fast charge and discharge, long lifetime, tolerance to a wide
temperature range, reliability, and being maintenance-free.
One approach to increasing energy density and power density
is to increase surface area and thus the volumetric capacitive
storage. Traditional supercaps or EDLCs (electrochemical
double layer capacitors) use activated carbon, a porous,
amorphous material. Nanomaterials such as carbon nanotubes,
graphene-based electrodes, and carbide-derived carbon
promise superlative increases in energy density in supercaps.
Carbon nanotubes have been used for many years but
integration with commercial processing, limited increase in
energy density, and cost concerns have limited their adoption
for commercial supercapacitors.
Graphene-based materials are currently enjoying intense
research and development attention. Graphene has a similar
atomic structure to carbon nanotubes but differs in being a
flat sheet of carbon atoms, connected into hexagons like a
honeycomb lattice. In being flat, it’s similar to silicon so that
engineers can process graphene with some of the familiar
techniques they use for silicon.
It will be a few years before the commercial and energy
density benefits of graphene are clear enough to warrant
adoption into mainstream supercap manufacturing processes.
Along with these new nanomaterials for electrodes, exploration
of higher temperature and voltage performance using new
electrolyte materials such as ionic liquid is also an active area
for industrial and academic research.
A second major approach, which also relies on new
electrode materials to boost a supercap’s energy density, is the
production of asymmetric or hybrid supercaps that have one
battery electrode and one supercap electrode.
Though hybrid supercapacitors are, in essence, a drop-in
replacement for Li-Ion batteries, they do have some notable
differences. They can work with the Constant-Current/Constant-Voltage charge regime traditionally used for Li-Ion batteries,
but the supercapacitor mechanism of energy storage can also
accept charge rates on the order of ten times higher. And for
safety, the EDLC and hybrid supercapacitors pose little risk in
the areas of fire, explosion, or toxicity.
Because supercaps are an electrostatic charge storage
media, which is significantly limited in terms of volumetric energy
storage as compared to an electro-chemical lithium battery, their role has
been limited to date.
Now, with energy densities
approaching that of lead-acid, the
possible applications are expanding.
With the introduction of nanomaterials,
which extend charge-carrying surface
area and cathode lithiation, it’s likely
that hybrid supercapacitors will
experience an increase in energy
density in the neighborhood of 20
percent per year for the next few years.
The research and development
combinations include nanoporous
nickel hydroxide and activated carbon, laser-scribed graphene
and manganese dioxide, one electrode formed with nanoporous
metal oxide and liquid crystal templating technology.
Among successful commercial approaches, ruthenium oxide
doped supercaps have been produced for specialized high
energy and peak power delivery applications for a number of
years. Another more popular approach in recent years is the
Lithium Ion Capacitor (LIC) as hybrid supercaps with lithium
salts doped into a carbon-based material of the negative
As far as safety, hybrid supercapacitors aren’t subject to
the same hazards as Li-Ion batteries. Most notably there is
no lithium metal deposition taking place during charge and
discharge and therefore no possibility of thermal runaway.
Supercapacitors are finding their way into a growing number
of applications as market segments, such as enterprise storage
and automotive, seek to improve efficiencies of batteries or, in
some cases, replace them entirely. Even within these markets,
there are clear choices among competing technologies (EDLC
vs. hybrid supercap) that answer the concerns of runtime anxiety
in volumetrically-efficient form factors.
A supercap’s materials represent a large fraction of their cost,
with the activated carbon in the electrode being particularly
costly. But as demand increases and production volumes grow,
costs will begin to scale downward.
As they become mainstream, the market for supercaps will
become increasingly diverse. Thanks to their fast charging
rates, supercaps can efficiently harvest power from distributed
electric motors when they are operating in braking mode. In
heavy vehicles that run for short distances, supercaps capture
energy ordinarily wasted during braking and release it during
Meanwhile, the increased energy density and high discharge
rates of hybrid supercapacitors will enable designers to size
battery packs in hybrid vehicles for continuous power instead
of peak power. Sizing an engine for average rather than peak
power results in a more efficient system with a size/cost
If their annual energy density growth continues to outpace
Li-Ion technologies, the “better battery” of the near-future will
actually be a supercapacitor.