Highly Efficient Oxygen Electrocatalyst for Rechargeable Zinc-Air Batteries: Surface-Oxidized Cobaltous Fluoride Nanocrystals Embedded in Carbon Nanofibers

The rational design and control of active sites are essential for achieving a highly active and stable electrocatalyst. Herein, we report a bifunctional oxygen electrocatalyst, nanocrystals of CoF2-inserted carbon nanofibers, which was prepared by electrospinning and a subsequent fluorination process. Due to the large specific surface area and hierarchical porous structure, carbon nanofibers provide rapid mass transfer and expose more active sites for electrocatalytic oxygen reactions. The surface-oxidized CoF2 nanocrystals show high electroactivity for both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in alkaline solution, marked by up to 0.822 V of the ORR half-wave potential and as low as 370 mV of the OER overpotential. Benefiting from the excellent electrocatalysis activity, this material exhibits an excellent application prospect in rechargeable zinc-air batteries. As an air electrode catalyst, it can endow the zinc-air battery with a high specific capacity (871.2 mAh g(-1)), a large power density (124.30 mW cm(-2)), and excellent stability (120 h continuous charge/discharge), dramatically outperforming that of commercial Pt/C and RuO2 electrocatalysts. Density-functional theory (DFT) calculations indicate that surface oxygen atoms on CoF2 play a key role in enhancing the activity of Co atoms by p-d orbital hybridization. This work provides a perspective for designing bifunctional oxygen electrocatalysts urgently needed by rechargeable zinc-air batteries.