Free-standing graphene-carbon as negative and FeCoS as positive electrode for asymmetric supercapacitor

Free-standing graphene was synthesized using atmospheric pressure microwave plasma and used as electrode for supercapacitors. The graphene based electrodes demonstrated good supercapacitive and electronic properties and displayed a high rate retention of 78.5% and a low time constant of 0.2 s. It was further studied as conductive agent where the electrode produced after mixing it with activated carbon (AC) in 47.5:47.5 ratio demonstrated a specific capacitance of 208 F g −1 at 2 A g -1 , rate capability of 70.8% at 10 A g -1 and a time constant of 4 s, significantly below the 25.1 s one obtained for electrodes made with commercial activated carbon and conductive agents. Electrodeposited FeCoS on stainless steel tested as positive electrode. The ratio of Fe to Co equal to 1:2 showed the best electrochemical performance with specific capacitance of 827.5 F g −1 at 1 A g -1 with a rate capability of 84.1% at 10 A g -1 and long cycling stability of 90.4% up to 5000 cycles. An asymmetric cell made of AC:Graphene – 47.5:47.5||Fe:Co-1:2, displayed good capacitive response in a potential window of 1.2 V , in 1 M KOH, stored a maximum energy density of 67.8 W h kg -1 at a power density of 1.6 kW kg −1 and retained 45.2 Wh kg −1 at a power density of 6.1 kW kg −1 . • Activated carbon with different content of plasma derived graphene as conductive agent is tested as negative electrode. • The equal activated carbon and graphene ratio showed short time constant of 4 s and high rate capability of 70.8%. • FeCoS having different morphologies have been directly grown on stainless steel substrate by electrodeposition process. • Fe and Co ratio of 1:2 delivered the high specific capacitance of 827.5 F g −1 at 1 A g -1 with a rate capability of 84.1%. • Asymmetric supercapacitor of developed materials delivered energy density of 67.8 W h kg -1 at a power density of 1.6 kW kg −1 .