Rational Design Of Metal-organic Complexes Derived Electrocatalyst For Efficient Oxygen Reduction Reaction

Oxygen reduction reaction（ORR）is a critical electrochemical reaction in those clean electrochemical energy conversion devices,including metal-air batteries and fuel cells.However,the ORR reaction kinetics is pretty slow,resulting in high cathodic overpotentials.It is necessary to use excellent electrocatalysts to speed up the reaction.Currently,the most efficient ORR electrocatalysts are Pt-based materials.Unfortunately,the scarcity and high cost of resources hinder their large-scale commercial applications.Pt-based materials also suffer from insufficient durability and poor poisoning resistance.Therefore,it is very necessary to develop economical,stable and excellent transition metals-based ORR catalysts to replace Pt-based materials.This dissertation mainly focus on non-precious metal catalysts.By using the transition metal complexes and carbonization at high temperature,catalysts with excellent single atom electrocatalytic activity were obtained.The active site and reaction mechanism of electrocatalyst were studied by DFT and experimental characterizations.We also investigated the application of as-developed electrocatalysts in zinc-air batteries.The main research contents are as follows:（1）we have developed a rationally designed strategy to fabricate Cu single atoms（SAs）in carbon nanosheets（Cu-SAs/NSs）from pyrolysis of two-dimensional（2D）Cu/Zn bimetallic MOF.Specifically,melamine was deliberately added to tune the p H and intrigue the self-assembly between the deprotonated H₂BDC-NH₂and metal cations(Zn²⁺and Cu²⁺).When these trapped nitrogen-rich molecules were decomposed,the in situ generated gases contributed to the formation of highly porous structure and abundant Cu-N₄active sites.This facile strategy was also successfully utilized to fabricate other transition metal catalysts（Fe-SAs/NSs,Co-SAs/NSs,Ni-SAs/NSs）,demonstrating the generality of this synthetic method.The as-synthesized CuSAs/NSs possess abundant Cu-N₄active sites in the hierarchical porous structure with superior ORR performance with high onset potential(E_onset=1.05 V vs.RHE)and half-wave potential(E_1/2=0.9 V vs.RHE),outperforming the Pt/C benchmark and most electrocatalysts reported so far.Furthermore,the Cu-SAs/NSs-based Zn air battery shows outstanding performance compared to the one assembled with Pt/C+RuO₂,demonstrating the potential application of Cu SAs/NSs in electrochemical energy conversion and storage technology.（2）Enlightened by the theoretical predication,which revealed the pivotal role of nitrogen dopant in carbon skeletons in serving an electron-withdrawing environment and boosting ORR activity of Fe-N₄sites.We report a facile synthesis of Fe-N₄single-atom electrocatalysts anchored in a hierarchical porous nitrogen-rich carbon support with tubular channels.This single atom electrocatalyst is derived from a complex of Fe and melamine.Anchoring the active Fe-SAs in the carbon support provides maximized Fe-N₄active sites,and the hierarchical porous structure with tubular channels provides excellent fast mass transport during ORR.Consequently,Fe-SAs@NCTCs exhibits remarkable ORR performances with high half-wave potential(E_1/2)of 0.91 V（vs.RHE）in 0.1 M KOH and E_1/2of 0.80 V（vs.RHE）in 0.1M HCl O₄.The fabricated rechargeable Zn–air battery displays promising performance,including large power density and high specific capacity,outperforming the Pt/C+RuO₂-based battery.Specifically,the Fe-SAs@NCTCs-based Zn-air battery can operate steadily for 650 hours with a narrow discharge/charge gap of 0.76 V.This work exemplifies an efficient SA-based electrocatalyst for ORR,and projects a rational strategy,which involves controlling the atomic configuration of active sites and optimizing the morphology of electrocatalyst,to boost the ORR activity of SAs.