Metal-organic framework-derived metal-free highly graphitized nitrogen-doped porous carbon with a hierarchical porous structure as an efficient and stable electrocatalyst for oxygen reduction reaction

Nitrogen-doped carbon materials are promising oxygen reduction reaction (ORR) electrocatalysts owing to high performance and stability. Herein, a three-dimensional porous bio-MOF-1, Zn8(Ad)4(Bpdc)6O·2Me2NH2 (Ad = adeninate; Bpdc = biphenyldicarboxylate), was used as precursor to fabricate N-doped porous carbon materials (NPC-1000-ts, where 1000 stands for the carbonization temperature and t represents the carbonization time, t = 2, 3 and 4 h) by simple carbonization under Ar atmosphere. The porous carbon materials had different contents of graphitic N and graphitization degrees of carbon. The catalytic activities of NPCs as metal-free ORR electrocatalysts were studied. The obtained NPC-1000-4 (pyrolysis under 1000 °C for 4 h) displayed outstanding ORR performance, with a positive onset potential (-0.012 V), a higher half-wave potential (E1/2) (-0.13 V) and a larger limiting current density (-5.76 mA/cm2) at -0.8 V (vs. Ag/AgCl) in KOH solution (0.1 M) than those of commercial Pt/C (20 wt%) catalyst (Eonset = -0.014 V, E1/2 = -0.14 V and -5.08 mA/cm2 at -0.8 V vs. Ag/AgCl). Obviously, the onset potential of NPC-1000-4 surpassed that of Pt/C, which was rare among currently available studies on metal-free nitrogen-doped porous carbon materials. Graphitic N significantly affected ORR catalytic performance besides graphitization degree of carbon. Meanwhile, NPC-1000-4 allowed an effective 4e--dominant ORR process, and most importantly, coupled with much higher long-term stability (89.5%) than that of commercial Pt/C (20 wt%, 65.8%) catalyst and higher resistance to methanol poisoning. The remarkable ORR activity of NPC-1000-4 can be ascribed to large surface area, considerable hierarchical porosity, high graphitization degree and synergism between enriched active sites and high portion of graphitic N. Overall, the findings guide the development of MOF-derived metal-free N-doped carbon materials as high-activity non-precious electrocatalysts for ORR.