Synthesis And Proton Conductivity Of MOFs Based On Aromatic Polycarboxylic Ligands

In the field of fuel cell proton exchange membranes,Nafion,as a perfluoroalkane sulfonic acid polymer membrane,has been widely used commercially.Its proton conductivity can reach10^-2 to 10^-1 S/cm at 30-80℃and 90%-98%relative humidity（RH）.However,the amorphous and complex structure of the polymer makes it difficult to clarify the relationship between the material structure and the proton transport pathway,and the high cost and temperature limitations make the development and research of alternative materials necessary.Relatively speaking,MOFs materials（Metal-Organic Frameworks,MOFs）have certain advantages.Due to the nature of their crystals,their internal structure and proton conduction are more easily manifested;MOFs materials also have good chemical stability,thermal stability and modifiable pores.It is possible to artificially design a superior proton conductor that operates in a wide temperature range and humidity range through pore modification,loading of small molecules,structural phase changes and manufacturing structural defects.In this paper,starting from the synthesis of new/classical MOFs and their composite materials,we are committed to studying their applications in proton conduction.The main research content is divided into the following three sections:The first section,using four ligands as organic linkers(p-terphenyl-3,5,3’’,5’’-tetracarboxylic acid,2,5-di（3’,5’-dicarboxylphenyl）pyridine,3-（2-pyridyl）-5-（4-pyridyl）-1,2,4-triazolyl and 3-（2-pyrazinyl）-5-（4-pyridyl）-1,2,4-triazolyl and Cadmium,Cobalt and Indium as metal sources,six crystalline materials were synthesized,including Cd-MOF-1,Cd-MOF-2,Co-MOF-1,Co-MOF-2,In-MOF-1 and In-MOF-2.Especially,the crystalline materials with the same metal ion are isostructural.Tested the proton conductivity of them and found that the conductivity（σ）of Cd-MOF-2(1.12×10^-1 S/cm)is about 16.2 times that of Cd-MOF-1(6.91×10^-3 S/cm),the conductivity（σ）of Co-MOF-2(1.92×10^-6 S/cm)is about 5.6 times that of Co-MOF-1(3.40×10^-7 S/cm)and the conductivity（σ）of In-MOF-2(3.03×10^-3 S/cm)is about7.4 times that of In-MOF-1(4.11×10^-4 S/cm)at 98%RH and 60℃,the former has a smaller activation energy（E_a）simultaneously.The analysis found that the main reason for the different proton conduction properties of isomorphic materials is the N active site in the three-dimensional pore structure of the crystal,the N atom in the ligand is located in the pore,which has an important effect on the adsorption of water molecules.Through the research in this part,it is found that the distribution of polar hydrophilic groups in the pores can adsorb and solidify free water molecules,and this structure has a beneficial effect on the proton conduction performance based on hydrogen bonds.The second section,the classic material UIO-66 is synthesized from terephthalic acid and Zr salt,and used it as a carrier to load[1,2,4]-triazole small molecules to synthesize a composite material Tz@UIO-66.Tested the proton conductivity of them and found that the conductivity（σ）of Tz@UIO-66(9.30×10^-7 S/cm)is about 9.7 times that of UIO-66(9.57×10^-8 S/cm)at 98%RH and 60℃,the former has a smaller activation energy（Ea）simultaneously.The analysis found that after loading[1,2,4]-triazole molecules,its number as a proton carrier has increased,the composite material showed better proton conductivity under the condition of water participation.The third section,the classic material UIO-67 is synthesized from terephthalic acid and Zr salt,and used it as a carrier to load ionic liquid（1-ethyl-3 methylimidazole hydrogen sulfate）to synthesize a composite material[EMIm][HSO₄]@UIO-67.Tested the proton conductivity of them and found that the conductivity（σ）of[EMIm][HSO₄]@UIO-67(1.26×10^-3 S/cm)is three orders of magnitude higher than that of UIO-67(1.32×10^-6/cm)at 98%RH and 60℃,the former has a smaller activation energy（Ea）simultaneously.Analysis found that after loading the ionic liquid,the composite material showed better proton conductivity under the condition of water participation.Then,tested the fitting internal resistance,conductivity,and activation energy（E_a）of[EMIm][HSO₄]@UIO-67 under anhydrous conditions.Its conductivity（σ）up to2.27×10^-5 S/cm（at 140℃）.The analysis results show that the ionic liquid acts as a proton（ion）carrier so that the composite material exhibits proton（ion）conductivity in a wide temperature range.