Dip-coating synthesis of rGO/α-Ni(OH)2@nickel foam with layer-by-layer structure for high performance binder-free supercapacitors

Abstract The conventional fabrication of composite electrodes often requires extraordinarily complex electrode preparation steps. We provide here a separate dip-coating method for ultrasmall α-Ni(OH)2 nanoparticles grown between reduced graphene oxide (rGO) sheets on nickel foam, forming a direct binder-free electrode of 3D rGO/α-Ni(OH)2@nickel foam (GNH@NF-n, layer number n= 5, 8, 10, 15, 20) with a layer-by-layer structure. At a charge and discharge current density of 1 A/g, the specific capacitance of GNH@NF-5 reaches 2118 F/g (capacity: 323.5 mAh/g). When the current density is changed from 1 to 10 A/g, the specific capacitance retains 74.8% of the maximum value (capacity retains 88.4%). Changes in structure and electrochemical performance are analyzed as the number of layers increases, i.e., as the mass loading increases. When the mass loading increases by a factor of 5.2 from 1.4 to 7.3 mg/cm2, the GNH@NF-n electrode maintains a very high specific capacitance, with a value as high as 89% of the capacitance (or capacity) for the lowest mass loading. Dipping separately in nickel-based and rGO solutions (separate dip-coating method) is more effective in achieving the layer-by-layer structure and high electrochemical performance than dipping in a mixed solution (mixed dip-coating method). Furthermore, asymmetric GNH@NF-5//rGO supercapacitors are assembled and exhibit a high specific energy of 109.9 Wh/kg at a specific power of 1.3 kW/kg and a high cyclic stability with capacitance retention of 86.1% after 10,000 charge and discharge cycles.