Preparation And Application Of Graphene-based Non-noble Metal Catalysts For Electrocatalytic Oxygen Reduction Reaction

Electrocatalytic oxygen reduction（ORR）is imperative to the development of a new generation of energy conversion and storage equipment.There are mainly two reaction pathways of it,the first one is the 4e pathway and convert oxygen to H₂O,while the other is the 2e pathway and convert oxygen to hydrogen peroxide（H₂O₂）.The ORR through 4e pathway can be applied to fuel cells and metal-air cells,both of which have high energy density and energy conversion efficiency.For fuel cells,ORR reaction takes place at the cathode and Hydrogen oxidation reaction（HOR）takes place at the anode.Similarly,the output energy capacities of metal air batteries also depend on the activity and stability of ORR cathode.However,the ORR kinetics at the cathode is about 6 orders of magnitude slower than the HOR at the anode,which seriously limits the large-scale commercialization of hydrogen fuel cell vehicles or metal-air batteries.For the ORR through 2e pathway,the H₂O_2,which is a valuable chemical product,can be synthesized from it.The in situ electrochemical synthesized H₂O₂are less-consuming and more environmental friendly.Researchers have focused on the electrochemical production of H₂O₂only recently,and it has not been employed for industrial use due to the lack of cheap and stable ORR cathodes.The current benchmark catalysts for fuel cells,zinc air cells and hydrogen peroxide production are all composed of highly dispersed platinum based or other carbon black supported noble metal nanoparticles.Therefore,it is of great significance to reduce the loading of precious metals or to use cheap non precious metal catalysts.This paper focuses on the preparation methods of non-precious metal doped carbon materials,through liquid phase synthesis and chemical vapor deposition method to synthesize nano carbon composites.The main research work includes the following three aspects:1、Surface-confinement engineered Fe-Nx moieties with dual nitrogen sources on carbon nanotube for boosting oxygen reduction reaction performance.Here in a typical Fe-N/C catalyst composed of nitrogen-rich porous carbon nanosheet with improved amounts of Fe-N_xsites on carbon nanotube（NR-CNT@Fe N-PC）was successfully prepared by using polyaniline（PANI）and dicyandiamine（DCD）as binary nitrogen sources coupled with silica-confined pyrolysis,which led to synergistically improving the formation of more micropores and nitrogen-containing Fe-N_xsites in the carbon matrix.Benefiting from the advanced composition and nanoporous structure to guarantee the sufficient accessible active sites for ORR,the NR-CNT@Fe N-PC catalyst under optimized conditions delivers prominent ORR performance with a half-wave potential（0.88 V vs.RHE）of 20 m V more positive than that of commercial Pt/C catalyst in alkaline electrolyte.When assembled in a home-made Zn-air battery device as cathodic catalyst,it achieved a maximum output power density of 246 m W cm^-2and a specific capacity of 719 m A h g_Zn^-1outperformed commercial Pt/C catalyst.It has a good application prospect in metal air battery.2、Single walled carbon nanotubes（SWNTs）were oxidized by different acids to improve their oxygen reduction activity and selectivity.SWNTs were treated with concentrated sulfuric acid,concentrated nitric acid,mixed sulfuric acid and nitric acid.In alkaline and neutral media,SWNTs with high concentration oxygen functional groups treated by mixed acid showed high activity and selectivity for the synthesis of H₂O₂under low overpotential.Combined with a variety of physical characterization analysis,it is shown that carbon oxide play an important role in the catalysis of 2e ORR,which provides a deep understanding of the defect carbon as the active catalytic site of 2e ORR.3、Based on the chemical vapor deposition（CVD）method,a kind of fluorine nitrogen Co doped graphene nanosheets was synthesized,and its hydrogen peroxide yield was tested.Graphene nanosheets with high F and N doping levels were synthesized by chemical vapor deposition using 2,2,3,3-tetrafluoropropanol as F source and pyrimidine as N source.The catalyst has good 2e ORR activity and acid stability.At 0.1V vs RHE,the highest yield of H₂O₂(3.18 mol g^-1h^-1)was obtained and the Faraday efficiency was 93.6%.The degradation of methylene blue（MB）was further explored.Under the experimental conditions,50 mg L^-1MB could be completely removed in 20minutes.The synergistic effect of F and N co doping was studied experimentally.It was proposed that both F and N are essential for the high activity of 2e ORR.The research results can provide reference for further material design and development.