NiCo2S4 nanocrystals anchored on nitrogen-doped carbon nanotubes as a highly efficient bifunctional electrocatalyst for rechargeable zinc-air batteries

The commercial development of metal-air batteries with remarkably high theoretical energy output is largely limited by the scarcity of low-cost and highly stable electrocatalysts with activities for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) comparable to precious metals and their alloys. Herein, a new inorganic-nanocarbon coupled hybrid, homogeneous NiCo2S4 nanocrystals anchored on nitrogen-doped carbon nanotubes (NiCo2S4/N-CNT), has been developed as an extremely efficient bifunctional catalyst to promote the sluggish ORR and OER kinetics for advanced rechargeable zinc-air battery. It is found that the highest activity is obtained by tailoring the crystal size of NiCo2S4 in the hybrid through tuning the construction of metal ammonia complexes. The optimized NiCo2S4/N-CNT nanocomposite exhibits extraordinary bifunctional activity through half reaction testing, displaying comparable ORR activity to state-of-the-art carbon supported platinum (Pt/C) and superior OER capability to RuO2 as well as much better durability. The resulting battery performance in generative Zn-air system further confirms its superb effective bi-functionality for catalyzing dual ORR and OER. Compared with those of well-known commercial Pt/C and RuO2, the synergetic NiCo2S4/N-CNT hybrid enables significantly reduced charge-discharge polarization (similar to 0.63 V), enlarged energy efficiency (similar to 67.2%) and prolonged cyclability up to 150 cycles at 10 mA cm(-2). The tremendously enhanced electrochemical behaviors arise from favorable factors including small sized, homogenous dispersed novel NiCo2S4 nanocrystals and coupling interaction between sulfide spinels and underlying N-doped CNT network, which not only provides efficient electron transfer pathway but also alters the electronic structure, thus facilitating the oxygen electrocatalysis during the discharge-charge processes.