Nanoengineering strategies for metal-insulator-metal electrostatic nanocapacitors.

Nanostructures can improve the performance of electrical energy storage devices. Recently, metal-insulator-metal (MIM) electrostatic capacitors fabricated in a three-dimensional cylindrical nanotemplate of anodized aluminum oxide (AAO) porous film have shown profound increase in device capacitance (100× or more) over planar structures. However, inherent asperities at the top of the nanostructure template cause locally high field strengths and lead to low breakdown voltage. This severely limits the usable voltage, the associated energy density (1/2 CV(2)), and thus the operational charge-discharge window of the device. We describe an electrochemical technique, complementary to the self-assembled template pore formation process in the AAO film, that provides nanoengineered topographies with significantly reduced local electric field concentrations, enabling breakdown fields up to 2.5× higher (to >10 MV/cm) while reducing leakage current densities by 1 order of magnitude (to ∼10(-10) A/cm(2)). In addition, we consider and optimize the AAO template and nanopore dimensions, increasing the capacitance per planar unit area by another 20%. As a result, the MIM nanocapacitor devices achieve an energy density of ∼1.5 Wh/kg--the highest reported.