(537g) Energy Dense Electrochemical Capacitors Via Advanced Nanomanufacturing and Designer Electrolytes

There is a growing demand for high cycle life, high rate, and high power energy storage devices, both on the micro-scale for microelectromechanical systems as well as on the macro-scale for large kilovolt-level power delivery applications. While batteries have been shown to provide a high energy density, they have very limited power and cycle-life capabilities. Electrochemical capacitors provide the greatest potential to meet the power and cycle life demands for new energy storage devices. In particular, electrochemical double-layer capacitors (EDLCs) or ultracapacitors are energy storage devices that show the most promise for maintaining a high power density and high cycle life that is achievable with electrostatic capacitors while providing a path to much greater energy density storage. While EDLCs have fast frequency response times and are capable of providing power for longer periods of time than conventional capacitors, their energy density is still much lower than that of batteries. The main focus of our current ultracapacitor research is on increasing the energy density through the use of advanced nanostructured electrode materials and new highly tailored electrolytes.

Since capacitance or energy storage is directly proportional to the accessible surface area of the electrode, we have developed new electrode structures with a tunable, highly controlled porosity and a very high surface area that is easily accessible to the electrolyte. By using a novel and scalable approach to fabricate carbon nanotubes (CNTs), we have developed a high surface area electrode with highly aligned vertically-oriented carbon nanotubes with a greater areal tube density than typical carpet-grown CNTs, which significantly improves the available surface area for charge storage. In addition, the fabrication approach allows for tuning of the electrode pore size to the specific electrolyte. Furthermore, we have preliminary evidence of a capacitive enhancement phenomenon, further increasing the capacitance, which can be attributed to the confinement of the electrolyte within the pore that results from the tailoring of the pore size to the electrolyte molecular size. While this is not well understood at this time, we are investigating the level of capacitive enhancement that can be achieved with optimization of the pore size.

Ionic liquids are popular for electronic applications requiring electrolytes with large accessible voltage window, and properties that can be tailored to a wide range of applications. Additionally, since they are nonflammable and have negligible vapor pressure, ionic liquids are ideal for micro- and on-chip devices. Thus, designer electrolytes based on pure and mixed ionic liquids can be used with the electrode pore size tailored specifically to accommodate the chosen electrolyte. The use of ionic liquid electrolytes also allows operation at higher cell voltages than the conventional aqueous or organic solution-based capacitors. Larger operation voltage further enhances the power and energy density. By combining and optimizing the interaction between these two technologies, namely Mainstream’s high surface area and anisotropic nanostructured carbon electrodes and designer ionic liquid electrolyte, we are able to produce an energy storage technology which combines the benefits of high energy density and high power density. This talk will detail electrode development, electrolyte selection, and application in both large plate capacitors and chip-scale capacitors.

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This work is supported by the US Air Force under Contract No. FA9302-17-C-0001.The views, opinions and/or findings contained in this report are those of the author(s) and should not be construed as an official Air Force position, policy or decision unless so designated by other documentation.