Synthesis And Application Research On Conjugated Polyphthalocyanine-based Electrocatalytic Materials

Oxygen reduction reaction(ORR)not only widely exists in the respiration of natural organisms,but is also a key component of a variety of emerging energy technologies(e.g.fuel cell,metal-air battery,and chlor-alkali electrolysis).In this context,the development of ORR is an important part for achieving sustainable energy demand.Traditionally,precious metal platinum and its alloys are used as electrocatalysts for oxygen reduction in order to improve its kinetics and reduce the reaction overpotential.However,limited by factors such as low reserves,high cost,poor selectivity,and poor stability,noble metal-based electrocatalysts and the related energy conversion devices are difficult to achieve large-scope popularization and utilization.Therefore,the study of non-precious metal catalysts to replace precious metal materials such as platinum is critical to the development and utilization of new energy conversion devices,and is also an effective way to realize the sustainable development of modern society.In this paper,we mainly focus on the polyphthalocyanine-based materials with two dimensional conjugated aromatic networks(CANs).We studied the influence of different central metals and the structural changes during low-temperature heat treatment(<500?)to figure out the active sites in the CANs.With pyrolysis-free and support-free strategy,we synthesized a series of two-dimensional CANs with ultra-thin conjugated layers.As molecular catalysts,this kind of materials can be used directly in the electrocatalysis.We are committed to improving the intrinsic activity and increasing the number of exposed active sites of electrocatalysts,by adjusting the central metals,the electronic structure,and the spatial structure of catalytic materials.Based on multiple characterization experiments and density functional theory,we provides a new way for the rational design and controllable preparation of conjugated polyphthalocyanine-based materials as non-noble metal oxygen reduction electrocatalysts.Moreover,the pyrolysis-free and support-free strategy is expected to be used for the preparation of high-performance electrocatalysts for other energy conversion and storage devices.The main conclusions are as follows:(1)A series of conjugated polyphthalocyanine materials with different central metals(Fe,Co,Ni,Cu)were prepared by solid-phase synthesis.The corresponding CANs were obtained by ball-milling method.The type of central metals has great impact on the ORR electrocatalytic activity.Among them,the samples with cobalt and iron as the central metals exhibit relatively excellent electrocatalytic performance for ORR with the half-wave potential of 0.76 V and approximative four-electron transfer processes;while the samples with nickel and copper as the central metals exhibit poor electrocatalytic activity.The difference in the electrocatalytic activity comes from the difference in the adsorption energy of oxygen molecules on the central metals.During the low-temperature heat treatment process,the peripheral carbon on the phthalocyanine ring is firstly removed,followed by the nitrogen coordinated with the central metal.When the heat treatment temperature is 300?,the Co-N4 in the CANs reach the highest content.Electrochemical tests show that the CAN-Pc(Co)-300 exhibits the most excellent catalytic activity for ORR with a half-wave potential of 0.83 V,which is equivalent to commercial Pt/C catalysts.In addtion,the heat treatment also improves the electrochemical stability of CANs.With the study of central metals and low-temperature heat treatment process,it is proved that the M-N4 is the active sites of CANs for ORR.(2)A new class of two-dimensional CANs with structurally well-defined iron and cobalt bimetallic polyphthalocyanine is constructed by a pyrolysis-free and support-free strategy.The CAN-Pc(Fe/Co)shows excellent electrocatalytic performance in alkaline electrolyte when directly used as ORR electrocatalysts.Structural characterization and theoretical calculations show that the improved electrocatalytic performance originate from the increased electron delocalization and different spin states of the contracted M-N4 structure.Benefit from the high density of M-N4 sites and the ultra-thin 2D layered structure,the zinc-air battery constructed with CAN-Pc(Fe/Co)exhibit a faster mass transfer process at a large current density,thereby achieving ultra-high mass and volume power density.(3)During the solid-phase synthesis of polyphthalocyanine,different kinds of aromatic acid anhydride are employed to successfully prepare edge-functionalized cobalt polyphthalocyanine.After ball milling,the CAN-Pc(Co)-p exhibits excellent electrochemical catalytic activity that surpasses benchmark Pt/C in alkaline solutions,along with excellent electrochemical stability and selectivity.The zinc-air battery assembled with CAN-Pc(Co)-p as the cathode catalysts exhibits ultra-high mass power density.It has been proved that the increase in steric hindrance of the edge groups can effectively increase the degree of exfoliation of the two-dimensional CANs,ledding to a higher specific surface area and electrochemically active surface area of the electrocatalyst.As a result,the number of exposed active sites greatly increased,which has a positive effect on electrocatalytic activity.(4)A self-sacrificial template synthesis strategy is employed to prepare cobalt-nitrogen-doped hierarchical porous carbon materials by pyrolysis treatment of cobalt polyphthalocyanine at different temperatures(Co-NHPC-T,T=800,900 and 1000).The prepared Co-NHPC-T electrocatalysts have excellent catalytic activity for hydrogen evolution in both alkaline and acidic solutions.Among them,the Co-NHPC-900 electrocatalyst shows the best performance with smallest overpotentials(-0.27 V and-0.23 V in alkaline and acid electrolytes to deliver 10 mA cm-2,respectively),fastest electrocatalytic kinetics and excellent catalytic durability.The excellent catalytic performance of Co-NHPC-900 is attributed to its Co-N-C sites and 3D porous structure,which can accelerate the transportation of reactants and increase the number of effective reactive sites.In addition,the high degree of graphitization for Co-NHPC-900 is also conducive to improving its catalytic durability.Due to its high-efficiency electrocatalytic performance,wide pH range and excellent catalytic durability,low-cost cobalt and nitrogen co-doped hierarchical porous carbon materials are expected to become potential substitutes for Pt-based catalysts in hydrogen evolution reactions.At the same time,this general synthesis method has greatly expanded the application of conjugated metal polyphthalocyanine materials in other electrocatalytic fields.