Researchers at North Carolina
State University
have discovered the means by which a polymer known as PVDF, polyvinylidene
fluoride, enables capacitors to store and release large amounts of energy
quickly. Their findings could lead to much more powerful and efficient electric
cars.
Capacitors are like batteries in that they store and release energy.
However, capacitors use separated electrical charges, rather than chemical
reactions, to store energy. The charged particles enable energy to be stored
and released very quickly. Imagine an electric vehicle that can accelerate from
zero to 60 mph at the same rate as a gasoline-powered sports car. There are no
batteries that can power that type of acceleration because they release their
energy too slowly. Capacitors, however, could be up to the job—if they
contained the right materials.
NC State physicist Vivek Ranjan, PhD, had previously found that capacitors
which contained the polymer PVDF in combination with another polymer called
CTFE were able to store up to seven times more energy than those currently in
use.
“We knew that this material makes an efficient capacitor, but wanted to
understand the mechanism behind its storage capabilities,” Ranjan says.
In research published in Physical
Review Letters, Ranjan; fellow NC State physicist Jerzy Bernholc, PhD; and Marco
Buongiorno-Nardelli, PhD, from the University
of North Texas, did
computer simulations to see how the atomic structure within the polymer changed
when an electric field was applied. Applying an electric field to the polymer
causes atoms within it to polarize, which enables the capacitor to store and
release energy quickly. They found that when an electrical field was applied to
the PVDF mixture, the atoms performed a synchronized dance, flipping from a
non-polar to a polar state simultaneously, and requiring a very small
electrical charge to do so.
“Usually when materials change from a polar to non-polar state it’s a chain
reaction—starting in one place and then moving outward,” Ranjan explains. “In
terms of creating an efficient capacitor, this type of movement doesn’t work
well—it requires a large amount of energy to get the atoms to switch phases,
and you don’t get out much more energy than you put into the system.
“In the case of the PVDF mixture, the atoms change their state all at once,
which means that you get a large amount of energy out of the system at very
little cost in terms of what you need to put into it. Hopefully these findings
will bring us even closer to developing capacitors that will give electric
vehicles the same acceleration capabilities as gasoline engines.”
