Geometry And Electronic Regulation Of Iron Phthalocyanine Compounds To Construct High-Efficiency Oxygen Reduction Electrocatalysts

Metal-air batteries work on the principle of metal oxidation and oxygen reduction,thus exhibiting a high energy density,and their typical semi-open structure ensures a continuous supply of oxygen.Considering the cost,resource storage,self-discharge,and safety,Zinc（Zn）-air batteries are one of the most promising metal-air batteries.However,the sluggish kinetics of the oxygen reduction reaction（ORR）at the air electrode leads to a severe limitation in the power density of the Zn-air battery.Although platinum and its alloys have been used as commercial catalysts,the low reserves,high price,and susceptibility to poisoning in the air have been limiting their large-scale applications.The development of efficient,stable,and economical noble metal-free ORR catalysts is of great significance.Presently,many materials exhibit favorable ORR activity after high-temperature treatment,the pyrolysis process still faces many unavoidable drawbacks,such as large energy consumption,rigorous elemental ratio of precursors,tanglesome structural evolution,wet-chemistry etching process,ultra-low metal atom yield,etc.In contrast,pyrolysis-free Fe phthalocyanine（Fe Pc）compounds with fully conjugated electronic structure and atomically dispersed N-ligand Fe sites exhibit good ORR activity.Therefore,this work focuses on the geometric and electronic structure regulation of Fe Pc compounds to prepare materials,such as the laminar Fe Pc-based polymer（PFe Pc）nanosheets covalently and longitudinally linked to graphene hierarchical nanosheets and bay-site N-doped Fe Pc functionalized graphene.And the relationship between accurate Fe site and catalytic activity was systematically investigated,and the obtained catalytic materials were applied to liquid and flexible quasi-solid-state Zn-air batteries.The specific studies are as follows:1.Theoretical calculations,ultraviolet photoemission spectroscopy,and band gap analysis prove that the free electrons can pass from PFe Pc with the work function（?）of 4.44 e V to graphene with a deeper?of 4.62 e V.The strong electronic interactions between graphene and PFe Pc lead to a further reduction of the charge density on the Fe centres.Therefore,improving the electrical conductivity while maintaining the electronic structure of the Fe centre is of enormous significance.Guided by theoretical calculations,the laminar PFe Pc nanosheets covalently and longitudinally linked to graphene（3D-G-PFe Pc）hierarchical nanosheets were prepared by in-situ growth of PFe Pc on the surface of on o-phthalodinitrile functionalized graphene,then exfoliating of the bulk PFe Pc by KOH solution.Such structural engineering qualifies 3D-G-PFe Pc with high site utilization and rapid mass transfer.Thence,3D-G-PFe Pc demonstrates efficient ORR performance with a high specific activity of 69.31μA cm^-2,a high mass activity of 81.88 A g^-1,and a high turnover frequency（TOF）of 0.93 e s^-1 site^-1 at 0.9V vs reversible hydrogen electrode（RHE）,outperforming the lamellar PFe Pc wrapped graphene counterpart and commercial Pt/C.Systematic electrochemical analyses integrating variable-frequency square wave voltammetry and in-situ scanning electrochemical microscopy further underline the rapid kinetics of 3D-G-PFe Pc towards ORR.In addition,the liquid and flexible quasi-solid-state Zn-air batteries assembled with 3D-G-PFe Pc exhibit excellent performance with high power density and long cycle life.2.Firstly,the effect of bay-site N doping on the electronic structure of Fe centres of Fe Pc was investigated by theoretical calculations.Calculation results showed that the increase in the number of introduced N atoms leads to stretching the length of the central Fe-N,resulting in an increase in the electronic delocalization capacity of the Fe centres.Thus,the Fe azaphthalocyanines with various amounts of bay-site N-doping were covalently grafted onto the graphene surface by microwave polymerization on the surfaces of o-phthalodinitrile functionalized graphene.Benefiting from the optimized Fe centres,unique longitudinal grafting,and rational mass transfer channels,the pyrazine-based bay-site N-doped Fe Pc functionalized graphene（G-Fe Az Pc-N2）exhibits excellent ORR activity and stability.Systematic electrochemical tests show that the ORR intrinsic activity and selectivity of the bay-site N-doped Fe Pc functionalized graphene increase to some extent with the increasing amounts of bay-site N-doping.G-Fe Az Pc-N2 exhibits the most positive onset potential（E₀）and half-wave potential(E_1/2)(E₀=1.03 V vs RHE,E_1/2=0.93 V vs RHE)and TOF value of0.52 e s^-1 site^-1,and the ORR performance of G-Fe Az Pc-N2 exceeds that of commercial Pt/C catalysts under alkaline conditions.Variable frequency square wave voltammetry indicates that G-Fe Az Pc-N2 exhibits faster electron transfer efficiency relative to pyridine-based bay-site N-doped Fe Pc functionalised graphene（G-Fe Az Pc-N1）and Fe Pc functionalized graphene（G-Fe Pc）.Both theoretical calculations and experimental observations show that bay-site N-doped Fe Pc functionalized graphene is more favorable to ORR than conventional Fe Pc functionalized graphene.In addition,the obtained G-Fe Az Pc-N2 demonstrates significant potential for efficient energy storage and conversion systems and flexible energy storage applications under extreme conditions.This study elucidates the effect of the amounts of bay-site N doping on the electronic structure of Fe centres in Fe Pc functionalized graphene and establishes a clear“structure-performance relationship”for the design of efficient Fe Pc-based ORR catalysts.