Hybrids of Co-based Nanocrystals and Carbon Nanomaterials as Highly Efficient Oxygen Electrocatalysts

Author:Ma Xiu Xiu

Supervisor:he xing quan


Degree Year:2018





Regenerative fuel cells are of potential promise due to its advantages of high efficiency,environmental friendliness,easy device and long-term development.Oxygen reduction reaction and oxygen evolution reaction are two fundamental reactions that decide the efficiency of regenerative fuel cells,however,they are controlled by their sluggish kinetics.So far,Pt-based,Ru-and Ir-based noble composites display high catalytic performance for oxygen reduction reaction and oxygen evolution reaction,respectively,nevertheless,they are of high cost and limited storage in Earth,which hinder their widely commercial applications.Hybrids of transition metal compounds and heteroatom doped carbon can be used as potential oxygen electrocatalysts.We have done the research work in the six aspects:(1)On the foundation of nitrogen doped carbon matrix,it’s an important promotor for a catalyst to achieve high electrocatalytic performance that exposing more active sites on the surface.Via an in-situ hydrothermal method combined with the heat-treatment procedure,CoO and SnO2 were anchored on the nitrogen-doped three dimensional graphene,and it was discovered that SnO2 with high energy surface was strongly interacted with CoO,and induced the formation of more oxygen vacancy and defects in CoO.As a result,the prepared catalyst displayed high oxygen reduction reaction and oxygen evolution reaction performance.(2)The interaction between the 3d orbit center of transition metal andπelectrons of graphene is an important factor for a catalyt to obtain high catalytic activity.Therefore,adjusting the 3d electrons of transition metal is meaningful.Herein,the target Al2O3@Co/NG-800 catalyst was prepared via an in-situ hydrothermal method and pyrolysis procedure.When Al2O3 was coated on nitrogen doped graphene,a flower-like structure with more exposure of defects was formed,which enabled controllable growth of Co nanoparticles.At last,the flower like structure disappeared.XPS characterization demonstrated the electronic tailoring of Co by Al elements.Electrochemical measurements indicated the high performance of the catalyst for both oxygen reduction reaction and oxygen evolution reaction.The onset potential for oxygen reduction reaction was as positive as 1.018 V vs.revisible hydrogen electrode,and the potential gap between oxygen evolution reaction and oxygen reduction reaction was 0.745 V.After electrochemical measurements,Co nanoparticles were leached,the flower-like structure appeared again,and the obtained material showed close electrocatalytic performance to the target catalyst,confirming the electronic tailoring effect of Co was transferred to the graphene support to further promote oxygen reduction raction and oxygen evolution reaction.(3)Heteratoms co-doping is effective to improve the catalytic activity of a catalyst,ammoninated graphene can address the protonization issues of nitrogen dopants,and the high charges of[PO4]3-promotes the arrangement of the combined cation to form stable structure.Herein,nitrogen and phosphorus were co-doped on graphene through an in-situ hydrothermal method,and a high efficient catalyst for oxygen reduction reaction was constructed by anchoring Co2(OH)PO4 on the graphene matrix.The purity of Co2(OH)PO4 was as high as94.06%.The onset potential and half-wave potential for oxygen reduction reaction were 1.002and 0.816V vs.revisible hydrogen electrode,respectively,and the limiting current density was6.49 mA cm-2,close to those of 20 wt%Pt/C.The excellent activity of the hybrid was mainly due to the synergistical effects between nitrogen and phosphorus,the formation of active nitrogen and phosphorus species,the high BET surface area and porous structure.(4)Different active componets of Co4S3,NixS6 and NiOOH were introduced on the nitrogen and sulfur co-doped carbon matrix,and the inter-transformation for the various valance states of cobalt and nickel was a promoter for a catalyst to exhibit high catalytic activity.The effective combination can not only maximumly play a role in catalysis for each other,but also promote the activity of the optimized catalyst due to the synergistic effects.Three dimensional interpenetrated structure was formed for the support of graphene and acid dealt carbon nanotubes,which not only increased the electronic conductivity,but also increased the BET surface area and stability.Consequently,the target catalyst showed high bifunctional performance for both oxygen reduction reaction and oxygen evolution reaction.(5)Different active species for oxygen reduction reaction,oxygen evolution reaction and hydrogen evolution reaction were grown on graphene efficiently via in-situ hydrothermal and pyrolysis methods.The prepared material(Co3C/Co-N-C/G)exhibited high performance for the three reactions,which was ascribed to the high BET surface area and porous structure,the strong interaction between cobalt and nitrogen,and the synergistically effects.(6)Co4S3/carbon hybrids were of bifunctional performance.Through a NaCl-guided in-situ hydrothermal method,Co4S3 was anchored on nitrogen and sulfur co-doped graphene uniformly.The catalyst was of high doping contents of nitrogen,sulfur and cobalt,high BET surface area and porous structure,and electrochemical measurements confirmed its excellent performance for both oxygen reduction reaction and oxygen evolution reaction,and the potential gap between the two reactions was 0.81 V.Additionally,cobalt atoms were replaced by iron to study their catalytic effects on catalysis.