Exploration Of The Proton Conductive Properties Of A Series Of Covalent Organic Frameworks Built By C-N/C=N Linkages

The potential applications of crystalline proton-conducting materials in proton exchange membrane fuel cells,sensing,and other fields have recently attracted intense attention.As a new type of highly crystalline solid-state materials,covalent organic frameworks(COFs)have received continuous attention in the fields of catalysis,gas adsorption and separation as a result of their high specific surface area,large pore size,outstanding chemical and thermal stability since its appearance in 2005.Nevertheless,the research in the field of proton conduction is still very limited.In view of this,we design and synthesize a series of COFs built by C-N/C=N linkages to explore their proton conductivities.The reasons are as follows:first,the long-range ordered porous structures and modifiable frameworks of such COFs are conducive to the construction of the rich H-bonded networks facilitating the efficient proton conduction,second,the high stability of such COFs is also beneficial to optimizing their proton conductivity by post-modification and other approaches.The main work of this dissertation is as follows:Ⅰ.Five COFs,TpBth,TpTta,TaBth,DaTta and TaTta were synthesized by using 1,3,5-triformylphloroglucinol(Tp),2,5-dihydroxyterephthalaldehyde(Da),4,4’,4"(1,3,5-triazine-2,4,6-triyl)-tribenzaldehyde(Ta),4,4’,4"-(1,3,5-triazine-2,4,6-triyl)trianiline(Tta)and benzene-1,3,5-tricarbohydrazide(Bth).TpBth and TpTta have βketoenamine structures bearing a large amount of-NH groups and intramolecular Hbonds.TaBth,DaTta and TaTta are imine COFs containing a large number of-NH groups,hydroxyl groups and intramolecular H-bonds,and a great number of C=N linkers,respectively.Subsequently,TpTta,DaTta and TaTta were post-modified by introducing imidazole,triazole and histamine molecules to obtain Im@TpTta,Im@DaTta and Im@TaTta,Tra@TpTta and His@TpTta.Finally,CuCl2@TpTta3 was obtained by anchoring CuCl2 into the framework(3 represents the percentage content of metal salts in CuCl2@TpTta-3).Ⅱ.The molecular structures of the five COFs were determined by theoretical simulation,XRD and XPS tests,etc.Thermogravimetric and XRD analyses show that all the five COFs have good thermal and chemical stability.The BET surface areas and pore sizes of the five COFs were determined by N2 adsorption/desorption test,which were 51.04 m2·g-1 and 1.3 nm(TpBth),641.31 m2·g-1 and 0.66 nm(TpTta),19.15 m2·g-1 and 1.7 nm(TaBth),1057.56 m2·g-1 and 3.06 nm(DaTta)and 777.46 m2·g-1 and 2.31 nm(TaTta),respectively.The water vapor adsorption-desorption test confirmed that each COFs have a large water vapor adsorption capacity,which were 146.77(TpBth),258.53(TpTta),247.60(TaBth),482.65(DaTta)and 621.17 mg·g1(TaTta),respectively.Ⅲ.Their proton conductivity was measured under different temperatures(30-100℃)and relative humidities(RHs)(68%-98%),and showed a certain degree of temperature dependence and humidity dependence(Except for DaTta no proton conductivity).Under 100℃ and 98%RH,the highest proton conductivities are 2.11 ×10-4(TpBth),2.57 ×10-5(TpTta),1.77 ×10-3(CuCl2@TpTta-3),1.14 × 10-2(Im@TpTta),3.82 ×10-4(Tra@TpTta),3.45 × 10-3(His@TpTta),6.17× 10-6(TaBth),9.09 × 10-3(Im@DaTta),1.65 × 10-5(TaTta)and 3.73 × 10-3 S·cm-1(Im@TaTta),respectively.Note that the proton conductivities of the post-modification products are basically increased by 1-2 orders of magnitude,denoting the advantages of the post-modification method.Finally,according to the calculated activation energy values,water vapor adsorption values and the structural characteristics of these COFs,their proton conduction mechanisms were compared and analyzed.Moreover,the high electrochemical stability of the obtained COFs was tested,and the potential application value was discussed.