This work presents a facile and general solvothermal approach to prepare hierarchical cobalt sulfides (Co_mS_n) microflowers, which are composed of curved nanosheets. The homogeneous phases of Co₉S₈, Co₃S₄ and Co_1−xS could be obtained just by adjusting the molar ratio of cobalt acetate to thiourea. The structure and morphology of the as-prepared cobalt sulfides were investigated by powder X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy (TEM) and high-resolution TEM, X-ray photoelectron spectroscopy and N₂-sorption measurements. The possible growth process was discussed on the basis of time-dependent experimental results. The Co_mS_n materials exhibited great electrocatalytic activities towards both oxygen reduction and oxygen evolution reactions, making them a kind of promising electrode materials for fuel cells and batteries.

A novel hybrid, with g-C₃N₄ embedded CoO particles covalently supported on a two-dimensional graphene sheet, is synthesized by a facile and scalable method towards the oxygen reduction reaction (ORR) for fuel cells. The composite hybrid with a suitable loading of g-C₃N₄@cobalt oxide on graphene exhibits excellent electrocatalytic activity and a dominant four-electron oxygen reduction pathway in basic solution. The kinetic-limiting current density (J_k) is 16.78 mA cm⁻² at −0.25 V, approaching that of 20% Pt–C (17.22 mA cm⁻²) at the same potential. The performance gap between the hybrid and 20% Pt–C in terms of the half-wave potential difference (ΔE_½) is 25 mV in alkaline solution. Furthermore, the hybrid is robust and methanol tolerant, making it a good candidate as a cathodic electrocatalyst in fuel cells.