Will supercapacitors enable an energy storage revolution? By David

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electrodes and some electrolyte, making it something
of a hybrid between a capacitor and a battery. The
most popular is the double layer capacitor, which
stores the energy in the double layer formed near the
carbon electrode surface.
The amount of energy a conventional capacitor can
hold is measured in microfarads – even nano or
picofarads. In contrast, supercapacitors typically store
farads, thousands of times more. Large commercial
ones can store as much as 5000F. The energy density of
today’s supercapacitors ranges from around 1 to
10Wh/kg, much higher than a standard capacitor but
still only about a tenth of an NiMH battery. But as we
shall see, this could change radically.
One of the main benefits of supercapacitors is that
they can be charged extremely quickly, in about 10s.
This means large devices can be used for regenerative
braking on vehicles. Hundreds of supercapacitors can
be connected in series to provide the energy storage
needed. Several transportation systems worldwide
are now using them in such a way, for example in
Shanghai and Mannheim.
Charging ahead
Will supercapacitors
enable an energy
storage revolution?
By David Boothroyd.
Centre:
Zenn’s Clifford:“I decided the
world needed a 180° turn from
petroleum and ... saw the
fundamental issue wasn’t drive
systems, but energy storage.”
14
I
nvented more than 250 years ago – in 1745 to be
exact – the humble capacitor has been a
mainstay of electrical and electronic
engineering, almost wherever energy needs to be
stored. Billions of circuits have been built containing
them. But a device that could transform the world?
Hardly.
However, if the potential of a new generation of
supercapacitors turns out to be as great as proponents
claim, the world really will become a different place.
Such devices could transform the entire nature of
electrical storage and make today’s most advanced
batteries look positively primitive. And it could
happen at both ends of the power storage spectrum:
tiny units for consumer electronic products; and
massive ones for the automotive industry. Both
markets might – possibly – be in for a true revolution.
Supercapacitors have been with us for several
decades, occupying relatively niche markets. Whilst a
standard capacitor consists of conductive foils and a
dry separator, the supercapacitor uses special
Another of the supercapacitor’s advantages over
batteries is that it can be recharged and discharged
effectively an unlimited number of times. There is
little wear and tear induced by cycling and age does
not affect them, with the result that a supercapacitor
only deteriorates to about 80% of peak performance,
even after 10 years.
Even though today’s supercapacitors have some
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Supercapacitors
clear advantages over batteries, they cannot match
them for electrical storage density: a typical
supercapacitor has only about 5 to 10% of the density
of the best batteries. But two developments taking
place could change that radically.
One centres on Texan company EEStor. It is reticent,
but is known to be developing an electrical energy
storage unit (EESU) based on a ceramic supercapacitor
with a barium-titanate dielectric. This, it says, achieves
extremely high specific energy – the amount of energy
in a given unit of mass.
It claims its system has a specific energy of about
280Wh/kg – compared with around 120 for lithiumion and 32 for lead gel batteries – and that it will
outperform lithium ion batteries in terms of price,
charge time and safety.
By modifying the composition of the bariumtitanate powders, a thousand fold increase in
supercapacitor voltage can be achieved, it says,
giving 1200V to 3500V and possibly higher.
Many are sceptical, but EEStor has attracted some
impressive partners. In January, Lockheed Martin
signed a deal to use EEStor supercapacitors for
military and homeland security applications. It is also
backed by Kleiner Perkins, a venture
capital company whose track
record includes Google,
Amazon and Sun.
EEStor’s closest partner is
the Zenn Motor Company of
Toronto, a supplier of low
speed electric vehicles
(LSVs), founded by its ceo Ian
Clifford in 2001. LSVs have a
top speed of around 25mph
and a restricted range,
suitable only for inner urban transport applications.
“I decided the world needed a 180° turn from
petroleum and we saw the fundamental issue wasn’t
drive systems, but energy storage,” Clifford says. “We
think a complete paradigm shift in energy storage is
needed and that is what EEStor is proposing.”
The two companies signed a technology agreement
in 2004 and are working closely together. The target is
the cityZENN, an all electric car scheduled for launch
in Europe by autumn 2009 (earlier than in North
America, where certification takes longer). If it
happens, it will revolutionise the automotive industry
at a stroke because its performance will dwarf
anything achieved so far: a top speed of 80mph, a
range of 250 miles, a recharge time as little as five
minutes, a competitive purchase price and operating
costs one tenth of a comparable ordinary car. And of
course, zero emissions and noise!
The first vehicles to feature EEStor’s supercapacitor
may even appear this year, Clifford says “EEStor has
committed publicly to commercialisation of its
technology in 2008. The LSVs we are building today are
‘EEStor ready’ and they will be the first to have the
EEStor systems. ZENN will not be building the highway
vehicle, but will work with global automotive partners –
that has always been our intention,” he adds. “With
EEStor storage and our drive system technology, we will
be an enabler.” ZENN will supply the complete electric
drive system, including the motor controller and
associated drive electronics.
Also available will be a ZENNergy drive train
powered by EEStor. A key target market here is
‘retrofitting’ standardised fleets, such as London cabs
– installed bases of vehicles, operating in controlled
environments. The aim is to enable them to convert to
all electric drive. Ultimately, Clifford envisages the
development of conversion centres, where consumers
could take their car and have it converted.
For recharging, there are several
possibilities. The simplest is to do it
by plugging into a standard
household socket, which
will achieve a charge in
about four hours,
certainly
overnight.
But
www.newelectronics.co.uk 13May 2008
Above:
Zenn says its electric vehicles
may be charged overnight
using a standard plug. But it
also envisions ‘charging
stations’, where the job could
be done in five minutes.
Below:
Whilst Zenn’s current LSV
has a top speed of 25mph
and restricted range, the
forthcoming cityZENN will
reach 80mph and have a
range of 250miles – if
EEstor’s supercapacitor
technology lives up to
expectations.
15
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Supercapacitors
dedicated charging centres may reduce this to less
than five minutes. More announcements are due soon
– ZENN’s agreement with EEStor calls for regular
milestones to be met.
Transportation is obviously a huge market, but by
no means the only one for EEStor’s technology. Grid
load levelling and consumer electronics are two other
potentially massive areas. Storage for wind and solar
power are other possibles. And Clifford says that,
“We have demonstrated that nanotube
forests can be grown on a conductive
substrate.” Prof Joel Schindall, MIT
unlike batteries, raw materials are no problem.
“Think of the impact of hundreds of millions of
electric cars. Take lithium: there are about 17million
tons available on the planet, most of it in three areas
that are relatively unstable. EEStor’s technology uses
barium-titanate and there is more than 2billion tons
of it. So, critically, it is a technology that, once
commercialised, is scaleable to a global level.”
Nanotechnology research
Top:
Production of Zenn’s LSVs is
underway at its Canadian facility.
Above:
The surface area of MIT’s ‘nano
forest’ should exceed the effective
surface area of activated carbon
devices by at least a factor of five.
MIT also anticipates higher
voltage operation.
16
EEStor’s approach to supercapacitors is not the only
one that could change the future. Nanotechnology
could do the same, if work at MIT and Cambridge
University fulfils its promise. The basis of this work is
to tackle the fundamental limitation of
supercapacitors, which is that storage capacity is
proportional to surface area.
Today’s supercapacitors still hold 25 times less energy
than similarly sized chemical batteries, despite using
electrodes made of activated carbon, which is extremely
porous and does have a large surface area. However,
the pores in the carbon are irregular in size and shape,
which reduces efficiency. If the surface area could be
further increased, and with more regular elements,
storage capacity could be increased significantly.
This is exactly what can be achieved by coating the
supercapacitor’s electrodes with millions of nanotubes,
each 100,000 times as long as they are wide. The
vertically aligned nanotubes in MIT’s supercapacitor
have a regular shape and are only several atomic
diameters in width. The result is a more effective
surface area, providing increased storage capacity.
It is done by growing a microscopic ‘forest’ of the
vertically aligned nanotubes on a silica substrate by
use of a thin catalyst layer and a chemical vapour
deposition process. The silica is coated by a
nanometre thick layer of a catalyst such as iron. When
the material is heated in a vacuum, the catalyst
breaks into tiny droplets. A hydrocarbon gas is then
passed over the substrate and the catalyst ‘grabs’
carbon atoms. A long nanotube then self assembles
and pushes upward from the catalyst droplet.
“We have demonstrated that nanotube forests can
be grown on a conductive substrate,” says Joel
Schindall, Professor of Product Design at MIT’s
Electrical Engineering and Computer Science
department. “Compared to activated carbon, this is
somewhat like a paintbrush, as opposed to a sponge.
The forest’s surface area should exceed the effective
surface area of activated carbon devices by at least a
factor of five and we anticipate being able to operate at
higher voltages, due to the nanotube’s inert chemistry.
“We have grown the required nanotube array and
are now testing it in small battery cells, with reasonable
performance – currently, about the same as existing
activated carbon systems. But we are still looking for
ways to increase the density of the nanotubes and, by
doing that, exceed today’s devices.”
At Cambridge University’s Electronics Power and
Energy Conversion (EPEC) Group, Professor Gehan
Amaratunga heads a team that has developed
nanoscale supercapacitors made from multiwalled
carbon tubes roughly 70nm wide, grown vertically from
nickel catalyst dots on niobium films. The nanotube
forest and its niobium floor was then covered with a
silicon nitride layer, then an aluminium film.
The resulting supercapacitor is made from niobium
and aluminium electrodes separated by an insulating
silicon nitride layer and carbon nanotubes. In
ongoing work, the team is looking at replacing the
dielectric, which could help increase the specific
capacity by up to three times.
The key target for the EPEC group’s supercapacitor
is portable electronic devices, where they will
potentially offer much greater performance. But any
area needing high current pulses, such as
communications, could also benefit.
“The main challenges are reproducibility and
scalability – showing the process can be used for
large volume production,” Prof Amaratunga says. ■
www.newelectronics.co.uk 13 May 2008