Bimetal–Organic Framework Self-Adjusted Synthesis of Support-Free Nonprecious Electrocatalysts for Efficient Oxygen Reduction

Abstract

The development of low-cost catalysts with oxygen reduction reaction (ORR) activity superior to that of Pt for fuel cells is highly desirable but remains challenging. Herein, we report a bimetal–organic framework (bi-MOF) self-adjusted synthesis of support-free porous Co–N–C nanopolyhedron electrocatalysts by pyrolysis of a Zn/Co bi-MOF without any post-treatments. The presence of initial Zn forms a spatial isolation of Co that suppresses its sintering during pyrolysis, and Zn evaporation also promotes the surface area of the resultant catalysts. The composition, morphology, and hence ORR activity of Co–N–C could be tuned by the Zn/Co ratio. The optimal Co–N–C exhibited remarkable ORR activity with a half-wave potential of 0.871 V versus the reversible hydrogen electrode (RHE) (30 mV more positive than that of commercial 20 wt % Pt/C) and a kinetic current density of 39.3 mA cm –2 at 0.80 V versus RHE (3.1 times that of Pt/C) in 0.1 M KOH, and excellent stability and methanol tolerance. It also demonstrated ORR activity comparable to and stability much higher than those of Pt/C in acidic and neutral electrolytes. Various characterization techniques, including X-ray absorption spectroscopy, revealed that the superior activity and strong stability of Co–N–C originated from the intense interaction between Co and N, the high content of ORR active pyridinic and pyrrolic N, and the large specific surface area.