Factors Of Affecting The Performance Of Oxygen Reduction Reaction Of Carbon-Supported Transition Metal Electrocatalysts

Energy conversion devices such as fuel cells have the advantages of no exhaust emission and high energy density.They have wide applications in transportation,electronic equipments and other fields.However,the sluggish kinetics of oxygen reduction reaction（ORR）limit the performance of fuel cells,and high efficient electrocatalyts are required to accelerate the reaction.Precious metal-based catalysts such as Pt/C has the disadvantage of high cost and easy poisoning,thus it is urgent to develop non-precious metal-based electrocatalysts with high activity and low cost which can be commercially produced.There are many studies about the mechanism and active sites of nitrogen-doped carbon-supported transition metal ORR catalysts,and it has been comfirmed that nitrogen-containing metal species are the primary active sites for ORR.Nevertheless,still lots of factors that affect the electorcatalystic performance are not well acknowledged.Herein,the effects of different factors,such as different organic ligands of precursors,the structure and morphologies of the carbon supports,and the type of nitrogen-free transition metal-based species,on the ORR activities are studied for the carbon supported transition metal catalysts.The possible active sites and reaction mechanisms were analyzed as well.The main contents include:（1）The effect of organic ligands of precursors on ORR performance of Co-N-C catalysts.Four precursors with different types of organic ligands（2-Methylimidazole（2-Me IM）,Benzimidazole（BIM）,Dicyandiamide（DCD）,and glucose（GLU））were prepared by direct grinding the mixtures of organic ligand and cobaltous nitrate.The Co-N-C catalysts and N-free Co-C catalyst were obtained by pyrolysis of the different precursors at 800°C.It is found that the organic ligands significantly affect the morphology and structure,specific surface area,metal content and nitrogen content of catalysts.Among these catalysts,Co-N-C（2-Me IM）electrocatalyst exhibits the best performance with an onsite potential（E₀）of 0.96 V,and a half-wave potential(E_1/2)of0.83 V,which due to the high nitrogen content,high specific surface area,and abundant microporous and mesoporous structure.（2）Regulation of the structure of bimetallic Fe_xCo_y-N-C catalysts.Metal-organic complexes were prepared via a simple Na OH-mediated method by the organometallic reaction of Fe³⁺/Co²⁺ions and benzimidazole in the presence of polyvinylpyrrolidone.Metal-organic complexes with either cuboid or cylindrical morphologies were obtained by modulating the initial ratio of Fe³⁺/Co²⁺.Fe Co containing N-doped carbon catalysts（Fe_xCo_y-N-C）decorated with Fe_xCo_y nanoparticles were then fabricated by a direct carbonization of the metal-organic complexes.Fe_xCo_y-N-C catalysts have hierarchical micro-/mesoporous-structure and retain the morphologies of microcuboids or microcylinders of the metal-organic complexes precursors.The Fe₁Co₇-N-C exhibits promising catalytic activity with an E₀ of 0.97 V,a E_1/2 of 0.82 V in 0.1 M KOH solution.The ORR catalytic activity should be attributed to the high specific surface area,special morphologies of carbon-based materials and bimetallic synergetic effect.Fe Co-N_xspecies are suggested to be the main active sites.The encapsulated Fe Co alloy nanoparticles activating the surrounding N-doped carbon layers which is important to the ORR performance as well.（3）The role of nitrogen-free transition metal species on ORR activity.It is generally believed that the ORR performance of carbon-supported transition metal catalysts can be significantly affected by transition metal itself and nitrogen-doped carbon,M-N_x species are suggested to be the main active sites.However,the researches about nitrogen-free species affecting the ORR performance of carbon-supported transition metal catalysts are still very limited.Using benzenetricarboxylic acid as the carbon source and metal acetate as the metal source,and the nitrogen-free carbon supported transition metal catalysts（M-C）were prepared by directly pyrolysized at1010°C.The results show that the type of transition metals largely affects the structures and performance of the catalysts,and the electron transfer number of ORR follows roughly an order of Cu>Co>Mn>Fe>Ni.It is found that Cu-C and Co-C exhibit promising ORR activity close to the four-electron transfer mechanism in 0.1 M KOH electrolyte.Fe-C and Mn-C show a mixture of two-electron and four-electron transfer pathways.Ni-C is an efficient catalyst for the electrochemical synthesis of H₂O₂ and the ORR process basically follows a two-electron transfer pathway.We found that the ORR activity of M-C is closely correlated to the O contents in the catalysts.The O-contained species such as M-O_x are suggested to play a crucial role in the four-electron transfer ORR mechanism with a sequential 2×2e^-process in M-C catalysts.