Transition-metal sulfide is pursued for replacing scare platinum-group metals for oxygen electrocatalysis and is of great importance in developing low-cost, high-performance rechargeable metal–air batteries. We report herein a facile cationic-doping strategy for preparing nickel-doped cobalt sulfide embedded into a mesopore-rich hydrangea-like carbon nanoflower. Nickel cations are introduced to induce the formation of Co3+-active species and more oxygen vacancies due to higher electronegativity and smaller ionic radius, thereby strengthening the intrinsic activity for oxygen electrocatalysis. Moreover, hydrangea-like superstructure composed of interconnected carbon cages provides abundant accessible active sites and hierarchical porosity. As a result, it shows excellent catalytic performance with a superior mass activity for the oxygen reduction reaction to the state-of-the-art Pt/C catalyst and a low overpotential of 314 mV at 10 mA cm–2 for the oxygen evolution reaction. When used as an air cathode for the rechargeable Zn–air battery, it delivers a peak power density of 96.3 mW cm–2 and stably operates over 214 h. This work highlights the importance of cationic doping in strengthening the electrocatalytic performance of 3d-transition-metal chalcogenides.

We report a self-template and facile pyrolysis method to synthesize Fe/Fe3C-decorated metal–nitrogen–carbon mesoporous nanospheres, of which preserved plum-like and hollow structures can be simply engineered via controlling the thickness of the outermost polydopamine layer in the precursors. The preserved plum-like structure is demonstrated to show a large electrochemically active surface area and facilitate fast charge transfer, in comparison with the hollow one. The catalytic activities of metal–nitrogen–carbon and nitrogen-doped carbon active sites in the outer carbon layer toward oxygen reduction are improved under the activation of the encased Fe species. Hence, preserved plum-like structures exhibit excellent catalytic kinetics toward the oxygen reduction reaction in alkaline media. The mass activity of 21.0 mA mgcatalyst –1 at 0.9 V vs RHE is achieved and the half-wave potential is 50 mV more positive than that of the Pt/C catalyst with the same mass loading. Moreover, the outer carbon layer endows the tolerance of strong acidic and alkaline environments, resulting in good durability. Our study proposes a simple strategy for the rational design of novel transition metal carbide-based catalysts, making it a promising candidate for replacing platinum-group metal catalysts in low-temperature fuel cells.

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