Percolation Effects in Supercapacitors with Thin, Transparent Carbon Nanotube Electrodes

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• 作者：Paul J. King ; Thomas M. Higgins ; Sukanta De ; Norbert Nicoloso ; Jonathan N. Coleman
• 刊名：ACS Nano
• 出版年：2012
• 出版时间：February 28, 2012
• 年：2012
• 卷：6
• 期：2
• 页码：1732-1741
• 全文大小：551K
• 年卷期：v.6,no.2(February 28, 2012)
• ISSN：1936-086X

文摘

We have explored the effects of percolation on the properties of supercapacitors with thin nanotube networks as electrodes. We find the equivalent series resistance, R_ESR, and volumetric capacitance, C_V, to be thickness independent for relatively thick electrodes. However, once the electrode thickness falls below a threshold thickness (100 nm for R_ESR and 20 nm for C_V), the properties of the electrode become thickness dependent. We show the thickness dependence of both R_ESR and C_V to be consistent with percolation theory. While this is expected for R_ESR, that the capacitance follows a percolation scaling law is not. This occurs because, for sparse networks, the capacitance is proportional to the fraction of nanotubes connected to the main network. This fraction, in turn, follows a percolation scaling law. This allows us to understand and quantify the limitations on the achievable capacitance for transparent supercapacitors. We find that supercapacitors with thickness independent R_ESR and C_V occupy a well-defined region of the Ragone plot. However, supercapacitors whose electrodes are limited by percolation occupy a long tail to lower values of energy and power density. For example, replacing electrodes with transparency of T = 80% with thinner networks displaying T = 97% will result in a 20-fold reduction of both power and energy density.

Keywords:

supercapacitor; nanotube; network; percolation; transparent