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Posted: Jul 04, 2011
Graphene supercapacitor: Keep it moist
(Nanowerk News) The exceptional thermal, electrical and mechanical properties of graphene make this single atomic layer of carbon a promising material for a range of applications, including energy storage. The high specific area of graphene and its unique electron transport properties make it particularly suitable for use in supercapacitors, especially if the graphene can be stacked into multiple separate layers. Unfortunately, when stacked in this way, graphene tends to form a graphite-like solid mass through strong inter-sheet attractions, which severely compromises the useful properties of the single graphene sheets. Now, Dan Li and colleagues from Monash University in Australia have shown a simple possible solution to the restacking problem: just keep chemically derived graphene wet ("Bioinspired Effective Prevention of Restacking in Multilayered Graphene Films: Towards the Next Generation of High-Performance Supercapacitors").
A photograph of a hydrated graphene multilayer film obtained from chemically derived graphene.
The team drew their conclusion from carefully examining the formation of graphene 'paper', which is made by filtering graphene suspended in water. They found that water trapped in as-filtrated graphene paper can act as a spacer between the individual layers to prevent restacking.
Li and his colleagues believe that because chemically derived graphene sheets are corrugated, the contact area between two sheets is much lower than if they were perfectly flat. In addition, the hydration of graphene generates repulsive forces strong enough to keep the graphene sheets well separated. The high specific surface area of the graphene sheets can therefore be harnessed effectively in a film form (pictured) suitable for integration into various devices.
The layer separation due to water is very effective. Li and his colleagues characterized the electrochemical response of supercapacitors constructed using their wet graphene films and observed a much higher capacitance that in the case of dried films. This enhanced capacitance was obtained even at very high frequency, meaning that the charging and discharging of the capacitor can occur at very high speed.
"Given the extraordinary supercapacitor performance, ease of cost-effective and scalable synthesis of such materials, our discovery will boost the development of the next generation of ultrafast energy storage devices", says Li. In addition to its great potential for capacitive energy storage, the researchers believe that other applications could also benefit from this bulk assembly, including battery electrodes, fuel cells, electrochemical sensors and actuators, water purification membranes and even biomedical devices.