Preparation And Properties Of Layered Double Hydroxide Modified Fluorenyl-containing Polysulfone-based Anion Exchange Membranes

Compared with proton exchange membrane fuel cells（PEMFCs）,anion exchange membrane fuel cells（AEMFCs）can avoid the use of precious metal catalysts in practical applications,and have the advantages of high electrode reactivity and low fuel permeability,which are favored by researchers.The problem of low conductivity of anion exchange membrane（AEMs）is often overcome by functionalizing polymer matrix,but this method will lead to the increase of the ion exchange capacity（IEC）and deterioration of dimensional stability of AEMs,which will greatly hinder their practical application.Therefore,how to balance the conductivity and dimensional stability of AEMs is a key problem in the structure design of AEMs.The alkaline stability and electrochemical properties of AEMs can be significantly improved by introducing inorganic nano-fillers into polymer matrix.In addition,building a crosslinked structure is also an effective way to balance the conductivity of AEMs with the dimensional expansion.Through cross-linking,a network structure is formed inside the AEMs to inhibit the excessive swelling of the membranes,thereby improving the dimensional stability and mechanical properties of the membranes.This thesis mainly focuses on the layered double hydroxides（LDHs）modified polymer composite anion exchange membranes,which is introduced from the following two parts:In the first part,MgAl-NO₃-LDH was synthesized by the co-precipitation method,and was stripped from the formamide solution to obtain 2D sheet-like LDHs.The flake LDHs was introduced into the as-synthesized fluorenyl-containing quaternized polysulfone matrix to construct LDH modified composite AEMs.It was found that a large number of hydroxyl groups and physically adsorbed water molecules on the surface of 2D flake LDHs could promote the formation of more hydrophilic ion clusters in the membrane,thus improving the ionic conductivity of composite AEMs.The ion conductivity of QPFSU-10%-LDH membrane could reach 87.4 mS cm^-1at 80 °C,which is much higher than that of QPFSU membrane(49.6 mS cm^-1).In addition,the sheet-like structure of LDHs and the physical cross-linked structure formed in the AEMs could effectively prevent the erosion of alkali,thereby improving the chemical stability of the composite AEMs.In the second part,3-（dimethylamino）-1-propylamine（DMAPA）and polyethyleneimine（PEI）were grafted to the surface of LDHs through mussel-inspired chemistry.The small molecule and macromolecule functionalized LDHs（DLDH and PLDH）as both filler and crosslinker were incorporated into the quaternized fluorenyl-containing polyethersulfone（QPFSU）matrix to get a series of new cross-linked composite anion exchange membranes（AEMs）.The as-prepared micro-crosslinked AEMs were found to form more hydrophilic conductive ion clusters by TEM observation owing to the generation of new quaternary ammonium groups during the crosslinking reaction.The micro-crosslinked AEMs have better dimensional stability while having higher OH^-conductivity.Especially the ion conductivity of CQPFSU-1%-PLDH membrane could reach 86.8 mS cm^-1at 80 °C with excellent dimensional and chemical stability（SR is 9.6% at 80 °C,21.8% decrease in OH^-conductivity at 60 °C in 1.0 M NaOH for 200 h）.In a word,the micro-crosslinking method for functionalized LDH modified AEMs achieves a balance between ion conductivity and dimensional stability of AEMs.