Formation of uniformly and finely dispersed nanoshells by carbonization of cobalt-coordinated oxine–formaldehyde resin and their electrochemical oxygen reduction activity

To obtain uniformly and finely dispersed nanoshell structures in nanoshell carbons (NSCs), we synthesized three oxine–polymer (OFR)/cobalt complexes with different ligand/metal ratios (L/M) (xOFR(Co), x = L/M = 1, 2, and 3) as the starting materials of NSCs by polycondensation of 8-hydroxyquinoline (oxine) and formaldehyde. The formation of polymeric structures and coordination bonds was confirmed by infrared spectroscopy. Thermogravimetry revealed that the thermal decomposition temperature increased with the L/M ratio. Carbonization of xOFR(Co) (x = 2 and 3) at 1000 °C resulted in the uniform formation of nanoshells with diameters of 30 nm, while carbonization of 1OFR(Co) resulted in the formation of nanoshells with a broader size distribution including some with diameters over several hundred nm. The results indicate that the fixation of metal atoms in the starting polymers is an effective method for the preparation of uniformly and finely dispersed nanoshells in NSCs. The carbon from 3OFR(Co), i.e., 3OFR(Co)/C, showed the highest electrocatalytic activity for the oxygen reduction reaction (ORR). The linear shape of the voltammograms were understood in terms of the distribution of active sites for ORR with different activation energies.