Preparation Of Multi-pyridine Derivatives And Their Application In Redox Flow Batteries

With the development of society,energy issues and environmental problems are becoming more and more serious,so developing new energy becomes the good choice of many countries.In large-scale energy storage,using batteries to store energy has become a research hot spot.Among them,redox flow batteries attracted more attention due to their advantages in diversity of active materials,high safety,high output and high capacity.However,in the process of commercialization of redox flow battery,there are some key issues,such as expensive materials,low solubility and low working voltage,etc.Therefore,this work baesd on pyridine derivatives as active center aims to solve these problems through modified functional groups.In the first part,we use terpyridine which has strong coordination ability as ligand and Fe2+ as the metal center to prepare bis(2,2':6',2''-terpyridine)iron complex(Fe-tpy)in a facile method.In the molecule design,because of strong electron-withdrawing ability of terpyridine,redox potential of Fe-tpy is higher than redox potential of the formal Fe2+/Fe3+.Cyclic voltammetry studies exhibit that the redox potential is 1.07 V vs.Ag/Ag Cl for Fe-tpy.And we try to explain this phenomenon by DFT in detail.According to the liner sweep voltammetry,diffusion constant and rate constant are4.1×10-6 cm2/s and 2.2×10-3 cm/s,respectively.Fe-tpy/Li half battery exhibits high working voltage which is about 4.0 V,excellent cycle performance and high active material utilization ratio of the initial cycle which is 90%.Even after 250 cycles,capacity retention per cycle is about 100%.In the second part,to adjust the properties of complex,we introduce functional groups to change the electron-withdrawing ability of multi-pyridines.At the end of themulti-pyridines group,we introduce a methyl formate group which will not only increase the degree of conjugation of the ligand but also enhance the electron-withdrawing ability of the ligand by an appropriate reaction method.multi-pyridines and Fe2+ will form tri(4,4'-dimethyl-2,2'-bipyridine)iron(Fe-bpyester).After functional modification,the molecule has obvious advantages.Solubility of Fe-bpyester is 1.5 M in EC/DMC(3:7)solvent.And cyclic voltammetry studies exhibit that the redox potential is 1.32 V vs.Ag/Ag Cl for Fe-bpyester.Liner sweep voltammetry shows that the diffusion constant and rate constant are 5.2×10-6 cm2/s and4.1×10-3 cm/s,respectively.In Fe-bpyester/Li half battery test,it exhibits good cycle performance at low concentration and current.In the third work,we use the first-principle simulation based on density functional theory.When viologen is modified by different functional groups,according to the theoretical simulation of frontier molecular orbital energy level,solvation free energy and redox potential,we finally choose-(CH2)5CH3 as functional group.The solubility of1,1'-dihexyl-4,4'-bipyridinium(Viol2+)is 1 M in 1 M Li TFSI EC/DMC(3:7)solvent.Cyclic voltammetry studies exhibit that the redox potential is-0.48 V vs.Ag/Ag Cl for Viol2+.According to the liner sweep voltammetry,diffusion constant and rate constant are 3.0×10-6 cm2/s and 4.1×10-3 cm/s,respectively.Viol2+/Li half battery exhibit excellent active material utilization ratio of the initial cycle which is 84%.During the 50 cycles,coulombic efficiency is almost 100%.When anolyte Viol2+ paired with catholyte Fe-tpy to form a full battery,the working voltage is about 1.5 V and the coulombic efficiency is almost 100%.All the materials used in this work are abundant on the earth and they will not only reduce the cost of redox flow batteries but also make a great contribution.