Capacitors are devices that store electric charge. Supercapacitors, more accurately known as electric double-layer capacitors or electrochemical capacitors, can store much more charge thanks to the double layer formed at an electrolyte-electrode interface when voltage is applied.

The conductive wrapping technique is very simple, say Zhenan Bao, Yi Cui and colleagues, and involves dipping a composite electrode made of graphene/manganese oxide into a solution containing either carbon nanotubes (CNTs) or a conductive polymer. The CNTs or polymer coat the electrode and greatly improve its electrical conductivity, so enhancing its specific capacitance (or its ability to store charge) by over 20% for the CNT coating and 45% for the polymer.

Higher specific capacitance
The specific capacitance obtained by the researchers (around 380 F/g) is comparable to other ternary MnO2-based electrodes, which typically have specific capacitances of between 250–400 F/g. However, it is much higher than that of existing commercial carbon-based supercapacitors that only have a specific capacitance of around 150–250 F/g.

As well as having high specific capacitance, the hybrid electrodes also show good rate capability – which means that the electrodes maintain their high capacitance at high charging and discharging rates. They can also be used over more than 3000 charge–discharge cycles while retaining over 95% of their capacitance – an important advance because conventional metal-oxide based electrodes usually show poor rate capability since they have low electronic and ionic conductivity.

Large-scale energy storage applications
"The hybrid electrode system we developed in this work shows promise for large-scale energy storage applications," team member Guihua Yu told nanotechweb.org. "From the perspective of materials selection, both graphene and MnO2 are attractive electrode materials given that both carbon and manganese are cheap and abundant. From the processing point of view, our coating method is solution-based and easy to scale up."

The team is now busy working on improving the performance of lithium-ion battery electrodes using its method. "Our novel approach could be applied to a wide range of energy storage electrode materials that have high energy density but which show limited performance because of their insulating nature," said Yu.

The results were reported in Nano Letters.