WO2018033912A1

WO2018033912A1 - Asymmetric supercapacitor electrode having a combination of carbon allotropes

Info

Publication number: WO2018033912A1
Application number: PCT/IL2017/050900
Authority: WO
Inventors: Ervin TAL-GUTELMACHER; Mordechay Moshkovich; Tamir Stein
Original assignee: POCell Tech Ltd.
Priority date: 2016-08-18
Filing date: 2017-08-16
Publication date: 2018-02-22
Also published as: IL247365B

Abstract

An asymmetric supercapacitor comprising a unique electrode composition utilizing a transition metal oxide or sulfide and a combination of three different carbon allotropes, and methods for the preparation thereof.

Description

ASYMMETRIC SUPERCAPACITOR ELECTRODE HAVING A COMBINATION OF

CARBON ALLOTROPES

FIELD OF THE INVENTION

The present invention is directed to an asymmetric supercapacitor comprising a unique electrode composition utilizing a transition metal oxide or sulfide and a combination of different carbon allotropes, and methods for the preparation thereof.

BACKGROUND OF THE INVENTION

Ongoing technological advances in such disparate areas as consumer electronics, transportation, and energy generation and distribution are often hindered by the capabilities of current energy storage/conversion systems, thereby driving the search for high-performance power sources that are also economically viable, safe to operate, and have limited environmental impact. Electrochemical capacitors (ECs), or an electric double-layer capacitor (EDLC), also termed supercapacitor s or ultracapacitors, are one class of energy- storage devices that fill the gap between the high specific energy of batteries and the high specific power of conventional electrostatic capacitors.

A basic EDLC cell configuration is a pair of highly porous electrodes, typically including activated carbon, disposed on opposite faces of parallel conductive plates known as current collectors. The electrodes are impregnated with an electrolyte, and separated by a separator consisting of a porous electrically-insulating and ion-permeable membrane. When a voltage is applied between the electrodes, negative ions from the electrolyte flow to the positive electrode while positive ions from the electrolyte flow to the negative electrode, such that an electric double layer is formed at each electrode/electrolyte interface by the accumulated ionic charges. As a result, energy is stored by the separation of positive and negative charges at each interface. The separator prevents electrical contact between the conductive electrodes but allows the exchange of ions. When the EDLC is discharged, such as by powering an external electrical device, the voltage across the electrodes results in current flow as the ions discharge from the electrode surfaces. The EDLC may be recharged and discharged again over multiple charge cycles.

While the amount of energy stored per unit weight is generally lower in an EDLC in comparison to electrochemical batteries, the EDLC has a much greater power density and a high charge/discharge rate. Furthermore, an EDLC has a far longer lifespan than a battery and can undergo many more charge cycles with little degradation (millions of charge cycles, compared to hundreds for common rechargeable batteries). Consequently, EDLCs are ideal for applications that require frequent and rapid power delivery, such as hybrid vehicles that are constantly braking and accelerating, while batteries are used to supply a larger amount of energy over a longer period of time. EDLCs are also environmentally friendly (have a long lifespan and are recyclable), safe, lightweight, and have a very low internal resistance (ESR). The charging process of an EDLC is also relative simple, as it draws only is the required amount and is not subject to overcharging.

The most widely available commercial supercapacitors is an electric double-layer capacitor (EDLC) based on a symmetric configuration of two high-surface-area carbon electrodes separated by an electrolyte. Charge is stored in the electric double-layer that arises at all electrode/electrolyte interfaces, resulting in effective capacitances of 10-40 uFcrn^-2 (for flat plates). On charge the anions are adsorbed on one electrode and the cations on the other one. Aqueous-based activated carbon (AC) supercapacitors are promising low cost devices for providing high power densities, since water is a low-cost and non-toxic material, aqueous electrolytes do not require specific manufacturing conditions, and possess relatively high conductivity. However, energy density of aqueous electrolyte supercapacitors is relatively low due to the limited cell voltage. An efficient way to improve the cell voltage in terms of the energy density is to use organic electrolytes with a wider electrochemical stability window than water. Organic electrolytes including the combination of a solvent with different salts could enable the maximum cell voltage to reach more than 3V, a value three times higher than the maximum cell voltage of aqueous-based supercapacitors. However, such improvements inevitably sacrifice the capacitance and equivalent series resistance (ESR), which precludes it from easily reaching-high power density. The organic electrolytes also suffer from toxicity, flammability and safety hazards and from high manufacturing costs. Additionally, organic electrolytes have low tolerance for high temperatures thus limiting the working conditions of a capacitor comprising such electrolytes.

Specific energy can be further enhanced by moving to asymmetric configurations and selecting electrode materials that store charge via rapid and reversible pseudo electron-exchange reactions on or near the electrode surface in addition to the electrical double-layer capacitance. The exact mechanism of charge storage is not well known. Such materials often express broad and symmetric charge-discharge profiles that are reminiscent of those generated by double-layer capacitance, thus the term "pseudocapacitance" is used to describe their charge- storage mechanism. Many transition metal oxides, metal nitrides, and conducting polymers exhibit pseudocapacitance. Pseudocapacitance-based charge storage is most effective in aqueous electrolytes, and the corresponding enhancements in charge- storage capacity can compensate for the restricted voltage window of water, resulting in energy densities for aqueous asymmetric (also termed hybrid) ECs that are competitive with non-aqueous conventional EDLCs. By using other electrode materials in addition to carbons, asymmetric EC designs also circumvent the main limitation of aqueous electrolytes by extending their operating voltage window beyond the thermodynamic 1.2 V limit to operating voltages approaching 2 V.

In the asymmetric AC/metal oxide electrochemical capacitors, one electrode stores charge through a reversible, non-faradaic reaction of ion adsorption/desorption on the surface of an active carbon, and the other electrode (also termed herein "pseudocapacitive electrode") utilizes a reversible pseudo-redox reaction in a transition metal oxide electrode.

Transition metal oxide exhibiting the highest pseudocapacitance is Ru(¾. However, since ruthenium is a noble metal, ruthenium oxides cannot be used in electrochemical capacitor applications on a large scale. An alternative metal oxide exhibiting capacitance- like behavior is manganese oxide, which is currently extensively used in the supercapacitor technology. In terms of specific capacitance, MnOx-based materials demonstrate a clear advantage compared to carbon-based materials that rely solely on double-layer capacitance.

Despite their attractive features, such as high pseudocapacitance and low cost, manganese oxide electrodes have several disadvantages. The capacitance of thick MnC electrodes is ultimately limited by the poor electrical conductivity of MnCh, while performance of a supercapacitor using a planar electrode ultrathin configuration is restricted because of low mass loading.

In order to increase energy and power density of the metal oxide-based supercapacitor electrodes, a carbonaceous material can be added to said electrode. For example, US Patent Application publication No. 2016/0042877 to some of the inventors of the present invention is directed to an electric double-layer capacitor (EDLC) and method for manufacturing thereof. The ELDC includes at least one capacitor cell with two parallel current collectors, two opposite polarity electrodes, a separator, and a rigid dielectric frame. The electrode material may include: activated carbon, a transitional metal oxide, a conductive polymer, and/or graphene.

US Application Publication No. 2011/0261502 is directed to a charge storage device structure, incorporating a double layer supercapacitor (DLS) material, electrochemical supercapacitor (ECS) material and/or battery material. DLS material may contain a network of (e.g., electrically conductive) nano wires., including carbon nanotubes, metallic nano wires, semiconducting nanowires, oxide nanowires, organic nanowires and inorganic nanowires. The DLS material can further include carbonaceous materials such as graphene flakes, activated carbon and carbon aerogel.

US Patent Application Publication No. 2011/0255214 is directed to charge storage devices with at least one electrode having combined double layer supercapacitor, electrochemical supercapacitor and/or battery functionalities, wherein the electrode, may be composed of an ECS material, a highly- structured DLS material and a less-structured DLS material. The ECS material can be selected from a polymer, oxide, metal oxide nanoparticle and metal oxide nanowire, the highly-structured DLS material can be selected from nanowires, nanostructured carbon, graphene and carbon nanotubes, and the less-structured DLS material can be selected from activated carbon and carbon black.

However, energy and powder densities of the commercially available supercapacitors are still below the anticipated threshold which would allow their widespread implementation in disparate technological fields, such as, for example, transportation and energy storage applications. There remains, therefore, an unmet need for high-performance, long cycle life and cost-efficient hybrid supercapacitors employing pseudocapacitive electrodes.

SUMMARY OF THE INVENTION

The present invention provides a unique electrode composition utilizing a transition metal oxide or sulfide and a specific combination of carbon allotropes, demonstrating an improved capacitance and wide temperature range stability. The beneficial electrode composition of the invention promotes the formation of a mechanically, chemically and thermally- stable electrode. The three carbon allotropes, which should be present in the supercapacitor electrode according to the principles of the present invention, include carbon nanotubes (CNTs), graphite and amorphous carbon.

The present invention is based in part on a surprising finding that a supercapacitor electrode comprising a transition metal oxide and a specific combination of at least three carbon allotropes is characterized by enhanced conductivity, cycle life and capacitance as compared to an electrode lacking at least one of the carbon allotropes or containing a different combination of carbon allotropes. Specifically, addition of amorphous carbon to the electrode comprising a metal oxide, CNTs and graphite increased the capacitance of the electrode by about 27% at higher currents and by more than 50% at lower currents, when tested at essentially similar conditions. Surprisingly, substitution of amorphous carbon by paracrystalline carbon (which is a different allotropic form of carbon) in the electrode composition as described above resulted in the decrease of about 45% in the electrode capacitance as measured at both higher and lower currents.

The electrode according to the principles of the invention can be substantially free of commonly used binder agents, gelling agents and other thickening agents. Additionally, the materials used in the unique electrode composition of the invention are environmentally friendly, and have no hazardous effect on humans or the surrounding environment. The unique electrode composition of the invention allows the production of cost-efficient supercapacitor with a prolonged cycle life.

The present invention further provides facile preparation methods of the electrode, enabling both small and large scale production possibilities, which can be tailored to the desired supercapacitor application.

Thus, according to one aspect, the present invention provides an asymmetric supercapacitor, comprising at least one capacitor cell comprising a first porous electrode, a second porous electrode, an aqueous electrolyte being in contact with both first porous and second porous electrodes, and a separator separating the first porous electrode from the second porous electrode, wherein the first porous electrode comprises a transition metal oxide or sulfide in a weight percent ranging from about 50 to 90 wt% of the total weight of the first electrode, and at least three allotropes of carbon, comprising carbon nanotubes (CNTs), graphite and amorphous carbon.

In some embodiments, the first porous electrode, as described above, comprises a metal oxide. In further embodiments, the transition metal oxide of the first porous electrode is selected from the group consisting of Mn_nO_x, TiO_x, NiO_x, CoO_x, SnO_x, and combinations thereof, wherein x ranges from 1.5 to 3, and n ranges from 1 to 2. Each possibility represents a separate embodiment of the invention. In a currently preferred embodiment, the transition metal oxide is MnCh. In an additional embodiment, the transition metal oxide is NiOx.

In some embodiments, the first porous electrode comprises a metal sulfide. In further embodiments, the transition metal sulfide of the first porous electrode is selected from the group consisting of FeS_y, MoS_y, NiS_y, CoS_y, MnS_y, TiS_y, SnS_y and combinations thereof, wherein y ranges from 1.8 to 2.2. Each possibility represents a separate embodiment of the invention. In some embodiments, the transition metal oxide is present in the first porous electrode in a weight percent ranging from about 50 to about 90 wt% of the total weight of the first electrode. In further embodiments, the transition metal oxide is present in the first porous electrode in a weight percent ranging from about 60 to about 85 wt% of the total weight of the first electrode.

Is some embodiments, the transition metal sulfide is present in the first porous electrode in a weight percent ranging from about 50 to about 90 wt% of the total weight of the first electrode. Is further embodiments, the transition metal sulfide is present in the first porous electrode in a weight percent ranging from about 60 to about 85 wt% of the total weight of the first electrode.

In some additional embodiments, the CNTs present in the first porous electrode as described above are in a weight percent ranging from about 1 to about 10 wt% of the total weight of the first electrode. Is one embodiment the CNTs are selected from the group consisting of single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs) and combinations thereof. In a currently preferred embodiment, the CNTs are MWCNT. In certain embodiment, the CNTs diameter ranges from about 5 to about 25 nm.

In one embodiment, the graphite present in the first porous electrode as described above is in a weight percent ranging from about 0.5 to about 40 wt% of the total weight of the first porous electrode.

In another embodiment, the amorphous carbon present in the first porous electrode as described above is in a weight percent ranging from about 0.5 to about 40 wt% of the total weight of the first electrode. In a related embodiment, the amorphous carbon comprises activated carbon. In some specific embodiments, the activated carbon has a surface area of at least about 500 m²/gr- In some other embodiments, the activated carbon has a surface area of at least about 1000 m7gr. In some related embodiments, the activated carbon has an ash content of above 10 wt%. In some other embodiments, the activated carbon has an ash content of above 20 wt%.

In some embodiments, the first porous electrode as described above is essentially free of paracrystalline carbon.

In an additional embodiment, the first porous electrode as described above, further comprises an additional carbon allotrope. The additional carbon allotrope can be graphene.

In some embodiments, the first porous electrode as described above is a binder-free electrode. In a related embodiment, the first porous electrode as described above is substantially free of gelling agents and/or thickening agents.

In one embodiment, the first porous electrode as described above comprises: from about 60 to about 85 wt% Mn0₂;

from about 0.1 to about 10 wt% CNTs;

from about 0.5 to about 15 wt% graphite; and

from about 0.5 to about 10 wt% activated carbon.

In a specific embodiment, the first electrode as described above consists essentially of Mn0₂, carbon nanotubes (CNT), graphite and activated carbon.

In another embodiment, the first porous electrode comprises:

from about 70 to about 90 wt% NiO_x;

from about 0.1 to about 10 wt% CNTs;

from about 0.5 to about 15 wt% graphite; and

from about 0.5 to about 10 wt% activated carbon.

In a specific embodiment, the first electrode as described above consists essentially of NiO_x, carbon nanotubes (CNT), graphite and activated carbon.

In some embodiments, the first porous electrode as described above has a thickness ranging from about 50 micron to about 5 mm. In a related embodiment, the first porous electrode as described above has a thickness ranging from about 300 to about 800 microns.

In some embodiments, the first electrode is impregnated with the electrolyte. In some related embodiments, the electrolyte comprises a dissolved salt selected from the group consisting of potassium hydroxide (KOH), sodium hydroxide (NaOH), lithium oxide (LiOH), potassium sulfate (K2SO4), sodium perchlorate (NaClC¼), sulfuric acid (H2SO4), hydrochloric acid (HC1), nitric acid (HNO₃), methanesulfonic acid (MSA, CH₃SO₃H), and tetrafluoroboric acid (HBF₄). Each possibility represents a separate embodiment of the invention. In one specific embodiment, the electrolyte is an alkaline electrolyte. In a currently preferred embodiment, the electrolyte comprises KOH.

In some embodiments, the first porous electrode acts as a cathode in the hybrid supercapacitor.

In some embodiments, the second porous electrode comprises a high surface area carbon material selected from the group consisting of amorphous carbon, graphite, CNTs, graphene, and combinations thereof. Each possibility represents a separate embodiment of the invention. In a currently preferred embodiment, the second porous electrode comprises amorphous carbon comprising activated carbon.

In some embodiments, the second porous electrode acts as an anode in the hybrid supercapacitor. According to another aspect, the present invention provides a method for preparing a supercapacitor electrode comprising a transition metal oxide or sulfide, and at least three allotropes of carbon comprising carbon nanotubes (CNTs), graphite and amorphous carbon. In one embodiment, the method as described above comprises the following steps:

(a) mixing the metal oxide or sulfide powder together with the powders of the at least three allotropes of carbon;

(b) adding to the mixture obtained in step (a) an electrolyte solution;

(c) mixing the composition obtained in step (b) to achieve a homogeneous paste; and

(d) processing the paste obtained in step (c) to form an electrode.

In one embodiment, the first porous electrode as described above, comprises a metal oxide. In further embodiments, the transition metal oxide is selected from the group consisting of Mn_nO_x, TiO_x, NiO_x, CoO_x, SnO_x, and combinations thereof, wherein x ranges from 1.5 to 3 and n ranges from 1 to 2. Each possibility represents a separate embodiment of the invention. In a currently preferred embodiment, the transition metal oxide is MnCh. In a certain embodiment, the transition metal oxide is NiO_x.

In some other embodiments, the porous electrode as described above, comprises a transition metal sulfide. In further embodiments, the transition metal sulfide is selected from teh group consisting of FeS_y, MoS_y, NiS_y, CoS_y, MnS_y, TiS_y, SnS_y and combinations thereof, wherein y ranges from 1.8 to 2.2. Each possibility represents a separate embodiment of the invention.

In another embodiment, the electrolyte added in step (b) of the method as described above is an alkaline electrolyte. In a specific embodiment, the electrolyte is KOH.

In an additional embodiment, either one of the mixing steps (a) or (c) or both of these steps are carried out utilizing a mixing apparatus selected from a mixer, a hand mixer, a mortar and pestle, a homogenizer and a sonicator. Each possibility represents a separate embodiment of the invention. In a currently preferred embodiment, the mixing in step (a) and in step (c) is carried out utilizing a hand mixer.

In an additional embodiment, the processing of electrode as described above in step (d) is carried out utilizing rolling.

In a specific embodiment, the weight ratio of the powder components mixed together in in step (a) ranges from about 60 to about 85 wt% Μη(¾; from about 0.1 to about 10 wt% CNTs; from about 0.5 to about 15 wt% graphite; and from about 0.5 to about 10 wt% activated carbon. In an additional embodiment, the weight ratio of the powder components mixed together in in step (a) ranges from about 70 to about 90 wt% NiO_x; from about 0.1 to about 10 wt% CNTs; from about 0.5 to about 15 wt% graphite; and from about 0.5 to about 10 wt% activated carbon.

Further embodiments and the full scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1: depicts a cyclic voltammogram of a supercapacitor comprising

Mn0₂:CNT:Graphite in a weight ratio of 85:5:10, respectively. Cyclic voltammetry scan rate used was 5mA/sec.

Figure 2: depicts a cyclic voltammogram of a supercapacitor comprising Mn0₂:CNT:Graphite in a weight ratio of 75:5:20, respectively. Cyclic voltammetry scan rate used was 5mA/sec.

Figure 3: depicts a cyclic voltammogram of a supercapacitor comprising Mn0₂:CNT:Graphite in a weight ratio of 65:5:30, respectively. Cyclic voltammetry scan rate used was 5mA/sec.

Figure 4: depicts a cyclic voltammogram of a supercapacitor comprising Mn02:CNT:Graphite:Activated Carbon in a weight ratio of 80:5:10:5, respectively. Cyclic voltammetry scan rate used was 5mA/sec.

Figure 5: depicts a cyclic voltammogram of a supercapacitor comprising Mn0₂:CNT:Graphite: Activated Carbon in a weight ratio of 70:5:10:15, respectively. Cyclic voltammetry scan rate used was 5mA/sec.

Figure 6: depicts a cyclic voltammogram of a supercapacitor comprising

Mn0₂:CNT:Graphite: Activated Carbon in a weight ratio of 60:5:10:25, respectively. Cyclic voltammetry scan rate used was 5mA/sec.

Figure 7: depicts a cyclic voltammogram of a supercapacitor comprising Mn0₂:CNT:Graphite:conductive carbon in a weight ratio of 80:5 :10:5, respectively. Cyclic voltammetry scan rate used was 5mA/sec. Figure 8: depicts a cyclic voltammogram of a supercapacitor comprising Mn0₂:CNT:Graphite:conductive carbon in a weight ratio of 70:5:10:15, respectively. Cyclic voltammetry scan rate used was 5mA/sec.

Figure 9: depicts a cyclic voltammogram of a supercapacitor comprising Mn0₂:CNT:Graphite:conductive carbon in a weight ratio of 60:5:10:25, respectively. Cyclic voltammetry scan rate used was 5mA/sec.

Figure 10: depicts a cyclic voltammogram of a supercapacitor comprising MnC^CNT: conductive carbon in a weight ratio of 80:5:15, respectively. Cyclic voltammetry scan rate used was 5mA/sec.

Figure 11: depicts a cyclic voltammogram of a supercapacitor comprising MnC^CNT: conductive carbon in a weight ratio of 70:5:25, respectively. Cyclic voltammetry scan rate used was 5mA/sec.

Figure 12: depicts a cyclic voltammogram of a supercapacitor comprising MnC^CNT: conductive carbon in a weight ratio of 60:5:35, respectively. Cyclic voltammetry scan rate used was 5mA/sec.

Figure 13: depicts a constant current (CC) profile of three different supercapacitors. a) represents the profile of supercapacitor (a) at 1.4V; b) represents the profile measured for supercapacitor (b) at 1.6V; c) represent a super capacitor with an electrode lacking amorphous carbon at similar conditions measured for supercapacitor (b).

Figure 14: summarizes the energy produced from different profiles measured for supercapacitors (a) and (b) measured at a discharge rate of 0.3mA/cm². I) constant current (CC) profile measured for super capacitor (b) charged to 1.8V; II) constant current constant voltage (CCCV) profile measured for super capacitor (b) charged to 1.6 V; III) CC profile measured for super capacitor (b) charged to 1.6V; and IV) CC/CCCV profiles measured for super capacitor (a) charged to 1.4V.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to an asymmetric supercapacitor demonstrating an improved capacitance and wide temperature range stability, having a unique electrode composition comprising a transition metal oxide or sulfide and a combination of different carbon allotropes. The beneficial electrode composition of the invention promotes the formation of a mechanically- stable free-standing electrode, which is substantially free of commonly used binder agents, gelling agents and commonly used thickening agents. Additionally, the specific electrode composition of the invention utilizes environmentally friendly materials, which have no hazardous effect on humans or the surrounding environment. Moreover, the unique electrode composition of the invention allows the production of cost-effective supercapacitor with a prolonged cycle life.

Thus, according to one aspect, the present invention provides a supercapacitor electrode comprising a transition metal oxide or sulfide, and at least three allotropes of carbon, comprising carbon nanotubes (CNTs), graphite and amorphous carbon.

According to another aspect, the present invention provides an asymmetric supercapacitor, comprising at least one capacitor cell comprising a first porous electrode, a second porous electrode, an aqueous electrolyte being in contact with both first porous and second porous electrodes, and a separator separating the first porous electrode from the second porous electrode, wherein the first porous electrode comprises a transition metal oxide or sulfide, and at least three allotropes of carbon, comprising carbon nanotubes (CNTs), graphite and amorphous carbon.

In one embodiment, the transition metal oxide of the first porous electrode as described above, is selected from the group consisting of Mn_nO_x, TiO_x, NiO_x, CoO_x, SnO_x and combinations thereof, wherein x ranges from 1.5 to 3, and n ranges from 1 to 2. Each possibility represents a separate embodiment of the invention. In a currently preferred embodiment, the transition metal oxide is MnC>2. In an additional embodiment, the transition metal oxide is NiO_x. In a further embodiment, said NiO_x is NiO_x(III).

In some embodiments, the transition metal sulfide of the first porous electrode as described above, is selected from the group consisting of FeS_y, MoS_y, NiS_y, CoS_y, MnS_y, TiS_y, SnS_y and combinations thereof, wherein y ranges from 1.8 to 2.2. Each possibility represents a separate embodiment of the invention.

Balancing the weight ratio between the transition metal component and the carbon components in the electrode composition of the invention is important for achieving high capacitance values while providing a wide operating voltage window. Thus, in some embodiments, the transition metal oxide or sulfide is present in the first porous electrode in a weight percent ranging from about 50 to about 90 wt% of the total weight of the first electrode. In other embodiments, the transition metal oxide or sulfide is present in the first porous electrode in a weight percent ranging from about 60 to about 85 wt% of the total weight of the first electrode. In certain embodiments, the first porous electrode comprises a metal oxide, which is present in a weight percent ranging from about 50 to about 90 wt% of the total weight of the first electrode. In further embodiments, the metal oxide is present in a weight percent ranging from about 60 to about 85 wt% of the total weight of the first electrode, from about 65 to about 85 wt , or from about 70 wt% to about 90 wt%. Each possibility represents a separate embodiment of the invention.

In certain embodiments, the first porous electrode comprises a metal sulfide, which is present in a weight percent ranging from about 50 to about 90 wt% of the total weight of the first electrode. In further embodiments, the metal sulfide is present in a weight percent ranging from about 60 to about 85 wt% of the total weight of the first electrode, from about 65 to about 85%, or from about 70 wt% to about 90 wt%. Each possibility represents a separate embodiment of the invention.

In some particular embodiments, the first porous electrode comprises Μη(¾, which is present in a weight percent ranging from about 50 to about 90 wt% of the total weight of the first electrode. In further embodiments, Μη(¾ is present in a weight percent ranging from about 60 to about 85 wt% of the total weight of the first electrode, or from about 65 to about 85 wt%. Each possibility represents a separate embodiment of the invention. In some exemplary embodiments, the first porous electrode comprises Mn0₂, which is present in a weight percent of about 80 wt% of the total weight of the first electrode. In further exemplary embodiments, the first porous electrode comprises Μη0₂, which is present in a weight percent of about 70 wt% of the total weight of the first electrode. In additional exemplary embodiments, the first porous electrode comprises Μη(¾, which is present in a weight percent of about 60 wt% of the total weight of the first electrode

In some particular embodiments, the first porous electrode comprises NiO_x, which is present in a weight percent ranging from about 50 to about 90 wt% of the total weight of the first electrode. In further embodiments, NiO_x is present in a weight percent ranging from about 60 to about 90 wt% of the total weight of the first electrode, or from about 70 to about 90 wt%. Each possibility represents a separate embodiment of the invention. In some exemplary embodiments, the first porous electrode comprises NiO_x, which is present in a weight percent of about 80 wt% of the total weight of the first electrode

As used herein, in various embodiments and in the claims, the term "weight percent" refers to the concentration as percentage by mass of the different components of the solid electrode based on the total dry mass of the electrode. The term "total dry mass", as used herein, refers to the total mass of the solid components of the electrode (i.e., the term does not include the mass of electrolyte solution, which may be present in the electrode).. For example, in some embodiments, the first porous electrode comprises a transition metal oxide in a weight percent ranging from about 50 to about 90 wt% of the total weight of the first electrode, thus, the total weight percent of the additional dry components (e.g. CNTs, graphite and amorphous carbon) ranges respectively, from about 50 to about 10 wt% of the total weight of the first electrode.

The term "porous", as used herein, refers to a structure of interconnected pores or voids such that continuous passages and pathways throughout a material are provided. In some embodiments, the porosity of the electrodes is from about 20% to about 90%, such as, for example, 30% - 80%, or 40% - 70% porosity. Each possibility represents a separate embodiment of the invention.

In some embodiments, the porous electrodes have a high surface area. The term "high surface area", as used in some embodiments, refers to a surface area in the range from about 1 to about 2000 m²/g, such as, for example, 10 - 100 m²/g or 50 -1500 m²/g.

In some embodiments, the terms "porous" and/or "high surface area" encompass materials having micro or nanoparticles.

CNTs

Without wishing to being bound by theory or mechanism of action, the use of CNTs as an integral part of the electrode composition improves the mechanical strength of the electrode, increases the electrical conductivity and capacitance, and provides a longer cycle life. The weight percent of CNTs in the electrode can be maintained balanced by utilizing other carbon- based components to achieve a cost effective, easy to fabricate and mechanically stable electrode composition. Thus, in some embodiments, the CNTs present in the first porous electrode as described above are in a weight percent ranging from about 0.1 to about 10 wt% of the total weight of the first electrode. In further embodiments, the CNTs present in the first porous electrode are in a weight percent ranging from about 0.5 to about 9 wt%, from about 2 to about 9 wt%, from about 3 to about 8 wt%, or from about 4 to about 7 wt% of the total weight of the first electrode. Each possibility represents a separate embodiment of the invention. In some exemplary embodiments, CNTs are present in the first porous electrode in a weight percent of about 5 wt% of the total weight of the first electrode. In another embodiment the CNTs are selected from the group consisting of single -walled carbon nanotubes (SWCNTs), multi- walled carbon nanotubes (MWCNTs) and combinations thereof. Each possibility represents a separate embodiment of the invention. In a currently preferred embodiment, the CNTs are MWCNTs. In a further embodiment, the CNTs average diameter is between about 5 to about 25 nm. In a currently preferred embodiment, the CNTs average diameter is between about 7 to about 15 nm. The CNTs can have an average length of about 0.5 to about 5 microns. In further embodiments, CNTs have an average length of about 1 to about 3 microns.

Graphite

Without wishing to being bound by theory or mechanism of action, it is contemplated that the use of graphite in combination with other carbon allotropes increases the conductivity of the porous electrode of the invention, and promotes high capacitance of the asymmetric supercapacitor. It was discovered by the inventors that graphite was extremely efficient in decreasing the equivalent series resistance (ESR) of the supercapacitor as compared to other highly conductive materials, such as, but not limited to paracrystalline carbon. In some embodiments, graphite is present in the first porous electrode as described above in a weight percent ranging from about 0.5 to about 40 wt% of the total weight of the first porous electrode. In further embodiments, graphite being present in the first porous electrode is in a weight percent ranging from about 1 to about 35 wt , from about 5 to about 25 wt , from about 7 to about 20 wt , or from about 8 to about 15 wt% of the total weight of the first electrode. Each possibility represents a separate embodiment of the invention. In some exemplary embodiments, graphite is present in the first porous electrode in a weight percent of about 10 wt% of the total weight of the first electrode. Without further wishing to being bound by theory or mechanism of action, it is contemplated that graphite loadings higher than mentioned hereinabove, decreased the capacitance of the pseudocapacitive electrode despite increasing its conductivity. Amorphous carbon

Without wishing to being bound by theory or mechanism of action, the use of amorphous carbon in combination with other carbon allotropes increases the surface area of the pseudocapacitive electrode of the invention and promotes high capacitance of the asymmetric supercapacitor. Additionally, it is contemplated that amorphous carbon decreases impedance and provides mechanical and/or electrical stabilization to the electrode, thereby increasing the life cycle of the asymmetric capacitor. In one embodiment, the amorphous carbon present in the first porous electrode as described above is in a weight percent ranging from about 0.5 to about 40 wt% of the total weight of the first electrode. In further embodiments, the amorphous carbon present in the first porous electrode is in a weight percent ranging from about 1 to about 30 wt , from about 2 to about 20 wt , from about 3 to about 15 wt , or from about 4 to about 10 wt of the total weight of the first electrode. Each possibility represents a separate embodiment of the invention. In some exemplary embodiments, amorphous carbon is present in the first porous electrode in a weight percent of about 5 wt of the total weight of the first electrode. In further exemplary embodiments, amorphous carbon is present in the first porous electrode in a weight percent of about 15 wt of the total weight of the first electrode. In additional exemplary embodiments, amorphous carbon is present in the first porous electrode in a weight percent of about 25 wt of the total weight of the first electrode.

In some related embodiments, the amorphous carbon comprises activated carbon. In some embodiments, the activated carbon has a surface area of at least about 500 m²/gr. In some specific embodiments, the activated carbon has a surface area of at least about 750 m²/gr. In some other embodiments, the activated carbon has a surface area of at least about 1000 m²/gr. In a currently preferred embodiment, the activated carbon has a surface area of at least about 1300 m /gr. Without further wishing to being bound by theory or mechanism of action, it is contemplated that increasing the amorphous carbon content above the values mentioned hereinabove decreases the electrochemical window of the supercapacitor, thereby reducing the efficiency of the asymmetric supercapacitor.

Moreover, in some embodiments, a low purity carbon source can be used as the amorphous carbon component which promotes a cost-effective manufacture process. The term "low purity carbon", as used herein, refers in some embodiment to the ash content of above about 5 wt . In other embodiments, the term refers to the content of impurities of above about 10 wt . In other embodiments, the term refers to the content of impurities of above about 20 wt . Said impurities can be selected from inorganic impurities, for example,, metals, oxides and ceramic materials (e.g., silicates). In some currently preferred embodiments, the activated carbon has an ash content of above 15 wt . In some other preferred embodiments, the activated carbon has an ash content of above 20 wt . In some specific embodiments, the activated carbon has an ash content of between about 20 to about 25 wt . Each embodiment represents a separate embodiment of the invention.

The present invention is based in part on the unexpected mechanical and electrochemical advantages achieved by utilizing a combination of a few different carbon allotropes together with the transition metal oxide or sulfide in the pseudocapacitive electrode of the invention. In some embodiments, the weight ratio between the transition metal oxide or sulfide and the combination of carbon allotropes in the first porous electrode, as described above, ranges from about 2:1 to about 6:1 , respectively. In further embodiments, the weight ratio ranges between about 3:1 to about 5:1.

According to some embodiments, the weight ratio between the transition metal oxide or sulfide and CNTs in the first porous electrode ranges from about 900:1 to about 5:1. In further embodiments, said ratio ranges from about 500:1 to about 9:1.

According to some embodiments, the weight ratio between the transition metal oxide or sulfide and graphite in the first porous electrode ranges from about 180:1 to about 1 :1. In further embodiments, said ratio ranges from about 100:1 to about 2:1.

According to some embodiments, the weight ratio between the transition metal oxide or sulfide and amorphous carbon in the first porous electrode ranges from about 180:1 to about 1 :1. In further embodiments, said ratio ranges from about 100:1 to about 2:1.

According to some embodiments, the weight ratio between the CNTs and graphite, as combined, and amorphous carbon in the first porous electrode ranges from about 100: 1 to about 1 :70. In further embodiments, said ratio ranges from about 5 :1 to about 1 :2.

In some embodiments, the weight ratio between graphite and amorphous carbon in the first porous electrode ranges from about 80:1 to about 1 :80. In further embodiments, said ratio ranges from about 5 :1 to about 1 :5 or from about 2:1 to about 1 :2. Each possibility represents a separate embodiment of the invention.

In some embodiments, the weight ratio between CNTs and graphite in the first porous electrode ranges from about 20:1 to about 1 :400. In further embodiments, said ratio ranges from about 1 :1 to about 1 :20 or from about 1 :3 to about 1 :6. Each possibility represents a separate embodiment of the invention.

In some embodiments, the weight ratio between CNTs and amorphous carbon in the first porous electrode ranges from about 20:1 to about 1 :400. In further embodiments, said ratio ranges from about 1 :1 to about 1 :20 or from about 1 :3 to about 1 :6. Each possibility represents a separate embodiment of the invention.

In certain embodiments, the first porous electrode as described above comprises transition metal oxide, CNTs, graphite and amorphous carbon in a weight ratio of about 13:2:3:2, respectively.

According to the principle of the present invention, the ratio between the different carbon allotropes in the first porous electrode as described above is beneficial to the mechanical and electrical properties of the electrode. Without being bound by theory or mechanism of action, it is understood that the physical characteristics of the different carbon allotropes such as crystallinity, affect the rigidity, conductivity, and/or mechanical and chemical stability of the electrode. For example, CNTs and graphite are both crystalline materials and they contribute to an increase in conductivity of the final electrode structure, while amorphous carbon increases the surface area of the electrode and increases the capacitance thereof. Thus, in some embodiments, the contents of the crystalline and amorphous carbon components in the first porous electrode are between about 0.6 to about 25 wt% and between about 0.5 to about 10 wt , respectively, of the total weight of the electrode. It is postulated that the concentration (as percentage by mass) of the carbon allotropes, as well as relative weight between crystalline and non-crystalline (amorphous) carbon components in a given electrode as described above, can be elucidated utilizing different methods such as thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Said methods can be further combined with microstructural investigation techniques, such as, but not limited to, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The concentration of the metal oxide or sulfide can be evaluated by the above characterization methods, as well as, energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), XPS (for definition of the oxidation state of the oxide), mass spectrometry analysis, i.e. inductively coupled plasma mass spectroscopy (ICP).

In one embodiment, the first porous electrode as described above comprises: from about 60 to about 85 wt% metal oxide; from about 0.1 to about 10 wt% CNTs; from about 0.5 to about 15 wt% graphite; and from about 0.5 to about 10 wt% activated carbon.

In one embodiment, the first porous electrode as described above comprises: from about 60 to about 85 wt% metal sulfide; from about 0.1 to about 10 wt% CNTs; from about 0.5 to about 15 wt% graphite; and from about 0.5 to about 10 wt% activated carbon.

In some currently preferred embodiments, the first porous electrode as described above comprises: from about 60 to about 85 wt% Mn(¾; from about 0.1 to about 10 wt% CNTs; from about 0.5 to about 15 wt% graphite; and from about 0.5 to about 10 wt% activated carbon. In certain embodiments, the first porous electrode comprises: from about 60 to about 80 wt% Mn0₂; about 5 wt% CNTs; about 10 wt% graphite; and from about 5 to about 25 wt% activated carbon. In a further embodiment, the first porous electrode comprises: about 80 wt% Mn(¾; about 5 wt% CNTs; about 10 wt% graphite; and about 5 wt% activated carbon. In a yet further embodiment, the first porous electrode comprises: about 70 wt% Μη(¾; about 5 wt% CNTs; about 10 wt% graphite; and about 15 wt% activated carbon. In a still further embodiment, the first porous electrode comprises: about 60 wt% Mn0₂; about 5 wt% CNTs; about 10 wt% graphite; and about 25 wt% activated carbon.

In additional embodiments, the first porous electrode as described above comprises: from about 70 to about 90 wt% NiO_x; from about 0.1 to about 10 wt% CNTs; from about 0.5 to about 15 wt% graphite; and from about 0.5 to about 10 wt% activated carbon. In a certain embodiment, the first porous electrode comprises: about 80 wt% NiO_x; about 5 wt% CNTs; about 10 wt% graphite; and about 5 wt% activated carbon.

Additional carbon-based components

In addition to the beneficial carbon allotrope combination comprising CNT, graphite and amorphous carbon, further carbon-based materials can be added to improve the conductivity and high temperature resistance of the supercapacitor. Thus, in one specific embodiment, the first porous electrode as described above further comprises graphene. It is understood that the addition of graphene may improve the conductivity of the electrode without decreasing the overall capacitance.

In some embodiments, the first porous electrode as described above is essentially free of paracrystallme carbon. It was surprisingly found that the addition of the paracrystallme carbon to the electrode instead of graphite increased ESR by about 40%. One non-limiting example of the paracrystallme carbon is conductive carbon black.

Additional electrode components

According to the principles of the invention, in some embodiments, the first porous electrode is a mechanically stable self-standing electrode. As used herein and in the claims the term "self- standing electrode" refers to an electrode that is made out of a paste comprising the transitional metal oxide and the beneficial combination of carbon allotropes as described above, wherein said paste can be easily processed to achieve a flat electrode configuration in a reproducible and cost-productive manner, without utilizing additional stabilizing reagents, such as binders, gelling agents, thickening agents, cross-linkers, silicates or combinations thereof. Thus, in some embodiments, the first porous electrode as described above is a binder-free electrode. Non-limiting examples of a binder include carboxymethyl cellulose (CMC), rubbers, PVDF, Teflon, LiPAA. In a related embodiment, the first porous electrode as described above is substantially free of gelling agents and/or thickening agents such as clay, sulfonates, saccharides and organosilicons. As used herein and in the claims, the term "substantially free of" gelling agents and/or thickening agents means that the amount of such agents is undetectable by conventional detection means (e.g., elemental analysis).

In another embodiment, the first porous electrode as described above has a thickness ranging from about 50 micron to about 5 mm. In a currently preferred embodiment, the first porous electrode as described above has a thickness ranging from about 300 to about 800 microns.

In some embodiments, the weight of the first porous electrode in the dry state thereof ranges from about 0.05 g to about 5 g. In further embodiments, the weight of the first porous electrode in the dry state ranges from about 0.1 g to about 1 g. In certain embodiments, the weight of the electrode ranges from about 0.5 g to about 0.8 g. The term "dry state", as used herein, refers to the electrode, which does not include electrolyte or is not impregnated with electrolyte.

In some embodiments, the area of the first electrode ranges from about 1 cm² to about 1000 cm². In further embodiments, the area of the first electrode ranges from about 1 cm² to about 100 cm . In still further embodiments, the area of the first electrode ranges from about 1 cm² to about 10 cm². In certain embodiments, the area of the first electrode is about 6 cm².

In a specific embodiment, the first electrode as described above consists essentially of Mn0₂, carbon nanotubes (CNT), graphite and activated carbon as solid components of the electrode. In an additional embodiment, the first electrode as described above consists essentially of NiO_x, carbon nanotubes (CNT), graphite and activated carbon as solid components of the electrode. In certain such embodiments, the term "consists essentially of refers to the dry contents of the electrode.

In certain embodiments, the first porous electrode is a cathode. Additional supercapacitor components

Electrolyte

Electrolyte generally comprises a solvent and dissolved chemicals that dissociate into positive cations and negative anions, making the electrolyte electrically conductive. In electrochemical capacitors electrolytes are the electrically conductive connection between the first porous electrode and the second porous electrode. Additionally, in electrochemical capacitors the electrolyte provides the ions for the formation of the double-layer and delivers the ions for pseudocapacitance. As mentioned hereinabove, in some embodiments, the asymmetric supercapacitor of the invention comprises a first porous electrode, a second porous electrode and an aqueous electrolyte being in contact with said two electrodes. In some embodiments, said first electrode is impregnated with the electrolyte. In further embodiments, said first electrode comprises the electrolyte. In certain embodiments, the electrode comprises from about XX to about YY wt% dry matter and from about XX to about YY wt% electrolyte. In some related embodiments, the electrolyte comprises a dissolved salt selected from the group consisting of potassium hydroxide (KOH), sodium hydroxide (NaOH), lithium oxide (LiOH), potassium su_lfate (K₂SO₄), sodium perchlorate (NaClO/ , sulfuric acid (H₂SO₄), hydrochloric acid (HC1), nitric acid (HNO₃), methanesulfonic acid (MSA, CH₃SO₃H), and tetrafluoroboric acid (HBF₄).

Second porous electrode

According to the principles of the invention, an asymmetrical electrode configuration may include a first porous electrode comprising a transition metal oxide or sulfide, and at least three allotropes of carbon, comprising carbon nanotubes (CNTs), graphite and amorphous carbon, and a second porous electrode, distinct in composition from the first porous electrode, comprising a high surface area carbon material.

According to the principles of the invention, an asymmetric electrode configuration may increase the energy density stored in the capacitor. Thus, in some embodiments, the second porous electrode comprises a high surface area carbon material selected from the group consisting of amorphous carbon, graphite, CNTs, graphene, and combinations thereof. Each possibility represents a separate embodiment of the invention. In a currently preferred embodiment, the second porous electrode comprises amorphous carbon comprising activated carbon.

In some related embodiments, the activated carbon of the second porous electrode may be prepared from raw materials such as charcoal, carbon, and coke.

In some embodiments, the second porous electrode comprises a backing layer, which may include a conductive support such as, but not limited to carbon paper, carbon felt, carbon - plastic conductive composites, thin metal, including nickel, stainless steel, matrix, sponge or felt. Each possibility represents a separate embodiment of the invention. The thin metal support can have a thickness of about 0.05 to 5 mm.

In certain embodiments, the second porous electrode is an anode. Separator

Typically, the separator comprises an inert, electrically-insulating and ion-permeable material. In some embodiments, the separator is porous. In some related embodiments, the separator is an inert membrane, which is ion-permeable (i.e., allowing the exchange of ions therethrough) and electrically-insulating (i.e., preventing the transfer of electrons therethrough). In an optional embodiment, the separator may include multiple layers (e.g., a number of separate ion-permeable and electrically-insulating membranes arranged successively).

In some embodiments, the first porous electrode comprises the transition metal oxide or sulfide, and at least three allotropes of carbon, comprising carbon nanotubes (CNTs), graphite and amorphous carbon, applied to a separator. In further embodiments, the first porous electrode comprises the combination of the transition metal oxide or sulfide, and the at least three allotropes of carbon supported on the separator.

Current collector

In some embodiments, the asymmetric supercapacitor further comprises a current collector. Typically, the current collector is made from a conductive material, such as a conductive polymer material, in which the electrical conductivity is anisotropic, such that the conductivity perpendicular to the surface of the current collector sheet is greater than the conductivity along the surface. Alternatively, the current collector can be made from a metal or other material which is inert to the chosen electrolyte as described above.

In some embodiments, the first porous electrode comprises the transition metal oxide or sulfide, and at least three allotropes of carbon, comprising carbon nanotubes (CNTs), graphite and amorphous carbon, applied to a current collector. In further embodiments, the first porous electrode comprises the combination of the transition metal oxide or sulfide, and the at least three allotropes of carbon supported on the current collector.

Stack

In some embodiments, the supercapacitor of the invention may be a supercapacitor stack, made up of a plurality of such capacitor cells connected in series. The stack can include from about 2 to about 200 capacitor cells. In certain embodiments, the stack comprises from 2 to 50 cells. Preparation of the first porous electrode

The present invention further provides facile preparation methods of the electrode composition of the invention, enabling both small and large scale production possibilities, which can be tailored to the desired technological application. Accordingly, in some embodiments, the present invention provides a method for preparing a supercapacitor electrode comprising a transition metal oxide or sulfide, and at least three allotropes of carbon comprising carbon nanotubes (CNTs), graphite and amorphous carbon. According to some currently preferred embodiments, said electrode is a free-standing electrode, which demonstrates mechanical stability, and is produced in a highly reproducible and facile manner.

Thus, in one embodiment, the method as described above comprises the following steps:

(b) adding to the mixture obtained in step (a) an electrolyte solution;

(c) mixing the composition obtained in step (b) to achieve a homogeneous paste; and (d) processing the paste obtained in step (c) to form an electrode.

In one embodiment, the transition metal oxide used for the preparation of the first porous electrode is selected from the group consisting of Mn_nO_x, TiO_x, NiO_x, CoO_x, SnO_x, and combinations thereof, wherein x ranges from 1.5 to 3 and n ranges from 1 to 2. In a currently preferred embodiment, the transition metal oxide is Μη(¾. In certain embodiments, the transition metal oxide is NiO_x. In further embodiments, NiO_x is NiO_x(III).

In some other embodiments, the porous electrode as described above, comprises a transition metal sulfide. In further embodiments, the transition metal sulfide is selected from the group consisting of FeS_y, MoS_y, NiS_y, CoS_y, MnS_y, TiS_y, SnS_y and combinations thereof, wherein y ranges from 1.8 to 2.2.

In an additional embodiment, either one of the mixing steps (a) or (c) or both of these steps are carried out utilizing a mixing apparatus selected from a mixer, a hand mixer, a mortar and pestle, a homogenizer and a sonicator. In a currently preferred embodiment, the mixing in step (a) and in step (c) is carried out utilizing a hand mixer. In some related embodiments, the mixing of step (c) is carried out at room temperature. In further embodiments, the mixing time ranges from about 15 minutes to about 1 hour. In an optional embodiment, heating of the mixture can take place during the mixing process, which in turn reduces the mixing time needed for obtaining a homogenized mixture.

In some embodiments, the processing of the paste as described above is carried out utilizing technique known in the art, such as, but not limited to, brushing, spraying, screen printing, and rolling. In some currently preferred embodiments, the processing of the paste as described above in step (d) is carried out utilizing rolling.

In a specific embodiment, the weight ratio of the powder components mixed together in in step (a) ranges from about 60 to about 85 wt% Μη(¾; from about 0.1 to about 10 wt% CNTs; from about 0.5 to about 15 wt% graphite; and from about 0.5 to about 10 wt% activated carbon.

As used herein and in the appended claims the singular forms "a", "an," and "the" include plural references unless the content clearly dictates otherwise. It should be noted that the term "and" or the term "or" is generally employed in its sense including "and/or" unless the content clearly dictates otherwise. As used herein, the term "about", when referring to a measurable value such as an amount, a temporal duration, and the like, is meant to encompass variations of +/-10%, more preferably +1-5%, even more preferably +/- 1 , and still more preferably +/-0.1 % from the specified value, as such variations are appropriate to perform the disclosed methods.

The following examples are presented in order to more fully illustrate some embodiments of the invention. They should, in no way be construed, however, as limiting the broad scope of the invention. One skilled in the art can readily devise many variations and modifications of the principles disclosed herein without departing from the scope of the invention.

EXAMPLES

Example 1 - preparation of porous MnO? - carbon alio tropes electrode

The preparation procedure included the following steps:

1 ) Calculating and weighing the desired amounts of Μη(¾, SWCNT, graphite and activated carbon according to the weight ratios depicted in Tables la, 2a, 3a, and 4a.

2) Placing the ingredients in a polypropylene beaker.

3) Mixing the ingredients for about 0.5 -1 hours, using a manual mixer, covered with a disposable coating per each batch.

4) Adding an electrolyte while mixing to form a homogeneous paste. A) Paste preparation: 280 g Mn0₂, 17.5 g activated carbon and 35 g graphite were inserted into a polypropylene beaker and were mixed together for about 0.5 hr at room temperature. 17.5 g MWCNT were added to the mixture at mixed together to obtain a homogenized mixture. 138 g 6.7M KOH electrolyte was added in a stepwise manner during stirring. The mixture was further mixed for about 0.5 hr more to achieve a homogenized paste material.

B) In one batch 15 Kg of electrode paste were prepared and rolled as a batch. The paste was allocated between two laminate layers/foils and pressed by the rolling process to the desired thickness. The process was done at room temperature environment. The rolled paste was cut by die cut to the required electrode shape and size. The laminate layers were removed upon cell assembly.

Example 2 - analysis of the different components and their content over the overall super capacitor performance

A series of different electrode formulations was prepared in order to analyze the overall effect of Μη(¾, graphite, activated carbon and paracrystalline conductive carbon over the capacitance, equivalent series resistance (ESR), electrochemical window and life cycle of the supercapacitor cells comprising said electrodes as a cathode. The electrode compositions were prepared as described in Example 1 , with the relevant weight percent modifications as depicted in tables la, 2a, 3a, and 4a hereinbelow.

The supercapacitor cell was constructed as follows. Said first rolled electrode (cathode) having an average weight of about 0.55 g was placed on top of a 6 cm² polymeric conductive film and a polymeric porous membrane separator was placed on top of the electrode. A second electrode (anode) comprising 40% (w/w) activated carbon and 60% (w/w) KOH having an average weight of about 0.7 g was placed on top of a second polymeric conductive film. A copper conductive terminal and a rubber isolating layer were attached to each electrode of the cell assembly and closed between plates to form a 6 cm² cell.

The supercapacitor cells were evaluated by cyclic voltammetry at a scan rate of 5 mV/sec. ESR was measured directly by 1 kHz AC-impedance Ohm-meter. The capacitance was calculated from the cyclic voltammograms as an average current during charge/discharge divided by the scan rate. A. Graphite:

Table la - Electrode composition comprising two carbon allotropes

• The values are given in (w/w) %.

Table lb shows the ESR and capacitance values of the electrodes presented in Table la.

Table lb - Electrochemical characterization of the electrodes comprising two carbon allotropes

As can be seen in Figures 1-3 and Table lb, the increase of the conductive graphite component at the expense of Μη(¾ in the electrode formulation gave rise to a 30% reduction of ESR values, but caused a 17% reduction of the overall capacitance values of the supercapacitor comprising such electrodes.

B. Activated carbon:

Table 2a - Electrode composition comprising three carbon allotropes

Mn0₂ CNT Graphite Activated

carbon

80 5 10 5

70 5 10 15

60 5 10 25 • The values are given in (w/w) %.

Results:

Table 2b shows the ESR and capacitance values of the electrodes presented in Table 2a.

Table 2b - Electrochemical characterization of the electrodes comprising three carbon allotropes

The addition of 5% activated carbon at the expense of Μη(¾ gave rise to an improved supercapacitor performance. A comparison between Figures 1, 2, 3 and Figures 4, 5, 6, respectively, demonstrates a 25% reduction in the ESR values and about 27% increase in the capacitance values, while a further increase of the activated carbon content (over 5%) at the expense of Μη(¾, limits the electrochemical window.

C. paracrvstalline conductive carbon:

Table 3a - Electrode composition comprising three carbon allotropes comprising polycrystalline carbon

• The values are given in (w/w) %.

Results:

Table 3b shows the ESR and capacitance values of the electrodes presented in Table 3 a. Table 3b - Electrochemical characterization of the electrodes comprising three carbon allotropes comprising polycrystalline carbon

The addition of paracrystallme conductive carbon does not improve the overall performance of the supercapacitor, compared to the results obtained with no additional carbon component (Figures 7, 8, 9, versus Figures 1, 2, 3, respectively). Therefore, although paracrystallme carbon is a conductive component added to the formulation, it surprisingly does not contribute to the electrochemical activity of the supercapacitor.

D. paracrystallme conductive carbon and graphite:

Table 4a - Electrode composition comprising two carbon allotropes comprising polycrystalline carbon

• The values are given in (w/w) %.

Results:

Table 4b shows the ESR and capacitance values of the electrodes presented in Table 4a. Table 4b - Electrochemical characterization of the electrodes comprising two carbon allotropes comprising polycrystalline carbon

The addition of paracrystalline conductive carbon at the expense of the graphite component (both are conductive) demonstrated a significant (-40%) rise of the ESR values, as can be seen from Figures 10, 11, and 12, as compared to Figures 1 , 2, and 3. Therefore, it appears that the use of paracrystalline conductive carbon does not promote the electrochemical performance of the supercapacitor in comparison to the combination of activated carbon and graphite.

Example 3 - monitoring electrolyte effect over the supercapacitor stability

The effect of the nature of the electrolyte over the stability of the supercapacitor including the electrode containing three carbon allotropes was measured using a constant current (CC) and a constant current constant voltage (CCCV) experiment. CC - dynamic charge and discharge profile at a constant current without rest or hold at a constant voltage between charge and discharge. CCCV - combined dynamic charge profile at constant current, followed by constant voltage charge until the current drops below 10% of the constant current value, and a dynamic discharge at constant current.

A comparison between two supercapacitors having an identical electrode composition of Mn0₂, SWCNT, graphite and activated carbon according to the corresponding ratio: 80%w/w ; 5%w/w ; 10%w/w ; and 5%w/w was carried, using the following samples and conditions:

(a) supercapacitor as described above, utilizing an alkaline electrolyte, 6.7M KOH. The measurements were carried out at a voltage of up to 1.4 V.

(b) supercapacitor as described above, utilizing neutral pH electrolyte, 0.5M K₂SO₄. The measurements were carried out at a voltage of up to 1.6 or 1.8 V.

According to the CC profile obtained for both supercapacitors, the energy obtained from supercapacitor (a) was higher for all current density values measured in comparison with those measured for supercapacitor (b) (Figure 13). As presented in Figure 14, the energy produced by supercapacitor (b) during CC profile at 1.6V was lower than the energy produced from supercapacitor (a) during CC profile measurements at 1.4 V.

According to the CCCV profile, which represents a deep charging mode of the capacitor, no change was measured between the energy produced from supercapacitor (a) in this deep charging mode than the energy produced from a regular CC profile.

ESR of supercapacitor (b) operated at each of the profiles at 1.6V and 1.8V increased by about 50%-150%. ESR of supercapacitor (a) increased by only 11 % at similar current profiles up to 1.4V.

It was further found that after 16 days of operation shunt current of supercapacitor (b) at 1.6 V the ESR was two- fold higher than the shunt current of supercapacitor (a) at 1.4V.

Example 4 - preparation of NiOx - based electrode comprising carbon allotropes

The preparation procedure included the following steps:

1) Calculating and weighing the desired amounts of NiOx, SWCNT, graphite and activated carbon according to the desired ratios as depicted in Table 5.

2) Placing the ingredients in a laboratory mortar and pestle kit.

3) Mixing the ingredients using the pestle.

4) Adding an electrolyte while mixing thoroughly with the pestle for 5 minutes to form a homogeneous paste.

The electrode comprising three carbon allotropes contained: 80% (w/w) NiOx (III); 5%

(w/w) graphite; 10% (w/w) CNTs; and 5% (w/w) activated carbon. The electrode paste was prepared as follows. 2.2 g NiOx, 0.138 g activated carbon, 0.28 g graphite and 0.132 g MWCNT were inserted into a mortar pot and were mixed together for 1 minutes using a pestle at room temperature to obtain a homogenized mixture. 3.5 g 6.7M KOH electrolyte was added in a stepwise manner during stirring. The mixture was further mixed for about 5 minutes to achieve a homogenized paste material.

Example 5 - analysis of the effect of different carbon allotropes on the conductivity of a supercapacitor comprising NiOx-based electrode

The pastes described in example 4 and shown in Table 5 were used to prepare the electrodes. About 1.5 g electrode paste was manually applied on top of a 6 cm polymeric conductive film to form a uniform electrode layer, and a polymeric porous membrane separator was placed on top of the electrode. A second electrode (anode) comprised 40% (w/w) activated carbon and 60% (w/w) KOH. 1.06 g of said electrode paste was applied on a second polymeric conductive film. A copper conductive terminal and a rubber isolating layer were attached to each electrode of the cell assembly and closed between plates to form a 6 cm cell.

A comparison between supercapacitors comprising different electrode contents of NiOx, SWCNT, graphite and activated carbon according to the weight ratios depicted in Table 5 was carried out, using the following experimental conditions. The supercapacitors were evaluated by running 0.5A CC charge - discharge cycles at a voltage of up to 1.4 V and a current of 0.5 A. ESR of the supercapacitors was measured before and after the cycles 1 kHz AC-impedance Ohm-meter.

Table 5 - ESR values of different NiOx-based electrodes

Introduction of the three carbon allotropes including activated carbon, graphite and CNTs to the NiOx(III) -based electrode resulted in about 30% decrease in ESR, as compared to the NiOx(III)-based electrode having the same weight percent of the metal oxide but only two carbon allotropes (graphite and CNTs). ESR values obtained with the electrodes comprising only one carbon allotrope (graphite or activated carbon) were higher than the ESR of the electrode including three carbon allotropes by about 70% and 100%, respectively.

It is appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and sub-combinations of various features described hereinabove as well as variations and modifications. Therefore, the invention is not to be constructed as restricted to the particularly described embodiments, and the scope and concept of the invention will be more readily understood by references to the claims, which follow.

Claims

1. An asymmetric supercapacitor, comprising at least one capacitor cell comprising a first porous electrode, a second porous electrode, an aqueous electrolyte being in contact with said first porous and second porous electrodes, and a separator separating the first porous electrode from the second porous electrode, wherein the first porous electrode comprises a transition metal oxide or sulfide in a weight percent ranging from about 50 to about 90 wt% of the total weight of the first electrode, and at least three allotropes of carbon, comprising carbon nano tubes (CNTs), graphite and amorphous carbon.

2. The asymmetric supercapacitor according to claim 1 , wherein the first porous electrode comprises a transition metal oxide.

3. The asymmetric supercapacitor according to claim 2, wherein the transition metal oxide is selected from the group consisting of Mn_nO_x, TiO_x, NiO_x, CoO_x, SnO_x and combinations thereof, wherein x ranges from 1.5 to 3, and n ranges from 1 to 2.

4. The asymmetric supercapacitor according to claim 3, wherein the transition metal oxide is Mn0₂.

5. The asymmetric supercapacitor according to claim 3, wherein the transition metal oxide is NiO_x.

6. The asymmetric supercapacitor according to claim 1 , wherein the first porous electrode comprises a transition metal sulfide.

7. The asymmetric supercapacitor according to any one of claims 1 and 6, wherein the transition metal sulfide is selected from the group consisting of FeS_y, MoS_y, NiS_y, CoS_y, MnS_y, TiS_y, SnS_y and combinations thereof, wherein y ranges from 1.8 to 2.2.

8. The asymmetric supercapacitor according to any one of claims 1 to 7, wherein the transition metal oxide or sulfide is present in the first porous electrode in a weight percent ranging from about 60 to about 85 wt% or from about 70 to about 90 wt% of the total weight of the first electrode.

9. The asymmetric supercapacitor according to any one of claims 1 to 8, wherein the CNTs are present in the first porous electrode in a weight percent ranging from about 0.1 to about 10 wt% of the total weight of the first electrode.

10. The asymmetric supercapacitor according to any one of claims 1 to 9, wherein the CNTs are selected from the group consisting of single-walled carbon nanotubes (SWCNTs), multi- walled carbon nanotubes (MWCNTs) and combinations thereof.

11. The asymmetric supercapacitor according to claim 10, wherein the CNTs are MWCNTs.

12. The asymmetric supercapacitor according to any one of claims 1 to 11 , wherein the CNTs diameter ranges from about 5 nm to about 45 nm.

13. The asymmetric supercapacitor according to any one of claims 1 to 12, wherein graphite is present in the first porous electrode in a weight percent ranging from about 0.5 to about 40 wt% of the total weight of the first porous electrode.

14. The asymmetric supercapacitor according to any one of claims 1 to 13, wherein the amorphous carbon is present in the first porous electrode is a weight percent ranging from about 0.5 to about 40 wt% of the total weight of the first electrode.

15. The asymmetric supercapacitor according to any one of claims 1 to 14, wherein the amorphous carbon comprises activated carbon.

16. The asymmetric supercapacitor according to claim 15, wherein the activated carbon has a surface area of at least about 500 m²/gr.

17. The asymmetric supercapacitor according to claim 16, wherein the activated carbon has a surface area of at least about 1000 m /gr.

18. The asymmetric supercapacitor according to any one of claims 15 to 17, wherein the activated carbon has an ash content of above 10 wt%.

19. The asymmetric supercapacitor according to claim 18, wherein the activated carbon has an ash content of above 20 wt%.

20. The asymmetric supercapacitor according to any one of claims 1 to 19, wherein the first porous electrode is essentially free of a paracrystalline carbon.

21. The asymmetric supercapacitor according to any one of claims 1 to 20, wherein the first porous electrode further comprises graphene.

22. The asymmetric supercapacitor according to any one of claims 1 to 21, wherein the first electrode is a binder-free electrode.

23. The asymmetric supercapacitor according to any one of claims 1 to 22, wherein the first electrode is substantially free of gelling agents and/or thickening agents.

24. The asymmetric supercapacitor according to claim 1 , wherein the first porous electrode comprises:

from about 60 to about 85 wt% Mn0₂;

from about 0.1 to about 10 wt% CNTs;

from about 0.5 to about 15 wt% graphite; and

from about 0.5 to about 10 wt% activated carbon.

25. The asymmetric supercapacitor according to claim 24, wherein the first electrode consists essentially of Μη(¾, carbon nanotubes (CNT), graphite and activated carbon.

26. The asymmetric supercapacitor according to claim 1 , wherein the first porous electrode comprises:

from about 70 to about 90 wt% NiO_x;

from about 0.1 to about 10 wt% CNTs;

from about 0.5 to about 15 wt% graphite; and

from about 0.5 to about 10 wt% activated carbon.

27. The asymmetric supercapacitor according to claim 26, wherein the first electrode consists essentially of NiO_x, carbon nanotubes (CNT), graphite and activated carbon.

28. The asymmetric supercapacitor according to any one of claims 1 to 27, wherein the first porous electrode has a thickness ranging from about 50 micron to about 5 millimeters.

29. The asymmetric supercapacitor according to claim 28, wherein the first porous electrode has a thickness ranging from about 300 to about 800 microns.

30. The asymmetric supercapacitor according to any one of claims 1 to 29, wherein the first electrode is impregnated with the electrolyte.

31. The asymmetric supercapacitor according to any one of claims 1 to 30, wherein the electrolyte comprises a dissolved salt selected from the group consisting of potassium hydroxide (KOH), sodium hydroxide (NaOH), lithium oxide (LiOH), potassium sulfate (K₂SO₄), sodium perchlorate (NaC104), sulfuric acid (H2SO4), hydrochloric acid (HCl), nitric acid (HNO3), methanesulfonic acid (MSA, CH₃SO₃H), and tetrafluoroboric acid (HBF₄).

32. The asymmetric supercapacitor according to claim 31 wherein the electrolyte is an alkaline electrolyte.

33. The asymmetric supercapacitor according to claim 31 or 32, wherein the electrolyte comprises KOH.

34. The asymmetric supercapacitor according to any one of claims 1 to 33, wherein the second electrode comprises a high surface area carbon material selected from the group consisting of amorphous carbon, graphite, CNTs, graphene, and combinations thereof.

35. The asymmetric supercapacitor according to any one of claims 1 to 34, wherein the second electrode comprises amorphous carbon comprising activated carbon.

36. A method for preparing a supercapacitor electrode comprising a transition metal oxide or sulfide, and at least three allotropes of carbon comprising carbon nanotubes (CNTs), graphite and amorphous carbon, the method comprising the steps of: (a) mixing the metal oxide or sulfide powder together with the powders of the at least three allotropes of carbon;

(b) adding to the mixture obtained in step (a) an electrolyte solution;

37. The method according to claim 36, wherein the transition metal powder in step (a) is a transition metal oxide.

38. The method according to claim 37, wherein the transition metal oxide is selected from the group consisting of Mn_nO_x, NiO_x, TiO_x, CoO_x, SnO_x and combinations thereof, wherein x ranges from 1.5 to 3 and n ranges from 1 to 2.

39. The method according to claim 38, wherein the transition metal oxide in Μη(¾.

40. The method according to claim 38, wherein the transition metal oxide in NiO_x.

41. The method according to claim 36, wherein the transition metal powder in step (a) is a transition metal sulfide.

42. The method according to claim 41 , wherein the transition metal sulfide is selected from the group consisting of FeS_y, MoS_y, NiS_y, CoS_y, MnS_y, TiS_y, SnS_y and combinations thereof, wherein y ranges from 1.8 to 2.2.

43. The method according to claim 36, wherein the electrolyte added in step (b) is an alkaline electrolyte.

44. The method according to claim 36, wherein the mixing in step (a) and in step (c) is independently carried out utilizing a mixing apparatus selected from a mixer, a hand mixer, a mortar and pestle, a homogenizer and a sonicator.

45. The method according to claim 44, wherein the mixing in step (a) and in step (c) is carried out utilizing a hand mixer.

46. The method according to claim 36, wherein the processing of step (d) is carried out utilizing rolling.