Researchers at North Carolina State University have identified the origin of the nonlinear dielectric response and high energy density of polyvinylidene-fluoride-based (PVDF) polymers enabling capacitors to store and release large amounts of energy quickly.
NC State physicist Dr. Vivek Ranjan had previously found that capacitors which contained PVDF in combination with the polymer chlorotrifluoroethylene (CTFE) were able to store up to seven times more energy than those currently in use.
In research published in the journal Physical Review Letters, Ranjan and colleagues used first-principles 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 flipped from a non-polar to a polar state simultaneously, 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. 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.—Vivek Ranjan
V. Ranjan, M. Buongiorno Nardelli and J. Bernholc (2012) Electric Field Induced Phase Transitions in Polymers: a Novel Mechanism for High Speed Energy Storage. Physical Review Letters doi: 10.1103/PhysRevLett.108.087802