Design And Synthesis Of Metal-free Conjugated Polymers For Oxygen Reduction Electrocatalytic Materials

With the development of science and technology,the demand for non-renewable energy is increasing,and the environmental pollution is becoming more and more serious.Therefore,it is inevitable to establish a green,renewable and pollution-free energy system.Because fuel cells can convert the chemical energy in fuel into electric energy,showing extremely high chemical-electrical conversion efficiency and energy density,they are considered to be one of the main energy sources for future transportation.The design of the electrocatalyst for oxygen reduction is the key factor affecting the performance of the battery.At present,as an alternative to precious metal platinum(Pt)based catalysts,metal-free carbon materials have rich sources,high stability,and excellent ORR performance,which has attracted great attention in recent years.Researchers used a variety of carbon material system,all kinds of miscellaneous dopant atoms and the different way of carbonization temperature range,through chemical doping and defects induced strategy to developt high catalytic efficiency of metal-free carbon oxygen reduction catalyst.But these methods lead to the uncertainty of the active site.The direct link of material structure and properties is not established.Therefore,the preparation of high performance and low cost electrocatalytic materials with adjustable structure and controllable active site is of great significance and challenge for the development of ORR.In this thesis,a series of conjugated polymer electrocatalytic materials containing bipyridine molecular framework were designed and developed by chemically controlled synthesis method.The main innovation points include:(1)The design idea of developing heterocyclic polymer electrocatalyst for oxygen reduction reaction with adjustable structure and controllable active site was put forward for the first time,and the new catalytic material system with high activity with clear mechanism was developed.(2)It was first discovered and proposed to optimize the catalytic activity of polymer electrocatalyst for oxygen reduction by regulating molecular weight,which provided a new idea for the study of polymer electrocatalytic system.(3)The concept of asymmetry was introduced into the structural design of oxygen reduction reaction electrocatalyst for the first time.The position and number of B?N bond and the length of alkyl side chain were adjusted by chemical synthesis strategy to achieve precise regulation of the asymmetric molecular structure.The specific research results are as follows:(1)The conjugated polymers containing phenyl(P-Ph),thiophene(P-T)and selenophene(P-Se)were obtained by cross-coupling reaction of boron-nitrogen conjugated pyridine unit with pure carbon ring(phenyl)and heterocyclic(thiophene and selenophene)units,respectively.The relationship between the molecular structure and catalytic activity of heterocyclic linear polymer electrocatalysts was studied by adjusting the types of copolymerization units.The results showed that the thiophene-containing P-T exhibited onset potential of 0.94 V and half-wave potential of 0.79 V,which were 80 m V and 120 m V higher than that of P-Se(0.86 V and 0.67 V),respectively,and both of them were higher than that of the pure carbon ring molecule P-Ph.These results indicate that the replacement of pure carbon ring with heterocyclic structure can improve its ORR performance.And for heterocyclic atoms of the same main group,S atom(P-T)with smaller atomic radius shows better catalytic activity than Se atom based polymer P-Se,which is one of the highest performance of metal-free organic polymer used as ORR catalyst at present.DFT calculations showed that the excellent ORR catalytic activity of P-T was attributed to the five-memeric thiophene heterocyclic structure,which resulted in a unique electron density distribution and increased active sites in the pyridin-based polymer skeleton.(2)By the copolymerization of bipyridine unit and thiophene unit,simple conjugated polymers with average molecular weight(M_n)of 72.35 k Da(PBIPYT_H),38.09 k Da(PBIPYT_M)and 13.11 k Da(PBIPYT_L)were synthesized by controlling the ratio of reactants.The influence of different molecular weight on catalytic activity was investigated for the first time.The results showed that PBIPYT_H with higher M_n showed better ORR performance,and the onset and half-wave potentials reached 0.92 V and 0.72V,respectively,which were much higher than that of PBIPYT_L(0.86 V and 0.63 V).Moreover,increasing the M_n of the polymer will lead to the increase of the maximum absorption peak red shift and the increase of the effective conjugate length,which will enhance the electron transfer performance.Theoretical calculations also show that the polymer PBIPYT_H with high M_n has delocalized molecular orbitals and larger dipole moments,which are also beneficial to charge separation and transfer.(3)The conventional doping and defect regulation showed high catalytic activity by changing the electronic structure of the carbon material itself.However,it is difficult to accurately control the structure of the active site.The understanding of the catalytic center and the active site is still very controversial,which limits the in-depth research on the catalytic mechanism.Based on this,we proposed for the first time to design catalytic active sites using asymmetric molecular strategy and realize charge redistribution through chemical means,which solved the disadvantage of the imprecision-difficult regulation of active sites.By introducing different amounts of boron-nitrogen coordination bonds(B?N)into the bipyridine-thiophene skeleton,the asy-PB with asymmetric main chain and the corresponding symmetric structure(sy-P)were synthesized.By changing the length of the side chain of the alkyl group,asy-PB-A and its symmetric structure(sy-PB)were synthesized.The charge distribution of the active center can be further adjusted by accurately designing the conjugated polymer catalysts with different asymmetry degrees.The results show that,compared with the symmetric sy-P and sy-PB,the asymmetric molecules asy-PB and asy-PB-A show superior ORR performance.The onset potential and half-wave potential of asy-PB-A with greater asymmetry(main chain asymmetry)are0.92 V and 0.74 V,respectively,which are higher than those of asy-PB-A with smaller asymmetry(side chain asymmetry)(0.91 V and 0.73 V).The asy-PB also showed a higher current density(67 m A cm^-2)and a maximum power density(0.132 W cm^-2)in zinc air cells.The theoretical calculation shows that the increase of dipole moment and the inhomogeneity of charge distribution caused by different degree of asymmetry make the central carbon atom of bipyridine have more efficient catalytic activity.These findings broaden our understanding of the mechanism of asymmetric ORR activity and provide new strategies for the design of high-performance non-metallic ORR electrocatalysts.