Development of well-designed bifunctional electrocatalysts with high activity for OER (oxygen evolution reaction) and ORR (oxygen reduction reaction) are a crucial topic owing to their promising applications in rechargeable Zinc (Zn)-air battery. Herein, a facile adsorption-pyrolyzation strategy is proposed for preparing ultrafine Ni nanoparticles anchored on carbon nanofiber (Ni/CNF), which derives from pyrolyzation of bacterial cellulose (BC) (with pre-adsorbed of Ni2+) via a two-step heat-treatment proce-dure (firstly 360 degrees C, and then 750 degrees C) (Ni/CNF-750) and used as an excellent oxygen electrocatalyst for flexible all solid-state Zn-air cell. The resultant ultrafine Ni/CNF-750 with plentiful pore structure and rel-atively high specific surface area of 449.0 m(2) g(-1), delivering overpotential of 293 mV at current density of 10 mA cm(-2) for OER, obtaining an onset potential of 0.93 V vs. RHE and half-wave potential of 0.76 V vs. RHE for ORR. Moreover, a home-made flexible all solid-state battery is constructed by using Ni/CNF-750 as air electrodes, which provides a power density of 56.8 mW cm(-2) and wonderful cycling durability with maintaining 50 cycles, and can drive a light-emitting-diode (LED) device. Our work may provides a reli-able approach for fabricating ultrafine metal nanoparticles anchored on carbon based substrate with high activity for next-generation energy conversion and storage devices. (C) 2020 Elsevier Inc. All rights reserved.