FeCo alloy nanoparticles encapsulated in hollow N-doped carbon as a bifunctional electrocatalyst for aqueous zinc–air batteries with a low voltage gap†

Developing bifunctional catalysts with low reversible oxygen reaction potentials (ΔE) to improve the energy conversion efficiency and cycling stability of rechargeable zinc–air batteries (ZABs) remains a huge challenge. Herein, a series of catalysts containing a hollow N-doped carbon cage with FeCo alloy nanoparticles encapsulated (Fe_xCo_y@N-C) are fabricated by a strategy of epitaxial growth followed by pyrolysis. Thereinto, owing to the relative balance and optimal synergy between the active sites of the FeCo alloy and the specific surface area of porous carbon, the Fe_xCo_y@N-C-0.3 catalyst displays a best electrocatalytic performance in both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with a small ΔE value of 0.686 V. Aqueous ZABs catalyzed by Fe_xCo_y@N-C-0.3 exhibit a high peak power density of 191 mW cm⁻² and the voltage gap only increased by about 30 mV after 345 h. Conspicuously, the aqueous ZABs still maintain an extremely small charge–discharge voltage gap of 0.73 V after 345 h, which is better than the performance of ZABs based on metal catalysts reported in many studies. Moreover, flexible ZABs based on the Fe_xCo_y@N-C-0.3 catalyst demonstrate an excellent cycling stability of 50 h and outstanding mechanical stability under different bending states.