Cobalt and cobalt oxides N-codoped porous carbon derived from metal-organic framework as bifunctional catalyst for...

Metal-organic framework (MOF)-derived transition metal/metal oxide-carbon hybrids are promising cost-effective electrocatalysts to replace noble metal catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, Co@CoO@Co₃O₄-N/C was prepared by two-step thermal treatment of Co-MOF ([Co(INA)₂]·0.5EtOH) (INA: isonicotinic acid). Firstly, Co-MOF, as precursor, was pyrolyzed at different temperatures in N₂ atmosphere to obtain Co-N/C-T (T = 700, 800, 900 °C) materials among which Co-N/C-800 shows remarkably high ORR activity. After oxidation treatment, Co-N/C-800 is transformed into Co@CoO@Co₃O₄-N/C which exhibits enhanced electrocatalytic activities for both ORR and OER. The as-obtained Co@CoO@Co₃O₄-N/C has more positive onset potential (-0.136 V vs. Ag/AgCl) and higher limit current density (4.9 mA cm^-2) than Co-N/C-800 (-0.143 V vs. Ag/AgCl and 3.9 mA cm^-2), as well as better tolerance to methanol and stability (80.0%) than those of Pt/C (63.2%) for ORR. Co@CoO@Co₃O₄-N/C also displays outstanding OER performances, with lower overpotential (450 mV) than that of Co-N/C-800 (492 mV) at a current density of 10 mA cm^-2. The excellent electrochemical performance of Co@CoO@Co₃O₄-N/C can be ascribed to uniformly dispersed Co-N_x active sites, strong synergistic effects between N-doped carbon support and Co@CoO@Co₃O₄ as well as ordered mesoporous structure, boosting mass transfer and accelerating electrocatalytic reaction.