Nanostructured capacitors for renewable energy storage

Fossil fuels are a convenient, portable, and high-density source of energy that can only be displaced by a cost-effective alternative. However, any new energy technology requires storage capability. In fact, energy storage is a key bottleneck slowing the proliferation of large-scale power generation from renewable sources such as wind and solar, which face geographical restrictions and intermittent output. To keep pace with growing world power demands, which are approaching 20 terawatts, we need a transformative change in our approach to storage, including the materials required.

Capacitive energy storage devices could be used to hold electrical energy generated by wind turbines and solar panels.¹ Nanostructured capacitors have high power density and fast charge/discharge times. Apart from renewable energy production, they could also find use in electronics, national defense and other applications.

The idea of storing energy in dielectrics is as old as dielectric materials themselves, but the approach has not been successful due to limitations in material properties.^{2, 3} Just like the multilayered capacitors used in power electronics, nanostructured devices are optimized for energy density. In such devices, extremely thin dielectric layers over large areas are desirable to get the highest capacitance.

We use a nanostructured composite of ceramic and glass in a geometry resembling a multilayer capacitor: see Figure 1. High volumetric and gravimetric energy density storage in capacitors requires novel materials that will withstand high electric fields (∼1MV/cm) and possess an extremely high dielectric constant (≤100, 000). Though these metrics have been individually exceeded in small single crystals, thin films, and bulk ceramics, the challenge is to realize the required density in a cost effective way in a prototype test device. To accomplish this goal, we use a perovskite ferroelectric (e.g., lead magnesium niobate) nanopowder, and low-melting, alkali-free glass for the composite layers: see Figure 2. Even though the smallest spatial dimension of our prototype device is not on the nanometer scale, the material features that we are tuning to achieve the highest possible energy densities are.

Figure 1. Schematic diagram of a proposed capacitor for high density energy and power storage.

Figure 2. Cross section SEM micrograph of a prototype nanostructured capacitor showing the individual dielectric/glass composite and metal layers. PMN: Lead magnesium niobate. Ag: Silver. Al₂O₃: Aluminum oxide.

We are now optimizing the properties of the ceramic and glass materials we use to make the nanocomposite. The material has to be processed into thin, uniform layers to form a capacitor. We plan to optimize the design of the composite and the fabrication process for both performance and future commercialization. The eventual goal is to extend this technology from the nanoscale to systems having megawatts of power, and to do this in a commercially viable way.

This work was supported by the U.S. National Science Foundation under the Emerging Frontiers for Research and Innovation RESTOR Program (for renewable energy storage), overseen by the NSF Engineering Directorate.