Obtaining pure nanocarbons with a high yield is a great challenge in the field of material science. Research into nanocarbons for bi-functional oxygen reduction reaction (ORR) and evolution reaction (OER) electrocatalysts are classified according to their dimensionality from 0D to 3D nanocarbons. Among them, 1D epitaxial heterostructures with modulated composition enable the generation of devices with diverse catalytic activities. Despite significant progress, these nanocarbons still suffer from insufficient activity and stability. To further enhance catalytic activities and chemical stability, metals with nitrogen are used as a dopant (M-N-C). In current stage, conventional synthesis (for example, chemical vapour deposition (CVD), arc discharge or laser ablation methods) for M-N-C catalysts leads to mixtures of metals and carbon where exposed metals on the carbon composites are easily degraded into highly concentrated alkaline electrolyte for practical applications. Therefore, the controlled growth in nanoscale building blocks with chemically incorporated M-N-C is crucial for the development of high performance ORR and OER electrocatalyst.
Herein we suggest novel bottom-up synthetic process for core-shell Fe/Cu@Carbon Nitride Nanotubes (FeCu@CNNT) via supercritical reaction where Fe and Cu atoms are encapsulated within carbon nitride nanotubes. The morphology of the FeCu@CNNT was directly visualized by using scanning transmission electron microscopy (STEM). Electron energy-loss spectroscopy (EELS) mapping image and spectra was used to clearly tract the Fe and Cu arrangements within CNNT. ORR and OER activities of catalysts were measured using a rotating ring disk electrode (RRDE), showing higher half-wave potential of 0.890 V for ORR than that of Pt/C (0.850 V) and current densities of 2.924 mA cm−2 at 1.60 V for OER (2.276 mA cm−2 for IrO2). Further, we fabricated the rechargeable Zn-air batteries and flow-assisted Al-air batteries with FeCu@CNNT electrocatalyst. For the Zn-air batteries, the FeCu@CNNT showed better bi-functional properties than the mixture of Pt/C and IrO2 under high depth of discharge (DOD) of ~ 29% (12 h per cycle). The intrinsic precipitation problems of the Al-air batteries have been solved by introducing new Al-air flow batteries system, which showed the specific energy of 2287 Wh kgAl−1 at a current density of 25 mA cm−2. We believe that the bottom-up growth of the FeCu@CNNT with a high yield of ~40% is promising strategy for the synthesis of high quality M-N-C catalysts and the practical mass production.