Doctor of Philosophy
Institute for Superconducting and Electronic Materials
Chidembo, Alfred Tawirirana, Advanced graphene-metal oxide nanostructured composites for supercapacitors, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, 2014. http://ro.uow.edu.au/theses/4151
The development of alternative energy storage systems is undoubtedly one of the greatest challenges facing our modern society as a result of emerging ecological concerns. The ever-increasing environmental concerns and the need for efficient energy storage devices have catapulted research in this field, mainly focused on electric double layer capacitors (EDLC’s), also known as supercapacitors, which consist of carbon based materials. The ultimate performance of these devices strongly depends on the intrinsic properties of their constituent materials and their eventual architectural design.
The main focus of this doctoral thesis is to develop high performance supercapacitor electrodes utilising simple and cost effective methods ideal for industrial and commercial use. The materials reported herein are therefore traditional pseudocapacitive metal oxides incorporated onto an electrical double layer material (graphene sheets and carbon nanotubes) to produce a synergistic network with high conductivity. The development of this work stems from previous work where we developed a graphene oxide-carbon nanotube composite for stable and high performance supercapacitor electrodes. It is against this background that the work reported herein was developed to incorporate metal oxides onto large graphene sheets and additionally a ternary system comprising metal oxide nanoparticles, carbon nanotubes and graphene.
In this work, an in-situ spray pyrolysis approach to fabricate different metal oxide-graphene composites with highly porous morphologies for cost-effective high performance supercapacitor devices was used for the first time. An efficient encapsulation of metal oxide particles with graphene oxide (GO) while simultaneously reducing the GO is reported. In the first experiment, a high surface area (BET surface area of 139 m2g-1) self–organized, micron sized urchin-like composite made up of reduced graphene oxide (rGO) and needle-shaped manganese oxide (rGO-Mn2O3/Mn3O4) is reported. Maximum capacitances of 425 Fg-1 at 5 mVs-1 from a three electrode set up and 133 Fg-1 at a current density of 0.2 Ag-1 are reported from an asymmetric two electrode set up with graphene as the anode. The composite material also shows capacitance retention of 83% over 1000 cycles. These remarkable performances are attributed to the high specific surface area due to the “urchin-like” hollow structures and synergy between the manganese oxide and reduced graphene oxide materials within the composite. Apart from the outstanding performance, the synthesis technique can be exploited further in the bulk synthesis of cost effective graphene-metal oxide hybrid materials for energy storage applications.
In a development of the first work, composites consisting of NiO and Co3O4 and rGO were synthesized and tested. As a result of spray pyrolysis, the composites exhibit unique globular lettuce-like structures comprising NiO or Co3O4 nanoparticles embedded between graphene sheets. An impressive overall performance arises from such structures by exploiting the pseudocapacitive nature of the metal oxide and conductive EDLC nature of the rGO. Specific capacitances as high as 687 and 656 Fg-1(three electrode set up) for rGO- Co3O4 and rGO-NiO composites were recorded respectively. Based on the same principle a more developed experiment, this time utilising reduced graphene oxide, functionalized multiwalled carbon nanotubes and nickel oxide nanoparticles is reported. Electrodes fabricated from this novel ternary system exhibit exceptionally high capacitance (2074 Fg-1 from a three electrode set up) due to the highly conductive network, synergistic link between graphene oxide and carbon nanotubes and achieving high surface area monodispersed NiO decorated rGO/CNTs composite employing the liquid crystallinity of GO dispersions. To further assess the practicality of this material for supercapacitor manufacture, we assembled an asymmetric supercapacitor device incorporating activated carbon as the anode. The asymmetric supercapacitor device showed remarkable capacity retention (>96%), high energy density (23 Whkg-1) and a coulombic efficiency of 99.5%. In all three experiments, the benefits of fabricating composites of EDLC and pseudocapacitive metal oxide nanoparticles are reflected by the high specific capacitances, high energy densities and long cycle life.