18-12-2012, 01:56 PM
A SURVEY OF ELECTROCHEMICAL SUPERCAPACITOR TECHNOLOGY
[attachment=44173]
Abstract
The electrochemical supercapacitor is an emerging technology that promises to
play an important role in meeting the demands of electronic devices and systems
both now and in the future. This paper traces the history of the development of
the technology, and explores the principles and theory of operation. The use of
supercapacitors in applications such as pulse power, backup sources, and others
is discussed and their advantages over alternative technologies are considered.
To provide examples with which to outline practical implementation issues,
systems incorporating supercapacitors as vital components are also explored.
Introduction
Electrochemical capacitors are currently called by a number of names:
supercapacitor, ultracapacitor, or electrochemical double-layer capacitor. The
list of different names is almost as large as the number of manufacturers, and
since the technology is only currently beginning to find itself a market a
universal term does not seem to have been agreed upon as yet. The term
‘supercapacitor’ finds itself in common usage, being the tradename of the first
commercial devices made by Nippon Electric Company (NEC), but
‘ultracapacitor’ is also commonly used, originating from devices made by the
Pinnacle Research Institute (PRI) for the US military. Within this thesis the
technology will be referred to as much as possible by the term ‘electrochemical
double-layer capacitor’, (EDLC), thus reducing reliance on the use of any
commercial names, but sometimes the term ‘supercapacitor’ will be used for the
sole purpose of reducing the tedium of repeated usage of the term ‘EDLC’. It
should be understood that the two terms are used interchangeably, and that they
both refer to a capacitor that stores electrical energy in the interface that lies
between a solid electrode and an electrolyte.
While electrostatic capacitors have been used as energy storage elements for
nearly a century, low capacitance values have traditionally limited them to lowpower
applications as components in analogue circuits, or at most as short-term
memory backup supplies. Recent developments in manufacturing techniques
have changed this, however, and with the ability to construct materials of high
surface-area and electrodes of low resistance has come the ability to store more
energy in the form of electric charge. This has combined with an understanding
of the charge transfer processes that occur in the electric double-layer to make
high-power electrochemical capacitors possible.
EDLCs therefore represent a new breed of technology that occupies a niche
amongst other energy storage devices that was previously vacant. They have the
ability to store greater amounts of energy than conventional capacitors, and are
able to deliver more power than batteries. The current position of the EDLC is
easily visualised by means of a Ragone plot (Fig. 1.1), which graphically
represents a device’s energy and power capabilities.
Figure 1.1 - Ragone plot of various energy storage devices [1].
Besides bridging the gap between capacitors and batteries, supercapacitors also
possess a number of desirable qualities that make them an attractive energy
storage option. The mechanisms by which EDLCs store and release charge are
completely reversible, so they are extremely efficient and can withstand a large
number of charge/discharge cycles. They can store or release energy very
quickly, and can operate over a wide range of temperatures.
3
Supercapacitors have only very recently begun to make themselves known as a
viable energy storage alternative, and while most electrical engineers may be
aware of the technology it is probable that few possess an understanding of the
processes involved and the applications that are possible. Ignorance of the full
capabilities of EDLCs will most likely lead to more conventional alternatives
being selected instead.
It is therefore the aim of this thesis to present a complete overview of
electrochemical double-layer capacitor technology for the purpose of fostering a
better understanding within the engineering community. This work will cover
various aspects of the technology relevant to the field of electrical engineering,
providing an introduction and a firm foundation from which to embark on
further endeavours. The content of this thesis is intended for use by practising
electrical engineers or engineering students who wish to become more informed
about this new technology.
This survey will cover the historical development of supercapacitors, and draw a
brief picture of the current state of the technology. The scientific principles
behind the device’s operation will be covered, and various theories and models
will be discussed. Applications which make good use of the EDLC’s best
properties will be considered, and their advantages over alternative technologies
will be presented. Design considerations will be outlined and illustrated with
examples, and conclusions about the future direction of the technology will be
drawn.
Historical background
The storage of electrical charge in the interface between a metal and an
electrolytic solution has been studied by chemists since the nineteenth century,
but the practical use of double-layer capacitors only began in 1957, when a
patent was placed by General Electric for an electrolytic capacitor using porous
carbon electrodes (Fig. 1.2) [2]. Although the patent admits that “it is not
positively known exactly what takes place when the devices… are used as
energy storing devices,” it was believed that energy was being stored in the
pores of the carbon, and it was noted that the capacitor exhibited an
“exceptionally high capacitance.” Later, in 1966, The Standard Oil Company,
Cleveland, Ohio (SOHIO) patented a device that stored energy in the doublelayer
interface (Fig. 1.3) [3]. At this time SOHIO acknowledged that “the
‘double-layer’ at the interface behaves like a capacitor of relatively high specific
capacity.” SOHIO went on to patent a disc-shaped capacitor in 1970 utilising a
carbon paste soaked in an electrolyte (Fig. 1.4) [4]. By 1971, however, a
subsequent lack of sales led SOHIO to abandon further development and license
the technology to NEC [5]. NEC went on the produce the first commercially
successful double-layer capacitors under the name “supercapacitor.” These low
voltage devices had a high internal resistance and were thus primarily designed
for memory backup applications, finding their way into various consumer
appliances [6].
By the 1980’s a number of companies were producing electrochemical
capacitors. Matsushita Electric Industrial Co., (otherwise known as Panasonic in
the Western world), had developed the “Gold capacitor” since 1978. Like those
produced by NEC, these devices were also intended for use in memory backup
applications [7]. By 1987 ELNA had begun producing their own double-layer
capacitor under the name “Dynacap” [8]. The first high-power double-layer
capacitors were developed by PRI. The “PRI Ultracapacitor,” developed from
1982, incorporated metal-oxide electrodes and was designed for military
applications such as laser weaponry and missile guidance systems [9]. News of
these devices triggered a study by the United States Department of Energy (DoE)
in the context of hybrid electric vehicles, and by 1992 the DoE Ultracapacitor
Development Program was underway at Maxwell Laboratories [10].
[attachment=44173]
Abstract
The electrochemical supercapacitor is an emerging technology that promises to
play an important role in meeting the demands of electronic devices and systems
both now and in the future. This paper traces the history of the development of
the technology, and explores the principles and theory of operation. The use of
supercapacitors in applications such as pulse power, backup sources, and others
is discussed and their advantages over alternative technologies are considered.
To provide examples with which to outline practical implementation issues,
systems incorporating supercapacitors as vital components are also explored.
Introduction
Electrochemical capacitors are currently called by a number of names:
supercapacitor, ultracapacitor, or electrochemical double-layer capacitor. The
list of different names is almost as large as the number of manufacturers, and
since the technology is only currently beginning to find itself a market a
universal term does not seem to have been agreed upon as yet. The term
‘supercapacitor’ finds itself in common usage, being the tradename of the first
commercial devices made by Nippon Electric Company (NEC), but
‘ultracapacitor’ is also commonly used, originating from devices made by the
Pinnacle Research Institute (PRI) for the US military. Within this thesis the
technology will be referred to as much as possible by the term ‘electrochemical
double-layer capacitor’, (EDLC), thus reducing reliance on the use of any
commercial names, but sometimes the term ‘supercapacitor’ will be used for the
sole purpose of reducing the tedium of repeated usage of the term ‘EDLC’. It
should be understood that the two terms are used interchangeably, and that they
both refer to a capacitor that stores electrical energy in the interface that lies
between a solid electrode and an electrolyte.
While electrostatic capacitors have been used as energy storage elements for
nearly a century, low capacitance values have traditionally limited them to lowpower
applications as components in analogue circuits, or at most as short-term
memory backup supplies. Recent developments in manufacturing techniques
have changed this, however, and with the ability to construct materials of high
surface-area and electrodes of low resistance has come the ability to store more
energy in the form of electric charge. This has combined with an understanding
of the charge transfer processes that occur in the electric double-layer to make
high-power electrochemical capacitors possible.
EDLCs therefore represent a new breed of technology that occupies a niche
amongst other energy storage devices that was previously vacant. They have the
ability to store greater amounts of energy than conventional capacitors, and are
able to deliver more power than batteries. The current position of the EDLC is
easily visualised by means of a Ragone plot (Fig. 1.1), which graphically
represents a device’s energy and power capabilities.
Figure 1.1 - Ragone plot of various energy storage devices [1].
Besides bridging the gap between capacitors and batteries, supercapacitors also
possess a number of desirable qualities that make them an attractive energy
storage option. The mechanisms by which EDLCs store and release charge are
completely reversible, so they are extremely efficient and can withstand a large
number of charge/discharge cycles. They can store or release energy very
quickly, and can operate over a wide range of temperatures.
3
Supercapacitors have only very recently begun to make themselves known as a
viable energy storage alternative, and while most electrical engineers may be
aware of the technology it is probable that few possess an understanding of the
processes involved and the applications that are possible. Ignorance of the full
capabilities of EDLCs will most likely lead to more conventional alternatives
being selected instead.
It is therefore the aim of this thesis to present a complete overview of
electrochemical double-layer capacitor technology for the purpose of fostering a
better understanding within the engineering community. This work will cover
various aspects of the technology relevant to the field of electrical engineering,
providing an introduction and a firm foundation from which to embark on
further endeavours. The content of this thesis is intended for use by practising
electrical engineers or engineering students who wish to become more informed
about this new technology.
This survey will cover the historical development of supercapacitors, and draw a
brief picture of the current state of the technology. The scientific principles
behind the device’s operation will be covered, and various theories and models
will be discussed. Applications which make good use of the EDLC’s best
properties will be considered, and their advantages over alternative technologies
will be presented. Design considerations will be outlined and illustrated with
examples, and conclusions about the future direction of the technology will be
drawn.
Historical background
The storage of electrical charge in the interface between a metal and an
electrolytic solution has been studied by chemists since the nineteenth century,
but the practical use of double-layer capacitors only began in 1957, when a
patent was placed by General Electric for an electrolytic capacitor using porous
carbon electrodes (Fig. 1.2) [2]. Although the patent admits that “it is not
positively known exactly what takes place when the devices… are used as
energy storing devices,” it was believed that energy was being stored in the
pores of the carbon, and it was noted that the capacitor exhibited an
“exceptionally high capacitance.” Later, in 1966, The Standard Oil Company,
Cleveland, Ohio (SOHIO) patented a device that stored energy in the doublelayer
interface (Fig. 1.3) [3]. At this time SOHIO acknowledged that “the
‘double-layer’ at the interface behaves like a capacitor of relatively high specific
capacity.” SOHIO went on to patent a disc-shaped capacitor in 1970 utilising a
carbon paste soaked in an electrolyte (Fig. 1.4) [4]. By 1971, however, a
subsequent lack of sales led SOHIO to abandon further development and license
the technology to NEC [5]. NEC went on the produce the first commercially
successful double-layer capacitors under the name “supercapacitor.” These low
voltage devices had a high internal resistance and were thus primarily designed
for memory backup applications, finding their way into various consumer
appliances [6].
By the 1980’s a number of companies were producing electrochemical
capacitors. Matsushita Electric Industrial Co., (otherwise known as Panasonic in
the Western world), had developed the “Gold capacitor” since 1978. Like those
produced by NEC, these devices were also intended for use in memory backup
applications [7]. By 1987 ELNA had begun producing their own double-layer
capacitor under the name “Dynacap” [8]. The first high-power double-layer
capacitors were developed by PRI. The “PRI Ultracapacitor,” developed from
1982, incorporated metal-oxide electrodes and was designed for military
applications such as laser weaponry and missile guidance systems [9]. News of
these devices triggered a study by the United States Department of Energy (DoE)
in the context of hybrid electric vehicles, and by 1992 the DoE Ultracapacitor
Development Program was underway at Maxwell Laboratories [10].