A vertical and cross-linked Ni(OH)2 network on cellulose-fiber covered with graphene as a binder-free electrode for advanced asymmetric supercapacitors

Nanostructured transition metal oxides are attractive pseudocapacitive materials with high theoretical specific capacitance, scale-up potential and environmental benignity. However, realizing high capacitance and excellent rate capability remains a critical challenge. Herein, a three-dimensional carbon support of cellulose-fiber covered with graphene (CFG) to induce the growth of a hierarchical nanostructured Ni(OH)2 (Ni(OH)2–CFG) is fabricated through a one-pot hydrothermal reaction without using any surfactants or hard templates. The resulting Ni(OH)2–CFG composite exhibits a special vertical and cross-linked network structure with a large surface area (425.9 m2 g−1, higher than that of unsupported Ni(OH)2, 366.9 m2 g−1) and appropriate pore size distribution of micro–mesopores, which offer fast electrolyte ion-transport and short ion-diffusion pathways. Electrochemical characterization demonstrates that the Ni(OH)2–CFG composite as a binder-free electrode reveals high mass capacitance (2276 F g−1, at 1 A g−1), good rate capability and excellent cycling stability (no capacitance decay after 1000 cycles at a high current density of 5 A g−1). In addition, an asymmetric Ni(OH)2–CFG//activated carbon supercapacitor exhibits a high cell-voltage of 1.6 V and a maximum specific capacitance of 191.3 F g−1 with an energy density up to 15.0 W h kg−1. The excellent performances of the Ni(OH)2–CFG composite demonstrate its promising potential for future capacitor based energy storage and conversion.