Nanostructured Metal Oxide and Composite Electrodes for Use in Ultracapacitors
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1113-F04-06
Nanostructured Metal Oxide and Composite Electrodes for Use in Ultracapacitors Michael T. Brumbach1, Todd M. Alam1, Paul G. Kotula2, Bonnie B. McKenzie2, Bruce C. Bunker1 1 Electronic and Nanostructured Materials, Sandia National Laboratories, Albuquerque, NM 87185, U.S.A. 2 Materials Characterization, Sandia National Laboratories, Albuquerque, NM 87185, U.S.A. ABSTRACT Maximizing power and energy densities of ultracapacitors requires configuring redoxactive materials in specific architectures that: 1) maximize electrolyte-electrode contact area, 2) minimize transport distances for both electrons and charge compensating species, and 3) minimize transport barriers. We have developed a simple solution-based method, using an organic template, that enables us to introduce hierarchical porosity in ruthenium oxide down to the nano-scale by controlling the oxidative crystal growth of RuO2. The high capacitances of the resulting nanostructured electrodes were found to be comparable to hydrous ruthenium oxide formed under dramatically different conditions. Materials characterization reveals that the organic template directs structure formation and promotes hydroxyl retention. INTRODUCTION Electrochemical capacitors are defined by their ability to achieve a combination of power and energy densities not available to either dielectric capacitors or batteries. Batteries can achieve high energy densities, while capacitors, with very low energy densities, are able to deliver high power.[1] Ultracapacitors, therefore, present the opportunity to move towards devices, and device architectures, for both high power and high energy for use in power regulation from renewable energy resources, for applications such as an all-electric vehicle, or as sources for pulsed-power. Electrochemical capacitors operate in modes between that of a dielectric capacitor, where charge is stored physically, and that of a battery, where charge is stored chemically via Faradaic reactions.[1,2] High surface area carbon electrodes dominate the area of power storage in electrochemical capacitors, a.k.a. supercapacitors, where charge is stored physically in the electrical double layer between the charged surface of the carbon electrode and an electrolyte solution. Alternatively, redox active electrochemical capacitors, a.k.a. pseudocapacitors, are exemplified by the charge storage properties of ruthenium oxide where ruthenium reduction/oxidation is accompanied by proton insertion/desorption. Despite its high cost, ruthenium oxide presents such remarkable pseudocapacitive behavior that attempts to understand this material and increase its utilization efficiency are important for further development of charge storage technologies. Connectivity to the electrolyte is critically important for both types of capacitors and a high surface area, hierarchically porous nanostructure is ideal. The charge storage properties of ruthenium oxides have been shown to be strongly affected by the degree of material crystallinity and the associated hydroxyl/water cont
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