Carbon nitride decorated nitrogen doped graphene hollow spheres loaded Ni/Co and corresponding oxides nanoparticles as reversible air electrode catalysts for rechargeable zinc-air batteries

ABSTRACT Bifunctional catalyst, which presents excellent electrocatalytic activity for oxygen reduction/evolution reaction (ORR/OER), is the key factor to explore the rechargeable Zn-air batteries (ZABs) with excellent performance and high stability. Herein, the polyaniline is coated on the outer surface of N-doped graphene hollow spheres (NGHS), which can further form the N-doped carbide (CNx) through the high temperature calcination process. The prepared CNx decorated N-doped graphene hollow spheres (CNx-NGHS) could enhance the conductivity of substrate material caused by the doping of N atoms, and the stretchy 3D hollow structure can prevent the deformation and collapse of fabricated electrode catalyst during the charge/discharge processes. Furthermore, the doped N atoms, acting as active sites, could facilitate the nucleation and growth of the core/shell metals/oxides nanoparticles on the surface of the CNx-NGHS, and at the same time enhance the interaction between metals/oxides nanoparticles and CNx-NGHS, both of which are helpful for promoting the electrocatalytic activity. Based on the investigation results, it can be concluded that CNx-NGHS loaded Co/CoO core/shell nanoparticles (Co/CoO/CNx-NGHS) show obvious better bifunctional electrocatalytic performance than that of CNx-NGHS supported Ni/NiO core/shell nanoparticles (Ni/NiO/CNx-NGHS). The synthesized Co/CoO/CNx-NGHS presents the lowest ΔE (~0.728) (ΔE= Ej10-E1/2), which indicates the better bifunctional electrocatalytic activity. The ZAB equipped with the Co/CoO/CNx-NGHS catalyst can deliver a maximum power density of 117 mW cm-2, which is larger than that of commercial 10% Pt/C (95 mW cm-2). The specific capacity of the Co/CoO/CNx-NGHS at both 100 and 200 mA cm-2 can reach ∼99% of the theoretical capacity (∼824 mAh g-1). Moreover, a minimum visible voltage loss can be seen for the synthesized Co/CoO/CNx-NGHS after cycled for 100 h, indicating the good durability and stability. All investigated results demonstrated that the outperformance of the fabricated Co/CoO/CNx-NGHS catalyst makes it a potential choice to replace the noble metal/carbon-based materials.