B,N Co-doped Carbon Materials Based On Polymer Precursor Design And Investigation Of Their Electrochemical Performances

Carbon-based materials have been widely investigated in energy-related field on account of their superior electrical conductivity,thermal stability,high specific surface area,environment friendly and broad source.The already commercialized carbon materials include carbon black,fullerene,carbon nanotube and graphene.However,their applications using as electrochemical catalysts or carrier materials have been greatly limited because of their highly electrical neutrality and hydrophobic surface.The introduction of heteroatoms might improve the wettability and electric charge distribution of carbon materials,thus providing sufficient active site for electrochemical reaction and the loading of noble metal nanoparticles.In view of the oxidative polymerization and condensation polymerization to form supramolecular assemblies,we have constructed the crosslinked polymer nanostructures containing N,B element,and TiO₂@polymer core-shell nanomaterials.These precursors are pyrolyzed to form N or N,B co-doped carbon and TiO₂@CNB composite materials.The structure and chemical composition of the hybrid materials could be controlled through a templating method,or tuning the building block of the polymer networks.We investigated the effect of the above mentioned factors on the improvement of the electrochemical performance.The detail contents of our research are listed as follows:?1?The poly?Schiff-base?nanoparticles were prepared by oxidative polymerization between polyethyleneimine?PEI?and 3,4-dihydroxybenzaldehyde.Similarly,the copolymerization of PEI,3,4-dihydroxybenzaldehyde and 4-formylphenylboronic acid afforded poly?Schiff-base?assemblies containing B element.These two kinds of poly?Schiff-base?assemblies firstly were carbonized at 650 ? to form N or N,B doped carbon nanosheets?N-CS or NB-CS?with the aid of water-soluble Na2SO4 template.To estimate the effect of temperature,the second carbonation was performed at 850 ?.N-CS and NB-CS obtained at 850 ? showed the improving activities of oxygen reduction reaction?ORR?and methanol tolerant performance due to the presence of heteroatoms and higher defect on carbon nanosheets.NB-CS indicated the highest ORR activities.Particularly,N-CS and NB-CS catalysts obtained at 850 ? were used as cathode of Zn-air battery,respectively.Their test results exhibited that the performances of NB-CS catalyst approximated those of commercial 20 wt%Pt/C,and its stability was higher compared to that of Pt/C.?2?Based on molecular design,a facile and efficient strategy has been developed to synthesize N,B co-doped carbon?CNB?materials through a nucleation-elongation polymerization between catechol and boromic monomer driven by B-N dative bond,and subsequent pyrolytic process.The morphology,surface area and heteroatoms content of CNB nanomaterials could be easily adjusted by altering molar ratios of catechol and boronic monomers.Also,their effects on the performance of supercapacitors and ORR were studied.The CNB materials obtained from equal molar of catechol and boronic monomers exhibited spherical shape,highest surface areas and higher disorder degree,which are favorable to shorten the diffusion pathway of ions transport,enhance the gathering of ions on the interface between electrode and electrolyte,as well as pseudocapacitance,respectively,leading to specific capacitance.The catalytic activities of CNB toward ORR gradually increased with the increase of boronic monomer.With 1:2 molar ratio of catechol monomer to boronic monomer,the obtained CNB materials using as ORR catalysts demonstrated higher electrocatalytic activity,excellent tolerance for methanol and long durability,which are comparable to that of commercial 20 wt%Pt/C.?3?The TiO₂@boronate polymer core-shell nanoparticles are created through a self-assembly route driven by the strong interaction between catechol group and TiO₂ nanoparticle,as well as B-N dative bond among polymer chains.The obtained TiO₂@-boronate polymer nanoparticles were then carbonized to transform the polymer shell into the B,N co-doped carbon layer to give TiO₂@CNB composite materials for the support of Pt nanoparticles?Pt/TiO₂@CNB?.In particular,the CNB shell thickness on TiO₂@CNB supports could be effectively tuned by controlling the total amounts of monomers.As the increasing of CNB shell thickness,the catalytic activities of Pt/TiO₂@CNB catalysts toward ORR gradually increased.This result can be attributed to the improved electronic conductivity of TiO₂ and the dispersibility of Pt nanoparticles.The ORR activity of Pt/TiO₂@CNB catalyst with a thicker CNB layer of 5.0 ± 0.5 nm was significantly higher than that of commercial Pt/C?20 wt%?catalysts.Moreover,Pt/TiO₂@CNB catalyst also showed the enhanced stability due to the strong metal-support interaction between Pt nanoparticles and TiO₂@CNB support.For the methanol oxidation reaction,the Pt/TiO₂@CNB catalyst with a moderate CNB shell thickness of 3.3 ± 0.3 nm demonstrated the highest activity and stability,which relied on the synergy between the dispersibility and stability of Pt nanoparticles,as well as the relatively easy transport of-OH species produced from TiO₂ nanoparticles to the surface of Pt nanoparticles surface.