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Developments in polymer-based materials for energy storage

Pennsylvania State University : 23 October, 2008  (Special Report)
Developments at Penn State, MIT and Delft are leading the way in fundamental research into new ways of managing electrical energy. Traditional ceramic materials have high weight and are very fragile, whereas mobile electronics need light weight electrical energy storage
Hybrid cars are a good target for ferroelectric polymer capacitors because for example, when coasting downhill, they convert mechanical energy to electricity and charge batteries for use at other times. Conventional batteries are often heavy and may not be able to deliver the power amounts needed for quick acceleration.

But the proliferation of solar, wind and even tidal electric generation and the rapid emergence of hybrid electric automobiles demands even more flexible and reliable methods of high-capacity electrical storage.

Now, a team of Penn State materials scientists is developing ferroelectric polymer-based capacitors that can deliver power more rapidly and are much lighter than conventional batteries. The National Science Foundation and the Office of Naval Research funded this research.

Qing Wang, associate professor of materials science and engineering at Penn State, is currently developing ferroelectric polymer-based capacitors that can deliver power more rapidly than conventional batteries. The polymer, with the addition of chlorotrifluoroethylene (a material commonly used as a refrigerant in cryogenic applications) had a very high dielectric permittivity at room temperature, producing a composite material with a large energy storage capacity.. Permittivity is a measure of how much charge is stored in a material for a given electric field and is an indicator of how effective a material will be when storing energy in a capacitor.

Wang and his research team reported at the 236th national American Chemical Society meeting in Philadelphia in two papers, on the development of power density tunable polymers and polymer ceramic nanocomposites as electric storage materials for capacitors. Currently, power conditioning is carried out by capacitors, but Wang believes that eventually properly tuned polymer capacitors could replace batteries.

They found that by altering the amounts of the various chemical components of the polymer, they could tune the dielectric property and energy density.

Wang and Li, report on a further modification of this ferroelectric polymer by adding nanoparticulate ceramics to further improve the energy density. Because ceramics often have higher permittivities than the polymers, they believed that combining polymers with high breakdown strength with ceramics of high permittivity would produce a composite material with a large energy storage capacity. Breakdown strength is a measure of the maximum electric field that an insulating material can withstand before it begins to conduct electricity. The higher the breakdown strength, the better a material is for a capacitor.

Unfortunately mixing nano particles of ceramic with polymers is not a simple action. The ceramic particles tend to clump and aggregate. If the two materials are not matched for electrical properties, their interface will breakdown at high electric fields and the ability of the composite to store energy will decrease, rather than increase. Wang and his team fine-tuned the dielectric particles to the polymer matrix by adding functionalised groups to the materials to match them. They also tried to control the mixing so that uniformly dispersed particles are spread through the matrix.

Dielectric polymers like the ones Wang creates cannot only be used as capacitors, but could also substitute for the dielectric silicon dioxide layer currently used in computers. Because polymers are processed at room temperature, they are easily fabricated and they are extremely flexible. Their use would open the way for flexible electronics applications, such as foldable screens and computers.

Because ceramics often have higher permittivities than the polymers, in a further modification of the ferroelectric polymer, the researchers added nanoparticulate ceramics to further improve the energy density. The process of mixing nanoparticles of ceramic with polymers is not simple, as the ceramic particles tend to clump and aggregate. If the two materials do not have matching electrical properties, their interface will break down at high electric fields and the ability of the composite to store energy will decrease rather than increase.

Wang and his team fine added functionalised groups to the materials to match the dielectric particles to the polymer matrix. Matching the permittivity and uniformly dispersing the ceramic nanoparticles is not easy. Both problems have to be tackled and solved at the same time for the material to have the desired characteristics.

Meanwhile, so-called virus-powered batteries created by researchers from MIT and conducting plastics developed by researchers from the Delft University of Technology in the Netherlands could together lead to a whole new way of making electronics.

Engineers at Massachusetts Institute of Technology (MIT) have developed a method to create and install tiny microbatteries which are the size of half a human cell. Using viruses to generate power, this new type of battery could one day power miniature devices by stamping the batteries onto the devicesí surface. The team has reported that they have successfully assembled and tested two critical components of the battery. They are currently working on a complete battery.

MIT professors Yet-Ming Chiang, Angela Belcher and Paula Hammond have jointly authored a paper detailing their virus-based method of creating and installing microbatteries by stamping them onto a variety of surfaces.

This is the first time that microcontact printing has been used to manufacture and arrange microbattery electrodes. Furthermore, it is also the first implementation of virus-based assembly in such a procedure. The technique itself does not require precise conditions (it can be performed at room temperature) and void of any expensive equipment.

The design of the battery comprises of two electrodes; an anode and cathode, which are divided by an electrolyte. At the present moment, the team at MIT have only developed the anode and the electrolyte. The process of creating the battery starts with a clear, rubbery material, where the team used a familiar technique labelled soft lithography to form a pattern of tiny posts either four or eight millionths of a meter in diameter. On top of these posts, several layers of two polymers, which act hand in hand as the solid electrolyte and battery separator, are deposited.

To form the anode, the engineers used viruses that are able to preferentially self-assemble a top the polymer layers on the posts. This idea was adapted from findings published in 2006, where Hammond, Belcher, Chiang, and colleagues reported on how to form the anode. Furthermore, the MIT team modified the virus' genes to enable it to produce protein coats that accumulate molecules of cobalt oxide to shape ultra-thin wires together with the anode.

The final step was to stamp each tiny post, which were covered with layers of electrolyte and the cobalt oxide anode. The testing of the current system was performed using lithium foil and by turning the stamp over, the transfer the electrolyte and anode to a platinum structure was made possible.

The latest findings are positive, with the team concluding that the resulting electrode arrays are able to display full electrochemical functionality. Future plans for this engineering team is to research more on using the battery stamp on curved surfaces and integrating the batteries with biological organisms.

The Americium Power Source is a new battery-like device developed in Israel, which includes a core of americium 242, which generates a very efficient fission reaction. Additional information on this virus powered battery can be obtained at MITís website.

In other developments, Sonyís sugar-powered batteries generate electricity from carbohydrates (sugar). The device was developed based on the same power generation principles found in living organisms.

Alberto Morpurgo and his team of researchers at Delft University of Technology in the Netherlands recently attached a micrometer-thick crystal of an organic polymer to a similarly thin organic crystal of a second polymer creating a thin but strongly conducting channel along the junction that acts like a metal. The discovery could lead to a whole new way of making electronics from non-metallic materials, and even new superconductors.

The thin, flexible crystals which conform to each othersí shape and stick together due to van der Waals forces are both electrical insulators. Morpurgo's team found that a 2nm thick strip along the interface between the two crystals conducts electricity as well as a metal. While it was known that a blend of the two materials could conduct electricity, but it does so relatively poorly.

The two materials are physically unchanged when laid side-by-side, but the way electrons behave is subtly altered along the interface where the different materials are in close proximity. Because the two plastics become more insulating at lower temperatures, during testing the combined materials were cooled down, expecting the odd behaviour to disappear. Surprisingly, the interface became a better conductor, just as metals offer less resistance to electricity when they are cooled.

While electrons inside each of the materials are usually unable to travel freely Morpurgo thinks that molecules at the interface are able to jump over to vacant spaces known as 'holes', so that they can travel freely, allowing current to flow. Such an electron-hole system is really something new and it may have interesting electronic properties.

This new interface conducts electricity much better than standard semiconductors and has the power to create new effects, from magnetism to superconductivity.

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