Supercapacitors
Supercapacitors are energy storage systems in which the energy is stored by pure electrosorption of ions on the surfaces of charged electrodes thus forming the double-layer (Figure 1). These devices occupy the area in the Ragone plot (the plot of power against energy density) between batteries and dielectric capacitors. Supercapacitors are designed for high power and long cycle life, but at the expense of energy density. Hence the research on supercapacitors is focused on improving their energy densities, i.e. the times of energy delivery. This can be achieved by using the electrolytes with wide potential window and/or by optimisation of the electrode structure.
Electrodes are the most important components of supercapacitors as the energy storage happens on their surfaces. Porous carbon has been the most studied electrode material for several good reasons: high surface area, good polarisability, chemical resistance, abundance, and low toxicity.
With regards to the commercial applications of supercapacitors, examples include memory back-up in toys, cameras, video recorders and mobile phones. More recently, supercapacitors have been employed in emergency doors on an Airbus A380 and as advanced energy storage for the Palmdale Water District’s Southern California water treatment plant. With a projected market of over $180 million in 2009, supercapacitors are early in commercialisation, though the technology is firmly in place.
Major achievements
The research at the ARC Centre of Excellence for Functional Nanomaterials mainly focuses on the optimisation of the porous structure and the surface chemistry of carbons for maximum energy storage. Development of the low-cost activated carbons from organic biowastes for application in supercapacitors is another key research topic carried out at the Centre.
In the last year we have achieved several important research breakthroughs including:
High-performance low-cost carbons have been synthesised from spent coffee grounds. The performances of supercapacitors constructed from such carbons in aqueous and organic electrolytes were superior to that of commercial and costly activated carbon (such as Maxsorb).
In collaboration with Prof Bandosz, the combined effect of nitrogen-, oxygen- containing functional groups and the porosity of the carbons on their electrochemical energy storage capacity has been understood for the first time. This will significantly aid tailoring the carbon electrode materials for maximum energy storage capacity.
In collaboration with Dr Puziy, the enhancing effect of phosphorus-containing functional groups on carbon on its electrochemical performance in supercapacitors has been revealed. Supercapacitor cells made from phosphorus-rich carbons were capable of operating sturdily at voltages larger than the theoretical decomposition of water in environmentally friendly water-based electrolyte. As a result the energy density of aqueous supercapacitor was greatly improved approaching the energy densities of supercapacitors.
In collaboration with Alexey Glushenkov and Prof Chen, a new type of crystalline vanadium nitrate for supercapacitors, with direct attachment of nitride grains and thus advanced rate capability has been developed.
Future plans and directions
The future research directions and challenges include:
Fundamental understanding of the energy storage mechanism in carbons with complex porous structure and surface chemistry. Quantum chemical calculations will be employed for the first time in clarifying the charge-transfer reactions between the electrolyte ions and the nitrogen-, oxygen-, and phosphorus-functional groups.
Preparation of low-cost carbons from various biowaste precursors and optimisation of the synthesis procedure for maximum energy storage at minimum cost.
Collaborations
Prof Teresa J Bandosz, City University, Department of Chemistry, New York, USA
Dr Alexander Puziy, National Academy of Sciences of Ukraine, Institute for Sorption and Problems of Endoecology, Kiev, Ukraine
