Development of Nano-Micro-Macro-Structured Porous Nickel Electrodes for use in Supercapacitors
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Development of Nano-Micro-Macro-Structured Porous Nickel Electrodes for use in Supercapacitors Jason Manning1,2, Roger Campbell1, Renee Woo1, Brenda O'Neil2,3, Leigh McKenzie2,3, Martin Gerard Bakker1,2, Velma Montoya4, and George Havrilla4 1 Chemistry, The University of Alabama, Box 870336, Tuscaloosa, AL, 35487-0336 2 Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, AL, 35487 3 Admiral Moorer Middle School, Eufaula, AL, 36027 4 Los Alamos National Laboratory, Los Alamos, NM, 87545 ABSTRACT The extremely high surface areas required for supercapacitors has limited the use of metal based electrodes, despite the other advantages such electrodes might have. Self-assembling surfactants and block co-polymers can be used as templates to produce nanostructured thin films that readily give 60-140 fold increases in surface area on both planar and three-dimensional substrates. However, even when relatively high surface area porous metal substrates such as nickel foam are used as a starting point, the resultant material still has surface area density well short of that available in other types of materials. Micro-emulsions offer a method of generating microstructure that bridges the gap between the 100 micron scale structures of foamed metals and the 10-50 nm scale structure of self-assembling block co-polymers. Electrodeposition of nickel and cobalt from micro-emulsions of Tween surfactants gives rise to structure on the 0.110 micron length scale. The scale of the microstructure is strongly influenced by the metal ion concentration and the electrodeposition potential. The nature of the metal ion also strongly effects the ease with which the microstructure can be generated and the distribution of the microstructured film on foamed nickel electrodes. For microstructured nickel films ten fold surface area increases can be achieved. The microstructured films are expected to be compatible with a number of the nanostructuring methods to yield cumulative surface area increases of 1000-2000 fold. INTRODUCTION There is considerable interest in the development of high surface area porous electrodes for applications ranging from sensors and fuel cells to electrochemical capacitors. Methods for fabricating carbon based electrodes are relatively well developed, with surface areas of 1000 m2/g being readily attainable. By comparison, methods for all metal electrodes are relatively less advanced. We have been interested in the use of soft-template based approaches combined with electrodeposition to develop high surface area porous electrodes. This approach has the advantage that after electrodeposition no further processing is necessary beyond removal of the template. Our chosen substrate is nickel foam, which is widely used as the current collector in Nickel/Cadmium and Nickel/Metal Hydride batteries. The foamed nickel used in this study consists of open cells with a diameter of 0.5 mm formed by triangular ligaments. The density of the material is low, so that the surface area is only 80
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