Taming transition metals on N-doped CNTs by a one-pot method for efficient oxygen reduction reaction

Various transition metals have been incorporated into nitrogen-doped carbon nanotubes (M-N-C/CNT, M = Fe, Co, Mn, and Ni) via a one-pot method using dopamine as nitrogen source and metal salts as precursors for oxygen reduction reaction (ORR). Raman spectra and XRD patterns of the catalysts were collected to characterize the graphitization degree and metal state. XPS was employed to determine atom state and element fraction. The electrochemical performance of the catalysts for ORR were evaluated in alkaline media at −0.8 V to 0.2 V. The E onset and E 1/2 with the values of −100 mV and −170 mV (vs Ag/AgCl) are achieved on Mn-N-C/CNT-800. The superior selectivity toward the 4e − pathway are obtained on Mn-N-C/CNT-800 and Co-N-C/CNT-800 with the transferred electron numbers per O 2 molecule of 4.12 and 3.94, respectively. Results show that the states of doped transition metal play a key role on determining ORR performance. The electron transfer number of Ni-N-C/CNT at −0.5 V is increased from 3.22 (Ni-N-C/CNT-800) to 3.99 (Ni-N-C/CNT-600) when the metallic Ni has been eliminated at lower pyrolysis temperature.