The Synthesis And Properties Of Carbon-based Multifunctional Electrocatalytic Materials Derived From Covalently Organic Frameworks

The energy crisis and environmental pollution are two major difficulties faced by the development of modern society.Efficient and environmentally friendly fuel cell and electrolyzed water technologies are ideal for achieving clean,sustainable energy conversion and mitigating current dilemmas.Among them,the oxygen reduction reaction?ORR?occurring at the cathode of the fuel cell,the hydrogen precipitation reaction?HER?at the cathode of the water decomposition cell,and the oxygen precipitation reaction?OER?at the anode are the key electrochemical processes for achieving the above energy conversion and storage.However,these electrochemical processes need to overcome the high overpotential both in terms of thermodynamics and kinetics.Therefore,high-efficiency electrochemical catalysts are required to reduce the overpotential of reaction and increase the rate of chemical reactions.The most effective electrocatalysts currently in use are mainly noble metal catalysts such as platinum and platinum alloys?for ORR and HER?and IrO₂,RuO₂ and their doped oxides?for OER?.Due to the high price and lack of resources of precious metals,their commercial applications are greatly limited.Therefore,the development of efficient,stable,low-cost non-precious metal catalysts to replace expensive precious metal catalysts is of great significance for promoting their commercial application.Covalent organic frameworks?COFs?are a new class of porous materials.The advantages of preparing highly active non-noble metal catalysts as precursors are as follows:1)The two-dimensional or three-dimensional spatial structure of COFs materials has a large specific surface area and a rich pore structure,thereby avoiding aggregation of active components and effectively increasing the active bit density..2)Its lower skeletal density can be generated as a carbon support at the same time as the active material.3)The compositional structure of COFs can be changed to facilitate the molecular creation and functional regulation of catalytic materials.These characteristics provide the possibility for COFs materials to derive carbon-based non-precious metal materials with multifunctional electrocatalytic properties.In view of the insufficient performance of non-noble metal catalysts for multiple catalytic processes,the paper proposes to design a series of novel porous covalent organic frameworks with open metal chelating sites,and regulate their structure,composition,morphology and chelation.The type of the bit,the type of metal,and the subsequent heat treatment process,etc.,to optimize the electrocatalytic performance of the derivatized carbon-based non-noble metal.Based on the structure-activity relationship between the structural composition of the covalent organic framework and the derived carbon materials and the corresponding electrocatalytic properties,an electrocatalytic material that can be applied to multiple electrocatalytic processes at the same time is obtained with high efficiency and stability,which reduces the cost of preparation of the electrodes.The large-scale application of high-efficiency energy conversion and storage technology based on fuel cells and electrolyzed water.The research contents and results are as follows:1)A new porous covalent porphyrin framework?CPF?filled with triphenylphosphine was designed and synthesized using the rigid tetrakis?p-bromophenyl?porphyrin?TBPP?and 1,3,5-benzenetriboronic acid trivalent alcohol ester as building blocks.The carbonization of this special CPF has afforded coupled Fe₂P and Fe₄N nanoparticles embedded in N-doped carbons?Fe₂P/Fe₄N@N-doped carbons?.This CPF serves as an“all in one”precursor of Fe,N,P,and C.The porous property and solid skeleton of the CPF endow Fe₂P/Fe₄N@N-doped carbons with porous structure and a high degree of graphitization.As a result,Fe₂P/Fe₄N@N-doped carbons exhibited highly efficient multifunctional electrocatalytic performance for water splitting and oxygen electroreduction.Typically,Fe₂P/Fe₄N@C-800,obtained at a heat-treatment temperature of 800°C,showed an ORR half-wave potential of 0.80 V in alkaline media and 0.68 V in acidic media,close to that of commercial Pt/C catalysts.Fe₂P/Fe₄N@C-800 also displayed efficient OER and HER activities,comparable to other phosphide and nitride electrocatalysts.The coupled Fe₄N and Fe₂P nanoparticles embedded in carbons exert unique catalytic efficiency for water splitting and fuel cells.2)Efficient and earth-abundant materials with multifunctional electrocatalytic properties are the new darlings for developing renewable and sustainable energy conversion and storage techniques.A novel porous covalent phenanthroline framework?Fe-Phen-COFs?that involved Fe-DMSO coordination complexes was successfully synthesized using 3,8-dibromophenanthroline?3,8-DBPL?and 1,3,5-benzenetriboronicacid trivalent alcohol ester?BTA?as a rigid building block via Suzuki coupling reaction.Precursor of Fe-Phen-COFs as a self-carrier enriched with Fe,S,N and C is pyrolyzed to produce N-S-codoping carbons with embedded core-shell Fe₃C and FeS hybrid nanostructures.Electrochemical measurement displays that the FeS/Fe₃C@N-C obtained at 800°C?FeS/Fe₃C@N-C-800?exhibited efficient trifunctional catalytic activity for oxygen reduction reaction?ORR?,oxygen evolution reaction?OER?and hydrogen evolution reaction?HER?in a wide pH electrolytes.Impressively,the ORR half-potential of FeS/Fe₃C@N-C-800 reached 0.87 V in 0.1 M KOH,more positive than the previously reported Pt-free electrocatalysts.This material also shows remarkable activity for the HER in 0.5 M H₂SO₄,affording the lower overpotential of-174 m V at a current density of 10 mA cm^-2.Thermal conversion of novel iron-modified covalent phenanthroline frameworks provide an effective strategy to prepare high-performance trifunctional electrocatalytic materials.3)A novel two-dimensional porous covalent polymer?Co-SCOF?containing Salen structural units was successfully synthesized by Suzuki coupling reaction,which was used as a precursor to derive Co@N-C electrocatalytic material for ORR,OER and HER.The larger specific surface area and homogeneous micro-mesoporous structure of the prepared Co@N-C electrocatalytic materials resulted in uniform distribution of active sites and higher effective active bit density.In addition,the higher degree of graphitization of the material increases the conductivity of the catalyst.The electrocatalytic properties of the material were investigated using cyclic voltammetry and rotating disk electrodes.The test results show that Co@N-C-800 has a half-wave potential of 0.56 V under acidic conditions and a half-wave potential of 0.84V under alkaline conditions,especially,which could comparable to commercial Pt/C and displayed excellent oxygen reduction catalytic performance.In addition to this,it also exhibits an oxygen precipitation catalytic performance comparable to that of the precious metal RuO₂ and a better hydrogen precipitation catalytic performance than the similar materials,which could be an efficient multifunctional electrocatalyst.