Modulation in Spin State of CoO Decorated Fe Single Atom Enables a Superior Rechargeable Zinc-Air Battery Performance.

High-performance bifunctional electrocatalyst for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is the keystone for the industrialization of rechargeable zinc-air battery (ZAB). In this work, the modulation in the spin state of Fe single atom on nitrogen doped carbon (Fe-NC) is devised by CoO (CoO@Fe-NC), and a mediate spin state is recorded. Besides, the d band center of Fe is downshifted associated with the increment in e filling revealing the weakened interaction with OH moiety, resulting in a boosted ORR performance. The ORR kinetic current density of CoO@Fe-NC is 2.0- and 5.6 times higher than Fe-NC and commercial Pt/C, respectively. Moreover, high spin state is found for Co in CoO@Fe-NC contributing to the accelerated surface reconstruction of CoO witnessed by operando Raman and electrochemical impedance spectroscopies. A robust OER activity with overpotential of 352 mV at 50 mA cm is achieved, decreased by 18 and 60 mV by comparison with CoO@NC and IrO. The operando Raman reveals a balanced adsorption of OH species and its deprotonation leading to robust stability. The ZAB performance of CoO@Fe-NC is 193.2 mW cm and maintains for 200 h. Furthermore, the all-solid-state ZAB shows a promising battery performance of 163.1 mW cm.