Controlled Ultrathin Films of Carbon Nanotubes for Electrochemical Applications
|Lee and colleagues report a process to incorporate functionalized multi-wall carbon nanotubes into highly tunable thin films using Layer-by-layer assembly.|
Reviewed by Jeff Morse, Ph.D., National Nanomanufacturing Network
Lee SW, Kim BS, Chen S, Shao-Horn Y, Hammond PT. 2009. Layer-by-Layer Assembly of All Carbon Nanotube Ultrathin Films for Electrochemical Applications J. Am. Chem. Soc. 131 (2): 671-679.DOI: 10.1021/ja807059k
Carbon nanotubes (CNT) have gained widespread interest for a range of electrochemical energy storage and conversion device applications—such as batteries, supercapacitors, and fuel cells—due to their unique properties including electrical conductivity, high surface area, and chemical and mechanical stability. In order to demonstrate the functional design of specific thin film architectures, precise control of CNT film thickness, density, and adhesion properties is necessary.
Films of vertically aligned CNTs prepared by chemical vapor deposition have previously been demonstrated, but their usefulness is limited by low density and high porosity. Solution processes for forming polymer-CNT nanocomposites are another method to form dense films, yet they suffer from precipitation into bundles due to strong van der Waals interactions between CNTs. Alternately, chemical functionalization processes to modify CNT surface properties have successfully been used to provide dispersed CNT solutions and are now a prevalent method to achieve improvements in control of solution coating processes.
Layer-by-layer (LBL) assembly is a versatile method to form dense thin films from dispersed solutions containing functionalized nanomaterials. The LBL approach consists of the repeated, sequential immersion of a substrate into aqueous solutions of functionalized materials having complementary charge, thereby producing conformal ultrathin films and controllable surface morphology using various nanomaterials. Electrostatic LBL assembly, in which CNTs have been alternated with polymers for various energy storage and conversion devices, has exhibited improved networks for electrochemical energy applications.
Recently, Lee et. al. reported the preparation of all multi-wall nanotube (MWNT) thin films based on the LBL assembly method. The authors prepared negatively and positively charged MWNTs by surface functionalization using carboxylic acid to yield MWNT-COOH and amine groups to yield MWNT-NH2 respectively. The complementary charged, functionalized MWNT dispersions enable the incorporation of MWNTs into highly controlled thin films using LBL assembly. The authors further reported that the pH dependent surface charge on the MWNTs gives this functional MWNT system the unique characteristics of LBL assembly of weak polyelectrolytes, thereby providing control of thickness and morphology with assembly pH conditions. Using AFM and SEM imaging studies, the authors reported that these MWNT thin films exhibit a randomly-oriented interpenetrating network structure with well developed nanopores, demonstrating the necessary control for optimization for electrochemical applications.
As an efficient electrode, the all-MWNT films exhibited high electronic conductivity in comparison with polymer composites with single wall nanotubes, and high capacitive behavior with the ability to precisely control the capacity. Electrical conductivity in the range 2-8 S/cm was measured, depending on process conditions. The high conductivity of the LBL electrodes was attributed to the binder-free, all carbon nanotube composition of the multilayer film and its highly interconnected morphology. Cyclic voltammetry studies of heat-treated MWNT thin films on ITO-coated glass assembled at pH 2.5 (+)/3.5 (-) were obtained in 1.0 M H2SO4 solution as a function of the number of LBL bilayers. The integrated surface charge from adsorbed and desorbed ions on MWNT thin film electrode was found to scale linearly as a function of film thickness. The average capacitance was considerably higher than those of vertically aligned or conventional CNT electrodes due to the high CNT densities and well developed nanopores in the LBL MWNT thin films.
This work indicates the potential to precisely control the charge and energy storage parameters in MWNT thin films by controlling the number of bilayers and film thickness in the LBL assembly. The authors note that the surface charge density of MWNTs play an key role in controlling film thickness and roughness by adjusting the charge reversal mechanism as well as the degree of interpenetration. The potential to achieve high capacitance in the MWNT thin films with precisely controlled capacity using LBL makes these MWNTs assemblies promising for supercapacitor electrodes. Further investigations of the charge/discharge properties of the LBL all MWNT electrodes structures will lead to optimization of specific process sequence and parameters for specific applications. Ultimately, precise control of the LBL system can be used to design ideal electrode materials for fuel cells, photoelectrochemical cells, batteries, supercapacitors, and gas and biosensors.
Image reproduced with permission from Lee SW, Kim BS, Chen S, Shao-Horn Y, Hammond PT. 2009. Layer-by-Layer Assembly of All Carbon Nanotube Ultrathin Films for Electrochemical Applications J. Am. Chem. Soc. 131 (2): 671-679. Copyright 2009 American Chemical Society.
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