Study On The Oxygen Reaction Electrocatalysis Based On Graphene And Metal-organic Frameworks

With the serious deterioration of environment and the rapid depletion of conventional fossil fuel,developing clean,low cost,and sustainable next generation energy storage and conversion technologies becomes the most optimal route in the future industry.The fuel cells,metal-air batteries and water splitting devices are the promising candidates to be used as the future renewable energy technologies.However,the implementing of these energy technologies in our daily life is still a big challenge because the sluggish kinetics of involved oxygen reaction including:oxygen reduction reaction?ORR?in fuel cells and both ORR and oxygen evolution reaction?OER?in rechargeable metal-air batteries,as well as the OER for water splitting devices.Noble metal based electrocatalysts such as platinum?Pt?,ruthenium oxide?RuO₂?/iridium oxide?IrO₂?exhibit the most efficient electrocatalytic activity for ORR and OER.However,the high prices,low reserve in the earth and poor long-term stability have seriously impeded their practical application.Developing highly efficient oxygen reaction electrocatalyst with low price is the main goal for the researchers.In the recent years,graphene and metal-organic framework?MOFs?based materials have been regarded as the most popular cadicates to replace the conventional noble metal electrocatalysts in oxygen reaction.However,it is usually difficult to contral the doping level in heteroatom-doped graphene,the pyrolyzed MOFs materials still lack of high conductivity and high temperature means high energy consumption et al.These problems urgently required to be solved.To address the above issues,we have launched a series of research based on the graphene and MOFs materials as oxygen reaction electrocatalysts.The detailed researches including:???There are great amount of sp² carbon atoms in the graphene carbon lattice.The doping reaction would happen when 4-nitrobenzene diazonium tetrafluoroborate?4-NBD?was mixed with graphene,and small organic molecule of nitrobenzene with strong electron-withdrawing could be doped into graphene lattice via covalent C-C bonding.After electrochemical measurement,it could be found that when the mass ratio of 4-NBD to graphene was 1:20,the best ORR electaocatalytic performance would be obtained.The strong electron electron-withdrawing ability of organic molecule induces the charge transfer happening between graphene and nitrobenzene which could further enhance the electrocatalytic ORR activity of graphene.This work broadens the development direction of metal-free ORR electrocatalyst.However,the electrocatalytic performance of the as-prepared NB-graphene is much poor than that of the commercial Pt/C electrocatalyst.Therefore,preparation of metal compound/graphene hybrid would be an effective strategy to enhance the electrocatalytic activity of graphene for oxygen reaction.???NH₃ plasma was applied to treat the Co₉S₈/graphene hybrid.The experimental results indicated the plasma process could not only lead to N atom doping into both Co₉S₈ and graphene,but also partially etch the surface of Co₉S₈ and graphene.The heteroatom doping could efficiently tune the electronic properties of Co₉S₈ and graphene,and the surface etching could expose more active sites for electrocatalysis,which can contribute significantly to the enhanced electrocatalytic performance for ORR and OER.The electrochemical results revealed that the etched and N-doped Co₉S₈/G shows excellent ORR activity,which is close to the commercial Pt/C catalyst,and great OER activity.???A novel bi-functional electrocatalyst of Co-embedded carbon nanotube/porous carbon?Co-CNT/PC?was prepared via a one-step pyrolysis of ZIF-67 encapsulated Co₃O₄ nanoparticles.The research was first focused on the effect of Co₃O₄ NPs when being pre-loaded into the ZIF-67.The results revealed that the presence of the pre-loaded Co₃O₄ NPs is essential for the successful preparation of Co-CNT/PC,and the pyrolysis process led to the reduction of Co₃O₄ to Co NPs,which could be used as catalysts for growth of CNTs.Meanwhile,during the pyrolysis process,the 2-methylimidazole ligands in ZIF-67 were partially carbonized to porous carbon and partially decomposed to active C and N species for the in-situ growth of N-doped CNTs on Co catalysts.The Co²⁺ from ZIF-67 would be converted to additional Co NPs and Co-Nx moieties as active sites for ORR electrocatalysis.Thanks to the precence of CNT,the as-prepared Co-CNT/PC exhibited extraordinary electrocatalytic performance towards both ORR and OER.The Co-CNT/PC also demonstrateed its promising application in Zn-air batteries which showed excellent charging/discharging voltage and better stability that the mixture of Pt/C and RuO₂ electrocaalyst.In spite excellent electrocatalytic performance of Co-CNT/PC,the high energy-comsuption of pyrolysis process could not be avoided.Thus,we will try to treat the pristine ZIF-67 at room temperature by plasma technology and study the enhancement of electrocatalytic activity.???O₂ plasma was also used to treat ZIF-67 at room temperature.The TEM,XRD,XPS,HADDF-STEM,and XANES et al.confirmed that atomic-scale CoOx species was on-site produced in ZIFs?denoted as CoOx-ZIF?as an efficient OER electrocatalyst.During the plasma treatment,the Co-N coordination bonds in the ZIFs were partially broken and the suspended Co species could be easily oxidized by the present O₂ in the system to obtain CoOx species locally.The as-obtained CoOx species in ZIF-67 show advanced electrocatalytic performance for OER in alkaline electrolyte.Coupling the CoOx-ZIF with conductive supports could bring even better activity than RuO₂.The excellent stability also suggested the further potential application in water splitting technology.The work realizes the targeted on-site formation of atomic-scale species for efficient electrocatalysis in MOFs matrix by plasma technology.