Enhanced supercapacitors from hierarchical carbon nanotube and nanohorn architectures

Abstract

Supercapacitors fill the power and energy gap between electrolytic capacitors and batteries. The energy density for commercial supercapacitors is currently limited to ∼5 Wh kg⁻¹. Enhancing the energy and power density of supercapacitors is of great interest as it would open up a much wider range of applications. In this work, thin film supercapacitors made of random networks of single-walled carbon nanotubes (SWNTs) were enhanced by the use of carbon nanoparticles of a size ideal to fill the pores in the SWNT network. These nanoparticles, termed carbon nanohorns (CNHs), provide a much enhanced surface area, whilst maintaining high permeability and porosity. We demonstrate the hierarchical use of carbon nanostructures in a controlled fashion, allowing an enhancement provided by both types of materials, high power density by the SWNTs and high energy density from the CNHs. SWNT films serve as an ideal template onto which CNHs are deposited, with a good size match, adhesion and charge transfer between particles of a single chemical species. This combination results in an enhanced specific capacitance and a reduced equivalent series resistance (ESR) compared to a capacitor made of either individual component. Additionally, the good binding properties of the hybrid material and the high electrical conductivity of the SWNTs permit the removal of both the binder and the charge collector, paving the way for thinner and lighter supercapacitors. These electrodes allow the fabrication of supercapacitors with novel properties. As an example, we demonstrate a semitransparent supercapacitor. These results demonstrate the possibilities that may be available for the enhancement of electrodes by tailoring and combining relevant materials hierarchically in multiple scales. Much potential remains in further enhancement through tailored hierarchical nanostructuring.