Efficient ORR activity of N-doped porous carbon encapsulated cobalt electrocatalyst derived from a novel bimetal-organic framework

Abstract

Transition metal nitrogen-doped carbon catalysts have been feasible substitutes for Pt-based electrocatalysts for oxygen reduction reaction (ORR). Herein, porous cobalt N-doped carbon materials with core-shell nanostructure were synthesized by direct carbonization of bimetal-organic framework CoxZn100-x(adeninate)4(biphenyldicarboxylate)6 (x = 0, 3, 5, 8) precursors at various temperatures (800, 900, 1000 °C) in Ar atmosphere. The as-prepared catalysts present core-shell nanostructure and highly graphitic hollow carbon-ring structure. Meanwhile, Co nanoparticles are evenly distributed in the nitrogen-doped carbon matrix. The effects of different molar ratios of Co/Zn and pyrolysis temperatures on ORR catalytic performance were evaluated. After replacement of 5 % Zn in the precursor by Co, the Co5-N-C-900 exhibited superior oxygen reduction activity to that of 20 wt% commercial Pt/C catalyst in 0.1 M KOH, as evidenced by higher half-wave potential (0.86 V vs. 0.85 V) and diffusion-limited current density (−5.79 mA cm−2 vs.−5.26 mA cm−2) vs. RHE. Additionally, the catalyst has long-time stability than that of Pt/C catalyst. Co core-shell nanostructure, large surface area, porous structure and Co-Nx active sites of Co5-N-C-900 were beneficial to the oxygen reduction property, which enlarged the contact area between catalyst and oxygen molecules, exposed sufficient active sites, and elevated mass diffusion and electron transfer rates. Overall, this study provides an appropriate strategy for the synthesis and application of bimetal-organic framework-derived transition metal nitrogen-doped carbon materials as oxygen reduction electrocatalysts.