Well-defined gradient Fe/Zn bimetal organic framework cylinders derived highly efficient iron- and nitrogen- codoped hierarchically porous carbon electrocatalysts towards oxygen reduction

Abstract

The development of highly active and durable non-precious metal electrocatalysts (NPMCs) for the oxygen reduction reaction (ORR) is crucial for the sustainable commercialization of fuel cell technologies. In this work, we have synthesized a new class of bimetallic zeolitic imidazolite framework (Fe,Zn-ZIF) materials with a cylindrical morphology and a well-defined Fe/Zn composition gradient. The Fe/Zn gradient in the Fe,Zn-ZIF cylinders—with a Zn-rich core and a Fe-rich shell—results from a balance of the kinetics and thermodynamics of Fe– and Zn–benzimidazole coordination reactions. Pyrolysis of Fe,Zn-ZIF results in evaporation of Zn from the core of the cylinders generating an inter-connected hierarchical microporous–mesoporous structure, and the concomitant formation of highly exposed and accessible Fe-N4 sites on the surface of the resulting carbonized material, which retains the cylindrical morphology of the precursor. The as-obtained iron- and nitrogen-codoped carbon material thus has both high porosity, favoring mass transport of reactants, and an abundance of sites that are known to be active for the ORR on its surface. Indeed, when used as an ORR electrocatalyst the material outperforms a commercial Pt/C catalyst in alkaline electrolyte (with a half-wave potential ~50 mV higher) and possesses comparable activity to that of Pt/C in acidic electrolytes. Extension of this strategy to other gradient MOFs with diverse compositions and morphologies offers a new avenue for the design and synthesis of a variety of carbon-based materials with targeted functionalities.