Mutual Self-Regulation of d-Electrons of Single Atoms and Adjacent Nanoparticles for Bifunctional Oxygen Electrocatalysis and Rechargeable Zinc-Air Batteries

Rechargeable zinc-air batteries(ZABs) are a promising energy conversion device,which rely critically on electrocatalysts to accelerate their rate-determining reactions such as oxygen reduction(ORR) and oxygen evolution reactions(OER).Herein,we fabricate a range of bifunctional M–N–C(metal-nitrogen-carbon) catalysts containing M–Nxcoordination sites and M/MxC nanoparticles(M=Co,Fe,and Cu) using a new class of γ-cyclodextrin(CD) based metal–organic framework as the precursor.With the two types of active sites interacting with each other in the catalysts,the obtained Fe@C-FeNC and Co@C-CoNC display superior alkaline ORR activity in terms of low half-wave(E1/2) potential(～0.917 and 0.906 V,respectively),which are higher than Cu@C-CuNC(～0.829 V) and the commercial Pt/C(～0.861 V).As a bifunctional electrocatalyst,the Co@C-CoNC exhibits the best performance,showing a bifunctional ORR/OER overpotential(ΔE) of ～0.732 V,which is much lower than that of Fe@C-FeNC(～0.831 V)and Cu@C-CuNC(～1.411 V),as well as most of the robust bifunctional electrocatalysts reported to date.Synchrotron X-ray absorption spectroscopy and density functional theory simulations reveal that the strong electronic correlation between metallic Co nanoparticles and the atomic Co-N4sites in the Co@C-CoNC catalyst can increase the d-electron density near the Fermi level and thus effectively optimize the adsorption/desorption of intermediates in ORR/OER,resulting in an enhanced bifunctional electrocatalytic performance.The Co@C-CoNC-based rechargeable ZAB exhibited a maximum power density of 162.80 mW cm-2at 270.30 mA cm-2,higher than the combination of commercial Pt/C+RuO2(～158.90 mW cm-2at 265.80 mA cm-2) catalysts.During the galvanostatic discharge at 10 mA cm-2,the ZAB delivered an almost stable discharge voltage of 1.2 V for～140 h,signifying the virtue of excellent bifunctional ORR/OER electrocatalytic activity.