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-Nx coordination sites and M/MxC nanoparticles (M = Co, Fe, and Cu) using a new class of gamma-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 (E-1/2) potential (similar to 0.917 and 0.906 V, respectively), which are higher than Cu@C-CuNC (similar to 0.829 V) and the commercial Pt/C (similar to 0.861 V). As a bifunctional electrocatalyst, the Co@C-CoNC exhibits the best performance, showing a bifunctional ORR/OER overpotential (Delta E) of similar to 0.732 V, which is much lower than that of Fe@C-FeNC (similar to 0.831 V) and Cu@C-CuNC (similar to 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-N-4 sites 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(-2) at 270.30 mA cm(-2), higher than the combination of commercial Pt/C + RuO2 (similar to 158.90 mW cm(-2) at 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 similar to 140 h, signifying the virtue of excellent bifunctional ORR/OER electrocatalytic activity.

Automated summary

We successfully fabricated bifunctional M-N-C catalysts for rechargeable zinc-air batteries using a new class of metal-organic framework. These catalysts exhibited excellent alkaline oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities, with lower overpotentials and half-wave potentials compared to commercial Pt/C catalysts. The strong electronic correlation between metallic Co nanoparticles and atomic Co-N-4 sites in the catalysts enhanced the adsorption/desorption of intermediates in ORR/OER, resulting in improved bifunctional electrocatalytic performance. The Co@C-CoNC-based rechargeable zinc-air battery showed high power density and stability during discharge.