Nanofoaming to Boost the Electrochemical Performance of Ni@Ni(OH)2 Nanowires for Ultrahigh Volumetric Supercapacitors.

Three-dimensional free-standing film electrodes have aroused great interest for energy storage devices. However, small volumetric capacity and low operating voltage limit their practical application for large energy storage applications. Herein, a facile and novel nanofoaming process was demonstrated to boost the volumetric electrochemical capacitance of the devices via activation of Ni nanowires to form ultrathin nanosheets and porous nanostructures. The as-designed free-standing Ni@Ni(OH)2 film electrodes display a significantly enhanced volumetric capacity (462 C/cm3 at 0.5 A/cm3) and excellent cycle stability. Moreover, the as-developed hybrid supercapacitor employed Ni@Ni(OH)2 film as positive electrode and graphene-carbon nanotube film as negative electrode exhibits a high volumetric capacitance of 95 F/cm3 (at 0.25 A/cm3) and excellent cycle performance (only 14% capacitance reduction for 4500 cycles). Furthermore, the volumetric energy density can reach 33.9 mWh/cm3, which is much higher than that of most thin film lithium batteries (1-10 mWh/cm3). This work gives an insight for designing high-volume three-dimensional electrodes and paves a new way to construct binder-free film electrode for high-performance hybrid supercapacitor applications.