Mussel-inspired facile synthesis of Fe/Co-polydopamine complex nanospheres: complexation mechanism and application of the carbonized hybrid nanospheres as an efficient bifunctional electrocatalyst

Carbon nanostructures containing binary transition metal compounds have shown great potential for applications such as electrocatalysis. In this article, we report a one-pot synthesis of Co(ii)-Fe(iii)-polydopamine (PDA) complex nanospheres as a facile approach to binary metal-doped carbon nanostructures. The complex nanospheres were prepared simply by polymerization of dopamine (DOPA) in a basic aqueous solution containing both types of transition metal ion. Results show that cobalt(ii) ions do not form complexes with DOPA, whereas can form coordination bonds with PDA oligomers after the initiation of polymerization, leading to irregular shaped complex particles. When both cobalt(ii) and Fe(iii) ions are present in the polymerization system, Fe(iii) ions, which are able to form complexes with DOPA before the initiation of polymerization, play a dominant role in determining the morphology of the complex nanostructure, leading to hybrid complex nanospheres. Furthermore, the Co(ii)-Fe(iii)-PDA complex nanospheres can be easily converted to mesoporous N-doped carbon nanospheres with embedded CoFe₂O₄/CoFe nanoparticles. Electrochemical studies suggest that these nanospheres exhibit enhanced bifunctional electrocatalytic performance over commercial Pt/C towards ORR and OER. They also show good stability with steady cycling performance for over 300 cycles when used as a bifunctional oxygen electrocatalyst in a rechargeable zinc-air battery cell.