AQUEOUS ELECTROLYTE

FIELD OF THE INVENTION

The present invention is directed to a supercapacitor comprising a carbonaceous electrode utilizing a low purity carbon source in combination with alkaline electrolyte, 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 supercapacitors 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. 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 relatively simple and is not subject to overcharging.

The most widely available commercial supercapacitor is an electric double-layer capacitor 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 μΈονη ^'2 (for flat plates). On charge the anions are adsorbed on one electrode and the cations on the other electrode.

Aqueous-based activated carbon supercapacitors are promising 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 3 V, 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.

At present, various carbon materials are used as active materials for EDLC electrodes, owing to their high conductivity, high surface area, a rich variety of dimensionality, good corrosion resistance, controlled pore structure, processability and compatibility in composites, and relatively low cost. A developed surface area and controlled distribution of pores for porous carbons, produced by well-established chemical and physical activation methods, determine the electrode/electrolyte interface in supercapacitor applications. The non-limiting examples of carbonaceous materials include activated carbons (ACs), carbon nanotubes, graphite, graphene, and carbide-derived carbons. Considering the essential criteria of low cost and high volumetric capacitance required by industrial applications, activated carbon remains the material of choice for EDLCs.

Yet, AC material used for the preparation of carbon-based electrodes often requires several purification stages prior the electrode processing step in order to achieve high power densities. These additional purification stages are economically inefficient and contribute to the overall costs of the electrode starting materials. Commercially available activated carbons which are designated for use in supercapacitors are typically of ultra-purity grades in order to minimize poisoning of the electrolyte and ensure high durability and cycle life. Purity of the carbon source is particularly important in aqueous supercapacitors, which do not include water-, acid- and/or alkaline-soluble materials, such as, for example, metals.

Furthermore, AC -based electrodes often require the use of stabilizing reagents such as binders, silicates or gelling agents which are incorporated into AC during the electrode preparation. Incorporation of said stabilizing reagents can influence electrochemical capacitance, add additional steps to the electrode manufacturing process and increase the overall costs thereof. Additionally, AC-based electrodes, which include stabilizing reagents are prone to mechanical degradation and thus have a shorter cycle life.

There remains, therefore, an unmet need for efficient supercapacitor electrode, which would be cost- and energy-efficient, mechanically and electrochemically stable, and made of environmentally friendly materials.

SUMMARY OF THE INVENTION

The present invention provides a unique electrode composition utilizing a low purity carbon source in combination with alkaline electrolyte, demonstrating improved energetic efficiency, capacitance and wide temperature range stability. The beneficial electrode composition of the invention utilizes low purity carbon source as the main component, thus promoting the use of environmentally friendly and economically efficient starting materials. The electrode of the present invention is mechanically, chemically and thermally-stable, and demonstrates an advantageous stability in the sub-zero temperatures. The electrode according to the principles of the invention can be substantially free of commonly used binder agents, gelling agents and other thickening agents and has no hazardous effect on humans or the surrounding environment. The inventors of the present invention have surprisingly found that a low purity carbon comprising above about 7% ash can beneficially be used to prepare a supercapacitor electrode, which provides high energetic efficiency and cost-efficient capacitance. Furthermore, when used in combination with an alkaline electrolyte, the low purity carbon-based electrode showed diminished leakage currents, as compared to the use with an acidic electrolyte, wherein said leakage currents are acceptable for commercial use. The unique electrode composition of the invention therefore allows the production of cost-effective alkaline supercapacitor with a prolonged cycle life. Performance of the supercapacitor comprising low purity carbon-based electrodes can be further increased by adjusting the electrode thickness and selecting suitable separator material .

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

Thus, in one aspect, the present invention provides a supercapacitor comprising at least one capacitor cell comprising a first porous electrode, a second porous electrode, an aqueous alkaline 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 low purity activated carbon having an ash content of above about 7 wt%, and wherein the first porous electrode is impregnated with an aqueous alkaline electrolyte.

According to some embodiments, the activated carbon has an ash content of above about

10 wt%. According to further embodiments, the activated carbon has an ash content of above about 15 wt%.

In certain embodiments, the activated carbon as described above has a surface area of at least about 500 m ² /gr. In some other embodiments, the activated carbon has a surface area of at least about 1000 m ² /gr. In some additional embodiments, the activated carbon as described above has a porosity/pore volume of about 0.3 to about 0.9 cc/gr.

In some embodiments, the aqueous alkaline electrolyte comprises a dissolved salt selected from the group consisting of potassium hydroxide (KOH), sodium hydroxide (NaOH) and lithium oxide (LiOH). Each possibility represents a separate embodiment of the invention. In some specific embodiments, the electrolyte comprises KOH. In some related embodiments, the aqueous alkaline electrolyte concentration is between about 20 and about 50 wt%. In some currently preferred embodiments, the aqueous alkaline electrolyte concentration is about 30 wt%. In certain embodiments, the first electrode as described above is a binder-free electrode. In some related embodiments, the first electrode is substantially free of gelling agents and/or thickening agents.

In some embodiments, the activated carbon is present in the first porous electrode in a weight percent ranging from about 35 to about 45 wt% of the total weight of the first electrode. In some currently preferred embodiments, the alkaline electrolyte is present in the first porous electrode in a weight percent ranging from about 55 to about 65 wt% of the total weight of the first electrode. In certain embodiments, the first porous electrode comprises a dry matter content (DMC) from about 35 to about 45 wt% activated carbon and an alkaline electrolyte in an amount of about 55 to about 65 wt%.

In some embodiments, the first electrode consists essentially of the activated carbon and aqueous alkaline electrolyte. In some specific embodiments, said aqueous alkaline electrolyte comprises KOH.

In some embodiments, the first porous electrode as described above has a thickness ranging from about 50 microns to about 5 millimeters. In further embodiments, the first porous electrode has a thickness ranging from about 50 microns to about 350 microns. In additional embodiments, the first porous electrode has a thickness ranging from about 300 microns to about 1.5 millimeters.

In some embodiments, the specific capacitance of the first porous electrode is at least about 45 F/g.

In some embodiments, the first porous electrode has a porosity of about 0.3 to about 0.9 cc/gr.

In some embodiments, the separator comprises a material selected from the group consisting of polyvinyl alcohol, polypropylene, polyethylene and combinations thereof. In further embodiments, the separator is selected from a polyvinyl alcohol-coated polyethylene separator and a polypropylene separator. Each possibility represents a separate embodiment of the invention.

In some specific embodiments, the separator comprises polyvinyl alcohol. In some specific embodiments, the supercapacitor is a polyvinyl alcohol coated polyethylene separator. In certain embodiments, the separator has a thickness of above about 30 micron.

In some embodiments, the supercapacitor of the invention is a symmetric supercapacitor, wherein the second electrode is substantially identical to the first electrode. In certain such embodiments, the second electrode comprises low purity activated carbon having an ash content of above about 7 wt%, and is impregnated with aqueous alkaline electrolyte.

In some embodiments, the supercapacitor of the invention is an asymmetric supercapacitor. In some specific embodiments, the second electrode comprises low purity activated carbon having an ash content of above about 7 wt%, and is impregnated with aqueous alkaline electrolyte, wherein the weight percent of activated carbon and/or the thickness of the second electrode are different than those of the first electrode. In some specific embodiments, the second electrode comprises a transition metal oxide or sulfide. In some related embodiments, the transition metal oxide or sulfide is selected from the group consisting of Mn _n O _x , TiO _x , NiO _x , CoO _x , SnO _x , FeSy, MoS _y , NiS _y , CoS _y , MnS _y , TiS _y , SnS _y and combinations thereof, wherein x ranges from 1.5 to 3, y ranges from 1.8 to 2.2 and n ranges from 1 to 2. Each possibility represents a separate embodiment of the invention. In certain embodiments, the second electrode comprises Mn0 ₂ . The second electrode can further include activated carbon. In some embodiments, said activated carbon is a low purity carbon comprising above about 7 wt% ash. In further specific embodiments, the second electrode comprises an additional carbonaceous material. In a certain embodiment, the second electrode comprises activated carbon, Mn0 ₂ , carbon nanotubes (CNTs), and graphite.

In another aspect, the present invention provides a method for preparing a supercapacitor porous electrode comprising a low purity activated carbon having an ash content of above about 7 wt%, wherein the electrode is impregnated with an aqueous alkaline electrolyte, the method comprising the steps of: (a) mixing the aqueous alkaline electrolyte with the low purity activated carbon, with optional stirring, to achieve a homogeneous paste; (b) heating the paste obtained in step (a); and (c) processing the paste obtained in step (b) to form a standalone homogeneous electrode.

In some embodiments, the activated carbon has an ash content of above about 10 wt%. In further embodiments, the activated carbon has an ash content of above 15 wt%.

In some other embodiments, the activated carbon has a surface area of at least about 500 m ² /gr. In yet some other embodiments, the activated carbon has a surface area of at least about 1000 m ² /gr. In some embodiments, the aqueous alkaline electrolyte comprises a dissolved salt selected from the group consisting of potassium hydroxide (KOH), sodium hydroxide (NaOH) and lithium oxide (LiOH). Each possibility represents a separate embodiment of the invention. In some currently preferred embodiments, the electrolyte comprises KOH.

In some additional embodiments, the mixing in step (a) is carried out in a stepwise manner. In some specific embodiments, the heating in step (b) is carried out at a temperature of about 50 to about 100 °C. In some related embodiments, the mixing in step (a) is carried out utilizing a mixing apparatus selected from a mixer, a hand mixer, a homogenizer or a sonicator. Each possibility represents a separate embodiment of the invention. In some currently preferred embodiments, the mixing in step (a) is carried out utilizing a mixer. In some specific embodiments, the mixing is performed at a speed of from about 20 to about 120 RPM. In some other embodiments, the mixing speed is increased throughout step (a).

In some embodiments, the weight ratio of activated carbon and the alkaline electrolyte in step (a) ranges from about 35 to about 45 wt% activated carbon and from about 55 to about 65 wt% electrolyte.

In some embodiments, the processing of step (c) is carried out utilizing a method selected from rolling, calendering, coating, casting, pressing, printing, 3D printing or a combination thereof. In certain embodiments, the processing of step (c) is carried out utilizing rolling. In some specific embodiments, the rolling as described above is performed on an inert polymeric laminate. In some currently preferred embodiments, the laminate is peeled off after the rolling process, thereby allowing the formation of a free-standing electrode. In some embodiments, the processing of step (c) further comprises pressing. In some currently preferred embodiments, the paste is pressed between two non-conducting polymer sheets.

In another aspect, the present invention further provides a method for the preparation of the supercapacitor as described above, comprising preparing the first supercapacitor electrode comprising (a) mixing aqueous alkaline electrolyte with low purity activated carbon having an ash content of above about 7 wt%, with optional stirring, to achieve a homogeneous paste; (b) heating the paste obtained in step (a); and (c) processing the paste obtained in step (b) to form a standalone homogeneous electrode. In further embodiments, the method comprises preparing a second porous electrode; separating the first porous electrode from the second porous electrode by a porous separator. In further embodiments, the method comprises filling the separator with the electrolyte, wherein the electrolyte is in contact with the first porous electrode and with the second porous electrode. 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 the specific capacitance of the alkaline and acidic sup ercapaci tors comprising low purity carbon- and high purity carbon-based electrodes (solid black bars - low purity carbon and acidic electrolyte, diagonal stripes bars - low purity carbon and alkaline electrolyte, solid grey bars - high purity carbon and acidic electrolyte, and checkerboard bars - high purity carbon and alkaline electrolyte).

Figure 2: depicts the energetic efficiency of the alkaline and acidic supercapacitors comprising low purity carbon- and high purity carbon-based electrodes.

Figure 3: depicts the leakage current of the alkaline and acidic supercapacitors comprising two distinct low purity carbon-based electrodes (anodes).

Figure 4: depicts the electrode capacitance cost-efficiency as a function of carbon ash content.

Figure 5: depicts the Equivalent Series Resistance (ESR) values measured for different electrode thicknesses in both symmetric and asymmetric supercapacitor configurations.

Figure 6: depicts the capacitance measured for symmetric supercapacitor configuration comprising varied electrode thicknesses.

Figure 7: depicts the capacitance measured for asymmetric supercapacitor configuration comprising varied electrode thicknesses.

Figure 8: depicts the ESR values measured at room temperature for different supercapacitors as a function of the separator used in each system. The corresponding thickness of each separator is indicated as a white circle. Grey squares represent ESR DC measured at 0.1 Amp; white squares represent ESR DC measured at 1 Amp; and black squares represent ESR AC.

Figure 9: depicts the ESR values measured at -30°C for different supercapacitors as a function of the separator used in each system. The corresponding thickness of each separator is indicated as a white circle. Grey squares represent ESR DC measured at 0.1 Amp; white squares represent ESR DC measured at 1 Amp; and black squares represent ESR AC.

Figures lOA-lOC: depict the ESR values measured over a wide temperature range for three distinct sup ercapaci tors utilizing different separators, namely, separators No: 1 (♦), 2 (■) and 8 (-). Figure 10A depicts the ESR DC measured at O. lAmp; Figure 10B depicts the ESR DC measured at 1 Amp; and Figure IOC depicts the ESR AC .

Figure 11A: depicts the ESR values measured at 65 °C for three distinct supercapacitors utilizing different separators, namely, separators No: 1 (♦), 5 (·) and 8(-), over the course of the charge/discharge experiment.

Figure 11B: depicts the capacitance measured at 65 °C for three distinct supercapacitors utilizing different separators, namely, separators No: 1 (♦), 5 (·) and 8(-), over the course of the charge/discharge experiment.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a unique electrode composition based on the beneficial combination of a low purity carbon source with an alkaline electrolyte, giving rise to a mechanically, chemically and thermally-stable free-standing electrode. Utilizing low purity carbon source is both cost-effective and environmentally friendly, as the electrode preparation does not require purifying steps and as such does not impose any hazardous effect on humans or the surrounding environment. According to the principles of the invention, the low-purity carbon source is an activated carbon, having a high surface area which promotes high capacitance of the supercapacitor of the invention.

The electrode according to some embodiments of the invention can be substantially free of commonly used binder agents, gelling agents and other thickening agents, allowing improved electrochemical performance without jeopardizing the overall mechanical stability of the electrode, thereupon enabling prolonged cycle life.

Thus, in one aspect, the present invention provides a supercapacitor comprising at least one capacitor cell comprising a first porous electrode, a second porous electrode, an aqueous alkaline 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 low purity activated carbon having an ash content of above about 5 wt%, and wherein the first porous electrode is impregnated with an aqueous alkaline electrolyte. According to some embodiments, the activated carbon has an ash content of above about 7 wt%. The term "low purity activated carbon", as used herein, refers in some embodiments to the ash content of above about 5 wt%. In further embodiments, the term "low purity activated carbon", refers to the ash content of above about 6 wt%. In some currently preferred embodiments, the term "low purity activated carbon" refers to the ash content of above about 7 wt%. The ash weight percent refers to the weight of ash relatively to the total weight of the activated carbon utilized in the electrode.

In some embodiments, the activated carbon has an ash content of above about 8 wt%. In further embodiments, the activated carbon has an ash content of above about 9 wt%. In still further embodiments, the activated carbon has an ash content of above about 10 wt%. In yet further embodiments, the activated carbon has an ash content of above about 11 wt%. In still further embodiments, the activated carbon has an ash content of above about 12 wt%. In yet further embodiments, the activated carbon has an ash content of above about 13 wt%. In still further embodiments, the activated carbon has an ash content of above about 14 wt%. In yet further embodiments, the activated carbon has an ash content of above 15 wt%. In some embodiments, the activated carbon does not contain an ash content of above 20 wt%. Without being bound by theory or mechanism of action, it is postulated that activated carbon having above 20 wt% ash will be more prone to go through parasitic reactions, which in turn might cause an increase in the internal resistance and leakage current of the supercapacitor comprising said impure activated carbon, and decrease the overall performance of the supercapacitor. In some embodiments, the activated carbon utilized in the first porous electrode is characterized in that the ash content is below about 20 wt%. In further embodiments, the ash content is below about 19 wt%, below about 18 wt%, below about 17 wt%, below about 16 wt%, or below about 15 wt%. Each possibility represents a separate embodiment of the invention.

In some embodiments, the activated carbon has an ash content ranging from about 7 wt% to about 20 wt%. In certain embodiments, the activated carbon has an ash content ranging from about 7 wt% to about 15 wt%. In further embodiments, the activated carbon has an ash content ranging from about 8 wt% to about 12 wt%. In additional embodiments, the activated carbon has an ash content ranging from about 15 wt% to about 20 wt%. In additional embodiments, the activated carbon has an ash content ranging from about 16 wt% to about 19 wt%. In some particular embodiments, the activated carbon has an ash content of about 10 wt%.

In additional particular embodiments, the activated carbon has an ash content of about 18 wt%. The term "ash", as used herein, refers in some embodiments to the impurities found in activated carbon. Said impurities can include inorganic impurities e.g. metals, metal oxides, metal phosphates, metal sulphates, and ceramic materials (i.e. silicates). Non-limiting examples of said impurities include calcium carbonate, potash, phosphate, iron, manganese, sodium, aluminum, strontium, zinc, and copper. In some embodiments, the acid/water-soluble ash content constitutes at least about 1 wt% of the total weight of ash in the carbon source and/or electrode. In further embodiments, the acid/water-soluble ash content constitutes at least about 2.5 wt% of the total weight of ash. In further embodiments, the acid/water-soluble ash content constitutes at least about 5 wt% of the total weight of ash. The term "acid/water-soluble ash", as used herein, refers in some embodiments, to metallic content of ash. The ash content of the activated carbon present in the electrode can be assessed as known in the art, for example, by Inductively coupled plasma mass spectrometry (ICP-MS) or Inductively coupled plasma Atomic Emission Spectroscopy (ICP-AES). Ash content of commercially available carbons is typically assessed by burning the carbon at the temperature of above about 500°C and weighting the product of the combustion reaction. The difference in the weight of the initial carbon and the reaction product is defined as the ash content.

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 m ² /g, such as, for example, 10 - 1000 m ² /g or 50 -1500 m ² /g.

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

In some embodiments, the activated carbon as described above has a surface area of at least about 500 m ² /gr. In further embodiments, the activated carbon has a surface area of at least about 600 m ² /gr. In still further embodiments, the activated carbon has a surface area of at least about 700 m ² /gr. In yet further embodiments, the activated carbon has a surface area of at least about 800 m ² /gr. In still further embodiments, the activated carbon has a surface area of at least about 900 m ² /gr. In yet further embodiments, the activated carbon has a surface area of at least about 1000 m ² /gr. In some additional embodiments, the activated carbon as described above has a porosity/pore volume of about 0.3 to about 0.9 cc/gr. In another embodiment, the porosity/pore volume is about 0.4 to about 0.8 cc/gr. In yet another embodiment, the porosity/pore volume is about 0.45 to about 0.75 cc/gr. Choice of electrolyte

The present invention is based in part on a surprising finding that the supercapacitor comprising a low-purity electrode comprising activated carbon having ash content of above about 7 wt% and an alkaline electrolyte had leakage current which was significantly lower than the one measured in a similar supercapacitor utilizing acidic electrolyte. Thus, according to the principles of the invention, the low purity activated carbon is combined with an aqueous alkaline electrolyte. It is to be understood that the terms "combined" and "impregnated" when used in connection with the electrode and electrolyte mean that the electrode comprises the electrolyte.

Without wishing to being bound by theory or mechanism of action, it is contemplated that alkaline environment enables the use of impure carbon source as the main component of the electrode, as a chemical degradation process involving the impurities present in the carbon source occurs to a lesser extent in alkaline conditions, compared to acidic conditions, which might promote oxidation reactions involving such impurities. Thus, in some embodiments, the aqueous alkaline electrolyte comprises a dissolved salt selected from the group consisting of potassium hydroxide (KOH), sodium hydroxide (NaOH) and lithium oxide (LiOH). Each possibility represents a separate embodiment of the invention.

The utilized electrolyte should be complimentary to the supercapacitor system in which it serves in. Studies showed a strong correlation between the electrode pore size and the electrolyte ion size. This correlation affects the conductivity of the electrode and addresses the need of tailoring the pore sizes to the ion size to promote optimum supercapacitor performance. Another important factor is the chemical interaction between the electrode material, i.e. active carbon, and the electrolyte.

As mentioned hereinabove, it was found that the KOH electrolyte has a lower tendency to react with the carbon impurities, as well as typical surface functional groups existing in activated carbon, in comparison, for example, to acidic electrolytes. The low reactivity of KOH prevents parasitic reactions, which in turn reduce the tendency for leakage current, self-discharge and energy/power loss of the supercapacitor device. Moreover, KOH electrolyte is more chemically ccoommppaattiibbllee wwiitthh tthhee ppaacckkaaggiinngg eeqquuiippmmeenntt aanndd mmaatteeririaallss ooff tthhee aasssseemmbblleedd ssuuppeerrccaappaacciittoorr ddeevviiccee aanndd pprreevveennttss ccoorrrroossiioonn ooff tthheessee mmaatteerriiaallss wwhhiicchh aarree iinn cclloossee pprrooxxiimmiittyy ttoo tthhee eelleeccttrroollyyttee ssoolluuttiioonn..

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According to some embodiments of the invention, the first electrode as described above is a free-standing and mechanically stable electrode. Additionally, the electrode can be essentially free of commonly used additives such as binder reagents for enforcing mechanical stability of an electrode. Thus, in certain embodiments, the first electrode as described above is a binder-free electrode. In some related embodiments, the first electrode is substantially free of gelling agents and/or thickening agents. As used herein the term "self-standing electrode" refers to an electrode that is made out of a paste comprising the low purity activated carbon and alkaline electrolyte 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. 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 terms "substantially free of gelling agents and/or thickening agents" and "essentially free of gelling agents and/or thickening agents" are used interchangeably and mean that the amount of such agents is undetectable by conventional detection means (e.g., elemental analysis). In some embodiments, the term "essentially free of gelling agents and/or thickening agents" refers to the presence of less than 0.5 wt% of said agents in the first porous electrode. In some other embodiments, said term refers to the presence of less than 0.1 wt% of said agents of the total weight of the electrode. In further embodiments, said term refers to an amount of less than 0.05 wt% of said agents of the total weight of the electrode. As described above, the low purity activated carbon is one of the main components in the electrode of the invention. Accordingly, in some embodiments, the activated carbon is present in the first porous electrode in a weight percent ranging from about 35 to about 45 wt% of the total weight of the first electrode. The second main component is the alkaline electrolyte as described above. Thus, in some currently preferred embodiments, the alkaline electrolyte is present in the first porous electrode in a weight percent ranging from about 55 to about 65 wt% of the total weight of the first electrode. In certain embodiments, the first porous electrode comprises a dry matter content (DMC) from about 35 to about 45 wt% activated carbon and an alkaline electrolyte in an amount of about 55 to about 65 wt%. In additional embodiments, the first electrode consists essentially of the low purity activated carbon having above about 7 wt% ash and aqueous alkaline electrolyte. In some specific embodiments, said aqueous alkaline electrolyte comprises KOH.

In some embodiments, the first porous electrode has a porosity of about 0.3 to about 0.9 cc/gr. In another embodiment, the porosity/pore volume is about 0.4 to about 0.8 cc/gr. In yet another embodiment, the porosity/pore volume is about 0.45 to about 0.75 cc/g.

The free-standing electrode as described above is mechanically stable and can be handled in a reproducible and efficient manner. In some embodiments, the first porous electrode as described above has a thickness ranging from about 50 micron to about 5 millimeters. In certain embodiments, the first porous electrode has a thickness ranging from about 50 micron to about 350 micron. In further embodiments, the first porous electrode has a thickness ranging from about 100 micron to about 300 micron. In additional embodiments, the first porous electrode has a thickness ranging from about 300 micron to about 1.5 millimeters. In further embodiments, the first porous electrode has a thickness ranging from about 500 micron to about 1 millimeter.

In some additional embodiments, the supercapacitor of the invention includes a porous electrode as described above, which further comprises a conductive material selected from the group consisting of carbon nanotubes (CNTs), graphite, graphene and paracrystalline carbon. Each possibility represents a separate embodiment of the invention.

Supercapacitor stabi lity

. The electrode of the invention demonstrates an improved overall performance at a wide range of temperatures. In some embodiments, the specific capacitance of the first porous electrode is at least about 45 F/g. According to the principles of the present invention, the supercapacitor as described above demonstrates an improved stability and capacitive properties at subzero temperatures. Without wishing to being bound by theory or mechanism of action, it is contemplated that the performance as subzero temperatures is affected by the separator composition and the specific combination of the unique composition of the electrode and 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).

Non-limiting examples of separator materials suitable for use in the supercapacitor of the invention include polyvinyl alcohol, polypropylene, polyethylene and combinations thereof. In certain embodiments, the separator is selected from a polyvinyl alcohol -coated polyethylene separator and a polypropylene separator. Each possibility represents a separate embodiment of the invention.

In some currently preferred embodiments, the separator comprises polyvinyl alcohol. In some specific embodiments, the separator has a core-shell structure in which the core in made of polyethylene and the shell is made of polyvinyl alcohol (i.e. polyvinyl alcohol coated polyethylene separator). It is further contemplated that the combination between the low purity activated carbon source and the polyvinyl alcohol-based separator in an alkaline media contributes to the low and stable equivalent series resistance (ESR) values provided by the supercapacitor of the invention.

In some embodiments, the first porous electrode comprises the activated carbon applied to the separator. In some additional embodiments, the 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. According to the principles of the invention, the low purity activated carbon in combination with alkaline electrolyte offers high energetic efficiency and capacitance, and relatively low leakage current of the supercapacitor comprising said electrode. The supercapacitor of the invention may be a symmetric or an asymmetric supercapacitor, including at least one electrode as described above. In some embodiments, the supercapacitor of the invention is a symmetric supercapacitor. In certain such embodiments, the second electrode is substantially identical to the first electrode. The term "substantially identical", as used herein, refers to electrodes, having the same composition, wherein the weight percent of each of electrode's constituents varies between the electrodes by no more than 10%. In further embodiments, the term "substantially identical" refers to electrodes, having the same composition, wherein the weight and/or thickness of the electrodes differs by no more than 10%. In further embodiments, the second electrode comprises low purity activated carbon having an ash content of above about 5 wt%, and is impregnated with aqueous alkaline electrolyte. In still further embodiments, the second electrode comprises low purity activated carbon having an ash content of above about 7 wt%, and is impregnated with aqueous alkaline electrolyte. In yet further embodiments, the second electrode comprises low purity activated carbon having an ash content of above about 10 wt%, and is impregnated with aqueous alkaline electrolyte. In still further embodiments, the low purity activated carbon of the first electrode has above about 7 wt% ash.

In some other embodiments, the supercapacitor of the invention is an asymmetric supercapacitor. The asymmetric electrodes configuration may increase the energy density stored in the supercapacitor. In some embodiments, the asymmetric supercapacitor of the invention has a first low-purity carbon based anodic electrode as described above and a second cathodic electrode comprising a distinct composition. The term "distinct composition", as used herein, refers in some embodiments to the presence or at least one additional constituent in the second electrode as compared to the first electrode or vice versa. In further embodiments, the term "distinct composition" refers to a difference in the weight percent of at least one of electrode's constituents which is above about 10%. In certain such embodiments, the weight ratio between the electrodes can range between about 1 : 1 to about 1 :5. In some embodiments, the weight ratio refers to the dry matter of the electrode (excluding the electrolyte). In further embodiments, the weight ratio refers to the total weight of the electrode (i.e., the electrode impregnated with the electrolyte).

In additional embodiments, the term "distinct composition" refers to the electrodes comprising the same constituents and/or essentially the same weight percent of said constituents, wherein a weight ratio between the electrodes ranges between about 1 : 1.1 to 1 :5. In some embodiments, the weight ratio refers to the dry matter of the electrode (excluding the electrolyte). In further embodiments, the weight ratio refers to the total weight of the electrode (i.e., the electrode impregnated with the electrolyte). In further embodiments, the term "distinct composition" refers to the electrodes comprising the same constituents and/or essentially the same weight percent of said constituents, wherein a thickness ratio between the electrodes ranges between about 1 : 1.1 to 1 :5.

In some specific embodiments, the second electrode comprises low purity activated carbon having an ash content of above about 5 wt%, and is impregnated with aqueous alkaline electrolyte, wherein the weight of the second electrode is different than that of the first electrode. In further embodiments, the second electrode comprises low purity activated carbon having an ash content of above about 5 wt%, and is impregnated with aqueous alkaline electrolyte, wherein the weight percent of activated carbon of the second electrode is different than that of the first electrode. In still further embodiments, the second electrode comprises low purity activated carbon having an ash content of above about 5 wt%, and is impregnated with aqueous alkaline electrolyte, wherein the thickness of the second electrode is different than that of the first electrode. In certain embodiments, the low purity activated carbon of the second electrode has above about 7 wt% ash. In additional embodiments, the low purity activated carbon of the second electrode has above about 10 wt% ash. In further embodiments, the low purity activated carbon of the second electrode has above about 15 wt% ash. In yet further embodiments, the low purity activated carbon of the first electrode has above about 7 wt% ash.

In some additional specific embodiments, the second electrode comprises a transition metal oxide. In some related embodiments, the transition metal oxide is selected from the group consisting of Mn _n O _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 certain embodiments, the second electrode comprises Mn0 ₂ .

In further specific embodiments, the second electrode comprises a transition metal sulfide. In some related 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. Each possibility represents a separate embodiment of the invention.

In some embodiments, the second electrode further comprises activated carbon. The activated carbon can be a high purity activated carbon or a low purity activated carbon. Each possibility represents a separate embodiment of the invention. In further specific embodiments, the second electrode comprises an additional carbonaceous material, such as, but not limited to, graphite, carbon nanotubes, and graphene..

In some currently preferred embodiments, the second electrode as described above comprises Mn0 ₂ , carbon nanotubes (CNTs), graphite and activated carbon.

In certain embodiments, the second electrode does not include a metal as an active ingredient.

Current collector

In some embodiments, the super capacitor further comprises one or more current collectors. 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. l lectrode preparation

In another aspect, the present invention further provides a method for the preparation of the low purity activated-carbon-based electrode as described hereinabove. Such electrode composition and preparation methodology is both economically and environmentally friendly, and gives rise to a straightforward and reproducible production of the beneficial electrode of the invention. Thus, in some embodiments, the present invention provides a method for preparing a supercapacitor electrode comprising a low purity activated carbon having an ash content of above about 5 wt%, wherein the electrode is impregnated with an aqueous alkaline electrolyte, the method comprising the steps of: (a) mixing the aqueous alkaline electrolyte with the low purity activated carbon, with optional stirring, to achieve a homogeneous paste; (b) heating the paste obtained in step (a); and (c) processing the paste obtained in step (b) to form a standalone homogeneous electrode.

In some embodiments, the low purity activated carbon has an ash content of above about 7 wt%. In some embodiments, the activated carbon has an ash content of above about 10 wt%. In further embodiments, the activated carbon has an ash content of above about 15 wt%. In some embodiments, the aqueous alkaline electrolyte comprises a dissolved salt selected from the group consisting of potassium hydroxide (KOH), sodium hydroxide (NaOH) and lithium oxide (LiOH). Each possibility represents a separate embodiment of the invention. In some currently preferred embodiments, the electrolyte comprises KOH.

The present invention is based in part on the unexpected results demonstrating an improved absorption of the electrolyte solution into the activated carbon material upon heating the mixture obtained in step (a). The heating process can be also considered as a degassing process, as the heating assists in the removal of unwanted gases which are physically adsorbed onto the carbon source. Additionally the removal of the unwanted gases releases pressure from the system and reduces the occurrence of abruption of the sealed capacitor system at later stages (e.g., during operation). In some additional embodiments, the mixing in step (a) is carried out in a stepwise manner. In some related embodiments, the mixing in step (a) is carried out utilizing a mixing apparatus selected from a mixer, a hand mixer, a homogenizer or a sonicator. Each possibility represents a separate embodiment of the invention. In some currently preferred embodiments, the mixing in step (a) is carried out utilizing a mixer. In some specific embodiments, the mixing is performed at a speed of from about 20 to about 120 RPM. In further embodiments, the mixing is performed at a speed of from about 40 to about 100 RPM. In some additional embodiments, the mixing speed is increased throughout step (a).

In some related embodiments, the weight ratio of activated carbon and the alkaline electrolyte in step (a) ranges from about 35 to about 45 wt% activated carbon and from about 55 to about 65 wt% electrolyte.

In some embodiments, the heating in step (b) is carried out at a temperature of about 50 to about 100 °C. In further embodiments, the heating in step (b) is carried out at a temperature of about 60 to about 90 °C or of from about 70 to about 80 °C. Each possibility represents a separate embodiment of the invention.

According to the principles of the present invention, the processing of the homogeneous paste obtained in step (a) can be achieved by several commonly used industrial methods. Thus, in some embodiments, the processing of step (c) is carried out utilizing a method selected from rolling, calendering, coating, casting, pressing, printing, 3D printing or a combination thereof. Each possibility represents a separate embodiment of the invention. In certain embodiments, the processing of step (c) is carried out utilizing rolling. In some specific embodiments, the rolling as described above is performed on an inert polymeric laminate. In some currently preferred embodiments, the laminate is peeled off after the rolling process to further allow the end-product electrode, which is a free-standing electrode. In some embodiments, the processing of step (c) further comprises pressing. In some currently preferred embodiments, the paste is pressed between two non-conducting polymer sheets.

In yet another aspect, the present invention provides a method for the preparation of the supercapacitor as described hereinabove, comprising preparing the first supercapacitor electrode comprising (a) mixing aqueous alkaline electrolyte with low purity activated carbon having an ash content of above about 5 wt%, with optional stirring, to achieve a homogeneous paste; (b) heating the paste obtained in step (a); and (c) processing the paste obtained in step (b) to form a standalone homogeneous electrode.

In some embodiments, the low purity activated carbon has an ash content of above about 7 wt%. In some embodiments, the activated carbon has an ash content of above 10 wt%. In further embodiments, the activated carbon has an ash content of above about 15 wt%. In further embodiments, the method comprises preparing a second porous electrode. In still further embodiments, the method comprises separating the first porous electrode from the second porous electrode by a porous separator. In further embodiments, the method comprises filling the separator with the electrolyte, wherein the electrolyte is in contact with the first porous electrode and with the second porous electrode.

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 : Electrode preparation

a) Paste preparation:

A free-standing active carbon-based electrode was prepared utilizing impure carbon source, and without the presence of stabilizing reagents such as binders, silicates or gelling agents. The preparation of 15Kg active carbon (AC) pate was performed as follows: 30 wt% KOH electrolyte solution was prepared using titration against a known standard solution. 6Kg of low purity AC having 10 wt.% ash content was mixed with 4.5Kg of electrolyte solution in a pot and mixed a mixer at a speed of about 40 RPM for ten minutes. Pressure buildup during the stirring caused by the release of gas adsorbed to the carbon was released in a controlled manner. An additional 4.5Kg of electrolyte solution was added to the pot and was mixed at a speed of about 80 RPM for another 15 minutes. The pot was heated during the exothermic process and was left for about 30 minutes to cool down. The paste was then heated in an oven at 80°C for 12 hours under KOH atmosphere.

b) Electrode processing:

15 Kg of electrode paste were prepared according to step (a) above and rolled as one batch. The paste was allocated between two laminate layers/foils and pressed by the rolling process to the desired 10 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: Energetic efficiency and capacitance of supercapacitors including different carbon sources and electrolytes.

In order to evaluate the effect of the carbon source purity and acidity of the aqueous electrolyte on the supercapacitor performance (including ESR, capacitance and energetic efficiency), four different types of supercapacitors were built (five 6 cm ² supercapacitor cells of each type) including the anodes and electrolytes presented in Table 1. Table 1 : Composition of the supercapacitors tested in the energetic efficiency experiment

The anodes were prepared as described in Example 1. The alkaline electrolyte utilized was a 30% KOH solution and the acidic electrolyte was a 4.2M sulfuric acid solution (H ₂ S0 ₄ ).

Cathodes of identical composition and weight/thickness were used in all the tested supercapacitors. The cathodes had the same composition as the anode (described above) as well as the same weight/thickness.

The ESR values of the supercapacitors were measured before the beginning of the cell operation and after 12 charge/discharge cycles. The ESR was measured by HIOKI mili-ohm- meter at a 1kHz frequency. The measured ESR values are summarized in Table 2. It can be seen that the resistance of the alkaline supercapacitors was higher than that of their alkaline counterparts both in the beginning and end of the experiment, but resistance of the alkaline capacitors decreases following cycling. Without wishing to being bound by theory or mechanism of action, it is contemplated that the decrease in the resistance values of the alkaline supercapacitors can be due to the penetration of the electrolyte into the carbon pores. The results of the ESR measurement further suggest that the charge/discharge cycling reduced the difference in the ESR of the low purity carbon-based anode and the high purity carbon-based anode from 18% (before cell operation) to only 10% (after 12 cycles). It can therefore be assumed that prolonged cell operation should further decrease the ESR of the low purity -based anode, making the difference between the high purity and the low purity carbon sources negligible. Table 2: ESR measurement of the supercapacitor cell before and after charge/discharge cycles

Following the initial ESR measurement, the cells were charged at constant current (CC) up to the voltage of 0.9V, then were left to rest for 5 seconds. Afterwards the cells were discharged at CC (same as charge CC) up to 0.1V and left to rest again for 5 seconds. Capacitance of the supercapacitors including different anodes and electrolytes was calculated from the slope of the discharge curve in the 0.3 V-0.55V potential range.

It was found that the capacitance of the high purity carbon-based anode in the acidic electrolyte was significantly lower than that of the low purity carbon-based anode and even in the alkaline electrolyte the capacitance of the high-purity carbon anode was essentially the same as that of the low-purity carbon anode, wherein the capacitance is normalized to the weight of the electrode paste (Figure 1). Accordingly, in the alkaline electrolyte there is no superiority to the high purity carbon over the low purity carbon source.

A charge/discharge experiment was performed in order to assess the energetic efficiency of the supercapacitors in terms of energy obtained versus energy invested. The cells were charged and discharged under a constant current of 0.1 A between 0.1V and varying upper potential limits (0.5V - 1.3 V). As can be seen from Figure 2, the highest energetic efficiency was obtained under alkaline conditions. Surprisingly, the most energy efficient supercapacitor was the alkaline cell including low purity carbon-based anode (B) when the cells were charged and discharged between 0.1V and 0.9V-1.3V. Its superiority over the high purity carbon-based anode-containing supercapacitor was particularly pronounced at the deeper charge/discharge profiles. The high energetic efficiency at wide potential window cycles is of high importance, as the energetic efficiency of the supercapacitor decreases with the increase of the potential window.

It can therefore be concluded that despite the potential parasitic reactions and relatively high ESR, the low-purity carbon source offers capacitance which is comparable to and energy efficiency which is even higher than that of the high-purity carbons typically employed in supercapacitor electrodes.

Example 3 : Leakage current of supercapacitors including different carbon sources and electrolytes

Leakage current is an indicator of the energy spent on parasitic reactions, which may lead to self-discharge of the supercapacitor. Low purity carbons are more prone to parasitic reactions due to the higher content of the impurities which can interact with the electrolyte. In order to further evaluate the use of low purity carbons in supercapacitor electrodes, the effect of the carbon source and the type of electrolyte on the leakage current was tested. Two distinct low purity carbon sources were used to manufacture alkaline and acidic supercapacitors as shown in Table 3.

Table 3 : Composition of the supercapacitors tested in the leakage current experiment

The anodes were prepared as described in Example 1. The cathodes had the same composition as the anode as well as the same weight/thickness.

The supercapacitors were tested utilizing both acidic and alkaline electrolytes in order to study the interaction of the electrolyte with the carbon source used in each capacitor. The alkaline electrolyte utilized was a 30% KOH solution and the acidic electrolyte was a 4.2M sulfuric acid solution (H ₂ SO ₄ ).

The leakage current was assessed as follows:

• The supercapacitor was kept at constant temperature of 23°C for at least 12 hours.

The supercapacitor was tested at Discharged condition.

• The supercapacitor was charged to required cell voltage for 120 min. · A 1000Ω resistor was connected to the supercapacitor circuit. • The voltage was measured on the resistor in the circuit and the leakage current (LC) was calculated using Formula I:

r ji measured voltaqe \V] „ , _T

LC \A \ = : Γ Formula I

known resistance [R]

Voltage measurement was continued for 7 days.

Results:

As can be seen in Figure 3, the measured leakage current of the acidic supercapacitors was significantly higher than that of the alkaline supercapacitors, for both carbon sources. Both in acidic and alkaline conditions the more contaminated carbon source (supercapacitors E and F) showed higher leakage currents than the less contaminated carbon. Without wishing to being bound by theory or mechanism of action, it is postulated that the high ash content carbon is more prone to parasitic reactions when utilizing acidic environment than an alkaline one. The leakage currents measured for the supercapacitors G and H containing anodes with 10 wt% ash were lower than the ones measured for the supercapacitors E and F containing anodes with 18 wt% ash. However, the difference in the leakage currents of the two anode types in alkaline supercapacitors was lower than the difference in the leakage currents in the acidic conditions (about 66% in alkaline conditions about 94% in acidic conditions) suggesting that alkaline supercapacitors are more tolerant towards the impurities found in the low purity carbon-based electrodes and wider range of low purity carbon sources can be employed in alkaline supercapacitors. It was therefore shown that the combination of a low purity carbon and alkaline electrolyte allows efficient operating conditions with low leakage current involved.

Example 4: Cost efficiency of different carbon -based anodes

In order to further assess the performance and cost efficiency of the electrodes including 10 wt% and 18 wt% ash-containing carbons, ESR and capacitance of alkaline supercapacitors comprising said anodes were compared to the corresponding data obtained from supercapacitors including anodes containing carbons with various higher purity grades.

The anodes were prepared as described in Example 1 and included carbons with 18 wt% ash, 10 wt% ash, 5 wt% ash, and less than 2 wt% ash. The electrolyte was alkaline, comprising a 30% KOH solution.

The cathodes had the same composition as the anode as well as the same weight/thickness.

The supercapacitors were 6 cm ² supercapacitor cells. The capacitance was measured as described in Example 2.

The measured electrode capacitance at 0,1 A was normalized to the price per gram of the carbon source (capacitance cost-efficiency). Table 4 represents characterization data of the sources and the acquired ESR and capacitance values.

Table 4: ESR and capacitance of different carbon -based electrodes

Figure 4 represents the electrode capacitance cost-efficiency as a function of carbon ash content. It can be seen that there is a direct correlation between the ash content and commercial viability of the electrode capacitance. The empirical linear relationship is shown in Formula II.

Capacitance (F/$)=535 ash wt% + 1739 Formula II

Activated carbons employed in commercially available supercapacitors (including aqueous and organic ones) were tested to assess their capacitance cost-efficiency. Typical capacitance provided by such carbons was found to be 80-100 F/g, wherein said carbon cost is 15 $/kg. Capacitance cost-efficiency of the carbon used in commercial supercapacitors therefore ranges between 5333 and 6667 F/$. Using Formula II it can be estimated that low purity carbon electrodes having above 7 wt% ash, according to the principles of the present invention, provide higher capacitance cost-efficiency than the currently available commercial supercapacitor carbon electrodes.

Accordingly, not only the low purity carbon sources and in particular activated carbons having ash content of above about 7 wt%, provide high energetic efficiency and acceptable leakage currents, they also enable production of electrodes with highly cost-efficient capacitance. Example 5: Electrode thickness effect over internal resistance and overall capacitance While low purity carbon-based electrodes were shown to be highly energetically- and cost= efficient, the performance of the supercapacitors comprising such electrodes can be further improved, for example by reducing the internal resistance. The present experiment was designed to study the effect of the low purity carbon-based electrode thickness on the internal resistance and overall capacitance of the supercapacitors. ESR and capacitance measurements were performed in two distinct electrode configurations including: a) symmetric supercapacitor comprising two similar low purity carbon-based electrodes of the invention; and b) asymmetric supercapacitor comprising a first low purity carbon-based electrode of the invention and a second electrode comprising Mn0 ₂ , carbon-nanotubes and active carbon. The low-purity carbon- based electrodes were prepared as described in Example 1. The mass of the electrodes in the symmetric supercapacitor were 0.32 gr for each carbon-based electrode and the mass of the electrodes in the asymmetric supercapacitor were 0.3 gr of the low purity carbon electrode and 0.38 gr of the Mn0 ₂ -based electrode. Both supercapacitors were 6 cm ² cells and were sealed under 10Kgf/cm ² . The electrode thicknesses which were evaluated in both supercapacitors were 600 μπι and 300μπι. The electrolyte used was 30% KOH.

Results:

As can be seen in Figure 5, the reduction in electrodes' thickness from 600 μπι to 300 μπι in the symmetric supercapacitor promoted the reduction in the internal resistance by 1 1%. In the asymmetric supercapacitor the same thickness reduction gave rise to a 40% reduction in the internal resistance values.

The decrease in the ESR values was however accompanied by the decrease in capacitance of the supercapacitor. As can be seen in Figure 6, the reduction in electrodes' thickness from 600 μπι to 300 μπι in the symmetric supercapacitor gave rise to a reduction of about 50% in capacitance under different current conditions. Similarly, the electrode thickness reduction performed in the asymmetric supercapacitor gave rise to a reduction of about 50% in the capacitance values measured under different current condition as can be seen in Figure 7.

Using thinner electrodes (about 300 μπι and below) can therefore be beneficial for decreasing internal resistance of the supercapacitors comprising low purity carbon-based electrodes. Example 6: Separator and separator-paste interaction effect on the capacitance

In order to improve the overall performance of the super capacitor comprising the low purity carbon-based electrode of the invention, and in particular to further reduce the internal resistance of the cells, several supercapacitors were built with distinct separator compositions were the main groups tested are as follows:

A) Polypropylene-based separators;

C) Polyethylene-based separators coated with polyvinyl alcohol; and

D) Cellulose-based separator.

The separators tested and their physical properties are presented in table 5 :

Table 5 : Physical properties of supercapacitors

* D - No Data

Anodes of identical composition and weight/thickness were used in all the tested supercapacitors. The anodes were prepared as described in Example 1. The alkaline electrolyte utilized was a 30% KOH. Cathodes of identical composition and weight/thickness were used in all the tested supercapacitors. Cathodes are identical to the anodes so they contain the same. The capacitance and internal resistance were measured for 10 different supercapacitors, each having a distinct separator with different characteristics as described in Table 5. Said electrochemical characterization tests were performed at different temperatures, ranging from - 30°C to 65°C in order to assess the electrodes' stability at a wide temperature range. The ESR DC was measured at 0.1 and lAmp for each supercapacitor, and ESR AC was measured in order to gain more information about the ionic resistance of the measured system.

Results:

The separator's thickness is an important factor affecting the supercapacitor's processibility. In order to allow effective production of the supercapacitor, a relatively thick separator is considered to be advantageous. According to Figures 8 and 9, supercapacitors comprising separators No. 2 and 8, both comprising polyvinyl alcohol coated polyethylene separators gave rise to low internal resistance in both room temperature measurements and at subzero measurements carried out at -30°C.

According to Figures lOA-lOC, supercapacitor comprising separator No. 8 provided the lowest internal resistance in a wide temperature range, and demonstrated superiority over supercapacitor comprising separator No. 2 (thinner separator).

Additionally, a charge-discharge experiment in which the supercapacitors were charged at a constant current density of 50 mA/cm ² , and later were discharged at the same current density to full depth of discharge (DOD), demonstrated the stability of a supercapacitor comprising low purity carbon-based anodes and polyvinyl alcohol coated polyethylene separators, giving a low and improved ESR values at 65°C, even after thousands of cycles as can be seen in Figure 11 A, and the overall capacitance as can be seen in Figure 11B. The low ESR values afforded by the PVA-coated polyethylene separators are particularly important when using the low purity carbon-based electrodes of the present invention.

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.