Unveiling the role of surface functional groups for the design of nickel manganese/reduced graphene oxide based composite electrode material for high performance asymmetric supercapacitor

Development of additive-free electrode material on conductive substrates is a cost-effective strategy and is essential for the fabrication of high-performance supercapacitor. Herein, the effect of transition metal ions on the preparation of bimetallic (Ni & Mn)/reduced graphene oxide (RGO) composites through multi-step electrodeposition technique was extensively analysed. Two different composites; nickel manganese oxide/RGO (NiMn-G/NF) and manganese nickel oxyhydroxide/RGO (MnNi-G/NF) were synthesized through step-wise electrodeposition technique. The NiMn-G/NF and MnNi-G/NF were prepared through interchanging the source of host transition metal ions (Ni and Mn). Various physicochemical characterization techniques confirmed that the crystal structure as well as the morphology and electrochemical performance of the composites were largely dependent on the host metal ions. The formation of surface functional groups may play a key role in the electrochemical performance of supercapacitors. The MnNi-G/NF electrode showed high specific capacitance of ~1525 F g⁻¹ at 1.5 A g⁻¹ current density compared to NiMn-G/NF (~1312.5 F g⁻¹) and other synthesized electrode materials. An asymmetric supercapacitor (ASC) device was fabricated using MnNi-G/NF as positive and thermally-RGO as negative electrode materials. The fabricated device exhibited highest specific capacitance of ~173.3 F g⁻¹ at 2 A g⁻¹ current density and maximum energy density of ~54.1 Wh kg⁻¹ and power density of 9 kW kg⁻¹.