Synthesis of mesoporous Co3O4 nanosheet-assembled hollow spheres towards efficient electrocatalytic oxygen evolution

Hollow spheres assembled by α-Co(OH)2 nanosheets were firstly synthesized through a templated self-assembly solvothermal method, where rationally involved polyvinylpyrrolidone (PVP) is acting as molecular template for nanocrystal geometrical manipulation and ethylene glycol (EG) is acting as both solvent and precipitant for metal cation coordination; followed by subsequent calcinations, mesoporous Co3O4 nanostructures were then topotactic transformed from the primary α-Co(OH)2 by preserving their original frameworks. Such monodisperse hierarchical structure, distributed in diameter of around 2.76-4.22 μm, is constructed orderly with several well-aligned nanosheet subunits in a high degree of orientation and separation; moreover, the shell thickness and the void space can be adjusted by changing the concentration of the PVP surfactant. When applying as electrocatalyst for oxygen evolution reaction (OER), the obtained Co3O4 delivered a low onset potential (e.g., around 1.438-1.481 V vs. RHE) with a low overpotential (e.g., around 290-310 mV at 10 mA cm−2) and relatively small Tafel slopes (e.g., around 86-97 mV dec−1) as well as long-term durability (e.g., up to 99.15% after 10 h) in an alkaline electrolyte. These fine catalytic activities can be attributed to the hierarchical structure with high specific surface area, ordered multilevel pores and void space, which facilitate the electrolyte accessibility and provide more active sites for redox reaction. Therefore, with the presented method, it would be promising for the synthesis of other transition metal oxides/hydroxides with such unique structure towards energy conversion and storage applications.