularly manganese oxide (IV), cobalt (III) oxide, nickel (III) oxide, and iron (III) oxide nanorods and their doped forms, within the open-ended hollow carbon nanostructures, which act as nanocontainers, improves the intrinsically low conductivity of the metal oxide since a strong interaction is created between the nanoparticles and the carbon surface.

[0016] Without being bound to theory, it is believed that as a result of the nanorod encapsulation within a confined space, the diffusion of Mn2+, Co2+, Ni2+, or Fe2+into the electrolyte is limited and their recapture by deposition as an oxide (e.g., MnC>2 when the transition metal is manganese) is favored when positive potentials are reached. Thus, because of the encapsulation of the transition metal oxide nanorods inside the open-ended hollow carbon nanostructures, the loss of active material at the electrode by dissolution, which is a direct cause of the low stability of the metal oxide, is minimized. This is illustrated in the examples for the precursor s-Mn3C>4@CNF, see FIG. 12a-b. By contrast, an analog material comprising nanoparticles (s-Mn3O4@CNF) deposited on the outside of the nanofibers was found to suffer from low stability (FIG. 13).

[0017] Unlike in the case of coated materials in which the increased stability is usually at the expense of their activity, for the hybrid material of the present invention, the dissolution of the material is not inhibited, but only the diffusion of Mn2+, Co2+, Ni2+, or Fe2+cations into the electrolyte is reduced. In the present invention, as illustrated in the examples, the encapsulation does not inhibit the intrinsic reactivity of the nanoparticles (FIG. 14), i.e., dissolution processes continue to take place.

[0018] The material of the invention can be conveniently obtained by electrosynthesis from a precursor material comprising spherical nanoparticles composed of a transition metal (II, III) oxide, particularly manganese (II, III) oxide, already encapsulated within open-ended hollow carbon nanostructures, herein also designated as s-RtM’”3-tO4@CN, wherein R and M’” are transition metals as defined herein; and CN represents the open-ended carbon nanostructure.

[0019] Electrochemical (or electrosynthesis) processes have proven to be quite versatile and an alternative to hydrothermal processes that require large amounts of solvent, high temperatures or pressures. The inventors have found that by controlling the electrochemical parameters, such as the sweep potential, current density or the composition of the electrolyte, it is possible to obtain the desired morphology of the material in the form of nanorods (FIG. 2).

[0020] The precursor material (s-RtM’”3-tO4@CN), in turn, may be synthesized by a very simple and fast process, since it includes a single step and may be typically performed in about 1-3 hours of reaction. These features facilitate the implementation of the process on an industrial scale. Furthermore, the process is cost-effective since non-expensive metals such as manganese or iron salts can be exclusively used, and the reaction temperature is rather low (typically around 90 °C), which allows using low melting point organic solvents (such as e.g., xylene) as opposed to other more common solvents in the synthesis of metal oxide nanoparticles such as diphenyl ether, 1 -octadecene, or trioctadecene, which have higher melting points (i.e., 260 °C, 317 °C and 365 °C, respectively).

[0021] Additionally, the process for preparing the materials of the invention is very versatile as it allows the incorporation of other transition metals into the crystalline structure of a first transition metal oxide, particularly manganese oxide, cobalt oxide, nickel oxide, and iron oxide, typically in low amounts, replacing Mn or M atoms by other cations and give rise to doped materials. As illustrated in the examples below by simply replacing part of the starting manganese salt used to prepare the precursor encapsulated spherical nanoparticles by other transition metal salts, such as for example cobalt, nickel, or iron salts, it is possible to obtain doped materials (FIG. 3-10), with different applicability depending on their reactivity (FIG. 11). The formation of doped nanorods inside the nanofibers after electrosynthesis (FIG. 18-20) results in materials with high stability and activity (FIG. 17), making it possible to generate a family of one-dimensional multi-metal nanostructures inside carbon nanofibers with applicability in various fields.

[0022] Therefore, a first aspect of the invention relates to a material comprising a plurality of nanorods encapsulated within open-ended hollow carbon nanostructures, wherein the plurality of nanorods is composed of either a) a transition metal oxide of the formula (A)

[0023] AzM’2yMni-xO2

[0024] (A) or alternatively, b) a transition metal oxide of the formula (B)

[0025] M”3m / nM2-mO3

[0026] (B) wherein: in the formula (A), A is a monovalent metal, M’ is a transition metal other than Mn in a divalent oxidation state; x is equal to (z / 4 + y) with 0 <y<1 and 0 <z<4; and manganese can adopt either an oxidation state of (IV) or a mixture of (IV) and (III), with the latter case exhibiting oxygen vacancies; in the formula (B), M is a transition metal in a trivalent oxidation state selected from iron, cobalt and nickel; M” is a transition metal equal to or different from M in an oxidation state n, wherein n is value from 2 to 6, being 0 <m<2; the transition metal oxide of the formula (A) or formula (B) is in an amount from 20 to 60% by weight with respect to the total material weight; and the volume of the nanorods encapsulated within hollow carbon nanostructures is equal to or less than 50% with respect of the total cavity volume of the hollow carbon nanostructures. In particular, the hollow carbon nanostructures are tubular and their internal average diameter is at least 2 times the average thickness of the nanorods.

[0027] A second aspect of the invention relates to an electrochemical process for preparing the material comprising a plurality of nanorods composed of a transition metal oxide as defined herein, which comprises the steps of: i) providing a modified working electrode by coating a conductive surface with a precursor material comprising a plurality of spherical nanoparticles composed of a transition metal (II, 11 l)-oxide of the formula (C)

[0028] RtM”’3-tO4

[0029] (C) which is encapsulated within open-ended hollow carbon nanostructures, wherein:

[0030] M’” is a transition metal selected from the group consisting of manganese, iron, cobalt, and nickel in a divalent and trivalent oxidation state;

[0031] R is a transition metal other than M’” in a divalent oxidation state; t is a value from 0 to less than 1 ; the transition metal oxide of the formula (C) is in an amount from 20 to 60% by weight with respect to the total material weight; and the volume of the spherical nanoparticles encapsulated within hollow carbon nanostructures is equal to or less than 50% with respect of the total cavity volume of the hollow carbon nanostructures; in particular, wherein the hollow carbon nanostructures are tubular and their internal average diameter is at least 2 times the average thickness of the nanorods; ii) carrying out electrosynthesis of an aqueous solution comprising an electrolyte selected from the group consisting of a halide salt of an alkali or alkaline-earth metal, or an alkali or alkaline-earth metal hydroxide, using a counter electrode, a reference electrode, and the modified working electrode obtained in step i) by: iia) applying 5000 galvanostatic cycles or less in the range from -0.2 to 0.8 V at a current density equal to or higher than 0.5 A per g of precursor material or alternatively, iib) applying 5000 potentiodynamic cycles or less in the range from 1 to 1.7 V at a scan rate equal to or higher than 40 mV / s.

[0032] In the above process when in the precursor material of formula (C) M’” is manganese, a material comprising a plurality of nanorods encapsulated within open-ended hollow carbon nanostructures is obtained, wherein the plurality of nanorods is composed of a transition metal oxide of the formula (A); and when in the precursor material of formula (C) M’” is a transition metal selected from the group consisting of iron, cobalt, and nickel, a material comprising a plurality of nanorods encapsulated within open-ended hollow carbon nanostructures is obtained, wherein the plurality of nanorods is composed of a transition metal oxide of the formula (B).

[0033] A third aspect of the invention relates to a precursor material comprising a plurality of spherical nanoparticles composed of a transition metal (II, 11 l)-oxide of the formula (C) RtM”’3-tO4

[0034] (C) which is encapsulated within open-ended hollow carbon nanostructures, wherein: M’” is a transition metal selected from the group consisting of manganese, iron, cobalt, and nickel in a divalent and trivalent oxidation state;

[0035] R is a transition metal other than M’” in a divalent oxidation state; t is a value from 0 to less than 1 ; the transition metal oxide of the formula (C) is in an amount from 20 to 60% by weight with respect to the total material weight; and the volume of the spherical nanoparticles encapsulated within hollow carbon nanostructures is equal to or less than 50% with respect of the total cavity volume of the hollow carbon nanostructures. In particular, the hollow carbon nanostructures are tubular and their internal average diameter is at least 2 times the average diameter of the spherical nanoparticles.

[0036] A fourth aspect of the invention relates to a process for preparing the precursor material as defined herein, which comprises the steps of: a) providing a mixture of a transition metal (II) salt, and optionally a second salt of a transition metal other than the first a transition metal with a divalent oxidation state, 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 other than the metal of the first salt, in the presence of a surfactant in a suitable solvent; b) adding the mixture of step a) to a suspension of open-ended hollow carbon nanostructures in a suitable solvent; c) heating the suspension to a temperature from 80 to 110 °C; and d) adding water to the suspension of step c) and heating to a temperature from 80 to 110 °C to obtain a precursor material of formula (C); wherein when step a) is carried out in the absence of another salt of a transition metal, a material of formula (C) is obtained wherein t is 0; and when step a) is carried out in the presence of a second salt of a transition metal, a precursor material of formula (C) is obtained wherein t is different from 0, and R is a transition metal other than M’” in a divalent oxidation state.

[0037] A fifth aspect of the invention relates to the use of the material comprising a plurality of nanorods as defined above as electrocatalyst.

[0038] Brief Description of Drawings

[0039] FIG. 1 shows a bright field Transmission Electron Microscopy (TEM) image (a), a dark field scanning mode (STEM) image (b), Energy Dispersive X-ray (EDX) Spectroscopy elemental mapping images for oxygen (c), carbon (d) and manganese (e) for s- Mn3C>4@CNF. Scale bar: (a) 100 nm and (b-e) 20 nm. FIG. 2 shows a bright field TEM image (a), a dark field STEM image (b), EDX elemental mapping images for carbon (c), oxygen (d), and manganese (e) and an EDX spectrum (f) (C: counts, E: energy, presence of Cu is due to the use of a Cu-made TEM grid) of manganese oxide nanorods within carbon nanofibers (r-MnC>2@CNF). Scale bar for all images is 100 nm. Inset in (a): an example of a rod with crystalline planes (interplanar spacing distance (d= 0.34 nm)) associated to MnC>2 crystal structure (pyrolusite-type).

[0040] FIG. 3 shows the Raman spectrum of (a) undoped manganese oxide nanoparticles in carbon nanofibers (s-Mn3O4@CNF), (b) cobalt doped manganese oxide nanoparticles in carbon nanofibers (s-Co / Mn3O4@CNF), (c) nickel doped manganese oxide nanoparticles in carbon nanofibers (s-Ni / Mn3O4@CNF), (d) iron doped manganese oxide nanoparticles in carbon nanofibers (s-Fe / Mn3O4@CNF), (e) undoped cobalt oxide nanoparticles in carbon nanofibers (s-Co3O4@CNF), (f) undoped iron oxide nanoparticles in carbon nanofibers (s-Fe3O4@CNF). C: counts, RS: Raman Shift.

[0041] FIG. 4 shows the X-ray powder diffractogram of (a) undoped manganese oxide nanoparticles in carbon nanofibers (s-Mn3O4@CNF), (b) cobalt doped manganese oxide nanoparticles in carbon nanofibers (s-Co / Mn3O4@CNF), (c) nickel doped manganese oxide nanoparticles in carbon nanofibers (s-Ni / Mn3O4@CNF), (d) iron doped manganese oxide nanoparticles in carbon nanofibers (s-Fe / Mn3O4@CNF), (e) undoped cobalt oxide nanoparticles in carbon nanofibers (s-Co3O4@CNF), (f) undoped iron oxide nanoparticles in carbon nanofibers (s-Fe3O4@CNF). I: intensity.

[0042] FIG. 5 shows the thermogravimetric analysis (TGA) measurements under air of (a) undoped manganese oxide nanoparticles in carbon nanofibers (s-Mn3O4@CNF), (b) cobalt doped manganese oxide nanoparticles in carbon nanofibers (s-Co / Mn3O4@CNF), (c) nickel doped manganese oxide nanoparticles in carbon nanofibers (s- Ni / Mn3O4@CNF), (d) iron doped manganese oxide nanoparticles in carbon nanofibers (s- Fe / Mn3O4@CNF), (e) undoped cobalt oxide nanoparticles in carbon nanofibers (s- CO3O4@CNF), (f) undoped iron oxide nanoparticles in carbon nanofibers (s-Fe3O4@CNF). w: weight, T: temperature, L: percentage of metal oxide remaining after annealing at 1000 °C.

[0043] FIG. 6 shows particle size distribution histogram of (a) undoped manganese oxide nanoparticles in carbon nanofibers (s-Mn3O4@CNF), (b) cobalt doped manganese oxide nanoparticles in carbon nanofibers (s-Co / Mn3O4@CNF), (c) nickel doped manganese oxide nanoparticles in carbon nanofibers (s-Ni / Mn3O4@CNF), (d) iron doped manganese oxide nanoparticles in carbon nanofibers (s-Fe / Mn3O4@CNF), (e) undoped cobalt oxide nanoparticles in carbon nanofibers (s-Co3C>4@CNF), (f) undoped iron oxide nanoparticles in carbon nanofibers (s-Fe3C>4@CNF). C: counts, D: nanoparticle diameter.

[0044] FIG. 7 shows a bright field TEM image (a), dark field EDX-STEM elemental mapping images for manganese (b), carbon (c), oxygen (d), and cobalt (e) and an EDX spectrum (f) (C: counts, E: energy, presence of Cu is due to the use of a Cu made TEM grid) of cobalt doped manganese oxide nanoparticles within carbon nanofibers (s- Co / Mn3C>4@CNF). Scale bar for all the images is 100 nm.

[0045] FIG. 8 shows a bright field TEM image (a), dark field EDX-STEM elemental mapping images for manganese (b), carbon (c), oxygen (d), and nickel (e) and an EDX spectrum (f) (C: counts, E: energy, presence of Cu is due to the use of a Cu made TEM grid) analysis of nickel doped manganese oxide nanoparticles within carbon nanofibers (s- Ni / Mn3O4@CNF). Scale bar for all the images is 100 nm.

[0046] FIG. 9 shows a bright field TEM image (a), dark field EDX-STEM elemental mapping images for manganese (b), carbon (c), oxygen (d), and iron (e) and an EDX spectrum (f) (C: counts, E: energy, presence of Cu is due to the use of a Cu made TEM grid) analysis of iron doped manganese oxide nanoparticles within carbon nanofibers (s- Fe / Mn3O4@CNF). Scale bar for all the images is 50 nm.

[0047] FIG. 10 shows the cyclic voltammetry of manganese oxide nanoparticles in carbon nanofibers (a) undoped (s-Mn3O4@CNF) and doped with (b) cobalt (s-Co / Mn3O4@CNF), (c) nickel (s-Ni / Mn3O4@CNF) and (d) iron (s-Fe / Mn3O4@CNF) in a nitrogen-saturated 1 M KOH solution at a scan rate of 50 mV / s. C: current density, P: potential.

[0048] FIG. 11 shows the linear scanning voltammetry of the oxygen evolution reaction (OER, from 1 to 1.8V) and the oxygen reduction reaction (ORR, from 1V to 0.3V) for manganese oxide nanoparticles in carbon nanofibers without doping (s-Mn3O4@CNF) and doped with cobalt (s-Co / Mn3O4@CNF), nickel (s-Ni / Mn3O4@CNF) and iron (s-Fe / Mn3O4@CNF) in O2- saturated 1 M KOH solution at a scan rate of 10 mV / s. C: current density, P: potential (RHE: reversible hydrogen electrode).

[0049] FIG. 12 shows bright field TEM images of unwashed carbon nanofibers (s-Mn3O4@CNF) containing manganese oxide nanoparticles on the nanocontainer exteriors (a) before and (b) after 500 cycles (between -0.2 and 0.8V at scan rate 50 mV / s) in a nitrogen-saturated 2M KOI solution. Scale bars are 100 nm. FIG. 13 shows the cyclic voltammetry (n = number of cycles) of unwashed carbon nanofibers (s-Mn3C>4@CNF) containing manganese oxide nanoparticles on the nanocontainer in a nitrogen-saturated 2M KCI solution. C: current density, P: potential.

[0050] FIG. 14 shows a bright field (a) and a dark field (b) TEM image, dark field EDX-STEM elemental mapping images for carbon (c), oxygen (d) and manganese (e) and an EDX spectrum (f) (C: counts, E: energy, presence of Cu is due to the use of a Cu made TEM grid) analysis of manganese oxide nanoparticles in carbon nanofibers (s-Mn3O4@CNF) after several charge and discharge cycles (1500) at 1 A / g in a nitrogen-saturated 2M KCI solution. Scale bar for all the images is 50 nm.

[0051] FIG. 15 shows (a) galvanostatic cycles of charge and discharge at 1 A / g of manganese oxide nanoparticles in carbon nanofibers before (s-Mn3O4@CNF, solid dotted line) and after (r-MnO2@CNF, dashed line) electrosynthesis consisting of 2500 cycles in a nitrogen- saturated 2M KCI solution. The solid black line is carbon nanofibers without manganese oxide nanoparticles, (b) Change in the capacitance retention (%) with respect to the number of cycles. The large (more than 4-fold) enhancement of capacitance retention is related to the formation of nanorods within carbon nanofibers. P: Potential, T: time, CR: capacity retention, C: number of cycles

[0052] FIG. 16 shows (a) the linear scanning voltammetry and (b) Tafel plot of manganese oxide nanoparticles on carbon nanofibers in an oxygen-saturated 1 M KOH solution before (s- Mn3O4@CNF, solid line) and after (r-MnO2@CNF, dashed line) electrosynthesis (2500 cycles in 2M KCI). C: current density, P: potential.

[0053] FIG. 17 shows the linear scanning voltammetry of manganese oxide nanoparticles on carbon nanofibers doped with (a) cobalt, (b) nickel, and (c) iron in an oxygen-saturated 1 M KOH solution before (solid line) and after (dashed line) electrosynthesis (2500 cycles in 2M KCI or 1M KOH). C: current density, P: potential.

[0054] FIG. 18 shows a bright field TEM image (a), dark field EDX-STEM elemental mapping images for manganese (b), carbon (c), oxygen (d), and cobalt (e) and an EDX spectrum (f) (C: counts, E: energy, presence of Cu is due to the use of a Cu made TEM grid) analysis of cobalt doped manganese oxide nanorods within carbon nanofibers (r- KCo / MnO2@CNF). Inset in (a): an example of a rod with crystalline planes (interplanar spacing distance (d= 0.49 nm) associated to MnO2 crystal structure (hollandite-type, inset) with a formula AzM’2yMni-xO2. Scale bar for all the images is 50 nm. FIG. 19 shows a bright field TEM image (a), dark field EDX-STEM elemental mapping images for manganese (b), carbon (c), oxygen (d), and nickel (e) and an EDX spectrum (f) (C: counts, E: energy, presence of Cu is due to the use of a Cu made TEM grid) analysis of nickel doped manganese oxide nanorods within carbon nanofibers (r-KNi / MnC>2@CNF). Inset in (a): an example of a rod with crystalline planes (interplanar spacing distance d= 0.24 nm) associated to MnO2crystal structure (pyrolusite-type, inset) with a formula M’2yMni.xO2. Scale bar for all the images is 50 nm.

[0055] FIG. 20 shows a bright field TEM image (a), dark field EDX-STEM elemental mapping images for manganese (b), carbon (c), oxygen (d), and iron (e) and an EDX spectrum (f) (C: counts, E: energy, presence of Cu is due to the use of a Cu made TEM grid) analysis of iron doped manganese oxide nanorods within carbon nanofibers (r-KFe / MnO2@CNF). Inset in (a): an example of an interplanar spacing distance (d= 0.49nm) associated to MnO2crystal structure with potassium ions intercalated (hollandite-type, inset) with a formula AzM’2yMni-xO2. Scale bar for all the images is 50 nm.

[0056] FIG. 21 shows the variation of the amount of cobalt doped Mn2O4 nanoparticles within CNF (s-Co / Mn3O4@CNF) when 5%, 10%, 15% and 20% mmol of manganese (II) acetate was replaced with cobalt (II) acetate.

[0057] FIG. 22 shows a bright field (a) and a dark field (b) TEM image, dark field EDX-STEM elemental mapping images for carbon (c), oxygen (d) and iron (e) and an EDX spectrum (f) (C: counts, E: energy, presence of Cu is due to the use of a Cu made TEM grid) analysis of iron oxide nanorods within carbon nanofibers (r-Fe2O3@CNF). Scale bar for all the images is 50 nm.

[0058] Detailed description of the invention

[0059] All terms as used herein in this application, unless otherwise stated, shall be understood in their ordinary meaning as known in the art. Other more specific definitions for certain terms as used in the present application are as set forth below and are intended to apply uniformly through-out the specification and claims unless an otherwise expressly set out definition provides a broader definition.

[0060] The term "about" or “around” as used herein refers to a range of values ± 10% of a specified value. For example, the expression "about 10" or “around 10” includes ± 10% of 10, i.e., from 9 to 11.

[0061] For the purposes of the present invention, any ranges given include both the lower and the upper endpoints of the range. Ranges given, such as weight, temperature, time, and the like, should be considered approximate, unless specifically stated.

[0062] The term “material” as used herein refers to a hybrid material which is the result of a combination of at least two different materials: nanoparticles of a transition metal oxide of formulas (A), (B) or (C), and open-ended hollow carbon nanostructures which encapsulate them.

[0063] The term “precursor material” refers to the spherical nanoparticles composed of a transition metal (II, III) oxide, particularly manganese (II, III) oxide, iron (II, III) oxide or cobalt (II, III) oxide, encapsulated within open-ended hollow carbon nanostructures (s- RtM”’3-tO4@CN). The abbreviation CN as used herein refers to an open-ended hollow carbon structure.

[0064] The term “nanoparticles” as used herein may generally refer both to the nanorods and the nanospheres as defined herein unless otherwise stated.

[0065] The term “plurality”, as used herein, refers to a population of nanoparticles, i.e. , a collection of two or more nanoparticles.

[0066] The term "encapsulated” or “confined” are used interchangeably and mean that the nanoparticles are placed inside the open-ended hollow carbon nanostructures. Thus, the nanoparticles are not placed on the outside surface of the hollow carbon nanostructures.

[0067] The term “room temperature” refers to a temperature of the environment, without heating or cooling, and it is generally comprised from 20 to 25 °C.

[0068] The expression "obtainable by” is used herein for defining the material or the precursor material of the invention by their preparation process and refer to the product that can be obtained through the preparation process disclosed herein. For the purposes of the invention, the expressions “obtainable”, “obtained” and similar equivalent expressions are used interchangeably and, in any case, the expression “obtainable” encompasses the expression “obtained”.

[0069] As mentioned above, a first aspect of the invention relates to a material comprising a plurality of nanorods encapsulated within open-ended hollow carbon nanostructures, wherein the plurality of nanorods is composed of either a) a transition metal oxide of the formula (A) AzM’2yMni-xO2

[0070] (A) or alternatively, b) a transition metal oxide of the formula (B)

[0071] M”3m / nM2-mO3

[0072] (B) wherein: in the formula (A), A is a monovalent metal, M’ is a transition metal other than Mn in a divalent oxidation state; x is equal to (z / 4 + y) with 0 <y<1 and 0 <z<4; and manganese can adopt either an oxidation state of (IV) or a mixture of (IV) and (III), with the latter case exhibiting oxygen vacancies; in the formula (B), M is a transition metal in a trivalent oxidation state selected from iron, cobalt and nickel; M” is a transition metal equal to or different from M in an oxidation state n, wherein n is value from 2 to 6, being 0 <m<2; the transition metal oxide of the formula (A) or formula (B) is in an amount from 20 to 60% by weight with respect to the total material weight; and the volume of the nanorods encapsulated within hollow carbon nanostructures is equal to or less than 50% with respect of the total cavity volume of the hollow carbon nanostructures. In particular, the hollow carbon nanostructures are tubular and their internal average diameter is at least 2 times the average thickness of the nanorods.

[0073] In one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the invention relates to a material comprising a plurality of nanorods encapsulated within open-ended hollow carbon nanostructures, wherein the plurality of nanorods is composed of a transition metal (IV) oxide of the formula AzM’2yMni-xO2 (A).

[0074] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the invention relates to a material comprising a plurality of nanorods encapsulated within open-ended hollow carbon nanostructures, wherein the plurality of nanorods is composed of a transition metal (III) oxide of the formula M”3m / nM2-mC>3 (B). In a more particular embodiment, the transition metal (III) oxide of the formula (B), M is iron. In another more particular embodiment, the transition metal (III) oxide of the formula (B), M is cobalt. In another more particular embodiment, the transition metal (III) oxide of the formula (B), M is nickel. According to another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the transition metal oxide is present in the material in an amount from 20 to 60%, more particularly from 25 to 55%, and even more particularly from 30 to 40% by weight with respect to the total material weight. The amount of transition metal oxide of the formula (A) or formula (B) can be determined by thermogravimetric analysis (TGA).

[0075] The transition metal oxide of formulas (A), (B) or (C) may be undoped or doped. For the purposes of the invention, the term “undoped” means that in the transition metal oxide no other metals are comprised as dopants. The term “doped” as used herein, refers to a metal oxide which comprises impurities (dopants) from different metals and eventually shows modified properties.

[0076] Non-limiting examples of transition metals than can be used in the present invention include manganese, iron, cobalt, nickel, cooper, chromium, molybdenum, titanium, vanadium, zinc and the like.

[0077] Non-limiting examples of alkali metals than can be used in the present invention include sodium, potassium, cesium, and the like.

[0078] According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the transition metal oxide of formula (A) is undoped, i.e., y and z are 0. In such case, the transition metal oxide of formula (A) has the formula MnC>2.

[0079] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the transition metal oxide is doped, i.e., in the transition metal (IV) oxide of the formula (A) at least one of y or z are different from 0, and therefore x is different from 0. More particularly, 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, cooper, 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 one more particular embodiment, y is 0 and z is different from 0. In another more particular embodiment, y is different from 0 and z is 0. In another more particular embodiment, y and z are independently different from 0. When z is 0 pyrolusite-type structures are obtained (e.g., see FIG. 1 and FIG. 19) while when z is different from 0 hollandite-type structures are obtained (e.g., see FIG. 18 and FIG. 20).

[0080] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the transition metal oxide is doped, i.e., in the transition metal (IV) oxide of the formula (A), x is a value higher than 0 and lower than 1. In a more particular embodiment, x is from 0.01 to 0.4, even more particularly x is from 0.05 to 0.09. Even more particularly, 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.

[0081] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the invention relates to a material comprising a plurality of nanorods encapsulated within open-ended hollow carbon nanostructures, wherein the plurality of nanorods is composed of a mixture of one or more transition metal oxide of the formula (A). More particularly, the plurality of nanorods is composed of a mixture of a metal oxide of the formula (A1)

[0082] M’2yMni.x’O2

[0083] (A1), and a metal oxide of the formula (A2),

[0084] AZ’M’2yMni-x O2

[0085] (A2) wherein M’ is a transition metal other than Mn in a divalent oxidation state; x’ is equal to y and x” = (z’ / 4 + y) with 0 <y<1 and 0 <z’<4, A is a monovalent metal; and manganese can adopt either an oxidation state of (IV) or a mixture of (IV) and (III), with the latter case exhibiting oxygen vacancies. The metal oxide of the formula (A1) corresponds to the metal oxide of the formula (A) when z is 0, and the metal oxide of the formula (A2) corresponds to the metal oxide of the formula (A) when z is different from 0.

[0086] In a more particular embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the weight ratio of the metal oxide of the formula (A1) to the metal oxide of the formula (A2) when y = 0 is from 100:0 to 50:50, more particularly from 80:20 to 60:40.

[0087] In another more particular embodiment, the weight ratio of the metal oxide of the formula (A1) to the metal oxide of the formula (A2) when y = 0 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. In a more particular embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the weight ratio of the metal oxide of the formula (A1) to the metal oxide of the formula (A2) when y is different from 0 is from 10:90 to 50:50, more particularly from 20:80 to 40:60. In another more particular embodiment, the weight ratio of the metal oxide of the formula (A1) to the metal oxide of the formula (A2) when y is different from 0 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, about 50:50.

[0088] According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the transition metal (III) oxide of formula (B) is undoped, i.e., m is 0.

[0089] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the transition metal (III) oxide is doped, i.e., in the transition metal (III) oxide of the formula (B) m is a value higher than 0 and lower than 2, and M” is a transition metal equal or other than M with a oxidation state from 2 to 6. 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, chromium, and molybdenum.

[0090] In a more particular embodiment, m is from 0.01 to 0.4, even more particularly m is from 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.

[0091] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, in the transition metal oxide of the formula (B) M” is a transition metal other than M.

[0092] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, in the transition metal oxide of the formula (B) M” is the same transition metal as M.

[0093] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, in the transition metal oxide of the formula (B) n is a value from 2 to 6, more particularly n is selected from the group metal with oxidation state consisting of 2, 3, 4, 5, and 6. The transition metal (IV) oxide of the formula (A) or the transition metal (III) oxide of formula (B) is in the form of nanorods. For the purposes of the invention, the term “nanorod” or “nanowire” are used interchangeably and refer to a nanoparticle having a narrow dimension (i.e., thickness) and a long dimension (i.e., length), where the ratio between the long dimension to the narrow dimension (i.e., the aspect ratio) is equal or higher than 2. The narrow dimension or diameter corresponds to the shortest dimension or cross-sectional thickness of the nanorod, and it is essentially constant along the length for a given nanorod. The long dimension or length corresponds to the longest dimension of the nanorod that is generally orthogonal to the diameter of the nanorod. The diameter and the length of the nanorods can be measured by methods well-known in the art such as transmission electron microscopy (TEM). Typically, average thickness values 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.

[0094] According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the nanorods have an average thickness of at least 5 nm, more particularly from 5 to 20 nm, even more particularly from 8 to 16 nm, even more particularly 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.

[0095] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the nanorods have an average length of at least 16 nm, more particularly from 16 to 180 nm, more particularly from 50 to 150 nm.

[0096] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the nanorods show an aspect ratio from 2 to 15, more particularly from 3 to 10.

[0097] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the nanorods have a surface area equal to or higher than 280 nm2, more particularly, equal or lower than 14500 nm2. The material of the invention further comprises open-ended 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 of the hollow, particularly the two ends, are open. In this way, the electrolyte liquid can easily flow inside the carbon nanostructures.

[0098] According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the hollow carbon nanostructure is a hollow tubular carbon nanostructure. As used herein, the term "hollow tubular carbon nanostructure” refers to a carbon material in the form of a tube which has a continuous closed wall along its length, wherein the tube is open on two opposite ends. Particularly, the hollow tubular carbon nanostructures referred to in this invention have a circular cross section.

[0099] In one particular embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the amount of open-ended hollow carbon nanostructures in the material is from 40 to 80%, more particularly from 45 to 75%, and even more particularly from 60 to 70%, by weight with respect to the total material weight.

[0100] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the open-ended hollow carbon nanostructures are selected from carbon nanotubes, graphitized carbon nanotubes, heteroatom-doped carbon nanotubes, carbon nanofibers, graphitized carbon nanofibers, and heteroatom-doped carbon nanofibers, more particularly, the hollow carbon nanostructures are hollow tubular carbon nanostructures selected from carbon nanotubes, and carbon nanofibers, even more particularly are carbon nanofibers.

[0101] For the purposes of the invention, the term "heteroatom-doped" or "heteroatom-doping" refers to the intercalation and / or incorporation of heteroatoms into the carbon nanostructure. Heteroatoms include, but are not limited to, nitrogen, sulfur, phosphorous, boron, silicon and aluminum.

[0102] The term “open-ended hollow carbon nanotube”, also referred herein simply to as CNT, refers to a hollow material which consists of graphene sheets rolled into cylindrical shapes, wherein graphene layers form more than one concentric cylinders along the tube axis.

[0103] The term “hollow carbon nanofiber”, also referred herein simply to as CNF, consists of graphene sheets which are tilted at a certain angle with respect to the main axes and thus create a stack of nanocones. CNF differs from carbon nanotubes by the fact that in the CNTs this angle is equal to zero.

[0104] CNTs and CNFs may be prepared by methods well-known in the art, for example by chemical vapor deposition (CVD) from hydrocarbons. They are also commercially available. Graphitized carbon nanofibers may be prepared for example from carbon nanofibers by heating under appropriate conditions, such as e.g., heating at about 1000 °C under inert (argon) atmosphere.

[0105] The diameter and the length of the CNTs and CNFs can be measured by methods well- known in the art such as transmission electron microscopy (TEM). Typically, average internal and external diameter values and average length values are given. The average internal diameter is lower than average external diameter. The term “average diameter” refers to the average value taken from at least three different diameter measurements. The term “average length” refers to the average value taken from at least three different length measurements.

[0106] In one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the hollow tubular carbon nanostructure has an internal average diameter from 20 to 180 nm or 30 to 180 nm, or 40 to 100 nm, more particularly from 60 to 80 nm, and even more particularly about 70 ± 20 nm.

[0107] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the hollow tubular carbon nanostructure has an external average diameter from 40 to 200 nm or 60 to 140 nm, more particularly from 80 to 130 nm, and even more particularly about 110 nm ± 30 nm.

[0108] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the hollow tubular carbon nanostructure has a length equal to or lower than 10 pm, more particularly from 1 to 10 pm, and even more particularly 1-2 pm.

[0109] In one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the hollow tubular carbon nanostructure has i) an internal average diameter from 20 to 180 nm or 30 to 180 nm, or 40 to 100 nm, more particularly from 60 to 80 nm, and even more particularly about 70 ± 20 nm: ii) an external average diameter from 40 to 200 nm, more particularly from 80 to 130 nm, or 60 to 140 nm, and even more particularly about 110 nm ± 30 nm, with the condition that the average internal diameter is lower than average external diameter; and iii) a length equal to or lower than 10 pm, more particularly from 1 to 10 pm, and even more particularly 1-2 pm.

[0110] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the invention relates to a material consisting of a plurality of nanorods composed of a transition metal oxide which is encapsulated within open-ended hollow carbon nanostructures as defined herein.

[0111] As mentioned above, in the material of the invention the plurality of the nanorods composed of a transition metal oxide is encapsulated within open-ended hollow carbon nanostructures. Typically, the average thickness of the nanorods is much smaller than the inner space of the hollow carbon nanostructures, i.e. , the nanorods do not occupy the entire internal carbon nanostructure cavity. In particular when the hollow carbon nanostructures are hollow tubular carbon nanostructures the average thickness of the nanorods is much smaller is much smaller than the inner diameter.

[0112] In the material of the invention, the internal cavity of the hollow carbon nanostructures is only partially filled with the plurality of nanorods composed of a transition metal (IV) oxide. In particular, the volume of the nanorods encapsulated within hollow carbon nanostructures is equal to or less than 50% with respect of the total cavity volume of the hollow carbon nanostructures.

[0113] In one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the volume of the nanorods encapsulated within hollow carbon nanostructures 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%, with respect of the total cavity volume of the hollow carbon nanostructures.

[0114] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the volume of the nanorods encapsulated within hollow carbon nanostructures is equal to or less than 5% with respect of the total cavity volume of the hollow carbon nanostructures for rods with a diameter in the range of from 8 to 14 nm confined in a hollow tubular carbon nanostructures with an internal diameter about 70 nm.

[0115] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the plurality of nanorods do not block the internal space of the hollow tubular carbon nanostructures.

[0116] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the internal average diameter of the hollow carbon nanostructures is at least 2 times the average thickness of the nanorods.

[0117] As a result of the filling volume equal to or less than 50% and / or the fact that the internal average diameter of the hollow carbon nanostructures is at least 2 times the average thickness of the nanorods, all the rods are accessible to the electrolyte, which translates into a larger surface area, and the flow / diffusion of the reactants / products is facilitated.

[0118] In one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the internal average diameter of the hollow carbon nanostructures is at least 2 times, more particularly from 2 to 20 times, even more particularly from 3 to 10 times, and even more particularly 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, the average thickness of the nanorods.

[0119] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the plurality of nanorods composed of a transition metal oxide is placed on the internal surface of the hollow carbon nanostructures. Thus, in this embodiment, the central part of the cavity of the hollow carbon nanostructures is empty.

[0120] A further advantage of having the nanorods placed on the internal surface of the carbon nanostructures is that intimate contact is created between the metal oxide nanoparticles and the support (carbon nanostructure), which improves electrical transfer due to the high conductivity of carbon. This intimate contact is the result of the preparation process of the nanorods, i.e., is the result of the electrochemically triggered in situ transformation process of the precursor. Thus, in one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the metal oxide nanoparticles and the hollow carbon nanostructure are in intimate contact.

[0121] As mentioned above, the material of the invention is prepared by an electrochemical process that have advantages over the conventional hydrothermal processes. Thus, according to a second aspect, the invention relates to an electrochemical process for preparing the material as defined herein, which comprises the steps of: i) providing a modified working electrode by coating a conductive surface with a precursor material comprising a plurality of spherical nanoparticles composed of a transition metal (II, II l)-oxide of the formula RtM’”3-tO4 (C) as defined herein which is encapsulated within open-ended hollow carbon nanostructures; ii) carrying out electrosynthesis of an aqueous solution comprising an electrolyte selected from the group consisting of a halide salt of an alkali or alkaline-earth metal, or an alkali or alkaline-earth metal hydroxide, using a counter electrode, a reference electrode, and the modified working electrode obtained in step i) by: iia) applying 5000 galvanostatic cycles or less in the range from -0.2 to 0.8 V at a current density equal to or higher than 0.5 A per g of precursor material or alternatively, iib) applying 5000 potentiodynamic cycles or less in the range from 1 to 1.7 V at a scan rate equal to or higher than 40 mV / s; wherein when in the precursor material of formula (C) M’” is manganese, a material comprising a plurality of nanorods encapsulated within open-ended hollow carbon nanostructures is obtained, wherein the plurality of nanorods is composed of a transition metal oxide of the formula (A); and when in the precursor material of formula (C) M’” is a transition metal selected from the group consisting of iron, cobalt, and nickel, a material comprising a plurality of nanorods encapsulated within open-ended hollow carbon nanostructures is obtained, wherein the plurality of nanorods is composed of a transition metal oxide of the formula (B).

[0122] For the purposes of the present invention, the term “working electrode” refers to the electrode in the electrochemical cell where the electrochemical transformation of interest, in this case oxidation, takes place when electrons are exchanged between the electrode and the electrolyte. The working electrode may be a carbon-based electrode.

[0123] The term “counter electrode” as used herein refers to an electrode in the electrochemical cell which is configured to balance an electrical current through the working electrode, i.e., the charge that flows to the counter electrode is necessarily of an opposite sign to that charge that flows to the working electrode. The counter electrode may be a platinum (Pt) electrode or a carbon-based electrode. Non-limiting examples of carbon-based electrodes include electrodes made of glassy carbon, pyrolytic carbon film, graphene, carbon black, carbon particles, carbon nanotubes, graphite, or the like.

[0124] The term “reference electrode” as used herein refers to an electrode refers to a nonpolarizable electrode with a known and highly reproducible potential. The reference electrode provides a stable reference point, against which the voltage of a working electrode is measured. The reference electrode specifically may comprise an Ag / AgCI electrode or a reversible hydrogen electrode (RHE).

[0125] According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the working electrode is a carbon-based electrode, more particularly a glassy carbon electrode.

[0126] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the counter electrode is selected from a platinum electrode and a carbon-based electrode, more particularly a platinum electrode.

[0127] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the reference electrode is selected from an Ag / AgCI electrode and a reversible hydrogen electrode (RHE).

[0128] In step i) of the electrochemical process the conductive surface may be a glassy carbon electrode, foam, cloth, buckypaper, or Indium Tin Oxide (ITO).

[0129] According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, 14 pg of the precursor material are coated per 1 cm2of the working electrode.

[0130] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, in step i) a suspension of the precursor material in a suitable solvent is used. The solvent may be for example hexane, ethanol, methanol, isopropanol, deionized water, dimethylformamide (DMF), pentane, among others. The electrochemical process is generally carried out at room temperature in the presence of an electrolyte, which may be a halide salt of an alkali or alkaline-earth metal, or alternatively an alkali or alkaline-earth metal hydroxide.

[0131] Non-limiting examples of halide salt of an alkali or alkaline-earth metal that can be used include sodium chloride, potassium chloride or lithium chloride, calcium chloride, or magnesium chloride. Non-limiting examples of alkali or alkaline-earth metal hydroxide that can be used include sodium hydroxide, potassium hydroxide or lithium hydroxide, calcium hydroxide, or magnesium hydroxide.

[0132] According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the electrolyte is a chloride salt of an alkali or alkaline-earth metal, particularly KCI, at a concentration equal to or more than 1 M, more particularly from 1 to 3 M, more particularly from 1 .5 to 2.5 M, and even more particularly about 2 M.

[0133] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the electrolyte is an alkali or alkaline-earth metal hydroxide, particularly KOH, at a concentration from equal to or more than 0.1 M, more particularly from 0.1 to 2M, more particularly from 0.5 to 1.5M, and even more particularly about 1 M.

[0134] In the electrochemical process of the invention , a transition metal oxide of the formula AzM’2yMni.xO2(A) may be formed wherein A is the alkali or alkaline-earth metal ion of the halide or hydroxide salt and z is different from 0.

[0135] According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, when the process is carried out by using spherical nanoparticles composed of a transition metal (II, 11 l)-oxide of the formula RtM’”3-tO4 (C), wherein t is 0 and M’” is Mn, i.e., the transition metal (II, II l)-oxide of the formula (C) has the formula Mn3C>4, and a halide salt of an alkali metal or an alkali metal hydroxide is used as electrolyte, the nanorods obtained are composed of a mixture of one or more transition metal oxide of the formula (A). More particularly, the plurality of nanorods is composed of a mixture of a metal oxide of the formula M’2yMni-x O2 (A1), and a metal oxide of the formula AZ’M’2yMni-X”O2 (A2), wherein x’= y and x”= (z’ / 4 + y) with y =0 and 0 <z’<4, A is a monovalent metal; and manganese can adopt either an oxidation state of (IV) or a mixture of (IV) and (III), with the latter case exhibiting oxygen vacancies; i.e., the plurality of nanorods is composed of a mixture of a metal oxide of the formula MnCh (A1), and a metal oxide of the formula AzMni.X”O2 (A2), wherein x”= z74, and 0 <z’<4.

[0136] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, when the process is carried out by using spherical nanoparticles composed of a transition metal (II, 11 l)-oxide of the formula RtM’”3-tO4 (C), wherein t is different from 0 and M’” is Mn, i.e., the transition metal (II, 11 l)-oxide of the formula (C) has the formula RtMn3-tO4, and a halide salt of an alkali metal or an alkali metal hydroxide is used as electrolyte, the nanorods obtained are composed of a mixture of one or more transition metal oxide of the formula (A). More particularly, the plurality of nanorods is composed of a mixture of a metal oxide of the formula M’2yMni-xO2 (A1), and a metal oxide of the formula AzM’2yMni-X”O2 (A2), wherein M’ is a transition metal other than Mn in a divalent oxidation state; x’ is equal to y and x”= (z’ / 4 + y) with 0 <y<1 and 0 <z’<4, A is a monovalent metal; and manganese can adopt either an oxidation state of (IV) or a mixture of (IV) and (III), with the latter case exhibiting oxygen vacancies.

[0137] The electrosynthesis may be carried out in two different ways: by iia) applying from 1000 to 5000 galvanostatic cycles in the range from -0.2 to 0.8 V at a current density from 0.5 to 5 A per g of precursor material; or alternatively, by iib) applying from 1000 to 5000 potentiodynamic cycles in the range from 1 to 1 .7 V at a scan rate from 40 to 400 mV / s.

[0138] The electrosynthesis may be carried out in two different ways: by iia) applying galvanostatic cycles wherein the current is kept constant, or alternatively, by iib) applying potentiodynamic cycles wherein the potential changes at a constant rate (potential sweep rate) by varying the current.

[0139] According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the process comprises applying from 1000 to 5000 galvanostatic cycles, more particularly from 1500 to 3500 cycles, and even more particularly about 2500 galvanostatic cycles.

[0140] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, in step iia) of the process the current density is from 0.5 to 5, more particularly from 0.7 to 1.3, and even more particularly about 1 A per g of precursor material.

[0141] According to another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the process comprises applying from 1000 to 5000 potentiodynamic cycles, more particularly from 1500 to 3500 cycles, and even more particularly about 2500 potentiodynamic cycles.

[0142] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, in step iia) of the process the current density is from 40 to 400 mV / s, more particularly from 100 to 300 mV / s, and even more particularly about 200 mV / s.

[0143] It also forms part of the invention a material comprising a plurality of nanorods encapsulated within open-ended hollow carbon nanostructures, wherein the plurality of nanorods is composed of either a) a transition metal oxide of the formula AzM’2yMni-xO2 (A) as defined herein; or alternatively, b) a transition metal oxide of the formula M”3m / nM2-mC>3 (B) as defined herein; which is obtainable by a process, which comprises the steps of: i) providing a modified working electrode by coating a conductive surface with a precursor material comprising a plurality of spherical nanoparticles composed of a transition metal (II, II l)-oxide of the formula RtM’”3-tO4 (C) as defined herein which is encapsulated within open-ended hollow carbon nanostructures; ii) carrying out electrosynthesis of an aqueous solution comprising an electrolyte selected from the group consisting of a halide salt of an alkali or alkaline-earth metal, or an alkali or alkaline-earth metal hydroxide, using a counter electrode, a reference electrode, and the modified working electrode obtained in step i) by: iia) applying 5000 galvanostatic cycles or less in the range from -0.2 to 0.8 V at a current density equal to or higher than 0.5 A per g of precursor material or alternatively, iib) applying 5000 potentiodynamic cycles or less in the range from 1 to 1.7 V at a scan rate equal to or higher than 40 mV / s; wherein when in the precursor material of formula (C) M’” is manganese, a material comprising a plurality of nanorods encapsulated within open-ended hollow carbon nanostructures is obtained, wherein the plurality of nanorods is composed of a transition metal oxide of the formula (A); and when in the precursor material of formula (C) M’” is a transition metal selected from the group consisting of iron, cobalt, and nickel, a material comprising a plurality of nanorods encapsulated within open-ended hollow carbon nanostructures is obtained, wherein the plurality of nanorods is composed of a transition metal oxide of the formula (B).

[0144] All embodiments mentioned herein for the electrochemical process for preparing the material of the invention also apply to the material obtainable by this process. A third aspect of the invention relates to the precursor material from which the materials of the invention can be obtained. The precursor material comprises a plurality of spherical nanoparticles composed of a transition metal (II, 11 l)-oxide of the formula (C)

[0145] RtM”’3-tO4

[0146] (C) which is encapsulated within open-ended hollow carbon nanostructures, wherein:

[0147] M’” is a transition metal selected from the group consisting of manganese, iron, cobalt, and nickel in a divalent and trivalent oxidation state;

[0148] R is a transition metal other than M’” in a divalent oxidation state; t is a value from 0 to less than 1 ; the transition metal oxide of the formula (C) is in an amount from 20 to 60% by weight with respect to the total material weight; and the volume of the spherical nanoparticles encapsulated within hollow carbon nanostructures is equal to or less than 50% with respect of the total cavity volume of the hollow carbon nanostructures. In particular, the hollow carbon nanostructures are tubular and their internal average diameter is at least 2 times the average diameter of the spherical nanoparticles.

[0149] In one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the invention relates to a precursor material consisting of a plurality of spherical nanoparticles composed of a transition metal (II, 11 l)-oxide of the formula (C) which is encapsulated within open-ended hollow carbon nanostructures as defined herein.

[0150] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, in the transition metal (II, III) oxide of the formula (C), M’” is manganese.

[0151] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, in the transition metal (II, III) oxide of the formula (C), M’” is iron.

[0152] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, in the transition metal (II, III) oxide of the formula (C), M’” is cobalt.

[0153] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, in the transition metal (II, III) oxide of the formula (C), M’” is nickel. The precursor material also encompasses undoped and doped materials with regard to the transition metal (II, III) oxide of the formula (C). According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the transition metal (II, III) oxide of the formula (C) is undoped, i.e., t is 0.

[0154] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the transition metal (II, III) oxide is doped, i.e., in the transition metal (II, III) oxide of the formula (C) t is a value higher than 0 and lower than 1, and R is a transition metal other than M’” with a divalent oxidation state. More particularly, R is selected from the group consisting of manganese, iron, cobalt, nickel, cooper, chromium, molybdenum, titanium, vanadium, and zinc. Even more particularly, R is selected from the group consisting of iron, cobalt, and nickel. In a more particular embodiment, t is from 0.01 to 0.4, even more particularly t is from 0.05 to 0.09. Even more particularly, 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.

[0155] All embodiments mentioned above for the material of the invention with respect to the hollow carbon nanostructures also apply for the precursor material.

[0156] The precursor material comprises spherical nanoparticles. For the purposes of the invention, the term “spherical” nanoparticles, refers to any shape that is either perfectly spherical or approaching a spherical shape. The spherical nanoparticles are characterized by a diameter, which refers to the length through the midpoint of the nanoparticle from surface end to surface end. The diameter can be measured by methods well-known in the art such as transmission electron microscopy (TEM). The term “average diameter” refers to the average value taken from typically eighty different diameter measurements.

[0157] According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the spherical nanoparticles have an average diameter from 2 to 20 nm, more particularly from 7 to 15 nm, and even more particularly about 10 ±2 nm. In another embodiment, the spherical nanoparticles have an average diameter 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, about 20 nm. In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the spherical nanoparticles have a surface area from 50 to 5000 nm2, more particularly from 600 to 2800 nm2, and even more particularly from 800 to 1800 nm2

[0158] The precursor material can be prepared by a process as defined in the fourth aspect, which comprises the steps of: a) providing a mixture of a transition metal (II) salt, and optionally a second salt of a transition metal other than the first a transition metal with a divalent oxidation state, 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 other than the metal of the first salt, in the presence of a surfactant in a suitable solvent; b) adding the mixture of step a) to a suspension of hollow carbon nanostructures in a suitable solvent; c) heating the suspension to a temperature from 80 to 110 °C; and d) adding water to the suspension of step c) and heating to a temperature from 80 to 110 °C to obtain a precursor material of formula (C); wherein when step a) is carried out in the absence of another salt of a transition metal, a material of formula (C) is obtained wherein t is 0; and when step a) is carried out in the presence of a second salt of a transition metal, a precursor material of formula (C) is obtained wherein t is different from 0, and R is a transition metal other than M’” in a divalent oxidation state.

[0159] The above process has the advantage that it may be carried out under ambient conditions (room temperature and atmospheric pressure (around 1 ,013.25 hPa). Furthermore, the process can be carried out in the presence of water.

[0160] Step (a) is carried out in the presence of a surfactant. Non-limiting examples of surfactants that may be used include acids such as (Ci-C2o)alkyl carboxylic acids such as oleic acid, lauric acid, stearic acid, myristic acid and hexadecanoic acid;

[0161] (Ci-C2o)alkylamines such as oleylamine, laurylamine, hexadecylamine, trioctylamine and dioctylamine; or mixtures thereof.

[0162] In one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the mole ratio metal: surfactant is from 1 :8 to 1 :14, particularly from 1:10 to 1:12.

[0163] For the purposes of the invention, the term “(Ci-Cn)alkyl” refers to a linear or branched saturated hydrocarbon group having from 1 to n carbon atoms. By way of example, mention may be made of methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, or tertbutyl, n-pentyl or n-hexyl groups and the like.

[0164] According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the surfactant is a (Ci-C2o)alkylamine, more particularly selected from the group consisting of oleylamine, laurylamine, hexadecylamine, trioctylamine, dioctylamine, and a mixture thereof.

[0165] Non-limiting examples of salts that may be used in step a) include nitrate, sulphate, chloride, or acetate salts. When a second salt is present, the anions of the salts may be the same or different.

[0166] According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the transition metal (II) salt of step (a) is selected from the group consisting of a nitrate salt, a sulphate salt, a chloride salt, and an acetate salt, more particularly is an acetate salt.

[0167] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the salt of the second transition metal with a divalent oxidation state is selected from the group consisting of a nitrate salt, a sulphate salt, a manganese salt, a chloride salt, and an acetate salt, more particularly is an acetate salt.

[0168] Step a) is generally carried out at room temperature and in a suitable solvent. Non-limiting examples of solvents which can be used include (Ce)aromatic solvents such as xylene, benzene, toluene, fluorobenzene, 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,

[0169] 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,

[0170] 1.4-diiodotoluene, 1 ,2,3-triiodotoluene, 1,2,4-triiodotoluene, or a combination thereof.

[0171] In one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the solvent used in step a) is a (Ce)aromatic solvent, more particularly is xylene. The solvent used in step b) may be the same or different from the solvent used in step (a). In one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the solvent used in step b) is the solvent used in step a).

[0172] In another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the solvent used in step b) is a (Ce)aromatic solvent, more particularly is xylene.

[0173] Step c) comprises heating the solution obtained in step b) to a temperature from 80 to 110 °C to obtain a material of formula (C). A convenient temperature ramp may be used, for example a temperature ramp from 1 to 10°C / min, more particularly about 5 °C / min.

[0174] In one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the temperature used in step c) is from 80 to 110 °C, more particularly is about 90 °C.

[0175] After step c) water may optionally be added at the same temperature of step c), particularly under vigorous stirring.

[0176] The precursor material may be isolated by centrifugation (for example at 6000 to 10000 rpm from 5 to 15 min), particularly at room temperature, followed by filtration, for example by using a Polytetrafluoroethylene (PTFE) membrane filter (0.45 pm pore size, 47 mm diameter), which may be washed with one or more solvents, such as hexane, ethanol, acetone or mixtures thereof.

[0177] Optionally the obtained precursor material may be re-suspended and sonicated in a suitable solvent such as hexane for example for a time period from 10 to 30 min, and filtered again to remove nanoparticles from the outer surface of the hollow carbon nanostructures.

[0178] Optionally in the obtained precursor material the organic caping layer (i.e. , surfactant) may be removed from the nanoparticle surface by heating at a temperature of at least 300 °C.

[0179] In one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the solvent used in step b) is the solvent used in step a). According to another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the invention relates to a process for preparing the precursor material as defined above, which comprises the steps of: a) providing a mixture of a transition metal (II) salt, and optionally a second salt of a transition metal other than the first a transition metal with a divalent oxidation state, 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 other than the metal of the first salt, in the presence of a surfactant in a suitable solvent; b) adding the mixture of step a) to a suspension of hollow carbon nanostructures in a suitable solvent; c) heating the suspension to a temperature from 80 to 110 °C; d) adding water to the suspension of step c) and heating to a temperature from 80 to 110 °C to obtain a precursor material of formula (C); e) optionally isolating the material obtained in step c) by centrifugation, particularly at 6000 to 10000 rpm from 5 to 15 min, particularly at room temperature, followed by filtration; f) optionally washing the isolated material of step d) with one or more solvents, particularly selected from the group consisting of hexane, ethanol, acetone, and mixtures thereof; g) optionally re-suspending and sonicating the obtained precursor material in a suitable solvent for a time period from 10 to 30 min, and filtering the precursor material to remove nanoparticles from the outer surface of the hollow carbon nanostructures; and h) optionally removing the organic caping layer (i.e. , surfactant) from the nanoparticle surface by heating at a temperature of at least 300 °C.

[0180] It also forms part of the invention a precursor material comprising a plurality of spherical nanoparticles composed of a transition metal (II, lll)-oxide of the formula RtM’”3-tO4 (C) as defined herein, which is encapsulated within open-ended hollow carbon nanostructures; which is obtainable by a process comprising the steps of: a) providing a mixture of a transition metal (II) salt, and optionally a second salt of a transition metal other than the first a transition metal with a divalent oxidation state, 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 other than the metal of the first salt, in the presence of a surfactant in a suitable solvent; b) adding the mixture of step a) to a suspension of open-ended hollow carbon nanostructures in a suitable solvent; c) heating the suspension to a temperature from 80 to 110 °C; and d) adding water to the suspension of step c) and heating to a temperature from 80 to 110 °C to obtain a precursor material of formula (C); wherein when step a) is carried out in the absence of another salt of a transition metal, a material of formula (C) is obtained wherein t is 0; and when step a) is carried out in the presence of a second salt of a transition metal, a precursor material of formula (C) is obtained wherein t is different from 0, and R is a transition metal other than M’” in a divalent oxidation state.

[0181] All embodiments mentioned herein for the electrochemical process for preparing the precursor material of the invention also apply to the precursor material obtainable by this process.

[0182] According to another embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the invention relates to a process for preparing the material comprising a plurality of nanorods composed of a transition metal oxide as previously defined, which comprises the steps of: a) providing a mixture of a transition metal (II) salt, and optionally a second salt of a transition metal other than the first a transition metal with a divalent oxidation state, 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 other than the metal of the first salt, in the presence of a surfactant in a suitable solvent; b) adding the mixture of step a) to a suspension of open-ended hollow carbon nanostructures in a suitable solvent; c) heating the suspension to a temperature from 80 to 110 °C; and d) adding water to the suspension of step c) and heating to a temperature from 80 to 110 °C to obtain a precursor material of formula (C); wherein when step a) is carried out in the absence of another salt of a transition metal, a material of formula (C) is obtained wherein t is 0; and when step a) is carried out in the presence of a second salt of a transition metal, a precursor material of formula (C) is obtained wherein t is different from 0, and R is a transition metal other than M’” in a divalent oxidation state; i) providing a modified working electrode by coating a conductive with the precursor material obtained in step c); ii) carrying out electrosynthesis of an aqueous solution comprising an electrolyte selected from the group consisting of a chloride salt of an alkali or alkaline-earth metal at a concentration equal to or more than 1 M, and an alkali or alkaline-earth metal hydroxide at a concentration equal to or more than 0.1 M, using a counter electrode, a reference electrode, and the modified working electrode obtained in step i) by: iia) applying 5000 galvanostatic cycles or less in the range from -0.2 to 0.8 V at a current density equal to or higher than 0.5 A per g of precursor material or alternatively, iib) applying 5000 potentiodynamic cycles or less in the range from 1 to 1.7 V at a scan rate equal to or higher than 40 mV / s; wherein when in the precursor material of formula (C) M’” is manganese, a material comprising a plurality of nanorods encapsulated within open-ended hollow carbon nanostructures is obtained, wherein the plurality of nanorods is composed of a transition metal oxide of the formula (A); and when in the precursor material of formula (C) M’” is a transition metal selected from the group consisting of iron, cobalt, and nickel, a material comprising a plurality of nanorods encapsulated within open-ended hollow carbon nanostructures is obtained, wherein the plurality of nanorods is composed of a transition metal oxide of the formula (B).

[0183] It also forms part of the invention the material as previously defined, which is obtainable by a process which comprises steps a), b), and c), i), and ii), iii) as defined herein.

[0184] As mentioned above the materials of the invention have application as electrocatalysts not only in the area of energy storage for batteries, but also in the area of energy conversion as cathodes in fuel cells and anodes in water splitting where the reduction reaction of oxygen to water and the oxidation of water to oxygen are involved, respectively.

[0185] Thus, another aspect of the invention relates to the use of the material comprising a plurality of nanorods composed of a transition metal oxide as defined above as electrocatalyst. According to one embodiment, optionally in combination with one or more features of the various embodiments described above or below throughout all the description, the invention relates to the use of the material as defined above as electrocatalyst, as cathodes in fuel cells or alternatively as anodes in water splitting.

[0186] Throughout the description and claims the word "comprise" and variations of the word, are not intended to exclude other technical features, additives, components, or steps. Furthermore, the word “comprise” encompasses the case of “consisting of”. Additional objects, advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention. The following examples and drawings are provided by way of illustration, and they are not intended to be limiting of the present invention. Furthermore, the present invention covers all possible combinations of particular and preferred embodiments described herein.

[0187] Examples

[0188] Materials: Carbon nanofibers (CNF), produced by chemical vapour deposition, were purchased from Pyrograph Products (external diameter: 110 ± 30 nm, internal diameter: 70 ± 20 nm.). Both pristine carbon nanofibers and milled carbon nanofibers were employed in this work so the nanofiber length ranged from 10 (pristine) to 2 (milled) pm. Milled carbon nanofibers were produced by mechanical ball milling using a Retsch MM400 ball mill instrument (600 rpm). In a typical experiment, 50 mg of CNF were placed into a stainless-steel container (5ml_) with a stainless-steel ball (10 mm diameter) and milled in air for 180 min at 600 rpm (98% yield). No further purifications steps were required. To produce graphitized carbon nanofibers, milled carbon nanofibers (CNF, 100 mg) were placed in an alumina crucible and heated under argon atmosphere. The sample was initially purged for 30 min at room temperature, heated up to 1000 °C with a 10 °C / min ramp, hold for 90 min and then cooled down under argon atmosphere All other reagents were purchased from Sigma-Aldrich and used without further purification.

[0189] The synthesis of undoped or doped nanorods inside hollow carbon nanofibers (r- AzM’2yMni-xO2@CNF or r-M”3m / nM2-mC>3@CNF) takes place in two steps: synthesis of s- RtM”’3-tC>4 NP inside CNF followed by electrochemical synthesis.

[0190] TEM was performed on a JEOL JEM F200 microscope equipped with a cold fieldemission gun (Cold-FEG) operated at 200 kV with an ultra-high-resolution pole piece. TEM images were acquired using a Gatan OneView camera. Energy Dispersive X-ray Spectroscopy (EDS) was performed with a Centurio Large Angle Silicon Drift Detector (SDD) that collects X-rays from a detection area of 100 mm2. TEM specimens were prepared by casting several drops of a suspension of the carbon material in hexane onto a copper-grid mounted “holey” carbon film before drying under a stream of nitrogen. Thermogravimetric analysis (TGA) was carried out on a TGA / DSC 3+ Mettler Toledo instrument over the range of 25-1000 °C in air with a scan rate of 5 °C / min. Raman spectra were performed with RENISHAW Raman microscope with laser Ion Ar (514 nm). Powder X-ray diffraction (XRD) patterns were recorded using a PANalytical diffractometer equipped with Cu Ka source (A = 1.5418 A). The data were analyzed and processed using the X’Pert Data software package.

[0191] Example 1

[0192] In situ synthesis of s-RtM’”3-tO4 NP inside CNF (s-RtM”’3-tC>4@CNF)

[0193] In a typical experiment, a mixture of manganese (II) acetate (1.1 mmol, 190.3 mg) and oleylamine (12.2 mmol, 5.1 mL ratio metal (Mn): surfactant 1 :11.1) in xylene (7 mL) was added to a suspension of carbon nanofibers (CNF, 25 mg), previously sonicated for 15 min in xylene (10 mL). To obtain doped MnsO4 nanoparticles inside CNF (s-RtM’”3- tC>4@CNF), 15% (mmol) of manganese (II) acetate was replaced by iron (II), cobalt (II) or nickel (II) acetate, respectively. Hence, a solution of manganese (II) acetate (0.935 mmol) and optionally iron (II), cobalt (II) or nickel (II) acetate (0.165 mmol), oleylamine (12.2 mmol, 5.1 mL) ratio metals (Mn + Fe or Mn + Co or Mn+Ni): surfactant 1 :11.1 in xylene (7mL) was added to a suspension of CNF (25 mg) in xylene (10 mL). In all the 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 under vigorous stirring, and the resulting suspension was aged at 90°C for 3 hours in air. The hybrid material (s-Mn3O4@CNF or s- RtMn3-tO4@CNF when iron (II), cobalt (II) or nickel (II) acetate was added) was isolated by centrifugation (8000 rpm, 10 min) at room temperature and filtration by using a PTFE (Polytetrafluoroethylene) membrane filter (0.45 pm pore size, 47 mm diameter), which was then thoroughly washed with hexane (50mL), ethanol (50 mL) and finally acetone (25mL) to yield a dark black solid. Additionally, the resulting solid was re-suspended and sonicated in hexane (25 mL) for 15 minutes and filtered again to remove nanoparticles from the outer surface of carbon nanofibers (CNF). The resulting black solid was heated in a furnace at 300 °C for 2 hours in air to remove the organic caping layer (i.e. , surfactant) from the nanoparticle surface to yield a dark black solid with undoped (s-Mn3O4@CNF, FIG. 1) or doped nanoparticles (s- RtMn3-tO4 @CNF) with cobalt (s-Co / Mn3O4@CNF, Coo.i5Mn2.8s04@CNF FIG. 7), nickel (s- Ni / Mn3O4@CNF Nio.i5Mn2.8s04@CNF, FIG. 8), or iron (s-Fe / Mn3O4@CNF, Feo.isMn2.8504@CNF, FIG. 9).

[0194] 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 FIG. 3. In all cases, the band associated with MnsO4 (i.e., 660 cm-1) was observed, confirming that the exchange of Mn atoms for Co, Ni, or Fe (i.e., doping) does not affect the crystal structure.

[0195] X-ray powder diffractograms of s-Mn3O4@CNF (a), s-Co / Mn3O4@CNF (b), s- Ni / MnsO4@CNF (c) and s-Fe / Mn3O4@CNF (d) are shown in FIG. 4. In all cases, the peak associated with MnsO4 (i.e., the most intense around 36.2 °) was observed, confirming that the exchange of Mn atoms for Co, Ni or Fe (i.e., doping) does not affect the crystal structure. The peak at 26.4 ° corresponds to the carbon nanofiber.

[0196] TGA measurements of (a) s-Mn3O4@CNF, (b) s-Co / Mn3O4@CNF, (c) s-Ni / Mn3O4@CNF), and (d) s-Fe / Mn3O4@CNF are shown in FIG. 5. It can be seen that the gradual oxidation of the CNF structure takes place in the same region (i.e., from 400 to 600 °C) for all the materials while the content of metal oxide in the hybrid (L) after annealing at 1000 °C varies from 13 to 30%.

[0197] Histograms of the nanoparticle size distribution for (a) s-Mn3C>4@CNF, (b) s- Co / Mn3C>4@CNF, (c) s-Ni / Mn3C>4@CNF), and (d) s-Fe / Mn3C>4@CNF are shown in FIG. 6. It can be seen that doping MnsOt with Ni or Fe does not change the nanoparticle size as much as doping with Co atoms. Further, in FIG. 10 it is appreciated that the incorporation of cobalt, nickel, or iron atoms in the crystal structure of manganese oxide modifies the oxidation and reduction potentials of manganese.

[0198] Additional experiments demonstrate the scalability of the synthetic procedure by increasing the amounts of reagents, for example, by a factor of ten times.

[0199] Additional experiments to obtain doped MnsOt nanoparticles inside CNF (s-RtMn3-tO4 @CNF) were carried out by replacing 5%, 10%, 15% and 20% mmol of manganese (II) acetate with cobalt (II) acetate (t from 0.05 to 0.20). It was found that analog materials could be obtained. Doping over 15% (t>0.15) nevertheless reduces the metal oxide loading in the hybrid material are obtained (FIG. 21).

[0200] Example 2

[0201] In situ synthesis of s-RtMn3-tC>4 NP inside a graphitized carbon nanostructure s-RtMn3-tC>4 NP inside a graphitized carbon nanostructure was obtained using similar procedure than example 1 but replacing pristine carbon nanofibers for graphitized carbon nanofibers. The resultant material was found to be analogous material to the material of example 1 (no changes in the composition and morphology of the NPs based on powder X-ray diffraction, thermal gravimetric analysis, Raman spectroscopy and Transmission electron microscopy characterization). More importantly, electrochemical impedance spectroscopy of the resultant material revealed a slight increase from 10 to 22Q between the electrode surface and the electrolyte (known as solution resistance) with respect to material of example 1. Such increase corroborates the increase in hydrophobicity of the material when using graphitized carbon nanofibers without affecting the electrocatalytic performance of the NPs.

[0202] Example 3

[0203] In situ synthesis of S-CO3O4 NP inside CNF (s-Co3C>4@CNF)

[0204] S-CO3O4 nanoparticles inside CNF were obtained following the same protocol as described in example 1 but using cobalt (II) acetate instead of manganese (II) acetate.

[0205] Raman spectra of s-Co3C>4@CNF is shown in FIG. 3(e). The band associated with CO3O4 was observed at 685.3 cm-1.

[0206] X-ray powder diffractogram of s-Co3O4@CNF is shown in FIG. 4(e). The peaks associated with CO3O4 were observed at 36.9, 31.3, 59.5 and 44.9 °. TGA plot of S-CO3C>4@CNF is shown in FIG. 5(e). The gradual oxidation of the CNF structure takes place between 350 and 600°C and the content of metal oxide in the hybrid (L, loading) is 67%.

[0207] Nanoparticle size distribution histogram of s-Co3O4@CNF is shown in FIG. 6(d) and indicates an average size of 15 ± 5 nm.

[0208] Example 4

[0209] In situ synthesis of s-Fe3O4 NP inside CNF (s-Fe3O4@CNF) s-Fe3O4 nanoparticles inside CNF were obtained following the same protocol as described in example 1 but using iron (II) acetate instead of manganese (II) acetate.

[0210] Raman spectra of s-Fe3O4@CNF is shown in FIG. 3(f). The bands associated with FesO4 were observed at 221 , 286 and 397 cm-1.

[0211] X-ray powder diffractogram of s-Fe3O4@CNF is shown in FIG. 4(f). The most intense peak associated with FesO4 was observed at 35.7°.

[0212] TGA plot of s-Fe3O4@CNF is shown in FIG. 5(f). The gradual oxidation of the CNF structure takes place between 400 and 650°C ant the content of metal oxide in the hybrid (L, loading) is 40%.

[0213] Nanoparticle size distribution histogram of s-Fe3O4@CNF is shown in FIG. 6(e) and indicates an average size of 7 ± 2 nm.

[0214] Additional experiments were carried out to obtain s-Fe3O4@CNF by varying the ratio metal / oleylamine to 1 :0.9 and 1 :0.75. It was found that a decrease in the amount of the surfactant did not significantly increase the size of the NP.

[0215] Example 5

[0216] Electrosynthesis of r-AzM’2vMni-xO2@CNF and r-M”3m / nM2-mO3@CNF

[0217] All electrochemical tests were performed at room temperature using a conventional three- electrode cell on an electrochemical station (Autolab PGSTAT302N) while the electrochemical synthesis was carried out using an electrochemical station (Autolab 201A). The Ag / AgCI and the reversible hydrogen electrode (RHE) were used as reference electrode and the counter electrode was carbon rod. The sample-coated glassy carbon electrode (GCE) and the rotating ring disk electrode (RRDE) acted as working electrode. The modified working electrode was prepared by dropping 5 pL of the catalyst ink (s- Mn3O4@CNF, Fe3O4@CNF or s-Ro.i5Mn2.8s04@CNF being R = Co, Ni or Fe) on the GCE (diameter of 3 mm) or 10 pL on the RRDE (diameter of 5 mm) and drying naturally. This process was repeated until the mass loading of the catalyst reached 0.1 mg / cm2of active catalyst on RRDE and 0.01 mg / cm2on GCE. Prior to use, GCE and RRDE were polished mechanically with aqueous slurries of alumina powder (0.05 pm), rinsed with Milli-Q water and acetone, and allowed to dry under nitrogen. The catalyst ink to coat the RRDE was prepared through dispersing the catalyst (2 mg) ultrasonically into a mixture solution composed of ethanol (980 pL) and Nation solution (20 pL, 0.5 wt%). The catalyst ink to coat the GCE was prepared through dispersing the catalyst (1 mg) ultrasonically in hexane (1 mL). Cyclic voltammogram (CV) measurements were performed in N2 saturated 2M KCI with a scan rate of 50 mV / s. The linear sweep voltammetry (LSV) measurements for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) were performed in O2 saturated 1 M KOH with a scan rate of 10 mV / s at rotation rate of 1600 rpm. Several galvanostatic or potenciodynamic cycles (i.e. 2500) were carried out between -0.2 and 0.8 V (vs Ag / AgCI) in KCI (2M) or 1 to 1.7 (vs RHE) in KOH (1 M) at either high current density (i. e. 1 A / g) or high scan rate (i. e. 200 mV / s). In these conditions, based on the dissolution-precipitation mechanism, undoped (r-K / MnO2@CNF, FIG. 2 and r-Fe2O3@CNF, FIG. 22 ) or doped nanorods with cobalt (r-KCo / MnO2@CNF, FIG. 18), nickel (r-KNi / MnO2@CNF, FIG. 19), or iron (r-KFe / MnO2@CNF, FIG. 20) were obtained.

[0218] To corroborate the morphology transformation, TEM images of s-Mn3O4@CNF after several charge and discharge cycles in 2M KCI were taken (FIG. 14). It can be seen that the electrochemical treatment induced the agglomeration of the nanoparticles (see for comparison before treatment FIG. 1), which is a preliminary phase to achieve a complete transformation of the morphology. This demonstrates that while the confinement provides high stability, it does not inhibit the intrinsic reactivity of the nanoparticles.

[0219] Furthermore, galvanostatic cycles of charge and discharge of manganese oxide nanoparticles in carbon nanofibers before (s-Mn3O4@CNF, solid dotted line) and after (r- MnO2@CNF, dashed line) electrosynthesis consisting of 2500 cycles in 2M KCI (FIG. 15 (a), the solid black line is carbon nanofibers without manganese oxide), and the evolution of capacitance with respect to stability cycles (FIG. 15 (b)), showed a large (more than 4- fold) enhancement of capacitance which is related to the formation of nanorods within carbon nanofibers, which means that the nanorod have a larger surface area than the spherical nanoparticles.

[0220] Electrocatalytic performance of manganese oxide nanoparticles in carbon nanofibers without doping (s-Mn3O4@CNF) and doped with cobalt (s-Co / Mn3O4@CNF), nickel (s- Ni / Mn3O4@CNF) and iron (s-Fe / Mn3O4@CNF) was evaluated from linear sweep voltammogram (LSV, FIG. 11) in an oxygen saturated KOH (1 M) solution at room temperature. It is observed that the incorporation of cobalt, nickel and iron atoms in the crystalline structure of Mn3O4 increases the catalytic activity both to reduce oxygen (ORR, in the range of potential less than 1 V, inset) and to oxidize water (OER, in the potential range greater than 1 ,4V).

[0221] The electrocatalytic activity of the nanomaterials of the invention was also measured by linear scanning voltammetry of manganese oxide nanoparticles on carbon nanofibers in an alkaline solution of 1 M KOH saturated with oxygen before (s-Mn3O4@CNF, solid line) and after (r-MnO2@CNF, dotted line) electrosynthesis (2500 cycles in 2M KOI) (FIG. 16). It is appreciated that the change in morphology produced an increase in the catalytic capacity to oxidize water (OER, decreasing the overpotential) and improving its kinetics (according to the low Tafel slope).

[0222] In the case of the manganese oxide nanoparticles on carbon nanofibers doped with (a) cobalt, (b) nickel, or (c) iron (FIG. 17), it is appreciated that the high electrocatalytic activity is maintained after several cycles (i. e. 2500) while the capacity to oxidize water (OER, in the potential range greater than 1.4V) in the case of s-Ni / Mn3O4@CNF improves noticeably.

[0223] Finally, to test the stability of the confined manganese oxide nanoparticles, manganese oxide nanoparticles (s-Mn3O4) deposited both on the outside of the nanofiber and inside (FIG. 12 (a)) was evaluated. The material comprising nanoparticles deposited on the outside of the nanofiber was synthesized in the same way as s-Mn3O4@CNF but without performing the purification step, thus leaving nanoparticles also on the outside of the nanofiber. The material with nanoparticles deposited on the outside of the nanofibers was found to suffer from low stability, showing a noticeable decrease in current after only 500 cycles during electrosynthesis in KCI (FIG. 13), which may be associated with the loss of material (nanoparticles adhered to the outside of the nanofibers). TEM images show only the dissolution of nanoparticles supported on the outside of the nanofiber (FIG. 12 (b)), confirming that the encapsulation of nanoparticles limits their dissolution.

[0224] For reasons of completeness, various aspects of the invention are set out in the following numbered clauses:

[0225] Clause 1. A material comprising a plurality of nanorods encapsulated within open-ended hollow carbon nanostructures, wherein the plurality of nanorods is composed of either a) a transition metal oxide of the formula (A) AzM’2yMni-xO2

[0226] (A) or alternatively, b) a transition metal oxide of the formula (B)

[0227] M”3m / nM2-mO3

[0228] (B) wherein: in the formula (A), A is a monovalent metal, M’ is a transition metal other than Mn in a divalent oxidation state; x is equal to (z / 4 + y) with 0 <y<1 and 0 <z<4; and manganese can adopt either an oxidation state of (IV) or a mixture of (IV) and (III), with the latter case exhibiting oxygen vacancies; in the formula (B), M is a transition metal in a trivalent oxidation state selected from iron, cobalt and nickel; M” is a transition metal equal to or different from M in an oxidation state n, wherein n is value from 2 to 6, being 0 <m<2; the transition metal oxide of the formula (A) or formula (B) is in an amount from 20 to 60% by weight with respect to the total material weight; and the volume of the nanorods encapsulated within hollow carbon nanostructures is equal to or less than 50% with respect of the total cavity volume of the hollow carbon nanostructures.

[0229] Clause 2. The material according to clause 1, wherein in the transition metal oxide of the formula (A), y and z are 0, and in the transition metal oxide of the formula (B) m is 0.

[0230] Clause 3. The material according to clause 1, wherein in the transition metal oxide of the formula (A) at least one of y or z are different from 0, and in the transition metal oxide of the formula (B) m is different from 0.

[0231] Clause 4. The material according to any of the clauses 1-3, wherein the plurality of nanorods is composed of a transition metal (IV) oxide of the formula AzM’2yMni-xO2 (A).

[0232] Clause 5. The material according to any of the clauses 1-3, wherein the plurality of nanorods is composed of a transition metal (III) oxide of the formula M”3m / nM2-mC>3 (B).

[0233] Clause 6. The material according to clause 5, wherein M is iron.

[0234] Clause 7. The material according to clause 5, wherein M is cobalt. Clause 8. The material according to clause 5, wherein M is nickel.

[0235] Clause 9. The material according to any of the clauses 1-8, wherein the nanorods have an average thickness from 5 to 20 nm, and an average length from 16 to 180 nm.

[0236] Clause 10. The material according to any of the clauses 1-9, wherein the nanorods have a surface area equal to or higher than 280 nm2.

[0237] Clause 11. The material according to any of the clauses 1-10, wherein the amount of hollow carbon nanostructures in the material is from 40 to 80% by weight with respect to the total material weight.

[0238] Clause 12. The material according to any of the clauses 1-11, wherein the hollow carbon nanostructures are selected from carbon nanotubes, graphitized carbon nanotubes, heteroatom-doped carbon nanotubes, carbon nanofibers, graphitized carbon nanofibers, and heteroatom-doped carbon nanofibers.

[0239] Clause 13. The material according to any of the clauses 1-12, wherein the hollow carbon nanostructure is a hollow tubular carbon nanostructure, particularly a carbon nanotube.

[0240] Clause 14. The material according to clause 13, wherein the internal average diameter of the hollow tubular carbon nanostructure is at least 2 times, more particularly from 2 to 20 times, the average thickness of the nanorods.

[0241] Clause 15. The material according to any of the clauses 13 or 14, wherein the hollow tubular carbon nanostructure has an internal average diameter from 20 to 180 nm, or 30 to 180 nm, or 40 to 100 nm, an external average diameter from 40 to 200 nm, or 60 to 140 nm, and an average length equal to or lower than 10 pm.

[0242] Clause 16. The material according to any of the clauses 1-15, wherein the volume of the nanorods encapsulated within hollow carbon nanostructures is equal to or less than 10% with respect of the total cavity volume of the hollow carbon nanostructures.

[0243] Clause 17. An electrochemical process for preparing the material as defined in clause 1 , which comprises the steps of: i) providing a modified working electrode by coating a conductive surface with a precursor material comprising a plurality of spherical nanoparticles composed of a transition metal (II, lll)-oxide of the formula (C) RtM”’3-tO4

[0244] (C) which is encapsulated within open-ended hollow carbon nanostructures, wherein:

[0245] M’” is a transition metal selected from the group consisting of manganese, iron, cobalt and nickelin a divalent and trivalent oxidation state;

[0246] R is a transition metal other than M’” in a divalent oxidation state; t is a value from 0 to less than 1 ; the transition metal oxide of the formula (C) is in an amount from 20 to 60% by weight with respect to the total material weight; and the volume of the spherical nanoparticles encapsulated within hollow carbon nanostructures is equal to or less than 50% with respect of the total cavity volume of the hollow carbon nanostructures; ii) carrying out electrosynthesis of an aqueous solution comprising an electrolyte selected from the group consisting of a halide salt of an alkali or alkaline-earth metal, or an alkali or alkaline-earth metal hydroxide, using a counter electrode, a reference electrode, and the modified working electrode obtained in step i) by: iia) applying 5000 galvanostatic cycles or less in the range from -0.2 to 0.8 V at a current density equal to or higher than 0.5 A per g of precursor material or alternatively, iib) applying 5000 potentiodynamic cycles or less in the range from 1 to 1.7 V at a scan rate equal to or higher than 40 mV / s.

[0247] Clause 18. The electrochemical process according to clause 17, wherein in the precursor material of formula (C) the internal average diameter of the hollow tubular carbon nanostructure is at least 2 times, more particularly from 2 to 20 times, the average diameter of the spherical nanoparticles.

[0248] Clause 19. A precursor material comprising a plurality of spherical nanoparticles composed of a transition metal (II, I ll)-oxide of the formula (C)

[0249] RtM”’3-tO4

[0250] (C) which is encapsulated within open-ended hollow carbon nanostructures, wherein:

[0251] M’” is a transition metal selected from the group consisting of manganese, iron, cobalt, and nickel in a divalent and trivalent oxidation state;

[0252] R is a transition metal other than M’” in a divalent oxidation state; t is a value from 0 to less than 1 ; the transition metal oxide of the formula (C) is in an amount from 20 to 60% by weight with respect to the total material weight; and the volume of the spherical nanoparticles encapsulated within hollow carbon nanostructures is equal to or less than 50% with respect of the total cavity volume of the hollow carbon nanostructures.

[0253] Clause 20. The precursor material according to clause 19, wherein M’” is manganese.

[0254] Clause 21. The precursor material according to clause 19, wherein M’” is iron.

[0255] Clause 22. The precursor material according to clause 19, wherein M’” is cobalt.

[0256] Clause 23. The precursor material according to clause 19, wherein M’” is nickel.

[0257] Clause 24. The precursor material according to any of the clauses 19-23, wherein the spherical nanoparticles have an average diameter from 2 to 20 nm.

[0258] Clause 25. The precursor material according to any of the clauses 19-24, wherein the spherical nanoparticles have a surface area from 50 to 5000 nm2.

[0259] Clause 26. The precursor material according to any of the clauses 19-25, wherein the amount of hollow carbon nanostructures in the material is from 40 to 80% by weight with respect to the total material weight.

[0260] Clause 27. The precursor material according to any of the clauses 19-26, wherein the hollow carbon nanostructures are selected from carbon nanotubes, graphitized carbon nanotubes, heteroatom-doped carbon nanotubes, carbon nanofibers, graphitized carbon nanofibers, and heteroatom-doped carbon nanofibers.

[0261] Clause 28. The precursor material according to any of the clauses 19-27, wherein the hollow carbon nanostructure is a hollow tubular carbon nanostructure, particularly a carbon nanotube.

[0262] Clause 29. The precursor material according to clause 28, wherein the internal average diameter of the hollow tubular carbon nanostructure is at least 2 times, more particularly from 2 to 20 times, the average diameter of the spherical nanoparticles.

[0263] Clause 30. The precursor material according to any of the clauses 28 or 29, wherein the hollow tubular carbon nanostructure has an internal average diameter from 20 to 180 nm, or 30 to 180 nm, or 40 to 100 nm, an external average diameter from 40 to 200 nm or 60 to 140 nm, and an average length equal to or lower than 10 pm. Clause 31. The precursor material according to any of the clauses 19-30, wherein the volume of the spherical nanoparticles encapsulated within hollow carbon nanostructures is equal to or less than 10% with respect of the total cavity volume of the hollow carbon nanostructures.

[0264] Clause 32. A process for preparing the precursor material as defined in clause 19, which comprises the steps of: a) providing a mixture of a transition metal (II) salt, and optionally a second salt of a transition metal other than the first a transition metal with a divalent oxidation state, wherein the metal of the first transition metal (II) salt is selected from manganese, iron, cobalt, and nickel, and the metal of the second salt is other than the metal of the first salt, in the presence of a surfactant in a suitable solvent; b) adding the mixture of step a) to a suspension of open-ended hollow carbon nanostructures in a suitable solvent; and c) heating the suspension to a temperature from 80 to 110 °C to obtain a precursor material of formula (C); wherein when step a) is carried out in the absence of another salt of a transition metal, a material of formula (C) is obtained wherein t is 0; and when step a) is carried out in the presence of a second salt of a transition metal, a precursor material of formula (C) is obtained wherein t is different from 0, and R is a transition metal other than M’” in a divalent oxidation state.

[0265] Clause 33. The process for preparing the precursor material according to clause 32, wherein after heating the suspension to a temperature from 80 to 110 °C in step c), step d) is carried out by adding water to the suspension of step c) and heating to a temperature from 80 to 110 °C to obtain a precursor material of formula (C).

[0266] Clause 34. Use of the material as defined in any of the clauses 1-17 as electrocatalyst.

[0267] Clause 35. The use according to clause 34, as cathodes in fuel cells or alternatively as anodes in water splitting.

Claims

Claims1. A material comprising a plurality of nanorods encapsulated within open-ended hollow tubular carbon nanostructures, wherein the plurality of nanorods is composed of either a) a transition metal oxide of the formula (A)AzM’2yMni-xO2(A) or alternatively, b) a transition metal oxide of the formula (B)M”3m / nM2-mO3(B) wherein: in the formula (A), A is a monovalent metal, M’ is a transition metal other than Mn in a divalent oxidation state; x is equal to (z / 4 + y) with 0 <y<1 and 0 <z<4; and manganese can adopt either an oxidation state of (IV) or a mixture of (IV) and (III), with the latter case exhibiting oxygen vacancies; in the formula (B), M is a transition metal in a trivalent oxidation state selected from iron, cobalt and nickel; M” is a transition metal equal to or different from M in an oxidation state n, wherein n is value from 2 to 6, being 0 <m<2; the transition metal oxide of the formula (A) or formula (B) is in an amount from 20 to 60% by weight with respect to the total material weight; the volume of the nanorods encapsulated within hollow tubular carbon nanostructures is equal to or less than 50% with respect of the total cavity volume of the hollow tubular carbon nanostructures; and the internal average diameter of the hollow tubular carbon nanostructures is at least 2 times the average thickness of the nanorods.

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

3. The material according to claim 1 , wherein in the transition metal oxide of the formula (A) at least one of y or z are different from 0, and in the transition metal oxide of the formula (B) m is different from 0.

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

5. The material according to any of the claims 1-4, wherein the nanorods have a surface area equal to or higher than 280 nm2.

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

7. The material according to any of the claims 1-6, wherein the hollow tubular carbon nanostructures are 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 of the claims 1-7, wherein the hollow tubular carbon nanostructures are carbon nanotubes.

9. The material according to claim 8, wherein the hollow tubular carbon nanostructure has an internal average diameter from 20 to 180 nm, an external average diameter from 40 to 200 nm, and an average length equal to or lower than 10 pm.

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

11. An electrochemical process for preparing the material as defined in any of the claims 1-10, which comprises the steps of: i) providing a modified working electrode by coating a conductive surface with a precursor material comprising a plurality of spherical nanoparticles composed of a transition metal (II, 11 l)-oxide of the formula (C)RtM”’3-tO4(C) which is encapsulated within open-ended hollow tubular carbon nanostructures, wherein: M’” is a transition metal selected from the group consisting of manganese, iron , cobalt and nickel in a divalent and trivalent oxidation state;R is a transition metal other than M’” in a divalent oxidation state; t is a value from 0 to less than 1 ;the transition metal oxide of the formula (C) is in an amount from 20 to 60% by weight with respect to the total material weight; the volume of the spherical nanoparticles encapsulated within hollow tubular carbon nanostructures is equal to or less than 50% with respect of the total cavity volume of the hollow tubular carbon nanostructures; and the internal average diameter of the hollow tubular carbon nanostructures is at least 2 times the average thickness of the nanorods; ii) carrying out electrosynthesis of an aqueous solution comprising an electrolyte selected from the group consisting of a halide salt of an alkali or alkaline-earth metal, or an alkali or alkaline-earth metal hydroxide, using a counter electrode, a reference electrode, and the modified working electrode obtained in step i) by: iia) applying 5000 galvanostatic cycles or less in the range from -0.2 to 0.8 V at a current density equal to or higher than 0.5 A per g of precursor material or alternatively, iib) applying 5000 potentiodynamic cycles or less in the range from 1 to 1.7 V at a scan rate equal to or higher than 40 mV / s; wherein when in the precursor material of formula (C) M’” is manganese, a material comprising a plurality of nanorods encapsulated within open-ended hollow carbon nanostructures is obtained, wherein the plurality of nanorods is composed of a transition metal oxide of the formula (A); and when in the precursor material of formula (C) M’” is a transition metal selected from the group consisting of iron, cobalt, and nickel, a material comprising a plurality of nanorods encapsulated within open-ended hollow carbon nanostructures is obtained, wherein the plurality of nanorods is composed of a transition metal oxide of the formula (B).

12. A precursor material comprising a plurality of spherical nanoparticles composed of a transition metal (II, lll)-oxide of the formula (C)RtM”’3-tO4(C) which is encapsulated within open-ended hollow tubular carbon nanostructures, wherein: M’” is a transition metal selected from the group consisting of manganese, iron, cobalt, and nickel in a divalent and trivalent oxidation state;R is a transition metal other than M’” in a divalent oxidation state; t is a value from 0 to less than 1 ; the transition metal oxide of the formula (C) is in an amount from 20 to 60% by weight with respect to the total material weight;the volume of the spherical nanoparticles encapsulated within hollow tubular carbon nanostructures is equal to or less than 50% with respect of the total cavity volume of the hollow tubular carbon nanostructures; and the internal average diameter of the hollow tubular carbon nanostructures is at least 2 times the average diameter of the spherical nanoparticles.

13. A process for preparing the precursor material as defined in claim 12, which comprises the steps of: a) providing a mixture of a transition metal (II) salt, and optionally a second salt of a transition metal other than the first a transition metal with a divalent oxidation state, 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 other than the metal of the first salt, in the presence of a surfactant in a suitable solvent; b) adding the mixture of step a) to a suspension of open-ended hollow tubular carbon nanostructures in a suitable solvent; c) heating the suspension to a temperature from 80 to 110 °C; and d) adding water to the suspension of step c) and heating to a temperature from 80 to 110 °C to obtain a precursor material of formula (C); wherein when step a) is carried out in the absence of another salt of a transition metal, a material of formula (C) is obtained wherein t is 0; and when step a) is carried out in the presence of a second salt of a transition metal, a precursor material of formula (C) is obtained wherein t is different from 0, and R is a transition metal other than M’” in a divalent oxidation state.

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

15. The use according to claim 14, as cathodes in fuel cells or alternatively as anodes in water splitting.

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