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The development of an electrocatalyst with high performance using nonprecious metals/metal oxides as well as its applications in flexible and rechargeable Zn−air batteries and water electrolyzers is in strong demand from industries. In this study, we have designed and synthesized a new (Fe 3 NiS 8−δ ) −4+δ carbon nanotube (CNT) hybrid electrocatalyst and revealed that the catalyst shows a very high oxygen evolution reaction (1.55 V at 10 mA/cm 2 ) and oxygen reduction reaction (E 1/2 = 0.82 V vs RHE) performances. Based on the analyses by in situ electrochemical X-ray diffraction together with structure analysis software, in situ electrochemical Fourier transform infrared spectroscopy, transmission electron microscopy, and computer simulations, such a high performance is derived from the sulfur vacancies that were formed via the self-doped d−p orbitals of Fe III in (Fe 3 NiS 8−δ ) −4+δ . Here, we describe an adequate explanation about the role of the iron doping in the nickel sulfides in the catalyst. Furthermore, the fabricated flexible and rechargeable Zn−air and water electrolyzer batteries using the catalyst show a low charge−discharge overpotential gap of 0.66 V and a 237 mA/cm 2 current density at 1.9 V, which is very important for the development of a rechargeable Zn−air battery and water electrolyzer with a high performance. First-principles calculations are employed to investigate the reaction mechanisms and elucidate the effect of the CNT support for the catalytic activity.

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