http://phys.org/news/2015-06-technique-ions-supercapacitor.html
New technique for 'seeing' ions at work in a supercapacitor
[2015-06-21]
Researchers from the University of Cambridge, together with French
collaborators based in Toulouse, have developed a new method to see
inside
battery-like devices known as supercapacitors at the atomic level. The
new
method could be used in order to optimise and improve the devices for
real-world applications, including electric cars, where they can be
used
alongside batteries to enhance a vehicle's performance.
By using a combination of nuclear magnetic resonance (NMR) spectroscopy
and
tiny scales sensitive enough to detect changes in mass of a millionth
of a
gram, the researchers were able to visualise how ions move around in a
supercapacitor. They found that while charging, different processes are
at
work in the two identical pieces of carbon 'sponge' which function as
the
electrodes in these devices, in contrast to earlier computer
simulations.
The results are published today (22 June) in the journal Nature
Materials.
Supercapacitors are used in applications where quick charging and power
delivery are important, such as regenerative braking in trains and
buses,
elevators and cranes. They are also used in flashes in mobile phones
and as
a complementary technology to batteries in order to boost performance.
For
example, when placed alongside a battery in an electric car, a
supercapacitor is useful when a short burst of power is required, such
as
when overtaking another car, with the battery providing the steady
power for
highway driving.
"Supercapacitors perform a similar function to batteries but at a much
higher power—they charge and discharge very quickly," said Dr. John
Griffin,
a postdoctoral researcher in the Department of Chemistry, and the
paper's
lead author. "They're much better at absorbing charge than batteries,
but
since they have much lower density, they hold far less of that charge,
so
they're not yet a viable alternative for many applications. Being able
to
see what's going on inside these devices will help us to control their
properties, which could help to make them smaller and cheaper, and that
might make them a high-power alternative to batteries."
At its most basic level, a battery is made of two metal electrodes (an
anode
and a cathode) with some sort of solution between them (electrolyte).
When
the battery is charged, electrolyte ions are stored in the anode. As
the
battery discharges, electrolyte ions leave the anode and move across
the
battery to chemically react with the cathode. The electrons necessary
for
this reaction travel through the external circuit, generating an
electric
current.
A supercapacitor is similar to a battery in that it can generate and
store
electric current, but unlike a battery, the storage and release of
energy
does not involve chemical reactions: instead, positive and negative
electrolyte ions simply 'stick' to the surfaces of the electrodes when
the
supercapacitor is being charged. When a supercapacitor is being
discharged
to power a device, the ions can easily 'hop' off the surface and move
back
into the electrolyte.
The reason why supercapacitors charge and discharge so much faster is
that
the 'sticking' and 'hopping' processes happen much faster than the
chemical
reactions at work in a battery.
"To increase the area for ions to stick to, we fill the carbon
electrode
with tiny holes, like a carbon sponge," said Griffin. "But it's hard to
know
what the ions are doing inside the holes within the electrode—we don't
know
exactly what happens when they interact with the surface."
In the new study, the researchers used NMR to look inside functioning
supercapacitor devices to see how they charge and store energy. They
also
used a type of tiny weighing scale called an electrochemical quartz
crystal
microbalance (EQCM) to measure changes in mass as little as a millionth
of a
gram.
By taking the two sets of information and putting them together, the
researchers were able to build a precise picture of what happens inside
a
supercapacitor while it charges.
"In a battery, the two electrodes are different materials, so different
processes are at work," said Griffin. "In a supercapacitor, the two
electrodes are made of the same porous carbon sponge, so you'd think
the
same process would take place at both—but it turns out the charge
storage
process in real devices is more complicated than we previously thought.
Previous theories had been made by computer simulations—no-one's
observed
this in 'real life' before."
What the experiments showed is that the two electrodes behave
differently.
In the negative electrode, there is the expected 'sticking' process and
the
positive ions are attracted to the surface as the supercapacitor
charges.
But in the positive electrode, an ion 'exchange' happens, as negative
ions
are attracted to the surface, while at the same time, positive ions are
repelled away from the surface.
Additionally, the EQCM was used to detect tiny changes in the weight of
the
electrode as ions enter and leave. This enabled the researchers to show
that
solvent molecules also accompany the ions into the electrode as it
charges.
"We can now accurately count the number of ions involved in the charge
storage process and see in detail exactly how the energy is stored,"
said
Griffin. "In the future we can look at how changing the size of the
holes in
the electrode and the ion properties changes the charging mechanism.
This
way, we can tailor the properties of both components to maximise the
amount
of energy that is stored."
The next step, said Professor Clare P. Grey, the senior author on the
paper,
'is to use this new approach to understand why different ions behave
differently on charging, an ultimately design systems with much higher
capacitances."
Explore further: Beyond the lithium ion—a significant step toward a
better
performing battery
More information: In situ NMR and electrochemical quartz crystal
microbalance techniques reveal the structure of the electrical double
layer
in supercapacitors, DOI: 10.1038/nmat4318
http://phys.org/journals/nature-materials/
Journal reference: Nature Materials search and more info website
http://phys.org/partners/university-of-cambridge/
Provided by University of Cambridge search and more info website
[© phys.org]
http://www.nanowerk.com/nanotechnology-news/newsid=40538.php
New technique for 'seeing' ions at work in a supercapacitor
[2015-06-23] The new method could be used in order to optimise and
improve
the devices for real-world applications, including electric cars, where
they
can be used ...
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