Designed formation of NiCo2O4 with different morphologies self-assembled from nanoparticles for asymmetric supercapacitors and electrocatalysts for oxygen evolution reaction

Spinel NiCo2O4 (NCO) as a promising electrode material, is essential for supercapacitors and electrocatalysts for oxygen evolution reaction (OER), which crucially related to its structure and morphology. Herein, hierarchical spinel NCO nanomaterials with different morphologies such as homogeneous NCO nanoparticles, one-dimensional NCO nanorods, heterogeneous NCO nanoflowers, and three-dimensional NCO microspheres assembled with nanoparticles are synthesized via facile coprecipitation method combined with calcination process. The effect of precipitating agents on designed formation mechanism of these NCO structures is further investigated. NCO microspheres are evaluated as supercapacitors and demonstrate excellent electrochemical performances with a high specific capacitance (225.07 C/g at 0.5 A/g) and cycling stability (86.9% at 2 A/g over 3000 cycles). Furthermore, the asymmetric supercapacitor is fabricated with NCO microspheres as the positive and graphene as the negative respectively, which exhibits a high energy density of 19.12 Wh/kg at a power density of 509.87 W/kg. As the electrocatalyst, NCO microspheres also show a superior OER electrocatalytic activity with a minimal Tafel slope of 70.32 mV dec(-1) due to more mesoporous and diffusion pathways than other NCO samples. These results provide a new strategy to develop affordable alternative OER catalysts and asymmetric supercapacitors. (C) 2018 Elsevier Ltd. All rights reserved.