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.

Time-varying energy profiles of renewable sources, electric vehicles, end user demands, portable devices, novel military applications and more, require high power as well as high energy density in storage systems. Electrochemical capacitors (EECs), with higher power than batteries, benefit from nanostructured geometries that further increase their power capability. Nanostructured electrostatic capacitors (ESCs) are known to have much higher power capability than EECs, though lower energy density. The physical and chemical mechanisms by which EECs and ESCs function in charge/discharge are completely different, as are their electrical specifications and constraints. We consider for the first time how the contrasting characters of electrochemical and electrostatic nanostructured capacitors might be combined in a heterogeneous hybrid circuit to achieve better power and energy performance than either device alone. While the benefits of hybrid circuits have been previously considered for electrochemical devices -i.e., battery and electrochemical capacitorthis perspective article demonstrates for the first time that hybrid storage circuits of heterogeneous devices can exploit the very high power of electrostatic devices, in concert with electrochemical devices. Using response surface models from our own experimental results with nanostructured ESC and ECC devices, we develop a hybrid simulation model combining the two types of devices, recognizing the intrinsic nonlinearities and constraints of each. We demonstrate that charge capture by the ESC and subsequent rapid transfer to the ECC compensates for the lower power capability of the ECC while avoiding energy loss by ESC leakage currents. Although more sophisticated models and simulations are warranted in the future, these initial results underscore the opportunity that ESC devices and hybrid circuits offer for storage applications which require ultrahigh power performance. Broader contextHigh to ultra-high power energy storage, along with suitably large energy density, is a critical focus of materials and systems research for renewable energy integration, power levelling, transportation, wireless and portable electronics applications and more. While new materials and new material combinations have been able to achieve signicant improvements, combinations of different devices with complementary performance parameters may be able to compensate for some of the limitations of each individual storage device type. In order to take advantage of recent research advances synergistic combinations of devices are needed which can manage widely varying mechanisms of charge storage, non-linearities, and different failure regimes. While hybrid combinations of batteries and supercapacitorsboth electrochemical deviceshave been studied before, no combination of an electrochemical and an electrostatic device has been reported. Its fundamental energy-power characteristics and prospective applications are explored and discussed for high to ultra-high power energy storage device in this p...

Electrical characteristics of 25 nm Al-doped TiO2 (ATO) dielectric films are investigated in an effort to access the benefits of TiO2’s high dielectric constant (κ) while minimizing leakage current as needed for nanocapacitor applications. Al-doped films with 0-3.9 at. % Al were deposited using atomic layer deposition (ALD). As-deposited films of all compositions were amorphous and had poor electrical performance. Annealing at 600 °C was implemented to modify the film structure and to increase electrical performance. Raman spectroscopy monitored phase changes as a result of annealing. The Raman-measured Eg mode provides a clear signature of the anatase crystallization structure and showed the rapid formation of crystalline anatase in all films. The electrical performance of annealed films was significantly improved for films containing Al. Incorporation of few percent Al and annealing created a dramatic drop in leakage current from 10−3–10−4 to 10−7 A/cm2, nearly that of pure Al2O3. Comparing features of the Raman Eg mode with Al-doping displays a strong correlation to leakage currents. Trends observed in the Raman lineshape which relate to microstructural variances are discussed. Raman spectroscopy thus provides a measure of structural changes in ATO films which correlate strongly with its electrical characteristics as doping and annealing are employed to optimize the properties of the high-κ films.