Phosphorus doped Co9S8@CS as an excellent air-electrode catalyst for zinc-air batteries

[Display omitted] •Phosphorus-doped Co9S8 and carbon sphere composite (Co9S8/P@CS) is prepared.•Co9S8/P@CS performs a high electrocatalytic activity for oxygen evolution reaction.•A super-stable Zn-air battery is assembled with Co9S8/P@CS as air-electrode catalyst.•Co9S8/P@CS shows a power density of 142 mW cm−2 superior to Pt/C-IrO2.•The relationships between structure and the catalytic performance is revealed. A novel phosphorus-doped Co9S8 and carbon nanosphere composite (Co9S8/P@CS) was successfully prepared by adding a certain amount of sodium hypophosphite during the thermal treatment process of preparing Co9S8. The results of XRD and XPS confirmed the incorporation of phosphorus into the lattice of Co9S8. Electrochemical tests demonstrated that Co9S8/P@CS performs a high electrocatalytic activity for oxygen evolution reaction (OER). Under the current density of 10 mA cm−2, Co9S8/P@CS-1:2 as the best catalyst has the lowest overpotential of 233 mV and Tafel slope of 48 mV dec–1, which is far superior to the current noble-metal catalyst IrO2 (414 mV, 96 mV dec–1). A rechargeable Zn-air battery was assembled with Co9S8/P@CS-1:2 as the air-electrode catalyst. Its maximum power density reached 142.50 mW cm−2 at 0.56 V and the current density was 255.47 mA cm−2, significantly higher than those of the battery made by Pt/C-IrO2 catalysts. Especially, after 350 h (1050 cycles) of charge and discharge, the performance of this assembled battery did not show a significant decrease, predicting a broad application prospect. In-situ Fourier transform infrared spectroscopy (FTIR) and density functional theory (DFT) calculations were used to analyze the active sites of catalyst and catalytic mechanism, revealing the relationships between structure and the catalytic performance for OER.