Graphene Anchored with NiO-MnO2 Nanocomposites for Use as an Electrode Material in Supercapacitors

Abstract

Supercapacitors have attracted great interest since they combine the advantages of the high specific power of dielectric capacitors and the high specific energy of rechargeable batteries. Electrode materials with electronic conductivity such as amorphous carbon, activated carbon, graphite, carbon black, carbon fiber, and carbon nanotubes 1,2 are generally used as EDLCs. Many transition metal oxides including Ru, Mn, Ni, Co, Mo, and Sn have also been used as electrode materials in pseudocapacitors. 3‐5 Compounds containing binary metal oxides perform better than single transition metal oxides when used as the electrode in supercapacitor. Mn-Ni oxide, 6 Mn-Co oxide, 7 Mn-NiCo oxide, 8 and metal oxide‐carbon composites have attracted much interest in recent years due to their potential applications in supercapacitors, which require both high energy and high power densities. Graphene, a monolayer of carbon, has been found to exist as a freestanding form exhibiting many intriguing physical properties. 9,10 Due to the high quality of the sp 2 carbon lattice, the electrons were found to move ballistically in the graphene layer even at ambient temperatures. 11,12 Graphene is an ideal single-atom thick substrate for the growth of functional nanomaterials to render them electrochemically active and electrically conductive to outside current collectors. Recent works have demonstrated Li-ion battery and supercapacitor applications of oxides and polymers coupled with reduced graphite oxide in addition to previously reported pseudocapacitive nanomaterials including β-Ni(OH)2 (1335 F g-1), 13 MnO2 (216 F g −1 ), 14 SnO2 (43.3 F g −1 ), 15 and their composites with graphene materials. In this paper, graphene/NiO-MnO2 composites were prepared by a simple chemical precipitation method using a chelating agent. Recently, we reported a NiO-MnO2/MWCNT (193.50 F g −1 ) 16 composite material for application as a supercapacitor. In the graphene compositematerial,NiO-MnO2 isselectivelyanddirectlygrownonhighly conducting graphene. We employed a simple chemical precipitation method to prepare morphologically uniform graphene/NiO-MnO2 composites and characterized their electrochemical behaviors. The capacitive properties of the composites were investigated by galvanostatic charge/discharge and cyclic voltammetry method. For comparison, the electrochemical properties of graphene and graphene/NiOMnO2 electrodes were evaluated.