CoFe Alloy Nanoparticles Embedded in N-doped Carbon Supported on Highly Defective Ketjenblack for Rechargeable Zn Air Battery

The severe crisis of environmental pollution and depletion of fossil fuels has stimulated an urgent demand for developing efficient, clean, and sustainable energy devices. Recently, rechargeable zinc-air batteries (ZABs) have emerged as promising renewable energy devices for next-generation owing to their high theoretical energy densities, cost-effectiveness, and zero-carbon emissions. The main obstacles of ZABs are the sluggish kinetic behaviors of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) involving a complex four-electron process. Precious metal-based electrocatalysts (e.g., Pt/C, IrO₂, and RuO₂) with high electrocatalytic performances for ORR and OER are commonly used as the state-of-the-art electrocatalysts. However, their high cost, scarcity, inferior multifunctionality, and low stability limit the large-scale commercialization of ZABs. Therefore, the development of effective non-precious metal-based electrocatalysts (NPMCs) is a key for enhancing the energy conversion efficiency and utilization of ZABs. Among various candidates, the transition metal-containing N-doped carbon (M-N-C, M = Fe, Co, Ni, etc. and their alloys) materials with enhanced electric conductivity and outstanding durability are considered as the most competitive electrocatalysts.<br/>Herein, we fabricated CoFe alloy nanoparticles embedded in N-doped carbon supported on highly defective ketjenblack (FeCoNC/D) via simple hydrothermal and impregnation method, followed by a high-temperature annealing process. First, the defect-rich ketjenblack (D-KB) was prepared by partial oxidation and etching of carbon atoms in ketjenblack during the hydrothermal procedure using H₂O₂, thereby introducing abundant carbon vacancies and nanopores. After that, metal ions and dicyandiamide (DCD) which has a high content of nitrogen were mixed and adsorbed on the D-KB. Finally, DCD was transformed to an N-doped carbon matrix with homogeneously embedded CoFe alloy nanoparticles on D-KB via in-situ thermal polymerization into g-C₃N₄, followed by the high-temperature carbonization process. The synthesized FeCoNC/D exhibits superior bifunctional electrocatalytic activity with a half-wave potential of 901 mV (vs. RHE) for ORR and an overpotential of 362 mV at a current density of 10 mA cm^-2 for OER. Moreover, the FeCoNC/D showed outstanding long-term durability for both of ORR and OER compared to commercial Pt/C and RuO₂. The excellent bifunctional oxygen electrocatalytic activity of FeCoNC/D encouraged us to assess its potential for air-cathode in rechargeable ZABs. The ZABs with FeCoNC/D display a narrow discharge/charge gap of 0.7 V, the higher power density of 157 mW cm^-2 at 240 mA cm^-2, and superior stability than Pt/C-RuO₂. The improved electrocatalytic performances toward ORR, OER, and ZABs of FeCoNC/D can be attributed to these advantages : a) outstanding properties of defect rich carbon support exhibiting excellent electric conductivity, abundant edge defects with high charge densities, and large surface area promoting exposure of catalytic active sites; b) uniform distribution of CoFe alloy nanoparticles, which can not only induce the internal electron redistribution but decrease the reaction energy barrier owing to synergistic effects induced by the inherent polarity of the alloy components; c) the encapsulated alloy nanoparticles inside the N-doped carbon layer, which are strongly coupled with outer carbon nanosheets, thereby preventing corrosion of metal during the electrocatalytic reaction.