Preparation Of Carbon-based Single Atom Catalysts Derived From Biomass And Their Cathodic Electrocatalytic Performance

Functionalized carbon-based materials are widely used in new energy devices as efficient electrode materials.It is an important subject in this field to develop efficient carbon-based materials with controllable structure and properties from biomass.In this paper,based on the cathode reactions in two new energy devices,the synthesis,structure and properties of biomass-derived carbon-based nanomaterials are studied by selecting suitable biomass precursors and synthesis strategies.In view of this,three different types carbon-based nanomaterials for ORR derived from biomass are prepared,including heteroatom doped carbon-based nanomaterials,carbon-supporting transition metal composite nanomaterials and atomically dispersed M-Nx-C materials.The results show that carbon-based nanomaterials derived from biomass have unique structure,composition characteristics and good ORR catalytic performance.When combined with transition metals(especially atomic-level metal active centers),the electrocatalytic performance of the naterials is further improved.The details are as follows:Firstly,N and F co-doped hierarchically porous carbon material is produced by one-step pyrolysis using tea residue as carbon and heteroatom sources.The morphology,pore structure,elemental composition of our sample as well as its electrocatalytic performance in ORR are studied.This material has high specific surface area(855.6 m2·g-1)and hierarchical pore structure,which can offer abundant active sites and meanwhile is favorable to ion diffusion in liquid electrolyte.In addition,nitrogen and fluorine elements are successfully embedded in the carbon skeleton structure,and the resulting charge redistribution firther promotes the electrocatalytic reaction.The electrochemical measurements show that the prepared T-NFC material undergoes a four-electron ORR process and has high electrocatalytic activity and excellent stability.The assembly between carbon materials and transition metals has been proved to be an important way to improve the activity of eletrocatalysts.Using kelp as biomass template,we synthesize a carbon-based supported catalytic material with special structure via introducing a small amount of cobalt salt.Firstly,the material is co-doped with nitrogen,phosphorus and sulfur,in which the nitrogen content is about 4.18 at.%,mainly in the form of pyridinic-N and graphitic-N.Secondly,cobalt nanoparticles in this material are successfully anchored to carbon substrate and encapsulated by thin graphene layers to form onion-like structure.The electrochemical studies show that the material has good electrocatalytic activity for both ORR and TIRR.Specifically,the onset potential and the half-wave potential of this material in 0.1 M KOH electrolyte are 0.90 V and 0.74 V vs.RHE,respectively.It is noteworthy that the limited current density of the composite is evidently higher than that of commercial Pt/C at 0.6 V(5.5 mA·cm-2 vs.5.0 mA·cm-2).The DSSC assembled by Co/KCM-1000 CE exhibit a comparable PCE(7.48%)to that of Pt CE(7.64%).More importantly,the unique structure of multilayer-graphene-encapsulated cobalt nanoparticles is reliable,thus improving the electrocatalytic performance and stability for ORR and TIRR.As the size of metal active components shrinks to atomic level,the electrocatalytic performance of the material will be further improved.We prove a facile and simple pyrolysis process for making well-dispersed Co-N sites material from chlorella precusor by taking advantage of the solvation effect for NaCl/KCl eutectie system at high temperatures.The atomically isolated Co can be discerned basing on aberration-corrected high-angle annular dark-field scanning transmission electron microscopy(AC-HAADF-STEM)images.Element-selective X-ray absorption fine structure(XAFS)measurements at Co K-edge are conducted to further study the chemical state and coordination environment of Co species at atomic level.The results show that the Co species are present in the form of isolated dispersed single Co atoms,and stabilized by nitrogen in CoN4 atomic structure.Furthermore,a series of comparison samples are synthesized by changing the type,dosage and proportion of molten salts,in which cobalt nanoparticles are observed.It is proved that reasonable selection of salt system plays an important role in the preparation of single atom catalysts(SACs)by molten salt-assisted pyrolysis.The porous structure with large specific surface area and atomically dispersed Co-N active sites make the material exhibit excellent electrocatalytic activity and stability for ORR.The electrochemical measurements show that the half-wave potential and limited current density of this material are 0.83 V vs.RHE and 5.5 mA·cm-2,respectively,which are comparable to those of commercial Pt/C.Next,we report the synthesis of hierarchically porous N-doped carbon anchoring atomically dispersed Fe-N4 moieties derived from pig liver by molten salt-assisted pyrolysis.The prepared SA-Fe-PLP material is ised as effective electrode materials for DSSCs and fuel cells.The Fourier-transformed k3-weighted extended X-ray absorption fine structure(EXAFS)spectrum of SA-Fe-PLP sample exhibits one main peak at ≈1.8 A,which corresponds to the Fe-N first coordination shell.EXAFS fitting parameters suggest that the coordination number of Fe atom is about 4,indicating that the Fe atom in this material is coordinated by four N atoms to form Fe-N4 active moieties.The results show that large surface area,abundunt porous structure,high graphitization degree and more accessible atomically dispersed Fe-N species endow this material excellent activity for ORR and TIRR.