3 Storage factors
1000
fuel cells
energy density (Wh per kg)
100
10h
10
1
conventional
batteries
1h
1s
supercapacitors
0.03 s
0.1
conventional
capacitors
giant BASF has even developed metal–organic “
nano-cubes” made from terephthalic acid and zinc oxide that
can store and release hydrogen to power fuel cells for
laptop computers and other portable electronic devices
for up to 10 hours.
Another approach being considered is to find a material that chemically bonds to the hydrogen. Using
this chemisorption technique, yields of 14% hydrogen
by weight have been achieved by covalently bonding
the hydrogen to carbon nanotubes. But as forming and
breaking chemical bonds requires the material to be
heated, several research groups are looking at developing an intermediate technique between physical adsorption and chemisorptions as a working compromise.
0.01
10
This “Ragone” diagram compares the performance of different energy-storage devices. Energy
density is how much energy is available and power density shows how quickly that energy can
be delivered: conventional capacitors, for example, do not store more energy but can deliver it
very quickly. A conventional lead-acid battery has an energy density of about 30–40 Wh per kg
and modern lithium-ion batteries about 160 Wh per kg. Nanotechnology is leading to new
kinds of supercapacitor that can store and release lots of energy quickly, as well as novel forms
of batteries. The times indicate how long the devices release their energy over.
10 000 100 1000
power density (W per kg)
Being able to
manipulate
matter at the
molecular level
has profound
implications in
almost every
area of human
endeavour
Save your energy
Inspired by developments in nanotechnology, solar
cells, wind turbines, fuel cells and other devices can all
help to reduce carbon-dioxide emissions, provided that
they are accompanied by a switch away from fossil fuels.
But another way to cut greenhouse-gas emissions is
to use less energy in the first place, and here nanotechnology can help too. For example, scientists at the Materials and Engineering Research Institute at Sheffield
Hallam University in the UK have developed more
durable, longer-lasting surfaces. Similar harder-wear-ing surfaces were first developed for bowling balls and
are now being used for the final lacquer on Mercedes-Benz cars. These materials consist of either alumina or
silica nanoparticles. By lasting longer, they save energy.
In the automotive industry, light but strong nanocomposites – consisting of nanoclays (
montmoril-lonites) – are well-established alternatives to metal
panels, and they reduce fuel consumption by saving
weight. Meanwhile, nanoparticulates have been used
to reduce diesel consumption. For example, the UK
company Oxonica has developed a technology based
on cerium-oxide nanoparticles called ENVIROX,
which are high-surface-area catalysts. Adding them to
diesel makes the fuel burn more efficiently – even at
low parts-per-million levels – lowering fuel costs and
reducing carbon-dioxide emissions.
Heat losses from buildings can be dramatically
reduced by using a variety of renewable insulating materials, for example fibreglass or rock wool. Nanotechnology can also make improved insulating “aerogels” –
highly porous, low-density nanomaterials with a very
high surface area and a high dielectric constant. They
are such good insulators that if you put your hand on
a sheet of the material you cannot feel the heat of a
Bunsen burner placed on the other side. Aerogels are
about eight times more effective than traditional insulation materials, and find applications where high
thermal insulation can protect and safeguard buildings,
thus lowering costs and saving energy.
Fuel for thought
The material that can store more energy per unit
weight than any other, however, is hydrogen, and it is
possible to envisage a world where our energy is provided not by burning fossil fuels but by this simple substance. The idea is to react the hydrogen with oxygen
in a fuel cell, releasing just energy and water, with no
pollution. (As little as 5 kg of the gas could drive a small
car more than 500 km.) The snag is that, compared with
fossil fuels, hydrogen gas (H2) has a low energy density
by volume (figure 4). The hydrogen can, of course, be
compressed by storing it at several hundred atmospheres, but this requires heavy, thick-walled tanks;
while liquefying the hydrogen means cooling it to 20 K,
which produces problems of its own.
A preferable solution is “chemical storage”, which
involves encapsulating hydrogen inside highly porous,
high-surface-area materials. It is, for example, possible to encapsulate hydrogen inside and onto the surface of hollow molecules, such as buckyballs (C60).
Researchers are also working on a range of nanostructure polymeric materials as hydrogen adsorbents. For
example, Martin Schröder and colleagues at the University of Nottingham in the UK have developed a
porous copper-based metal–organic polymer made up
of three polyhedral molecular cages that fit together to
provide a hollow framework. When cooled to 77 K, it
can store 10% of its own mass as hydrogen at a pressure of 77 bar – one of the highest uptakes so far for this
type of porous material. Low temperatures are needed
to strengthen the intermolecular forces to hold the
hydrogen within the material, and further work will be
needed to allow these materials to store hydrogen at
room temperature.
Meanwhile, Adam Phillips and Bellave Shivaram of
the University of Virginia in the US have been able
to store up to 12% by weight of hydrogen in complex
metal hydrides containing magnesium, sodium, calcium, lithium, aluminium and boron. German chemical
Grasp the solution
Being able to manipulate matter at the molecular level
has profound implications in almost every area of
human endeavour – from energy production and transport to medicine and textiles. Unfortunately, our rapid
acquisition of knowledge often outstrips our ability to
exploit it. If nanotechnology is to provide the answer to
our energy crisis, then we do not just need more invest-
