Supercapacitors to fuel the future

Professor Sean Li, Dr Dewei Chu and postgraduate student Zhemi Xu

Electric vehicles and portable electronic devices are increasingly in demand due to the heightened awareness of climate change and the need to find alternatives to fossil fuels. Electrochemical energy storage devices are the key components of these new technologies. Currently, the most promising rechargeable electrochemical energy storage systems are lithium-based batteries. However, they are not very applicable for the large, multi-cell modules required, due to their limited cycle life and problematic safety features.

Supercapacitors are an emerging technology that can store far more energy than traditional capacitors yet release it quickly. They can repeat this many times without loss of storage capacity. One disadvantage of state-of-the-art supercapacitors is their relatively high fabrication cost compared with lithium-ion batteries. Researchers at the University of New South Wales (UNSW) are investigating a new material for superconductors: exceptionally thin sheets of cobalt oxide (Co3O4).

It is known that only the surface-most atoms of active electrode materials play a key role in the supercapcitor function, and that the electrochemical activities of electrodes are closely related to their microstructures. Therefore manipulating those microstructures enables the design of better materials specifically for energy conversion and storage. A range of different Co3O4 morphologies, including nano-spheres, nano-needle and nano-sheets, have been successfully fabricated and reported so far. However, among such morphologies, it is the porous Co3O4 nano-sheets, attached to their supporting material by their edges, which show the greatest capacitive behavior.

The high fabrication cost of current super-capacitors is due primarily to their graphene support. Although carbon is cheap and available, it is demanding and costly to make it into graphene, a single-atom layer form of carbon. Porous carbon foam, with the same excellent electric properties as other carbon-based materials, could therefore be an ideal replacement. Owing to its network of tiny holes all through the structure, similar to a sponge, porous carbon foam provides large surface area on which the supercapacitive material can be deposited.

Electrochemical deposition of metal hydroxides has been the usual way of making supercapacitors. This is suitable for large-scale industry, since it is simple and convenient, but is not stable in the long term. Also the chemistry of the process does not lend itself to effective deposition of cobalt oxide. To enable the safe and efficient deposition of Co3O4, postgraduate student Zhemi Xu, under the guidance of Prof. Sean Li and Dr. Dewei Chu at the UNSW used ultraviolet (UV) irradiation at room temperature to bring about the necessary chemical reactions. UV treatment gives an easy and effective solution. The method leads to a network of cobalt oxide nano-sheets attached to the carbon foam along their edges. The researchers used both transmission and scanning electron microscopy in the AMMRF at UNSW, to confirm aspects of the nanocrystalline structure of the Co3O4 nano-sheets and their compositional homogeneity. The new technique with its simple synthetic procedures and reaction conditions is suitable for industrial-scale production of Co3O4 nano-sheets on carbon foam bringing the promise of more commercially viable supercapacitors.

AMMRF News Vol. 25, March 2014