Nickel Nanoparticles Encapsulated in Nitrogen-Doped Carbon Nanotubes as Excellent Bifunctional Oxygen Electrode for Fuel Cell and Metal–Air Battery

Developing low cost, high-performance, and durable bifunctional catalysts for oxygen reduction and oxygen evolution reactions is critical for a commercial application of fuel cells and metal–air batteries. Nitrogen-doped carbon nanotubes encapsulated nickel nanoparticles are prepared through a simple pyrolysis procedure with melamine and nickel chloride hexahydrate as precursors. The catalyst is featured by nickel nanoparticles encapsulated inside nitrogen-doped carbon nanotubes, with abundant surface nitrogen doping. The optimized catalyst exhibits proximate oxygen reduction activity to platinum/carbon catalyst, comparable oxygen evolution activity to ruthenium dioxide catalyst, and better stability to noble metal catalysts in alkaline medium. The oxygen electrode activity parameter (the gap between the potential of oxygen evolution at 10 mA cm –2 and the half-wave potential of oxygen reduction) of the as-prepared catalyst is 0.754 V, which is among the state-of-the-art bifunctional electrocatalysts reported to date. To explore the active sites, a series of catalysts with different bulk nickel and surface nitrogen contents are synthesized and served as the oxygen reduction and oxygen evolution reactions catalysts. The results reveal that the oxygen reduction activity of this catalyst arises from the doped nitrogen, while the oxygen evolution activity originates from the encapsulated nickel nanoparticles.