Coupling MnS and CoS Nanocrystals on Self-Supported Porous N-doped Carbon Nanofibers to Enhance Oxygen Electrocatalytic Performance for Flexible Zn-Air Batteries

Seeking highly efficient, stable, and cost-effective bifunctional electrocatalysts of rechargeable Zn-air batteries (ZABs) is the top-priority for developing new generation portable electronic devices. For this, the rational and effective structural design, interface engineering, and electron recombination on electrocatalysts should be taken into account to reduce the reaction overpotential and expedite the kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, we construct a MnCo-based metal organic framework-derived heterogeneous MnS-CoS nanocrystals, which are anchored on free-standing porous N-doped carbon fibers (PNCFs) by the in situ growth method and vulcanization process. Benefiting from the abundant vacancies and active sites, strong interfacial coupling as well as favorable conductivity, the MnS-CoS/PNCFs composite electrode delivers a mentionable oxygen electrocatalytic activity and stability with a half-wave potential of 0.81 V for ORR and an overpotential of 350 mV for OER in the alkaline medium. Of note, the flexible rechargeable ZAB using MnS-CoS/PNCFs as binder-free air cathode offers high power density of 86.7 mW cm-2, large specific capacity of 563 mA h g-1, and adapts to different bending degree of operation. In addition, the density functional theory calculation clarifies that the heterogeneous MnS-CoS nanocrystals reduces the reaction barrier and enhances the conductivity of the catalyst and the adsorption capacity of the intermediates during the ORR and OER process. This study opens up a new insight to the design of the self-supported air cathode for flexible electronic devices.