Self-assembled hollow urchin-like NiCo2O4 microspheres for aqueous asymmetric supercapacitors

Self-assembled hollow urchin-like microspheres have been synthesized by a low temperature coprecipitation approach by employing weakly alkaline NaHCO3 as a generator of gas bubbles and precipitant and CTAB as a structure modifier. The as-prepared hollow precursor can be easily calcined into crystalline NiCo2O4. The crystal structures, morphologies, sizes and porous properties, and the formation mechanism of the hollow urchin-like microspheres (HUMs) have been investigated carefully as well as the role of CTAB. Compared with the NiCo2O4 HUMs without the assistance of CTAB, the CTAB-assisted NiCo2O4 HUMs (denoted as NC-C) applied in a supercapacitor exhibit enhanced capacitance properties and application potential. An asymmetric supercapacitor (ASC) has been successfully constructed with NC-C as positive electrode and activated carbon (AC) as negative electrode. The fabricated NC-C//AC ASC with an extended stable operational voltage of 1.6 V can deliver a relatively high specific capacitance of 95 F g(-1) at a current density of 1 A g(-1) and still maintain 63 F g(-1) at 20 A g(-1). What's more, the NC-C//AC ASC demonstrates remarkable energy density (similar to 36 W h kg(-1) at 852 W kg(-1)), high power density (similar to 11.9 kW kg(-1) at 16.5 W h kg(-1)) and moderate cycleability towards long term charge-discharge tests.