  Supercapacitors operating in aqueous solutions are low cost energy storage
devices with high cycling stability and fast charging and discharging
capabilities, but have suffered from low energy densities. Here, we grow
Ni(OH)2 nanoplates and RuO2 nanoparticles on high quality graphene sheets to
maximize the specific capacitances of these materials. We then pair up a
Ni(OH)2/graphene electrode with a RuO2/graphene electrode to afford a high
performance asymmetrical supercapacitor with high energy and power densities
operating in aqueous solutions at a voltage of ~1.5V. The asymmetrical
supercapacitor exhibits significantly higher energy densities than symmetrical
RuO2-RuO2 supercapacitors and asymmetrical supercapacitors based on either
RuO2-carbon or Ni(OH)2-carbon electrode pairs. A high energy density of
~48Wh/kg at a power density of ~0.23kW/kg, and a high power density of ~21kW/kg
at an energy density of ~14Wh/kg have been achieved with our Ni(OH)2/graphene
and RuO2/graphene asymmetrical supercapacitor. Thus, pairing up
metal-oxide/graphene and metal-hydroxide/graphene hybrid materials for
asymmetrical supercapacitors represents a new approach to high performance
energy storage.
