Percolation Effects in Supercapacitors with Thin, Transparent Carbon Nanotube Electrodes

By King, Paul J.; Higgins, Thomas M.; De, Sukanta; Nicoloso, Norbert & Coleman, Jonathan N.
Published in ACS Nano NULL 2012

Abstract

We have explored the effects of percolation on the properties of supercapacitors with thin nanotube networks as electrodes. We find the equivalent series resistance, RESR, and volumetric capacitance, CV, to be thickness independent for relatively thick electrodes. However, once the electrode thickness falls below a threshold thickness (∼100 nm for RESR and ∼20 nm for CV), the properties of the electrode become thickness dependent. We show the thickness dependence of both RESR and CV to be consistent with percolation theory. While this is expected for RESR, 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 RESR and CV 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.

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