Batteries, particularly lithium-ion, store large amounts of energy but have more difficulty with high power or fast recharge. Electrostatic capacitors are characterized by high power but energy storage is limited because only surface charge is used. Conventional electrochemical supercapacitors store charge in electric double layers or in faradic reactions, permitting larger energy density storage on the electrode surfaces. However, power density is more limited in these devices because of the requirement for mass transport of ions and/or redox reactions.

The Maryland/KAIST research team’s new devices are electrostatic nanocapacitors which increase the energy storage density of such devices by a factor of 10 over that of commercially available devices without sacrificing the high power they traditionally characteristically offer. This advance brings electrostatic devices to a performance level competitive with electrochemical supercapacitors and introduces a new player into the field of candidates for next-generation electrical energy storage.

For the nanocapacitors described in this Letter, the nanostructure significantly enhances capacitance density. The nanocapacitors demonstrate the high power (up to ~1 x 10⁶ W kg^-1) typical of electrostatic capacitors while achieving the much higher energy density (~0.7 Wh kg^-1) characteristic of electrochemical supercapacitors. As a result, electrostatic nanocapacitors are attractive for high-burst-power applications requiring the energy density of supercapacitors.

—Banerjee et al. (2009)

The work was led by Professor Gary Rubloff, director of the University of Maryland’s NanoCenter and his collaborator Professor Sang Bok Lee, associate professor in the Department of Chemistry and Biochemistry at the College of Chemical and Life Sciences (University of Maryland) and WCU (World Class University Program) professor at KAIST.

The fabrication strategy makes use of AAO nanopore templates in combination with metal–insulator–metal (MIM) structures deposited in the nanopores by ALD. The anodization process produces an ultrahigh density (~1 x 10¹⁰ cm^-2) of hexagonally arranged, uniform, self-assembled nanopores in AAO film on Al tens of micrometers deep. MIM capacitor structures are then formed by the application of successive ALD layers of metal (TiN, titanium nitride), insulator (Al₂O₃) and metal (also TiN) with atomic layer thickness control within the AAO nanopores.

Lee and Rubloff emphasize that they are developing the technology for mass production as layers of devices that could look like thin panels, similar to solar panels or the flat panel displays we see everywhere, manufactured at low cost. Multiple energy storage panels would be stacked together inside a car battery system or solar panel. In the longer run, they foresee the same nanotechnologies providing new energy capture technology (solar, thermoelectric) that could be fully integrated with storage devices in manufacturing.

The researchers suggest that using of high-κ dielectrics such as HfO₂ and perhaps higher-aspect-ratio AAO pores may enable higher capacitance values, further boosting the performance of the new devices.

Resources

• Parag Banerjee, Israel Perez, Laurent Henn-Lecordier, Sang Bok Lee and Gary W. Rubloff (2009) Nanotubular metal–insulator–metal capacitor arrays for energy storage. Nature Nanotechnology Published online: 15 March 2009 | doi: 10.1038/nnano.2009.37