Study On Covalent Organic Framework/Graft Modified Chitosan Composite Proton Exchange Membrane

Chitosan(CS),having the advantages of high methanol barrier properties,low cost,non-toxicity,and good film formation properties,is considered as a potential proton exchange membrane(PEM)material.However,the shortcomings of proton conductivity and high swelling limit application of CS in fuel cells.The methods of grafting and blending modification can improve these two properties simultaneously,but there are still problems of limited performance improvement and poor controllability of proton transport groups.In order to further solve these problems,researchers use metal organic framework(MOF)materials with regular pores and adjustable structures to improve both dimensional stability and proton conductivity,but MOF has the problem of poor hydrolytic stability.The covalent organic framework(COF)has controllable pores and good hydrolysis resistance.Therefore,this paper is mainly based on the two methods of CS graft modification and blending COF,striving to improve the proton conductivity and dimensional stability simultaneously.The specific research is as follows:1.Graft modification of CS.Firstly,butyl sultone(BS)is used to graft CS in powder(CS-BS)and solution(SCS)state,respectively.The impact of the modification method and the feed ratio on the structure and performance.Although the proton conductivity of SCS has been improved,the dimensional stability is poor.In contrast,CS-BS can improve both performances.When the feed ratio of BS to CS is 1:1(CS-BS100),the improvement effect is the most obvious.In addition,in order to provide more cross-linking sites,serine was used to modify the amination of CS(NCS)to study the effect of group type on structure and performance.The results show that the performance of NCS is greatly affected by temperature and has poor dimensional stability.At 70?,NCS bursts due to excessive water absorption,which makes it impossible to continue the test.2.Blending CS and COF.A ketoenamine covalent organic framework(TpPa-SO₃H)with the sulfonic acid groups(-SO₃H)and secondary amino(-NH-)is added to the CS matrix.The introduction of sulfonic acid groups increases the proton conductivity.Simultaneously,the acid and base groups of TpPa-SO₃H can participate in the cross-linking reaction,which improves the degree of cross-linking and limits its swelling by water absorption.The adsorption of CS molecular chains by COF channels further improves the dimensional stability.In this paper,the effect of the pore structure is verified by the method of blocking the pore channel with an inert substance(1,4-Dimethoxybenzene).3.The graft modified CS is blended with COF.The combination of graft modification and blending method further improves the proton conductivity and dimensional stability.It is found that when the mass ratio of TpPa-SO₃H to CS-BS100 is5:100,the improvement effect is the most obvious.Compared with the pure CS membrane,the proton conductivity increases by 57.0%and the volume swelling ratio decreases by51.5%,at 25?.After TpPa-SO₃H is blended with NCS,the dimensional stability is poor.Although it will not burst due to excessive water absorption,it cannot be tested due to excessive swelling.