Co3O4@carbon with high Co2+/Co3+ ratios derived from ZIF-67 supported on N-doped carbon nanospheres as stable bifunctional oxygen catalysts

Abstract

Abstract Zeolite imidazole framework-67 (ZIF-67) derivatives are increasingly used for oxygen reduction and evolution reactions (ORR/OER). Many fundamental issues concerning their structure-bifunctional activity relationships still remain unknown. Here we use nitrogen-doped carbon nanospheres (NCS) to support ZIF-67-derived Co3O4@carbon to obtain Co3O4@Z67/NCS catalysts with the assistance of polyvinylpyrrolidone (PVP). Carbonization temperatures have great effects on the structure and active-component changes of Co3O4@Z67/NCS. Co3O4@Z67/NCS-850 (850 °C) with the Co2+/Co3+ ratio of 1.58 shows the promising bifunctional (ORR/OER) activity (ΔE=Ej=10 (1.55 V)-E1/2 (0.812 V) =0.738 V) and long-term stability via the 4e− process in alkaline electrolytes. Co3O4@Z67/NCS-850 exhibits a comparable half wave potential of 0.812 V with a better ORR durability (an activity decline of 16.3 % after 30000 s) and methanol resistance, by comparing with commercial Pt/C (0.817 V and 29.1 %). For OER, Co3O4@Z67/NCS-850 obtains a low overpotential of 0.32 V at 10 mA cm-2 and a low charge transfer resistance of 9.69 Ω. The active-sites (Co2+ (ORR) and Co3+-CoOOH (OER)) on Co3O4 are well protected by carbon shell (Z67), which can hinder the fast deactivation (corrosion and agglomeration) of Co3O4 during electrocatalysis. The high porosity (387.88 m2 g-1) of Z67/NCS-850 should facilitate the mass transfer through the pores to promote the in-situ electrochemical O2 reduction/evolution on the Co and/or N active sites. These results not only find the relationships between structure/components and ORR/OER activities, but also indicate a direction on promoting ZIFs-derived catalysts.