DISCLOSURE / FILE
DIA Defense Intelligence Reference Document DIA-08-1011-005, dated 01 November 2010, surveying ultracapacitor technology for commercial and military energy storage applications under the AAWSA Program.
DIA AATIP Defense Intelligence Reference Documents (38 DIRDs), FOIA Release, DIRD_32-DIRD_Ultracapacitators_as_Energy_and_Power_Storage_Devices.pdf
DIA Defense Intelligence Reference Document DIA-08-1011-005, dated 01 November 2010, surveying ultracapacitor technology for commercial and military energy storage applications under the AAWSA Program.
Authored under the Defense Intelligence Agency's Advanced Aerospace Weapons System Applications (AAWSA) Program in FY 2010, this 34-page reference document (ICOD 20 July 2010) reviews ultracapacitor (ultracap) technology as a high-power energy storage platform. It contrasts ultracaps with lithium-ion batteries, citing roughly 5 Wh/kg specific energy versus 100-200 Wh/kg for lithium ion but cycle life above 500,000 versus under 5,000. The report covers electrolytes, carbon electrodes, carbon nanotubes, thin films, and magnetic capacitors, and identifies pulsed-power storage for electric propulsion and directed-energy weapons as the ultimate military application.
Ultracapacitors (ultracaps) are energy storage devices capable of extremely rapid charge and discharge rates with the ability to be cycled hundreds of thousands of times.p.5
This product is one of a series of advanced technology reports produced in FY 2010 under the Defense Intelligence Agency, (b)(3):10 USC 424 Advanced Aerospace Weapons System Applications (AAWSA) Program.p.2
Charge storage is a physical mechanism rather than a chemical phase change, so these are theoretically capable of cycling an infinite number of times.p.6
In 1966, Standard Oil Company of Ohio (SOHIO) invented the device in the format now commonly used, which forms the basis for hundreds of patents and thousands of journal articles.p.10
Magnetic capacitors are a recent development, manufactured using semiconductor processes on silicon wafers.p.5
Due to their durability, their use in aerospace has been for munitions fusing and missile power load leveling.p.5
Extracted text · Page 1
Page 1 · vision OCR
UNCLASSIFIED//FOR OFFICIAL USE ONLY Defense Intelligence Reference Document Defense Futures 01 November 2010 ICOD: 20 July 2010 DIA-08-1011-005 Ultracapacitors as Energy and Power Storage Devices for Commercial and Military Applications UNCLASSIFIED//FOR OFFICIAL USE ONLY
Page 2 · vision OCR
UNCLASSIFIED//FOR OFFICIAL USE ONLY Ultracapacitors as Energy and Power Storage Devices for Commercial and Military Applications The Defense Intelligence Reference Document provides nonsubstantive but authoritative reference information related to intelligence topics or methodologies. Prepared by: (b)(3):10 USC 424 Defense Intelligence Agency Authors: (b)(6) (U) COPYRIGHT WARNING: Further dissemination of the photographs in this publication is not authorized. This product is one of a series of advanced technology reports produced in FY 2010 under the Defense Intelligence Agency, (b)(3):10 USC 424 Advanced Aerospace Weapons System Applications (AAWSA) Program. Comments or questions pertaining to this document should be addressed to (b)(3):10 USC 424;(b)(6) AAWSA Program Manager, Defense Intelligence Agency, ATTN: (b)(3):10 USC 424 Bldg 6000, Washington D.C. 20340-5100 i UNCLASSIFIED/FOR OFFICIAL USE ONLY
Page 34 · vision OCR
UNCLASSIFIED//FOR OFFICIAL USE ONLY
43 J. Leis, M. Arulepp, A. Kuura, M. Latt, E. Lust, Carbon 44, 2122 (2006).
44 T. Goto, T. Kimura, G. Lawes, et. al. Ferroelectricity and Giant Magnetocapacitance in Perovskite Rare-earth
Manganites. Physical Review Letters, 92, 257201 (2004).
45 N. Hur, S. Park, P. A. Sharma, S. Guha, and S-W. Cheong, Colossal Magnetodielectric Effects in DyMn2O5,
Physical Review, Lett. 93, 107207 (2004).
46 R. F. Mamin, T. Egami, Z. Marton, and S. A. Migachev, Giant dielectric permittivity and Magnetocapacitance in
La0.875Sr0.125MnO3 Single Crystals, Phys. Rev. B 75, 115129 (2007).
47 J. Wan, Qi Lu, Bo Chen, Fengqi Song, J. Liu, J.g Dong, and G.Wang, Giant Room-Temperature
Magnetocapacitance in Co2+ Doped SnO2 Dielectric Films, Applied Physics Letters, 95, 152901 (2009)
29
UNCLASSIFIED//FOR OFFICIAL USE ONLYConcatenated page-by-page transcript. Born-digital pages came through pdf.js; scanned pages were transcribed by Claude vision OCR. Pages marked unreadable failed multiple OCR retries (heavy redaction, microfilm artifacts, or blank separators) and are kept in place for audit.
UNCLASSIFIED//FOR OFFICIAL USE ONLY Defense Intelligence Reference Document Defense Futures 01 November 2010 ICOD: 20 July 2010 DIA-08-1011-005 Ultracapacitors as Energy and Power Storage Devices for Commercial and Military Applications UNCLASSIFIED//FOR OFFICIAL USE ONLY
UNCLASSIFIED//FOR OFFICIAL USE ONLY Ultracapacitors as Energy and Power Storage Devices for Commercial and Military Applications The Defense Intelligence Reference Document provides nonsubstantive but authoritative reference information related to intelligence topics or methodologies. Prepared by: (b)(3):10 USC 424 Defense Intelligence Agency Authors: (b)(6) (U) COPYRIGHT WARNING: Further dissemination of the photographs in this publication is not authorized. This product is one of a series of advanced technology reports produced in FY 2010 under the Defense Intelligence Agency, (b)(3):10 USC 424 Advanced Aerospace Weapons System Applications (AAWSA) Program. Comments or questions pertaining to this document should be addressed to (b)(3):10 USC 424;(b)(6) AAWSA Program Manager, Defense Intelligence Agency, ATTN: (b)(3):10 USC 424 Bldg 6000, Washington D.C. 20340-5100 i UNCLASSIFIED/FOR OFFICIAL USE ONLY
UNCLASSIFIED//FOR OFFICIAL USE ONLY
Contents
Summary.................................................................................................................iv
Chapter 1: Concept Overview ................................................................................. 1
Batteries and Ultracaps........................................................................................ 3
History ................................................................................................................ 5
Chapter 2: Materials Technology ............................................................................ 8
Electrolytes ......................................................................................................... 8
Electrodes ........................................................................................................... 9
Chapter 3: Applications ....................................................................................... 13
Electronics and Telecommunications................................................................... 13
Industrial ........................................................................................................... 13
Transportation ................................................................................................... 14
Aerospace .......................................................................................................... 15
Chapter 4: Recent Developments ......................................................................... 17
CNTs and Advanced Carbons .............................................................................. 17
Thin Films .......................................................................................................... 20
Magnetic Capacitors ........................................................................................... 21
Chapter 5: Future Developments.......................................................................... 25
Chapter 6: Conclusions........................................................................................ 27
Chapter 7: Endnotes............................................................................................ 28
Figures
Figure 1. Ultracaps in Various Configurations. .................................................... 1
Figure 2. Operation of an Ultracapacitor..............................................................3
Figure 3. Ragone Plot.........................................................................................5
Figure 4. Electrolytic Capacitor as Designed and Patented by SOHIO....................5
Figure 5. Activated Carbon Porosity..................................................................10
Figure 6. Ultracapacitor Use in a Forklift...........................................................14
Figure 7. CNT Forest........................................................................................18
Figure 8. Transmission Electron Microscope Image of Graphene .........................19
Figure 9. Relationship Between Average Pore Size and Normalized Specific
Capacitance........................................................................................................ 20
ii
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY Figure 10. CDC Synthesis and Electrochemical Test Cell Preparation Schematic. 21 Figure 11. Volumetric Capacitance of the Films in (A) TEABF4 and (B) H2SO4...... 21 Figure 12. MCap Structure................................................................................. 22 Figure 13. Preliminary MCap Test Results.. ........................................................ 23 Figure 14. Summary Chart of MCap Results to Date.. .......................................... 23 Figure 15. Specific Energy and Estimated Costs vs. fGMC ................................... 24 Figure 16. Evolution of Ultracapacitors .............................................................. 26 Tables Table 1: Contrasting Properties of Batteries and Ultracapacitors. .......................... 4 Table 2: Manufacturers of Ultracapacitors............................................................ 7 Table 3: Properties of Various Electrolytes Used in Ultracapacitors. ...................... 8 Table 4: Properties of Various Materials used in Electrochemical Capacitor Electrode Materials..............................................................................................9 iii UNCLASSIFIED/FOR OFFICIAL USE ONLY
UNCLASSIFIED//FOR OFFICIAL USE ONLY
Ultracapacitors as Energy and Power Storage Devices for
Commercial and Military Applications
Summary
Ultracapacitors (ultracaps) are energy storage devices capable of extremely
rapid charge and discharge rates with the ability to be cycled hundreds of
thousands of times. These unique capabilities make ultracaps attractive for a
number of applications. The advantages over lithium ion batteries are
somewhat mitigated by the fact that they have less than 10% of the specific
energy (Wh/kg) and require a power converter to regulate their voltage. The
high capacitance is achieved by the enormously high surface area of the
carbon electrodes compared to planar electrolytic capacitors. Ultracapacitors
are commonly used to provide voltage stabilization of power converters and to
supplement peak-power loading of electrical systems. Their application has
expanded from electronics and telecommunications to industrial load leveling
and transportation. Due to their durability, their use in aerospace has been for
munitions fusing and missile power load leveling. Ultimately, application will
be pulsed power storage and delivery for electric propulsion and directed-
energy weapons.
Recent developments in ultracapacitors have focused on different active
materials and electrode designs. By using advanced carbons, polymers, and
metal oxides, increased power density and energy density can be achieved.
Oxide-based thin-film ultracaps have demonstrated high-performance energy
and power density, approaching theoretical limits. Carbon nanotubes and
advanced carbons have been used as additives to and solely as electrode
materials. Nanostructured materials and thin-film manufacturing process
improvements will generate breakthroughs in mass production. Magnetic
capacitors are a recent development, manufactured using semiconductor
processes on silicon wafers. They possess a different storage mechanism and
will transform energy storage and pulsed-power applications. Ultracapacitors
are reaching widespread adoption and they are now available to support
electronics through transportation. New advancements in materials will enable
high-energy density devices with exceptional power capabilities.
iv
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
Chapter 1: Concept
Overview
Ultracapacitors, also known as
supercapacitors, hybrid capacitors,
electrochemical capacitors, electrochemical
double-layer capacitors, or ultracaps, are
energy-storage platforms that offer energy
storage capable of extremely rapid charge
and discharge rates. Ultracapacitors also
have the ability to be cycled hundreds of
thousands of times. The phenomenal
charge and discharge capabilities make
them ideal for supporting volatile memory
and computing applications, energy
efficiency/capture processes, and high-
power applications. Ultracaps store their
charge in the electrical double layer
between the electrode and the electrolyte.
Charge storage is a physical mechanism
rather than a chemical phase change, so
these are theoretically capable of cycling an
infinite number of times.2 One of the
disadvantages of ultracaps can be a high
self-discharge rate.3 The charge and discharge voltage output for an ultracap is a
sloping linear curve, which allows for straightforward and accurate state-of-charge
monitoring.4 The real advantage of an ultracapacitor is the ability to deliver or accept
bursts of power in a short time.
Figure 1. Ultracaps in Various Configurations.1
Ultracaps were introduced in 1966 and found initial use 12 years later as backup power
devices for volatile memory and clocks. Over the last 30 years, numerous advances
have been made that have led to many uses of ultracaps, from transportation to
portable electronics and more. Ultracaps are becoming more affordable as activated
carbon electrodes and manufacturing improvements have driven costs down. Several
companies now make ultracapacitors to fill a broad spectrum of applications. Figure 1
shows a number of ultracaps with various capacities to fill a range of functions.
Transportation, microelectronics, and aerospace markets are some of the many areas
where ultracaps have become enablers. Advanced materials and improved cell designs
will lead to improvements in both power density and energy density. The improved
performance is expected to make ultracaps important components for efficient power
leveling and high-power receiving and delivery. The unique signature of ultracapacitors
must be understood for aerospace and military applications.
As an electrochemical capacitor, ultracapacitors store energy within the electric double
layer formed at the interface between the electrode and electrolyte. In a conventional
capacitor, the energy is stored by moving charge carriers from one plate to another,
and the charge separation creates a potential. Voltage differentials in a conventional
capacitor are dependent upon the dielectric material separating the plates. In the
ultracap, the electrical double layer is the separation of charge in a vanishingly thin gap
between two plates. Figure 2 depicts the components and basic design of the ultracap,
1
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
showing the electrochemical double layer responsible for the characteristic
performance.5 The basic principles of operation have not changed with improved
technologies, only the materials and cell design.
The two electrode plates are traditionally the same material, which is usually a
carbonaceous material, such as activated carbon. Each layer is capable of storing a low
voltage, and multiple layers serve to increase the potential. Individual cells are placed
in series to create a higher voltage, in much the same manner as a battery. Recent
advances in ultracapacitors have moved toward employing dissimilar electrodes, which
create a higher potential; since the electrodes are now different, these are sometimes
referred to as "pseudo" or hybrid capacitors. The pseudocapacitor uses a battery-like
electrode to replace one of the carbon electrodes, yielding a high-energy-storage
electrode and a highly capacitive electrode within the same system.
The capacitance of an ultracapacitor can be determined by the Helmholtz equation
(equation 1), which describes the relationship between the electrolyte and the electrode:
C = εA/d (1)
Here, ε is the dielectric constant of the electrolyte, A is the available surface area, and d
represents the distance between the center of the double layer and the electrode
surface. Increasing the double-layer capacitance in an ultracap is generally
accomplished by either manipulating the electrode (carbon) surface area or the
electrolyte. Energy density (equation 2) is the product of the capacitance and the
square of the voltage:
E = ½ CV² (2)
Strategies for increasing the stored energy target improvements to both the electrodes
and the electrolyte. Changing from an aqueous electrolyte to an organic electrolyte with
a higher dielectric constant will increase the voltage from approximately 1 volt to more
than 2.5 volts. Increasing the surface area of the electrode is another approach to
increasing storage capability. There is a tradeoff between porosity and surface area that
must be considered when constructing an electrode with an extremely high surface area.
An activated carbon approaching a measured surface area of 3,000 m²/g may have less
than half that as accessible or useable area.6 Increasing the pore size sacrifices surface
area but provides more accessible material. By providing more interfacial area between
the electrode and the electrolyte, a better electrochemical double layer can be produced,
yielding a better ultracapacitor.
2
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
Activated
carbon
electrodes
Electrolyte Separator soaked in electrolyte
Activated
carbon
electrodes
Material selection
and construction
■ Electrodes
■ Separator
■ Electrolyte
■ Wound component
■ Case
Figure 2. Operation of an Ultracapacitor. A simple ultracapacitor utilizes two similar electrodes (typically
carbon) with a separator and electrolyte to create the electrochemical double layer that governs operation.7
BATTERIES AND ULTRACAPS
For an ultracapacitor, energy is stored within the electrical double layer, which is a
physical storage mechanism. Contrast this with batteries, where charge and discharge
take place through a chemical (Faradaic) reaction. Since charge is stored physically,
there is no direct degradation mechanism that limits cycle life. Side reactions do occur,
which prevents infinite cycle life, but ultracaps can often be cycled tens of millions of
times.8 Batteries store their energy through chemical reaction; consequently, cycle life
is much shorter and heavily dependent on the depth of discharge. In contrast, there are
no limitations to the discharge depth with an ultracapacitor. Table 1 addresses some of
the general characteristics of batteries and ultracapacitors, which can help to determine
the preferred system for a specified application. The values are based upon 2008 data,
so some numbers are outdated, but the relative values remain consistent. There are
now hybrid devices that blur the boundaries further by introducing a battery-like
electrode to replace one of the two identical electrodes in a traditional ultracapacitor.
3
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
Table 1: Contrasting Properties of Batteries and Ultracapacitors9
Characteristic
State of the Art Lithium Ion Battery Ultracapacitor
Charge time ~3-5 minutes ~1 second
Discharge time ~3-5 minutes ~1 second
Cycle life <5,000 @ 1C rate >500,000
Specific Energy (Wh/kg) 100-200 5
Specific power (kW/kg) 0.5 -1 5-10
Cycle efficiency (%) <50% to >90% <75 to >95%
Cost/Wh $1-2/Wh $10-20/Wh
Cost/kW $75-150/kW $25-50/kW
The voltage curve for an ultracapacitor is proportional to the depth of discharge,
whereas batteries deliver a relatively constant voltage over a long discharge period. The
sloping voltage curve of an ultracap can be advantageous for state-of-charge
determinations. However, this is also responsible for the decreased energy density
available in an ultracapacitor. For applications requiring energy to be delivered over a
longer time scale or delivered at a constant voltage, the relatively flat voltage curve
found in most batteries would be the preferred option. In instances where power is to
be delivered or received quickly, or where many cycles are required, an ultracap is ideal.
To better understand the differences between batteries and ultracaps, it helps to think
of batteries as storing watt-hours of energy and ultracaps as storing watts of power. A
Ragone plot illustrates the distinctions by plotting power versus energy in a logarithmic
scale. As can be seen in Figure 3, ultracaps make an excellent option where high power
density is required. The times shown are rough estimates for a full charge or discharge,
which are estimates to help understand the relationship between energy density and
power density. Batteries have made significant improvements to power delivery
recently, but batteries remain the high-energy-density solution. Ultracapacitors are
often thought of as a stop-gap between conventional capacitors and batteries. Recent
developments of hybrid capacitors have led to considerable progress toward higher
energy density. However, there is a tradeoff between high energy density and high-
power devices, and the distinctions between the two require consideration of application
requirements when choosing the energy storage platform for a system. The differences
between ultracaps and batteries do not make the two mutually exclusive. There are
applications where either a battery or an ultracap is the preferred energy storage
platform. Quite often, these two can be used together in systems to perform
complementary roles.
4
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
[FIGURE: Ragone Plot graph showing Energy density (Wh/kg) on Y-axis (0.01 to 1000) vs Power density (W/kg) on X-axis (10 to 10000). Areas labeled: Fuel cells (top left), Conventional batteries, 10 hours, 1 hour, 1 second (ultracaps region, dark oval), 0.03 second, Conventional Capacitors (bottom right).]
Figure 3. Ragone Plot. The graph illustrates the areas where ultracaps and batteries dominate in the power
density versus energy density relationship.
HISTORY
The concept of storing electrical energy in the electric double layer that is formed at the
interface between an electrolyte and a solid has been known since the late 1800s. In
1966, Standard Oil Company of Ohio (SOHIO) invented the device in the format now
commonly used, which forms the basis for hundreds of patents and thousands of
journal articles. Figure 4 shows a drawing from the electrochemical double-layer device
invented by SOHIO. NEC introduced the SuperCapacitor™ in 1978 under license from
SOHIO.
[FIGURE: Diagram of electrolytic capacitor showing: CONDUCTOR (left and right), POROUS GRAPHITE ELECTRODE (left and right), SAT. Al1(SO4)3 SOLUTION, ION PERMEABLE MEMBRANE]
Figure 4. Electrolytic Capacitor as Designed and Patented by SOHIO.
From this point on, ultracaps have rapidly evolved through several generations of
designs. In the 1980s, Matsushita Electric Company developed a method of
manufacturing ultracapacitors with improved electrodes. Initially, they were used as
backup power devices for volatile clock chips and complementary metal-oxide-
semiconductor (CMOS) computer memories. As the technology became more
understood, more and more applications were developed for ultracaps. Many other
applications have emerged over the past 30 years, including wireless communication,
power quality, and improved energy efficiency through regenerative energy capture
processes, as found in hybrid electric vehicles.10
5
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
Initial ultracaps used aqueous electrolytes made with Al2(SO4)3 (aluminum sulfate),
H2SO4 (sulfuric acid), or KOH (potassium hydroxide) . While these electrolytes have
excellent ionic conductivity, the operating voltage is limited to 1.2 volts using carbon
electrodes. Organic electrolytes have lower ionic conductivity, but the higher dielectric
constant increases the nominal cell voltage up to as high as 3 volts. Carbon has been
used as a high-surface-area electrode material since the inception of the
electrochemical capacitor. It is still the material of choice for many ultracapacitors; one
of the primary reasons is the low cost of carbon materials. However, there are many
types of carbon that are available for use as an electrode material. In addition to
carbonaceous electrodes, metal oxides and conductive polymers are finding increasing
use in ultracap design. Advancements in the understanding of the electric double-layer
and ultracapacitor behavior have led to better materials utilization and, consequently,
improved devices.
Early electrochemical capacitors were rated at a few volts and had capacitance values
measured from less than one farad up to several farads. Today cells range in size from
small devices with exceptional pulse-power performance in the millifarad range up to
devices rated at several kilofarads. There are even some specialized ultracapacitor cells
now in production that have ratings of more than 100 kF. The technology is
experiencing increasingly broader use, replacing batteries in some cases and in others
complementing their performance. Ultracap technology has grown into an industry with
sales of several hundred million dollars per year that is poised for rapid growth in the
near term due to expansion of power quality needs and the emerging energy
management/conservation applications.11
Advancements in ultracapacitors have led to numerous devices from an array of
manufacturers. Table 2 compares the various products on the market, showing voltage,
capacity, power density, and additional energy-storage characteristics. Ultracaps have
moved away from aqueous electrolytes and are typically organic electrolytes due to the
increased voltage performance. Electrodes vary from carbon/carbon systems to hybrid
systems using metal oxides or conductive polymers paired with a carbon electrode.
Packaging and sizes of ultracapacitors covers a large range as these are now used from
cellular communications and small electronics to power delivery and management for
seaport cranes. Ultracapacitor technology development is focusing on delivering better
energy density, and this is being approached by improved carbon electrodes, better
electrolytes, and alternative electrodes that provide pseudocapacitive behavior,
including battery-like electrodes.12
6
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
Table 2: Manufacturers of Ultracapacitors13
V C RC Wh/kg W/kg W/kg
Device rated (F) R (mOhm) (sec) (1) (95%) Match. Wgt. Vol.
Imped. (kg) lit.
Maxwell* 2.7 2,885 .375 1.08 4.2 994 8,836 .55 .414
Maxwell 2.7 605 .90 .55 2.35 1,139 9,597 .20 .211
Apowercap** 2.7 55 4 .22 5.5 5,695 50,625 .009 ---
Apowercap** 2.7 450 1.4 .58 5.89 2,574 24,595 .057 .045
Ness 2.7 1,800 .55 1.00 3.6 975 8,674 .38 .277
Ness 2.7 3,640 .30 1.10 4.2 928 8,010 .65 .514
Ness (cyl.) 2.7 3,160 .4 1.26 4.4 982 8,728 .522 .38
Asahi Glass 2.7 1,375 2.5 3.4 4.9 390 3,471 .210 .151
(pc) (est.)
Panasonic 2.5 1,200 1.0 1.2 2.3 514 4,596 .34 .245
(pc)
EPCOS 2.7 3,400 .45 1.5 4.3 760 6,750 .60 .48
LS Cable 2.8 3,200 .25 .80 3.7 1,400 12,400 .63 .47
BatScap 2.7 2,680 .20 .54 4.2 2,050 18,225 .50 .572
Power Sys. 2.7 1,350 1.5 2.0 4.9 650 5,785 .21 .151
(AC, pc) **
Power Sys. 3.3 1,800 3.0 5.4 8.0 486 4,320 .21 .15
(GC, pc) **
3.3 1,500 1.7 2.5 6.0 776 6,903 .23 .15
Fuji Heavy 3.8 1,800 1.5 2.6 9.2 1,025 10,375 .232 .143
Industry-
hybrid
(AC/GC) **
JSR Micro 3.8 1,000 4 4 11.2 900 7,987 .113 .073
(AC/GC)**
3.8 2,000 1.9 3.8 12.1 1,038 9,223 .206 .132
(1) Energy density at 400 W/kg constant power, Vrated - 1/2 Vrated
(2) Power based on P=9/16*(1-EF)*V²/R, EF=efficiency of discharge
* Except where noted, all the devices use acetonitrile as the electrolyte (pc = propylene
carbonate)
AC = Activated Carbon; GC = Graphitic Carbon
** Laminated pouches; all other devices are packaged in metal containers
7
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
Chapter 2: Materials Technology
The standard electrochemical capacitor is made of two electrodes, an electrolyte, and a
separator, packaged in either a metal container or a laminated pouch. Ultracapacitors
are often packaged in a fashion similar to a typical battery configuration. The separator
is an ultrathin material that allows ion transport but is electrically insulating. There are
several polymer separators on the market that have the desired porosity needed to
create fast ion transport. The electrodes are traditionally the same material in an
ultracapacitor—this is in stark contrast to a battery. Electrodes are typically carbon; this
is advantageous in making a cost-effective energy/power-storage device. Activated
carbon has a very high surface area, vital to storing energy in the electrical double
layer between the electrode and the electrolyte. This thin double layer is responsible for
the impressive high-power-storage capabilities of the ultracapacitor. The electrolyte's
resistivity and dielectric constant play a vital role in behavior of the double layer;
consequently, the choice of electrolyte impacts performance.
The earliest electrochemical capacitors were introduced 30+ years ago; they were
symmetric designs (two identical electrodes) in aqueous electrolyte. While these
electrolytes have excellent ionic conductivity, the operating cell voltage was limited to
~1.2 V/cell and these had a nominal cell rating of ~0.9 volts. In the second generation
of electrochemical capacitors, the use of organic electrolyte led to an increase of the
rated cell voltage from about 0.9 V/cell to 2.3-2.7 V/cell. Today, ultracapacitors using
an organic electrolyte are the most popular.14
ELECTROLYTES
Initial ultracaps used aqueous electrolytes, saturated with Al2(SO4)3, or 30%
concentrations of H2SO4 or KOH. While these electrolytes have excellent ionic
conductivity, the operating voltage is limited to 1.2 volts using carbon electrodes.
Improvements to ultracapacitor performance were made by transitioning from aqueous
electrolytes to an organic medium. Organic electrolytes have lower ionic conductivity,
but the higher breakdown potential increases the nominal cell voltage. An increase in
cell voltage up to as high as 3 volts has been realized by use of an organic electrolyte.
These electrolytes are typically an ammonium salt dissolved in an organic solvent, such
as propylene carbonate or acetonitrile. Table 3 highlights the comparison between the
various electrolytes used in ultracapacitors. Ionic liquids are beginning to become more
common in ultracapacitors, as they allow for an increase in cell voltage to as high as 4
volts. These ionic liquids have a higher resistivity, especially at lower temperatures. The
tradeoff in resistivity for voltage may be beneficial for some high-power applications.
Table 3: Properties of Various Electrolytes Used in Ultracapacitors15
Electrolyte Density Resistivity Cell
(gm/cm3) (Ohm-cm) Voltage
KOH 1.29 1.9 1.0
Sulfuric acid 1.2 1.35 1.0
Propylene carbonate 1.2 52 2.5-3.0
Acetonitrile 0.78 18 2.5-3.0
125 (25°C) 4.0
Ionic liquid 1.3-1.5
28 (100°C) 3.25
8
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
ELECTRODES
Carbon has been the optimal electrode material for ultracapacitors since their
commercial introduction over 30 years ago. Pseudocapacitor systems use a combination
of a carbon electrode and a battery-like electrode, such as a conductive polymer or a
metal oxide. The high surface area of carbon makes it an extremely attractive option
for ultracap electrodes. Carbon chemistry is quite well known, and high surface area
and low cost ensures carbon's continued use for the near future. While carbon has been
used in electrochemical systems for many years, there are many nuances that make its
properties and performance vary significantly amongst the many types of carbon. The
use of a carbon electrode makes ultracaps affordable for use in many systems.
Activated carbon is readily available, can be made from many source materials, and is
inexpensive. Materials science activities for electrodes are focused on improving the
carbon electrodes and introducing alternative materials to create an asymmetric
electrode.
Types of carbon can include activated carbon, carbon cloth, aerogels, porous carbon,
carbon nanotubes, and graphene. The properties of carbon can change dramatically
based upon processing, having a major impact to the porosity and active surface area.
Activated carbon has an extremely high surface area, is inexpensive, and is produced at
a global scale for use in a number of applications. For these reasons, activated carbon
is the traditional carbon of choice for ultracapacitor applications. Recent advances in
carbon materials development have led to a number of options for ultracapacitor
electrodes. Table 4 demonstrates a number of electrode materials in use today and the
corresponding performance of these ultracaps.
Table 4: Properties of Various Materials Used in Electrochemical Capacitor
Electrode Materials16
Material Density Electrolyte F/g F/cm3
(g/cm³)
Activated Carbon 0.7 KOH 160 112
Organic 100 70
Carbon Cloth 0.35 KOH 200 70
Organic 100 35
Aerogel Carbon 0.6 KOH 75
Organic 125 84
Porous Carbon from SiC 0.7 KOH 175 122
Organic 100 70
Porous Carbon from TiC 0.5 KOH 220 110
Organic 120 60
Anhydrous RuO2 2.7 Sulfuric Acid 150 405
Hydrous RuO2 2.0 Sulfuric Acid 650 1,300
Doped Conductive Polymer 0.7 Organic 450 315
9
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
Activated carbons are synthesized by a heat treatment of carbon-rich organic
precursors in a controlled atmosphere. This carbonization process can be performed
from natural sources, such as fruit shells, wood, pitch, or coke. Activated carbons can
also be produced from synthetic precursors, such as select polymers. The heat
treatment process uses a controlled partial oxidation of the precursor combined with a
high-temperature processing. The high-temperature processing can be done in an inert
atmosphere, an oxidizing environment, or with a chemical modification. The activated
carbons contain a distributed porous network throughout, as shown in Figure 5.17 These
materials can have an extremely high surface area, upward of 3,000 m²/g, but process
conditions to create activated carbons have little control over pore size distribution,
resulting in incomplete utilization of the surface area. Carbon cloth electrodes use
processing conditions similar to those used to create activated carbons. The advantage
with a fabric composed of activated carbon is these materials are directly used as active
electrodes, requiring no binder or additional processing. However, these tend to be
expensive to produce, which has thus far limited their use to specialized applications.
[FIGURE: Diagram showing activated carbon porosity with labels:
- macropores >50nm
- micropores <2nm
- mesopores 2-50nm]
Figure 5. Activated Carbon Porosity. These can be produced with a distribution of pore sizes ranging from less
than 2 nm to greater than 50 nm.
Continued improvements to the carbon electrodes are expected to provide additional
performance enhancement to ultracaps. Current research is leading toward carbon
materials with higher specific capacitance (F/g). Improved materials, such as carbon
nanotubes and tailored porous carbons show promise as the next generation of carbon
materials. Graphene is another advanced carbon material that shows promise to
providing increased capacitance. One of the key design issues revolves around
improved understanding of the relationship between the carbon pore size and the
electrolyte ion. The nanostructured materials allow fine tuning of the porous structure
to increase capacitance.
Carbon nanotubes (CNTs) are commonly produced as powders where they can be cast
as a distributed network to form an electrode, and they can be used as an additive to
increase conductivity. Additionally, CNTs can be grown as a "forest," where the CNTs
are grown perpendicular to a substrate. These forests can be used as is or can be
modified or coated to change specific structural or electronic properties. CNTs are
grown by a number of methods, but most methods used currently are a derivative of a
chemical vapor deposition (CVD) process. The specific properties of the nanotubes are
extremely tailorable. CNT length, diameter, the number of tubes, and the electronic
properties can all be tailored by growth conditions. The catalysts used to grow CNTs can
affect tube properties, as well as CNT temperature and precursor variables. Carbon
nanotubes are finding utility in both battery and ultracapacitor applications, where they
are used for conductivity enhancement and energy storage. Their capacitive behavior
suggests they may have a fundamentally different storage mechanism than traditional
carbon. Storage potential includes CNT outer walls, inner diameter, and interwall spaces
10
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
of some CNT types. Carbon nanotubes have been a rich area of study for many
applications, which has translated to a broad understanding of synthesis techniques to
produce CNTs with tailorable properties.
Graphene can be thought of as a sheet of graphite, either a single layer or a few layers
thick. The two-dimensional nature of graphene leads to extremely high electron
transport across the surface, interesting magnetic properties, high strength, and a large
accessible surface area.18 Graphene is generally made by exfoliation/separation of
graphite or grown by CVD processes. Derivations from both of these methods can
produce single-layer graphene or few-layer graphene. These methods include, but are
not limited to, CVD methods, epitaxial growth, solvothermal synthesis, micromechanical
exfoliation, and colloidal synthesis.19 Graphene has generated an enormous amount of
attention due to its potential for transforming electronics. A number of approaches to
graphene synthesis are being used that consider purity, scalability, and cost.
Carbide-derived carbons are a unique class of porous carbons with extremely fine
control over pore size.20 These porous carbons are derived by chlorination of metal
carbides at very high temperatures. At temperatures of 500–1000°C, metals and
metalloids are removed as chlorides, leaving behind a finely tunable nanoporous carbon.
These porous carbons have a pore-size distribution that is tunable within
0.05 nm. The extremely accurate tunability has allowed these systems to serve as a
model for the relationship of pore size and capacitance, thereby allowing for design of
materials to be tailored for maximum capacitance within a specific system. The local
maximum in capacitance is dependent on the ion solvation sphere, which is a product of
the electrolyte ion, solvent, and interaction with the pores of the electrode. These
findings will lead to new electrode materials and designs.
Activated carbon and other porous carbon derivatives are commonly used for
symmetric ultracapacitors. Metal oxides such as ruthenium oxide and conductive
polymers replace one of the electrodes to create an asymmetric system. Asymmetric
electrodes greatly increase the storage capacity of ultracapacitors. These are a different
class of capacitor, as they store their charge both in the electrical double layer and
using a surface redox (faradaic) reaction (in the same manner as a battery). While
these undergo electron transfer reactions, they behave in a capacitive fashion. Often
termed pseudocapacitor, this class of materials includes a variety of metal oxides and
conductive polymers. The capacitance of an asymmetric electrode is twice that of a
symmetric design. The electron transfer electrode essentially has a fixed potential,
whereas the potential of the carbon electrode changes with state of charge. Additional
storage capacity arises from the higher working voltage, due to the different rest
potentials of the different electrodes.
Asymmetric electrodes are becoming increasingly common due to the increased energy-
storage capability. As energy is proportional to the voltage squared, the higher
operating voltage has profound effects on the capacity. Ruthenium oxide (RuO2) has
been shown to give capacitance of as high as 1300 F/cm3. By comparison,
carbonaceous materials in an aqueous electrolyte have a capacitance of 110-125 F/cm3.
There are many additional metal oxides being investigated for insertion into the
ultracapacitor system. Many of these have their roots in lithium ion battery cathode
materials. As ultracaps gain wider acceptance, market size is increasing rapidly and is
expected to grow at an increasing pace. Investigations are being conducted into
alternative metal oxide systems to ensure that cost, supply, and performance can be
met on a global scale. Manganese oxide (MrO2) and nickel metal oxides/hydroxides are
11
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
leading alternative candidates for asymmetric ultracaps. Using a pseudocapacitive
design will most likely not be able to deliver the cycle life of a symmetric ultracap, but it
may be possible to achieve tens of thousands of cycles out of a battery-like electrode.
Better capacitance and higher operating voltage of the asymmetric electrode design
allow ultracapacitors to begin approaching batteries in terms of energy density.
Additional ultracapacitor improvements will come from materials and cell design. Cell
design factors include optimization and assembly techniques. Thinner electrodes and
current collectors will be one of the primary paths for better performance. Electrolytes
that provide cell voltages of at least 3 volts will be standard. Improvements such as
these will likely lead to improvements in commercially available ultracaps from 4 Wh/kg
to 5-6 Wh/kg.21
12
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
Chapter 3: Applications
ELECTRONICS AND TELECOMMUNICATIONS
The most straightforward application of an ultracapacitor is to stabilize dc voltages,
which is where they have found widespread use. Areas where a dip in voltage may
occur and negatively impact performance is another natural fit for ultracaps. Initially,
they were used as backup power for clock chips and CMOS memory; now, they find
utility in power smoothing, flash photography, and digital cameras. Flash for mobile
phone cameras is an ever-increasing market. Nearly one billion phones equipped with
cameras were sold in 2009. Ultracaps enable flash photography integrated with mobile
communication. Today's market is driving toward electronics devices with multiple
functions to eliminate the need for multiple specialized electronics that each performs
one task. Integrating components into one electronic device requires the use of
ultracapacitors to help stabilize the short power spikes from the individual components.
Ultracaps also serve to reduce thermal loading from power dissipation.
Ultracaps have found extensive application in wireless communication, where they are
used in wireless data cards for GSM, GPRS, and WiMAX data transmission. The peak
current required during data transmission can exceed what is available under USB or
PC-card standards, and the excess current is delivered with an ultracapacitor.
Automated meter reading is another application where ultracaps have become an
integral component, as the power required to send and receive the meter information is
handled by an ultracapacitor. The mobile telecomm industry is currently using these in
a number of ways; it is likely that additional uses for ultracaps will spring up along with
needs to balance power requirements for increasingly complex and multifunctional
devices contained within smaller areas.
INDUSTRIAL
Industrial applications are becoming much more ubiquitous as ultracaps are being used
as a platform to deliver a large amount of power quickly. These are becoming
increasingly important in cranes, forklifts, elevators, and power tools. As an example,
power is stored by an ultracapacitor in hybrid forklifts every time a loading fork
descends. The stored power is then delivered to the forklift when heavy lifting is
required. Figure 6 shows the power requirements for a hybrid forklift. The peak power
pulses are areas where using an ultracap to deliver or to receive the power greatly
improves the operational efficiency. The blue shaded area represents the power
distribution handled by a fuel cell, and the tan regions above and below the blue area
represents the output from and the input to the ultracapacitor, respectively.22 In cranes
and elevators, a similar application is used. Peak power is supplied for lift operations by
the ultracap. During descent of the elevator or lowering of the loaded crane, generated
power can be used to recharge the ultracapacitor. Power tools are becoming
increasingly disconnected from an outlet. Many of these cordless power tools require
significant amounts of peak power, which many batteries cannot adequately supply.
The ultracapacitor used in conjunction with a high energy battery supplies the pulse
power required during heavy use.
13
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
[FIGURE: Line graph titled "Lift truck operating profile" showing Power (kW) on Y-axis (-10 to 35) vs Time (hr) on X-axis (0 to 2). Labels on graph: "Ultracapacitor output", "Peak power", "Fuel-cell output", "generation", "Ultracapacitor Input". Source: General Hydrogen]
Figure 6. Ultracapacitor Use in a Forklift. Ultracaps provide energy storage upon descent of a load and deliver
peak power to lift heavy loads in fuel cell powered forklifts.23
Grid storage management is gaining more visibility as energy conservation and
efficiency are becoming increasingly important. Many utility power sources generate a
relatively continuous supply of electricity. The generated power is matched to expected
use statistics. Daytime use and nighttime rest periods can cause power shortages or
generate unused power if not managed efficiently. This is especially true during the hot
summer months or heat waves, where air conditioning loads can tax a system.
Ultracapacitors can store excess power generated in the night, which can then be
delivered when insufficient power is available to match the daytime needs. Distributed
power sources such as wind and solar require a higher level of operational efficiency
with a fluctuating power supply. Wind farms experience power fluctuations due to wind
shear or lack thereof. These spikes in power generation may be wasted on a system ill-
equipped to handle the burst of power delivered. During periods where wind may be
lacking, ultracapacitors can be used to help create a more stabilized power supply.
TRANSPORTATION
Earliest uses of ultracapacitors were in motor startup for tanks and submarines. Early
adoption by the military led to cost reductions, which was a critical enabler for
incorporation into diesel trucks and railroad locomotives. Ultracapacitors are finding use
in transportation in engine startup, braking systems, acceleration, and waste-energy
harvesting. Ultracaps have been placed in rail cars to generate power for acceleration
and recapture it during braking. In some cases, they are placed alongside the tracks,
and the power management is hosted by the rail stations.
The ability to absorb and discharge energy rapidly makes ultracapacitors far better than
batteries for regenerative braking schemes. Most of these applications have been in
public transportation. Regenerative braking can recuperate as much as 38% of the
propulsion energy when used in an ultracap/battery hybrid configuration. Electric hybrid
public buses were some of the first adopters of ultracapacitors for automotive
transportation. These buses also use ultracaps for acceleration, which enables a much
faster acceleration.
14
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
AEROSPACE
Ultracapacitors are suitable for applications requiring large bursts of power for relatively
short periods of time, with the ability to be recharged rapidly. Ultracapacitors are
finding ubiquitous use in electronics and transportation, and there are a number of
applications in aerospace ranging from backup power to high pulse-power delivery. The
electronics systems in military vehicles require failsafe operation; ultracaps can be
designed to provide a continuous power source in the event of a power interruption.
Fire-control systems are another example where continuous power is required,
including higher power capability. Airbags require a backup power source to ensure
they are deployed as required with or without a powered system. Small portable
communications systems have ultracaps for backup power and to retain memory.
Memory retention is critical in many systems; for example, the bridge power used
during a transfer from ground to onboard power in aircraft systems can be supplied by
an ultracap. Ultracaps are excellent for stabilizing the power that is either delivered or
received. For example, communication systems may require additional power during
transmission, which can be offset by using an ultracap to supplement the system. Bus
voltages may need peak current stabilization.
The high-power capabilities of ultracaps make them advantageous for several systems.
The fast discharge is ideal for delivering the significantly higher power required for
starting cold engines. Active suspension systems in vehicles require bursts of power
that are delivered in response to the terrain; an ultracap can store the necessary power,
deliver as necessary, and recharge quickly. GPS-guided missiles and projectiles may
use ultracapacitors to provide the power required for communication and navigation.
High-power discharge for naval systems has been identified as an area where ultracaps
are especially useful, often used in hybrid systems in tandem with high-energy
batteries. Extremely high power delivery makes ultracapacitors a potential vehicle for
delivering the power required for directed-energy weapons. As pseudocapacitive
systems reach higher energy densities, along with improvements to the materials and
cell composition, ultracapacitors will have capabilities of delivering large amounts of
power in a very short time, contained within an increasingly smaller footprint.
Signatures and Vulnerabilities
As mentioned above, ultracapacitors could be important in the development of
ultrawideband energy weapons. Previously, this market has been dominated by bulk
acoustic semiconductor switch (BASS) devices, but by stringing several capacitors
together in parallel, a more formidable discharge can be created. Because
ultracapacitors have very high discharge rates, they will emit a distinctive broadband RF
signature when discharged. A modest wideband receiver may be able to detect this
discharge; its proximity will depend upon the construction of the target device.
Furthermore, the architecture of such a device may yield a unique signature that, with
the proper equipment, can be identified and certainly warrants further study. When
employed in a particular electronic device, this signature may be altered by its
immediate environment, and this may further give an indication of both the type of use
and device employed. When searching for ultracaps in the field that are not in use, the
large electrostatic potentials may provide a clue.
15
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
Stimulated Discharge
Depending on the application and the implementation and construction of ultracaps in
sensitive or destructive applications, vulnerabilities likely exist and warrant study in
areas of research such as circuit vulnerability analysis, reversible manufacturing and
construction techniques used in fabrication, stimulation of nondestructive and controlled
discharge evaluations, and laboratory stimulated discharge experiments.
16
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
Chapter 4: Recent Developments
There have been several recent advances in ultracapacitors focused on increasing
energy density by using different active materials24 and designs,25 activated carbons,26
polymers,27 and metal oxides.28 Conducting polymers have good performance,29 but
lack microfabrication protocols. Oxide-based thin-film ultracaps have shown high
performance, approaching the theoretical limit for capacitance (~1,000 F/g for MnO2).30
However, the poor electrical conductivity and high impedances associated with surface
intercalation redox reactions of these oxides have limited practical film thicknesses to a
few microns. Carbon nanotubes have been added to the films to increase the electrical
conductivity, but the complexity of manufacturing limits practical applications of such
composite electrodes. In this section we will discuss advances in application of CNT and
advanced carbons to ultracapacitor materials, the application of thin-film manufacturing
processes, and a revolutionary new super capacitor using a giant magneto capacitive
effect.
CNTS AND ADVANCED CARBONS
Carbon electrodes constitute both electrodes in a symmetric ultracapacitor and one of
the electrodes in an asymmetric, or pseudocapacitive, design. Improvements to the
carbon electrode rely upon increasing the specific capacitance (in Farads per gram).
These improvements come by tailoring the surface area and porosity to achieve the
best balance that maximizes the interaction with the electrolyte. There is a linear
relationship to the surface area and the capacitance up to a point where capacitance
plateaus with activated carbons. By controlling the porosity and surface area, it is
possible to increase the capacitance beyond this plateau. Carbon nanotubes could
provide performance increases with aligned CNT forests of tailored sizes. The
characteristics of an ultracapacitor are highly dependent on the nanostructure of the
carbon used for the thin-film electrodes. Advanced carbons will provide better control
over the pore size and distribution, leading to an expected 50- to 100-percent
improvement over the carbons in use today.
Carbon nanotubes can be produced with a wide variety of properties. Depending on
synthesis parameters, nanotubes can be single walled or multiwalled, with varying
numbers of tubes. CNT diameters can be tailored from a few nanometers to tens of
nanometers, with lengths up to hundreds of microns. CNTs can be grown in random
orientations or as aligned forests. CNTs have a fully accessible surface area and very
high electrical conductivity. Methods for incorporating CNTs into electrodes for ultracaps
include using CNTs as an additive for conductivity enhancement, creating dense mats of
randomly oriented tubes, and creating electrodes from vertically aligned forests of
tubes. Initial results of CNT-enabled ultracaps tended to show much lower capacitance
than expected, which has been attributed to the hydrophobic nature of the CNT walls.
Surface functionalization is a common approach to mitigating the hydrophobicity issues
and thus enabling higher capacitance. Another benefit of the functionalization is the
ability to introduce and control pseudocapacitance.
Most efforts in CNT ultracaps are directed toward vertically aligned forests. It is possible
to controllably grow a dense, aligned forest that is perpendicular to the current collector.
The size and density of the tubes and the number of walls can be controlled with
catalyst design and reaction parameters. Manipulation of the CNT forest leads to
increased capacitance by fine-tuning the distance between tubes. Additionally, fine-
17
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
tuning the inner diameter may provide further enhancements. Figure 7 shows a
vertically aligned CNT forest produced at Lockheed Martin's Advanced Technology
Center (LMATC). Lockheed Martin has worked with a number of catalysts and surfaces
to grow tailored CNT forests for a variety of applications. Select locations throughout
Lockheed Martin have been developing CNT-based technologies, utilizing vertically
aligned CNTs, CNTs dispersed onto surfaces, and CNTs dispersed into other media.
LMATC possesses expertise in CNT growth processes and characterization and has
capabilities for both materials development and ultracap testing. Lockheed Martin's
NEARLab facility produces CNT-coated glass fibers, which may provide a cost-effective
power storage solution that has structural elements built in. Ultracaps assembled from
CNT forests appear extremely promising for use in microelectronics.
[Figure 7 - CNT Forest image]
Figure 7. CNT Forest. CNT forests grown in Lockheed Martin's laboratories can be tailored for specific sizes,
lengths, and densities.
Graphene is a relatively new discovery amongst carbonaceous materials. There are a
number of types of graphene that can be characterized by the number of layers of
graphene, the functionalization, or the oxidation status. Most graphene for
ultracapacitor applications is going to be few-layer graphene and large-area flakes. The
fewer the layers, the higher the active surface area will be. Single layer is ideal, but
manufacturing considerations make single layer difficult, even at the laboratory scale.
Functionalization will be directed toward improving capacitance or modifications to
enable battery-like performance.
Theoretical values of graphene indicate it could become an important material for the
next generation of ultracapacitors. Graphene may be used as the sole electrode
material, or it could be used as a conductive additive that also provides capacitance.
The surface area is calculated to be as high as 2,600 m2/g, the thermal conductivity is
5,000 W/m·K, and the charge carrier mobility is 200,000 cm2/V·s. High surface area
values and great conductivity are ideal properties for creating an electrode with very
high capacitance and extremely favorable rate capabilities. Reported capacitances
range from 135 to 205 F/g in aqueous electrolytes.31, 32 Figure 8 shows a transmission
electron microscope image of graphene flakes used for ultracapacitor electrodes. These
measurements come from few-layer graphene, rather than single layer, which suggest
18
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
there is room for improvement on these values. Optimization of graphene synthesis to
produce finer control is an area receiving increased attention in research and
development activities in universities, national labs, and industrial research facilities.
Lockheed Martin is actively engaged in research investigating graphene and applications
where graphene could be a critical differentiator. There are a number of companies that
supply graphene made from various methods, as well as emerging startups geared
toward producing graphene-based ultracapacitors or materials designed for ultracaps.
[Figure 8 - Transmission Electron Microscope image of graphene flakes]
Figure 8. Transmission Electron Microscope Image of Graphene. This could be an excellent ultracapacitor
electrode material with its high surface area and excellent conductivity. 33
Manipulation of the porosity of high surface area carbons leads to the largest difference
in specific capacitance. Understanding the relationship between the electrolyte ion size
and the carbon pore size is critical to improving performance. There are a number of
strategies being investigated for fine control over the pore-size distribution to increase
the specific capacitance. The most common methods used currently are template
methods and carbide-derived carbons. Template methods create a controlled
mesoporous structure with a fairly narrow range. These structures have pores that
range from 2 to 10 nanometers and are maximized to pore sizes roughly twice that of
the solvated ions. The template process involves filling the pores of an inorganic
template host with a carbon precursor (such as an alumina template). The template is
removed after carbonization by acid treatment. The pore size is then dictated by the
template as the pores are the remaining void space once occupied by the template.
Similar methods have shown that smaller pores, including those less than two
nanometers, may provide high specific capacitance. The realization that smaller pores
contribute to charge storage in an electric double layer has led to the need to develop a
better understanding of the charge storage mechanism.
Carbide-derived carbons have a unique pore-size distribution that is tunable with sub-
angstrom accuracy. These have served as models to study the charge storage behavior
and ion adsorption in pore sizes ranging from 0.6 nm to 1.1 nm.34 The normalized
capacitance decreases with decreasing pore size until a critical value is reached.
Figure 9 shows the relationship between average pore size and the normalized specific
capacitance. Pore sizes smaller than one nanometer significantly contribute to the
charge storage despite the fact that the solvated ion size is larger than the pore
diameter. The capacitance increase is explained by a distorted ion shell model. The ion
solvation shell is perturbed such that it is capable of a closer approach of the ion and
the carbon surface. The discoveries at Drexel University that utilize the fine control to
create tailored porous carbon structures can maximize specific capacitance for a given
ultracapacitor system.
19
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
[Figure 9 - graph: Relationship Between Average Pore Size and Normalized Specific Capacitance]
Figure 9. Relationship Between Average Pore Size and Normalized Specific Capacitance. As the average
pore size decreases below 1 nm, the specific capacitance increases due to distorted electrolyte ion solvation.
THIN FILMS
CNT films have been used in 2D thin-film ultracapacitors having respectable gravimetric
and volumetric capacitance. Activated carbon powders have a better performance in
terms of energy per unit area,35 but they need to be processed into films before use,
and this limits their choice for microdevice fabrication, which allows mass production of
capacitors on semiconductor wafers. Carbide-derived carbon (CDC) is a class of carbon
materials produced by selectively etching metals from metal carbides using chlorine at
elevated temperatures in a process similar to current dry-etching techniques used in
MEMS and microchip fabrication. CDC has been shown to have excellent performance as
the active material in traditionally processed ultracaps36 since it can have its
microstructure precisely tuned by tailoring the synthesis conditions for a particular
electrolyte.37
For microfabricated supercapacitors, CDC is attractive for several reasons. The
precursor carbides are conductive and can be deposited in uniform thin and thick films
by well-known chemical and physical vapor deposition (CVD and PVD) techniques.38 In
addition, the chlorination process can be performed at temperatures at least as low as
200°C,39 and the resulting coatings are well-adhered with an atomically perfect
interface,40 which minimizes device impedance. This technology can be used to produce
the microfabricated ultracaps on the same chip as the integrated circuits, which they
are powering (shown in Figure 10).41 Chlorine-containing plasma etching of materials in
semiconductor manufacturing is a well-established technique and is similar to the
chlorination procedure in CDC manufacturing. Continuous porous carbon films cannot
be produced by conventional CVD, PVD, or other techniques, and the high-temperature
activation needed to produce the microstructures necessary for ultracapacitor
performance in CVD carbons would destroy the devices they were intended to power.
20
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
Sintered TiC plate TiC plate CDC film
Teflon plates
Electrolyte + separator
[microscopy image of CDC surface, 5 μm scale bar]
1. Chlorination @ 500 °C TiC-CDC film
2. Cell assembly
Figure 10. CDC Synthesis and Electrochemical Test Cell Preparation Schematic.42
Electrochemical measurements of CDC films were carried out in a three-electrode
configuration with a large, overcapacitive activated carbon counter-electrode in both 1M
TEABF4 and 1M H2SO4, as well as two-electrode cells with symmetric bulk CDC film
electrodes. Volumetric capacitance was calculated for each of the different film
thicknesses (shown in Figure 11). For both TEABF4 and H2SO4, the volumetric
capacitance decreases with increasing coating thickness. This is especially pronounced
in the organic electrolyte, where there is a huge increase in volumetric capacitance as
the film thickness decreases from 200 to ~2 mm.
[Figure 11 - two graphs A and B showing Volumetric capacitance (F/cm3) vs. Coating thickness (μm)]
Figure 11. Volumetric Capacitance of the Films in (A) TEABF4 and (B) H2SO4.43
MAGNETIC CAPACITORS
Giant magneto capacitance (GMC) is a quantum mechanical effect observed in thin-film
structures composed of alternating ferromagnetic and nonmagnetic layers.44, 45 Such
materials have been reported to have increased permittivity by 109.46 This material
behavior normally occurs at temperatures well below 273K and has only recently been
reported in the literature above this temperature.47 However, since 2007, Northern
Lights Semiconductor Corp. (NLSC) has been developing a magnetic capacitor (MCap)
that utilizes a material which exhibits the GMC above 300K.
Structured like the basic flat-plate capacitor, an MCap nanocell features a proprietary
dielectric layer sandwiched between two magnetized layers. Each magnetic material
layer is made of multiple nanometer-thick thin-film layers. Millions of MCap nanocells
are created and linked using a semiconductor thin-film process, producing the MCap cell
on a wafer (Figure 12).
21
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
MCap nanocell
0.55μm 1.6μm
Magnetic materials ─┐
Dielectric layer ─┤ MCap cell
Magnetic materials ─┘ Made up of millions
(all proprietary compositions) MCap nanocells
Figure 12. MCap Structure. MCap nanocell and MCap cell are fabricated using traditional semiconductor
fabrication processes.
MCap cells are then packaged into larger MCap modules. The capacitance of the device
is given by the equation , C = ε0 k A/d, where A is the area of the plate, d is the
separation between plates, ε0 the permittivity of free space, and k is the dielectric
constant (or relative permittivity) of the material. The MCap increases capacitance by
increasing the dielectric constant k through the GMC effect (fGMC) given by the
relationship, k' = k*fGMC. GMC acts like a charge trap that brings electrons closer, thus
increasing electron densities at the plates. Based on quantum theory, GMC brings about
a capacitance which, to date, has been measured to be 109 times larger than that
observed in electrostatic capacitors. As shown earlier in equation (2), energy is
proportional to the capacitance and the voltage squared. Moreover, capacitor leakage
and self-discharge are essentially eliminated as electrons are "trapped" in the magnetic
field.
Due to the above effects, the available energy for a large range of storage devices, like
smart cards and other products, can be increased substantially by simply packaging
them in series-parallel connected modules to meet the energy and voltage
requirements. Coupled with Lockheed Martin's extensive nanomaterials and device
physics experience, it is likely that NLSC will be able to use its MRAM development
expertise to accomplish this.
Experimental Results
Over the last year, NLSC has had MCap structures and nanocells tested by independent
parties with results shown in Figure 13. These unbiased tests demonstrate the validity
of the high-capacitance claims. Figure 14 quantifies the progress in improved
performance throughout the development of the device. The number of devices that
demonstrate the GMC behavior has increased substantially in 2009, which is reportedly
due to improved materials development rather than an increased production load. The
performances of the nanocells as well as the variability are plotted, which shows that
potential improvement in the materials could lead to an increased GMC factor. There is
promising evidence that improvements may allow a higher level of performance, which
could match lithium ion batteries at ambient temperature and perhaps exceed them at
low and high temperatures.
22
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
[Figure 13 - two graphs]
Left graph: 4284A-pad 01. Cp
X-axis: Frequency, [kHz] (0.01 to 100)
Y-axis: Cp [uF] (0 to 180000)
Legend lines: AC 1ms 0Vdc, AC 1ms 1Vdc, AC 1ms 3Vdc, AC 1ms 5Vdc
Right graph: Frequency vs Capacitance
AC500mV, DC0V to 4V
X-axis: Frequency [kHz] (0.01 to 10000)
Y-axis: Capacitance [pF] (0.0 to 5.0)
Legend lines: f=0.1_41A_4V, f=0.1_41A_2V, f=0.1_41A_0V, f=0.1_41A_-2V, f=0.1_41A_2V, f=0.1_41A_4V
Figure 13. Preliminary MCap Test Results. These graphs demonstrate NLSC capabilities: MCap microstructure
tested at different VDC (by Taiyo Yuden) (left) and single nanocell tests (right).
[Figure 14 - Summary Chart]
Y-axis left: Number of Mcap nanocells showing GMC behavior (50 to 350)
Y-axis right: fGMC - a measure of technology (Max-Min line and Median point) (1E+00 to 1E+12)
X-axis: Q1 Q2 Q3 Q4 / Q1 Q2 Q3 Q4 / Q1 Q2 Q3 spanning 2007, 2008, 2009
Annotations on chart:
Maximum GMC attained
Replace Li-ion battery
Replace HV Capacitor / Ultracapacitor
Few Mcap nanocells with GMC due to processing & equipment issues
Figure 14. Summary Chart of MCap Results to Date. Right Axis: High-Low value lines in red, Median value in
black dot. Left Axis: number of nanocells exhibiting GMC phenomenon in blue bar.
Figure 15 shows an estimate that has been made by NLSC of the potential specific
energies and projected manufacturing costs of the MCap for comparison to current
energy storage devices.
23
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
[Figure 15 - graph: Specific Energy and Estimated Costs vs. fGMC]
Y-axis left: Mcap Attainable Energy Density (Wh/g)
Values on left axis: 10,000.00 / 1,000.00 / 100.00 / 10.00 / 1.00 / 0.10 / 0.01 / 0.00
Y-axis right: Mcap Projected Nominal Cost ($/Wh)
Values on right axis: $10 / $1 / $0.1 / $0.01 / $0.001
X-axis: fGMC (1E+06 to 1E+11)
Annotations:
Mcap nanocell #2 (labeled "2")
P Production target initial
P
2
Li-ion price
Pb-Acid price
Fuel cell
Li-Ion
Mcap nanocell #1 (labeled "1")
Trend (arrow)
Figure 15. Specific Energy and Estimated Costs vs. fGMC
The mass production target of MCaps on a wafer of fGMC of ~109 would enable the
achievement of exceeding the specific energy of lithium ion batteries at a significantly
reduced cost. MCaps at these production targets would disrupt both the lithium ion
battery and the ultracapacitor market.
24
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
Chapter 5: Future Developments
There have been significant advancements in ultracapacitor technology in the last few
years. Increased understanding of the physical mechanism behind this technology,
combined with advancements in materials science, particularly on the nanoscale, has
led to a rapid increase in capability. Improvements to ultracaps in the next 10 years
(2010-20) will focus on electrodes, better electrolytes, packaging, and alternative
designs. Electrode improvements will most likely include migration away from activated
carbon, for both electrodes. Asymmetric electrodes will eliminate carbon on one side,
and new carbon materials will provide better performance at a competitive price.
Electrolytes will likely move toward additional organic materials and ultracaps with high
temperature performance requirements likely will use ionic liquids.
In the near term, a transition to the asymmetric design is expected. Continued
improvements to the electrodes and cell design will provide better capacitance. Many of
the short-term improvements will likely be directed toward manufacturing capacity.
Ultracapacitors are becoming more common, and as improvements to performance and
cost make them more accessible to multiple applications, this trend will continue. Over
the next few years, it is expected that the market for ultracaps will increase
dramatically. Trends in materials for ultracaps will continue forward with improvements
and utilization of activated carbons, thinner current collectors, and improved cell design
and packaging. Research being done at universities and national labs will continue, but
these will not become common materials for ultracapacitors in the next few years.
However, as the demand for ultracapacitors increases, specialty materials will begin to
see utility for some applications, driving down manufacturing costs, which in turn will
enable their use in more systems. Significant use will be made in integrating into power
converters to reduce size, mass, and cost. Both thin-film and MCap devices will become
prevalent. If MCaps achieve lithium ion specific energy their adoption will be rapid and
revolutionary as has occurred with LiFePO4 (lithium-iron-phosphate) batteries for both
transportation and extremely high pulsed-power systems such as lasers.
Within the next 10 years, ultracaps will begin to see the incorporation of advanced
carbon materials and electrodes designed with features on the nanometer scale. Carbon
nanotubes, graphene, and porous carbons all have extremely high potential to unseat
activated carbon as the electrode of choice. The replacement of carbon systems being
used today (including activated carbon, aerogels, and carbon cloths) will occur as cost
reductions take place in the manufacturing of nanostructured carbonaceous materials.
The tradeoff between surface area and pore size can be exploited at the nanoscale.
Optimization of these parameters will likely yield fairly significant improvements to the
capacitance and result in higher power and better energy density.
The mid-range (2020-30) development of ultracapacitors will most likely be the full
incorporation of advanced carbons and hybrid systems. It is hard to anticipate which
technology has the most to offer, as there are unique benefits and hurdles for each.
Hybrid systems will be common, utilizing a battery-like electrode combined with one of
the advanced carbon electrodes. The pseudocapacitive electrode options will become
diversified, with manufacturers using unique materials to differentiate their product. An
alternative electrode to the ruthenium oxide (RuO2) used today will be used for both
performance and economical reasons. Supply issues will drive electrode materials
toward those with larger availability.
25
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
In the 2020-2030 timeframe, ultracapacitor cell designs will be different. Simple plate
designs, coin cells, and laminated pouches will still be used for some applications.
However, advanced designs and form-factors will be used to fit specialized applications.
Ultracapacitors will be designed for incorporation into chip architectures. Several
options will exist for micro- and nano-electronics. Electrolyte options will most likely be
different from those used today. New electrolyte solvents and better salts will be
discovered that produce higher voltage cells while maintaining good ionic conductivity.
Ionic liquids and complex solvents that withstand higher voltages are likely candidates.
Three dimensional architectures may begin to be introduced, which will greatly improve
both energy density and power density. Manufacturing techniques will enable small-
and medium-format three-dimensional electrodes, where the electrical double layer is
stored perpendicular to the current collector. The manufacture and assembly of large-
format ultracapacitors with extremely high power capabilities will be enabled by three-
dimensional architectures of nano-engineered carbons and current collectors.
Beyond 2030 it is difficult to project the future developments of ultracapacitors since
materials science is experiencing a nanomaterials revolution. It can be expected,
however, that nearly all microelectronic circuitry will incorporate thin-film ultracaps or
MCaps, and laptop computers will become the size of an iPhone with mostly voice-
activated functions. As depicted in Figure 16, advancements will occur on a much faster
timescale than what has happened over the last 50 years.
In the energy storage area, particularly for PHEV and EV batteries, the major impact for
ultracapacitors may allow not only reduced mass and size but also order-of-magnitude
reduction in costs.
[Figure 16 - Evolution of Ultracapacitors timeline images]
2010 2020 2030 2050
ULTRACAPS [image] ────────► [image] ────────────────────────► [image]
[image] ──────► [image] ────────────────────────► [image]
[image] ──────► [image] ────────────────────────► [image]
Figure 16. Evolution of Ultracapacitors
26
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
Chapter 6: Conclusions
Ultracapacitors are becoming increasingly important in applications ranging from
portable electronics to cargo transport cranes. Their introduction in electronic circuits
for conversion and storage assistance will become the next major milestone. The
virtually unlimited cycle life combined with high power capabilities has made them an
integral component to many energy-management schemes. Early adoption of ultracaps
by the military was an important differentiator for vehicle performance and a
technology driver. Improvements to activated carbons and manufacturing techniques
have brought costs down. The ultracapacitor is relatively young and is expanding
rapidly as ultracaps are reaching a more widespread audience. Increased adoption has
led to recognition of the benefits, and the attention has allowed researchers to develop
a better understanding of the power-storage mechanisms.
New materials and cell designs will produce increased capacitance and higher voltages,
which will in turn give ultracapacitors better energy density. Advanced carbons show
great promise to generate a carbon electrode with much higher capacitance than the
activated carbons in use today. These include porous carbons, carbon nanotubes, and
graphene. Capacitance increases by the carbon electrode alone could double the energy
density. Additional advantages in conductivity and the ability to functionalize these
materials will generate improvements in power density as well as energy density.
Hybrid systems utilizing a battery-like electrode are beginning to gain acceptance.
These pseudocapacitive systems maintain the high power density of a symmetric
ultracap and also have substantially higher energy density. While these systems
sacrifice some cycle-life capability, it appears as though they will survive tens of
thousands of cycles.
Ultracapacitors are a solution for generating or absorbing high pulse power. They also
are excellent devices for backup power and stabilization of fluctuating power
requirements. Increased power densities and energy densities will open up new
applications for ultracaps, replacing fuel and batteries in some cases, supplementing
them in others. Power management and generation for military systems will see
increasing uses for ultracaps. Their unique capabilities will enable future designs. Their
specific performance capabilities must be optimally utilized and we must have a keen
understanding and awareness of the unique spectral signatures these devices generate
when storing high power and in charge or discharge modes. Development of these
advanced ultracaps must be performed, understood, and adopted by the U.S.
government and those who are developing the systems to be used by the government.
27
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
Chapter 7: Endnotes
1 Tecate Group website
2 B. E Conway, Electrochemical Supercapacitors: Scientific, Fundamentals, and Technological Applications, Kluwer,
New York (1999).
3 Wikipedia Commons
4 J. R. Miller, P. Simon, Fundamentals of Electrochemical Capacitor Design and Operation; ECS Interface, 17, Spr.
2008, 31-33
5 EPCOS White Paper on UltraCap Technology
6 P. Simon, A. Burke, Nanostructured Carbons: Double-Layer Capacitance and More; ECS Interface, 17, Spr. 2008,
38-43
7 EPCOS White Paper on UltraCap Technology
8 D. Tuite, Get the Lowdown On Ultracapacitors; Electronic Design, Nov 2007.
9 J. R. Miller, A. F. Burke, Electrochemical Capacitors: Challenges and Opportunities for Real-World Applications;
ECS Interface, 17, Spr. 2008, 53-57
10 J. R. Miller, A. F. Burke, Electrochemical Capacitors: Challenges and Opportunities for Real-World Applications;
ECS Interface, 17, Spr. 2008, 53-57
11 J. R. Miller, A. F. Burke, Electrochemical Capacitors: Challenges and Opportunities for Real-World Applications;
ECS Interface, 17, Spr. 2008, 53-57
12 K. Naoi, P. Simon, New Materials and New Configurations for Advanced Electrochemical Capacitors; ECS
Interface, 17, Spr. 2008, 34-37
13 Burke, Ultracapacitor Technologies and Application in Hybrid and Electric Vehicles; International Journal of
Energy Research; July 2009
14 J. R. Miller, P. Simon, Fundamentals of Electrochemical Capacitor Design and Operation; ECS Interface, 17, Spr.
2008, 31-33
15 P. Simon, A. Burke, Nanostructured Carbons: Double-Layer Capacitance and More; ECS Interface, 17, Spr. 2008,
38-43
16 EPCOS White Paper on UltraCap Technology
17 P. Simon, Y. Gogotsi, Charge Storage Mechanism in Nanoporous Carbons and Its Consequence for Electrical
Double Layer Capacitors, Phil. Trans. R. Soc. A, 368, 3457 (2010)
18 C. N. R. Rao, A. K. Sood, R. Voggu, K. S. Subrahmanyam, Some Novel Attributes of Graphene, J. Phys. Chem.
Lett., 1, 572 (2010)
19 S. Park, R. Ruoff, Chemical Methods for the Production of Graphenes; Nature Nanotechnology; 4, 217 (2009)
20 J. Chmiola, G. Yushin, Y. Gogotsi, C. Portet, P. simon, P. L. Taberna, Anomalous Increase in Carbon Capacitance
at Pore Sizes Less than 1 Nanometer; Science, 313, 1760 (2006)
21 P. Simon, A. Burke, Nanostructured Carbons: Double-Layer Capacitance and More; ECS Interface, 17, Spr. 2008,
38-43
22 D. Tuite, Get the Lowdown On Ultracapacitors; Electronic Design, Nov 2007.
23 D. Tuite, Get the Lowdown On Ultracapacitors; Electronic Design, Nov 2007.
24 A. S. Aricò, P. Bruce, B. Scrosati, J.-M. Tarascon,W. van Schalkwijk, Nat. Mater. 4, 366 (2005).
25 S. W. Lee, B.-S. Kim, S. Chen, Y. Shao-Horn, P. T. Hammond, J. Am. Chem. Soc. 131, 671 (2009).
26 D. Pech et al., J. Power Sources 195, 1266 (2010).
27 W. Sun, X. Y. Chen, Microelectron. Eng. 86, 1307 (2009).
28 J. Wang, J. Polleux, J. Lim, B. Dunn, J. Phys. Chem. C 111, 14925 (2007).
29 K. Naoi, P. Simon, New Materials and New Configurations for Advanced Electrochemical Capacitors; ECS
Interface, 17, Spr. 2008, 34-37
30 H. J. Ahn, W. B. Kim, T. Y. Seong, Electrochem. Commun. 10, 1284 (2008).
31 C. N. R. Rao, A. K. Sood, R. Voggu, K. S. Subrahmanyam, Some Novel Attributes of Graphene, J. Phys. Chem.
Lett., 1, 572 (2010)
32 M. D. Stoller, S. Park, Y. Zhu, J. An, R. S. Ruoff, Graphene-Based Ultracapacitors, Nano Lett, 8, 3498 (2008)
33 M. D. Stoller, S. Park, Y. Zhu, J. An, R. S. Ruoff, Graphene-Based Ultracapacitors, Nano Lett, 8, 3498 (2008)
34 J. Chmiola, G. Yushin, Y. Gogotsi, C. Portet, P. simon, P. L. Taberna, Anomalous Increase in Carbon Capacitance
at Pore Sizes Less than 1 Nanometer; Science, 313, 1760 (2006)
35 D. Pech et al., J. Power Sources 195, 1266 (2010).
36 J. Leis, M. Arulepp, A. Kuura, M. Latt, E. Lust, Carbon 44, 2122 (2006).
37 A. Jänes, E. Lust, J. Electrochem. Soc. 153, A113 (2006).
38 E. N. Hoffman, G. Yushin, B. G. Wendler, M. W. Barsoum,Y. Gogotsi, Mater. Chem. Phys. 112, 587 (2008).
39 R. K. Dash, G. Yushin, Y. Gogotsi, MicroporousMesoporous Mater. 86, 50 (2005).
40 Z. G. Cambaz, G. N. Yushin, Y. Gogotsi, K. L. Vyshnyakova, L. N. Pereselentseva, J. Am. Ceram. Soc. 89, 509
(2006)
41 J. Chmiola, C. Largeot, P. Taberna, P.Simon, Y.Gogotsi, Monolithic Carbide-Derived Carbon Films for Micro-
Supercapacitors, Science,328, 480 (2010).
42 A. Jänes, E. Lust, J. Electrochem. Soc. 153, A113 (2006).
28
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY
43 J. Leis, M. Arulepp, A. Kuura, M. Latt, E. Lust, Carbon 44, 2122 (2006).
44 T. Goto, T. Kimura, G. Lawes, et. al. Ferroelectricity and Giant Magnetocapacitance in Perovskite Rare-earth
Manganites. Physical Review Letters, 92, 257201 (2004).
45 N. Hur, S. Park, P. A. Sharma, S. Guha, and S-W. Cheong, Colossal Magnetodielectric Effects in DyMn2O5,
Physical Review, Lett. 93, 107207 (2004).
46 R. F. Mamin, T. Egami, Z. Marton, and S. A. Migachev, Giant dielectric permittivity and Magnetocapacitance in
La0.875Sr0.125MnO3 Single Crystals, Phys. Rev. B 75, 115129 (2007).
47 J. Wan, Qi Lu, Bo Chen, Fengqi Song, J. Liu, J.g Dong, and G.Wang, Giant Room-Temperature
Magnetocapacitance in Co2+ Doped SnO2 Dielectric Films, Applied Physics Letters, 95, 152901 (2009)
29
UNCLASSIFIED//FOR OFFICIAL USE ONLY