Electrodeposition of Mesoporous Silica on 3-D Scaffolds as Templates for 3-D Porous Metal Electrodes
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Electrodeposition of Mesoporous Silica on 3-D Scaffolds as Templates for 3-D Porous Metal Electrodes Nikolas Cordes1 and Martin G. Bakker1,2 1 Department of Chemistry, The University of Alabama, Tuscaloosa, AL 35487-0336, U.S.A. 2 Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, AL 35487-0209, U.S.A. ABSTRACT Supercapacitors and advanced batteries capable of rapid charge and discharge need conductive three dimensional porous electrodes. The high conductivities of porous metal electrodes are attractive. However, the surface areas of such electrodes are still well short of those achievable in carbon. One approach to formation of high surface area porous metal electrodes is to electrodeposit metal into nanostructured templates on 3-D scaffolds such as nickel foam. By careful control of composition and voltage thin films of mesoporous silica can be deposited onto these 3-D templates. Removal of the templating surfactant produces a very high surface area mesoporous coating. Metal can then be plated into the mesoporous silica, which, after removal of the silica, leaves a high surface area 3-D porous electrode. INTRODUCTION There are many uses for very high surface area porous metals including as current collectors in batteries and supercapacitors. Nickel foam1 is a widely used, cheap material, which has a relatively low surface area of only 5 m2/g. Given the commercial availability of this material, there is interest in ways of significantly increasing the surface area. Mesoporous silica is formed by incorporation of a surfactant such as hexadecyltrimethylammonium bromide (CTAB) in a sol-gel silica mixture and gives rise to surface areas of 500-1500 m2/g. We have previously shown that dipcoating nickel foam with mesoporous silica provides a suitable template for electrodeposition of metal nanowires2, resulting in 150 fold surface area increases. Building on recent development of electrochemically assisted deposition of mesoporous silica3,4, we are exploring the use of mesoporous silica thin films produced by this method as templates for metal nanowire formation. Electrochemically assisted deposition of mesoporous silica makes use of the electrochemical reduction of nitrates and of hydrogen ions in the sol-gel mixture which produces an increase in the local pH. This leads to significantly increased condensation rates for silica sol-gel systems producing precipitation of a silica film on the electrode surface. Our expectation was that such electrodeposited films would be more adherent and even than dipcoated films. EXPERIMENTAL Mesoporous silica films on three dimensional electrodes were fabricated by electrochemically assisted self-assembly (EASA). The silica sol-gel solution consisted of 50 mL
of aqueous 0.1 M NaNO3 , 50 mL of ethanol, 7.54 mL of tetraethylorthosilicate (TEOS), and 3.97 g of hexadecyltrimethylammonium bromide (CTAB) corresponding to a CTAB/TEOS ratio of 0.32. Hydrochloric acid was added to the sol-gel solution to give a pH of 3. The sol-gel solution wa
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