Synthesis Of High Efficiency Carbon Materials In The System Of Oxygen Reduction To Hydrogen Peroxide

The efficient electrocatalytic production of hydrogen peroxide（H₂O₂）through the oxygen reduction reaction（ORR）in fuel cells has the advantage of being able to produce low concentration H₂O₂ solutions in situ,which can greatly reduce the cost of H₂O₂ synthesis,storage and transportation.For this system,the current focus is on the development of efficient and inexpensive catalysts,while the commonly used platinum-carbon catalysts have many disadvantages such as high price of precious metals and easy deactivation.As a petroleum distillation product,vacuum residuum（VR）is viscous and enriched in S,N,Ni and V,which make it difficult to utilize.Meanwile,VR has the advantages of high temperature fluidity,high carbon content and aromatic content,which can be used to synthesize efficient materials in ORR systems.In this paper,different types of carbon material catalysts（e.g.carbon fibers,hollow carbon cubes）were prepared by combining high temperature treatment and template method using VR as carbon source,and their ORR performance for H₂O₂preparation was investigated to deeply understand the structure-effect relationship between structure and reaction performance.The related research results have some implications for the design,synthesis and application of carbon catalysts prepared from VR.Firstly,the mixed VR containing Fe（NO₃）₃,CTAB and F127 was made into carbon fiber material with length over 100nm and diameter about 20nm by one-pot method.The influence mechanism of the synthesis method on the structure of carbon fiber materials was elucidated by varying the parameters of roasting temperature and raw material ratios,combined with characterization techniques.The results show that the sample prepared at 800^oC has the highest selectivity 60%and the most regular morphology,which can ensure the reaction performance with low reaction temperature and energy consumption.The prepared materials can have high electrical conductivity under alkaline conditions with an onset potential of0.8133 V and an ultimate current density of 3.75 m A cm^-2 in the ORR reaction,which contains heavy metal impurities that may inhibit the performance of the materials.Secondly,the preparation method of hollow carbon cubic materials was developed.The highly symmetric hollow carbon cube material was successfully fabricated by using Na Cl as a hard template and continuously depositing a carbon film on the liquid phase of the crystal particle surface with the assistance of CTAB surfactant,after high temperature carbonization and water washing to remove the Na Cl template agent.The TEM and SEM results show that the carbon cubes with special hollow structure are 100～300 nm in size,with an average wall thickness of 10 nm and an approximate aspect ratio of 1:1.It is noteworthy that the metal elements such as Na and Fe are negligible in this material,indicating that the heavy metal elements in VR are not incorporated into the carbon skeleton and only some light components play a synthetic role.Meanwhile,its high degree of graphitization makes it have strong electrical conductivity and stability,and the oxygen-containing,sulfur and nitrogen functional groups on the surface work together to coordinate and influence the electrochemical properties of the material.Besides,the material possesses a more stable and relatively high oxygen reduction selectivity（80%）in the potential range of 0.4～0.6 V in alkaline solutions.Finally,we find method to improve the H₂O₂ selectivity of carbon fiber and hollow carbon cubic materials.The elemental content,material morphology and surface functional groups were purposely modified by adding N-containing compounds,potassium hydroxide activation and nitric acid oxidation.The laws of the influence of different preparation methods on the selectivity were summarized,and the maximum selectivity of the tested samples could reach 83%with a low electron transfer number of 2.32,which showed good oxygen reduction performance.The gas diffusion electrode was prepared by using hollow carbon cubic composites as the powder catalyst diffusion layer and catalytic layer,and the H₂O₂ solution with 50 mg/L concentration could be synthesized under 1h in 1M KOH electrolyte with additional-0.7 V（0.385 V vs.RHE）potential.