Nonprecious Transition Metal Based Oxygen Electrocatalysts Derived From Polydopamine-metal Ion Complex:Synthesis And Electrocatalytic Performance

The electrochemistry of molecular oxygen including the oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?has been intensively investigated for more than a century.Unfortunately,both reactions still possess as primary bottlenecks for the commercial applications of some renewable energy conversion/storage devices such as fuel cell,metal air battery,and water electrolyzer.Noble metal platinum?Pt?and ruthenium/iridium?Ru/Ir?oxides have been recognized as the state-of-the-art ORR and OER catalysts,respectively,but their high cost and limited reserves severely hinder their large-scale implementation in these devices.Therefore,it is imperative to explore sustainable and efficient nonprecious-metal based electrocatalysts.Among various electrocatalysts studied so far,earth-abundant first-row transition metal-nitrogen decorated carbon?M-N-C,M stands for transition metals,such as Fe,Co,etc.?catalysts have been extensively investigated as the most promising candidates for ORR/OER.However,it remains as an open question as how to rationally design and controllably synthesize highly efficient M-N-C catalysts.Further,the nature of the active sites in this type of catalysts and the correlation between the structure and the activity are still far from being clealy understanding.In this thesis,a facile and effective approach is developed to synthesize M-N-C catalysts by using bio-inspired polydopamine?PDA?as a universal matrix for loading of transition metal ions as well as N-rich source.The main contents of this paper are as follows:?1?Mesoporous Fe/N co-doped graphene with encapsulated Fe₃C nanoparticles?Fe₃C@Fe/N-graphene?is synthesized as an efficient ORR electrocatalysts.A precursor composed of Fe ion-incorporated PDA modified graphene oxide?Fe-PDA-GO?is prepared via a simple one-pot reaction in an aqueous solution at room temperature,followed by therm annealing in Argon?Ar?atmosphere,the precursor is converted to Fe₃C@Fe/N-graphene.Chemical compositions and electrochemical performances suggest that Fe₃C nanoparticles are surrounded by additional carbon layers.The as-prepared catalyst shows high ORR catalytic activity with overwhelming four-electron pathway,long term durability and high methanol tolerance in alkaline media.?2?Mesoporous Co/N co-doped graphene with encapsulated Co/CoO_x nanoparticles?Co/CoO_x@Co/N-graphene?is synthesized as an efficient ORR electrocatalysts.A composite precursor composed of Co ion-incorporated PDA modified graphene oxide?Co-PDA-GO?is first prepared,followed by therm annealing in Argon?Ar?atmosphere to obtain the Co/CoO_x@Co/N-graphene electrocatalyst.This catalyst contains abundant Co/N active sites and Co/CoO_x nanoparticles and displays commercial Pt/C-comparable ORR activity in alkaline medium.Control experiments further unveil the nature of ORR active sites in the M-N-C catalysts,both the M-N_x sites and Co/CoO_x nanoparticles contribute to the excellent ORRcatalysts.?3?Metallic Co/spinel CoFe₂O₄ nanoparticles supported on nitrogen-doped graphene is synthesized as an efficient bifunctional oxygen electrocatalyst.Fe and Co binary transition metal ions are introduced to PDA film to form the precusor?Fe/Co-PDA-GO?,followed the calcination process,Co/CoFe₂O₄ nanoparticles are formed via a solid-state reaction and are entrapped by the PDA-derived N-doped carbon layer.This method enables simultaneous N-doping of graphene and in situ growth and entrapment of metal/spinel oxide nanoparticles with intimate contact.After careful optimization of Fe/Co concentration and other experimental parameters,the resulting Co/CoFe₂O₄@N-graphene exhibits highly efficient catalytic activity and excellent stability for both ORR and OER in alkaline solution.In summary,this thesis develops a facile and universal way to synthesize M-N-C nonprecious metal oxygen electrode catalysts and it shows great potential in the development of highly efficient electrocatalysts.By changing the transition metal type,concentration and using binary transition metal,three efficient oxygen electrocatalysts are synthesized and their catalytic behaviors,the nature of active sites and the carrelation between the structure and the activity are investigated in details.This thesis may offer valuable insights into the rational design and controllable synthesis of nonprecious electrocatalysts with high catalytic performance.