Design of 3‑Dimensional Hierarchical Architectures of Carbon and Highly Active Transition Metals (Fe, Co, Ni) as Bifunctional Oxygen Catalysts for Hybrid Lithium–Air Batteries

Flexible power sources and efficient energy storage devices with high energy density are highly desired to power a future sustainable community. Theoretically, rechargeable metal–air batteries are promising candidates for the next-generation power sources. The rational design of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts with high catalytic activity is critical to the development of efficient and durable metal–air batteries. Herein, we propose a novel strategy to mass synthesize nonprecious transition-metal-based nitrogen/oxygen codoped carbon nanotubes (CNTs) grown on carbon-nanofiber films (MNO-CNT-CNFFs, M = Fe, Co, Ni) via a facile free-surface electrospinning technique followed by in situ growth carbonization. With a combination of the high catalytic activity of Fe-catalyzed CNTs and the efficient mass-transport characteristics of 3D carbon fiber films, the resultant flexible and robust FeNO-CNT-CNFFs exhibit the highest bifunctional oxygen catalytic activities in terms of a positive half-wave potential (0.87 V) for ORR and low overpotential (430 mV @ 10 mA cm^–2) for OER. As proof-of-concept, newly designed hybrid Li–air batteries fabricated with FeNO-CNT-CNFFs as air electrode present high voltage (∼3.4 V), low overpotential (0.15 V), and long cycle life (over 120 h) in practical open-air tests, demonstrating the superiority of the freestanding catalysts and their promising potential for the applications in fuel cells and flexible energy storage devices.