DOW-UAP-D147, AAWSAP DIRD, Ultracapacitors as Energy and Power Storage Devices, November 2010

This document is a Defense Intelligence Reference Document (DIRD), a technical reference format used by the Defense Intelligence Agency (DIA) to capture baseline knowledge on a specific topic for later analytic use. DIRDs are best understood as reference and synthesis products rather than as original research. It is one of 38 DIRDs produced under the Advanced Aerospace Weapon System Applications Program (AAWSAP) between 2009 and 2011. Because AAWSAP’s scope permitted a broad range of supporting topics, not every DIRD in the series directly concerns aerospace systems or future threat assessment. The following summary reflects the DIRD’s scope and framing at the time of writing and should not be read as implying current validation of the concepts discussed. This DIRD surveys ultracapacitors as high-power energy-storage devices and argues that their main value lies in extremely rapid charge and discharge, very long cycle life, and usefulness in systems that need short bursts of power rather than sustained energy delivery. The report explains how ultracapacitors differ from batteries, reviews the materials and cell designs that determine their performance, and emphasizes that they are especially useful for power stabilization, backup power, load leveling, regenerative braking, and other applications where fast energy transfer matters more than total stored energy. It also notes their drawbacks, including lower energy density, self-discharge, and voltage-management requirements that limit their usefulness as stand-alone replacements for batteries in many applications. The document presents ultracapacitors as a maturing and increasingly important technology whose most credible aerospace and military uses lie in pulsed-power, missile and munitions systems, electric propulsion support, and other hybrid power architectures rather than in long-duration primary energy storage.

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Defense
Intelligence
Reference
Document
Defense Futures
01 November 2010
!COD: 20 July 2010
DIA-08-1011-005
Ultracapacitors as Energy and
Power Storage Devices for
Commercial and Military
Applications
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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 intelli ence to ics or methodolo ies.
Prepared by:
Technology Warning Division (DW0-4)
Defense Warning Office
Directorate for Analysis
Defense Intelligence Agency
Authors:
AAP Person 84, AAP Person 85
(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, Defense Warning Office's Advanced Aerospace Weapons System
Applications (AAWSA) Program. Comments or questions pertaining to this document should be
I
addressed to !MP Person 1 AAWSA Program Manager, Defense Intelligence Agency,
ATTN: JUIAF - DI/DWO-3, Bldg 6000, Washington D.C. 20340-5100
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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
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Figure 10. CDC Synthesis and Electrochemical Test Cell Preparation Schematic. 21
Figure 11. Volumetric Capacitance of the Films in (A) TEABF4 and (B) H2S04...... 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. foMc ...................................... 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
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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.
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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
Figure 1. Ultracaps in Various Configurations.'
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.
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,
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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 = EA/d (1)
Here, £ 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 cv 2 (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.
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Material selection
and construction
Activated
carbon 0000000000 ■ Electrodes
electrodes
■ Separator
■ Electrolyte
Electrolyte Separator soaked in electrolyte ■ Wound component
0000000000 ■ Case
Activated
carbon
electrodes
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 an