NiCo2S4 nanocrystals anchored on nitrogen-doped carbon nanotubes as a highly efficient bifunctional electrocatalyst for rechargeable zinc-air batteries
Abstract 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 NiCo 2 S 4 nanocrystals anchored on nitrogen-doped carbon nanotubes (NiCo 2 S 4 /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 NiCo 2 S 4 in the hybrid through tuning the construction of metal-ammonia complexes. The optimized NiCo 2 S 4 /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 RuO 2 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 RuO 2 , the synergetic NiCo 2 S 4 /N-CNT hybrid enables significantly reduced charge-discharge polarization (~0.63V), enlarged energy efficiency (~67.2%) and prolonged cyclability up to 150 cycles at 10mAcm −2 . The tremendously enhanced electrochemical behaviors arise from favorable factors including small sized, homogenous dispersed novel NiCo 2 S 4 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. Highlights NiCo 2 S 4 /N-CNT hybrid has been prepared by nucleation-crystallization strategy. The NiCo 2 S 4 /N-CNT exhibits extraordinary bifunctional activity for ORR/OER. The Zn-air cell delivers low polarization, high efficiency and long cyclability. The enhanced performance is due to the synergy between NiCo 2 S 4 and N-CNT. Graphical abstract [DISPLAY OMISSION]
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