Design And Oxygen Reduction Properties Of Carbon-based Transition Metal Nanocomposites

Fuel cells and metal-air batteries are the most promising clean energy conversion devices to replace fossil fuels to alleviate the energy crisis and environmental pollution.As the key reaction on the cathode,the oxygen reduction reaction(ORR)still requires effective precious metal catalysts(commercial Pt/C)to enhance its sluggish dynamic process.However,the high cost,limited reserve,and poor long-term durability severely restrict the commercial production and application of precious metal-based energy equipment.Therefore,it is important to design cheap,efficient,and stable carbon-based catalysts to replace precious metal-based catalysts.This dissertation focuses on the design of carbon-based transition metal nanocomposites,including transition metal carbides,nitrides,sulfides,phosphides/carbon composites,and isolated transition metal-doped carbon.By regulating the electron conductivity,surface area,porous structures,defects,and doping effect to achieve excellent ORR performances and application to Zn-air batteries.The five main parts are listed as following:1.Co9S8 nanoparticles/N,S-codoped defect-rich carbon nanotubes(Co9S8/N,S-CNTs)were fabricated by an ingenious template method.CdS nanowires can be removed simultaneously during the carbonization process.The large Brunauer-Emmet-Teller(BET)surface area(661.2 m2 g-1)and pore volume(1.49 cm3 g-1)of Co9S8/N,S-CNTs can largely expose more active sites and facilitate the mass transportation.The abundant defects produced by the evaporation of Cd and S doping can provide more active centers.Co9Sg/N,S-CNTs performs excellent ORR and oxygen evolution reaction(OER)activities in alkaline solution.2.Strongly coupled ultrasmall-Fe7C3/N-doped porous carbon hybrids(u-Fe7C3@NC)were designed by ZnO template method as ORR electrocatalyst.The u-Fe7C3@NC materials feature with relatively large BET surface area(321 m2 g-1),uniformly dispersed ultrasmall-Fe7C3 nanoparticles,and hierarchical pores.The strong interaction between u-Fe7C3 nanoparticles and NC promotes the charge transfer,O2 adsorption,and etching resistance during the ORR process.Benefitting from all these advantages,u-Fe7C3@NC shows excellent ORR activity,durability,and high performances for Zn-air batteries.3.A morphology-controlled strategy to fabricate uniform Co4N nanoparticles anchored on N-doped carbon(Co4N@NC-m)was proposed.The diameter and distribution of Co4N nanocrystals can be tuned to be homogeneous by abundant N sources in melamine.Moreover,thanks to the advantages of higher nitrogen doping content,better electrical conductivity,higher degree of graphitization,and larger electrochemical surface area,Co4N@NC-m performs excellent ORR and OER activities in basic solution.The Zn-air battery fabricated with Co4N@NC-m exhibits the great potential in practical applications.4.We proposed a simple grinding-calcination method to synthesize Fe2P anchored on 3D N,P-codoped porous carbon(Fe2P/NPC).The rich porous channels and large BET surface area(1288 m2 g-1)are generated after the ZnO removal during calcination,which boost the mass transfer and contact between active sites and electrolyte.The 3D porous carbon framework exhibits robust ORR activity in 0.10 M KOH.In addition,Fe2P/NPC-based Zn-air battery displays the larger open-circuit voltage(1.469 V)and power density(111.6 mW cm-2)than the Pt/C battery.5.A simple one-step strategy was proposed to prepare Fe,Co atomically supported on N-doped nanocarbon(FeCo-IA/NC)as ORR electrocatalyst.Both the electrochemical results and density functional theory(DFT)calculations indicate that the synergetic effect between Fe-N4 and Co-N4 accounts for the enhanced ORR activity.Benefiting from the large BET surface area(748 m2 g-1),microporous feature,and high content(85%)of pyridinic and graphitic N,FeCo-IA/NC owns better ORR activity and Zn-air battery performances.