Abstract:
A nanoporous templating substrate, which is an anodically oxidized alumina (AAO) substrate, is employed to form a pseudocapacitor having high stored energy density. A pseudocapacitive material is deposited conformally along the sidewalls of the AAO substrate by atomic layer deposition, chemical vapor deposition), and/or electrochemical deposition employing a nucleation layer. The thickness of the pseudocapacitive material on the walls can be precisely controlled in the deposition process. The AAO is etched to form an array of nanotubes of the PC material that are cylindrical and structurally robust with cavities therein. Because the AAO substrate that acts as scaffolding is removed, only the active PC material is left behind, thereby maximizing the energy per mass. In addition, nanotubes may be de-anchored from a substrate so that free-standing nanotubes having randomized orientations may be deposited on a conductive substrate to form an electrode of a pseudocapacitor.

Description:
BACKGROUND 
       [0001]    The present disclosure relates to an energy storage device, and particularly to an energy storage device including at least one nanostructure electrode having a large surface area of a pseudocapacitive material for pseudocapacitive energy storage, and methods of manufacturing the same. 
         [0002]    Ultracapacitors or electrochemical double layer capacitors (EDLC&#39;s) provide the highest energy density among commercially available devices employing capacitive energy storage. Although such EDLC&#39;s are capable of operation at considerably higher power than a battery, the energy density of even high performance EDLC&#39;s is lower than the energy density of high performance batteries by a factor of 10˜20. A traditional ultracapacitor consists of two electrodes that are fabricated from highly porous activated carbon sheets that provide very large surface area, which is typically on the order of 1000 square meters/gram of material. These porous activated carbon-based electrodes are immersed in an electrolyte. When a voltage is applied across a porous activated carbon-based electrode and the electrolyte, energy is stored in the electric field set up in the double layer formed between the carbon surface and the electrolyte. No charge is transferred across the interface between the porous activated carbon-based electrode and the electrolyte. 
         [0003]    The capacitance of an EDLC is thus limited by the area of the surface of the activated carbon sheets. Increasing this area is not only difficult, but also produces only minimal increases in stored energy. To date, this constraint has limited the energy density of an ultracapacitor to below 10 Wh/kg. This value has not changed appreciably in more than 10 years. 
         [0004]    Another means of increasing the energy density is to store charge through redox (reduction/oxidation) chemistries at the surface of certain metals and metal oxides. This Faradaic process involves the actual transfer of electrical charges between the surface of the metal oxide and the electrolyte. The change in the stored electrical charges varies continuously as a function of an externally applied voltage in a manner similar to a conventional capacitor. Thus, this phenomenon is called pseudocapacitance. Pseudocapacitive energy storage refers to the method of energy storage employing the phenomenon of pseudocapacitance. 
         [0005]    While pseudocapacitance (PC) can store about ten times more charge than a standard EDLC in theory, there are no commercial pseudocapacitors that have demonstrated anything remotely approaching this energy density level to this date. The problem can be found in the microscopic nature of the electrode—the electrode must have a very large surface area in order to be able to take advantage of the potential for high energy density. Further, a proper PC material and electrolyte or ionic liquid is required as well. Still further, a high energy density pseudocapacitor must be constructed of lightweight, low cost, non-toxic materials in order to be commercially viable. So far, all known methods for creating a PC electrode involve coating of a PC material onto an inactive substrate, which only adds mass without contributing to energy storage and reduces the stored energy density. 
         [0006]    U.S. Pat. No. 7,084,002 to Kim et al. describes a similar templating method employing sputtering of a metal onto the anodized aluminum oxide template, a method that will not work for the ultrahigh aspect ratios of the nanoscale pores required for the electrode to work properly and to its highest energy storage potential due to the directional nature of the deposition process and shadowing effect of a deposited material upon any structure underneath. In addition, U.S. Pat. No. 7,084,002 requires electrochemical deposition of appropriate metal oxides, which cannot not occur on insulating aluminum oxide templates. Similarly, U.S. Pat. No. 7,713,660 to Kim et al. describes wet chemical processes that cannot achieve the wall thickness control or arrayed attachment to a conductive substrate. Further, capillary and surface tension effects limit the tube diameters to dimensions greater than hundreds of nanometers under this method. 
       BRIEF SUMMARY 
       [0007]    A nanoporous templating substrate, which is an anodically oxidized alumina (AAO) substrate, is employed to form a pseudocapacitor having high stored energy density. A pseudocapacitive material is deposited conformally along the sidewalls of the AAO substrate by atomic layer deposition, chemical vapor deposition, and/or electrochemical deposition employing a nucleation layer. The thickness of the pseudocapacitive material on the walls can be precisely controlled in the deposition process. The AAO is etched to form an array of nanotubes of the PC material that are cylindrical and structurally robust with cavities therein. Because the AAO substrate that acts as scaffolding is removed, only the active PC material is left behind, thereby maximizing the energy per mass. In addition, nanotubes may be de-anchored from a substrate so that free-standing nanotubes having randomized orientations may be deposited on a conductive substrate to form an electrode of a pseudocapacitor. 
         [0008]    According to an aspect of the present disclosure, an energy storage device includes an electrode, which has a plurality of pseudocapacitive nanocylinders located on a conductive substrate. Each pseudocapacitive nanocylinder includes a pseudocapacitive material and has a cavity therein. 
         [0009]    According to another aspect of the present disclosure, a method of manufacturing a plurality of pseudocapacitive nanocylinders includes: depositing a pseudocapacitive material layer on an anodized aluminum oxide substrate having a plurality of holes therein; exposing surfaces of the anodized aluminum oxide substrate; and removing the anodized aluminum oxide substrate. A plurality of pseudocapacitive nanocylinders is formed from remaining portions of the pseudocapacitive material layer. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0010]      FIG. 1  is a top-down scanning electron micrograph (SEM) of the surface of an anodized aluminum oxide (AAO) substrate, coated with TaN via atomic layer deposition, having a regular hexagonal array of ˜30 nm diameter pores. 
           [0011]      FIG. 2  is a scanning electron micrograph (SEM) showing a cross-sectional view of a broken piece of AAO substrate that has a coating of TaN grown via ALD. 
           [0012]      FIG. 3  is a bird&#39;s eye view of a stack of an AAO substrate and a conductive substrate, in which the AAO substrate includes an array of cylindrical holes, according to a first embodiment of the present disclosure 
           [0013]      FIG. 4  is a vertical cross-sectional view of the stack of the AAO substrate and the conductive substrate of  FIG. 3  along the plane Z. 
           [0014]      FIG. 5  is a vertical cross-sectional view of the stack of the AAO substrate and the conductive substrate after deposition of a pseudocapacitive material layer according to the first embodiment of the present disclosure. 
           [0015]      FIG. 6  is a vertical cross-sectional view of the stack of the AAO substrate and the conductive substrate after removal of top portions of the pseudocapacitive material layer according to the first embodiment of the present disclosure. 
           [0016]      FIG. 7  is a vertical cross-sectional view of the conductive substrate and an array of pseudocapacitive nanocylinders after removal of the AAO substrate according to the first embodiment of the present disclosure. 
           [0017]      FIG. 8  is a bird&#39;s eye view of the conductive substrate and the array of pseudocapacitive nanocylinders of  FIG. 7 . 
           [0018]      FIG. 9  is a stack of an AAO substrate and a disposable substrate, in which the AAO substrate includes an array of cylindrical holes, according to a second embodiment of the present disclosure. 
           [0019]      FIG. 10  is a vertical cross-sectional view of the stack of the AAO substrate and the disposable substrate after deposition of a pseudocapacitive material layer according to the second embodiment of the present disclosure. 
           [0020]      FIG. 11  is a vertical cross-sectional view of the AAO substrate and pseudocapacitive material layer after removal of a disposable substrate according to the second embodiment of the present disclosure. 
           [0021]      FIG. 12  is a bird&#39;s eye view of the AAO substrate and pseudocapacitive material layer of  FIG. 11 . 
           [0022]      FIG. 13  is a vertical cross-sectional view of the AAO substrate and pseudocapacitive material layer after flipping over and placement on a conductive substrate according to the second embodiment of the present disclosure. The pseudocapacitive material layer may, or may not, be attached to the conductive substrate at this step. 
           [0023]      FIG. 14  is a bird&#39;s eye view of the AAO substrate and pseudocapacitive material layer and the conductive substrate of  FIG. 13 . 
           [0024]      FIG. 15  is a vertical cross-sectional view of the conductive substrate and an array of pseudocapacitive nanocylinders after removal of the AAO substrate according to the second embodiment of the present disclosure. 
           [0025]      FIG. 16  is a bird&#39;s eye view of the conductive substrate and the array of pseudocapacitive nanocylinders of  FIG. 15 . All pseudocapacitive nanocylinders are connected to one another through a sheet of a planar pseudocapacitive material layer. 
           [0026]      FIG. 17  is a vertical cross-sectional view of pseudocapacitive nanocylinders and the AAO substrate after removal of the top portions of the pseudocapacitive material layer of  FIG. 11  according to a third embodiment of the present disclosure. 
           [0027]      FIG. 18  is a bird&#39;s eye view of the conductive substrate and the array of pseudocapacitive nanocylinders of  FIG. 17 . 
           [0028]      FIG. 19  is a bird&#39;s eye view of a random stack of pseudocapacitive nanocylinders on a conductive substrate that are obtained by removing the AAO substrate of  FIG. 18  and letting pseudocapacitive nanocylinders fall on a conductive substrate. 
           [0029]      FIG. 20  is a schematic view of an energy storage device employing pseudocapacitive nanocylinders. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    As stated above, the present disclosure relates to an energy storage device including at least one nanostructure electrode having a large surface area of a pseudocapacitive material for pseudocapacitive energy storage, and methods of manufacturing the same, which are now described in detail with accompanying figures. It is noted that like reference numerals refer to like elements across different embodiments. 
         [0031]    Referring to  FIG. 1 , a top-down scanning electron micrograph (SEM) of the surface of an anodized aluminum oxide (AAO) substrate shows a regular hexagonal array of ˜60 nm diameter pores. It is known in the art that acidic anodizing solutions produce pores in an anodized coating of aluminum. Examples of acids that can be employed to anodize aluminum include, but are not limited to, phosphoric acid and sulfuric acid. The pore size and the pitch depend on the type of anodizing carried out, the anodization temperature, and the forming voltage. The pores can be about 10 nm to 200 nm in diameter, and the wall thickness (the distance between adjacent pores) can be between 10 nm and 200 nm, although lesser and greater diameters and wall thicknesses may be obtainable under suitable anodization conditions. The pore length can be orders of magnitude longer than the pore diameter, and can be as much as about 25,000 times the diameter. 
         [0032]    Referring to  FIG. 2 , a scanning electron micrograph (SEM) shows a cross-sectional view of a broken piece of AAO substrate that has a coating of TaN layer grown by atomic layer deposition (ALD). The TaN layer prevents charge accumulation and/or arcing on the insulating material of the AAO substrate. The vertical cylindrical pores extend through the entire cross-section of the AAO substrate, and the aspect ratio, i.e., the ratio of the length of the pore to the diameter of the pore, can be up to 25,000 or more as discussed above. 
         [0033]    Referring  FIGS. 3 and 4 , a first exemplary structure according to a first embodiment of the present disclosure is shown in a bird&#39;s eye view in  FIG. 3  and in a vertical cross-sectional view in  FIG. 4 . The Z plane of  FIG. 3  is the vertical cross-sectional plane of  FIG. 4 . 
         [0034]    The first exemplary structure includes a stack of a conductive substrate  10  and an anodized aluminum oxide (AAO) substrate  20 . The AAO substrate  20  is a sheet of aluminum foil that is anodically oxidized to be converted into aluminum oxide layer that includes a self-assembled array of vertical pores therein. An AAO substrate  20  can be formed employing methods known in the art. The AAO substrate  20  includes an array of “nanopores”  21 , which refers to pores having a diameter less than 1 micron. The diameter of individual nanopores  21  and the pitch of the array of nanopores  21  can be controlled by altering anodization parameters. 
         [0035]    Typically, the diameter of each nanopore  21  is from 10 nm to 200 nm, although lesser and greater diameters may be practicable depending on optimization of process conditions in the future. The thickness of the AAO substrate  20  is at least 50 times the diameter of the nanopores  21 , and can be up to, or exceed, 25,000 times the diameter of the nanopores  21 . Typically, the thickness of the AAO substrate  20  is from 10 microns to 5 mm, although lesser and greater thicknesses can also be employed. Each nanopore  21  is a cylindrical hole extending from the topmost planar surface of the AAO substrate  20  to the bottommost surface of the AAO substrate  20  that contacts a planar topmost surface of the conductive substrate  10 . Thus, the AAO substrate  20  includes a plurality of holes therein, which are a plurality of nanopores  21  therein. The plurality of holes may form a two-dimensional periodic array such as a hexagonal array. 
         [0036]    The AAO substrate  20  is placed on a conductive substrate  10 , which includes a conductive material such as elemental metal, an intermetallic alloy of at least two elemental metals, a conductive oxide of a metal, a conductive nitride of a metal, a heavily doped semiconductor material, or an alloy or a stack thereof. The material of the conductive substrate is selected to withstand an etch process that is subsequently employed to remove the AAO substrate  20  without loss of structural integrity. The thickness of the AAO substrate  20  can be from 50 microns to 1 mm, although lesser and greater thicknesses can also be employed. The bottommost surface of the AAO substrate  20  contacts a planar topmost surface of the conductive substrate  10 . While the AAO substrate  20  and the conductive substrate  10  maintains a planar interface at a microscopic scale, the assembly of the AAO substrate  20  and the conductive substrate  10  can be bent on a macroscopic scale as needed. Preferably, the conductive substrate  10  is a thin lightweight substrate in order to maximize stored energy density per total mass of an energy storage device. 
         [0037]    Referring to  FIG. 5 , a pseudocapacitive material layer  30 L is conformally deposited on the stack of the AAO substrate and the conductive substrate after deposition of according to the first embodiment of the present disclosure. As used herein, a “pseudocapacitive material” refers to a material that can store energy through a reversible reduction/oxidation reaction on a surface thereof. Pseudocapacitive materials include some metals and some metal oxides. The phenomenon of a pseudocapacitive material storing and releasing energy through the reversible reduction/oxidation reaction is referred to as “pseudocapacitance.” Pseudocapacitive materials include, but are not limited to, manganese oxide (MnO 2 ), ruthenium oxide (RuO 2 ), nickel oxide (NiO), and a combination thereof. 
         [0038]    Typically, the extremely high aspect ratio of the nanopores  21  (which is at least 50) necessitates the use of atomic layer deposition (ALD) in order to produce a conformal coating of a pseudocapacitive material on the sidewalls of the nanopores  21  in the AAO substrate  10 . The AAO substrate  20  functions as a template for deposition of the pseudocapacitive material layer  30 L. 
         [0039]    In atomic layer deposition, a monolayer of a first material is deposited in a self-limiting reaction that saturates upon formation of the monolayer of the first material by flowing a first reactant into a deposition chamber. After removing the first reactant, a monolayer of a second material is deposited in another self-limiting reaction that saturates upon formation of the monolayer of the second material by flowing a second reactant into the deposition chamber. The first reactant and the second reactant are flowed into the same deposition chamber alternately with a pumping period between each round of deposition of a monolayer. In the case of a pseudocapacitive material in the form of a metal oxide, a metal precursor is deposited in a self-limiting reaction that saturates upon formation of the monolayer of metal atoms by flowing metal-containing reactant into a deposition chamber. After removing the metal-containing reactant, a monolayer of oxygen is deposited in a self-limiting reaction that saturates upon formation of the monolayer of oxygen atoms by flowing oxygen gas into the deposition chamber. The oxygen is then pumped out from the deposition chamber. The steps of flowing the metal-containing reactant, pumping of the metal-containing reactant, flowing oxygen gas, and pumping the oxygen gas are repeatedly cycled to deposit a metal oxide layer exhibiting the characteristics of pseudocapacitance, i.e., a “pseudocapacitive” metal oxide layer. The pseudocapacitive material layer  30 L is deposited on the exposed surfaces of the conductive substrate  10  at the bottom of each nanopore  21 . 
         [0040]    The thickness of the pseudocapacitive material layer  30 L can be precisely controlled with atomic level accuracy. Further, the thickness of the pseudocapacitive material layer  30 L is identical throughout the entirety of the pseudocapacitive material layer  30 L with atomic precision due to the self-limiting nature of the reactions in the ALD process. The thickness of the pseudocapacitive material layer  30 L is selected to be less than one half of the diameter of the nanopores  21  so that a cavity  21 ′ having a lesser diameter than the diameter of the nanopores  21  is present within each recessed portion of the pseudocapacitive material layer  30 L. The entirety of the pseudocapacitive material layer  30 L is contiguous at this step. As such the inner diameter of the nanotube can be exquisitely controlled down to, and below, 1 nanometer where substantial increases in capacitance have been reported. See, for example, J. Chmiola, G. Yushin, Y. Gogotsi, C. Portet, P. Simon, and P. L. Taberna, “Anomalous increase in carbon capacitance at pore sizes less than 1 nanometer,” Science 313, 1760 (2006). 
         [0041]    In general, atomic layer deposition is required to achieve the required high level of conformity and overall geometrical control in order to form a contiguous pseudocapacitive material layer  30 L that extends to the bottom portions of the nanopores  21 . Attempts to employ electroplating faces two problems. The first problem is that the AAO substrate  20  cannot be employed as an electrode for electroplating because the AAO substrate  20  is an insulator. In order to employ electroplating, the exposed surfaces of the AAO substrate  20  must be converted to a conductor surface by first forming a uniform coating of a conductive material. Thus, atomic layer deposition is required anyway even to form a conductive seed layer for the purpose of employing electroplating. The second problem is that the diameters of the nanopores  21  are too small and the aspect ratio of the nanopores  21  is too high to employ electroplating even if a conductive seed layer were to be successfully provided. The plating liquids and the electric fields cannot penetrate to the lower portion of the nanopores  21  because of the small diameters of the nanopores  21  and the high aspect ratio (at least 10, and typically greater than 50) of the nanopores  21 , thereby rendering electroplating impracticable. 
         [0042]    Chemical vapor deposition (CVD) is a generic gas phase process in which cracking of the precursor occurs on a heated surface. While the method of chemical vapor deposition could in principle work, chemical vapor deposition does not possess the exquisite thickness control that atomic layer deposition provides. Currently, no chemical vapor deposition process is available that can reliably reach to the bottom of nanopores  21  given the small diameter of the nanopores  21  and the high aspect ratio of each nanopore  21 . In practice, atomic layer deposition is currently the only viable method of forming a conformal layer of a pseudocapacitive material that contacts the bottommost portions of the nanopores  21 . The use of atomic layer deposition provides the capability to coat the sidewalls of the nanopores  21  and to form a single contiguous pseudocapacitive material layer  30 L given the length, diameter, and pitch of the array of the nanopores  21 . The thickness of the pseudocapacitive material layer  30 L can be from 1 nm to 75 nm, and typically from 3 nm to 30 nm, although lesser and greater thicknesses can also be employed. 
         [0043]    Referring to  FIG. 6 , the top surfaces of the AAO substrate  20  are exposed by removing distal planar portions of the pseudocapacitive material layer  21 . The distal portions of the pseudocapacitive material layer  21  refer to the contiguous planar portions of the pseudocapacitive material layer  21  located on and above the topmost surfaces of the AAO substrate  20 . The distal portions of the pseudocapacitive material layer  30 L can be removed, for example, by chemical mechanical planarization or by an anisotropic etch such as a reactive ion etch. If chemical mechanical planarization is employed, the distal portions of the pseudocapacitive material layer  30 L can be removed by polishing, in which chemical slurry is employed as needed. If an anisotropic etch is employed, the etchants in a gas phase impinges on the distal portions of the pseudocapacitive material layer  30 L with directionality, i.e., along the vertical direction. Typically, the etchants do not etch the bottommost portions of the pseudocapacitive material layer  30 L that contact the conductive substrate  10  inside the cavities  21 ′ due to the high aspect ratio of the cavities  21 ′, which is greater than the aspect ratio of unfilled nanopores  21 . (See  FIG. 4 .) 
         [0044]    Referring to  FIGS. 7 and 8 , a plurality of pseudocapacitive “nanocylinders”  40  is formed by removing the AAO substrate  20 . As used herein, a “nanocylinder” refers to a structure including a cylindrical tube having an outer diameter that does not exceed 1 micron. Typically, the outer diameter of a nanocylinders is from 10 nm to 200 nm, although lesser and greater outer diameters (less than 1 micron) can also be employed. The alumina, i.e., the aluminum oxide, in the AAO substrate can be etched away, for example, by utilizing standard wet etching methods such as immersion in aqueous chromic acid. The result is the plurality of pseudocapacitive nanocylinders  40  is formed as an array of pseudocapacitive nanocylinders  40 , which are nanotubes of the pseudocapacitive materials that are structurally robust. In other words, the plurality of pseudocapacitive nanocylinders  40  is formed from remaining portions of the pseudocapacitive material layer  30 L after removal of the AAO substrate  20 . Prior to removal, the AAO substrate  20  functions as scaffolding for the two-dimensional periodic array of pseudocapacitive nanocylinders  40 . Upon removal of the AAO substrate  20 , only an assembly of the conductive substrate  10 , the array of pseudocapacitive nanocylinders  40 , and an outer pseudocapacitive wall  42  is left. 
         [0045]    The advantage of removal of the AAO substrate  20  is manifold. First, the removal of the AAO substrate  20  forms a two-dimensional ordered array of pseudocapacitive nanocylinders  40  that can be employed as parts of an electrode having an exceptionally high specific area. A “specific area” refers to a surface area per unit mass. For example, a two-dimensional ordered array of pseudocapacitive nanocylinders  40  can have an areal density up to 10 16 /m 2  and a specific area about 500 m 2 /g. The specific area could be two to three times higher depending on the specific morphology of the sidewalls of the pseudocapacitive nanocylinders  40 , e.g., if the surfaces of the pseudocapacitive nanocylinders  40  is roughened or textured. 
         [0046]    Second, the removal of the AAO substrate  20  reduces the total mass of the first exemplary structure by reducing the parasitic mass, i.e., the total mass of materials that do not contribute to charge storage. In other words, the energy to mass ratio of the first exemplary structure is enhanced by completely removing all materials, i.e., the alumina in the AAO substrate  20 , that do not contribute to the storage of energy. The reduced mass of the assembly ( 10 ,  40 ,  42 ), which includes all remaining portions of first exemplary structure at this step, can be subsequently advantageously employed to provide a lightweight electrode including the assembly of the conductive substrate  10  and the array of pseudocapacitive nanocylinders  40 . 
         [0047]    Third, the removal of the AAO substrate  20  more than doubles the total surface area of the pseudocapacitive material, thereby doubling the specific capacitance, i.e. the capacitance per unit mass. Because the exposed outer sidewall surfaces of the cylinder portions of the pseudocapacitive nanocylinders  40  add to the total surface area, the total capacitance of the assembly ( 10 ,  40 ,  42 ) increases correspondingly. When the assembly ( 10 ,  40 ,  42 ) functions as an electrode, the upper portion ( 40 ,  42 ) of the electrode is fully optimized to store electrical charges via Faradaic processes, i.e., via charge transfer processes that employ oxidation and reduction. In this case, the conductive substrate  10  functions as a portion of the electrode upon which the array of pseudocapacitive nanocylinders  40  is structurally affixed. 
         [0048]    Thus, the electrode can employ a plurality of pseudocapacitive nanocylinders  40  located on a conductive substrate  10 . Each pseudocapacitive nanocylinder  40  includes a pseudocapacitive material and has a cavity  21 ′ therein. The cavity  21 ′ in each pseudocapacitive nanocylinder  40  is not encapsulated by that pseudocapacitive nanocylinder  40 , but each pseudocapacitive nanocylinder  40  has an opening at one end thereof. The opening at one end is contiguously connected to the cavity  21 ′ in each pseudocapacitive nanocylinder  40 . 
         [0049]    Each pseudocapacitive nanocylinder  40  includes an end cap portion  40 E that does not include a hole therein at an opposite end of the opening contiguously connected to the cavity  21 ′. The entirety of each pseudocapacitive nanocylinders  40  has a uniform (same) thickness throughout including the end cap portion  40 E that includes an outer end surface. The outer end surface of each pseudocapacitive nanocylinder  40  is contiguously connected to an entire periphery of sidewalls of that pseudocapacitive nanocylinder  40 . Further, the entirety of the end surface of each pseudocapacitive nanocylinder  40  contacts, and is attached to, the conductive substrate  10 . 
         [0050]    The plurality of pseudocapacitive nanocylinders  40  is formed as an array of pseudocapacitive nanocylinders  40  having sidewalls that are perpendicular to the top surface of the conductive substrate  10 . Each pseudocapacitive nanocylinder  40  does not contact any other pseudocapacitive nanocylinder  40 , i.e., is disjoined from other pseudocapacitive nanocylinders  40 . Thus, each pseudocapacitive nanocylinder  40  is laterally spaced from any other of the plurality of capacitive nanocylinders  40 . 
         [0051]    Optionally, functional molecular groups may be coated on the outer sidewalls and/or inner sidewalls of the plurality of pseudocapacitive nanocylinders  40 . The functional groups include an additional pseudocapacitive material that can add to the charge storage of the plurality of pseudocapacitive nanocylinders  40 . Exemplary functional groups include, but are not limited to, polyaniline which is a conducting polymer. The coating of the functional groups can be effected in at least another atomic layer deposition process or processes that utilize vapor deposition or wet chemical deposition. The coatings on the inner sidewalls and the outer sidewalls may be performed at the same processing step or at different processing steps. For example, the coating of the inner and outer sidewalls may be performed after removal of the AAO substrate  20 . Alternately, the inner sidewalls of the plurality of pseudocapacitive nanocylinders  40  can be coated prior to removal of the AAO substrate  20 , and the coating of the outer sidewalls of the nanocylinders  40  can be coated after removal of the AAO substrate  20 . The coating materials and the coating processes known in the art can be employed to coat the outer sidewalls and/or inner sidewalls of the plurality of pseudocapacitive nanocylinders  40 . See, for example, Stewart, M. P.; Maya, F.; Kosynkin, D. V.; Dirk, S. M.; Stapleton, J. J.; McGuiness, C. L.; Allara, D. L; Tour, J. M. “Direct Covalent Grafting of Conjugated Molecules onto Si, GaAs, and Pd Surfaces from Aryldiazonium Salts,” J. Am. Chem Soc. 2004, 126, 370-378. 
         [0052]    Referring to  FIG. 9 , a second exemplary structure according to a second embodiment of the present disclosure includes a stack of an AAO substrate  20  and a disposable substrate  99 . The AAO substrate  20  can be the same as in the first embodiment. The disposable substrate  99  can include a conductive material, a semiconducting material, an insulating material, or a combination thereof. The material of the disposable substrate  99  is selected for easy removal thereof selective to the material of the AAO substrate  20 , i.e., without removing the material of the AAO substrate  20 , by a method to be subsequently employed. The method of removal of the disposable substrate  99  can be a mechanical removal method, a chemical mechanical removal method, or a chemical removal method. The thickness of the disposable substrate  99  can be from 10 microns to 500 microns, although lesser and greater thicknesses can also be employed. 
         [0053]    Referring to  FIG. 10 , a pseudocapacitive material layer  30 L is deposited on the stack of the AAO substrate  20  and the disposable substrate  99 . The deposition of the pseudocapacitive material layer  30 L can be effected employing the same method, i.e., atomic layer deposition, as in the first embodiment. Portions of the pseudocapacitive material layer  30 L at the bottom of each cavity  21 ′ contacts the top surface of the disposable substrate  99 . 
         [0054]    Referring to  FIGS. 11 and 12 , the disposable substrate  99  is removed and the bottom portions of the pseudocapacitive material layer  30 L are removed to form an assembly of the AAO substrate  20  and the remaining portions of the pseudocapacitive material layer  30 L. The removal of the disposable substrate  99  selective to the assembly of the AAO substrate  20  and the pseudocapacitive material layer  30 L can be effected, for example, by a mechanical removal method such as grinding, a chemical mechanical removal method such as chemical mechanical planarization, a chemical removal method such as a wet etch or a dry etch, or a combination thereof. The bottommost surfaces of the pseudocapacitive material layer  30 L, which are the same as the outer end surfaces of the end cap portions  40 E in  FIG. 7 , and the bottommost surfaces of the AAO substrate are exposed once the disposable substrate  99  is removed. 
         [0055]    Subsequently, the bottommost portions of the AAO substrate  20  and the bottommost horizontal portions of the pseudocapacitive material layer  30 L that correspond to the end cap portions  40 E in  FIG. 7  are removed employing a non-selective removal method such as grinding or chemical mechanical planarization or a non-selective etch process. Once the bottommost horizontal portions of the pseudocapacitive material layer  30 L are removed, each cavity  21 ′ extends from the topmost surface of the assembly ( 20 ,  30 L) of the AAO substrate  20  and the pseudocapacitive material layer  30 L to the bottommost surface of the assembly ( 20 ,  30 L) with an opening at the top and another opening at the bottom. A portion of the pseudocapacitive material layer  30 L around each cavity  21 ′ constitutes a prototypical pseudocapacitive nanocylinder  40 P. The entirety of the pseudocapacitive material layer  30 L is contiguous because each prototypical pseudocapacitive nanocylinder  40 P is contiguously connected all other prototypical pseudocapacitive nanocylinders  40 P through the upper horizontal portions of the pseudocapacitive material layer  30 L located between each neighboring pair of prototypical pseudocapacitive nanocylinders  40 P. 
         [0056]    Referring to  FIGS. 13 and 14 , the assembly ( 20 ,  30 L) of the AAO substrate  20  and the pseudocapacitive material layer  30 L is flipped over. Optionally, the assembly ( 20 ,  30 L) can be placed on a conductive substrate  10 , which can have the same composition and thickness as the conductive substrate  10  of the first embodiment. If a conductive substrate  10  is employed, the pseudocapacitive material layer  30 L may, or may not, be attached to the conductive substrate  10  at this step. In one embodiment, the bottom surfaces of the pseudocapacitive material layer  30 L are permanently attached, for example, employing a conductive adhesive material (not shown). In another embodiment, the assembly  20 ,  30 L) of the AAO substrate  20  and the pseudocapacitive material layer  30 L is placed without attachment or with temporary attachment to the conductive substrate  10  to enable subsequent detachment of the pseudocapacitive material layer  30 L. 
         [0057]    Referring to  FIGS. 15 and 16 , the AAO substrate  20  is removed employing the same removal process of the first embodiment corresponding to  FIGS. 7 and 8 . If a conductive substrate  10  is employed, the planar pseudocapacitive material layer  30 P contacts the top surface of the conductive substrate  10 . The outer sidewalls of the prototypical pseudocapacitive nanocylinders  40 P become exposed as the AAO substrate  20  is removed, and a plurality of prototypical pseudocapacitive nanocylinders  40 P become a plurality of pseudocapacitive nanocylinders  40 ′. All pseudocapacitive nanocylinders  40 ′ are connected to one another through a sheet of a planar pseudocapacitive material layer  30 P. 
         [0058]    The remaining portions of the pseudocapacitive material layer  30 P include the plurality of pseudocapacitive nanocylinders  40 ′ and the planar pseudocapacitive material layer  30 P, which are of integral construction and have the same thickness and composition throughout. Thus, each of the plurality of capacitive nanocylinders  40 ′ is contiguously connected to one another through the planar pseudocapacitive material layer  30 P at a bottom end of each capacitive nanocylinder  40 ′. The planar pseudocapacitive material layer  30 P has at least as many number of holes therein as the total number of pseudocapacitive nanocylinders  40 ′ among the plurality of pseudocapacitive nanocylinders  40 ′. The plurality of pseudocapacitive nanocylinders  40 ′ is formed as an array of pseudocapacitive nanocylinders having the same two-dimensional periodicity as the nanopores in the AAO substrate  20  (which is no longer present at this step; see  FIG. 9 ). If a conductive substrate  10  is present, the array of pseudocapacitive nanocylinders  40 ′ has vertical sidewalls that are perpendicular to the top surface of the conductive substrate  10 . 
         [0059]    Each pseudocapacitive nanocylinder  40 ′ includes a pseudocapacitive material and has a cavity  21 ′ therein. The cavity  21 ′ in each pseudocapacitive nanocylinder  40 ′ is not encapsulated by that pseudocapacitive nanocylinder  40 ′. Each pseudocapacitive nanocylinder  40 ′ has two end surfaces each including a hole therein. Each pseudocapacitive nanocylinder  40 ′ has two openings that are located at end portions of that pseudocapacitive nanocylinder  40 ′. Specifically, each pseudocapacitive nanocylinder  40 ′ has an opening at a top end, i.e., a top opening, and another opening at a bottom end, i.e., a bottom opening. Each of the top opening and the bottom opening is contiguously connected to the cavity  21 . The top opening contiguously extends to the ambient. The bottom opening can also contiguously extend to the ambient if a conductive plate  10  is not employed, or can be blocked by the top surface of a conductive plate  10  if the conductive plate  10  is employed. If a conductive plate  10  is present, the sidewalls of the plurality of pseudocapacitive nanocylinders  40 ′ are perpendicular to the top surface of the conductive substrate  10 . 
         [0060]    The second exemplary structure can be employed as an electrode of an energy storage device. In one embodiment, the electrode includes a plurality of pseudocapacitive nanocylinders  40 ′, a planar pseudocapacitive material layer  30 P, and a conductive substrate  10 . In another embodiment, the electrode includes a plurality of pseudocapacitive nanocylinders  40 ′ and a planar pseudocapacitive material layer  30 P, but does not include a conductive substrate  10 . Optionally, appropriate functional groups can be coated employing the same methods as in the first embodiment. 
         [0061]    Referring to  FIG. 17 , a third exemplary structure according to a third embodiment of the present disclosure can be derived from the second exemplary structure of  FIGS. 11 and 12  by removing the topmost planar portion of the pseudocapacitive material layer  30 L to expose the surfaces of the AAO substrate  20 . Alternately, the third exemplary structure can be derived from the second exemplary structure of  FIG. 10  by first removing the topmost planar portion of the pseudocapacitive material layer  30 L to expose the surfaces of the AAO substrate  20  and then removing the disposable substrate  99  and the bottom portions of the pseudocapacitive material layer  30 L. An assembly ( 20 ,  40 ′) of the AAO substrate  20  and a plurality of pseudocapacitive nanocylinders  40 ″ is formed. Each pseudocapacitive nanocylinder  40 ″ is a cylindrical tube that is topologically homeomorphic to a torus, and has an exposed inner vertical sidewall, an exposed top end surface with a hole therein, and an exposed bottom end surface with a hole therein. The outer vertical sidewall of each pseudocapacitive nanocylinder  40 ″ contacts the AAO substrate  20 , which holds the plurality of pseudocapacitive nanocylinders  40 ″ in place at this step. The surfaces of the AAO substrate  10  are exposed at the top and at the bottom. 
         [0062]    Referring to  FIG. 19 , the assembly ( 20 ,  40 ′) of the AAO substrate  20  and a plurality of pseudocapacitive nanocylinders  40 ″ is placed on a conductive substrate  10  or a temporary substrate (not shown), and the AAO substrate  20  is removed employing the same removal process of the first embodiment corresponding to  FIGS. 7 and 8 . All pseudocapacitive nanocylinders  40 ″ are detached from one another as the AAO substrate  20  is etched away, and fall down on the conductive substrate  10  or on the temporary substrate. 
         [0063]    As the pseudocapacitive nanocylinders  40 ″ fall down, the orientations of the pseudocapacitive nanocylinders  40 ″ become “randomized,” i.e., the orientations become “random.” As used herein, “random” orientations or “randomized” orientations refer to a lack of alignment among elements, and includes geometries that include a short range order or an accidental long range trend. For example, the orientations of the pseudocapacitive nanocylinders  40 ″ are considered “random” even if a particular orientation has a higher probability of occurrence, for example, due to tilting of the conductive substrate  10  or the temporary substrate during the etch process to induce a fall in a preferred orientation because the process of falling inherently introduces uncertainty in the final orientation of each pseudocapacitive nanocylinders  40 ″. 
         [0064]    The plurality of pseudocapacitive nanocylinders  40 ″ may be affixed to the conductive substrate  10 , for example, employing a thin layer of conductive adhesive. If a temporary substrate is employed, the plurality of pseudocapacitive nanocylinders  40 ″ can be poured onto a conductive substrate  10  coated with a thin layer of conductive adhesive so that the plurality of pseudocapacitive nanocylinders  40 ″ is affixed to the conductive substrate. The orientations the plurality of pseudocapacitive nanocylinders  40 ″ are randomized upon placement on the conductive substrate  10  either by directly falling onto the conductive substrate  10  or by falling on a temporary substrate and subsequently being poured onto the conductive substrate  10 . 
         [0065]    Each pseudocapacitive nanocylinder  40 ″ includes a pseudocapacitive material and has a cavity  21 ′ therein. Each pseudocapacitive nanocylinder  40 ″ has two openings that are located at end portions of that pseudocapacitive nanocylinder  40 ′. Each opening is within an end surface of a pseudocapacitive nanocylinder  40 ″. Each opening is contiguously connected to the cavity  21 ′. Thus, the cavity  21 ′ in each pseudocapacitive nanocylinder  40 ″ is not encapsulated by that pseudocapacitive nanocylinder  40 ″. 
         [0066]    The third exemplary structure can be employed as an electrode of an energy storage device. In this case, the electrode is a “randomized nanocylinder electrode” in which the orientations of the pseudocapacitive nanocylinder  40 ″ are randomized in a two-dimensional plane parallel to the local portion of the conductive substrate  10 . The electrode can be bent as needed along with the pseudocapacitive nanocylinder  40 ″ therein. Optionally, appropriate functional groups can be coated employing the same methods as in the first and second embodiments. 
         [0067]    Referring to  FIG. 20 , an exemplary energy storage device employing pseudocapacitive nanocylinders is schematically illustrated. The exemplary energy storage device includes a first electrode that employs one of the first, second, and third exemplary structures described above. The exemplary energy storage device includes a second electrode that does not contact the first electrode. The second electrode includes an electrically conductive material such as porous activated carbon or a nanostructured material that is not a pseudocapacitive material. The exemplary energy storage device further includes a separator, which is a membrane that is ionically conductive but is a barrier to electrons. To reiterate, ions move through the separator under applied electrical bias across the first electrode and the second electrode. However, the separator prevents movement of electrons therethrough. In one embodiment, a robust paper may be used for the separator. The robust paper is an electron insulator, but becomes ionically conductive when saturated with electrolyte. An electrolyte solution is provided between the first and second electrodes such that the separator is embedded in the electrolyte solution. 
         [0068]    The unique structures and processes described above can be employed to provide an ultracapacitor electrode that could double or triple the energy density presently achievable, and replace lead-acid battery technology in a host of applications such as automotive batteries and backup batteries in telecommunications. The disclosed electrodes employing pseudocapacitive nanocylinders can achieve similar energy densities as, but also enables charge/discharge cycling life that is 100˜1000 times that of a typical battery. 
         [0069]    While the disclosure has been described in terms of specific embodiments, it is evident in view of the foregoing description that numerous alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the disclosure is intended to encompass all such alternatives, modifications and variations which fall within the scope and spirit of the disclosure and the following claims.