Engineering carbon-shells of M@NC bifunctional oxygen electrocatalyst towards stable aqueous rechargeable Zn-air batteries

Non-precious transition-metal based materials have attracted increasing attention as an efficient oxygen electrocatalysts for Zn-air battery although their unsatisfactory durability in harsh charging/discharging cycling condition makes their practical application still challenging. Herein, we report a facile strategy for the synthesis of high-density and well-dispersed FeCo nanocrystals encapsulated in tunable N-doped carbon shells (FeCo@NC) on porous carbon support. The morphology and structure of the carbon shells are intentionally engineered to achieve two types of catalysts, one with complete and well-graphitized carbon shells (FeCo@NC-g) and the other with disordered and defect-rich carbon shells (FeCo@NC-d), to investigate their influences on battery performance. Systematic experiments suggest that although two catalysts hold similar physiochemical features including nanoparticle size and loading, surface area, N-doping content as well as ORR or OER activity in halfcell measurements etc., they exhibit distinct stability during ORR/OER cycling. The complete and graphitized carbon shells were found to be able to keep FeCo nanocrystal core unchanged and maintain its ORR activity thus the battery performance during cycling. However, the disordered and defect-rich carbon shells cannot prevent the nanoparticles from conversion into hydroxide or oxyhydroxide, leading to the irreversible sharp decay of ORR activity and battery performance. As a result, the Zn-air battery with FeCo@NC-g as air catalysts demonstrates superior charging/discharging durability with much higher power density of 190.2 mW cm-2 compared with that with FeCo@NC-d. These findings might inspire the strategies for the construction of efficient and durable non-precious metal based bifunctional electrocatalysts for advanced metal-air batteries and other energy devices.

Automated summary

The morphology and structure of carbon shells significantly affect the performance of FeCo catalysts in Zn-air batteries. Well-graphitized carbon shells can maintain the stability and ORR activity of FeCo nanocrystals, while disordered and defect-rich carbon shells lead to a decrease in activity.