Patent Application
20120236467
The present invention relates generally to electrochemical capacitors, and more particularly to ultracapacitors that utilize a hybrid of carbon nanotubes and nanoparticles of an active material as electrodes, and methods of manufacturing and applications of the same.
Ultracapacitors, also known as supercapacitors, are electrochemical capacitors with relatively high energy density. Generally, an ultracapacitor has the energy density which is hundreds of times greater than that of a conventional electrolytic capacitor. Carbon nanotube (CNT) forests perform well as ultracapacitor electrodes because of their high electrical conductivity, large surface area, polarizability, and chemical and thermal stability. Despite these advantages, the specific capacitance of pristine-CNT ultracapacitors is mediocre (<40 F/g, where F/g refers to the specific capacitance of the electrode material). Methods to improve the CNT capacitance via mixing with pseudocapacitive materials have been reported.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.
In one aspect, the present invention relates to an ultracapacitor. In one embodiment, the ultracapacitor has a first plate, a second plate and a separator sandwiched between the first plate and the second plate. Each of the first plate and second plate includes a substrate, first nanostructures formed on the substrate, and second nanostructures, being different from the first nanostructures, attached to the first nanostructures. In one embodiment, the separator is porous. The first plate and the second plate are adapted to be symmetrically or asymmetrically arranged.
The substrate can be a rigid conducting substrate or a flexible conducting substrate. In one embodiment, the substrate comprises a doped silicon substrate.
In one embodiment, the first nanostructures are grown vertically on the substrate. In one embodiment, the first nanostructures comprise carbon nanotubes (CNTs) or carbon fibers/nanofibers (CFs). The CNTs or CFs have diameters or thicknesses in a range of about 1.0-1,000.0 nm.
In one embodiment, the first nanostructures are grown in a continuous film on the entire substrate or over the region of interest of the substrate. In another embodiment, the first nanostructures are grown in a pre-determined array pattern on the substrate.
In one embodiment, the second nanostructures comprise nano-particles of an active material, having diameters or sizes in a range of about 1.0-1000.0 nm, wherein the active material comprises MnO2, Ag2O, FeS, RuO2, NiOx, CoOx, V2O5 or a mixture thereof.
In one embodiment, the ultracapacitor also includes an electrolyte solution filled in spaces among the first nanostructures and the second nanostructures in the first plate and the second plate.
In one embodiment, the ultracapacitor further has one or more insulation layers disposed on at least one of the first plate and the second plate, such that rolling over the ultracapacitor defines a cylindrical-type multi-layered ultracapacitor cell.
In another aspect, the present invention relates to an electrical energy storage device comprising at least one ultracapacitor claimed above, where the first plate and the second plate are formed with materials and with dimensions such that the specific capacitance is greater than 500 F/g.
In yet another aspect, the present invention relates to an ultracapacitor cell having a plurality of ultracapacitors electrically parallel-connected to each other. Each ultracapacitor comprises a first plate, a second plate and a separator sandwiched between the first plate and the second plate. Each of the first plate and second plate includes a substrate, first nanostructures formed on the substrate, and second nanostructures, being different from the first nano structures, attached to the first nano structures. In one embodiment, the separator is porous.
In one embodiment, the first nanostructures comprise carbon nanotubes (CNTs) or carbon fibers/nanofibers (CFs), wherein the CNTs or CFs are grown on the substrate. In one embodiment, the first nanostructures are grown in a continuous film on the entire substrate or over the region of interest of the substrate. In another embodiment, the first nanostructures are grown in a pre-determined array pattern on the substrate.
In one embodiment, the second nanostructures comprise nano-particles of an active material, where the active material comprises MnO2, Ag2O, FeS, RuO2, NiOx, CoOx, V2O5 or a mixture thereof.
The ultracapacitor cell may further include a first conducting track member and a second conducting track member positioned apart from the first conducting track member to define a space therebetween, such that the plurality of ultracapacitors is stacked in the space and parallel-connected through the first and second conducting track members.
Additionally, the ultracapacitor cell may also have an electrolyte solution filled in spaces among the first nanostructures and the second nanostructures in the first plate and the second plate.
In a further aspect, the present invention relates to an ultracapacitor cell. In one embodiment, the ultracapacitor cell has a first conducting track member and a second conducting track member positioned apart from the first conducting track member to define a space therebetween.
The ultracapacitor cell also has a plurality of first plates electrically coupled to the first conducting track member, and a plurality of second plates electrically coupled to the second conducting track member. The plurality of first plates and the plurality of second plates are alternately positioned in the space defined between the first conducting track member and the second conducting track member. Each of the plurality of first plates and the plurality of second plates comprises a conducting substrate, first nanostructures formed on the conducting substrate, and second nanostructures, being different from the first nanostructures, attached to the first nanostructures formed on the conducting substrate.
In one embodiment, the first nanostructures comprise carbon nanotubes (CNTs) or carbon fibers/nanofibers (CFs), where the CNTs or CFs are grown on the substrate. The second nanostructures comprise nano-particles of an active material.
Additionally, the ultracapacitor cell further includes a plurality of separators, where each separator is sandwiched between a respective first plate and its adjacent second plate in the space. In one embodiment, each separator is porous.
In yet a further aspect, the present invention relates to a method of fabricating an ultracapacitor. In one embodiment, the method has the steps of forming a first plate and a second plate, where each of the first and second plates comprises a substrate, first nanostructures formed on the substrate, and second nanostructures, being different from the first nanostructures, attached to the first nanostructures, and disposing a separator between the first plate and the second plate. In one embodiment, the separator is porous. The substrate comprises a rigid conducting substrate or a flexible conducting substrate.
In one embodiment, the step of forming each of the first plate and the second plate comprises the steps of growing vertically the first nanostructures on the substrate, and attaching the second nanostructures to the first nanostructures grown on the substrate.
The first nanostructures in one embodiment comprise carbon nanotubes (CNTs) or carbon fibers (CFs).
In one embodiment, the first nanostructures are grown in a continuous film on the entire substrate or over the region of interest of the substrate. In another embodiment, the first nanostructures are grown in a pre-determined array pattern on the substrate.
In one embodiment, the substrate is a doped n-type silicon substrate. The growing step includes the steps of oxidizing the silicon substrate to form a layer of SiO2 on the silicon substrate, spin-coating a layer of photoresist on the SiO2 layer, patterning the photoresist layer to expose regions of the SiO2 layer in accordance with the pre-determined array pattern, wet-etching back of the exposed regions of the SiO2 layer to expose the corresponding regions of the silicon substrate, depositing a buffer layer in the corresponding exposed regions of the silicon layer, lifting off the photoresist on the SiO2 layer, and growing vertically aligned CNTs or CFs in the regions at which the buffer layer are present so as to form the array of the vertically aligned CNTs or CFs on the substrate in accordance with the pre-determined array pattern. In one embodiment, the buffer layer comprises titanium and a catalyst of cobalt.
In one embodiment, the growing step is performed with an MPECVD (microwave plasma enhanced chemical vapor deposition) process or a HFCVD (hot filament chemical vapor deposition) process or a thermal chemical vapor deposition process.
In one embodiment, the second nanostructures comprise nano-particles of an active material, and wherein the active material comprises MnO2.
In one embodiment, the attaching step comprises the steps of preparing a suspension of the nano-particles of the active material in a liquid medium, dripping the suspension into the first nanostructures grown on the substrate, and drying the suspension to attach the nano-particles of the active material onto the first nanostructures. The liquid medium comprises acetone or water or other liquid media.
In another embodiment, the attaching step comprises the steps of providing a solution containing potassium permanganate (KMnO4) and water, and performing in-situ electrodeposition of the solution in the CNTs or CFs grown on the substrate so as to impregnate MnO2 directly on the CNTs or CFs.
In one embodiment, the method may also have the step of filling an electrolyte solution in spaces among the first nanostructures and the second nanostructures in the first plate and the second plate.
These and other aspects of the present invention will become apparent from the following description of the preferred embodiment taken in conjunction with the following drawings, although variations and modifications therein may be affected without departing from the spirit and scope of the novel concepts of the disclosure.
The accompanying drawings illustrate one or more embodiments of the invention and together with the written description, serve to explain the principles of the invention. Wherever possible, the same reference numbers are used throughout the drawings to refer to the same or like elements of an embodiment.
The present invention is more particularly described in the following examples that are intended as illustrative only since numerous modifications and variations therein will be apparent to those skilled in the art. Various embodiments of the invention are now described in detail. Referring to the drawings, like numbers indicate like components throughout the views. As used in the description herein and throughout the claims that follow, the meaning of “a”, “an”, and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein and throughout the claims that follow, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise. Moreover, titles or subtitles may be used in the specification for the convenience of a reader, which shall have no influence on the scope of the present invention. Additionally, some terms used in this specification are more specifically defined below.
The terms used in this specification generally have their ordinary meanings in the art, within the context of the invention, and in the specific context where each term is used. Certain terms that are used to describe the invention are discussed below, or elsewhere in the specification, to provide additional guidance to the practitioner regarding the description of the invention. For convenience, certain terms may be highlighted, for example using italics and/or quotation marks. The use of highlighting has no influence on the scope and meaning of a term; the scope and meaning of a term is the same, in the same context, whether or not it is highlighted. It will be appreciated that same thing can be said in more than one way. Consequently, alternative language and synonyms may be used for any one or more of the terms discussed herein, nor is any special significance to be placed upon whether or not a term is elaborated or discussed herein. Synonyms for certain terms are provided. A recital of one or more synonyms does not exclude the use of other synonyms. The use of examples anywhere in this specification including examples of any terms discussed herein is illustrative only, and in no way limits the scope and meaning of the invention or of any exemplified term. Likewise, the invention is not limited to various embodiments given in this specification.
It will be understood that when an element is referred to as being “on” another element, it can be directly on the other element or intervening elements may be present therebetween. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.
It will be understood that, although the terms first, second, third etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention.
Furthermore, relative terms, such as “lower” or “bottom” and “upper” or “top,” may be used herein to describe one element's relationship to another element as illustrated in the Figures. It will be understood that relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. For example, if the device in one of the figures is turned over, elements described as being on the “lower” side of other elements would then be oriented on “upper” sides of the other elements. The exemplary term “lower”, can therefore, encompasses both an orientation of “lower” and “upper,” depending of the particular orientation of the figure. Similarly, if the device in one of the figures is turned over, elements described as “below” or “beneath” other elements would then be oriented “above” the other elements. The exemplary terms “below” or “beneath” can, therefore, encompass both an orientation of above and below.
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
As used herein, “around”, “about” or “approximately” shall generally mean within 20 percent, preferably within 10 percent, and more preferably within 5 percent of a given value or range. Numerical quantities given herein are approximate, meaning that the term “around”, “about” or “approximately” can be inferred if not expressly stated.
As used herein, if any, the term “scanning electron microscope” or its abbreviation “SEM” refers to a type of electron microscope that images the sample surface by scanning it with a high-energy beam of electrons in a raster scan pattern. The electrons interact with the atoms that make up the sample producing signals that contain information about the sample's surface topography, composition and other properties such as electrical conductivity.
As used herein, if any, the term “transmission electron microscopy” or its abbreviation “TEM” refers to a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through. An image is formed from the interaction of the electrons transmitted through the specimen; the image is magnified and focused onto an imaging device, such as a fluorescent screen, on a layer of photographic film, or to be detected by a sensor such as a CCD camera.
As used herein, if any, the term “X-ray photoelectron spectroscopy” or its abbreviation “XPS” refers to a method used to determine the composition of the top few nanometers of a surface. It involves bombarding the surface with x-rays above a threshold frequency which leads to generation of photoelectrons from the core-level of the atoms, leaving behind holes. Based on conservation of energy, the kinetic energy can be given by ½ mev2=hυ−EB−φ, where me is the mass of the electron, v is the electron velocity after ejection, h is the Plank's constant, υ is the frequency of incident x-ray, EB is the electron binding energy and φ is the work function of the material being studied. The binding energy can be plotted versus photoelectrons' intensity, and the peaks observed are characteristic of the elements.
As used herein, “nanoscopic-scale”, “nanoscopic”, “nanometer-scale”, “nanoscale”, “nanocomposites”, “nanoparticles”, the “nano-” prefix, and the like generally refers to elements or articles having widths or diameters of less than about 1 μm, preferably less than about 100 nm in some cases. In all embodiments, specified widths can be smallest width (i.e. a width as specified where, at that location, the article can have a larger width in a different dimension), or largest width (i.e. where, at that location, the article's width is no wider than as specified, but can have a length that is greater).
As used herein, a “nanostructure” refers to an object of intermediate size between molecular and microscopic (micrometer-sized) structures. In describing nanostructures, the sizes of the nanostructures refer to the number of dimensions on the nanoscale. For example, nanotextured surfaces have one dimension on the nanoscale, i.e., only the thickness of the surface of an object is between 0.1 and 1000 nm. Nanotubes have two dimensions on the nanoscale, i.e., the diameter of the tube is between 0.1 and 1000 nm; its length could be much greater. Finally, sphere-like nanoparticles have three dimensions on the nanoscale, i.e., the particle is between 0.1 and 1000 nm in each spatial dimension. A list of nanostructures includes, but not limited to, nanoparticle, nanocomposite, quantum dot, nanofilm, nanoshell, nanofiber, nanoring, nanorod, nanotube, and so on.
As used herein, “plurality” means two or more.
As used herein, the terms “comprising”, “including”, “carrying”, “having”, “containing”, “involving”, and the like are to be understood to be open-ended, i.e., to mean including but not limited to.
This invention discloses, among other things, a novel hybrid electrochemical ultracapacitor that combines desirable attributes such as extremely high energy-power density, excellent life-cycle reliability and safety characteristics, with low production cost and has the potential for widespread deployment in energy delivery/storage applications, and innovative methods/approaches of making the same. In the innovative approach, carbon nanotubes (CNTs) or carbon fibers/nanofibers (CFs) are grown directly on conducting flexible (or rigid) substrates to reduce contact resistances. The CNTs or CFs have excellent electrical conductivity and provide extremely large effective surface area essential for generating higher capacitance values. The more controllable CNT or CF nanoarchitectures for optimum attachments of inexpensive pseudocapacitive manganese-dioxide (MnO2) nanoparticles to enhance charge efficiency and energy-power capacity are also exploited. The approaches in one embodiment employ a “green” electrolyte that increases cell voltages.
In one aspect of the invention, the ultracapacitor utilizes an advanced nanoarchitectured pseudocapacitive electrode to enhance energy density with a three dimension (3D) nanostructured high-surface-area CNT or CF conductor electrode for optimum pseudocapacitive MnO2 nanoparticle impregnation, and an ionic electrolyte to operate with greater cell voltage. When charging, the positive cations in the electrolyte are attracted to the negative electrode, and the negative anions to the positive electrode. Both negative and positive ions in the electrolyte accumulate at the electrode surface to compensate for the electronic charge. Hence, electrical energy is stored inside the electrochemical double layer (the Helmholtz layer) at the electrode-electrolyte interface. This effect is further enhanced by the fast reversible redox faradaic pseudocapacitance induced by the MnO2 nanoparticles. The thickness of the double layer (varies between 5 and 10 angstroms) is usually determined by the electrolyte concentration and the ion size. Since the capacitance is proportional to the electrode area, C=εA/d, higher capacitance can be obtained by making electrodes from nanostructured CNTs of CFs for attachment of the nanoscaled MnO2 to optimize the pseudocapacitive effect with very large effective surface areas.
According to the invention, the ultracapacitor stores electrical charges at an electrode-electrolyte interface, with CNTs as a current conductor and MnO2 pseudocapacitor nanoparticles as electrodes. The nanoarchitectured network structure formed by nanotubes allows the attachment of nano-sized MnO2 particles to them, so that each individual nanotube may be covered with a very thin layer of MnO2. Therefore, each nanotube serves as a miniature current collector while the attached MnO2 thin coating serves as tiny electrodes. Because of the excellent electronic conductivity of CNTs, the total equivalent series resistance (ESR) is significantly reduced and the power density is enhanced via P=V2/4R. The fine tailoring of the nano-scale attachment of the electrode material has resulted in optimal performance in terms of energy, power, and cycling capabilities. Moreover, this system can utilize potassium chloride aqueous solution as the electrolyte, which is more environmentally friendly than other alternatives. Accordingly, a specific capacitance of the CNT/MnO2 electrode 100-1000× higher than as-grown CNT-only thin-film electrodes and >50× larger energy density than conventional electrochemical capacitors are achieved. With further optimization of electrode configurations and the cell voltage, the specific capacitance is increased and energy-power are maximized, thereby achieving a transformational energy storage/delivery system both fundamentally and technologically.
In one aspect, the present invention relates to an ultracapacitor utilizing a novel approach, in which as-grown nanostructured CNTs (first nanostructures) provide an excellent conductor network for the plate with high surface area for the attachment of nano-particles of an active material, such as pseudocapacitive nano-particles of MnO2 (second nanostructures). The resulting enhanced surface area maximizes the charge efficiency and the power density, and at the same time the series resistance is reduced. In the following description, the MnO2 pseudocapacitive nano-particles are used as an exemplary active material to describe the invention for the benefits of readers but not as a limitation. However, it is understood that other active materials, including Ag2O, FeS, RuO2, NiOx, CoOx, V2O5 or a mixture thereof, can be utilized to practice the invention.
In some embodiments, the structure of the ultracapacitor can be modified for the purpose of various applications. For example, in one embodiment, the substrate may include a flexible conducting plate such that the ultracapacitor can be formed in different shapes, such as a rolled shape or a cylindrical shape. In one embodiment, a plurality of plates is alternately aligned and electrically connected to two different potentials to form a stacked ultracapacitor. In one embodiment, the ultracapacitor is used in an electrical energy storage device, and the materials and dimensions of the plates are specifically designed such that the specific capacitance is greater than 500 F/g.
According to the invention, the fabrication of the ultracapacitor begins with growth of current collectors, e.g., CNTs or CFs directly on substrates. The substrates can be rigid substrates such as highly doped silicon substrates, or flexible substrates, such as metallic foils (e.g. molybdenum), and very inexpensive graphite foil. The CNTs or CFs growth can be achieved using catalyst assisted MPECVD (microwave plasma enhanced chemical vapor deposition) or HFCVD (hot filament chemical vapor deposition) or thermal chemical vapor deposition processes. As such, the contact resistance can also be minimized.
In one embodiment, the CNTs and CFs are present as a continuous film on the entire substrate or over the region of interest, which is corresponding to a configuration of planar CNTs, CFs. In another embodiment, pre-determined array patterns of CNTs or CFs are created on the silicon substrates, for example, by using typical silicon microfabrication technology, which is corresponding to a configuration of micropatterned CNT, CF arrays. The individual array element can be circular, square or rectangular, other geometrical shapes and is separated from the nearest neighbor by a finite distance, with a thin layer of silicon dioxide providing electrical isolation. After catalyst deposition, CNTs and CFs are grown in selective regions based on the array design.
Further, two processes are utilized for the pseudocapacitive material attachment or incorporation onto the CNTs and CFs. One is a dripping process, which is implemented by using a suspension of MnO2. The suspension is made in different liquid media, for example, acetone or water. The other process is in-situ electrodeposition of MnO2 on the current collectors, where an aqueous solution of KMnO4, is used and MnO4− is directly reduced to MnO2 on the CNTs and CFs surface.
These and other aspects of the present invention are more specifically described below.
Without intent to limit the scope of the invention, exemplary methods and their related results according to the embodiments of the present invention are given below. Note that titles or subtitles may be used in the examples for convenience of a reader, which in no way should limit the scope of the invention. Moreover, certain theories are proposed and disclosed herein; however, in no way they, whether they are right or wrong, should limit the scope of the invention so long as the invention is practiced according to the invention without regard for any particular theory or scheme of action.
As shown in
The separator 110 is positioned between the first plate 120 and the second plate 130 to form the partition between the first and second plates 120 and 130. In one embodiment, the separator 110 can be porous. The separator 110 may be a piece of thin filter paper or an anodized Al2O3 perforated film or other polymeric separator films, which have respectively been proven to perform well as porous separators.
Each of the first and second plates 120 and 130 includes a substrate 126/136, first nanostructures 122/132 formed on the substrate 126/136 and second nanostructures 124/134 different from the first nanostructures 122/132. The second nanostructures 124/134 are attached to the first nanostructures 122/132. Further, an electrolyte solution 140 fills in the space between the substrate 126/136, the first nanostructures 122/132, and the second nanostructures 124/134 such that the electrolyte solution 140 is in contact with the first nanostructures 122/132, the second nanostructures 124/134 and the substrate 126/136 in each plate 120/130.
The two substrates 126 and 136 are positioned outside to bind the respective first and second plates 120 and 130. In one embodiment, each substrate 126/136 can be a rigid conducting substrates including highly doped silicon. In another embodiment, the substrates 126/136 can be a flexible conducting film such as metallic foils (e.g., Molybdenum), graphite or corrosion resistant metal foils.
The first nanostructures 122 and 132 of the first and second plates 120 and 130 are adapted as current collectors, and can be carbon-based nanoscale materials, such as carbon nanotubes (CNTs) or carbon fibers/nanofibers (CFs), which have excellent electrical conductivity and provide extremely large effective surface area essential for generating higher capacitance values. These CNTs or CFs can be grown directly on the substrates, as shown in
The second nanostructures 124 and 134 of the first and second plates 120 and 130 are nano-particles of an active material, i.e., pseudocapacitive material. In one embodiment, the active material includes MnO2. Thus, the first nanostructures 122 and 132 and the second nanostructures 124 and 134 form the CNT/MnO2 electrode structures 125 and 135 for the first and second plates 120 and 130, respectively. In certain embodiments, the active material can be silver oxide, iron sulfide, RuO2, NiOx, CoOx, V2O5 or mixtures thereof, or any other pseudocapacitive materials. In one embodiment, sizes/diameters of the nano-particles of the active material are in the range of 1.0-1000.0 nm.
In one embodiment, the second nanostructures (active material) 124 and 134, e.g., MnO2, are incorporated or impregnated into the first nanostructures 122 and 132, e.g., vertically grown CNTs or CFs, as shown in
For the first plate 120, a partially enlarged view is provided at the top right of
Similarly, for the second plate 130, a partially enlarged view is provided at the top left of
The ultracapacitor structure 100 shown in
In one embodiment, fabrication of the ultracapacitor begins with the growth of CNTs or CFs on a rigid or flexible conducting substrate (for example, graphite or corrosion resistant metal foil). In this configuration, the CNTs and CFs are present as a continuous film on the entire substrate or over the region of interest on the substrate. The grown CNTs or CFs may have diameters or thicknesses of about 1.0-1,000.0 nm. The CNT or CF growth is followed by the deposition of MnO2 nanoparticles, using a dripping/wetting process or other methods including electrodeposition. Electrical leads are connected to the corresponding conducting substrates. Thin filter paper and anodized Al2O3 perforated film or other polymeric separator films have each performed well as porous separators. The assembled package is then housed in a container and immersed in an electrolyte medium. The resulting structure constitutes a basic form of electrochemical ultracapacitor from which various cell configurations can be derived. The growth of CNTs directly on a conducting substrate and the intimate contact between the MnO2 nanoparticles and CNTs provides a large capacitance per unit volume, low internal resistance, and negligible leakage current. When the conducting substrate happens to be flexible graphite foil, the growth of CNTs achieves similar atomic bonding with high mechanical strength.
Referring to
The first plate 220 includes a substrate 226, a first CNT/MnO2 electrode structure 225, and a conducting rod 228. The second plate 230 includes a substrate 236 and a second CNT/MnO2 electrode structure 235. The first and second CNT/MnO2 electrode structures 225 and 235 are similar to the CNT/MnO2 electrode structures 125 and 135 of the ultracapacitor 100 shown in
As shown in
In one embodiment, an electrolyte solution (not shown) can be injected and filled in the space of the first and second CNT/MnO2 electrode structures 225 and 235 such that the first and second CNT/MnO2 electrode structures 225 and 235 can be soaked and immersed in the electrolyte solution, and the electrolyte solution can be in contact with the arrays of the first nanostructures with the coatings of the second nanostructures.
It should be noted that the materials and the dimensions of the first plate 220 and the second plate 230 determine the electrical performance of the electrical energy storage device. For example, the specific energy density of the electrical energy storage device Wh/kg is determined by the following formula:
Wh/kg=(1/8)(F/g)(V2/3.6)
where F/g is the specific capacitance of the electrode material, and V is the cell voltage, which is dependent primarily on the electrolyte solution used in the electrical energy storage device. In one embodiment, the first plate 220 and the second plate 230 are formed with materials and with dimensions such that the specific capacitance is greater than 500 F/g.
Referring to
The first plate 320 includes a first substrate 326 and a first CNT/MnO2 electrode structure 325. The second plate 330 includes a second substrate 336 and a second CNT/MnO2 electrode structure 335. The first and second CNT/MnO2 electrode structures 325 and 335 are similar to the CNT/MnO2 electrode structures 125 and 135 of the ultracapacitor 100 shown in
It should be noted that
As shown in
In one embodiment, the first and second substrates 326 and 336 are flexible conducting plates such that they can be deformed and rolled as shown in
In one embodiment, an electrolyte solution (not shown) is filled in the space of the first and second CNT/MnO2 electrode structures 325 and 335 such that the electrolyte solution can be in contact with the arrays of the first nanostructures with the coatings of the second nanostructures.
It should be appreciated that the multi-stack structure of the symmetric ultracapacitor in
As shown in
Each of the first and second plates 420 and 430 comprises a conducting substrate, and an array of first nanostructures with a coating of second nanostructures different from the first nanostructures formed on the conducting substrate. For example, an enlarged view of one of the second plates 430 is shown in
Further, the separators 410 are positioned such that at least one separator 410 is positioned between any first plate 420 and its adjacent second plate 430. In other words, there is at least one separator 410 between any pair of adjacent first plate 420 and second plate 430.
In one embodiment, an electrolyte solution (not shown) is injected and filled in the space of the CNT/MnO2 electrode structures such that the CNT/MnO2 electrode structures can be soaked and immersed in the electrolyte solution, and the electrolyte solution can be in contact with the arrays of the first nanostructures with the coatings of the second nanostructures.
As shown in
In one embodiment, an electrolyte solution (not shown) can be injected and filled in the space of the first and second CNT/MnO2 electrode structures 525 and 535 such that the first and second CNT/MnO2 electrode structures 525 and 535 can be soaked and immersed in the electrolyte solution, and the electrolyte solution can be in contact with the arrays of the first nanostructures with the coatings of the second nanostructures.
When the ultracapacitor 500 is assembled, the first and second substrates 526 and 536 are respectively coupled to the two electrical terminals 504 and 506 such that the two different potentials can be provided to the first and second plates 520 and 530. The separator 510 is positioned between the first and second plates 520 and 530, and the seal rings 508 are positioned to enclose the first and second CNT/MnO2 electrode structure 525 and 535 to form a package of the ultracapacitor 500, thus ensuring isolation and preventing from leakage of the electrolyte solution.
Further,
According to the invention, the fabrication of an ultracapacitor begins with growth of current collectors, e.g., CNTs or CFs directly on substrates. The CNTs or CFs have excellent electrical conductivity and provide extremely large effective surface area essential for generating higher capacitance values. The substrates can be rigid substrates such as highly doped silicon substrates, or flexible substrates, such as metallic foils (e.g. Molybdenum), and very inexpensive graphite foil. The CNTs or CFs growth can be achieved using catalyst assisted MPECVD (microwave plasma enhanced chemical vapor deposition) or HFCVD (hot filament chemical vapor deposition) or thermal chemical vapor deposition processes. As such, the contact resistance can also be minimized.
There are two different and distinct configurations possible:
(a) Planar CNTs, CFs: In this configuration, the CNTs and CFs are present as a continuous film on the entire substrate or over the region of interest; and
(b) Micropatterned CNT, CF arrays: By using conventional silicon microfabrication technology, pre-determined array patterns are created on the silicon substrates. The individual array element can be circular, square or rectangular, other geometrical shapes and is separated from the nearest neighbor by a finite distance, with a thin layer of silicon dioxide providing electrical isolation. After catalyst deposition, vertically aligned CNTs and CFs are grown in selective regions based on the array design.
In one embodiment, for the growth of the micropatterned structures of CNTs, a highly doped n-type silicon substrate is thermally oxidized to grow a 0.5 μm thick layer of SiO2. After spin coating a layer of photoresist, conventional UV photolithography is used to define the outlay of the final structure. This is followed by a wet-etch back of the exposed regions of SiO2 by BOE solution (buffered oxide etch). Thin layers of titanium (buffer layer) (about 15 nm) and cobalt (catalyst) (about 5 nm) are deposited by DC sputtering technique followed by a photoresist lift-off step. The thickness of the buffer and catalyst layers can be tailored depending upon the CVD process being used for growing CNTs/CFs. Hot filament CVD process is used to grow vertically aligned CNTs in regions where the Ti/Co layers are present. Alternative processes such as thermal CVD or MPECVD can also be used to grow the CNTs/CFs. The SiO2 layer acts as an insulator to provide isolation between regions where CNTs had been grown. The schematic process of the growth of the micropatterned structures of CNTs is illustrated in
As shown in
Then, an etching process is performed on the exposed regions of the SiO2 layer 690 so as to expose the corresponding regions of the silicon substrate, in accordance with the pre-determined array pattern, as shown in
After the etching process, a buffer layer 694 is deposited in the corresponding exposed regions of the silicon layer, which are corresponding to the array of the first nanostructures, as shown in
As shown in
Finally, an MPECVD (microwave plasma enhanced chemical vapor deposition) process or a HFCVD (hot filament chemical vapor deposition) or thermal chemical vapor deposition process is used to grow vertically aligned CNTs or CFs 622 in the regions at which the buffer layer 694 are present so as to form the array of the vertically aligned CNTs or CFs 622 on the substrate 626 in accordance with the pre-determined array pattern, as shown in
As to the formation of the second nanostructures, various methods can be used to provide nano-particles of the active material with the array of the first nanostructures.
After the growth of the current collectors, e.g., CNTs or CFs, on the substrate, an active material (pseudocapacitive material) such as MnO2 is attached to the CNTs or CFs. The process is extremely important according to the invention, because it requires optimum attachment of the active material to the current collectors. If it is added in excess, then the ultracapacitor/device impedance increases. If it is added less, then one cannot maximize the capacitance obtainable from such a device.
According to the invention, two processes are utilized for the pseudocapacitive material attachment or incorporation. One is a dripping process, which is shown in
At first, the active material, MnO2, is prepared, for example, from reduction of KMnO4 which is a strong oxidizing agent. MnO2 nanoparticle precipitates are then collected to obtain MnO2 powder.
In one embodiment, the MnO2 powder is added into acetone and ultra-sonicated to form a uniform suspension 710. The MnO2/acetone suspension is then dripped onto CNT thin film formed on the substrate 726 and dried at room temperature to form the CNT/MnO2 ultracapacitor.
In another embodiment, the MnO2 powder is added into DI-water, and ultra-sonicated to form the uniform suspension 710. The MnO2/acetone suspension is then dripped onto CNT thin film formed on the substrate 726 and dried at greater than about 75° C. to form the CNT/MnO2 ultracapacitor.
The substrate 726 can be a flexible conducting plate on which the CNTs or CFs are formed. In one embodiment, the substrate 726 is disposed on a feeding machine such that the dripping and drying process can be performed by feeding the substrate 726 forward in a direction, as shown in
The other process for the pseudocapacitive material incorporation is in-situ electrodeposition of MnO2 on the current collectors, which is shown in
KMnO4 is a strong oxidizing agent. However, it can be electrochemically reduced directly on the conducting substrate by the following reaction.
MnO4−+2H2O+3e−→MnO2+4OH−
Thus, the improvement in the capacitance of the MnO2/CNT electrode structure can be achieved due to the intercalation/deintercalation of the electrolyte cations in the bulk of the MnO2.
MnO2+K+e−MnOOK
As shown in
In one embodiment, cyclic voltammetry is used at room temperature to electrochemically deposit MnO2 nanoparticles directly on the CNT network grown on a highly doped silicon substrate and flexible graphite foil (substrate) 826.
After clamping the target substrate in the flat cell, 25 ml of 10 mM KMnO4 solution in de-ionized water was added. The cyclic voltammetry process involved application of a potential scan between −1V to +1V at a scan rate of 100 mV/s. This was repeated after an interval of 120 seconds. The number of repetitions determines the MnO2 layer thickness. After the electrodeposition, the KMnO4 solution can be recycled for electrodeposition on other substrates.
Accordingly, a controlled amount of MnO2 nanoparticles can be provided, and it is achievable that diameters of the nano-particles of the active material are in the range of 1.0-1000.0 nm.
Referring to
To examine the effect of MnO2 on the capacitance of CNT electrodes, 5-droplet, 15-droplet, and 30-droplet samples, along with a pristine CNT control sample, are tested as working electrodes in 0.1M KCl electrolyte.
To examine their cyclic stabilities and to further quantify their capacitances, galvanostatic charging-discharging tests were performed.
In the following examples, four different designs are disclosed for producing MnO2/CNT based ultracapacitors. The first design is a square array consisting of 25,600 (160×160) circular elements, each of 3 μm diameter and 30 μm spacing (pitch). The second design is of a rectangular array of 858 (33×26) elements, each 75 μm long and 6 μm wide with a spacing of 30 μm. The third design is an inter-digitated structure with fingers 400 μm long and 40 μm wide. Finally, the fourth design is a device including CNTs grown on a flexible substrate, graphite foil (125 μm thick), and then wrapped around a Teflon rod.
In the first design, 25,600 circular elements of CNT structures, each of 3 μm diameter and 30 μm pitch, were arranged in a square array, and 10 μm tall CNTs were grown by a hot filament CVD process. All the CNTs are isolated from each other by the isolation layer of SiO2. The two electrodes 802 and 804 are Ag/AgCl (3M KCl) (as a reference electrode) and a platinum wire (as a counter electrode). CVs were recorded in 0.1M KCl as the electrolyte at different scan rates (10 mV/s, 50 mV/s, 100 mV/s, 200 mV/s and 500 mV/s), before and after any MnO2 deposition step.
The CV measurements were taken in a flat cell in a 3 electrode configuration, which is similar to the device shown in
The electrochemical deposition of MnO2 was done by using cyclic voltammetry in a 10 m M KMnO4 solution under ambient conditions. The deposition was achieved in 10 cycles (20 sweep segments) with an interval of 60 s between each cycle. CVs recorded during MnO2 deposition can be seen in
The active area based on the CNT footprint will be used for current density measurements and can be calculated as follows:
Total Area=160×160×3.14×(1.5)2×10−8 cm2=0.00181 cm2.
Total volume=0.00181 cm2×10×10−4 cm=1.81×10−6 cm3.
The CVs recorded in 0.1M KCl with and without MnO2 at 100 mV/s and 50 mV/s scan rate are shown in
The device in the first design was also baked at 100° C. for 1 hour under ambient conditions after the 10 cycles of MnO2 deposition.
The second design was of a rectangular array of 858 (33×26) elements, each 75 μm long and 6 μmwide with a spacing of 30 μm. 75 μm tall CNTs were grown by hot filament CVD process, as shown in
The CV measurements were taken in a flat cell in a 3 electrode configuration, which is similar to the device shown in
The electrochemical deposition of MnO2 was done by using cyclic voltammetry in a 10 mM KMnO4 solution under ambient conditions. The deposition was achieved in 15 sets of 2 cycles each (4 sweep segments) for a total of 30 cycles with an interval of 120 s between each cycle. CVs recorded during MnO2 deposition (#30) is shown in
The active area based on the CNT footprint will be used for current density measurements and can be calculated as follows:
Total Area=33×26×75×10−4×6×10−4 cm2=0.00386 cm2.
Total volume=0.00386 cm2×75×10−4 cm=2.9×10−5 cm3.
CVs recorded in KCl before MnO2 deposition and after 8, 16, 24 and 30 cycles of MnO2 deposition at 100 mV/s and 50 mV/s scan rates are shown in
The third design was of interdigitated electrodes with each finger 400 μm long and 40 μm wide and having a gap of 40 μm between the fingers. The fingers of each electrode are connected by an 80 μm wide “path”. 25 μm tall CNTs were grown by hot filament CVD process, as shown in
The CV measurements were taken in a flat cell in a 3 electrode configuration, which is similar to the device shown in
The electrochemical deposition of MnO2 was done by cyclic voltammetry in a 10 mM KMnO4 solution under ambient conditions. The deposition was achieved in 3 sets of 2 cycles each (4 sweep segments) for a total of 6 cycles with an interval of 120 s between each cycle. CVs recorded during MnO2 deposition is shown in
CVs recorded in KCl after 2, 4 and 6 cycles of MnO2 deposition at 100 mV/s and 50 mV/s scan rates are shown in
The fourth design was of a device consisting of CNTs grown on a flexible substrate, graphite foil (125 μm thick), and then wrapped around a Teflon rod. The dense CNT forest was grown using hot-filament CVD process.
The CV measurements were taken in a flat cell in a 3 electrode configuration, which is similar to the device shown in
The deposition was achieved in 2 sets, one of 30 cycles (60 sweep segments) and the second of 10 cycles (20 sweep segments) for a total of 40 cycles with an interval of 120 s between each cycle. CVs recorded during MnO2 deposition are shown in
The assembly used to perform the capacitance measurements and MnO2 deposition is shown in
The active area based on the CNT footprint will be used for current density measurements and can be calculated as follows:
Total Area=1×1.2 cm2=1.2 cm2.
Total volume=1.2 cm2×3×10−4 cm=3.6×10−4 cm3.
The CVs recorded in 0.1M KCl after 40 cycles, 30 cycles and without MnO2 at 100 mV/s and 50 mV/s scan rate are shown in
In sum, the present invention provides, among other things, a novel hybrid electrochemical ultracapacitor that combines desirable attributes such as extremely high energy-power density, excellent life-cycle reliability and safety characteristics, with low production cost and has the potential for widespread deployment in energy delivery/storage applications. According to the present invention, CNTs or CFs were grown directly on conducting flexible (or rigid) substrates to reduce contact resistances, and the more controllable CNT or CF nanoarchitectures for optimum attachment of inexpensive pseudocapacitive MnO2 nanoparticles to enhance charge efficiency and energy-power capacity was exploited. The approach employs “green” electrolyte that increases cell voltage. Accordingly, a specific capacitance of the CNT/MnO2 electrode 100-1000 times higher than as-grown CNT-only thin-film electrodes and >50× larger energy density than conventional electrochemical capacitors is achieved. With further optimization of electrode configurations and cell voltage, it is expected to further increase the specific capacitance and maximize energy-power, achieving a transformational energy storage/delivery system both fundamentally and technologically.
Among other things, the advantages of the present invention are (a) better control of CNT growth on supporting substrate, including flexible conducting foils, (b) enhanced application of the MnO2 nanoparticles, and (c) progress in the choice of large-scale fabrication techniques. The technology is robust and easily integrated into all types of energy storage/delivery applications. It is demonstrated that this technology is amenable to simple and ‘foldable’ packaging which supports the prototypes cell described herein. The prototypes made according to various embodiments of the present invention described can be easily integrated into many applications, including high-voltage applications in which numerous ultracapacitors are configured into parallel-connected stacks or banks.
A further benefit of this technology is that it utilizes completely benign materials such that damage, destruction and disposal are each inconsequential and environmentally neutral. The novel ultracapacitor technology is scalable in performance.
Moreover, the diversity and flexibility in design and material selections allow an electrochemical ultracapacitor to cover a broad range of power vs. energy plot, and bridge the performance gap between the high power densities offered by conventional capacitors and high energy densities of batteries. The push to obtain higher energy without sacrificing power is the central focus of electrochemical ultracapacitor research. Ruthenium dioxide (RuO2) has attracted much attention as a suitable component for CNT composite due to its outstanding pseudocapacitance and excellent reversibility, but the low availability and extremely high cost limit its practical use. Moreover, it is an environmentally hazardous material. On the other hand, active materials such as MnO2 are inexpensive, readily available, environmentally friendly, and demonstrates good pseudocapacitance behavior.
The described invention for achieving extreme high energy and power density ultracapacitor is based on symmetric cell design, but is also applied to asymmetric cell design as well. The ultracapacitor utilizes an advanced nanoarchitectured pseudocapacitive electrode to enhance energy density with a 3D nanostructured high-surface-area CNT conductor electrode for optimum pseudocapacitive MnO2 nanoparticle impregnation, and ionic electrolyte to operate with greater cell voltage. When charging, the positive cations in the electrolyte are attracted to the negative electrode, and the negative anions to the positive electrode. Both negative and positive ions in the electrolyte accumulate at the electrode surface to compensate for the electronic charge. Hence, electrical energy is stored inside the electrochemical double layer (the Helmholtz layer) at the electrode-electrolyte interface. This effect is further enhanced by the fast reversible redox faradaic pseudocapacitance induced by the MnO2 nanoparticles. The thickness of the double layer (varies between 5 and 10 angstroms) is usually determined by the electrolyte concentration and the ion size. Since capacitance is proportional to the electrode area, C=εA/d, higher capacitance can be obtained by making electrodes from nanostructured CNT for attachment of the nanoscaled MnO2 to optimize the pseudocapacitive effect with very large effective surface areas.
Using the innovative nanostructuring approach realized in the ultracapacitor of the present invention, MnO2 has been incorporated directly onto the surface of CNTs carpet by a simple liquid media (‘dripping’) method. In such a configuration, the CNT surface serves as high-surface-area, 3D current collector for MnO2 coatings, and defines the electrode's internal pore structure to facilitate the infiltration and rapid transport of electrolyte to the nanoscopic MnO2. Although such 3D CNT/MnO2 nanostructures can still being further optimized, results by practicing the present invention indicate this approach is far superior to other electrochemical ultracapacitor approaches as per the Ragone curve, >50× in energy density. The experiment results according to the present invention also predict that efficient utilization of the incorporated MnO2 phase can achieve >2,000 F/g. Furthermore, based on the experimental results according to the present invention and the electrochemical ultracapacitor theory, using an asymmetric cell configuration and ionic liquid with cell voltage of ˜3V, the projected performance of the hybrid ultracapacitor of the present invention may far exceed the power-energy density provided by the battery technology.
The preliminary accomplishments of the present invention can be, among other things, highlighted below:
The foregoing description of the exemplary embodiments of the invention has been presented only for the purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the above teaching.
The embodiments were chosen and described in order to explain the principles of the invention and their practical application so as to enable others skilled in the art to utilize the invention and various embodiments and with various modifications as are suited to the particular use contemplated. Alternative embodiments will become apparent to those skilled in the art to which the present invention pertains without departing from its spirit and scope. Accordingly, the scope of the present invention is defined by the appended claims rather than the foregoing description and the exemplary embodiments described therein.
This application claims priority to and the benefit of, pursuant to 35 U.S.C. §119(e), U.S. provisional patent application Ser. No. 61/453,473, filed Mar. 16, 2011, entitled “ENHANCED ELECTROCHEMICAL ULTRACAPACITOR, METHODS OF MAKING AND APPLICATIONS OF THE SAME,” by Weng Poo Kang, Supil Raina and Siyu Wei, the disclosure of which is incorporated herein in its entirety by reference. Some references, which may include patents, patent applications and various publications, are cited and discussed in the description of this invention. The citation and/or discussion of such references is provided merely to clarify the description of the present invention and is not an admission that any such reference is “prior art” to the invention described herein. All references cited and discussed in this specification are incorporated herein by reference in their entireties and to the same extent as if each reference was individually incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61453473 | Mar 2011 | US |
