Hybrid materials derived from polydopamine-transition metal complexes : formation mechanism and electrocatalytic functions

• Main Author: Ang, Jia Ming
• Other Authors: Lu Xuehong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2018
• Subjects: DRNTU::Engineering::Materials::Energy materials
• Online Access: http://hdl.handle.net/10356/74174
• Institution: Nanyang Technological University

In this PhD study, polydopamine (PDA)-transition metal hybrids synthesized via in situ polymerization of dopamine (DOPA) with the presence of transition metal species are studied. Iron(III) ions and cobalt(II) ions were chosen as the model systems used for this PhD study. Substantial efforts have been devoted to understand the interactions between DOPA/PDA and iron(III)/cobalt(II) ions, and the effects of the transition metal ions on oxidation polymerization and self-assembly behaviours of the hybrids. For the iron(III) ions system, it was found that the oxidative polymerization of dopamine and Fe(III)-PDA complexation co-contributed to the in situ “polymerization” process. During the polymerization process, the morphology of the complex nanostructure transformed from sheet-like to spherical due to the decrease in hydrophilic groups caused by the covalent polymerization, resulting in re-self-assembly of the PDA oligomers to reduce surface area. For the cobalt(II) ions system, cobalt(II) ions formed complex with hydroxyl ions and not DOPA monomers. With the initiation of oxidation, cyclization and polymerization of DOPA, the hydroxyl ions were then displaced by the oxidized DOPA units or PDA oligomers. When both iron(III) ions and cobalt(II) ions are added into the system, iron(III) ions were observed to play a more dominant role during the in situ polymerization process. The transition metal/PDA hybrids could be converted into transition metal/carbonized polydopamine (C-PDA) nanocomposites via a facile annealing process. These transition metal/C-PDA nanocomposites, Fe3O4/C-PDA and CoFe2O4/CoFe/C-PDA, were then investigated as oxygen electrocatalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in air cathode of primary and rechargeable zinc-air batteries (ZnABs). In the last part of this PhD study, the fabrication of a free-standing three dimensional (3D) carbon nanofibrous macrostructure embedded with CoFe/CoFe2O4 core/shell nanoparticles was reported. The carbon nanofibrous macrostructure was fabricated by the combination of electrospinning of polyacrylonitrile (PAN) and in situ polymerization of DOPA followed by carbonization. The CoFe2O4/CoFe/C-PDA nanofibers showed good ORR and OER electrocatalytic activity and was employed as a binder- and additive-free air cathode in rechargeable ZnAB. This PhD study has provided insights into the underlying mechanisms for the formation of PDA-transition metal hybrid nanostructures during the in situ polymerization process. With the knowledge obtained from this PhD study, it is possible to better predict and control the morphologies of the transition metal/PDA hybrids and transition metal/C-PDA nanocomposites that can be utilized as efficient oxygen electrocatalysts and also for other electrochemical reactions.