Ultra-small cobalt nanoparticles embedded into N-doped hierarchical porous carbon derived from Ion-Exchange MOFs as high-efficient bifunctional catalysts for rechargeable Zn-air battery

Rational design and accessible fabrication of bifunctional electrocatalysts with high electrocatalytic activity, low price and good durability for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are urgently required for metal-air batteries, but it remains as a difficult and hot subject. Herein, we report a novel cobalt-ion-exchange and pyrolysis strategy with the well-designed bio-MOF-1 as the self-template to prepare ultra-small cobalt nanoparticles embedded into N-doped hierarchical porous carbon (Co@N-HPC-800) with excellent electrical conductivity, high surface area and abundant Co-N-4 active sites. The electrochemical test results verify the outstanding bifunctional electrocatalytic activity and stability of Co@N-HPC-800 in alkaline condition, even outperforming the commercial Pt/C + RuO2 electrocatalysts. The in situ Raman technique demonstrates the ORR and OER catalytic process occur through the redox of Co(II) and Co(III) species. The rechargeable Zn-air battery (ZAB) assembled with Co@N-HPC-800 cathode deliveries an open circuit voltage of 1.42 V, a specific capacity of 129 mAh cm(-2), a peak power density of 89.1 mW cm(-2), and stable cyclability over 85 h at 5 mA cm(-2) by refreshing the electrolyte. The Co@N-HPC-800 cathode in a flexible quasi-solid-state ZAB exhibits good flexibility and stability under different bending states. This work paves a simple and valuable way for designing non-noble high-efficient bifunctional electrocatalysts in Zn-air batteries and related energy storage and conversion technologies.