Graphene/MnO2 Supercapacitors, Maher F. El-Kady; pnas

Engineering three-dimensional hybrid supercapacitors and microsupercapacitors for high-performance integrated energy storage
Supercapacitors now play an important role in the progress of
hybrid and electric vehicles, consumer electronics, and military and
space applications. There is a growing demand in developing
hybrid supercapacitor systems to overcome the energy density
limitations of the current generation of carbon-based supercapacitors.
Here, we demonstrate 3D high-performance hybrid supercapacitors
and microsupercapacitors based on graphene and MnO2
by rationally designing the electrode microstructure and combining
active materials with electrolytes that operate at high voltages.
This results in hybrid electrodes with ultrahigh volumetric
capacitance of over 1,100 F/cm3
. This corresponds to a specific capacitance
of the constituent MnO2 of 1,145 F/g, which is close to
the theoretical value of 1,380 F/g. The energy density of the full
device varies between 22 and 42 Wh/l depending on the device
configuration, which is superior to those of commercially available
double-layer supercapacitors, pseudocapacitors, lithium-ion capacitors,
and hybrid supercapacitors tested under the same conditions
and is comparable to that of lead acid batteries. These hybrid
supercapacitors use aqueous electrolytes and are assembled in air
without the need for expensive “dry rooms” required for building
today’s supercapacitors. Furthermore, we demonstrate a simple
technique for the fabrication of supercapacitor arrays for high-voltage
applications. These arrays can be integrated with solar cells for
efficient energy harvesting and storage systems
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