InfoChem
Clean energy
In this issue
Silver nitrate
Tom Westgate meets some chemists working
towards a fossil fuel-free future
Photography and mirrors
EMMA MCKENDRICK
so materials chemists can develop
our understanding of how these
materials work and in turn, help
design better devices.
Flat battery
Chemists have already provided
one great advance by developing
the thin, small, light, but powerful
rechargeable lithium batteries that
power our mobile phones, laptops
and other gadgets. According to
Saiful Islam, the next challenge is
to make even more powerful, faster
charging versions to run electric cars.
He describes the classic
lithium battery as being like an
electrochemical sandwich, where
the ‘bread’ is the electrodes, and the
Interstitial oxide ions (red) squeeze through channels in the fuel cell structure to reach the fuel
‘filling’ is the electrolyte. When the
The world’s population is growing, meaning we need more
battery is plugged in to charge, lithium
and more energy to drive our cars, light and heat our homes, ions (Li+) move from the positive electrode (cathode)
and power our high-tech gadgets. To produce this energy we through the electrolyte to the negative electrode (anode).
mostly burn fossil fuels, but our supplies will start to run out
As it does this, Li+ captures electrons from the power
in the next few decades. Fossil fuel energy also comes at a
source, and stores their energy. In the charged battery, the
cost, producing the greenhouse gas carbon dioxide which
lithium is ready to move in the opposite direction (anode to
has been blamed for changes in the global climate.
cathode), releasing the electrons giving power to whichever
device is connected.
We will need cleaner, sustainable ways to generate and
store energy, and we will need them soon. Chemists are
playing their part in helping to develop the clean energy
technologies that could allow us to kick the fossil fuel habit,
before we run out for good.
‘One single technology will not be the solution.’ says
Professor Saiful Islam of the University of Bath, a chemist
who is working on materials for the next generation of
sustainable energy production and storage devices. ‘The
performance of the devices relies on the materials,’ he says,
ISSUE 130 | SEPTEMBER 2011
Most batteries currently use cathodes made of LiCoO2,
but Co is expensive and toxic. To make batteries more
powerful, affordable and safe, better cathode materials are
needed. Saiful Islam says the ‘hottest’ candidate, already
being used in some new electric cars, is LiFePO4, which is
cheaper and contains strong P–O bonds making it safer for
use on the road.
He believes there is still room for improvement in battery
cathode materials and explains that their operation relies
Apollo 13
Lithium hydroxide saves
the day
Backyard chemistry
The power of atmospheric
pressure
A day in the life of...
Adam Hunt – passionate
about chemistry
Plus…
Puzzles and competitions
Editor
Karen J Ogilvie
Assistant editor
David Sait
Science correspondent
Josh Howgego
Layout
Scott Ollington
Publisher
Bibiana Campos-Seijo
InfoChem is a supplement to
Education in Chemistry and is
published six times a year by
the Royal Society of Chemistry,
Thomas Graham House,
Cambridge, CB4 0WF.
01223 420066
email: [email protected]
www.rsc.org/infochem
© The Royal Society of Chemistry,
2011. ISSN: 1752-0533
www.rsc.org/infochem
Registered Charity Number 207890
0511INFO - FEATURE_Clean Energy.indd 1
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electron
flow
Key
load
oxygen, O2
oxide ion, O2-
oxygen, O2
proton, H+
hydrogen, H2
water, H2O
excess, O2
cathode electrolyte
Generating electricity
in a solid oxide fuel cell
(SOFC)
anode
on efficient transport of lithium ions. ‘We are trying to
understand how this happens, on an atomic scale, so
we can maximize the lithium diffusion properties in new
materials,’ he says.
Li+ is a very small and light ion, making it difficult to
pinpoint in a crystal using standard experiments with
beams of x-rays or neutrons. So his group turns to
what he describes as ‘virtual microscopes’: powerful
supercomputers. ‘We know the physics and chemistry of
the structure and bonding, so these rules can be used to
calculate the forces within the material,’ he explains.
Pollution-free power
The efficient, powerful batteries
of the future will still need a clean
source of power to charge them
up. Solid oxide fuel cells (SOFCs)
generate electricity from hydrogen
and oxygen, and produce only
hydrogen, H2
water as a by-product. Like lithium
batteries, they too are made up
of an electrolyte sandwiched
between two electrodes. At the
cathode, oxygen gas picks up
water, H2O
electrons and is reduced to oxide
ions (O2-). The O2- ions then
migrate through the electrolyte
to the anode, where they react
with hydrogen gas to form water,
generating electrons that flow back
around an external circuit towards
the cathode providing useful electrical power that can be
used for homes and other buildings.
Cathode Reaction: O2 + 4e–  2O2–
Anode Reaction: 2H2 + 2O2–  2H2O + 4e–
Overall Cell Reaction: 2H2 + O2  2H2O
‘The key material in the fuel cell is the electrolyte,’ says
Dr Peter Slater of the University of Birmingham. They are
usually made from inorganic, crystalline materials which
need very high temperatures (above 800°C) to operate. Part
of the challenge for chemists is to find new materials that
can operate at lower temperatures (approx 500–700°C), so
they may become cheaper to run and more durable.
Peter Slater and Saiful Islam’s teams work together in
the search for ideal electrolyte materials. Saiful Islam’s
calculations reveal the exact path the ions take through
the materials Peter Slater’s team produce. ‘It’s important to
understand why certain materials conduct ions well, so we
can learn how to make better materials,’ he says.
Lithium ions (gold)
squeeze their way in a
curved path through the
LiFePO4 structure.
Saiful Islam’s group used the supercomputers’ numbercrunching to calculate the paths of lithium ions through
LiFePO4, with a surprising result: ‘We were the first to
predict that it does not go in a straight path, but in
curves,’ He said. This is very valuable information to help
understand how the electrode works and to develop new,
better materials.
2
He compares moving O2– through the electrolyte to trying
to get from one side of a packed concert hall to the other.
One way is to keep some ‘seats’ empty, so there is always a
space to move into. This can be done by introducing defects
where a few O2– ions are missing from the electrolyte
crystal. But the two teams have shown that some structures
also have room for the O2– to squeeze through gaps
between atoms, like someone pushing their way between
full rows of seats in the hall. Understanding how oxide ions
travel through electrolyte materials will help chemists to
design better fuel cells.
EMMA MCKENDRICK
Peter Slater’s team prepare new electrolyte compounds
with built-in channels between the atoms, to guide the O2–
through the material. They compare the materials they make
by measuring the conductance, or how fast the O2– flows
through. They also examine how the conductivity changes
at different temperatures, and in different atmospheres such
as air or hydrogen.
InfoChem
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Sun worshippers
SOFCs are a promising source of clean energy for buildings,
but they can’t provide for all our needs. Renewable
sources such as solar power will also have a role to play in
providing us with cheap, clean energy. More energy from
the sun strikes the surface of the earth in an hour than the
entire population of the planet needs in a year. Finding
materials to cheaply and efficiently turn more of this energy
into electricity is one of the biggest prizes for materials
chemists.
Line up!
Another important challenge for materials chemists in solar
power is to control how individual molecules combine with
Chemists working on
solar power materials can
control the wavelength
of light that is absorbed
by their molecules (and
create any colour of the
rainbow...)
WEI YOU
Solar cells are another kind of electrochemical sandwich.
This time, the light absorbing material forms the sandwich
Building up these blocks into long polymer chains means
they can easily be processed into thin sheets like clingfilm,
using processes that are already used for conventional
plastics. Plastic solar cells also do not require the same
purity as silicon in manufacturing, keeping costs lower.
These lightweight, flexible solar panels of the near future
would be highly portable, and could be built into many
more settings, offering free power even when we are on the
move.
filling between two electrodes. In commercially available
solar panels that you can see on the roofs of buildings, the
filling is semiconducting crystalline silicon. This material
works because its electrons are free to absorb the sun’s
light energy and move into a high-energy state. The excited
electrons carry a negative charge, while they leave behind
positive charges called ‘holes’. These opposite charges
can migrate to opposite electrodes, creating a voltage and
giving electrical power.
FERNANDO URIBE-ROMO AND WILLIAM DICHTEL
But manufacturing the silicon into solar cells is extremely
expensive because of the high purity needed: even one
atom of impurity can affect performance. However, the
electronic structure of semiconducting silicon can be
replicated using other semiconducting materials, such
as nanoparticles of cadmium telluride (CdTe), or even
polymers which contain a large number of alternating
single and double carbon to carbon bonds.
These materials work as solar cells but are not as efficient
as silicon and are currently much more expensive. The
task of making them better and cheaper ‘really goes to
fundamental control of their chemistry,’ said Professor
William Dichtel of Cornell University, New York. First, he
explains, they need to be designed to absorb a wide range
of wavelengths of light. Next, the material must have a way
to keep the electrons and holes apart so they can migrate
to the electrodes. ‘These are simple processes, but new
materials need to do them well,’ he said. He points out
that chemists can contribute a lot to the search for lightabsorbing materials, because they have been designing
coloured dyes for hundreds of years by using different
arrangements of C=C and C-C bonds.
Professor Wei You of the University of North Carolina
describes his group’s approach to this challenge as like
molecular lego, building with different structures that all
contain alternating C-C and C=C bonds. ‘You can make
whatever you like, putting the building blocks together,
understand what works well, and use this as a rationale to
make them better,’ he says.
one another in the solid state. This is as important to the
solar power material as the structures of the molecules
themselves. The charges generated by light have to be able
to hop easily from one molecule to the next on the way to
the electrodes. If the molecules are randomly ordered, the
charge hopping will be restricted.
William Dichtel’s group tackles this problem by designing
large three-dimensional networks of light absorbing units
joined by covalent bonds to lock them in place. Other
researchers try to make their molecules or polymer chains
line up in an orderly fashion by adding specific groups
to the molecules to control the forces between them. His
materials also have built-in spaces to accommodate a
second material that can accept the excited electrons and
channel them to the electrode.
‘It’s unclear which type of material will win out’ in the
race to replace silicon solar cells, he says. Polymers,
nanocrystals, or hybrid combinations are all vying to be
the best, but he believes they will all be useful: ‘there will
probably be enough niche applications suitable for all of
these technologies’.
Batteries
included
Join Saiful Islam for a
fascinating lecture
Watts new with clean
energy? Batteries
included on the evening
of 23 November in
London.
Saiful will illustrate how
scientists use structural
and modelling techniques
to help understand the
fascinating properties of
crystalline materials,
which are used to create
greener technologies.
rsc.org/mcdschools
Molecular building blocks
are designed to assemble
on electrodes into
ordered networks ideal for
transporting charge
InfoChem
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