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

Abstract

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 (∼36 W h kg⁻¹ at 852 W kg⁻¹), high power density (∼11.9 kW kg⁻¹ at 16.5 W h kg⁻¹) and moderate cycleability towards long term charge–discharge tests.