We know, E=1/2 CV2, E can be increased if c or V is increased, to achieve the higher energy density, recent research mainly focusses on constructing asymmetric supercapacitors (ASCs), which extends the operating
potential window in aqueous electrolytes.
please read recent papers like
"In-situ encapsulate Fe3O4 nanosheet arrays with graphene layers as anode for high-performance asymmetric supercapacitors"