Highly Reversible Zn-Air Batteries Enabled by Tuned Valence Electron and Steric Hindrance on Atomic Fe-N 4 -C Sites.

The bifunctional oxygen electrocatalyst is the Achilles' heel of achieving robust reversible Zn-air batteries (ZABs). Herein, durable bifunctional oxygen electrocatalysis in alkaline media is realized on atomic Fe-N 4 -C sites reinforced by Ni x Co 3- x O 4 (Ni x Co 3- x O 4 @Fe 1 /NC). Compared with that of pristine Fe 1 /NC, the stability of the oxygen evolution reaction (OER) is increased 10 times and the oxygen reduction reaction (ORR) performance is also improved. The steric hindrance alters the valence electron at the Fe-N 4 -C sites, resulting in a shorter Fe-N bond and enhanced stability of the Fe-N 4 -C sites. The corresponding solid-state ZABs exhibit an ultralong lifespan (>460 h at 5 mA cm -2 ) and high rate performance (from 2 to 50 mA cm -2 ). Furthermore, the structural evolution of Ni x Co 3- x O 4 @Fe 1 /NC before and after the OER and ORR as well as charge-discharge cycling is explored. This work develops an efficient strategy for improving bifunctional oxygen electrocatalysis and possibly other processes.