Template-free synthesis of 3D hierarchical nanostructured NiCo2O4 mesoporous ultrathin nanosheet hollow microspheres for excellent methanol electrooxidation and supercapacitors

Abstract

A facile template-free solvothermal method without any additional alkali was developed to fabricate a 3D hierarchical hollow microsphere precursor, followed by annealing in air, leading to a novel 3D hierarchical NiCo₂O₄ hollow microsphere material, which is composed of mesoporous (16.1 nm) ultrathin nanosheets (∼11–21 nm) consisting of ultrafine NiCo₂O₄ nanoparticles (11.9 nm). This 3D hierarchical NiCo₂O₄ nanosheet hollow microsphere material possesses a high specific surface area (93.4 m² g⁻¹) and mesoporosity, and thus superior electrochemical performance as an advanced electrode material. For methanol electrooxidation, the 3D hierarchical NiCo₂O₄ nanosheet hollow microsphere displays much higher electrocatalytic activity (95 A g⁻¹, at 0.6 V), lower overpotential (0.27 V, vs. SCE), and higher stability compared with the 3D hierarchical NiCo₂O₄ nanosheet solid microspheres, Co₃O₄ and NiO microspheres. For supercapacitors, the NiCo₂O₄ hollow microsphere exhibits excellent specific capacitance of 1701 F g⁻¹ at 1 A g⁻¹, excellent rate capability (61.5% retention at 15 A g⁻¹), and good electrochemical stability with 78.2% retention after 1000 charge–discharge cycles even at a high current density of 10 A g⁻¹. These findings can be explained by the unique integral characteristics of 3D NiCo₂O₄ hollow spheres with high electron conductivity, large surface area and numerous open spaces between neighboring mesoporous ultrathin nanosheets, which can offer many facile diffusion paths for ion/electrolyte and greatly improve the electron/ion transfer within the electrode and at the electrode–electrolyte interfaces.