Performance Enhancement of Activated Carbon Based Supercapacitors By Incorporation of Fullerene Self-Assemblies
Authors: Sridhar Deepak , Balakrishnan Kaushik , Raghavan Srini , Muralidharan Krishna 
Published: February 2020

The deployment of renewable energy based power-grids, the development of next-generation high-efficiency hybrid/fully electric-vehicles with extended per-charge mileage, and the design and fabrication of cutting-edge energy-efficient portable electronic devices, all require robust, long cycle-lifetime energy storage systems with exceptional energy-density and power-density properties. In this context, supercapacitors could play a potential role in satisfying the above needs.

Currently, activated carbon (AC) is the electrode material of choice, though there is a big push towards developing newer carbon nanostructure based electrodes due to some pitfalls associated with AC. Specifically, AC suffers from low electrical conductivity, and additionally demonstrates a broad range of porosities, which is shown to adversely affect the device performance. As a consequence, conductive additives are used for reducing resistance, but they adversely affect the available specific surface area of the electrode. Further, AC has also been combined with other carbon nanostructures such as graphene and carbon nanotubes (CNT) in an effort to increase electrode conductivity. In addition, it was seen that the nanostructures also increase available surface area, leading to an increase in power density, as well as specific capacitance. However, the increase in capacitive performance of AC systems due to graphene and CNT requires significant amount (30-50 %) of an addition of these structures, which has a huge impact on the overall cost of fabrication. Further, in contrast to the numerous investigations on AC-graphene and AC-CNT composite based electrodes, not much work has focused on the addition of fullerene to AC.

In this work, the ability to further increase the energy and power density performance of AC based supercapacitors by the incorporation of carbon nanostructures is demonstrated. Specifically, using an easy-to-fabricate method that enables rapid self-assembly of fullerene (C60) nanotubes and nanorods, light weight supercapacitor electrodes that also incorporate commercially available AC were developed. The presence of graphene over the current collector and fullerene self-assembly over the graphene grown copper current collectors were also studied. Of note is the fact that the developed method relies on simple wet-chemistry based procedures, and is therefore completely scalable as well as compatible with existing device fabrication methods.

A specific capacitance enhancement of 50 % was observed due to the addition of the fullerene self-assemblies along with graphene grown current collectors as compared to fullerene-free systems on just bare current collectors. These electrode systems were also accompanied by a simultaneous increase in max power density by 20%, 94% capacitance retention even after 5000 cycles interestingly, the mass fraction of the fullerenes as compared to activated carbon is less than 1%. The substantial increase in performance is attributed to the meso and micro-porosity of the fullerene self-assemblies. While we have focused on enhancing the performance of low-cost AC based systems in this work, the inclusion of the fullerene self-assemblies within any carbon based electrode should lead to similar performance enhancements.