Improving capacitor storage capacity

Dr. Neil Canter, Contributing Editor | TLT Tech Beat January 2016

A new dielectric capacitor combines superior energy storage with a higher breakdown voltage.

KEY CONCEPTS
• Capacitors currently in use do not adequately store energy.
• A new capacitor based on nanoporous anodic aluminum oxide exhibits significantly higher energy storage than any other dielectric capacitor.
• This new capacitor also displays a higher breakdown voltage, which is another feature in current capacitors that needs to be increased.

IN MOVING TO ALTERNATIVE ENERGY SYSTEMS, DELIVERY OF ENERGY and storage of energy are two very important parameters. A good deal of attention has been paid to developing batteries that typically exhibit good storage capacity but are limited from the standpoint of energy delivery.

But there are alternatives to batteries. In a previous TLT article, applications for the flywheel are discussed (1). The flywheel is a mechanical device that stores rotational energy as a rotor spins on its axis. Applications where flywheels are used include a lightweight, hybrid electric transit bus, a train locomotive system and in-mobile gantry cranes used to move shipping containers in ports.

Another energy storage and delivery device is the capacitor. Bingqing Wei, professor of mechanical engineering at the University of Delaware in Newark, Del., says, “Traditionally there are two types of capacitors known as the dielectric capacitor and the supercapacitor.”

A dielectric capacitor is composed of an insulator material such as glass or a plastic sandwiched by two metal plates. The supercapacitor functions in a similar manner except that an electrochemical double layer is formed between the electrode and electrolyte.

Wei says, “The most common problem for capacitors is their ability to store energy. This parameter is known as capacitance, which is directly proportional to the dielectric constant (relative permittivity) of the dielectric materials and the surface area of the metal plates and is inversely proportional to the distance between the two electrodes.”

Efforts to increase the capacitance have involved selecting materials with a higher dielectric constant, increasing the surface area of the metallic electrodes and decreasing the distance between the electrodes. One other factor that also has been a problem with capacitors is increasing the breakdown voltage that represents the voltage at which a short circuit will form across the dielectric material. It is desirable to have the breakdown voltage much higher than the working voltage.

A new type of dielectric capacitor has now been developed that combines superior energy storage with a higher breakdown voltage.

3D INTERDIGITAL ELECTRODE
Wei worked with a group of researchers in developing a new type of dielectric capacitor based on nanoporous anodic aluminum oxide (AAO). He says, “We used aluminum oxide because it exhibits good insulation, a reasonably large degree of permittivity and has the added advantage of displaying a very high breakdown voltage.”

Anodization of high purity aluminum foil is accomplished through treatment with an oxalic acid solution at 10 C under 50 volts for six hours. This step is followed by mild anodization, hard anodization and chemical etching to form two sets of diameter pores. Carbon nanotube (CNT) arrays are then grown inside the two sets of pores in the AAO membrane through the pyrolysis of acetylene for five hours at a temperature of 550-600 C inside a tube furnace. The carbon nanotubes are present as the electrodes. 

As shown in Figure 4, the result is the preparation of a 3D interdigital electrode structure. In this arrangement each large diameter CNT pore is surrounded by six small diameter CNT pores in a hexagonal arrangement. Wei says, “The interdigital electrode is similar to interwoven fingers between two hands with gloves. The result is that the distance between the two electrodes is reduced to a large extend leading to a higher level of capacitance.”


Figure 4. A new capacitor based on a 3D interdigital electrode structure produces greater capacitance through maximizing the interface and minimizing the distance between the two electrodes. (Figure courtesy of the University of Delaware.)

When a potential is applied, a static electric field forms across the dielectric enabling negative charges to become present on the large diameter CNT pores and positive charges to be present on the small diameter CNT pores or the reverse (see Figure 4).

Wei says, “We determined the 3D interdigital electrode structure through the use of scanning electron microscopy and evaluated the performance by using cyclic voltammetry.”

Capacitance measurements were made using an inductance capacitance resistance meter at 100 Hz. For a six-micron thick nanoporous AAO membrane, a capacitance of 47 microfarad per square centimeter was achieved. Under the same conditions, the breakdown voltage is 15 volts that corresponds to an electric field of 7.5 megavolts per centimeter. 

The researchers believe that the energy density of the capacitor can be maximized to two watt-hours per kilogram through optimization of the process used to prepare the 3D interdigital electrodes. Wei says, “This result is significantly higher than any other dielectric capacitor and approaches the performance of supercapacitors.”

This is the first time that 3D interdigital electrodes have been used in preparing an energy storage device. Future work will involve scaling up the device so that it can be used commercially and for optimizing performance. 

Wei says, “One aspect that may help us improve performance is the use of a higher quality aluminum oxide starting material. We are also going to be evaluating the durability of the capacitor by running charge/discharge cycles.”

Additional information can be found in a recent article (2) or by contacting Wei at weib@udel.edu. 

REFERENCES
1. Canter, N. (2009), “Flywheels: Improving energy efficiency,” TLT, 65 (7), pp. 12-13.
2. Han, F., Meng, G., Zhou, F., Song, L., Li, X., Hu, X., Zhu, X., Wu, B. and Wei, B. (2015), “Dielectric capacitors with three-dimensional nanoscale interdigital electrodes for energy storage,” Science Advances, 1 (9), e1500605.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.