WO2014200442A1

WO2014200442A1 - Method of preparing a metal oxyhydroxide nanostructured material

Info

Publication number: WO2014200442A1
Application number: PCT/SG2014/000285
Authority: WO
Inventors: Qingyu Yan; Hui Teng TAN
Original assignee: Nanyang Technological University
Priority date: 2013-06-14
Filing date: 2014-06-16
Publication date: 2014-12-18

Abstract

A method of preparing a metal oxyhydroxide nanostructured material is provided. The method comprises depositing a metal oxyhydroxide precursor on a carbon-based fibrous ubstrate; and heating the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor under hydrothermal conditions to obtain the metal oxyhydroxide nanostructured material. A metal oxyhydroxide nanostructured material and a supercapacitor electrode are also provided.

Description

METHOD OF PREPARING A METAL OXYHYDROXIDE NANOSTRUCTURED

MATERIAL

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of US provisional application No. 61/835,079 filed on 14 June 2013, the content of which is incorporated herein by reference in its entirety for all purposes.

TECHNICAL FIELD

[0002] The invention relates to a metal oxyhydroxide nanostructured material, and supercapacitor electrodes including the metal oxyhydroxide nanostructured material.

BACKGROUND

[0003] Supercapacitors have emerged as alternatives to complement batteries in commercial market, due to their attractive characteristics of higher power densities and faster charging rates. Insatiable market demand for high-efficiency supercapacitors has triggered high levels of research interest in exploring advanced electrode materials fulfilling criteria of being low-cost, environmental friendly and excellent performance.

[0004] To this end, transition metal oxides such as Ru0₂, Mn0₂, and V₂0₅ which exhibit multiple valence states and provide higher pseudocapacitance are promising candidates. Among various pseudocapacitive materials, manganese oxide and its derivatives have been regarded as good electro-active materials due to their high theoretical capacitance, natural abundance, low costs, and lower toxicities.

[0005] In contrast to manganese oxide (Mn0₂), use of manganese oxyhydroxide (MnOOH) as electro-active material is less reported. This is because, even though MnOOH may be synthesized using methods such as solvothermal method, electrochemical deposition, and pulsed-induced deposition, synthesis of MnOOH with controlled geometry and dimension using the methods to render it suitable as an electro-active material presents a great challenge.

[0006] In view of the above, there remains a need for improved methods to prepare materials that may be used as electrodes that overcomes or at least alleviates one or more of the above-mentioned problems. SUMMARY

[0007] In a first aspect, the invention refers to a method of preparing a metal oxyhydroxide nanostructured material. The method comprises

a) depositing a metal oxyhydroxide precursor on a carbon-based fibrous substrate; and

b) heating the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor under hydrothermal conditions to obtain the metal oxyhydroxide nanostructured material.

[0008] In a second aspect, the invention refers to a metal oxyhydroxide nanostructured material prepared by a method according to the first aspect.

[0009] In a third aspect, the invention refers to a supercapacitor electrode comprising a metal oxyhydroxide nanostructured material prepared by a method according to the first aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:

[0011] FIG. 1 is a graph showing X-ray diffraction (XRD) pattern of the as-prepared samples for (a) bare graphite felt (GF); and (b) MnOOH (3 hr). Y-axis: intensity (a.u.); x-axis: 2Θ (degree).

[0012] FIG. 2(A), (B), and (C) are scanning electron microscopy (SEM) images of MnOOH nanotubes. FIG. 2(D), (E), and (F) are transmission electron microscopy (TEM) images of MnOOH nanotubes. FIG. 2(G) is a high-resolution transmission electron microscopy (HRTEM) image of MnOOH nanotubes. Inset of FIG. 2(C) illustrate the digital photograph image of the bended composite, showing the high flexibility of the supercapacitor electrode. Regions 1 and 2 marked in FIG. 2(D) were magnified in FIG. 2(E) and FIG. 2(F) respectively. Scale bar in FIG. 2(A) and (B) denotes a length of 10 μιη. Scale bar in FIG. 2(C) and (D) denotes a length of 500 nm and 100 nm, respectively. Scale bar in FIG. 2(E) and (F) denotes a length of 20 nm. Scale bar in FIG. 2(G) denotes a length of 5 nm. [0013] FIG. 3(A) to (D) show (A) SEM image; (B) and (C) TEM images; and (D) HRTEM image of the Mn0₂ nanosheets. FIG. 3(E) to (H) show (E) SEM image; (F) and (G) TEM images; and (H) HRTEM images of the MnOOH nanorods.

[0014] FIG. 4 is a graph showing XRD pattern of the as-prepared samples for (A) nanosheets (0.5 h); and (B) nanorods (6 h).

[0015] FIG. 5 is a graph showing nitrogen absorption/desorption isotherm of the as- prepared samples of graphite felt (GF), nanosheets, nanotubes, and nanorods.

[0016] FIG. 6 shows TEM images of (A) and (B) rolling (1 h); and (C) and (D) infilling (4 h) processes.

[0017] FIG. 7 is a schematic diagram showing proposed formation mechanism of metal oxyhydroxide nanosheets, nanotubes, and nanorods. The embodiment shown is MnOOH.

[0018] FIG. 8 shows electrochemical measurements of the as-prepared samples: (A) cyclic voltammagram of the as-prepared samples at scan rate of 50 mV s^"1; (B) galvanic charged/discharged curves of the as-prepared samples at current density of 1 A .g^'1. Specific capacitance of the as-prepared samples as a function of current densities: (C) capacitance contributed from the MnOOH/GF composites; (D) capacitance contributed from the MnOOH nanostructures only.

[0019] FIG. 9 are graphs showing (A) cycling performance of the as-prepared MnOOH/GF composites; and (B) Ragone plot.

[0020] FIG. 10 are cyclic voltammagram of the as-prepared samples at scan rate of (A) 2 mV s^"1; and (B) 100 mV s^"1. Galvanostatic charge/discharge curves of (C) nanosheets; (D) nanotubes; and (E) nanorods at different current densities.

DETAILED DESCRIPTION

[0021] Although metal oxyhydroxide nanostructured materials such as MnOOH possess high energy densities due to their pseudo-capacitive characteristics, their semi conductive properties result in poor electronic conductivity and render them less capable of withstanding tensile strain. Consequently, electrodes or electronic devices prepared using metal oxyhydroxide nanostructured materials have poor rate capabilities and shorter cycle life.

[0022] Using a method disclosed herein, whereby metal oxyhydroxide nanostructured materials are formed in-situ on carbon-based fibrous substrates such as graphite felt (GF), constraints that hinder metal oxyhydroxides from realizing their full potential are mitigated. As disclosed herein, metal oxyhydroxides and carbon-based fibrous materials are synergistically coupled into a hybrid system, which has demonstrated improvements in terms of energy and power densities. Depending on user requirements, different metal oxyhydroxide nanostructured materials such as nanosheets, nanorods and nanotubes may be formed. Advantageously, a binder is not required to attach the metal oxyhydroxide to the carbon-based fibrous substrate.

[0023] With the above in mind, the invention refers in a first aspect to a method of preparing a metal oxyhydroxide nanostructured material.

[0024] As used herein, the term "metal oxyhydroxide" refers to compounds having the general formula MOOH, where M denotes a metal. In various embodiments, the metal oxyhydroxide is manganese oxyhydroxide (MnOOH).

[0025] Metal of the metal oxyhydroxide may generally be selected from Period 4 of d- block element of the Periodic Table of elements. For example, metal of the metal oxyhydroxide may be a transition metal selected from the group consisting of scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), and alloys thereof. In specific embodiments, metal of the metal oxyhydroxide is manganese.

[0026] The term "nanostructured material" as used herein refers to a material having at least one dimension that is in the nanometer range. At least one dimension of the nanostructured material may be less than 100 nm. In various embodiments, a nanostructured material has a dimension typically ranging from 1 nm to 100 nm (where 10 angstrom = 1 nm = 1/1000 micrometer).

[0027] Nanostructured materials may be classified into the following dimensional types:

[0028] Zero dimensional (0D): nanospherical particles (also called nanoparticles);

[0029] One dimensional (ID): nanorods, nanowires (also called nanofibers) and nanotubes; and

[0030] Two dimensional (2D): nanoflakes, nanoflowers, nanodiscs and nanofilms.

[0031] Examples of nanostructured material may include nanotubes, nanoflowers, nanowires, nanofibers, nanoflakes, nanoparticles, nanodiscs, nanofilms, and combinations of the aforementioned. [0032] In various embodiments, the metal oxyhydroxide nanostructured material is selected from the group consisting of nanosheets, nanorods, and nanotubes.

[0033] The method includes depositing a metal oxyhydroxide precursor on a carbon-based fibrous substrate.

[0034] The term "fibrous substrate" as used herein refers to a material having filaments or fibers. The filaments or fibers in the fibrous substrate may be discrete and/or separable. For example, the fibrous substrate may be a woven or non-woven felt, cloth, sheet, mat, to name only a few. The term 'fibrous substrate" also includes a material that is in the form of a fiber, or a plurality of fibers. Accordingly, the term "carbon-based fibrous substrate" refers to a material having filaments or fibers that contain carbon.

[0035] Carbon-based fibrous materials such as carbon nanotubes (CNTs), graphenes, activated carbon textiles, carbon felts, and graphite felts, are environmentally benign, and are able to provide mechanical support for the metal oxyhydroxide nanostructured material. Their use as electrode materials is particularly advantageous as they possess high electronic conductivities. Furthermore, their conductive scaffold and unique three dimensional meshlike structures may enhance electrolyte ions penetration.

[0036] The carbon-based fibrous substrate may be carbon felt. As used herein, the term "carbon felt" refers to a textile material that predominantly comprises randomly oriented and intertwined carbon filaments or fibers. Depending on the material used and carbonization conditions, the carbon felt may be graphitic or amorphous.

[0037] In various embodiments, the carbon-based fibrous substrate is graphite felt. The term "graphite felt" refers to carbon felt that has been subjected to a graphitisation process, which may involve heat treating the carbon felt at high temperatures, such as in the range of about 2600 °C to about 3300 °C. During the graphitising process, the randomly oriented and intertwined carbon filaments or fibers may be converted into a three-dimensionally ordered graphite structure, and may translate into improved electrical conductivity of the graphite felt.

[0038] Depositing a metal oxyhydroxide precursor on a carbon-based fibrous substrate may include immersing a carbon-based fibrous substrate in an aqueous solution comprising the metal oxyhydroxide precursor.

[0039] The metal oxyhydroxide precursor may comprise or consist of an oxyacid salt. In various embodiments, the oxyacid salt is selected from the group consisting of manganates, permanganates, borates, titanates, tantalates, molybdates, vanadates, metavanadates, chromates, zirconates, sulfates, phosphates, polyphosphates, silicates, nitrates, chlorides, and combinations thereof.

[0040] In specific embodiments, the oxyacid salt comprises or consists of a permanganate (Mn0₄ ^~) salt. In these embodiments, the metal oxyhydroxide precursor may be sodium permanganate, potassium permanganate, or mixtures thereof.

[0041] The metal oxyhydroxide precursor may be added in form of a solution and in a dropwise manner into the aqueous solution in which the carbon-based fibrous substrate is immersed under agitation. By adding the metal oxyhydroxide precursor in a dropwise manner into the aqueous solution under agitation, this allows improved dispersion of the metal oxyhydroxide precursor in the aqueous solution and its subsequent deposition on the carbon- based fibrous substrate.

[0042] In various embodiments, the agitation comprises sonication. Sonication may be carried out for any suitable time period that allows dispersion of the metal oxyhydroxide precursor in the aqueous solution. In various embodiments, sonication is carried out for a time period in the order of minutes, such as in the range of about 10 minutes to about 60 minutes, about 10 minutes to about 30 minutes, about 20 minutes to about 40 minutes, or about 10 minutes, about 20 minutes, about 30 minutes, or about 40 minutes.

[0043] Method of the first aspect includes heating the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor under hydrothermal conditions to obtain the metal oxyhydroxide nanostructured material. In various embodiments, heating the carbon- based fibrous substrate comprising the metal oxyhydroxide precursor under hydrothermal conditions to obtain the metal oxyhydroxide nanostructured material is carried out in an autoclave.

[0044] The temperature at which the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor is heated may depend on the type of metal oxyhydroxide precursor used. In various embodiments, heating the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor under hydrothermal conditions comprises heating the carbon- based fibrous substrate comprising the metal oxyhydroxide precursor, preferably in an autoclave, at a temperature in the range of about 120 °C to about 190 °C.

[0045] For example, heating the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor may be carried out at a temperature in the range of about 140 °C to about 190 °C, about 160 °C to about 190 °C, about 170 °C to about 190 °C, about 120 °C to about 170 °C, about 120 °C to about 150 °C, about 120 °C to about 130 °C, about 130 °C to about 160 °C, about 140 °C to about 170 °C, about 150 °C, about 170 °C, or about 190 °C.

[0046] As mentioned above, different metal oxyhydroxide nanostructured materials, such as nanosheets, nanorods, and nanotubes, may be formed using a method disclosed herein. This may be carried out by varying the time period at which the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor is heated under hydrothermal conditions.

[0047] In various embodiments, heating the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor under hydrothermal conditions includes heating the carbon- based fibrous substrate comprising the metal oxyhydroxide precursor for a time period in the range of about 30 minutes to about 8 hours.

[0048] For example, heating the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor may be carried out for a time period in the range of about 1 hour to about 6 hours, about 3 hours to about 8 hours, about 5 hours to about 8 hours, about 45 minutes to about 6 hours, about 1 hour to about 3 hours, or about 2 hours to about 6 hours.

[0049] In various embodiments, the metal oxyhydroxide nanostructured material obtained comprises or consists of nanosheets. The nanosheets may be obtained by heating the carbon- based fibrous substrate comprising the metal oxyhydroxide precursor for 1 hour or less. In specific embodiments, the nanosheets are obtained by heating the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor in an autoclave for about 45 minutes.

[0050] In further embodiments, the metal oxyhydroxide nanostructured material obtained comprises or consists of nanotubes. The nanotubes may be obtained by heating the carbon- based fibrous substrate comprising the metal oxyhydroxide precursor for more than 45 minutes and less than 6 hours, preferably about 2 hours to about 4 hours. For example, the nanotubes may be obtained by heating the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor for more than 1 hour and less than 6 hours. In specific embodiments, the nanotubes are obtained by heating the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor in an autoclave for about 3 hours.

[0051] In some embodiments, the metal oxyhydroxide nanostructured material obtained comprises or consists of nanorods. The nanorods may be obtained by heating the carbon- based fibrous substrate comprising the metal oxyhydroxide precursor for at least 6 hours. In specific embodiments, the nanorods are obtained by heating the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor in an autoclave for about 6 hours.

[0052] According to the structure-activity dependent relationship, morphology of the active materials in electrodes plays a vital role in determining their electrochemical performances. By controlling the time period at which the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor is heated under hydrothermal conditions, metal oxyhydroxides with different morphologies may be obtained.

[0053] For example, by heating the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor under hydrothermal conditions, the metal oxyhydroxide that is formed may cover a surface of the carbon-based fibrous substrate. With increase in heating time, a thicker layer of metal oxyhydroxide may be formed, and/or more of the carbon-based fibrous substrate may be covered. As a result, nanosheets of metal oxyhydroxide may be obtained.

[0054] With further heating, the metal oxyhydroxide nanosheets may eventually scroll into a roll, to form nanotubes. Without wishing to be bound by theory, it is postulated that formation of the metal oxyhydroxide nanotubes relates to the tendency for reduction in surface energy and obtaining saturation level of the free bonds from the nanosheets boundary under elevated temperatures and pressures during the hydrothermal treatment.

[0055] When heating time is further increased, metal oxyhydroxide nanorods may be obtained due to infilling process of nanotubes with nanoparticles, where interior of the nanotubes are filled with the newly formed metal oxyhydroxide nanoparticles.

[0056] Using a method disclosed herein, therefore, by controlling the time duration at which the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor is heated, nanosheets, nanotubes, or nanorods of metal oxyhydroxide may be obtained.

[0057] In specific embodiments, the hydrothermal heating process is controlled such that the metal oxyhydroxide nanostructured material obtained comprises or consists of nanotubes. In this regard, the unique structural properties of nanotubes are advantageous for surface- mediated reaction, which render them suitable for pseudo-capacitor applications. As compared to their solid core counterparts such as nanorods, for example, nanotubes have higher surface area due to presence of their inner walls for electrochemical reactions. Furthermore, hollow interiors of the nanotubes allow enhanced accessibility of the active surface area of electrodes thus formed to electrolyte ions due to good permeability of the nanotubes.

[0058] In various embodiments, each nanotube has a diameter in the range of about 20 nm to about 50 nm, such as about 20 nm to about 40 nm, about 20 nm to about 30 nm, about 30 nm to about 50 nm, about 40 nm to about 50 nm, about 25 nm to about 45 nm, or about 30 nm to about 40 nm.

[0059] Each nanotube may have a length of about 1 μπι to about 10 μιτι, such as about 1 μιη to about 8 μιη, about 1 μηι to about 6 μιη, about 1 μηι to about 4 μιη, about 3 μηι to about 10 μιτι, about 5 μιτι to about 10 μιη, about 7 μηι to about 10 μιη, about 2 μιη to about 8 μηι, or about 3 μηι to about 7 μηι.

[0060] Wall thickness of each nanotube may be in the range of about 10 nm to about 20 nm, such as about 10 nm to about 18 nm, about 10 nm to about 15 nm, about 12 nm to about 20 nm, about 15 nm to about 20 nm, about 12 nm to about 15 nm, about 14 nm to about 16 nm, about 14 nm, about 15 nm, or about 16 nm. In specific embodiments, each nanotube has a wall thickness of about 15 nm.

[0061] The metal oxyhydroxide nanostructured material may be conformally coated on the carbon-based fibrous substrate. The metal oxyhydroxide nanostructured material may be attached to the carbon-based fibrous substrate. Advantageously, using a method disclosed herein, a binder for attaching the metal oxyhydroxide nanostructured material to the carbon- based fibrous substrate is not used.

[0062] The invention refers in a second aspect to metal oxyhydroxide nanostructured material prepared by a method according to the first aspect. Examples of metal oxyhydroxide nanostructured material have already been mentioned above. In various embodiments, the metal oxyhydroxide is manganese oxyhydroxide (MnOOH).

[0063] In a third aspect, a supercapacitor electrode comprising a metal oxyhydroxide nanostructured material prepared by a method according to the first aspect is provided.

[0064] Examples of metal oxyhydroxide nanostructured material have already been mentioned above. As mentioned above, unique structural properties of metal oxyhydroxide nanotubes provide them with excellent electrochemical performances in terms of specific capacitance, energy and power densities. By hybridizing the carbon-based fibrous substrate with metal oxyhydroxide species using the binder-free concept in the scalable approach disclosed herein, the carbon-based fibrous substrate comprising the metal oxyhydroxide nanostructures may be used as flexible electrodes in supercapacitors.

[0065] Hereinafter, the present invention will be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, lengths and sizes of layers and regions may be exaggerated for clarity.

[0066] As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

[0067] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0068] The invention illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms "comprising", "including", "containing", etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the inventions embodied therein herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention.

[0069] The invention has been described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

[0070] Other embodiments are within the following claims and non- limiting examples. In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group. EXPERIMENTAL SECTION

[0071] In exemplary embodiments, successful attempt has been made on controlled synthesis of tubular MnOOH with nano-dimension on high electronic conductivity graphite felt (GF) as flexible supercapacitor electrode. As a fundamental study, time-dependent kinetics was investigated to interpret its formation mechanism, which can be depicted as the curling of 2-dimensional precursor into 1 -dimensional structure with hollow interior. With the nanotubes structure, the active surface area of the MnOOH can be completely accessible to electrolyte ions besides of having shorter charge transport length and greater ability to withstand the structural deformation. Hence, the hollow-structured MnOOH shows great promise in electrochemical system which can be reflected from its high specific capacitance of 1156 F g^"1 at 1 A g^"1. On the other hand, the high energy density of 1 125 W h kg^"1 and power density of 5.05 kW kg^"1 also revealed the outstanding energy storage behaviour of MnOOH/GF as the supercapacitor electrode.

[0072] Example 1: Synthesis of the as-prepared samples

[0073] Commercially available graphite felt (GF) was cut into 1 cm x 5 cm and immersed in 3 M nitric acid to remove the impurities deposited on the surface. After that, the GF was rinsed with deionized water for several times and dried in vacuum oven overnight for further usage. The GF was subsequently used as support for MnOOH deposition. [0074] MnOOH nanotubes may be prepared by immersing the GF in 35 mL of deionized water, followed by dropwise addition of 2 mL of 0.1 M potassium permanganate (KMn0₄) solution under sonication (30 min) in a 50 mL Teflon lined pressure vessel. The Teflon lined autoclave was then hydrothermally treated in an electric oven preheated to 190 °C for 3 h. For the synthesis of nanoflakes and nanorods, the hydrothermal duration was set at 45 min and 6 h respectively.

[0075] After the allowed reaction duration, the autoclave was cooled to room temperature naturally and the as-prepared samples were rinsed with considerable amounts of deionized water to get rid of the remaining reactants and undesirable side products. The samples were then dry in vacuum oven overnight to remove residual water.

[0076] Example 2: Material characterization

[0077] X-ray diffraction (XRD) was performed on Shimadzu Thin Film diffractometer with Cu Ka radiation (λ = 0.15406 nm) for compositional analysis. The diffraction pattern was collected within 10° to 70° (2Θ). JEOL JSM-7600F scanning electron spectroscopy (SEM) and JOEL JEM 2100 transmission electron microscope (TEM), operating at 200 kV were employed for morphology investigation. Accelerated Surface Area and Porosimetry System (ASAP 2020) was used for the measurement of nitrogen adsorption/desorption isotherms were at -196 °C. Before the measurement, all the as-prepared samples were degassed at 150 °C for 6 h under vacuum. The specific surface areas were determined by Brunauer-Emmet-Teller (BET) method.

[0078] Example 3: Electrochemical characterization

[0079] The electrochemical characterizations were carried out on Solartron analytical equipment (Model 1470E). A two electrode cell configuration was employed in cyclic voltammetry (CV) and galvanostatic charge/discharge voltammetry. The as-prepared samples were directly used for measurements without undergoing any fabrication steps or addition of carbonaceous conductive matrix. To ensure better penetration of the electrolyte into the inner region of the electrode, the electrodes were immersed in the electrolyte, 1 M of LiC10₄/PC solution for 15 min at ambient condition before the electrochemical measurements were conducted.

[0080] Specific capacitance of the cell was derived from the galvanostatic charge/discharge curve using the following equation:

[0081]

[0082] where / is the discharge current, is the slope of the discharged curve after IR dt

drop, m is the total mass of the active materials. Since a symmetry electrode configuration was used in the measurement, the single electrode capacitance is twice of the total capacitance obtained.

[0083] The capacitance contribution of MnOOH can be derived from the following equation.

[0084]

composite„ _ _<~> GF MnOOH

l^sp X m_comp_OSite— ^sp ^x niGF + ^sp ^{x m}MnOOH

[0085] where c_sp ^composite, C_sp ^GF, C_sp ^MnOOH represent the specific capacitance of the MnOOH/GF composite, GF and MnOOH respectively. m_composite, HIGF, m_MnooH represent the mass of the MnOOH/GF composite, GF and MnOOH respectively.

[0086] The energy (E) and power (P) density can be calculated by using equation (3) and (4) respectively.

[0087] E (Whkg^'') = - C(AV)² x (3)

2 3600

[0088] P (Wkg ') =— x 3600 (4)

At

[0089] where C, AV, At are the specific capacitance, voltage range and discharged, time after the IR drop respectively.

[0090] Example 4: Characterization of the as-prepared samples

[0091] In-situ growth of MnOOH on GF can be achieved via a facile hydrothermal method.

[0092] Initially, the acid treated GF was soaked in a precursor solution of KMn0₄ with vigorous sonication to introduce homogeneous dispersion of the Mn0₄ ^~ anion onto the GF surface. The core-shell structure was formed by reduction of Mn0₄ ^" anion on GF under hydrothermal condition, finally leading to the formation of MnOOH. XRD diffraction patterns shown in FIG. 1(b) provide the crystallographic information of the sample after 3 h hydrothermal treatment, which can be referred to manganite γ-ΜηΟΟΗ phase (JCPDS 41- 1379). The main diffraction peaks located at 26.4°, 34.2°, 35.9°, 37.5°, 40.0°, 41.4°, 51.7°,

54.2°, 55.4°, 56.6°, 62.4°, 65.5°, 65.7° can be indexed to the (Ϊ 1 1), (020), (1 1 1), (200), ( Ϊ21),

( 202), (210), (022), ( 2 22), (3 11), (131), (131), (202) planes respectively. In addition, the broad diffraction peaks at 25° and 44° shown in FIG. 1(a) may be ascribed to the (002) and (100) planes of graphite felt, respectively (JCPDS 41-1487).

[0093] No crystalline impurity phase was detected other than the alumina holder peak observed at 38° (JCPDS 04-787). Morphology of the sample was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Before undergoing hydrothermal treatment, the carbon scaffold with 10 μπι diameter with smooth surface, shown in FIG. 2(A), functions as a support for nucleation of the manganese species. The SEM images in FIG. 2(B) illustrated the surface of GF after deposition of MnOOH. Rougher texture may be observed and the thickness was apparently greater than the uncoated GF, indicating the conformal coating of nanostructured MnOOH on the carbonaceous skeleton. The foldable capability of the composite shown in the inset of FIG. 2(C) indicated that it is strong enough to withstand structural deformation and applied stress. FIG. 2(C) showed a mat of interwoven 1-D nanostructures with the length of several micrometers grown on the GF surface.

[0094] Investigating by TEM, these 1-D nanostructures were nanotubes with diameter in the range of 20 nm to 50 ran and their hollow interiors can be clearly seen (FIG. 2(D)). The high magnification TEM images (FIG. 2(E) to (F)) indicated that the nanotubes are mostly open-ended with the wall thickness of about 15 nm. The high-resolution TEM (HRTEM) image (FIG. 2(G)) of one nanotube revealed its polycrystalline nature and the lattice information further confirmed the formation of MnOOH, where the 0.24 nm and 0.34 nm lattice d-spacings matched the (002) and ( 1 1 1) planes of its crystallographic structure, respectively.

[0095] Having a strong correlation between the surface area and the capacitance, BET measurement was conducted to provide detailed information on the surface texture of the active materials. The specific surface area of the as-prepared samples was summarized in TABLE 1.

[0096] TABLE 1 Physical properties of the as-prepared samples.

[0097] Bare graphite that constructed in micron size can only provide BET surface area of 10.2 m² g^"1. After incorporation of MnOOH nanotubes, the BET surface area increased

2 1

significantly to 153.7 m g^" , showing that the tailored morphology can give rise to higher surface-to- volume ratio.

[0098] Example 5: Morphological evolution of MnOOH

[0099] The time-dependent kinetics was investigated to understand the formation mechanism of the MnOOH sample. It indicated that the morphology and surface structures of MnOOH could be tuned by varying the reaction duration. When the reaction time was set to be 45 min, lamellar structure evolved and densely covered the GF surface (FIG. 3(A) to (C)). It is noted that the nanosheets (FIG. 3(B)) tend to stack together and formed aggregates, which is consistent with the SEM observation. Further magnifying the view of the edge region as shown in FIG. 3(C), ultrathin nanosheets with submicrometers in lateral size could be clearly seen.

[00100] The HRTEM (FIG. 3(D)) revealed that the nanosheets were polycrystalline with grain size of about 5 nra. In this regard, XRD peaks in FIG. 4 of the nanosheets sample could be attributed to the δ-Μη0₂ (JCPDS 80-1089) that has a layered structure. The 2D nanosheets were believed to originate from the aggregation of Mn0₆ octahedra in the solution and the condensation of these monomers has driven them packing into nanosheet morphology by following an oriented attachment mechanism.

[00101] The nitrogen adsorption/desorption isotherm (FIG. 5) showed a typical type IV isotherm with a pronounced hysteresis loop. This suggested the mesoporous characteristics of the nanosheets, which constructed the interspaces between the small cystallites. The BET surface area of the nanosheets (105.5 m² g^"1) is lower than that of the nanotubes, indicating the intimate stacking of nanosheets has blocked some of the active-site accessibilities. With increasing reaction time to 3 h, the Mn0₂ was reduced to MnOOH.

[00102] The TEM observations (FIG. 6B) depicted the intermediate stage that hierarchical nanosheets were eventually scrolled into a roll. The rolling of the nanosheets is related to the tendency for reduction in surface energy and obtaining saturation level of the free bonds from the nanosheets boundary under elevated temperature and pressure during the hydrothermal treatment. Previously, the rolling mechanism was also suggested in explaining the transformation mechanism of 2-D nanosheets into 1 -D structures.

[00103] Further extending the reaction time to 6 h, the morphology of the sample changed to rod-shape with length of about 1 μιη and diameter of about 100 nm, as shown in FIG. 3(A) to (C). The HRTEM image of one nanorod, shown in FIG. 3(H), displayed that it is polycrystalline with the grain size of 5 nm to 10 nm. This observation indicates the infilling process of nanotubes with nanoparticles, which was evidenced by the observation of the sample (FIG. 6(D)) taken in intermediate state (4 hours reaction time). This phenomenon was related to the heterogeneous nucleation of nanocrystallites on the surface of the nanotubes and ultimately the tubular interior was filled with the newly formed nanoparticles. Meanwhile, the MnOOH species had strong tendency to arrange into monoclinic packing driven by the external heat energy and the crystallinity of the as-prepared samples was improved significantly with increasing reaction time suggested by the enhanced XRD peak intensity (FIG. 4).

[00104] From the BET analysis of the nanorods, the capillary condensation effect of N₂ (FIG. 5) became less apparent and adopted a type II isotherm, hinting gradual loss of the mesoporous structure as time goes. The obtained BET surface area of the nanorods reduced

2 1

to 109.3 m g^' because the densification process had sacrificed a number of active sites from the interiors. The schematic representation of the morphological evaluation of MnOOH is summarized in FIG. 7.

[00105] Example 6: Electrochemical performances

[00106] The electrochemical performances of the as-prepared samples were investigated using cyclic voltammetry (CV) and galvanostatic charge/discharge measurements. The redox reaction between various valence states of manganese ions upon chemisorption or physisorption can be used to represent the charge storage reaction at the electrode, as follows:

[00107] (MnOOH)_sulface + C⁺ + e <→ (MnOOH^" C⁺)_surface (5)

[00108]

MnOOH + A^" <-> Mn0₂ + AH + e^" (6)

[00109] where C⁺ and A^" represent the cation and anion in the electrolyte respectively.

[00110] It is known that the energy stored in a capacitor is a function of capacitance (C) and working potential (V), represented by the equation: E = ½ CV². To overcome the shortages of aqueous electrolyte that easily undergoes water electrolysis and turn into evolution of gaseous products (hydrogen and oxygen) beyond 1.2 V, organic electrolytes become the attractive alternatives. Hence, the double-electrodes cell was measured in LiC10₄/propylene carbonate (PC), which is thermodynamically stable in the range of 2.5 V.

[001 1 1] FIG. 8(A) showed the CV curves of the as-prepared MnOOH/GF samples at 50 mV s^"1. At this scan rate, all the as-prepared samples maintain an almost rectangle CV loop. It hint the good kinetic reversibility and low contact resistance of the active materials, where the electrolyte could penetrate beyond the surface region and the diffusion rate was able to keep pace with the chemical reaction occurred on the electrode surface.

[001 12] Although the intrinsic features of graphite felt have prompted enhanced reversibility in the graphite felt, its specific capacitance derived from the electrostatic interaction is limited. The area of the CV loop of the pure graphite felt that represent the total number of electron transferred during the electrochemical reaction was significantly smaller than those with MnOOH attached. No obvious redox peaks were observed from the voltammograms in FIG. 8(A), evidencing the unique electrochemical properties of manganese species in performing charge/discharge processes at a pseudo-constant rate.

[001 13] The galvanostatic charge/discharge curves were measured (FIG. 8(B)) to evaluate their specific capacitance. In brief, the as-prepared samples can be arranged in the order of increasing capacitance as follows: GF < nanosheets < nanorods < nanotubes. The specific capacitance of the as-prepared MnOOH/GF composites as a function of current densities was illustrated in FIG. 8(C). Since both the GF and MnOOH are electro-active species, the specific capacitance of MnOOH alone can be evaluated by extracting the contribution of GF using Eq. 2, as shown in FIG. 8(D). In this case, the bare graphite felt can only deliver a specific capacitance of 1 18 F g^"1 at 1 A g^"1 without addition of pseudocapacitive materials.

[001 14] In contrast, the specific capacitance was significantly enhanced for MnOOH samples, e.g. 1 156 F g^"1 for MnOOH nanotubes, 347 F g^"1 for MnOOH nanosheets and 575 F g^"1 for MnOOH nanorods at 1 A g^"1. This manifested the structural effect on the electrochemical performances, where nanotubes that possess the highest BET surface area can achieve the highest specific capacitance. Along this line, the tubular structure was beneficial for maximizing the utilization of the active materials by activating the inner walls as the active sites for electrochemical reactions. Furthermore, the hollow interiors can shorten the diffusion length of the charge transport by providing electrolyte ions through their interior tunnels and replenishing the electrolyte depleted region effectively. The MnOOH nanotubes also exhibited excellent rate capability by delivering 675 F g^"1 at a high current density of 5 A g^"1. Comparatively, GF, nanosheets, nanorods can only achieve specific capacitances of 68 F g^"1, 128 F g^"1, 72 F g^"1, respectively, at a current density of 5 A g^~\ Again, these results had proved the positive impacts obtained from the nanotubes morphology.

[00115] Further investigation was also done on the cycling stability of the as-prepared MnOOH/GF composites. Apart from the conventional fabrication method, the GF was used as the active materials as well as the current collector. Therefore, the electrical double layer charge storage reaction of GF that governed by the electrostatic charge separation has to be taken into consideration. As shown in FIG. 9(A), the GF composites with MnOOH nanosheets, nanotubes, nanorods exhibited capacitance retention of 90 %, 92 % and 70%, respectively, upon charge and discharge after 1000 cycles. Minimum capacitance decay of the nanotubes composites could be attributed to their capability to withstand the structural deformation during the electrochemical reaction through their well-defined tubular structure. The concentric expansion and contraction force exerted on the tubular structure results in more symmetric deformation, making it remained under the threshold stress for structural collapse upon long-term cycling. Here, the contributions of GF as the scaffold for mechanical support and conductive "highway" to preserve electrical integrity of the MnOOH species should not be omitted also.

[00116] The energy and power densities of the symmetrical two-electrode cell evaluated at different current densities can be gleaned from the Ragone plot shown in FIG. 9(B). The samples with manganese loading exhibited higher in both energy and power densities than the GF. Precisely tuning the morphology of manganese species enabled high power density of 5.05 kW kg^-1 to be obtained from the MnOOH nanotubes while maintaining a relatively high energy density of 1125 W h kg^"1.

[001 17] Example 7: Conclusion [001 18] In summary, the MnOOH nanotubes were synthesized via a facile hydrothermal method. The underlying mechanism may be depicted as the rolling mechanism that transforms the 2-D nanosheets to 1-D nanotubes. The subtle interplay between the morphology and dimension is responsible for the remarkable properties of MnOOH as supercapacitor. In this regard, the tubular structure of MnOOH has contributed to different aspects: (1) both the exterior and interior surface of the active materials were subjected to chemical reactions; (2) reduced the mass and charge transport; (3) allowed more freedom of expansion to accommodate their volume change during redox transformation. As a result, the electrochemical performances of the MnOOH nanotubes surpassed the nanosheets and nanorods.

[001 19] The significance of the present work also falls on the fundamental understanding on the mechanistic transformation process of MnOOH, which provide new platform for rational materials design. Coupling the binder-free concept in this scalable approach, fabrication of flexible supercapacitor electrode can be easily achieved, making it feasible for industrial use.

[00120] Various embodiments refer to a facile hydrothermal synthesis of freestanding nanostructured MnOOH/GF composites as efficient electrochemical capacitor. With the architecture of ίη-situ growth MnOOH on filamentary microstructure GF flexible substrate, non-conductive binders that induce internal resistance and block the reaction surface area of the electrode may be excluded. Nonetheless, successful synthesis of MnOOH nanotubes has been demonstrated herein and the significant capacitive enhancement was observed attributing to their unique tubular structure.

[00121] More remarkably, shape-controlled synthesis of MnOOH has been demonstrated and the formation mechanism was studied in detail, which may be described as a rolling process followed by a nanoparticles infilling process to transform the nanosheets to nanorod as a function of reaction time. Furthermore, the binder-free strategy was developed in preparing the flexible and freestanding electrode to reduce the reliability on conventional fabrication procedures. By assembling the MnOOH/GF freestanding electrodes and tested them in organic electrolyte (up to 2.5 V), superior electrochemical performances in terms of energy and power densities were observed, implying the feasibility and practical application of this hybrid composite as high-efficiency supercapacitor. [00122] This invention is not to be limited in scope by any of the specific embodiments described herein. These embodiments are intended for the purpose of exemplification only. Functionally equivalent elements, constituents (e.g. other metal oxyhydroxides) and preparation methods/parameters are also intended to be within the scope of the invention as described herein.

[00123] While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.

Claims

Method of preparing a metal oxyhydroxide nanostructured material, the method comprising

Method according to claim 1, wherein the carbon-based fibrous substrate is graphite felt.

Method according to claim 1 or 2, wherein depositing a metal oxyhydroxide precursor on a carbon-based fibrous substrate comprises immersing a carbon -based fibrous substrate in an aqueous solution comprising the metal oxyhydroxide precursor.

Method according to claim 3, wherein the metal oxyhydroxide precursor is added in form of a solution and in a dropwise manner into the aqueous solution in which the carbon-based fibrous substrate is immersed under agitation.

Method according to claim 4, wherein the agitation comprises sonication.

Method according to any one of claims 1 to 5, wherein heating the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor under hydrothermal conditions comprises heating the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor, preferably in an autoclave, at a temperature in the range of about 120 °C to about 190 °C.

Method according to any one of claims 1 to 6, wherein heating the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor under hydrothermal conditions comprises heating the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor for a time period in the range of about 30 minutes to about 8 hours.

Method according to any one of claims 1 to 7, wherein the metal oxyhydroxide nanostructured material is selected from the group consisting of nanosheets, nanorods, and nanotubes.

Method according to any one of claims 1 to 8, wherein the metal oxyhydroxide nanostructured material comprises or consists of nanosheets.

Method according to claim 9, wherein heating the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor under hydrothermal conditions comprises heating the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor in an autoclave for 1 hour or less.

Method according to any one of claims 1 to 8, wherein the metal oxyhydroxide nanostructured material comprises or consists of nanorods.

Method according to claim 1 1 , wherein heating the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor under hydrothermal conditions comprises heating the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor in an autoclave for at least 6 hours.

Method according to any one of claims 1 to 8, wherein the metal oxyhydroxide nanostructured material comprises or consists of nanotubes.

Method according to claim 13, wherein heating the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor under hydrothermal conditions comprises heating the carbon-based fibrous substrate comprising the metal oxyhydroxide precursor in an autoclave for more than 45 minutes and less than 6 hours, preferably about 2 hours to about 4 hours. Method according to claim 13 or 14, wherein each nanotube has a diameter in the range of about 20 nm to about 50 ran.

Method according to any one of claims 13 to 15, wherein each nanotube has a length in the range of about 1 μιη to about 10 μτη.

Method according to any one of claims 13 to 16, wherein each nanotube has a wall thickness in the range of about 10 nm to about 20 nm.

Method according to any one of claims 13 to 17, wherein each nanotube has a wall thickness of about 15 nm.

Method according to any one of claims 1 to 18, wherein the metal oxyhydroxide nanostructured material is conformally coated on the carbon-based fibrous substrate.

Method according to any one of claims 1 to 19, wherein a binder for attaching the metal oxyhydroxide nanostructured material to the carbon-based fibrous substrate is not used.

Method according to any one of claims 1 to 20, wherein metal of the metal oxyhydroxide is selected from Period 4 of d-block element of the Periodic Table of elements.

Method according to any one of claims 1 to 21 , wherein the metal oxyhydroxide is manganese oxyhydroxide (MnOOH).

Method according to any one of claims 1 to 22, wherein the metal oxyhydroxide precursor comprises or consists of an oxyacid salt.

Method according to claim 23, wherein the oxyacid salt is selected from the group consisting of manganates, permanganates, borates, titanates,_. tantalates, molybdates, vanadates, metavanadates, chromates, zirconates, sulfates, phosphates, polyphosphates, silicates, nitrates, chlorides, and combinations thereof.

Method according to claim 23 or 24, wherein the oxyacid salt comprises or consists of a permanganate (Mn0₄ ^~) salt.

Method according to claim 25, wherein the metal oxyhydroxide precursor is sodium permanganate, potassium permanganate, or mixtures thereof.

Metal oxyhydroxide nanostructured material prepared by a method according to any one of claims 1 to 26.

Supercapacitor electrode comprising a metal oxyhydroxide nanostructured material prepared by a method according to any one of claims 1 to 26.