Hierarchical N-doped CNTs grafted onto MOF-derived porous carbon nanomaterials for efficient oxygen reduction.

ABSTRACT

The rational design and preparation of nonprecious metal-based oxygen reduction reaction (ORR) catalysts to facilitate electron and mass transport are of great significance in oxygen-involved energy applications. Herein, a stepwise approach to synthesize a type of hierarchically porous N-doped carbon nanotubes (CNTs) grafted onto zinc-based coordination polymer derived carbon nanomaterials (M-NCNT, M = Fe/Co/Ni) is proposed. At first, an isostructural zinc-based metal-organic framework (MOF) to HKUST-1(Cu) (ZnHKUST-1) is solvothermally prepared, and then under pyrolysis to obtain MOF-derived porous carbon. After the secondary calcination, the in-situ formed N-doped CNTs are efficiently catalyzed by iron group metal-based nanoparticles (Fe/Co/Ni), which are thermally reduced by porous carbon together with additional urea. The synergistic effect between ultrahigh porosity, large surface area, suitable N-doping, high graphitization degree, and ultrafine metal particles prompts M-NCNT series to exhibit satisfactory electrocatalysis in oxygen reduction. Among them, Fe-NCNT owns the optimal ORR activity with high positive onset potential (0.987 V), half-wave potential (0.860 V) and large diffusion-limited current density (4.893 mA cm-2). Meanwhile, it shows a high current retention of 90.7% after the 24-hour stability, and the obtained Zn-air battery by Fe-NCNT with open-circuit voltage of 1.44 V owns moderate capacity and satisfying stability. The demonstrated method to prepare hierarchically porous N-doped carbon nanomaterials stemmed from MOF precursors unfolds a new route for the facile construction of efficient nanocatalysts for advanced energy applications.