Optimization Of Active Sites Of Nanosized Carbon-based Materials And Their Electrocatalytic Performances

Rapid oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)are critical important for the development of clean and efficient energy systems.In this study,carbon-based electrocatalytic materials were prepared from the perspective of optimizing active sites by adjusting the composition and structure of various carbon-based materials.The prepared materials were characterized and analyzed by various techniques,such as SEM,TEM,AFM,Raman,XPS,XRD,FT-IR,and BET.Electrochemical workstation and rotating disk device were used to test their electrocatalytic performances,and zinc-air batteries were assembled and tested to explore potential applications of the prepared materials.(1)The preparation and electrochemical performance of graphene oxide quantum dots with high oxygen content.In a strong acid environment,original graphene oxide(GO)sheets are cut and oxidized by etching to obtain graphene oxide quantum dots(GOQDs).The GOQDs are in uniform size distribution.By optimizing post-treatment process,GOQDs are quickly separated from acidic environment to enter aqueous solution.This method solves a big difficult problem of traditional method of preparing quantum dots which were complex and time-consuming.The obtained GOQDs are very densely distributed in both TEM and AFM fields of view without agglomeration.The GOQDs have a quasi-spherical shape of about 2.5 nm.GOQDs have more oxygen-containing functional groups than those of original GO.And a large number of carbon defects can provide more catalytically active sites.Heat treatment is used to reduce GOQDs into GQDs.GQDs show enhanced ORR and OER catalytic performance than RGO,which proves their potential as electrocatalytic materials.(2)For the design of electrocatalysts,the combination between components and material structures tend to be neglected,giving rise to the constraint of catalytic performances and durability.Herein,we developed graphene oxide quantum dots(GOQDs)with enhanced oxygen content by a one-step cutting method.Then,one-dimensional(1D)carbon nanotubes and two-dimensional(2D)reduced graphene oxide are crosslinked and self-assembled,thus attracting unsaturated-bond-riches GOODs(0D)to uniformly attach to the skeleton,simultaneously achieving nitrogen and sulfur co-doping.To the best of our knowledge,there is no report to prepare bifunctional electrocatalyst with GOQDs.Electrochemical tests show that even without metal-doping,the novel non-metal bifunctional electrocatalyst(N,S-GOQD-RGO/CNT)exhibits a higher half-wave potential(0.84 V)and enhanced limiting current density(5.88 m A cm^-2)than commercial Pt/C catalyst.The density functional theory is implemented to reveal the coordination of nitrogen and sulfur co-doping on GOQDs,which results in the improvement of overall catalytic active sites.Furthermore,the rechargeable zinc-air battery based on N,S-GOQD-RGO/CNT exhibits a maximum power density of 134.3m W cm^-2,open circuit potential of 1.414 V,which are better than Pt/C+Ru/C mixed material.The obtained N,S-GOQD-RGO/CNT will provide a perspective application in Zn-air batteries.(3)Metal-free and metal-based electrocatalysts are designed according to different principles,but there should be an intimate connection between them.In this work,a simple yet cost-effective strategy is proposed to etch and disperse graphene oxide(GO)and porous hollow carbon shells loaded with Co S₂(PHCS-Co S₂)into a porous skeleton similar to"highway"to effectively"trap"graphene oxide quantum dots(GOQDs)via a one-step process.Due to interconnected networks and size effects,N-S codoped porous graphene carbon sheets along with GOQDs(N,S-PGCS(QDs))have eminent ORR and OER electrocatalytic properties.Furthermore,the metal-free system can be transformed to a metal-based electrocatalyst(N,S-PGCS(QDs)-Co_1-xS/Co₉S₈)loaded with Co_1-xS/Co₉S₈ nanohybrids,which demonstrates superior electrocatalytic characteristics than Pt/C+Ru O₂ in terms of half-wave potential(0.85 V),current density,methanol tolerance,the potential difference between E_ORR1/2 and E_OER10(0.68 V),and long-term stability.The Zn-air battery assembled with N,S-PGCS(QDs)-Co_1-xS/Co₉S₈ shows a large peak power density of 168.2 m W cm^-2,a capacity performance up to 817.9 m Ah g^-1,an ultra-high specific energy density of 973.3 Wh kg _Zn^-1 and considerable durability.The findings above may provide a scalable synthetic guidance and a novel direction for the design of metal-free and metal-based electrocatalysts.