Of all the science reporting nowadays on advances in electrical energy storage, the overwhelming majority is devoted to batteries. Capacitors, which have been known for centuries and are key to numerous applications and commercial products, tend to draw little press attention. Even so, researchers are making substantial progress in boosting the capacity of these charge-storage devices by designing novel organic polymers, organic-inorganic nanocomposites, and other unconventional capacitor materials with specialized properties tailored for storing energy.
Capacitors are ubiquitous. They are found in consumer electronics, electric-grid utility stations, pulse-driven instruments such as lasers and cardiac defibrillators, hybrid electric vehicles, and many other applications. One of the simplest forms of capacitors, consisting of a pair of electrically conducting plates separated by an insulating material referred to as a dielectric, was studied in the early days of electrical research by notables such as Benjamin Franklin. Applying a voltage to the conductors causes an electric field to develop across the dielectric medium, which causes positive charge to collect on one plate and negative charge on the other plate.
The ability of capacitors to store energy depends on how well the dielectric can stabilize the charge separation, which depends on its intrinsic properties and on the way the capacitor is used. According to J. Paul Armistead, a program officer at the Office of Naval Research (ONR), the maximum energy that can be stored in a capacitor is proportional to the permittivity (or dielectric constant), which is a type of resistance that describes the interaction of the dielectric with an electric field, and the square of the breakdown field, which is determined by the highest voltage the dielectric can tolerate before it allows the capacitor to discharge or short-circuit.
“The Navy has major needs for advanced energy storage technology,” Armistead says, especially for pulse-power systems that can rapidly deliver large bursts of energy. Such power systems are needed to run electric ships or drive the electromagnetic launch systems that propel airplanes from aircraft carriers. To increase the energy that can be stored in capacitors for use in such power systems, ONR is supporting research aimed at finding materials with greater permittivity and higher breakdown fields than those associated with the conventional materials found in today’s commercial capacitors.
A variety of materials have been used as dielectrics. The most common ones today are ceramics, such as barium titanate and other metal oxides, and polymers, often polypropylene and polystyrene. Ceramics tend to have high permittivity but low breakdown strength, according to Michele L. Anderson, an ONR program officer. They also tend to be fragile, bulky, and heavy. In contrast, she says, conventional polymers are generally endowed with high breakdown strength, but they have low permittivity values.
One advantage of polymer-based capacitors, Armistead says, is that in the event of a dielectric breakdown, “they fail gracefully,” meaning they just lose some capacitance, as opposed to cracking and failing catastrophically, as often occurs with ceramic devices. Those characteristics and others have motivated materials chemists to search for new types of lightweight and flexible organic-based compounds with useful dielectric properties.
At the University of Michigan, Ann Arbor, for example, chemistry professor Theodore Goodson III, then-graduate student Meng Guo, and coworkers developed a one-pot method for synthesizing novel hyperbranched polymers. Initially the team, which includes colleagues at Tokyo Institute of Technology, was investigating the materials as part of a study in optoelectronics, but the researchers found that one family of hyperbranched copper phthalocyanine polymers, HBCuPc, exhibited a dielectric constant greater than 46. Traditional polymers have much smaller dielectric constants (~3). The dendrimeric compound is also thermally stable and has other useful traits including a high breakdown voltage and low dielectric loss, which is a measure of resistance to deterioration (J. Phys. Chem. B, DOI: 10.1021/jp205428j). These compounds and the know-how needed to fabricate flexible energy storage devices form the cornerstone of Wolverine Energy Solutions & Technology, a start-up company recently launched by Goodson, devoted to developing these materials for military, medical, and automotive applications.
Another approach to designing new dielectrics is based on forming hybrid materials that combine the strengths of polymers and ceramics. In years past, researchers had tried various methods for physically blending metal oxide particles into polymer matrices, according to Armistead. But the particles were not uniformly distributed, and the rough hybrid products often showed no enhancement in dielectric properties. Nowadays, researchers have attained much finer control over the hybridization process by using chemical methods that yield nanoscale composites.
In one such example, researchers working with materials science professor Qing Wang at Pennsylvania State University take advantage of phosphonic acid end groups on polyvinylidene fluoride (PVDF) molecular chains to covalently attach zirconium oxide nanoparticles. As the team reported in Chemistry of Materials in 2010, the direct-coupling method leads to a highly uniform distribution of ceramic particles throughout the PVDF matrix, which has an inherently high dielectric constant as a result of the dipole moment (charge separation) associated with C–F bonds (DOI: 10.1021/cm101614p). The team measured stored energy (energy density) values that exceed those reported for traditional polymer-ceramic composites and under some conditions represent a 60% increase relative to the neat polymer.
Meanwhile, Tobin J. Marks’s chemistry research group at Northwestern University prepares high-permittivity and low-dielectric-loss nanocomposites by following a different tack. The team treats agglomeration-prone nanoparticles of barium titanate (and in separate experiments, zirconium oxide) with an aluminum-based polymerization cocatalyst, thereby breaking apart the clumps of particles and forming alumina-metal oxide core-shell nanoparticles. Then, after depositing metallocene catalyst molecules on the alumina shells, they treat the particles with propylene. That sequence triggers polymerization directly on the particle surfaces and ensures that the particles end up well dispersed in the matrix that grows around them (Chem. Mater., DOI: 10.1021/cm1009493).
When it comes to materials for increasing energy storage in capacitors, there’s plenty of opportunity to think outside the box, Marks says. “Let’s wipe the blackboard clean and use our imaginations, synthetic talents, and materials processing skills to make new generations of advanced materials.”