Fe<sub>0.64</sub>Ni<sub>0.36</sub>@Fe<sub>3</sub>NiN Core-Shell Nanostructure Encapsulated in N-doped Carbon Nanotubes for Rechargeable Zinc-Air Batteries with Ultralong Cycle Stability

Abstract: Rechargeable zinc-air batteries (ZABs) have been extensively investigated owing to their high power density and environmental friendliness. However, the slow kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) processes limit their practical application. Currently, IrO2 and RuO2 are considered the optimal OER electrocatalysts, and Pt/C is the most effective ORR electrocatalyst. However, the practical application of Pt, Ir, and Ru in ZABs is severely limited owing to their low natural abundance and high cost. Therefore, the fabrication of inexpensive and high-performance bifunctional catalysts is essential for the development of rechargeable ZABs. Transition-metal alloys have a high electrical conductivity and low energy barrier for the reaction of oxygen, and thus they are considered promising ORR electrocatalysts. Transition-metal nitride-transition-metal alloy core-shell nanostructures can be fabricated to improve the bifunctional electrocatalytic activity. In this study, a bifunctional electrocatalyst with Fe0.64Ni0.36@Fe3NiN core-shell structures encapsulated in N-doped carbon nanotubes (Fe0.64Ni0.36@Fe3NiN/NCNT) was designed for highly efficient rechargeable ZABs. Fe0.64Ni0.36@Fe3NiN/NCNT was synthesized by pyrolyzing the nickel-iron-layered double hydroxide (NiFe-LDH) precursor, followed by ammonia etching of the Fe0.64Ni0.36 alloy. The core-shell structure produced more ORR/OER active sites. The Fe0.64Ni0.36 core exhibited high electrical conductivity, which facilitates charge transfer. The Fe3NiN shell enhanced the OER performance and improved the bifunctional performance. Moreover, the NCNT structures not only efficiently enhanced the mass transfer efficiency and intrinsic electrical conductivity, but also provided a large electrochemical active surface area. The high anticorrosion property of the Fe3NiN shell effectively protected the Fe0.64Ni0.36 core, which consequently enhanced electrocatalyst stability during the electrochemical processes. The protective carbon layer and the superior chemical stability of the Fe3NiN shell resulted in the ultrahigh stability of Fe0.64Ni0.36@Fe3NiN/NCNT. The catalyst exhibited an excellent bifunctional oxygen electrocatalytic performance, with a half-wave potential of 0.88 V for the ORR and low OER overpotential of 380 mV at 10 mA cm−2. Moreover, the catalyst exhibited electrochemical stability (92.8% current retention after 8 h). In addition, the Fe0.64Ni0.36@Fe3NiN/NCNT-based ZAB exhibited a higher peak power density (214 mW·cm−2) than the ZABs based on Pt/C+IrO2 (155 mW·cm−2) and Fe0.64Ni0.36/NCNT (89 mW·cm−2). Moreover, the Fe0.64Ni0.36@Fe3NiN/NCNT-based ZAB delivered a high capacity of 781 mAh·g−1, while the ZABs based on Fe0.64Ni0.36/NCNT and Pt/C+IrO2 reached capacities of 688 and 739 mAh·g−1, respectively. Furthermore, the Fe0.64Ni0.36@Fe3NiN/NCNT-based ZAB exhibited ultralong cycling stability (cycle life > 1100 h), which exceeded those of Pt/C (50 h) and Fe0.64Ni0.36/NCNT (450 h). We propose that this study will facilitate the design of novel catalysts for highly stable and efficient ZABs.