Soft-confinement conversion of Co-Salen-organic-frameworks to uniform cobalt nanoparticles embedded within porous carbons as robust trifunctional electrocatalysts

Co nanoparticles with about 10 nm particle size embedded in nitrogen-doped carbons (Co@N-C) were successfully synthesized by a soft-confinement conversion strategy. A special Co(II)-Salen-organic framework was prepared as the precursor, which isolated a few Co(II) ions with thick aromatic polymer pore walls. These Co(II) ions were thermally converted into fine Co nanoparticles highly dispersing in porous nitrogen-doped carbons. The as-prepared Co@N-Cs showed efficient electrocatalytic performance for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), together with better catalytic durability. In alkaline solution, the optimized Co@N-C-800 exhibited a positive ORR half-wave potential (E-1/2) of 0.85 V, superior to the benchmark Pt/C (E-1/2 = 0.84 V). It also had a low overpotential (E-eta) of 0.35 V at a current density of 10 mA cm(-2) for OER, close to IrO2 (E-eta = 0.36 V) catalysts. It could be also utilized as the outstanding air electrode materials in Zn-air batteries, which exhibited higher peak power density and cycling stability than Pt/C-based counterpart. By this unique soft-confinement conversion of Co(II)-Salen-COFs, the derived Co@N-Cs possessed the optimized local strcutures (high porosity and graphization degree) and the multiple active sites especially including high-density Co nanoparticle-activitated carbon layers, which contributed to their impressive trifunational electrocatalytic properities. (C) 2019 Elsevier Ltd. All rights reserved.