Quantum capacitance tuned flexible supercapacitor by UV-ozone treated defect engineered reduced graphene oxide forest

Abstract

A forest like 3D carbon structure formed by reduced graphene oxide (RGO) was prepared to use as an electrode material for a highly power efficient supercapacitor. To improve the specific energy of the electrode, pore like defects were incorporated on the RGO forests by atomic oxygen etching, during the UV-ozone treatment. The modified surface helps to increase the net capacitance by permitting the electrolyte to the inner core of the active material and improving the minimal quantum capacitance. Density functional theory based first principle studies were carried out to find DOS at the Fermi level of defect induced RGO sheet and hence to validate the effect of quantum capacitance on net capacitance. Specific capacitance of RGO forest was increased by almost 150% after introduction of the defects. The best performing material exhibits 18.87 mF cm−2 areal capacitance at 2 mA cm−2 current density which is equivalent to 70 F cm−3 at 3.7 A cm−3 current density, and it was used to fabricate the supercapacitor. Two supercapacitors were fabricated, (i) on graphite sheet (non-flexible) and (ii) on scotch tape (flexible). Here PVA-KOH gel soaked filter paper was used as electrolyte-separator. Both the prepared supercapacitors on graphite sheet and scotch tape are able to transfer electrical energy with ultra high specific power (656.25 mW cm−3 and 164.06 mW cm−3 respectively) while maintaining moderate energy densities. The first device can withstand its primary capacitance by 90% even after 10 K charge–discharge cycles and the flexible device was able to hold 96% of its capacitance after 1 K bending cycles.