CNT DC microgrid History of engineering & technology KOH aqueous electrolyte energy storage device NiMoO4 NPs NiMoO4/3D-rGO nanocomposite activated carbon ball milling coin-cell prototype composite copper electric double-layer capacitor electrochemical performance electrode energy density energy efficiency energy management energy storage hybrid capacitor prototype case study hybrid power system nickel-cobalt hydroxide porous structure power density specific capacitance starch supercapacitor