Reduced Graphene Oxide Based Layer - By- Layer Inorganic-Organic Hybrid Material for Asymmetric Supercapacitor Device

Synthesis of novel layered material of large surface area is of significant interest in the fabrication of energy storage devices such as batteries and supercapacitors. In this recent years, the development of inorganic-organic hybrid layer-by-layer material has received much attention for supercapacitor application. The stacking of layered metal oxide such as Ni(OH)₂ over reduced graphene oxide (rGO) sheets can significantly improve the capacitive performance of the layered oxide materials. The unique combination of such two layer-structured materials would deliver high specific capacitance and long life cycle.

Herein, we describe a facile single-step synthesis of novel layer-by-layer hybrid material of α-Ni(OH)₂ by non-hydrothermal route for supercapacitor application. The hybrid material rGO/α-Ni(OH)₂ was synthesized using glucose as templating agent. Glucose acts as a bifunctional template, it partially reduces graphene oxide (GO) to rGO and assists the growth of α-Ni(OH)₂ layers in between the rGO sheets. The layer-by-layer stacking of α-Ni(OH)₂ and rGO is confirmed by electron microscopic measurements. The X-ray diffraction pattern reveals the increase in the interlayer distance of α-Ni(OH)₂ due to formation of layer-by-layer assembly. The supercapactive performance was evaluated by voltammetric, electrochemical impedance and charge-discharge measurements in alkaline pH in terms of specific capacitance, internal resistance and capacitance retention. The performance of the hybrid material is superior to that of rGO, free α-Ni(OH)₂ and the physical mixture of rGO and free α-Ni(OH)₂. Specific capacitance as high as 1671.67 Fg^-1 was obtained at a current density of 1A/g. The hybrid material retains 92 % of its initial capacitance after 1000 repeated charge-discharge cycles at a current density of 20A/g. The large surface area and high electronic conductivity of rGO improves the pseudocapacitive performance of Ni(OH)₂. The hybrid material has a high specific surface area and the layer structure assures the easy diffusion of electrolytes ions. The layer-by-layer structure of the hybrid material is believed to act as an ion buffering reservoir.

An assymmetric supercapacitor device (ASD) was fabricated by pairing the hybrid material with rGO. The hybrid ASD shows a wide potential window 0 to 1.6 V in aqueous 1M KOH electrolyte. ASD could deliver a specific capacitance of 94 F/g at a current density of 1 A/g. It exhibits good recyclability and it retains 100% initial specific capacitance after 500 consecutive charge-discharge cycles. It could deliver an energy density of 42.67 Wh/kg at a power density of 0.4 kW/kg and also retains 15.64 Wh/kg at a power density of 4.02 kW/kg, which is significantly higher than the existing metal oxide-based devices.

Keywords:inorganic-organic hybrid material; layer-by-layer; assymmetric supercapacitor device; reduced graphene oxide

References:

[1] B. E. Conway. Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic/ Plenum Publisher, New York, 1999.

[2] S. Bag, C. R. Raj, J. Mater Chem. A, 2014, 42, 17848.