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

Abstract

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.