Microwave Synthesis for Trimetallic Needle Structures coupled with N-Doped Carbon for Highly Boosted Bifunctional Oxygen Catalysts for Zn–Air Batteries

Energy storage systems play a crucial role in expanding the capabilities of sustainable energy sources and addressing existing energy issues associated with environmental pollution concerns. While versatile renewable energy sources have been developed over the past decade, their large-scale implementation inherently requires the use of highly efficient yet robust energy storage solutions. Metal-air batteries have recently been attracting attention as a promising candidate for next-generation storage cells due to their high theoretical capacity and eco-friendly use of oxygen. Among various types of metal-air batteries, the Zn-air battery has notably demonstrated a high theoretical energy density of 1860 Wh/kg, while ensuring cost-effective production, safety, and environmental compatibility. The primary emphasis in the development of rechargeable Zn-air batteries has been on improving stability and energy efficiency. The challenge of achieving high energy efficiency lies in the substantial overpotential of the constituent materials, which impacts the effectiveness of both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Noble metal-based electrocatalysts such as Pt/C and Ir/C exhibit remarkable electrochemical performances in ORR and OER. However, its widespread application is limited due to its high cost and stability issue.<br/><br/>In this work, we report a facile synthesis route of trimetallic bifunctional catalysts with N-doped carbon, with their low overpotential and high stability comparable to precious metals. The formulation of the precursor solution is conducted by precisely mixing 0.3 M iron nitrate hexahydrate (Fe(NO₃)₃6H₂O), 0.3 M nickel nitrate hexahydrate (Ni(NO₃)₃6H₂O), 0.3 M cobalt nitrate hexahydrate (Co(NO₃)₂6H₂O), 1.2 M urea (CO(NH₂)₂), and 0.4 M ammonium fluoride (NH₄F) in 100 ml of deionized (DI) water. The melamine foam was soaked in this solution and it was subjected to heating in an oven at 130 degrees celsius for a duration of 5 h as an integral step in the synthesis process. Then, the catalyst is directly fabricated using microwave heating for 20 seconds at 1000 W power. Scanning electron microscopy images illustrate the morphology transition of the catalysts at each stage of the synthesis process. Optimizing the microwave heating exposure leads to the creation of uniform metal oxides, as validated through transmission electron microscopy analysis. The composition and chemical state of the elements constituting the catalyst surfaces are preciously examined using X-ray photoelectron spectroscopy and X-ray powder diffraction.<br/><br/>The electrochemical performance of the developed catalysts is examined using a potentiostat in a three-electrode electrochemical cell with a rotating ring disk. The optimized trimetallic (Fe-Ni-Co) catalysts exhibit a low overpotential (0.071 V), similar to that of Pt/C and Ir/C. Under the full cell test, the catalysts obtain stable performances for 300 h in repetitive charge/discharge cycles for an hour interval, and the specific capacitance reaches 769 mAh g^-1 at 10 mA cm^-2. We believe that this rational synthesis strategy toward a facile fabrication process will contribute to developing novel active materials with mesoporous structure and large surface area, potentially useful for diverse electrochemical energy storage systems. Furthermore, the outcome of this work will inspire novel design of high performance electrocatalysts and other energy-related hybrid materials using transition metals.