Metal-organic framework-derived carbon nanotubes with multi-active Fe-N/Fe sites as a bifunctional electrocatalyst for zinc-air battery

Sustainable metal-air batteries demand high-efficiency, environmentally-friendly, and non-precious metal-based electrocatalysts with bifunctionality for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). In this research, novel functional carbon nanotubes with multi-active sites including well-dispersed single-atom iron throughout the walls and encapsulated ultrafine iron nanoparticles were synthesized as an electrocatalyst (FeNP@Fe-N-C) through one-step pyrolysis of metal-organic frameworks. High-resolution synchrotron powder X-ray diffraction and X-ray absorption spectroscopy were applied to characterize the unique structure of the electrocatalyst. In comparison to the commercial Pt/C and RuO2 electrodes, the newly prepared FeNP@Fe-N-C presented a superb bifunctional performance with its narrow potential difference (Egap) of 0.73 V, which is ascribed to the metallic Fe nanoparticles that boosts the adsorption and activation of oxygen on the active sites with an enhanced O2 adsorption capacity of 7.88 cm3 g−1 and synergistically functionalizes the iron atoms dispersed on the nanotubes. A rechargeable zinc-air battery based on FeNP@Fe-N-C exhibited a superior open-circuit voltage (1.45 V), power density (106.5 mW cm−2), and stable cycling performance. The green technique developed in this work for the fabrication of functional nanotubes raises the prospect of making more efficient electrocatalysts for sustainable energy cells.