Structure Design And Controllable Preparation Of Carbon Matrix Composites And Their Applications In The Field Of Batteries

Structural engineering carbon support in controlling the dopant concentrations or types and their homogeneity is an effective way to modulate the catalytic properties of metal–carbon materials.In this dissertation,a series of carbon-based composite nanomaterials were designed and prepared.Various characterization instruments were employed to investigate the formation mechanism of their microstructures.The resulting nanomaterials were used as electrocatalysts for fuel cells,electrode materials for lithium ion batteries and sodium ion batteries.The main results of this work are summarized as follows:（1）Controllable synthesis and electrocatalytic properties of Pt nanoparticles loaded on carbon black modified by nitrogen doped graphene quantum dotsHerein,we reported a convenient and scalable strategy to incorporate pyridinic and pyrrolic N components into carbon matrix via quantum dots anchoring pathway.After secondary Hummers oxidation of colloidal graphite,NGQDs were obtained by hydrothermal reaction in the DMF solvent.The newly designed carbon can realize accurately regulating the N contents,and importantly,expose numerous functional edge sites for growing and stabilizing Pt nanoparticles with a very narrow size distribution.The nitrogen atoms on the support surface provide abundant nucleation sites for the growth of platinum nanoparticles,effectively reducing the particle size of platinum nanoparticles in the final product（Pt/NCB）and improving their dispersion on the support surface.The results of electrocatalytic oxidation of methanol under acidic conditions showed that Pt/NCB-10 exhibited excellent electrocatalytic performance,giving an ECSA value of 76.3 m²g^-1and an I_F/I_Bof 1.63 for Pt/NCB-10,exhibiting high catalytic activity and anti-poisoning ability.Similar results were also obtained under alkaline conditions.These properties are mainly attributed to the strong interaction between nitrogen atoms on the carrier surface and Pt,which is conducive to charge transfer,thereby improving the electrocatalytic activity of Pt catalyst for methanol oxidation,enhancing its anti-poisoning performance,inhibiting the Ostwald ripening effect in the process of methanol electrocatalytic oxidation,and improving the stability of the catalyst.Theoretical evaluation and experimental validation identified that a strengthened metal-support interaction was afforded on this Pt/NCB architecture,which significantly optimizes its catalytic behaviors toward the methanol electrooxidation process.（2）Controllable synthesis and electrocatalytic properties of nitrogen doped carbon black supported platinum palladium nanomaterialsBased on the unique physical and chemical properties of Pd and Pt precursors in the ethylene glycol/hydrazine hydrate system,Pt-Pd nanoparticles can be loaded on the surface of nitrogen-doped carbon black via a simple approach with two steps for the electrocatalytic oxidation of methanol.The formation mechanism of Pd _X@Pt/NCB is speculated as follows:due to the enrichment area of platinum complex generated by electrostatic adsorption,the contact probability of platinum complex with palladium particles is significantly increased.Through the galvanic replacement reaction,platinum ions can be deposited on the surface of the palladium core,and the replaced palladium is re-deposited on the surface by hydrazine hydrate reduction.When the concentration of platinum complex near the nucleus is reduced,the platinum complex is re-enriched driven by the concentration gradient.With this process repeated,the Pd _X@Pt/NCB is eventually formed.The results of structural characterization show that Pd _X@Pt/NCB has a unique‘Pt-rich shell’structure,which enables Pd₄@Pt/NCB to enrich Pt on the surface of alloy particles in the case of low Pt content,significantly increasing the Pt concentration on the surface and improving utility rate of Pt,thus giving a high electrochemical activity specific surface area（ECSA）and excellent methanol catalytic oxidation activity and stability.The methanol oxidation catalytic process of Pd _X@Pt/NCB catalyst can be described as follows:methanol molecules have different adsorption sites on the surface of Pd _X@Pt/NCB catalyst,including two top sites（Pd-top and Pt-top）,three bridge sites（Pt₂-bridge,Pt Pd-bridge and Pd₂-bridge）and four hollow sites（Pt₂Pd-fcc,Pt Pd₂-fcc,Pt Pd₂-hcp and Pt₂Pd-hcp）.The most stable adsorption structure of the intermediate product of methanol oxidation reaction on the Pt Pd（111）crystal plane indicates that CH₃OH molecules first undergo the initial C-H bond cleavage,followed by the dehydrogenation in turn until CHO oxidation.（3）Controllable synthesis and electrocatalytic properties of PtM（M=Co,Ni）nanomaterials supported on nitrogen doped carbon materialsA bottom-up method was proposed and NCQDs were fabricated by in-situ polymerization of N-methylpyrrolidone（NMP）via ultrasonic heating method.The resulting NCQDs can be used for surface modification of carbon black and carbon nanotube.Pt Co/NCB and Pt Ni/NCNT were prepared by using the above nitrogen-doped carbon materials.It is found that NCQDs can effectively improve the electronic distribution on the surface of carbon black,provide plenty of anchor sites for the nucleation and growth of metal nanoparticlesand enhance the dispersion of metal nanoparticles as well as reduce the size of nanoparticles.The introduction of metal Co or Ni can also help nanoparticles to have smaller sizes.The Pt Co/NCB catalyst exhibited excellent electrocatalytic activity for methanol oxidation with ECSA value of 91.2 m²g^-1and I_F/I_Bvalue of 2.32.According to the electrochemical test results of Pt Ni/NCNT and comparison samples in acidic methanol solution,it is inferred that methanol molecules first dehydrogenation occurs on the metal platinum surface,and then gradually dehydrogenation generates intermediate products(-CH₂OH_ads,-CHOH_ads,-CHO_ads,-CO_ads,-COOH,)in turn,and finally decomposes on the platinum surface to form-COads,preventing the continuation of the methanol electrocatalytic reaction.With the increase of scanning potential,the nickel atoms on the surface of Pt nanoparticles located on the high-energy crystal plane were oxidized and gradually formed Ni-OH_ads.Pt-OH_adscan oxidize-CO_adsadsorbed by adjacent Pt atoms to form carbonates and then desorb them to release active sites.Under the participation of Ni-OH_ads,the highest catalytic activity and the highest current density of the corresponding potential in the CV curve can be reached,indicating that nickel is involved in the bifunctional catalysis of the system.（4）Design and construction of graphitic/amorphous hetero-phase porous carbon with lotus-leaves-like surface microstructure for high performance Li-ion and Na-ion batteriesWe demonstrate a facile two-step approach to synthesize graphitic/amorphous hetero-phase porous carbon with lotus-leaves-like surface microstructure using cellulose paper as single carbon source and K₂Fe O₄as activator.The results show that there are abundant graphitic granular bulges with an average size of 40 nm and large numbers of micropores on the surface of amorphous carbon,which is similar to the surface microstructure of lotus leaves.This surface microstructure also endows the graphitic/amorphous carbon with a large BET surface area of 372 m²g^-1and remarkable electrical conductivity of 14.3 S cm^-1.As a result of the lotus-leaves-like surface microstructure,the graphitic/amorphous carbon can be directly used as the freestanding and binder-free anode for LIBs and SIBs with significantly enhanced electrochemical performances.The graphitic/amorphous carbon is directly used as a freestanding and binder-free anode with a high surface mass of 2.6 mg cm^-2,which delivers a high specific capacity of 335.9 m Ah g^-1and 193 m Ah g^-1at 100 m A g^-1for Li-ion battery and Na-ion battery,respectively.Moreover,the graphitic/amorphous carbon freestanding anode exhibits superior long-term cycling stability（1000 cycles）and excellent rate performance for both Li-ion and Na-ion batteries.