Constructions And Properties Of Phthalocyanine-based Covalent Organic Frameworks Bifunctional Electrocatalytic Materials

The rapid consumption of fossil fuels and the resulting increased costs are forcing researchers to look for sustainable solutions for energy collection,storage and conversion.Zinc-air batteries are considered to be one of the promising candidates for the next generation of energy devices due to their high energy density,eco-friendliness,and excellent safety.The air cathode of a zinc-air battery undergoes oxygen reduction reactions（ORR）and oxygen precipitation reactions（OER）involving gases during the discharge and charging steps,so in zinc-air batteries the air cathode is one of the most important battery components and a key factor in determining the performance of zinc-air batteries.However,the redox kinetics of ORR/OER are very slow,thus requiring efficient bifunctional electrocatalysts to promote further development of zinc-air batteries.So far,extensive research has been conducted on various advanced oxygen electrocatalysts.Nevertheless,some existing electrocatalysts have not yet been commercialized due to limitations in functionality.Generally,Pt-and Ru/Ir-based oxide significantly improve the electrocatalytic performance of ORR and OER,respectively.Yet,their practical application is hindered due to limited resources,high cost,poor durability,and poor bifunctional activity.Therefore,designing and synthesizing non-precious metal bifunctional electrocatalysts that can replace precious metals is of great significance for promoting the commercial development of zinc-air batteries.Covalent organic frameworks（COFs）/polymers（COPs）are a class of emerging crystalline porous polymers.Due to their unique characteristics,including tunable pore size,abundant accessible active sites,comprehensive controllability,and pre-designed topological structure,COFs/COPs have been widely applied in catalysis,energy storage and conversion,gas adsorption,and optoelectronics.Moreover,the incorporation of phthalocyanine（Pc-）structural units into COFs/COPs endows them with unique structural properties and excellent optical and electrical performance,leading to outstanding performance in areas such as gas storage,optoelectronics,and photo/electrocatalysis.This paper synthesized phthalocyanine-based covalent organic framework materials with open metal coordination sites by using metal phthalocyanine as a precursor and optimizing the metal type,composition,and specific building units.To address the issue of poor electrical conductivity of the material,multi-walled carbon nanotubes were selected as conductive carbon carriers.The resulting composite material exhibited excellent bifunctional electrocatalytic performance for ORR/OER and outstanding performance in practical applications in zinc-air batteries.The specific content of this paper is as follows:（1）Using multi-walled carbon nanotubes（MWCNTs）as a substrate,a novel two-dimensional skeleton-quinone-based covalent organic framework(CoFe-COF_bpda/CNT)was synthesized by coordinating the building block 2,2’-bipyridine-5,5’-diformaldehyde（BPDA）and tetraaminocobalt phthalocyanine onto the surface of the carbon nanotubes（CNTs）.This was used as a precursor to optimize the pyrolysis process to produce a Co/Fe/N-tridoped ORR/OER bifunctional electrocatalyst(CoFe-COF_bpda/CNT-700).Among them,a reasonable selection of bipyridine which can strongly cooperate with metal ions(Fe²⁺in this paper)is incorporated into the main chain of COFs,which limits the ion migration in the pyrolysis process,so as to form small Co/Fe clusters or nanoparticles rather than aggregates.And the synergistic effect of metal elements Fe and Co promoted the electrocatalytic performance of CoFe-COFbpda/CNT-700.Electrochemical performance testing shows that CoFe-COF_bpda/CNT-700 exhibits higher ORR catalytic activity than 20%Pt/C in 0.1 M KOH solution,with a half-wave potential of 0.862 V and a limiting current density of 5.199 mA cm^-2.For OER,CoFe-COF_bpda/CNT-700 shows electrocatalytic performance comparable to that of IrO₂ in 0.1M KOH solution,with an overpotential of 1.67 V at 10 mA cm^-2 and a lower Tafel slope.So,it can be concluded that CoFe-COF_bpda/CNT-700 exhibits excellent ORR/OER bifunctional performance（ΔE=0.808 V）.In addition,compared with 20%Pt/C-IrO₂,CoFe-COF_bpda/CNT-700 shows superior performance(peak power density of 208 mW cm^-2)and higher cycling stability in a zinc-air battery application.（2）Using multi-walled carbon nanotubes（MWCNTs）as the substrate,a new bimetallic phthalocyanine-based covalent organic polymer（CoFe/NDI-COP@CNT）was prepared by introducing the building unit 1,4,5,8-naphthalene tetracarboxylic dianhydride（NTCDA）and tetraaminocobalt phthalocyanine and aminoferric phthalocyanine using a simple solvent-thermal method.CoFe/NDI-COP@CNT has abundantπ-conjugated NTCDA units with oxidation-reduction activity,a high surface area,a porous structure,and coupling of bimetallic Fe/Co-N,which can effectively utilize redox active sites and exhibit excellent structural stability and rapid ion diffusion,thus exhibiting excellent ORR/OER bifunctional catalytic activity and stability even without thermal decomposition.In 0.1 M KOH alkaline solution,for ORR,the half-wave potential of CoFe/NDI-COP@CNT is as high as 0.902 V,with a peak current density of 5.362 mA cm^-2,which is better than 20%Pt/C.For OER,the catalyst shows OER activity similar to IrO₂ at 10 mA cm^-2,with an overpotential of 1.69 V.Based on this,CoFe/NDI-COP@CNT exhibits better bifunctional performance（ΔE=0.788 V）than 20%Pt/C-IrO₂.When applied in a zinc-air battery,the peak power density of CoFe/NDI-COP@CNT reaches 219 mW cm^-2,and it exhibits long-term cycle discharge stability.